Conductive Mechanism of AntistaticPoly(ethylene terephthalate)/ZnOw Composites

Ke Chen, Chuanxi Xiong, Liubin Li, Liu Zhou, Youan Lei, Lijie DongSchool of Materials Science and Engineering, Wuhan University of Technology,Wuhan 430070, People’s Republic of China

Antistatic composites of poly(ethylene terephthalate)(PET) and tetrapod-shaped ZnO whisker (ZnOw), thesurface of which was treated with a coupling agent,were prepared by melt-mixing using a twin screw ex-truder. The structure and morphology of the blendswere examined by FTIR and SEM. Antistatic perform-ance and the effect of ZnOw loading levels were inves-tigated. The conductive mechanism of PET/ZnOw com-posites is presented, which can be classified into tun-nel effect, discharging effect at the pinpoint andconductive network. The simple formula of critical vol-ume fraction (vc) for ZnOw forming conductive networkis proposed, based on the actual processing conduc-tion. Comparison of the theory value to the experimen-tal data showed a close agreement. POLYM. COMPOS.,30:226–231, 2009. ª 2008 Society of Plastics Engineers

INTRODUCTION

Poly(ethylene terephthalate) (PET) is widely used in

various fields such as packaging materials, electrical and

electronic parts, and automotive components because of

its good mechanical and thermal properties. PET is inher-

ently an electrical insulator, exhibiting the surface resis-

tivity of 1015–1016 O. The static charge that easily builds

up on such molded parts by contact or rubbing may cause

an electrostatic discharge, which becomes a serious prob-

lem because there may be electrostatic damage to sensi-

tive semiconductor devices and interference with circuit

operation. To solve these problems, a lot of antistatic

materials are often incorporated into the polymer at the

molding process or applied to the surface of the products

in a finishing process to control the resistivity [1].

According to the properties of antistatic materials, they

can be divided into the following categories:

(1) Conductive additives, such as carbon black [2–4],

metal powder [5, 6], or conductive fibers [7, 8], can

be blended with polymer matrix to dissipate the elec-

tronic charge.

(2) The substance whose molecular structure is originally

conductive, such as conducting polymer, can be used

to prepare antistatic materials [9–12].

(3) Antistatic agents, such as nonionic surfactants or

hydrophilic substance, can be added to the surface of

products, or blended into raw materials to form anti-

static composites [13–15].

Because of its excellent properties and lower price,

carbon black is blended into polymer materials to provide

the conductivity of materials. Although carbon black can

serve as a good conductive additive, it will darken the

surface of polymer products, a major defect making color-

ing of polymers more difficult.

Many studies have indicated that polymer composites,

prepared as a mixture of a conjugated conductive polymer

and a classical nonconductive polymer, can show quite a

good electrical conductivity at a relatively low content of

the conducting phase. However, there have been some

problems in the application of the commercial production

of antistatic composites and films because conductive poly-

mers are not cost effective.

The antistatic effect of surfactants or hydrophilic sub-

stance is realized by the equilibrium moisture adsorbed

on the surfactant, so excellent antistatic effect is not

achieved under low humidity. Also, the surfactant can be

removed by rubbing or washing back and forth, so the

antistatic effect disappears easily.

ZnO, a compound semiconductor with a wide direct-

band gap of 3.3 eV, is a good candidate in some applica-

tion fields, especially, tetra-shaped ZnO whisker (ZnOw).

It is a new kind of micro-fiber consisting of a single crys-

tal and possesses good properties such as high strength,

good wear resistance, microwave absorption, and antibac-

terial properties. It can be widely used as both functional

and structural materials [16]. Research indicates that

ZnOw is an effective agent for antistatic of resin materi-

als. In this study, ZnOw fillers are dispersed into PET ma-

trix, the structure and morphology of these composites are

discussed. The antistatic performance and electronic con-

duction mechanism are studied in detail.

