Charging of Solids and Powders

Journal of Electrostatics, 30 (1993) 167-180 Elsevier

167

Charging of Solids and Powders A. G. Bailey Electrostatics Research Group, Department of Electrical Engineering, University of Southampton, Southampton, SO9 5NH, United Kingdom

Abstract

The electrostatic charging of materials such as webs, films and powders due to contact and triboelectric phenomena is reviewed. Some of the problems that may arise in industry due to material charging are discussed. Particular operations such as charging of webs moving over rollers, pneumatic transfer of powder along pipes and into a silo, and the charging of single polymer particles impacting onto a metal surface are reviewed. Some of the problems that arise due to charge accumulation on the surfaces of materials are considered. Powder flow may be affected and electrical discharges which cause fires and explosions are examples of some of these problems.

1. INTRODUCTION

During the handling of solids and powders, especially in industry, considerable charge generation may occur leading to undesirable and sometimes catastrophic effects. Charge generation is in reality charge separation accompanied by charge accumulation on surfaces. The simplest manifestation of the process is the contact charging that occurs when two dissimilar surfaces are brought together and then separated, resulting in oppositely charged surfaces. In practice, the contacting and separating of surfaces is rarely simple, as sliding, rolling and energetic impact with surface deformation normally occur. The term triboelectrification is then used and implies that significant charge separation and accumulation occur when dissimilar surfaces are rubbed together. When the two dissimilar materials are metals, the charge separation process is reasonably well understood and is explained in terms of electron transfer due to the difference in work function between the metals. Insulator/metal contacts and insulator/insulator contacts result in charge transfer that is incompletely understood despite a considerable amount of experimental work and an abundance of theory. Difficulties arise for a number of reasons. For charge transfer between a metal and an insulator to occur, donor or acceptor sites must exist at or near the insulator surface. This implies dislocations and/or the presence of impurities. If small amounts of surface impurity, perhaps due to adsorbed mono layers from 0304-3886/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

168 the atmosphere, significantly affect the contact charging process, it is necessary to work under ultra-high vacuum to achieve reproducible data; such data has little relevance to the triboelectric charging that arises in an industrial plant when handling material under atmospheric conditions. The triboelectric charging behaviour of some materials is sensitive to an atmospheric environment whilst other materials are insensitive. When charge transfer occurs a surface of contact is established. The process can only be properly quantified if the true area of contact is known. This depends not only on surface roughness, but also on contact pressure and the nature of the contact and separation process. When a surface becomes charged, by triboelectrification or any other means, the maximum surface charge density that can be sustained usually depends on the breakdown field strength of the atmosphere adjacent to the surface. For example, a charged plane surface in air would be expected to have a breakdown threshold, under normal atmospheric conditions, of 3.10 6 V/m. A field of this value terminating normally on a charged surface corresponds to a surface charge density of 26.5 #C/m 2. It is interesting to compare this value with the data of Hays and Watson [1], and Hays [2]. They investigated the charging of polymers when contacted by a mercury surface. Hays carried out measurements with polyethylene, polypropylene and polystyrene, and found that contact charge exchange was sensitive to the oxidative states of the polymer surfaces. Freshly prepared films of unsaturated polyethylene exhibited low contact charge densities of about 10/~C/m2. Exposure of film to air near a corona discharge, or ozone led to a significant increase in contact charging to charge densities of about 200 /~C/m 2, which is well above the predicted maximum possible. The increased charging was believed to be due to the formation of ozonides at the unsaturated bond sites which were then capable of electron acceptance when contacted by the mercury. Ozonization of saturated polyethylene did not significantly increase contact charge exchange. Another very important situation in which relatively high surface charge densities may be sustained arises with surface curvature, in particular when dealing with charged particles. The existence of surface charge on a particle gives rise to an electric field which has a maximum intensity at the particle surface and decays inversely with the square of distance away from the surface. However due to the highly non-uniform nature of the field in the vicinity of a charged particle, much higher fields and higher surface charge densities can be supported than 3.106 V/m and 26.5 /~C/m2 respectively. The values sustainable depend on surface curvature, i.e. particle size. Harper [3] has modified an empirical formula of Schaffer quoted by Schumann [4] which gives the maximum sustainable particle surface field E in terms of the radius r of a spherical particle as: E = 9.3 x 10~ r °3 V/m The surface field calculated from this expression for a charged particle of radius 0.02 m is 3.10 ~ V/m, the "normal" breakdown field strength, and so for all particles of radius less than 0.02 m higher values of surface field may be sustained with maximum sustainable field increasing as particle size decreases. The maximum surface charge density o corresponding to the above expression is given by:

