*Article* **Polylactic Acid Chemical Foaming Assisted by Solid-State Processing: Solid-State Shear Pulverization and Cryogenic Milling**

**Philip R. Onffroy , Nathan T. Herrold, Harrison G. Goehrig, Kalie Yuen and Katsuyuki Wakabayashi \***

Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837-2029, USA

**\*** Correspondence: kw025@bucknell.edu; Tel.: +1-(570)-577-3778

**Abstract:** A chemical foaming process of polylactic acid (PLA) was developed via the solid-state processing methods of solid-state shear pulverization (SSSP) and cryogenic milling. Based on the ability of solid-state processing to enhance the crystallization kinetics of PLA, chemical foaming agents (CFA) are first compounded before foaming via compression molding. Specifically, the effects of the pre-foaming solid-state processing method and CFA concentration were investigated. Density reduction, mechanical properties, thermal behavior, and cell density of PLA foams are characterized. Solid-state processing of PLA before foaming greatly increases the extent of PLA foaming by achieving void fractions approximately twice that of the control foams. PLA's improved ability to crystallize is displayed through both dynamic mechanical analysis and differential scanning calorimetry. The solidstate-processed foams display superior mechanical robustness and undergo low stress relaxation. The cell density of the PLA foams also increases with solid-state processing, especially through SSSP. Additionally, crosslinking of PLA during the pre-foaming processing step is found to result in the greatest enhancement of crystallization but decreased void fraction and foam effectiveness. Overall, SSSP and cryogenic milling show significant promise in improving chemical foaming in alternative biopolymers.

**Keywords:** solid-state shear pulverization; cryogenic milling; polylactic acid; foams; processing; semicrystalline polymers; compression molding

#### **1. Introduction**

Polymer foams have widespread valuable applications, including packaging, safety padding, and insulation [1]. Polymer foams are created by incorporating pressurized gas into a molten polymer and subsequently solidifying the polymer-gas composite. In the case of semicrystalline polymers, gas is captured both by entanglement in the polymer chains and by polymer crystallites [2–4]. Today, nearly all polymers in commercial foams are derived from non-renewable fossil fuels and do not degrade easily [5]. Their ubiquitous use can be an environmental challenge. In the pursuit of developing bio-based and/or biodegradable polymers to replace petroleum-based polymers in foams, a variety of strategies have been taken, ranging from plant-based materials to microorganism-produced polymers [6–9]. One of the most studied bio-based polymers is polylactic acid (PLA), a condensation polymer derived through the fermentation of sucrose from cornstarch into lactic acid [10–12]. PLA, known to be more compostable than petroleum-based plastics in accordance with ASTM D6691 [13,14], is becoming a prevalent sustainable material of choice in biomedical, packaging, and additive manufacturing applications [11].

Polymer foams can be created through physical pressurized gas injection or by incorporating gas generated from chemical reactions. To date, most PLA-foaming studies with high levels of success are limited to the former physical foaming method [2,15,16]. However, physical foaming tends to produce unevenly distributed foams with less versatility in product shape than chemical foaming and can be expensive due to the need for high-pressure gas sources and precision transport systems [17,18]. Chemical foaming,

**Citation:** Onffroy, P.R.; Herrold, N.T.; Goehrig, H.G.; Yuen, K.; Wakabayashi, K. Polylactic Acid Chemical Foaming Assisted by Solid-State Processing: Solid-State Shear Pulverization and Cryogenic Milling. *Polymers* **2022**, *14*, 4480. https://doi.org/10.3390/ polym14214480

Academic Editor: Cristina Cazan

Received: 30 September 2022 Accepted: 17 October 2022 Published: 22 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

which uses small molecular additives known as chemical foaming agents (CFAs) that break down into gas when heated above their activation temperature [2,19], can circumvent the physical foaming concerns and enable a thoroughly consistent, in situ foam through common polymer processing methods, such as extrusion, injection molding, and compression molding [3]. However, the limited success of chemical foaming of PLA has been reported to date [18,20,21].

Creating an effective PLA foam is challenging partly due to PLA's slow crystallization kinetics, which allows the foaming gas to predominantly escape from the system rather than being secured by PLA's crystalizing chains as the temperature cools to shape a product [2,22]. Additionally, PLA is shear-sensitive in the melt state, suffering from molecular, viscosity, and physical property degradation compared to petroleum-based analogs [2]. Incorporating other polymers, such as polyethylene into a blend with PLA overcomes these challenges, but significantly lessens the sustainable nature of the output [23]. Another potential solution is crosslinking PLA chains via chemical crosslinking agents; however, the crosslinking often still must be accompanied by blending PLA with another polymer such as poly(butylene succinate) to achieve an adequate foam [24].

In previous studies, an alternative processing method called solid-state shear pulverization (SSSP) has shown promising results in increasing the crystallization kinetics of PLA [25–27], which is considered key for better control of the foam cell structure [2]. SSSP is a form of twin screw extrusion conducted under chilled conditions, and it has previously been used to modify homopolymers [28,29], compatibilize polymer blends [30–32], disperse additives [33,34], and create nanocomposites [35–39]. The foci of SSSP have been at the forefront of polymer sustainability, ranging from mechanical recycling [40,41] and natural fiber/renewable feedstock composites [42–44], to PLA/starch blends [45] and PLA crystallinity studies [25,26]. Specifically, the mechanochemistry of SSSP leads to scission and imperfections in PLA chains, which increase the material's rate of nucleation and growth [25]. Another solid-state processing method called cryogenic milling (cryomilling), has also been employed alongside SSSP [46–48] and has contributed to previous sustainable PLA processing research [49,50].

This is the first study in the literature to use SSSP and cryomilling to facilitate the chemical foaming of PLA, aiming to develop a more sustainable biopolymer foam. PLA foams are prepared by first incorporating a CFA with neat polymer pellets via a solidstate process, and subsequently compression-molding it into a specimen. These SSSP and cryomill techniques are compared to a control prepared via manual blending. An additional set of crosslinked PLA foams processed through cryomilling is introduced to investigate the combination of crosslinking and solid-state processing. Void fractions for the different sets of PLA foams are first measured. The foam morphology characterization through scanning electron microscopy (SEM) imaging is followed by thermal analysis of the foams via differential scanning calorimetry (DSC) and mechanical property evaluation with static compression testing and dynamic mechanical analysis (DMA). The processingstructure-property relationships of pre-foaming solid-state compounding of the CFA and the biopolymer are explored.

#### **2. Materials and Methods**

#### *2.1. Materials*

The PLA material used in this study was Ingeo Biopolymer 2003D with an L-lactide to D-lactide ratio of 96/4, supplied by NatureWorks, LLC [51]. This extrusion grade material is reported by the manufacturer to have a density of 1.24 g/cm<sup>3</sup> , a melt flow rate of 6 g/10 min at 210 ◦C, tensile yield strength of 60 MPa, and a heat distortion temperature of 55 ◦C [51]. Due to PLA's hygroscopic nature, it was dried for at least 2 h at 90 ◦C in a Moretto XD1 Dryer before all procedures.

The CFA used for this study was an azodicarbonamide (ADCA)-based CFA custom formulated by Avient Corporation. This CFA came in viscous liquid form and its formulation consisted of 54 wt% ADCA with the remainder being carrier, active, surfactant, and

clay thickener. This ADCA CFA activates and releases nitrogen (N2), carbon monoxide (CO), and ammonia (NH3) in an exothermic event [18]. The activation temperature of approximately 205 ◦C, determined via in-house thermogravimetric analysis testing, is key, as the activation temperature is higher than PLA's measured melting point of around 170 ◦C; the higher activation temperature ensures that the PLA is molten and able to contain and dissolve the gas [52]. The crosslinking agents used in the final portion of this study were triallyl isocyanurate (TAIC) and dicumyl peroxide (DCP) [53], purchased from Sigma Aldrich. (CO), and ammonia (NH3) in an exothermic event [18]. The activation temperature of approximately 205 °C, determined via in-house thermogravimetric analysis testing, is key, as the activation temperature is higher than PLA's measured melting point of around 170 °C; the higher activation temperature ensures that the PLA is molten and able to contain and dissolve the gas [52]. The crosslinking agents used in the final portion of this study were triallyl isocyanurate (TAIC) and dicumyl peroxide (DCP) [53], purchased from Sigma Aldrich.

of 55 °C [51]. Due to PLA's hygroscopic nature, it was dried for at least 2 h at 90 °C in a

The CFA used for this study was an azodicarbonamide (ADCA)-based CFA custom formulated by Avient Corporation. This CFA came in viscous liquid form and its formulation consisted of 54 wt% ADCA with the remainder being carrier, active, surfactant, and clay thickener. This ADCA CFA activates and releases nitrogen (N2), carbon monoxide

*Polymers* **2022**, *14*, x FOR PEER REVIEW 3 of 15

Moretto XD1 Dryer before all procedures.

#### *2.2. Pre-Foaming Processing Methods 2.2. Pre-Foaming Processing Methods*  Both SSSP and cryomilling were used as the primary processing methods for com-

Both SSSP and cryomilling were used as the primary processing methods for compounding CFA with PLA before foaming. PLA pellets manually blended with CFA were designated as the third control formulation, modeling a traditional process where polymer pellets and additives were directly fed into a molding machine without any solid-state preprocessing step. The fourth formulation of crosslinked PLA was prepared by first crosslinking PLA through single-screw melt extrusion followed by cryomill-compounding with CFA. For the balance of this paper, the SSSP-processed foam set will be referred to as SP, the cryomill-processed set as CM, the melt blended control set as CT, and the crosslinked/cryomill-processed set as XL. For each of the four sets of pre-foam processing modes, a CFA content parametric study was carried out to determine the relationships between the weight percentage of CFA and the physical properties of the resulting foams. The nominal concentrations of CFA for the six series tested were 0.5 wt%, 1.0 wt%, 2.0 wt%, 3.5 wt%, 5.0 wt%, and 6.5 wt%. pounding CFA with PLA before foaming. PLA pellets manually blended with CFA were designated as the third control formulation, modeling a traditional process where polymer pellets and additives were directly fed into a molding machine without any solidstate preprocessing step. The fourth formulation of crosslinked PLA was prepared by first crosslinking PLA through single-screw melt extrusion followed by cryomill-compounding with CFA. For the balance of this paper, the SSSP-processed foam set will be referred to as SP, the cryomill-processed set as CM, the melt blended control set as CT, and the crosslinked/cryomill-processed set as XL. For each of the four sets of pre-foam processing modes, a CFA content parametric study was carried out to determine the relationships between the weight percentage of CFA and the physical properties of the resulting foams. The nominal concentrations of CFA for the six series tested were 0.5 wt%, 1.0 wt%, 2.0 wt%, 3.5 wt%, 5.0 wt%, and 6.5 wt%.

For foam set SP, CFA was compounded with PLA pellets through SSSP. The SSSP processing method is based on a KraussMaffei Berstorff ZE25-UTX intermeshing, corotating twin screw extruder with a screw diameter of 25 mm and the length-to-diameter ratio of 34. The extruder barrels were chilled to low temperatures using a circulation of −12 ◦C-ethylene glycol/water solution, provided by Budzar Industries BWA-AC10 chiller. Figure 1 outlines the screw configuration, taken from a previous study [25], which employed a balance of harsh and mild screw elements to disperse the CFA additives while preventing premature polymer decomposition during the SSSP process. PLA pellets were manually coated in the liquid CFA and fed into the SSSP barrel using a Brabender Technologie Volumetric RotoTube feeder with the assistance of pressurized air through the center of the feeder hopper to ensure a continuous flow of 50 g/h. The SSSP screw speed was set to 200 rpm based on a previous parametric study on SSSP processing-structure-property relationships [54]. For foam set SP, CFA was compounded with PLA pellets through SSSP. The SSSP processing method is based on a KraussMaffei Berstorff ZE25-UTX intermeshing, co-rotating twin screw extruder with a screw diameter of 25 mm and the length-to-diameter ratio of 34. The extruder barrels were chilled to low temperatures using a circulation of −12 °C-ethylene glycol/water solution, provided by Budzar Industries BWA-AC10 chiller. Figure 1 outlines the screw configuration, taken from a previous study [25], which employed a balance of harsh and mild screw elements to disperse the CFA additives while preventing premature polymer decomposition during the SSSP process. PLA pellets were manually coated in the liquid CFA and fed into the SSSP barrel using a Brabender Technologie Volumetric RotoTube feeder with the assistance of pressurized air through the center of the feeder hopper to ensure a continuous flow of 50 g/h. The SSSP screw speed was set to 200 rpm based on a previous parametric study on SSSP processing-structureproperty relationships [54].

**Figure 1.** The SSSP screw design used in this study contains 9 bilobe kneading discs distributed among conveying elements for size reduction, mixing, and pulverization purposes. **Figure 1.** The SSSP screw design used in this study contains 9 bilobe kneading discs distributed among conveying elements for size reduction, mixing, and pulverization purposes.

For foam set CM, the cryomill processing method achieved a similar low-temperature mechanochemical compounding effect as SSSP, in a batch setting [55]. Each cryomill run was composed of a 12-g total sample of PLA with CFA, run through a SPEX SamplePrep 6870 Freezer/Mill. The cryomill procedure started with a 15-min cooldown period followed by 5 cycles of 4 min of pulverization and 4 min of cooldown between each cycle. After the final cycle, the sample contents were thawed to room temperature and stored.

For control foam set CT, PLA pellets were manually blended with CFA with a 20-g total sample size in a glass container. This mixture was prepared and stored at room temperature.

Foam set XL followed a two-step preparation process. The first part involved meltcompounding PLA pellets with 0.1 wt% TAIC and 0.1 wt% DCP crosslinking agents through a Killion Model KLB075 single-screw extruder. The screw speed was set to 15 RPM, and an extruder temperature of 180 ◦C was used because that is above both the melting temperature of PLA and the activation temperature of the crosslinking agents. The crosslinked polymer extrudate was cooled to ambient temperature and pelletized. The second step was to compound the crosslinked PLA with CFA in a cryomill in the same manner as foam set CM.

#### *2.3. Compression Molding Foaming Process*

After the four pre-foaming preparation methods were completed, the foaming procedure was carried out in a consistent fashion using compression molding. A 5.0 g sample of each formulation was added into a custom, cylindrical stainless steel mold with a 7.6 cm inner diameter and 6.4 cm height. The mold was loaded into an automated Carver Auto-Four 30-15 HC Press. Under an initial 5 MPa of pressure, the sample was pressed at 220 ◦C and held isothermally for 8 min; during this process, pressure increase was observed inside the mold as CFA activated between 190–210 ◦C. The pressure was released, and the mold was cooled at an average rate of approximately 10 ◦C/min on a steel cooling surface with convective air cooling from two AC Infinity Model AI-MPF120P2 dual fans. After at least 20 min of cooling and resting, the foam sample was removed from the mold and stored.

#### *2.4. Foam Analysis Methods*

The density reduction measurements of the foam samples were conducted following the ASTM D792 standard using an OHAUS Density Determination Kit and Adventurer Model AX324 scale. The density of the sample was first calculated as:

$$
\rho\_{foam} = \frac{A}{A - B} (\rho\_0 - \rho\_L) + \rho\_L \tag{1}
$$

where *A* is the weight of the sample in air, *B* is the weight of the sample in water, air density (*ρL*) equals 0.00119 g/cm<sup>3</sup> , and water density (*ρ*0) equals 0.997 g/cm<sup>3</sup> at 25 ◦C. The void fraction (*φ*) of the foam samples, which is the volume expansion ratio of the material caused by foaming [21,56], was then calculated using the following equation:

$$\phi = \frac{V\_{\text{void}}}{V\_{\text{sample}}} = 1 - \frac{\rho\_{foam}}{\rho\_{PLA}} \tag{2}$$

In Equation (2), *ρf oam* is the density of the foam sample calculated via Equation (1), PLA density (*ρPLA*) equals 1.24 g/cm<sup>3</sup> , *Vvoid* represents the volume taken up by gas cells inside the sample, and *Vsample* is the overall volume of the sample.

Scanning electron microscopy (SEM) was conducted using a Hitachi SU 5000 Field Emission Scanning Electron Microscope. Surfaces of cryogenically fractured PLA foam samples were sputter-coated with gold using a Denton Desk IV. SEM images were taken under a high vacuum with an electron beam voltage of 3.0 kV and at a magnification of ×70. These SEM images were used to quantitatively compare the gas cell distribution in samples using the software program ImageJ [57]. The cell density (*NC*), defined as the number of cells per volume of non-foamed base PLA material, was calculated for each SEM micrograph following Equation (3):

$$N\_{\mathbb{C}} = (\frac{n \ast M^2}{A})^{\frac{3}{2}} \ast \frac{1}{1 - \phi} \tag{3}$$

In Equation (3), *φ* is the void fraction, *n* is the number of cells counted in each micrograph image, *A* is the cross-sectional area of the foam in the image, and *M* is the magnification factor of the image.

Differential scanning calorimetry (DSC) was performed on foam samples using a TA Instruments Q2000, calibrated with indium. A standard heat-cool-reheat run between 0 ◦C and 220 ◦C was programmed with a ramp rate of 10 ◦C/min. Differential scanning calorimetry (DSC) was performed on foam samples using a TA Instruments Q2000, calibrated with indium. A standard heat-cool-reheat run between 0 °C and 220 °C was programmed with a ramp rate of 10 °C/min.

1− (3)

number of cells per volume of non-foamed base PLA material, was calculated for each

∗<sup>ଶ</sup> <sup>ሻ</sup> ଷ <sup>ଶ</sup> <sup>∗</sup> <sup>1</sup>

In Equation 3, is the void fraction, *n* is the number of cells counted in each micrograph image, *A* is the cross-sectional area of the foam in the image, and *M* is the magnification

= ሺ

Dynamic mechanical analysis (DMA) was conducted in a compression mode where a 12.7 mm diameter cylindrical cut-out of each foam sample was placed on a custom stainless-steel platform and subjected to oscillatory compressive stress by a cylindrical steel plunger. The compression deformation mode was chosen because it most closely resembles the mechanical strain a polymer foam material would undergo in applications such as packaging. Each compression DMA run was conducted at an oscillation frequency of 1 Hz in a dynamic temperature ramp mode between −20 ◦C and 170 ◦C. Additionally, static compression and stress relaxation runs for each sample were conducted at room temperature. The compression strain rate was 0.003/s and the initial static load for stress relaxation was set at 200 kPa [58]. Dynamic mechanical analysis (DMA) was conducted in a compression mode where a 12.7 mm diameter cylindrical cut-out of each foam sample was placed on a custom stainless-steel platform and subjected to oscillatory compressive stress by a cylindrical steel plunger. The compression deformation mode was chosen because it most closely resembles the mechanical strain a polymer foam material would undergo in applications such as packaging. Each compression DMA run was conducted at an oscillation frequency of 1 Hz in a dynamic temperature ramp mode between −20 °C and 170 °C. Additionally, static compression and stress relaxation runs for each sample were conducted at room temperature. The compression strain rate was 0.003/s and the initial static load for stress relaxation was set at 200 kPa [58].

#### **3. Results and Discussion 3. Results and Discussion**

*Polymers* **2022**, *14*, x FOR PEER REVIEW 5 of 15

SEM micrograph following Equation (3):

factor of the image.

#### *3.1. Density Reduction 3.1. Density Reduction*

Density reduction measurements were conducted on the compression-molded samples to obtain average *φ* values, which are presented as a function of CFA concentration in Figure 2. As the CFA content increased, the void fraction increased for all samples up to a maximum plateau value. The plateau in each set indicates that there is an upper limit to the number of gas cells that a compression-molded PLA foam can successfully contain upon foam expansion, even as an increasing amount of gas is released inside the polymer melt. The different plateau values shown in Figure 2 for each of the four sets reveal that the solid-state processing of PLA before foaming makes a significant difference to the maximum void fraction a foam sample can achieve with compression molding. The manually blended control set CT reached a void fraction plateau of about 35% at a relatively low CFA concentration of 1.0 wt%, whereas the SSSP and cryomill sets (SP and CM, respectively) reached void fraction plateaus of approximately 70% at a CFA concentration of 5.0 wt%. Density reduction measurements were conducted on the compression-molded samples to obtain average values, which are presented as a function of CFA concentration in Figure 2. As the CFA content increased, the void fraction increased for all samples up to a maximum plateau value. The plateau in each set indicates that there is an upper limit to the number of gas cells that a compression-molded PLA foam can successfully contain upon foam expansion, even as an increasing amount of gas is released inside the polymer melt. The different plateau values shown in Figure 2 for each of the four sets reveal that the solid-state processing of PLA before foaming makes a significant difference to the maximum void fraction a foam sample can achieve with compression molding. The manually blended control set CT reached a void fraction plateau of about 35% at a relatively low CFA concentration of 1.0 wt%, whereas the SSSP and cryomill sets (SP and CM, respectively) reached void fraction plateaus of approximately 70% at a CFA concentration of 5.0 wt%.

