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thermal dual-curable polyurethane acrylate
Journal Pre-proof Preparation and properties of ultraviolet/thermal dual-curable polyurethane acrylate Yongxia Ren, Yunsheng Dong, Ruoning Yang, Youwei Yao PII:
S0143-7496(20)30041-5
DOI:
https://doi.org/10.1016/j.ijadhadh.2020.102580
Reference:
JAAD 102580
To appear in:
International Journal of Adhesion and Adhesives
Received Date: 1 November 2019 Accepted Date: 3 February 2020
Please cite this article as: Ren Y, Dong Y, Yang R, Yao Y, Preparation and properties of ultraviolet/ thermal dual-curable polyurethane acrylate, International Journal of Adhesion and Adhesives, https:// doi.org/10.1016/j.ijadhadh.2020.102580. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Preparation and properties of ultraviolet/thermal dual-curable polyurethane acrylate Yongxia Ren, Yunsheng Dong, Ruoning Yang, Youwei Yao* Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China *Corresponding author Youwei Yao Tel and Fax: +86-0755-26036796 E-mail: [email protected] Postal address: Room 1510, Division of Energy and Environment, Tsinghua Campus, The University Town, Shenzhen 518005, PR China
Abstract Ultraviolet (UV)/thermal dual-curable polyurethane acrylate (PUA) mixtures were prepared using a PUA oligomer, tripropylene glycol diacrylate (TPGDA), benzoyl peroxide (BPO), 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (TPO), and cobalt isooctanoate. For purposes of comparison, a second UV-curable PUA mixture was prepared by mixing the PUA oligomer, TPGDA, and TPO. When placed under a UV lamp (400 W, 50 mW/cm2), the surface drying time of each curable mixture was approximately 2 s. The content of volatiles during UV curing (the intensity of the lamp was 100 mW/cm2) was less than 0.3 wt.%, and the content of potential volatiles (the test condition was heating at 110 °C for 1 h after UV curing) was below 1.0 wt.%. Compared with the UV-cured sample, the UV/thermal dual-cured sample containing cobalt isooctanoate had more favorable properties, that is: (1) better acid and alkali resistance; and (2) better thermal stability and higher gel content. The temperature at 5 wt.% weight loss (T5wt.%) and the gel fraction of the UV-cured sample (the curing condition was 1 min of UV irradiation) were 269.4 °C and 94.6%, respectively. The T5wt.% and the gel fraction of the UV/thermal dual-cured sample (cured at 100 °C for 10 min after 1 min of UV irradiation) were 318.2 °C and 99.8%, respectively. BPO and cobalt isooctanoate had little effect on the surface drying rate of the curable mixtures, and the performance of the cured samples was improved by thermal treatment following irradiation with UV. Considering the dyeing effect of BPO, this system provides a more practical approach to the complete curing of colored UV-curable resins.
Keywords: polyurethane acrylate; UV/thermal dual-curing treatment; content of volatiles; surface drying time
1. Introduction In recent decades, ultraviolet (UV) curing techniques have attracted considerable attention because of the characteristic of less environmental pollution, fast curing rate, low energy consumption [1-3]. Polyurethane acrylate (PUA) oligomers are among the
most important oligomers used in UV-curable coatings, and curable PUA mixtures have been widely used in, for example, leather, printing, and inks because they have excellent properties including resistance to wear, weather, and solvents [4, 5]. Researchers have made various attempts to improve the performance of PUA systems such as by reducing their surface drying time [6-8], increasing their thermal stability [9], and enhancing their flame-retarding properties [10, 11]. However, research into the curing of colored coatings, which are aesthetically valuable for art [12] for example, is relatively rare. Moreover, UV curing systems also have some disadvantages [13-15]. According to Park et al. [13], the curing depth is limited, and shaded portions cannot be cured completely with UV irradiation alone, which restricts the use of such systems in printed circuit boards, ultra-thick coatings, colored coatings, and irregularly shaped devices. Thermal treatment following UV irradiation could eliminate such defects, because thermal curing affects those areas that cannot be irradiated with UV light. Bi et al. [16] prepared a series of dual-curable adhesives. When the proportion of C=C bonds was relatively low, the addition of a thermal curing agent significantly increased the gel fraction. When 20 parts per hundred resin (phr) of a thermal curing agent was added, the gel content reached 100%, regardless of the proportion of C=C bonds. Furthermore, the storage modulus and glass transition temperature of the UV/thermal dual-cured samples were higher than those of a UV-cured sample. Adding a thermal curing agent and a curing accelerator to UV-curable mixtures is a common method of preparing UV/thermal dual-curing systems. The method is simple, and increases the degree of crosslinking, which for example improves the mechanical strength, solvent resistance, and thermal stability of the cured sample [17, 18]. Zhi et al. [19] synthesized three dual-cure PUAs with different contents of double bonds, and compared the properties of the resulting UV/thermal-cured samples with those of a UV-cured sample. They found that the pendulum/pencil hardness, flexibility, and abrasion resistance improved when heat treatment was applied after UV curing. However, the effect of the addition of additives on the curing rate of UV/thermal dual-curable mixtures requires further exploration and analysis. The
specific volatiles content of UV- or thermal/UV-curable mixtures, which must be considered carefully before they are used in actual situations, has also rarely been reported. In the present study, we prepared a UV/thermal dual-curable system based on the UV-curable PUA mixtures reported in our previous study, to which we added a thermal curing agent (benzoyl peroxide (BPO)) and a curing accelerator (cobalt isooctanoate). The UV/thermal dual-curable mixtures are partially cured when exposed to UV light, and completely cured during subsequent heating. The surface drying time of each curable mixture was determined, and the gel content, chemical resistance, thermal stability, and dynamic mechanical thermal analysis (DMTA) data were obtained for each cured sample.