169 = 8.2 x 106 r -°3 C/mL An important parameter which determines the dynamic behaviour of a charged particle when subjected to electrostatic and gravitational fields is the particle charge-to-mass ratio. For a spherical particle of density p charged to the air breakdown limit this is readily shown to be: Charge-to-mass ratio = 2.45 x 10.5/pr 1"3 C/kg In practice, during powder handling operations the above limit is sometimes attained especially with polymeric materials. It is instructive to consider some fairly typical data. A 50/~m diameter, spherical particle for example, may have a charge-to-mass ratio of 10"4C/kg, which is fairly typical in powder handling procedures. If this particle is of density 1000 kg/m 3 then it has a mass of approximately 6.5 x 10H kg. Particle charge and surface area are therefore 6.5 x 10-15 C and 7.5 x 10 -9 m 2 respectively. It follows that the particle surface charge density is 1/~C/m 2. This charge density corresponds to an electron number density of approximately 1013 electrons/m 2 Now consider a typical molecular surface density for an insulator. Suppose the insulator molecular diameter is 10 ,~ (10 -9 m). In 1 m2 of insulator surface there are 10TM molecules. Hence for every 1018 molecules of surface there are 10~a sites charged by the donation or acceptance of one electron, i.e., for every charged molecule or site there are 10s uncharged molecules. If charge is associated with surface impurities, as is often assumed, then surface impurites need only to be present in a concentration of 1 part: 105 parts of surface to achieve the level of particle charging considered. This corresponds to a mass or volume impurity concentration of 1 part impurity: 108 parts material.

2. CONTACT C H A R G I N G AND TRIBOELECTRIFICATION OF SOLIDS

In principle contact charging between two surfaces is relatively simple compared with triboelectrification. Nevertheless contact charging itself is a complex process depending upon many parameters and considerable confusion exists in the literature as clear differentiation between the two related charging processes is not always made. Contact charging involves dissimilar surfaces and much experimental data has been obtained under well-controlled conditions, usually under vacuum. Several workers have classified various insulators in terms of "pseudo-work functions" by measuring the charge transfer between very clean metal probes of known work functions and samples of insulators. Davies [5], for example, tested materials such as PVC, PTFE, polystyrene and nylon, by contacting specimens with a wheel made from five different metals of known work functions. The "pseudo-work functions" he determined ranged from 4.08 to 4.85 eV for a set of seven different polymeric materials. Later work by Strella [6] considerably extended the number of polymers tested but the pseudo-work function values obtained were somewhat higher than Davies' values with an

170 Table 1 The triboelectric/contact charging series

Friction Reference RH (%) +

Mica Wool PVA

Contact

a b c 52-69 33

de 15

J

30 (10-50

\

* \

i

--

PMMA

PVA Silk (woven) Ebonite Cellulose acetyl/ butyrate Cotton PVC Ebonite PS PET PS Polyvinylbutyrate Polyacrylonitrile Sulphur PVC,h PE PTFE Polyimide PVC

h i 18

*%

PMMA

PA66 Wool (knitted) Silk Viscose Cotton Silk PVAc

f g

-

.ag~

, . . *

/

I I

I

i

i i /

*--::: * * * *

* -4.

Reprinted from Cross [7]. All references are in Bauser [8]. a, Sasaki (1969); b, Hersh and Montgomery (1956); c, Ballou (1954); d, Fukuda and Fowler (1958); e, Henniker (1962); f, Webers (1963); g, Rose and Ward; h, Silsbee (1924); i, Coste (1970); j, Davies (1969).

