Preparation And Surface Modification Of Magnesium Hydroxide Sulfate Hydrate And Magnesium Hydroxide Whiskers

Magnesium salt whiskers have the similar high strength and anticorrosionperformance with ceramic whiskers such as carbide whiskers and nitride whiskers.They are a kind of materials with super enforcing performance. These whiskers whichare modified by surface modification agents have a good compatibility with polymers,and they can enhance the tensile strength and flame retardancy of the polymers.In this paper, magnesium hydroxide sulfate hydrate （MHSH） whiskers weresuccessfully prepared using brine as the main raw material, and the surface of thewhiskers were modified by stearic acid and KH-570. Then magnesium hydroxidewhiskers were successfully prepared using MHSH whiskers as the precursor. X-raypowder diffraction （XRD）, fourier transform infrared spectrum （FT-IR）, scanningelectron microscope （SEM） and thermal analysis （TG-DTA） were employed to studythe preparing procedure of MHSH whiskers and magnesium hydroxide whiskers. Thesurface modification effect was studied by characterizing the hydrophobic value of themodified whiskers.MgSO₄·5Mg（OH）₂·2H₂O whiskers with the diameter of0.1¹μm and the aspectratio of60¹20were prepared by one-step hydrothermal synthesis using brine, NaOH（or NH₃·H₂O） and MgSO₄as raw materials. The experimental results suggested thatusing NaOH as the precipitator, the length of MHSH whiskers could reach to100μm,while using NH₃·H₂O as the precipitator, the length of MHSH whiskers was just20²5μm. In consideration of the properties of whiskers, energy saving andapplication requirements, experiments were carried out to optimize the preparingtechnique. When the concentration of Mg²⁺was1.2mol·L^-1, the molar ratio of SO₄^2- to Mg²⁺was0.3, the hydrothermal reaction temperature was180℃, and thehydrothermal reaction time was5h, whiskers with high aspect ratio and gooddispersity were obtained and the energy consumption under the condition was low.The utilization ratio of Mg²⁺in brine was also analyzed in our experiments, and theresults showed that the utilization ratio of magnesium in brine could reach above80%.The results of surface modification experiments indicated that using stearic acid asthe surface modifier, the optimized conditions of the wet method were: the amount ofstearic acid was8%of MHSH whiskers, the modification temperature was75℃, andthe modification time was30min; the optimized conditions of the dry method were:the amount of stearic acid was5%of MHSH whiskers, the modification temperaturewas60℃, and the modification time was30min. Using KH-570as the surfacemodifier, the optimized conditions of the wet method were: the amount of stearic acidwas6%of MHSH whiskers, the modification temperature was40℃, and themodification time was90min; the optimized conditions of the dry method were: theamount of stearic acid was2%of MHSH whiskers, the modification temperature was150℃, and the modification time was120min. Overall, using stearic acid as thesurface modifier, the surface modification effect was better than that of usingKH-570; the surface modification effect of the wet method was better than that of thedry method.Magnesium hydroxide whiskers with hexagonal crystal structure, high aspect ratio,good dispersity and high purity were successfully prepared using MHSH whiskers asthe precursor. The results of the crystal convertion experiments indicated that theconverting reaction of MHSH whiskers to magnesium hydroxide whiskers wasobviously affected by the reaction temperature, the dosage of NaOH and the reactiontime. When the reaction temperature was80℃, the molar ratio of MHSH to NaOHwas1:5, and the hydrothermal reaction time was5h, magnesium hydroxide whiskerswith high aspect ratio and good disparity were obtained. The existence of the stearicacid also affected the converting reaction, if the stearic acid was added into the reaction system during the conversion procedure, the reaction process could behampered on a certain degree.