Stress cracking of nylon polymers in aqueous salt solutions

Abstract

The susceptibility of nylon polymers to environmental stress cracking in aqueous salt solution was investigated. The time to fail (complete rupture) was recorded for razor-cut precracked samples which were totally immersed in water, sodium chloride, or calcium chloride at various initial stress intensity levels and temperatures. Ductile failures were observed in both water and aqueous sodium chloride at all temperatures. However, in saturated aqueous calcium chloride the nylon failure mode changed from ductile tearing, at room temperature, to crazing and cracking at elevated temperatures (50 to 100 ° C). Lithium and magnesium chlorides were also observed to be potent cracking agents at the elevated temperatures. The temperature interval over which cracking occurred corresponded to the onset of the glass transition temperature, Tg, of the nylon. Thus, nylon 6, having a lower Tg, was more susceptible to cracking than nylon 6,6 at any given temperature. On the other hand, nylon 11, having a lower ratio of amide to methylene groups, was not susceptible to salt-induced cracking. For the nylon 6 and nylon 6,6, the failure kinetics in salt solutions were similar, regardless of whether dry samples or previously moisturized samples were examined. Thus a simple temperature-moisture content equivalence does not apply for salt-induced stress cracking in aqueous solutions.