THERMAL ANALYSIS OF THERMOPLASTIC ELASTOMER MATERIALS DUE TO ELECTRICAL TRACKING PHENOMENA

Zainab Abdullah

Department of Electrical Engineering Politeknik Tuanku Syed Sirajuddin

Pauh Putra, 02600, Arau, Perlis, Malaysia e-mail: zainababdullah72@yahoo.com

ABSTRACT

Thermoplastic Elastomers (TPE) has been used as electrical insulation since the 1960s. The combination of thermoplastic and an elastomer material is to be responded into high voltage system application particularly as an outdoor electrical insulation. However, the largest problem of the outdoor insulation is the result of tracking and erosion that leads to insulation failure. Tracking is a carbonaceous process that created on the surface a continuous film of water known as drybands. The surface of the material slowly loses its hydrophobic, allowing high leakage current to flow and its influence under wet and contaminated conditions. The insulation failure occurs when the dryband bridge between the electrodes. In order to understand the tracking process in the insulation structure, the thermal analyses are carried out. The current experimental work is to determine the thermal properties of thermoplastic elastomer (TPE) due to electrical tracking phenomena under AC voltage. The thermoplastic elastomer of high density polyethylene (HDPE) and epoxidized natural rubber (ENR50) filled with different types of nanofillers which are montmorillonite (MMT), aluminium oxide (Al2O3) and silicon dioxides (SiO2) were introduced. An electrical tracking test is conducted to study the performance of surface tracking properties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are used to analyze the thermal properties of the materials before and after undergoing tracking test. These techniques measured the heat flows and weight changes as a function of temperature, respectively. It is concluded that the surface degradation due to electrical tracking phenomena altered the thermal properties of materials. Keywords: Thermoplastic elastomer, Nanofillers, Electrical tracking, Thermal analysis, TGA, DSC. 1.0 INTRODUCTION Polymeric insulators offers significant advantages over porcelain and glass insulators in high voltage outdoor insulators. The history of polymeric insulators began in the 1940s. Their application has gradually become more acceptable for outdoor used in the 1950s. In the late 1960s to 1970s no significant developments of this material were observed. The studies of polymeric materials were coming back and attract many researchers to continue the development in 1980s. As reported by James F. Hall (1993), there are various types of polymer were used in the manufacture of composite polymeric insulators such as ethylene propylene rubber (EPR) which were made by Ceraver of France (1975), by Ohio Brass of United State of America (USA) (1976), by Sedivar of USA (1997) and Lap of USA (1980). The development of

silicon rubber was manufactured by Rosenthal in Germany (1976) and by Reliable of USA (1983). The cycloaliphatic epoxy was manufactured in 1997 by Transmission Development of the United Kingdom (UK). The good characteristics of polymeric materials such as weight reduction, excellent hydrophobic capabilities, high mechanical strength, vandalism resistance and better contamination performance makes them achieved the highest rank in high voltage application (H.M. Schneider et. al. 1989, J. Mackevich and M. Shah, 1997, R. Hackham, 1998, R. Sarathi, 2006). Lightweight contributes 42% which is the largest percentage of reasons for applying polymeric insulators (Yasin Khan, 2009). Figure 1 shows the detail breakup of reasons for applying polymeric insulators obtained during the survey.

FIGURE 1: Reasons for applying polymeric insulators (Yasin Khan, 2009). For long term application, the materials properties are important to be considered. Environmental stresses like ultraviolet (UV) radiations, heat, contamination and moisture affect its service life. The development of tracking and erosion problem is influence under wet and contaminated condition. It leads to formation of continuous film or dry-band due to leakage current and promote insulation failure when the carbonized tracks bridge the distance between the electrodes (Sarathi et al. 2004). Up to now, the interests of using polymeric insulator for high voltage application attract many researchers doing research on them. In recent year, with the fastest growing sector in the polymer science and technology, a new class of materials called polymeric nanocomposites is gained more attraction as electrical insulation in power delivery systems. Introducing nanofiller into polymer materials is called polymer nanocomposites or nano dielectrics. The potential property changes that would benefit electrical insulation due to nano-sized inclusion were introduced in 1994 (T.J. Lewis, 1994). This experimental study is to determine the thermal properties of thermoplastic elastomer (TPE) material due to electrical tracking phenomena under AC voltage. The thermoplastic elastomer

