Currently, the main method for producing lightweight calcium carbonate is to produce calcium carbonate through the carbonation method after calcining limestone. As a product obtained through chemical synthesis, its original particle size is very small, its surface energy is high, and it is in a thermodynamically unstable state, making it very easy to agglomerate, thus affecting its practical use.

Currently, the main method for producing light calcium carbonate is to produce calcium carbonate through the carbonation method after calcining limestone. As a chemically synthesized product, its original particle size is very fine, with high surface energy, and it is in a thermodynamically unstable state, easily aggregating, thus affecting the actual use effect.

　　In addition, since calcium carbonate is a hydrophilic inert powder, there are no active groups on the surface that can combine with polymers. Therefore, corresponding surface activation treatment is necessary to reduce the surface energy, increase the surface active groups, improve the wettability of the interface with the polymer and the interaction with the polymer, and the high and low activation index directly affects the physical properties of the polymer.

　　Therefore, the dispersion and modification effects of calcium carbonate directly affect its use value and application fields. The dispersion and modification technology of light calcium carbonate, as a key technology for the development of the calcium carbonate industry, its status 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 the calcium carbonate in it to coat its surface, and finally dry it. This is generally done in light calcium carbonate or nano-calcium carbonate production enterprises.

　　After wet modification treatment, the surface energy of calcium carbonate particles is reduced. Even after filter pressing and drying to form secondary particles, only weakly bonded soft agglomerates are formed, effectively avoiding the hard agglomeration phenomenon caused by the formation of chemical bond oxygen bridges in dry modification.

　　This method is a traditional surface treatment method for calcium carbonate, suitable for water-soluble surfactants. The advantages of this method are uniform coating and high production quality. However, drying needs to control certain temperatures and conditions. Some surface treatment agents are insoluble in water or easily decompose in water, and the use of other organic reagents has 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 belongs to anionic surfactants. The structure of the long-chain alkyl group at one end of the molecule is similar to the polymer structure, which is a lipophilic group, so it has good compatibility with the polymer matrix. The other end is a water-soluble polar group such as a carboxyl group, which can physically and chemically adsorb to 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 components such as Ca2+ to generate calcium fatty acid precipitates, which coat the surface of calcium carbonate, making the surface properties of the particles change from hydrophilic to lipophilic.

　　(2) Phosphate and condensed phosphoric acid

　　Fatty acids (esters) such as phosphates are used for surface modification of calcium carbonate. After surface modification of calcium carbonate using special structured polyphosphate esters (ADDP), the surface of calcium carbonate particles becomes hydrophobic and lipophilic, the average agglomerate particle size in oil is reduced, and the modified calcium carbonate is filled into the PVC plastic system, which can significantly improve the processing performance and mechanical properties of the plastic. The mixed use of stearic acid and sodium dodecylbenzenesulfonate to surface treat light calcium carbonate can improve the effect of surface modification.

　　(3) Quaternary ammonium salts

　　Quaternary ammonium salts are a kind of cationic surfactant. One end with a positive charge is adsorbed on the surface of calcium carbonate through electrostatic adsorption, and the other end can be cross-linked with the polymer to achieve 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, then add a surface modifier, and use the mixer and a certain temperature to make the modifier uniformly adsorbed on the surface of calcium carbonate particles to achieve the modification effect.

　　The key technical requirements of the dry modification process are: fast mixing to facilitate the uniform coating of the coupling agent on the surface of calcium carbonate particles, suitable temperature to facilitate the reaction and adsorption, and calcium carbonate drying without water to prevent the coupling agent from reacting with water first instead of with the -OH on the surface of calcium carbonate, affecting the modification effect.

　　Surface modifiers are generally coupling agents. The surface modification of calcium carbonate by coupling agents utilizes the fact that one end of the coupling agent can react with the surface of calcium carbonate to form strong chemical bonds, while the other end of the coupling agent can undergo some chemical reaction or mechanical entanglement with organic polymers, thus closely combining calcium carbonate and organic polymers, two materials with extremely different properties. Currently, the main coupling agents on the market 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 cost, and can be directly packaged after discharge. However, compared with the wet method, the activation degree is poor, and it is difficult to uniformly coat the surface of the primary particle size calcium carbonate particles. Therefore, the dry activation process is currently suitable for the modification of filler-grade calcium carbonate, and further improvement is needed for functional nano-calcium carbonate.