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Journal of Electrostatics 71 (2013) 513e516

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Journal of Electrostatics

journal homepage: www.elsevier .com/locate/elstat

Predicting the electrostatic charging behavior of insulating materials withoutcharging tests

Ulrich von Pidoll a,*, Kanchan Chowdhury b

a Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, GermanybCryogenic Engineering Centre, Indian Inst. of Technology, Kharagpur 721302, West Bengal, India

a r t i c l e i n f o

Article history:Received 15 August 2012Received in revised form2 November 2012Accepted 18 November 2012Available online 1 December 2012

Keywords:Electrostatic test methodsSurface roughnessMould release agentsWaxSiliconChargeabilitySurface analysis

* Corresponding author.E-mail address: ulrich.v.pidoll@ptb.de (U. von Pido

0304-3886/$ e see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.elstat.2012.11.018

a b s t r a c t

It is well known that experiments of charging and later discharging of materials with gas ignition probesor hand-coulombmeters show that some insulating materials do exist that cannot be hazardouslycharged by manual rubbing in spite of their high surface resistance. In this work, effects of variations ofphysical and chemical properties as well as the structure of these materials have been systematicallyinvestigated. The results obtained help to explain this behavior and propose approaches to predict thecharging behavior of such insulating materials without any charging test at all.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

PTB has been testing insulating materials such as plastics,ceramics and composite materials concerning their electrostaticproperties for 12 years with the method of transferred charge [1].During this test period many properties were identified thatinfluence the electrostatic chargeability of these materials. Theseproperties include:

1.) Presence of conductive tips on the surface (carbon nanotubes,metal flakes etc.),

2.) Surface structure of the material (grooves and projections etc.),3.) Thickness of the material (the thicker the material, the lower its

chargeability),4.) Fall-off of the surface resistance at voltages exceeding 1000 V,5.) Surface roughness of the material (some materials were so

smooth and slippery that they cannot be charged by rubbingbut only by spraying electrons on them).

With this knowledge we started to investigate manufacturingand measuring parameters of insulating materials systematically in

ll).

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order to identify non-chargeable insulating materials suitable forcertain specific applications. The results obtained are shown in thefollowing sections.

2. Experimental

The samples were obtained from their manufacturers and sub-jected to charging tests according to IEC 60079-0:2007 [2]. Mean-while, this testmethod has been shifted to IEC 60079-32-2:2012 [3]and its threshold limits to IECTS60079-32-1:2012 [4]. To investigatethe five different properties shown in the introduction we used:

1.) Plastic samples containing metal powder or antistatic additivesin their total volume: polyoxymethylene (POM) with conduc-tive additives [5], polytetrafluorethylene (PTFE) with 60%bronze [6], polytetrafluorethylene with 50% stainless steel [7]and e for comparison e polytetrafluorethylene with 25%carbon [8], see Section 3.1.

2.) Grooved samples of square shape with 150 mm on each side,2 mm thick, with slits of 1 mm depth and width, prepared frompolytetrafluorethylene, polypropylene and polyvinylchloride,see Section 3.2.

3.) Additional plain samples with a thickness of 2 mm, 10 mm,40 mm and 60 mm, see Section 3.3.

Fig. 1. Foaming created by silicon on plastic surfaces after shaking them in toluene p.a.

Table 2Transferred charges from charged grooved samples (1 mm depth, 1 mm width).

Material/distanceof grooving

Plain 20 mm 10 mm 5 mm 5 mm � 5 mm 2.5 mm

PVC 1 212 nC 100 nC 114 nC 84 nC 106 nC 87 nCPVC 2 218 nC 100 nC 83 nC 74 nC 54 nC 51 nCPTFE 133 nC e e 58 nC 48 nC 61 nC

U. von Pidoll, K. Chowdhury / Journal of Electrostatics 71 (2013) 513e516514

4.) Insulating antistatic ceramic tiles [9] and the samples of 1.), seeSection 3.4.

5.) Smooth samples from transparent polystyrol and poly-carbonate, normal polycarbonate, acrylonitrile styrene acrylat,polyacrylat and polyvinylchloride [8], see Section 3.5.

