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The viability of waste copper chromium and arsenic treated timber reinforced polypropylene as a sustainable building material

The aim of this thesis was to investigate the viability of wood-plastic composites (WPCs) produced from wood flour and fibre derived from waste copper chromium and arsenic (CCA) treated timber for use as a sustainable material in the construction sector. The research included an assessment of mechanical properties and the leaching of heavy metals from the wood-plastic composites, and the results were compared to material requirements in the building sector provided through literature and New Zealand building standards. Mechanical and chemical methods were used to prepare wood flour and fibre samples from waste CCA treated timber for use in wood-plastic composites. Composites were produced through compounding using a twin screw extruder or a custom sigma blade compounder and then injection moulded for material testing. Composites produced using an extruder with 40 wt.% bleached fibres and 4 wt.% MAPP, increased tensile strength (TM) from 18.5 MPa for the polypropylene used for the matrix to 27.6 MPa; Young's modulus (YM) was increased from 0.84 GPa to 2.33 GPa. Comparison of mechanical performance with commercial WPC properties suggests that waste CCA treated timber could be used as a raw material to produce WPC for semi-structural applications. The modulus (2.33 GPa, in the highest performing composites) was roughly three times lower than commonly used commercial timbers used in structural applications. The creep performance was not measured for a time frame long enough to be indicative of the 50 years required for structural application. Therefore, further research is required to investigate whether the low modulus and creep performance of the WPC is acceptable for a structural material in the construction sector. One of the largest concerns with using waste CCA treated timber as a reinforcement material in WPC was the inclusion of heavy metals and the potential for leaching. Arsenic was considered the most toxic of the heavy metals in the CCA treated timber. Processing of the waste CCA treated timber to obtain fibre was found to reduce arsenic concentration by up to 99.94 % in the best scenario. Furthermore, arsenic in leachate from composites was found to decrease from 41.29 to 0.07 ppb when comparing wood flour composites to digested and bleached fibre composites. The concentration of copper, chromium, and arsenic in leachate from composites produced with chemically obtained fibre was found to be significantly lower than the maximum allowable drinking levels in New Zealand. Therefore, indicating the application of composites produced with wood fibre from waste CCA treated timber can be regarded as safe even in scenarios that involve human contact with the material. A life cycle analysis study found that the use of an alkali digestion process to obtain fibre significantly increased the environmental impact compared to the processing required to obtain wood flour. In a study of a potential commercial scale process, ultrasonic treatment and hydrogen peroxide bleaching as secondary fibre treatments were the largest contributors to environmental impact in the preparation of wood fibre. Thus, the results suggest that chemical treatments should only be employed to produce wood fibre when the highest mechanical performance or lowest leaching levels are required for the application of the wood-plastic composites.
Type of thesis
The University of Waikato
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