Development of shellac from source available in thailand as an alternative polymer for postharvest treatment

Abstract

Shellac, which is abundant in Thailand, had been thought as an obsolete polymer for a few decades because of its poor solubility and instability. The purpose of study was to solve the problems by structure modification at carboxyl groups and hydroxyl groups of shellac molecules. The modification of carboxyl groups was conducted by salt formation. Shellac salts were prepared by dissolving shellac with ammonia or meglumine. The solid-state reaction was applied for esterification of shellac molecules at hydroxyl groups. Shellac was co-ground with cyclic anhydrides and then annealed at various temperatures to obtain shellac esters. The results demonstrated that percent insoluble solid of shellac salts (less than 12% w/w) was significantly decreased as compare to native shellac (40% w/w), suggesting the stability enhancement after salt formation. The conversion of carboxylic acid to carboxylate, after salt formation, could protect the polymerization which was the major cause of instability. However, the solubility was still a problem. The problem was further clarified by esterification shellac with succinic and phthalic anhydrides. The succinate and phthalate esters of shellac demonstrated better aqueous solubility as compared to native shellac which made them more ready for using by dissolving in water. In addition, the stability enhancement was also observed for the shellac esters because of protection of hydroxyl groups. In conclusion, the findings could give the solution for problems of shellac and made it more potential as a coating polymer for postharvest treatment of agricultural products in the future.