Paint matting agent silica

Evonik’s Coatings Additives business line has launched a new patented precipitated silica, SPHERILEX® DP-0115, for use as a matting powder in powder coating products.

It achieves the desired controlled matting effect in powder coatings, maintains a uniform and consistent appearance, and improves the hardness and flexibility of the final coating. The matting effect is not affected by the curing temperature and is suitable for a wide range of chemicals and formulations. This product is primarily used in ultra-durable polyester and clear acrylic resins for general industrial, automotive and architectural powder coating applications.

Schematic diagram of emulsion polymerization

For the emulsion polymerization process carried out in a fully mixed batch reactor, it can be divided into 4 stages according to the reaction mechanism, namely, the dispersion stage, the latex particle generation stage, the latex particle growth stage and the polymerization completion stage.

1. Dispersion stage
When the concentration of the emulsifier added to the reactor is greater than the critical micelle concentration (CMC), micelles are formed. When the monomer is added to the system, the monomer will be dispersed into monomer beads under the action of stirring. At the same time, because the interior of the micelle is hydrophobic, the oil-soluble monomer will decompose from the monomer droplets formed by emulsification, and enter the micelle through the migration of the water phase. There are three forms of emulsifier at this stage: dissolved in water, formed micelles and adsorbed on the surface of monomer droplets; and monomers also have three destinations: monomer droplets, dissolved in water and solubilized in micelles. There is a dynamic balance water based wax additive between these three forms of emulsifiers and monomers.

2. Latex particle formation stage
When the initiator is added, the initiator decomposes into primary free radicals in the water phase, and reacts with a small amount of monomers dissolved in water to form oligomer free radicals with surfactant properties. After this type of oligomer free radicals with surfactant properties enter the micelles, they will immediately react with oil-soluble monomers to grow into latex particles.

3. The latex particles grow up
In this type of typical emulsion polymerization reaction, there are 1013 monomer droplets per liter of emulsion, with an average size of 1 pm. Compared with the same period, there are about 1018 micelles with a diameter of about 5nm. The total interface area of ​​micelles is about three times larger than the area of ​​monomer droplets. Therefore, the oligomer free radicals in the water phase are very likely to diffuse into the monomer-containing micelles instead of monomer droplets. Therefore, the polymerization reaction only occurred in Jiaodong and the latex particles formed later, and the consumed monomer was supplemented by the decomposition and migration of monomer droplets in the water phase. The micelles gradually transformed into latex particles with a diameter of about 100 nm.