Properties of Rubber

NATURAL RUBBER

(Polyisoprene)

The original natural material which has been in commercial use since the turn of the century. The most widely developed rubber with a huge range of compounds available. It also usually has the lowest price.

Natural rubber is an environmentally desirable material and comes from a naturally replenishable source. During its production as a tree sap (latex), it constantly absorbs carbon dioxide (a greenhouse gas) from the air. At the end of their working lives, the rubber trees are used to make furniture and are replaced with young trees for further production. Natural rubber itself is readily biodegradable and non-toxic.
Properties

• widest range of hardnesses
• very strong (naturally self-reinforcing) and extremely resilient
• good compression set
• good resistance to inorganic chemicals

Limitations

• lack of resistance to oil and organic fluids
• relatively low maximum temperatures (75°C continuous, 100°C intermittent)
• poor ozone resistance, with tendency to perish in open air (can be improved to some extent by careful compounding).

Typical Applications

• components which are protected from constant air changes - i.e. inside machinery - and which do not come into contact with any oil or oil based fluids
• applications requiring strength and resistance to abrasion
• sealing and shock absorption

EPDM

(Ethylene Propylene Diene Monomer)

Originally developed in 1950s for tyre applications. Became more widely used because of its suitability for outdoor use.

Properties

• the most water resistant type of rubber - also very resistant to most water based chemicals
• very inert structure, remains stable over long periods of time
• can withstand temperatures of up to 130°C for extended periods of time (months)
• very good weathering resistance
• easily compounded and processed

Limitations

• will not resist oil or oil based products
• compression set not as good as some other rubbers, but can be improved by careful compounding

Typical Applications

• general engineering without exposure to oil.

NEOPRENE*

(Polychloroprene)

One of the first synthetic rubbers developed in the search for oil resistant rubber. Widely used due to its combination of useful properties and comparatively low price.
Properties

• resistant to a wide range of hostile environments
• resistant to oils and chemicals
• weather and water resistant
• can withstand temperatures from -30°C to 95°C
• easy to process and compound, offering cost benefits
• flame retardant
• can be produced in any colour required

Limitations

• unsuitable for applications requiring contact with fuels
• tendency to tear once there is initial damage

· some Neoprenes may crystallise during storage or use causing temporary stiffening (increase in modulus/hardness). If parts are deformed during crystallization, they may take on a set. However, crystallization is a readily reversible phenomenon and can be removed by warming over 80°C. It can be prevented by the use of special grades.

Typical Applications

• most general mechanical applications without contact with fuel
• particularly useful in marine environments due to good ozone resistance.

HYPALON

(Chlorosulphonated Polyethylene)

Developed in the 1950s as a speciality rubber for rugged applications. Best described as a "super" Neoprene, with similar but better developed characteristics.

Properties

• resistant to oil and fluids, especially at higher temperatures (+125°C)
• extremely resistant to ozone and weathering - can withstand harsh outdoor conditions for up to 15 years

Limitations

• cannot be used where there is contact with fuels

Typical Applications

• situations where there is likely to be heavy weather conditions or exposure to hot liquids and/or gases

NITRILE

(Acrylonitrilebutadiene)

Another early development in the search for an oil resistant rubber. The most suitable rubber for applications requiring resistance to petroleum based fluids (there are rubbers with higher degrees of resistance but these are much more expensive).

Properties

• very good resistance to petroleum based fluids
• good high temperature resistance - up to 100°C (120°C with EV cure systems)
• economical to compound and produce
• very low level of permeability to gases

Limitations

• poor resistance to outdoor weathering without special compounding
• comparatively low strength
• flammable and burns with toxic fumes

Typical Applications

• sealing in enclosed spaces where there is contact with petroleum based fluids
• sealing against gases

SILICON

(Polymethlysiloxane)

Synthetic rubber with a wide temperature range and outstanding resistance to weathering. Characterised by clean, smooth appearance with good flexibility.

Properties

• wide temperature range
• extremely good resistance to weathering
• excellent electrical properties
• good resistance to oils
• easily coloured
• low level of toxicity

Limitations

• not a very strong material
• poor resistance to fuels
• expensive compared to other rubbers

Typical Applications

• anywhere where complete resistance to weather/fluids is required
• electrical applications

FLUOROCARBON

(Viton (R)*)

The best material for resistance to hostile chemical and oil environments at normal and elevated temperatures.

Properties

• strong
• good resistance to water
• good resistance to fuels, oils and most chemicals

Limitations

• limited use at low temperatures, -20°C being the limit for flexibility
• expensive
• does not resist Ketone solvents

Typical Applications

• situations requiring resistance to hostile fluids at high temperatures
• Viton (R) is a registered trade mark of Du Pont

SBR

(Styrene Butadiene Rubber)

A synthetic rubber which is easy to process in large quantities. Widely used in the footwear and tyre industries.

Properties

• good physical strength
• good tear and abrasion resistance
• range of colours
• one of the cheaper rubbers

Limitations

• does not resist oil or fuels
• prone to weathering

Typical Applications

• non-mechanical high-volume products such as shoe soles and heels or car tyres.