Microvoid calcined clay for improved opacity

Transcription

1 Microvoid calcined clay for improved opacity A new type of calcined clay for the coatings market. Robert McGuffog. An entirely new type of calcined clay has recently been developed which contains sealed voids to give high dry film opacity in paints both below and above CPVC and a significant contribution to wet opacity. For many years, calcined clays have been widely used as extenders in decorative paints, with roughly 250,000 tonnes being consumed annually by the paint industry world-wide. Their main use is in interior matt emulsion paints, where they give a combination of high opacity, good scrub resistance and low sheen [1]. Origin and production of calcined clays Calcined clays are derived from the crystalline mineral, kaolinite, a naturally occurring hydrated aluminium silicate with the chemical composition Al 2 O 3-2SiO 2-2H 2 O. In the calcining process, a natural clay is heated to about 1000 C. For conventional calcined clays, this heating takes place over about 30 minutes and causes sintering of the platy clay particles to form an irregularly-shaped aggregate with tiny surface voids, as shown in Figure 1. These external voids are open and will be filled with binder or resin in paints below the critical pigment volume concentration (CPVC). However, in paints above the CPVC, there is insufficient binder to fill all the voids in the dry film and some of them will be occupied by air. These tiny air voids are very effective in scattering light and giving dry film opacity. The open nature of the voids means that the opacity benefits of conventional calcined clays are seen only above the CPVC, which explains why their use is mainly restricted to interior matt emulsion paints. An entirely new type of calcined clay has recently been developed which contains sealed voids to give high dry film opacity in paints both below and above CPVC and a significant contribution to wet opacity. This new type of opacifying extender is known as "Super Microvoid Calcined Clay" or SMCC. Microvoid calcined clay This new type of extender is produced by a very rapid calcination technique, which creates sealed voids within the particles. It is commercially available from Imerys Minerals under the name "Opacilite". Natural clays contain hydroxyl (OH) groups as part of their crystal structure. On heating above 500 C, the hydroxyl groups disassociate from the crystal structure and are evolved from the particles as steam. In the case of conventional calcined clays produced by a gradual heating process, the steam escapes slowly to the outside of the particles and simply evaporates. In the case of SMCC, the clay temperature is increased to 1000 C in a fraction of a second and there is insufficient time for the steam to escape. Steam pressure increases rapidly within the particles causing them to expand or bloat like popcorn [2]. Expansion is facilitated by the presence of natural fluxes in the clay, which soften at high temperatures and allow the particles to expand without bursting. The resultant particles contain large numbers of completely sealed internal voids, as shown in the transmission electron micrograph in Figure 2. The formation of the internal voids reduces the specific gravity of the particles from 2.6 to 2.05 [3]. The success of the calcination process in creating large numbers of internal voids depends on achieving a very fast rate of temperature rise. The patented process used for SMCC has been specially developed to achieve this aim [4]. The choice of feed clay is also critical to success of the process. Additionally, the natural fluxes in the clay cause the clay particles to sinter together and form irregular-shaped aggregated particles, just as in the case of conventional calcined clays. These aggregates are extremely strong and are not broken down by subsequent paint dispersion processes. Consequently, Super Microvoid Calcined Clay has a unique combination of sealed internal and open external voids. Effect of voids on dry film opacity SMCC contributes strongly to opacity in a wide range of paints via its internal and external voids. The sealed internal voids contribute to opacity of paints both below and above the CPVC. These voids have a refractive index of 1.0, which is significantly lower than both the surrounding clay particle shell and the surrounding paint film. As light passes through the voids, it is refracted at the void boundaries and eventually totally internally reflected and scattered back out of the paint film. The open external voids contribute to the opacity of paints above the CPVC, where there is insufficient binder in the dry film to fill all the voids. Again, this is due to a difference between the refractive index of the void and the surrounding paint film. The light scattering effect from both the internal and external voids is shown in Figure 3. Effect of voids on wet opacity The sealed internal voids in SMCC are completely resistant to penetration by resins, solvents or water in the liquid paint. Consequently, these voids contribute to the wet film opacity of the paint. The difference in wet opacity between SMCC and a conventional calcined clay is shown in Figure 4. Properties below the critical PVC To compare the effect of SMCC and other extenders in paints below the CPVC, a series of emulsion paints were prepared. Starting with a paint at 20% PVC containing TiO 2 as the only pigmentation, an extender was added in 5% PVC increments, so increasing the total PVC of the paints in steps up to a maximum of 45%. The TiO 2 level was maintained at 20% by volume. The extenders compared were: - Super Microvoid Calcined Clay - "Opacilite" - Opaque polymer - Conventional calcined clay - Fine CaCO 3 (d mm) The effect of these extenders on dry film opacity is shown in Figure 5. The conventional calcined clay contributes little to opacity below the CPVC, because it has a similar refractive index to the binder in the paint and does not improve TiO 2 spacing (5). On the other hand, the two extenders which contain sealed voids, opaque polymer and SMCC, both strongly increase the opacity of these paints.

