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Following their mechanical extraction, the fibers coming from the banana pseudo-stem are coarse and the inter fiber cohesive substances need to be removed before their transformation into textile y...

Resistance to impact thrust is one of the most important requirement along with many others for defining suitable applications of any composite materials. In this present work, the influence of composite architecture and composite interfacial bonding strength on the impact performance of flax reinforced PP composites have been investigated. In this regard two different composite architecture through conventional film stacking method and through DREF (DR. E. Fehrer open-end friction spinning process) yarn consolidation method have been produced and to alter the fibre–matrix interfacial bonding the flax fibres are treated with MAgPP (Maleic Anhydride grafted Polypropylene). Finally, the notch Izod impact behaviour of all composite samples are evaluated and compared.

International audience; Linseed flax is a multipurpose crop. It is cultivated for its seeds and particularly for its oil. The main contributors for this crop are Canada, France and Belgium. In general, straws of linseed flax are buried in the fields or burnt. However, these solutions are not good practices for the environment and from an economical point of view. In this study, straws of linseed flax (six batches in total) with different dew retting durations and harvesting techniques were studied to possibly use them for producing innovative geotextiles. Two different fibre extraction processes were investigated. A first process (A) involved horizontal breaker rollers and then a breaking card. A second one (B) consisted in using vertical breaker rollers, and an “all fibre” extraction device (fibre opener) followed by sieving. The chemical composition of fibres in parietal constituents appeared to be globally equivalent to the one of textile flax with a pectic content decreasing as a function of the dew retting duration. This contributed to an increase in the cellulose content. The fibre content was situated in a range from 29% to 33%, which corresponds to a good yield for linseed flax fibre. The level of purity can reach values of up to 90% for method A (without extra-sieving) and 96% for method B (with extra-sieving), and the length of the fibres (larger for method A than for method B) and their tensile properties make them suitable for structural geotextile yarn manufacturing.

International audience; Retting of fibrous plants such as flax is an essential step in the extraction of fibre bundles and their transformation into textiles and reinforcement fabrics for use in garments and composites. Dew-retting is traditionally performed from Northwest France to the Netherlands, as the climate is highly favourable for this process. Hemp is a plant that can be grown almost all over Europe with a low environmental impact. A retting step is also required to facilitate the separation of the hemp fibres before their transformation into textiles for garments or for 1D to 3D reinforcement composites, which requires thoroughly separated fibres. Dew-retting is currently used in flax production zones. The present work demonstrates that dew retting can be conducted under different climates on different soils, from dry Mediterranean environments up to the cooler regions of eastern France. If the ternary combination of moisture, temperature and solar radiation is appropriate, field retting (dew-retting) can be as short as about three weeks. In less favourable conditions, such as in dryer areas or when retting is performed late in the season after seed maturity (cooler temperatures), it lasts longer, but it can reach suitable levels. When conducted with care and with proper monitoring of the retting level, the dew-retting process does impact neither the tensile properties of elementary hemp fibres (by degrading crystalline cellulose I) nor the tensile properties of unidirectional and injected composite materials. Consequently, if extracted with a suitable process such as scutching and hackling, fibres suitable for load-bearing composites can be produced from dew-retted hemp stems produced in a wide range of climates and locations, therefore not limited to the conventional “dew retting zone” of flax production areas.

