Chunping Dai

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Bamboo and its composites have been increasingly used to supplement timber-based products owing to their fast growth and superior strength attributes. To make laminated products, green bamboo strips must be dried to moisture contents below 10%. Drying determines both the productivity and quality of laminated bamboo, but is currently very inefficient due to limited understanding of many critical phenomena. This thesis investigates the fundamentals and industrial practice of bamboo strip drying, and the effects of different heat treatment processes on their morphology and properties.In the first part, green strips were subjected to different steam pretreatments called caramelization and dried in a step-wise schedule in lab. Strip deformations and flexural strength were evaluated. Mould resistance was tested using controlled inoculation experiments, and starch modification was observed by scanning electron microscopy (SEM). The results showed that the 160 °C saturated-steam caramelization reduces bamboo strength and causes longitudinal bowing, transverse deformation, and node shrinkage. The caramelization does not appear to improve mould resistance in the long term, but it delays the onset of mould colonization, which may be explained by the continued presence of modified starch.In the second part, the drying department of a typical laminated bamboo factory was investigated. Drying involves five steps in chronological order: caramelization, primary drying, ambient conditioning, steam reconditioning, and secondary drying. From monitoring in-situ environmental conditions, the step-wise drying schedule can adequately simulate the progressive tunnel kiln drying process. Statistical analysis suggests reconditioning rather than caramelization is the key process for controlling strip moisture content and dimensional changes. Bamboo cell collapse was investigated using SEM to elucidate the patterns and severity of the cellular deformation under different reconditioning regimes. Collapse occurs in parenchyma cells, and the patterns in the cross-section are complex and governed by the spatial locations relative to fiber bundles and the inner or outer edge of the culm. Bamboo requires saturated steam at higher temperatures (>100 °C) than wood for effective collapse recovery.This thesis pointed to real potential for improving industrial bamboo drying by eliminating pre-caramelization and reducing cell collapse. Further research is needed to implement the findings in bamboo industry.

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Bamboo, a hierarchically organised material with a distinct cylindrical shape, exhibits anisotropic behaviour due to its unique anatomical structure. Bamboo culm walls are reinforced by vascular bundles densely in the outer region and sparsely in the inner region, resulting in a gradient structure along the radial direction of the culm. This research investigates the elastic behaviour of bamboo at four different scales, namely nanoscale, microscale, mesoscale and macroscale, by combining microstructural investigations, image analysis of bamboo microstructure and finite element modeling (FEM) techniques. The focus is to understand the orthotropic elastic properties at each specific scale. This study utilises representative volume element (RVE) homogenisation to extract nine independent elastic parameters and analyse stress and strain contours, providing valuable insights into the elastic deformation behaviour of bamboo. Moreover, to ensure robust validation, the experimental data are compared with the predicted elastic properties at each scale, aiming to achieve a satisfactory level of agreement. The results reveal that among the different cell wall layers, the broad layer of fiber exhibits the highest homogenised elastic properties due to its lower microfibril angle (MFA) and higher cellulose content. Layer-structured composition of different cell walls within a fiber, parenchyma, and vessel are modeled. Consequently, fiber cells exhibit the highest elastic properties among these three cell types. Anatomical-based homogenisation of vascular bundles at the mesoscale level reveals that increasing the fiber content from 8% to 56% results in 2.6 times increase in longitudinal Young's modulus and 1.7 times increase in transverse Young's modulus. Experimental results at the macroscale level, using a non-contact, in-situ digital image correlation (DIC) system for strain measurements, reveals a high accuracy of 94.7% for estimating the experimental Young’s modulus. Overall, this research contributes to a better understanding of bamboo's elastic behaviour at different scales, offering valuable insights for bamboo structural analyses, material designs, and potential applications.

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Sustainable development of bamboo-wood composites requires a better understanding and optimization of bonding. This thesis investigates the bonding process and performance of structural bamboo-wood laminates. A polylamellate cell wall structure, low tissue porosity and permeability, and poor surface wettability all hamper bamboo bonding with most softwood adhesives. Adhesive modification must be optimized in conjunction with more efficient material utilization and processes. Understanding the bonding mechanism between wood/bamboo and adhesives is essential for the development of modified adhesives for advanced hybrid composite. Phenol formaldehyde (PF) resins with two different molecular weights (MW) were tested for bamboo-wood bonding. Optical microscopy was used to observe the penetration of resin into the surface of solid or veneered wood (Douglas fir) and milled or flattened bamboo (Moso). The results showed that on the bamboo substrate, high MW PF largely remained in the glueline and only entered the lumina of cut or damaged cells near the bondline. Low MW PF penetrated cell wall corners of Moso bamboo but not the uncut lumens. Undamaged lumens seem impermeable to PF resins. Results from dry and wet bond shear tests showed that applying low MW PF to the bamboo and high MW PF to the wood surface separately significantly improved the bonding performance. The work also evaluated PF mixed with extenders and fillers for bonding veneer-type hybrid wood-bamboo composites with different glue application rates. The mixed plywood PF was comparable in both dry and wet bond shear strength and wood failure to using pure PF even at the same glue application rate due to its good gap filling capability. The findings indicate that plywood resin with fillers is a viable, lower cost-effective adhesive for veneer-type wood-bamboo composites.In conclusion, hybrid bamboo-wood composites are promising cost-effective approaches for the engineered bamboo industry, leading to viable building products. Bond qualification standards for wet bond criteria of plywood will need to be modified and adapted to accommodate the different resins and materials used in wood-bamboo composites. Further modifications are required to produce a stronger adhesive than the bamboo outer wall tissue in order to improve wet shear fiber failure rates.

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The demand for biodegradable molded pulp packaging materials has increased significantly due to the ban on single-use plastics in many countries. Current dominant commercial molded pulp food containers impart oil resistance with the addition of Per- and Polyfluoroalkyl Substances (PFAS), but recent research has confirmed the toxicological effects of PFAS on human health. This strongly motivates the industry to find an alternative oil repellent that is food-grade safe, environmentally-friendly and effective. This thesis investigated a new modified starch-based oil repellent (SOR) as a wet-end additive.Non-wood fibers were selected to be the substrate to quantify the effects of additives due to the advantages of low cost, rapid renewability and abundant resources. The fiber morphology of bamboo, bagasse, wheat straw and miscanthus grass was characterized to select the suitable fibers for producing packaging materials. Due to the short fiber length and high coarseness, miscanthus grass was excluded from the subsequent experiments. Fiber blend effects on tensile index and paper porosity for bamboo, bagasse and wheat straw were investigated. Five commercial molded pulp food containers were characterized in terms of their physical and mechanical properties to benchmark laboratory fabrication and testing of prototypes. The quantitative effects of two key performance additives: alkyl ketene dimer (AKD) and SOR, and fiber blend were studied through mechanical and barrier property tests. I concluded that: (1) Bamboo fiber has the longest fiber length and can act as a reinforcement by mixing with short wheat straw and bagasse fibers; (2) Bamboo and bagasse fibers with thin cell wall can facilitate the formation of dense fiber network structure, which are better suited for food packaging manufacturing; (3) High sheet density and thin fiber cell wall are two critical factors to reduce the liquid penetration for paper substrate; (4) AKD and SOR are effective to provide water and oil resistance for molded pulp products, but the current bio-based oil repellent appears to not perform as well as PFAS additives; (5) Fiber blend can lead to more synergistic interaction with performance additives and result in better overall mechanical and barrier properties.

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