Preparation And Surface Modification Of Magnesium Hydroxide Sulfate Hydrate Whiskers

Magnesium hydroxide sulfate hydrate (MHSH) whiskers have attracted much attention because of their potential application as resin additives of flame retardant, fillers, or reinforcers. In this thesis, we report preparation of MHSH whiskers using one-step hydrothermal synthesis with magnesium chloride (or bittern), ammonia and magnesium sulfate as raw materials and no additional template. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), thermal analysis (TG-DTA), and scanning electron microscope (SEM) were employed to characterize the phase transition induced by temperature and time, morphology evolution and growth mechanism during the hydrothermal process. The products were modified to enhance the compatibility with polypropylene (PP). The surface modification methods were evaluated by investigating the changes of surface contact angle of whiskers, mechanical and flame retardant properties of the composites.The MHSH whiskers prepared at 190℃for 5 hours exhibited a randomly arranged, fanlike morphology with an average diameter of 0.5-2μm and aspect ratios between 50 and 200. The SAED and EDS analysis showed that the elementary composition at the intersection point was consistent with that in the main body of the fanlike whiskers. The morphology of whiskers prepared using magnesium chloride or bittern as raw materials was very similar without obvious difference. Moreover, no distinct difference was observed for products from one-step and two-step techniques.When using ethanol as crystal control agent, most of the whiskers presented as single fibers, while the whiskers aggregated more severely with increased aspect ratios when adding acetic acid as crystal control agent.In the synthesis process, many two-dimensional steps were observed at tips of the whiskers, as seen from SEM images. It was speculated that these steps were formed during crystal growth as a result of crystal dislocations. The growth of MHSH whiskers was essentially the process of Mg-06 entering the crystal lattice through the interface. It was exactly the extension of dislocations that made the growth of the whiskers.From a thermodynamic view, the formation of MHSH whiskers might follow the dissolution-recrystallization mechanism in which the interaction of Mg2+, SO42- and OH- ions, after the dissolution of the initially formed particulate Mg(OH)2, resulted in the production of the whiskers. From a dynamic point of view, the mechanism for crystal growth was different when the products were synthesized at different temperatures. When the reaction occurred at 170℃,180℃and 190℃, the crystallization was controlled by multi-core surface growth, while it changed to be controlled by single-core surface growth when the reaction temperature was 200℃or 210℃.Thermo-gravimetric analyzer was used to characterize the thermal behavior of the whisker products. It was indicated from Satava method that the first step of the thermal decomposition of the MHSH whiskers was in accord with the equation of Avrami-Erofeev with integral form of G(a)=[-ln(1-a)]1/2. The second step was in accord with the equation of Prout-Tomkins with integral form of G(a)=ln[a/(1-a)], and the final step was Mampel power with integral form of G(a)=a1/2. The corresponding mechanism was nucleus formation and growth, branching nuclei and nucleus formation, respectively. Based on DTA data, the kinetic parameters for each step were calculated using a non-isothermal Kissinger method. The activation energy of the first decomposition step was 277.6 kJ/mol, indicating the crystal water moieties can be easily lost. The activation energies of the second and third decomposition steps were 492.3 and 797.6 kJ/mol.This paper fist reported the method of surface-initiated in-situ polymerization to prepare PMMA modified MHSH whiskers with a well defined core-shell hybrid structure. SEM, EDS, TG and FT-IR were used to characterize the effect of surface modification. Compared with the modified effect of stearic acid (SA) and KH-570, the surface contact angle of the PMMA modified whiskers increased most greatly and the whiskers surface was all changed from hydrophilic to lipophilic after modification. The mechanical and flame retardant properties of the composites were investigated in order to evaluate the three different modification method. It was found that the mechanical properties of PP composites can be improved by 80 percent after adding modified whiskers, and the reinforce effect of modified whiskers was better compare to those unmodified. While the elongation at break of PP composites decreased because of the brittleness of MHSH whiskers. The flame retardant effect was better when the content of whiskers was higher. The main impact of surface modification on the composite materials was improvement of mechanical properties, and the effect on flame retardancy of the whisker was little.The research above showed that MHSH whiskers prepared using bittern, ammonia and magnesium sulfate as starting raw materials had wide potential application prospect due to its good flame retardant effect and reinforce properties for polymer matrix.In this paper, the MHSH whiskers were prepared for the first time using one-step method with bittern as raw material. It also provided a new approach for multi-channel development and utilization of magnesium in the bittern resources and production of magnesium products with high additional value.