Journal of Colloid and Interface Science244,200–207 (2001)doi:10.1006/jcis.2001.7909, available online at http://www.idealibrary.com on

The Mechanisms of Hydrophobic Recovery of PolydimethylsiloxaneElastomers Exposed to Partial Electrical Discharges

Jongsoo Kim,∗ Manoj K. Chaudhury,∗,1 Michael J. Owen,† and Tor Orbeck,‡∗Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015;†Dow Corning Corporation,

Midland, Michigan 48686-0994; and‡East Jordan, Michigan 49727

Received June 4, 2001; accepted August 10, 2001; published online October 18, 2001

Silicone elastomers exposed to electrical discharges can be ren-dered hydrophilic, and the loss of water repellency is related to theenergy level and number of the discharge pulses on the surface.However, the silicone surface has the unique ability to recover thehydrophobicity and enhance the performance of a polymer insu-lator. Most of the studies of the loss and recovery of hydrophobic-ity on elastomer surfaces have been carried out at high dischargeenergies experienced under severe service conditions. Despite nu-merous studies, the mechanisms contributing to the loss and re-covery of hydrophobicity under lower energy partial dischargesare not fully elucidated. The current study was done with par-tial discharge pulses in the range 10–10000 pC and by using aneedle-to-plane electrode configuration. At very low levels of en-ergy pulses, the recovery is primarily caused by the migration ofpreexisting fluids from the bulk to the surface of the elastomer. Athigher levels of partial discharges, the fluids no longer play thesame role, but the dominant mechanism in the recovery becomesthe migration of in situ produced low molecular weight (LMW)species in the elastomer. These studies were done under dry and wetconditions. C© 2001 Academic Press

Key Words: polydimethylsiloxane; PDMS; partial discharge;preexisting fluid effect; hydrophobic recovery mechanism.

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I. INTRODUCTION

Polymer insulators are gaining market share in high-voltainsulation applications mainly due to their easy installation, ligweight, and superior vandalism resistance compared to trtional inorganic materials (1). Compared to most polymematerials, silicone elastomers have better thermal stabilityresistance to UV radiation because of the strong Si-O bondergy and excellent water repellency resulting from their lowtermolecular interactions. Although the silicone elastomerthese highly desirable properties for high-voltage applicatioits surface inevitably may lose its hydrophobicity when exposto partial electrical discharges as do other polymeric materiSurface oxidation by electrical discharges such as dry banding is accelerated in the presence of water and air-borne con

1 To whom correspondence should be addressed.

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200021-9797/01 $35.00Copyright C© 2001 by Academic PressAll rights of reproduction in any form reserved.

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inations. However, the silicone elastomer has the unique chacteristic of readily recovering its original hydrophobicity oncdischarge ceases, and thereby continuing to suppress the leacurrents and prevent insulator flashover. This surface restrucing of silicone elastomers, often referred to as “hydrophobrecovery,” has been extensively studied in the developmennew high-voltage insulators.

The following mechanisms for hydrophobic recovery havbeen suggested:

(a) Reorientation of polar groups from the surface to the buphase or reorientation of nonpolar groups from the bulk to tsurface (2, 3).

(b) Diffusion of preexisting low-molecular-weight (LMW)silicone fluid from the bulk to the surface (2, 4–20).

(c) Condensation of the surface hydroxyl groups (3).(d) Migration of in situ created LMW species during dis-

charge to the surface (7–10, 21, 22).

Even though most researchers have pointed out the imptance of the migration of LMW fluid preexisting in the bulk(mechanism b), other mechanisms may also contribute tohydrophobic recovery of oxidized PDMS elastomers. In oprevious papers (21, 22), we suggested that when silicone etomers are exposed to discharge pulses exceeding 1500 pCkey mechanism is the migration of the LMW species producin situduring discharge from the bulk to its surface (mechanisd). We also found that preexisting fluid in the bulk does not sinificantly affect the hydrophobic recovery if silicone elastomeare subject to higher energy discharges that may occur under ctaminated service conditions. However, only a limited numbof insulators are exposed to severe contamination. Most insutors are only exposed to low-energy discharge pulses duringservice conditions. It is important to study the loss and recoveof the hydrophobicity of these discharge levels. This promptus to study the behavior of polydimethylsiloxane (PDMS) elatomers exposed to partial electrical discharges of various intenties (10–10000 pC). The contact angle changes on the surfacePDMS elastomers were monitored intermittently during and/after partial discharge to study the dominant mechanisms ofdrophobic recovery. This study also investigated the roles of ffluid on the restructuring behavior of silicone elastomers af

