Bio-based tetrafunctional crosslink agent from gallic acid and its enhanced soybean oil-based UV-cured coatings with high performance

Abstract

The utilization of soybean oil-based UV coatings depends on the introduction of petroleum-based comonomers or crosslink agents. Thus, in this paper, a bio-based crosslink agent (GACA) for UV curable coatings was synthesized from gallic acid and its chemical structure was confirmed by FT IR, ¹H NMR and ¹³C NMR. Crosslinked networks with high biobased content of more than 88% were obtained after co-photopolymerization between acrylated epoxidized soybean oil (AESO) and GACA. The thermal, mechanical and coating properties of these GACA crosslinked AESO networks were investigated and a commonly used crosslink agent triallyl isocyanurate (TAIC) was used as the control. GACA exhibited more functional groups and better copolymerization with AESO than TAIC, resulting in the higher gel content, crosslink density, tensile strength and modulus as well as much better coating properties (reflected by the higher pencil hardness, better wear resistance and adhesion) of GACA crosslinked AESO networks than TAIC crosslinked AESO networks. These results indicated that GACA exhibited great potential to replace petroleum-based crosslink agents such as TAIC, and high-performance soybean oil-based UV-cured coatings with high biobased content could be achieved after introducing GACA.

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1 Introduction

Due to the increasing concern about the depletion of fossil reserves, greenhouse gas emission, and other environmental problems (e.g. waste), there is an increasing interest in utilization of biorenewable raw materials in the chemical industry.^1,2 Coatings with an output of 39.82 million tons in 2012 are an important sector of the modern chemical industry and with the global economic rebound, rapid industrialization in emerging economies and increased demand from such sectors as automotive and construction, the coatings market is still growing.^3,4 Driven by the rising raw material cost and the stricter environmental regulations, the coatings industry is transforming to a “greener” industry by incorporating more alternative renewable raw materials and “greener” technologies such as UV cure which has the capability to produce high-performance coatings with high productivity, low energy consumption and extremely low VOC emissions.^5–8

Plant oils, as one of the most important renewable raw materials for the chemistry industry, has been widely used for surfactants, cosmetic products, and lubricants, flooring materials, coating and resin applications.⁹ Soybean oil is one of the most readily available and inexpensive vegetable oil in the world today (with a global production of about 45 million tons in 2013), making it a very attractive alternative renewable raw material to fossil resources in certain applications.^10,11 Its derivatives epoxidized soybean oil (ESO) and acrylated epoxidized soybean oil (AESO) have been used in the UV curable coatings. However, due to the flexible long aliphatic chain of soybean oil, rigid compounds such as cycloaliphatic epoxy resin,¹¹ styrene and divinylbenzene,^12–14 triallyl isocyanurate (TAIC)¹⁵ and sol–gel silica¹⁶ were often used together to achieve the necessary rigidity and strength required for some applications. Although they were partially bio-based, their biobased contents were low and some rigid compounds such as styrene and divinylbenzene were toxic. Thus it is necessary to develop high-performance soybean oil-based coatings with high biobased content.

In order to obtain soybean oil-based materials with high biobased content, efforts have been made to develop bio-based comonomers in recent years.^5,17–20 For example, Chen et al.⁵ used bio-derived acrylated sucrose monomers and tetrahydrofurfural acrylate to co-photopolymerize with AESO, coatings with enhanced properties and high biobased content was obtained. Jang et al.¹⁷ synthesized a 2,5-furan diacrylate from 2,5-furan dicarboxylic acid which was identified by U.S. Department of Energy as one of the top 12 potential bio-based platform chemicals.¹⁸ The crosslinked polymers from the co-photopolymerization between AESO and 2,5-furan diacrylate showed relatively high tensile strength of 14 MPa about twice as much as that without 2,5-furan diacrylate. Moreover, Liu et al.¹⁹ developed a tannic acid-based hyperbranched methacrylates (TAHAs) from tannic acid and glycidyl methacrylate. Using TAHAs as the comonomer, improved coating performance and environment degradability were obtained for AESO based coatings. In our previous work, rosin based comonomers were also prepared to serve as alternatives to some petroleum-based rigid monomers to strengthen the thermosetting resins derived from AESO.²⁰ Compared with DVB, the rosin-based comonomers, divinyl acrylicpimaric acid and trivinyl compound, were more effective in improving the mechanical and thermal properties of cured AESO, especially the T_g, tensile strength and tensile modulus. Although the soybean oil-based crosslinked networks with the bio-based comonomers above showed improved properties as well as high biobased content, the improvement of the properties were limited. More attentions should be paid to soybean oil-based materials with high performance.

