Rubber Materials & Properties
A standardised colour coding system is used within AVMR to differentialte between different rubber compounds in use, as after moulding it is very difficult to identify materials without complex analysis. Products whcih are normally only available in one compond are not colour coded.
Origins of Rubber
The rubber tree (Hevea Brasiliensis) is a tropical tree originating in the rain forests of South America where early rubber was collected from ‘wild’ trees. Caoutchouc was the name for rubber used by the Indians of Central and South America. A natural substance that had been used for centuries before being ‘discovered’ by Columbus and introduced to western culture.
Caoutchouc is derived from the Indian word “cahuchu,” which meant “weeping wood.”
Early civilisations such as the Incas probably invented rubber. They made rubber by treating latex with sulphur and heat, and used it to water-proof cloth and produce shoe soles, hats, and tarpaulins. They also made rubber balls, ropes, cups, and bowls. By smoking latex impregnated cloth over a fire, they clearly ‘discovered’ how to add carbon to reduce sunlight degradation and the vulcanisation process to improve the physical properties of the material.
In 1876 seeds were taken from Brazil by Henry Wickman, a local entrepreneur, at the behest of Sir Joseph Hooker, an eminent Victorian botanist and director of Kew Gardens, where the seeds were subsequently germinated.
The germinated seeds were sent initially to Ceylon (Sri Lanka) and then seedlings to Singapore. By 1900, bud grafting techniques had been developed enabling large numbers of identical trees to be produced allowing the British in Malaysia and the Dutch in Indonesia to create plantations in vast areas of cleared rain forest.
The liquid Latex is often described as the sap of the Hevea tree but this is not strictly true since the sap runs deeper inside the tree, beneath the cambium layer. Latex runs in the latex ducts immediately outside the cambium in millions of capillary vessels within the soft outer bark. This distinction is crucial for the tapper since, if the cambium is cut, growth and latex production will be reduced.
All natural rubber originates in the Hevea tree, although it is technically possible also from the Ficus Elastica (indoor rubber plant in the UK)
Tapping of the tree involves removing a thin layer of bark in a downward spiral, say 300 mm long. This will eventually heal over, but by making successive cuts parallel to the first many such tappings are possible before returning to the area of the first cut. At the bottom of the cut a small cup is attached into which the latex flows to be collected a few hours later. The bark is left to heal usually for 2 or 3 days before the process is repeated. If raw material for rubber production is required the latex is allowed to coagulate in the collection cup (cup lump). However, for latex production, the latex must be kept in liquid form by the addition of a stabilising agent such as ammonia to the cup.
The English name "rubber" originated from the use of the natural substance as a pencil eraser that could “rub out” pencil marks.
Besides pencil erasers, early rubber was used for many other products, but these did not stand up well to extreme temperatures and became brittle in winter.
Properties of Rubber
For engineering applications we now only refer to the material that has been processed and vulcanised. The outstanding strength and resilience of natural rubber elastomers has maintained its position as the preferred material in many engineering applications. With long fatigue life, high strength and low cost without the need for re-inforcing fillers the only notable disadvantages are poor resistance to certain oils, solvents and ozone. However, some of these disadvantages are ameliorated chemically.
Rubber is very ductile, has good creep and stress relaxation resistance and elongation in excess of 1000% is possible. Within such extreme strain, the material remains elastic. In contrast, metals have very low strain below their elastic limit. Generally when a material is subjected to a load (stress) below its elastic limit, the deformation (strain) will be proportional (Hooke’s Law). This does NOT apply to rubber under either tension or compression. As a result, rubber does not have a constant modulus of elasticity except over relatively small strains
Processing within AVMR
Transfer injection moulding is usually employed as this is the most efficient method available for the production of rubber to metal bonded products. The rubber compound of the required mass is loaded into a heated chamber (approx 165°C) and then forced through a network of orifices into the heated mould cavities which have the mating metal parts accurately located. A range of such presses is available up to 500 tonnes clamping pressure. Smaller presses are available for small batch production permitting the use of lower cost tooling.
In its natural state, or as a processed compound, rubber is plastic, easily deformed and weak. In 1843, Charles Goodyear (of tyre fame) discovered that if you removed the sulphur from rubber and then heated it, it would retain its elasticity and other characteristics would be improved. This process which he patented on June 24th 1844 (Pat No 3633), called vulcanization made rubber waterproof and winter-proof and opened the door for a enormous market for rubber goods.
The material comprises randomly oriented (spaghetti – like) long hydrocarbon chains (long chain molecules) which are cross linked by heating (vulcanised) to achieve the properties required – flexibility, resilience, strength and abrasion resistance. Rubber processed in this way can be considered as a long chain thermoset.
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