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HYDROPHOBIC RECOVERY OF POLY

exposure to different levels of partial discharge. Additionalthe effect of humidity on the hydrophobic recovery of PDMelastomers was explored to assess the role of hydrolytic depmerization in the formation of LMW species. Contact angle msurement and angle-resolved X-ray photoelectron spectrosc(XPS) were used to study the hydrophobic recovery of PDMelastomers exposed to partial discharge. On the basis of tresults, different dominant restructuring mechanisms of PDelastomers are proposed.

II. MATERIALS AND EQUIPMENT

1. Materials

Two-component high-temperature vulcanized silicone eltomer (SYLGARD 184, Dow Corning Corp.) was used for thstudy. Silicone elastomers are normally made of several mrials such as a base polymer, a cross-linker, a catalyst, filland other additives. SYLGARD 184 is supplied in two separkits with a vinyl-terminated silicone base polymer in one kit awith a cross-linker and a catalyst in the other kit. These materwere thoroughly mixed according to the manufacturer’s instrtions, and air bubbles trapped in the compound were removedder vacuum. After the platinum-catalyzed hydrosilation react(75◦C, 2 h) of vinyl functional groups of the base polymer wihydrogen functional groups of the crosslinker was completthe elastomer was Soxhlet-extracted for 12 h with chloroformremove unreacted oligomers. In other experiments, pre-exisfluid samples were prepared by mixing linear methyl-terminasilicone fluid (Mw= 236, Mw= 950, Gelest Inc.) or fluorine-containing silicone fluid (3,3,3-trifluoropropylmethylsiloxanMw= 950, Gelest Inc.) with the compound before curing to evuate the influence of these additives on the hydrophobic recovFluid was added in the ratio of 5 weight parts to 100 weight paof the base polymer.

2. High-Voltage Test and Measuring Equipment

A high-voltage AC corona test set (HIPOTRONICS, Mod750-5CTS B/S) with a 50-kV noise free transformer was eployed to generate and measure the partial electrical dischaon the surfaces of PDMS elastomers. This model is suitafor measurement of pulses in the range 10–10000 pC. Thecharge magnitude was calibrated by using the calibration pgenerator module and the switchable amplifier module prioconducting the main experiments. The calibration pulse geator module is specially designed for the indirect calibrationdischarge detection circuits, by injecting a known magnitupulse in the input unit to provide a quantitative comparison wthe magnitude of discharges in a test specimen. After calibrais completed against a pulse of known value, voltage is graally increased until partial discharges reach desired values.magnitude meter module then reads the height of the higpulse among the numerous partial discharges appearing odisplay ellipse during the experiment. A strip chart recorder w

used to observe the variation of the discharge with time.

DIMETHYLSILOXANE ELASTOMERS 201

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A sample sheet (thickness, 1 mm) was placed on a circglass plate (diameter, 10 mm; thickness, 3 mm). The platerequired as a rigid support to prevent any flexing during hdling. Microcracking caused by bending is a significant facin the hydrophobic recovery (4, 6). A needle electrode (tipameter, 0.065 mm) was employed to obtain various levelsdischarge. The cylindrical electrode (rod diameter, 1 cm) uin our previous high discharge studies (22) produces relativlarger oxidized areas that allow easier measurement of thetact angles, but it proved impossible to sustain a stable electdischarge at lower intensities with this electrode configuratiFigure 1 shows the design of the test cell made of aluminum wbrass ground electrodes. The cell is designed to test eight sples at a time, but only one sample was tested with one electbecause of the difficulty in obtaining the same levels of parelectrical discharges for all samples tested. The aluminum bis made to circulate cooling water (0◦C) to form water conden-sation on the samples as experienced in the field applicawhen high-voltage insulators are exposed to a temperaturelow the dew point of the surrounding air. Experiments were aperformed at varying discharge levels to study the effect of dcharge intensity on the hydrophobicity loss and recovery durand after partial discharge at constant humidity levels. Mostperiments were carried out under normal laboratory conditiat 23◦C and 50% RH. A few tests were made under the vdry conditions of 15% RH to study the effect of the enviromental humidity on the surface modification and hydrophorecovery of PDMS elastomers. Two gap distances between

FIG. 1. The diagram of a partial electrical discharge test unit. The point-to-plane electrode configuration was used to sustain a low stable discharge.

