The invention relates to a cable wrapping strip of nonwoven fabric whose purpose is to render the space between the jacket and core in an electrical cable waterproof lengthwise. For this purpose the wrapping strip contains particles of a material which swells greatly upon the entry of water but is insoluble in water, and is referred to hereinafter as a "superabsorber." The swelling of such a wrapping when exposed to moisture leads to the sealing off of the space between cable and core, and the shielding wires if any, so that propagation of the water lengthwise of the cable is prevented and the damage remains limited to a short length of the cable.
The swellable powder consists as a rule of a crosslinked sodium salt of a polyacrylic acid.
Without such a swellable wrapping, short circuits develop upon the penetration of moisture into electrical high-voltage cables or communication cables due to often unavoidable damage done to the jacket when they are laid (cutting and kinking). It can happen that paper insulation in multi-conductor cables becomes wet, or entering water propagates between the cable core and jacket and between the conductors themselves. Due to insulation defects, these wires can then become grounded through the water in the area of splices and taps.
To stop this lengthwise propagation of the water it is necessary that it be absorbed as quickly as possible by the wrapping.
In the case of medium- and high-voltage cables, a wrapping strip is also expected to provide, by means of a partially conducting property produced by carbon black, an electrical contact between a partially conducting intermediate layer present in the cable, which is made of carbon-filled polyethylene, and the conductive metallic shielding. The contact must be maintained even upon the expansion or contraction of the cable due to extreme heating or cooling. Such effects are produced in these cables by inductively produced temperature fluctuations of 80.degree. to more than 200.degree. C.
The partially conductive intermediate layer is intended to prevent corona discharges within the cable due to unequal potential distribution on the insulation. In addition to the above-named thermal effects, the brittleness of the partially conductive layer can cause spalling and cracking when the cable is flexed, and this leads to undesirable ionization. In these cases too the wrapping strip is intended to have an electrical bridging effect so as to assure that the intermediate layer has no electrical potential different from that of the metal shielding.
In the production of these cables the wrappings are wound very tightly on the cable either helically or lengthwise. The width of a strip amounts often to 1 to 15 cm, but special other methods of application are conceivable. The thickness can vary from 0.3 to 1 mm, although lesser or greater thicknesses can be used in special cases, e.g., for cables of lower quality with larger interior cavities. Also the application of more than one layer of wrapping has already been practiced. Frequently needed specific weights are between 90 and 200 grams per square meter.
DE-PS No. 1,790,202 discloses a cable wrapping strip in which the superabsorber powder is embedded in an open-pored, water-repellent foam matrix. This embodiment assures a uniform fixing of the absorber particles on the wrapping. However, the time it takes for the powder to swell is disadvantageously lengthened by its being surrounded on all sides by the water-permeable foam. For the same reason any very great increase in the volume of the absorber particles is hampered. Since the penetration of water through the open-pored foam is additionally hampered by surface tension effects, it takes too long overall for the mass to react and swell to block off the area of the water leakage, and during this time the water penetrates unhampered longitudinally within the cable. Another disadvantage is that it is technically very difficult to make such a foam material electrically semiconductive.
Accordingly, swellable wrappings are being marketed in which the superabsorber is placed on a relatively impermeable nonwoven layer which it cannot penetrate and which, in some cases, is impregnated with carbon black. The sandwich of materials is often consolidated with a water-impermeable binding agent.
This construction assures that if the winding is performed in an obligatory direction with the thin nonwoven layer out and damage occurs, the water will immediately reach the swelling powder as required, although only from one side, namely through the nonwoven covering. Therefore the swelling is very slow to start, and the wrapping achieves only a small final volume. The absorbent particles are bound so tightly between the nonwoven layers that first a sufficient swelling pressure is needed, and hence a considerable amount of time, before the fiber bonds of the nonwoven covering are destroyed and additional volumetric swelling of the swelling agent becomes possible. The swelling powder is not at all able to break up the basic nonwoven that is on the other side.
Other disadvantages are the considerable leakage of the superabsorber through the thin covering nonwoven during manufacture and assembly, the prescribed winding direction with the covering nonwoven out, the great danger of damage to the delicate, thin covering fabric, and the interruption of the electrical conductivity in the cross section of the wrapping strip by the layer of swelling powder, i.e., the excessively high specific resistance, which amounts to more than 10,000 ohms per square centimeter. Cable manufacturers are generally expected to provide values of 3,000.OMEGA..multidot.cm.sup.2 or less.
The invention is addressed to the problem of devising a swellable cable wrapping strip in which complete swelling is achieved within a shorter interval of time than heretofore upon contact with water, in which the water can enter from both sides (no prescribed winding direction), in which the swelling agent does not leak out, and which can be provided with or without carbon black, and if it is provided with carbon black it must be possible to achieve specific resistances of 3,000.OMEGA..multidot.cm.sup.2 or less. The wrapping strip must be able to withstand without damage the high longitudinal traction stresses occurring in the manufacture of cables when they are reeled up. Furthermore, a method is to be found in which the manufacture of the wrapping strips according to the invention can be changed over from carbon-free to carbon-containing products without further measures. Moreover, the method is to indicate how to proceed so that the finished cable wrapping strip no longer will appear to be laminated and thus, when a carbon black-containing binding agent is added, will have the required specific resistance.