Insights into biobased epoxidized fatty acid isobutyl esters from biodiesel:Preparation and application as plasticizer

Xiaojiang Liang,Fengjiao Wu,Qinglong Xie,Zhenyu Wu,Jinjin Cai,Congwen Zheng,Junhong Fu,Yong Nie,*

1 Biodiesel Engineering Lab of China Petroleum&Chemical Industry Federation,and Zhejiang Province Key Lab of Biofuel,Zhejiang University of Technology,Hangzhou 310014,China

2 Zhejiang Jiaao Enprotech Stock Co.,Ltd,Tongxiang 314000,China

Keywords:Biodiesel Transesterification Isobutyl esters Epoxidized isobutyl esters Biobased plasticizer

ABSTRACT Biodiesel was used to prepare epoxidized fatty acid isobutyl esters(Ep-FABEs)as a biobased plasticizer in this work.Transesterification of biodiesel with isobutanol catalyzed by tetrabutyl titanate was carried out in a gas-liquid tower reactor.The conversion achieved nearly 100%within 5 h under the reaction temperature,the mass ratio of catalyst to fatty acid methyl esters(FAMEs),and isobutanol to FAMEs total molar ratio of 180 °C,0.4 %(mass),and 5.4:1,respectively.In addition,kinetic model of the transesterification reaction was developed at 150–190 °C.The calculated activation energy was 48.93 kJ?mol-1.Then,the epoxidation of obtained fatty acid isobutyl esters (FABEs) was conducted in the presence of formic acid and hydrogen peroxide.The Ep-FABEs was further analyzed for its plasticizing effectiveness to replace dioctyl phthalate (DOP)and compared with conventional epoxy plasticizer epoxidized fatty acid methyl esters(Ep-FAMEs).The results indicated that the thermal stability and mechanical properties of PVC films with Ep-FABEs plasticizer were significantly improved compared with those plasticized with DOP.In addition,the extraction resistance and migration stability of Ep-FABEs were better than those of Ep-FAMEs.Overall,the prepared Ep-FABEs via structural modification of biodiesel proved to be a promising biobased plasticizer.

1.Introduction

Biodiesel is a mixture of long-chain fatty acid methyl esters(FAMEs)synthesized by the transesterification of triglycerides with methanol [1,2].Nowadays,biodiesel has become one of the most promising alternatives to the mineral diesel fuel due to its environmental friendliness and renewable resources such as soybean oil[3],Jatropha oil [4],waste vegetable oil [5],waste cooking oils[6,7].Moreover,biodiesel is considered as an important green source to produce high value-added products,e.g.,biobased plasticizers [8],biobased surfactants [9],biobased lubricant [10] due to its special structure containing unsaturated C-C bonds,αprotons,and esters groups.Hence,the issue of how to utilize biodiesel in an efficient way is becoming one of the focuses of current biodiesel industry.

With the restricted use of phthalates plasticizers (DOP,DBP) in food package,medical equipment,and cosmetics,biobased plasticizers as a promising alternative to phthalates plasticizers have been increasingly and widely used in various PVC products,due to their renewability,low toxicity,and biodegradability [11,12].To date,biobased plasticizers mainly include epoxidized fatty acid methyl esters (Ep-FAMEs) [13],aliphatic diesters [13],epoxidized triglycerides [14],and citrate plasticizers [15].Among them,Ep-FAMEs have received considerable attention because of its high plasticizing efficiency and low cost,which can be directly acquired from unsaturated fatty acid methyl esters by using percarboxylic acids or organic/inorganic peroxides[16–19].Normally,the formic acid-hydrogen peroxide autocatalytic method is a compromise between process safety,energy integration,and kinetics.

Furthermore,Ep-FAMEs can be used as an efficient heat stabilizer for PVC,since the epoxy groups in the Ep-FAMEs can absorb and neutralize the hydrogen chloride generated during light or thermal degradation of PVC,which limits the continuous decomposition of PVC to some extent [20,21].Thus,Ep-FAMEs are now considered as a crucial biobased coplasticizer in the industry.However,Ep-FAMEs cannot be used in PVC formulations alone due to their strong tendency to migrate [22].Therefore,it is urgent to modify the structure of Ep-FAMEs to efficiently suppress migration.

