Thermosetting powder coatings are obtained by mixing, extruding, cooling, crushing, pulverizing, and sieving reactive film-forming mixtures (resins and crosslinkers) with pigments, fillers, leveling agents, antistatic agents (dispersants), and appropriate accelerators in an extruder at a reaction temperature; M's powder particles. The resin used in thermosetting powder coatings is a low degree of polymerization prepolymer with a low molecular weight, so the coating has good leveling and decorative properties. After curing, the low molecular weight prepolymer can form a network of cross-linked macromolecules, thus the coating has good corrosion resistance and mechanical properties.

In 2014, the global sales of thermosetting powder coatings exceeded 8 billion US dollars. The varieties and proportions of thermosetting powder coatings in China in 2014 are shown in Table 1.1 Resin and curing agent

The resins widely used in thermosetting powder coatings include epoxy resin, polyester resin, and acrylic resin, and different resins are combined with different curing agents.

1.1 Epoxy resin and curing agent

Epoxy resin refers to high molecular weight prepolymers with two or more epoxy groups as the main chain, including aliphatic, cycloaliphatic, or aromatic groups. A typical example is bisphenol A epoxy resin. Epoxy resin has good flexibility, chemical resistance, corrosion resistance, and adhesion. The molecular weight of bisphenol A-type epoxy resin used in thermosetting powder coatings is generally between 1400 and 2200, with a polymerization degree of around 4 and an average epoxy value of 0.12. This is known as Type 4 solid resin, or E-12, with a softening point of 88-98 ℃. Common curing agents include dicyandiamide, imidazole, anhydride, and dihydrazide.

1.2 Polyester resin and curing agent

The commonly used polyester resins for thermosetting powder coatings include those with carboxyl groups and those with hydroxyl groups. Compared with epoxy resin, polyester resin has good weather resistance, among which polyester resin with carboxyl group has better weather resistance than polyester resin with hydroxyl group, but the price is correspondingly higher. Polyester resins with carboxyl groups generally use TGIC and HAA as curing agents, while polyester resins with hydroxyl groups (such as polyurethane) generally use IPDI or 1,3-diazetidine-2,4-dione (also known as urea diketone) as curing agents.

TGIC has been banned and restricted due to toxicity issues, and most Western European countries have already achieved full replacement of TGIC with HAA. In 1994, HAA was industrialized in China, and both TGIC and HAA are used in China. Compared with TGIC, HAA has a price advantage, but there are certain performance drawbacks, such as susceptibility to yellowing, pinholes in thick coatings, and powder loss in humid weather.

In recent years, as an alternative to IPDI, the use of urea diketone has also been increasing. During the curing process of urea diketone, isocyanates can be released to achieve cross-linking, but volatile organic compounds will not be released.

1.3 Polyester/Epoxy Resin

Polyester/epoxy resin can obtain products with better economic efficiency than pure epoxy resin powder coatings and pure polyester resin powder coatings by changing the ratio of the two. When epoxy resin and polyester resin with carboxyl groups are used together, no curing agent is needed, and cross-linking can be achieved only through the mutual reaction between the two resins. The powder coating of 50% epoxy resin and 50% polyester resin is suitable for electrical appliances and large household appliances; 60% polyester resin and 40% epoxy resin are suitable for industrial use; 70% polyester resin and 30% epoxy resin, as well as 80% polyester resin and 20% epoxy resin, are suitable for furniture and general industrial facilities with low anti-corrosion and decorative requirements.

1.4 Other resins

Acrylic resin functionalized with glycidyl methacrylate is also used as thermosetting powder coating resin, and the curing agents used are lauric acid, acid anhydride, etc. Acrylic resin has excellent properties such as light color, light resistance, light retention, color retention, and stain resistance. The paint film made from it has high glossiness and good decorative properties, making it one of the preferred varieties of resin for outdoor high decorative powder coatings. Organic silicon/polyester resin is used for high-temperature resistant kitchen appliances, and fluorinated polymers have excellent weather resistance and durability. They have been used in the construction industry, but their applications are not yet widespread. In addition, polyetheretherketone resin powder coating can withstand high temperatures above 260 ℃.