In the charging test, the samples were laid on an eartheddissipative wooden desk, charged ten times each by rubbing withsheep felt cloth, polyamide cloth and polyethylene table cloth, thencarefully lifted from the table and discharged to a hand-coulombmeter [10]. Additionally, the samples were charged tentimes by corona from a tip of a�70 kV high-voltage stick [11], liftedand discharged. For comparison, some experiments were made thesame way but with a highly insulating foil between sample anddissipative desk.

All experiments were executed in a climate chamber at(25 � 3) �C and (15 � 5)% rel. humidity. The measuring range of thehand-coulombmeter was 8 nCe200 nC with a measuringuncertainty of �2 nC for discharges up to 100 nC. Thisuncertainty is negligible compared to a much higher statisticalscatter of the measured values.

The smooth samples had mirror-like surfaces (Ra < 40 nm)obtained by forming in moulds with mirror-like surface. Some ofthem contained silicon oil or polyvinylalcohol as mould releaseagent on their surface.

The roughness Ra of the samples was measured with a pocketsurf meter with a diamond tip of 10 mm in radius and a stylus forceof max. 15mN [12]. Becausewewere concerned that the measuringstylus may deform the plastic surfaces to a smooth (and possiblyundetected) groove, the instrument was verified with Ra value ofplastic reference probes of known roughness.

Table 1Samples with conductive or antistatic additives.

Dissipative desk Insul

Maximal. transferred charge after charging by Maxi

Rubbing Corona Rubb

POM/antistatics 0 nC 0 nC 18 nCPTFE/60% bronze 0 nC 0 nC 19 nCPTFE/50%steel 0 nC 0 nC 0 nCPTFE/25%carbon 0 nC 0 nC 0 nC

a value depends on the capacitance of the sample. Given value is for C ¼ 10 pF.

The silicon oil was detected by shaking a sample of about5 mm � 10 mm size in a test-tube with a grease-free cork stop(rubber stops usually contain silicon) for about 30 s with 5 mltoluene pro analysis andwatching for foam production [13]. Insteadof the health-hazardous trichloroethylene as proposed by [13], theuse of toluene pro analysis is recommended which was found towork the same (Fig. 1). Other mould release agents, e.g. poly-vinylalcohol, did not show any foaming in our experiments.

Some antistatic plastic samples were aged by an aging testaccording to IEC 60079-0:2011 [14] (four weeks at 70 �C and 90%rel. humidity) and then tribocharged and discharged as describedabove.

The surface resistance was measured according IEC 60079-0:2011 [15] with two stripe electrodes of smooth conductiverubber, 100 mm in length and 10 mm in distance. However, insteadof the required 500 V, an increased measuring voltage up to10,000 V was used. This gave additional information helping theelectrostatic evaluation of the sample. The measuring uncertaintywas negligible compared to the much higher statistical scatter ofthe measured values.

3. Results and discussion

3.1. Insulating materials with conductive additives

There are many types of insulating materials in the marketwhich cannot be charged by manual rubbing or corona toa hazardous level. One group of these materials consists of plasticmaterials filled with conductive additives like carbon flakes, carbonnanotubes, metal powders and antistatic agents.

All four samples (ivory POM with antistatic additives, brownPTFE with 60% bronze and gray PTFE with 50% stainless steel, blackPTFE with 25% carbon) could not be charged by manual rubbing orcorona when in contact with the dissipative desk during thecharging process despite the high surface resistance of the POM/antistatics and PTFE/bronze samples.

However, when charged in isolated condition, transferredcharges could be detected after rubbing the insulatingmaterials butnot after corona charging. The opposite behavior was found for theconductive PTFE/steel composite and the carbon filled PTFE sample.

The results for the insulating samples can be explained ifa resistance fall-off at high voltages is assumedwhichwould lead toa leakage path between sample and desk during the rubbingprocess. In contrary, as expected, the conductive samples could notbe charged by rubbing but charged by corona when isolated from

ating foil on dissipative desk Surface resistance at 1000 V

mal. transferred charge after charging by

ing Corona

0 nC >3 TU0 nC >3 TU100 nCa <1 MU

120 nCa <1 MU

Fig. 2. Charge distribution after a provoked brush discharge from a charged grooved(1 mm depth and width, 5 mm apart) insulator obtained with Bürkers chargingpowder [16].

Table 4Results of measuring resistance fall-off at different voltages.

