A study to define the relationship of bulk resistivity and paint transfer efficiency using a conductively modified thermoplastic resin

摘要

Electrostatic painting of exterior body components is considered standard practice in the automotive industry. The trend toward the use of electrostatic painting processes has been driven primarily because of environmental legislation and material system cost reduction efforts. When electrostatically painting thermoplastic body panels, side by side with sheet metal parts, it is imperative that the thermoplastic parts paint like steel. Electrostatic painting of thermoplastics has traditionally required the use of a conductive primer, prior to basecoat and clearcoat application. The use of conductive plastics eliminates the need for this priming step, while improving paint transfer efficiency and first pass yield. These elements provide an obvious savings in material and labor. The most significant benefit, is the positive environmental impact that occurs through the reduction in the emission of volatile organic compounds (VOCs). VOC emissions reduction is achieved by 1) eliminating the conductive primer and 2) through potentially increased paint transfer efficiency of the basecoat and clearcoat process as compared to conventional conductive primed material systems. During the product development and manufacturing process the material supplier needs a way to predict, and control how paintable the material will be in an electrostatic painting process. Historically, the metric that has been used to capture this attribute has been substrate bulk resistivity. Bulk resistivity is a measurement of the conductivity of the substrate, and will vary directly with conductive additive concentration. A higher conductive additive content will increase the conductivity of the substrate which will result in higher paint transfer efficiency numbers. However, the paint applicator is concerned with paint transfer efficiency, not with bulk resistivity. A rigorous highly statistical "six sigma" process has been used for defining the relationship between paint transfer efficiency and material bulk resistivity. The performance criteria of six sigma means that a customer will experience no more than 3.4 defects per million opportunities. This six sigma methodology utilizes a four step process in order to achieve six sigma capability, 1) measure, 2) analyze, 3) improve and 4) control. Some of the six sigma tools that were used as a result of this product development process are illustrated and referenced in this paper.