Size, size distribution, shape, and integrity of core particles have an impact on the process and coating requirements. Careful consideration of this impact is an important aspect of process and formula optimization.

Size

The Wurster process is applicable to materials that can be fluidized and confined in a coating unit. In general, this includes solid particles with a particle size or diameter ranging from near 40 µm to several centimeters.

Size Distribution

A size distribution within a 1:4 range (smallest : largest particle size in bulk core material) is ideal for the Wurster process, but up to 1:10 is acceptable. Beyond 1:10, fluid bed uniformity is lost and agglomeration and poor coating uniformity become problems.

Shape

If it can be fluidized, it can be coated. Spherical particles are ideal for the Wurster coating process due to a minimum surface area/weight ratio and the curvature of the surface. Shapes that are crystalline, jagged, flat, and amorphous can also be uniformly coated.

Integrity

Friable particles can break or chip throughout the coating process leaving exposed and poorly coated surfaces in the coated product. This can be an important concern depending on the function of the coating.

Size/Coat Percentage/Thickness Relationship

Coating requirements depend on particle size. For a given weight of material, the surface area for small particles is higher than that for large particles. Higher surface area means more coating material is needed to attain a desired coat thickness. Assuming equivalent density of core and coating material and a spherical particle, the table below correlates size, coat thickness, and weight% coating.

The importance of particle size is clearly evident from these data. When particle size can be controlled, optimal coating can be realized by maximizing the particle size for the end use of the product.