International audience; A versatile twin-screw extrusion process to provide an efficient thermo-mechano-chemical pre-treatment on lignocellulosic biomass before using it as source of mechanical reinforcement in fully bio-based fiberboards was developed. Various lignocellulosic crop by-products have already been successfully pre-treated through this process, e.g., cereal straws (especially rice), coriander straw, shives from oleaginous flax straw, and bark of both amaranth and sunflower stems.The extrusion process results in a marked increase in the average fiber aspect ratio, leading to improved mechanical properties of fiberboards. The twin-screw extruder can also be fitted with a filtration module at the end of the barrel. The continuous extraction of various chemicals (e.g., free sugars, hemicelluloses, volatiles from essential oil fractions, etc.) from the lignocellulosic substrate, and the fiber refining can, therefore, be performed simultaneously.The extruder can also be used for its mixing ability: a natural binder (e.g., Organosolv lignins, protein-based oilcakes, starch, etc.) can be added to the refined fibers at the end of the screw profile. The obtained premix is ready to be molded through hot pressing, with the natural binder contributing to fiberboard cohesion. Such a combined process in a single extruder pass improves the production time, production cost, and may lead to reduction in plant production size. Because all the operations are performed in a single step, fiber morphology is better preserved, thanks to a reduced residence time of the material inside the extruder, resulting in enhanced material performances. Such one-step extrusion operation may be at the origin of a valuable industrial process intensification.Compared to commercial wood-based materials, these fully bio-based fiberboards do not emit any formaldehyde, and they could find various applications, e.g., intermediate containers, furniture, domestic flooring, shelving, general construction, etc.

International audience; The tow-sliding defect also referred to as gapping that appears during complex shaping of woven reinforcementsis due to a loss of cohesion in the fabric. The cohesion of the fabric tends to keep the tows from sliding undertension. This cohesion depends on the frictional behavior of the tows mainly. Other factors such as the weaveof the fabric, the tensions and shear state of the fabric during forming play an important role as well. An analysisof the frictional behaviors of the tows coupled to out of plane pull-out tests to determine the influence theaforementioned parameters on the cohesion of the reinforcements was then carried out. It was showed that thecohesion of the fabric, limiting the risk of appearance of tow sliding defect, can be improved by increasing thetransverse tow tension in low shear region, and by the in-plane shear itself in highly sheared zones.

International audience; This work assesses the potential of nettle (Urtica dioica L.) fibers produced on contaminated lands for composite applications. The nettles studied in this work grew spontaneously and in a prevalent manner in poplar short rotation coppice planted for the phytomanagement of a land contaminated by traces of metals. Results show that the contaminant contents in nettle bast fibers are low: only traces were measured. It makes it possible to consider this biomass for material use. The measured matter yield is lower than those obtained with traditional fiber crops cultivated in Europe on agricultural lands but the tensile properties of the bast fibers are equal to or better than those of hemp and flax, making spontaneous nettle an interesting supplement to traditional European fiber crops for composite applications

International audience; The significant industrial development of non-woven biocomposites requires the implementation of environmentally and economically coherent end-of-life recycling solutions. In this study, we studied the recycling of a non-woven poly-(propylene)-flax composite by injection but also by thermo compression. For comparison, a material with the same architecture but reinforced by glass fibres was studied. Both recycling methods showed strong specificities. Injection recycling leads to efficiently homogenised microstructures of the parts but also to drastically reduced lengths of the fibres, up to 10 times lower than with compression moulding. This method globally promotes high failure strengths while compression moulding, by preserving the length of the fibrous reinforcements, guarantees higher stiffness. This work also highlights the impacts of the length and division of the fibre elements on the microstructure of the injected parts; thus, after a series of compression recycling cycles, injected parts exhibit an important skin-core effect larger than after initial injection recycling cycles, whether in terms of orientation or local fibre volume fraction. As a consequence, after a series of recycling by compression, a new injection cycle has for effect to improve the tensile mechanical performances. For example, the strength and modulus of PP-flax composites are increased by 103% and 75%, respectively. These results highlight the technical feasibility and relevance of implementing these two recycling methods, depending on the volumes or equipment available and the final properties to promote, as they enable the production of new highperformance parts.

