Synthesis of eugenol-based multifunctional monomers via a thiol–ene reaction and preparation of UV curable resins together with soybean oil derivatives

Abstract

Two kinds of UV-curable monomers (EM2G and EM3G) were synthesized from eugenol via a thiol–ene reaction. Their chemical structures were confirmed by FT-IR, ¹H-NMR and ¹³C-NMR before they were employed to copolymerize with acrylated epoxidized soybean oil (AESO). Bio-based UV curable resins were prepared and their thermal and mechanical properties were investigated by tensile testing and dynamic mechanical analysis (DMA). Their coating properties on tinplate were also studied. The results showed that the tensile strength, tensile modulus and glass transition temperatures of the cured AESO were significantly improved after the introduction of eugenol-based monomers. In addition, the UV-cured resins could be well coated on the surface of tinplate and good coating properties, such as hardness, flexibility, adhesion, solvent resistance and water absorption were demonstrated.

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1. Introduction

Due to the fast depleting petroleum reserves, global warming and other environmental problems, there is an increasing interest in the utilization of bio-based feedstock in both academic and industrial laboratories.¹ The conversion of biomass into useful polymeric materials is believed to have considerable environmental and economic values.^2,3 In the last two decades, a large quantity of polymers derived from renewable resources were successfully commercialized^3–5 and the potential of bio-based materials has been widely recognized. UV-curing is an effective and economic measure that has been widely used in coatings, printing inks, optoelectronics, dental restoration and composites.^6–11 Up to now, the application of UV to cure the thermosetting resins has been becoming more and more popular because of its high efficiency, low process cost and less environmental damage. Therefore, the combination of renewable resource and UV-curing method will provide us a “green + green” strategy to prepare the thermosetting resins derived from biomass, which should have a bright future.

Soybean oil, with a global annual production of about 45 million tons, has been used as an ideal substitute for petroleum-based compounds to prepare polymers.^12,13 Especially, its derivatives epoxidized soybean oil (ESO) and acrylated epoxidized soybean oil (AESO) have been widely applied in the UV-curable coatings.^14,15 As we know, soybean oil contains three long aliphatic chains, which will result in low mechanical or thermal properties. In order to improve its performance, as much as 50% or even more rigid compounds, such as styrene,^16,17 acrylate acid¹⁸ and divinylbenzene,¹⁹ were employed as the comonomers to make their mechanical or thermal properties acceptable for end use. Recently, with the rapid progress on bio-based materials, more and more bio-based monomers were also taken to strengthen the polymers derived from soybean oil so as to ensure both better performance and higher bio-based content.^13,20–22 For example, acrylated sucrose monomers and tetrahydrofurfural acrylate were copolymerized with AESO to obtain a coating with higher bio-based content in Chen's work.²⁰ Liu et al.²¹ synthesized hyperbranched methacrylates (TAHAs) from tannic acid to improve the properties of AESO-based coatings. Cai et al. reported the bio-based films with good coating properties derived from AESO and myrcene derivatives.²² However, it is easy to notice that the volatile organic compounds (VOC) emission during the synthesis process is inevitable and eager to be overcome.

Eugenol (EG) is an aromatic compound extracted from the essential oils of clove, basil, cinnamon and nutmeg etc. And nowadays, some investigations have shown that eugenol could also be obtained from pyrolysis^23,24 or depolymerization²⁵ of lignin. Considering its potential abundant production and unique structure, which includes methoxy-substituted phenolic ring and allyl group, it has been regarded as an ideal candidate for petroleum-based compounds to synthesize thermosetting acrylic resins,^26–28 benzoxazines resins,^29–34 cyanate ester resins³⁵ and epoxy resins.^36–38

In our previous work, the rosin based divinyl and trivinyl compounds³⁹ as well as polyesters synthesized from itaconic acid⁴⁰ have been synthesized to strengthen the AESO. However, the chemical reactivity of the unsaturated groups (allyl group or pendant double bond) in above compounds was relatively lower when compared with that of acrylate groups in AESO, which will result in an inadequate crosslink so as to affect the end properties. In addition, their viscosities appeared a little high and the practical process was difficult. As a continuous work, in this study we newly synthesized two UV-curable monomers (EM2G and EM3G) from EG via thiol–ene coupling reaction and epoxide ring-opening reaction. These two compounds demonstrated low viscosity and high reactivity when copolymerized with AESO. Their effects on the cure kinetics, mechanical and thermal properties as well as coating performance of the cured AESO were investigated. The objective of this work is to explore an ideal bio-based compound to strengthen the thermosetting resins derived from soybean oil through a more environmental friendly method.

2. Experimental section

2.1 Raw materials

Eugenol, benzoin dimethyl ether (DMPA) and triethanol amine were purchased from Zhejiang Guoguang Biochemistry Co., Ltd, China. Acrylated epoxidized soybean oil (AESO) was supplied by Jiang Su Li Tian Science and Technology Co. Ltd, China. It is a yellowy transparent liquid with the acid number <13 mg KOH g⁻¹, which has approximately two double bonds in each triglyceride molecule determined by ¹H-NMR. Triphenylphosphine (TPP), 3-mercaptopropionic acid, glycidyl methacrylate (GMA) and 4-methoxyphenol (MEHQ) were all obtained from Aladdin Reagent, China. 1-Hydroxycyclohexyl phenyl ketone (Irgacure 184) and thiomalic acid were purchased from JK Chemical Ltd. All chemicals were used as received without further purification.

