Inorganicflame retardantThese include aluminum hydroxide, magnesium hydroxide, expanded graphite, borates, aluminum oxalate and zinc sulfide based flame retardants. Aluminum hydroxide and magnesium hydroxide are the main varieties of inorganic inhibitor, they have the characteristics of non-toxic and low smoke. Due to the thermal decomposition to absorb a large amount of heat in the combustion zone, the temperature of the combustion zone is reduced to below the critical temperature of combustion and self-extinguishing. Most of the metal oxides generated after decomposition have high melting point, good thermal stability, and cover the surface of the combustion solid phase to block heat conduction and Heat radiation, thereby playing a flame retardant effect. At the same time, the decomposition produces a large amount of water vapor, which can dilute the combustible gas and also play a flame retardant role.

Hydrated alumina has good thermal stability, heating at 300 ℃ for 2h can be converted into AlO(OH), will not produce harmful gases after contact with the flame, and can neutralize the acid gas released during polymer pyrolysis, less smoke, low price and other advantages, so it has become an important variety of inorganic flame retardants. Hydrated alumina is heated to release chemically bound water, which absorbs the heat of combustion and reduces the combustion temperature. In the play the role of flame retardant, mainly two crystal water play a role, in addition, the water loss product is activated alumina, can promote some polymer in the combustion of condensed ring carbonization, so it has a condensed phase flame retardant effect. From this mechanism, it can be seen that the use of hydrated alumina as a flame retardant should be added in a large amount.

The main variety of magnesium element flame retardant is magnesium hydroxide, which is a kind of flame retardant being developed at home and abroad in recent years. It starts to undergo endothermic decomposition reaction to generate magnesium oxide at about 340 ℃, and the weight loss reaches the maximum at 423 ℃, and the decomposition reaction is terminated at 490 ℃. From the calorimetric method, the reaction absorbs a large amount of heat energy (44.8KJ/mol), and the generated water also absorbs a large amount of heat energy, reducing the temperature and achieving flame retardant. Magnesium hydroxide has better thermal stability and smoke suppression ability than hydrated alumina, but because of its large surface polarity and poor compatibility with organic matter, it needs to be surface treated to be an effective flame retardant. In addition, its thermal decomposition temperature is high, which is suitable for the flame retardant of polymers with high decomposition temperature such as thermosetting materials.

At high temperatures, the embedded layer in the expandable graphite is easily decomposed by heat, and the generated gas rapidly expands the layer spacing of the graphite to tens to hundreds of times the original. When expandable graphite is mixed with a polymer, a tough carbon layer can be generated on the surface of the polymer under the action of flame, thereby playing a flame retardant effect.

Borate flame retardants are borax, boric acid and zinc borate. At present, the main use is zinc borate. Zinc borate starts to release crystal water at 300 ℃. Under the action of halogen compounds, boron halides and zinc halides are generated, free hydroxyl groups are inhibited and captured, and combustion chain reaction is prevented. At the same time, a solid layer is formed to isolate the surrounding oxygen, prevent the flame from burning and have smoke suppression effect. Zinc borate can be used alone or in combination with other flame retardants. At present, the main products are fine zinc borate, heat-resistant zinc borate, anhydrous zinc borate and high water zinc borate.

Aluminum oxalate is a crystalline material derived from aluminum hydroxide with low alkali content. Containing aluminum oxalate polymer combustion, the release of H2O, CO and CO2, without generating corrosive gases, aluminum oxalate can also reduce the smoke density and smoke speed. Due to the low alkali content of aluminum oxalate, it does not affect the electrical properties of the material when using its flame retardant wire and cable cladding material.

Five zinc sulfide-based flame retardants have now been developed, four of which are for rigid PVC and one for soft PVC, polyolefin and nylon. This kind of flame retardant can improve the anti-aging performance of the material, and has good compatibility with glass fiber and improve the thermal stability of polyolefin.