Fibres from hemp stems can be extracted through different mechanical processes following dew or water retting etc. Extraction processes generally have a significant impact on mechanical and morphological properties of the fibres. In this study, hemp fibres are extracted following three different ways. In the first route hemp fibres are extracted from FUTURA 75 variety stems by performing scutching, hackling and microwave degumming. A second batch of fibres of the same variety was extracted by scutching and hackling after an initial microwave degumming treatment. In the third route, the same variety of hemp fibres are extracted from dew retted stems grown at Piacenza (Italy). Finally, the mechanical properties of single fibres as well as the fineness of technical fibres of all types of extracted fibres are evaluated and compared and the interest of the microwave degumming for hemp stem is evaluated.

National audience; In this study, fiberboards were produced from shives collected after continuous fiber mechanical extraction from oleaginous flax straw. Fiberboards were produced through thermo-pressing, and their mechanical and thermo-mechanical properties were studied, as well as their water resistance. The influence of one pretreatment for shives and lignin addition was investigated on the different properties. Boards obtained were all cohesive hardboards. The optimal board was obtained from fibers extruded from shives and without addition of any supplementary lignin amount. Looking at its characteristics and standard NF EN 312, the latter perfectly complied with the requirements for type P1 boards, i.e. boards for general uses in dry conditions.; Dans cette étude, des panneaux agglomérés de fibres ont été produits à partir d’anas collectés après extraction mécanique des fibres libériennes de paille de lin oléagineux. Les panneaux ont été mis en oeuvre par thermopressage. Leurs propriétés mécaniques et thermo-mécaniques ont été étudiées ainsi que leur comportement vis-à-vis de l’eau. L’influence d’un prétraitement thermo-mécanique des anas bruts à l’aide d’une extrudeuse bi-vis a été étudiée ainsi que l’ajout de lignine exogène. Tous les panneaux mis en oeuvre possèdent une tenue suffisante pour être manipulés à la main. Le panneau ayant obtenu les propriétés optimales a été obtenu à partir des anas extrudés et sans ajout supplémentaire de lignine. Au regard de ses caractéristiques et du standard NF EN 312, ce dernier remplit complètement les critères d’un panneau aggloméré de type P1 (panneau d’usage général utilisé en milieu sec).

Flax-PP based thermally bonded roving (TBR) has a unique structure where the flax fibres remain twist-free and fully aligned along the roving axis. The present study describes an experimental investigation on the low velocity impact (LVI) behaviour of the TBR based woven fabric composites and compares the same with plain woven glass fabric reinforced PP composites (GRPC). Two different fabric architectures namely plain woven (PW) and unidirectional (UD) are fabricated using flax/PP based TBR. These TBR based woven fabrics and the glass fabric/PP sheets are consolidated in a compression moulding machine and the resultant composite-laminates are tested for their LVI behaviour. The impact test results revealed that the glass/PP composites absorb more energy and exhibit a higher peak load than both TBR based PW and UD fabric composites. However, the specific load and energy of all flax/PP composites are higher than the glass/PP composite. The damage tolerance of all composite laminates are evaluated by comparing their flexural strength before and after the impact. It is observed that the proportionate loss in flexural strength due to impact thrust is larger in case of glass/PP composites than all flax-PP composites.

International audience; This work assesses the potential of stinging nettle (Urtica dioica L.) growing on trace element contaminated soils to produce fibres for material applications. The nettles studied in this work grew spontaneously and dominated the vegetation cover in poplar short rotation coppices planted for the phytomanagement of lands contaminated by trace elements. Two sites were studied, contaminated by Hg for the first one and a mix of As, Cd, Pb and Zn for the second one. Results show that, for the considered soils, the contaminant contents in nettle bast fibres were at low levels, comparable to those collected at unpolluted control areas. It makes it possible to consider this biomass for material use. The measured matter yield was lower than those obtained with traditional fibre crops cultivated in Europe on agricultural lands. However, the tensile properties of the bast fibres mechanically extracted without field retting or prior alkaline treatment were equal to or better than those of industrial hemp and flax, making spontaneous nettles an interesting supplement to traditional European fibre crops for material applications.

