Development of Scratch Resistance Matte Black PU Coatings for Automotive Interior Parts

Qiu Shixin (Zhongshan City Hengmei Ink & paint Co.Ltd.ZhongshanGuangdong 528441,China

Abstract:To solve the problem of lower scratch resistance of matte black PU coatings,according to the characteristics and quality demand for automotive interior parts coatings,an excellent anti-scratch single-coat matte black PU coatings for automotive interior parts was developed by selection of different hydroxy acrylic resins,suitable additives and optimized formulation design,which was formulated with a blended binder of medium hydroxyl value acrylic resin,and a specific functional resin and mixed with matting agents,magnesium powder,wax etc.The overall performance of the new coatings was tested.The results showed that the matte black PU coatings developed provided better comprehensive performance,especially the scratch resistance. It could be scaled up for further applications for automotive interior parts.

With the continuous and rapid development of China’s society and economy, the number of car ownership continues to grow rapidly. As of the end of 2018, the country’s car ownership reached 240 million units, an increase of 22.85 million units over 2017 and an increase of 10.51%. From the perspective of vehicle types, the number of small passenger cars reached 201 million, exceeding 200 million for the first time, an increase of 20.85 million or 11.56% over 2017. The huge car ownership and the rapid growth of the market means that China has a huge consumer market for automotive coatings. The automotive interior coatings market is an important part of the coatings industry, and of course it is no exception. Due to the high light reflectivity of high-gloss coatings, it is easy to cause confusion to human vision; at the same time, consumers are increasingly inclined to pursue leisure and personalization, coupled with environmental protection, raw material prices and other factors, resulting in single visual effects with soft visual effects. The market demand for matt PU coatings, especially for single-coat matt black PU coatings, has increased dramatically. However, in use, it has been found that the single-coated matte PU coating, especially the matt black PU coating widely used in automotive interior paint, has not been scratched after the coating film is scratched by sharp foreign objects such as nails. However, it is easy to leave a shiny mark that is not the same as the one where the gloss is not scratched. At present, the industry generally uses a method of selecting a high Tg resin and adding wax powder to improve the scratch resistance. Although it has some effects, it is not ideal. Therefore, how to solve the problem of bright marks generated after scratching is particularly prominent. In this study, the resin was matched with micro-nano-improved resin to improve scratch resistance. In terms of additives, the synergistic anti-scratch effect of wax powder and magnesium strong powder was used. The wax powder and magnesium strong powder were added to the formula to improve The purpose of anti-scratch performance, through the selection and formulation testing of formulated resins and additives, put forward an effective way to improve the problem of bright marks, and provide a certain reference for the research of interior matte coatings.

1 Experimental section
1. 1 Main raw materials
LF-3120-6HV hydroxyl acrylic resin: Richtek; HA3616C hydroxyl acrylic resin: Hitachi Kasei; ZH-220 hydroxyl acrylic resin, HY-1238 hydroxyl acrylic resin: customized; 6900-10X polyamide wax: Nanben; ZN-38 wax slurry : Gifu; TF1792 polytetrafluoroethylene wax powder: Lubrizol; BYK331 leveling agent: Becker auxiliaries; TIN-22 drier: Hemings; N3390 BA / SN curing agent: Covestro; nano-grade SiO2 : Aladdin Reagent (Shanghai) Co., Ltd .; Micron-grade SiO2: Homemade according to literature [2]; OK520 matting powder (particle size 6. 5 μm), OK607 matting powder (particle size 4. 4 μm): Evonik; E- 1011 Matting powder (particle size 1.5 μm): Tosoh; Ultra-fine magnesium powder: Monkey Saint; Black color paste: Homemade; Ethyl acetate, n-butyl acetate, methyl isobutyl ketone, diacetone alcohol, Diisobutyl ketone:
Preparation of commercially available

 1.2 special modified functional resins.
Nanometer and micron SiO2 were dispersed in ethanol, electrically stirred for 0.5 h until the solution was uniformly dispersed, and ultrasonic vibration was 0.5 h; 2% silane coupling agent in ethanol solution, add a small amount of water to hydrolyze the silane coupling agent, and stir for 10 min; The ethanol dispersion solution of rice and micron SiO2 was mixed with the silane coupling agent ethanol solution, stirred for 1 h until homogeneous, suction filtered, and dried at 80 ° C under vacuum, and then washed again with ethanol, centrifuged, and the bottom sediment was taken and dried at 80 ° C under vacuum. Dry to constant mass to obtain nano and micro SiO2 with surface modification treatment. The modified micron-sized SiO2 and nano-sized SiO2 are mixed at a mass ratio of 4: 1, and then the mixed modified SiO2 which accounts for 0.5% of the total mass is added to HA3616C hydroxyacrylic resin, and magnetically stirred for 20 minutes to make it uniformly dispersed, 50 The modified functional resin was prepared by sealed ultrasonic dispersion at ℃ for 2 h.

