GB2183365A - Hydrogen occlusion in optical cables - Google Patents

Abstract

In an optical fibre cable, hydrogen generated is absorbed by the incorporation in the cable of a silver substituted zeolite. The zeolite may be dispersed in a plastics material or in a water blocking compound with which the cable is filled. The optimum absorbing capacity is obtained with a silver substitution level in the zeolite of 15%. At this level a gel having 0.05 weight percent zeolite can absorb the hydrogen generated in a cable under average service conditions. <IMAGE>

Description

SPECIFICATION Hydrogen occlusion in optical cables This invention relates two methods of occluding hydrogen in optical fibre cables and to cables treated bythese methods.

It has been foundthatthe generation oftrace quantities of hydrogen within an optical cable can cause degeneration of the transmission properties of the optical fibre elements. This problem is particularly acute with submarine cables where it is necessary to provide a working lifetime of at least twenty yea rs.

Attempts have been madeto design cables from materials which do not generate hydrogen overthe lifetime of the cable. However, this has proved costly and, for many applications, impractical.

The object of the present invention isto minimise orto overcomethis disadvantage.

According to one aspect of the invention there is provided an optical fibre cable, including a transmission package, in contact with a material incorporating a particulate silver substituted zeolite in sufficientconcentration as to provide hydrogen occluding properties for at least the service life of the cable.

According to another aspect of the invention there is provided a method of manufacturing a hydrogen occluding silver substituted water blocking gel for a cable, the method including dispersing undervacuum a particulate hydrogen occluding zeolite in an oil, and thickening the oil to form a gel by the incorporation of hydrophobic silica or bentonite.

Thezeolite may be incorporated in a water blocking gel with which a cable isfilled or it may be dispersed in thethermoplastics material extruded on to the individual films oron to the transmission package.

Typically the zeolite is treated by exchanging some of the light metal cations for silver ions. Zeolites are crystalline alluminosilicates having micropores as features of the crystal structure and possessing cation exchange properties. The pores are generally in the range 0.4-1.0 nm and exclude molecular species larger than their critical pore size which is characteristic of the particularzeolite structure. This property has given rise to the term "molecular sieves" to describe zeolites. The concentration of exchangable cations in a zeolite is determined by the aluminium concentration of the crystalline framework. More than 30 naturally occurring zeolite minerals are known and well over 100 synethetic zeolite species have been described.Avaluable compedium of information on zeolites is "Zeolite Molecular Sieves" by D.W. Breck, Wiley-lnterscience, New York 1974 while more recently a large numberof newzeolite phases have been described.

These materials include sodium aluminium silicates which may be rendered hydrogen absorbent by re placement ofat least some ofthe sodium by an active metal,typicallysilver. Typically we employzeolitey which has the composition Na54,7AI54.7Sia37,30384241 H20 Some or all of the sodium in this material is replaced by silver and the material is then dehydrated by calcining. The composition is then Agn Na547-n n Al54.7 Ski137.3 0384 The.silver substitution level is defined as the proportion of sodium atoms that are replaced by silver. For example, if halfthe sodium atoms are replaced, this corresponds to a silver substitution level of 50%.

The treated zeolite may be dispersed either in a thixotropic water blocking gel or in a thermoplastics material in contact with the transmission package of the cable.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross-section ofthetransmission package of an optical fibre cable; Figure2 is a cross-section of a finished cable containing the transmission package of Figure 1; and Figure 3 illustrates the relationship between zeolite concentration and hydrogen absorption.

Referring to Figures 1 and 2, the transmission package comprises a set of plastics packaged optical fibres 11 stranded around a king wire 12, and optionally held together as a bundle byatapewrapping 13, e.g. of a polyester material. Each fibre package 11 has at its core a glass fibre 14, typically of silica having an internal optical waveguiding structure. The glass core 14is surrounded by plastics material 15 provided in one or more layers to give a measure of protection for the underlying glass fibre and to provide sufficient solutions for the necessary stranding operations involved in the manufacture of the cable.Around this transmission package is extruded a seamless aluminium tube 16which may be formed by continuous aluminium tube 16 which may be formed by continuous fictional extrusion. Suitable process for extruding the tube 16 is described in our co-pending application No.8229562(2128358 A) (C.S. Parfree - K. Taylor - P.A. Jones 20-6-1).

