Resins

Resins, term applied to a group of sticky, liquid, organic substances that usually harden, upon exposure to air, into brittle, amorphous, solid substances. Natural resins are secreted by many plants, appearing on the external surface of a plant after a wound. The resins form protective coatings over the plant wounds, preventing the entrance of pathogenic micro-organisms and also excessive loss of sap from the wound. In obtaining natural resins commercially, cuts are made in the tree bark, and the globules of liquid resin that flow from the cut are directed by troughs into collecting buckets. Amber is one of many fossil resins that are often collected from ground deposits. Natural resins are yellow to brown in colour. They burn with a smoky flame and exude an aromatic odour. Chemically, they differ from one another in detail, but they all contain carbon, hydrogen, and oxygen. All resins are insoluble in water, as distinguished from the water-soluble gums. Resins are also soluble in alcohol, ether, and other organic solvents. Synthetic resins resemble natural resins (see Plastics). The natural resin known as lac is not a plant exudate but is formed by the tiny scale insect, Laccifer lacca, indigenous to south-eastern Asia. Lac is deposited on trees and is harvested for the production of shellac.

The many natural resins are classified according to hardness and chemical constitution into three principal categories: hard resins, oleoresins, and gum resins. The hard resins, among which are amber, copals, mastic, and sandarac, are hard, brittle, odourless, and tasteless and exhibit a glasslike fracture. Hard resins are obtained either as fossils or as distillation products of the oleoresins. The most important of the hard resins, and possibly the most commercially important of all the resins, is rosin, which is used in sizing paper, in soapmaking, as a constituent of varnishes and paints, and as a friction-producing coating for the bows of stringed instruments. Rosin is obtained by distillation of the oleoresin turpentine. The oleoresins are sticky, amorphous semi-solids that contain essential oils. Among the oleoresins are the balsam, dragon's blood, and copaiba; turpentine is possibly the most widely used oleoresin. The essential oil of turpentine is used as a solvent for paints and varnishes and is employed in the manufacture of shoe polish and sealing wax. During the era of sailing vessels, the crude oleoresin turpentine was much used for caulking and waterproofing. Resins such as frankincense, myrrh, benzoin, and asafoetida contain gums and are called gum resins.