Factors Affecting High Temperature Relaxation Behaviors of Expanded PTFE Gaskets

High temperature behaviors of expanded PTFE gaskets have been studied using the hot blow-out test (HOBT) method. The results suggest that the operating stress and temperature measurements of a tested gasket form a linear relationship on a logarithmic plot, and that higher assembly stress causes proportionally larger degrees of stress relaxation. In contrast, the cooling curve follows a linear relationship in a semi-logarithmic plot (only stress converted). The new formulation and methodology so derived has permitted a better understanding of external and internal factors on relaxation behaviors of expanded PTFE-based gaskets. For example, for a gasket design incorporating a corrugated metal insert, the relaxation curve stays relatively flat until the temperature reaches about 90–100°F, where the stress reduction starts to follow a linear trend parallel to, but above the relaxation curve of the expanded PTFE gasket without the insert. Essentially, the metal insert “delays” the effect of temperature on relaxation, and produces an “effective assembly stress” that is about 13% higher than the actual assembly stress. The use of Belleville washers has shown a similar phenomenon, but with a longer delay and higher effective assembly stress. Finally, the effect of in-process retightening, or hot retorquing, is quantitatively assessed. The contrasting relaxation behaviors of the material by different retightening methods (hot retorque versus post-process retorque in a cooled state) will be discussed.