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FIG. 2. The advancing contact angle of water does not depend on the volof the drop. The sample was an extracted silicone elastomer exposed to 300of partial discharge for 30 min. The oxidized area corresponds to∼50 mm2.

sample and the electrode rod were used, 0.4 and 0.05 mmis generally observed that a stable, low partial discharge caobtained more consistently as the gap becomes narrowerconstant humidity.

4. Surface Analysis

Contact angle measurement is a useful and inexpensive tnique extensively employed in characterizing plasma, or corotreated polymer surfaces in conjunction with other surface anysis techniques such as XPS and SIMS. Since the contact anof liquid droplets on surface-treated polymers are closely asciated with chemical compositions of the surface, this methovery useful in studying the aging of the surface-modified pomers. One of the most commonly used methods for contact ameasurement is the sessile drop technique (23, 24). By usicontact angle goniometer (Ram`e-Hart. Inc., Model No. 100-00-115), the advancing contact angle of a sessile water droplethe surface was measured at least four times while mainting the volume of a drop as constant as possible. For nonidpolymer surfaces, surface defects lead to numerous metasstates of the system, showing hysteresis of contact angles31). Significant dependency of the contact angle on the dvolume has also been found when the line tensions are notligible (32). In our studies, however, we noticed no effectthe liquid drop size on the contact angle when the drop vume varied from 0.1 to 2µl (Fig. 2). This result indicates thathe patchwise heterogeneities of the types observed by Gand Koo (32) in their studies on the drop size dependent conangles are absent here. The result also shows that there is noical variation of wettability within the entire area (∼50 mm2) ofthe PDMS elastomer that was exposed to the partial dischaIn the subsequent experiments, a water volume of the or

of 0.2µl was chosen. The angle-resolved XPS technique walso used to quantitatively analyze the atomic composition

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III. RESULTS AND DISCUSSION

1. Hydrophobicity Loss

Water-in-air advancing contact angles of PDMS elastomwere monitored during experiments to study the hydrophobiloss by partial electrical discharge in a point-to-plane electrconfiguration. The needle electrode was suitable for susting a low, stable discharge with a small gap of 0.05 mmtween the electrode and the sample. The hydrophobicityof PDMS elastomers was observed as a function of the exsure time of the discharge that varied from 10 to 250 pC aconstant ambient humidity (50%). As shown in Fig. 3, the etracted sample exposed to the lowest discharges (10–60gradually loses its hydrophobicity as discharge time increabut the contact angle does not become less than ca. 60◦ evenafter 100 h of discharge. Possibly, very small amounts of LMspecies may be produced during this low discharge, which ssequently migrate to the surface to minimize high surface eneresulting from the oxidation of methyl groups attached on siloane backbones. For the fluid-containing sample, the advansessile contact angle decreases only slightly (109◦ to 89◦) afterwhich it remains unchanged, indicating that a dynamic equirium is maintained at the interface at this low discharge. Sa dynamic equilibrium can be reached when the rate of thegration to the surface of fluids preexisting in the polymernally becomes the same as the rate of its oxidation by elect

FIG. 3. The hydrophobicity losses of the silicone elastomers exposedifferent levels of discharge at a constant environmental humidity of 50% inpoint-to-plane electrode configuration with a gap of 0.05 mm. The inset shthe data obtained within the first 10 h; (r) Extracted, 10–60 pC; (j) extracted,75–125 pC; (m) extracted, 150–250 pC; (e) 5% fluid (Mw= 236), 10–60 pC;

asof(h) 5% fluid (Mw = 236), 75–125 pC; and (n) 5% fluid (Mw = 236), 150–250 pC.

