Sequential heteropolymer dispersion and a particulate materal obtainable therefrom, useful in coating compositions as a thickening and/or opacifying agent

A process of making an aqueous dispersion of water insoluble core/sheath polymer particles in which the core contains sufficient acid groups to render the core swellable by neutralization with a volatile base to at least twice its volume and the sheath is permeable to said base, comprising

• (a) sequentially emulsion polymerizing at a temperature of from about 10.degree. to about 100.degree. C. in an aqueous medium containing a free radical initiator:
• one or more monoethylenically unsaturated core monomers having a group of the formula --HC.dbd.C<, at least one of which has a carboxylic acid group, emulsified in the medium, the core monomer(s) comprising a hydrophilic monomer selected from (i) a non-acid monomer copolymerized with at least 5% of a carboxylic acid monomer, and (ii) 15-100% of said acid monomer when no other hydrophilic monomer is present, based on the weight of the monomers utilized to prepare the core polymer particles to thereby form dispersed core polymer particles, said core particles having an average diameter of from about 0.05 to 1 micron, the core monomers having no more than 40% of butyl acrylate or no more than an amount of an equivalent monomer which would give a comparable T.sub.i when using the same comonomer combination, and
• (b) then adding to the polymer dispersion resulting from (a) at least one monoethylenically unsaturated sheath monomer having no ionizable group to form a sheath polymer on the core particles, any monoethylenically unsaturated carboxylic acid in the sheath monomer mixture being present in an amount of no more than 10% by weight of the sheath monomers, the proportion of carboxylic acid in the sheath monomer mixture being less than 1/3 the proportion thereof in the core monomer mixture, the core/sheath particles having an average diameter before neutralization and swelling of from about 0.07 to 4.5 microns, relative amount of core-forming monomer(s) and sheath-forming monomer(s) being such that the ratio of the weight of the core to the weight of the total polymer in the resulting dispersed particles is from about 1:4 to 1:100, said sheath being permeable at 20.degree. C. to an aqueous volatile base selected from ammonia and amines, and essentially impermeable at 20.degree. C. to fixed or permanent bases including sodium, potassium, calcium or magnesium hydroxide, the exterior sheath having a Ti of below about 25.degree. C. or below or about the ambient temperature of application, said sheath being film forming, the core polymer being relatively hard or crosslinked, the particles being film forming:
• (c) neutralizing with ammonia or amine so as to swell said core.