**Figure 2.** Void fraction of SSSP-processed (SP), cryomilled (CM), manually blended control (CT), and crosslinked/cryomilled (XL) PLA foam samples. **Figure 2.** Void fraction of SSSP-processed (SP), cryomilled (CM), manually blended control (CT), and crosslinked/cryomilled (XL) PLA foam samples.

Polymer foaming technology often employs crosslinking to effectively capture gas cells and impart prototypical slow recovery foam behavior [59,60]. This study included a crosslinked analog of set CM to investigate the combination of crosslinking and cryogenic milling. Figure 2 reveals that the crosslinked foam set XL resulted in significantly lower void fraction values than set CM. PLA is a relatively brittle polymer at room temperature, and crosslinking may have constrained the chains of the material to such a great extent

that fewer gas cells could be contained, as the XL set reached a void fraction plateau of approximately 30%. that fewer gas cells could be contained, as the XL set reached a void fraction plateau of approximately 30%.

Polymer foaming technology often employs crosslinking to effectively capture gas cells and impart prototypical slow recovery foam behavior [59,60]. This study included a crosslinked analog of set CM to investigate the combination of crosslinking and cryogenic milling. Figure 2 reveals that the crosslinked foam set XL resulted in significantly lower void fraction values than set CM. PLA is a relatively brittle polymer at room temperature, and crosslinking may have constrained the chains of the material to such a great extent

*Polymers* **2022**, *14*, x FOR PEER REVIEW 6 of 15

#### *3.2. Foam Morphology 3.2. Foam Morphology*

The cross-sectional gas cell morphology was evaluated with SEM, for different concentrations of CFA across four pre-foaming processing methods. Figure 3 displays how the four processing methods resulted in different gas cell shapes and size distributions, in a representative comparison of the 6.5 wt% CFA loading series. The non-crosslinked, solid-state-processed foams in Figure 3(SP) and Figure 3(CM) showed similarly high areas of coverage by closed cells. The SP samples displayed smaller gas cells than the CM samples across different CFA concentrations despite similar cell area coverage and *φ* values from an earlier analysis. When comparing the SP and CM samples to the CT sample in Figure 3, it appears the control foam also exhibited closed cells. However, cell area coverage in CT foams was lower than those of the solid-state-processed foams, revealing one major reason why the control foams had lower void fraction values. In addition, the cells in the CT foams were concentrated in clusters around the sample rather than distributed evenly, for example, clustering at the top of Figure 3(CT). The cross-sectional gas cell morphology was evaluated with SEM, for different concentrations of CFA across four pre-foaming processing methods. Figure 3 displays how the four processing methods resulted in different gas cell shapes and size distributions, in a representative comparison of the 6.5 wt% CFA loading series. The non-crosslinked, solid-state-processed foams in Figures 3(SP) and 3(CM) showed similarly high areas of coverage by closed cells. The SP samples displayed smaller gas cells than the CM samples across different CFA concentrations despite similar cell area coverage and values from an earlier analysis. When comparing the SP and CM samples to the CT sample in Figure 3, it appears the control foam also exhibited closed cells. However, cell area coverage in CT foams was lower than those of the solid-state-processed foams, revealing one major reason why the control foams had lower void fraction values. In addition, the cells in the CT foams were concentrated in clusters around the sample rather than distributed evenly, for example, clustering at the top of Figure 3(CT).

**Figure 3.** SEM images of SSSP-processed (SP), cryomilled (CM), manually blended control (CT), and crosslinked/cryomilled (XL) PLA foams with 6.5 wt% CFA. **Figure 3.** SEM images of SSSP-processed (SP), cryomilled (CM), manually blended control (CT), and crosslinked/cryomilled (XL) PLA foams with 6.5 wt% CFA.

A combination of the results so far indicates that PLA compounded with CFA in SP and CM methods were able to be compression-molded into consistent and physically expanded foams, containing a greater amount of gas in closed cells, compared to the CT foams. One explanation is that the mechanochemical modification of the PLA chains enabled enhanced crystallization kinetics [25], leading to a higher effect in trapping gas in closed cells upon solidification. Another explanation is that the intimate and homogeneous mixing in SSSP and cryomilling increased the CFA distribution and its contact level with PLA prior to the foaming process [36,37,42,47]. A combination of the results so far indicates that PLA compounded with CFA in SP and CM methods were able to be compression-molded into consistent and physically expanded foams, containing a greater amount of gas in closed cells, compared to the CT foams. One explanation is that the mechanochemical modification of the PLA chains enabled enhanced crystallization kinetics [25], leading to a higher effect in trapping gas in closed cells upon solidification. Another explanation is that the intimate and homogeneous mixing in SSSP and cryomilling increased the CFA distribution and its contact level with PLA prior to the foaming process [36,37,42,47].

The crosslinked foams, such as shown in Figure 3(XL), displayed cross-sectional morphology significantly different from the other sets in that open cells were formed instead of closed cells. Open cell structure is a common characteristic of polymer foams with high void fractions [61]. Despite the apparent network structure and moderate open cell concentration, the XL foams did not expand in the mold as much as the SP and CM foams. This indicates that the crosslinking agents made the material too strong and tough to be able to contain as much gas as the other foams, contributing to its significantly lower *φ* values [62]. able to contain as much gas as the other foams, contributing to its significantly lower values [62]. Quantitatively, the cell density for each foam was calculated using Equation (3). The

The crosslinked foams, such as shown in Figure 3(XL), displayed cross-sectional morphology significantly different from the other sets in that open cells were formed instead of closed cells. Open cell structure is a common characteristic of polymer foams with high void fractions [61]. Despite the apparent network structure and moderate open cell concentration, the XL foams did not expand in the mold as much as the SP and CM foams. This indicates that the crosslinking agents made the material too strong and tough to be

Quantitatively, the cell density for each foam was calculated using Equation (3). The average *N<sup>C</sup>* values are plotted as a function of CFA concentration in Figure 4. For a given CFA concentration, the cell density values generally reflect the visual trends observed in the SEM images. However, the standard deviation ranges overlap for many data points in Figure 4, and therefore we refrain from making definitive remarks but rather provide general observed trends. average *NC* values are plotted as a function of CFA concentration in Figure 4. For a given CFA concentration, the cell density values generally reflect the visual trends observed in the SEM images. However, the standard deviation ranges overlap for many data points in Figure 4, and therefore we refrain from making definitive remarks but rather provide general observed trends.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 7 of 15

**Figure 4.** Calculated cell density versus CFA concentration for SP, CM, CT, and XL foams. The error bars indicate one standard deviation. The data points that do not show error bars have deviation ranges smaller than the point size. **Figure 4.** Calculated cell density versus CFA concentration for SP, CM, CT, and XL foams. The error bars indicate one standard deviation. The data points that do not show error bars have deviation ranges smaller than the point size.

Both sets of solid-state-processed foams displayed greater cell density than the CT and XL foam sets in most cases, confirming the enhanced ability of the pre-foaming solidstate processing to generate and capture the gas in closed cells. The SP foams tended to have greater gas cell density than CM foams, particularly at low CFA concentrations. At higher CFA content, the CM set began to achieve similarly high *NC* values as the SP set. Perhaps the shearing nature of SSSP is more conducive to dispersing CFA than impactbased cryomilling at low concentrations. As CFA concentration increases, this nuanced difference becomes less relevant because the amount of gas being released is high and the effect of enhanced solidification rate dominates the level of CFA dispersion in these materials. The XL samples experienced the most inconsistent trend, with the majority of *NC* values remaining low except for high jumps observed for 3.5–6.5 wt% CFA. The inconsistency of the XL foam results can be attributed to the open-cell nature of the crosslinked foams causing less consistent cell formation compared to the other closed cell foams. Both sets of solid-state-processed foams displayed greater cell density than the CT and XL foam sets in most cases, confirming the enhanced ability of the pre-foaming solid-state processing to generate and capture the gas in closed cells. The SP foams tended to have greater gas cell density than CM foams, particularly at low CFA concentrations. At higher CFA content, the CM set began to achieve similarly high *N<sup>C</sup>* values as the SP set. Perhaps the shearing nature of SSSP is more conducive to dispersing CFA than impact-based cryomilling at low concentrations. As CFA concentration increases, this nuanced difference becomes less relevant because the amount of gas being released is high and the effect of enhanced solidification rate dominates the level of CFA dispersion in these materials. The XL samples experienced the most inconsistent trend, with the majority of *N<sup>C</sup>* values remaining low except for high jumps observed for 3.5–6.5 wt% CFA. The inconsistency of the XL foam results can be attributed to the open-cell nature of the crosslinked foams causing less consistent cell formation compared to the other closed cell foams.

Lastly, it might be expected that the plateauing trend of in Figure 2 would correspond to a similarly plateauing trend of cell density in Figure 4. This may be occurring for the SP and CT sets but is not the case for the CM set. Perhaps the fine powder nature of CM formulations after cryomilling enabled gas cell formation more consistent with CFA content than other sets. In contrast, the XL samples showed a delayed increase in cell density while is relatively steady in Figure 3. Further investigation on the relationship between and cell density is warranted, but one definitive takeaway is that consistent PLA foams with practical density reduction are reliably achievable with CFA concentrations at Lastly, it might be expected that the plateauing trend of *φ* in Figure 2 would correspond to a similarly plateauing trend of cell density in Figure 4. This may be occurring for the SP and CT sets but is not the case for the CM set. Perhaps the fine powder nature of CM formulations after cryomilling enabled gas cell formation more consistent with CFA content than other sets. In contrast, the XL samples showed a delayed increase in cell density while *φ* is relatively steady in Figure 3. Further investigation on the relationship between *φ* and cell density is warranted, but one definitive takeaway is that consistent PLA foams with practical density reduction are reliably achievable with CFA concentrations at around 5–6.5 wt%. These CFA loadings are considerably higher than a typical industry polymer foam CFA concentration of around 1.0 wt% [63].

#### *3.3. Differential Scanning Calorimetry*

We turned to thermal characterization by DSC to examine the PLA crystal development that occurred in the compression-molded foams. Figure 5 compares the thermograms of the first heat of as-compression-molded foam samples of the 6.5 wt% CFA concentration

*Polymers* **2022**, *14*, x FOR PEER REVIEW 8 of 15

polymer foam CFA concentration of around 1.0 wt% [63].

*3.3. Differential Scanning Calorimetry* 

around 5–6.5 wt%. These CFA loadings are considerably higher than a typical industry

We turned to thermal characterization by DSC to examine the PLA crystal development that occurred in the compression-molded foams. Figure 5 compares the thermograms of the first heat of as-compression-molded foam samples of the 6.5 wt% CFA concentration grouping. The key thermal events occurring during the first heat curves are the glass transition at *Tg* = 60 °C, cold crystallization at *Tcc* = 100 °C, and melting at *Tm* = 150 °C. The thermogram shape of the XL foams at *Tg* is a typical step change expected for reversible glass transition whereas the CT, CM, and SP foams record a *Tg* overshoot peak in their thermograms at ~60 °C. These overshoots were caused by the devitrification of additional mobile amorphous phase PLA in the sample after cooling during the foaming

Significant cold crystallization exotherms occurred beginning at ~100 °C for the SP and CM foams. Conversely, the CT samples displayed only a shallow cold crystallization peak, and the peak shifted to a higher temperature range than the solid-state-processed samples. The XL samples showed no cold crystallization. These findings indicate that solid-state-processed foams show a higher potential to crystallize whereas the CT and XL samples either have a lower capacity to crystallize or have already crystallized to their full extent before 100 °C. The melt peak characteristics reveal more about which of these is occurring. Clear differences can immediately be seen between the different melting endotherms at *Tm* ~150 °C. The melting peaks for the solid-state-processed samples were much larger than for the CT sample. This indicates that the solid-state-processed samples underwent a significant level of total crystallization prior to melting, whereas the CT samples were less able to crystallize comprehensively, resulting in a small melting peak. The double peak nature of the SP sample melt peaks has been attributed to reflecting the recrystallization and reorganization process in a previous study [25]. Despite the XL samples also displaying little evidence of cold crystallization, they still had large melting peaks, indicating that any crystallization in the XL foams happened during the initial foaming

process [25]. The reasoning behind this will be explained later in this section.

grouping. The key thermal events occurring during the first heat curves are the glass transition at *T<sup>g</sup>* = 60 ◦C, cold crystallization at *Tcc* = 100 ◦C, and melting at *T<sup>m</sup>* = 150 ◦C. process rather than the DSC's first heat run. These contrastive thermogram features between SP, CM, CT, and XL samples were observed at all CFA concentrations.

**Figure 5.** DSC first heat curves for the SP, CM, CT, and XL foams with 6.5 wt% CFA (exo up). **Figure 5.** DSC first heat curves for the SP, CM, CT, and XL foams with 6.5 wt% CFA (exo up).

The differences in the relative latent heat of melting (∆*Hm*) vs. cold crystallization (∆*Hcc*) are worth further investigating. Table 1 lists the two latent heats from the first heat thermograms, and further calculates the effective latent heat of "melt crystallization" (∆*Hmc*) during the respective foaming process, i.e., the measure of the extent that the PLA The thermogram shape of the XL foams at *T<sup>g</sup>* is a typical step change expected for reversible glass transition whereas the CT, CM, and SP foams record a *T<sup>g</sup>* overshoot peak in their thermograms at ~60 ◦C. These overshoots were caused by the devitrification of additional mobile amorphous phase PLA in the sample after cooling during the foaming process [25]. The reasoning behind this will be explained later in this section.

Significant cold crystallization exotherms occurred beginning at ~100 ◦C for the SP and CM foams. Conversely, the CT samples displayed only a shallow cold crystallization peak, and the peak shifted to a higher temperature range than the solid-state-processed samples. The XL samples showed no cold crystallization. These findings indicate that solid-stateprocessed foams show a higher potential to crystallize whereas the CT and XL samples either have a lower capacity to crystallize or have already crystallized to their full extent before 100 ◦C. The melt peak characteristics reveal more about which of these is occurring. Clear differences can immediately be seen between the different melting endotherms at *T<sup>m</sup>* ~150 ◦C. The melting peaks for the solid-state-processed samples were much larger than for the CT sample. This indicates that the solid-state-processed samples underwent a significant level of total crystallization prior to melting, whereas the CT samples were less able to crystallize comprehensively, resulting in a small melting peak. The double peak nature of the SP sample melt peaks has been attributed to reflecting the recrystallization and reorganization process in a previous study [25]. Despite the XL samples also displaying little evidence of cold crystallization, they still had large melting peaks, indicating that any crystallization in the XL foams happened during the initial foaming process rather than the DSC's first heat run. These contrastive thermogram features between SP, CM, CT, and XL samples were observed at all CFA concentrations.

The differences in the relative latent heat of melting (∆*Hm*) vs. cold crystallization (∆*Hcc*) are worth further investigating. Table 1 lists the two latent heats from the first heat thermograms, and further calculates the effective latent heat of "melt crystallization" (∆*Hmc*) during the respective foaming process, i.e., the measure of the extent that the PLA was able to crystallize during the cooling step of the compression molding after the CFA has been activated [25]. This value was calculated by subtracting ∆*Hcc* from ∆*Hm*. Table 1 reveals that in every case, ∆*Hmc*, ∆*Hcc*, and ∆*H<sup>m</sup>* are all greater for the SP and CM samples than for the CT samples. The ∆*Hmc* values for SP and CM foams were recorded in the range of 5.0–8.5 J/g, compared to 0.5–3.0 J/g for CT foams. While there appears to be no significant correlation between CFA concentration and enthalpy values, the higher ranges in SP and CM confirm substantial PLA crystallite development during their cooling process

in the compression mold, which led to more effective containment of chemically induced gas in the foams.

**Table 1.** Crystallization characteristics extrapolated from DSC first heat curves, with the top number in each cell being ∆*Hm*, the middle, subtracted number being ∆*Hcc*, and the resulting number (shaded) being ∆*Hmc*. Note: all values are reported in J/g.


Interestingly, the XL foams did not display large cold crystallization peaks but still exhibited significantly large melting curves, suggesting that much of the crystallization occurred during foam cooling, to an extent even larger than those of SP and CM samples. Despite the significant crystallization enhancement caused by crosslinking, Section 3.1 showed how the XL foam void fraction values remain significantly lower than the noncrosslinked analogs in the CM set. This suggests that while a moderate amount of crystallization during foaming is desirable, exemplified by solid-state processing cases, excessive crystallization inhibits foaming by over-stiffening the PLA matrix. A similar inhibition of PLA foaming by excessive crystallinity has also previously been observed in physical foaming contexts [62,64].

#### *3.4. Dynamic Mechanical Analysis Results*

Temperature ramp DMA was conducted to observe the mechanical properties of the foams as a function of temperature, as well as to verify the crystallization behavior that was inferred from the DSC study above. We first focus on the changes in storage modulus (*E'*) in Figure 6 based on a representative set. The 6.5 wt% CFA samples were selected because their substantial density reductions provided the highest contrast of DMA curves between the four contrastive foam samples within the series. The same thermal transition events and relative *E'* position trends were observed in other series of CFA concentrations. We limit the following discussion to qualitative comparisons.

The stiffness of the PLA foams remained relatively constant from room temperature up to the *T<sup>g</sup>* ~60 ◦C, above which the foams lose stiffness as their chains become mobile. Note that the relative *E'* positions of the four samples switch between the pre- and post-*T<sup>g</sup>* plateaus in Figure 6. In the region between glass and cold crystallization temperatures, the SP and CM samples exhibited lower *E'* values. With higher void fraction and cell density, the two solid-state-processed foams displayed a suppressed solid-like behavior, especially because their crystallinity during this region was only modest. In contrast, the XL sample did not experience a drastic decrease in stiffness after *Tg*, as it was supported by the crosslinks and significant crystallinity that had already developed.

**Figure 6.** Representative plots of E' versus temperature for SP, CM, CT, and XL foams with 6.5 wt% CFA. **Figure 6.** Representative plots of E' versus temperature for SP, CM, CT, and XL foams with 6.5 wt% CFA.

The stiffness of the PLA foams remained relatively constant from room temperature up to the *Tg* ~60 °C, above which the foams lose stiffness as their chains become mobile. Note that the relative *E'* positions of the four samples switch between the pre- and post-*Tg* plateaus in Figure 6. In the region between glass and cold crystallization temperatures, the SP and CM samples exhibited lower *E'* values. With higher void fraction and cell density, the two solid-state-processed foams displayed a suppressed solid-like behavior, especially because their crystallinity during this region was only modest. In contrast, the XL The 100–120 ◦C region corresponds to cold crystallization. A gradual modulus recovery correlates with the increasing number of developing crystals, as the crystalline phase is stiffer than the amorphous component above the *T<sup>g</sup>* [65]. Figure 6 reveals that the solid-state-processed SP and CM samples experienced significant cold crystallization, to a level higher than any CT stiffness value and even surpassing their own original *E'*, having raised their crystalline potential [25]. On the other hand, the CT and XL samples showed little to no cold crystallization, corroborating the DSC results.

sample did not experience a drastic decrease in stiffness after *Tg*, as it was supported by the crosslinks and significant crystallinity that had already developed. The 100–120 °C region corresponds to cold crystallization. A gradual modulus recovery correlates with the increasing number of developing crystals, as the crystalline phase is stiffer than the amorphous component above the *Tg* [65]. Figure 6 reveals that the solidstate-processed SP and CM samples experienced significant cold crystallization, to a level higher than any CT stiffness value and even surpassing their own original *E'*, having raised their crystalline potential [25]. On the other hand, the CT and XL samples showed little to no cold crystallization, corroborating the DSC results. Often, one of the most valuable properties of foam material is its ability to absorb energy [66,67]. The tan *δ* plot of the temperature ramp DMA can be used to observe the material damping factor of the samples [68,69]. The higher the value of tan *δ* at a given temperature, the more the material will absorb energy [68]. Figure 7 compares tan *δ* curves between the 1.0 wt% and 6.5 wt% CFA series of the four foam sets. A major peak in tan *δ* at *Tg* associated with PLA devitrification was observed in each sample, as expected from a previous study on compression DMA of polymer foams [70]. The height of the tan *δ* Often, one of the most valuable properties of foam material is its ability to absorb energy [66,67]. The tan *δ* plot of the temperature ramp DMA can be used to observe the material damping factor of the samples [68,69]. The higher the value of tan *δ* at a given temperature, the more the material will absorb energy [68]. Figure 7 compares tan *δ* curves between the 1.0 wt% and 6.5 wt% CFA series of the four foam sets. A major peak in tan *δ* at *T<sup>g</sup>* associated with PLA devitrification was observed in each sample, as expected from a previous study on compression DMA of polymer foams [70]. The height of the tan *δ* peak varied slightly depending on the pre-foaming processing method. The most noticeable difference was between the XL foams and the SP, CM, and CT foams, which suggests that the XL foams remained too rigid through the *T<sup>g</sup>* and deviated from a typical foam behavior in its mechanical response to the oscillatory motion. A peak height difference was also observed between the 6.5 wt% and 1.0 wt% samples of a given foam set. The fact that the higher CFA content foams constantly displayed a higher damping factor in the SP, CM, and CT sets confirms the effectiveness of employing higher CFA loading in preparing PLA foams. Again, the XL foams did not follow the same trend because their foam structure and rigidity properties are fundamentally different from the other sets.

peak varied slightly depending on the pre-foaming processing method. The most noticeable difference was between the XL foams and the SP, CM, and CT foams, which suggests that the XL foams remained too rigid through the *Tg* and deviated from a typical foam behavior in its mechanical response to the oscillatory motion. A peak height difference was also observed between the 6.5 wt% and 1.0 wt% samples of a given foam set. The fact that the higher CFA content foams constantly displayed a higher damping factor in the SP, CM, and CT sets confirms the effectiveness of employing higher CFA loading in preparing PLA foams. Again, the XL foams did not follow the same trend because their foam structure and rigidity properties are fundamentally different from the other sets. A second, shallower, and broader tan *δ* peak appeared around 100 ◦C most distinctly in the solid-state-processed samples. The CT foams showed a continuously gradual increase without peaking, while the XL foams did not show any evidence of a significant second peak. As discussed above with *E'* transitions, the SP, CM, and CT samples developed more liquid-like and damping behaviors above their devitrification points. This typical and desired foam property caused tan *δ* to remain high until cold crystallization occurs in the respective sample, at which point stiff solid-like behavior returns and lowers the damping factor. The CT curve continued to display high tan *δ* due to a lack of cold crystallization. The XL foams were already stiff and crystalline before *Tg*, causing the tan *δ* curve to remain low.

