*Review* **Processing of Functional Composite Resins Using Deep Eutectic Solvent**

## **Jing Xue, Jing Wang \*, Daoshuo Feng, Haofei Huang and Ming Wang**

School of Chemistry and Chemical Engineering, Shandong University of Technology, 266 Xincun Road, Zibo 255000, China; xuejingsdut@163.com (J.X.); fengdaoshuosdut@163.com (D.F.); 1982hhf@163.com (H.H.); wangmingmw@sdut.edu.cn (M.W.)

**\*** Correspondence: wjing@sdut.edu.cn

Received: 24 August 2020; Accepted: 23 September 2020; Published: 24 September 2020

**Abstract:** Deep eutectic solvents (DESs)—a promising class of alternatives to conventional ionic liquids (ILs) that have freezing points lower than the individual components—are typically formed from two or more components through hydrogen bond interactions. Due to the remarkable advantages of biocompatibility, economical feasibility and environmental hospitality, DESs show great potentials for green production and manufacturing. In terms of the processing of functional composite resins, DESs have been applied for property modifications, recyclability enhancement and functionality endowment. In this review, the applications of DESs in the processing of multiple functional composite resins such as epoxy, phenolic, acrylic, polyester and imprinted resins, are covered. Functional composite resins processed with DESs have attracted much attention of researchers in both academic and industrial communities. The tailored properties of DESs for the design of functional composite resins—as well as the effects of hydrogen bond on the current polymeric systems—are highlighted. In addition to the review of current works, the future perspectives of applying DESs in the processing of functional composite resins are also presented.

**Keywords:** deep eutectic solvent; composite resins; hydrogen bond

## **1. Introduction**

Composite resins play a significant role in industrial and domestic applications because of their advantages of lightness, plasticity and economic feasibility. The solvents, which plays multiples roles such as diluent, monomer and viscosity modifier, is indispensable in processing of traditional composite resins. However, the usage of volatile organic solvents (VOC) in the processing of traditional composite resins induces serious environmental problems which limit the industrial development of this type of macromolecular material [1,2]. Modifications of traditional composite resins have become the focus of research in recent years. Modified composite resins have brought great convenience to production and life [3].

In the past few decades, research is increasingly focusing on green processing of resin composites in order to deal with the hazards of VOCs to make them more environmentally friendly. Abbott et al. [4] firstly introduced deep eutectic solvent (DES) by mixing choline chloride (ChCl) with urea which has advantages of easy preparation, low cost, non-toxicity and biodegradability. DESs have been regarded as a new generation of ionic liquids (ILs) with a variety of applications in polymer processing, biomedicine and nanotechnology [5]. DESs are usually composed of quaternary ammonium salts and metal salts or hydrogen-bond donors (HBDs), which could be prepared by simply mixing [6]. Before the practice of DESs, ILs took the domination of green solvent for various applications. Tang et al. [7] studied a hydrophobic IL-modified thermo-responsive molecularly imprinted monolith and N-isopropylacrylamide as a thermo-responsive monomer for selective recognition and separation of tanshinones. Due to the porous and low-pressure

nature of monolith, the separation of the five tanshinones was achieved via the thermo-responsive hydrophilicity/hydrophobicity transformation in water. Tang et al. [8] also synthesized CO2-induced switchable ILs with reversible hydrophobic/hydrophilic conversion, and they applied the novel ILs for lipid extraction and separation from wet microalgae by bubbling CO2. Because of the physiochemical similarity between DESs and ILs, DESs started taking place of ILs. Similar to ILs, due to their solvent properties, DESs can also dissolve CO2, metal oxides and versatile organic species. The solubility strongly depends on the pressure and temperature [9]. Moreover, DESs have also been applied to the field of catalysis, such as base-catalyzed reactions, acid-catalyzed reactions and transition-metal-catalyzed reactions because of their designable chemical structures and excellent solvent properties [10]. The formation of eutectic mixtures has also provided a new extraction method by modifying the chemical structures of HBDs where DES is formed in situ and extracted, and this method used DESs as extractants for separations of liquid, solid and gaseous phases [11,12]. Lou et al. [13] investigated the extraction of lignin nanoparticles from herbaceous biomass (wheat straw) with ChCl–lactic acid DES. It was found that DES could extract high purity lignin (up to 94.8%) and the water content in biomass affected the hydrogen bond interaction between lignin and DES, which influenced the lignin extraction yield. Tang et al. [14] synthesized novel DESs modified molecularly imprinted polymers (DESs–MIPs) using acrylamide as function monomer, alcohol-based DESs as auxiliary function monomer and chloramphenicol (CAP) as the template. The adsorption results that the ChCl/ethylene glycol DESs-based MIPs had stable interactions with CAP. In addition to the applications in catalysis and separations, DESs have also played an important role in polymer synthesis and processing [15,16], which provided hints and inspirations for the green processing of functional composite resins. Despite of the fact that a relatively smaller number of publications about composite resin processing with DESs are available currently [17,18], it is of great significance to both academia and industry for the green production of composite resins by encompassing research about processing of various composite resins with DESs. This paper covers the main categories of composite resins including epoxy resin, phenolic resin, acrylics, polyester resin and imprinted resin. It is in a trend that DES is playing an all-in-one role in the green processing of composite resins and is providing a promising future on the development of green synthesis and production, which is illustrated in the end as the perspective of this review.

