Etude des mécanismes de fluoration de fibres naturelles destinées à la fabrication d'éco-composites

Vegetable fibers are increasingly used to substitute glass fibers for polymer strengthening to make eco-composites. Indeed, in addition to the fact that vegetal fibers and glass fibers present equivalent specific properties, the use of the former allows to valorize bio-based and local resources while lightening the overall weight and reducing the cost of composites Therefore, all these advantages provide them to be more and more used in the transport industry (aeronautics, automotive, etc.), and this is expected to increase in light of current environmental issues and the emerging context of the bio-economy aimed at continuing economic growth while preserving the environment and earth resources.However, one of the main difficulties arising when using these natural compounds as polymer matrix reinforcement is their incompatibility with a large part of the latter. Indeed, the hydrophilic nature of vegetal fibers makes them sensitive to moisture absorption and difficult to wet by hydrophobic resins (which represent the majority of polymer matrices used). Thus, to obtain the optimal mechanical performance of these composites, it is necessary to compatibilize these fibers with the polymer matrix to avoid the formation of cavities during the manufacturing process, which would greatly weaken the composite. Nowadays, several chemical and/or physical methods were developed with the aim to achieve this compatibilization (acetylation, alkaline treatment, treatment using peroxides, etc.). However, these techniques are generally harmful to the environment through the use of toxic products and solvents, and can even degrade the initial material (and consequently the mechanical properties).It is in this context that fluorination treatment takes place. Indeed, a treatment under molecular fluorine (F2) of wood or vegetal fibers allows to covalently graft fluorine atoms on the outmost surface of lignocellulosic material surface in substitution of hydroxyl groups responsible for hydrophilicity, and this in a fast and controlled reaction. This grafting, (proved, inter alia, by FT-IR spectroscopy and 19F NMR) allowed to significantly increase the hydrophobicity of fibers without modification of their bulk’s mechanical performance as it is only located on the material’s surface. This treatment thus reduces the gap between the surface energies of the fibers and the matrices; in other words, the wetting of the fiber by the polymer is improved. Thereby, porosity of the composite thus formed is significantly reduce, increasing its mechanical performance, without any chemical coupling agent harmful to humans and the environment.Les fibres végétales sont de plus en plus utilisées comme bio renfort de matériaux composites, car elles permettent non seulement de valoriser des ressources naturelles, locales et renouvelables, mais aussi de diminuer le poids global de ces matériaux ainsi que leur coût, pour des propriétés mécaniques spécifiques équivalentes aux fibres de verre. En outre, l’utilisation de ces matériaux étant plus respectueuse de l’environnement que l’usage de fibres synthétiques, elle s’inscrit parfaitement dans la problématique environnementale du XXIème, avec l’idée de fabriquer des « éco-composites », présentant une faible empreinte écologique.Toutefois, l’une des principales difficultés rencontrées lors de l’utilisation de ces composés naturels comme renfort de matrice polymère est leurs incompatibilités avec une grande partie ces dernières. En effet le caractère hydrophile des fibres végétales les rend difficiles à mouiller par les résines hydrophobes (qui représentent la majorité des matrices polymères utilisées) induisant une porosité micrométrique qui fragilise le matériau. Ainsi, pour obtenir les performances mécaniques optimales de ces composites, une comptabilisation de ces fibres avec la matrice est nécessaire. À l’heure actuelle, plusieurs méthodes chimiques et/ou physiques sont employées (acétylation, traitement alcalin, peroxydes, etc.). Cependant, elles s’avèrent généralement nocives pour l’environnement de par l’emploi de produits et solvants toxiques, ce qui va à l'encontre de l'idée même de réaliser des éco-composites, censés être plus respectueux de l'environnement.C’est dans ce contexte que le traitement de fluoration intervient. En effet, un traitement sous fluor moléculaire F2 de matériaux ligno-cellulosiques naturels tels que les fibres de lin a permis de greffer de manière covalente des atomes de fluor en substitution de groupements hydroxyles, responsables de l’hydrophilie, de manière rapide et contrôlée. Ce greffage, prouvé, entre autres, par spectroscopie infra-rouge, RMN du 19F et XPS, permet de réduire significativement l’hydrophilie des fibres, sans pour autant dégrader leurs performances mécaniques, car étant uniquement localisé en surface. Ce traitement permet ainsi de réduire l’écart entre les énergies de surface des fibres et des matrices ; en d’autres termes, le mouillage de la fibre par le polymère est amélioré, diminuant significativement la porosité du composites formés à partir de ces renforts fluorés et in fine, augmentant ses performances mécaniques, sans agent de couplage chimique nocif pour les Hommes et l’environnement

