What Factors Affect Static Electricity

Among the many factors that affect the generation and maintenance of a static charge are humidity, the type of material, repetition and change in temperature.

Type Of Material
Some materials are more readily charged than others. For example materials such as acetate will gain a charge very readily whilst glass will gain a charge less readily. Also the relative position of materials on the Triboelectric Series will determine whether a material charges positively or negatively dependent on the other material with which it has come into contact. For example hard rubber, when rubbed against nylon, will become negatively charged whilst when it is rubbed against polythene will become positively charged.

Humidity
Generally speaking the dryer the environment, the higher the level of static charge and conversely the higher the humidity, the lower the static charge. In relative terms water is a significantly better conductor of electricity than most plastics. Atmospheric humidity deposits small quantities of water on all surfaces in their environment and hence surface static charges on materials have a tendency to dissipate to earth by current flow through the surface moisture.
Repetition
Repeated actions such as friction or separation will increase the level of charge found on a material. For example a plastic web moving over a series of Teflon rollers will increase its surface charge after every roller.
Battery Effect
The combination of many charged items can lead to extremely high charges. For instance individual sheets of plastic with relatively low surface charges when stacked together can generate extremely high voltages.
Change In Temperature
As a material cools down it has a tendency to generate charge. The action of the cooling is to leave a net charge on the material throughout its entire volume. If the material is a very good insulator the internal (volumetric) static charge can be maintained for extremely long periods of time. However over time this charge normally migrates to the surface at which point it becomes a surface static charge. An example of this is an injection moulding which is seemingly neutral when hot but can subsequently be found to have a large surface charge once cool.

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