#### **2. Deep Eutectic Solvent**

DESs could be defined as a general formula R1R2R3R4N<sup>+</sup>X−Y<sup>−</sup> [19], and they can be divided into three types. When it was discovered that metal salts coupled with alcohols and amides can also form DESs (MCl<sup>χ</sup> + RZ; M=Al, Zn; Z=CONH2, OH), a fourth type of DES was added [20]. Among the four types of DES, Type III DES (Figure 1a) is the most attractive to researchers, which is typically formed through hydrogen bond, where the charge delocalization occurring through hydrogen bonds between the halide anions and the HBDs leads to the decrease in the freezing point of the mixture (Figure 1b) [17,21]. Taking DES of ChCl/urea as an example, through molecular dynamics simulation and spatial distribution function analysis, some researchers have found that after formation of DES, increases were observed on ions disorder and charge density, while decreases were observed on lattice energy and freezing point [22].

**Figure 1.** (**a**) Illustration of Type III deep eutectic solvent with hydrogen bond formed; (**b**) principle behind Type III deep eutectic solvents.

Structural characterization of DES is usually carried out by using Fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrum (NMR), dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Wang et al. [23] prepared an antibacterial DES based on benzalkonium chloride (BC) and acrylic acid (AA), and they determined the formation of DES by FTIR and NMR. FTIR result showed that the hydrogen bond mainly effected the position of C=O stretch between 1700–1725 cm−<sup>1</sup> in AA. Single component of AA showed peaks at 1700 cm−<sup>1</sup> and 1725 cm−1, but the peak at 1700 cm−<sup>1</sup> diminished and at the shoulder of the peak at 1725 cm−<sup>1</sup> for AA-derived eutectic mixtures. Similarly, the chemical shift in the NMR of -COOH proton was at δ = 11.38 for AA, and the chemical shift moved to a lower value (δ = 10.58) after mixing with BC. The formation of hydrogen bond for BC–AA had been evidenced by FTIR and NMR. Gautam et al. [24] found that the vibrational peaks in the FTIR spectra of DES formed with ChCl and carboxylic acid shifted to a longer wavelength compared to its individual component, which is a sign of hydrogen bond formation. They further investigated the hydrogen bonds location and length through hydrogen bond sites and confirmed the existence of hydrogen bonds by quantum theory of atoms and the reduced density gradient. Troter et al. [25] reported the effect of temperature on dynamic viscosity of DESs of the binary ChCl-based DESs by DMA at a temperature range of 293.15–363.15 K. the results showed that the viscosity–temperature curve of the DES with ChCl followed the Arrhenius equation, and it was demonstrated that the viscosity of DES was affected by the formation of hydrogen bond. Aroso et al. [26] studied the thermal and rheological properties of DESs, and these DES systems had Newtonian behavior as well as viscosity decrease with temperature and water content. DSC characterization confirmed that for water content at 1:1:1 molar ratio, the mixture retained its single-phase behavior. Tomai et al. [27] prepared a low transition temperature mixture (LTTM) by mixing ChCl and acetylsalicylic acid in a molar ratio 1:2. DSC result showed that only a Tg at −37 ◦C was observed and the sample did not undergo a phase transition, crystallization or melting, and therefore it was suggested to be defined as a LTTM instead of DES. Francisco et al. [28] found that hydrogen bonding can be evidenced by the shifts in the FTIR and a shift in the resonance signal can also be noticed to lower field in NMR. Choi et al. [29] observed intermolecular interaction between the sucrose and the malic acid mixture by H–H-nuclear, implying that molecules of these compounds in the liquid were aggregated into larger structures. Stefanovic et al. [30] reported that the trends in the solvation structure of poly ethylene glycol (PEO) is closely related to the density of hydrogen bond network within DES and the extent of disturbance induced by the polymer solute. It was indicted that the incorporation of polymers as solute into DES would destruct the complex solvation environment leading to a transformation in

polymer structures with relatively static spiral shape. The interactions between the hydrogen bonds within DES and the polymers as solute in DES-based solutions is remaining an interesting topic when using DES to process composite resins.

