Choosing the right wetting agent improves coating performance.
Architectural paints are composed of a few basic ingredients: an emulsion or latex, pigments, and additives. Each material plays a vital role in the overall performance of the finished paint, so selection of raw materials is critical as improper selection of any of these may drastically affect the properties of the paint. For example, choosing the wrong binder may result in poor durability, excessive dry times, or lack of adhesion to the substrate. Similarly, a poorly chosen wetting agent or surfactant can lead to poor dispersibility of the pigment, resulting in loss of gloss and poor film aesthetics. For architectural paints, surface active agents, also known as surfactants, are used to lower surface or interfacial free energies of the aqueous phase to achieve good pigment wetting for efficient grinding and consistent substrate wetting for defect-free coatings.
The process of preparing a pigment dispersion is typically defined as the sum of three distinct processes. The first process involves wetting of the dry pigment to displace air and any other impurities from the pigment surface. Once the pigment particles are sufficiently wetted out, grinding occurs to reduce the large pigment aggregates and agglomerates to their primary particle size. The last step involves stabilizing the dispersion to enhance color and viscosity stability as well as shock resistance and letdown compatibility.
Traditional wetting agents include alkylphenol ethoxylates, which have an excellent ability to wet out the pigment surfaces. Due to growing health concerns surrounding APEs, there is an increasing trend towards replacing these materials in coatings applications. Because of their hydrophilic nature, APEs tend to stabilize foam and can cause water sensitivity issues in the final film. Use of these surfactants also leads to the use of strong defoamers to control foam which, in turn, can cause defects such as craters and pinholes. As shown in Figure 1, choosing the right wetting agent for pigment wetting will not only provide the most efficient milling and color development, but also may aid in preventing foam generation during manufacture and application (see Figure 2). Foam created during the manufacture of coatings causes insufficient grinding as well as handling problems. Figures 3 and 4 illustrate how a non-foaming, non-APE-containing surfactant promotes efficient pigment grinding as well as improved gloss leveling and durability in the finished paint film.
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In Figure 4, a semigloss paint formulated with an APE-containing surfactant is compared to a paint formulated with a non-APE-containing surfactant. The paint prepared with the non-APE-containing surfactant not only has better flow and leveling properties as seen in Figure 1 but also enjoys a 27% increase in scrub resistance over the APE-containing paint due to better wetting of the substrate.
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Jeanine M. Snyder
Air Products and Chemicals, Inc.
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|Title Annotation:||FORMULATOR'S CORNER|
|Author:||Snyder, Jeanine M.|
|Date:||Sep 1, 2010|
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