*1.2.3. Alumina*

Hydrous aluminum oxide is probably the most common treatment agent of TiO<sup>2</sup> pigments. Various reagents can be used for deposition, e.g., sodium aluminate, which reacts with acid (see **Figure 4**), or aluminum sulfate, which reacts with base (see **Figure 5**).

Alumina can also be deposited from a combination of sodium aluminate and aluminum sulfate, which neutralize each other (**Figure 6**).

presented in **Figure 7**. From literature, we know that the structure of the alumina layer on TiO<sup>2</sup> pigment varies with the pH, i.e., the alumina tends to deposit as a pseudoboehmite in basic solutions. Alumina deposits in an amorphous form in the acid solutions. The transition pH is

Parameters such as temperature, pH, coating reagent concentration, core particle concentration and particle surface characteristics significantly affect precipitation coating process [26]. Hydrolysis polymerization, a precipitation process of the coating reagent as well as the coating morphology is influenced by pH and temperature. At high pH values, hydrolysis of Al3+ is accelerated and more multinuclear OH─Al species are formed compared to the situation at low pH [27]. The number of Al3+ ions depends on pH and Al3+ concentration, while the structure of the OH─Al species depends on the process conditions, e.g., concentration of Al3+, temperature and stirring strength [28]. pH also affects the protonation and deprotonation

particle surface occurs

particles through random collisions. Formation of

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temperature-sensitive and tends to shift to lower pH at higher temperature [1].

Condensation between OH─Al species and –OH groups on the TiO<sup>2</sup>

**Figure 6.** Sodium aluminate and aluminum sulfate as a source of aluminum hydroxide.

reactions on the core particle surface [29].

**Figure 5.** Aluminum sulfate as a source of aluminum hydroxide.

when the OH─Al species collide with TiO<sup>2</sup>

Irrespective of the reagent used, under controlled conditions of surface treatment, a thin, even layer, which entirely encapsulate the surface of TiO<sup>2</sup> is formed.

#### *1.2.4. Coating process of the Al2 O3 layer*

By coating the surface of TiO<sup>2</sup> uniformly with alumina, coated particles behave in some ways similar to pure alumina. Alumina coating increase the amount of ─OH groups on particle surface and consequently improve dispersibility of particles in aqueous solution and provide more active sites for further organic modification [24].

The level of alumina depends on the gloss, tint strength and opacity needed in the pigment. Low alumina levels yield better gloss but lower tint strength compared with higher levels [10].

The morphology of the alumina layer depends on the deposition conditions. Aluminum oxide and hydrous alumina have many different structures, which can give many different properties to coated particles. Formation of aluminum layers as a function of the suspension's pH is

**Figure 4.** Sodium aluminate as a source of aluminum hydroxide.

**Figure 5.** Aluminum sulfate as a source of aluminum hydroxide.

species exist as single silicate anions, or less aggregated siliceous micelles with very small particle size, which should be more negatively charged. The single silicate anions and the highly

Raising the temperature of the reaction affects the covering extent and causes the formation of

Various reagents can be used for deposition, e.g., sodium aluminate, which reacts with acid

Alumina can also be deposited from a combination of sodium aluminate and aluminum sul-

Irrespective of the reagent used, under controlled conditions of surface treatment, a thin, even

similar to pure alumina. Alumina coating increase the amount of ─OH groups on particle surface and consequently improve dispersibility of particles in aqueous solution and provide

The level of alumina depends on the gloss, tint strength and opacity needed in the pigment. Low alumina levels yield better gloss but lower tint strength compared with higher levels [10]. The morphology of the alumina layer depends on the deposition conditions. Aluminum oxide and hydrous alumina have many different structures, which can give many different properties to coated particles. Formation of aluminum layers as a function of the suspension's pH is

is formed.

uniformly with alumina, coated particles behave in some ways

surface

pigments.

coating layers forms on the TiO<sup>2</sup>

powder.

negatively charged siliceous micelles do not react with the negatively charged TiO<sup>2</sup>

Hydrous aluminum oxide is probably the most common treatment agent of TiO<sup>2</sup>

due to the strong electrostatic repulsion. Therefore, no SiO<sup>2</sup>

coating layer on the surface of the rutile TiO<sup>2</sup>

(see **Figure 4**), or aluminum sulfate, which reacts with base (see **Figure 5**).

surface at this high pH value.

