Note: Descriptions are shown in the official language in which they were submitted.
- 1 200386S
A thixotropic ~el and its us_ as
a fillinq compound for fibre-oDtic cables
The invention relates to a thixotropic gel comprising a synthetic
hydrocarbon oil, a thixotropic agent and also optionally an organic
thickener, a mineral oil and other additives. The in~ention also
relates to the use of such a thixotropic gel for fibre-optic
cables as a core-filling compound, i.e. a compound for filling the
thin cladding surrounding each individual optical fibre in a
fibre-optic cable, and also as a cable-filling compound, i.e. a
compound for filling the space between the outer tubing of a
fibre-optic cable and the cladded optical fibres which it contains.
lO Thixotropic gels and their use as a filling compound in fibre-optic
cables are known.
In DE-A 27 28 642 a longitudinal water-tight fibre-optic cable with
a loose casing is described, in which a viscous substance which
does not flow or drip in the cable is introduced into the loose
15 casing. A lightly cross-linked silicone resin, a polyester resin,
a thermoplastic polyurethane rubber or an expanded polystyrene in
oil is used as the filling compound.
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EP-B Q 029 198 describes a fibre-optic cable in which at least one
lightguide in the form of a fibre is arranged in the inside of a
20 protective casing and the remaining inner space is filled with a
gelatinous substance. The gelatinous substance consists of a
mixture of an oil and a thixotropic agent. It additionally
contains an organic thickener which consists wholly or partly of
halogenated and/or halogen-free hydrocarbon polymers. Aromatic or
25 aliphatic hydrocarbon oils, paraffin oil, silicone oil and
halogenated, especially chlorinated biphenylene are used as the
oil.
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2003865
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A filling compound for fibre-optic cables is described in US-A
4 701 016, which consists of an oil or a mixture of oils, colloidal
particles and, in some cases, organic thickeners. Paraffin oil,
naphthenic oil, polybutene oil, vegetable oil based on
triglyceride, polypropylene oil, chlorinated paraffin oil and
polyesters are used as the oils. The colloidal particles consist
of hydrophobized pyrogenic silicic acid, precipitated silicic acid
and clay.
The filling compound in the fibre-optic cable is intended to ensure
that if possible no tensile or compressive forces are exerted on
the sensitive optical fibre, because its attenuation increases in
an undesirable manner under mechanical stress. For this reason
thixotropic gels are preferably used as the filling compound which
do not flow at rest but when under mechanical stress liquefy
temporarily, so that the optical fibre is buffered from the
external influence.
Furthermore, the filling compound serves to prevent the penetration
of water into the intermediate space between the optical fibre and
cable casing. The filling compound must finally be compatible with
the coatings used as protective coatings on the optical fibres and
with the protective casing of the core and possibly also with the
outer cable casing.
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Filling compounds based on silicone oil have the disadvantage that,
due to its tendency to creep with the passing of time, the silicone
oil coats the cable connection elements and electronic components,
and this can lead to faults. For this reason filling compounds
which contain silicone oil are hardly used today.
Filling compounds which contain halogenated hydrocarbons have the
disadvantage that the halogen content can have a corrosive effect.
2003865
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For these reasons, gels based on hydrocarbons are preferred for
the production of fibre-optic cables. The known gels based on
hydrocarbons are however unsatisfactory in various respects.
Gels based on paraffin oil have the disadvantage that the paraffin
crystallizes at low temperatures, e.g. in the winter. The
formation of crystals produces undesirable mechanical stress on the
optical fibres. Many of the known gels based on hydrocarbons, and
particularly those products which are suitable for processing at
a higher temperature, lose their thixotropic properties at low
temperatures. Below the pour point of the fluid phase, in
fibre-optic cables these gels result in a considerable
deterioration in the attenuation behaviour. For this reason a gel
based, for example, on a white oil with a pour point of -25C,
as described in US-A 4 701 016, is not suitable for use in regions
with severe winters.
The use of gels based on hydrocarbons with low pour points as a
filling compound for fibre-optic cables has also been tried. These
gels however are problematic with regard to their processing.