International audience; This work proposes to analyse the different aspects that should be taken into account from the fibre extraction from plants to the forming process to achieve correct part forming. The study will focus in a first extent on the impact of the textile operations leading to the yarn manufacturing. The properties of the flax fabric will then be associated to its behaviour during experimental forming and related to the possible appearence of defects such as tow buckling and solutions to prevent its appearance is widely discussed.

Colloque avec actes et comité de lecture. Internationale.; International audience; The composite manufacturing process occupies a more prominent place in the aerospace and automotive industries due to the lightweight and high performance of the fibre reinforced polymers. The first step in this manufacturing process consists in forming a flat textile reinforcing structure into a designed (tailored) form. The woven textile preform is widely used in the composite manufacturing for its good draping and flexibility properties. The quality of the final woven composite part depends on the fibre distribution and orientation. It also depends on the absence or presence of forming defects. The reasons of occurrence of defects are related to tool geometry, process parameters, textile characteristics, relative plies orientation, inter-ply friction and fabric-tools interaction. Loss of cohesion in the woven fibre network (intra-ply yarn sliding) is a frequent defect in the forming process and it is expected when the cohesion between the yarns is weak or when the blank holder pressure is high. However, the mechanism of formation of this defect is not fully understood. In the present study, forming experiments with friction-based holder have been conducted for one layer 2x2 twill woven carbon fabric (HexForce 48600 C 1300) in three fabric orientations and also for two plies of this fabric with different relative plies orientation. The occurrence of the intra-ply yarns sliding has been observed in the different configurations and as a function of the blank holder pressure. A correlation between the occurrence of this defect and the fabric orientation has been observed. Otherwise, the effect of the fabric orientation, number of plies, relative plies orientation and blank holder pressure on the recorded forming force and on the fabric in-plane shear is also reported and analysed. That permits to better understand the multilayer woven fabric behaviour during forming and this leads to a better understanding of the loss of cohesion defect within the woven fibre network (intra-ply yarn sliding). The mechanisms associated to the appearance of this defect and their dependency on the process parameters and the textile preform characteristics will be discussed and presented in this work.

International audience; Cultivated primarily for its seeds, oleaginous flax could also be valued for the different fractions that can be extracted from the straw. However, as the straws are not harvested with the same technique and care than for the textile flax, the classical scutching technique cannot be used. As a consequence, an “all fibre” device was used to perform the separation of the different constituents of the oleaginous flax straws. The different fractions were quantified for two retting levels and for two degrees of rewetting of the stems. The physical and mechanical properties of fibres were then evaluated. It appears that the relative amount of fibres extracted from oleaginous flax straw is comparable to the one from textile flax (i.e. 40% of the stem dry mass), and their tensile properties are situated in the lower part of the textile flax range. This work shows that the individual fibre length of oleaginous flax (between 3 and 6 cm) is comparable to that of the scutched textile flax fibres. This makes them suitable for the production of carded aligned fibre yarns for technical reinforcement textiles (e.g. composites or geotextiles). These results demonstrate the interest and the potential added value of harvesting the stems for technical fibre applications.

International audience; Geotextiles are widely used to stabilize river banks from erosion when these ones are restored into vegetal covered areas as mentioned by European regulations. For these applications imported coir (coconut shell fibres) based geotextiles are generally used because coir fibres show a good resistance to soil degradation. In Europe, flax and hemp plants are already grown for textile, building or oil applications. By-products of these industries such as flax tows and short hemp fibres were used to manufacture yarns. The resistance to degradation via the measurement of the mechanical properties of these yarns submitted to enzymatic (cellulase) and microbial attacks (Cellvibrio gandavensis) mimicking soil degradation was evaluated. Large decreases in mechanical properties were observed, even though these ones were still higher than the as received reference coir material. After impregnation by chitosan of the fibres, the tensile properties of the yarns globally remained unchanged after severe attacks. The chitosan acts as a protection against the soil microorganism attacks. As a consequence, flax and hemp by-products could be good candidates for local manufacturing of biodegradable geotextiles.

