Investigation Of Dielectric Behaviors Of Nanoclay Filled Epoxy And PP/NYLON66

Nanocomposites For Cable Insulation Application

Rashmia, N. M. Renukappaa, and Siddaramaiahb

aDepartment of Electronics & Communication Engineering, Sri Jayachamarajendra College of Engineering, Mysore - 570 006, India.

bDepartment of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore – 570 006, India.

Abstract. High performance polymer nanocomposites are emerging as a new class of materials for its demanding applications as insulating material. The outstanding properties of nanoclay make them an attractive candidate for preparing advanced composite materials with multi functional features for electrical and electronics applications. A series of nanoclay incorporated epoxy and polypropylene/nylon66 (50/50 blend) nanocomposites have been prepared via chemical and melt mixing methods respectively. The fabricated nanocomposites have been characterized for dielectric behaviors such as dielectric constant (εr) and dissipation factor (tan δ). The effect of filler content, frequency, temperature and sea water ageing on dielectric behavior of nanocomposites has been investigated. The variation in the diffusion coefficient (D) of the material aged in water at different temperature with different percentage of nanoclay loaded epoxy and PP/nylon66 nanocomposites were calculated. It is observed that at increase in ageing temperature relatively increases the diffusion coefficient of the material. The measured dielectric results of the nanocomposites reveals that a significant influence of frequency and sea water ageing and marginal change with temperature. Higher dielectric constant was noticed for epoxy nanocomposites as compared to PP/nylon66 composites

Keywords: Nanoclay, nanocomposites, dielectric behavior, diffusion coefficient PACS:

1. INTRODUCTION

Polymer nanocomposites with better dielectric and electrical insulation properties are slowly emerging as excellent functional materials for dielectric and electrical insulation application and the term “nanodielectrics” for such material is increasingly becoming popular. Although the technology of addition of additives, agents and fillers are often used for improving dielectric properties has been in existence for several decades [1-3]. The effect of filler size on the dielectric property of the polymer composites has not been understood fully. It is with the advent of nanotechnology leading to the availability and commercialization of nanoparticles that polymer nanocomposite technology started to gain momentum.Polymer nanocomposite have

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been found to exhibit enhanced physical, thermal and mechanical properties when compared to the traditional polymer materials and that too at low nanofiller concentration (1-10%) [4-6].The present work looks into two of the dielectric properties in epoxy nanocomposites- dielectric constant and dissipation factor. Dielectric constant determines the charge storage capacity of the materials as well as dictates the dielectric field distribution in a composite insulation system whereas the tan δ indicates the dielectric losses possible in an insulating material. For many electrical insulation systems, a low tan δ value is always desired in the dielectric material, whereas the desired εr of the material can be higher or lower depending on the end application. Based on these facts, the current experimental investigation attempts to understand the behavior of dielectric properties and loss tangent in epoxy and PP/nylon66 nanocomposites with clay nanofiller at low filler concentration, temperature, frequency (> 1MHz) and sea water ageing.

2. MATERIALS AND SAMPLE PREPARATION

Fig. 1. Flowchart showing the processing of epoxy/clay nanocomposites.

Table 1. Materials used in the present study

Materials Supplier

Epoxy resin Epon 828 Miller-Stephenson Chemical Company, Inc.

Nanoclay Nanomer 1.30 E Nanocor, Inc.

Diethyltolunediamine Epikure W curing agent Miller-Stephenson Chemical Company, Inc.

Table 2. Sample designation and composition of ingredients used in the study Sample Designation Composition by % wt.

Epoxy Particulate filler nanoclay A0 100 - A2 98 2 A5 95 5 A7 93 7

2.1. Compounding

Before compounding, the polymer granules and fillers were dried at 80 ˚C for 10 hours in an air circulated oven and then dry mixed with nylon66 and other additives. Composition shown in table 3 was mixed and extrudated in a co-rotating twin extruder. The L/D ratio of the screw is 40:1. Mixing speed of 60 rpm was maintained

Nanoclay +

Epoxy resin +

Curing agent

1KA high shear mixing

at 24000 RPM for 45 min

Degas and Cure

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for all the compositions. The extrudate from the die were quenched in a tank at 20-30 ˚C and then palletized. For melt blending the temperature profile of the extrusion were Zone 1( 205 oC ) Zone2 ( 235 oC ) Zone 1( 245 oC ) Zone 1( 255 oC ) and Die ( 265 oC ). The extrudate of the composition was palletized in palletizing machine. The rpm of the pelletizer was maintained between the ranges of 60-80 rpm.

