The objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics. / text

Actuellement, 80 à 85% des cuirs sont tannés au chrome. Ce procédé permet d'obtenir d'excellentes propriétés physico-mécaniques dans des conditions économiques optimales. Les sels de chrome sous forme trivalente utilisés lors de la fabrication du cuir ne sont pas nocifs; cependant sous certaines conditions et associés à d'autres substances, le chrome trivalent peut se transformer en chrome hexavalent, très allergisant et pouvant s'avérer dangereux pour la santé du consommateur. Depuis le 1er mai 2015, la présence de chrome hexavalent dans le cuir est soumise à la législation européenne; la teneur en chrome hexavalent ne doit pas excéder plus de 3 mg/kg de poids sec total du cuir (REACh ANNEXE 17, §47). A ce jour, de nombreux travaux de recherche ont été réalisés et des premières hypothèses ont été avancées pour expliquer la formation du chrome hexavalent dans le cuir. L'objectif de ce projet de recherche a été de rationaliser et d'approfondir ces différentes études pour essayer de mieux appréhender cette problématique. Ce projet de thèse a permis d'améliorer la spécificité et la fiabilité la méthode d'analyse pour la détermination de la teneur en chrome hexavalent en introduisant la chromatographie ionique. Ensuite, l'influence de la composition du cuir (matière grasse, indice d'iode, pH, chrome total et chrome extractible) sur la formation de chrome hexavalent dans le cuir a été étudiée afin d'expliquer l'origine et d'éventuels mécanismes d'oxydation du chrome trivalent (en chrome hexavalent). Enfin, cette étude s'est intéressée aux facteurs environnementaux (température, humidité relative, lumière et durée d'exposition), aux conditions de fabrication du cuir ou encore au cycle de vie du cuir (transport, transformation en produit fini, utilisation par le consommateur) afin de déterminer l'impact de ces paramètres sur la formation du chrome hexavalent. A l'issue de ce travail, une méthode de vieillissement "artificiel" représentative de l'évolution réelle d'un cuir a été proposée, et ce quelles que soient les caractéristiques et la nature du cuir / Currently, 80 to 85% of leather is tanned with chrome. This process allows to obtain excellent physico-mechanical properties with optimum economic conditions. The chromium salts in trivalent form used in the manufacturing of leather are not harmful; however under some conditions and associated with other substances, trivalent chromium can be transformed into hexavalent chromium, highly allergenic and potentially harmful to the health of the consumer. Since 1st May 2015, the presence of hexavalent chromium in leather is subject to European legislation; the hexavalent chromium content must not exceed 3 mg/kg of the total dry weight of leather (REACh ANNEX 17, § 47). To date, a great amount of research has been done and initial hypothesis have been able to explain the formation of hexavalent chromium in leather. The objective of this research project was to rationalize and to deal with these various studies in order to understand this problem in more detail. In this thesis project, the introduction of ion chromatography has improved the specificity and reliability of the analysis method for the determination of hexavalent chromium. Secondly, the influence of leather composition (fat liquor, iodine value, pH, total chromium and extractible chromium) on the formation of hexavalent chromium in leather was studied. The objective was to explain the potential oxidation mechanisms of trivalent chromium (in hexavalent chromium). The third part of this study deals with environmental factors (temperature, relative humidity, light and duration of exposure), leather manufacturing conditions or the life cycle of leather (transport, transformation into finished product, consumer use). The aim was to determine the impact of these parameters on the formation of hexavalent chromium. To conclude, an "artificial" ageing method representing the natural evolution of leather, and which corresponds to the characteristics and the nature of the leather, has been proposed