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Performance of UV curable lignin based epoxy acrylate coatings
Progress in Organic Coatings 116 (2018) 83–89
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Performance of UV curable lignin based epoxy acrylate coatings a
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Ru Yan , Dalei Yang , Niaona Zhang , Qi Zhao , Baijun Liu , Wei Xiang , Zhaoyan Sun , Rui Xu , ⁎ Mingyao Zhanga, Wei Hua, a b c
College of Chemical Engineering, Changchun University of Technology, 2055 Yan’an Street, Changchun 130012, PR China College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Epoxy acrylate UV-curable Coatings
In this work, lignin based epoxy acrylate (LBEA) oligomer with various percentage of lignin was in situ synthesized with epoxy and acrylic acid. The first step was the etherification between lignin and epoxy to produce lignin based epoxy (LBE), and the second step was the esterification between LBE and acrylic acid to synthesize LBEA. Epoxy value and acid value were measured to determine the reaction conditions of the two steps, respectively. The structure of prepared LBEA was confirmed by Flourier infrared spectroscopy (FTIR) characterization. LBEA coatings were prepared through ultraviolet radiation curing method. The gel content, chemical resistance, mechanical properties and thermal properties of coatings were investigated. It was found that the mechanical properties and chemical resistance were much improved than those of epoxy acrylate (EA) resin with the addition of lignin. Thus, lignin was proved to be a new promising biomaterial which could be applied in biobased EA coatings.
1. Introduction Ultraviolet radiation (UV) curing is recognized as the most effective method to transform liquid oligomer into a three-dimensional crosslinking solid polymeric material and no evaporative volatile solvents applied or produced as compared to the conventional materials [1–3]. UV-curing coating has been popular for decades because of its lower energy consumption, less environmental pollution, lower process costs, excellent film quality, fast reaction rates, solvent-free and high efficiency in production. Due to the foreseeable exhaustion of fossil feedstock and the increasing environmental concerns, the exploitation of biorenewable resources in UV-curable coatings provides a “green + green” solution to the current coating industry [4–8]. Epoxy acrylate (EA) resin is a kind of resin for UV curing coatings because of its excellent performance, such as outstanding adhesion, flexibility, hardness and chemical resistance [9]. Epoxy acrylate is produced by introducing vinyl ester groups and carbon–carbon double bonds into the epoxy resin [4], and can be cured with radical photoinitiators. The application of EA resins is limited in the light of its disadvantages, such as poor light aging resistance. Currently, in order to improve the performance and widen the application of EA resins, a variety of monomers and oligomers have already been explored to modify EA resins. The active epoxide or hydroxyl groups of EA can be used as active sites to react with other functional monomers [5]. ⁎
With the increasing attention to environmental issue, people have begun to pay more attention to the application of natural polymers. Lignin is the second most sufficient natural macromolecule next to cellulose [10,11]. There are plenty of aromatic units in the lignin structure. It is also a kind of macromolecular compounds with high reactivity because of its ample functional groups [12]. Due to its advantages, such as abundant sources, low cost, biodegradability and renewability, lignin has received extensive attention [13]. Diane Schorr [14], José C. del Río [15] and other researchers has analyzed and characterized the structure of lignin in detail. However, the highly complex amorphous three-dimensional structure of lignin in which abundant aromatic rings link together through CeOeC and CeC bonds has not been completely elucidated although the primary structure has been well depicted [16–19]. So far, only small amount of lignin and its derivatives has been applied [20–22]. Chao [23] applied alkali lignin to prepare waterborne UV-curable polyurethane. They took the advantage of the phenol group on lignin to replace the polyol. The results showed that proper dosage of the lignin could change the micro-phase separation structure, and could improve the mechanical properties of the product. Tuan [24] used anhydrides to modify sodium lignosulfonate and applied the modified lignin to react with glycerol diglycidyl ether and ethylene glycol diglycidly ether to synthesize bio-based epoxy resins. Lignin has also been applied to prepare lignin based composite materials [25].
Corresponding author. E-mail address: [email protected] (W. Hu).
https://doi.org/10.1016/j.porgcoat.2017.11.011 Received 24 April 2017; Received in revised form 17 September 2017; Accepted 14 November 2017 0300-9440/ © 2017 Published by Elsevier B.V.
Progress in Organic Coatings 116 (2018) 83–89
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So far, there have been seldom studies to insert lignin into EA to prepare biobased UV-curable coatings, whose chemical resistance and mechanical properties could be much improved. In this paper, unpurified commercial organosolv lignin was in-situ synthesized with epoxy to get lignin based epoxy, and then react further with acrylic acid to produce lignin based epoxy acrylate (LBEA) oligomer, which was UVcured into membrane coatings thereafter. The prepared biobased coatings were characterized in detail, and were proved to be promising in coating industry. 2. Experimental 2.1. Materials The industrial grade organosolv lignin was received from Jinan Yanghai chemical. Its purity was 88% according to the acid-alkali purification method [26]. The industrial grade Epon827 epoxy resin was from the Shell Oil Company. Hydroquinone was from Aladdin Reagent (Shanghai) (99.0%). Dimethylformamide (DMF) and Acrylate (AA) were from Tianjin Guangfu Chemical with the purity of 99.5% and 99.0%, respectively. Pyridine was received from Tianjin Damao Chemical Reagent (99.5%). The industrial grade Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) and Phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (819) was from Nanjing Wali chemical, the industrial grade Isobornyl acrylate (IBOA) and Tripropylene Glycol Diacrylate (TPGDA) was from Nanjing Jiazhong chemical.
Fig. 1. Curing schematic diagram of LBEA.
