Itaconic Acid as a Green Alternative to Acrylic Acid ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Itaconic Acid as a Green Alternative to Acrylic Acid for Producing a Soybean Oil-Based Thermoset: Synthesis and Properties Peng Li,†,‡ Songqi Ma,*,† Jinyue Dai,† Xiaoqing Liu,† Yanhua Jiang,† Sheng Wang,† Jingjing Wei,†,§ Jing Chen,† and Jin Zhu† †

Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201, People’s Republic of China ‡ Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan 030051, People’s Republic of China § Northwestern Polytechnical University, Xi’an, Shanxi 710072, People’s Republic of China ABSTRACT: In this paper, itaconic acid was monomethylated with methanol to produce monomethyl itaconate, and then monomethyl itaconated epoxidized soybean oil (IESO) was obtained by melt ring-opening esterification of monomethyl itaconate with ESO. For comparison, acrylated epoxidized soybean oil (AESO) was also synthesized from ESO and acrylic acid with the same synthetic method. The chemical structures of monomethyl itaconate, IESO, and AESO were characterized in detail by differential scanning calorimetry (DSC), FTIR, and 1H NMR. The isothermal thermogravimetric analysis at 30, 60, and 90 °C indicated that monomethyl itaconate exhibited extremely low volatility even at 90 °C, while acrylic acid volatilized very fast even at 30 °C, which suggested that monomethyl itaconate could be used as a green alternative to acrylic acid. Under UV radiation, IESO showed good copolymerization ability with reactive monomers such as glycidyl methacrylate modified itaconic acid (IG), trimethylolpropane triacrylate (TMPTMA), and styrene. The cured neat IESO exhibited similar properties to the neat AESO, and the IESO/reactive monomers systems showed higher glass transition temperature (Tg), modulus, and comparable or better tensile strength and coating properties than the AESO/reactive monomers systems. KEYWORDS: Biobased, VOC, Coatings, Volatility, Epoxidized soybean oil

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INTRODUCTION Due to the concern about the finite and rising price of fossil resources, climate change from CO2 emission, and other environmental problems, polymers from biorenewable raw materials with a wide variety of biomass resources with low price and enhanced environment benefits are receiving increasing attention.1−4 Soybean oil is the most readily available and inexpensive vegetable oil, with a global production of around 52 million tons in 2015,5 making it a very attractive alternative renewable raw material to fossil resources in polymeric materials.6−10 The carbon−carbon double bonds in soybean oil’s structure show fairly low reactivity for typical free radical polymerization, which limits the direct utilization of soybean oil in polymeric materials.11−13 Acrylated epoxidized soybean oil (AESO) owns easily polymerizable groups and has been widely reported as the biobased resin for coatings14−17 and composites.18−20 Although the polymerizable groups can be conveniently introduced through acrylation between epoxidized soybean oil (ESO) and acrylic acid,12 the reaction cannot proceed completely, and it is hard to remove all the unreacted acrylic © 2016 American Chemical Society

acid, especially during the industrial production of AESO; as a result, the commercially obtained AESO often contains an offensive odor from the small amount of acrylic acid residue. Besides the offensive odor, acrylic acid is severely irritating and corrosive to the skin and the respiratory tract. Eye contact can result in severe and irreversible injury, and high exposure can trigger pulmonary edema.21 Thus, the main objective of this work was to resolve this issue and to make soybean oil-based thermoset “greener”. Itaconic acid (IA), one of the top 12 value added chemicals from biomass promulgated by the U.S. Department of Energy,22 possesses two carboxyl groups and one carbon− carbon double bond. Besides the ability to produce polyester by polycondensation,23 it can also be polymerized by free radical polymerization just like acrylic acid.24 Despite the similar structure and reactivity to acrylic acid, itaconic acid has no VOC problem as acrylic acid on account of its solid state, which Received: November 2, 2016 Revised: November 28, 2016 Published: December 1, 2016 1228