filled with different types of nanofillers which are montmorillonite (MMT), aluminium oxide (Al2O3) and silicon dioxides (SiO2) were introduced in the present study. An electrical tracking test is conducted to study the performance of surface tracking properties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are used to analyze the thermal properties of the materials before and after undergoing tracking test. These techniques measured the heat flows and weight changes as a function of temperature, respectively. 2.0 LITERATURE REVIEW A blend of high density polyethylene (HDPE) and epoxidised natural rubber (ENR-50) was used as the base polymer, with three types of nanofiller. The effects of nanofillers concentration in the blend were observed in understanding the behavior of TPE material. As a consequence, thermal properties of this material as well as the electrical properties are investigated by performing thermal analysis techniques due to electrical tracking on the material surface.

2.1 Thermoplastic Elastomer (TPE) Thermoplastic Elastomer (TPE) materials are a blend of a plastic and a rubber. Having this in view, in this present work, the formulated TPE of polyethylene (PE) and epoxidized natural rubber (ENR-50) is used as the base polymer. This new material gives a better balance of properties than obtainable with a single polymer (Mat Uzir, 2006). TPE become widely acceptable at a growing rate of 11% per year because the unique properties of both materials (Salman Amin and Muhammad Amin, 2011). Several studies about the TPE materials was reported by Roy and De in 1991, that investigated about natural rubber (NR) blended with low density polyethylene (LDPE). Asaletha et al. (1998) investigated the morphology and mechanical properties of polystyrene (PS) and natural rubber (NR) blend. Other authors, Ibrahim and Dahlan (1998) have been reported various group of polymer blends such as PP/NR (polypropylene/natural rubber), PE/ENR (polyethylene/epoxidized natural rubber) and PVC/ENR (polyvinyl chloride/epoxized natural rubber). The NR and waste high density PE (WHDPE) blended was reported by Abd-El Messieh et al. (2000). Dahlan et al. (2002) and Piah et al. (2005) studied the blends of linear LDPE (LLDPE) and NR. The characteristics of degradation of polymer insulator were studied by Noronha et al. (2005). According to their studied, the factor of water has been shown the most critical cause of degradation. It gives impacts in electrical, physic-chemical, mechanical, thermal and rheological properties. In addition, the temperature and UV radiation were the most aggressive factors. Salman Amin et al. (2008) studied the multi stress aging of TPE when exposed to polluted industrial areas. They also observed that the hydrophobicity of TPE significantly loss when exposed to polluted condition. In general, they expected that TPE insulators may arc or fail at a later stage of aging. It can be seen that the most of these studies focused on the compatibility of the composites, electrical properties, thermal and chemical stability and mechanical properties of the composites. Therefore, the current study introduced the inclusion of nanofiller into the base polymer , is expected to provide better understanding of material properties that to be responded as the high voltage insulation (T.Tanaka et. al.,2004). 2.2 Nanofillers

Nanoparticles offer a potential advantage over micro-sized fillers because of the small amount (normally less than 10wt%), they would benefit electrical insulation (T.J. Lewis, 1994). In spite of

