Abstract
By changing the ratio of resin to hardener, a series of epoxy resin samples has been produced with differing network structures and different retained chemical functionalities. The resulting materials were characterized by thermal analysis, dielectric spectroscopy, DC conductivity, and DC and AC breakdown strength measurements, to explore the effect of network structure and chemical composition on molecular dynamics and electrical properties. Differential scanning calorimetry showed that the glass transition temperature is primarily determined by the crosslinking density and indicates that, under the range of conditions employed here, side reactions, such as etherification or homopolarization, are negligible. Conversely, changes in DC conductivity with resin stoichiometry appear to occur as a result of changes in the chemical content of the system, rather than variations in network structure or dynamics. Specifically, we suggest that the DC conductivity is markedly affected by the residual amine group concentration in the system. While DC conductivity and DC breakdown appear broadly to be correlated, AC breakdown results indicated that this parameter does not vary with changing stoichiometry, which suggests that the AC and DC breakdown strengths are controlled by different mechanisms.
Highlights
EPOXY resins (ERs) are thermosetting polymers, which are commonly used as adhesives or coatings in a wide range of different industries, e.g. in aerospace and automotive industries [1, 2]
Investigating the impact of changing the resin/hardener stoichiometry on the unfilled epoxy would be beneficial for identifying any stoichiometric effect which might be imparted by the addition of the nanofiller and could help in determining the underlying mechanisms that control the behavior of nano-filled epoxy networks
A thin layer of gold was sputter coated onto both sides of each specimen, in order to improve the contact between the sample and the electrodes of the measurement cell
Summary
EPOXY resins (ERs) are thermosetting polymers, which are commonly used as adhesives or coatings in a wide range of different industries, e.g. in aerospace and automotive industries [1, 2]. Better understanding of the mechanisms which control the electrical properties of epoxy networks can help in tailoring and optimizing such materials to fit a particular application Such understanding is fundamental for improving the dielectric performance of these materials. Investigating the impact of changing the resin/hardener stoichiometry on the unfilled epoxy would be beneficial for identifying any stoichiometric effect which might be imparted by the addition of the nanofiller and could help in determining the underlying mechanisms that control the behavior of nano-filled epoxy networks. This study, as a precursor to considering the impact of nanofiller surface chemistry on epoxy curing, and consequent material properties, set out to investigate the effect of resin/hardener stoichiometry and to consider the influence of the chemical content (represented by amine, epoxy and hydroxyl groups) and the network structure on the electrical behavior of the resulting material. All samples were stored under dry conditions in a vacuum desiccator for at least two weeks before any measurement, to remove any absorbed water
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