High-performance Soybean Oil-based Epoxy Acrylate Resins: âGreen

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High-performance Soybean Oil-based Epoxy Acrylate Resins: “Green” Synthesis and Application in UV-Curable Coatings Qiong Wu, Yun Hu, Jijun Tang, Jing Zhang, Cuina Wang, Qianqian Shang, Guo dong Feng, Chengguo Liu, Yonghong Zhou, and Wen Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00388 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Sustainable Chemistry & Engineering

High-performance Soybean Oil-based Epoxy Acrylate Resins: “Green” Synthesis and Application in UV-Curable Coatings Qiong Wu2, Yun Hu1, Jijun Tang3, Jing Zhang3, Cuina Wang2, Qianqian Shang1, Guodong Feng1, Chengguo Liu1, *, Yonghong Zhou1, Wen Lei2

1

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry;

National Engineering Lab for Biomass Chemical Utilization; Key Lab on Forest Chemical Engineering, State Forestry Administration; Key Lab of Biomass Energy and Material, Jiangsu Province; 16 Suojin Wucun, Nanjing 210042, P. R. China 2

College of Science, Nanjing Forestry University, 159 Longpan Road, Nanjing

210037, P. R. China 3

Department of Corrosion Prevention and Polymer Materials, College of Materials

Science and Engineering, Jiangsu University of Science and Technology, 2 Huancheng Road, Zhenjiang 212003, P. R. China

Corresponding author: *Chengguo Liu. E-mail: [email protected]. Tel.: +86-25-85482520; Fax: +86-25-85482520.

1

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ABSTRACT Novel soybean oil (SBO)-based epoxy acrylate (EA) resins were developed via ring-opening reaction of epoxidized soybean oil (ESO) with hydroxyethyl methacrylated maleate (HEMAMA) precursor, a synthesized unsaturated carboxylic acid having two active C=C groups and a side methyl group. Experimental conditions for the synthesis of the precursor and the SBO-based EA (ESO-HEMAMA) product were studied and their chemical structures were confirmed by FT-IR, 1H NMR,

13

C

NMR, and gel permeation chromatography. Subsequently, the volatility of HEMAMA was studied and compared with acrylic acid (AA). Furthermore, gel contents and ultimate properties of the UV-cured ESO-HEMAMA resins were investigated and compared with a commercial acrylated ESO (AESO) resin. At last, UV-curing behaviors of the SBO-based EA resins were determined by real-time IR. It was found that the HEMAMA precursor showed much lower volatility than AA, and the optimal pure ESO-HEMAMA resin possessed a C=C functionality of 6.02 per ESO and biobased content of 58%. Meanwhile, the obtained ESO-HEMAMA biomaterials exhibited much superior properties than the AESO resin. For instance, the obtained pure ESO-HEMAMA material possessed a storage modulus at 25 °C of 1.00 GPa, glass transition temperature (Tg) of 70.1 °C, and tensile strength and modulus of 13.4 MPa and 592.1 MPa, which were 9.4, 3.6, 6.9, and 15.7 times the values of the pure AESO material, respectively. The resulting biomaterial with 30% of hydroxyethyl methacrylate diluent even reached a tensile strength of 28.4 MPa and Tg of 89.0 °C. Therefore, the developed SBO-based EA resins are very promising to be applied in 2


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UV-curable coatings.

KEYWORDS: Soybean oil, epoxy acrylate, UV-curable coatings, “green” synthesis

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INTRODUCTION UV-curing coatings are widely been employed in modern industry thanks to their advantages such as low energy consumption, fast curing rate, low capital investment, and low volatile organic chemical (VOC) emissions.1-2 However, due to the dramatic price fluctuations of fossil resources and the stricter environmental regulations, researchers is increasingly being directed to develop the coatings from renewable resources such as plant oils, carbonhydrates, and cardanol.3-13 The utilization of biorenewable resources in UV-curable coatings could offer a “green + green” solution to the present coating industry. In all the biobased feedstocks, plant oils are the most frequently-used one owing to their abundance, low toxicity, biodegradability, and triglyceride structures suitable for the preparation of major constituents of UV-curable coatings.3,

9-10, 14-20

For

example, soybean oil (SBO), with a global production of about 38 million tons in 2009/2010,21 are mainly used for food applications, but it can also be employed in industrial applications if abundant. Approximate 80% of fatty acids (FAs) in SBO triglycerides are unsaturated FAs such as oleic acids, linoleic acids, and linolenic acids. Thus it can be readily formulated to epoxidized soybean oil (ESO), which is used directly in cationic UV-curable systems,22-23 or applied in free-radical type UV-curable systems by further modification.24-25 Plant oil-based epoxy acrylate (EA) is an important type of free-radical UV-curable prepolymers, which has the merits of facile synthesis, low energy consumption, less pollution, etc. It is usually prepared through ring-opening reaction of epoxidized plant oils with acrylic acid (AA). 4


