Ester Laguna-Gutierrez

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.

Low Density Non-crosslinked Closed/Open Cell Polypropylene Foams with High Mechanical Properties

Low density polypropylene based foams with different cellular structures have been produced by the improved compression molding route using a high melt strength polypropylene as polymer matrix. In addition, different types of nanoparticles have been introduced in the formulation (multi-wall carbon nanotubes, organomodified nanoclays and natural nanoclays) to modify the structure and properties. The results have showed a clear correlation between the open cell content of the foams and the mechanical properties in compression. In the unfilled polypropylene high specific mechanical properties are only achievable with low values of open cell content. In comparison, for an equal value of the interconnectivity between cells, the samples containing nanoclays present much higher specific properties. This result is attributed to the reinforcement of these nanoparticles in the solid matrix, due to an improved exfoliation during the foaming process and the presence of a bimodal cellular structure. The produced foams have interesting properties with stiffness similar to those of commercial polymer foams used for the core of sandwich panels.

Extensional rheology, cellular structure, mechanical behavior relationships in HMS PP/montmorillonite foams with similar densities

The main goal of this work is to analyze the relationships between the extensional rheological behavior of solid nanocomposites based on high melt strength polypropylene (HMS PP) and montmorillonites (MMT) and the cellular structure and mechanical properties of foams produced from these materials. For this purpose two systems have been analyzed. The first one incorporates organomodified MMT and a compatibilizer and the second system contains natural clays and is produced without the compatibilizer. Results indicate that the extensional rheological behavior of both materials is completely different. The strain hardening of the polymer containing organomodified clays decreases as the clay content increases. As a consequence, the open cell content of this material increases with the clay content and hence, the mechanical properties get worse. However, in the materials produced with natural clays this relationship is not so clear. While no changes are detected in the extensional rheological behavior by adding these particles, the nano-filled materials show an open cell structure, opposite to the closed cell structure of the pure polymer, which is caused by the fact of having particle agglomerates with a size larger than the thickness of the cell walls and a poor compatibility between the clays and the polymer.

Improving the extensional rheological properties and foamability of high-density polyethylene by means of chemical crosslinking

Obtaining high-density polyethylene-based microcellular foams is a topic of interest due to the synergistic properties that can be obtained by the fact of achieving a microcellular structure using a polymer with a high number of interesting properties. However, due to the high crystallinity of this polymer, the production of low-density microcellular foams, by a physical foaming process, is not a simple task. In this work, the proposed solution to produce these materials is based on using crosslinked high-density polyethylenes. By crosslinking the polymer matrix, it is possible to increase the amount of gas available for foaming and also to improve the extensional rheological properties. In addition, the foaming time and the foaming temperature have also been modified with the aim of analyzing and understanding the mechanisms taking place during the foaming process to finally obtain cellular materials with low densities and improved cellular structures. The results indicate that cellular materials with relative densities of 0.37 and with cell sizes of approximately 2 µm can be produced from crosslinked high-density polyethylene using the appropriate crosslinking degree and foaming parameters.

Gas Dissolution Foaming as a Novel Approach for the Production of Lightweight Biocomposites of PHB/Natural Fibre Fabrics

The aim of this study is to propose and explore a novel approach for the production of cellular lightweight natural fibre, nonwoven, fabric-reinforced biocomposites by means of gas dissolution foaming from composite precursors of polyhydroxybutyrate-based matrix and flax fabric reinforcement. The main challenge is the development of a regular cellular structure in the polymeric matrix to reach a weight reduction while keeping a good fibre-matrix stress transfer and adhesion. The viability of the process is evaluated through the analysis of the cellular structure and morphology of the composites. The effect of matrix modification, nonwoven treatment, expansion temperature, and expansion pressure on the density and cellular structure of the cellular composites is evaluated. It was found that the nonwoven fabric plays a key role in the formation of a uniform cellular morphology, although limiting the maximum expansion ratio of the composites. Cellular composites with a significant reduction of weight (relative densities in the range 0.4–0.5) were successfully obtained.

Sorption behavior and microcellular foaming of low-density polyethylene/clay nanocomposites by gas dissolution under sub-critical conditions:

Two of the most interesting properties of polymer composites are their improved gas barrier properties, which make them useful for packaging and protective applications and their ability to generate microcellular structures with interesting applications in the construction sector. In this paper, the batch gas dissolution method has been employed to analyze both the sorption behavior and the microcellular foamability of different formulations based on blends of low-density polyethylene, polyethylene grafted with maleic anhydride, and layered clays. Blends of low-density polyethylene and polyethylene grafted with maleic anhydride have been also produced with the aim of clearly distinguish the role played by the clays. The batch gas dissolution process, employing CO2 as gaseous phase, has been intensively used for the foaming of amorphous polymers; however, it is not common in the case of semi-crystalline polymers like low-density polyethylene. Results indicate that the presence of clays reduces slightly the diffusivity. However, the reduction in diffusivity when the clay content increases from 3 wt.% to 5 wt.% is not as high as expected. The existence of an interface between the clays and the polymer, which allows gas molecules to find faster ways across the composite, could be the reason of this behavior. Regarding the cellular materials, in general the relative density increases when adding nanoparticles and also a deterioration of the cellular structure is observed. The slight improvements in the sorption behavior have not been high enough to overcome the effects related to the worsening of the extensional rheological properties of the polymer composites.

Microcellular foaming by using subcritical CO2 of crosslinked and non-crosslinked LDPE/clay nanocomposites

The semicrystalline character of low density polyethylene adds severe difficulties to its foamability by a batch process in which the gas is dissolved into the polymer matrix under subcritical conditions. To improve the low density polyethylene foamability, two strategies have been used: the addition of nanoclays and a partial crosslinking of the polymer matrix. On the one hand, the use of nanoparticles is suggested because they act as heterogeneous nucleating sites reducing the cell size and increasing the cell density. On the other hand, crosslinking is also adopted as a solution because both the crystallinity (and hence, the gas solubility and diffusivity) and the extensional rheological properties of the polymer matrix are highly influenced by the crosslinking degree achieved. Results indicate that despite the fact that the presence of nanoclays deteriorates the rheological behaviour of the nanocomposites and, hence, the later foaming behaviour, the use of partially crosslinked polymer matrices allows achieving high expansion ratios (around 7.5) as well as enhanced cellular structures with cell sizes of approximately 15 µm.