YuanLei Little knowledge | Application of kaolin in coatings

China is a major producer of kaolin in the world. The production of kaolin in 2020 will be about 6.5 million tons, with a year-on-year increase of 30%. It is expected that the production of kaolin in 2021 will also be stable at about 6.4 million tons.

At present, the main export provinces and cities of kaolin are Guangdong Province, Fujian Province, Shanxi Province and Jiangsu Province. In 2021, Guangdong Province will export the most kaolin with 236,600 tons, accounting for 23.8% of the total kaolin export, followed by Fujian Province with 227,900 tons. 136,400 tons in Shanxi Province; 86,000 tons of Jiangsu Province, etc.

In 2015, the forged kaolin consumption in China was about 4.37 million tons, and the largest consumption was about 4.05 million tons in the three industries of ceramics, papermaking and coatings, among which the consumption in the field of coating was 400,000 tons, accounting for 9%.

Kaolin is mainly used as a filling skeleton in the production of paint coatings. At the same time, because kaolin has excellent dispersibility, stable chemical properties, corrosion resistance and fire resistance, it can be used as a coating additive to improve the adsorption capacity and covering capacity of coatings.

The commonly used raw material in the paint industry is titanium dioxide, which is expensive and easy to produce chemical pollutants. Replacing titanium dioxide with natural mineral kaolin with stable chemical properties can reduce the cost and moderate pollution.

(1) Application in architectural coatings

Architectural coating as a building decoration, protection material coating on the surface of the building, it is required to have a certain fineness, adhesion, hiding power, stain resistance, wash resistance, aging resistance, heat resistance, leveling, short drying time, suitable for low film forming temperature. Due to the plasticity, viscosity, high strength and chemical stability of non-metallic minerals, people in the preparation and modification of building coatings, the commonly used non-metallic minerals are bentonite, diatomite, kaolin and rectorite.

With the development of the real estate industry, the global demand for architectural coatings is growing, and people have higher and higher requirements for the quality and functionality of architectural coatings. High-performance, high-grade, cost-effective coatings are becoming more and more popular. Especially for the improvement of environmental protection requirements, it is necessary to develop building coatings with anti-pollution and pollutant control performance. Kaolin has a special use in architectural coatings and it will have a broad application prospect in the modification of architectural coatings.

(2) in the paper coating industry application

The main pigments in paper coating are kaolin and CaCO. And TiO2, of which kaolin accounts for about 90%.

All kaolin used for coating is selected by either dry or wet methods, and the wet-selected product is more uniform than the dry-selected product, contains fewer impurities, and has better brightness and whiteness. The coated paper presents a strong pearlized luster for offset printing, mainly used in boutique packaging, business cards, high-end calendars.

(3) Application in latex coatings and paints

Kaolin has long been used as a filler in latex coatings and paints. This is due to its chemical inertness, good fluidity, white color and low price. At the same time, the surface is easy to modify, so that its suspension in the coating can be improved.

Kaolin is suitable for a wide range of particle sizes in the coating industry. For example, coarse-grained kaolin is used in matte or plain paints, and fine-grained kaolin is used in high-gloss paints or latex paints.

In the coating industry, kaolin is often used in two categories, one is washed ultrafine kaolin, the other is calcined ultrafine kaolin. Calcined kaolin is a new functional filler developed in recent years. As a filler for coatings, it not only has higher whiteness and opacity, but also provides better stability and color in polymers. It also has better hiding power, soft and wear-resistant, and has the characteristics of abrasion resistance and non-shrinkage.

As a white filler, kaolin itself does not have masking power. But if it is added to the white paint in a certain proportion, it can play the role of increment agent and improve the hiding power of the paint. Add appropriate amount of kaolin, reduce the amount of the main pigment, still can make the latex paint or paint cover power unchanged, and the cost of the product decreased.

(4) The application of water-based coatings

Kaolin has long been used as a filler for paints and paints because of its white color, low price, good fluidity, stable chemical properties and large cation exchange capacity on the surface. Kaolin used for paints generally has two categories: washed ultra-fine kaolin and forging ultra-fine kaolin. Among them, forged kaolin is just developed in recent years. As a new functional additive, it has good whiteness, covering, softness, wear resistance, opacity, non-shrinkage, etc., and makes the coating maintain good color and stability in high polymer. Kaolin itself is white, and it has no coverage when used as an additive. However, if it is properly added to white paint or paint, it can enhance the luster and improve its coverage ability.

It should be noted that the ratio between the substrate of water-based coating and the powdery material in the coating also has a great influence on the hiding power of kaolin. The reason is that the refractive index of the coating substrate is close to kaolin. When the amount of substrate is large, the kaolin particle interface will be more covered by it, thus reducing the refraction, reflection and scattering generated by the interface, and thus reducing the hiding power.