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Titanium Dioxide is an inorganic white pigment mainly composed of titanium dioxide, which can be divided into three types according to crystal morphology: titanium dioxide, anatase, and rutile. Due to its advantages such as high refractive index, strong fading power, high whiteness, non-toxicity, and good stability, titanium dioxide is widely used in industries such as coatings, plastics, paper making, and ink. Among them, the coating industry accounts for the largest amount, accounting for about 60%.
Whether solvent based or water-based coatings, if titanium dioxide is used, its function is not only to cover and decorate, but also to improve the physical and chemical properties of the coating, enhance chemical stability, and ultimately improve covering power, color fading power, corrosion resistance, light resistance, and weather resistance, enhance the mechanical strength and adhesion of the paint film, prevent cracks, prevent the penetration of ultraviolet light and water, thereby delaying aging and extending the lifespan of the paint film. At the same time, it can also save materials and increase variety.
2、 The Effect of Particle Size of Titanium Dioxide on the Covering Power of Coatings
The different shapes and sizes of titanium dioxide particles result in varying degrees of light scattering, which is a key factor affecting the shielding power of titanium dioxide. Research has shown that under the same conditions, when the particle size of titanium dioxide is 160-350nm, which is about 0.4-0.5 times the wavelength of visible light, it has a strong scattering ability towards light and will directly affect the covering power of the coating in application.
In the coating system, if the film-forming material is not sufficient to completely cover titanium dioxide particles, it will cause mutual contact and agglomeration between titanium dioxide particles, which is equivalent to an increase in the particle size of titanium dioxide and a decrease in the coating covering power.
3、 The Effect of Dispersion of Titanium Dioxide on the Covering Power of Coatings
In the field of coatings, the degree of dispersion of powder particles largely determines product performance. During the production of coatings, the dispersion of titanium dioxide powder requires wetting, sanding, and dispersion processes.
The stable suspension state of titanium dioxide in the coating can improve the covering power of the coating. However, due to the certain activity of titanium dioxide, the system environment used can have a certain impact on its dispersion, and it is prone to adverse dispersion states such as flocculation, sedimentation, and suspension. Therefore, the degree of dispersion also affects the covering power of the coating.
4、 The Effect of Dispersant Dosage on the Covering Power of Coatings
When titanium dioxide is dispersed, its particles tend to agglomerate due to their relatively small size compared to the filler. Therefore, the selection and dosage of dispersants will affect the dispersibility of titanium dioxide and also affect the covering power of the paint film. Experiments have shown that the degree of dispersion of pigments and fillers increases with the increase of dispersant dosage. The increase in dispersion narrows the distribution range of pigment and filler particle size, resulting in an increase in the covering power of the paint film.
5、 Sustainable Development Approaches of Titanium Dioxide for Coatings
Titanium dioxide, as an efficient light scattering pigment, provides excellent whiteness and covering power for coatings. With the rapid rise of the automotive 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 environment are becoming increasingly prominent. Improving the sustainable development capacity of the titanium dioxide industry is urgent.
In addition to promoting the development of new processes and technologies for titanium dioxide production, coating manufacturers also need to actively explore how to improve the efficiency of titanium dioxide use or seek new alternatives to reduce the amount of titanium dioxide used.
1. Improving the efficiency of titanium dioxide use
In practical applications, the agglomeration or flocculation phenomenon of titanium dioxide leads to the inability to achieve ideal coverage even in high titanium dioxide content. Therefore, improving the light scattering efficiency of titanium dioxide has become a hot topic of research. Michael combined Monte Carlo simulation method to explain that when using equal volume of fine fillers to replace coarse fillers in coating formulations, more spatial barriers will be obtained between titanium dioxide particles, effectively improving the covering power of the coating film.
As the particle size of the filler decreases, titanium dioxide pigments are better separated, improving the light scattering efficiency of titanium dioxide. This means that obtaining the same covering power will reduce the amount of titanium dioxide used. The spatial barrier of this titanium dioxide powder is also known as the "pigment dilution" effect. However, there is also a possibility of re aggregation of diluted titanium dioxide particles. In 2013, Dow Chemical won the US President's Green Chemistry Challenge Award for successfully developing EVOQUE pre composite polymer technology.
If the distance between titanium dioxide particles in ordinary coatings is too close, overlapping light scattering areas will occur, resulting in a decrease in efficiency. The pre composite polymer is fixed on the surface of titanium dioxide particles in the coating, forming an effective spatial barrier, thereby improving the distribution and light scattering efficiency of titanium dioxide particles in the coating, improving the covering power of the coating film, and reducing the amount of titanium dioxide in the coating formula by 20%, achieving the same or even better covering effect with less cost.
In addition, the addition of pre polymerized composites also helps to improve the stain and corrosion resistance of coatings. The application of this technology can significantly reduce energy consumption. According to third-party validation life cycle assessment (LCA) results, EVOQUE pre composite polymers can reduce carbon emissions and water consumption of coating products by more than 22% and 30%, respectively.
