Cellulose consolidation under high-pressure and high-temperature uniaxial compression
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Materials based on cellulose cannot be obtained from thermoplastic processes. Our aim is to prepare all-cellulose materials by uniaxial high pressure thermocompression of cellulose. The effect of moisture content (0–8 w/w%) and temperature (175–250 °C) was characterized through the mechanical properties (bending and tensile), morphology (scanning electron microscopy, X-ray tomography) and microstructure (viscometric degree of polymerization, Raman spectroscopy, X-ray diffraction, solid-state NMR) of the specimens. The specimens were mechanically stronger in bending than in tension. They exhibited a more porous heart, a dense but very thin skin on the faces (orthogonal to the compression axis) and thick and extremely dense sides. During thermocompression severe friction between fibers caused a decrease in molecular weight while heating above the glass transition temperature was responsible for water migration towards the specimen heart. Most of the cohesion came from the small sides of the test samples (parallel to the compression axis) and seemed mainly related to the entanglement of amorphized cellulose at the interface between particles. Around 200 °C water accumulated and provoked delamination upon pressure release, but at higher temperatures water, in a subcritical state, may have been consumed during the hydrolysis of amorphous cellulose regions. The all-cellulose material with the best mechanical properties was obtained at 2% moisture and 250 °C. This work shows that thermocompression at high temperature with limited moisture may be viable to produce renewable, sustainable all-cellulose materials for application in biobased plastic substitutes including binderless boards.
KeywordsAll-cellulose materials Compression Mechanical properties
The French National Research Agency (ANR) and the Competitive Cluster for the Agricultural and Food Industries in South-West France (AGRIMIP) financed this study under the aegis of the HYPMOBB (High Pressure Molding of Biopolymers and Biocomposites) project. PC thanks the Academic Development Program at WSU for his stay at INP Toulouse. The authors are grateful to Brigitte Dubreuil and Manuel Marcoux for their precious help with Raman spectroscopy and X-ray tomography, respectively. The authors thank the Advanced Materials Characterisation Facility (AMFC) at Western Sydney University (WSU) for the use of the X-ray diffractometer and the scanning electron microscope (SEM), Matthew Van Leeuwen (WSU) for discussions on deconvolution of XRD data, as well as the School of Science and Health (WSU), and Dr James Hook, Dr Aditya Rawal (Mark Wainwright Analytical Centre, University of New South Wales) for the use of solid-state NMR spectrometers. MH thanks Conseil Régional Midi-Pyrénées for financial help for her stay at WSU.
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