Three different classes of thickeners were examined with both an extensional and a rotational rheometer: CMC (CarboxyMethylCellulose) thickener, acrylate thickener, and associative thickener. In the elongation experiments, the various thickeners displayed characteristic differences. The acrylate and associative thickeners displayed an exponential decrease in the filament diameter as a function of time, the typical elongational behavior of an elastic fluid. The CMC solutions displayed the elongational behavior of a more viscous fluid, i.e. the filament diameter decreased according to a power law. The thickeners that had zero viscosity in the shear experiment displayed a longer string life than the associative thickeners, with the shear viscosity increasing in the low-shear range. The differences that were found for associative and non-associative thickeners correlated with the spray coating properties of water-based automotive coatings.
A water-based automotive coating and an aqueous solution of an associative thickener as a rheological additive were examined in the elongational and shear experiments. As expected, in the elongational experiment the decrease in the filament diameter as a function of time displayed the exponential curve for the pure thickener, while the complete formulation followed the pattern of the power law function. Surprisingly however, the filament break-up time for the automotive coating was significantly longer than for the pure thickener, even though the shear viscosity of the pure thickener, up to a speed of 100 s-1, is approximately one decade above that of the automotive coating.
In the case of paper finishing, coating is applied via rollers in most cases. When the coating is transferred to raw paper, unwanted drop formation, or misting, can occur. In case of this phenomenon, the elongational viscosity plays a decisive role.
For the two paper coatings tested, the filament break-up times of the coating with acrylate thickener were significantly longer than in the case of the recipe containing CMC. This correlated with practical experience in machine experiments. Coatings thickened with acrylate tend to spray and mist more rarely than those containing CMC.
In the elongation experiment, it was apparent that a pure acrylate thickener has a clear tendency to form filament. The coating formulations without thickener displayed no filament formation at all. However, if the formulation contained only 0.35% of this acrylate thickener, string formation could be observed. This is related to the interaction between the dissolved thickener molecules, pigments, and binder particles. It could be seen that the filament diameter for all paper coating compounds decreased by mathematical power quantities. In contrast, the pure thickener displayed an exponential decrease in the filament diameter. In the shear experiment, it was barely possible to differentiate between the different mixtures of the paper coatings, which means that it is not possible to draw conclusions from this data about the application behavior during the coating process.
The elongational and shear experiments on paper coatings and automotive coatings indicate that the behavior of these substances cannot be predicted by the simple characterization of the pure thickener. The properties of these complex formulations are controlled by the balance of the interactions between the dissolved thickener polymers and the pigments or the binder. The shear viscosities often do not permit any clear conclusions to be drawn about the application behavior. Elongational experiments can often better differentiate between various products and allow correlations with the process behavior with regard to the coating processes described above.
Read The Influence of Thickeners on the Application Method of Automotive Coatings and Paper Coatings – Rheological Investigations with the HAAKE CaBER 1, which discusses the rheological results of elongational and shear experiments on three different classes of thickeners.