Enhancing the Properties of Sodium Alginate with a Glycerol–Silicate Plasticizer

Abstract

The impact of a glycerol–silicate (GS) plasticizer on the mechanical, thermal and hydrophobic properties pertaining to sodium alginate (NaAlg) and calcium alginate (CaAlg) films were investigated. Spectroscopic and physio-chemical analysis were conducted to evaluate the effects of the GS incorporation. The results determine that both NaAlg and CaAlg films exhibited poor mechanical properties which only improved by increasing the GS loading (up to 25 wt%), after which it declined. CaAlg exhibited the highest tensile strength after 25 wt% GS loading was incorporated. The elongation at break varied, with NaAlg films showing a ~10-fold increase, while the CaAlg films remained relatively unchanged. Thermal gravimetric analysis (TGA) revealed that GS reduced the onset decomposition temperature of NaAlg films, whereas CaAlg films maintained a greater onset decomposition temperature. The advancing contact angle measurements indicated a nearly linear decrease (from 54° to 39°) in hydrophobicity for the NaAlg films while the hydrophobicity for CaAlg films initially increased from 65° to 74°, and then became more hydrophilic with greater GS loading. These findings highlight the potential of GS plasticization to enhance and tailor alginate film properties, providing insights into the development of sustainable bioplastics with improved mechanical properties.

1. Introduction

Alginates are naturally occurring non-toxic, film forming biopolymers derived from brown sea algae, known as Sargassum, and can exhibit a combination of favorable physiochemical properties such as biocompatibility, biodegradability and hydrogel formation [1,2]. These versatile properties enable their use in various fields including food, biomedicine, textile and wastewater treatment industries [3,4,5]. Sodium alginate is composed of D-mannuronic acid (M) and L-guluronic acid (G) units, where the polysaccharide chain can exist as homopolymer blocks (M and G) or with regions of alternating sequences (MG blocks) [6]. The M/G ratio can impact the polymer’s overall properties. Due to the abundance of hydroxyl groups, alginates are predominantly hydrophilic and form hydrocolloids with water and hydrogels from ionotropic gelation when crosslinked cation ions are present [7]. These ionic cations can range from mono, di, tri and tetravalent cations of Al, Pb, Cu, Cd, Ba, Sr, Ca, Co, Ni, Zn and Mn [8]. Calcium ions (Ca²⁺) are the preferred crosslinkers for alginates due to their stability and non-reactive nature [8].

Alginate has been gaining significant attention as a promising alternative for biodegradable bioplastic manufacturing, driven by the need to address climate change and reduce reliance on finite resources such as fossil fuels. For the past century, there has been an overdependence on the use of fossil fuels to generate energy and produce petrochemical-based plastics [9]. Alginate-based plastics can be a viable alternative to these materials. Alginate films are usually produced on a laboratory scale in which alginate and other additives are dissolved in water and solvent cast into clear films [10]. However, due to their high water solubility, poor thermal stability and deficient mechanical strength properties, the application of alginate-based films in industry are limited [11]. These properties could be improved with select plasticizers or crosslinking agents [11].

Plasticizers are often low molecular weight additives used to enhance polymer plasticity and flexibility [12]. This is achieved by increasing the free volume between polymer chains which allows the chain segments to move and rotate more freely [13]. The increased movement of polymer chains with respect to each other also decreases the glass transition temperature (T_g) and melt viscosity [14,15,16,17]. Common plasticizers used for bioplastics include glycerol, sorbitol and polyethylene glycol [17]. The hygroscopic nature of alginate salts plays a role in their interaction with plasticizers, as their ability to absorb and retain moisture influences the polymer’s physical properties [7,18]. Incorporating salts such as Na⁺ and Ca²⁺ into the polymeric matrix increases moisture content, prompting hydration interactions that expand free volume mirroring the behavior observed in methylcellulose-derived films [18,19]. This mechanism parallels glycerol’s effect, which inserts into the alginate chains, creating secondary bonds, and filling the empty space [6]. Studies have shown that incorporating an increasing amount of glycerol into alginate often improves the elongation up to a threshold before it decreases, while the tensile strength consistently decreases. The addition of glycerol causes intermolecular forces to weaken within the alginate chains, impacting both tensile strength and elongation [11,20].