Correspondence to: Chuanxi Xiong; e-mail: polymerlab@mail.whut.edu.cn

DOI 10.1002/pc.20679

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2009

EXPERIMENTAL

Materials

Poly(ethylene terephthalate) pellets (0.8 dl/g with

intrinsic viscosity and 1.3 g/m3 with density) used in this

work were commercial polymers manufactured by Far

Eastern Industries (Shanghai). ZnOw, in the form of a

white fluffy powder in appearance and with a tetrapod-

shaped structure in microimage (as shown in Fig. 1) was

purchased from Advanced Technologies and Crystal-wide.

The surface resistivity is 103–104 O, with the length of

the whisker needle of about 5–200 lm, the diameter at

the base of the whisker needle of 0.1–10 lm, real density

and apparent density of 5.8 and 0.01–0.5 g/cm3, respec-

tively. Spherical ZnO (s-ZnO) powders were purchased

from Tianjin Tianda Purified Materials Fine Chemical.

APTMS (c-aminopropyltrimethoxysilane) was supplied by

Wuhan University Silicone New Materials. The other

materials were used without any purification.

Modifying the Surface With Coupling Agents

Two kinds of ZnO powders were dried at 608C for 10 h

and put into to a glass flask with a mechanical agitator,

water bath, condenser, and thermometer. The coupling

agents APTMS was used with 1 wt% ZnO and mixed with

acetone. The mixture was stirred and refluxed at 588C for

2 h. Then the suspension solution was deposited for 1 h at

room temperature and then filtered. The treated ZnO was

dried to constant weight in a vacuum drying oven.

Preparation of PET/ZnO Composites

Binary blends of PET pellets and ZnO were melt

mixed by using a SJJ-06 twin screw extruder (Ruiming

Plastics Machine Company) with a screw diameter of

11.5 mm and L/D (length–diameter ratio) of 10. The bar-

rel was set with temperature of 2608C and a screw speed

of 60 rpm. The mixture was stirred about 10 min and

then the extruded strand was cut into pellets for injection

molding. The extruded pellets were molded into the

standard specimens using a WZM-1 micro injection mold-

ing machine (Jilin University Science & Teaching Instru-

ment Plant). Characterization.

FTIR spectra of the samples were recorded between

4,000 and 500 cm21 on a Nicolet Nexus Fourier Trans-

form Infrared Spectrometer (Thermo Nicolet Corporation,

USA). Thin films were prepared, by mixing the finely

ground solid sample with powdered potassium bromide,

and the mixture was pressed under high pressure. The

morphology of the fractured surface was examined by a

JSM-5610LV SEM (Japan Electron Optics Laboratory).

Before observation the samples were gold-coated in a

vacuum chamber to make the surfaces conductive.

The resistivities of the composites were tested on a

ZC-36 model super high resistance meter (Shanghai Cany

Precision Instrument) according to the Chinese standard

of GB/T 1410-2006. The test was conducted with a volt-

age of 500 V, charging of 15 s, temperature of 238C, andrelative humidity of 65%.

RESULTS AND DISCUSSION

FTIR Analysis and Morphology

The FTIR spectra of PET/ZnOw composites and pure

PET are presented in Fig. 2. There was a little difference

between the two spectra pictures, and the new peak in the

PET/ZnOw at 3,300 and 1,080 cm21 corresponded to the

N��H and Si��O vibration of APTMS silane coupling

agent, respectively. This indicated that the silane coupling

agent was grafted onto the surface of ZnOw and inter-

acted well with PET matrix.

Figure 3 presents the morphologies of the PET/ZnO

composites. From the micrographs of bulk samples, it can

be seen that the distribution of ZnOw is basically uniform

in the PET matrix although they are randomly distributed.

At the same time, it can be observed that a small quantity

of ZnOw agglomeration phenomena occurred in the

blends system. The length of some ZnOw particles is

more than 30 lm. When the composites were in the proc-FIG. 1. SEM image of ZnOw.

FIG. 2. FTIR spectra of PET/ZnOw and pure PET.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 227

essing of the melting extrusion, some amounts of ZnOw

needles were broken away from the matrix and some

small cavities formed that could be seen on the fracture

surfaces. Most of the ZnOw remained in situ in the PET,

an interface between the dispersion phase (ZnOw) and the

continuous phase (PET) could be observed. These results

suggest that PET is compatible with ZnOw modified by

the coupling agents to some degree.

