Titanium Dioxide Use in the Paints Sector

1. Titanium Dioxide (TiO2)

Titanium dioxide (TiO2) is an important white pigment used in the paints industry. It is used to provide opacity, referred to as "whiteness and hiding power (covering power)" to paints, inks and plastics. There are two reasons for this: • TiO2 particles of suitable particle size scatter visible light with wavelengths of λ ≈ 380 - 700 nm (nanometres), because TiO2 has a high refractive index (R.I=2.73). • It is white because it does not absorb visible light. Titanium dioxide is commercially available in two different crystal structures: anatase and rutile. Rutile TiO2 pigments are preferred over anatase pigments because they scatter light more efficiently, and are more stable and durable. On the other hand, the pigment becomes expensive when used on a volume cost basis in systems. Most paint and ink companies purchase raw materials on a weight basis and sell their products on a volume basis. Since TiO2 has a relatively high density, ρ ≈ 4 g/cm3, the raw material contributes significantly to the volume cost of the system. To understand why TiO2 (especially rutile TiO2) offers such major advantages in hiding power, it is sufficient to compare only the refractive indices of rutile and anatase with other commercial white pigments (Table 1). The greater the difference between the refractive index of the pigment and the refractive index of the medium in which it is dispersed, the greater the scattering of refracted light. Table 1. Refractive indices (R.I.) for pigments used in paint production [caption id="attachment_130362" align="aligncenter"] R.I. = speed of light in vacuum / speed of light in matter
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2. TiO2 Pigment Production

Several processes are applied for TiO2 pigment production. Rutile TiO2 occurs naturally. This is because the crystal structure of rutile is the thermodynamically stable form of titanium dioxide. In chemical processes, natural TiO2 is purified to obtain synthetic TiO2. The pigment can be obtained from titanium-rich ores extracted from the earth's crust. Two chemical methods are applied in obtaining both rutile and anatase TiO2 pigments.

2.1. Sulfate Process

The sulfate process was commercialized with the production of anatase TiO2 in 1931 and later (1941) rutile TiO2. In this process, titanium-rich ore is reacted with sulfuric acid (H2SO4) to obtain TiOSO4. Pure TiO2 is obtained from TiOSO4 in several steps via TiO(OH)2. Depending on the chemical method selected, rutile or anatase TiO2 is obtained. The crystal structure (anatase or rutile) is controlled by nucleation and calcination (Figure 1). [caption id="attachment_130363" align="aligncenter"] Figure 2. Chloride process flow diagram[/caption] In both processes, the particle size of the pigment particles and the final chemical steps after processing are adjusted with precise controls.

3. Ways to Optimize Titanium Dioxide Use for Paints 3.1. Optimizing the Dispersion Process

Each individual primary TiO2 particle should be used as efficiently as possible. TiO2 pigment light scattering reaches maximum efficiency when all particles are separated from one another and distributed throughout the system (Figure 3). [caption id="attachment_130364" align="aligncenter"] Figure 3. Dispersion process: separation and stabilization of solid particles in a liquid[/caption] The strong attraction of solid particles to one another presents a difficulty in this respect. This has two implications: 1. During the dispersion process, considerable effort is required to separate particles from one another. Separation is achieved by using high-energy dispersion equipment as follows: • Disk disperser (dissolver) or • Bead mill. The shear forces in a dissolver arrangement are too weak to separate all primary pigment particles from one another. More effective separation is achieved by using a bead mill. 2. Particles must be stabilized against flocculation, that is, agglomeration caused by attractive forces between particles. Stabilization against flocculation is achieved by adsorbing a stabilizer called a dispersant on the surface of solid particles immediately after they are separated from one another. The dispersant causes particles to repel one another. This way particles remain separated from each other. Two mechanisms can be used for this: Electrostatic stabilization: Occurs when all particles have the same electrostatic charge. Steric stabilization: Results from polymeric tails that are part of dispersant molecules dissolved in the liquid phase surrounding the particles.

3.2. Distributing Particles

The scattering efficiency of the expensive titanium dioxide pigment reaches maximum when all primary particles are separated from one another, stabilized against agglomeration, and distributed throughout the entire system. The distance between individual pigment particles should be as large as possible. This means that pigment particles must be spaced (with gaps) in the system (Figure 4). [caption id="attachment_130365" align="aligncenter"] Figure 4. Spacing and agglomeration of TiO2[/caption] Spacing can be achieved when TiO2 particles are combined with filler particles of comparable size. Filler particles do not scatter light but prevent mixing of titanium dioxide particles. Unwanted agglomeration of TiO2 particles occurs when they combine with filler particles that are larger in size compared to TiO2 particles.

4. Laboratory Tests

Important laboratory tests include: • Particle size distribution (PSD) analysis using laser light scattering technique, • X-ray fluorescence (XRF) analysis for precise process control of surface treatments, • Dispersion measured by Hegman fineness, • Dry color measurement of compressed TiO2 pigment in CIE L*a*b* color space, • pH measurement, • Blue/red reflectance ratio of a mixture of 20 units TiO2 in silicone oil with carbon black undertone and 1 unit carbon black, • Oil absorption rate similar to ASTM D281-31.

Sources / References

1.https://www.tipure.cn/-/media/files/tipure/legacy/titanium-dioxide-for-coatings.pdf?rev=70975dd1245f4082841a13ef41058282(18.10.2021) 2.https://coatings.specialchem.com/selection-guide/complete-guide-on-titanium-dioxide (18.10.2021)

 

Prof. Dr. Öner Yusuf Toraman Niğde Ömer Halisdemir Üniversitesi Mining Engineering Department