ABSTRACT: Based on the surface properties of titanium dioxide, the dispersion mechanism of titanium dioxide in solvent-based coatings was studied, and the factors affecting the dispersion of titanium dioxide and the ways to improve the dispersion were analyzed.
1. Preface Titanium dioxide is a kind of white powder pigment with good quality and good light scattering ability, so it has good whiteness, high coloring power and strong covering power. It also has high chemical stability and good weather resistance, non-toxic and tasteless, and has no stimulating effect on human body. It is widely used in coatings, plastics, papermaking and ink, etc. Industry. In solvent-based coatings, titanium dioxide, as a white pigments, is the secondary film-forming substance of coatings. It not only gives bright and white color to the surface of the coated objects, but also plays a certain role in the weatherability, powder resistance and rheological properties of coatings. The oil dispersibility of titanium dioxide is very concerned in solvent coatings and plastics. Its advantages and disadvantages will greatly affect the production process, production equipment and product quality. Based on the surface properties of titanium dioxide and the composition of solvent-based coatings, the dispersion mechanism of titanium dioxide in solvent-based coatings is discussed, and the factors affecting its shadow and mouth orientation are analyzed, and the improvement measures are put forward. This provides a theoretical basis for improving the dispersion of titanium dioxide in solvent-based coatings, and has a certain reference significance.
2. Surface property of titanium dioxide Titanium dioxide is an inorganic compound which is neither soluble in water nor oil, but can be dispersed in water or oil medium. Titanium dioxide has very small particle size and high specific surface energy, so it shows a very strong coagulation characteristics in a single solid phase to reduce surface energy and form stable sub-state aggregated particles. The results show that the surface of titanium dioxide adsorbed water in the air will form alkaline-ldquo; end-rdquo; hydroxyl and acidic-ldquo; bridge-rdquo; hydroxyl. The surface electronegativity of titanium dioxide is very strong (Zeta potential of anatase titanium dioxide is about-50mv), forming an adsorption layer with the same charge, and it has a strong electronegativity. Titanium dioxide exhibits strong dispersion and surface adsorption in both phases.
Dispersion mechanism of 3-titanium dioxide in solvent-based coatings
3.1 The basic composition of solvent-based coatings solvent-based coatings mainly include film-forming base oil or resin, volatile organic solvents and pigments in dispersing media. Among them, the film-forming base material is the main film-forming substance of coatings, while the pigments are the secondary film-forming substances which give the object the function of coloring and covering. Dispersion medium is to make the film-forming base pigments and pigments well dispersed to form a viscous liquid. Therefore, the dispersing mechanism of titanium dioxide in solvent-based coatings is to discuss the wetting and dispersing process of titanium dioxide in film-forming base material and dispersing medium (volatile organic solvents).
3.2 The interaction between titanium dioxide, resin and solvent oil solvents oil can dissolve resin completely and form mixed paint. The strong surface adsorption of titanium dioxide can absorb resin and solvent oil. In the process of coating dispersion, titanium dioxide powder is required to be adsorbed on the surface of titanium dioxide as much as possible and firmly as possible, while solvent oil is not adsorbed as much as possible. This is conducive to the solubility of solvent oil to resin, and improves the solubility of resin. At the same time, the coating formed with titanium dioxide has a high degree of stability. Therefore, titanium dioxide, resin and solvent oil interact and restrict each other. Any abnormal component will affect the performance of the whole system.
3.3 Dispersion mechanism of titanium dioxide in solvent coatings. Solvent oils are generally non-polar solvents or solvents with weak polarity. The electric properties of solvents are very weak. The electric double layer formed on the surface of titanium dioxide is very weak, which is not enough to form a strong double layer effect on the whole system to reduce surface energy and reach a stable state. 。 In this system, titanium dioxide adsorbs polymer resins, and the polymer is solvated by solvent oil. Therefore, the adsorbed resins all have solvated long chains, which are arranged neatly on the surface of titanium dioxide particles and form a shielding layer in a certain area. When two solvated long chains encapsulated titanium dioxide particles are close to each other, the solvated long chains interpenetrate with each other. In the process of interpenetration, the concentration of polymer resin in the interpenetrating area increases, and the osmotic pressure difference is formed with the adjacent area. Because of the effect of osmotic pressure difference, the solvent oil is forced to flow out to the area with low pressure difference, and the particles are separated to reach titanium dioxide. The purpose of powder dispersion in mixed paint of resin and solvent oil. Therefore, the dispersion mechanism of titanium dioxide in solvent-based coatings can be explained by the surface adsorption shielding effect.
4. The factors affecting the dispersion of titanium dioxide in solvent-based coatings. When the resin and solvent oil are determined, the dispersion and stability of titanium dioxide in mixed paint depend on its application performance and surface characteristics.
4.1 The average particle size and particle size distribution of titanium dioxide are comprehensive indicators reflecting the properties and application properties of titanium dioxide pigments. A large number of experiments show that when the mass percentage of titanium dioxide particle size is less than 200 nm is about 30%, the Zeta potential is about - 70 mv, if the mass percentage is less than 20%, the Zeta potential is about - 40 mv, which indicates that the particle size of titanium dioxide seriously affects its surface electrical properties, thus affecting the surface adsorption performance. However, the particle size can not be too small, otherwise it will affect the performance of other pigments. Therefore, it is necessary to control the appropriate particle size range, and generally control the particle size in (0.15-0.3) & mu; M.