Gallic acid (Fig. 1), also known as 3,4,5-trihydroxybenzoic acid, is widely present both free and as part of hydrolyzable tannins in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants.²¹ Gallic acid has been widely used in pharmaceuticals industry, antioxidants, ink dyes, photography and paper manufacture.²² Owing to its rigid benzene ring and three phenolic hydroxyl groups and one carboxylic group, it also showed great potential to enhance the physicomechanical properties of materials²³ and produce high-performance epoxy resins.^24–26 Thus, in the present paper, a tetra functional gallic acid-based crosslink agent (GACA) was synthesized to co-photopolymerize with AESO and to reinforce the AESO based coatings. For easy evaluation, a commonly used petroleum-based crosslink agent triallyl isocyanurate (TAIC, as shown in Fig. 1) was also employed here as the control crosslink agent and co-photopolymerized with AESO under the same conditions. The objective of this work was to formulate a novel bio-based crosslink agent from gallic acid to copolymerize with AESO and obtain UV-cured coatings with high performance as well as high biobased content.

Fig. 1 Chemical structures of the gallic acid, triallyl isocyanurate (TAIC) and gallic acid-based crosslink agent (GACA).

2 Experimental

2.1 Raw materials

Acrylated epoxidized soybean oil (AESO) was purchased from JiangSu LiTian Science and Technology Co. Ltd, China. AESO is a yellowy transparent viscous liquid with acid number <13 mg KOH per g, which has approximately three double bonds in each triglyceride molecule determined by ¹H NMR. Triallyl isocyanurate (TAIC) and triethanolamine were obtained from Sinopharm Chemical Reagent Co., Ltd, China. Gallic acid, allyl bromide, acryloyl chloride and 2,2-dimethoxy-2-phenylacetophenone (DMPA) were from Aladdin Reagent, China.

2.2 Synthesis of the gallic acid-based crosslinking agent

5.10 g (30 mmol) gallic acid, 7.26 g (60 mmol) allyl bromide, 100 g (1724 mmol) acetone and 18.24 g (132 mmol) potassium carbonate were placed in a three-necked round-bottomed flask with a magnetic stirrer, a thermometer and a reflux condenser. After the reactants were mixed vigorously at room temperature for 10 min, it was heated and refluxed for 2 h. Then it was cooled to room temperature and 8.96 g (99 mmol) acryloyl chloride was added dropwisely during 30 min, and it was heated to 40 °C and kept at this temperature for 12 h. The solution was filtered and the gallic acid-based crosslinking agent (GACA) was obtained with a yield of 86% after removing the unreacted allyl bromide, acryloyl chloride and acetone on a rotary evaporator. The obtained product GACA was light yellow transparent oil liquid. The synthetic route was shown in Fig. 2. Gallic acid was also suitable to introduce four allyl groups, while the allyl ether group on the phenolic group has low reactivity during the UV curing process. Thus, in this paper three acrylate groups was incorporated on the phenolic groups of gallic acid after placing an allyl group on the carboxylic acid.

Fig. 2 The synthetic route of gallic acid-based crosslinking agent (GACA).

FT IR (KBr, cm⁻¹): 3083 cm⁻¹ (Ph–H), 3035 cm⁻¹ (C–H of –CH=CH₂), 1600 cm⁻¹ (C=C of benzene ring), 1630 cm⁻¹ (C=C of –CH=CH₂), 2990 cm⁻¹ and 2948 cm⁻¹ (CH₂–CH=CH₂), 1760 cm⁻¹ (Ph–O–C=O), 1727 cm⁻¹ (Ph–C=O).

