Powder coatings require basically no cross-linking during production, no caking and good stability during storage; coalescence, degassing and leveling at the lowest possible baking temperature , and comprehensive and balanced flow and leveling properties; Low temperature and shortest possible crosslinking time to achieve acceptable appearance and protection properties in the required film thickness range. Coatings that flow easily before crosslinking will form a flat film, but flow driven by differences in surface gravity caused by faster heating of the corners will cause them to flow away from the corners.
Flow and leveling, the Tg is high enough to prevent powder coating from caking, and the Tg is low enough to coalesce and level at the lowest possible temperature. If a powder coating resin has high reactivity and the baking temperature is much higher than the Tg of the paint film, it can be baked at a low temperature for a short time but this composition will cause early crosslinking during the production of powder coatings, and in the powder coating When melting, the viscosity increases rapidly, so that the coating cannot coalesce and level well, so the powder coating needs to have an appropriate Tg and reactivity.
A rough rule of thumb is: the lowest baking temperature is 50°C higher than the melt extrusion temperature for powder coating production, and about 70-80°C higher than the Tg of the solidified powder. In this way, the minimum baking temperature of the powder with a Tg of 55°C is about 125-135°C. The T of the resin is controlled by changing the chemical composition and molecular weight of the resin. When T is the same, the resin with higher molecular weight and greater , because the higher the molecular weight, the better the storage stability, and the greater the flexibility of the chain, the better the fluidity during baking. The use of supercritical CO2 as an additive enables the production of powder coatings at lower temperatures, allowing the use of more reactive formulations.
After the powder is melted, the viscosity immediately rises to a high level, but as the temperature increases, the viscosity drops sharply. After the cross-linking reaction begins, the molecular weight of the resin increases, and the viscosity tends to be stable, and then increases rapidly as the coating approaches gelation. The fluidity of the paint film depends on the minimum viscosity achieved and the time spent at this viscosity, which is called the flow window.
The relationship between viscosity and temperature does not follow the Arrhenius relationship, but depends on the free volume of the paint film, and the important factor controlling the free volume is T-Tg Dynamic mechanical analysis (DMA) shows that the widely used decorative powder coatings The Tg value of epoxy/polyester hybrid coating, TGIC polyester coating and blocked isocyanate-polyester) coating film is 89-92°C, and the average relative molecular weight M between crosslinking of the coating film after curing. In the narrow range of 2500~3000. The T of the cured film of the protective epoxy powder coating crosslinked with modified dicyandiamide was 117°C and the Mc was 2200. Powder coatings have a higher Tg than liquid coatings , but have a lower crosslink density. DMA studies have shown that TiO2 has essentially no effect on Tg and that the pigment-binder interaction is weaker for powder coatings compared to liquid coatings .
Low melt viscosity promotes leveling. Surface tension is the main driving force for leveling thinner and/or liquid films, but thicker and/or slightly viscous films level better than expected with surface tension. The flow with different surface tension can cause the shrinkage cavity of the powder coating , and it is necessary to add a small amount of additives such as polyoctyl acrylate derivatives to eliminate the shrinkage cavity.
Many powder coatings contain 0.1%~1% benzoin (melting point 133~134°C), which acts as an anti-pinhole agent and a degassing aid . Benzoin can plasticize the melt and increase the flow window of the polyester-glycouril formulation, that is, it melts when the powder is molten, reduces Tg, promotes coalescence and degassing, and improves leveling. Other solid plasticizers, such as aluminum stearate and a solid ethylene glycol surfactant dispersed on a solid silica carrier had similar effects to benzoin.
Benzoin acts as a solid swimming agent, which can keep the coating film open for a long enough time, so that the air can have enough time to release from the coating film, but it has a tendency to yellow.