2 leveling agents

The function of leveling agent is to improve the appearance of the coating, eliminate surface defects such as orange peel, shrinkage pores, pinholes, and shrinkage edges. The mechanism of action is to reduce or change the surface tension, interfacial tension, and promote the uniformity of surface tension during the curing process to eliminate surface defects of the coating. Commonly used leveling agents include acrylic homopolymers and copolymers, modified polydimethylsiloxane.

Hydrogenated castor oil and epoxidized soybean oil are used for thermosetting powder coatings, which have a good effect on eliminating orange peel, but can cause shrinkage; Cellulose acetate butyrate has good anti orange peel and anti shrinkage effects, but it is costly and prone to yellowing; Polyacrylates have good comprehensive effects and cost advantages, making them widely used leveling agents.

The dosage of polyacrylate leveling agent varies slightly in different types of thermosetting powder coatings, such as 0.5% to 1% in epoxy thermosetting powder coatings, 0.75% to 1.5% in epoxy/polyester and polyester coatings, and about 2% in acrylic ester coatings.

Leveling agents have a certain degree of incompatibility with the base resin, and will quickly migrate to the surface of the coating during the curing process, affecting the glossiness of the coating. In recent years, two types of functional leveling agents have been newly developed, namely high molecular weight polymers with active functional groups and organic fatty acid ester polymers.

3 Pigments and Fillers

Pigments and fillers endow thermosetting powder coatings with color, improve the mechanical and chemical properties of coatings, and reduce coating costs. The pigments and fillers used in thermosetting powder coatings are required to not undergo chemical reactions with other components in the coating, have good dispersibility (optimal dispersion particle size is 0.2-0.9 μ m), good thermal stability (at least temperature resistance of 160 ℃ or above), good light and weather resistance, moderate oil absorption, and good color fastness.

Thermosetting resins have good wetting properties for pigments and fillers. In thermosetting powder coatings, while ensuring the smoothness, brightness, and physical and mechanical properties of the coating, more pigments and fillers can be added appropriately in the order of polyester resin>epoxy resin>acrylic resin. For example, the amount of pigments and fillers in white high gloss polyester powder coatings can reach up to 40%.

The commonly used inorganic pigments in powder coatings include titanium dioxide, carbon black, iron oxide, chromium based pigments, ultramarine, etc. Organic pigments include phthalocyanine blue BGS, phthalocyanine green G, perpetual yellow, benzidine yellow, sun resistant yellow, dispersed yellow, pigment yellow, perpetual red, sun resistant bright red, toluidine red, perpetual purple, and quinacridone purple, etc; Metal powder pigments include aluminum powder, copper gold powder, and pearl pigments; Functional pigments include luminescent pigments, conductive pigments, high-temperature pigments, fluorescent pigments, etc. The commonly used fillers for powder coatings include precipitated barium sulfate, light calcium carbonate, white carbon black, wollastonite, and quartz powder.

Thermosetting powder coatings have a wide range of applications, including household appliances, office furniture, automotive parts, transportation facilities, electrical equipment, various pipes, steel bars, building materials, and others (including toys, springs, bicycles, and agricultural machinery). The main areas that have been applied include metal coating, household appliances, aluminum materials, automobiles, etc. Potential new application areas include wooden furniture, plastics, packaging containers, and semi cured sheets.

In recent years, China's thermosetting powder coating industry has developed rapidly, accounting for over 50% of the global thermosetting powder coating market. The sales revenue of thermosetting powder coatings from 2008 to 2014 is shown in Table 2.

“ Twelfth Five Year Plan; During this period, the sales growth rate of thermosetting powder coatings in China first slowed down and then increased. In 2011, it was 9.9%, in 2012, it was 4.5%, in 2013, it was 6.7%, and in 2014, it rebounded significantly, reaching 11.2%. In 2015, it also reached double digits, at 10.4%. At present, the technical content of resin and additives for powder coatings in China is not high. In the future, targeted molecular design and development of resin for powder coatings suitable for near-infrared curing of high molecular weight polyester and epoxy should be carried out, as well as research and development of leveling agents and enhanced high-efficiency degassing agents suitable for high transparency powder raw materials.