EP0039867B1 - Longitudinally water-blocked cable, especially communication cable - Google Patents

Description

The invention relates to a longitudinally watertight cable, in particular a communication cable, in the interior of which a filling compound containing water-repellent substance is provided, into which gas bubbles are embedded, the filling compound containing an admixture of thermoplastic rubber or rubber-like thermoplastics which melts at its processing temperature as part of the cable filling. whose condition in the operating temperature range of the cable by linking, hooking or a connection via thermoplastic blocks with a correspondingly low, ie the melting range lying just below the filling temperature is solidified and the gas bubbles are held in place by the rubber network thus formed in the filling compound in such a way that its connecting points can take over the buoyant forces of the gas bubbles without tearing.

A longitudinally sealed cable of this type is known from DE-A 2243615. When gas bubbles are stored, their position in the filling compound is not readily stable, and there is therefore a risk of gas bubbles forming at certain points due to migration of the gas bubbles, which adversely affect the electrical properties of the cable. For this reason, the known arrangement provides for the position of the air pockets to be stabilized by adding a stiffening agent to such an extent that at most longitudinal cavities with a length of a few centimeters can form.

For the selection of the admixture serving as a stiffening agent, there is the requirement that it should be as easy to process as possible and that the spatial lattice structure formed by linking, hooking or connection via thermoplastic blocks is designed in such a way that the gas bubbles are held as firmly as possible is achieved. This securing of the gas bubbles against movement is necessary in order to prevent the gas bubbles from moving over a longer period of time and possibly under the influence of the prevailing temperature and pressure conditions and combining to form larger gas bubbles.

The invention is based on the object of specifying an admixture which is particularly advantageous both in terms of its processability and in terms of the spatial lattice structure formed by it. According to the invention, this object is achieved in the case of a longitudinally watertight cable of the type mentioned at the outset, in that the admixture consists of a polyolefin powder, in particular a polyethylene powder, with a grain size of between 20 and 600 11 m, that the proportion of the polyolefin powder is between 5 and 30 percent by weight of the filler mass is chosen that the volume fraction of the gas bubbles in the filling mass is chosen between 10 and 80% and that the diameter of the gas bubbles is between 1 and 1500 µm.

The use of a powdery admixture with the specified grain sizes and percentages by weight has the advantage that it dissolves more quickly in the filling compound, while at the same time ensuring that the admixture is distributed evenly throughout the entire filling compound. On the basis of this initial situation, the formation of the spatial lattice structure from the admixture as a result of the smaller particles of the admixture and their better mixing with the filling compound is also favorably influenced. The longitudinally watertight cable thus constructed shows particularly favorable properties with regard to the strength and temporal stability of its lattice structure and the manufacturing possibilities. The specified diameter range and the volume fraction make a decisive contribution to securing the position of the gas bubbles.

Polyolefins with a chain length of 25 to 45 carbon atoms can preferably be used as thermoplastic blocks, in particular low molecular weight polyethylene (PE) or paraffin wax.

Particularly advantageous values with regard to the electrical and mechanical properties as well as with regard to the material consumption of filling compound and admixture are obtained when the gas bubbles make up a volume fraction between 50 and 70% of the filling compound.

When using a polyethylene powder, it is expedient to select a polyethylene whose starting material has between about 5000 and 200,000 carbon atoms per molecule.

The invention further relates to a method for producing a longitudinally watertight cable, which is characterized in that the polyolefin powder is added to the filling compound heated to 140 to 150 ° C. with stirring and this is homogeneously distributed, that gas at a pressure of about 1.5 to 15 barü and at a temperature of about 140 ° C is introduced into the filler containing the molten powder, and that after relaxing and cooling to 80 to 120 ° C gas bubbles are formed in a fine, uniform distribution.

