Electret Properties of Polypropylene Fabric

8/6/2019 Electret Properties of Polypropylene Fabric

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*Corresponding author.

E-mail address: [email protected] (E. Motyl).

Journal of Electrostatics 51}52 (2001) 232}238

Electret properties of polypropylene fabrics

Boz ena |owkis, Edmund Motyl*

Institute of Electrical Engineering Fundamentals (I}7), Wroc!aw University of Technology,

WybrzezRe Wyspian&skiego 27, 50-370 Wroc!aw, Poland

Abstract

Results of investigations on electret properties of polypropylene (PP) fabrics formed by high

voltage corona are presented here. The equivalent voltage of samples was measured in function

of temperature that was increasing linearly in time. The half decay temperature and lifetime of

stored charges were estimated. The "ltration index of PP fabrics in function of surface charge

was investigated. The pictures of the structure of PP fabrics using scanning electron microscope

were also shown. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Polypropylene; Fabrics; Electret "lter; Lifetime; Filtration index

1. Introduction

Wide investigation on applications of electrets to aerosol "lters was done by van

Turnhout [1}4]. The electrical forces attracting the aerosol particles as well as

mechanical forces condition work of such kind of"lters. The charged particles are

drifting in the electric "eld generated by electret "bres. The neutral particles are

polarized and move in gradient of the electric "eld. Both kinds of forces are verye!ective. Considerable large diameter of"bres gives low gradient of pressure along the

"lter, and high "lter e$ciency is maintained. The fact that submicron particles can be

easily catched is advantageous for electret "lters in comparison to mechanical con-

structions in which "ne nets with submicron meshes have to be used. The e!ective

"ltration of the submicron particles is of great importance, because such kind of

particles constitute a threat against health. Although electret "lters are produced in

a large way, new realizations are searched for improvements in collecting dust

particles.

0304-3886/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 5 3 - 5


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Fig. 1. Surface of P1 fabric, magni"cation;250.

In this paper, the results of investigations with two kinds of PP fabrics (MALEN

PF-401), P1 and P2 are presented. Fabrics were formed with corona from high voltage

string electrode under potential#25 kV. The lifetimes of formed fabrics were esti-

mated and compared using the thermally stimulated equivalent voltage character-istics. An attempt was made to obtain a relation between the equivalent surface charge

q

and the "ltration index K. The results were complemented by observation of

fabrics in scanning electron microscopy.

2. Samples

Rectangular samples (10 cm;10 cm) were cut from randomly selected places of

electret fabrics P1 and P2. The surface equivalent charge q

at samples is nonuniform.

The positive equivalent charges q from 100 to 1600 pC/cm were measured on theside faced to the charging positive corona electrode in P1 samples. Negative equiva-

lent charges q

from!2500 to!750 pC/cm were observed from the opposite sides.

Samples selected from P2 fabric have also nonuniform surface charge distribution

with the negative equivalent charges q

from!1000 to!100 pC/cm at both surfa-

ces. Investigated fabrics have chaotically distributed "bres. Fibres of P2 fabric are

thinner in comparison to P1, which makes the fabric more nappy. Microscopic

pictures of surface and cross-section of P1 fabric are shown in Figs. 1 and 2. Diameters

of "bres are between 3 and 20 m. Thicknesses of fabrics were estimated as 1 and

1.6 mm for P1 and P2, respectively.

B. %owkis, E. Motyl/ Journal of Electrostatics 51}52 (2001) 232}238 233


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Fig. 2. Cross-section of P1 fabric, magni"cation;650.

3. Results

3.1. Investigation of PP fabrics lifetimes

Thermally stimulated equivalent surface voltages were measured at linear heating

rate b"2 K/min in a temperature range from 293 to 390 K. Temperature runs of

equivalent voltages ;

() are shown in Figs. 3 and 4, respectively for fabrics P1

and P2.

Decay of homocharge, during thermally stimulated discharge at linear rate can be

approximated according to the following equation [5]:

q2R (

)

q exp

!

k

b=

exp

!=

k

, (1)

where q

is the initial equivalent surface homocharge density, = is the activation

energy, k is the Boltzmann constant,

is a preexponential factor in relaxation time

and is temperature. At temperature

, the thermally stimulated run ofq2R

() has

a bending point, which can be used to calculate activation energy, using the equation

="!k

q(

)

dq2R

(t)

d 22

. (2)

The lifetime at temperature was estimated from

()+k

b=

exp=

k 1

!

1

, (3)

234 B. %owkis, E. Motyl/ Journal of Electrostatics 51}52 (2001) 232}238


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Fig. 3. Temperature run of equivalent voltage for P1 electret fabric.

Fig. 4. Temperature run of equivalent voltage for P2 electret fabric.

