Degradation of high barrier ethylene-vinyl alcohol copolymer under mild thermal-oxidative conditions studied by thermal analysis and infrared spectroscopy

A study of the thermal-oxidative degradation of a high barrier ethylene-vinyl alcohol copolymer with 32 mol% of ethylene (EVOH) has been carried out by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) under mild temperature conditions above melting. It was found that time exposures of up to 11h at temperatures between 9 and 33 degreesC above the EVOH melting point resulted in polymer weight losses of up to 3.6% with colour formation. The weight loss was faster at short times and slowed down with increasing exposure time. DSC showed a small decrease in crystallinity and melting point, melting-peak broadening and a slight increase in the glass transition temperature of the samples subjected to the more severe thermal-oxidative treatment. The FTIR experiments showed transformation of the vinyl alcohol hydroxyl groups into carbonyl groups and creation of double bonds. Changes in degradation kinetics and perhaps in mechanisms are thought to occur with increasing exposure time. Moreover, FTIR measurements suggest that transformation of the hydroxyl groups leads to a weakening of the overall hydrogen bonding strength in the degraded samples, and therefore a reduction in intermolecular cohesion can be anticipated. (C) 2001 Society of Chemical Industry.