One-Step Synthesis of Novel Renewable Vegetable Oil-Based Acrylate Prepolymers and Their Application in UV-Curable Coatings

With the rapid development of social economy, problems such as volatile organic compound (VOC) pollution and the excessive consumption of global petroleum resources have become increasingly prominent. People are beginning to realize that these problems not only affect the ecological environment, but also hinder the development of the organic polymer material industry based on raw fossil materials. Therefore, the modification and application of bio-based materials are of theoretical and practical significance. In this study, a series of vegetable oil-based acrylate prepolymers were synthesized by one-step acrylation using palm oil, olive oil, peanut oil, rapeseed oil, corn oil, canola oil, and grapeseed oil as raw materials, and the effect of different double bond contents on the product structure and grafting rate was investigated. Furthermore, the as-prepared vegetable oil-based acrylate prepolymers, polyurethane acrylate (PUA-2665), trimethylolpropane triacrylate (TMPTA), and photoinitiator (PI-1173) were mixed thoroughly to prepare ultraviolet (UV)-curable films. The effect of different grafting numbers on the properties of these films was investigated. The results showed that as the degree of unsaturation increased, the acrylate grafting number and the cross-linking density increased, although the acrylation (grafting reaction) rate decreased. The reason was mainly because increasing the double bond content could accelerate the reaction rate, while the grafted acrylic groups had a steric hindrance effect to prevent the adjacent double bonds from participating in the reaction. Furthermore, the increase in grafting number brought about the increase in the structural functionality of prepolymers and the cross-linking density of cured films, which led to the enhancement in the thermal (glass transition temperature) and mechanical (tensile strength, Young’s modulus) properties of the cured films.


Introduction
According to different curing temperatures, traditional curing methods can be divided into room temperature curing and heat curing. Curing coatings by heating at high temperature is often time-consuming, energy-intensive, and not suitable for heat-sensitive substrates [1]. Compared with traditional thermosetting coatings, ultraviolet (UV)-curable coatings have developed rapidly due to their advantages of the "5Es" (Efficient, Environmentally friendly, Energy saving, Enabling, and Economical) [2,3]. The above advantages of UV-curable coatings can be specifically explained as Shandong Luhua Marketing Co. Ltd. (Yantai, China). Palm oil (PaO, chemical pure) was purchased from Guangzhou Duode Chemical Co. Ltd. (Guangzhou, China). Olive oil (OO, analytical pure) and corn oil (CoO, analytical pure) was purchased from Shanghai Maclean Biochemical Technology Co. Ltd. (Shanghai, China). Acrylic acid (AA, analytical pure), n-hexane (analytical pure), sodium bicarbonate (analytical pure), and anhydrous magnesium sulfate (analytical pure) were obtained from Tianjin Fuchen Chemical Reagent Co. Ltd. (Tianjin, China). Boron trifluoride diethyl ether solution (BF 3 , 46.5%) was bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Polyurethane acrylate (PUA-2665, chemical pure, with a molecular weight of 3000 g·mol −1 and a viscosity of 1000 mPa·s) was supplied by Zhaoqing Power Dream Chemical Co. Ltd. (Zhaoqing, China) [32]. Trimethylolpropane triacrylate (TMPTA, chemical pure) and 2-hydroxy-2-methylpropiophenone (photoinitiator, denoted as PI-1173, chemical pure) were received from BASF (China) Co. Ltd. (Shanghai, China). The chemical structures of PI-1173, TMPTA, and PUA-2665 employed in this work are shown in Scheme 1. All of the above chemical reagents were used without further treatment.

Synthesis of Vegetable Oil-Based Acrylate Prepolymers
As shown in Scheme 2, certain amounts of vegetable oils, acrylic acid, and boron trifluoride ether solution wereaddedin a 500 mL single-necked flask.The molar ratio of the carbon-carbon double bond to acrylic acid was1:4, and the molar ratio of the carbon-carbon double bond to boron trifluoride ether was 1:0.6. The mixture was reacted at 80 °C under stirring for 2h. Afterward, the mixture was poured into a separating funnel and extracted with n-hexane. A saturated aqueous solution of sodium bicarbonate was added to remove the unreacted acrylic acid and boron trifluoride ether solution. The cessationof bubbles indicated that the excess reactants had been washed away. The upper orange-yellow organic phase was transferred to a beaker,andan appropriate amount of anhydrous magnesium sulfate was added to dry overnight and then filtered. Finally, the solvent was removed by vacuum distillation and driedto obtain a yellow liquid vegetable oil-based acrylate prepolymer.Vegetable oil-based acrylate prepolymers prepared from palm oil (PaO), olive oil (OO), peanut oil (PeO), rapeseed oil (RSO), corn oil (CoO), canola oil (CaO), and grapeseed oil (GSO) weredenoted as APaO, AOO, APeO, ARSO, ACoO, ACaO, and AGSO, respectively.

