145

Chapter 6

Effect of Seawater Ageing on Dielectric Properties of

Epoxy Nanocomposites

6.1 Introduction

Nanocomposites exhibit novel performance due to incorporation of fillers which are

nano scales and high specific surface area. Significant and high quality of research has

been carried out in recent years by incorporating different nano particles into existing

dielectric systems in a cost effective manner, to derive improved benefits over

conventional composites. However, the effect of seawater which is important for

marine applications and condition of high humidity on nanocomposites is still far

from being well established. The weakness imparted by seawater has a detrimental

effect on the mechanical and electrical behavior of nanocomposites. For example,

epoxy resin can consume up to a few wt.%, of water and this may lead to an overall

degradation of the dielectric performance. This could be further compounded by

additional absorption by nanofillers caused by their tendency to increase the free

volume (e.g. through cracks, voids or through inhibiting cross-linking). In practical

applications, water absorbance in materials is inevitable. The absorbed water can

bring many negative influences to insulating materials, such as distortion of electric

field distribution, inducing water trees, creating structural damage etc. Therefore, it is

very important to understand the effect of absorbed water on the dielectric

performance of epoxy nanocomposites.

In this chapter, the dielectric performance of epoxy nanocomposites in terms

of seawater absorption, hydrophobicity of the material, diffusion co-efficient, Tg, free

volume, dielectric properties, and percolation phenomenon of epoxy matrix and

epoxy nanocomposites is discussed.

6.2 Seawater absorption of epoxy nanocomposites

For better understanding the effect of seawater absorption on the epoxy

nanocomposites, water absorption measurement were carried out and the results of

weight gain as a function of time in hours are shown in Figure 6.1(a-c). The epoxy-

SiO2/Al2O3/ZnO nanocomposites absorb less water except at 20wt.% filler loading

than unfilled epoxy. The rate of water absorption is comparatively higher up to 100h

and there after the absorption rate reduces up to 200h and beyond this the

sorption/absorption rate becomes negligible. The absolute weight gain of the unfilled

epoxy is 45% at 100h but for nanocomposites it has an average value of 33% for

epoxy SiO2 system, (5 to 20wt.%), 20% for epoxy-Al2O3 system (5 to 20wt.%) and

23% for epoxy-ZnO system (over 5 to 15wt.%) for 100h of ageing.

146

Figure 6.1. Plots of weight gain versus duration of seawater ageing for a) epoxy-SiO2,

b) epoxy-Al2O3 and c) epoxy-ZnO nanocomposites

147

From the Figure 6.1(a-c) it can be observed that the water sorption rate

saturates in epoxy-SiO2 nanocomposites at 23% in epoxy-Al2O3 nanocomposites at

53% and in epoxy-ZnO system at 46% as compared to pure epoxy value of 43%.

Among three nanocomposite systems, water absorption is least in epoxy-Al2O3

system.

This may be due to the high specific interfacial area (particle area per

composite volume) where water may be localized due to congregate and increase free

volume [34, 255]. Moreover, the equilibrium water content depends not only on the

free volume in the matrix, but also on the concentration of hydrogen bonds formed

between water and network polar groups [310] and the interfacial effects.

From the water sorption results shown in Figure 6.1(b) it can be seen that the

water sorption characteristics of epoxy and the nanoAl2O3 filled epoxy characteristics

are almost similar at 10wt.%. However at 5, 15 and 20wt.% of filler loading, some

difference are observed. This shows that the incorporation of Al2O3 nanoparticles and

its concentration do not influence the basic water diffusion mechanism in epoxy and it

is independent of duration of water treatment. Water absorption shows decrease with

increasing nanoAl2O3 content and this can be explained by two factors: 1) The

volume of epoxy for water diffusion is reduced with increasing nanoAl2O3content. 2)

The presence of the nanoparticles can increase the path length for diffusion of water

molecules [311]. The reduction in weight gain due to water absorption in epoxy-

SiO2/Al2O3/ZnO nanocomposites system may be due to the inability of spherical nano

particles in comparison to the layered silicates in introducing torturous-paths for

diffusion of water molecules [312].

6.3 Hydrophobicity characterization of nanocomposites

The measure of contact angle of a water droplet is an indirect measure of

hydrophobicity of the material.

6.3.1 Effect of filler loading

Figure 6.2 shows variation in contact angle of epoxy-SiO2/Al2O3/ZnO

nanocomposites at room temperature. It is observed that the contact angle increases

up to10wt,% of SiO2/Al2O3 loading in comparison to pure epoxy. However a higher

value of contact angle of 28% is observed at 15wt.% of ZnO loading and this

indicates an increase in hydrophobicity of epoxy-ZnO nanocomposites. It is well

known that increase in contact angle of the specimen reduces the wettability of the

surface [313]. With the addition of 15wt.% and 20wt.% of SiO2 and Al2O3 nanofillers

in epoxy, a reduction of 24% in contact angle is observed as compared to 10wt.% of

SiO2/Al2O3 filled epoxy nanocomposite. Compared to epoxy-SiO2/Al2O3/ZnO

nanocomposite systems, the lowest value of contact angle is observed in epoxy-SiO2

system. The hydrophobicity of any material is not only dependant on chemical

composition and nature of the surface, but also on the surface morphology and

roughness. Therefore, by increasing surface heterogeneity and roughness of material,

hydrophobicity increases.

