Influence analysis of selected variables on wet-milling dispersion process for automotive base coats production at PPG Quattordio Plant

Several coating layers are applied over car body shells, both for protective and decorative purposes. Among them, the colored base coat is one of the most significant from a commercial point of view. PPG Industries plant in Quattordio (Alessandria, Italy), where this master’s degree thesis project has taken place, is one of leader plants in tinting pastes production, which are necessary for the manufacturing of colored base coats. For tinting pastes, which are mixtures of resins, solvents, additives and pigments, the pigment dispersion step is the most crucial within the production process, strictly linked with PPG industrial knowledge. The ability to obtain the proper quality of tinting paste for each batch is essential to make it possible to manufacture colored base coats, which should always ensure the same colorimetric characteristics. The object of this study has been an influence analysis of selected variables on wet-milling dispersion of the pigment with a stirred bead mill, where dispersion is ensured by friction stresses caused by small colliding beads. As a first step, pre-dispersion time with a Cowles impeller and tinting paste flow rate through the mill have been analyzed with two One-Factor-At-Time experiments. After choosing their proper working values, a Design Of Experiments has been proposed to evaluate the influence of four different variables: mill tip speed, filling ratio of beads inside the grinding chamber, dimension of the beads and concentration of the pigment in the tinting paste formulation. The main analysis focused on the development of color strength, quantitively linked to how visible spectrum interferes with dispersed particles inside the tinting paste. OFAT 1 and OFAT 2 pre-dispersion time and flow rate have been set, respectively, to 30 minutes and to the lowest operative value of the available pump. DOE results, on which even a statistical analysis with Minitab has been done, have outlined how all the selected variables influence, in different ways, the color strength of the tinting paste. Notably, the main effect on the results has been provided by mill tip speed.