From the apparent index and application system of titanium dioxide, some application problems of titanium dioxide in water-based coatings and the connection between the index and application of titanium dioxide are explained. It is advocated to choose the appropriate titanium dioxide variety to design the formula of water-based coatings.
In recent years, with the strengthening of China's national environmental protection policy and the improvement of people's living standards, water-based architectural coatings have developed by leaps and bounds. At the same time, with the development of science and technology and social productivity, human beings pay more and more attention to the living environment. In particular, China strictly controls the solvent-based coatings of "triphenylbenzene". Coating manufacturers have made a lot of improvements in product varieties, quality, environmental protection, etc. to adapt to new forms and have achieved great development. It is imperative to gradually eliminate raw materials that are harmful to human body and environment. Titanium dioxide is a non-toxic and harmless chemical raw material, which has been widely used in coatings and paint industries. Titanium dioxide is pure white in color, has high tinting power and hiding power, and is light-resistant, heat-resistant and anti-powdering. It is an indispensable raw material for manufacturing coatings and paints. As an important raw material for coatings, the quality of titanium dioxide directly affects the application performance of water-based coatings. The properties and dosage of titanium dioxide particles; the dosage and type of dispersants are all important factors affecting the quality of water-based coatings. Therefore, each coating manufacturer should choose a special titanium dioxide according to the properties of different titanium dioxides. As we all know, there are few special titanium dioxide varieties in China's domestic titanium dioxide enterprises. In the process of variety construction and design, it is best to have a multifunctional general type, which can not only save storage space, but also reduce confusion in market development, but it is difficult to achieve such a requirement. Because the application range of titanium dioxide is quite wide, it is difficult to apply it to multiple fields with a general brand. Therefore, designing different titanium dioxide varieties according to the characteristics of each industry is the direction of titanium dioxide enterprises' efforts in the future and even for a long time, but multifunctionalization is still a trend. Water-based coatings have the characteristics of high solid content and low volatile organic compounds, and are developing rapidly now. Water-based paint also has the advantages of being applicable without restrictions on occasions, easy cleaning of painting tools, and resource conservation. However, it also has its own disadvantages, mainly that the latent heat of evaporation of water in water-based paint is high, and higher temperatures and longer times are required for drying. The large number of hydrophilic groups and low relative molecular weight in the paint reduce the water resistance and corrosion resistance of the paint film. Therefore, designing a reasonable formulation system and minimizing production costs are also the main directions of competition for paint companies.
1. Dispersion of titanium dioxide in water-based paint
In the process of paint production, the dispersion of titanium dioxide directly affects the storage stability, coating appearance and coating operation difficulty of water-based paint. As we all know, the dispersion of titanium dioxide includes three processes: wetting, mechanical crushing and dispersion stability. In the production process of water-based paint, the wetting process of titanium dioxide pigment is a process in which the titanium dioxide-air and titanium dioxide-titanium dioxide particle interfaces are replaced by the titanium dioxide-resin solution interface. The factors that determine the wetting effect are mainly the adsorption of resin solution on the surface of titanium dioxide pigment particles and the penetration of resin into the gaps between titanium dioxide pigment aggregates. The crushing process is the process in which the aggregates or agglomerates of titanium dioxide pigment particles are destroyed into fine particles by shear force or impact force. It is not only related to the equipment, but also to the aggregation state of titanium dioxide pigment. Dispersion stability is the process in which repulsion is generated between particles to prevent reaggregation between particles.
1.1 Double layer theory Double layer theory is an electrostatic stabilization mechanism. By adjusting the pH value or adding electrolytes, a certain amount of surface charge is generated on the particle surface to increase the double layer thickness and the Zeta potential value of the particle surface, so that a larger repulsive force is generated between the particles, thereby achieving stable dispersion of the particles. Although the Zeta potential of titanium dioxide contributes to dispersion stability, its effect is still quite small compared with the repulsive force of steric hindrance. The usual process requires that the slurry has a high solid content, low viscosity and good stability. The stability of the suspension is its basic property: first, the particles cannot be too large, otherwise gravity will cause rapid sedimentation; another important factor is the attraction between the particles. Regardless of whether there are other forces, van der Waals forces exist between particles. If the attraction between the particles is large enough, they will adhere to each other, resulting in rapid sedimentation of the particle beam (such as flocculation). The commonly used method to prevent flocculation is to introduce mutually repulsive forces on the particle surface: repulsive force between charges (electrical stability), repulsive force between adsorbed polymers (steric stability), or a combination of the two (steric stability). The double layer stabilization mechanism is to adjust the pH value so that the surface of the particles carries a certain surface charge to form a double layer; the repulsive force between the double layers greatly reduces the attraction between the particles, thereby achieving the dispersion of the powder. The surface electrical properties of titanium dioxide are determined by the charged ions on the surface of the powder, such as OH-, H+, etc. When titanium dioxide is dispersed in the solution, the electrical properties are also related to one of the pH values of the solution, that is, the ion type of the solution. The charge on the surface of the powder affects the particles, the particles and inorganic ions, the surfactant ions and other chemicals.
