Titanium Dioxide E-Book

Jochen Winkler

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Serie: European Coatings TECH FILES
• Sprache: Englisch

Seitenzahl: 200

Veröffentlichungsjahr: 2014

Beschreibung

This eBook, second, completely revised edition, contains a wealth of information on the properties and use of titanium dioxide pigments. It gives the reader a comprehensive insight into how titanium dioxide works and its possible applications, as well as discussing the current state of development and its use in various forms for UV absorbers, effect pigments and catalytic materials. Practitioners will especially welcome the fact that Chapter 4 now includes information on the scope for, and limitations on, replacing titanium dioxide pigments in formulations.

Leseprobe

Vincentz Network GmbH & Co. KG

Jochen Winkler

Titanium Dioxide

Production, Properties and Effective Usage

Cover: Sachtleben Chemie, Duisburg/Germany

Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Jochen Winkler

Titanium Dioxide: Production, Properties and Effective Usage

Hanover: Vincentz Network, 2013

EUROPEAN COATINGS TECH FILES

ISBN 3-86630-833-7

ISBN 978-3-86630-833-6

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EUROPEAN COATINGS TECH FILES

Jochen Winkler

Titanium Dioxide

Production, Properties and Effective Usage

EUROPEAN COATINGS TECH FILES

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Preface

The first edition of “Titanium Dioxide” was published in 2003. In the meantime, it has been sold out and the decision was made to bring out a second edition. This was taken as an incentive to revise all of the chapters in view of new information that has appeared since that time or, otherwise, has only yet come to the attention of the author.

For the practitioner, this book is conceived as a source of information on the properties and use of titanium dioxide pigments. Aspects concerning pigmentary titanium dioxide are primarily dealt with in the Chapters 2 to 6 of this booklet. For the sake of scrutiny, Chapter 4 was complemented by some information on the possibilities and limits of replacing titanium dioxide pigments in formulations.

In the remaining chapters, the current state of the development and use of titanias as UV absorbers and effect pigments as well as catalytic materials is outlined. Whereas this can only be a momentary picture, the same is especially true for the utilization of TiO2 in photovoltaic cells and as stationary phases in chromatography. Nevertheless, for the sake of completeness, these fields of potential use are at least briefly mentioned in the last two chapters of this revised edition of the book and some references are given.

My gratitude goes to Mr. Dirk Marschke for revising the information on the legislative requirements for titanium dioxide pigments and for discussions on the impact of the REACH legislation on the titanium dioxide industry as well as industry as a whole. Furthermore, I would like to especially thank Dr. Bernd Proft for calculating the scattering efficiencies of air bubbles in composite materials in comparison to titanium dioxide pigments. The helpful input I received from many of my colleagues at crenox GmbH, now Sachtleben Pigment GmbH, is also gratefully acknowledged.

Jochen Winkler

Krefeld/Germany, September 2012

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Contents

1Introduction

2Physical, chemical and toxicological properties

2.1Physical properties of titanium dioxide

2.2Chemical properties of titanium dioxide

2.3Toxicological properties of titanium dioxide

2.3.1Oral intake

2.3.2Parenteral uptake

2.3.3Percutaneous uptake

2.3.4Hypodermic (through the skin) injection

2.3.5Inhalation

3Production

3.1Raw materials

3.2Sulfate process

3.3Chloride process

3.4Inorganic and organic surface treatments

4Optical properties

4.1Fundamentals of color measurement

4.1.1CIELAB-values of titanium dioxide pigments

4.2Electromagnetic waves (“radiation”)

