7. Shellac
Synonyms Lacca; Lac.
Biological Sources Shellac is the resinous excretion of the insect Laccifer (Tachardia) lacca Kerr, order Homoptera belonging to family: Coccidae. The insects usually suck the juice of the tree and exerete ‘stick-lac’ more or less continuously. The various host trees are, namely: Butea frondosa Koen. ex. Roxb. (Family: Leguminosae) and Butea monosperma (Lam.) Kuntze; Aleurites moluccanna (L.) Willd. (Family: Euphorbiaceae)-Varnish Tree; Ficus benjamina Linn., (Family: Moraceae); Zizyphus jujuba (Lam.) (Family: Rhamnaceae). However, the whitest shellac is produced while the Kusum tree is the host i.e., Schleichera trijuga (Willd.) (Family: Sapindaceae).
Preparation The resin which is stuck on the smaller twigs and branches is normally serapped by means of knives. The resulting resin is subsequently powdered and extracted either with water or with alkaline solution so as to remove the colouring matter. The residual product is dried, melted in narrow bags suspended over a fire. The contents of the bags i.e., the molten shellac, are squeezed out mechanically so as to force the liquid shallac through the cloth on to a previously cleaned surface of tiles to obtain the product as flat cakes. The product may also be obtained as thin sheets by streching the semi-cooled product on the tiles with the help of a scrapper (or spreader). The thin sheets thus obtained get hardened after cooling and are subsequently broken up to obtain the flakes of shellac for the commercial market.
Characteristic Features Shellac is a brittle, yellowish, transparent/translucent sheets or crushed pieces or powder. It does not has any specific odour and taste. Its mp is 115-120°C and d 1.035-1.140. Its solubility in alcohol is 85-95% (w/w) (very slowly soluble); in ether 13-15%; in benzene 10-20% and in petroleum ether 2-6%. It is sparingly soluble in oil of turpentine. It is practically insoluble in water, but soluble in alkaline solutions, in aqueous solution of ethanolamines and in borax solutions with slightly purple colouration.
Chemical Constituents The major component of shellac is a resin that on being subjected to mild hydrolysis yields a complex mixture of aliphatic and alicyclic hydroxy acids and their polyesters respectively. Interestingly, the composite of the resultant hydrolysate solely depends on the source of shellac and the time of collection.
The major component of the aliphatic fraction is aleuritic acid, while the major component of the alicyclic fraction is shellolic acid.*
However, it also contains the isomers of shellolic acid along with small amounts of kerrolic acid and butolic acid. The colouring matter is due to the presence of laccaic acid, which is watersoluble, as given below:
Note Laccaic acid-A is the major component while the rest are present in relatively smaller quantities.
Uses
1. It is used chiefly in laquers and varnishes.
2. It is also employed in the manufacture of buttons, sealing wax, cements, inks, grinding wheels, photograph records, paper.
3. It also finds its use in electrical machines and for stiffening hats.
4. It is also used for finishing leather.
5. It is extensively used for coating tablets and confections.
6. It has also been used for preparing sustained release medicament formulations-----------------------------------------------
* Notes Field Tetrahedron, 26, 3135 (1970)
