Colloidal silica, viscosity

Commonly used enamels contain multiple ingredients. Typically silica, alumina and other metal oxides such as calcia arc the major ingredients. In addition, some organic additives such as dispersants and viscosity modifiers (e.g., polyvinyl alcohol) arc used to ensure that the starting slip consists of finely divided and dispersed particles in the submicron range in order to seal the pores on the end surfaces. The pores can be filled with very fine ceramic particles (Garcera and Gillot, 1986] or calcined colloidal silica... 

Increased viscosity of the base - achieved for example, by addition of 2% aluminium monostearate or colloidal silica (Aerosil) - decreases the rate of release of soluble dmgs but has little effect on the release of the less-soluble materials. (The action of aluminium stearate and Aerosil in gelling oils is discussed in section 7.4.5.)... 

DBTDL was used as a catalyst in the frontal polymerization of 1,6-hexanediisocyanate with ethylene glycol. In frontal polymerization the polymerization is locally initiated and the exotherm of the reaction propagates the polymerization throughout the system. Pyrocatechol was used to avoid spontaneous polymerization. Pyrocatechol chelates tin and depresses the catalytic activity at room temperature without affecting catalysis at the higher temperature. To achieve a uniform advancing reactive front, and to avoid fingering, the viscosity of the blend was increased with colloidal silica. 

Field tests of both colloidal silica and polysiloxane showed that these materials could be successfully placed with conventional grouting equipment, had good gel time control (from minutes to hours) and reduced formation permeabilities by two or more orders of magnitude. Reports prepared for the meeting noted above described other unusual grout properties These materials have a viscosity less than that of water and... 

Silicsol is a commercial product introduced in Europe this past decade. It is described as an activated silica liquor with a calcium-based reagent. As opposed to sodium silicate (colloidal silica particles dispersed in soda), silicsol is claimed to be a true solution. Viscosity and penetrability are similar to sodium silicate, but the reaction is different, resulting in a stronger end product more resistant to creep. There is no syneresis associated with silicsol. 

Colloidal silica serves two functions in the process. Since it is a low viscosity water-based liquid it is used as the vehicle to slurry the ceramic powders which make up the ceramic shell. The viscosities of slurries used in the process are usually similar to a thin crepe or pancake batter. They must be thin enough to flow into the surface details of the wax patterns such as the narrow grooves in a golf club head, but thick enough to leave a continuous... 

Because of this Roberts [7] took a different approach and determined the strength of slurries made at the same viscosity with binders containing different levels of colloidal silica. The binders were pre-diluted to the desired level and refractories added to make slurries. The strength of each was measured, plotted, and the optimum binder silica content determined from the plot. In all cases, the slurries were weak at low binder silica levels, went through a maximum and then became weaker when binder having higher silica contents were used. 

When silica is used, it should be considered that the release of aqueous soluble to very soluble active substances may be reduced. Silica increases the viscosity of the base which may impair the transport of the active substance to the interface between water and fat, thereby impairing active substance release. For water-soluble active substances, crossing the interface is rate determining (Sect. 16.2.4). For this reason the amount of silica has to be limited to 1 % of the weight of the active substance. A negative influence of anhydrous colloidal silica on active substance release is evident for amounts above 1.5-2 % of the suppository base [8i]. Concentrations above 1.5-2 % are used only for suppositories with an intentionally delayed release profile. 

Enhancing the viscosity of an aqueous suspension is often necessary to obtain a reasonable physical stability (see Sect. 18.4.2.2). Apart from the already mentioned viscosity enhancers, also mineral viscosity enhancing substances, such as anhydrous colloidal silica (2—4 %, usually 2 %), and colloidal aluminium magnesium silicate (2—4 %, usually 2 %) or bentonite (1-2 %) are used. All percentages refer to the final amount of the preparation. 

Oleogels may be mixtures of liquid paraffin with polyethylene (Hydrophobic Base Gel DAC [43]) or fatty oils combined with anhydrous colloidal silica, aluminium or zinc soaps as viscosity enhancer. Hydrophobic gels are prepared by mixing the oil phase with the viscosity enhancer. If the... 

Schwartz and co-workers [52, 63] applied a silica layer on the inner surfaces of polymeric, copper, and steel capillaries of 4-400 m length and =0.5 mm inside diameter. The authors [62] ran into difficulties in using a suspension containing micron-size particles and therefore they decided in favor of true colloidal silica solutions. They used [52] 22% silica sol in [water-2-propanol] system commercially available under the trade mark Nal-coag 1092. Such colloidal silica sols are produced for preparing adsorption and antistatic impregnated systems. They have a low viscosity and therefore can readily pass through a capillary to form a thin... 

