Microstructural design and properties of high performance recycled cellulose fibre reinforced polymer eco-nanocomposites

Abstract

In recent years, cellulose fibre-reinforced polymer composites have been gaining a great attention in several engineering applications due to their desirable properties, which include low density, low cost, renewability and recyclability as well as good mechanical properties. Moreover, cellulose fibres are environmentally friendly, non-toxic and renewable materials. Therefore, manufacturing industries especially packaging, building construction, automotive and furniture have been encouraged to use cellulose fibres in their applications instead of the more expansive and non-renewable synthetic fibres. However, one of the major drawbacks that has limited the use of cellulose fibres as reinforcement in polymer composites is their susceptibility to moisture absorption due to the hydrophilic nature of cellulose fibres. Moisture absorption can result in a reduction of mechanical properties and dimensional stability of composites. Several studies in plant fibre reinforced polymer composites have reported an enhancement in mechanical properties, fibre-matrix interfacial bonding and fibre resistance to moisture via various chemical or physical treatments. In this project, a novel approach has been used to enhance the resistance of cellulose fibre reinforced polymer composites to water absorption and to improve the mechanical properties by introducing a nano-filler that provides good resistance to water diffusion and enhances fibrematrix interfacial bonding for better mechanical properties.In this study, epoxy eco-nanocomposites reinforced with both recycled cellulose fibres (RCF) and different nano-fillers such as nanoclay platelets (Cloisite 30B), halloysite nanotubes (HNT) and silicon carbide nanoparticles (n-SiC) were synthesized. The influence of RCF/nano-filler dispersions on physical, thermal, mechanical and fracture properties was investigated in terms of water absorption, flexural strength, flexural modulus, impact strength, fracture toughness, impact toughness and thermal stability. The effect of water soaking on the mechanical properties of composites was also investigated. Different analytical methods such as wide angle X-ray scattering (WAXS), synchrotron radiation diffraction (SRD), transmission electron microscopy (TEM), Fourier transforms infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to examine the nano and microstructures of these materials.First, multi layered recycled cellulose fibre (RCF) reinforced epoxy composites were fabricated with fibre loadings of 19, 28, 40, 46 and 52wt%. Results indicated that flexural strength, flexural modulus, facture toughness and impact strength increased as the fibre content increased. Water absorption and diffusion coefficient were found to increase with an increase in fibre content. Mechanical properties such as flexural strength, modulus and fracture toughness were found to decrease after water treatment due to the degradation of bonding at the fibre–matrix interfaces. The thermal stability of samples was determined using thermo-gravimetric analysis (TGA). Results indicated that the presence of cellulose fibres led to a reduction in the maximum decomposition temperature (Tmax) of epoxy. However, composites with cellulose fibre showed better thermal stability than neat epoxy at high temperature (≥ 600 oC). SEM observations showed a variety of toughness mechanisms such as crack bridging, fibre pullouts and fibre fracture and matrix cracking on the fracture surface of RCF/epoxy composites, which led to good fracture properties for samples reinforced by RCF layers.Second, epoxy-based nanocomposites reinforced with organo-clay platelets (Cloisite 30B), halloysite nanotubes (HNT) and nano-silicon carbide (n-SiC) were prepared by mixing the epoxy resin with three different filler loadings (1, 3 and 5 wt%) using a high speed mechanical mixer for 10 minutes with rotation speed of 1200 rpm. WAXS results showed that nanoclay platelets were intercalated by the epoxy resin. The d-space of the peak (001) of nanoclay increased from 1.85 to 3.4 nm after mixing with epoxy. TEM results showed a major intercalated structure with some exfoliated regions. Based on TEM results, the basal spacing of (001) varied from 2.65 to 7.98 nm. SRD results of HNT and n-SiC showed no change in the peak position after mixing with epoxy. TEM results of epoxy filled with nanoclay, HNT and n-SiC indicated that the dispersion of nanofiller was quite homogenous with some particle agglomerations that found to increase as filler content increased due to an increase in matrix viscosity. The addition of nano-fillers enhanced the mechanical properties of epoxy matrix.Maximum improvements in flexural strength, modulus and fracture toughness were achieved at 1 wt% of nano-filler loading, while the addition of 5 wt% of nano-filler displayed the maximum impact strength and toughness. The presence of nano-filler was found to have insignificant effect on the thermal stability of neat epoxy. Water absorption was found to decrease as the filler content increased. After six months of water treatment, there was a reduction in flexural strength and modulus, but an improvement in fracture toughness and impact strength. SEM results showed that nanocomposites had rougher fracture surfaces than that of neat epoxy. Several toughness mechanisms such as crack deflection, crack pinning, particle debonding, plastic void growth, plastic deformation and particle pullouts were observed.Finally, epoxy-based nanocomposites filled with nano-filler (i.e. nanoclay, HNT and n-SiC) were successfully used as a matrix for fabrication of multi-layers RCF/nano-filler reinforced epoxy econanocomposites. The presence of nano-fillers was found to have insignificant or modest effect on flexural strength, modulus and fracture toughness when compared to unfilled RCF/epoxy composites. Impact strength and impact toughness increased due to the presence of nano-fillers. The addition of nano-fillers increased the rate of the degradation by decreasing the maximum decomposition temperatures by about 8-9 oC compared to unfilled RCF/epoxy composites.However, the thermal stability of nano-filler filled RCF/epoxy eco-nanocomposites was found to increase at high temperature (≥ 500 oC). The presence of nano-fillers led to a significant decrease in maximum absorbed water compared to unfilled RCF/epoxy composites. Exposure to water for six months severely reduced the mechanical properties of wet composites when compared to dry composites. However, the addition of nano-fillers enhanced the mechanical properties of nanofiller reinforced RCF/epoxy eco-nanocomposites compared to unfilled RCF/epoxy composites in wet condition. SEM results showed that water absorption led to degradation in cellulose fibres and weakening of the bonding at fibres-matrix interfaces. Enhanced barrier and mechanical properties of nanocomposites were more pronounced for composites filled with n-SiC as compared to those filled with nanoclay platelets and halloysite nanotubes.The success in this project indicated that this approach of ‘designing for recycling’ or ‘ecodesign’ to develop environmentally friendly composite materials is achievable. Moreover, this project may provide a great momentum for a ‘cradle to grave’ approach in the eco-design of fully ‘green’ or biodegradable and environmentally friendly composite materials through the use of nanoclay and recycled cellulose fibres as reinforcement for bio-resins.