148

The increase in contact angle for 5, 10 and 15wt.% of epoxy nanocomposite

indicates that the surface of the nanocomposite remains hydrophobic. It is therefore

evident that the electrical performance of insulation can be considered as acceptable

because of the control over leakage current. On the other hand, at 15wt.% and 20wt.%

of SiO2/Al2O3 filled epoxy nanocomposite, contact angle reduces indicating decrease

in hydrophobicity.

Figure 6.2. Variation of contact angle with filler loading of epoxy nanocomposites

This may lead to increase in leakage current. Existence of Si-O and hydrogen

bonds at the surface may result in good adhesion between epoxy and nanofiller. Due

to surface functionalization and lower surface energy of nanocomposite; higher

hydrophobicity will be achieved.

6.3.2 Effect of seawater ageing on contact angle

The variation in contact angle of the pure epoxy and epoxy nanocomposites aged in

seawater at 25C are shown in Figure 6.3. It can be observed from Figure 6.3 that

there is an average decrease in contact angle of 8% in epoxy-SiO2 nanocomposites

after seawater ageing for 240h at 25C. Comparing the contact angle of the pure

epoxy resin with epoxy-SiO2 nanocomposite, it is observed that increase in filler

content leads to increase of contact angle and therefore higher hydrophobicity results.

From Table 6.1 an average decrease in contact angle of 5% and 6% in epoxy-

Al2O3 and epoxy-ZnO nanocomposites after seawater ageing for 240h at 25°C is

observed. The variation in contact angle of the seawater aged epoxy nanocomposite is

less as compared to that of dried epoxy nanocomposite. This implies that the

nanocomposite retains its hydrophobicity even after seawater ageing for 240h and

hydrophobic filler does not permit water to form a continuous film on the surface.

149

Figure 6.3. Effect of seawater ageing of contact angle in epoxy-SiO2 nanocomposites

at 25°C

Table 6.1. Contact angle of dried and saturated for epoxy-Al2O3/ZnO nanocomposites

Composition

(wt.%)

Contact angle (degree)

epoxy-Al2O3 epoxy-ZnO

Dried Saturated Dried Saturated

0 71.09 65.27 71.09 65.27

5 76.82 71.12 73.18 68.34

10 86.47 79.48 80.43 75.80

15 75.86 70.13 90.71 85.61

20 69.94 64.86 Not measured

The absorption of the seawater and adhesion of salts to the surface increases

the surface free energy of epoxy, epoxy-SiO2/Al2O3/ZnO nanocomposites and

therefore there is decrease in contact angle. Similarly, the recovery of the contact

angle is also related to the change in the surface free energy. Further, surface

degradation due to oxidation at high salinity and higher temperature has a permanent

effect and is likely to result in a permanent increase in surface free energy. This

causes a decrease in the contact angle.

6.4 Diffusion co-efficient of nanocomposites

The diffusion co-efficient of epoxy, epoxy-SiO2/Al2O3/ZnO nanocomposites was

computed using, the equation (6.1) and the results are shown in Table 6.2

2

0 5

0 564

.

.

LD

t

(6.1)

Here, D is the diffusion co-efficient, L0.5 is thickness of the specimen, t0.5 =

m(t)/m(∞), m(t) is the initial time of water absorption and m(∞) is the saturation

time of water absorption.

150

From Table 6.2, it is observed that with SiO2/Al2O3 loading of epoxy there is

an increase in diffusion co-efficient by 16% and 44% up to 20wt.%, at 25C. In

epoxy-ZnO nanocomposites the diffusion co-efficient shows decrease at 5wt.% where

as at 10wt.% of nanofiller loading, the value is enhanced to 1.155x10-12

m2/s. With

further increases in filler loading to 15wt.% diffusion co-efficient decreases to

0.894x10-12

m2/s with respect to the basic value of pure epoxy. It is observed that

epoxy-Al2O3 nanocomposite have higher diffusion co-efficient values, Further it is

observed that the diffusion co-efficient of nanocomposite increases with increase in

temperature in epoxy-SiO2/Al2O3/ZnO nanocomposite systems.