4.3Light absorption, light scattering, reflection and diffraction

4.3.1Particle size dependency of light absorption

4.3.2Particle size dependency of light scattering

4.4Mie theory

4.4.1PVC – depency of light scattering

4.5Kubelka-Munk theory – relative scattering power

4.6Determination of the spectral scattering coefficient

4.7Hiding power

4.7.1Standardized methods for determining hiding power

4.7.2Hiding power of sulfate and chloride grade pigments

4.8Lightening power

4.9Color tint

4.10Gloss and haze

4.11Substitute materials for titanium dioxide pigments

5Photo catalytic properties

5.1Chalking cycle

5.2Photo activity of anatase and rutile

5.3Accelerated and natural weathering

5.4Accelerated tests for monitoring photo activity

6Dispersing

6.1Steps taking place during dispersion

6.2Wetting

6.3Mechanical de-agglomeration

6.3.1Mill base formulation

6.3.2Agglomerate strength

6.3.3Relationship between time length of dispersion, mechanical power input and dispersion success

6.4Stabilization against flocculation

6.4.1Electrostatic stabilization

6.4.2Zeta potential

6.4.3Stabilization by adsorption of polyelectrolytes

6.4.4Adsorption of ions

6.4.5Steric (entropie) stabilization

6.5Rub out effect and Benard cells

6.6Final remarks

7Nano-titanium dioxide

7.1Production of nano-titanium dioxides

7.2Properties of titanium dioxide nano-particles

7.3Nano-titanium dioxides as UV absorbers

7.4Nano-titanium dioxides as effect pigments

8Titanium dioxide in catalysis

8.1DeNOx catalysts

8.2Diesel catalysts

8.3Titanium dioxide in photo catalysis

8.4Titanium dioxide as a catalyst for the production of biodiesel

9Titanium dioxide in photovoltaic cells and chromatography

Author

Index

1Introduction

Titanium dioxide (TiO2) pigments make up about 60 % of the global pigment production. In the year 2011, approximately 5.5 Million tons of titanium dioxide pigments were manufactured. The annual increase of titanium dioxide pigment production averaged at about 2 to 5 % in the past. Future growth rates of about 2 % per year are predicted for the US and Europe, whereas the demand for titanium dioxide pigments will probably grow at a rate of 6.5 % per annum in China and India. By the year 2015 the global demand is predicted to be at about seven million tons.

Whereas in former times sulfuric acid was considered to be the indicator for economic growth, this position is today held by titanium dioxide pigments. The per capita consumption of TiO2 can be taken as an indicator for the standard of living in a country. In 2011, the industrialized countries used an amount of about 4 kg of titania per person versus approximately 0.5 kg in India or Mainland China.

Nevertheless, titanium dioxide is a fairly new industrial product that gained importance only after the Second World War. Because of its superior light scattering properties, it replaced lithopone, a co-precipitated product consisting of zinc sulfide and barium sulfate, as the standard white pigment.

This development was, however, not predetermined. Apart from having a high refractive index, which is desirable for a white pigment, it also has an undesirable property: titanium dioxide is a photo semiconductor. That is to say, it absorbs ultraviolet light and transfers the energy of the absorbed photons into electrochemical reactions that lead to the disintegration of the organic media in which it is normally embedded. The quest to overcome this problem led to the development of new technologies of pigment lattice doping and inorganic treatments of titanium dioxide surfaces. These techniques are today gradually utilized and refined in the context of problems concerning the production of other pigments, but also in heterogeneous catalyst development.

“Transparent” titanium dioxides make use of the particle size dependency of light scattering of titania. Nano-scaled TiO2 particles scatter visible light far less than TiO2 pigments, yet still have the intrinsic property of UV light absorption. Today already, there is hardly a sun screen lotion with a high light protection factor that does not rely upon nano-scaled titania. Contrarily, the use of titania nanoparticles as photo catalysts to avoid the contamination of surfaces by dirt or bacteria is still at an early stage and may not gain economical importance at all.

2Physical, chemical and toxicological properties

2.1Physical properties of titanium dioxide

Titanium dioxide with the chemical formula TiO2 exists in three modifications with different crystal lattice structures and therefore alternating physical properties. These are rutile, anatase and brookite.

Stability

Rutile is thermodynamically the most stabile form. For that reason, anatase and brookite rearrange monotropically to rutile at elevated temperatures of 750 °C (brookite) or 915 °C (anatase), respectively. This modification is stabile all the way up to its melting point at approximately 1830 °C to 1850 °C. The rearrangement from anatase to rutile is exothermic and generates 12.6 kJ of heat per mole.