**Figure 7.** Representative plots of tan *δ* versus temperature for the four contrastive foams with 1.0 wt% and 6.5 wt% CFA. **Figure 7.** Representative plots of tan *δ* versus temperature for the four contrastive foams with 1.0 wt% and 6.5 wt% CFA.

#### A second, shallower, and broader tan *δ* peak appeared around 100 °C most distinctly *3.5. Static Compression Testing*

in the solid-state-processed samples. The CT foams showed a continuously gradual increase without peaking, while the XL foams did not show any evidence of a significant second peak. As discussed above with *E'* transitions, the SP, CM, and CT samples developed more liquid-like and damping behaviors above their devitrification points. This typical and desired foam property caused tan *δ* to remain high until cold crystallization occurs in the respective sample, at which point stiff solid-like behavior returns and lowers the damping factor. The CT curve continued to display high tan *δ* due to a lack of cold crystallization. The XL foams were already stiff and crystalline before *Tg*, causing the tan *δ* curve to remain low. *3.5. Static Compression Testing*  As polymer foams are likely used in practical applications at ambient temperatures, room-temperature static compression tests were carried out to determine the stress-strain relationships and stress relaxation tendencies. Based on the representative foam set of 6.5 As polymer foams are likely used in practical applications at ambient temperatures, room-temperature static compression tests were carried out to determine the stress-strain relationships and stress relaxation tendencies. Based on the representative foam set of 6.5 wt% CFA, Figure 8a reveals that the SP and CM samples both displayed stress-strain relationships with higher slopes than the CT set. The solid-state-processed PLA foam samples were significantly stiffer and more mechanically robust than manually blended foam samples at room temperature, due to their higher as-molded crystallinity, as observed earlier by the ∆*Hmc* values. The stiffness difference may also reflect that in the foam morphology, as Section 3.2 established that the solid-state-processed samples displayed higher cell density and more spatially consistent closed cell structure. The XL samples exhibited high stiffness because of their enhanced crystallinity through crosslinking and cryomilling; crosslinking has previously been shown to make PLA stiffer [53]. However, the XL sample's stress-strain curve had a lower slope than the SSSP and cryomill stress-strain plots perhaps due to the open-cell nature of the crosslinked foam [71]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 12 of 15

context of static loading, foams that undergo less stress relaxation are better able to retain their initial shape after being compressed to some reasonable deformation level, enabling sustainable usage in various applications. For certain packaging applications, an ideal foam can be defined as one that continuously applies a consistent force on an object [72], **Figure 8.** Results from the static compression testing displaying the (**a**) stress-stain curves (truncated at a compressive stress of 200 kPa) and (**b**) stress relaxation curves of the four foam samples with 6.5 wt% CFA at room temperature. **Figure 8.** Results from the static compression testing displaying the (**a**) stress-stain curves (truncated at a compressive stress of 200 kPa) and (**b**) stress relaxation curves of the four foam samples with 6.5 wt% CFA at room temperature.

which the solid-state-processed foams exhibit. While the XL foams had high crystalline properties, their open cell foam structure nonetheless caused significant stress relaxation to occur. **4. Conclusions**  An effective PLA chemical foaming method has been established through solid-state processing, via either SSSP or cryomilling, followed by compression molding. Solid-stateprocessing PLA achieved foams with void fraction values approximately double those of The stress relaxation results in Figure 8b reveal that SP and CM foams relaxed to a lesser extent than CT foams when subject to a constant initial static load of 200 kPa. In the context of static loading, foams that undergo less stress relaxation are better able to retain their initial shape after being compressed to some reasonable deformation level, enabling

the control foams (70% versus 35%) and consistently higher cell density. Though unusual, a relatively high CFA loading of around 6 wt% is recommended with solid-state processing, as increasing CFA concentration resulted in a corresponding increase in void frac-

pulverization effects of solid-state processing resulted in enhanced melt crystallization and cold crystallization enthalpies. Additionally, solid-state-processed foams proved more robust and displayed less stress relaxation than crosslinked and control foam sets, enabling better reusability for sustainable applications. The crosslinked foams, which were also solid-state processed, achieved the highest level of melt crystallization but achieved low void fraction values (~30%) and inconsistent cell density, disproving that combining solid-state processing and crosslinking is an effective strategy for PLA foam

In the future, a better understanding of the optimal compression molding foaming heating and cooling rates should be established to ensure the most effective foaming method for PLA foams. One potential route is an in-depth investigation into the interplay between foaming and crystallization at different solidification rates. The chemical foaming method developed in this study complements existing physical foaming methods for PLA and contributes toward the widespread application of sustainable foams in our soci-

**Author Contributions:** Conceptualization, P.R.O. and K.W.; methodology, P.R.O., H.G.G., and K.W.; software, P.R.O., K.Y., and K.W.; validation, N.T.H. and K.W.; formal analysis, P.R.O. and K.W.; investigation, P.R.O., N.T.H., H.G.G., K.Y., and K.W.; resources, K.W.; data curation, P.R.O. and K.W.; writing—original draft preparation, P.R.O. and K.W.; writing—review and editing, N.T.H., H.G.G., and K.Y.; visualization, P.R.O.; supervision, K.W.; project administration, K.W.; funding acquisition, K.W. All authors have read and agreed to the published version of the manu-

**Funding:** The SSSP instrument was funded by a National Science Foundation Major Research In-

development.

ety.

script.

strumentation grant, CMMI-0820993.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable

sustainable usage in various applications. For certain packaging applications, an ideal foam can be defined as one that continuously applies a consistent force on an object [72], which the solid-state-processed foams exhibit. While the XL foams had high crystalline properties, their open cell foam structure nonetheless caused significant stress relaxation to occur.

#### **4. Conclusions**

An effective PLA chemical foaming method has been established through solid-state processing, via either SSSP or cryomilling, followed by compression molding. Solidstate-processing PLA achieved foams with void fraction values approximately double those of the control foams (70% versus 35%) and consistently higher cell density. Though unusual, a relatively high CFA loading of around 6 wt% is recommended with solid-state processing, as increasing CFA concentration resulted in a corresponding increase in void fraction up until a plateau value. DSC and DMA findings indicated that the shearing and pulverization effects of solid-state processing resulted in enhanced melt crystallization and cold crystallization enthalpies. Additionally, solid-state-processed foams proved more robust and displayed less stress relaxation than crosslinked and control foam sets, enabling better reusability for sustainable applications. The crosslinked foams, which were also solidstate processed, achieved the highest level of melt crystallization but achieved low void fraction values (~30%) and inconsistent cell density, disproving that combining solid-state processing and crosslinking is an effective strategy for PLA foam development.

In the future, a better understanding of the optimal compression molding foaming heating and cooling rates should be established to ensure the most effective foaming method for PLA foams. One potential route is an in-depth investigation into the interplay between foaming and crystallization at different solidification rates. The chemical foaming method developed in this study complements existing physical foaming methods for PLA and contributes toward the widespread application of sustainable foams in our society.

**Author Contributions:** Conceptualization, P.R.O. and K.W.; methodology, P.R.O., H.G.G. and K.W.; software, P.R.O., K.Y. and K.W.; validation, N.T.H. and K.W.; formal analysis, P.R.O. and K.W.; investigation, P.R.O., N.T.H., H.G.G., K.Y. and K.W.; resources, K.W.; data curation, P.R.O. and K.W.; writing—original draft preparation, P.R.O. and K.W.; writing—review and editing, N.T.H., H.G.G. and K.Y.; visualization, P.R.O.; supervision, K.W.; project administration, K.W.; funding acquisition, K.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** The SSSP instrument was funded by a National Science Foundation Major Research Instrumentation grant, CMMI-0820993.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Acknowledgments:** The authors are grateful to the Avient Corporation for providing the chemical foaming agent. PRO was supported by the Presidential Scholars Program at Bucknell University. We additionally acknowledge our Bucknell colleagues Seth A. Pletcher and Prism Li for preliminary background experiments, Ethan Blumer for valuable technical discussions, and Diane Hall, Dan Johnson, and Tim Baker for supporting the laboratory instruments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Review* **Microencapsulation of Essential Oils: A Review**

**Vânia Isabel Sousa , Joana Filipa Parente , Juliana Filipa Marques, Marta Adriana Forte and Carlos José Tavares \***

> Physics Center of Minho and Porto Universities (CF-UM-PT), Campus of Azurém, University of Minho, 4804-533 Guimarães, Portugal; vaniafernandesousa@gmail.com (V.I.S.); joanacp\_17@hotmail.com (J.F.P.); juliana.g.marques@hotmail.com (J.F.M.); martadrianaff@gmail.com (M.A.F.)

**\*** Correspondence: ctavares@fisica.uminho.pt

**Abstract:** Essential oils (EOs) are complex mixtures of volatile compounds extracted from different parts of plants by different methods. There is a large diversity of these natural substances with varying properties that lead to their common use in several areas. The agrochemical, pharmaceutical, medical, food, and textile industry, as well as cosmetic and hygiene applications are some of the areas where EOs are widely included. To overcome the limitation of EOs being highly volatile and reactive, microencapsulation has become one of the preferred methods to retain and control these compounds. This review explores the techniques for extracting essential oils from aromatic plant matter. Microencapsulation strategies and the available technologies are also reviewed, along with an in-depth overview of the current research and application of microencapsulated EOs.

**Keywords:** essential oils; extraction techniques; microencapsulation; controlled release; microcapsules; pharmacology

#### **1. Introduction**

Essential oils (EOs) are liquid products present in plants and can be defined as complex natural mixtures of volatile secondary lipophilic metabolites that give plants and spices their essence and colour [1,2]. These compounds can be obtained by hydrodistillation, solvent extraction, and supercritical CO<sup>2</sup> extraction, among other methods that can also be used for essential oil extraction [1]. These oils can be extracted from different parts of the plant, such as the flowers, leaves, stems, roots, fruits, and bark, and have different biological and pharmaceutical properties [3]. Due to their versatile nature, the oils can be utilised for several purposes, from contact toxicant and fumigant to attractive or repellent applications [4]. However, there are many factors that affect the chemical composition of essential oils, including genetic variation, type or variety of plants, plant nutrition, fertilizer applications, geographical location of the plant, climate, seasonal variations, stress during growth or maturation, as well as post-harvest drying and storage.

EOs that have antimicrobial properties are alternatives to the use of antibiotics and chemical additives [5]. As they have been used worldwide in many industries, their prices differ due to the supply of raw materials, issues related to harvesting, climate factors, and extraction yields. Some of these EOs also have antioxidant properties, with studies reporting that EOs from celery, citronella, cloves, oregano, parsley, tarragon, and thyme seeds were able to inhibit 50% of the 2,2-diphenylpicliryl-hydrazil (DPPH) radical elimination activity [6]. The application of EOs as antioxidants has been evaluated in different types of foods, and research is currently being conducted to optimise the process [7].

Essential oils have gained renewed interest in several areas over the years. Its use was expanded to the medical field due to its biocidal activities (bactericides, viricides, and fungicides) and medicinal properties [8]. The use of natural compounds has become popular in the food industry, with EOs being used as preservatives and food additives due to their antioxidant and antimicrobial properties and pleasant flavour. EOs are included

**Citation:** Sousa, V.I.; Parente, J.F.; Marques, J.F.; Forte, M.A.; Tavares, C.J. Microencapsulation of Essential Oils: A Review. *Polymers* **2022**, *14*, 1730. https://doi.org/10.3390/ polym14091730

Academic Editor: Cristina Cazan

Received: 18 March 2022 Accepted: 18 April 2022 Published: 23 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in the composition of many dosage forms in pharmaceutical products. Studies have been carried out on the many biological activities of essential oils (Figure 1) and their components, and particular interests have also been established to elucidate their modes of action, allowing for the improved and targeted intervention in new drugs [9]. carried out on the many biological activities of essential oils (Figure 1) and their compo‐ nents, and particular interests have also been established to elucidate their modes of ac‐ tion, allowing for the improved and targeted intervention in new drugs [9].

EOs are unstable and highly susceptible to changes caused by external factors, such as light, temperature, oxygen, and humidity [10]. The high volatility and reactivity of these compounds represent challenges for the application of essential oils in several in‐ dustries [11]. To overcome these limitations, the microencapsulation technique is often used to maintain the functional and biological characteristics of these compounds and to EOs are unstable and highly susceptible to changes caused by external factors, such as light, temperature, oxygen, and humidity [10]. The high volatility and reactivity of these compounds represent challenges for the application of essential oils in several industries [11]. To overcome these limitations, the microencapsulation technique is often used to maintain the functional and biological characteristics of these compounds and to control their release [12].

control their release [12]. Microencapsulation is a technology based on the coating of solid, liquid, or gaseous particles through an encapsulating agent that acts as a barrier, completely isolating the core material from the external environment [13]. Most microcapsules have a diameter within the range 1–1000 μm [14]. The shell material can be a film of a natural, semi‐syn‐ thetic, or synthetic polymer and its choice has a key role in the stability of core material [15]. Arabic gum, agar, alginate, proteins, and dextrins are some of the materials used as Microencapsulation is a technology based on the coating of solid, liquid, or gaseous particles through an encapsulating agent that acts as a barrier, completely isolating the core material from the external environment [13]. Most microcapsules have a diameter within the range 1–1000 µm [14]. The shell material can be a film of a natural, semi-synthetic, or synthetic polymer and its choice has a key role in the stability of core material [15]. Arabic gum, agar, alginate, proteins, and dextrins are some of the materials used as encapsulating agents in the microencapsulation process [16].

encapsulating agents in the microencapsulation process [16]. Due to the enormous interest of the scientific community and the industry in the mi‐ croencapsulation of active substances, several microencapsulation methods have been de‐ veloped over time. Encapsulation processes are usually divided into three main catego‐ ries: physicochemical, mechanical, and chemical processes [17]. In this review article, some of these methods, which are used in EO microencapsulation, are described and an outlook of scientific works developed in this area is approached. Due to the enormous interest of the scientific community and the industry in the microencapsulation of active substances, several microencapsulation methods have been developed over time. Encapsulation processes are usually divided into three main categories: physicochemical, mechanical, and chemical processes [17]. In this review article, some of these methods, which are used in EO microencapsulation, are described and an outlook of scientific works developed in this area is approached.

#### **2. Essential Oils**

**2. Essential Oils** Essential oils are defined, according to the *European Pharmacopoeia,* as an "odorous product, usually of complex composition, obtained from a botanically defined plant raw material by steam distillation, dry distillation, or a suitable mechanical process without heating. EOs are usually separated from the aqueous phase by a physical process that does not significantly affect their composition" [18]. EOs are extracted from aromatic plant ma‐ terials such as oily aromatic liquids, and they can be biosynthesised in different plant or‐ gans as secondary metabolites, such as flowers, herbs, buds, leaves, fruits, branches, bark, Essential oils are defined, according to the *European Pharmacopoeia,* as an "odorous product, usually of complex composition, obtained from a botanically defined plant raw material by steam distillation, dry distillation, or a suitable mechanical process without heating. EOs are usually separated from the aqueous phase by a physical process that does not significantly affect their composition" [18]. EOs are extracted from aromatic plant materials such as oily aromatic liquids, and they can be biosynthesised in different plant organs as secondary metabolites, such as flowers, herbs, buds, leaves, fruits, branches, bark, zest, seeds, wood, rhizomes, and roots.

zest, seeds, wood, rhizomes, and roots. Essential oils are complex mixtures of highly volatile aromatic compounds named Essential oils are complex mixtures of highly volatile aromatic compounds named after the plant from which they are derived. Within the different species of plants, only 10%

after the plant from which they are derived. Within the different species of plants, only

contain EOs and are called aromatic plants. These natural products exert the function of protecting the plants, guaranteeing the growth of the plant and the propagation of species. Essential oil provides the essence, odour, or flavour of the plant and some of the functions that it performs in plants can also be made in living organisms [19]. EOs are generally liquid at room temperature and are hydrophobic (immiscible with water) and lipophilic (miscible with other oils and organic solvents) substances [20]. In general, essential oils are a mixture of compounds with their own physicochemical characteristics that, when combined, give the oil a particular odour. The different aroma of oils is fundamentally due to variations in the volatility and relative concentration of its constituents [21]. cies. Essential oil provides the essence, odour, or flavour of the plant and some of the functions that it performs in plants can also be made in living organisms [19]. EOs are generally liquid at room temperature and are hydrophobic (immiscible with water) andlipophilic (miscible with other oils and organic solvents) substances [20]. In general, es‐ sential oils are a mixture of compounds with their own physicochemical characteristics that, when combined, give the oil a particular odour. The different aroma of oils is funda‐ mentally due to variations in the volatility and relative concentration of its constituents [21].

10% contain EOs and are called aromatic plants. These natural products exert the function of protecting the plants, guaranteeing the growth of the plant and the propagation of spe‐

*Polymers* **2022**, *14*, x FOR PEER REVIEW 3 of 45

#### *2.1. Chemistry of Essential Oils 2.1. Chemistry of Essential Oils*

The chemical composition of EOs can be complex due to the number of different components, which can have promising chemical and biological properties [22]. Essential oils are complex mixtures that can contain over 300 different compounds. Most EOs are characterised by two or three main components in reasonably high concentrations (20–70%) compared to other components present in small amounts [8]. The organic constituents have a low molecular weight, and their vapour pressure (at atmospheric pressure and at room temperature) is high enough for them to be partially in vapour state [23]. The chemical composition of EOs can be complex due to the number of different components, which can have promising chemical and biological properties [22]. Essential oils are complex mixtures that can contain over 300 different compounds. Most EOs are characterised by two or three main components in reasonably high concentrations (20– 70%) compared to other components present in small amounts [8]. The organic constitu‐ ents have a low molecular weight, and their vapour pressure (at atmospheric pressure and at room temperature) is high enough for them to be partially in vapour state [23]. Chemically, EOs mainly belong to two classes of compounds: terpenes and phe‐

Chemically, EOs mainly belong to two classes of compounds: terpenes and phenylpropanoids (Table 1). The terpene family is predominant, and phenylpropanoids, when they appear, are responsible for the characteristic odour and taste [24]. nylpropanoids (Table 1). The terpene family is predominant, and phenylpropanoids, when they appear, are responsible for the characteristic odour and taste [24].

#### 2.1.1. Terpenoids 2.1.1. Terpenoids

Terpenes, also called terpenoids, constitute the largest class of natural products with several structurally diversified known compounds [25]. Their structures contain carbon skeletons and are formed by isoprene units, being classified according to the number of these units that compose their structure. They can be classified as hemiterpenes (1 isoprene unit; 5 carbons), monoterpenes (2 isoprene units; 10 carbons), sesquiterpenes (3 isoprene units; 15 carbons), diterpenes (4 isoprene units; 20 carbons), triterpenes (6 isoprene units; 30 carbons), and tetraterpenes (8 isoprene units; 40 carbons), among others. Monoterpenes and sesquiterpenes are mostly found in volatile essential oils. Terpenes can present aromatic, aliphatic, and cyclic structures and can contain oxygen atoms, being called terpenoids (Figure 2) [26]. Terpenes, also called terpenoids, constitute the largest class of natural products with several structurally diversified known compounds [25]. Their structures contain carbon skeletons and are formed by isoprene units, being classified according to the number of these units that compose their structure. They can be classified as hemiterpenes (1 iso‐ prene unit; 5 carbons), monoterpenes (2 isoprene units; 10 carbons), sesquiterpenes (3 iso‐ prene units; 15 carbons), diterpenes (4 isoprene units; 20 carbons), triterpenes (6 isoprene units; 30 carbons), and tetraterpenes (8 isoprene units; 40 carbons), among others. Mono‐ terpenes and sesquiterpenes are mostly found in volatile essential oils. Terpenes can pre‐ sent aromatic, aliphatic, and cyclic structures and can contain oxygen atoms, being called terpenoids (Figure 2) [26].

