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Abstract: Nanoindentation tests were performed on flax fibers or stems sections in order to study the influence of the maturity of the fibers and the one of the location of the fibers within a bundle on their mechanical properties. The first results evidence the interest of this technique to approach differences into mechanical properties of plant fibers and highlight some questions linked to the specific structure of fibers. [Copyright &y& Elsevier]

Abstract: Polypropylene/hemp or sisal fibre composites exhibit interesting recyclability [Bourmaud A, Baley C. Investigations on the recycling of hemp and sisal fibre-reinforced polypropylene composites. Polymer Degradation and Stability 2007;92(6):1034–45]. The obtained results prove that the tensile modulus of these polypropylene/vegetal fibre composites is well conserved with the number of reprocessing cycles. In this work, we investigated the relationship between the mechanical properties of the fibres and those of the composites by taking the influence of the recycling into account. In the first part of this study we carried out nanoindentation and tensile tests on vegetal fibres to obtain transversal and longitudinal Young''s moduli. The experimental values were then introduced into micromechanical models, taking the aspect ratio changes into account, to estimate the stiffness of the PP/vegetal fibre injected composites before and after recycling. The first results show an interesting correlation between experimental results and model predictions; however a general underestimation of tensile stiffness of composites can be noticed. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Developing new natural fibre composites is the focus of many studies today. Indeed, they are made out of renewable resources and, therefore, have a lower environmental impact in comparison to mineral fibre composites. The mechanical performances of stinging nettle fibres are measured and compared to flax and other lignocellulosic fibres. The stress/strain curve of stinging nettle fibres (Urtica dioica) shows they have a linear behaviour. The average tensile properties are a Young''s modulus equal to 87 GPa, a tensile strength equal to 1594 MPa, and a strain at failure equal to 2.11%. [Copyright &y& Elsevier]

• This paper provides a unique review of a large quantity of data from hundreds of tests on flax fibers • Data are discussed in terms of testing procedures and sample geometry • Variability of composite properties is discussed in terms of fibre variability. 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. [ABSTRACT FROM AUTHOR]

Highlights • The influence of lodging on flax fiber performances is explored. • Flax gravitropic response is investigated at key stages of plant development. • Fibers from the pulling side of tilted stems show changes in mechanical properties. • Only slight dissimilarities in fiber performances remain at plant maturity. • Fiber properties stay adequate for composite reinforcement in case of lodging. Abstract 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. [ABSTRACT FROM AUTHOR]

Highlights • Conventional and greenhouse cultivation of Bolchoï textile flax were compared. • Changes of flax stem architecture attributed to seismomorphogenesis were explored. • Much taller plants were obtained under greenhouse conditions. • The two types of cultivations led to similar fiber number and morphology. • Interestingly, stem and fiber moduli are unaffected despite architectural changes. Abstract Flax fibers (Linum usitatissimum L.) are of great interest for textile and composite applications. Thus, their use for industrial purposes requires increasing quantities and constant quality. In general, plants can change their morphology and mechanical properties when submitted to stress, particularly in the case of the reaction of plants to wind, a phenomenon known as seismomorphogenesis. In this paper, the influence of greenhouse or field cultivation on plant architecture and anatomy as well as the fiber yield and mechanical performances of flax fibers are investigated. The results highlight the development of much taller plants under greenhouse conditions, but similar fiber length and number of fibers per plant with both types of cultivation. Finally, the bending stiffness of stems is estimated by three-point bending tests and fiber performances are measured by tensile tests; in terms of mechanical properties at the stem level but also at the fiber scale, there are no statistically significant differences between greenhouse and field cultivated plants. In conclusion, despite the increased plant height under greenhouse conditions (44% increase in total height), fiber yield and properties are unchanged compared to field cultivation. Hence, the greenhouse cultivation of flax does not appear to favor higher fiber yields or quality, but nevertheless maintains compliance with these essential criteria. [ABSTRACT FROM AUTHOR]

