Performance of Silicone Rubber Composites with SiO2 ... - IEEE Xplore

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge

Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge M. Tariq Nazir, B.T. Phung School of Electrical Engineering and Telecommunications University of New South Wales Sydney, NSW 2052, Australia and Mark Hoffman School of Materials Science and Engineering University of New South Wales Sydney, NSW 2052, Australia

ABSTRACT Outdoor insulators are often subject to corona discharges and the problem is becoming more prevalent with the increasing use of higher transmission voltage levels. For polymeric insulators, exposure to such discharges can alter the chemical structure of basic polymer and degrade surface properties. This paper investigates the effect of micro and/or nano fillers in silicone rubber composites in suppressing such damage. Four different types of samples are fabricated: pristine silicone rubber (PR), 30wt% micron–sized silica/silicone rubber (MC), 27.5wt% micron + 2.5wt% nano silica/silicone rubber (NMC), and 5wt% nano silica/silicone rubber (NC) composites. Samples are exposed to AC corona using a needle to ground-plane electrode setup. Experimental results are analyzed based on five different measurement methods: phase-resolved partial discharge (PD), hydrophobicity loss-recovery, Scanning Electron Microscopy (SEM), Surface roughness and Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that NC shows a strong resistance to partial discharges and hydrophobicity loss. In the area below the needle tip, higher hydrophobicity loss and higher recovery are observed as compared to the vicinity region. Variations in surface roughness, appearance of crackles, voids, pits, surface splitting into blocky structures and damages to chemical structure of silicone rubber are appreciably retarded in NC as compared to PR, MC and NMC. Based on NMC results, it is found that addition of nano–sized silica can be an attractive approach to improve the corona resistance of micron–sized silica filled silicone rubber. Index Terms Outdoor insulation, polydimethylsiloxane (PDMS) micro/nanocomposites, AC corona, hydrophobicity, partial discharge, degradation.

1 INTRODUCTION AMONG polymeric materials for outdoor insulation applications, silicon dioxide (silica) and aluminum trihydrate (ATH) filled silicone rubber is a popular choice which provides excellent electrical and mechanical characteristics [1]. Silicone rubber surfaces are highly hydrophobic and resist water flow because of presence of low surface energy polar methyl group in the side chain. Better pollution flashover performance due to its hydrophobicity is a major factor which makes it superior over ceramic and glass insulators [2, 3]. On the other hand, the organic structure of silicone rubber exhibits gradual deterioration and undergoes ageing due to Manuscript received on 7 February 2016, in final form 2 June 2016, accepted 6 July 2016.

occurrence of chemical changes caused by partial and arc discharges [4] and accelerated weathering conditions during operation in the field [5-7]. Electrical discharges produce highly energized UV radiation, ions species and ozone. This high energy plasma assembled the charges on the insulator surface and such a charge injection initiates drastic chemical changes on the insulator surface. These chemical changes reduce surface hydrophobicity due to formation of the hydrophilic hydroxyl (–OH) and silanol (Si-OH) groups and increased oxygen contents on surface [8-12]. The problem of corona partial discharge (PD) is becoming more common in view of increasing trend in UHV transmission lines [13-15]. In previous studies of polymeric insulators for HV outdoor insulation applications, many authors investigated the effect of corona on hydrophobicity loss-recovery properties and

DOI: 10.1109/TDEI.2016.005858

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23, No. 5; October 2016

degradation behavior of polymer on its long term performance. Hillborg et al [11-12] explained the hydrophobicity loss and recovery mechanism of PDMS after corona exposure. Yong et al presented physicochemical analysis of HTV silicone rubber [8, 16] and ATH filled EVA [17] by means of FTIR and SEM. It is reported that, after corona exposure, formation of some slight crackles is observed in both types of samples. Major cracks with high degree of degradation are noticed under the HV electrode needle tip. Also with respect to crack formation, EVA undergoes more ageing and deterioration as compared to silicone rubber. Liang et al [18-21] reported a series of investigations on impact of corona with different intensity on hydrophobicity loss and corona exposure durations on hydrophobicity recovery characteristics of HTV silicone rubber. It is reported that hydrophobicity recovery pace is dependent not only on exposure time but also on the physical and chemical state of surface. Moreno and Gorur utilized a pin to ground plate electrode configuration to investigate the impact of long term corona on degradation of EPDM, silicone rubber and a blend of both [22-26]. Partial discharge (corona) magnitude and pulse repetition rate were monitored and it was found that increase in relative humidity along with mechanical stress could accelerate the corona discharge mechanism. In recent years, there has been a global interest in application of nanodielectrics in the area of electrical insulation and numerous outcomes proclaimed that polymeric nanocomposites can be advantageous for outdoor insulation applications. When appropriately chosen, a small amount of nano-fillers dispersed in the base polymer can considerably improve the mechanical and electrical characteristics of the resultant composite. El-Hag [27] compared the erosion resistance of nano-sized fumed silica filled samples with micro-sized ground silica and improvement in erosion resistance of silicone rubber nanocomposites was reported. Venkatesulu and Vas [28, 29] reported the impact of nano filler loading on surface properties silicone rubber under AC and DC corona exposure and suggested that 3wt% nano particle loading showed a much better performance relative to unfilled silicone rubber. Recently, Reddy reported the impact of clean cold fog [30] and acidic fog [31] on the corona resistance of RTV and ATH-filled SiR samples under AC and DC stress. Iyer et al. [32, 33] reported that nano-sized particles improve the corona resistance of epoxy nanocomposites and micro + nanofilled epoxy offer better resistance to partial discharges. To date, the literature on the role of combined nano and micro filler addition on microscopic degradation and PD characteristics of silicone rubber micro/nano composites is scarce. The present work investigates the impact of nano filler addition in pristine and micro filled silicone rubber composites on their AC corona resistance and PD characteristics. The average PD magnitude and PD repetition rate in both positive and negative half-cycles are compared with respect to different AC corona ageing times. Hydrophobicity loss-recovery characteristics at two different locations on sample surface are correlated with corona exposure duration. Moreover, material characterization

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techniques including SEM, surface roughness and FTIR were used to analyze degradation level. Finally, the role of filler size, inter-particle spacing and specific surface area of fillers in the composites are presented to justify the results on corona resistance.

