*2.1.2. Optical density*

The problem with particle size distribution measurements is that the data cannot be directly related to the scattering power without using a theoretical model. An optical density test has been developed to provide a direct measure of the actual pigment scattering potential, which can then be used to evaluate performance in end-use applications [31–33].

A lab-dispersed sample in water is accurately diluted and analyzed for its total light transmission *T*. We have used the 'DuPont optical density', which is defined as –log(total *T*)/concentration and defines the scattering potential of the pigment. Compared to standard end-use tests of scattering power such as tint strength, optical density (OD) measurement offers the advantage of faster turn-around and improved precision with standard deviations less than 0.5% of the mean value.

The optical density was determined by UV-Vis spectrophotometer. To determine the optical density, 1 L of the diluted suspension of pigment particles TiO<sup>2</sup> in ethanol (c TiO<sup>2</sup> = 30 mg/L) was prepared. During the analysis, the suspension was subjected to ultrasound stirring. The

**Figure 14.** Particle size distribution of highly coated TiO<sup>2</sup> in slurry after controlled (narrow curve) and uncontrolled (wider curve) surface treatment.

distribution. Different populations are evident, meaning that the sample consists of many

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 alu-

dency for particle agglomeration can be attributed for the higher average particle diameter.

mine not only the mean particle size but also the whole distribution.

can then be used to evaluate performance in end-use applications [31–33].

density, 1 L of the diluted suspension of pigment particles TiO<sup>2</sup>

**Figure 14.** Particle size distribution of highly coated TiO<sup>2</sup>

(wider curve) surface treatment.

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 deter-

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

The problem with particle size distribution measurements is that the data cannot be directly related to the scattering power without using a theoretical model. An optical density test has been developed to provide a direct measure of the actual pigment scattering potential, which

A lab-dispersed sample in water is accurately diluted and analyzed for its total light transmission *T*. We have used the 'DuPont optical density', which is defined as –log(total *T*)/concentration and defines the scattering potential of the pigment. Compared to standard end-use tests of scattering power such as tint strength, optical density (OD) measurement offers the advantage of faster turn-around and improved precision with standard deviations less than 0.5% of the

The optical density was determined by UV-Vis spectrophotometer. To determine the optical

was prepared. During the analysis, the suspension was subjected to ultrasound stirring. The

in ethanol (c TiO<sup>2</sup> = 30 mg/L)

in slurry after controlled (narrow curve) and uncontrolled

surface. The higher degree of surface modification and greater ten-

small and many over-sized particles (**Figure 14**).

432 Titanium Dioxide - Material for a Sustainable Environment

mina imparted on the TiO<sup>2</sup>

particles (**Figure 15**).

*2.1.2. Optical density*

mean value.

**Figure 15.** Particle size distribution of micronized highly coated TiO<sup>2</sup> after controlled (narrow curve) and uncontrolled (wider curve) surface treatment.

optical density method determines the optical properties of the particles. This method determines the dispersibility or degree of particles agglomeration.

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 agglomerated particles should exhibit different OD value.

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

**Figure 16.** Light scattering efficiency of TiO<sup>2</sup> particles coated under controlled and uncontrolled process conditions.

uncontrolled conditions (black curve). Consequently, differences in undertone for both samples were evident.

Smaller particles (≤0.2 μm) scatter the light of the short wavelength more strongly; therefore, they have a slightly blue undertone, while the larger particles scatter the light of a longer wavelength, i.e., they have a yellow undertone.