The varietal selection of flax ( Linum Usitatissimum L.) has always focused on specific criteria fulfilling requirements of farmers and textile workers. Thus, the current development of composites using flax as reinforcement presents new challenges for flax breeders in terms of fiber quantities and quality. However, the impact of the varietal selection on the mechanical properties of resulting fibers is yet to be determined. In the present study, several architectural characteristics of flax stems are defined. Stem transverse sections from four varieties selected from the 1940s to 2011 are compared. Anatomical changes over time are highlighted. The most important ones involve the gap between fiber bundles and the amount of fibers which can be improved with the selection (from 7.8% to 13.4% of the tissue area per section). This trend coincides with the increase in biomass production over time expected from the selection work. Moreover, this study demonstrates that flax fibers preserve their good mechanical performances in spite of the anatomical differences. Thus, through varietal selection, it is possible to increase the biomass yield while preserving the excellent specific mechanical properties of flax fibers. Finally, flax fibers can compete with glass fibers to reinforce composite materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 . [ABSTRACT FROM AUTHOR]

Climate change is characterized by rising temperatures and an increased risk of spring water deficit, both of which threaten the cultivation of textile flax. With the multiplication and intensification of these phenomena, fibre plant yields have become increasingly uncertain in recent years, leading to shorter stems as well as reduced yields. In this context, winter flax cultivation emerges as a solution to sustain the exploitation of this plant. Sown before winter, it can resist periods of spring water stress, owing to the early development of it root system, and is also protected from the summer heat. In this study, we investigate the stem architecture and fibres properties of two winter flax varieties, in comparison to spring flax. Cross-sections reveal that both winter and spring flax have a similar plant structure. The part located at mid-height of the winter stem contains the most fibres, accounting for around 23% of the biomass. At the middle of the stem, the elementary fibres of the Cirrus and Olga varieties have an average length of 20.9 ± 7.7 mm and 30.0 ± 10.8 mm, respectively. At this same plant height, winter flax fibres of the Cirrus and Olga varieties have an average Young's modulus of 63.6 ± 18.8 GPa and 57.6 ± 20.8 GPa, and an average strength at break of 1248 ± 554 MPa and 1187 ± 627 MPa, respectively. As a conclusion, winter flax has a similar length and mechanical properties to spring flax and therefore offers the same potential for textile applications. In addition, as demonstrated in this study, winter flax can be similarly useful as a reinforcement in composite materials. • Two varieties of winter flax were grown for two consecutive years. • The internal structure of winter flax stems was analyzed. • The mechanical properties of elementary winter flax fibres were assessed. • Winter flax properties are close to spring flax literature data. • The reinforcement potential of winter flax fibres in composites was demonstrated. [ABSTRACT FROM AUTHOR]

To take advantage of the mechanical performance of plant fibers and avoid their degradation, it is necessary to develop biocomposites by working on the least aggressive process conditions possible. The use of thermoplastic polymers with low processing temperatures is one possible way. In this study, tests were performed on poly-(butylene-succinate) (PBS) flax composite, extruded and injected at 140 °C. They have a good level of tensile or impact properties compared to poly-(propylene) (PP) or l -poly-(lactide) (PLLA) based biocomposites. Nanoindentation measurements were performed in situ on the composites. Despite the low Young's modulus of PBS, it was shown that the use of a moderate process temperature limits the downward stiffness of the flax cell walls. Finally, it was demonstrated that the PBS could be associated with PLLA for making flax fiber reinforced biocomposites. The introduction of PBS, with adjustable volume fractions, improves elongation at break and impacts on the behavior of PLLA–flax composites, whilst retaining high performance mechanical properties. Thus, it is possible to elaborate fully biodegradable composites with the desired mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract: The integration of the environmental problem in the design of industrial products leads us to incorporate vegetal fibres and recycled polymers into composite materials. The aim of this work is to study the behaviour and the environmental interest of a recycled PP/hemp fibre after several injection cycles. The mechanical and rheological behaviour of recycled PP/hemp composite was first studied by using tensile, dynamical mechanic analysis and rheological measurements. Then, to better understand the influence of the recycling, a morphology study was carried out on composites by using optical and electron microscopy. Finally, we investigated the environmental advantages of our composite thanks to a simplified environmental assessment. Our results highlighted the environmental interest of using a recycled matrix to prepare composites reinforced with vegetal fibres and the interesting properties of this material after recycling. [Copyright &y& Elsevier]

