Hello and welcome to this new Rule of Thumb post. Today we discuss three effective ways to achieve Electrostatic discharge (ESD) properties with thermoplastics.
Overview and introduction to ESD
In Europe the ATEX Directive (from the French "ATmosphère EXplosive") 94/9/EC contains regulations for the use of components and systems in potentially explosive atmospheres.
If an electrical discharge in a potentially explosive area (so-called EX area) produces the necessary ignition energy, a spark may be generated which ignites the explosive substance.
Thermoplastics with specific conductive properties can discharge static charges in a controlled and permanent manner. They are used in many industrial sectors such as electric & electronics, semiconductor industry, medical technology, chemical and pharmaceutical industry, and other industries handling dusty bulk materials, combustible substances. It is important during polymer material selection to ensure the maximum safety for these applications by modifying the selected plastic.
Plastics are insulating and their electrical conductivity ranges between 10^-18 to 10^-12 Siemens per meter (S/m). In general plastics have an eclectic surface resistance of >10^15 Ohm and can be electrostatic loaded. Anti Static conductive behavior is achieved from 10^-9 to 10^-3 S/m with a corresponding resistance of 10^6 to 10^10 Ohm. For electromagnetic shielding (EMI), electrical conductivity of 10^3 to 10^9 S/m is needed.
3 ways to have ESD properties in thermoplastics
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| 3 effective ways to achieve ESD properties in thermoplastics |
Stainless steel filler
Stainless steel fillers are an effective way to provide thermoplastic compounds with conductive properties. They can be directly added during injection moulding by a master batch or in a continuous way during compounding.
Achieving ESD properties, 0.25-0.5 vol.% of steel fiber (4 w%) needs to be added. This is enough to have a volume resistivity of <10^2.
Inner anti static additives
Fatty acid esters and Fatty acid amides are added either directly or over a masterbatch to the polymer compound. They migrate onto the surface of the finished plastic part and create a hydrophilic layer which in turn takes water up. The result is an electrically conductive outer layer. Anti static masterbatch concentration is between 3 to 50 w%. Also, hydrophilic polymers such as Polyamidcopolymer can be added which results in a surface and volume conductivity.
Conductive carbon black
Conductive carbon black in a concentration up to 15 w% is often used in polyolefin compounds. Specific volume resistance between 10^2 to 10^5 Ohm cm can be achieved. Apart of conductive carbon black, carbon fibers can be used too.
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Literature:
[1] https://www.roechling.com/industrial/characteristics/esd-plastic
[2] https://www.bekaert.com/en/products/basic-materials/materials-plastic/esd-protected-plastics
[3] Sachtling Kunststoff Taschenbuch, Hanser
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