RUBBER: PRODUCTION AND MANUFACTURE. Between 190o and 1911 the world's crude rubber production in creased at the average rate of about 3,00o tons per year. For the period between 1911 and 1929, mainly because of the develop ment of automotive transportation, the rate of increase was much greater, averaging about 44,00o tons per year. World economic conditions after 1929 caused a decrease in production ; but in 1934 the figure for 1929 was again attained and exceeded. In 1937 more rubber was imported into manufacturing countries than ever before. (See Table I.) Since 1925, when 73% of the world supply was imported into the United States, increasing imports by other countries have low ered this proportion. Since 1929 the United States has imported slightly more than half the total. For some years preceding 1929 the annual value of rubber products made there exceeded a bil lion dollars ; and since 1934, varied between 2 and a billion. In 1935 wage earners numbered 115,000 and salaried employees about 15,00o in 466 establishments. The manufacture of rubber products is, therefore, one of the large industries of the U.S.
The most important underlying development of the industry was the discovery of hot vulcanization of rubber made in 1839 by Charles Goodyear, of New Haven, Conn. Another important de velopment was the art of compounding—the blending with crude rubber of various mineral powders, oils, waxes, tars, asphalts, fibres, etc., to secure special properties in the finished goods. A third prominent development was the art of combining rubber compositions with other structural materials, such as textile fab rics, metals and alloys, concrete, wood, glass, or asbestos, to secure in commercial articles the combined benefits of the rubber por tion and the superior structural strength or rigidity of the aux diary materials. Various processes of reclaiming scrap vulcan ized rubber have resulted in supplying a plastic rubber material which can be processed with the equipment used for crude rubber, which blends smoothly with crude rubber, and which makes eco nomically possible the manufacture and sale of many articles that would not find a ready market if only compositions of crude rub ber were available.
Since 1906 organic accelerators of vulcanization have con stantly increased in use, saving millions of dollars annually to the industry and to consumers through economies in manufacture and improvement in quality of goods. A later major development is
the retarding of deterioration of rubber compositions by admix ture with them of small quantities of age-resisters. Finally, rub ber manufacturers have improved the quality of goods designed for special types of service by replacement of rubber, wholly or in part, by synthetic rubber-like materials, such as Neoprene, Koro seal, or some of the butadiene group of polymers.
The main original source of rubber materials today is the latex of certain plants (see RUBBER : BOTANY, CULTIVATION AND CHEM ISTRY). Preserved latex alone or compounded with other ingredi ents to cause vulcanization or to modify the characteristics of the rubber has long been used for impregnating the cord fabric used in pneumatic tires. Toy balloons and surgeons' gloves are pro duced by dipping into latex compounded with water dispersions of pigments, sulphur and accelerator, porous forms which absorb water, leaving a thin film of vulcanizable stock on the form. The articles thus formed are then dried and vulcanized, giving a prod uct of excellent quality. Philip Schidrowitz, of London, developed and patented in 1922 a process for vulcanizing the rubber while still dispersed in the form of latex, of forming articles from the vulcanized suspension and drying them. The anode process, by which latex compositions are deposited on forms either by the electric current or by coagulating solutions, is also used for mak ing balloons, surgeons' gloves, ice bags, and rubber covered metal articles. Concentrated latices have been placed on the market. That prepared by centrifuging contains 6o% to 65% of rubber ; the product prepared by evaporation of some of the water, after addition of stabilizing agents, may contain as much as 85%. Latex concentrated by addition of chemical agents which cause the rubber to form a cream at the surface is also now a commercial product. Large quantities of latex have recently been used in the manufacture of light-weight sponge rubber for seat cushions, mat tresses and similar uses. Increasing importations of latex are indicated by the figures in Table III.