Processing conductive polymer compounds

Conducting compounds can be obtained by means of incorporating conductive particles into the polymer matrix. These compounds allow the substitution of metallic parts, which means a weight reduction of as much as eight times that of some metals. Today there is an increasing desire for making electrically active compounds, such as their use in portable consumer devices and appliances and electronics in vehicles. The main applications are to protect parts using electrostatic shielding and electrostatic discharge. Processing conditions are a very important factor when achieving good electrical properties, as getting the right dispersion and distribution of conducting particles during the compounding process and applying the right injection moulding conditions can increase the electrical properties of a part. Begoña Galindo and Luis Roca, from the Compounding Department at Aimplas, explain to BP&R the best conditions for processing conductive polymer compounds.

The development of conductive compounds allows the substitution of metal parts in many industrial applications. The advantages of employing conductive plastics include weight and cost reduction, easy extrusion or injection of products with complex shapes and adaptable formulation to increase efficiency, as high conductivity is not needed for every application.

Nevertheless, conductive compounds have some limitations, which must be taken into account. The most important is the use temperature, which is low in comparison to metals. Some applications in which conductive compounds can be used include EMI Shielding, which consists of protecting an electronic circuit from external electrostatic fields and Electrostatic discharge (ESD), which takes place when two objects with different electrical potential are in contact. Static charge can be discharged in a controlled way by increasing the surface conductivity of plastics parts.

To obtain conductive compounds (10¹ – 10⁶ Ω/sq) there are two possibilities:

1. Polymer blends including between 15-20 percent of intrinsically conductive polymers (ICPs) and 85-80 percent of a low melting polymer as polyethylene or polypropylene. The disadvantages of these polymers are their low viscosity, they degrade easily with temperature and their cost is very high. Therefore, the option of including conductive particles is more realistic at an industrial level.
2. Conductive compounds including carbon or metal-based particles in the formula. High electrical properties can be obtained and adapted to the final application.

This article aims to describe the most influencing processing parameters to obtain the optimum electrical conductivity with a carbon-based conductive compound.

The most important property of a conductive particle is the specific surface and the aspect ratio. As a general rule, the higher the specific surface and the aspect ratio are, the higher the particle efficiency. The most common example is the difference between carbon black and carbon nanotubes.

The percolation threshold of carbon nanotubes is much lower than the percolation of carbon black. This is because carbon nanotubes have a higher specific surface area and high aspect ratio. Therefore, low percentage of carbon nanotubes is needed to ensure the formation of the conductive net. The main objective during processing is to achieve this conductive net, meaning that conductive particles must be close enough to allow the electron transfer.

Processing Steps

Processing conductive compounds consists of different phases:

• Compounding: conductive formula is developed
• Quality control: electrical resistivity measurement
• Injection moulding or extrusion à ensuring conductive net

Processing conditions are very important to achieve good electrical properties. Electrical resistivity can vary several orders of magnitude depending on the processing conditions. Conductive compounds can be optimised knowing the influence of processing parameters on the final conductivity.

Compounding Process

The objective is to obtain a proper dispersion and distribution of conductive particles within the polymer matrix. A homogeneous compound is critical to achieve reproducible injected or extruded parts.

Processing parameters that influence on the electrical properties are:

• Extruder type: A co-rotative twin-screw extruder is the most suitable extruder available on the market due to its high mixing capacity.
• Screw configuration: Mid-high shear stress is needed to disperse conductive particles. Therefore, the screw configuration must have large number of kneading blocks.
• Screw speed: Better electrical properties are achieved working at medium screw speed.
• Temperature profile: High screw profile temperatures increase the melt flow index improving the wettability and dispersion of conductive particles.

Injection Moulding

The objective of the injection moulding process is to achieve the conductive net within the polymer matrix. Several processing factors must be taken into account:

• Injection speed: This is the most influencing factor. High injection speed gives lower surface conductivity, however, better volume resistivity and electromagnetic reflection is achieved working at lower injection speeds. Injection speed can change the final conductivity in several orders of magnitude.
• Mould Temperature: High mould temperature improves volumetric resistivity by giving time to the conductive particles to reorganise the conductive net.
• Injected part thickness: Lower thickness improves electromagnetic shielding properties.
• Working with masterbatch: The melt flow index of the conductive masterbatch must be similar to the one of the dilution polymer. In case it is not possible to develop a custom masterbatch, processing aids like erucamides can be used in case of polyolefins.

Film Extrusion

Conventional film extrusion process in a single screw extruder can be optimised in a “one step process”. The one step process consists of adapting a film extrusion die in a co-rotative twin-screw extruder.

Comparing a film produced in a one step process with a conventional extrusion process, the electrical conductivity can be improved in three orders of magnitude. Table 1 compiles the results obtained with a polypropylene filled with four percent carbon nanotubes (CNTs). Surface resistivity is improved in the one step process.

Conclusion

Processing conditions of conductive compounds have high influence on the final electrical properties. During compounding a good particulate dispersion must be achieved to guarantee the homogeneity of the compound. Extrusion and injection moulding parameters must be controlled to achieve optimum electrical properties. The same formulation can change the electrical properties depending on the processing parameters.

With regards to formulation, nanocomposites are an alternative at industrial level. Some benefits are: low percolation threshold (low percentage is needed) and lower influence on the mechanical properties of the neat polymer. Furthermore, prices tend to decrease.