2 Although, the SMCC and opaque polymer have a similar effect on opacity, they are very different in their effect on the gloss of these paints, as shown in Figure 6. SMCC strongly reduces gloss and is similar to the conventional calcined clay in this property. The matting effect is due to its very irregular particle shape, which induces micro-roughness at the film surface. This means that SMCC is not suitable for high gloss paints. However, it also means that SMCC is ideal for paints below CPVC, where gloss is not required, such as satin finishes and exterior wall paints. Properties above critical PVC To demonstrate the performance above the critical PVC, the following opacifying extenders were compared: - Super Microvoid Calcined Clay - "Opacilite" - Opaque polymer - Conventional Calcined Clay - Fine CaCO 3 (d µm) Starting with a paint at 75% PVC, containing 10% PVC TiO 2 and 65% PVC of a 4 µm mean size CaCO 3 as the pigmentation, a series of paints were made with the opacifying extenders replacing the CaCO 3 in 5% PVC steps. The total % PVC and volume solids of the paints were kept constant. The effect on dry film opacity of replacing the CaCO 3 by the different opacifying extenders is shown in Figure 7. Above the CPVC, SMCC gives much higher opacity than either opaque polymer or the conventional calcined clay, due to the combined effect of the internal and external voids. Another important benefit of SMCC is its effect on scrub resistance. Mineral extenders usually rely on a high binder absorption to give increased dry film opacity (dry hiding), but the high binder absorption has a detrimental effect on scrub resistance. Super Microvoid Calcined Clay is different, because the internal voids, which provide opacity, do not absorb binder. Additionally, calcined clay particles are relatively hard and contribute to the abrasion resistance of the dry film. Consequently, SMCC gives a unique balance of high opacity and high scrub resistance, as shown in Figure 8. This shows the effect of various extenders on the opacity and scrub resistance of a 77% PVC matt emulsion paint. Scrub resistance was measured using the BS 7719 test. The weight loss from the film is measured in this test. The less the weight loss, the better the scrub resistance. In Figure 8, the dry hiding effect of normal mineral extenders (natural CaCO 3, precipitated CaCO 3 and talc) is inversely proportional to their scrub resistance, so the points for these extenders lie roughly on a straight line. The point for the conventional calcined clay lies above the line, due to its greater particle hardness giving improved scrub resistance. The point for SMCC lies even further above the line. It gives similar scrub resistance to the conventional calcined clay, but higher opacity, due to its internal voids. Reducing titanium dioxide levels without any loss in performance A key benefit of Super Microvoid Calcined Clay is that it gives excellent dry and wet film opacity, without any adverse effect on other properties, such as scrub resistance. This benefit can be utilised to reduce the TiO 2 levels in a wide range of paints, without any loss in performance. Considerable reductions in TiO 2 levels can be achieved, as shown in the following examples. Example 1: Saving titanium dioxide in an alkyd undercoat Alkyd undercoats are designed for use on wood substrates to provide a smooth surface for a high gloss alkyd topcoat. They are formulated below CPVC to ensure film flexibility. Key requirements are good opacity and enamel holdout (minimal effect on topcoat gloss). Undercoats usually have relatively high TiO 2 levels to ensure good opacity. The starting formulation for a white alkyd undercoat is shown in Table 1. SMCC was tested in the undercoat formulation as a partial replacement for the CaCO 3 extender. The formulation changes made and their effect on undercoat properties are shown in Table 2. As shown in Table 2, replacing 8% of the CaCO 3 by SMCC (Experimental Paint 1) gives a large improvement in opacity and whiteness. This allows the TiO 2 level to be reduced from 20% to 15.5% (Exp. Paint 2) to give similar opacity, better whiteness and equal enamel holdout to the Standard Paint. So SMCC gives a 22% saving in TiO 2 level, without any loss in performance. The specific gravity of the paint is reduced with SMCC, due to its low density. The combination of the TiO 2 saving and the reduced paint specific gravity would lead to an estimated cost saving of approximately 5 Euro cents/litre. Example 2: Saving titanium dioxide in a matt interior emulsion paint Matt emulsion paints are normally pigmented at significantly above the CPVC to take advantage of the dry hiding effect from extenders. The starting formulation for a good quality white matt emulsion paint is shown in Table 3. SMCC was evaluated in this formulation as a total replacement for the talc extender. The formulation changes made and their effect on the properties of the matt emulsion paint are shown in Table 4. In Table 4, replacing the talc extender in the Standard Paint by SMCC (Experimental Paint 1) gives large increases in both opacity and whiteness and improved scrub resistance. This allows the TiO 2 level to be reduced. In Experimental Paint 2, the TiO 2 level is reduced from 14% to 11%, which results in higher dry film opacity, but similar wet opacity to the Standard. Further reductions in TiO 2 level cannot be made without reducing the wet opacity. However, since the dry film opacity is still higher than the Standard, savings can be made in opaque polymer. Opaque polymer makes virtually no contribution to wet opacity. In Experimental Paint 3, the opaque polymer is completely replaced by water. Although this causes a drop in dry opacity, the hiding is still at least as good as the Standard. In fact, all the properties of Experimental Paint 3 are equal or better than Standard, despite the 21% reduction in TiO 2 level and the complete replacement of opaque polymer. These replacements, together with the reduction in paint specific gravity, would result in a cost saving of about 6 Euro cents/litre. References [1] R. M. McGuffog, Farben und Lacke optisch verbessern mit Kaolinen, FARBE&LACK, (1999), 6 [2] T. W. Davies, Exfoliation of Kaolinite by Partial Rapid Dehydroxylation monograph, High Tech Ceramics, Pt. C, Elsevier Science Publishers BV, Vol. 38C (1987) [3] T. W. Davies, Density Reduction of Kaolinite by Flash Heating, Chem Eng Res Des, Vol. 63 (1985) [4] J. A. Kostuch, US Patent 6,334,894 B1 (2002) [5] F. B Stieg, Effect of Extender Crowding of Titanium Pigment, J. Coat. Tech., Vol. 61, No. 778, (1989),11 Benefits at a glance - Super Microvoid Calcined Clay is a new high performance