¹H NMR (400 MHz, acetone-d6, δ ppm): 7.94 (s, 2H), 6.13–6.60 (m, 9H), 6.04–6.12 (m, 1H), 5.27–5.46 (m, 2H), 4.84–4.88 (m, 2H).

¹³C NMR (400 MHz, acetone-d6, δ ppm): 162.68 (1C), 162.60 (2C), 161.47 (1C), 143.80 (2C), 138.97 (1C), 134.18 (1C), 133.54 (2C), 132.35 (1C), 127.43 (1C), 126.78 (2C), 126.11 (1C), 121.94 (2C), 117.78 (1C), 65.84 (1C).

2.3 Preparation of the crosslinked networks

The predetermined AESO, GACA/TAIC, 3 wt% 2,2-dimethoxy-2-phenylacetophenone (DMPA) and 2 wt% triethanolamine (on the basis of the total weight of AESO and GACA/TAIC) were stirred together until homogeneous mixture were obtained. After that, they were degassed in a vacuum oven at 30 °C for at least 15 min. Then the gas free mixtures were poured into a preheated stainless steel mould to obtain samples with dimension of 80 mm × 8 mm × 0.5 mm, and coated on the zinc iron sheet to get the coatings with thickness of about 50 μm and cured for 45 min using a high-pressure mercury lamp (500 W) at 366 nm with a distance of 8 cm from lamp to the surface of samples under air at room temperature. The crosslinked networks with different contents of GACA/TAIC were prepared and their compositions and codes were listed in Table 1. All the samples were cured under the same conditions.

Table 1 The composition, biobased content and gel content for the different systems

Samples	AESO/GACA/TAIC weight ratio	Biobased content (wt%)	Gel content (wt%)
Neat AESO	100/0/0	95	94.3
AESO/GACA10	90/10/0	94	95.7
AESO/GACA20	80/20/0	92	95.9
AESO/GACA30	70/30/0	91	96.1
AESO/GACA40	60/40/0	89	96.7
AESO/GACA50	50/50/0	88	96.5
AESO/TAIC10	90/0/10	86	94.8
AESO/TAIC20	80/0/20	76	94.9
AESO/TAIC30	70/0/30	67	94.9
AESO/TAIC40	60/0/40	57	94.4
AESO/TAIC50	50/0/50	48	93.6

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2.4 Characterization

The infrared spectrum (FT IR) was recorded with NICOLET 6700 FT IR (NICOLET, America). ¹H NMR and ¹³C NMR were performed on a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with acetone-d6 as a solvent. The gel contents were measured using acetone extraction. The cured samples weighing between 0.42 and 0.50 g were precisely weighed (W₁), extracted with acetone for 48 h under reflux using a Soxhlet extractor, and finally dried and weighed (W₂). The gel content was calculated as W₂/W₁. 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 0 to 130 °C at a heating rate of 3 °C min⁻¹ and a frequency of 1 Hz. Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA/DSC1 Thermogravimetric Analyzer (METTLER TOLEDO, Switzerland) with high purity nitrogen as purge gas at a scanning rate of 20 °C min⁻¹ from 50 °C to 600 °C. 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 × 0.5 mm were used for this evaluation. The data was taken from an average of at least five specimens for accuracy. The pencil hardness of coatings with the thickness of 40–50 μm on the steel substrate was measured according to ASTM D 3363-00. The friction coefficient of coatings was measured on a JLTB-02 friction wear testing machine (J&L Tech, Korea) with the load of 3 N at a linear speed of 25.0 mm s⁻¹. The adhesion of coatings on the steel substrate was evaluated using the ASTM D 3359 crosshatch adhesion method.