171 upper value of 5.75 eV for F r F E (Davies' value for P'ITE was 4.26 eV). The aforementioned results illustrate the variability of contact charging data amongst various workers, there being no absolute values for particular materials although trends are clearly in evidence. On the basis of contact charging experiments, insulating materials have been ranked into "triboelectric series". Many materials have been tested both in contact charging and frictional (triboelectric) charging experiments. Cross [7] has tabulated these as shown in Table 1. Materials higher up in a triboelectric series should always charge positively when contacted by another material lower down, the magnitude of the charge transfer relating to the distance apart in the series of the materials. Note that the relative humidity is specified for many of the test results, especially where frictional charging has been investigated, as this normally is carried out under atmospheric conditions. The positions of some materials, notably wool, PVA, PMMA, silk and cotton vary considerably from worker to worker. Considering that contact electrification and the more complex process of triboelectrification depend upon electron transfer (also ion transfer in some instances) one would expect a dependence on temperature and surface electric field. Zimmer [9] found during experiments in which a metal surface was brought into simple contact with various polymers that their natural tendency to charge negatively (i.e. accept electrons) at room temperature could be reversed at elevated temperatures as shown in Table 2. Table 2 Change in Polarity of Contact Charging with Temperature

Polymer

Charge density Charge density (at room temp.) Transition (at transition temp.) (xl0 -5 Cm -2) temperature (°C) (x 10"s Cm 2)

ABS (Novodur W) ABS (Novadur PM) PMMA (Plexiglass) Polyester Polyphenylenoxide ABS (Novadur PK) Polyvinylchloride

-4.7 -16.5 -70.5 -70.5 -70.5 -185 -235

160 130 90 180 150

+5.6 +7.05 +3.76 +9.4 + 4.7

Reprinted from Cross [7]. It is not surprising therefore that when surfaces are rubbed and heat is generated that charging becomes much less predictable. Rubbed surfaces experience temperature gradients with considerable variations in temperature from one region to another. The work of Bertein [10] illustrates well how the polarity of charge across rubbed surfaces may vary. She showed that the charge due to rubbing is distributed across the surface more or less evenly, but is spotted with charges of opposite polarity; the latter charges usually being distributed over regions of limited dimensions. These latter charges were explained, not by temperature effects due to

172

rubbing, but rather by regions subjected to glow discharge which arose when the electric field in the air space between the insulator and the withdrawn rubbing material reached its breakdown value. The polarity of charge on a surface was highlighted by "powdering" with a mixture of sulphur and red lead; the sulphur adhered to regions of positive charge and the red lead to negative regions, beautiful patterns often being formed. Triboelectrification occurs even when like materials are rubbed together and the polarity of charge on each surface depends on whether the contact areas are points or lines, i.e. which surface is rubbed (new surface continually involved) and which surface rubs (same surface repeatedly involved). Even during relatively simple contact experiments data may be difficult to reproduce, especially when contact is made repeatedly to different regions of a surface. When one considers that the surface of an insulator probably only accepts charge at regions where imperfections or impurities are present this result is not surprising. However, when a particular region of surface is repeatedly contacted the charge transfer often is dependent on the number of prior contacts, tending to decrease to a saturation level.

3. E L E C T R I F I C A T I O N OF FILMS AND WEBS

Coste and Pechery [11] studied the way that contact charging of insulator films of polyethyleneglycol terephthalate (PET) and melinex depends upon film surface microroughness and contact pressure. Their results were explained in terms of accurate assessments of contact area between the test film and metallic surfaces. Tests with five samples of melinex of various measured values of surface roughness showed that the most rough samples (variations - + 2.5/~m about mean) when subjected to a load of 105 kg/m 2 charged to about 0.2 #C/m 2. The smoothest sample (variations - + 0.1 #m about mean) when tested under the same conditions charged much more, to about 1.7/~C/m 2.

charge density

I~\\

t ime :~

/\ ri'

[\ .---.... /

.

Fig. 1 Contact and frictional charging of films (after Coste and Pechery [11]).

173 Coste and Pechery also studied the contact charging of PET and cellulose triacetate films when the contact was subject to friction. The importance of frictional heating and relative humidity was demonstrated. The experimental procedure consisted of drawing a film of test material over a metal roller. The roller could be adjusted to move either at the same speed as the film or at speeds below or above the film such that surface slippage occurred. Surface charge densities after roller contact were determined from the measured values of electric field above the film surface. Three modes of charging were found, as shown in Fig. 1. In mode 1, charging increased to a saturation level with time and was always unipolar. This mode occurred when there was no slippage between the film and roller surfaces. In mode 2, charging increased with time to a maximum value and then decreased and changed polarity to eventually attain an equilibrium level. This mode was always associated with film slippage and was more likely if mechanical stress and relative humidity were high. It was noted that even at relative humidity values as high as 70% very high surface charge densities could develop. A third mode in which charge density rose to a maximum value and subsequently decayed to a lower constant level of the same polarity was sometimes observed. Coste and Pechery found that after charge reversals had been induced, when testing PET films subjected to slippage, the original unipolar charging behaviour could be recovered if slippage was reduced to zero. However, slippage tests with cellulose triacetate seemed to cause irreversible changes in behaviour, although partial reversion to original behaviour occurred at very high relative humidity values. They concluded that surface states could be significantly modified by surface friction. Their work demonstrates the fundamental role that water may play in charge transfer processes, due to surface adsorption.