2 EXPERIMENTAL 2.1 MATERIALS Low viscosity two part RTV 615 (pure PDMS) was selected as base matrix and supplied by DC products Pty Ltd, Victoria, Australia; RTV 615 (Part A: cross linker, Part B: platinum catalyst) does not contain any added particles and possesses similar excellent properties to heat curing silicones used in the fabrication of outdoor insulators [34]. Nano precipitated and micron sized silica particles are selected in this work. Their principal characteristics are given in Table 1. Table 1. Characteristics of SiO2 particles [35, 36]. Filler

Particle diameter

Density

Supplier

Precipitated Silica

~20 nm

2.6 g/cm3

Sigma Aldrich

Ground Silica

~5 µm

2.6 g/cm3

US Silica

2.2 SAMPLE FABRICATION Firstly, the required amounts of RTV silicone rubber parts A and B were obtained using a precise digital balance and then degassed through a vacuum pump to remove moisture and air from the liquid matrix. Pertinent quantity of fillers was kept overnight in an oven set at 150ºC to remove diffused vapors. In order to achieve a more uniform dispersion of nano-sized particles in the matrix, two different processing methodologies of mechanical stirring followed by sonication were adopted as in previous works [15, 37]. Initially, the fillers were mixed with a mechanical mixer for 5 minutes and then the matrix was evacuated. Next, sonication was carried out for 80 minutes in a water tank at 40 kHz frequency with intermittent stirring. In the last stage of sample fabrication, the RTV B component was added at a ratio of 10:1 and mixed again as described earlier. Thereafter, the matrix was degassed and poured into molds. Initially, the molds were kept at room temperature for 24 hours and after that cross-linked composites were unmolded and heated at 85ºC for 4 hours. The cast samples were thin plate shape of dimension 30 ×30 × 3 mm3. Four different types of samples were fabricated with the composition shown in Table 2 [38]. Table 2. SiO2 particle contents in four kinds of composites. Composite

Nano-SiO2 wt %

Micro-SiO2 wt%

Acronym

Pristine silicone rubber

0

0

PR

0

30

MC

2.5

27.5

NMC

5

0

NC

Micro silica/ silicone rubber composite Nano Micro silica/ silicone rubber composite Nano silica/ silicone rubber composite

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge

voltages up to 120 kV. A 100 nm thin section of silicone rubber filled with nano–sized silica was obtained by means of Leica EM FC6 Cryo-Ultramicrotome – a low temperature sectioning system working at -160 °C. The image of Figure 2 shows the nano silica dispersion in NC. It can be seen that a uniform distribution of the nano-particles in the matrix was achieved. However, a few particle agglomerates are present in sub-micron range in the NC. Nevertheless, previous work reported that agglomerates of such dimension did not provoke any major chemical bonding instability between the filler and silicone rubber [27, 37].

Figure 1. Schematic of the experimental setup used for AC corona degradation studies of silicone rubber composites.

2.3 CORONA DISCHARGE TEST CELL Corona was produced using a point to plane copper electrode geometry. The setup was housed in a glass cell under a controlled gaseous environment as shown in Figure 1 [37, 38]. The point electrode has a tip diameter of 1.6 mm and works as a corona source. The bottom round plate electrode (diameter: 50 mm, thickness: 10 mm) is connected to ground and used as a platform for the test sample. The vertical gap between point corona source and sample surface was controlled at 1mm with the help of standard gauge block. The test cell was ventilated to ensure that there is no ozone accumulation in the cell and the humidity along with temperature was monitored. All the tests were performed at room temperature (20-25°C) and relative humidity of RH=50±10%. Each test was run for 96 hours under continuous application of 8 kVrms AC voltage. Each composite type was tested 5 times with a new sample each time for reproducibility of results. 2.4 TRANSMISSION ELECTRON MICROSCOPY (TEM) The degree of dispersion of nano–sized silica particles in silicone rubber composites was studied using a JEOL 1400 transmission electron microscope (TEM) at accelerating

Figure 2. TEM image of NC at magnification of 20 k.

2.5 PARTIAL DISCHARGE MEASUREMENT Partial discharge measurement was carried out according to the IEC 60270 Standard [39]. The circuit is shown in Figure 3. The high voltage was provided from a 240V/50kV/10kVA step-up transformer with the low voltage side energized from 240 V mains via a variac. For protection purposes, a resistor was inserted in the circuit to limit the fault current in case of flashover. 8 kVrms 50 Hz AC high voltage was applied to the corona source to initiate surface degradation of the test sample (Cx). Over the duration of corona exposure, phase resolved PD patterns of each sample were recorded at the end of 0, 48 and 96 hours. The PDs were continuously recorded by means of 1000 pF blocking capacitor (Cb), quadripole measuring impedance Z and PD detector (OMICRON MPD 600 system). Before PD measurement, offline calibration was done by injecting 50 pC across the sample using MPD 542 calibrator. The PD threshold was adjusted at 100 pC and each PD pattern was recorded 5 times for 30 s duration. For ageing evaluation, the average PD amplitude and PD repetition rate (ratio of number of PD events over duration) are analysed. The same parameters for characterisation were adopted by Moreno [22, 25] along with the same 1 mm air gap between electrode tip and sample surface.

Figure 3. Experimental setup for partial discharge measurement.