Synthesis of Vegetable Oil-Based Acrylate Prepolymers
As shown in Scheme 2, certain amounts of vegetable oils, acrylic acid, and boron trifluoride ether solution were added in a 500 mL single-necked flask. The molar ratio of the carbon-carbon double bond to acrylic acid was1:4, and the molar ratio of the carbon-carbon double bond to boron trifluoride ether was 1:0.6. The mixture was reacted at 80 • C under stirring for 2 h. Afterward, the mixture was poured into a separating funnel and extracted with n-hexane. A saturated aqueous solution of sodium bicarbonate was added to remove the unreacted acrylic acid and boron trifluoride ether solution. The cessation of bubbles indicated that the excess reactants had been washed away. The upper orange-yellow organic phase was transferred to a beaker, and an appropriate amount of anhydrous magnesium sulfate was added to dry overnight and then filtered. Finally, the solvent was removed by vacuum distillation and dried to obtain a yellow liquid vegetable oil-based acrylate prepolymer. Vegetable oil-based acrylate prepolymers prepared from palm oil (PaO), olive oil (OO), peanut oil (PeO), rapeseed oil (RSO), corn oil (CoO), canola oil (CaO), and grapeseed oil (GSO) were denoted as APaO, AOO, APeO, ARSO, ACoO, ACaO, and AGSO, respectively.

Preparation of UV-Curable Films
Different vegetable oil-based acrylate prepolymers obtained above were mixed with polyurethane acrylate (PUA-2665), trimethylol propanetriacrylate (TMPTA), and photoinitiator (PI-1173). The mass fraction of vegetable oil-based acrylate prepolymerswas 50 %, the mass fraction of TMPTA was 20 %, the mass fraction of PUA-2665 was 27 %, and the mass fraction of PI-1173 was 3 %. The mixtures were poured on the surface of tinplates, and the wet film samples were prepared by using a 500 μm wet film coater. Thereafter, the wet films were placed under a 365 nm UV-LED light (the radiation intensity wasabout 378.3 mW·cm −2 ) and irradiated for 10 s to obtainUV-cured films.

Characterization
The functional groups of the samples were analyzed using Thermo-Nicolet iS10 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The scanning measurement range of Fourier transform infrared (FT-IR) spectra was 4000 to 400 cm −1 .
The proton nuclear magnetic resonance ( 1 H-NMR) characterization of the sampleswas performed using a Bruker AV600spectrometer (Bruker Corporation, Billerica, MA, USA). Tetramethylsilane (TMS) was used as the internal standard, and deuterated chloroform (CDCl3) was used as the solvent to analyze the molecular structure of the samples.
A DMA 242E dynamic thermo-mechanical analyzer (Erich NETZSCH GmbH & Co. Holding KG, Selb, Bavaria, Germany) was used to analyze the dynamic thermo-mechanical properties of the samples. The tensile bracket was selected, the samplesize was 20.0mm (length)×6.0mm (width)×0.5mm (thickness), and the oscillation frequency was 1 Hz. Sampleswere first cooled to −80 °C with liquid nitrogen for 3 min, then the temperature was increased to 180 °C at a rate of 5 °C·min −1 . Glass transition temperature (Tg)of the cured filmswas obtained from the peak of the tanδ curves.
Mechanical properties of the samples were studied employing a Shimadzu AGS-X 1 kN universal testing machine (ShimadzuCorporation, Kyoto Prefecture, Kyoto, Japan). Tensile bracket was selected,the size of the sampleswas 40.0 mm (length)×10.0 mm (width)×0.5 mm (thickness), and the crosshead speed was 10 mm·min −1 . For accuracy, each sample was measured three times.
The NetzschSTA 449C thermo-gravimetric analyzer (Erich NETZSCH GmbH & Co. Holding KG, Selb, Bavaria, Germany) was used to investigate the thermal stability of the samples. Under the protection of nitrogen at a flow rate of 60 mL·min −1 , the temperature of the samples increased from 35to 650 °C at a rate of 10 °C·min −1 .

