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Resins

Natural Resins, Thermosetting ResinsSynthetic resins, Thermoplastic resins

Historically, the term resin has been applied to a group of substances obtained as gums from trees or manufactured synthetically. Strictly speaking, however, resins are complex mixtures, whereas gums are compounds that can be represented by a 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. (A polymer is a macromolecular compound made up of a large number of repeating units,

**TABLE 1. THERMOSETTING SYNTHETIC RESINS**
Synthetic Resin	1994 U.S. Sales (in million of pounds)	Major Applications
phenolics	3222	electrical products such as ovens and toasters, wiring devices, switch gears, pulleys, pot and cutlery handles
unsaturated polyesters	1496	construction and transportation industries
polyurethanes	1102	building insulation, refrigeration
amino resins	2185	wiring devices, molded products, electrical parts, adhesives and bonding agents
epoxy resins	602	coatings, reinforcement, electrical and electronic applications, adhesives, flooring, and construction

**TABLE 2. GUM RESINS**
Resin	Source	Applications
galbanum	gum resin from perennial herb of western Asia	medicinal uses
myrrh	gum resin from small trees of India, Arabia, and northeast Africa	incense and perfumes; medicinal tonics, stimulants, antiseptics
asafetida	gum resin from perennial herb	Asian food flavoring; used for medicines and perfumes in the United States.
creosote bush resin	amber-colored, soft, and sticky gum resin from the leaves of the greasewood bush or creosote bush of the desert regions of Mexico and the southwestern United States	adhesives, insecticides, core binders, insulating compounds, pharmaceuticals
okra gum	gum resin from the pods of a plant native to Africa but now grown in many countries	foodstuffs, pharmaceuticals; used for its antioxidizing and chemically stabilizing properties, and as a gelation agent
ammoniac resin	gum resin from the stems of a desert perennial plant of Persia and India	adhesives, perfumes, medicinal stimulants

**TABLE 3. THERMOPLASTIC SYNTHETIC RESINS**
Synthetic resin	1994 U.S. Sales (in million of pounds)	Major applications
polyethylene	25,683	packaging and non-packaging films
polypropylene	9752	fibers and filaments
polystyrene	5877	molded products such as cassettes, audio equipment cabinets; packaging film; food-stock trays
acrylonitrile/butadiene/styrene (ABS)	1489	injection-molded automotive components
polyethylene terephthalate (PET)	—	food packaging
polyvinyl chloride	11,123	flooring; pipes and conduits; siding
polycarbonate	695	compact discs and optical memory discs
nylon	921	transportation industry products
thermoplastic elastomers	867	automotive, wire and cable, adhesive, footwear, and mechanical goods industries
liquid crystal polymers	—	chemical pumps, electronic components, medical components, automotive components
acetals	214	transportation industry products
polyurethane	1790	flexible foams in the transportation industry
thermoplastic polyester	3441	engineering plastics

called mers.) 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.

Synthetic resins are polymeric materials, which are better known as plastics. The term plastic better describes polymeric material to which additives have been added. There are two important classes of synthetic resins: thermosetting resins and thermoplastic resins.

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. Table 3 lists some commercially important synthetic thermoplastic resins, their uses, and their levels of consumption.

Resources

Books

Brady, G. S., and H.R. Clause. Materials Handbook. New York: McGraw Hill, Inc. 1991.

Engineered Materials Handbook. Metals Park, OH: ASM International, 1988.

Randall Frost

KEY TERMS

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Gum: —A viscous secretion of some trees and shrubs that hardens upon drying.
Synthetic: —Referring to a substance that either reproduces a natural product or that is a unique material not found in nature, and which is produced by means of chemical reactions.
Thermoplastic: —A high molecular weight polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.
Thermoset: —A high molecular weight polymer that solidifies irreversibly when heated.

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