Powder coating compositions

Abstract

Powder coating compositions incorporate, by dry blending, a wax-coated silica additive in finely divided form. Further additives that may be incorporated by dry blending include aluminum oxide and/or aluminum hydroxide. The compositions offer advantages, for example, in terms of fluidity and in terms of first-time deposition of fine particles.

What is claimed is:  
     
         1 . A dry-blended powder coating composition comprising a film-forming polymer and wax-coated silica in finely divided form, with the film-forming polymer present in an amount sufficient to form a coating when deposited on a substrate and the wax-coated silica being present in an amount sufficient to enhance the deposition or uniformity of the coating.  
     
     
         2 . The powder coating composition of    claim 1   , wherein the silica comprises a precipitated silica or a silica gel, and wherein the silica is coated with an animal wax, a vegetable wax, a petroleum wax, or a synthetic wax.  
     
     
         3 . The powder coating composition of    claim 1   , wherein the silica is coated with a microcrystalline wax.  
     
     
         4 . The powder coating composition of    claim 1   , wherein the silica is coated with a wax which comprises an ester of an aliphatic alcohol having at least 16 carbon atoms with a fatty acid having at least 16 carbon atoms.  
     
     
         5 . The powder coating composition of    claim 1   , wherein the silica is coated with a wax which comprises a salt of a fatty acid having at least 16 carbon atoms.  
     
     
         6 . The powder coating composition of    claim 5   , wherein the silica is coated with a wax which comprises aluminum stearate.  
     
     
         7 . The powder coating composition of    claim 1   , wherein: 
 the film-forming polymer is the majority component of the composition;    the wax-coated silica is present in an amount from 0.002 to 2.0% by weight, based on the total weight of the composition without the silica; and    the wax coated onto the silica is present in an amount from 2 to 10% by weight, based on the weight of the wax and the silica.    
     
     
         8 . The powder coating composition of    claim 1   , further comprising aluminum oxide, aluminum hydroxide, or both, wherein the relative weight ratio of wax-coated silica to aluminum oxide, aluminum hydroxide, or both, is from 99:1 to 1:99.  
     
     
         9 . The powder coating composition of    claim 8   , wherein the total amount of the wax-coated silica and the further aluminum oxide, aluminum hydroxide, or both, is from 0.1 to 5% by weight, based on the total weight of the composition without the wax-coated silica and the further aluminum oxide, aluminum hydroxide, or both.  
     
     
         10 . The powder coating composition of    claim 8   , wherein the wax-coated silica, the aluminum oxide, and the aluminum hydroxide are all present in relative proportions by weight of from 1 to 98% SiO 2 , from 1 to 98% Al 2 O 3 , and from 1 to 70% Al(OH) 3 .  
     
     
         11 . The powder coating composition of    claim 8   , wherein the particle size of each of the wax-coated silica and the further aluminum oxide, aluminum hydroxide, or both, is between 0.1 and 10 microns.  
     
     
         12 . The powder coating composition of    claim 1   , wherein the film-forming polymer comprises a carboxy-functional polyester resin, a hydroxy-functional polyester resin, an epoxy resin, a functional acrylic resin, or a mixture thereof.  
     
     
         13 . The powder coating composition of    claim 1   , wherein the film-forming polymer comprises a thermosetting polymer and the composition also includes a curing agent.  
     
     
         14 . The powder coating composition of    claim 1   , wherein the particle size distribution of the composition is such that one or more of the following criteria is satisfied: 
 95-100 vol % of the particles are smaller than 50 μm;    90-100 vol % of the particles are smaller than 40 μm;    45-100 vol % of the particles are smaller than 20 μm;    5-100 vol % of the particles are smaller than 10 μm;    1-80 vol % of the particles are smaller than 5 μm;    d(v) 50  is from 1.3 μm to 32 μm; or    a combination thereof, d(v) 50  being the particle size below which 50% by volume of the particles in the composition is found.    
     
     
         15 . The powder coating composition of    claim 1   , which further comprises one or more coloring agents.  
     
     
         16 . A process for forming a coating on a substrate, which comprises applying the composition of    claim 1    to the substrate, such that the composition adheres to, and forms a continuous coating over at least part of, the substrate.  
     
     
         17 . The process of    claim 15   , wherein the coated substrate is subjected to a heat treatment or radiant energy to cure the composition and adhere it to the substrate.  
     
     
         18 . The process of    claim 17   , wherein the coating thickness is no more than 150 microns.