1.3 Preparation of the samples
Prepare the coatings according to the formula in Table 1. Spray the test film with W-101 spray gun. The dry film thickness should be controlled at 20 ~ 25 μm. Leave it at room temperature for 2 ~ 5 minutes and put it in an air drying oven at 80 ℃. Continue baking for 8 hours, and then let it cool naturally for 24 hours after taking out to test the film performance.

Coating formula

2 Results and discussion
2.1 Selection of the main resin and its influence
on the performance of the coating film In single-coated automotive interior PU coatings, acrylic resins with medium and low hydroxyl values ​​are usually used as the main reason, mainly due to: (1) acrylic resins with low and medium hydroxyl values Resin has the characteristics of good weather resistance, excellent water and chemical resistance, and fast drying; (2) has good adhesion on the surface of ABS, PC, PC + ABS materials. The scratch-resistant matte black PU coating prepared in this article needs to meet all the performance requirements of Volkswagen’s interior coatings. Based on the formula in Table 1, three different resins (ZH-220: 50% solids, hydroxyl value) were selected for the experiment. 15 mgKOH / g; HY-1238: solid content 50%, hydroxyl value 60 mgKOH / g; LF-3120-6HV: solid content 50%, hydroxyl value 50 mgKOH / g), tested according to Volkswagen standard TL 226 Several key performance requirements are shown in Table 2.

Film properties of different resins

Note: (1) —No color or feel change is required, the adhesion is 0, scratch resistance is not allowed, and marks on the base layer due to load are allowed; (2) gloss is allowed to increase without color and feel change. Focus on 0, scratch resistance does not allow cracks, and allows imprints on the substrate due to load. Same below.It can be known from Table 2 that the ZH-220 hydrolytic storage and skin cream test failed, the main reasons may be its low crosslink density and poor resistance to moisture and heat damage to the coating film structure. HY-1238 resin and LF-3120-6HV both passed key performance tests. Among them, HY-1238 resin has the highest hydroxyl value among the three resins and has a high gloss. When the coating film needs to be reduced to the same gloss, The formulation of HY-1238 resin needs to add more matting powder for matting, which may cause the matting powder to aggregate on the surface, thereby reducing the synergistic anti-scratch effect of wax powder, wax slurry and magnesium powder in the system. LF-3120-6HV resin has high hardness, many cross-linked structures, good skin cream resistance, hydrolysis resistance, storage stability, and easy matting. Therefore, LF-3120-6HV was selected as the main resin in the formulation.

2.2 The effect of the amount of modified functional resin on the performance of the coating film

The homemade modified functional resin has flexible components, which can well balance the change of internal stress during film formation, and give the coating film a certain degree of resilience. High reactivity can make the coating film form a higher cross-link density, thereby improving the chemical resistance of the coating film. The self-made modified functional resin was added to the coating formula. Based on the formula in Table 1, the amount of modified functional resin was changed, and the hydrolytic storage stability and skin cream resistance of the coating film were tested according to TL 226 standards. The scratch resistance of the coating film was also tested. The results are shown in Table 3.

Effect of Modified Functional Resin Amount on Film Properties

Note: (1) —10 N load, the test pin is tested according to the Bosch standard (tip point is 0.75 mm) using an Erichsen test rod of model 318, and the marks on the paint surface caused by the load are visually judged, the same applies hereinafter.
As can be seen from Table 3, the formula without modified functional resin cannot pass the hydrolysis storage and skin cream test. The scratch resistance is not good, and the bright marks after shaving are obvious. When 5% modified functional resin is added At the same time, the scratches were improved, and the skin cream test could pass, but the hydrolysis storage test failed. When the modified functional resin accounted for 10% of the formula, the coating film showed excellent skin cream resistance and hydrolysis. Storage stability and scratch resistance. This shows that in the matte system, the resin has a certain effect on the lightness of the coating film after being scratched. The reason may be that micro-nano materials have special properties such as surface effects, small size effects, optical effects, and quantum size effects, which can effectively improve the tribological, mechanical, and mechanical properties of polymer materials and their coatings.


2.3 Effect of matting powder on scratch resistance of coating film
Based on the formula in Table 1, different matting powders were selected to examine the effects of different types of matting powder on coating film performance. The results are shown in Table 4.