Structure Analysis of Vegetable Oil-Based Acrylate Prepolymers
Scheme 2. Synthetic route to vegetable oil-based acrylate prepolymers.

Preparation of UV-Curable Films
Different vegetable oil-based acrylate prepolymers obtained above were mixed with polyurethane acrylate (PUA-2665), trimethylol propanetriacrylate (TMPTA), and photoinitiator (PI-1173). The mass fraction of vegetable oil-based acrylate prepolymers was 50%, the mass fraction of TMPTA was 20%, the mass fraction of PUA-2665 was 27%, and the mass fraction of PI-1173 was 3%. The mixtures were poured on the surface of tinplates, and the wet film samples were prepared by using a 500 µm wet film coater. Thereafter, the wet films were placed under a 365 nm UV-LED light (the radiation intensity was about 378.3 mW·cm −2 ) and irradiated for 10 s to obtain UV-cured films.

Characterization
The functional groups of the samples were analyzed using Thermo-Nicolet iS10 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The scanning measurement range of Fourier transform infrared (FT-IR) spectra was 4000 to 400 cm −1 .
The proton nuclear magnetic resonance ( 1 H-NMR) characterization of the samples was performed using a Bruker AV600spectrometer (Bruker Corporation, Billerica, MA, USA). Tetramethylsilane (TMS) was used as the internal standard, and deuterated chloroform (CDCl 3 ) was used as the solvent to analyze the molecular structure of the samples.
A DMA 242E dynamic thermo-mechanical analyzer (Erich NETZSCH GmbH & Co. Holding KG, Selb, Bavaria, Germany) was used to analyze the dynamic thermo-mechanical properties of the samples. The tensile bracket was selected, the sample size was 20.0 mm (length) × 6.0 mm (width) × 0.5 mm (thickness), and the oscillation frequency was 1 Hz. Samples were first cooled to −80 • C with liquid nitrogen for 3 min, then the temperature was increased to 180 • C at a rate of 5 • C·min −1 . Glass transition temperature (T g ) of the cured films was obtained from the peak of the tanδ curves.
Mechanical properties of the samples were studied employing a Shimadzu AGS-X 1 kN universal testing machine (Shimadzu Corporation, Kyoto Prefecture, Kyoto, Japan). Tensile bracket was selected, the size of the samples was 40.0 mm (length) × 10.0 mm (width) × 0.5 mm (thickness), and the crosshead speed was 10 mm·min −1 . For accuracy, each sample was measured three times.
The Netzsch STA 449C thermo-gravimetric analyzer (Erich NETZSCH GmbH & Co. Holding KG, Selb, Bavaria, Germany) was used to investigate the thermal stability of the samples. Under the protection of nitrogen at a flow rate of 60 mL·min −1 , the temperature of the samples increased from 35 to 650 • C at a rate of 10 • C·min −1 .