International audience; The use of natural fibres for technical advanced products such as composites is widely increasing with the view to reduce the impact of the material throughout its life cycle on the environment. Some work has been performed on natural fibre based reinforcement textiles for composite materials. The mechanical and the formability behaviours of woven fabrics has particularly been characterised. However, few research work concerns the forming aptitude of nonwoven fabrics despite promising preliminary studies. In the present work, the mechanical characterizations of flax-fibre nonwoven reinforcements are carried out firstly. Then the forming tests of the nonwoven fabrics are performed to quantify their formability behaviour. The tensile and forming tests showed very different mechanical behaviours in comparison to the ones observed on woven fabrics due to the non-uniformity of nonwoven fabric. The high deformation potential of the nonwoven fabrics is established. The specific behaviour of the nonwoven fabrics is studied by analysing the local and global deformation mechanisms of the reinforcement during forming. Moreover, the manufacturing defects experienced in nonwoven fabric forming are demonstrated. The slippage/damage of network is a typical problem in the nonwoven fabric forming, which depends strongly on the fibre density (area density) of fabric and blank-holder pressure.

Natural cellulosic fibres such as flax fibres present interesting mechanical properties as well as biodegradability, and by-products, such as short flax fibres, could be used to produce geotextiles to stabilise soils. Today, geotextiles are often made of coir fibres, which have a high lignin percentage leading to their slow degradation in soil. Fibres with a high cellulosic content, such as those of flax, exhibit lower resistance to soil degradation. This study investigates solutions to improve this parameter with a view to increasing their service life and therefore their credibility compared to coir fibres for geotextile applications. For this purpose, a cellulose coating of yarns made of short flax fibres was performed and its stability under a water flow was assessed. The ability to form a cellulose sheath was estimated by chromaticity measurements of flax fibres after applying a dye specific to lignin. Infrared spectrometry analysis to monitor the level of protection against degradation by cellulolytic enzymes was also carried out. It appears that the cellulose coating provides an efficient physical protection, preventing access of these enzymes to their fibrous substrate. Then, the possibility of conferring antibacterial properties on the cellulose coating by chelating phytoalexin molecules such as gramine on it was assayed and proven to be effective against soil cellulolytic bacteria such as Cellvibrio fulvus and Cellvibrio vulgaris. This study therefore establishes that coating flax yarns with cellulose associated with antibacterial molecules could contribute to obtaining a longer service life in soil for geotextiles manufactured from flax fibres.

International audience; Forming complex shape composite parts with a good production rate/cost ratio is of particular importance for the automotive industry. The sheet forming of woven reinforcements is a promising technique, especially if complex shapes with singularities such as case corners can be obtained. Due to the more and more important recycling needs, the use of flax fibre based reinforcements may be considered for structural or semi-structural parts. During the sheet forming of a tetrahedron shape, the tows constituting the architecture of the reinforcement material are submitted to tensile strains. When using glass or carbon fibre tows, strains to failure are generally not reached. When flax-based fabrics are considered, the failure/degradation strength of the tows constituting the fabric may be reached. Even if no apparent failure is visible when observing the tows during forming, the strains measured by a mark tracking method indicate that the degradation limit of particular tows of the preform has been reached. This could lead to local lack of fibre density and to possible zones of weakness for the composite part. As a consequence, it is essential to improve the tensile performances of the tows constituting the fabric without losing their good impregnation characteristics and good ability to reach high mechanical properties for the composite part.

International audience; This work investigates the tow buckling defect that may take place during the forming of complex shapes. The defect is studied independently of the process with a device specially designed. A specific instrumentation was associated to the device. Structure light interferometry was chosen to measure the elevation of the tows exhibiting the buckling defect all along its growth. The device and its instrumentation were validated in this work and a preliminary study was performed to investigate the origin of the tow buckle’s appearance and its growth kinematic. The growth kinematic of the buckle’s appearance consisting on a double simultaneous rotation of the tow exhibiting the buckle around the Z and the Y axis was established. It was shown that the in-plane bending of the tow is a key parameter that can probably be considered as a preliminary criterion that conditions the appearance and the growth of the tow buckling defect.

