Types, principles and processes of inorganic coating of titanium dioxide
  • 07 Jun, 2022
  • By EPIC Powder
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Types, principles and processes of inorganic coating of titanium dioxide

Studies have shown that the application performance of titanium dioxide depends on the type of inorganic coating on its surface. The surface-coated alumina can be used to improve the dispersion stability in the aqueous system of the product, and the coated silica can be used to increase the weather resistance of titanium dioxide products. performance, the coated zirconium dioxide layer can be used to improve the light resistance of titanium dioxide. Coating a single type or multiple types of inorganic film layers on the surface of titanium dioxide can meet the application performance requirements of titanium dioxide in different application fields. According to the difference of coating composition, inorganic coating can be divided into unit inorganic coating and multi-component inorganic coating.

1. Alumina coating

Coating principle: When the surface of titanium dioxide is coated with alumina, hydrated alumina (Al2O3·nH2O) slowly forms a film on the surface of titanium dioxide particles to form a coating layer.

Under different reaction pH conditions, the hydrated alumina (Al2O3·nH2O) of the coating layer presents different phases. In an acidic environment, an amorphous gel is formed; in an alkaline environment, it is boehmite or bayerite.

Jacobson et al. showed in the phase study of hydrated alumina in the coating layer that 50%-70% exists in the form of boehmite (AlOOH), and the rest is amorphous hydrogel. With the change of reaction pH and neutralization speed, the compactness of the coating changed significantly. The neutralization speed is fast, forming a loose spongy coating layer; the neutralization speed is slow, forming a dense coating layer. Under acidic conditions, the precipitation rate of primary particles of hydrated alumina is greater than its own flocculation rate, and it precipitates rapidly and continuously on the surface of titanium dioxide particles to form a dense coating; on the contrary, under alkaline conditions, a loose coating is formed.

Coating process: The titanium dioxide coating process steps are generally as follows: slurrying and dispersing the titanium dioxide, using NaAlO2 or Al2(SO4)3 as the coating agent, and the coating amount of alumina is 1%-5% of the mass of the titanium dioxide. The co-current method is used to control the reaction pH value and reaction temperature, the coating time is 1-5h, and the curing time is 2h to obtain a titanium dioxide coated product.

Through the above process, Li et al. prepared alumina-coated titanium dioxide products under different reaction pH conditions. The results show that when the reaction pH value is 4, an amorphous dense alumina coating layer is formed, and when the reaction pH value is 8.5 When , a boehmite-type loose coating is formed.

2. Silica coating

Coating principle: When amorphous hydrated silica is formed, sodium silicate acidifies and precipitates orthosilicic acid in the form of Si(OH)4. The solution only contains orthosilicic acid hydrolysis products H3SiO4- and H3SiO42-, and there is no metasilicon. acid ions. However, H3SiO4- and H3SiO42- monomers are extremely unstable, and the condensation and polymerization reactions proceed rapidly to generate condensed silicic acid with silicon-oxygen bonds.

The density of hydroxyl groups on the surface of titanium dioxide particles is relatively high, and orthosilicic acid is adsorbed on the surface of the particles by electrostatic adsorption, and dehydrates and condenses with the hydroxyl groups on the surface of the particles to form silicic acid polycondensates. The silicic acid polycondensate grows uniformly and continuously on the surface of titanium dioxide particles, and through further dehydration condensation, a dense amorphous hydrated silica coating layer is formed. Therefore, the process of coating silica on the surface of titanium dioxide is not only a single physical adsorption process, but also has a certain chemical bonding effect. Cui Aili et al. studied the coating of silica on the surface of titanium dioxide and pointed out that the coating process is a sol-gel process. Godnjavec, Park et al. used infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis techniques to study the film formation mechanism of silica-coated titanium dioxide. Si-O-Ti bonds are formed between them.

By controlling the reaction pH, reaction temperature and neutralization speed, silica coating layers with different morphologies can be formed. At low reaction temperature and high neutralization speed, the deposition rate of silicic acid is fast, and island-like coating is easily formed; at high reaction temperature and low neutralization speed, the coating is continuous and dense. The study by Ge Chen et al. to control the coating morphology by controlling the pH value of the reaction showed that when the pH value of the reaction was 7-8.5, a dense island-shaped discontinuous coating layer was formed, and when the pH value of the reaction was 9-10, the coating layer was continuous and dense.

Coating process: The process steps of coating silica on the surface of titanium dioxide are generally: use Na2SiO3 as coating agent, adopt co-current method, maintain pH value of 8-10, coating temperature of 80-100ºC, and coating amount of 3%- 5%, ripening for 1-3h.

3. Zirconium dioxide coating

When the titanium dioxide unit is coated with zirconium dioxide, the coating agents are mainly zirconium sulfate, zirconium tetrachloride, zirconium oxychloride and zirconium nitrate. Among them, zirconium sulfate and zirconium oxychloride have the advantages of low cost and less environmental pollution during use. , has been widely used in industry.