Mechanism Analysis of Electric Conduction

Figures 4 and 5 illustrate the surface resistivity (qs) andvolume resistivity (qv) of two kinds of PET/ZnO compo-

sites plotted against the weight content of ZnO. It can be

found that the resistivity of PET/ZnOw composites

decreased slowly before 8 wt% of ZnOw content in the

PET matrix. When the content of ZnOw reached between 8

and 12 wt%, qs and qv had decreased rapidly from 2.8 31013 O to 3.6 3 108 O and 4.3 3 1014 O � cm to 4.2 3 109

O � cm, respectively. The blends of PET/ZnOw show the re-

sistivity low enough for achieving better antistatic perform-

ance. With addition of the ZnOw amount above 12 wt%,

there was not an obvious decrease for two kinds of resistiv-

ities value. For the PET composites filled with s-ZnO, the

magnitude of qs and qv were about 1012 even 20% of s-

ZnO content, which had no obvious influence on the PET

resistivities compared with ZnOw.

ZnOw plays a significant role in reducing the resitivity

of polymer composites and is an effective antistatic agent

for resin materials. The mechanism of electric conduction

in the composites containing ZnOw can be described as

the following reason:

Tunnel Effect [17]

When addition of only a small quantity of ZnOw is

available, there is not a continuous conductive network

formed in the polymer composites. Nevertheless, the

resistivities of the composites decrease by several orders

comparing with the matrix. Tunneling theory must be

taken into account. In the electron tunneling theory, the

electrical conduction is believed to take place by electrons

FIG. 3. SEM image of PET/ZnOw composites with various content: (a) 4 wt% ZnOw, (b) 8 wt% ZnOw, (c) 12 wt% ZnOw, and (d) 16 wt% ZnOw.

FIG. 4. Volume and surface resistivity vs. ZnOw content in PET.

228 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

jumping across a gap or tunnel through energy barriers

between conducting elements in the polymer matrix. The

tunnel effect phenomenon can be described by Fig. 6.

The following equation regarding the charge carrier

and penetrating probability explains this phenomenon:

g ¼ exp�4pah

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2mðu0 � EÞ

p� �ð1Þ

where g is the penetrating probability, h equals 6.63 310234 J � s and is Planck’s constant, a is the distance

between the charge carriers.

It can be found that the tunnel effect decreases expo-

nentially with the increase of the potential barrier. When

charge carriers are electron and (u0 2 E) ¼ 1.6 3 10219

J in the electron system, according to (1), the relation of

between g and a is shown in Table 1.

It can be seen from Table 1 that leakage current gener-

ating from the tunnel effect has become obvious only

when the distance between the needles of the whiskers is

less than 0.2 nm. In actual composites, the related situa-

tion certainly exists, so the tunnel effect appears in the

PET/ZnOw composites. This explains that although the

content of ZnOw is relatively low not to form a continu-

ous conductive network, polymer/ZnOw composites ex-

hibit certain electric conductivity.

Discharging Effect at the Pinpoint [18]

When the composites are loaded by an electric field,

there exists high charge density on the pinpoint of the

needle because the pinpoint of the needles of whiskers is

very sharp, exciting strong electric fields on the vicinity

and resulting in the breakdown of polymer medium

locally. The appearance of those local breakdown areas

can be also regarded as decreasing the width of potential

barrier, thereby causing the discharging effect become

obvious.

If a ball-like particle is considered, the surface density

of electric charge is expressed as:

rs ¼ Q

4pR2ð2Þ

where Q is the electric charge on the surface and R is the

radius of the ball-like particle.

