INDUSTRIAL USES OF SHELLAC
Due to the versatility of the lac resin, it finds innumerable uses in different industries. From the standpoint of industrial uses of shellac, one or more of the following properties are of great importance:
Shellac dissolves in a wide variety of alkaline or rapidly drying alcoholic solvents, but is resistant to a number of other solvents, particularly hydrocarbons.
Its films show excellent adhesion to a wide variety of surfaces, posessing high gloss, hardness and strength.
shellac is a powerful bonding material with low thermal conductivity and a small
coefficient and expansion. Its thermal plasticity and capacity of absorbing large amounts of filters are noteworthy.
Its electrical properties include high dielectric strength, low dielectric constant and characteristic freedom from tracking.
It is resistant to the action of ultraviolet rays.
Shellac is non-poisonous.
Scientific and technical developments in the field of synthetic resins have been responsible for lac being replaced in many of its traditional fields, but at the same time science has helped to elucidate the complex nature of lac resin and open up new potentialities.
The investigations of different groups of scientists have shown that it is a polyester type of resin formed from hydroxyl fatty sesquiterpene acids. the main fatty acid - aleuritic acid - and a mixture of sesquiterpene acids are present in approximately equal amounts constituting 70-75% of the total resin. An average lac resin molecule has five free hydroxyl and one carboxyl group accounting for its special properties.
Lac resin or its component acids can be modified with phenol and some accelerators mainly organic imides to give a new type of elastomerand plastomer which has shown immense possibility in leather finishes as pigment binder. It has also shown promising properties to be used as aqueous medium for decorative and protective coating for wood, plastic, metal etc. Its satisfactory adhesive and electrical properties make it suitable for its use in assembling mica for electrical insulation. As the material can be masticated and extruded directly on to wires, it may be used for cable insulation. On thermal polymerisation, the material is unaffected by common solvents.
The future of shellac however depends on its recognition as additive to synthetics. The compatibility of lac with epoxy and the resultant improvement in properties have been subjected to several patents. One such example is the fluidised bed method of coating with lac/epoxy mixtures containing as much as
75% lac. Lac has been successfully compounded with acrylates polyesters and isocynates. Grafting and copolymerisation of lac with ethylene oxylated lac with various synthetic monomers are being extensively investigated in different research laboratories.
Some of the present important and popular use of shellac are:
Shellac is unrivalled for wood finishing and furniture polishing. The art of polishing with shellac consists of producing on a carefully prepared surface a very thin film of the resin from its solution in alcohol. Two or three coats, applied with a pad, result in a rich glossy and durable finish. Shellac may also be brushed or sprayed. Bleached shellac is used to produce a transparent finish to bring out the natural beauty of the light coloured wood. Large quantities of bleached shellac are used for floor polishes in the United States, specially for making so-called No-Rub polishes that can be applied by brushing, spraying or even by wiping with a short cloth. Shellac is also used as sealer and undercoat.
During the recent years use of shellac in printing ink industry has considerably increased. Various types of inks particularly flexographic ink and water-proof ink contain shellac. The former is fast drying aniline ink, transparent or opaque, much used for rotary printing of numerous types of
packing materials ranging from foils to plastics. The chief characteristics of shellac
are strong adhesion and excellent binding power for dyes and pigments, superior flow, gloss, slip and abrasion resistance and good definition in printing. It is also quick-drying, free from any objectionable odour and possessing long shelf life. In addition, where colour permits, shellac enhances colour intensity and strength of print.
Transparent dye inks based on shellac being true solution for low viscosities need some modification. The addition of pigments to shellac solution to obtain opaque light fast inks greatly increases the viscosity which is sometimes necessary for the ink maker to modify the basic resin solution in order to bring viscosity within workable range.
The excellent electrical properties of shellac, its freedom from tracking, its good adhesion and thermoplastic properties have
established for this resin a very important place in electrical industry. In all its uses it is either applied dissolved in alcohol as an insulating varnish or in the molten state as a binder in large moulded articles. It is mainly used in the manufacture of insulating varnishes and cements, moulded insulators, laminated and
moulded mica products (micanite and micafolium), laminators, paper boards, tubes and coated or ompregnated paper, cloth and silk.
For the manufacture of micanite, mica splittings are bonded with shellac varnish to the required thickness and then pressed at 150-160oC for two hours under high pressure. In another process, mica sheets and shellac powder are dropped down a tower until necessary thickness is obtained and then consolidated. Micanite is one of the most valuable materials for electrical insulation. Micafolium is a wrapping material consisting of one or two thin layers of mica splittings bonded to strong kraft paper with shellac. This is a general purpose insulator for large electrical appliances and is applied by a hgot ironing process. Hot moulding micanite needs to be flexible when hot and to achieve this, large flakes of mica are used and shellac bond increases to more than 10%. This type of micanite is used for commutator cones, V-rings, transformer rings, tubes etc. A very fluid shellac is required.