The integration of hybrid sol–gel technology with glycerol can enhance these properties. Sol–gel materials, which involve the transition of a system from a liquid into a solid “gel” phase, can be used to incorporate glycerol into various matrices, leading to improvements in mechanical strength, thermal stability and biocompatibility [21].

The study aims to assess the mechanical properties, thermal stability and hydrophobicity of sodium and calcium alginates with an increasing load of a silica–glycerol plasticizer. The structural morphology and advancing contact angle of these modified materials was contrasted between the system with the sodium and calcium alginates. Additionally, the novel sodium and calcium alginate hybrids were fully characterized by Fourier Transform Infrared (FTIR), thermogravimetric analysis (TGA), tensile strength and elongation measurements.

2. Materials and Methods

2.1. Materials

Sodium alginate from brown algae (Low viscosity, Molecular weight: 26 KDa, and M:G ratio of 1.56), glycerol, tetraethyl orthosilicate (TEOS) and CaCl₂ were obtained from MilliporeSigma Canada Ltd. (Oakville, ON, Canada).

2.2. Glycerol–Silicate (GS) Precursor Synthesis

Tetraethyl orthosilicate (TEOS) was added dropwise to a glycerol solution at a ratio of 1-part TEOS to 6 parts glycerol while stirring for 12 h at 90 °C in a round bottom flask with a condenser under nitrogenous conditions. The ethanol produced as a byproduct of the condensation reaction was removed using a rotary evaporator under reduced pressure at 60 °C, to yield a transparent, viscous, glycerol–silicate precursor [22].

2.3. Alginate Bioplastic Composite Preparation

Sodium alginate (NaAlg) solution was prepared by adding 4 g of dried NaAlg in 96 g of DI water and mixed at ambient temperature to make a 4% w/w solution. Alginate glycerol–silicate biohybrids were prepared by adding 20 mL of NaAlg solution and varied percentages of a glycerol–silicate (GS) precursor to a 50 mL beaker and stirred at ambient temperature for 4 h to make 5–50 wt% sodium alginate glycerol–silicate (NaAlgGS) biohybrids, as shown in Scheme 1a. Then, 20 mL of the solution was cast into a 100 × 20 mm Petri dish and left to air dry overnight, and stored in a desiccator until testing. Transparent films averaged ≈0.15 mm thickness and were measured with a Mastercraft 58-6800-4 calipers. Calcium alginate (CaAlg) films were obtained by steeping the NaAlg and NaAlgGS films in 1 M CaCl₂ solution for ~10 min, blotted with a paper towel, then left to air dry overnight and stored in a desiccator until testing (Scheme 1b). The films were labeled based on the series and percentage of precursor added. For example, the sodium alginate glycerol–silicate biohybrid (NaAlgGS) with 5% precursor was labeled as GS 5.0% (Na⁺) and the corresponding calcium alginate glycerol–silicate (CaAlgGS) was labeled as GS 5.0% (Ca²⁺).

2.4. Fourier Transmission Infrared Spectroscopy (FTIR)

Samples of dried sodium alginate (1 mg) and alginate bioplastic films were used for FTIR analysis. FTIR spectroscopy was performed on Cary 630 Fourier Transform Infrared Spectroscopy (Agilent, Santa Clara, CA, USA). All spectra were recorded at ambient temperatures in the range of 650–4000 cm⁻¹ using 256 scans at a resolution of 4 wavenumbers.

2.5. Mechanical Properties

The films subjected to tensile strength testing were dried at room temperature (RT) and stored in a desiccator until testing. The films were cut into dog-bone shaped specimens of 75 mm × 5 mm × 0.15 ± 0.04 mm following the ASTM D882 [23] procedure and measured at room temperature with an Instron 34SC-1 extensometer (Instron, Esslingen, Germany).

2.6. Thermogravimetric Analysis (TGA)

The films subjected to thermogravimetric analysis (TGA) were dried until reaching constant weight and stored in a desiccator until testing. TGA was conducted using an SDT Q600 TA instrument (TA Instruments, New Castle, DE, USA) with a flow of 60 mL/min of air. Approximately 10 mg of sample was placed in the TGA platinum pan, equilibrated at 80 °C and then heated to 700 °C at a rate of 10 °C/min.

