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Introduction of Quality Control of Titanium Dioxide (2)

Introduction of Quality Control of Titanium Dioxide (2)2020.09.22

China's GB / t1706-2006 "Titanium Dioxide Pigments" is modified by the international standard ISO591-1:2000 "Titanium Dioxide Pigments for Color Paints Part 1: Specifications and Test Methods". It is the direction of technical requirements, test methods and inspection rules of titanium dioxide. The main control indexes are as follows: mass fraction of TiO2, mass fraction of volatile matter at 105 , mass fraction of water soluble matter, mass fraction of sieve residue (45 μ m), color, scattering power, oil absorption, resistivity of extraction solution, etc.

The application characteristics of different industries are different, the requirements for titanium dioxide are not the same, and the test methods will be different.

Requirements for titanium dioxide in powder coatings industry

In the application of powder coatings, titanium dioxide is mainly used for coloring, and plays a role of skeleton support when the amount of white is large. According to the process characteristics of powder coating industry, the quality control indexes of titanium dioxide mainly include: color, volatile, oil absorption, 300 mesh (48 μ m) screen residue, particle size distribution, covering power, heat resistance, weather resistance, crystal structure, composition, etc.


1. Color

According to the theory of subtraction method, the higher the whiteness, the better. When it contains black or other mottled colors, it is easy to turn gray and the whiteness will decrease. The more mixed with the variegated pigments, the grayer and not the whiter. In general, we use the sample as the standard sample after the first feeding is confirmed to be qualified, as the standard for later evaluation. It can be compared visually or made into dry board under the same conditions. The color difference can be compared with the result of instrument measurement.


2. Dispersion

Dispersion matter is one of the important indexes to control titanium dioxide in powder coatings. The dispersion includes the dispersion of water and gases produced by the reaction at the temperature of 95 ~ 200 . Excessive dispersion will lead to pinholes on the coating surface and affect the gloss. The smaller the dispersion, the better. It can be measured by weight change before and after baking.


3. Oil absorption

The oil absorption amount is the amount of refined linseed oil absorbed by 100g sample under specified conditions. It can be expressed in ml / 100g or g / 100g. The amount of oil absorption is related to the wetting and dispersion of the coating, too large will affect the smoothness and luster of the coating. Suitable surface treatment (organic coating) process can improve the wetting and dispersion of titanium dioxide, and can properly control the oil absorption of titanium dioxide.


The method of measuring oil absorption: put the sample on the plate, drip refined linseed oil with a burette, the amount of oil added each time is no more than 10 drops. After adding, press and grind the oil with adjusting knife to make the oil penetrate into the tested sample, and continue to add it at this speed until the oil and sample surface form a lump. From then on, each drop is fully ground with a knife. When a paste of uniform consistency is formed, which just does not crack and can adhere to the plate, it is the end point. Record the fuel consumption, and all operations should be completed within 20-25min.


4. 300 mesh (48 μ m) sieve residue

The particle defect of coating film is one of the difficult problems in powder coating production. Therefore, it is better to prevent in advance. According to the thickness requirements of the powder coating, it is appropriate for titanium dioxide to pass through the 300 mesh standard sample separation screen when it is wet sieved in water, and hard particles can not be felt when brushing in the sieve by hand. If a hard particle is felt, even if it can pass the sieve completely, it will produce the defect that the coating has particles.


5. Particle size distribution

Theoretically, the narrower the particle size distribution, the better. If we want to effectively scatter light, the diameter of titanium dioxide pigment particles should be slightly less than half of the wavelength of scattered light. Because the naked eye is most sensitive to yellow and green light (the wavelength is about 0.55 μ m), the particle size of titanium dioxide for coating is between 0.2 and 0.3 μ M.

However, in the completely covered non light absorbing white film, changing the particle size of the pigment has no effect on the color; but in the film containing light absorbing material, reducing the particles of titanium dioxide will enhance the blue degree. Titanium dioxide produced by sulfuric acid method has large particle size and red color.

Color is not very white, gray or yellow. The results show that the titanium dioxide produced by chlorination method has fine particle size, regular particle size distribution, pure white color, stable color phase and blue phase. Therefore, the titanium dioxide produced by chlorination is the trend of the coating industry.

Due to the poor dispersion of titanium dioxide dry powder, it is recommended to use wet particle size analyzer for measurement.

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