Formulators Forum—How Clear Powder Coatings are Made and How They’re Used

A powder clear coat is as simple as it sounds—a powder coating that is transparent.

While powder coating technology dates back to the 1940s, clear powder coatings were not introduced until the 1980s. Today, they are commonly used in automotive, consumer, industrial, and architectural applications.

Clear powder coatings offer high levels of transparency and clarity, while permitting light to penetrate to the surface. This layer provides durability while retaining the natural finish of the substrate beneath it. Conversely, the opacity of pigmented powder coatings block light from penetrating to the substrate. Unlike their clear counterparts, these opaque coatings cover the underlying substrate.

The resins and crosslinker materials used in powder clear coats are similar to the ingredients used in opaque powder coatings. They can be formulated in nearly any powder coating resin system, but some systems are higher in clarity than others. The most commonly available clear coat systems are polyester resin based. However, epoxies and acrylics can also be used as the primary resin or blended with polyesters to provide different properties, adhesion strength, and other mechanical features. Additional materials used in powder clear coat formulations need to be selected carefully to maintain the optically transparent nature of a clear coat.

Clear powders can be used as a stand-alone one-layer system applied directly to a substrate or as an additional layer in other coating systems, including stacked on top of powder basecoats or liquid coating systems. This versatility makes them suitable for a broad range of applications.

When are powder clear coats used?
Clear coat layers are used in both decorative and functional coating systems but the three primary reasons to utilize them are durability, specialized coating features, and design aesthetics.

Durability is brought to a powder coating system in many ways. For exterior use products, a clear coat brings added protection to limit the adverse effects of outdoor weathering like salt, acids, and environmental pollutants. When a clear coat is used as a single layer, the film will protect the substrate underneath from environmental degradation. When using clear coats as a second or additional coating layer, they also help to protect the coating system underneath from ultraviolet light degradation, protecting the color fastness and integrity of the layering system.

Durability can be enhanced with a clear coat for interior products by adding a layer that can strengthen the paint system against material handling, such as work surfaces. Commonly, clear coats also help to protect against cleaning chemicals or interaction with common foods when used on consumer appliances. Powder clear coats can also be added to layering systems for enhanced functional features like microbial resistance for medical end uses or enhanced cleanability on applications like dry erase surfaces.

Powder clear coats can elevate design by adding depth and can be produced in various levels of gloss that range from low or matte finishes to high gloss reflective looks. The clarity, or level of transparency, of a clear coat layer can also be important in high end finishes needed for decorative coatings. Clarity in high gloss decorative coatings is sometimes referred to as DOI, or distinctness of image. This is a quantification of how clear an image is reflected through the coating. For instance, DOI is an important property if a clear coat has been used to protect and maintain the brilliance of a metallic or highly chromatic coating underneath.

Lastly, powder clear coats can be used to enhance color properties of decorative coating systems. Clear coats can be tinted with special effect pigments or dyes, providing more “pop” to a coating stack with a transparent coating formulated in various shades of the color wheel.

How do you make a powder clear coat?
Making powder clear coats is not drastically different than the process for making opaque powder coatings. The ingredients are premixed, extruded, and the chip is finely ground into usable powder paint. The primary difference in manufacturing powder clear coats comes down to the preparation of the processing equipment.

Preparing to make a clear powder coat requires meticulous cleaning to avoid picking up any leftover material in the extruders and milling equipment. Even low levels of stray opaque pigments and additives can tint a clear coat or cause defects like craters or incompatibility in the clear coat finish. For these reasons, many coating manufacturers have dedicated equipment to produce clear coats to avoid the unusually long cleaning cycles and the need for rework due to defects.

Sarah Mueller is powder technical manager for PPG.