Valentina Volpe

Injection molding of magneto-sensitive polymer composites

Abstract Magneto sensitive parts made of a thermoplastic elastomer reinforced with iron microparticles were prepared by imposing a magnetic field during injection molding. In particular, an aluminum mold was designed to host an electro-magnet able to apply a magnetic field during the injection of the material into the mold and its subsequent solidification. Samples obtained in the presence of magnetic field were characterized by a peculiar, aligned distribution of iron particles along the magnetic field lines, and columnar structures of variable length were obtained. The mechanical characterization showed that the samples in which the iron particles were aligned had higher modulus compared to samples in which the particles were randomly dispersed. The magnetic field induced an anisotropic structural reinforcement that imparted to composite samples a magnetostrictive feature, namely the capability to sense the magnetic field and to react with a shape change under the application of the magnetic field.

Effect of processing condition on properties of polylactic acid parts obtained by foam injection molding

Foam injection molding is a processing technology particularly interesting for biodegradable polymers, which present a very narrow processing window, with the suitable processing temperatures close to the degradation conditions. The addition of a physical blowing agent, besides decreasing the final part weight, reduces both the viscosity and the glass transition temperature of the polymer melt, allowing the processability of these materials at lower temperatures. In this work, structural foams of polylactic acid with nitrogen as physical blowing agent were obtained by foam injection molding. In particular, the effects of back pressure, namely the pressure imposed inside of the cylinder when the screw is returning back to prepare a new amount of material to be injected, and of the injection flow rate on foaming and mechanical properties of the molded parts was assessed. It was found that the samples molded adopting a higher injection flow rate are shorter than those injected at lower flow rate, and this result was ascribed to the large compressibility of the injected shot. As far as the mechanical properties of the foamed parts, it was found that the modulus decreases with decreasing density. However, the density reduction is not the only significant parameter, but also the morphology of the foams should be taken into account in order to justify the differences between tensile and flexural modulus.

PLA-Based Nanobiocomposites with Modulated Biodegradation Rate

The disposal of polymeric waste is increasingly becoming an issue of international concern. The use of biodegradable polymers is a possible strategy to face most of the problems related to the disposal of the durable (non-biodegradable) polymers. Among biodegradable polymers, polylactic acid (PLA), obtained from renewable sources, is a very attractive one, due to its relatively good processability, biocompatibility, interesting physical properties. Hydrolysis is the major depolymerization mechanism and the rate-controlling step of PLA biodegradation in compost. The propensity to degradation in the presence of water significantly limits specific industrial applications such as automotive, biomedical, electronic and electrical appliances, agriculture. Therefore the control of biodegradation rate is somewhat even more important than the characteristic of biodegradability itself. In this scenario, it is critical to find additives able to modulate the biodegradation rate of biodegradable polymers, in relationship to the expected lifetime. Since the kinetics of hydrolysis strongly depend on the pH of the hydrolyzing medium, in this work some fillers able to control the pH of water when it diffuses inside the polymer were added to PLA. In particular, fumaric acid, a bio- and eco- friendly additive, and magnesium hydroxide, a common antiacid, were used. These fillers were added to the material using a melt-compounding technique, suitable for industrial application. The results obtained are encouraging toward the possibility of effectively controlling the degradation rate.

Effect of processing conditions on the cell morphology distribution in foamed injection molded PLA samples

Foam injection molding uses physical blowing agents under high pressure and temperature to produce structural foams having a cellular core and a compact solid skin. This technology is particularly interesting for biodegradable polymers, which often present a very narrow processing window, with the suitable processing temperatures close to the degradation conditions. The addition of a supercritical gas can lead to the reduction of both the viscosity and the glass transition temperature of the polymer melt, which therefore can be injection molded adopting lower temperatures and pressures. In this work, the effect of different processing parameters on foam morphology of Poly(lactic) Acid, PLA, was studied. In particular, two commercial grades of PLA having different rheological properties were adopted to obtain foamed parts by injection molding process with nitrogen as a physical blowing agent. For both PLA grades, the effect of mold temperature on the crystallinity and the resulting cell morphology was asse...

Effects of an External Magnetic Field on Polymeric Foam-Ferromagnet Composites

Composite lightweight materials based on a polymeric matrix with embedded magnetic micro-particles have been developed. The application of a magnetic field (MF) during the foaming of samples induced the alignment of magnetic particles along the MF lines, forming reinforcing chain-like structures. The presence of aligned micro-particles imparted an anisotropic mechanical behavior along the particle alignment direction, thus strongly improving mechanical stiffness and strength compared to randomly filled systems. The application of a MF on pre-strained samples during the magneto-mechanical characterization resulted in a direct relationship between the measured variation of the elastic modulus of the foam and the time dependent intensity of the applied MF (also for a magnetic field strength as low as 200 kA/m). In particular, all reinforced samples pre-strained in the linear elastic region of the stress-strain curve exhibited a magneto-strictive response (negative variation of the measured stress). On the contrary, a positive variation of the measured stress (strengthening effect) was detected in samples with aligned particles at pre-strains above the yield point. This behavior has been related to the tendency of chain-like aggregates in buckled cell edges to re-align along the MF lines.

Effect of pressure on viscosity at high shear rates by using an injection molding machine

The difficulties in performing accurate measurements of the effect of pressure on the viscosity of the polymeric materials results in a shortage of relevant experimental data in the literature. In this work, an atactic polystyrene has been characterized to determine the effect of pressure on viscosity during the injection molding process. In particular, the nozzle of the injection molding machine has been modified to host a slit rheometer that allows obtaining in-line rheological measurements by means of two pressure transducers. Slits with two different geometries have been adopted, in order to obtain a wide range of shear rates (102-105 s−1). Experiments have been conducted at increasing injection flow rates, corresponding to different shear rates. By analyzing the measured pressures it has been possible to determine the coefficient β, which describes the effect of pressure on viscosity. Results show that the effect of pressure on viscosity decreases on increasing the shear rate until a plateau is reach...

Foam injection molding of elastomers with iron microparticles

In this work, a preliminary study of foam injection molding of a thermoplastic elastomer, Engage 8445, and its microcomposite loaded with iron particles was carried out, in order to evaluate the effect of the iron microparticles on the foaming process. In particular, reinforced samples have been prepared by using nanoparticles at 2% by volume. Nitrogen has been used as physical blowing agent. Foamed specimens consisting of neat and filled elastomer were characterized by density measurements and morphological analysis. While neat Engage has shown a well developed cellular morphology far from the injection point, the addition of iron microparticles considerably increased the homogeneity of the cellular morphology. Engage/iron foamed samples exhibited a reduction in density greater than 32%, with a good and homogeneous cellular morphology, both in the transition and in the core zones, starting from small distances from the injection point.