As a solvent-cast polymeric coating dries, each part reaches a concentration at which it solidifies and develops elastic modulus. Thereafter, as further solvent departs, that part shrinks out-of-plane, but not in-plane, if the coating adheres to its substrate. Hence, it develops in-plane elastic stress. If the stress grows large enough, the stress-free state may yield, which reduces the final stress level. A theoretical model of diffusion and mass transfer, large shrinkage-induced deformation, and elastic stress, together with yielding and postyielding viscous deformation, was developed to predict stress evolution in one-dimensional drying of polymer coatings. Concentration 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 evolving stress in various solvent-cast polymer coatings and aqueous gelatin coatings by a cantilever-deflection method.
Abstract— Many of the films used in polarizer assemblies in LCDs (e.g., triacetyl cellulose or TAC) are produced by a solvent‐casting process, which is known to impart optical anisotropy to the film expressed as finite out‐of‐plane birefringence. This feature of the film could have a significant impact on the optical performance of the display and it needs to be accounted for in any compensation scheme for the LC cell. This paper reviews the origin of this optical anisotropy, and it presents a viscoelastic model that links this property to the solvent‐casting process and to some key material parameters. The model results are compared with experimental data generated for polystyrene films cast from toluene, and generally good agreement is demonstrated.
Stress development during drying is a critical factor that affects the final structure and properties of a coated fiber or spherical product. Stress development during drying of the coating is due to nonuniform shrinkage and physical constraints. In this study, a large deformation elasto-viscoplastic model is developed to predict stress development in drying fibers and spheres after the coatings solidify. From the model, stress evolution in the drying fibers/spheres can be predicted by a partial differential equation of diffusion in one dimension, a first-order partial differential equation of pressure distribution, and two ordinary differential equations on local evolution of the stressfree state. The system of equations is solved by the Galerkin/finite element method in the one dimensional axial/ spherical symmetric coatings. Solutions show changes in solvent concentration and viscous stress as the coating dries.
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