Shape evolution and stress development during latex-silica film formation

The shape evolution and stress development of composite films of deformable acrylic latices and rigid silica spheres were studied using noncontact laser profilometry and a controlled environment stress apparatus that simultaneously monitors optical clarity, drying stress, and weight loss. Their shape evolution was strongly influenced by capillary forces experienced during drying, latex T-g, and the ratio of deformable/ rigid particles in the film. Their stress histories exhibited three distinct regions: (1) a period of stress rise stemming from capillary tension exerted by the liquid on the particle network, (2) a maximum stress, and (3) a period of stress decay. Significant differences in stress histories were observed between the deformable latex and rigid silica films. Latex films exhibited a gradual stress rise, a maximum stress of similar to0.1 MPa, and only a slight stress decay. In contrast, silica films displayed a sharp stress rise and a stress maximum of similar to1 MPa, followed by a decay to a nearly stress-free state at the culmination of the drying process. Composite films exhibited a marked transition from a deformable to a rigidlike response as their silica content increased above 40 vol %. The highest maximum stress was observed for composite films with 55% silica, which was near their critical pigment volume concentration. Such films also exhibited the highest amount of residual stress of all films studied.