Rubber, Natural

caoutchouc, crude rubber, a polymer of plant origin, the vulcanization of which produces processed rubber.

Natural rubber is one of the elastomers, a group of macromolecular compounds that possess the capacity for considerable reversible deformation at room temperatures and lower. Natural rubber is contained primarily in the milky sap, or latex, of rubber-bearing plants, individual inclusions also being found in the cells of the bark and leaves. Natural rubber is obtained primarily from the latex of the Para rubber tree (Hevea brasilien-sis), which is grown on plantations in tropical countries. Malaysia is the largest producer of natural rubber (more than 40 percent of world production).

The name “caoutchouc” derives from cauchu, which the inhabitants of Brazil applied to the product obtained from the Para rubber tree, which grows naturally on the banks of the Amazon River (from cau, “tree,” and uchu, “to leak” or “to cry”). The history of natural rubber is usually taken as beginning in 1738, when the French researcher C. de la Condamine presented to the Academy of Sciences in Paris specimens of caoutchouc, articles made from it, and a description of the methods used in South America to obtain it. The industrial use of natural rubber became possible after the discovery of the process of vulcanization (C. Goodyear in the United States in 1839 and T. Hancock in Great Britain in 1843). Basic data on the structure of natural rubber were obtained beginning in the 1870’s by G. Bouchardat, H. Staudinger, and the German scientist C. Harries. Extensive research on the vulcanization of natural rubber was carried out by B. V. Byzov, B. A. Dogadkin, I. I. Ostromyslenskii, and the American scientist E. H. Farmer. The Soviet scientists A. P. Aleksandrov, V. A. Kargin, and P. P. Kobeko and the American researchers E. Guth, L. R. G. Treloar, and F. T. Wall have investigated the physical properties of natural rubber and developed a theory of its elasticity.

The latex is extracted by tapping the bark of the tree. The natural rubber is isolated by coagulation with formic, oxalic, or acetic acid. The loose mass (coagulum) that forms is washed with water and rolled on a mill, producing sheets that are then dried and usually smoked in special chambers. Smoking makes the natural rubber resistant to oxidation and microorganisms.

According to the International Standard for Quality and Packing (1969), natural rubber is divided into eight international types, including 35 international grades. The basic types of natural rubber are ribbed smoked sheet (a product of light amber color) and pale crepe (a product of light creamy color; special bleaching agents, such as sodium bisulfite, are added to the latex before isolation and the rubber is not smoked). The quality of natural rubber of international types and grades is judged both by external examination and by comparison with a standard. There is also a system of classification for technical standards that regulates the content of impurities in the rubber. Along with general-purpose natural rubber, special types of rubber are also produced (for example, with improved technical or mechanical properties, prepared in a powderlike form).

Extensive experimental work and research is being carried out with the object both of improving the quality of natural rubber and of raising the productivity of rubber-bearing plants.

The principal component of natural rubber is the rubber hydrocarbon (91-96 percent), a polyisoprene with the general formula (C₅H₈)n. Natural rubber also contains 2.2-3.8 percent proteins and amino acids; 1.5-4.0 percent acetone-soluble substances (acetone extracts: oleic acid, stearic acid, linoleic acid, carotene); compounds of metals of variable valence, such as copper (to 0.0008 percent), manganese (to 0.001 percent), iron (to 0.01 percent); and sand and certain impurities. Natural rubber is one of the stereoregular polymers; 98-100 percent of the isoprene monomers in its macromolecule are joined in the cis = 1, 4 configuration:

The molecular weight of natural rubber is between 1.4 million and 2.6 million, and the content of double bonds in the macromolecule comes to 95-98.5 percent of the theoretical value. The density is 0.91-0.92 g per cu cm; the refractive index, 1.5191; the glass transition temperature, from —70° to — 72°C; the specific heat, 1.880 kilojoules per (kg • °K)[0.449 calories per (g°C)]; the thermal conductivity, 0.14 watts per (m. °K) [0.12 kilocalories per (m • hour • °C)]; the dielectric constant at a frequency of 1 kilohertz, 2.37-2.45; and the specific electric conductivity, 25.7 • 10~¹⁸ ohm^-1. cm^-1.

Rubber is unaffected by water but easily soluble in benzene, toluene, xylene, gasoline, carbon tetrachloride, chloroform, carbon disulfide, and cyclohexane. It is amorphous at temperatures above 10°C. Prolonged storage at lower temperatures or stretching at room temperatures cause its partial crystallization. Among the valuable properties of natural rubber is its high cohesive strength; to a large extent it is this property that has made natural rubber irreplaceable in the production of certain tire parts. A production drawback of natural rubber, related to its high molecular weight, is the necessity of mastication before introducing the ingredients of the rubber stock.

Sulfur is the most widely used vulcanizing agent for natural rubber. Vulcanization accelerators include 2-mercaptobenzo-thiazole (Kaptaks), its sulfenamide derivatives (for example, Santokiur), dibenzthiazyl disulfide (AFtaks), and tetramethylthi-uram disulfide (thiram). Radiation vulcanization of natural rubber and vulcanization with organic peroxides or alkyl-phenol-formaldehyde resins are also possible.

Crystallization of natural rubber causes the high degree of strength in stretching vulcanized rubbers based on this process. Introducing active fillers hardly alters the strength of processed rubbers, but certain other mechanical properties rise substantially (see Table 1). Processed rubbers made from natural rubber are characterized by a high degree of elasticity, durability, and frost resistance, by high dynamic properties, and by a low level of resistance to solvents and oils and lower heat resistance and weather resistance than in certain synthetic rubbers.

Tire production is the principal consumer of natural rubber, which is also used in producing industrial articles (conveyor belts, transmission belts, shock absorbers, and seals), electric insulating materials, consumer goods, and rubber cements. A certain amount of natural rubber is also used in the form of latex. The volume of production of natural rubber came to about 3 million tons in 1971. Owing to the development of stereoregular synthetic rubbers and a wide assortment of special-purpose synthetic rubbers, the demand for natural rubber in certain industrial sectors is declining.