Process for coating a substrate with a coating including a fluoropolymer, thermoplastic polymer, and filler
A process for coating a substrate with a coating comprising sequential steps of coating the substrate with mixtures of powders of fluoropolymers, thermoplastic polymers, perfluoropolymers and inorganic fillers of certain particle sizes. The coated substrate has the property of strongly adhering to a topcoat comprising a perfluoropolymer.
1. A process for coating a substrate with a coating comprising a fluoropolymer, said process comprising the sequential steps of:
a. Preparing a solid mixture comprising one or more fluoropolymers and one or more thermoplastic polymers thermally stable at temperatures in excess of 400° C.;
b. Melt blending and extruding said solid mixture at a temperature of from about 250° C. to about 400° C. to achieve homogeneity;
c. Subjecting the extrudate to mechanical means to obtain a powder of up to about 100 microns average particle size;
d. Blending the resulting powder with an inorganic filler with an average particle size between about 5 and 100 microns, and a perfluoropolymer powder with a particle size between about 5 and 100 microns to the blend of step c. to obtain a powder coating; said inorganic filler and perfluoropolymer being blended with the powder of step c. at the same time, or the inorganic filler blended before the perfluoropolymer, or the perfluoropolymer blended before the inorganic filler;
e. Applying said powder coating onto said substrate;
f. Heating said substrate to a temperature sufficient to cause said powder to partially coalesce;
g. Applying a perfluoropolymer powder coating to the coated substrate from step f; and
h. Again heating said substrate to cause the powder to become sufficiently fluid and bond as a second layer to the first layer.
2. The process of claim 1 wherein said powder coating of step e, is applied electrostatically in a layer of about 20-100 microns thick.
3. The process of claim 2 wherein the average particle size of said inorganic filler is equal to or greater than the thickness of the coating on the substrate following step f.
4. The process of claim 1 wherein said inorganic filler comprises a pigment.
5. The process of claim 4 wherein said pigment comprises aluminum flakes.
6. The process of claim 1 wherein the particle size of the powder of step c. is similar to the particle size of the inorganic filler of step d.
7. The process of claim 1 wherein the particle size of the powder that forms the second layer is similar to the particle size of the powders that form the first layer.