When DES is employed to modify the composite resins, proper chosen chemical structures of individual components within DES could significantly affect the properties of the original composite resins which may include the hydrophobicity/hydrophilicity, viscosity and conductivity. Tiecco et al. [31] have demonstrated that the hydrophobicity/hydrophilicity of DESs corresponded to the chosen HBDs instead of hydrogen-bond accepters (HBAs). For example, even when the DES was prepared with a highly water-soluble HBAs, it would still be easily separated from water if the chosen HBD is highly hydrophobic. Tang et al. [32] applied fatty acid/alcohol-based hydrophobic DESs in the extraction of levofloxacin and ciprofloxacin in water by liquid–liquid microextraction. The mechanism of extraction was that the targets can be transferred from the water phase to the DES phase by hydrogen bonding. the extraction efficiency of alcohol-based hydrophobic DESs was higher than that of the fatty acid-based DESs, and because that the long carbon chain structure of fatty acid-based DES increased the distance between the hydroxyl group and HBA, which weakens the hydrogen bond strength. Tang et al. [33] further constructed a polarity controlled biphasic extraction system by combining a hydrophilic DES phase (hexafluoroisopropanol–choline chloride) and a hydrophobic DES phase (menthol–tricaprylylmethylammonium chloride), and a DES-based biphasic system was used to extract and separate high/low polarity compounds. Therefore, the polarity control of DES-based biphasic system can be used to regulate hydrophobicity/hydrophilicity properties of extraction system. Tang et al. [34] also employed a choline salt–aniline DES to remove aniline from organic waste liquid, and the mechanism of extraction was to remove aniline by forming a choline salt–aniline DES and choline-based DESs were insoluble in the organic solvent due to the differences in polarity. The result showed that a choline salt–aniline DES had higher extraction capacity (>95%) than traditional extraction agents. From this point, it would be made possible to adjust the hydrophobicity/hydrophilicity of composite resins through the incorporation of DES. Research on viscosity and conductivity of DESs is also inevitable. Sas et al. [35] measured the viscosity of DES formed with ChCl and levulinic acid, and it was found that the viscosity of DES obeyed the Arrhenius equation. Moreover, the conductivity of DES was related to its viscosity. Abbott et al. [36] further summarized the hole theory claiming that the viscosity and conductivity of DESs were closely related to the dimension of species and the free volume and increased size of free volume was correlated to decreased surface tension. Hole theory is helpful for the design of DESs with favorable viscosity and conductivity, which set the basis for the processing of composite resins with DESs on certain circumstances.

Knowledge of thermal stability of DESs is important for applying them to process composite resins at high temperatures. Chen et al. [37] used thermal gravimetric analysis (TGA) to study the thermal stability of various DESs. By comparing the onset decomposition temperatures of DESs, the reasons of decomposition and weight loss were considered to be due to the destruction of hydrogen bond after being heated. The analysis of the thermal decomposition behavior of DESs provided a basis for preparing DESs with adjustable thermal stability. In order to use DESs for a variety of industrial applications, Ghedi et al. [38] investigated the thermal stability of DESs in a certain temperature range, and they found that the thermal stability of DESs increased with the increase of alkyl chain length of the HBD. They further used FTIR to investigate the vibration modes of hydrogen bond and analyzed the intaeractions of functional groups on HBDs and HBAs. Troter et al. [25] compared the difference of the physical and thermodynamic properties of a series of binary DESs at the temperature range of 293.15–363.15 K. With the increase of temperature, the density and viscosity of all DESs in this study decreased while their conductivity increased. Saputra et al. [39] investigated the thermophysical properties of highly stable novel ammonium-based ternary deep eutectic solvents (TDESs) by combining glycerol and two kinds of HBAs (ethyl ammonium chloride and zinc chloride), and a similar trend to the binary DESs in the previous work [25] was observed. It was because of the

adjustable thermal stability of DESs, either binary or ternary, that made them suitable for processing composite resins in some specific conditions.

The investigation of the toxicity of DESs is indispensable for the assessment of safety, health and environmental impacts, and the toxicity of DESs must be taken into consideration before applying them in the processing of new materials. It was found that the phosphonium-based DESs were toxic on bacteria, while no toxicity was observed for ammonium-based DESs [40,41]. Hayyan et al. [42] investigated the cytotoxicity of ammonium-based DESs on five human cancer cell lines and one normal cell line. The DESs had inhabitation effect on cancer cell growth at certain condition and suggested that the toxicity of DESs may be related to the type of HBD and molar ratio of HBD/salts. Torregrosa-Crespo et al. [43] monitored the toxicity of DESs to *Escherichia coli*, and no toxic effect was observed at low DES concentration. When the concentration of DES was raised, the toxicity effect became obvious, which was considered due to the dual effect of both chemical composition of the DES and the high acidification of the media caused by DES hydrolysis during cellular growth. Wen et al. [44] assessed the toxicity of ChCl-based DESs comprising ChCl and choline acetate (ChAc) as the salts and urea, acetamide, glycerol and ethylene glycol as the HBDs on different living organisms. The effect of DESs on different living organisms was considered to be associated with their interactions with the cellular membranes. The toxic investigation on DESs provides meaningful ecological basis for the applications of DESs in the processing of composite resins.

#### **3. Composite Resins Processing with DES**

Since Type III DES formed through hydrogen bond interactions processed conveniently adjustable properties on hydrophobicity/hydrophilicity, viscosity, conductivity and thermal stabilities, the applications of DESs in the green processing of composite resins have attracted the interest of many researchers worldwide [21,45]. Table 1 showed the applications of DESs in various composite resin processing, which is nowadays a hot research area with a focus on the literature published in recent years.


