Natural rubber

Rubber is a macromolecule or polymer of repeated chains of carbon and hydrogen atoms. Its unique properties of extensibility, stretchability, toughness, and resilience have made it a useful commodity in applications ranging from tires to clothing. The name “rubber” originates from its ability to erase pencil marks; its chemical designation is polyisoprene with several isomers.

Background

When Christopher Columbus arrived in Haiti in 1492 he found Indians playing a game with a ball made from the latex of rubber. Indians were also known to have used latex for making footwear, bottles, and cloaks. By 1735, latex had been described as caoutchouc by a French geographical expedition in South America. Thus, in the seventeenth and eighteenth centuries rubber and rubber products were already in use.

The role that rubber could play in clothing and footwear attracted the attention of chemists and inventors throughout the world in the eighteenth and nineteenth centuries. Charles Macintosh and Thomas Hancock, working as colleagues, discovered two separate means of using rubber in fabrics and footwear. Macintosh found that placing rubber and naphtha between layers of fabric resulted in a fabric with no sticky and brittle surfaces, and Hancock developed the rubber masticator, which welded rubber scraps to be used for further manufacturing.

The dramatic increase in the use of rubber that occurred in the twentieth century is attributable largely to the development of the automobile industry (and the resultant increase in tire production) and advances in industrial technology. Although rubber’s percentage of use compared with other elastomers decreased from the end of World War II to about the late 1970s, the development of radial automobile tires in Europe in the late 1940s and early 1950s and their popularization in the United States in the late 1960s and 1970s resulted in increased use of natural rubber.

The Origin of Rubber

The early use of rubber involved all-natural rubber formed from a number of different plant species belonging to the Euphobiaceae family, of which the rubber tree (Hevea brasiliensis), native to Brazil, has become the exclusive commercial source of natural rubber. As a coagulated milk substance, rubber is obtained from a fluid in latex vessels located in the bark of the tree. A number of other tropical and subtropical plant species also contain such latex vessels, including Manihot species, Castilla species, the Russian dandelion, guayule (Parthenium argentatum), and Funtumia elastica. In fact, both the Russian dandelion and guayule were widely used during World War II. Research continued on guayule, a plant native to the southwestern United States and northern Mexico. Similarly, Funtumia elastica, native to West and Central Africa, received some research attention. Guayule, used by American Indians, is an alternative rubber source to synthetic rubber (or “para rubber”) in North America, particularly the southwestern United States.

The production of natural rubber is based on Hevea brasiliensis, which is grown mostly in tropical and subtropical environments. While production is concentrated in developing countries, occurs mostly in the industrialized countries.

Until about 1913 Brazil was the major producer of natural rubber, which was obtained mostly from wild rubber trees growing in the jungles of the Amazon River basin. However, around the beginning of the twentieth century, plantation rubber for commercial production began, based on the work of Henry Riley in Singapore around 1890. Riley developed the “tapping” method for extracting latex from Hevea brasiliensis. This method has since been improved upon; improvements have included the mechanization (motorization) of the tapping knife.

During tapping, a slice of bark is systematically removed from one side (panel) of the tree, starting from an upper left corner and shaving to a lower right corner; care is taken not to damage the cambium. The cut usually has an angle of 25 to 30 degrees. Once the cut is made, latex flows into a collecting cup through an inserted spout on the tree. Generally, tapping is done from just before sunrise to about 10:00 a.m. to take advantage of maximum turgor pressure within the tree in the early morning hours. Stoppage of latex flow is attributable to a coagulum that plugs latex vessels.

The Growing of Rubber Plants

Commercial rubber plantations are vegetatively propagated by means of bud grafting. The bud from a high-yielding tree is cut and inserted under the bark of a rootstock. Upon a successful take, the bud grows and the rootstock is topped or removed at the point of growth. The bud is then transplanted from the nursery to the field. The tree is ready for tapping in five to seven years, when tree girth reaches 50 centimeters at 1.60 meters from ground level. Crown budding may also be done before budded stumps are transferred to the field. This type of budding is used to provide a crown that is tolerant of or resistant to disease or wind damage. Stand in rubber plantations ranges from 250 to 400 trees per at an average spacing of about 6 square meters.

Rubber performs best on deep, well-drained soils with a pH of less than 6.5 (in the 4.5 to 7.5 range). However, Hevea brasiliensis can be grown on a wide range of soils. Thus, while in China rubber plantations are found on latosols and lateritic red soils, in Brazil—the primary center of diversity—rubber grows on red yellow podosols. In Malaysia, at one time the world’s leading producer, rubber grows on lateritic red soils.

With regard to climatic requirements, P. Sanjeeva Rao and K. R. Vijayakumar, in Natural Rubber: Biology, Cultivation, and Technology (1992, edited by M. R. Sethuraj and N. M. Mathew), summarize the optimum conditions as follows: a rainfall of 2,000 millimeters or more, evenly distributed without any marked dry season and with 125 to 150 rainy days per year; a maximum temperature of about 29° to 34° Celsius, a minimum temperature of about 20°, and a monthly mean of 25° to 28° Celsius. There should be high atmospheric humidity in the order of 80 percent with moderate wind, and bright sunshine amounting to about 2,000 hours annually, at the rate of 6 hours per day in all months. These conditions exist in the major rubber-producing countries of the world.

Anatomy and Physiology of Latex

Latex is obtained from latex vessels called secondary laticifers in Hevea. The quantity of laticiferous tissue in the tree is determined by a number of anatomical factors such as vessel rings, size of laticifers, girth of trees, and the distribution of latex and latex vessel rows. The flow of latex and, subsequently, the yield of a rubber tree is dependent on these anatomical features.