DOI: 10.1021/acssuschemeng.6b02654 ACS Sustainable Chem. Eng. 2017, 5, 1228−1236

Research Article

ACS Sustainable Chemistry & Engineering ensures the feasibility of preparing a “greener” soybean oilbased thermoset from epoxidized soybean oil and itaconic acid instead of acrylic acid. More interestingly, itaconic acid has no rigid structures such as a benzene ring, a furan ring, and a rosin ring, while itaconic acid−based polymers showed high modulus, strength, and glass transition temperature (Tg),25−29 which is in favor of producing a soybean oil-based thermoset with improved properties by replacing acrylic acid with itaconic acid. Thus, in this paper, monomethyl itaconate was synthesized from itaconic acid to be a green alternative to acrylic acid, followed by reacting with epoxidized soybean oil, and monomethyl itaconated epoxidized soybean oil (IESO) was obtained. Volatility of monomethyl itaconate and acrylic acid was investigated to evaluate the impact of monomethyl itaconate and acrylic acid residues in resins on the environment or the human body. Thermal, mechanical, and coating properties of the UV-cured IESO and AESO were investigated. Owing to the flexible long aliphatic chains of soybean oil, petroleum-based rigid compounds such as styrene and divinylbenzene,30−32 bisphenol A type vinyl ester,33 triallyl isocyanurate (TAIC),34 and sol−gel silica,35 as well as biobased rigid compounds such as acrylated sucrose monomers and tetrahydrofurfural acrylate,16 2,5-furan diacrylate,36 tannic acidbased hyperbranched methacrylates,37 myrcene,38 rosin,39 and gallic acid17 derived monomers, etc. were often applied together with AESO to achieve adequate rigidity and strength for applications. Hence, in order to investigate IESO’s copolymerization properties, several reactive monomers such as glycidyl methacrylate modified itaconic acid (IG), trimethylolpropane triacrylate (TMPTMA), and styrene were employed together with IESO to prepare UV-cured materials. For comparison, AESO was also synthesized from epoxidized soybean oil and acrylic acid by the same synthetic method, and the properties were investigated as well.

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Scheme 1. Synthetic Route of Monomethyl Itaconate

then IESO was obtained after removing the solvents with rotary evaporator. The synthetic route is shown in Scheme 2.

Scheme 2. Synthetic Routes of Monomethyl Itaconated Epoxidized Soybean Oil (IESO) and Acrylated Epoxidized Soybean Oil (AESO)

Preparation of Acrylated Epoxidized Soybean Oil. 6.57 g (0.09 mol) of acrylic acid, 24.00 g of epoxidized soybean oil (ESO) (0.09 mol of epoxy group), 0.31 g (1 wt %) of triphenylphosphine as the catalyst, and 0.06 g (0.2 wt %) of hydroquinone as the inhibitor were placed in a 100 mL three necked round-bottomed flask with a magnetic stirrer, a thermometer, and a reflux condenser, heated to 80 °C, stirred at 80 °C for 0.5 h, then heated to 120 °C, and kept at 120 °C for 2 h. As IESO, AESO was also purified by being dissolved in dichloromethane and being washed with 1 wt % NaHCO3 aqueous solution and distilled water followed by removing the solvents with rotary evaporator. The synthetic route is shown in Scheme 2. Preparation of Cross-Linked Networks. The compositions of the cross-linked networks are listed in Table 1. For all the samples, IESO or AESO with or without monomers such as IG, TMPTMA, and styrene were mixed together with photoinitiators (5% total weight of resins and monomers, the weight ratio of TPO to HM is 2/3.) After homogeneous mixtures were obtained, they were coated on cleaned smooth finish steel panels using a drawdown bar with gaps of 50 μm to get the coatings with thickness of around 50 μm and poured into stainless steel molds to obtain samples with dimensions of 80 mm × 8 mm × 0.6 mm. Then, they were cured at room temperature for 5 and 30 min under air, respectively, using a high-pressure mercury lamp (1000 W) at 366 nm with a distance of 15 cm from the lamp to the surface of samples. Characterization. 1H NMR was examined by a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with acetone-d6 as solvent. The infrared spectrum (FTIR) was measured with a NICOLET 6700 FTIR (NICOLET, America). The gel content of the cured samples was measured using acetone extraction. The cured samples weighing around 0.5 g were precisely weighed (m1), extracted with acetone for 24 h under reflux using a Soxhlet extractor, and finally dried at 60 °C under vacuum for 24 h and weighed (m2). The gel content was calculated as 100% × m2/m1. Differential scanning calorimetry (DSC) measurements were performed on a

