Why is Compression Set measurement important?

Manufacturer: We are manufacturing some new rubber parts for engineering applications using SMR rubber from Malaysia. Part of the specification ask for very low compression set values. Can you please explain briefly the term "set" and how best could we achieve a low value?


JohnWoon: After having been deformed under force by either compression or tension for a period of time, most finished rubber products would almost recover to their original shape and size if the force is released and taken off.

As we all know, such recovery is not 100%. Some permanent deformation is to be expected and this is referred to as the "permanent set" because the rubber has "set" itself to the deformed state. It is known as "compression set" if the rubber is under compression such as bridge bearings and "tension set" when the rubber is under stretch or tension as in the case of rubber bands, rubber threads and beltings.

There are National and International Standards for measuring these two properties. For applications such as those mentioned above, this measurement is very important for obvious reason.

The degree of the "set" could be controlled with judicious design of the curative recipes. For maximum reduction of the permanent set, you should use a recipe design without elemental sulphur as far as possible. Sulphur-donors could be used in place of free sulphur. At the same time, you should aim at achieving as high a cure state as practically possible as this would ensure a high degree of crosslinking which is
also one of the prerequisites for low permanent set.

The downside of this approach is a loss of tensile strength although the heat ageing resistance would be enhanced. Hence a compromise should be reached.

Perhaps I should point out that there is another type of set caused by low temperature crystallisation during the service life of the rubber. The recipe approach I recommend above could in fact deteriorate this type of set. I am of course assuming that you do not have this problem as your products are being used at room or elevated temperatures ?

The Joule Effect

Glove Consumer: I wonder if others have experienced what I have with the donning of latex medical gloves. If I stretch the glove to some extent in the process of donning it, I feel an obvious warming of my skin by the glove. But when I release the stretch to allow it to retract immediately, I could feel a cooling sensation.

Have you experienced this yourself? Can you explain this unexpected observation of mine?

JohnWoon: Yes, I've experienced this indeed but I expected it. This is what rubber technologists refer to as the "Joule Effect".

When natural rubber is stretched, before or after vulcanization, it produces heat and becomes warm as opposed to some metals which tend to cool when stretched. Also, a stretched rubber is expected to retract on heating i.e. when it absorbs heat.

The effect is more pronounced if you stretch the glove more quickly.

Some early reports speculated that as rubber was stretched, the orientation and orderly alignment of the rubber molecules coupled with the inherent van der Waals forces resulted in the immobility of the rubber molecules. Hence the mobility could only be reversed as the temperature increases. The deduction was that if rubber is elongated quickly it releases heat and as the stretching is quickly terminated, it absorbs heat, hence your reporting of the "cooling" sensation.

Having said this, I'd like to draw your attention to the fact that at low elongation, you might actually get a cooling effect. Also, the heat evolved as you've observed has nothing to do with the evolution of heat through repeated cycle of flexing (deformation) of rubber. In the latter case, the heat is a result of "hysteresis". Let me know if you don't understand the meaning of this term.