GB2217868A

GB2217868A - Fibre optic cables

Info

Publication number: GB2217868A
Application number: GB8808031A
Authority: GB
Inventors: John Robert Ivor Bury; Stephen Roy Kendall
Original assignee: SYNTHETIC TECHNOLOGY Ltd
Current assignee: SYNTHETIC TECHNOLOGY Ltd
Priority date: 1988-04-06
Filing date: 1988-04-06
Publication date: 1989-11-01
Also published as: GB8808031D0

Abstract

In the formation of a filled loose-tube fibre optic cable, a blowing agent is incorporated in the filling compound 3. This is activated by contact of the filling compound with the interior wall of the extruded tube 2 so that small voids are formed adjacent the wall and as far away as possible from the fibre 1. The filling compound 3 containing a chemical blowing agent is claimed per se. <IMAGE>

Description

FIBRE OPTIC CABLES This invention relates to fibre optic cables and in particular to loose-tube fibre optic cables.

The term "loose-tube fibre optic cable" is defined as a fibre optic cable in which one or more fibres are surrounded by a support member in the form of a tube, normally produced by extrusion, the length of the fibre usually exceeding the length of the tube and the internal diameter of the tube usually exceeding the external diameter of the fibre, whereby the fibre can follow a sinuous path within the tube and thus be protected from any stress resulting from tension on the tube.

Such cables are in common use and are generally required to be filled with a hydrophobic compound to protect the fibre(s). The filling compound must be added continuously at the point of manufacture of the tube.

Using commercially available filling compounds, it is virtually impossible to ensure complete filling of the tube and filling deficiencies in the region of 2 1/2 to 7 1/2X are typical of those experienced. This gives rise to problems which will be explained in more detail in relation to Figures 1 and 2 of the accompanying drawings which are in the form of longitudinal sections through two filled loose-tube fibre optic cables.

Referring to Figure 1, the fibre 1 is loosely contained in a tube 2 which has been filled as completely as possible with a filling compound 3.

As a result of stretching of the tube during manufacture and the differential thermal contractions of the tube polymer and the filling compound, "vacuum" voids 4 are apt to form in the vicinity of the fibre.

These voids are undesirable from a cabling viewpoint in that the fibre or fibres are locally exposed in the areas of the voids and are subject to rapid ageing through stress corrossion when the atmosphere within the void becomes wet, due to entrained moisture.

In contrast to the fibre shown in Figure 1, it is assumed that the fibre shown in Figure 2 has been pre-coated with filling compound with the result shown, namely, the formation of large voids such as 4a. The formation of these voids can encourage irregular overfeeding of the fibre with the result of poorer fibre performance especially during temperature cycling. The effect of these voids on the fibre is of course proportionately greater than that of the voids shown in Figure 1 and it is all the more desirable to avoid their formation.

It is an object of the present invention to provide a compound for filling a loose-tube fibre optic cable by the use of which such voids as are likely to be present are kept as far away from the fibre as possible.

According to the invention in its broadest aspect, there is provided a filling compound containing a chemical blowing agent which will be activated by heat during the processing of one or more optical fibres into a loose tube extrudate, thereby creating small gas bubbles on the inside of the tube wall to relieve internal stress.

According to another aspect of the invention, there is provided a method of filling a loose-tube fibre optic cable comprising extruding a thermoplastic tube loosely around the fibre or fibres and injecting into the tube a filling compound containing a chemical blowing agent adapted to be activated by contact with the interior wall of the tube while it is subject to the heat of extrusion whereby small gas bubbles are formed close to the interior wall of the tube and away from the fibre or fibres.

An embodiment of the invention will now be described with reference to Figures 3 and 4 of the accompanying drawings of which: Figure 3 shows a section through the extrusion die at the point of manufacture and filling of the cable, and Figure 4 is a longitudinal section through the finally produced cable.

Referring to Figure 3, the fibre 1 passes through the centre of an extrusion die 5 and molten polymer 6 is extruded from the peripheral portion of the die to form the tube 2. The rate of feeding of the fibre and the rate of extrusion of the polymer is such that the fibre is loosely contained within the tube, as previously explained.

Filling compound, typically in the form of a thixotropic hydrophobic gel is continuously supplied to the centre of the die around the fibre and substantially fills the space between the tube and the fibre.

Incorporated in the filling compound is a blowing agent which may comprise an azodicarbonamide or derivative, together with an initiator, with or without an accelerator. This blowing agent becomes active at between 1000 and 1500 centrigrade and the only area of the cable in which such temperatures are likely to occur is the interior wall of the tube 2 which is subject to the heat of extrusion. As a result, small gas bubbles or voids 7 are formed adjacent the interior wall of the tube 2, as far away as possible from the fibre 1.

The blowing agent is stable at room temperature and its performance during the extrusion process can be controlled to match the parameters involved.

The voids 7 grow under the influence of the vacuum set up within the tube and thereby act to relieve the stresses which would otherwise result in the formation of the large voids such as that shown at 4a in Figure 2. The final form of the cable at the conclusion of the manufacturing process is shown in Figure 4.

Claims

CLAIMS:

1. A filling compound containing a chemical blowing agent which will be activated by heat during the processing of one or more optical fibres into a loose tube extrudate, thereby creating small gas bubbles on the inside of the tube wall to relieve internal stress.

2. A compound as claimed in claim 1 which is in the form of a thixotropic, hydrophobic gel and in which the blowing agent comprises an azodicarbonamide or derivative and an initiator with or without an accelerator.

3. A compound as claimed in claim 1 or 2 in which the blowing agent becomes active at between 1000C and 1500C

4. A. method of filling a loose-tube fibre optic cable comprising extruding a thermoplastic tube loosely around the fibre dr fibres and injecting into the tube a filling compound containing a chemical blowing agent adapted to be activated by contact with the interior wall of the tube while it is subject to the heat of extrusion whereby small gas bubbles are formed close to the interior wall of the tube and away from the fibre or fibres.

5. A method of filling a loose-tube fibre optic cable substantially as described with reference to Figures 3 and 4 of the accompanying drawings.

6. A filled loose-tube fibre optic cable made by a method according to claim 4 or 5.

7. A filling compound substantially as described.