Before coating, the tinplate substrates were polished with abrasive paper and rinsed with acetone and ethanol, and dried in the oven at 80 °C. The LBEA oligomers were casted on a tinplate with a wiper. The thickness of the coating membrane was about 20 μm. UV chamber (UVLED light curing machine, Shanghai Maixin photoelectricity technology) was used to cure the membrane. The curing time was 60 s and the wavelength range of 300–545 nm was applied. The power of the UV light was 100 W, and the distance between sample and light bulb was 10 cm. Fig. 1 shows the curing process. 2.4. Characterization
2.2. Synthesis of LBEA
The epoxy value of LBE was determined according to Chinese Standard GB/T1677-2008. The epoxy group content of residual epoxy after the reaction was determined by the method of hydrochloric acidacetone method. The acid value of the reaction system to synthesize LBEA was determined according to Chinese Standard GB2895-82. The measurement method was KOH-ethanol solution titration method. The FTIR spectra were recorded using NEXUS-670 (Nicolet, USA) equipped with an attenuated total reflectance (ATR) accessory. The resolution of the spectra recorded was 4 cm−1. 256 scans were performed for each sample in the range of 4000–400 cm−1 at room temperature. Gel content was determined by the following equation.
The synthesis of LBEA was performed through two steps. In the first step, LBE was prepared by etherification reaction between lignin and epoxy with different percentages (0, 5, 10, 15, 20 and 25%, w/w) of lignin in the presence of DMF at the temperature of 80 °C ∼ 100 °C. The reaction was carried out in 3-necked flask reaction kettle equipped with mechanical stirrer and water condenser for 1 ∼ 4 h to obtain the LBE. The reaction process of the first step is shown in Table 1. The reaction route is shown in Scheme 1. In the second step, LBEA was prepared by esterification reaction between LBE and acrylate with the presence of 0.2% hydroquinone and 0.2% pyridine at 80 °C. The required amount of acrylate was dropped in during a period of 30 min with stirring. The reaction was carried out for 0.5 ∼ 1.5 h to obtain the LBEA, and the reaction time is shown in Table 1. The reaction route is shown in Scheme 2.
Gel content (%) = (m2/m1) × 100% where m1 is the weight of the cured film sample; m2 is the residual weight of the cured film. The cured film was immersed in a beaker filled with acetone, and was kept for 48 h. The film was then dried at 60 °C until its weight was constant to get the weight m2. Thermos gravimetric analysis (TGA) was performed on a TGA/DSC1 STAR e System over the temperature range of 50–600 °C under the nitrogen atmosphere with a heating rate of 10 °C/min. The weight of samples were 5–10 mg. The pencil hardness was measured using a Pencil coating hardness tester (Shanghai PuShen chemical machinery, China) according to the Chinese Standard GB/T 6379-1996. The film flexibility was measured according to the Chinese Standard GB/T 1731-93 with the coating film iron panels. The adhesion was measured according to the Chinese Standard GB/T 9286-1998 using the lattice notch method (Shanghai PuShen chemical machinery, China). The chemical resistance of UV curing coatings was studied by immersing the cured membrane into 5% HCl, 5% NaOH solutions and ethyl alcohol for a period of time, respectively. The appearance of film was observed and recorded. A scanning electron microscope (JSM5600, Japan) was employed to observe the morphology of the lignin powder and the fractured surface
2.3. Curing of LBEA The obtained LBEA were blended with photo initiator 819 (2.5 wt %), TPO (7.5 wt%), defoaming agent (5 wt%), flatting agent (5 wt%), diluent IBOA (15 wt%), TPGDA (15 wt%) at ambient temperature, and the mixture was stirred for several minutes to ensure a complete homogeneous mixing. Table 1 Reaction conditions to prepare LBE and LBEA with different lignin content. Sample
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
LBE
LBEA
Reaction time at 80 °C (min)
Reaction time at 100 °C (min)
Reaction time at 80 °C (min)
— 120 100 100 100 60
— 125 75 70 — —
355 90 70 55 40 35
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reaction time for the synthesis of LBE with different lignin content. It is clear that epoxy value of the reaction system decreased with reaction time. It is shown in Table 1, there were two reaction stages for the reaction between lignin and epoxy varied with the lignin content in the range of 5%–15%. The reaction temperature for the first stage was 80 °C, which seemed to be an activation process. For different lignin ratio, the epoxy value decreased with the reaction time with different slope tendency. The lower lignin ratio, the smaller slope of the declining tendency was, as shown in Fig. 3, which meant the longer activation process of the reaction between lignin and epoxy was. When the lignin ratio was 5%, the epoxy value of the second reaction stage at 100 °C still declined slowly, and the turning point happened at about 210 min. When temperature increased to 100 °C, the higher energy made the reaction faster and epoxy value decline more quickly. When lignin content was 20%–25%, the reaction rate was very fast, and the epoxy value decreased quickly even at 80 °C. The whole reaction time was only 60 min for the reaction of LBE with 25% lignin. This was because the hydroxyl group amount in the system was increased with the increase of lignin content, the collision probability between the epoxy groups and hydroxyl groups was increased, and thus the reaction was accelerated. In the second reaction step, LBE reacted with acrylic acid to produce LBEA. The acid value in the reaction system could illustrate the residual acid of the reaction. The smaller acid value, the higher reaction extent was, and the sticker solution was obtained. In our case, the reaction was determined to be finished when the viscosity of the system became constant. Fig. 4 shows the scattering curves of acid value as a function of reaction time for the synthesis of LBEA with different lignin content. For the case of the epoxy reacted with the acrylic acid, the reaction time was as long as six hours, and the final acid value was 102. As the content of lignin increased, ring opening reaction between the epoxide group on LBE and the acrylic acid was accelerated, and the reaction time was shortened. When lignin content was 25%, the reaction time was only 35 min, and the final acid value was 113. The viscosity of LBE system was larger than epoxy. The reaction speed of LBE with acrylate acid became faster than that of the reaction between epoxy and acrylate because of the higher collision probability.