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Acrylated ESO (AESO), perhaps the most common oil-based EA, has been utilized in UV-curing systems since 1970s.26 Nevertheless, several disadvantages were disclosed during the long study of AESO. First, ultimate properties such as stiffness and glass transition temperature (Tg) of the cured AESO polymer are far inferior to those of petroleum-based EA polymers. For example, due to the lack of rigid structure in AESO, the pure AESO resin only showed a tensile modulus of around 3 MPa and Tg of about 20 oC,7 while the bisphenol A-type epoxy acrylate resins could reach a tensile modulus of 300–500 MPa and Tg of about 75oC.24, 27 Second, the use of volatile AA may cause severe and irreversible injuries of human’s biotic tissues such as skin, eyes, and respiratory tract. Hence, developing new AESO-like prepolymers that doesn’t have such limitations is essential for the current and future coating application. It has been widely reported that the stiffness and Tg of oil-based unsaturated polyester-like thermosets can be apparently enhanced by the employment of unsaturated ester (UE) macromonomers or oligomers with high C=C functionality.28-34 The reason for this improvement lies in that highly functional UEs can improve crosslink densities of the resultant polymer matrices. Meanwhile, steric structure can also provide a positive effect on the stiffness and thermal properties of EA resins. For example, by replacing AA with methacrylic acid or monomethyl itaconate, stiffness and Tg of the obtained biobased EA materials were improved accordingly.35-37 Nevertheless, up to now, there is barely any report on developing new biobased EA resins in consideration of both the two aspects. In this paper, a new SBO-based EA was developed and utilized in UV-curable 5



coatings in an effort to overcome the above obstacles of AESO. The EA-like prepolymer was synthesized in two basic steps (Scheme 1): First, hydroxyethyl methacrylate (HEMA) was used to react with maleic anhydride (MA) and an unsaturated carboxylic acid containing two active C=C groups and a side methyl group, hydroxyethyl methacrylated maleate (HEMAMA), was produced; second, HEMAMA was utilized to modify ESO, and the ESO-HEMAMA was obtained. It should be noted that the HEMAMA precursor can bring more C=C and side methyl groups into the biobased EA product, thus may leading to superior performance for the resultant biomaterials. Although some unauthorized patents38-40 have mentioned the synthesis of HEMAMA and petroleum-based epoxy acrylates with HEMAMA, the detailed information on the synthesis and structural characterization of them were still unknown. Therefore, experimental conditions for the synthesis of the precursor and prepolymer were investigated and their structures were characterized by FT-IR, 1

H NMR,

13

C NMR, and gel permeation chromatography (GPC). Volatility of

HEMAMA was further evaluated and compared with AA. Moreover, gel contents and ultimate properties of the UV-cured biomaterials with different types of diluent were studied in order to establish the structure–property relationships for the new biobased EA resins. As indicated in Scheme 2, four diluents were employed, which can be divided into two groups. The first group involves hydroxyethyl acrylate (HEA), triethylene glycol diacrylate (TEGDA), and trimethylolpropane triacrylate (TMPTA), which possess a C=C functionality of 1 to 3, respectively; the second group contains HEA and HEMA, which are both monofunctional but with or without steric hindrance. 6


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For comparison, the properties of a commercial AESO product were investigated as well. At last, UV-curing behaviors of the SBO-based EA resins were studied using real-time IR (RT-IR).

Scheme 1 Synthesis route for (a) HEMAMA and (b) ESO-HEMAMA.

Scheme 2 Chemical structures of the used diluents.

EXPERIMENTAL SECTION Materials. ESO was supplied by Shanghai Aladdin Chemistry Co., Ltd. (China). The

7



ESO was functionalized with about 3.99 epoxy groups per triglyceride and had a molar mass of about 950 g/mol. AESO was obtained from Shouguang Lukehuagong Co., Ltd. (China), which had a viscosity of 37500 mPa·s. The HEA (≥97%) and HEMA (≥97%) were obtained from Macklin Chemical Reagent Co., Ltd. (China). MA (≥99.5%) was purchased from Nanjing Chemical Reagent Co., Ltd. (China). N,N-Dimethyl benzyl amine (≥98%) was supplied by Tianjin Chemical Reagent Institute Co., Ltd. (China). Hydroquinone (≥99%) was supplied by Guoyaojituan Chemical Reagent Co., Ltd. (China). The TEGDA (98%), TMPTA (99%), and Darocur 1173 (98%) were obtained from Sahn Chemical Technology Co., Ltd. (China). The HEA, HEMA, TEGDA, TMPTA, and N,N-dimethyl benzyl amine were all dried by molecular sieves for at least one week before use.

Synthesis of HEMAMA. 52.1 g (0.4 mol) of HEMA, 39.2 g (0.4 mol) of MA, and 0.18 g of hydroquinone were added into a 250 mL four-neck round-bottom flask equipped with a mechanical stirrer, refluxing condenser, thermometer, and an inlet for nitrogen (N2) gas. Under the N2 protection, the reaction mixture was heated to 70 °C by an oil bath and agitated at this temperature until MA solid completely melted. After that the mixture was heated to 90 °C and reacted at this temperature for 5 h. Finally, the HEMAMA product was obtained, which was a light yellow, transparent liquid at room temperature. 1

H NMR (CDCl3, δ ppm): 11.12 (s), 7.22 – 6.96 (m), 6.91 – 6.22 (m), 6.13 (t),

5.76 (m), 4.62 – 4.08 (m), 3.96 – 3.63 (ddd), 1.92 (d). 8


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13

C NMR (CDCl3, δ ppm): 164.34 – 167.22 (m), 135.64 (m), 130.09 (m), 126.39

(d), 77.94 – 76.56 (m), 62.05 (m), 18.06 (s).

