Titanium Dioxide in Coatings, Dispersion
In any surface coating, the full potential of the pigment in terms of color, opacity, tint strength, gloss, durability and flow cannot be realized without optimum dispersion.
Millennium Inorganic Chemicals Tiona® titanium dioxide pigments are surface treated to facilitate rapid and complete dispersion in the widest possible range of coatings binders. This is demonstrated by the coatings exhibiting:
Dispersion Fundamentals
Dispersing TiO2
Nowadays, it is common practice to disperse modern pigments using only a high-speed disperser mill (HSDM) to make most types of paints. However, the overall suitability of the HSDM to disperse a given binder/solvent/pigment system will depend more on the binder/solvent blend than on the pigment. Most modern pigments are easily dispersed in suitable media.
While there are numerous different designs of HSDM equipment ranging from small laboratory sizes, to units capable of manufacturing in excess of 20,000 liters of paint, the basic principles of design and operation are essentially the same. Best results are obtained using the mill geometry shown in Figure 1. It is essential that the correct blade speed (20 to 30 m/s) be maintained at all times. Therefore when purchasing a new HSDM you should ensure that a constant speed motor, rather than a constant torque motor is installed.
When the mill is operated correctly, you should see a typical "rolling doughnut" type of circulation induced by the rotation of the blade. You should also be able to see a small inner portion of the blade. Correct dispersion is critical to you obtaining the best performance from the pigment.
The HSDM process can generate considerable heat, leading to solvent loss and viscosity reduction if the millbase is not correctly formulated. Both of these changes will reduce the efficiency of your dispersion stage. It is generally accepted that you will obtain the best dispersion with a millbase resin solids content of between 30% and 40% non-volatile, the exact level depending on the type and quality of resin used. Your ideal pigment loading is usually 75-80% pigment by weight or even higher depending on the binder. If it is not possible to load the pigment to this level, you should give serious consideration to the use of a more suitable dispersing vehicle.
Compared to most conventional products, it is often possible to incorporate up to 10% more of Millennium Inorganic Chemicals leading multipurpose pigments into a millbase due to their low vehicle and water demands. This can be achieved without incurring excessive viscosity or increasing the amount of energy necessary to reach the required dispersion level. Thus power demand can be kept to a minimum and a potential overload of the power unit can be avoided. This increased loading also means that you make more paint per mill load, again leading to significant savings.
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Millbase Formulation and Optimization
The ideal millbase should be slightly dilatent. The viscosity/shear relationship can then be optimized whilst maintaining millbase mobility. Dilatency will also help to ensure that "dead spots" around the side of your milling vessel are avoided. If the viscosity of the fully loaded millbase is too low, the addition of pigment, rather than the use of a more viscous resin or a lower solvent component, is the better way for you to correct it. The aim is to increase the shear applied to the pigment particles, not to increase resin viscosity. Maximizing the amount of pigment does this best.
Increasing the pigment content increases shear with less influence on viscosity. It also reduces heat generation by reducing the amount of energy wasted in stirring high viscosity resin solution. This maintains the millbase viscosity and temperature at safe levels; minimizing solvent loss and maximizing pigment content. Maximized pigment content ensures that dispersing efficiency is optimized gaining full opacity, tint strength and gloss from the pigment. Millennium Inorganic Chemicals has been able to demonstrate both in the laboratory and under practical plant conditions that a fully optimized millbase gives dispersion which frequently allows a lower pigment content to be used in the final paint. We regularly see a 5% gain in opacity and tint strength following such a millbase reformulation. Significantly higher savings have been obtained from time to time depending on the efficiency of the original millbase.
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