Titanium dioxide, also known as titanium (IV), is an oxide of titanium that occurs naturally. Its chemical formula is TiO2 and its pigment color index is CI 77891. It has many uses outside of paint, which include food, paper coatings, plastics, cosmetics and others. It is primarily obtained from ilmenite, rutile and anatase, but also occurs as brookite. Additionally, titanium dioxide can occur in high-pressure conditions as a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form.
Titanium is mainly sourced from ilmenite ore, which is the most widespread form of titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98 percent titanium dioxide in the ore. The metastable anatase and brookite phases convert irreversibly to the equilibrium rutile phase upon heating above temperatures in the range of 600 to 800°C (1,112 to 1,472°F).
There are two main processes for TiO2: In 1916, the Titanium Pigment Corporation of Niagara Falls, New York, and the Titan Company A/S, of Norway simultaneously began commercial TiO2 production with the sulfate process. In 1951 DuPont introduced the chloride process.
As per Figure 2 and Figure 3, the chloride process has been the predominant production method but has decreased over the past 25 years due to the influx of Chinese sulfate-grade TiO2. However, the chloride process still dominates the global market.
In addition to the raw processing, the pigment is treated in various ways for specific uses. Some organic treatments are provided to improve dispersibility or the reduction of degradation by ultraviolet light. Inorganic treatments consist of other metal oxides that also provide protection from UV. The pigments are ground to a specific particle size (median and distribution) for optimal opacity as well. Dependent upon the treatment and particle size, TiO2 can range from a yellow to a blue undertone, which has to be considered by formulators of base paints for customized colors.
As stated previously, there are many applications of TiO2, but paint is by far the largest (Figure 4). Of all white-hiding pigments, titanium dioxide, and specifically rutile, affords the best opacity/hiding (Figure 5).
TiO2 does possess the highest opacity of paint pigments, but it is not an inexpensive pigment, nor does it work alone for optimal usage. Extender pigments such as silica, silicates and carbonates are used as “spacers” in a paint, so that the TiO2 pigment particles are less agglomerated and are more readily amenable to optimum hiding. This is more easily achieved in higher PVC (pigment volume concentration) paints, as opposed to gloss paints, because the extenders can have a deleterious effect on gloss. The extender pigments may also be functional, providing improvement to touch-up and burnishing, scrub resistance and cleanability6.
In the past 15 years, nanomaterials have found greater utilization in many applications. This has been true also for TiO2, where the very small particles are very useful in cosmetics as well as catalytic self-cleaning. In cosmetics, the move away from organic ultraviolet absorbers has led to the use of nano-sized zinc oxide (ZnO) and TiO2. Both are highly effective in absorbing UV sunlight.
Nano-TiO2 has also found utilization in the remediation of pollutants and deposited materials on skyscraper windows and other structures not easily accessed. The schematics in Figure 7 and Figure 8 represent what occurs.
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