Marcelo Antunes

Vegetable fibres from agricultural residues as thermo-mechanical reinforcement in recycled polypropylene-based green foams

Novel lightweight composite foams based on recycled polypropylene reinforced with cellulosic fibres obtained from agricultural residues were prepared and characterized. These composites, initially prepared by melt-mixing recycled polypropylene with variable fibre concentrations (10-25 wt.%), were foamed by high-pressure CO(2) dissolution, a clean process which avoids the use of chemical blowing agents. With the aim of studying the influence of the fibre characteristics on the resultant foams, two chemical treatments were applied to the barley straw in order to increase the α-cellulose content of the fibres. The chemical composition, morphology and thermal stability of the fibres and composites were analyzed. Results indicate that fibre chemical treatment and later foaming of the composites resulted in foams with characteristic closed-cell microcellular structures, their specific storage modulus significantly increasing due to the higher stiffness of the fibres. The addition of the fibres also resulted in an increase in the glass transition temperature of PP in both the solid composites and more significantly in the foams.

Multifunctional nanocomposite foams based on polypropylene with carbon nanofillers

This work considers the preparation and characterization of polypropylene foams with variable concentrations of graphene and carbon nanofibres, focussing on the influence of the foaming process and the nanofillers on the microstructural and dynamic-mechanical-thermal properties of the foams. Great differences were found in terms of foam morphology depending on the type of foaming process, with foams prepared by physical foaming showing a vertically deformed cell structure, while chemical foams presented an isotropic-like cellular structure. The addition of graphene resulted in foams with higher cell densities and more uniform cellular structures when compared to the ones with nanofibres. Direct result of the combination of their particular cellular structure and higher expansion, polypropylene foams obtained by physical foaming presented a higher orientation of the α-monoclinic polypropylene crystal perpendicular to the foam’s surface and higher exfoliation of the nanofillers, resulting in foams with improved mechanical properties. All these considerations are of extreme importance, as some of the most promising applications of these polymer foams require a good electromagnetic interference shielding efficiency, which greatly depends on the developed foam morphology.

Study of the Influence of the Pressure Drop Rate on the Foaming Behavior and Dynamic-Mechanical Properties of CO2 Dissolution Microcellular Polypropylene Foams

This article presents the preparation of microcellular polypropylene foams produced by a CO2 batch-foaming process and their characterization regarding the influence of the pressure drop rate on the foaming behavior and dynamic—mechanical properties. A polypropylene-based material was prepared by melt-mixing in a twin-screw extruder, cooled, and pelletized and later compression-molded in a hot-plate press to solid discs. These discs were finally foamed inside a high pressure vessel by dissolving CO2 and carefully controlling its sudden decompression drop. The dynamic—mechanical properties of the different expansion ratio-produced PP foams were studied, analyzing the influence of the pressure drop rate and residual pressures on the cellular structure and subsequent dynamic—mechanical behavior of the foams. With increasing the sudden pressure drop by reducing the residual pressure value, higher expansion ratio PP foams were obtained, reaching a maximum value of 3. Only slight differences were observed between foams regarding the cell size (maximum cell size ≈ 100 μm), the foams presenting slightly lower specific storage moduli than that of the solid material, indicating the efficiency of this process in nucleating and generating relatively high expansion ratio foams with a closed-cell type of structure and cell sizes in the micrometer range.

Novel polycarbonate-graphene nanocomposite foams prepared by CO2 dissolution

Polycarbonate foams reinforced with 0,5 wt% of graphene were obtained by firstly melt-mixing the polycarbonate and graphene in an internal mixer, compression-moulding the melt-compounded grinded material and lastly dissolving CO2 inside a high pressure vessel. The CO2 desorption behaviour in the unfilled polycarbonate and nanocomposite was studied in terms of the CO2 saturation concentration and desorption diffusion coefficient, with the graphene-filled nanocomposite displaying a higher CO2 loss rate when compared to the neat polycarbonate. The cellular structure of the foams was found to be highly dependent on the saturation/foaming temperature, with smaller cell sizes being obtained with decreasing the temperature. Another parameter that had an important influence was the residual pressure, with higher residual pressure values resulting in foams with more uniform and regular cells.

Foaming behaviour, structure, and properties of polypropylene nanocomposites foams

This work presents the preparation and characterization of compression-moulded montmorillonite and carbon nanofibre-polypropylene foams. The influence of these nanofillers on the foaming behaviour was analyzed in terms of the foaming parameters and final cellular structure and morphology of the foams. Both nanofillers induced the formation of a more isometric-like cellular structure in the foams, mainly observed for the MMT-filled nanocomposite foams. Alongside their crystalline characteristics, the nanocomposite foams were also characterized and compared with the unfilled ones regarding their dynamic-mechanical thermal behaviour. The nanocomposite foams showed higher specific storage moduli due to the reinforcement effect of the nanofillers and higher cell density isometric cellular structure. Particularly, the carbon nanofibre foams showed an increasingly higher electrical conductivity with increasing the amount of nanofibres, thus showing promising results as to produce electrically improved lightweight materials for applications such as electrostatic painting.

