When a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time.
How to Determine if a Physical or a Chemical Process is responsible for the Relaxation?
At normal or low temperatures and/or at short time applying stress, the stress relaxation is dominated by physical processes. At high temperatures and/or at long time applying stress, it is dominated by chemical processes. The figure below presents a curve, showing the stress applied and its relaxation over time.