Stress cracking of polyurethanes by absorbed steroids

Circumferentially stretched polyurethanes, Pellethane 2363-80A and 2363-55D, were studied following exposure to three steroids of differing polarity; cholesterol, cholesteryl acetate and cortisone. Equilibrium uptakes, diffusion kinetics and environmental stress cracking (ESC) were investigated. Cholesterol reached equilibrium uptake by 16 h, whereas cortisone and cholesteryl acetate did not attain equilibrium uptake until 96 h, suggesting different processes. All uptakes were less than 0.6%. The cortisone diffusion curves showed an inflection at 36 h suggesting some sort of structural change in 55D. The glass transition temperature, T g , of the soft segment as a function of exposure time to cortisone and water shows a steady increase with cortisone content until after 36 h at which time the T g falls almost to the value for water equilibrated samples. This shows that cortisone is able to affect the mobility of soft segments. The relationship between circumferential strain, steroid uptake and ESC was investigated. Samples were stretched at 50% increments from 0 to 250%; after 12days exposure steroid uptakes were determined and the samples examined with SEM. Cholesterol promoted ESC in 80A and 55D at strains of 200 and 150%, respectively. Pellethane 55D was resistant to ESC in the presence of cortisone at all strains, while 80A cracked only at strains above 200%. The equilibrium uptake of cholesterol acetate was most sensitive to strain, particularly for 55D. Cholesterol acetate was the most effective ESC agent, cracking both 80A and 55D after 12days exposure with an uptake under 0.25% and a strain of only 50%.