Structure Analysis of Vegetable Oil-Based Acrylate Prepolymers
As the FT-IRand 1 H NMR spectra of all vegetable oil-based acrylate prepolymers were similar, corn oil (CoO) and corn oil-based acrylate prepolymer (ACoO) were selected as representative examples. The FT-IR spectra of CoO and ACoO are shown in Figure 1. In the spectrum of CoO, the peak at 3010 cm −1 corresponded to the C−H stretching vibration of C = C − H on the fat chain [33]. The peak at 1747 cm −1 corresponded to the carbonyl groups on glyceride, and the peak at 1655 cm −1 indicated the presence of unsaturated carbon-carbon double bonds in corn oil. From the spectrum of ACoO, it was revealed that the peak at 3010 cm −1 disappeared after acrylation, indicating that unsaturated double bonds participated in the reaction. At the same time, a new peak appeared at 1724 cm −1 , corresponding to the C = O stretching vibration in acrylate groups after grafting. The peaks at 1637 and 1619 cm −1 corresponded to the C = C stretching vibration in acrylate groups, which were much stronger than the peak at 1655 cm −1 , corresponding to the unsaturated double bonds in CoO [34]. The peak at 1406 cm −1 corresponded to the CH 2 vibration of CH 2 = C in the acrylate groups [24,34]. The peaks at 1296 and 1272 cm −1 corresponded to the CH bending vibration of CH= in the acrylate groups. The peaks at 984 and 966 cm −1 corresponded to the in-plane rocking vibration of CH 2 = in the acrylate groups. The above analysis showed that the corn oil-based acrylate prepolymer (ACoO) was successfully synthesized by the grafting of acrylate to the unsaturated double bonds of corn oil.
Polymers 2020, 12, x FOR PEER REVIEW 6 of 13 As the FT-IRand 1 H NMR spectra of all vegetable oil-based acrylate prepolymers were similar, corn oil (CoO) and corn oil-based acrylate prepolymer (ACoO) were selected as representative examples.
The FT-IR spectra of CoO and ACoO are shown in Figure 1. In the spectrum of CoO, the peak at 3010 cm −1 corresponded to the C−H stretching vibration of C=C−H on the fat chain [33]. The peak at 1747 cm −1 corresponded to the carbonyl groups on glyceride, and the peak at 1655 cm −1 indicated the presence of unsaturated carbon-carbon double bonds in corn oil. Fromthe spectrum of ACoO, it was revealed that the peak at 3010 cm −1 disappearedafter acrylation, indicating that unsaturated double bonds participated in the reaction. At the same time, a new peak appeared at 1724 cm −1 , corresponding tothe C=O stretching vibration in acrylate groups after grafting. The peaks at 1637 and 1619 cm −1 corresponded to the C=C stretching vibration in acrylate groups, which were much stronger than the peak at 1655 cm −1 , corresponding to the unsaturated double bonds in CoO [34]. The peak at 1406 cm −1 corresponded to the CH2vibration of CH2=C in the acrylate groups [24,34]. The peaks at 1296 and 1272 cm −1 corresponded to the CH bending vibration of CH= in the acrylate groups. The peaks at 984 and 966 cm −1 corresponded to the in-plane rocking vibration of CH2= in the acrylate groups. The above analysis showedthat the corn oil-based acrylate prepolymer (ACoO) was successfully synthesizedby the grafting of acrylate to the unsaturated double bonds of corn oil. The proton nuclear magnetic resonance ( 1 H NMR)spectra of CoO and ACoOareshown in Figure 2. In the spectrum of CoO, the peaks at Hd (5.30-5.50 ppm) and He (5.25-5.30 ppm) corresponded to the protonsbound to the unsaturated double bonds and the methine proton of the triglyceride, respectively [22], although these two peaks overlapped slightly. The peak at Hg (4.05-4.35 ppm) corresponded to the methylene protonsof triglyceride [22]. The peak at Hh (2.70-2.75 ppm) corresponded to the methylene proton which sandwiched between two double bonds [35]. The peak at Hi (2.20-2.40 ppm) corresponded to the methylene protonsadjacent to carbonyl groups, and it was selected as internal standard due to its good stability in the reaction and its obvious appearance in 1 H NMR spectrum [22]. The peak at Hj (1.90-2.10 ppm) corresponded to the methylene protonsadjacentto the double bonds [35]. Through normalization,it was found that the peak area ratio of Hito Hgwas 6:4, indicating that CoO contained a triglyceride structure. The double bond content in CoO could be determined by comparing the peak areas of Hd and Hi. However, since there was a certain overlap between the peaks of Hd and He, the peak area of Hd could be determined by subtracting the peak