Multi-stage opacifying polymer particles containing non-polymeric acid absorbed therein

Claim 1

A process for preparing opacifying polymer particles containing one or more void space(s) within said particles comprising: (A) sequentially emulsion polymerizing polymer particles comprising two or more polymer stages wherein (1) each of said polymer stages is emulsion-polymerized from a monomer system comprising one or more ethylenically-unsaturated monomer(s) containing one or more --HC═C< group(s); (2) the final polymer stage of said particles has a glass transition temperature of about 50° C. or greater; (3) at least one essentially low-acid, earlier polymer stage, other than said final polymer stage (2), is prepared from a monomer system which contains less than about 5% by weight of monomers containing carboxylic acid functionality; (4) each polymer stage is different from any adjacent polymer stage by either about a one percent difference by weight in the amount of any ethylenically unsaturated monomer used therein or by a difference in number average molecular weight between said stage and said adjacent stage of at least about a factor of 2; and (5) the polymer stages of said particles having a Tg of about 50° C. or greater comprises at least about 60% by weight of said particles; (B) contacting said polymer particles with a non-polymeric alipharic or aromatic carboxylic acid(s) having 6 to 12 carbon atoms containing at least one carboxylic acid group before, during or after polymerization of said final polymer stage (2) to pemit said acid-containing compound to be absorbed into said low-acid polymer stage (3); and (C) swelling the resulting multi-stage polymer particles containing said absorbed acid-containing compound with water, an optical plasticizing solvent, at or above the glass transition temperature of the neat polymer or glass transition temperature of the optionally plasticized polymer of said final polymer stage (2) by contacting said particles with base at a pH sufficient to swell said polymer particles, so as to produce particles which, when dried, contain one or more void space(s) within said particles.