Prediction of environmental stress cracking in plastics with Hansen solubility parameters

The results reported here confirm that environmental stress cracking phenomena in plastics depend on the size and shape of the test liquid's molecules as well as their Hansen solubility parameters (HSP) relative to those of the polymer. The behavior of other, untested liquids can be estimated from HSP correlations based on a limited number of test liquids. A simple test has been developed for the cracking tendency of immersed polymer tubes having different levels of externally applied, controlled stresses. This uses standard tapered NS Teflon (most glass is too rough) stoppers placed on a toploader analytical balance. Polymer tubes are pressed onto the stoppers until the balance shows the given desired force. The tube and stopper are then immersed in a test liquid and observed at regular time intervals for cracking. It has been assumed that the Teflon stoppers are inert in this application. The experiments described here used a COC (cyclo-olefinic copolymer) type polymer (Topas 6013, Ticona). The samples cracked in a number of solvents without external stress. Methyl isobutyl ketone gives rapid cracking of tubes of this polymer at low external stress while olive oil, for example, is somewhat slower to give cracking and requires moderate external stress and longer times.