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International audience; PLA-flax non-woven composites are promising materials, coupling high performance and possible degradation at their end of life. To explore their ageing mechanisms during garden composting, microstructural investigations were carried out through scanning electron microscopy (SEM) and atomic force microscopy (AFM). We observe that flax fibres preferentially degrade ‘inwards’ from the edge to the core of the composite. In addition, progressive erosion of the cell walls occurs within the fibres themselves, ‘outwards’ from the central lumen to the periphery primary wall. This preferential degradation is reflected in the decrease in indentation modulus from around 23 GPa for fibres located in the preserved core of the composite to 3–4 GPa for the remaining outer-most cell wall crowns located at the edge of the sample that is in contact with the compost. Ageing of the PLA matrix is less drastic with a relatively stable indentation modulus. Nevertheless, a change in the PLA morphology, a significant decrease in its roughness and increase of porosity, can be observed towards the edge of the sample, in comparison to the core. This work highlights the important role of intrinsic fibre porosity, called lumen, which is suspected to be a major variable of the compost ageing process, providing pathways of entry for moisture and microorganisms that are involved in cell wall degradation.

Surfactants offer a tunable approach for modulating the exposed surface area of a nanoparticle. They further present a scalable and cost-effective means for suspending single-walled carbon nanotubes (SWCNTs), which have demonstrated practical use as fluorescence sensors. Though surfactant suspensions show record quantum yields for SWCNTs in aqueous solutions, they lack the selectivity that is vital for optical sensing. We present a new method for controlling the selectivity of optical SWCNT sensors through colloidal templating of the exposed surface area. Colloidal nanotube sensors were obtained using various concentrations of sodium cholate, and their performances were compared to DNA-SWCNT optical sensors. Sensor responses were recorded against a library of bioanalytes, including neurotransmitters, amino acids, and sugars. We report an intensity response towards dopamine and serotonin for all sodium cholate-suspended SWCNT concentrations. We further identify a selective, 14.1 nm and 10.3 nm wavelength red-shifting response to serotonin for SWCNTs suspended in 1.5 and 0.5 mM sodium cholate, respectively. Through controlled, adsorption-based tuning of the nanotube surface, this study demonstrates the applicability of sub-critical colloidal suspensions to achieve selectivities exceed- ing those previously reported for DNA-SWCNT sensors.

International audience; Injection moulded composite materials reinforced with wood fibres or particles can offer a credible alternative to other plant-derived fibres, particularly from an economic and environmental point of view. In this study, we compared the recycling behaviour of poly-(propylene) composites reinforced with ground wood particles versus wood and flax fibre reinforced composites. An experimental and numerical approach was used. In spite of the mechanical and morphological performances of fibres which were initially poorer than those of long fibres, the wood particles demonstrated their reinforcement potential (+84 % and +14 %, for the Young’s modulus and the strength at max, respectively, compared to the pure matrix), due to their good adhesion with the matrix and also to the stability of their aspect ratio after recycling (-24 % after 5 injection cycles against -66 % and -61 % for long flax and wood fibres, respectively). The numerical model developed and fed by the mechanical and morpho-logical measurements carried out on the reinforcements made it possible to compare the experimental and nu-merical values. Overall, a close match was obtained between the two sets of parameters. The small differences obtained can be explained by the increase in fibre dispersion as the recycling process progresses, as well as by the possible increase in wood stiffness after several processing cycles.

Liquid composite moulding (LCM) of plant fibre composites has gained much attention for the development of structural biobased composites. To produce quality composites, better understanding of the resin impregnation process and flow behaviour in plant fibre reinforcements is vital. By reviewing the literature, we aim to identify key plant fibre reinforcement-specific factors that influence, if not govern, the mould filling stage during LCM of plant fibre composites. In particular, the differences in structure (physical and biochemical) for plant and synthetic fibres, their semi-products (i.e., yarns and rovings), and their mats and textiles are shown to have a perceptible effect on their compaction, in-plane permeability, and processing via LCM. In addition to examining the effects of dual-scale flow, resin absorption, (subsequent) fibre swelling, capillarity, and time-dependent saturated and unsaturated permeability that are specific to plant fibre reinforcements, we also review the various models utilised to predict and simulate resin impregnation during LCM of plant fibre composites.

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; In this study, a gradually increased hydro-mechanical treatments duration were applied to native hemp bast fibres with a traditional pulp and paper beating device (laboratory Valley beater). There is often a trade-off between the treatment applied to the fibres and the effect on their integrity. The multimodal analysis provided an understanding of the beating impact on the fibres at multiple scales and the experimental design made it possible to distinguish the effects of hydro- and hydro-mechanical treatment. Porosity analyses showed that beating treatment doubled the macroporosity and possibly reduced nanoporosity between the cellulose microfibrils. The beating irregularly extracted the amorphous components known to be preferentially located in the middle lamellae and the primary cell walls rather than in the secondary walls, the overall increasing the crystallinity of cellulose from 49.3 to 59.1%, but a non-significant change in the indentation moduli of the cell wall was observed. In addition, beating treatments with two distinct mechanical severities showed a disorganization of the cellulose conformation, which significant dropped the indention moduli by 11.2 GPa and 8.4 GPa for 10 and 20 min of Valley beater hydro-mechanical treatment, respectively, compared to hydro-treated hemp fibres (16.6 GPa). Pearson’s correlation coefficients between physicochemical features and the final indentation moduli were calculated. Strong positive correlations were highlighted between the cellulose crystallinity and rhamnose, galactose and mannose as non-cellulosic polysaccharide components of the cell wall.

