Cross-linking shellac

Shellac [124,125] is a natural resin used in very old times for varnishes and moulding compounds. The resin secreted by the lac insect, Kerriar paca, is collected by scraping the shellac-encrusted trees found in southern parts of Asia. It consists of a complex mixture of cross-linked polyesters derived from hydroxy acids, principally aleuritic acid (9,10,16-trihydroxyhexadecanoic acid). From a structural point of view, it appears that this material can be used as a crosslinking agent and/or as a monomer for developing dentrite-like polymers. The question is whether it is possible to produce this material from shellac by controlled hydrolysis. 

Shellac, which was used by Edison for molding his first photograph records and is still used as an alcoholic solution (spirit varnish) for coating wood, is a cross-linked polymer consisting largely of derivatives of aleuritic acid (9,10,16-trihydroxyhexadecanoic acid). Shellac is excreted by small coccid insects Coccus lacca), which feed on the twigs of trees in Southeast Asia. Over 2 million insects must be dissolved in ethanol to produce 1 kg of shellac. 

Natural polymers such as albumin, gelatin, fish glue, shellac, and gum arabic received attention up to the early 1900 s, after which time other materials such as bichromate, diazo compounds, iron salts, and silver halides were added to resins in order to hasten or accomplish their crosslinking. These systems were neither photopolymerization nor photocrosslinkable polymers. Instead, the added second component, when excited by light, produced a species which itself caused the formation of radical sites on a prepolymer or polymer chain which then led to cross-linking. 

Naturally occurring miscellaneous polymers, like, for instance, shellac, a resin secreted by the Lac insect. This is a complex cross-linked polyester of 9,10,16-trihydroxy-exadecanoic acid (aleuritic acid). The structure also includes some unsaturated long-chain aliphatic acids together with other compounds [1]. 

The polymer industry traces its beginning to the early modifications of shellac, natural rubber (NR, an amorphous cis-l,4-polyisoprene), gutta-percha (GP, a semicrystalline fran -l,4-polyisoprene), and cellulose. In 1846, Parkes patented the first polymer blend NR with GP partially co-dissolved in carbon disulfide. Blending these two polyisoprene isomers resulted in partially cross-linked (co-vulcanized) materials whose rigidity was controllable by composition. The blends had many applications ranging from picture frames, tableware, ear trumpets, to sheathing the first submarine cables. 

Shellac is a varnish of animal origin, and its composition is quite different from that of plant resins. It is secreted by the female lac bug in India and Thailand and has been used as a stain, a varnish, and in artifacts such as 78 rpm record discs. While it has been replaced by man-made polymers for most uses, it is still used in French polishing of furniture and on musical instruments. Hydrolysis gives a mixture of hydroxy acids such as aleuritic acid, R-6-hydroxyhexadecanoic acid, jalaric acid, and the colorant laccaic acids (Figure 21.30). The material is composed of low molecular weight, cross-linked polyesters. 

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