Preparation and weathering resistance of fluorinated polyester for coating powder

Polyester powder coating for outdoor use is a widely used thermosetting coating, which currently accounts for about 70% of the total output of powder coatings. It has the advantages of energy saving, environmental friendliness, and long service life, and is widely used on the surface of various outdoor parts coating [1-2]. With the expansion of the powder coating market, people have higher and higher requirements for its weather resistance [3], such as aluminum profiles for high-rise buildings and anti-corrosion of cross-sea bridges, etc. Therefore, further expanding its weather resistance has become the top priority for the polyester industry. Task. FEVE fluorocarbon resin (F—C) is mainly obtained by free radical block polymerization of chlorotrifluoroethylene and alkyl vinyl ether. Because the C—F bond in fluorocarbon resin is extremely short and the bond energy is large[4], the fluorine atoms are closely arranged around the polymer carbon chain, which plays a good role in protecting the C—C bond and the resin matrix, making FEVE Fluorocarbon resin has excellent super weather resistance. However, the high cost of pure FEVE fluorocarbon resin, and poor mechanical properties and heat resistance [5] limit its application in some fields.

In this study, the polyester resin containing fluorine in the molecular chain was prepared by adding FEVE fluorocarbon resin and vacuum polycondensation during the synthesis stage of polyester resin. The secondary performance shows that the prepared powder coating has excellent weather resistance and impact performance. . It shows that this method can be used to prepare super weather-resistant polyester resin for powder coatings.

1 Experimental part

1.1 Main raw materials

Neopentyl glycol (NPG), 2-methyl-1,3 propanediol (MPDI), trimethylolpropane (TMP), butylpropanediol (BEPD), 1,4 cyclohexanedimethanol (CHMD), ethyl Glycol (EG), terephthalic acid (PTA), isophthalic acid (IPA), adipic acid (ADA), 1,4 cyclohexanedicarboxylic acid (CHDA), trimellitic anhydride (TMA), fumaric acid ( FCC), monobutyl tin oxide (FC4100), titanium dioxide, barium sulfate, leveling agent, benzoin, etc., are all industrial products. Fluorocarbon resin: A certain type of FEVE fluorocarbon resin, its theoretical fluorine content is (23% ± 2)%, the hydroxyl value is (50 ± 5) mgKOH/g, and the acid value is (1.0 ± 0.5) mgKOH/g.

1.2 Main experimental equipment

A set of 5L glass reaction kettle, small powder-making equipment such as φ30 twin-screw extruder, electrostatic spraying equipment, impact Tester, gloss meter, film thickness meter, artificial accelerated aging meter and other secondary performance testing equipment.

1.3 Synthesis process of fluorine-containing polyester resin

按配比将多元醇和多元酸、FC4100 加入到 5L 的球形玻璃反应釜中， 缓慢升温至 240~255 ℃并维持，取样检测酸值(AV)达到 5~15 (单位 mgKOH/g 树脂，下同)时加入二次酸 IPA、ADA、CHDA、TMA 和 FCC 等进行酸解封端。待聚酯的酸值达到 40~60 时降温至 230~240 ℃并真空缩聚一段时间， 使得聚酯的酸值维持在25~35 左右。 随后降温至 200~230 ℃下投氟碳树脂并维持 0.5~3 h， 之后降温至 180~230 ℃投助剂并维持5~30 min 之后放料。 通过这种方法制备纯树脂以及有效氟含量是 1%和 2%的含氟树脂， 分别标记为 PE、PE+1%F 和 PE+2%F。

1.4 粉末涂料及其涂层的制备

按照表 1 的基本配方制备粉末涂料， 工艺流程为： 配料→预混→挤出→压片→粉碎→过筛→产品。随后将制备得到的粉末涂料用静电喷涂的方式将粉末涂料喷涂于样板上，然后置于烘箱中 200 ℃下烘烤10 min 固化并得到涂层。 纯氟碳树脂和含氟聚酯在制备粉末涂料的配方见表 1。

1.5 粉末的二次性能检测

胶化时间(gt)：采用自制的胶化时间测定仪，取适量粉末置于 200 ℃的铜板表面并不断搅拌拉伸，从开始融化到不能拉出丝时的时间定义为胶化时间，以秒(s)来表示。样板的光泽度采用比克光泽仪获得； 厚度通过Positest DFT 膜厚仪获得；冲击性能通过国营天津仪器试验机厂的冲击仪获得；200 ℃烘烤实验是在 binderFP 烘箱中进行。 采用 Xenon 人工加速老化试验机测试得到样板的耐候性数据，主要采用的主要技术参数为：光照温 度 65 ℃、光照+喷淋温度：65 ℃、辐照强度 0.51 w/m2 (340 nm )、相对湿度 50%、箱体温度 38 ℃；循环方式为：连续运行，18 min 润湿，102 min 干燥。

