It is known to use thixotropic cable gels as filling material in optical fibre cables.

DE 36 22 211 describes a filling material which contains from 50 to 99 wt. % polypropylene glycol, especially having a molar mass from 2000 to 3500, and from 50 to 1 wt. % highly disperse silicon dioxide as thixotropic agent.

US 4,701, 016 describes a filling material which contains from 77 to 95 wt. % of an oil or of a mixture of oils and from 2 to 15 wt. % colloidal particles. They may additionally contain up to 15 wt. % of an elastomer. There are used as oils paraffin oil, naphthenic oil, polybutene oil, triglyceride-based vegetable oil, polypropylene oil, chlorinated paraffin oil and polyesters. The colloidal particles consist of hydrophobic fumed silica, precipitated silica and clay.

In US 5,348, 669 there is described a filling material which contains from 75 to 95 wt. % of a polypropylene glycol having a molar mass of at least 3000, and from 2 to 20 wt. % of either a hydrophilic or hydrophobic fumed silica. The filling material may additionally contain an antioxidant in order to improve temperature resistance, and an elastomer in order to reduce oil separation.

US 5,433, 872 describes a filling material which contains from 75 to 95 parts of a liquid to semi-solid polyol having a molar mass of at least 4000, small amounts of an unsaturated compound, and from 1 to 15 wt. % of a fumed silica as thickener. The filling material may additionally contain a thermoplastic elastomer in order to reduce oil separation.

DE 38 39 596 describes a thixotropic gel and its preparation based on a synthetic hydrocarbon oil and a hydrophobic thixotropic agent. The cable gel is suitable as filling material in the production of optical fibre cables.

It is also known from the series of documents Pigmente No. 63, Degussa AG, 1995, page 28 to 30, that compressing hydrophilic fumed silica enables the silica to be incorporated into the binder more rapidly, for example using a planetary mixer or a planetary dissolver.

Incorporation is understood to be the time required until the finely divided silica has disappeared completely from the surface of the binder and is wetted with the binder.

Depending on the size of the batch, the dispersing device and the recipe, the incorporation time can be up to several hours and is accordingly the step that determines the speed of production of the product in many applications.

Furthermore, because of the increased density of the silica, reduced dust formation of the silica is achieved.

The smaller volume of the compressed silica can also be of advantage when planning the system of dispersing devices.

It is a disadvantage that the viscosity of the silica- containing binder falls in most applications as the tamped density increases. Likewise, the dispersibility of the silica deteriorates. A phenomenon which manifests itself in the form of stippling, for example, in the case of a silicone sealing composition and higher surface roughness.

Both effects are significant disadvantages. For that reason, compressed hydrophilic fumed silica can be used only in relatively limited fields of application.

Accordingly, the object is to produce cable gels in which those disadvantages do not occur.

The invention provides cable gels which are characterised in that they contain, in addition to polypropylene glycol

and polyol, from 1 to 15 wt. % of a compressed fumed silica which has been rendered hydrophobic with silicone oil.

The silica used can have a tamped density of from 60 to 150 g/l.

Fumed silicas are known from Ullmann's Enzyklopädie der technischen Chemie, 4th edition, Volume 21, page 464 (1982).

The pyrogenically prepared silica can be rendered hydrophobic by means of silicone oil in a known manner. For example, the following silicas may be used as the pyrogenically prepared silicas which have been rendered hydrophobic with silicone oil: æROSIL R 202 W 60 æROSIL R 202 W 90.

The physicochemical data of those silicas are listed in Table 1: Table 1 AEROSIL R202 W60 AEROSIL R202 W 90 Behaviour towards hydrophobic hydrophobic water Appearance loose white powder BET surface areal) m2/g 100 +-20 100 +-20 Average primary particle size nm 14 14 Tamped density g/1 60 90 approx. value Loss on drying3) % < 0.5 < 0.5 Ignition loss4' '% 4-6 4-6 C content % 3.5-5. 0 3. 5-5.0 pH value5) 4-69) 4-69) sio2) > 99. 8> 99.8 Al2O37) % < 0. 05 < 0.05 Fe2O37) % < 0.01 < 0.01 TiO27) % < 0.03 < 0.03 HCl7)8) % < 0. 025 < 0.025 Drum size kg 10 15

1) following DIN 66131 2) following DIN ISO 787/XI, JIS K 5101/21 3) following DIN ISO 787/11, ASTM D 280, JIS K 5101/21 4) following DIN 55921, ASTM D 1208, JIS K 5101/21 5) following DIN ISO 787/IX, ASTM D 1208, JIS K 5101/24 6) based on material dried for 2 hours at 105°C 7) based on material ignited for 2 hours at 1000°C 8) HCl content is a constituent of the ignition loss 9) in water: ethanol = 1 : 1

In a preferred embodiment of the invention, a silica known from document DE 199 61 933 Al can be used.

Using that silica it is possible according to the invention markedly to shorten the time required for the production of the cable gel compared with an uncompressed fumed silica rendered hydrophobic with a silicone oil, while the rheological and application-related properties of the thixotropic cable gel remain equally as good. This saving in terms of time when producing the cable gel enables the costs to be reduced. A further advantage achieved with that more highly compressed fumed silica is reduced dust formation during incorporation into the binder.

