Colloidal sihca

The 3M Company manufactures a continuous polycrystalline alurnina—sihca—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal sihca and dimethylform amide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extmded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-p.m diameter aluminum borosihcate fibers, which are suitable for fabrication into textiles for use at temperatures up to 1427°C. 

Advincula et al. used the same initiator type (DPE activated with u-BuIi) to perform LASIP from colloidal sihca [265] or clay [266, 267]. The spacer between the DPE unit and the surface active group (quatemized amine for clay and chlorodi-methylsilane for silica) was a long n-alkyl chain. In all cases, a relatively broad polydispersity for the prepared polystyrene bmsh (PDI = 1.2-2) was observed. 

Fig. 11. Effects of pH in the colloidal siHca—water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the silica particle.

Schwarz S, Dragan S (2004) Nonstoichiometric interpolyelectrolyte complexes as colloidal dispersions based on NaPAMPS and their interaction with colloidal sihca particles. Macromolecular Symposia 210(1) 185... 

Schwarz S, Lunkwitz K, Kefiler B, Spiegler U, Killmann E, Jaeger W (2000) Adsorption and Stability of Colloidal Sihca. Colloids and Surfaces A 163 17... 

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkah-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal Si02 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al usually present in colloidal sihca 33). 

Hamilton, S. K., Sippel, S. J., and Bunn, S. E. (2005). Separation of algae from detritus for stable isotope or ecological stoichiometry studies using density fractionation in colloidal sihca. Limnol. Oceanogr. Methods 3, 149-157. 

Davis, K., Russel, W., and Glantschnig, W., Disorder-to-order transitions in settling suspensions of colloidal sihca x-ray measurements. Science, 245, 507, 1989. Bartlett, R, Pusey, R, and OttewiU, R., Colloidal crystallization under time-averaged zero gravity, Langmuir, 1, 213, 1991. 

Sears, G.W., Determination of specific surface area of colloidal sihca by titration with sodium hydroxide. An /. Chem., 28, 1981. 1956. 

Sonnefeld, J., An analytic expression for the particle size dependence of the surface acidity of colloidal sihca, J. Colloid Interf. Sci., 155, 191, 1993. 

Kasseh, A. and Keh E., Transfers of colloidal sihca from water into organic solvents of intermediate polarities,/. Colloid Interf. Sci., 197, 360, 1998. 

Bauer, D. et al., Stabihty of colloidal sihca, sikron and polystyrene latex influenced by the adsorption of polycations of different charge density, Colloids Surf A, 156, 291, 1999. 

Thereafter, sihca assumes different characteristics and can be stabilized as cohoidal sihca sols in the pH range 8-10. Thus, colloidal sihca as opposed to PSA is made of highly polymerized species or particles larger than about 5 nm. 

This could have some ramifications in the manufacture of colloidal sihca. Typically, sodium silicate is diluted prior to being deionized and polymerized. It is likely that the equilibrium distribution of oligomers will be different depending on whether the solution is freshly made or has been allowed to stand diluted for some time. While it is not clear that this variation in silica sources could have a significant effect on the properties of the colloidal silica made from them, it is a variable... 

All colloidal sihcas have a concentration at which they will gel. The smaller the particle size, the lower the gelling concentration. Typically, this gel concentration will be about 35% silica solids for a 7-nm sol, 45% for a 12-nm sol and 55% for a 22-nm sol. Since the silica is more than twice as dense as water, the % volume solids of... 

If one examines the effects of colloidal silica on the optimum flocculation point, one finds that the amount required is always related to the surface area of the colloidal silica used. If it takes 5 grams of colloidal silica with a smface area of400 m /gm to optimize the flocculation, then it will take 10 grams of colloidal sihca with a smface area of 200 m /gm and 20 grams if the surface area is 100 m /gm. One must supply the same total smface area to achieve optimal results. However, there is a major difference in the activity of the colloidal silica... 

The colloidal sihca-resin, e.g., polyvinyl acetate components of the binder can be used in the form of a stable liquid mixture, the carbonaceous material being optionally present. Thus uniform mixtures containing colloidal silica and polyvinyl acetate within the relative amounts specified in this invention, such as 1.94 parts by weight of 40% aqueous colloidal silica and 2 parts by weight of 55% polyvinyl acetate aqueous dispersion, can be made by mixing the two components in a beaker. The mixture is stable and uniform and can be used within the working day. Overnight the mixture tends to separate in two layers and can be stirred up to make it uniform. 

