Zinc borate has the functions of diluting and blocking combustible gases, foaming, absorbing heat, dehydrating, promoting carbonization, and producing a glassy coating, which can produce a flame retardant effect. The decomposition temperature of zinc borate is similar to that of engineering plastics, PVC, etc. It can release a large amount of water vapor at high temperatures, absorb the heat generated by the material, and reduce the surface temperature of the plastic material. The flame retardant mechanism of zinc borate can be roughly understood from the following aspects.
1. Zinc borate reacts with organic halides at high temperatures to generate boron trihalides that can be flame retardant in the gas phase. When ZB reacts with hydrogen halides produced by the decomposition of flame-retardant polymers or halogen flame retardants, zinc compounds and boron compounds can be generated according to the following reaction.
2ZnO·3B2O3+12HC1-Zn(OH)C1+ ZnCl2 + 3BCl3 + 3HBO2 + 4H2O, the generated non-volatile zinc compounds and boron compounds promote carbonization and play a condensed phase flame retardant role.
2. The dehydration temperature of ZB is similar to the decomposition temperature of some high-temperature engineering plastics. When ZB decomposes at high temperature, most of the boron and zinc remain in the carbon layer.
3. When ZB flame retards PVC, only a small part of the boron may be converted into volatile boron halides and lost. A small amount of boron halides is not only insufficient for flame retardancy, but will destroy the integrity of the carbon layer. In halogen-free systems, the role of anhydrous ZB is to improve the quality of the carbon layer, while hydrated ZB is mainly flame retardant by dehydration mechanism. If the flame retardant system contains aluminum hydroxide, at about 550°C, zinc borate can form a porous ceramic layer as a barrier for heat and mass transfer. Boron compounds can delay the oxidation of graphite structure because boron can inactivate some oxidation-sensitive reaction points on the graphite surface. Graphite usually exists in the carbon layer at the same time as amorphous carbon.
4. When zinc borate is used in combination with halogen flame retardants, it can play a role in both the gas phase and the condensed phase. Because the generated boron halide and water vapor can dilute combustibles and absorb heat to cool down, they can play a gas phase flame retardant role. The generated zinc halide can catalyze the dehydrochlorination and cross-linking of certain polymers (such as PVC) in the condensed phase, and has the effect of suppressing flames in the gas phase (only a small amount of zinc halide evaporates into the gas phase). Boron oxide generated by the reaction of acid and ZB can stabilize the carbon layer and suppress the smoldering of materials. However, it should be pointed out that the boride and zinc compound that escape into the gas phase are very limited. Boron halide is a flame inhibitor, and its function of capturing free radicals is of the same order of magnitude as hydrogen halide.
Zinc borate can be used as a flame retardant in cables, rubber, textiles, plastics, coatings, anti-rust paints, etc. Because it also has insect-proof, anti-corrosion and mildew-proof effects, it also has important uses in the protection of wood and paper. Zinc borate can be used alone or in combination with other phosphorus and nitrogen flame retardants, and it can also improve the smoke suppression performance of the material.
