11. OTHER SYNTHETIC RESINS
The synthetic resins already discussed are those in substantial commercial production but, by no means, the only ones known or produced. Several thousand new ones have been reported and the search continues in laboratories throughout the world. A successful new product must be one made from inexpensive raw materials or must possess some property or advantage that will permit its sale at a price level above that of other resins.
No attempt is here made to list the host of less important resins. Certain ones of unusual interest or possessing unique properties are described below. These include resins obtained from adipic acid, aniline, citric acid, diphenyl, furfural, lignin, sugar, and sulphonamide.
The resins from adipic acid are classed as alkyd resins. Those obtained by the condensation of adipic acid and glycerin are soft and rubbery and are used to some extent in surface coatings and in photographic films. In these the resin is formed in three stages as in other alkyd types: A soluble liquid, a viscous rubbery product, and a form insoluble in the usual solvents.
Commercial domestic production of these resins was reported for the first time in 1935 and the output has increased each year since then.
Resins obtained by condensing aniline and formaldehyde have been developed in recent years. Much of the research on this type of resin was done in Switzerland by the Ciba Co., which holds a number of patents on it. The Swiss product, called Cibanite, has excellent electrical and mechanical properties. At least one domestic manufacturer is licensed under the Swiss-owned patents.
Considerable interest has recently been manifest in synthetic resins derived from citric acid. The sharp decline in the price of citric acid, as a result of large scale synthesis from sugar has placed it within the realm of possibility as a raw material for synthetic resins.
The citric acid resins, classed as alkyd resins, are obtained by condensing citric acid and glycerin. Commercial production is said to have started in Europe, but there is no known domestic production as yet.
A series of products known as Aroclors and made by chlorinating diphenyl are available in commercial quantities.
Diphenyl was commercially produced for the first time by Swann Research, Inc., at Anniston, Ala., about 1928. The demand for it as a heat-transfer medium resulted in large scale output. Later it was found that certain of the chlorinated compounds of diphenyl possess valuable resin properties.
The Aroclors range from a clear mobile oily liquid to an amber colored transparent solid. They are thermoplastic, do not polymerize or oxidize, and are therefore nondrying. They may be dissolved in varnish oils, such as tung oil and linseed oil, to give varnishes which are resistant to alkali and water. The diphenyl resins are good adhesives on metal and glass and give strong joints between such surfaces. They have a high dielectric constant, resistivity, and a low power factor. Their chief use is in wire insulation.
The domestic production of chlorinated diphenyls is, at present, solely by the Monsanto Chemical Company, St. Louis, Mo.
Large scale commercial production of furfural, an aldehyde obtained from oat hulls and other farm waste, has made it available for synthetic resin manufacture.
Tar-acid furfural resins possess certain outstanding properties, such as great dimensional accuracy, great reaction speed to the infusible solid stage, and unusual strength and toughness. They are available in dark shades only. Printing plates as large as those of metropolitan daily papers are molded from them as are radio tube bases, all sorts of electrical parts, and machined parts requiring great dimensional accuracy. Other uses are in abrasive wheels, varnishes, and adhesives.
Probably the largest domestic maker of furfural resins is the Durite Plastics Division of Stokes and Smith Company, Philadelphia, Pa.
Many attempts have been made to utilize sugar as a raw material for synthetic resins. United States Patent No. 1,949,831, dated March 6, 1934, claims a process for the manufacture of molding compounds by condensing saccharide with aldehydes and urea. Pure sucrose yields a clear, colorless, nonresilient resin, while molasses and cane sugar give dark-colored resins. The trade name Sakaloid is used to designate certain of these resins; there is, however, no known domestic production. Sucrolite is the trade name of a brand of resins from sugar produced in Europe.
The sulphonamide resins were developed from para toluenesulphonamide, a byproduct obtained in the manufacture of saccharin (synthetic sweetening agent).
Para toluenesulphonamide, condensed with formaldehyde or other aldehyde, forms a viscous mass which, on heating, is converted to a hard colorless resin. Such resins are compatible with cellulose acetate or nitrocellulose in lacquers, the combination yielding clear, colorless lacquers of good gloss and adhesion. Other possible uses are as an adhesive in safety glass, in certain molding compositions, in insulating materials, and to deluster artificial silk.
Domestic production of sulphonamide resin is entirely by the Monsanto Chemical Co., St. Louis, Mo. It is marketed under the trade name Santolite.