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Application of titanium dioxide in coatings and sustainable development approach

application of titanium dioxide in coatings and sustainable development approach

April 18, 2019

application of titanium dioxide in coatings and sustainable development approach

Ji Xinghong

(Jotun coatings (Zhangjiagang) Co., Ltd., Suzhou, Jiangsu 215634)

Abstract: the main production processes, advantages and disadvantages of titanium dioxide for coatings are described, The covering mechanism of titanium dioxide in coatings is introduced, the research progress of improving the use efficiency of titanium dioxide and reducing the amount of titanium dioxide is summarized, and the sustainable development path of partial replacement of titanium dioxide in coatings is pointed out

key words: titanium dioxide; Production process; Hiding power; Titanium dioxide substitution; Sustainable development

0 introduction

titanium dioxide is an inorganic white pigment. Its main component is titanium dioxide. According to the crystal form, it can be divided into three types: brookite, anatase and rutile. Due to its high refractive index, strong achromatic power, high whiteness, non toxicity and good stability, titanium dioxide is widely used in coatings, plastics, papermaking, ink and other industries, of which the largest amount is used in the coating industry, accounting for about 60% of China's coatings. Titanium dioxide, as an efficient light scattering pigment, provides excellent whiteness and hiding power for the film. With the rapid rise of automobile industry, construction industry and water-based coating market, the overall demand for titanium dioxide is also increasing sharply. However, due to the shortage of raw titanium ore resources, the supply of titanium dioxide is tight and the price is rising, which also causes huge cost pressure on coating manufacturers. Moreover, the high energy consumption and unnecessary by-products brought by the titanium dioxide production process are also increasingly contrary to the current concept of low carbon footprint. Therefore, improving the use efficiency of titanium dioxide, reducing the use of titanium dioxide or seeking an effective substitute for titanium dioxide will also become a new way for the sustainable development of the coating industry

1 titanium dioxide production process according to the different types of raw materials and process flow, the more mature titanium dioxide production process can be divided into sulfuric acid process and chlorination process

1.1 sulfuric acid process the sulfuric acid process began in 1918 and was commercialized to produce titanium dioxide in 1931. The production raw materials in the sulfuric acid process mainly come from ilmenite or acid soluble iron slag, and then react with sulfuric acid to decompose into TiOSO4 solution, then crystallize, precipitate, filter and wash, and finally obtain titanium dioxide through calcination. The crystal form of titanium dioxide is mainly controlled by crystallization and calcination process. The raw materials in the sulfuric acid process are cheap and easy to obtain, and the synthesis technology is relatively mature, so the production cost is low. However, due to the intermittent operation of sulfuric acid process, the process flow is long. In addition, the consumption of sulfuric acid and water in the reaction process is large, resulting in more wastes and by-products, which is harmful to the environment

1.2 chlorination method rutile titanium dioxide was first commercially produced by DuPont in the 1950s. The chlorination method uses titanium rich ore as raw material, which reacts with chlorine to produce titanium tetrachloride, and then oxidizes it in gas phase to obtain titanium dioxide. Whether it is sulfuric acid method or chlorination method, the titanium dioxide obtained from the reaction still needs further grinding and separation. In addition, titanium dioxide used in the coating also needs inorganic surface treatment for titanium dioxide particles, usually aluminum oxide or silicon oxide. This is because in the presence of ultraviolet light, titanium dioxide can produce active free radicals and degrade the coating. Surface treatment as a barrier can prevent the production of these free radicals. The oxidation process in the chlorination process controls the particle size distribution and crystal form of the finished product. Compared with the sulfuric acid process, the chlorination process is a continuous production process with short process and easy to control the quality of finished products. The chlorination process uses titanium rich ore as raw material, which produces less waste and has relatively little impact on the environment. However, the chlorination process has high requirements for raw materials, complex synthesis technology, and strong corrosive substances such as chlorine and hydrochloric acid also have a great test on the anti-corrosion performance of the equipment

generally speaking, the production cost and technical requirements of chlorination method are relatively high, but the titanium dioxide produced has higher purity, narrower particle size distribution and better performance

2 the covering effect of titanium dioxide in paint the covering power of paint refers to the minimum amount of paint used to evenly coat the color paint on the surface of the object so that the background color will no longer appear. In the paint industry, the paint is usually coated on a black and white background, and the corresponding reflectance is measured by a spectrophotometer. The ratio, i.e. the contrast ratio, is used to characterize the coverage. The higher the contrast ratio, the stronger the hiding power. Obscuration or opacity can be achieved in two ways: light absorption and light scattering. For white paint, the absorption of light is very weak, and the main contribution of covering comes from light scattering. Light scattering usually occurs at the interface of two substances with different refractive indices. The coating contains numerous pigment particles. At the interface between the pigment and the resin, light is scattered or bent due to the difference of refractive index, so that the light is completely reflected before reaching the substrate to produce an opaque coating. The refractive index of the resin usually has little difference, so the refractive index of the pigment particles determines the hiding power. The larger the refractive index of the pigment particles, the stronger the covering power of the film. The refractive index of rutile titanium dioxide is higher than that of anatase, which is one of the important reasons why rutile titanium dioxide is more widely used in the field of coatings. The refractive index affects the propagation path of light in the film

