Production and optimisation of all cellulose composites using native and regenerated cellulose fibres.

All-cellulose composites (ACCs) are gaining interest as renewable material alternatives to traditional fossil fuel derived composites. In contrast to traditional composite materials, whereby the matrix and reinforcing components are formed from different materials, ACCs feature matrix and reinforcing components comprising entirely of the renewable biopolymer, cellulose. The ACCs studied and discussed throughout this thesis were produced via a partial dissolution method, using alternating textile layers of either cotton or regenerated cellulose fibres and an interleaved cellulosic film immersed in solvent solution, and heating them under an applied pressure. The solvent used was a mixture of the ionic liquid 1-Ethyl-3-methylimidazolium acetate ([C2MIM][OAc]) combined with a co-solvent dimethyl sulfoxide (DMSO).

Firstly, for the ACCs based on the cotton textile layers, the addition of the interleaved film was found to contribute to a significant improvement in interlayer bonding when compared to processing without the film, as quantified by peel strength measurements. With an optimised [C2MIM][OAc] and DMSO ratio of 80/20, Young’s modulus was improved from 2.2 +/- 0.2 GPa to 4.2 +/- 0.2 GPa and peel strength reached as high as 917 +/- 73 N/m. The increased interlayer adhesion was additionally found to improve uniformity between longitudinal and transverse mechanical properties. Optimization of the processing variables for producing ACCs was carried out using statistical design of experiments (DoE). A full factorial design (23) was applied to explore the effects of dissolution temperature, pressure, and time on ACC mechanical properties, which were then optimised using Response Surface Methodology (RSM). A relationship between Young’s modulus and processing conditions was revealed and used to identity optimum process condition to maximise this property. Temperature and time settings of 101 °C and 97 minutes respectively, were identified, from which a Young’s modulus of 3.3 GPa was predicted to yield. In-lab validation samples were found to exhibit a very similar Young’s modulus of 3.4 ± 0.2 GPa, confirming the adequacy of the predictive model. The optimized samples had an average tensile strength and peel strength of 72 ± 2 MPa and 811 ± 160 N/m respectively, as well as a favourable density resulting from excellent consolidation within the material microstructure.

ACCs were then produced using a regenerated cellulose fibre-based textile, Tencel, and DoE was again applied to investigate the effect of process conditions of temperature, time, and [C2MIM][OAc] concentration, as well as the benefits of the interleaf film. A full factorial screening design was expanded to a central composite face-centred (CCF) design which captured the process using the film more strongly. It was found that the film remained in between the textile layers, rather than penetrating through the fibre assembly, as observed in previous work on cotton-based ACCs, offering insights into the structural differences between Tencel and cotton. Multi-response optimization led to a prediction for Young’s modulus and strain-to-failure to be obtained. An optimized processing temperature of 30 °C with 70 % IL / 30 % DMSO was identified to yield a Young’s modulus and strain-to-failure of 5.3 GPa and 3.5 % respectively. In-lab samples were made and found to exhibit a Young’s modulus and strain-to-failure of 4.9 ± 0.2 GPa, and 3.3 ± 0.3 % respectively.