Characterisation of photo-oxo-degradable polyethylene films

crop propagation films - a tool to enhance crop growing conditions by minimising the effects of climate fluctuation. It is an economical method to reduce the use of irrigation, pesticide, and fertiliser and to increase crop yield. However, the removal of non-degradable PE films is laborious and the remaining films become a waste problem. A potential solution, such as photo-oxo-degradable PE films, is available but their rate of degradation is not well controlled and would not be suitable for varying crop cycle lengths and farm sites. PE degradation is affected by many factors - the primary structure of PE, type and concentration of pro-degradants used, film processing method and processing conditions, cold drawing, and ageing method (accelerated or natural ageing). One of the key tasks of developing a time-controlled PE degradable film is to understand how various factors play a role in the rate of PE degradation and to understand the change in film properties as the film becomes brittle - termed the embrittlement point. This is the critical feature for crop propagation films where PE transitions from being ductile to brittle, allowing the crop to penetrate and grow through the film. This project identified the dominant factors that influence the degradability of PE - by studying a commercial linear low density PE (LLDPE) grade, Dowlex 2045G catalysed by a Ziegler-Natta catalyst (1-octene/ethylene co-polymer) and a commercial pro-degradant, Ampacet 30091, an iron-stearate based pro-degradant. The Dowlex-Ampacet system was compared with an Elite-Ampacet system to understand the effect of short-chain branching distribution (SCBD), where Elite 5400, is also a 1-octene/ethylene LLDPE catalysed by a metallocene catalyst but with a narrower SCBD than Dowlex. The effects of processing method and cold drawing were also studied. Films were cast extruded and aged in an accelerated ultraviolet light device, QUV, until embrittled. Films were characterised at various stages of degradation by evaluating their mechanical properties, molecular mass, oxidative products, crystal morphology and surface properties. From these results, a conceptual model was developed to understand how PE degraded and became brittle. In a semi-crystalline system, it is well established that the amorphous phase and tie chain molecules are the most susceptible to photo-oxo-degradation, compared to the crystalline phase that is too dense for oxygen diffusion. When PE with added pro-degradant was exposed to UV, free radicals were generated from the decomposition of hydroperoxides initiating chain scission reactions, along with some crosslinking and oxidation. Over time, small chain fragments formed from chain scission diffused together, where this process is known as recrystallisation. Results demonstrated that film embrittlement had little correlation with its oxidative products (as indicated by the carbonyl index calculated from infrared spectroscopy spectra) but correlated well with its crystal morphological properties (from differential scanning calorimetry). Further studies via small angle and wide angle X-ray diffraction showed the inter-lamellar distance decreased significantly as the system became denser, with the critical inter-lamellar spacing reaching about 5 to 6 nm when the film became brittle, regardless of Ampacet pro-degradant concentration. This result correlated well with the changes in double yield points seen in the tensile data, where the absence of the second yield point signified that the tie molecules at the lamellar interface underwent chain scission and could no longer transfer the tensile stress to reach c-axis slip of the lamellar crystals. The key factor that influenced the rate of PE degradation was Ampacet, followed by the initial degree of crystallinity, crystal morphology, chain mobility and regularity of the small chain fragments (formed from chain scission). The main factors affecting the crystal morphology were co-monomer concentration, SCBD, processing method, and cold drawing. This fundamental study on a commercial grade of PE with Ampacet pro-degradant has allowed to gain a better understanding of PE photo-oxo-degradation. This knowledge could assist to design a time-controlled crop propagation PE film suitable for varying crop cycle lengths and for different crop sites across Australia. Not only that, the knowledge gained from this project could be transferred to develop other degradable PE film products such as plastic bags, magazine wraps, and other packaging products