Brushstrokes - October 2011 - Surface Coatings Association of New ...

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The French process uses purified metal which is melted and vaporized at 1650 to 1850°F under controlled conditions. The main difference in the American process is that this process uses ore instead of refined metal. However, the effects for which it is used in paints can be summarized, together with the known or suggested explanations for the results: “Use of Zinc Oxide in Paints (Nelson-Mattiello) To aid mixing and grinding - Consistency control - Penetration control and sealing - Drying - Hardening or solidifying the film Gloss and gloss retention - To minimize discoloration and yellowing - Reduced chalking - Color and tint retention Dirt shedding (self-cleansing), washability, and ease of polishing - Mildew control - Resistance to moisture and to blistering under moist conditions - Neutralizing and inhibiting” Leaded zinc oxide was first introduced into the paint industry about 1896 as a result of developments on smelting Colorado complex zinc and lead ores. With improvements in the manufacturing process the product was standardized. Several grades were developed containing 5, 20 and 35 per cent lead sulfate and basic lead sulfate, the remainder being zinc oxide. The more recent developments have been the introduction of a 50 per cent grade of leaded zinc and the manufacture of leaded zinc oxides by blending white basic lead sulfate and lead-free zinc oxide. A considerable portion of the leaded zinc oxide on the market today is entirely or in part a mechanical mixture. Practically the entire production of leaded zinc oxide is used by the paint industry. It is used in conjunction with other white pigments in exterior house paints. When leaded zinc oxide is used in a paint it usually furnishes the entire amount of zinc oxide required in the particular paint. Co-fumed leaded zinc is made in the same general manner as American process lead-free zinc oxide. Dry-mixing zinc oxide and basic lead sulfate in the proper proportions yields blended leaded zinc oxide. Lithopone was not very important for some time after its introduction. This is often characteristic, and some attribute this condition to the fact that people do not like to make changes. However, the fact that lithopone was not light-stable prior to about 1920 no doubt had a definite effect on its acceptance. The improvement in lithopone at this time was very marked and made it possible for the material to be used much more widely than previously. Lithopone is primarily used in paints, rubber, textiles, paper and printing inks. It is receiving very stiff competition from titanium dioxide and extended titanium dioxide. High strength lithopones are made by blending lithopone with zinc sulfide or titanium dioxide. The former are made in several “strengths,” the 50 to 55 per cent zinc sulfide grade being the most popular. This is also known as zinc sulfide-barium pigment. Blends with titanium dioxide are called “titanated lithopones,” and usually contain about 15 per cent of titanium dioxide. Also in this same class are the zinc sulfide-calcium and zinc sulfide-magnesium pigments; these are blends of zinc sulfide with calcium sulfate or magnesium silicate. 4 SURFACE COATINGS ASSOCIATION OF NEW ZEALAND Zinc sulfide is a relatively high-strength pigment, but it has not been used as widely as the blends with lithopone or extenders. Extended titanium pigments were introduced shortly after World War I. First to be produced was titanium-barium pigment, consisting of 25 per cent titanium dioxide and 75 per cent barium sulfate. Later came titanium-calcium pigment, with 30 per cent titanium dioxide, and a 30 per cent titanium-barium pigment. Relatively recently, titanium-magnesium pigment (30 per cent titanium dioxide and 70 per cent magnesium silicate) was made for use in house paints. These pigments, particularly straight titanium dioxide, met great resistance because of their relatively high price per pound, and a long educational program was required to convince the pigment users that these prices were really low when considered on the basis of cost per unit of hiding power developed in the finished product. Fig. 4 Titanium dioxide pigment Titanium dioxide has the highest hiding power of any of the white pigments, but its manufacture is more complicated and more expensive than any of the other pigments which have been considered. Figure 4 shows the manufacture of titanium dioxide pigment. Examination of this flow sheet shows how complicated the manufacture of this pigment really is. In order to manufacture pigment of a high quality today, extreme care must be taken in all steps, and impurities controlled very accurately. Previously the subject of hiding power was mentioned. It is this quality of titanium dioxide that makes it very important in the paint field. The comparison of the hiding power of various pigments is given in Figure 5. There may be some disagreement in the actual values of hiding power. This disagreement is due primarily to the fact that different conditions are used in measuring it. However, in general, the order given in Figure 5 will be found under usual conditions. Fig. 5 Comparison of hiding power of various pigments.