the path of light passing through the coating containing high refractive index pigments is shorter. Both films are white with good covering. However, when the film thickness is reduced to x, part of the light in the film containing low refractive index pigments will be absorbed by the substrate, resulting in a decrease in the hiding power. However, the coating containing high refractive index pigments still retains good coverage. Therefore, the film thickness also plays a key role in covering the film. Paint manufacturers prefer to obtain ideal covering through lower film thickness as far as possible, so as to reduce costs. Compared with other white pigments, titanium dioxide has higher refractive index and stronger light scattering ability. Moreover, when the particle size of titanium dioxide is about half of the wavelength of incident light, i.e. the particle size is 0.2~0.3 µ m, the optimal light scattering efficiency can be generated. This is because the incident light diffracts near the pigment particles, and its scattering cross-sectional area is equivalent to 4~5 times of the actual geometric cross-sectional area of the particles

as mentioned above, the light scattering efficiency is determined by the particle size and refractive index of scattering particles, thus affecting the hiding power of the coating film. In addition, the closeness of pigment particles also affects the light scattering efficiency to a certain extent. When two titanium dioxide particles are close to each other, their light scattering ability will be reduced. This phenomenon is called agglomeration effect. This means that the scattering ability per unit mass of titanium dioxide decreases with the increase of the volume concentration of titanium dioxide in the coating

with the increase of titanium dioxide PVC, the hiding power will not increase all the time, and its change process can be divided into five areas: a-e. In area a, the volume content of titanium dioxide in the formula is small, which is effectively separated by space, and the light scattering efficiency is very high. At this time, the hiding power increases with the increase of titanium dioxide content, showing an approximate linear relationship. With the further increase of the volume content of titanium dioxide, the titanium dioxide particles become closer. As shown in region B, the slope of the curve is lower than that in region a, which means that the agglomeration effect reduces the light scattering efficiency of titanium dioxide particles per unit mass. However, due to the increase of light scattering particles, the lost part of the light scattering effect is compensated, so the total hiding power is still rising. In area C, the covering power reaches the extreme value. At this time, the titanium dioxide PVC continues to increase, and the agglomeration effect becomes very obvious, offsetting the contribution of the increase of light scattering particles. Therefore, the covering power begins to decline, and an inflection point appears at the critical volume concentration (CPVC) in area D. When adding titanium dioxide PVC, there is not enough resin to wrap the pigment, resulting in voids inside the film after drying. The refractive index of air is about 1.0, which is much smaller than that of resin. The difference between the refractive index of pigment and air becomes larger, resulting in stronger light scattering at the interface. Therefore, in area E, the hiding power increases rapidly with the increase of titanium dioxide PVC

3 sustainable development path of titanium dioxide for coating as an indispensable white pigment in coating formulation, the demand for titanium dioxide is also rising, resulting in more and more serious constraints on resources, energy consumption and environment. It is urgent to improve the sustainable development ability of titanium dioxide industry. 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 use efficiency of titanium dioxide, or seek new substitutes to reduce the amount of titanium dioxide

3.1 improve the use efficiency of titanium dioxide in practical application, the agglomeration or flocculation of titanium dioxide leads to the failure to obtain an ideal cover even when the content of titanium dioxide is high. Therefore, improving the light scattering efficiency of titanium dioxide has become a hot topic. Michael combined with Monte Carlo simulation method to explain that when the coarse filler is replaced by the fine filler in the coating formulation, more space barriers will be obtained between titanium dioxide particles, so as to effectively improve the covering power of the coating film

with the decrease of filler particle size, titanium dioxide pigments are better separated, and the light scattering efficiency of titanium dioxide is improved. This means that with the same hiding power, the use of titanium dioxide will be reduced. This spatial barrier of titanium dioxide is also known as the "pigment dilution" effect

however, the diluted titanium dioxide particles also have the possibility of re aggregation. 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 common coatings is too close, the light scattering areas will overlap, which will reduce the efficiency. The precomposite polymer is fixed on the surface of titanium dioxide particles in the coating to form an effective space barrier. Its environmental protection benefit is 10 points significant, which improves the distribution and light scattering efficiency of titanium dioxide particles in the coating, improves the covering power of the film, and can reduce the amount of titanium dioxide in the coating formula by 20% to achieve the same or even better covering effect at less cost. In addition, the addition of prepolymerized composites also helps to improve the stain resistance and corrosion resistance of the coatings. The application of this technology can significantly reduce energy consumption. According to the third-party validation life cycle assessment (LCA) results, evoque pre composite polymer can reduce the carbon emissions and water consumption of coating products by more than 22% and 30%

1997, Virtanen proposed a titanium dioxide particle embedding technology, which takes titanium dioxide particles as the core and calcium carbonate as the shell, forming a functional pigment with core-shell structure as shown in Figure 8. The outer layer of calcium carbonate provides a space barrier between titanium dioxide particles that needs to be removed and cleaned at this time, which improves the light scattering efficiency. The carbon footprint is about 70% lower than that of ordinary titanium dioxide, which can be partially replaced by titanium dioxide. This kind of pigment has been commercialized in FP segments

similarly, Kemu company has developed a surface treated titanium dioxide ts-6300. Conventional surface treatment is often to reduce the photocatalytic activity of titanium dioxide and improve the dispersion. And ts-6300

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