Synthesis and Properties of Isosorbide-Based Eco-friendly Plasticizers for Poly(Vinyl Chloride)

In this study, isosorbide-based eco-friendly and effective plasticizers were synthesized and characterized. Isosorbide esterification was conducted using fatty acids with different alkyl-chain lengths; the optimal degree of isosorbide esterification was obtained after 6 h reaction at 220 °C with a short alkyl-chain fatty acid in the presence of the catalyst Ti(OBu)4. Isosorbide-based plasticizers of C12 or more alkyl-chain lengths exhibited low compatibility and did not form PVC sheets. An esterified isosorbide with long alkyl-chain length was epoxidized to enhance its plasticizing properties and enable low-cost production; the high epoxidation degree (91%) indicated efficient epoxidation by formic acid and hydrogen peroxide. The synthesized plasticizers, according to the alkyl-chain length, were used to fabricate PVC sheets; subsequently, their mechanical and thermal properties were analyzed. As the alkyl-chain length of the synthesized plasticizer increased, the tensile strength and modulus increased, while the elongation decreased. Furthermore, as the plasticizer content increased, the tensile strength and elongation decreased, while the plasticization efficiency increased. As the alkyl-chain length of the plasticizer increased, the molecular weight increased and the smaller molecules generated more free volume in the PVC chain. The thermal stability of the synthesized isosorbide-based plasticizers improved on increasing their alkyl-chain length, and was higher than that of a commercial plasticizer. Thus, the plasticizers synthesized in this study, particularly isosorbide di epoxidized oleate, could be used as eco-friendly and effective plasticizers for practical PVC applications.


Introduction
Poly(vinyl chloride) (PVC) is a popular thermoplastic for various applications, such as cable insulation, pipes, construction, packaging, window profiles, and automobile interior materials, because of its low-cost production, easy fabrication, high chemical stability, and good mechanical properties [1][2][3]. However, applications of PVC products are limited, owing to the rigidity and brittleness imparted by dipoles in the C-Cl structure of PVC, hindering chain mobility [4]. Therefore, to process flexible PVC products, such as, cable insulation, and automobile interior materials, plasticizers are required as additive materials to reduce the melt viscosity, and glass transition temperature (T g ) during PVC processing [5,6].
To the best of our knowledge, this is the first study to introduce isosorbide di epoxidized oleate as a green plasticizer for PVC processing. Here, isosorbide was esterified with a series of fatty acids, and the isosorbide diester plasticizer with short alkyl chain length showed high compatibility with PVC. However, fatty acids with short alkyl chain length are not suitable for industrial applications because of high material costs. Therefore, the isosorbide diester with long alkyl chain length was epoxidized to enhance its plasticizing properties. The resulting isosorbide di epoxidized oleate, as a plasticizer for PVC sheets, was characterized by analyzing its plasticizing, chemical, mechanical, and thermal properties. As confirmed by precise analytical results, the isosorbide di epoxidized oleate plasticizer exhibits potential as a promising alternative for practical PVC applications.

Synthesis of Isosorbide Dioctanoate
Isosorbide (122 g, 0.8 mol) and toluene (100 mL) were stirred at room temperature in a 1 L 4-necked flask, equipped with a Dean-Stark column, under N 2 . Subsequently, octanoic acid (277.25 g, 1.92 mol) and titanium (IV) butoxide (1 g, 0.3 mol%) were added and kept at 130 °C for 5 h. After toluene removal, the reaction was maintained at 220 °C, for 10 h, and on reaction completion, the mixture was stirred under vacuum (10 hPa) at 220 °C until the pH was < 3, followed by natural cooling to 70 °C, under ambient conditions. The mixture was neutralized with a saturated NaOH aqueous solution until pH 0, and then, it was extracted using a separatory funnel. Isosorbide dioctanoate (ISODO) (261 g, 0.70 mol) was obtained at 120 °C, under 10 hPa, after the removal of residual solvent and moisture (Fig. 1a).

PVC Sheet Processing
The PVC resin, plasticizer (50-60 phr), and stabilizer (2.5 phr) were mixed at 90 °C, using a Henschel mixer (5 L, Jooshin Industrial Co.), and the mixture was compounded at 170 °C for 5 min in a test mixing roll (TOYOSEIKI 287, rotation ratio: 14:19.6 rpm). The PVC sheet was fabricated using a hot-press machine (Carver, Auto Series Presses) at 175 °C for 3 min under 5 ton pressure and then cooled in

Characterization
Fourier-transform infrared spectroscopy (FT-IR, Nicolet 5700, Thermo) was used to analyze changes in the isosorbide structure. Solubility parameters (δ) of the plasticizers were calculated according to Small's cohesive energies [35], and epoxy values of the synthesized plasticizers were calculated according to ASTM D1652-04. The PVC specimens were prepared as type IV dumbbell-type, according to ASTM D638. The tensile stress, elongation, and 100% modulus were measured using a universal test machine (UTM, INSTRON 3369), according to ASTM D882. The hardness was measured using a Shore A-type hardness meter (e-Asker), according to ASTM D2240. Thermogravimetric analysis (TGA, TG209 F3, NETZSCH) was performed in the range of 30-700 °C at a heating rate of 10 °C/min under N 2 atmosphere.