10 V 250 V 500 V 1000 V 2500 V 5000 V 10 000 V

POM/antistatics e >3 TU >3 TU >3 TU >3 TU >3 TU 2.5 TUPTFE/60% bronze e >3 TU >3 TU >3 TU >3 TU >3 TU >3 TUPTFE/50% steel e 2 TU 50 GU <1 MU <1 MU <1 MU <1 MU

PTFE/25% carbon <1 MU <1 MU <1 MU <1 MU <1 MU <1 MU <1 MU

Travertin tile e 200 GU 200 GU 200 GU 200 GU 100 GU 1.6 GUDolapix tile e 1 TU 1 TU 1 TU 1 TU 1 TU 100 GU

Table 5Smooth and rough samples [8] with and without silicon mould release agent.

Material RoughnessRa

Ra <40 nm Silicon onsurface

Max.transferred

U. von Pidoll, K. Chowdhury / Journal of Electrostatics 71 (2013) 513e516 515

ground. Both samples can easily be identified by thousands ofchippings with a stereoscopic microscope having amagnification ofat least 10.

3.2. Grooved samples

It was anticipated that grooves would act as a barrier for elec-trostatic discharges limiting the dischargeable area and thusgrooving of insulating plastic samples may drastically reduceelectrostatic discharges from such surfaces. An investigation of thetransferred charge of charged grooved plastics indeed showeda reduction of the transferred charge, but this reduction was notsubstantial (Table 2).

Fig. 2 explains this result by showing the typical charge distri-bution after a brush discharge from a grooved charged area: In themid of the discharged area remain positive discharged channels,surrounded by an uncharged white region, and then a non-discharged negative charged area. The grooves deform the nor-mally circular discharged area only to an ellipsoid and not toa rectangle bordered by the grooves.

3.3. Samples of different thickness

It is well known that increasing the thickness of an insulatingcharged solid material decreases the transferred charge ofa provoked discharge.

Table 3 shows this behavior and indicates that only withsamples of at least 40 mm in thickness non-incendive electrostaticdischarges for normal organic solvents (<60 nC) are to be expected.Additional experiments with a PTFE plate rubbed in air without any

Table 3Maximum transferred charge of discharges from charged samples(150 mm � 150 mm) of different thickness.

Material Type of charging 60 mm 40 mm 20 mm 10 mm 2 mm

PVC Rubbing ondissipative desk

58 nC e e 161 nC 212 nC

PTFE Rubbing ondissipative desk

e 34 nC 98 nC 111 nC 178 nC

PTFE Rubbing in air e e 38 nC 53 nC e

PP Corona charging ondissipative desk

e 41 nC e 121 nC 220 nC

PTFE Corona charging ondissipative desk

e 13 nC e 128 nC 131 nC

object in the vicinity and the same sample halved in thickness andrubbed on the same side as before indicate that the reduction oftransferred charges by increased thickness is mainly caused bythickness and only partly by charge binding effects of a dissipativedesk surface.

3.4. Samples with resistance fall-off above a certain voltage

There exist materials which have a strong resistance fall-offcurve above a certain voltage. This behavior is often observed inmixed plastic/conductor composites or ceramic tiles and leads totheir antistatic behavior in spite of a high surface resistance atlower voltages. Table 4 summarizes somemeasured values for suchmaterials unable to produce any provoked discharge by manualrubbing.

One such example is the PTFE/stainless steel compositedescribed in Section 3.1. Its surface resistance of 2 TU at 250 V dropsdown more than six orders of magnitude to less than 1 MU ataround 600 V. This behavior was not observed for the PTFE/bronzecomposite even at voltages up to 10,000 V but observed for nonchargeable insulating ceramic tiles: E.g. the brown Travertin tile [8]has a surface resistance of 200 GU at 2500 V, which drops down to100 GU at 5000 V and 1.6 GU at 10,000 V. The Dolapix tile [8] passedthe 100 GU-limit at 10,000 V too. However, most tiles on themarketdo not show this behavior.