2.2 Measurements

¹H-NMR was performed on a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with CDCl₃ as a solvent. The infrared spectrum was recorded with NICOLET 6700 FTIR (NICOLET, America). The gel content was determined after Soxhlet extraction. Specimens of about 0.5 g were cut from bulk resins and extracted with 250 ml of refluxing acetone using a Soxhlet extractor until the dry weight of resins did not change (48 h was enough in all of the cases). The extracted resins were then dried in vacuum oven at 60 °C for 24 h. Gel content was calculated from the weight difference of the dry samples before and after extraction. The final gel content was taken from an average of three specimens for accuracy. The viscosity of samples was measured by the isothermal rheological analyzer Physica MCR-301 of Rheometric (Anton Paar, Austria) with parallel plate tools. The test was performed at 30 °C for 2 min with a shear rate of 5 s⁻¹. Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA/DSC Thermogravimetric Analyzer (METTLER TOLEDO, Switzerland) with the high purity nitrogen as purge gas. The scanning rate is 20 °C min⁻¹ with the scanning range from 50 °C to 600 °C. Dynamic mechanical analysis (DMA) was carried out on Mettler-Toledo DMA/SDTA861e using a tension fixture. All the samples with the dimension of 20 mm × 7 mm × 0.5 mm were tested from −30 °C to 150 °C at a heating rate of 3 °C min⁻¹ and a frequency of 1 Hz. Mechanical properties were measured by Instron 5569A Universal Mechanical Testing Machine with a crosshead speed of 5 mm min⁻¹. The tensile properties of each sample were reported as the average of five measurements. UV curing behaviors of the resins were determined by real-time infrared (RTIR) spectroscopy. RTIR measurements were performed using a Nicolet 6700 FTIR spectrometer from Thermo Scientific with an extended KBr beam-splitter and an MCT/a detector. Uncured sample was exposed to UV light for 5 min at intensity of 30.0 mW cm⁻². The radiation intensity was measured using a UV light meter (UV-integrator 140, 250–410 nm, Kühnast Radiation Technology Limited Liability Company, Germany). All the samples were cured at room temperature and performed without air. The reaction progress was followed by monitoring the change in absorption of the double bond at 810 cm⁻¹. The pencil hardness of coatings with the thickness of about 50 μm on tinplate was measured according to ASTM D 3363-00. The flexibility of the coatings was measured by T-Bend Test according to ASTM D 4145-10. The adhesion on tinplate was evaluated using ASTM D 3359-09 crosshatch adhesion method. The solvent resistance of coatings was determined by the double rub method according to the modified ASTM D 5402-06.⁴¹ The UV-cured films were rubbed with cotton gauze using ethanol and MEK as solvent. The results were reported as the minimum number of double rubs at which the films were observed to fail or “>250” (if no change happened for the film after 250 double rubs. It was the maximum number of double rubs in the test).

2.3 Synthesis of eugenol-based compounds

2.3.1 Synthesis of EM2. In a 100 ml flask, eugenol (20.0 g, 0.12 mol), 3-mercaptopropionic acid (12.93 g, 0.12 mol) and DMPA (0.33 g, 1 wt% based on the total weight of mixture) were mixed homogeneously without adding any solvent. The mixture was stirred constantly and irradiated with a UV lamp (the lamp had a low power of 36 W at the wavelength of 365 nm) at room temperature. The distance from UV lamp to the mixture surface was about 15 cm and a reaction time of 1 h was taken to ensure the complete curing reaction. The target compound was identified by ¹H NMR.

¹H-NMR (400 MHz, CDCl₃), δ (ppm): 6.66–6.84 (m, 3H, –CH–CH–), 3.86 (s, 3H, –CH₃), 2.76–2.79 (t, 2H, –S–CH₂–CH₂–), 2.62–2.66 (t, 2H, HOOC–CH₂–CH₂–), 2.52–2.55 (t, 4H, –S–CH₂–CH₂–, –CH₂–CH₂–benzene), 1.84–1.91 (m, 2H, –CH₂–CH₂–CH₂–).

¹³C-NMR (400 MHz, CDCl₃), δ (ppm): 178.19 (–COOH), 146.52 (–C–OCH₃), 143.81 (–C–OH), 133.47 (–C–CH₂–), 121.04 (–CH–CH– of benzene), 114.37 (–CH–CH– of benzene), 111.19 (–CH–CH– of benzene), 55.98 (–OCH₃), 34.74 (–CH–CH–CH₂–), 34.42 (–CH₂–CH₂–), 31.47 (–CH₂–COOH), 31.30 (–CH₂–CH₂–S–), 26.56 (–S–CH₂–CH₂–).

FT-IR (KBr, ν/cm⁻¹): 3437 (–OH), 2935 and 2849 (–CH₂–), 1710 (C=O), 817 and 1600 (CH=CH of benzene).