this, nanofillers which are derived from nanotechnology process were introduced in polymeric material and it can modified the dielectric properties by alter the fundamental structure of the material, so that the material can be superhydrophobic. In recent years, various types of fillers in nanoparticles can be used for a range of potential application with different properties. Incorporation of fillers into the polymer material can improve the physical and mechanical properties in relation of the polymer matrix. Due to their nano-size dimension, fillers have rapidly used because of their good processibility, reduce costs and also found to generate new properties from the filler to the composite materials. Many researchers have been reported on the effect of nanofillers in material properties. Previous work done by Varlow et al. (2003) used of barium titarate as filler to modify the permittivity of epoxy resin. The mechanical resistance increases with contents of filler as well as the electrical conductivity studied by (Oliveira et al. 2005). Guo et al. (2009) reported on the strength and heat distortion are improved by the impact of nanocomposite and incorporation of appropriate nanofiller in the polymers material, demonstrate good impact on the mechanical characteristics and electrical properties was done by Arroyo et al. (2006). The effect of filler size and filler surface chemistry on pore nucleation in silica/polymethylmethacrylate (PMMA) nanocomposites was studied by Goren et al. (2010). As a consequence, to control the stress problem in polymeric material as insulating materials, appropriate fillers can be introduced for material properties improving. The type, size, geometrical structure, surface and dispersion of the particles determine the electrical conductivity of polymeric materials. 2.3 Tracking Effects Based on analysis by various means, there were other problem of polymeric insulators such as tracking and erosion susceptibility especially under environmental agents (humidity, pollution, UV radiation, etc.) (W. Pinheiro et. al., 1996). Electrical tracking is a peculiar phenomena that occur on the insulator surface because of the creepage discharge resulting from surface contamination (Sarathi et al. 2002). As a result, a continuous film of water normally known as dry-band or carbonized track is created on the surface. It varies with surface intensity, surface current magnitude and the state of electrical discharges (surface wetting and the degree of contamination). The insulation failure occurs when dry-band bridge between the electrodes. In order to avoid degradation due to leakage current activity, the test method and solution are used to allow for the determination of the qualitative data in understanding the behaviour of TPE materials. Then, the good formulation of material and environmental conditions also can be determined. The inclined-plane test is one of the methods can be used for evaluating tracking and erosion resistance of polymeric materials for outdoor insulation. In this study the inclined-plane test according to IEC 60587 was performed. The tracking process of the material surface as shown in Figure 2 (Sarathi et al. 2004). The inclined-plane test used to analysis the electrical activity was studied by Chang and Linas Mazeika (2000). They used four types of sample in order to collect the result of analysis. The four representative materials are: (a) a silicone with high loadings of alumina trihydrate (ATH); (b) a silicone with no ATH; (c) a polyethylene vinyl acetate (EVA); (d) a glazed porcelain. The results showed that the electrical activities were observed depended on the materials. The wettability of material surface affected water evaporation and dry band formation. The interesting result was shown that the presence of ATH is not absolutely necessary for achieving excellent tracking and erosion resistance for silicone elastomers. (Chang and Linas Mazeika, 2000).

FIGURE 2: Schematic representation of tracking process (Sarathi et al. 2004). 2.4 Thermal Properties

Thermal analysis is a technique to study the characteristics of materials as a function of temperature. In this study, the thermal properties of the surface with exposed to the tracking test have been characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Differential Scanning Calorimetry (DSC) is a thermoanalytical technique that measures the enthalpy changes in which the difference in the amount of heat required to increase the temperature of a sample and reference material are measured as a function of temperature. It is possible to evaluate the heat capacity changes, heat absorbed or emitted by a sample during several thermal events, including glass transition (Tg), melting point (Tm), crystallization, crosslinking, chemical reaction, evaporation and chemical decomposition. Thermogravimetry analysis (TGA) is an instrument to simply measure weight changes against temperature. It is a very effective technique to study chemical and physical phenomena as a function of temperature. TGA provides definitive data for materials and product design and aging stability information with short test time (<60 min). For measurement, specimens were cut from the samples as small pieces. The specimen is heated at a constant rise of temperature. The sample weight is continuously monitored by computer screen. Sarathi et al. (2002), in their research on investigations of surface modifications in EPDM concluded the results from differential scanning calorimetry (DSC) study showed that the thermal characteristics of the material near the tracking zone are altered and tracking is a surface degradation process. In the study by Asaletha et al. (1997), which used natural rubber and polystyrene blends, and the results showed that the blending has improved the thermal properties. Ehsani et al. (2004) in their study on investigation on outdoor polymeric insulators reported the results from thermogravimetric analysis (TGA) showed the tracking and erosion occur with thermal degradation as a result of high temperatures caused by dry band arcing.

