Applications of Shellac
Shellac resin boasts unique and excellent properties. Its films, formed from alcohol or alkaline solutions, are hard, glossy, and tough. It exhibits strong adhesion to various materials such as wood, glass, metal, paper, and mica, and is resistant to acids and hydrocarbon solvents; it also offers moisture resistance, rust prevention, oil resistance, electrical insulation, and good thermoplastic properties, without being toxic or irritating to humans. Therefore, it is a versatile natural resin used across a broad range of industries, including defense, electronics, food, coatings, pharmaceuticals, machinery, rubber, plastics, leather manufacturing, papermaking, and printing. However, shellac resin also has some inherent drawbacks, such as brittleness, low softening point, and poor resistance to water, heat, and chemicals, which limit its applications in certain areas. To expand the range of uses for shellac resin and to meet the special requirements of different industrial sectors, numerous experiments and studies have been conducted both domestically and internationally in recent years. These efforts have involved testing various formulations and adding different substances to chemically modify shellac resin, producing a wide array of modified shellac resin products with special functions, achieving significant success.
Fruit preservation and storage is an extension of agricultural production. Utilizing shellac's colorless, odorless, non-toxic properties, and its excellent film-forming capabilities with good water resistance, moisture resistance, acid resistance, and oil resistance, the introduction of nano SiOx powder materials improves the gas barrier properties of bleached shellac films. By incorporating natural fungicides, antioxidants, and growth regulators, a bleached shellac fruit preservative is created. When applied to the surface of citrus fruits, pears, apples, and melons, it not only enhances the gloss and appearance of the fruit but also retains moisture, preserves freshness, reduces rot, maintains the original flavor, and extends the storage period, shelf life, and increases the rate of marketable products and selling points. Under ambient temperature conditions: apples can be stored for 90 days, pears for 80 days, citrus for 50 days, melons for 25 days, and crisp plums for 15 days. The appearance of the fruit remains plump, the skin color pure, with a commercial rate of 90%, and a moisture loss rate of ≤12%.
The direct use of shellac resin products in military technology and formulations is often classified as military secrets and rarely published. Indirect uses include insulation materials for military electrical equipment (electron tubes, detonators, instruments, and radio equipment), protective varnishes for military hardware (guns, shell casings, wings, hulls, etc.), and as moisture-proof agents. It is applied inside and outside the shell casings and at the junction of the bullet and casing to prevent ammunition from becoming damp and ineffective during storage. It is also used to manufacture moisture-proof paper and cloth for packaging ammunition and as an adhesive for powder granulation.
Inks made from shellac resin as the main ingredient are durable, smooth, bright, quick-drying, and wear-resistant, with strong adhesion to glossy surfaces like glassine paper and aluminum foil. Shellac-based inks have been widely adopted in the ink industry abroad.
Shellac inks are divided into alcohol-soluble and water-soluble types. The former is mainly used for printing on glassine paper, celluloid, packaging paper, lampwick paper, holiday paper, and printed products; the latter is mainly used for black waterproof drafting ink, colored waterproof drafting, and advertising inks.
4. Fertilizers
Due to its excellent waterproof, moisture-proof, and sealing properties, shellac resin is used to manufacture and improve the packaging materials for fertilizers. Fertilizers like ammonium chloride, calcium superphosphate, and dipotassium phosphate often deteriorate during storage and transportation due to moisture absorption or dehydration. Applying modified shellac varnish to packaging materials makes them heat-resistant (≤100°C), waterproof, leak-proof, dust-proof, and also resistant to chemical corrosion.
Shellac resin, with its excellent electrical insulation properties, adhesiveness, anti-carbonization, and thermoplasticity, is widely used in the electronics industry for insulating varnishes, laminated insulators, and molded insulators.
Due to its superior insulation and non-conductivity after exposure to electric arcs, shellac resin can be directly used in the electrical industry to manufacture high-voltage resistant laminated mica sheets and other insulating materials' adhesives. It can also be esterified and modified with ethylene glycol, tartaric acid, etc., to prepare laminated mica sheet adhesives with improved strength and flexibility. Additionally, by blending with semi-drying oils such as castor oil, linseed oil, fatty acids, and glycerin, modified shellac can be used to produce black insulating adhesives.
There is a wide variety of insulating varnishes, including those used for impregnating or coating instruments, small motor coils, and bare wires, or for bonding paper strips and fibrous strips on enameled conductors, thereby improving the dielectric properties and mechanical strength of the conductors. Some are used for protective varnishes for electrical equipment to resist heat, water, and cold; others are used for insulating coatings on enameled steel wire, providing flexibility, heat resistance, and corrosion resistance.
