Hierarchical cellulosic reinforcement for composites: enhanced resistance to moisture and compatibility with polymers

Abstract: Cellulosic fibres (flax, hemp, regenerated cellulose) possess decent mechanical properties and they are gaining interest as an alternative to synthetic reinforcement (e.g. glass fibres) in polymers to reduce the petroleum consumption and pollution. In particular, manmade Regenerated Cellulose Fibres (RCF) have been extensively studied as potential reinforcement in polymer composites. For high performances where stability is highly required, RCF among the cellulosic fibres are well qualified due the advantage of being continuous with regular cross section. However, the hydrophilic character and the sensitivity to moisture reduce the use of fibres based cellulose in composite applications. Indeed, the moisture absorption and the low compatibility leading to weak fibre/matrix interface are major factors behind the less interest of utilizing cellulosic fibres in composite intended for high performances. The short term objective of this thesis was to improve the resistance to moisture and the adhesion of regenerated cellulose fibres (RCF) commercialized under the trade name CORDENKA 700 super 3 to Epoxy matrix through chemical treatments by cellulose nano-crystals via silane coupling agents. In Paper I chemical treatments of cellulose nano-crystals using (CNC) esterification and amidification to attach long aliphatic chains is studied. The treatment was successfully achieved as confirmed by spectroscopic characterisations and led to a decrease of the moisture absorption. Contact angle measurement showed hydrophobic of CNC after treatment. In Paper II, CNC extracted from wastes of date palm tree were grafted on RCF fibres to create hierarchical structure. The effect of grafting CNC on RCF was evaluated by tensile tests both in static and loading-unloading. In fact, treatments were revealed to change slightly the microstructure where the orientation of both crystalline and amorphous phases where re-oriented as X-ray analysis showed. Grafted fibres based unidirectional composite were manufactured and transversally tested. Both mechanical properties and resistance to crack were significantly increased by fibre modification. Another approach for chemical modification of RCF fibres was developed in Paper III. In this paper, the process of modification of RCF by CNC is more environmentally friendly. The γ-methacryloxypropyltrimethoxysilane (MPS) was used as coupling agent to attach the CNC onto the fibres. This treatment involves a mixture of water and ethanol as solvents and was run at relatively low temperature. The impact of the treatment on fibres was scrutinised after each treatment basically by MPS and after grafting CNC. Results showed that the modification by silane decreased the stiffness and strength of fibres while the strain at failure was increased. However, after grafting CNC, stiffness and strain at failure were recovered while the strength remained at the same order of magnitude as for fibres treated only by the coupling agent. The effect of these treatments on moisture absorption and on the adhesion with epoxy matrix was the focus of the Paper IV. In this paper, it was shown that at high relative humidity (RH=64%) the treatment by CNC decreased water uptake by factor of two compare to untreated fibres. Besides, the treatments by CNC at different concentrations lessened the impact of moisture on stiffness and strength of fibres after exposure to the same humidity level (RH=64%). Moreover, the pull-out test performed on fibre bundles showed that the adhesion between fibre and matrix is less affected by moisture (samples conditioned at RH=64%) for CNC grafted fibres compare to untreated fibres. The treatment process by MPS was Scaled-up to Non-Crimp Fabric in Paper V and the interlaminar properties of composites reinforced with RCF were studied. Double cantilevered beam (DCB) test was used to characterize fracture toughness, under static and fatigue loading. Regenerated cellulose fibres exhibit highly nonlinear behaviour and strongly influence the performance of their composites. The obtained fracture toughness values were significantly high compared to those of synthetic fibre reinforced composites. However, due to the high nonlinearity, a concrete conclusion was not easy to make on the effect of fibre treatment on the materials performance. Thus, scanning electron microscopy studies were carried out on fracture surfaces which confirmed the treatment effect, qualitatively, on the improvement of interfacial adhesion.