The biological function of rubber for the plants is not fully known. However, it has been shown that rubber can help plants to heal after they are damaged, by covering wounds and stopping the bleeding.
This blocks the entry of harmful bacteria and viruses into the plants. The properties of rubber include high strength and the capability to be stretched many times without breaking. Natural rubber compounds are exceptionally flexible, good electrical insulators, and are resistant to many corrosive substances [ 1 ].
Synthetic man-made rubber can be produced through a chemical process, but people have not been able to produce a synthetic rubber that has all the properties of natural rubber. So, natural rubber cannot be replaced by synthetic rubber in most of its applications.
This is why natural rubber is still very important to human society [ 2 ]. As far back as B. It was not until the conquest of America that the use of rubber reached the western World. Christopher Columbus was responsible for finding rubber in the early s. Natives from Haiti played football with a ball made of rubber, and later, in , Fray Juan de Torquemada wrote about indigenous and Spanish settlers of South America wearing shoes, clothing and hats made by dipping cloth into latex, making these items stronger and waterproof.
But rubber had some problems: it became sticky in response to warm weather and it hardened and cracked with cold weather. There, he found two different trees containing latex: Hevea brasiliensis Figure 1B and Castilla elastica [ 3 ], but only the first became important as a natural rubber source. The reason why the Hevea tree succeeded over the Castilla tree was the way its latex was transported along the trunk.
The Hevea tree has connected latex tubes Figure 1A that form a network, whereas the Castilla tree does not form a connected system. Thanks to its connected system, the Hevea tree bleeds latex when a special incision is made in its trunk Figure 2. Without the latex tube connections, the Castilla tree does not bleed latex, making harvest of rubber more difficult. In , Charles Goodyear invented the vulcanization process, solving many of the problems associated with rubber.
Vulcanization is the process of treating rubber with sulfur and heat, to harden it while keeping its elasticity. It prevents rubber from melting in the summer and cracking in the winter. A few years after this important discovery, in , Dunlop invented the air-filled rubber tire, making rubber an extremely important raw material worldwide.
Rubber became an essential material for the Industrial Revolution. From to , businessmen were pushing entrepreneurs and traders to increase the amount of rubber extracted from Amazonian trees. During this period, the Brazilian Amazon was the only source of rubber and they controlled the price, making rubber expensive.
At the same time, as more and more industry was developing in Europe and USA, more uses for rubber were being found [ 4 ]. Rubber was such an important material for Brazilians that they prohibited the export of rubber seeds or seedlings. However, in , H. Wickham managed to smuggle 70, rubber seeds, hidden in banana leaves, and brought them to England.
From those seeds, only 1, seedlings survived and were sent to Malaysia to start the first rubber plantations in Asia.
While working there, he found the first 11 rubber trees that were planted in Malaysia and he started promoting the establishment of rubber tree plantations. Sometime later, he developed a revolutionary method for harvesting latex from the Hevea tree by continuous tapping.
Tapping is the process of removing the latex from the tree. The new plantations were more competitive in price, so from the end of the nineteenth century until the First World War, rubber collection from wild sources in tropical America declined tremendously.
During the war, the supply of rubber was cut off. The USA, Germany, and Russia started searching for alternative rubber sources, either natural or synthetic, since the Amazonian trees were not supplying enough rubber for their needs [ 3 ]. Several research programs started in these countries, but, after the war, the supply of rubber from Malaysian plantations started again and the effort to look for new rubber sources almost disappeared. In recent years, the search for alternative sources of rubber has begun again.
There are three main reasons for this:. First of all, the rubber trees are exposed to several diseases and since Asian rubber plantations started from only a handful of seeds, all the trees are genetically very similar.
Less genetic variation means lower ability to fight against plant diseases. If one tree becomes sick, the illness can rapidly spread to the entire plantation. Today, the most important and dangerous disease that Hevea brasiliensis suffers from is called South American leaf blight disease.
This disease can cause the devastation of an entire plantation. It is still confined to the tropical Americas, but if it arrives in Asia, it could mean the end of the rubber plantations. Under natural conditions, rubber trees commonly grow with a lot of space between them.
In nature, serious damage to Hevea from South American leaf blight is unusual, because the other kinds of trees growing in between the rubber trees are not susceptible to the disease and act as barriers. But, on plantations where rubber trees grow very closely together, it can become lethal. Second of all, an important threat to the natural rubber market is the very competitive and fast-growing market for palm oil and its side products.
There is an increasing demand for both rubber and palm oil but, in Malaysia, the area in which Hevea brasiliensis is being grown is not decreasing, however, the area dedicated to grow oil palm is increasing. If the continuous growth of oil palm plantations does not stop, either the natural forest or the Hevea plantations will have to get smaller to make room for new crops of oil palms. And last but not least, rubber tapping is a not well-paid job and it is difficult work.
Natural rubber is made up of long isoprene polymer chains, loosely connected. The chains reattach themselves when pulled apart; this gives rubber its elasticity. Natural rubber has flexibility and strength, as well as impurities and vulnerability to environmental conditions and hydrocarbons.
Additionally, it has a lot of tensile strength and adheres easily to other materials. Natural rubber must first be tapped from a rubber tree, then processed with chemicals and heat to be used in manufacturing. The tree is cut and the sap drips into a cup. A third of the latex is rubber at this point, held in a colloidal suspension, and another third is water. To refine the latex into rubber, latex is mixed with formic acid to make it the rubber coagulate into curds, which are then washed and pressed into blocks or pressed into sheets that are then smoked.
Next the rubber is put through masticating machinery to make it more workable, then mixed with chemicals to improve its properties. Vulcanization involves heating the rubber with some sulfur in a pressure cooker at around degrees F, although it can also be steam cured, microwaved, or sent through a fluidized bed or molten metal salts to be vulcanized. The vulcanization cross-links molecular chains of polyisoprene to add strength and chemical resistance, and remove the stickiness of raw rubber.
Even though artificial rubber was invented in the s, natural rubber is still widely used today, making up a little under half the market. Natural rubber is used in applications requiring a high level of wear and heat resistance.
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