MICROSTRUCTURE EVOLUTION IN LATEX COATINGS

Experiments of rewetting latex films by water show that latex particle deformation in the film formation processes can be reversible under c ertain conditions. Water roughens latex films and raises their topogra phic contrast. Sodium dodecyl sulfate promotes redispersion of latex p articles. Fronts of rewetting are observed. Water-soluble materials ap pear on rewetted film. The consolidation and pore-emptying or ''dryout '' fronts of dip-coated model latex-pigment coatings are recorded. Nea r the consolidation front the suspension reaches the first critical co ncentration; near the ''dryout front,'' the suspension passes through the second critical concentration. The microstructure development in l atex-pigment coatings is continuous behind the consolidation front, al though the dryout front has been observed there. The slightly carboxyl ated poly (styrene-butadiene) latex particles deform in seconds in coa tings of precipitated calcium carbonate with latex binder. The highly carboxylated poly (styrene- butadiene) latex deformed only slightly in dir-dried coating. The results indicate that latex particle deformati on is not finished dt the second critical concentration.