International audience; Flax (Linum usitatissimum L.) is a plant of industrial interest. Its fibres have traditionally been used for textile applications and more recently, for composite reinforcement. To increase fibre yields, varietal selection has been used to develop varieties having high fibre content while retaining good resistance to lodging. This selection process has led to impressively slender structures of flax compared to other herbaceous plants. The present study focuses on the mechanical stability of flax related to its specific architecture. An anatomical study of transverse sections provides information about the architecture of flax stems, including the repartition of the internal reinforcing tissues being phloem fibres and xylem. Then, by using three-point bending tests, flexural modulus is evaluated along the stem. The safety factor (SF) against buckling for the plant was estimated based on Greenhill's model, taking into account gradients in diameter, load, and elastic modulus. Although flax plants have an unusually slender structure, they are mechanically stable. The stability of the plant is ensured by a high stem flexural modulus. This originates from an external ring composed of high-performance fibres, while an inner thick porous xylem provides the plant with a high resistance to local buckling. This is useful information for breeders, demonstrating that it is possible to keep increasing fibre yield without jeopardising plant stability.

Flax fibers (Linum Usitatissimum L.) are currently used for textile applications and composite reinforcement. Due to its industrial importance, flax is the subject of a varietal selection work in view of obtaining varieties with higher fiber yields, but also exhibiting a greater lodging resistance. Indeed, lodging sometimes happens within flax fields, complicating plant harvest and compromising yields. Interestingly, it sometimes occurs that flax stems restore from lodging through a gravitropic reaction. Depending on the time of lodging, variations in elementary fiber mechanical performances, monitored by tensile tests appeared to be more or less pronounced, being greater in the earliest stage of the experiment, and also depend on the studied side of the stem curvature. Namely, the pulling of the stems provides fibers with the most emphasized changes, in terms of strength at break, filling rate (presence of a fiber lumen) as well as cell wall tangent modulus. Finally, differences between tilted and control fibers diminish as the plant maturity progresses, with only slight remaining dissimilarities at plant maturity. Thus, flax fibers are involved in the plant gravitropic reaction and maintain their efficient mechanical characteristic despite lodging, through the adjustability of their cell wall performances over fiber thickening, which is a major result for fiber suppliers and composite manufacturers.

Pre-treatments for plant fibres are very popular for increasing the fineness of bundles, promoting individualisation of fibres, modifying the fibre-matrix interface or reducing water uptake. Most pre-treatments are based on the use of chemicals and raise concerns about possible harmful effects on the environment. In this study, we used physical pre-treatments without the addition of chemical products. Flax tows were subjected to ultrasound and gamma irradiation to increase the number of elementary fibres. For gamma pre-treatments, a 20% increase in the number of elementary fibres was quantified. The biochemical composition of pre-treated flax tows exhibited a partial elimination of sugars related to pectin and hemicelluloses depending on the pre-treatment. The hygroscopic behaviour showed a comparable decreasing trend for water sorption-desorption hysteresis for both types of pre-treatment. Then, non-woven composites were produced from the pre-treated tows using poly-(lactid) (PLA) as a bio-based matrix. A moderate difference between the composite mechanical properties was generally demonstrated, with a significant increase in the stress at break observed for the case of ultrasound pre-treatment. Finally, an environmental analysis was carried out and discussed to quantitatively compare the different environmental impacts of the pre-treatments for composite applications, the environmental benefit of using gamma irradiation compared to ultrasound pre-treatment was demonstrated.

International audience; A variety of techniques were used to investigate flax yarns sampled from four selected Italian paintings on canvas dated between the 17th and 18th centuries and compare them with a modern flax yarn. The goal was to establish their state of preservation and highlight the critical issues thanks to a combined approach that used SEM, atomic force microscopy, nanoindentation, nuclear magnetic resonance, second harmonic generation imaging microscopy and FTIR spectroscopy. The condition of the flax fibres was assessed, and the results showed that two of the four canvases were in a generally good state of preservation, while the other two exhibited signs of biological attack and brittleness due to a previous relining intervention.

International audience; Over the last decades, bio-based composite materials have been developed as an ecological alternative to synthetic fibre-reinforced composites, and flax fibres are one of the most commonly used fibres for this purpose. The secondary cell wall (S2) and the microfibril angle (MFA) of plant fibres are the main factors responsible for the mechanical behaviour of the fibres and, consequently, for the properties of the final biocomposite material. However, the MFA values reported in the literature are obtained through heavy, time-consuming methods and often without resolution at the scale of the elementary fibres. In the present paper, for the first time, the MFAs of elementary flax fibres are measured with the alternative method of second-harmonic generation imaging under controlled polarised light (P-SHG); cotton trichomes are also investigated as a homogeneous and well-known cellulose fibre with expected contrasted MFAs compared to flax. To estimate the MFA, we analysed the images collected that clearly show the microfibrils. The values found are in line with the literature data obtained with conventional techniques. However, new important details of the microfibrils orientation of elementary flax fibres and cotton trichomes are highlighted, such as inhomogeneities in a single flax fibre, leading to MFAs varying between 0 and 10 degrees along the fibre with an average value around 5 degrees. The results obtained give an important contribution to the knowledge of the plant fibre ultrastructure, giving some structural details never provided with measurements of fibre bundles.

International audience; Prompted by environmental legislation and citizens’ awareness induced by global warming ef-fects, the market for plant-fibre reinforced composites has been growing steadily for the past 10–20 years, as observed by the substantial increase in academic and industrial research de-velopments. However, the transition to larger production still requires several uncertainties to be overcome. Among these uncertainties, defects in plant fibres are known to decrease the me-chanical properties at the composite scale. It is therefore of interest to better understand the defects nature, origin and consequences at the fibre scale to monitor the use of plant fibres as reinforcement. In recent decades, finite element modelling has emerged in various scientific fields as an interesting tool that complements experimental characterization. Finite element modelling is even more critical for small and intricate elements such as plant fibres where standard me-chanical tests require substantial adjustments and investments due to their complex ultrastructure compared to synthetic materials. The main objective of this review is to provide a novel overview of defects found in plant fibres and their influence on the mechanical properties of plant fibres based on experimental and modelling work. Through a top-down and multi-scale approach, we first describe the flax fibre ultrastructure with a focus on defects. Then, advanced testing methods and emerging numerical approaches that capture the complex mechanical behaviour of plants, especially flax fibres, are addressed.

