Effect of cold extrusion and heat treatment on the mechanical properties of polypropylene

Polypropylene rods are cold extruded through a die with three nominal area reductions of 18, 40 and 64%. These extrudates are subject to subsequent heat treatments at 100°, 120° and 140 °C for two hours after which this is followed by either air cooling or water quenching. The effect of cold extrusion increases the tensile strength, the elastic moduli in tension and compression as well as the specific impact energy absorption. The 0.2% offset yield strength in tension decreases slightly for the 18 and 40% extrudates but increases above the value of the as-received polymer at 64% cold work. However, cold extrusion decreases the compressive yield strength and the density which is a measure of crystallinity of the cold worked polymer. Cold extrusion followed by heat treatment reduces the elastic moduli but raises the yield strengths of the extrudates. The impact energy absorption shows a sharp increase with annealing temperature and the fracture surfaces display increasing orientation effect with increasing amounts of cold work. Air-cooled and quenched samples do not have any significant differences in these macroscopic mechanical properties. It is apparent that heat treatment increases the density and hence the crystallinity of the cold worked polymer. The improvement is marginally larger for the air-cooled than for the quenched samples. The results of this investigation suggest that it is possible to obtain a combination of mechanical properties of polypropylene by a suitable extrusion-heat treatment process.