Fig. 2. The SEM image for organosolv lignin.
of coating films. The sample was fractured in liquid nitrogen, and fresh cross-sectional cryogenic fractures of the composite membranes were vacuum sputtered with a thin layer of Au prior to SEM examination. All the samples should be dried at 50 °C for 12 h before tests. 3. Results and discussion 3.1. Morphology of lignin Fig. 2 shows the morphology of lignin. It is clear that organosolv lignin applied in this study was with irregular shape. Its size was not uniform and was between several to dozens of micrometers. The coarse surface of organosolv lignin possessed big specific surface area. 3.2. Reaction conditions Epoxy value can reflect the reaction degree between the lignin and epoxy, according to which the optimum reaction time could be determined. It also can provide the necessary acrylic acid dosage for the next reaction step. In the light that the structure of lignin is complexed with three dimensional linking, the aliphatic hydroxyl and phenol hydroxyl group exist randomly in the lignin structure [13,27]. The hydroxyl groups at the different position of lignin could react with the epoxide group of epoxy. Such reaction can randomly happen on the same lignin, which finally makes the lignin be a crosslinking point. Thus, the crosslinking reaction can happen if the reaction extent is not controlled properly. In our case, the reaction was tried to stop just before the crosslinking reaction happened, and the final reaction time was determined according to the result as shown in Fig. 3. Fig. 3 shows the scattering curves of epoxy value as a function of
3.3. FTIR spectroscopy The successful synthesis of LBE and LBEA was confirmed using FTIR spectroscopy. FTIR spectra of lignin, epoxy resin, LBE and LBEA with different content of lignin are shown in Fig. 5. FTIR spectra of lignin presented a broad absorption band centered at 3245 cm−1 that can be attributed to the phenolic and aliphatic OeH groups. Similarly, the
Fig. 3. The scattering curves of epoxy value as a function of reaction time for the synthesis of LBE with different lignin content.
Fig. 4. The scattering of acid value with the reaction time to synthesize LBEA with different lignin content.
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increased C]C bond concentration in epoxy acrylate oligomers. The amount of the C]C double bonds had an effect on the degree of crosslinking [4,7,28]. In our case, the content of reacted epoxy functional group decreased with the increase of lignin content in the first reaction step to produce LBE. Acrylic acid amount applied in the second step thus decreased, which resulted in a decreased gel fraction. In addition, the impurity content of the lignin increased with the lignin content. Therefore, the gel content of UV-cured LBEA films gradually reduced with the increase of lignin content.
3.5. Mechanical properties Hardness, adhesion and flexibility are usually used to evaluate the property of the UV-cured coating. Table 2 shows the properties of cured EA and LBEA coatings depending on the lignin content. As shown in Table 2, the hardness of EA coating was HB, and the LBEA coatings always had higher hardness than EA coating. LBEA with 5% and 10% lignin showed the maximum hardness of 3H. The lignin presents the three dimension structure with plenty of phenyl groups, which should be helpful to the hardness of LBEA. When the lignin content was 15%–25%, the hardness of LBEA coatings decreased from 2H to H. Cross-linking density of the network are the vital factors for the hardness. The hardness decreased with the cross-linking density of the system [29]. As shown in Table 2, the acrylic acid amount applied in LBEA decreased with the increase of lignin content. Thereafter, the cross-linking density decreased due to the decreasing C]C content. Furthermore, the amount of impurity in the lignin also increased with the lignin content, which also affected the hardness of resin. The adhesion of coatings is related to the hydroxyl content in the system. This is because the hydroxyl groups on molecular chain can improve the interaction and adhesion between films and the substrate [30]. It can be seen from Table 2, suitable addition content of lignin to EA resin can significantly increase the adhesion of the coatings. This was because the hydroxyl groups on the lignin could react with the epoxide group of epoxy to produce secondary hydroxyl group. Thus, hydroxyl content of LBEA was increased, and the adhesion was improved. When lignin content was 15%–25%, the adhesion was decreased with the increase of lignin content. Secondary hydroxyl group content in LBEA oligomer increased with the lignin content, leading to the formation of inter-molecule hydrogen bond as well as the weaker interaction force between LBEA and substrate. When lignin content was 5% ∼ 25%, the flexibility of film was 4 ∼ 8 mm, which was much better than that of EA (12 mm). Especially, the flexibility of the LBEA-5 film with 5% of lignin could be 4 mm. Lignin contained a large number of bulky benzene ring, methyl groups and a small amount of aliphatic side chain. The aliphatic side chain in the structure of lignin could help to improve the flexibility of coatings [31]. Thus, moderate amount of lignin can improve the flexibility of membranes. However, when the lignin content was increased to be higher than 5%, the content of benzene ring and methyl groups was also increased, which would lead to the decreasing flexibility [3,32].
Fig. 5. FTIR curves of lignin, epoxy, LBE and LBEA.
peaks corresponding to the CeH stretching vibrations in methyl and methylene groups was observed in the range of 3050–2800 cm−1. Aromatic skeletal vibrations in lignin were typically found at 1606 cm−1 and 1510 cm−1. Syringyl units was demonstrated by the presence of a peak at 1121 cm−1. Similarly, guaiacyl units was demonstrated by the presence of a peak at 1034 cm−1 [17,22]. Thus, organosolv lignin contained both syringyl and guaiacyl structure. In the FTIR spectra of epoxy and LBE, it is clear that the band of epoxide group existed at 913 cm−1 [5]. The intensity of the characteristic absorption peak of epoxy was more intensive than that of LBE because one epoxide group of epoxy reacted with lignin to produce secondary hydroxyl group. Thus, the residual epoxide group content decreased with the increase of lignin content. The broad band near 1100 cm−1 showed the presence of secondary hydroxyl group on LBE spectra [22], which proved the first reaction step did happen as shown in Scheme 1, and LBE was prepared successfully by etherification reaction between lignin and epoxy. In the second step, LBEA was synthesized according to Scheme 2. The residual epoxide groups in the system reacted with acrylic acid. It is shown in Fig. 5, the characteristic peak at 913 cm−1 of epoxide group of LBEA with different lignin content disappeared completely. The new peak at 1730 cm−1 appeared, which was the characteristic peak of carbonyl on the ester group formed during the acylation of LBE [2]. This proved LBEA was synthesized successfully. 3.4. Gel content Table 2 shows the gel content of UV-cured LBEA films with different lignin content. The results show that the gel content gradually reduced with the increase of the lignin content. Young-Jun Park [4] reported that the gel fraction gradually increased with the concentration of acrylic acid applied for the end capping of epoxy resin because of the
Scheme 1. Synthesis reaction of LBE.