3.0 METHODOLOGY 3.1 Samples Preparation The HDPE used in this work was obtained from Titan Petchem (M) Sdn. Bhd., Malaysia. This resin was originally in the form of extruded pellets. The melt flow index (MFI) at 190oC and 2.16 kg load and the density of HDPE were 26 g/10 min and 0.941 g/cm3, respectively. The ENR-50 used in this work was obtained from Research Rubber Institute of Malaysia (RRIM). The ENR-50 was received in bale form. Three different types of nanofillers were used in to study the electrical tracking resistance. They are organoclay (MMT), silicon dioxide (SiO2) and aluminium oxide (Al2O3). The organoclay-montmorillonite (MMT) grade Nanomer 1.30P were commercial product from Nanocor Inc. USA that has mean dry particle size is 16 - 20 microns. The silicon dioxide typically known as silica (SiO2) received in form of powder was produced by Sigma-Aldrich, UK. Mean dry particle size of the silicon dioxide is 5 – 15 nm (TEM). The aluminum oxide typically known as alumina (Al2O3) was produced by Sigma-Aldrich, UK. With mean dry particle size of the alumina is < 50 nm (TEM). The blend composition of HDPE/ENR50/Nanofillers is shown in Table 1.

TABLE 1: Test samples investigated

Designation Composition Wt% phr*

HDPE ENR-50

MMT Al2O3 SiO2

H8E2 HDPE/ENR-50

80 20 - - -

H8E2-MMT HDPE/ENR-50/MMT

80 20 2 - -

H8E2-AO HDPE/ENR-50/Al2O3

80 20 - 2 -

H8E2-SD HDPE/ENR-50/ SiO2

80 20 - - 2

3.2 Electrical Tracking Test The tracking test was set up based upon the general consideration laid down in the IEC-60587 test method, which is adopted for alternative current (A.C.) voltage application. A schematic diagram of the experimental setup and the electrode configuration used in the present work are shown in Figure 3. The sample was mounted at an angle of 45o with the flat surface on the underside. The stainless steel electrodes are separate of 50 mm apart. A test voltage of 2.5 kV rms was applied for 4 hours with a contaminant flow rate 0.15 ml/min which flowing continuously from peristaltic pump. Eight layers of filter paper are clamped between the top electrode and the sample to act as an electrolyte reservoir for uniform flow of electrolyte on the sample surface. The test was stopped after 4 hours or earlier if major erosion is observed on the material surface. In this experiment, the range of leakage current was recorded for

measurement. The leakage current counts were measured for 4 hours. The electrical performance was quantified by counting the number of peak leakage current values for every 15 minutes.

FIGURE 3: Tracking test configuration IEC-60587 (M.A.M. Piah et al. 2005).

3.3 Thermal Analysis

3.3.1 Differential Scanning Calorimetry (DSC)

The melting behaviour testing of the blends was carried out using a Perkin-Elmer Differential Scanning Calorimeter (DSC). All the DSC measurements were performed under nitrogen purge and the sample weight was about 6 - 12 mg. The samples were heated at a rate 10oC/min from 30oC to 200oC. The melting temperatures (Tm) were taken as those corresponding to the peak values of the melting endotherms. The degree of crystallinity (equation 1) was calculated as the ratio of the melting enthalpies, ΔHsc divided by weight fraction wi of the respective component in the blend (Naranjo et al. 2008).

(1)

3.3.2 Thermogravimetry Analysis (TGA) TGA was performed with a Perkin Elmer TGA 7 thermal analysis system. This technique gives the useful information on the thermal decomposition of polymers. The TGA scans were recorded at 10oC/min heating rate under a nitrogen atmosphere from 30 to 900oC. The amount of sample used for the experiment was approximately 6 to 12 mg. Each sample was placed in an open platinum sample pan. The purge gas was nitrogen at a flow rate of 20 cm3/min. The