• 77 % Paraffin-Öl
• 19,9% Polypropylen
• 3 % feindisperse Kieselsäure
• 0,1% Stabilisator (zum Schutz gegen Oxydation)

Gemäss der Erfindung wird die Stabilisierung der Gasbläschen in der beschriebenen Füllmasse durch eine pulverförmige Zugabe von Polyolefin, insbesondere Polyäthylen, mit einem Dichtebereich von 0,915 bis etwa 0,96 g/ml erreicht. Diese erfolgt in fein verteilter Form, d.h. mit etwa 20-600 pm Korngrösse. Die Zugabe erfolgt bei einer Temperatur der übrigen Mischungskomponenten, die über dem Kristallit-Schmelzpunkt des zugesetzten Polyolefins, d.h. oberhalb ca. 135 °C, liegt. Dann wird unter entsprechendem Überdruck Gas, beispielsweise C0₂, N₂ oder Luft in der heissen Mischung gelöst. Beim gleichzeitigen Abkühlen und Entspannen (d.h. Überdruck wird aufgehoben) der Mischung bilden sich nunmehr feinverteilte Gasblasen aus, die durch das zusammenhängende flexible Gerüst aus im wesentlichen Kohlenwasserstoffen fixiert werden.A filling compound for the admixture of the powdery additive can be constructed, for example, as follows:

• 77% paraffin oil
• 19.9% polypropylene
• 3% finely dispersed silica
• 0.1% stabilizer (to protect against oxidation)

According to the invention, the stabilization of the gas bubbles in the filling composition described is achieved by adding powdered polyolefin, in particular polyethylene, with a density range from 0.915 to about 0.96 g / ml. This takes place in a finely divided form, ie with a grain size of about 20-600 pm. The addition of the other mixture components is carried out at a temperature above the crystallite melting point of the polyolefin added, ie above about 135 ° C. Then under appropriate over Pressure gas, for example C0 ₂ , N ₂ or air dissolved in the hot mixture. As the mixture cools down and relaxes (ie overpressure is released), finely divided gas bubbles are now formed, which are fixed by the coherent, flexible framework of essentially hydrocarbons.

Example:

• 69,8% Paraffin-Öl
• 18,1% (ataktisches) Polypropylen
• 2,7% (feindisperse) Kieselsäure
• 0,3% Stabilisator (Schutz gegen Oxydation u.a. des PE)
• 9,1 % Polyäthylen-Pulver

Bei einer so aufgebauten Mischung kristallisiert die Beimengung nicht wieder in diskreten Bereichen oder ungleichmässig aus, sondern bildet eine die ganze Füllmasse weitgehend gleichmässig erfüllende räumliche Gitterstruktur, welche die Gasbläschen gegen Bewegung sichert.

• 69.8% paraffin oil
• 18.1% (atactic) polypropylene
• 2.7% (finely dispersed) silica
• 0.3% stabilizer (protection against oxidation, inter alia, of the PE)
• 9.1% polyethylene powder

In the case of a mixture constructed in this way, the admixture does not crystallize again in discrete areas or unevenly, but instead forms a spatial lattice structure which largely fulfills the entire filling mass and which secures the gas bubbles against movement.

The lowest limit for adding a powder is about 5% of polyolefin or polyethylene, while the upper limit is about 30% (percent by weight). A suitable range in percentages by weight for the powdery admixture is between 6 and 20%, with optimal values being achieved by admixture between 8 and 10 percent by weight.

No restrictive conditions per se have to be observed for the filling compounds. It is only important to ensure that the filling compound does not prevent the formation of the spatial lattice structure when added. For example, compositions which consist entirely or in mixtures of petrolates, hydrocarbon waxes, aliphatic or cycloaliphatic paraffins or polymeric olefins can advantageously be used.

During manufacture, the filling compound is first heated to 140-150 ° C. The required amount of polyolefin powder, in particular polyethylene powder, is added with constant stirring. The stirring process is complete when the powder is melted and homogeneously distributed in the filling compound. In the case of direct further processing, the mass is placed in a conventional foaming device and a gas (C0 ₂ , N ₂ ) is dissolved under pressure (1.5-15 barg) at about 140 ° C. After relaxing and at the same time cooling the mass, the gas bubbles correspondingly finely divided then have a diameter in the range of 1-1500 μm, preferably between 20 and 200 11 m. The gas bubbles are advantageously formed in a temperature range of 80-120 ° C. The described foaming of the mass can take place either by means of a filling pipe in the cable core or in the stranding point.

However, it is also possible to abort the process after introducing and stirring the powder and to soak the mass in the cooled state.