Synthesis of ESO-HEMAMA. The synthesis of ESO-HEMAMA was conducted under different feed ratios, reaction temperatures, and reaction times, as shown in Table 1. Typically, 50.0 g (0.0526 mol) of ESO, 43.5 g (0.191 mol) of HEMAMA, 0.94 g of N,N-dimethyl benzyl amine, and 0.094 g of hydroquinone were added into a 250 mL four-neck round-bottom flask equipped with a mechanical stirrer, refluxing condenser, thermometer, and an inlet for N2. Under the N2 atmosphere, the mixture was gradually heated to 100 °C and agitated at this temperature for 5 h. In the work-up procedures, the obtained ESO-HEMAMA product was washed three times with a hot 10wt%NaCl/water solution in a separating funnel, dissolved in dichloromethane, dried by Na2SO4, and evaporated via rotary evaporation. The obtained ESO-HEMAMA product (with a yield of 92.4%) was a light red, viscous liquid resin at room temperature. 1

H NMR (CDCl3, δ ppm) 6.97 – 6.78 (m), 6.27 (s), 6.12 (s), 5.58 (s), 5.30 (s),

5.27 – 5.18 (m), 5.03 – 4.84 (m), 4.52– 3.89 (dd), 3.88 – 3.52 (m), 2.30 (s), 1.94 (s), 1.79 – 1.14 (dd), 0.88 (s). 13

C NMR (CDCl3, δ ppm) 173.56 – 170.77 (m), 166.91 – 165.85 (m), 165.85 –

162.90 (m), 136.55 – 133.76 (m), 131.30 – 126.61 (m), 126.61 – 123.49 (m), 77.94 – 75.49 (m), 73.59 – 72.53 (m), 72.02 – 71.13 (m), 69.07 – 67.28 (m), 66.44 – 64.88 (m), 63.82 – 59.13 (m), 54.22 – 50.20 (m), 34.35 – 32.62 (m), 31.72 – 30.16 (m), 9



29.44 – 27.20 (m), 26.14 – 23.69 (m), 22.63 – 20.39 (m), 18.83 – 16.71 (m), 14.81 – 11.69 (m).

Curing of SBO-based EA Resins. The UV-curable samples were prepared by blending the obtained prepolymer, the diluent (30% of the weight of the prepolymer), and Darocur 1173 photoinitator (1.5% of the total weight of the prepolymer and dilute) at room temperature for 20 min. For the pure ESO-HEMAMA and AESO samples, no diluents were added. All the mixtures were degassed to remove bubbles, then poured into homemade polytetraﬂuoroethylene (PTFE) molds or coated on polished tinplate sheets with a coating apparatus. Finally, the prepared resin samples were cured using an Intelli-Ray 400W UV light-curing microprocessor (Uvitron International Corporation, USA) with an exposure intensity of 100 mw/cm2 at room temperature. The exposure time was 20 min for all the testing samples.

Characterization. Acid Value (Av). The Av values of samples were detected according to the procedures specified in GB/T 2895–1982, as described in our previous works.29-30 Infrared (IR) Spectrometry. FT-IR spectra of samples were recorded on a Nicolet iS10 IR spectrometer (Thermo-Fisher Corporation, USA) in a scanning range of 650–4000 cm-1 with a resolution of 4 cm-1. Nuclear Magnetic Resonance (NMR). 1H NMR and

13

10


C NMR spectra of samples

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were recorded on a DRX-300 Advance NMR spectrometer (Bruker Corporation, Germany) by using CDCl3 as solvent. Gel Permeation Chromatography (GPC). GPC measurements of samples were conducted on a HPLC system equipped with a 2414 refractive index (RI) detector (Waters Corporation, USA). Waters Styragel HR1 and HR2 (300 mm×7.8 mm) columns were employed for separation and the columns’ temperature was maintained at 35 °C. The flow rate of tetrahydrofuran (THF) eluent was 1.0 mL/min. The samples were dissolved in THF with a concentration of 15–25 mg/mL. The calibration curve, determined by a series of narrowly-distributed polystyrene (PS) standards with a molar mass of 580–19600, was employed to calculate the relative molar masses of samples. Gel Content (Cgel). The Cgel tests for the UV-cured samples were conducted via Soxhlet extraction. The cured samples with a mass of around 0.50 g were weighed precisely (denoted as m0), extracted with acetone for 24 h, dried in vacuum at 60oC for 24 h, and weighed again (denoted as m1). The Cgel values were determined as m1/m0. Dynamic Mechanical Analysis (DMA). The DMA tests were performed on a Q800 solids analyzer (TA Corporation, USA) in stretching mode with an oscillating frequency of 1 Hz. The cured samples with a size of 40×6×1 mm3 were examined at a heating rate of 3 °C/min. Thermogravimetric Analysis (TGA). The TGA tests was conducted on a STA 409PC thermogravimetry instrument (Netzsch Corporation, Germany) at a heating rate of 11



