Stress development in drying coatings

As a solvent-cast polymeric coating dries, each part reaches a concentratio n at which it solidifies and develops elastic modulus. Thereafter, as furth er solvent departs, that part shrinks out-of-plane, but not in-plane, if th e coating adheres to its: substrate. Hence, it develops in-plane elastic st ress. If the stress grows large enough, the stress-free state may yield, wh ich reduces the final stress level. A theoretical model of diffusion and ma ss transfer, large shrinkage-induced deformation, and elastic stress, toget her with yielding and postyielding viscous deformation, was developed to pr edict stress evolution in one-dimensional drying of polymer coatings. Conce ntration varies only perpendicularly to the substrate, the coating shrinks only in that direction, and the stress varies only in that direction but is in-plane isotropic. The predictions are compared with measurements of evol ving stress in various solvent-cast polymer coatings and aqueous gelatin co atings by a cantilever-deflection method. (C) 2001 John Wiley & Sons, Inc.