2.7. Advancing Contact Angle

Films were cut into 2 × 2 cm squares and placed on the stage of the optical tensiometer. Using a micropipette, 10 μL of DI water was dispersed on the film without disrupting the integrity of the droplet. An image was captured and analyzed using the Ossila Contact Angle Goniometer (Ossila, Sheffield, UK) to determine the advancing contact angle by measuring the angle formed between the baseline of the droplet and the tangent at the droplet’s edge. Four replicates were used to calculate the average advancing contact angle and determine the standard deviation.

3. Results

3.1. Film Preparation and Morphology

Biopolymer films were prepared using NaAlg as the primary polymer, blended with varying concentrations of a GS plasticizer to evaluate the material’s mechanical properties, thermal stability and hydrophobicity. Additionally, Ca²⁺ ions were incorporated to induce further ionic crosslinking, providing comparison to assess the plasticizer’s effect.

The biopolymeric films were prepared through the dissolution of NaAlg in water, followed by the addition of the glycerol–silicate at various concentrations ranging from 5–50% by weight, were then casted to form flexible biopolymeric plastic films. Additionally, the effect of ionic crosslinking was evaluated by treating the plasticized NaAlg films with 1 wt% of aqueous CaCl₂ solution to form divalent ionic crosslinks (Ca²⁺) with the alginate polymers.

All the prepared films displayed visual transparency with a slight yellow tint, consistent across the NaAlg and CaAlg complexes, as seen in Figure 1. The coloration of the films is due to the partial oxidation of the alginate resulting in a slight coloration. The transparency of the films indicates that the films are homogeneous. The plasticized alginate films were subsequently analyzed for mechanical properties, thermal stability and hydrophobicity for bioplastic applications.

3.2. Spectrometric Analysis

Fourier Transmission Infrared (FTIR) spectroscopy analysis of the glycerol silicate plasticizer of both the NaAlg and CaAlg films is shown in Figure 2. The GS plasticizer shows successful formation of the glycerol silicate near 3300 cm⁻¹ and 1034 cm⁻¹ attributed to O-H stretching and C-O vibrations in Figure 2a [22]. The formation of Si-OH bonds, replacing the Si-O-Et bonds, is indicated by the absorption at 850 cm⁻¹ [22]. This is further supported by the absence of the Si-O-Et peak at 1069 cm⁻¹ in the glycerol–silicate spectrum, which is otherwise present in the TEOS spectrum [22]. The Si-O-Si shoulder was also detected, providing evidence of a silicate network formed within the plasticizer seen in Figure 2a.

In the study of the interactions between NaAlg and CaAlg with the GS plasticizer, FTIR spectroscopy analysis was conducted on both the unplasticized films and plasticized films. The results, shown in Figure 2b,c, correspond to the NaAlg and CaAlg complexes, respectively. For the sodium alginate (Figure 2b), a peak at 3253 cm⁻¹ and 2920 cm⁻¹ is observed, corresponding to the stretching vibrations of O-H and C-H bonds. Notably, these peaks increase with higher concentration of the GS plasticizer [24]. The peaks at 1595 cm⁻¹ and 1406 cm⁻¹ are attributed to the -COO⁻ functional group, while those at 1081 cm⁻¹ and 1024 cm⁻¹ represent the stretching vibrations of C-O-C bonds [5]. Additionally, the peak near 1027 cm⁻¹ is associated with the coupling of C-O, C-C and C-O-H vibrations within the carbohydrate region, typically found between 1200 and 870 cm⁻¹ [25]. Peaks observed at 817 cm⁻¹ and 935 cm⁻¹ are assigned to the C-C stretching, which is a characteristic of polysaccharides [26].

The CaAlg complex (Figure 2c) exhibits similar structural features to the NaAlg complex, with subtle differences due to Ca²⁺ crosslinking. Specifically, the O-H stretching peak shifts slightly from 3254 cm⁻¹ to 3231 cm⁻¹ [27,28]. This slight shift in O-H stretching can be due to the increase in ionization strength which results in the intermolecular interaction of the ion–dipole type between the hydroxyl groups of the CaAlg and the ions of the dissociated salts, thus weakening the O-H covalent bonds and reducing its absorption frequency [18,19]. The -COO⁻ bands also show a slight shift from 1591 cm⁻¹ to 1583 cm⁻¹ and from 1406 cm⁻¹ to 1413 cm⁻¹, indicating the presence of Ca²⁺ crosslinking within the films [5,27,28]. Both the NaAlg and CaAlg complexes exhibit characteristic peaks associated with alginate, glycerol and tetraethyl orthosilicate (TEOS), with clear integration of the Si-O-Si network evident at 1108 cm⁻¹. These results confirm the successful crosslinking and integration of the GS plasticizer within the alginate matrices.

