How is lightweight calcium carbonate surface modified?

　2021 Powder Surface Modification Technology Advanced Seminar will be held in Nanjing, Jiangsu Province, 2021.4.22~23, newspaper-name-1-8-3-0-1-2-1-1-6-6-0-1, applicable objects: non-metallic ore powder enterprises: calcium carbonate, silicon micropowder, talc, barite, wollastonite, kaolin, bentonite, dolomite, limestone, wollastonite, silica limestone, wollastonite, mica, diatomaceous earth, sepiolite, tourmaline, etc.; functional powder enterprises: magnesium hydroxide, aluminum hydroxide, alumina, titanium dioxide, silica black, Iron oxide red, pearlescent mica, zinc oxide, fly ash, nano powder, etc.; pharmaceutical and equipment enterprises; powder filler application enterprises; other enterprises that need powder surface modification.

　At present, the production method of lightweight calcium carbonate is mainly limestone calcination and then produced by carbonization method, as a chemical synthesis method to obtain products, its original particle size is very small, the surface energy is high, in a thermodynamic unstable state, it is easy to accumulate into clumps, thus affecting the actual use effect.

　　

　　In addition, since calcium carbonate as a hydrophilic inert powder, the surface does not exist with polymer binding active groups, so it must be treated with corresponding surface activity, reduce surface energy, increase surfactant groups, improve the wetness with the polymer interface and interaction with the polymer, the level of activation index, directly affect the physical properties of the polymer.

　　Therefore, the dispersion and modification effect of calcium carbonate directly affects its use value and application field. As the key technology of the development of calcium carbonate industry, the dispersion and modification technology of lightweight calcium carbonate even exceeds 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), in which calcium carbonate is stirred to cover its surface, and finally dried, which is generally done in light calcium carbonate or nano calcium carbonate production enterprises.

　　After the wet modification of calcium carbonate particles, their surface can be reduced, even if the secondary particles are formed after compression filtering and drying, only the soft agglomeration with weak binding force is formed, which effectively avoids the hard agglomeration phenomenon caused by the formation of chemical bond oxygen bridge in dry modification. This method is a traditional calcium carbonate surface treatment method, suitable for water-soluble surfactants, the advantages of this method are uniform coating, high production quality. However, drying needs to control a certain temperature and conditions, some surface treatment agents are insoluble in water or easy to decompose in water, and the use of other organic reagents has costs and safety issues.

　　(1) Stearic acid (salt) surfactant

　　Stearic acid (salt) surfactant is one of the commonly used surface treatment agents for calcium carbonate modification, which belongs to anionic surfactants, the structure of the long-chain alkyl group at one end of the molecule is similar to the structure of the polymer, it is an oil-loving group, so it has good compatibility with the polymer binder, and the other end is a water-soluble polar group such as carboxyl group, which can be physically and chemically adsorbed with 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 form a fatty acid calcium precipitate, which is coated on the surface of calcium carbonate, so that the surface properties of the particles change from hydrophilic to lipophilic.

　　(2) Phosphate and condensation phosphate surfactants

　　Fatty acids (esters) such as phosphate are used for surface modification of calcium carbonate, which is the use of special structure polyphosphate (ADDP) to modify the surface of calcium carbonate, calcium carbonate particles surface hydrophobic and lipophilic, the average agglomeration particle size in the oil is reduced, and the modified calcium carbonate is filled with PVC plastic system can significantly improve the processing performance and mechanical properties of plastics. Surface treatment of lightweight calcium carbonate using a mixture of stearic acid and sodium dodecylbenzenesulfonate can improve the effect of surface modification.

　　(3) Quaternary amine salt surfactants

　　Quaternary amine salts are a kind of cationic surfactant, which is positively charged at one end by electrostatic adsorption on the surface of calcium carbonate, and the other end can be crosslinked with polymers to achieve surface modification of calcium carbonate.

　　2. Light calcium carbonate dry modification

　　Dry modification is to put the calcium carbonate powder into the high-speed mixer, and then put in the surface modifier, with the help of the mixer and a certain temperature, the modifier is uniformly 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 coupling agent to be uniformly coated on the surface of calcium carbonate particles, a suitable temperature to facilitate the reaction and adsorption, calcium carbonate drying does not contain water to avoid the coupling agent from reacting with water first, rather than reacting with the -OH reaction on the surface of calcium carbonate, affecting the modification effect.

　　The surface modifier is generally a coupling agent, the coupling agent for surface modification of calcium carbonate, is the use of coupling agent at one end of the group can react with the surface of calcium carbonate, the formation of a strong chemical bond, and the other end of the coupling agent can have a certain chemical reaction or mechanical entanglement with the organic polymer, so as to closely combine the two materials with great differences in properties of calcium carbonate and organic polymer. At present, the coupling agents on the market mainly include titanate coupling agents, aluminate coupling agents, borate coupling agents and phosphate coupling agents.

　　Dry modification process process is simple, production equipment investment and production cost is low, can be directly packaged after discharge, but compared with the wet method, the degree of activation is not good, and it is difficult to uniformly coat the calcium carbonate particles with a particle size, therefore, the dry activation process is currently suitable for filler-level calcium carbonate modification treatment, for functional nano calcium carbonate needs to be further improved.

　　Source: Shang Kai. Research on the energy-saving and consumption-saving process of nano calcium carbonate co-production of ultrafine calcium carbonate [D].East China University of Science and Technology, 2018", edited and sorted by [Powder Technology Network], please indicate the source!