Table 3. Materials used in the present study

Materials Supplier Polyamide-66 (PP) DuPont Co.Ltd Nylon66 IPCL, India. Maleic anhydride grafted PP (MAgPP) DuPont Co.Ltd Nanoclay Sigma-Aldrich Inc.

Table 4. Sample designation and composition of ingredients used in the study Sample Designation

Composition by % wt. PP Nylon 66 Particulate filler nanoclay

B 100 - - B0 50 50 - B2 48 50 2 B5 45 50 5 B7 43 50 7

2.2. Injection Molding

The granules of the extrudates were pre dried in air circulated oven at 80 ˚C for 10 hours and injection moulded in a microprocessor based injection moulding machine fitted with a master mould containing the cavity for tensile strength, flexural and impact specimens. After its ejection from the mould, specimens were cooled in ice-water. Processing parameters are Zone 1 (200 ˚C) Zone 2 (235 ˚C) Zone 1(260 ˚C).

3. MEASUREMENTS

The electrical capacitance (c) and dissipation factor (tanδ) was measured in the frequency range of 0.1MHz to 30MHz and temperature range of 25 to 180 °C at constant ac supply of 1V using HP 4285A multi frequency LCR meter as per ASTM D-150 for all the samples. From the measured value of capacitance, the dielectric constant of the specimen was calculated. The effect of dielectric constant and dissipation factor on sea water ageing of the nanocomposites for the duration of 96 hours were studied

4. RESULTS AND DISCUSSION

4.1. Effect of frequency and filler on dielectric constant

The plot of the dielectric constant as a function of log frequency is shown in figure 2. It is realized that �r decreases with increase in frequency. Dielectric constant is a frequency dependent parameter in polymer systems. In a typical epoxy system, based on an epoxy resin cured with an amine hardner as in the present case, the epoxy

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component of dielectric constant is governed by the number of orientable dipoles present in the system and their ability to orient under an applied electric field [7]. Usually, the molecular groups which are attached perpendicular to the longitudinal polymer chain attribute to the dielectric relaxation mechanisms. At lower frequencies of applied voltage, all the free dipolar functional groups in the epoxy chain can orient themselves resulting in a higher �r value at these frequencies. As the electric field frequency increases, the bigger dipolar groups find it difficult to orient at the same pace as the alternating field, so the contributions of these dipolar groups to the �r goes on reducing resulting in a continuously decreasing �r of the matrix system at higher frequencies. Similarly, the inherent dielectric constant of nanoclay particles also decreases with increases in frequency of the applied field [8-9]. This combined decreasing effect of the �r for both matrix and filler particles result in a decrease in the effective �r of the epoxy and PP/nylon66 composites also when the frequency of the applied field increases. The variation of nanocomposite �r in figure 2 shows that there is a significant effect of the matrix type. In PP/nylon66 filled nanoclay composites, a lower permittivity is observed as compared to epoxy filled nanoclay composites. At nanoclay filler loadings of 7% wt. or less, the influence of the filler on the �r variations with respect to frequency can be considered to be very minimal, the slope of the �r

between 103 to 107 Hz is almost the same as that seen for pure epoxy and PP/nylon66 blend. The above observation suggests apart from the influence of filler dielectric constant, there is probably an occurrence of interfacial polarization in the epoxy nanocomposite system.

Fig. 2. Variation of dielectric constant with frequency of insulating

nanocomposites

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Fig. 3. Variation of dielectric constant with temperature for insulating

nanocomposites

4.2. Effect of temperatureon dielectric constant

From figure 3 firstly, �r increases with the temperature increasing, which is the result of more rapid thermal motion of dipolar polarization at elevated temperatures. Secondly, upon loading clay nanoparticles, �r increases over the entire tested temperature range with the particle content increasing. This is because that, for the nanocomposites, there is a great amount of interfacial areas formed between the nanoparticles and the polymer matrix. In this case, therefore, the interfacial polarization takes place, which contributes to the increases in �r [10]. Also found that, at a particular frequency (1MHz), the value of �r continuous to increase with the increase in temperature.

4.3. Effect of sea water aging on dielectric constant

The plot of �r with sea water ageing of nanocomposites is shown in figure 4. It can

be noted that increase in �r is much more significant for the samples after water treatment. One reason is ascribed to the stronger dipoles of water compared to the base matrix and the nanocomposites. Besides, the absorbed water may damage the polymer main chains and increase the segmental mobility by plasticization, increasing free volume or even breaking weak bonds, and thus can increases the total dipole strength of the nearby segments [11]. Singh et al. [12] also found that, at a particular frequency, the value of �r continues to increase as the relative water content increases for their epoxy laminate samples.