2.6 HYDROPHOBICITY LOSS AND RECOVERY STUDIES The surface hydrophobicity is one of the key factors to assess the degradation of polymeric materials used for outdoor insulation. Corona discharge produces highly energized species which can initiate changes on the insulating surface and result in chemical damage to basic polymer structure, chalking, erosion and high roughness. The synergetic impact of the above mentioned changes destroy the surface hydrophobicity and increase the surface energies of insulating materials. Silicone rubber is highly hydrophobic due to the presence of low surface energy nonpolar –CH3 group. As discussed earlier, by-product silica like the layer formed on

2.7 SURFACE ROUGHNESS It has been reported that the surface roughness is gradually changed with the passage of deterioration of polymeric materials [2, 28-29]. A TIME TR-200 handheld surface roughness tester was used to measure the surface roughness after AC corona exposure. The area of the test sample directly below the needle tip was scanned ten times and the average value of surface roughness for each sample is reported with error bars. 2.8 SCANNING ELECTRON MICROSCOPY (SEM) SEM was used to analyze the filler dispersion in MC and NMC as shown in Figure 4. Also, the changes on the composite surface after continuous corona exposure were studied through SEM. A Hitachi S3400 SEM was used for imaging in this work. Before SEM analysis, all the samples were kept in a drying chamber for 24 hours. Moreover, a 15 nm thin layer of gold was deposited on each sample in an argon atmosphere prior to SEM analysis using an Emitech K550X sputter coater.

Discharge Magnitude (nC)

the silicone rubber surface as a result of corona exposure and such a layer retards the hydrophobicity recovery mechanism [18]. In this paper, hydrophobicity loss and recovery of silicone rubber composites deteriorated by AC corona exposure is quantified by means of static water contact angle measurement. This was performed using a Kyowa goniometer DMs-200. The deionized water drop volume was controlled to 2.5 µl and average of ten measurements is reported for each sample. The hydrophobicity loss of each composite was evaluated at the end of 48 h and 96 h of corona treatment. Subsequently, each sample was allowed to stand in air and recovery was evaluated at the end of 100 h.

Discharge Magnitude (nC)

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(a)

(b)

(c)

(d)

20 15 10 5 0 25 20 15 10 5 0 0

50 100 150 200 250 300 350 0

50 100 150 200 250 300 350

Phase Position (Degree)

Figure 5. Phase-resolved PD patterns of (a) virgin PR (b) 96h-degraded PR (c) virgin NC and (d) 96h-degraded NC recorded in 30 s.

paper, absorption spectra are recorded using Perkin Elmer Spectrum 100-FTIR spectrometer and relative changes in different absorption peak heights of the functional group are correlated with degradation level.

3 RESULTS 3.1 PHASE-RESOLVED PD PATTERNS The phase-resolved PD patterns of PR and NC at virgin and after 96 hours of corona exposure are shown in Figure 5. All 14

Positive Half Cycle

12 10

(a)

(b)

Figure 4. SEM images showing the micro silica distribution in (a) MC and micro + nano silica in (b) NMC.

2.9 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR) ATR-FTIR is a spectroscopy technique which has often been used for degradation assessment of polymeric materials. FTIR gives the transmission or absorption spectra of samples with respect to wave numbers (in cm-1 unit) ranging from 500 to 4000. Every wave number relates to some specific functional group inside the polymeric material. The transmission and absorption spectra are simply the reciprocal of each other and thus either can be used to estimate the ageing and degradation level of the samples [40, 41]. In this

Average PD Magnitude (nC)

8 6 4 1.0

Negative Half Cycle

0.8 0.6 0.4 0.2 0.0

PR

MC

NMC

Virgin State 48h-Degraded 96h-Degraded

NC

Figure 6. Average PD magnitude of composites at different degradation states.

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge 300

Positive Half Cycle

200

Negative Half Cycle

2000

(b)

(a)

1600

Figure 8. Top view of corona aged sample (a) and a sketch (b) showing the regions where hydrophobicity loss and recovery are evaluated.

1200 800

Total

2000 1600 1200 800 400 PR

MC

NMC

NC

Virgin State 48h-Degraded 96h-Degraded

Figure 7. PD repetition rate of composites at different degradation states.

the patterns were recorded for 30 s of duration. The discharge initially appeared only in the negative AC half-cycle and then also in the positive half-cycle with higher PD magnitude as the HV supply was increased further to 11.3 kV peak. For degradation evaluation, the average PD magnitude and PD repetition rates for each half-cycle and overall of all composites are compared at three different surface ageing states of composites. Average PD magnitudes of all composites in the two AC half-cycles at virgin, 48 h and 96 h of corona exposure are shown in Figure 6. At virgin state, the values are 8.2 nC, 6.2 nC, 9.1 nC and 5.5 nC in positive halfcycle for PR, MC, NMC and NC, respectively. The most striking result is that in PR, at the end of 48 h and 96 h of corona exposure, the average PD magnitude has increased considerably to 12 nC. A similar trend is also noticed in MC. The lowest values are from NMC and NC. On the other hand for the negative half-cycle, no pronounced variations are observed in average PD magnitude of composites with corona exposure. Experimental findings on repetition rate are summarized in Figure 7. In all cases, it can be seen that the discharge activity is much higher in the negative half-cycle and thus dominates the overall results (combined 2 half-cycles). The general trend is consistent; all cases showed clear increase in the repetition rate by the end of corona exposure. The repetition rate of PR increased most sharply (~2000 pps) whereas NC showed the least, followed by NMC and MC. 3.2 HYDROPHOBICITY EVALUATION Figure 8a shows NC after 96 hours of corona treatment and Figure 8b highlights the two main regions where

hydrophobicity loss and recovery is evaluated. The variations in water static contact angles before and after the corona treatment below the needle tip and in the vicinity for 48 and 96 hours are shown in Figure 9. The initial contact angle of all virgin composites was measured at 112º with slight variations. Experimental results clearly indicate that hydrophobicity significantly declines for all composites as a result of corona exposure. In all cases, the contact angle declined more in the area below the needle tip than in the vicinity area. After 48 hours of corona treatment, the contact angle lessened by 66º, 75º, 63º and 56º and 52º, 56º, 47º and 40º below the tip and in the vicinity for PR, MC, NMC and NC, respectively. The higher reduction in contact angle was observed in PR and MC 130

