Titanium dioxide

Colorant Summary

Amina Tariq

We might not even realise it, but titanium dioxide (Tio2, CIP White 6) is an essential part of our everyday life. It is an integral component of large number of consumer goods and products of daily use such as cosmetics, paints, dyes, varnishes, textiles, paper, plastics, food, printing inks, rubber, adhesives, coatings, ceramics, roofing materials, water treatment agents, automotive products, self-cleaning glass, paving stones, Gratzel Cells, tattoos and catalyst systems [1]. The uses of this wonderful pigment mentioned above by no means are exhaustive; rather they just give an idea of type of industry and products where it is used on a daily basis. Fig 1; Ground TiO 2 [5]

Titanium dioxide, the oxide of metal titanium, is an inorganic white solid substance (shown in fig 1), and is generally considered to be chemically inert. What makes TiO2 an excellent choice as a pigment is properties like: thermal stability; poor solubility; non-flammability and non-toxicity. Titanium dioxide is a polymorphic compound as it occurs in three crystal modifications: rutile; anatase; and brooklite (for crystal structures see fig 2 & 3 below). From commercial point of view only rutile and anatase crystal forms are of industrial value as brooklite is difficult to produce, and has no known industrial applications. In 1897, Dr. J. Overton was the first person to introduce titanium compounds as pigments using a mixture of rutile with the medium as the resistant paint on ships’ bottoms [10].

Fig 2; Crystal structure of rutile [5] Fig 3; Crystal structure of anatase [5]

The two most important titanium- ore minerals are ilmenite (FeTiO3) and natural rutile (TiO2), and both are used in the manufacture of different end products [9]. Even though titanium dioxide is naturally occurring pigment, high demand has led to synthetic production to meet the supply of ever growing use of this pigment. In 2003, the biggest producers of TiO2 were Australia, South Africa and Canada [11]. The world’s largest rutile mine, the Sierra Rutile Mine, is in Sierra Leone. In 1995, this mine was reported to produce 25% of world’s rutile production [11]. On commercial scale it is produced using sulphate and chloride processes [6] [7].

The great versatility of titanium dioxide is due to its various pigment grades, shapes and size. TiO2 can be used either in microscale or nanoscale (ultrafine) particle form [3]. As a nanoparticle it is used in cosmetics particularly sun creams because of its unparalleled ability to reflect, scatter and absorb harmful UV light- hence protecting the skin beneath it [2]. TiO2 is a physical filter for UVA light (wavelength of 315-400nm) and UVB (wavelength of 280-315nm) radiation. As it is chemically inert it will not decolourise under UV light. TiO2 nanoparticles have to be laminated with inorganic surface modifiers such as alumina and silica, because particles that come into contact with water produce free radicles such as hydroxyl radicals, which are potentially harmful to humans. Coating blocks the particles’ contact with the skin and water by improving dispersion and providing photo stability- hence rendering titanium dioxide very safe to use [2] [4]. TiO2 as nanomaterial is transparent, and is not used as a colorant because fundamentally its properties are different from pigment size particles, and does not impart colour or opacity to a product such as sun cream [1]. At present, sun protection factor has been made possible using just nanoscale TiO2

[3].

As a bright white pigment, TiO2 is extensively used in paints and surface coatings. For these applications, It has some excellent properties, such as high opacity, good dispersability, high refractive index, strong light scattering, incident-light reflection, high UV resistance, excellent tinting strength and high hiding power [3] [4] [8]. White titanium dioxide pigments currently in use are mostly rutile, in the micrometres size range, as anatase form is prone to yellowing and has poor weather fastness. Particle size is of paramount importance to optimise the light scattering in the visible range, this is achieved by using the primary particle size of roughly half the wavelength of the light to be scattered, that is half of 400-700nm for visible light [1]. This scattering of light which results in high opacity is caused by high refractive index of TiO2 (rutile, 2.70), the incident light enters through the crystal slowly and its path is considerably changed as opposed to the air, if the alignment of the small particles is in the random directions, this will result in high refractive index leading to scattering of light and high opacity, as not much light passes through [8].

TiO2 in a true sense is a white pigment of 21st century. It plays a vital part in a range of industries in the form of raw material, intermediate or as an end product. In any case the properties of the pigment, such as particle size, surface area, shape, dispersibility and rheology need to be tailored to fulfil the requirements of a specific application.

Bibliography

1) Titanium Dioxide Manufacturer Association. About Titanium Dioxide. [Online]. 2012. [Accessed 22 November 2015]. Available from : http://www.cinkarna.si/filelib/tio2/tdma_about_tio2__20120914.pdf

2) The Royal Society of Chemistry. Titanium dioxide. [Online][Accessed 3 December 2015]. Available from : http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/titanium_dioxide.asp

3) Steinbach, C. Nanoparticles & Nanomaterial- Knowledge Base Nanomaterial. [Online]. 2009. [Accessed 20 November 2015]. Available from : http://nanopartikel.info/en/nanoinfo/materials/titanium-dioxide/material-information#literatur

4) DuPont Ti-Pure titanium dioxide. [Online]. [Accessed 20 November 2015]. Available from: https://www.chemours.com/Titanium_Technologies/en_US/tech_info/literature/Coatings/CO_B_H_65969_Coatings_Brochure.pdf

5) Douma, M. Curator. In Pigments through the ages- detailed pigment histories. [Online]. 2008. [Accessed 18 November 2015]. Available from : http://www.webexhibits.org/pigments/indiv/technical/titaniumwhite.html

6) Christie, M. R. Colour Chemistry. [Online]. Cambridge: Royal Society of Chemistry, 2001 [Accessed 4 December,2015]

7) Buxbaum, G. ed. Industrial inorganic pigment. 2nd, completely rev. ed. Chichester: Wiley-VCH, 1998.

8) Royal Society of Chemistry.TiO2 Photocatalysis: Uses of Titanium Dioxide. [Online]. [Accessed on 22 November 2015]. Available from: file:///E:/Masterclass-TiO2_6_Uses%20of%20titanium%20dioxide.pdf

9) Miller, A. J. Titanium; a materials survey. [Online]. 1957. [Accessed 6 December, 2015]. Available from: http://hdl.handle.net/2027/mdp.39015078459594

10) Tyler, M.P. Trends in white-pigment consumption. [Online]. 1889. [Accessed 6 December, 2015]. Available from: http://hdl.handle.net/2027/mdp.39015077569278?urlappend=%3Bseq=9

11) Royal Society of Chemistry.TiO2: Manufacture of Titanium Dioxide. [Online]. [Accessed on 1 December 2015]. Available from: http://www.rsc.org/learn-chemistry/resource/res00001267/tio2-photocatalysis-manufacture-of-titanium-dioxide