WO1993005424A1

WO1993005424A1 - Optical cable

Info

Publication number: WO1993005424A1
Application number: PCT/SE1991/000592
Authority: WO
Inventors: Håkan LENNARTSSON; Laurence Jones; Brian Anthony Perrett
Original assignee: Neste Oy; Telephone Cables Limited
Priority date: 1991-09-10
Filing date: 1991-09-10
Publication date: 1993-03-18

Abstract

An optical cable with improved tracking resistance combined with good mechanical properties is described. The cable contains at least one optical fibre and an outer sheath of polymeric material, characterized in that this polymeric material in its matrix consists of a linear polyethylene, preferably an MDPE, optionally blended with up to 20 % of an LDPE based on the weight of the MDPE, and that the sheath in addition contains 15 to 30 %, and preferably 18-25 % of magnesium hydroxide or aluminium hydroxide based on the weight of the total composition. Preferably the sheath also contains 1-5 % by weight of carbon black.

Description

OPTICAL CABLE

The present invention relates to an optical cable having simultaneously good tracking resistance and mecha- nical properties,and more especially to cables typically designed to be freely suspended between supports spaced up to 500 metres apart.

Optical cables are defined as cables containing at least one optical fibre for transmission of optical signals for telecommunication, data or control. A type of optical cable is described in GB A2 2 193 583, which patent application hereby is included as a reference. Besides one or more optical fibres an optical cable also has an outer sheath protecting the optical transmission medium but otherwise can have different configurations. The cable sheath normally consists of a polymeric materi¬ al, such as polyethylene, i.e. a plastic material based on polyethylene or a copolymer of ethylene in which the ethy- lene monomer constitutes the greater part of the total mass of the polymer. The requirements on the sheath are that it should be easy to produce and have good mechanical properties, such as tensile strength and elongation at break, especially at increased temperatures, even up to the neighbourhood of 100°C. It should also have low heat deformation and good tracking resistance.

The last of these requirements arises because of the fact that for practical reasons optical cables are frequently installed together with power cables. This may be done without excessive problems at voltages up to about 130 kV. At higher voltages, problems related to degrada¬ tion of the optical cable sheath will appear, however. The reason is the influence of the neighbouring power cables and more exactly of the strong electrical fields such cables give rise to. These fields induce electric voltages across regions of the sheaths of the optical cable, which voltages in turn cause creeping currents leading to tracking and consequential local breakdown of the cable. Breakdown is particularly prone to occur under damp condi¬ tions, manifested by the phenomenon commonly referred to as dry band arcing.

In order to avoid this problem the optical cable has to be protected against creeping currents. A solution to the problem must, however, not be bought at the expense of a deterioration in the other properties of the sheath material. Thus it is important that a good processability of the cable sheath material is retained so that the sheath can be produced in an efficient and economical way. Further, it is important that the mechanical properties, such as tensile strength and elongation at break, and also the temperature resistance of the finished cable have not deteriorated. .Through DE Al 3 504 041 it is known to fit an optical cable to be used in conjunction with power cables with a sheath consisting of a material made self-extinguishing and tracking resistant by the addition of 30-60 % by weight of-a metal hydroxide. The metal hydroxide preferen- tially is aluminium hydroxide but alternatively zinc hydroxide or magnesium hydroxide can be used. Due to the high percentages of metal hydroxide, necessary to use in order to make the sheath self-extinguishing, the mecha¬ nical properties of the sheath are considerably reduced. From the Japanese Patent Application 63-322299 it is further known a track-resistant material to be used especially in the form of shaped insulations applied for joints of outdoor cables etc. This track-resistant mate¬ rial consists of 100 to 500 parts by weight of a thermo- plastic rubber blended with 100 parts by weight of a thermoplastic resin, wherein 20 to 50 parts by weight of magnesium hydroxide are admixed to 100 parts by weight of the said blend of thermoplastic rubber and thermoplastic resin. Due to the contents of thermoplastic rubber, this composition, however, has comparatively poor values for Young's modulus, tensile strength and elongation at break, especially at increased temperatures. Further the heat deformation is unacceptably high.