EXPERIMENTAL SECTION

Materials. Itaconic acid, methanol, benzoyl chloride, toluene, petroleum ether, and styrene were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Epoxidized soybean oil, triphenylphosphine, hydroquinone, trimethylolpropane triacrylate (TMPTMA), and glycidyl methacrylate were obtained from Aladdin Reagent, China. 2-Hydroxy-2-methylpropiophenone (HM) and 2,4,6trimethylbenzoyldiphenyl phosphine oxide (TPO) were supplied by Nanjin Jiazhong Chemical Co., Ltd., China. Glycidyl methacrylate modified itaconic acid (IG) was prepared as previously reported.40 Preparation of Monomethyl Itaconate. 39.00 g (0.3 mol) of itaconic acid, 42.6 mL (0.9 mol) of methanol, and 3 mL of benzoyl chloride were placed in a 100 mL three necked round-bottomed flask with a magnetic stirrer, a thermometer, and a reflux condenser, heated to 65 °C, and kept at 65 °C for 0.5 h. The unreacted methanol was removed by a rotary evaporator, and the product was obtained after recrystallization with toluene/petroleum ether component solvent with a volume ratio of 1:1. The yield of the product is 86%. The synthetic route is shown in Scheme 1. Preparation of Monomethyl Itaconated Epoxidized Soybean Oil (IESO). 20.46 g (0.14 mol) of monomethyl itaconate, 37.18 g of epoxidized soybean oil (0.14 mol of epoxy group), 0.58 g (1 wt %) of triphenylphosphine as the catalyst, and 0.12 g (0.2 wt %) of hydroquinone as the inhibitor were placed in a 250 mL three necked round-bottomed flask with a magnetic stirrer, a thermometer, and a reflux condenser, heated to 80 °C, stirred at 80 °C for 30 min, then heated to 120 °C, and kept at 120 °C for 2 h. In order to remove the impact of unreacted monomethyl itaconate on the properties of the cured samples and simplify the systems in this work, purification was applied for this product. It was dissolved in dichloromethane and washed with 1 wt % NaHCO3 aqueous solution and distilled water; 1229


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ACS Sustainable Chemistry & Engineering Table 1. Feed Compositions for the Cured Samples

a

sample

IESOa (g)

neat IESO IEIG20 IEIG40 IETA20 IETA40 IEST20 IEST40 neat AESO AEIG20 AEIG40 AETA20 AETA40 AEST20 AEST40

3 2.4 1.8 2.4 1.8 2.4 1.8

AESOb (g)

IGc (g)

TMPTMAd (g)

styrene (g)

0.6 1.2 0.6 1.2 0.6 1.2 3 2.4 1.8 2.4 1.8 2.4 1.8

0.6 1.2 0.6 1.2 0.6 1.2

TPOe (g)

HMf (g)

0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06

0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09

Monomethyl itaconated epoxidized soybean oil. bAcrylated epoxidized soybean oil. cGlycidyl methacrylate modified itaconic acid. Trimethylolpropane triacrylate. e2,4,6-Trimethylbenzoyldiphenyl phosphine oxide. f2-Hydroxy-2-methylpropiophenone.