The results indicate that the epoxy-SiO2 nanocomposite are more hydrophobic

than pure epoxy because of ceramic nature, surface functionalization of SiO2 and

better interfacial adhesion between the matrix and SiO2 filler. NanoSiO2 loading of

20wt.%, causes a slight increase in diffusion coefficient of epoxy-SiO2

nanocomposites. This is attributed to local inhomogeneities in adhesion, pores,

defects and pin holes that exist at the interface. A slight increase in diffusion

coefficient of epoxy-Al2O3 nanocomposites may be attributed to the presence of water

content, free charge carriers on the surfaces of Al2O3 nanoparticles and higher density

of Al2O3 nanoparticles as discussed under section 4.2.2 on FTIR characterization of

Al2O3 nanoparticles.

Table 6.2. Variation of diffusion co-efficient of epoxy nanocomposites

Composition (wt.%) Diffusion co-efficient (m

2/s)10

-12

25°C % change

(1.0)

50°C % change

(1.0)

75°C % change

(1.0)

epoxy-SiO2

0 0.935 - 1.160 - 2.332 -

5 0.860 8.02 1.075 7.32 2.151 7.76

10 0.914 2.20 1.141 1.64 2.283 2.10

15 0.917 1.96 1.146 1.20 2.293 1.67

20 1.025 8.78 1.281 9.37 2.562 8.97

epoxy-Al2O3

5 0.767 21.9 0.958 17.4 1.917 17.7

10 1.179 20.6 1.470 21.1 2.948 20.8

15 1.294 27.7 1.617 27.9 3.235 27.9

20 1.371 31.8 1.713 32.0 3.427 31.9

epoxy-ZnO

5 0.799 14.5 0.999 14.6 1.998 14.3

10 1.155 19.0 1.441 19.4 2.886 19.1

15 0.894 4.35 0.352 69.8 2.235 4.15

6.5 Effect of seawater on glass transition temperature

Most epoxy manufactures declare that the Tg of epoxy resin does not or only slightly

decrease with moisture. Some researchers have claimed that absorbed water

molecules which form double hydrogen bonds would cause an increase in Tg [314].

On the contrary, the absorbed water in epoxy materials would lead to a decrement of

glass transition temperature due to the plasticizing effect of water [315].

151

Figure 6.4. The plots showing the variation of Tg on a) epoxy-SiO2, b) epoxy-Al2O3

and c) epoxy-ZnO nanocomposites

152

To confirm this finding, DSC experiments were carried out and results are

discussed. Figure 6.4 shows the Tg of the epoxy nanocomposites in both “dried” and

“240h seawater aged” samples. It can be seen that the seawater saturated epoxy

nanocomposites have lower Tg as compared with the dried ones. For epoxy

nanocomposites loaded with 5wt.% of nanoSiO2 fillers, the difference of Tg between

dried and seawater saturated sample is 16°C, whereas for 10, 15 and 20wt.% samples,

there is a difference of ~17°C . In epoxy-Al2O3 system, the difference of Tg is ~12°C

and in epoxy-ZnO system it is ~13°C between dried and saturated samples. The study

shows that the water absorbed by epoxy nanocomposites tends to accumulate at the

interface between polymer molecular and the organic nanosize fillers [316].

The water absorption of epoxy nanocomposites is enhanced by the presence of

nanosize fillers as a result of hydration. The bound water will disrupt the Vander

Waals force between molecular chains and the hydrogen bonds as well. In such a case

the chain mobility will increase, resulting in lower Tg. Polymer composites used

above their Tg, will tend to degrade in terms of physical and mechanical properties

[314, 315].

DSC results also show that the depression of Tg caused by water is

recoverable. Thus drying of samples helps in recovery of Tg. It seems likely that

reversible interactions have occurred between water and the resin under the

experimental conditions used in this study. However, exposure to moisture at elevated

temperatures is expected to produce irreversible effects, which can be attributed to the

chemical degradation of the matrix and to the attack on the fillers/resin interfaces.

This causes an increase of the internal voids of the entangling polymer chain,

promoting chain expansion and the microcracks formation into the polymer matrix

[317].

6.6 Effect of seawater on free volume of nanocomposites

The variation of free volume content or fractional free volume (Fv), free volume size

(Vf) and longest life time (3) with different wt.% of SiO2 in epoxy nanocomposites

are shown in Table 6.3. It is observed that,3 shows increase from 1.62 to 1.67ns, Vf

and Fv also increase from 63.74 to 68Å3

and 2.20 to 2.23% respectively, but for

20wt.% of filler loading, all these parameters show decreasing trend.

With increase in seawater sorption up to 240h, the free volume parameters Vf,

3 and Fv in epoxy-SiO2/Al2O3/ZnO nanocomposites show decreasing tendancy as in

Table 6.3. These changes can be attributed to the size of water molecules ingressed

into the sample which is ~1.5Å in radius. This is less than the existing free volume

size ~2.5Å in radius (measured from PALS) in the nanocomposite. Therefore, as the

seawater gets sorbed in to the nanocomposite (up to saturated level of 240h) the water

molecules certainly occupy the existing free volume space present in the

nanocomposite and this results in the decreasing tendency of free volume parameters.

