tert-Butyl hydroperoxide → Butyl acrylate vinyl acetate

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0.3 parts by weight (ppw) of the anionic polyelectrolyte dispersant used in Example 1, 1.6 ppw of the non-ionic surfactant solution quoted at the foot of Table 4, 0.1 ppw of the cellulose "Cellosize" QP 300 and 2.8 ppw vinyl pyrrolidone were dissolved at ambient temperature (20° to 25° C.) in 54.8 ppw water contained in a glass polymerisation vessel fitted with a stirrer, reflux condenser and inlets for reactants. The vessel was fluxed with nitrogen and then maintained under an atmosphere of nitrogen. The stirrer was started and the solution was heated to 55° C. 0.05 ppw of tertiary butyl hydroperoxide free radical initiator dissolved in 0.05 ppw water were added to the heated solution followed by 0.06 ppw of sodium formaldehyde sulphoxylate dissolved in 0.5 ppw water and the temperature of the contents of the vessel rose to 60° C. indicating exothermic polymerisation of the vinyl pyrrolidone. The temperature was maintained at 60° C. for 50 minutes and then 19.1 ppw vinyl acetate, 8.2 ppw butyl acrylate and 0.6 ppw tertiary butyl hydroperoxide were each added at a steady rate over a period of two hours simultaneously with a steady addition of a solution of 0.6 ppw sodium formaldehyde sulphoxylate and 3 ppw of the nonionic surfactant used in Example 1 dissolved in 7.3 ppw water. A copolymerisation occurred which formed a colloidally stable dispersion of film-forming particles of vinyl acetate/butyl acrylate copolymer with which chains of polyvinyl pyrrolidone had chemically bonded. The dispersion was allowed to cool to ambient temperature, stirring was stopped and the cooled dispersion was filtered. The filtrate was found to consist of the colloidally (at least partially sterically) stable dispersion of film-forming copolymer particles bonded to polyvinyl pyrrolidone water-soluble compound. The filtrate which will be referred to as Latex 6. The copolymer particles in Latex 6 had a number average particle size of 90 nm, a minimum film-forming temperature of 285K and at least one or other of the amino or carbonyl groups of the polyvinyl pyrrolidone were adsorbable onto the titanium dioxide particles of Millbase 6. The pH of Latex 6 was adjusted to 8 as in Example 1.

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