Combining Renewable Eleostearic Acid and Eugenol to Fabricate Sustainable Plasticizer and Its Effect of Plasticizing on PVC

The recent studies on sustainable plasticizer mainly focus on raw material source, synthesis method and plasticization, but the effect of chemical functional groups (epoxy group and ester group) of sustainable plasticizer on compatibility and thermal stability of plasticized polyvinyl chlorid (PVC) materials has been ignored. In this study, two kinds of sustainable plasticizers (eleostearic acid eugenol ester (EAEE) and epoxidized EAEE) were synthesized to plasticize PVC films using eleostearic acid and eugenol. Compared to EAEE, PVC plasticized with epoxidized EAEE showed more flexible and thermal stability. Due to forming more hydrogen bonds between PVC chains and epoxidized EAEE than that of PVC chains, the efficient plasticizing on PVC with epoxidized EAEE was more than EAEE. Because of the flexible alkane chains and polar group (ester groups and epoxy groups) in the structure of epoxidized EAEE, the intermolecular interaction force with PVC was stronger than EAEE, and there was homogeneous and smooth surface of plasticized PVC films.


Introduction
Polyvinyl chlorid (PVC) has been widely used in many different fields as early as 1926 [1]. Due to its good physical properties, excellent thermochemical resistance and low cost, it is second only to polyethylene (PE) and polypropylene (PP) in the global plastics market [2]. Some specific additives are often added to PVC to obtain better performance, such as thermal stability, flexibility, flame retardancy, etc. Plasticizer is one of the most common plastics additives [3], which is used to make PVC obtain better performance, softer and more ductile [4].
Phthalates are commonly used as plasticizers, in particular, the dioctyl phthalate (DOP) and dibutyl phthalate (DBP), accounting for 80% of the global plasticizer market [5]. Phthalates are fully derived from fossil oil [6], which are easy to diffuse out of PVC products, reduce the performance of PVC, and increase the harm to the human body and the environment. Studies have shown that it destroys the body's normal endocrine, and causes carcinogen [7][8][9]. When plastic is discarded and flowed into rivers and lakes, it will be broken into small pieces under solar radiation, temperature changes and biological action, which is also known as microplastics [10]. These microplastics can adsorb pollutants in the water, and when unfortunately enter the human body, which will cause inflammation, immune damage and even death [11]. In order to solve the existing problems, Navarro and his team had developed various non-migration plasticizers by connecting with plasticizing molecules on the PVC chain through highly active functions [12][13][14]. Chung and his group using a plasticizer composed of alkylterminal high-branched polyglycerol to produce non-toxic, phthalate-free flexible of PVC [15]. Because of its harmfulness, various countries' governments have restricted the addition of DOP in PVC products, including children's toys, food packaging, etc. [9,16]. As early as two decades ago, both European and American regulations had defined six banned phthalates derivatives [17]. Due to recent stricter bans, the use proportion of phthalate is expected to dropping from 65% (2017) to 60% (2022) [18].
Safe bio-based plasticizers have become the mainstream of research, which are derived from vegetable oils, citrates, and sugar derivatives [4], and have the advantages of sustainable, largely available and low cost. There is a lot of research on the use of soybean oil [19][20][21]. Tung oil [22], sunflower oil [23,24], cardanol [25,26], waste cooking oil and other bio-based plasticizers. Dekai Liu reported that oleic acid based eater derived from vegetable oil was a renewable environmentally friendly plasticizer with good cold resistance [27]. Before using vegetable oil to synthesize plasticizer, it must be modified, because vegetable oil has limited compatibility with PVC. Cardanol is often modified and used as a potential plasticizer, and its unsaturated bonds are often epoxidized. Greco and his team developed an epoxidation method, the general m-chloro benzoic acid and dichloromethane were discarded, and the environmentally friendly reagents were used to modify cardanol. The experimental results showed that the strength of PVC with 70 phr cardanol plasticizer was similar to that of 50 phr DEHP. However, the performance droped rapidly after aging, and the waist anti-migration performance was poor [28]. In most cases, natural unsaturated bonds are usually epoxidized to obtain better heat resistance. At this time their anti-migration performance is improved [29].
However, due to the complex and cumbersome reaction and harsh conditions, it is difficult to apply to large-scale industrial production [28]. Foreign chemical giants have launched related products, such as EASTMAN's TIXB (2,4-trimethyl-1,3-glycol diolates) and other environmentally friendly plasticizer products. Witold Brostow compared the performance of toxic plasticizers and non-toxic bio-based plasticizers in dynamic friction, and found that non-toxic plasticizers were comparable to traditional toxic plasticizers [30].
Eleostearic acid and eugenol are derived from biomass materials, the molecular structure of eleostearic acid contains a conjugated double bond, and the molecular structure of eugenol contains benzene ring and phenolic hydroxyl. The structure of esterification product with eleostearic acid and eugenol (EAEE) contains benzene ring and ester group, which is similar to commercial petroleum based plasticizer DOP. And there are many epoxy groups in the structure of epoxidized EAEE, which is similar to commercial bio-based plasticizer epoxidized soybean oil. Therefore, EAEE and epoxidized EAEE can be used as PVC plasticizers to obtain excellent plasticizing properties.
Herein, in this study, eleostearic acid and eugenol were used to synthesize two kinds of bio-based plasticizers (EAEE and epoxidized EAEE) to plasticize PVC in different proportions. The performance of the plasticized PVC films will be investigated. The effect of chemical functional groups (epoxy group and ester groups) of biomass based plasticizer on compatibility and thermal stability of plasticized PVC materials are investigate in detail.

