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Tracking an d erosio n resistance stabi l i ty of h igh ly

f i l led s i l icone and alloy m ater ials against e lect r ical

and enviro nm ental s t resses

S

Kumagai and

N.

Yoshimura

Abstraft: Tracking and erosion resistances of highly filled silicone rubbers (SIRs) and of polymer

alloys made from SIR and ethylene vinyl acetate copolymer (EVA) were evaluated after being aged

by the stress of acid rain, W , corona or water absorption. It was demonstrated whether or not

high-level fillers could sufficiently protect tracking and erosion resistances against ageing. Acid rain

dissolved alumina trihydrate filler at the surface layers of various polymers tested, thereby reducing

the tracking and erosion resistances of those polymers. UV and corona stresses affected the basic

polymers rather than the fillers, decreasing the polymers' resistances to tracking and erosion, while

highly filled SIR and alloys maintained their tracking and erosion resistances after absorbing large

amounts of water.

1

Introduction

Nonceramic (composite) insulators are begnning to be used

as alternatives to ceramic and glass insulators in many

power lines. Nonceramic insulators made

of

polymeric

housing materials and fibre-reinforced glass cores offer

several advantages: they are lightweight, easy to handle,

perform better when contaminated, resist vandalism and,

for all those reasons, reduce costs. The types of housing

materials that strongly affect these nonceramic insulators'

electrical performance and longevity should he determined

with care. Silicone rubbers (SIRs)perform, in general, much

better electrically in wet and contaminated conditions than

do polyolefins such as ethylene propylene diene terpolymers

(EPDMs) and ethylene vinyl acetate copolymers (EVAs).

On the other hand, polyolefins are superior

to

SIRs in

material productivity, cost and several mechanical proper-

ties

[I]. SIRs

and polyolefins can compensate for each

other's shortcomings when they are combined into polymer

alloys. Such alloys are useful in lightly and moderately

contaminated conditions. Highly filled polymers have k e n

in outdoor use worldwide to prevent accidents resulting

from tracking and erosion and to reduce material cost

[ 2 4 ] .

The authors have shown that the tracking and erosion

resistance

of

unfilled

SIR

can decrease by exposure to

electrical or environmental stresses [5]. However, although

the addition of fillers to SIRs may enhance tracking and

erosion resistance to ageing by electrical or environmental

stresses, such enhancements have not been examined. The

role of ambient stresses on the tracking and erosion

resistances of

SIRs

and alloys that are highly filled, and

which are therefore expected to perfom sufficiently, should

he understood well so that the reliability of housings for

nonceramic insulators can he improved. This study targets

O I E E ,

2U3

I proceeding.^

online

no

0330502

Publication dale: 2ist Mag 2003. Paper first received

10th

July 2032

The

authors

are with

th Dcpaltment of Elecitical and Electronic Engineering.

Akita University. 1-1

Tegatagiiken-machi.

Akita 010-8502, Japan

392

doi: n. 049/ipgld:20030502

practical materials whose properties for outdoor applica-

tions are sufficiently improved by certain methods, such as

the addition of filler. In this study, these materials are

subjected to acid rain, W corona and water absorption

stresses, after each of which their tracking and

erosion

resistances are evaluated.

2

Experimental methods

Several types of high temperature vulcanising silicone

rubbers (HTV-SIRS) and alloys made by combining

HTV-SIR and EVA are prepared for the tests. Each

sample is described in Table 1. The procedures for applying

the stresses are described here. Referring

to

the survey of the

Environment Agency of Japan, synthetic acid rain is

prepared. The acidity

of

the synthetic acid rain: whose

ingredients are shown in Table

2,

is pH 1.9, while actual

rain in Japan averages pH 4.8 (Honshu; Japan, 1989-1992).

The acid concentration of the acid rain used in this test is

about 500 times that of actual rain in Japan. The acid rain

stress is applied

at

room temperature by statically immer-

sing the samples in the synthetic rain. To suppress the action

of absorbed water and take only the chemical effects of acid

rain into account, the samples are completely desiccated at

23k2 C for more than

30

days. U V stress

is

applied by

using a xenon short arc lamp (Ushio, UXL-500D). It is well

known that the spectra of xenon lamps approximate that of

actual sunlight. Fig. 1shows the UV radiation system and

the radiation spectrum. The power of the xenon lamp used

is

500W, and the temperature at the sample surface is

60C.

Corona stress is produced by the parallel-plane electrode

method shown in Fig. 2.The corona discharge attacks the

whole surface area of samples exposed between the glasses.