426 Titanium Dioxide - Material for a Sustainable Environment

*1.2.4. Coating process of the Al2*

By coating the surface of TiO<sup>2</sup>

fate, which neutralize each other (**Figure 6**).

layer, which entirely encapsulate the surface of TiO<sup>2</sup>

more active sites for further organic modification [24].

**Figure 4.** Sodium aluminate as a source of aluminum hydroxide.

*O3 layer*

a dense SiO<sup>2</sup>

*1.2.3. Alumina*

presented in **Figure 7**. From literature, we know that the structure of the alumina layer on TiO<sup>2</sup> pigment varies with the pH, i.e., the alumina tends to deposit as a pseudoboehmite in basic solutions. Alumina deposits in an amorphous form in the acid solutions. The transition pH is temperature-sensitive and tends to shift to lower pH at higher temperature [1].

Parameters such as temperature, pH, coating reagent concentration, core particle concentration and particle surface characteristics significantly affect precipitation coating process [26]. Hydrolysis polymerization, a precipitation process of the coating reagent as well as the coating morphology is influenced by pH and temperature. At high pH values, hydrolysis of Al3+ is accelerated and more multinuclear OH─Al species are formed compared to the situation at low pH [27]. The number of Al3+ ions depends on pH and Al3+ concentration, while the structure of the OH─Al species depends on the process conditions, e.g., concentration of Al3+, temperature and stirring strength [28]. pH also affects the protonation and deprotonation reactions on the core particle surface [29].

Condensation between OH─Al species and –OH groups on the TiO<sup>2</sup> particle surface occurs when the OH─Al species collide with TiO<sup>2</sup> particles through random collisions. Formation of

**Figure 6.** Sodium aluminate and aluminum sulfate as a source of aluminum hydroxide.

preferred. Since a large amount of ─OH groups on the particle surface could provide protons at higher pH, there were many OH─Al species that condense with the ─OH groups on the particle surface; neighboring condensed OH─Al species easy condense each other, forming a

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When the pH is lower than the IEP, the number of ─OH groups on the particle surface that could provide protons is small and condensation with the OH-Al species hardly occurs; here,

At high pH, the OH─Al species are large, having a certain shape, and their sedimentation

The layer morphology mainly depends on the sedimentation speed and the directed growth speed of the OH─Al species [30]. When the temperature is in the middle range, the sedimentation speed and the directed growth speed of the OH─Al species are about the same. At increased temperature, the directed growth speed of the OH─Al species dominates the precipitation

continuous film coating, as shown in **Figure 9**.

leads to a loose floccule or flake-like layer (**Figure 11**).

**Figure 9.** TEM image of a continuous film aluminum hydroxide coating.

particles with dotted alumina layer.

**Figure 10.** Image of TiO<sup>2</sup>

homogeneous nucleation or the dotted layer is preferred (**Figure 10**).

**Figure 7.** Formation of aluminum layers as a function of the suspension's pH.

Ti─OvAl bond formed via condensation of ─OH groups from TiO<sup>2</sup> surface and OH─Al species is presented in **Figure 8**.

When the pH is higher than the isoelectric point, the TiO<sup>2</sup> particles carry negative charge [11]. The number of ─OH groups on the particle surface that could provide protons is large; the surface is easy to provide protons, which facilitated the condensation between the OH─Al species and the ─OH groups on the particle surface. In such a case, heterogeneous precipitation is

**Figure 8.** At pH higher than the IEP, the number of ─OH groups on the particle surface that could provide protons is large and the condensation with the OH─Al species easily occurs [13].

preferred. Since a large amount of ─OH groups on the particle surface could provide protons at higher pH, there were many OH─Al species that condense with the ─OH groups on the particle surface; neighboring condensed OH─Al species easy condense each other, forming a continuous film coating, as shown in **Figure 9**.

When the pH is lower than the IEP, the number of ─OH groups on the particle surface that could provide protons is small and condensation with the OH-Al species hardly occurs; here, homogeneous nucleation or the dotted layer is preferred (**Figure 10**).

At high pH, the OH─Al species are large, having a certain shape, and their sedimentation leads to a loose floccule or flake-like layer (**Figure 11**).

