7.3 Electrostaticity

The electrical charge capacity of an item depends on the condition of its surface, on the dielectric constant, the surface resistivity, and the relative humidity of the surrounding atmosphere. While the charge capacity is inversely proportional to the dielectric constant and relative humidity, it is directly proportional to surface resistivity. The sign of the electrical charge is invariably from the material with the higher to the one with the lower dielectric constant. An electrostatic discharge event can produce sufficient energy to ignite some materials, resulting in fire or explosion.

Due to their composition, most plastics are powerful insulators, a property that makes them indispensable materials for high frequency equipment (radar). An associated drawback of the electrical insulation characteristic is the accumulation of static electricity, which is not discharged fast enough due to the low surface conductivity of most plastics, a difference between plastics and metals. The material, which gains electrons, becomes negatively charged, while the material that loses electrons acquires a positive charge [97]. In an explosive environment, this sudden transfer of charge may be energetic enough to serve as an ignition source [98-100].

There are three basic routes to modify the static discharge behaviour of plastics used in building or other applications:

• Incorporating electro-conductive fillers into the plastic compound

• Incorporating migrating, internal antistatic agents

• Treating the finished product with a coating (painting) of an external antistatic agent [97, 101, 102].

Internal antistatic agents are of interfacially active character, and...