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| Rubber tree |
The name "rubber" originates from the material’s ability to erase pencil marks; its chemical designation is polyisoprene with several isomers. About 60 to 65 percent of the rubber produced today is synthetic.
When explorer Christopher Columbus arrived in Haiti in 1492 he found Indians playing a game with a ball made from the latex of rubber. American 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.
The role that rubber could play in clothing and footwear attracted the attention of chemists and inventors throughout the world in the late eighteenth and mid-nineteenth centuries. Charles Macintosh and Thomas Hancock, working as colleagues, discovered two separate means of using rubber in fabrics and footwear.
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Macintosh found that placing rubber between layers of fabric resulted in a fabric with no sticky and brittle surfaces. 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 advances in industrial technology.
Although rubber’s percentage of use compared with other elastomers decreased from the end of World War II to the late 1970’s, the development of radial automobile tires in Europe in the late 1940’s and their popularization in the United States in the late 1960’s resulted in increased use of natural rubber.
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. As a coagulated milk substance, rubber is obtained froma 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, Castilla, the Russian dandelion, guayule (Parthenium argentatum), and Funtumia elastica.
Both the Russian dandelion and guayule were widely used during World War II. Research has continued on guayule, a plant native to the south-western United States and northern Mexico.
Similarly, Funtumia elastica, native to West and Central Africa, has received some research attention. Guayule, used by American Indians, is still considered a possible alternative rubber source to synthetic rubber in North America, particularly the southwestern United States.
Today 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, consumption occurs mostly in industrialized countries. Between 1955 and 1988 production of rubber more than doubled, with Malaysia the leading world producer and the United States the world’s largest consumer.
Growing Rubber Plants
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| commercial rubber plantation |
Trees for commercial rubber plantations are vegetatively propagated bymeans 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.
It 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 density in rubber plantations ranges from 250 trees to 400 trees per hectare at an average spacing of about 6 meters by 6 meters.
Rubber grows best in deep,well-drained soil but can be grown on a wide range of soils. Rainfall should exceed 2,000 millimeters yearly, evenly distributed without anymarked dry season.
Temperature should average 25 to 28 degrees Celsius, with high (80 percent) humidity and bright sunshine of about six hours per day year-round. These conditions exist in the major rubber-producing countries of the world.
In Hevea, latex is obtained from latex vessels called secondary laticifers. 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.
Latex Processing
Until about 1913 Brazil was the major producer of natural rubber, obtained mostly from wild rubber trees growing in the jungles of the Amazon basin.
In the early twentieth century, plantation production of rubber began, based on the work of Henry N. Riley in Singapore around 1890. Riley developed the “tapping” method for extracting latex from Hevea. Later improvements to this method included the mechanization 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, with care being 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 a spout inserted 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 flowis attributable to a coagulum that plugs latex vessels.
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 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).
A number of fundamental weaknesses associated with manufactured rubber were resolved in 1839 with the development of vulcanization by Charles Goodyear, an American 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. Current vulcanization technology is a modification of Goodyear’s invention.
New forms of vulcanization are available based on diurethanes, which are stable at processing temperatures as high as 200 degrees Celsius or more. Vulcanized rubber can then be processed into a wide range of applications, including tires, fabrics, bridge constructions, and other latex products such as adhesives and footwear.
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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, projections for new uses range from snowplow blades to earthquake-resistant building construction materials.
Although many proposed uses are engineering applications, there are other applications in the area of wood products that may eventually make the large acreages of rubber plantations important sources for environmental restoration, given the increasing deforestation that is taking place in the natural rubber-producing areas of the world.
In rubber plantations that are more than forty years old, the regeneration of secondary forests with associated wildlife species occurs frequently. Thus, natural rubber 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 allmonomers for synthetic rubber are derived from petroleum 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, synthetic rubber is produced mainly in industrialized countries. The United States is the world’s leading producer.
Theoretically, synthetic rubber production dates back to 1826, when scientist 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. Great interest in using synthetic rubber as a substitute for natural rubber began during World War II, when the Germans were looking for alternatives to natural rubber.
The severe shortages of natural rubber during and immediately after the war stimulated significant research in synthetic rubber and its technology. Today, synthetic rubber is used in a wide range of applications, and it constitutes 60 to 65 percent of the total rubber produced and consumed.