Table 1

Activation energies and lifetimes at 293 K of two kinds of PP fabrics P1 and P2

Kind of fabric

(K) ;

(V) = (eV) (s) (yrs)

P1 376 390 2.03 4.2;10 1.3

P2 377 465 2.16 8.36;10 2.7

where, at

the surface equivalent charge equals q

/e and e"2.7182 is the radix of

natural logarithm. The calculated values of activation energies and lifetimes at 293 K

for two kinds of PP fabrics P1 and P2 are shown in Table 1. Results are averages from

eight measurements. As it can be seen from Table 1, the lifetime for P2 fabric is

considerably longer than for P1 fabric.

3.2. Ewect of humidity on electret properties of xltering fabrics

Filtering fabrics change their properties in environmental surroundings. High

humidity a!ects the value of equivalent surface charges. It can be seen from Fig. 5 for



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Fig. 5. Decay of equivalent surface charge in humid atmosphere and its recovery after the samples P1 were

exposed to atmospheric conditions. 1,2,3: three di!erent specimens.

Fig. 6. Decay of equivalent surface charge in humid atmosphere and its recovery after the P2 samples were

exposed to atmospheric conditions. 1,2,3: three di!erent specimens.

negative charged samples. The P1 fabrics were placed in a high humidity chamber(100%) and surface equivalent voltage was measured. Equivalent voltage decays

slowly in time of storage in the chamber. After 14 days samples were taken out of the

humidity chamber and remained at atmospheric conditions. After that the partial

recovery of equivalent voltage was observed.

Similar behaviour has been observed for P2 fabric samples (Fig. 6). In case of P2

samples, the voltage decays more slowly (14%) than in case of P1 samples (30%).

3.3. Filtration index

The "ltration index was measured with the use of aerosol particles of sodiumchloride. The photometric method of measurement was applied. Solid particles of

sodium chloride were obtained from dispersed solution of NaCl by evaporation of



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water using compressed air. A stream of aerosol particles NaCl is burned to a cinder in

a hydrogen #ame. The fumes are conducted through a PP fabric electret sample. The

intensity of yellow fumes was measured behind and before fabric sample. The "ltra-

tion index was calculated according to the relation

K"

I!I

I!I

100%, (4)

where I

is the self-indication of the photometer (mV), I

is the indication of the

photometer that corresponds to the aerosol concentration input of the sample (mV)

and I

is the indication of the photometer that corresponds to the aerosol concentra-

tion behind the sample (mV).

It was found that "ltration index for P1 samples changes from 4% to 7% and

practically does not depend on surface charge of the sample. The "ltration index is

lower than 2% in case of P2 fabric.

4. Conclusions

The average value of equivalent surface charge density is not related to the "ltration

index. However, it cannot be concluded that charges have no in#uence on the

"ltration process. Filtration e$ciency depends on local "elds between charged "bres.

Higher e$ciency of P2 fabric in comparison to P1 is probably connected not only

with electrostatic charges, but with lower diameter of"bres, higher nappiness of theP2 fabric and its greater thickness. Fibres with lower diameters have greater surfaces

for dust collecting. Electrical "eld is higher with higher gradient, which favours the

catching of the noncharged particles.

The charge storage in humid atmosphere depends on fabric structure. The nappy

materials are more humidity resistant, but recovery of charge after removal of the

sample to normal atmospheric condition is slower.

The presented results are consistent with those published by van Turnhout [6], who

was pioneering in construction of new generation of electret "lters using thin "bres.

The lifetime of PP electret charges depends on the kind of woven and is above two

times longer for P2 in comparison to P1.

Acknowledgements

This work was carried out as a statutory project supported by the State Committee

for Scienti"c Research (KBN), Warsaw, Poland.

References

[1] J. van Turnhout, C. Van Bachove, J. van Veldhuizen, Electret "bres for high-e$ciency "ltration of

polluted gases, Sonderdruck aus Staub-Reinhaltung der Luft 36 (1) (1976) 36}39.



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[2] J. van Turnhout, J.H.M. Albers, W.J. Hoeneveld, J.W.C. Adams, L.M. van Rossen, Non-woven

electret "bre: a new "ltering medium of high e$ciency, Proc. 5th Int. Conf. on Static Electri"cation,

London, 1979, Inst. Phys. Conf. Ser., No. 48, 1979, pp. 337}349.

[3] P.H. de Haan, J. van Turnhout, K.E.D. Wapenaar, IEEE Trans. Electrical Insul. EI-21 (1986) 465}472.

[4] J. van Turnhout, P.J. Droppert, M. Wubbenhorst, PP-based blends for electret "lters-an appraisal,

VIII International Symposium of Electrets, Paris, 7}9 September 1994, pp. 961}966.

[5] J. van Turnhout, P.H. Ong, Proceedings of the International Conference on the Investigation of Solid

Dielectrics and Methods of their Testing, Wroc"aw, 1977, pp. 133}140.

[6] J. van Turnhout, W.J. Hoeneveld, J.W.C. Adams, M. van Rossen, IEEE Trans. Ind. Appl. IA-17 (1981)

240}248.


Electret Properties of Polypropylene Fabric

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