Continuous metallized regenerated cellulose fibers for advanced applications (e.g. multi-functional composites) are produced by electroless copper plating. Copper is successfully deposited on the surface of cellulose fibers using commercial cyanide-free electroless copper plating package commonly available for manufacturing of printed wiring boards. The deposited copper is found to enhance the thermal stability, electrical conductivity and resistance to moisture uptake of the fibers. On the other hand, involved chemistry results in altering the molecular structure of the fibers as is indicated by the degradation of their mechanical performance (tensile strength and modulus).

A novel testing methodology for the characterization of viscoplastic (VP) deformation of unidirectional (UD) composites under transverse tensile loading is described. The law of transverse VP deformation, which, due to specimen failure, is almost impossible to obtain using UD specimens, is identified from curvature changes of an unsymmetrical CF/EP [0/90] specimen. An initially thermally curved specimen is loaded in the axial direction to different high levels of tensile strain, with a specified holding time. After unloading and waiting for recovery of the viscoelastic strains, the curvature of the specimen had changed due to the irreversible transverse VP strain and microdamage had developed in the 90° ply. The damage effect is incorporated into the model by introducing the“effective stiffness” of the damaged layer, which is a function of the crack density and the local delamination length. A model based on the classical laminate theory is used to construct a relationship between the measured curvature after a loading step and the accumulated VP strain, temperature, and the effective stiffness. The back-calculated VP strains are analyzed, and their functional dependence on the stress and time in the viscoplasticity law is obtained. The model and the methodology are validated by comparing calculation results with test data for [902/0] laminates subjected to a similar loading. The methodology suggested could be very accurate on using a laminate with ultrathin plies in the test.

The effect of treatment of regenerated cellulose fibres by cellulose nanocrystals on the moisture absorption of the fibres as well as on fibre–epoxy resin adhesion has been investigated. Nanocrystals were deposited on the fibres using γ-methacryloxypropyltrimethoxysilane (MPS) as coupling agent. Water absorption tests performed on fibres showed that, at 64% relative humidity, treatment by the coupling agent decreased the water uptake by a factor of two compared with untreated fibres, whereas deposition of cellulose nanocrystals (CNC) on fibres treated with MPS (FMMPS) did not further increase the water absorption despite the hydrophilic character of the CNC. This result was confirmed by monitoring fibre swelling using contact angle measurements. Indeed, it was found that FMMPS presented the same contact angle with glycerol before and after CNC deposition, being higher than that obtained for untreated fibres. The tensile strength and stiffness of fibres were not affected by moisture after either treatment, but nanocrystal deposition enhanced the fibre–epoxy resin adhesion, as revealed by results of pull-out tests performed on fibre bundles at 64% relative humidity.

A simple and innovative new route, with less negative impact on the environment, for depositing and hope-grafting cellulose nano-crystals onto the surface of regenerated cellulose fibres (Cordenka 700 Super 3), using γ-methacryloxypropyltrimethoxysilane as coupling agent, is presented. Hierarchical cellulosic structure involving micro-scale fibres and nano-scale cellulose crystal network was created as verified by the scanning electron microscopy. The fibres were initially oxidised by optimized concentration of cerium ammonium nitrate to generate radicals on the cellulose backbone in order to polymerize the coupling agent at the surface. Infrared spectroscopy and scanning electron microscopy confirmed the chemical polymerisation of MPS onto regenerated cellulose fibres without enabling to show the chemical bonding between silane and nano-crystals. However, tensile test which was performed to study the impact of different treatments on mechanical properties of regenerated cellulose fibres, revealed that the modification by silane decreased the stiffness and strength of fibres (22% and 10% decrease, respectively) while the strain at failure was increased. These changes were attributed to the treatment conditions which may have induced the disorder and the misalignment of the structure of cellulose fibres (e.g. axial orientation of molecular chains and crystalline phase of the fibre has been reduced). This assumption is supported by the results from successive loading-unloading test of the fibre bundle. However, after depositing cellulose nano-crystals onto the fibre’s surface, the stiffness was recovered (20% increase in comparison to MPS treated fibres) while the strength and strain at failure remained at the same order of magnitude as for fibres treated only by the coupling agent.

