temperature and hydrophobicity of silicon rubber

7/29/2019 Temperature and Hydrophobicity of Silicon Rubber

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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 1, February 2013

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TEMPERATURE AND HYDROPHOBICITY

OF SILICON RUBBER

1Syed Aamir Raza Naqvi,

2Syed Kashif Imdad

1Water and power development authority [email protected]

2Department of Electrical Engineering, HITEC University Taxila, Pakistan

[email protected]

ABSTRACT

Among new insulating materials used in high voltage power systems, silicon rubber materials are

extremely important. During last 20 years, these materials have been applied for manufacturing outdoor

insulators. Silicon rubber is organic material. It decomposes under different environmental conditions. Its

characteristics change with time, the most important among these characteristics is hydrophobicity.

Hydrophobicity is the formation of water beads on the surface of insulator, which resists the flow of water

in the form of continuous tracks. Hydrophobicity is due to Low Weight Molecules in the bulk of silicon

rubber that skim up to the surface due to difference in diffusion density. With the passage of time,

hydrophobicity reduces and then recovers back. In other words, hydrophobicity goes through cyclic

changes. Temperature slows down the reduction rate of hydrophobicity and increases recovery rate of

hydrophobicity. This phenomenon is experimentally investigated in this research.

KEYWORDS

Polymeric insulators, Hydrophobicity, degradation, SEM, FTIR, STRI

1. INTRODUCTIONHydrophobicity of any material is its resistance to the flow of water on its surface, denying theformation of a continuous stream of water [1]. Hydrophobic nature splits the water layer in the

form of separate drops. Reduction in hydrophobicity for an electrical insulator increases surface

leakage current activity [4, 5, 8, 9]. This is a major silicon rubber insulation performance factor,

because it causes serious effects as [2,13]:

a) Reduction in electrical insulationb) Influence on the aging processIn this research paper, the effects of temperature on hydrophobicity are experimentally

investigated. Temperature has great influence on hydrophobicity [11]. It is thought by many, that

atmospheric temperature depolymerises long chains of silicon rubber. In fact depolymerisationcannot take place at atmospheric temperature [12]. Temperature only speeds up the transfer of


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low molecular weight components from the inner bulk to the surface. The main reason forhydrophobicity transfer is the diffusion of low molecular weight components (LMW) from the

inner bulk to the surface [18]. This increases the rate of rise of hydrophobicity. Depolymerisation

of silicon rubber molecules occurs by hydrolysis only [17].

The hydrophobicity is measured by:

a) Leakage Current Methodb) STRI Classificationc) Fourier Transform Infrared Spectroscopya) Leakage Current:The Leakage current is directly proportional to the hydrophobicity loss [4, 5, 8, 9]. That is why

the leakage current in this test was measured after every 15 days to show the effects oftemperature on hydrophobicity. For this purpose leakage current was measured using a high

precision digital multimeter. The leakage current is shown in the table-2. For this purpose, rubberpieces were kept energized by a 10kv single phase transformer on one side, whereas the other

ends of rubber pieces were kept grounded. Each piece was kept in its individual wooden box.

Leakage current values were taken after every 15-days. The leakage current reading is the current

going over the surface towards ground.

b) STRI Classification:It is a simple procedure for manually obtaining a collective measure of the hydrophobicity

properties of insulator. For practical purpose, the degree of water repellency of an insulator

surface may be divided into seven hydrophobicity classes according to STRI classification guide[3, 14]. HC-1 is the most hydrophobic class and HC-7 is the most hydrophilic class. Manualmethod is to keep the insulator at 35 degrees from horizontal and spray water on the surface.

Leave insulator for 45 seconds and then take snaps of insulator and compare them with the STRI

guide to assign a hydrophobicity class between HC-1 to HC-7 [4, 6, 7].

c) Fourier Transform Infrared Spectroscopy:It is a material analysis technique, which provides us structural information and compound

identification. It is used for quantitative measurement as well. Mostly it is used to identify organic

compounds but in some cases inorganic compounds are also identified. In this technique, the

sample under test is exposed to infrared radiations. The sample absorbs those frequencies that

match with the vibrations of its atoms. A dip is obtained at these frequencies in infrared spectrum.The infrared spectrum is then matched with standard curves stored in computerized reference

library to identify the degradation in the material. The FTIR curves are called transmittance

curves between transmittance in % and wave number in 1/cm.