**Table 1.** Summary of applications of deep eutectic solvents (DESs) in composite resin processing.


**Table 1.** *Cont.*


**Table 1.** *Cont.*

#### *3.1. Epoxy Resin*

Structural adhesives are widely used in the repair of degraded concrete or bonding fiber reinforced laminates to strengthen the concrete components. Among high-performance polymers used for the formulation of structural adhesives, epoxy resins are one of the most important materials to enhance their mechanical properties, adhesion performances and longevity [83,84]. However, the production of epoxy resins usually starts with bisphenol-A, a harmful compound to the human reproductive system, which also affects the curing kinetics when preparing epoxy resins [22,29,85]. Chen et al. [86] prepared a protocatechuic acid and epichlorohydrin-based epoxy resin instead of bisphenol-A-based, which had higher transition temperature than that of a cured commercial bisphenol-A epoxy resin, and its coefficient of thermal expansion was much lower. Hsu et al. [87] reported an epoxy resin reinforced with citric acid-modified cellulose (CAC) with improved tensile strength, Young's modules and toughness than either pure epoxy resins or cellulose/epoxy composites. Herein, CAC was dispersed homogeneously in the composite, and the combination of terminal carboxyl groups of CAC and epoxy resin had a positive influence on the curing behavior of epoxy system in their work. Biomass had been proved to be an acceptable alternative to bisphenol-A, but it had limited functionalities to facilitate the curing reactions when preparing epoxy resins.

ILs have played multiple roles in the development of polymers, and the number of IL-based polymers has been increasing steadily [88]. ILs have been utilized in polymer science as curing agents to contribute to polymerization and additives to modify certain properties of polymers. Carvalho et al. [89] developed a new IL (1-butyl-3-methyl imidazolium tetrafluoroborate)-based epoxy resin used for coating. Compared with epoxy resins cured with conventional hardeners (anhydride or aromatic amine), the thermal stability of epoxy resins cured with the IL was found to be better. It was suggested that side reaction and plasticizing effect of IL may decrease the Tg of epoxy resins, which is unfavorable in the cases that high thermal stability of epoxy resins are required. Nguyen et al. [90] reported a new way to synthesize epoxy resins using a variety of phosphonium-based ILs and ILs displayed a high reactivity, curing properties to epoxy resins in all cases. The excellent thermal stabilities obtained in their study was attributed to the application of ILs as plasticizer in the curing of epoxy resins. Sidorov et al. [91] reported the chemical structures of some imidazolium and pyrrolidinium-based ILs and their influences on the curing behavior of epoxy resins, and it was shown that ILs possessed high catalytic activities without affecting the physical and chemical properties of current epoxy resins. Donato et al. [92] studied several epoxy composite resins using ILs (1-decyl-3-methylimidazolium tetrafluoroborate, 1-triethylene glycol monomethyl ether-3-methylimidazolium tetrafluoroborate and 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate) as additives to control the interphase interaction in order to produce a fine dispersed morphology. It was indicated in their work that the thermomechanical properties of epoxy resins were improved due to the strong hydrogen bonds within the epoxy network.

The application of ILs in the curing of epoxy resins has paved a way to DES for processing of epoxy resins with green chemistry. Generally, metal chlorides are known as cationic catalysts for epoxy resins, most which are difficult to handle due to their cytotoxicity and instability in the water. Doolan et al. [93] found that ZnCl2 and SnCl2 exhibited lower cytotoxicity compared to other metal halides and preparing them into DESs may take use of their environmentally benign advantages [40]. Maka et al. [48] prepared ZnCl2 and SnCl2 into the form of DES by mixing them with a certain molar ratio of ChCl and used DESs (ChCl/ZnCl2 and ChCl/SnCl2) as curing cationic catalysts in the epoxy resins. It was observed that using DES of ChCl/SnCl2 could produce higher curing efficiency for the epoxy resin than using DES of ChCl/ZnCl2. Aromatic amines are another important class of curing agent used in the processing of epoxy resins. When mixing aromatic amines with epoxy resin, an elevated temperature is typically required to lower the viscosity of the mixture in order to get better homogeneity. M-phenylenediamine (MPDA) is one of the most commonly used aromatic amines in the curing of epoxy resin and Staciwa et al. [49] mixed it with ChCl to get a DES. After making MPDA into DES, the mixing process was made convenient because the viscosity of DES was lower than that of epoxy resin at the same temperature. Maka et al. [94] prepared guanidine-derived DESs (ChCl/guanidine thiocyanate (GTC) and ChCl/1-(o-tolyl) biguanide (TBG)). By using GTC and TBG as curing agent for the polymerization of the epoxy resin, the pot life values of the final products were shortened compared to that cured with ChCl/urea DES, and the viscosity jump because of the eutectic phenomenon was considered as the main affecting factor. The eutectic phenomenon is providing a hint for the processing of aromatic amines or epoxy resin curing agents with similar chemical structures, at lower temperature. In addition to the facilitation in lowering viscosities, preparing a certain component into DES can also decrease the interparticle aggregation and get better morphology control, which provided a new approach for the preparation of non-aqueous sol–gel-based epoxy resins. Lionetto et al. [95] studied a ChCl/urea-based epoxy resin and found that the addition of DESs can lower the Tg of epoxy resins and contribute to the dispersion of materials. Although in recent years, many researchers have found that DESs can replace the ILs in some applications, the use of DESs still has certain limitations. Mka et al. [47] investigated epoxy resins crosslinked with IL of 1-ethyl-3-methylimidazolium chloride ([EMIM] Cl) and DES of imidazole/ChCl, and they found that epoxy resins cured with IL exhibited more elastic properties and better mechanical features than those cured with DES.

