Introduction
Rubber is a generic word referring to a number of different types and families of polymeric materials.
In the USA, but not in other English-speaking countries, the word is synonymous with 'condom'.
Most materials described as 'rubber' have the following attributes in common.
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Soft (typically below 70 in the Shore D hardness range)
- Stretchy (typically with elongation at break ranging between 50 percent and 500 percent)
- Springy (compression set typically in the range 30 percent or less).
In addition, most rubbery materials are thermosetting—they are cured in a chemical reaction called vulcanisation, and on subsequent exposure to heat will degrade, burn, or otherwise become denatured. This contrasts with thermoplastic polymers, which can typically be melted and frozen repeatedly at temperatures in the 100°C to 200°C range.
Confusion often arises between the cured and uncured state of rubber. Most people only encounter cured rubber compounds in the form of finished products (tyres, condoms, balloons, hoses, conveyor belts, etc). In all such products, the pure rubber polymer has been mixed with other ingredients, such as fillers (carbon black, silica, etc), curing agents (sulphur, peroxide and others), and anti-degradents, such as anti-oxidants, and anti-ozonants to make a compound. Typically, rubber polymer represents around 50 percent by weight of such compounds, but in some highly-filled compounds, rubber polymer can contribute as little as 20 percent of the total weight. Dipped latex goods (gloves, condoms and balloons) are close to 100 percent rubber polymer, but they also include a curing package and some anti-degradents.
Therefore it is often misleading to speak of a rubber component being made from polyisoprene, or SBR or any of the other rubber families (see below). It is more correct to say it is made from a compound based on that polymer.
The broad families of rubber polymer fall into two clear groups: natural rubber (NR) and synthetic rubber (SR). NR originates as latex in trees cultivated for the purpose. Almost all (99.9 percent) of these are the species Hevea Brasiliensis, and the vast majority (some 90 percent) are located in south- and south-east Asia. Thailand is the biggest producer of NR in the world, followed by Indonesia, Malaysia, India, China, Vietnam, Sri Lanka and a number of other countries, all of which lie within a band lying from 5° to 15° north or south of the equator. The world produces around 6.5 million tonnes of NR each year.
Hevea trees are typically cloned, with the rootstock, trunk and crown each coming from a different genetic stock. Thus the agronomic value of the crop can be maximised by having a root structure which is resistant to infection; a trunk which is either fast-growing (for maximum biomass) or high-yielding (for latex production) and a fast-growing, dense crown, to generate the energy and absorb the oxygen necessary for rapid growth.
Natural rubber comes in many grades, but the most important distinction is between latex (the liquid which comes out of the tree) and solid grades. Solid grades are made from latex which has coagulated either in the field or in a factory. Latex for the manufacture of condoms and other dipped goods is stabilised with ammonia, and then centrifuged until the solids content reaches around 60 percent.
Synthetic rubber is made from oil in large chemical plants. The world makes around 11.5 million tonnes of SR each year, with the USA being the largest producer by far, followed by China, the EU and Japan.
Within synthetic types, there are a lot of different varieties, but the most common are as follows:
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- Styrene-butadiene rubber (SBR)*
- Polybutadiene rubber (BR)*
- Butyl rubber (IIR)(with a sub-group halo-butyl—CIIR, BIIR)*
- Ethylene-propylene diene monomer (EPDM)
- Acrylonitrile rubber (NBR) and the hydrogenated kind (HNBR)
- Polychloroprene rubber (CR)
- Polyisoprene rubber (IR)
And a whole dictionary of others, including fluoro-elastomers, silicones, polynorbornenes, acrylic rubber, chlorosulphonated polyethylene etc. Visit http://www.IIRSP.com if you want a full list
Anything above marked with an asterisk (*) is used primarily in tyres.
Application areas
The main application areas for rubber are as follows:
- Tyres (truck, bus, car, aircraft, off-roaders, agricultural, 2-wheelers, and implement tyres)
- General automotive (hoses, belts, door seals, oil seals, gaiters and boots)
- General industrial (more hoses, belts, seals etc)
Interesting applications include:
Earthquake bearings, which isolate the structure of a building from the ground below, and allow the building to remain nearly stationary, even though the ground may displace by up to 500mm during an earthquake.
Condoms: made thick or thin, and in different sizes (yes, and with flavourings and texture). Although the industry tries to avoid selling condoms directed at specific user groups, it does market extra-thick condoms, designed to provide reliable protection during particularly energetic use. Market research suggests the main consumers of these products are gay men, but the industry is reluctant to talk about this market. And, err, size does matter. Condoms produced for the Asian market are a little smaller than those produced for the US and European markets, but the marketing people advise against marking the packs 'large' 'medium' and 'small'
Oven gloves: Yes, those square pads of rubbery stuff that you can use to take hot dishes out of the oven really are rubber. Made from silicone rubber, they can withstand temperatures up to 250°C easily, and better quality ones will go up to 300°C sustained use before they start to disintegrate.