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2. Experimental SetupFive wooden boxes of size 2X2X2 feet were taken. A 100 watt bulb was fitted inside every box

and temperature was controlled using thermostat. The temperature maintained in different boxes

is tabulated in table-1. The size of silicon rubber piece suspended in the boxes was 6.5X1.2 sq.

inch. The material of samples was High Temperature Vulcanized Silicon Rubber with Alumina

tri-hydrate and Silica. The experiment continued for 5000 hrs as per IEC criterion (from 16/02/09

to 12/09/09). The samples were energized at 10kv using a single phase transformer. The setup is

shown in fig-1. The leakage current was measured by placing 100 resistor placed in series with

every sample, using high precision digital multimeter. The record of leakage current of different

samples is given in table-2.

Box No 1 2 3 4 5

TemperatureC

30 40 45 50 55

At 30C, fig-2 At 40

C, fig-3

The graphs between leakage current in micro amperes and time in days are:

At 45C, fig-4 At 50

C, fig-5

0

5

10

15

0thd

ay

30thd

ay

60thd

ay

90thd

ay

120thd

ay

150thd

ay

190thd

ay

209thd

ay

0

5

10

15

0thd

ay

30thd

ay

60thd

ay

90thd

ay

120thd

ay

150thd

ay

190thd

ay

209thd

ay

0

5

10

15

0thd

ay

30thd

ay

60thd

ay

90thd

ay

120thd

ay

150thd

ay

190thd

ay

209thd

ay

02468

10

0thd

ay

30thd

ay

60thd

ay

90thd

ay

120thd

ay

150thd

ay

190thd

ay

209thd

ay


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fig-1

3. Leakage Current AnalysisThe leakage current readings are

At

On 12/09, fig-10

( Amps)

Temperature ( C )

30 40 45 50 55 Date

6.5 6.4 6.4 6.6 6.6 16/02/099.5 9.0 9.1 9.0 8.9 02/03

10.5 9.8 9.6 9.4 9.4 17/03

10.8 9.9 9.7 9.4 9.3 31/03

9.6 9.2 9.0 8.9 8.9 15/04

9.2 9.0 8.9 8.8 8.6 30/04

8.5 8.3 8.2 8.0 7.7 15/05

8.2 7.8 8.0 7.4 7.0 30/05

7.5 7.0 7.4 6.9 6.7 14/06

7.1 6.6 6.3 6.7 6.4 29/06

6.9 6.6 6.4 6.6 6.4 14/07

6.5 6.4 6.3 6.5 6.5 29/07

6.6 6.5 6.4 6.6 6.5 13/08

7.0 6.9 6.9 6.8 6.6 28/087.2 7.0 7.1 6.9 6.8 12/09/09


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55C, fig-6

Also the graphs between temperature in centigrade and leakage current in micro amps for different samples

are shown below as:

On 16/02, fig-7 On 31/03, fig-8

On 14/07, fig-9 On 12/09, fig-10

To analyze the data, method of curve fitting and regression analysis is used. A regression

parabola is fitted to show the trend of variation of leakage current with time. It is interesting to

note that hydrophobicity cyclically changes with time. If the number of readings is odd as in ourcase then take the middle reading as a starting point and assign to it a zero. Then assign the

number 1,2,3,4,5, to the succeeding readings, and -1,-2,-3,-4,.... to the preceding values so that

the sum of the series x is zero, where x = ..,-3,-2,-1,0,1,2,3,4,5,.. This shortens the

method. The data required to derive mathematical relationship is given in the table-3.

6.6

6.7

6.8

6.9

7

7.1

7.2

7.3

30 40 45 50 55

6.2

6.4

6.6

6.8

30 40 45 50 55

8

9

10

11

30 40 45 50 55

6

6.2

6.4

6.6

6.87

30 40 45 50 55

6.6

6.8

7

7.2

7.4

30 40 45 50 55


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The normal equations are:

y = na + bx + cx2

where n is a positive integer

xy = ax + bx + cx

xy = ax + bx + cx4

From above tables, we have:

x = 0, y = 121.6, xy = -60.1, x = 280, xy =

2199.4, x4 = 9352

x = 0

Putting these results in normal equations we have:

y = na + cx (1)

xy = bx (2)

xy = ax + cx4 (3)

And finally we have:

121.6 = 15a + 280c (4)

-61.1 = 280b (5)

2199.4 = 280a + 9352c (6)