PR

120

MC

NMC

NC

110 100

Contact Angle ()

400

90 80 70 60 50 40 30 20 Virgin

48h-Tip

48h-Vicinity

96h-Tip

96h-Vicinity

Degradation State

Figure 9. Reduction in hydrophobic water contact angle of composites due to corona discharge treatment. 150

PR

MC

NMC

NC

140 130

Contact Angle ()

Repetition rate (pps)

100

120 110 100 90 80 70 48h-Tip

48h-Vicinity

96h-Tip

96h-Vicinity

Recovery state

Figure 10. Hydrophobic water contact angle recovery of composites rest in air for fixed 100 hours duration.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23, No. 5; October 2016

samples. There was a significant improvement in NMC and NC in impeding the reduction of hydrophobic contact angle. The degraded NC showed a lesser reduction of 10º, 19º and 7º at tip and 12º, 16º and 7º at vicinity with respect to degraded PR, MC and NMC. The experimental results on hydrophobicity loss also revealed that degraded NMC showed an acceptable improvement in blocking the reduction of water angle as compared to MC but there was no major difference observed with respect to PR. For degraded NMC, contact angle declined by 3º, 12º and 5º, 9º below the electrode tip and in vicinity region as compared to PR and MC, respectively. At the end of 96 hours of corona exposure, the level of damage to hydrophobicity is quite high as shown in Figure 9. The contact angle below the tip declined below 30º for all samples. There was a reduction of 90º, 85º, 88º and 83º below the tip and 66º, 65º, 66º and 64º in the vicinity area, respectively. Moreover, NC offered better resistance to hydrophobicity loss below the tip as compared to PR. NC suppressed the contact angle reduction by 7º, 2º and 5º relative to PR, MC and NMC. This slight difference might be due to permanent physical damage to surface of the composite and building of a hydrophilic hydroxyl layer on the samples. As for the vicinity area, no major difference was observed among all samples at the end of 96 hours of degradation. After 48 and 96 hours of corona treatment, all the samples were rested in air at room temperature for 100 hours duration to measure hydrophobicity recovery as shown in Figure 10. Interestingly, the contact angle at needle tip spots was significantly higher than its virgin values for all composites. Here, the recovery contact angles were 129º, 122º, 112º and 116º for 48 hours and 131º, 132º, 131º and 124º for 96 hours aged PR, MC, NMC and NC, respectively. Relatively, the hydrophobic contact angle recovered to lower degrees in the vicinity regions and did not reach its values at virgin state. The angle for 48 hours aged composites recovered to 101º, 102º, 100º and 101º for PR, MC, NMC and NC, respectively. For the case of 96 hours, recovery angles in vicinity were 102º, 92º, 103º and 102º for PR, MC, NMC and NC. Note that the recovery angle of MC was low and this might be due to occurrence of serious chemical changes on the sample. 2.5 Virgin state Degraded State

Surface Roughness (µm)

2.0

1.5

1.0

0.5

0.0 PR

MC

NMC

NC

Composites Figure 11. Surface roughness values of composites after 96 hours of corona exposure.

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Possible reasons on hydrophobicity loss-recovery and the impact of micro and nano filler addition on hydrophobicity are discussed in Section 4. 3.3 SURFACE ROUGHNESS MEASUREMENT The surface roughness variations of all the composites before and after 96 hours of corona exposure are compared in Figure 11. It shows clearly that there is a significant increase in the surface roughness of all composites after corona exposure. Initially, the measured values of surface roughness were 0.17, 0.19, 0.19 and 0.18 µm for PR, MC, NMC and NC, respectively, with minor variations shown by error bars. After 96 hours of ageing, the values increased to 1.54, 0.57, 0.63 and 0.34 µm for PR, MC, NMC and NC, respectively. In terms of percentage change, the increase is 805%, 200%, 231% and 89%, respectively. Experimental results proclaimed that the addition of fillers to pristine silicone rubber retards the increase in surface roughness due to corona discharge, resulting in improved performance of silicone rubber under AC corona. Physical material damage below the needle tip is a fundamental factor that increases the roughness. As seen from the results, NC offered remarkably high resistance to changes in surface roughness, comparatively. The percentage increment in roughness of NC is 716%, 111% and 142% lower relative to roughness values of aged PR, MC and NMC, respectively. As far as surface roughness of micro composites is concerned, the performance of MC and NMC is clearly better as compared to PR. The percentage increase in surface roughness of MC and NMC is 605% and 574% lower as compared to PR. Also, it should be mentioned that NMC roughness is 31% higher relative to MC and this might be due to presence of higher number of filler contents in NMC. Based on experimental findings, it can be concluded that after 96 hours of corona exposure, NC gives the best performance in terms of resistance to surface roughness increase, followed by MC, NMC and PR in that order. 3.4 SEM STUDIES Figure 12 shows the surface morphology of composites below the needle tip after 96 hours of corona exposure at different magnification. It can be seen from the SEM images on the leftmost column of Figure 12 that after corona exposure, the damaged area appeared in a form of two layers round craters for all composites [4]. The inner region showed a high degree of damage while the outer ring is a band shaped boundary. Between two rings, a wavy shaped white powder layer appeared in all composites. At the end of 96 hours of corona exposure, there is no relation found between corona ring diameter and filler addition. The middle and rightmost columns of Figure 12 show the magnified images of specified regions inside the inner ring where high degree of damage and crackles are observed. Degradation in PR appeared in the form of severe cracks and surface splits into individual blocky structures. Between the individual blocks, a large number of holes and defects appeared in degraded PR. Holes and voids with significant defects and damage in the depth direction are also observed. The interior surface of degraded PR also revealed formation of cracks. Moreover, the outer and internal surface of PR undergoes different degree of corona

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge

(Below needle tip)

(Voids, pits and surface splitting)

(Formation of crackles)

(PR)

