Making plastic conduct

Electrically conductive plastic simplifies designs and reduces costs.

One of the things we like to talk about here at Engineering Materials is the possibilities new technology and new materials offer in terms of replacing the status quo. From steel to aluminium, aluminium to composite, and perhaps back again, material science is continually evolving what we can do.

One of the most exciting prospects is that of conductive plastics. While the science behind it has been well known for some time, getting the compounds right, and actually getting the material to market – and in to products - is relatively recent and its exploitation is something we will see dramatically increase over the next five years.

So what is it? Quite simply they are plastic compounds that allow electrical conductivity. This is achieved in a number of ways but the most common is to use a conductive filler of some kind like steel fibres or carbon nanotubes.

Finnish plastic specialist, Premix - represented by Plastribution in the UK, is actively involved in the area and is encouraging customers to challenge the possibilities. "Every material has a function," continues Dr. Helminen. "It might be a support for another part or to protect contents. Or it might simply be to give a nice feel or look. What we do is take functionalities from other materials, such as electrical conductivity from metals, and add that in to the plastic to open up new possibilities."

"We are able and willing to develop new recipes for customer requirements," says Dr. Antti Helminen, research and development manager at Premix. "If you need particular properties, we will try and find the right the material or create a new one. We are all familiar with normal thermoplastic materials, which have wonderful properties. They are quite lightweight, suitable for mass manufacture; they are corrosion resistance and so on.

Premix helped one of its customers improve a simple push-button electrical switch which had relied on metal to electrical signal transfer. The switch used a piezo-electro mechanism and needed to be water tight.

Like many common electrical components there is heavy competition, and the metal part inside the plastic housing was labour intensive to assemble the 200,000 parts a year. Premix offered the solution by suggesting using a highly conductive thermoplastic material as a signal transfer medium.

By using normal polypropylene in the injection moulding, the part was then covered with Premix's special Preseal material. Preseal is a highly conductive thermoplastic compound primarily intended for EMI gasket applications, but it can also be used as a conductor or in grounding applications. The conductivity is achieved with nickel-coated graphite and it can be injection-moulded making mass production possible with co-injection moulding. So in one injection moulding the switch could be produced without the need for much assembly work.

"The good thing about this material choice is that Preseal is inherently adherent to polypropylene so no primers were needed," says Dr. Helminen. "The conductivity achieved was around 0.5ohms [measured from piezo element to contact pins], so it's a really functional resistivity of the switch."
This example highlights the possibilities of what can be achieve by clever design and where functional plastics can be used to make products that are easier or cheaper to manufacture, easier to assemble, but just as functional.

This can be seen elsewhere in industry. European automotive supplier Jet Press has used a conductive plastic to find a cost-effective method of securing a fuel pipe to an automotive body in such a way that there is no risk of damage to the pipe, and the pipe was earthed through the fastener. A metallic material usually performs the earthing function but in this case a complex solution that combined a plastic and metal would push the in-place cost too high.

The solution developed and provided by Jet Press involved the use of carbon nanotubes (CNT). When dispersed in the main polymer mix (in this case, impact modified Nylon 66) the carbon nanotubes allowed any part moulded to achieve electrically conductivity. Getting the best from this material required specially modified and controlled moulding techniques to make the fasteners with a constant conductivity.

"Products manufactured in this way offer major advantages over other conductive solutions based on carbon black, carbon fibres or metals," says Martin Belcher, technical manager at Jet Press. "They provide, a decreased risk of chemical contamination, homogeneous electrical conductivity at low loading, processing benefits, elimination of residual voltage hot spots, decreased weight, and retention of key mechanical properties.

"In this particular application, a metal clip would risk damage to the fuel line and would need a secondary fixing such as a screw to hold the clip in place. This bespoke solution developed by us was able to provide the customer with all the properties they required of the product, at the lowest in-place cost."

Plastic bearing and energy chain manufacturer, igus, has also been able to utilise conductive plastic in a similar way. It offers a range of its e-chains made from igumid material which has been designed for applications where any electrostatic discharges (ESD) are potentially damaging, for example manufacturing or handling of sensitive electronic products and component parts.

Permanent and reliable discharging of energy chains and cable management systems is especially important to avoid this electrostatic build up. Motion between machine parts can cause static electricity to build up and problems occur when these parts come in to contact with an earthed body causing the voltage to be discharged. To avoid any damage, all machine components need continuous discharging and the entire system needs to be earthed throughout.

Electrostatic discharge is not only dangerous for electronic component parts, it can cause combustion of gases, dusts or vapours in potentially explosive areas. Wherever such conditions exist, for example in the chemical industry, material bulk handling, or in coating lines, the ESD energy chains can be used to avoid electrostatic discharges.

The igumid ESD is proven in thousands of applications and with its special additives, the material is conductive right through the chain, from the first to the last link, unlike products with only temporarily conductive layers. The optimised design also guarantees maintenance-free conductivity.

These examples highlight the possibilities of what can be achieved by using conductive plastics as a replacement material for metal. In conjunction with clever design products can often be more straight forward and cheaper to manufacture, but just as functional.

Justin Cunningham

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Do you have any comments about this article?
Very good information.
This can avoid one operation of applying conductive primer for plastics,before electrostatic application of paint in OE industries.

Comment P B Deshmukh, 06/10/2014

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