2. Material and methods 2.1 Materials Benzoyl peroxide (BPO) were purchased from Aladdin Reagent (China). 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide (TPO) and tripropylene glycol diacrylate (TPGDA) were purchased from Energy Chemical (China). All of the above reagents were of analytical reagent grade. Cobalt isooctanoate with 10 wt.% of cobalt were supplied by the Shanghai Yongyan Chemicals Co., Ltd. 2.2 Synthesis of the PUA oligomer The PUA oligomer was synthesized in our previous report [11, 20]. And the synthesis process is shown in Fig. 1.
Fig.1. The synthesis scheme of PUA oligomer [11, 20]. 2.3 Preparation of UV/thermal dual-curable mixtures The oligomer (70 wt.%), TPGDA (30 wt.%) and TPO (5 wt.%) were mixed and stirred with a mechanical stirrer for 4 h, then the sample 3 was obtained. When mixing sample 3 with 1 wt.% of BPO and 0.5 wt.% of cobalt isooctanoate and stirring the mixture until the mixture was transparent, the sample 1 will be obtained. When only blending 1 wt.% of BPO with sample 3, the sample 2 was prepared. The detailed formulations of sample 1-3 were listed in Table 1. Table 1 The detailed formulations of sample 1-3. Samples/ingredients(g)
1
2
3
PUA oligomer
7.0
7.0
7.0
TPGDA
3.0
3.0
3.0
TPO
0.5
0.5
0.5
BPO
0.1
0.1
0
Cobalt isooctanoate
0.05
0
0
2.4 Preparation of UV/thermal dual-cured sample The UV cured samples were prepared by casting the curable mixtures onto the poly(tetrafluoro-ethylene) (PTFE) mold and curing under UV lamp. The UV/thermal dual-cured samples were obtained by adding a thermal treatment for 10 min at 80
,
100
or 120
after UV radiating. The parameters of the UV lamp were listed as
follows: the main wavelength (365 nm); lamp power (400 W); distance between the curable mixtures and the center of the UV lamp (20 cm); and typical intensity (50%, 50 mW/cm2). 2.5 Characterization For the determination of volatile contents, the experiments and calculations were carried out according to the GB/T 33374-2016 standard. Firstly, the mixtures were put into PTFE molds and the weight (m0) were measured. The mixtures were cured with a UV lamp for 1 min and then were placed at room temperature for 15 min. Then we recorded the remaining weight (m1). In this step, the volatile content during the curing process(w1) were calculated. Then the cured samples were placed in an oven at 110 for 1 h and put for another 1 h at room temperature. The final weight (m2) were obtained. In this step, the potential volatile contents(w2) were calculated. The sum of w1 and w2 were the total volatile content of the sample. − = × 100% − =
× 100%
For the surface drying tests, the surface drying time in three cases were determined according to the GB/T 1728-1979 standard. If there was no trace by pressing a finger on the surface, the time was recorded as the surface drying time. (1) Surface drying time under UV lamp: the curable mixtures 1-3 were put on a tinplate sheet and irradiated by a UV lamp. The sizes of the curable mixtures were 120 mm × 25 mm × 15 µm. (2) Surface drying time at room temperature: The curable mixtures 1-3 were placed in a PTFE mold (the thickness of the groove was 1 mm) at room temperature. The temperature and the humidity also were noted. (3) Surface drying time at different temperatures: using the same mold used in (2), the curable mixtures 1/2 were placed in the mold and cured at 80 120
, 140
, 100
,
.
For the gel fraction tests, the cured samples 1-3 were cut into 1 cm × 1 cm
squares and we measured the initial weights (W0). Then the cured samples were immersed in acetone for 60 h. The rest weights (W) of cured samples were obtained after drying for 36 h at 60 °C. The gel content G were calculated according to the following formula: =
× 100%
The water resistance and solvent resistance of the cured samples were tested according to GB/T 1733-1993 and GB/T 9274-1988, respectively. The cured sample were immersed in deionized water, sodium hydroxide solution (NaOH, 5 wt.%), sodium chloride solution (NaCl, 5 wt.%), phosphoric acid solution (H3PO4, 5 wt.%) for 36 h at the room temperature. Then the states of the sample were recorded. The hardness of the cured sample was measured by a LX-D rubber hardness tester according to the GB/T 2411-2008 standard. Measurements were done five times for each sample and the average value were calculated. Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA/DSC1 analyzer. The cured samples (about 9-12 mg) were placed in alumina pans and heated from 25 to 600 °C at a heating rate of 10 °C/min in Ar atmosphere (60 mL/min). Dynamic mechanical thermal analysis was carried out on a Mettler Toledo TMA/SDTA 2+/600 analyzer at a heating rate of 5 °C/min in the range of -50 °C to 225 °C. The glass transition temperature (Tg) was defined as the peak of tan δ curve.
3. Results and discussion 3.1 Surface drying time 3.1.1 Surface drying time under a UV lamp For PUA coatings, the surface drying time of a film of the material provides a general evaluation of the surface drying rate. The surface drying times of curable mixtures 1–3 under UV irradiation are listed in Table 2. Table 2 Surface drying times of various mixtures under a UV lamp.
Samples
The surface drying time of different mixtures /s
1
2-3
2
1-2
3
1-2
As shown in Table 2, curable mixture 3 dried in a few seconds, in agreement with our previous report; mixtures 1 and 2 had rapid surface drying rates. Therefore, adding BPO and cobalt isooctanoate for the purpose of subsequent thermal treatment did not affect the surface drying rate. 3.1.2 Surface drying time at room temperature Under ambient conditions (24 °C; 64% humidity; irradiation with natural light), the mixtures took a long time to dry. The surface drying times of various curable mixtures at room temperature (i.e., 24 °C) are shown in Fig.2.