4. T R I B O E L E C T R I F I C A T I O N OF POWDERS

It is often not realised that metal/metal contacts can result in significant triboelectrification. As long ago as 1932 Vollrath [12] described an electrostatic dust generator capable of generating 260 kV by blowing the metal powders of iron and antimony through copper tubes. When metal powders are blown against metal surfaces, the charge levels that result are comparable with those attained when insulating powders are used. Generally, during powder handling, the greater the energy of the operation the greater is the observed triboelectric charging. For example, the relatively feeble operations of powder sieving and pouring charge powder up to between 10tl and 10.9 C/kg. Energetic operations such as micronising and pneumatic transfer generate charge levels in the range 1 0 -7 - l0 ~ C/kg. Bailey [13] describes a full-scale experimental silo system which was built to study the electrostatic charging characteristics and possible ignition hazards during pneumatic conveying and storage of powders. Several powders, covering a range of electrical resistivity, have been tested in the silo facility. Resistivity is an important parameter which determines the rate at which charge leaks away when powder collects in bulk at the bottom of a container such as

174 a silo. For high resistivity powders, high surface potentials may develop on powder heaps leading to energetic and hazardous discharges. A summary of powders tested is given in Table 3 Table 3 Southampton silo data. Product

Powder resistivity (tim)

Minimum ignition energy (mJ)

Approx. Non-incendive Vapour ignitions maximum discharges (charge injection silo promoted from in use) currents* (no charge injection) Bulk Cloud/ Bulk Cloud/ (/xA) filling filling column column

HDPE (fines)

> 1015

10

-7.0

Yes (one)

_b

Yes

b

Wheat

4-6 x 107

50

+ 11.0

No

Yes

No

No

Sugar

108-10~°

30

+3.5

Yesc

Yes

No

No

Phenol formaldehyde

1013-10tS

10

-8.0

Yes

Yes

No

No

Starch

0.3-1.0x109 25

-13.5

No

Yes (one)

No

Yes

Reprinted from [25]. ' It should be noted that for many products silo current is dependent on ambient relative humidity. The dependence is particularly noticeable for polyethylene when powder charging is usually negative, especially at high relative humidity (above about 40%), but may be positive under dry conditions. b NO data collection c Very few observed

Some of the data presented is discussed later in section 6.2. The silo was grounded via an electrometer, so that the recorded silo currents were observed continuously during the collection of powder that had been pneumatically transported to the silo along a 10 cm diameter steel pipe. The ratio of silo current to powder mass-flow rate corresponds to the powder charge-to-mass ratio. The recorded currents correspond to charge-to-mass ratios of a few/xC/kg. Note that two of the materials tested charged positively. The tests carried out with high density polyethylene (HDPE) were with material type Rigidex 002-55P, supplied

175 by BP. Particles were in the size range 10 - 1000 #m. Cartwright et al. [14] reported experimental findings which illustrated the importance of relative humidity. It was found that when relatively dry polyethylene powder was transferred into the test silo, bipolar charging occurred. Coarse particles charged positively, and fines negatively. Conversely, when relative humidity was high, all particles regardless of size charged negatively. At an RH of about 35 %, when bipolar charging occurred, the net charge level was approximately zero so the danger of discharges and fire hazard was low. Bipolar charging of powder particles has been observed in a number of powders and is dependent on particle size [15] and contacting surface types as well as relative humidity. Singh and Hearn [16] developed a microprobe which enabled the charge on individual powder particles to be measured. Tests were carried out using commercial, granulated suspension PVC with particles in the size range 20 - 500/~m diameter. Figs 2 and 3 show the probe voltage waveforms obtained by scanning a number of these particles with the microprobe. Fig. 2 shows bipolar charging resulting from agitation of the SPVC powder in a plastic container; monopolar charging resulted from rubbing the powder against a metal surface (Fig. 3). *3

*'?: ÷£

÷2

m

6-1 -2

-3

Fig. 2 Bipolar charge on SPVC particles.