See Table 1. The effect of average particle size and particle size distribution of titanium dioxide on oil dispersion.
Effect of Table L Average Particle Size and Particle Size Distribution on Oil Dispersion
The average particle size of titanium dioxide at the production site is nmd, and the oil dispersibility /& Mu is m.
Panzhihua 273.50.102 35
Henan 467.7 0.323 75
Yunnan 378.20.502 85
Table 1: Oil dispersibility of titanium dioxide powders with different particle size distributions is tested. It is found that the average particle size and the width of particle size distribution (normal distribution of mass percentage) have a great influence on the oil dispersibility of titanium dioxide powders. According to a large number of experimental data statistics, we can draw the following conclusions: when the particle size distribution width of titanium dioxide is below 0.1, the scraper fineness meter is used to detect the fineness of titanium dioxide, which is (40-30) p.m.When the peak width is 0.2, the fineness is about 60 GM. If the distribution width is larger than 0.3, the product can not be used in solvent-based coatings basically, and treatment measures must be taken. Therefore, the particle size distribution should be as uniform as possible, increase the grinding strength of the product, reduce the distribution width and improve the oil dispersion.
4.2 The content of soluble substances in titanium dioxide seriously affects the application performance of titanium dioxide. In the dispersion system of mixed paint, soluble salts are generally inorganic ions with strong polarity, which can offset part of the charge on the surface of titanium dioxide, but can not form a strong electric double layer. On the contrary, the adsorption of titanium dioxide on the surface is weakened and the resin can not be firmly adsorbed on the surface of titanium dioxide particles. The degree of dispersion stabilization is low, which leads to the coarsening of titanium dioxide in solvent-based coatings. In addition, in some insulating paint coatings, the soluble matter is higher, which reduces the resistivity of the system and the insulating property of the surface of the coated object. At the same time, the electrochemical characteristics of the coatings are obvious, which reduces the chemical resistance and corrosion resistance. See Table 2. The effect of soluble matter of titanium dioxide on oil dispersion.
Table 2 Effect of Soluble Substances of Titanium Dioxide on Oil Dispersion
Water soluble substance /% distribution width oil dispersion /& mu in sample purified water;
Sample 1 Deionized water 0.450.122 40
Sample 2 produces water 0.82 0.150 60
Sample 3 Deionized water 0.350.102 30
As can be seen from the data in Table 2, water-soluble substances do have a certain effect on the oil dispersion of the product. According to the quality assurance requirements of titanium dioxide products, the content of soluble substances must be controlled less than 0.5%. Therefore, deionized water must be used to remove impurity ions in the purification process of titanium dioxide.
4.3 The experiment of oil absorption of titanium dioxide shows that if the titanium dioxide has high oil absorption, the dispersion of titanium dioxide in solvent oil is poor or unstable, and it is easy to coagulate. In solvent coatings, titanium dioxide powder with high oil absorption is proportioned and dispersed by grinding according to the original formula. A part of solvent oil is absorbed by titanium dioxide, which makes the oil of dissolving resin insufficient and reduces the solvation degree of resin. This affects the adsorption of titanium dioxide on resin, resulting in the decrease of dispersion and dispersion stability.
5. Ways to Improve Dispersibility of Titanium Dioxide in Solvent-based Coatings
5.1 Strictly controlling the particle size and distribution of titanium dioxide is complicated. Firstly, the hydrolysis process and technology are improved to ensure the homogeneity of the primary particles. Secondly, we should strengthen the control of the key temperature points of calcination to make the titanium dioxide particles grow uniformly. Lastly, the crushing technology of titanium dioxide enterprises mostly adopts Raymond system. It can also use airflow pulverizer with high efficiency and good classification effect to further control the particle size of titanium dioxide and stabilize its surface characteristics.
5.2 The control of oil absorption and water-soluble matter is mainly related to calcination atmosphere or calcination temperature at high temperature. If improperly controlled, the lattice defects and irregular shape of titanium dioxide particles will be caused. Therefore, appropriate calcination temperature must be controlled, and the reasonable matching of feed rate and kiln speed must be adjusted to reduce the amount of titanium dioxide particles. The lattice defects of few particles and the regular geometric shape of particles.
5.3 Surface treatment can treat the surface of titanium dioxide or add a certain dispersant, which can firmly adsorb on the surface of titanium dioxide and provide a long chain of resin solvation, which is more conducive to the adsorption of titanium dioxide and resin, and increase the degree of dispersion stability. Generally, the composition of dispersants with this characteristic is complex, and most of them are polycarboxylic alkyl ammonium salts.
At present, most domestic manufacturers produce low-grade anatase products, which are generally poorly dispersed in solvent-based coatings. Therefore, rutile products are gradually replaced by rutile products in the coatings industry. The surface properties of anatase titanium dioxide can be changed by surface treatment, which is comparable to the oil dispersion of rutile products to a certain extent, and the price is low and rutile products have certain market competitiveness. The most popular surface treatment agents in China are organosilicon, azo-coupling reagent or some polyphosphates. Among them, organosilicon is widely used and has been successfully applied in industry.
In conclusion, based on the surface characteristics of titanium dioxide, the role and dispersion mechanism of titanium dioxide in solvent-based coatings are discussed, and the factors affecting the dispersion of titanium dioxide in solvent-based coatings are analyzed, and a new way to improve the dispersion of titanium dioxide in solvent-based coatings is proposed.