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 paddleover-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. INTRODUCTION Shellac is the purified product of lac, a natural resinous oligomer (MW ≈ 1000 D) secreted by the parasitic insect Kerria lacca on various host trees in India, Thailand, and Myanmar. Shellac consists of polyesters of mainly aleuritic acid, shellolic acid, and a small amount of free aliphatic acids (1, 2). The composition varies depending on the insect species as well as the host tree from which the raw material is obtained. After harvesting, the so-called “stick lac” is chopped and separated from wood and resin. A washing step extracts the water-soluble dye, laccaic acid, yielding the raw material “seed lac.” There are three different processes used for refining, resulting in different shellac qualities: The melting filtration process, where melted seed lac is filtered through a cotton hose, leads to wax containing shellac. Bleached shellac is obtained by treating the dissolved polymer with sodium hypochlorite. The most suitable type of refining is the solvent-extraction process, in which the raw material is dissolved in alcohol, decolorized by treatment with activated carbon, filtered, and cast to a film. After cooling, the film breaks into flakes giving shellac its typical appearance (3). The solvent-extraction process is a gentle process that does not change the chemical structure of the material. Careful selection of the raw material ensures shellac qualities with narrow specifications. Due to its acidic character, shellac is used primarily as an enteric coating. Other applications are sustained release (4), colon targeting (5), and microencapsulation (6). Shellac is non-toxic and physiologically harmless (7); it is therefore listed as GRAS by the FDA. This makes shellac suitable for use even in foods or confectionaries. Shellac has excellent film-forming properties, high gloss, and poor permeability to gases and water vapor (8, 9). Despite these advantages, the use of shellac as a pharmaceutical excipient has significantly declined, because historically, shellac was mainly used in alcoholic solutions. Shellac films prepared from alcoholic solutions show pronounced hardening induced by a continuing polymerization process. This results in a loss of gastric resistance and a decrease in intestinal fluid solubility, both leading to major changes in drug dissolution profiles. These are disadvantages when compared with synthetic or partially synthetic polymers such as polymethacrylates and cellulose derivatives. Nevertheless, it could be shown that shellac films prepared from ammoniacal solutions lack these material changes during storage (10, 11). Thus, aqueous shellac solutions could regain importance in pharmaceutical applications. Although shellac films prepared from aqueous solutions show better stability, the raw material is still prepared by solvent extraction, which results in the unstable acid form. Therefore, besides origin and type of refining process, further processing to an aqueous formulation has a major effect on the quality of the material. In the present study, shellac batches of different ages and origins were investigated with regard to their dissolution properties at various pH values, and the dissolution rates were correlated to the acid values and thermal properties. MATERIALS AND METHODS Materials The shellac types are listed below. All other reagents were of analytical grade. Corresponding author. diss-16-02-05.indd 33 5/14/2009 9:00:23 AM dx.doi.org/10.14227/DT160209P33