3.3. Mechanical Properties

The effect of Ca²⁺ crosslinking and effect of the GS plasticizer concentration on the comparative tensile strength (MPa) and elongation (%) was measured at RT for the NaAlg and CaAlg complexes shown in Figure 3a,b.

In Figure 3a, NaAlg films exhibit brittle behavior due to the nature of their molecular structure consisting of long chains of alternating M and G blocks [29]. These alternating M and G blocks create a relatively stiff polymer chain that lacks significant flexibility [30]. In the absence of plasticizers, the polymer chains are unable to move or slide past each other, resulting in their brittle nature [6]. It can also be seen that CaAlg films exhibit a tensile strength of 3.53 MPa, whereas NaAlg similarly exhibits a tensile strength of 5.78 MPa. CaAlg films are formed through the ionic cross-linking of alginate with Ca²⁺ ions. The Ca²⁺ ions create strong ionic bridges between G-block regions of adjacent chains, resulting in relatively similar tensile strength [30].

The increase in GS concentration to 25% improves the tensile strength of both NaAlg and CaAlg due to the plasticizers creating strong hydrogen bonding and increasing intramolecular spacing by reducing internal hydrogen bonding. The addition of the GS plasticizer reduces the brittle nature of both NaAlg and CaAlg films while proving a desired extent of flexibility [6,10,11,31,32]. However, after the incorporation of loadings greater that 25% GS, the tensile strength of CaAlg decreases, owing to the structural integrity of its ionic crosslinked structure, providing considerable mechanical strength and stability to the material [6,33]. NaAlg shows a less pronounced decrease due to the absence of rigid crosslinks [6,33].

The rigid structure caused by ionic crosslinking restricts the flexibility of the plasticized polymer matrix, leading to a material with less flexibility, as seen in Figure 3b. The incorporation of GS plasticizer at concentrations 5–25% dramatically improves the NaAlg elongation by ~10 fold, whereas the CaAlg does not follow a similar pattern. For the NaAlg complex, the increase is likely due to the disruption of intermolecular hydrogen bonding, which reduces stiffness, leading to an increase in elongation [11]. This is very typical for plasticizers containing glycerol as the concentration increases [11]. However, at low loadings of GS, the elongation of CaAlg is improved by reducing the polymers’ rigidity and enhancing flexibility. The incorporation at higher concentrations of GS disrupts the cohesion of the polymeric network, resulting in the decline in elongation. This nonlinear and synergic behavior is common among CaAlg and different plasticizers [6]. This demonstrated the effectiveness of GS plasticizers in enhancing the tensile strength of both NaAlg and CaAlg, while only NaAlg exhibited an improvement in elongation.

3.4. Thermal Properties

The effect of GS plasticization on the thermal properties of NaAlg and CaAlg was evaluated by TGA. Figure 4a,b depicts the weight percentage profile of the samples during TGA, while

Table 1. Decomposition temperatures of the NaAlg and CaAlg samples and their respective ratios with the plasticizer.

Sample	Tonset (°C)	Tdmax (°C)	Sample	Tonset (°C)	Tdmax (°C)
NaAlg	216	238.3	CaAlg	206.4	215.6
GS 5%	206.3	218.1	GS 5%	209.5	218.1
GS 10%	206.3	214.1	GS 10%	217.9	250.4
GS 25%	204.3	210.6	GS 25%	206.9	215.7
GS 35%	203.5	208.4	GS 35%	206.1	213.5
GS 50%	202.0	208.7	GS 50%	203.8	210.2

summarizes the onset temperature (T_onset) and maximum decomposition temperature (T_dmax) for both the NaAlg and CaAlg plasticized complexes. For all samples, the first peak at ~100 °C is attributed to the strong water-binding capacity of polysaccharides [11,34]. Crosslinked films (e.g., CaAlg) exhibited less moisture due to Ca²⁺ binding, leading to lower weight loss compared to the un-crosslinked films. The second weight loss is around ~200 °C (T_onset), while the maximum decomposition temperature (T_dmax) occurs slightly after 208–250 °C, corresponding to the decomposition of the polysaccharide by the fracture of glycosidic bonds, decarboxylation, decarbonylation and loss of bonded water [34]. The final decomposition occurred at ~260 °C, owing to the formation of intermediate compounds in the second stage and char formation [34]. The initial results of NaAlg demonstrate good thermal stability compared to CaAlg, with onset decomposition temperatures of 216 °C and 206.4 °C, respectively. The overall trend observed reveals the impact of GS on the thermal stability for both NaAlg and CaAlg complexes.