The gas bubbles can be mixed in (as in cell PE production) in a known manner either by gas injection, ie they are added to the mass flow under high pressure as nitrogen or free gas before the actual filling pipe. However, it is also possible to generate the gas bubbles by means of a blowing agent having a suitable temperature-pressure dependence. It is important that the gas bubbles in the filling apparatus are still vanishingly small (this is achieved by adding Ti0 ₂ or Si0 ₂ as a kicker) and can therefore be transported evenly into all gussets of the cable core, but after the mass has been released to normal pressure, ie After the filled core exits the filling tube or the downstream extruder, expand to its final size.

When the cable is cooled, e.g. After the downstream extruder, as already mentioned, a rubber network forms in the cable filling compound, the nodes of which are mainly formed by the then solidified thermoplastic blocks. The gas bubbles in the oil mass are held in place by the network threads. For this, the proportion of the polyolefin powder must be sufficiently high, namely between 3 to 30 percent by weight. In addition, the filling compound must be sufficiently viscous, and clearly above 5 Pa s.

To explain the invention, reference is made to a drawing, in which a gas bubble G B is shown, which has formed in a filling compound FM of a cable. This gas bubble GB, which is a few tenths to a maximum of one millimeter, is held in the filling compound FM by rubber-elastic molecular threads GF which are formed by admixing polyolefin powder. However, since these threads GF would not be sufficiently stable in position within the filling compound FM containing oil and / or wax, and consequently migration of the gas bubbles, e.g. would allow due to the buoyancy, thermoplastic blocks TB are provided, which e.g. be formed by PE waxes. These thermoplastic blocks link the existing rubber-elastic molecular threads of the rubber network, which in turn has the effect that the gas bubbles present in the filling compound are held in place.

The short time after production, in which there are still higher temperatures (until cooling) and in which thermoplastic blocks TB have not yet formed as a result of cooling, is harmless, because in these small periods an undesirably large migration of the gas bubbles does not occur. The thermoplastic rubber composition forming the network is produced by block polymerization of thread-like, rubber-elastic molecules with the thermoplastics melting at about 60 to 80 ° C. Below their melting temperature, these thermoplastic blocks attach to one another and thus contribute to the formation of networks.

• Ein Gasbläschen mit Radius r wird durch die Auftriebskraft K bei einer Viskosität η mit einer Geschwindigkeit
  
  in einem fliessfähigen Körper bewegt. Mit dem Auftrieb (p = Dichte; g = Fallbeschleunigung)
  
  folgt
  

With regard to the achievable long-term stability of foam-like filling materials formed in this way, the following considerations must be observed:

• A gas bubble with radius r is at one speed by the buoyancy force K at a viscosity η
  
  moved in a flowable body. With the buoyancy (p = density; g = gravitational acceleration)
  
  follows
  

The velocity therefore increases with the square of the bubble size and decreases with the viscosity η. The bubbles must be as small as possible, ideally less than a tenth of a mm, and the viscosity η should be as high as possible. It is not the dynamic viscosity that is important, but the resting viscosity that arises at very low shear rates and speeds. This rest value is the greater than the dynamic value (which determines the processability) if the «liquid» contains more thread-like, elongated components. The resting value can become very large to infinite if the threads can form a real gel by cross-linking. The mass then behaves like a solid body of extremely low strength for small mechanical loads. The mass of rubber mixtures would roughly correspond to this picture, while the masses of non-crosslinked polymers with rubber-like properties should rather be regarded as liquids with a high resting viscosity. In addition to increasing the resting viscosity, the introduction of thread-like components has another advantage with regard to the stability of the bladder: the bubbles suffer constant deflections and changes in direction during their movement, so that the effectively covered distance is significantly smaller than the actually covered distance. If we assume, for example, that an actual path of 1 mm is allowed in 30 years for this reason, then (3) results in a necessary rest viscosity of over 10 ⁶ Pas for a bubble diameter of approximately 0.05 mm.

