Modification of shellac coating using Jeffamine® for enhanced mechanical properties and stability

Shellac is a unique natural resin derived from the secretion of insects. It is widely used as a preservative coating or a confectionary glaze in the food industry and as an enteric coating in the pharmaceutical industry. Despite the high potential of shellac as an environmentally friendly protective coating, its utilization is still limited due to its mechanical brittleness and poor stability caused by self-polymerization reactions. This study is aimed at exploring a new strategy that will reduce the drawbacks of shellac coating and enhance the mechanical properties and stability of the material. The suggested approach is based on compounding shellac with the biocompatible modifying agents Jeffamine®, aiming to utilize two stabilization mechanisms: hydrophilic plasticizing and amine interaction with shellac carboxyl groups. The modified coating materials exhibited excellent film forming properties when applied on a variety of surfaces and superior mechanical strength of up to one order of magnitude compared to native shellac. The new approach has demonstrated to achieve preservation of solubility in ethanol during storage, as well as mechanical properties and thermal stability over a period equivalent to 18 months at standard storage conditions. Based on Fourier Transform Infrared experiments, these beneficial effects were attributed to the formation of amide bonds and acid-base interactions between shellac and Jeffamine®. Hence, the Jeffamine® modification succeeds in hindering shellac self-polymerization occurring in native shellac and provides a significant improvement in the stability and shelf-life of shellac coating.