15 °C/min under N2 gas with a flow rate of 100 mL/min. HEMAMA and AA liquid samples with a weight of about 20 mg were placed into the aluminum crucible without covers and used to evaluate their volatility at 40oC and 110oC. Cured polymer samples with a weight of around 10 mg were heated in a range of 35–600oC. Mechanical Properties. Tensile properties of the UV-cured polymer materials were detected by a SANS7 CMT-4304 universal tester from Shenzhen Xinsansi Jiliang instrument corporation (China) with a gauge length of 50 mm at a cross-head speed of 5.0 mm/min. Specimens within a size of 80×10×1 mm3 were evaluated. Five specimens were tested for each sample to calculate the average values. Coating Properties. Adhesion tests were carried out using an adhesion test machine (Tianjin Shiboweiye Glass Instrument Corporation, China) based on the procedures specified in GB 1720–79(89). Typically, a prepared tinplate sheet with a film thickness of about 100 µm were fixed on the substrate of test machine, circled clockwise by a needle in a distance of 7–8 cm, and graded with a magnifying glass. The grade of adhesion ranged from 1 (best) to 7 (worst). Pencil hardness was tested by a QHQ-A pencil hardness tester (Tianjin Litengda Instrument Corporation, China) according to the procedures specified in GB/T 6739–2006. After placing the coated tinplate sheets horizontally, the pencil hardness tester was set with a pencil of known hardness and then pushed along the coating surface in a speed of about 1 mm/s. The sample’s grade was confirmed until the film was not scratched for at least two times. The hardness involved the classes of 6H, 5H, 4H, 3H, 2H, H, HB, B, 2B, 3B, 4B, 5B, and 6B (from hardest to softest). Coating’s flexibility was measured on a QTY-32 12


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paint film cylindrical bending machine (Tianjin Litengda Instrument Corporation, China) based on the procedures specified in GB/T 1731–93. When testing the tinplate sheets was placed tightly under the cylindrical shafts of the machine and then bended. The rate of flexibility was determined based on the diameter of the shaft used, including 2 mm, 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, etc. (the smaller the better). Swelling Properties. Swelling measurements were carried out in different solvents (water, ethanol, acetone, and toluene). Typically, the cured samples were dried in an oven at 50 oC for 24 h, and then immersed into solvents for 48 h at room temperature. Finally, the immersed samples were dried with filter papers and weighed. The swelling of solvent was calculated using the following equation

S=

W 1 −W 0 × 100% W1

(1)

where W0 and W1 are the weights of materials before and after being immersed into solvent, respectively. UV-curing Behavior. UV photo-curing kinetics of the liquid resins were conducted on a modified Nicolet 5700 spectrometer (Thermo–Nicolet Instrument Corporation, USA). The C=C conversion was measured by monitoring the intensity of C=C peak at about 810 cm-1.

RESULTS AND DISCUSSION Synthesis

and

Characterization

of

ESO-HEMAMA. 13


The

synthesis

of


ESO-HEMAMA prepolymer involved two steps, as shown in Scheme 1. In the first step, with a molar ratio (HEA/MA) of 1:1 and reaction temperature of 90 oC, HEMAMA precursor was synthesized. The reaction was carried out without any catalyst, and was monitored by the Av of reaction mixture. After a 5 h reaction, the Av reached a stable value of 242.5 mgKOH/g (Fig. S1), which was approaching to the theoretical value of 245.3 mgKOH/g. In the second step, the ESO-HEMAMA prepolymer was synthesized by monitoring the Av of reaction mixture as well. Table 1 provides the effects of molar ratio (epoxy/carboxyl), reaction temperature, and reaction time on conversion of HEMAMA (αHEMAMA). The αHEMAMA was determined with the expression of 1-(Av)t/(Av)0, where (Av)t and (Av)0 are the Av values at the instant time and initial time, respectively. The introduced C=C functionality per ESO (NC=C) for the ESO-HEMAMA products was estimated by 1H NMR, which would be discussed later. It can be seen that the conversion increased as the increase of feed ratio and reached a stable value at around 5 h. At the ratio of 1.4:1, the conversion achieved a maximum value of 81.8% at 3 h, but the reaction mixture soon gelled. The reason for the gelation would be discussed later. Therefore, in consideration of both the values of αHEMAMA and NC=C, the feed ratio of 1.1/1 was selected for further investigation. Next, the effect of reaction temperature on the αHEMAMA was studied, and the optimal temperature was 110 oC. Hence, the optimal conditions for the synthesis of ESO-HEMAMA included the feed molar ratio of 1.1:1, reaction temperature of 110 oC, and reaction time of 5 h, at which the NC=C could achieve a value of 6.02, which was much higher than that of the commercial AESO product 14


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with a value of 2.16 (as indicated in Fig. S2). Besides, the biobased content of the optimal ESO-HEMAMA product was about 58%. Table 1 Reactions of ESO and HEMAMA, Conversion of HEMAMA, and Introduced C=C Functionality of ESO-HEMAMA αHEMAMAb %

Entry

Molar ratioa (Epoxy/Carboxyl)

Temperature o C

1h

3h

5h

1 2 3 4 5 6

1.00/1.00 1.10/1.00 1.20/1.00 1.40/1.00 1.10/1.00 1.10/1.00

120 120 120 120 100 110

63.0 67.0 66.4 75.6 62.9 66.3

64.9 68.8 71.4 81.8 68.3 70.3

65.9 69.8 71.4 Gel. 69.4 70.5

a

NC=Cc 6.05 5.88 5.51 / 5.76 6.02

Molar ratio of epoxy group on ESO to carboxyl group on HEMAMA. b Conversion of

HEMAMA. c Introduced C=C functionality per ESO.