-2

-3

Fig. 3 Monopolar charge on suspension PVC particles (after Singh and Hearn [16]).

5. SINGLE PARTICLE CHARGING

During powder handling, individual particles charge during collision with metal or plastic surfaces. An understanding of the charging process for a single particle colliding with a solid surface is a basic requirement for the development of a theory of triboelectrification of powders during handling. Several studies have been reported in which single particles have been fired at targets and the charge exchanges measured. For example, Masui and Murata [17] fired particles of nylon and PMMA of a few mm in diameter at a chromium-plated brass target. The data for charge variation with impact angle followed a cosine law and they found that charge was proportional to the normal component of impact velocity. One surprising result was that the shape and size of impact area did not vary with impact angle. This implied that no rolling or sliding occurred even at oblique strike angles. These data appear to contradict the finding of Cole et al. [18] who reported that in the case of pneumatic transport of powder through a pipe, the pattern of scars left on the pipe walls consisted of lines rather than points.

176 Yamamoto and Scarlett [19] working with nylon and polystyrene particles impacting onto brass, nickel and aluminium plates confirmed that charge exchange depends primarily upon the normal component of impact velocity; there was some slight dependence upon tangential velocity. Of great importance was the observation that impact charge could be significantly affected by the pre-impact charge of particles. The authors found that rolling and sliding effects rose to a maximum at intermediate impact angles before tailing off again. They suggested that whilst increases in the normal component of impact velocity pushed up charging levels this effect was partially offset by the shorter duration of contact. Particle charging was found to increase linearly with impact velocity by all investigators cited. During many powder handling operations, individual powder particles charge up so that a significant space charge field is generated at container wall surfaces. It is therefore important to understand how charge transfer between a particle and a wall during impact is influenced by the presence of an electric field. Furthermore, in many industrial operations, powder handling occurs above ambient temperatures. The relevance of temperature to charge exchange is therefore also important.

200

charge [pC]

150 100f NI ~ ~~ ~ ~ ~ , ~ Polyacetal -50 -100

-150[ -200" -200

~ dSO

~ -100

~ -50

~ 0

L

~

,

50

100

150

electric field [kVIm]

Z00

Fig. 4 Pellet charge dependence on target electric field (after Bailey and Smedley [20]). Bailey and Smedley [20] fired spherical pellets of nylon, polyacetal and teflon at a brass target and monitored the ensuing charge exchanges. Impact velocity and target angle were measured and to extend the work of the above-mentioned authors, experiments were also carded out with the brass target heated above ambient temperature and with the brass surface subjected to an applied electric field. Fig. 4 [20] shows how charge transfer between a pellet and target was influenced by a superimposed field at the surface of the target. An applied negative field (target negative potential with respect to ground) caused the pellets to charge negatively and vice versa. The data obtained for pellet-target charge exchange at target temperatures up to 230o C were no different from normal ambient temperature data. The contact time of a pellet with the target was of the order of microseconds, so it was assumed that the pellet did not heat up above ambient temperature. The relatively small increase of

177 temperature, from ambient to 2300 C, of the metal target would have had a relatively minor influence on the work function, i.e. the contact potential of the metal. It seems likely that with a metal/insulator impact, the insulator would play the dominant part in determining the amount of charge transfer, with an Arrhenius like relationship enabling the charge transfer to occur.