Previously,several methods have been reported to modify the structure of plasticizer,including hyperbranched polymerization from specific feedstocks [23],modification of Ep-FAMEs structure with citric acid [24],covalently attaching the plasticizers to PVC by click chemistry [25],and ester-branched from the ester feedstocks by Claisen condensation [26].Recently,our group modified the molecular structure of Ep-FAMEs through transesterification reaction with 2-ethyl-1-hexanol.The obtained epoxidized fatty acid 2-ethylhexyl esters showed better migration resistance than that of Ep-FAMEs [27].The transesterification reaction proved to be a simple and efficient method to increase the molecular weight of Ep-FAMEs and hence improve their migration resistance.Nowadays,tetrabutyl titanate catalyst has attracted considerable attention in the esterification/transesterification reaction owing to its properties including relatively high activity,low cost,availability,and fewer side reactions [28–30].Compared with other basic catalysts (KOH,K2CO3),tetrabutyl titanate was beneficial to reduce the generation of unwanted soaps during transesterification reaction,which can improve the yield of product and facilitate the separation process.

Isobutanol is a widely used reagent in transesterification reactions,forming a well-soluble homogeneous system with oils and esters[31].Isobutanol can be easily obtained from renewable biomass by fermentation.Therefore,it is promising to modify the molecular structure of Ep-FAMEs using isobutanol as the transesterification reagent due to its renewability and relatively low cost.However,few studies on the preparation and application of epoxidized fatty acid isobutyl esters (Ep-FABEs) as the biobased plasticizer in PVC were reported.

In this paper,a two-step approach,including transesterification and epoxidation,was adopted to prepare Ep-FABEs using biodiesel as the raw material,as shown in Fig.1.Firstly,transesterification of biodiesel with isobutanol catalyzed by tetrabutyl titanate to synthesize fatty acid isobutyl esters (FABEs) was carried out in a self-design tower reactor.Operating parameters such as reaction temperature,catalyst amount,and total molar ratio of isobutanol to FAMEs were researched.The kinetic of the transesterification reaction was further investigated.Secondly,Ep-FABEs were obtained by formic acid-hydrogen peroxide autocatalytic method.The plasticizing properties of Ep-FABEs for PVC were evaluated and compared with Ep-FAMEs,including the glass transition temperature,mechanical properties,thermal stability,extraction resistance,and migration stability.

2.Materials and Methods

2.1.Reagents and materials

FAMEs(Iodine value 127.24 g I2/100 g oil)and hydrogen peroxide (50 %(mass)) were obtained from Zhejiang Jiaao Enprotech Stock Co.,Ltd.The main composition of FAMEs is shown in Table 1.Isobutanol (99.0 %(mass)),tetrabutyl titanate (99.0 %(mass)),formic acid (99.0 %(mass)),PVC (K–value 65),dioctyl phthalate(DOP,99.0 %(mass)),zinc stearate (Zn 10.0%–12.0 %(mass)),calcium stearate (Ca 6.6%–7.4 %(mass)),and kerosene(99.0%(mass)),were purchased from Aladdin Industrial Corporation,China.

Table 1 Composition of FAMEs

2.2.Experimental procedure and analysis

2.2.1.Transesterification of FAMEs with isobutanol

The transesterification of FAMEs with isobutanol was investigated in a self-design gas-liquid tower reactor [32].The reactor was mainly consisted of a 500 mL four-neck flask,a glass tube(20 mm × 500 mm),a glass condenser,and peristaltic pumps,as shown in Fig.2.The transesterification reaction took place in the four-neck flask.A peristaltic pump was used to circulate the FAMEs,and another micro-peristaltic pump was used to add the isobutanol to the reaction section.The glass tube filled with glass spring packings as the mass transfer section was used for removal of produced methanol and dissolution of isobutanol vapor in the FAMEs.Methanol and excess isobutanol were condensed in the glass condenser.

