Hierarchically structured paper-based composite laminates
Creators: name: Binti Daud, Noorol Jannah
Publisher: name: Imperial College London
Year: 2019

description: In this study, refined dissolving bagasse fibers was used to reinforce poly(lactic acid) (PLA) to produce hierarchically structured paper-based composite laminates. Initially, a recirculating colloidal mill was used to modify the surface of virgin cellulosic fibers from dissolving bagasse fibers. Accordingly, the gap between refiner disks was kept constant while the refining times were varied. Through this, “hairy” fibers with micron and sub(micron) dimensions was successfully liberated from the surface of the primary fibers at short time (e.g. after 6 min). This promoted areas for hydrogen thus produced dense and strong fiber networks (termed as re-engineered paper). As the use of the recirculating colloidal mill was found useful to produce “hairy” fibers, I extended the approach as a route to manufacture hierarchically structured paper-based composite laminates. My first strategy was to create the multi-layered composite laminates to manufacture ordered composites. Using laminating and papermaking like process, I studied composites with different magnitude of the hierarchy: 1, 2, and 4 denoted the number of re-engineered papers used in the composite laminates. Through morphological, mechanical, and thermal analysis of the multi-layered composites, the composites made perform better as the layers of the papers used increased. However, the limitation was the thinner re-engineered paper suffered from local stress concentration because of poor dispersibility henceforth creating gradient fibers distribution. This, without careful control, caused the properties to fall, hence, the synergism effect of the hierarchy structure was not achieved. Also, “hairy” fibers enhanced hydrogen bonding upon drying, hence, residual stress in re-engineered paper built-up creating micro-compression. Albeit, the effect not distinct on the mechanical performance of the papers and their composites. However, it is important to consider that when micro-compression overrides the bond strength of fibers loss of strength and modulus may occur. In another strategy, I combined cellulose nanofibers (e.g. cellulose nanofibrils (CNFs) and bacterial cellulose (BC)) with modified cellulosic fiber 7 to obtain fiber networks and composites with ordered higher. This allowed studies on the nano-confinement effect in the modified fiber networks. The composites structure utilising a simple laminate structure whereby the re-engineered (nano)paper sandwiched in between PLA films. The CNFs enhanced the mechanical and thermal properties of the composites, whereas with BC the performances were poor. This is due to BC agglomeration in the fiber networks owing to difficulty in disrupting three-dimensional network of BC fibers in pulp suspension. Consequently, preventing adjacent fibers for hydrogen bonding hence the advantages of nanofibers reinforcement effect not attained. To sum up, it is possible to produce hierarchically structured paper-based composite laminated using the aforementioned strategies. Introducing the recirculating colloidal proved useful as an alternative to existing mechanical equipment to fibrillate cellulosic fibers. Through “hairy” fibers hydrogen bonds were enhanced thereby creating dense and strong fiber networks. Nevertheless, the modified cellulosic fibers introduce itself as a hierarchical internal structure fiber networks model. This, in combination with the laminating technique, proved to level up the complexity of multiscale length in the hierarchy strategy. Nevertheless, nano-reinforcement effect abled to increase the order of hierarchy in composites, however, good dispersibility shall be attained to effect overall performances.