Cutting the cost of opacity

Transcription

1 Seite/Page: 1 Cutting the cost of opacity An important strategy in producing decorative paints economically is that of adding extremely fine extenders to space the titanium dioxide particles. A new naturally occurring complex carbonate was compared with existing materials for this purpose in a matt interior wall paint. This material produced the highest contrast ratio of all the materials tested along with a high brightness. Natural complex carbonate reduces TiO2 requirements in decorative paints Simon Bussell* Mart Verheijen In the first quarter of 2009 the price of titanium dioxide (TiO2) stood at USD 2,400/tonne for the USA and Asia [1]. Since that time regular increases have seen the price rise dramatically to USD 2,750-2,950/tonne by the end of Prices are even higher in Europe at up to EUR 2,780/tonne [2]. At the same time, supply has been severely limited, with manufacturers rationing deliveries to customers. There are several reasons 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 TiO2in 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 on a natural mixture of calcium and magnesium carbonates and hydrated carbonates. This 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. The work described below will show that the mixture, which occurs naturally as sub-micrometre, platy crystals, is an ideal TiO2 extender, maintaining and improving brightness and hiding power in comparison to other ultrafine extenders. How titanium dioxide efficiency in use has been improved Titanium dioxide is responsible for two principal features of paints and coatings, whiteness and opacity. It 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 TiO2particle. Reduction of TiO2 content is a balancing act between the need to reduce costs and the need to maintain quality. To be effective, a TiO2extender must allow levels of the primary pigment to be reduced while maintaining whiteness and opacity levels. In the 1970s 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 1ashows 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. In any case, 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 1b). Typically, particles of around twice the diameter of TiO2particles ( µm) 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 as optical extenders: finely ground calcite (GCC), precipitated calcium carbonate (PCC), fine hydrous kaolin and calcined kaolin, at two different TiO2volume concentrations. This work was undertaken at the SSME paint lab in Maastricht using a formulation (Table 1) based on a styreneacrylic emulsion. Initially two coatings were prepared using 7 % and 3.5 % TiO2 by volume. Selected coatings from these trials were re-tested using a higher binder level. With the PVC above critical and a low level of TiO2, these coatings were considered to represent a general use interior wall paint. Key properties of extender material The complex carbonate extender evaluated here 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 mines and partially processes the material in Greece 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 the company has developed proprietary processing techniques to achieve an optimum particle size distribution without affecting the unique morphology. The deposit is fine and white with a high carbonate content and an iron content of only 0.03 %. It has a refractive index of and a BET surface area of 18 m2/g. Other details of the morphology and physical properties can be seen in Figure 2 and Table 2. It occurs naturally as an aggregate of platy crystals with sides of approximately µm and an aspect ratio of 10:1. The defining properties are its high brightness, low particle size and the high oil absorption/surface area. The advantages of these properties will be shown in the following work. The properties of the other test fillers (as measured by SSME laboratories) are also summarised in Table 2. Where quoted, the manufacturer"s D50 particle size is also given. Measurements were made using the Sedigraph technique which is useful for platy materials such as hydrous kaolin, but can give discrepancies for round or needle shaped particles such as PCC. A lower value would be expected for

