Titanium dioxide is a widely used white pigment. Due to its strong reflection of all wavelengths of visible light, it appears white and is widely applied in industries such as ink, coatings, and papermaking. White opacity is one of the important indicators of titanium dioxide. Because of its high white content, it is called the "King of White".

The whiteness of titanium dioxide has a significant impact on the application scope of titanium dioxide. In industries such as coatings, papermaking, and printing inks, strict color requirements are imposed. If the whiteness of titanium dioxide is too low, it will limit its application range.

Recently, researchers from the Titanium Dioxide Product Application Research Center analyzed and discussed the relationship between the dry powder whiteness of titanium dioxide and its application whiteness in various application fields from four aspects: system pollution, titanium dioxide addition amount, production processing temperature, and system compatibility. The study showed that the main factor affecting the application whiteness of titanium dioxide in different application systems is the system compatibility, and there is no necessary correlation between the dry powder whiteness of titanium dioxide and its application whiteness.

The dry powder whiteness of titanium dioxide and its influencing factors

The dry powder whiteness of titanium dioxide is measured by filling the titanium dioxide powder into an aluminum cup under constant pressure until the surface of the powder becomes a mirror-like finish. Then, the whiteness of the powder cake is detected using a whiteness meter or a spectrophotometer. Currently, the factors influencing the dry powder whiteness of titanium dioxide include impurities, particle size and particle size distribution, particle shape, titanium content, etc.

The influence of impurities

In the titanium dioxide production process, especially in the sulfuric acid method for titanium dioxide production, most of the operations are aimed at removing impurities from the product, as these impurities significantly affect the application performance of titanium dioxide, particularly the whiteness. Color-developing metal oxide impurities can affect the whiteness even at extremely low concentrations. These elements include iron, manganese, chromium, copper, etc.

The iron content has a significant impact on whiteness. As the iron content increases, the whiteness decreases. When the iron content exceeds 50 x 10-6, the effect becomes particularly noticeable. It is recommended to control the iron content below 30 x 10-6.

Particle size and particle size distribution

Particle size and particle size distribution are also the main factors affecting the whiteness of titanium dioxide. It mainly influences through the reflection and scattering of light by the titanium dioxide particles. The smaller the particle size of titanium dioxide, the higher the whiteness value. This is mainly because the smaller the particle size of titanium dioxide, the larger the surface area, and the enhanced reflection and diffuse reflection of light.

According to relevant experiments, the particle size of titanium white products has a significant impact on the product's whiteness. Samples with an SEM particle size ranging from 220 to 240 nm, and a proportion of 150 to 350 nm greater than 75% have better whiteness.

Particle shape

The particle shape refers to the existence state of titanium dioxide particles, which can be classified as spherical, rod-shaped and layered. These three particle shapes are commonly found in titanium dioxide. The spherical shape is the ideal state, but due to factors such as coating or powder entrainment, rod-shaped and layered shapes may occur.

The whiteness of the layered titanium dioxide is slightly lower. The whiteness of the spherical and rod-shaped particles is very close. Therefore, although the shape of titanium dioxide particles can slightly affect the whiteness, it is not the main factor. In production, the shape of the particles mainly affects the gloss, covering power, and dispersion of the titanium dioxide.

The application whiteness of titanium dioxide and its influencing factors

The whiteness of titanium dioxide application refers to the white degree of the paint film, plastic products, paper, etc. that are produced when titanium dioxide is added to various downstream application systems. This is not only related to the particle size and distribution of titanium dioxide or the content of metallic elements in titanium dioxide, but also related to the compatibility of titanium dioxide in the application system. Because the more evenly the titanium dioxide is dispersed, the more scattered light can be presented and the better the whiteness will be, and vice versa.

The researchers believe that this is the main difference between the dry powder whiteness of titanium dioxide and its application whiteness, and it is also the main factor that leads to the lack of a necessary correlation between the two.

Production system pollution

During the production and processing of titanium white powder into coatings, inks, plastic products, decorative papers, etc., the color changes caused by contamination from dispersion equipment, dispersion medium, etc., result in a decrease in whiteness and are considered as abnormal controllable factors. Through experiments using the same brand of titanium white product, it was found that as the iron content in the titanium white powder increases, the Hunter whiteness of it in PP boards decreases accordingly. Thus, it can be concluded that the amount of iron content in titanium white powder is one of the main factors affecting the application whiteness of titanium white powder.

Increase the amount of titanium dioxide

Within a certain range, as the amount of titanium dioxide increases, its whiteness value also increases. However, beyond a certain range, due to the decline in the dispersion of the system, the ability of individual particles to scatter light decreases, and its whiteness value will stop increasing or even decrease. The user's application formulas are all mature formulas. Considering both application performance and production costs, there is generally no situation where the amount of titanium dioxide is used excessively.

The researchers conducted experiments using products of the same brand of titanium dioxide and found that when the addition amount of titanium dioxide was less than 10%, as the addition amount increased, its Hunter whiteness in the PP board also improved; when the addition amount of titanium dioxide was more than 10%, its Hunter whiteness in the PP board showed no significant change, and even there was a phenomenon of a slight decrease. Thus, it can be concluded that within a certain range, increasing the addition amount of titanium dioxide can improve its application whiteness. However, when the addition amount exceeds the critical value, the effect is not obvious, and even because the dispersion state of titanium dioxide becomes relatively worse, its application whiteness will be reduced.

Production processing temperature

The maximum production temperature of titanium dioxide is around 1000℃, which is much higher than the maximum heat curing temperature of coatings (around 200℃), inks (around 160℃), plastics (around 320℃), and decorative papers (around 170℃). Therefore, titanium dioxide that has not undergone surface treatment will not change color due to excessive application temperature.

However, the organic treatment agents (alcohols: trimethylolpropane, trimethylolethane; amines: triethanolamine, 1,3-propanediamine; silane coupling agents: 3-aminopropyl triethoxy silane, octyl triethoxy silane; polyorganosiloxanes: polydimethylsiloxane, etc.) that form the surface coating have significant differences in their high-temperature resistance and anti-yellowing properties.

System compatibility

Most titanium dioxide manufacturers will coat the surface of the titanium dioxide with an organic treatment agent according to the product design characteristics, so as to make the titanium dioxide exhibit hydrophilic or hydrophobic properties. Due to differences in system compatibility, the application performance of titanium dioxide in different application systems is also different, among which includes the application whiteness.

The researchers conducted a comparative analysis of the dry powder whiteness of titanium dioxide and the whiteness of multiple applications (application detection system: coatings, plastics, inks, papermaking; maximum detection temperature: 200℃). They found that there was no inevitable correlation between the dry powder whiteness of titanium dioxide and the application whiteness; the main factor influencing the application whiteness of titanium dioxide in different application systems is the compatibility of the system.