The effect of sample thickness on the relative breakdown strength of epoxy systems M Reading*, Z Xu, A S Vaughan and P L Lewin University of Southampton, Southampton, UK

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The effect of sample thickness on the relative
breakdown strength of epoxy systems M Reading,
Z Xu, A S Vaughan and P L Lewin University of
Southampton, Southampton, UK
AC Electrical Breakdown Results
Introduction

• For AC ramp breakdown testing a Phenix AC
  Dielectric Test Set, Type 600C was used with a
  custom built test cell.
• The test cell used mushroom electrodes held
  horizontally covered in silicone oil to prevent
  flashover. The electrodes were checked
  frequently for signs of pitting.
• 12 breakdown sites were then chosen for each
  thickness material with the voltage at breakdown
  recorded and processed using the Reliasoft
  Weibull 7 software package.
• Figure 3. The Weibull plot for absolute
  Figure 4. The Weibull plot for relative
• breakdown voltages for all samples
  breakdown voltages for all samples

Since it is difficult to safely generate the very
high voltages requird to give breakdown data on
final-geometry samples, especially without
causing excessive damage to equipment, smaller
samples are often produced for testing to give an
indication of the materials properties. In
recent work, 100 mm thick samples were created to
provide breakdown strength data for a range of
epoxy-based systems the quantitative effect of
scaling up from the limited sample thickness to
technologically realistic values needs to be
considered. Volume and area effects are intrinsic
to Weibull analysis since they affect the
probability of a defect or impurity being in the
breakdown path. This investigation aims to
analyse the effect of sample dimensions on the
experimental breakdown strength of epoxy systems
with varying sample thickness. Using a proven
sample production technique, thin epoxy films
with thicknesses varying from 50 um up to 1 mm
have been produced. These samples were then
electrically tested using a specialised
electrical breakdown instrument and data
processed using Weibull statistics. This paper
analyses the breakdown characteristics of the
samples relative to their thicknesses in order to
(a) test the validity of the Weibull distribution
and (b) to provide estimates of the optimum
sample dimensions for different material
formulations.
Sample Production and Materials

• DER 332 epoxy resin cured with Jeffamine D-230
  was chosen due to the large amount of interest in
  such thermosetting materials of late.
• Samples were produced with a stoichiometric rate
  of 1000 resin to 344 hardener and cured at 100 0C
  for 4 hours followed by gradual cooling for 10
  hours.
• Samples were produced using a gravity fed
  pre-made mould technique established previously,
  shown in Figure 1 with a 1 mm aluminium spacer
  and sample. A QZ13 release agent was used to aid
  in removal of the polymer film from the mould.
• Sample thickness was varied using Melinex
  spacers obtained from DuPont to produce the
  samples listed in Table 1. Example spacers are
  shown in Figure 2.
• Figure 1. Pre-made mould produced
  Figure 2. Melinex spacers
• 1 mm thick sample
• Table 1. Samples produced and Melinex spacers
  used

Sample Thickness of spacer / µm Sample Thickness of spacer / µm
A 50 F 300
B 70 G 350
C 120 H 500
D 190 I 1000
E 250
Weibull Analysis
Conclusions

• The breakdown strength dependence of a polymeric
  insulating material with respect to sample
  thickness was investigated.
• The absolute breakdown strength of the epoxy
  increased with thickness, but fall below a linear
  relationship, being well described by an
  exponential rise to maximum function.
• The relative breakdown voltage of the samples
  was seen to decrease in an exponential decay,
  suggesting that addition of further material was
  proving less effective at increasing the
  breakdown strength.

M Reading, mdr_at_ecs.soton.ac.uk University of
Southampton, Highfield, Southampton, SO17 1BJ, UK
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