The layer morphology mainly depends on the sedimentation speed and the directed growth speed of the OH─Al species [30]. When the temperature is in the middle range, the sedimentation speed and the directed growth speed of the OH─Al species are about the same. At increased temperature, the directed growth speed of the OH─Al species dominates the precipitation

**Figure 9.** TEM image of a continuous film aluminum hydroxide coating.

Ti─OvAl bond formed via condensation of ─OH groups from TiO<sup>2</sup>

The number of ─OH groups on the particle surface that could provide protons is large; the surface is easy to provide protons, which facilitated the condensation between the OH─Al species and the ─OH groups on the particle surface. In such a case, heterogeneous precipitation is

**Figure 8.** At pH higher than the IEP, the number of ─OH groups on the particle surface that could provide protons is

When the pH is higher than the isoelectric point, the TiO<sup>2</sup>

**Figure 7.** Formation of aluminum layers as a function of the suspension's pH.

large and the condensation with the OH─Al species easily occurs [13].

cies is presented in **Figure 8**.

428 Titanium Dioxide - Material for a Sustainable Environment

surface and OH─Al spe-

particles carry negative charge [11].

**Figure 10.** Image of TiO<sup>2</sup> particles with dotted alumina layer.

**Figure 11.** TEM image provides an excellent view of the shell covering of a crystalline core of TiO<sup>2</sup> with a loose floccule or flake-like layer.

process regardless of the pH and size of the OH─Al species, which results in a directed growth [10]. Layer morphology changes from uniform and continuous film to loose floccules.

Temperature has a significant effect on the direction and growth speed of the OH─Al species in gel precipitation or coating, while also affecting the self-assembly of the OH─Al species in the aging process. Under high pH conditions, the OH─Al species form a large particle size, which facilitates the formation of boehmite Al(OH)<sup>3</sup> gels or a floccule/flake coating. Amorphous Al(OH)<sup>3</sup> gel forms only under conditions when the precipitation and aging proceed at a low temperature and pH; it can be converted to boehmite under high pH or temperature conditions.

However, boehmite gel cannot be converted to the amorphous form when the pH and temperature are low in the aging process [10].

#### **2. TiO2 grades**

#### **2.1. Highly coated TiO2 grades**

There is a special route for improving TiO<sup>2</sup> light scattering efficiency that is closely related to targeted spacing—the encapsulation of the TiO<sup>2</sup> particles by a thick, porous material. This coating material, which in practice is aluminosilicates, needs to be thick enough to effectively prevent close contact of the TiO<sup>2</sup> portion of these pigments and highly porous because a solid coating would unnecessarily dilute the TiO<sup>2</sup> content of the pigment. Even with high porosity, these coatings dilute the weight percent of TiO<sup>2</sup> in the pigment to roughly 80%.

It is important to manage the process under controlled conditions. **Figures 12** and **13** indicate differences between two highly coated TiO<sup>2</sup> , produced under controlled and uncontrolled conditions. Surface of TiO<sup>2</sup> , coated under appropriate conditions is uniform with coatings covering the entire surface of the particles (**Figure 13**). Increased SiO<sup>2</sup> loading up to 10 wt.% resulted in thicker layers. Coating TiO<sup>2</sup> particles under neutral conditions yields fluffy coatings with coating thickness up to 50 nm. Pigment particles are separated from each other, showing no agglomeration.

*2.1.1. Particle size distribution*

**Figure 13.** Highly surface treated TiO<sup>2</sup>

**Figure 12.** Highly surface treated TiO<sup>2</sup>

The distribution of highly surface-treated TiO<sup>2</sup>

The particle size relevant for inorganic pigments stretch between several tens of nanometers for transparent pigment types to approximately 2 μm. For practical applications, it is desir-

ticle size distribution, while an uncontrolled process yields material with broad particle size

in a controlled process indicates narrow par-

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able to determine not only the mean particle size but also the whole distribution [3].

(controlled conditions).

(uncontrolled conditions).

On the contrary, uncontrolled coating process yielded incomplete coatings (**Figure 12**). Homogeneous coating was not attained and some particles were not coated with the silica layer.

**Figure 12.** Highly surface treated TiO<sup>2</sup> (uncontrolled conditions).

**Figure 11.** TEM image provides an excellent view of the shell covering of a crystalline core of TiO<sup>2</sup>

facilitates the formation of boehmite Al(OH)<sup>3</sup>

430 Titanium Dioxide - Material for a Sustainable Environment

perature are low in the aging process [10].

