An end to overcrowding

Transcription

1 An end to overcrowding Hydrous kaolin optimises paint opacity by improving TiO 2 spacing. Although the refractive index of most extenders is too low to directly affect the opacity of coatings, small particle size materials can improve the spacing and therefore the optical efficiency of titanium dioxide. Evaluations are presented which show that fine particle size kaolins are more effective than calcium carbonate or talc for this purpose. David Stewart.* * Contact: David Stewart, Imerys Minerals Ltd., Par Moor Centre, Par Moor Road, Par, Cornwall, PL24 2SQ, United Kingdom, Tel: , Fax: , david.stewart@imerys.com Extenders are used to increase the volume of paint and to improve their technical properties, including optical properties [1]. A variety of carbonate and silicate materials are typically used in decorative emulsion paint to influence these properties. Generally, the difference in bulk refractive index between any extender and the surrounding medium is insufficient to significantly affect the scattering efficiency of the system. Most extenders can only exert an indirect influence on the optical properties of a pigmented paint by influencing the state of dispersion of a pigment [2]. Size and shape of extenders affects TiO 2 efficiency Titanium dioxide (TiO 2 ) is often poorly dispersed and crowded by the extender particles in decorative emulsion paint [3], particularly at higher pigment volume concentrations (PVC). Consequently, the main aim of the formulator when selecting an extender or, more typically, a combination of extenders, should be to make the most efficient use of the TiO 2 present in the decorative emulsion paint. TiO 2 crowding can be significantly reduced by reducing the particle size of the extender used in high PVC matt emulsions; however, this usually implies that some matting efficiency must be sacrificed. The particle shape of the extender selected should also be taken into consideration since a nodular particle can exert a greater adverse effect on TiO 2 crowding than a similar lamellar particle of equivalent spherical particle size. Hydrous kaolin is an example of a lamellar extender and is typically used in decorative emulsion paint for its contribution to optical properties. The benefit of using hydrous kaolin in decorative emulsion paint, specifically matt emulsion paint, will be considered in this article. Structure and mineralogy of kaolins Kaolin is a hydrous aluminium, layered silicate with chemical composition Al 2 O 3 2 SiO 2 2 H 2 O. Its structure can be depicted as alternate layers of SiO 4 tetrahedrons and Al(O,OH) 6 octahedrons, that are held together by hydrogen bonds and dipole interactions [1]. This structure is illustrated in Figure 1. The particle shape is typically a hexagonal plate that has an oxygen-rich surface on one side and a hydroxyl-rich surface on the other. Kaolin is hydrophilic, inert to acids and alkalis and is water dispersible [4]. The hydrous kaolin used in decorative emulsion paint is water washed to remove mineral impurities and can be chemically bleached and/or subjected to magnetic separation to improve brightness and colour. Kaolin that is extracted from primary kaolinite deposits and processed in Cornwall, UK was formed by the hydrothermal degradation of granite and is renowned for its high purity, good colour and influence on low shear rheology. Extenders are compared in a matt emulsion The influence that particle size and shape have on TiO 2 utilisation in a decorative matt emulsion paint can be evaluated by comparing the properties of paint containing calcium carbonate (CaCO 3 ), talc and hydrous kaolin, taking note that these comparisons are based on equivalent spherical particle size. The physical properties of the extenders evaluated are listed in Table 1. The paint used in this work was a 70% PVC styrene-acrylic matt decorative emulsion containing 10% TiO 2, 18% of 2.5 µm CaCO 3, 6% of 9 µm talc and 10% of the extender under evaluation. This formulation is shown in Table 2. The comparative dry film properties of these paints containing various grades of CaCO 3, talc and hydrous kaolin are detailed in Tables 3 and 4. Factors affecting the optical properties Small particle sizes give best opacity The results of the analysis show that as the equivalent spherical particle size of the CaCO 3, talc and hydrous kaolin decreases, so the opacity of the matt emulsion paint increases as a result of the improved utilisation of the TiO 2. Lamellar particles have a much greater effect on the reduction of the TiO 2 crowding than nodular particles such as CaCO 3. For example, hydrous kaolin provides a 12% higher spreading rate than talc and a 22% higher spreading rate than CaCO 3 when comparing extenders with an equivalent mean spherical particle size of 2.5 µm. The finest hydrous kaolin tested (d 50 = 0.4 µm) provided a 1.6 m 2 /l spreading rate advantage compared to the finest CaCO 3 (d 50 = 0.9 µm) tested and a 0.8 m 2 /l spreading rate advantage compared to the finest talc (d 50 = 1.0 µm) tested. A comparison of all these results is given in Table 3. Coarser particles give more effective matting Hydrous kaolin provides lower gloss at 85 than CaCO 3 and talc when comparing extenders with equivalent spherical particle size. The matting efficiency of the coarsest hydrous kaolin tested (d 50 = 3.5 µm), is matched by CaCO 3 and talc with an equivalent mean spherical particle size of 5.0 µm and 4.5 µm respectively. In this series of work, hydrous kaolin provided more than double the matting efficiency of CaCO 3 and talc per equivalent mean spherical micron of extender. The lowest gloss at 85, 1.4 gloss units, was achieved with the coarsest CaCO 3 (d 50 = 10 µm) tested. The matt emulsion paint containing the finest grade of kaolin had a gloss of 5.8 gloss units at 85 and was the highest value recorded. A comparison of all these results is provided in Figure 3. Calcium carbonate provides the best colour At all particle sizes of CaCO 3 tested, it gave the best overall paint colour in the matt emulsion paint. Only one talc (d 50 = 2.5 µm) and the two finest grades of kaolin (d 50 = 0.4 µm and d 50 = 0.7 µm) were able to match the whiteness (L*) of the paints containing CaCO 3. The effect that the choice of extender has on the final paint colour is thus dependent on the extender colour. Kaolin improves the structure and sag resistance The wet film properties of the 70% styrene acrylic matt emulsion comparing various equivalent spherical particle

