Powder Particles

Particle size

One of the main physical requirements of a powder, is that the particle size and distribution should be suitable for the application considered.

As most thermosetting powders are applied by electrostatic spraying, the powder must be receptive to accepting an electrical charge.  This must be sufficient for the powder particles to be electrostatically attracted to the surface being coated.  The electrostatic charge must be adequate for cover over edges and for ‘wrap-around’, but the deposit of electrostatically applied powder should not be so insulating as to inhibit film build-up.

The fluidity of the powder is also of particular importance as it must meet various transport demands, eg. from manufacturer to customer, from storage container to storage vessel.  Blocking, which occurs when the powder ‘cold flows’ to form a solid or partially solidified mass, must be avoided at all costs.

Powder fluidity depends upon the shape and size of the particles as well as on their material composition and ambient powder storage conditions.

Under certain adverse conditions such as high ambient temperature some polymerised materials exhibit a tendency to deform, so causing them to block.

The tendency of a powder to be free-flowing depends to a considerable extent on the particle size distribution.  At the present time most thermosetting powders are applied by electrostatic spray and most commercial powders would have a particle size within 2-100µ, with the peak between 30-40µ.  These powders have reasonable flow characteristics.  Powders with a wide particle size spectrum are less satisfactory as regards powder recovery than those with a narrow one.  The wider the particle size range the more likely is the particle size distribution of the recovered powder to vary in relation to the virgin powder.

Film thickness

The relationship of the powder particle size and ultimate cured film thickness is important.  Currently all thermosetting powders consist of irregularly shaped, multi-sided particles of varying size and particle size distribution.  Graph 1 illustrates a typical particle size distribution of a commercial pigmented epoxy powder.

Figure 1 illustrates the relationship of powder particle size and film thickness.  It can be seen that it would be extremely difficult to obtain a 50µ powder coating film with particles larger than 50-75µ, unless the polymer has exceptionally good flow out.

Ideally for the formulation of smooth, non-porous films, the powder should be as densely packed as possible on the surface prior to fusion and cross-linking so that shrinkage and formation of voids, pinholes and orange peel is minimised.

It is generally easier to obtain a smooth film with a narrow particle size band distribution, especially at low particle size cuts, eg. 10-20µ, than it is at wider or higher particle size distribution.  However, powder of the lower particle sizes have to be specially formulated to apply using existing spray application equipment.

Because of these factors, the melt-flow index and rate of cure of the powder are critical properties in achieving surface smoothness and maintaining adequate edge cover.

Obviously therefore the powder particle size, shape and size distribution influence the quality and appearance of the resultant powder coating.  Smaller particle sizes accept a higher charge relative to their weight than larger particles.  The attractive force between the charged particle and the workpiece being coated is therefore greater, which usually leads to preferential deposition and wrap-around with smaller particle size.

However because of the Faraday cage effect the smallest particles are less apt to penetrate down into openings and recessed areas.

Coating thickness can be increased by preheating the workpiece as recorded in Table 1.  Table 2 summarises the basic difference between fine powder (10-40µ) and a coarse powder (50-100µ).