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Scheme 2. Synthesis reaction of LBEA.
Table 2 The data of gel fraction, hardness, adhesion and flexibility of UV-cured films. Sample
Gel fraction/%
Hardness
Adhesion
Flexibility (mm)
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
98.7 96.4 94.3 90.6 87.5 83.2
HB 3H 3H 2H 2H H
5 1 1 2 3 4
12 4 5 6 6 8
3.6. Chemical resistance The results of chemical resistance of EA and LBEA to H2SO4, NaOH and ethyl alcohol are shown in Table 3. It is apparent from the data that the chemical resistance of LBEA enhanced with the increasing lignin content. LBEA with 25% lignin content could be unchanged more than 10 days to the three chemicals, while EA could only remain unchanged within 2 days to H2SO4 and NaOH, 6 days to ethyl alcohol, respectively. On the one hand, the benzene ring amount increased with lignin content, the structure of films was compact, which can help to improve solvent resistance of films [33]. On the other hand, lignin contains some inorganic impurities [34]. The content of inorganic impurities increased with lignin content, which can also help to improve solvent resistance of films [35]. 3.7. Thermal stability Thermal gravimetric analysis is one of the most commonly techniques for the evaluation of thermal stability and the decomposition of polymers at various temperatures. The TGA and DTG curves of EA and LBEA films are shown in Fig. 6. Temperature of 5% weight loss (T5), 10% weight loss (T10) and two weight loss temperatures with the maximum degradation speed (Tmax1, Tmax1,) are tabulated in Table 4. The T5 is usually used to evaluate the thermal stability of the material. T5 of EA was 283 °C, which was higher than that of LBEA-5 (214 °C). This was attributed to the decomposition of the lignin segment. The degradation mechanism of lignin at around 200 °C was a dehydration reaction of hydroxyl groups in alkyl groups, and heterolysis and hemolysis dissociation of β-aryl ether bonds [36,37]. However, the crosslinking density also plays an important role
Fig. 6. The thermos gravimetric graph of UV-cured LBEA films a: graph of TGA; b: graph of DTG.
Table 4 The data of thermal properties for UV-cured EA and LBEA films.
Table 3 The data of chemical resistance for UV cured EA and LBEA films. Sample
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
5%H2SO4
5%NaOH
ethyl alcohol
Time of resistance(d)
Time of resistance(d)
Time of resistance(d)
<2 <4 <6 <8 < 10 > 10
<2 <4 <6 <8 > 10 > 10
<6 <8 < 10 > 10 > 10 > 10
Sample
T5 (°C)
T10 (°C)
Tmax1(°C)
Tmax2(°C)
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
283 214 262 273 238 205
337 269 334 331 277 265
292 281 375 372 282 285
416 410 407 407 407 408
on the thermal stability of resin. EA presented a high crosslinking content of 98.7%, which was higher than that of LBEA-5. When lignin content was 10% and 15%, T5 and T10 of the films increased with the lignin content. This was attributed to the less content of hydroxyl group in lignin, which was consumed by the reaction with epoxy. As we know, the degradation mechanism of lignin at around 200 °C was a dehydration reaction of hydroxyl groups in alkyl groups [37]. Furthermore, the 87
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Fig. 7. The SEM photograph of the fractured surface of EA and LBEA film. a: EA; b: LBEA-5; c: LBEA-10; d: LBEA-15; e: LBEA-20; f: LBEA-25.
impurities. Impurities content affects greatly the property of membranes.
increasing content of benzene ring structure of lignin also played a certain role to some extent on the thermal stability of LBEA [38,39]. When lignin content was 20% and 25%, T5 and T10 of LBEA were decreased, which was because the crosslinking density decreased. It is apparent from Fig. 6(b) that the decomposition process contains two stages. The first stage of decomposition located between 230 and 330 °C, which was due to the side-chain breakage of the polymers. The first stage of decomposition temperature was highest with 10% and 15% lignin content. Tmax1 reached around 375 °C. This was because that they had suitable rigid structure and cross-linking density. The result was similar with those of the mechanical properties of membranes. The second stage of decomposition happened at the temperature higher than 400 °C, which was due to the fragmentation of the macromolecules [3].
4. Conclusion In this study, lignin based epoxy acryliate oligomer (LBEA) with different lignin content was in-situ successfully synthesized by two steps: the lignin reacted with epoxy to produce lignin based epoxy (LBE), and LBE reacted with acrylic acid to produce LBEA. The synthesis result was confirmed by FTIR analysis. The prepared LBEA was mixed with additives and then UV-cured to obtain the biobased coatings. The pencil hardness, flexibility, adhesion and chemical resistance of EA coatings were improved with the lignin insertion. Specifically, the hardness of LBEA with 10% lignin increased to 3H, adhesion was 1, and flexibility increased to 5 mm. These much improved properties of LBEA coatings were attributed to the structure of lignin and chemical bonding between lignin and epoxy acrylate. LBEA membranes also possessed good thermal stability according to the TGA analysis. This study provided a method to develop high performance UV-curable LBEA resins. The performance of the prepared membranes proved the lignin to be a kind of promising reinforcing biomaterial for UV-cured coatings.
3.8. Morphology of fractured membrane surface The SEM images of the fractured membrane surface are shown in Fig. 7. The surface of EA was smooth, as shown in Fig. 7(a), which proved its brittleness. The fractured surface of LBEA membranes became coarser, especially when the lignin content was 15%, as shown in Fig. 7(c). This proved the improved toughness of the membranes. In addition, the big particles in the membranes could be observed more clearly with the increased lignin content. Fig. 7(d) shows the obvious impurities with big size in the membranes with 25% lignin, which will also weaken the mechanical properties of the resin. Lignin was extracted from black liquor of paper industries, and it has some
Acknowledgments The authors acknowledge the financial support for this project from the National Natural Science Foundation of China (No. 21404013), the Science and Technology Development Plan of Jilin Province, China (No. 88
Progress in Organic Coatings 116 (2018) 83–89
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20160101323JC and 20170101110JC), Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and the Research Project of Science and Technology of the Education Department of Jilin Province during the 12th Five-year Plan Period (No. 2015-78).
