Chapter 9

Electrical Properties

Abstract

The electrical properties of banana fibre reinforced phenol formaldehyde

composites have been studied with special reference to fibre loading, fibre

treatments and hybridization with glass fibres. The dielectric constant

decreased with frequency and fibre loading. Treatments with silanes,

NaOH and acetylation, latex treatment, heat treatment and cyanoethylation

decreased the dielectric constant value. The dielectric constant is higher

for banana/PF composites than that of glasslPF compos~tes. In hybrid

composites the dielectric constant decreased with increase in glass fibre

concentration. The volume resistivity of banana fibre reinforced phenol

formaldehyde composites decreased with frequency and fibre loading.

Chemical modifications in the fibre increased the volume resistivity of the

composites. Volume resistivity of bananalglass hybrid composites increased

with increase in glass fibre content. Layered composites with glass fibre at

periphery were found to have higher volume resistivity than other layering

patterns. The loss factor decreased by the chemical treatments and in hybrid

composites it decreased with increase in volume fraction of glass fibres.

Results of this chapter have been communicated forpublication in Journal of Applied Polymer Sciences

246 Chapter 9

The most desirable combination of characteristics such as ease of

fabrication, low cost, light weight, and excellent insulation properties have

made plastics one of the most desirabte materials for electrical

applications. The use of plastics in electrical applications was limited to

nonload bearing general-purpose applications. Fibre . reinforced plastic

materials not only act as effective insulators, but also provides mechanical

support for field carrying conductors. Phenol formaldehyde resin is well

known for its electrical insulator characteristics. But it is very brittle and has

poor mechanical properties. l ncorporation of fibrous reinforcements in

polymer matrices leads to high performance composite materials having

very good mechanical properties and at the same time suitable for

electrical applications. They can be used as terminals, connectors,

industrial and house hold plugs, switches and printed circuit boards.

Incorporation of fillers and reinforcement changes the electrical properties

of polymers. The shape of filler particles, dispersion, orientation of the

particles, etc. also affects the electrical properties of the composite. It is

reported that anisotropy in electrical conductivity is observed in carbon

black, carbon fibre, steel fibre and aluminium flake filled polyethylene

composite [I]. The processing techniques as well as processing conditions

influence the electrical property of the end product [2-51. Generally the

introduction of conductive fillers to matrix resin is found to develop

conductivity to the final composite product [6-81.

The use of the composite materials in electrical applications makes the

study of the electrical properties of composites like dielectric constant,

volume resistivity and loss factor important. Electrical properties of

polybithiophene- polystyrene composites were analyzed by Morsli et al. [9].

Kortschot and Wood hams [I 01 reported that for maximum efficiency of

conductive com posites very high aspect ratio flakes are recommended and

Electrical properties 247

these particles will be most effective when they have random orientation in

the composite.

Wu and Tung [I I ] did dielectric studies of mineral filled epoxy composites

containing different dielectric fillers. The results indicated that the dielectric

constant increases with the addition of the dielectric filler and with an

increase of dielectric constant of the filler.

Composite materials based on undoped conjugated polymers and

conductive filler (Graphite and single-walled nanotubes) were synthesized

and their electrophysical and electrochemical properties were investigated

by Tchrnutin et al. [12]. Conjugated polymers such as polyaniline (PANI),

polyacetylene (PA) and typical polymer dielectric, polypropylene (PP) were

used as matrices. The investigation of the dependencies of conductivity on

filler concentration showed that in contrast to PP and PANI, PA becomes

conductive due to injected charge carriers from filler particles. It was

shown that a conjugated polymer matrix allows composite materials with

new electrophysical and electrochemical properties to be produced.

