Development and application of GMA acrylic powder

2022-10-18
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Development and application of low temperature curing GMA acrylic powder coating

development and application of low temperature curing GMA acrylic powder coating

January 16, 2019

Abstract: epoxy acrylic resin was modified and cured with dicarboxylic acid. Through mixing, melt extrusion, grinding and screening, a 120 ℃ low temperature curing epoxy acrylic powder coating suitable for plastic substrate coating was prepared. Combined with infrared curing method, the curing time can be shortened. After the surface treatment of the polypropylene automobile bumper, the low-temperature curing epoxy acrylic powder coating is coated on the surface treated automobile bumper to improve the adhesion of the coating on the bumper. The test results show that the coating has excellent appearance, adhesion and weather resistance, which reduces the VOC emission of bumper coating at present

acrylic powder coating has excellent decoration, weather resistance, pollution resistance, corrosion resistance, humidity and heat resistance, excellent hardness and wear resistance, and is widely used in the fields of automobiles, household appliances and high-end decoration; Especially in the automotive field, the application prospect is very good, which has aroused research interest at home and abroad. At present, it has rapidly become an important variety of powder coatings at home and abroad. GMA acrylic powder coating is an important branch of powder coating. It is a powder coating made of GMA acrylic resin, dicarboxylic acid, pigment and various functional additives. Under the condition of hot air circulation curing, the conventional powder coating has high curing temperature and long curing time, and the polypropylene of automobile bumper is prone to deformation. In this paper, a low-temperature curing epoxy acrylic powder coating for polypropylene automobile bumper was prepared. The system can realize low-temperature curing at 120 ℃. Combined with infrared curing, it can shorten the curing time and effectively avoid plastic deformation. The adhesion between the coating and polypropylene was solved by treating the surface of the bumper

1

experimental part

1.1

raw materials

epoxy acrylic resin A, B, C: imported; Dodecyl dicarboxylic acid (DDDA), tetradecyl dicarboxylic acid (C14): Shandong Kaisai biological; Eicosyl dicarboxylic acid (C20): imported; Degassing agent: Troy; Leveling agent: estron; Onium salt accelerator: jietonda; Antioxidant: BASF; Adhesion promoter of plastic substrate: self made; Carbon black: Orion

1.2

test equipment

parallel twin screw extruder: zs-35c, Yangzhou Yuanfa; Differential scanning calorimeter (DSC): TA - 200, Ta company; Xenon lamp aging instrument: xe-3, q-sun

1.3

preparation of acrylic powder coating and coating

GMA acrylic resin, dibasic acid curing agent, degassing agent, curing accelerator and leveling agent are mixed in a high-speed mixer for 3min, melt extruded, ground, and sieved through 200 meshes to prepare acrylic powder coating (extrusion temperature: 30 ℃ → 120 ℃ → 90 ℃; feeding speed: 22r/min; screw speed: 45r/min). Spray with electrostatic gun, the spraying voltage is 40 ~ 50kV, and the coating film thickness is 60 ~ 80 μ m. Cure according to the curing conditions in the text

1.4

analysis and testing

1.4.1

thermal analysis

analyze the thermal changes of powder coatings during curing with DSC. The mass of the powder is 5 ~ 10mg, and it is heated from 25 ℃ to 300 ℃ at a heating rate of 10 ℃/min. The glass transition temperature Tg, the peak temperature TP of the exothermic peak, and the reaction enthalpy h are analyzed. Reduce to room temperature again, and then raise the temperature to 150 ℃ at the heating rate of 10 ℃/min, and analyze the Tg of the coating

1.4.2

performance test

chemical resistance test: soak the coating in acid-base solution at 50 ℃ for 5h respectively, and observe the color change of the coating. The volume concentration of nitric acid used in the experiment is 10%; The mixed acid used in the experiment was prepared from H3PO4 solution, HNO3 solution and HF solution with a volume concentration of 10% in the ratio of 1:1:1 by volume; The alkali solution used in the experiment is NaOH solution with ph=13. The results are described as follows: ×— Severe discoloration; √ - obvious discoloration; √√√ - slight discoloration; √√√√ - no color change

