Research progress and development trend of inorgan

2022-09-21
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Research progress and development trend of inorganic coated titanium dioxide

research progress and development trend of inorganic coated titanium dioxide

March 17, 2021

Dong xiongbo 1, 2 Zhang Xiangwei 1, Liu Xiaorui 1, Zheng Shuilin 1

1 School of chemistry and environmental engineering, China University of mining and Technology (Beijing)

2. China University of Geosciences (Wuhan) Engineering Research Center for nano mineral materials and applications, Ministry of Education Abstract: inorganic coating on the surface of titanium dioxide is one of the indispensable processes in the post-treatment process of titanium dioxide. Analyzing the inorganic coating process mechanism, structure-activity relationship and application performance improvement mechanism of titanium dioxide has important theoretical significance and application value for the design and preparation of titanium dioxide coated products with excellent application performance. Based on the inorganic coating of titanium dioxide surface units (alumina, silicon dioxide, zirconia, aluminum phosphate, etc.), the research status of inorganic coating microstructure regulation on titanium dioxide surface, the structure-activity relationship between coating microstructure and application properties, and the application performance improvement mechanism are reviewed. The development trend of basic theoretical research on inorganic coating of titanium dioxide is prospected

key words: titanium dioxide; Inorganic coating; Basic theory; Research progress; Development trend

0 preface

since the discovery of titanium at the end of the 18th century to the preparation of commercial titanium dioxide by sulfuric acid at the beginning of the 20th century, it has been more than 100 years of titanium dioxide preparation and commercial history. Titanium dioxide, as the best white pigment, has the advantages of stable chemical properties, high whiteness and high coverage. It is widely used in coatings, plastics, papermaking, printing ink, cosmetics, rubber and other fields

due to certain lattice defects in uncoated titanium dioxide, it has strong photocatalytic activity, and is easy to produce active groups with high oxidation capacity under sunlight, resulting in oxidative degradation of organic resins in its surrounding environment and reducing the service life of products. At the same time, uncoated titanium dioxide particles have high surface energy and serious self agglomeration, resulting in low dispersion stability, which reduces the stability of the product in practical application. In addition, when uncoated titanium dioxide is exposed to the external environment, temperature changes and acid rain erosion cause titanium dioxide to be powdered, reducing the service life of the product. In the application of coatings and cosmetics industry, titanium dioxide needs to have excellent dispersion stability and light resistance; In the application of plastics, papermaking and rubber industry, titanium dioxide needs to have excellent weather resistance and light resistance; In the application of ink industry, titanium dioxide needs to have excellent dispersion stability. In order to meet the requirements of the actual industrial application of titanium dioxide, which will still show a good development trend in China's automotive industry in the future, it is necessary to treat titanium dioxide with inorganic coating (alumina, silica, zirconia, aluminum phosphate, etc.) to improve its dispersion stability, light resistance and weather resistance

compared with developed countries, China's titanium dioxide industry started late, and the application performance and stability of titanium dioxide products have a certain gap compared with developed countries. Facing the blockade of developed countries on the inorganic coating process and mechanism of titanium dioxide, it is of great significance to develop titanium dioxide products with excellent application performance and stability and analyze the inorganic coating mechanism of titanium dioxide

for titanium dioxide inorganic coated products, its application performance largely depends on the microstructure of the coating. Realizing the precise control of the microstructure of the inorganic coating on the surface of titanium dioxide is an important technical basis for optimizing the application performance of titanium dioxide products. In addition, clarifying the mechanism of inorganic coating on the surface of titanium dioxide to improve the dispersion stability, light resistance and weather resistance has important theoretical significance for the design and preparation of titanium dioxide coated products with excellent application properties

in recent years, the author and his research group have used liquid-phase chemical precipitation method and advanced characterization methods of material interface structure and performance to deeply study the precise regulation of coating process on the microstructure of coating layer and the formation process of nano coating layer, obtained the optimized coating process conditions of alumina, silicon dioxide, zirconia and aluminum phosphate coated titanium dioxide, and constructed the microstructure of coating layer The structure-activity relationship between physical and chemical properties and application properties reveals the mechanism of improving the application properties of titanium dioxide by the coating layer. Based on the above work and combined with the research status at home and abroad, the author summarized the research progress and development trend of nano inorganic coating modified titanium dioxide from the perspectives of coating microstructure and regulation, structure-activity relationship, application performance improvement mechanism and so on

1 alumina coated titanium dioxide

alumina coated titanium dioxide is one of the common means to improve the dispersion and stability of titanium dioxide aqueous system in industry, and it is an indispensable process in the post-treatment process of titanium dioxide. Among them, the microstructure of the alumina coating on the surface of titanium dioxide has a direct impact on the surface properties of its particles, such as hydroxyl density, surface free energy, surface potential, steric hindrance and so on, and then affects its dispersion stability in the aqueous system. The microstructure of the alumina coating can be adjusted and optimized by adjusting the coating process factors

1.1 microstructure and regulation of coating layer

in the process of coating alumina on the surface of titanium dioxide, the phase structure of hydrated alumina primary particles and its film-forming process on the surface of titanium dioxide particles jointly determine the microstructure of the coating layer. Therefore, the study of the influence of coating process factors (such as reaction pH, reaction temperature, slurry concentration, alumina coating amount, reaction time, aging time, salt solution type and content, pre dispersant type and content, etc.) on the phase structure and growth process of hydrated alumina primary particles is the key to achieve the precise control of the microstructure of the coating. The regulation rules of various coating process parameters on the microstructure of alumina coating are summarized as follows

