Aqueous polymer latex of film-forming copolymers suitable as binder in waterborne coating compositions

An aqueous polymer latex of a copolymer obtained by aqueous emulsion polymerisation of ethylenically unsaturated monomers M, which comprise

• i. 5 to 40% by weight, based on the total amount of monomers M, of a monomer M1, which is a methylene-γ-butyrolactone;
• ii. 20 to 95% by weight, based on the total amount of monomers M, of at least one monomer M2, which is selected from C2-C20-alkyl esters of acrylic acid, except for tert-butyl acrylate, and C5-C20-alkyl esters of methacrylic acid and mixtures thereof;
• iii. 0 to 40% by weight, based on the total amount of monomers M, of at least one monomer M3, which is selected from tert.-butyl acrylate, C1-C4-alkyl esters of methacrylic acid, cyclopentyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate and monovinyl aromatic monomers and mixtures thereof,
• iv. 0.05 to 5% by weight, based on the total amount of monomers M, of at least one monomer M4, which is selected from monoethylenically unsaturated monomers having an acidic group;
• where the total amount of monomers M1 and M3 is in the range of 5 to 70% by weight, based on the total amount of ethylenically unsaturated monomers M, and where the total amount of monomers M1, M2 and M3 is at least 85% by weight, based on the total amount of ethylenically unsaturated monomers M.