Rubbers have found many applications in industrial and consumer goods because they are the only group of materials able to provide elastic properties across a wide range of temperatures.
The rubber family includes a diverse range of materials - as varied as "metals" or "plastics".
A hundred years of research and development cannot be fully covered in a small guide, but the main properties are summarized below with examples of actual applications.
Designers choose rubber because of its wide range of properties:

• It can be used over a temperature range from -80°C to +300°C

• It is available in a wide range of colours

• It can be electrically insulating, conductive or anti static

• It can withstand extremes of weather and outdoor environments indefinitely

• It can withstand exposure to fuels, oils and chemicals while retaining its properties

• It can be made flame retardant and self extinguishing, with halogen free and smoke suppressant types available

• It can maintain tension and compression forces indefinitely - for example in seals

• It is conformable, adaptable and can accommodate movement, shock, thermal changes tolerances and roughness

• It can absorb vibration and noise and act as an insulator

• It can be gas tight and used as a fluid seal or separator

• It has low thermal conductivity and can be used to reduce heat transfer

• It has friction properties similar to human skin and is comfortable to grip

• It can have a clean, smooth surface which is non-stick and suitable for hygienic applications

• It is compatible with other engineering materials (e.g. metals, plastics and ceramics) and can be combined with them in many different ways, including bonding.
   

Many of these properties can be combined by suitable compounding, although no single material is "best" in every aspect. Some properties are only available in one type of rubber.

Let us look at some of these properties in more detail. (Shortcut to Rubber Data Table)

RESISTANCE TO HOSTILE ENVIRONMENTS

The development of synthetic rubbers stemmed from the need to create materials with greater resistance to fuels and oils. Aggressive chemicals, hydraulic oils, food substances and refrigerants all have to be contained and rubbers have to be carefully formulated and tested to ensure safe and predictable service lives.

Typical Applications

Rubber is used for seals and gaskets in almost any chemical environment and for mechanical components in machinery of all kinds. It is also suitable for parts which must be reasonably resistant to normal contaminants, such as printed circuit board components which will be solvent cleaned.
 

Major Materials

• For moderate resistance to oils and fuels - Neoprene, Hypalon

• Good resistance to oils and fuels - Nitrile, Viton (R), Silicones

• Extreme resistance to many chemicals - Viton (R), Acrylic, Fluorosilicones

• For a comprehensive list of suggested rubber types for resistance to named chemicals, consult ISO Technical Report 7620 or the Fluid Sealing Association Technical Handbook.

Examples of Components

Nitrile rubber bearing cup to insulate the commutator bearing of electric motors. This material must withstand high temperature, ozone and grease, as well as copper and carbon dust.

 

WEATHER RESISTANCE
Suitable rubbers will retain their properties indefinitely under all weather conditions - hot, cold, dry, wet or humid.

Typical Applications

The combination of weather resistance and the ability to maintain sealing forces indefinitely makes rubber highly suitable for electronic instruments seals. Examples include case gaskets, shaft and cable seals, bellows and operating keypads. Rubber components are also used in the harsh environments of marine applications, sealing delicate electronics against the elements.
 

Major Materials

EPDM rubbers have exceptional resistance to water and ozone attack while Silicones are unaffected by extremes of weather.
 

Examples of Components

Silicone rubber seal used to waterproof a submersible electronic compass, operating at depths of over one metre.

EXTREMES OF TEMPERATURE
Apart from Silicone, rubbers are essentially hydrocarbon materials and perform within a limited range of temperatures. Where working temperatures are quoted, these represent the range within which the rubber's properties are maintained more or less indefinitely. Temperatures lower than the minimum will always stiffen the material (although it will relax as the temperature rises) and extremely low temperatures may turn it brittle. Temperatures higher than the maximum will degrade the rubber, ultimately destroying it.

Typical Applications

Where service temperatures are known, the best types of material can be selected to provide adequate life under those conditions. Temperature guidelines are provided in the Data Chart (see pages 14 & 15), covering the range from - 80°C to + 300°C.