The transmission package is contained in an outer sheathing structure comprising one or more layers of steel wires 27 (Figure 2) and a layer of a plastics material 28. In some applications an external armouring (not shown) may also be applied.

To prevent ingress of moisture the void space of the transmission package may contain a water blocking compound 18. Typically we employ a thixotropic gel forthis purpose. Preferably the gel comprises an oil base fluid thickened by the addition of hydrophobic silica or bentonite. Such materials are described in our co-pending application No.8429077 (W.E. Simpson -J.R.I. Bury- D. Joiner 19-3-1). With such a composition we preferto incorporate the particulatezeolite in the oil base fluid priorto addition ofthe thickener as this has been found to provide significantly better dispersion of the zeolite within the gel than is obtained by adding thezeolitetothefinished gel.

In a typical process a sodium containing zeolite, e.g. zeolite X, zeolite Y or Mordenite, is treated with silver nitrate solution to exchange some of the sodium cations for silver. The treated material is dried and then calcined at 340 to 360into drive off any remaining moisture. The zeolite is then dispersed under vacuum in the base oil ofthe gel. Preferably the oil is heated to 50 -80 Cto lower its viscosity thus facilitating dispersion. Thickening agents, e.g. hydrophobic silica, are then added to form a thixotropic gel.

In an alternative embodiment, the treated zeolite is dispersed in a thermoplastic, e.g. a copolyester such as HYTREL,which isthen extruded on to the transmission package. We havefound thatthe zeolite is notdegra- ded at the extrusion temperatures employed for such materials. In a further embodiment the zeolites may be dusted on to the transmission package priorto extrusion coating or gel filling.

Where 15% silver substituted zeolite Y is dispersed in a water blocking gel we prefer two employ 0.05to 1% and typically 0.1 to 0.5% zeolite based on the weight ofthe gel. For dispersion of this zeolite in a thermoplastics we prefer two employ similar proportions based on the weight of the copolymer. It will be appreciated thatthese proportions correspond to a 15% silver substituted zeolite. For higher or lower silver substitution levels the quantities of the zeolite required will be proportionally lower or higher respectively.

The rate at which hydrogen is absorbed by the zeolite is related to its silver content. However, this relationship is non linear,the efficiency of hydrogen absorption decreasing with increased silver substitution levels.

For maximum silver range, a low substitution level is desirable. However, at very low silver levels, thequantity of zeolite required may become excessive and can havea deleterious effect on the thixotropic properties of the blocking compound. Conversely, although high silver substitution levels minimise the quantity of zeolite required to provide a given degree of hydrogen absorption, the significant reduction in absorption efficiency at high silver levels results in wastage of silver. Between these two extremes we have found that the most advantageous use of silver is obtained with a substitution level in the range to 5to Typicallywe employasilversubstitution levelofl5%.

The potential quantity of hydrogen that can be absorbed is related to the concentration of the zeolite in the gel, but in a distinctly non-linear manner. This effect is illustrated in Figure 2 ofthe accompanying drawings.

As can be seen from Figure 2, the hydrogen absorption capacity of a 15% silver substitute zeolite increases with zeolite concentration up to a concentration of about 0.5%. Above this level, and particularly above 1%, further increases in the zeolite concentration produce only a small increase in the capacity to absorb hydrogen. Thus, for maximum efficiency of silver usage we prefer two employ a maximum zeoliteconcentra tion in the gel of k/i by byweightwhere kisthe silver substitution level. For most applications we havefound that a maximum zeolite concentration of k/30% provides adequate hydrogen absorption. Indeed, effective protection is provided for less exacting requirements and concentrations as low ask/300%.

In a typical example, an optical fibre cable containing 4kg of blocking compound per kilometre is expected to generate 11 cc hydrogen per kilometre over the 25 year service life ofthe cable. A 0.05% concentration of a 15% silver substituted zeolite in the gel has a hydrogen absorbing potential that is morethen adequate for this quantity of hydrogen and in fact provides a safety factor of about 2.