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Fig. 4. Variation of dielectric constant with sea water ageing in insulating

nanocomposites

Fig. 5. Variation of dissipation factor with frequency for insulating nanocomposites

Fig. 6. Variation of dissipation factor with temperature in insulating nanocomposites

4.4. Effect of frequency and filler on dissipation factor

From figure 5., it was noted that tan � increases with increase in log frequency from 103 to 107 Hz in epoxy/clay systems whereas, in PP/nylon66/clay nanocomposites, the tan � values marginally increases with the increasing frequency with the occurrence of a peak around 106 Hz and slowly starts to decrease beyond 106 Hz. Tan � depends on

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the electrical conductivity in the epoxy and PP/nylon66 composites. The electrical conductivity in turn depends on the number of charge carriers in the bulk of the material, the relaxation time of the charge carriers and the frequency of the applied electric field. Since the measurement temperatures are maintained constant, their influence on the relaxation times of the charge carriers is neglected. Over the current frequency range of measurement, charge transport will be mainly dominated by lighter electronic species.

The effect of filler loading on tan � was observed from figure 5. With increasing the filler loading, the tan � increases than the unfilled matrix in both the systems. Usually the introduction of inorganic fillers to a polymer matrix enhances the tan � values of the composites as there is an enhancement in the sources of charge carriers in the system. The number of nanoparticles causes an increase in the electrical conductivity at this filler loading which in turn influences the tan � behaviors [13].

4.5. Effect of temperature on tanδδδδ

From the figure 6, it is evident that tan � increases with the content of the nanoclay particles for matrix systems. Further, a single peak corresponding to the glass transition of the matrix systems can be clearly seen, which is consistent with the result from the DMA. However, At T < Tg it can be seen that tan � increases, on the other hand at T > Tg, the tan � decreases. The possible reason may be at temperatures T < Tg, the thermal motions of small side-groups are dominant; while segmental motions of the molecular chains are dominant at temperatures T > Tg [10]. The dispersion of nanoclay prevented the tan � from increasing at temperature above glass transition temperature.

Fig. 7. Variation of dissipation factor with sea water ageing of 96hrs for insulating

Nanocomposites.

4.6. Effect of seawater aging on tanδδδδ

The effect of tan � after sea water absorption of 96 hrs is shown in figure 7. It can be seen that tan � increases dramatically, meaning the water absorption produced significant effect in changing the tan �. The water absorption is high in epoxy system as compared to PP/nylon66 system [10].

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4.7. Diffusion coefficient of Epoxy/Clay and PP/nylon66/clay nanocomposites

Tables 5 and 6 shows the variation in the diffusion coefficient of the material aged in water at different temperatures. It is observed that at increase in ageing temperature relatively increases the diffusion coefficient of the material. Also increase in percentage of nanofiller in epoxy and PP/nylon66 shows reduction in diffusion coefficient of the material, for a given temperature. This indicates that the epoxy and PP/nylon66 nanocomposite is hydrophobic in nature with increase in percentage content of nano-filler in the base resin material [14]. The Diffusion coefficient is determined by the equation.

D = π L20.5 / 64 t0.5 (1)

Table 5. Diffusion coefficient of Epoxy/clay nanocomposites Sample At 30 °C

(× 10-10 m2/s) At 60 °C (× 10-10 cm2/s)

At 90 °C (×10-10 cm2/s)

A0 3.86 33.11 93.29 A2 2.51 26.20 67.39 A5 1.20 10.34 50.61 A7 1.12 9.73 40.78

Table 6. Diffusion coefficient of pp/nylon 66/clay nanocomposites Sample At 30 °C

(× 10-10 cm2/s) At 60 °C (× 10-10 cm2/s)

At 90 °C (×10-10 cm2/s)

B 1.39 12.02 33.88 Nylon 66 3.01 23.58 66.45 B0 2.68 22.98 64.72 B2 2.55 21.87 61.45 B5 2.34 20.07 56.59 B7 2.17 18.61 52.48

Where D is the Diffusion coefficient, L is the thickness of the specimen, t is the time of absorption.

5. CONCLUSION

The �r and tan � of the epoxy and PP/nylon66 nanocomposites shows the inverse relationship with frequency. With increase in nanoclay loading, �r and tan � of the nanocomposite system increases due to increase in total interfacial area. The epoxy/clay nanocomposite shows slightly higher �r and tan � as compared with PP/nylon66/clay nanocomposites. The epoxy and PP/nylon66 nanocomposite is hydrophobic in nature with increase in percentage content of nano-filler in the base resin material. Higher the water absorption in epoxy/clay system as compared to PP/nylon66 system.

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