resin, any of a class of amorphous solids or semisolids. Resins are found in nature and are chiefly of vegetable origin. They are typically light yellow to dark brown in color; tasteless; odorless or faintly aromatic; translucent or transparent; brittle, fracturing like glass; and flammable, burning with a smoky flame. Resins are soluble in alcohol, ether, and many hydrocarbons but are insoluble in water. When heated, they soften and finally melt. Their chemical composition varies, but most are mixtures of organic acids and esters. Resins are generally classified according to their source or by such qualities as hardness or solubility. Natural resins are found as exudations, often as globules or tears, on the bark of various trees (mostly pines and firs) or on other living plants; they also occur as fossils or as exudations from the bodies of certain scale insects (see lac). Some natural resins, called oleoresins, contain both a resin and an essential oil; they are often viscid, sticky, gummy, or plastic. Other resins are exceedingly hard and resistant to most solvents, softening only at high temperatures. The primary uses for most resins are in varnish, shellac, and lacquer, in medicine, in molded articles (e.g., pipe mouthpieces), and in electrical insulators. See amber; balsam; benzoin; Canada balsam; copaiba; dragon's blood; mastic; rosin; turpentine.

Any natural or synthetic organic compound consisting of a noncrystalline (amorphous) solid or viscous liquid substance or mixture. Natural resins are usually transparent or translucent yellow to brown and can melt and burn. Most are exuded from trees, especially pines and firs (see conifer), when the bark is injured or stripped. The fluid secretion usually dries out and hardens into a material that can be worked. Natural resins have been used in perfumes and medicines (e.g., balsams), in paints and varnishes (e.g., turpentine and shellac, the latter derived from the secretion of an insect), and in decorative ware (e.g., amber, Oriental lacquer). Synthetic resins are all plastics; the term resin, though still used in the modern industry, dates from the years when synthetics began to replace natural resins. Thermoplastic resins are plastics such as polyethylene that can be shaped repeatedly on reheating, whereas thermosetting resins are plastics such as epoxy that set permanently and cannot be reshaped.

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Any of a wide variety of synthetic polymers containing positively or negatively charged sites that can interact with or bind to an ion of opposite charge from a surrounding solution. Light, porous solids in granules, beads, or sheets, they absorb the solution and swell as they attract the target ions; when exhausted, they are removed from use and regenerated by an inexpensive brine or carbonate solution. A solid support of styrene-divinylbenzene copolymer to which are attached sulfonic or carboxylic acid groups is often used to attract and exchange cations (e.g., ions of hydrogen or metals). Quaternary ammonium groups on the solid matrix are used to attract anions (e.g., ions of chlorine). Industrially, the resins are used to soften hard water, purify sugar, and concentrate valuable elements (gold, silver, uranium) from their ores. In the laboratory they are used to separate and concentrate substances and sometimes as catalysts. Zeolites are minerals with ion-exchange properties.

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