oven was continuously purged with nitrogen during the experiment to remove the release moisture and maintain dry conditions. The onset temperature of a 10 % weight loss deviation from the baseline of thermogravimetric (TG) curve was used as an indicator of the composite’s thermal stability. 4.0 DATA ANALYSIS 4.1 Analysis of Tracking Studies The electrical tracking test according to IEC 587 is conducted. During the test, the range of leakage current was recorded after 4 hours or earlier if major erosion is observed on the material surface. The electrical performance was quantified by counting the number of peak leakage current values for every 15 minutes. Electrical properties are determined from leakage current level on the surface of samples. Leakage current had started when the contaminant flow on the samples surface. Experimental results of tracking test showed that the different nanofiller concentrations in the compositions as well as the condition of material surface clearly affected the characteristics of leakage current activity. Several differences of leakage current were found for all the samples. The absorption of contaminated water sometimes has caused seriously affects the material properties of polymeric insulating materials. This study shows that the additional of monmorillonite (MMT) in the compounding indicated no degradation has occurred for 4 hours electrical tracking test. However, the sample with aluminum oxide experienced failed due to burning takes place just before 4 hours test and has to stop early. For the sample with silicon dioxide was observed degraded zone on the surface and the sample without filler kept unchanged during the tracking test. The characteristic of the material near the tracking zone used to identify further analysis to investigate the thermal properties which are to determine either this material are acceptable for use as outdoor insulating material. 4.2 Analysis of Thermal Properties In this section, the discussion is focusing on the thermal properties of the thermoplastic elastomer blends with different types of nanofillers (HDPE 80%, ENR50 20% and 2 phr nanofiller). The thermal degradation of the samples was measured by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimeter (DSC) analyzer. The effect of HDPE, rubber functionality and filler content on a formulation are also discussed. Based on a melting temperature and peak degradation an optimal formulation of the thermoplastic elastomer blend is suggested. 4.2.1 Thermogravimetric Analysis (TGA)

In general, all the thermogravimetric (TG) traces show a shift of the weight loss towards higher temperature. During the thermal degradation, the TG curves displayed a single step degradation process for all samples produced. The sample weight percentage shifts continuously to a residual level after the temperature has commenced. The initial thermal stability (thermal stability can be referred to peak degradation, 10 % weight loss or char residue) is characterized by temperatures at 10 % weight loss, which denoted as T-10%. The degradation of the sample depends on material blends. The residue remaining is calculated using the following equation:

(2)

The detail variations of data at 10% initial weight loss (T-10%) for all blends are summarized in Table 3 and Table 4. Thermal stability of blends increases (in certain fillers) as adding of filler in the composition; act as an insulator and mass transport barriers.

TABLE 3: Sample without tracking test

COMPOSITION

Peak

degradation

Tdpeak

Temperature of

composition (oC) at

10% weight loss

(T-10%)

Char residue

(Wt %)

H8E2 501.082 424.701 0

H8E2-SD 499.280 426.283 1.496

H8E2-AO 501.608 414.991 1.597

H8E2-MMT 503.914 413.668 1.394

TABLE 4: Sample with tracking test

COMPOSITION

Peak

degradation

Tdpeak

Temperature of

composition (oC) at

10% weight loss

(T-10%)

Char residue

(Wt %)

H8E2 500.950 419.060 0

H8E2-SD 497.947 416.484 1.492

H8E2-AO 503.841 424.719 1.899

H8E2-MMT 502.550 428.284 1.478

4.4.2 Differential Scanning Calorimeter (DSC)

There are two steps involved when measuring the flow of heat through the material. They are heating and cooling step. The heating step is to investigate the thermal history of the material during processing step, usually denominated melting temperature, Tm (Dahlan et al. 2002). Tm is observed as an endothermic peak in the heat capacity or in the heat flow, so a peak temperature and a peak area (enthalpy change during heating) can be determined. At a certain temperature the crystalline structure is obtained due to the rearrangement process and it represents by Tc. Tc is observed as an exothermic peak in the heat capacity or in the heat flow, so a peak temperature and a peak area (enthalpy change during crystallization) can be determined (Naranjo et al. 2008). The glass transition as denoted by Tg could not be detected by this DSC analyser because this instrument is only sensitive to a broader scope of molecular motion. So, in this study DSC has been used to analyze the melting temperatures of the blends only and indirectly the degree of crystallization can be calculated. Figure 4 illustrated DSC thermograms of all TPE material. As mentioned in the previous discussions, these materials have been exposed to tracking test in order to obtain the information about surface degradation. It was found the melting characteristic of the material in the tracking formed zone causing free radicals, confirming that the tracking process is the degradation process ( R. Sarathi et. al., 2004).. The melting temperatures of the samples which are denoted by Tm were taken as the endothermic peaks. The DSC thermogram shows the sample start to melt around 125-130oC during heating as increasing in heat flow. It also believed that the melting behavior of a material depends on several factors such as thermal history and the heating rate selected during the measurement. Data of Tm and χc (before and after tracking test) can be seen as in Table 5. It is clear that the thermogram DSC studies indicated that the insulation surface structure due to ageing process slightly change the melting point by types of filler. This is believed that the decrease in χc indicates that the effect of the cross-link formation in the rearrangement process of the crystalline structure (Asaletha et al., 1997, Dahlan et al., 2002).