International audience; Over the past decades, the use of plant fibre reinforced composites has increased significantly due to their many attractive attributes such as high specific strength and modulus, wide availability, low cost and high environmental credibility compared to their synthetic counterparts. These attributes are especially attractive for lightweight applications in automotive, marine, aerospace and sporting goods sectors. This growth is expected to continue in the future. To improve the design and performance of bio-based composites, an improved understanding of processing-structure-property relations in such bio-based composites is required, the fibres being the key component of the composite to obtain performing properties. This is due to the sensitivity of the constituent plant fibres to mechanical stress (pressure), temperature, water and other parameters. The purpose of this review is to critically synthesise literature on the impact of composites processing steps on plant fibre cell wall structure and properties. The impact of plant fibre composites processing steps from the polymer impregnation stage right through to the end-of-life recycling stage is reviewed. Additionally, mechanical, morphological and hygroscopic properties of plant fibres are considered in conjunction with process times, temperature and shear rate. This review will aid process and product designers to develop new performing plant fibre composite products, taking into account the process parameters to select the most optimised process and (their effects on) plant fibres. Considering how fibre properties change with biocomposites processing steps is indeed essential to understanding the links between the micro and macro scales, and to be able to design optimised plant fibre composite materials.

Coral microbiomes are critical to holobiont functioning, but much remains to be understood about how prevailing environment and host genotype affect microbial communities in ecosystems. Resembling human identical twin studies, we examined bacterial community differences of naturally occurring fire coral clones within and between contrasting reef habitats to assess the relative contribution of host genotype and environment to microbiome structure. Bacterial community composition of coral clones differed between reef habitats, highlighting the contribution of the environment. Similarly, but to a lesser extent, microbiomes varied across different genotypes in identical habitats, denoting the influence of host genotype. Predictions of genomic function based on taxonomic profiles suggest that environmentally determined taxa supported a functional restructuring of the microbial metabolic network. In contrast, bacteria determined by host genotype seemed to be functionally redundant. Our study suggests microbiome flexibility as a mechanism of environmental adaptation with association of different bacterial taxa partially dependent on host genotype., The microbiomes associated with reef corals are complex and diverse. Here, the authors investigate fire coral clones naturally occurring in distinct habitats as a model system to disentangle the contribution of host genotype and environment on their microbiome, and predict genomic functions based on taxonomic profiles.

Looking at taxa that could dominate Anthropocene reefs, the fire corals, Millepora spp., represent compelling alternative because of their similar role than scleractinian corals. However, species identification has been the subject of much debate over the past 150 years. As they may turn as among the last refuges for reef biodiversity, it is critical to accurately delimit their species boundaries to appreciate better their resilience to future environmental conditions. Combining morphological and molecular approaches, we investigated Millepora species delimitation in Reunion Island where three species co-occur: Millepora exaesa, Millepora platyphylla and Millepora tenera. Our results showed that both pore characters and polyp morphologies successfully distinguished species. Gastropore and dactylopore numbers and diameters should be used as standard traits in Millepora. Regarding molecular delineation, the mitochondrial gene clearly segregated the species, whereas the nuclear gene showed shared haplotypes between species, likely influenced by ancestral polymorphism. Confirming the results of a recent study on Red Sea fire corals showing their distinction from the Indo-Pacific ones, we emphasize that formal re-descriptions of the Indo-Pacific M. cf. platyphylla and M. cf. exaesa are needed. Our study highlights the importance of a trait-based qualitative and quantitative approach coupled to molecular delineation, especially for structurally important reef species.

International audience; Today, plant fibers are considered as an important new renewable resource that can compete with some synthetic fibers, such as glass, in fiber-reinforced composites. In previous works, it was noted that the pectin-enriched middle lamella (ML) is a weak point in the fiber bundles for plant fiber-reinforced composites. ML is strongly bonded to the primary walls of the cells to form a complex layer called the compound middle lamella (CML). In a composite, cracks preferentially propagate along and through this layer when a mechanical loading is applied. In this work, middle lamellae of several plant fibers of different origin (flax, hemp, jute, kenaf, nettle, and date palm leaf sheath), among the most used for composite reinforcement, are investigated by atomic force microscopy (AFM). The peak-force quantitative nanomechanical property mapping (PF-QNM) mode is used in order to estimate the indentation modulus of this layer. AFM PF-QNM confirmed its potential and suitability to mechanically characterize and compare the stiffness of small areas at the micro and nanoscale level, such as plant cell walls and middle lamellae. Our results suggest that the mean indentation modulus of ML is in the range from 6 GPa (date palm leaf sheath) to 16 GPa (hemp), depending on the plant considered. Moreover, local cell-wall layer architectures were finely evidenced and described.

The automotive and aeronautic industries have become strategic sectors for North African countries. However, the future growth of these sectors will likely depend on the availability of raw materials such as plant fibres to reinforce polymers. In the framework of this paper, we reviewed the research that focused on alfa fibres, which are deemed to be one of the most available fibres in the Mediterranean area. This plant also plays an essential role in protecting the Mediterranean countries from desertification and desert encroachment. Throughout this review, we discussed and expounded the morphological, chemical compositions and the mechanical properties of the alfa fibres and we compared them to other plants fibres. Moreover, we surveyed several academic research works that used different alfa fibres as reinforcement of thermoset and thermoplastic matrices. In particular, we outlined that alfa fibre can constitute a potential reinforcement of polymer matrices in the North Africa countries.

