Charge it up!

Last week, on April Fools’ Day no less, I was informed that a book chapter I’d written last year had finally been published online.  The article is about electrostatic discharge (ESD) and how it impacts a particular class of RFID tags.  While I’ve gotten pretty comfy dealing with RFID, at least the ultra-high frequency (UHF) variety, I am actually far more interested in ESD as area of study.  The physics of ESD is fascinating, primarily because it’s so difficult to get anything resembling a quantitative model.

ESD generally begins as a build-up of charge, which can happen in many ways.  One of the most common is through frictional transfer.  More simply, one object rubs against another.  The model is fairly intuitive: two materials come in contact with each other, and as they move across each other, electrons that are held too loosely by one material jump to the other material.  The reason that ESD is so difficult to quantify is because there are several factors that can affect how many electrons are transferred: different materials will have different affinities for keeping or releasing electrons, the speed of rubbing and the pressure will affect how many are released, high humidity can lower the amount of charge on a surface while low humidity will have little impact, and separating the material slowly will allow electrons to tunnel through the gap between materials while fast separation increases the physical barrier too quickly for charge to redistribute.

In light of all these issues, one of the most useful tool to come from studying ESD is the triboelectric series.  It is a qualitative chart displaying the affinities for positive or negative charge of various materials.  The items near the center do not have a strong affinity, while the materials at either end have strong preferences to gain or release electrons.

The problem with the triboelectric chart is that it is relative.  One has to know where both materials that are being rubbed together are placed on the chart to understand how charge builds up.  However, casting that issue aside, there is also the issue that it’s very limited in scope: it usually contains 20-30 materials, which is certainly not comprehensive.

In my book chapter, I cite a paper by Diaz and Felix-Navarro discussing triboelectric ordering of organic polymers.  The authors of this paper attempt to improve the traditional triboelectric chart.  They took four different triboelectric charts containing different polymers and collated them into one chart.  Surprisingly enough, the charts were fairly consistent with only a couple materials out of order.  To rectify the problem, the authors then used quantitative measurements of how much charge was generated by rubbing certain materials on different types of metal.  These measurement helped to quantify the ionization from each material and therefore verify the correct order for the series.  Once this was done, the authors could extract the relative behaviors of a number of organic polymers for comparison.  Finally, they examined the properties of the functional groups in the polymers and discussed how they tended to interact triboelectrically.  Surprisingly, some of the transfer of charge may be from ion transfer rather than electrons!

While most people are interested in ESD as a problem in electronics manufacturing and use, this new table has potential beyond that.  For instance, in medical manufacturing, it’s important to avoid contaminants.  Charge build-up on a surface can often attract contaminants in the air.  Knowing which materials are likely to develop charge, and thus attract contaminants, can help manufacturers decide if the want to use a particular material in their process or for packaging or if there is a suitable alternative.

Diaz, A., & Felix-Navarro, R. (2004). A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties Journal of Electrostatics, 62 (4), 277-290 DOI: 10.1016/j.elstat.2004.05.005