Process for preparing protective-colloid-stabilized polymers by continuous emulsion polymerization

A process for preparing polyvinyl-alcohol-stabilized polymers based on vinyl esters, ethylene and optionally further comonomers, in the form of their aqueous polymer dispersions or water-redispersible polymer powders, by means of free-radical-initiated continuous emulsion polymerization, without employing a seed latex, and in the case of polymer powders, drying the resultant polymer dispersion, comprising

• a) emulsion polymerizing ethylene, at least one vinyl ester, and optionally further comonomers in at least two pressurized reactors arranged in series at a temperature of from 40 to 100° C. and at a pressure of from 5 to 100 bar, to a residual monomer content of less than 3% by weight, and continuing polymerizing in at least one unpressurized reactor at a temperature of from 20 to 60° C. and at a pressure of less than 1.0 bar;
• b) initiating polymerization using a redox system comprising an oxidizing component and a reducing component, where at least part of the reducing component is introduced within the first pressurized reactor;
• c) metering-in polyvinyl alcohol in the first pressurized reactor such that the proportion is always less than or equal to 6% by weight, based on the monomers continuously introduced to the first pressurized reactor;
• d) reserving 10 to 60% by weight of the total vinyl ester monomer for introduction into the second pressurized reactor;
• e) maintaining a ratio by weight of the oxidation component of the redox system to monomer of from 0.03 to 0.07 in the first pressurized reactor;
• and
• f) establishing a residence time of 2.5 hours or less in the first pressurized reactor.