Process for applying fluoropolymer powder coating as a primer layer and an overcoat
View Patent ↗The invention relates to use of a tetrafluoroethylene/perfluoroolefin copolymer applied as a primer powder in conjunction with a powder overcoat of tetrafluoroethylene/perfluoro (vinyl alkyl ether) copolymer, also known as perfluoroalkoxy polymer (PFA), which when baked onto a substrate gives superior and more durable adhesion of the coating system to the substrate. The quality of the adhesion is measured by a boiling water peel test.
1. A process for forming a release surface on a substrate, comprising (a) applying a primer powder to said substrate to form a primer layer thereon, (b) applying an overcoat powder on said primer layer to form an overcoat layer on said primer layer, said primer containing a tetrafluoroethylene/perfluoroolefin copolymer and polymer binder and said overcoat containing tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (c) baking said primer layer and said overcoat layer to form said release surface.
2. The process of claim 1 wherein said perfluoroolefin contains 3 to 8 carbon atoms.
3. The process of claim 2 wherein said perfluoroolefin is hexafluoropropylene.
4. The process of claim 1 in which the alkyl group of said perfluoro(alkyl vinyl ether) contains 1 to 5 carbon atoms.
5. The process of claim 1 in which said perfluoro(alkyl vinyl ether) is perfluoro(propyl vinyl ether).
6. The process of claim 1 in which the said primer powder is applied by electrostatic spraying.
7. The process of claim 1 in which the said overcoat powder is applied by electrostatic spraying.
8. The process of claim 1 wherein said polymer binder is selected from the group consisting of polyether sulfones, polyphenylene sulfides, and polyaryleneetherketones.
9. The process of claim 2 wherein said polymer binder comprises polyphenylene sulfide.
10. The process of claim 2 wherein said primer powder further comprises up to 60 wt % of tetrafluoroethylene/perfluoro(alkyl vinyl ether).
11. The process of claim 1 wherein said primer powder contains 5 to 90 wt % polymer binder based on the combined weight of said fluoropolymer and said polymer binder.
12. The process of claim 1 wherein said primer contains 10 to 20 wt % of inorganic filler based on the combined weight of said fluoropolymer, polymer binder, and filler.
13. The process of claim 1 wherein the powder of said primer powder contains particles containing both said fluoropolymer and said polymer binder.
14. The process of claim 1 wherein said primer powder has been formed by spray drying.
15. The process of claim 1 wherein said substrate is selected from a group consisting of carbon steel, aluminum or aluminized steel.
16. The process of claim 1 wherein said substrate is at a temperature of 15 to 25° C. when steps (a) and (b) are carried out.
17. The process of claim 1 wherein said applying of said overcoat powder on said primer layer is carried out prior to baking said primer layer, whereby said baking of said overcoat layer also bakes said primer layer.
18. The process of claim 1 wherein said applying of said overcoat powder on said primer layer is carried out after baking said primer layer and said overcoat layer is baked after application of said overcoat powder.
19. The process of claim 1 wherein said primer layer is less than 2 mils (50 micrometers) thick and said overcoat layer is no greater than 25 mils (650 micrometers).
20. The process of claim 17 wherein said primer layer is less than 1.5 mils thick.
21. The process of claim 17 wherein said overcoat layer is 1.5 to 3 mils thick (38 micrometers to 76 micrometers).