Développement, caractérisation et optimisation de matériaux composites hybrides

Climate change will require a more sober industry, in which “green” materials may play a key role. This also applies to composite materials, and every possibility to reduce their carbon footprint might be considered, while maintaining acceptable performance and durability levels.Partial replacement of carbon fibres by flax fibres is for instance a way to reduce the environmental impact of these composite materials. Surface energies of the fibres being quite different, the properties of the composite materials resulting of such a hybridisation might not be optimised. Moreover, flax fibres are hydrophilic, which rises two problems: they are poorly compatible with hydrophobic polymers, and can lead to a decrease of the durability of the composite, more susceptible to absorb water.In this study, a treatment by direct fluorination by molecular fluorine F2 has been implemented, aiming at level the surface energies of the fibres and increase their affinity with the matrix polymer. This work is mainly dedicated to the fluorination of carbon fibres, while another simultaneous doctoral project focused on flax fibre fluorination.First, a state of the art on the fluorination of carbon fibres has been realised, and published as a literature review, where the versatility of the carbon-fluorine bond and the experimental conditions leading influencing it were carefully described.It has then been shown by a wide variety of physicochemical analyses that room-temperature fluorination of carbon fibres was a way to fluorinate their sizing, and thus to tailor their surface energy. The same treatment was carried on a wide temperature scale, successively highlighting a sizing fluorination and hyperfluorination, and then a fluorination of the carbon fibre, leading to a huge increase of its hydrophobicity, and a decrease of its polar component.These fluorinated carbon and flax fibres were then integrated in an epoxy matrix by infusion process, with several stacking sequences. Mechanical tests do not show an improvement of the fibre-matrix compatibility thanks to fluorination, which might be explained by the high-quality interface allowed by sizing for the non-fluorinated fibres. Wet ageing tests are however promising, as composites made with fluorinated fibres absorb less water than composites made with non-fluorinated fibres.Le changement climatique va exiger une industrie plus sobre, dans laquelle les matériaux “verts” peuvent jouer un rôle prépondérant. Les matériaux composites n’échappent pas à cette règle, et tout effort pour diminuer leur empreinte écologique doit être entrepris, tout en conservant des niveaux de performance et de durabilité acceptables.Le remplacement partiel de fibres de carbone par des fibres de lin est par exemple une possibilité de réduction de l’impact environnemental de ces matériaux composites. Les énergies de surface des deux fibres étant relativement différentes, les propriétés des matériaux composites issus de cette hybridation ne sont potentiellement pas optimales. De plus, les fibres végétales sont hydrophiles, ce qui pose deux problèmes : d’une part, cela les rend incompatibles avec nombre de polymères, hydrophobes. D’autre part, cela peut engendrer une réduction de la durabilité du composite, plus susceptible de se gorger en eau.Dans cette étude, un traitement par fluoration directe par du fluor moléculaire F2 a été mis en place, afin de niveler les énergies de surface des deux types de renfort et de favoriser leur interaction avec la matrice. Ces travaux se sont focalisés sur la fluoration du renfort carbone, tandis qu’une autre thèse menée en parallèle s’est focalisée sur la fluoration du lin.Dans un premier temps, un état de l’art sur la fluoration des fibres de carbone a été proposé sous la forme d’une revue de littérature. La versatilité de la liaison carbone-fluor et les conditions expérimentales permettant de jouer dessus ont été décrites en détail.Dans un deuxième temps, il a été montré par un large panel d’analyses physico-chimiques que la fluoration à température ambiante de fibres de carbone permettait de fluorer leur ensimage, et donc d’ajuster l’énergie de surface des fibres de carbone. Le même traitement a été ensuite mené sur une large gamme de températures, montrant successivement une fluoration et une hyperfluoration de l’ensimage, puis une fluoration de la fibre de carbone elle-même, avec une augmentation drastique de son hydrophobie et une diminution de sa composante polaire. Ces fibres de carbone et de lin fluorées ont ensuite été intégrées par infusion dans une matrice époxyde avec différentes séquences d’empilement. Les essais mécaniques sont peu concluants vis-à-vis d’une éventuelle amélioration de la compatibilité due à la fluoration, ce qui s’explique par l’optimisation d’interface permise par l’ensimage dans le cas de fibres non-fluorées. Les tests de vieillissement sont quant à eux prometteurs, puisque les matériaux intégrant des fibres fluorées absorbent moins d’eau que ceux renforcés de fibres non-fluorées

