(641h) Examining the Thermo-Oxidative Degradation of Ethylene Vinyl Alcohol (EVOH) By Time-Resolved Rheology

The objective of this investigation was to understand the effects of thermo-oxidative conditions on ethylene vinyl alcohol (EVOH) utilizing time-resolved rheology. In this work, the viscoelastic properties of EVOH were monitored over 200 minutes to examine shifts in rheological behavior with time and frequency. From this approach, the degradation and crosslinking phenomena can be understood by examining polymer relaxation, terminal behavior, and other rheological signatures, which determine its industrial processing conditions and recyclability near its end of life uses.

EVOH is a copolymer made of ethylene and vinyl alcohol units. Because of EVOH’s great gas barrier properties arising from strong intra- and intermolecular interactions, it is commonly applied in multilayer food packaging systems traditionally placed between structural polymers like polyolefins and generally processed under thermo-oxidative conditions. Rheology is commonly used to understand the viscoelasticity and flow behavior of polymers for application to industrial settings. Unfortunately, EVOH is quite susceptible to thermo-oxidative degradation which makes studying the viscoelasticity of EVOH by rheology quite difficult. To address this shortcoming of traditional rheological tests such as frequency and amplitude sweeps, time-resolved rheology was applied to EVOH to understand its time-dependent viscoelasticity and degradation above its melting point. Time-resolved rheology utilizes a series of frequency sweeps (100 to 0.1 rad/s) to monitor viscoelastic shifts in the polymeric melt over a fixed time period. Monitoring frequency dependent properties like storage and loss moduli over time can elucidate information about short and long chain contributions to the overall molecular relaxation process and structure, leading to a better understanding of the fundamental degradation process of EVOH. Surprisingly, there is a lack of time-resolved rheology studies present in the literature, especially so for EVOH. Some work has been established in the field investigating the thermo-oxidative degradation of EVOH largely by spectroscopic and thermal methods, but still, neat EVOH has not been extensively studied by means of time-resolved rheology.

Through rheological analysis of various EVOH grades, it was found that the storage moduli of the long chain contributions were highly time-dependent and showed dramatic increases in elasticity. Similarly, tanδ and the loss moduli also showed long chain evolution over time, while the short chains were much less affected. It was also determined that EVOH transitioned from a Carreau fluid to a power law fluid over the testing time. From these results, the onset of crosslinking between long chains of EVOH form quickly with respect to chain scission degradation. An arrest of the relaxation process was observed from the Han plots, where the storage moduli plateaued through ~2 orders of magnitude in the loss moduli halting terminal behavior. Additionally, Cole-Cole plots showed that the initial relaxation process of EVOH evolved into a long, complex process that dominated the initial relaxation. Given this arrest of terminal behavior, full relaxation of the polymer was not reached due to the formation of the aforementioned crosslinked structure. The approach to the terminal flow plateau vanished over the testing period, as observed in the van Gurp Palmen plots and indicated a significant change in microstructure, likely owing to the crosslink formation. From this work, it was evident that a densely crosslinked polymer structure was formed in the thermo-oxidative environment. This microstructural shift was attributed to the formation of a “double carbonyl” between EVOH chains by conversion of vinyl alcohol hydroxyl groups. This work showed that EVOH experiences significant viscoelastic changes under thermo-oxidative environment in relatively little time, which will directly affect EVOH’s reprocessibility and recyclability near its end of life uses.