Surface modification is an important processing technology to improve the application performance of calcium carbonate, improve the applicability, expand the market and the amount of the necessary, at present, there are mainly inorganic surface treatment, organic surface modification, polymer surface modification, mechanical force chemical modification and high-energy surface modification and other five categories of methods.

1, Calcium carbonate inorganic surface treatment

Calcium carbonate has the disadvantages of poor acid resistance and high pH value, so that its application range and usage are greatly limited. Calcium carbonate by inorganic surface modification treatment, can make the calcium carbonate surface produce double electrostatic layer, thereby reducing the mutual repulsion between particles, reduce the accumulation phenomenon. At the same time, the hydrophilicity of calcium carbonate is increased, so that calcium carbonate can be well wetted in water, thereby increasing the dispersion performance and expanding the scope of application.

Do Su Kim et al. made amorphous silicon precipitate on the surface of calcium carbonate by hydrolysis of silicofluoric acid ions, which improved the acid resistance of calcium carbonate.

2, calcium carbonate organic surface modification

Organic modification mainly refers to the chemical or physical method, the organic modifier wrapped in the surface of calcium carbonate, so that the surface properties of calcium carbonate change, so as to increase the interface compatibility, reduce the powder reunion, is the most mainstream surface modification method of calcium carbonate.

Commonly used surface modifiers are: fatty acids (salts), phosphate esters, quaternary ammonium salts, silane coupling agents, titanate coupling agents, aluminate coupling agents and composite coupling agents.

3. Surface modification of calcium carbonate polymer

Polymer surface modification refers to the formation of a core-shell polymer layer on the surface of calcium carbonate particles. There are mainly two cases of polymer modified calcium carbonate: one is that the monomer on the surface of calcium carbonate forms polymer chain segments through polymerization reaction; the other is that the polymer is dissolved in an appropriate solvent to form a polymer solution, and calcium carbonate is added to it. When the polymer is gradually adsorbed on the surface of calcium carbonate, the solvent is removed to form a coating. In this way, the polymer can be adsorbed to the surface of calcium carbonate, form an effective adsorption layer, reduce the agglomeration of calcium carbonate particles, improve the dispersion, improve the shortcomings of poor dispersion performance of calcium carbonate in the process of use, and achieve the purpose of surface modification.

4. Mechanical and chemical modification of calcium carbonate

Mechanical and chemical modification is mainly the use of strong mechanical force to activate the surface of calcium carbonate particles, the crystal structure of the surface of calcium carbonate changes, so that the lattice moves to a certain extent, so that the reactivity with other substances is enhanced.

This method is mainly aimed at the effect of calcium carbonate with relatively large particles, but for nano calcium carbonate, due to its small particle size, the improvement effect of mechanical force is not very good, but it can activate some active sites and groups on the surface and enhance the interaction with organic surface modifiers. Therefore, nano calcium carbonate can be modified by a combination of mechanical chemistry and other modification methods.

5. High-energy surface modification of calcium carbonate

High-energy surface modification mainly uses high-energy rays, plasma and other methods to modify the surface of inorganic powders. The method mainly relies on high-energy rays and plasma sources to bombard and touch the surface of calcium carbonate, so that some reactive sites are generated on the surface of calcium carbonate, and then an unsaturated monomer (such as a vinyl monomer) is added. The unsaturated monomer can react with the active sites on the surface to form a coated organic film on the surface of the inorganic particles.

However, the cost of this method is high, the effect after modification is not very stable, and the application is limited to a certain extent.

The surface modification of calcium carbonate powder is mainly to solve three problems:

① Dispersion of calcium carbonate

The problem of interface bonding with organic polymer materials

③ Functionalization and specialization after surface modification

Therefore, in the choice of surface modifier should consider the above points, and according to the structure of the surface modifier, properties and its mechanism of action with calcium carbonate powder, should also consider the organic polymer products of the base material properties, formula, process technology requirements and the physical and chemical properties of calcium carbonate, the correct and targeted choice. In addition, in the selection of modifiers, not only the coupling agent for surface modification, but also according to the properties of calcium carbonate powder and the formulation and process technology requirements of organic polymer products, a certain amount of other auxiliary modifiers and additives are added to cooperate with the coupling agent for more effective surface modification, so that the performance and quality of modified calcium carbonate powder products meet higher requirements and the cost of products is lower.

The calcium carbonate powder modified by the coupling agent will produce a small part of the agglomerated particles, which will have a certain impact on the quality of the product, so it must be effectively classified, and only in this way can the quality of the product be fully guaranteed.

Common surface modifiers of calcium carbonate include stearic acid (salt), titanate coupling agent, aluminate coupling agent, etc.

　　01

Stearic acid (salt)

Stearic acid (salt) is the most commonly used surface modifier of calcium carbonate, dry process can be directly added to stearic acid, wet process to first saponify stearic acid or use stearate, such as sodium stearate.

In order to make stearic acid better dispersed and uniformly interact with calcium carbonate particles, stearic acid can also be diluted with a solvent (such as anhydrous ethanol) in advance, and other additives can also be added in appropriate amounts during modification.

Activated calcium carbonate modified with stearic acid (salt) is mainly used to fill PVC plastics, cable materials, adhesives, inks, coatings, etc.

　　02

titanate coupling agent

The molecular structure of titanate coupling agent is divided into six functional areas, each of which has its own characteristics. After understanding its characteristics, it can be based on the characteristics of the powder to be treated and the application field, to choose the flexibility to meet the various requirements of the titanate coupling agent.

Titanate coupling agents are divided into single alkoxy type, chelating type, coordination type:

(1) the characteristics of the single alkoxy type is multi-functional, wide range of adaptation, mainly to adapt to the treatment of dry calcium carbonate powder.

(2) Chelating type contains ethylene glycol chelating group, which is suitable for surface modification of calcium carbonate powder with certain water content.

(3) The coordination type has good water resistance, most of which are insoluble in water and do not undergo ester exchange reaction. It is suitable for surface modification of various powders.

Calcium carbonate treated with titanate coupling agent has good compatibility with polymer molecules. At the same time, because the titanate coupling agent can form molecular bridges between calcium carbonate molecules and polymer molecules, the interaction between organic polymers or resins and calcium carbonate can be enhanced, and the mechanical properties of thermoplastic filled composites can be improved, such as impact strength, tensile strength, bending strength and elongation. Compared with the untreated calcium carbonate filler or the calcium carbonate treated with stearic acid (salt), the properties of the modified calcium carbonate coated with titanate coupling agent are obviously improved.

　　03

aluminate coupling agent

Aluminate coupling agent is rarely used before because of easy hydrolysis. In recent years, the aluminate produced by manufacturers has adopted a special structure that partially meets the coordination number of the central aluminum atom, which greatly improves the quality of its products. The aluminate coupling agent is light in color, has high thermal stability coordination effect and lubrication plasticization effect, and has a wide range of applications.