Ultra-Fine Silica Surface Modification And Its Properties

The suspension of ultra-fine silica (SiO₂)/PEG (200) has shear-thinning and shear-thickening phenomena, so it has a great potential application as the soft amour materials. The rheological properties of suspension are determined by silica surface nature to some degree. The aim of this article is to study the effect of silica surface modification on the rheological properties of this suspension, in order to supply experimental basis for its application.In this article, ultra-fine silica (SiO₂) particles were synthesized by the hydrolysis of tetraethoxy silane (TEOS) in alcohol-water solutions using ammonia (NH₃·H₂O) as a catalyst. Then the ultra-silica particles were modified by different modifiers and were used to make suspensions in order to investigate the effect of ultra-silica surface modification on the rheological properties of its suspension. The modified SiO₂ were characterized by laser particle size analyzer, scanning electron microscopy (SEM), contact angle measuring instrument, thermal analyzer and adsorption test. It is found that the nature of ultra-fine SiO₂ has changed after modifyed. The weaker polar functional group of modifiers(KH540,TPM,WD-10) was found to be beneficial to the dispersion and hydrophobic of ultra-fine SiO₂ particles, indicating that polar component would strengthen the force between particles and not conducive to dispersion. It can decrease the dispersion and increase the hydrophobic of ultra-fine SiO₂ as the modifier molecular chain increasing for the long chain modifiers(PEG400,PEG2000,PEG6000), indicating that long-chain molecules entangling on the surface of SiO₂, this will make the dispersion decline on one hand, and make the particle surface show non-polar hydrophobic on the other hand.Milling method was used to prepare susupension containing the modified SiO₂ and PEG200, and the the static and dynamic rheological properties of the suspensions were measured by a rotating rheometer. It is found that surface properties of SiO₂ have a great impact on the rheological properties of the suspensions. The static rheological test results exhibit that SiO₂ powder treated with a modifier containing characteristic functional group has better dispensability as the polarity of function group was reduced, therefore the force points between silica and dispersion media increase, and the viscosity of suspension increases. When the polarity of functional groups carried by the modifier attached to SiO₂ surface is strong, the force between silica and dispersion medium is strong, so the dispersion viscosity is great. When SiO₂ particle size is large, the friction between particles and the hinder effect of network structure is great, so the viscosity is great. If molecular chain connected to the SiO₂ surface is long, the winding effect between dispersion medium and modifier is strong, and then the viscosity is high. As the size of silica become small, the size distribution become broad and the force between silica and dispersion media become weak, the critical shear rate get larger.Storage modulus and its change under shearing are influenced by the network and its transformation. The stronger the network structure in suspension, the greater the storage modulus is. Loss modulus and its change under shearing are influenced by the friction between particles and the hinder effect of network structure to the particles, and they are proportional to these two factors.