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Performance of UV curable lignin based epoxy acrylate coatings a
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T c
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Ru Yan , Dalei Yang , Niaona Zhang , Qi Zhao , Baijun Liu , Wei Xiang , Zhaoyan Sun , Rui Xu , ⁎ Mingyao Zhanga, Wei Hua, a b c
College of Chemical Engineering, Changchun University of Technology, 2055 Yan’an Street, Changchun 130012, PR China College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Epoxy acrylate UV-curable Coatings
In this work, lignin based epoxy acrylate (LBEA) oligomer with various percentage of lignin was in situ synthesized with epoxy and acrylic acid. The first step was the etherification between lignin and epoxy to produce lignin based epoxy (LBE), and the second step was the esterification between LBE and acrylic acid to synthesize LBEA. Epoxy value and acid value were measured to determine the reaction conditions of the two steps, respectively. The structure of prepared LBEA was confirmed by Flourier infrared spectroscopy (FTIR) characterization. LBEA coatings were prepared through ultraviolet radiation curing method. The gel content, chemical resistance, mechanical properties and thermal properties of coatings were investigated. It was found that the mechanical properties and chemical resistance were much improved than those of epoxy acrylate (EA) resin with the addition of lignin. Thus, lignin was proved to be a new promising biomaterial which could be applied in biobased EA coatings.
1. Introduction Ultraviolet radiation (UV) curing is recognized as the most effective method to transform liquid oligomer into a three-dimensional crosslinking solid polymeric material and no evaporative volatile solvents applied or produced as compared to the conventional materials [1–3]. UV-curing coating has been popular for decades because of its lower energy consumption, less environmental pollution, lower process costs, excellent film quality, fast reaction rates, solvent-free and high efficiency in production. Due to the foreseeable exhaustion of fossil feedstock and the increasing environmental concerns, the exploitation of biorenewable resources in UV-curable coatings provides a “green + green” solution to the current coating industry [4–8]. Epoxy acrylate (EA) resin is a kind of resin for UV curing coatings because of its excellent performance, such as outstanding adhesion, flexibility, hardness and chemical resistance [9]. Epoxy acrylate is produced by introducing vinyl ester groups and carbon–carbon double bonds into the epoxy resin [4], and can be cured with radical photoinitiators. The application of EA resins is limited in the light of its disadvantages, such as poor light aging resistance. Currently, in order to improve the performance and widen the application of EA resins, a variety of monomers and oligomers have already been explored to modify EA resins. The active epoxide or hydroxyl groups of EA can be used as active sites to react with other functional monomers [5]. ⁎
With the increasing attention to environmental issue, people have begun to pay more attention to the application of natural polymers. Lignin is the second most sufficient natural macromolecule next to cellulose [10,11]. There are plenty of aromatic units in the lignin structure. It is also a kind of macromolecular compounds with high reactivity because of its ample functional groups [12]. Due to its advantages, such as abundant sources, low cost, biodegradability and renewability, lignin has received extensive attention [13]. Diane Schorr [14], José C. del Río [15] and other researchers has analyzed and characterized the structure of lignin in detail. However, the highly complex amorphous three-dimensional structure of lignin in which abundant aromatic rings link together through CeOeC and CeC bonds has not been completely elucidated although the primary structure has been well depicted [16–19]. So far, only small amount of lignin and its derivatives has been applied [20–22]. Chao [23] applied alkali lignin to prepare waterborne UV-curable polyurethane. They took the advantage of the phenol group on lignin to replace the polyol. The results showed that proper dosage of the lignin could change the micro-phase separation structure, and could improve the mechanical properties of the product. Tuan [24] used anhydrides to modify sodium lignosulfonate and applied the modified lignin to react with glycerol diglycidyl ether and ethylene glycol diglycidly ether to synthesize bio-based epoxy resins. Lignin has also been applied to prepare lignin based composite materials [25].
Corresponding author. E-mail address: [email protected] (W. Hu).
https://doi.org/10.1016/j.porgcoat.2017.11.011 Received 24 April 2017; Received in revised form 17 September 2017; Accepted 14 November 2017 0300-9440/ © 2017 Published by Elsevier B.V.
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So far, there have been seldom studies to insert lignin into EA to prepare biobased UV-curable coatings, whose chemical resistance and mechanical properties could be much improved. In this paper, unpurified commercial organosolv lignin was in-situ synthesized with epoxy to get lignin based epoxy, and then react further with acrylic acid to produce lignin based epoxy acrylate (LBEA) oligomer, which was UVcured into membrane coatings thereafter. The prepared biobased coatings were characterized in detail, and were proved to be promising in coating industry. 2. Experimental 2.1. Materials The industrial grade organosolv lignin was received from Jinan Yanghai chemical. Its purity was 88% according to the acid-alkali purification method [26]. The industrial grade Epon827 epoxy resin was from the Shell Oil Company. Hydroquinone was from Aladdin Reagent (Shanghai) (99.0%). Dimethylformamide (DMF) and Acrylate (AA) were from Tianjin Guangfu Chemical with the purity of 99.5% and 99.0%, respectively. Pyridine was received from Tianjin Damao Chemical Reagent (99.5%). The industrial grade Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) and Phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (819) was from Nanjing Wali chemical, the industrial grade Isobornyl acrylate (IBOA) and Tripropylene Glycol Diacrylate (TPGDA) was from Nanjing Jiazhong chemical.
Fig. 1. Curing schematic diagram of LBEA.