Abstract: Plant fibers have several aspects, such as low energetic production costs, biodegradability and great mechanical properties, which make them very attractive in the field of composite reinforcement. The aim of this study was to compare the impact of two eco friendly water washings on the thermal behavior, surface morphology and mechanical properties of flax fibers (Electra variety). As a first step, we have carried out scanning electronic microscopy (SEM) to evaluate surface properties and performed thermogravimetric analysis (TGA) to estimate the effects of the treatments on the fiber composition. Secondly, tensile tests were performed on elementary fibers. The results show no negative impact of water treatment on the mechanical properties of the elementary fibers. Moreover, after water washing for 72h at 23°C, the Young''s modulus values appeared to increase slightly, possibly due to better surface state and an easier extraction of flax fibers. Finally, an analysis of eluted sugars suggests some negative effect of the boiling treatment on the fiber structure. [Copyright &y& Elsevier]

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). • Mechanically speaking, the studied biocomposites are, at least, as good as flax/PP. • Degradation appears in surface and/or at flax fibers/matrix interface. • Interface degradation occurs at the first stage of degradation, impacting strength. • After six months, biocomposites and flax/PP lose 50% of their initial strength. [ABSTRACT FROM AUTHOR]

• The impact of flax fines particles within an injected biocomposite was studied. • A range of matrices and fibre and fines volume fractions were explored. • The negative impact of fines on composite mechanical properties was demonstrated. • Results vary according to the thermal and rheological properties of the matrix used. 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. [ABSTRACT FROM AUTHOR]

This study aims to investigate the evolution of hygromechanical properties of flax nonwoven PP composites in a wide range of environmental Relative Humidity (RH) conditions (10, 33, 50, 75 and 98 % RH). The influence of the fibre nature (glass or flax) and porosity (Φ=5, 30, 50 %) is assessed to improve our understanding of the in-service mechanical behaviour of these composites. Flax nonwoven/PP have a complex mechanical behaviour which only becomes amplified at high moisture contents. However, Relative Humidity variations do not affect the performances of glass/PP composites mechanical, whereas the tangent modulus and strain at rupture of flax/PP composites are strongly modified. Interestingly, stress at yield exhibits an improvement with increasing moisture content. [ABSTRACT FROM AUTHOR]

Highlights • The impact of field retting time was studied on six flax fibres samples. • AFM measurements revealed a significant increase of cell wall stiffness with retting. • XRD and NMR investigations showed an increase of cellulose crystallinity with retting. • NMR evidenced a compaction of inaccessible polymers for most retted sample. • This densification can be a possible explanation of the indentation modulus increase. Abstract 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 compaction 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. [ABSTRACT FROM AUTHOR]

For the past 15 years, there has been tremendous interest and technological development concerning biocomposites. Plant fibres can be derived from a multitude of natural agro-sources, with the preferred choice as a composite reinforcement often being driven by abundance, geographical location, and historical use. While from a product designer's or engineer's point of view, all plant cell walls are 'similar', they have indeed substantial morphological and mechanical diversity linked to their structure, biochemical composition and the plant growing conditions. Here, we provide a holistic overview of the main types of plant cell walls used as polymer reinforcements. The relationship between their structures and properties, in constant link with potential associated composite, is specifically discussed. Then, the fibre extraction and cultivation modes are compared, through an environmental assessment. We also show how a scientist's point of view on cell wall structure and associated experimental approach lead to distinct results; following a critical review, we make recommendations on appropriate characterisation. A final discussion highlights the pertinent parameters that accurately define a composite reinforcement fibre. The review will serve as a handbook reference for researchers and designers in the field of biomaterials for appropriate selection of plant cell walls for specific composite applications. [ABSTRACT FROM AUTHOR]

