By Michael Krayer and Sean W. Bullis, BASF Corporation, USA

Over the past few decades, the performance of water-based architectural paint has consistently improved. However, latex polymer dispersions used in water-based coatings are typically limited to around 50 wt % polymer solids. This is because most polymer dispersions used for architectural coatings have a fairly narrow, single particle-size distribution (monodisperse), and when the monodisperse polymer particles approach their close packing limit, there is a sharp increase in viscosity at solids contents much beyond 50%.1,2,3

On the other hand, polymer dispersions with a bimodal particle-size distribution can achieve a significantly higher solids content due to the increased random close packing limit of polydisperse systems (Figure 1).3 BASF has extensive experience in developing and producing higher solid polymer dispersions, especially for pressure-sensitive adhesive applications, achieving solids content up to 70%. Recently, BASF has developed a 60 wt % polymer dispersion with a bimodal particle-size distribution for water-based architectural coating applications.

FIGURE 1 Comparison of the random close packing limit and practical solids content of monomodal versus bimodal polymer dispersions.

Compared to architectural paints formulated with conventional polymer dispersions, paints formulated with bimodal high-solid dispersions can achieve a higher solids content, resulting in an increased dry film thickness. For instance, in a paint formulation that uses a monodisperse latex binder, increasing the paint solids content beyond 50 wt % (or 40 vol%) causes the formulation to quickly turn into a paste, making it unsuitable as a paint. However, paint formulations that use a bimodal high-solid latex can achieve solids contents of more than 60 wt % (or 50 vol%).

Furthermore, the rheological properties of paints formulated with high-solid dispersions allow for a more favorable balance between high-shear viscosity, sag resistance, and flow and leveling. A certain high-shear viscosity is typically desired to achieve the desired amount of paint applied. However, current high-shear rheology modifiers, used in conventional paint formulations, have a significant viscosity contribution to the mid- and low-shear regions (see Figure 2). When targeting a desired high-shear viscosity by adding high-shear rheology modifiers, one quickly increases the midand low-shear viscosity beyond the desired points. The mid- and low-shear regions of paint are responsible for the sagging and flow and leveling behavior of the paint. While a certain mid-shear rheology is desirable to prevent sagging, the optimal flow and leveling is achieved when the low-shear viscosity is kept low (Figure 3). Compared to paint formulated with a monodisperse latex, bimodal latex systems have a lower low- and mid-shear viscosity response when high-shear rheology modifiers are added. Therefore, they allow for higher high-shear viscosities while being able to achieve the desired mid- and low-shear viscosities. In other words, bimodal latex binders allow for the formulation of paints with a more Newtonian rheology profile.

FIGURE 2 Examples of Brookfield (low-shear), KU (mid-shear), and ICI (high-shear) viscosity contributions of various BASF rheology modifiers used in conventional paint formulations.

FIGURE 3 Illustration of different shear rates and applications associated with them. A) Increasing high-shear viscosity typically has an undesirable impact on mid- and low-shear viscosity. B) Mid- and low-shear rheology need to be balanced for optimum sag resistance and flow and leveling.

By combining a higher paint solids content, resulting in thicker dry films by itself, with a favorable rheology profile that enhances paint transfer and increases both wet and dry film thickness, the resulting thicker films can significantly improve hiding (Figure 4).

FIGURE 4 Illustration of dry film thickness of a A) conventional architectural paint vs. B) architectural paint formulated with high-solid polymer dispersions.

These value propositions have been thoroughly explored and demonstrated by our team in Europe and were published in the European Coatings Journal earlier in 2023.4 However, the development and formulation work in Europe focused primarily on brush application to mimic application properties of solventborne alkyd wood and trim paints. In this report, we focus on exploring the same value propositions for roller application in the North American market. Our objective is to demonstrate that bimodal high-solid polymer dispersions can be formulated in paint to improve application feel and one-coat-hide properties when applied with a roller to a vertical wall.

Continue reading in the July-August digital issue of CoatingsTech