It is surprising according to the invention that the compressed fumed silica which has been rendered hydrophobic with a silicone oil is markedly different from the compressed hydrophilic fumed silica as regards rheological and application-related parameters. There is thus no appreciable fall in the viscosity of a cable gel as the degree of compression of the hydrophobic silica increases.

Nor is there any deterioration in other application-related parameters of the cable gel, such as, for example, oil separation. Incorporation of the compressed hydrophobic silica into the polypropylene glycol of the cable gel is markedly shorter than in the case of an uncompressed hydrophobic silica. Accordingly, the cable gel can be produced in a shorter time. This means an improvement to the prior art. Further advantages which are obtained with the use of a compressed hydrophobic silica are, as in the case of compressed hydrophilic silicas, reduced dust formation during incorporation into the binder.

The cable gels according to the invention can be used in the production of glass fibre cables.

Examples: Irganox L 57 and Irganox L 135 are used as antioxidants.

Irganox L 57 is: CAS Number Product name Content Symbols R sets 068411-46-1 Aniline, N-> 99 % N R 51/53 phenyl-, reaction products with 2,4, 4- trimethylpentene 000122-39-4 Diphenylamine < 1 % N-T R 23/24/25- R 33-R 50/53 Irganox L 135 is a high molecular weight liquid antioxidant of the formula Example 1: Production of cable gels using a dissolver and æROSIL R 202 and AEROSILe R 202 W 90 Recipe: 89.3 wt. % polypropylene glycol having a molar mass of 3850 g/mol.

1.4 wt. % Irganox L57 (2 parts) and Irganox L135 (1 part) in the form of a mixture 9.3 wt. % æROSILE R 202 or æROSIL R 202 W 90

Procedure: The polypropylene glycol is placed into the container of the dissolver. The antioxidant Irganox is added with gentle stirring. The silica is added in portions with gentle stirring, and the incorporation time is measured until the silica has disappeared completely from the surface of the polypropylene glycol and is completely wetted with the polypropylene glycol. Dispersion is then carried out in vacuo for 10 minutes at 3000 rpm.

After 1 day's storage at room temperature, the viscosity of the cable gel is measured using a cone/plate rheometer at 23°C and 20/s.

Likewise after storage of the cable gel for 1 day, the oil separation of the cable gel is measured by the following wire cone method: The cable gel is introduced by means of a spatula, as far as possible without bubbles, into a nickel wire cone (60 mesh sieve) and suspended over a glass beaker by a wire. The glass beaker is placed in an oven at 80°C for 24 hours. The oil separation of the cable gel, i. e. the pure oil that has dripped onto the bottom of the glass beaker during storage at high temperature, is determined gravimetrically by re-weighing and is indicated in percent.

Results: Description Incorporation Viscosity in Oil separation time in s Pa s in % Cable gel 81 23.9 0.65 with AEROSIL R 202 Cable gel 36 23.3 0.77 with æROSIL R 202 VV 90

Evaluation : The incorporation time when æROSIL R 202 W 90 is used is markedly shorter than when æROSIL R 202 is used, with almost identical viscosities and identical oil separations.

Production of the cable gel can therefore be shortened by using æROSIL R 202 W 90.

Example 2: Production of cable gels by means of a 3-roller mill using æROSILE R 202, AEROSIL@ R 202 W 90 and AEROSILe 200 Recipe: 89.1 wt. % polypropylene glycol having a molar mass of 2700 g/mol.

1.4 wt. % Irganox L57 (2 parts) and Irganox L135 (1 part) in the form of a mixture 9.5 wt. % æROSIL R 202 or æROSIL R 202 VV 90 or AEROSILO 200 Procedure : The polypropylene glycol is placed into the container of the dissolver. The antioxidant Irganox is added with gentle stirring. The silica is added in portions with gentle stirring, and the incorporation time is measured until the silica has disappeared completely from the surface of the polypropylene glycol and is completely wetted with the polypropylene glycol. The cable gel is then milled twice in each case by means of a 3-roller mill.

After 1 day's storage at room temperature, the viscosity and the flow limit are measured using a cone/plate rheometer at 23°C and 20/s. Likewise after storage of the cable gel for 1 day, the oil separation of the cable gel is measured by the following wire cone method: The cable gel is introduced by means of a spatula, as far as possible without bubbles, into a nickel wire cone (60 mesh sieve)

and suspended over a glass beaker by a wire. The glass beaker is placed in an oven at 80°C for 24 hours. The oil separation of the cable gel, i. e. the pure oil that has dripped onto the bottom of the glass beaker during storage at high temperature, is determined gravimetrically by re- weighing and is indicated in percent.

Results: Description Incorporation Flow limit Viscosity in Oil time in s in Pa Pa s separation in % Cable gel with 16 154 22.7 2.0 AEROSILO R 202 Cable gel with 8 122 20. 0 2. 1 AEROSILO R 202 W 90 Cable gel with 28 0.01 35 97 æROSIL 200 Evaluation: æROSIL R 202 W 90 can be incorporated into the polypropylene glycol markedly more rapidly than æROSIL R 202, with approximately comparable rheological properties and oil separations.

The hydrophilic fumed silica AEROSILe 200 is not suitable for the production of a polypropylene glycol-based cable gel, because the oil separation of 97 % in that case is much too high.