**Figure 2.** Structures of terpenes and terpenoids: acyclic monoterpenes (**2a**), cyclic monoterpenes (**2b**), diterpenes (**2c**), triterpenes (**2d**), and terpenoids (**2e**). **Figure 2.** Structures of terpenes and terpenoids: acyclic monoterpenes (**2a**), cyclic monoterpenes (**2b**), diterpenes (**2c**), triterpenes (**2d**), and terpenoids (**2e**).

#### 2.1.2. Phenylpropanoids

Phenylpropanoids are natural substances commonly found in plants and consist of a six-carbon aromatic ring joined to a three-carbon side chain. This side chain contains a double bond and the aromatic ring may be substituted. These compounds are biosynthesised

from shikimic acid, which forms the basic units of cinnamic and *p*-coumaric acids. These units, through enzymatic reductions, produce propenylbenzenes and/or allylbenzenes and, through oxidations with side chain degradation, generate aromatic aldehydes [27,28].


**Table 1.** Composition of compounds found in essential oils [29].

#### *2.2. Extraction Methods*

Aromatic herbs or parts thereof, such as leaves, flowers, bark, seeds, and fruits, are subjected to extraction processes after being collected at specific stages of maturity and stored under controlled conditions (light, temperature, and humidity).

Extraction techniques are essentially divided into classical and conventional methods and innovative methods. Classical methods are based on the distillation of water by heating, to extract the EOs from the plant matter. Hydrodistillation, steam distillation, hydrodiffusion, organic solvent extraction, and cold pressing are some of these methods. New extraction technologies have been developed in order to overcome some of the disadvantages of conventional methods. Methods such as ultrasound-assisted extraction and microwave-assisted extraction use energy sources that make the process more environmentally friendly. On the other hand, methods such as supercritical fluid extraction and subcritical liquid extraction allow the non-polar components from the material to be extracted [30].

#### 2.2.1. Hydrodistillation

Hydrodistillation is the oldest and simplest method for extracting OEs. This method is characterised by direct contact between the solvent and the plant material, that is, the raw material is submerged in boiling water (Figure 3) [31]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 5 of 45

> In this procedure, the cell walls are broken, and the oil is evaporated together with the water, and then condensed into a mixture of water vapour and volatile compounds of

> Steam distillation is one of the preferred methods of extracting EOs. The extraction procedure is based on the same principles as hydrodistillation. The difference essentially lies in the absence of contact between the substrate to be extracted and the water, which

> The sample is placed in a column where the bottom part is connected to a flask with water under heating (Figure 4). The top part is connected to a condenser, where the steam produced passes through the sample, taking essential oils to the condenser. This process causes the condensation of the water–oil mixture, and this mixture can be separated by

> immiscible, rendering possible an additional separation according to the difference in density [32]. This technique is inexpensive, but, at the same time, it is not selective because of the waste of large amounts of the compound in the solvent (part of the extract can be lost in the aqueous phase) and can provide low yields [33,34]. Despite being the oldest method, hydrodistillation is still used today for extracting oils from different matrices. Essential oils from *Rosmarinus officinalis* L. [35], *Ziziphora clinopodioides* L. [36], *Citrus lati‐ folia Tanaka* [37], and *Zingiber officinale* [38] are some of the medicinal plants where EOs

**Figure 3.** Schematic representation of hydrodistillation. **Figure 3.** Schematic representation of hydrodistillation.

can be extracted by hydrodistillation.

causes a reduction in the extraction time.

2.2.2. Steam Distillation

liquid–liquid extraction [39].

In this procedure, the cell walls are broken, and the oil is evaporated together with the water, and then condensed into a mixture of water vapour and volatile compounds of vegetable raw material. However, these two phases (volatile compounds and water) are immiscible, rendering possible an additional separation according to the difference in density [32]. This technique is inexpensive, but, at the same time, it is not selective because of the waste of large amounts of the compound in the solvent (part of the extract can be lost in the aqueous phase) and can provide low yields [33,34]. Despite being the oldest method, hydrodistillation is still used today for extracting oils from different matrices. Essential oils from *Rosmarinus officinalis* L. [35], *Ziziphora clinopodioides* L. [36], *Citrus latifolia Tanaka* [37], and *Zingiber officinale* [38] are some of the medicinal plants where EOs can be extracted by hydrodistillation.

#### 2.2.2. Steam Distillation

Steam distillation is one of the preferred methods of extracting EOs. The extraction procedure is based on the same principles as hydrodistillation. The difference essentially lies in the absence of contact between the substrate to be extracted and the water, which causes a reduction in the extraction time.

The sample is placed in a column where the bottom part is connected to a flask with water under heating (Figure 4). The top part is connected to a condenser, where the steam produced passes through the sample, taking essential oils to the condenser. This process causes the condensation of the water–oil mixture, and this mixture can be separated by liquid–liquid extraction [39]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 6 of 45

**Figure 4.** Experimental setup used in steam distillation. **Figure 4.** Experimental setup used in steam distillation.

This method is applied commercially and on a large scale in the extraction of essential oils from hops [40] and in the extraction of several EOs such as lavender [41] and patchouli essential oil [42]. This method is applied commercially and on a large scale in the extraction of essential oils from hops [40] and in the extraction of several EOs such as lavender [41] and patchouli essential oil [42].

#### 2.2.3. Organic Solvent Extraction 2.2.3. Organic Solvent Extraction

Some essential oils (such as rose and jasmine) have low thermal stability and are un‐ able to withstand high temperatures. In these cases, organic solvents that have a low boil‐ Some essential oils (such as rose and jasmine) have low thermal stability and are unable to withstand high temperatures. In these cases, organic solvents that have a low boiling temperature, are chemically inert, and have low cost can be used.

ing temperature, are chemically inert, and have low cost can be used. In organic solvent extraction, the sample is placed in contact with the organic solvent (which can be hexane, benzene, toluene, or petroleum ether, among others) for a period that allows the transfer of the soluble content of the sample. The extracted matrix is con‐ centrated by evaporating the solvent present in the liquid phase. This method allows the sample to be permanently in contact with a quantity of fresh solvent and, at the end of the process, it is not necessary to carry out filtration, as long as there are high yields [34]. In organic solvent extraction, the sample is placed in contact with the organic solvent (which can be hexane, benzene, toluene, or petroleum ether, among others) for a period that allows the transfer of the soluble content of the sample. The extracted matrix is concentrated by evaporating the solvent present in the liquid phase. This method allows the sample to be permanently in contact with a quantity of fresh solvent and, at the end of the process, it is not necessary to carry out filtration, as long as there are high yields [34]. Solvent extraction

Solvent extraction is the most‐used conventional method in the cosmetic industry [43–45]. Figure 5represents the extraction of organic solvents through a Soxhlet extraction [32,46].

**Figure 5.** Schematic representation of organic solvent extraction using the Soxhlet method.

is the most-used conventional method in the cosmetic industry [43–45]. Figure 5 represents the extraction of organic solvents through a Soxhlet extraction [32,46]. Solvent extraction is the most‐used conventional method in the cosmetic industry [43–45]. Figure 5represents the extraction of organic solvents through a Soxhlet extraction [32,46].

This method is applied commercially and on a large scale in the extraction of essential oils from hops [40] and in the extraction of several EOs such as lavender [41] and patchouli

Some essential oils (such as rose and jasmine) have low thermal stability and are un‐ able to withstand high temperatures. In these cases, organic solvents that have a low boil‐

In organic solvent extraction, the sample is placed in contact with the organic solvent (which can be hexane, benzene, toluene, or petroleum ether, among others) for a period that allows the transfer of the soluble content of the sample. The extracted matrix is con‐ centrated by evaporating the solvent present in the liquid phase. This method allows the sample to be permanently in contact with a quantity of fresh solvent and, at the end of the process, it is not necessary to carry out filtration, as long as there are high yields [34].

ing temperature, are chemically inert, and have low cost can be used.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 6 of 45

**Figure 4.** Experimental setup used in steam distillation.

essential oil [42].

2.2.3. Organic Solvent Extraction

**Figure 5. Figure 5.**Schematic representation of organic solvent extraction using the Soxhlet method. Schematic representation of organic solvent extraction using the Soxhlet method.

#### 2.2.4. Cold Pressing

Essential oils are mechanically removed by cold pressing, where the oil glands are broken and volatile oils are released. In this process, an aqueous emulsion is formed, where the oil present can be obtained through centrifugation, decantation, or fractional distillation [13]. The cold pressing method is essentially used to extract oils from citrus fruits [47–49].

#### 2.2.5. Supercritical Fluid Extraction (SCFE)

Supercritical fluid extraction is an efficient, environmentally friendly, and clean technique for isolating EOs. In this technique, supercritical fluids are used as extraction agents due to the supercritical state of fluids, conferring excellent characteristics for the extraction process, such as low viscosity, high density (close to that of a liquid), and high diffusivity (high penetration power).

Several substances can be used as supercritical solvents, such as water, carbon dioxide (CO2), methane, ethylene, and ethane. However, CO<sup>2</sup> is the most-used solvent due to its critical point being easily reached (low temperature and pressure, 31.2 ◦C and 72.9 atm, respectively), low toxicity and reactivity, low cost, and non-flammability. After selecting the ideal temperature and pressure for extraction, supercritical fluid passes through the sample and the oils are dissolved and extracted. Subsequently, the extraction solution is maintained at a pressure below the critical point and as the pressure decreases, the supercritical fluid passes to the gaseous state and loses its solvating capacity, being recycled [50]. This method is increasingly used commercially, being applied in the extraction of EOs from the leaves of laurel [51], rosemary [52], sage [53], flowering plants [54], and horseradish tree [55].

#### 2.2.6. Microwave-Assisted Extraction (MAE)

Due to the need to use more ecological and energy-efficient extraction methods, microwave-assisted extraction has become an alternative to conventional methods. The sample is placed in a microwave reactor without any solvent, where the electromagnetic energy that is converted into heat increases the internal temperature of sample cells due to the evaporation of the moisture present. The internal pressure increases, the glands rupture, and the essential oil is released [56]. Several EOs were extracted from plant matrices through this technique, such as orange [57], laurel [58], lemon [59], mint [60], rosemary [61], and basil [62].

#### 2.2.7. Ultrasound-Assisted Extraction (UAE)

Ultrasound energy allows the intensification of EO extraction [63]. Therefore, it is usually combined with other extraction techniques in order to accelerate the extraction process and increase the speed of mass transfer. The sample is submerged in a solvent while being subjected to ultrasound. This method, through rapid solvent movements, induces a mechanical vibration of the walls and membranes of the sample that causes the release of essential oils. In some areas, it is already considered a large-scale application method, such as in the medical and food industry, where it is used to increase the quality of the extracted substrate, reduce working time, and increase the yield [64].

In general, alternative methods have emerged to overcome some of the disadvantages and limitations of conventional methods. Traditional methods have long extraction times (4 to 6 h), high energy consumption, and use solvents that increase environmental pollution. Furthermore, they can cause chemical changes to the EOs that are thermally unstable, causing a decrease in the quality of the extracted oils and changes in the chemical nature of compounds. The 'greener' alternatives are more sustainable and economical due to reduced water and energy consumption and reduced CO<sup>2</sup> emissions. However, these methods are not easily accessible, and the initial investment is higher.

Therefore, currently, the hydrodistillation method continues to be the most-used extraction technique in laboratory due to its accessibility, simplicity, and lower cost [46]. Table 2 summarises the advantages and disadvantages of the different EO extraction methods.

#### *2.3. Essential Oils Application*

#### 2.3.1. Essential Oils in Plants

EOs are stored in specific parts of plants, acting in extraordinarily different ways. Some aromatic plants have been widely explored due to their properties, such as bay laurel (*Laurus nobilis*). This plant is an aromatic tree, and laurel oil is extracted from the dry leaves and branches, appearing as a greenish yellow liquid with a powerful medicinal odour. In addition to being used in cuisine, the laurel tree leaves are used in medicine for having antioxidant [65], antibacterial [65,66], and antifungal [22] properties. According to the literature, laurel has also been proven to be an insect repellent [67,68]. However, it can cause dermatitis in some individuals, and due to the possible narcotic properties attributed to methyleugenol, this oil should be used in moderation.

*Cymbopogon nardus*, commonly known as citronella, is an aromatic and perennial herb. Citronella oil can also be produced from Java or Maha Pengiri citronella (*C. winterianus*) [69]. Citronella leaves are used for their aromatic and medicinal value in many cultures, such as in the treatment of fever, intestinal parasites, and digestive and menstrual problems, as well as for use as an insect stimulant and repellent [69–72]. Citronella is also used in traditional Chinese medicine for rheumatic pain, and it has antifungal [73], antioxidant, and antibacterial [74] properties. It is non-toxic and non-irritating, but it can cause dermatitis in some people [69].

Regarding the medical properties of hops (*Humulus lupulus*), these are better known for treatments associated with nerves, insomnia, nervous tension, neuralgia, and for sexual neurosis in both sexes [5]. It has antibacterial [75,76], antifungal [76], anti-cancer [76,77], and repellent [78,79] properties. In China, it is used for pulmonary tuberculosis and cystitis treatment. It can also be used to make beer. It is non-toxic and non-irritating, but it can cause sensitivity in some individuals, and people with depression should avoid this oil [76].

Lemon balm (*Melissa officinalis*) is a herbaceous perennial from the mint family and it has antibacterial, antifungal [80], sedative, antipyretic, antispasmodic, anti-hypertensive, anti-Alzheimer, and antiseptic properties [81]. In addition to the treatment of several gastrointestinal, liver, and nervous system disorders, it has also been reported that lemon balm is useful in the treatment of asthma, bronchitis, coughs, and several pains [82]. Furthermore, this plant is notably marked by its antimicrobial applications in different medicines, exemplified by its use in insect bites (wasps and bees) and poisonous or infectious bites [81,83].


**Table 2.** Advantages and disadvantages of each essential oil extraction method [46].

*Azadirachta indica*, better known as neem, is an ancient tree that has been used for centuries for the most varied purposes. The plant provides a great number of secondary metabolites with biological activity. The plant has gained great importance in several areas, such as agriculture, livestock, and medicine [84]. It is used as an insecticide [85], antiviral [86], antibacterial [87], and antimicrobial [88], among others. Neem oil is very effective for acne, psoriasis, and eczema treatments, but it can also be applied as a support in the treatment of topical fungal or viral conditions, such as nail fungus, athlete's foot, warts, or wounds. The natural antihistamines contained in neem oil are effective in relieving the itching and burning caused by, for example, bee, mosquito, and spider bites. The main constituent of neem is azadiractin, found in the leaves, fruits, and seeds.

*Mentha pulegium*, better known as pennyroyal or mint (Brazil), is one of the bestknown species of the genus *Mentha*. Pennyroyal extracts are good insect repellents [89–92]. There are several studies that show that these extracts also have other properties, such as antimicrobial [92–94], antioxidant [92,93,95], antibacterial [95,96], and anti-tumour [96] uses. It is still current in the *British Herbal Pharmacopoeia*, indicated for flatulent dyspepsia, intestinal colic, common cold, delayed menstruation, skin rashes, and gout [69].

*Illicium verum*, popularly known as star anise, is a plant considered a spice for medicinal and culinary use. The extraction of *Illicium verum* has carminative, stomach, stimulating, and diuretic properties and is used as a pharmaceutical supplement [97]. The extracted

[69].

[69].

[69].

[69].

[69].

[69].

logical properties.

logical properties.

logical properties.

logical properties.

logical properties.

logical properties.

logical properties.

logical properties.

logical properties.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 45

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regardingtherapeutic indications,

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications, it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications, it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

large doses, it is narcotic and slows down circulation, which can lead to brain disorders

than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

[105,106]. There are also studies that demonstrate its antibacterial [107] and antimicrobial

[108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐

[108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐

[108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐

[108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐

[108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications,

shikimic acid is one of the main ingredients of the antiviral drug Tamiflu® (oseltamivir) that is used to treat symptoms caused by avian influenza [98]. It has also been reported to have antimicrobial properties [99] and antioxidant properties [100], as well as significant anti-cancer potential [101]. There are studies in which star anise has been used as an insect repellent, such as for the Indian flour moth (*P. interpunctella larvae*) [102,103]. The main constituent of star anise is trans-anethole (Table 3) (80–90%) and when used in large doses, it is narcotic and slows down circulation, which can lead to brain disorders [69]. it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping [105,106]. There are also studies that demonstrate its antibacterial [107] and antimicrobial [108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐ scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping [105,106]. There are also studies that demonstrate its antibacterial [107] and antimicrobial [108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐ scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping [105,106]. There are also studies that demonstrate its antibacterial [107] and antimicrobial [108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐ scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ [108] properties. It is non‐toxic, non‐irritating, and can cause sensitisation. Table 3 de‐ scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oilplants, species, and main components, and their molecular structure and bio‐ **Chemical Struc‐ Some Biologi‐** scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ **Chemical Struc‐ Some Biologi‐** scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ **Chemical Struc‐ Some Biologi‐** scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ **Chemical Struc‐ Some Biologi‐** scribes the main components ofthe EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ **Plant Species EO Major Com‐ Chemical Struc‐ Some Biologi‐** scribes the main components of the EOs present in plants, as well as their chemical struc‐ tures and some of the biological properties. **Table 3.** Essential oil plants, species, and main components, and their molecular structure and bio‐ **Plant Species EO Major Com‐ Chemical Struc‐ Some Biologi‐**

**Table 3.** Essential oil plants, species, and main components, and their molecular structure and biological properties. **Chemical Struc‐ Some Biologi‐ Chemical Struc‐ Some Biologi‐ Chemical Struc‐ Some Biologi‐ Plant Species EO Major Com‐ ponents tures of EOs Components cal Properties Plant Species EO Major Com‐ ponents tures of EOs Components cal Properties Plant Species EO Major Com‐ ponents tures of EOs Components cal Properties Plant Species EO Major Com‐ ponents tures of EOs Components cal Properties ponents tures of EOs Components cal Properties ponents tures of EOs Components cal Properties**


Valerian *Valeriana of‐*

Valerian *Valeriana of‐*

Valerian *Valeriana of‐*

Valerian *Valeriana of‐*

Valerian *Valeriana of‐*

Valerian *Valeriana of‐*

*ficinalis*

2.3.2. Pharmacological and Medical Applications

*ficinalis*

*ficinalis*

*ficinalis*

*ficinalis*

*ficinalis*

*ficinalis*

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

2.3.2. Pharmacological and Medical Applications

anti‐inflammatory [112], and antiparasitic activities [113].

anti‐inflammatory [112], and antiparasitic activities [113].


#### **Table 3.** *Cont. Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45 *Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45 Geraniol, citral,

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

Geraniol, citral,

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 11 of 45

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐ ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

Essential oils have a wide range of biological properties, and there has been a grow‐ ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐ ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

Essential oils have a wide range of biological properties, and there has been a grow‐

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

ing interest in clinical applications (Table 4). Some of the properties include the chemo‐ preventive effects of cancer[109], antifungal [110], antiviral [111], antimicrobial, analgesic,

Antibacterial, antimicrobial, antifungal, antioxidative, sedative

antimicrobial, antifungal, antioxidative, sedative

antifungal, antioxidative, sedative

antifungal, antioxidative, sedative

antimicrobial, antifungal, antioxidative, sedative

antifungal, antioxidative, sedative

Antibacterial, antimicrobial, antifungal, antioxidative, sedative

Antibacterial,

Antibacterial, antifungal, an‐ timicrobial, sedative, anti‐

Antibacterial, antifungal, an‐ timicrobial, sedative, anti‐

Antibacterial,

Borneol, cam‐ phene, α and β‐ pinene, valera‐

phene, α and β‐ pinene, valera‐

pinene, valera‐ none, valerenol

Borneol, cam‐ phene, α and β‐ pinene, valera‐

Borneol, cam‐ phene, α and β‐ pinene, valera‐

phene, α and β‐ pinene, valera‐ none, valerenol

phene, α and β‐ pinene, valera‐ none, valerenol

Valerian is a perennial flowering plant with many chemical constituents, with more than 150 constituents identified in its essential oil [104]. Regarding therapeutic indications, it is advisable for people with nervous agitation, mild anxiety, and difficulties in sleeping [105,106]. There are also studies that demonstrate its antibacterial [107] and antimicrobial [108] properties. It is non-toxic, non-irritating, and can cause sensitisation. Table 3 describes the main components of the EOs present in plants, as well as their chemical structures and some of the biological properties.

