Occurring in both static and dynamic seals, O-ring hardening is chiefly caused by exposure to high temperatures. Hardening results when exposure extends for a period sufficient to 1) cause additional cross-linking among the material’s macromolecular chains, 2) evaporate plasticizers in the compound, and 3) promote oxidation. Hardening of an O-ring in service dramatically reduces its resilience, and, as a result, severely limits its ability to act as an effective seal.

Unwanted cross-linking of the rubber can also be caused by chemical attack. For example, sulfur-containing compounds can cause nitrile rubber to cross-link. Amine exposure will lead to cross-linking and hardening in fluorocarbon elastomers.

Progressive hardening of the seal has two phases: surface cracking and/or pitting, followed by hardening of the entire cross-section. Compressed seals will also undergo high degrees of compression set as they harden. Figure 133 is an example of O-ring failure due to hardening.

Lowering the system’s operating temperature will help avoid or correct this problem. Use of materials that can withstand higher temperatures and that are resistant to chemical attack will also be beneficial.