(MC)

(NMC)

(NC)

Figure 12. SEM images of 96 hours corona aged samples at different scale and mentioned locations.

degradation. No blocky structures appeared in the aged MC but defects appeared in the form of pits. A large number of small voids and holes appeared in and on the boundary of the pit. The voids and holes in MC are significantly smaller as compared to those found in aged PR. The width and depth of cracks on MC are considerably larger. And, there are no blocky structures and pit formation observed in degraded NMC. Major aged areas appeared in the form of white powder layer with slight cracks on it. Crack formation is significantly suppressed and its width is comparatively narrow. Furthermore, surface structure analysis of degraded NC suggested that appearance of holes, voids and blocky shaped damage structures is subdued as compared to aged PR, MC and NMC. A compact white layer on the surface is observed with minor voids on it. Cracks appearance, width

and penetration into the sample interior are significantly reduced in NC. Based on morphological results of degraded composites, it can be concluded that due to absence of fillers in PR, the deterioration of silicone rubber is accelerated and it results in larger cracks, voids, holes and surface splitting. Later, the damage penetrated deeper into the interior of the sample. MC offered a better resistance to damage as compared to PR due to the presence of micro silica. As far as crack formation is concerned, there is an improved performance observed in NMC compared to MC. Among all, the performance of NC is excellent and it might be due to better barrier resistance to corona discharges and high UV reflectivity of NC due to the presence of nano–sized silica [37].

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23, No. 5; October 2016

Absorbance

3.5 ATR-FTIR ANALYSIS Figure 13 shows the ATR-FTIR spectra of virgin and degraded composites after 96 hours of corona treatment. Corresponding wave numbers are indicated on each peak in FTIR spectra. Table 3 shows the IR absorption characteristics of different functional groups present in silicone rubber. Functional groups and their corresponding wave number are indicated in the table with the absorption peak heights at virgin and degraded states of all composites. Also, the percentage increase and decrease in peak heights of all functional groups with respect to virgin state values (considered as 100%) is also indicated to show the extent of chemical structural changes on the surface of the composites. The hump shaped peak between 3200 cm-1 to 3300 cm-1 in aged composites points to H bonded OH stretch which is hydrophilic in nature and appeared as a byproduct of corona treatment. The increase in OH peak heights in degraded MC, NMC is quite high as compared to degraded PR and NC. The lowest increase of 365% with respect to its virgin state of –OH is observed in degraded NC. The peak at 2962 cm-1 indicates CH stretch in CH2 and CH3 in the spectra and peak heights reduced to 90.5%, 83% and 96% with respect to virgin state values. Higher reduction of 17% is observed in degraded 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

Virgin-PR Degraded-PR

Virgin-MC Degraded-MC 1350

1413

3244

Virgin-NMC Degraded-NMC

787

1258 700

2962

500

1000

1500

2000

2500

3000

3500

MC with respect to its virgin value. Against the trend of degraded PR, MC and NMC, there is an increment of 24% in degraded NC with respect to its virgin state. The peak heights at 1413 cm-1 and 1350 cm-1 represent CH and NO (Nitro) functional groups. A significant increase in peak heights of both functional groups is found at the end of corona exposure. The absorption peak intensity of CH increased up to 323%, 417%, 295% and 222% in degraded PR, MC, NMC and NC, respectively. Furthermore, a sharp increase in absorbance intensity of nitro is perceived in all aged composites which indicate the presence of nitride compounds on the degraded composites [39]. The absorbance intensity of nitro in degraded composites shoots up to 1396%, 1503%, 990% and 533% in PR, MC, NMC and NC, respectively. There is a noteworthy reduction in formation of nitro groups on the surface of NMC and NC. From Table 3, the absorbance intensities of Si-CH3, Si-O-Si and Si (CH3)2 are found to be in descending order for all composites at the end of 96 h of corona exposure. Si-CH3 declined to 68.78%, 92.23%, 74.35% and 97.32% for PR, MC, NMC and NC, respectively. The highest resistance to breakage of side chain (Si-CH3) in composites is offered by NC and a significant stability of Si-CH3 is observed in MC as compared to NMC. The main chain (Si-O-Si) dwindled to 49%, 64%, 84% and 93% in PR, MC, NMC and NC, respectively. Similarly, the lowest and highest Si-O-Si bond breakage is noticed in NC and PR like Si-CH3. In contrast to Si-CH3, the Si-O-Si is found to be more stable in NMC than MC. A sharp reduction in peak heights at 787cm-1 in Figure 9 indicates major damage to Si (CH3)2 in the composites due to corona exposure. The peaks of Si (CH3)2 drop down to 40%, 61%, 70% and 91% in degraded PR, MC, NMC and NC. The reduction in absorbance intensities of Si (CH3)2 is found to be in same order like the main chain Si-O-Si of composites. A minor increment in absorbance intensity of Si (CH3)3 is noticed in all composites. Si (CH3)3 peak height increased by 16% in degraded PR but just 6%, 6% and 5% in MC, NMC and NC, respectively. It indicates improved resistance to deformation of Si (CH3)3 offered by MC, NMC and NC.

4 DISCUSSION

Virgin-NC Degraded-NC

1010

2811

400

-1 Wavenumber[cm ] Figure 13. FTIR spectra of composites at virgin and 96h-degraded states with peak heights of functional groups at different wave numbers.

4.1 HYDROPHOBICITY Experimental results on hydrophobicity indicate that hydrophobicity loss and recovery angles are different in the two regions (below needle tip and vicinity). Corona discharge increases the oxygen concentration on the surface of silicone rubber and this phenomenon results in the formation of silica like hydrophilic layer and polar silanol groups on the PDMS surface. According to mechanism described in [11], severe oxidation deforms the hydrophobic –CH3 to –CH2 and generates the hydroxyl (Si-CH2OH) along with peroxides (SiCH2OOH) due to reaction between oxygen contents and PDMS. Below the needle tip, the higher corona discharge intensity might result in more severe surface oxidation and formation of hydrophilic layer. FTIR results in Table 3 illustrated the formation of –OH group below the needle tip for all composites. Furthermore, the hydrophobicity recovery

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge Table 3. Variation in absorbance peak heights of different functional groups at virgin and degraded states of composites.