Mica is one of the best insulators in the electrical industry. By peeling mica sheets and bonding them with shellac resin, different additives (such as tartaric acid, oxalic acid, zinc oxide, chlorinated biphenyl, ethylene glycol, naphthenate, etc.) can be incorporated into the shellac to prepare adhesives with varying properties. This process produces laminated mica sheets, rigid laminated insulators, laminated insulating tubes, and flexible laminated insulating materials with various characteristics.
Shellac is also a commonly used thermoplastic adhesive for molding insulators. These molded insulators are characterized by their non-conductivity after exposure to electric arcs (i.e., they do not cause surface carbonization and significantly weaken insulation performance due to short-term electric arcs or sparks passing over the surface). Typically, shellac is mixed with asbestos powder, mica powder, rubber, rosin, ethanol, talc, sulfur, and fibers, among others, using various formulations to produce acid-resistant insulating materials, flame-retardant molding materials, fire-resistant and heat-resistant insulating materials, and waterproof insulating materials. These materials are then molded into various electrical insulating parts, such as knobs, buttons, switch bases, switch plates, switch handles, and arc shields.
A putty made by adding marble powder and phenolic resin to a shellac resin alcohol solution can be used for bonding light bulbs, fluorescent lamps, and vacuum tubes.
In the pharmaceutical industry, shellac can be used to coat tablet or sugar-coated tablet surfaces, providing an attractive gloss while protecting against moisture and quality degradation. Utilizing the acid resistance and lack of alkali resistance of shellac resin, enteric coatings that do not dissolve in the stomach but dissolve in the intestines can be prepared. This regulates the drug release rate and timing, enhancing the stability and effectiveness of the medication while reducing gastrointestinal irritation. Coating medications not only helps mask unpleasant odors, making them easier to swallow, but also facilitates transportation, storage, and use through improved handling properties.
Shellac is also used in health supplements. Applying a shellac ethanol solution to the exterior of health supplement capsules can prevent royal jelly capsules from dissolving in the stomach and protect the capsules from oxidation, degradation, and discoloration during storage and transport.
When combined with other drugs, shellac exhibits strong antibacterial activity, showing effectiveness in preventing and treating dental caries. A mixture of shellac with anhydrous ethanol, formic acid esters, and lavender oil, in specific proportions, can be uniformly applied to the tooth surface to form an antibacterial film. This film is effective in preventing and treating dental caries and periodontal caries.
A mouthwash with high antibacterial activity can be produced by mixing shellac with ethanol, glycerin, licorice-salt acid, sodium hydroxide, etc. Shellac can also be used to make tooth powder, which prevents periodontal caries and absorbs bacteria on the surface of the tooth. For teeth whitening, shellac can be mixed with methyl polysiloxane solution in proportion to create a teeth whitening agent. When applied to teeth, this whitening agent can maintain its effect for 24 hours, providing a cosmetic whitening benefit. Additionally, the excellent film-forming properties of shellac can be utilized in formulations for treating athlete's foot and tinea corporis.
In the food industry, shellac resin products are used to enhance the appearance and gloss of candies and pastries, preventing them from becoming moist and sticky. This application leverages shellac's easy film formation as a coating material and its derivation from natural products, which facilitates approval for food use. Additionally, when combined with other ingredients, shellac can improve the texture and appearance of food. Generally, only shellac that has been dewaxed, carbon-black filtered for decolorization, and not chemically modified is suitable for food applications. If necessary, it can also be modified with edible acids, phenols, or extracts from natural berries containing these substances before use. For example, mixing sugar, starch, eggs, water, and shellac-coated chocolate in certain proportions, then rolling, cutting, and baking at 200°C for a few minutes, produces beautifully finished, tasty biscuits. Shellac and starch can also be used as base materials to produce edible internal packaging films through modification, offering good water resistance, oil resistance, and moisture barrier properties, high mechanical strength, non-toxicity, harmlessness, absorbability by the human body, and natural degradability. Coating bread with shellac can effectively prevent mold and extend its shelf life.