The aim of this paper is to analyze the relationship between the tensile properties of flax and jute fibres and the mechanical performance of associated grafted polypropylene (PP-MAPP) biocomposites. Firstly, we analyzed the differences of the morphological and mechanical properties of these two types of fibres. Flax fibres demonstrated better mechanical properties. Secondly, polypropylene matrix unidirectional (UD) composites were manufactured and tensile tested. We noticed a strong correlation between the experimental Young's modulus and the estimated modulus with the rule of mixtures. Finally, short fibre PP-MAPP composites were extruded and injected and their tensile behavior was analyzed. Despite the lower tensile properties of the jute fibres, the PP-MAPP/jute composite exhibited better mechanical performances. A detailed analysis of the microstructure was conducted; it highlighted the preponderant role of fibre orientation on the mechanical properties of injected composites. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, we explore the interest of recycling PA11 and PP unidirectional flax fibre composites. After describing the mechanical performance of the various components of the composite, its tensile properties are investigated before and after recycling. The results indicate a good stability of the modulus and strength of the composites during the multiple recycling steps, which also applies to the fibre properties, as highlighted by nanoindentation tests throughout the recycling process. However, interestingly, the elongation at break of the PA11 composites shows a significant increase with the number of injection cycles, ranging from 5.2 to 14.7%. Further analyses show that the PA11 matrix has highly stable thermal, rheological and mechanical properties, so the matrix cannot be involved in this increased elongation at break. Flax fibers display a steady decrease in length with recycling, especially in the case of PA11 matrix compared to PP matrix. This drastic decline is explained by the high viscosity of the matrix, but above all by the presence of defects (kink bands) in the flax fibers, spaced at intervals of 60 μm, which is consistent with the residual lengths measured at the end of recycling. PA11 and PP flax UD composites prove to be very efficient after recycling, while PA11 biocomposites exhibit an increase of elongation after recycling that makes it a material of first choice in the biocomposites industry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 nanoindentation 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: An elementary plant fiber could be assimilated to a laminate, mainly constituted of the secondary wall S2 layer, made of a few non-crystalline polysaccharides reinforced by cellulose fibrils organized in a helix, with a microfibrillar angle (MFA) around 10° relative to the longitudinal fiber axis. This paper investigates the relationships between the MFA, the mechanical properties and the biochemical composition of different varieties of flax. To conduct this study, tensile tests on elementary flax fibers, X-ray diffraction, and solvent extractions have been carried out. Within the different varieties of flax, Young''s modulus was found to be negatively correlated with the MFA. The results showed that the ratio between hemicelluloses (matter extracted with alkali) and pectins (hydrolyzed with acids) is highly correlated with the tensile properties; concurrently, we showed the great influence of pectic acids on the fiber’ Young''s modulus, and on the orientation of the microfibrils. [Copyright &y& Elsevier]

Abstract: Many works deal with the mechanical properties of flax fibers cultivated for textile applications and today used for the reinforcement of polymers. Nevertheless, quantities of oleaginous flax fiber are obtained each year and not promoted. The aim of this work is to study the mechanical properties of single linseed flax fiber as a function of variety, culture year, dew-retting degree and agronomic factors. Five varieties of oleaginous flax have been characterized by tensile tests on elementary fibers and compared to four varieties of textile flax. These tensile experiments have been carried out on with the same equipment, experimental protocol and environmental conditions. The results show that interesting mechanical properties were obtained with the oleaginous variety and are close of those of textile varieties, such as Agatha or Electra. Considering the diameters and specific properties of these oleaginous fibers, we evidenced that they are good candidates for the substitution of glass fibers in composite materials. To increase the development of flax fibers, it is important to have a better control of the spread of their mechanical properties. This point could be observed with the Everest variety cultivated for 4 years and no conclusion could be made. We have evidenced that the retting degree has no influence on the diameters and mechanical properties of the fibers; the same conclusion is obtained with agronomic factors such as seeding rate and plant height. [Copyright &y& Elsevier]

Abstract: Natural fibers appear to be a promising alternative to glass fibers for the reinforcement of polymer matrix composites. However, the wide dispersion of their mechanical properties slows down their development. The aim of this study was to evaluate the influence of the sampling area of the stem on the mechanical properties of hemp fibers. Tensile tests were carried out on fibers extracted from the bottom, the middle, and the top of one stem. The results show there is only slight variations between the different areas: fibers from the middle exhibit higher tensile strength and ultimate elongation than top and bottom fibers, but there are no differences in terms of stiffness. A strong dependence of the fiber mechanical properties on their diameter is observed. This dependence induces more dispersion of the properties than the sampling area, thus it seems relevant to consider the whole stem to extract fibers, without defining distinct areas. [Copyright &y& Elsevier]

