Toxic Shock: Reducing Electrical Discharge With Antistats
One of the great joys of being a five-year-old is the ability to inflict a bright blue static discharge on your younger siblings merely by rubbing your feet along the carpet. Fast forward twenty years, and the novelty had a different flavour, as t...
One of the great joys of being a five-year-old is the ability to inflict a bright blue static discharge on your younger siblings merely by rubbing your feet along the carpet. Fast forward twenty years, and the novelty had a different flavour, as the much smaller static charge on my finger tips fried a then-expensive processor I.C. before I could even get it into the PLC’s main board.
It goes without saying that molders of electronic products need to consider static, as do many processors of packaging materials. Whether it’s a nuisance that makes parts stick, or a serious issue that kills precision electronics, understanding static and how to fight it begins with a little thought about what static electricity really is. It’s simple stuff — the same stuff, electrons, as what we commonly think of as electricity. Electrons are just the outer parts of any and every atom, whether it’s hydrogen or plutonium, and their main feature is that they carry a negative charge. The charge is balanced by an equal number of positively charged particles in the atom’s core called protons. Electrons move relatively easily, so if you collect an excessive number of them on a surface you can build a considerable charge. How much? You can gather two or three thousand volts of charge by walking across a carpet on a winter day. A wall outlet supplies 110 volts, so how come you don’t get killed? Because your body has a very small capacitance, i.e. it can’t store enough charge to flow out of you at a rate that would interfere with the other electrical equipment in your body, like your heart.
PVC and PE are at the top of what scientists call the triboelectric series, meaning that unlike your body, they’re just about the most perfect materials for holding static charge. Antistatic additives are a solution. Beating static is all about bleeding it away fast enough to keep it from building up in the first place. Antistat-equipped resins are called ESD polymers (electro static discharge) and they’re basically resins with a controlled surface conductivity.
There are two ways for additives to do this: make the polymer intrinsically more conductive, or attract moisture to the surface and let the water act as the conductive path. Carbon black is a classic (and cheap) example of the former, with carbon nanotubes and exotic graphites at the Cadillac end of the spectrum. Amphiphilic antistatic additives have molecules that have segments that are attracted to the polymer and others that are attracted to water. At the surface they act like Velcro, holding the water molecules in place to provide a dissipative current path for the static charge. Modern additives in this class can work with humidity as low as 15%.
Effective loadings can be as low as fractions of a percent, up to the low single digits. Low loadings are great, but they do mean that good dispersion and consistency are critical, which in turn means buying compounded ESD resins or masterbatches. Also, if you need to control static, you need some way to assess how effective your resin will be. The most common measure is surface resistivity, measured in ohms just like regular current resistance. ESD polymers have surface resistivities in the neighbourhood of 1 Meg ohm to one million Meg ohms. Another way to measure it is to specify a discharge half-life of 60 seconds or less, meaning that at least half the charge is dissipated in the first minute.
As usual, there’s far more to say about this subject, but the key is to think about surface resistivity and remember that even in commodity resins, there will be a slight change in physical properties when switching to an ESD grade, so allow a little time to climb the learning curve.