d

Mettler-Toledo MET DSC apparatus (METTLER TOLEDO, Switzerland) under a nitrogen atmosphere. Monomethyl itaconate weighting 5−7 mg was heated to 110 °C at a heating rate of 10 °C min−1 and held there for 5 min to eliminate thermal history. Then it was cooled to 0 °C at a cooling rate of 10 °C min−1 and held there for 5 min followed by heating again to 110 °C at a heating rate of 10 °C min−1. The melting point of monomethyl itaconate was obtained from the peak temperature of the second heating curve. Thermogravimetric analysis (TGA) was performed using a Mettler-Toledo TGA/DSC1 Thermogravimetric Analyzer (METTLER TOLEDO, Switzerland) with high purity nitrogen as purge gas at a flow rate of 50 mL min−1. Monomethyl itaconate and acrylic acid weighing 5−7 mg were put into the aluminum crucible without covers and kept at 30, 60, and 90 °C for 3 h, respectively, to evaluate their volatility. Tensile properties were evaluated by an Instron 5567 Electric Universal Testing Machine (Instron, America) with gauge length of 50 mm at a cross-head speed of 5 mm min−1. The specimens of 80 mm × 8 mm × 0.6 mm were used for this evaluation. The data was taken from an average of at least five specimens for accuracy. Dynamic mechanical analysis (DMA) was carried out on a TA Instruments Q800 DMA in tension mode to measure the dynamic mechanical properties of the cured samples. Rectangular samples with dimensions of 12 mm (length) × 5 mm (width) × 0.6 mm (thickness) were prepared and tested from −10 to 150 °C at a heating rate of 5 °C min−1 and a frequency of 1 Hz. Coating properties examination: Pencil hardness and flexibility were measured according to ASTM D3363 and ASTM D 4145, respectively. The adhesion of the coatings was evaluated using the ASTM D 3359 crosshatch adhesion method. Methyl ethyl ketone (MEK) double rub test was carried out following ASTM D 5402.

Figure 1. 1H NMR spectrum of monomethyl itaconate.

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RESULTS AND DISCUSSION Synthesis and Characterization of Monomethyl Itaconate, IESO, and AESO. The chemical structure of monomethyl itaconate was characterizaed by 1H NMR and melting point measurement. The melting point of monomethyl itaconate obtained by DSC measurement is 68.4 °C. The 1H NMR spectrum of monomethyl itaconate in Figure 1 presents the characteristic peaks of protons H1, H2 on the carbon− carbon double bond at 6.30 and 5.82 ppm, peak of H3, H4 on the −CH2− next to the carbon−carbon double bond at 3.36 ppm, and peak of H5, H6, H7 on the −C(O)O−CH3 at 3.64 ppm. Figure 2 shows the FTIR spectra of IESO, AESO, and their raw material ESO. As can be seen, peaks for the −OH at around 3300−3500 cm−1 and peaks for the carbon−carbon double bonds at around 1630 cm−1 appeared, and the peak for

Figure 2. FTIR spectra of IESO, AESO, and ESO.

the epoxy group at about 827 cm−1 disappeared for IESO and AESO, which demonstrates the occurrence of esterification between ESO and monomethyl itaconate and acrylic acid. The FTIR spectra of IESO and AESO both have strong peaks at around 2930 cm−1, 1860 cm−1 belonging to the characteristic peaks for CH3, CH2 and a stronger peak at around 1740 cm−1 belonging to the characteristic peaks for CO relative to the spectrum of ESO. Figure 3 shows the 1H NMR spectra of IESO and AESO. In the 1H NMR spectrum of IESO (Figure 3a), the peaks at 6.31 and 5.81 ppm belong to the protons H1, H2 on 1230


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ACS Sustainable Chemistry & Engineering

Figure 3. 1H NMR spectra of (a) IESO and (b) AESO.