The reaction process was shown as follows:firstly,a proportion of FAMEs and tetrabutyl titanate catalyst were added to the fourneck flask.The mixture and the packing section were heated to the reaction temperature.Meantime,FAMEs were pumped and sprayed from top to bottom in the tower reactor,with the flow rate and spray density of FAMEs being 19.2 mL?min-1and 3.67 m3?m-2?h-1,respectively.Then,isobutanol was continuously added into the four-neck flask by the micro-peristaltic pump to start the reaction.Since the reaction temperature was higher than the boiling point of isobutanol,the added isobutanol was heated and immediately evaporated from the flask to the glass tube.The contact and mass transfer between isobutanol vapor and FAMEs were realized on the glass spring packings.The isobutanol vapor can be absorbed by the dropped FAMEs.Meanwhile,methanol as a product was quickly removed with the continuous vapor of isobutanol,shifting the transesterification reaction to the right side.Finally,the mixture vapor of methanol and excess isobutanol was condensed by the glass condenser.During the reaction,a small amount of sample(about 0.5 mL)was taken out of the reactor at a regular interval and analyzed by gas chromatography (GC).The effects of reaction temperature (150–190 °C),mass ratio of tetrabutyl titanate to FAMEs (0.2%–1.6 %(mass)),and total molar ratio of isobutanol to FAMEs (2.7:1–8.1:1) on the FAMEs conversion were investigated.After the reaction,excess isobutanol in the liquid product was removed in a rotary evaporator.Then,FABEs were purified by high vacuum distillation for subsequent epoxidation reaction.

Fig.1.Scheme for the synthesis of epoxidized fatty acid isobutyl esters (Ep-FABEs).

Fig.2.Semi-batch tower reactor for transesterification of FAMEs with isobutanol.

The transesterification products were determined using SHIMADZU 2014C GC (SHIMADZU,Kyoto,Japan) equipped with a flame ionization detector.A capillary column (DB-5,30 m × 0.25 mm × 0.25 μm) (Agilent Technologies Lnc.,Palo Alto,California,USA) was used.The carrier gas was nitrogen and the flow rate was 30 mL?min-1.Besides,the flow rates of air and hydrogen were 400 and 40 mL?min-1,respectively.The oven temperature was initially held at 120 °C for 1 min,and then programmed at a rate of 10 °C?min-1to 140 °C and held at 140 °C for 1 min,and finally increased to 280 °C at a rate of 20 °C?min-1where it remained for 10 min.The temperatures of both injector and detector were kept at 290 °C and the injection size was 1 μl with a split ratio of 100:1.

In the semi-batch tower reactor above,the FAMEs conversion could be calculated by the following equation:

whereCFAMEs,0was initial FAMEs concentration (mol?L-1),CFAMEs,twas FAMEs concentration at reaction timet(h).

2.2.2.Epoxidation of fatty acid isobutyl esters

The epoxidation of FABEs was carried out in the presence of formic acid and hydrogen peroxide according to the procedure described in Wuet al[33].Briefly,FABEs (100 g) and formic acid(5.09 g)were placed into a 250 ml flask equipped with an agitator and a dripping device for hydrogen peroxide.The flask was heated in a preheated oil bath to 60 °C and the stirring rate was kept at 400 r?min-1during the 6-hour process.Afterwards,hydrogen peroxide (51.10 g) was added to the mixture to start the epoxidation reaction,which was added dropwise within 30 min to maintain the reaction temperature stable.When the reaction was complete,the product was quickly separated into oil phase and aqueous phase by centrifugation.The oil phase was washed repeatedly with deionized water to neutrality,and then dewatered in a rotary evaporator to obtain the Ep-FABEs product.

The product was then analyzed to measure its epoxy value(EV)and iodine value (IV) by Chinese Standard GB/T 1677–2008 and GB/T 1676–2008,respectively.Fourier transform infrared (FT-IR)spectra of products were recorded using a Nicolet 6700 spectrometer (Thermo Nicolet Corporation,USA) at the scanning range of 400–4000 cm-1.Nuclear magnetic resonance (NMR) spectra of products were collected on a Bruker Avance Щ-400 NMR spectrometer(Bruker,Swiss)using deuterated chloroform as a solvent.

2.2.3.Preparation and analysis of plasticized PVC films

PVC resin and thermal stabilizers (calcium stearate and zinc stearate,1:1) were evenly mixed with different plasticizers.The mixture was thoroughly compounded in double-roller blending rolls at 180°C for 8 min.Subsequently,the mixture was preheated by a curing press at 180 °C for 2 min,pressed at a pressure of 10 MPa for 2 min.The final PVC films with the thickness of 1 mm were obtained after cooling for 2 min.The formulation of different plasticized PVC films was shown in Table 2.

