Thermal Analysis

1. The Effect of Nanofiller on Polyethylene SystemK. Y. Lau1, 2,*, A. S. Vaughan1, G. Chen1 and I. L. Hosier11University of Southampton, Southampton, UK2Universiti Teknologi Malaysia, Johor Bahru, Malaysia Introduction Morphological Analysis The topic of polymer nanocomposites remains an active area of research in terms of its potential applications in dielectric and electrical insulation applications. Although more than a decade has passed since Lewis first considered these systems as dielectric materials, the precise effects of incorporating nanofillers into different polymers are yet to be confirmed. This paper reports on an investigation into nano-filled polyethylene system prepared via a solution blending route. A blend of polyethylene containing both high and low density polyethylene was used as the base polymer, with nanosilica as the filler. The strategy employed for material preparation involves the initial dissolution of the polymer in nonpolar xylene and the dispersion of the nanosilica in relatively polar methanol – a non-solvent for polyethylene. Mixing together the two components results in the rapid gelation of the polymer, including the nanoparticles. The crystallisation behaviour and morphology of differently processed materials have been evaluated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The influence of nanofiller dispersion on breakdown behaviour is also described. • Figure 2 illustrates the morphological structure for the materials investigated. • In the unfilled polyethylene system that has been isothermally crystallised at 115 ºC (Figure 2(a)), spherulites can be clearly observed through POM. Crystallisation at 113 ºC (Figure 2(b)) and 117 ºC (Figure 2(c)) also show clear evidence of spherulites in this system. • However, as shown in Figure 2(d), the observation of spherulites was less pronounced due to nano-inclusion. Therefore, the addition of nanosilica appears to have dramatically perturbed spherulitic development. • In both unfilled (Figure 2(e)) and nano-filled (Figure2(f)) systems that have been quenched, no spherulites can be observed by POM. Materials Preparation and Experimental Setup • The desired amount of nanosilica was added into methanol and sonicated. Concurrently, the proper amount of high density polyethylene (HDPE) and low density polyethylene (LDPE) were dissolved in xylene under heating and stirring. • The hot xylene/polyethylene mixture was poured onto the methanol/nanosilica mixture quickly with vigorous stirring. The nanocomposites precipitated out as a white mass. • Upon filtering and drying, the resulting nanocomposite was melt pressed at 150 ºC and vacuum dried at 100 ºC. • Samples for different tests were prepared by melt pressing at a temperature of 150 ºC, followed by direct quenching into water or isothermal crystallisation. • For comparison purpose, unfilled polyethylene system were prepared in the same way. • The types of materials investigated are summarised as in Table 1. (a) PEA/0/115 (b) PEA/0/113 (c) PEA/0/117 (d) PEA/5S1M/115 (e) PEA/0/Q (f) PEA/5S1M/Q Table 1: Materials investigated Figure 2: Optical micrographs taken under crossed polarisers Breakdown Strength Analysis Table 3: Weibull parameters • Figure 3 and Figure 4 show that the breakdown strength of PEA/5S1M/Q and PEA/5S1M/115 were significantly lower than that of PEA/0/Q and PEA/0/115, respectively. • Table 2 shows the Weibull parameters obtained from the breakdown test conducted for each of the materials. • Polarised optical microscopy was used to evaluate the morphology of the materials. • Differential scanning calorimetry was used to determine the thermal behaviour of the materials. The experiment was performed in a nitrogen atmosphere at a scan rate of 10 ºC/min, with sample weight of about 5 mg. • Dielectric breakdown strength measurements were conducted based upon the general consideration laid down in the ASTM Standard D149-87. The sample thickness was about 85 µm. An AC voltage of 50 Hz and a ramp rate of 50 V/s was applied until failure. The breakdown data were statistically analysed using the two-parameter Weibull distribution analysis. • Therefore, in this preparation route, the addition of nanosilica caused reduced breakdown strength in both quenched and isothermally crystallised polyethylene systems. Thermal Analysis • The DSC melting behaviour for the materials investigated is shown in Figure 1. • There are two melting peaks observed in each material, with the lower peak associated with the LDPE-rich phase and the upper peak associated with the HDPE-rich phase. • The melting behaviour of PEA/5S1M/Q is similar to PEA/0/Q, with a lower peak of ~114 ºC and an upper peak of ~124 ºC. • The lower and upper melting peaks of PEA/5S1M/115 were ~105 ºC and ~124 ºC, respectively, with no significant difference from PEA/0/115. • The DSC thermal traces indicate that there were no thermal changes caused by nanosilica inclusion. Figure 3: Breakdown strength of PEA/5S1M/Q and PEA/0/Q Figure 4: Breakdown strength of PEA/5S1M/115 and PEA/0/115 Summary and Future Work • The melting behaviour of both the quenched and isothermally crystallised polyethylene systems was not altered by nano-inclusion. • In the isothermally crystallised polyethylene systems, the addition of nanosilica disrupted spherulitic development, as observed by POM. • The breakdown strength was significantly reduced due to nano-inclusion. It could be related to the preparation or morphology of the materials, but more detailed analysis, such as the use of scanning electron microscopy, is required to provide such understanding. kyl1g10@ecs.soton.ac.uk University of Southampton, Highfield, Southampton, SO17 1BJ, UK Figure 1: DSC melting traces * Contact details : Acknowledgements One of the authors (K. Y. Lau) would like to acknowledge Ministry of Higher Education, Malaysia and Universiti Teknologi Malaysia for the financial sponsorship.