the carbon−carbon double bond from the monomethyl itaconate structure, and peaks at 3.66 and 3.40 ppm come from the protons H8, H9, H10, H11, and H12 on the monomethyl itaconate structure. The peaks at 4.91, 4.32, 4.19, 2.33, and 0.89 ppm are from the protons H3, H4, H5, H6, H7, H13, H14, H15, H16, and H17 of the soybean oil structure. From the integral area of the peak belonging to proton H1 on the carbon−carbon double bond at 6.31 ppm (Idouble bond) and the integral area of the peak at 0.89 ppm corresponding to the protons H15, H16, and H17 on terminal −CH3 of soybean oil (I−CH3), the grafting ratio of the carbon−carbon double bond GCC was calculated by the equation GCC = 9 × Idouble bond/ I−CH3, and the value of GCC for IESO is 2.39. From the 1H NMR spectrum in Figure 3b, the GCC of AESO could also be calculated with the same equation, and the value is 2.41 which is similar to the GCC of IESO. This demonstrates that monomethyl itaconate had similar reactivity toward ESO, and the obtained AESO is very suitable to be used as the control for IESO. Volatility of Monomethyl Itaconate and Acrylic Acid. The volatility of monomethyl itaconate (MMI) and acrylic acid (AA) was evaluated by isothermal TGA at 30, 60, and 90 °C, and the curves are shown in Figure 4. Obviously, the volatility of the MMI is much lower than AA. For AA, even at room temperature it volatilized very fast, after 120 min virtually all the AA volatilized, and it volatilized completely within 20 min at 60 °C and within 5 min at 90 °C. While for MMI, the volatilization is extremely slow, at 30 °C for 150 min, no weight loss was

Figure 4. Isothermal TGA curves of monomethyl itaconate (MMI) and acrylic acid (AA) at 30, 60, and 90 °C.

detected. A rise in temperature accelerated the volatilization rate, but even at 90 °C for 150 min, there is only 26.7 wt % of MMI volatilized. This can be explained by the state of MMI and AA. MMI is solid at room temperature, so the volatilization could not be detected during the testing time. At 60 °C, the temperature was still below the melting point of MMI, so it still volatilized very slowly. While for AA, it is a volatile liquid, the saturated vapor pressure is much higher than MMI, and, when put in air or in other flowing gases, it would volatilize fast. This manifests that AA is a volatile organic compound (VOC), while 1231


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ACS Sustainable Chemistry & Engineering MMI is not. Using MMI as the alternative to AA, the VOC emission issue of AA containing polymers or products will be resolved. Gel Content of the Cured Samples. Gel content is a crucial indicator of the formation of a well-cross-linked system and has a direct relationship with the properties of the thermoset. Thus, the gel content of the cured samples was examined and shown in Figure 5. Obviously, all the systems

Table 2. DMA Data of the Cured Samples sample neat IESO neat AESO IEIG20 AEIG20 IEIG40 AEIG40 IETA20 AETA20 IETA40 AETA40 IEST20 AEST20 IEST40 AEST40

Tg (°C)

modulus at 25 °C (MPa)

modulus at Tg + 60 °C (MPa)

cross-link density (mol m−3)

47.8 47.8

548 575

19 53

2029 5612

60.0a 63.0b 93.5 88.1 80.8 82.8 98.0 94.1 88.3 65.7 91.5 76.2

1280 1002 1787 1752 1603 1299 2417 2160 1627 1037 2195 1760

103 136 207 241 193 173 553 496 27 40 20 21

10504 13714 19492 22921 18656 16628 51402 46537 2561 4065 1855 2023

a Tg values of IEIG20 were determined by the average values of two peak temperatures of the tan delta curves. bTg values of AEIG20 were determined by the average values of two peak temperatures of the tan delta curves.