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discharge. As discharge increases from 10–60 to 75–125the effect of preexisting fluids on hydrophobicity loss becommore pronounced. The extracted sample loses its hydrophity fast while the fluid-containing sample suffers a moderateof hydrophobicity (Fig. 3). Apparently, preexisting fluids plaa more important role in preserving the hydrophobicity ofsurface thanin situ produced LMW species during these lodischarge intensities. At more severe discharge levels (1250 pC), the fluid-containing sample loses its hydrophobimuch faster, but it also shows a much faster recovery thanextracted sample due to the migration of preexisting fluids adischarge. Probably, the discharges in the range 10–250 pnot produce enough LMW species to overwhelm the effecthe preexisting fluid. This migration process takes place duthe active discharge as well as after the discharge period. Hever, at discharges higher than 1500 pC, the migration durindischarge does not make any impact on keeping the surfacdrophobic. When samples are exposed to the severe dischboth extracted samples and fluid-containing samples totallytheir original hydrophobicity within one hour. The dischargeso intense that an inorganic, silica-like layer is rapidly createdthe outermost surface, which may provide the resistance todiffusion of both preexisting fluid andin situ produced LMWspecies to the surface. In addition, any nonpolar groupsgrating to the surface during the discharge seem to be quoxidized by harsh discharges. Therefore, immediately afterdischarge, a water drop spreads on the surface of the PDelastomer, and after the cessation of electrical discharge, thidized PDMS elastomers recover their hydrophobicity throuthe migration of LMW species from the bulk to the surface. Edently, the hydrophobicity loss of extracted and fluid-containsamples is highly dependent on discharge intensity. We hshown how the recovery behavior of PDMS elastomers issignificantly affected by the intensity of electrical discharge aenvironmental humidity.

2. Hydrophobic Recovery

Figure 4 shows the hydrophobic recovery of the extracand the fluid-containing samples in air after they were expoto different ranges of discharge (10–10000 pC) for 1 h. Thetracted sample exposed to the very low discharge (10–60exhibits a high initial contact angle (60◦) immediately after118 h of this low discharge and then recovers its hydrophobvery slowly. Most probably, at a mild discharge, the extracsample undergoes a mild oxidation of pendent methyl groupssimultaneously produces very small amount of LMW specThe subsequent hydrophobic recovery is then possibly duthe migration ofin situ produced species from the bulk to thsurface of the elastomer. At discharges higher than 1500although the extracted samples rapidly lose their hydrophoity, they show a faster recovery than those exposed to lodischarge intensities. Interestingly, as discharge becomes

severe, the rate of the recovery of the extracted samplescreases and then eventually becomes comparable to that of fl

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FIG. 4. The hydrophobic recovery of the samples exposed to partial elecal discharges. The discharge experiment was performed at a humidity ofin the point-to-plane configuration with a gap of 0.05 mm. All of the sampwere aged in air after 1 h discharge except two (r, e). The contact angles ofthe samplesr ande are measured after 118 and 92 h discharge, respectiv(r) Extracted, 10–60 pC; (j) extracted, 100–400 pC; (m) extracted, 4000–10000 pC; (e) 5% fluid (Mw = 236), 10–60 pC; (h) 5% fluid (Mw = 236),150–300 pC; and (n) 5% fluid (Mw= 236), 4000–8000 pC.

containing samples. This suggests that the high level of LMspecies produced beneath the silica-like layer controls thecovery process by migrating to the surface (22). When bothextracted and the fluid-containing samples are compared at adischarge, the latter exhibits a superior hydrophobic recovto the former. The initial contact angle of the fluid-containinsample measured immediately after exposure to 150–300for 1 h is 17◦, which is higher than that (8◦) observed on theextracted sample exposed to 100–400 pC (Fig. 4). Moreoa pronounced increase of the recovery rate is observed influid-containing sample. Presumably, at the mild discharge, pexisting fluids contribute to maintaining the hydrophobicityPDMS elastomers by migrating to the surface during dischaas well as after discharge. It follows that the migration of preisting fluid from the bulk to the surface is a key mechanismthis recovery process. However, as the discharge increases150–300 to 4000–8000 pC, the contribution of the preexistLMW silicone fluid to the restructuring process becomes lesignificant. While the preexisting fluid has no great effect onhydrophobic recovery of samples exposed to a discharg4000–8000 pC, the extracted sample shows a marked incrof recovery as the discharge increases from 100–400 to 4010000 pC. At high discharge intensities, the rate of hydropbic recovery of the extracted samples is also comparable tofluid-containing sample. Accordingly, at the severe dischalevel, the key mechanism seems to be the migration of thin

