Lightweight calcium carbonate surface modification commonly used agents, processes and effect evaluation indicators

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　　Lightweight calcium carbonate due to small particle size, high surface energy, intermolecular force, electrostatic action, hydrogen bond, oxygen bridge, etc. caused by calcium carbonate particles are easy to agglomerate, as a filler, will affect the actual use effect; in addition, calcium carbonate surface has a strong hydrophilic -oh, alkaline, is a hydrophilic powder, unevenly dispersed in polymers, so the application must be modified on its surface, reduce surface energy, increase surfactant groups, improve the wetness of the polymer interface and interaction with the polymer.

　　

　　The physical properties of the polymer are affected by the degree of activation, and the level of activation, in addition to being related to the modifier, the key point is whether the calcium carbonate particles are really dispersed. Therefore, the degree of dispersion of calcium carbonate and the quality of the modification effect directly affect its use value and application field.

　　1. Wet modification of commonly used agents and processes

　　Wet activation is to add an activator to a solvent (such as water), in which calcium carbonate is stirred to make its surface coated, and finally a thousand dry, 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.

　　Yue Linhai et al. reported the preparation of composite calcium carbonate by co-precipitation by using sodium stearate saponification solution as a medium. Jin Ruidi et al. studied sodium stearate in situ modified calcium carbonate, under the condition of the presence of modifiers, calcium hydroxide was carbonized to prepare modified calcium carbonate, indicating that the hydrophobicity is due to the fact that sodium stearate binds to the surface of calcium carbonate in the form of ionic bonds to form insoluble calcium stearate.

　　(2) Phosphate and condensation phosphate surfactants

　　Fatty acids (esters) such as phosphate for surface modification of calcium carbonate, is the use of special structure of polyphosphate (addp) after the surface modification of calcium carbonate, calcium carbonate particles surface hydrophobic and lipophilic, the average agglomeration particle size in the oil is reduced, the modified calcium carbonate filled in 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.

　　Zhang Zhihong et al. used the new cationic surfactant cetyl dimethyl allyl ammonium chloride (cdaac) to organically modify calcium carbonate, and the modified product was used as a rubber filler to obtain good results.

　　2. Dry modification of commonly used agents and processes

　　The dry modification process 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, the appropriate 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

　　3. Evaluation of the modified effect of calcium carbonate

　　The evaluation of the effect of modified calcium carbonate can be roughly divided into two categories: direct method and indirect method. The indirect method refers to the modified calcium carbonate filler and the application system to determine the application performance of the application system. The direct method refers to the determination of the surface physicochemical properties of modified calcium carbonate, such as activation, specific surface area, oil absorption value, coating amount and surface structure, morphology, etc.

　　(1) Degree of activation

　　Inorganic fillers are generally relatively dense, and the surface is hydrophilic and settles naturally in water, while the surface modified inorganic fillers change from hydrophilic to hydrophobic. This water-repellent fine particle, due to the huge surface tension, floats unsinkable in the water. Based on this phenomenon, the concept of activation degree is proposed, which is expressed in ω.

　　ω = weight of the floating part of the sample (g) / Total weight of the sample (g)

　　The change process of ω from 0-100% reflects the degree of surface activation of modified calcium carbonate from small to large.

　　The test method is as follows, weighing about 5g of specimens, accurate to 0.01g, 250ml of dispensing funnel, adding 200ml of water, vibrating back and forth at a speed of 120 times / min for 1min, gently placed on the funnel rack, standing for 20-30min, after obvious layering, the sinking calcium carbonate will be put into a constant weight (accurate to 0.001g) glass sand crucible pre-105±5 °C, pumping and filtering water, placed in a constant temperature drying box, and dried to constant weight at 105±5 °C. Accurate to 0.001g.

　　(2) Specific surface area

　　In addition to improving activity, the surface modification process can also effectively prevent secondary agglomeration. Unmodified nano-calcium carbonate particles are prone to hard agglomeration between them, and the specific surface area is small. After the surface modification, the agglomeration phenomenon of calcium carbonate particles has been greatly improved, and the specific surface area has increased significantly. The larger the specific surface area, the better the dispersion and dispersion of the particles. This is because the surface of the modified nano-calcium carbonate particles is coated with a layer of modifier, and the surface energy is reduced, so that the particles are in a stable state. Even if some particles are reunited together, their reunion with each other is a soft reunion, and this reunion is easier to open.

　　(3) Oil absorption value

　　The oil absorption value is related to the size, dispersion, degree of agglomeration, specific surface area and surface properties of calcium carbonate particles. The oil absorption value is an important quality that affects the practical application of modified calcium carbonate, especially for the coatings, plastics, and ink industries. If the oil absorption value is large, the application to the paint and ink industry will increase the viscosity, and the application to the plastic industry will increase the consumption of plasticizers, so the oil absorption value should be low.

　　Source: Shang Kai. Study on energy saving and consumption reduction process of nano calcium carbonate co-production of ultrafine calcium carbonate[d].East China University of Science and Technology, 2018.