Dutta et al. [I31 studied the mechanical and electrical anisotropy of

pineapple fibres. They found a sharp increase of dielectric constant and

fall of loss factor along fibre direction compared with that of the transverse

direction due to increase in crystallinity along the fibre direction. Electrical

properties of pineapple fibre reinforced pol yet hylene composites were

studied by George et al. [14]. They found an increase in the dielectric

constant of composite with fibre loading due to increased orientation and

interfacial polarization. The electrical properties of natural fibres were

studied by Kulkarni et al. [15]. Effect of sisal fibre orientation on electrical

properties of sisal fibre reinforced epoxy composites were analyzed by

Chand and Jain [16]. Paul and Thomas [I71 compared the electrical

248 Chapter 9

properties of sisal fibre reinforced tow-density polyethylene composites

with that of carbon black and glass fibre filled low-density polyethylene

(LDPE) composites. At low frequencies, the dielectric constant varied in

the order: neat LDPE<glass-LDPE<sisaI-LDPE<coir-LDPE< carbon black-

LDPE. Paul and Thomas [I81 also analyzed the .effect of surface

treatments on the electrical properties of low-density polyethylene

composites reinforced with short sisal fibres. They found that treatments

such as alkali, CTlDlC (urethane derivative of cardanol), stearic acid,

peroxide permanganate and acetylation decreased the dielectric strength

of the composites as these treatments decreased the hydrophilicity of the

composites.

In the present work, the electrical properties of phenol formaldehyde resin

and banana fibre reinforced phenol formaldehyde composites are

analyzed. The effect of chemical treatments with alkali, silanes,

acetylation, latex treatment, heat treatment and cyanoethylation on the

dielectric constants, volume resistively and loss factor are studied. The

electrical properties of bananalglass/ hybrid fibre reinforced composites

with varying hybrid ratios and layering patterns are also analyzed.

9.1. Dielectric constant (E)

9. I. 1. Banana fibre reinforced PF composites

A plot of dielectric constant as a function of log frequency for neat PF and

banana fibre reinforced PF composites at different fibre loadings are given in

Figure 9.1. The dielectric constant decreases with increase in banana fibre

loading for entire range of frequencies.

Electrical properties 249

--e- 18% banana fibre PA-- 28% banana fibre

log Frequency ( Hz)

Figure 9.7. Plot of dielectric constant as a function of frequency for neat PF and banana fibre reinforced composites at different fibre loadings (volume %)

This decrease is higher for low frequencies and lower for very high

frequencies. The variation of E value with banana fibre loading at a low

frequency (10 Hz) is given in Figure 9.2. The highest dielectric constant

value is obtained for neat PF. The dielectric constant decreases with

increase in frequency for neat PF as well as for all the composites. The

change of dielectric constant with frequency is the highest at very low

frequencies. The change of t: with frequency can be explained in terms of

interfacial polarization.

250 Chapter 9

0 10 20 30 40

Banana fibre loading (volume %)

Figure 9.2. The variation of E value with banana fibre loading at low frequency ( 10 Hz) for banana fibre reinforced phenol formaldehyde composites

The dielectric constant of a material depends upon the polarisability of the

material. If there is greater polarisability of the molecule, E will be high. The

dielectric constant of a polymeric material depends on interfacial, dipole,

electronic and atomic polarization 1141. The atomic and electronic

polarizations are instantaneous polarization components, the effect of

which is seen only at high frequencies. The dipole or orientation

polarization occurs due to the presence of polar groups in the material.

The interfacial polarization arises due to heterogeneity, which is highest at

lower frequency. Due to the presence of polar groups in PF resin, c value

is found to be high. Since composite is heterogeneous, the interfacial

polarization also exists. Interfacial polarization influences the low

frequency E values. The frequency dependence of dielectric constant is

due to the fact that at low frequencies complete orientation of the molecule

is possible. But at medium frequencies there is only little time for

Electrical properties 251

orientation. Orientation of molecule is not possible at all at very high

frequencies. The frequency dependence of dielectric constant due to

interfacial polarization is similar to those corresponding to dipole

polarization. The E value of lignocellulosic fibre is lower than that of phenol

formaldehyde resin [19]. So the incorporation of fibres will decrease the

dielectric constant.