Fig.

1

demonstrates

the

FT-IR

spectra

of

HEMAMA,

ESO,

and

ESO-HEMAMA. In the spectrum of HEMAMA, several typical peaks were shown: carboxyl (2500–3400 cm-1), ester carbonyl (1713 cm-1), and C=C groups (1633 and 814 cm-1). In the spectrum of ESO, the characteristic peaks at 1738 cm-1 and 824 cm-1 were shown, which represented ester carbonyl and epoxy groups, respectively. However, in the spectrum of ESO-HEMAMA, the peaks of ester carbonyl and C=C groups shifted to 1720 and 1638 cm-1, respectively. The peak of epoxy group at 824 cm-1 almost disappeared. Moreover, a new but small peak occurred at 3521 cm-1, which was corresponding to the hydroxyl groups generated due to the opening reaction of epoxy groups. All these changes indicated that ESO was successfully grafted with HEMAMA.

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Fig. 1 FT-IR spectra of HEMAMA, ESO, and ESO-HEMAMA.

Fig. 2 shows the 1H NMR spectra of HEMAMA and ESO-HEMAMA. The 1H NMR spectrum of ESO is shown in Fig. S3. In the spectrum of HEMAMA, the peak at 11.1 ppm corresponded to the protons on the generated carboxyl groups. The peaks at 6.3, 6.9, and 7.1 ppm were assigned to the maleate, fumarate, and unreacted MA vinyl protons, respectively. The peaks at 5.6 and 6.1 ppm were corresponding to the two protons on methacylate. The small peaks at 3.6–3.9 ppm represented the hydroxyl-functional methylene protons of unreacted HEMA. The peak at 1.9 ppm denoted the methyl protons on methacylate and the corresponding proton number was three. Its intensity should not be altered during the synthesis of HEMAMA since it is inert. With this peak as reference, the C=C functionality of HEMAMA product can be estimated from the peaks at 5.6–6.9 ppm and was 2.00, which was exactly equal to the theoretical value. The amounts of unreacted HEMA and MA were determined as 0.10 and 0.02 per HEMAMA, respectively. As illustrated in Scheme 3, the small amounts

16


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Fig. 2 1H NMR spectra of (a) HEMAMA and (b) ESO-HEMAMA.

Scheme 3 Possible (a) polyether and (b) polyester oligomerization of epoxy groups during the synthesis of ESO-HEMAMA.

of HEMA and MA residues in the HEMAMA product can lead to the oligomerization of epoxy groups.25, 29 When the epoxy/acid ratio increased from 1.0/1.0 to 1.4/1.0, the 17



amount of epoxy groups consumed by HEMAMA would decrease, while the left amount of epoxy groups consumed by HEMA and MA would increase. Therefore, at the ratio of 1.4/1.0, the extent of oligomerization in the reaction system reached the gel point. In the spectrum of ESO, the most important characteristic peaks were at 2.8–3.2 ppm, which represented the protons on epoxy groups. In plant oil triglycerides the peak at about 0.9 ppm showing the terminal methyl protons of fatty acids is always taken as reference, since the intensity of the peak should not be altered throughout all the reactions. Thus the amount of epoxy groups could be determined by using this referenced peak and was about 3.99. In contrast, in the spectrum of ESO-HEMAMA, the peaks of epoxy groups totally disappeared. The reason lies in that the reaction between epoxy groups and HEMA belongs to the polyether oligomerization (Scheme 3(a)), which mainly consumes epoxy groups, not the HEMA. A new but very small peak occurred at around 4.9 ppm, which corresponded to the methine protons at the connecting structure of HEMAMA onto ESO. The peaks at 5.6–6.9 ppm, representing the protons on C=C groups, were all from the HEMAMA precursor. Hence, the introduced C=C functionality could be determined using the following equation

N C =C =

A5.6−6.9 ppm / 2

=

9 A5.6− 6.9 ppm

A0.9 ppm / 9

2 A0.9 ppm

(2)

The corresponding results are listed in Table 1. Fig. 3 indicates the

13

C NMR spectra of HEMAMA and ESO-HEMAMA. The

13

C NMR spectrum of ESO is shown in Fig. S4. In the spectrum of HEMAMA, the

peaks at around 164.3–167.2 ppm and 126.4–135.6 ppm were corresponding to the 18


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carbonyl and unsaturated carbons, respectively. The peak at 18.1 ppm represented the methyl carbons from HEMA. In the spectrum of ESO-HEMAMA, the peaks at 170.8–173.6 and 162.9–166.9 ppm were corresponding to the carbonyl carbons from ESO and HEMAMA, respectively. The peaks at 125.1–135.2 ppm and 16.7–18.8 ppm corresponded to the unsaturated and methyl carbons from HEMAMA, respectively. The peaks at 67.3–69.1 and 11.7–14.8 ppm denoted to the tertiary carbons at the polyols backbones and the methyl carbons of triglycerides. In addition, new peaks at 71.1–73.6 and 64.9–66.4 ppm occurred, which represented the tertiary carbons at the grafted structure of HEMAMA and the fatty acid carbons connecting to the formed secondary hydroxyls, respectively.29, 41 The peaks at 51.6–54.7 ppm, representing the carbons of epoxy groups in the spectrum of ESO, almost disappeared in the spectrum of ESO-HEMAMA.

