PRÜFEN UND MESSEN

TESTING AND MEASURING

Rubber blend  mechanical  thermal 

electrical properties  environmental
Evaluation of SiR / EPDM Blends
conditions.
for High Voltage Insulators
Silicon rubber (SiR) and ethylene propy-
lene diene rubber (EPDM) were blended
in different ratios and their mechanical,
thermal, and electrical properties were
studied. The results showed that the
tensile strength and the elongation at Polymers are currently being used as a tance and lower thermal stability, but its
break increased by increasing of SiR construction material for the high volta- volume resistivity is higher than that of
content in the blends. Also the thermal ge outdoor insulators in distribution and silicone and it has excellent electrical in-
properties were found to be improved. transmission power systems. In the early sulating properties [13]. Thus a suitable
Whereas by increasing the percent of days, outdoor insulators were made of blend of (Q) and EPDM can give a product
EPDM the dielectric strength values of ceramic and glass materials. Recently, with generally superior properties to eit-
the blends increased and the tracking polymeric insulators were developed and her rubber.
time decreased. the improvements in design and manu- In present work our objective is to
In addition the effects of different envi- facturing have made them more and prepare a suitable (Q) / EPDM blends
ronmental conditions, (water, 5 and more attractive for utilities. Moreover that may lead to a compound with ap-
10 % NaCl and UV irradiation) on those they have better dielectric properties, propriate properties and lower cost that
properties were investigated. light weight and low costs when compa- can be used as high voltage insulator.
red to the porcelain or glass insulators.
When selecting the insulator, there are Experimental
Auswertung von SiR / EPDM choices that encompass both material Material
properties and design. There is an increa- Ethylene-propylene-diene rubber (EPDM,
Mischung als Hochspannung-
sing interest in power industry worldwi- EP96) with Mooney Viscosity (ML 1+4,
isolator de, to understand the degradation pro- 257 °F) was obtained from EGYPT AFRICA
Kautschukmischung  mechanische  cess and the performance characteristics S.A.E.
thermische  elektrische Eigenschaften of polymer insulating material in severe Silicone rubber (SiR) was supplied by EGY
 Umgebungverhälnisse pollution conditions [1, 2]. Gates International trading S.A.E.
Polymers, such as ethylene propylene Dicumyl peroxide 4-methoxy phenol and
Silicon-Kautschuk (SiR) und Ethylen- diene rubber (EPDM) and silicon rubber NaCl were purchased from Merck, Ger-
Propylen-Dien-Kautschuk (EPDM) wur- (Q) are widely used in the wire and cable many.
den in verschiedenen Verhältnissen ge- industry because of their high perfor-
mischt und die mechanischen, thermi- mance under wet and contaminated Blend preparation
schen und elektrischen Eigenschaften conditions. The mixing of EPDM with (SiR) in diffe-
wurden untersucht. Die Ergebnisse EPDM due to its combination of good rent ratios 0, 25, 50, 75 and 100 wt%
zeigten, dass die Zugfestigkeit und electrical properties, costs, flexibility was carried out on a laboratory two roll
Bruchdehnung mit Zunahme des SiR - over a wide temperature range and its mill (470 mm diameter and 300 mm
Gehaltes in der Mischung ebenso ver- resistance to moisture and weather is working distance). The gap between the
bessert wurden wie die thermischen Ei- often used as an electrical insulating two rolls ranged from 3 mm to 1 mm
genschaften. Bei Erhöhung des EPDM- material [3, 4]. according to the mixing conditions. The
Anteils in der Mischung nimmt die di- The intrinsic properties of silicone po- speed of the slow roll is 24 rpm with a
elektrische Stabilität zu und die lymers include flexibility, chemical, ther-
Trackingzeit ab. Darüber hinaus wurden mal and UV resistance, broad tempera-
die Auswirkungen von unterschiedli- ture dynamic behavior and long term
chen Umgebungverhältnissen (wie stability. These properties have led to
Wasser, Salzwasser: 5 und 10 % NaCl their use in numerous commercial appli-
cations such as sealants, gaskets, rubber
Authors
und UV-Strahlungen) auf die Eigen-
schaften untersucht. moldings, insulators housing and advan- J. N. Asaad, A. A. Youssef, L. S.
ced composites for thermal protection Nasrat, Cairo, Egypt
[5- 8].
Blending of two or more polymers for Corresponding author:
property improvement and economic ad- J. N. Asaad
vantage has gained considerable impor- National Research Centre
tance. A large number of polymer blends Polymers and Pigments
has been proposed for commercial pur- Department
poses. Blends can be tailored to meet the El Behooth St.
requirements of specific applications 12311 Dokki Cairo, Egypt
[9- 12]. Tel. +202-33335978
Figures and Tables:
By a kind approval of the authors.
Comparing with silicone rubber, EP- E-mail:
DM has lower UV and weathering resis- na_jeannette@hotmail.com