Coating principle: When the zirconium dioxide unit is coated, the coating layer exists in the form of hydroxy hydrated zirconia, which has high surface activity and strong adsorption performance. It condenses with the hydroxyl groups on the surface of titanium dioxide particles to form Zr-O-Ti bonds, which are firmly bonded on the surface of titanium dioxide particles.

Under the condition of low neutralization rate, the generation rate of sol in the solution is slow, and the collision probability between sol and titanium dioxide particles is much greater than that between sols, which is conducive to the formation of a dense coating on the surface of titanium dioxide particles; on the contrary, the neutralization rate is fast, The nucleation rate of the sol itself is fast, and the hydrated zirconia tends to homogeneously nucleate and adsorb on the surface of the titanium dioxide particles to form an island-shaped discontinuous coating.

Coating process: The general process conditions for coating zirconia on the surface of titanium dioxide are: use Zr(SO4)2 or ZrOCl2 as the coating agent, the reaction pH is 9-10, the coating temperature is 50-70ºC, and the coating time is 1-5h, and the aging time is 1-3h.

4. Silica-alumina composite coating

Cao Hongqing et al. used one-time co-precipitation coating, using sodium silicate as silicon source, aluminum chloride and sodium metaaluminate as acid and alkaline precipitants to prepare high-dispersion, high-weather-resistant titanium dioxide products, reducing production steps, reducing production costs.

Bettler adopts the liquid phase co-precipitation method and uses citric acid as the precipitant to coat silica and alumina in turn to obtain a highly dispersive titanium dioxide pigment.

Li et al. coated silica at a reaction pH of 4.0 and a reaction temperature of 60ºC, and coated alumina at a reaction pH of 8.0 and a reaction temperature of 60ºC, and obtained good compactness through secondary coating. Silicon-alumina coated titanium dioxide composite product, the results show that there is Al-O-Si bond in the coating layer, and an aluminosilicate layer is formed between the Si-Al coating layers.

Zhang et al. used the co-precipitation coating method to coat silica under the conditions that the reaction pH was 9.5, the reaction temperature was 90ºC, and the molar ratio of the coating agent sodium silicate to titanium dioxide was 1:15. The reaction temperature was 80ºC, and the performance of the composite-coated samples was optimized by adjusting the pH value of the reaction and the molar ratio of the coating agent sodium metaaluminate to titanium dioxide. The results show that when the reaction pH is 5, a continuous dense silica-alumina coating layer is formed on the surface of the titanium dioxide particles, and the composite coated samples have excellent dispersion stability. Al-O-Si bonds are formed between alumina and particles through Si-O-Ti bonds.

Wei et al. adopted the co-precipitation coating method, adding 1.29mol/L sodium silicate, 0.5mol/L aluminum sulfate and a certain concentration of sodium hydroxide solution to the initial slurry of titanium dioxide at the same time, and controlling the reaction temperature to 60ºC, the dioxide The coating amount of silicon is 3.0%, the coating amount of alumina is 2.0%, and the pH value of the solution is gradually decreased from 10 to 5.

The titanium dioxide composite-coated sample prepared by the above method has excellent weather resistance.

5. Zirconia-alumina composite coating

Su Zhenning et al. through two precipitations, coating dense zirconium dioxide at a reaction pH of 5.0-6.5 and a reaction temperature of 50ºC-90ºC, and using sodium metaaluminate and aluminum sulfate co-precipitation to form an alumina coating layer, prepared Titanium dioxide-zirconia-alumina composite coating product with high gloss, whiteness, light resistance and dispersion stability.

Li Kun et al. explored the optimized process conditions for titanium dioxide composite coating of zirconia and alumina. The XPS spectrum shows that there are Zr-O-Ti and Al-O-Ti bonds in the coating layer, and both zirconium dioxide and aluminum oxide are chemically bonded to the surface of titanium dioxide particles.

Through the control of different coating processes, Li Li et al. effectively improved the hiding power, brightness and other properties of the product, and obtained the optimized coating agent dosage: the mass ratio of zirconia and titanium dioxide is 0.35:100, and the mass ratio of alumina and titanium dioxide is 0.35:100. is 3.0:100.

6. Ternary inorganic coating

Hou Qinglin and others used the sol-gel method to develop zirconia-silica-alumina coated titanium dioxide ternary inorganic coated composite products with high dispersion stability, low loss of light and high whiteness. The process of each cladding process is optimized.

Yang Zhen et al. used the co-precipitation method to prepare a ternary composite product of zirconia-silica-alumina. The obtained product also has excellent dispersion stability, light resistance and weather resistance, and its application performance can reach internationally renowned products. s level.

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