| As concerning about environment increases, more and more stringent regulations will implemented to restrict the application of halogenated flame retardant in each country like polybrominated biphenyls and polybrominated diphenyl ether. As new fiber materials of environmental protection, magnesium salt whiskers have better performance than other materials in use of flame retardant and reinforcement of organic material. Therefore, it has a good extensive application prospect.In this paper,the preparation technology of magnesium hydroxide sulphate hydrates (MHSH) whisker was researched. The process of the conversion of MHSM whiskers to Mg(OH)2 whiskers was also investigated. The influence of the hydrothermal reaction conditions on the composition and morphology of hydrothermal products were discussed. Chemical analysis, thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) were employed to analyze the structural features, morphology, chemical composition and thermal behavior of the products.MHSH whiskers with zonal morphology was obtained via hydrothermal synthesis using magnesium sulphate and ammonia or sodium hydroxide as reactants. The composition of MHSH whiskers as MgSO4·5Mg(OH)2·2H2O (152MHSH) was confirmed by chemical analysis, TG-DTA and XRD.The effect of operational parameters such as reaction temperature, reaction time, material ratio and base source on the composition and morphology of the products was studied. The optimal synthesis conditions were: the concentration of magnesium sulphate:1.0 mol·L-1, material ratio: 1:2, reaction temperature: 200℃, reaction time: 6h, base source: ammonia. The whiskers with zonal morphology had an aspect ratio over 100.The variation of the concentrations of SO42- was studied under different conditions. It was found out that the increase of the reaction time and temperature and the decrease of the pH values of system suitably were beneficial to the consumption of SO42- and the growth of the whiskers, comprehensively considering the results of SEM and XRD.The aspect ratio and length of MHSH whiskers choosing ammonia as precipitant were much higher than the ones using sodium hydroxide as precipitant. These phenomenon was caused by the different alkaline between the two precipitants and the chemical complexation of NH3 and Mg2+.152MHSH could also be obtained by two-step method using magnesium chloride and ammonia and magnesium sulphate as reactants. Through the route of two-step method, the morphology of the products would have no obvious changes when reaction time is more than 4h at 180℃, and the length of the products would reduce as the concentration of magnesium sulphate is 2.0 mol·L-1. The increase of reaction temperature was beneficial to the formation of whiskers while enhanced agglomeration at the same time. The optimal synthesis conditions were: the concentration of magnesium sulphate:0.5 mol·L-1,reaction temperature: 180℃,reaction time: 4h。Magnesium hydroxide whiskers could be formed by treating 152MHSH whiskers in an sodium hydroxide solution, which has an concentration more than 0.5mol·L-1, at 200℃for 3 hours or more. The XRD pattern and SEM image indicated that: The magnesium hydroxide whiskers had no impurity and the morphology was similar to its precursor. When the concentration of sodium hydroxide solution were 0.5mol·L-1 and 0.7mol·L-1, the morphologies of the whiskers had no observable difference between each other. But fracture of whiskers would take place when the concentration was as high as 1.0mol·L-1. The same phenomenon would appear as the time was prolonged to 6h in 0.5mol·L-1 NaOH. This fact shows that 152MHSH whisker was thermodynamic metastable state. OH- would react with Mg2+ produced by the dissolution of MgSO4 in 152MSHS in certain condition and 152MHSH would convert to magnesium hydroxide. The addition of an amount of crystal habit modifiers would make the size of the products more homogeneous.
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