The unique nanostructure of shellac films

Shellac is a biocompatible resin of natural origin, comprising a mixture of polyesters and single esters, commonly used as a protective coating in the food and pharmaceutical industries. Remarkably, the physicochemical characteristics of shellac, in particular its excellent film forming, low vapor permeability, and good adhesion properties, are very untypical to low molecular weight materials. This study was aimed at exploring the nanostructure of shellac and gaining insights into the structure-property relations alongside the effect of its aging phenomena. Small angle x-ray scattering and high resolution electron microscopy experiments revealed a unique bicontinuous nanostructure in shellac. Using manipulations of shellac's composition and studies of shellac solutions in ethanol, we could identify the hydrophilic phase, the lipophilic phase, and the amphiphile residing on the interface between them. The bicontinuous morphology, typified by an extremely large interfacial area, was used to explain shellac's physicochemical characteristics and mechanical properties. Although significant effort has been devoted to generating and stabilizing bicontinuous molecular organization in man-made systems, it is surprising to discover that the conditions required for this unique morphology exist in native shellac. Understanding this exceptional morphology may open the way for further manipulation that will enable better control over shellac's properties and its utilization for novel applications, such as a coating material in drug or pesticide delivery systems.