The first stage: coalescence (1 min-3 min, 40-60 C)
During the curing process, with the increase of temperature, the surface of powder coatings particles began to melt, and then the melted periphery aggregated with each other.
The second stage: melt spreading and leveling (3-7 min, 80-100 C)
In leveling stage, because of the low softening point of leveling agent itself, the softening point of the whole system decreases when leveling agent is added to powder coatings. The leveling agent migrates rapidly to the surface of the melt system, forming a molecular layer with locally uniform surface tension.
During the spreading stage, the whole powder coating system becomes melting state, and the leveling agent has migrated to the surface of the melting coating. Therefore, the wetting effect between the resin and filler pigments in the powder is not obvious, so another kind of acrylate polymer with higher softening point is needed, which is not easy to migrate to the surface quickly. The surface tension between the low melting coating and the base material, pigments and fillers promotes the spreading of the melting system to the base material and the infiltration of pigments and fillers. This additive is a brightener (also known as brightener, wetting promoter).
In this process, with the increase of temperature, the viscosity of powder coatings decreases sharply, that is, the driving force of leveling and wetting is much greater than the hindrance caused by viscosity, so this process is the main stage of wetting and leveling. The surface tension of the melt coating decreases due to the combined action of brightener and leveling agent, so the resin is easier to infiltrate the pigments and fillers, and the volatile substances such as water in the melt fluid are accelerated to overflow from the surface (adding benzoin can accelerate bubble bursting and overflow ). The overflowed bubbles will act as surface tension. When used, it becomes a flat surface.
The third stage: Curing (7min-15min 130-200 C)
When the powder solidifies at high temperature, the viscosity of the system increases gradually. When the surface tension remains unchanged, the flow resistance increases rapidly, so the powder will not leveling at this time.