823 Natural and synthetic rubbers

Synthetic materials which have been developed as substitutes for natural rubber and have been utilized for tyre construction are listed with natural rubber as follows:

b) Chloroprene (Neoprene) rubber (CR)

c) Styrene-butadiene rubber (SBR)

d) Polyisoprene rubber (IR)

e) Ethylene propylene rubber (EPR)

f) Polybutadiene rubber (BR)

g) Isobutene-isoprene (Butyl) rubber (IIR)

Natural rubber (NR) Natural rubber has good wear resistance and excellent tear resistance. It offers good road holding on dry roads but retains only a moderately good grip on wet surfaces. One further merit is its low heat build-up, but this is contrasted by high gas permeability and its resistance to ageing and ozone deterioration is only fair. The side walls and treads have been made from natural rubber but nowadays it is usually blended with other synthetic rubbers to exploit their desirable properties and to minimize their shortcomings.

Chloroprene (Neoprene) rubber (CR) This synthetic rubber is made from acetylene and hydro chloric acid. Wear and tear resistance for this rubber compound, which was one of the earliest to compete with natural rubber, is good with a reasonable road surface grip. A major limitation is its inability to bond with the carcass fabric so a natural rubber film has to be interposed between the cords and the Neoprene covering. Neoprene rubber has a moderately low gas permeability and does not show signs of weathering or ageing throughout a tyre's working life. When blended with natural rubber it is particularly suitable for side wall covering.

Styrene-butadiene rubber (SBR) Compounds of this material are made from styrene (a liquid) and butadiene (a gas). It is probably the most widely used synthetic rubber within the tyre industry. Styrene-butadiene rubber (SBR) forms a very strong bond to fabrics and it has a very good resistance to wear, but suffers from poor tear resistance compared to natural rubber. One outstanding feature of this rubber is its high degree of energy absorption or high hysteresis and low resilience. It is these properties which give it exceptional grip, especially on wet surfaces. Due to the high heat build up, SBR is restricted to the tyre tread while the side walls are normally made from low hysteresis compounds which provide greater rebound response and run cooler. Blending SBR with NR enables the best properties of both synthetic and natural rubber to be utilized so that only one rubber compound is necessary for some types of car tyres. The high hysteresis obtained with SBR is partially achieved by using an extra high styrene content and by adding a large proportion of oil to extend the compound, the effects being to increase the rubber plastic properties and to lower its resilience (i.e. reduce its rebound response).

Polyisoprene rubber (IR) This compound has very similar characteristics to natural rubber but has improved wear and particularly tear resistance with a further advantage of an extremely low heat build up with normal tyre flexing. These properties make this material attractive when blended with natural rubber and styrene-butadiene rubber to produce tyre treads with very high abrasion resistance. For heavy duty application such as track tyres where high temperatures and driving on rough terrains are a problem, this material has proved to be successful.

Ethylene propylene rubber (EPR) The major advantage of this rubber compound is its ability to be mixed with large amounts of cheap carbon black and oil without destroying its rubbery properties. It has excellent abrasive ageing and ozone resistance with varying road holding qualities in wet weather depending upon the compound composition. Skid resistance on ice has also been varied from good to poor. A great disadvantage, however, is that the rubber compound bonds poorly to cord fabric. Generally, the higher the ethylene content the higher the abrasive resistance, but at the expense of a reduction in skid resistance on ice. Rubber compounds containing EPR have not proved to be successful up to the present time.

Polybutadiene rubber (BR) This rubbery material has outstanding wear resistance properties and is exceptionally stable with temperature changes. It has a high resilience that is a low hysteresis level.

When blended with SBR in the correct proportions, it reduces the wet road holding slightly and considerably improves its ability to resist wear. Because of its high resilience (large rebound response), if mixed in large proportions, the road holding in wet weather can be relatively poor. It is expensive to produce. When it is used for tyres it is normally mixed with SBR in the proportion of 15 to 50%.

Isobutene-isoprene (Butyl) rubber (IIR) Rubber of this kind has exceptionally low permeability to gas. In fact it retains air ten times longer than tubes made from natural rubber, with the result that it has been used extensively for tyre inner tubes and for linings of tubeless tyres. Unfortunately it will not blend with SBR and NR unless it is chlorinated, but in this way it can be utilized as an inner tube lining material for tubeless tyres. The resistance to wear is good and it has a high hysteresis so that it responds more like plastic than rubber to distortion at ground level. Road grip is good for both dry and wet conditions. When mixed with carbon black its desirable properties are generally improved. Due to its high hysteresis tyre treads made from this material do not generate noise in the form of squeal since it does not readily give out energy to the surroundings.

8.2.4 Summary of the merits and limitations of natural and synthetic rubber compounds

Some cross-ply tyres are made from one compound from bead to bead, but the severity of the carcass flexure with radial ply tyres encourages the manufacturers of tyres to use different rubber composition for various parts of the tyre structure so that their properties match the duty requirements of each functional part of the tyre (i.e. tread, side wall, inner lining, bead etc.).

Side walls are usually made from natural rubber blended with polybutadiene rubber (BR) or styr-ene-butadiene rubber (SBR) or to a lesser extent Neoprene or Butyl rubber or even natural rubber alone. The properties needed for side wall material are a resistance against ozone and oxygen attack, a high fatigue resistance to prevent flex cracking and good compatibility with fabrics and other rubber compounds when moulded together.

Tread wear fatigue life and road grip depends to a great extent upon the surrounding temperatures, weather conditions, be they dry, wet, snow or ice bound, and the type of rubber compound being used. A comparison will now be made with natural rubber and possibly the most important synthetic rubber, styrene-butadiene (SBR). At low temperatures styrene-butadiene (SBR) tends to wear more than natural rubber but at higher temperatures the situation reverses and styrene-butadiene rubber (SBR) shows less wear than natural rubber. As the severity of the operating condition of the tyre increases SBR tends to wear less relative to NR. The fatigue life of all rubber compounds is reduced as the degree of cyclic distortion increases. For small tyre deflection SBR has a better fatigue life but when deflections are large NR provides a longer service life. Experience on ice and snow shows that NR offers better skid resistance, but as temperatures rise above freezing, SBR provides an improved resistance to skidding. This cannot be clearly defined since it depends to some extent on the amount of oil extension (plasticizer) provided in the blending in both NR and SBR compounds. Oil extension when included in SBR and NR provides similarly improved skid resistance and in both cases becomes inferior to compounds which do not have oil extension.

Two examples of typical rubber compositions suitable for tyre treads are:

High styrene butadiene rubber

31%

Oil extended butadiene rubber

31%

Carbon black

30%

Oil

6%

Sulphur

2%

Styrene butadiene rubber

45%

Natural rubber

15%

Carbon black

30%

Oil

8%

Sulphur

2%

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