Cellulose nanopaper as reinforcement for sustainable polymer composites

Abstract

The use of cellulose nanopapers as reinforcement to produce high performance polymer composites is investigated in this thesis. Cellulose nanopapers are dense networks of nanofibrils that uses the hydrogen bonding ability of cellulose nanofibres. Both microbially synthesised cellulose nanofibres (known as bacterial cellulose or BC) and wood derived cellulose nanofibrils (CNF), were used in this work. This thesis starts with the investigation of the influence of test specimen geometries on the measured tensile properties of both CNF and BC nanopapers is investigated. Miniaturised specimens are often used for the tensile testing of cellulose nanopapers as there are no standardised test geometries to evaluate their tensile properties Four test specimen geometries were studied: (i) miniaturised dog bone specimen with 2 mm width, (ii) miniaturised rectangular specimen with 5 mm width, (iii) standard dog bone specimen with 5 mm width and (iv) standard rectangular specimen with 15 mm width. It was found that the tensile moduli of both CNF and BC nanopapers were not significantly influenced by the test specimen geometries if an independent strain measurement system (video extensometer) was employed. The average tensile strength of the cellulose nanopapers is also influenced by test specimen geometries. It was observed that the smaller the test specimen width, the higher the average tensile strength of the cellulose nanopapers. This can be described by the weakest link theory, whereby the probability of defects present in the cellulose nanopapers increases with increasing test specimen width. The Poisson’s ratio and fracture resistance of nanopapers are also discussed. The use of (ultra-)low grammage nanopaper as polymer reinforcement is also investigated. Bacterial cellulose (BC) nanopapers of 5, 10, 25 and 50 g m-2 were manufactured. Vacuum filtration to produce a 5 g m-2 nanopaper was found to be 3 times faster than that of a 50 g m-2. Low grammage nanopapers possessed a tensile modulus and strength as low as 2.4 GPa and 31 MPa respectively, against 11.8 GPa and 111 MPa for the 50 g m-2. Laminated composites containing 10, 5, 2 and 1 layer(s) of 5, 10, 25 and 50 g m-2 nanopapers were produced using a polylactide (PLLA) matrix. With a fibre loading fractions of vf ≥ 39%, the manufactured composites all possessed a tensile modulus and strength of ~10 GPa and ~100 MPa, respectively. The porosity of the nanopapers increased from 48% to 78% from 50 g m-2 to 5 g m-2. The porosity of the composites was ~10% independently of the layup. Finally, SEM images of the fracture surfaces of the composites revealed a layered morphology with little or no impregnation. The mechanical response of PLLA reinforced with multiple layers of BC nanopaper is then discussed. Laminated composites consisting of 1, 3, 6 and 12 sheet(s) of BC nanopaper were produced. It was observed that increasing the number of BC nanopaper led to an increase in the porosity of the resulting BC nanopaper-reinforced PLLA laminated composites. The tensile moduli of the laminated composites were found to be ~12.5 – 13.5 GPa, insensitive to the number of sheets of BC nanopaper in the composites but the tensile strength of the laminated composites decreased by up to 25% (from 121 MPa to 95 MPa) when the number of reinforcing BC nanopaper increased from 1 to 12 sheets. This was attributed to the presence and severity of the scale-induced defects increased with increasing number of sheets of BC nanopaper in the PLLA laminated composites. Finally, the environmental impacts of BC- and CNF-reinforced epoxy composites were evaluated using life cycle assessment (LCA). Neat polylactide (PLA) and 30% randomly oriented glass fibre-reinforced polypropylene (GF/PP) composites were used as benchmark materials for comparison. A cradle-to-gate LCA showed that BC- and CNF-reinforced epoxy composites have higher global warming potential (GWP) and abiotic depletion potential of fossil fuels (ADf) compared to neat PLA and GF/PP even though the specific tensile moduli of the nanocellulose-reinforced epoxy composites are higher than neat PLA and GF/PP. However, when the use phase and the end-of-life of nanocellulose-reinforced epoxy composites are considered, their “green credentials” are comparable to that of neat PLA and GF/PP composites. The life cycle scenario analysis showed that the cradle-to-grave GWP and ADf of BC- and CNF-reinforced epoxy composites could be lower than neat PLA when the composites contains more than 60 vol.-% nanocellulose. The LCA model suggests that nanocellulose-reinforced epoxy composites with high nanocellulose loading is desired to produce materials with “greener credentials” than the best performing commercially available bio-derived polymer.