The electrical field applied is A C 10kV/cm. The concentra-

tion of evolved ozone was measured to be about 20ppm by

a detecting tube (GASTEC, ozone). The temperature at the

sample surface during the corona treatment is maintained at

3G35 C. Water absorption stress

is

applied by immersing

the samples into boiling distilled water (0.5 ~ S / c mt 23C);

boiling water is used because it increases absorbance and

accelerates it considerably as compared with lukewarm

I E E P w : G m e r .

Tmusm D h d i l ~ . .

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Table 1: Descriptions or sample materials

Sample Supplier Basic polymer

Curing system Filler and level Average filler particle

diameter.

om

~

H N 40 A (Japan) HN-SIR'

peroxide curing

A T H ~

0

wt

-1

H l V 650

A (Japan) HN-SIR C

peroxide curing ATH 50

wl

-1

MSR A50

B (Japan)

EVAd: HN-S IR=9 :1

(in

weight) peroxide curing

ATH 50%

wt

-1

MSR C40

C (Japan)

EVA HN -S IR =l :l (in weight) peroxide curing

ATH 40 wt - 1

Not es aHigh emperature vulcanising silicone rubber. 'alumina trihydrate. 'having

a

different formulation o f the basic polymer of H N 40,

dethylene vinyl acetate copoly mer

Table2: Ingredients of synthetic acid rain

Ingredient Concentration,

dl

NH&I

0.50

NaCl

1.23

KCI 0.09

HN03 0.45

MgSOi 0.53

CaSO,. 2 H Z 0 0.45

starter

XS-501

OPAA-A

power supply

for

lg lIl0

XB-5010tAA-A

0.011'

, ' I

' ' I

* ' I

, z ' , ~ , i ' . , L i J

200

300

400

500 6 7

8

wavelength, nm

b

Fig. 1

a

Arrangement of

W radiation equipment

h 7 he radiation spectrum of

the

xenon lamp used

E.xperimenlu/ detaik

of

UV

radiurivn

.system

water. The amount of absorbed water is represented by

water absorbance A (x),which is calculated using the

following equation.

/

\

sample HV copper electrod

-

Fig. 2 Airon:lemenr of corona generation

equ@nent

where

W

and Wire the weight of the sample that absorbs

water and the initial weight of the sample, respectively.

After the artificial ageing treatments are applied, the

tracking and erosion resistances

of

the samples are

evaluated by using the IEC

587

inclined-plane(IP) test.

The definitions of the material failure and the tracking and

erosion resistance quantifications are described in detail in

[ 5 ] ; a brief description is presented here. During the IP test,

the material surfaces are continuously wetted by a nonionic

wetting agent added to the contaminant electrolyte. The use

of a nonionic wetting agent can verify a material's original

resistance to tracking and erosion, except for the hydro-

phobic effect. This test well controls the discharges from

leakage current, thereby shortening the test time and

increasing reproducibility. The experimental circuit, the

sample arrangement, and discharges and the tracking

produced'during the test are presented in Fig. 3. Each

sample slab (120 x 50 x

5 M 0mm3

has an inclination of

45 , and the contaminant electrolyte flows

from

the top to

the bottom electrode. Deionised water containing 0.1

w i

ammonium chloride and.

0.02

wt nonionic wetting agent

(ToritonTM

X-100,

Wako Chemicals) is used as a

contaminant. The water's conductivity is

2400

pS/m at

2 3 T , and the applied voltage is

AC 4.5

kV. The resistance

of the series resistor and the flow rate of solution are fixed

at

33k

ohm and 0.6ml/min, respectively. The distance

between the electrodes, which are stainless steel, is 50

The effective current value varies from

5 to I 5 mA

when AC

4.5

kV

is applied. The resistances to tracking and erosion

are quantified by time to failure. Failure is defined as either:

(a) a track lengthens to at least

25

mm or (b) erosion breaks

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S

=

power supply Switch

VT = variable ratio

t ransformel

T =

high-voltage transformer

R

=

series resister

V = voitmetei

Sp

= specimen

F =

overcurrent device

a

track is initiating

b

Fig. 3

U Circuit diagram of IP test

h Sample arran_rment

and

tracking produced d u n n g test

through the thickness of the sample

[5,

61. The publication

IEC 587 suggests that the test be stopped at 6 h [7]. It is very

inconvenient if the IP test is not stopped at that prescribed

time because if highly tracking-resistant materials are tested,

their time to failure may become extremely long and huge

dispersions may appear. To evaluate both highly tracking-

resistant materials and others (i.e. aged materials), the test is

stopped at 6 h. The value of expected time to failure (ETF),

which is obtained by dividing the average time to failure of

the samples that failed within 6h by the probability that

samples failed within 6 h, is used to deal with the tracking

and erosion resistances

of

both the materials that allowcd

failures within 6 h and those that did not. A detailed

description of ETF is presented along with the 1P test

results. The ETF equation can treat any number of test

samples.

