“White King” Titanium Dioxide
Titanium dioxide is the most widely used white pigment. Because of its strong reflection of light waves of all wavelengths in visible light, it appears white and is widely used in ink, paint, paper and other industries. Whiteness is one of the important indicators of titanium dioxide. Because of the high whiteness of titanium dioxide, it is called "the king of white". The whiteness of titanium dioxide has a great influence on the application range of titanium dioxide. In the coating, paper, ink and other industries, the requirements for color are very strict. If the whiteness of titanium dioxide is too low, its application range will be reduced.
Recently, the editor of China Powder Network learned from the Titanium Dioxide Product Application Research Center of Panzhihua Iron and Steel Group that researchers focused on the four aspects of system pollution, titanium dioxide addition, production and processing temperature, and system compatibility. The relationship between the whiteness of the titanium dioxide and its application in various application fields has been analyzed and discussed. The research shows that the main factor affecting the whiteness of the titanium dioxide application of the same brand of titanium dioxide in different application systems is the compatibility of the system. There is no necessary correlation between the whiteness of the dry powder of titanium dioxide and the whiteness of the application.
The whiteness of dry powder of titanium dioxide and its influencing factors
The whiteness of dry powder of titanium dioxide is a powder cake that is pressed to the surface into a mirror surface with an aluminum cup at a constant pressure, and the whiteness of the powder cake is measured with a whiteness meter or a spectrophotometer. At present, the factors affecting the whiteness of dry titanium dioxide powder include impurities, particle size and particle size distribution, particle shape, and titanium content.
Impact of impurities
In the titanium dioxide process, especially the sulfuric acid titanium dioxide process, most of the operations are to remove impurities in the product, because the impurities seriously affect the application performance of the titanium dioxide, especially the whiteness. Color metal oxide impurities can affect the whiteness at very low content. These elements include iron, manganese, chromium, copper and so on.
The iron content has a significant effect on the whiteness. As the iron content increases, the whiteness decreases. When the iron content exceeds 50x10-6, the effect is particularly obvious. It is recommended to control the iron content below 30×10-6.
Particle size and particle size distribution
Particle size and particle size distribution are also the main factors affecting the whiteness of titanium dioxide, which are mainly affected by the reflection and scattering of light by titanium dioxide particles. The smaller the particle size of the titanium dioxide, the higher the whiteness value, which is mainly due to the smaller the particle size of the titanium dioxide, the larger the surface area, and the enhancement of light reflection and diffuse reflection.
According to related experiments, the particle size of titanium dioxide products has a significant impact on the whiteness of the product. The SEM particle size is 220~240nm, and the sample with a ratio of 150~350nm greater than 75% has better whiteness.
The particle shape refers to the state of existence of titanium dioxide particles, which can be divided into spherical, rod-shaped and layered. These three particle shapes are common in titanium dioxide. The spherical shape is the ideal state, but rod-like and layered shapes may be produced due to factors such as coating or gas powder.
The whiteness of layered titanium dioxide is slightly lower, and the whiteness of spherical and rod-shaped 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, hiding power, dispersibility and other indicators of titanium dioxide.
Application whiteness of titanium dioxide and its influencing factors
The application whiteness of titanium dioxide refers to the whiteness of the paint film, the whiteness of plastic products, and the whiteness of paper that the titanium dioxide is added to the downstream application systems. This is not only related to the size and distribution of the titanium dioxide particles or the content of non-ferrous metals in the titanium dioxide, but also related to the compatibility of the titanium dioxide in the application system. Because the more uniform the dispersion, the more scattered light that can be presented, the better the whiteness, and vice versa. Researchers believe that this is the main difference between the whiteness of dry titanium dioxide and its application, and it is also the main factor leading to the lack of an inevitable correlation between the two.
Production system pollution
In the process of titanium dioxide production and processing into coatings, inks, plastic products, decorative paper, etc., the hue changes caused by the pollution of dispersion equipment and dispersion media, resulting in a decrease in whiteness, are abnormally controllable factors. Test with the same grade of titanium dioxide, as the iron content in the titanium dioxide increases, the Hunter whiteness in the PP board decreases. It can be seen that the iron content in the titanium dioxide is one of the main factors affecting the whiteness of the application of titanium dioxide.
Titanium dioxide dosage
Within a certain range, as the amount of titanium dioxide added increases, its whiteness value will also increase, but after a certain range value, due to the decline in the dispersion of the system, the ability of a single particle to scatter light decreases, and its whiteness value will be reduced. Stop growth, or decline. The user application formulas are all mature formulas. Regardless of application performance or production cost considerations, there is generally no excessive use of titanium dioxide.
The researchers used the same brand of titanium dioxide to test and found that when the addition of titanium dioxide was less than 10%, as the addition of titanium dioxide increased, the Hunter whiteness in the PP board increased; when the addition of titanium dioxide When it is more than 10%, the Hunter whiteness in the PP board has no obvious change, and there is even a partial decrease. It can be seen that increasing the amount of titanium dioxide within a certain range can improve its application whiteness, but when the addition exceeds the critical value, the effect is not obvious, and it may even reduce its application whiteness due to the relative deterioration of the dispersion state of titanium dioxide. .
Production processing temperature
The maximum production temperature of titanium dioxide is about 1000℃, which is much higher than coatings (the highest thermal curing temperature is about 200℃), inks (the highest thermal curing temperature is about 160℃), plastics (the highest production temperature is about 320℃) and decorative paper ( The highest processing temperature is about 170℃), so the titanium dioxide before surface treatment will not cause color change due to high application temperature.
However, the organic treatment agent coated on the surface (alcohols: trimethylolpropane, trimethylolethane; amines: triethanolamine, 1,3-propanediamine; silane coupling: 3-aminopropyl Triethoxysilane, octyltriethoxysilane; polysiloxanes: polydimethylsiloxane, etc.) have large differences in high temperature and yellowing resistance.
Most of the titanium dioxide manufacturers will coat a layer of organic treatment agent on the surface of the titanium dioxide according to the design characteristics of the product to promote the hydrophilic or lipophilicity of the titanium dioxide. Due to the difference in system compatibility, the application performance of titanium dioxide in different application systems is also different, including application whiteness.
Researchers made a comparative analysis of the whiteness of the dry powder of titanium dioxide and the whiteness of multiple applications (application detection system: coatings, plastics, inks, paper, the highest detection temperature is 200 ℃), and found that the whiteness of the dry powder of titanium dioxide is different from the whiteness of the application. There is no inevitable correlation between them; the main factor affecting the whiteness of titanium dioxide in different application systems of the same brand of titanium dioxide is system compatibility.