A crosslinking material, the crosslinking material comprising the reaction product of a reaction mixture comprising: (i) a cyclic unsaturated acid anhydride and/or diacid derivative thereof; (ii) an ethylenically unsaturated monomer; and (iii) an alcohol, amine and/or thiol, wherein the cyclic unsaturated acid anhydride and/or diacid derivative thereof is partially-esterified by reaction with the alcohol, amine and/or thiol; and wherein the wherein the crosslinking material has an acid number of at least 100 mg KOH/g.