Fig. 3 13C NMR spectra of (a) HEMAMA and (b) ESO-HEMAMA. 19



Fig. 4 shows the GPC chromatographs of ESO and ESO-HEMAMA. The peaks of ESO-HEMAMA were clearly broader than that of ESO and shifted to shorter retention times owing to the growth of molar mass. With the PS calibration curve, the molar masses of them can be calculated. The ESO-HEMAMA product possessed a number-average molar mass of 3716 and polydispersity index of 2.52, which were apparently higher than those of ESO (1273 and 1.40), respectively. Moreover, in the ESO curve, two peaks with molar masses of 1452 and 2945 were probably corresponding to monomers and dimers of ESO, respectively. In the curve of ESO-HEMAMA, the molar masses of the two peaks moved to 2268 and 4187, suggesting ESO was grafted by HEMAMA. Apart from the two peaks, there was still an area with much larger molar masses, indicating the occurrence of oligomers. This result was in good agreement with the 1H NMR analysis.

Fig. 4 GPC curves of ESO and ESO-HEMAMA.

Volatility of HEMAMA and AA. The volatility of HEMAMA and AA was evaluated by isothermal TGA at 40 °C and 110 °C, and the corresponding curves are plotted in 20


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Fig. 5. After a 2 h heating at 40 °C and 110 °C, the left weights of AA were only 21.2% and 12.0%, respectively. However, the left weights of HEMAMA were 99.4% and 87.3%, respectively, indicating the volatility of HEMAMA was much lower than that of AA.

Fig. 5 Isothermal TGA curves of HEMAMA and AA at 40 °C and 110 °C.

Gel contents of the UV-cured SBO-based Resins. Gel contents have a direct impact on the final properties of thermosetting polymers. In this work the Cgel values of the UV-cured SBO-based resins were determined by Soxhlet extraction and the corresponding data are summarized in Table 2. Firstly, the neat ESO-HEMAMA material showed a higher Cgel value than the neat AESO material, indicating the crosslink extent of the new EA material was higher. This improvement probably results from the higher C=C functionality of the ESO-HEMAMA product. Secondly, the ESO-HEMAMA materials containing TEGDA or TMPTA diluent demonstrated larger Cgel values than the pure ESO-HEMAMA material, suggesting the employment of the diluent with high functionality could improve the crosslink extent of the new 21



biomaterials.

Properties of the UV-cured SBO-based Resins. Dynamic Mechanical Analysis. Fig. 6 demonstrates DMA curves of the UV-cured SBO-based resins with or without diluents, and the related results are listed in Table 2. The Tg values were determined from the peak temperatures of tan δ curves. Firstly, the pure ESO-HEMAMA resin showed a storage modulus at 25 °C (E′25) of 1.00 GPa and Tg of 70.1 °C, which were 9.4 and 3.6 times those values of the pure AESO resin, respectively. As is known to us, the thermo-mechanical properties of thermosets usually have a close correlation with structural factors, for example, crosslink density (νe). According to the kinetic theory of rubber elasticity, the experimental νe of copolymers can be estimated from the rubbery modulus by the following equation29-30, 33-34

E ' = 3ν e RT

(3)

where E′ represents the storage modulus of crosslinked copolymers in the rubbery plateau region, R is the gas constant, and T is the absolute temperature. In this work the rubber modulus at Tg+50 °C was selected for the calculation of νe. It can be seen that the νe value of the pure ESO-HEMAMA resin was much higher than that of the pure AESO resin, which was in good accordance with the analysis of gel contents. Therefore, with the combined effects of νe and steric structure, the new pure SBO-based EA resin demonstrated much better thermo-mechanical properties than the pure AESO resin. Besides, as the C=C functionality of diluent increased from 1 to 3, 22


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Fig. 6 (a) Storage modulus and (b) loss factor of the UV-cured ESO-HEMAMA and AESO resins.

Table 2 Gel contents and thermal properties of the UV-cured SBO-based resins Samples

Cgela (%)

E′25b (GPa)

Tgc (°C)

νed (mol/m3)

Tie (°C)

Tpf (°C)

wcharg (%)

ESO-HEMAMA AESO ESO-HEMAMA/30% HEA ESO-HEMAMA/30% HEMA ESO-HEMAMA/30% TEGDA ESO-HEMAMA/30% TMPTA

97.2 95.5 95.9 99.2 99.8 97.7

1.00 0.106 1.06 1.41 1.23 1.27

70.1 19.3 56.2 89.0 78.2 66.8

10092 3713 4056 3209 13241 45358

360.8 361.1 368.1 367.1 377.3 390.4

408.7 399.4 416.7 437.6 423.2 445.6

4.73 0.11 3.32 3.06 4.25 5.09

a

Gel content. b Storage modulus at 25 °C. c Glass transition temperature. d Crosslink density. e Initial decomposition temperature. f Peak temperature at the curves of weight loss rate. g Char yield.

