Conductive polymers & compounds

Additives for electrically conductive compounds. Electrically conductive modified thermoplastics have been available since the seventies. Thanks to tighter regulations on radiation and safety, and the greatly increased use of electronics in the eighties, such plastics have undergone very strong growth.

Three categories

Electrically conductive compounds can be divided into roughly three categories:

• Antistatic – the prevention of static buildup;
• ESD (Electrostatic Discharge)– the prevention of high energy sparking by allowing the fast and controlled dissipation of static electricity;
• EMI (Electromagnetic Interference)– the prevention of disruption to the surroundings by electromagnetic radiation from electronics or vice versa. Typically electrically conductive compounds are used in computers, telecommunications and networking systems, inkjet and laser printers, ATMs, fuel systems and filters, internal transport systems, air filter/cleanroom systems and medical equipment, but also in packaging films and such.

Methods to make plastics conductive

Below is a list of the most commonly used additive systems to make thermoplastics electrically conductive for ESD protection and EMI shielding.

Migrating antistatic agents (cationic, anionic and non-ionic)

Conductivity range: Antistatic agent 1014 – 1010 ohm/sq
Additive percentage (% by weight): 0.1 – 3

Advantages:

• Low-cost additive systems
• Easy to handle and suitable for almost all plastics
• Also works as a processing aid (release agent)
• Many different possibilities in terms of conductivity and sustainability
• Virtually all colours possible

Limitations in processing:

• Stability at higher temperatures
• Compatibility with base polymer

Restrictions on use:

• Antistatic properties disappear over time
• Requires a certain air humidity to function
• Can cause problems with post-operation, such as printing and bonding
• Only works at RH% > 10-15

There are many different migrating antistatic agents available that vary in migration speed and migration time. By carefully combining various types of fast and slow migrating antistatic agents with each other, these can also be made suitable for technical applications. However, there should be a careful examination of the requirements regarding conductivity and durability. In other words, consult with your supplier of the antistatic agents.

Polymeric antistatic agents (for example, Pebax, polyaniline)

Conductivity range: ESD 1012 – 109 ohm/sq
Additive percentage (% by weight): 5 – 15

Advantages:

• Relatively pricey compared with carbon black
• Easy handling of the compound (not to be dosed yourself during injection moulding)
• Virtually all colours possible
• Also works well at very low RH%

Limitations in processing:

• Stability at higher processing temperatures
• Compatibility with base polymer

Restrictions on use:

• Not suitable for all polymers
• Relatively expensive
• Limited conductivity in compounds

Carbon Black (soot)

Conductivity range: ESD 109 – 106 ohm/sq
Additive percentage (% by weight): Max. ~40

Advantages:

• cheap additive, a cost-efficient conductive compound
• applicable in many plastics

Limitations in processing:

• Risk of reduced surface quality

Restrictions on use:

• Releases ‘carbon dust’ (carbon sloughing); therefore unsuitable for cleanroom applications
• Only possible colour is black
• Flow properties and surface quality decrease at higher % of carbon black
• Especially suitable for ESD applications
• Mechanical properties are not optimal

PAN carbon fibre

Conductivity range: ESD-EMI 106 – 100 ohm/sq
Additive percentage (% by weight): 7 – 40

Advantages:

• no carbon sloughing
• Good mechanical properties (high rigidity, and so on) of the compound
• Combines well with other fillers
• Applicable in all polymers
• Good workability
• Not as abrasive as glass fibre
• Independent of RH%

Limitations in processing:

• Process carefully in connection with sensitivity to shear; the fibre length determines the shielding effectiveness
• Wall thickness has an influence on the shielding effectiveness
  Restrictions on use:
• Only dark colours are possible
• High modulus (very rigid)
• Carbon sloughing can cause problems (depending on the quality of the carbon fibre)
• Expensive

Stainless steel fibres

Conductivity range: ESD-EMI 106 – 100 ohm/sq
Additive percentage (% by weight): 5 – 15

Advantages:

• no carbon sloughing
• Mechanical properties of the base polymer remain reasonably well preserved in the compound
• Most colours are possible
• Reasonable freedom of base polymer
• Independent of RH%

Limitations in processing:

• Process carefully in connection with sensitivity to shear; the fibre length determines the shielding effectiveness
• Wall thickness has an influence on the shielding effectiveness
• Difficult to combine with other fillers in connection with the softness of the stainless steel fibre

Restrictions on use:

• Low modulus; combinations with glass fibre reduce the shielding effect
  Expensive

Aluminium flakes, metal powders, metal-coated fibres and spheres (glass and carbon)

Conductivity range: ESD – EMI 1010 – 102 ohm/sq
Additive percentage (% by weight): 10 – 60

Advantages:

• More freedom with regard to compound properties can often be combined with other fillers
• Colours are possible
• Reasonably good processing
• Independent of RH%

Limitations in processing:

• Metal powders are abrasive for processing equipment
• Fibres are very sensitive to shear
• Restrictions on use:

Very expensive, such fillers are typically used in specific cases

Miniaturisation

Thanks to the progressing miniaturisation of (electronic) components, the use of electrically conductive plastics is becoming more and more interesting in comparison with conventional materials, such as metal and standard plastics (whether or not with an electrically conductive coating).

Costs

In the use of electrically conductive compounds, it is very important to get an understanding of the costs of various alternatives, such as metallised/painted plastic, metal and similar. The cost statement must include all expenses in terms of production, logistics, post-processing, the environment, waste and production outages and the supply chain.

Most successful applications

To date, when replacing metal and plastic with a conductive coating, electrically conductive compounds have proved particularly successful in products with the following properties:

• Complex shapes (3D curved, whether or not with pillars, ribs, recesses, snap fits, and the like)
• Relatively small products; a rule of thumb is that electrically conductive plastics are of interest for components up to a maximum of 80-100 cm3
• If the conductive coating is not applied by the plastics processor. In that case, logistics costs and risks begin to play a part.