Abstract: Vegetable fibers have several positive aspects, such as low production costs, biodegradability and good mechanical properties, which make them very attractive in the field of composite reinforcement. The aim of this study is to introduce into a l-poly-(lactic-acid) (PLLA) – hemp biocomposite a new compatibilizer, inspired from the mussels'' adhesive capacity, to improve the fiber-matrix bonding. As a first step, we carried out atomic force microscopy (AFM) and surface energy measurements in order to prove the compatibilizer''s presence. Secondly, tensile tests were performed. The results show a significant increase of Young''s modulus and the breaking strength when using polydopamine. Observations made through scanning electronic microscopy (SEM) confirmed an improvement of the fiber-matrix coupling in the presence of poly-(dopamine). [Copyright &y& Elsevier]

Flax/Poly-(lactide) non-woven composites are an alternative to conventional glass/poly-(propylene), offering a potentially lower environmental impact solution for the automotive industry. To understand this complex material, its detailed architecture and void distribution are examined through 3D microtomography. Anisotropy in fibre orientation is also observed, further verified through off-axis tensile tests. Then micro-mechanics and laminate theory are used, taking into account fibre orientation distribution, to predict mechanical properties and compare with experimental measurements. Even though some deviation is observed at off-axis angles greater than 45°, we report that off-axis tensile tests (property) can be used to predict fibre orientation (structure) in industrial production facilities as a simple means of quality control and tailored design of new non-woven preforms. [Display omitted] • Detailed microstructure analysis of flax/PLA non-woven composite. • Preferential orientation of the fibres is observed inside the composite. • This anisotropy is observed on tensile stiffness (transversal = 60% longitudinal). • Off-axis tensile test can predict to indirectly the fibre orientation distribution. [ABSTRACT FROM AUTHOR]

Abstract: Thermoplastics reinforced by natural fibres are mainly used for fitting-up products in the automotive industry. The aim of this work is to study the tensile properties of natural fibre-biopolymer composites in order to determine whether or not, biocomposites may replace glass fibre reinforced unsaturated polyester resins. The materials used are flax fibre, polylactic acid (PLA), l-polylactide acid (PLLA), poly(3-hydroxylbutyrate) (PHB), polycaprolactone and starch thermoplastic (MaterBi® Z), poly(butylene succianate) (PBS) and poly(butylene adipate-co-terephtalate) (PBAT). The tensile properties of the flax fibres have already been determined [C. Baley, Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Comp Part A 2002;33:939–948]. The composites are manufactured using a film stacking technique. After studying the processing parameters, these are then adapted to each thermoplastic composites. Test samples are cut out from the composites to test their mechanical properties under tensile loading conditions. These tensile properties are then compared to those of similar polypropylene flax composites. Preliminary results show that the tensile properties are improved with the fibre volume fraction. The tensile strength and Young’s modulus of PLLA and PLA flax composites are greater than those of similar PP/flax fibre composites. The specific tensile strength and modulus of flax fibre/PLLA composite have proved to be very close to those of glass fibre polyester composites. [Copyright &y& Elsevier]

• The impact of granulometry flax shives was assessed in injected PP biocomposites. • Morphology, crystallinity and biochemical composition of flax shives were analysed. • Mechanical results vary according to flax shive aspect ratio and volume fraction. • A model was established to evaluate the role of hollow reinforcing flax cell walls. 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. [ABSTRACT FROM AUTHOR]

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. • Evolution of hygromechanical properties of flax/PP non-woven composites was investigated. • A large range of porosity (5, 30, 50%) and humidity (10–98%) was explored. • The kinetic sorption behaviour was greatly impacted by porosity content. • Tensile behaviour and properties are slightly changed over a range of 10–75%RH. [ABSTRACT FROM AUTHOR]