International audience; Despite the wide development of biocomposites, their compressive behaviour is still not well understood. In this paper, the longitudinal compressive and tensile behaviour of unidirectional natural fibres is studied through a parametric analysis taking into account the nature of the fibre (flax or jute), the matrix (thermoplastic and thermoset PP, PP/MAPP, PA11, epoxy or acrylic), the fibre volume fraction and the fibre/matrix bond strength. In parallel with this approach, the quasi-static tensile behaviour is also investigated to allow comparisons. At low strains, the compressive and tensile moduli are closely similar. On the other hand, the compressive strength is systematically lower than the tensile strength whatever the fibre and matrix used. With a PP matrix, use of a coupling agent (MA) to improve the Interfacial Shear Strength (IFSS) leads to an increase of strength, which highlights the importance of this parameter. The compressive strength increases with the fibre volume fraction, but the maximum value remains lower than 140 MPa. Back calculation allows us to estimate the compressive strength of the flax fibres as 240 MPa, which appears as a current limit for the dimensioning of biocomposite structures.

International audience; Flax fibres are a promising reinforcement in the development of biocomposites and are finding new applications in transport structures. However, there is a perceived problem with plant fibres related to the variability of the properties of these natural materials. This paper describes the factors which affect variability, from plant growth conditions to fibre sampling and testing. A large number of test results are presented (characterization of elementary fibres, bundles, assemblies of bundles, and unidirectional composites), and it is shown that provided fibre supply is carefully controlled, characterization procedures are appropriate, and manufacturing processes are optimal then excellent composite properties can be achieved with low variability.

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.

Hemp (Cannabis Sativa) and flax (Linum Usitatissimum L) are the most commonly cultivated fibre plants in Europe. The use of their fibres covers a wide range of applications and requires a precise knowledge of their mechanical properties. The aim of this study is to investigate the contribution of fibres to the stiffness of flax and hemp dry stems subjected to bending during a simple three-point flexural test. The analysis is based on measuring the difference of bending stiffness between stems with fibres and stems without fibres after removal by manual peeling. Mechanical characterization shows that hemp and flax fibres contribute 54% and 71%, respectively, on average, to the flexural stiffness of a stem. Then, a microstructural analysis of stems is used to determine fibre distribution and geometry, which leads us to obtain longitudinal moduli of 33.9 ± 13.0 GPa and 55.8 ± 14.5 GPa for hemp and flax, respectively. These values are close to those obtained by tensile tests on elementary fibres (30.0 ± 13.3 GPa and 54.7 ± 13.2 GPa). These results encourage to propose the three-point bending test on stems as a standard test for the estimation of bast fibre stiffness.

International audience; This work proposes a multi-scale study of the properties of leaf sheath date palm fibres currently considered as agricultural waste. Firstly, by using optical and electronic microscopy, two main types of bundles were identified which have profoundly different structures. Biochemical analysis and X-ray diffraction (XRD) revealed a low degree of crystallinity but a significant lignin content of about 17% giving the bundles a very cohesive structure as well as a good thermal stability in addition to a singular behaviour in dynamic vapour sorption. An average cell wall stiffness in the order of 16 GPa was highlighted by Atomic Force Microscopy in mechanical mode but tensile tests on bundles have revealed low stiffness and strength but a high elongation. These results combined with the cellular structure of these bundles, provides the potential of these wastes as cost effective and environmentally friendly composite reinforcements for high energy absorption and improved acoustics functions. (C) 2017 Elsevier Ltd. All rights reserved.

Robust cost optimization is the challenging task of fitting a large number of parameters to data points containing a significant and unknown fraction of outliers. In this work we identify three classes of deterministic second-order algorithms that are able to tackle this type of optimization problem: direct approaches that aim to optimize the robust cost directly with a second order method, lifting-based approaches that add so called lifting variables to embed the given robust cost function into a higher dimensional space, and graduated optimization methods that solve a sequence of smoothed cost functions. We study each of these classes of algorithms and propose improvements either to reduce their computational time or to make them find better local minima. Finally, we experimentally demonstrate the superiority of our improved graduated optimization method over the state of the art algorithms both on synthetic and real data for four different problems.

Fibres from annual plants are a sustainable alternative to glass fibres in composite manufacturing. However, both synthetic fibres, such as carbon fibres, and cellulosic fibres exhibit heterogeneities along their lengths, which appear as localised morphological misorientations known as kink-bands. In plant fibres, kink-bands occur due to growing conditions under abiotic stress during the retting stage and the fibre extraction process. Many studies have been conducted to identify the origin of such kink-bands and their impact on the mechanical behaviour of composites, but their characteristics and fine ultrastructure remain unclear. The presence of cavities in transition zones was assessed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) and confirmed by low-intensity second harmonic generation microscopy (SHG) signals, especially when large kink-bands were considered. Moreover, the transverse indentation modulus was obtained by AFM in peak force mode; no substantial differences were observed between the kink-band and defect-free regions, with average values ranging from 6.2–7.3 GPa. Additionally, important MFA changes were measured through SHG imaging, especially in large kink-bands with local misorientation up to 47°. Thus, this in-depth investigation of kink-band areas reveals ultrastructure heterogeneities and the presence of local defects.

The last 80 years have seen significant changes in industrial technologies, with the development of composite materials being particularly striking. These were hardly present before the Second World War but were made possible by the availability of new polymers, fibre reinforcements and innovative manufacturing techniques. In parallel, the traditional flax farming practices for the textile industry have benefited from major improvements in seed selection, retting and fibre extraction methods. This aims of this paper are twofold; first to describe the early work to develop flax fibre reinforced composites and compare their mechanical properties with those of contemporary flax fibres. And second, to understand the impact of significant efforts made recently to improve flax fibre production. The simple substitution of synthetic fibres is no longer sufficient to justify their use; their environmental benefits must be demonstrated, through life cycle analyses to support sustainable societal choices, and these are also discussed.

