Low foaming paper surface sizing composition

A paper surface sizing composition comprising a copolymer having:

• a) structural units derived from styrene;
• b) structural units derived from monomers selected from esters of ethylenically unsaturated mono-carboxylic acids, half-esters of ethylenically unsaturated dicarboxylic acids and mixtures thereof bearing a substituent R linked to the oxygen atom of the ester groups that is at each occurrence selected from:
• (i) polyalkyleneoxide groups corresponding to the formula:
• wherein R1 is independently at each occurrence selected from hydrogen and C1-C4 alkyl, R2 is C1-C44 hydrocarbyl, t is 0 or 1 and m is an integer in the range of 5 to 200, with the proviso that if R2 is C1-C5 alkyl the polyalkyleneoxide group does not contain more than 50 weight percent ethyleneoxide moieties based on the weight of the polyalkyleneoxide group;
• c) structural units derived from ethylenically unsaturated monomers selected from acrylic acid, methacrylic acid, maleic acid, the salts thereof, maleic anhydride and mixtures thereof;
• and optionally
• d) structural units derived from alkyl substituted alkyl acrylates or methacrylates.