area of He from the total peak area. Due to its triglyceride structure, the peak area ratio of Heto Hgwas 1:4. The following formulacanbe used to calculate the number of double bonds contained in each CoO molecule [30]: The proton nuclear magnetic resonance ( 1 H NMR) spectra of CoO and ACoO are shown in Figure 2. In the spectrum of CoO, the peaks at H d (5.30-5.50 ppm) and H e (5.25-5.30 ppm) corresponded to the protons bound to the unsaturated double bonds and the methine proton of the triglyceride, respectively [22], although these two peaks overlapped slightly. The peak at H g (4.05-4.35 ppm) corresponded to the methylene protons of triglyceride [22]. The peak at H h (2.70-2.75 ppm) corresponded to the methylene proton which sandwiched between two double bonds [35]. The peak at H i (2.20-2.40 ppm) corresponded to the methylene protons adjacent to carbonyl groups, and it was selected as internal standard due to its good stability in the reaction and its obvious appearance in 1 H NMR spectrum [22]. The peak at H j (1.90-2.10 ppm) corresponded to the methylene protons adjacent to the double bonds [35]. Through normalization, it was found that the peak area ratio of H i to H g was 6:4, indicating that CoO contained a triglyceride structure. The double bond content in CoO could be determined by comparing the peak areas of H d and H i . However, since there was a certain overlap between the peaks of H d and H e , the peak area of H d could be determined by subtracting the peak area of H e from the total peak area. Due to its triglyceride structure, the peak area ratio of H e to H g was 1:4. The following formula can be used to calculate the number of double bonds contained in each CoO molecule [30]: where A d,e , A g , and A i represent the peak area of H d,e , H g , and H i , respectively.
whereAa, Ab, Ac, and Airepresent the peak area of Ha, Hb, Hc, and Hi, respectively. The peaks at Ha, Hb, Hc, and Hf became more obvious as the reaction proceeded, and the peaks at Hd, Hh, and Hj did not disappear completely, but became weaker, which revealed that the unsaturated double bonds of CoO did not react completely [38]. The acrylationgrafting rate ofACoOcanbe calculated by the following formula: Table 1 shows the acrylate grafting number and acrylationgrafting rate of different vegetable oil-based acrylate prepolymers. As the number of double bonds in the vegetable oils increased, the grafting number increased, but the grafting rateshowed a downward trend. Even though the acrylate grafting number increased with the increase in double bonds due to the steric hindrance of the already grafted acrylate species, further acylation of the adjacent unsaturated double bonds would be restricted, leading to the lowering of the acylation rate [30].   Each CoO molecule contained 4.36 double bonds. In the 1 H NMR spectrum of ACoO, the peaks at H a (6.30-6.50 ppm), H b (6.10-6.20 ppm), and H c (5.75-5.90 ppm) corresponded to the three double bond protons on the acrylate groups, respectively (Scheme 2) [36]. The ratio of peak areas was about 1:1:1. The peaks at H d , H h , and H j weakened significantly after the reaction. The newly emerging peak at H f (4.90-5.00 ppm) corresponded to the methine proton attached to the acrylate groups [37]. These changes indicated the successful grafting of the acrylate to the unsaturated double bonds of CoO through acrylation to obtain the acrylate prepolymer ACoO. The grafting number can be calculated from the following formula [30]: where A a , A b , A c , and A i represent the peak area of H a , H b , H c , and H i , respectively. The peaks at H a , H b , H c , and H f became more obvious as the reaction proceeded, and the peaks at H d , H h , and H j did not disappear completely, but became weaker, which revealed that the unsaturated double bonds of CoO did not react completely [38]. The acrylation grafting rate of ACoO can be calculated by the following formula:  Table 1 shows the acrylate grafting number and acrylation grafting rate of different vegetable oil-based acrylate prepolymers. As the number of double bonds in the vegetable oils increased, the grafting number increased, but the grafting rate showed a downward trend. Even though the acrylate grafting number increased with the increase in double bonds due to the steric hindrance of the already grafted acrylate species, further acylation of the adjacent unsaturated double bonds would be restricted, leading to the lowering of the acylation rate [30].