9.1.2. Effect of chemical treatment

Figure 9.3 shows the variation of dielectric constant with frequency for

chemically treated banana fibre reinforced phenol formaldehyde

composites. We have seen in Chapter 4 that sodium hydroxide

treatment removes wax, cuticle and alkali soluble constituents of

hemiceflulose and increases the crystallinity of the fibre thus decreasing

the water sorption capacity of the fibre. So moisture content is

decreased by the alkali treatment of fibre. Hence the polarization due to

the presence of water molecule is decreased. So dielectric constant of

alkali treated composite is lower than that of untreated fibre. The

dielectric constant value of acetylated banana1PF composite is lower

than untreated composite. This is because the hydrophilic nature of the

fibre is decreased by acetylation. The sodium hydroxide pretreatment

before acetylation results in partial removal of hydrogen bonding in

cellulose chains and make the fibre surface more reactive for

acetylation.

Similarly the silane treatment also reduces the moisture content, which

decreases the dielectric constant. In the case of heat-treated corn posites

the interface is not affected. But what happens is that there is an increase

in crystallinity of the fibre [20], which can improve the composite

properties. Similarly, the decomposition of less stable components and

volatile components occurs by the heating thus reducing the moisture

252 Chapter 9

content. So the dielectric constant is lower for heat treated composite also.

The very low value of dielectric constant for cyanoethylated and latex

treated fibre composites is also due to the decrease in orientation

polarization resulting from the increased hydrophobicity of the fibre. The

moisture content decreases due to the presence of. hydrophobic latex

coating in the latex treated fibre.

-A- Aminosilane treated -r- Heat treated -+- Latex treated -+- NaOH treated -x- Vinyl silane treated ----I- Untreated

---_

m----- -- +---

log Frequency ( Hz)

Figure 9.3. Effect of chemical treatments on dielectric constant versus log frequency for banana fibre reinforced PF composites

9.1.3. Effect of hybridization

Effect of hybridization using glass fibres on the dielectric constant of

banana fibre reinforced phenol formaldehyde composites is shown in

Figure 9.4. The dielectric constant value is found to be higher for banana

fi brelPF corn posites than that of glass fi brelPF corn posites. A plot + of

dielectric constant versus glass fibre content in bananalglass hybrid

Electrical properties 253

composites is given in Figure 9.5. In hybrid composites the dielectric

constant decreases with the increase in glass fibre content. This is due to

the hydrophobic nature of glass fibre whilst banana fibre is hydrophilic.

Relative volume fraction of glass fibre - 0, - 0.1 2,+ 0.26 + 0.35, + 0.5, - 0.75

0.5 1 .O I .5 2.0 2.5 3.0 3.5 4.0 4.5

log frequency(H2)

Figure 9.4. Effect of hybridization using glass fiber on the dielectric constant vs. log frequency curve of banana fibre reinforced phenol formaldehyde composites

The part of the orientation polarization, which is due to the presence of

polar water molecules, is very low in glass fibre composites when

compared with banana fibre composites. So low c value is obtained for

glasslPF composites. But as the banana fibre content is increased, the

polarization increases and the E value increases.

254 Chapter 9

I I I I I I 1 I I 0 20 40 60 80 100

Glass fibre content (volume %)

Figure 9.5. Variation of dielectric constant with glass fibre content in banana/glass hybrid composites (Frequency 10 Hz)

9.1.4. Effect of hybrid layering pattern

The effect of hybrid layering pattern on the dielectric constant value of

bananalglass1 hybrid fibre reinforced composites are given in Figure 9.6.

The dielectric constant values of hybrid composites with layering patterns

GB, BGB and intimate mix of B and G are found to be higher than that of

GBG and GBGBG layering patterns. This is because of the presence of

banana fibre in the outer layers of composites, which will increase the

dielectric constant of hybrid composites with layering patterns GB, BGB

and intimate mix of B and G. But in GBG and GBGBG arrangements the

banana fibre is in the core and glass fibre mat at the periphery, resulting in

low interfacial polarization due to polar water molecules.