Figure 5. Gel content of the cured samples.

where E′ is the storage modulus of the thermoset in the rubbery plateau region, R is the gas constant, and T is the absolute temperature. As can be seen in Table 2, the neat IESO and AESO showed similar Tg and modulus at 25 °C, while the neat IESO’s crosslink density is much lower than the neat AESO’s cross-link density. The lower cross-link density of the neat IESO than that of the neat AESO is attributed to the relatively lower reactivity of carbon−carbon double bonds in IESO than those in AESO,26 which is in accordance with the results from the gel content investigation. The Tg and the modulus of the crosslinked polymers have a close tie with their cross-link density and the rigidity of the chain segment structure.43−46 The higher the cross-link density of the thermosets and rigidity of their chain segment are, the higher Tg and modulus at glass state are. The neat IESO owns lower cross-link density but similar Tg and modulus compared with the neat AESO, which suggests that IESO is more rigid than AESO. All the monomers IG, TMPTMA, and styrene enhanced the Tg and modulus at 25 °C of both IESO-based samples and AESO-based samples. Both IG, TMPTMA modified IESO and AESO systems exhibited higher cross-link density than the neat IESO and AESO, leading to the higher Tg and modulus of IG, TMPTMA modified IESO and AESO systems relative to those of the neat IESO and AESO. Styrene has only one reactive group, resulting in the decreased cross-link density of styrene modified IESO and AESO systems except IEST20. So the higher Tg and modulus of styrene modified IESO and AESO systems than the neat IESO and AESO is attributed to the increased rigidity of the chain segment after incorporation of rigid styrene. IEST20 showed relatively higher cross-link density than the neat IESO, which can be explained by the fact that the high reactivity of styrene made more IESO reacted and cross-linked during the curing process. The increased cross-link density of IEST20 by more IESO reacted relative to the neat IESO during the curing process exceeded the decreased cross-link density of IEST20 by incorporating styrene into the cross-linked network, corresponding to the higher cross-link density than the neat IESO. The decreased cross-link density by incorporating styrene into

exhibited high gel content above 95%, which indicates that the systems were well cured under UV radiation. Although IESO owns a similar amount of reactive groups relative to AESO, IESO systems showed slightly lower gel content (95.1%− 97.8%) than AESO systems (95.3%−98.4%). This might be due to the fact that IESO owns the carbon−carbon double bond with relatively lower reactivity than AESO;26 as a result, a relatively higher amount of unreacted IESO remained in the IESO networks than that of unreacted AESO that remained in the AESO networks. Meanwhile, for both IESO systems and AESO systems, the gel content rose with the introduction of reactive monomers (IG and TMPTMA), which is ascribed to the increased cross-link density of the systems after copolymerizing with these monomers17 (Table 2), while the styrene modified IESO and AESO systems exhibited similar gel content to the neat IESO and AESO. Styrene is a monofunctional monomer; during the free radical polymerization, the chain propagation proceeded linearly. As a result, introducing styrene would increase the length of chain segment and lower the cross-link density of the systems (Table 2), which would decrease the gel content, while the high reactivity of styrene would make more resins be cross-linked in the networks, which would raise the gel content. These two competition factors led to the similar gel content of styrene modified systems to the neat systems. Thermal and Mechanical Properties of the Cured Samples. DMA was used to determine the glass transition temperature (Tg), modulus, and cross-link density of the cured samples. The curves of storage modulus and tan delta of the cured samples as a function of temperature are shown in Figure 6, and the data are summarized in Table 2. The peaks of the tan delta curves were used to determine the Tg of the cured samples. The modulus values at Tg + 60 °C were utilized to calculate the cross-link density of the cured samples according to the following equation41,42 ve =

E′ 3RT

(1) 1232


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Figure 6. DMA curves of the cured samples.