As compared to epoxy-SiO2 nanocomposites, for epoxy-Al2O3/ZnO

nanocomposites, both in dried and water saturated conditions, a decrease in the free

volume parameters is observed. This might be due to the interaction between the

matrix-filler interface regions and also the filler size of Al2O3/ZnO nanoparticles (30

153

to 45nm) as compared to SiO2 nanoparticles (≤20 nm). The reduction may also be

attributed to the structural changes of epoxy-SiO2/Al2O3/ZnO nanocomposites which

is evident from the results of X-ray diffraction described under section 2.8. There

appears to be slight difference in the observed variation in free volume parameters for

epoxy filled with different ZnO nanoparticles. This can be ascribed to the varying size

of the nanofillers.

Table 6.3. Variation of fractional free volume (Fv), free volume size (Vf) and longest

life time (3) in epoxy nanocomposites

Composition

(wt.%)

Parameters epoxy-SiO2 epoxy-Al2O3 epoxy-ZnO

Dried Saturated Dried Saturated Dried Saturated

0

Vf ± 0.002 (Å3) 68.80 63.74 68.80 63.74 68.80 63.74

Fv ± 0.06 (%) 2.58 2.20 2.58 2.20 2.58 2.20

3 ± 0.02 (ns) 1.68 1.62 1.68 1.62 1.68 1.62

5

Vf ± 0.002 (Å3) 70.52 67.11 60.85 58.74 61.60 59.82

Fv ± 0.06 (%) 2.56 2.33 2.09 1.99 2.11 2.01

3 ± 0.02 (ns) 1.70 1.66 1.59 1.50 1.60 1.52

10

Vf ± 0.002 (Å3) 67.92 65.45 65.76 63.49 63.27 61.19

Fv ± 0.06 (%) 2.39 2.17 1.87 1.68 2.08 1.98

3 ± 0.02 (ns) 1.67 1.64 1.65 1.56 1.62 1.51

15

Vf ± 0.002 (Å3) 72.2 68.00 60.10 58.24 67.03 65.61

Fv ± 0.06 (%) 2.62 2.23 2.09 1.88 1.76 1.59

3 ± 0.02 (ns) 1.72 1.67 1.58 1.51 1.66 1.59

20

Vf ± 0.002 (Å3) 71.35 66.24 Due to certain limitations

measurements could not be performed. Fv ± 0.06 (%) 2.52 2.09

3 ± 0.02 (ns) 1.71 1.65

6.7 Effect of seawater ageing on resistivity

When nanocomposites used in microelectronic devices are operated under marine

environment, they are bound to be affected by environmental factors. Hence this

study focuses reason for understanding of the influence of seawater ageing on ρv and

ρs. The results of ρv and ρs are presented in Figure 6.5 for 96h of ageing. As shown in

figure, there is certainly a significant influence of seawater on the electrical resistivity.

For epoxy and epoxy-SiO2 nanocomposites, ρv decreases by 82 and 91% at 48 and

96h, respectively, whereas ρs decreases by 87 and 67% at 48 and 96h, respectively. It

is observed that the decrease in ρs is very dominant in the 10wt.% filled composite up

to 48h of seawater ageing, and further changes are minimal. This is mainly due to the

surface adhesion of the ions of seawater.

The free volume results further show that 10wt.% filled nanocomposites show

largest free volume as compared to the others, and hence, a processes of faster

absorption and diffusion of seawater are possible.

154

Figure 6.5. The plots of variation of volume and surface resistivity with seawater ageing

of a) epoxy-SiO2, b) epoxy-Al2O3 and c) epoxy-ZnO nanocomposites

155

For seawater sorption, the rate of sorption is initially higher because of the

ionic nature of water, which results in increase in conductivity and increase/decrease

of resistivity [338]. As shown in Figure 6.5, ρv shows a continuous decrease up to

96h, but ρs remains more or less constant after 48h of ageing. The constancy of ρs

after 48h of ageing suggests a possible saturation of the surface with ionic species,

whereas ρv does not show such effects because it is the bulk parameter and the ions

require more time for diffusion. The mechanism leading to decrease in the dc

resistivity results may be explained as follows:

The absorbed water is not confined to one layer but is formed in different

layers of the epoxy matrix. Thus transfer of charge carriers occurs more rapidly and

the conductivity starts increasing. The first few water molecules might be firmly

bound to the nanoparticle. Other water molecules may be loosely bound by Vander

Waals forces. Therefore, concentration of water in the matrix may be sufficient for

conduction through a channel for charge carriers. This results in a decrease in ρv.

Further, high polarity of water molecules attracts charge carriers, particularly at the

filler-matrix interface, and this leads to significant decrease in resistivity [318].

Though the experiments were carried out up to 240h, results are shown only up to 96h

since saturation is observed beyond 96h.