#### *3.2. Phenolic Resin*

Phenolic resin is a type of product obtained via the condensation polymerization of acid compounds and aldehyde compounds. Optimizing the reactivity and heat resistance of phenolic resins is urgently needed in order to meet the requirements of production and manufacturing. From Figure 2, lignin has functional groups such as phenolic hydroxyl groups and alcoholic hydroxyl groups, which was similar to the chemical structure of phenols. Therefore, a similar method for modifying lignin can be used for the modification of phenolic resins. Taking the advantages of DESs into consideration, the lignin

modified by DESs could also be used for the modification of phenolic resins [96,97]. Cao et al. [55] reported that lignin could replace phenol to prepare phenolic resins, and lignin in their work was modified with DES (ChCl/ZnCl2 = 1:2) to improve the reactivity. By analyzing the structural change of lignin before and after modification, modified lignin may undergo partial demethylation reaction compared with lignin. The methoxy group on its aromatic ring was oxidized to phenolic hydroxyl group under the condition of Lewis acid, forming a small amount of active catechol structure. Although the bond strength of the DES-modified lignin used to prepare phenolic resins decreased due to the limited amount of modified lignin used for phenolic resins, the bond strength of phenolic resins could still reach the national standard. Lian et al. [53] introduced ZnCl2 and urea based on DES into phenolic resin and an increase in Tg of phenolic resin was reported with the increase of Zn content. The onset of thermal degradation of modified phenolic resins (filled with relignin) occurred at higher temperatures than those resins with unmodified lignin and without it, showing superior thermal properties. DESs impacted the interaction between the intermolecular and intramolecular in the lignin network due to the action of the ionic environment, and the chemical structure of lignin was found to be influenced by Zn by comparing the FTIR spectra of lignin and relignin, where a similar conclusion was drawn by Sun et al. [98]. Hong et al. [54] compared the phenolic resins modified by lignin (LPF), DES/lignin (DLPF) and DES-modified lignin (MLPF) and reported that the curing time was in a trend of DLPF > MLPF >LPF. MLPF exhibited higher bond strength and shorter curing time than the phenolic resin. It indicated that modification with DES was the most efficient approach to improve the mechanical, reactivity and thermal properties of phenolic resin in their study.

**Figure 2.** Chemical structure of (**a**) monolignol and (**b**) phenolic resin.

Nowadays, carbonaceous materials have been widely used for various purposes, such as catalysis, gas separating or capture and electrode of supercapacitor, and phenolic resin-based carbon is one of the most studied carbonaceous materials. Due to pollution and recovery issues, surfactants or block-copolymers are no longer suitable as structure directing agents for carbon materials with controlled pore characteristics [99,100]. Deng et al. [57] have used DESs instead of conventional pore structure directing agents to reduce pollution and solve recovery problems. The phenolic resin was selected as the raw material, which was mixed with DES composed of ZnCl2 and urea to obtain a carbonaceous material. Compared to the traditional ways, DES-based carbonaceous materials could drastically decrease the time of the whole manufacturing process, increase the specific surface area and optimized electrochemical properties. Moreover, Zhong et al. [56] used DES (urea/ZnCl2 = 1:1) and phenolic resins as raw materials via direct carbonization process to manufacture nanoporous carbon materials. The results showed that modified nanoporous carbon materials had large specific capacitance, superior cycling stability and excellent energy density in alkaline KOH electrolyte. It is

a commendable idea to use DESs as new green solvents to assist in the synthesis of carbonaceous materials or to modify carbonaceous materials.

#### *3.3. Acrylic Resins*

Acrylic resins are a well-known class of pH-responsive polymers that have been used as absorbers, anti-fouling agents and carriers, and they have excellent resistance to weather, heat and chemical. To date, researchers from worldwide are dedicated on the issues corresponding to raw materials, production technology and product quality of acrylic resins [101–103]. DESs exhibit similar properties to ILs that make them suitable for the processing of acrylic resins, which not only prevents the use of VOCs, but also offers a green approach for the functionalization of acrylic resins.

