Patent ID: 7220803

Claim:
Process for obtaining an aqueous polymer dispersion having the following properties: a solids content of from 65% to 75% by weight; Brookfield viscosity of less than 3 000 mPa·s, measured at 23° C. under the following conditions: up to 400 mPa·s using a No. 2 spindle at 100 rpm, from 400 to 700 mPa·s using a No. 2 spindle at 20 rpm, from 700 to 3 000 mPa·s using a No. 3 spindle at 20 rpm; content of micro-aggregates of less than 500 ppm; content of wet macro-aggregates, relative to the weight of the final dispersion, of less than 0.2% by weight; the particles of the said dispersion having a polymodal distribution of the mean diameter, and the following distribution of the mean diameters: mean diameter of less than 300 nm: from 0 to 40% by weight, mean diameter from 300 to 500 nm: from 0 to 40% by weight, mean diameter of greater than 500 nm: from 40 to 95% by weight, the sum of the percentages of the particles being 100% by weight, the said process being carried out by free-radical polymerization in aqueous emulsion of unsaturated monomers, according to the following steps: (a) forming of a pre-emulsion of monomers in water using surfactants, where a water-soluble salt can be optionally added from this stage, said salt being soluble in water at 25° C. to at least 1% by weight; (b) transferring into the polymerization reactor with stirring, in the following order, of the following aqueous phases: reaction stock consisting of a solution of a surfactant, in an amount by weight relative to the weight of final polymer dispersion, of from 1 to 15%; optionally with an added salt which is soluble in water at 25° C. to at least 1% by weight and is inert under the reaction conditions; the salt/surfactant (s/s) weight ratio in the reaction stock being less than 70; a portion of the pre-emulsion prepared in the previous phase, in an amount of from 1% to 10% by weight of the initial pre-emulsion; C.1 initiating the polymerization, by adding free-radical initiators which is activated thermally, chemically or by UV irradiation; C.2 polymerizing the monomer mixture, by gradually adding into the reactor, separately, the initiator solution and the remaining portion of the pre-emulsion with an added salt, which is soluble in water at 25° C. to at least 1% by weight and is inert under the reaction conditions, in an amount so as to have a salt concentration in the added pre-emulsion, expressed as a percentage by weight, of from 0.02 to 0.25%; the polymerization temperature generally being from about 30° C. to about 90°C.; C.3 Scrubbing of the residual monomers.