2.3.2 Synthesis of EM3. In a 100 ml flask, eugenol (20.0 g, 0.12 mol), thiomalic acid (18.02 g, 0.12 mol) and DMPA (0.38 g, 1 wt% based on the total weight of mixture) were mixed homogeneously without adding any solvent. Similar to the synthesis of EM2, the mixture was stirred constantly and irradiated with a UV lamp at room temperature. The lamp power was 36 W with the wavelength of 365 nm. The distance from UV lamp to the mixture surface was about 15 cm and the irradiation was lasted for 1 h to ensure the complete curing reaction. The target compound was identified by ¹H NMR.

¹H-NMR (400 MHz, CDCl₃), δ (ppm): 6.64–6.83 (m, 3H, –CH–CH– of benzene), 3.85 (s, 3H, –CH₃), 3.61–3.65 (m, H, –S–CH–), 2.98–3.05 (m, 1H, HOOC–CH–CH–), 2.61–2.75 (m, 5H, HOOC–CH–CH–, –S–CH₂–CH₂–, –CH₂–CH₂–benzene), 1.87–1.93 (m, 2H, –CH₂–CH₂–CH₂–).

¹³C-NMR (400 MHz, CDCl₃), δ (ppm): 177.30 (–COOH), 176.34 (–COOH), 146.53 (–C–OCH₃), 143.75 (–C–OH), 133.04 (–C–CH₂–), 121.06 (–CH–CH– of benzene), 114.46 (–CH–CH– of benzene), 111.19 (–CH–CH– of benzene), 66.95 (–OCH₃), 55.92 (–S–C–), 40.99 (–CH₂–COOH), 34.22 (–C–CH₂–CH₂–), 31.13 (–CH₂–CH₂–CH₂–), 30.73 (–CH₂–CH₂–S–).

FT-IR (KBr, ν/cm⁻¹): 3437 (–OH), 2935 and 2858 (–CH₂–), 1712 (C=O), 818 and 1609 (CH=CH of benzene).

2.3.3 Synthesis of EM2G. The mixture of EM2 (20.0 g, 0.078 mol) and GMA (22.2 g, 0.156 mol) in the molar ratio of 1 : 2, TPP (0.21 g, 0.5 wt% based on the total weight of mixture) as catalyst for epoxide ring–opening reaction and MEHQ (0.04 g, 0.1 wt% relative to the total weight of mixture) as the free radical polymerization inhibitor were charged into a four-necked round-bottom flask equipped with a mechanical stirrer, a thermometer, a reflux condenser and a nitrogen inlet. After the mixture was stirred at 95 °C until the homogeneous solution was formed, it was maintained at this temperature for 30 min. Then the temperature was increased to 110 °C and the reaction was conducted at this temperature for 2 h followed by at 130 °C for another 1 h. At last, the mixture was cooled to room temperature and the target compound EM2G was obtained.

¹H-NMR (400 MHz, CDCl₃), δ (ppm): 6.65–6.83 (m, 3H, –CH–CH–), 6.13 (s, H, =CH₂), 5.61 (s, H, =CH₂), 3.86 (s, 3H, –CH₃), 3.59–4.42 (m, 10H, 2-O–CH₂–CHOH–CH₂–O–), 2.75–2.78 (t, 2H, –S–CH₂–CH₂–), 2.50–2.73 (m, 8H, –OC–CH₂–CH₂–, –OC–CH₂–CH₂–, –S–CH₂–CH₂–, –CH₂–CH₂–benzene), 1.94 (s, 6H, 2-CH₃), 1.86–1.89 (t, 2H, –CH₂–CH₂–CH₂–).

¹³C-NMR (400 MHz, CDCl₃), δ (ppm): 171.99 (–COO–), 167.44 (–C–COO–), 146.48 (–C–OCH₃), 143.81 (–C–O–), 135.86 (CH₃–C–), 133.13 (–C–CH₂–), 126.47 (–C–CH₂), 120.98 (–CH–CH– of benzene), 114.35 (–CH–CH– of benzene), 111.06 (–CH–CH– of benzene), 65.31–68.21 (–O–CH₂–CH–, –CH₂–C–CH₂–, –COO–CH₂–C, C–CH₂–COO–), 48.31 (–C–CH₃–), 26.89–35.95 (–OCH₃, –CH–CH₂–CH₂–, –CH₂–CH₂–CH₂–, –CH₂–CH₂–S–, –CH₂–COO–, –S–CH₂–CH₂–).

FT-IR (KBr, ν/cm⁻¹): 3451 (–OH), 2956 and 2852 (–CH₂–), 1719 (C=O), 814 and 1635 (C=CH₂ and CH=CH of benzene).