Biocomposites have major advantage in term of weight saving thanks to lower densities compared to conventional materials; this property also allows a reduction of environmental impacts which is beneficial for the automotive industry. In addition, they maintain rather good mechanical properties after recycling cycles which makes possible to consider other end-of-life scenarios than incineration or landfill. In this work, we studied two potential routes for the recycling of moulding wastes from poly (propylene)-flax nonwovens. By representing close to 25%-wt, these wastes are a major industrial problem, inducing additional recycling or treatment costs. In a first step, the parts were grinded and reincorporated at different weight fractions into virgin nonwovens. Up to 30%-wt of reincorporated wastes, results showed good mechanical performances for recycled nonwovens, especially in bending mode. In a second stage, scraps were grinded and then compounded for injection moulding applications. Their good rheological behavior and ability to conserve fibre lengths during extrusion made possible to formulate competitive materials in terms of tensile performances. (C) 2017 Elsevier Ltd. All rights reserved.

International audience; Blends of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly (butylene succinate) (PBS) (50:50 w/w) were prepared by melt mixing. 5 wt% of compatibilizing agent, obtained by grafting of maleic anhydride onto PHBV, was used to improve the miscibility between the blend components. The influence of the presence of both sepiolite and compatibilizer was studied by evaluating the effect of repeated extrusion cycles (up to 6) on the morphology and the functional properties of materials. Changes in thermal, morphological, rheological and mechanical properties were investigated. All the results showed that PHBV is the more sensitive component towards recycling due to thermomechanical degradation, but PBS as well as sepiolite and compatibilizer play a stabilizing role on PHBV due to improved morphology. The mechanical characteristics measured at different scales remain more or less constant compared to the initial ones revealing the preservation of the mechanical properties of the different samples after reprocessing. This paper highlights a synergistic effect induced by the presence of both compatibilizer and sepiolite leading to an improved recyclability of the resulting materials compared to neat polymers. (C) 2017 Elsevier Ltd. All rights reserved.

The natural character of plant fibres was often cited as a disadvantage which results in the scattering of their properties. This work aims at studying the influence of this scattering on the stiffness of an epoxy-flax fibres twill fabric ply. Elastic coefficients were estimated using an analytical model in an elliptical configuration based on the use of the classical laminate theory. Most of flax fibre elastic coefficients were obtained in the literature; in addition, a flax fibre Poisson's ratio of 0.498 was estimated from an experimentally inverse approach. Then, the use of the analytical model evidenced that the Young's moduli of woven composite were strongly correlated to longitudinal and transversal fibre ones; moreover, the fibre transversal modulus had a significant impact on the Poisson's ratios of the composite. As regards to the yarn configuration and architecture, we showed that the mean twisting angle significantly affected stiffness and Poisson's ratio of the composite; in the same way, the importance of waviness described by its waviness ratio was underlined whereas the scattering into yarn morphology had low influence on composite performances when the waviness ratio was high. Keywords: Flax fibre, Scattered properties, Woven composite, Elastic properties, Analytical model

International audience; Peak-Force Quantitative Nano-Mechanical property mapping (PF-QNM) was applied to explore the nano-mechanical properties of flax fibre cell walls in cross-section. After validation of the ability of PF-QNM to determine stiffness gradients in aramid fibres, measurements were performed on developing flax bast fibres. The presence of two layers with different indentation moduli implies their progressive development during thickening of the secondary cell wall. Finally, measurements were carried out on technical flax fibre cell wall; but, in this case, no significant stiffness gradient could be identified in the secondary S 2 layer.

This study investigates the evolution of hygromechanical properties of flax/PP nonwoven composites in a wide range of environmental Relative Humidity (RH) conditions from 10 to 98% RH. The influence of microstructure with various porosity content ( Φ = 5, 30, 50%) on the mechanical and hygroscopic behaviours is studied and compared to glass/PP composites as a baseline. No significant changes in the moisture content and mechanical properties of glass-fibre reinforced composites are observed. For flax fibre nonwoven composites, the porosity greatly impacts the kinetic of sorption with moisture saturation varying from 9 h to 15 days with decreasing voids. Hygroscopic expansion is hardly modified by the porosity content while anisotropic expansion is always observed. Tensile behaviour and properties are slightly changed over a range of 10–75%RH but negatively impacted between 75% and 98% RH. Interestingly, unlike the tangent tensile modulus and strain at rupture of flax/PP composites, the yield strength exhibits a non-monotonic trend with an optimal value over 50% RH; compressive stresses at the fibre/matrix interface induced by flax fibres hygroexpansion are proposed to explain this trend.

The hygro-mechanical behavior of single hemp fiber/epoxy interface was investigated through its characterization at the microscopic scale. Microdroplet debonding tests were developed in controlled humidity chambers to study the interfacial properties and adhesion mechanism of hemp/epoxy systems in in-use environmental conditions (from 9% to 98% relative humidity). The increase in environment relative humidity and therefore moisture content in the fiber/matrix system engenders a non-monotonic evolution of interfacial shear strength (IFSS) and post-debonding strength. Capillary adhesion illustrated with interfacial meniscus and attractive capillary force contribute to interfacial bond strength until RH = 50%. Increasing RH above 50%, corresponding to a specific amount of water molecules in the system, provokes a reduction of interactions between the hydroxyl groups on the fiber surface and the resin. Debonding behavior analysis associated to fiber/matrix interface fracture observation highlights a modification of the stress state at the interface when increasing relative humidity. The residual hygroscopic stresses generated during storage at various humidity are shown to be the major contributor of the optimal interface performances at 50% relative humidity (RH). The sensitivity to moisture of natural fibers could therefore be beneficial for improving biocomposite interface properties.

International audience; This paper investigates biochemical, morphological and mechanical properties of a large range of plant fibres explored with the same methods. Biochemical results clearly exhibit strong differences between gelatinous, i.e. flax and hemp, and xylan type, i.e. jute and kenaf, cell walls. These differences into parietal composition have an impact on cell wall stiffness, highlighted through nanoindentation and atomic force microscopy measurements, but also on fibre individualisation, due to variations into fibre bundles cohesion. In addition, the morphology and particularly the lumen size induces apparent density differences. Moreover, the influence of fibre morphology and properties is demonstrated on UD materials. Finally, longitudinal Young's modulus of each plant fibre batches is back-calculated from UD stiffness by an inverse method; the results obtained are in accordance with the values in the literature values, proving the interest of this method to estimate longitudinal Young's modulus of short plant fibres.

