Two-coat single cure powder coating
Methods and systems for coating metal substrates are provided. The methods and systems include sequential application of low flow and high flow powder coatings followed by a single heating step to provide a cured coating. The methods and systems include a marker that allows coating uniformity to be monitored and assessed during application. The described methods provide coatings with optimal surface smoothness and edge coverage.
1. A coated article made by a process, the process comprising:
providing a metal substrate;
applying at least a first powder coating composition onto the metal substrate, wherein the at least first powder coating composition has a first flow of about 20 mm to no more than about 40 mm to a surface of the metal substrate;
applying at least a second powder coating composition onto the at least first powder coating, wherein the at least second powder coating composition has a second flow of at least about 50 mm; and
heating the first powder coating composition and second powder coating composition simultaneously in a single step to form a continuous cured coating on the metal substrate, thereby forming the coated article,
wherein the at least first and second powder coating compositions both comprise a polymeric binding resin with a transition temperature (Tg) of about 55° C. to about 70° C., and
wherein the second powder coating composition is critically color-matched to the first powder coating composition, having a ΔE of less than about 2 units, as assessed on an L*a*b scale.
2. The article of claim 1 , wherein the first powder coating composition has a flow of about 20 to 35 mm.
3. The article of claim 1 , wherein the second powder coating composition has flow of greater than about 70 mm.
4. The article of claim 1 , wherein the second powder coating composition has flow of greater than about 75 mm.
5. The article of claim 1 , wherein the cured coating has edge coverage equal to about 10% of the face coverage.
6. The article of claim 1 , wherein the metal substrate is unprimed, clean blasted, or pretreated.
7. The article of claim 1 , wherein the metal substrate may be heated prior to applying the first powder coating composition.
8. The article of claim 1 , wherein the first powder coating composition is applied at ambient temperature to the metal substrate.
9. The article of claim 1 , wherein the first powder coating composition and second powder coating composition are heated in a single step at a temperature of about 180° C. to 200° C.
10. The article of claim 1 , wherein the first powder coating composition and second powder coating composition are heated in a single step at a temperature of about 140° C. to 170° C.
11. The article of claim 1 , wherein the first powder coating composition and second powder coating composition are heated in a single step at a temperature of about 120° C. to 135° C.
12. The article of claim 1 , wherein the polymeric binding resin is a carboxyl-functional polyester resin.
13. The article of claim 1 , wherein the cured coating is a corrosion-resistant coating.
14. The article of claim 1 , wherein the first powder coating composition or the second powder coating composition comprises a marker visible during the application process.