Resins are solid or semisolid complex amorphous mixtures of organic compounds that cannot be characterized by definite melting points and which show no tendency to crystallize. Historically, however, the term resin has been applied to a group of substances obtained as gums from trees or manufactured synthetically. But strictly speaking, resins are complex mixtures, whereas gums are compounds that can be represented by a single chemical formula.
The word gum was originally applied to any soft sticky product derived from trees; for example, the latex obtained from Hevea trees, which is the source of natural or gum rubber. Natural rubber, i.e, chemically unsaturated polyisoprene, is a polymeric material that can also be produced synthetically. Thus, although the term resin when applied to polymers actually antedates the understanding of the chemistry of polymers and originally referred to the resemblance of polymer liquids to the pitch on trees, it has by association also come to refer to synthetic polymers.
The term natural resin usually refers to plant products consisting of amorphous mixtures of carboxylic acids, essential oils, and isoprene-based hydrocarbons. These materials occur as tacky residues on the bark of many varieties of trees and shrubs. In addition, natural resins have also come to describe shellac, which is a natural, alcohol-soluble, flammable material made from deposits on tree twigs left by the lac insect in India and other parts of Asia; amber, which is a fossilized polymeric material derived from a coniferous tree; and natural liquid substances such as linseed and similar drying oils.
A varnish is a solution of a hard resin, drying oil, and a solvent. It is transparent as it does not contain the pigments present in paint. Shellac varnish is a solution of shellac, in denatured alcohol. As a point of reference, by 1000 B.C., the Egyptians are known to have used a varnish of the gum of the acacia tree.
Vegetable-derived natural resins generally fall in one of four categories: rosins, which are resinous products obtained from the pitch of pine trees, and are used in varnishes, adhesives, and various compounds; oleoresins, which are natural resins containing essential oils of plants; fossil resins, which are natural resins from ancient trees that have been chemically altered by long exposure such as amber and copal and gum resins, which are natural mixtures of true gums and resins. Examples of gum resins along with their traditional or modern uses are galbanum (obtained from a perennial herb of western Asia and having medicinal uses); myrrh obtained from small trees of India, Arabia, and northeast Africa and used for incense and perfumes; medicinal tonics, stimulants, and antiseptics); asafetida (obtained from a perennial herb and used as an Asian food flavoring; and for medicines and perfumes in the U.S.; creosote bush resin (obtained from the leaves of the greasewood bush or creosote bush of the desert regions of Mexico and the southwestern United States and used in adhesives, insecticides, core binders, insulating compounds, and pharmaceuticals); okra gum (obtained from the pods of a plant native to Africa but now grown in many countries, and used for foodstuffs, pharmaceuticals; also as an antioxidant and chemical stabilizer, and as a gelation agent); ammoniac resin (obtained from the stems of a desert perennial plant of Persia and India and used in adhesives, perfumes, and medicinal stimulants).
Synthetic resins are polymeric materials, which are better known as plastics. There are two important classes of synthetic resins: thermosetting resins and thermoplastic resins.
Thermosetting resins cure to an infusible and insoluble mass with either the application of heat or a catalyst. The thermosetting resins are dominated by phenolics, polyesters, polyurethanes, and amino resins. Although the first polyester resin was made by the Swedish chemist Jacob Berzelius (1779-1848) in 1847, the first thermosetting unsaturated polyester resins did not appear on the market until nearly 100 years later in 1946 when they found use in the construction and transportation industries. American Leo Hendrik Baekeland (1863-1944) prepared the first phenolic resin in 1909, but did not succeed in commercializing it until 1932 when it found use in electrical products such as ovens and toasters, wiring devices, switch gears, pulleys, pot and cutlery handles.
Thermoplastic resins are polymeric materials that can be softened and resoftened indefinitely by the application of heat and pressure, provided that the heat that is applied does not chemically decompose the resin. They include polyethylene (first synthesized in 1933; commercialized in 1939) used in packaging and non-packaging films; polystyrene (synthesized 1839; commercialized 1930) used in molded products such as cassettes, audio equipment cabinets, packaging film, and food-stock trays; acrylonitrile/butadiene/styrene (ABS) (synthesized 1953; commercialized 1954) used in injection-molded automotive components; polyethylene terephthalate (synthesized 1941; commercialized 1954) used in food packaging, soda pop bottles, textiles; polyvinyl chloride (synthesized 1912; commercialized 1927) used in flooring, pipes and conduits, siding; polycarbonate (synthesized 1953; commercialized 1959) used in compact discs and optical memory discs; nylon (synthesized 1935; commercialized 1940) used in transportation industry products; polyacetal (synthesized 1956; commercialized 1959) used in transportation industry products; silicone resins (synthesized in the 1930s; commercialized 1943); Teflon (synthesized 1938; commercialized 1949) used in coatings; and poly(methyl methacrylate) (synthesized 1933; commercialized 1936) used as a substitute for silica glass.
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