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Wollastonite is a calcium metasilicate mineral with the chemical formula of Ca [SiO3], containing 48.3% CaO and 25.7% SiO2. Natural wollastonite is usually white to grayish white, with glass to pearl
發布時間: 2016-10-31 01:14:51
Wollastonite is a calcium metasilicate mineral with the chemical formula of Ca [SiO3], containing 48.3% CaO and 25.7% SiO2. Natural wollastonite is usually white to grayish white, with glass to pearl luster, density of 2.78 ～ 2.91g/cm3, hardness of 4.5 ～ 5.0, usually needle shaped, radial, fibrous aggregates, and even tiny particles still maintain fiber structure.
Wollastonite products can be divided into two categories: one is wollastonite products with high length to diameter ratio, which mainly uses its needle like performance as a reinforcing agent for fillers, especially in plastics, rubber, paint, coatings, etc. Wollastonite can increase the hardness, bending strength, impact resistance, improve the electrical properties of plastics, and improve the thermal stability and dimensional stability of materials; the other is finely ground wollastonite powder, which mainly uses its mineral chemical properties, Used in ceramic and metallurgical industries, the CaO and SiO2 components in wollastonite provide it with low thermal expansion and impact resistance.
1 Reinforcing materials
Wollastonite is used as reinforcement material by taking advantage of the fibrous structure of its crystalline powder. The fibrous wollastonite powder is uniformly mixed with polymer matrix materials such as plastics and rubber, and then is orderly dispersed in the matrix material in the form of fibers. The fibrous crystalline wollastonite powder can resist large radial damage stress under external force, and has high thermal stability. The mechanical properties The aging property and dimensional stability can be enhanced. Under the same performance requirements, the amount of polymer matrix materials with higher price can be reduced accordingly, which can significantly reduce the manufacturing cost of composite materials. At present, the matrix materials with good reinforcement effect of wollastonite include nylon 6, nylon 66, unsaturated polyester, PVC, natural rubber, etc.
Liu Weiping modified wollastonite with silane coupling agent first and then used it to modify nylon 6, which can completely or partially replace glass fiber. The test results are shown in Table 1.
It can be seen from Table 1 that when the mass ratio of wollastonite to nylon 6 is 40/60, the tensile strength, notch impact strength, bending modulus and hot deformation temperature of the resin are increased by 23.64%, 62.96%, 141.2% and 84.62% respectively compared with those of pure nylon 6 resin; When the mass ratio of wollastonite/glass fiber/nylon 6 is 20/20/60, the tensile strength, notched impact strength, bending modulus and hot deformation temperature of nylon 6 resin are increased by 109.1%, 185.2%, 324.7% and 204.6% respectively.
Wollastonite also has good application effect in unsaturated polyester. Hu Shan et al. modified 325 mesh ordinary wollastonite with KH-570 silane coupling agent at the dosage of 0.8%~1.0%, and filled unsaturated polyester with the dosage of 40%~50%. The tensile strength and bending strength of the material can both be increased by more than one time, while the shrinkage of the product decreases.
Wu Xueming et al. used wollastonite modified by methacrylic acid to modify rigid PVC. When the dosage of wollastonite/PVC is 50/50, the impact strength and tensile strength of the product are respectively 128% and 9% higher than those of pure PVC resin materials.
2 Ultra white powder material
Wollastonite has a high whiteness, which can generally reach 95, and the highest whiteness can reach 102~104. It can be used as a special white powder material for paint, filler and other systems. It has the characteristics of low oil absorption, easy dispersion, stable storage of the system, good leveling property of the coating and strong adhesion, and can partially replace expensive titanium white; It can completely replace talc powder, precipitated barium sulfate and zinc oxide. In the United States, about 9% of wollastonite is used for white semi gloss enamel and polyvinyl acetate emulsion paint, and about 13% is used for primer and sealer.
3 Bioactive ceramic powder materials
The high-purity wollastonite prepared by the solution reaction roasting method can be used as the precursor powder material for bioactive ceramics. The wollastonite ceramics can be obtained by forming the powder under 10~20MPa pressure and calcining at 1350 ℃/4h. This kind of ceramics has the characteristics of high strength, good biological activity and harmless to human body. For example, the prepared bioceramics can quickly form a carbonated hydroxyapatite layer on their surface in vitro bioactivity evaluation, which is convenient for the growth of tissues and blood vessels after implantation into the human body, thus strengthening the bonding strength between the implanted human materials and the surrounding tissue interface, accelerating the reconstruction of human tissues and wound healing, and the bioactive ceramics can be used for the repair of human hard tissues (bones, teeth).
Lu Shengliang and others studied the application of wollastonite in high-voltage electric porcelain. The research shows that the sintering temperature decreases obviously with the addition of wollastonite, and the greater the amount of wollastonite added, the greater the degree of decline. However, with the increase of wollastonite addition, the sintering temperature becomes narrow, which can be eliminated by adding a certain amount of powdered quartz or bauxite. Adding about 5% wollastonite to ordinary high voltage electric porcelain and medium and high strength high voltage electric porcelain has achieved good results. The sintering temperature is reduced by more than 60 ℃, and the performance reaches or exceeds the national standard.
4 Building materials
The application of low-grade wollastonite in building materials is an important way. The firing temperature and firing time of glazed tiles made of wollastonite can be reduced by 150~180 ℃ and 23~40h respectively compared with ordinary glazed tiles, and the cost can be reduced by 16%~23% accordingly; Colored glass, colored glass mosaic and other products fired with wollastonite have good optical and alkali resistance, and the product cost is low.
Hao Desheng et al. used the mineral raw materials of the wollastonite limestone mixed deposit to produce white cement. The experiment shows that it is completely feasible to produce white cement by using wollastonite instead of high-quality clay and introducing SiO2. The white cement clinker produced has high C3S content and features rapid setting and early strength. The introduction of wollastonite into the production of SiO2 white cement, roughly calculated from the theoretical point of view, will consume less energy than the ordinary method in the production of white cement.
5 Preparation of silicon dioxide
Wollastonite resources in China are very rich, characterized by high calcium metasilicate content and few colored impurities. It is easy to react with inorganic acids to generate precipitated silica, which is an ideal natural silicon raw material for preparing white carbon black. There are two methods: one is to react wollastonite with acid and then directly separate the solid from the liquid to produce white carbon black; The other is to convert wollastonite into sodium silicate and neutralize it with acid to form white carbon black.
Wollastonite is used in the metallurgical industry for metallurgical mold powder and continuous casting mold powder, in the welding electrode coating, and has a certain application market in the fields of asbestos free friction plate and environmental friendly adsorbent. It can be expected that some new uses will be found continuously with the deepening of research.