(249f) Determination of Drying Stresses and Fracture Energy in Tablet Coating Films

Thin coatings of polymer films find numerous technological applications in various industrial sectors related to protective and functional coatings. Tablets in the pharmaceutical industry may be coated for branding, to improve appearance or taste, increase stability or to achieve sustained or delayed release of the drug. In this study, these coatings are initially cast as thin films of polymer solutions, which dry to form a solid film. The solidification process due to the removal of the solvent phase is often accompanied by shrinkage of the film. If the polymer adheres to the substrate, the film can only shrink in the film-normal direction since the substrate prevents contraction in the transverse plane. In such cases, drying is accompanied by the development of transverse tensile stress in the film. If the tensile stress exceeds a critical value, the polymer film may develop cracks, thereby compromising the integrity of the film. Alternatively, if the adhesion between the film and the substrate is weak, the film may debond instead of cracking. It is essential to understand these mechanisms to obtain defect-free films.

In the present work, transverse stress was measured during the drying of the film using glass as a cantilever substrate for solutions of 10% and 20% total polymer solids (Figure 1, left panel). Two polymer systems used in this study were CA (cellulose acetate): PEG (polyethylene glycol) and CA (cellulose acetate): HPC (hydroxypropyl cellulose). It was found that final drying stress remains the same for both the polymer systems irrespective of concentration and coating layers except for 10% CA: HPC films. Transverse stresses in the latter films were found to be high and suggest a higher propensity for cracking (Figure 1, right panel). Cross-sectional SEM images were taken to relate the microstructure to the stress. Ultimate tensile strength of polymeric films was also determined using a tensile testing instrument. The tensile strength decreases with flaw size, in accordance with the Griffith’s criterion. A simple mathematical model was developed to predict the critical thickness and critical stress beyond which the film would crack.