6.8 Effect of seawater on dielectric constant

The variations in dielectric constant as a function of frequency in case of epoxy

nanocomposites with treated SiO2/Al2O3/ZnO nanofillers are shown in Figure 6.6(a-

d). It can be seen from the Figure 6.6 that there is an increase in by 6.4%, 36% and

60% in epoxy-SiO2/Al2O3/ZnO nanocomposite systems after seawater ageing as

compared to dried samples. It is also observed from the results that effective dielectric

constant of epoxy nanocomposites is strongly influenced by the presence of inorganic

nanofillers and their surface characteristics.

It is also observed from Figure 6.6(b-d) that the real part of dielectric constant

is highest in case of epoxy-SiO2 and the lowest in epoxy-ZnO as compared to epoxy-

Al2O3 nanocomposites.

In SiO2 systems, it can be seen from Figure 6.6(b) that at 5 and 10wt.% of

filler loadings, the ' at all frequencies are lower than that of pure epoxy, but at 15 and

20wt.% of filler loading, ' shows higher values. On the other hand, in Al2O3 and

ZnO filled systems results of which are shown in Figure 6.9(c-d), ' over the entire

frequency range is lower than the corresponding values of pure epoxy up to 15wt.%.

The difference in dielectric constant is a result of the reduction of mobility of

the dipolar groups within the composites which will reduce the polarization within the

composites [71]. When the nanofillers are introduced into the polymer materials, an

interaction region between polymer matrix and nanofiller is formed. The multi-core

model proposed by Tanaka [16] can be applied in this case to explain the dielectric

constant variations, details of which are discussed under Chapter 4.

156

Figure 6.6. Plots of variation of dielectric constant with frequency after seawater

ageing of a) epoxy, b) epoxy-SiO2, c) epoxy-Al2O3 and d) epoxy-ZnO

nanocomposites

157

The reasons for increase in dielectric constant with seawater ageing are:

i) Stronger dipoles of water as compared to epoxy.

ii) Damage of polymer chains and,

iii) Increase in segmental mobility due to plasticization increase in free volume and

breaking of weak bonds

Reasons for decrease in dielectric constant with seawater ageing are:

i) Increase in free volume from loose structure.

ii) Increase in free volume due to mobile interlayer nanovoids.

iii) Restricted molecular movements -OH radicals.

These factors result in increase in total dipole strength of the nearby segments [319,

320].

The decrease in dielectric constant of the epoxy matrix with different SiO2

content is due to increase in free volume resulting in a loose structure of polymer

matrix [321]. Due to increase in free volume, a highly mobile interlayer or nanovoid

can be formed around the nanofiller [322]. When silane is used as a coupling agent,

the molecular movement is restricted through OH radicals due to silane coupling. The

surface treatment of SiO2/Al2O3/ZnO nanofillers also leads to a better particle

dispersion rate in the base polymer materials and results in a contribution to the

volume fraction. Therefore, in the present study, the epoxy-SiO2/Al2O3/ZnO

nanocomposites loaded with treated nanofillers have slightly lower dielectric constant.

It can also be observed that the dielectric constant of epoxy and epoxy-Al2O3

nanocomposites, the effect of seawater ageing is minimal.

6.9 Effect of seawater on dissipation factor

The variation of seawater ageing on tanδ value in epoxy-SiO2/Al2O3/ZnO

nanocomposites with different concentrations are presented in Figure 6.7(a-c). It can

be seen from the Figure 6.7 that there is an increase in tanδ by 50%, 84% and 74% in

epoxy-SiO2/Al2O3/ZnO nanocomposite systems after seawater ageing as compared to

dried specimen. In fact, water sorption is higher in epoxy-ZnO system as compared to

epoxy-SiO2/Al2O3 systems. The reasons for increase in tanδ with seawater ageing are

as follows:

i) The tanδ increases because water molecule is polar and the increase in water

content of the nanocomposites leads to increase in the orientation polarization

processes. In addition other physical and chemical degradations may lead to

increase in polarization and conduction processes.

ii) The increase in tanδ is relatively lower in SiO2/ZnO nanocomposite systems as

compared to epoxy-Al2O3 nanocomposites when aged in seawater. This can be

explained on the basis of different morphologies of the two fillers. For epoxy-

Al2O3 nanocomposite, water shell is likely to be located around the lamellae, to

create path which according to literature [314, 323] can increase conduction

current leading to increase in tanδ.

158

Figure 6.7. Graphs showing variation of tanδ with frequency after seawater ageing of

a) epoxy-SiO2, b) epoxy-Al2O3 and c) epoxy-ZnO nanocomposites

159

6.10 Effect of seawater on breakdown strength of nanocomposites

The ac breakdown strength of seawater saturated unfilled epoxy resin, and epoxy-

SiO2/Al2O3/ZnO nanocomposites (for 5, 10, 15 and 20wt.%) are evaluated and the

results are shown in Figure 6.8(a-c).