E.tperbiienrii1 deruils

u I P

trucking rind erosioi i re51

3 Results

and discussion

3 7

rain immersion

After the samples of HTV A40 and

MSR

AS0 have been

immersed in synthetic acid rain and then completely

desiccated in air at room temperature. they are subjected

to the IP tracking and erosion test. Fig.

4

shows the time to

failure and calculated ETF for HTV A40 and for

MSR

A50

as a function of acid rain immersion time. In the figures for

IP test results, the squares represent the times to failure of

the failed samples. A sample that did not fail: a sample

survived both the development of a 25-mm-long conductive

path and the progression of erosion completely through the

thickness,

is

marked with a circle at 360min. Failure is

defined

as

either

a

track

I O )

or erosion (01) failure.

here indicated in Boolean terms as either true= ( , or

false=

O ) . .

In this work, samples often failed in a

394

racking and erosion resistance after acid

0

time to failure

0

no

ailure within 360 min

A

FTF

1000

36

c

.-

E

6

100

E

-

-

I

I

I

0 30 60 90

immersion time. day

a

.-

E

.-

-

0

30

60 90

immersion time, day

b

Fig.

4

hfluence

ofacid ruin on friickbigvndcrosioii resisrmices of

higlr vJilled HTV-SIR malpo vmer uUoy ma& from HTV SIR

unci

EVA.

The ir re.risrunces

10

rrurking

und ero.?ionwere eualuured ufler

being imrnrrsedin ucid ruin

and

completely rlesicmted or

more

rhun

30 d u p . The concentmion of acid ruiii used

ahour

500

rhar

of

ucriml 1-00 in J q u n

combination of modes, in which case ( 1) is used. In the

figures for

IP

test results, a square

is

shown for each case of

either (01) or I O ) . No cases

of ( 1 1 )

are observed,

because the test is stopped once a

single

failure occurs. Case

< O O ) is marked as a circle. ETF is thus defined as the ratio

of the average failure time of the samples (shown

by

squares) to the fraction of failed samples. For example,

when a total of

3

samples failed at 100, 150 and 200 min and

two samples survived the test, the ETF (min) is

I*)+

1 5 0 t Z L n

ETF = = 250

3

5

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In figures for IP test results. ETF is indicated by a filled

triangle. It is found from the ETF that acid rain immersion

reduces the tracking or erosion resistance of MSR A50 hut

not of HTV AM. Attenuated total reflection-Fourier

transform infra-red spectroscopy (ATR-FTIR) was em-

ployed to demonstrate chain scissions and recombinations

of

the basic polymer of HTV

A40

and that of

MSR

A50.

However, at either surface_no significant changes resulting

from the immersion into synthetic acid rain were observed.

In acidic solution, alumina trihydrate (ATH) filler can he

dissolved through the following chemical reaction.

AI(OH), + 3H'

-

I3+ + 3Hz0

(3)

The dissolution amount of ATH in synthetic acid rain

increases with time and, finally, the saturation amount

appears: showing approximately the following function,

which is not theoretically verified but is empirical and

apparent [SI.

Q = D &

(4)

where

Q,

and

f

are the dissolution amount of ATH, a

variable depending on the content of ATH in synthetic acid

rain and time. respectively. ncreases with the content of

ATH in synthetic acid rain, but not in direct proportion.

ATH existing near the surface and thus being exposed to

acidic compounds is dissolved prior to the dissolution in the

hulk. X-ray diffractometry (XRD), whose general purpose

is to analyse the crystal structure of objective materials, is

used to verify the dissolution of ATH near the surfaces of

HTV A40 and

MSR

ASO. Energy-dispersive X-ray analysis

(EDX) or other techniques capable of detecting the

composition of AI at the material surfaces are also available.

However, these cannot identify whether or not detected AI

truly exists as ATH, because several states of AI are possible

in strongly acidic conditions. The concentration of crystal-

line substance is proportional to the intensity of the

diffracted X-rays

[9].

