The Language of Rubber Properties and Behaviour – Permanent Set

Manufacturer: We are a group of university students from India. Can you please explain in simple language what "Permanent Set" of rubber means? We've been arguing this with our supervisor. We hope your explanation can put to rest this argument. Sorry to trouble you.

John Woon (Senior Latex Consultant): When rubber is distorted for a given duration, the degree of deformation after the release of the distortion force is referred to as the 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.

Go to the following link for the video on how Compression Set test is carried out:

(The Rubber samples are put into the fixture and spacers are used to compress to the appropriate size. The samples are then left for a given period at room temperature or elevated temperature. After the test is complete the height of the rubber samples is measured and the before-and-after heights are compared)

https://tinyurl.com/y9l5pget  or http://bit.ly/2CFZ2jT