Figure 6.8. Weibull plots of dielectric strength after saturation for a) epoxy-SiO2,

b) epoxy-Al2O3 and c) epoxy-ZnO nanocomposites

160

Table 6.4. Weibull parameters of nanocomposites after saturation

Composition (wt.%) Shape parameter

()

Scale Parameter

(Eo) kV/mm

epoxy-SiO2

0 6.75 8.09

5 7.85 10.69

10 8.40 11.21

15 9.62 12.48

20 5.63 8.08

epoxy-Al2O3

5 9.84 13.2

10 5.63 9.65

15 10.59 13.69

20 6.79 10.87

epoxy-ZnO

5 10.82 9.18

10 6.26 8.03

15 9.01 8.41

The Weibull analysis is used to determine the shape (β) and scale parameters

(E0). These values are listed in Table 6.4. The results show that there is a reduction in

shape parameter and scale parameter for seawater saturation after 240h for pure epoxy

and epoxy-SiO2/Al2O3/ZnO nanocomposite in comparison to dried samples.

The breakdown strength of the seawater saturated epoxy-5wt.%Al2O3 sample

is higher as compared to the breakdown strength of seawater saturated pure epoxy

sample. The breakdown strength increases with increase in filler loading

concentration in epoxy-SiO2 samples. However, the breakdown strength reduces with

increase in filler loading concentration. On the other hand, in epoxy-ZnO

nanocomposites, the breakdown strength of the saturated 10wt.% loading

concentration is lowest as compared to epoxy with 5 and 15wt.% fillers.

The above results indicate that the presence of water in epoxy nanocomposites

has a significant effect on the breakdown strength of the nanocomposite. Other

researchers have already pointed out that the rapid decrease in breakdown strength is

due to the presence of water in the epoxy resin which helps the charges to conduct

across the sample. In epoxy nanocomposites, water tends to form a layer surrounding

the nanoparticles and under saturated condition, the water content is high enough so

the water layers surrounding the nanoparticles may overlap with each other to form a

conductive path for charge carriers to travel through the bulk of the materials. As a

result, seawater saturated 20wt.% SiO2 and 10wt.% ZnO samples show lower

breakdown strength as compared to seawater saturated pure epoxy sample. Increase in

filler loading concentration will reduce the inter-particle distance and result in higher

possibility for water layers surrounding the nanoparticles to overlap with nearby

layers. Thus a further decrease in breakdown strength is observed in seawater

saturated 10 and 15wt.% epoxy-ZnO nanocomposites.

6.11 Water shell surrounding the nanoparticles

The water shell model is depicted in Figure 6.9. Water existing in the matrix is

defined as “free/liquid water”. The results of hydration measurements show that the

161

nanocomposites absorb water up to 23% in epoxy-SiO2, 53% in epoxy-Al2O3 and

46% in epoxy-ZnO, which is less than the pure epoxy matrix.

In Figure 6.4, the glass transition temperature (Tg) of epoxy-SiO2/Al2O3/ZnO

nanocomposites, measured by DSC, shows reduction in Tg from dried to fully water-

saturated specimens. This shows that in epoxy-SiO2/Al2O3/ZnO nanocomposites, the

amount of water in the resin is almost independent of the concentration of filler.

However, it is observed that there is up to 23% extra water in SiO2 nanofilled

specimens. It is not possible for the extra water to have been located in the bulk of the

epoxy since its Tg would be reduced further for the water saturated case. The most

obvious implication is that the extra water is located around the nanoparticles, most

likely at the interfaces.

Under this assumption, some water will surround each nanoparticles and the

rest of the water exists in the epoxy matrix in the form of “free” or “liquid” water. In

any case, the link between silica and epoxy is more likely to be broken by water and

the adhesion of the epoxy to the silica also becomes weak so as to reduce mechanical

strength of the nanocomposite.

Since the density of silica is higher than that of epoxy matrix, its volume

percent is ~0.79%. In other words, the distance between one nanoparticle and its

neighbor particle is about seven times larger than the average size of particles under

conditions of dispersion. If the size of nanoparticles is same, e.g., 21nm, the distance

between one particle and its neighbors will be 105-141nm. For a given humidity,

assuming that pure epoxy absorbs x% of water and nanofilled epoxy absorbs y% of

water, extra water absorbed by the nanofilled epoxy is therefore (y-x) %. Hence, the

thickness of each water shell can be calculated using this concept.

6.11.1 Water shell model

The “water shell model” is used to describe the real situation describing how water

exists in polymer nanocomposites this is shown in Figure 6.9. This concept was

originally proposed by Lewis [22] and developed as a multi-core model by Tanaka

[16], with nanoparticle playing the role of “core”. In Figure 6.9, the nanoparticle is

shown in grey color at the centre, surrounded by layer I of water.

