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Effects of nanoparticle and stoichiometry on properties of an epoxy-based system
Van Nguyen1, A. S. Vaughan1, P. L. Lewin1, A. Krivda2
1 University of Southampton, Southampton, UK 2 ABB Corporate Research, Baden, Switzerland
Glass transition temperature Breakdown strength
Introduction Preparation of samples
Discussions
Conclusions and future work
University of Southampton, Highfield, Southampton, SO17 1BJ, UKContact details :
From the presented Tg, permittivity and breakdown strength results, the ratio of 100:85:1 seems to be optimum for neat epoxies.
Less hardener than optimum stoichiometry leads to a reduced effective molecular weight and a reduced Tg and increased permittivity. However, excess epoxy groups can react together and can be considered as defects, decreasing breakdown strength due to heterogeneity.
Excess hardener may restrict molecular conformations due to the rigidity of double bonds, such that Tg decreases somewhat. Excess hardener can result in the formation of low density heterogeneous regions, which can be considered as defects that reduce the breakdown strength.
Nano-silica may create more free volume and also catalyse homo-polymerization of epoxy, leading to chemical imbalance , such that Tg and the breakdown strength decrease, while the permittivity increases.
Nano-silica may catalyse homo-polymerization of excess epoxy, thus chemical structure of material becomes more homogeneous, leading to an increase in breakdown strength. Homo-polymerization cannot greatly increase the degree of crosslinking, thus Tg decreases and permittivity increases due to free volume.
Glass transition temperature (Tg) was measured using differential scanning calorimetry method. DSC was run from 50 0C to 200 0C at heating rate of 10 0C/min, followed by fast cooling to 50 0C and re-run to remove enthalpy relaxation. Tg values were determined based on the heat flow diagram of the second run.
Nanopox E 470 (from Nanoresins) was used to provide the nano-filler. The silica phase consists of surface-modified synthetic SiO2 with the average diameter of 20 nm, and maximum diameter of 50 nm.
Table 1: Samples investigated
Samples were subjected to AC breakdown test at a ramp rate of 50 V/s.
Breakdown voltages were recorded and process to obtain a Weibull probability distribution of breakdown strength with 90% confidence limits.
Stoichiometric ratio plays an important role in determining material properties.
Besides creating more free volume, introduction of nano-silica can catalyse homo-polymerization , leading to changes in the chemical structure of neat epoxies, deteriorating material properties.
Introduction of nano-silica into neat epoxies of other ratios than stoichiomtric one will be conducted to provide a more comprehensive understanding of material behaviour and interfacial surfaces between nano-silica and epoxy matrix.
Epoxy-based systems are used widely as dielectrics in electrical applications, especially under high temperature conditions. The chosen stoichiometry is important in determining the nature of the network that forms and hence the physical properties of the final system; a stoichiometric formulation with the optimal chemical balance between reactants will introduce good performances. However, addition of nanoparticles with large interfacial areas into epoxy systems may introduce additional chemical reactions between moieties on nanoparticle surfaces and reactants which may lead to chemical unbalance, and thus deteriorate the performance of the final nanocomposite system. This study set out to investigate the effects of stoichiometry and the introduction of nano-silica (Nanopox) on the thermal and electrical properties of an epoxy system, based on a diglycidyl ether of bisphenol-A (DGEBA) type resin, which was cured using an anhydride-based hardener and coupled with a tertiary amine accelerator. The properties considered include the glass transition temperature (Tg), electric breakdown and dielectric response.
Sample code Resin ratio % nano filler (%wt)
Ep70 100:70:1 0
Ep80 100:80:1 0
Ep85 100:85:1 0
Ep90 100:90:1 0
Ep100 100:100:1 0
Pox85-1% 100:85:1 1
Pox85-2% 100:85:1 2
Pox85-5% 100:85:1 5
Pox60-5% 100:60:1 5
Pox70-5% 100:70:1 5
Pox80-5% 100:80:1 5
Pox90-5% 100:90:1 5
Fig1: Tg of neat epoxies Fig2: Tg of nanocomposites (100:85:1)
Fig3: Tg of nanocomposites with 5% nano-filler content
Relative permittivity
The dielectric response was measured at room temperature using a Solartron 1296 system.
Sample code
Thickness (µm) Alpha Beta
Ep70 50 202 16.8
Ep80 50 205 26.2
Ep85 50 205.7 27.4
Ep90 50 198.7 23.5
Ep100 50 191.7 21.6
Ep85 70 185.2 35.1
Pox85-1% 70 177.4 31.5
Pox85-2% 70 177 26.5
Pox85-5% 70 169.3 29.8
Pox60-5% 70 177.9 30.1
Pox70-5% 70 173.2 30.9
Pox80-5% 70 173.9 25.3
Pox90-5% 70 164 39.5