Function Groups

Wave Number (cm-1)

OH stretch, H bonded

3200-3300

CH3(C-H)

2960-2962

CH

1410-1440

N-O

1345-1385

Si-CH3

1240-1270

Si-O-Si

1000-1100

Si(CH3)2

790-840

Si(CH3)3

700

Virgin PR 0.0052 (100%) 0.0908 (100%) 0.029 (100%) 0.0087 (100%) 0.4078 (100%) 1.0158 (100%) 1.2855 (100%) 0.2188 (100%)

FTIR peak absorbance heights of composites at different degradation states Degraded Virgin Degraded Virgin Degraded Virgin PR MC MC NMC NMC NC 0.0262 0.0059 0.0578 0.0059 0.0854 0.0089 (503%) (100%) (967%) (100%) (1425%) (100%) 0.0822 0.098 0.0814 0.0987 0.0949 0.0906 (90.5%) (100%) (83%) (100%) (96%) (100%) 0.0938 0.0391 0.1633 0.0393 0.1160 0.0304 (323%) (100%) (417%) (100%) (295%) (100%) 0.1215 0.0134 0.2019 0.0134 0.1331 0.0099 (1396%) (100%) (1503%) (100%) (990%) (100%) 0.2804 0.4151 0.3828 0.4211 0.3131 0.3999 (68.78%) (100%) (92.23%) (100%) (74.35%) (100%) 0.5009 1.0024 0.6448 1.0037 0.8443 1.0212 (49%) (100%) (64%) (100%) (84%) (100%) 0.5241 1.2627 0.7719 1.2677 0.8943 1.2712 (40%) (100%) (61%) (100%) (70%) (100%) 0.2551 0.2231 0.2372 0.2232 0.2367 0.2215 (116%) (100%) (106%) (100%) (106%) (100%)

mechanism after corona exposure involves the reorientation of polar groups, condensation of silanol and migration of low molecular weight (LMW) components from bulk to surface of PDMS [18]. According to hydrophobicity recovery results as shown in Figure 10, the hydrophobic water contact angle of unfilled PR is higher than filled composites below the needle tip region after 48 h of corona exposure. This could be due to easier transportation of LMW components from bulk to surface in PR. In filled composites, the fillers may act as a shield that retards the diffusion of LMW components. Moreover, the hydrophobicity recovery in NC is found to be lower than MC and this is because of the enhanced shielding effect in NC due to the presence of nano fillers. This improved shielding effect may enhance PD resistance of NC but it provokes a negative impact on hydrophobicity recovery. Moreover, the hydrophobic water contact angle below the needle tip is significantly higher than in the vicinity region after 100 h of air rest for both cases of 48 h and 96 h corona exposure. The contact angle below the needle tip is remarkably higher at 130º, 131º, 130º and 124º for PR, MC, NMC and NC than virgin state of composites. Lower recovery of hydrophobicity in vicinity region could be due to permanent and uncracked formation of hydrophilic layer on composites. This intact and non-deformed hydrophilic layer might retard the migration of LMW from

Small circles: Nano-fillers, Big circles: Micro-fillers

r Figure 14. Corona discharge degradation mechanism of silicone rubbe composites.

Degraded NC 0.0414 (465%) 0.1125 (124%) 0.0677 (222%) 0.0528 (533%) 0.3892 (97.32%) 0.9532 (93%) 1.1568 (91%) 0.2338 (105%)

bulk to surface in the vicinity region. MC recovered its angle to 91º in the vicinity regions after 96 h of corona exposure which is ~20º less than its virgin state value. The higher contact angle recovery below needle tip can be due to increase in surface roughness of composites and crack formation on surface. Formation of cracks on surface might be one reason behind higher contact angle recovery because such defects provide less resistive route to LMW components of PDMS to diffuse from bulk to surface. Higher hydrophobicity loss and higher recovery occurred below the needle tip. Lower hydrophobicity loss and lower recovery of contact angle were measured in the vicinity region. Hillborg et al [43] reported that a minor mechanical stress can crack the hydrophilic layer and resulted in faster recovery. Gubanski et al [44] suggested that deformation of PDMS surface due to corona initiates higher pace of angle recovery. But, high surface roughness and crack formation might be a demerit for outdoor insulators because it can increase the chances for pollution accumulation and moisture absorption. The role of different size fillers addition on surface roughness and crack formation is discussed in detail in the following section. 4.2 ROLE OF INTER-PARTICLE DISTANCE, SIZE AND SURFACE AREA Experimental results on AC corona degradation of silicone rubber indicated that the introduction of micro and nano– fillers improves its performance in terms of PD resistance, hydrophobicity loss, surface roughness, crack/void formation and damage to chemical structure. In the experiment, the corona source electrode did not touch the sample surface and spacing of 1 mm is maintained as illustrated in Figure 14. The composite surfaces are eroded by corona discharges but no evidence of electrical treeing observed. The silicone rubber composites are degraded largely in its base polymer locations. And, the earlier reported [45] that addition of micro and nano fillers assists in suppressing the PD activity and it could be due to higher permittivity of silica particles than that of base matrix. Moreover, FTIR analysis also indicates the presence of –OH functional group on the surface of silica particles in the range of 2945 cm-1 to 3400 cm-1 [28] and it can initiate the

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23, No. 5; October 2016 Table 4. Inter-filler spacing and total surface area of fillers in composites.

