THE PHYSICAL AND CHEMICAL PROPERTIES OF SHELLAC By H. S. COCKEKAM and S. A. LEv•E* Presented May 12, 1961, New York City Revived interest in shellac as a hair wave setting material points up a need for wider dissemination of technical information on shellac. It also reveals the widely scattered state of shellac literature. In most cases, gathering information is quite difficult, as there is no modern mono- graph or similar comprehensive source. A natural result of this situation is that most technical people regard shellac as a commodity. When we look at shellac as the complex of chemicals which it is, we find many points both interesting and unexpected. Of prime interest to the cosmetic chemist are purity and toxicity. Bleached shellac, either regular or wax-free, is nontoxic. It may be ingested without harm. Considerable amounts of bleached shellac are employed as coatings in the confectionery and pharmaceutical industries. There have been no substantiated cases of allergy or sensitization. It is free from arsenic and rosin. In our own plant it is specially handled to insure its acceptability as an ingredient of foods, drugs and cosmetics, in accordance with all the regulations of the Federal Food, Drug and Cosmetics Act, as amended. From the chemical standpoint, shellac is a mixture of polyhydroxy polycarboxylic esters, lactones and anhydrides (1). On theoretical grounds it is most probable that polyesters predominate. These may be both linear and cross linked. The esterification of the constituent acids takes place during and after the exudation of a resinous substance from the body of the lac insect (Tachardia lacca Kerr). When first exuded the lac resin is a viscous, almost colorless liquid. It gradually liardens and darkens, forming "sticklac." This is the primary raw material from which orange shellac and commercial bleached shellac are manufactured. When }leated, shellac po]ymerizes further to an insoluble, in fusible product. Shellac appears to be a rather tough, springy network of hydroxy fatty acid esters and sesquiterpene acid esters. The interstices orerain a * Acme Shellac Products Co., Newark, N.J. 316

PHYSICAL AND CHEMICAL PROPERTIES OF SHELLAC 317 mixture o( lower molecular weight fatty acid esters. These latter sub- stances serve as natural plasticizing agents. They may be separated from the hard, less soluble network by solvent extraction with benzene, toluene, xylene, trichloroethylene and like solvents. Considerable effort has been devoted to the separation, properties and uses of the less soluble "hard lac resin" (2-6). This resin has been found to be more adherent than shellac to most substrates, but it is more brittle. By extracting 20 per cent of soft lac from whole shellac, the hard lac obtained had a melting point only 7øC. higher. Hard lac resin has some attractive properties. The economic problem of how to dispose of the useless soft resin has so far blocked commercial development. Shellac may be dispersed in alcohols or aqueous alkalies, or a mixture of these. These dispersions deposit films which may be permanent or removable. The character of the film depends on the dispersion in- gredients, and on the amount and kind of softeners or other modifying agents employed. The flexibility of shellac films may very well be due to the chains of seven methylene groups which are present in, for example, aleuritic acid, one of the constituent acids: HO--CH,.(CH•) 5CHOH--CHOH--(CH0 7COOH This is indicated by some recent work done in the adhesives field by Stivala, Powers and others (7-9). At the Indian Lac Research Institute, Vetman and others (10-12) have measured many of the basic physical properties. All the details would be superfluous here. You are referred to the original papers. Clarke (13) found interesting x-ray diffraction patterns for shellac. There was a weak ring characteristic of an amorphous structure, and a relatively sharp ring corresponding to a crystalline structure. When the observed sample was heated in an inert atmosphere, the diffused ring became more intense, and the sharp ring weaker, disappearing entirely when a completely polymerized state was reached. Unfortunately, Clarke gave n6 further details. Another investigator, Houwink (14), found x-ray patterns corresponding to long chain groups in shellac. In Table 1 is presented the percentage composition of shellac in terms of the individual acids and fractions which have been isolated up to the present time. It will be noticed that aleuritic acid and shellolic acid and its CH= homologues together make up 73 per cent of the lac complex. With this information, work on modifying shellac or preparing derivatives can be carried out on a more secure foundation than was possible only a few years ago. Futhermore we can better visualize the relationship between structure and properties.