Development of composites materials from waste paper and plastic
Creators: name: James, Aaron
Publisher: name: Swinburne
Year: 2024

description: The paper recycling process generates a source of waste as the paper collected is refined to a quality required for recycled paper manufacturing. These waste materials contain a high paper fibre content with various types and proportions of contaminants, and as such have limited other economic uses. The basis of this project is to investigate a method to re-use this waste, through the addition of a polymer, to produce a filled polymer composite. To satisfy this goal this project has focused on the study of the material properties of a discrete range of highly-filled polymer composites from two different waste-paper filler types, Recycled Paper Waste (RPW) and Liquid Paperboard (LPB). The sources were primarily identified for the study based on their abundance, availability and low-cost as a waste product. Stretch-Wrap (SR) as a post-consumer product was chosen as the polymer matrix. The composites were studied under the condition of maximised fractional utilisation of the waste paper filler, balanced against the deleterious affects on some of the composite material properties, and allowing for the potential future large scale production. Investigation of the base raw materials identified particular aspects which drew attention to the composites materials in the study as being novel. The RPW is a diverse range of pre-treated wood fibres with a high level of non-paper contaminants and the LPB is a composite of high-grade clay coated paper, polymer and Aluminium layers. The SR matrix presents as a unique source as is a mixture of varied blends of barrier coating plastics and non-polymer inclusions, as well as adhesion promoting agents. The composites were produced utilising a pre-processing stage, compounded by twin-screw extrusion then moulded to the required form. The raw RPW and LPB fillers were both observed to take the form of short non-aligned fibres, evenly dispersed fibres in the matrix. Across the project a comprehensive range of material properties were studied to characterise the material behaviour, grouped to mechanical (short-term), thermal and melt rheological, optimisation of injection moulding processing and performance (long-term) properties, including creep and water absorption. The fillers were shown to provide reinforcement to the compliant matrix, resulting in the composites exhibiting significant increases in modulus values over the unfilled matrix as a function of the filler volume fraction. A constitutive model was developed to model the cantilever modulus as a function of the frequency of excitation and temperature. Reinforcement from the filler was noted to be more efficient in loading in flexure and cantilever. Young's modulus was shown to be well modelled by the modified Einstein model, with the flexural and cantilever modulus shown to be modelled using a Nielson-Tsai-Halpin model, which exhibited more efficient reinforcement from the filler. Modulus properties were shown to fit to mid-range Hashin-Shtrikman boundary model, using the modulus value for the filler as a typical value for paper sheets. Material maximum stress in tension, elongation at maximum stress and impact properties were observed to be limited to the strength and ductility of the unfilled matrix, and were observed to be subject to the degrading effect of the filler as a source of stress concentrations in the material. Yield stress and strength in flexure were both observed to increase with filler volume fraction, commented to be the result of being a function of both the positive effect of reinforcement in stiffness and degrading effect of the filler as a source of stress concentrations, able to be well modelled by the Piggott and Liedner model. Using thermo-gravimetric analysis the thermal limits of the filler under different heating rates was defined, with upper limits identified to be in the order of 240°C for both fillers, higher for the matrix. A particular noted behaviour, which would impact significantly on the processing of the composite, was identification of the water holding characteristics of the filler phase, which was observed to hold absorbed moisture up to 100°C, indicating the ability of the wood fibre to bind to water within the chemical structure. The melt rheological properties of the composite materials were measured and modelled as a function of filler volume fraction, temperature and frequency of excitation, with a constitutive model presented that is able to be applied to model the composite properties as a function of each of the respective variables. The composites were shown to exhibit the melt rheological behaviour of the unfilled polymer matrix, altered by the flow retardation effects of the filler. As a function of frequency of excitation, the composites were observed to display a power-law relation, with a decreasing power-law exponent and increasing consistency index with filler content, which was able to be adequately modelled using an Ostwald-De Waele power-law model. The viscosity of the composites was observed to be less thermally sensitive than the unfilled matrix. As a function of filler volume fraction, the viscosity of the composites was shown to increase significantly, at orders of magnitudes greater than the unfilled matrix, representing a major finding of the study. The effect of the filler content on composite viscosity was shown to fit to an exponential model. Optimisation of the injection moulding processing conditions was achieved using a second-order rotatable full central composite design method with the operating parameters the barrel temperature, injection pressure and dwell time. Factors noted to be a function of the processing parameters were the composite polymer-rich skin thickness, and to a lesser extent the packing density, viscosity of the melt and volatile emissions. The filler was observed to improve the creep resistance of the unfilled matrix, however providing reinforcement to a different characteristic behaviour to the modulus results, showing far less sensitivity to the filler volume fraction. The different behaviour in creep in comparison to the short-term modulus was believed to be due to the long loading time in creep allowing significantly more re-arrangement of the polymer chains, affecting the flow retardation effect of the fillers. The creep properties of the composites were successfully modelled using a two-element discrete relaxation spectrum model, with two-distinctive relaxation times, common to the unfilled polymer and the composites, and partial relaxation modulus values showing similar linear relation to the filler volume fraction observed in the short-term mechanical modulus results. A useful predictive model observed to fit to the experimental data was the elastic filler creep model. The water absorptive properties of the composites were experimentally measured and modelled as a function of temperature and surface preparation. The water absorbed as a function of the filler volume fraction at saturation was observed to be linearly dependent with an offset value above the water resistant matrix, noted to be different between the filler types. The differences between the fillers was believed to be due to differences in the compounded structure as the characteristic filler absorptive properties were measured to be equivalent. Saturation water absorption was shown to be independent of temperature. The rate of water absorption was modelled using a single diffusion rate Shen-Springer model, which showed good fit to the experimental results beyond the initial absorption. The effect on the mechanical properties of water absorption at saturation was shown to be a proportional reduction in both stiffness and strength, noted to have a proportionately greater effect on the stiffness properties, commented to be a function of the effect of the softening of the filler phase.