Fumed silica density

Figures 8.13 and 8.14 show the HRR and mass loss rate of the polypropylene samples with each silica additive. The addition of low density, large surface area silica, such as fumed silicas and silica gel, to PP and PEO significantly reduced the HRR and mass loss rate. However, the addition of fused silica did not reduce the flammability properties as much as other silicas.

The authors proposed a mechanism that accounts for the reduction in flammability properties, which depends on physical processes in the condensed phase rather than chemical reactions. Three factors are critical in determining the silica behavior during the combustion process the density and surface area of the additive, the melt viscosity of the polymer. The interplay between these factors can determine whether the silica will accumulate near the surface or sink through the polymer melt. Fumed silica and silica gel provide examples for the first case where the silica particles accumulated on the surface and formed an insulating layer that provide protection to the underlying polymer. This is in contrast to the other case where the fused silica particles sank through the polymer melt. 

Abstract. Nanocarbon materials and method of their production, developed by TMSpetsmash Ltd. (Kyiv, Ukraine), are reviewed. Multiwall carbon nanotubes with surface area 200-500 m2/g are produced in industrial scale with use of CVD method. Ethylene is used as a source of carbon and Fe-Mo-Al- mixed oxides as catalysts. Fumed silica is used as a pseudo-liquid diluent in order to decrease aggregation of nanotubes and bulk density of the products. Porous carbon nanofibers with surface area near 300-500 m2/g are produced from acetylene with use of (Fe, Co, Sn)/C/Al203-Si02 catalysts prepared mechanochemically. High surface area microporous nanocarbon materials were prepared by activation of carbon nanofibers. Effective surface area of these nanomaterials reaches 4000-6000 m2/g (by argon desorption method). Such materials are prospective for electrochemical applications. Methods of catalysts synthesis for CVD of nanocarbon materials and mechanisms of catalytic CVD are discussed. 

MWNT are produced in form of black powder with bulk density of 15-40 g/dm3. Experiments carried out by us have shown that low bulk density MWNT samples are preferable for application in composite materials, particularly for PTFE-MWNT composites. So, we tried to produce MWNT with the bulk density as low as possible. This was achieved by use of co-precipitated Fe203Mo03-Al303 catalysts containing aerosil (fumed silica) as a pseudo-liquid diluent of growing nanotubes [11, 12],... 

Keywords fumed silica alumina/silica, titania/silica alumina/silica/titania Ni(II) Cd(II) Pb(II) polyethylene glycol) poly(vinyl alcohol) adsorption potentiometric titration surface charge density... 

Fumed silica possessing relatively large specific surface area can adsorb different metal ions up to 100% at pH > 7 (Figure 1), when the contribution of different hydroxy species of these ions increases and the surface charge density of silica becomes more negative. Therefore, a tenfold increase in the initial CpbdD value leads to a tenfold increase in the plateau adsorption (Figure 2). 

Fumed silica appears as a fluffy white powder characterized by an extremly low bulk density down to the range of about 20-50 g f. In contrast, the submicron fumed silica particle consists of amorphous silicon dioxide and, hence, its true density is about 2200 g 1 ... 

Any discussion of fumed silica particle structure has to take into account this enormous difference. The approach of mass fractal dimension may provide a rough but helpful estimation. A real mass fractal is limited by the size of the cluster as an upper limit and the size of the particles as a lower limit. Then, the density of the cluster pduster be calculated from the true density of the particle Pparticio fhe ratio of the cluster size c/ciuster to the particle size /particle and the mass fractal dimension /) of the cluster (Eq. 2) ... 

By Eq. 2 and D = 2.5 from DLA we estimate the apparent density of the aggregate to be 700 g 1 . This is the density found experimentally, when powdered fumed silica is pressed to a solid disc. > = 2.1 from RLCA then gives an agglomerate density of about 20 g 1 - close to the bulk density of the freshly produced fluffy product. 

Under suitable reaction conditions the acid-base reaction provides a hydroxyl capacity or silanol group density of fumed silica of about 2 SiOH per nm. This veilue falls well between the total amount of silanol groups on fumed silica of about 2.5 SiOH per nm [11] and 1.7 SiOH per nm as reported for the content of reactive isolated silanol groups [12]. Titration seems to give a good estimation about the content of chemically reactive and available silanol groups. 

There are a variety of other silicas. These include the metastable silica-W, which has a very low density (1.97 gcm ), and whose structure consists of edge-sharing chains of Si04 tetrahedra, the microcrystaUine diatomaceous earths, various forms of noncry stalline silica inclnding vitreous silica (glassy silica), and the finely divided amorphous silicas such as dry silica gel, fumed silicas, and colloidal silicas. 

There are many similarities between oxide CMP and poly-Si CMP. The main difference between the two processes is that poly-Si CMP slurries contain less abrasives and are, in general, more chemically active. Therefore, the poly-Si CMP process is by nature very sensitive to the polishing temperature. Temperature has a direct effect on removal rate, topography removal, and defect density (pitting and voids). Most poly-Si CMP slurries use colloidal silica that is less likely to form large aggregates than the fumed silica. 

Such a hybrid behavior has been noted on various materials amongst the most porous and is not peculiar to low density xerogels. It is the case of carbon black [6] and of some silica precipitated from alkaline silicates. The fumed silica synthesized in gas phase exhibits, depending on their bulk density, either a collapse behavior similar to the one of aerogels or a hybrid behavior similar to the one of low density xerogels [7]. 

That observation is consistent with both the high manufacturing temperature of the fumed silica and its low hydroxyl surface density ( 3.65 OH groups per square nanometer ... 

Surface of a Fumed Silica. Several results obtained for silica A, as received and after some contact with air, can be rationalized in the following way. The low silanol surface density (about 3.65 OH per square nanometer, internal silanols excluded), the comparatively high fraction of geminal sites (/g = 0.21), and the presence of a rather strong D2 band in the Raman spectrum indicate an only partial and selective hydrolysis of the surface after the manufacturing of silica A at high temperature. 

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