Fabrication of thermally stabilized shellac through solid state reaction with phthalic anhydride

Shellac has been regarded as an obsolete polymer due to its poor thermal stability. The purpose of the study was to solve the problem by solid-state synthesis of shellac phthalate (SHL-PHT). Shellac was ground with phthalic anhydride and then thermally activated at various conditions. The solid-state esterification of shellac was clearly observed after annealing. As indicated by an increase of acid value, the esterification was more pronounced after the increase of annealing temperature and time. The formation of SHL-PHT was confirmed by FTIR spectra and other characterization techniques. The SHL-PHT demonstrated to improve the thermal stability as compared to native shellac. After aging at high temperature, percent insoluble solid and acid value of the native shellac dramatically changed while those of SHL-PHT were relatively the same as initial value, especially for the more esterified SHL-PHT. The protection of hydroxyl groups of shellac by phthalate moieties might be a possible explanation for the improved stability. In conclusion, the study may give an eco-friendly way to synthesize the shellac derivatives for future applications.