A common ZnOw is composed of two components: an

octahedron of the central nucleus and four hexagonal pyr-

amids of the cross section [19]. Here, a needle of the

whisker can be approximately regarded as half an ellip-

soid, and the surface electric density of a rotating ellip-

soid is introduced to simulate the needle of whiskers.

r ¼ Q

4pab2� 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

x2

a4 þ y2þz2

b4

q ð3Þ

where a is the length of the ellipsoid’s longer axis, b is

the length of the ellipsoid’s shorter axis and a/b is the as-

pect ratio of the ellipsoid. The surface density of electric

charge at the needle tip (x ¼ a, y ¼ z ¼ 0) can be

obtained as:

rw ¼ Q

4pb2ð4Þ

The volume of a rotating ellipsoid is 83pab2. If a tetra-

pod-shaped whisker occupies the same volume as a ball-

like particle, and the aspect ratio is 40, we obtain

R ¼ 4:3b ð5Þrw=rs ¼ 18:5 ð6Þ

The results of the above analysis demonstrate that the

density of electric charge at the pinpoint of a whisker

needle(whose aspect ratio is 40) is about 20 times higher

than that of general spherical particle at the same volume

fraction, which indicates an obvious charge concentration

at the tips of whisker. The effect of the related charge

concentration at the tips is so obvious that the charge con-

FIG. 5. Volume and surface resistivity vs. s-ZnO content in PET.

FIG. 6. Tunnel effect phenomenon.

TABLE 1. The relation of between g and a.

a/nm g a/nm g

0.1 0.36 0.5 0.006

0.2 0.13 0.6 3.9 3 1025

0.3 0.047 2.0 1.0 3 1029

0.4 0.017

DOI 10.1002/pc POLYMER COMPOSITES—-2009 229

centrating effect may have caused the polymer matrix

between the trips of the whiskers to be broken down,

therefore lowering the resistivity of the composites.

Conductive Network

When a sufficient amount of filler is loaded, the filler

particles get closer to each other and form linkages,

which result in continuous chains (conductive network) in

the polymer matrix. There is a critical content at which

the system experiences the transition from an insulator to

conductor. At the critical content, the conductive network

in the matrix will form and a large increase in conductiv-

ity results from a minor increase of the fillers in the ma-

trix. The corresponding filler content is called the percola-

tion threshold. When the content of conductive fillers

overpasses this critical value, the conductivity of the sys-

tem is mainly determined by the conductive network

formed by the fillers. Since the conductive network al-

ready exists, the conductivity will increase marginally

against the increase of filler content.

This phenomenon can be explained by percolation

theory [20] which has been successfully used for describ-

ing the electrical conductivity of mixtures of conductors

and insulators in the following form:

r ¼ r0ðv� vcÞt ð7Þwhere r is the conductivity of the mixture, r0 is the con-

ductivity of the filler particles, vc is the critical volume

fraction (percolation threshold) of formation of conductive

network, v is the volume fraction of conductive phase

above vc. The critical exponent t expresses the rate of

conductivity change depending on the conductive compo-

nent concentration. From the Fig. 4, it can be observed

that at about 10 wt% of ZnOw which corresponds to 2.4

vol% in the composites, the resistivity value decreases

drastically by several orders of magnitude.

It is understandable that the content to obtain the con-

ductive network is much lower for ZnOw than for s-ZnO.

This can be ascribed to the tetrapod-shaped needle struc-

ture of ZnOw, which will lead to the formation of the

conductive network more easily.

Garboczi et al. [21] investigated an idealized model of

conductive composites built up from freely rotating ellip-

soid particles randomly placed in polymer matrix. This

percolation theory model is that completely permeable fil-

ler objects, whose free interconnecting as more and more

objects uniformly added a matrix, eventually results in a

geometrically continuous conductive network. By means

of theory deduction and simulation study, the critical vol-

ume fraction of ellipsoids of revolution, which is also

called percolation threshold, may be expressed as:

vc ¼0:6

b

aðprolateÞ

1:27a

bðoblateÞ

8><>: ð8Þ

Tetrapod-shaped ZnO whisker can be approximately

expressed as two prolate ellipsoids of revolution, therefore

the critical volume fraction of ZnOw in polymer matrix is

given by vc ¼ 0:3 ba in theoretical distribution.

For the actual processing PET/ZnOw system of the

composites, other important influencing factors must be

taken into accounts [22]:

(a) Covering effect: In the actual composites system, the

shorter whiskers below 12la (la is the average length of

whisker) don’t contribute to the formation of an effec-

tive conductive network. Moreover, those whiskers

above 2la probably overlap the space that other

whiskers have occupied in the composites system.