Développement, caractérisation et optimisation de matériaux composites hybrides

Climate change will require a more sober industry, in which “green” materials may play a key role. This also applies to composite materials, and every possibility to reduce their carbon footprint might be considered, while maintaining acceptable performance and durability levels.Partial replacement of carbon fibres by flax fibres is for instance a way to reduce the environmental impact of these composite materials. Surface energies of the fibres being quite different, the properties of the composite materials resulting of such a hybridisation might not be optimised. Moreover, flax fibres are hydrophilic, which rises two problems: they are poorly compatible with hydrophobic polymers, and can lead to a decrease of the durability of the composite, more susceptible to absorb water.In this study, a treatment by direct fluorination by molecular fluorine F2 has been implemented, aiming at level the surface energies of the fibres and increase their affinity with the matrix polymer. This work is mainly dedicated to the fluorination of carbon fibres, while another simultaneous doctoral project focused on flax fibre fluorination.First, a state of the art on the fluorination of carbon fibres has been realised, and published as a literature review, where the versatility of the carbon-fluorine bond and the experimental conditions leading influencing it were carefully described.It has then been shown by a wide variety of physicochemical analyses that room-temperature fluorination of carbon fibres was a way to fluorinate their sizing, and thus to tailor their surface energy. The same treatment was carried on a wide temperature scale, successively highlighting a sizing fluorination and hyperfluorination, and then a fluorination of the carbon fibre, leading to a huge increase of its hydrophobicity, and a decrease of its polar component.These fluorinated carbon and flax fibres were then integrated in an epoxy matrix by infusion process, with several stacking sequences. Mechanical tests do not show an improvement of the fibre-matrix compatibility thanks to fluorination, which might be explained by the high-quality interface allowed by sizing for the non-fluorinated fibres. Wet ageing tests are however promising, as composites made with fluorinated fibres absorb less water than composites made with non-fluorinated fibres.Le changement climatique va exiger une industrie plus sobre, dans laquelle les matériaux “verts” peuvent jouer un rôle prépondérant. Les matériaux composites n’échappent pas à cette règle, et tout effort pour diminuer leur empreinte écologique doit être entrepris, tout en conservant des niveaux de performance et de durabilité acceptables.Le remplacement partiel de fibres de carbone par des fibres de lin est par exemple une possibilité de réduction de l’impact environnemental de ces matériaux composites. Les énergies de surface des deux fibres étant relativement différentes, les propriétés des matériaux composites issus de cette hybridation ne sont potentiellement pas optimales. De plus, les fibres végétales sont hydrophiles, ce qui pose deux problèmes : d’une part, cela les rend incompatibles avec nombre de polymères, hydrophobes. D’autre part, cela peut engendrer une réduction de la durabilité du composite, plus susceptible de se gorger en eau.Dans cette étude, un traitement par fluoration directe par du fluor moléculaire F2 a été mis en place, afin de niveler les énergies de surface des deux types de renfort et de favoriser leur interaction avec la matrice. Ces travaux se sont focalisés sur la fluoration du renfort carbone, tandis qu’une autre thèse menée en parallèle s’est focalisée sur la fluoration du lin.Dans un premier temps, un état de l’art sur la fluoration des fibres de carbone a été proposé sous la forme d’une revue de littérature. La versatilité de la liaison carbone-fluor et les conditions expérimentales permettant de jouer dessus ont été décrites en détail.Dans un deuxième temps, il a été montré par un large panel d’analyses physico-chimiques que la fluoration à température ambiante de fibres de carbone permettait de fluorer leur ensimage, et donc d’ajuster l’énergie de surface des fibres de carbone. Le même traitement a été ensuite mené sur une large gamme de températures, montrant successivement une fluoration et une hyperfluoration de l’ensimage, puis une fluoration de la fibre de carbone elle-même, avec une augmentation drastique de son hydrophobie et une diminution de sa composante polaire. Ces fibres de carbone et de lin fluorées ont ensuite été intégrées par infusion dans une matrice époxyde avec différentes séquences d’empilement. Les essais mécaniques sont peu concluants vis-à-vis d’une éventuelle amélioration de la compatibilité due à la fluoration, ce qui s’explique par l’optimisation d’interface permise par l’ensimage dans le cas de fibres non-fluorées. Les tests de vieillissement sont quant à eux prometteurs, puisque les matériaux intégrant des fibres fluorées absorbent moins d’eau que ceux renforcés de fibres non-fluorées

Fluorination of carbon fibre sizing without mechanical or chemical loss of the fibre