Dynamic Mechanical Analysis (DMA)
The glass transition temperature (T g ), storage modulus (E ), and cross-linking density (v e ) of a series of UV-cured films were studied by dynamic mechanical analysis (DMA). Figure 3 shows the trend of the loss factor (tanδ) and storage modulus (E ) of the films with temperature. Table 2 lists the relevant T g , E , and v e data. Generally, the temperature at the peak of the tanδ curve is the T g of the cured film. As shown in Figure 3b, in the low temperature region, all films showed a high E value, and E dropped rapidly with increasing temperature due to the glass transition. In this study, the vegetable oil-based prepolymers with a higher number of acrylate grafting had a higher double bond density, so it was easier to obtain a higher cross-linking density, so their storage modulus was increased. In addition, it can be seen from Figure 3a that all films only had one tanδ peak, indicating that the formulated systems had good compatibility between the different components. In order to further study the thermo-mechanical properties of the films, the cross-linking density (v e ) of the films can be calculated by the following formula [18,36]: where T represents the absolute temperature of the films in the rubber state (T g + 30 • C), E represents the storage modulus of the films at T , and R is the gas constant.

Dynamic Mechanical Analysis (DMA)
The glass transition temperature (Tg), storage modulus (E'), and cross-linking density (ve) of a series of UV-cured films were studied by dynamic mechanical analysis (DMA). Figure 3 shows the trend of the loss factor (tanδ) and storage modulus (E') of the films with temperature. Table 2 lists the relevant Tg, E', and ve data. Generally, the temperature at the peak of the tanδ curve is the Tg of the cured film. As shown in Figure 3b, in the low temperature region, all films showed a high E'value, and E' dropped rapidly with increasing temperature due to the glass transition. In this study, the vegetable oil-based prepolymers with a higher number of acrylate grafting hada higher double bond density,so it was easier to obtain a higher cross-linking density, so their storage modulus was increased. In addition, it can be seen from Figure 3athat all films only had one tanδ peak, indicating that the formulated systems had good compatibility between the different components.In order to further study the thermo-mechanical properties of the films, the cross-linking density (ve) of the films canbe calculated by the following formula [18,36]: where T'represents the absolute temperature of the films in the rubber state (Tg + 30 °C), E' represents the storage modulus of the films at T', and R is the gas constant.    The cross-linking density (v e ) values of different vegetable oil-based cured films are shown in Table 2. It was found that as the grafting number increased, the v e value of these films increased from 2.0 × 10 3 to 13.3 × 10 3 mol·m −3 , and the glass transition temperature T g increased from 30.3 to 50.0 • C, which indicated that the films became harder. This is because the vegetable oil-based prepolymers with a high grafting number had a higher double bond density, and it was easier to obtain a higher cross-linking density.

Thermal Stability
The thermal decomposition curves of different vegetable oil-based cured films are shown in Figure 4. Char yield was measured at 650 • C. Table 3 lists the thermal decomposition temperature of all films at the weight loss of 10% (T 10% ) and 50% (T 50% ). Two degradation stages were demonstrated in the thermal decomposition curves. Under a nitrogen atmosphere, the films were relatively stable between 50 and 150 • C. The first degradation stage occurred between 150 and 350 • C, which was mainly due to the decomposition of the PI-1173 and residual raw materials. The second degradation stage occurred between 340 and 460 • C mainly due to the molecular chain decomposition and carbonization of the cross-linked polymers. In addition, as shown in Table 3, when the grafting number increased, the values of T 10% , T 50% , and char yield also increased, indicating that the vegetable oil-based prepolymers with a higher grafting number had a higher double bond density, and it was easier to obtain a higher cross-linking density, thereby improving the thermal stability of the films [36]. Overall, all cured films had excellent thermal stability, indicating that they could be used at high temperatures.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 13 The cross-linking density (ve) values of different vegetable oil-based cured films are shown in Table 2. It was found that as the grafting number increased, the ve value of these films increased from 2.0×10 3 to 13.3×10 3 mol·m −3 , and the glass transition temperatureTgincreased from 30.3 to 50.0°C, which indicated that the films became harder. This is because the vegetable oil-based prepolymerswith a high grafting number had a higher double bonddensity, and it was easier to obtain a higher cross-linking density.

Thermal Stability
The thermal decomposition curves of different vegetable oil-based cured filmsare shown in Figure 4. Char yield was measured at 650 °C. Table 3 lists the thermal decomposition temperature of all films at the weight lossof 10% (T10%) and 50% (T50%). Two degradationstages were demonstrated in the thermal decomposition curves. Under a nitrogen atmosphere, the filmswere relatively stable between 50 and 150 °C. The first degradation stage occurred between 150 and 350 °C, which was mainly due to the decomposition of the PI-1173 and residual raw materials. The second degradation stageoccurred between 340 and 460 °Cmainly due to the molecular chain decomposition and carbonization of the cross-linked polymers. In addition, as shown inTable 3, when the grafting number increased, the values of T10%, T50%, and char yield also increased, indicating that the vegetable oil-based prepolymers with a higher grafting number had a higher double bond density, and it waseasier to obtain a higher cross-linking density, thereby improving the thermal stability of the films [36]. Overall, all cured films had excellent thermal stability, indicating that they could be used at high temperatures.

