|Ph.D Student||Shiran Garnai Hirsch|
|Subject||Development and Characterization of Methods for Studying|
and Predicting Polyethylene Degradation
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Segal Ester H.|
Photo-oxidative degradation of polyethylene triggers significant deterioration in the polymer properties, which can lead to its premature failure. Consequently, much interest is aimed at characterizing and possibly predicting photo-oxidative damages as early as possible during product life, prior to the occurrence of profound changes in its performance in general, and in its mechanical properties in particular. This study presents a multidisciplinary approach towards achieving a comprehensive toolbox for identifying degradation level in low-density polyethylene (LDPE) films.
Thin films of LDPE, with varying molecular weights, were exposed to accelerated and to natural weathering conditions. Surface deterioration was monitored using apparent contact angle (CA) measurements, Fourier-transform infrared spectroscopy (FTIR), tensile tests and electron microscopy. A significant decrease in the films’ CA values during weathering was observed. We found that composition changes, specifically the evolution of polar groups, to be the dominant parameter affecting the CA behavior of the weathered films. The results of the CA measurements were found to coincide well with FTIR analysis, which is used to evaluate the extent of degradation during weathering by assessing the carbonyl index (CI) value. The deterioration in mechanical properties was also characterized and was found to present poor sensitivity and high variability (between samples) at these early stages. Thus, relatively simple CA measurements could potentially be used as a qualitative indicator for evaluating the aging/weathering impact on LDPE.
In the next stage, the effect of carbon black (CB) addition on the extent of degradation was investigated. Degradation was characterized by FTIR spectroscopy, oscillatory rheology studies, differential scanning calorimetry (DSC), successive self-nucleation and annealing (SSA), as well as by hot-stage microscopy. Rheology studies displayed a profound decrease in the complex viscosity of weathered neat films, suggesting that these samples have undergone degradation accompanied by a significant reduction in their molecular weight. However, addition of 0.5wt% of CB reduced this effect. DSC analysis showed an increase in the dregree of crystallinity with weathering time, originating from chain scission, for both neat and CB-filled samples. Tensile analysis also demonstrates the high effectivity of CB as a UV stabilizer, where addition of only 0.5wt% CB to the LDPE increases its "half-life" by fourfold in comparison to the neat films.
Finally, we studied the effects of natural and accelerated weathering on the activation energy for decomposition (E), as measured by modulated thermal gravimetric analysis (MTGA). Surprisingly, we found that weathering had no effect on the calculated E values. In contrast, we showed that the extent of LDPE photo-degradation is linearly correlated to the onset value, termed here as the ‘volatilization energy’ (E'), associated with the measured activation energy curve. We also demonstrated that there is a strong correlation between the degradation degree, as determined by the traditional CI, and the amount of water evaporated up to the temperature at which E’ is measured.
The conclusions derived from this work can also be readily applied for assessing the extent of degradation of polyethylene-based recycled materials, paving the way for predicting service life and increasing the potential usage of recycled plastics.