(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.

ypical 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

ACRYLIC

(Polyacrylic)

A synthetic rubber which is particularly resistant to hot oil.

Properties

• excellent resistance to oxidation and weathering

• particularly resistant to oil at high temperatures

Limitations

• low temperature applications limited to -20°C

• poor chemical resistance to acids and bases

• very low resilience below 70°C

Typical Applications

• anywhere where resistance to hot oil or fuel is required.

VAMAC

(Ethylene Acrylic)

Provides good oil resistance over a wide temperature range. More expensive than Nitrile but considerably cheaper than Silicone.

Properties

• good resistance to oxidation and weathering

• good oil resistance

• strong and abrasion resistant

Limitations

• no resistance to fuels

THERMOPLASTIC RUBBERS (TPRs)

e.g. Santoprene

A proprietary brand of EPDM and Polypropylene, readily available in a range of hardnesses. Flame retardant and coloured grades can also be obtained.

Properties

• easy to process

• reasonable resistance to fuels and oils

• available in hardnesses ranging from 60° to 99° Shore A

Limitations

• limited maximum usable temperature
  Actual maximum temperatures depend upon the properties required - only the softer grades are "elastic". At temperatures over 80°C Santoprene cannot take stress since it softens and creeps, leading to permanent distortion. Other TPRs have even lower softening temperatures. Advice should be sought if the parts are load bearing (significant tension or compression).

Typical Applications

• Vast range of flexible parts used at room temperature e.g. Gaiters, handles, covers etc.

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

FLUOROSILICONE

The best rubber to use in hostile environments where fuel, oil, chemicals or low temperatures are encountered.

Properties

• performs excellently in the temperature range -60°C to 200°C

• resistant to oils and fuels (particularly at low temperatures)

• good electrical strength

Limitations

• not physically strong

• very expensive

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

SPECIALIST ELASTOMERS

THERBAN

(HNBR)

Hydrogenated nitrile rubber provides good all round performance at a compound cost between Nitrile and Fluoroelastomer. Its highly saturated main chain provides good resistance against thermal oxidation and chemical attack.

Properties

• good physical properties, including abrasion resistance, at high temperatures

• good dynamic behaviour and flex cracking resistance at elevated temperatures

• excellent heat, ageing and ozone resistance

• outstanding resistance to steam and hot water

Limitations

• no inherent flame retardency

• poor electrical properties

• unsuitable in contacts with aromatic and polar organic solvents

Typical Applications

• diaphragms requiring chemical and heat resistance

• chain tensioners and seals in vehicle engines

• oil exploration and production.

EPICHLOROHYDRIN

A synthetic rubber designed for more extreme heat and oil resistance applications. Best described as a "super" Nitrile.

Properties

• very good high temperature resistance, up to 150°C

• good low temperature properties

• good ozone resistance

Limitations

• expensive

• may require higher cost tooling

• relatively low physical strength

Typical Applications

• automotive applications such as fuel systems

• printing rollers

LIQUID SILICONE

An addition cured Silicone which is processed in modified plastic injection moulding presses. Liquid Silicone is similar in properties to normal Silicone but varies in its processing characteristics. It is purchased as a two part raw material with a viscosity similar to Vaseline. The two materials are pumped from a drum through a mixing head and injected into the cavities. Very low pressures are involved and very fast cures (typically 20-45 seconds).

Properties

• Similar to those of normal Silicone rubber, plus:

• full range of hardnesses can be achieved

• extremely clean process enabling colour matching

• fast cycles and low material usage offers significant cost advantages on long runs

Limitations

• lower strength, particularly where harder grades are used

• tooling is very expensive (up to 10x normal rubber tool cost) due to low viscosity and the need to remove air and prevent flash

• short runs are not viable due to the difficulties of setting up. Liquid silicone should only be considered for long runs, particularly where the weight to part size ratio is low

Typical Applications

• baby bottle teats

• thin diaphragms

• syringe plungers