Electrical properties 255

- GB + GBG ---A- GBGBG --t-- Intimate mix - BGB

log Frequency (Hz)

Figure 9.6 Effect of hybrid layering patterns on the dielectric constant vs. log frequency cun/e of bananaMass hybrid fibre reinforced phenol formaldehyde composites

9.2. Volume resistivity

9.2.1. Banana fibre reinforced PF composites

The resistivity of lignocellutosic fibres depends on the moisture content,

crystalline and amorphous component present, presence of impurities,

chemical composition, cellular structure, microfibrillar angle etc. The shape

of reinforcement determines the interparticle contact, which affects the

conductivity of the system. Fibres and flakes having elongated shapes

enhance electrical conductivity effectively [21]. The moisture content in fibres

increases conductivity and the composite defects during processing will

increase the resistivity [15]. A plot of volume resistivity as a function of log

frequency for banana fibre reinforced phenol formaldehyde composite is given

256 Chapter 9

in Figure 9.7. In all the composites the volume resistivity decreases with

increase in frequency.

--t- Neat PF & 18% banana fibre

1- A 28% banana fibre l-._ 32 O/O banaan fibre

-1. -. + 48% banana fibre =-- -- \--_-- -a

log frequency (Hz)

Figure 9.7. Log volume resistivity versus log frequency curves of banana fibre reinforced PF composites at different fibre loadings.

As the frequency increases the resistivity shows a decrease due to the

interfacial polarization arising due to the heterogeneity of the system. The

maximum volume resistivity is obtained for neat PF. A plot of log volume

resistivity versus banana fibre loading is given in Figure 9.8. The

incorporation of banana fibre decreases the volume resistivity of the

composite. In other words the electrical conduction increases with increase

in banana fibre loading.

Electrical properlies 257 .-

Banana fibre loading (%)

Figure 9.8. Variation of volume resistivity with banana fibre loading in banana fibre reinforced PF composjtes

The hydrophilicity of the cellulose fibre is responsible for the greater

conductivity of the composite. In the case of polymeric materials most of

the current flow through the crystalline regions and noncrystalline region

allows current to pass through it mainly when moisture is present [-17]. The

phenol formaldehyde is an amorphous polymer and it has high volume

resistivity. But banana fibre is semicrystalline. The chemical composition

and structure of lignocellulosic banana fibre is very complex. The hydroxyl

groups in the hydrophilic fibre can absorb moisture and hence the

presence of the natural fibre increases the conductivity of the resin.

9.2.2. Effect of chemical treatment

The log volume resistivity versus log frequency curves of untreated, alkali

treated, cyanoethylated, acetylated, latex treated, silane treated and heat

treated banana fibre reinforced phenol formaldehyde composites are given

in Figure 9.9.

258 Chapter 9

-4- - Acetylated, -@- NaOH treatment, -A- Latex treatment, -r-- Vinyl silane treatment, -+- Heat treatment, - - -+- Untreated -x- Amino silane, -m- Cyanoethylated

tog Frequency (Hz)

Figure 9.9. Effect of chemical treatments on log volume resistivity versus log frequency for banana fibre reinforced PF composifes at different fibre loadings.

Volume resistivity values of all the treated composite samples remained

higher than that of the untreated composites. In fact, conductivity of the

cellulose fibre is mainly due to the presence of water molecules absorbed

by fibre surface.

Cyanoethylation, silane treatments, acetylation and latex treatment reduce

the water absorption of fibre in the composite considerably. Heat treatment

removes the moisture content in the fibre. So the resistivity of the

composite is increased by chemical treatments i'

Electrical properties 259

9.2.3. Effect of hybridization

The effect of hybridization using glass fibres on the volume resistivity of

banana fibre reinforced phenol formaldehyde composites is given in Figure

9.10. The volume resistivity of glass fibre reinforced composites is higher

than that of banana fibre composite. This is due to the high hydrophilicity

of banana fibre compared to glass fibre. A plot of volume resistivity versus

glass fibre content in bananalglass hybrid composites is given in Figure

9.1 1. The volume resistivity values of hybrid composites do not show a

marked difference. But there is a slight increase in volume resistivity with

increase in glass fibre content.

