Oxo-biodegradation of clay-based polyethylene nanocomposites

Rezaei, M 2012, Oxo-biodegradation of clay-based polyethylene nanocomposites, Doctor of Philosophy (PhD), Civil, Environmental and Chemical Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Oxo-biodegradation of clay-based polyethylene nanocomposites
Author(s) Rezaei, M
Year 2012
Abstract Polyethylene is the most widely used polymer in various industrial and day-to-day consumer applications. However, the widespread use of polyethylene has led to a problem called ‘plastic pollution’. Among the several strategies used to solve the plastic pollution, the most effective one seems to be the production of degradable plastics. Degradable plastics are made by blending polymer with additives such as metal ions and naturally biodegradable polymers which enhance the degradability of plastics. Degradable polyethylene disintegrates into smaller fragments in the presence of heat, light and oxygen. However, these small fragments remain in the environment for very long period and contaminate soil and water. To make polyethylene completely biodegradable, the smaller fragments must be made consumable to microorganisms which convert them into CO2 and water during their metabolism. Current knowledge on the mechanisms of polyethylene biodegradation is far from complete and therefore more research is required in this area. Previous studies on polyethylene degradability have shown that degradability is enhanced remarkably by incorporating metal ions called pro-oxidant into the polymer matrix. Recent studies showed that clay-based polyethylene nanocomposites have better biodegradability. The role of clay, however, on the degradation of polyethylene has not been completely understood yet. The main objective of this work is to investigate the combined effects of manganese stearate as pro-oxidant and nanoclay on polyethylene biodegradation. It is also aimed to develop a mathematical model for predicting the biodegradation rate of oxo-biodegradable polyethylene nanocomposites (nanocomposites with pro-oxidant) using the experimental data. Films of polyethylene nanocomposites and oxo-biodegradable polyethylene nanocomposites were prepared using melt intercalation followed by film blowing. Biodegradation study was carried out in two stages: abiotic stage and biotic stage. Abiotic degradation was achieved by subjecting the film samples to heat at 70°C.

Thermally degraded samples were then subjected to biodegradation in a closely monitored composting system according to AS-ISO 14855. Thermally degraded samples were subjected to TGA and FTIR analysis. Biodegradation was monitored by measuring the volume of CO2 produced as a function of time. Biodegraded film samples were also subjected to ESEM (bio-film growth) and FTIR analysis. Experimental results show that manganese stearate helps in different aspects of both abiotic and biotic stages of biodegradation. However, the presence of clay itself has no significant effect on the thermal degradation of polyethylene nanocomposites. But nanoclay has been found to enhance the overall degradation by either catalysing the thermal oxidation or making the material suitable for the growth of microorganisms. Extensive degradation observed in oxo-biodegradable nanocomposites indicates that co-existence of manganese stearate and nanoclay in polyethylene structure is vital to achieve an effective biodegradation process. CO2 evolution data obtained in biodegradation studies were used to develop a mathematical model that describes the biodegradation behaviour of polymer samples used. It has been found that biodegradation of oxo-biodegradable polyethylene can be represented by a mathematical model similar to the one that represents bacterial growth. This model equation will be a useful tool to analyse the kinetics of biodegradation and estimate the biodegradation potential of polyethylene compounds.

Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Civil, Environmental and Chemical Engineering
Keyword(s) Polyethylene
Nanocomposites
Degradation
Biodegradation
Thermal degradation
Modeling
Composting
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