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ELECTROSTATIC DISCHARGE
Electrostatic discharge measurement for conductive textile materials • R Perurnalra],
E Tholkappiyan,
J Kanimozhi, P Jayashree
Bannari Amman Institute of Technology, Sathyamanglam
M
easurements are needed to show the electrostatic condit ions I hat are present, 10 '--'oelp understand why these conditions are present, to help confirm that control measures put in place are continuing to work and to help select and develop suitable materials. Without measurements it is all guesswork. The classical groundwork in electrosta tics used instruments such as t he gold lear electroscope and the quadrant electrometer. These arc perfectly valid measuring instruments - but they belong to the physics laboratory.During production. static electricity can cause overload of electrical equipment. explosion in an operating room and there is always shock risk. In textile industry, especially for nonwovens. it can increase tbe production time. Static clectrlcity CHn cause the product slick to machine or other parts. and make it unable to move easily. 'I'hts can cause break apart or textile products and this will be a reason of higher process Lime. Also. during production. staLic electricity on the material can cause breakage of fibres. This will decrease t he l'abr ic
strength", These descriptions draw attention to the design features needed (.0 ensure that save important impacts on polymeric materials. While there are useful attributes of static charge in some areas such as flocking and electrets filters, there also exist some negative properties during fibre processing. The manufacturers arc concerned about the controlling of static charge generated during the modern. high-speed textile processes.
638401,
and R Ramya
Tamil Nadu, E-mail:
[email protected]
for example, static accumulation during the unwinding process of a large roll of polymer sheet could be so intense that a sudden discharge will cause very dangerous situation if no proper static control device was adopted+Consumers arc also very aware or issues such as the static charge gener-ated during walking on carpet or cling of clothing during wear: static charge created when getting out of vehicles may even cause sparks that is strong enough to initiate major fires at gas stations. Given (he growing Significance of electrostatic phenomena in textiles and
The presence of the static charge in textiles can be a major hazard in explosives, paper, printing, electronics, plastics and the photographic industry. textile pr ocessin g. a better understanding of this property is critical to the future of the textile industry.There is a general consensus that when surfaces come into contact with one another. they exchange electrons. [I' the surfaces arc conductive. the excess charge will be dissipated 10 ground. But if the surfaces are insulators, the charge will be retained and. as the surfaces are separated, will appear as static charge. After separation. this charge will attempt 10 dissipate and the mechanisms for Asian Textile Journal.
May 2011 .58
dissipation have been well-documented. The generation mechanism and molecular surface location of static charge. on low energy polymer surfaces. is still a matte]' of speculation. While there has been some recent research in thls area, there are no systematic studies that provide either a practical or a theoretical understanding in this area 1.
Static electricity [lor many years it was known that if a piece of amber was rubbed it acquired the ability to attract small bits of straw. paper and such. According to electron •••• •• .. •••• ••~Orbit
...
electron
I
II
Electrical conductivity of materials is a property which spans a very wide range.
I
semiconductors k'l at room temperature is sufficient to move the electrons from valance band to conduction band 1M.
Temperature
e
Ii
Ir rna ter ial is cooler tban the surrounding air. moisture will condense on it and its insulation resistance will deteriorate. If it is warmer. it will tend to dry out and its resistance will become high. even if the relative humidity of the surroundings is high. Also. as material cools down, it has a tendency to generate charge. A rise in temperature causes an increase in perruittivity in solid materials.
Conductivity of fibres Textile fibres are electrical insulators. As explained above. all electrons are bound to the nucleus core or shared in the covalent bonds. No electrons are free to move. Electrical conductivity of materials is a property
which spans a very Wide range. it may occur through the movement of either electrons or ions.Conducuvtty values show how good can a material conducts charge. Fig 8 shows some textile materials' conductivity values".
