Surface treatments of wood by chemically modified shellac

Shellac is an animal resin, which has been widely used as a decorative lacquer for wooden handicrafts in the last centuries. It still finds application as wood sealer and finisher because of its peculiar properties. However, there are also some limitations, such as sensitivity to alcoholic solvents and to pH variations and considerable tendency to chromatic variations upon ageing. The aim of this work is to improve some features of shellac by inducing a cross-linking in its structure. Two new materials were obtained by addition of a polyamine and a polyaziridine to shellac, and their performance were studied by different experiments. The varnish obtained by addition of polyaziridine shows a considerably improved performance when compared to plain shellac. In particular, it preserves almost unaltered the chromatic properties of shellac varnish and it shows an increased resistance to alcohols, to basic agents and to artificial ageing caused by a long lasting UV irradiation