Table 2 Formulation of different plasticized PVC films

Table 3 Fitted observed rate constants k at different temperatures

The glass transition temperature,mechanical properties,thermal stability,extraction resistance,and migration stability of PVC films with different plasticizers were determined according to the methods described in our previous work [27].

3.Results and Discussion

3.1.Optimization of transesterification of FAMEs with isobutanol

3.1.1.Effect of temperature on FAMEs conversation

The effect of reaction temperature on the FAMEs conversion was researched in the range of 150–190 °C.As shown in Fig.3,the reaction rate generally increased with increasing reaction temperature.The FAMEs conversion after 2 h reaction was only 54.3%at 150 °C,which reached up to 71.7% at a reaction temperature of 190°C.When the temperature was lower than 160°C,the reaction temperature was the dominant factor affecting the FAMEs conversion and hence the FAMEs conversion obviously increased with the increasing temperature.However,when the temperature was higher than 160°C,the concentration of isobutanol would become an important factor affecting the FAMEs conversion.The solubility of isobutanol vapor in the FAMEs was reduced at a higher temperature,leading to a decrease in the concentration of isobutanol in the reaction mixture.The average concentration of isobutanol was significantly reduced from 0.65 mol?L-1at 160 °C to 0.38 mol?L-1at 190 °C,which was unfavorable for the conversion of FAMEs to FABEs.As a result,only a slight increase in FAMEs conversion was observed when the temperature increased from 170°C to 190°C.Since no obvious change in FAMEs conversion was found at the reaction temperature of higher than 180°C,the optimal temperature for the transesterification reaction was considered to be 180 °C.

Fig.3.Effect of reaction temperature on FAMEs conversion (Reaction conditions:isobutanol to FAMEs total molar ratio of 5.4:1,catalyst amount of 0.4 %(mass)).

3.1.2.Effect of the catalyst amount on FAMEs conversation

The influence of catalyst amount varying from 0.2% (mass) to 1.6% (mass) (based on FAMEs mass) on the FAMEs conversion is shown in Fig.4.The reaction rate obviously increased with increasing catalyst amount.The tetrabutyl titanate catalyst can be dissolved in the FAMEs,which eliminated the mass transfer resistance.It can also be observed that the FAMEs conversion after 5 h reaction reached higher than 99.5% when the catalyst amount was larger than 0.4% (mass).It indicated that catalyst amount of 0.4% (mass) was sufficient to completely convert FAMEs to FABEs.

3.1.3.Effect of isobutanol to FAMEs total molar ratio on FAMEs conversion

Fig.4.Effect of catalyst amount on FAMEs conversion (Reaction conditions:reaction temperature of 180 °C,isobutanol to FAMEs total molar ratio of 5.4:1).

Fig.5.(a) Effect of isobutanol to FAMEs total molar ratio on FAMEs conversion(Reaction conditions:reaction temperature of 180 °C,catalyst amount of 0.4 %(mass)).(b) Effect of isobutanol to FAMEs total molar ratio on isobutanol concentration in the reaction mixture.

The effect of isobutanol to FAMEs total molar ratio ranging from 2.7:1 to 8.1:1 on the FAMEs conversion is shown in Fig.5 (a).The reaction rate generally increased with the increase in total molar ratio of isobutanol to FAMEs.The FAMEs conversation after 5 h reaction increased from 91.0% to 99.7% when the total molar ratio of isobutanol to FAMEs increased from 2.7:1 to 5.4:1.The main reason lied in the increasing isobutanol to FAMEs total molar ratio would enhance the solubility of isobutanol vapor in the FAMEs and hence increase the isobutanol concentration in the reaction mixture.As shown in Fig.5 (b),the isobutanol concentration was significantly increased from 0.26 mol?L-1at isobutanol to FAMEs total molar ratio of 2.7:1 to 0.47 mol?L-1at the total molar ratio of 5.4:1.However,higher isobutanol to FAMEs total molar ratio would cause larger energy consumption in isobutanol evaporation and recycle.Therefore,considering both FAMEs conversion and energy cost,the optimal total molar ratio of isobutanol to FAMEs was 5.4:1,where the isobutanol flow rate was 2.02 mL?min-1.

