Coating formulation for the interior surfaces of cans

Abstract

The invention relates to a water-based can inner coating comprising a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and at least one water-dispersed or water-soluble curing agent. Inventive can inner coatings are characterized in that due to the good crosslinking of the copolymer or of the copolymer mixture with the curing agent, the cured film on the inner surfaces of metal cans possesses excellent properties in regard to hardness, abrasion resistance and resistance towards hot liquids.

1 . A water-based can inner coating comprising, in addition to water,
 a) a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and   b) at least one water-dispersed or water-soluble curing agent selected from an aminoplast, a carbodiimide and combinations thereof.   
     
     
         2 . The water-based can inner coating according to  claim 1 , wherein the acid groups of the copolymer or of the copolymer mixture in water-dispersed form are at least partially neutralized. 
     
     
         3 . The water-based can inner coating according to  claim 2 , comprising a neutralization agent for neutralizing the acid groups of the copolymer or of the copolymer mixture in water-dispersed form, said neutralization agent being selected from the group consisting of ammonia, amines, metallic Al, metallic Zn, water-soluble oxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II), water-soluble hydroxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II) and combinations thereof. 
     
     
         4 . The water-based can inner coating according to  claim 3 , wherein the neutralization agent is selected from ammonia, morpholine, hydrazine, hydroxylamine, monoethanolamine, diethanolamine, triethanolamine, dimethylethanolamine, diethylethanolamine, and combinations thereof. 
     
     
         5 . The water-based can inner coating according to  claim 1 , wherein the copolymer or the copolymer mixture has a glass transition temperature of not more than 80° C. 
     
     
         6 . The water-based can inner coating according to  claim 1 , wherein the aliphatic and acyclic alkene is selected from ethene, propene, 1-butene, 2-butene, isobutene, 1,3-butadiene, 2-methylbuta-1,3-diene and combinations thereof. 
     
     
         7 . The water-based can inner coating according to  claim 1 , wherein the α,β-unsaturated carboxylic acids are selected from cinnamic acid, crotonic acid, fumaric acid, itaconic acid, maleic acid, acrylic acid, methacrylic acid and combinations thereof. 
     
     
         8 . The water-based can inner coating according to  claim 1 , wherein a weight fraction of the aliphatic and acyclic alkenes in the copolymer or in the copolymer mixture is at least 40 wt. %, but not more than 95 wt. %. 
     
     
         9 . The water-based can inner coating according to  claim 1 , wherein the copolymer or the copolymer mixture additionally comprises comonomers that are selected from esters of α,β-unsaturated carboxylic acids. 
     
     
         10 . The water-based can inner coating according to  claim 9 , wherein the esters of α,β-unsaturated carboxylic acids comprise linear or branched alkyl esters of acrylic acid and/or methacrylic acid, said esters having an aliphatic group of not more than 12 carbon atoms, wherein the copolymer or the copolymer mixture has an acid number of less than 100 mg KOH/g. 
     
     
         11 . The water-based can inner coating according to  claim 1 , wherein water dispersed polymeric constituents of the water-based can inner coating have a D90 value of not more than 100 μm and a D50 value of not less than 1 μm. 
     
     
         12 . The water-based can inner coating according to  claim 1 , comprising at least 40 wt. % water and
 a) 4-30 wt. % of the copolymer or of the copolymer mixture,   b) 2-20 wt. % of the at least one curing agent,   c) not more than 5 wt. % of emulsifiers selected from non-ionic amphiphiles with an HLB value of at least 8;   d) not more than 40 wt. % of water-miscible organic solvents;   e) not more than 10 wt. % of auxiliaries selected from wetting agents, leveling agents, defoamers, catalysts, film-formers, stabilizers and neutralizing agents.   
     
     
         13 . A method of coating inner surfaces of a can comprising applying the water-based can inner coating according to  claim 1  to inner surfaces of a can and drying said water-based can inner coating. 
     
     
         14 . The method of  claim 13 , wherein the can is a tin plate can and the inner coating is deposited in a dry film thickness of at least 5 g/m 2 , but not more than 50 g/m 2 , on to the inner surface. 
     
     
         15 . The method of  claim 13 , wherein the can is an aluminum can and the inner coating is deposited in a dry film thickness of at least 1.5 g/m 2 , but not more than 50 g/m 2 , on to the inner surface. 
     
     
         16 . The method of  claim 13 , wherein the water-based can inner coating is deposited in a spraying process.