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The aim of this study is to investigate the dimensional variation of polypropylene composites reinforced with UD flax fibers, and the modification of its dynamical mechanical properties after moisture absorption. The effects of fiber content, coupling agent and moisture content were also examined. Results have shown that the swelling increases significantly when increasing the fiber content and the moisture content. It was also concluded that the addition of a coupling agent reduces the swelling of the composite, by improving the adhesion at the fiber/matrix interface. It is interesting to note that the coefficient of hygroscopic swelling of the composites presents a non-linear evolution, with respect to moisture content. Moreover, it was proved that the addition of fiber and the presence of a coupling agent improve the dynamical mechanical properties of the material. However, following moisture absorption, the storage modulus seems to be decreasing, which might be due to the micro-cracks created as a result of the internal swelling of fibers. Additionally, the coupling agent was found to increase the storage modulus of the composite at dry state, whereas following moisture absorption, it was found that the coupling agent does not modify the storage modulus of the material. Moreover, the results reveal that the dynamic mechanical properties of the composites are hardly dependent on the quality of the material, such as the state of the adhesion at the fiber/matrix interface, a possible misalignment of the fibers within the material and the moisture content within the material at the testing time.

Natural fibers have been approved as an excellent alternative for traditional reinforcements in polymer composites. However, the main disadvantage of natural fibers as reinforcements is their poor ...

Natural fiber-reinforced composites are increasingly being used in the industry. The fiber–matrix interfacial properties of the composites are influenced by many factors, including chemical treatment of the natural fiber, type of polymer matrix, composites fabrication method, and process and the service environment of the composites. In this paper, a modified shear-lag model based on a cohesive fiber/matrix interface is proposed and applied to the analysis of the stress–transfer characteristics and the tensile properties of unidirectional short flax fiber-reinforced composites. The model takes into account of the interfacial shear stiffness, bonding strength between fiber end face and matrix, fiber aspect ratio and fiber volume fraction. 3D finite element models of the composites using a cohesive zone method are used to verify the accuracy of the modified shear-lag model. The fiber tensile strength and the composite tensile elastic modulus are significantly influenced by the interfacial shear stiffness, fiber aspect ratio, and fiber volume fraction. The bonding strength between the fiber end face and the matrix only has an effect when the interfacial shear stiffness is low. The predicted results from the modified shear-lag model show good agreement with the finite element analysis and experimental results in the literature. The modified cohesive shear-lag model provides a simple and effective method for analyzing fiber axial stress, shear stress in the fiber/matrix interface, and tensile elastic modulus of the final composite.

Surface free energy of a fiber is an important parameter for predicting the interfacial bond strength of fiber-matrix adhesion of composite materials. For mechanical characterization of bio-composite materials, the measurement of the surface energy of individual single microfibers is complicated due to their surface roughness, formation of chemical bonds, wicking characteristics, etc. This paper demonstrates a novel method for determining the dispersive component of surface free energy [Formula: see text] of single flax fiber by directly measuring the adhesion force between the probe tip of an atomic force microscope and the fiber surface. Johnson–Kendal–Roberts theory was employed to correlate the adhesion force with the surface energy and tip radius. Finally, the value of [Formula: see text] was determined, and its significance with respect to other methods was analyzed.

Tracking the variability of natural fiber-based fabrics properties, such as local areal weight, fiber volume fraction, and therefore permeability, is crucial to optimize the parts processing of the bio-composites. This paper aims at developing a cost-effective and efficient optical method in order to predict the permeability of flax fabrics used in liquid composite molding processes. This method using an LCD monitor as light source and a reflex camera as a measurement device is based on light transmission measurement through fabric thickness. The raw data given by the camera are gray scale maps, transformed into areal weight maps. FEM software based on levelset method is finally used to highlight the influence of the local variability of the fiber volume fraction, and of the related fabrics porosity and permeability on the mold filling time. The proposed method can be directly implemented on the manufacturing line of the composites. It can be used to optimize, part-to-part, the resin consumption by predicting the resin flow through perform. Interestingly, this novel optical method is auto-calibrated and does not depend on picture resolution. © The Author(s) 2018.

The potential of flax fabric in the production of ‘green’ composites based on polypropylene (PP) was evaluated. The laminate composites were prepared by compression molding of flax fabric (with and without treatment) and PP films (using different formulations of PP). The morphology and crystallization of the PP/flax composites were characterized using scanning electron microscope, X-ray diffraction, and differential scanning calorimetry. The mechanical performance of the composites was also evaluated. The results demonstrate that fiber treatment and PP formulation play important roles on the performance of composites. This study proved that the addition of the coupling agent maleic anhydride grafted PP to the composites improves the interfacial adhesion between hydrophilic flax fibers and the hydrophobic PP matrix and thus the mechanical properties. The combination of the coupling agent and reactive additive provides superior strength, modulus and brings further fire resistance to the PP/flax composites.

This work investigates the mechanical and aging properties of flax/glass fibers reinforced polypropylene hybrid composites. The mechanical properties as a function of reinforcement content show that adding glass fiber to polypropylene/flax composites improves tensile modulus and strength as well as impact resistance and hardness, but has negligible effect on strain at yield and elongation at break. Water uptake at 85℃ and variations in mechanical behavior are determined after water, thermal, and accelerated UV aging tests. The results indicate that glass fibers enhance water resistance of polypropylene/flax composites. Thermal aging at 85℃ reveals that irrespective of filler type and content the composites are thermally resistant. According to accelerated UV aging tests, the presence of glass fiber accelerates the degradation of the polypropylene matrix, but flax fiber can protect the composites. Finally, a partial least-squares model is built to correlate the composite composition to the properties of aged and unaged specimens.

In the past decade, natural fiber composites with thermoplastic matrices have been embraced by many composite manufactures for its properties such as lightweight and low cost. Reinforced cowrap spinning yarns and thermoplastics offer a potential for low-cost manufacturing of complex-shaped and good, drapable composite parts, due to reduced impregnation times and resin flow distance during processing. In this article, the cowrap spinning yarns were produced by cowrap spinning method with flax and polypropylene (PP) and then woven to fabric preforms. The natural flax fibers were twistless in yarns and preforms, and PP served as a carrier for flax and prevented damage to flax fibers during processing, and became the polymer matrix in the final composites. To get an even distribution of fibers and thermoplastic resins in preforms and different flax fiber content, the main spinning parameters (hollow spindle twist and hollow spindle rotational speed) were analyzed and optimized. Besides fabric structure, the parameters affecting the mechanical properties of flax/PP preforms and composites were investigated. The fabric density, fabric structure (including plain, twill, and basket weave), and yarn tensile orientation (at 0°, 90°, and 45°) had a great influence on the tensile strength and elongation of preforms. Different pretreatments on flax/PP were investigated, and the microstructure and mechanical properties of thermoplastic composites showed improved wettability of impregnation and good adhesion and bonding interface between flax fiber and PP matrix in composites.

Flax fibers possess high specific strength and stiffness, and thus are competitive in terms of mechanical properties with traditional reinforcing fibers used in polymer-matrix composite materials. For environmental and economical benefit, it would be preferable to apply nontextile grade fibers in composites provided their mechanical characteristics are acceptable. Elementary fibers have been extracted from long line flax, used as high-quality raw material for textile industry, and flax tow, and their strength distribution and damage level determined. It is shown that the elementary flax fibers coming from short flax fiber are not inferior to those of textile-quality flax in terms of strength and damage.