Figure 6.9. Water shell model

162

A higher concentration of water is present in layer II as an interconnected

region which is indicated by white region. Outside this layer, which is layer III, water

is in lower concentration and may not be conductive.

Quasi-dc behavior at low frequency is typically observed when carriers have

some limited freedom of movement and under the influence of the electric field,

follow tortuous paths that do not allow transport right through the material. At low

temperature and low hydration, water cannot provide the channels for carriers to go

through the bulk of nanocomposites because of the low activity/mobility (at low

temperature) or low content (at low hydration) of the water, where the carriers can go

through. However, at high temperature (above Tg) and high hydration, the situation is

similar to the prediction by the water shell model shown in Figure 6.10. The paths for

charges and carriers are formed by the overlapping of water layers surrounding

nanoparticles. It seems likely that the carriers are moving through partially

interconnected interaction zones that surround the particles.

Figure 6.10. The water shell as the path for carriers in nanocomposites (lower

concentration)

The water shell model for low filler concentration is shown in Figure 6.10.

Under dried conditions, the gap between two particles is too far for their interaction

zones to overlap and only conduction current is possible in nanocomposites by charge

carriers going through the matrix. However, when enough of water enters the

nanocomposites, the water layers surrounding nanoparticles may overlap and provide

the paths for charges and carriers. This causes the QDC behavior at low frequencies.

The higher the content of nanoparticles, the shorter is the distance between particles,

and the lower is the hydration required to make the water shells around nanoparticles

overlap.

6.12 Percolation phenomenon in epoxy nanocomposites

In an epoxy nanocomposite in which the particles are well dispersed, unless they are

arranged to be “self-repelling”, the distance between the particles will be distributed

163

according to Poisson distribution [19]. The probability P, that one interaction zone

will overlap with one of its neighbor‟s (for low concentration) is then given by

equation 6.2 for spherical particles and is as shown in Figure 6.11. Here„t‟ is the zone

thickness and„d‟ is the average particle separation [23]. For a 50% probability of

overlap, t/d would be 0.345 and a 20nm particle would require an interaction zone

thickness of 9.078nm [23].

21

tP exp

d

(6.2)

For charge carriers to percolate through overlapping shells, the volumetric

concentration zone and spherical particle must exceed 19% [324]. Figure 6.11 shows

the required ratio of shell thickness to particle radius as a function of volumetric

concentration of particle for complete percolation to occur [19]. For 10wt.%, of the

volumetric concentration, it is approximately 5%. Thus, for these 10nm radius

particles, the thickness of layers 1 and 2 must exceed 10nm for full percolation [19].

One would therefore expect extended charge carrier movement but at a sub-

percolation levels [19].

Figure 6.11. Probability of interacting particle zones

At low levels of RH and at lower concentration of filler, the shells would not

be expected to overlap much and conduction could be largely determined by the

conduction through the epoxy matrix between the particles. This would result in a low

frequency dielectric characteristic in which is directly proportional to 0

and is

directly proportional to -1

. This is observed in epoxy-SiO2/Al2O3/ZnO

nanocomposite systems at lower RH levels, the higher activation energies observed at

low levels of RH in the matrix are similar to that for 5wt.%, in which the volumetric

concentration of particles is ~1.5%. In 5wt.%, percolation through overlapping water

shell will be less common.

164

In Figure 6.12, the value of zone thickness or particle radius as a function of

filler content is shown. At lower levels of relative humidity and lower filler

concentration shells do not overlap and conduction is through the matrix between

filler particles as seen in low frequency response. This accounts for behavior at

5wt.%. Hence percolation through overlapping water shell is less possible. Thus drift

velocity for charge transport will be determined by the carrier movement through the

epoxy rather than through percolation overlapping in water shell. However, in

10wt.%, percolation is much more likely.

Figure 6.12. Percolation occurs on the “ ” side of the line

If both epoxy matrix and inorganic filler (with their interfacial phase) are

considered as capacitive, it can be assumed that a series model is applicable to epoxy

composites as shown in Figure 6.13.

Figure 6.13 The equivalent circuit of epoxy nanocomposites

According to Figure 6.13, the following relation can be obtained

1 1 2 2

1 1

' " ' " ' "

T T

P P

i i i (6.3)

Here P (0 < P < 1) is a ratio coefficient to describe the contribution of epoxy part. The

contribution of particle part is (1-P). Hence,

165

2 2 1 1

1 1 1

1

' " ' " ' "

T T

P

i P i i

1 1 1 1 1 12 2 2

1 1

1

' ' " " ' ' " ' ' " " "' T T T T T T

' ' " "

T T

( )( P ) ( )( P )( P ).

( P ) ( P )

(6.4)

1 1 1 1 1 12 2 2

1 1

1

" ' ' " ' ' ' ' " " " "" T T T T T T

' ' " "

T T

( )( P ) ( )( P )( P ).