The present invention has as the object to design an optical cable having good tracking resistance in combi¬ nation with good mechanical properties. In particular the invention has as the object to design an optical cable having sufficient tracking resistance at the high electric field strengths occurring in the vicinity of power cables of voltages from 100 to 400 kV.

The object of the invention is reached with an opti- cal cable, containing at least one optical fibre and a sheath of polymeric material, and the sheath of which is characterised by containing 15-30 % by weight, preferen¬ tially 18-25 % by weight, of magnesium hydroxide or aluminium hydroxide. Other aspects of the invention will appear from the following description and the appendant claims.

As earlier described, the cable according to the invention contains at least one optical fibre and has an outer sheath but may otherwise be of varying design. As an example, the cable may have the design described in GB A2

2 193 583, which hereby is included as a reference. As shown and described in this reference the cable contains one or more optical fibre elements in the form of optical fibre packages. Besides the optical fibre packages the cable contains one or more non-metallic strength members, such as glass fibre reinforced plastic composite, and the strength members and the optical fibre packages are sur¬ rounded by an outer sheath of polymeric material, such as polyethylene. The cable sheath suitably has a thickness of one or a few millimeters. The interstices within the cable may be filled with a suitable water blocking compound.

According to the present invention the optical cable differs from prior art in that the sheath contains 15-30 % by weight of magnesium hydroxide or aluminum hydroxide. This is a distinctly lower percentage of magnesium hydroxide than according to patent application DE Al

3 504 041. In spite of this, such a low percentage of magnesium hydroxide has been found to result in a superior level of tracking resistance.- Simultaneously, it has been found that the blending with such a relatively low content of magnesium hydroxide does not lead to inferior mechani- cal properties. The cable thus shows satisfying values for ^■tensile strength, elongation at break and the like. It must be considered surprising that a cable according to the invention thus simultaneously complies with the conflicting requirements on good tracking resistance and good mechanical properties.

The sheath of the optical cable according to the invention consists of a polymeric material and more pre¬ cisely of a linear polyethylene. The linear polyethylene may be chosen from among linear low density polyethylene {LLDPE), linear medium density polyethylene (MDPE) and high density polyethylene (HDPE) or any mixture of these alternative polyethylenes. The preferred polymeric material according to the invention has a density of the

3 base polymer matrix of 0.93-0.95 kg/dm and the most

3 preferred material has a density of around 0.94 kg/dm . It is possible also to include a smaller amount of branched low density polyethylene (LDPE) into the composition, but at the most 20 % by weight, though, in order not to lose too much of thermal resistance. The magnesium hydroxide is mixed into the polymeric material of the polymeric matrix of the sheath in connec¬ tion with the compounding of this material or may, alternatively, be mixed into a master batch already before this compounding. The magnesium hydroxide is in the form of a fine powder that is added in the proportion wanted. In order to get the best result and to get the magnesium hydroxide mixed as homogeneously into the sheath material as possible, the powder should have as low particle size as possible, and a mean particle size of 0.3-0.4 μ is preferred. The amount of magnesium hydroxide that is added into the polymeric material of the sheath should be such that the sheath will contain 15-30 % by weight, preferably 18 - 25 % by weight and most preferably around 21 % by weight thereof. As an alternative to the magnesium hydroxide aluminium hydroxide may be used and then in similar amounts, with similar particle size and compounded in a similar way.

Besides the magnesium hydroxide the sheath material should also, preferably, contain a certain amount of carbon black as a protection against ultraviolet light. The amount of carbon black may be up to about 5 % by weight, is suitably about 1-5 % by weight and preferably 2-3 % by weight, such as 2.5 % by weight.