3 Results and discussion

3.1 Chemical characterization of GACA

The chemical structure of GACA was determined by FT IR, ¹H NMR and ¹³C NMR. Fig. 3a shows the FT IR spectrum of GACA. As can be seen, the peaks at 3083 cm⁻¹ and 3035 cm⁻¹ were corresponding to the stretching absorptions of Ph–H and C–H of –CH=CH₂, the peaks at 1600 cm⁻¹ and 1630 cm⁻¹ belonged to the stretching absorptions of C=C of benzene ring and –CH=CH₂, respectively. Other characteristic absorption peaks of GACA were at 2990 cm⁻¹ and 2948 cm⁻¹ (CH₂–CH=CH₂), 1760 cm⁻¹ (Ph–O–C=O) and 1727 cm⁻¹ (Ph–C=O). In order to further identify its structure, the ¹H NMR and ¹³C NMR spectra of GACA were also displayed in Fig. 3b and c. As can be seen in Fig. 3b and c, the peaks of protons and carbons were all in accordance with the characteristic peaks of protons and carbons of target compound GACA. These results demonstrated that the target compound was synthesized successfully.

Fig. 3 The FT IR (a), ¹H NMR (b) and ¹³C NMR (c) spectra of GACA.

3.2 Biobased content of the crosslinked networks

The United States Department of Agriculture defines the biobased content of a product as “the amount of biobased carbon in the material or product as a percent of the weight (mass) of the total organic carbon in the product”. According to this definition, the biobased content of the raw materials AESO, GACA, TAIC, DMPA and TEA was 100%, 84.2%, 0%, 0% and 0%, respectively (acrylic acid was counted as a bio-derived chemical in the calculations²⁷). And the biobased content of the crosslinked networks (based on the biobased content of the raw materials) was shown in Table 1. Obviously, AESO/GACA systems showed high biobased content (≥88%) which was much higher than AESO/TAIC systems with the same content of AESO.

3.3 Gel content of the crosslinked networks

Gel content which is an important factor for the UV-cured coatings has a direct relationship with the properties of the coatings. Thus, the gel content of the crosslinked networks were examined and shown in Table 1. As can be seen, the gel content of the crosslinked networks increased with increasing GACA content, and the AESO/GACA systems showed higher gel content of 95.7–96.7% than the neat AESO network with the gel content of 94.3%, which was owing to the increased crosslink density of AESO network after introducing GACA. Due to the same reason, the AESO/TAIC networks exhibited slightly higher gel content than the neat AESO network when the TAIC content was 10% and 20%. However, the further increase of TAIC content did not further increase the gel content of network, and the high amount of TAIC (40% and 50%) decreased the gel content of network.

3.4 Dynamic mechanical properties of the crosslinked networks

The dynamic mechanical analysis (DMA) was used to determine the glass transition temperature (T_g) and crosslink density (ν_e) of the cured systems. Fig. 4a and b shows the temperature dependence of storage modulus (E′) and loss factor (tan δ) of the crosslinked networks. The T_gs of the crosslinked networks obtained from the peak temperatures of the curves of tan δ as a function of temperature were shown in Table 2. As can be seen, after introducing GACA, the T_g increased from 21 °C for the neat AESO system to 71 °C for the AESO system with 50 wt% GACA (shown as AESO/GACA50). This increase is as expected because the addition of crosslinking agent (GACA) is accompanied by an increase in the crosslinking density, which decreases the mobility of the copolymer. As the mobility decreased, the T_g of the material increases.²⁸ As shown in Table 2, TAIC could also increase the T_g of the AESO network when the TAIC content was 10% and 20%. While the further increase of TAIC content increased the heterogeneity of the crosslinked networks, corresponding to the unclear peak temperatures of the tan δ curves for AESO/TAIC30, AESO/TAIC40 and AESOTAIC50. This result coincides with the gel content of AESO/TAIC networks.

Fig. 4 Storage modulus (E′) versus temperature (a) and tan δ versus temperature (b) for the GACA crosslinked AESO networks and storage modulus (E′) versus temperature (c) and tan δ versus temperature (d) for the TAIC crosslinked AESO networks.

