Wollastonite is a calcium metasilicate mineral, often white and off-white, with a glass luster to pearl luster. It is widely used as a reinforcing filler for polymer matrix composites because of its non-toxicity, chemical resistance, thermal stability and dimensional stability, excellent mechanical and electrical properties, and reinforcing effects. However, the compatibility of the wollastonite powder with the polymer base is poor, so the direct dispersibility is not good. After the surface modification treatment, the compatibility with the polymer base material can be improved, and the reinforcing effect is enhanced. The mechanical properties of the filled polymer-based composite material are better.
First, wollastonite modification method
The surface modification methods of wollastonite generally include surface modification of coupling agent, surface chemical modification of mechanical force and surface inorganic coating.
Second, wollastonite modification process
There are two main processes for the modification of wollastonite, namely wet modification and dry modification. The process flow chart is as follows:
Wet modification process
Dry modification process
Third, the surface modifier commonly used in wollastonite:
At present, the surface modifiers commonly used for wollastonite mainly include silane coupling agents, titanate and aluminate coupling agents, surfactants and methyl methacrylate.
1. Silane coupling modification
Silane coupling modification is one of the commonly used surface modification methods for wollastonite powder. The dry modification process is generally employed, and the amount of the coupling agent is related to the required coverage and the specific surface area of ​​the powder. When the wollastonite is treated with aminosilane, the amount is about 0.5% by weight of wollastonite; the amount of methacrylic oxysilane is 0.75% by weight of wollastonite, and the two modified products are filled with nylon 6 and polyester, respectively. Replacing 30% glass fiber can significantly improve the mechanical properties of the product.
2. Surfactant modification
The treatment of wollastonite with a silane coupling agent can greatly improve the compatibility with the polymer and enhance the filling effect, but the production cost of the silane coupling agent modification is high. Therefore, under certain application conditions, less expensive surfactants such as stearic acid (salt), quaternary ammonium salt, polyethylene glycol, higher fatty alcohol polyoxyethylene ether (nonionic surfactant), etc. may be used. The surface modification of the wollastonite powder. These surfactants cover the surface of the particles by the action of polar groups and the surface of the particles, which greatly enhances the lipophilicity of the wollastonite filler.
3. Modification of organic monomer polymerization
The polymerization test results of organic monomers in wollastonite powder/water suspension show that the polymer can be adsorbed on the surface of the particles, which changes the surface properties of the wollastonite powder without affecting its particle size and BaiDu. The wollastonite powder is used as a filler for the coating to reduce the settling property and enhance the dispersibility of the coating. The monomer currently selected for polymerization in a wollastonite powder/water suspension is methyl methacrylate.
Fourth, the factors affecting the modification of wollastonite:
1. Modifier type and dosage
The type and amount of modifier are important factors affecting the modification effect. Taking stearic acid as an example, the effect of the amount of modifier on the modification effect is as follows. As the amount of modifier increases, the settling height of the modified wollastonite in benzene also increases. When the amount of the modifier is 1.5%, the sedimentation height is the largest, that is, the modification effect at this time is optimal. Subsequently, the amount of the modifier is increased, and the modification effect is rather deteriorated.
Relationship between modifier dosage and sedimentation height
2, modification time
The effect of modification time was discussed by using 1.5% stearic acid as a modifier to keep other conditions constant. It was found that the modification effect was better than the modification effect of 10 minutes when the modification time was 15 minutes. However, as the modification time increases with time, the modification effect decreases, and it is more obvious.
3, modification temperature
Temperature also has a great influence on the effect of wet modification. Appropriate temperature is beneficial to the dispersion of the modifier in water, but the temperature is too high, but the modification effect is reduced.
4, pulp concentration
The slurry concentration during wet modification is also an important factor affecting the modification effect of wollastonite. A reasonable slurry concentration can both improve the modification effect and save costs. The slurry concentration is generally 40% is the best value. When this value is exceeded, the fluidity of the slurry is poor, which is not conducive to the sufficient dispersion of the modifier.
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