3.5. Smooth samples with remaining mould release agents

Some insulating materials can hardly be charged by manualrubbing due to a very smooth and slippery surface [17]. A closerinvestigation of these materials shows that these materials, incombination with the remaining mould release agent on theirsurface, have a mirror-like surface which may act as a sliding wax.As an indication for this theory other chargeable smooth plasticsamples could be made non-chargeable by manual rubbing simply

chargeafter manualrubbing

ASA Starex� WR-9120 12 nm Yes Yesa <10 nCASA Starex� WR-9120 12 nm Yes No >60 nCPolycarbonate 11 nm Yes Yes <10 nCPolypropylene 28 nm Yes Yes <10 nCPVC 57 nm No No >60 nCPVC 57 nm No Yesa >60 nCPVC 10 nm Yes Yes <10 nCPolyacryle 1.3 nm Yes No >60 nCPolyacryle 1.3 nm Yes Yesa <10 nCPolystyrole 1.1 nm Yes No >60 nCMakrolon� 2856 11 nm Yes No >60 nCMakrolon� 2856 11 nm Yes Yesa <10 nC

a Manually applied with a silicon oil damped cloth [18].

Table 6Aging test (70 �C, 90% rel. humidity) of smooth insulating samples [8] with silicon oilmould release agent.

Start After 7 days After 14 days After 28 days

Transferred charge after manual rubbingASA 0 nC 10 nC 26 nC 30 nCPolycarbonate 0 nC 0 nC 0 nC 10 nCPolystyrol

transparent0 nC <10 nC 10 nC 20 nC

Foaming time of the qualitative silicon testASA 20 s e e 2 sPolycarbonate 15 s e e 5 sPolystyrol

transparent25 s e e 20 s

U. von Pidoll, K. Chowdhury / Journal of Electrostatics 71 (2013) 513e516516

by rubbing them oncewith a cloth wetted in a very small amount ofsilicon oil (Table 5):

To ascertain whether silicon oil wetted surfaces remain stablefor a long term, a standard aging test for plastic equipment inexplosive atmosphere [14] was performed. The results aresummarized in Table 6. It turns out that the surfaces wetted withsilicon oil had limited long term stability.

It is known that adsorption of surfactants on non-polar polymersurfaces leads to electrostatic charge dissipation [19]. As silicon oilis a surfactant, this effect may additionally play a role on the elec-trostatic charging of the samples.

4. Conclusion

The results obtained clearly show the influence of differentparameters on the antistatic properties of some insulating mate-rials. For example, if the material contains insulating grainboundaries with an electrical breakthrough at only a few kilovolts,it cannot be charged by manual rubbing and other charge gener-ating processes. Today, such materials with a resistance fall-off atvoltages below 10 kV are already in use for fuel lines of motor carsand FIBC type B.

Another example of non chargeable material is a polymer filledwith conductors or antistatic agents. The first material can beidentified by their metallic chipping under a stereoscopic micro-scope with a magnification of at least 10. An indication for thesecond is its somewhat reduced surface resistance compared topure plastic materials. However, the results in Table 1 indicate thatthe antistatic effect of such materials may be due to a resistancefall-off at voltages exceeding 10 kV.

A third example is a sample produced with silicon wax/oil asa mould release agent in combination with a highly polishedmould. Suchmaterials can be identified visually by their mirror-likesurface in combinationwith silicon on their surface, detected by thesimple shaking test described in Section 2. Unfortunately thesematerials show a limited antistatic stability for a long-term, whichhas to be taken into account in the event of an antistatic approval ofsuch material. Because surfactants like silicon oil are expelled from

insulating surfaces by high humidity [19] and the aging test per-formed in Section 3.5 is very severe (28 days at 70 �C, 90% rel.humidity), one may discuss whether such samples are still suitablefor indoor use under certain circumstances.

However, as we are only at the very nascent stage of investi-gations in this field, there is a strong need for additional work. Itshould especially be investigated whether additional materialparameters leading to antistatic behavior need to be considered,which threshold limit for fall-off resistances lead to antistaticbehavior (e.g.<100 GU at 10 kV and 30% rel. humidity) and inwhichsituations smooth insulating surfaces slightly wetted with siliconoil may be acceptable for antistatic applications.

References

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[2] International Standard IEC 60079-0, Explosive Atmospheres, Part 0: GeneralRequirements, Section 26.14 Charging Tests (2007).

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[16] Triturate 1 vol carmine powder with 5 vol flowers of sulphur. Then add 3 vollycopodium. Mix by shaking and strew the resulting mixture on the chargedobject. Negatively charged parts will become yellow and positive ones red,best seen on dark samples K. Bürker, Ueber ein Dreipulvergemisch zurDarstellung elektrischer Staubfiguren, Ann. Phys. 306 (3) (1900) 474e482.

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