Ultrafine kaolin refers to hydrated kaolin having an average particle diameter of between 0.2 and 0.25 μm and a cumulative mass percentage of less than 1 μm of 95% or more, also referred to as high gloss kaolin.

Ultrafine kaolin gives coated paper and paperboard a higher paper surface gloss, smoothness, smaller coating pore size, and improved ink drying rate. So far, the particle size of domestic kaolin has not reached such a distribution, and the particle size distribution curve of foreign ultrafine kaolin is shown in the following figure.

Fig.1 Particle size distribution curve of ultrafine kaolin in foreign countries

1. American ultrafine kaolin production process and performance index

The ultrafine kaolin produced by KaMin Company of the United States has a narrow particle size distribution, flat particles, high whiteness and good rheology. It is especially suitable for high solid content coating formulations and high speed coating machines, giving coated paper and paperboard high gloss and Good opacity.

Figure 2 KaMin company ultra-fine kaolin production process

Table 1 Main physical properties of high whiteness ultrafine kaolin and standard whiteness ultrafine kaolin from American kaolin production company

Figure 3 Electron microscope magnified image of American ultrafine kaolin

2. Production process of Brazilian superfine kaolin

The ultra-fine kaolin produced by CADAM Kaolin Company of Brazil has narrow particle size distribution, high viscous concentration and good rheology. It is especially suitable for high solid content coating formula and high speed coating machine, which gives high gloss to coated paper and paperboard.

Figure 4 Kaolin kaolin clay production process

Table 2 Main physical properties of Brazilian ultrafine kaolin

Figure 5 Electron microscope magnified image of Brazilian ultrafine kaolin

3. Effect of ultrafine kaolin on coating, coating running performance and coated paper quality

(1) Effect of ultrafine kaolin on coating properties

Ultrafine kaolin can produce a higher solids content of the slurry, giving a higher solids content of the coating. Ultrafine particles give slower dehydration or better water retention than coarser particles. The low shear viscosity of the suspension of ultrafine particles is slightly higher than the low shear viscosity of the suspension of coarser particles. Due to differences in raw ore and production processes, the rheology of ultrafine kaolin produced in Brazil is superior to the rheology of ultrafine kaolin produced in the United States.

(2) Effect of ultrafine kaolin on coating cost

Due to the complexity of the processing steps, the production cost of the ultrafine kaolin is higher than the production cost of the coarse particle kaolin. Higher coating solid content, reducing drying requirements, saving drying energy; less adhesive migration, reducing printing spots and reducing the amount of adhesive; thus, coating cost can be reduced.

(3) Effect of ultrafine kaolin on coating runability

Ultrafine kaolin is very good for coating operation, not only has a high coating solid content, but also has less streaks applied by the blade. High speed coater for all coating methods.

(4) Effect of ultrafine kaolin on the quality of coated paper

Ultrafine kaolin gives coated paper and paperboard a higher paper gloss, lower delta gloss, lower napping speed and ink absorbency. Ultrafine kaolin can impart a higher coating solids content, faster coating cure, improved coating coverage; less adhesive migration and reduced print spots. Ultrafine kaolin increases the coating bulk density, reduces the micropore diameter of the coating and increases the number of micropores, speeding up the ink drying rate. Ultrafine kaolin produced in the United States gives the coated paper and paperboard better fiber coverage, higher whiteness and blue-white tone than the ultrafine kaolin produced in Brazil.

As an important natural new environmentally friendly pigment, ultrafine kaolin has been widely used in the field of papermaking and has an increasingly important position, which not only can significantly improve the quality of paper, increase the added value of products, but also meet the excellent quality and special Requirements for paper products. For a long time to come, ultrafine kaolin will be an ideal choice for high-grade coated paper pigments.

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