Before coating, the tinplate substrates were polished with abrasive paper and rinsed with acetone and ethanol, and dried in the oven at 80 °C. The LBEA oligomers were casted on a tinplate with a wiper. The thickness of the coating membrane was about 20 μm. UV chamber (UVLED light curing machine, Shanghai Maixin photoelectricity technology) was used to cure the membrane. The curing time was 60 s and the wavelength range of 300–545 nm was applied. The power of the UV light was 100 W, and the distance between sample and light bulb was 10 cm. Fig. 1 shows the curing process. 2.4. Characterization
2.2. Synthesis of LBEA
The epoxy value of LBE was determined according to Chinese Standard GB/T1677-2008. The epoxy group content of residual epoxy after the reaction was determined by the method of hydrochloric acidacetone method. The acid value of the reaction system to synthesize LBEA was determined according to Chinese Standard GB2895-82. The measurement method was KOH-ethanol solution titration method. The FTIR spectra were recorded using NEXUS-670 (Nicolet, USA) equipped with an attenuated total reflectance (ATR) accessory. The resolution of the spectra recorded was 4 cm−1. 256 scans were performed for each sample in the range of 4000–400 cm−1 at room temperature. Gel content was determined by the following equation.
The synthesis of LBEA was performed through two steps. In the first step, LBE was prepared by etherification reaction between lignin and epoxy with different percentages (0, 5, 10, 15, 20 and 25%, w/w) of lignin in the presence of DMF at the temperature of 80 °C ∼ 100 °C. The reaction was carried out in 3-necked flask reaction kettle equipped with mechanical stirrer and water condenser for 1 ∼ 4 h to obtain the LBE. The reaction process of the first step is shown in Table 1. The reaction route is shown in Scheme 1. In the second step, LBEA was prepared by esterification reaction between LBE and acrylate with the presence of 0.2% hydroquinone and 0.2% pyridine at 80 °C. The required amount of acrylate was dropped in during a period of 30 min with stirring. The reaction was carried out for 0.5 ∼ 1.5 h to obtain the LBEA, and the reaction time is shown in Table 1. The reaction route is shown in Scheme 2.
Gel content (%) = (m2/m1) × 100% where m1 is the weight of the cured film sample; m2 is the residual weight of the cured film. The cured film was immersed in a beaker filled with acetone, and was kept for 48 h. The film was then dried at 60 °C until its weight was constant to get the weight m2. Thermos gravimetric analysis (TGA) was performed on a TGA/DSC1 STAR e System over the temperature range of 50–600 °C under the nitrogen atmosphere with a heating rate of 10 °C/min. The weight of samples were 5–10 mg. The pencil hardness was measured using a Pencil coating hardness tester (Shanghai PuShen chemical machinery, China) according to the Chinese Standard GB/T 6379-1996. The film flexibility was measured according to the Chinese Standard GB/T 1731-93 with the coating film iron panels. The adhesion was measured according to the Chinese Standard GB/T 9286-1998 using the lattice notch method (Shanghai PuShen chemical machinery, China). The chemical resistance of UV curing coatings was studied by immersing the cured membrane into 5% HCl, 5% NaOH solutions and ethyl alcohol for a period of time, respectively. The appearance of film was observed and recorded. A scanning electron microscope (JSM5600, Japan) was employed to observe the morphology of the lignin powder and the fractured surface
2.3. Curing of LBEA The obtained LBEA were blended with photo initiator 819 (2.5 wt %), TPO (7.5 wt%), defoaming agent (5 wt%), flatting agent (5 wt%), diluent IBOA (15 wt%), TPGDA (15 wt%) at ambient temperature, and the mixture was stirred for several minutes to ensure a complete homogeneous mixing. Table 1 Reaction conditions to prepare LBE and LBEA with different lignin content. Sample
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
LBE
LBEA
Reaction time at 80 °C (min)
Reaction time at 100 °C (min)
Reaction time at 80 °C (min)
— 120 100 100 100 60
— 125 75 70 — —
355 90 70 55 40 35
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reaction time for the synthesis of LBE with different lignin content. It is clear that epoxy value of the reaction system decreased with reaction time. It is shown in Table 1, there were two reaction stages for the reaction between lignin and epoxy varied with the lignin content in the range of 5%–15%. The reaction temperature for the first stage was 80 °C, which seemed to be an activation process. For different lignin ratio, the epoxy value decreased with the reaction time with different slope tendency. The lower lignin ratio, the smaller slope of the declining tendency was, as shown in Fig. 3, which meant the longer activation process of the reaction between lignin and epoxy was. When the lignin ratio was 5%, the epoxy value of the second reaction stage at 100 °C still declined slowly, and the turning point happened at about 210 min. When temperature increased to 100 °C, the higher energy made the reaction faster and epoxy value decline more quickly. When lignin content was 20%–25%, the reaction rate was very fast, and the epoxy value decreased quickly even at 80 °C. The whole reaction time was only 60 min for the reaction of LBE with 25% lignin. This was because the hydroxyl group amount in the system was increased with the increase of lignin content, the collision probability between the epoxy groups and hydroxyl groups was increased, and thus the reaction was accelerated. In the second reaction step, LBE reacted with acrylic acid to produce LBEA. The acid value in the reaction system could illustrate the residual acid of the reaction. The smaller acid value, the higher reaction extent was, and the sticker solution was obtained. In our case, the reaction was determined to be finished when the viscosity of the system became constant. Fig. 4 shows the scattering curves of acid value as a function of reaction time for the synthesis of LBEA with different lignin content. For the case of the epoxy reacted with the acrylic acid, the reaction time was as long as six hours, and the final acid value was 102. As the content of lignin increased, ring opening reaction between the epoxide group on LBE and the acrylic acid was accelerated, and the reaction time was shortened. When lignin content was 25%, the reaction time was only 35 min, and the final acid value was 113. The viscosity of LBE system was larger than epoxy. The reaction speed of LBE with acrylate acid became faster than that of the reaction between epoxy and acrylate because of the higher collision probability.
