1. Thermoplastic Polymers Act as Insulators
By their very nature, thermoplastic polymers, like PE, PS, PA, PC act as insulators, a useful property in many applications. However, in certain applications, only electrically conductive materials should be used. This is the case with ESD applications (electrostatic discharge), i.e., where problems due to electrostatic discharge must be minimised. For these applications, plastics with permanent electrical conductivity are required.
2. Units of Measurement
The electrical conductivity of materials is expressed in Siemens. In practice, however, it is not the electrical conductivity of the plastics that is measured, but rather their electrical resistance, which is the inverse of their electrical conductivity.
The unit of measurement of electrical resistance is the Ohm. Ordinary plastics are insulating, and have an electrical resistance of 1014 Ohm. Metals are located at the other end of the range with electrical resistance values of 10-4 to 10-8 Ohm.
In order to use plastics for ESD applications, their electrical resistance must be reduced. Either conductive materials (electrical resistance <105 Ohm) or dissipative materials (electrical resistance between 106 and 109 Ohm) are used. The resistance values mentioned must not alter over time. In order to achieve these resistance values, carbon black is added to plastics for ESD applications.
3. ESD: Problems and Solutions
Electrostatic charges are generated by friction between two material surfaces. The actual ESD problem is not the formation of these electrostatic charges, however, but their discharge. The discharge occurs when two bodies with different electrostatic potential come into contact, or when they experience an induction by an electrostatic field (definition according to CECC00015).
A practical example: A person walking on a synthetic-fibre carpet, who becomes charged without even realising it. If that person then touches a door-handle (at 0 volt), he or she will receive an electrostatic shock. In this case, it is not dangerous. However, in certain circumstances, a discharge can have catastrophic effects.
Formation and consequences of ESD: The formation of electrostatic charges cannot be avoided. However, violent electrostatic discharges can be prevented by using conductive materials instead of insulating materials. On the conductive materials, the electrostatic charges are not localised, but immediately after their formation, are channelled to an earthing point. This means there is no longer any voltage differential.
ESD can have catastrophic consequences if the intensity of the discharge is so high that it causes spontaneous combustion of powders or liquids, leading to fires or explosions. For example, when an explosive fuel/air mixture is present, an electrostatic discharge can set fire to a petrol station by generating a spark. Another example of this kind is an explosion in a grain silo.
For this reason, conductive compounds are used in safety applications. Typical applications are pipes and hoses for areas at risk from explosion, such as mines, chemical plants, etc. Another application example is packaging for combustible powders or liquids into blow moulded containers, e.g., spare fuel cans, conductive fibres for big bags and technical injection moulded components.
ESD in the electronics industry.
Another area at risk from ESD is electronics. A person walking across a synthetic-fibre carpet can charge himself up to 35,000 volts! If this person then picks up an electronic component, this could be damaged to a greater or lesser extent. Voltages of 30 - 7,000 volts are enough to destroy computer chips. Since the trend in the electronics industry is to manufacture ever-smaller and higher-performance chips, the risks from ESD are growing all the time.
Therefore, conductive plastics have the following applications in the electronics industry: transport systems and storage boxes for electronic circuit boards, bags for electronic components, profiles and blister packs for chips and thermoformed sheets for trays.
All of these articles are required in EPAs (or ESD protected areas). In the electronics industry, the assembly of circuit boards or electronic devices is carried out in such EPAs. All equipment and materials used are conductive and earthed, to avoid an electrostatic discharge.
4. ESD Compounds
Cabot offers a comprehensive range of CABELEC® ESD products for use in ESD applications. These compounds are based on conductive carbon black that is built into a modified polymer matrix. The compounds, which are based on various polymer types like PE, PP, PS, PC, PA, POM, have been specifically developed for extrusion, injection moulding and blow moulding processes. The types of conductive carbon blacks used are characterised by their small particle size, high specific surface area and high structure.
Addition of carbon black
The addition level of carbon black in the polymer must be high enough so that the carbon black particles touch, or are less than 10 nm away from each other. The relationship between the quantity of carbon black added and the electrical resistance achieved is shown in the percolation curve below. This curve is carbon black and polymer specific.The carbon black must be worked homogeneously into the polymer matrix. The shear forces during compounding must guarantee optimum dispersion of the carbon black without destroying its structure. The properties of the basic polymer are altered by the addition of carbon black (see table below). Therefore, the role of the compounder is critical in creating a conductive compound with the desired electrical and mechanical properties.
As carbon black is added, one can see:
| An increase in : | A decrease in : |
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Processors should pre-dry carbon black-filled compounds since, in most cases, the carbon black used is hygroscopic. CABELEC ESD compounds can be processed with the usual production equipment. However, low shear forces should be used in order to achieve the required electrical characteristics.
