Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates, and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems, which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent based coating compositions also suffer from the disadvantage of relatively poor percent utilization; in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
In addition to exhibiting good gloss, impact strength, and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet steel that is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles, in the course of which the sheet metal is flexed or bent at various angles.
Formation of a powder coating composition typically entails the dry mixing of flakes or granules of resin with the cross-linking agent and other ingredients, extruding the mixture at temperatures in the range of about 80.degree. to 130.degree. C., cooling the extrudate, and then chipping and grinding the resulting solid into particles of suitable size. This pulverizing operation typically produces a powder in which the particles are characterized by irregular shape and a broad size distribution.