Clay-containing polyolefin blends and nanocomposites for high voltage applications

Abstract

This thesis presents attempts made to prepare novel insulating materials based on polyolefin tuned through blending and nanotechnology for high voltage applications, mainly the insulating materials in high voltage power cables. An organically modified natural clay, montmorillonite, was used in different loadings as the nanoreinforcement to be dispersed in low-density polyethylene (LDPE) and polypropylene (PP). Furthermore, polystyrene-bpoly(ethylene-co-butylene)-b-polystyrene (SEBS) triblock copolymer was used as the secondary polymer to form an immiscible blend with the polyolefin to selectively accommodate and disperse the nanofiller. A high degree of dispersion of clay platelets was observed when clay was incorporated into either LDPE or PP. Clay incorporation has led to a solid-like rheological behavior with a shoulder in storage modulus in low frequencies. The neat blends of polyolefin with SEBS were observed to have a co-continuous morphology. The addition of clay to those blends resulted in a decrease of the dispersed domains. Dielectric spectra of nanocomposites clearly showed two main relaxation processes representing Maxwell-Wagner-Sillars (MWS) interfacial polarization process at low frequency and dipolar relaxation at high frequency. A new relaxation peak was observed for blend nanocomposites that was hypothesized to be related to the elastomer phase. The 3D network of clay in nanocomposites facilitates the charge carriers flow when subjected to high electric fields resulting in a higher DC conductivity comparing to polyolefin homopolymers. This was shown to enable the nanocomposites to prevent the accumulation of space charge by persistently allowing the temporary stored charges to flow across the material. However, at elevated temperatures the current flow exceeds a threshold above which high amount of charges are injected and as a result the electric field is heavily distorted. All nanocomposites showed significant improvement in the AC breakdown strength comparing to the neat polyolefin. The intercalated/exfoliated clay layers create a tortuous path for charge carriers to flow and thus distribute the electric stress and prolong the breakdown time. However, LDPE nanocomposites showed diminished DC breakdown strength most probably due to the thermal instability brought by clay.

Date	21 Feb 2019
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Éric David (Supervisor) & Nicole R. Demarquette (Co-supervisor)