Titanium dioxide has a wide range of uses, and its application range covers coatings, plastics, papermaking, inks, cosmetics, rubber and other fields.
Since different titanium dioxide manufacturers have determined different production processes according to their own characteristics, this also leads to differences in the application performance of titanium dioxide from different manufacturers. These differences are most prominent in the dispersion of titanium dioxide in this field.
The problem of dispersion is the first problem that needs to be solved for every titanium dioxide production enterprise and user enterprise. As far as the production and application process of titanium dioxide is concerned, the quality of dispersibility is often an extremely important indicator for evaluating the quality of the product. Rutile titanium dioxide is widely used in the field of coatings, and its dispersibility is the focus of attention.
1. Dispersion principle of titanium dioxide in water-based paint
The basic principle of the dispersion of titanium dioxide in the water phase: we can roughly divide the grinding and dispersion process of titanium dioxide into three steps of mutual influence and mutual restriction:
A. Wetting; B. Separation; C. Stabilization.
In the actual grinding and dispersing process, these three steps are not step-by-step, but interlocking, complementing and restricting each other.That is, if one of the three fails to achieve the purpose, the grinding efficiency will decrease, and even the product cannot reach the required level, or it will return to coarse.
In the process of coating production, the quality of titanium dioxide dispersion directly affects the storage stability of water-based coatings, the appearance of coating films and the difficulty of coating operations.
In the production process of water-based paint, the wetting process of titanium dioxide pigment is a process in which “titanium dioxide-air”, “titanium dioxide-titanium dioxide particle interface” is replaced by “titanium dioxide-resin solution interface”,the factors that determine the wetting effect are mainly the adsorption of the resin solution on the surface of the titanium dioxide pigment particles, and the penetration of the resin into the gaps of the titanium dioxide pigment aggregates. The pulverization process is the process in which the agglomerates or agglomerates of titanium dioxide pigment particles are broken into fine particles by shear force or impact force. It is not only related to equipment, but also related to the coagulation state of titanium dioxide pigments. Dispersion stabilization is a process in which repulsion is generated between particles to prevent re-agglomeration between particles.
2. Factors and mechanism affecting the dispersibility of titanium dioxide
(1) Salt content (resistivity)
According to the general principle of the electric double layer, the electric double layer of particles with negative charges on the surface will be compressed by anions or anion groups in the system, which will lead to a decrease in the Zeta potential of the particles and directly lead to a decrease in the stability of the dispersion system.It is especially obvious that the influence of high-valent anions or anion groups on the Zeta potential of the dispersed particles is more prominent.
During the surface treatment of rutile titanium dioxide, it is usually unavoidable to introduce a large number of impurity ions, among which the common ones are SO42+, Cl- and other anions and Na+ and other cations. Although the production process conditions of each manufacturer are different, a certain amount of impurities introduced by surface treatment will have a negative impact on the water dispersibility of the final product. Therefore, it is necessary to remove impurities after surface treatment, and this index is often expressed in resistivity.
(2) PH value of the system
Although the pH value of titanium dioxide in the application system is not determined by the titanium dioxide manufacturer, it is still necessary to fully understand the impact of this indicator. Because in the preparation process of titanium dioxide, the range of pH value itself is relatively large, understanding the impact of this index on dispersibility will help us control the pH value of titanium dioxide within a more effective range and improve it as much as possible. Dispersion performance of titanium dioxide in water phase system.
In the water phase system that is close to the ideal state, the pH value of the system will also become an important factor affecting the dispersion of rutile titanium dioxide, and the main mechanism of its influence may be attributed to the electric double layer. Due to the difference in surface properties of different types of titanium dioxide, the so-called isoelectric points are very different.
(3) Surface chemical properties
In fact, in the water-based coating system, the ξ potential increases rapidly with the increase of the unit adsorption amount of titanium dioxide, but when the ξ potential rises to a certain level, the increase rate gradually slows down. This shows that titanium dioxide particles must reach a certain amount of adsorption to have a certain charge stability, which has far-reaching significance for the actual formulation design of water-based coatings.
As titanium dioxide particles become smaller, the proportion of surface atoms increases. However, the surface atoms are subjected to asymmetric forces, that is, the asymmetric valence bond forces of adjacent internal atoms and the van der Waals forces of other atoms, and the valence bonds are unsaturated.
According to the pH value of titanium dioxide in aqueous solution, it can be charged positively, negatively and neutrally. When the pH value is relatively low, M-OH2 is formed on the surface of the particle, which leads to positive charge on the particle surface; when the pH value is high, M-O bonds are formed on the particle surface, which makes the particle surface negatively charged; if the pH value is in the middle value, the surface forms M-OH , the particles are electrically neutral. At different pH values, the surface chemistry of powders dispersed in water is determined by the H+ and OH- particles adsorbed to the surface of the particles.
(4) Effect of water-based coating dispersant on the properties of titanium dioxide powder
Under the condition that the content of titanium dioxide remains unchanged, the adsorption capacity per unit is directly proportional to the content of dispersant. According to the DLVO theory of colloidal stability, there is a van der Waals attraction between colloidal particles, and when the particles are close to each other, repulsion occurs due to the overlap of the electric double layer,the stability of a colloid depends on the relative size of the attraction and repulsion between the particles.
The dispersibility and fluidity of the suspension have a great relationship with its ξ potential value. Generally, the larger the absolute value of the ξ potential, the more conducive to the dispersion of the suspension. In the preparation process of the slurry, in order to obtain a slurry with high dispersion and stability, some polymer electrolytes are usually selected to improve its suspension performance.
After the dispersant is introduced, the absolute value of the particle surface ξ potential increases. This is because after the dispersant is added, the negatively charged dispersant anion groups are adsorbed on the surface of the titanium dioxide particles, making the particle surface more negatively charged.
The charge density on the surface of the particles is the highest, and the largest electrostatic repulsion is generated between the particles, so the slurry has the best dispersion. Therefore, the optimum pH value for preparing titanium dioxide slurry with high solid content should be 8-9.
(5) Effect of water-based coating dispersant on rheological properties of titanium dioxide slurry
The rheology of the suspension is determined by the solids content of the slurry and the interactions between the particles. Titanium dioxide powder has a high specific surface energy in water medium, so it is easy to agglomerate or flocculate, which affects the stability and uniformity of the slurry.
The addition of a dispersant can not only improve the surface properties of the powder, but also have a greater impact on the rheological behavior of the titanium dioxide suspension. By adding a dispersant, a coating slurry with low viscosity, high stability and good uniformity can be obtained.
The viscosity of the slurry is the highest when the dispersant is not added. With the increase of the amount of dispersant, the surface charge density of the particles increases, and the repulsion between ions also increases accordingly. The viscosity of the slurry begins to decrease, the fluidity improves, and the dispersibility of the system is improved.
Further increasing the amount of dispersant, the viscosity of the slurry does not change much, which is due to the saturation of the surface charge of the particles. However, if the amount of dispersant is too large, the excess dispersant molecules will bridge with each other to form a network structure, which will restrict the movement between particles and make the rheology worse.
Post time: Jul-19-2023