23



the E′25 value of the resulting ESO-HEMAMA material increased from 1.06 to 1.27 GPa, which can be also attributed to the increase of νe. The Tg increased firstly from 56.6 to 78.2 °C and then decreased to 66.8 °C. This decrease is possibly caused by the growing content of unincorporated TMPTA or ESO-HEMAMA in the crosslinked structure, as indicated by the gel content. As the diluent changed from HEA to HEMA, the E′25 and Tg grew from 1.06 to 1.41 GPa and from 56.2 to 89.0 °C, respectively. However, the νe value dropped when the diluent with steric structure was used, suggesting the steric structure of diluent played a much more important role than the νe in determining the thermo-mechanical properties of the new biomaterials.

Thermogravimetric Analysis. TGA thermograms and their derivative curves of the cured SBO-based resins are plotted in Fig. 7. The corresponding data of the obtained biomaterials, including initial decomposition temperature (Ti), peak temperature at the curves of weight–loss rate (Tp), and char yield (wchar), are summarized in Table 2. Compared with the pure AESO resin, the pure ESO-HEMAMA resin showed a comparable Ti but higher Tp and wchar values, indicating its thermal stability was superior to the AESO resin. In addition, when the functionality of diluent increased from 1 to 3, all the data of the obtained biomaterials increased: Ti from 368.1 °C to 390.4 °C, Tp from 416.7 °C to 445.6 °C, and wchar from 3.32% to 5.09%. These results indicated the incorporation of the diluent with higher functionality can enhance the thermal stability of the novel UV-cured biomaterials. When the diluent changed from HEA to HEMA, the values of Ti and wchar slightly decreased while the Tp increased apparently from 416.7 °C to 437.6 °C, meaning the incorporation of the diluent with 24


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steric structure can also improve the thermal stability of the ESO-HEMAMA materials.

Fig. 7 (a) TGA curves and (b) their derivatives of the UV-cured ESO-HEMAMA and AESO resins.

Mechanical Properties. Fig. 8 presents the typical tensile stress–strain curves of the UV-cured ESO-HEMAMA and AESO resins, and the related data are summarized in Table 3. All the samples broke without reaching yielding points, suggesting they were rigid materials. Remarkably, the pure ESO-HEMAMA material exhibited a tensile 25



Page 26 of 38

Fig. 8 Typical tensile stress–strain curves of the UV-cured ESO-HEMAMA and AESO resins.

Table 3 Mechanical and coating properties of the UV-cured SBO-based resins Samples

σa (MPa)

Eb (MPa)

εc (%)

Ad.d

P.H.e

Fl.f (mm)


13.4±1.5 1.93±0.12 17.8±1.1 28.4±1.2 14.2±0.9 8.35±0.79

592.1±48.8 37.7±1.8 620.2±16.8 991.9±58.2 741.6±38.3 915.8±40.1

4.7±0.4 5.9±0.2 10.1±0.4 6.2±0.4 3.2±0.1 2.2±0.3

3 3 1 2 3 4

HB 4B H 2H H B

4 2 2 2 2 8

a

Tensile strength. b Young’s modulus. c Tensile breaking strain. hardness of coatings. f Flexibility of coatings.

d

Adhesion of coatings. e Pencil

strength and modulus of 13.4 MPa and 592.1 MPa, which were 6.9 and 15.7 times as large as the values of the pure AESO resin, respectively. These results were also apparently better than those of similar pure SBO-based EA resins.7,

37

The large

improvement can also be attributed to both the effects of crosslink density and steric hindrance. Furthermore, the resultant UV-cured biomaterial with 30% of HEMA demonstrated a tensile strength of 28.4 MPa, although the HEMA diluent is 26


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monofunctional. In addition, as the C=C functionality of diluent increased from 1 to 3, the tensile strength and breaking strain decreased from 17.8 to 8.35 MPa and from 10.1% to 2.2%, respectively. However, the tensile modulus increased from 620.2 to 915.8 MPa, indicating the stiffness of the cured ESO-HEMAMA material was greatly enhanced by the incorporation of highly functional diluents. The reason can be attributed to the growth of νe. As is known to us, the increase of νe usually leads to the improvement of stiffness but the drop of flexibility and toughness for thermosets, and vice versa.28-31,

33-34

As the diluent varied from HEA to HEMA, both the tensile

strength and modulus grew apparently, meaning that the steric hindrance of diluent was very effective in raising the mechanical properties of the ESO-HEMAMA resins.

Coating Properties. Coating properties of the UV-cured SBO-based resins were measured and the related results are listed in Table 3. Compared with the neat AESO coating, the neat ESO-HEMAMA coating demonstrated inferior flexibility but superior hardness, which can be also ascribed to the effects of high functionality and steric hindrance. Moreover, as the C=C functionality of diluent increased from 1 to 3, all the coating properties dropped accordingly: the adhesion from 1 to 3 grade, the pencil hardness from H to B, and the flexibility from 2 to 8 mm. The drop of adhesion probably results from the decrease of hydroxyl concentration in the cured polymers, the decrease of flexibility can be ascribed to the increase of νe, and the change of hardness may be ascribed to the final C=C conversions (shown later). As the diluent varied from HEA to HEMA, the adhesion dropped from 1 to 2 grade, the flexibility stayed unchanged at the best value of 2 mm, while the pencil hardness grew from H to 27



Page 28 of 38

2H. Both the changes of adhesion and pencil hardness may be attributed to the steric structure from HEMA.

