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Powder coating and coating
Information technology dry goods industry group
Pay attention to the synthesis and application of acrylic resin for high gloss powder coating
Huang Wei, Guo Changpei
Nanfang resin Guangzhou industry association polymer Co., Ltd
Abstract: carboxyl acrylic resin for high gloss powder coating was synthesized by solution polymerization. The influence of various factors on the performance of the resin, as well as the coating curing and powder coating performance were discussed. The results show that the acrylic powder coating has the advantages of high hardness, strong solvent resistance, water boiling resistance, acid alkali resistance and weather resistance.
1 foreword
In recent years, China's powder coating industry has developed rapidly. The production of powder coating ranks in the forefront of the world, and the varieties are gradually complete. Acrylic resin is a kind of high decorative and weather resistant resin, which has good aging resistance, corrosion resistance and hardness, high mechanical strength, good light and color retention, excellent wear resistance and so on. GMA type acrylic resin and its powder coating have been industrialized, but at present, GMA acrylic resin in China is mainly used in the powder coating for low gloss workpieces by its matting effect after curing with carboxyl polyester resin. With the expansion of the application of acrylic powder coating, some areas also put forward new requirements for the acrylic powder coating with good weather resistance - high gloss. Therefore, based on the original GMA type acrylic resin, our company developed the carboxyl type acrylic resin and solidified it with the epoxy resin to obtain the acrylic powder coating with high gloss and high weather resistance, which greatly reduced the cost of the powder coating, filled in the blank of the resin for the high gloss and high performance acrylic powder coating in China, and provided a price for developing the domestic acrylic powder coating market High specific resin raw materials.
2 synthesis of acrylic resin
The solid content of acrylic resin for powder coating must be more than 99%, and the relative molecular weight must be 2500-10000. Our company adopts solution polymerization method to prepare acrylic resin. The process is simple and stable. The reaction conversion, relative molecular weight and distribution of resin are easy to control. The acrylic resin obtained is suitable for powder coating.
2.1 test raw materials
See Table 1 for raw materials for synthetic acrylic resin.
2.2 synthesis process
Adding solvent, monomer and initiator into the reaction vessel, the free radical polymerization was carried out under the protection of nitrogen at 100-130 ℃ and a certain stirring speed for 3-6h. After the polymerization, the solvent was removed to obtain solid acrylic resin, No. ar-8823. The synthesis mechanism is as follows:
Functional acrylic monomers + (meth) acrylate monomers + other unsaturated monomers → acrylic resin
Due to the different reaction mechanism, the synthesis process of acrylic resin and traditional polyester resin is quite different. The reaction activity of various acrylic monomers is different, so the reaction rate between the comonomers is not consistent, resulting in the uneven distribution of monomer units in the chain, which has a great impact on the performance of powder coating prepared by synthetic resin, so the synthesis process control is very important.
2.3 ar-8823 performance
We tested the properties of ar-8823 acrylic resin, and the test results are shown in Table 2.
2.4 synthesis process of acrylic resin
The introduction of functional monomer (m) AA provides the necessary functional group for the reaction of acrylic resin and curing agent. If the amount of (m) AA is large, it can improve the hardness and mechanical strength of the acrylic powder coating, and significantly improve the solvent resistance and salt spray resistance of the coating; however, if the amount of (m) AA is too large, it will lead to the excessive crosslinking density of the coating, the poor leveling of the coating, serious orange peel, and the decrease of the gloss of the coating with the increase of the acid value.
When the acid value is fixed, the size and distribution of resin molecular weight (functionality) have a great influence on the final performance of powder coating. When the number of acrylic resin is very small, the film surface has good leveling and high gloss; when the molecular weight of resin is too high, the film has poor leveling, orange peel is serious, but the film has improved solvent resistance. Considering all the factors, the molecular weight of the resin can be adjusted from 3000 to 10000 by changing the amount of initiator and the polymerization temperature, so that the comprehensive performance of the coating is better.
3 powder coating and coating performance
Acrylic resin with carboxyl side chain can react with bisphenol A epoxy resin to form film. As shown in Figure 1, adding appropriate amount of catalyst 2-methylimidazole is conducive to the ring opening of epoxy group, improving the reaction speed and curing conversion rate of epoxy group and carboxyl group, which is indispensable in the powder coating formula. The amount of catalyst determines the gelation time of the powder coating. Too short gelation time will lead to poor smoothness of the coating. If the gelation time is too long, the coating will have defects such as sagging, poor edge coverage and so on.
