Dispersion Of Fumed Silica Research Articles

Morphology and surface properties of fumed silicas

Several series of fumed silicas and mixed fumed oxides produced and treated under different conditions were studied in gaseous and liquid media using nitrogen and water adsorption–desorption, mass spectrometry, FTIR, NMR, thermally stimulated depolarization current (TSDC), photon correlation spectroscopy (PCS), zeta potential, potentiometric titration, and Auger electron spectroscopy methods. Aggregation of primary particles and adsorption capacity ( V p) decrease and hysteresis loops of nitrogen adsorption–desorption isotherms becomes shorter with decreasing specific surface area ( S BET). However, the shape of nitrogen adsorption–desorption isotherms can be assigned to the same type independent of S BET value. The main maximum of pore size distribution (gaps between primary nonporous particles in aggregates and agglomerates) shifts toward larger pore size and its intensity decreases with decreasing S BET value. The water adsorption increases with increasing S BET value; however, the opposite effect is observed for the content of surface hydroxyls (in mmol/m 2). Associative desorption of water (2( Si OH) → Si O Si + H 2O) depends on both the morphology and synthesis conditions of fumed silica. The silica dissolution rate increases with increasing S BET and pH values. However, surface charge density and the modulus of zeta-potential increase with decreasing S BET value. The PCS, 1H NMR, and TSDC spectra demonstrate rearrangement of the fumed silica dispersion depending on the S BET value and the silica concentration ( C SiO 2 ) in the aqueous suspensions. A specific state of the dispersion is observed at the C SiO 2 values corresponding to the bulk density of the initial silica powder.

Rheological properties of fumed silica filled Bis‐GMA dispersions

AbstractThe rheological properties of Bis‐GMA dispersions filled with fumed silica were investigated to optimize the manufacturing process and mechanical properties by using a Rheometrics Mechanical Spectrometer (RMS). Steady and dynamic measurements on the RMS were carried out to obtain shear viscosity and dynamic mechanical properties. The effect of several factors on the rheological properties of fumed silica dispersions was also examined. The factors were a concentration of a silane coupling agent (γ‐MPS), the methods of surface treatment of fillers, silica content, diluent concentration, shear rate, and operating temperatur. From these studies, it was observed that shear viscosity showed an asymptotic phenomenon at a higher concentration than a uniform multi‐layer coverage concentration of γ‐MPS. The silane coupling agent had a significant role in the reduction of tan δ, resulting from a decrease of loss modulus, while fillers decreased tan δ by increasing the storage modulus. In cases where the silica content and diluent concentration increased simultaneously, the Barcol hardness of Bis‐GMA/silica composites was increased, but there was no change in the viscosity of dispersions and diametral tensile strength of those composites.

The rheological properties optimization of fumed silica dispersions using statistical experimental design and taguchi method

The rheological properties of fumed silica filled Bis-GMA dispersions were analyzed using statistical experimental design and Taguchi method. Viscosity and shear thinning property were considered as the rheological properties. Silica content, silane concentration, diluent concentration, shear rate, and temperature were chosen as significant factors. Statistical experimental design using analysis of variance and orthogonal array was applied to optimize the viscosity of the dispersions, and Taguchi's parameter design was simultaneously performed to analyze the shear thinning property of the dispersions. Shear rate, silica content, and γ-MPS concentration showed major effects on the viscosity, and silica contents and γ-MPS concentration also had principal effects on shear thinning property of the fumed silica dispersions. Diluent concentration was determined as the only insignificant factor concerning the viscosity, while the temperature was the sole factor not to influence the shear thinning property of the dispersion. Optimum viscosity condition and less shear thinning condition were also determined using statistical experimental design and Taguchi method.

Monitoring Flocculation in Situ in Sterically Stabilized Silica Dispersions Using Rheological Techniques

Flocculation of sterically stabilized colloidal dispersions is a technologically important and complex phenomenon. Often, a state of controlled, weak flocculation is desired to slow the aging of dispersions due to irreversible flocculation and subsequent sedimentation. Using rheological techniques, we have characterized flocculation in situ in sterically stabilized fumed silica dispersions toward identifying those variables important to the control of the flocculated network strength. Both poly(methyl methacrylate) homopolymers and polystyrene/poly(methyl methacrylate) block copolymers have been used as stabilizers. Dynamic oscillatory measurements have been used to quantify the elastic moduli and hence the strength of the flocculated networks as a function of polymer surface coverage, molecular weight, architecture, and solvency as well as particle volume fraction. The results show that the flocculated network strength is weaker for complete surface coverage and when diblock copolymers are used as stabilizers, as expected. In addition, a scaling of the elastic modulus with particle volume fraction demonstrates weaker flocculation with increasing polymer molecular weight. This work illustrates the utility of rheological techniques in studying the dynamic flocculation process

Rheology of fumed silica dispersions for fiber‐optic cables

AbstractFumed silica dispersions in mineral or silicone oil are used to fill fiber‐optic cables to prevent macro‐bending of the fibers and to provide environmental protection. Adequate control of the rheology is essential for the effectiveness of the filling compounds; the materials must be sufficiently viscous to minimize stresses on the fibers but have considerable elasticity so as not to drip with time or temperature. The effects of shear history, aging, temperature, and water contamination on the rheological properties of two commercially available systems are studied using steady and dynamic measurements. The results are interpreted in terms of the microstructure resulting from the aggregation of submicron silica particles and their interactions with different solvents.