( P ) ( P )

(6.5)

The dielectric constant T*

of filled epoxy composites, at 5 and 10wt.% at 25°C

and 100% RH is known, compared to the data of matrix (1*

) at 25°C and 100% RH.

From this dielectric constant of particles in composites can be calculated using

equations 6.4 and 6.5.

Figure 6.14. The dielectric constant of epoxy-SiO2 filler predicted by effective media

theory a) 5wt.% and b) 10wt.%

166

By adjusting the value of P, and using the raw data of epoxy matrix (1 and

1) and nanocomposites (T

and T

) at 100% RH and 25°C, the dielectric constant of

particle part in nanocomposites (the interfacial part is also included as the particle

part) at 25°C and 100% RH, can be calculated using equations 6.4 and 6.5. Under the

real condition, 2 > 0, 2

> 0, and the value of P should be chosen appropriately to

maintain this requirement.

Figure 6.14 shows, calculated real and imaginary dielectric constant at 25°C

and 100% RH for (a) 5wt.% nanoparticle system and (b) 10wt.% nanoparticle system

on the basis of equation 6.4 and 6.5. From Figure 6.14(a-b), it is possible to observe

QDC behavior for all possible values of P. At high temperatures, the nanoparticles are

surrounded by water shell and they behave similar to the epoxy matrix.

6.13 Ageing

Rowe [325] has proposed an “alternative scenario” for ageing of composite materials

in which the interfaces between the particles and the host material gradually weaken

as shown in Figure 6.15.

Figure 6.15. Representation of the “Alternative Ageing Scenario”, showing water

degraded, filler interfaces [325]

In this case, water may weaken the interfaces between the epoxy and silica.

Thus accumulation of water modifies the mechanical strength, the dielectric

properties and the electrical strength of these tiny regions [325]. Since this

degradation is diffused and occurs at all interfaces at the same time, it can be thought

of as ageing in the true sense [34]. Gradually a network of semi-interconnected

pathways builds up through the labyrinth of filler particles. If percolation through

these graded regions occurs, (Figure 6.15), an electrical pathway is formed and leads

to breakdown. This “alternative ageing scenario” for highly filled HV insulation is

fundamentally different from other concepts of ageing. In particular, the ageing

precursor is not dependent upon the electrical field, charge, etc. It is only after the

damage has been initiated by water, which is inevitably present, that electrical

degradation starts to occur [325].

167

6.14 Conclusions

In this work, the effect of water uptake on the dielectric properties of epoxy materials

has been studied. Both theoretical considerations and experimental results show that

some water content can exist in epoxy nano structures because of relatively larger free

volume.

i) The weight gain due to water absorption in samples increases in epoxy-ZnO

system as compared epoxy-SiO2/Al2O3 systems.

ii) The contact angle increases up to 10% in epoxy-SiO2/Al2O3 systems, but in

epoxy-ZnO systems it increases up to 15wt.%.

iii) Diffusion coefficient, free volume and breakdown strength show variations

with increase in filler loading in the case of epoxy-ZnO systems.

iv) Dielectric constant and tanδ show increase and glass transition temperature

decreases in epoxy-SiO2/Al2O3/ZnO nanocomposite systems due to water

absorption. Volume and surface resistivity show similar trends in variation due

to water absorption.

v) In particular electrical conductivity and dielectric loss increases while electric

strength shows a considerable decrease due to water absorption. The

difference in the electrical properties of the composites due to water

absorption can be associated directly to different filler morphological

characteristics like the nanoparticle aspect ratios.

vi) With increasing temperature, once the hydration in epoxy nanocomposites

reaches a certain level, the dielectric properties at the low frequencies will

transform into a quasi-dc/LFD process due to conduction process. This

phenomenon is explained by a "water shell" model, which assumes that a

water shell around nanoparticles provides a channel for charges.

vii) The dielectric behavior of epoxy composites are influenced by low

concentration of nanofillers. Thus small amount of fillers can be used to tailor

the properties of composites. However in epoxy composites, it is important to

control hydration in epoxy materials.

viii) The influence of water on the dielectric properties of epoxy composites can

be explained by "water shell" model. Experimental results show that nanofiller

water absorption leads to increase in dielectric constant and tanδ, on the other

hand, breakdown strength and resistivity shows reduction in the epoxy-

nanocomposites.

This investigation, aims at analyzing nanocomposites for proper selection of

filler for achieving desired modifications in electrical properties of nanocomposites,

in relation to its morphology and water absorption characteristics. This will aid the

design and manufacturing processes of nanostructured materials for optimization of

its performance.

Electrical insulation failures are invariably due to surface stresses leading to

wear and bulk stresses leading to mechanical breakdowns. The mechanical

characteristics of the nanocomposites are presented and discussed in Chapter 7.