Rubber Production and Manufacture

A

successful method has been brought into practical use for the production of aqueous dispersions of crude rubber or of rubber compositions containing vulcanizing and compounding ingredients. These dispersions have found commercial applications in the manufacture of artificial leather and of rugs made of sisal fibre. They have also been used for impregnating fabric prior to the application of a thin rubber coating by the usual process of cal endering (see below).

Materials Used in Dry Mixing.

The ordinary practice of mixing rubber compositions is carried out with dry materials,— coagulated rubber, reclaimed rubber, mineral powders, softeners (These amounts are included in the table of imports of crude rubber, on p. 6o5) such as oils or tars or waxes, accelerators of vulcanization, age resisters, and sulphur. Hundreds of materials are used in rubber compounding and thousands of combinations are in commercial use. Materials may be classified into relatively few groups.

I. Rubber Materials.—A. Crude Rubber.—The sources and commercial forms are discussed in the article RUBBER: BOTANY, CULTIVATION AND CHEMISTRY.

B. Reclaimed Rubber.—Vulcanized rubber scraps are plas ticized and are available for reprocessing by the ordinary methods of rubber goods manufacture. The process of scrap treatment is known as reclaiming and the product as reclaimed rubber, re claim, or shoddy. Many methods have been devised but the one most extensively used is that of heating ground scrap with a dilute solution of caustic soda at temperatures about F. for I2 to 20 hours. This is the alkali process of A. H. Marks, patented in 1889. Between 1924 and 1927 the use of reclaimed rubber in the United States increased 137%, whereas the increase in crude rub ber consumption was only 31%. Since 1927, because of lower rubber prices, the relative consumption of reclaimed rubber has decreased. During the last ten years consumption of reclaimed rubber averaged 29% of the amount of crude rubber used.

C. Scrap.—Unvulcanized trimmings containing no fabric or other materials are not waste. They may be blended with fresh stocks of the same composition and utilized with no loss of ma terial value. Unvulcanized waste containing fabric may be treated to pulverize the fabric and utilized in rubber compositions with some reduction in value. Even vulcanized waste is incorporated in certain goods.

II.

Dry Pigments.—A. Reinforcing and Filling Pigments.— Powdered materials are blended with rubber materials in order to modify the stiffness, strength, resistance to abrasion or chemical action of the vulcanized rubber. "Reinforcing" pigments possess

the property of stiffening and strengthening rubber compositions so that the total energy necessary to extend a strip of the com pound to its breaking point is greater than that necessary to stretch a similar mix containing only rubber, sulphur, and acceler ator. Fillers, though they may stiffen the vulcanized compound, do not increase the total energy of rupture. Carbon black (gas black), zinc oxide, certain clays, and magnesium carbonate are common reinforcing pigments. Whiting and barytes are exten sively used as fillers.

B. Colours.—Most of the colours for rubber goods are used in powdered form, and few colours soluble in rubber are used. For white goods, zinc oxide, lithopone, titanium oxide, and zinc sul phide are used. Reds, blues, yellows and other colours and shades are secured with pigments such as ferric oxide, ultramarine blue, and zinc chromate. The most recent development of special col ours for the rubber industry is the utilization of metallic salts of organic azo- or other coloured compounds. These are pro duced in many beautiful shades, have high tinting values, and are not changed in colour by sulphur or heat during vulcanization.

III.

Other Compounding Ingredients.—A. Softeners.—For mod ifying the characteristics of the vulcanized rubber mixtures and for improving their properties for ease in processing, many kinds of softening materials are incorporated in rubber stocks. Petroleum products from oils to paraffin wax, tars, oxidized petroleum. residues (mineral rubber), rosin, pine tar, fatty acids or their zinc salts, and many others are common to the industry.

B. Vulcanizing Agents.—By far the commonest vulcanizing agent is sulphur, used in the form of ground brimstone. The pro portion used depends upon the character of the product required. Soft rubber goods carry from 21% to i o% of sulphur, which in most compounds is nearer the lower figure. Compositions unus ually resistant to natural deterioration are now produced contain ing less than 1% of sulphur. Hard rubber compositions carry 20% to 5o%. Some organic sulphur compounds which liberate sulphur at vulcanizing temperatures have been used in special cases without addition of sulphur itself. Selenium and tellurium will produce vulcanization also, and have been used to some extent, usually, however, with some sulphur.