In a typical measurement procedure, a paint film was drawn down on a glass plate using a 400 μm applicator blade and immediately transferred to a Haake Mars 60 rheometer. This novel measuring system has been used to investigate the influence of several Clariant additives on the drying process of aqueous polymer dispersions and paints. FIGURE 1 » Novel rheological measuring system 12 pins fixed on a ring. Moreover, undisturbed water evaporation from the film is guaranteed due to the open design of the measuring system, as the pins are attached to a ring and not to a condensation-prone plate which might create a local humidity environment. It also reduces the influence of inhomogeneities in the paint film, thanks to the multiple point measurement technique in comparison to the one-point measurement. The increase in the number of pins provides a higher torque response and therefore a higher sensitivity towards viscosity changes in the film during drying. The new system consists of 12 evenly distributed 2mm thick pins attached to a ring, as shown in Figure 1. In this paper, a novel rheological measuring system adapted from the previously referenced needle-plate system is presented. The main disadvantage of this method is that it relies only on a one-point measurement and therefore may be sensitive to film inhomogeneities. This has been described in previously published literature.
A more intuitive, but less sophisticated method to characterize the drying of paint films can be provided by a rheometer, using a needle affixed to the classic plate geometry, which slowly rotates through the paint film. It often fails in performance differentiation when comparing the effect of paint additives on drying. In practice, this method is not reproducible and consistent due to non-standardized factors, such as the force applied during brushing, among other things.
The current ASTM D7488 method relies on subjective visual assessment of repainted marks on drawdown paint film. This part of the painting process is still a challenging task due to the lack of an objective and stable method for its evaluation. This is particularly relevant for the identification of additives for the prolongation of open time, which refers to the period of time remaining after paint application to make corrections, without leaving visible defects in the film. In addition to sophisticated analysis, we still see the need for a simple and easily accessible method which allows for quantitative differentiation of drying behavior of aqueous paints and, moreover, one that will help formulators to select the most favorable ingredients for specific products and applications. Among the technical challenges that have arisen due to the switch from solvent to waterborne paints - driven by the regulatory push towards a more sustainable coatings industry - the control over the drying and film forming process of paint dispersions has been a matter of intense investigation in recent years.Īdvanced analytical methods and imaging techniques have been used to gain insights into the mechanisms driving the film forming process and to identify factors which influence open time.
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