ESD Systems - Questions And Answers

Does the term “antistatic” depend on the materials in question’s relative location in the triboelectric series? If so, how is it that a material can be antistatic to all other materials? If not, how does such a material resist building up a charge over 200 or so volts? Does it simultaneously bleed the charge off? Can a material be antistatic and also be a strong insulator? Thanks again.

This is a tough question, and I’m still not convinced I’ve properly answered it.This phenomena is really not well understood in general and by me in specific. From what I understand, some materials, such as conductors are naturally ‘antistatic’ (now referred to as low tribocharging) because they just don’t charge up well, probably due to the abundance of surface free electrons which tend to bring a charge imbalance back to balance to some extent (i.e., a conductor will distribute it’s charge imbalance over it’s whole surface area unlike an insulator which will keep it’s imbalance at the selective area of contact). For this reason, I don’t believe it common for a strong insulator to be ‘antistatic’. For a material to be classified as ‘antistatic’, the minimum charging voltage acceptable is about +/- 200 volts (depending on the referring standard – in this case EIA-625). For EIA-625, the level of voltage generation is dependent on the materials used or in question. I.e., applying or removing static generating tapes and labels to antistatic tubes, magazines and carriers shall not be done with ESDS devices inside while the generated static voltage exceed +/- 200 volts. I believe the ‘test’ would follow the ESD Association’s ADV11.2-1995, Triboelectric Charge Accumulation Testing where a ‘triboelectric series neutral’ material such as brass is used to determine the amount of charge generation from triboelectric generation process. Of course is Teflon or asbestos where used, an extreme result may occur. In trying to standardize the testing process (evaluating the comparative performance of materials), one material should be used to test other materials, in this case Brass was chosen in ADV11.2. The excerpt below is from Bill Beaty’s Web Site: http://www.eskimo.com/~billb/ “Note: the term "frictional electrification" is misleading. Separation of charges is not accomplished by friction. Surface charge imbalance comes from the same place that friction does: adhesion on the molecular level. Surfaces stick together because chemical bonds form. When surfaces in contact are separated, the bonds rupture, and any asymmetrical bonds will tend to leave imbalanced charges behind. Other effects are important too. This subject is not very well studied in science. Go search for info about surface charge, contact electrification, and Atomic Force Microscopes. Since most materials are not flat enough or flexible enough to attain large-area contact, most materials don't become very strongly electrified when simply touched together then separated. For example, when fur is used on plastic it helps greatly if you drag the filaments of hair across the plastic surface. Among other things, it acts to increase the total contact area. Some surfaces, such as adhesive tape or plastic sheets, CAN attain intimate contact over a large area, and DO exhibit strong charging when they are simply touched to another surface and pulled away. This phenomenon is called "contact charging" or "electrification by contact," and it's better to use these terms and avoid giving the idea that the mechanism for the electrification is CAUSED by friction. I also like to say that "surface charging is caused by peeling," since the scotch-tape demo works so well, and because "peeling" always implies a preexisting intimate contact between surfaces. Caveat: contact electrification is not well understood. Friction DOES play a part. For example, the ordering of the triboelectric series is different when surfaces are rubbed rather than simply touched. The order also changes when surfaces of differing roughness are rubbed together, and IDENTICAL substances can even generate a charge-imbalance if one surface is rough. This is probably a major reason why different references give different ordering of the series: the experiment must guarantee that no rubbing occurs, otherwise results will vary from trial to trial. “

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