Relative volume fraction of glass fibre 0, --t-- 0.12, A 0.26

- --w 0.5, + 0.75, 1

log frequency (Hz)

Figure 9.10. Volume resistivity versus log frequency for banana glass hybrid fibre reinforced PF composites at different relative volume fractions of glass

260 Chapter 9

Glass fibre content (volume%)

Figure 9.1 1. Variation of volume resistivity wifh glass fibre content in banana/glass hybrid composites (Frequency 7 0 Hz).

9.2.4. Effect of hybrid layering pattern

Effect of fibre layering patterns on the volume resistivity of banana/glass

hybrid composites containing banana: glass volume ratio 0.25:0.75 is

given in Figure 9.12.

-. - . . . . . .- -

-.&- GB - GBG

A - GBGBG r Intimate --t- BGB El

log frequency (Hz)

Figure 9.12. Volume resistivity versus log frequency curves of banana/glass hybrid fibre reinforced PF composites having different layering patterns.

Electrical properties 26 1

The composite having intimate mixture of banana fibre and glass fibre is found

to have minimum volume resistivity value. The layered composites are having

higher volume resistivity than that of intimate mix. The hybrid composites

having glass fibre at periphery (GBG and GBGBG) are found to have higher

volume resistivity values than other arrangements (GB and -BGB). This is due to

high volume resistivity value of glass fibre compared to banana fibre. The

resistivity of hybrid composites with layering patterns Intimate mix, GB and BGB

arrangements are low due to the presence of banana fibre in the periphery in

these composites.

9.3. Dielectric loss factor Figures 9.13. Shows the loss factor as a function of logarithm of frequency for

banana fibre reinforced PF composites at varying fibre loadings. In banana fibre

composites, at low frequencies as the fibre loading increases, the loss factor

also increases. But at high frequencies a reverse behavior is observed and the

values come closer. This is due to polarization of the fibres at low frequencies,

which is absent at higher frequencies. As the fibre loading increases, the

heterogeneity of the system increases and the polarization increases.

I I I I I I 1 2 3 4

log Frequency (Hz)

18% banana fibre 1 i 28°/~ banana fibre 1 32% banana fibre

-#- 48% banana fibre

Figure 9.13. The variation of loss factor as a function of logarithm of frequency for banana fibre reinforced composites with different fibre contents (volume %).

Figure 9.14 shows the variation of loss factor with frequency for banana

fibre reinforced composites after chemical treatments.

-.. . -- - - Acetylated - Amino silane. treated -- --L- Cyanoethylated --r-- Heat treatment + Latex treatment -+ NaOH treated '..

m-. '.. \--.. .'. - Vinyl silane treated '*\. * Untreated I I

log Frequency ( Hz)

Figure 9.14. Effect of chemical treatments on loss factor versus log frequency for banana fibre reinforced PF composites.

In this study, thus we see that all types of treatments except the NaOH

treatment decrease the loss factor. This is due to reduced orientation

polarization as a result of chemical treatment. The loss factor is found to

be very low for acetylated and cyanoethylated corn posites.

Figure 9.15. is the loss factor versus log frequency plots of bananalglassl

hybrid composites. Here maximum loss factor is obtained for banana fibre

composites.

Electrical properties - 263

log frequency (Hz)

2.0

1.5-

1.0-

0.5 -

0.0 -

Figure 9.1 5. Effect of hybridization with glass fiber on loss factor versus log frequency for banana fibre reinforced PF composites.

-- . -

Relative volume fraction of glass fibre

V- - 0.5, -+-- 0.75, -+- 1 =. v:.,

!

----.

This is due to the higher degree of polarization of banana fibre, which is

very low for glass fibres. As the glass fibre loading increases the loss

factor also increases.

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