Natural Rubber (NR)
This is the original rubber and in many ways is an ideal polymer for dynamic or static engineering applications. It has excellent dynamic properties, with a low hysteresis loss, and good low temperature properties, it can be bonded well to metal parts, has high resistance to tear and abrasion and it is relatively easy to process. It also has excellent low temperature properties (with a Tg of approximately -70°C).
Unfortunately it has a relatively high reactivity with its environment, with oxygen and particularly ozone. Ozone causes surface cracking that can rapidly penetrate the component when even a low threshold value of tensile stress is applied. However, in components of fairly large cross sectional area, whilst there may be extensive surface reaction, depending upon the external stress pattern, actual penetration of the oxygen and ozone can be low, with the inside being protected by the degraded exterior.
Articles in shear or compression remain unaffected provided that the surface itself does not enter a tension mode. This property can be ensured by design. One hundred year old seals from Victorian water and drainage systems demonstrate this very effectively as the seals still function. In tension, ozone cracking can propagate quite rapidly through an otherwise satisfactory sample. This means that the lives and performance of thin and thick items made of the same material in the same environment can be very different.
Oils and Solvents
Attack by contact with oils is usually restricted to a thin surface layer due to slow diffusion rates. Lighter solvents will attack the rubber more rapidly, with actual rates dependent on the type of solvent and the type of rubber. Both oils and solvents will cause a loss of physical strength, with thin articles being the worst affected.
Advantages of Natural Rubber
The major advantage of Natural Rubber, which makes it dominant in many engineering applications, is its dynamic performance. It has a low level of damping, and its properties remain fairly constant over the range 1 to 200Hz, and show only slight increase to 1000Hz. Its combined dynamic properties generally out perform any synthetic rubbers or combinations available to date. Despite proliferation of general and special purpose synthetics, Natural Rubber still holds a significant market share between 30 and 40%.
Although Natural Rubber, with the benefit of modern compounding, is very satisfactory for many applications, it is also a strategically important material, a natural crop only produced in tropical countries and has relatively poor ageing properties. Therefore synthetic materials have been developed to replace Natural Rubber in a wide range of applications.
There is now a wide range of synthetics available able to cope with high and low temperatures, contact with fluids of various types (including at high pressures), and aggressive or corrosive environments.
The main Synthetic Rubbers are outlined below.
Styrene Butadiene Rubber (SBR)
A general purpose rubber, which, when compounded with carbon black, behaves similarly to NR (Tg is higher at about -55°C).
Butadiene Rubber (BR)
A non-polar rubber like NR and SBR, with a very low Tg (approximately -80°C). Very high resilience (very low loss) rubber used in ‘superballs’, but also commonly used in combination with NR and SBR in long life rubber tyre treads. Difficult to process unless blended with another elastomer.
Chloroprene Rubber (CR)
A polar polymer with improved resistance to attack by non-polar oils and solvents. It has high toughness, good fire resistance, good weatherability, and is easily bonded to metals.
(Acrylo) Nitrile Butadiene Rubber (NBR)
A variation of the Acrylonitrile (ACN) content from 18 to 50% controls polarity and other properties. High resistance to non-polar oils and fuels (e.g. used in seals, fuel lines, hydraulic pipes) but high Tg. Improved versions of this much used polymer are becoming available.
Iso Butylene Isoprene (Butyl) Rubber (IIR)
This material has a low Tg but has very little ‘bounce’. It has excellent ageing properties and has a very low permeability to gases, so it is often used as a tubeless tyre liner, as well as for reservoir linings and other membranes. Chemically modified forms are frequently used.
Ethylene Propylene Rubber (EPDM or EPR)
This is a commonly used non-polar rubber in applications that require good ageing properties, such as in heater and radiator hoses, car door water and draught seals. The structure of the polymer can be altered to give a fairly wide range of properties and uses.
Other more expensive varieties are generally designed to increase the working temperature range, especially at the high end, and usually contain chemical elements such as fluorine to increase the stability of the carbon backbone.
Silicone rubber is unique in not having a carbon backbone, being –Si-O-Si-O-, and this extends the useful temperature range noticeably. It has a Tg as low as -127°C depending on type, and can be used in service at temperatures of 200°C or more for several years. Further modification with fluorine will give even better performance. Several other special purpose rubbers are available, including polyurethanes.
Chloroprene rubber, an early synthetic rubber, has been used in many outdoor applications due to its superior weathering properties and oil resistance. It performs well compared with Natural Rubber in many ways but can suffer from long term stiffening (change in properties) and its low temperature performance is not as good as Natural Rubber.