A Naturally Occurring Sub-Micron Titanium Dioxide Extender for Decorative Paints Introduction By Simon Bussell*, Technical Sales Manager, Sibelco Speciality Minerals Europe * Corresponding author Mart Verheijen, Application Development Manager, Sibelco Speciality Minerals Europe In the first quarter of 2009 the price of titanium dioxide (TiO2) stood at $2,400/tonne for the US and Asia. Since that time regular increases have seen the price increase dramatically to $2,750-2,950/tonne by the end of 2010. Prices are even higher in Europe at up to €2,780/tonne.‚ At the same time supply has been severely limited with manufacturers rationing deliveries to customers. There are several causes for this, shortages of titanium ores such as ilmenite and rutile as well as other raw materials such as sulphuric acid, increased energy costs, increased demand from Asia and capacity reduction by suppliers. This situation has forced formulators to look again at the level of TiO2 in their coatings and consider options for reducing the quantities used. One such option is a complex carbonate extender produced by the Sibelco Group company Ankerpoort based a mixture of calcium and magnesium carbonates and hydrated carbonates. The material is marketed by Sibelco Speciality Minerals Europe (SSME) a new commercial group focused on the supply of functional fillers for coatings, polymers and adhesives. This work will show that the mixture, which occurs naturally as sub-micron, platy crystals, is an ideal TiO2 extender, maintaining and improving brightness and hiding power in comparison to other ultrafine extenders. Experimental Titanium Dioxide is responsible for two principle features of paints and coatings, whiteness and opacity. TiO2 is the best pigment for these properties because of its high refractive index (2.75). This gives the highest level of scattering when light crosses the boundary between binder and TiO2 particle or air and TiO2 particle. Reduction of TiO2 content is a balancing act between the need to reduce cost and the need to maintain quality. To be effective a TiO2 extender must allow TiO2 levels to be reduced while maintaining whiteness and opacity levels. In the 1970’s a wall paint could contain more than 18% TiO2 by volume. 3 This has been steadily reduced since that time, typically by improving the dispersion and separation of TiO2 particles in the film. Figure 1 shows diagrammatically how this can be done by reducing the average particle size of the filler material. There is a limit to how effective this method can be. Beyond a certain particle size suitable finely ground fillers may not be available, and if they are binder demand, dispersant levels and dispersion time would all be increased. An alternative method is to use ultrafine particles to space the TiO2 particles in the interstitial voids between large filler particles (figure 2). Typically Figure 1 – improved TiO2 distribution by reduction of filler particle size (not to scale). particles of around twice the diameter of TiO2 particles (0.5- 0.6µm) are found to be most effective. The purpose of this work was to determine how effective this complex carbonate mineral is as a TiO2 extender. To do this it was compared with four other materials commonly used for TiO2 extension, finely ground calcite, precipitated calcium carbonate, fine hydrous kaolin and calcined kaolin at two different TiO2 volume concentrations. Figure 2 - TiO2 spacing by ultrafine extender (not to scale) Materials This work was undertaken at the SSME paint lab in Maastricht using a formulation (table 1) based on a styrene-acrylic emulsion. Initially two coatings were prepared using 7% TiO2 by volume and 3.5% TiO2 by volume. Selected coatings from these trials were repeated using a higher binder level. With the PVC above critical and a low level of TiO2 these coatings were felt to represent a general use interior wall paint. Fillers The complex carbonate extender is a mixed alkaline earth carbonate with the general formula Mg3Ca(CO3)4. It is formed by the weathering of magnesite or dolomite and subsequent deposition. Ankerpoort mine and partially process the material in Greece (figure 3) where the raw material is screened and beneficiated to provide a reasonably pure feedstock for further processing. The material is then shipped to The Netherlands where Ankerpoort have developed proprietary processing SURFACE COATINGS ASSOCIATION OF NEW ZEALAND 5
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