Table 1 Formulas of fluorocarbon resin and fluoropolyester resin in the process of powder preparation

2 Experimental results

2.1 Acid number and viscosity data and resin color

It is found from Table 2 that with the addition of fluorocarbon resin, the overall viscosity (η, the unit is mPa s/200 °C, the same below) presents an increasing trend. system is more evident. According to the analysis, it is because the hydroxyl group on the side chain of the fluorocarbon resin participates in the reaction with the carboxyl group in the polyester system, which increases the chain entanglement density of the polyester system, so its viscosity increases significantly.

Table 3 Relative molecular mass and distribution (PDI) summary data of fluorocarbon resin and polyester data

Since fluorocarbon resin is copolymerized by chlorotrifluoroethylene and alkyl vinyl ether, the presence of alkyl vinyl ether will cause a small amount of oxidative degradation of polyester resin at high temperature, and induce its color from colorless to light Yellow to deep yellow transition.

2.2 Relative molecular mass data of fluorocarbon resin and polyester resin

Fig. 1 is the distribution curve of the eluting time of different polyesters during the GPC test. It can be found from the efflux time data of different samples that the main chain of the fluorocarbon resin has been successfully grafted on the molecular chain of the polyester resin, indicating that a chemical polycondensation reaction has occurred between the two. Table 3 is a summary of the number-average molecular mass (Mn), weight-average molecular mass (Mw) and distribution (PDI) data of fluorocarbon resins and fluorine-containing polyesters with different fluorine contents. Quantitative relative molecular mass distribution data show that with the increase of fluorine content, the relative molecular mass distribution becomes wider, and the analysis suggests that part of the vinyl segment reacts with the polyester molecular segment, resulting in a wider distribution, which is consistent with the above mentioned The obtained viscosity data are in good agreement. The above data show that the fluorocarbon resin was successfully grafted onto the polyester main chain.

Figure 1 The retention time and mass ratio distribution curves of different resins: (a) pure polyester; (b) polyester with 1% fluorine content; (c) polyester with 2% fluorine content; (d) pure fluorocarbon resin.

2.3 Secondary properties of powder samples

表 4 为不同型号聚酯树脂二次性能数据汇总。 相对于纯氟碳树脂，含氟聚酯的表面光泽度较高，如有效氟含量在 2% 时， 样板的光泽度可以保持在 70%~85% ；含氟聚酯制备的样板冲击优异，如有效氟含量在2% 时，样板的冲击可以达到 100++ ；烘烤实验显示含氟聚酯制备得到的样板在保光率和色差都和纯聚酯树脂比较接近，显示其具有优异的耐热性。 含氟聚酯的胶化时间和纯聚酯和的胶化时间接近，使得其相对于氟碳树脂而言具有优异的流平性能。 上述二次性能显示含氟聚酯的流平、冲击和耐烘烤性能等常规性能已经接近常规聚酯，且优于氟碳树脂。

2.4 人工老化

表 5 为纯聚酯和含氟树脂在人工老化过程中不同阶段的保光率数据汇总。 从表格中可以观察发现纯聚酯在 886 h 总有的保光率在 55%~60% ， 氟含量为1% 的聚酯保光率是 60%~65% ，而氟含量是 2% 的聚酯保光率是 65%~70% 。 同样观察发现，在 1 119 h 出含氟聚酯树脂的保光率显著上升，且上升的幅度随着氟含量的增加而显著增加。 因此可以判断氟碳树脂的引入显著的增强了纯聚酯的耐候性， 且随着氟含量越高，耐候性增加的幅度越大。

表 4 不同型号树脂样板的二次性能数据汇总

表 5 不同型号树脂样板在不同时间段的保光率

3 结语

The polyester resin containing fluorine in the molecular chain was successfully prepared by adding FEVE fluorocarbon resin into the resin during the synthesis stage of the polyester resin. Experimental tests show that its leveling, impact performance and heat resistance are close to those of pure polyester resin. Artificial aging shows that compared with pure polyester, the weather resistance of fluorine-containing polyester resin is significantly increased, and the increase range increases with the fluorine content. increased by the increase. Fluorine-containing polyester resin for super weather resistance can be successfully prepared by this method.