Flax (Linum usitatissimum L.) Fibers for Composite Reinforcement: Exploring the Link Between Plant Growth, Cell Walls Development, and Fiber Properties

Due to the combination of high mechanical performances and plant-based origin, flax fibers are interesting reinforcement for environmentally friendly composite materials. An increasing amount of research articles and reviews focuses on the processing and properties of flax-based products, without taking into account the original key role of flax fibers, namely, reinforcement elements of the flax stem (Linum usitatissimum L.). The ontogeny of the plant, scattering of fiber properties along the plant, or the plant growth conditions are rarely considered. Conversely, exploring the development of flax fibers and parameters influencing the plant mechanical properties (at the whole plant or fiber scale) could be an interesting way to control and/or optimize fiber performances, and to a greater extent, flax fiber-based products. The first part of the present review synthesized the general knowledge about the growth stages of flax plants and the internal organization of the stem biological tissues. Additionally, key findings regarding the development of its fibers, from elongation to thickening, are reviewed to offer a piece of explanation of the uncommon morphological properties of flax fibers. Then, the slenderness of flax is illustrated by comparison of data given in scientific research on herbaceous plants and woody ones. In the second section, a state of the art of the varietal selection of several main industrial crops is given. This section includes the different selection criteria as well as an overview of their impact on plant characteristics. A particular interest is given to the lodging resistance and the understanding of this undesired phenomenon. The third section reviews the influence of the cultural conditions, including seedling rate and its relation with the wind in a plant canopy, as well as the impact of main tropisms (namely, thigmotropism, seismotropism, and gravitropism) on the stem and fiber characteristics. This section illustrates the mechanisms of plant adaptation, and how the environment can modify the plant biomechanical properties. Finally, this review asks botanists, breeders, and farmers’ knowledge toward the selection of potential flax varieties dedicated to composite applications, through optimized fiber performances. All along the paper, both fibers morphology and mechanical properties are discussed, in constant link with their use for composite materials reinforcement

Nanostructured hydrogels by blend electrospinning of polycaprolactone/gelatin nanofibers

Nanofibrous membranes based on polycaprolactone (PCL) have a large potential for use in biomedical applications but are limited by the hydrophobicity of PCL. Blend electrospinning of PCL with other biomedical suited materials, such as gelatin (Gt) allows for the design of better and new materials. This study investigates the possibility of blend electrospinning PCL/Gt nanofibrous membranes which can be used to design a range of novel materials better suited for biomedical applications. The electrospinnability and stability of PCL/Gt blend nanofibers from a non-toxic acid solvent system are investigated. The solvent system developed in this work allows good electrospinnable emulsions for the whole PCL/Gt composition range. Uniform bead-free nanofibers can easily be produced, and the resulting fiber diameter can be tuned by altering the total polymer concentration. Addition of small amounts of water stabilizes the electrospinning emulsions, allowing the electrospinning of large and homogeneous nanofibrous structures over a prolonged period. The resulting blend nanofibrous membranes are analyzed for their composition, morphology, and homogeneity. Cold-gelling experiments on these novel membranes show the possibility of obtaining water-stable PCL/Gt nanofibrous membranes, as well as nanostructured hydrogels reinforced with nanofibers. Both material classes provide a high potential for designing new material applications

Caractérisation multi-échelle des tiges et fibres de lin : structure et performances mécaniques

Flax (Linum usitatissimum L.) is a plant with multiple interests. Its stem provides fibers, which have long been used in the textile industry. The economic potential of flax explains its varietal selection, aiming at developing varieties exhibiting higher fiber yields as well as greater resistance toward diseases and lodging. More recently, flax fibers have been dedicated to the reinforcement of composite materials due to their outstanding mechanical and morphological properties. These singular characteristics are related to fiber development and functions within the stem. Thus, the present work offers a multi-scale characterization of flax, from the stem to the fiber cell wall, in order to understand the link between plant growth parameters, the development of its fibers and their properties. The general architecture of a flax stem is investigated, as well as the impact of the varietal selection on this structure and on fiber performances. Moreover, changes in mechanical properties of fiber cell walls over plant growth and retting process are characterized. In addition, the fiber contribution to the stem stiffness is highlighted, as well as the fiber role in the resistance of the stem to buckling. The influence of culture conditions on stem architecture and fiber features is also studied through cultivations in greenhouse and by simulating a lodging event. This original approach emphasizes the uncommon characteristics of flax, which make this plant an instructive model toward future bioinspired composite materials.Le lin (Linum usitatissimum L.) est une plante aux intérêts multiples. Sa tige est source de fibres, depuis longtemps utilisées dans le domaine du textile. Ce potentiel économique justifie la sélection variétale du lin en vue de développer des variétés plus riches en fibres et offrant une meilleure résistance aux maladies et la verse. Plus récemment, les fibres de lin ont vu leur utilisation s’étendre au renfort de matériaux composites grâce à leurs étonnantes propriétés mécaniques et morphologiques. Ces propriétés singulières s’expliquent grâce à leur développement et à leurs fonctions dans la tige. Ainsi, ce travail de thèse propose une caractérisation multi-échelle du lin, de la tige jusqu’à la paroi cellulaire de la fibre, afin de comprendre le lien entre les paramètres de croissance de la plante, le développement des fibres et leurs propriétés. L’architecture générale d’une tige de lin est explorée, ainsi que les conséquences de la sélection variétale sur cette structure et sur les propriétés des fibres. De plus, l’évolution des propriétés mécaniques des parois de fibres au cours de la croissance de la plante et de la phase de rouissage est caractérisée. En complément, la contribution des fibres à la rigidité en flexion d’une tige est mise en évidence, de même que leur rôle dans la résistance des tiges au flambage. Enfin, l’influence des conditions de culture sur les architectures des tiges et propriétés des fibres est étudiée par le biais de cultures en serre ou encore en simulant un phénomène de verse. Cette approche originale met en valeur les caractéristiques remarquables du lin qui en font un modèle de bioinspiration pour les matériaux composites de demai