Synthesis of Eleostearic Acid Eugenol Ester (EAEE)
Eleostearic acid (50 g, 0.18 mol) and 150 ml dichloromethane were dissolved in 500 ml three-necked flask equipped with a magnetic stirrer and a reflux condenser. Then oxalyl chloride (23 g, 0.18 mol) was added in the flask, and the reaction was continued at 25 °C for 3 h. After the mixture of eugenol (27 g, 0.16 mol), trimethylamine (18 g, 0.18 mol) and 50 ml dichloromethane was slowly added into the system at 25 °C. Then the reaction was heated to 50 °C for 12 h. Finally, precipitate was removed by suction filtration, and the filtrate washed several times with 7 wt% sodium bicarbonate solution, and saturated NaCl solution cleaned to neutral pH by a separating funnel. The organic layer was collected and dried with anhydrous sodium sulfate and filtered. The methylene chloride was removed by a rotary evaporator and stored in a refrigerator at 4 °C for later use.

Synthesis of Epoxidized EAEE
EAEE (50 g) and ethyl acetate (100 ml) were added into a 500 ml flask, and perform magnetic stirring in an ice-water bath (< 5°C). 3-chloroperoxybenzoic acid (92 g) was dissolved in 200 ml of ethyl acetate and dropped into the reaction system through a constant pressure separatory funnel, and the temperature was controlled to be lower than 5 °C. Then the reaction was heated to 25 °C for 2 h, benzoic acid precipitated during the reaction. The product was washed several times with 7 wt% sodium bicarbonate solution, and saturated NaCl solution cleaned to neutral pH by a separating funnel. The organic layer was collected and dried with anhydrous sodium sulfate and filtered. The methylene chloride was removed by a rotary evaporator to obtain epoxidized EAEE.

Preparation of Plasticized PVC Materials
PVC and plasticizer (epoxidized EAEE and EAEE) was dissolved in 40 ml of THF at 40 °C. The mixture was fully dissolved by magnetic stirring, and then sonicated for 30 min to remove air bubbles. The samples were poured into petri dishes (d = 12 cm) followed by slowly air drying at ambient temperature for 1 days then under reduced pressure for 1 days to remove traces of THF and to obtain thin films. The formulations are shown in Table 1.

Characterization
FTIR spectroscopy was performed on a Nicolet IS 10 IR spectrometer (Nicolet Co., USA) in a range of 4000-500 cm -1 and the resolution of 4 cm -1 . 1 H NMR spectra were confirmed by a Bruker ARX 300 nuclear magnetic resonance spectrometer with CDCl 3 as the solvent and tetramethylsilane as the internal standard. TGA measurements were performed NETZSCH TG 209F1 (Netzsch Instrument Crop., Germany) in a nitrogen atmosphere was used with a heating rate of 10 °C/min, and the temperature ranged from 35 to 600 °C. Microstructure of PVC films was investigated using Leica DM750M (Leica Co., Germany). The glass transition temperature (T g ) of PVC films was investigated using a NETZSCH differential scanning calorimeter (DSC) 200 PC analyzer under N 2 atmosphere. The temperature ranged from − 40 to 120 °C at a heating of 10 °C/min. Tensile properties test was measured by a CMT4000 universal testing machine (according to ISO 527-2:1993) with stretching rate of 20 mm/min. Each named sample need test at least five times (Fig. 1).