Results and Discussion
The esterification of isosorbide was conducted using various catalysts for synthesizing bio-based plasticizers. The conversion yields of a water-soluble acid catalyst (H 2 SO 4 ), and organic catalysts (TsOH, and Ti(OBu) 4 ) were analyzed to optimize the esterification of isosorbide with octanoic acid. The conversion results are summarized in Table 2. The conversions of isosorbide with H 2 SO 4 and TsOH catalysts were 32% and 63%, respectively, indicating inefficient conversion of the -OH moieties at the syn-position due to steric hindrance of the isosorbide. The highest conversion (87%) was obtained with 1 mol/L Ti(OBu) 4 catalyst after reaction at 220 °C for 6 h, indicating improved conversion efficiency of the -OH group at the syn-position. Therefore, Ti(OBu) 4 was used as an organic catalyst. To elucidate the influence of the alkyl chain length, a series of isosorbide diesters were synthesized, using fatty acids having different numbers of carbons (C8-C18), such as, octanoic acid (ISODO), decanoic acid (ISODD), lauric acid (ISODL), myristic acid (ISODM), palmitic acid (ISODP), stearic acid (ISODS), and oleic acid (ISODOL). Changes in the chemical structure of isosorbide before and after esterification were analyzed using FT-IR spectroscopy (Fig. 2a). The broad peak at 3392 cm -1 (attributed to the -OH group of fatty acids (C8, C10, and C18)) disappeared, indicating efficient conversion of the -OH group, during esterification [36]. The peak intensity of the -OH group for isosorbide (at 1421 cm -1 ) decreased, indicating esterification of the -OH groups located at the anti-and synpositions in the isosorbide with the acid group of octanoic acid [37]. Thus, isosorbide efficiently reacted with the fatty acids, and remained in the system. The peak intensities at 2925 and 2856 cm -1 for ISODO were stronger than those of isosorbide, confirming the formation of CH 2 and CH 3 , respectively, in ISODO [34]. The intensity of the characteristic peak assigned to C=O at 1737 cm -1 increased significantly after esterification [38]. Furthermore, the characteristic peaks at 1159 and 1097 cm -1 exhibited enhancement after esterification, owing to C-O-C group formation [39]. FT-IR spectra were analyzed to study changes in the chemical structure before and after epoxidation. As shown in Fig. 2b, ISODOL exhibited characteristic peaks at 3009 and 1600 cm -1 , for C=C and C-H groups, respectively [40]. After epoxidation (ISODEOL), the peak intensity at 3009 cm -1 (for the C=C group) decreased, while the characteristic peak of the epoxy group appeared at 820 cm -1 , indicating synthesis of the epoxidized plasticizer [41,42].
The epoxidation of ISODOL was investigated as a function of reaction time to obtain the highest yield, and the calculated results are shown in Fig. 3. The epoxy value and conversion increased with increasing reaction time, with the highest values of 3.9% and 91%, respectively, obtained after 12 h reaction time, indicating effective transformation of the ISODOL double bonds into the epoxy group [43]. Thus, ISODEOL was epoxidized from ISODOL, using formic acid and hydrogen peroxide, in agreement with the FTIR results (Fig. 2b). The ISODEOL plasticizer could improve PVC compatibility due to its high degree of conversion [44].
Compatibility is an important parameter for evaluating the processability and mechanical properties of PVC. Therefore, compatibilities of the synthesized plasticizers were evaluated and compared with those of a commercial DEHP plasticizer. To conduct the measurement in the form of a sheet, PVC was mixed with a plasticizer (50 phr) and stabilizer (2.5 phr) at 90 °C using a Henschel mixer. The solubility parameter is an index related to the polarity; similar values for PVC and plasticizer indicate good compatibility [45]. Compatibility between PVC and the plasticizer was evaluated in terms of the solubility parameter (δ) ( Table 3). The δ value of PVC was 9.7, and the δ values of isosorbide-based plasticizers  The plasticizers with > C12 were not absorbed because of high molecular weight [46]. Thus, plasticizers with short alkyl chain length (< C10) exhibit enhanced compatibility with PVC. The ISODOL plasticizer was not absorbed into PVC before epoxidation, while the ISODEOL plasticizer was absorbed into the PVC resin within 6 min 16 s after epoxidation. Additionally, the δ value increased from 8.4 to 8.7 after epoxidation, moving closer to the δ value (8.9) of DEHP. Thus, the PVC compatibility of ISODOL was improved by epoxidation [47]. To evaluate processability, PVC compounds were prepared with different plasticizers, using a two-roll mill, at 170 °C. As shown in Table 3, the PVC resins with DEHP, ISODO, and ISODD plasticizers melted, whereas the resins with ISODL, ISODM, ISODP, and ISODS plasticizers did not melt. Thus, processability of the PVC compounds depended strongly on the alkyl chain length of the plasticizers, in agreement with results of the compatibility assessment [48]. Considering the compatibility and processability of PVC, the synthesized ISODEOL, with the functionalized epoxy group, is a suitable plasticizer for PVC. It is a non-toxic, bio-based, and low-cost material, with efficient plasticizing ability. Using this bio-based plasticizer, the final PVC product could be employed in real applications. To assess the mechanical properties of plasticized PVC sheets with different plasticizers, the tensile stress, elongation at break, 100% modulus, and hardness were measured. As shown in Table 4, the tensile stress and 100% modulus of the PVC sheets increased with increasing alkyl chain length of the plasticizer, while the elongation at break decreased. A plasticizer improves the processability of a polymer, without distorting its basic structure and physical properties. The plasticizer penetrates the polymer chains, weakens the interchain attraction, and increases the free volume, facilitating chain movement. Thus, plasticizer addition increases the chain mobility, and the PVC exhibits a relatively low melting temperature [49]. Therefore, results for mechanical properties could be attributed to the relatively high molecular weight of the plasticizer, which increased with increasing alkyl chain length, whereas relatively small molecules could form the free volume in PVC. Compared to DEHP, ISODO exhibited higher tensile stress and 100% modulus, which could be attributed to formation of different degrees of free volume in the PVC, owing to the structural characteristics of ISODO (linear C8 alkyl chain) and DEHP (branched alkyl ester). Hardness of the PVC sheets increased with increase in alkyl chain length of the plasticizer and decreased slightly with increase in plasticizer content. Mechanical properties of PVC are attributed to free volume formation due to the alkyl chain length, and amount of plasticizer. Thus, ISO-DEOL was confirmed to be the most efficient plasticizer for PVC sheets, compared to other bio-based novel plasticizers in this study.
PVC products formed using a plasticizer should be thermally stable. TGA thermograms were used to elucidate the thermal degradation behaviors of PVC sheets with different  plasticizers (Fig. 4). The initial thermal degradation in the 190-350 °C range was due to the dehydrochlorination of PVC [50]. The second region at 350-430 °C was due to the formation of an aromatic structure, by polyene cyclization [30]. Finally, at temperatures above 430 °C, the polyene structure was thermally degraded [51]. Table 5 summarizes the characteristic temperature points for TGA thermograms of PVC: the initial degradation temperature (T i ), weight loss temperatures of 10% (T 10 ) and 50% (T 50 ), and the residue at 700 °C (%, R 700 ). The T i values of DOP, ISODO, ISODD, and ISODEOL were 190, 200, 210, and 236.8 °C, respectively. The values of T i , T 10 , T 50 , and R 700 for ISODEOL were higher than those of DOP, ISODO, and ISODD. Notably, the R 700 value of ISODEOL significantly increased to 11.37%, which was significantly higher than those of the others. Thus, higher thermal stability of the epoxy groups within ISODEOL enhanced the thermal stability of PVC.

