Hydroxy acrylic resins were synthesized from methyl methacrylate, butyl acrylate, N,N−dimethylaminoethyl methacrylate (DMAEMA) and hydroxyethyl methacrylate (HEMA), and then 3−isopropyl− Dimethylbenzyl isocyanate (TMI) is grafted to introduce unsaturated bonds to prepare ultraviolet (UV) curable resin, which is used to prepare cathodic electrophoretic paint. The influence of the amount of DMAEMA, HEMA and TMI on the properties of resin and paint film was discussed and the better amount was determined: DMAEMA accounted for 10% ~ 14% of the total monomer mass, HEMA 15% ~ 20%, TMI 14% ~ 19% . The performance of the prepared coating is equivalent to that of similar foreign products.
As a kind of water-based environment-friendly coatings, electrophoretic coatings have been widely used. At present, electrophoretic coatings are formed by thermal curing and cross-linking. Not only the curing temperature is high (usually 170 ~ 180 °C), but the curing time is long (more than 20 min). Even low-temperature curing cathodic electrophoretic paints have a curing temperature higher than 140 °C, which limits their application on heat-sensitive substrates [1-3]. Moreover, the widely used blocked isocyanate curing agents release small molecules, some of which are hazardous air pollutants (HAPs), such as ε-caprolactam and butyl cellosolve blocked diphenylmethane diisocyanate in traditional cathodic electrocoating. Ultraviolet (UV) curing not only has low curing temperature (especially suitable for heat-sensitive substrates), fast curing rate (improved production efficiency), solvent-free (less environmental pollution), low energy consumption, but also can adjust resin and activity The ratio of monomer diluent to change the properties of the coating film. The research and development of light-curing cathodic electrophoretic paint with the advantages of both cathodic electrophoretic paint and UV-curable paint can not only further broaden the application range of electrophoretic paint, but also conform to the development trend of environmental friendliness, which has become a new direction for the development of electrophoretic paint [4 -7]. In this paper, 3-isopropyl-dimethylbenzyl isocyanate (TMI) was used as raw material[8] to graft hydroxyacrylic resin, and unsaturated bonds were introduced into the resin chain segment to prepare a resin with UV curing function ( The synthesis principle is shown in Figure 1), and formulated into highly decorative cathodic electrophoretic coatings. In this way, the drawbacks of the electrophoretic paint film caused by relying on the addition of UV-cured small molecule additives are avoided. This paper also examines the influence of the dosage of the main components on the performance of the final paint film, and determines the best raw material ratio of the matrix resin.
1 experiment
1. 1 Materials and instruments
甲基丙烯酸甲酯(MMA)、丙烯酸丁酯(BA)、甲基丙烯酸 N,N−二甲氨基乙酯(DMAEMA)、甲基丙烯酸羟乙酯(HEMA)、偶氮二异丁腈(AIBN，半衰期 30 ~ 45 min)、二月桂酸二丁基锡(DBTL)、乳酸(中和剂)均为化学纯，3−异丙基−二甲基苄基异氰酸酯(TMI)、助溶剂丙二醇甲醚醋酸酯(PMA)、光引发剂 TPO、光引发剂 184均为工业品，蒸馏水自制。
(a) 羟基丙烯酸树脂的合成
(b) 涂料基体树脂的合成
图 1 树脂的合成路线
SUNRAY 400 SM UV 实验型固化机，深圳市慧烁机电有限公司；Z-100 电泳试验机，深圳市志邦科技有限公司；202-0A 数显电热恒温干燥箱，湖南力辰仪器科技有限公司；QND-1 型黏度计，天津市世博伟业化玻仪器有限公司；JND-A60 型光泽度仪，福安市钧能达电子科技有限公司；1 kW 手提 UV 固化机，东莞市蓝盾机电设备科技有限公司。
