SYNTHETIC RESINS Synthetic or artificial resins possess most of the physical char acteristics of natural resins, and in addition may have, in certain types, the unique property of becoming infusible and insoluble after heat treatment. Chemically, a certain degree of resemblance is traceable between the phenolic synthetic resins and Japan lacquer and acaroid among natural resins, compounds of phenolic character having been shown to be present in the latter products. Although produced by the chemical interaction of substances of definite composition, synthetic resins are usually of complicated and indeterminate composition. They are of industrial interest as substitutes for natural resins in the varnish industry, as insulating material in the electrical industry, and as a basis for the produc tion of moulded articles for decorative purposes, such as umbrella handles, buttons, beads, brush backs, door-plates, knobs, etc.
It is possible to group synthetic resins according to solubility. There are resins which always remain soluble and those which are initially soluble but become finally insoluble under the action of heat, so called "heat-harden ing" resins. The resins from formaldehyde and phenol, such as Bakelite, are of the latter class. Resins in the permanently solu ble class may be further divided into (a) alcohol- or spirit-solu ble resins, and (b) benzene- and oil-soluble resins.
The resins suitable for varnishes are either spirit-soluble (chiefly of formaldehyde-phenolic type), forming substitutes for shellac and the softer varnish resins; or oil-soluble (chiefly the coumarone type, and also modified formaldehyde-phenolic types), forming substitutes for hard natural resins. The modified oil-soluble formaldehyde-phenolic types referred to are produced from the spirit-soluble phenolic resins, by a process of melting with a natural resin akin to the "running" process adopted with natural resins.
Artificial resins are used extensively in preparing moulding compositions. Moulding resins, usually of heat-hardening phenolic type, are used either as pure unmixed resin, producing as a rule transparent mouldings, or incorporated with some inert "filling" material, such as sawdust, asbestos or clay, and pressed in heated moulds ; or they may be used for impregnating paper or woven fabrics. By submitting these prep
arations to a process of pressing in heated moulds, highly durable mouldings in almost any desired form can be obtained. In order to give them maximum resistance to heat, chemical action and electrical forces, the mouldings, on removal from the moulds, are baked further, thus completing the chemical reaction in the resin. A material of considerable value for insulating pur poses in the electrical industry is thereby obtained. The elec trical properties are somewhat similar to those of ebonite.
Moulding compositions can also be prepared in a form suitable for the production of domestic articles, cups and saucers, bowls and plates, and the like. Textile materials can be decorated with beads made from artificial resin. Numerous other decorative uses are continually being found for artificial resins.
Two main classes may be dis tinguished, (a) condensation resins, such as those of the formalde hyde-phenolic type, condensation being, in its simplest form, the chemical process whereby molecules unite, with elimination of water ; (b) polymerization resins, such as those of the coumarone type. Polymerization is the chemical process in which relatively simple molecules of a compound become complex by combination amongst themselves. The term "polymerization resin" is used to distinguish a resin which is formed directly by the polymerization of a chemical compound, without passing through a preliminary stage of condensation. Polymerization is usually initiated by the action of light, heat, strong acids or alkalis.
Resins.—Bakelite, invented by L. H. Baekeland in 1909, is the leading representative of one main class of synthetic resins produced to-day in most countries in one form or another. The preparation involves the interaction of phenolic substances, such as phenol (see CARBOLIC ACID) and cresol (q.v.), with aldehydes, particularly formaldehyde (q.v.). The output of such resins and their derivative moulding composi tions is a growing one. In America alone nearly 7,00o tons were manufactured in 1925, representing double the production of 1922. Other countries, of which Germany, France and Great Britain are the chief, manufacture something like an equal amount between them.