Latex, from Hevea tree

Natural Rubber Latex (from Hevea tree)... 

Derivation From latex obtained from Hevea trees, coagulated with acetic or formic acid. Also made synthetically. 

The latex as obtained from Hevea trees by tapping (called field latex) is a dilute aqueous dispersion of latex particles. It contains a host of organic molecules... 

Effect of Precipitating Agents.—In practice we are interested in the effect of precipitating agents on the latex as it comes from the tree, but in order to find the scientific basis for this effect it is necessary to eliminate all the dissolved salts already present in the natural milk. This was effected by Henri on the latex from Hevea braziliensis by means of dialysis through collodion filters, see page 36. His results may be summarized as follows ... 

Natural rubber is obtained as a latex from a tree called Hevea Braziliensis. It consists predominantly of cis-lA-polyisopropene (Fig. 3.1). The word rubber is derived from the ability of this material to remove marks from paper, which was noted by Priestley in 1770. Rubber materials are not, however, restricted to natural rubber. They include a great variety of synthetic polymers of similar properties. An elastomer is a polymer which exhibits rubber elastic properties, i.e. a material which can be stretched to several times its original length without breaking and which, on release of the stress, immediately returns to its original length. That is to say, its deformation is reversible. 

The rubber industry has a long and colorful history. Natural rubber is produced from latex, a milky fluid found in cells that lie between the bark and the wood of many plants. You may have seen latex flow from the broken stalks of milkweed plants, but the source of commercial rubber is the Hevea tree, a native of Brazil. When the bark of this tree is slashed, its milky white sap oozes out and can be collected in cups mounted on the tree s trunk. The people of the Amazon jungle made bouncing balls, shoes, and water Jars out of rubber, and Portuguese explorers sent waterproof boots and a rubber-coated coat back to their king. The first commercial exports included some rubber shoes shipped to Boston in 1823. 

A term applied to clones of Hevea trees which give a much higher yield of latex than ordinary rubber trees. The average yield in the 1950s was under 500 kg of rubber per hectare per annum. It is now more common to achieve a two- to threefold increase on these figures from modem planting material. 

Renewable raw materials are made or derived from short-term renewable sources (one to a few years or a few tens of years) such as plants, trees, wood wastes and other agricultural products. Not all these materials are necessarily biodegradable. Natural rubber, for example, comes from the latex of a tree (Hevea brasiliensis) and is not biodegradable. Renewable materials are often considered as opposites to fossil sources such as petroleum that are not renewable on a human timescale. On the other hand, some synthesized plastics such as certain polyesters are biodegradable. 

Natural rubber can be found as a colloidal emulsion in a white, milky fluid called latex and is widely distributed in the plant kingdom. The Indians called it wood tears. It was not until 1770 that Joseph Priestly suggested the word rubber for the substance, since by rubbing on paper it could be used to erase pencil marks, instead of the previously used bread crumbs. At one time 98% of the world s natural rubber came from a tree, Hevea brasiliensis, native to the Amazon Basin of Brazil which grows to the height of 120 ft. Today most natural rubber is produced on plantations in Malaysia, Indonesia, Singapore, Thailand, and Sri Lanka. Other rubber-bearing plants... 

The absence of dimethylallyl-group in NR indicates that the initiating species for rubber formation in Hevea tree is not FDP, but FDP modified at the dimethylallyl-group, which is abbreviated here as (o [103,109,110]. This was confirmed by 13C-NMR analysis of in vitro polymerised rubber by incubation of the bottom fraction of fresh latex and isopentenyl diphosphate (IDP) [111]. The newly synthesised in vitro rubber formed in the presence of FDP and IDP showed the dimethylallyl group derived from FDP. On the other hand, no dimethylallyl group was detected in the in vivo rubber prepared without the addition of FDP [112]. 

Natural rubber, a terpene composed of repeating isoprene units, is isolated from latex, a sticky white fluid that oozes from Hevea brasiliensis, the South American rubber tree, when its bark is cut. The word rubber was first suggested by English chemist Joseph Priestly in 1770, when he used latex to rub out pencil markings. Rubber is a polymer containing 1,5-diene units in which all the double bonds have the Z configuration. In Chapter 30, we learn about the synthesis and properties of polymers like rubber. 

NR occurs as particles dispersed in an aqueous serum, i.e., latex, which are contained in bundles of tubular vessels in the trunk of the tree [7]. Latex is harvested from the tree by a process called tapping. The latex of the Hevea braziliensis tree has been described as a cytoplasmic system containing rubber and nonrubber particles dispersed in an aqueous serum phase. Freshly tapped Hevea latex has a pH of 6.5-7 and a density of 0.98 gcm . The total solids of fresh field latex vary typically from 30 to 40%, depending on weather and tapping frequency and other factors. The dry rubber content is approximately 3 wt% less than the total solids [8]. 

X. Gidrol, H. Chrestin, H. L. Tan, and A. Kush, Hevein, a lectin-like protein from Hevea brasiliensis (rubber tree) is involved in the coagulation of latex, J. Biol. Chem., 269 (1994) 9278-9283. 

The modem elastomer industry was founded on the naturally occurring product isolated from the latex of the tree Hevea brasiliensis. It was first used by indigenous South Americans and was called caoutchouc, but, later, simply rubber, when it was discovered by Priestley that the material robbed out pencil marks. 

Section 6.3 deals with purely phenomenological theories. In this section the predictions of a theory based on the microstructure of a rubber are considered. By 1788 at the latest the term rubber was being applied to the material obtained from the latex of the tree Hevea braziliensis because of its ability to remove pencil marks from paper. The first printed account of this use for wiping off from paper the marks of black lead pencil was given by Joseph Priestley as early as 1770. This material is now called natural rubber and its chemical structure is shown in fig. 6.9. 

Natural rubber of the best quality is prepared by coagulating the latex of the Hevea brastliensis tree that is primarily cultivated in the Far East. However, there are other sources such as the wild rubbers of the same tree growing in Central America, guyayule rubber coming from shrubs grown mostly in Mexico, and balata. Balata is a resinous material and carmot be tapped like the Hevea tree sap. The balata tree must be cut down and boiled to extract balata that cures to a hard, tough product used as golf ball covers. 

We begin with two trees, both cultivated on plantations in Southeast Asia. One, Hevea brasiliensis, is a source of natural rubber and was innported from Brazil in the nineteenth century. The other, Isonandra gutta, is native to Sumatra, Java, and Borneo and gives a latex from which gutta-percha is obtained. 

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