Vera Realinho

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.

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.

Microstructure anisotropy in polyolefin flexible foams

The use of polyolefin flexible foams with typical thicknesses between 1 and 3 mm produced by a physical foaming extrusion process is nowadays quite widespread in the packaging sector. Their high flexibility and closed-cell structure allows them to show good energy absorption properties under low loading conditions. Although the compressive response of these materials is well known, the inner microstructure developed during processing induce a high anisotropy that is responsible for their direction-dependent tensile and fracture behaviours. In this work, two different polyolefin-based foams, with densities ranging from 20 to 45 kg/m3, were studied. The induced microstructure anisotropy was characterized by micro-Raman. With this technique, the relative orientations of both crystalline and amorphous phases in the foams base polymer could be determined and thus related to their mechanical properties measured in the different directions.

Diffusion of CO2 in Polymer Nanocomposites Containing Different Types of Carbon Nanoparticles for Solid-State Microcellular Foaming Applications

This work considers the study of the diffusion of carbon dioxide in polypropylene and amorphous polymers containing carbon nanoparticles, particularly carbon nanofibres and graphene, as well as nanoclays, to be used in microcellular foaming. The diffusion of CO2 out and into the nanocomposites was studied during high pressure CO2 dissolution, as the amount of CO2 dissolved into the nanocomposite and CO2 desorption rate are crucial in order to have a proper control of foaming. Comparatively, platelet-like nanoparticles slowed down the desorption of CO2 out of the nanocomposites by means of a physical barrier effect, enabling a higher concentration of CO2 to remain in the polymer and be used in foaming. As a consequence of the higher amount of CO2 retained in the polymer and the cell nucleation effect promoted by the nanoparticles, polymer nanocomposite foams presented finer microcellular structures, in the case of PMMA even sub-microcellular, and higher specific moduli and electrical conductivities when compared to their pure counterparts.

Porous membranes based on polypropylene-ethylene copolymers. Influence of temperature on extrusion, annealing and uniaxial strain stages

In this study, block and random copolymers of polypropylene–ethylene are selected to prepare porous membranes through the melt extrusion-annealing-uniaxial stretching technique (MEAUS), at a constant draw ratio. In some cases, these copolymers were blended with a homopolymer grade. The variation of temperature in the stages of extrusion, annealing and uniaxial strain was analysed. Several characterisation techniques were employed to study this influence. The crystalline orientation was analysed by polarised infrared spectroscopy (FT-IR), and crystalline features were studied by differential scanning calorimetry (DSC). The thermal stability of the membranes was checked by thermogravimetric analysis (TGA). Tensile tests were performed to ascertain the stiffness and ductility of the produced samples. The results were correlated with the porous morphology, global porosity, and permeability to air. A close relationship was found between crystalline characteristics, porous morphology and the trends registered. An improved pore distribution along the membrane surface was found when copolymers were employed.