Etude des mécanismes de fluoration de fibres naturelles destinées à la fabrication d'éco-composites

Vegetable fibers are increasingly used to substitute glass fibers for polymer strengthening to make eco-composites. Indeed, in addition to the fact that vegetal fibers and glass fibers present equivalent specific properties, the use of the former allows to valorize bio-based and local resources while lightening the overall weight and reducing the cost of composites Therefore, all these advantages provide them to be more and more used in the transport industry (aeronautics, automotive, etc.), and this is expected to increase in light of current environmental issues and the emerging context of the bio-economy aimed at continuing economic growth while preserving the environment and earth resources.However, one of the main difficulties arising when using these natural compounds as polymer matrix reinforcement is their incompatibility with a large part of the latter. Indeed, the hydrophilic nature of vegetal fibers makes them sensitive to moisture absorption and difficult to wet by hydrophobic resins (which represent the majority of polymer matrices used). Thus, to obtain the optimal mechanical performance of these composites, it is necessary to compatibilize these fibers with the polymer matrix to avoid the formation of cavities during the manufacturing process, which would greatly weaken the composite. Nowadays, several chemical and/or physical methods were developed with the aim to achieve this compatibilization (acetylation, alkaline treatment, treatment using peroxides, etc.). However, these techniques are generally harmful to the environment through the use of toxic products and solvents, and can even degrade the initial material (and consequently the mechanical properties).It is in this context that fluorination treatment takes place. Indeed, a treatment under molecular fluorine (F2) of wood or vegetal fibers allows to covalently graft fluorine atoms on the outmost surface of lignocellulosic material surface in substitution of hydroxyl groups responsible for hydrophilicity, and this in a fast and controlled reaction. This grafting, (proved, inter alia, by FT-IR spectroscopy and 19F NMR) allowed to significantly increase the hydrophobicity of fibers without modification of their bulk’s mechanical performance as it is only located on the material’s surface. This treatment thus reduces the gap between the surface energies of the fibers and the matrices; in other words, the wetting of the fiber by the polymer is improved. Thereby, porosity of the composite thus formed is significantly reduce, increasing its mechanical performance, without any chemical coupling agent harmful to humans and the environment.Les fibres végétales sont de plus en plus utilisées comme bio renfort de matériaux composites, car elles permettent non seulement de valoriser des ressources naturelles, locales et renouvelables, mais aussi de diminuer le poids global de ces matériaux ainsi que leur coût, pour des propriétés mécaniques spécifiques équivalentes aux fibres de verre. En outre, l’utilisation de ces matériaux étant plus respectueuse de l’environnement que l’usage de fibres synthétiques, elle s’inscrit parfaitement dans la problématique environnementale du XXIème, avec l’idée de fabriquer des « éco-composites », présentant une faible empreinte écologique.Toutefois, l’une des principales difficultés rencontrées lors de l’utilisation de ces composés naturels comme renfort de matrice polymère est leurs incompatibilités avec une grande partie ces dernières. En effet le caractère hydrophile des fibres végétales les rend difficiles à mouiller par les résines hydrophobes (qui représentent la majorité des matrices polymères utilisées) induisant une porosité micrométrique qui fragilise le matériau. Ainsi, pour obtenir les performances mécaniques optimales de ces composites, une comptabilisation de ces fibres avec la matrice est nécessaire. À l’heure actuelle, plusieurs méthodes chimiques et/ou physiques sont employées (acétylation, traitement alcalin, peroxydes, etc.). Cependant, elles s’avèrent généralement nocives pour l’environnement de par l’emploi de produits et solvants toxiques, ce qui va à l'encontre de l'idée même de réaliser des éco-composites, censés être plus respectueux de l'environnement.C’est dans ce contexte que le traitement de fluoration intervient. En effet, un traitement sous fluor moléculaire F2 de matériaux ligno-cellulosiques naturels tels que les fibres de lin a permis de greffer de manière covalente des atomes de fluor en substitution de groupements hydroxyles, responsables de l’hydrophilie, de manière rapide et contrôlée. Ce greffage, prouvé, entre autres, par spectroscopie infra-rouge, RMN du 19F et XPS, permet de réduire significativement l’hydrophilie des fibres, sans pour autant dégrader leurs performances mécaniques, car étant uniquement localisé en surface. Ce traitement permet ainsi de réduire l’écart entre les énergies de surface des fibres et des matrices ; en d’autres termes, le mouillage de la fibre par le polymère est amélioré, diminuant significativement la porosité du composites formés à partir de ces renforts fluorés et in fine, augmentant ses performances mécaniques, sans agent de couplage chimique nocif pour les Hommes et l’environnement