The design of acrylic acid (AA) and methacrylic acid (MA)-based DESs paved the way for the development of frontal polymerization in which DESs were first used as monomers. Fazende et al. [59] prepared a series of acrylic resins with DESs of acrylic acid (AA) and methacrylic acid (MA) as HBDs as well as ChCl as HBA through frontal polymerization and the talc, dimethyl sulfoxide, lauric acid and stearic acid were used to replace ChCl to determine the impact of DESs in acrylic resins polymerization. It was observed that the polymerization in the DESs was well controlled and evenly propagated, and the introduction of ChCl increased the chemical reactivity of AA and MA. Mota-Morales et al. [60] further took advantage of the capability of DESs as solvents to disperse carbon nanotube and reported the preparation of poly acrylic acid (PAA)-carbon nanotube composites by incorporating carbon nanotube in the polymerizable DES (AA/ChCl), and carbon nanotube played a role of filler in the acrylic resin. A desired macroporosity of the acrylic resin was formed due to the strong interaction between carbon nanotube and PAA. Furthermore, Mota-Morales et al. [61] reported a series of acrylic resins based on the DESs which were formed through hydrogen bonds of the acrylic monomers and ammonium salts. These DESs played an all-in-one role, such as monomer, solvent and fillers, in acrylic resins, and the adjustment of viscosities and double bonds in DESs favored the extraordinary conversion of the monomers by frontal polymerization. In their work, they prepared DES acrylic resin loaded with lidocaine hydrochloride (ammonium salts) as HBA and it was observed that the conversion rate of AA was 100% at a mild temperature and the release of lidocaine hydrochloride could be controlled conveniently utilizing the swelling behavior of PAA affected by pH. A similar explanation on kinetics was reported by Sánchez-Leija et al. [104], and it was claimed that the release of lidocaine hydrochloride was controlled by multiple factors of pH, ionic strength and solubility in the polymer coupled with the swelling of polymers and the specific interactions between the lidocaine hydrochloride and the polymer.

DESs were also applied for acrylic resin-based functional polymers to give novel properties to meet certain requirements. Li et al. [62] reported a flexible tactile/strain sensor based on the photopolymerization of AA/ChCl DES with favorable transparency (transmittance of ~81%), elasticity (strain up to 150%) and conductivity (~0.2 S m−1). The transparency of the materials was able to be triggered by complex cross-linked molecular networks. The elasticity was ascribed to the flexible macromolecular chains consisted of easily changing molecular configuration of AA parts within the DES, while the conductivity was owing to the moving positive/negative ions of ChCl parts within the DES. Moreover, the sensor was sensitive to an external strain which may be due to weak hydrogen bond contact between ChCl and AA. Li et al. [63] further studied a green fabrication of conductive paper with stable electrical conductivity and increased elasticity by polymerization of ChCl and AA-based DES. An increase on the elasticity of conductive paper was observed, and it was considered due to the poly DES (acrylic resin) elastomers bridging neighboring cellulose fibers. Similarly, Wang et al. [64] reported a novel transparent and conductive wood using renewable wood as substrate and polymerizable DES of ChCl and AA as backfilling agent. The cellulose orientation and strong interactions between the cellulose template and the polymerizable DES contributed to excellent stretch ability of the wood. Wang et al. [23] reported two acrylic resins synthesized with polymerizable DESs, which were designed by using benzalkonium chloride (BC) as the HBA and AA/MA as the HBDs. The obtained acrylic resins

exhibited not only adjustable flexibility, but also antimicrobial properties. In this work, BC within the DES, as an antimicrobial agent, was in charge of antimicrobial functionality, while AA or MA, two commonly used unsaturated acrylic acid, performed the polymerization reaction. The properties and functions of DESs can be easily tailored through the selection of HBDs and HBAs and tuning on their molar ratios, which provided more possibilities for the processing of acrylic resins [105]. Moreover, the uses of DESs as monomers and polymerization medium in acrylic resins have become a spotlight, and it is believed that the multiples roles of DESs in the processing of acrylic resins should be highlighted.

## *3.4. Polyester Resin*

Polyester resin is a kind of polymer containing unsaturated bonds. It is conventionally processed by two routes: (1) transesterification and polycondensation of dimethyl ester and diol to form polymer; (2) esterification of the diacid and diol and then polycondensation [106]. It is a relatively important material used in manufacturing fibers, concrete and coating [107,108]. Researchers have modified polyester resins by various means, taking the factors of mechanical performance, environmental protection and biocompatibility into consideration.