Conclusion
In this study, an alternative isosorbide-based PVC plasticizer was synthesized using a commercially available bio-based raw material, dialkyl isosorbide, and analyzed. Firstly, the synthesis conditions of an isosorbide-based linear fatty-acid ester plasticizer were optimized using a Ti(OBu) 4 catalyst at 220 °C for 6 h; the optimized conversion was 87%. Secondly, the synthesized isosorbide fatty-acid ester-based plasticizer with short alkyl-chain length exhibited absorption and melting trends similar to those of the conventional DEHP plasticizer, while those with long alkyl-chain length exhibited lower compatibility as plasticizers due to the solubility parameter. The solubility parameters of ISODO and ISODD (8.7) were similar to that of DEHP (8.9), while those of ISODL and ISODS decreased to 7.5 due to the increase in alkyl-chain length, causing lower compatibility. Furthermore, the synthesized plasticizer was epoxidized with a long-chain oleic acid for cost effectiveness and increased compatibility, as indicated by the increased solubility parameters of ISODOL (8.4) and ISOEOL (8.7). The increase in solubility indicated an enhancement in the absorption and melting of PVC. Subsequently, the mechanical properties of the PVC fabricated using a synthesized plasticizer was analyzed; the tensile strength and 100% modulus increased, while the elongation-at-break decreased. On increasing the plasticizer content, the tensile strength and 100% modulus of the PVC decreased, while the elongation-at-break increased. A thermal-stability analysis according to the alkyl-chain length through TGA indicated that an increase in the alkylchain length enhanced thermal stability. Thus, the ecofriendly plasticizers synthesized using bio-based isosorbide could be applied for PVC processing.