Multimodal Polyethylene-Graphene Nanocomposite via Melt Extrusion
The production of an innovative, high-performance graphene-based multimodal-high-density polyethylene (HDPE)polymer nanocomposite using cost-effective techniques was pursued in this study. This study reports on the reinforcement of high molecular weight multimodal-high-density polyethylene HDPE by a microwave-induced plasma graphene, via melt intercalation, using a closely self-wiping co-rotating twin-screw compounding extruder. The tailored process included designing a suitable screw configuration, paired with coordinating extruder conditions and blending techniques. This enabled the polymer to sufficiently degrade, predominantly through thermo-mechanical degradation, as well as thermo-oxidative degradation, which subsequently created a suitable medium for the graphene sheets to disperse readily and distribute evenly within the polymer matrix. This project was conducted on about two metric tons of polymer and more than three kilograms of graphene, on a semi-industrial scale extrusion system that can be scaled up to full industrial scale production for an application. The well-dispersed and uniformly distributed graphene platelets throughout a multimodal-polymer matrix, with a strong interfacial bonding between the platelets and the matrix, provided an optimal nanocomposite system for industrial interest. A commercial carbon black/multimodal-HDPE product, based on the same polymer matrix, was used as a benchmark in this study for the sake of comparison. Different microscopy techniques were employed to prove the effectiveness. This was then qualitatively assessed by Raman spectroscopy, X-ray diffraction, density measurements, and electrical testing, confirming both the originality, as well as the effectiveness of the process. Likewise, microscopic examination of the cryofractured surfaces of the nanocomposite, stress-induced Raman band shifts, mechanical, and thermal expansion testing, showed strong interfacial bonding between the graphene sheets and polymer matrix. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy revealed that in the presence of graphene sheets, the polymer has been slightly oxidised, probably due to the presence of the oxygen molecules introduced along with the as-received graphene. Further, gel permeation chromatography and rheology measurements revealed that the neat polymer has been degraded predominantly by chain scission. However, the homogeneous dispersion and distribution of graphene platelets within the polymer matrix, as well as the strong adhesion bonding, led to the formation of an interconnected 3D network in the polymer which accordingly increased the thermal stability of the polymer by forming a continuous network-structured protective layer. Consequently, the reinforced polymers exhibited a greater melt strength during thermoforming, whereby the sagging resistance, onset degradation temperature, and oxidative induction time, have been greatly improved. The effect of a microwave-induced plasma graphene on the crystallisation kinetics of the multimodal- high-density polyethylene (HDPE) was also studied under non-isothermal conditions. The results suggested that the non-isothermal crystallisation behaviour of multimodal-HDPE/graphene nanocomposites relied significantly on both the graphene content and the cooling rate. The addition of graphene caused a change in the mechanism of nucleation and crystal growth of the multimodal-HDPE. The mean activation energy barrier, required for the transportation of the molecular chains from the melt state to the growing crystal surface, gradually diminished as the graphene content increased, which is attributable to the nucleating agent effect of graphene platelets. Besides, moisture absorption testing revealed a reduction by more than 100% when replacing carbon black by graphene. Hence, the results of this research are expected to provide greater insight into melt intercalation, affecting the multimodal HDPE-graphene nanocomposite performance and criterion for effectively producing the next generation of black multimodal-polyethylene compounds for use in high-pressure pipes, automotive, and energy cable applications.