Static Electricity

Static electricity causes productivity, quality and safety problems throughout industry. In the from of electrostatic attraction or repulsion it generates dust contamination and product misbehaviour. As an electrastatic discharge it gives shocks to operators, starts fires and destroys electronic components by the million.

What is Static Electricity?
When a material or object hold a net electrical charge, either positive or negative, it is said to have a static charge. Static charges usually decrease over a period of time which is dependent, amongst other things, on the resistance of the material and on ambient conditions. Taking plastics and metals as examples, plastics generally haver very high resistances which allow them to maintain static charges for long periods of time. Metals have very low resistances, and an earthed metal object will hold its charge for an imperceptibly short period of time.

Static electricity is usually measured in Volts. Whilst mains voltages of 220V are considered dangerous, levels of static electricity of 100kV are common.

The voltage present on a material is dependent on two factors: the amount of charge on the material and the capacitance of the material. The simple relationship is Q=CV where Q is the charge, C the capacitance and V the voltage. Metals tend to have high capacitive values and therefore a relatively high charge will produce low voltages. The reverse is true of plastics, which is why problems with static electricity are most noticeable when working with plastics.

Thre are two main types of static electricity, volumetric and surface. Volumetric static charges are charge imbalances within the body of a materia, whereas surface static electricity is only present on the very outher surface of a material. In practice nearly all the static electricity problems found in industry relate to surface charges.

How is Static Created
There are three main causes of statis electricity:
. Friction
. Seperation
. Induction

Friction: As two materials are rubbed together the electrons associated with the surface atoms come into very close proximity and can move from one material to another. The harde and/or faster the two materials are rubbed together, the greater the exchange of electrons and the higher the charge generated. This process is caused by the surface electrons gaining heat energy generated by friction. This extra energy allows them to break their atomic bonds and transfer to other atoms.

The direciton of electron movement depends on the position of the two materials in the Triboelectric Series (see right). Materials on the positive side of the series tend to give up their surface electrons and become positively charged, whilst materials on the negative side tend to gain electrons and become negatively charged.

Seperation: The method of charging by separation is similar to that of friction. When two materials are in contact, the surface electrons are in close proximity to each other and upon separation have a tendency to adhere to one material or the other, depending on their relative positions in the Triboeeltric Series. The faster the separation of the materials, the higher the charge generated. A common example is of PVC web moving over a Teflon-coated roller. As the materials seperate, the electrons will tend to adhere to the Teflon, generating a net negative charge on the Teflon and a net positive charge on the PVC.

Induction: Static charges can be generated when materials are in the presence of a strong electric field. Charging is caused by ionisation of the air between the surface of the material and the voltage source which causes surface electrons to be carried away from the material to the source. A common example of induction is an operator working near charged materials. The operator will become charged and on touching an earthed object will discharge to it, giving himself/herself an electric shock.

Factors Affecting static Electricity
A number of factors affect the generation and maintenance of a static charge including:

Humidity
Atmospheric humidity deposits small quantities of water on all surface and hence surface static charges on materials have a tendency to dissipate to earth by current flow through the surface moisture. For example paper generally has a relatively high moisture content and does not maintain particularly high levels of static. However if the papar is particularly dry, static can become a severe problem.

Type of Material
Some materials are more readily charged than others. For example acetate will gain a charge very readily compared to glass. Also the relative position of a material on the Triboelectric Series will determine whether it charges positively or negativelywhen in contact with another material. For example when hard rubber is rubbed against nylon, it becomes negatively charged, but when rubbed against polythene it becomes positively charged.

Repetition
Repeated actions such as friction or separation increase the level of charge on a material. For example a plastic web moving over a series of Teflon rollers increases its surface charge after every roller.

Change in Temperature
As a material cools down it has a tendency to generate a net charge throughout its entire volume. If the material is a good insulator, the internal (volumetric) static charge can persist for very long periods of time. However over time the charge normally migrates to the surface to become a surface static charge. An exampşe of this phenomenon is an injection moulding which appers neutral when hot but can have a large surface charge when cool.