Cellulose is an excellent resource for bio-based materials production due to its high strength, high stiffness, low density, availability, and biodegradability. An incompatibility issue experienced by many of the bio-composites has led to the development of a new class of biobased materials aka all-cellulose composite (ACC). ACC is produced from cellulose for both reinforcement and matrix phases. Non-derivatising solvents and coagulants are used to dissolve and regenerate cellulose, respectively. The excellent mechanical properties of ACCs compared to other bio-based materials have been reported. Recently, solvent infusion process (SIP) has shown a promising processing route for ACCs production. As a result, extensive scientific research has been focussed on investigating the best processing route for ACCs production. Hygrothermal effects are the effects of water/moisture, temperature or the combination of both factors on the structures and properties of bio-based materials. A potential problem for ACCs is their propensity for absorbing water due to the use of cellulose for both reinforcement and matrix phases. Cellulose is known to have numerous hydroxyl groups, resulting in strong cellulose-cellulose and cellulose-water interaction. There are very limited studies of hygrothermal effects on the mechanical properties of ACCs. Hence, the aim of the present study was to investigate hygrothermal effects on the structure and properties of ACCs.

This thesis investigated the production of ACCs using two types of solvents (BmimAc and NaOH/urea) and a precursor cellulose (woven rayon textile). Both types of ACCs were manufactured using SIP. A design of experiment methodology (DOE) and a statistical analysis based on the Taguchi approach were used to study the multivariable control factors including the dissolution time (td), dissolution temperature (Td), pressure at hot press (PHP), and pressure vii at SIP (PSIP) incorporated with a wide range of its levels during SIP. The effect of control factors on the mechanical properties of ACCs was investigated using the DOE Taguchi approach, resulting in the best combination of control factors and their levels in maximising the mechanical properties of ACCs. Multiple regression analysis (MRA) was used to determine the accuracy of the experimental and theoretical mechanical properties. Interestingly, td was the most significant control factor for both ACCs, followed by PHP. The effect of td on phase characterisation, viscoelastic properties, density and microstructure was also investigated.

The water absorption behaviour of ACCs was investigated. The effects of water bath temperature and td on the water absorption behaviour were examined. The water absorption behaviour was also associated with the changes in the structure and phase composition of ACCs. The relationship between experiments and theory for water concentration (Mt/Mm) and thickness swelling (TS) was investigated. Finite element-based simulation work using ANSYS was used to (i) determine the effect of water bath temperature on Mt/Mm at different thicknesses, and (ii) observe the correlation of saturation time between theoretical calculations and experimental data.

The effect of varying water contents on the mechanical properties, glass transition temperature (Tg), and structural changes of ACCs were studied. The tensile testing, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and Fourier-transformed infrared spectroscopy (FTIR) were used for ACC characterisation. A decreasing Young’s modulus (E) and ultimate tensile strength (UTS) were observed with an increasing water content of ACCs. An increase in the strain to failure (ɛf) was also observed with the increasing water content. A higher water threshold associated with increasing freezing bound water was found in ACCs produced via NaOH/urea than that produced via BmimAc. The presence of free water was observed beyond the water threshold for both types of ACC. A decrease in Tg was also observed due to plasticisation effects, caused by softening of the amorphous phase in ACCs. The experimentally determined values of E were within ±15% of the theoretical value of E as calculated from the C-O-C region in the FTIR spectrum. Interestingly, the structural changes in the C-O-C and OH regions were observed to affect the mechanical properties of ACCs due to the increasing strain.

The hygrothermal effect on the creep performance of ACCs was investigated. The time temperature superposition (TTS) principle was used to construct master creep curves using short-term creep tests. The results of experimental long-term creep testing was compared with the TTS master curve in order to investigate the physical aging of ACCs. The Bürgers model for creep could be used to describe the experimental creep of ACCs. Increasing creep strain in ACCs was also observed with the increasing water content and temperature. Higher creep strains were observed for ACC produced via NaOH/urea compared with BmimAc due to slippage effects. The results suggest that physical aging in all of the ACCs took place at the temperature of 30 °C. Physical aging was negligible at temperatures between 150 and 200 °C. As a conclusion, td was found as the most significant control factor in ACCs production via SIP method. The optimum td for ACCs produced via BmimAc and NaOH/urea were 30 and 15 min, respectively. A higher water diffusivity was found in the ACCs as compared with other bio-based composite materials due to cellulose being used for both the reinforcing and matrix phases. Furthermore, a decrease in the mechanical properties was reported with the increasing absorbed water into the ACCs. These results were due to the presence of bound and free water at a lower and higher water content, respectively. An increase in creep strain was also observed at a higher water content that was due to the decreasing hydrogen bonding density and increasing molecular-level slippage effects.