Tapping, which causes injury to laticifers, does not expel the nucleus or mitochondria as part of the latex; that is, latex itself is a true cytoplasm. Hence latex reconstitution occurs following a complex phenomenon that results in the plugging of the wound. Several biosynthetic processes are responsible for the formation of latex from initial monomers through a glycolytic pathway. Increased yield can be obtained by using chemical stimulants on the bark of trees. The most effective and commonly used stimulant (2-chloroethane phosphonic acid) is commercially known as Ethrel or Ethephon. This chemical keeps latex flowing by delaying the plugging mechanism, and through its use certain clones can be made to yield twice as much latex. Present in the latex are three types of suspended particles, two of which are nonrubber (10 to 15 percent of the latex) and the third of which is dry rubber (40 to 60 percent of the latex, depending on clonal characteristics, conditions of cultivation and tapping, and other environmental factors).

Latex Processing and the Grading of Commercial Rubber

About four to five hours after tapping, the latex is collected from the trees. Field latex or cuplumps and “tree-lace” latex (strips or sheets of latex coagulated on a tapping cut) are collected and taken to a factory, laboratory, or small-holder processing center. At the factory or processing center, latex is sieved to remove foreign objects such as stones, branches, and leaves and is then blended by the addition of water or dilute acetic or formic acid. About 10 percent of the latex is shipped as latex concentrate following blending. Concentrates of natural rubber latex are obtained by the process of centrifugation and creaming. Meanwhile, the remainder of the latex and field coagulum are processed, either into conventional types of rubber or into technically specified rubber (TSR).

Conventional grades of dry rubber include ribbed smoked sheets (RBS), air-dried sheets (ADS), michellin sheets (MS), skim rubber (SR), pale crepe (PC), sole crepe (SC), and brown and blanket crepe (BBC). These conventional grades are based on visual examination that is dependent on criteria set by the Rubber Manufacturers’ Association, headquartered in Washington, D.C. These criteria include the presence or absence of extraneous foreign matter, bubbles, uniformity, intensity of color, and mold and rust spots. The major drawback to this method of grading is the lack of technological basis or quantifiable assessment.

The limitations of the conventional grading of natural rubber led to the development of technically specified rubber (TSR) systems. Although the use of TSRs dates back to the 1950s, the concept was first introduced into the market by Malaysia as Standard Malaysian Rubber (SMR) in 1965. The use of TSR gradings has been facilitated by developments in processing technologies, notably the heveacrumb and communition processes. The former is a chemical-mechanical process, while the latter is a mechanical process with no chemical additives.

Technically specified rubber has the advantages of quality assurance, consistency, reduced storage space, and ease of handling. TSR classification varies from country to country. In Malaysia there are at least ten different grades, and in Indonesia there are more than five. Other rubber-producing countries have also adopted the TSR grading system.

Besides TSRs and conventional types of rubber, there are at least ten other grades of natural rubber, including technically classified rubber (TCR), oil-extended natural rubber (OENR), tire rubber (TR), deproteinized natural rubber (DPNR), peptized rubber (PTR), powdered rubber (PR), skim rubber (SR), superior processing rubbers (SP), heveaplug MG rubber (MG), and thermoplastic natural rubber (TPNR). There are also other minor grades, currently not of commercial significance.

Vulcanization

In 1839, a number of fundamental weaknesses associated with manufactured rubber were resolved with the development of vulcanization by Charles Goodyear, a US inventor. Vulcanization is the process of treating natural rubber with sulfur and lead and subjecting the compounds to intense heat, resulting in what Goodyear first called “fire proof gum” but later called vulcanized rubber. Present vulcanization technology is simply a modification of Goodyear’s invention. Other forms of vulcanization are available based on diurethanes, which are stable at processing temperatures that may be as high as 200° Celsius or more. Vulcanized rubber can then be processed into a wide range of applications, including tires, fabrics, bridge constructions, condoms, and other latex products such as adhesives and footwear.

Future Uses of Natural Rubber

There is continuing interest and effort on the part of research scientists and natural rubber producers to find new uses for natural rubber. Thus, in addition to conventional uses, especially in tire production, projections for further uses range from snowplow blades to uses in earthquake-resistant buildings. Although many proposed uses are engineering applications, there are others in the area of wood and wood products that may eventually make rubber plantations important sources for environmental restoration, given the increasing that has taken place in the natural rubber-producing areas of the world. It should be noted that in rubber plantations that are more than forty years old, the regeneration of secondary forests with associated wildlife species occurs frequently. Thus, natural rubber (Hevea brasiliensis) is both an important industrial crop species and a major renewable resource.

Synthetic Rubber

Much of what people typically consider rubber today is actually synthetic rubber. Synthetic rubber is a polymer of several hydrocarbons; its basis is monomers such as butadiene, isoprene, and styrene. Almost all monomers for synthetic rubber are derived from and petrochemicals. The emulsion polymerization process occurs at very high temperatures. There are different types of synthetic rubbers, three of which are dominant in the rubber industry. These are styrene-butadiene rubber (SBR), polyisoprene rubber (IR), and polybutadiene rubber (BR). Unlike natural rubber, with a few exceptions, synthetic rubber is produced mainly in industrialized countries.

Theoretically, synthetic rubber production dates back to 1826, when Michael Faraday indicated that the empirical formula for synthetic rubber was (C₅H₈)_x. The technology for synthetic rubber production was not developed until 1860, however, when Charles Williams found that natural rubber was made of isoprene monomers. Significant interest in using synthetic rubber as a substitute for natural rubber developed only during World War II, when the Germans were looking for alternatives to natural rubber. The severe shortages of natural rubber during and immediately after World War II stimulated research on synthetic rubber and its technology. Today, synthetic rubber is used in a wide range of applications, and it constitutes about three-quarters of the total rubber produced and consumed.

Bibliography

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