The rubber-like mass described behaves differently than the flowable mass of non-crosslinked polymers. There is a network of rubber threads, the wide mesh of which is filled with the oil or wax components. When analyzing this 2-phase system, both the network that provides stability at rest and the added liquid must be taken into account. In the idle state, however, the network can only absorb tensile and shear stresses up to a very low tear limit, while the liquid phase remains mobile within the mesh and approximates the laws of hydrostatics. The bubbles embedded in the liquid in turn experience a buoyancy which is transmitted to the network via the surface tension in the vicinity of the bubble and puts it under tensile, shear and possibly also compressive stresses. Instead of the viscosity analysis carried out above, the fracture mechanics of the network must be used here. The tensile stress a _z below the bladder is approximate, for example

Apart from the acceleration due to gravity g and the density difference p, it only depends on the bubble radius. For r = 0.05 mm there follows a tensile stress of only 2.7 ₂ - The exact analysis must include the complete state of stress (including shear stress) around the bladder and will lead to lower local stresses. A mass dimensioning according to (4) is therefore initially sufficient. For each steadfastness (influenced by the type and proportion of the network) follows from (4) a maximum permitted bubble size, or, if this is technologically specified, (4) results in an at least necessary quantity ratio for the admixture of a given network former.

Since the size of the buoyancy depends on the diameter of the gas bubbles, the buoyancy K can thus be adjusted in a particularly simple manner by selecting the size of the bubbles so that the network structure cannot be torn apart by this driving force K. In addition, the still permissible bubble size for a given substance can be determined simply by creating samples with gas bubbles of different sizes and by observing which diameter value no longer occurs.

Claims (11)

1. A longitudinally watertight cable, in parti- cufar a communication cable, in the interior of which there is provided a filling material which includes a waterrepelling substance and in which gas bubbles are incorporated, wherein the filling material (FM) contains an addition consisting of thermoplastic rubber or rubber-like thermoplastics which melts at its processing temperature during the cable filling, the condition of which is rendered firmer in the operating temperature range of the cable, by linking, interlooking, or a connection through thermoplastic blocks with a melting region which is suitably lower, i.e. which lies just below the filling temperature, and wherein the gas bubbles (GB) are so held by the rubber network which is formed in this way in the filling material, that the connection points of the rubber network can take up the buoyant force of the gas bubbles without breaking, characterised in that the additions consists of a polyolefine powder, in particular a polyethylene powder, having a grain size of between 20 and 600 ₁₁m; that the amount of the polyolefine powder is selected to be between 5 and 30% by weight of the filling material; that the amount by volume of the gas bubbles in the filling material is selected to be between 10 and 80%; and that the diameter of the gas bubbles lies between 1 and 1500 ₁₁m

2. A cable according to Claim 1, characterised in that as an addition, polyolefines having a chain length of 25 to 45 C-atoms, in particular low-molecular polyethylene, or paraffin wax, are used.

3. A cable according to Claim 1 or Claim 2, characterised in that the amount by volume of the gas bubbles is between 50 and 70% of the filling material.

4. A cable according to one of the preceding Claims, characterised in that, when using a polyethylene powder, a material is selected which has between about 5000 and about 200,000 C-atoms per molecule.

5. A cable according to one of the preceding Claims, characterised in that the powder used as the addition lies in a density range of between 0.915 and 0.96 g/ml.

6. A cable according to one of the preceding Claims, characterised in that the crystallite melting point of the added polyolefine powder lies above the processing temperature of the remaining components of the filling material.

7. A cable according to one of the preceding Claims, characterised in that the powdered addition lies between 6 and 20% by weight, preferably between 8 and 10% by weight.

8. A cable according to one of the preceding Claims, characterised in that the diameter of the gas bubbles lies between 20 and 200 µm.

9. A cable according to one of the preceding Claims, characterised in that the cable filling material consists of a mixture of waxes and oils.

10. A cable according to one of the preceding Claims, characterised in that the filling material has a viscosity appreciably above 5 Pa s.

11. A method for the production of a longitudinally watertight cable according to one of the preceding Claims, chraracterised in that the polyolefine powder is added with stirring to the filling material, which is heated to 140 to 150 °C, and the polyolefine powder is homogenously distributed; that the gas is introduced into the filling material containing the molten powder at a pressure of about 1.5 to 15 barü and at a temperature of about 140 °C; and that, after expansion and cooling to 80 to 120 °C, gas bubbles are formed with a fine, uniform distribution.

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