Yuan Lei's Little Knowledge | Requirements of Calcined Kaolin in Coatings

Release time:

2024-01-29 09:35

Calcined kaolin used in the coating industry can reduce the amount of titanium dioxide, so that the film has good characteristics, can improve the coating processing, storage and application performance. The amount of calcined kaolin in the middle and high grade coating is 10 ~ 30%, and the calcined kaolin used is mainly 70 ~ 90% of-2um. So what are the indicators of calcined kaolin in the use of coatings?


The coating requirements for calcined kaolin are whiteness, particle size, hiding power, dispersion, 325 mesh sieve residue, oil absorption value, sedimentation volume, pH value, weather resistance, acid and alkali resistance.

01  Whiteness:Whiteness is one of the main parameters of kaolin process performance. Kaolin with high purity is white, so the higher the whiteness, the better, at least greater than 90%, and some manufacturers will require greater than 93%. The stability of whiteness is also very important, if the fluctuation is very large, it will affect the optical performance of the coating, so that the same with different batches of products appear color difference.

02  Granularity:For the calcined kaolin production plant, the inspection index of the finished product should be-2μm at about 80%. Of course, the performance of products produced by different raw materials will be different. If other indicators are qualified, the particle size requirement can be reduced. The detection method of calcined kaolin particle size usually has centrifugal sedimentation method, gravity sedimentation method, laser analysis method and so on.

The particle size is mainly affected by the grindability of the raw ore and the calcination temperature. The grindability is determined by the quality of the raw ore, which mainly affects the particle size of the raw meal stage, and the calcination temperature mainly affects the particle size of the kiln. Calcined kaolin coating application temperature calcination (1000~1200 ℃), select the best calcination temperature is in the amorphous phase has just transformed into silica alumina spinel phase, at this time to produce porous bulky structure, resulting in pore development, the wear value of the product is relatively low.

03  Coverability:Covering power is one of the most important indicators of calcined kaolin. Covering power refers to the ability that when an object is coated with a certain paint, the pigment in the paint can cover the background color of the surface of the coated object, so that the background color can no longer be revealed through the paint. If the same surface is to be painted, the greater the hiding power of the pigment used, the less the amount of pigment needed becomes.

04  Dispersion:Although the calcined kaolin has undergone ultrafine grinding, the average particle size can reach ≤ 1μm, but in the high viscosity mother liquor of the coating, the particles are not separate but agglomerated with each other.

The ability of calcined kaolin to disperse in the coating system is called dispersibility. Generally, the dispersibility of calcined kaolin is related to the final effect of grinding. If particles of 45 μm or larger are contained during ultra-fine processing, they will not be dispersed in the coating system. Therefore, the requirement for this indicator is less than 45μm, and the smaller the better. The dispersion test can be used with the Hagman particle sizer.

05  325 mesh sieve residue:The coating industry's requirement for kaolin 325 mesh screen residue is <0.02%, and the smaller the better.

06  Oil absorption value:The coating requires that the oil absorption value of calcined kaolin is 50-60g/100g, which is moderate. If the oil absorption is high, the viscosity of the coating made of the same formula will be large. Therefore, if you want to reduce the viscosity, you must increase the amount of paint base and increase the cost. If the oil absorption value is low, the prepared coating will easily settle, resulting in poor can opening performance.

07  Sedimentation volume:The volume reduction reflects the sedimentation performance of calcined kaolin in the coating and directly affects the can opening performance of the coating. The indicator should be the higher the better, the higher the pigment is not easy to settle, it will maintain good uniformity in the coating.

08  pH:The pH value reflects the acidity and basicity of the calcined kaolin. The coating requires a value between 6 and 8, essentially close to neutral.

09  Weather resistanceThe ability of pigments to resist the above changes is called weather resistance. After adding calcined kaolin in the coating, it can shield ultraviolet radiation, improve the weather resistance of the coating, delay the pulverization and discoloration, and enhance the comprehensive mechanical properties and stain resistance of the coating.

10  Acid and alkali resistance:Calcined kaolin does not react with any acid or alkali at room temperature, nor does it react chemically with any other substance, so calcined kaolin has good acid and alkali resistance and can be well mixed with other substances.


Coatings on the other indicators of calcined kaolin requirements, as well as mixing performance, wetting performance, density, etc., these are all calcined kaolin in common, some are not digital indicators.


Requirements, calcination, kaolin, coatings, performance, particle size, index, pigment, sedimentation

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