Processing of All Cellulose Composites via an Ionic Liquid Route

Newly developed all-cellulose composites (ACCs) can overcome the chemical incoherence between cellulose and other polymers by dissolution and regeneration of a portion of cellulose to create a chemically identical matrix phase. New “close to industry”-processing ways for ACCs were developed to create “thick” ACCs (>1 mm thickness) based on composite processes already used in the composite industry. The ionic liquid (IL) 1-Butyl-3-Methylimidazolium Acetate (BmimAc) is a strong solvent for both, native cellulose and cellulose II. The dissolution process is strongly depended on the temperature and viscosity of the IL-cellulose solution. Next to complete dissolution, rayon fibre can be dissolved partially to achieve the formation of a matrix phase in situ. The highly hydrophilic cellulose based materials show different amounts of shrinkage after composite processing when the coagulant necessary to regenerate the dissolved cellulose is removed by evaporative drying. Multilayered, “thick” composite laminates could be produced by a simple hand-impregnation of rayon and linen textiles with the solvent and partial dissolution of the cellulosic textiles. A solvent infusion process (SIP) based on vacuum assisted resin infusion was successfully developed to process ACCs. The application of pressure during SIP is crucial to achieve good interlaminar adhesion. The SIP based laminates showed improved tensile strength and stiffness compared to the hand impregnation process. An analysis of the processing parameters showed that the drying process used to remove the coagulant is important to achieve good fibre-matrix-bonding as harsh evaporative drying causes shrinkage induced cracks in the created matrix phase. Using ethanol as a coagulant instead of water reduced composite swelling and corresponding shrinkage, but leads to a strong reduction in crystallinity of the regenerated cellulose, as shown X-ray diffraction and solid state NMR measurements. Regeneration in distilled water, followed by drying at room temperature produced the best ACC laminate. The SIP based laminates showed high flexural and impact strength compared to other biocomposites. The composites were also found to be easily compostable especially compared to a PLA-rayon composite. The rayon fibre was processed on an ITA 3D rotary braiding machine, generally used for the processing of stronger and stiffer glass and carbon fibres. A rectangular profile was produced and analysed. The fibre strength and Young’s modulus were unaffected by the braiding process. The braid could be processed into an ACC by immersion in IL for 60 min at 100 °C. The so produced ACCs showed further improvements in tensile and impact strength due to improved through the thickness strength