1. 2 UV 固化丙烯酸阴极电泳漆的制备
1. 2. 1 基体树脂的合成
在反应器中加入 40 ~ 60 g PMA，搅拌并加热，升温到 85 °C 时滴加混合单体(20 ~ 25 g MMA、30 ~ 35 g BA、15 ~ 25 g DMAEMA和15 ~ 25 g HEMA)以及1.5 ~ 2.0 g自由基聚合引发剂AIBN，用时2 ~ 3 h，然后升温至90 °C，补加少量引发剂，保温 2 h，直至单体转化率大于 98%。降温至 85 °C，分别按 m(─NCO)/m(─OH)为 0.3、0.4、0.5、0.6、0.7 和 0.8 加入 20 ~ 30 g TMI，并保温反应约 2 h，待检测到 NCO 为零时，降温至 50 °C 以下，用 6 ~10 g 乳酸中和至中和度为 90%，并加入 20 ~ 30 g 水调节固含量至 60% ± 2%。得到 UV 固化型丙烯酸阴极电泳漆专用树脂，其外观为淡黄色透明液体。
1. 2. 2 阴极电泳涂料的制备
Taking the preparation of UV electrophoretic varnish as an example, the specific process is as follows: in the stirring state, according to the effective solid content of the UV resin, first add 2% of the resin mass UV initiator [m(TPO):m(184) = 2:1] , and slowly add pure water, then put it into a high-speed stirring container, stir until uniformly dispersed, and prepare a bath solution with a solid content of 12% to 15% (mass fraction) that can be used for electrophoresis. In a small plastic electrophoresis tank, a 120.00 mm × 25.00 mm × 0.25 mm tinplate test piece that had been degreased and cleaned with acetone was connected to the cathode, and placed in a stainless steel anode plate, and electrophoresis was performed at 50 V for 30 s. After rinsing, drying and flashing at 80 °C for 10 minutes, use a 1 kW UV testing machine to carry out UV curing for 2 minutes under the condition that the distance between the test piece and the UV light source is 100 mm, and the electrophoretic coating is obtained. 10 ~ 15 μm thick.
1.3 Analysis test
1. 3. 1 Resin
The ─NCO group content in the resin was determined by di-n-butylamine back titration. The structure of the prepared resin was analyzed in the total reflection mode with the Nicolet IN10 Fourier transform microscopic infrared spectrometer (FT-IR) of Nicolet Company of the United States, and the HP1100 effective gel permeation chromatography (GPC) of Agilent Company was used to determine the resin's density. Molecular weight and its distribution. The light transmittance at a resin concentration of 10% was measured by an SP-754PC UV-Vis Spectrophotometer from Shanghai Spectrum Instrument Co., Ltd., the reference sample was distilled water, and the wavelength was 600 nm.
1. 3. 2 Electrophoretic coating
Respectively refer to GB/T 9286-1998 "Cross-cut test of paint and varnish film", GB/T 6739-2006 "Determination of paint film hardness by pencil method of paint and varnish", GB/T 13452.2-2008 "Paint and varnish Determination of Varnish Film Thickness", GB/T 1743-1979 "Determination of Paint Film Gloss", GB/T1731-1993 "Determination of Paint Film Flexibility" and GB/T 23989-2009 "Determination of Coating Solvent Wipe Resistance 》Test the adhesion, pencil hardness, thickness, gloss, flexibility and solvent (MEK) resistance of the coating.
2 Results and discussion
2.1 Infrared spectrum analysis of UV polymer
2.1.1 Infrared spectrum of hydroxyacrylic resin
阳离子型羟基丙烯酸树脂的红外光谱见图 2。3 458 cm −1 处由于氢键的存在，有宽而强的羟基伸缩振动峰。2 950 cm −1 和2 855 cm −1 处为甲基与亚甲基的伸缩振动峰。1 752 cm −1 处出现酯基中C═O的吸收峰。1 465 ~ 1 161 cm −1酯的特征谱带。1 161 cm −1 处为酯基中 C─O 的伸缩振动峰。而丙烯酸酯单体在 1 680 ~ 1 620 cm −1 处的 C═C双键伸缩振动吸收峰消失，表明所有单体完成聚合，得到预定结构的丙烯酸树脂。
2. 1. 2 UV 电泳漆专用树脂的红外谱图