6. C H A R G E ACCUMULATION - SOME PROBLEMS

Charge may accumulate on any insulating or isolated surfaces that are handled. Insulating webs or films being unwound or passing over rollers may charge to such an extent that surface discharges occur. These may prove hazardous in flammable atmospheres of gases or vapours or when dust clouds are present. They may damage or destroy products, as in the case of photographic films, or merely be a nuisance as may arise when the acetate rolls of overhead projectors are unfurled, for example. Even simple acts such as unwrapping of cellophane from packages can result in visible electrostatic discharges that might possibly present a hazard, as for example in coal mines where methane may sometimes be present. When powders are handled in industry severe problems sometimes arise. Particle charging may cause agglomeration so that powder does not flow smoothly or complete blocking of flow may occur. Particle size is often of great importance when these problems are encountered and the proportion of fines is especially significant. Fine material in the form of a flow agent may be used sometimes to alleviate flow problems. The opposite problem of powder particle repulsion leading to reduction in powder density also occurs. This effect may prevent the full load of powder being delivered to bags or containers. The following two sections outline some problems of powder flow and also the hazards that can occur due to powder charging. 6.1 Powder Flow Problems An experimental hopper was used by Harpavat and Frantz [21] to study the flow of charged xerographic developer powders. A similar set up consisting of two flat metal plates set at an angle of 60o was used by Liu [22] to study polymeric powders. Figs. 5, and 6 show some data obtained at Southampton. The powders tested were charged prior to flow tests by agitation in a metal drum. Agitation time, which determines the triboelectric charge on the powder was used as the control parameter. Fig. 5 illustrates how powder flow rate is decreased as charge levels due to triboelectric charging increase. For the particular powder tested (PVC) flow rate was halved. For other powders the effects of charge upon flow rate ranged from negligible to complete flow blockage. A potential difference across the plates of the hopper sets up an electric field which permeated the powder especially in the hopper aperture regime. Powder particles polarized and cohesive particle-particle forces arose. Powder flow rate was controlled by gate voltage. In the extreme case of a relatively high voltage, flow of powder from the hopper could be completely arrested.

178 flow rite (g/s)

2,5

21

1.5

I

0

0

0

0.5

i

0

i

50

i

i

ioo ~5o agitation time (s)

zoo

zso

Fig. 5 The influence of powder charge, due to agitation, on flow (after Liu [22]).

1.6

flow rate (g/s]

1.4( 1.2 1 0.8 0,6 0.4 0.2 i

i

L

i

i

1

2

3

4

5

6

gale v o l t a g e {kV)

Fig. 6 Control of powder flow by gate voltage (after Liu [22]). 6.2 Hazards due to Powder Charge

When powder charges during handling, there is a danger that ensuing discharges may ignite the powder cloud. Powder clouds usually have minimum ignition energies of 10 mJ and above, with fine powder clouds being the most easily ignited. If flammable solvents or gases are present, ignition energies can be significantly lower so that hazard is increased. The pneumatic transport of powder is common in industry and high levels of charge are often generated. A particularly hazardous situation may arise when powder is pneumatically conveyed into a silo. Within such a silo there may be discharges within the powder cloud generated during filling, especially in the vicinity of the transport pipe outlet. As the powder cloud settles and forms a heap at the bottom of a silo the charged powder particles pack

179 closely together and very high space charge densities arise, especially with insulating powders. Energetic discharges from the bulk powder to the silo metal walls then occur with possibly catastrophic results if the powder cloud is ignited. During the Southampton silo research programme the energies of discharges within the silo were determined by introducing an earthed spherical metal probe into the powder cloud in the vicinity of the transport pipe and also just above the powder heap. The probe was based on a design by Gibson and Lloyd [23]. It was shrouded in an atmosphere of propane/air mixture which had a known ignition energy. The ignition energy was adjusted by changing the propane/air ratio. When discharges were drawn to the probe in the silo an ignition of the gas mixture signified that the discharge energy was at least equal to that of the gas mixture (usually about 0.4 mJ). Two ignitions were recorded, one in a starch dust cloud and the other when a discharge was drawn from the surface of a heap of polyethylene powder. It was concluded that the most hazardous situations were likely when discharges arose from heaps of insulating powder. The resistivity of the powder is a key parameter in determining how quickly charge leaks from the powder heap during the silo-filling operation. At the completion of the silo project in 1988, guidelines [24] for the assessment of electrostatic hazards during pneumatic conveying and storage of flammable powders were drawn up. Included in the Guidelines is a procedure in the form of a flow chart which enables electrostatic hazards to be assessed. An understanding of the charge buildup and discharge processes in powder heaps is far from complete. Work is continuing at Southampton on a laboratory-scale silo in which it has been found that the surface potential of a settled powder, which is directly related to the occurence of static discharges, is higher for polyethylene pellets than for fine powder. Consequently, electrical discharges are more likely to occur with coarse powder rather than fine. Although the handling of coarse insulating pellets alone, in the absence of a dust cloud or flammable gases, presents a very low explosion risk, the situation becomes hazardous if fine particles are present. There are numerous reports on dust explosions due to static and they are almost invariably concerned with fine powders. They all assume that a convex surface of unipolar charge builds up during powder settlement. Although experimental evidence confirms that such conditions normally exist, it has been found recently that this will not always be the case especially when coarse particles or pellets are present. Recent work at Southampton [25], with pellets of HDPE of about 3 mm diameter has shown that a concave surface may be formed on a powder heap, consisting of both positively and negatively charged particles despite the fact that only negative particles were conveyed.

