Conductive TPO for Optimum Electrostatic Painting Efficiency

Conductively modified thermoplastic olefins (TPOs) have been developed using low levels of conductive carbon fillers, <6 wt.%, without sacrificing the favorable mechanical and rheological properties exhibited by these materials for the automotive market. The bulk electrical properties of these materials exhibit traditional percolation behavior when the samples are compression molded. Conductively modified injection molded materials exhibit a high surface resistivity, typically greater than 1016 ohm cm, which is compensated by a relatively low resistance interior, defined as a core resistance. The threshold for electrostatic paintability has been identified based on core resistance measurements, electrostatic dissipation results, and electrostatic painting transfer efficiency data. The core resistance must be less that 10⁹ ohm/cm for charge dissipation to occur and for any significant increase in paint film builds to be observed. Using these conductively modified materials, high-solids paint coating film builds and transfer efficiencies equivalent to steel substrates have been achieved in plaque studies. Results acquired from electrostatic painting of production rear bumper fascias at Utica Plant have shown transfer efficiency increases for adhesion promoter, primer, basecoat, and clearcoat when conductive TPO parts are used. In addition, these plant trials were also used to examine the effect of lightweight fixturing on painting efficiency. Using conductive TPO parts, only a small effect on transfer efficiency was observed with these fixtures. For non-conductive parts, the use of tubular wire-frame supports decreased painting transfer efficiencies up to 50% relative to full cast-aluminum supports.