Understanding and predicting environmental effects on degradation of oxo-degradable polyethylene blown films

Abstract

Polyethylene (PE) based films are commonly used in agriculture for different applications such as mulching, walk-in tunnel and mini greenhouse. In the case of mini greenhouse, the thin film creates a confined environment providing favourable conditions for crop growth, such as increased temperature and moisture retention, hence reducing the need for irrigation and agro-chemical treatments. However, in this specific application, it is necessary to control the rate of degradation of the film above ground to enable healthy crop growth. Several studies have investigated the photo- and/or thermo-oxidation of PE in sunlight and when buried in soil or compost. However the degradation rate is not well controlled and the degradation mechanisms are not fully understood. The service lifetime of polymer films is controlled by the chemical reactions leading to chain scission and mediating environmental factors. For application in agricultural cropping films, a controlled accelerated degradation is required. The rate of PE film degradation can be influenced by its intrinsic properties (such as polymer grade, concentration and/or type of pro-degradants) but also by environmental factors, such as UV dose, temperature and humidity. However, in practice, these are not the only factors controlling the polymer lifetime, with poor translation from model (laboratory based) accelerated ageing studies to in-field application. This creates a challenge in accurately predicting the useful lifetime of a given film. Due to the confined environment created by the thin films, the ultimate service lifetime of the film may be affected by unexpected environmental factors such as the composition and photo-chemistry of soil components. The evaluation of the impact of these environmental factors, including the effect of soil type on the above-ground degradation of PE, has not previously been addressed. This project aims to better understand the effect of such factors on the PE photo-degradation processes, by correlating the film lifetime to key soil parameters such as soil type, chemistry and properties. The above-ground degradation of oxo-degradable PE thin films as well as a control film, were investigated over a variety of soils, and over isolated organic substances such as humic and fulvic acids.Outdoor weathering field trials performed using oxo-degradable PE thin films were conducted across several sites around Australia. It was found that a site factor, that was apparently independent of total solar dose and temperature, significantly impacted the rate and extent of photo-oxidation. This finding was confirmed by controlled laboratory-based accelerated ageing trials of both PE film with no pro-degradant as well as with oxo-degradable PE films, which revealed that the rate and extent of PE photo-oxidation did not correlate with temperature under the film nor UV exposure, but was soil dependent. Under accelerated photo-oxidative conditions, the time to reach embrittlement for a PE film aged over the soil with a high content of organic matter (OM 8.4%) was halved (at 24.5 days) when compared to similar film aged over air (at 48 days). Further investigation revealed that humic acids and fulvic acids within soil organic matter may contribute to the change in the oxidation rate of PE photo-oxidation, possibly through the formation of volatile reactive oxygen species that may form under photo-oxidative conditions. In addition, the presence of water also had a significant impact on the rate of photo-oxidation.Overall, additional environmental factors to solar dose and temperature were influencing the rate of PE photo-degradation. The key factors were found to be the presence of condensed water and soil type. For the latter factor, the specific soil properties of importance with respect to film degradation appeared to be associated with organic matter, although the impact of soil on PE photo-oxidation was found to be complex and likely dependent at least in part on soil components that varied between different soil types, consequently influencing their photo-chemistry. This knowledge could assist in designing a modelling tool to recommend the appropriated time-controlled PE film suitable for a specific crop cycles at specific sites across Australia. These findings could also be transferred to other degradable PE film applications, such as mulch films, plastic bags and other packaging products.