1) by adjusting the reaction pH, on the one hand, the formation rate, surface potential, migration to the surface of titanium dioxide particles and heterogeneous nucleation rate of hydrated alumina primary particles or polymeric particles can be adjusted, so as to control the continuity and agglomeration state of the coating layer; On the other hand, the physical structure of the coating layer (boehmite, bayerite and amorphous hydrated alumina) can be adjusted and controlled, and then the pore structure characteristics of the coating layer can be adjusted and controlled. The porosity of alumina coating layers with different physical phases from large to small is boehmite, bayerite and amorphous hydrated aluminum oxide

2) by adjusting the reaction temperature, firstly, the crystallinity and grain size of boehmite in the coating can be adjusted, and then the porosity of the coating can be adjusted; Second, it can regulate the dehydration and condensation rate of primary particles and the collision probability between primary particles and titanium dioxide particles, and then regulate the agglomeration state and porosity of the coating

3) hydrated alumina primary particles first form heterogeneous nucleation on the surface of titanium dioxide particles to form a continuous dense coating, and then homogeneous nucleation to form a loose flocculent porous coating. By adjusting the coating amount of alumina, the continuous dense (inner layer) - loose flocculent porous (outer layer) boehmite coating on the surface of titanium dioxide particles can be accurately adjusted

4) adjusting the reaction time can regulate the generation rate and concentration of hydrated alumina primary particles in the suspension, change the nucleation state of primary particles (homogeneous nucleation or heterogeneous nucleation), and adjust the continuity of the coating

5) adjusting the curing time can adjust the hydrolysis and coating process of hydrated alumina primary particles, adjust the proportion of alumina on the surface of titanium dioxide particles and free alumina between particles, and change the porosity and continuity of alumina coating on the surface of titanium dioxide particles

6) adjusting the slurry concentration can adjust the concentration of hydrated alumina primary particles in the suspension, change the nucleation form of hydrated alumina (heterogeneous or homogeneous nucleation), and then adjust the continuity of alumina coating on the surface of titanium dioxide particles

7) the oxidized primary product of titanium dioxide is obtained by sulfuric acid method or chlorination method. Chloride ion or sulfate radical can enter the coating process with the oxidized primary product slurry. The results show that NaCl can reduce the concentration of hydrated alumina primary particles in suspension by forming [alcl4] coordination compounds, and improve the porosity and continuity of alumina coating during the process of alumina coating titanium dioxide; On the other hand, it can change the viscosity of the initial slurry of titanium dioxide, regulate the nucleation form of hydrated alumina primary particles (heterogeneous or homogeneous nucleation), and reduce the continuity of alumina coating. However, Na2SO4 has no significant effect on the coating structure in the process of alumina coating rutile titanium dioxide

8) the effect of different dispersants on the microstructure and dispersion stability of the coating layer of alumina coated titanium dioxide samples is inconsistent with its effect on the surface potential of titanium dioxide particles. Among them, the dispersant with long carbon chain length is easy to induce the growth of boehmite crystal nucleus and form fibrous coating. Through the adjustment of dispersant, the growth of boehmite crystal nucleus can be induced, and the formation of fibrous alumina coating with high porosity can be controlled to improve the steric hindrance between titanium dioxide particles

1.2 structure activity relationship

when the alumina coating is boehmite structure, and the higher its continuity and porosity, the more significant the flocculent structure is, and the better the dispersion stability of titanium dioxide coated alumina samples in aqueous system. When the reaction pH is 9, the reaction temperature is 70 ℃, the mass ratio of alumina to titanium dioxide is 3.2%, the reaction time is 60min, the curing time is 120min, the slurry concentration (solid mass fraction) is 25%, the amount of NaCl is 2.5% (mass fraction), and the amount of pre dispersant is 0.3% (mass fraction), the flocculent and highly continuous boehmite coating layer is formed on the surface of the prepared alumina coated rutile titanium dioxide sample, It has excellent dispersion stability of aqueous system

1.3 performance improvement mechanism

the mechanism of alumina coating significantly improving the dispersion and stability of titanium dioxide samples in the aqueous system: 1) the formation of flocculent or fibrous hydrated alumina coating on the surface of titanium dioxide particles hinders the collision and agglomeration between titanium dioxide particles and maintains the spatial stability of titanium dioxide particles in the aqueous system; 2) The formation of continuous flocculent hydrated alumina on the surface of titanium dioxide particles significantly increased the hydroxyl content on the particle surface, increased the wettability of the particle surface, and accelerated its dispersion in the aqueous system; 3) The formation of continuous flocculent coating on the surface of titanium dioxide particles increases the zeta potential on the surface of titanium dioxide particles, enhances the electrostatic repulsion between particles, and hinders the agglomeration between particles

2 silica coated titanium dioxide

the silica coating on the surface of titanium dioxide can hinder its direct contact with the surrounding media and the external environment, and improve the weatherability of titanium dioxide. The microstructure of silica coating on the surface of titanium dioxide directly determines the area of titanium dioxide particles exposed to the external environment or surrounding media, and then affects its weather resistance. In recent years, the research progress of microstructure and regulation, structure-activity relationship and performance improvement mechanism of silica coated titanium dioxide coating is summarized as follows

2.1 microstructure and regulation of coating

the phase structure of silica coating on the surface of titanium dioxide is amorphous hydrated silica, and its microstructure mainly depends on the adsorption, film formation and polymerization process of hydrated silica primary particles on the surface of titanium dioxide particles. The regulation rules of various coating process parameters on the microstructure of silica coating are as follows

1) adjust the reaction pH: ① it can adjust the hydrolysis rate of sodium silicate and control the hydrolysis of silica coating

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