#### 2.3.2. Pharmacological and Medical Applications

Essential oils have a wide range of biological properties, and there has been a growing interest in clinical applications (Table 4). Some of the properties include the chemopreventive effects of cancer [109], antifungal [110], antiviral [111], antimicrobial, analgesic, anti-inflammatory [112], and antiparasitic activities [113].


**Table 4.** Some pharmacological actions of essential oils [114].

An extensive range of EOs have antibacterial activity against Gram-positive and Gram-negative bacteria, along with antifungal properties. These compounds have been studied and have shown very promising results in salmonella, staphylococci, and other oral pathogens, and can be an alternative to antibiotics providing they are properly studied for these effects [115,116]. EOs that have shown antibacterial potential are basil [117], manuka oil (more potent among eucalyptus oil, rosmarinus, lavandula, and tea tree) [118], melaleuca oil [119], and essential leaf oils *P. undulatum* and *Hedychium gardnerianum*.

With regard to antifungal activity, melaleuca oil showed positive results for all of its constituents, especially against dermatophytes and filamentous fungi [120]. In a reported study, germinated conidia of *Aspergillus niger* were more susceptible to non-germinated ones, with EOs of *Melaleuca ericifolia*, *Melaleuca armillaris*, *Melaleuca leucadendron*, and *Melaleuca styphelioides* exhibiting good activity against this fungus [120]. These same oils were evaluated for their antiviral activity in African green monkey kidney cells through the plaque reduction assay in the herpes simplex virus type 1 [121]. Other plants that have good antifungal activity are *M. piperita*, *Brassica nigra*, *Angelica archangelica*, *Cymbopogon nardus, Skimmia laureola, Artemisia sieberi*, and *Cuminum cyminum* [122–127]

Regarding antioxidant activity, the essential oil of the seeds of *Nigella sativa L*. showed considerable activity in the elimination of hydroxyl radicals. The essential oil of *M. armillaris* has marked antioxidant potential, changing the parameters of superoxide dismutase, and improving the concentrations of vitamin E and vitamin C [128]. However, there have been promising insect repellency/toxicity results from the essential oils of *Nepeta parnassica*, in *Culex pipiens molestus* [129]. Geranial, neral, geraniol, nerol, and trans-anethole are well established to stimulate the estrogenic response, and citrus (a combination of geraniol, nerol, and eugenol) is effective in replacing [3H] 17β-estradiol at the oestrogen receptors in recombinant yeast cells [130,131].

Cancer is a growing health problem worldwide and is the second leading cause of death. Essential oil constituents play an important role in cancer prevention and treatment as alternatives to synthetic drugs. Mechanisms such antioxidant, antimutagenic, and antiproliferative properties, enhancement of immune function and surveillance, enzyme induction and enhancing detoxification, modulation of multidrug resistance, and the synergistic mechanism of EO constituents are accountable for their chemo-preventive properties [132]. It has been reported that mitochondrial damage and apoptosis/necrosis in the yeast *Saccharomyces cerevisiae* were reduced by essential oils [133]. Recently, some studies demonstrated that certain EOs exhibited antimutagenicity towards mutations caused by UV light [8]. Jaganathan et al. reported that the active constituent eugenol from *Syzygium aromaticum* (cloves), nutmeg, basil, cinnamon, and bay leaves showed antiproliferative activity against several cancer cell lines in animal models [134].

In addition to medicinal and pharmacological applications, essential oils are used in perfumes, cosmetics, hygiene products, disinfectants, repellents, candles, phytochemicals, preservatives, and food additives.

#### 2.3.3. Food Applications

In food, cosmetics, and personal care products, EOs are used as a natural aroma due to their chemical properties. In the food industry, EOs are being used as a food preservative because one of the main concerns is the preservation of food to prolong its useful life, ensuring safety and quality [11]. An expiration date is defined as the period of time during which the food product will remain safe. This ensures the maintenance of sensory, chemical, physical, microbiological, and functional characteristics. For example, spices can be encapsulated to extend their shelf life, maintain their properties, and inhibit reactions with other compounds [135]. Cinnamaldehyde, the aromatic agent present in cinnamon, has antimicrobial properties, and when encapsulated can slow the growth of yeasts in bakery products. Thus, the use of cinnamon in encapsulated form allows the product to be flavoured without interfering with the leavening process [136].

As the unpleasant taste and instability limit the application of EOs, the use of these encapsulated compounds can allow their application for several purposes. One of them is the intensification of the flavour of food products, where capsules can be used that release the product quickly when introduced into the mouth [20].

The packaging has the function of delaying deterioration, maintaining the quality and safety of packaged foods. For the packaging material to be satisfactory, it must be inert and scratch resistant, and must not allow molecular transfer to or from the packaging materials. Active packaging technologies extend the shelf life and control the quality of food products, decreasing microbial, biochemical, and enzymatic reactions through different strategies, such as adding chemical additives/preservatives, removing oxygen, controlling humidity and/or temperature, or a combination of these [137]. Oregano oil contains a high amount of carvacrol and is considered one of the most active plant extracts against pathogens due to its antimicrobial activity. Therefore, it has been used to preserve a variety of foods such as pizza, fresh beef [138], and cheddar cheese [139]. For the same purpose, limonene is reported for the preservation of strawberries [140], rosemary in chicken breast cuts [141], and cinnamon in pastries [142].

#### 2.3.4. Cosmetic and Cleaning Applications

In the detergent and cosmetics industry, microcapsules of essential oils are used in many products such as perfumes, creams, and deodorants where the controlled release of EOs is essential, increasing the duration of fragrance and the properties of the EOs [45]. Aroma ingredients such as patchouli (*Pogostemoncablin*), citronella (*Cymbopogon winterianus*), sandalwood (*Santalum álbum*), bergamot (*Citrusaurantium*), rosemary (*Rosmarinus officinalus*), mint (*Mentha piperita*), and vetiver (*Chrysopogon zizanioides*) are frequently

used [4]. Regarding the EOs from flowers, *Lavandula officinalis*, rose, jasmine, tuberose, narcissus, and gardenia are those most commonly exploited for cosmetic applications [143]. Products such as detergents, soaps, shampoos, and softeners are largely produced using these natural compounds.

Over the years, EOs have also been used against nosocomial infections, as a cleaning liquid for disinfecting equipment and medical surfaces [9], or as an aerosol in operating rooms and waiting rooms to limit contamination [10].

#### 2.3.5. Agrochemical Applications

The loss of quality of agricultural products is caused by the presence of insect pests. The presence of these pests leads to reduced quality, low yield, and economic losses. Furthermore, human and animal health is compromised due to the production of carcinogenic secondary metabolites. To overcome this problem, chemical insecticides were used to excess. Despite being highly efficient, their overuse caused physiological resistance in several insect species and irreversible damage to the environment. Essential oils have emerged as a natural plant alternative to protect agricultural products from pests [144]. The use of EOs has intensified, mainly in gardens and homes, for pest control (Table 5), being important due to their toxic (pesticide) effect. EOs can be inhaled, ingested, or absorbed through the skin of insects. Monoterpenoids are an important group of chemical compounds in essential oils that interfere with the octopaminergic system of insects, which represent a target for insect control. As vertebrates do not have octopamine receptors, most chemicals in EOs are relatively safe to use. The special regulatory status together with the availability of essential oils has made the commercialisation of EO-based pesticides possible. Microencapsulation technology is used to produce these natural pesticides in order to mimic chemical compartmentalisation in plants, by protecting essential oils from degradation [145].


**Table 5.** Pests/pesticides and their corresponding essential oil [145].

#### 2.3.6. Textile Applications

Essential oils are used in medical and technical fabrics. The technique used in industrial processes is encapsulation, which is used to give finishes and properties to textiles that were not possible or economical. The main application for encapsulation is durable fragrances and skin softeners, and other applications may include insect repellents, dye, vitamins, microbial agents, and phase-change materials, and medical applications, such as antibiotics, hormones, and other medications.

The functionalisation of textiles with EOs with anti-mosquito repellent properties is a revolutionary way to protect human beings from insect bites and, thus, from many diseases such as malaria and dengue [146]. Plants, whose OEs have been reported to have repellent properties, include citronella, cedar, geranium, pine, cinnamon, basil, thyme, garlic, and mint. Khanna et al. performed the synthesis of a modified cyclodextrin host (β-CD CA) for inclusion complexation with the essential oils of cedarwood, clove, eucalyptus, peppermint, lavender, and jasmine for the assessment of repellent efficacy against *Anopheles Stephensi* in cotton. It was concluded that jasmine EO is the weakest against mosquitoes, as it worked as an attractant simulating flower nectar. Eucalyptus and clove are the feeding deterrents. On the other hand, lavender and peppermint are potential mosquito repellents, and cedarwood is an effective mosquito killer [147]. Soroh et al. reported that textiles treated with the Litsea and lemon EO microemulsion showed potential mosquito-repellent properties [148]. In general, citronella remains the most promising as an insect repellent and, therefore, is the most-incorporated EO in tissue functionalisation for this purpose. Specos et al. demonstrated citronella essential oil's mosquito-repellent action, especially against *Aedes aegypti* [148]. Microcapsules with citronella are commonly incorporated into matrices such as cotton and polyester [149]. Another report determined that bio-based citronella oil has a better insect repellent effect than synthetic agents. Sariisik et al. concluded that, after washing, the insect repellent activity of the printing and coating method was increased, and the fabrics still showed repellency after five washing cycles [150]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 15 of 45 are the feeding deterrents. On the other hand, lavender and peppermint are potential mos‐ quito repellents, and cedarwood is an effective mosquito killer [147]. Soroh et al. reported that textiles treated with the Litsea and lemon EO microemulsion showed potential mos‐ quito‐repellent properties [148]. In general, citronella remains the most promising as an insect repellent and, therefore, is the most‐incorporated EO in tissue functionalisation for this purpose. Specos et al. demonstrated citronella essential oil's mosquito‐repellent ac‐ tion, especially against *Aedes aegypti* [148]. Microcapsules with citronella are commonly incorporated into matrices such as cotton and polyester [149]. Another report determined that bio‐based citronella oil has a better insect repellent effect than synthetic agents. Sar‐ iisik et al. concluded that, after washing, the insect repellent activity of the printing and coating method was increased, and the fabrics still showed repellency after five washing cycles [150].

#### **3. Microencapsulation 3. Microencapsulation** Microencapsulation is the protection of small solid, liquid, or gaseous particles

Microencapsulation is the protection of small solid, liquid, or gaseous particles through a coating system (1–1000 mm) [151]. The encapsulated material is called the core and the material that forms the coating of particle is the wall or encapsulating agent [152]. Wall material can be a natural, synthetic, or semi-synthetic polymeric coating. In this technology, microparticles are formed, which can be classified in relation to their size and morphology, according to the encapsulating agent and microencapsulation method used [153]. through a coating system (1–1000 mm) [151]. The encapsulated material is called the core and the material that forms the coating of particle is the wall or encapsulating agent [152]. Wall material can be a natural, synthetic, or semi‐synthetic polymeric coating. In this tech‐ nology, microparticles are formed, which can be classified in relation to their size and morphology, according to the encapsulating agent and microencapsulation method used [153].

Microparticles can be distinguished according to their form: they are classified as a reservoir-type system, 'microcapsules', when the core (encapsulated material) is concentrated in the central region, coated by a continuous wall material (encapsulating agent); or a monolithic system, 'microspheres', when the active agent (core) is dispersed in a matrix system (Figure 6). In general, the main difference is that in microspheres, part of the encapsulated material is exposed on the surface of the microparticle [154]. Microparticles can be distinguished according to their form: they are classified as a reservoir‐type system, 'microcapsules', when the core (encapsulated material) is concen‐ trated in the central region, coated by a continuous wall material (encapsulating agent); or a monolithic system, 'microspheres', when the active agent (core) is dispersed in a ma‐ trix system (Figure 6). In general, the main difference is that in microspheres, part of the encapsulated material is exposed on the surface of the microparticle [154].

**Figure 6.** Schematic representation of a microcapsule and a microsphere. **Figure 6.** Schematic representation of a microcapsule and a microsphere.

The physicochemical characteristics of the microcapsule are defined by the encapsu‐ lating agent and the active agent. The wall material must form a cohesive film that bonds with the encapsulated material [155]. Several materials can be used for the coating, with The physicochemical characteristics of the microcapsule are defined by the encapsulating agent and the active agent. The wall material must form a cohesive film that bonds with the encapsulated material [155]. Several materials can be used for the coating, with

proteins, carbohydrates, and lipids being frequently used. Furthermore, the materials must be chemically compatible and the encapsulating agent chemically inert, so as not to

particularly used to protect the core active agent's sensitivity to oxygen, light, and mois‐ ture, or to prevent interaction with other compounds. However, the most important rea‐

son for encapsulating an active agent is to obtain a controlled release [157].

react with the core [156].

proteins, carbohydrates, and lipids being frequently used. Furthermore, the materials must be chemically compatible and the encapsulating agent chemically inert, so as not to react with the core [156].

Microencapsulation technologies achieve several objectives (Figure 7) and they are particularly used to protect the core active agent's sensitivity to oxygen, light, and moisture, or to prevent interaction with other compounds. However, the most important reason for encapsulating an active agent is to obtain a controlled release [157]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 16 of 45 *Polymers* **2022**, *14*, x FOR PEER REVIEW 16 of 45

**Figure 7.** Objectives of microencapsulation. **Figure 7.** Objectives of microencapsulation. **Figure 7.** Objectives of microencapsulation.

The process of defining a microencapsulation system is mainly dependent on the purpose of the microcapsules. Characteristics such as shape, size, permeability, biodegra‐ dability, or biocompatibility are defined depending on the application of this material. Other physical and mechanical properties of the microcapsule, such as strength and flex‐ ibility, must also be defined [158]. The process of defining a microencapsulation system is mainly dependent on the purpose of the microcapsules. Characteristics such as shape, size, permeability, biodegradability, or biocompatibility are defined depending on the application of this material. Other physical and mechanical properties of the microcapsule, such as strength and flexibility, must also be defined [158]. The process of defining a microencapsulation system is mainly dependent on the purpose of the microcapsules. Characteristics such as shape, size, permeability, biodegra‐ dability, or biocompatibility are defined depending on the application of this material. Other physical and mechanical properties of the microcapsule, such as strength and flex‐

One of the great advantages of microencapsulation is the mechanism of the con‐ trolled, sustained, or targeted release of the active agent. This release can occur at a certain defined time or not, through a mechanism of diffusion through or rupture of the wall. The release can be activated through temperature variations, solubility, pH changes, or even the biodegradability of the wall material [159]. One of the great advantages of microencapsulation is the mechanism of the controlled, sustained, or targeted release of the active agent. This release can occur at a certain defined time or not, through a mechanism of diffusion through or rupture of the wall. The release can be activated through temperature variations, solubility, pH changes, or even the biodegradability of the wall material [159]. ibility, must also be defined [158]. One of the great advantages of microencapsulation is the mechanism of the con‐ trolled, sustained, or targeted release of the active agent. This release can occur at a certain defined time or not, through a mechanism of diffusion through or rupture of the wall. The release can be activated through temperature variations, solubility, pH changes, or even the biodegradability of the wall material [159].

Depending on the nature of the interaction of the encapsulating and encapsulated material, microencapsulation methods can be distinguished as chemical, physicochemi‐ cal, and mechanical (Figure 8) [160]. In general, a microencapsulation method must be fast, easy, reproducible, and easily scalable for industry. The most‐used microencapsula‐ tion methods are spray drying and coacervation, and these approaches will be mentioned in more detail below. Depending on the nature of the interaction of the encapsulating and encapsulated material, microencapsulation methods can be distinguished as chemical, physicochemical, and mechanical (Figure 8) [160]. In general, a microencapsulation method must be fast, easy, reproducible, and easily scalable for industry. The most-used microencapsulation methods are spray drying and coacervation, and these approaches will be mentioned in more detail below. Depending on the nature of the interaction of the encapsulating and encapsulated material, microencapsulation methods can be distinguished as chemical, physicochemi‐ cal, and mechanical (Figure 8) [160]. In general, a microencapsulation method must be fast, easy, reproducible, and easily scalable for industry. The most‐used microencapsula‐ tion methodsare spray drying and coacervation, and these approaches will be mentioned in more detailbelow.

**Figure 8.** Main microencapsulation methods. **Figure 8.** Main microencapsulation methods. **Figure 8.** Main microencapsulation methods.

*3.1. Emulsification*

*3.1. Emulsification*

#### *3.1. Emulsification* systems, consisting of: 1. Oil‐in‐water emulsion (O/W);

Emulsification is a fundamental step in oil microencapsulation, being used in a wide variety of food and pharmaceutical products. It is applied for the encapsulation of bioactive substances in aqueous solutions, which can be used directly in liquid or dried (spray-or freeze-drying) to form powders. 2. Water‐in‐oil emulsion (W/O); 3. Oil‐in‐water‐in‐oil emulsion (O/W/O); 4. Water‐in‐oil‐in‐water emulsion (W/O/W). In these systems, the droplet diameters can vary from 0.1 to 100 μm [161] and have

Emulsification is a fundamental step in oil microencapsulation, being used in a wide variety of food and pharmaceutical products. It is applied for the encapsulation of bioac‐ tive substances in aqueous solutions, which can be used directly in liquid or dried (spray‐

An emulsion consists of at least two immiscible liquids, with one of the liquids being dispersed as small spherical drops in the other. As can be seen in Figure 9, there are four

An emulsion consists of at least two immiscible liquids, with one of the liquids being dispersed as small spherical drops in the other. As can be seen in Figure 9, there are four systems, consisting of: been extensively revised by scientists [162]. The O/W emulsion consists of small oil drop‐ lets that are dispersed in an aqueous medium, being the droplets wrapped in a thin inter‐ facial layer. Its advantages are the ease of preparation and low cost, with some disad‐

1. Oil-in-water emulsion (O/W); vantages such as physical instability and limited control [162]. Through modifications of

or freeze‐drying) to form powders.


*Polymers* **2022**, *14*, x FOR PEER REVIEW 17 of 45

**Figure 9.** Illustration of emulsion systems. **Figure 9.** Illustration of emulsion systems.

A straightforward method for obtaining small droplets with a stratum size distribu‐ tion is the evaporation/extraction of the emulsifying substance. This method is used in the preparation of biodegradable and non‐biodegradable polymeric microparticles and in the microencapsulation of a wide variety of liquid and solid materials [164]. However, it is an expensive method with a low encapsulation efficiency, leading to residual solvent amounts [165]. *3.2. Coacervation* In these systems, the droplet diameters can vary from 0.1 to 100 µm [161] and have been extensively revised by scientists [162]. The O/W emulsion consists of small oil droplets that are dispersed in an aqueous medium, being the droplets wrapped in a thin interfacial layer. Its advantages are the ease of preparation and low cost, with some disadvantages such as physical instability and limited control [162]. Through modifications of the emulsifiers, features can be added, such as the use of Maillard reaction products. These products can increase encapsulation efficiency and are able to protect the microencapsulation oil and other oils from oxidation [163].

Coacervation is one of the most widely used microencapsulation techniques. The technique is based on oppositely charged polyelectrolyte polymers that interact and form a wall covering the active agent. The coacervation process can be classified as simple and complex if one or two (or more) polymers are used, respectively. Generally, this technique is defined by the separation of two liquid phases in a colloidal solution, where one phase is rich in polymer (coacervated phase) and the other phase does not contain polymer A straightforward method for obtaining small droplets with a stratum size distribution is the evaporation/extraction of the emulsifying substance. This method is used in the preparation of biodegradable and non-biodegradable polymeric microparticles and in the microencapsulation of a wide variety of liquid and solid materials [164]. However, it is an expensive method with a low encapsulation efficiency, leading to residual solvent amounts [165].