A Rigaku RAD-1A is used for

XRD analysis of 20 15 x mm3 sections cut from the

surfaces

of

both the unaged samples and the samples

immersed for 90 days. The depth o f analysis is about I-

IOpm. The XRD patterns of ATH filler at the surface of

MSR A50 before and after the immersion for 90 days are

shown in Fig. 5 .

All

the peaks correspond to the ghbsite

structure ATH [lo]. The immersed MSR AS0 samples show

decreased intensities of X-rays diffracted at the surface,

c

....

500 cps

1

immersed

or

90 days

indicating that the synthetic acid rain dissolves

ATH

in the

basic polymer of MSR

A50.

The quantity of ATH at

the surface is reduced to about 50%. ATH inorganic filler

at the surhce can be dissolved, thereby reducing MSR

0 time to failure

no failure within

360

min

+

.)

T . . _ . . :I-

000

. . . . .

y ..

360

t o

0 250 500

UV radiation time, hour

.-_ .. ..

UV

radiation ime, hour

1000

360

c

.-

E

E

< 100

.-

-

Uv radiation time, hour

Fig.

6

Dependence o t m ck i n y and

w mi o n

resistancm of

liiqhly

filled

HTV-SIR\

andpolymer a1loy.s made of

IITV-SIR

and EVA

on

UV radiarion

time

SW

W xenon lamp was wed as U V

source

95

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A50's ability to resist tracking and erosion. On the other

hand, the intensity difference between the unagcd and the

aged HTV A40 was observed to he minor, maintaining the

resistance to tracking and erosion even after exposure to

acidic conditions.

The minor dissolution of ATH at the

surface

or

HTV A40 suggests that the solubility of ATH

depends

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groups finally form carboxyl 0= C-OH) groups. Ester

linkages in EVA are also subject to hydrolysis, forming

carboxyl groups

(151.

In this case, water is a hyproduct of

autoxidation

1151.

Simultaneously. free radicals appearing in

autoxidation would cross link and branch polymer chains,

thereby producing tertiary carbon atoms and partial chars

stripped of hydrogen. After exposure to U V or corona

stress, EVA contains many carboxyl groups. as stated in

[ I I ,

121. When subjected to dry-band arcing, carbon atoms

in carboxyl groups leave the polymer as volatile species,

such as CO and COz 15]. Other carbon atoms- particularly

tertiary carbon and partial char, would convert to

carbonaceous tracks. very readily. because the previous

autoxidation had reduced the overall heat (activation)

energy available to be spent on dehydrogenation and on the

formations of volatile species. Hence, both U\' and corona

stresses seriously decrease the tracking and erosion

resistances of MSR

A50

and MSR C40 including EVA.

3.3

water absorption

After absorbing water for

I20

or 240 h, HTV A40 and

MSR A50 are subjected to the IEC 587 IP test. Table

3

shows the water absorbance of each material at 120 and at

240h. Fig.

8

shows the tracking and erosion resistances of

racking and erosion resistance during

Table3: Water absorbance against water absorption time

for H N 40 and

MSR A50

Water absorption time lh) Water absorbance %

HlV A40 MSR A50

0

120

240

0 0

1.2 0 5

1.5 0.7

HTV A40 and MSR A50 with absorbed water.

No

tracking

or erosion was observed during the test. In [ 5 ] t was shown

that the absorption

of

water dccreases the tracking and

erosion resistances of RTV-SIR. For RTV-SIR, the

expansion force of water while boiling promoted the

progression o f erosion. This suggests that the filler enhances

mechanical strength and that the proper choice of basic

polymer formulation can stabilise the material against

boiling of absorbed water, thus preventing water-induced

erosion.

4 Conclusions

The tracking and erosion resistances of

SIRS

and

of

alloys

made from SIR and EVA are assessed after aging by each

of several electrical and environmental stresses. The

mechanisms underlying the changes in tracking.and erosion

resistances are also discussed. Acid rain dissolves ATH filler

at the material surface, and this dissolution can reduce

tracking and erosion resistance. The soluhilily of ATH at

the surfaces of materials seems to depend on the properties

of the formulations of basic polymers and on the surface

treatment of the ATH filler. It is shown that U V as well as

corona stress can reduce the tracking and erosion

resistances of SIRS and of the alloys made from SIR and

EVA. In

SIRS,

reductions in tracking and erosion resistance

can he caused by the formation of byproducts that burn at

low temperatures: for EVA, resistatice can he reduced by

the formation of tertiary carbons and partial chars stripped

f Pro

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3

4

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398

References

Zhao, T. , and

&mstorf.

R.A.: A g h g test of polymeric housing

inaterials

for

nonceramic insulators'. IEEE

El