Usually when an optical fibre core or cable is manufactured, the
filling compound and the plastic covering are co-extruded. The
filling compound is exposed to a high temperature of, for example,
200C in the extruder. This processing temperature lies above the
flash point of the synthetic polybutene oil with a pour point of
-35C which is used according to US-A 4 701 016. An additional
disadvantage is that the known gels based on hydrocarbons with low
pour points have an undesirably high vapour pressure at the high
processing temperature.
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The object of the invention is to improve a thixotropic gel of the
type mentioned above in such a way as to overcome the
200386~
aforementioned disadvantages. Firstly, the gel must also be
suitable as a filling compound for fibre-optic cables in regions
with cold winters. And secondly, it must be possible for it to be
processed problem-free at a high temperature.
The object is achieved with a thixotropic gel of the type mentioned
above, in which the synthetic hydrocarbon oil comprises a hydrated
polyalkylene, which is a polymer of l-octen, l-nonen, 1-decen, 1-
undecen and~or l-dodecen or a mixture of these polymers. Preferred
embodiments of the invention result from the sub-claims.
The gels according to the invention are surprisingly well-suited
; for the production of fibre-optic cables. They behave neutral to
the other construction materials used in fibre-optic cables. They
have good oxidation stability. They can be easily processed at
high temperatures. They keep their thixotropic properties down to
temperatures below -50C. Because the gels according to the
invention essentially contain only hydrophobic materials, they
prevent the penetration of water into a fibre-optic cables fil led
with them.
The synthetic hydrocarbon oils used according to the invention have
a special, particularly highly branched structure. Because of this
branching these oils have a low temperature dependence as regards
their viscosity. The length of the branching also has a
considerable effect on the pour point, which generally lies below
-40C and preferably below -50C.
Hydrated polyalkylenes, as used according to the invention, are
commercially available. They are usually produced by
polymerisation, either thermal or catalytic, of the monomers in
the presence of a di- tert.-alkylperoxide or a Friedel-Crafts
catalyst such as aluminium chloride or boron trifluoride.
200~86~`
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According to the invention the preferred average value of the chain
length of the monomers is about 10 carbon atoms. Particularly
preferred are l-decen, or olefin mixtures which are rich in this
olefin.
Usefully, the synthetic hydrocarbon oil used according to the
invention has a dynamic viscosity at room temperature of 5 to I o4
mPa.s The dynamic viscosity i5 preferably from 200 to 400 mPa.s.
The kinematic viscosity of the suitable, co~nercially available
products is from 10 to S00 mm2/sec. at 40C.
These branched hydrocarbon polymers have good shear stability.
They can therefore be worked into gels in high-speed mixers with
the other constituents without any problem.
They are easily miscible with mineral oils and many natural and
synthetic polymers. It is therefore possible to modify the gels
` 15 in the invention by adding further substances.
A suitable thixotropic agent is pyrogenic silicic acid. In
particular hydrophobic, pyrogenic silicic acid is the preferred
thixotropic agent when the gel according to the invention is used
as a core filling compound in fibre-optic cables. In this silicic
acid the surface is chemically modified with silane. In addition
hydrophobized precipitated silicic acid also comes into
consideration.
Another suitable thixotropic agent is hydrophobized clay such as,
for example, tetra-alkylammonium derivatives of montmorillonite.
Finally, metallic soaps such as aluminium soaps of long-chain fatty
acids are also suitable thixotropic agents.
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200;~865
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In general the thixotropic agents have a B.E.T. surface area of
about 50 to about 400 m2/g.
In order to ensure that the oil remains fixed in the gel phase even
when under extreme conditions for a long period of time, it is
useful to add an organic thickener. Suitable thickeners are
described in EP-A 29 198 and US-A 4 701 016. A typical thickener
is a hydrocarbon polymer with a viscosity of 103 to 105 mPa.s
at 180 degrees C.
Other suitable thickeners are thermoplastic rubbers, in particular
10 styrene-(ethylene-butylene) blockcopolymers and styrene-(ethylene-
propylene) blockcopolymers. These materials are used in proportions
below 10~ by weight, based on the total weight of the gel.