3.1 powder coating reference formula
For comparison, we use ar-8823 and polyester resin to prepare powder coating respectively, and the basic formula is shown in Table 3.
3.2 pulverizing and coating
According to the formula in Table 3, dry powder is premixed, extruded by twin-screw extruder at 100-110 ℃, cooled, preliminarily crushed, high-speed crushed by crusher, and powder coating is obtained through 180 mesh screen. The powder coating is electrostatically sprayed on the surface treated tinplate, baked and solidified in an oven at 190 ℃ for 15 minutes, and then taken out and cooled for testing.
It should be noted that the compatibility between acrylic resin and other powder coating resin base materials is poor, and strict isolation measures should be taken in the process of powder production and coating. In the process of experimental extrusion, the extruder must be cleaned to avoid defects such as coating particles and shrinkage cavity. It is better to use a special production line to prepare acrylic powder.
3.3 coating performance
See Table 4 for the performance test results of the powder coating prepared by ar-8823 and ordinary polyester.
The test results in Table 4 show that the leveling property of ar-8823 formula high gloss powder coating is very good, which is equivalent to that of ordinary polyester formula. 94% high gloss can meet the technical requirements in many fields, and the hardness of coating film is up to 2h, while the hardness of most polyester powder can only reach H. The solvent resistance, boiling water resistance and alkali resistance of the coating are the most outstanding advantages of the acrylic powder coating in this study. The MEK Resistance of the coating is up to 30 times, while the MEK Resistance of the common polyester formula is up to 5 times; the ar-8823 formula powder coating can be boiled continuously for 32H without losing light, and the light loss of the common polyester formula powder coating is very serious after 32H; the alkali resistance of the ar-8823 formula powder It is very excellent, but the alkali resistance of polyester powder is very poor.
With the increase of the acid value of the resin, the crosslinking density, hardness, luster and solvent resistance of the coating increased. For example, the acid value of ar-8845 is as high as 206, the gloss of the coating film prepared with ar-8845 is reduced to 4%, the hardness is as high as 3h, and the MEK Resistance is more than 100 times. Most of the properties of the powder coating prepared by ar-8830 resin are between ar-8823 and ar-8845.
The gelation time has a great influence on the properties of coatings and films. It is found that in the test formula in Table 3, if the dosage of 2-methylimidazole is 0.2%, the gelation time of the coating is 32S; when the dosage of 2-methylimidazole is 0.1%, the gelation time of the coating is extended to 60s. In this gelation time range, the properties of powder coatings have not changed significantly. However, when the amount of 2-methylimidazole is reduced to 0.05%, the gelation time of the coating is extended to 4 min; if the catalyst is not used completely, the curing reaction of the coating is very slow, and it can not be gelatinized for more than 10 min. Therefore, 2-methylimidazole catalyst is essential in the formulation.
According to the impact resistance of the coating, the impact performance of ar-8823 is improved, and the coating is intact and does not crack during positive impact, which well balances the contradiction between flexibility and hardness of the coating. However, due to the inherent characteristics of acrylic resin - hard and brittle, the impact resistance of acrylic powder coating is not satisfactory. Therefore, we suggest that acrylic powder coating is best used for powder coating of molding products without tailoring after coating.
The powder coating is made up of carboxylic acrylic resin ar-8823 and epoxy resin. Because the super weatherability of acrylic resin makes up for the poor weatherability of epoxy resin to some extent, the weatherability of the powder coating obtained is not as good as that of pure polyester powder, but better than that of polyester / epoxy powder coating.
4 Conclusion
With BPO as initiator, MMA, EA, Ba, St, i-boma, and (m) AA as monomers, we synthesized carboxyacrylic resin with excellent properties by semi continuous dropping reaction at 100-130 ℃ for 3-6h, adjusting the amount of initiator, solvent, monomer ratio and reaction temperature. The powder coating based on ar-8823 and epoxy resin has good comprehensive properties, such as high gloss, solvent resistance, boiling water resistance, acid and alkali resistance and strong weather resistance.
The powder coating made of acrylic resin has a series of advantages, such as high hardness, good chemical resistance, good adhesion, good weather resistance, good electrostatic coating effect and thin coating. In Europe and America, the proportion of acrylic powder coating is not small. It is widely used in household appliances, kitchen cookers, machinery, automobiles, bicycles, motorcycles, aerospace, ships, chemical anti-corrosion, road guardrails, metal furniture, office supplies, construction and decoration industries. With the further improvement of China's powder coating structure and the development of export trade, acrylic powder coating has become a research hotspot in recent years. In the future, acrylic powder coating will surely get greater development.
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