Titanium dioxide is the naturally occurring oxide of titanium, chemical formula TiO2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. Generally it is sourced from ilmenite, rutile and anatase. It has a wide range of applications, from paint to sunscreen to food coloring. When used as a food coloring, it has E number E171.
Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite, and additionally as two high-pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form, both found at the Ries crater in Bavaria. It is mainly sourced from ilmenite ore. This is the most wide spread form of titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The metastable anatase and brookite phases convert to rutile upon heating.
The production method depends on the feedstock. The most common method for the production of titanium dioxide utilizes ilmenite. Ilmenite is mixed with sulfuric acid. This reacts to remove the iron oxide group in the ilmenite. The by-product iron (II) sulfate is crystallized and filtered-off to yield only the titanium salt in the digestion solution. This product is called synthetic rutile. This is further processed in a similar way to rutile to give the titanium dioxide product. Synthetic rutile and titanium slags are made especially for titanium dioxide production.The use of ilminite ore usually produces pigment grade titanium dioxide. Another method for the production of synthetic rutile from ilminite utilizes the Becher Process.
Rutile is the second most abundant mineral sand. Rutile found in primary rock cannot be extracted hence the deposits containing rutile sand can be mined meaning a reduced availability to the high concentration ore. Crude titanium dioxide (in the form of rutile or synthetic rutile) is purified via converting to titanium tetrachloride in the chloride process. In this process, the crude ore (containing at least 70% TiO2) is reduced with carbon, oxidized with chlorine to give titanium tetrachloride; i.e., carbothermal chlorination. This titanium tetrachloride is distilled, and re-oxidized in a pure oxygen flame or plasma at 1500–2000 K to give pure titanium dioxide while also regenerating chlorine. Aluminium chloride is often added to the process as a rutile promotor; the product is mostly anatase in its absence. The preferred raw material for the chloride process is natural rutile because of its high titanium dioxide content.
Titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index, which is surpassed only by a few other materials. Approximately 4.6 million tons of pigmentary TiO2 are consumed annually worldwide, and this number is expected to increase as consumption continues to rise. When deposited as a thin film, its refractive index and color make it an excellent reflective optical coating for dielectric mirrors and some gemstones like “mystic fire topaz”. TiO2 is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets) as well as most toothpaste. In paint, it is often referred to offhandedly as “the perfect white”, “the whitest white”, or other similar terms. Opacity is improved by optimal sizing of the titanium dioxide particles.
In ceramic glazes titanium dioxide acts as an opacifier and seeds crystal formation.
Titanium dioxide has been shown statistically to increase skimmed milk’s whiteness, increasing skimmed milk’s sensory acceptance score.
In cosmetic and skin care products, titanium dioxide is used as a pigment, sunscreen and a thickener. It is also used as a tattoo pigment and in styptic pencils. Titanium dioxide is produced in varying particle sizes, oil and water dispersible, and in certain grades for the cosmetic industry.
Titanium dioxide is found in almost every sunscreen with a physical blocker because of its high refractive index, its strong UV light absorbing capabilities and its resistance to discoloration under ultraviolet light. This advantage enhances its stability and ability to protect the skin from ultraviolet light. Nano-scaled titanium dioxide particles are primarily used in sunscreen lotion because they scatter visible light less than titanium dioxide pigments while still providing UV protection. Sunscreens designed for infants or people with sensitive skin are often based on titanium dioxide and/or zinc oxide, as these mineral UV blockers are believed to cause less skin irritation than other UV absorbing chemicals.
This pigment is used extensively in plastics and other applications not only as a white pigment or an opacifier but also for its UV resistant properties where the powder disperses the light – unlike organic UV absorbers – and reduces UV damage, due mostly to the extremely high refractive index of the particles. Certain polymers used in coatings for concrete or those used to impregnate concrete as a reinforcement are sometimes charged with titanium white pigment for UV shielding in the construction industry, but it only delays the oxidative photodegradation of the polymer in question, which is said to “chalk” as it flakes off due to lowered impact strength and may crumble after years of exposure in direct sunlight if UV stabilizers have not been included.