Composites

PR MC NMC NC

Interfiller spacing among microfillers (nm) 2753 3032 -

Interfiller spacing among nanofillers (nm) 55.64 39.72

Total surface area of microfillers (km2/m 3 ) 0.169 0.152 -

Total surface area of nanofillers (km2/m3)

Total surface area of fillers in composite (km2/m3)

2.904 5.90

0.169 3.056 5.90

hydrogen bonding between base silica particles and PDMS. A possible model of hydrogen bonding between silica particle and base PDMS is shown in Figure 15 by considering that there is no moisture on particle. Hydrogen bonding could assist the silica particles to grip the base PDMS matrix and improve the barrier resistance of micro/nano filled PDMS. Experimental results on PD activity, hydrophobicity, SEM images and FTIR analysis clearly show that there is a significant improvement in the performance of silicone rubber with addition of nano–silica (NC) and micro + nano silica (NMC). Only in case of surface roughness, NMC performance is found slightly lower than MC. For resistance against corona degradation, experimental findings indicate that NC is the best amongst the composites tested. Moreover, it can also be concluded that the addition of nano–silica in antecedent micro–filled silicone rubber (NMC) offered better corona resistance than PR and MC. In order to understand the better performance of NMC and NC over PR and MC, it is important to consider the interparticle spacing and total surface area of fillers in the composites. By assuming ideal distribution of fillers according to model proposed by Tanaka [46], these parameters are calculated and shown in Table 4. It can be seen that the interparticle spacing in NC is reduced by 98.5% and total surface area of fillers is increased by ~3391% with respect to MC. Moreover, the total surface area of fillers in NMC is increased by ~1708% relative to MC. Based on these results, it can be said that barrier resistance of NC and NMC is much improved than MC. Due to the large increase in surface area of fillers

Figure 15. Model of hydrogen bonding of –OH groups on silica particles with PDMS.

2813

and massive reduction in inter-particle spacing in NC and MNC, the probability of covering the base matrix by fillers is increased. Therefore, it can increase the barrier resistance and improve the resistance to partial discharges which are clearly demonstrated from PRPD results.

5 CONCLUSIONS Major experimental findings are: 1. From phase-resolved PD measurement, the most striking result is that in PR, at the end of 96 h of corona exposure, the average PD magnitude in the positive half-cycle has increased considerably to 12 nC. A similar trend is also noticed in MC whereas the lowest values are from NMC and NC. On the other hand for the negative half-cycle, no pronounced variations are observed in average PD magnitude of composites with corona exposure. 2. In terms of discharge repetition rate, the activity is much higher in the negative half-cycle and thus dominates the overall results. The trend is consistent; all cases showed clear increase in the repetition rate by the end of corona exposure. Higher PD resistance is given by NC followed by NMC, MC and PR. 3. Higher hydrophobicity loss is noticed below the needle tip rather than in the vicinity region. After 48 h of corona exposure, NC showed a lower hydrophobicity reduction of 10º, 19º and 7º below the tip and 12º, 16º and 7º at vicinity with respect to degraded PR, MC and NMC, respectively. After 96 h of corona degradation, the contact angle below the tip declined below 30º for all samples but measured 10º higher in vicinity with respect to the tip for all composites. 4. Higher hydrophobicity recovery angle is measured below the needle tip than in the vicinity area. The hydrophobicity of composites prior exposed to 96 h of corona recovered its angle to 131º, 132º, 131º and 124º below the needle tip and 102º, 92º, 103º and 102º in vicinity location for PR, MC, NMC and NC, respectively. Appearance of micron–sized crackles and increase in roughness may be the reasons behind higher recovery. 5. SEM images show that the initiation of micron–sized crackles, voids, pits and surface splitting into blocky structures is stopped in NC. More damage is observed in PR, particularly in the specimen depth direction. It can be proclaimed that NC offered higher resistance against damages, followed by NMC and MC. 6. Higher surface roughness occurred in PR followed by NMC, MC and NC at the end of 96 h of corona exposure. 7. NC offered appreciable resistance to contraction in absorbance peak heights of Si-CH3, Si-O-Si and Si (CH3)2 comparative to NMC, MC and PR. All the functional groups are found stable in NMC with respect to aged MC and PR. Only Si-CH3 showed a slightly higher reduction in NMC as compared to MC. From the above key findings, it can be summarized that NC evinces strong resistance to PD activity and hydrophobicity loss. Furthermore, higher hydrophobicity loss and recovery is noticed in the area below the needle tip. The results also

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M. T. Nazir et al.: Performance of Silicone Rubber Composites with SiO2 Micro/Nano-filler under AC Corona Discharge

reveal that morphological and organic structural damages are considerably impeded in NC as compared to PR, MC and NMC. Finally, addition of nano–sized silica can be a viable approach for enhancing the corona resistance of micron–sized silica filled silicone rubber.