图 3 给出了 UV 固化型丙烯酸阴极电泳漆专用树脂的红外谱图。由图 3 可知，2 962 cm −1 与 2 858 cm −1 处为─CH 3 、CH 2 中饱和 C─H 的伸缩振动吸收峰，在 1 729 cm −1 处的吸收峰是异氰酸酯接枝后形成的氨基甲酸酯键结构的伸缩振动吸收峰，且在 2 250 cm −1 附近无─NCO 吸收峰，表明接枝反应很完全。1 632 cm −1 处又重新出现 C═C双键伸缩振动吸收峰，1 300 ~ 1 100 cm −1 处为酯基中 C─O─C 的伸缩振动吸收峰，1450 cm −1 处为─CH 3 、─CH 2变形振动吸收峰。以上都符合目标聚合物的结构。
图 2 羟基丙烯酸树脂的红外光谱图
图 3 涂料基础树脂的红外光谱图
2. 2 所得羟基丙烯酸树脂的分子量
所制羟基丙烯酸树脂的分子量及分布如图 4 所示。可见其数均分子量(M n )为 4 958 g/mol，质均分子量(M w )为 12 483 g/mol，分散度 D 为 2 518。
图 4 羟基丙烯酸树脂的 GPC 测试结果
2. 3 DMAEMA 含量对羟基丙烯酸树脂水溶性的影响
DMAEMA 为设计的树脂提供了阳离子成盐水溶的可能，其用量对树脂水溶性起了决定性的作用。表 1 列出了一组 HEMA 用量为 15%，M n 约为 5 500、TMI 接枝率相同[m(─NCO)/m(─OH)= 0.5]而 DMAEMA 含量(占共聚单体总量的质量分数)不同的丙烯酸共聚物的水溶性及透光率测试结果。由表 1 可知，DMAEMA 用量越多，树脂的水溶性越好，其水溶液的透光度越高。但是 DMAEMA 含量过高将导致电泳槽液的泳透力下降，同时氨基易生成着色基团，用量过多会导致树脂颜色加深，最终造成电泳漆膜泛黄。DMAEMA 的很好的用量为 10% ~ 14%。
表 1 DMAEMA 含量对羟基丙烯酸树脂水溶性的影响
2. 4 HEMA 含量对羟基丙烯酸树脂黏度的影响
羟基丙烯酸树脂中的羟基作为功能性基团，可以与氨基或─NCO 基团交联固化，同时为树脂提供辅助的水溶性作用。合成了一系列不同羟基含量的丙烯酸树脂，考察了 HEMA 含量(羟值)对树脂黏度的影响，结果见图 5。
图 5 HEMA 含量对羟基丙烯酸树脂黏度的影响
从图 5 可知，随着 HEMA 用量增加，黏度几乎呈线性地增加，这是因为大量的─OH 增强了分子链间的氢键作用。当 HEMA 含量大于 20%以后，黏度增长的趋势有所变缓。过多的羟基残留会影响漆膜的耐盐雾、耐溶剂等性能，在满足 TMI 接枝的前提下，HEMA 不宜超量过多，控制在 15% ~ 20%，即羟值在 65 ~ 85 范围为宜。
2. 5 TMI 接枝比例对漆膜性能的影响
羟基树脂接枝的 TMI 越多，引入双键的比例越大，UV 固化的能力越强，可固化的交联点越多，交联密度也越大，相应的漆膜硬度越高。但是过度交联会使漆膜变脆，耐冲击性、韧性等机械性能大大降低。表 2 为 HEMA占反应单体的质量分数为 20%时，不同 TMI 接枝比例对漆膜性能的影响。
表 2 m( (─NCO) )/m( (─OH) ) 对漆膜性能的影响
It can be seen from Table 2 that when m(─NCO)/m(─OH) is 0.5 ~ 0.7, that is, when the amount of TMI is 14% ~ 19% of the active ingredients of hydroxyacrylic resin, the amount of double bonds introduced is more appropriate. When m(─NCO)/m(─OH) is 0.6, the physical property index of the paint film reaches a satisfactory state. When m(─NCO)/m(─OH) is greater than 0.8, the flexibility of the paint film decreases significantly, and the yellowing tendency of the paint film increases.
2.6 Comprehensive performance of electrophoretic paint and paint film
The UV acrylic cathodic electrophoretic coating prepared under good conditions (DMAEMA accounts for 10% ~ 14% of the total monomer mass, HEMA 15% ~ 20%, TMI 14% ~ 19%) was tested and produced with Japan Shimizu Co., Ltd. Compared with similar products, the results are shown in Table 3. It can be seen from Table 3 that the photocuring rate of the prepared UV acrylic cathodic electrophoretic coating and the main physical properties of the paint film are equivalent to those of "Nippon Shimizu" products, but the dispersibility of the paint, the fullness of the paint film, and the tendency of yellowing resistance There is still room for improvement.
Table 3 Performance comparison between self-made products and similar foreign products
3 Conclusion
UV-curable acrylic cathodic electrophoretic coating resin was synthesized by grafting the unsaturated double bond of TMI onto the chain segment of hydroxyl acrylic resin, and introducing UV-curable "double bond" groups into the chain segment of acrylic resin. After the paint test, the performance of the paint film is equivalent to that of similar foreign products. The obtained paint film has a smooth appearance and good mechanical properties. It can be used in the surface coating industry with high requirements such as furniture hardware, metal spectacle frames, and decorative aluminum profiles.
