Bentonite
Bentonite is a lamellar silicate, the main component of which is montmorillonite. The cations between the interlayers are easily exchanged, and it has a large ion exchange capacity. "Bentonite" comes from the place name of the clay producing area in Wyoming, USA. It is also called bentonite or "bentonite rock". Pure bentonite is rare, and most of them contain varying amounts of impurities, such as quartz, feldspar, mica, zeolite, calcium carbonate, etc. Bentonite is usually white, but also light gray, green, blue, red, brown, brick red, brown, black, variegated, etc. It has a self-slip luster, a fibrous luster or an earthy luster, and a shell-like or serrated fracture. Bentonite is known as "universal clay" because of its good physical and chemical properties. It can be used as a binder, suspending agent, thixotropic agent, stabilizer, purifying and decolorizing agent, filler, feed, catalyst, etc. It is widely used in metallurgy, petroleum, casting, food, chemical industry, environmental protection and other industrial sectors. The content of bentonite is generally above 65%, so its properties determine the properties of bentonite. The main components of bentonite are silicon dioxide and aluminum oxide, and contain a small amount of iron, calcium, magnesium, sodium, iron and other ions. Its chemical formula is: (Na,Ca)0.33(Al,Mg,Fe)2(Si,Al)4O10(OH)2·nH2O. Its structure is a 2:1 type crystal structure composed of two silicon oxygen tetrahedrons sandwiching a layer of aluminum oxygen octahedron.
Five advantages of bentonite in coating applications
1. It has a thickening effect:
Bentonite forms flocculent substances in water and has a thickening effect. Its molecular groups can combine with the organic base material in the coating, which can enhance the water resistance of the coating and improve the strength and adhesion of the coating.
2. Suspension and dispersibility:
Bentonite has good suspension and dispersibility in water, which makes the paint not precipitate, not easy to stratify, and uniform in color, thereby improving the stability of the paint suspension, and has good brushability, and can form a flat, uniform, smooth coating. Based on this, a certain amount of polyamide powder can be added to the paint to reduce costs.
3. Water resistance and moisture resistance:
Bentonite has certain water resistance and moisture resistance, and is inexpensive. Adding it to polyvinyl alcohol series glue can greatly reduce the amount of polyvinyl alcohol and its fillers (such as ultrafine calcium carbonate), thereby reducing product costs.
4. Hydrophilicity, plasticity, swelling, and adhesion:
Bentonite has good hydrophilicity, plasticity, swelling, and adhesion. It can combine with an appropriate amount of water to form a colloid. It can release charged ions in water. The electrical repulsion between ions makes it act as a dispersant in the coating. In waterproof coatings, appropriate bentonite particles are needed to collide and cut with asphalt to break and gelatinize the asphalt. The polarity of the gelled asphalt particles helps to adsorb the bentonite particles, so that a colloidal water film is formed on the surface of the asphalt particles, reducing the interfacial tension between asphalt and water, preventing the asphalt particles from re-condensing, thereby forming a stable dispersed phase and playing the role of an emulsifier.
5. Anti-mildew:
Bentonite is anti-mildew, and the prepared coating can still be used at lower temperatures in winter.