2.3.4 Synthesis of EM3G. The mixture of EM3 (20.0 g, 0.067 mol) and GMA (28.6 g, 0.2 mol) in the molar ratio of 1 : 3, TPP (0.24 g, 0.5 wt% based on the total weight of mixture) as catalyst for epoxy ring-open esterification and MEHQ (0.05 g, 0.1 wt% relative to the total weight of mixture) as the free radical polymerization inhibitor were charged into a four-necked round-bottom flask equipped with a mechanical stirrer, a thermometer, a reflux condenser and a nitrogen inlet. After the mixture was stirred at 95 °C until the homogeneous solution was formed, it was maintained at this temperature for 30 min. Then the reaction was conducted at 110 °C for 2 h and at 130 °C for another 1 h. At last, the mixture was cooled to room temperature and the target compound EM3G was obtained.

¹H-NMR (400 MHz, CDCl₃), δ (ppm): 6.67–6.82 (m, 3H, –CH–CH–), 6.14 (s, H, =CH₂), 5.61 (s, H, =CH₂), 3.87 (s, 3H, –CH₃), 3.59–4.28 (m, 11H, 2-O–CH₂–CHOH–CH₂–O–, H, –S–CH–), 2.93–3.06 (t, 2H, –S–CH₂–CH₂–), 2.52–2.76 (m, 6H, –OC–CH₂–CH–, –S–CH₂–CH₂–, –CH₂–CH₂– benzene), 1.78–1.94 (m, 11H, 2-CH₃, –CH₂–CH₂–CH₂–).

¹³C-NMR (400 MHz, CDCl₃), δ (ppm): 167.72 (–CH₂–COO–), 167.41 (–C–COO–), 167.37 (–CH–COO–), 146.49 (–C–OCH₃), 143.85 (–C–O–), 135.83 (CH₃–C–), 132.93 (–C–CH₂–), 126.37 (–C–CH₂), 120.99 (–CH–CH– of benzene), 114.34 (–CH–CH– of benzene), 111.10 (–CH–CH– of benzene), 63.41–70.25 (–O–CH₂–CH–, –CH₂–C–CH₂–, –COO–CH₂–C, C–CH₂–COO–), 55.90 (–OCH₃), 41.33 (–S–CH–), 29.04–36.29 (–CH–CH₂–CH₂–, –CH₂–CH₂–CH₂–, –CH₂–CH₂–S–, CH–CH₂–COO–), 18.28 (–C–CH₃).

FT-IR (KBr, ν/cm⁻¹): 3453 (–OH), 2929 and 2852 (–CH₂–), 1715 (C=O), 813 and 1635 (C=CH₂ and CH=CH of benzene).

2.4 Preparation of the UV-curable coatings

The detailed formulations of different UV-curable systems are shown in Table 1. In our experiments, Irgacure 184 was used as the photoinitiator and triethanol amine was employed here to increase the UV curing speed. After the tinplates were cleaned with acetone, the homogenized mixtures of AESO, EM2G or EM3G, predetermined Irgacure 184 (3 wt% relative to the total weight of mixtures) and triethanol amine (2 wt% based on the total weight of mixture) was casted on them using a spreader to form the thin films with the thickness of 50 μm. Then, the films casted on the tinplates were cured at room temperature for 5 min using a medium-pressure mercury lamp (500 W) at 365 nm. The distance from the lamp to the sample surface is about 15 cm. The coating properties in terms of hardness, flexibility, adhesion and solvent resistance were investigated on the tinplates substrate.

Table 1 Feed composition, bio-based content and gel content for different systems

Code	Weight ratio (%)		Bio-based content (wt%)	Gel content (wt%)
	Comonomer	AESO
AESO	0	100	95.0	91 ± 1
AESO–EM2G10	10	90	88.6	92 ± 1
AESO–EM2G30	30	70	75.2	93 ± 1
AESO–EM2G50	50	50	61.9	92 ± 1
AESO–EM3G10	10	90	89.5	93 ± 1
AESO–EM3G30	30	70	78.1	93 ± 1
AESO–EM3G50	50	50	66.7	93 ± 1

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For the tensile properties evaluation, the degassed homogenous mixtures were poured into a stainless steel mold with the cavity dimensions of 80 mm × 8 mm × 0.5 mm. After they were cured by the same curing procedures, the films were removed carefully from the mold and applied for test.

3. Results and discussion

3.1 Synthesis and chemical structure identification of UV curable monomers

The synthetic route and chemical structures of EM2G and EM3G are shown in Scheme 1. In our study, the target unsaturated monomers were synthesized from eugenol, mercaptan carboxylic acid and glycidyl methacrylate (GMA) via the thiol–ene coupling reaction and epoxide ring-opening reaction (chemical structures characterization of the compounds were shown in the ESI†). It was noted that all the reactions were conducted under a mild condition without any organic solvents. In addition, neither by-product nor VOC emission was detected during the whole process. Obviously, the syntheses of EM2G and EM3G were finished in an environmental-friendly way and they were all in line with the basic principles of Green Chemistry.

Scheme 1 The synthetic route of eugenol-based unsaturated monomers.

