(533e) Cracking in Drying Films of Polymer Solutions

Thin coatings of polymer films find numerous industrial applications including tablet coatings in the pharmaceutical industry. The tablet may be coated for improving its appearance or taste, for increasing its stability or for achieving sustained or delayed release of the drug. Drying and the accompanying solidification process of the wet polymer film are often accompanied by shrinkage of the film. If the polymer film adheres strongly to the tablet surface, the latter prevents the film from shrinking leading to transverse tensile stress in the film. If the stress exceeds a critical value, the polymer film may develop cracks thereby compromising the integrity of the film.

In the present work, thin films of a polymer solution were cast on substrates of varying stiffness and investigated for cracking as a function of film thickness and substrate modulus. We measured the tensile stress and the maximum crack-free thickness. The measurements agree well with predictions of a model based on fracture mechanics of the drying film that balances the elastic energy released due to cracking to the increase in surface energy. These results provide guidelines to the formulator for designing crack-free coatings.