Calcined kaolin can be used in the coating industry to reduce the amount of titanium dioxide, make the coating film have good characteristics, and improve the processing, storage and application performance of the coating. The amount of calcined kaolin used in medium and high-end coatings is 10-30%, and the calcined kaolin used is mainly -2um with a content of 70-90%. So what are the index requirements for calcined kaolin in the use of coatings?

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

Whiteness: Whiteness is one of the main parameters of kaolin process performance. High-purity kaolin 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 large, it will affect the optical properties of the coating, causing color differences in different batches of products under the same formulation.

Particle size: For calcined kaolin production plants, the inspection index of finished products should be -2μm at about 80%. Of course, the performance of products produced from different raw materials will be different. If other indicators are qualified, the particle size requirements can be lowered. The detection methods for calcined kaolin particle size usually include centrifugal sedimentation, gravity sedimentation, laser analysis, etc.

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 in the raw material stage, and the calcination temperature mainly affects the particle size after the kiln is broken up. Calcined kaolin for coatings should be calcined at medium temperature (1000~1200℃). The best calcination temperature is when the amorphous phase has just transformed into the silicon-aluminum spinel phase. At this time, a porous expanded structure is produced, resulting in pore development, and the wear value of the product is relatively low.

Hiding power: Hiding power is the most important indicator for coatings for calcined kaolin. Hiding power refers to the ability of the pigment in the paint to cover the base color of the surface of the painted object when an object is coated with a certain paint, so that the base color can no longer be revealed through the paint. If the same surface is to be coated, the greater the hiding power of the pigment used, the less pigment is needed.

Dispersibility: Although the calcined kaolin has been ultra-finely crushed and the average particle size can reach ≤1μm, in the high-viscosity mother liquor of the paint, the particles are not separate but agglomerated with each other.

The ability of calcined kaolin to disperse in the paint system is called dispersibility. Generally, the dispersibility of calcined kaolin is related to the final effect of grinding. If it contains 45μm or larger particles during the ultra-fine processing, it will not be dispersed in the paint system anyway. Therefore, the requirement for this indicator is less than 45μm, and the smaller the better. The dispersion test can be done with a Hegman particle size analyzer.

325 mesh sieve residue: The paint industry requires kaolin 325 mesh sieve residue to be <0.02%, and the smaller the better.

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

Sedimentation volume: The sedimentation volume reflects the sedimentation performance of calcined kaolin in the coating, which directly affects the can opening performance of the coating. The higher the index, the better. The higher the better. The less likely the pigment is to settle, and it will maintain good uniformity in the coating.

pH value: The pH value reflects the acidity and alkalinity of calcined kaolin. The coating requires that its value should be between 6-8, which is basically close to neutral.

Weather resistance: The ability of the pigment to resist the above changes is called weather resistance. After adding calcined kaolin to the coating, it can shield ultraviolet light, improve the weather resistance of the coating, delay powdering and discoloration, and enhance the comprehensive mechanical properties and stain resistance and self-cleaning properties of the coating.

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

Other requirements of coatings for calcined kaolin include mixing performance, wetting performance, density, etc. These are common characteristics of all calcined kaolins, and some are not digital indicators.