Introduction to Rubber
What do we mean by the word ‘Rubber’?
In this Rubber Technology series we define it as ‘a material that can be stretched or compressed and when the force is removed, will return quickly to its original shape’ without permanent deformation.
The main chemical building blocks of rubber are elastomers, or “elastic polymers.” These are large chainlike molecules, which when cured (vulcanized) form chemical crosslinks between the polymer chains. The ability of the chains to stretch and spring back is a measure of the rubber’s elasticity and resilience under load. The first common elastomer was Polyisoprene, from which natural rubber is made.
In general, rubber materials are characterised by their low modulus of elasticity. They are resistant to water, alkalis and weak acids. It is also a good electrical insulator, and can be used as a bonding agent.
There are two types of rubber: natural and synthetic. Natural is produced from the latex (milky juice) of the Hevea Brasiliensis tree. It is, therefore, a renewable resource unlike synthetic, which is manufactured from petrochemicals.
Natural rubber is harvested as latex – a natural source of isoprene, called caoutchouc or India rubber – which is dried out for commercial processing. This dried out form of natural rubber is the chemical Polyisoprene. Although this has some elasticity, it is normally mixed with chemicals and vulcanized to produce the finished product. Natural rubber is both elastic and viscous making it an ideal polymer for dynamic and static engineering applications.
Natural properties include:
- Ease of processing
- Excellent dynamic performance with a low hysteresis loss
- Limited high temperature resistance (max. +70C)
- Good low temperature properties (-50C)
- Ability to bond strongly with metal parts
- High resistance to tear and abrasion, can be self-healing
- Poor resistance to sunlight, oxygen, ozone, solvents and oils
Natural rubber has the following advantages over synthetic rubber:
- Good dynamic performance
- Low level of damping
- Excellent resistance to heat buildup during flexing
- Greater resistance to tearing when hot
Despite the availability of specialist synthetic rubbers, natural rubber retains a 30-40% market share, and performs well in many applications such as:
- Gaskets, seals and rolls
- Electrical components
- Hose and tubing
- Vibration isolators
- Shock mounts
- Drive couplings
Synthetic rubber is manufactured from oil by-products using either solution or emulsion polymerisation techniques. By applying polymer chemistry techniques, they allow greater opportunities for customization of physical properties and ability to resist temperature, chemicals and solvents, than are possible with natural rubber.
The properties of synthetic rubber are far wider than those of natural rubber. There are over 20 different classes of synthetic rubber giving a far wider choice of rubbers to meet the desired mechanical properties, and temperature and chemical resistance, required for a specific application.
The major classes of synthetic rubber are:
- Styrene-butadiene copolymer (SBR)
- Nitrile elastomers (poly (butadiene-acrylonitrile) NBR / HNBR
- Butyl (IIR)
- Neoprene (polychloroprene) (CR)
- Ethylene-propylene (EPDM)
- Urethane elastomers (EU)
- Silicone rubbers VMQ)
- Fluoroelastomer (FKM)
- Fluorosilicone (FVMQ)
Synthetic rubber can have the following advantages over natural rubber:
- More resistant to oil, certain chemicals and oxygen
- Better aging and weathering resistance
- Resilience over a wider temperature range
If you would like further information on rubber, it’s properties or the products manufactured by Metflex please contact us on 01254 884 171.