Titamiun Dioxide in Coatings, Opacity

The development of opacity by titanium dioxide pigments is governed by a number of factors. However, as paint manufacturers, you should always ensure that you do not waste the potential of your TiO₂ pigments during formulation and manufacture of your paints.

Opacity is not a standalone property, but depends on a number of other factors during paint formulation and manufacture:

• Dispersion
• Stabilization
• Cost and Opacity
• TiO₂ Content

Opacity measurement is not a straightforward operation and there are a number of ways to express it using various techniques. A few of these are described below:

• Contrast Ratio
• Hiding Power
• Scattering Coefficient

Probably the most fundamental way of ensuring opacity is completely developed is by fully dispersing the pigment. Millennium Inorganic Chemicals Tiona^® manufacturing process  ensures that the particle size and distribution are optimized for maximum opacity. However, during transport and storage, agglomeration of the pigment takes place. This effectively changes the particle size and distribution so that the pigment is no longer optimized. The dispersion process must ensure that the pigment is returned as closely as possible to its initial particle size and distribution.

With modern pigments, it is recommended that you use high pigment loading in your millbase to ensure maximum dispersion. Remember the grind gauge only shows particles greater than approximately 5000 nanometers (5 microns). It is at particle sizes of 250 nanometers (0.25 microns) that opacity is developed. Therefore the grind gauge should only be used as a guide to the fact that the pigment is dispersing, not as a measure of the particle size achieved.

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Good, stable dispersion of a TiO₂ pigment is a prerequisite to developing high opacity. In the production of coatings, the pigment needs to be fully dispersed and fully utilized in the minimum time. However, it is also necessary  to ensure that your millbase formulation is robust enough to withstand the stresses of the let down stages of manufacture. This is best done by first stabilizing the millbase by an interim stabilization stage prior to final let down. As a guide, with solvent borne coatings, stability is usually achieved at a pigment:binder ratio in the concentrated, but stabilized, millbase of between 5:1 and 4:1. If this is achieved by reducing the concentration of the millbase under efficient stirring, then the stabilized millbase should be capable of being let down to final paint concentration without flocculation. With water borne millbases, starting with approximately 80% TiO_2, reducing the pigment content to approximately 60% by the addition of water and thickener, or even polymer, will stabilize your millbase dispersion. These guidelines will require confirmation in your millbases as the needs of stability change from system to system. Control of flocculation means that you will obtain maximum opacity from your pigment. As the main property required from a titanium dioxide pigment is opacity then careful manufacturing techniques will repay the trouble of careful stabilization.

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Fundamental studies in relation to optimum cost effective formulation have been carried out at Millennium Inorganic Chemicals and a short summary follows. Figure 2 shows the classical relationship between opacity and pigment volume concentration (PVC). At PVC's greater than 10%, opacity no longer has a linear relationship to TiO₂ content. At these higher levels, the rate of increase in opacity diminishes with increasing PVC until it reaches zero at approximately 25% PVC. This relationship will vary slightly with individual resins and pigments.

These changes are due to pigment crowding effects and as PVC increases further, hiding power decreases. However, at lower, more efficient PVC's the raw material costs are higher. Therefore an optimum balance between cost and opacity will need to be struck over a certain PVC range as shown in Figure 3. Therefore an optimum balance between cost and opacity will need to be struck over a certain PVC range. This range will vary depending upon the system and the relative costs of the key raw materials. Fundamental analysis of formulating technique can lead to cost effective formulation.

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The TiO₂ content of a pigment is an important factor in the opacity the pigment can develop. The TiO₂ content of surface treated pigments can vary between 80 and 95%. The higher the level of TiO₂, the greater the opacity for a given pigment content. Consideration must be given to the comparison between surface treatment and opacity when selecting a pigment but end use of the pigment will modify choices at times.

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Contrast ratio is the ratio of the reflectance of a paint film measured over a black substrate compared to its reflectance measured over a white substrate.

It includes all contributions to the opacity of the paint film from light scattering sources and light absorption sources and thus measures the "total" opacity of the film. Due to the inclusion of absorption, it is possible to influence the contrast ratio by the inclusion or omission of a tinter (usually yellow oxide, black or blue) The contrast ratio will also be influenced by a change in raw material whose color differs from the one of the original. Contrast ratio is only suitable for use with white paints. The introduction of colored pigments adversely impacts on the measure. Contrast ratio has no units.

Measuring Opacity Top

The hiding power of a paint is defined as the area that a unit volume of wet paint will cover at a film thickness sufficient to produce a contrast ratio of 98% when the film has dried. 98% contrast ratio is used, as it is the limit beyond which any further increase in contrast ratio is not visually detectable by a trained eye. This, therefore, represents a standard definition of full opacity. Because of its basis on contrast ratio, this function also gives a measure of "total opacity" including contributions from scattering and absorption. Units of hiding power are m².L^-1 or ft² .gal^-1.

Measuring Opacity Top

This function is a measure of the amount of light scattered at pigment/vehicle interfaces within the paint film. This coefficient is calculated via Kubelka Munk theory using the same reflectance measurements as contrast ratio. This function is influenced very little by contributions from light absorption and therefore most effectively expresses the performance of TiO2 in a paint film. If a pigment gives a high scattering coefficient, then the potential opacity of a tinted paint containing that pigment is high. This function is expressed in terms of "per unit film thickness" usually corrected to a thickness that is relevant to real paint films.

Measuring Opacity Top