From equation 5, we have:

b = -61.1/280 = -0.21

Simultaneously solving equation 4 and 6, we have:

c = -0.01764

a = 8.44

The parabolic equation between leakage current and time is:

y = a + bx + cx

y = 8.44 0.21x 0.017x at 30 CParabolic equations at remaining temperatures are:

y = 8.02 0.21x 0.014x2 at 40 C


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y = 8.06 0.18x 0.019x2 at 45 C y = 7.84 0.17x 0.010x2 at 50 C y = 7.57 0.18x 0.004x2 at 55 C

Similarly, the parabolic relation between leakage current and temperature was established as

follows: We use the data of 16/02, which is:

Temperature (C): 30 40 45 50 55

L/ current: 6.5 6.4 6.4 6.6 6.6

Origin

The parabolic expression is:

y = a + bx + cx

Assuming the middle temperature as origin we have:

The normal equations are:

y = na + bx + cx (1)

xy = ax + b x + c x (2)

xy = a x + b x + c x4

(3)

From the above table, we have:

x = 0, y = 32.5, xy = 0.4, x = 10, xy = 65.4, x4

= 34

From above results equations (1), (2) and (3) become:

y = na + cx

32.5 = 5a + 10c (4)

xy = b x

0.4 = 10b (5)


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Table-2

xy = a x + b x + c x4

65.4 = 10a + 34c (6)From equation (5), we have:

b = 0.4/10 = 0.04

Solving equations (4) and (6) simultaneously, we have:

c = 0.0286

Date X Y XY X2

X2Y X

4

16/02/09 -7 6.5 -45.5 49 318.5 2401

02/03 -6 9.5 -57 36 342 1296

17/03 -5 10.5 -52.5 25 262.5 625

31/03 -4 10.8 -43.2 16 172.8 256

15/04 -3 9.6 -28.8 09 86.4 81

30/04 -2 9.2 -18.4 04 36.8 16

15/05 -1 8.5 -8.50 01 8.5 01

30/05 0 8.2 0 00 0 0

14/06 1 7.5 7.5 01 7.5 1

29/06 2 7.1 14.2 04 28.4 16

14/07 3 6.9 20.7 09 62.1 81

29/07 4 6.5 26 16 104 256

13/08 5 6.6 33 25 165 625

28/08 6 7.0 42 36 252 1296

12/09/09 7 7.2 50.4 49 352.8 2401


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Putting the value of c in equation (6), we have:

a = 6.4428The parabolic expression is:

y = a + bx + cx

Putting the values of a,b,c in above equation, we have a relationship between leakage current and

temperature:

y = 6.442 + 0.04x + 0.028x on 16/02/09Parabolic equations on remaining dates are:

y = 9.014 0.12x + 0.042x2 on 02/03/09 y = 9.940 0.26x 0.100x2 on 17/03/09 y = 10.03 0.35x 0.107x2 on 31/03/09 y = 8.990 0.17x + 0.064x2 on 15/04/09 y = 8.900 0.14x + 0.000x2 on 30/04/09 y = 8.182 0.19x 0.021 on 15/05/09 y = 7.794 0.28x 0.057x2 on 30/06/09 y = 7.142 0.17x 0.021x2 on 14/06/09 y = 6.448 0.13x + 0.086x2 on 29/06/09 y = 6.494 0.10x + 0.042x2 on 14/07/09 y = 6.368 + 0.01x + 0.040x2 on 29/07/09 y = 6.477 0.01x + 0.021x2 on 13/08/09 y = 6.768 0.78x + 0.036x2 on 28/08/09 y = 6.986 0.09x 0.007x2 on 12/09/09


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4. STRI Classification Hydrophobicity Analysis

Fresh plate, HC-1, fig-11 at 30 C, HC-1, on 16/02, fig-12 at 30 C, HC-2, on 31/03, fig-13

The hydrophobicity classification of all the samples on different dates is shown:

Table-3

at 30 C, HC-1, on 14/07, fig-14 at 40 C, HC-1, on 16/02, fig-15 at 40 C, HC-2 , on 31/03, fig-16

Temperature X Y XY X2

X2Y X

4

30 -2 6.5 -13 4 26 16

40 -1 6.4 -6.4 1 6.4 1

45 0 6.4 0 0 0 0

50 1 6.6 6.6 1 6.6 1

55 2 6.6 13.2 4 26.4 16


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at 40C, HC-1, on 14/07, fig-17 at 45 C, HC-1, on 16/02, fig-18 At 50 C, HC-1, on 16/02, fig-21

