Physicochemical Properties of Various Shellac Types

Shellac in its acid form undergoes aging, resulting in the change of its physicochemical properties. Therefore, various shellac types were investigated as free films prepared from ammoniacal solutions and as micronized powder in its acid form. Due to its acidic character, shellac shows a pH-dependent solubility. The dissolution properties of shellac films prepared from ammoniacal solution were investigated at various pH values using a dissolution apparatus with basket holder. Micronized shellac in its acid form was analyzed using the intrinsic dissolution method (Ph. Eur.) with a paddle-over-disk apparatus. The dissolution properties of the investigated shellac types were correlated with their acid values and their thermal properties. Aging of shellac results in an increase in the glass transition temperature and a decrease in the acid value and the solubility. However, the extent of this change in physicochemical properties depends on the type of shellac, its origin, and type of refining process. Besides the acid value and the glass transition temperature, the intrinsic dissolution rate is an important parameter for the characterization of different shellac types.