Creep of Nylon 66 in Concurrently Changing Humidity

The purpose of this investigation was to study a hygroscopic polymer, Nylon 66, to see whether it exhibited similar anomalous creep behaviour during concurrent moisture changes as has been observed in wood.
This investigation was therefore a study of the relations, in Nylon 66, between the variables of stress, time, moisture and dimensional changes; no direct study being made of structural changes, if any. The anomalous creep was not observed, but on the contrary it was found that the creep behaviour in changing humidities could be approximately predicted from creep data at constant stress and humidity.
Among the relatively new and successful pieces of equipment designed and constructed were an environmental chamber, a width extensometer, a zero-load control machine, a machine for creep tests on samples of thickness 50 μm or less, and apparatus for measuring the diffusion coefficients of thin stressed sheets.
The well-known Crank-Nicholson method of solving the diffusion equation was further developed to successfully predict moisture changes for one- and two-dimensional flow during sorption, desorption and cycling; and led to a rapid method of conditioning samples which was first simulated on the computer and then checked in the laboratory. A method of distorting the logarithmic-time scale was found to allow the accurate superposition of creep curves even when they each covered six decades of time, so giving improved creep predictions. A method of predicting creep with changing humidity was developed by assuming that the predicted instantaneous mean moisture content controlled the behaviour. This was compared with the very lengthy "explicit" method and found to give almost identical results, which fitted fairly closely to the experimental values.
Evidence was obtained that Nylon 66 slowly "age hardens" following conditioning at 90% r.h.
Anomalous changes in surface dimensions as shown by thickness measurements during sorption and desorption were also studied but no explanation could be found.