The Language of Rubber - Part 3 (Creep, Stress Relaxation)

When subjected to a constant stress, rubber (and most elastomers) gradually increases in deformation over time. This is referred to as "Creep". When the deformation or strain is held constant over a period of time at a given temperature, the force or stress required to maintain this deformation gradually reduce.

This gradual decay or loss of stress is referred to as the "Stress Relaxaton". The higher the Permanent Set (discussed in Part 2), the higher would be the Creep and hence the Stress Relaxation. It is usually expressed as the percentage of the remaining stress after a given time and at a given temperature.

Stress Relaxation would usually lead to product failure, for instance in the case of a rubber sealing ring, the rubber works by virtue of its force against the flanges of the pipe between which the seal is being sandwiched and compressed. This force would fall when the Stress Relaxation is excessive resulting in the leaking of the fluid from the pipe.

As in the case of permanent set, the degree of cross-linking plays an important part in stress relaxation and under-vulcanisation would make it worst. On the other hand the so-called "efficient vulcanisation" system would give lower creep.

Such usually unwanted inherent creep or stress relaxation is taken advantage of by certain Nitrile examination glove manufacturers to claim that the gloves could "relax" and conform to the hands of the wearers after a short period of donning. However if the fingers are in constant movement during use, an excessive stress relaxation would result in poor tactility which is a prerequisite of a good quality glove.

Careful selection and maintaining of the level of Budadiene in Nitrile rubber and its (butadiene's) cross-linking could minimise the degree of stress relaxation.

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