The method presently preferred of installing optical fibre cables into ducts is to blow the cable into the duct by means of compressed air. This method produces the advantage that the installation force is distributed over the entire length of that part of the cable lying within the tube, as a result of which significant tensional forces are not applied to the leading end of the cable, which would be the case if the cable were to be installed by pulling. As a result, the cable can be made smaller and more lightweight because it does not need to be reinforced to the same extent as a cable installed by means of pulling. A method of installing such a cable by blowing is disclosed in EP0108590.

One of the disadvantages which arise during installation of fibre optic cables by blowing is the generation of static electricity which causes an attractive force between the cable and the duct, which impedes the blowing process.

An attempt to overcome the problem of generation of static electricity is disclosed in EP0432171, in which a duct assembly comprises a plurality of ducts, each duct having an inner layer suitable for fibre blowing, such as slip enhanced medium density polyethylene. The duct can be formed by means of a single stage extrusion process if the melting temperatures of the inner and outer layers are matched, and it is found that if high-density polyethylene is used to form the inner layer, it is necessary to incorporate solid lubricants such as anti-static grade carbon into the high-density polyethylene. However, this has the disadvantage that the carbon is not very effective as a static reducing agent, and renders the blowing tube opaque, which in many situations is undesirable.

In addition, known anti-static agents commonly use highly water soluble organic amines such as Armostat 400'available from Akzo Nobel Polymer Chemicals LLC 300 South Riverside Plaza Chicago IL 60606 USA. This particular material is a Coco bis (2-hydroxyethyl) amine, which can provide a highly effective surface anti-static effect. However'materials of this type suffer two disadvantages. Firstly, they require the presence of water, and in the case of the above material in this application, this renders them ineffective at relative humidities below approximately 20o. Secondly, their water solubility means that if the blowing tube becomes wet there is a danger that some of the compound may be removed into the water causing a loss of anti-static property.

A further anti-static arrangement is disclosed in EP752604, which discloses a duct for an optical fibre assembly in which an anti- static agent in the form of conventional polyglycol ether is used. However, this arrangement has the disadvantage that the anti-static agent requires the addition of a high boiling point organic liquid to be effective.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the above prior art.

According to the present invention, there is provided a tube for installation of at least one flexible cable by means of fluid flow, the tube comprising:- a first layer of thermoplastic material ; and a second layer of thermoplastic material, inwards of said first layer, said second layer including at least one friction reducing material and at least one anti-static material comprising a blend of polyamide and polyether block amides.

The addition of at least one anti-static material comprising a blend of polyamide and polyether block amides gives rise to a conductive percolating network within the tube liner which has been found to be especially advantageous in this application.

Also, by providing an anti-static agent of the type defined above, this provides the advantage of creating a fixed anti- static property which is not removed by water contamination of the tube. Additionally, it enables the friction reducing material to be pre-compounded with the thermoplastic material of the second layer, as a result of which the entire tube can be formed as a single step extrusion, thus reducing the cost of manufacture of the tube. Also, by avoiding the use of carbon as an anti- static material, this makes the anti-static properties of the tube more effective and avoids making the tube opaque.

Furthermore, the addition of a high boiling point organic liquid to the anti-static material is no longer necessary with the tube of the present invention, and the blowing performance of the tube is found to be significantly improved compared with the prior art.

Said first layer may include polyethylene.

Said first layer may include low-density polyethylene.

Said first layer may include medium density polyethylene.

Said first layer may include high-density polyethylene.

Said second layer may include at least one polyolefin material.

The second layer may comprise a material which provides a permanent anti-static effect throughout the bulk of the material and which operates at relative humidities below 20% and is substantially unaffected by water exposure.

A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which :- Figure 1 is a schematic cross sectional view of a tube embodying the present invention; Figure 2 shows a comparison of the blowing performance of the tube of Figure 1 with that of the prior art; and Figure 3 shown a comparison of the speed of removal of a cable from the tube of Figure 1 compared with that from prior art tubes.

Referring to Figure 1, a co-extruded tube 2 for receiving fibre optic cables by blowing comprises an outer layer 4 co extruded with an inner layer 6. The tube 2 has an outer diameter of 5mm; an inner diameter of 3.5mm and the radial thickness of the inner layer lining material is approximately 0. lmm. The outer layer 4 is formed of low-density polyethylene (LDPE) (grade BP28D780, available from BP Chemicals, PO Box 21, Bolness Road Grangemouth Stirling, FK3 9XH, UK). The inner layer 6 is a high-density polyethylene (HDPE) carrier containing a blend of polyamide and polyether block amides with a silicone slip agent. The blend is produced by combining these ingredients using heat and pressure and a thermoplastic compounding line in a manner which will be familiar to persons skilled in the art.

The tube 2 of Figure 1 was made by means of a conventional co extrusion process using a 45mm extruder for the first layer and a 60,80 or 100mm extruder for the second layer, as will be familiar to persons skilled in the art. The extrusion conditions for the second layer were as follows: Zone Temperature, C Rear Zone ; 140 Rear Zone 2 180 Rear Zone 3 185 Rear Zone 4 190 Heated Zone 1 200 Heated Zone 2 205 Heated Zone 3 205 The tube 2 produced by this process has been tested for its fibre blowing ability under conditions which are specifically designed to induce static charging of the tube and fibre unit. If the air delivered to the tube is significantly cooler than the tube and the air has a suitably low dew point then static charging occurs.

A de facto standard exists where the temperature difference (AT) is set to-25C and the Dew Point of the delivered air is-25C.

Under these conditions a prior art product using a water-soluble amine agent only just passes a standard blowing test. The standard blowing test requires that a fibre unit is blown around a drum of 0.5m diameter in a time not exceeding 21 minutes. The results of this test are set out in figure 2, in which sample A' was a prior art product and sample B'was a tube 2 as shown in Figure 1. The conditions chosen were slightly worse than the standard test and failure of the prior art product was observed whilst the product embodying the present invention easily blew under the worsened conditions. Figure 2 illustrates the results obtained under the following conditions: TestDew Point (°C) At (°C) A trial 1-25-26 A trial 2-25-27 B trial 1-27-26 B trial 2-26-27 A further indication of the effectiveness of an anti-static tube is the time taken to remove a unit from the route applied in Fig 2 above. The greater the withdrawal time, the greater the hindrance caused by static charging. Figure 3 shows that the new tube performs significantly better in this respect. For this exercise the standard static conditions were applied (AT and the delivered air dew point were both-25C).

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.