Two component powder coating system and method for coating a heat sensitive substrate therewith

Abstract

.[.Wood.]. .Iadd.A heat sensitive substrate .Iaddend.is .[.electrostatically.]. coated with a thermosetting powder coating system in which a mixture of a self-curing epoxy resin and a catalyst therefore is extruded and .Iadd.pulverized and .Iaddend.a low temperature curing agent .[.are both.]. .Iadd.a .Iaddend.pulverized and the powders are blended with conventional additives to make a coating powder which is deposited on .[.a wooden.]. .Iadd.the .Iaddend.substrate and heated to cure. The mixture of resin and catalyst does not cure within the extruder but it is made to cure at low temperatures by the separate addition of the curing agent. A small amount of the low temperature curing agent, insufficient to cause substantial curing during extrusion may be used in place of the catalyst.

The subject matter claimed is: 
     
       1. A method for coating wood comprising electrostatically spraying a coating powder selected from the group consisting of (A) a blend comprising (i) a powdered, extruded mixture of a self-curing epoxy resin and a catalyst therefore, and (ii) a powdered, low temperature curing agent; and   (B) a blend comprising (i) a powdered, extruded mixture of a self-curing epoxy resin and an amount of a low temperature curing agent insufficient to cause substantial curing of the resin during extrusion, and (ii) a sufficient amount of the same or different low temperature curing agent in powder form to complete the curing of the resin onto a surface of the wood to a thickness of from about 3 to about 6 mils, and curing the powder at a temperature of from about 180° F. up to but not including the decomposition temperature of the blend.   
     
     
       2. The method of claim 1 wherein the low temperature curing agent is an epoxy resin adduct of an aliphatic polyamine having a primary amino group. 
     
     
       3. The method of claim 1 wherein the catalyst is an epoxy adduct of an imidazole having the general formula: ##STR2## wherein R 1 , R 2 , R 3 , and R 4  are independently hydrogen or any substituent which is not reactive with the epoxy resin. 
     
     
       4. The method of claim 1 wherein the electrostatic spraying is triboelectric. .Iadd. 
     
     
       5.  A method for coating a heat sensitive substrate comprising applying a coating powder selected from the group consisting of (A) a blend comprising (i) a powdered, extruded mixture of an epoxy resin and a catalyst therefore, and (ii) a powdered, low temperature curing agent; and     (B) a blend comprising (i) a powdered, extruded mixture of an epoxy resin and an amount of a low temperature curing agent insufficient to cause substantial curing of the resin during extrusion, and (ii) a sufficient amount of the same or different low temperature curing agent in powder form to complete the curing of the resin onto a surface of the substrate to a thickness of at least about 1 mil, and curing the powder at a temperature of from about 180° F. up to but not including the decomposition temperature of the blend..Iaddend..Iadd.       
     
     
       6.  The method of claim 5 wherein the low temperature curing agent is an epoxy resin adduct of an alipbatic polyamine having a primary or secondary amino group..Iaddend..Iadd.7. The method of claim 5 wherein the powder is electrostatically sprayed onto the surface..Iaddend..Iadd.8. The method of claim 5 wherein the catalyst is an imidazole having the general formula: ##STR3## wherein R 1 , R 2 , R 3 , R 4  are independently hydrogen or any substituent which is not reactive with the epoxy resin..Iaddend..Iadd.9. The method of claim 2 wherein the catalyst is an epoxy adduct of an imidazole having the general formula: ##STR4## wherein R 1 , R 2 , R 3 , R 4  are independently hydrogen or any substituent which is not reactive with the epoxy resin..Iaddend..Iadd.10. The method of claim 2 wherein the catalyst is an imidazole having the general formula: ##STR5## wherein R 1 , R 2 , R 3 , R 4  are independently hydrogen or any substituent which is not reactive with the epoxy resin..Iaddend.