Fig.2. Surface drying times of various mixtures at room temperature (i.e., 24 °C). As shown in Fig.2, curable mixture 3 took the longest time (311 min) to cure, whereas mixture 1 only took 167 min. The explanation for this result is that cobalt isooctanoate helps BPO produce radicals at lower temperatures, and the radicals subsequently induce polymerization of the C=C groups in the oligomer. 3.1.3 Surface drying times at various heating temperatures The time required by BPO to form the free radicals that initiate polymerization
varies according to the heating temperature. To measure the surface drying times when the mixtures were cured only by heating, mixtures 1 and 2 were heated at 80, 100, 120, and 140 °C, and the corresponding surface drying times were recorded. The data are shown in Fig.3.
Fig.3. Surface drying times of various mixtures at various heating temperatures. At 80 °C, the surface drying times of curable mixtures 1 and 2 were 16 min and 25 min, respectively. The thermal curing accelerator in sample 1 promoted the generation of free radicals by BPO. At higher temperatures (100, 120, and 140 °C), there was little difference between the drying times of samples 1 and 2. For example, at 140 °C, the surface drying time of sample 2 (260 s) was greater than that of sample 1 (187 s). Curing accelerators help BPO generate free radicals at low temperatures, so the effect of a curing accelerator may not be obvious when BPO is quickly activated and generates free radicals readily at high temperatures. 3.2 Gel fraction The gel fraction represents the proportion of material in a cured sample that is insoluble in an organic solvent. We chose acetone as the organic solvent. The gel fractions of UV-cured sample 3 and UV/thermal dual-cured samples 1 and 2 are listed in Table 3.
Table 3 Gel fractions of the various samples. Samples
Curing condition
The gel fraction/%
1
UV 1 min+100
10 min
99.8±0.05
2
UV 1 min+100
10 min
99.5±0.14
UV 1 min
3
94.6±0.65
As shown in Table 3, the gel fraction of UV-cured sample 3 was 94.6%, and the gel fractions of UV/thermal dual-cured samples 1 and 2 were above 99%. For curable mixtures 1 and 2, the thermal curing agent (BPO) was able to generate free radicals during subsequent thermal treatment, and the free radicals promoted the polymerization of the remaining C=C moieties. Therefore, some areas that could not have been cured completely under UV were cured during thermal treatment, which resulted in a higher gel content. To determine the effect of the temperature on the degree of curing during thermal treatment, sample 1 was heated at 80, 100, and 120 °C for 10 min after UV curing for 1 min. The gel fractions of UV/thermal dual-cured sample 1 were determined under various curing conditions, and the data are summarized in Table 4. Table 4 Gel fractions of sample 1 under various curing conditions. Sample
Curing condition
The gel fraction/%
1
UV 1 min+80
10 min
99.8±0.21
1
UV 1 min+100
10 min
99.8±0.05
1
UV 1 min+120
10 min
99.7±0.16
Following additional thermal treatment at various temperatures after UV curing, the gel fraction consistently exceeded 99.5%. Therefore, the temperature at which the thermal treatment took place did not have a significant effect on the degree of crosslinking. 3.3. Content of volatiles To eliminate the possibility of insufficient UV light intensity during the determination of the contents of volatiles, the highest intensity of UV light (100
mW/cm2) was used in this part of the study. Reactive diluents can participate in the curing reaction during the exposure of UV-curable coatings to UV light. Therefore, we determined the contents of both volatiles and potential volatiles during the curing process. The experimental data are shown in Fig.4.
Fig.4. Contents of volatile organic compounds in the various samples. As shown in Fig.4, each mixture had a total content of volatiles below 1.30 wt.%. The contents of volatiles during UV curing were all below 0.30 wt.%. 3.4. Chemical resistance performance The chemical resistance of a UV-cured material is a useful performance indicator, and is an important consideration for specific applications, such as architectural coatings. The samples (0.2-0.3 mm thick) were prepared by UV curing, then stored at room temperature for 24 h. The states of cured samples 1–3 after immersion in various chemical solutions are presented in Tables 5–7. Table 5 States of UV/thermal dual-cured sample 1. Loss of light
discoloration foaming
Wrinklin g
shedding rusting
Water
No
No
No
No
No
No
NaCl
No
No
No
No
No
No
NaOH
No
No
No
No
No
Yes
H3PO4
No
No
No
No
No
No
Table 6 States of UV/thermal dual-cured sample 2. Loss of light
discoloration foaming wrinkling shedding rusting
Water
No
little
No
No
No
No
NaCl
No
No
No
No
No
Yes
NaOH
No
No
No
No
No
Yes
H3PO4
Yes
Yes
No
No
No
Yes
Table 7 States of UV-cured sample 3. Loss of light
discoloration foaming wrinkling shedding rusting
Water
No
No
No
No
No
No
NaCl
No
No
No
No
No
Yes
NaOH
No
No
No
No
No
Yes
H3PO4
Yes
Yes
No
No
No
No
Fig.5. Digital photographs of the various samples. As shown in Fig.5, cured samples 1–3 had favorable water and salt resistance.
After immersion in NaOH solution, all the samples showed signs of mild corrosion. However, the degree of “rusting” in sample 1 was lower than in samples 2 and 3. After immersion in H3PO4, samples 2 and 3 were obviously corroded, whereas the surface of sample 1 was largely unaffected. Therefore, the addition of BPO and cobalt isooctanoate improves the acid and alkali resistance of the PUA system. 3.5. Hardness The hardness of each sample is presented in Table 8. UV-cured sample 3 was the hardest of the three samples, with a Shore D hardness value of 78. Table 8 Hardness of the various samples. Sample
Curing condition
Hardness/shore D
1
UV 1 min+100
10 min
68±3.0
2
UV 1 min+100
10 min
75±3.1
3
78±3.5
UV 1 min
To determine the effect on hardness of the temperature during thermal treatment, sample 1 was heated at 80, 100, and 120 °C for 10 min after UV curing for 1 min. The hardness data are summarized in Table 9. Table 9 Hardness of sample 1 under various curing conditions. Sample
Curing condition
Hardness/shore D
1
UV 1 min+80
10 min
71±3.6
1
UV 1 min+100
10 min
68±3.0
1
UV 1 min+120
10 min
72±2.3
As shown in Table 9, the hardness varied as the temperature at which the heat treatment was performed after UV curing varied. The hardness values of sample 1 following thermal treatment at 80, 100, and 120 °C for 10 min after UV curing were 71, 68, and 72, respectively. 3.6. Thermal stability The thermal stabilities of cured sample 3 and UV/thermal dual-cured samples 1 and 2 were investigated by thermogravimetric analysis (TGA) in a nitrogen
atmosphere. The TG and differential TG (DTG) curves are shown in Fig.6, and the related data are summarized in Table 10.
