CNTs | Application
Figure 1a: Paint transfer efficiency of ABS/PC blend and TPO compunds containing NC7000 MWCNTs, normalised to ABS/PC or TPO with typical conductive primer. The red dashed line indicates the acceptable normalised transfer efficiency for key automotive suppliers
ing the L*a*b* parameters (CIE LAB) at different measurement angles (as required for metallic colours) compared to the reference conductive primer samples. A* and b* parameters (color channels) were almost unaffected, and lightness (L*) remained within the typical tolerance for all measurement angles. Other parameters such as gloss, short waves and long waves (which are related to surface quality) were also within specification and similar to reference samples. Paint adhesion was evaluated using cross cut and
high pressure water jet tests according to Volkswagen standards (and shown in Figure 2). All the MWCNT –con- taining samples passed the tests, as did the primer references. The stone-chip impact test was also found to be positive. The adhesion tests (which were performed
Figure 1b: General evolution of VOC emissions as a function of paint transfer efficiency
before and after ageing) showed that paint adhesion is determined by the combination of base resin, paint and, when applicable, flaming conditions; the presence of NC7000 MWCNTs had no detectable influence. NC7000-containing plastics can be used with
conventional injection moulding equipment. Critical mechanical properties (tensile strength, thermal expan- sion, mould shrinkage, heat deflection temperature, impact strength) and compound density are either not or are minimally affected by the MWCNTs.
Counting the cost Aside from technical performance, cost is a critical component in material selection. Today, unlike fenders, many exterior parts are based on lower cost resins such as PP-impact or ABS. These currently mainly use a con- ductive primer before application of an electrostatic base coat and clear coat as the global additional cost of a MWCNT solution was considered too great. However, the higher cost of NC7000-based materials can be compensated for by other benefits: primer cost savings; elimination of primer spraying booth/oven and associ- ated drying energy and VOC emissions; and simplified grounding bucks and painting sequences (no need to start painting near a grounding clip). Conductive NC7000 compounds can also result in fewer rejects due to improved paint wrap at corners and edges (conduc- tive primers can cause conductivity gradients and lead to local paint accumulation at edges). The value of any of these benefits in a particular
Figure 2: Selected adhesion tests on non-aged samples (VW specfication) show the different conductive substrates pass the three main tests. The damaged area for the stone chip impact test is smaller than 5.5%
64 COMPOUNDING WORLD | September 2016
project will vary depending on the paint-shop of the automotive Tier 1, including whether it is using solvent or waterborne paints, the age of the equipment and other factors. This variation is represented by standard
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