Application of titanium dioxide in coatings

I. Introduction

Titanium dioxide (TiO2) is an inorganic white pigment primarily composed of titanium dioxide crystal structures, including anatase, rutile, and brookite. Renowned for its high refractive index, strong tinting strength, high whiteness, non-toxicity, and excellent stability, titanium dioxide finds extensive applications in industries such as coatings, plastics, paper, and inks. Among these, the coatings industry is the largest consumer, accounting for approximately 60% of total usage.

Whether in solvent-based or waterborne coatings, the incorporation of titanium dioxide serves more than mere coverage and decoration. Its paramount role lies in enhancing the physicochemical properties of coatings, improving chemical stability, and boosting attributes like hiding power, tinting strength, corrosion resistance, lightfastness, mechanical strength of the film, and adhesion. Additionally, it prevents cracks, UV penetration, and moisture ingress, thereby delaying aging and extending the lifespan of the coated film. Coatingol.com offers more information on coatings.

II. Impact of Titanium Dioxide Particle Size on Coating Hiding Power

The diverse particle shapes and sizes of titanium dioxide significantly influence the scattering of light, a key factor in its hiding power. Research indicates that, under similar conditions, when the particle size of titanium dioxide ranges from 160 to 350 nm, approximately 0.4 to 0.5 times the wavelength of visible light, it exhibits strong light-scattering capabilities. This directly affects the hiding power of coatings.

In coating systems, inadequate coverage of titanium dioxide particles by the film-forming material can lead to contact and agglomeration between particles. This is akin to an increase in the effective particle size of titanium dioxide, resulting in a decrease in the hiding power of the coating.

III. Effect of Titanium Dioxide Dispersion on Coating Hiding Power

In the field of coatings, the degree of dispersion of powder particles significantly determines product performance. During coating production, the dispersion of titanium dioxide involves wetting, grinding, and dispersing processes. Maintaining titanium dioxide in a stable suspended state within the coating enhances the coating's hiding power. However, due to the inherent reactivity of titanium dioxide, the dispersal environment can influence its dispersion, leading to undesirable states such as flocculation, precipitation, or suspension. Therefore, the quality of dispersion also affects the hiding power of coatings.

IV. Influence of Dispersant Dosage on Coating Hiding Power

During the dispersion of titanium dioxide, its small particle size relative to fillers tends to result in particle aggregation. Hence, the choice and dosage of dispersants become crucial in affecting the dispersion of titanium dioxide and, consequently, the hiding power of the coating. Experiments demonstrate that an increase in dispersant dosage enhances the dispersion of color fillers, narrowing the distribution range and reducing particle size. Consequently, this improvement in dispersion leads to an increase in the hiding power of the coating film.

V. Sustainable Development Paths for Titanium Dioxide in Coatings:

Titanium dioxide, as an efficient light-scattering pigment, provides excellent whiteness and hiding power to coatings. With the rapid growth of industries such as automotive and construction, along with the rise of waterborne coatings, the overall demand for titanium dioxide has increased significantly. This surge in demand poses challenges related to resources, energy consumption, and environmental concerns, highlighting the urgent need to enhance the sustainable development capability of the titanium dioxide industry.

In addition to advancing new production processes and technologies for titanium dioxide, coating manufacturers also need to explore ways to improve its efficiency or seek new alternatives to reduce its usage.

1. Improving Titanium Dioxide Efficiency:

In practical applications, the phenomenon of titanium dioxide agglomeration or flocculation often prevents the attainment of ideal hiding power even at higher titanium dioxide concentrations. Therefore, enhancing the light-scattering efficiency of titanium dioxide has become a focal point of research. Michael, using Monte Carlo simulation methods, explained that incorporating fine fillers to replace coarse fillers in coating formulations creates more spatial separation between titanium dioxide particles, effectively improving the coating's hiding power.

As filler particle size decreases, titanium dioxide pigments are better separated, enhancing the light-scattering efficiency. This means that achieving the same hiding power requires less titanium dioxide. This spatial separation of titanium dioxide is also known as the "pigment dilution" effect. However, there is a possibility of re-aggregation of diluted titanium dioxide particles. In 2013, Dow Chemical won the U.S. Presidential Green Chemistry Challenge Award for successfully developing the EVOQUE pre-composite polymer technology.

Conventional titanium dioxide particles in coatings tend to overlap in the light-scattering region, reducing efficiency. Pre-composite polymers fixed on the surface of titanium dioxide particles in coatings create effective spatial separation, improving the distribution and light-scattering efficiency of titanium dioxide particles, enhancing coating hiding power. This technology can reduce titanium dioxide usage in coatings by 20%, achieving the same or even better hiding effects at a lower cost.