Swelling properties. Swelling properties of the UV-cured ESO-HEMAMA and AESO resins were investigated and the corresponding results are provided in Table 4. Compared to the pure AESO resin, the pure ESO-HEMAMA resin possessed superior resistance to ethanol and toluene but inferior resistance to acetone. Furthermore, as the C=C functionality of diluent increased from 1 to 3, all the uptake of solvents dropped dramatically: water from 3.75% to 0.22%, ethanol from 5.36% to 0.01%, acetone from 21.5% to 0.05%, and toluene from 4.07% to 0.06%. The improvement can be mainly attributed to the large growth of νe. As the diluent varied from HEA to HEMA, all the absorbed amounts of solvent decreased, reflecting the employment of the diluent with steric structure can enhance the swelling properties of the new biomaterials.

Table 4 Swelling properties and final C=C conversions of the UV-curable SBO-based resins Samples


Swelling (%) Water

Ethanol

Acetone

Toluene

Final C=C conversion (%)

1.01 1.02 3.75 2.39 1.51 0.22

5.43 9.36 5.36 4.85 0.09 0.01

31.7 20.8 21.5 17.3 1.25 0.05

11.2 27.2 4.07 1.43 0.22 0.06

50.7 91.5 74.7 69.4 60.1 43.6

28


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Photopolymerization Kinetics of the SBO-based Resins. The RT-IR technique was used to study the photopolymerization kinetics of the SBO-based EA resins. Fig. 9 illustrates the C=C conversions of the SBO-based resins and the related results are listed in Table 4. Compared with the neat AESO resin, the neat ESO-HEMEMA resin showed much lower C=C conversions throughout the UV curing, which may be attributed to the steric hindrance for both maleates and acrylates on ESO-HEMAMA. However, as mentioned above, the crosslink density of the ESO-HEMAMA resin was much larger than that of the AESO resin. The reason probably lies in that the ESO-HEMAMA resin generates more crosslinking sites than the AESO resin when crosslinked. As shown in Scheme 4, the ideal crosslinked ESO-HEMAMA and AESO resins can generate 8 and 3 crosslinking sites (indicated by red dashed circles), respectively. By multiplying the value of crosslinking site with the final C=C conversion for each sample, it can be deduced that the real crosslinked ESO-HEMAMA resin increases 4.1 crosslinking sites, while the real crosslinked AESO resin only increases 2.7 sites. Furthermore, when the C=C functionality of diluent increased from 1 to 3, the final C=C conversion decreased from 74.7% to 43.6%, indicating the use of the diluent with higher functionality would hamper the C=C conversions of the ESO-HEMAMA resins. When the diluent changed from HEA into HEMA, the final C=C conversion dropped from 74.7% to 69.4%, meaning the steric structure of diluent would also have a negative effect on the final C=C conversion.

29



Fig. 9 Double bond conversions of the SBO-based EA resins determined by RT-IR.

Scheme 4 Ideal crosslinked structures of (a) ESO-HEMAMA and (b) AESO.

CONCLUSIONS Through the synthesis and employment of the HEMAMA precursor, the traditional method of synthesizing SBO-based EA resins has been successfully upgraded. The modifying process avoids the use of volatile acrylic acid and doesn’t involve any

30


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solvent, which follows the principles of green chemistry. Meanwhile, the ultimate properties of the UV-cured pure ESO-HEMAMA resin, such as stiffness and Tg, were greatly improved compared with the pure AESO resin, meaning our theoretical hypothesis that the development of biobased EA resins with both high C=C functionality and steric structure can improve these properties were well supported. Furthermore, the properties of the resulting UV-cured biomaterials could be further modulated by the employment of different diluents. For example, the stiffness and Tg of the resulting ESO-HEMAMA materials were enhanced by the use of the diluent with high C=C functionality or steric hindrance. In general, this work provides a new and effective strategy in the development of biobased EA resins, and the obtained SBO-based EA resins exhibit great potential in the coating industry.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. Variation of acid value with reaction time for the synthesis of HEMAMA (Figure S1), 1H NMR spectrum of AESO (Figure S2), and 1H NMR and 13C NMR spectra of ESO (Figures S3–S4) (PDF).

AUTHOR INFORMATION 31



Corresponding Authors *C. Liu. E-mail: [email protected]; Tel.: + 86-25-85482520; Fax: + 86-25-85482520. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors greatly appreciate the Fundamental Research Funds of CAF (CAFYBB2017QB006), the National Natural Science Foundation of China (31770615), and the Natural Science Foundation of Jiangsu Province (BK20161122) for financial support.

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For Table of Contents Use Only

Novel soybean oil-based epoxy acrylate resins were synthesized with a “green” precursor and exhibited much better performance than similar bioresin after UV curing.

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