Study of carbon-flax hybrid composites modified by fibre fluorination

International audienceKnowing that fibre reinforced polymers are sensitive to moisture through their interphases, carbon and flax fibres sized with Bisphenol A diglycidyl ether were fluorinated under a N2/F2 atmosphere. Because of differences in reactivity of gaseous F2 between fibres and sizing, only the latter has been fluorinated, resulting in a covalent grafting of fluorine atoms onto the fibres, that has been evidenced by Infrared spectroscopy, and X-Ray Photoelectron Spectroscopy. Fluorinated fibres exhibit enhanced mechanical properties, as highlighted by tensile tests. Hybrid and non-hybrid composite materials were then fabricated with vacuum infusion process, using non-fluorinated and fluorinated carbon and flax fibres. Their hydrothermal behaviour and mechanical properties were investigated, in order to study the impact of both hybridisation and fibre fluorination on the composite properties. A better water resistance was observed for polymer materials reinforced with fluorinated fibres, whereas their mechanical properties were slightly lower than polymers reinforced with non-fluorinated fibres

Carbon fibre fluorination: Surface and structural properties

International audienceKnowing that carbon fibre reinforced polymers are sensitive to moisture through their interphases, carbon fibres sized with Bisphenol A diglycidyl ether were fluorinated under a N2/F2 atmosphere, which confers them a hydrophobic behaviour, as highlighted by wettability measurements. Low fluorination temperatures ( 250°C) result in a carbon lattice fluorination with a complete desizing of the fibre through decomposition, that has been evidenced through X-Ray Diffraction and Raman spectroscopy studies. For both fluorination mechanisms, 19F Nuclear Magnetic Resonance, X-Ray Photoelectron and infrared spectroscopies have been used to study the nature of fluorine bonds. Scanning Electron Microscope and Energy-Dispersive X-Ray Spectroscopy have evidenced a core–shell structure for the fluorinated fibres with a fibre degradation at the highest temperatures, also highlighted by tensile tests, Atomic Force Microscopy and Electron Paramagnetic Resonance

Fluorination of flax fibers for improving the interfacial compatibility of eco-composites

International audienceBecause of their specific properties, vegetal fibers are increasingly used as sustainable polymer reinforcementsfor eco-composites. Nevertheless, their polar character hinders them from being used more frequently at in-dustrial scale due to their incompatibility with mostly dispersive polymers (the cheapest and most commonones). In this study, direct fluorination treatment was carried out to covalently graft fluorine atom at the outmostsurface of flax fibers. Such a grafting has been proved by FT-IR, 19F NMR and XPS spectroscopies, and thesecharacterizations allowed to understand chemical change due to the treatment. This chemical modificationinduced an augmentation of the dispersive character of flax fibers, by significantly lowering the polar componentof surface energy without significant change of the dispersive component. Young’s modulus was also maintained.Thereby, treated fibers become perfectly compatible with hydrophobic polymer, and an improvement in themechanical performance of the resulting composite is expected according to the literature