According to the double layer theory, we can clearly see that the so-called Zeta potential is not the interface potential of the particles, but the potential outside the adsorption layer. The Zeta potential is very close to the potential of the tight layer of the particles and can be approximately equal. The thermodynamic potential is always greater than the Zeta potential. The thicker the adsorption layer, the lower the gross potential. If the number of negative charges on the surface of the particles is equal to the number of positive charges adsorbed by the fixed layer, the Zeta potential becomes zero, and the corresponding pH value of the solution is called the isoelectric point. When the pH value of the solution is greater than the isoelectric point, the surface of the powder is negatively charged, and when it is less than the isoelectric point, it is positively charged. Of course, there are many factors that affect the HI and Zeta potential, such as the chemical composition of the powder, pH value, surface defects, solvent, particle size distribution, etc. The isoelectric points of some inorganic powders are shown in Table 1.
1.2 Surface chemical properties In fact, in the water-based coating system, the Zeta potential increases rapidly with the increase of the unit adsorption amount of titanium dioxide, but when the Zeta potential rises to a certain degree, its increase gradually slows down. This shows that titanium dioxide particles must reach a certain adsorption amount to have a certain charge stability, which has far-reaching significance for the actual design of water-based coating formulas. As the titanium dioxide particles become finer, the proportion of surface atoms increases. However, the surface atoms are subject to asymmetric forces, that is, the asymmetric valence bond forces with the adjacent internal atoms and the van der Waals forces of other atoms, and their valence bonds are unsaturated. Depending on the pH value of titanium dioxide in the aqueous solution, it can be positively charged, negatively charged, and electrically neutral. When the pH is relatively small, M-OH2 is formed on the particle surface, resulting in a positive charge on the particle surface; when the pH is high, M-O bonds are formed on the particle surface, making the particle surface negatively charged; if the pH value is at an intermediate value, M-OH is formed on the surface, and the particle is electrically neutral. At different pH values, the surface chemical properties of the powder dispersed in water are determined by the H+ and OH- particles adsorbed to the surface of the particles.
1.3 Effect of waterborne coating dispersants on the properties of titanium dioxide powder When the content of titanium dioxide remains unchanged, the unit adsorption amount is proportional to the content of dispersant. The adsorption amount on the unit titanium dioxide can be derived according to the formula a=Ax/(1+Ay)=x/(A-1+Y). According to the DLVO theory of colloid stability, there is van der Waals attraction between colloid particles, and when the particles approach each other, they repel each other due to the overlap of the double electric layer. The stability of the colloid depends on the relative size of the attraction and repulsion between the particles. The dispersibility and fluidity of the suspension are closely related to its Zeta potential value. Generally speaking, the larger the absolute value of the Zeta potential, the more conducive it is to the dispersion of the suspension. In the preparation process of slurry, in order to obtain a slurry with high dispersibility and good stability, some polymer electrolytes are usually selected to improve its suspension performance. After the introduction of the dispersant, the particle surface. The absolute value of Zeta potential increases. This is because after adding the dispersant, the negatively charged anionic groups of the dispersant are adsorbed on the surface of the titanium dioxide particles, making the surface of the particles more negatively charged, the charge density of the particle surface is the highest, and the maximum electrostatic repulsion is generated between the particles, so the slurry has the best dispersibility. Therefore, the optimal pH value should be selected to prepare titanium dioxide slurry with a high solid content, which should be around 8-9.
1.4 Effect of water-based coating dispersant on the rheological properties of titanium dioxide slurry The rheological properties of the suspension are determined by the solid content of the slurry and the interaction between particles. Titanium dioxide powder has a high specific surface energy in the water medium, so it is easy to agglomerate or flocculate, affecting the stability and uniformity of the slurry. The addition of dispersant can not only improve the surface properties of the powder, but also has a great influence on the rheological behavior of the titanium dioxide suspension. By adding dispersant, a coating slurry with low viscosity, high stability and good uniformity can be obtained. The viscosity of the slurry is the largest when no dispersant is added. As the amount of dispersant increases, the surface charge density of the particles increases, 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 each other to form a network structure, restricting the movement of particles and making the rheology worse.