Poly (ethylene terephthalate) (PET) is one of the most important polyesters widely used in the manufacturing of plastic bottles and fabrics. PET bottles recycling and waste management have become an especially critical issue for environmental protection. Modifications of PET are helpful to improve the fiber suitability to be used as fabrics. There is a trial stage for DESs to take charge in the modifications taking advantage of their design flexibility. ChCl/urea-based DES [65] and ChCl/ethylene glycol-based DES [66] have been tried for hydrophilicity/hydrophobicity surface modifications of PET with microwave irradiation. Researchers have found that DES was an ideal solvent that can not only functionalize the PET surface with desired properties, but also help avoid side-reactions with more environmentally benign results. Depolymerization of PET is one of the easiest approaches to deal with PET; DESs have been utilized to facilitate the processing. Glycolysis of PET is an effective approach to transform PET back to its monomer of bis(hydroxyethyl)terephthalate (BHET) after recycling in order to remake qualified products, and DES has played multiple roles to make the process greener. Liu et al. [109] explored the possibility and processing efficiency of glycolysis of PET with IL of choline acetate ([Ch][OAc]), and they proposed that the enhancement of the glycolysis reaction was due to the formation of hydrogen bonds between PET and the IL. The formation of hydrogen bonds is a notable characteristic of Type III DES, so replacing ILs with DES for the glycolysis of PET is considered feasible in theory. Choi et al. [67] have utilized DES of ChCl/glycerol to replace conventional solvent used in PET depolymerization, with the assistance of microwave irradiation, the processing of PET was simplified and improved in energy efficiency. Sert et al. [68] synthesized five DESs to catalyze the glycolysis of PET, and the DES of potassium carbonate and ethylene glycol was considered to be the most efficient catalyst with highest monomer product yield. Liu et al. [69] also explored the possibility to use DES of 1,3-dimethylurea/Zn (OAc)2 for PET glycolysis, and they attributed the high catalyze efficiency to the synergistic effect of acid and based formed within DES. A similar result was obtained by Wang et al. [70] when using DES of urea/ZnCl2 to catalyze the glycolysis of PET, in which the role of hydrogen bonds within DES was addressed. Instead of obtaining BHET, Zhou et al. [71] utilized a DES of ChCl/Zn (Ac)2 in the alcoholysis of PET to produce dioctyl terephthalate (DOTP), which is a plasticizer used in polymer industry. They also found that the hydrogen bonds within DES were important to accelerate the degradation process. Aminolytic depolymerization of PET has provided other ways to recycle and use of the waste plastic bottles. Musale et al. [72] proved that DESs of ChCl/urea and ChCl/ZnCl2 are efficient catalysts for the aminolytic depolymerization of PET, yielding pure products of N1, N1, N4, N4-tetrakis (2-hydroxyethyl)-terephthalamide (THETA) of 82%, terephthalic acid (TPA) of 83% and bis (2-hydroxy ethylene) terephthalamide (BHETA) of 95%, respectively.

Some other polyesters may include polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycaprolactone (PCL) and polyacrylate (PAR). Rare published works were found concerning the processing of PBT and PEN with DES, but more frontier research has been conducted regarding the processing of PCL and PAR. García-Argüelles et al. [73] reported that polycaprolactones (PCLs) was able to be synthesized by using DES of methane sulfonic acid (MeSO3H) and the guanidine 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) as catalyst, where DES replaced other solvents and initiators minimizing the presence of harmful chemical reagents. PCL is sometimes used as carrier for drug deliveries and Pradeepkumar et al. [74] designed a DES-mediated drug carrier where DES influenced the formation of folic acid (FA)-tagged g-β-alanine-co-PCL polymer (DES@FA-g-β-alanine-co-PCL). This amphiphilic polymer is expected to have great potential for pharmaceutical applications owing to the controlled release of drug from DES@FA-g-β-alanine-co-PCL as the carrier. Functionalization of polyesters is attractive to researchers for expanding their applications; antimicrobial properties are favorable when polyesters were used for biomedical purposes. Zhou et al. [110] added quaternary ammonium compound (QAC) as antibacterial agent to synthesize a new kind of polyester and an excellent antibacterial effect for *Escherichia coli* and *Staphylococcus aureus* were observed. However, directly adding QACs to polyesters may have the problem of burst release due to the lack of bonding between QACs and the polymer network, which greatly affect the long-term antimicrobial efficiency of the modified polyester [111]. García-Argüelles et al. [75] prepared the antimicrobial agents of quaternary ammonium or phosphonium salt into the form of DES by using 1,8-octanediol as HBA and acquired antibacterial properties were obtained with stability and biocompatibility. Hydrogen bonds between the halide anion of quaternary nitrogen or phosphonium salt and the hydroxyl groups in 1,8-octanediol were considered as the main reason to stabilize the antimicrobial groups within the polyester network. Wang et al. [76] studied the polymerization of methyl methacrylate by metal Fe catalysts with three types of DESs as solvents, or ligands. The polymerization process could be well controlled and more environmentally friendly. DES could also be easily separated from the polymerization system. Wang et al. [77] incorporated DES (benzalkonium chloride/acrylic acid) into a dental composite resins with PARs as the main components, and the incorporation of DES produced better maintenance of the mechanical properties, biocompatibility than adding benzalkonium chloride as antimicrobial agent directly. Hydrogen bond within DES was considered as the main factor that contribute to high mechanical strength and limited releasing of BC within the polymer network. Proper utilization of the eutectic phenomena and the hydrogen bonds within DES is inspiring the processing of polyester resins for a variety of future applications.

#### *3.5. Imprinted Resin*

Molecularly imprinted polymers (MIPs), also known as "plastic antibodies", is a new research area developed based on molecular recognition theory that has been growing rapidly in recent years. A typical approach to prepare MIP may include the following steps [82,112–114]: (1) synthesizing a complex through non-covalent bonds or covalent methods under a certain temperature or pressure with a template and the functional monomer; (2) cross-linking the functional monomer through polymerization reaction with the formed polymer surrounding the template; (3) removing the template from the polymer by a specific method with MIP remained. For the moment, MIPs can selectively recognize the template molecules or their analogs though the specific recognition site or the three-dimensional cavity with a spatially complementary structure formed with the template molecule and the functional monomer during preparation (Figure 3). MIPs have shown broad application prospects in molecular recognition, purification and material processing and have become a research hotspot in the field of polymeric materials [115,116]. In order to solve environmental pollution, high costs and hydrophilic issues of traditionally imprinted polymers, DESs have been applied to the molecular imprinting processing.

