Dynamic mechanical analysis of untreated and silane treated talc filled High Density Polyethylene (HDPE) composites
Tsuwi, Julius Kazungu
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The molecular dynamics above glass transition temperature in two systems of injection - moulded talc filled High-Density Polyethylene (HDPE) composites has been studied using Dynamic Mechanical Analyzer (DMA). The composite systems were silane treated talc - filled HDPE and talc - filled HDPE without any treatment. The time scales ranging from 0.04 to 2 seconds and temperature range of 298 K and 353 K were covered. The dynamic glass transition process characterized by Vogel - Fulcher - Tamman (VFT) temperature dependence of the mean relaxation time was observed. Talc tends to slow down chain motion due to the induced stiffness in the HDPE matrix. This is attributed to the increase in length scale of cooperative dynamics responding to the mechanical stress. The induced stiffness is confirmed by the increase in intensities of the melt endotherm in differential scanning calorimetry (DSC) measurements. The intensity of the loss modulus increased with increase in talc concentration more significantly at high concentrations. The glass transition temperature was found to increase by an average of 0.52% for every 5% talc increase. The loss modulus perpendicular to melt flow is less than that along the direction of flow in pure HDPE while the reverse effect is observed in talc filled composites. This anisotropy in the loss modulus intensities is attributed to the injection - moulding process. The loss modulus for samples near and perpendicular to the injection point (NP) is higher than those far from the injection point (FP) for pure sample, while the reverse holds for composites. This shows that both the pure and the composite samples exhibit inhomogeniety. The inhomogeniety is explained in terms of the skin - core - skin morphology introduced by the injection - moulding process. For pure NP samples, there is favourable interchain interaction introduced by the skin - core - skin morphology that restricts the sample from relieving mechanical stress. As for the composites, there is favourable interparticle interaction in the FP samples due to the disorder in the skin zone that makes the sample unable to relieve stress, hence giving a higher loss modulus. Addition of silane coupling agent increased the loss intensities for all composites of various talc concentrations. The molecular dynamics shifted from a relaxation behaviour characterized by VFT to Arrhenius behaviour. This shows that the process is confined to localized motions. The glass transition temperature was found to reduce by an average of 12.2% for every 5% talc filled silane treated samples.