Process for the preparation of a finely divided vinyl chloride graft copolymer and its use as viscosity-reducing agent and flatting agent

Process for the preparation of a finely divided vinyl chloride graft copolymer having an average particle size of 20 to 30 .mu.m by a suspension polymerization process in the presence of monomer-soluble free radical initiators and further conventional additives, such as buffer substances or emulsifiers, consisting essentially of polymerizing a graft monomer in the presence of 1.0 to 5.0% by weight, relative to the graft monomer content, of an alkyl acrylate polymer which is initially introduced in a aqueous emulsion having a solids content of 30 to 60% by weight, and 0.1 to 1.0% by weight, relative to the graft monomer content, of a cellulose ether having a methoxy content of 15 to 25% and a hydroxypropyl content of 3 to 10% and having a viscosity of 30 to 150 mPa.s according to ASTM D 1363, which is initially introduced or metered in after addition of the graft monomer.