Formulators Forum—Impacts of Specific Gravity on Powder Coating Formulations

Specific gravity in the context of powder coating refers to the density of the powder relative to the density of water. In other words, it’s a measure of how heavy the powder is compared to an equal volume of water. For powder coatings, this concept is important for several reasons. For starters, it affects the application characteristics of the powder. Powders with higher specific gravity may be heavier and potentially more challenging to handle, whereas their lighter counterparts might be easier to apply but may require more volume to achieve the desired coverage. Different powder coating formulations, which include the types and ratios of resins, pigments, fillers, and other additives, will influence the specific gravity. For example, zinc-rich powders often contain more than 50% zinc powder to achieve robust corrosion resistance. This has a big impact on the specific gravity because zinc is a very heavy element. Maintaining consistent specific gravity in powder formulations ensures uniform application and finish, while variations can lead to inconsistencies in coating thickness, coverage, and appearance.

Now, let’s discuss the impact powder formulation can have on specific gravity for the finished product. Different resins have different densities. A formulation with a high-density resin will have a higher specific gravity. The choice of resin will affect the final properties of the coating, like hardness and flexibility. Pigments and fillers can also impact the specific gravity. For example, titanium oxide (TiO2), metallic pigments, and heavy fillers will increase it. These components are often added to achieve aesthetic effects, performance characteristics, or to reduce cost of the product. Other additives used to improve flow, cure, or enhance certain properties can also affect the powder’s density. For instance, the addition of certain types of flow agents, waxes, and other additives might slightly alter the specific gravity. Formulators will consider using ingredients that provide the benefit of increased coverage that comes with lower specific gravity formulations.

Specific gravity has a big impact when it comes to theoretical coverage and applied cost. A lighter specific gravity powder will yield higher coverage than a higher specific gravity powder. However, while a lower specific gravity powder will cover more square feet per pound because it is less dense, the hiding capabilities may be lower. Depending on the desired coating performance, a lighter specific gravity powder may require a thicker film build, ultimately resulting in having to use more powder per square foot.

Specific gravity for a powder coating is often listed on the technical datasheet. Theoretical coverage of a powder coating can be calculated with a simple formula. It may be the case that a more expensive powder per pound could be cheaper in terms of applied cost depending on the specific gravity of the two products. In Table 1, two powders are compared for theoretical coverage, which illustrates the impact of specific gravity on theoretical coverage. Note that the number 192.3 is a standard used to represent the square footage coverage per one pound of powder at a specific gravity of 1.0, at a thickness of 1 mil, and 100% transfer efficiency.

In summary, specific gravity is a key factor in powder coating as it affects application, coverage, and the final finish. The formulation of the powder, including the choice of resins, pigments, fillers, and additives, directly impacts its specific gravity and consequently its performance in the coating process.

Aric Yarosz is a technical service representative, powder coatings at AkzoNobel Powder Coatings.