Fig.6.(a) Arrhenius plot for calculation of activation energy and frequency factor.(b) Relative deviation between FAMEs conversion obtained in experiments and using the model.

3.2.Kinetic model of the transesterification of FAMEs with isobutanol catalyzed by tetrabutyl titanate

To establish a kinetic model of the transesterification of FAMEs with isobutanol catalyzed by tetrabutyl titanate,the experimental data were obtained at the isobutanol to FAMEs total molar ratio of 5.4:1 and catalyst amount of 0.4 %(mass).The kinetic study of the transesterification reaction was performed at a reaction temperature of 150–190 °C.

The following assumption was used in the development of the kinetic model:

According to the GC results,no methanol was detected in the liquid reaction mixture.It suggested that the produced methanol was quickly removed from the system,and hence the transesterification reaction was regarded as irreversible.

According to the above assumption,the following equation for the transesterification reaction of FAMEs with isobutanol was obtained.

The reaction rate can be expressed as Eq.(3).

wherekrepresents the observed rate constant;CA,CBrepresent the molar concentrations of FAMEs and isobutanol,respectively;α,β represent the reaction order of FAMEs and isobutanol,respectively.

The mole balance could be expressed as Eq.(4).

Combining Eqs.(3) and (4) yields Eq.(5) as follows.

Therefore,based on the concentrations of FAMEs and isobutanol at different reaction time,the values of α,β,andkcan be determined by a least squares method.The mathematic fitting results showed the values of α and β were 0.81 and 1.0,respectively.The observed rate constants(k)at different temperatures are listed in Table 3.

Table 4 Mechanical and thermal properties of different plasticized PVC films

Based on the results,the Arrhenius equation (Eq.(6)) was used to determine the activation energy(Ea)and frequency factor(A)of the transesterification reaction.As shown in Fig.6(a),the graph of lnk vs.1/Twas plotted with a slope ofEa/R.Accordingly,the activation energy in the reaction system was calculated to be 48.93 kJ?mol-1,which was consistent with the typical values(33–84 kJ?mol-1) for the base catalyzed transesterification reactions [34].The frequency factor was calculated to be 21279.1 L?mol-1?min-1?g-1.

Fig.7.FT-IR spectra of (a) Ep-FABEs,(b) FABEs.

Therefore,the observed rate constant could be expressed as follows:

Fig.8.1H NMR spectra of (a) FABEs,(b) Ep-FABEs.

Fig.9.13C NMR spectra of (a) FABEs,(b) Ep-FABEs.

The experimental results of the transesterification of FAMEs with isobutanol at 150,160,170,180,and 190 °C were compared with the fitting results using the kinetic model.As shown in Fig.6 (b),the calculated FAMEs conversion agreed well with the experimental results,with the error being within ±10%.Furthermore,experiments at 165 °C and 175 °C were carried out to validate the kinetic model.The results in Fig.6 (b) show that the kinetic model could predict the FAMEs conversion well,and the average relative deviation is about 7%.Therefore,the established kinetic model can be used to describe and predict the transesterification of FAMEs and isobutanol catalyzed by tetrabutyl titanate.

3.3.Characterization of FABEs and Ep-FABEs

Fig.10.DMA curves of different plasticized PVC films.(S1:100%(mass)DOP,S2:70%(mass) DOP and 30 %(mass) Ep-FABEs,S3:40 %(mass) DOP and 60 %(mass) Ep-FABEs,S4:100 %(mass) Ep-FABEs,S5:40 %(mass) DOP and 60 %(mass) Ep-FAMEs,S6:100 %(mass) Ep-FAMEs).

The Ep-FABEs with an epoxy value of 4.8%was synthesized.The transesterification and epoxidation products were characterized by FT-IR and NMR techniques.The FT-IR spectra of FABEs and Ep-FABEs are shown in Fig.7.Absorption peaks at 1739.5 cm-1and 1176.2 cm-1were attributed to the stretching vibrations of C=O and C-O-C,respectively.Moreover,there is an absorption peak at around 1243.9 cm-1corresponding to the C-O stretching asymemetric in the spectra of FABEs and Ep-FABEs.The results indicated the existence of ester group in FABEs and Ep-FABEs.The peak of C=C bending vibration at 3012.3 cm-1in the spectrum of FABEs was noticed,which disappeared in the spectrum of Ep-FABEs.Besides,a new peak at 826.8 cm-1corresponding to epoxy groups was detected in the Ep-FABEs spectrum.The results indicated that the disappearance of carbon-carbon double bonds and the formation of epoxy groups after the epoxidation reaction.

