Resin is a hydrocarbon secretion formed in special resin canals of many plants, from many of which (for example, coniferous trees) it is exuded in soft drops from wounds, hardening into solid masses in the air. It may be obtained by making incisions in the bark or wood of the secreting plant. It can also be extracted from resin-bearing plants by leaching of the tissues with alcohol.
Plants produce resins for various reasons whose relative importances are debated. It is known that resins seal the plant's wounds, kill insects and fungi, and also allow the plant to eliminate excess metabolites.
Resin as produced by most plants is a viscous liquid, typically composed mainly of volatile fluid terpenes, with lesser components of dissolved non-volatile solids which make resin viscous and sticky. The commonest terpenes in resin are the bicyclic terpenes alpha-pinene , beta-pinene , delta-3 carene and sabinene , the monocyclic terpenes limonene and terpinolene , and smaller amounts of the tricyclic sesquiterpenes longifolene , caryophyllene and delta-cadinene . The individual components of resin can be separated by fractional distillation.
A few plants produce resins with different compositions, most notably Jeffrey Pine and Gray Pine, the volatile components of which are largely pure n-heptane with little or no terpenes. The exceptional purity of the n-heptane distilled from Jeffrey Pine resin, unmixed with other isomers of heptane, led to its being used as the defining zero point on the octane rating scale of petrol quality. As heptane is explosively inflammable, distillation of resins containing it is very dangerous; before Jeffrey Pine was distinguished as a separate species from the similar, but terpene-producing Ponderosa Pine in the mid to late 19th century, some resin distilleries in California exploded as a result.
Solidified resin from which the volatile terpene components have been removed by distillation is known as rosin. Typical rosin is a transparent or translucent mass, with a vitreous fracture and a faintly yellow or brown colour, non-odourous or having only a slight turpentine odour and taste. It is insoluble in water, mostly soluble in alcohol, essential oils, ether and hot fatty oils, softens and melts under the influence of heat, is not capable of sublimation, and burns with a bright but smoky flame. This comprises a complex mixture of different substances including organic acids named the resin acids . These are closely related to the terpenes, and derive from them through partial oxidation. Resin acids can be dissolved in alkalis to form resin soaps , from which the purified resin acids are regenerated by treatment with acids. Examples of resin acids are abietic acid (sylvic acid), C20H30O2, and pimaric acid , C20H35O2, a constituent of gallipot resin . Abietic acid can also be extracted from rosin by means of hot alcohol; it crystallizes in leaflets, and on oxidation yields trimellitic acid , isophthalic acid and terebic acid . Pimaric acid closely resembles abietic acid into which it passes when distilled in a vacuum; it has been supposed to consist of three isomers.
Some resins when soft are known as oleo-resins , and when containing benzoic acid or cinnamic acid they are called balsams. Other resinous products are in their natural condition mixed with gum or mucilaginous substances and known as gum resins . Many compound resins have distinct and characteristic odours, from their admixture with essential oils.
The hard transparent resins, such as the copals, dammars , mastic and sandarach , are principally used for varnishes and cement, while the softer odouriferous oleo-resins (frankincense, elemi, turpentine, copaiba ) and gum resins containing essential oils (ammoniacum , asafoetida, gamboge, myrrh, and scammony) are more largely used for therapeutic purposes and incense.
Certain resins are obtained in a fossilized condition, amber being the most notable instance of this class; African copal and the kauri gum of New Zealand are also procured in a semi-fossil condition.
Synthetic resins are according to DIN 55958 (December 1988) epoxy resins, which are manufactured through polymerization—polyaddition or polycondensation reactions. They can be modified by natural substances, e.g. vegetable or animal oils and/or natural resins, or be manufactured by veresterung or soaping of natural resins.