Sequential emulsion polymerization process

A process for preparing an aqueous copolymer latex of colloidally dispersed, substantially spheroidal, heterogeneous copolymer particles which are individually composed of a relatively hydrophobic polymer core portion having a relatively hydrophilic, polymeric portion preferentially oriented toward the outer surface thereof, said process comprising the steps of:

• (a) first preparing an initial aqueous latex of a substantially linear hydrophilic polymer by emulsion copolymerizing at a pH of from about 2 to about 6 in the substantial absence of a cross-linking agent an initial monomer charge comprising (1) one or more vinyl esters of a non-addition polymerizable carboxylic acid and/or one or more esters of an addition polymerizable carboxylic acid and (2) one or more relatively hydrophilic comonomers in an amount which renders the copolymer of said initial latex sufficiently hydrophilic that it ultimately becomes preferentially oriented toward the outer surface of the finished copolymer particles of the final copolymer latex product, said relatively hydrophilic comonomer having a solubility in water of at least about 3 weight percent on a total weight basis at 25.degree. C. and with the proviso that such hydrophilic comonomer can be excluded when the monomer component of step (a)(1) is exclusively vinyl acetate;
• and
• (b) thereafter emulsion polymerizing, in the presence of from about 5 to about 50 parts by weight on a polymer solids basis of the initial hydrophilic polymer latex of step (a) and at a pH of from about 2 to about 6, from about 50 to about 95 parts by weight of a second monomer charge comprising (1) an aliphatic conjugated diene and/or an addition polymerizable monovinylidene monomer selected from the group consisting of monovinylidene aromatic monomers and vinylidene halide monomers and (2) from 0 to about 10 weight percent based upon the weight of such second monomer charge of a monoethylenically unsaturated carboxylic acid monomer.