Cass salt spray test of coating shall be carried out according to gb/t

10125-2012; Test the impact resistance of the coating according to gb/t 1732-1993; Test the pencil hardness of the coating according to gb/t 9275-2008; Test the interlayer adhesion of the coating according to gb/t 9286-1998; The xenon lamp aging performance of the coating shall be tested according to gb/t

1865-2009

2

results and discussion

2.1

synthetic monomer of acrylic resin

resin for acrylic powder coating is copolymerized by a variety of monomers. According to the difference of molecular structure and the different role in the resin, it can be divided into three categories: hard monomer, soft monomer and cross-linked monomer. See Table 1 for the introduction of various monomers

in resin synthesis, fox formula [such as formula (1)] can be used to predict Tg of resin

in the formula, TG - glass transition temperature of copolymer, K; TGN - glass transition temperature of homopolymer formed by monomers participating in polymerization, K; Wn - mass fraction of each monomer

according to Fox formula, the Tg of homopolymer of hard monomer is high, and if its content is high, the Tg of acrylic resin is high; On the contrary, the Tg of homopolymer of soft monomer is low. If its content is low, the Tg of acrylic resin is low. Therefore, the appropriate composition of soft and hard monomers under TG can be estimated according to Fox formula

the content of crosslinking monomer determines the number of epoxy groups involved in the curing reaction, which affects the amount of curing agent in powder coatings and the crosslinking density of the film, and ultimately affects the performance of the film. The more crosslinked monomers, the more epoxy groups available for reaction, and the better the compactness of the film. The chemical resistance and weather resistance of the film with high density will be improved, but too dense will cause the film to be very brittle and poor physical and mechanical properties. Therefore, it is necessary to comprehensively consider the physical and chemical properties of the film to determine the content of crosslinking monomer

2.2

gma acrylic resin on the film properties

this paper investigated the effect of epoxy acrylic resin as the main film-forming material on the film properties. Choose gbt15831 rotating fastener mechanical property test. What should we pay attention to? Take the representative epoxy acrylic resin A, B, C in the market. See Table 2 for the properties of three resins

it can be seen from table 2 that the properties of resin A and B are similar. Resin A is mainly used in colored acrylic powder coatings, which has good wettability to pigments and high price; B resin is used in ordinary transparent powder coatings, which is cheap. Resin C has high Tg and high melt viscosity

according to the formula in Table 3, the above three resins are mixed, extruded, ground and sieved to prepare powder coatings, sprayed on aluminum or steel substrates, and then cured at 175 ℃/20min to prepare coatings. The effects of resins on coating properties are investigated. The results are shown in Table 4

it can be seen from table 4 that the properties of powder coatings made of resins A and B are similar; Due to the high Tg, high viscosity, high relative molecular weight and high content of hard monomers of resin C, the interlayer adhesion and impact resistance of the coating are poor, but the powder coating prepared by it has high Tg and high coating hardness

the acrylic powder coating made of resin A or resin B has low Tg and the powder is easy to agglomerate, but by adding high Tg resin, the Tg of the powder coating increases significantly, and the coating performance will not be affected. The reason is that the high Tg resin has high relative molecular weight, high content of hard monomers, and the chain segment is not easy to move; The low Tg resin has been integrated with the high Tg resin through interpenetrating structure through melt extrusion; The high Tg resin segment is not easy to move, which also prevents the movement of the low Tg resin segment, which shows that the Tg of powder coating increases and the storage stability improves

it is difficult to make the powder coating have excellent physical and chemical properties at the same time by relying on a certain resin alone. Two or more kinds of trees can be used to mix the grease when the metal object approaches the sensing surface, so that the powder coating has the properties of several resins, showing the effect of "1+1> 2". Considering the cost and performance comprehensively, the formula of 5 × 5 (combining resin B and resin C with a mass ratio of 1:1) is the best

