Optimising high speed coating processes.

The coil coating process of strip steel is a continuous and highly automated process in which the aim is to apply a paint layer of uniform thickness onto metal sheet at high operational speed. However, the performance of the coil coating process has been limited due to the occurrence of coating defects, such as ribbing and fat edges, which leads ultimately to a reduction in the operational speed of coating. Therefore, in order to enhance coating productivity, the complex flow behaviour of commercial paints needs to be fully understood. Consequently, the main objective of this work was to obtain rheological parameters for the commercial paints using rotational Rheometers, Capillary Break-up Rheometer and Capillary Extrusion Rheometer to allow the characterisation of the complex flow behaviour. This research project was tailored over its course to generate a solid knowledge foundation of the rheological properties of polyester resin based paints prior to the defining of different experimental procedures so as to guarantee the reproducibility of the data measured with various Rheometers. Once these rheological characteristics were fully understood for one paint, the project was moved forward by selecting other commercially available paints which were similarly analysed to allow a comparison of the various rheological behaviours. One key finding was that the commercial paints behaved in either a Newtonian or a Non Newtonian fashion. The Non Newtonian paints possessed a high internal microstructure which was reflected in the degree of elasticity exhibited, whereas the Newtonian paint did not seem to exhibit any elasticity. Furthermore, a mutual, ideal operability window for the Capillary Break up Rheometer could be derived from data which was systematically collected by varying the input parameters including opening time, stretch profile and Hencky strain. Capillary Extrusion Rheometer provided extensional and shear information at very high strain rates. At the end, a steady state flow curve for both shear and extensional viscosity could be determined for a strain rate range of 10-1 to 5 x 104, which is unique for the polyester resin based paints analysed in this work