High Temperature Effects
All rubber is subject to deterioration at high temperature. Volume change and compression set are both greatly influenced by heat. Hardness is influenced in a rather complex way. The first effect of increased temperature is to soften the compound. This is a physical change, and will reverse when the temperature drops. However, it must be considered in high pressure applications because a compound that is sufficiently hard to resist extrusion at room temperature may begin to flow and extrude through the clearance gap as the temperature rises, due to this softening effect.
With increasing time at high temperature, chemical changes slowly occur. These generally cause an increase in hardness, along with volume and compression set changes as mentioned above. Changes in tensile strength and elongation are also involved. Being chemical in nature, these changes are not reversible.
Parker has applied a realistic temperature range with a margin of safety when setting the general operating temperature range for seal compounds. The maximum temperature recommendation for a compound is based on long term functional service. If it is subjected to this temperature continuosly, it should perform reliably for 1,000 hours. Time at less than maximum temperature will extend life. Similarly, higher temperature will reduce it.
The high temperature limits assigned to compounds in the figure below are conservative estimates of the maximum temperature for 1,000 hours of continuous service in the media the compounds are most often used to seal. Since the top limit for any compound varies with the medium, the high temperature limit for many compounds is shown as a range rather than a single figure. This range may be reduced or extended in unusual fluids.
Since some fluids decompose at a temperature lower then the maximum temperature limit of the elastomer, the temperature limits of both the seal and the fluid must be considered in determining limits for a system.