Nylon 6.6 accelerating aging studies: II. Long-term thermal-oxidative and hydrolysis results

Long-term (greater than 5 year exposures), low-temperature (as low as 37 degrees C) accelerated oven aging results were obtained for Nylon 6.6 fibers under thermo-oxidative conditions (air aging with an oxygen partial pressure of 13.2 cmHg in Albuquerque). To assess the importance of humidity on aging, experiments were also conducted under a combination of 100% RH plus 13.2 cmHg of oxygen partial pressure at temperatures ranging from 138 degrees C to 64 degrees C plus an additional experiment at 70% RH and 80 degrees C. The low-temperature tensile strength results showed that the Arrhenius activation energy under the pure oxidative degradation conditions dropped from similar to 96 kJ/mol above similar to 100 degrees C-similar to 30 kJ/mol below this temperature, indicative of a transition in the oxidative chemistry at low temperatures. Earlier work by our group on the same material concluded that hydrolytic degradation effects dominated oxidation effects at higher aging temperatures. However, the current long-term, low-temperature comparisons lead to the conclusion that humidity is not an important aging factor below 50 degrees C. By extrapolating time-temperature superposed oxidative degradation data using the low-temperature activation energy, we obtain predictions at 21 degrees C. At this temperature, we estimate that a tensile strength loss of 50% takes on the order of 70 years. The 21 degrees C predictions are shown to be reasonably consistent with long-term (up to 38 year) ambient results on similar Nylon materials removed from field-aged parachutes. Although the estimated average exposure temperature varies from parachute to parachute, the highest average temperature is estimated to be on the order of 21 degrees C. (C) 2010 Elsevier Ltd. All rights reserved,