Synthetic Resins

The process of manufacture is one of condensation, usually facilitated by the use of a small addition of acid or alkali. The details of one method of procedure are as follows : Equal parts of pure phenol and formaldehyde solution (formalin) with a small quantity of caustic soda are heated together in a steam jacketed pan with stirring gear. The mixture gradually becomes cloudy and finally an oily layer separates. This layer is removed, and from it, by further heating, sometimes under diminished pressure, a molten resin is obtained. This resin, on cooling, sets to a brittle, transparent product. In this condition it is ready for dissolving in alcohol for use as an impregnating solution for sawdust or other "filler," to form moulding compositions. Mouldings made by pressing such a composition are then baked to bring about the final change to the inert resistant condition.

The simplest condensation products of phenol and formalde hyde are saligenin or o-hydroxybenzyl alcohol and p-hydroxy benzyl alcohol. Further mutual condensation of these compounds leads to a syrupy product which, on continued heating, gives a resin somewhat akin to Bakelite. No definite conclusions have yet been made as to chemical constitution, owing to the small reactivity of the resin in its final form. The fully polymerized formaldehyde-phenolic resin is insoluble in all the usual solvents and chemical reagents, except nitric acid and caustic alkali. It is more than probable that the final fully polymerized_ resin, pre pared industrially, contains, in addition to a polymerized molecule of high molecular weight, a mixture of other substances, which are retained by the resin during hardening, such as free phenols and formaldehyde and crystalline intermediates formed during the condensation reaction. A number of mouldings are made with resins of the Bakelite type. A variety of useful mouldings are obtained from wood-meal-resin moulding composition, and also from laminated resin-paper and fabric compositions. The ad vantages of this type of material consist in its power to withstand temperatures at which rubber, ebonite, celluloid and natural resins soften and liquefy, or even decompose, together with the accuracy and high finish of the mouldings. For electrical insula tion it is used in wireless apparatus, telephones, electric lamp sockets, generators, transformers and other electrical equipment. Paper or woven fabric compositions have been used for silent transmission gears and also, experimentally, for aeroplane pro pellers. Grinding wheels containing resin and emery, and self lubricating bearings containing resin and graphite, can be built up. Laboratory apparatus for handling corrosive acids, especially hydrofluoric acid, has also been made.

Modified Formaldehyde-phenolic Resins.

The German Albertols and American Amberols are soluble in linseed oil and are prepared from formaldehyde-phenolic resins by heating with common rosin. They are now being further modified by the inventors by neutralizing the acidic part of the rosin component with glycerin. One such modified resin softens at oo° C and melts at i2o° C, whereas the rosin used in its preparation softened at 59° C and melted at 65° C. These resins are used mostly as substitutes for oil-soluble natural resins in varnishes, although some varieties are proposed for the replacement of shellac in the preparation of bonded mica sheets, such as micanite. Certain formaldehyde-phenolic resins containing sulphur have some ap plication in electrical insulation ; thiolite, a resin of French origin, is prepared by the action of sulphur chloride on a condensation product of formaldehyde and cresol. It contains 12% of sulphur.

Formaldehyde-urea Resins.

Formaldehyde may also be con densed with urea, or its sulphur analogue, thiourea. Thiourea can be made from a by-product in gas manufacture. Opaque or transparent colourless resins are produced which, by continued heating, become insoluble. They are finding application in Great Britain, particularly the Beetle resin, in moulding powders from which table and decorative ware, very artistically tinted with delicate colours, can be produced. The transparent variety, particularly the German Pollopas, is proposed for use as a substi tute for glass in motor-car wind-screens. Efforts are being made to render them suitable for electrical insulation. The moulded table ware can be washed with soap or weak soda solution without ill effect, and will stand temperatures of I C. The specific gravity varies from 1.4-1.5. The chemical basis of this form of resin is probably a dimethylol derivative of urea or thiourea. Glycerin Resins.—Glycerin and phthalic anhydride (q.v.) react to give a resin of industrial interest, which in America is known as Glyptal. It is transparent, pale yellow and soluble in acetone, but becomes insoluble on heating to 220° C. In the insoluble condition it retains some degree of flexibility and finds application in the electrical industry, especially for amalgamating mica flakes to form insulating sheets. Such sheets when properly prepared compare favourably in electrical properties with similar sheets in which shellac is used as the binding resin. Pure acrolein, obtained from glycerin, is polymerized in the cold by the addi tion of an alkali, forming a white powder melting between 8o° C and ioo° C. It can be dissolved in alcohol and used as a varnish. Acrolein also reacts with phenol to form a hard resin in the presence of about 1% of caustic soda. These resins of French origin have electrical properties of the order of those of ebonite. An American resin, acrolite, can be prepared from glycerin and phenol by heating to between 160° C and 190° C, in the presence of a small quantity of sulphuric acid.

Coumarone Resins.—Coumarone resins are prepared from certain fractions distilled from coal-tar naphtha, and are used in varnishes, and also to some extent as softening agents in rubber mixing. Their method of production involves no preliminary condensation but consists in direct polymerization of coumarone, indene and unsaturated cyclic compounds of this class, contained in the naphtha distillate, by means of strong sulphuric acid. They vary in colour from light yellow to black. They are miscible with drying oils and will dissolve in benzene but not in alcohol.

BIBLIOGRAPHY.—General: C. Ellis, Synthetic Resins and their Plastics (1923) ; Clement and Riviere, Matieres Plastiques (1924) ; Barry, Drummond and Morrell, The Chemistry of the Natural and Synthetic Resins (1926). Manufacture and Application: E. Hemming, Plastics and Moulded Electrical Insulation (5923) ; H. W. Rowell, Jour. Soc. Chem. Ind. t1927). Constitution of Phenolic Resins: L. H. Baekeland, Industrial and Engineering Chemistry (5909 and 1925). Physical Properties: A. V. Mory, Industrial and Eng. Chem. (1927). Viscosity and Solubility: A. A. Drummond, Jour. Soc. Chem. Ind. (1924) ; Jour. Oil and Colour Chemists' Assoc. (1927). (A. A. D.)