Introduction

By far most commonly during winter when the atmosphere is dry, we are likely to experience triboelectric charging. Getting out of a car or removing a jacket made of synthetic fabric can lead to a startling electric shock. Nuisance electrostatic charging has become more and more common in the modern world, due primarily to widespread use of synthetic fabrics and plastics. These materials are good insulators and do not dissipate charge once it has been separated. Examples of such charging, such as those below, can be used effectively in electrostatic demonstrations.

Conventional rubbing cloths and materials

The commonest demonstration of triboelectrification in the lecture hall involves rubbing a rod, a sheet, or other solid object with a cloth, and then showing that the rod has become electrified. There are many choices for materials these days, and so only a few of the more reliable and readily available materials are listed below.

• Silk cloth & glass rod: glass should be Pyrex for best results. The glass charges positively.
• Neoprene rubber & PVC pipe: Neoprene rubber gloves are handy here. The PVC charges negatively.
• Wool cloth & Plexiglas^TM (or Teflon^TM): Wool charges both these polymers negatively.
• Insulating fabric & electrically isolated aluminum rod: Use polypropylene filter media as the cloth. Aluminum is triboactive, usually charges positively (because it freely gives up electrons), and will retain charge as long as it is electrically isolated from ground.

Because of their conductive properties, rubbing cloths made of wool, silk, cotton, and most other natural fibers usually retain no charge because of rapid leakage to ground through the body. Exceptions include certain synthetic fabrics, such a polyethylene filter media, etc.

In all but the purest of materials, the nature of triboelectrification is very complex. Part I of Harper's classic book [Harper, 1967] provides an excellent discussion of the phenomenology.

Charging due to a CO₂ fogger

Charging of a plastic desoldering tool (solder sucker)

Desoldering tools apply strong suction to remove excess molten solder from a printed circuit board or connection terminal. These tools come in a variety of sizes and types and are likely to be constructed of plastic. The device is essentially a spring-loaded piston which is first cocked and then released to create a temporary partial vacuum in the chamber which sucks up the excess molten solder. Refer to the figure below. The nozzle area of these tools can become triboelectrically charged by contact with bits of solder and other debris being drawn at very high speed through the aperture into the piston. If the charging is sufficient and the nozzle makes contact with a wire or circuit element, a small electrostatic discharge can occur. Sensitive electronic components can be damaged by these small electrostatic discharges.

Desoldering tools made with antistatic plastic are available. The slightly conductive plastic probably does not significantly decrease triboelectric charging, but does more rapidly dissipate this charge from the nozzle, reducing the chance of damage to electronic components.

Charging of plastic tape

Charging of nested plastic buckets

When stackable, plastic objects such as chairs, buckets, or funnels are pulled apart, a separation of electrostatic charge often occurs so that opposite sign charge accumulates on the surfaces being separated. This phenomenon, depicted below, has been implicated in a number of ESD ignitions involving flammable liquids [Lüttgens, 1985]. Such incidents are likely to be serious because of the high probability of direct worker exposure in the event of an ignition of flammable material. In the incident mentioned by Lüttgens, a worker was burned when the flammable vapors of the solvent caught fire.

A good question to ask is why buckets made of the same material could ever become charged when separated. The explanation is that plastic is neither a pure nor a homogeneous material, so the two surfaces being separated are NOT identical and thus can become charged. Even if the two buckets were identical, their surfaces become contaminated with dirt, so triboelectrification is still likely to occur.

A plastic bucket with an insulating grip on the metal handle is more likely to stay charged when held by its handle. Note that, if the bucket contains water or has moisture on its inside, then it may behave like a Leyden jar, storing the charge at lower potential and possibly increasing the likelihood that any capacitive spark might be incendive.

Automobile- and truck-related static problems

Neutralizing charged objects

To demonstrate triboelectrification of different materials in a lecture, it is very important to start each test with materials that are uncharged. Starting with zero net charge is necessary to convince student that it is the rubbing that causes the separation of the charge. Many easily charged materials, such as PVC or Teflon, are very highly insulating and retain their charge long after being rubbed. Therefore, using them more than one time requires that the lecturer be able to discharge them quickly. One fairly effective way to discharge objects is with the small open flame of a Bunsen burner or alcohol lamp. But this solution is often inconvenient and may present a safety risk. A better solution is to use a small ionizing air blower. These simple devices are essentially fans with a high voltage AC voltage source and a set of corona points to produce equal amounts of positive and negative ions. Even the smallest model blower will effectively neutralize almost any charged insulating object within a few seconds.