Fig.8 showed the1H NMR spectra of FABEs (a) and Ep-FABEs(b).Compared with FABEs,peaks at 5.284–5.394 range which referred to olefinic hydrogen(-CH=CH-)disappeared in the spectrum of Ep-FABEs.In addition,peaks at 2.82–3.13 range corresponding to epoxide products (-CH-O-CH-) were observed in the Ep-FABEs spectrum.Fig.9 showed the13C NMR spectra of FABEs(a)and Ep-FABEs(b).Peaks at 127.64–130.58 range referring to olefinic carbons were observed in the spectrum of FABEs,which completely disappeared in the Ep-FABEs spectrum.The presence of new peaks in the spectrum of Ep-FABEs at 53.65–56.78 range was attributed to epoxy carbons.The above results confirmed that nearly all carbon-carbon double bonds were completely converted to epoxy groups.

3.4.Application of Ep-FABEs in PVC

3.4.1.Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) results of DOP,Ep-FABEs,and Ep-FAMEs plasticized PVC films,including the storage modulus(E′)and the glass transition temperature(Tg),were summarized in Fig.10.TheTgvalue was an important parameter to evaluate the effect of plasticizer,which was related to the main peak of tanδ curve of PVC film.As shown in Fig.10 (b),all PVC films had only a single tanδ peak,meaning that all plasticizers had good compatibility with PVC resin.Fig.10(a)shows that the storage modulus of S4 was obviously lower than that of S1,indicating that Ep-FABEs made PVC material more flexible.The mixing of Ep-FABEs with DOP can give lower storage modulus compared with PVC plasticized with only DOP,leading to better processability of the plastic products.It can be noticed in Fig.10(b)that theTgvalues of S2,S3,and S4 were 31.49 °C,26.87 °C,and 35.08 °C,respectively,which were significantly lower than that of pure PVC without plasticizer which was 85 °C [35].Ep-FAMEs have higher oxirane content due to lower molecular weight,thus S6 had the lowestTgvalue.The branched-chain Ep-FABEs molecules could be wedged between PVC molecular chains,which increased the distance and weaken the interaction force among PVC molecules and hence theTgvalue was reduced.Overall,Ep-FABEs exhibited good compatibility with PVC and remarkable processability by effectively reducing theTgvalue and storage modulus.

Fig.11.TGA curves of different PVC films.(S1:100 %(mass) DOP,S2:70 %(mass)DOP and 30 %(mass) Ep-FABEs,S3:40 %(mass) DOP and 60 %(mass) Ep-FABEs,S4:100 %(mass) Ep-FABEs,S5:40 %(mass) DOP and 60 %(mass) Ep-FAMEs,S6:100 %(mass) Ep-FAMEs).

Fig.12.Degree of migration and extraction loss of various plasticized PVC films.(S1:100%(mass)DOP,S2:70%(mass)DOP and 30%(mass)Ep-FABEs,S3:40%(mass)DOP and 60 %(mass) Ep-FABEs,S4:100 %(mass) Ep-FABEs,S5:40 %(mass) DOP and 60 %(mass) Ep-FAMEs,S6:100 %(mass) Ep-FAMEs).

3.4.2.Mechanical properties

The mechanical tests were performed to determine the tensile strength,modulus of elasticity,and elongation at break.As shown in Table 4,it could be noticed that the amount of Ep-FABEs had a significant impact on the mechanical properties of PVC films.The elongation at break of PVC films increased,and modulus of elasticity decreased with the increase in Ep-FABEs addition.As the ratio of Ep-FABEs to DOP increased,the tensile strength was found to increase first and then slightly decrease.The results indicated that the mechanical properties of PVC films with Ep-FABEs plasticizer were significantly improved compared with those plasticized with DOP.This might be attributed to the presence of longer alkyl chains and epoxy groups in Ep-FABEs.Therefore,Ep-FABEs can be used as an efficient plasticizer for PVC materials,which enhance the flexibility and motion ability of PVC molecular chains.