(a)
(b)
Fig.6. (a) Thermogravimetric (TG) and (b) differential thermogravimetric (DTG) curves associated with the various samples Table 10 Thermogravimetric analysis (TGA) data for the various samples. Sample 1
2
Curing condition UV 1 min+100℃ 10 min
UV 1 min+100℃ 10 min
3
UV 1 min
T5wt.%/°Ca
Tmax/°Cb
Residues /wt.% c
318.2
405.2
7.7
288.7
407.1
7.0
269.4
408.1
8.7
a
Temperature at 5% weight loss.
b
Maximum degradation rate temperature.
c
Char yield at 600 °C.
For UV-cured sample 3, the temperature at 5 wt.% weight loss (T5wt.%) was 269.4 °C, the temperature at the maximum degradation rate (Tmax) was 408.1 °C, and there was a large amount of char residue at 600 °C (8.7 wt.%). UV/thermal dual-cured sample 2 had a higher T5wt.% (288.7 °C) and less char residue (7.0 wt.%) than UV-cured sample 3. For UV/thermal dual-cured sample 1, T5wt.% increased to 318.2 °C, the amount of char residue at 600 °C was 7.7 wt.%, and Tmax changed little. The TGA tests revealed that UV/thermal dual-curing increased the T5wt.%, and UV/thermal dual-cured sample 1, which contained cobalt isooctanoate, had greater thermal
stability. This result may be attributed to the higher degree of crosslinking in cured sample 1. 3.7. Dynamic mechanical thermal performance Dynamic mechanical thermal analysis (DMTA) was carried out on UV-cured sample 3 and UV/thermal dual-cured samples 1 and 2 to determine the effect of the dual treatment on dynamic mechanical performance. The pertinent data are summarized in Table 11, and the tan δ and storage modulus curves are shown in Fig.7.
(a)
(b)
Fig.7. (a) Tan δ and (b) storage modulus curves of the samples Table 11 Dynamic mechanical thermal analysis (DMTA) data relating to the various samples. Sample
Curing condition
Tg( )
E′(MPa)1
1
UV 1 min+100℃ 10 min
99.4
2
UV 1 min+100℃ 10 min
3
UV 1 min
E′′(MPa)2
E′′(MPa)3
827.1
6321.0
395.5
91.4
1239.6
8857.5
428.8
94.2
1828.4
13819.1
756.7
Tg: the temperature corresponding to the peak in the tan δ curve; E′(MPa)1: the storage modulus at Tg; E′(MPa)2: the storage modulus at 0 °C; E′(MPa)3: the storage modulus at Tg+50 °C.
The glass transition temperature (Tg) of UV-cured sample 3 was 94.2 °C. Sample 1 had a higher Tg (99.4 °C), whereas sample 2 had a slightly lower Tg (91.4 °C). The results demonstrate that additional thermal treatment after UV curing could increase the degree of crosslinking. The storage moduli of samples 1 and 2 were much lower
than the storage modulus of sample 3. The PUA oligomer used in the present study had a linear structure, in which isophorone diisocyanate (IPDI) provided the hard segments and poly(glycidyl methacrylate) (PGMA) provided the soft segments. Therefore, the thermal treatment after UV curing induced the reaction of the remaining acrylate double bonds and more PGMA is connected in the PUA oligomer, which lead to the decrease of the storage modulus.
4. Conclusions In the present study, UV/thermal dual-curable polyurethane acrylate mixtures were prepared by mixing a PUA oligomer, TPGDA, TPO, BPO, and cobalt isooctanoate. UV-curable polyurethane acrylate mixtures were prepared by mixing the same PUA oligomer mentioned above, TPGDA, and TPO. The surface drying time of each curable mixture subjected to UV irradiation (400 W) was approximately 2 s. Compared with the corresponding performance of the UV-cured sample (cured for 1 min by UV radiation), the UV/thermal dual-cured samples containing cobalt isooctanoate (cured by heating at 100 °C for 10 min, after UV irradiation for 1 min) were more resistant to acids and alkalis, and had higher glass transition temperatures and gel fractions. The total volatiles content of each curable mixture was lower than 1.3 wt.%. In summary, the UV/thermal dual-curable system performed better than the UV-curable system whilst retaining the rapid surface drying rate of the latter.
Acknowledgments This work was supported by the Special Development Program of the Strategic Emerging Industries of Shenzhen City (JSGG20160331104855391). References: [1] Qiu F, Xu H, Wang Y, Xu J, Yang D. Journal of Coatings Technology and Research 2012;9(5):503-514. [2] Kim J, Cho B, Kweon J, Noh S. Progress in Organic Coatings 2014;77(11):1695-1700. [3] Wang D. UV curable materials - theory and application. Beijing: Science Press, 2001. [4] Xu J, Jiang Y, Qiu F, Dai Y, Yang D, Yu Z, Yang P. Polymer Bulletin 2018. [5] Xu H, Qiu F, Wang Y, Wu W, Yang D, Guo Q. Progress in Organic Coatings 2012;73(1):47-53.