Heat Transfer of Mineral-Filled Polypropylene Foams

The thermal conductivity of unfilled polypropylene foams produced using different foaming processes has previously been demonstrated to be mainly affected by the foam’s bulk density [1]. The influence of adding inorganic particles is now studied, with the thermal conductivity of the mineral-filled PP foams being determined using the Transient Plane Source Method (TPS). To this end, two different fillers were used. The incorporation of high amounts (50 and 70 wt.%) of magnesium hydroxide resulted in considerably higher thermally conductive foamed materials, with interesting thermal anisotropies being observed for the higher expansion ratio foams. On the contrary, adding montmorillonite (MMT) nanoparticles did not considerably alter the thermal conductivity of the foams, their value being mainly affected by the relative density.

Thermal Conductivity of Carbon Nanofibre-Polypropylene Composite Foams

Carbon nanofibre-reinforced polypropylene nanocomposites containing from 5 to 20 wt.% of carbon nanofibres and a chemical blowing agent were melt-compounded and later foamed using compression-moulding. Alongside their foaming behaviour analysis and cellular characterization, foams showing an increasingly finer isometric cellular structure with increasing the amount of nanofibres, their thermal conductivity was determined using the Transient Plane Source Method (TPS). Contrarily to the electrical conductivity, which has previously been shown to rise with increasing the amount of carbon nanofibres [1], the addition of the nanofibres did not significantly alter the thermal conductivity of the PP foams, their value being mainly affected by the relative density, only slight differences being assessed for the higher expansion ratio PP-CNF foams.

Characterization of highly filled magnesium hydroxide-polypropylene composite foams

Magnesium hydroxide—filled polypropylene foams were prepared by a compression-molding chemical foaming process and studied considering the effects of foaming and the presence of the particles on the microstructure (cellular structure and induced particle and polymer orientations), dynamic mechanical, and flame retardancy of the polypropylene composites. Two magnesium hydroxide concentrations, 50 and 70 wt%, as well as different foam densities, were considered. Results are discussed in terms of the observed anisotropy-induced cellular and particle and crystal orientations and their effects on the direction-dependent dynamic mechanical and flame behavior results. Preliminary flame retardancy characterization of the several solid and foamed composites showed interesting results due to foaming, foams globally exhibiting a higher extinguishability than the respective solid composites.

Enhanced electrical conductivity in graphene-filled polycarbonate nanocomposites by microcellular foaming with sc-CO2

Abstract Electrically conductive polycarbonate (PC) foams containing a low concentration of graphene nanoplatelets (0.5 wt.%) were produced with variable range of expansion ratio by applying a high-pressure batch foaming process using sc-CO2. The structure of the foams was assessed by means of SEM, AFM and WAXS, and the electrical conductivity was measured in the foam growing direction. Results showed that electrical conductivity of PC composite foams remarkably increased when compared to that of non-foamed PC composite, with both the electrical conductivity and the main cell size of the foams being directly affected by the resultant expansion ratio of the foam. This interesting result could be explained by the development of an interconnected graphene nanoparticle network composed by increasingly well-dispersed and reoriented graphene nanoplatelets, which was developed into the solid fraction of the foam upon foaming by sudden depressurising of the plasticised CO2-saturated PC preform. Some evidences of morphological changes in the graphene nanoplatelets after foaming were obtained by analysing variations in graphene’s (0 0 2) diffraction plane, whose intensity decreased with foaming. A reduction of the average number of layers in the graphene nanoplatelets was also measured, both evidences indicating that improved dispersion of graphene nanoparticles existed in the PC composite foams. As a result, foams with a proper combination of low density and enhanced electrical conductivity could be produced, enabling them to be used in applications such as electromagnetic interference shielding.

Mechanical Properties and Morphology of Multifunctional Polypropylene Foams

One of the actual trends in polymer foams consists in the development of new materials by combining density reduction through foaming with the incorporation of functional fillers. This would enable to obtain materials with improved specific properties and added functionalities. There is also a growing interest in the use of environmentally-friendly materials obtained from renewable sources, thus coming as a logical step to combine both in order to create novel biocomposite foams. This work presents an overview of our recent results regarding the preparation and structural and thermo-mechanical characterizations of rigid polypropylene-based composite foams, with the main goal of developing new lightweight materials with tailor-made properties (multifunctional foams). Several reinforcements have been considered, from renewable cellulose fibres to nanometric-sized reinforcements such as silicate-layered nanoclays and carbon nanofibres.