Method for making polymodal aqueous synthetic resin dispersions

A method for making an aqueous synthetic resin dispersion comprising at least two particle families of different average particle size, the largest particle family having an average particle diameter of 0.6 micron or less and the average particle size of the next smaller particle family being at most two-thirds of the average particle size of the largest particle family, which method comprises

• (a) gradually adding an ethylenically unsaturated monomer difficultly soluble in water or a mixture of ethylenically unsaturated monomers forming a polymer which is insoluble under the conditions of polymerization, as such or in the form of an aqueous emulsion, to an aqueous phase containing an emulsifier and a water soluble polymerization initiator and having a temperature at which polymerization of said monomers is effected, whereby synthetic resin particles are formed;
• (b) adding a seed latex containing synthetic resin particles to the polymerization mixture in the course of the polymerization after 2 weight percent, but before more than 40 weight percent, of said monomers have been converted, the particles of said seed latex being smaller by a factor ranging from 2 to 15 than are the particles already formed by the emulsion polymerization of step (a) (the size of the particles being calculated as average particle diameters), the amount by weight of the particles in the seed latex being not greater than 10 percent by weight of the monomers, the ratio by weight of particles of said seed latex to the weight of the monomers already added being from 1:4 to 1:500, and the amount of emulsifier used in step (a) being such that no free emulsifier is present in the polymerization mixture when said seed latex is added;
• (c) continuing the monomer addition under emulsion polymerization conditions after the addition of said seed latex and terminating said monomer addition and emulsion polymerization before the average particle diameter of the largest particle family is larger than 0.6 micron.