[6] Gao Q, Li H, Zeng X. Journal of Coatings Technology and Research 2011;8(1):61-66. [7] Qin L, He Y, Liu B, Jian Y, Li C, Nie J. Progress in Organic Coatings 2013;76(11):1594-1599. [8] Zhang T, Wu W, Wang X. Progress in Organic Coatings 2010:201-207. [9] Chen LJ, Tai QL, Song L, Xing WY, Jie GX, Hu Y. Express Polymer Letters 2010;4(9):539-550. [10] Wu G, Li J, Luo Y. Polymer Degradation and Stability 2016;123:36-46. [11] Ren Y, Dong Y, Zhu Y, Xu J, Yao Y. Progress in Organic Coatings 2019;129:309-317. [12] Kim Y, Kang S. Applied Physics A 2010;98(1):245-248. [13] Park C, Lee S, Park J, Kim H. Reactive and Functional Polymers 2013;73(4):641-646. [14] Studer K, Decker C, Beck E, Schwalm R, Gruber N. Progress in Organic Coatings 2005;53(2):126-133. [15] Zhi J, Nie J, He Y. Adhesion 2009(1):48-52. [16] Bi Y, Li Z, Wang N, Zhang L. International Journal of Adhesion and Adhesives 2016;66:9-14. [17] Park Y, Lim D, Kim H, Park D, Sung I. International Journal of Adhesion and Adhesives 2009;29(7):710-717. [18] Studer K, Decker C, Beck E, Schwalm R. European Polymer Journal 2005;41(1):157-167. [19] Zhi J, He Y, Xiao M, Nie J. Progress in Organic Coatings 2009;66(1):35-39. [20] Zhu Y, Dong Y, Chen L, Yao Y, Hong Y, Tang G. Fine Chemicals 2018(01):17-23.
S0143-7496(20)30041-5
DOI:
https://doi.org/10.1016/j.ijadhadh.2020.102580
Reference:
JAAD 102580
To appear in:
International Journal of Adhesion and Adhesives
Received Date: 1 November 2019 Accepted Date: 3 February 2020
Please cite this article as: Ren Y, Dong Y, Yang R, Yao Y, Preparation and properties of ultraviolet/ thermal dual-curable polyurethane acrylate, International Journal of Adhesion and Adhesives, https:// doi.org/10.1016/j.ijadhadh.2020.102580. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Preparation and properties of ultraviolet/thermal dual-curable polyurethane acrylate Yongxia Ren, Yunsheng Dong, Ruoning Yang, Youwei Yao* Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China *Corresponding author Youwei Yao Tel and Fax: +86-0755-26036796 E-mail: [email protected] Postal address: Room 1510, Division of Energy and Environment, Tsinghua Campus, The University Town, Shenzhen 518005, PR China
Abstract Ultraviolet (UV)/thermal dual-curable polyurethane acrylate (PUA) mixtures were prepared using a PUA oligomer, tripropylene glycol diacrylate (TPGDA), benzoyl peroxide (BPO), 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (TPO), and cobalt isooctanoate. For purposes of comparison, a second UV-curable PUA mixture was prepared by mixing the PUA oligomer, TPGDA, and TPO. When placed under a UV lamp (400 W, 50 mW/cm2), the surface drying time of each curable mixture was approximately 2 s. The content of volatiles during UV curing (the intensity of the lamp was 100 mW/cm2) was less than 0.3 wt.%, and the content of potential volatiles (the test condition was heating at 110 °C for 1 h after UV curing) was below 1.0 wt.%. Compared with the UV-cured sample, the UV/thermal dual-cured sample containing cobalt isooctanoate had more favorable properties, that is: (1) better acid and alkali resistance; and (2) better thermal stability and higher gel content. The temperature at 5 wt.% weight loss (T5wt.%) and the gel fraction of the UV-cured sample (the curing condition was 1 min of UV irradiation) were 269.4 °C and 94.6%, respectively. The T5wt.% and the gel fraction of the UV/thermal dual-cured sample (cured at 100 °C for 10 min after 1 min of UV irradiation) were 318.2 °C and 99.8%, respectively. BPO and cobalt isooctanoate had little effect on the surface drying rate of the curable mixtures, and the performance of the cured samples was improved by thermal treatment following irradiation with UV. Considering the dyeing effect of BPO, this system provides a more practical approach to the complete curing of colored UV-curable resins.
Keywords: polyurethane acrylate; UV/thermal dual-curing treatment; content of volatiles; surface drying time
1. Introduction In recent decades, ultraviolet (UV) curing techniques have attracted considerable attention because of the characteristic of less environmental pollution, fast curing rate, low energy consumption [1-3]. Polyurethane acrylate (PUA) oligomers are among the
most important oligomers used in UV-curable coatings, and curable PUA mixtures have been widely used in, for example, leather, printing, and inks because they have excellent properties including resistance to wear, weather, and solvents [4, 5]. Researchers have made various attempts to improve the performance of PUA systems such as by reducing their surface drying time [6-8], increasing their thermal stability [9], and enhancing their flame-retarding properties [10, 11]. However, research into the curing of colored coatings, which are aesthetically valuable for art [12] for example, is relatively rare. Moreover, UV curing systems also have some disadvantages [13-15]. According to Park et al. [13], the curing depth is limited, and shaded portions cannot be cured completely with UV irradiation alone, which restricts the use of such systems in printed circuit boards, ultra-thick coatings, colored coatings, and irregularly shaped devices. Thermal treatment following UV irradiation could eliminate such defects, because thermal curing affects those areas that cannot be irradiated with UV light. Bi et al. [16] prepared a series of dual-curable adhesives. When the proportion of C=C bonds was relatively low, the addition of a thermal curing agent significantly increased the gel fraction. When 20 parts per hundred resin (phr) of a thermal curing agent was added, the gel content reached 100%, regardless of the proportion of C=C bonds. Furthermore, the storage modulus and glass transition temperature of the UV/thermal dual-cured samples were higher than those of a UV-cured sample. Adding a thermal curing agent and a curing accelerator to UV-curable mixtures is a common method of preparing UV/thermal dual-curing systems. The method is simple, and increases the degree of crosslinking, which for example improves the mechanical strength, solvent resistance, and thermal stability of the cured sample [17, 18]. Zhi et al. [19] synthesized three dual-cure PUAs with different contents of double bonds, and compared the properties of the resulting UV/thermal-cured samples with those of a UV-cured sample. They found that the pendulum/pencil hardness, flexibility, and abrasion resistance improved when heat treatment was applied after UV curing. However, the effect of the addition of additives on the curing rate of UV/thermal dual-curable mixtures requires further exploration and analysis. The
specific volatiles content of UV- or thermal/UV-curable mixtures, which must be considered carefully before they are used in actual situations, has also rarely been reported. In the present study, we prepared a UV/thermal dual-curable system based on the UV-curable PUA mixtures reported in our previous study, to which we added a thermal curing agent (benzoyl peroxide (BPO)) and a curing accelerator (cobalt isooctanoate). The UV/thermal dual-curable mixtures are partially cured when exposed to UV light, and completely cured during subsequent heating. The surface drying time of each curable mixture was determined, and the gel content, chemical resistance, thermal stability, and dynamic mechanical thermal analysis (DMTA) data were obtained for each cured sample.