(a) Without tracking test

(b) With tracking test

FIGURE 4: Summarized of DSC heating scan for with and without tracking test for H8E2

blends by different type of nanofiller.

TABLE 5: Value of Tm, and χc for HDPE/ENR-50 blends before (pure) and after (tracked)

tracking test.

Melting Temperature, T m

(oC)

Degree of Crystallinity,

χc (%)

pure tracked pure tracked

H8E2 132.33 132.67 59.95 50.68

H8E2-SD 131.83 133.00 49.16 61.46

H8E2-AO 131.67 131.50 42.63 56.99

H8E2-MMT 132.67 133.17 49.08 57.55

5.0 CONCLUSION AND DISCUSSION HDPE/ENR-50 compounding (neat, with organoclay, with aluminum oxide and with silicon dioxide), the filler plays an important role in the thermal properties of materials. Based on the results and discussions of the thermal behavior of TPE materials, the following conclusions could be made:

This study shows that the additional of monmorillonite (MMT) in the compounding indicated no degradation has occurred for 4 hours electrical tracking test. However, the sample with aluminum oxide experienced failed due to burning takes place just before 4 hour test and has to stop early. For the sample with silicon dioxide was observed degraded zone on the surface and the sample without filler kept unchanged during the tracking test. This confirms that the characteristic of the materials alters due to material preparation process. The surface degradation or formation of tracked zone on the materials surface due to electrical tracking phenomena relatively changed the thermal properties. The thermal studies showed the characteristic of the material is different for the non-ageing (without tracking test) and ageing (with tracking test) materials. Indirectly, it also found that the thermal properties of the HDPE and ENR-50 compositions of 80% and 20% respectively were found to be strongly dependent on the nanofiller used. The incorporation of 2 phr of nanofiller in the compositions results some changes in melting point and peak degradation value. However, the effect of nanofillers depends on the types of filler. The TGA results showed that the reduction of weight for H8E2 and H8E2-SD samples were observed change especially at initial 10% weight loss after the samples experienced surface discharge. However, this degradation temperature (T10%) increased by adding of filler aluminum oxide and monmorillonite into the H8E2 content. This indicated that the effect of fillers plays an important role in the thermal property of a material. Therefore, the types of filler need to well known before introduce to the composite systems because they are found to be accelerate the thermal degradation of the composites. The DSC was used to measure the melting point (Tm) of the material. From the investigation on DSC analysis, it was shown that the melting temperature of the samples is slightly shifted as well as the degree of crystallinity (χc) due to tracking phenomena on the surface samples. Once again, it is because the effect of adding fillers into the blends. The flow of heat through a material represents the energy required to change the temperature of the material. The results indicate that at the certain amount of the heat which is determined by heat of fusion, the materials are melted. In this condition, the melting point is observed. When the Tm is determined, the heat flow was jumped to the lowest value. As a consequence, the thermal conductivity is decreased as increasing in the temperature. This study clearly indicates that the thermal stability of the samples is different for the sample without and with tracking test. The data presented from thermogravimetry analysis shows that the thermal stability of the sample with MMT and AO concentration are improved particularly at peak degradation temperature, while in DSC analysis shows that the crystallinity of H8E2-AO obtained is lower and the compound of H8E2-MMT presented the highest crystallinity. From this standpoint, it is believed that the compound of H8E2-MMT is the main factor contributing to the good surface condition due to high in melting temperature as well as degree of crytallinity. REFERENCES Abd.-El-Messieh, S. L., Younan, A. F. and Abd-El-Nour, K. N. (2000). Dielectric and Mechanical Properties of Waste Polyethylene-Natural Rubber Blends. Dielectric Materials, Measurement and Applications Conference Publication. IEE (No. 473), 170 – 176.

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