Results and Discussions
The obtained biomass based plasticizers (EAEE and epoxidized EAEE) were characterized with FTIR and 1 H NMR, as seen from Fig. 2, and compared with eugenol and eleostearic acid. As seen from Fig. 2a, the peaks at 3533 cm −1 , 3010 cm −1 , 1515 cm −1 were attributed to -OH, -CH=CH-and -CH 2 -O-CH 2 of eugenol respectively [31,32]. Figure 2b shows the FTIR of eleostearic acid, the strong peak at 1796 cm −1 was attributed to C=O group. In the Fig. 2c, the peaks at 6.8-6.9 ppm and 5.0-6.0 ppm were attributed to the protons of benzene ring and -CH 2 -CH 2 -respectively, the other peaks at 3.43 ppm and 3.91 ppm was attributed to other protons of -CH 2 - [31,32]. As seen from Fig. 2d, the peaks at around 5.4-5.9 ppm were corresponded to the protons of conjugated double bond from eleostearic acid, the peaks at 4.2-4.4 ppm were attributed to the protons of -CH 2 -connecting to the conjugated  [33]. After esterification, as seen from Fig. 3, the characteristic peaks of -OH at 3550 cm −1 was almost disappeared in the FTIR of EAEE, and the peak of ester group was observed at around 1761 cm −1 , which indicated that EAEE was obtained. Compared with the spectrum of EAEE, the characteristic peaks of -CH=CH-at 3010 cm −1 was disappeared, and the characteristic peak of epoxy group was observed around 969 cm −1 in the FTIR of epoxidized EAEE, the result showed that epoxidized EAEE was synthesized [33,34]. 1 H NMR of EAEE and epoxidized EAEE were also characterized to investigate their chemical structure. As seen from Fig. 4, the peaks of EAEE at 6.72-70 ppm were attributed to the protons of benzene ring, the peaks at 5.31-5.5 ppm were corresponded to the protons of conjugated double bond, which indicated that EAEE was obtained. In the 1 H NMR of epoxidized EAEE, the proton peaks of conjugated double bond at 5.31-5.5 ppm were disappeared, and the new peaks of epoxy group were observed at 2.80-3.12 ppm [33,35,36], which also indicated that epoxidized EAEE was synthesized successfully.
In the processing of PVC resin, epoxidized plasticizer not only has plasticizing effect on PVC, but also has the function of stabilizer, because the epoxy group can absorb the hydrogen chloride degraded by light and heat, so as to prevent the   [37,38]. In addition, epoxidized plasticizer has a cross-linking and toughening effect on PVC products, which can improve PVC's resistance to light, heat, aging, impact and lubrication. In this study, TGA was used to evaluate the thermal stability of PVC materials plasticized with EAEE and epoxidized EAEE. Figures 5  and 6 show the TGA and DTG curves, and the related TGA data 5%, 60% mass loss temperatures (T −5% , and T −60% ), and char yield at 600 °C are summarized in Table 2. With the addition of more epoxidized EAEE and EAEE in PVC films, T −5% increased from 171.0 to 237.3 °C for plasticized PVC with EAEE, and T −5% increased from 171.0 to 244.1 °C for plasticized PVC with epoxidized EAEE. In addition, T −60% of PVC-50 wt% EAEE and PVC-50 wt% epoxidized EAEE reached 349.1 °C and 372.9 °C, respectively. The results showed that the thermal stabilities of plasticized PVC films with epoxidized EAEE and EAEE were improved, and epoxidized EAEE had more effective for improving the thermal stability of PVC films than EAEE. This was mainly because the epoxy group of epoxidized EAEE could react with HCl, which could absorb the hydrogen chloride degraded by heat, so as to prevent the decomposition of PVC dehydrochlorination [35,39,40]. In addition, epoxidized EAEE had a crosslinking and toughening effect on PVC films, which could improve PVC's resistance.
It has been reported that plasticized PVC films with lower T g will show more excellent plasticizing property and compatibility [30]. In this study, DSC was employed to investigate the T g of plasticized PVC films. Figure 7 shows the DSC curves of PVC plasticized with epoxidized EAEE and EAEE. T g value for PVC films was 82.5 °C, which had been reported in our previous study [30]. T g value for PVC-25 wt% EAEE, PVC-50 wt% EAEE, PVC-25 wt% epoxidized EAEE and PVC-50wt% epoxidized EAEE were 36 °C, 27 °C, 24 °C and 7 °C respectively, which showed that both plasticizers played efficient plasticizing effect on PVC, and the plasticizing effect of epoxidized EAEE was more efficient than EAEE. In addition, all of the plasticized PVC