Research Of Synthesis And Modification Of Magnesium Hydroxide For Flame-retardance

Magnesium hydroxide, one of inorganic additive flame retardants, is a kind of promising green flame retardant and has attracted much attention because of its good thermal stability, nontoxicity, fume suppression, char-forming promotion, and no formation of acid and corrosive gas product during burning process. However, its poor dispersibility in and compatibility with polymer matrices would decrease the mechanical properties of the filled polymer. In this paper, therefore, the preparation and surface modification of ultrafine and nanometer magnesium hydroxide were investigated aiming at the problems above-mentioned.Magnesium hydroxide nanoneedles and nanorods were synthesized by reverse precipitation in the presence of polyethylene glycol (PEG1000) at ambient temperature. The obtained magnesium hydroxide were characterized in terms of morphology, particle size, crystal structure and thermal stability by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and thermogravimetric analysis-differential thermal analysis (TGA-DTA). The experimental results showed that the growth of magnesium hydroxide crystals and dispersivity of nanoparticles were greatly influenced by polymer dispersant. The mechanism of the formation of magnesium hydroxide nanorods and nanoneedles was also proposed.The lamellar ultrafine magnesium hydroxide with diameter of 300-500 nm was obtained from hydrothermal route at 150°C by adding a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), with pH value of 8 and Mg²⁺/CTAB ratio of 80. The resultant magnesium hydroxide was characterized by X-ray powder diffraction (XRD), thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) and field-emission scanning electron microscopy (FESEM). The mean particle size of sample was determined by size distribution. The effects of pH value and Mg²⁺/CTAB ratio on particle size were investigated. The results showed that appropriate amount of CTAB could promote the dissolution and precipitation of magnesium hydroxide in hydrothermal system, resulting in the well-defined morphology, narrow size distribution and good crystallinity of ultrafine particles at relatively lower hydrothermal temperature. The influence mechanism of CTAB on morphology of magnesium hydroxide was discussed.Nanoscaled magnesium hydroxide was first synthesized by w/o microemulsion process by blowing NH3 into the microemulsion zones solubilized by MgCl₂. The typical quaternary microemulsions of cyclohexane/Triton X-100/hexanol/water were used as space-confining microreactors for the nucleation, growth and crystallization of magnesium hydroxide nanoparticles. The obtained particles were characterized by means of XRD, IR, TGA-DSC and FESEM. The results indicated that as-prepared magnesium hydroxide particles were in the diameter of 50-80nm with even size and fine dispersibility, and thermal stability of these particles were also improved, compared with those prepared through precipitation in aqueous solution.In order to solve the heavy aggregation among magnesium hydroxide nanoparticles, a novel surface modification method was proposed by means of grafting polymethyl methacrylate (PMMA) onto the surface of nanoparticles. The functional double bonds were firstly introduced on the surface of nanoparticles by modification with oleic acid (OA), followed by dispersion polymerization on the particles surface in ethanol solution using methyl methacrylate (MMA) as monomer, benzoyl peroxide (BPO) as initiator and Polyvinylpyrrolidone (PVP) as stabilizer to graft PMMA on the surface of modified magnesium hydroxide. The obtained composite particles were characterized by XRD, IR, TGA, FESEM and energy dispersive spectra (EDS), and the compatibility with organic solution was determined by sedimentation test. The results showed that the organic macromolecule PMMA could be successfully grafted on the surface of OA-modified magnesium hydroxide nanoparticles, the dispersibility of PMMA-grafted magnesium hydroxide was greatly improved, and the dispersion stability and the compatibility of modified magnesium hydroxide particles in organic solution were better than those of unmodified ones.