CAB-O-SIL®Untreated Fumed Silica Properties & Functions
CAB-O-SIL®untreated fumed silica is fumed silicon dioxide, a material which is generally regarded as unique in industry becau of its unusual particle characteristics. CAB-O-SIL fumed silica’s extremely small particle size, its enormous surface area, its high purity and its chain-forming tendencies t it apart in a class of its own.
So uful has CAB-O-SIL become—in so wide a variety of products—that a CAB-O-SIL sample is a standard item on most laboratory shelves. Almost every day, someone finds a new way to u
CAB-O-SIL fumed silica.
cayaForemost among CAB-O-SIL fumed silica’s veral contributions to product performance is its ability to modify a product’s flow properties by building a three-dimensional network which alternately forms and disrupts in respon to the degree of shear forces prent—thereby controlling flow characteris-tics in accordance with the formulator’s exact requirements.
At the same time, the success of CAB-O-SIL fumed silica in simultaneously enhancing veral different properties of a formulated product makes possible distinctive chemical creations which would be difficult to achieve with any other single material—or even with the costly addition of veral materials. Combinations of thickening, thixotropy, suspension of solids, and optical clarity, for
example, have been obtained in such diver products as coatings, adhesives, cosmetics, inks, plastics, and rubbers. CAB-O-SIL fumed silica’s action to promote free flow of dry powders, granules, and pellets has been beneficial in foods, cosmetics, and chemicals.This general brochure is intended to (1) introduce the rearcher to CAB-O-SIL fumed silica and the properties it can impart to both liquid and dry systems, and (2) aid him or her in evaluating CAB-O-SIL in his or her own formulations. For certain industries and functions, special brochures have been prepared to aid in identifying the areas where CAB-O-SIL can help improve product and process effectiveness.
Light, fluffy, pure white CAB-O-SIL®untreated fumed silica,
piled on a laboratory micro-spatula.
Introduction
Manufacture
CAB-O-SIL®untreated fumed silica is produced by the vapor pha hydrolysis of silicon tetrachloride in a hydrogen oxygen flame. The reactions are:
2Hbbc china
2+ O
2
2H
2xiangyue
zp
O
SiCl
4+ 2H来吧宝贝英文
2
costa ricaO SiO
2
+ 4HCl
(Overall reaction):
SiCl
4+ 2H
2
+ O
2strikes
SiO
2
react+ 4HCl
The combustion process creates silicon dioxide molecules which conden to form particles. The particles collide, attach and sinter together.The result of the process is a three-dimensional branched chain aggregate with a length of approxi-mately 0.2–0.3 microns. Once the aggregates cool below the fusion point of silica (1710°C), further collisions result in mechanical entanglement of the chains, termed agglomeration. Fig. 1 illustrates this process. Further agglomeration takes place in the collection system to yield a fine white fluffy powder with an agglomerate size of less than 325 US Mesh (44 microns). Calcining reduces the hydrogen chloride level to less than 60 ppm.
Several CAB-O-SIL fumed silica grades are available in dend form. The mechanical compres-sion increas the bulk density from approximately 2.5–3 pounds per cubic foot pour density to approxi-mately 5 pounds per cubic foot pour density.
Properties of CAB-O-SIL®
Untreated Fumed Silica
Figure 2: Electron micrograph of CAB-O-SIL ®fumed silica
machiwhich has been highly disperd in collodion to isolate thestings
individual aggregates. Original magnification 7,938 X; final magnification 25,000 X (from a negative print).Figure 3: Electron micrograph of a CAB-O-SIL ®fumed silica aggregate showing the outlines of individual primary particles. Original magnification 62,300 X; final magnification 350,000 X (from a ne
gative print).Figure 4: Electron micrograph of CAB-O-SIL ®fumed silica
which has been shadowed with vaporized gold to highlight
the three-dimensional random branching of the aggregates.Final magnification 50,000 X.Figure 5: Scanning electron micrograph of gold-coated CAB-O-SIL ®fumed silica showing the sphericity of the individual particles in the aggregates. Final magnification 50,000 X.
Appearance
CAB-O-SIL®untreated fumed silica is referred to as a fumed silica becau of its smoke-like appear-ance as it forms in the flame. The term “pyrogenic”silica, formerly ud, has been discontinued becau of the ambiguity resulting from other common definitions of the word pyrogenic, i.e.,“flammable” and “fever-producing.” CAB-O-SIL fumed silica has neither of the properties.
The fine, particulate nature of CAB-O-SIL can be en in high magnification electron micrographs of the material. Fig. 2 is a typical electron micrograph of CAB-O-SIL, well disperd in collodion to show the branched chain aggregates. There is virtually no mechanical entanglement (agglomeration) of the fumed silica aggregates in this micrograph. Fig. 3 is a much greater magnifica
tion of a single aggregate. It shows that the aggregate chain is in fact compod of many primary particles which have fud together.The maximum lengths of the aggre-gates are a few tenths of a micron. The electron micrographs of gold-shadowed CAB-O-SIL samples (Figs. 4 and 5) give an impression of the three-dimensional branching of the aggregates. They also show the extremely loo packing of the uncom-presd aggregates.
Purity
The purity of the starting materials, the complete-ness of the flame process, and careful handling of the product result in a very high degree of chemical purity for CAB-O-SIL untreated fumed silica. The lack of significant amounts of ionic impurities gives CAB-O-SIL excellent dielectric properties, so it is ud frequently in reinforcement applications in polymeric systems for electrical insulation—including organic rubbers, vinyl polymers, and silicones.
Density
The true density of the aggregate is 2.20 grams per milliliter, but the bulk density of uncompresd CAB-O-SIL untreated fumed silica is much lower, being approximately 2.5–3 pounds per cubic foot pour density. In the compresd grades this is incread to approximately 5 pounds per cubic foot p
our density.
Compaction of the CAB-O-SIL results in the formation of large agglomerates, formed through mechanical entanglement of the aggregates. In order to obtain optimum utilization of CAB-O-SIL in its various applications, the agglomerates must be reduced in particle size through the u of suitable dispersion equipment.Gas adsorption studies of CAB-O-SIL fumed silica have shown that the material is non-porous. Thus adsorption and desorption process are rapid since molecules are only held physically on exterior surfaces, not entrapped in pores. However, the clustered nature of the aggregates may make it difficult for large molecules—of polymers for example—to gain access to all surfaces which can be reached by the gas molecules ud in the surface area measuring procedure.
Structure
The amorphous nature of CAB-O-SIL untreated fumed silica is shown by the abnce of lines in its X-ray diffraction pattern. For comparison, this pattern is shown with the patterns of some well-known crystalline forms of silica, i.e., α-quartz,
β-cristobalite, and β-tridymite, in Fig. 6. The amorphous form is caud by the extremely rapid coolin
g of the silica aggregates, which takes place in only a few thousandths of a cond.
Refractive index
The refractive index of CAB-O-SIL untreated fumed silica is 1.46—very clo to that of many organic solvents and other materials. Glycerine, butyl alcohol, certain hydrocarbons, oils, and many other liquids can be thickened without loss of transparen-cy. Similarly, silicone rubbers and other transparent polymers can be reinforced with CAB-O-SIL with little loss of clarity.