Influence de l'évolution de l'architecture de la plante sur les propriétés mécaniques des fibres de lin

International audienc

Conventional or greenhouse cultivation of flax What influence on the number and quality of flax fibers?

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

Varietal selection of flax over time: Evolution of plant architecture related to influence on the mechanical properties of fibers

International audienceThe 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. (C) 2016 Elsevier B.V. All rights reserved

Flax stems: Using the plant architecture to provide models of bioinspired composite structures

International audienc

Flax stems from a specific architecture to an instructive model for bioinspired composite structures

International audienceThe present paper proposes to carefully study and describe the reinforcement mechanisms within a flax stem, which is an exceptional natural model of composite structure. Thanks to accurate microscopic investigations, with both optical and SEM method, we finely depicted the flax stem architecture, which can be view as a composite structure with an outer protection, a unidirectional ply on the periphery and a porous core; each component has a specific function, such as mechanical reinforcement for the unidirectional ply and the porous core. The significant mechanical role of fibres was underlined, as well as their local organisation in cohesive bundles, obtained because of an intrusive growth and evidenced in this work through nanomechanical AFM measurement and 3D reconstruction. Following a biomimetic approach, these data provide a source of inspiration for the composite materials of tomorrow

The remarkable slenderness of flax plant and pertinent factors affecting its mechanical stability

International audienceFlax (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

Caractérisation mécanique de la paroi cellulaire des fibres de lin par AFM : de la biomécanique aux effets des procédés de mise en forme des composites bio-sourcés

National audienceL'intérêt industriel pour les fibres libériennes végétales utilisées comme renforts de matériaux composites augmente. Cette tendances’explique notamment par leur faible impact sur l'environnement et leurs propriétés mécaniques spécifiques très intéressantes qui lesrendent attractives pour les applications dans les transports. Les propriétés en traction des fibres élémentaires sont bien décrites dansla littérature, mais des informations complémentaires à l'échelle de la paroi cellulaire sont nécessaires pour mieux comprendre larelation entre leur ultrastructure et leurs performances mécaniques. Le but de ce travail est de présenter les résultats issus decaractérisations mécaniques effectuées en microscopie à force atomique (AFM), avec le mode PeakForce QuantitativeNanomécanique (PF-QNM) qui permet de réaliser des cartographies de module élastique d'indentation (ou de contact) à des échellesde l'ordre de quelques dizaines de nanomètres.Dans un premier temps, cet outil a été utilisé afin de vérifier la présence ou non de gradient mécanique de module d'indentation ausein des couches de la paroi cellulaire des fibres de lin, en utilisant des essais de nanoindentation instrumentée et des fibres desynthèse (Kevlar) comme mesure de référence. Des cartographies de module d'indentation ont ainsi été effectuées à l'échellenanométrique, à la fois sur des parois cellulaires à différentes phases de développement dans la plante (biomécanique des tiges de lin)et matures. Ces cartographies ont permis de mettre en évidence la présence de sous couches au sein de la paroi cellulaire secondairedes fibres en cours en développement, avec des différences significatives de propriétés mécaniques. En revanche aucun gradient demodules d'indentation n’a pu être décelé dans les sections de fibres matures.De plus, cette démarche a été utilisée pour mieux comprendre l'évolution des propriétés mécaniques et morphologiques des fibres delin au cours du procédé de rouissage. Les mesures par AFM ont été complétées par une analyse de l'évolution de l'ultrastructure de laparoi cellulaire des fibres à l'aide de mesures DRX et RMN (cristallinité de la cellulose).Enfin, l’impact d’un cycle thermique, subi lors de l’élaboration d’un composite, sur les performances des parois végétales de lin a étéétudiée à l'échelle de la paroi cellulaire. Des mesures de module d'indentation ont été effectuées, à la fois par nanoindentation et PFQNM, sur des sections de fibres de lin traitées thermiquement à différentes températures (de 140 à 250°C) au sein d'une matricepolymère (PBS ou PP-MAPP) ou en l'absence de tout confinement (fibres isolées)