As calcium carbonate is a hydrophilic inert powder, there is no active group on the surface that binds to the polymer, so corresponding surface active treatment must be done to reduce the surface energy, increase the surface active group, and improve the interface with the polymer. The wettability and the interaction with the polymer, the level of the activation index, directly affects the physical properties of the polymer.
Therefore, the dispersion and modification effect of calcium carbonate directly affect its use value and application field. As the key technology for the development of the calcium carbonate industry, the dispersion and modification technology of light calcium carbonate even surpasses the production process of calcium carbonate itself.
1. Wet modification of light calcium carbonate
Wet activation is to add an activator to a solvent (such as water), stir calcium carbonate in it to coat the surface, and finally dry it, which is generally done in light calcium carbonate or nano-calcium carbonate production enterprises.
After the calcium carbonate particles are modified by the wet method, their surface energy is reduced. Even if they are filtered and dried to form secondary particles, they only form soft agglomerates with weak binding force, which effectively avoids the chemical bond oxygen in the dry modification. hard agglomeration caused by the formation of bridges.
This method is a traditional calcium carbonate surface treatment method, which is suitable for water-soluble surfactants. The advantages of this method are uniform coating and high production quality. However, drying needs to control certain temperature and conditions. Some surface treatment agents are insoluble in water or easily decomposed in water, and other organic reagents have cost and safety issues.
(1) Stearic acid (salt)
Stearic acid (salt) is one of the commonly used surface treatment agents for calcium carbonate modification. It is an anionic surfactant. The structure of the long-chain alkyl group at one end of the molecule is similar to that of the polymer structure, and it is a lipophilic group. It has good compatibility, and the other end is a water-soluble polar group such as a carboxyl group, which can physically and chemically adsorb on the surface of inorganic fillers such as calcium carbonate.
The specific reaction mechanism of stearic acid (salt) modified calcium carbonate is that under alkaline conditions, ROOH- reacts with Ca2+ and other components to form fatty acid calcium precipitates, which are coated on the surface of calcium carbonate, so that the surface properties of the particles are changed from pro- Water becomes lipophilic.
(2) Phosphate and condensed phosphoric acid
Fatty acid (ester) such as phosphate is used for surface modification of calcium carbonate. The particle size of the agglomeration is reduced, and the processing and mechanical properties of the plastic can be significantly improved by filling the modified calcium carbonate in the PVC plastic system. Mixed use of stearic acid and sodium dodecyl benzene sulfonate for surface treatment of light calcium carbonate can improve the effect of surface modification.
(3) Quaternary ammonium salts
Quaternary ammonium salt is a kind of cationic surfactant, its positively charged end is electrostatically adsorbed on the surface of calcium carbonate, and the other end can be cross-linked with polymer to realize the surface modification of calcium carbonate.
2. Dry modification of light calcium carbonate
Dry modification is to put calcium carbonate powder into a high-speed mixer, and then add a surface modifier. With the help of the mixer and a certain temperature, the modifier is evenly adsorbed on the surface of the calcium carbonate particles to achieve the modification effect.
The key technical requirements of the dry modification process are: rapid mixing to facilitate the uniform coating of the coupling agent on the surface of the calcium carbonate particles, appropriate temperature to facilitate the reaction and adsorption, and drying of the calcium carbonate without moisture to avoid the coupling agent It reacts with water first, instead of reacting with -OH on the surface of calcium carbonate, which affects the modification effect.
The surface modifier is generally a coupling agent. The coupling agent modifies the surface of calcium carbonate by using the group at one end of the coupling agent to react with the surface of calcium carbonate to form a strong chemical bond. The other end of the organic polymer can undergo some chemical reaction or mechanical entanglement, so that the two materials with very different properties of calcium carbonate and organic polymer are closely combined. At present, the coupling agents on the market mainly include titanate coupling agents, aluminate coupling agents, borate coupling agents and phosphate coupling agents.
The dry modification process has a simple processing process, low investment in production equipment and production costs, and can be packaged directly after discharging. Therefore, the dry activation process is currently suitable for the modification of filler-grade calcium carbonate, and it needs to be further improved for functional nano-calcium carbonate.