2. Performance of rutile titanium dioxide in water-based coatings
2.1 Resistivity of titanium dioxide According to the theory of double electric layer, the double electric layer of the loaded powder particles will be compressed by the anions or anion groups in the system, resulting in a decrease in the Zeta potential of the system, which in turn leads to a significant reduction in the dispersion stability of the entire system. Because the process of inorganic surface treatment and organic treatment of titanium dioxide is actually a process of introducing other elements to improve product performance. If it is reasonable, the performance of the entire product can be greatly improved and excellently improved; if it is unreasonable, it will lead to a fatal blow, making it a harmful impurity element, reducing the performance of the product or making it unusable. Due to the different processing processes and storage technologies of various manufacturers, the anions SO4-, Cl-, etc. and cations K+, Na+, etc. of their powders are very different, resulting in a huge difference in product application performance. However, the higher the resistivity of titanium dioxide, the better. Too high a resistivity has no practical significance, but increases the economic cost of titanium dioxide production, and is also inconsistent with the current national policies and energy conservation and consumption reduction. The resistivity of titanium dioxide also reflects the impurities in titanium dioxide to a certain extent.
2.2 pH value of titanium dioxide
The pH value of titanium dioxide directly affects its application performance in water-based coatings. In theory, the pH value of titanium dioxide powder should be near neutral, but due to different production processes, calcination conditions, coating state, chemical composition and many other factors, the pH of titanium dioxide deviates from neutral. The pH value of titanium dioxide directly affects the stability of coating production. Therefore, stable titanium dioxide powder will be of great significance to coating manufacturers. In addition, the acidic pH value of titanium dioxide can easily cause the curing and drying speed of coating production to slow down, the dispersibility to deteriorate, and in severe cases, the coating will gel.
2.3 Moisture content of titanium dioxide Since the surface free energy of titanium dioxide pigment is particularly high, it is inevitable that the pigment surface absorbs water. Due to the presence of water, the grinding and dispersibility of the pigment in the coating production process is affected. Therefore, controlling the appropriate pigment moisture to prevent anti-coarsening and flocculation in the coating production process has a significant effect.
2.4 Hiding power of titanium dioxide Hiding power refers to the ability of titanium dioxide in the coating to cover the coated object. Hiding power is an important economic indicator of titanium dioxide in the application field. Hiding power depends not only on the ability to scatter light, but also on the ability to absorb light, and also on the difference in the refractive index between titanium dioxide and the base material. The larger the refractive index, the greater the hiding power, and the smaller the particle size, the higher the hiding power. However, when the particle size is less than half the wavelength of visible light, the hiding power decreases instead. Hiding power is also directly related to the particle size of titanium dioxide. Generally speaking, in a coating system with a high pigment volume concentration, titanium dioxide with a larger particle size has a higher hiding power; in a coating system with a low pigment volume concentration, titanium dioxide with a smaller particle size has a higher hiding power. The main factors that affect hiding power due to the pigment properties of titanium dioxide are the refractive index of titanium dioxide and the refractive index of the medium, the particle size and particle size distribution of titanium dioxide, and the dispersibility of titanium dioxide after dispersion.
2.5 Tinting strength of titanium dioxide The tinting strength of titanium dioxide is a method to strictly measure the efficiency of titanium dioxide in scattering light in coatings. Tinting strength is to mix titanium dioxide with blue and black raw materials, and then test the reflectivity of the system to green light. As long as titanium dioxide does not flocculate, tinting power and covering power are parallel to each other. Tinting power emphasizes the reflectivity of titanium dioxide pigment to light, and should not overly consider and focus on the color tone of titanium dioxide.
2.6 Titanium dioxide gloss retention and powder resistance in water-based coatings The essence of powdering of coatings and paints is that the organic resin base material on the surface of the paint film has been induced and stimulated by external factors, mainly referring to photochemical degradation after being affected by sunlight, moisture, temperature changes and environmental pollution, and the organic resin degradation paint film surface becomes dull and rough. Because titanium dioxide itself is a powder, there is no problem of powdering or not. It is the powdering of the surrounding medium where titanium dioxide is located.
2.7 Titanium dioxide's own flocculation Due to the free energy of the titanium dioxide surface, titanium dioxide particles form loose block-like agglomerates in the fluid. This block is easy to open under the action of moderate shear force. If the shearing is not thorough enough, it is easy to cause the paint's hiding power and tinting power to decrease, the gloss is dull, and the brightness is worse.
3. Conclusion
In the application of titanium dioxide in water-based paint and the design of water-based paint, it is important to select the appropriate titanium dioxide variety to design the water-based paint formula. According to the different application ranges of the paint, it is most important to select the appropriate titanium dioxide product and the appropriate grade. If the titanium dioxide variety is properly selected, it can not only improve the performance of the product, but also reduce the production cost of water-based paint. Therefore, it is most important to design the paint formula according to users and social development and select titanium dioxide.