In 1997, Virtanen proposed a titanium dioxide particle embedding technology, which uses titanium dioxide particles as the core and calcium carbonate as the shell to form a core-shell structured functional pigment.
The outer layer of calcium carbonate provides effective spatial barriers between titanium dioxide particles, improving light scattering efficiency. Compared to ordinary titanium dioxide, the carbon footprint is about 70% lower, which can achieve partial replacement of titanium dioxide. This pigment has been commercially produced by FP Pigments. Similarly, Kemu Company 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 its dispersibility. The highly processed technology in TS-6300 creates additional barrier space between titanium dioxide particles, reduces the agglomeration effect between titanium dioxide particles, and thus improves light scattering efficiency. Moreover, this surface treatment increases the oil absorption of titanium dioxide particles, reduces the level of CPVC, and enables the use of air in the coating to improve light scattering efficiency at lower PVC levels.
2. Introduce air
The presence of air in the coating film can reduce the refractive index of the Resin/air mixture, thereby increasing the difference in refractive index with Titanium Dioxide Pigment and improving the light scattering ability of the coating film. In coatings, there are usually three types of voids that help improve coverage, namely the air inside the resin, the air inside the filler particles, and the air at the interface between the resin and the pigment.
A typical example of improving air coverage within a resin is the hollow polymer microspheres first developed by Kowalski et al. in 1984, which were commercialized by Rohm&Haas and named ROPAQUE.
The latex particles containing carboxylic ACID groups were selected to undergo polymerization reaction with hard monomers such as styrene to obtain latex particles wrapped in a high glass transition temperature (Tg) polymer hard shell. Then, the system temperature is raised above the Tg of the shell layer, and alkali is used to neutralize and dissolve the carboxyl groups in the core to expand the core. Then, the temperature is lowered to shape the shell, producing microspheres filled with water. During the drying process of the coating, water evaporates through the shell of the polymer and is gradually replaced by air. In order to compare the effects of hollow polymers on wet and dry covering, experiments have shown that coatings containing only titanium dioxide have a higher initial wet covering, which gradually decreases with increasing drying time until reaching a stable state of dry covering.
Coatings containing both titanium dioxide and hollow polymers have similar initial wet cover, and then the cover gradually decreases during the drying process. After reaching the lowest point, due to the evaporation of water in the hollow polymer, the cover gradually increases to a stable state.
When the content of titanium dioxide in the coating is reduced and combined with hollow polymers, the initial wet coverage is poor, but after the coating film is dried, it can obtain the same dry coverage ability as the coating film containing only titanium dioxide. Therefore, hollow polymers can be used to partially replace titanium dioxide and serve as effective spatial barriers like ultrafine fillers, improving the efficiency of titanium dioxide. In addition, hollow polymers can also improve the stain resistance, stain resistance, and scrub resistance of coatings, as well as provide excellent outdoor color retention. Similar to hollow polymers, the air inside the filler also helps to cover the film. The focused particle beam image at the cross-section of microporous kaolin particles produced by Omia Company contains many micropores in its structure. This type of kaolin containing closed micropores is prepared through a rapid calcination process.
In the traditional calcination process of kaolin, natural hydrated aluminum silicate is slowly heated to 1000 ° C within 30 minutes, resulting in the formation of irregular shaped aggregates of flaky clay particles. The heating process of this calcined kaolin containing closed micropores only takes a few seconds. The hydroxyl groups in natural aluminum silicate undergo dissociation at temperatures up to 500 ° C and are released in the form of steam. Due to the rapid heating rate, the steam cannot be released in time, causing the pressure inside the particles to increase and expand, ultimately forming many micropores. The volume of voids within the particles accounts for about 20%, reducing the density of kaolin from 2.60 to 2.06.
The enclosed air in microporous kaolin completely resists the penetration of resin, solvent, or water in liquid coatings, so these voids help to improve both wet and dry coverage of the coating. And it can provide high coverage for the coating film when it is lower or higher than the CPVC of the coating, saving up to 20% of titanium dioxide usage.
Among them, in the formula below CPVC, the initial point is set as the volume content of titanium dioxide at 20%, without any other pigments and fillers. Then, three contrast substances are added in a gradient of 5% PVC, namely PVC gradually increases from 20% to 45%. The entire process is maintained by replacing the resin with equal volume to maintain the volume content of titanium dioxide unchanged. It can be seen that traditional calcined kaolin has a minimal effect on the covering power of the coating film, because its refractive index is not significantly different from that of the resin. The two substances containing closed pores, microporous kaolin and hollow polymer, greatly improve the covering power of the coating film. Although the contributions of the two to the film coverage are similar, their effects on the gloss are not the same. Microporous kaolin has a matte effect due to its slightly rough surface structure, while hollow polymers are beneficial for improving the gloss of the coating film.
In formulas higher than CPVC, the starting point is 75% PVC, which includes titanium dioxide powder with 10% PVC and calcium carbonate with an average particle size of 4 µ m with 65% PVC. Then, replace calcium carbonate with a gradient of 5% PVC and maintain the total PVC and volume solid content unchanged.
