PROPERTIES OF SHELLAC
Shellac, the only known commercial resin of animal origin, is a hard, tough, amorphous and easily brittle resinous solid. It is practically odourless in the cold, but
evolves a characteristic smell on heating or melting . Superior grades are light yellow in colour, while the inferior grades range from deep orange brown to almost dark red. The resin is associated with colouring matter, odoriferous principles and varying quantity of wax. Lac resin entirely free from these associates is unknown although commercially available dewaxed and decolourised lac is the closest approximation.
The lac resin is not a single compound, but consists of intimate mixture of several components. The manner in which these components are linked togetjer to build up the shellac components. The manner in which these components are linked together to build up the shellac complex has baffled intensive chemical research during the last few decades.
When slowly heated, it softens at 65-70oC and melts between 1.14 to 1.21. It has a molecular weight 1006, acid value 65-75 and saponification value 220-230.
shellac is insoluable in water, glycerol, hydrocarbon solvents and esters, but dissolves readily in alcohol and organic acids. The most popular solvent of shellac is spirit.
Aqueous solution of inorganic alkalis readily dissolve shellac. Usually the milder alkalis - ammonia borax and sodium carbonate are employed to prepare aqueous solution.
Shellac can be split into two fraction by ether. The ether soluable portion which is about 30% called the soft resin is stickly and viscous. The ether insoluable portion called the hard resin or the pure resin has higher softening and melting points (84oC and 94oC) than Shellac.
When lac is heated between 120-150oC and maintained over a prolonged period, the resin gradually becomes more viscous and rubbery due to polymerisation, and becomes insoluble in the standard solvent.
The shellac produced is normally allowed to cool off after stretching for a few hours and then transferred to a cool-shedded godown. It is best stored in air-conditioned ware-houses which maintains a temperature between 14-18oC.
Storage under such conditions maintains the quality of lac although its main function is to prevent shellac from blocking in the hot weather which is encountered for most of the year in India. The blockiness of shellac is quite a problem in many industries and as a result the use of air-conditioned storage in ships has increased very largely in order to ensure arrival of shellac at its destination in free and good condition.