Figures 7-8. SEM images of unfilled epoxy (left) and a single SD particle in SD-SON (right)

Figures 9-10. A standard SEM image of the MMT-SON sample (left) and a backscattered image highlighting the highly dispersed MMT particles (right)

The importance and methods of dispersing fillers into epoxy resin M Reading*, Z. Xu, A S Vaughan and P L Lewin

University of Southampton, Southampton, UK

ResultsIntroduction

Dispersion methods and materials

Conclusions

There are many methods available to achieve a good dispersion of fillers, however it is important to ensure that these methods do not alter the polymer matrix resulting in misleading results. This investigation looks at several methods of dispersing three chosen fillers within a polymer matrix and the resulting electrical properties with regard to the dispersion state of the fillers. Also, the same processes will be performed on unfilled materials to investigate any effects they may have on the host material.

For this investigation an epoxy system (EP) was chosen as the host polymer with aluminium pillared montmorillonite (MMT), micro spheres of silicon dioxide (SD) and nano spheres of silicon dioxide (NSD) as fillers. The dispersion of the fillers was quantified by use of a scanning electron microscope (SEM) and inferred by use of electrical breakdown tests, which also revealed any effect on the electrical breakdown strength of the materials.

M Reading, mdr@ecs.soton.ac.uk

University of Southampton, Highfield, Southampton, SO17 1BJ, UKContact details :

It is clear from SEM that regimes A and B produce poorly dispersed composites resulting in low β values and agglomeration of the fillers, as expected.

Magnetic stirring is seen to improve the dispersion of the fillers within the host polymer, achieving an acceptable level of dispersion seen by SEM and higher β values.

Ultrasound and Sonication methods are both seen to further improve the dispersion of the materials by use of SEM, with sonication found to produce particularly uniform materials having broken up the larger agglomerations of filler. However β values do not appear to reflect this.

Ultrasound and Sonication were actually seen to act to increase the breakdown strength of the unfilled epoxy slightly, an effect worth investigating.

DER 332 epoxy resin cured using a Jeffamine D-230 hardener chosen as the host polymer matrix. The stoichiometric ratio was calculated to be 1000:344 for resin:hardener.

Samples produced using a previously established gravity fed pre-made mould technique using a QZ13 release agent. This method produced thin film samples of ~ 80 µm.

Samples were de-gassed at 50 0C for 20 minutes before being cured at 100 0C for 4 hours followed by a gradual cool over 10 hours.

Figure 1. Pre-made mould Figure 2. An example thin film sample

0.25 g of MMT, SD and NSD added pre-curing and 5 different mixing regimes employed to create 20 samples, detailed in Table 1 with the mixing regimes detailed in Table 2. Samples were de-gassed for 20 minutes prior to mould introduction.

Table 1. Samples generated

Magnetic stirring was performed on a standard Heat stirrer plate at 50 0C using a magnetic stirrer bar

For Ultrasound an UltraWave DP201 Precision Ultrasonic bath was used at 50 0C

For Sonication a Hielscher UP200S Ultrasonic processor was used at 50 % amplitude.

Table 2. Mixing regimes

For electrical breakdown characterisation a custom built AC breakdown kit was used following the ASTM D149 standard (50 Hz, 50 V/s).

Approximately 12-15 breakdown sites were performed for each sample using 6.3 mm ball bearing electrodes covered in silicone oil to prevent flashover.

Figures 3-6. Weibull plots of Eb data Table 3. Weibull parameters for samples

for samples containing no filler (top left),

SD filler (top centre), NSD filler (top right)

and MMT filler (bottom left)

For scanning electron microscope (SEM) a Carl Zeiss AG Evo 50 was usedSample Filler Mixing regime

EP NM - A

EP HM - B

EP MS - C

EP US - D

EP SON - E

MMT NM 5% MMT A

MMT HM 5% MMT B

MMT MS 5% MMT C

MMT US 5% MMT D

MMT SON 5% MMT E

Sample Filler Mixing regime

SD NM 5% micro SD A

SD HM 5% micro SD B

SD MS 5% micro SD C

SD US 5% micro SD D

SD SON 5% micro SD E

NSD NM 5% nano SD A

NSD HM 5% nano SD B

NSD MS 5% nano SD C

NSD US 5% nano SD D

NSD SON 5% nano SD E

Regime Details

A Resin, hardener and filler combined with no mixing

B Hand mixed for 20 minutes

C Hand mixed for 20 minutes followed by

magnetic stirring for 20 minutesD Hand mixed for 20 minutes, magnetically stirred

for 20 minutes and ultrasound for 20 minutesE Hand mixed for 20 minutes, magnetically stirred

for 20 minutes and sonicated for 20 minutes

Breakdown Strength / kVmm-1

100 120 140 160

We

ibu

ll P

rob

ab

ility

/ %

0.0

0.1

1.0

5.0

10.0

20.0

50.0

70.0

95.099.099.9

UnmixedHand mixedMagnetically stirredUltrasoundSonication

Breakdown Strength / kVmm-1

100 120 140 160

We

ibu

ll P

rob

ab

ility

/ %

0.0

0.1

1.0

5.0

10.0

20.0

50.0

70.0

95.099.099.9

UnmixedHand mixedMagnetically stirredUltrasoundSonication

Breakdown Strength / kVmm-1

100 120 140 160

We

ibu

ll P

rob

ab

ility

/ %

0.0

0.1

1.0

5.0

10.0

20.0

50.0

70.0

95.099.099.9

UnmixedHand mixedMagnetically stirredUltrasoundSonication

Breakdown Strength / kVmm-1

100 120 140 160

We

ibu

ll P

rob

ab

ility

/ %

0.0

0.1

1.0

5.0

10.0

20.0

50.0

70.0

95.099.099.9

UnmixedHand mixedMagnetically stirredUltrasoundSonication

Sample Weibull Parameter

Sample Weibull Parameter

α β α βEP NM 135.8 9.1 SD NM 129.8 9.88EP HM 141.3 14.3 SD HM 127 14.62EP MS 143.8 26.5 SD MS 132.5 16.2EP US 147.5 21.9 SD US 138.3 13.1

EP SON 152 22.17 SD SON 141.5 15.7MMT NM 128.9 6.1 NSD NM 129.2 5.6MMT HM 137.8 7.52 NSD HM 135.3 10.18MMT MS 139.2 14.9 NSD MS 139.5 12.3MMT US 138.5 29.9 NSD US 145.9 18.4

MMT SON 141.8 11.52 NSD SON 138.5 19.9

Unfilled epoxy SD filled NSD filled

MMT filled