**Figure 3.** Procedure for preparing molecularly imprinted polymers (MIPs).

The monomers and templates in MIPs are generally two separate constitutions, which may lead to complications in processing and high costs for MIPs polymerization. The combination of both can greatly improve production efficiency and save resources. Fu et al. [78] reported a ternary DES of ChCl/caffeic acid/ethylene glycol that played dual roles of template and functional monomer in the preparation of MIPs. It was illustrated in their work that MIPs prepared with the ternary DES had a good recognition ability for polyphenols, and this MIP was stable at room temperature because of the increased recognition sites between template and monomer polymer. The choice of the elution solution in MIPs is also of great importance, because it may significantly affect the production efficiency and manufacturing costs. Li et al. [79] prepared MIPs with caffeic acid as a template and a mixture of methanol and DES (ChCl/glycerol = 1:2) as elution solution, which were used for rapid purification of caffeic acid from hawthorn. The template could be extracted easily by the elusion solution according to the "like dissolves like" theory, which provided a new approach to improve the preparation efficiency through the choice of elution solution. Other drawbacks of MIPs may include poor compatibility and molecule-recognition ability in the aqueous phase—especially in the cases where various aqueous biologic and environmental samples require MIPs to be compatible with aqueous media. The introduction of hydrophilic groups into MIPs is an effective approach to adjust the hydrophilicity/hydrophobicity of MIPs. Tang et al. [80] introduced hydrophilic resorcinol and melamine as monomers and DESs (ChCl/ethylene glycol, tetramethylammonium bromide/ethylene glycol and tetramethylammonium chloride/ethylene glycol) as solvents into MIPs targeting to recognize quinolones (ciprofloxacin and ofloxacin), and the hydrophilicity of MIPs was increased owing to the hydrophilic groups from both the monomers and DESs. It was observed that the MIPs prepared with DESs—especially the DES of ChCl and ethylene glycol—had excellent recoveries and purification of quinolones in wastewater. Liang et al. [81] synthesized molecularly imprinted phloroglucinol–formaldehyde–melamine resin (MIPFMR) using a hydrophilic resin and MIPs technology in the DES (ChCl/ethylene glycol = 1:2) to enhance the affinity of the MIPFMR for analytes in aqueous media. MIPFMR was prepared by using phenylephrine phloroglucinol as dummy template, melamine as bifunctional monomer and formaldehyde as cross-linker. Hydroxyl groups, ether linkages and amino groups were able to be introduced to MPIFMR by using phloroglucinol and melamine, which could interact with the template in DES via hydrogen bonds and π–π interactions. Moreover, MIPFMR prepared in DES showed higher adsorption property and hydrophilicity than resins prepared in alcoholic solvent systems, because polar solvents were not favorable to the formation of hydrogen bonds between templates and monomers. There have been many studies to introduce DESs in the preparation of MIPs for modification, purification and functionalization, but the expansions of the roles of DESs in the processing of MIPs still need further exploration by researchers.

#### **4. Conclusions and Outlook**

Composite resins are widely used in public transportation, construction supplies, biomedicine and industrial productions. Advancement in processing of composite resins to make them better suited for certain applications is of great interest to both academic and industrial communities. DESs have been found to be a greener and more economical alternative to conventional organic solvents or ILs and can be easily prepared by mixing two or more components through the formation of hydrogen bonds. This review addressed the development of DESs in the processing of composite resins. For example, DESs can be used for the synthesis and modifications of various polymeric materials. Different DESs can be also applied to a certain type of composite resin to meet specific requirements. Previous successful practices have demonstrated the feasibility of using DESs as solvents, monomers and catalysts in polymerization reactions, which has provided new clues for the development of resin preparations. Taking advantage of the designable properties of DESs through the selection of HBDs and HBAs, it has been possible to adjust the physiochemical properties of DESs to better match certain applications during the processing of resin composite. Previous works have shown that DESs can play multiple roles in the processing of functional composite resins, and DESs still hold great potentials in the development of polymer sciences.

The development of DES-based resin composites can be a promising approach for further improvement of resin composites. In addition, profound knowledge of resins and DESs are keys to better understand the behavior of resin composites in certain applications. However, due to the limitations of the design principles of DESs, it has been difficult to gain a clear understanding on the intermolecular forces when DESs are introduced to a new polymeric matrix. Therefore, in-depth research is necessary to clarify the interactions between DESs and the original resin composites—especially in terms of the functionality of hydrogen bonds within DESs in the complexed composite resins. This review aimed to provide a summary of previous works towards the processing of composite resins with DESs, and hopefully an inspiration of low-cost technology to high-tech products was able to be made.

**Author Contributions:** Writing—original draft preparation, J.W., J.X., D.F.; writing—review and editing, H.H., M.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the University City Fusion Project from Science and Technology Bureau of Zhangdian District (Zibo, China), Grant Number 9001-118246 (J.W.) and the startup fund from Shandong University of Technology (J.W. and M.W.).

**Acknowledgments:** J.W. and M.W. would like to specially acknowledge the startup fund from Shandong University of Technology that were used in partial support of this work.

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

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