In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high mositure level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment may cause problems and should be prevented. Further, in some climates, the development of ice within an optical fiber cable may have a crushing influence on the optical fibers in the core which may affect adversely the attenuation thereof.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and into the core. For example, a metallic shield which often times is used to protect a cable against electromagnetic interference is provided with a sealed longitudinal seam. However, because lightning strikes may cause holes in the metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Filling materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material causes housekeeping problems, inhibits line speeds because of the need to fill carefully interstices of the core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core.
Presently, many commercially available cables also include a water-swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two cellulosic tissues. Further included may be a polyester scrim which is used to provide tensile strength for the laminated tape. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
What is needed and what does not appear to be available in the marketplace is a tape which is relatively thin and relatively inexpensive. Such a tape should be one which is compressible and which has acceptable tensile properties. Also, because in some optical fiber cables, the tape is engaged by helically wound metallic strength members, it should be able to conform to the configurations of those members and to allow those members to become bedded therein. If the tape has this capability, commonly used strength member wires will not move about and will provide torsional stability from layer to layer. On the other hand, if the tape does not have this capability and if all the wires were to assume positions on one portion of the periphery, the cable would not be balanced torsionally and would be very difficult to bend.
Care also must be taken to avoid problems caused by what is referred to as bleed-through of molten plastic jacketing material. With a tape comprised of a highly porous substrate material, the greater the line-speed the greater the flow of the molten plastic material into the tape and the more difficult it becomes to strip the jacket to expose the core. As a result, it has been known that the use of such a highly porous substrate material severely limits the line speed.
Seemingly, the prior art does not disclose a cable which is provided with a water arresting tape having all the hereinbefore-identified properties and which is relatively easy to incorporate into a cable. The solution to this problem should bring rewards in that smaller size cables and higher line speeds will be achievable.