Cristina Saiz-Arroyo

Moulded polypropylene foams produced using chemical or physical blowing agents: structure–properties relationship

Polypropylene (PP) foams have become essential items due to their excellent properties. Nevertheless, obtaining net-shaped PP foams with medium relative densities is a complicated issue. In this article, two processes able to produce moulded PP foams in this density range are presented. One of them is based on a modification of the pressure quench foaming method and therefore uses a physical blowing agent (CO2). The second one is the improved compression moulding technique which uses a chemical blowing agent (azodicarbonamide). PP foams with relative densities in the range between 0.25 and 0.6 and cylindrical shape were prepared using these foaming techniques. A common PP grade (instead a highly branched one) was used to obtain the samples, showing, that by combining the appropriate foaming technique, the adequate moulds, suitable blowing agent and proper foaming parameters, net-shaped PP foams with excellent properties can be produced starting from a conventional PP grade. Samples were characterized by analyzing their cellular structure and their mechanical properties. Results have showed that depending on the chosen foaming route isotropic or anisotropic structures with cell sizes ranging from 40 to 350 μm and open cell content in the range between 0 and 65% can be obtained. Moreover, mechanical properties are highly influenced by the production route and chemical composition of the foams. For instance, the stiffer materials at relative densities higher than 0.4 are the ones produced using the chemical blowing agent while at relative densities lower than 0.4 are the ones produced using the physical blowing agent.

Improving the Structure and Physical Properties of LDPE Foams using Silica Nanoparticles as an Additive

Low density polyethylene/silica nanocomposites have been produced by melt blending and foamed in a high pressure autoclave by gas (CO2) dissolution. Different amounts of nanosilica, (from 1 wt% to 9 wt%) were used in order to analyze the influence of silica content on thermal and mechanical properties of both foamed and un-foamed composites. It has been proved that the presence of silica nanoparticles modifies the structure and properties of both foamed and un-foamed materials. An increase in the crystallinity of the polymeric matrix as well as a decrease in cell size (for low contents of silica) have been observed. In addition it was found a significant increment of melt strength, thermal stability and mechanical properties for both solids and foams. From the obtained results, it has been concluded that silica particles play a multifunctional role in this system, and in addition to this, it has been demonstrated that the use of nanoparticles can produce synergetic effects, increasing foam mechanical properties in a greater extend than that of the solids.

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.