Titanium dioxide is an inorganic white pigment, the main component is TiO2, which can be divided into brookite, anatase and rutile according to the crystal form. Due to the advantages of high refractive index, strong decolorizing power, high whiteness, non-toxicity and good stability, titanium dioxide is widely used in coatings, plastics, papermaking and ink industries, among which the largest amount is used in the coatings industry, accounting for about 60%.
Regardless of solvent-based or water-based paint, if titanium dioxide is used, its role is not only to cover and decorate, but more importantly, to improve the physical and chemical properties of the paint, enhance chemical stability, and improve hiding power, color reduction, and corrosion resistance. , light resistance, weather resistance, enhance the mechanical strength and adhesion of the paint film, prevent cracks, prevent the penetration of ultraviolet rays and moisture, thereby delaying aging and prolonging the life of the paint film. At the same time, it can also save materials and increase varieties.
Effect of Particle Size of Titanium Dioxide on the Hiding Power of Coatings
The shape and size of titanium dioxide particles are different, and the degree of scattering of light is very different, which is the key factor affecting the hiding power of titanium dioxide. Studies have shown that under the same conditions, when the particle size of titanium dioxide is 160-350nm, that is, when the particle size is about 0.4-0.5 times the wavelength of visible light, it has a strong ability to scatter light,It will directly affect the hiding power of the paint in application.
In the coating system, if the film-forming substance is not enough to completely cover the titanium dioxide particles, the titanium dioxide particles will contact each other and agglomerate, which is equivalent to the increase of the particle size of the titanium dioxide to a certain extent, which will reduce the covering power of the coating.
Influence of Dispersion of Titanium Dioxide on the Hiding Power of Coatings
In the field of coatings, the degree of dispersion of powder particles determines the performance of the product to a large extent. During the production of coatings, the dispersion of titanium dioxide must go through the processes of wetting, sanding and dispersion.
The stable suspension state of titanium dioxide in the coating will improve the hiding power of the coating. However, due to the certain activity of titanium dioxide, the system environment used will have a certain impact on its dispersion, and it is prone to poor dispersion states such as flocculation, precipitation, and suspension. Therefore, The degree of dispersion also affects the hiding power of the paint.
The effect of the amount of dispersant on the hiding power of paint
When titanium dioxide is dispersed, its particles tend to agglomerate because its particles are relatively small compared to fillers. Therefore, the choice and amount of dispersant will affect the dispersibility of titanium dioxide, and at the same time affect the hiding power of the paint film. Experiments have shown that the degree of dispersion of pigments and fillers increases with the increase in the amount of dispersant, and the increase in dispersibility reduces the distribution range of the particle size of pigments and fillers, and the particle size becomes smaller, thereby improving the hiding power of the paint film.
The Sustainable Development Approach of Titanium Dioxide Used in Coatings
As an efficient light-scattering pigment, titanium dioxide provides excellent whiteness and hiding power for the coating film. With the rapid rise of the automobile industry, construction industry and water-based paint market, the overall demand for titanium dioxide is also increasing rapidly.
The resulting constraints on resources, energy consumption and the environment are becoming more and more prominent. Improving the sustainable development capability of titanium dioxide industry is imminent.
In addition to the need to promote the development of new production processes and technologies for titanium dioxide, paint manufacturers also need to actively explore how to improve the efficiency of titanium dioxide use, or seek new substitutes to reduce the use of titanium dioxide.
1. Improve the use efficiency of titanium dioxide
In practical applications, the agglomeration or flocculation of titanium dioxide makes it impossible to obtain ideal coverage even when the content of titanium dioxide is high. Therefore, improving the light scattering efficiency of titanium dioxide has become a hot topic of research. Michael combined the Monte Carlo simulation method to explain that when the fine filler is used to replace the coarse filler in the coating formula, more space barriers will be obtained between the titanium dioxide particles, thereby effectively improving the hiding power of the coating film.
As the particle size of the filler decreases, the titanium dioxide pigment is better blocked, which improves the light scattering efficiency of the titanium dioxide. This means that to obtain the same hiding power, the amount of titanium dioxide used will be reduced. This steric barrier of titanium dioxide is also known as the "pigment thinning" effect. However, there is also the possibility of re-agglomeration of the diluted titanium dioxide particles.
If the distance between the titanium dioxide particles of ordinary coatings is too close, the light scattering area will overlap, which will reduce the efficiency. The pre-composite polymer is fixed on the surface of titanium dioxide particles in the coating to form an effective space barrier, thereby improving the distribution of titanium dioxide particles in the coating and the light scattering efficiency, improving the hiding power of the coating film, and being able to formulate the coating The amount of medium titanium dioxide is reduced by 20%, achieving the same or even better covering effect with less cost.
In addition, the addition of pre-polymerized compounds also helps to improve the stain resistance and corrosion resistance of the coating. The application of this technology can significantly reduce energy consumption. According to third-party verified life cycle assessment (LCA) results, EVOQUE pre-compound polymers can reduce carbon emissions by more than 22% and water consumption by 30% in coating products.
In 1997, Virtanen proposed a pre-embedded technology of titanium dioxide particles, which used titanium dioxide particles as the core and externally wrapped calcium carbonate as the shell to form a functional pigment with a core-shell structure.
The calcium carbonate in the outer layer provides an effective space barrier between the titanium dioxide particles and improves the light scattering efficiency. The carbon footprint is about 70% lower than that of ordinary titanium dioxide, which can partially replace titanium dioxide. This pigment has been commercially produced by FP pigments. Similarly, Kemu has developed a surface-treated titanium dioxide TS-6300. Conventional surface treatment is often aimed at reducing the photocatalytic activity of titanium dioxide and improving dispersibility. The highly processed technology in TS-6300 creates additional barrier space between titanium dioxide particles, reduces the agglomeration effect between titanium dioxide, and thus improves the light scattering efficiency. Moreover, this surface treatment increases the oil absorption of titanium dioxide particles and reduces the level of CPVC, so that the presence of air in the coating film can be used to improve the light scattering efficiency in the case of lower PVC.
