Automotive bumpers and like plastic substrates are generally coated by spray coating, such as air spray coating or airless spray coating. In recent years, however, electrostatic coating, which shows excellent deposition efficiency and thus emits only a small amount of environmentally harmful substances, has been finding wider application.
Since plastic substrates generally have high electrical resistance (usually about 1012 to about 1016 Ω/sq.), it is extremely difficult to electrostatically apply a paint to plastic substrate surfaces directly. Therefore, before electrostatic application, conductivity is imparted to plastic substrates themselves or their surfaces so that the substrates have a surface electrical resistance below 109 Ω/sq.
For example, before electrostatic application of a paint to a plastic substrate, a conductive primer may be applied to impart conductivity to the substrate. A coating material containing a resin component and conductive filler is conventionally used as such a conductive primer.
Particles of conductive carbon, metals, conductive metal oxides, etc. have been heretofore used as conductive fillers. The form or shape of such conductive filler particles is usually a powder, needles, fibers, spheres or the like.
When a carbon powder or carbon fibers are added to a coating material as a conductive filler, although a relatively small amount can impart conductivity, the resulting coating layer has reduced whiteness, i.e., reduced brightness, and thus affects the color properties, such as brightness, of the upper coating layer to be formed thereon.
Metal powders have high conductivity, but need to be added in large amounts, since the particles of metal powders need to be in contact with one another to form an electrical conduction path in a coating layer. Thus, use of a metal powder as a conductive filler impairs the whiteness of the coating layer and stability of the coating material.
Japanese Unexamined Patent Publication No. 2001-311047 proposes a conductive coating composition containing a specific sulfonium salt compound. Reportedly, this composition can make plastic substrates suitable for electrostatic coating, and has little influence on the color tone of the upper coating layer when forming a multilayer coating film.
However, the sulfonium salt compound in this coating composition adversely affects the environment when baking the coating of the composition or recycling the coated plastic substrates. Further, the coating film formed from the composition has insufficient whiteness.
Japanese Unexamined Patent Publication No. 2002-179948 proposes a conductive resin composition prepared by blending, with a resin, a white conductive powder comprising white inorganic pigment particles having on their surfaces a tin dioxide covering layer containing tungsten.
However, this publication does not describe a specific formulation of the conductive resin composition, and mentions nothing about primer coating compositions. Thus, it cannot be known what formulation of the white conductive powder, when used, gives a conductive primer coating composition that can form, on a plastic substrate, a coating layer with high brightness and sufficient conductivity for electrostatic application of an upper coating layer thereto.