Amphoteric and anionic polymer dispersions, process for their preparation and use thereof

A process for producing an amphoteric aqueous polymer dispersion by two-step radical copolymerization, the process comprising:

• a. decomposing a starch by one of treatment with an enzyme effective therefor or acid hydrolysis to provide a starch solution comprised of decomposed starch and/or derivatives thereof, with the decomposed starch and/or derivatives thereof having an average molecular weight ranging from 500 to 2000 g/mole and with the starch solution having an internal viscosity<0.1 dl/g;
• b. polymerizing a monomer mixture comprised of ethylenically unsaturated monomers containing up to 30% by weight of monomers of anionic character (monomers A) selected from the group consisting of ethylenically unsaturated carbonic, sulphonic and phosphonic acids in the presence of from an effective amount up to 40% by weight of the starch solution, with the weight ratio of starch to monomer mixture ranging from 20:1 to 1:20 and with the so-obtained anionic polymer dispersion having a solids content ranging from 15 to 45% by weight and having a zeta-potential ranging from -3 to -70 mV;
• and
• c. polymerizing the anionic polymer dispersion of step (b) with another monomer mixture which contains the ethylenically unsaturated monomers and up to 35% by weight of monomers of cationic character (monomers B), with the weight ratio between the anionic polymer dispersion of step (b) and the another monomer ranging from 10:1 to 1:10 and with so-obtained amphoteric aqueous polymer dispersion having a solids content ranging from 20 to 55% by weight and having a uniform particle size distribution.