Development and application of low temperature cured GMA acrylic powder coatings

Abstract:
modified the epoxy acrylic resin, and cured with binary pyric acid, through mixing, melting extrusion, grinding powder, screening to make suitable for plastic substrate coating of 120 degrees C low-temperature cured epoxy acrylic powder coating. Combined with infrared curing, curing time can be reduced. After surface treatment of polypropylene-based automotive bumpers, a low-temperature cured epoxy acrylic powder coating is applied to surface-treated automotive bumpers to enhance the adhesion of the coating to the bumper . The test results show that the coating has excellent appearance, adhesion and weather resistance, which reduces VOC emissions when the bumper is applied.
Acrylic powder coating has excellent decoration, weather resistance, pollution resistance, corrosion resistance, moisture and heat resistance, with excellent hardness and wear resistance, widely used in automotive, home appliances and high-grade decoration fields; GMA acrylic powder coating is an important branch of powder coating, it is made of GMA acrylic resin, binary pyric acid, pigments and a variety of functional additives made of powder coating. Conventional powder coatings have high curing temperature and long time under the condition of hot air cycle curing, and polypropylene in automobile bumpers is prone to deformation. In this paper, the polypropylene automobile bumper with low temperature curing epoxy acrylic powder coating, the system can achieve low temperature curing at 120 degrees C, combined with infrared curing, can shorten the curing time, effectively avoid the deformation of plastic. By treating the bumper surface, the adhesion of coating and polypropylene is solved.
1
Experimental Part
1.1 Raw Materials
Epoxy Acrylic Resin A, B, C: Imports; Biology; twenty-alkyl dichloric acid (C20): Imported; Degassing agent: Troy; leveling agent: Estron;Salt Promoter: Jietonda; Antioxidant: BASF; Plastic substrate adhesion promoter: Homemade; Carbon Black: Orion.
1.2 Test Equipment
Parallel Twin Screw Extruder: ZS-35C, Yangzhou Far Away;
1.3 Preparation of acrylic powder coatings and coatings
GMA acrylic resin, binary acid curing agent, degassing agent, curing promoter, leveling agent through high-speed mixer mix 3min, melt extrusion, grinding powder, and by 200 mesh screening, acrylic powder coating (extrusion temperature: 30 degrees C→120 degrees C→90 degrees C; feeding speed: 22r/min; screw speed: 45r/min). Spray with electrostation gun, spray voltage of 40 to 50kV, coating film thickness of 60 to 80 m, with reference to the curing conditions in the text for curing.
1.4 Analysis and Testing
1.4.1 Thermal Analysis
using DSC to analyze thermal changes during curing of powder coatings. The powder has a mass of 5 to 10 mg, and heats up from 25 degrees C to 300 degrees C at a heating rate of 10 degrees C/min, and analyzes its glassy transition temperature Tg, heating peak peak temperature TP, and reaction H. Then drop to room temperature, then heat up to 150 degrees C at a heating rate of 10 degrees C/min to analyze the coating Tg.
1.4.2 Performance Testing
Chemical Resistance Test: Soak the coating in a 50-degree-C acid-base solution to observe the color changes of the coating. The volume concentration of nitric acid used in the experiment was 10%, the mixed acid used in the experiment was made of H3PO4 solution, HNO3 solution and HF solution with a volume concentration of 10%, and the alkali solution in the experiment was made of NaOH solution with pH-13. The results were described as follows: × - severe color change, √ - significant change of color, √√√ - slight change of color, √√√√ - unchanged color.
CASS salt spray experiment with GB/T 10125-2012, impact resistance by GB/T 1732-1993 test coating, GB/T 9275 - Pencil hardness of 2008 test coating, inter-layer adhesion of GB/T 9286-1998 test coating, aging performance of xenon lamp by GB/T 1865-2009 test coating.
2
results and discussion
2.1 acrylic synthetic monomer
acrylic powder coating resin resin is composed of a variety of monomers, according to molecular structure differences and the role in the resin can be divided into three categories: hard monomer, soft monomer, cross-linked monomer. A description of the various types of monosomes can be found in Table 1.
in resin synthesis, the Tg of the resin can be predicted using the Fox formula (e.g. formula (1).
, the glass transition temperature of Tg-co-polymer, K; Tgn - glass transition temperature of the oxide formed by the polymerized monomers involved in the polymerization, K; Wn - Mass score for each monosome.
From the Fox formula, it can be seen that: the average polymer Tg of the hard monomer is high, if its content is high, then the Tg of the acrylic resin is high; Therefore, according to the Fox formula, the composition of the soft and hard monosome under the appropriate Tg can be estimated.
content of the cross-linked monosome determines the number of epoxy groups involved in the curing reaction, thus affecting the amount of curing agent in the powder coating and the cross-link density of the coating film, and ultimately affecting the performance of the coating film. The more cross-linked monobodies, the more epoxy groups are available for reaction, and the better the film tightness. High density coating resistance, weather resistance and other performance will be improved, but too dense will cause the coating film is very brittle, physical mechanical performance is very poor. Therefore, the physicochemical properties of the coating film should be considered to determine the content of the cross-linked monosome.
effect of 2.2 GMA acrylic resin on coating performance
This paper examines the effect of epoxy acrylic resin as the main film-forming substance on film-coating performance. The representative epoxy acrylic resins A, B and C on the market were selected. The properties of the three resins are in Table 2.
can be seen from Table 2, resin A and B performance is similar, resin A is mainly used in colored acrylic powder coating, good wetting of pigments, high price; B resins are used in ordinary transparent powder coatings and are inexpensive. The Tg of resin C is high and the viscosity of the melt is large.