This article provides deeper insight into moisture sorption over a wide range of relative humidity (RH), from 9% to 98%, and its effect on the evolution of the tensile behaviour and performance of hemp/epoxy unidirectional biocomposites. Moisture sorption is associated with orthotropic hygroscopic expansion which is also related to the influence of fibre reinforcement distribution and the generation of residual stresses due to manufacturing. Tensile behaviour is significantly modified by moisture content, with a non-linear behaviour becoming more evidenced. The increase of moisture content causes exponential decay of tensile modulus, but a non-monotonic evolution of tensile strength and strain at break shows optimal properties at about 3%. The hygroscopic property evolution observed is responsible for the creation of compressive radial stresses at the fibre/matrix interface, which promotes load transfer and benefits the interface performance.

International audience; Flax (Linum usitatissimum L.) fibres are commonly used as reinforcement of composite materials. Nevertheless, literature shows that the compressive strength of flax-based composites is rather modest compared with materials reinforced by synthetic fibres. The present article investigates the compressive strength of flax fibre bundles both within the stems and in unidirectional (UD) composites. In this way, an optimised arrangement of fibre bundles inside the plant is assumed. Damage mechanisms are found to be similar in the stem and within flax-based UD materials, namely by buckling of fibre bundles, a typical failure mechanism of UD composites. Inside the stems, this phenomenon is highlighted by nanotomography, which underlines the key role of the woody core in the buckling resistance of the plant. For UD, failure can also be studied by scanning electron microscopy (SEM). The same ranges of average compressive strength values are estimated for flax fibre bundles, being 206 MPa within the stem and 242 MPa within UD composites. Finally, this study highlights that, if a flax stem is an optimised natural structure, the compressive strength of flax fibre bundles seems to be a limiting factor for structural applications of flax-based composite materials.

Plant fibres are increasingly used for composite reinforcement, but valorization of by-products such as flax shives still needs to be developed. Also, additive manufacturing, such as Fused Deposition Modelling (FDM), is strongly progressing and can be of great interest to implement biocomposite solutions. The present work focuses on the development of innovative and fully biodegradable filaments for FDM. Three polymer matrices were tested: PLLA, PLLA/PBS 50/50%-wt and PBAT. A filler content of 30-wt% has been achieved using flax shives with PBAT. Flax fibres and shives reinforced 3D printed parts exhibit promising performances and printability, highlighting their potential to successfully develop fully compostable filaments.

International audience; Flax retting is a major bioprocess in the cultivation and extraction cycle of flax fibres. The aim of the present study is to improve the understanding of the evolution of fibre properties and ultrastructure caused by this process at the plant cell wall scale. Initially, investigations of the mechanical performances of the flax cell walls by Atomic Force Microscopy (AFM) in Peak Force mode revealed a significant increase (+33%) in the cell wall indentation modulus with retting time. Two complementary structural studies are presented here, namely using X-Ray Diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of the cellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increase of 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. In addition, NMR investigations show a com-paction of inaccessible cell wall polymers, combined with an increase in the relaxation times of the C4 carbon. This densification provides a structural explanation for the observed improvement in mechanical performance of the secondary wall of flax fibres during the field retting process.

International audience; The use of biomass in injection moulded or extruded thermoplastic composites is an important issue, especially when trying to add value to low-cost co-products. The objective of this work was to conduct a complete study on the morphological characterisation and carbohydrate analysis of a range of co-products obtained during the processing of flax straw. Thus, the morphology of (i) cut flax fibres, (ii) fragmented shives, and (iii) scutching and carding dusts is characterised using a dynamic image analyser with a sieving approach. These different fractions are then used to produce injection moulded composite materials. Their mechanical performances are discussed in relation to the morphology of the reinforcements, as well as their carbohydrate compositions and fine particle contents. Co-products, based on their reinforcement properties, can be classified into three categories. In all cases, a reinforcing effect is demonstrated for the tensile Young’s modulus with an increase from + 24 to + 137% depending of the material. A linear relationship was observed between the cellulose content of reinforcing material and the tensile strength at break of the injection moulded composites. The results are promising for adding value to all flax co-products in plastics processing, targeting industrial applications in line with their intrinsic performances.

The objective of the investigation described in this work was to study the reprocessing effects on the alfa fiber reinforced polyvinylchloride composites with and without maleic anhydride-grafted polyvinyl chloride used as compatibilizer. The material was characterized after each extrusion using tensile tests, scanning electron microscopy (SEM), dynamic mechanical, and thermal analysis. Results indicated that generally after four cycles, the recycled composites had considerably higher modulus as compared with the original composites, which were attributed to changes in physical and chemical properties of the composites induced by the recycling process. This effect was enhanced for the compatibilized samples. Increase of the modulus strength of the poly(vinyl chloride) (PVC) matrix is detected due to the molecular chain cross-linking resulting from degradation. In addition, it was found that the reprocessing cycles increase in glass transition temperature of PVC and PVC/alfa composites.

This paper reports the mechanical behaviour of novel biocomposites made up from PA11 and flax fibres. We find that, using various flax fibre loadings (i.e. volume fractions), it is possible to obtain bio-based composites with good and reliable tensile properties compared to other biocomposites such as PLA/Flax. Our investigations reveal that the thermal cycle process used for composite moulding has a significant impact on the fibre mechanical properties. Indeed, a 210 °C thermal cycle for 8 min induces a decrease in fibre stiffness and strength of 32.8 and 12.9%, respectively. A simple micromechanical model, i.e. mixing rule, is applied to estimate composite stiffness and strength based on heat-treated fibre properties. In the case of composite stiffness, our results indicate an underestimation of the Young’s modulus compared to experimental data, especially when the composite stiffness is taken in the first part of the stress–strain curve where the stiffness exhibit unstable values. This implies that the mechanical behaviour of flax fibre bundles compared to fibre bundles constrained within the polymer matrix is not the same, probably due to rotational locking, which prevents the micro fibrillar realignment of the cellulose leading to higher stiffness values.

