S. Kohjiya

Kyoto University, Japan

Y. Ikeda

Kyoto Institute of Technology, Japan

Introduction to the unique qualities of natural rubber

Natural rubber is in widespread daily use. It is unique among types of rubber, biopolymers and other materials in general use [1−10]. Its unique qualities may be summarised as follows:

Hevea brasiliensis is the botanical name of a commercially grown plant producing natural rubber [8−10]. Hevea is the generic name and brasiliensis is one of the 11 species of the genus Hevea, in accordance with Linnaean nomenclature. Other plants growing in the wild have been used for the extraction of natural rubber. These include Castilla elastica (commonly known as Castilloa), which is grown in Central and South America, and Manihot glaziovii (Ceara), grown in Brazil. Ficus elastica is widespread in tropical Asia. The genera Landlphia (vine rubbers) and Funtumia are common in mid-western African jungles. These are known as typical rubber producing trees [8−10]. More types of rubber yielding trees are described in Chapter 15.

American scientists continue to work on Parthenium argentatum (Guayule) [10, 12−16] (see Chapter 1), a shrub found in Mexican deserts. This was cultivated in the United States during the Second World War, when synthetic rubbers underwent rapid development due to the scarcity of natural rubber [16−18].

Russian scientists cultivated Taraxacum kok-saghyz (Russian dandelion rubber) during the 1930s and 1940s. Thomas Alva Edison (1847–1931), with the support of Henry Ford, investigated many types of Goldenrods as possible sources of rubber in addition to Cryptostegia grandiflora [16]. (Goldenrods are a group of weeds widely found in the United States which are now abundant in other countries, including Japan, as non-native plants.)

In addition to Hevea, more than 2,000 plants are now known to yield rubber, though the quality and quantity are inferior. The superiority of Hevea was recognised as early as the middle of the nineteenth century, although the well-known Collins report [19] failed to state this clearly. Natural rubber from Hevea brasiliensis has historically been preferred (see Chapter 15).

Neither the reasons for, nor the significance of, so many plants being rubber-yielding has yet been fully elucidated. When one of the present authors visited RRIC (Rubber Research Institute of Ceylon, now RRISL) in 1977, he asked bio-related officers (including a physiologist), why plants produce rubber. The reply was that there is as yet no evidence for the physiological function of rubber in plants. Although rubber appears to be of no use to the plants themselves, they enable the bio-synthesis of highly stereo-regular cis-1,4-polyisoprene, the perfect stereo-regularity of which has not yet been achieved by chemical synthesis [10, 20]. This unsolved puzzle as to why cis-1,4-polyisoprene (a polymeric isoprenoid) is produced in plants or in vegetables may be a unique quality associated with natural rubber.

The history of natural rubber

Rubber was first used during the Olmec civilisation (circa 1300–300 BC), and its use continued among the Mayans (mainly on the Yucatan peninsula in Mexico, from circa 300 BC to AD 1500), the Incas (the Andes highlands around Peru, from circa AD 1100 to 1500), and the Aztecs (from the twelfth century in central Mexico) until the Spanish destruction of the Central and South American civilisations. The Olmec had been known to tap plants, most probably Castilla elastica, and to have made rubber goods.’Olmec’ may mean ‘rubber people’.

One of the most notable usages of rubber was the manufacture of balls. These were thought to have been used in a game [2, 10, 21] which was considered an important religious and political event, in which victory or defeat was used to determine the outcome of wars. Figure 0.2 shows an athletic field at Chichen Itza, a well-known site of the Mayan civilisation. A stone ring attached to the wall at a height of about seven metres is assumed to be a goal. This game is thought to symbolise the harmonious nature of civilisations in South and Central America. In the twentieth century, rubber became an important military material, but remained a symbol of peace for the people associated with its origin.

The discovery by Columbus of the ‘New World’, which may have marked the end of the Middle Ages in Europe, was the beginning of a European invasion of that new continent by a military force, despite there being few counter-attacks due to the peaceful nature of the local Indians. The rubber ball which Columbus observed during his second voyage may be assumed to have been manufactured by Olmec craftsmen using rubber obtained from Castilla elastica trees [10, 22]. A Spanish priest, P. Martyre d’Anghiera, attached to the invading army, first wrote about rubber in his book ‘De Orbo Novo’, which was published in 1530. Further literature was published, but the useful application of rubber remained unknown among Europeans for nearly 200 years.

A breakthrough on rubber came from two French scientists [23]. F. Fresneau (1703–1770) was an agricultural scientist working at the colonial office in French Guiana. While travelling in Guiana and the Amazon in search of economically useful plants, he became interested in rubber-producing trees on which he prepared a report. The other scientist, C. M. de la Condamine (1701–1774), was a geographer, and a member of the expedition to Quito (1735–1745) whose task was to measure longitude just below the equator. While in Cayenne in French Guiana, he obtained the report authored by Fresneau, and later gave a lecture on rubber at the meeting of the French Academy of Science in Paris. This was the first scientific report on rubber [10]. (Historically, this achievement may be attributed to Fresneau [10, 23, 24].)

In 1765, an encyclopedia entitled ‘Encyclopedie, ou dictionaire raisonne des sciences, des arts et des métiers’ was published in France. It included the term ‘caoutchouc’ – the French word for rubber. It is probable that one of the editors, Denis Diderot...