In accordance with the formula of Table 3, the above three resins are mixed, extruded, milled, screened to prepare powder coating, sprayed on aluminum or steel substrates, and then cured by 175 degrees C/20min preparation coating, to investigate the effect of resin on coating performance, the results as shown in Table 4.
From Table 4, it can be seen that the performance of the powder coating made from resin A and B is close, because the Tg of the resin C is high, the viscosity is large, the relative molecular quality of the resin is high, the content of the hard monoid is high, so the inter-layer adhesion and impact resistance of the coating is poor, but the preparation of the powder coating Tg is high and the coating hardness is high.
Tg low in acrylic powder coatings made from resin A or resin B, the powder is easy to lump, but by adding high Tg resins, the Tg of the powder coating increases significantly without affecting coating performance. The reason is that the resin of high Tg is relatively high molecular mass, the content of hard monosome is high, the chain segment is not easy to move, while the low resin of Tg is already integrated with the high Tg resin through the interoperability network structure through melting extrusion, and the high Tg resin chain segment is not easy to move, thus preventing the movement of the resin chain segment of low Tg resin, thus showing the rise of powder coating Tg, storage stability is improved.
it is difficult to make powder coating with excellent physical and chemical properties at the same time, can be mixed with 2 or several resins, so that powder coatings have the performance of several resins, showing the effect of "1 plus 1>2". Taking into account the cost and performance, formula 5 is best (combining resin B and resin C with a mass ratio of 1:1).
2.3 Effect of carbon chain length of curing agent on coating film performance
Epoxy acrylic resin curing is usually cured with binary acid, commonly used in tyrannyl diacetic acid (DDDA). This paper examines the effect of binary acid curing agents of different carbon chain lengths on coating properties, and the results are shown in Table 5.
table 5 shows that the more carbon in the curing agent carbon chain, the longer the carbon chain, the greater the flexibility of the carbon chain, the better the impact resistance of the coating. The coating cured by 20 alkyl nicolic acid has excellent impact resistance, but the coating has slightly poor acid resistance and is rarely sold on the market, so GMA acrylic curing agents choose 12 carbons or 14 carbon binary acids.
2.4 Study of curing behavior of low-temperature cured GMA acrylic powder coatings
In order to prepare low-temperature cured GMA acrylic powder coatings, B, C2 acrylic resins andsalt promoters, antioxidants are squeezed out by extruder melting out, making modified GMA acrylic resins. The modified GMA acrylic resin and DDDA, degassing agent, leveling agent, carbon black according to table 6 formula, mixing, extrusion, grinding powder to make low temperature curing GMA acrylic powder coating, and then the powder sprayed the model in 120 degrees C infrared curing furnace, hot air oven curing, the resulting coating performance as shown in Table 7.
As can be seen from Table 7, according to the Tg test results, the low temperature cured GMA acrylic powder coating prepared by modified low temperature cured acrylic resin can be cured completely in the oven 120 degrees C curing 30min, in the infrared curing furnace 120 degrees C curing 8min can be cured completely. The coatings are chemically resistant under these two curing conditions.
low-temperature cured GMA acrylic powder coating in infrared curing conditions can achieve 120 degrees C/ 8min curing, achieve rapid curing of low temperature, expand the application range of powder coating, plastics, carbon fiber composites, MDF plates and other thermal substrates can be applied with this powder.
2.5 Low temperature curing GMA acrylic powder coating application on automotive bumpers
automotive bumpers by modified polypropylene (PP) molding by injection molding, due to PP and other conventional plastic parts temperature difference, conventional powder coating due to high curing temperature, easy to cause plastic parts deformation, so the current bumper is mostly liquid coating. In view of the low temperature curing of GMA acrylic powder coating in this study, the application effect in automobile bumper coating is examined.
modified polypropylene is a non-polar plastic, and whether solvent-based coatings, water-based coatings or powder coatings are made of polar resins, the two are not compatible. And PP is an insulator, paint spray is difficult to powder. Therefore, before coating, the bumper needs to be treated to improve the coating's powdering capacity and coating adhesion on the plastic substrate, the specific process is as follows: PP automotive bumper →fire treatment to improve surface polarity→ spray homemade adhesion promoter → drying→ coating.
In this study, after the above-mentioned process processing bumper with 2.4 prepared low-temperature cured GMA acrylic powder coating for coating, and in 120 degrees C/8min infrared conditions curing, coating performance as table 8, coated car bumper photo as shown in Figure 1.
can be seen from Tables 8 and 1, the low-temperature cured GMA acrylic powder coating is applied to automotive insurance coating, resulting in excellent coating performance to meet GM standards.
3

by analyzing the structure of the resin used in GMA acrylic powder coatings, and modifiing the GMA acrylic resin to improve its low-temperature curing properties, while selecting the appropriate curing agent, the resulting storage performance is good, can achieve rapid curing of the powder coating at 120 degrees C/8min infrared low temperature conditions. The powder coating was successfully applied to the automobile bumper coating. The results show that the automotive bumper coating coated with low temperature cured GMA acrylic powder coating has excellent performance, and the process reduces VOC emissions from the car bumper coating process.
/ Pan to the east, Ba Xumin
Asda Huajia Coatings (Yellow Mountain) Co., Ltd