For NaAlg, an increase in GS concentration led to a gradual decrease in T_onset and T_dmax, indicating that the plasticizer reduces the material’s thermal resistance by weakening the forces between polymeric chains. On the contrary, CaAlg initially shows enhanced thermal stability at lower additions of the GS plasticizer, with the T_onset and T_dmax increasing due to improved interactions between Ca²⁺ ions and the plasticizer. However, with excessive amounts of GS plasticizer incorporated, the stabilizing effect diminishes and thermal stability begins to decline. This declining effect that is noticeable in both the NaAlg and CaAlg complexes is due to the incompatibility between organic and inorganic phases or by hydrolysis [35,36]. Polymeric backbone scission, catalyzed by hydrolysis, can result in the disruption of the skeletal structure, reducing the decomposition temperature [18]. This suggests that moderate-to-low concentrations of the GS plasticizer, combined with ionic crosslinking, and enhanced structural integrity, resulted in a more favorable thermally stable polymeric material.

3.5. Advancing Contact Angle

Advancing contact angle analysis was used to evaluate the surface hydrophobicity of the NaAlg and CaAlg materials with varying amounts of GS plasticizer, which can be seen in Figure 5a,b.

The NaAlg complex shows a decreasing contact angle trend with an increasing GS plasticizer concentration. The GS plasticizer is expected to show the same effect as glycerol, where it is incorporated between the NaAlg chains, reducing the matrix integrity by limiting the intramolecular hydrogen bonding [37]. The pure NaAlg films exhibit a contact angle of 54.2° ± 1.13°, similar to that reported by Sharma et al. on the development of sodium alginate/glycerol/tannic acid-coated cotton as antimicrobial system, which displays a contact angle value of 46.2° ± 2.2° [33,38]. Their results also showed that increasing glycerol concentrations resulted in a decrease in contact angle, further supporting our findings [38]. The decline in the contact angle can be attributed to the plasticization effect due to the excessive hydroxyl groups in the GS plasticizer.

When analyzing the CaAlg complex (Figure 5b), the contact angle shows an initial increase, up to 10% GS addition, followed by a decline as the GS plasticizer concentration increases. The initial increase is attributed to the ionic crosslinking of the Ca²⁺ and the plasticizer folding in-between the chemical bridge [39]. This would allow for an increased flexibility with the bonus of having a polymeric matrix that is still strong. As you increase the concentration of the GS plasticizer added, the GS starts to leak out from in-between the chains, allowing for excessive hydroxyl groups and an incompatibility between organic and inorganic phases [22]. The decrease in contact angle is similar to the NaAlg complex, where excessive hydroxyl groups make the material hydrophilic [38]. These results indicate a moderate increase in advancing contact angle as you increase the GS plasticizer to a certain loading, after which it declines.

4. Conclusions

Spectroscopic and physio-chemical characterization demonstrated the effects of GS as a plasticizer on NaAlg and CaAlg films. Both NaAlg and CaAlg initially exhibited poor mechanical properties, which improved with the incorporation of GS. Among the films, CaAlg after 25 wt% GS loading exhibited the highest tensile strength with an increase of ~1591% compared to the CaAlg film which exhibited 3.53 MPa. The elongation revealed distinct trends, showing that NaAlg films increased ~700% with the plasticizer loading up to 50 wt%, while CaAlg films mirrored the tensile strength trend, peaking to a threshold and then decreasing. The thermal stability analysis revealed that, with more GS loadings, the NaAlg films decreased in decomposition onset and maximum temperatures, while CaAlg films maintained higher thermal stability overall. The advancing contact angle measurements indicated that GS loading reduced hydrophobicity in NaAlg films, whereas the CaAlg films exhibited an initial increase after which it decreased. These findings exhibit the tunable properties of alginate films through GS plasticization, with increased mechanical, thermal and hydrophobicity properties for tailored applications in suitable materials.