International audience; The development of new plant fibre composites is a key point in the development of semi-structural biodegradable or biobased parts, especially for automotive applications. The aim of this original and innovating work is to study, at different scales, the recycling ability of a fully biodegradable L-Poly-(lactide)-Poly-(butylene-succinate)-flax (PLLA-PBS-flax) biocomposite. The biocomposites were manufactured by twin-screw extrusion followed by injection moulding, then the recycling behaviour was studied during successive injection moulding cycles. Firstly, we investigated the length of the flax fibre after compounding and injection, as well as the cell wall stiffness and hardness, by in-situ nano indentation tests. Secondly, we focused on the effects of recycling on thermal, rheological and tensile properties. We highlighted a severe evolution of the cell wall properties, especially concerning the polysaccharidic matrix after the first thermal cycle, nanoindentation properties remaining quasi-stable after this first drop. Furthermore, the biocomposites did not show any significant evolution of their mechanical performances during cycle three or four of the first injection cycles; after this plateau, the tensile strength and strain as well as impact energy were significantly altered due to the conjugated fibre length decrease and degradation of the PLLA, the latter being emphasized when the flax fibre is embedded. Nevertheless, this fully biodegradable composite exhibits a suitable recycling behaviour for 3 or 4 cycles, which is sufficient for industrial applications.

Commingled plant and polypropylene fibres (PP) nonwovens are attractive for automotive industry because of their interesting mechanical performances combined with good acoustical properties. This double function is achieved thanks of controlled porosity within the composite material. Indeed, from a same material, different functions (acoustic or mechanical) can be obtained by only varying the compaction rate during moulding. This study aims to highlight the necessity of a very large porosity volume fraction (60%) to reach good acoustic properties and understand the mechanical effect behind it. By combining tensile testing, acoustic absorption measurement and scanning electron imaging analysis, the microstructure, acoustic and mechanical properties have been investigated and were found to be intimately related to the material porosity content. As expected, when increasing porosity level from 5 to 60%, the material behaviour change and its tensile properties (modulus, strength and elasticity domain) drop drastically to a point where the material is no longer elastic (70% porosity) due to a modification of the material microstructure involving different failure mechanisms. (C) 2016 Elsevier Ltd. All rights reserved.

Automated fibre placement (AFP) has been used to manufacture flax/polypropylene biocomposites for the first time. This required preparation of tape with a calibrated width from impregnated fibre sheets. The unidirectional tapes showed polymer-rich zones on the surface. During manufacture the polypropylene (PP) is melted locally with a laser. A consolidation step in a hot press is then required to reduce void content. The flax fibres, composed of polysaccharides, have limited thermal resistance so optimization of the thermal cycle is necessary, but subsequent characterization of mechanical behaviour showed no evidence of property loss in spite of an additional melting cycle. AFP appears to be a promising manufacturing method for biocomposites. Keywords: Structural composites, AFP, Biocomposites, Flax, PP

Throughout this study, different plant fibres, used as polymer reinforcement, were tested by quasi-static tensile and nanoindentation tests to obtain their modulus using these two methods. Major differences of stiffness were found due to their different structures and morphologies. As example, tensile and nanoindentation moduli for sisal fibres were and 25.0±12.9 GPa and 10.2±1.7 GPa, respectively, whereas they were 52.4±13.2 GPa and 18.9±1.8 GPa for Eden flax fibres. A correlation between these two moduli was established; thus by an inverse method, we estimated the longitudinal stiffness of softwood fibre, for which the tensile test is delicate, mainly due to the short length of the fibre.

Cultivation of flax fibres ( Linum usitatissimun ) for composite reinforcement must provide fibres with good mechanical properties, but also a high fibre yield and lodging stability in order to ensure a stable and satisfactory income for the farmer. This work proposes a study of the impact of the seeding rate on these key parameters. We studied the Aramis variety with 4 different seeding rates (1200, 1500, 1800 and 2500 seeds/m 2 ). The results indicate the significant impact of the seeding rate on the stem’s morphological parameters; its increase induces a progressive decrease of the scutched fibre length and of the stem diameter. At the same time, the higher seeding rates obtained improved the scutched fibre’s yield (+11% between 1200 and 2500 seeds/m 2 ) but, conversely, induced a drop in the elementary fibre’s tensile properties and in the flax stem’s lodging stability, mainly due to the large decrease in the stem’s diameter. This work shows that a compromise must be found to optimize the fibre yield, the mechanical performance and the plant’s stability; it underlines the relevance of using a conventional seeding rate, close to 1800 seeds/m 2 .

International audience; Fibre orientation is an essential factor governing the mechanical properties of composite materials. This study proposes an original method based on gray-level granulometry to analyse the fibre orientation distribution (FOD) of synthetic and natural fibre reinforcements aiming composite applications. An orientation maps is computed from SEM images and frequency of fibre orientation is graphically illustrated for each angular direction. First, glass fibre nonwoven and unidirectional preforms were analysed as a model to validate the method before testing their flax fibre counterparts. Differences in structural organisations were found between flax and glass fibre reinforcement FOD due to the specific structure and mechanical behaviour of plant fibres but also to the preform manufacturing process. Promising results were obtained confirming the reliability of this novel numerical method for fibre orientation determination.