2. Material and methods 2.1 Materials Benzoyl peroxide (BPO) were purchased from Aladdin Reagent (China). 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide (TPO) and tripropylene glycol diacrylate (TPGDA) were purchased from Energy Chemical (China). All of the above reagents were of analytical reagent grade. Cobalt isooctanoate with 10 wt.% of cobalt were supplied by the Shanghai Yongyan Chemicals Co., Ltd. 2.2 Synthesis of the PUA oligomer The PUA oligomer was synthesized in our previous report [11, 20]. And the synthesis process is shown in Fig. 1.
Fig.1. The synthesis scheme of PUA oligomer [11, 20]. 2.3 Preparation of UV/thermal dual-curable mixtures The oligomer (70 wt.%), TPGDA (30 wt.%) and TPO (5 wt.%) were mixed and stirred with a mechanical stirrer for 4 h, then the sample 3 was obtained. When mixing sample 3 with 1 wt.% of BPO and 0.5 wt.% of cobalt isooctanoate and stirring the mixture until the mixture was transparent, the sample 1 will be obtained. When only blending 1 wt.% of BPO with sample 3, the sample 2 was prepared. The detailed formulations of sample 1-3 were listed in Table 1. Table 1 The detailed formulations of sample 1-3. Samples/ingredients(g)
1
2
3
PUA oligomer
7.0
7.0
7.0
TPGDA
3.0
3.0
3.0
TPO
0.5
0.5
0.5
BPO
0.1
0.1
0
Cobalt isooctanoate
0.05
0
0
2.4 Preparation of UV/thermal dual-cured sample The UV cured samples were prepared by casting the curable mixtures onto the poly(tetrafluoro-ethylene) (PTFE) mold and curing under UV lamp. The UV/thermal dual-cured samples were obtained by adding a thermal treatment for 10 min at 80
,
100
or 120
after UV radiating. The parameters of the UV lamp were listed as
follows: the main wavelength (365 nm); lamp power (400 W); distance between the curable mixtures and the center of the UV lamp (20 cm); and typical intensity (50%, 50 mW/cm2). 2.5 Characterization For the determination of volatile contents, the experiments and calculations were carried out according to the GB/T 33374-2016 standard. Firstly, the mixtures were put into PTFE molds and the weight (m0) were measured. The mixtures were cured with a UV lamp for 1 min and then were placed at room temperature for 15 min. Then we recorded the remaining weight (m1). In this step, the volatile content during the curing process(w1) were calculated. Then the cured samples were placed in an oven at 110 for 1 h and put for another 1 h at room temperature. The final weight (m2) were obtained. In this step, the potential volatile contents(w2) were calculated. The sum of w1 and w2 were the total volatile content of the sample. − = × 100% − =
× 100%
For the surface drying tests, the surface drying time in three cases were determined according to the GB/T 1728-1979 standard. If there was no trace by pressing a finger on the surface, the time was recorded as the surface drying time. (1) Surface drying time under UV lamp: the curable mixtures 1-3 were put on a tinplate sheet and irradiated by a UV lamp. The sizes of the curable mixtures were 120 mm × 25 mm × 15 µm. (2) Surface drying time at room temperature: The curable mixtures 1-3 were placed in a PTFE mold (the thickness of the groove was 1 mm) at room temperature. The temperature and the humidity also were noted. (3) Surface drying time at different temperatures: using the same mold used in (2), the curable mixtures 1/2 were placed in the mold and cured at 80 120
, 140
, 100
,
.
For the gel fraction tests, the cured samples 1-3 were cut into 1 cm × 1 cm
squares and we measured the initial weights (W0). Then the cured samples were immersed in acetone for 60 h. The rest weights (W) of cured samples were obtained after drying for 36 h at 60 °C. The gel content G were calculated according to the following formula: =
× 100%
The water resistance and solvent resistance of the cured samples were tested according to GB/T 1733-1993 and GB/T 9274-1988, respectively. The cured sample were immersed in deionized water, sodium hydroxide solution (NaOH, 5 wt.%), sodium chloride solution (NaCl, 5 wt.%), phosphoric acid solution (H3PO4, 5 wt.%) for 36 h at the room temperature. Then the states of the sample were recorded. The hardness of the cured sample was measured by a LX-D rubber hardness tester according to the GB/T 2411-2008 standard. Measurements were done five times for each sample and the average value were calculated. Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA/DSC1 analyzer. The cured samples (about 9-12 mg) were placed in alumina pans and heated from 25 to 600 °C at a heating rate of 10 °C/min in Ar atmosphere (60 mL/min). Dynamic mechanical thermal analysis was carried out on a Mettler Toledo TMA/SDTA 2+/600 analyzer at a heating rate of 5 °C/min in the range of -50 °C to 225 °C. The glass transition temperature (Tg) was defined as the peak of tan δ curve.