In all three crystal modifications, the titanium atoms are surrounded octahedrally by six oxygen atoms in a distorted fashion. However, the octahedra in rutile, anatase and brookite vary with respect to their spacing relative to each other. Of technical importance are only rutile and anatase, which differ from each other in their physical properties.

Crystal structure

The crystal structures of rutile and anatase may be described on the basis of the spacing of the oxygen atoms [1]

Rutile:

Hexagonal close packing of the oxygen atoms, in which half of the octahedral spaces are filled with titanium atoms.

Anatase:

Cubic close packing of the oxygen atoms, in which half of the tetrahedral spaces are filled with titanium atoms.

Figures 2.1 to 2.3 compare the elementary lattice cells of rutile, anatase and brookite. The crystal structure is developed by extending one elementary cell by others in the front, back, left, right and above as well as below. When doing this, one will recognize that, in the case of rutile, the octahedra which belong to the titanium atoms in the centers of the cells form chains connected via two of their edges. These chains develop in the c-direction of the elementary cell.

Figure 2.1: Elementary cell of rutile; black spheres: titanium atoms; white spheres: oxygen atoms

Figure 2.2: Elementary cell of anatase; black spheres: titanium atoms; white spheres: oxygen atoms

Figure 2.3: Elementary cell of brookite; black spheres: titanium atoms; white spheres: oxygen atoms

Figure 2.4 shows a section of the crystal lattice of rutile as an arrangement of octahedra, viewed in the c-direction. On the outsides, the rows of octahedra connected by two edges (two octahedra in this case) can be seen. The chain of octahedra in the middle connects the outer octahedra chains via mutually shared corners. The chain of octahedra in the middle is displaced by the length c/2 relative to the outer chains.

Figure 2.4: Structure of rutile

For anatase, the same procedure leads to a crystal lattice segment as shown in Figure 2.5. In this case, the viewing direction runs along the b-axis of the elementary cell. It is seen that the octahedra of one layer are connected to each other by four corners. To the bottom and the top, every octahedron shares two edges with its neighbors. Finally, Figure 2.6 demonstrates the circumstances in the case of brookite.

Table 2.1 compares the crystallographic data of rutile, anatase and brookite as well as their Mohs’ hardness and densities.

Figure 2.5: Structure of anatase

Hardness

Compared to rutile, anatase has a lower Mohs’ hardness of 5.5 to 6 instead of 6.5 to 7. For that reason, anatase is predominantly used where its lower hardness has at echnical advantage. This is especially the case in the textile industry, where anatase pigments are used for the matting of synthetic fibers. But also in plastics, where weather stability may not be so important in all cases, anatase is used because of its lower level of wear on machinery. In paper and in rubber industry, the softer anatase is widely used as a white pigment because of its lower abrasiveness, making it easier on cutting tools. A further reason for using anatase lies in its UV absorption which occurs at lower wavelengths than in the case of rutile (see below). This enables the use of optical fluorescence brighteners that change UV light into visible blue radiation. Rutile absorbs the UV light necessary for that to occur.

Figure 2.6: Structure of brookite

Table 2.1: Crystallographic and physical properties of rutile, anatase and brookite

Figure 2.7: Wavelength dependency of the refractive indices for the ordinary and extraordinary rays of rutile and anatase

Refractive indices

The high refractive index of titanium dioxide, in combination with the absence of absorption in the visible range of the spectrum between 380 nm and 700 nm wavelength, is the reason for its usefulness as a white pigment.

Both rutile and anatase form birefringent crystals, in which the refractive indices of the ordinary (no) and extraordinary (ne) rays are wavelength dependent. As shown in Figure 2.7 (values from [2]) the refractive indices rise towards the absorption edges. This behavior, termed “optical dispersion”, is completely normal for a dielectric substance1 like titania. Whereas in rutile the extraordinary ray possesses higher refractive index values, the opposite is true for anatase. Rutile is called a “positive crystal” (no < ne) and anatase a “negative crystal” (no > ne). In the case of anatase, the refractive indices of both rays differ less than those of rutile.