3 extender produced by a special rapid calcination process that creates sealed voids within the particles. These sealed voids scatter light and give opacity. - SMCC gives a number of important advantages in paints: High dry film opacity, Good wet film opacity, Good paint whiteness, Excellent scrub resistance, Reduced paint specific gravity. - The contributions to dry and wet film opacity mean that SMCC can allow significant reductions to be made in TiO 2 levels. - These benefits can be achieved in a wide range of paints, both below and above the critical PVC, both water and solvent based. The only restriction is that SMCC has a matting effect, so is not suitable for high gloss coatings. LIFELINE -> After studying chemistry, Robert McGuffog joine Imerys Minerals (formerly ECC International) in their main laboratories, working initially on the development of minerals for various applications. Subsequently, he joined the paint technical service team and became Technical Service Manager 13 years ago. He currently holds the position of Technical Manager at Imerys Minerals and is responsible for technical support for all Imerys' products sold to the paint and ink industries.

4 Figure 1: Scanning Electron Micrograph of Calcined Clay Particles.

5 Figure 2: Transmission Electron Micrograph of Super Microvoid Calcined Clay showing Internal Voids.

6 Figure 3: Light Scattering from both Internal and External Voids.

7 04/2003 Ausgabe/Issue: 143 Seite/Page: Figure 4: Effect on Wet Opacity. SMCC film on the left. Conventional calcined clay on the right.

8 Figure 5: Effect of extenders on opacity below CPVC.

9 Figure 6: Effect of extenders on gloss.

10 Figure 7: Effect of extenders on opacity above CPVC.

11 Figure 8: Opacity v scrub resistance.

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