Multifunctional sandwich structure for electromagnetic absorption and mechanical performances

A sandwich panel based on a multiscale architectured material is developed for structural and EM absorption performances. At the nanoscale level, carbon nanotubes are dispersed in a polymer to obtain a conductive material. This composite is then foamed into a micro porous solid to improve EM absorption and to decrease the density. The foam is inserted in a millimeter scale hexagonal metallic honeycomb lattice. The combination of the metallic honeycomb and the polymeric foam provides high bending, impact and crushing performances and a moderate thermal conductivity. This hybrid is used as core for sandwich panels, produced by the addition of two EM transparent face-sheets made of glass fiber reinforced polymers. EM absorption around 90% is achieved in the 10-40 GHz frequency band with a 8.8 mm thick sandwich panel

Melt processing and characterization of cellulose fibres based composites and nanocomposites

Cellulose is one of the structural elements of wood, cotton, flax… It has a so-called “microfibrillar” structure made from a meticulous assembly of parallel chains resulting from the biosynthesis of cellulose. Microfibrils have a section of nanoscopic dimension combined with a high length conferring them a high aspect ratio (=length/diameter). They exhibit also interesting mechanical properties (Young’s modulus around 150 GPa), a low density, biodegradability, wide availability, etc. making them quite attractive filler compared to a glass fibre. The objective was to investigate the dispersion of cellulose microfibrils in polymers using melt processing technique. Microfibrils were envisaged to be released from cellulose fibres either during extrusion assisted by water injection under pressure or in suspension via homogenization (producing microfibrillated cellulose or MFC) before extrusion. Limiting cellulose thermal degradation under melt processing conditions was another challenge. The impact of the good specific properties and the gel effect of microfibrillated cellulose on the composite properties were also aimed. In-situ defibrillation during extrusion step was not reasonably reached. However it was shown that high shearing forces improved the dispersion of cellulose fibres in polyolefins. The most spectacular effects came from the water injection under pressure during extrusion. The dispersion of the fibres was enhanced whatever the shear applied. The yellowing of the composites resulting from the thermal degradation of cellulose during processing was reduced. The incorporation of microfibrils in polymers was realized by first defibrillating cellulose by homogenization and then by introducing the dried MFC in the polymer using melt compounding. Addition of surfactant, suspension of polypropylene grafted on maleic anhydride (PP-g-MA) or polyethylene glycol (PEG) to the MFC gel was necessary for preserving microfibrils from agglomeration during drying. Thermal properties of dried MFC were found to be lower than starting cellulose. The anticipated degradation was ascribed to promoted dehydration reactions with as consequence an increase of char. Coating MFC was found to limit the anticipated dehydration reactions. Dispersion of PP-g-MA coated MFC into polyolefins and PEG coated MFC into poly(lactic acid) was obtained and microfibrils and microfibril bundles were located. The efficiency of the dispersion of MFC modified the viscoelastic behavior of the system due to the formation of a percolating network.(FSA 3) -- UCL, 201

Polyolefins–biofibre composites: A new way for an industrial production

International audienc

The study of temperature and radiation induced degradation of cable polymers: A comparison between the mechanical properties of industrial and neat EPDM

AbstractThe mechanical properties of industrial and neat Ethylene Propylene Diene Monomer (EPDM) polymers, aged under γ-irradiation at different temperatures, are studied. The focus is given to the dose rate effects in polymer insulation materials, so the ageing is performed in the wide range of dose rates, doses and temperatures. Industrial EPDM samples are extracted from the cables in use in Belgian Nuclear Power Plants (NPP), and the neat EPDM samples are produced in the laboratory. The mechanical tests of non-aged and aged polymers are performed, and the methodology of estimating the polymer life time is discussed. The ultimate tensile stress and elongation at break are found to be strongly affected by both irradiation condition and temperature. The ultimate tensile stress clearly exhibits the dose rate effect observed through the shift of the crossover between cross-linking to chain scission process as a function of the dose. This crossover shifts to high dose for large dose rates, while the opposite is observed by increasing the temperature. Dose rate effect is less evident in the elongation at break data, probably because both cross-linking and chain scission affect the elongation at break in the same way, by decreasing it. In comparison to industrial EPDM aged under the same conditions, the cross-linking to chain scission crossover appears at lower dose in neat polymer and the elongation at break decreases faster by increasing the dose. In addition, the elongation at break experimental results can be modeled by changing single parameter, namely pre-exponential factor of the irradiation rate constant. This confirms that both aging processes, cross-linking and chain scission affect the elongation at break in a similar way, by deteriorating network structure responsible for polymer elastic properties. Irradiation rate constant is found to follow the square root dependence for industrial EPDM, while the linear dependence is observed for the neat EPDM. This indicates the existence of different degradation processes in these two polymers

Polyolefins-biofibre composites: A new way for an industrial production

Low density polyethylene (LDPE) composites based on cellulose fibres have been processed by high shear extrusion with water injection to help dispersion of fibres and release nanofibres from cellulose. Influence of extrusion parameters as shear, residence time, storage conditions of the matrix, and effect of water injection on the morphological properties of the composites have been studied using microscopy. Optimization of the extrusion parameters is necessary to reach a dispersion of the fibres. Increasing shearing forces and residence time allows limiting the presence of large aggregates of cellulose fibres. Use of powdered LDPE, even for short residence time and low shear, is efficient to produce well-dispersed composites. Injection of water during the extrusion also improves the quality of the dispersion. However, no nanofibres are observed. The main effect is a spectacular decrease of the discoloration (yellowing) due to cellulose degradation. Mechanical properties of the composites have been investigated. Young modulus increases with cellulose content and reinforcing effect is more important above 10% by weight. For well-dispersed composites, the extrusion parameters have no significant influence on the stiffness of the composites. However, due to the weakness of the interface, the ductility of composites is reduced compared with LDPE