Application of antioxidants in high-temperature powder coatings

In recent years, with the continuous strengthening and deepening of China's environmental protection policy, higher standards of environmental protection requirements have been put forward for the paint industry, and the domestic paint industry has been transformed and upgraded in the direction of water-based, high solids, and solvent-free, and powder coatings have developed rapidly.

Compared with traditional industrial solvent coatings, powder coatings are more economical and environmentally friendly, and have excellent chemical resistance, corrosion resistance and high mechanical strength. In the process of processing and curing, powder coating is subjected to mixed thermal processing and high-temperature baking, and if the protection of the molecular chain is insufficient in the production process, it is easy to undergo thermal and oxidative aging and decomposition of the molecular structure, which will affect the performance and color of the coating. And more powder coatings are increasingly being applied to high-temperature environments, such as rice cookers, microwave ovens and other small household appliances coatings, these factors put forward higher requirements for the high-temperature stability of powder coatings. As a common anti-aging additive, antioxidants provide excellent anti-thermal and oxidative aging protection and long-term stability for polymer materials, and are an indispensable component of powder coating solution system, and more and more application requirements are expected to improve the high-temperature stability of powder coatings through the optimization of antioxidant systems. At present, the traditional antioxidants used in powder coatings mainly include hindered phenolic antioxidants 1010 and 1076, phosphite antioxidants 168 and 626, etc., which basically cannot meet the thermal stability of powder coatings under high temperature conditions. In this paper, a composite antioxidant system for powder coatings is developed, which can effectively prevent the degradation of powder coatings by thermal and oxidative aging at high temperatures, and provide excellent protection for coating colors, which can expand the development and application of powder coatings in the field of high-temperature home appliances.

Experimental part

2.1 Preparation of specimens

Polyester resin, curing agent, antioxidant, barium sulfate, titanium dioxide and other additives are batched in a certain proportion, premixed, the mixture is melted and extruded, broken after tableting, and the powder coating product with a certain particle size is obtained after sifting the crushed powder.

2.2 Test equipment

High temperature oven (binder), twin screw extruder (Deteng), colorimeter (X-Rite).

2.3 Test methods

2.3.1 Antioxidant thermal stability test

The antioxidants used in the four formulations of ABCD were placed in a 230°C oven for 0.5h to test their color difference and weight loss rate.

2.3.2 Powder coating thermal stability test

The powder coating sample in 2.1 was electrostatically sprayed and cured at 200°C for 10min to obtain powder coating. In order to evaluate the effect of antioxidants on the heat resistance of powder coatings, the powder coating was put into a high-temperature aging oven with a heat aging condition of 230°C for 2h. Test the gloss retention and color difference of the coating.

Results and Discussion

3.1 Mechanism of action of antioxidants

The reticulated polymer coated in the paint is made up of countless molecular segments linked together by chemical bonds, which contain hydrogen, oxygen, double bonds, etc.; They are susceptible to heat and oxygen attack, producing large amounts of free radicals and hydroperoxides, which can damage the polymer segments and eventually cause the coating to begin to age, with gloss loss, surface chalking, and discoloration. The addition of antioxidants to powder coatings can effectively inhibit the thermal-oxidative aging behavior of polymers. Antioxidants are divided into two main categories: primary antioxidants and secondary antioxidants. The main antioxidants mainly include aromatic amine antioxidants and hindered phenolic antioxidants, and their mechanism of action is to capture the free radicals R▪ and ROO▪ formed in the chain reaction stage and terminate the chain reaction. Auxiliary antioxidants mainly include phosphite antioxidants and thioester antioxidants, and their mechanism of action is to decompose ROOH to produce stable substances. The combination of main and auxiliary antioxidants can play a certain synergistic role to achieve a better antioxidant effect.

3.2 Study on the thermal stability of antioxidants

The thermal stability of the antioxidant itself determines whether it can provide good protection for the resin during high-temperature processing and use. If the antioxidant itself has poor thermal stability and decomposes at high temperatures, it loses the properties of the antioxidant and cannot provide good protection for the resin from thermal and oxidative aging. As can be seen from Table 2, the color change and weight loss rate of the four antioxidants under high temperature aging are not serious, indicating that the thermal stability of the four antioxidants is good. Among them, formula A is a single hindered phenolic antioxidant. Formula B is a single phosphite antioxidant with a high weight loss rate, indicating that B has undergone a certain degree of decomposition. Formula C is a commonly used compound antioxidant system on the market. Formula D is a special composite antioxidant system developed for powder coatings, and the experimental results show that the color change and heat loss rate of formula D are excellent at high temperature.

3.3 Effect of different antioxidants on the heat resistance of powder coatings

It can be seen from the data in Table 3 that after 2 hours of high temperature aging at 230 °C, the powder coating was damaged to varying degrees, and formula A and B were both single antioxidant systems, and the protection effect of powder coating was slightly poor, and the protection of formula C against thermal and oxidative aging of the coating was significantly better than that of A and B. Among them, the D formulation, as an antioxidant formulation specially developed for powder coatings, has the best performance for the pulverization and color protection of the coating, so that the high temperature resistance of the powder coating has been significantly improved.

epilogue

The results show that: 1. There are differences in the thermal stability of different antioxidants at high temperature, among which the thermal stability of formula B (phosphite antioxidants) is poor, and formulations A, C and D all have excellent thermal stability. 2. It is found that the protection of the composite antioxidant system against the thermal and oxidative aging of powder coatings is better than that of the single-component hindered phenols or phosphite antioxidants, and the D formula of the composite antioxidant system developed for powder coatings has better protection against the thermal and oxidative aging of the coating under high temperature conditions.