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1 2

Fig. 1: Tensile strength values of (SiR) / EPDM blends before and Fig. 2: Elongation at break of SiR/ EPDM blends before and after
after exposed to UV exposed to UV

1 Tab. 1: TGA data of SiR / EPDM blends mm). A continuous flow of contaminant
Temp. at characteristic weight loss (°C) saline water (10 % NaCl) was used. The
SiR / EPDM 25 wt% 50 wt% 75 wt% Chare yield (%) distance between the top and bottom
wt % 700 °C electrode was 50 mm. An AC voltage (4
KV) was applied over the 50 mm distance.
0/ 100 430 470 480 0
25/ 75 420 470 500 10
Results and Discussion
50/ 50 500 500 530 18 In the present investigation, different ma-
75/ 25 500 530 580 25 terials for outdoor polymeric insulators,
100/ 0 550 580 – 33 namely silicone rubber (SiR), ethylene–
propylene–diene rubber (EPDM) and their
blends, were prepared and studied in view
gear ratio of 1:1.4. At first, EPDM and ISO 105 B01. The UV source was a stan- of their mechanical, thermal and electrical
(SiR) were masticated and then 2.5 wt % dard lamp, 500W, manufactured in Eng- properties under different environmental
of dicumyl peroxide 4-methoxy phenol land. The environmental condition was conditions, namely water, saline water (5
was added. The obtained compounds maintained at 60 ± 5 % relative humidity and 10% NaCl) and UV irradiation.
were then passed through a milling roll and 25 ± 5 °C.
for banding and left overnight before Mechanical properties of SiR / EPDM
vulcanization. The vulcanization was car- Thermogravimetric Analysis (TGA) The mechanical properties of (SiR) / EP-
ried out in a heated platen press, under a TGA was measured using macro/micro DM blends such as tensile strength and
pressure of about 40 kg-force/cm2 and a thermogravimetric analyzers (TGA 50) elongation at break % before and after
temperature of 152 ±1 °C for 10 min. from Shimadzu Tokyo, Japan. The samp- subjecting to UV radiation for 300 and
The investigated samples were cut les were heated from 25 to 800 °C at a 500 hr are presented in Figures 1 and 2.
and prepared with dimensions that best heating rate of 10 °C min-1. Generally, the presence of (SiR) in the
suited to each testing technique. Dielectric properties blends enhances the mechanical pro-
The dielectric strength measurements we- perties that means with increasing the
Technique re performed according to IEC 60156 at (SiR) content, the tensile strength, and
Mechanical properties room temperature under different envi- the elongation at break of samples in-
Tensile strength and elongation at break ronmental conditions, namely water, sali- creased [13].
of dumbbell-shaped specimens were de- ne water (5 and 10 % NaCl) and UV irradia- The results depicted in Figures 1 and 2
termined at room temperature (25 ± 1°C) tion. The imposed voltage was AC 50 Hz. indicate the decrease in mechanical
and after aging using a Zwick tensile The test samples were in the form of a strength values of all samples under in-
testing machine (model-1425, Munchen, disc with a thickness of 1mm. The com- vestigation after UV irradiation. The pos-
Germany) and at a crosshead speed of 50 position of electrodes was a sphere of sible explanation of this behavior is the
mm/min according to ASTM D 412. The 100 mm in diameter on high and low degradation of the polymeric insulators.
mean of five measurements was taken voltage sides. The mechanical properties of rubber can
into consideration. be changed due to UV aging either by
Tracking resistance test creating or destroying the cross-links
Ultraviolet aging Tracking is carried out according to IEC that join the rubber molecules and con-
The samples were subjected to UV radia- 60587 test method. The samples were in sequently the tensile strength and the
tion for 300 and 500 hours according to the form of slab shaped (12 cm × 5 cm × 6 hardness of rubber can be affected, as

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3 4
Dielectric strength (KV/mm)

Dry
20 Water
5 % NaCl
10 % NaCl
15

10

0
0 25 50 75 100
SiR wt %

Fig. 3: TGA curves of SiR / EPDM blends Fig. 4: Dielectric strength of SiR / EPDM blends in wet conditions