Problems that static electricity causes In the textile industry. static electricity may cause many problems if it. is not under control. These problems may be: • Electronic equipment can be overloaded and break down • Static discharge in an operating room may lead to an explosion • Risk of shock • Increased production lime: In some areas very long textile materials are produced. Because of the static charge on it. roll should he used for some time to allow static charge dissipation. • Fibre breakage and decreased fabric
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strength: Spunbond machine during production, before transferring the web to calender bonding, the fabric sticks to the belt. This may cause fabric breaks during product.on and higher process time. Basically static charge buildup is caused by one of two processes: either by friction bel ween two surfaces (called trlboclectnflcationl or by proximity to an electrostatic field (called induction charging). When su bsrances become charged by triboelcctriflcation. electrons migrate from the surface of one material to the surface of the other (Figs 9 and 10). lJpon. separ auon of the two surfaces. one surface loses electrons and becomes positively charged. The other surface gains electrons and becomes negatively charged. Triboelectric charging (a transfer of electronsloccurs when two materials are in contact and are then separated. As the pressure or speed of contact and separation (friction) increases. the amount of the static charge buildup (voltage level) increases. Rapidly moving materials - such as plastic trim ill a pneumatic conveyor or a converted film web - can quickly develop charges of more than 25,000 volts. The second means by which an object or material may be charged is by induction. A highly charged object is surrounded by a static charge field. If an isolated or ungrounded conduct ive object enters lnto this static field. it too will become charged. This creates the possibility of electrostatic discharge to somc other conductive object. which could result in an arc of sufficient energy to ignite combustibles or destroy sensitive electronic cornponents "
Basic principle to control static discharge Fig 9 : Iriboelectric
charging
CONTACT+PRESSURE+SEPARATIOI\
Fig 70 : Friction, pressure and separation are the major causes of industrial SIalic electricity
Asian Textile Joumal • May 2011 • 61
Conductors and insulator Materials are divided into two basic groups: conductors and in su lators. Within a conductor, electrons move freely throughout the entire substance. Therefore, when an u ngrou nded conductor becomes charged, the entire volume of the conductive body assumes a charge of the same voltage and polarity.
r ELECTROSTATIC DISCHARGE
INSULATORS
VS CONDUCTORS
INSULATORS
CONDUCTORS
1
1
fig " 1 : tnsutstor« and conductors hev« diiicrent suriaco charge and grounding cnpebilitie«
A
conductor can be neutr alrsed by connecting it to earth ground, A charged insulator can remain charged for many hours. Opposite polarity charges can exist on an insulator at thc same time. Charges will not migrate on insulators. Grounding insulators neither removes nor prevents surface charges. Charge of one polaritj can remain on a conductor as long as it is Isolated from ground- I. An insulator reacts much differently to st at ic electricity and cannot be neutralized by simple grounding techniqucs (Fig II). Within an Insulator, the Ilow of electrons is very limited. Because of this. an insulator may retain several static charges of different polarities and potentials at various areas on its surface. This accounts for why certain areas of a material may stick together and of hers may repel each other. Connecting the insulator to ground will not result in an exchange of electrons as is the case with conductive substances; therefore. other means must be used for neutralizing static on insulators. charged
matter what we do.static charge will try to find a way to discharge:" • Design In Immunity - the first principle i~ to design products and assemblies to be as immune as is reasonable [rom the effects of 1:5D. This involves such steps as using less staticsensitive devices or providing appropriate input protection on devices. boards. assemblies. and equipment. For engineers and designers. the paradox is IhM adva nc in g product technology requires smaller and more complex gcornetrres that often are more susceptible to HSl). • Eliminate and Reduce Generation obviously. product design is not the whole answer. The second principle of control is to eliminate or reduce the generation and accumulation of electrostatic charge in the first place.
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,
An insulator reacts much differently to static electricity and cannot he neutralized by simple grounding techniques. Fir!>t by reducing as many stauc generating processes or materials. such as the contact and separation of dissimilar materials and common pili sues. as possible from the work environment. The other processes and m a tert als at the same electr ost.attc potential. Electr ost atlc discha rgc does not occur bet ween materials kept at the same potentia! or at zero potential. The ground paths. such llS wrist straps. nooring and work surfaces. to reduce charge generation and accumulation. • Dissipate and Neutralize - because it is not simple to eliminate all generation of static in the environment. our third
principle is tu safely dissipate or neutralize those clccrrostauc charges that do occur. Proper grounding and the lise of conductive or dissipative materials play major roles. For example, workers who 'carry' a charge into the work environment can rid themselves of Ihat charge when they au arh a wrist strap or when they step on in productivity, machinery jams. fires. and explosions. Static charges can also cause severe damage to fa brics of differ en t compositions and blends. sensitive electronic
Sometimes, controlling electrostatic discharge in the electronics environment seems to be a formidable challenge. components, requiring costly rework and/or field service repair. Thus rubbing tests can be used to study the effect of rubbing speed. pressure, and distance bet ween the rubber and the sen sur on the charge generation dissipation from the fabrics to analyze the static behavior of the fabrics in a precise manner,
References Marcus, Abraham 2
Dr Paul Holdstock Services,
(2007).
Manc hestvr
Te'( hnic al STG,
M22
United Kingdom. Electrostatic
WIlson
ior measuring charge transfer
system
electrostatic
dischargr-,'.
in
of
lournal
Physics Vol 22, pg 64-70 .~ K"usik Bal & V K Kothari (june 2009). 'Measurement
of
textile
o{ dielectric
properties
materials
,lppliciltion
