(300d) Scaling the Long-Term Shear Stability of Aqueous Pigment Dispersions

Industrial application of paints today involves the handling of millions of tons of pigment dispersions. Shear and mechanical stress during pumping, storage or processing significantly affect color and shelf life. At present, mainly empirical test series and pilot scale process models are used to assess the stability of such dispersions and provide a marginal guidance when developing or testing large scale paint applications. Scaling the stability against shear stress is particularly important for water based metal pigment dispersions where damage to the pigment flakes causes their deterioration. The present work investigates the scaling of shear stress from production line (>1000 t/y) to laboratory scale (100 g scale). We analytically show why the integrated shear stress on a specific dispersion can be deconvoluted into a series of individual shear stress contributions. Based on the fluid mechanics (laminar flow) during the transport process these individual contributions can be accurately reproduced in the laboratory using a Couette-type shearing setup similar to a classical rheometer. As a representative example, a stable and a shear-sensitive paint from an automotive production line where sampled over 4 month production. The evolving deterioration, color changes and dispersion instability could be accurately reproduced in a single week using the here outlined scaling method. The combination of fluid mechanic computation and corresponding analytical methods offers a time and cost effective lab-scale alternative for the development of automotive metallic aqueous paints with high shear stress stability.