As we know, bio-based content is another key factor to evaluate the bio-based material. In order to regulate the development of bio-based polymeric products, some international standards have been established for the determination of the bio-based content. The United States Department of Agriculture (USDA) defines the bio-based content of a product as“the amount of bio-based carbon in the material or product as a percent of the weight (mass) of the total organic carbon in the product”.⁴² Following this definition, the bio-based content of the raw materials AESO, EM2G, EM3G, Irgacure 184 and TEA was 100%, 30%, 40%, 0% and 0%, respectively (acrylic acid was regarded as a bio-derived chemical in the calculations⁴³). And all the curing systems showed a bio-based content higher than 60% (Table 1). According to the USDA BioPreferred® Program, the minimum biobased content required for USDA BioPreferred Label is 25% and all the UV-curable systems listed in Table 1 were well-qualified.

3.2 Viscosity comparison between AESO, EM2G and EM3G

The viscosity of precursor is one of the key factors to determine the end use of thermosetting resin. For the previously reported styrene–AESO systems,^16–19 the styrene acted as not only a comonomer for mechanical properties enhancement, but also a diluent agent for easy process. The viscosity comparison between AESO, EM2G and EM3G as a function of time is shown in Fig. 1. Obviously, the viscosities of EM2G and EM3G were stable and lower than that of AESO. Especially, the EM2G demonstrated the viscosity of only about 0.7 Pa s at 30 °C, which was 20 times lower than AESO. The lower viscosity of comonomer will undoubtedly play a positive role in improving its overall performance.

Fig. 1 Viscosity of AESO, EM2G and EM3G as a function of time at 30 °C.

3.3 UV curing behaviors of AESO-based UV-cured systems

The carbon–carbon double bond conversion along with UV-curing time for the different systems were determined by monitoring the peak intensity change at around 800–820 cm⁻¹ in real-time infrared (RTIR) spectroscopy and the results are illustrated in Fig. 2. The measurement can provide useful information of the photo-polymerization rate as well as the final conversion of the functional groups. From Fig. 2, it was easy to notice that the carbon–carbon double bond conversion degrees for all the systems were increased sharply to above 70% within the initial 30 s, which indicated the high UV-curing activities of the systems. In fact, based on their chemical structures, AESO and EM2G as well as EM3G contain the same functional groups, acrylate groups, when applied to the UV-curing. They should demonstrate the similar conversion rates under the fixed condition and these results were reasonable. After the reaction was continued for 30 s, the conversion degrees were increased slowly and it might be due to the significantly decreased double bond density and the increased viscosity of the cured systems.

Fig. 2 UV curing behaviors of the different UV-cured systems (a: AESO and AESO–EM2G systems; b: AESO–EM2G and AESO–EM3G systems).

Compared with AESO–EM2G and AESO–EM3G, the pristine AESO system exhibited higher double bond conversion and the similar results were also reported in the previous literatures.²¹ As we know, when T_g of the cured resin reaches the curing temperature, the curing system will turn into a glassy state and make it a diffusion-controlled system.⁴⁴ For the AESO system, it demonstrated a lower ultimate T_g, which minimized the vitrification effects during the curing reaction. After the introduction of EM2G and EM3G, the T_g of the cured system increased higher than the curing temperature and the reaction was controlled by molecular diffusion, which led to more pronounced vitrification effects and thus lower the double bond conversion. When more carbon carbon double bond was introduced and the functional groups density was increased, the degree of their conversion will predictably increase. For the systems of AESO–EM2G and AESO–EM3G, it could be found that the double bond conversion of AESO–EM3G systems was higher than that of AESO–EM2G systems when the two monomers, EM2G and EM3G, were introduced in the same content.

3.4 Gel contents

Gel content is one of the crucial factors to determine the performance of the UV-curable coatings. Table 1 shows the gel content values for different UV-curable systems. As could be seen, the gel content of the UV-curable systems was increased with the increasing EM2G content, and the AESO–EM2G systems showed higher gel content of 92–93% than the pristine AESO network with the gel content of 92%, which was owing to the increased crosslink density after the introduction of EM2G. For the same reason, the AESO–EM3G networks also exhibited higher gel content than the neat AESO network. As for the AESO–EM3G and AESO–EM2G systems, the former possessed predictable higher gel content. This was because of the fact that AESO–EM3G systems had higher functionality than AESO–EM2G systems, which would lead to a higher crosslink density.

3.5 Tensile properties

Fig. 3 shows the typical tensile stress–strain curves of different systems and their tensile properties are summarized in Table 2. All of the cured systems exhibited rigid tensile behaviors without a yield point. The breaking elongation of all samples was below 10%, indicating that the co-polymers possessed the characteristics of rigid materials.

Fig. 3 Tensile curves for the different UV-cured systems (a: AESO and AESO–EM2G systems; b: AESO–EM2G and AESO–EM3G systems).