EXAMPLE 1

A cable sheath material has been produced according to the invention and the mechanical properties have been tested giving the values shown in Table 1 below. The cable sheath material consisted of an MDPE having a density of

3 0.939 kg/dm , containing 21 % by weight of magnesium hydroxide with a mean particle size of 0.4 μm and, in addition, 2.5 % by weight of carbon black (The percentages by weight based on the total composition). The properties tested were as follows:

Table 1 Young's.modulus (tape; at 23°C; ISO 6239 Al) 0.83 GPa Ditto (tape; at 70°C; ISO 6239 B2) 0.15 GPa

Yield strength (tape; at 23°C; ISO 6239 Al) 19.8 MPa Ditto (tape; at 70°C; ISO 6239 B2) 8.6 MPa

Tensile strength (tape; at 23°C; ISO 6239 Al) 32.3 MPa Ditto (tape; at 70°C; ISO 6239 B2) 22.5 MPa

Elongation at break (tape; at 23°C; ISO 6239) 648 % Ditto (tape; at 70°C; ISO 6239) 882 % Shore D hardness (ASTM D 2240) 59

Heat deformation (115°C/6 h, IEC 811-3-1) 4.2 % Environmental stress cracking 10000 h;

(10 % Igepal; ESCR, ASTM D 1693, 40 samples) no cracks Tracking resistance test: Salt fog test (IEC Draft 88/32990, Section 63)

Optical aerial cable according to GB A2 2 193 583, sheathed with the compound according to the example was subjected to this weathering and tracking test for (a) 1000 h at 25 kV

(b) 1000 h at 50 kV

(c) 1000 h at 50 kV with 10 kN tensile load

(d) 1000 h at 60 kV with 15 kN tensile load Condition (a) corresponds to 25 years of exposure in a 132 kV system and condition (d) corresponds to 25 years of exposure in a 400 kV system. The sheath according to the invention withstood all these tests whereas a conventional black polyethylene sheath failed badly already after 1000 h at 25 kV without any tensile load.

In general it can be stated that an optical cable with a sheath according to the invention, consisting of a linear MDPE mixed with about 15-30 % by weight of magne¬ sium hydroxide and about 1-5 % by weight of carbon black, has a Young's modulus of 0.8-1.2 MPa, a yield strength of 15-25 MPa, a tensile strength of 20-30 MPA and an elonga¬ tion at break of more than 400 % (all values measured at 23°C). EXAMPLE 2

A cable sheath material was produced, consisting

₃ of 78.8 % by weight of a linear MDPE of density 939 kg/m ,

21.0 % by weight of aluminium hydroxide and 0.2 % by weight of an antioxidant. The material was tested with the following results:

Table 2 Young's modulus (tape; at 23°C)

Ditto (tape; at 70°C)

Tensile strength (tape; at 23°C)

Ditto (tape; at 70^CC)

Elongation at break (tape; at 23°C) Ditto (tape; at 70^CC)

Heat deformation (115°C/6h)

Claims

1. Optical cable with improved tracking resistance, which cable contains at least one optical fibre, and an outer sheath of polymeric material, c h a r a c t e ¬ r i s e in that the polymeric material forms a matrix consisting of a linear polyethylene blended with 0 to at the most 20 % by weight of a branched LDPE, based on the weight of the linear polyethylene, and in that the total composition of the sheath material contains 15-30 % by weight of magnesium hydroxide or aluminium hydroxide.

2. Optical cable according to claim 1, c h a r a c ¬ t e r i s e d in that the sheath contains 18-25 % by weight of magnesium hydroxide or aluminium hydroxide.

3. Optical cable according to claim 2, c h a r a c ¬ t e r i s e d in that the sheath contains about 21 % by weight of magnesium hydroxide.

4. Optical cable according to any of the preceding claims, c h a r a c t e r i s e d in that the polymeric ^" matrix of the sheath material consists of a linear polyethylene of medium density (MDPE) i.e. with a density

₃ from 0.93-0.95 kg/dm .

5. Optical cable according to any of the preceding claims, c h a r a c t e r i s e d in that the sheath material contains 1-5 by weight of carbon black.

6. Optical cable according to claim 5, c h a r a c ¬ t e r i s e d in that the sheath material contains 2-3 % by weight of carbon black.

7. Optical cable according to any of the preceding claims, c h a r a c t e r i s e d in that it contains magnesium hydroxide with a mean particle size of 0.3-0.4 μm.