All-Cellulose Composite Laminates: The Processing-Structure-Property Relationships from the Macro- to the Nanoscale

Cellulose is an excellent resource for the manufacture of sustainable materials, due to its availability and biodegradability. All-cellulose composites (ACCs) are an emerging class of bio-based composites in which both the fibre and matrix phase consist of cellulose. Thereby, ACCs overcome the chemical incompatibility often encountered when hydrophilic cellulose is used as reinforcement of hydrophobic polymer matrices in bio-based composites. The mechanical properties of ACCs are reported to exceed those of traditional bio-based composites, which makes ACCs a promising material in the search for an alternative to petrochemical-derived thermoplastics. However, the manufacture and characterisation of ACCs has been limited to thin films (< 1 mm). Recently, solvent infusion processing (SIP) based on partial dissolution of cellulose fibres in an ionic liquid (IL) has been developed. SIP presents a pathway that allows the manufacture of thick ACC laminates (> 4 mm), which widens the range of potential applications. The aim of this work was the characterisation of the structure and properties of ACC laminates from the macroscopic laminate scale down to the individual fibre and matrix phases on the microscopic scale. The occurrence of size effects in composites reported in the literature poses the question whether increasing the dimensions of ACC laminates impairs the mechanical properties. In this work the effect of increasing thickness on the structure and mechanical properties of ACC laminates based on a woven rayon textile and manufactured by SIP was investigated. A positive size effect of increasing strength with increasing thickness was found. Ultimate tensile strength increased from 80MPa in a single lamina of 0.42mm thickness to 106MPa in an ACC laminate of 8 laminae with a thickness of 3.36mm. A strengthening mechanism for ACC laminates based on a woven rayon textile is proposed. Furthermore, a transition from low-strain failure to tough and high-strain failure with increasing thickness and a scale effect of increasing crystallinity towards the core of thick ACC laminates was observed. SIP has been developed using imidazolium-based ILs, which offer a high cellulose solubility and facilitate controlled dissolution by adjusting the processing temperature. However, ILs are also known to be toxic and non-biodegradable, making them non-ideal solvents for manufacturing a green material. In this work the use of an aqueous 7 wt.% NaOH/12 wt.% urea solution (NaOH/urea) as cellulose solvent for SIP has been explored as an environmentally friendly and cost-effective alternative to ILs. The effect of infusion temperature, dissolution time and cooling during processing were investigated. NaOH/urea facilitated rapid processing of ACC laminates with partial dissolution achieved in 5 min and when compared to IL-processed laminates a similar Young’s modulus in the range of 7 to 8 GPa and a 28% increase in ultimate tensile strength to 123MPa was found. Cooling the SIP setup and the solvent to -12 °C prior to infusion and continuous cooling during infusion were required to achieve homogeneous and optimum mechanical properties. Fourier-transformed infrared spectroscopy (FTIR) and elemental analysis were utilised to confirm the complete removal of IL and NaOH/urea from thick ACC laminates by washing in distilled water. Measuring the conductivity of the washing bath was established as a measure of the solvent content and to determine completion of solvent removal from ACCs. Micromechanical characterisation of the individual fibre and matrix phases by nanoindentation revealed a lower modulus of the matrix in comparison to the fibres, indicative of structural changes with ACC processing. FTIR-microspectroscopy and transmission electron microscopy suggest a more amorphous matrix in comparison to the fibres in ACC laminates. A significant decrease in modulus from 9.5 GPa of as-received fibres to values in the range of 7.9 to 8.9 GPa of fibres in ACC laminates measured by nanoindentation leads to the conclusion that not only the surface but also the core of the cellulose reinforcement is affected by processing