Table 2 Mechanical and thermal properties of the different cured systems

Samples	σ^a (MPa)	E^b (MPa)	ε^c (%)	Tg^d (°C)	E′^e (MPa)	νe^f/10^3 (mol m^−3)	Td10% (°C)	R600 (%)
a Tensile strength.b Young's modulus.c Elongation at break.d Glass transition temperature determined by DMA.e Storage modulus at 25 °C.f Crosslink density.
AESO	2 ± 1	20 ± 4	7 ± 1	10 ± 1	40 ± 10	2 ± 0.5	340 ± 4	1 ± 0.5
AESO–EM2G10	4 ± 1	40 ± 5	10 ± 1	16 ± 1	70 ± 8	3 ± 0.3	360 ± 5	2 ± 0.4
AESO–EM2G30	6 ± 1	90 ± 5	10 ± 2	30 ± 1	170 ± 9	3 ± 0.4	350 ± 4	2 ± 0.3
AESO–EM2G50	11 ± 1	260 ± 9	8 ± 1	46 ± 1	430 ± 15	4 ± 0.2	350 ± 3	3 ± 0.5
AESO–EM3G10	4 ± 1	50 ± 4	14 ± 2	22 ± 1	120 ± 12	3 ± 0.3	360 ± 6	1 ± 0.3
AESO–EM3G30	7 ± 2	145 ± 3	10 ± 1	26 ± 1	260 ± 10	4 ± 0.5	350 ± 4	2 ± 0.2
AESO–EM3G50	17 ± 3	390 ± 10	9 ± 1	66 ± 1	690 ± 30	5 ± 0.6	340 ± 5	3 ± 0.4

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It could be seen that the tensile strength and modulus of AESO systems were significantly improved after UV-curable monomers EM2G and EM3G were introduced. For the AESO–EM3G50, its tensile strength (17 MPa) and modulus (390 MPa) were 11 and 18 times higher than those of the cured pristine AESO, respectively. Although the ultimate tensile properties were not high enough for a lot of coating applications due to the long aliphatic chain in AESO, compared with the previous literatures,^39,40,45 the properties enhancement was attractive after the addition of EM2G and EM3G. For example, Ornella Zovi et al. strengthened epoxidized linseed oil (ELO) by a bio-derived diacid cross linker (Pripol 1009) and the highest tensile strength was only about 1.6 MPa when the weight ratio was ELO : Pripol = 50 : 50.⁴⁵ In our previous work, the rosin derivatives³⁹ and polyesters derived from itaconic acid⁴⁰ were also employed to copolymerize with AESO, and the results indicated that they were not as good as EM2G and EM3G in improving the tensile properties of cured AESO. As we know, the higher crosslink density always results in the increased strength, modulus and brittleness. In these systems, the synthesized EM2G and EM3G undoubtedly acted as the crosslink agents. In the meantime, the breaking elongation of AESO–EM2G and AESO–EM3G systems was about 10%, showing that the UV-cured systems possess good mechanical strength without sacrificing its flexibility. The reason for this phenomenon might be that EM2G and EM3G contained long soft aliphatic chain and soft sulfide bond. Due to the higher density of unsaturated groups in EM3G, the crosslink density of AESO–EM3G30 and AESO–EM3G50 were predictably higher than those of AESO–EM2G30 and AESO–EM2G50, which contributed to their improvement in tensile strength and tensile modulus.

3.6 Dynamic mechanical properties

Fig. 4 shows the temperature dependence of storage modulus (E′) and loss factor (tan δ) of the UV-cured systems. All the cured systems in this study demonstrated similar DMA behaviors and their modulus as well as glass transition temperature (T_g) data were summarized in Table 2. At room temperature, the storage modulus of UV-cured AESO systems was significantly increased after the addition of EM2G and EM3G. And when the content of EM2G and EM3G were increased further, their storage moduli were increased correspondingly. The T_g of the UV-cured systems in this study was determined by the peak temperature of tan δ shown in Fig. 4b and d. As we know, the T_g of a polymer network is affected by the crosslink density and monomers architecture. After the introduction of EM2G and EM3G, the crosslink density, intermolecular force and rigidity of UV-cured AESO systems were all increased, which decreased the mobility of chain segment, corresponding to the elevated T_g for all the AESO–EM2G and AESO–EM3G systems.⁴⁶ When the content of EM2G reached 50%, the T_g was increased from 10 °C for the pristine AESO system to 46 °C for the AESO–EM2G50. Compared with AESO–EM2G systems, with the increase of EM3G content, the T_g could be further increased to 66 °C for AESO–EM3G50. For the soy-bean oil based materials, the relatively lower T_g is one of the big shortcomings need to be overcome. Obviously, the synthesized unsaturated monomers based on eugenol, EM2G and EM3G, were effective agents for the T_g and mechanical properties elevation of AESO.

Fig. 4 DMA curves for UV-cured systems (a: E′ of AESO and AESO–EM2G systems; b: tan δ of AESO and AESO–EM2G systems; c: E′ of AESO–EM2G and AESO–EM3G systems; d: tan δ of AESO–EM2G and AESO–EM3G systems).

The crosslink density (ν_e) of the UV-cured systems could be calculated by the following equation derived from the theory of rubber elasticity:^47,48

where E′ is the storage modulus after T_g in the rubbery plateau region (E′ at the temperature of T_g + 40 K at which all crosslinked networks were in a rubbery state was chosen in this study), R is the gas constant and T is the absolute temperature. Based on this method, the calculated crosslink density data for the UV-cured systems is presented in Table 2. It could be seen that the crosslink density of UV-cured AESO systems was increased obviously with the addition of EM2G/EM3G because they all served as crosslinkers in the UV-cured systems. And these results were consistent with the above mechanical properties investigation.

