Titanium dioxide can be utilized in various applications such as a pigment base in paints, coatings, plastics and laminates. During both the production process at the TiO₂ producer and the manufacturing process of paints, coatings and composite materials, avoiding the coagulation and agglomeration of the TiO₂ dispersion is important. Surface chemistry of the TiO₂ dispersion can be controlled during the measurement of Electrokinetic Sonic Amplitude (ESA) by regulating the pH value, e.g. for avoiding flocculation of dispersion. This means the zeta potential of the dispersion should not be or close to zero. Zeta potential is a dimension of the effective surface charge of particles and the interaction between the particles with ions in solution and particles among each other. Characterizing the absolute surface charge (in an aqueous dispersion) of the TiO₂ pigment, negative or positive in the unit (mV), gives a decisive parameter for an ultimate application of the product. The zeta potential depends on the kind of solvent, the nature and amount of the ions in solution (specific conductivity) and the pH value. It is also the main factor determining the stability of the whole dispersion.

Materials, Measurement Technique and Devices

The applied materials and measurement techniques are listed as follows. The measurement of the TiO₂ dispersion was carried out at TINOX Chemie GmbH Düsseldorf. Five TiO₂ pigments grades with different post treatments in aqueous dispersion were tested.

Materials 

Aqueous TiO₂ dispersion 10% wt, 0.1 M HCl and 0.1 M NaOH (p. A.)

Technique

• ESA measuring system with programmable titrator. Hyper-sensitive probe with 500 kHz measuring frequency.
• ESA measuring cell, pH sensor, conductivity and temperature sensor, adjustable stirrer to prevent precipitation during measurement.

Results and Discussion

Measuring the Electrokinetic Sonic Amplitude ESA delivers a clear distinction of the electrostatic properties of various grades of TiO₂ in aqueous dispersions.

The measurement results in Figure 1 show that different TiO₂ grades differ in isoelectric point IEP and with regard to their zeta potential significantly from each other. Handling TiO₂ dispersions in a range beyond the IEP avoids any kind of flocculation. Choosing pH values in an extreme area could be a disadvantage regarding both the electrostatic property of the pigment surface and the ionic charge in the dispersion. With increasing ratio of Al₂O₃/SiO₂ posttreatment of TiO₂ base material the isoelectric point IEP shifts to alkaline pH. With increasing SiO₂ posttreatment the positive surface charge decrease.

Experienced users and specialists know well that, for example, Al₂O₃ and SiO₂ compounds have been used for decades in TiO₂ posttreatment processes. However, improvements have been made repeatedly in the type of post-treatment, for example in the targeted control of shell texture (dense, continuous), structure (heterogeneous, homogeneous), layer thickness, et cetera.

The added value of the current study lies in the method. TINOX Technical-Service uses measurement systems for highly concentrated technical formulations. During technical processes, dispersions are often found to be highly concentrated, muddy, colored, tempered, or electrostatically affected by additive formulations. Sedimentation is often inhibited by powerful agitators in large-scale treatment containers. All these process conditions have been reported in the stability analysis showed in this article.

Surface chemistry of TiO₂ dispersion can be controlled during the measurement in a simultaneously and recording manner by regulating the pH value, adding additives and stirring conditions like in process. Therefore the ESA technique is a valuable tool for TINOX TiO₂ product intensification.

Conclusion 

Electrokinetic Sonic Amplitude (ESA) measurements of highly concentrated dispersions are special measurement methods. The ESA technique has been developed for a range of different applications.

During the technical process, dispersions are often found to be highly concentrated, muddy, colored, tempered or electrostatically affected by additive formulations. Sedimentation is often inhibited by powerful agitators in the large scale treatment containers. All these process conditions mentioned before can be involved in the stability analysis via ESA. The results of the analyzed, highly concentrated samples correlate directly with the electro kinetic properties of the dispersed particle in both the raw material and the final product.

The ESA Measurement Procedure

The ESA method is an electro acoustic measuring technique for characterizing the charge stability of particles in dispersion. An oscillating voltage, generated by an AC source, is applied to a suspension, dispersion or emulsion. Charged particles in dispersion vibrate with the frequency of an applied electric field. One or more frequencies can be applied. Sound waves are generated by the particle oscillation at these frequencies. The amplitude of these sound waves gives the Electrokinetic Sonic Amplitude (ESA). The ESA signal is proportional to the dynamic mobility of the particle, which in turn is proportional to the zeta potential of the particles in dispersion. For the use of this effect, a certain density difference between the dispersion medium and the particle is required. For evaluable signals the density difference has to be at least 0.2 g/cm³. The measurements can be carried out with dispersions in aqueous and alcoholic solutions and paste-like formulations.

See the original article at https://tio2.info/2019/07/29/Research-on-TiO2-pigment-posttreatment-and-it%C2%B4s-influence-on-Electrostatic-Stability-of-technical-dispersion-at-TINOX-Chemie-GmbH/. For more information, visit tinoxchem.com.

By TINOX CHEMIE GMBH, Düsseldorf, Germany