Table 2 The thermal properties of the crosslinked networks

Samples	Tg	E′ at 130 °C (MPa)	νe (10^3 × mol m^−3)	Td5% (°C)	Td30% (°C)	Ts	Char yield (%)
Neat AESO	21	35	3.5	326	388	178	1.74
AESO/GACA10	29	49	4.9	315	385	175	2.53
AESO/GACA20	37	74	7.4	301	381	171	4.14
AESO/GACA30	54	135	13.4	301	379	170	5.92
AESO/GACA40	63	201	20.0	295	374	168	8.48
AESO/GACA50	71	276	27.5	293	371	167	11.12
AESO/TAIC10	29	49	4.9	327	393	180	1.53
AESO/TAIC20	35	63	6.3	332	400	183	1.68
AESO/TAIC30	—	118	11.7	339	413	188	2.20
AESO/TAIC40	—	190	18.9	330	422	189	2.82
AESO/TAIC50	—	241	24.0	337	442	196	3.77

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The storage modulus (E′) obtained from DMA in the rubbery plateau region is often used to calculate the crosslink density (ν_e) based on the following formula:²⁹

where E′ is the storage modulus of thermoset in the rubbery plateau region at 130 °C (The storage modulus in the rubbery plateau region was determined at generally 60 °C above the T_g,^6,30 while the T_gs of AESO/TAIC30, AESO/TAIC40 and AESOTAIC50 could not be obtained, so we chose the temperature of 130 °C at which all crosslinked networks were in a rubbery state.), R is the gas constant, and T is the absolute temperature.

The data for E′ at 130 °C and the calculated ν_e are shown in Table 2. Obviously, the E′ at 130 °C and ν_e of the crosslinked networks increased with the increase of GACA and TAIC content, which was due to the multifunctional and tight structures of GACA and TAIC. And the AESO/GACA networks exhibited higher E′ at 130 °C and ν_e relative to AESO/TAIC networks because of the more functional groups of GACA and better copolymerization of GACA with AESO than those of TAIC.

3.5 Thermal degradation behaviors of the crosslinked networks

The thermal stability and degradation behaviors of the crosslinked networks were evaluated by thermogravimetric analysis (TGA). The TGA curves under nitrogen atmosphere are shown in Fig. 5.

Fig. 5 TGA curves of (a) GACA modified AESO networks and (b) TAIC modified AESO networks.

The values of the initial degradation temperature for 5% weight loss (T_d5%), the degradation temperature for 30% weight loss (T_d30%) and the residual weight percent at 600 °C (R₆₀₀) are shown in Table 2. To specify the thermal stability of the cured resins, the statistic heat-resistant index (T_s) was used. It is determined from T_d5% and T_d30% of the sample by thermogravimetric analysis (TGA). The T_s is calculated by the following equation:^26,31

T_s = 0.49[T_d5% + 0.6(T_d30% − T_d5%)] (2)

Based on Fig. 5 and Table 2, the T_d5% and T_s of the AESO/GACA systems were both lower than those of the neat AESO system, and decreased with the increase of GACA content. On the contrary, the T_d5% and T_s of the AESO/TAIC systems were higher than those of the neat AESO system, and increased with the increase of TAIC content. These results indicated that the thermal stability of the AESO networks decreased after introducing GACA and increased after introducing TAIC. The increased thermal stability of the AESO/TAIC systems was due to the increased crosslink density of the networks after introducing TAIC. While the thermal stability not only has a relationship with the crosslink density of the network, it also has close ties with the chemical structure of the network.³² On the one hand, the incorporation of GACA increased the crosslink density of the AESO networks, which would bring about an increase in the thermal stability; on the other hand, the easily thermal cleavable ester bonds of GACA reduced the thermal stability of the AESO/GACA systems.³³ These two competing effects finally led to the lower thermal stability of the AESO/GACA systems than the neat AESO system. However, the char yield at 600 °C (R₆₀₀) of the AESO network increased with the increase of GACA content, from 1.74% for the neat AESO network to 11.12% for the AESO/GACA50, and the increase of R₆₀₀ was much bigger than that from the introduction of TAIC into the AESO network with the highest R₆₀₀ of 3.77% for AESO/TAIC50. This was due to that the incorporation of benzene ring into the polymers is beneficial to the char formation during the thermal degradation and burning processes,³⁴ the AESO/GACA networks contained more benzene ring than the neat AESO network and AESO/TAIC networks, leading to the higher R₆₀₀ of AESO/GACA networks.