In vehicles, under-bonnet components are required to perform reliably in a high temperature environment while being exposed to hot oil, brake fluid and other chemicals. In other countries, the same components must function even when subjected to high wind chill factors - in Scandinavia for example sometimes reaching - 50°C.

Examples of Components

Furnace rod control seals, operating at continuous temperatures of 250°C.

Telescope eyepieces which must remain flexible and comfortable even in Arctic conditions.

HARDNESS AND SOFTNESS
The property of hardness is easily recognised, but in design it must be specified to achieve a given objective.
Solid rubbers range from 20° to 98° Shore A, where 20° is extremely soft like foam and 98° is as hard as bakelite or nylon. As a reference, the ball of the human thumb is 25°, a Staedtler white rubber eraser 55° and a bath plug 95° Shore A.

The hardness of rubber is measured in a number of ways, described in more detail on page 24.

Typical Applications

Designers use rubber in its whole range of hardnesses and each application has to be individually considered. Once a mould has been produced, it is relatively easy to make the same part in other colours and hardnesses to suit different functions.

Whatever the hardness required, it may still be necessary for a rubber component to deform in order to seal against an uneven surface or to resist abrasion.

Major Materials

All rubber types can be compounded to cover most of the range of hardnesses.

Examples of Components

Hardness is required in a part designed to grip paper rolls. It must resist abrasion and not distort in operation. Conversely, rubber suckers used to lift paper sacks have to be very soft to conform to the rough and porous surface.

ELASTICITY

The ability to expand greatly and to return quickly is what distinguishes a rubber from a plastic. This property not only makes possible the catapult but also allows designers to use rubbers to supply constant forces, either in tension or compression.

Typical Applications

High quality rubber compounds will remain elastic for their full design lives, virtually irrespective of the movement cycles they undergo. However, all rubbers will relax to some extent under constant deformation and this should be specified if significant.

Where rubber is to be used continuously in tension, consideration should be given to the effects of failure and trials carried out as required.

Major Materials

All rubber types are elastic. Natural rubbers are tough and strong but may have limited life if exposed to ozone or sunlight. Thermoplastic rubbers generally have lower elasticity and the softer grades relax when deformed, giving rise to permanent set.

ELECTRICAL PROPERTIES

Rubbers can have a wide variety of electrical properties (including piezo electric and magnetic) and by suitable compounding can be made highly conductive or totally insulating.

Typical Applications

Conductive rubber is used in electronic equipment for switching, touchpads and continuity as well as static dissipation. Insulating rubbers are used extensively in electrical termination and switchgear components, grommets and weather seals.

Major Materials

All types of rubber can have varied electrical properties and a wide range of compounds can be produced for different applications. Silicone rubbers can be made highly conductive by adding silver particles or, more normally, carbon.

RESILIENCE AND ENERGY CONTROL

Resilience is the property of absorbing energy by deformation and returning a proportion of it on rebound. Depending upon the rubber type and compound, some of the energy will be converted into heat within the material. A high resilience material returns almost all the energy - for example a superball - while a low resilience material has a low rebound, "dead" feel, such as a squash ball or high performance tyre.
 

Typical Applications

Rubbers have always been used for energy control purposes. These range from the simple - buffers, elastic bands and sports equipment - to the complex, such as car suspension systems or keyswitches, where rubber provides that delicate, precise "feel".

Rubber is also valued for its vibration control. It is extensively used in flexible couplings where rubber "spiders" allow misalignment, reduce jamming and have the resilience to damp out vibration.
 

Major Materials

All rubber types can be used for energy control and can be compounded to vary their fundamental resilience to the exact requirements of the designer. Fine tuning of the characteristics can be achieved by small changes to the shape of the moulding.
 

Examples of Components

Keypads which have to be designed and moulded to the closest tolerances in order to achieve precise force/travel characteristics over millions of operations.