Figure 2.8: UV-Vis spectra of pressed powder tablets of rutile and anatase

Since titania pigments consist of submicroscopic particles of mean particle sizes between 0.2 μm and 0.3 μm, it does not make much sense to distinguish between the refractive indices of the ordinary and extraordinary rays. Normally, a mean value of both refractive indices is therefore assigned to every wavelength. According to the “average index approximation” the mean refractive index of a uniaxial, birefringent crystal which crystallizes tetragonally is given by [3].

The average is weighted because the ordinary refractive index is found for light polarized parallel to both the a- and the b-axis of the elementary cell whereas the extraordinary refractive index only comes into effect for light polarized parallel to the c-axis.

In practice, it has become customary to use one mean value as a characteristic estimate for the whole visible range of the spectrum, namely 2.75 as the refractive index of rutile and 2.55 as that of anatase, respectively. This corresponds roughly with the mean refractive index calculated from Equation 2.1 at a wavelength of 550 nm, at which the human eye is the most sensitive to light.

Polarisibility

The high refractive index is a consequence of the easy polarisibility of the binding electrons in the crystal lattice. These are mainly assorted around the oxygen atoms in the crystal.

Absorption spectra

With the equation

the energies E of these absorption processes can be calculated: they are 50 · 10-20 Joule for rutile and 52.7 · 10-20 Joule for anatase.

Photo activity

Due to the light absorption in the near UV, electrons are hoisted from the energy level of the valence band of TiO2 into that of the conduction band, thus leaving a positively charged hole in the valence band. The separated electron/hole pair is called “exciton”. The generation of excitons is the cause for the light induced semiconductor properties of TiO2. The photo activity of titania is generally disliked, since the excitons can have an oxidizing influence (see Chapter 5) and, for example, destroy the surrounding polymer matrix in which it is imbedded. Therefore, the titanium dioxide pigment industry goes through some efforts to reduce the photo activity of titanium dioxide pigments. On the other hand, this property is purposefully utilized in titania photo catalysts (see Chapter 8.3).

Figure 2.9: UV-Vis/NIR spectra of pressed powder tablets of rutile and anatase

Figure 2.10: IR-spectra of pressed powder tablets of rutile and anatase

Figures 2.11: X-ray diffraction spectrum of rutile

Blue tint

The additional remission of anatase taking place in the barely visible part of the spectrum (above 385 nm) leads to a slightly bluer appearance in comparison to a rutile pigment of comparable purity.

For enhancing the blue tint, trace metals that obtain higher oxidation states (e.g. Sb5+, Mo6+, W6+ or Ta5+) can be added as dopants to the calcination step. They supposedly act in the way that they reduce some of the Ti4+ to Ti3+, which is blue in color.

Reflectivity

Figure 2.8 also shows that anatase has a slightly lower remission in the red part of the visible spectrum, say above 650 nm, than rutile. Figure 2.9 shows the reflectance of anatase and rutile in the near infrared (NIR) from 700 nm to 2500 nm in addition to the UV-Vis range. In the NIR range, the slightly enhanced absorption of anatase in comparison to rutile remains manifest. Rutile itself starts to absorb at wavelengths exceeding 1300 nm. Neither anatase, nor rutile possess characteristic absorption bands in the NIR region of the spectrum.

Figures 2.12: X-ray diffraction spectrum of anatase

The further course of the reflectance spectra of anatase and rutile at wavelengths between 2.5 μm and 25 μm (infrared, IR) is presented in Figure 2.10. At 5 μm (corresponding to a wave number of 2000 cm-1), approximately 27 per cent of the light is reflected. The next light absorption of nearly 100 per cent (equivalent to about 0 % reflection) is found at 10.75 μm (930 cm-1) or 11.1 μm (900 cm-1) for anatase and rutile, respectively. Following the reflectance curves to higher wavelengths from there on, both anatase and rutile once again have higher reflectivities up to wavelengths of approximately 20 μm.