Moreover, the addition of pre-polymerized composites also helps improve coating properties such as resistance to staining and corrosion, significantly reducing energy consumption. Third-party verified life cycle assessment (LCA) results indicate that EVOQUE pre-composite polymers can reduce carbon emissions by over 22% and water usage by 30% in coating products.

In 1997, Virtanen proposed a titanium dioxide particle embedding technique, creating a functional pigment with a core-shell structure by embedding titanium dioxide particles as the core and externally coating them with calcium carbonate as the shell.

The outer calcium carbonate layer provides effective spatial separation between titanium dioxide particles, enhancing light-scattering efficiency. Compared to conventional titanium dioxide, this pigment has a 70% lower carbon footprint and can partially replace titanium dioxide. This pigment has been commercially produced by FP pigments. Similarly, Kobo Products developed a surface-treated titanium dioxide, TS-6300. Traditional surface treatments are often aimed at reducing the photocatalytic activity of titanium dioxide and improving its dispersion. The highly treated TS-6300 introduces additional separation spaces between titanium dioxide particles, reducing aggregation effects and enhancing light-scattering efficiency. Additionally, this surface treatment increases the oil absorption of titanium dioxide particles, lowers the CPVC (critical pigment volume concentration), allowing for improved light-scattering efficiency in coatings with lower PVC.

2. Introducing Air:

The presence of air in coatings can lower the refractive index of the resin/air mixture, increasing the refractive index difference with titanium dioxide pigments, thus improving the coating's light-scattering ability. In coatings, three types of voids typically contribute to improved hiding: air within the resin, air within filler particles, and air at the resin-pigment interface.

For air within the resin, a typical example of enhancing hiding is the development of hollow polymer microspheres first introduced by Kowalski in 1984 and later commercialized by Rohm and Haas as ROPAQUE. This involved the use of latex particles with carboxylic acid groups reacting with hard monomers like styrene to form polymer shells encapsulating the latex particles with a high glass transition temperature (Tg). The system was then heated above the Tg of the shell, using alkali neutralization and dissolution of the carboxyl groups inside, causing the core to expand, and subsequent cooling led to the formation of water-filled microspheres. During the drying process of the coating, water evaporates through the polymer shell, gradually being replaced by air. In comparison between wet hiding and dry hiding effects, it was observed that coatings with only titanium dioxide exhibited higher initial wet hiding, gradually decreasing with drying time until reaching a stable state.

Coatings containing both titanium dioxide and hollow polymer microspheres showed similar initial wet hiding, with hiding power gradually decreasing during drying. However, after reaching the lowest point, the presence of water in the hollow polymer microspheres contributed to an increase in hiding power to a stable state. When the titanium dioxide content was reduced in coatings, and hollow polymer microspheres were introduced, the initial wet hiding was lower. Still, after drying, the coatings achieved the same dry hiding capability as those with only titanium dioxide. Thus, hollow polymer microspheres can partially substitute titanium dioxide and act as effective spatial barriers, improving titanium dioxide efficiency. Additionally, hollow polymer microspheres contribute to enhanced stain resistance, soil resistance, scrub resistance, and excellent outdoor color retention in coatings. Similar to hollow polymer microspheres, air within filler particles also contributes to coating hiding. Microporous kaolin particles, captured by the imaging of focused particle beams at the cross-section, have many internal micropores. This kaolin with enclosed micropores is prepared through a rapid calcination process.

In traditional kaolin calcination processes, natural hydrated aluminum silicate is slowly heated to 1000°C over 30 minutes, forming irregularly shaped aggregates of clay particles. In contrast, the rapid calcination process for kaolin with enclosed micropores takes only a few seconds. Hydroxyl groups in natural aluminum silicate dissociate at 500°C, releasing steam. Due to the rapid heating, steam cannot escape in time, increasing internal particle pressure, causing expansion, and eventually forming many micropores. The void volume inside the particles accounts for about 20%, reducing the density of kaolin from 2.60 to 2.06.

Enclosed air in microporous kaolin completely resists the penetration of resin, solvent, or water in liquid coatings, aiding in simultaneously improving wet and dry hiding. Moreover, it provides high hiding power to the coating at both lower and higher PVC than the coating's CPVC, saving up to 20% of titanium dioxide usage.

When above CPVC, microporous polymers outperform hollow polymers and traditional calcined kaolin. This is due to the simultaneous action of voids inside and outside the microporous kaolin particles at this stage. Additionally, since the oil absorption capacity of microporous kaolin is lower than that of traditional calcined kaolin, it does not adversely affect scrub resistance. Furthermore, Nguyen et al. synthesized a composite nano-sandwich structure of polymer and titanium dioxide (TiO2) through free radical emulsion polymerization technology.

In this structure, TiO2 particles are first embedded in a water-swelling hydrophilic inner layer polymer, then coated with a hydrophobic outer layer, and finally, the internally hydrophilic polymer layer swells in an alkaline solution, forming a sandwich structure containing air and TiO2 particles.

