As environmental requirements and green production requirements become more stringent, the service life of various durable products will be greatly improved. Powder coatings have attracted more and more attention as a protective layer for protecting various substrates and improving the aging properties of powder coatings. An important component of powder coatings - titanium dioxide due to the lattice defects of titanium dioxide, it is reduced to unstable titanium oxide under the action of sunlight, especially ultraviolet radiation and water and other catalysts, while releasing the initial ecological oxygen, this Oxygen oxidizes the organic matter as a lacquer base, causing the chain scission and degradation of the polymer to become a soluble or volatile substance, which destroys the continuity of the paint film and has an important influence on the aging property.
2 Titanium white crystal type introduction
Titanium dioxide is a polycrystalline compound. In nature, there are mainly rutile titanium dioxide, anatase titanium dioxide and titanium titanium dioxide. Among them, the titanium-titanium dioxide crystal is unstable, so the market is The other two crystal forms are dominant. The anatase-type single crystal is composed of four carbon dioxide molecules, the crystal lattice is relatively large, the bonding is loose, the crystal arrangement is directional, and a mica-like sheet-like structure is uniformly formed on the polycrystalline composite, so the lattice stability is achieved. Poor on the surface, small phase contrast, and low crystal hardness. The single crystal of rutile titanium dioxide is composed of two Ti02 molecules, the lattice is relatively small, the binding is tight, and the lattice arrangement has no directionality, but is arranged in a divergent space. The polycrystalline combination is a dense spatial network. The structure is so dense and the hardness is large in the stability of the crystal lattice. The rutile titanium dioxide crystal is relatively slender and prismatic, usually twin, while the anatase titanium dioxide is mostly an regular octahedron. The respective x-ray diffraction characteristic peaks are also different. The diffraction angle of the anatase titanium dioxide is 25.3°, and the rutile titanium dioxide is 27.4°.
3 Analysis of the aging performance mechanism of titanium dioxide crystal form
The catalytic ability of Ti02 depends to a large extent on the degree of separation of photogenerated electron-hole pairs, but since the rutile type of TiO2 has a lower adsorption capacity for oxygen than the anatase type, the ratio of adsorbed oxygen on the surface is higher than that of anatase. With less crystal form, the surface adsorbed oxygen has the function of trapping photons during photocatalysis, which can inhibit the recombination of photogenerated electron-hole pairs, and can also oxidize the already hydroxylated reactants as an oxidant.
Rutile-type Ti02 absorbs light and can excite electrons, but it has a faster compounding rate and a low photon ratio for oxidative contamination. Studies have shown that the catalytic effect of pure anatase crystal form Ti02 is better than that of pure rutile type. When the content of anatase crystal form in mixed crystal sample is higher, the catalytic effect is better than that of pure anatase type Ti02.
How titanium dioxide compensates for lattice defects and reduces the direct contact of Ti02 with oxygen in the air is the key to improving weatherability. In order to improve the weather resistance of titanium dioxide paint film in industrial production, a small amount of salt treatment agent is added before the metatitanic acid is calcined, or it is coated to block the photoactivation point, and the direct contact between titanium dioxide and light (ultraviolet light) is isolated. . At present, the surface modification methods of titanium dioxide are: silicon aluminum coating, zirconium aluminum coating and the like. Since titanium dioxide is a product of titanium dioxide coating, the content of titanium dioxide is not 100%, which is between 93% and 95%. If the content of titanium dioxide is high, the degree of coating may be sacrificed, resulting in aging resistance. The performance is weakened. If the titanium dioxide content is low, the hiding rate of the titanium dioxide is low, and the weather resistance of the coating is also lowered.
Titanium dioxide plays a role as a pigment and filler in powder coatings and plays a vital role in weatherability. When using titanium dioxide, it is necessary to pay attention to the rutile crystal form conversion rate, titanium dioxide content, and coating material of titanium dioxide. When crystallographic analysis is carried out by X-ray diffractometer, if there is a distinct characteristic peak of anatase, the titanium dioxide is not suitable for outdoor powder coating, otherwise it will be rapidly pulverized; the conversion rate in rutile crystal form is 98%. When the coating is in a natural exposure environment, slight pulverization will occur in about 2 years. After that, the gloss retention rate will remain basically unchanged, and slight pulverization will occur. The coating will be free of rust and peeling, and will not affect. The coating protects the substrate.
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