Other properties

Rutile and anatase are clearly distinguished from each other by their X-ray diffraction spectra. This is seen in Figures 2.11 (rutile) and 2.12 (anatase). Both show the diffraction signals up to 2-theta values of 70°. Table 2.2 offers detailed information on the location and the relative intensities of the X-ray diffraction signals based on the main peak (equivalent to 100 % intensity).

Titanium dioxide is basically an electrical isolator. The relative dielectric constants εr of titanias are comparatively high with values near 100. That is why they are used for producing electrically insulating ceramics. Titanium dioxide of high purity is also used to make barium titanate for capacitors. Dielectric properties found by measurements on titania powders are, however, probably not relevant for practical purposes because of the difficulties involved in preparing suitable powder compacts.

The heat capacity of TiO2 is 0.69 J/g K. Titanium dioxide is weakly paramagnetic. This allows its magnetic separation from ferromagnetic ores in beneficiation processes.

Titanium dioxide is thermally relatively stabile although it easily releases small amounts of oxygen. At 100 °C, for example, this causes a slight gray color – due to the generation of Ti3+ – which is however reversible. At 400 °C, titanium dioxide appears yellow, caused by the expansion of the crystal lattice. This is reversible as well. As stated earlier, the melting point of rutile lies between 1830 °C and 1850 °C.

Table 2.2: Location and relative intensities of the 2-theta peaks of the X-ray diffraction spectra of rutile and anatase

2.2Chemical properties of titanium dioxide

Titanium itself is a relatively un-noble metal which (similar to aluminum) is only stable in the atmosphere because of an oxide passivation layer. It has a pronounced tendency to form oxides, which are very stable. The far most preferred oxidation state is that of Ti4+. Expectedly, titanium dioxide is chemically hardly reactive.

Solubility

Titanium dioxide is insoluble in water. Calcined titania, especially in the rutile modification, is insoluble or only moderately soluble even in hot, concentrated acids. In acidic or basic melts, however, both anatase and rutile are dissolved. Titanium dioxide can therefore be used for producing highly refractive glass.

Reduction reactions

Small amounts of oxygen are easily withdrawn from titanium dioxide. In that case, Ti3+ is generated, that leads to a grayish color due to the fact that Ti3+ within the TiO2 lattice is dark blue, almost black. This chemical reduction of the surface of the titania particles is, however, reversible. Even the oxygen in the air suffices to recover Ti4+, which is not colored. When illuminated with UV light in the presence of reducing agents such as mandelic acid, glycerine or SnCl2, yet in absence of oxygen, TiO2 also obtains a bluish-gray color. The differences in their tendency to become gray under these conditions are used as an indicator for the photo stability of different samples. With glycerine, the reduction reaction leads to the formation of Ti2O3. Especially anatase pigments of lower purity exhibit some graying, even in the absence of reducing agents, when subjected to intensive illumination. This reversible behavior is known as “phototropy”. Before the advent of flash photometers, this made the measurement of remission spectra of anatase pigments a difficult task at times.

Unlike, for example, iron oxide, titania is not turned into titanium metal even under strongly reducing conditions. In the presence of nitrogen, carbon, halogen or sulfur, titanium nitride, titanium carbide, titanium halide and titanium sulfide are formed.

The reaction behavior against gaseous chlorine is utilized in the chloride process of titanium dioxide production, in which the titanium in the ore is transferred into TiCl4. In pig iron production, ilmenite and other titanium bearing synthetic products help to extend the lifetime of blast furnaces. The titanium carbides and nitrides that are generated condense on the heat insulating refractory bricks inside the furnaces, thus regenerating them [5, 6].

Even the reaction with hydrogen under dramatic conditions (100 bar, 2000 °C) only leads to the suboxide TiO in which titanium has the oxidation state of Ti2+. Only when TiO2 is annealed in the presence of elementary silicium, calcium or magnesium, titanium metal is formed along with the corresponding oxides of silicium, calcium or magnesium.

Titanium metal is appreciated either by itself or as an alloy component for its low density (4.5 g m-3