Ashley and Innes (151) in 1952 pointed out that the low viscosity of colloidal silica at high concentrations clearly showed that the particles were spherical in shape, since any other shape, chainlike or even acicular, would cause a much higher viscosity. The conclusion is that silica in some way polymerizes to dense particles of more or less spherical shape, at least above neutrality. 

For stable sols such as colloidal silica, forced circulation evaporators have been generally used. Special precautions must be taken not to permit the sol to become too concentrated or to reach dryness at any point on the equipment walls and especially on the heat exchange surfaces. If this occurs, a layer of hard adherent silica is built up. The problem becomes acute as the silica approaches the final high concentration with increasing sol viscosity. 

Particles of colloidal silicates of polyvalent metals, for example, clay and asbestos, adsorb colloidal silica. Addition of colloidal silica improves dispersion and lowers viscosity of dispersions of asbestos in water without Introducing strong alkali such as sodium silicate (688, 689). 

Winslow (1949) reported that silica gel in a low-viscosity oil showed this effect under an electric field of 3 kV/mm. The fluid can be sheared with a force proportional to the square of the electrical field. For example, a 25% by volume of hydrophobic colloidal silica spheres of 0.75 pm diameter in 4-methylcyclohexanol showed ER responses at 40-4,000 Hz, although dc fields are also viable. Dispersants are often added to the suspension in order to prevent the settling of the solids. 

Figure 19-11. Reduced viscosities as a function of the reduced shear stress of colloidal silica suspensions (diameter of 100 nm) in the presence of addedpolymer (polystyrene). The solvent used is decalin which is a near theta solvent for polystyrene. The size ratio of the polymer radius of gyration to the colloid radius (Rg/R) is 0.02S. The colloid volume fraction ((f>) is kept fixed at 0.4. In the absence of added polymer (Cp/c = 0), the particles behave as hard spheres and as more polymer is added to the system, the particles begin to feel an attraction. The colloid-polymer suspensions at (p of 0.4 shear thin between a zero rate viscosity of r o and a high shear rate plateau viscosity r]x,. The shear thinning behavior (in the absence and presence of polymer) is well captured by equation (19-10) with n = 1.4. Note rjo, rjao and cTc are functions of volume fraction and strengths of attraction but weakly dependent on range of attraction (Shah, 2003c Rueb, 1997).

One can deduce from Eq. (233) that for x < 0.5, particle coverage should increase linearly with the dimensionless time, which, is indeed, confirmed by the experimental results shown in Fig. 59. This has a practical significance, indicating that in the case of the ballistic model, the particle adsorption rate remains constant for a much broader range of 0 than for the RSA model. This allows one to determine in a precise way the viscosity of the medium just by measuring the number of particles (of a known size) as a function of time. This approach was indeed exploited in Ref 193 to measure the viscosity of colloid silica solutions, by using melamine latex of size 1.68 pm. 

A. Imhof, A. van Blaaderen, G. Maret, J. Mellema, and J. K. G Dhont. A comparison between the long-time self-diffusion and low shear viscosity of concentrated dispersions of charged colloidal silica spheres. / Chem. Phys., 100(1994),2170-2181. 

A third system that is claimed to behave as a model hard sphere fluid is a dispersion of colloidal silica spheres sterically stabilized by stearyl chains g ted onto the surface and dispersed in cyclohexane ". Experimental studies of both the equilibrium thermodynamic and structural properties (osmotic compressibility and structure factor) as well as the dynamic properties (sedimentation, diffusion and viscosity) established that this system can indeed be described in very good approximation as a hard sphere colloidal dispersion (for a review of these experiments and their interpretation in terms of a hard sphere model see Ref. 4). De Kruif et al. 5 observed that in these lyophilic silica dispersions at volume fractions above 0.5 a transition to an ordered structure occurs. The transition from an initially glass like sediment to the iridescent (ordered) state appears only after weeks or months. 

It is difficult to clearly establish the role of additives since they can produce competitive or synergic effects affecting the rheological properties of the slurry. The most commonly used additives are binders and surfactants. Usual binders are colloidal silica or alumina. Alumina is more thermostable but can increase the acidity of the catalyst. Whatever the chosen binder, the minimum amount should be added [75]. Almeida et al. [9] studied the influence of the slurry composition on viscosity, load, and adherence of washcoated Fecralloy monoliths. They verified how colloidal alumina increased viscosity... 

---