Fig. 2. The SEM image for organosolv lignin.
of coating films. The sample was fractured in liquid nitrogen, and fresh cross-sectional cryogenic fractures of the composite membranes were vacuum sputtered with a thin layer of Au prior to SEM examination. All the samples should be dried at 50 °C for 12 h before tests. 3. Results and discussion 3.1. Morphology of lignin Fig. 2 shows the morphology of lignin. It is clear that organosolv lignin applied in this study was with irregular shape. Its size was not uniform and was between several to dozens of micrometers. The coarse surface of organosolv lignin possessed big specific surface area. 3.2. Reaction conditions Epoxy value can reflect the reaction degree between the lignin and epoxy, according to which the optimum reaction time could be determined. It also can provide the necessary acrylic acid dosage for the next reaction step. In the light that the structure of lignin is complexed with three dimensional linking, the aliphatic hydroxyl and phenol hydroxyl group exist randomly in the lignin structure [13,27]. The hydroxyl groups at the different position of lignin could react with the epoxide group of epoxy. Such reaction can randomly happen on the same lignin, which finally makes the lignin be a crosslinking point. Thus, the crosslinking reaction can happen if the reaction extent is not controlled properly. In our case, the reaction was tried to stop just before the crosslinking reaction happened, and the final reaction time was determined according to the result as shown in Fig. 3. Fig. 3 shows the scattering curves of epoxy value as a function of
3.3. FTIR spectroscopy The successful synthesis of LBE and LBEA was confirmed using FTIR spectroscopy. FTIR spectra of lignin, epoxy resin, LBE and LBEA with different content of lignin are shown in Fig. 5. FTIR spectra of lignin presented a broad absorption band centered at 3245 cm−1 that can be attributed to the phenolic and aliphatic OeH groups. Similarly, the
Fig. 3. The scattering curves of epoxy value as a function of reaction time for the synthesis of LBE with different lignin content.
Fig. 4. The scattering of acid value with the reaction time to synthesize LBEA with different lignin content.
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increased C]C bond concentration in epoxy acrylate oligomers. The amount of the C]C double bonds had an effect on the degree of crosslinking [4,7,28]. In our case, the content of reacted epoxy functional group decreased with the increase of lignin content in the first reaction step to produce LBE. Acrylic acid amount applied in the second step thus decreased, which resulted in a decreased gel fraction. In addition, the impurity content of the lignin increased with the lignin content. Therefore, the gel content of UV-cured LBEA films gradually reduced with the increase of lignin content.
3.5. Mechanical properties Hardness, adhesion and flexibility are usually used to evaluate the property of the UV-cured coating. Table 2 shows the properties of cured EA and LBEA coatings depending on the lignin content. As shown in Table 2, the hardness of EA coating was HB, and the LBEA coatings always had higher hardness than EA coating. LBEA with 5% and 10% lignin showed the maximum hardness of 3H. The lignin presents the three dimension structure with plenty of phenyl groups, which should be helpful to the hardness of LBEA. When the lignin content was 15%–25%, the hardness of LBEA coatings decreased from 2H to H. Cross-linking density of the network are the vital factors for the hardness. The hardness decreased with the cross-linking density of the system [29]. As shown in Table 2, the acrylic acid amount applied in LBEA decreased with the increase of lignin content. Thereafter, the cross-linking density decreased due to the decreasing C]C content. Furthermore, the amount of impurity in the lignin also increased with the lignin content, which also affected the hardness of resin. The adhesion of coatings is related to the hydroxyl content in the system. This is because the hydroxyl groups on molecular chain can improve the interaction and adhesion between films and the substrate [30]. It can be seen from Table 2, suitable addition content of lignin to EA resin can significantly increase the adhesion of the coatings. This was because the hydroxyl groups on the lignin could react with the epoxide group of epoxy to produce secondary hydroxyl group. Thus, hydroxyl content of LBEA was increased, and the adhesion was improved. When lignin content was 15%–25%, the adhesion was decreased with the increase of lignin content. Secondary hydroxyl group content in LBEA oligomer increased with the lignin content, leading to the formation of inter-molecule hydrogen bond as well as the weaker interaction force between LBEA and substrate. When lignin content was 5% ∼ 25%, the flexibility of film was 4 ∼ 8 mm, which was much better than that of EA (12 mm). Especially, the flexibility of the LBEA-5 film with 5% of lignin could be 4 mm. Lignin contained a large number of bulky benzene ring, methyl groups and a small amount of aliphatic side chain. The aliphatic side chain in the structure of lignin could help to improve the flexibility of coatings [31]. Thus, moderate amount of lignin can improve the flexibility of membranes. However, when the lignin content was increased to be higher than 5%, the content of benzene ring and methyl groups was also increased, which would lead to the decreasing flexibility [3,32].
Fig. 5. FTIR curves of lignin, epoxy, LBE and LBEA.
peaks corresponding to the CeH stretching vibrations in methyl and methylene groups was observed in the range of 3050–2800 cm−1. Aromatic skeletal vibrations in lignin were typically found at 1606 cm−1 and 1510 cm−1. Syringyl units was demonstrated by the presence of a peak at 1121 cm−1. Similarly, guaiacyl units was demonstrated by the presence of a peak at 1034 cm−1 [17,22]. Thus, organosolv lignin contained both syringyl and guaiacyl structure. In the FTIR spectra of epoxy and LBE, it is clear that the band of epoxide group existed at 913 cm−1 [5]. The intensity of the characteristic absorption peak of epoxy was more intensive than that of LBE because one epoxide group of epoxy reacted with lignin to produce secondary hydroxyl group. Thus, the residual epoxide group content decreased with the increase of lignin content. The broad band near 1100 cm−1 showed the presence of secondary hydroxyl group on LBE spectra [22], which proved the first reaction step did happen as shown in Scheme 1, and LBE was prepared successfully by etherification reaction between lignin and epoxy. In the second step, LBEA was synthesized according to Scheme 2. The residual epoxide groups in the system reacted with acrylic acid. It is shown in Fig. 5, the characteristic peak at 913 cm−1 of epoxide group of LBEA with different lignin content disappeared completely. The new peak at 1730 cm−1 appeared, which was the characteristic peak of carbonyl on the ester group formed during the acylation of LBE [2]. This proved LBEA was synthesized successfully. 3.4. Gel content Table 2 shows the gel content of UV-cured LBEA films with different lignin content. The results show that the gel content gradually reduced with the increase of the lignin content. Young-Jun Park [4] reported that the gel fraction gradually increased with the concentration of acrylic acid applied for the end capping of epoxy resin because of the
Scheme 1. Synthesis reaction of LBE.
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Scheme 2. Synthesis reaction of LBEA.
