Resins

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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, 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.

Natural resins

The term natural resins 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; amber, which is a fossilized polymeric material derived from a coniferous tree; and natural liquid substances such as linseed and similar drying oils.

Table 1. Thermosetting Synthetic Resins . (Thomson Gale.)
Thermosetting synthetic resins
Synthetic resin	1994 U.S. sales (in million of pounds)	Major applications
phenolics	3,222	electrical products such as ovens and toasters, wiring devices, switch gears, pulleys, pot and cutlery handles
unsaturated polyesters	1,496	construction and transportation industries
polyurethanes	1,102	building insulation, refrigeration
amino resins	2,185	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 . (Thomson Gale.)
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 the leaves of the greasewood bush or cresosote bush of the desert regions of Mexico and the southwestern United States	adhesives, insecticides, core binders, insulating from 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

Vegetable-derived natural resins generally fall in one of four categories:

1) Rosins, which are resinous products obtained from the pitch of pine trees. Rosins are used in varnishes, adhesives, and various compounds.

2) Oleoresins, which are natural resins containing essential oils of plants.

3) Gum resins, which are natural mixtures of true gums and resins including natural rubber, gutta percha, gamboge, myrrh, and olibanum (Table 2).

4) Fossil resins, which are natural resins from ancient trees that have been chemically altered by long exposure. Examples of fossil resins include amber and copal.

Synthetic resins

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.

Thermosetting resins

Thermosetting resins form a highly diverse, versatile, and useful class of polymeric materials. They are used in such applications as moldings, lamination, foams, textile finishing, coatings, sealants, and adhesives (Table 1).

A thermosetting resin cures to an infusible and insoluble mass with either the application of heat or

Table 3. Thermoplastic Synthetic Resins . (Thomson Gale.)
Thermoplastic synthetic resins
Synthetic resin	1994 U.S. sales (in million of pounds)	Major applications
polyethylene	25,683	packaging and non-packaging films
polypropylene	9,752	fibers and filaments
polystyrene	5,877	molded products such as cassettes, audio equipment cabinets; packaging film; food-stock trays
acrylonitrile/butadiene/styrene (ABS)	1,489	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	1,790	flexible foams in the transportation industry
thermoplastic polyester	3,441	engineering plastics

KEY TERMS

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.

a catalyst. The thermosetting resins are dominated by phenolics, polyesters, polyurethanes, and amino resins. Together, these account for about 70% of the commercially important thermosets.

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.