E. Solórzano

Characterization of the cellular structure based on user-interactive image analysis procedures

This work addresses the development of a novel user-interactive image analysis computed technique to determine the main characteristics of the cellular structure of cellular materials with any density, morphology, and/or cell size from micrographs obtained from any source—scanning electron microscopy, optical microscopy, etc. The specific analyzed parameters are cell size (mean cell diameter), cell anisotropy, cell size distribution, and cell density. A comparison with several conventionally used methods is also presented in this work. The development of such application offers a quick (<3 min per micrograph) and complete characterization of the cellular structure, with reproducible results and good agreement with the typical manual measurement methods.

In situ evidence of the nanoparticle nucleating effect in polyurethane–nanoclay foamed systems

This paper presents a comparative study on the nanoadditives (nanoclays) effect during the foaming process of rigid polyurethane (PU) foams. A non-standard technique, high-resolution microfocus X-ray radioscopy, has been applied for the first time on these materials to study the mechanisms (nucleation and coalescence) occurring during the reactive foaming process. Using this technique in combination with image analysis it has been possible to determine the evolution of density and cell size throughout the whole foaming process. Thus, cell density values have been calculated from the beginning of the expansion and, using the cell density data, it has been possible to demonstrate that nucleation at initial foaming stages strongly dominates over the coalescence mechanism – not observed – in the nanocomposite systems under study. These results confirm the theories assuming that nanoparticles act as heterogeneous nucleation sites for this type of materials, which were not undoubtedly confirmed via ex situ studies.

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.

Metal foams – towards microcellular materials

Abstract Various techniques to manufacture low-density metallic foams containing sub-millimetre or even micrometre-sized pores are discussed and first trial experiments presented. Three strategies are evaluated: use of an intrinsic blowing agent, foaming under high pressure and foam control by mechanical pressure manipulation. In all three cases, average pore diameters well below 1 mm could be achieved for some aluminium or zinc-based foams while keeping the relative density in a range between 20 % and 50 % of the full metal density.

Effect of Microstructural Anisotropy of PM Precursors on the Characteristic Expansion of Aluminum Foams

This work investigates the causes of the anisotropic early expansion (below the melting point) of powder metallurgical (PM) aluminum foam precursors by evaluating the crystallographic anisotropy induced during the production of the precursor materials. A varied group of precursors prepared using different parameters and techniques (direct powder extrusion and hot uniaxial compression) has been investigated. Multidirectional foaming expansion has been registered in situ by means of the optical expandometry technique, while X-ray diffraction has been used to characterize the preferred crystallographic orientation (texture) of the pressed powders. The results point to a clear correlation between the expansion anisotropy and the microstructural crystallographic anisotropy of the precursors. Although this correlation is not a direct cause–effect phenomenon, it is a good indicator of intrinsic precursor characteristics, such as densification and powder interparticle bonding, which govern the expansion behavior during the early stages when the material is still in a solid or semisolid state.

Effect of carbon nanofillers on flexible polyurethane foaming from a chemical and physical perspective

The effect of carbon nanoparticles (CNPs) on the physical and chemical events taking place during the foaming evolution of flexible polyurethane (FPU) foams is analysed by in situ X-ray time-resolved imaging. The differences observed in the cellular structure and density evolution of nanocomposite foams are explained in terms of the type of nanoparticles and the functional groups on their surface. The presence of certain types of particles enhanced the bubble nucleation at the beginning of the process although some others did not. The chemical interaction seems to produce delays in the blowing reaction process and promotes coalescence events during foam evolution as regarding the cell density results obtained. This study on the kinetics of polymerisation and morphology development of reactive PU nanocomposite foams contributes to understanding the physical phenomena occurring as a consequence of the CNP–FPU chemical interaction.

Early anisotropic expansion of aluminium foam precursors

This work reports a detailed study on the early expansion (before the melting point) of powder metallurgical (PM) aluminium foam precursors and its influence on the intrinsic anisotropy existing in the final cellular structure of PM foams. Hot uniaxial compressed tablets, rectangular and cylindrical extruded profiles and thixocast PM precursors have been considered to evaluate the effect of the processing technique on the early expansion behaviour. An optical device has been used to register, in situ, the dissimilar expansion of the precursors in the three spatial directions. Cellular morphology has been examined by X-ray microtomography and correlated to expansion behaviour. Results demonstrated a high influence of the processing technique and its correlation with powder debonding in preferential directions which, in combination with the early TiH2 decomposition, generates elongated cracks and causes an anisotropic expansion at early foaming stages. As a consequence, a remaining structural anisotropy is found in the final solid cellular material, even at high porosities. A discussion of the possible factors affecting this early expansion behaviour, complemented with possible solutions to minimize it, is provided in the paper.

Solid Skin Characterization of PMMA/MAM Foams Fabricated by Gas Dissolution Foaming over a Range of Pressures

A new model for solid skin formation on submicrocellular foams based on PMMA is presented. Structural foams has been produced by the solid state foaming technique using as precursor material a blend of poly (methyl methacrylate) PMMA and a triblock copolymer [poly (methyl methacrylate)-block poly (butyl acrylate)-block poly (methyl methacrylate), MA. An analysis on the skin thickness and densification has been carried based on x-ray high resolution radiography. The obtained results have been related to the fabrication parameters, i.e. MAM content of the blend and foaming pressure, and provide us a new vision about the processes that control the formation of a solid unfoamed, or partially unfoamed, skin in these materials.

Pore connectivity of aluminium foams: effect of production parameters

This work studies the effect of some production parameters on the pore connectivity grade (i.e., the open-cell content associated cracks and missing cell walls) of aluminium foams produced via powder metallurgy route. Two types of precursors, extruded and hot uniaxially compressed, were used to create a varied group of Al–Si and Al–Si-Mg alloy-based foams in a wide porosity range. The cellular structure and defects were characterized by gas pycnometry and X-ray tomography. The analysis performed points to a high pore connectivity in all foam specimens, despite these materials are classified as closed celled due to their appearance, and a significant dependence on all the parameters varied. These dependences and the related mechanisms are discussed in the paper in terms of (i) the dissimilar foam evolution at initial stages (effect of precursor processing technique), (ii) the solidification shrinkage of each alloy (effect of composition) and (iii) the cell wall thinning (effect of foam porosity and local drainage). In addition, it has been observed that the interconnections are preferably located in the central parts of the samples, thus suggesting the possible effect of the cooling conditions on defect generation.