The Language of Rubber – Part 2 (Permanent Set)
If the deformation is caused by tension, it is known as Tension Set.
On the other hand if it is caused by compression, it is described as Compression Set.
Permanent Set is therefore defined as the amount of residual displacement after the distortion load is removed. Permanent Set is worse at elevated temperatures.
Generally, the lower the permanent set, the higher would be the resilience and elasticity but the lower would be the tensile strength although there are exceptions.
A poorly or insufficiently vulcanized rubber would usually exhibit high permanent set. Choice of the vulcanizing system also plays an important part in determining the degree of permanent set. If the lowest possible permanent set is required for certain applications especially at elevated service temperatures, the sulphur to accelerator ratio should be low and sometimes the elemental sulphur is totally omitted in the compounding recipe.
Permanent set is closely related to another important property of rubber products, namely the Stress Relaxation which would be discussed in Part 3.
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