2 sizes of CaCO 3, talc and hydrous kaolin are detailed in Table 5. Gel strength is a measure of the degree of structure and it is related to the in-can appearance of the paint. Low shear viscosity (0.3 s -1 ) is indicative of the levelling and flow behaviour of the paint and can give an indication of sag resistance. The low shear rheology and gel strength of the matt emulsion paints containing hydrous kaolin are significantly higher than those of either the paints containing CaCO 3 or those using talc. This is illustrated in Figures 4 and 5 respectively. Fine kaolin optimises opacity and rheology Hydrous kaolin facilitates more effective TiO 2 utilisation and is also a more effective matting extender than CaCO 3 or talc of an equivalent spherical particle size. The improved low shear rheology provided by hydrous kaolin will improve the storage stability, anti-sag and application properties of matt emulsion compared to paints containing an equivalent quantity of CaCO 3 or talc. The matt emulsion paint containing the finest hydrous kaolin tested, provided the highest opacity and this is therefore the most effective extender tested to reduce 'crowding' of TiO 2. REFERENCES [1] D Gysau, Fillers for paints, Vincent Network, 2006, 16-17,32-33 [2] H Dörr, F Holzinger, Kronos titanium dioxide in emulsion paint: optimal formulation for quality and efficiency, Kronos International, 1990, 25 [3] P F Dietz, Spacing for better effects: influence of fine particle size extenders and entrapped air on utilisation of TiO 2 in emulsion paint, European Coatings Journal, 7-8/2003, [4] P Ciullo, S Robinson, Kaolin clay: functional optical additives, Paint and Coatings Ind. (USA), 2003, Vol 19(8), ACKNOWLEDGEMENTS The author would like to thank J. Trewolla, M. Julian and K. Thomas for their contributions to this project. Results at a glance - Although common extenders have a refractive index which is too low to increase opacity directly, fine particle size materials can reduce 'crowding' of titanium dioxide and so improve its optical efficiency. - Both the particle size and shape are significant in determining these effects. Plate-like materials such as hydrous kaolin can be more effective than nodular materials. - The performance of calcium carbonate, talc and kaolin of several different particle sizes were evaluated in a matt decorative emulsion paint. - It is concluded that while calcium carbonate is capable of producing the best overall colour, fine particle size kaolin provides the best optical efficiency. Kaolin can also improve rheological properties and has the better matting efficiency, if compared with CaCO 3 and talc extenders of a similar particle size. The author: -> David Stewart is the Coatings Technical Service Manager for Imerys Minerals Ltd, based in Par, Cornwall, UK. He has over 15 years experience in the coatings industry and has a BSc degree in chemistry and physics and a MBA in international management.

3 Figure 1: The structure of hydrous kaolin.

4 Figure 2: Effect of extender particle size and shape on opacity. Figure 3: Effect of extender particle size and shape on gloss. Figure 4: Effect of extender particle size and shape on low shear viscosity.

5 Figure 5: Effect of extender particle size and shape on gel strength.

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