Abstract
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 )i n 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.
Highlights
Polypropylene and polyethylene-ethylene block copolymer foams of various densities were considered. Polymeric foams such as polypropylene foams are extensively used in energy absorption applications such as automotive crash safety systems, or personal safety
The yield stress σy as a function of foam density is plotted in Fig. 4 for tests performed in an electromechanical universal machine and using the Hopkinson bar, for each kind of polymer: PP, BC1, BC5, BC7 and BC8
Yield stress of solid and microcellular foamed polypropylene and polypropylene-ethylene block copolymers has been measured in compression over a wide range of strain rates
Summary
Polymeric foams such as polypropylene foams are extensively used in energy absorption applications such as automotive crash safety systems, or personal safety (helmets, knee-pads, etc.). The mechanical properties of cellular foams has been extensively investigated [1,2,3,4,5], but these materials exhibit densities of the order of 30 to 100 kg/m3, i.e. more than 10 times lower than the density of the starting solid polymer. These foams have shown significant improvements in mechanical properties, such as strength-to-weight ratio, but it has been proved that a reduction in cell size can yield further improvement in properties. Copolymers of different content of ethylene and various densities were considered
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