3.4.3.Thermogravimetric analysis (TGA)

The thermal properties of different plasticized PVC films were studied by thermogravimetric analysis (TGA).As presented in Fig.11,all thermal degradation curves exhibited almost the same pattern which was a three–stage process.Hence,Ep-FABEs had similar thermal stability as DOP as a plasticizer.All PVC films were found to exhibit great thermal stability in nitrogen atmosphere below 200 °C.The degradation rate was the fastest in the temperature range of 230–440°C,which was mainly due to the release of HCl gas and chlorinated hydrocarbon compounds from the dechlorination of PVC.The polymer was cracked to form hydrocarbons with lower molecular weight,causing major mass loss in the temperature range of 440–480 °C.Tonset,T10,andT50listed in Table 4 represented the temperature at the beginning of step 2,the degradation temperature for 10% mass loss and 50% mass loss,respectively.As the ratio of Ep-FABEs to DOP increased,Tonsetrisen from 212.42 to 221.27 °C,T10risen from 227.89 to 238.58 °C andT50risen from 274.15 to 280.83 °C.It was mainly because the epoxy bonds in Ep-FABEs could scavenge hydrogen chloride generated during thermal degradation of PVC,which reduced the decomposition rate of PVC to some extent.Moreover,the thermal stability of Ep-FABEs plasticized PVC films was similar to that of Ep-FAMEs plasticized PVC films.Therefore,the addition of Ep-FABEs as a plasticizer can effectively enhance the thermal stability of PVC products.

3.4.4.Extraction resistance and migration stability

The extraction and migration resistance of plasticizer were affected by the compatibility with polymer,plasticizer polarity and structure,and contact media conditions.The extraction resistance of PVC films with different plasticizers (DOP/ Ep-FABEs/Ep-FAMEs) was tested in three solvents (distilled water,1% soap solution and kerosene).As shown in Fig.12 (b) and (c),when immersed in distilled water,all PVC films presented low plasticizer exudation and all plasticizers exhibited relatively poor extraction resistance in kerosene.The mass loss of Ep-FABEs in 1%soap solution was 0.41%,which was much lower than 0.90%for Ep-FAMEs.A similar trend was observed in kerosene.Ep-FABEs had higher molecular weight and higher number of polar bonds,which resulted in better compatibility with PVC resin.Another possible reason was that the short branched esters of Ep-FABEs contributed to additional polar interaction and physical interdigitation with PVC at the molecular level,resulting in enhanced extraction resistance.

Plasticizers can leach out of PVC films not only in solvents but also at elevated temperatures.The volatile mass losses of PVC films with different plasticizers after migration tests are shown in Fig.12(a).It can be noted that the mass loss of Ep-FABEs (0.20%) was slightly lower than that of Ep-FAMEs (0.22%),indicating that Ep-FABEs had better heat resistance and compatibility with PVC resin than Ep-FAMEs.By comparison with Ep-FAMEs,the molecular weight of Ep-FABEs was higher,which gave it more stability and reduced the migration loss of the plasticizer.Consequently,compared with Ep-FAMEs,Ep-FABEs had better extraction resistance and migration stability.

4.Conclusions

In this study,an eco-friendly and biodegradable plasticizer Ep-FABEs was synthesized using biodiesel as the feedstock through two steps of transesterification and epoxidation reactions.The transesterification of FAMEs with isobutanol was conducted in the self-design gas-liquid tower reactor.The highest FAMEs conversion could reach nearly 100% after 5 h reaction.The kinetic study of the transesterification reaction was performed,and the calculated activation energy was 48.93 kJ?mol-1.The obtained FABEs were then used as the raw materials to synthesize Ep-FABEs as a plasticizer to substitute DOP.Compared with DOP,the thermal stability and mechanical properties of PVC films were significantly improved with the addition of Ep-FABEs.Moreover,Ep-FABEs exhibited better resistance to extraction and migration than another biobased plasticizer Ep-FAMEs.In summary,Ep-FABEs derived from renewable sources have good potential as an environmentally friendly plasticizer in PVC industry.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support provided by the National High-tech Research and Development Program of China (2014AA022103) is greatly acknowledged.