There is a special route for improving TiO<sup>2</sup>

tively prevent close contact of the TiO<sup>2</sup>

differences between two highly coated TiO<sup>2</sup>

to targeted spacing—the encapsulation of the TiO<sup>2</sup>

a solid coating would unnecessarily dilute the TiO<sup>2</sup>

porosity, these coatings dilute the weight percent of TiO<sup>2</sup>

the entire surface of the particles (**Figure 13**). Increased SiO<sup>2</sup>

 **grades**

[10]. Layer morphology changes from uniform and continuous film to loose floccules.

process regardless of the pH and size of the OH─Al species, which results in a directed growth

Temperature has a significant effect on the direction and growth speed of the OH─Al species in gel precipitation or coating, while also affecting the self-assembly of the OH─Al species in the aging process. Under high pH conditions, the OH─Al species form a large particle size, which

temperature and pH; it can be converted to boehmite under high pH or temperature conditions. However, boehmite gel cannot be converted to the amorphous form when the pH and tem-

coating material, which in practice is aluminosilicates, needs to be thick enough to effec-

It is important to manage the process under controlled conditions. **Figures 12** and **13** indicate

coating thickness up to 50 nm. Pigment particles are separated from each other, showing no

On the contrary, uncontrolled coating process yielded incomplete coatings (**Figure 12**). Homogeneous coating was not attained and some particles were not coated with the silica layer.

gel forms only under conditions when the precipitation and aging proceed at a low

or flake-like layer.

Al(OH)<sup>3</sup>

**2. TiO2**

 **grades**

**2.1. Highly coated TiO2**

ditions. Surface of TiO<sup>2</sup>

agglomeration.

in thicker layers. Coating TiO<sup>2</sup>

with a loose floccule

gels or a floccule/flake coating. Amorphous

light scattering efficiency that is closely related

portion of these pigments and highly porous because

, produced under controlled and uncontrolled con-

, coated under appropriate conditions is uniform with coatings covering

particles under neutral conditions yields fluffy coatings with

particles by a thick, porous material. This

content of the pigment. Even with high

in the pigment to roughly 80%.

loading up to 10 wt.% resulted

**Figure 13.** Highly surface treated TiO<sup>2</sup> (controlled conditions).

#### *2.1.1. Particle size distribution*

The particle size relevant for inorganic pigments stretch between several tens of nanometers for transparent pigment types to approximately 2 μm. For practical applications, it is desirable to determine not only the mean particle size but also the whole distribution [3].

The distribution of highly surface-treated TiO<sup>2</sup> in a controlled process indicates narrow particle size distribution, while an uncontrolled process yields material with broad particle size distribution. Different populations are evident, meaning that the sample consists of many small and many over-sized particles (**Figure 14**).

Surface modification treatment in the suspension brings a significant shift in the pattern toward the higher particle diameter region due to high hydroxide loadings of silica and alumina imparted on the TiO<sup>2</sup> surface. The higher degree of surface modification and greater tendency for particle agglomeration can be attributed for the higher average particle diameter.

If we take into account only the mean particle sizes, we can conclude that we have two very similar samples. But, if we look at the whole distribution, it is obvious that the samples consist of very different populations. With this, we confirmed the fact that it is significant to determine not only the mean particle size but also the whole distribution.

After the grinding operation, particle size distribution again shifted to the lower particle diameter region. The sample produced by an uncontrolled process again contain over-sized particles (**Figure 15**).

optical density method determines the optical properties of the particles. This method deter-

after controlled (narrow curve) and uncontrolled

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The results of the optical density determination indicated the differences between particles coated under different process conditions, controlled and uncontrolled (**Figure 16**). The light scattering highly depends on the particle size and the distribution of the size and degree of agglomeration of the material. Agglomeration always reduces the effectiveness of pigment light scattering. With the results obtained, we gained information about the light scattering efficiency, which is most likely a consequence of the particle size distribution and the degree of dispersion. Differences in dispersion between differently coated particles are the result of a different particle size distribution, the degree of milling step and the controlled coating mode, where the particle surface was completely coated with a layer of hydroxide. Surface treatment is important in determination of the physical properties of the particle surface and thus, affects the dispersion in a particular medium. The quality of surface treatment defines how the pigment will perform when incorporated into a particular medium. Light scattering efficiency (LSE) will depend on how well the pigment will be dispersed. Differently agglom-

Results of the OD method indicate that coating the surface in a controlled manner resulted in particles with higher LSE (gray curve) in comparison with the particles coated under

particles coated under controlled and uncontrolled process conditions.

mines the dispersibility or degree of particles agglomeration.

**Figure 15.** Particle size distribution of micronized highly coated TiO<sup>2</sup>

(wider curve) surface treatment.

erated particles should exhibit different OD value.

**Figure 16.** Light scattering efficiency of TiO<sup>2</sup>