7. CONCLUSIONS During the handling of films, webs and powders electrostatic charging often occurs due to the processes of contact and triboelectric charging. The physics of these processes is not completely understood and depends upon many parameters, which are often ill-defined in industrial situations. The triboelectric series of materials is not unique but gives good

180 guidance to expected charging behaviour when different combinations of material are handled. Electrostatic charging depends upon the energy involved during handling operations, energetic operations such as micronising and pneumatic transfer leading to particularly high levels of charge accumulation on surfaces. Charge accumulation may lead to electrical discharges, which can ignite dust clouds and flammable materials. Charge may also affect powder handling operations as flow may be retarded or even in extreme cases clogging may occur.

8. REFERENCES

1. D.A. Hays and P.K. Watson, Soc. of Photographic Scientists and Engineers, 2nd Int. Conf. on Electrophotography, (197) 108. 2. D.A. Hays, Inst. Phys. Conf. Ser. No 48 (1979) 265. 3. W.R. Harper,'Contact and frictional electrification', OUP (1967). 4. W.O. Schumann, 'Elektrische Durchbruchfeldstaerke von Gasen', Springer, Berlin (1923). 5. D.K. Davies D K, J. Phys. D.: Appl. Phys. 2 (1969) 1533. 6. Strella 1970, unpublished results cited in Seanor 1972. Seanor D A Polymer Science Vol. 2 ed. A D Jenkins, Amsterdam, North Holland (1972) 1187. 7. J.A. Cross, 'Electrostatics: Principles, problems and applications', Adam Hilger, 1987 8. H. Bauser, 'Static electrification of organic solids,' Dechema Mono. vol. 72 (Frankfurt: Verlag Chemie} (1976) 11. 9. E. Zimmer, Kunststoffe 60 (1970) 465. 10. H. Bertein, Inst. Phys. Conf. Set. (1967) 11. 11. J. Coste and P. Pechery, 3rd Int. Cong. on Stat. Elec. Grenoble (1977) 4a-4f. 12. R.E. Vollrath, Phys. Rev. 42 (1962) 298. 13. A.G. Bailey, Inst. Phys. Conf. Ser. No 85 (1987) 1. 14. P. Cartwright, Sampuran Singh, A.G. Bailey and L.J. Rose, IEEE Trans. on IAS, vol. 1A-21, No 2 (1985) 541. 15. C.F. Gallo and W.L. Lama W L, J. of Electrostatics, 2 (1976) 145. 16. Sampuran Singh and G.L. Hearn, J. of Electrostatics, 16 (1985) 353. 17. N. Masui and Y. Murata, Jap. J. of Appd. Phys. 22, No 6 (1983) 1057. 18. B. Cole, M. Baum and F. Mobbs, Proc. Inst. Mech. Engrs. 184 Pt 3c (1969) 77. 19. H. Yamamoto and B. Scarlett B, Particle Characterization, 3 (1986) 117. 20. A.G. Bailey and C.J.A. Smedley C J A, Adv. Pow. Tech. Vol 2, No 4 (1991) 277. 21. G.L. Harpavat and C.L. Fran, IAS 76 Annual (1976) 923. 22. S.H.J. Lin, Final Year Project Report. Dept. of EE., Univ. of Southampton,(1985). 23. N. Gibson and F.C. Lloyd, Brit. J. Appl. Phys. vol. 16 (1965) 1619. 24. Wolfson Electrostatics Unit. Guidelines on the electrostatic hazards during pneumatic conveying and storage of flammable powders. J. Loss Prey. Process Ind. vol. 4 (1991) 211 25. W.L. Cheung. Proc. Ann. Mtg. of the Inst. of Electrostatics Japan. (1992) 69.