Non-woven composites reinforced with plant fibers are widely used in the automotive and construction sectors. The vast majority is composed of petroleum-based, non-compostable polyolefins, which are no longer a viable solution in an environmental context where the end-of-life management of industrial products is becoming a major societal issue. Here, fully green composites are produced by reinforcing three bio-based and biodegradable matrices – poly-(hydroxyalkanoate) (PHA), poly-(butylene-succinate) (PBS) and poly-(lactide) (PLA) – with non-woven flax fiber preforms. Notably, their mechanical performance was observed to be at least equivalent to the industry reference – poly-(propylene) (PP) reinforced non-woven flax. These composites were then buried in an instrumented garden compost, and the evolution in microstructure and mechanical properties was studied over a period of six months. Microtomography studies revealed that evolution in composite microstructure principally depended on the polymer matrix: surface degradation was predominant for PBS and PHA biocomposites, whereas rapid fiber-matrix interface degradation in the core was observed for PLA biocomposites. Interestingly, even after six months in the compost, all composites exhibit tensile strengths of at least 50% of their initial value. Moreover, the strength reduction in biodegradable composites was of the same magnitude as the industry reference, flax/PP composite. These results demonstrate the potential of biocomposites in resolving the ‘biodegradation paradox’: flax composites with biopolymers like PLA, PHA and PBS can be designed to have adequate mechanical performance for industrial products, even after ageing in harsh conditions, and yet offer an alternative end-of-life route to the typical incineration (with or without energy recovery).

Flax shives (FS) represent approximately 50 % in weight of dry flax stems, making it the main by-product of the flax scutching industry. Being an available and low-added value lignocellulosic resource, flax shives are an interesting candidate for thermoplastic composite reinforcement. In this study, raw flax shives were fragmented by knife milling using two grids of 500 and 250 μm respectively, while a third batch, with a targeted particle size below 50 μm, was obtained by an attrition beads mill. The fragmentation methods used do not modify the biochemical composition of FS but do reduce their crystallinity due to both crystalline cellulose allomorph conversion and amorphization. The poly-(propylene) and 4%-wt maleic anhydride modified poly-(propylene) injection moulded composites produced with these reinforcing materials have a maximum tensile strength that evolves linearly with particle aspect ratio after processing. The tensile Young’s modulus of the composites reinforced by coarser particles is 3268 ± 240 MPa, which is almost 90 % that obtained for a reference 1mm flax fibre reinforced composite. Furthermore, a basic micromechanical model was applied highlighting the reinforcing capacity of cell wall-like small tubular structures (e.g. flax shives). This study underlines the reinforcing potential of low-value by-product flax shives for value-added composite applications.

International audience; Ultrasound-assisted synthesis of diglycidyl ethers of isosorbide (DGEI) with different epoxy equivalent weight (EEW) with isophorone diamine (IPD) hardener have been performed. Interestingly, it was found that one specific epoxy formulation exhibits high reactivity with amine hardener inducing high glass transition and indentation stiffness. Associated with flax fibres, this new bio-based resin has made it possible to develop performing and promising unidirectional biocomposites with characteristics comparable to commercial petroleum-based epoxy resins.

International audience; AimTo investigate whether the populations of 17 widespread hydroid species are connected on a demographic level (one to few generations) or are mostly connected on evolutionary timescales.LocationCoral reefs of the South‐western Indian Ocean.Taxon17 widespread hydroid species (Cnidaria, Hydrozoa).MethodsUsing mitochondrial (16S) and nuclear (Calmodulin) sequences, we analysed 17 co‐distributed hydroid species with contrasting life history traits (with or without a pelagic life history phase). We used DNA‐based species delimitation methods, haplotype networks, analysis of molecular variance, and Bayesian skyline plots to investigate diversification and connectivity patterns in the region.ResultsPutative cryptic species were revealed in half of the species and the 17 species resulted in 54 cryptic lineages. The high isolation of hydroid populations in the region is underlined by the rarity of shared haplotypes among localities in most of the species studied. In contrast, haplotypes shared between specimens collected in different ocean basins were found in three species and suggest recent introductions via human activities. The presence of a short pelagic dispersal phase did not seem to increase the connectivity levels of the studied populations, as no significant differences in genetic structure were detected between species with contrasting life history traits.Main conclusionsThese results highlight once again our incomplete knowledge of the marine invertebrate fauna, as we might be underestimating hydroid diversity by two‐thirds, and are discussed in terms of diversification processes, dynamics of the Indo‐Pacific biodiversity and the role of its peripheral marine provinces.

Whatever in pulp or biocomposite sectors, the elements called fines coming from plant fibres have generally a length lesser than 200 μm. Their mechanical impact has long been debated in short plant fibre thermoplastic composites. Are they solely a filling agent or on the contrary, have they a potential of reinforcement in such composites depending on their numbers, their length and aspect ratio? This work proposes an original experimental approach to explore the mechanical role of fine flax particles. Based on controlled milling of an initially homogeneous flax fibre batch that provides a population of fines (99% under 200 μm, average Lw of 147 μm), we devised a set of composites made of an increasing content of flax fines (0, 3.1, 5.6, 12.3, 20.4, 34.6 and 40.2%-vol) mixed with poly(propylene) (PP) and maleic anhydrid grafted PP (PP-PPgMA). Reference composites reinforced with chalk and also with cut flax fibre (Lw of 2000 μm) were also manufactured and studied. Results demonstrate that, despite a low aspect ratio (5 ± 0.2 for 20.4%-vol), fines can act more than as just a simple filler, a slight modulus reinforcement is depicted but only beyond a high threshold of about 20%-vol (+29% compared to raw PP for 20.4%-vol). In addition to PP, we then investigated the mechanical influence of the flax fines in two representative matrix thermoplastic families Poly(amide)-11 (PA11) and poly(butylene-succinate) (PBS). We highlighted an increase of the Young’s modulus of PA11 and PBS fines composites (+41% and +115, respectively for 20.4%-vol), whereas strengths were lower compared to the respective neat polymers, exhibiting the possible negative role of fines on composite mechanical properties.