2 Seite/Page: 2 the calcined kaolin, suggesting that this sample was highly agglomerated. Manufacture and test methods summarised All coatings were produced according to the formulations in Table 1using a high speed disperser with a toothed blade. The water, dispersants, calcium scavenger, biocide, antifoam and thickener were dispersed at low speed until fully dissolved. The pigment, fillers and extender were added and dispersed at high speed for 30 minutes until a grind of 5 Hegman or better was achieved. Following this, the binder and thickener were added and dispersed at low speed for a further 30 minutes until a uniform dispersion was achieved. Brightness and contrast ratio were measured on coatings drawn down onto a "Leneta No. 2" opacity chart using a Bird type applicator to give 150 µm wet film thickness and allowed to dry for seven days. Brightness was measured as L* using a spectrophotometer. Contrast ratio was measured as the ratio of reflected light from the coating over the black and white portions of the card (RB/RW) expressed as a percentage. Viscosity was measured using a "Brookfield" viscometer fitted with spindle 5. High and low shear measurements were made at 100 rpm and 10 rpm. New extender gives highest contrast ratios The results for all test measurements are shown in Table 3. There was no significant difference in the brightness measured for all formulations, being in the range 95.0 ± 0.6, although it should be mentioned that the highest brightness for each series was given by the complex carbonate extender. As might be expected the brightness of the coatings containing 7 % TiO2 was higher than those containing 3.5 % TiO2although in reality the difference was on average only half a point. Gloss measurements were also very similar for all samples, ranging from 2.4 to 3.8. All coatings would thus be suitable for low sheen applications. The variation in contrast ratio was far greater, ranging from a low of 90.1 to a high of Once again the complex carbonate formulations had the highest value in all test series, in some cases by a significant margin, being at least two points above the average value for each series of tests. The results for the fine GCC and the PCC were very similar and it would be difficult to place one above the other in terms of contrast ratio. The lowest contrast ratio was given by the hydrous and calcined kaolins, which were also very similar. In terms of particle size and shape the hydrous kaolin was closest in morphology to the complex carbonate; however the results show it is not as effective as a TiO2extender as the complex carbonate is. There was some variation between the contrast ratio of the higher and lower TiO2 contents. It would be expected that reducing the TiO2 level or increasing the binder content would reduce the contrast ratio and this is observed for the complex carbonate, the hydrous kaolin and with available PCC results. Deviation from this pattern for the fine GCC and calcined kaolin may be due to dispersion effects as this is a controlling factor for TiO2 extension. Higher viscosity may be advantageous The area which showed the greatest difference between the complex carbonate and the other extenders was viscosity. The GCC, PCC and hydrous kaolin give reasonably similar viscosities; however, the complex carbonate gives a noticeably higher viscosity and the calcined kaolin a noticeably lower viscosity. The highly thixotropic nature of dispersions containing the complex carbonate can be an advantage in that settling is less pronounced and thickener levels can be reduced. If lower viscosities are required then subsequent work has shown that alternative combinations of dispersant and thickener are effective in controlling the rheological behaviour. The relatively low viscosity of the mixes containing the calcined kaolin, despite having the highest oil absorption, may be further evidence of the difficulty in deagglomerating the material and achieving a fine dispersion. This in turn would account for its poor performance in terms of contrast ratio. New extender provides highest optical efficiency SSME s complex carbonate extender is an ultrafine, platy extender that has been compared in this series of tests against ultrafine carbonate extenders and other platy extenders. Overall the results show that it works well as a TiO2extender. Brightness in the test formulations was slightly better than the competitive materials; however, the most important result was contrast ratio. In formulations above the critical PVC it is relatively easy to achieve a good brightness using white fillers but dry hiding power can be difficult to maintain as TiO2 levels are reduced. This work shows that increasing the level of the complex carbonate extender in a formulation allows the TiO2level to be reduced while actually increasing the dry hiding power to a level not achieved by the other fillers. This has advantages both in terms of cost reduction and quality improvement. In addition the rheological behaviour makes this extender highly suitable for viscous materials such as sealants, underbody coatings and putties. REFERENCES [1] Titanium dioxide price pressure, Industrial Minerals, 2009, March. [2] Titanium dioxide market most "buoyant" for a decade, Industrial Minerals, 2011, January. [3] Dietz P. F., The effect of fine particle size extenders and entrapped air on TiO2 in emulsion paints, PCI Magazine, 2003, September. Results at a glance Titanium dioxide is an extremely effective opacifier, but expensive and sometimes in short supply. The quantity required to achieve adequate whiteness and opacity in decorative paints has therefore been progressively reduced over time by improving dispersion efficiency and by the use of fine 'spacing' extenders. A newly marketed naturally occurring complex carbonate was compared with standard materials for this purpose in a matt interior wall paint formulation. The complex carbonate produced the highest opacity (contrast ratio) of all the materials tested along with a high brightness. It also produced a high viscosity, making it ideal for sealants and other high viscosity materials, but it can be used satisfactorily in paints by modifying the thickener addition. * Corresponding author: Simon Bussell Sibelco Europe T

3 Seite/Page: 3 simon.bussell@sibelco.com

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5 Seite/Page: 5 Figure 1: Improving TiO2 distribution by (a) reducing filler particle size and (b) incorporating ultrafine extender (not to scale)

6 Seite/Page: 6 Figure 2: Morphology of complex carbonate extender

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8 Seite/Page: 8 Figure from Cutting the cost of opacity

9 Seite/Page: 9 Figure from Cutting the cost of opacity

10 Seite/Page: 10 Figure from Cutting the cost of opacity