Application | CNTs
New work carried out at Nanocyl shows that conductive compounds containing MWCNTs can be more cost effective than use of conductive primers for painting plastic car body parts. Dr Marie Hurtgen and Michaël Claes explain why
CNTs spell the finish for primer
Electrostatic painting is commonly used in the automotive industry to minimise paint waste. Well suited to use with metallic parts, which can be easily grounded, many millions of automotive exterior parts today are made of electrically- insulating plastics. Achieving the necessary grounding with non-conducting polymers means the application of a conductive primer, which adds cost. The incorporation of electrically conductive additives such as multi-walled carbon nanotubes (MWCNTs) into the polymer provides an alternative option. Over the past 20 years, a number of plastic body parts have been produced in serial production based on CNT technology. Today, additional painted exterior parts using CNTs, including body panels, bumpers, wind deflectors, mirrors and fuel filler flaps, are under evaluation or qualification in the industry. The driver for this growing interest is the significant fall in CNT prices as a result of economies of scale and expanded production capacities. This article details the latest work carried out by Nanocyl (the Belgium-based producer of MWCNTs) to determine the performance and cost savings for automotive producers in adopting TPOs and ABS/PC blends containing its NC7000 MWCNTs for applications such as bumpers and tailgate skins. Conductive primers ensure high paint transfer efficiency onto plastic parts (defined as the amount of paint applied to the substrate compared to the total amount of paint sprayed towards it). This work com- pares the transfer efficiencies of plastics coated with a typical conductive primer and plastics containing NC7000. The amount of NC7000 in the ABS/PC and TPO formulations was optimised to give similar transfer efficiencies as the conductive primer. Figure 1a shows the transfer efficiency (TE) of plastic/NC7000 com- pounds (normalised to that of the reference plastic with conductive primer) as a function of CNT wt%. Low
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amounts of NC7000 (1.0-1.5 wt.%) are sufficient to allow dissipation of the electrostatic charges and ensure high transfer efficiency. Conductive primers generally impart a surface -105
resistance of 104 ohm/sq. NC7000-based plastics
provide in-spec transfer efficiencies even with a surface resistance as high as 1012
ohm/sq, provided there is
some electrical conductivity in the bulk of the material. Both polymers containing NC7000 exhibited a nor- malised TE of more than 80% compared to plastics coated with a conductive primer, which ensures minimal painting cost. Figure 1b shows the general relationship between transfer efficiency and Volatile Organic Compounds (VOC) emissions and related cost of disposal of waste. VOC emissions and related costs can be seen to be less sensitive to changes in transfer efficiencies at the latter approaches 100%. In addition to transfer efficiency, paint appearance and adhesion are of critical importance in automotive exterior applications. Colour was evaluated by measur-
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Falling prices and improved processing
expertise mean CNT-loaded compounds
could eliminate conductive primers for automotive parts
September 2016 | COMPOUNDING WORLD 63
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