#### (equilibrium phase) [46]. *3.2. Coacervation*

Complex coacervation involves the interaction of two oppositely charged colloids, where the neutralisation of charges induces a phase separation. A polysaccharide and a protein are usually used as the different polymers. Wall material systems that are most widely investigated include gelatin/gum arabic, gelatin/alginate, gelatin/glutaraldehyde, gelatin/chitosan and gelatin/carboxymethyl cellulose [166]. In the process of the microencapsulation of hydrophobic materials (Figure 10), the emulsification of the encapsulated agent in an aqueous solution containing two different Coacervation is one of the most widely used microencapsulation techniques. The technique is based on oppositely charged polyelectrolyte polymers that interact and form a wall covering the active agent. The coacervation process can be classified as simple and complex if one or two (or more) polymers are used, respectively. Generally, this technique is defined by the separation of two liquid phases in a colloidal solution, where one phase is rich in polymer (coacervated phase) and the other phase does not contain polymer (equilibrium phase) [46].

polymers occurs, usually at a temperature and pH above the gel and isoelectric point of the protein. Then, the separation into two liquid phases (polymer‐rich phase and aqueous Complex coacervation involves the interaction of two oppositely charged colloids, where the neutralisation of charges induces a phase separation. A polysaccharide and a protein are usually used as the different polymers. Wall material systems that are most widely investigated include gelatin/gum arabic, gelatin/alginate, gelatin/glutaraldehyde, gelatin/chitosan and gelatin/carboxymethyl cellulose [166].

In the process of the microencapsulation of hydrophobic materials (Figure 10), the emulsification of the encapsulated agent in an aqueous solution containing two different polymers occurs, usually at a temperature and pH above the gel and isoelectric point of the protein. Then, the separation into two liquid phases (polymer-rich phase and

aqueous phase) follows, which results from the electrostatic interaction of the polymers. Subsequently, a microcapsule wall is formed as the deposition of the polymer-rich phase occurs around the hydrophobic particles of the active agent, due to controlled cooling below the gelation temperature. Finally, the microcapsule walls harden through the addition of a crosslinking agent [167]. ciated cost, as cheap inorganic salts are used to induce the separation phase, while expen‐ sive hydrocolloids are applied in the complex method. Furthermore, complex coacerva‐ tion is more sensitive to small variations in pH. However, compared to other microencap‐ sulation methods, complex coacervation is a simple, scalable, inexpensive, reproducible, and solvent‐free method, enabling its industrial use [166]. crosslinking agent [167]. Simple coacervation has advantages over complex coacervation in terms of the asso‐ ciated cost, as cheap inorganic salts are used to induce the separation phase, while expen‐ sive hydrocolloids are applied in the complex method. Furthermore, complex coacerva‐ tion is more sensitive to small variations in pH. However, compared to other microencap‐ sulation methods, complex coacervation is a simple, scalable, inexpensive, reproducible,

Simple coacervation has advantages over complex coacervation in terms of the asso‐

phase) follows, which results from the electrostatic interaction of the polymers. Subse‐ quently, a microcapsule wall is formed as the deposition of the polymer‐rich phase occurs around the hydrophobic particles of the active agent, due to controlled cooling below the gelation temperature. Finally, the microcapsule walls harden through the addition of a

phase) follows, which results from the electrostatic interaction of the polymers. Subse‐ quently, a microcapsule wall is formed as the deposition of the polymer‐rich phase occurs around the hydrophobic particles of the active agent, due to controlled cooling below the gelation temperature. Finally, the microcapsule walls harden through the addition of a

**Figure 10.** Schematic illustration of the coacervation method. **Figure 10.** Schematic illustration of the coacervation method.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 18 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 18 of 45

*3.3. In Situ Polymerisation* In situ polymerisation (Figure 11) is based on the formation of a wall through the addition of a reagent inside or outside the core material [168], becoming one of the most‐ used methods in the preparation of microcapsules and functional fibres. Polymerisation takes place in the continuous phase and not on both sides of the interface between the core material and the continuous phase. Microcapsule formation occurs through an oil emul‐ Simple coacervation has advantages over complex coacervation in terms of the associated cost, as cheap inorganic salts are used to induce the separation phase, while expensive hydrocolloids are applied in the complex method. Furthermore, complex coacervation is more sensitive to small variations in pH. However, compared to other microencapsulation methods, complex coacervation is a simple, scalable, inexpensive, reproducible, and solvent-free method, enabling its industrial use [166]. **Figure 10.** Schematic illustration of the coacervation method. *3.3. In Situ Polymerisation* In situ polymerisation (Figure 11) is based on the formation of a wall through the addition of a reagent inside or outside the core material [168], becoming one of the most‐ used methods in the preparation of microcapsules and functional fibres. Polymerisation takes place in the continuous phase and not on both sides of the interface between the core

#### sion in a solution of melamine–formaldehyde resin and a sonication process to emulsify *3.3. In Situ Polymerisation* material and the continuous phase. Microcapsule formation occurs through an oil emul‐

crosslinking agent [167].

the oil in the aqueous phase. Then, resin is added under agitation and the pH is adjusted, with the formation of shells, thus promoting the reaction of the melamine with the for‐ maldehyde at the interface of the oil droplets. This type of microcapsule is used in fra‐ grances, insect repellents, food packaging, and footwear. The microcapsules result in smooth surface morphologies and are able to preserve the encapsulated scented oils for a sufficient period of time. They also have good thermal and controlled release properties [168,169]. Using a polymer as a microcapsule wrapper is considered a good addition due to its high strength and stability [170]. On the other hand, using a copolymer to prepare micro‐ capsules with a low molecular weight of formaldehyde–melamine avoids the toxicity of formaldehyde [171]. In situ polymerisation (Figure 11) is based on the formation of a wall through the addition of a reagent inside or outside the core material [168], becoming one of the mostused methods in the preparation of microcapsules and functional fibres. Polymerisation takes place in the continuous phase and not on both sides of the interface between the core material and the continuous phase. Microcapsule formation occurs through an oil emulsion in a solution of melamine–formaldehyde resin and a sonication process to emulsify the oil in the aqueous phase. Then, resin is added under agitation and the pH is adjusted, with the formation of shells, thus promoting the reaction of the melamine with the formaldehyde at the interface of the oil droplets. This type of microcapsule is used in fragrances, insect repellents, food packaging, and footwear. The microcapsules result in smooth surface morphologies and are able to preserve the encapsulated scented oils for a sufficient period of time. They also have good thermal and controlled release properties [168,169]. sion in a solution of melamine–formaldehyde resin and a sonication process to emulsify the oil in the aqueous phase. Then, resin is added under agitation and the pH is adjusted, with the formation of shells, thus promoting the reaction of the melamine with the for‐ maldehyde at the interface of the oil droplets. This type of microcapsule is used in fra‐ grances, insect repellents, food packaging, and footwear. The microcapsules result in smooth surface morphologies and are able to preserve the encapsulated scented oils for a sufficient period of time. They also have good thermal and controlled release properties [168,169]. Using a polymer as a microcapsule wrapper is considered a good addition due to its high strength and stability [170]. On the other hand, using a copolymer to prepare micro‐ capsules with a low molecular weight of formaldehyde–melamine avoids the toxicity of formaldehyde [171].

**Figure Figure 11. 11.** Schematic Schematic illustration in situ polymerisation method (adapted from [ illustration in situ polymerisation method (adapted from [168]). 168]).

Using a polymer as a microcapsule wrapper is considered a good addition due to its high strength and stability [170]. On the other hand, using a copolymer to prepare microcapsules with a low molecular weight of formaldehyde–melamine avoids the toxicity of formaldehyde [171].

In situ polymerisation is a method of rapid and easy expansion [172] and, at the same time, provides high encapsulation efficiency. However, the polymerisation reaction is

difficult to control [173] and requires a large amount of solvent, making the monomers non-biodegradable and/or non-biocompatible [174]. Spray drying is the most‐used technology in the microencapsulation of essential oils. It is mainly used on an industrial scale, as it allows simple, reproducible, continuous, and low‐cost production. Being used more frequently in the food industry, this process is also

In situ polymerisation is a method of rapid and easy expansion [172] and, at the same time, provides high encapsulation efficiency. However, the polymerisation reaction is dif‐ ficult to control [173] and requires a large amount of solvent, making the monomers non‐

#### *3.4. Spray Drying* utilised in the cosmetics, pesticides, and pharmaceutical industries [175]. This technique

*3.4. Spray Drying*

*Polymers* **2022**, *14*, x FOR PEER REVIEW 19 of 45

biodegradable and/or non‐biocompatible [174].

Spray drying is the most-used technology in the microencapsulation of essential oils. It is mainly used on an industrial scale, as it allows simple, reproducible, continuous, and low-cost production. Being used more frequently in the food industry, this process is also utilised in the cosmetics, pesticides, and pharmaceutical industries [175]. This technique allows encapsulated and powdered Eos to be obtained due to the ability to dry them in just one operation. In this process, the atomisation of emulsions occurs in a drying chamber with relatively high temperatures, where the evaporation of the solvent takes place and, consequently, microcapsules are formed [176]. allows encapsulated and powdered Eos to be obtained due to the ability to dry them in just one operation. In this process, the atomisation of emulsions occurs in a drying cham‐ ber with relatively high temperatures, where the evaporation of the solvent takes place and, consequently, microcapsules are formed [176]. The spray‐drying technique involves four steps (Figure 12), where the preparation of dispersion first occurs, i.e., the wall materials are dissolved in water with agitation and controlled temperature. Still in the same step, the addition of the EOs follows and, if nec‐ essary, the emulsifier can be added. Afterwards, the dispersion is homogenised to be in‐

The spray-drying technique involves four steps (Figure 12), where the preparation of dispersion first occurs, i.e., the wall materials are dissolved in water with agitation and controlled temperature. Still in the same step, the addition of the EOs follows and, if necessary, the emulsifier can be added. Afterwards, the dispersion is homogenised to be injected into the equipment through an atomising nozzle, where small droplets are formed. In the third step, emulsion atomisation occurs, where the formed droplets enter the drying chamber with a flow of hot air present. Finally, the dehydration of the atomised microparticles is done through the evaporation of the solvent, which dries the microparticles, which can them be recovered in the form of powder in a collector or filter [177]. jected into the equipment through an atomising nozzle, where small droplets are formed. In the third step, emulsion atomisation occurs, where the formed droplets enter the drying chamber with a flow of hot air present. Finally, the dehydration of the atomised micro‐ particles is done through the evaporation of the solvent, which dries the microparticles, which can them be recovered in the form of powder in a collector or filter [177]. The main limitations of this technique are related to the wall material, which must have good water solubility, and to the number of encapsulating agents available. In addi‐ tion, some materials may be sensitive to the high temperatures introduced in the atomi‐ sation process. In addition, the production of microcapsules in fine powderform can cause agglomeration and an additional process may be required [166].

**Figure 12.** Schematic representation of spray drying. **Figure 12.** Schematic representation of spray drying.

The main limitations of this technique are related to the wall material, which must have good water solubility, and to the number of encapsulating agents available. In addition, some materials may be sensitive to the high temperatures introduced in the atomisation process. In addition, the production of microcapsules in fine powder form can cause agglomeration and an additional process may be required [166].

#### *3.5. Freeze Drying 3.5. Freeze Drying*

Freeze drying, also known as lyophilisation, is a simple process (Figure 13) that is used to dehydrate most materials sensitive to heat and aromas such as oils. Sublimation is the major principle involved in this drying process, where water passes directly from a solid state to a vapour state without passing through the liquid state. Before starting this process, the oil is dissolved in water and frozen [178]. Afterwards, the pressure is reduced and heat is added to allow the frozen water to sublimate the material directly from the solid phase to the gas phase. Freeze-dried materials appear to have the maximum retention of volatile compounds compared to spray drying, and this technique is used to microencapsulate some oils, with high yields [179]. This method helps to better preserve the EO content in many types of herbs and spices compared with other preservation techniques [180]. Lyophilisation is simple and easy to operate, showing that lyophilised samples are more resistant to oxidation and less efficient in microencapsulation [181]. The process disadvantages include high energy use, long processing time, and high production costs [182]. Freeze drying, also known as lyophilisation, is a simple process (Figure 13) that is used to dehydrate most materials sensitive to heat and aromas such as oils. Sublimation is the major principle involved in this drying process, where water passes directly from a solid state to a vapour state without passing through the liquid state. Before starting this process, the oil is dissolved in water and frozen [178]. Afterwards, the pressure is reduced and heat is added to allow the frozen water to sublimate the material directly from the solid phase to the gas phase. Freeze‐dried materials appear to have the maximum reten‐ tion of volatile compounds compared to spray drying, and this technique is used to mi‐ croencapsulate some oils, with high yields [179]. This method helps to better preserve the EO content in many types of herbs and spices compared with other preservation tech‐ niques [180]. Lyophilisation is simple and easy to operate, showing that lyophilised sam‐ ples are more resistant to oxidation and less efficient in microencapsulation [181]. The process disadvantages include high energy use, long processing time, and high produc‐ tion costs [182].

**Figure 13.** Schematic diagram of a freeze dryer (adapted from [163]). **Figure 13.** Schematic diagram of a freeze dryer (adapted from [163]).

#### *3.6. Supercritical Fluid (SCF) Technology 3.6. Supercritical Fluid (SCF) Technology*

Many pharmaceutical, cosmetic, and food industries use supercritical fluid technol‐ ogy (Figure 14) to form the microcapsules of essential oils due to their inherent ad‐ vantages. The use of a wide variety of materials that produce controlled particle sizes and morphologies, the easy solvent removal, the non‐degradation of the product, and being a non‐toxic method are some of the many advantages of SCF technology. Many pharmaceutical, cosmetic, and food industries use supercritical fluid technology (Figure 14) to form the microcapsules of essential oils due to their inherent advantages. The use of a wide variety of materials that produce controlled particle sizes and morphologies, the easy solvent removal, the non-degradation of the product, and being a non-toxic method are some of the many advantages of SCF technology.

The methods used for supercritical fluids are the precipitation of gas anti‐solvent, particles of saturated gas solutions, the extraction of fluid emulsions, and the rapid ex‐ pansion of supercritical solutions [183,184]. The supercritical solvent impregnation pro‐ cess has proven to be successful in a wide variety of substances (essential oils, fragrances, active pharmaceutical compounds, and dyes) and matrices (wood, polymers, cotton, and contact lenses). The methods used for supercritical fluids are the precipitation of gas anti-solvent, particles of saturated gas solutions, the extraction of fluid emulsions, and the rapid expansion of supercritical solutions [183,184]. The supercritical solvent impregnation process has proven to be successful in a wide variety of substances (essential oils, fragrances, active pharmaceutical compounds, and dyes) and matrices (wood, polymers, cotton, and contact lenses).

An alternative to spray drying (that degrades oils at high temperatures) is impregna‐ tion with supercritical solvent, as it is an ecological process where supercritical carbon dioxide is used as a green solvent. An alternative to spray drying (that degrades oils at high temperatures) is impregnation with supercritical solvent, as it is an ecological process where supercritical carbon dioxide is used as a green solvent.

The food, cosmetic, and pharmaceutical industries use a new technology to encapsu‐ late oils, called coaxial electrospraying (Figure 15) [185,186]. This system is used in two phases, with external and internal solutions being sprayed coaxially and simultaneously

In the electrospray process, the Taylor cone is composed of a core‐shell structure that is formed at the top of the spray nozzle, ending up with the polymeric solution encapsu‐

*Polymers* **2022**, *14*, x FOR PEER REVIEW 21 of 45

**Figure 14.** Flow chart of supercritical fluid technology. **Figure 14.** Flow chart of supercritical fluid technology. particle, or even prevent its aggregation. On the theoretical side, many existing process models are empirical or semi‐quantitatively empirical. The simulated results are not

**Figure 14.** Flow chart of supercritical fluid technology.

through two feed channels separated by a nozzle.

*3.7. Coaxial Electrospray System*

#### *3.7. Coaxial Electrospray System 3.7. Coaxial Electrospray System* enough for the quantitative control of the process, as numerical simulations, such as com‐

The food, cosmetic, and pharmaceutical industries use a new technology to encapsu‐ late oils, called coaxial electrospraying (Figure 15) [185,186]. This system is used in two phases, with external and internal solutions being sprayed coaxially and simultaneously through two feed channels separated by a nozzle. The food, cosmetic, and pharmaceutical industries use a new technology to encapsulate oils, called coaxial electrospraying (Figure 15) [185,186]. This system is used in two phases, with external and internal solutions being sprayed coaxially and simultaneously through two feed channels separated by a nozzle. putational fluid dynamics modelling, have been used to simulate the formation of the liq‐ uid cone and atomisation in a single axial electrospray process [188]. In summary, more experimental and theoretical study is needed to better understand the physical nature of coaxial electrospray and to provide quantitative guidance for process control.

particle, or even prevent its aggregation. On the theoretical side, many existing process models are empirical or semi‐quantitatively empirical. The simulated results are not enough for the quantitative control of the process, as numerical simulations, such as com‐ **Figure 15.** Schematic representation of microencapsulation process by coaxial electrospraying (adapted from [144]).

putational fluid dynamics modelling, have been used to simulate the formation of the liq‐ uid cone and atomisation in a single axial electrospray process [188]. In summary, more experimental and theoretical study is needed to better understand the physical nature of coaxial electrospray and to provide quantitative guidance for process control. In the electrospray process, the Taylor cone is composed of a core-shell structure that is formed at the top of the spray nozzle, ending up with the polymeric solution encapsulating the internal liquid. This method is distinguished by its ease and efficiency, and the maximum speed of the core material. The coaxial electrospray system provides a uniform size distribution, a high encapsulation efficiency, and an effective protection of bioactivity. However, the encapsulation efficiency and the stability of the microcapsules are affected by the wall materials [186]. Furthermore, controlling the process in coaxial electrospraying is difficult to some extent [187].

In experimental terms, the reported work on coaxial electrospray is based on individual laboratory experiments, consisting of specific combinations of materials and empirical process parameters. The fabrication of polymeric microparticles and nanoparticles is hampered by the lack of standard protocols. Regarding the collection of particles, the methodology cannot facilitate the hardening of the shell or maintain the morphology of particle, or even prevent its aggregation. On the theoretical side, many existing process models are empirical or semi-quantitatively empirical. The simulated results are not enough for the quantitative control of the process, as numerical simulations, such as computational fluid dynamics modelling, have been used to simulate the formation of the liquid cone and atomisation in a single axial electrospray process [188]. In summary, more experimental and

theoretical study is needed to better understand the physical nature of coaxial electrospray and to provide quantitative guidance for process control. the Wurster system, uses a coating chamber that has a cylindrical steel nozzle (used to spray the coating material) and a cribriform bottom plate, coating small particles (100 μm). This multilayer coating procedure helps to reduce particle defects, although it is a time‐

**Figure 15.** Schematic representation of microencapsulation process by coaxial electrospraying

Fluidised bed coating is one of the most efficient coating methods, in which the in‐ gredients can be mixed, granulated, and dried in the same container. Consequently, the handling and processing time of the material is reduced. This approach was recently used to encapsulate fish oil by spraying and coating it (Figure 16) [189]. Fluidised bed coating is carried out by suspending the solid particles of the core material by an air stream under controlled temperature and humidity and then sprayed, building, over time, a thin layer on the surface of the suspended particles. This material must have an acceptable viscosity for atomisation, and the pumping should be able to form an appropriate film and be ther‐

There are several methods used in fluidised bed coating, including top spray, bottom spray, and tangential spray methods. In the top spray system, the coating solution is sprayed in the opposite direction with air in the fluid bed. The opposite flows lead to an increase in the efficiency of encapsulation and the prevention of agglomerates formation, achieving microcapsules with a size between 2 and 100 μm. The bottom spray, known as

#### *3.8. Fluidized Bed Coating* consuming process. On the other hand, tangential spray consists of a coating chamber

(adapted from [144]).

mally stable [190].

*3.8. Fluidized Bed Coating*

Fluidised bed coating is one of the most efficient coating methods, in which the ingredients can be mixed, granulated, and dried in the same container. Consequently, the handling and processing time of the material is reduced. This approach was recently used to encapsulate fish oil by spraying and coating it (Figure 16) [189]. Fluidised bed coating is carried out by suspending the solid particles of the core material by an air stream under controlled temperature and humidity and then sprayed, building, over time, a thin layer on the surface of the suspended particles. This material must have an acceptable viscosity for atomisation, and the pumping should be able to form an appropriate film and be thermally stable [190]. with a rotating bottom of the same diameter as the chamber. During the process, the drum is raised to create a space between the edge of the chamber and the drum. A tangential nozzle is placed above the rotating drum, where the coating material is released. Then, the particles move through the space into the spray zone and are finally encapsulated [191]. During this process, there are three mechanical forces, namely, centrifugal force, lifting force, and gravity. The particles to be coated must be spherical and dense, and must have a narrow size distribution and perfect fluidity, with the non‐spherical particles having the largest pos‐ sible surface area and requiring more coating material.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 22 of 45

**Figure 16.** Schematic representation of bottom spray fluidised bed coating process (adapted from [166]). **Figure 16.** Schematic representation of bottom spray fluidised bed coating process (adapted from [166]).

