Dielectric properties of modified epoxy resin systems: A novel approach for developing materials for new generation technologies

Saeedi, Istebreq (2020) Dielectric properties of modified epoxy resin systems: A novel approach for developing materials for new generation technologies. University of Southampton, Doctoral Thesis, 298pp.

Record type: Thesis (Doctoral)

Abstract

Epoxy resins are thermosetting polymers that features attractive electrical, thermal and mechanical properties, which made them widely used as a primary insulating material in many electrical applications. Over the past decade, the increase in the energy demands led to increase in the size and change in the operation conditions of the electrical equipments, where their insulating materials are expected to withstand the continuous shift in the operation conditions. Consequently, polymer nanocomposite materials were introduced as a potential solution for producing high-performance insulators, and tailor the properties that effect field distribution in epoxy based materials. Later on, it was reported that some of the fillers are suggested to cause deterred properties, for example, some nano-fillers were difficult to disperse in the host material, which limited the advantages of these nanocomposites. An alternative and/or complementary approach is to use liquid functional network modifiers FNM. These FNM contains epoxide groups within their chemical structure which allows the FNM to contribute to the curing process, thus, the FNM modifier become incorporated in the cured network structure of the modified systems and influence the final properties of the system.

This research was set out to provide insights into the effect of functional network modifiers on the properties of epoxy resin systems with the hope of engineering novel epoxy based insulating material for the future. While using liquid functional network modifiers FNM approach has previously been used as a means for altering the mechanical properties of epoxy resins. This research investigated this approach in depth, and also the first study that explores this approach as means of integrating different functional groups into a thermosetting polymers, in order to effectively customise its properties.

The research started by providing a background for epoxy curing mechanisms and the method used for modifying the properties of epoxy resin systems reported in the literature. In addition to review of studies investigated the concept of voltage stabilisers that is based on which an explanation or interpretation was provided for the body of knowledge about FNM behaviour.

Prior to investigating the effect of any FNM on the properties of the modified systems, the effect of the of the FNM on the stoichiometric ratio of epoxy resins was established by running a series of experiments (FTIR, DSC, dielectric properties and AC breakdown strength of the modified systems). These experiments were carried out on a simple amine-cured epoxy resin system by using compensated and uncompensated systems. In the compensated systems, the FNM was added by precisely calculating the number of the epoxide groups being supplied by the FNM, where the equivalent number of epoxide groups provided by the resin are removed, ensuring the same number of epoxide groups present in the system after the addition of the FNM. While, in the uncompensated systems the FNM was added without changing the stoichiometry of the system. By comparing the variation in the properties of the compensated and uncompensated systems, it was concluded that the stoichiometric ratio of the FNM plays a vital role in determining the properties of the final material. In addition, the effect of the FNM on the T_g, dielectric spectroscopy and the breakdown strength is found to be attributed to several factors. In these experiments, the most dominant factor was the structure of the functional groups of the FNM.

By investigating the effect of the chemical structure of the FNM on the FTIR, DSC, dielectric properties, AC breakdown strength and DC conductivity of epoxy resin systems modified by the inclusion of varied concentrations of different FNM modifiers, it was found that the properties of the modified systems are influenced by both the chemical structure of the functional groups and the concentration of the FNM in the system. For example, the use of liquid epoxy resin and low concentration FNM (4%) suggested to produce FNM modified epoxy resins with improved properties compared to that of the neat epoxy, which is suggested to offer a new means of engineering novel materials to meet current and future needs in an adaptable way.

This research also investigated the effect of the curing mechanisms on the behaviour of the modified systems. It was found that the presence of the functional network modifier influenced the chemical, thermal and electrical properties of the amine and anhydride cured systems. This was followed by a a detailed analysis study of the molecular origin and the activation energy of each relaxation process. This analysis was needed because of the lack detailed analysis of the dielectric behaviour of anhydride cured resins in literature. Also, the presence of the FNM made prediction of the behaviour more challenging. The study analysed the temperature dependent dielectric properties along with Havriliak-Negami deconvoluted peaks, as well as the Arrhenius plots and the associated activation energies of the different molecular relaxations. It was concluded that the type of the functional group of the FNM plays a vital role in determining the network topology and the dielectric molecular dynamics of the final material.

The work reported in this research proves that functional network modifiers constitute a complementary and alternative approach for nanotechnology for designing materials with controlled dielectric properties. The systems researched, combined with the applied research method and the described experimental setup provides a complete guide for future studies to replicate the results and experiment with different FNM combinations.