By way of example 0.8% ofthe Zeolite-Ywith 15% sodium ions replaced by silver ions was mixed intim atelywith the water blocking compound Rheogel 210 (Synthetic Technology Ltd). Identical samples of gas containing 10% hydrogen were injected into cells containing a sample ofthe gel. After reaction at400Cfor differenttimes,the gas in the cell was sampled and analysed for hydrogen. In this way the amountof hydrogen absorbed by the gel was measured. The results appear in the following Table.

Wtgel Hrs UnderTest Hydrogen Hydrogen at40" Measured absorbed (ppm) in m Moles ggel blank 0 6060 1.80 1 5390 9.14x10-4 2.64 4 3180 16.56x104 2.63 6 2070 21.0x104 2.40 17.5 2540 2.56 48 2810 18.1 x10-4 2.13 162 2890 21.8x10-4 The above example refers two silver-exchanged Zeolite Y, but hydrogen absorption has also been demonstrated from silver exchanged Zeolite X and silver-exchanged mordenite. Also although a level of silver exchange of 15% has been employed, we have also demonstrated hydrogen absorption in the Zeolites exchanged with silver ions at 16, 38, 50 and 60%.

As a further example, 5% of the Zeolite Y with 15% sodium ions exchanged by silver ions was dispersed into Hytrel 40D copolyester using a compounding machine. A sample of gas containing 10% Hydrogen was injected into a cell containing the Hytrel/Zeolite dispersion and reacted at40'C. After 18 hours the gas in the cell was sampled and analysed for hydrogen. The amount of hydrogen was measured as 3850 ppm, corresponding to an absorption of 30x40-4 m Moles/gram filled thermoplastic. This compares with a blank reading of 6060 ppm for an empty cell. It will be appreciated that, to facilitate measurement, a relatively large quantity of zeolite has been employed and that, in'practice, smaller proportions, as employed with blocking gels, could be used. The result demonstrates a potential for hydrogen absorption within a cable from a thermoplastic coating forthefibres ofthe king wire, containing silver exchanged zeolite as afiller.

Claims (12)

1. An optical fibre cable, including a transmission package, in contact with a material incorporating a particulate silver substituted zeolite in sufficient concentration as to provide hydrogen occluding properties for at least the service life of the cable.

2. Acable as claimed in claim 1,wherein said zeolite is dispersed in athixotropicwater blocking gel with whichthecableisfilled.

3. Acable as claimed in claim 2, wherein the zeolite comprises 0.05 to 1.0 weight percent of the gel.

4. Acable as claimed in claim 3, wherein the zeolite comprises 0.1 to 0.05 weight percentofthegel.

5. A cable as claimed in claim 1 ,wherein the zeolite is dispersed in a thermoplastic material surrounding said transmission package.

6. An optical fibre cable, including a transmission package material in awater tight sheath, and athixotropicwater blocking compound with which the cable is filled so asto inhibitthe ingress of moisture,wherein the blocking compound has dispersed therein a particulate hydrogen absorber material comprising a silver substituted zeolite, and wherein the weight concentration of the silver substituted zeolite in the gel is less than or equal to k/1 5 percent where k is the silver substitution level as hereinbefore defined.

7. An optical fibre cable as claimed in any one of claims 1 to 6, wherein the silver substitution level ofthe zeolite is less than or equal to 50 percent.

8. An optical fibre cable substantially as described herein with reference to the accompanying drawings.

9. A method of manufacturing a hydrogen occulding silver substituted water blocking gel for a cable, the method including dispersing under vacuum a particulate hydrogen occluding zeolite in an oil, and thickening the oil to form a gel by the incorporation of hydrophobic silica orbentonite.

10. A method as claimed in claim 9, wherein the zeolite comprises not more than 5 weight percent ofthe blocking gel.

11. A method as claimed in claim 9 or 10, wherein the silver substitution level, as herein before defined, of the zeolite is less than or equal to 50%.

12. A method of manufacturing a water blocking gel as claimed in claim 10 and substantially as described herein.