films showed only one endothermic peak, which illustrated   that epoxidized EAEE and EAEE were all compatible with PVC, there was no free plasticizer in PVC films. Figure 8 shows the stress-strain curves of all PVC films. With the addition of more plasticizers in PVC films, there was the same change trend of plasticized PVC, the elongation at break was increased, but the tensile stress was decreased. Which indicated that epoxidized EAEE and EAEE could improve the flexibility and movement ability of the PVC molecular chain to a certain extent. When the addition dosage of EAEE and epoxidized EAEE in PVC was same, epoxidized EAEE showed more flexible than EAEE, which indicated that epoxidized EAEE played more efficient plasticizing effect on PVC than EAEE.
Plasticizing mechanism was explored with optical microscope. As seen from Fig. 9, it could be observed that rough and irregular microstructure on surface including pits and bulges of PVC-25 wt% EAEE, with the addition of more EAEE in PVC film, PVC was dissolved in EAEE, the surface of PVC-50 wt% EAEE was less rough and irregular than PVC-25 wt% EAEE, and compatible area appeared on the surface of PVC-50 wt% EAEE. For epoxidized EAEE, there was less rough and more smooth microstructure compared with PVC-50 wt% EAEE, and the compatible area on the surface of all epoxidized EAEE presented more than PVC-50 wt% EAEE, which was attributed to the strong interaction of the polar groups of epoxidized EAEE (ester group and epoxy groups) with polar groups of PVC. The plasticizing mechanism based on compatibility could be explained that epoxidized EAEE containing the flexible alkane chains and polar group (ester groups and epoxy groups) had stronger intermolecular interaction force than EAEE. PVC was easier to dissolve in epoxidized EAEE than EAEE, which caused that the microstructure of PVC-50 wt% epoxidized EAEE was more homogeneous and smooth than PVC-25 wt% EAEE and PVC-50 wt% EAEE.
The compatibility between PVC and plasticizers (epoxidized EAEE and EAEE) was investigated by FTIR. It has been reported that FTIR is a facile method to investigate the compatibility of PVC and plasticizers [41,42]. Dipole-dipole interaction between polar groups (ester carbonyl groups and epoxy groups) and α-hydrogen of PVC resulted in a lower carbonyl group infrared absorption intensity of plasticizer (epoxidized EAEE and EAEE). As seen from Fig. 10, with the addition of more plasticizer (epoxidized EAEE and EAEE) in PVC films, the position shift of ester group of EAEE shifted from 1758 to 1756 cm −1 , while the position shift of ester group of epoxidized EAEE shifted from 1757 to 1755 cm −1 , which indicated that the miscibility between epoxidized EAEE and PVC was better than EAEE.
The formation of hydrogen bond between plasticizer (epoxidized EAEE and EAEE) and PVC chains would reduce interaction between PVC macromolecule themselves, it could be observed in Fig. 11, the distance of PVC chains would be increased due to the formation of hydrogen, which would promote the mobility of PVC chains easily. Due to more epoxy groups in the structure of epoxidized EAEE, more hydrogen bonds were formed between PVC chains and epoxidized EAEE than that of EAEE, which indicated that epoxidized EAEE played more efficient plasticizing effect on PVC than EAEE.

Conclusion
In this study, two kinds of biomass based plasticizer (EAEE and epoxidized EAEE) were synthesized. The performance of plasticized PVC with EAEE and epoxidized EAEE were investigated. The results showed that epoxidized EAEE had more effective to improve the thermal stability of PVC  films than EAEE, because the epoxy group could react with HCl and absorb the HCl degraded by heat, so as to prevent the decomposition of PVC dehydrochlorination. T g value for PVC-25 wt% EAEE, PVC-50 wt% EAEE, PVC-25 wt% epoxidized EAEE and PVC-50 wt% epoxidized EAEE were 36 °C, 27 °C, 24 °C and 7 °C respectively. Combing with the tensile proprieties, the results showed that epoxidized EAEE and EAEE played efficient plasticizing effect on PVC. More hydrogen bonds were formed between PVC chains and epoxidized EAEE than that of EAEE, which indicated that epoxidized EAEE played more efficient plasticizing effect on PVC than EAEE. The efficient plasticizers derived from eleostearic acid and eugenol would have wide application prospect in PVC products.