Curvature of unsymmetrical [0/90] specimens caused by thermal stresses changes if the specimen is subjected to large axial strains introducing intralaminar cracks in the 90-layer. It is shown that the large curvature reduction can not be explained by cracking related stress release only. The large irreversible viscoplastic strains introduced during the axial tensile loading (with 5 min holding at high strain for crack counting) give the main contribution to the curvature change. The effect of transient viscoelasticity (VE) was found to be of minor significance. Simple approach based on effective damaged layer stiffness and constant irreversible strain is used in the framework of laminate theory to extract the viscoplastic and VE strains from experimental curvature data. The obtained fitting expressions for viscoplastic- and VE-strain development are successfully used to describe curvature change in [0/90₂] laminate subjected to the same test procedure. It is suggested that the used curved beam tests could be efficient to characterize the viscoplastic strain development in the thin 90-layers

This paper describes the effect of surface modification of regenerated cellulose fibers by chemical treatment and deposition of cellulose nano-crystals onto fibers. The effect of this modification on themoisture absorption by fibers and interface properties with epoxy matrix has been studies. The preliminary results show positive trends in reducing moisture uptake by fibers and improving interfacial shear strength.The deposition of cellulose nano-crystals at different concentrations onto regenerated cellulose fibers creates a network covering surface of fibers and interconnecting them. This resulted in rather significant reduction of the moisture absorption compare to untreated fibers (8% vs 12% respectively)and improving interfacial shear strength of regenerated cellulose fiber/epoxy system. The increase of the interfacial shear strength measured from bundle pull-out test on fibers conditioned at 64% relative humidity has been observed. Thus, the hierarchical structure created by grafting nano-crystals onmicro-sized cellulose fibers resulted in improvement of fiber/matrix adhesion by reducing water absorption.

This paper presents environmentally friendlier technique for deposition of cellulose nano-whiskers onto the surface of regenerated cellulose fibres using γ-methacryloxypropyltrimethoxysilane as coupling agent. The result of this treatment is hierarchical reinforcement consisting of micro-scale fibres and nano-scale cellulose crystal network. In order to evaluate influence of treatment on fibre performance, tensile tests of fibre bundles were carried out. The results show that there is significant impact on stiffness of fibres only by first modification by silane, whereas grafting of cellulose nanowhiskers onto the surface of the fibre allowed recovery of initial properties. It is assumed that thetreatment may have induced the misalignment of macromolecular chains and crystalline cellulose phase with respect to the fibre axis.

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.

Microdamage, viscoplastic and viscoelastic strain development in 90-layers of cross-ply laminates subjected to tensile loading is studied on unsymmetrical GF/EP laminates measuring the thermal curvature change. All three phenomena partially compensate for the effect of the thermal mismatch reducing the residual stress (specimen curvature). The viscoplastic strain contribution to curvature change is the largest whereas the effect of transient viscoelasticity is the smallest. Damage is included in the analysis through its effect on the effective transverse modulus of the 90- layer.

The interfacial shear strength of short regenerated cellulose fibre/polylactide composites was characterized by means of an industry-friendly adhesion test method. The interfacial shear strength was back-calculated from the experimental tensile stress-strain curves of composites by using a micro-mechanical model. The parameters characterizing the microstructure of the composites, e.g. fibre length and orientation distributions, used as input in the model were obtained by micro-tomography. The investigation was carried out on composites with untreated and surface treated fibres with various fibre weight contents (5wt%, 10wt%, and 15wt% for untreated fibres, and 15wt% for treated fibres). The properties of fibres were measured by an automated single fibre tensile test method. Based on these results, the efficiency of the fibre treatment to improve fibre/matrix adhesion is evaluated, and the applicability of the method to measure the interfacial shear strength is discussed. The results are compared with data from previous work, and with other results from the literature