International audienc

A review about the fluorination and oxyfluorination of carbon fibres

International audienceDepending both on structure and properties of the starting carbonaceous material, and on the fluorination conditions, fluorine can bond to carbon fibres in various ways. This review aims to investigate the versatility of the C-F bonding and its influence on the fibre properties. Morphology and surface properties, such as diameter, Scanning Electron Microscopy observations, specific surface area, wettability and surface acidity are discussed according to the fluorine content and the fluorination route. Structural and bulk properties of fluorinated carbon fibres, regarding Raman spectroscopy, X-ray diffraction analysis, electrical and mechanical properties, and bonding with polymers are also studied. 114 papers are reviewed in order to both extract general trends on those characteristics and highlight applications as fillers in composites and sensitive materials in gas sensing

From the Understanding of Fluorination Process to Hydrophobic Natural Fibers

International audienc

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

Surface modification of sized vegetal fibers through direct fluorination for eco-composites

International audienceNatural fibers are frequently used as alternatives of glass fibers as polymer matrix reinforcement to form composite materials. However, their natural hydrophilicity prevents them from being easily compatible with hydrophobic polymers, which represent the majority of matrices. To solve this, direct fluorination of sized natural fibers (flax fibers sized with DGEBA) was performed. EDX, FTIR and 19F NMR analysis evidenced the chemical grafting of fluorine on DGEBA structure and the ablation of the oxiranes rings on this molecule. Chemical modifications of DGEBA have induced a hydrophobic character of this layer, by reducing at 0 the polar component of surface tension. Thereby, treated fibers are supposed to be perfectly chemically compatible with the hydrophobic polymer matrices (e.g. polypropylene). Additionally, the fluorination time also allows the dispersive component of surface tension and the rugosity of fibers to be tailored in order to perfectly adjust these characteristics and fit with the polymer. Moreover, this chemical modification was achieved without altering the mechanical properties of fibers for short fluorination times

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

Towards more efficient and environmental friendly flax-based eco-composite through direct F2 fluorination as a compatibilization treatment

International audienceIn light of current environmental issues and the emerging context of bio-economy aimed atcontinuing economic growth while preserving the environment and earth resources, vegetal fibers areincreasingly used to substitute glass fibers for polymer strengthening to make eco-composites. Indeed,the use of natural fibers allows bio-based and local resources to be valorized while lightening the overallweight and reducing the cost of composites.Flax fibers are usually considered as adequate bio-reinforcements for composites. However, theirhydrophilic character makes them sensitive to moisture sorption and difficult to wet by hydrophobicresins. This incompatibility can induce a defective interface and micro-porosity between fibers andmatrix that would greatly weaken the eco-composite’s mechanical performance[1][2][3]. Several wayshave been explored to improve wettability of bio-based reinforcements towards resins. However, thesetechniques can damage the initial material and/or are generally harmful to the environment through theuse of toxic products and solvents, which is against the idea of making environmental friendly eco-composites.It is in this context that fluorination treatment takes place. Indeed, a fast and controlled reactiontreatment under molecular fluorine (F2 ) of wood[4] or vegetal fibers allows fluorine atoms to becovalently grafted on the outmost surface of lignocellulosic material in substitution of hydroxyl groupsresponsible for hydrophilicity. This grafting, achieved on flax fibers, allowed the fibers’ polarity to besignificantly decreased without modification of their bulk’s mechanical performance. This reduce thegap between the surface energies of the fibers and different polymer matrix (bio-based epoxy andElium®). In other words, the wetting of the fiber by the polymer is improved during the infusion process.Thereby, porosity into the composite thus formed is significantly reduced, increasing its mechanicalperformance, its health, and its life span during a humid environment aging, without any chemicalcoupling agent harmful to the environment [3][5].[1] F. M. AL-Oqla, M. S. Salit, Materials Selection for Natural Fiber Composites, 2017, 23-48[2] P.-J. Liotier, M.F. Pucci, A. Le Duigou, A. Kervoelen, J. Tirilló, F. Sarasini, S. Drapier, Compos. B.Eng. 163, 2019, 86–95.[3] M.F. Pucci, P.-J. Liotier, D. Seveno, C. Fuentes, A. Van Vuure, S. Drapier, Compos. A: Appl. Sci.Manuf. 97, 2017, 31–40.[4] M. Pouzet, M. Dubois, K. Charlet, A. Béakou, J.-M. Leban, M. Bada, 133, 2019, 133–141.[5] F. Saulnier, M. Dubois, K. Charlet, L. Frezet, A. Beakou, Carbohydr. Polym. 94, 2013, 642–646

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