This technique has a low operating cost and a high thermal efficiency process, allow‐ ing total temperature control. However, it can be time consuming, which becomes a dis‐ advantage [173]. There are several methods used in fluidised bed coating, including top spray, bottom spray, and tangential spray methods. In the top spray system, the coating solution is sprayed in the opposite direction with air in the fluid bed. The opposite flows lead to an increase in the efficiency of encapsulation and the prevention of agglomerates formation, achieving microcapsules with a size between 2 and 100 µm. The bottom spray, known as the Wurster system, uses a coating chamber that has a cylindrical steel nozzle (used to spray the coating material) and a cribriform bottom plate, coating small particles (100 µm). This multilayer coating procedure helps to reduce particle defects, although it is a timeconsuming process. On the other hand, tangential spray consists of a coating chamber with a rotating bottom of the same diameter as the chamber. During the process, the drum is raised to create a space between the edge of the chamber and the drum. A tangential nozzle is placed above the rotating drum, where the coating material is released. Then, the particles move through the space into the spray zone and are finally encapsulated [191]. During this process, there are three mechanical forces, namely, centrifugal force, lifting force, and gravity.

The particles to be coated must be spherical and dense, and must have a narrow size distribution and perfect fluidity, with the non-spherical particles having the largest possible surface area and requiring more coating material.

This technique has a low operating cost and a high thermal efficiency process, allowing total temperature control. However, it can be time consuming, which becomes a disadvantage [173].

#### *3.9. Layer-by-Layer Self-Assembly 3.9. Layer‐by‐Layer Self‐Assembly*  Layer‐by‐layer (LbL) is a relatively simple and promising technique for the encapsu‐

Layer-by-layer (LbL) is a relatively simple and promising technique for the encapsulation, stabilisation, storage, and release of several active compounds [192]. This method consists of alternating the adsorption of oppositely charged wall materials through many intermolecular interactions onto a charged substrate (Figure 17). The microcapsules have good chemical and mechanical stability through a formation mechanism constituted by irreversible electrostatic interactions that allow the adsorption of successive layers of polyelectrolytes [193]. The adsorption of the layers is normally carried out by immersing the suspension in alternate solutions of cationic and anionic polymers, with washing processes being carried out after the deposition of each layer [194]. This technique has significant advantages over other microencapsulation methods, because it allows the control of the permeability, morphology, composition, size, and wall thickness of the microcapsules by adjusting the number of layers and experimental conditions [195]. Controlling these parameters allows a better adaptation of the microcapsule to its functionality in the target application. However, most LbL systems have some restrictions in terms of biocompatibility [196]. lation, stabilisation, storage, and release of several active compounds [192]. This method consists of alternating the adsorption of oppositely charged wall materials through many intermolecular interactions onto a charged substrate (Figure 17). The microcapsules have good chemical and mechanical stability through a formation mechanism constituted by irreversible electrostatic interactions that allow the adsorption of successive layers of poly‐ electrolytes [193]. The adsorption of the layers is normally carried out by immersing the suspension in alternate solutions of cationic and anionic polymers, with washing pro‐ cesses being carried out after the deposition of each layer [194]. This technique has signif‐ icant advantages over other microencapsulation methods, because it allows the control of the permeability, morphology, composition, size, and wall thickness of the microcapsules by adjusting the number of layers and experimental conditions [195]. Controlling these parameters allows a better adaptation of the microcapsule to its functionality in the target application. However, most LbL systems have some restrictions in terms of biocompati‐ bility [196].

*Polymers* **2022**, *14*, x FOR PEER REVIEW 23 of 45

**Figure 17.** Layer‐by‐layer (LbL) self‐assembly microcapsules (adapted from [197]). **Figure 17.** Layer-by-layer (LbL) self-assembly microcapsules (adapted from [197]).

#### **4. Microencapsulation of Essential Oils 4. Microencapsulation of Essential Oils**

Microencapsulation is an alternative that can be utilised to overcome several limita‐ tions in the application of essential oils. This application is profoundly affected by the high volatility and chemically unstable nature of EOs [198]. In addition, EOs are compounds that can be easily degraded due to interactions with other chemical components and ex‐ posure to several factors such as light, temperature, and oxygen [166]. Microencapsulation is an alternative that can be utilised to overcome several limitations in the application of essential oils. This application is profoundly affected by the high volatility and chemically unstable nature of EOs [198]. In addition, EOs are compounds that can be easily degraded due to interactions with other chemical components and exposure to several factors such as light, temperature, and oxygen [166].

Essential oils can be "trapped" in microcapsules, which act as micro‐reservoirs, en‐ suring excellent protection [199]. The encapsulation process, where small particles are en‐ closed in solid carriers to increase their protection, has the ability to reduce evaporation, promote easier handling, and control the release of essential oils during storage and ap‐ plication [199]. Furthermore, through microencapsulation, it is possible to change the ap‐ Essential oils can be "trapped" in microcapsules, which act as micro-reservoirs, ensuring excellent protection [199]. The encapsulation process, where small particles are enclosed in solid carriers to increase their protection, has the ability to reduce evaporation, promote easier handling, and control the release of essential oils during storage and application [199]. Furthermore, through microencapsulation, it is possible to change the appearance of EOs (which behave like a powder), without changing their structure and properties [177].

pearance of EOs (which behave like a powder), without changing their structure and properties [177]. In EO microencapsulation, the first step is often to emulsify or disperse the essential oils in an aqueous solution of a wall material, which also acts as an emulsifier. This process happens because the EOs exist in liquid form at room temperature. Then, the resulting microcapsules must be dried under controlled conditions, so that the loss of the encapsu‐ lated material by volatilisation is reduced [177]. One of the areas that has also aroused interest in the microencapsulation of EOs is in the agrochemical industry. Yang et al. pre‐ pared and characterised microcapsules based on polyurea, containing essential oils as an active agent for possible applications in the controlled release of agrochemical compounds [200]. The microcapsules were synthesised by O/W emulsion interfacial polymerisation and the synthetic conditions that showed the best results were used to encapsulate four essential oils (lemongrass, lavender, sage, and thyme), capable of interfering with the seed In EO microencapsulation, the first step is often to emulsify or disperse the essential oils in an aqueous solution of a wall material, which also acts as an emulsifier. This process happens because the EOs exist in liquid form at room temperature. Then, the resulting microcapsules must be dried under controlled conditions, so that the loss of the encapsulated material by volatilisation is reduced [177]. One of the areas that has also aroused interest in the microencapsulation of EOs is in the agrochemical industry. Yang et al. prepared and characterised microcapsules based on polyurea, containing essential oils as an active agent for possible applications in the controlled release of agrochemical compounds [200]. The microcapsules were synthesised by O/W emulsion interfacial polymerisation and the synthetic conditions that showed the best results were used to encapsulate four essential oils (lemongrass, lavender, sage, and thyme), capable of interfering with the seed germination and root elongation of some plants. In cases of pest control, biological pesticides must be more effective than synthetic pesticides.

germination and root elongation of some plants. In cases of pest control, biological pesti‐ cides must be more effective than synthetic pesticides. Bagle et al. reported success in encapsulating neem oil, an effective biological insecticide, in phenol formaldehyde (PF) microcapsules [201]. The microcapsules were obtained using an in situ polymerisation process in an O/W emulsion and their size was determined

using a particle size analyser. Controlled release was monitored by measuring optical observations in the UV range. Figure 18 shows scanning electron microscopy (SEM) micrographs of PF microcapsules containing neem oil. It was possible to visualise that the PF microcapsules were spherical and globular, with diameters between 30 and 50 µm at 400–500 rpm. The microcapsules' surface was considered quite smooth and can be useful regarding the protection and sustained release of the neem oil inside. μm at 400–500 rpm. The microcapsules' surface was considered quite smooth and can be useful regarding the protection and sustained release of the neem oil inside. The chemical constitution of synthesised microcapsules was confirmed by Fourier‐ transform infrared spectroscopy (FTIR), and it was found to be a good thermal stability of MCs needed for the long‐term preservation of the core, and it was concluded that neem oil can be better preserved in PF microcapsules. determined using a particle size analyser. Controlled release was monitored by measuring optical observations in the UV range. Figure 18 shows scanning electron microscopy (SEM) micrographs of PF microcapsules containing neem oil. It was possible to visualise that the PF microcapsules were spherical and globular, with diameters between 30 and 50 μm at 400–500 rpm. The microcapsules' surface was considered quite smooth and can be useful regarding the protection and sustained release of the neem oil inside. The chemical constitution of synthesised microcapsules was confirmed by Fourier‐

Bagle et al. reported success in encapsulating neem oil, an effective biological insec‐ ticide, in phenol formaldehyde (PF) microcapsules [201]. The microcapsules were ob‐ tained using an in situ polymerisation process in an O/W emulsion and their size was determined using a particle size analyser. Controlled release was monitored by measuring optical observations in the UV range. Figure 18 shows scanning electron microscopy (SEM) micrographs of PF microcapsules containing neem oil. It was possible to visualise that the PF microcapsules were spherical and globular, with diameters between 30 and 50

Bagle et al. reported success in encapsulating neem oil, an effective biological insec‐ ticide, in phenol formaldehyde (PF) microcapsules [201]. The microcapsules were ob‐ tained using an in situ polymerisation process in an O/W emulsion and their size was

*Polymers* **2022**, *14*, x FOR PEER REVIEW 24 of 45

*Polymers* **2022**, *14*, x FOR PEER REVIEW 24 of 45

**Figure 18.** SEM micrographs (**a**–**c**) of phenol formaldehyde microcapsules containing neem oil [201]. **Figure 18.** SEM micrographs (**a**–**c**) of phenol formaldehyde microcapsules containing neem oil [201].

The controlled release behaviour of PF microcapsules containing neem oil was stud‐ ied and the experimental data are shown in Figure 19. A release of about 30% was ob‐ served after 6 h, confirmed by the decrease in absorbance over time. The chemical constitution of synthesised microcapsules was confirmed by Fouriertransform infrared spectroscopy (FTIR), and it was found to be a good thermal stability of MCs needed for the long-term preservation of the core, and it was concluded that neem oil can be better preserved in PF microcapsules. **Figure 18.** SEM micrographs (**a**–**c**) of phenol formaldehyde microcapsules containing neem oil [201].

The controlled release behaviour of PF microcapsules containing neem oil was studied and the experimental data are shown in Figure 19. A release of about 30% was observed after 6 h, confirmed by the decrease in absorbance over time. The controlled release behaviour of PF microcapsules containing neem oil was stud‐ ied and the experimental data are shown in Figure 19. A release of about 30% was ob‐ served after 6 h, confirmed by the decrease in absorbance over time.

**Figure 19.** Controlled release of core material over time [201]. **Figure 19.** Controlled release of core material over time [201].

Like neem oil, other essential oils also have insecticidal properties, such as *Rosmarinus officinalis* and *Zataria multiflora* (Lamiaceae), that can be used as pesticides for storedproduct pests. In the study carried out by Ahsaei et al., these oils were encapsulated in octenyl succinic anhydride (OSA) starch to test their insecticidal activity against *Tribolium confusum*. The microcapsules were obtained using an O/W emulsion and dried using the

spray drying technique [202]. The solid formulations were characterised by particle size, encapsulation efficiency, and water activity. The release rate under storage conditions was measured over a period of 40 days, and the insecticidal activity against *T. confusum* was determined using specific bioassays. It was concluded that the encapsulation efficiency depends directly on the surfactant-to-oil ratio. Regarding the morphology of microcapsules loaded with OEs, SEM micrographs reveal the presence of oval and spherical microcapsules with irregular surfaces. The microcapsules appear to be devoid of cracks or fractures, which is an advantageous feature for protecting the oil. The results also showed an optimised release of pesticides from controlled release formulations, which maximises their biological activity for a longer time. spray drying technique [202]. The solid formulations were characterised by particle size, encapsulation efficiency, and water activity. The release rate under storage conditions was measured over a period of 40 days, and the insecticidal activity against *T. confusum* was determined using specific bioassays. It was concluded that the encapsulation efficiency depends directly on the surfactant‐to‐oil ratio. Regarding the morphology of microcap‐ sules loaded with OEs, SEM micrographs reveal the presence of oval and spherical micro‐ capsules with irregular surfaces. The microcapsules appear to be devoid of cracks or frac‐ tures, which is an advantageous feature for protecting the oil. The results also showed an optimised release of pesticides from controlled release formulations, which maximises their biological activity for a longer time.

Like neem oil, other essential oils also have insecticidal properties, such as *Rosma‐ rinus officinalis* and *Zataria multiflora* (Lamiaceae), that can be used as pesticides for stored‐ product pests. In the study carried out by Ahsaei et al., these oils were encapsulated in octenyl succinic anhydride (OSA) starch to test their insecticidal activity against *Tribolium confusum*. The microcapsules were obtained using an O/W emulsion and dried using the

*Polymers* **2022**, *14*, x FOR PEER REVIEW 25 of 45

The food sector is probably the sector where the microencapsulation of essential oils is most explored, with the encapsulation of flavours being one of the great interests of this industry. Flavours are necessary for some foods, to promote consumer satisfaction and the consumption of those products. Nevertheless, the flavour stability in foods has been a challenge for this sector in order to achieve quality and acceptability. The food sector is probably the sector where the microencapsulation of essential oils is most explored, with the encapsulation of flavours being one of the great interests of this industry. Flavours are necessary for some foods, to promote consumer satisfaction and the consumption of those products. Nevertheless, the flavour stability in foods has been a challenge for this sector in order to achieve quality and acceptability.

For the encapsulation of a flavour, Fernandes et al. evaluated, by spray drying, the effects of the partial or total substitution of arabic gum with modified starch, maltodextrin, and inulin in the encapsulation of rosemary essential oil [203]. For the encapsulation of a flavour, Fernandes et al. evaluated, by spray drying, the effects of the partial ortotal substitution of arabic gum with modified starch, maltodextrin, and inulin in the encapsulation of rosemary essential oil [203].

Regarding the characterisation of microcapsules, moisture content, wettability and solubility, density and apparent density, and oil retention was determined. From SEM observations (Figure 20), the authors found that there was no evidence of cracking in the particles using any of the encapsulating formulations, ensuring low gas permeability and thus better protecting the EO of rosemary. Differences were observed in the surface of each type of particle, showing that the particles have a spherical shape. It was concluded that the total substitution of arabic gum with modified starch or a mixture of modified starch and maltodextrin did not affect the efficiency of the encapsulation, increasing the possibility of developing new formulations of encapsulants. With the addition of inulin, the oil retention of particles decreased. However, the combination of modified starch and inulin was shown to be a viable substitute for arabic gum in foods. Regarding the characterisation of microcapsules, moisture content, wettability and solubility, density and apparent density, and oil retention was determined. From SEM observations (Figure 20), the authors found that there was no evidence of cracking in the particles using any of the encapsulating formulations, ensuring low gas permeability and thus better protecting the EO of rosemary. Differences were observed in the surface of each type of particle, showing that the particles have a spherical shape. It was concluded that the total substitution of arabic gum with modified starch or a mixture of modified starch and maltodextrin did not affect the efficiency of the encapsulation, increasing the possibility of developing new formulations of encapsulants. With the addition of inulin, the oil retention of particles decreased. However, the combination of modified starch and inulin was shown to be a viable substitute for arabic gum in foods.

**Figure 20.** Scanning electron micrographs of the particles containing rosemary essential oil [203]. (A: arabic gum; B: arabic gum/maltodextrin; C: arabic gum/inulin; D: starch; E: modified starch/maltodextrin; F: modified starch/inulin). **Figure 20.** Scanning electron micrographs of the particles containing rosemary essential oil [203]. (**A**): arabic gum; (**B**): arabic gum/maltodextrin; (**C**): arabic gum/inulin; (**D**): starch; (**E**): modified starch/maltodextrin; (**F**): modified starch/inulin.

A group of researchers compared the release properties of three different microcapsules, namely gelatin microcapsules loaded with holy basil essential oil (HBEO) (designated as UC), UC coated with aluminium carboxymethylcellulose (CC), and UC coated with aluminium compound carboxymethyl cellulose–beeswax (CB) [204]. To be applied as a feed additive, the HBEO was encapsulated in order to be a potential alternative to antibiotic growth promoters (AGP). However, its benefits depend on the available amount in the gastrointestinal tract.

The SEM technique was used to characterise the internal and external factors of the microcapsule surface morphology. According to Figure 21, UC microcapsules (Figure 21a) are almost spherical in shape and after coating, the CC (Figure 21b) and CB (Figure 21c) microcapsules are more spherical. Upon magnification of these micrographs, it was possible to verify that UC microcapsules have a spongy structure (Figure 21d) and that CC (Figure 21e) and CB (Figure 21) microcapsules are denser. When cut transversely, UC microcapsules seem to have a gelatinous morphology (Figure 21g), whereas the CC (Figure 21h) and CB microcapsules (Figure 21i) reveal a thicker and more compact outer coating layer with a honeycomb structure. This method of encapsulation demonstrated an effective process for improving HBEO efficacy for pathogen reduction in the distal region of the intestine. are almost spherical in shape and after coating, the CC (Figure 21b) and CB (Figure 21c) microcapsules are more spherical. Upon magnification of these micrographs, it was pos‐ sible to verify that UC microcapsules have a spongy structure (Figure 21d) and that CC (Figure 21e) and CB (Figure 21) microcapsules are denser. When cut transversely, UC mi‐ crocapsules seem to have a gelatinous morphology (Figure 21g), whereas the CC (Figure 21h) and CB microcapsules (Figure 21i) reveal a thicker and more compact outer coating layer with a honeycomb structure. This method of encapsulation demonstrated an effec‐ tive process for improving HBEO efficacy for pathogen reduction in the distal region of the intestine.

A group of researchers compared the release properties of three different microcap‐ sules, namely gelatin microcapsules loaded with holy basil essential oil (HBEO) (desig‐ nated as UC), UC coated with aluminium carboxymethylcellulose (CC), and UC coated with aluminium compound carboxymethyl cellulose–beeswax (CB) [204]. To be applied as a feed additive, the HBEO was encapsulated in order to be a potential alternative to antibiotic growth promoters (AGP). However, its benefits depend on the available amount

The SEM technique was used to characterise the internal and external factors of the microcapsule surface morphology. According to Figure 21, UC microcapsules (Figure 21a)

*Polymers* **2022**, *14*, x FOR PEER REVIEW 26 of 45

in the gastrointestinal tract.

**Figure 21.** SEM micrographs of UC, CC, and CB gelatin‐based microcapsules: (**a**) whole UC; (**b**) whole CC; (**c**) whole CB; (**d**) external surface of UC; (**e**) external surface of CC; (**f**) external surface of CB; (**g**) inner edge of UC; (**h**) inner edge of CC; (**i**) inner edge of CB [204]*.* **Figure 21.** SEM micrographs of UC, CC, and CB gelatin-based microcapsules: (**a**) whole UC; (**b**) whole CC; (**c**) whole CB; (**d**) external surface of UC; (**e**) external surface of CC; (**f**) external surface of CB; (**g**) inner edge of UC; (**h**) inner edge of CC; (**i**) inner edge of CB [204].

Regarding food safety, the use of antimicrobial packaging materials offers the poten‐ tial to retard the growth rate of spoilage microorganisms. The physical and antimicrobial properties of nanofibres manufactured for active packaging systems were studied by Munhuweyi et al. [205]. Microcapsules and active nanofibres derived from the precipita‐ tion of β‐cyclodextrin (β‐CD) with essential oils of cinnamon and oregano were developed Regarding food safety, the use of antimicrobial packaging materials offers the potential to retard the growth rate of spoilage microorganisms. The physical and antimicrobial properties of nanofibres manufactured for active packaging systems were studied by Munhuweyi et al. [205]. Microcapsules and active nanofibres derived from the precipitation of β-cyclodextrin (β-CD) with essential oils of cinnamon and oregano were developed and their antifungal activity in vitro against *Botrytis* sp. was examined. To induce microencapsulation, the solutions were subjected to co-precipitation. It was verified that cinnamon microcapsules have greater antimicrobial efficacy when compared to oregano. As food preservatives, this microencapsulation system could have promising applications in the development of active packaging systems.

Using the thermogravimetric analysis (TGA) technique, the initial weight loss for simple β-CD occurred at ~100 ◦C and the greatest weight loss at ~330 ◦C (Figure 22a). The degradation temperature of β-CD in the CIN/β-CD and OREG/β-CD complexes decreased from ~330 ◦C to ~270 ◦C (Figure 22b,c). Comparing the TGA curves, there is

*Polymers* **2022**, *14*, x FOR PEER REVIEW 27 of 45

the development of active packaging systems.

a difference between them, demonstrating the presence of chemical and guest molecule interaction in the complex. between them, demonstrating the presence of chemical and guest molecule interaction in the complex.

and their antifungal activity in vitro against *Botrytis* sp. was examined. To induce micro‐ encapsulation, the solutions were subjected to co‐precipitation. It was verified that cinna‐ mon microcapsules have greater antimicrobial efficacy when compared to oregano. As food preservatives, this microencapsulation system could have promising applications in

Using the thermogravimetric analysis (TGA) technique, the initial weight loss for simple β‐CD occurred at ~100 °C and the greatest weight loss at ~330 °C (Figure 22a). The degradation temperature of β‐CD in the CIN/β‐CD and OREG/β‐CD complexes decreased from ~330 °C to ~270 °C (Figure 22b,c). Comparing the TGA curves, there is a difference

**Figure 22.** TGA curves of (**a**) plain β‐cyclodextrin (β‐CD), (**b**) microencapsulated cinnamon (CIN/β‐ CD), and (**c**) oregano (OREG/β‐CD) [205]. **Figure 22.** TGA curves of (**a**) plain β-cyclodextrin (β-CD), (**b**) microencapsulated cinnamon (CIN/β-CD), and (**c**) oregano (OREG/β-CD) [205].