Leather and Footwear industry
Tanners use shellac, suitably pigmented, to produce a flexible,
waterproof and glossy finish on leather. Shellac is also applied for s final durable non-tacky and glossy finish to leather articles. Shoe polishes can be made with shellac wax.
Shellac for Pharmaceutical, Confectionary Glazes and Fruit Coating
Superior grades of shellac dissolved in special denatured alcohol are the glazes employed for coating pharmaceutical tablets and confectionaries by the Pan method. In confectionaries, these glazes are sometimes applied by brush or spray.
In pharmaceutical tablet coating, the functions of shellac are:
To serve as a moisture barrier protecting the co-ingredients
As a enteric coating
To control disintegration
As a granulating agent
As a finishing coat over wax prior to the printing of trade mark.
The main use of shellac in the confectionary industry are for coating chocolate goods such as extruded chocolates, chocolate covered nuts and raisins and similar products. Shellac has become an important
ingredient for fruit coating composition. Substantial quantity of Shellac is consumed in fruit coating industry.
The coating for confectionary of fruit performs one or more of the following functions:
Enhances and protects the gloss
Serves as a barrier to moisture either entering or leaving the confection.
Prevents blocking together
Extends shelf life.
Shellac has a worldwide use in hair spray and hair lacquer industry. It contains all the chemical and physical properties required by the cosmetic chemist in formulating hair-grooming products. The ability of shellac to hold the hair is an obvious basic requirement and its non-hygroscopic nature ensures that the hair keeps well
groomed in high humidity or when exposed to rain. Its solubility in alcohol, the usual cosmetic colvent makes for easy formulation and its solubility in mild alkalis makes for easy removal by shampoos. Shellac has a wide range of compatibility with other resins, plasticisers and softeners used in hair lacquers. Normally decolourised or bleached shellac is used in this industry.
Enormous quantities of paper varnishes are used for glazing paper, wrappers, labels, display cards etc. where adhesion on high gloss are imperative and consequently light coloured shellac is an essential ingredient. Glossy silky finish on superior quality playing cards is obtained by giving the blanks a coat of shellac varnish before the final cutting. There is nothing quite like shellac for this application.
Although hat is no longer an essential article of dress, but still hat trade even nowadays is a big consumer of shellac. Both fur and wool felts are made by similar processes. The hood is treated with an aqueous solution of shellac - a process known as proofing.
Shellac varnish is used to protect negative and sound tracks on talky film. Shellac is also used for preparing dry mounting paper and in the cold top enamel process of photo engraving and etching.
When shellac is compounded with rubber, it toughens the latter and makes it more resistant to wear as in rubber soles and heels, flow tiles and moulded rubber articles. Varnishes for finishing rubber articles are also made from shellac.
A number of formulations have been developed on the use of shellac in the paint industry. Primers for undercoat having excellent protective capacity have been formulated with shellac. Enamel and emulsion paints based on shellac are also being produced. A very important application of shellac is in the road marking paint.
In grinding wheels, shellac as a bond for abrasive grains such as aluminium oxide, silicon, carbide and emery. Shellac bonded grinding wheels have considerably elasticity and are specially useful for precision work such as final grinding of camshafts, hardened steel, chilled rolls and for polishing of lenses and razor blades. During grinding the heat of friction slightly softens the shellac and the dulled abrasive grains break off to expose a fresh, sharp cutting surface.
Most important use of shellac in the automobile industry is the gasket cement. Shellac is insoluble to petroleum and this property is utilised in shellac-based gasket cement.
Shellac is a typical thermo-plastic material. In moulding operation, thermo-plastic compounds have the disadvantage that the mould requires to be cooled before opening. but on the other hand thay have the disadvantage that scraped and spoiled articles can be reworked. ordinary shellac even with its limitations to heat resistance is excellent moulding resin capable of very high degree of acuracy. The fludity of the base persists during moulding operation and therefore takes perfect impression of the mould.
The moulding properties of shellac are utilised in the manufacture of various
The manufacture of gramophone records was formerly the largest single outlet for shellac. But in recent years shellac has been displaced almost completely by
synthetic resins. However, records of 78 r.p.m are still based on shellac.
Dental plate blanks are plastic compositions comprising of shellac fatty acids and waxes with filters and colouring matter. Sufficiently softened in warm water, they take an impression from which the hand mould is made.
The widely known sealing wax contains shellac turpentine, rosin and fillers including colouring matter. Shellac is essential for good adhesion, toughness, tenacity and preventing discolouration on melting.
Other moulded articles:
Inspite of extensive use of bakelite and other synthetic resins for a wide variety of plastic mouldings, shellac is still used for making some