To optimize a paint formulation for the best cost-performance, the choice of the kaolin-based extender is crucial, and the choice is dependent on the type of coating to be produced.

Paints with PVC/CPVC

Paints with high pigment volume concentration (PVC) levels above CPVC are economical and usually have high opacity at low to very low TiO2 concentration. Opacity is a crucial property in high PVC paints and is controlled by both TiO2 concentration and an extender/filler package. At a given TiO2 concentration, the surface area of the solid-air interface of the extender/filler package is the major opacity controller in the dry paint film. Extenders with voids in their structure, such as calcined kaolins, provide high surface-air interface and contribute to high dry opacity of the paint film.

Paints with PVC below critical, such as semi- and high-gloss paints, have no entrapped air in the dry film. The entire solid surface is wetted by the binder and opacity is obtained solely by TiO2 particles. At any given TiO2 concentration, opacity is improved or optimized by ensuring maximum surface exposure of the TiO2 particles to light. Using fine hydrous kaolins with a particle size similar to TiO2 (0.2 to 0.4um) will give optimum spacing of TiO2 particles, prevent further agglomeration and ensure maximum exposure to light.

The choice of extender is related directly to the main properties to be enhanced or controlled in a paint formulation.

Above the CPVC, paints are considered matte. The degree of matteness is determined by 85 deg gloss, or what is commonly known as the sheen. Below the CPVC, paints are glossy and are considered as either semi-gloss or high gloss. Gloss is measured at 60 deg or 20 deg angles.

This article discusses the use of appropriate kaolin extenders in matte low sheen (also called flat paints), matte medium sheen (also known as satin or eggshell paints), and gloss paints. Emphasis will be on improving opacity while maintaining or enhancing sheen/gloss properties.

Low Sheen Matte Paints

To reach low sheen in high PVC systems, large particle-size fillers/extenders are used, which adversely affect opacity.

To avoid compromising opacity for lower sheen, a fine particle-size calcined kaolin was developed. The chemically structured surface scatters the light efficiently, resulting in the reduction of sheen. Besides low sheen and high opacity, this surface structure exhibits excellent touch-up properties (minimum sheen difference at different paint thickness).

A fine particle-size (1.4E) extender gives good TiO2 spacing resulting in improved opacity. The unique surface structure scatters light efficiently, thereby producing low sheen.

Medium Sheen Matte Paints

With the increased demand for cleaner tints and whiter whites, new high-brightness calcined kaolins were developed. With brightness above 95 and a particle size as low as 0.8E, a bluish undertone with excellent opacity can be obtained at controlled formulation cost. The table shows the main physical differences between a conventional calcined kaolin and two high-brightness, high-surface-structure calcined kaolins.

In addition to the bluish undertone, the high brightness products have a finer particle size, higher void structure and higher oil absorption than the standard grades. As a result, at one-to-one replacement levels, opacity increases significantly, and an increase in sheen and decrease in scrub resistance is produced. The increase in opacity allows a reduction in TiO2 concentration. Although replacement levels up to 30% of TiO2 are possible, it is not usually applied in practice due to wet opacity limitations. In other words, high TiO2 replacement leads to a bigger wet-dry film opacity difference. A typical TiO2 replacement level is 10-15% without affecting the wet-dry opacity significantly. The amount of fine extender needed to replace the conventional types while allowing 10-15% TiO2 replacement is usually lower than that of the original amount used with the conventional grades. The difference in pigment volume is then replaced by coarse fillers, which in turn will lower the sheen to its original value. It is essential to realize that through increased opacity, one gains more latitude in formulating possibilities (i.e., less TiO2, less fine extender, and more coarse fillers than with conventional extender grades). The increase in concentration of the coarse filler will bring back the scrub resistance, due to its lower oil absorption. Lower TiO2 levels using less of the fine extender is the cost advantage in this type of reformulation. The performance difference between the 1.2E and the 0.8E high-brightness-structured calcined kaolins is slightly higher opacity and higher sheen for the finer grade. Both grades produce clean tints with a bluish undertone.

Gloss Paints

In gloss paints, fine particle-size extenders are used. The fine smooth structure is essential for gloss retention and efficient TiO2 spacing. The average particle size of TiO2 is 0.2-0.3E. An extender with a similar size to TiO2 will fit in perfectly between the dispersed TiO2 particles/agglomerates. In other words, the extender works as a TiO2 spacer, which maximizes exposure of the TiO2 surface to light, resulting in higher opacity. A new ultrafine hydrous kaolin grade was developed for gloss coating systems. The 0.2u average particle size allows 8-12% replacement of TiO2. The low density of 2.5 g/cm3, and the laminar structure of this ultrafine hydrous kaolin, improves the overall stability of the TiO2 dispersion. Here the efficient spacing of TiO2 particles increases the light exposure of the TiO2 surface leading to optimized opacity.

Conclusion

Paint formulations can be optimized to give the best opacity at a controlled or lower formulation cost by the proper choice of the extender package. The cost of a formulation is highly influenced by the amount of TiO2 used. Being able to formulate with a suitable kaolin-based extender for improved opacity at lower TiO2 levels favors this cost reduction of formulation without compromising other essential paint properties. Understanding the relationship between particle size, particle shape, surface structure and oil absorption gives the formulator the ability and the flexibility in obtaining the right quality at the right cost.

This article was originally published in Surface Coatings International.

For more information, contact Engelhard Corp., Appearance and Performance Technologies, 101 Wood Avenue, PO Box 770, Iselin, NJ 08830-0770; phone 732/205.5000; fax 732/321.0250; visit www.engelhard.com; or e-mail Ashok.khokhani@engelhard.com.