Various methods are currently used to obtain matt or satin powder paints but, to a greater or lesser extent, all of them have limitations.
Some methods have recourse to overpigmentation or the addition of polymers which are incompatible with the binder system in intimate homogenization but this leads to a diminution in the chemical and mechanical resistance of the paints.
Other methods use intimate homogenization of additives or special catalysts which, however, can only be used with specific systems of binders.
Other methods, always with intimate homogenization, are based on the difference in reactivity between resins of the same type of reactive group in a single binder system.
Still other methods do not use intimate homogenization but cold physical mixing of one or more paints extruded separately and with different viscosity or reactivity or systems of binders as described in international patent application PCT/EP 89/00022 in the name of the same applicant, published under number WO 89/06674.
These last methods are much less limiting than the preceding ones because they can employ practically all the systems of binders as long as they have different viscosities or different types of cross linking. However they are less practical because, with cold mixing methods, manufactures of powder paints normally extrude the paints separately, cool the extrusions, crush or grind them, sieve them and then weigh them out in the various pre-established ratios and mix them in a mixing machine. This means that the process is discontinuous and thus more time, space, materials and manpower are required.
There is also the danger of partial separation of the two paints during the various stages of grinding and sieving because of the difference in their specific weights or a different conformation of the granules of powder. Finally, at the spraying stage, the two paints can be deposited non-uniformly because of the different charges accumulated by the two binders.