3.7 Thermal properties of the cured systems

The TGA curves for the UV-cured systems under nitrogen are shown in Fig. 5 and the corresponding data are summarized in Table 2. Obviously, all of the cured resins exhibited one-step thermal degradation and were thermally stable at temperatures lower than 300 °C. In this work, the degradation temperature for 10% weight loss (T_d10%) was taken as the indicator to evaluate the thermal stabilities of different UV-cured systems. As could be seen from Table 2, T_d10% of AESO–EM2G and AESO–EM3G systems were all decreased slightly with the increasing EM2G or EM3G content, while the char yield at 600 °C (R₆₀₀) were increased. As we know that the ester linkage has a tendency to be easily cleaved and more benzene rings usually result in a higher char yield under elevated temperature. Therefore, the decreased T_d10% and increased R₆₀₀ of AESO–EM2G and AESO–EM3G system is reasonable because of the increased ester linkages and benzene rings after the addition of EM2G or EM3G in the systems.

Fig. 5 TGA curves of the different UV-cured systems (a: AESO and AESO–EM2G systems; b: AESO–EM2G and AESO–EM3G systems) under nitrogen.

3.8 Coating properties

The coating properties in terms of pencil hardness, adhesion, flexibility and solvent resistance of the UV-cured systems on tinplates sheet were investigated and the results are summarized in Table 3. Obviously, after the introduction of EM2G and EM3G, the pencil hardness of the UV-cured coatings was increased from 2B for the pristine AESO system to 2H for the AESO–EM2G50 and AESO–EM3G50 systems. The flexibility of all the coatings performed the highest level, which may be due to the flexible long soft aliphatic chain and soft disulfide bond structure of AESO/EM2G/EM3G. As could be seen, the adhesion of AESO coating on tinplates substrate was also improved significantly by the introduction of EM2G and EM3G. Especially, the adhesion of AESO–EM2G50 and AESO–EM3G50 system to tinplate was as high as 5B, which was the highest grade of adhesion according to the ASTM D3359-09 crosshatch adhesion method. It is well known that more polar groups in the cross-link systems might do a positive contribution to its adhesion on the substrate so as to increase the adhesive force. The polar hydroxyl groups in EM2G and EM3G were undoubtedly the adhesion promoter in this work. The solvent resistance is another important factor to determine the end use of coatings. Obviously, all the coating films showed good resistance to alcohol after 250 times double rubs on the surface. But in the MEK double rubs experiment, the neat AESO system exhibited relatively poor MEK resistance due to its lower cross-link density compared with other systems.

Table 3 The coatings performance of the UV-cured systems

Samples	Pencil hardness	Flexibility	Adhesion	MEK double rub resistance	Ethanol double rub resistance
AESO	2B	0T	0B	110	>250
AESO–EM2G10	B	0T	0B	>250	>250
AESO–EM2G30	HB	0T	4B	>250	>250
AESO–EM2G50	2H	0T	5B	>250	>250
AESO–EM3G10	B	0T	0B	>250	>250
AESO–EM3G30	H	0T	4B	>250	>250
AESO–EM3G50	2H	0T	5B	>250	>250

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4. Conclusions

Two kinds of UV curable monomers (EM2G and EM3G) were synthesized from renewable eugenol, mercaptan carboxylic acid and glycidyl methacrylate (GMA) under a mild condition without any organic solvents or VOC emission. They demonstrated high UV-curing reactivity and low apparent viscosity. The mechanical and thermal properties as well as coating performance of the AESO-based coatings could be significantly improved after the introduction of EM2G and EM3G. The results indicated that the unsaturated monomers derived from eugenol were effective co-monomers to enhance the properties of soybean oil-based UV coatings.

Acknowledgements

The authors are grateful for the financial support from Natural Sciences Foundation of Ningbo City (No. 2014A610110), the Research Project of Technology Application for Public Welfare of Zhejiang Province (No. 2014C31143), National Natural Science Foundation of China (NSFC No. 51373194; NSFC No. 51203176).