One method of providing a stable, pourable mixture of colloidal sihca-polyvinyl acetate with or without the carbonaceous material, e.g., pitch, is to make the liquid phase slightly thixotropic but not viscous. In other words, to make it so that it sets to a weak gel structure at once when undisturbed (to maintain all particles in uniform suspension) but when stirred, or even tilted to pour, the yield point is so weak as to permit ready transfer of the material and easy blending with the sand. 

If an alkah metal polysilicate solution is used as a binder, it should be mixed directly with the sand. If on the other hand colloidal sihca and sodium silicate solution are added separately to the sand, it is preferable to add the sihca sol first and to mix it thoroughly with the sand before adding the sodium silicate. Once the sodium silicate is added, the mix should not be kept too long in the mixer. A period of 2 min stirring is generally optimum for the sodium silicate. 

Dry colloidal sihcas such as pyrogenic amorphous sihca do not mix well with the sand and in addition they... 

Average particle size and particle size distribution are very important properties of colloidal silica that influence the behavior of such materials in a broad number of applications. When colloidal sihcas are aggregated in gels, precipitates, or fumed products, their texture can be described in terms of pore volume distributions. The texture of the aggregate also influences its behavior in applications. The chapters in this section describe methods used to determine particle size and pore volume distributions in silicas. 

Many of these applications are practiced today. Other apphcations exist only for specialized situations. For example, sihca sols are a desirable source of sihca for catalyst substrates. For catalyst preparations, however, colloidal sihca is generally used only for specialized apphcations. 

Griessbach [17] showed that five companies produced sihca sols in 1933. Most products were very dilute, and only one could be considered concentrated. The most concentrated sol available at that time was a product called Kieselsol I.G. (made by I.G. Farbenindustries), which contained 10% sihca and was stabilized with ammonium hydroxide. The characteristics of most of the sols were undetermined. Moreover, the difficulty of making a reproducible product that would perform in a predictable manner for specific uses was technically impossible at that time. This difficulty, most likely, was the major reason that colloidal sihca apphcations did not increase rapidly. Her [18] pointed out that colloidal sihca was not accepted for wide commercial use until methods were discovered for producing sols with high concentrations that would not gel or settle with time. The first steps to achieving that goal occurred in 1941 (discussed in the preceding section on preparation procedures). 

Dilute silica sols were prepared and studied over 70 years ago. Their uses as binders in catalyst preparation, as glazes on ceramics, as coatings on concrete and plaster of Paris, as agents for treating paper and textiles, and several other applications were investigated [1]. These early sihca sols contained less than 10% by weight of silica, were fairly unstable and did not have reproducible properties. Her [2] predicted that colloidal sihca would not be accepted for wide commercial use before these shortcomings were remedied. 

Otterstedt and Brandreth [4] discusses functions that can be achieved by using colloidal sihca in various applications. Most of these functions depend on the presence of a high specific surface area of a special chemical nature. 

The cement pastes, with or without colloidal sihca, had a water to cement ratio of 0.35 and contained sulfo-nated naftalene formaldehyde resin (NSF) superplasticizer. The colloidal silica was added to the concrete mixture after the superplasticizer so as to minimize prema-mre gelling. The compressive strength of the concrete samples are shown in Table 57.5. 

Amorphous silica-sodium silicate binder composition of Si02/Na20 ratio 5 1 can be formed directly on the sand by addition of colloidal sihca sol of uniform particle diameter about 14 nm to the sand, mixing, and then adding sodium silicate Si02/Na20 molar ratio 3.25 1 and mixing for 2 min. 

S.A. Johnson, P.J. OUivier, and T.E. Mallouk, Ordered mesoporous polymers of tunable pore size from colloidal sihca templates. Science, 283(5404), 963 965 (1999). 

Fig. 5. Sample plot showing the MOR values for some of the batches extmded into rods, and fired at three different temperatures, 300, 600 1000°C, for 3hrs. MOR values increase with firing temperature, which may be due to the onset of sintering. This hypothesis is corroborated by the reduction in specific surface area observed. For comparison, MOR value for fired Cordierite rods, of comparable porosity, is shown by the dashed line. The five batches shown here differ in compositional details like the particle size distribution (PSD) of the filler phase used and the amount of the fiber phase, colloidal sihca phase used.

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