3. Results and discussion 3.1 Surface drying time 3.1.1 Surface drying time under a UV lamp For PUA coatings, the surface drying time of a film of the material provides a general evaluation of the surface drying rate. The surface drying times of curable mixtures 1–3 under UV irradiation are listed in Table 2. Table 2 Surface drying times of various mixtures under a UV lamp.
Samples
The surface drying time of different mixtures /s
1
2-3
2
1-2
3
1-2
As shown in Table 2, curable mixture 3 dried in a few seconds, in agreement with our previous report; mixtures 1 and 2 had rapid surface drying rates. Therefore, adding BPO and cobalt isooctanoate for the purpose of subsequent thermal treatment did not affect the surface drying rate. 3.1.2 Surface drying time at room temperature Under ambient conditions (24 °C; 64% humidity; irradiation with natural light), the mixtures took a long time to dry. The surface drying times of various curable mixtures at room temperature (i.e., 24 °C) are shown in Fig.2.
Fig.2. Surface drying times of various mixtures at room temperature (i.e., 24 °C). As shown in Fig.2, curable mixture 3 took the longest time (311 min) to cure, whereas mixture 1 only took 167 min. The explanation for this result is that cobalt isooctanoate helps BPO produce radicals at lower temperatures, and the radicals subsequently induce polymerization of the C=C groups in the oligomer. 3.1.3 Surface drying times at various heating temperatures The time required by BPO to form the free radicals that initiate polymerization
varies according to the heating temperature. To measure the surface drying times when the mixtures were cured only by heating, mixtures 1 and 2 were heated at 80, 100, 120, and 140 °C, and the corresponding surface drying times were recorded. The data are shown in Fig.3.
Fig.3. Surface drying times of various mixtures at various heating temperatures. At 80 °C, the surface drying times of curable mixtures 1 and 2 were 16 min and 25 min, respectively. The thermal curing accelerator in sample 1 promoted the generation of free radicals by BPO. At higher temperatures (100, 120, and 140 °C), there was little difference between the drying times of samples 1 and 2. For example, at 140 °C, the surface drying time of sample 2 (260 s) was greater than that of sample 1 (187 s). Curing accelerators help BPO generate free radicals at low temperatures, so the effect of a curing accelerator may not be obvious when BPO is quickly activated and generates free radicals readily at high temperatures. 3.2 Gel fraction The gel fraction represents the proportion of material in a cured sample that is insoluble in an organic solvent. We chose acetone as the organic solvent. The gel fractions of UV-cured sample 3 and UV/thermal dual-cured samples 1 and 2 are listed in Table 3.
Table 3 Gel fractions of the various samples. Samples
Curing condition
The gel fraction/%
1
UV 1 min+100
10 min
99.8±0.05
2
UV 1 min+100
10 min
99.5±0.14
UV 1 min
3
94.6±0.65
As shown in Table 3, the gel fraction of UV-cured sample 3 was 94.6%, and the gel fractions of UV/thermal dual-cured samples 1 and 2 were above 99%. For curable mixtures 1 and 2, the thermal curing agent (BPO) was able to generate free radicals during subsequent thermal treatment, and the free radicals promoted the polymerization of the remaining C=C moieties. Therefore, some areas that could not have been cured completely under UV were cured during thermal treatment, which resulted in a higher gel content. To determine the effect of the temperature on the degree of curing during thermal treatment, sample 1 was heated at 80, 100, and 120 °C for 10 min after UV curing for 1 min. The gel fractions of UV/thermal dual-cured sample 1 were determined under various curing conditions, and the data are summarized in Table 4. Table 4 Gel fractions of sample 1 under various curing conditions. Sample
Curing condition
The gel fraction/%
1
UV 1 min+80
10 min
99.8±0.21
1
UV 1 min+100
10 min
99.8±0.05
1
UV 1 min+120
10 min
99.7±0.16
Following additional thermal treatment at various temperatures after UV curing, the gel fraction consistently exceeded 99.5%. Therefore, the temperature at which the thermal treatment took place did not have a significant effect on the degree of crosslinking. 3.3. Content of volatiles To eliminate the possibility of insufficient UV light intensity during the determination of the contents of volatiles, the highest intensity of UV light (100
mW/cm2) was used in this part of the study. Reactive diluents can participate in the curing reaction during the exposure of UV-curable coatings to UV light. Therefore, we determined the contents of both volatiles and potential volatiles during the curing process. The experimental data are shown in Fig.4.
Fig.4. Contents of volatile organic compounds in the various samples. As shown in Fig.4, each mixture had a total content of volatiles below 1.30 wt.%. The contents of volatiles during UV curing were all below 0.30 wt.%. 3.4. Chemical resistance performance The chemical resistance of a UV-cured material is a useful performance indicator, and is an important consideration for specific applications, such as architectural coatings. The samples (0.2-0.3 mm thick) were prepared by UV curing, then stored at room temperature for 24 h. The states of cured samples 1–3 after immersion in various chemical solutions are presented in Tables 5–7. Table 5 States of UV/thermal dual-cured sample 1. Loss of light
discoloration foaming
Wrinklin g
shedding rusting
Water
No
No
No
No
No
No
NaCl
No
No
No
No
No
No
NaOH
No
No
No
No
No
Yes
H3PO4
No
No
No
No
No
No
Table 6 States of UV/thermal dual-cured sample 2. Loss of light
discoloration foaming wrinkling shedding rusting
Water
No
little
No
No
No
No
NaCl
No
No
No
No
No
Yes
NaOH
No
No
No
No
No
Yes
H3PO4
Yes
Yes
No
No
No
Yes
Table 7 States of UV-cured sample 3. Loss of light
discoloration foaming wrinkling shedding rusting
Water
No
No
No
No
No
No
NaCl
No
No
No
No
No
Yes
NaOH
No
No
No
No
No
Yes
H3PO4
Yes
Yes
No
No
No
No
Fig.5. Digital photographs of the various samples. As shown in Fig.5, cured samples 1–3 had favorable water and salt resistance.
