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

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

Steam explosion pretreatment and thermal degradation of cellulose fibers

LIGNOFUE

Influence of steam explosion and thermal degradation of cellulose

The aim of the present work is to compare the effect of different steam explosion pretreatments on the thermal degradation of a bleached cellulose where components like hemicelluloses and lignin have already been removed by acid and alkaline treatments. The results of this study show that thermal degradation of cellulose fibres, studied by TGA, is still limited for a temperature process below 240 °C. However, derivative TGA show that thermal stability of cellulose obtained by these conditions decreases with the increase of temperature. For temperatures above 250°C, char level is higher at the end of the pyrolysis. According to the literature, the increase of the char level is correlated to an increase of the degradation product1. Determination of the degradation products in the liquor obtained after the pretreatment show an important increase of furfural and 5-hydroxymethylfurfural concentration with the temperature in agreement with the increase of the char level. These results confirm the important degradation of the cellulose fibres.LIGNOFUE