Hydrophobic silica
Hydrophobic, pyrogenically produced silica having a tamped density of 55 to 200 g/l is produced by hydrophobizing pyrogenically produced silica and then compacting it. The silica may be used for the production of dispersions.
This application claims the benefit of provisional application 60/171.667 filed Dec. 27, 1999 which is relied on and incorporated by reference.
INTRODUCTION AND BACKGROUNDThis invention relates to a hydrophobic, pyrogenically produced silica, to a process for the production thereof and to the use thereof.
It is known to compact hydrophilic, pyrogenically produced silica (EP 0 280 854 B1). Disadvantageously, as tamped or bulk density increases, thickening action declines in a linear manner. Dispersibility also falls as density increases. This results in unwanted speckling. Thus, once compacted, a hydrophilic, pyrogenically produced silica may only be used for a limited number of applications.
It is therefore an object of the present invention to avoid the problems of compacted, hydrophobic, pyrogenically produced silica of the past.
SUMMARY OF THE INVENTIONThe above and other objects of the present invention can be achieved by developing a hydrophobic, pyrogenically produced silica having a tamped density of 55 to 200 g/l.
The tamped density is preferably from 60 to 200 g/l.
A feature of the present invention is a process for the production of the hydrophobic, pyrogenically produced silica having a bulk density of 55 to 200 g/l, which process is characterised in that pyrogenically produced silica is hydrophobized using known methods and then compacted.
Hydrophobing can preferably be performed by means of halogen-free silanes. The chloride content of the silica can be less than or equal to 100 ppm, preferably from 10 to 100 ppm.
Compaction can be performed by means of a roller compactor. Compaction can preferably be performed by means of a belt filter press according to EP 0 280 851 B1, which reference is relied on and incorporated by reference.
The hydrophobic, pyrogenically produced silica used for purposes of the present invention can be, for example, the silicas known as:
Aerosil R 812 or Aerosil R 812S, having the group -0-Si (CH3)3
Aerosil R 202, Aerosil MS 202, Aerosil MS 202, Aerosil R 106
or Aerosil R 104 having the group
Aerosil R 805 having the group
These are commercially available products from Degussa Hüls AG.
The hydrophobic, pyrogenic silica according to the invention having a tamped density of 55 to 200 g/l exhibits the following advantages:
Transport costs are distinctly lower as a result of the higher tamped density.
Once dispersed, the silica according to the invention is in the form of relatively small aggregates.
In other words, the dispersions are more finely divided because the silica according to the invention is more readily dispersible.
The dispersions produced using the silica according to the invention exhibit a lower Grindometer value.
Both UV transmission transparency and visual transparency of the dispersions are distinctly improved by using the silica according to the invention.
Dispersions containing the silicas according to the invention exhibit distinctly increased stability because the tendency towards settling is distinctly lower.
Another advantage of the silica according to the invention is reduced dustings during incorporation and the distinctly reduced incorporation or wetting time in, for example, liquid systems.
In comparison with hydrophobic, pyrogenic silica of a lower bulk density, the hydrophobicity of the silica according to the invention is unchanged. Thickening action is also unchanged.
DETAILED DESCRIPTION OF THE INVENTIONThe present invention will be farther understood with reference to the following detailed embodiments thereof.
Example 1Various hydrophobic, pyrogenically produced silicas are investigated, wherein different compaction states are compared.
The following definitions apply:
bulk=pulverulent, unmodified silica
CF=silica compacted with a Carter filter
VV 60=silica compacted to a tamped density of approx. 60 g/l
VV 90=silica compacted to a tamped density of approx. 90 g/l
Aerosil grades R 202, US 202, US 204, R 812, R 812S and R 805 are investigated. The results are shown in Table 1.
As evaluated by the Corning Glass methanol wettability method, the degree of compaction has virtually no appreciable influence on hydrophobicity. Viscosity also exhibits no clear systematic dependency upon tamped density. Especially for R 812, dispersibility improves with increasing density. R 812 S, which contains more SiOH groups than R 812, exhibits the above phenomenon less markedly.
US 202 and US 204 have very comparable rheological properties and are inferior to AEROSIL R 202.
Surprisingly, the compacted variants, in particular of R 812, R 202 and US 202/4, exhibit an incorporation time reduced by up to half. The compacted silicas moreover exhibit reduced dusting.
Investigation of the Influence of Higher Compaction on Applicational Properties
1. Determined on standard sample (UB 3391)
RHE in the above table indicates the Rheinfelden plant located in Germany.
Rheological Testing:
Polymer: Araldit M (biphenol-1-expoxy resin by Ciba-Geogy, in the form of clear yellow liquid).
Thixotroping agent: R 202 and R 812 Additive:
Compaction may amount to a type of predispersion. Accordingly, effectiveness values rise with tamped density. i.e. the particles effectively present in the ethanol dispersion become smaller and the compacted samples exhibit distinctly less settling. Any suitable organic solvent can be used to form the dispersion.
The compacted samples accordingly have a more favourable Grindometer value in Araldit. However, since the larger particles have a decisive influence on thickening action, the property declines slightly on compaction.
It may be seen from the graph of effectiveness values that, while the highly compacted AEROSIL R 812 sample may indeed still be broken up with the Ultra-Turrax mixer (0965), it can no longer be broken up with the high speed mixer (0955). Due to the smaller surface area of AEROSIL R 202 (and to the consequently theoretically improved dispersibility), this phenomenon hardly occurs with AEROSIL R 202.
As compaction rises, the particles effectively present in an ethanol dispersion thus become smaller and 90° angle scattering rises due to Rayleigh scattering. Total scattering (over all angles), however, falls and the samples become distinctly more transparent on visual inspection, as is also substantiated by the UV transmission spectra.
Compaction has no influence on hydrophobicity, which in each case substantially corresponds to that of the standard sample.
Example 3 Investigation of the Influence of Higher Compaction on Applicational Properties.
1) Determined on standard sample (UB 3391)
3) Guide value
The compacted AEROSIL R 202 samples behave in a similar manner to the compacted AEROSIL R 812 samples.
Reference is thus made to Example 2 with regard to the discussion.
The parameter of “effectiveness” reported in the tables herein relates to the high degree of fineness of the particle. This is therefore an indicator of high transparency and good stability of the resulting dispersions.
Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.
German priority application filed Dec. 22, 2000 199 61 933.6 is relied on and incorporated herein by reference.
Claims
1. A finely divided, readily dispersible hydrophobic, pyrogenically produced silica in the form of aggregates, having a tamped density of 55 to 200 g/l and which has been produced by a process comprising:
- (1) hydrophobicizing by reaction of the silica with a halogen-free silane and
- (2) compacting the silica by compacting means consisting of a roller compactor or a belt filter press.
2. The finely divided, readily dispersible hydrophobic, pyrogenically produced silica according to claim 1 having a tamped density of 60 to 200 g/l.
3. (canceled)
4. The hydrophobic, pyrogenically produced silica according to claim 1 wherein said silica has a chloride content of less than or equal to 100 ppm.
5.-9. (canceled)
10. A dispersion of a hydrophobic, pyrogenically produced silica according to claim 1.
Type: Application
Filed: Apr 27, 2007
Publication Date: Aug 30, 2007
Inventors: Gunther Michael (Karlstein), Volker Kasack (Niedergrundau), Rudiger Nowak (Kahl/Main)
Application Number: 11/741,381
International Classification: C01B 33/12 (20060101);