Using the simple coacervation method, Leimann et al. encapsulated lemongrass, which is known for its broad spectrum antimicrobial activity [206]. Poly(vinyl alcohol) crosslinked with glutaraldehyde was used as the wall‐forming polymer. The influence of the agitation rate and the fraction of oil volume on the microcapsule size distribution was evaluated. Sodium dodecyl sulphate (SDS) and poly(vinyl pyrrolidone) were tested to prevent the agglomeration of microcapsules during the process. The microcapsules did not show agglomeration when 0.03% by weight of SDS was used. The composition and antimicrobial properties of the encapsulated oil were determined, demonstrating that the microencapsulation process did not deteriorate the encapsulated essential oil. Using the simple coacervation method, Leimann et al. encapsulated lemongrass, which is known for its broad spectrum antimicrobial activity [206]. Poly(vinyl alcohol) crosslinked with glutaraldehyde was used as the wall-forming polymer. The influence of the agitation rate and the fraction of oil volume on the microcapsule size distribution was evaluated. Sodium dodecyl sulphate (SDS) and poly(vinyl pyrrolidone) were tested to prevent the agglomeration of microcapsules during the process. The microcapsules did not show agglomeration when 0.03% by weight of SDS was used. The composition and antimicrobial properties of the encapsulated oil were determined, demonstrating that the microencapsulation process did not deteriorate the encapsulated essential oil.

Cyclodextrins (CDs) are important supramolecular microcapsule hosts in foods and other fields, and the essential oil of *Laurus nobilis* (LEO) has natural antioxidant properties in food due to its main constituents being terpenic alcohols and phenols. Forthese reasons, Li et al. isolated LEO by microwave‐assisted hydrodistillation [207]. The authors prepared chitosan (CS) microcapsules loaded with citrus essential oils (CEOs: Ɗ‐limonene, linalool, a‐terpinene, myrcene, and a‐pinene) using six different emulsifiers (Tween 20, Tween 40, Tween 60, Tween 60/Tween 20/Span 80 1:1, Tween 20/sodium dodecyl benzene sulfonate (SDBS) 1:1, Span 80) through an emulsion gelation technique [208]. After preparing β‐ cyclodextrin (β‐CD) microcapsules and their derivatives, several affecting factors were examined in detail. Cyclodextrins (CDs) are important supramolecular microcapsule hosts in foods and other fields, and the essential oil of *Laurus nobilis* (LEO) has natural antioxidant properties in food due to its main constituents being terpenic alcohols and phenols. For these reasons, Li et al. isolated LEO by microwave-assisted hydrodistillation [207]. The authors prepared chitosan (CS) microcapsules loaded with citrus essential oils (CEOs: D-limonene, linalool, a-terpinene, myrcene, and a-pinene) using six different emulsifiers (Tween 20, Tween 40, Tween 60, Tween 60/Tween 20/Span 80 1:1, Tween 20/sodium dodecyl benzene sulfonate (SDBS) 1:1, Span 80) through an emulsion gelation technique [208]. After preparing βcyclodextrin (β-CD) microcapsules and their derivatives, several affecting factors were examined in detail.

Figure 23 shows the total antioxidant activity of LEO. LEO caused Mo (VI) to be de‐ oxidised to become Mo (V) through a mechanism of total antioxidant activity. Mo (V) ex‐ hibits maximum absorption at 695 nm and has a stronger antioxidant activity; the greater the concentration of Mo (V) solution, the greater the absorbency of the solution. With the Figure 23 shows the total antioxidant activity of LEO. LEO caused Mo (VI) to be deoxidised to become Mo (V) through a mechanism of total antioxidant activity. Mo (V) exhibits maximum absorption at 695 nm and has a stronger antioxidant activity; the greater the concentration of Mo (V) solution, the greater the absorbency of the solution. With the increase in absorbance of the solutions, there was an increase in the concentrations of the sample, causing the antioxidant activity to increase significantly.

sample, causing the antioxidant activity to increase significantly.

increase in absorbance of the solutions, there was an increase in the concentrations of the

The microcapsules were analysed and the results indicate that the choice of emulsi‐ fier significantly affects the size and effectiveness of incorporating the microcapsules. The microcapsules were analysed and the results indicate that the choice of emulsifier significantly affects the size and effectiveness of incorporating the microcapsules.

Figure 24a presents the FTIR spectra observed in CS, CEOs, and four groups of mi‐ crocapsules prepared with different emulsifiers. In the CEOs curve, the peak at 886 cm−<sup>1</sup> corresponds to the absorption of limonene. The strong methylene/methyl band occurs at 1435 cm−1, and at 1646 cm−1, the C=O stretching vibration appears. Peaks corresponding to the asymmetric and symmetrical modes of the CH2 elongation vibration appear for Span 80 and Tween 60, and the new connections can be seen at 2922 cm−1. Through these results, it was possible to observe that the CEOs were incorporated in the microcapsules, showing benefits for inhibiting them from oxidation and volatilisation. Figure 24a presents the FTIR spectra observed in CS, CEOs, and four groups of microcapsules prepared with different emulsifiers. In the CEOs curve, the peak at 886 cm−<sup>1</sup> corresponds to the absorption of limonene. The strong methylene/methyl band occurs at 1435 cm−<sup>1</sup> , and at 1646 cm−<sup>1</sup> , the C=O stretching vibration appears. Peaks corresponding to the asymmetric and symmetrical modes of the CH<sup>2</sup> elongation vibration appear for Span 80 and Tween 60, and the new connections can be seen at 2922 cm−<sup>1</sup> . Through these results, it was possible to observe that the CEOs were incorporated in the microcapsules, showing benefits for inhibiting them from oxidation and volatilisation. *Polymers* **2022**, *14*, x FOR PEER REVIEW 29 of 45

**Figure 24.** (**a**) FTIR spectroscopy, (**b**) X‐ray diffraction of pure chitosan (CS), control group (CK), Tween 60, Tween 20/Span 80, and Span 80 [208]. **Figure 24.** (**a**) FTIR spectroscopy, (**b**) X-ray diffraction of pure chitosan (CS), control group (CK), Tween 60, Tween 20/Span 80, and Span 80 [208].

Mehran et al. carried out a study of the microencapsulation of spearmint essential oil

The infrared spectra of pure SEO, pure matrix (containing inulin and arabic gum), and microcapsules are shown in Figure 25. In the SEO spectrum, the characteristic peaks at 801 cm−<sup>1</sup> and 894 cm−<sup>1</sup> are ascribed to =CH vibrations. The C‐O‐C elongation corre‐ sponds to the peak at 1109 cm−<sup>1</sup> and the C=O elongation corresponds to the peak 1675 cm−1. In the matrix spectrum, a wide band at 3392 cm−<sup>1</sup> is related to the hydroxylated group. In relation to the peak at 1030 cm−1, it can be associated with the strong absorption bands of the C‐O‐C elongation. In the microcapsule spectrum, it can be observed that it is quite similar to the matrix, and that the peaks related to the SEO disappear or are absent, which may be related to the overlap of the peaks of the matrix and SEO due to the low weight fraction of SEO in the total weight of the microcapsules. Through this spectrum, it was possible to verify the successful encapsulation of the SEO (peaks at 1673 cm‐ <sup>1</sup> and 900 cm‐

crocapsules was spray drying. The microcapsules were characterised for oil retention, en‐ capsulation efficiency, hygroscopicity, and carbon content, having as ideal conditions 35% solid wall, 4% essential oil concentration, and 110 °C inlet temperature, with maximum retention of 91% of oil. To confirm that the SEO was encapsulated, this group of research‐ ers used differential scanning calorimetry (DSC) and FTIR characterisation techniques.

1).

A second step in the characterisation of the microcapsules was the analysis of the crystallographic structure. Through X-ray diffraction (XRD) analysis (Figure 24b), it was possible to observe that CS exhibits a diffraction pattern with a broad band centred at 2θ 20◦ , thus indicating the existence of an amorphous structure. Comparing the CS with the microcapsule groups, the latter exhibit a significant reduction in this broad band. This reduction in intensity is due to the destruction of the CS structure, which can be attributed to a change in the arrangement of the molecules in the crystalline chain.

To develop a new use of functional EOs, Karimi Sani et al. studied the influence of process parameters on the characteristics of microencapsulated essential oil *Melissa officinalis* using whey protein isolate (WPI) and sodium caseinate (NaCS). The impacts of these variables were examined using the response surface methodology. Smaller particle sizes were obtained for higher amounts of WPI with the lowest level of applied sonication power. The results of the desirability function indicate that the maximum amount of WPI with an ultrasound power of 50 W led to the smallest particle size and the lowest zeta potential and turbidity. In this study, the ultrasonic technique showed potential in the use of milk proteins to produce microparticles with OEs. The obtained results showed that the microcapsules loaded with *Melissa officinalis* can preserve the bioactive compounds and induce flavour stability, enabling their use in food formulations and pharmaceutical products.

Mehran et al. carried out a study of the microencapsulation of spearmint essential oil (SEO), using a mixture of inulin and arabic gum as wall material in order to be used in the food and pharmaceutical industry [209]. The technique used for the formation of the microcapsules was spray drying. The microcapsules were characterised for oil retention, encapsulation efficiency, hygroscopicity, and carbon content, having as ideal conditions 35% solid wall, 4% essential oil concentration, and 110 ◦C inlet temperature, with maximum retention of 91% of oil. To confirm that the SEO was encapsulated, this group of researchers used differential scanning calorimetry (DSC) and FTIR characterisation techniques.

The infrared spectra of pure SEO, pure matrix (containing inulin and arabic gum), and microcapsules are shown in Figure 25. In the SEO spectrum, the characteristic peaks at 801 cm−<sup>1</sup> and 894 cm−<sup>1</sup> are ascribed to =CH vibrations. The C-O-C elongation corresponds to the peak at 1109 cm−<sup>1</sup> and the C=O elongation corresponds to the peak 1675 cm−<sup>1</sup> . In the matrix spectrum, a wide band at 3392 cm−<sup>1</sup> is related to the hydroxylated group. In relation to the peak at 1030 cm−<sup>1</sup> , it can be associated with the strong absorption bands of the C-O-C elongation. In the microcapsule spectrum, it can be observed that it is quite similar to the matrix, and that the peaks related to the SEO disappear or are absent, which may be related to the overlap of the peaks of the matrix and SEO due to the low weight fraction of SEO in the total weight of the microcapsules. Through this spectrum, it was possible to verify the successful encapsulation of the SEO (peaks at 1673 cm−<sup>1</sup> and 900 cm−<sup>1</sup> ).

The double barrier release system is a method used for essential oils that have antifungal activity, even against drug-resistant fungi. However, there are some limitations due to the sensitivity to pH, temperature, and light. Adepu et al. encapsulated three essential oils (thymol, eugenol, and carvacrol) in a polylactic acid shell with high encapsulation efficiency to achieve their synergistic antifungal activity using the coacervation phase separation method. These were incorporated into bacterial cellulose (a nanofibre fibrous hydrogel) [210]. An antifungal test was performed on the *Candida albicans* fungus model (a cause of common oral and vaginal infections). Another test was carried out—a transvaginal drug release study in vitro—to compare the release of microcapsules like colloids and composites, where the latter exhibited a controlled release. Through several studies, such as the SEM technique, it was found that the average size and size distribution of the microcapsules depends on the concentration of the used polymer (poly(lactic acid)) and surfactant (poloxamer).

SEM images of BC loaded with microcapsules demonstrate a regular distribution and spherical shape, appearing to be well separated and stable in the stages of the preparation

process (Figure 26). From the highest magnification image, it was observed that the microcapsules were anchored to the nanofibre matrix. *Polymers* **2022**, *14*, x FOR PEER REVIEW 30 of 45

**Figure 25.** FTIR spectra of (**A**) pure SEO, (**B**) pure matrix, and (**C**) inulin and arabic gum‐based microcapsules [209]. **Figure 25.** FTIR spectra of (**A**) pure SEO, (**B**) pure matrix, and (**C**) inulin and arabic gum-based microcapsules [209].

The double barrier release system is a method used for essential oils that have anti‐ fungal activity, even against drug‐resistant fungi. However, there are some limitations due to the sensitivity to pH, temperature, and light. Adepu et al. encapsulated three es‐ sential oils (thymol, eugenol, and carvacrol) in a polylactic acid shell with high encapsu‐ lation efficiency to achieve their synergistic antifungal activity using the coacervation phase separation method. These were incorporated into bacterial cellulose (a nanofibre fibrous hydrogel) [210]. An antifungal test was performed on the *Candida albicans* fungus model (a cause of common oral and vaginal infections). Another test was carried out—a Repellent essential oils are becoming increasingly widespread due to their low toxicity and customer approval. Its application in textile materials has been widely developed. To optimise their application efficiency, it is important to develop long-lasting repellent textilesusing OEs. Specos et al. obtained citronella-loaded gelatin microcapsules through thecomplex coacervation method, which were applied to cotton fabrics in order to study the repellent effectiveness of the obtained fabrics [70]. The release of citronella by the treated tissues was monitored and the repellent activity evaluated by exposing a human hand and arm covered with the treated tissues to *Aedes aegypti* mosquitoes.

transvaginal drug release study in vitro—to compare the release of microcapsules like colloids and composites, where the latter exhibited a controlled release. Through several studies, such as the SEM technique, it was found that the average size and size distribution of the microcapsules depends on the concentration of the used polymer (poly(lactic acid)) and surfactant (poloxamer). SEM images of BC loaded with microcapsules demonstrate a regular distribution and spherical shape, appearing to be well separated and stable in the stages of the preparation process (Figure 26). From the highest magnification image, it was observed that the mi‐ crocapsules were anchored to the nanofibre matrix. It was found that the tissues treated with citronella microcapsules present greater and more lasting protection against insects in comparison to fabrics sprayed with an ethanol solution of essential oil. Repellent textiles were obtained by filling cotton fabrics with microcapsule sludge, using a conventional drying method. This methodology does not require additional investments for the textile finishing industries, which is a desirable factor in developing countries. Figure 27A shows the morphology of blackberry-type microcapsules in a fresh paste with diameters ranging from 25 to 100 µm, while Figure 27B shows SEM micrographs of spray-dried microcapsules revealing two types of structures, with small spherical units of less than 10 µm and clusters ranging from 25 to 100 µm.

Repellent essential oils are becoming increasingly widespread due to their low tox‐ icity and customer approval. Its application in textile materials has been widely devel‐ oped. To optimise their application efficiency, it is important to develop long‐lasting re‐ pellent textiles using OEs. Specos et al. obtained citronella‐loaded gelatin microcapsules through the complex coacervation method, which were applied to cotton fabrics in order to study the repellent effectiveness of the obtained fabrics [70]. The release of citronella by the treated tissues was monitored and the repellent activity evaluated by exposing a hu‐ man hand and arm covered with the treated tissues to *Aedes aegypti* mosquitoes. In 2016, Ribeiro et al. investigated the functionalisation of photocatalytic titanium dioxide nanoparticles on the surface of polymeric microcapsules as a way to control the release of citronella by solar radiation, thus obtaining a release of a repellent without mechanical intervention [211]. These authors used a modified hydrothermal sol-gel method to synthesise TiO<sup>2</sup> nanoparticles. Through several characterisation techniques, these authors were able to observe the surface of the microcapsules and the release efficiency. Using in vitro biological assays with live mosquitoes, the controlled release repellence effect of these photocatalytic microcapsules was reinforced by the inhibition of these vectors. According to the results, it was shown that functionalising the microcapsules with photocatalytic nanoparticles on the surface, and then exposing them to ultraviolet radiation, effectively increased the emission of citronella into the air, repelling mosquitoes. Table 6 shows an overview of illustrative examples of EO microencapsulation oils, wall materials, and microencapsulation methods with industrial importance.

**Figure 26.** Low‐ and high‐magnification SEM micrographs of (**a**,**a'**) BC‐PLA1.5‐Pol5.0, (**b**,**b'**) BC‐ PLA1.5‐Pol2.5, (**c**,**c'**) BC‐PLA3.0‐Pol5.0, and (**d**,**d'**) BCPLA3.0‐Pol2.5 [210]. **Figure 26.** Low- and high-magnification SEM micrographs of (**a**,**a'**) BC-PLA1.5-Pol5.0, (**b**,**b'**) BC-PLA1.5-Pol2.5, (**c**,**c'**) BC-PLA3.0-Pol5.0, and (**d**,**d'**) BCPLA3.0-Pol2.5 [210].

It was found that the tissues treated with citronella microcapsules present greater and more lasting protection against insects in comparison to fabrics sprayed with an eth‐ anol solution of essential oil. Repellent textiles were obtained by filling cotton fabrics with microcapsule sludge, using a conventional drying method. This methodology does not require additional investments for the textile finishing industries, which is a desirable fac‐ tor in developing countries. Figure 27A shows the morphology of blackberry‐type micro‐ capsules in a fresh paste with diameters ranging from 25 to 100 μm, while Figure 27B shows SEM micrographs of spray‐dried microcapsules revealing two types of structures, with small spherical units of less than 10 μm and clusters ranging from 25 to 100 μm.

Complex Coacervation

Gelatin/sodium algi‐ nate

**Figure 27.** (**A**) Optical micrographs of gelatin microcapsules containing citronella essential oil (100x magnification) and (**B**) SEM microphotographs of spray‐dried microcapsules containing citronella essential oil (500× magnification) [70]. **Figure 27.** (**A**) Optical micrographs of gelatin microcapsules containing citronella essential oil (100× magnification) and (**B**) SEM microphotographs of spray-dried microcapsules containing citronella essential oil (500× magnification) [70].

In 2016, Ribeiro et al. investigated the functionalisation of photocatalytic titanium dioxide nanoparticles on the surface of polymeric microcapsules as a way to control the release of citronella by solar radiation, thus obtaining a release of a repellent without me‐ **Table 6.** Overview of essential oil microencapsulation, methods, wall materials, and industrial applications.


Gelatin/gum arabic Lavender Cosmetics [219]

Citronella Anti‐mosquito textile [218]


**Table 6.** *Cont.*

#### **5. Conclusions**

This review summarises different types of EO structures and describes their extraction and application methodology. In addition, different techniques for microencapsulating essential oils are described and some reports are presented to provide a basis for research and industrial development.

As described in this paper, EOs are used in several applications in the pharmaceutical, cosmetic, agricultural, and food industries, as they are natural metabolites produced by plants with interesting properties. Furthermore, EOs are being explored as an alternative to synthetic products due to their ecological factors and the fact that their characteristics are different from the corresponding synthetic product. For example, synthesised oil may have the same odour as natural oils, but may not have the same therapeutic characteristics.

Currently, there is growing interest in the application of EO microencapsulation, making it an effective and important tool in the preparation of high-quality products, improving their chemical, oxidative, and thermal stability. Besides these advantages, the shelf life, biological activity, functional activity, controlled release, physicochemical properties, and general quality of oils can also be improved with microencapsulation technology.

Based on the scientific studies available and presented throughout this paper, it can be concluded that the microencapsulation of EOs is an emerging trend for industrial applications. However, this development has limitations, such as the low diversity of wall materials and their incompatibility with microencapsulation methods. Many of the encapsulating agents available present a high cost for production on an industrial scale. In future research, microencapsulation must also be directed to encapsulate a different mixture of oils by different techniques, in order to disguise the flavour of the oils and to improve safety, quality, and nutritional value.

**Funding:** Vânia I. Sousa and Joana F. Parente are grateful to the Project ReleaseME-POCI-01-0247- FEDER-033268, for their research grants from the Agência Nacional de Inovação, co-funded by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalisation (COMPETE 2020), under the PORTUGAL 2020 Partnership Agreement. Juliana F. Marques and Marta A. Forte are grateful to the Fundação para a Ciência e Tecnologia (FCT) of Portugal for their Ph.D. grants, SFRH/BD/112868/2015 and PD/BD/128491/2017, respectively. The authors also acknowledge the funding from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020-2023. This research was funded by the project Repel+: New solutions for mosquito repellence as an application for malaria control (project number 47036) from the Agência Nacional de Inovação, co-funded by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalisation (COMPETE 2020), under the PORTUGAL 2020 Partnership Agreement.

**Data Availability Statement:** The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