International audience; The mechanical behaviour of flax/polylactic acid (flax/PLA) and flax/polypropylene (flax/PP) bio-composites exposed to a medical environment (disinfectant and cleaning treatments) is studied for a use as a semi-structural part. Immersion tests in the different cleaning products were performed for both bio-composites in order to measure the absorption kinetic and monitor the evolution of the mechanical properties in the severe conditions. Pseudo-Fickian and Fickian behaviour were used to describe the absorption of the bio-composites. The absorption of the cleaning product results in a major loss of the mechanical properties. It was found that Terralin is the most aggressive cleaning product followed by Surfanios and water. However, coating the surfaces of these bio-composites could be a practical solution for increasing their durability in wet environments by reducing the sorption kinetics. The use of environmental friendly coatings such as layers of PLA should be favoured in an eco-design approach. In order to quantify in terms of absorption, the cleaning operation used in the medical environment and cleaning cycles were reproduced during a week. It was shown that the weight gained by the bio-composite during one cleaning cycle is very low, 1.98% of the initial mass in the worst case. Moreover, it was also shown that the initial weight is recovered before each cleaning cycle. Therefore, the saturated state is never reached in service condition, and only a low reduction of the mechanical properties ( 5.3% and 7.6% of the tensile strength and modulus, respectively) has been observed. This shows that the flax/PLA and flax/PP bio-composites submitted to disinfectant and cleaning cycles can be used in a medical environment. Based on these conditions, a dimensioning strategy has been proposed for the parts submitted to the periodic cleaning treatments.

International audience; A flax fibre plain weave fabric has been used to form a complex tetrahedron shape. The global shape has been obtained. Even if no apparent defects are visible when observing the yarns, the strain of the tightest yarns of the preform has been measured and compared to the biaxial strains of the fabric determined independently of the process. The results show that the strains in the yarns close to the triple point of the shape (top of the tetrahedron) are higher than the strain at failure. This could lead to local lack of fibre density and to possible zones of weakness for the composite part. It is therefore necessary to increase the tensile performances of the yarns constituting the fabric.

International audience; This paper presents one of the first experimental investigations to analyze the formability of dry woven preform made of flax yarns. A defect analysis at the preform scale, such as wrinkling or buckling is conducted. This work indicates the mechanisms at the origin of these defects. To form complex shapes without generation of defects, the constitution and the architecture of the yarns constituting the woven fabric should be optimised. Keywords

International audience; Nowadays, the market demand for environmentally friendly materials is rapidly increasing. Biodegradable fibres and biodegradable polymers mainly extracted from renewable resources are expected to be a major contribution to the production of new industrial high performance biodegradable composites, partially solving the problem of waste management. At the end of its lifetime, a structural biodegradable composite can be crushed and recycled through a controlled industrial composting process. Bodros et al. [1] showed that biodegradable PLLA (L-polylactide acid)/flax fibres mat composites exhibiting specific tensile properties equivalent to glass fibre polyester composites can be manufactured by an un-optimised film stacking process. In our study, the process has been investigated more extensively. Indeed, the compaction of flax mats requires a higher load than for glass mats of similar areal weight. The transverse permeability of flax mats has also been shown to be lower than for glass mats. In both cases, this is due to a higher degree of entanglement of the flax fibres within the mat. However, the range of permeability and compressibility values of the flax mats are well within the values that allow a good through-thethickness impregnation. Flax fibres cannot sustain long exposures at the impregnation temperature of the mats by PLLA resin. Through-the-thickness impregnation of flax mats processes such as film stacking are more suitable than in-plane impregnation processes such as Resin Transfer Molding because the flow of resin is limited on short distances and allows short times of impregnation.