Design and development of clay-based graphene/polymer composites for extruded cable application

Abstract

Different strategies and techniques are reported in this thesis to meet appropriate properties for conductive polymeric composites applicable in extruded cables. In this regard, the following efforts have been conducted to develop and modify several lightweight and flexible conductive polymeric composites. Dealing with composites, manufacturing aspects such as easy processability, easy accessibility and designing low-cost materials are of the key elements that need to be considered. Therefore, to response the industrial needs in HV cable applications, two commodity polymers (low-density polyethylene and polyethylene vinyl acetate) with several carbon-based conductive fillers were selected for this Ph.D. project. Our objectives were defined to increase the electrical and thermal conductivity of several conductive polymeric composites using naturally based graphene hybrids, obtained from clay and sucrose. To achieve our objectives, low-density polyethylene was combined with graphene-like filler by melt compounding technique, and the electrical properties, characterized by broadband dielectric spectroscopy, revealed the formation of a conductive network of graphene-like above 30 wt% of filler content. As benchmark, low-density polyethylene/carbon black (LDPE/CB) with several CB filler content was prepared via melt mixing. A significant increase in electrical conductivity was achieved at filler contents 15-20 wt%. The nanostructure morphology of the composite with well dispersion and distribution of sphere-shape carbon black led to adequate particle-particle contacts in which charge carrier pathways were formed as the consequence. LDPE/CB composite was found to show electric field-dependency and hysteresis behavior. The shift of interfacial polarization peak toward the higher frequencies was observed and related to the further intra-cluster connection at higher fields. Loading of 5 wt% of CB to the LDPE resulted in a 10% increase in dielectric breakdown which makes this material a good choice for electric insulating applications. Noticeable increase in thermal conductivity of the LDPE/CB composite was achieved with the addition of 20 wt% CB. By changing the host polymer from a non-polar to a polar-polymer, ethylene vinyl acetate was mixed by graphene-like by means of solvent casting. The investigation of the electrical properties of EVA/graphene-like showed a percolation threshold between 25-30 wt% of the filler content. To compare the electrical conductivity of the graphene-like filler in EVA polymer, (EVA) composite with two carbonaceous fillers such CB and commercially available graphene (G) was found to be conductive at filler content of higher than 5 and 15 wt%, respectively. Selecting solvent-casting and nanosize CB particles, led to the formation of a conductive network at relatively low filler content (5 wt%), while filler agglomeration for microsize graphene flakes hindered conductive network formation up to 15 wt%. Addition of carbon black and graphene to the EVA polymer continuously increased the thermal conductivity of the composites. Considering the role of graphene-like filler in LDPE and EVA polymers, then low-density polyethylene/ethylene vinyl acetate (LDPE/EVA) was blended with graphene-like filler via solvent casting. The LDPE/EVA/graphene-like composite was found to be conductive at 17.5 wt% of the filler content. The annealing of the LDPE/EVA/graphene-like composite was found to influence the electrical conductivity of the composite at the percolation threshold. Indeed, one order of magnitude increase in electrical conductivity was obtained thanks to better conductive network formation during the annealing. Dielectric response of the LDPE/EVA/graphene-like composite was scanned over a wide range of frequency (10-1-106Hz) and temperature from room temperate to near the melting point. Composites at subpercolation threshold revealed a frequency dispersion at low frequencies and elevated temperature. The effective permittivity of the LDPE/CB composite, simulated numerically, was found to be in relatively agreement with experimental values at low filler contents (approximately up to 15 wt%). The arrangement of the particles within the medium was simulated and the results evidenced negligible difference between the ordered and the random morphology when the filler content was likely low content. Water absorption by hydrophilic CB fillers was found to increase the effective permittivity of the composite remarkably. The utilization of the graphene-like filler, obtained from renewable resources (clay and sugar), resulted in a rewarding candidate for production of polymeric composites for extruded cable application.

Date	10 May 2019
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Claudiane Ouellet-Plamondon (Supervisor), Éric David (Co-supervisor) & Michel F. Fréchette (Co-supervisor)