8. Furniture
Shellac varnish, as a protective and decorative coating, is widely applied in the furniture industry for furniture, tables, chairs, cabinets, doors, windows, floors, musical instruments, vehicles, and various industrial wood products (countertops, radio casings, cultural goods, etc.). Shellac coatings are simple to prepare, easy to use, have strong adhesion, dry quickly, and produce a smooth, hard, transparent, elastic, and uniformly glossy film. Common types include ordinary varnish, colored shellac varnish, colored shellac enamel, and shellac nitrocellulose lacquer. They can be used for transparent finishes (to highlight wood grain) or opaque finishes (to cover the grain), and serve as an excellent sealer for knots and aniline dyes. Resin from wood knots can erode the surface coating of wood, causing peeling, discoloration, and the appearance of dark spots or even sticky droplets. Shellac paint provides good sealing properties. Additionally, when general varnish or paint is applied over a dye, it may redissolve aniline dyes, whereas a shellac varnish layer can seal or prevent oil-based dyes from diffusing into the top layer of varnish or paint.
Due to its exceptional adhesiveness, especially on smooth surfaces such as glass, mica, glazed ceramics, and polished metals, shellac resin has long been used as an ingredient in adhesives and bonding materials.
Shellac resin products can also be used to bond abrasive particles (such as aluminum oxide, silicon carbide, and corundum) to form grinding wheels of specific shapes. Grinding wheels are essential equipment in the mechanical industry for grinding various tools and cutlery. Shellac-based grinding wheels are highly wear-resistant, offer excellent cutting quality, do not deform, generate minimal heat on the object being cut, and can produce a high gloss on the material being ground. They are commonly used for grinding bearings, rollers, cutlery, camshafts, races for bearing balls, cutting high-speed tool steel, and for the precision processing of stainless steel strips, brass strips, and non-ferrous metal strips. They are also used for cutting and polishing agate, marble, and granite.
Shellac is also used in the manufacture of water sandpaper. The paper is first soaked in light mineral oil, then coated with a shellac-based mixture, after which the abrasive is uniformly adhered to the surface and gently baked.
Furthermore, shellac is used in the production of laminated paperboard. Paper is impregnated in a shellac mixture (shellac reacted with urea and formaldehyde in an alcohol solution), dried, and then hot-pressed into boards.
Hydrolyzed shellac reacting with maleic anhydride, followed by esterification with allyl alcohol, can produce a strong water-resistant adhesive suitable for bonding glass to glass and metal to metal.
Shellac resin is commonly applied to various metal sheets, foils, wires, and their products to prevent corrosion and rust, preserving the original appearance and luster of the metals and extending their lifespan.
l Acrylic and Acrylonitrile-Butadiene-Styrene (ABS) Modified Shellac Varnish: Enhances adhesion to metal surfaces, producing a tough coating that is heat and solvent resistant.
l Melamine Modified Shellac Varnish (40%): When sprayed on metals like copper, brass, and silver, it forms a hard, smooth, and shiny film with excellent heat and water resistance.
l Water-Soluble Resin and Drying Oil Modified Shellac Varnish: Offers good adhesion to metals. After baking, the film can resist 10% solutions of lactic acid, acetic acid, oxalic acid, hydrochloric acid, acetone, and lubricating oil.
l Butylated Urea-Formaldehyde Resin Modified Dewaxed Shellac Varnish (20%): The film is water-resistant, chemical-resistant, and weather-resistant.
Another major use of shellac resin products in metal coatings is as a primer. Phosphoric acid and chromium trioxide modified shellac phosphating primers significantly improve adhesion and water spotting resistance on stainless steel, cold-rolled steel, galvanized steel, and aluminum sheets. Urea and dibutyl phthalate modified shellac phosphating primers are effective on any metal surface, especially on aluminum, zinc, magnesium, copper, and other light metals and alloys that are generally difficult to paint. The characteristics of phosphating primers include quick drying, metal surface passivation, strong adhesion, and providing an excellent adhesive base for subsequent painting.
Modified shellac resin with olefin-branched phenolic compounds, added with dioctyl phthalate and glycerol for plasticization, and polyethylene glycol to reduce stickiness, can be made into a sealing glue. This sealant exhibits excellent resistance to oil, acid, water, and organic solvents other than alcohols, with a short curing time, easy film formation, non-stickiness to sealing surfaces, good elasticity, and shape adaptability to the sealed surfaces, making it convenient for disassembly and assembly of sealed parts. Thus, it is used in mechanical manufacturing, construction windows and doors, petrochemical, and electronics industries. Shellac and shellac polymers combined with hydroxy esters, hydroxy acids, hydroxy ketones, phosphate esters, and acidic phosphate esters can be made into sealing agents for glass containers, allowing treated glass bottles to be reused.
Shellac can also be used with ethanol, realgar resin, n-amyl acetate, benzaldehyde, and acid-resistant bituminous materials to make letter correction fluid; combined with pigments, adhesives, pearl essence, surfactants, and wetting agents, it can produce smooth, glossy, and even nail coatings.