For reasons of economy it may be desirable for the gel according
to the invention to contain mineral oil or another natural or
15 synthetic polymer as a further constituent. When these additional
oils are used the advantages of the invention are in some
circumstances only obtained to a lesser degree. For gels containing
mineral oil as a further constituent it is particularly preferred
to use thermoplastic rubber as a thickener.
20 It can be useful to add an antioxidant to the thixotropic gel.
In addition, colorants can be added to the gel for labelling
purposes.
Inorganic fillers, e.g. kaolin, chalk or the like can also be
admixed, particularly when the gel is used used as a core-filling
25 compound.
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The following Table 1 gives an indication of the quantities in
which the basic constituents can be contained in the gel according
to the invention. The contents in Table 1 are given in parts by
weight.
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Table 1
5 Branched synthetic
hydrocarbon oil,
e.g. poly-l-decen 99 - 80 10 - 89 98 - 60 10 - 88
'thixotropic agent 1 - 20 1 - 20 1 - 20 1 - 20
organic
10 thickener - - 1 - 20 1 - 20
mineral oil - 89 - 10 - 88 - 10
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The invention also relates to the use of the thixotropic gel for
the production of fibre-optic cables. The gel according to the
invention is suitable both as a core-filling compound and as a
15 cable-filling compound.
In the following, the production of the thixotropic gel according
to the invention is described with the aid of an embodiment which
is explained in detail. In the comparative examples materials
according to the prior art are compared.
20 The cone penetratLon given in the examples has been determined
according to DIN ISO 2137. The ~lash point of the gels was
determined according to DIN ISO 2592.
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2003865
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Example 1
A core-filling compound according to the invention for universal
use was produced from the following constituents:
92.5 parts by weight of a synthetic hydrocarbon oil rich in
poly-1-decen with a kinematic viscosity at 40C of 62 mm2~sec. and
a density at 15C of 0.833 g/cm3.
7.5 parts by weight of a silane-modified pyrogenic silicic acid
with a B.E.T. surface area above 150 mZ/g.
The oil was placed in a mixing vessel and heated to about 80C.
The silicic acid was added in portions under vigorous agitation.
Then it was mixed for about 1 hour at a high rate of agitation (800
to 1000 r.p.m.). After this, it was de-gassed for 30 minutes at
a medium rate of agitation (400 r.p.m.) under a low vacuum. Then
it was re-evacuated for a further 30 minutes under an increased
vacuum (approx. 200 mbar).
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; 15 The properties of the gel obtained in this way are given in Table
2.
Comparative ~m~le 2
For the production of a core-filling compound for regions with a
cold climate according to the prior art, 92.5 parts by weight of
a mineral oil were used with a kinematic viscosity at 40C of 13
mm2/sec. and a density at 15C of 0.850 g/cm3. The procedure in
Example 1 was then followed. The properties of the gel are shown
in Table 2.
20038~i5
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ComParative ExamPle 3
For the production of a core filling compound according to the
prior art for regions with a Central European climate, a mineral
oil was used with a kinematic viscosity at 40C of 100 mm2~sec. and
a density at 15C of 0.885 g/cm3. The procedure in Example 1 was
then followed. The properties of the gel obtained in this way are
shown in Table 2.
Table 2
Example 1 Comp. Comp.
Example2 Example 3
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Flash point (C) 240 155 250
Loss from evaporation at 1.5 30 0.5
150C/24 hr. (% by weight)
Cone penetration (1/10 mm)
at 25C 360 360 360
' at -45C 255 200 20
at -60C 188 145 5
~ As Table 2 shows, the gel in Comparative Example 3 is unusable at
,, low temperatures as a core-filling compound for fibre-optic cables.
f The gel in Comparative Example 2 can only be processed to produce
a fibre- optic cable with difficulty, because its flash point lies
20 below the usual processing temperature and it is very volatile at
a high temperature. In contrast the gel according to the invention
is satisfactory in every respect. It can be processed at a high
temperature, e.g. 200C without any problem. It does not set even
at temperatures down to -60C.
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