High pH process resistant coating for metal food containers
View Patent ↗Disclosed is an improved coating composition for food containers that is resistant to high pH processing environments. The improved coating composition is based upon an epoxy-amino resin coating composition that has been modified with other components to improve its performance in high pH environments balanced with flexibility, formability and abrasion resistance. Typically, the coating composition includes an epoxy resin, an amino resin cross-linking agent, a blocked polyisocyanate and a saturated polyester.
1. A can coating composition comprising, by weight, based on total solid resin weight, about 75% to about 95% of an epoxy resin, about 4% to about 15% of an amino resin cross-linking agent, about 1% to about 3% of a blocked polyisocyanate and about 1% to about 4% of a saturated polyester.
2. The can coating composition of claim 1 , wherein the epoxy resin comprises a blend of at least two high molecular weight epoxy resins in which one epoxy resin has an epoxide equivalent weight of about 1350-2000 and the other epoxy resin has an epoxide equivalent weight of about 2200-5500.
3. The can coating composition of claim 2 , wherein the one epoxy resin has an average degree of polymerization (n) of n=9 and the other epoxy resin has an average degree of polymerization (n) of n=12.
4. The can coating composition of claim 3 , wherein the weight ratio of the two high molecular weight epoxy resins comprises a 60:40 ratio of the lower molecular weight epoxy resin to the higher molecular weight epoxy resin.
5. The can coating composition of claim 2 , wherein the solids weight ratio of the epoxy resin to the amino resin is about 94:6 to about 90:10, wherein the blocked polyisocyanate is based on isophorone diisocyanate (IPDI) and is blocked with ε-caprolactam, wherein the saturated polyester has a molecular weight (Mn) in the range of about 500-10,000 Da and has a hydroxyl number in the range of about 3-80 mg KOH/g, and wherein the saturated polyester and blocked polyisocyanate are present in amounts such that the stoichiometric ratio of polyester OH to blocked polyisocyanate NCO is about 1:1.
6. The can coating composition of claim 5 , additionally comprising an organic solvent in an amount such that the composition has a total solids content of about 35% to 45% by weight of the total composition.
7. The can coating composition of claim 6 , wherein the amino resin cross-linking agent comprises an n-butylated benzoguanamine-formaldehyde resin having an acid value of about 1-5 mg KOH/g.
8. The can coating composition of claim 1 , wherein the epoxy resin is a diglycidyl ether of bisphenol A and epichlorohydrin.
9. The can coating composition of claim 1 , wherein the amino resin cross-linking agent comprises an n-butylated urea-formaldehyde resin having an acid value of about 0-5 mg KOH/g.
10. The can coating composition of claim 1 , wherein the amino resin cross-linking agent comprises an n-butylated benzoguanamine-formaldehyde resin having an acid value of about 1-5 mg KOH/g.
11. The can coating composition of claim 1 , wherein the solids weight ratio of the epoxy resin to the amino resin is about 94:6 to about 90:10.
12. The can coating composition of claim 1 comprising, by weight, based on total solid resin weight, about 80% to 90% of the epoxy resin and about 5% to 10% of the amino resin cross-linking agent.
13. The can coating composition of claim 1 , wherein the blocked polyisocyanate is based on isophorone diisocyanate (IPDI).
14. The can coating composition of claim 13 , wherein the blocked polyisocyanate is blocked with ε-caprolactam.
15. The can coating composition of claim 1 , wherein the saturated polyester has a molecular weight (Mn) in the range of about 500-10,000 Da, and has a hydroxyl number in the range of about 3-80 mg KOH/g.
16. The can coating composition of claim 15 , wherein the saturated polyester comprises 1.5-2.5% OH and has an OH equivalent weight of about 850.
17. The can coating composition of claim 1 , wherein the saturated polyester and blocked polyisocyanate are present in amounts such that the stoichiometric ratio of polyester OH to blocked polyisocyanate NCO is about 1:1.
18. The can coating composition of claim 1 , additionally comprising an organic solvent in an amount such that the composition has a total solids content of about 35% to 45% by weight of the total composition.
19. The can coating composition of claim 1 , additionally comprising one or more components selected from a blocked acid catalyst, a lubricant, a flow modifying agent and a foam control agent.
20. The can coating composition of claim 1 , wherein all of the components comply with FDA 175.300 regulations.