Nano-graphite as a functional additive for rotationally mouldable polymers and phase change materials for energy storage

Abstract

The overall aim of this study was to obtain a facile method of synthesizing graphite nanoplatelets from commercial expandable graphite and use these as functional fillers in rotational moulding applications and phase change materials for energy storage. Two commercial expandable graphites were evaluated as precursors for the synthesis of graphite nanoplatelets. Microwave radiation treatment was shown to be more efficient in exfoliating expandable graphite than furnace heating. The expandable graphite with better exfoliating characteristics was selected. XRD results of this graphite showed that it was a high stage graphite intercalation compound. Graphite nanoplatelets with an average particle size of 13 μm and an estimated thickness of about 76 nm were prepared by microwave exfoliation and ultrasonication-assisted liquid phase exfoliation in isopropanol from the selected expandable graphite. Prior to the selection of isopropanol as the ultrasonication media, various exfoliation media that encompassed different solvents and water with various surfactants had been evaluated, on the basis of their acoustic cavitation characteristics. The graphite nanoplatelets were used as a functional additive to fabricate linear low density polyethylene (LLDPE) and poly(ethylene-co-vinyl acetate) (EVA) based nanocomposites using the rotational moulding (rotomoulding) process. The dry blending approach yielded surface resistivities within the static dissipation range (antistatic) at filler loadings as low as 0.25 wt.% (0.1 vol.%). However, even at this low graphite content, impact properties were significantly reduced compared to the neat polymers. Bilayer mouldings via the double dumping method proved to be a feasible approach to achieve both acceptable mechanical properties and antistatic properties. This was achieved by rotomoulding nanocomposites with a 1 mm outer layer containing the filler and a 2 mm inner layer of neat LLDPE. Excellent fire resistance, in terms of cone calorimeter testing, was achieved when the outer layer also contained 10 wt.% expandable graphite. Pseudo binary mixtures of stearyl alcohol/commercial triple pressed stearic acid where prepared and characterized as a new phase change material (PCM) for energy storage. A facile method of preparing highly thermally conductive stearyl alcohol/stearic acid phase change material/graphite nanoplatelets (GNPs) nanocomposites was developed. Inclusion of the GNPs in the PCM matrix reduced the enthalpy of melting and crystallization marginally. However, the PCM/nanocomposite exhibited negligible super cooling. At 10 wt.% loading, the graphite nanoplatelets enhanced the thermal conductivity of the PCM by close to 600 % and 1200 % in the solid and molten states, respectively. Thermal conductivity modelling showed that the substantial thermal conductivity enhancement was as a result of relatively low interfacial thermal resistance between the PCM matrix and GNPs. The PCM/GNPs nanocomposites also showed excellent thermal reliability after being subjected to accelerated thermal cycling tests of 100 melting and freezing cycles. Settling tests showed the PCM/GNPs nanocomposite with 10 wt.% GNPs was stable after 60 days, with no apparent separation between the PCM matrix and the graphite nanoplatelets.