well as the distance that it can stretch suffer from loss of hydrophobicity, decre- irreversible damage of the surface and
before breaking [14]. ased tracking and erosion resistance, and consequently, the dielectric strength va-
degradation of their surface under envi- lues decrease in the same manner.
Thermal properties of SiR / EPDM ronmental conditions [2]. All samples were also subjected to UV
The thermal stability of the pure (SiR), Insulators accumulate deposits of airbor- radiation for 300 hours which was exten-
EPDM and their blends in different ratios ne solid contaminants when subjected to ded to 500 hours. As reported in litera-
(0, 25, 50, 75 and 100 wt%) was determi- polluted atmospheres. These contami- ture, 200 hours of exposure is equivalent
ned in terms of percentage of weight nants usually contain some ionic substan- to one year of actual outdoor exposure
loss at various temperatures as illustra- ces that form electrolytes and give rise to assuming that the UV wavelength in the
ted in Figure 3. It is clear that all samples surface leakage currents under moist con- range 300–400 nm causes deterioration
show thermal stability up to 300 °C. ditions such as fog, rain, dew, or mist. of polymers [17].
The pure (SiR) and EPDM samples ha- Figure 4 shows the dielectric strength The dielectric strength of the different
ve one stage weight loss from (400 to values of different (SiR) / EPDM blends in samples before and after UV aging is
580oC) and (340 to 470 °C) respectively. dry, wet, and salt solutions (5 and 10 % measured and the results are summa-
Whereas the (SiR) / EPDM blends display NaCl). It was observed that in dry condi- rized in Table 2 and Figure 5.
two distinct stages of degradation. As tion the pure EPDM has the highest diel- It is quite evident that the dielectric
can be deduced from Table 1, the pre- ectric strength value (22 KV/mm), while strength values of the aged samples de-
sence of (SiR) in the blends improves the the pure (SiR) has the lowest one (10 KV/ crease as compared to those of the una-
thermal stability of the blends and the mm). As can be deduced from Figure 4, ged ones and the increasing time of ex-
temperature at which 25 %, 50 % and the dielectric strength values of the posure results in more reduction in the
75 % weight losses occurred are shifted blends decrease by increasing the per- strength. The degradation of the polyme-
towards higher temperature by increa- cent of silicone rubber. ric insulators due to the photon energy
sing the (SiR) content. The (SiR) / EPDM blends show good of short wave length is probably the
This may be attributed to its proper- breakdown voltage strength compared main reason for the reduction in dielec-
ties which maintain high temperature to (SiR) [15]. tric strength values after UV irradiation.
and excellent thermal stability compa- It is worth noting that after exposing to As can be seen from the calculated per-
ring to EPDM [14]. The char yield at high water and NaCl solutions all samples cent of dielectric loss shown in Table 2
temperature 750 °C was 0 and 33 % for described here exhibit lower dielectric the UV withstand capability of the insu-
the pure EPDM and (SiR) respectively. strength values than those in dry condi- lators increases by increasing the amount
Whereas (SiR) / EPDM blend have inter- tion. of (SiR) in the blends [18]
mediate chare yield percents. These cha- It has been reported [16] that the pre-
re yield percent increase from 0 to 25 % sence of moisture and salt has a deleteri- Tracking resistance time of SiR / EPDM
in (SiR) / EPDM blends by increasing the ous effect on the hydrophobicity of the Tracking is the principal factor limiting the
(SiR) content. surface of the polymeric insulators and safe working electric stress of modern low
hydrolysis reaction will take place. There- loss insulation in power system applica-
Dielectric strength of SiR/ EPDM fore the main reason for the failure of tions. Knowledge of the factors involved in
The dielectric strength of an insulating these insulators can be related to loss of the deterioration and breakdown of insu-
material can be defined as the voltage hydrophobicity. Due to the decrease of lation by tracking is essential for the relia-
gradient or dielectric stress through the hydrophobicity, the surface of the insula- ble and economic design and operation of
material at which electrical failure or tor becomes conductive through absorp- electrical equipment. When high voltage
breakdown occurs. Polymeric insulators tion of water, resulting in flashover and is connected to the top electrode, current

www.kgk-rubberpoint.de KGK · 5 2013 45

 PRÜFEN UND MESSEN
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5 The aged samples have a lower mechani-

Fig. 5: Dielectric
cal and dielectric strength compared to
Dielectric strength (KV/mm) strength of SiR /
EPDM blends those of the unaged ones.
Unaged before and after
20 aged 300 hr exposed to
aged 500 hr

