Process And Materials Effect In Organic Coating Of Architectural Steels

Coil roll coating is an industrially mature process to produce pre-coated steels at high speeds.30% sales by revenue for Tata steel colors, UK is attributed to the manufacture of architectural steels which is primarily used for composite panels for cladding systems. A research programme has been carried out which has examined the mechanisms by which the coating is picked up from the coating trough by the roll, with a reference to a defect where an insufficient coating is transferred leading to a defect named paint misses (PMD). A comprehensive study of a pair of coatings, one which demonstrated PMD and one which did not identify that their performance difference could be attributed to differences in rheology. The coating extensional viscosity and elasticity were identified as indicators of the behaviour. The identification of these primary rheological characteristics was corroborated through a blind test of a further pair of coatings where PMD had been observed on the coating line. The ability of the pickup roller to pull the coating from the trough and apparent wall slip (AWS) near the surface were identified as the dominant mechanisms. These mechanisms were also identified in a laboratory-scale simulation of the pickup cylinder rotating in the coating trough. Model PVC coatings were manufactured where the pigment concentration and solvent level was varied independently to develop the further underlying relationship between material rheology and coating pick up. The Mooney analysis method used to examine the slip behaviour was found to be inadequate for describing the behaviour of the liquid coatings. The deviation between idealised and real behaviour increased as polymer and pigment mass fraction increased. The quantity of coating picked up by the roll was found to be difficult to measure accurately due to the presence of the ribbing phenomena, which was particularly evident on the commercial coating systems. Additionally, liquid coating pickup tended to reduce with roll speeds and decreased with increasing pigment fraction (and hence viscosity). The study recommends an updated Quality Control (QC) system for checking the liquid coatings prior to being introduced to the process coating line.