Also known as china clay, kaolin clay is a soft-textured white clay essential in porcelain and china manufacturing. Besides that, this kind of clay is also used to make beauty, paper, paint, and other products. Historically, the name “kaolin” is derived from a hill in China called Gaoling, where the mineral was mined for hundreds of years. For the first time, kaolin samples were then delivered to Europe by a Jesuit missionary from France in the 18th century.
Today, kaolin or kaolinite is mined not only in its original places, China and France, but also in some other countries such as Malaysia, Vietnam, Pakistan, Brazil, the US, Australia, Bulgaria, Germany, Bangladesh, the UK, Iran, India, South Korea, the Czech Republic, Spain, Tanzania, South Africa, and of course Indonesia.
Kaolin in its natural state is some powder that is white and soft. It mainly consists of kaolinite, a mineral that resembles flaky layers of hexagonal crystals when seen through an electron microscope. The size of the crystals ranges between 0.1 micrometre and 10 micrometres, or possibly even larger. Sometimes, they may look vermicular and size in millimetres.
The mineral may contain a variety of other minerals like feldspar, muscovite, quartz, and anatase. There is also crude kaolin which tends to look yellowish as it is stained by pigments of iron hydroxide. The stained kaolin is usually chemically bleached for commercial use to get rid of the iron pigment and other minerals.
Under thermal treatment, kaolin clay undergoes several transformations as follows:
At the temperature of 550–600 °C, kaolinite begins its endothermic dehydration, resulting in irregular metakaolin. However, it turns out that at 900 °C, the mineral experiences constant hydroxyl loss. This form is amorphous solid (non-crystalline) or widely known as glassy solid.
The milling process means mechano-chemically morphing a mineral. Like metakaolin, kaolinite milling turns it into an amorphous solid, even though its properties are different. It requires a lot of energy to transform this type of kaolin into metakaolin.
Being exposed to some dry air means the kaolin has been transformed below 100 °C. The ultimate result of this process is frequently called “leather dry”, while under 550 °C, the mineral will be drained from any remaining water. Thus, the latter often results in what is known as “bone dry.” However, the water content may be reversible if the kaolinite is exposed to it.
When heated up to 950 °C, metakaolin will be transformed into a spinel with aluminium-silicon characteristics.
5. Needle Mullite
At 1400 °C, the kaolinite will appear to be a “needle” mullite form. It offers heat resistance and structural strength to the transformed mineral.
6. Platelet Mullite
Above 1050 °C, the thermal treatment will produce a spinel phase, transforming into platelet mullite. It means it has high characteristics of crystals.
Kaolin clay is mainly used to produce glossy material or coating on some grades of paper. Also known for medical uses, kaolinite is capable of healing blood clots. In addition, the multifunctional mineral is also used to produce:
- a component of light bulbs
- medical products
- paints, coatings and many more.
Kaolin Clay as Paints & Coatings
It has been proven that kaolin is an excellent additional substance for white (titanium dioxide) pigment contained in paint. The mineral’s hardness also adds fire resistance and durability to the paint film. The following list tells how kaolin can function in the paint industry.
1. Architectural Coatings
For architectural coating, kaolin offers the strength and opacity of the paint, resistance against scrubs and stains, and an improvement in pigment suspension. As an extender, the mineral can also help keep the paint and coating manufacturing costs down.
2. Industrial Coatings
Having been a great TiO2 extender, kaolin clay as the paint is also used for equipment, machinery and metal finishing to wooden furniture. The “clay” is reputable for becoming color and special effect pigments and a premium-quality mineral additive as an industrial coating.
3. Automotive Coatings
For many vehicles, kaolinite plays a significant role in giving their paint some excellent qualities such as film smoothness, tank stability, resistance against corrosion and film uniformity.
A good ink color or effect should provide brightness and resistance. It should be able to make many packages, products and labels to stand out. For the printing industry, inks made with kaolinite will also save costs while performing as high as you want. Other advantages of using kaolinite as inks are low abrasion, pigment extender, durability, rheology control and quick drying process. Most importantly, the kaolin-extended ink could improve ink quality, increase company profit and add efficiency for manufacturing costs.
The Advantages of Using Kaolinite as Paint
As mentioned before, kaolin is a variety of natural minerals turned into filling materials. After the shredding process and separation, the mineral will be turned into insoluble paint when applied to any medium. As for filling material, kaolin can essentially contribute well to the quality and mechanism of the paint it extends. In the meantime, it can also reduce formulation costs. Other advantages of adding kaolin as an element of paints and coatings are as follows.
- Improving film formation
- Preventing rapid collapse prevention
- Increasing resistance against the exposure of weathers
- Reducing passage of moisture
- While reducing titanium dioxide cost, kaolin shares the same quality features
- Despite being economical, it contributes value to paint quality and performance
- The mineral can be a part of either water-based or solvent-based paint and electrophoretic inks and paints.
- Its whiteness provides uniformity to the paint particle distribution
- Giving out high gloss and coating that may add production quality
Kaolin clay has plenty of advantages to offer, especially to the paint industry. It provides high quality and mechanism to the paint it extends, but it can also keep production costs down. Not to mention that both the mineral and titanium dioxide nearly share similar features. Furthermore, kaolin is also not harmful to people exposed to it in their workplace. They can breathe in the powder form of kaolin without any worries.