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a hexane extraction and heating cycle, have demonstratedLMW chain regeneration and hydrophobic recovery occur evafter the initial supply of LMW species is depleted. The cotribution of preexisting LMW silicone fluid to the restructurinis not significant under severe discharge conditions, but its rbecomes more important at milder discharge conditions.

3. Humidity Effect

Moisture in air may participate in the hydrolytic depolymeization reaction and the production of LMW species by the hgenerated during a severe discharge according to the reascheme (33, 34)

Longer siloxane chains+H2O

Thermal depolymerization−−−−−−−−−−−−−→ shorter siloxane chains.

Since these species could markedly change the hydrophrecovery behavior of PDMS elastomers, experiments wereried out to assess the humidity effect on the restructuringextracted PDMS elastomers after exposure to different levof discharge in the needle-to-plane configuration withelectrode gap of 0.4 mm. Contact angles were measuredexposure to varying discharges for one hour at a conshumidity of 15 or 50% (Fig. 5). When the samples are sujected to the low discharge range of 80–650 pC, the recovernot highly affected by humidity. However, at the high discharrange of 4000–10000 pC, the effect of humidity on the recovis significant, suggesting that air-borne water acceleratesformation of LMW species during discharge. For compariso

FIG. 5. The effect of humidity on the hydrophobic recovery of extractesamples after exposure to different discharge intensities. Exposure time isand the gap between the sample and the needle tip is 0.4 mm. These rshow that humidity affects the hydrophobic recovery more significantly un

severe discharge conditions: (r) 400–650 pC, 15%; (j) 4000–6000 pC, 15%;(e) 80–150 pC, 50%; and (h) 4000–10000 pC, 50%.

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the samples discharged at 15% humidity recover their hydropbicity very slowly compared to those discharged at 50% humdity. It is well known (5, 6) that severe discharge under dry coditions results in the rapid formation of an inorganic silica-liklayer on the outermost surface that could act as a barrierthe migration of LMW species trapped beneath the layWater does not have enough time to penetrate through the rapformed inorganic silica layer in this dry condition, which maeventually limit the production of LMW species compared twhat happens under humid conditions. In spite of little moiture in air, LMW species may also be produced beneathlayer in the absence of oxygen after the thin, brittle, inorgansilica-like layer develops on the outermost surface by oxidatcross-linking. Many studies have demonstrated that the therdegradation of PDMS under vacuum or inert atmosphere caua mixture of cyclic oligomers (35–40). As Hillborg and Geddstate in their recent review (41), it is generally believed ththe aging of silicone elastomers in oxygen causes both oxitive cross-linking and chain scission, whereas oxygen-free agleads to chain scission. Therefore, when silicone elastomerssubject to electrical discharges in dry conditions, both oxidaticrosslinking and chain scission take place simultaneously,the formation of the cross-linked layer is favored during a sevedischarge. LMW species may be producedin situ beneath thelayer by which oxygen is excluded but to a lesser extent ththose produced by discharge in humid conditions. The humity effect is more pronounced as the discharge becomes msevere.

