Kolloidchemische Untersuchungen zur Optimierung von Autoserienlacken
The starting point of the research was a water-based paint system which showed changes of the colour during the application process. Colloidchemical examinations of the system were executed to analyse the problem. As a result the formulation and the production process could be optimized.The composition and properties of the paint components were analyzed: the pigments by measurement of BET-isotherms and zetapotentials; the polymer dispersions by GPC-, PCS-, PCD-, zetapotential- and turbidity measurement; the dispersing and thickening additives by IR-and 1H-NMR-spectroscopy.The interactions between the pigments and the additives were examined by the change of their zetapotential at different additive concentrations. It appeared that the properties of the pigment dispersions are determined by an interaction with the thickener. This interaction depends on the surface properties of the pigment and the chemical composition of the dispersant. In the case of the dispersant applied in the formulation an interaction of the thickening additive with the pigment dispersions appeared only below the isoelectric point (IEP) of the pigments. Charge neutralization and the adsorption of the thickening additive favoures a sterical mechanism of the stabilization in this pH-area. A pressure-induced agglomeration of the TiO2-pigments below their IEP was proved by determination the change of the colour of the paint during shear and pumping experiments. Above the IEP no change of the colour appeared because of the additional electrostatic repulsion. The formulation could be optimized by a pigment exchange very elegantly without changing the concentrations of the raw materials and the pH-value of the paint system. Production disturbances could be minimized by a modification of the order in the production process on account of the qualities of the polymer dispersions and the additives. The use of a new high pressure homogenizer in the grinding step effectively increased the colour strength of the paint with the same pigmentation height (about 50%) which leads to economic advantages.