15

10 References
[1] R. Sarathi and U. M. Rao, Bull. Mater. Sci., 24
(2001) 587.
5 [2] B-H. Youn and C-S. Huh, IEEE Trans. Dielectr.
Electr. Insul., 12 (2005) 1015.
[3] Y. Kurata, IEEE. Conf. Electr. Insul. Dielectr.
0 Pheno [1] Y mena (CEIDP), (1995) 471.
0 25 50 75 100
[4] R. R. Prabu, S. Usa, K .Udayakumar, M. Abdul-
Q wt %
la Khan and S.S.M. Abdul Majeed , Part I IEEE
Dielectr. Electr. Insul., 14 (2007) 1207.
[5] S. Simmons, M. Shah, J. Mackvich and R.J.
2 Tab. 2: Dielectric strength under AC voltage of RiS / EPDM blends before and after Chang, IEEE Electr. Insul. Mag., 13, (1997) 25.
UV aged [6] A. Genovese, R. A. Shanks, Composites: Part
SiR / EPDM Dielectric strength (KV/mm) Dielectric strength loss (%) A 39 (2008) 398.
wt % [7] D. Wilson, D Beckley and J.H Koo, High Per-
unaged UV 300 hr UV 500 hr UV 300 hr UV 500 hr
form Polym , 6 (1994) 165.
0/ 100 22 18 14 18 36 [8] C. A. Harper, Handbook of plastics, elasto-
25/ 75 18 15 13 17 28 mers and composites, (1996) McGraw-Hill.
50/ 50 15 13 12 13 20 [9] D. E. EL-Nashar, E. Gomaa, S. L. Abd-El-Mes-
sieh, J. Polym. Sci Part B: Polymer Physics, 47
75/ 25 12 11 10 8 17
(2009) 1825.
100/ 0 10 9 8 10 20
[10] M. G. Oliveira and B. G. Soares, J. Appl. Po-
lym. Sci., 91 (2004) 1404.
3 Fig. 3: The tracking resistance time of content in the blends. When the amount [11] S. Jia, Z. Zhang, Z. Du, R. Teng and Z. Wang,
SiR / EPDM blends of (SiR) is increased gradually from 0 to Radiat. Phys. Chemi., 66 (2003) 349.
SiR / EPDM The tracking resistance 25, 50, 75 and 100 wt %, the increasing [12] A. M. Omran, A. M.Youssef, M. M. Ahmed,
wt% time (minutes) percentage of tracking resistance time is and E. M. Abdel-Bary, KGK · Mai (2010) 197.
0/ 100 0 50, 100, 312, 400 % respectively. [13] M. Mohseni, M. Geramian and H. Mirzadeh,
It can be concluded from the combined 17th International Conference on Electricity
25/ 75 12
results obtained in the above discussion Distribution, Barcelona, 12-15 May (2003).
50/ 50 16
that the increasing trend in tracking time [14] C. Zhang, K. Pal, J-U. Byeon, S-M. Han and J.
75/ 25 33 K. Kim, J. Appl. Polym. Sci., 119 (2011) 2737.
as the (SiR) increases could be attributed
100/ 0 40 to its high thermal stability compared to [15] M. Ehsani, H. Borsi , E. Gockenbach , J. Mors-
EPDM. The presence of high bond energy hedian and G.R. Bakhshandeh, Europ. Po-
flows in the conductive path, (formed by of Si-O-Si bonds in silicone rubber is res- lym. J., 40 (2004) 2495.
the contaminant between top and bottom ponsible for its high thermal stability [16] M. Ehsani, H. Borsi, E. Gockenbach, G. R.
electrode) resulting in partial evaporation [15]. This result was further supported Bakhshandeh and J. Morshedi Adv. Polym.
of the contaminant that leads to dry band by the above thermal properties analysis. Tech., 24 (2005) 51.
in the gap. The high temperature of the arc [17] S. Jeon and S. Hwang-bo, 4th International
in the gap is considered to be responsible Conclusions Conferences on Properties and Applications
for erosion of material followed with tra- The presence of (SiR) in the blends en- of Dielectric Materials, Brisbane, Australia,
cking process. Once tracking occurs, the hances the mechanical properties and 2 (1994) 511-514.
surface electrical insulation property is the thermal stability. Also it increases the [18] M. Ehsani,y, G. R. Bakhshandeh, J. Morshe-
lost completely and it never recovers [19]. tracking time of the blends. dian, H. Borsi, E. Gockenbach and A. A.
Tracking resistance is the quantitative The dielectric strength values of the Shayegani, Euro. Trans. Electr. Power, 17
expression of the voltage and the time blends increase by increasing the percent (2007) 47.
required to develop a track under speci- of EPDM i.e. impart well break down vol- [19] R. R. Prabu,  S. S. A. Majeed,  S. Usa,
fied conditions. Tracking resistance time tage strength to the blends. and T. Thyagarajan, Tracking resistance
of the different samples under salt wet Blending of (SiR) with EPDM is found to behaviour of the blends of silicone and
conditions and AC voltage was measured be an effective way to improve mechani- EPDM polymeric insulators, Electrical In-
and the results are given in Table 3. cal, thermal and electrical properties and sulating Materials, (ISEIM 2001). Procee-
As illustrated in Table 3, the tracking resis- the suitable percentage was found to be dings of 2001 International Symposium
tance time increases with increasing (SiR) 50/50 (SiR) / EPDM. (2001) 309 - 312. 

46 KGK · 5 2013 www.kgk-rubberpoint.de