This paper represents the first demonstration that a small laboratory system can simulate the circulation of a paint in an automotive manufacturring plant. The effect that different types of pumps and back pressure regulators (BPR) commonly used in paint circulation systems have on the flow characteristics exhibited by a non-metallic, waterborne basecoat was determined. Both rotary circumferential piston and centrifugal pumps were found to progressively change the rheological properties of the basecoat, while a double acting, reciprocating piston pump had little effect. A smaller, secondary change in the basecoat's rheological properties was found to be induced by the use of a conventional BPR as compared to a low shear BPR. The rheological changes observed to occur during circulation did not cause any significant variation in the final appearance properties exhibited by the applied automotive finish. If a measurement of a paint's viscosity is to be used by the automotive industry as a quality control tool, these measurements must be obtained at both a constant strain rate and temperature. Due to the extreme shear thinning characteristics exhibited by a waterborne basecoat, the strain rate at which this measuremnt is taken should be relatively high (i.e., >300 sec(sup)-1). The routine monitoring of viscosity at low strain rates is not recommended because of the inherent inconsistency in the measurements caused by the thixotropic nature of the paint coupled with differences in shear histroy. The use of in-line viscometry should be considered as a potential method of monitoring viscosity changes and paint quality.
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