2.3

effect of carbon chain length of curing agent on film properties

dibasic acid is usually used as curing agent for epoxy acrylic resin curing, and dodecyl dicarboxylic acid (DDDA) is commonly used. In this paper, the effects of dicarboxylic acid curing agents with different carbon chain lengths on the film properties were investigated, and the results are shown in Table 5

it can be seen from table 5 that the more carbon in the carbon chain of the curing agent, the longer the carbon chain, the greater the flexibility of the carbon chain, and the better the impact resistance of the coating. The coating cured by eicosyl dicarboxylic acid has excellent impact resistance, but the mixed acid resistance of the coating is slightly poor, and it is rarely sold in the market. Therefore, the curing agent of GMA acrylic resin selects 12 carbon or 14 carbon dicarboxylic acid

2.4

study on the curing behavior of low-temperature curing GMA acrylic powder coating

in order to prepare low-temperature curing GMA acrylic powder coating, B and C2 acrylic resins, onium salt accelerators and antioxidants were melt extruded in an extruder to prepare modified GMA acrylic resin. The modified GMA acrylic resin, DDDA, degassing agent, leveling agent and carbon black are weighed, mixed, extruded and powdered according to the formula in Table 6 to prepare a low-temperature curing GMA acrylic powder coating, and then the powder sprayed samples are cured in 120 ℃ infrared curing furnace and hot air oven respectively. The properties of the coating are shown in Table 7

it can be seen from table 7 that according to the TG test results, the low-temperature curing GMA acrylic powder coating prepared from modified low-temperature curing acrylic resin can be cured completely after curing at 120 ℃ for 30min in the oven, and it can be cured completely after curing at 120 ℃ for 8min in the infrared curing furnace. Under these two curing conditions, the coating has good chemical resistance

low temperature curing GMA acrylic powder coating can achieve 120 ℃/8min curing under infrared curing conditions, realizing low temperature rapid curing, and expanding the application range of powder coating. Plastic, carbon fiber composite, MDF board and other thermosensitive substrates can be coated with this kind of powder

2.5

application of low temperature curing GMA acrylic powder coating on automobile bumpers

automobile bumpers are injection molded from modified polypropylene (PP). Due to the poor temperature resistance of conventional plastic parts such as PP, conventional powder coating is easy to cause deformation of plastic parts due to high curing temperature, so at present, bumpers are mostly painted with liquid paint. In view of the low temperature curing of GMA acrylic powder coating in this study, the application effect of GMA acrylic powder coating in automobile bumper coating was investigated experimentally

modified polypropylene is a non-polar plastic, and whether solvent based coating, water-based coating or powder coating are made of polar resin, their compatibility is poor. And PP is an insulator, so it is difficult to powder when spraying paint. Therefore, the bumper needs to be treated before coating to improve the coating powder application ability and the adhesion of the coating on the plastic substrate. The specific process is as follows: PP automobile bumper → flame treatment to improve the surface polarity → spraying self-made adhesion promoter → drying → coating

in this study, the bumper treated by the above process is coated with low-temperature curing GMA acrylic powder paint prepared in 2.4 and cured under the infrared condition of 120 ℃/8min. The coating performance is shown in Table 8, and the photos of the coated car bumper are shown in Figure 1

it can be seen from table 8 and figure 1 that the low-temperature curing GMA acrylic powder coating is applied to the coating of automobile safety shoulder, and the coating has excellent performance, meeting the requirements of general motors standards

3

conclusion

by analyzing the structure of the resin used for GMA acrylic powder coating, and modifying GMA acrylic resin to improve its low-temperature curing performance, and selecting a suitable curing agent, a powder coating with good storage performance and fast curing under the infrared low temperature condition of 120 ℃/8min is prepared. The powder coating was successfully applied to the coating of automobile bumper

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