The covering coefficient a1 is used to express the per-

centage of the shorter (�12la) and longer (‡2la)

whiskers that will lead to the increase of vc value.

vc ¼ 0:3ð1þ a1Þba

ð9Þ

where a1 depends upon the uniformity of ZnOw.

(b) Obstruction effect: In PET/ZnOw composites system,

some amounts of thicker PET insulating layers may

exist between separate ZnOw particles. Thus, the

spaces between noncontacting ZnOw particles are suf-

ficiently large, and the assumption of no continuous

conductive network in some areas is valid. It is diffi-

cult for electrical charges to transport beyond insulat-

ing layers because there is obstruction formation of

PET matrix layer. Therefore, the critical volume frac-

tion is given by

vc ¼ 0:3ð1þ a1Þbð1� a2Þa ð10Þ

where a2 is the obstruction fraction of the PET matrix.

(c) Agglomeration effect: For the case of the process of

the PET/ZnOw composites, a certain amount of aggre-

gates of ZnOw might occur. The agglomeration of

ZnOw particles into clusters permits the ZnOw par-

ticles to conduct at higher weight percentage, resulting

in the appearance of a larger volume fraction. So, the

value of the critical volume fraction has the following

form:

vc ¼ 0:3ð1þ a1Þbð1� a2Þð1� a3Þa ð11Þ

where a3 is the agglomeration coefficient of ZnOw.

(d) Breakage effect: Processing parameters, especially

those involving considerable shear of the PET/ZnOw

mixtures, have a profound effect on the conductive

properties of the ultimate conductive polymer compo-

sites. To ensure the conductive ZnOw dispersed uni-

formly throughout the polymer matrix, the composites

should be stirred sufficiently. On the other hand, how-

ever, this mixing condition will generate greater

shearing forces and lead to heavy breakage of the

whiskers, which will reduce their tendency to form

conductive network in the polymer matrix. The break-

230 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

age coefficient c1 and c2 are introduced to explain the

influence of the breakage effect on the lengths of nee-

dles and the volume fraction, respectively. Consider-

ing the above mentioned factors, the critical volume

fraction should be modified as:

vc ¼ 0:3ð1þ a1Þð1þ c2Þbð1� a2Þð1� a3Þð1� c1Þa

ð12Þ

The critical volume fraction for forming conductive

network in the composites is mainly determined by aspect

ratio, but the influences of covering, obstruction, agglom-

eration, and breakage effects during the processing should

be considered. In the composites of PET/ZnOw, let a/b ¼40, a1 ¼ 0.2, a2 ¼ 0.15, a3 ¼ 0.2, c1 ¼ 0.3, c1 ¼ 0.3 we

get vc ¼ 2.6%, which basically coincides with 2.4% of

the experimental result.

CONCLUSIONS

Silane coupling agent was used to improve the compat-

ibility of PET and ZnOw. The difference in the FTIR

spectra of PET-filled ZnOw suggested that coordination

interaction appeared between the surface of ZnOw and

function group of PET matrix. The morphologies of the

PET/ZnOw showed that the distribution of ZnOw is basi-

cally uniform and random in the PET matrix, which is in

favor of the formation of continuous conductive network.

The conductive mechanism of PET filler ZnOw was

explained by tunnel effect, discharging effect at the pin-

point and conductive network. At the lower loading con-

tent of ZnOw, if the interval distances between whisker

particles may be within 0.2 nm, there exist tunnel effect

phenomena. When the composites are loaded by an elec-

trical field, the charge concentration at the tips of the

whisker is distinct. The thinner layer of polymer matrix

between the tips of the whiskers will be broken down,

resulting in discharging effect at the pinpoint. When suffi-

cient ZnOw fillers are added to PET matrix to cause con-

tinuous conductive network, the resistivity of resultant

composites can be varied by several orders of magnitude

as the ZnOw concentration is increased beyond a certain

critical volume fraction. The critical value is basically

determined by aspect ratio, but the influences of covering,

obstruction, agglomeration, and breakage effects also play

an important role in the actual processing systems.

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