Investigation of Ageing Mechanisms in Thermoplastic Polyurethanes by Means of IR and Raman Spectroscopy

Abstract

SummaryWithin the present study FTIR and Raman spectroscopy are evaluated as to specific strengths in terms of analysing deterioration in TPU induced by UV radiation or thermal load. A conventional ester based TPU grade formulated from 4,4‘‐methylene diphenyl diisocyanate, 1,4‐butanediol and polycaprolactone was investigated. Exposure to UV was done in a Suntest CPS+ at different exposure times. Thermal treatment was done in an air circulation oven at 180 °C and different exposure times. Exposed samples were characterized as to changes in molecular structure with complementary methods ATR and Raman spectroscopy. To establish interrelationships between structural changes and macroscopic material characteristics UV/Vis/NIR spectroscopy and DSC were applied.UV/Vis/NIR spectroscopy clearly indicated yellowing, and thus an increase in absorption in the visible wavelength range, of TPU after exposure to UV or to thermal load. Yellowing correlated well with formation of chromophoric groups detected by ATR and Raman spectroscopy. ATR spectroscopy turned out to be very sensitive in detection of changes in chemical structure induced by UV radiation. Raman spectroscopy was found to be very sensitive in detection of changes in chemical structure induced by thermal load. No distinct effect of UV exposure on morphology of TPU was ascertained. In contrast, a strong effect of thermal treatment on (para)crystalline morphology was observed.