On top of CPVC, microporous polymers are superior to hollow polymers and traditional calcined kaolin. This is due to the simultaneous action of internal and external voids in microporous kaolin particles at this time. Moreover, due to the lower oil absorption of microporous kaolin compared to traditional calcined kaolin, it will not have an adverse impact on its scrub resistance. In addition, Nguyen et al. synthesized the composite nano sandwich of polymer and titanium dioxide through free radical lotion polymerization technology.
In this structure, titanium dioxide particles are first embedded through a hydrophilic inner layer polymer with water swelling, followed by a hydrophobic outer layer. Finally, the hydrophilic polymer layer inside swells in an alkaline solution, forming a sandwich structure containing air and titanium dioxide particles.
This structure provides cover in three ways: firstly, titanium dioxide particles; The second is air; The third is the space barrier provided by the outer layer.
In summary, in the coating formulation, according to different performance requirements, reducing the aggregation of titanium dioxide to improve its light scattering efficiency, or introducing air to increase additional light scattering, can achieve better covering power of the coating film, achieve partial replacement of titanium dioxide, reduce carbon emissions, and enhance the sustainable development ability of titanium dioxide.
6、 Application of titanium dioxide powder
Titanium dioxide is widely used in industries such as coatings, plastics, rubber, ink, paper, chemical fiber, ceramics, daily chemicals, pharmaceuticals, food, etc.
The coating industry is the largest user of titanium dioxide, especially the rutile type titanium dioxide, which is mostly consumed by the coating industry. Coatings made of titanium dioxide have bright colors, high covering power, strong coloring power, low dosage, and a wide variety. They can protect the stability of the medium, enhance the mechanical strength and adhesion of the paint film, prevent cracks, prevent UV and water penetration, and extend the lifespan of the paint film.
The plastic industry is the second largest user. Adding titanium dioxide to plastics can improve the heat resistance, light resistance, and weather resistance of plastic products, improve their physical and chemical properties, enhance their mechanical strength, and extend their service life.
The paper industry is the third largest user of titanium dioxide powder, mainly used as a paper filler in high-end and thin paper. Adding titanium dioxide to the paper can make it have good whiteness, good luster, high strength, thin and smooth, non penetrating during printing, and light weight. Titanium dioxide used in papermaking generally uses untreated anatase titanium dioxide, which can act as a fluorescent whitening agent and increase the whiteness of the paper. However, laminated paper requires the use of surface treated rutile titanium dioxide to meet the requirements of light and heat resistance.
Titanium dioxide is also an indispensable white pigment in advanced ink. Ink containing titanium dioxide is durable and does not change color, has good surface wettability, and is easy to disperse. The titanium dioxide used in the ink industry includes both rutile and anatase types.
The textile and chemical fiber industries are another important application field of titanium dioxide. Titanium dioxide used in chemical fibers is mainly used as a matting agent. Due to the fact that the anatase type is softer than the gold red type, the anatase type is generally used. Titanium dioxide powder for chemical fiber generally does not require surface treatment, but some special varieties require surface treatment to reduce the photochemical effect of titanium dioxide and avoid fiber degradation under the photocatalytic effect of titanium dioxide.
The enamel industry is an important application field of titanium dioxide. Enamel grade titanium dioxide has the advantages of high purity, good whiteness, fresh color, uniform particle size, strong refractive index, and high color extinction. It also has strong opacity and opacity, making the coating thin, smooth, and acid resistant after coating. In the enamel manufacturing process, it can be mixed evenly with other materials, not agglomerated, and easy to melt.
The ceramic industry is also an important application field of titanium dioxide. Ceramic grade titanium dioxide has the characteristics of high purity, uniform particle size, high refractive index, excellent high temperature resistance, and keeping the ash unchanged for 1 hour under high temperature conditions of 1200 ℃. High opacity, thin coating, light weight, widely used in materials such as ceramics, architecture, and decoration.
The titanium dioxide industry in China began in the mid-1950s. With the continuous development of the titanium dioxide industry, more and more brands are recognized by humans. Data shows that China's total titanium dioxide production capacity accounts for 30% of the world's total, making it the largest titanium dioxide production country. In 2011, the total national titanium dioxide production capacity was about 2.6 million tons. In addition, China has also become the world's largest consumer of titanium dioxide. From 1999 to 2011, the consumption of titanium dioxide in China increased from 248000 tons/year to 1.65 million tons/year, with a compound annual growth rate of 17.11%, far exceeding the GDP growth rate.
There are over 50 titanium dioxide production enterprises in China, distributed throughout the country, with a large number of small and medium-sized enterprises concentrated in the eastern region, which lacks resource advantages. However, China's titanium ore resources are mainly concentrated in the southwest region, and the entire industry is still in a relatively dispersed development period.
China has transitioned from relying mainly on sulfuric acid based anatase titanium dioxide to rutile titanium dioxide. The production capacity of rutile titanium dioxide has exceeded 70%, and this proportion is still increasing. However, products are still concentrated in mid to low grades, and high-end titanium dioxide still relies on a large amount of imports
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