P.I. Dolez, R. Mohamed, E. David, M. Weltrowski
Keywords: nanocomposite, nanoclay, HDPE, geomembranes, thermal expansion
Summary:Polymer membranes have progressively replaced traditional building materials to perform waterproofing functions and control the advective migration of pollutants in civil engineering structures. Either during deployment or in service, geomembranes, 50% of which being made of high density polyethylene (HDPE), may sustain large temperature variations. Yet, polymer materials generally display very high thermal expansion coefficients. This has been resulting in various problems with geomembranes, in particular the formation of interconnecting wrinkles leading to increased risk and extent of leakage. The addition of nanoparticles has been proposed as a way to reduce thermal expansion in polymers. The effect is attributed to a reduction in the mobility of polymer chains trapped in nanoscale zones formed between the nanoparticles. Surprisingly, very few results are published on HDPE, and none with nanoclay. Yet, a preliminary assessment revealed the potential of nanoclay for reducing the thermal expansion of HDPE. HDPE nanocomposite samples were prepared with various ratios of nanoclay and the addition of five different compatibilizing agents. The pellets were premixed manually and fed in a twin-screw extruder. The extruded blends were water-cooled and pelletized. The pellets were further homogenized in a two-roll mill. Finally, plates were prepared by compression molding. The coefficient of linear thermal expansion was measured using the standard test method ASTM E831 and a dynamic mechanical analyser. The thermal expansion coefficient was computed between 30 and 45°C. A complementary characterization of the degree of exfoliation of the nanoclay particles in the samples was assessed by transmission electron microscopy (TEM) on specimens prepared with an ultra-microtome. For samples without compatibilizing agent and with some of the compatibilizing agents like polyethylene-grafted maleic anhydride (PE-g-MA), an initial increase in thermal expansion with nanoclay content was observed; the expansion coefficient reached a maximum at 2% nanoclay, then decreased to reach about 70% of the value for pure HDPE at 10% nanoclay. The presence of the compatibilizing agent in the formulation did not appear to produce a significant effect on the sample thermal expansion. On the other hand, samples containing oxidized ethylene-vinyl acetate (EO-VA) displayed a higher thermal expansion at 0% nanoclay compared to pure HDPE. Then they experienced a steady decrease in thermal expansion to reach also about 70% of the value for pure HDPE at 10% nanoclay. The same trend was obtained with 10% compatibilizing agent. The degree of exfoliation of the nanoclay particles in the samples was investigated by TEM. They showed that the nanoclay particles exhibited an intercalated configuration, which persisted when a compatibilizing agent was added in the formulation. These results show that nanoclay intercalation alone can produce a significant reduction in HDPE thermal expansion. Different behaviours were observed depending on the compatibilizing agent used. A better understanding of the mechanisms involved in the action of the nanoclay particles and compatibilizing agents on HDPE thermal expansion, and the achievement of full nanoclay exfoliation should allow further improvements in HDPE thermal expansion reduction. This solution offers great promises for geotechnical applications, in particular for geomembranes.