Conductive modification of injection molded thermoplastics: electrical properties and electrostatic paintability

Conductively modified thermoplastic substrates have been developed using low levels of conductive carbon fillers, <6 weight percent, 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 resistivity interior, less than 10⁸ ohm cm. The threshold for electrostatic paintability has been identified based on core resistivity measurements and electrostatic dissipation results. This core resistivity must be less that 10⁹ ohm cm for charge dissipation to occur and for any significant increase in paint film builds to be observed. The surface to core resistivity transformation is intimately related to the material cooling rate and thus the distance of the sampled area to the injection molding tool surface. Using these conductively modified materials, high solids paint coating film, builds and transfer efficiency have been increased in excess of 100 percent relative to unmodified samples. The use of conductively modified thermoplastic substrates in the electrostatic painting process can eliminate the need for conducting primers when adhesion between the base coat and substrate surface is maintained