Effects of hot roll process on the morphology and tensile properties of injection-molded polypropylene

Changes in the high-order structure of injection-molded polypropylene and the tensile properties caused by a hot roll process were investigated. The following main results have been obtained. (1) A dimension recovery of the test specimen is usually found in a cold roll process. However, in a hot roll process there is an excellent dimensional stability and the maximum roll ratio may reach to 80%. (2) The roll process generally causes a decrease in the crystallinity of the test specimen, but the degree is much less in the case of a hot roll process. (3) Up to 40% of roll ratio, the molecular orientation and E' of a hot rolling are lower than these of a cold rolling. However, when the roll ratio over than 60% both the molecular orientation and E' are improved rapidly. (4) The stress level of a hot-rolled specimen in the tensile plastic-deformation range is high compared to that of a cold-rolled one. Especially when the roll ratio is over 60%, a very high tensile strength and good ductility property are demonstrated. Such results show that up to 40% of roll ratio under any rolling temperature no improvement in the mechanical properties of test specimen can be expected. However, the hot roll process is still an effective method for improving the material quality, the strength and the ductility, since a high roll ratio may be obtained by the process.