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[21] Y. Liang, L. Ding, C. Li, K. Yang and Y. Tu, “A method to detect the deterioration of HTV silicone rubber under corona discharge”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.509-512, 2006. [22] V.M. Moreno and R.S. Gorur, “Effect of long-term corona on nonceramic outdoor insulator housing materials”, IEEE Trans. Dielectr. Electr. Insul., Vol. 8, No. 1, pp. 117-128, 2001. [23] V.M. Moreno and R.S. Gorur, “An experimental approach to the estimation of the long term corona performance of non-ceramic insulator housings”, IEEE Int’l. Sympos. Electr. Insul. (ISEI), pp. 193-196, 2000. [24] V.M. Moreno and R.S. Gorur, “Impact of corona on the long-term performance of nonceramic insulators”, IEEE Trans. Dielectr. Electr. Insul., Vol. 10, No. 1, pp. 80-95, 2003. [25] V.M. Moreno and R.S. Gorur, “Accelerated corona discharge performance of polymer compounds used in high voltage outdoor insulators”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.731-734, 1999. [26] V.M. Moreno and R.S. Gorur, “Corona-induced degradation of nonceramic insulator housing materials”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.640-643, 2001. [27] A.H. El-Hag, L.C. Simon, S.H Jayaram and E.A. Cherney, “Erosion resistance of nano-filled silicone rubber”, IEEE Trans. Dielectr. Electr. Insul., Vol. 13, No. 1, pp. 122-128, 2006. [28] B. Venkatesulu and M. J. Thomas, “Corona aging studies on silicone rubber nanocomposites,” IEEE Trans. Dielectr. Electr. Insul., Vol.17, No.2, pp. 625-634, 2010. [29] J. V. Vas and M. J. Thomas, “Surface degradation of silicone rubber nanocomposites due to DC corona discharge,” IEEE Trans. Dielectr. Electr. Insul., Vol. 21, No.3, pp. 1175-1182, 2014. [30] B. Subba Reddy and Shakthi Prasad D, “Effect of Coldfog on the Corona Induced Degradation of Silicone Rubber Samples”, IEEE Trans. Dielectr. Electr. Insul., Vol. 22, No. 3, pp. 1711-1718, 2015. [31] B. Subba Reddy and Shakthi Prasad D, “Corona Degradation of the Polymer Insulator Samples under Different Fog Conditions”, IEEE Trans. Dielectr. Electr. Insul., Vol. 23, No. 1, pp. 359-367, 2016. [32] G. Iyer, R.S. Gorur, A. Krivda and V.H. Camara, “Corona resistance of epoxy nanocomposites”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.310-313, 2011. [33] G. Iyer, R.S. Gorur and A. Krivda, “Corona resistance of epoxy nanocomposites: experimental results and modeling”, IEEE Trans. Dielectr. Electr. Insul., Vol. 19, No. 1, pp. 118-125, 2012. [34] I. Ramirez, E.A Cherney, S. Jayaram and M. Gauthier, “Nanofilled silicone dielectrics prepared with surfactant for outdoor insulation applications”, IEEE Trans. Dielectr. Electr. Insul., Vol. 15, No. 1, pp. 228-235, 2008. [35] http://www.sigmaaldrich.com/catalog/product/aldrich/637238?lang=en& region=AU [36] http://www.ussilica.com/products/fine-ground-silica [37] M. Tariq Nazir, B.T. Phung and M. Hoffman, “Effect of AC corona discharge on hydrophobic properties of silicone rubber nanocomposites”, IEEE Int’l. Conf. Properties and Applications of Dielectr. Materials (ICPADM), pp.412-415, 2015. [38] M. T. Nazir and B.T. Phung, “AC corona resistance performance of silicone rubber composites with Micro/Nano silica fillers”, IEEE 1st Int. Conf. on Dielectrics (ICD), Montpellier, France, 2016. [39] High-Voltage Test Techniques - Partial Discharge Measurements, IEC 60270, 2001. [40] S. Amin, “Comparative natural aging of thermoplastic elastomeric and silicon rubber insulators in Pakistan,” J. Elastomers and Plastics, Vol. 44, No. 2, pp. 115-125, 2012. [41] R.S. Gorur, G.G. Karady, A. Jagota, M. Shah and A.M. Yates, “Aging in silicone rubber used for outdoor insulation”, IEEE Trans. Power Delivery, Vol. 7, No.2, pp. 525-538, 1992. [42] Y. Gao, J. Wang, X. Liang, Z. Yan, Y. Liu and Y. Cai, “Investigation on permeation properties of liquids into HTV silicone rubber materials” , IEEE Trans. Dielectr. Electr. Insul., Vol. 21, No. 6, pp. 2428-2437, 2014. [43] H. Hillborg and U.W. Gedde, “Hydrophobicity recovery of polydimethylsiloxane after repeated exposure to corona discharges. Influence of crosslink density”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.751-755, 1999. [44] S.M. Gubanski, “Modern Outdoor Insulation—Concerns and Challenges”, IEEE. Electr. Insul. Mag., Vol. 21, No. 6, pp. 5-11, 2005.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23, No. 5; October 2016 [45] Z. li, K. Okamoto, Y. Ohki and T. Tanaka, “Effects of nano-filler addition on partial discharge resistance and dielectric breakdown strength of micro-Al2O3/epoxy composite”, IEEE Trans. Dielectr. Electr. Insul., Vol. 17, No. 3, pp. 653-661, 2010. [46] T. Tanaka, M. Kozako, N. Fuse and Y. Ohki, “Proposal of a multi-core model for polymer nanocomposite dielectrics,” IEEE Trans. Dielectr. Electr. Insul., Vol. 12, No. 4, pp. 669-681, 2005. Muhammad Tariq Nazir (S’15) was born in Punjab province, Pakistan, on 3 September 1986. He received the B.Sc. and M.Sc. degrees in electrical engineering from the University of Engineering and Technology, Taxila, Pakistan and Chongqing University, Chongqing, China in 2009 and 2012, respectively. Currently, he is working towards his Ph.D. degree in electrical engineering from the University of New South Wales, Australia. His research interests in high voltage and its insulation includes composites materials used for outdoor insulation, Partial discharge measurement, Nano-dielectrics, breakdown testing and dielectric failure mechanisms. B.T. (Toan) Phung (M’87-SM’12) gained a Ph.D. in electrical engineering in 1998 and is currently an Associate Professor in the School of Electrical Engineering & Telecommunications at the University of New South Wales, Sydney, Australia. He has over 30 years of practical research/development experience in partial discharge measurement, and in on-line condition monitoring of high-voltage equipment. His research interests include electrical insulation (dielectric materials and diagnostic methods), high-voltage engineering (generation, testing and measurement techniques), electromagnetic transients in power systems, and power system equipment (design and condition monitoring methods).

2815 Mark Hoffman gained his BE (1989) and Ph.D. (1994) degrees in mechanical engineering from the University of Sydney. Following research positions at UC Berkeley, Tokyo Metropolitan University and the Technical University Darmstadt, Germany he joined the University of New South Wales in 1997. He specializes in structural integrity of materials and his research interests include polymer nanocomposites, piezoelectric ceramics, hybrid composites, biological materials and thin films. He is currently Dean of Engineering at the University

of New South Wales.