Value of Rubber
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 (well above and below the temperature ranges of plastics and other elastic materials).
The rubber family includes a diverse range of materials - as varied as metals. Currently we use 250 grades of rubber across 14 different polymer types (each as different as lead, iron, copper, silver or gold).
A hundred years of research and development cannot be fully covered in a short guide, but the main properties are summarised 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 insulatorIt 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.
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 a wide range of rubbers have been developed to resist most fluids and chemicals. Rubbers have to be carefully selected, formulated and tested to ensure safe and predictable service lives.
Major Materials
- For moderate resistance to oils and fuels - Neoprene, Hypalon®
- Good resistance to oils and fuels - Nitrile, Acrylics, Silicones
- Extreme resistance to many chemicals - Acrylics, Fluoroelastomers, 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.
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 again 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, 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 & Softness
The property of hardness is easily recognised, but in design terms 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 wood or nylon. For rough 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, including IRHD and is described in more detail here.
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 from 30-90° Shore A. Lower and higher hardnesses require special compounding.
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. True rubbers maintain this elasticity (and ability to seal) across a wide range of temperatures.
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. Rubber is thermoset and resists permanent deformation across a wide temperature range. Considerable care should be taken when considering thermoplastic rubbers (plastics with rubbery properties rather than true rubbers) which relax significantly under stress, particularly at temperatures over 70°C.
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. Most antistatic or conductive rubbers are based on carbon incorporated into the rubber. Harboro Rubber has the only known coloured antistatic rubber which was developed in its laboratories.
Typical Applications
Conductive rubber is used in electronic equipment for switching, keypads and continuity as well as static dissipation. Insulating rubbers are used extensively in electrical termination and switchgear components, grommets and weather seals. Anti-static rubbers are commonly used in operating theatres, explosive factories and electronics manufacturing areas.
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.
Resiliance & 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 which gives over 90% bounce - while a low resilience material has a low rebound, giving a “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 parts - buffers, elastic bands and sports equipment - to the more complex parts such as car suspension systems or keypads, 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.
The ability to deform or extend greatly and to return quickly to its original shape is what distinguishes a rubber from a plastic.
Weather Resistance
Suitable rubbers will retain their properties indefinitely under all weather conditions - hot, cold, dry, wet or humid. Many can be quoted for a 50 year life exposed to weather.
Major Materials
EPDM rubbers have exceptional resistance to water and ozone attack while Silicones are unaffected by extremes of weather. Hypalon® and Fluoro based rubbers also resist weather indefinitely.
Temperatures lower than the minimum will always stiffen the material
Testing the elasticity of rubber