Notes and references

1. A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411–2502.
2. C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48, 1–10.
3. M. Eissen, J. O. Metzger, E. Schmidt and U. Schneidewind, Angew. Chem., Int. Ed., 2002, 41, 414–436.
4. F. Xie, P. Halley and L. Avérous, Prog. Polym. Sci., 2012, 37, 595–623.
5. G. Chen and M. K. Patel, Chem. Rev., 2012, 112, 2082–2099.
6. J. Pascault, R. Hofer and P. Fuertes, in Polymer Science: A Comprehensive Reference, ed. M. Mller and K. Matyjaszewski, Elsevier, Amsterdam, Oxford, Waltham, 1st edn, 2012, vol. 10, pp. 59–82.
7. J. V. Crivello and E. Reichmanis, Chem. Mater., 2013, 26, 533–548.
8. E. Andrzejewska, Prog. Polym. Sci., 2001, 26, 605–665.
9. K. Takada, H. B. Sun and S. Kawata, Appl. Phys. Lett., 2005, 86, 071–122.
10. J.-P. Fouassier, Photoinitiation, Photopolymerization, and Photocuring: Fundamentals and Applications, Hanser, 1995.
11. C. G. Roffey, Photopolymerization of Surface Coatings, Wiley, Chichester/New York, 1982.
12. P. Zhang, J. Xin and J. Zhang, ACS Sustainable Chem. Eng., 2013, 2, 181–187.
13. S. Ma, Y. Jiang, X. Liu, L. Fan and J. Zhu, RSC Adv., 2014, 4, 23036–23042.
14. R. Raghavachar, G. Sarnecki, J. Baghdachi and J. Massingill, J. Coat. Technol., 2000, 72, 125–133.
15. Y. Gan and X. Jiang, Photo-cured Materials From Vegetable Oils, in Green Materials From Plant Oils, 2014, pp. 1–27.
16. J. Lu, S. Khot and R. P. Wool, Polymer, 2005, 46, 71–80.
17. E. Can, R. P. Wool and S. Küsefoğlu, J. Appl. Polym. Sci., 2006, 102, 1497–1504.
18. F. Li, M. Hanson and R. Larock, Polymer, 2001, 42, 1567–1579.
19. F. Li and R. C. Larock, J. Appl. Polym. Sci., 2001, 80, 658–670.
20. Z. Chen, J. F. Wu, S. Fernando and K. Jagodzinski, Prog. Org. Coat., 2011, 71, 98–109.
21. R. Liu, J. Zhu, J. Luo and X. Liu, Prog. Org. Coat., 2014, 77, 30–37.
22. J. Cai, W. Shu, L. Yun and L. Jun, Coating Industry, 2006, 36, 12–15.
23. G. Telysheva, G. Dobele, D. Meier, T. Dizhbite, G. Rossinska and V. Jurkjane, J. Anal. Appl. Pyrolysis, 2007, 79, 52–60.
24. K. Kuroda, Y. Inoue and K. Sakai, J. Anal. Appl. Pyrolysis, 1990, 18, 59–69.
25. P. Varanasi, P. Singh, M. Auer, P. Adams, B. Simmons and S. Singh, Biotechnol. Biofuels, 2013, 1, 14.
26. J. Deng, B. Yang and C. Chen, et al., ACS Sustainable Chem. Eng., 2015, 4, 599–605.
27. L. Rojo, B. Vázquez and J. San Román, et al., Dent. Mater., 2008, 12, 1709–1716.
28. L. Rojo, B. Vazquez and J. Parra, Biomacromolecules, 2006, 10, 2751–2761.
29. L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman and P. Dubois, J. Mater. Chem. A, 2015, 3, 6012–6018.
30. L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman and P. Dubois, Eur. Polym. J., 2015, 67, 494–502.
31. A. Van, K. Chiou and H. Ishida, Polymer, 2014, 55, 1443–1451.
32. P. Thirukumaran, A. Shakila Parveen and M. Sarojadevi, ACS Sustainable Chem. Eng., 2014, 2, 2790–2801.
33. P. Thirukumaran, A. Shakila and S. Muthusamy, RSC Adv., 2014, 4, 7959–7966.
34. C. Wang, J. Sun, X. Liu, A. Sudo and T. Endo, Green Chem., 2012, 14, 2799–2806.
35. B. Harvey, C. Sahagun, A. Guenthner, T. Groshens, L. Cambrea, J. Reams and J. Mabry, ChemSusChem, 2014, 7, 1964–1969.
36. J. Wan, B. Gan, C. Li, J. Molina-Aldareguia, Z. Li, X. Wang and D. Wang, J. Mater. Chem. A, 2015, 43, 21907–21921.
37. J. Qin, H. Liu, P. Zhang, M. Wolcott and J. Zhang, Polym. Int., 2014, 63, 760–765.
38. J. Wan, B. Gan, C. Li, J. Molina-Aldareguia, E. Kalali, X. Wang and D. Wang, Chem. Eng. J., 2016, 284, 1080–1093.
39. Q. Ma, X. Liu, R. Zhang, J. Zhu and Y. Jiang, Green Chem., 2013, 15, 1300–1310.
40. J. Dai, S. Ma, Y. Wu, L. Han, L. Zhang, J. Zhu and X. Liu, Green Chem., 2015, 17, 2383–2392.
41. F. Liao, X. Zeng, H. Li, X. Lai and F. Zhao, J. Cent. South Univ., 2012, 19, 911–917.
42. G. Norton and S. Devlin, Bioresour. Technol., 2006, 97, 2084–2090.
43. http://cen.acs.org/articles/91/i46/Hunting-Biobased-Acrylic-Acid.html.
44. J. Enns and J. Gillham, J. Appl. Polym. Sci., 1983, 28, 2567–2591.
45. O. Zovi, L. Lecamp, C. Loutelier-Bourhis, C. Lange and C. Bunel, Green Chem., 2011, 13, 1014–1022.
46. S. Oprea, J. Mater. Sci., 2010, 45, 1315–1320.
47. L. Hill, Prog. Org. Coat., 1997, 31, 235–243.
48. G. Palmese and R. McCullough, J. Appl. Polym. Sci., 1992, 46, 1863–1873.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01420h

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