brief history
The Hindi word "Lahk" comes from Sanskrit Lak-sha, which literally translates as100,000, meaning: the substance secreted by thousands of glue bugs. China's "lac" is named after the purple color of gum. In China's ancient books, it was named "Purple Mushroom", "Purple 498443" and "Red Gum". As early as 15 BC to 10 BC, the plasticity and uses of lac insects, their host plants and lac were recorded in the Sanskrit Vedas and the epic Mahabharata. Lu Wu, written by Jaco (265-289) of Jin Dynasty in China, describes the spread and secretion of lac insects on host plants and the use of lac as medicine and dye. Compendium of Materia Medica, written by Li Shizhen in Ming Dynasty, describes and summarizes the origin, insects, host plants, dyeing, cementation and medicinal properties of lac in detail.
/kloc-At the beginning of the 9th century, the production of lac in India has reached a considerable scale, with an annual output of 2,200 tons, which are exported to western countries. /kloc-At the end of 0/9th century, lac developed rapidly because of its unique properties and various uses, and occupied an important position in natural resins and industrial raw materials. In the first half of the 20th century, lac industry rose and flourished. Before 1957, India monopolized the world lac market, and almost all raw rubber from Thailand was imported to India and then processed and exported. Therefore, during this period, India's total export volume can represent the world lac consumption, with an annual average of 24,000 tons and the highest annual export volume reaching 39,000 tons. After 1957, the export volume of lac from India and Thailand decreased day by day. 1975 ~ 1985 was only 16000 tons on average, which was 50% of that in the early 1950s. The reason is that the development of new petrochemical products and the change of market demand mode make lac face the severe situation of competition with synthetic products. For example, PVC and PVA completely replaced lac in the record industry. Others, such as coatings and electrical insulation materials, are also replaced by composite materials to varying degrees.
China began to use lac as an industrial product in 1950s. 1956, Kunming, Yunnan established the first lac processing factory produced by indigenous method. In the early 1960s, a number of state-owned lac orchards were established, and the output of lac increased rapidly. By 1988, the original lac output reached 1482 tons. 1965, the mechanized production of hot filtration technology completely replaced the indigenous production, and a modern lac industry was established in all parts of the south.
Lac variety
(1) Raw lac: The dried lac block is peeled off from the purple stem, which is the raw material for processing lac. (2) Granular shellac: Granular shellac after primary purification of original shellac through crushing, screening, washing and drying is the raw material for processing purple film or decoloring, dewaxing and bleaching shellac. ③ Purple film: After the granular rubber is cleaned, it is hot-pressed into thin lac. There are many specifications and grades according to color, impurity content and other properties. ④ Lac bleaching: bleaching with sodium hypochlorite solution to remove the alcohol-soluble red lac in lac. Containing a certain amount of wax is called ordinary bleaching glue; Removing wax before or after bleaching is called refined bleached lac or dewaxed bleached lac. ⑤ Decolorization and dewaxing of shellac: After granular shellac is dissolved in alcohol, red shellac in shellac is removed by activated carbon adsorption with decolorized shellac; After granular shellac is dissolved in alcohol, shellac wax is removed by freezing or other methods.
Lac attribute
Lac is a non-toxic, hard and amorphous substance, which is slightly heavier than water. It softens at 65 ~ 70℃ and melts at 75 ~ 80℃. When heated above the melting point, it will gradually lose its fluidity, become rubber, and then become a completely insoluble substance. Lac is soluble in aqueous solution of alcohol, organic acid, inorganic base and organic base, and also soluble in acetone in the presence of a small amount of polar solvents (such as water and alcohol). Insoluble in ester, ether, hydrocarbon and water, it is dispersed in water with sodium carbonate, borax, ammonia, morpholine or triethanolamine, indicating that lac contains hydroxyl, carboxyl and carbonyl groups. The existence of polar groups in lac resin makes lac have strong adhesion on smooth surface, which is the orientation of these polar groups. These polar groups will adsorb polar solvents and reduce the water resistance of lac. Lac can still maintain stable insulation under the action of ultraviolet rays and electric arcs, but it is flammable. Lac is thermoplastic when heated for a short time, but it will polymerize when heated for a long time, and its viscosity will increase accordingly. When it becomes rubber, it will condense into horny, alcohol-insoluble blocks and will not melt. This phenomenon is called curing, which is the result of polymerization of lac resin. Polymerized shellac is also called cured shellac. Partially polymerized alcohol-soluble shellac resin can disperse shellac wax in alcohol solution.
Chemical Composition and Structure of Lac Resin
Lac resin is composed of hard resin and soft resin, and is composed of hydroxy acid and terpenoid acid. Among hydroxy acids, piperic acid is the most important, accounting for about 25%, followed by echinaconic acid, accounting for about 8%; Terpene acids are mainly chitin aldehyde acids, accounting for about 25%, and others are chitin acids, watchcase acids account for about 8%, and shellac acids account for about 8%. Lac resin also contains free fatty acid 10.72% and free terpene aldehyde acid 2.7%.
The ether insoluble part of lac resin is hard resin (or pure resin), accounting for 70% of lac, which dominates most characteristics of lac. The hard resin per 1 molecule consists of 4 terpenoid acids [3 shellac acids and 1 shellac acids] and 4 piperic acids. The structural formula is as follows:
The ether-soluble part of lac resin is soft resin, which is a soft and sticky balsam substance. It is yellow-orange or brown due to the existence of red lac, and plays the role of plasticizer in lac resin. Soft resins are mostly composed of shellac-ⅰ,-ⅱ and shellac-ⅰ,-ⅱ acid esters.
use
There are mainly the following aspects: ① film-forming substances. Household varnish and lac alcohol varnish can be used as wood primers to close the pits of wood and prevent wood resin from oozing out. Quick-drying, easy to polish, enhance the adhesion and wear resistance of topcoat. Lac anti-corrosion and moisture-proof primer is used to coat various non-ferrous metals, such as bullets, shell casings, moisture-proof gunpowder, etc. In addition, lac, titanium dioxide and carbon black are used as primer or quick-drying enamel; Lac aqueous solution of weak alkali such as sodium carbonate, borax, ammonia, morpholine and triethanolamine is used for leather finishing and polishing of floor, paper and packaging. ② plastic. Generally used for die casting grinding wheel, wax, etc. And a small amount for recording at 75 rpm. ③ Insulation material. Lac is used to bond insulating board and insulating paint of laminated mica board by its high dielectric strength, high adhesion and stability under the action of electric arc. ④ Adhesives and adhesives. It can be used as adhesive for laminated cardboard, jute board, gem processing, gunpowder, sandpaper and emery cloth, and can also be used as oil-gas sealing lining, gasket cement, optical glass and light bulb cement. ⑤ Others. It is also widely used in food, medicine and cosmetics industries, as well as in various ink, ink and hat making industries.
trend
Lac is a renewable resource, with no pollution in the production process, wide use and safe use, especially suitable for food, medicine, cosmetics and military industry, and has broad development prospects. However, lac has color, low softening point, poor water resistance and solvent resistance, and lacks some special properties required by modern technology, which limits its utilization. Therefore, the modification of lac to improve its properties has become an important topic to be studied and solved in the future lac processing.