Mechanical Properties
The stress-strain curves of different vegetable oil-based cured films are shown in Figure 5. Accordingly, Table 4 presents the data about their tensile strength, elongation at break, and Young's modulus. The stress-strain curves of all cured films showed that the stress continued to increase with the increase of strain, and they broke before reaching the yield point, indicating their rigid characteristics. In addition, as the grafting number increased, the corresponding tensile strength increased from 0.62 to 8.94 MPa, and the Young's modulus increased from 13.93 to 468.07 MPa, but the elongation at break decreased from 5.12 to 1.97%. The changing trend of mechanical properties can be explained by the inherent chemical composition and cross-linking density. In the cured systems, the increase in the grafting number resulted in an increase in the functionality of their structure, which was conducive to the formation of high cross-linking density and helped to increase the rigidity of the cured films, thereby enhancing the mechanical properties of the cured films [39,40]. The stress-strain curves of different vegetable oil-based cured films are shown in Figure 5. Accordingly, Table 4presentsthe data abouttheir tensile strength, elongation at break, and Young's modulus. The stress-strain curves of all cured films showed that the stress continued to increase with the increase of strain, and they broke before reaching the yield point, indicating their rigid characteristics. In addition, as the grafting number increased, the corresponding tensile strength increased from 0.62 to 8.94 MPa, and the Young's modulus increased from 13.93 to 468.07 MPa, but the elongation at break decreased from 5.12to 1.97%. The changing trend of mechanical properties canbe explained by the inherent chemical composition and cross-linking density. In the cured systems, the increase in the grafting number resulted in an increase in the functionality of their structure, which was conducive to the formation of high cross-linking density and helped to increase the rigidity of the cured films, thereby enhancing the mechanical properties of the cured films [39,40].  In addition, the characteristics of different vegetable oil-based UV-cured films were compared. As shownin Table 5, at least two steps were needed to prepare the vegetable oil-based prepolymer in other work, while our work used a one-step method to synthesize the prepolymer, which greatly simplified the synthetic route. Furthermore, these UV-cured films exhibited properties comparable  In addition, the characteristics of different vegetable oil-based UV-cured films were compared. As shown in Table 5, at least two steps were needed to prepare the vegetable oil-based prepolymer in other work, while our work used a one-step method to synthesize the prepolymer, which greatly simplified the synthetic route. Furthermore, these UV-cured films exhibited properties comparable to other films including high T g , 50% thermal decomposition temperature, and tensile strength.

Conclusions
In this study, palm oil, olive oil, peanut oil, rapeseed oil, corn oil, canola oil, and grapeseed oil were used as raw materials to synthesize different vegetable oil-based acrylate prepolymers by one-step acrylation. The as-prepared vegetable oil-based acrylate prepolymers, PUA-2665, TMPTA, and PI-1173 were mixed thoroughly to prepare the UV-curable films. The results revealed that as the number of double bonds in vegetable oil increased, the grafting number corresponding to the products increased from 1.13 to 2.17, and the grafting rate decreased from 66.47 to 47.90%. This was because the increased double bonds could speed up the reaction rate, whereas the branched acrylic groups had steric hindrance effect, preventing adjacent double bonds from participating in the reaction. Furthermore, the increased grafting number led to an increase in their functionality, which increased the cross-linking density from 2.0 × 10 3 to 13.3 × 10 3 mol·m −3 , thereby enhancing the thermal stability and mechanical properties of the cured films. Glass transition temperature increased from 30.3 to 50.0 • C, 50% thermal decomposition temperature increased from 408.9 to 428.3 • C, and tensile strength increased from 0.62 to 8.94 MPa. A series of cured films from soft to tough were prepared by using different vegetable oils, among which grapeseed oil-based UV-cured film had the best thermal (glass transition temperature) and mechanical (tensile strength, Young's modulus) properties. The adjustable properties of vegetable oil-based prepolymers make them a potential candidate to be used in UV-curable coatings. This work summarizes the inherent relationship between the structure of vegetable oils and the properties of UV-cured films, and it provides guidance for the synthesis of vegetable oil-based acrylate prepolymers and their applications in UV-curable coatings.