Two stage powder application method

Coating processes – Solid particles or fibers applied – Plural particulate materials applied

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Details

427202, 427203, 427421, B05D 102

Patent

active

057471061

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to an improved method for powder coating.


BACKGROUND OF THE INVENTION

In many powder coating applications, the average particle diameter applied to the surface to be coated varies depending on the thickness of the coating. For instance, if a relatively thick coating is desired, particles having a thick diameter are used. If a relatively thin coating is desired, particles having a thin diameter are used.
However, with large sized particles, the applied coating or film does not always appear level and smooth, even if heating is used to coalesce the particles after contact on the surface. This poor leveling of large particles is particularly acute when the coating material is a thermosetting polymer. In some cases, the coating looks uneven, like the external surface of a lemon, an effect sometimes referred to in the industry as the "orange peel" phenomenon.
In contradistinction, leveling is more easily achieved with smaller sized particles. However, per particle, the ratio of surface area to weight becomes high, and bulk density drops. When the particles are applied by spraying, the effect of air resistance makes it difficult to maintain a straight flight path for long distances. The ability to maintain a straight flight path is referred to as linearity. For coating objects with hollow or recessed portions, penetration of the small powder particles to these surfaces is poor. This poor coverage necessitates an increase in powder ejection air pressure. However, this increased ejection air pressure results in increased flow volume, so that while this additional air pressure carries the particles farther it tends also to carry particles away from the surface when the transport air rebounds away from the surface.
Additionally, powder having a relatively small or fine particle size has a high manufacturing cost. Thus, despite its well known coating qualities, the industry has generally not used powders of relatively small size particles.
It is an objective of this invention to improve the leveling and the aesthetic appearance of a powder coating applied to a surface, without compromising the delivery of the powder particles to the surface being coated or the adhesion of particles to the surface.


SUMMARY OF THE INVENTION

The present invention achieves the above-stated objectives by a two step particle application method which involves initially applying large size particles to a coating surface, followed by applying small particles to the large size particles. The average particle diameter for the powder applied during the first stage (Stage A) is 15 to 120 microns, and the average particle diameter for the powder applied during the second stage (Stage B) is 0.1 to 15 microns.
Preferably during both stages, the particles are applied by spraying. The larger sized particles applied during the first stage may be temporarily heated to cause partial melting and increased adherence. Subsequently, when the second stage particles are applied, heat may again be used to melt the applied powder particles. Stages A and B may be repeated as necessary, to achieve the desired coating thickness.
One advantage that results from this two stage powder application method is increased adhesion efficiency. Also, by initially applying the larger sized particles, the desired film thickness can be achieved, while subsequent application of the smaller sized particles improves overall leveling and surface appearance.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawings.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view which schematically shows a powder coating achieved with the two stage powder application method of this invention, wherein the coated surface is flat.
FIG. 2 is a cross-sectional view, similar to FIG. 1, which schematically shows a powder coating achieved with the two stage powder coating method of this invention, wherein the coated surface has a recess.
FIG. 3 is a cross-

REFERENCES:
patent: 3348995 (1967-10-01), Baker et al.
patent: 4542539 (1985-09-01), Rowe et al.

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