Powder coating compositions having a substantially non-zinc containing primer
Corrosion and chip-resistant coatings for high tensile steel components, such as automotive coil springs, are formed from a coating composition comprising a substantially non-zinc containing primer and a topcoat. The primer includes an epoxy resin having an epoxy equivalent weight of about 860 to about 930, a polyhydroxyl functional phenolic curing agent having a hydroxyl equivalent weight of about 200 to about 500, and a filler material. The topcoat includes an epoxy resin having an epoxy equivalent weight of about 520 to about 930, an elastomer-modified epoxy resin having an epoxy equivalent weight of about 1000 to about 1600, a carboxyl functional polyester with an acid number of about 45 to about 75 mg KOH/g, a foaming agent and a reinforcing fiber.
1. A dual-coat powder coating system comprising:
(I) an epoxy thermoset primer comprising:
(i) an epoxy resin having an EEW of about 860 to about 930;
(ii) a polyhydroxyl functional phenolic curing agent having a hydroxyl equivalent weight of about 200 to about 500; and
(iii) a filler material,
wherein said epoxy thermoset primer is substantially zinc-free, and
(II) an overlying epoxy-polyester thermoset topcoat comprising:
(i) an epoxy resin having an EEW of about 520 to about 930;
(ii) an elastomer-modified epoxy resin having an EEW of about 1000 to about 1600;
(iii) a carboxyl functional polyester resin with acid number of about 45 to about 75 mg KOH/g;
(iv) a foaming agent; and
(v) a reinforcing fiber.
2. The dual-coat powder coating system of claim 1 , wherein the epoxy thermoset primer further comprises an effective amount of an epoxy resin having an EEW of about 1250 to about 1400.
3. The dual-coat powder coating system of claim 1 , wherein said epoxy resin in said epoxy thermoset primer comprises a bisphenol A epoxy resin.
4. The dual-coat powder coating system of claim 1 , wherein said epoxy resin in said epoxy thermoset primer is present in an amount from about 60 to about 95 phr.
5. The dual-coat powder coating system of claim 1 , wherein said polyhydroxyl functional phenolic curing agent further comprises 2-methylimidazole.
6. The dual-coat powder coating system of claim 1 , wherein said polyhydroxyl functional phenolic curing agent is present in an amount from about 5 to about 40 phr.
7. The dual-coat powder coating system of claim 1 , wherein said filler material comprises complex aluminosilicate (mica), calcium metasilicate (wollastonite), micronized talc, zinc oxide powder, zinc dust, or a combination thereof.
8. The dual-coat powder coating system of claim 7 , wherein said complex aluminosilicate (mica) is present in an amount from about 10 to about 30 phr and has a median particle size of from about 10 to about 35 microns.
9. The dual-coat powder coating system of claim 7 , wherein said calcium metasilicate (wollastonite) is present in an amount from about 10 to about 30 phr and has a median particle size of from about 2 to about 6 microns.
10. The dual-coat powder coating system of claim 7 , wherein said micronized talc is present in an amount from about 1 to about 5 phr and has a median particle size of from about 0.5 to about 3.0 microns.
11. The dual-coat powder coating system of claim 1 , wherein said epoxy resin in said epoxy-polyester thermoset topcoat comprises a bisphenol A epoxy resin.
12. The dual-coat powder coating system of claim 1 , wherein said epoxy resin in said epoxy polyester thermoset topcoat is present in an amount from about 30 to about 65 phr.
13. The dual-coat powder coating system claim 1 , wherein said elastomer-modified epoxy resin comprises a bisphenol A epoxy resin.
14. The dual-coat powder coating system of claim 1 , wherein said elastomer-modified epoxy resin is present in an amount from about 5 to about 20 phr.
15. The dual-coat powder coating system claim 1 , wherein said carboxyl functional polyester resin is present in an amount from about 45 to about 70 phr.
16. The dual-coat powder coating system of claim 1 , wherein said foaming agent imparts a 20-50% reduction in density in said topcoat from that of the theoretical coating density without porosity.
17. The dual-coat powder coating system of claim 1 , wherein said foaming agent is present in an amount from about 0.5 to about 2.0 phr.
18. The dual-coat powder coating system of claim 1 , wherein said reinforcing fiber comprises aluminosilicate, calcium metasilicate (wollastonite), aramid, carbon, or a combination thereof.
19. The dual-coat powder coating system of claim 1 , wherein said reinforcing fiber is present in an amount from about 20 to about 70 phr.
20. The dual-coat powder coating system of claim 1 , wherein said epoxy thermoset primer contains no zinc.
21. A high tensile steel alloy coated with a composite coating layer formed from the dual-coat powder coating system of claim 1 .