Table 4

5. Fourier Transform Infrared Spectroscopic Analysis:The samples were sent for FTIR analysis at the end of the experiment. The Fourier Transform

Infrared Spectroscopy results between transmittance in % and wave number in 1/cm for original

rubber sample is shown in figure-27. The initial readings were used as reference and the

degradation/restoration found is shown in table-7

Spectrum of Original Rubber Sample, fig-21

TemperatureC Absorption ratio Change %30 0.6111 1.972

40 0.6097 2.291

45 0.60317 3.3381

50 0.60162 3.682

55 0.5984 4.10


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0

2

4

6

30 40 45 50 55

Virgin = 0.624 (absorption ratio)

6. Comparison and Analysis:a) Leakage Current:As is mentioned above, that hydrophobic loss is directly proportionally to leakage current [4, 5, 8,9]. It is worth noting that leakage current was almost the same for all the fresh pieces of rubber

initially on 16/02/09. The leakage current increased in the first one and half month and showed

maximum increase on and around 31/03/09. Increase of leakage current, for the rubber piece at

minimum temperature (30C) is maximum and for the piece at maximum temperature (55C) is

minimum. This proves that temperature rise decreases the hydrophobicity

reduction rate. The main reasonof this is that, at high temperature, the mobility or vibration of

low weight molecules increases. This helps the LMW to rapidly diffuse through the bulk and

reach the surface in large quantity at rapid pace to not only increase hydrophobicity, but also

increase the rate of rise of hydrophobic nature. Temperature only stimulates mobility of LMW.

Temperature cannot depolymerise long chains of silicon-oxygen (siloxane) bonds present in SiR.

The breaking energy of silicon-oxygen bond is 445kj/mole [12]. This energy can be provided at

or above 350C without catalyst and at or above 150C with catalyst [12]. Such high temperaturesdo not exist in atmosphere. Therefore temperature cannot depolymerise/break long chains of SiR.

The diffused LMW cause the water droplets to take circular shape on the surface of SiR. The

circular shape of drops denies the continuous stream of water and consequently the leakage

current. The leakage current readings clearly show a cyclic behavior. Initially leakage current

increased and then decreased later on, until consequent recovery of silicon rubber back to its

original. This is the principal reason of the long life of silicon rubber insulators used in power

systems. The speed of recovery is directly proportional to the temperature. This shows that the

higher the temperature, the greater the transfer rate of LMW is and the speedier is the recovery of

hydrophobicity.

a) STRI Classification:Initially on 16/02(date of energization), all the rubber pieces were falling in HC-1 category as is

clear from pictures. But afterwards, LMW components started disappearing from the surface of

rubber and hydrophobicity started decreasing and maximum decrease was noted on and around

31/03 as is clear by the STRI pictures. Then afterwards, the hydrophobicity started increasing

again and reached HC-1 as shown by pictures taken on 14/07. Water sprayed on the surface of


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silicon rubber takes certain shape. This shape decides in which class the droplet pattern falls.

When the surface is highly hydrophobic, the water droplets do not spread on the surface. Insteadthey take small circular shape [15,16]. In this state, all the droplets form beads and deny

continuous stream of water. As the hydrophobicity of SiR reduces, the water droplets start

spreading on thesurface, taking elliptical shapes. When this happens, the droplets join each other andform a continuous stream of water. Under such conditions, hydrophobicity class of SiR falls.

a) Fourier Transform Infrared Spectroscope:FTIR transmittance curve analysis shows that the high the temperature, the more will be the

physical degradation. But in general the speed of degradation is quite slow. This physical

degradation is surface roughness, fissures and small cracks due to temperature/heat. This

deterioration of surface reduces hydrophobicity [1]. Pollution particles trap in these cracks and

mask hydrophobicity of SiR surface.

7. Conclusions:The above results clearly show:

1. That silicon rubber show cyclic behavior for its hydrophobicity. Its hydrophobicitydecreases with use but later on recovers back as shown by leakage current readings and STRI

classification.

2. Temperature increases the transfer rate of low weight components, as the high the

temperature is the more speedy will be the recovery of hydrophobicity.

3. FTIR transmittance curve analysis shows, that the high the temperature is, the more will

be the physical degradation i.e. surface roughness. But in general the speed of degradation is quite

slow. That is why, all SiR insulators preserve good hydrophobicity properties even after 15 to 20year of service.

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