Preparation of polymer dispersions which form block-resistant films

Claim 1

A process for the preparation of an aqueous polymer dispersion which has a low film-forming temperature and forms films having a high block resistance, by multistage emulsion polymerization of predominant amounts of monoolefinically unsaturated monomers A, whose homopolymers have a glass transition temperature of from 0° to -72° C., with monoolefinically unsaturated monomers B, whose homopolymers have a glass transition temperature of from 80° to 140° C., from 0.2 to 6% by weight, based on the total amount of monomers, of monoolefinically unsaturated monomers C which possess carboxyl and/or carboxamide groups, and from 0 to 10% by weight, based on the total amount of monomers, of olefinically unsaturated monomers D which effect cross-linking, wherein first (a) from 1 to 10% by weight of a monomer mixture (I) consisting of from 75 to 98 parts by weight of monomer A, from 2 to 25 parts by weight of monomer B, from 0.5 to 10 parts by weight of monomer C and from 0 to 10 parts by weight of monomer D are initially taken and polymerized in 0.5-4% strength aqueous emulsion in the presence of from 0.1 to 2% by weight, based on the total amount of monomers (in monomer mixtures I and II), of an anionic emulsifier and from 0 to 1% by weight, based on the total amount of monomers, of a nonionic emulsifier, and in the presence of a water-soluble free radical polymerization initiator, then (b) the remainder of the monomer mixture (I) is fed in at the rate at which the emulsion polymerization progresses, and finally, when the addition of the monomer mixture (I) is complete, (c) a monomer mixture (II) which consists of from 75 to 98 parts by weight of monomer B, from 2 to 25 parts by weight of monomer A, from 0 to 10 parts by weight of monomer C, from 0 to 10 parts by weight of monomer D and/or from 0 to 15 parts by weight of monoolefinically unsaturated ureido compounds or tertiary amino compounds is added at the rate at which it is consumed, while the emulsion polymerization is continued, the amount of monomer mixture (I) being from 40 to 65% by weight, based on the total amount of the two monomer mixtures, monoolefinically unsaturated carbonyl compounds being used as monomers D, the monomer mixtures (I) and (II) being fed into the polymerization vessel as such or in the form of an aqueous emulsion, so that the resulting aqueous polymer dispersion has a polymer content of from 40 to 60% by weight, based on the polymer dispersion, and a content of anionic emulsifiers of from 0.5 to 3% by weight, based on the polymer, and, when the polymer contains monomer D as copolymerized units, a hydrazide of an aliphatic dicarboxylic acid is added to the ready-prepared polymer dispersion in an amount of from 0.5 to 1 mole per mole of carbonyl monomer.