3.6 Mechanical properties of the crosslinked networks

Mechanical properties of the crosslinked networks are shown in Fig. 6. As shown in Fig. 6a and b, both GACA and TAIC increased the tensile strength and modulus of the AESO network. The tensile strength and modulus of the neat AESO network were 3.5 MPa and 35 MPa, respectively. After introducing GACA into the AESO system, the tensile strength and modulus of the crosslinked networks increased up to 27.0 MPa and 744 MPa, respectively, which were much higher than those of AESO/TAIC networks (up to 23.3 MPa and 513 MPa, respectively). While the elongation at break of the AESO system was reduced by incorporating GACA and TAIC, and the decrease of the AESO/GACA networks was more than that of AESO/TAIC networks, as shown in Fig. 6c. This was due to the higher crosslink density of AESO/GACA networks than AESO/TAIC networks, resulting in the higher tensile strength and modulus and lower elongation at break of AESO/GACA networks relative to AESO/TAIC networks.

Fig. 6 Tensile strength (a), modulus (b) and elongation at break (c) of the crosslinked networks.

3.7 Coating properties of the crosslinked networks

Hardness, wear resistance and adhesion of coating are three important factors in determine its end use. Thus the hardness, friction factor and adhesion of the coatings on the zinc iron sheet were examined, and the data were shown in Table 3. Obviously, after introducing GACA, the hardness of the AESO coatings increased from B for the neat AESO coating to 4H for the AESO/GACA50 coating. TAIC also improved the hardness of the AESO coating, but the increase was limited (the hardness of AESO/TAIC50 coating was only H). This was ascribed to the reason that GACA and TAIC could both increase the crosslink density and rigidity of the AESO network, while the TAIC could not copolymerize well with AESO, more TAIC or AESO self-polymerized in AESO/TAIC systems relative to AESO/GACA systems, resulting in the relatively lower hardness of AESO/TAIC systems. For the same reason, the wear resistance of AESO/GACA systems was better than AESO/TAIC systems reflected by the lower friction factor of AESO/GACA systems relative to AESO/TAIC systems. As can be seen, the adhesion of the AESO coating was improved by introducing GACA and TAIC due to the increased polarity. The polarity of TAIC might be not as big as that of GACA, corresponding to the lower adhesion of AESO/TAIC coatings relative to that of AESO/GACA coatings. According to the above results, we can conclude that the AESO networks with GACA as crosslink agent could obtain good coating properties, much better than the TAIC crosslinked AESO networks.

Table 3 The coating properties of the crosslinked networks

Samples	Hardness	Friction factor	Adhesion
Neat AESO	B	0.768	0B
AESO/GACA10	H	0.433	1B
AESO/GACA20	2H	0.274	2B
AESO/GACA30	3H	0.168	2B
AESO/GACA40	3H	0.118	3B
AESO/GACA50	4H	0.103	3B
AESO/TAIC10	HB	0.693	0B
AESO/TAIC20	HB	0.634	0B
AESO/TAIC30	H	0.657	1B
AESO/TAIC40	H	0.602	1B
AESO/TAIC50	H	0.703	2B

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4 Conclusions

A bio-based tetrafunctional crosslink agent GACA was successfully synthesized from gallic acid. GACA showed more functional groups and better co-photopolymerization with AESO relative to TAIC. As a result, GACA crosslinked AESO networks showed higher gel content, crosslink density, tensile strength and modulus as well as much better coating properties than TAIC crosslinked AESO networks. GACA has already shown great potential to act as alternative to the petroleum-based crosslink agents, such as TAIC; and high-performance soybean oil-based UV coatings with high biobased content could be gotten after introducing GACA.

Acknowledgements

The authors acknowledge financial support from Project 51203176 supported by the National Natural Science Foundation of China, the National Basic Research Program of China (973 Program, 2010CB631100), the China Postdoctoral Science Foundation funded project (2013M540504), the Postdoctoral Science Foundation of Zhejiang province (Bsh1201011) and the Director Funds of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Y20224QF06).

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