Web Release Date: May 15,
Shape Evolution and Stress Development during Latex-Silica Film Formation
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Materials Science and Engineering Department, Chemical Engineering Department, and The Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received September 27, 2001
In Final Form: March 28, 2002
Abstract:
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 Tg, 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 ~0.1 MPa, and only a slight stress decay. In contrast, silica films displayed a sharp stress rise and a stress maximum of ~1 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.
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