Surface modification of magnesium hydroxide by -aminopropyltriethoxysilane
LUO Wei( ), FENG Qi-ming( ), OU Le-ming( ), LIU Kun( )
(School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China)
Abstract: Magnesium hydroxide(MH), which is commonly used as a halogen-freeflame retardant filler in composite materials, was modified by silanization reaction with -aminopropyltriethoxysilane ( -APS) in aqueous solution at different pH values (pH range from 8.0 to 12.0). The surface properties of grafted -APS on MH surface as a function of solution pH value were studied using elemental analysis, Fourier transform infrared spectroscopy and zeta potential measurement. Theresults show that hydrolysis and condensation of -APS are activated in alkaline solution and lead to multilayer adsorption of -APS molecules on the surface of MH. The type of adsorption orientation of -APS on MH surface is a function of coverage density that is altered by changing solution pH value. At low coverage density (e.g. 55 nmí2), -APS molecules are preferentially adsorbed to the surfacewith the silicon moiety towards the surface, and increasing coverage density (e.g. 90 nmí2) leads to parallel orientation. At an even higher coverage density (e.g. 115 nmí2), -APS molecules bond to the surface with the amino moiety towards the surface. Key words: magnesium hydroxide; -aminopropyltriethoxysilane; surface modification; orientation
Magnesium hydroxide(MH) has beenapplied in numerous areas including antiacids, absorbents, fertilizer additives, filler materials, precursors for magnesium oxide, flame retardants and smoke suppressants. In the field of halogen-free flame retardant composites, MH is widely used by virtue of its high decomposition temperature (300í320 ). The disadvantage of this filler, however, is the high level (more than massfraction of 60%) required to achieve the desired flame retardant effect. Such high level causes filler-matrix interfacial properties to be the major determinant of composite mechanical properties. Addition of fillers in such high contents affects negatively the rheological and mechanical properties of the composite, such as increasing melt viscosity, reducing elongation at break and impactstrength. In order to minimize this effect, surface modification of MH is an important method to improve their compatibility. It has been reported that MH can be successfully modified with fatty acids, unsaturated polymeric acids, titanate coupling agents and silanes. Although there have been some reports about the preparation of silane-modified MH particles, the study of the surfaceproperties of this kind of material still remains to be a challenge. In addition, the effect of functional groups on the surface properties of the particles is not clearly
understood. In this work, MH particles were modified by -aminopropyltriethoxysilane ( -APS) and various experimental techniques were used to investigate the surface properties of the modified particles.
2.1Materials and procedures 2.1.1 Materials MH powder ( 98%, mass fraction) from Shandong Haihua Company Limited was further dried in a vacuum oven at 100 for 24 h prior to each silanization experiment in order to remove any physisorbed water from the surface. The average particle size with an average surface area of 32.23 m2/g was determined to be 9.83 μm by CILAS laser particle size analyzer based onthe BET method using nitrogen as carrying gas. -APS 95% purchased from Wuhan Huachang Applied Technology Institute was used as received. All inorganic chemicals were of analytical grade. Deionized water was used in all of the experiments. 2.1.2 Synthesis of modified magnesium hydroxide In a 500 mL three-neck flask, the initial pH value of 100 mL water was adjusted by NaOH solution. Then an excess...