Effect of matting powder on film properties

The surface gloss of an object is closely related to the degree of roughness of the surface of the object. According to the knowledge of micro-surface theory, it can be calculated according to formula (1):
h = λ / cosα
where: h — roughness; λ — wavelength of incident light; α—angle of incidence.
When the angle of incidence is 60 °, it can be obtained that h = 1. 1 μm, that is, when the surface roughness of the object is greater than 1.1 μm, the surface of the object will appear uneven, and the gloss will decrease. Matting powder’s particle pore volume, average particle size and particle size distribution, dry film thickness, and whether the particle surface has been treated will all affect its matting effect. Studies have shown that when the average particle size of the matting powder is between 3 and 5 μm, a coating film with better matting effect can be obtained. It can be seen from Table 4 that E-1011 has the smallest particle size and the worst matting effect. The maximum amount of E-1011 is maintained while maintaining similar gloss, which means that more matting powder will be accumulated on the surface of the coating film, making it scratch resistant. Decrease; OK520 has the lowest usage and good matting effect, but because of its larger particle size, the surface of the coating film is rougher under the same conditions, that is, the matting powder significantly protrudes from the surface, resulting in a corresponding reduction in its scratch resistance; OK607’s scratch resistance The performance is the best, indicating that its average particle size, particle size distribution, pore volume and other factors are more suitable for the application of anti-scratch matte paint. The final formula selected OK607 as the main matting agent.


2. 4 Effect of magnesium powder and wax powder on the anti-scratch performance of coating film
According to the “floating theory” of micronized wax, during the coating film formation process, the wax will drift toward the coating film surface and form a microsphere on the coating film surface. Form a film with a low coefficient of friction; at the same time, the magnesium powder with a flake-like microcrystalline structure has a low coefficient of friction, has good scratch resistance and excellent suspension properties. When the micronized wax floats upward, and the magnesium strong powder has a good suspending effect, the magnesium strong powder will form a blending dense arrangement layer with the wax particles in the low recesses and lower parts of the wax arrangement layer. Therefore, together with the wax powder, it has the effect of enhancing the anti-scratch performance. The schematic diagram of the effect is shown in FIG. 1. Based on the formula in Table 1, the amount of magnesium powder was changed to test the adhesion of the coating film before and after hydrolysis and storage. The test results are shown in Table 5.

Effect of Magnesium Strength Powder on Hydrolytic Storage Performance of Coating Film

It can be known from Table 5 that when the magnesium powder is added in 2% and 4%, the conventional adhesion of the coating film and the adhesion after hydrolysis and storage are both 0, and the scratch resistance of 4% is better; when added When it is 6%, the conventional adhesion of the coating film is grade 0, and the scratch resistance is good. The adhesion decreases slightly after hydrolytic storage. With the addition amount increasing to 8%, the conventional adhesion and scratch resistance of the coating film are still good, but After hydrolysis, the adhesion dropped sharply to level 3. Magnesium powder is a pigment with excellent scratch resistance and low price. The larger the amount of magnesium powder is, the lower the formula cost will be. However, the adhesion of the coating film after hydrolysis and storage will decrease accordingly. The reason may be that with the magnesium powder consumption The increase in the volume concentration of magnesium strong powder in the coating film increases the resin’s encapsulation. At the same time, more magnesium powder particles with reduced encapsulation will be collected at the interface with the substrate, resulting in the coating film resistance to medium. Permeability has declined. In the final experiment, 4% magnesium powder was added.


2.5 Influence of thinner on anti-scratch performance of coating film

The thinner for anti-scratch matt black PU coating for automotive interior is formulated from a variety of organic solvents. When formulating the thinner, full consideration must be given to the volatilization rate of each component solvent, Surface tension, solubility and other parameters make it have good wettability to various components and materials of the coating, good solubility to the resin, and suitable overall evaporation gradient, so that the dried coating will be leveled, the surface is dense and delicate, and the matting agent Arrange evenly and have strong scratch resistance. Table 6 shows the diluent formulations.

Thinner formula

2. 6 Coating performance test results

Table 7 shows the coating film performance test results.

Coating film performance test results
Coating film performance test results

It can be known from Table 7 that various performances of the coating film have passed the performance requirements for the coating film of Volkswagen interior trim parts. At the same time, the problem of bright marks after the coating film has been scratched has also been effectively improved.

3 Conclusions
The focus of this research is to ensure high scratch resistance of low-gloss coatings on the premise of meeting the requirements for high chemical resistance of automotive interiors. By optimizing the selection and ratio of the main resin and the modified functional resin, the selection and matching application of matting powder, wax powder, wax slurry, and magnesium powder in the matting system have been fully considered, so that it has a good light scattering and surface Uniform hard coating surface to improve the scratch resistance as much as possible.

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