Dynamic strain rate and relative density effect on compression behavior of PP and PP/PE copolymers foamed by microcellular injection molding

Isotactic polypropylene–polyethylene copolymer microcellular foams of various relative densities were prepared using nitrogen (N2) in supercritical solid state. Also, series of copolymers with different polyethylene weight percentages were prepared. Mechanical properties (compression and impact conditions) dependence on relative density, PE content, and strain rate was studied. Results showed that yield stress gradually decreased with lower density. The yield stress of PP copolymer foams decreased with the relative densities, and was always smaller than that of the solid PP copolymer. Nevertheless, the sensitivity to strain rate effect decreases at impact velocities. Analytical models were developed including the effects of both relative density and strain rate

Fractura de elastómeros EPDM cargados con microesferas de vidrio mediante el trabajo esencial de fractura

In this work, the essential work of fracture concept (EWF) has been applied to untreated and silane- treated glass bead-filled EPDM. It has been proved that this theory is not applicable to pure EPDM due to its high elastic character, but being possible in glass bead-filled EPDM, thus the presence of these particles induces certain plasticity on the EPDM. The results show that the higher adhesion between matrix and particles, the higher value of plastic work of fracture, and the higher final instability of crack propagation. Moreover the influence of processing temperature on the mechanical characteristics of these materials has been studied, noticing a marked drop of stiffness and strength from a temperature processing of 200 ºC. The higher adhesion obtained between EPDM matrix and glass beads through surface treatment has been confirmed by scanning electron microscopy.<br><br>En el presente trabajo se ha aplicado el concepto de trabajo esencial de fractura (EWF) a una serie de compuestos de EPDM y microesferas de vidrio, sin tratamiento superficial y tratadas con silanos. Se ha comprobado que esta teoría no es aplicable al EPDM puro debido a su elevado carácter elástico, pudiéndose en cambio aplicar a sus compuestos cargados con microesferas de vidrio, pues la presencia de estas partículas induce cierta plasticidad en el EPDM. Los resultados indican que una mayor adhesión entre partícula y matriz se traduce en un mayor valor de trabajo plástico de fractura, así como en una mayor inestabilidad final en la propagación de la grieta. Asimismo se ha estudiado la influencia de la temperatura de procesado sobre las características mecánicas de estos materiales, observándose una caída acusada de la rigidez y de la resistencia a la tracción en todos los compuestos a partir de una temperatura de procesado de 200 ºC. La mayor adhesión que se consigue entre la matriz de EPDM y las microesferas de vidrio mediante el tratamiento superficial con silanos se ha confirmado a través de la microscopía electrónica de barrido

High strain rate behaviour of polypropylene microfoams

Microcellular materials such as polypropylene foams are often used in protective applications and passive safety for packaging (electronic components, aeronautical structures, food, etc.) or personal safety (helmets, knee-pads, etc.). In such applications the foams which are used are often designed to absorb the maximum energy and are generally subjected to severe loadings involving high strain rates. The manufacture process to obtain polymeric microcellular foams is based on the polymer saturation with a supercritical gas, at high temperature and pressure. This method presents several advantages over the conventional injection moulding techniques which make it industrially feasible. However, the effect of processing conditions such as blowing agent, concentration and microfoaming time and/or temperature on the microstructure of the resulting microcellular polymer (density, cell size and geometry) is not yet set up. The compressive mechanical behaviour of several microcellular polypropylene foams has been investigated over a wide range of strain rates (0.001 to 3000 s−1) in order to show the effects of the processing parameters and strain rate on the mechanical properties. High strain rate tests were performed using a Split Hopkinson Pressure Bar apparatus (SHPB). Polypropylene and polyethylene-ethylene block copolymer foams of various densities were considered

Morphology and Mechanical Characterization of ABS Foamed by Microcellular Injection Molding

AbstractIn the present work ABS was used to inject cylindrical test bars, obtaining solid and foamed specimens. By varying the gas content, two levels of weight reduction were achieved. Morphology analysis revealed the presence of solid skin-foamed core structure in foamed samples. SEM micrographs showed a nucleus zone having bigger cells and irregular cell distribution, surrounded by a microcellular area with finer cell structure. Foamed bars with 10% and 17% of weight reduction presented similar values of cell size, cell density and solid skin thickness. On the other hand, results provided by simulation software were consistent with the experimental analysis. Mechanical properties were determined through tensile tests. Tensile strength and elastic modulus gradually decreased with decreasing apparent density. Experimental results were related to relative density and morphology parameters, and prediction models were employed to compare the estimated values to the experimental data