After immersion in NaOH solution, all the samples showed signs of mild corrosion. However, the degree of “rusting” in sample 1 was lower than in samples 2 and 3. After immersion in H3PO4, samples 2 and 3 were obviously corroded, whereas the surface of sample 1 was largely unaffected. Therefore, the addition of BPO and cobalt isooctanoate improves the acid and alkali resistance of the PUA system. 3.5. Hardness The hardness of each sample is presented in Table 8. UV-cured sample 3 was the hardest of the three samples, with a Shore D hardness value of 78. Table 8 Hardness of the various samples. Sample
Curing condition
Hardness/shore D
1
UV 1 min+100
10 min
68±3.0
2
UV 1 min+100
10 min
75±3.1
3
78±3.5
UV 1 min
To determine the effect on hardness of the temperature during thermal treatment, sample 1 was heated at 80, 100, and 120 °C for 10 min after UV curing for 1 min. The hardness data are summarized in Table 9. Table 9 Hardness of sample 1 under various curing conditions. Sample
Curing condition
Hardness/shore D
1
UV 1 min+80
10 min
71±3.6
1
UV 1 min+100
10 min
68±3.0
1
UV 1 min+120
10 min
72±2.3
As shown in Table 9, the hardness varied as the temperature at which the heat treatment was performed after UV curing varied. The hardness values of sample 1 following thermal treatment at 80, 100, and 120 °C for 10 min after UV curing were 71, 68, and 72, respectively. 3.6. Thermal stability The thermal stabilities of cured sample 3 and UV/thermal dual-cured samples 1 and 2 were investigated by thermogravimetric analysis (TGA) in a nitrogen
atmosphere. The TG and differential TG (DTG) curves are shown in Fig.6, and the related data are summarized in Table 10.
(a)
(b)
Fig.6. (a) Thermogravimetric (TG) and (b) differential thermogravimetric (DTG) curves associated with the various samples Table 10 Thermogravimetric analysis (TGA) data for the various samples. Sample 1
2
Curing condition UV 1 min+100℃ 10 min
UV 1 min+100℃ 10 min
3
UV 1 min
T5wt.%/°Ca
Tmax/°Cb
Residues /wt.% c
318.2
405.2
7.7
288.7
407.1
7.0
269.4
408.1
8.7
a
Temperature at 5% weight loss.
b
Maximum degradation rate temperature.
c
Char yield at 600 °C.
For UV-cured sample 3, the temperature at 5 wt.% weight loss (T5wt.%) was 269.4 °C, the temperature at the maximum degradation rate (Tmax) was 408.1 °C, and there was a large amount of char residue at 600 °C (8.7 wt.%). UV/thermal dual-cured sample 2 had a higher T5wt.% (288.7 °C) and less char residue (7.0 wt.%) than UV-cured sample 3. For UV/thermal dual-cured sample 1, T5wt.% increased to 318.2 °C, the amount of char residue at 600 °C was 7.7 wt.%, and Tmax changed little. The TGA tests revealed that UV/thermal dual-curing increased the T5wt.%, and UV/thermal dual-cured sample 1, which contained cobalt isooctanoate, had greater thermal
stability. This result may be attributed to the higher degree of crosslinking in cured sample 1. 3.7. Dynamic mechanical thermal performance Dynamic mechanical thermal analysis (DMTA) was carried out on UV-cured sample 3 and UV/thermal dual-cured samples 1 and 2 to determine the effect of the dual treatment on dynamic mechanical performance. The pertinent data are summarized in Table 11, and the tan δ and storage modulus curves are shown in Fig.7.
(a)
(b)
Fig.7. (a) Tan δ and (b) storage modulus curves of the samples Table 11 Dynamic mechanical thermal analysis (DMTA) data relating to the various samples. Sample
Curing condition
Tg( )
E′(MPa)1
1
UV 1 min+100℃ 10 min
99.4
2
UV 1 min+100℃ 10 min
3
UV 1 min
E′′(MPa)2
E′′(MPa)3
827.1
6321.0
395.5
91.4
1239.6
8857.5
428.8
94.2
1828.4
13819.1
756.7
Tg: the temperature corresponding to the peak in the tan δ curve; E′(MPa)1: the storage modulus at Tg; E′(MPa)2: the storage modulus at 0 °C; E′(MPa)3: the storage modulus at Tg+50 °C.
The glass transition temperature (Tg) of UV-cured sample 3 was 94.2 °C. Sample 1 had a higher Tg (99.4 °C), whereas sample 2 had a slightly lower Tg (91.4 °C). The results demonstrate that additional thermal treatment after UV curing could increase the degree of crosslinking. The storage moduli of samples 1 and 2 were much lower
than the storage modulus of sample 3. The PUA oligomer used in the present study had a linear structure, in which isophorone diisocyanate (IPDI) provided the hard segments and poly(glycidyl methacrylate) (PGMA) provided the soft segments. Therefore, the thermal treatment after UV curing induced the reaction of the remaining acrylate double bonds and more PGMA is connected in the PUA oligomer, which lead to the decrease of the storage modulus.
4. Conclusions In the present study, UV/thermal dual-curable polyurethane acrylate mixtures were prepared by mixing a PUA oligomer, TPGDA, TPO, BPO, and cobalt isooctanoate. UV-curable polyurethane acrylate mixtures were prepared by mixing the same PUA oligomer mentioned above, TPGDA, and TPO. The surface drying time of each curable mixture subjected to UV irradiation (400 W) was approximately 2 s. Compared with the corresponding performance of the UV-cured sample (cured for 1 min by UV radiation), the UV/thermal dual-cured samples containing cobalt isooctanoate (cured by heating at 100 °C for 10 min, after UV irradiation for 1 min) were more resistant to acids and alkalis, and had higher glass transition temperatures and gel fractions. The total volatiles content of each curable mixture was lower than 1.3 wt.%. In summary, the UV/thermal dual-curable system performed better than the UV-curable system whilst retaining the rapid surface drying rate of the latter.
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