Dynamics mechanical analysis of unirradiated and gamma ray radiated injection molded virgin and recycled high density polyethylene
King'ori, Gladys Wangechi
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Modification in polymeric molecular structure can be brought about by either the conventional chemical means, usually involving silanes and peroxides, or by exposure to ionizing radiation, from either radioactive sources or highly accelerated electrons. In this work ionizing radiation from a radioactive source, in this case a standard ray from a Cobalt 60 source was used to irradiate injection molded samples of both recycled high density polyethylene (RHDPE) and virgin high density polyethylene (VHDPE). The samples were then studied using a Dynamic mechanical analyzer (DMA) and the Differential Scanning Calorimetry (DSC). The DMA measurements were done in a temperature range of 298 K to 355 K in the frequency range of 0.5 Hz to 13 Hz. One relaxation process was observed which follows the Vogel Fulcher Tammann (VFT) law of the temperature dependence of the mean relaxation for both VHDPE and RHDPE. The process is assigned to chain motions between the crystals. It was noted that doses of at least 12 kRad had no effect on the frequency of the relaxation process of VHDPE or RHDPE. This suggests irradiation effect on a small (local) scale. Another observation was that irradiation resulted in an increase of the loss modulus intensities, for VHDPE and a decrease in the RHDPE. For both the gamma ray radiated and unirradiated samples the intensities of the samples cut far from the injection point were higher than for samples cut near the injection point. As regards anisotropy of the samples, a comparison of the intensities of the loss modulus for samples cut parallel and those cut perpendicular to the polymer melt flow directions indicate that irradiation has a stronger influence on the samples cut perpendicular to the melt flow direction than those cut parallel. Anisotropy between the samples cut near and those cut far from the injection point was unaffected by irradiation. DSC thermograms were obtained at a heating rate of 10 K/min from 270K to 410K. From the DSC thermogram it was observed that irradiating RHDPE to a dose of up to 6 krad decreases its melting temperature, however for a dose of up to 12 krad, the melting temperature of VHDPE was insignificantly affected.