4. Effect of Water Condensation

In another experiment, moisture was allowed to condensea sample by circulating cooling water (0◦C) into the aluminumblock, which produces numerous tiny water drops on the sface. The extracted sample shows a contact angle of 23◦ im-mediately after exposure to 80–150 pC of discharge for 1 h at50% (without condensation), but it exhibits a contact angle62◦ immediately after exposure to 100–300 pC for 1 h undcondensation conditions (Fig. 6). At the low discharges, the etracted sample with water condensation also recovers faster tthat without condensation. The water drops condensing onsurface are believed to accelerate the formation of LMW speceven at a mild discharge by a depolymerization reaction wwater. Subsequently the migration of thein situproduced LMWspecies through the inorganic silica-like layer, which is poland is easily cracked by internal and external stresses, is tresponsible for the faster recovery. As the intensity of partdischarge increases, such a striking effect of water condention is not observed, but the sample with water condensatexhibits a higher initial contact angle (20◦) than that (spreading)of the sample without water condensation, subsequently shing a slightly fast recovery. When the samples are exposedsevere discharges at an environmental humidity of 50%, the

polymerization reaction probably takes place actively regardlessof water condensation.

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FIG. 6. The effect of water condensation on hydrophobic recovery oftracted samples after 1 h of discharge. Point-to-plane gap is 0.4 mm. Numtiny water drops formed on the surface during cooling water circulation, wsignificantly affect hydrophobic recovery at a low discharge: (r) 100–300 pC,with water condensation; (j) 1000–4000 pC, with water condensation; (e) 80–150 pC, without water condensation; and (h) 4000–10000 pC, without watecondensation.

5. Hydrophobic Recovery Mechanism

The hydrophobic recovery of silicone elastomers has lbeen studied, but there is still no general agreement due, into the variety of experimental conditions and discharge southat have been used. The migration of preexiting fluid andreorientation of the oxidized side group are believed to bemain mechanisms of hydrophobic recovery. Recently (22)showed that when samples are exposed to severe dischargditions with discharges of 1500–12000 pC and corresponapplied voltages of 6–12 kV in the cylinder-to-plane confiration, the recovery process is dominated by the migratioin situ produced species. Preexisting fluid does not markaffect recovery. Angle-resolved XPS results also supportsame mechanism by showing a higher concentration of undized carbon and silica at 15◦ take-off angle than measurementaken at 90◦ take-off angle (Table 1). This XPS analysis wperformed about 2 h after the extracted sample was expoto 6000–10000 pC for 1 h in thepoint-to-plane configuratiowith a gap of 0.4 mm and 50% humidity. Table 2 also shoatomic compositions of an extracted sample after exposupartial electrical discharges of intensity 10–20 pC for 94.5 hhumidity of 50%. The concentration of unoxidized carbon asilica increases with decreasing take-off angle, implyingsmall amounts ofin situproduced LMW species migrate to thsurface during and after discharge. These results may indthat the extracted samples are governed by the same mnism (migration ofin situ produced LMW species); howeve

the two samples exhibit different hydrophobicity loss andcovery behaviors depending upon the discharge intensity.

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after Exposure to 6000–10000 pC for 1 h at 50% Humidity in thePoint-to-Plane Configuration with an Electrode Gap of 0.4 mma

Take-off angle

Element 15◦(%) 90◦(%)

C(unoxidized) 9.0 2.8C(oxidized) 2.0 0.7Si(unoxidized) 4.5 1.0Si(oxidized) 27.0 32.1O 57.5 63.4

a The atomic compositions of the unoxidized sample are as follows(45.3%), Si (30.0%), and O (24.7%).

extracted sample exhibits spreading of water immediately aexposure to 6000–10000 pC for 1 h, whereas a contact angabout 50◦ is observed on samples immediately after exposto 10–20 pC for 94.5 h. The discharges in the range 10–20are not intense enough to make the extracted sample totallydrophilic. Here, thein situ produced LMW species, most probably, contribute to the high initial contact angle of the extracsample. Under severe discharge conditions, the extracted saloses and regains its hydrophobicity rather fast. The extracsample subject to low discharges (<200 pC) shows a slowerecovery than that exposed to severe discharges (>1500 pC).We believe that the amount of LMW species produced durdischarge increases with the discharge intensity. Therefore,lower discharge, the higher concentration of unoxidized carand silicon at the outmost surface is likely due to the migtion of in situ produced LMW species and the mild oxidatioof methyl groups during discharge; however, the hydrophorecovery is slow due to the limited production of LMW specieAt a higher discharge level, even though initial atomic compsitions of unoxidized carbon and silicon are lower than thoexposed to a low discharge, the samples show much fastedrophobic recovery. This could be due to the productionlarge amounts of LMW species at high discharge intensitwhereby the flux to the surface is enhanced. ConsequentlyLMW species should play an important role in the restructur