depends on the cross-link density of the thermosets; consequently, it is easy to see the lower modulus at Tg + 60 °C of the IESO systems than that of the AESO systems. Mechanical Properties of the Cured Samples. The mechanical properties of the cured samples were examined by tensile test, and the results are shown in Figure 7, Figure 8, and Figure 9. As can be seen from Figure 7 and Figure 8, for both IESO and AESO systems, the tensile strength and modulus were improved by incorporating reactive monomers IG,

the cross-linked network exceeded the increased cross-link density by more IESO and AESO reacted relative to the neat IESO and AESO during the curing process for IEST40, AEST20, and AEST40, corresponding to the lower cross-link density of IEST40, AEST20, and AEST40 than that of the neat IESO and AESO, respectively. Meanwhile, all the modified IESO systems showed higher Tg and modulus at glass state, which is attributed to the higher rigidity of IESO than that of AESO. The modulus at elastic state (at Tg + 60 °C) mainly 1233


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resulting in the higher modulus of modified systems than the neat systems. As shown in Figure 9, the neat IESO showed much higher elongation at break than the neat AESO, which might be from the lower cross-link density of the neat IESO than that of the neat AESO network. After copolymerization with IG and TMPTMA, the cross-link density and rigidity of the systems heightened and the molecular mobility reduced; as a result, the elongation at break declined for both IESO and AESO systems. Interestingly, the mechanical properties of the styrene modified systems are much different from those of the IG and TMPTMA modified systems. The styrene modified IESO systems showed much higher tensile strength and modulus than the styrene modified AESO systems and much higher elongation at break than IG and TMPTMA modified IESO systems, and the styrene modified AESO systems showed higher elongation at break than the neat AESO network. As shown in Table 2, styrene modified systems showed exceedingly low cross-link density; as a result, the polar groups between different chain segments are easier to connect by intermolecular force than those in IG and TMPTMA systems with high cross-link density. IESO has more polar groups (ester bonds) than AESO, corresponding to the much higher tensile strength and modulus of the styrene modified IESO systems than the styrene modified AESO systems. The much higher molecular mobility of styrene modified IESO systems from the much lower cross-link density than IG and TMPTMA modified IESO systems can explain the much higher elongation at break of styrene modified IESO systems than IG and TMPTMA modified IESO systems. After introducing styrene, the crosslink density of AESO systems reduced a great deal, and intermolecular force did not increase too much; as a result, molecular mobility of styrene modified AESO systems was higher than that of the neat AESO, corresponding to their higher elongation at break than the neat AESO. Coating Properties of the Cured Samples. The coating properties including adhesion, pencil hardness, flexibility, and solvent resistance of the cured samples were investigated, and the data are shown in Table 3. All the cured samples showed excellent solvent resistance with MEK double rubs of above 250, which also illustrates that they were well cross-linked, in agreement with the high gel content of the samples. Both the neat IESO and AESO based coatings showed outstanding

Figure 7. Tensile strength of the cured samples.

Figure 8. Modulus of the cured samples.

Table 3. Coating Properties of the Cured Samples sample neat IESO neat AESO IEIG20 AEIG20 IEIG40 AEIG40 IETA20 AETA20 IETA40 AETA40 IEST20 AEST20 IEST40 AEST40

Figure 9. Elongation at break of the cured samples.

TMPTMA, and styrene. The tensile strength of IESO systems (8.5 MPa-28.6 MPa) is close to even higher than that of AESO systems (8.1 MPa-30.0 MPa). The neat IESO showed lower modulus (87 MPa) than the neat AESO (130 MPa); after introducing the monomers, the modulus was enhanced for both IESO and AESO systems, and the IESO systems exhibited higher modulus than AESO systems. Among IG, TMPTMA, and styrene, TMPTMA showed the best enhancement for both IESO and AESO systems. The lower cross-link density of the neat IESO than that of the neat AESO led to the lower modulus of the neat IESO. Introducing reactive monomers, the cross-link density and rigidity of the systems were increased, 1234

thickness (μm)

