Wollastonite is a fibrous silicate mineral with a needle-like structure and high whiteness. It has a low coefficient of thermal expansion, excellent chemical stability, and flame-retardant properties. It also has high electrical insulation, making it suitable for various applications. However, natural wollastonite is hydrophilic. To enhance resource utilization and add value, modified wollastonite is necessary.
The Significance of Modified Wollastonite Powder
Wollastonite, especially ultrafine high aspect ratio wollastonite, is widely used in plastics, rubber, and fibers. It is also used as a filler and pigment in coatings, adhesives, and composite materials. As an inorganic material, wollastonite lowers cost and enhances stability, rigidity, flame resistance, and insulation. However, its poor compatibility with organic polymers causes difficulties in dispersion and interface bonding. Excessive addition may decrease mechanical strength and lead to brittleness.
Therefore, physical, chemical, or mechanical methods are required to modify its surface. This modification improves compatibility with polymers and resins and enhances dispersion. It also increases mechanical strength and optical performance in paints and coatings.
Surface Modification Method of Wollastonite
There are various methods for surface modification of mineral powders, classified into physical, chemical, and coating methods. Based on specific processes, surface modification methods include dry, wet, and hybrid methods. According to the nature, means, and objectives of comprehensive modification, methods include coating, precipitation reaction, surface chemistry, grafting, and mechanochemical methods.
Common modification methods for wollastonite include inorganic precipitation, mechanochemical reaction, and organic reaction coating.
Inorganic Precipitation Reaction Modification Method
This method uses chemical reactions to form a modification layer on the needle-like wollastonite surface. The functional structural units of the modification layer improve the surface properties of the wollastonite. It is a simple, easy-to-operate method that achieves modification at a relatively low cost.
Mechanochemical Reaction
This is a composite improvement method, where mechanical stirring and shearing first crush and granulate the wollastonite particles. The mechanical force significantly increases the aspect ratio of the needle-like wollastonite from tens to around a thousand, improving uniformity. Afterward, chemical modification methods are used to obtain modified wollastonite materials. This approach effectively reduces modification costs, simplifies the process, and requires relatively simple equipment and conditions. It shows promising application prospects and market value.
Organic Reaction Coating Method
The organic reaction coating method uses the functional groups of organic compounds to react with the surface of wollastonite. This results in a coating on the wollastonite particles and improves their surface properties. Typically, coupling agents with both oleophilic and polar groups are chosen as modifiers. The oleophilic groups enhance compatibility between wollastonite and polymers in composite fillers, while polar groups stabilize the modifier on the surface.
Common coupling agents include silane coupling agents, stearic acid, sebacic acid, and titanates. Different coupling agents with varying functional groups can significantly affect the filling rate and uniformity of inorganic fillers. Using two or more different types of coupling agents in varying concentrations and ratios can noticeably improve the properties of inorganic fillers.
Conclusion
In recent years, the demand for wollastonite has continued to grow both domestically and internationally, with prices steadily increasing. Wollastonite is increasingly linked to high-tech industries such as energy conservation, environmental protection, electronics, biotechnology, aerospace, and new materials and energy. Research on surface modification of wollastonite and ultrafine-grade wollastonite powders should continue to expand, aiming to explore new application areas for wollastonite.
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