2. Introduce air
The existence of air in the coating film can reduce the refractive index of the resin/air mixture, so that the refractive index difference with the titanium dioxide pigment becomes larger, and the light scattering ability of the coating film is improved. In coatings, there are generally three types of voids that contribute to hiding, air within the resin, air within filler particles, and air at the resin-pigment interface.
A typical example of air-enhanced covering in resins is the hollow polymer microsphere first developed by Kowalski et al. in 1984 and commercialized by Rohm and Haas under the name ROPAQUE.
The latex particles containing carboxylic acid groups are selected for polymerization reaction with hard monomers such as styrene to obtain latex particles covered by a polymer hard shell with a high glass transition temperature (Tg). Then raise the temperature of the system to above the Tg of the shell, and then use alkali to neutralize and dissolve the carboxyl groups in the core to expand the core, and then lower the temperature to shape the shell to produce water-filled microspheres. During the drying process of the coating film, water evaporates through the polymer shell and is gradually replaced by air. In order to compare the effect of hollow polymers on wet hiding and dry hiding, the test showed that the coating containing only TiO2 had higher initial wet hiding, and gradually decreased with the increase of drying time until reaching a plateau of dry hiding.
Coatings containing both TiO2 and hollow polymers had similar initial wet hiding, then the hiding power gradually decreased during the drying process, and after reaching the lowest point, thanks to the volatilization of water in the hollow polymer,The hiding power gradually increased to a stable state.
When the content of titanium dioxide is reduced in the coating and the hollow polymer is used, the initial wet hiding is poor, but the dry hiding ability of the coating film with only titanium dioxide can be obtained after drying. Therefore, hollow polymers can be used to partially replace titanium dioxide, and can act as an effective space barrier like ultrafine fillers to improve the efficiency of titanium dioxide. In addition, hollow polymers can improve the stain resistance, stain resistance and scrub resistance of the coating film, as well as provide excellent outdoor color retention. Similar to hollow polymers, the air inside the filler also contributes to the hiding of the coating film. Focused particle beam image of the microporous kaolin particle cross-section produced by Omiya, which contains many micropores in its structure. The kaolin with closed micropores is prepared by a rapid calcination process.
In the traditional kaolin calcination process, the natural hydrated aluminum silicate is slowly heated to 1000°C in 30 minutes, so that the flaky clay particles form irregularly shaped agglomerates. The heating process of this kind of calcined kaolin with closed micropores only takes a few seconds. The hydroxyl groups in natural aluminum silicate dissociate when the temperature reaches 500°C and are released as vapor. Because the heating rate is too fast, the steam is too late to release, which increases the pressure inside the particles and expands, eventually forming a lot of micropores. The volume of voids in the particles accounts for about 20%, which reduces the density of kaolin from 2.60 to 2.06.
The enclosed air in microporous kaolin completely resists the penetration of resins, solvents or water in liquid paints, so these voids help to improve both wet hiding and dry hiding of paints. And it can provide high hiding power to the coating film when it is lower or higher than the CPVC of the paint, and can save up to 20% of titanium dioxide.
Among them, in the formula lower than CPVC, the initial point is set as the volume content of titanium dioxide is 20%, and does not contain any other pigments and fillers, and then three contrasting substances are added with a gradient of 5% PVC,
That is, PVC is gradually increased from 20% to 45%, and the volume content of titanium dioxide is kept constant by replacing the resin with equal volume during the whole process. It can be seen that the traditional calcined kaolin has little effect on the hiding power of the coating film, because its refractive index is not much different from that of the resin. The two substances containing closed pores, microporous kaolin and hollow polymer, greatly improve the hiding power of the coating film. Although both contribute similarly to film hiding power, they do not have the same effect on gloss. Microporous kaolin has a matte effect due to its micro-rough structure on the surface, while hollow polymers are beneficial to improve the gloss of the coating film.
In formulations above CPVC, the starting point is 75% PVC containing titanium dioxide with 10% PVC and 65% PVC with calcium carbonate with an average particle size of 4 µm. Then replace the calcium carbonate with a gradient of 5% PVC, and keep the total PVC and volume solid content unchanged.
Microporous polymers outperform hollow polymers and traditional calcined kaolin when above CPVC. This is due to the simultaneous action of the internal and external voids of the microporous kaolin particles. And because the oil absorption of microporous kaolin is lower than that of traditional calcined kaolin, it will not have adverse effects on scrub resistance. In addition, Nguyen et al. synthesized a composite nano-sandwich body of polymer and titanium dioxide through free radical emulsion polymerization.
In this structure, titanium dioxide particles are first embedded in a water-swellable hydrophilic inner layer polymer, and then coated with a hydrophobic outer layer, and finally the inner hydrophilic polymer layer swells in an alkaline solution. , forming a sandwich structure containing air and titanium dioxide particles.
This structure provides coverage in three ways: one is titanium dioxide particles; the other is air; the third is the space barrier provided by the outer layer.
To sum up, in the coating formulation, according to different performance requirements, improving the light scattering efficiency by reducing the agglomeration of titanium dioxide, or increasing additional light scattering by introducing air, can make the coating film obtain better hiding power. Realize the partial replacement of titanium dioxide, reduce carbon emissions, and improve the sustainable development ability of titanium dioxide.
Post time: Jun-19-2023