Emulsion polymers for modifying impact strength

Claim 1

A multistage emulsion polymer adaptable to inclusion in a transparent methyl methacrylate thermoplastic molding compound to improve the impact strength thereof without loss of transparency and without stress whitening, said polymer comprising (a) a hard, nonelastomeric core polymer having a glass transition temperature above 50° C., produced by the emulsion polymerization of a monomer component consisting of at least 80 percent by weight of methyl methacrylate and up to 20 percent by weight of at least one other, different, ethylenically unsaturated monomer copolymerizable therewith; (b) an elastomeric intermediate polymer having a glass transition temperature below 0° C., produced by the emulsion polymerization, in the presence of the hard core polymer (a), of a monomer mixture consisting of (i) at least 90 percent by weight of at least one monoethylenically unsaturated monomer of which at least 50 percent by weight is at least one alkyl ester of acrylic acid having from 1 to 18 carbon atoms in the alkyl group or at least one alkyl ester of methacrylic acid having from 4 to 18 carbon atoms in the alkyl group, (ii) 0.2 to 3 percent, by weight of the mixture of stage (b), of at least one polyfunctional monomer having three or more acrylic or methacrylic groups, and (iii) the balance being at least one further monoethylenically unsaturated monomer such that the refraction index of polymer (b) matches that of polymethyl methacrylate; and (c) a hard, nonelastomeric final polymer having a glass transition temperature above 50° C., produced by the emulsion polymerization, in the presence of the elastomeric polymer (b), of a monomer component consisting of at least 80 percent by weight of methyl methacrylate and up to 20 weight percent of at least one other monoethylenically unsaturated monomer copolymerizable therewith.