Table 2 The data of gel fraction, hardness, adhesion and flexibility of UV-cured films. Sample
Gel fraction/%
Hardness
Adhesion
Flexibility (mm)
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
98.7 96.4 94.3 90.6 87.5 83.2
HB 3H 3H 2H 2H H
5 1 1 2 3 4
12 4 5 6 6 8
3.6. Chemical resistance The results of chemical resistance of EA and LBEA to H2SO4, NaOH and ethyl alcohol are shown in Table 3. It is apparent from the data that the chemical resistance of LBEA enhanced with the increasing lignin content. LBEA with 25% lignin content could be unchanged more than 10 days to the three chemicals, while EA could only remain unchanged within 2 days to H2SO4 and NaOH, 6 days to ethyl alcohol, respectively. On the one hand, the benzene ring amount increased with lignin content, the structure of films was compact, which can help to improve solvent resistance of films [33]. On the other hand, lignin contains some inorganic impurities [34]. The content of inorganic impurities increased with lignin content, which can also help to improve solvent resistance of films [35]. 3.7. Thermal stability Thermal gravimetric analysis is one of the most commonly techniques for the evaluation of thermal stability and the decomposition of polymers at various temperatures. The TGA and DTG curves of EA and LBEA films are shown in Fig. 6. Temperature of 5% weight loss (T5), 10% weight loss (T10) and two weight loss temperatures with the maximum degradation speed (Tmax1, Tmax1,) are tabulated in Table 4. The T5 is usually used to evaluate the thermal stability of the material. T5 of EA was 283 °C, which was higher than that of LBEA-5 (214 °C). This was attributed to the decomposition of the lignin segment. The degradation mechanism of lignin at around 200 °C was a dehydration reaction of hydroxyl groups in alkyl groups, and heterolysis and hemolysis dissociation of β-aryl ether bonds [36,37]. However, the crosslinking density also plays an important role
Fig. 6. The thermos gravimetric graph of UV-cured LBEA films a: graph of TGA; b: graph of DTG.
Table 4 The data of thermal properties for UV-cured EA and LBEA films.
Table 3 The data of chemical resistance for UV cured EA and LBEA films. Sample
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
5%H2SO4
5%NaOH
ethyl alcohol
Time of resistance(d)
Time of resistance(d)
Time of resistance(d)
<2 <4 <6 <8 < 10 > 10
<2 <4 <6 <8 > 10 > 10
<6 <8 < 10 > 10 > 10 > 10
Sample
T5 (°C)
T10 (°C)
Tmax1(°C)
Tmax2(°C)
EA LBEA-5 LBEA-10 LBEA-15 LBEA-20 LBEA-25
283 214 262 273 238 205
337 269 334 331 277 265
292 281 375 372 282 285
416 410 407 407 407 408
on the thermal stability of resin. EA presented a high crosslinking content of 98.7%, which was higher than that of LBEA-5. When lignin content was 10% and 15%, T5 and T10 of the films increased with the lignin content. This was attributed to the less content of hydroxyl group in lignin, which was consumed by the reaction with epoxy. As we know, the degradation mechanism of lignin at around 200 °C was a dehydration reaction of hydroxyl groups in alkyl groups [37]. Furthermore, the 87
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Fig. 7. The SEM photograph of the fractured surface of EA and LBEA film. a: EA; b: LBEA-5; c: LBEA-10; d: LBEA-15; e: LBEA-20; f: LBEA-25.
impurities. Impurities content affects greatly the property of membranes.
increasing content of benzene ring structure of lignin also played a certain role to some extent on the thermal stability of LBEA [38,39]. When lignin content was 20% and 25%, T5 and T10 of LBEA were decreased, which was because the crosslinking density decreased. It is apparent from Fig. 6(b) that the decomposition process contains two stages. The first stage of decomposition located between 230 and 330 °C, which was due to the side-chain breakage of the polymers. The first stage of decomposition temperature was highest with 10% and 15% lignin content. Tmax1 reached around 375 °C. This was because that they had suitable rigid structure and cross-linking density. The result was similar with those of the mechanical properties of membranes. The second stage of decomposition happened at the temperature higher than 400 °C, which was due to the fragmentation of the macromolecules [3].
4. Conclusion In this study, lignin based epoxy acryliate oligomer (LBEA) with different lignin content was in-situ successfully synthesized by two steps: the lignin reacted with epoxy to produce lignin based epoxy (LBE), and LBE reacted with acrylic acid to produce LBEA. The synthesis result was confirmed by FTIR analysis. The prepared LBEA was mixed with additives and then UV-cured to obtain the biobased coatings. The pencil hardness, flexibility, adhesion and chemical resistance of EA coatings were improved with the lignin insertion. Specifically, the hardness of LBEA with 10% lignin increased to 3H, adhesion was 1, and flexibility increased to 5 mm. These much improved properties of LBEA coatings were attributed to the structure of lignin and chemical bonding between lignin and epoxy acrylate. LBEA membranes also possessed good thermal stability according to the TGA analysis. This study provided a method to develop high performance UV-curable LBEA resins. The performance of the prepared membranes proved the lignin to be a kind of promising reinforcing biomaterial for UV-cured coatings.
3.8. Morphology of fractured membrane surface The SEM images of the fractured membrane surface are shown in Fig. 7. The surface of EA was smooth, as shown in Fig. 7(a), which proved its brittleness. The fractured surface of LBEA membranes became coarser, especially when the lignin content was 15%, as shown in Fig. 7(c). This proved the improved toughness of the membranes. In addition, the big particles in the membranes could be observed more clearly with the increased lignin content. Fig. 7(d) shows the obvious impurities with big size in the membranes with 25% lignin, which will also weaken the mechanical properties of the resin. Lignin was extracted from black liquor of paper industries, and it has some
Acknowledgments The authors acknowledge the financial support for this project from the National Natural Science Foundation of China (No. 21404013), the Science and Technology Development Plan of Jilin Province, China (No. 88
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20160101323JC and 20170101110JC), Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and the Research Project of Science and Technology of the Education Department of Jilin Province during the 12th Five-year Plan Period (No. 2015-78).
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