This structure provides coverage through three means: TiO2 particles, air, and the spatial barrier provided by the outer layer.

In summary, in coating formulations, addressing different performance requirements, either by reducing TiO2 aggregation to enhance its light scattering efficiency or by introducing air to increase additional light scattering, can improve the coverage of coatings. This achieves partial substitution of TiO2, reduces carbon emissions, and enhances the sustainable development capability of titanium dioxide.

VI. Applications of Titanium Dioxide

Titanium dioxide is widely used in industries such as coatings, plastics, rubber, inks, paper, chemical fibers, ceramics, daily chemicals, pharmaceuticals, food, etc.

The coatings industry is the largest consumer of titanium dioxide, especially the rutile titanium dioxide, which is mostly consumed by the coatings industry. Coatings made with titanium dioxide have vibrant colors, high covering power, strong tinting strength, low usage, diverse varieties, and can protect the stability of the medium. They enhance the mechanical strength and adhesion of the coating film, prevent cracking, resist ultraviolet rays and moisture penetration, and prolong the life of the coating film.

The plastics industry is the second-largest consumer, where adding titanium dioxide to plastics can improve their heat resistance, light resistance, and weather resistance, enhancing the physical and chemical properties of plastic products, and extending their lifespan.

The paper industry is the third-largest consumer of titanium dioxide, mainly used as a filler in high-grade paper and thin paper. Adding titanium dioxide to paper gives it good whiteness, gloss, high strength, thinness, smoothness, non-penetration during printing, and light weight. Untreated rutile titanium dioxide is generally used for papermaking as it acts as a fluorescent whitening agent, increasing the whiteness of the paper. However, laminated paper requires the use of surface-treated anatase titanium dioxide to meet the requirements of light and heat resistance.

Titanium dioxide is also an indispensable white pigment in high-grade inks. Inks containing titanium dioxide remain durable and non-discoloring, with good surface wetting properties and easy dispersion. The inks industry uses both rutile and anatase titanium dioxide.

The textile and chemical fiber industry is another important application area for titanium dioxide. In chemical fibers, titanium dioxide is mainly used as a matting agent. Due to the softness of anatase compared to rutile, anatase is generally used. Surface treatment is generally not required for chemical fiber-grade titanium dioxide, but for certain special varieties, to reduce the photochemical action of titanium dioxide and avoid fiber degradation under the photocatalytic action of titanium dioxide, surface treatment is necessary.

The enamel industry is an important application area for titanium dioxide. Enamel-grade titanium dioxide has high purity, good whiteness, bright color, uniform particle size, strong refractive index, high covering power, and high color reduction power. It has strong turbidity and opacity, making the coating thin, smooth, acid-resistant, and easy to mix with other materials in the enamel manufacturing process, without caking, and easy to melt.

The ceramics industry is also a significant application area for titanium dioxide. Ceramic-grade titanium dioxide has high purity, uniform particle size, high refractive index, excellent high-temperature resistance, and maintains its characteristics without discoloration for 1 hour at 1200°C. High opacity, thin coating, light weight, widely used in ceramics, construction, decoration, and other materials.

China's titanium dioxide industry started 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, making it the world's largest producer of titanium dioxide. In 2011, the total titanium dioxide production capacity in China was approximately 2.6 million tons. In addition, China has also become the world's largest consumer of titanium dioxide. From 1999 to 2011, China's titanium dioxide consumption increased from 248,000 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 more than 50 titanium dioxide production enterprises in China, distributed throughout the country, with a large number of small and medium-sized enterprises concentrated in the resource-deficient eastern region. China's titanium ore resources are mainly concentrated in the southwest, and the entire industry is still in a relatively decentralized development period.

China has shifted from the past, where sulfate process rutile titanium dioxide was dominant, to mainly anatase-type and rutile-type titanium dioxide. The production capacity of rutile titanium dioxide has exceeded 70%, and this proportion is still increasing. However, products are still concentrated in the mid-to-low-end range. High-end titanium dioxide still relies heavily on imports, and the main advantages of domestic products are still in several large titanium dioxide enterprises. In addition, the national production capacity of chloride process titanium dioxide is only 30,000 tons, which is negligible in the total of 1.47 million tons. In the next step, four or five companies plan to develop chloride process titanium dioxide business, planning a capacity of more than 600,000 tons. Therefore, this period is a key period for transforming product structure, improving China's technological level, introducing and digesting technologies.

VII. Conclusion

With the increasingly serious ecological and environmental issues and the strengthening of people's awareness of environmental protection, the titanium dioxide industry, with its high pollution and high energy consumption, faces enormous challenges and tests. Titanium dioxide, as a crucial white pigment in coatings, improving its efficiency, reducing usage, and developing new materials for partial substitution are necessary measures for achieving its sustainable development.