adhesion

pencil hardness

flexibility

MEK double rub resistance

51 ± 6 48 ± 3

0B 0B

3B 2B

0T 0T

>250 >250

± ± ± ± ± ± ± ± ± ± ± ±

0B 0B 0B 0B 0B 0B 0B 0B 2B 0B 3B 0B

B B H H B B H H B HB 2H 2H

2T 5T 3T 5T 2T 3T 3T 4T 0T 0T 0T 0T

>250 >250 >250 >250 >250 >250 >250 >250 >250 >250 >250 >250

60 36 68 50 47 48 49 47 28 46 26 29

6 2 3 0 2 6 5 6 4 8 3 5



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flexibility of 0T but low pencil hardness of 3B and 2B, respectively. With incorporating monomers IG, TMPTMA, and styrene, the pencil hardness of both IESO-based coatings and AESO-based coatings were upgraded to 2H, and the styrene modified systems showed the highest pencil hardness of 2H. This is due to the increased cross-link density and rigidity of chain segment after incorporating monomers. Both IG and TMPTMA decreased the flexibility of the coatings, while styrene modified samples still presented excellent flexibility of 0T. Flexibility has a close tie with cross-link density.45,46 The increased cross-link density of IG and TMPTMA modified samples relative to the neat samples led to the decreased flexibility, and styrene modified samples exhibited exceedingly low cross-link density even lower than the neat IESO and AESO samples, corresponding to their high flexibility. IESO and AESO have long nonpolar aliphatic chains, and part of their polar groups such as ester bonds and hydroxyl groups might be buried in the matrix resins and had a low chance to connect with the polar groups on the surface of the substrate; as a result, almost all of the samples showed poor adhesion toward steel panels except for IEST20 and IEST40. Copolymerization with styrene reduced the cross-link density of the samples and made more polar groups such as ester bonds and hydroxyl groups have a chance to connect with the substrate. Although styrene modified AESO systems showed low cross-link density, they still showed poor adhesion of 0B for the limited polar groups in AESO. IESO has more polar groups than AESO, corresponding to the better adhesion property. These results imply that IESObased coatings possessed comparable even higher properties than AESO-based coatings.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-574-86685186. E-mail: [email protected], [email protected]. ORCID

Songqi Ma: 0000-0002-9652-1016 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from Project 51473180 supported by the National Natural Science Foundation of China, Research Project of Technology Application for Public Welfare of Zhejiang Province (No. 2014C31143), Natural Science Foundation of Zhejiang Province (LY15E030004), and Ningbo Natural Science Foundation (No. 2016A610254).

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REFERENCES

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CONCLUSIONS

Monomethyl itaconate was synthesized successfully from itaconic acid and methanol to react with epoxidized soybean oil (ESO), and the obtained IESO showed a similar amount of functional groups (around 2.39 per IESO molecule) to AESO (about 2.41 per AESO molecule), which indicates that monomethyl itaconate and acrylic acid had similar reactivity toward ESO. Owing to the solid state with a melting point of 68.4 °C, monomethyl itaconate exhibited extremely low volatility even at 90 °C, which means that the offensive odor problem of AESO can be resolved by replacing acrylic acid with monomethyl itaconate during the preparation. The IESO systems are supposed to show higher modulus, strength, Tg, and coating properties than AESO systems, on account of the more polar groups in IESO than those in AESO. Nevertheless, because of the relatively lower reactivity of the carbon−carbon double bond from the itaconic acid structure than that from the acrylic acid structure, the neat IESO was not cured as well as the neat AESO under UV radiation, leading to the similar other than higher properties of the neat IESO relative to those of the neat AESO. The reactive monomers made IESO be crosslinked more completely during the curing process; as a result, IESO/reactive monomers systems exhibited improved properties exceeding AESO/reactive monomers systems. The success of synthesizing IESO affords the opportunity of using monomethyl itaconate as a green alternative to acrylic acid to react with other epoxy resins and produce substitutes for epoxy acrylates. 1235


Research Article

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Itaconic Acid as a Green Alternative to Acrylic Acid ... - ACS Publications

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