Abstract: Ordinary household appliance powder coatings do not have good temperature and oil resistance due to the formula system design not considering long-term exposure to high temperatures, which makes the coating prone to yellowing and peeling.

This article conducts experimental tests on the combination of different types of resins to improve the cross-linking density of coatings while also considering their physical properties. By using heat-resistant pigments, titanium dioxide, and curing accelerators, the temperature and oil resistance of the product can be improved, and the powder coating can meet customer requirements in terms of conventional performance, color retention, heat resistance, oil resistance, and detergent resistance.

Powder coatings have grown rapidly in the global market due to their solvent-free, easy to apply, and high utilization characteristics. In the 1980s, organic silicon high-temperature resistant powder coatings were first applied in ovens in the United States, and rapidly grew in the European and American markets in the mid-1990s.

As China gradually becomes the main production base for stoves, ovens, and other products worldwide, the demand for high-temperature resistant powder coatings in the market is also increasing.

Usually, powder coatings for household appliances are mainly used for air conditioning outdoor units, washing machines, refrigerators, etc. The surface temperature of these household appliances does not exceed 100 ℃, and the surface coating performance does not involve high temperature resistance or oil resistance.

As the outer coating of household appliance kitchen range hood products, powder coating needs to have grease resistance and temperature resistance (160-180 ℃). The use of silicone resin high-temperature resistant powder has excess quality and does not meet customers' requirements for grease. Therefore, a new powder coating that meets the requirements needs to be developed.

This study starts with the formulation framework system, and on the basis of not changing the main resin types and controlling costs, tests the combination of different resin curing agents and other base material systems, as well as the selection of pigments, fillers, and various additives.

Improve the physical and chemical properties of the paint film, including temperature resistance, yellowing resistance, impact resistance, bending resistance, and flow resistance, and develop high-temperature resistant powder coatings that meet customer requirements.

1 Experimental section

1.1 Main raw materials

Hydroxypolyester resin 1: Industrial grade, Dixing Resin (Kunshan) Co., Ltd; Hydroxypolyester resin 2: Industrial grade, Zhanxin Resin (Shanghai) Co., Ltd; Hydroxypolyester resin 3: industrial grade, produced by Meitu Corporation in the United States; Two step epoxy resin: industrial grade, Guodu Chemical (Kunshan) Co., Ltd; Closed isocyanate curing agent: industrial grade, Evonik company (model B1530); General additives for powder coatings (leveling agents, degassing agents, benzoin, etc.): Industrial grade, Ningbo Nanhai Chemical Co., Ltd; General pigments for powder coatings: industrial grade, Langsheng Company; Rutile titanium dioxide 1: Industrial grade, Anhui Annada Titanium Industry Co., Ltd; Rutile titanium dioxide 2: industrial grade, DuPont, USA; Temperature resistant pigment: industrial grade, Ferro company; Antioxidant: Industrial grade, product of Lu'an Jietongda Chemical Co., Ltd; Flexibility additive: Industrial grade, Shenzhen Xinhai Technology Co., Ltd.

1.2 Performance Testing

Refer to GB/T 11185‐ 2009, ISO6860:2006 Determination of flexibility of paint film; Refer to GB/T 1732‐ 1993 Determination of impact resistance of paint film; Refer to GB/T 1735‐ 2009 Determination of heat resistance of paint film.

2 Results and Discussion

2.1 Research on Performance Improvement of White High Temperature Powder

Limitations of the formula framework for ordinary powder coatings

The powder coating extruded according to the formula in Table 1 was sprayed to prepare a sample (film thickness of 70-80 μ m, curing conditions: 200 ℃/15min), and performance tests were conducted according to the standard. According to the standards provided by the customer, the ordinary powder coating was tested for oil resistance, stain resistance, detergent resistance, heat resistance, hardness, etc., but the performance did not pass.

2.1.2 The impact of formula adjustment on physical and chemical properties: According to actual needs, the coating film also needs to meet requirements such as crosslink density, temperature resistance, grease resistance, flexibility, impact resistance, 9N scratch hardness, leveling, color difference, etc.

Among these requirements, oil resistance, flexibility, impact resistance, and 9N scratch hardness are key indicators. Some of the above requirements are contradictory to each other, and it is necessary to comprehensively balance the various properties of the formula.

Polyurethane coatings have excellent leveling and solvent resistance, as well as high crosslinking density. However, due to their high crosslinking density, the flexibility and impact resistance of the coating film are poor.

To improve flexibility, a small amount of epoxy and hydroxyl polyester with high hydroxyl value, low viscosity, and acid value of 7-11mgKOH/g are added to the polyurethane base formula. With a slight decrease in crosslinking density, the flexibility of the coating is improved.

Due to the high hydroxyl value of high hydroxyl value resins, multiple functional groups involved in the reaction, and high crosslinking density, the coating has high hardness and good chemical resistance, but poor leveling and impact resistance;

Medium hydroxyl value resin, with good leveling and excellent comprehensive performance; Low hydroxyl value resin has good mechanical properties, but poor hardness and chemical resistance. Therefore, according to customer requirements, the main resin in the formula is selected as medium hydroxyl value resin.

When selecting a medium hydroxyl value resin, considering the leveling of the coating, the hydroxyl polyester resin 2 with a narrow viscosity range is chosen as the final resin from the two pre selected medium hydroxyl value resins.

After determining the base resin and curing agent, in order to meet the requirements of temperature and oil resistance, the leveling agent, pigment, and additives in the formula were adjusted, and inorganic temperature resistant pigments were selected.

The base material system composed of hydroxy polyester resin 2, hydroxy polyester resin 3, two-step epoxy resin, and blocking isocyanate curing agent was used as the preliminary formula framework for the heat-resistant powder coating. The formula is shown in Table 3, and the performance test results are shown in Table 4. This heat-resistant powder coating can be used for household appliances.

Through formula adjustment, the gloss and leveling of heat-resistant powder coatings are lower than those of ordinary household appliance powder coatings, but meet customer requirements. The temperature resistance, grease resistance, impact resistance, 9N scratch resistance, hardness, and color difference have all been significantly improved.

2.1.3 Effects of formula adjustment on yellowing resistance and leveling To determine the cause of yellowing, four sets of comparative tests were conducted by adding antioxidants and different titanium dioxide powders to the formula in Table 3. The corresponding formulas are shown in Table 5. The three formulas were powdered, sprayed, and then placed in a 230 ℃ oven for 2 hours to test the color change of the coating before and after baking