Shellac is defined in the 7th edition of the Merck Index as a "resinous secretion of the insect Laccifer(Tachardia) lacca Kerr, order Homptera, family Coccidae". The lac insects in feeding upon the vegetable sap of trees exude the lac resin which is then recovered in crude form from the encrusted twigs; purification by washing, melting and straining yields the orange shellac, a form commonly marketed and used by industry. Bleached shellac may be made from orange shellac by bleaching an aqueous alkaline solution followed by precipitation from solution with a dilute acid, such as sulfuric acid; additionally, if dewaxing is desired, it can be accomplished prior to the bleaching step.
Shellac, historically, has enjoyed usage as components or organic molding compounds, as resinous binder for laminated forms, for electrical insulating components, and as coatings from alcoholic solutions and aqueous emulsions. However, the application of shellac as coreactants have been limited by its restriction in the types of compounds most effective as solvents. It is an object of this invention to expand the applications beyond the conventional uses to include the utilization of isocyanates as curing agents; in attaining this objective, shellac is modified to alter its solubility from alcoholic to include ketone and estertype solvents, while maintaining functional group characteristics reactive with isocyanates.
Although knowledge as to the structure of shellac is still incomplete, data has been gathered, particularly in recent times, that represents major contributions to our understanding of its fundamental nature. Supplementing the elucidation of structure, functional group and physical constant information, which are readily obtained, is additionally useful in explaining the rationale of my invention; thus, by knowing the acid and hydroxyl numbers, calculations can indicate the average number of carboxylic acid and hydroxyl groups per given weight of shellac, forming the basis for establishing weight-to-weight relationships for the modified shellac reactants and the polyisocyanate curing agents.