TABLE 2Surface Atomic Compositions of an Extracted PDMS Elastomer

after Exposure to 10–20 pC for 94.5 h at 50% Humidity in the Point-to-Plane Configuration with an Electrode Gap of 0.05 mm

Take-off angle

Element 15◦(%) 30◦(%) 90◦(%)

C(unoxidized) 41.9 33.6 22.7C(oxidized) 1.6 1.7 1.3Si(unoxidized) 17.6 13.5 0.9Si(oxidized) 10.8 16.0 9.0

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FIG. 7. Key mechanisms underlying hydrophobic recovery of silicone etomers after partial discharge. Silicone elastomers exhibit different restructubehaviors depending on the presence of preexisting LMW fluid and dischintensity. At low discharge, preexisting fluid considerably contributes todrophobic recovery, but at severe dischargein situcreated LMW species becommore important.

of high-voltage silicone elastomers during service. Onother hand, especially at low discharges (<200 pC), preexist-ing fluid significantly contributes to hydrophobic recovery afdischarge, suggesting that the dominant mechanism in thisis the migration of preexisting fluid from the bulk to the suface (Fig. 7). To address the issue of surface migration, weperformed XPS analysis of a silicone elastomer in which aorinated siloxane (3,3,3-trifluoropropylmethyl siloxane, Mw=950) was added at 5% by weight. Before discharge, the surconcentration of fluorine observed at both 15◦ and 90◦ take-offangles is∼0.5%, which indicates that most of the fluorine moeties are present in the bulk. However, after a 30-min expoto a discharge of 7000–12000 pC, the fluorine concentratiothe surface of the elastomer increases markedly to 21% regless of the sample depth probed with XPS (i.e., independentake-off angle). The dramatic increase in fluorine concentraoffers clear evidence that the hydrophobic recovery is totdominated by the diffusion of fluid from the bulk to the surfacThe fluorine-containing species that diffuse to the surface mbe the ones producedin situ by a chemical depolymerizatio

FIG. 8. The effect of preexisting fluid on the hydrophobic recovery of eltrically discharged PDMS elastomers. Discharge experiments were perfoat 50% humidity in the point-to-plane configuration. Exposure time is 1

and the gap is 0.4 mm: (r) 5% fluorinated siloxane fluid (Mw = 950), 7000–11000 pC; and (j) 5% dimethlysiloxane fluid (Mw = 950), 7000–10000 pC.

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of both pendent groups and chain backbones during electridischarge. Consistent with the XPS results, the samples ctaining the fluorinated siloxane fluid (Mw= 950) also exhibit amuch faster hydrophobic recovery than either the extracted saples or those containing free methyl-terminated silicone flu(Mw=950) (Fig. 8). Six hours after exposure to 4000–11000 pfor 1 h, the fluorine-containing sample recovers 88% of its oriinal contact angle, whereas both the methyl-containing andextracted sample show only 45% recovery. This result shothat the fluorine-containing fluid segregates faster to the surfacompared to a methyl-containing fluid. Likely, the fast recoveof the fluorine-containing samples results from the diffusionspecies produced by the fragmentation of the fluorinated siloane rather than the fluid itself. This is, however, the subjectfuture studies.

IV. SUMMARY

The partial discharge experiments with a point-to-plane eletrode configuration show that hydrophobic recovery dependssuch factors as the discharge intensity, preexisting fluid, andvironmental humidity. When the PDMS elastomers are exposto mild discharges, the silicone fluid preexisting in the elastomplays an important role in the hydrophobic recovery of the oxdized elastomers. However its effect is less significant thanmigration of thein situ produced LMW species at severe discharge levels. As the discharge becomes severe, the dominmechanism of hydrophobic recovery is the migration of theinsitu produced LMW species from the bulk to the surface of thelastomer. Additionally, moisture in air promotes the prodution of LMW species under severe discharge conditions, whiconsequently leads to a faster hydrophobic recovery.

ACKNOWLEDGMENT

The authors gratefully acknowledge financial support by Dow Corning Co

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