How to eliminate pinholes in powder coatings with wax?

2022-01-09   Pageview:142

Powder coatings do not contain solvents, the product utilization rate is high, and the coating line can achieve high production efficiency, so it has environmental performance and economic type. Compared with liquid coatings, powder coatings are easy to form pinholes, which affects the appearance of the paint film as well as its performance, and is a problem that powder coatings must overcome.

Why do powder coatings tend to form pinholes?

Powder coatings are mainly applied by electrostatic spraying method. When spraying, powder coating particles are attached to the workpiece under the combined action of airflow thrust and electric field thrust. Unlike liquid coatings, air will exist between the powder particles and on the rough substrate surface. Upon entering the oven, the loosely packed powder particles are converted from a granular solid to a molten liquid state. As the film-former is cross-linked and cured, the viscosity rises sharply and some of the air is trapped in the molten coating until it breaks through the cured film and escapes under high temperature and pressure, and forms pinholes in the final film.

What is the difference between benzoin and degassing wax?

The degassing mechanism of benzoin is widely discussed in the industry, and one of the more widely accepted is that the oxidation reaction of benzoin occurs during the film formation process, which consumes oxygen in the bubble to reduce the diameter of the bubble, and there are also statements such as benzoin evaporates with heat to take away the gas, benzoin promotes the dissolution and diffusion of gas in the resin. However, no matter what the principle is, benzoin is easy to make the coating yellow while providing degassing effect, which is more obvious in light color or white coating, and the problem is more prominent in HAA system.

The key to the principle of using degassing wax lies in the heat-curing film-forming phase of the coating, which is the stage where the powder particles are transformed from solid to liquid phase by heat and cross-linked to cure. When the melting temperature of the wax is lower than the curing temperature of the film-former, it is transformed into the liquid phase before cross-linking and curing. The wax in the liquid phase is not involved in cross-linking and is not soluble in the molten film-forming material. Due to the low viscosity and low density, the wax will migrate to the coating surface, and the migration process creates a small channel one by one, allowing the gas to exit the paint film in time to form a smooth surface and reduce pinholes before the coating finishes curing.

For the sake of economy and effect, we suggest using benzoin and degassing wax together to achieve the best effect!

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