GB1599106A

GB1599106A - Manufacture of insulated wires and cables

Info

Publication number: GB1599106A
Application number: GB16614/78A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1978-04-26
Filing date: 1978-04-26
Publication date: 1981-09-30

Description

(54) MANUFACTURE OF INSULATED WIRES AND CABLES (71) WE, BICC LIMITED, a British Company, of 21 Bloomsbury Street, London WC1B 3QN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for the manufacture of insulated electric wires and cables and to the products of the method. More particularly it is concerned with wires and cables insulated and/or sheathed with a crystallisable polymeric material, by which is meant a polymeric material which, after suitable processing, has a crystalline structure in at least part of its volume.

The physical and chemical properties of melt processable crystallisable polymer used for insulation and sheathing of electric cables, are critically dependent on the degree of crystallinity and on crystalline orientation.

It is known that the properties of fibres or filaments made of many crystallisable polymers can be substantially improved by introducing a controlled degree of crystalline orientation in the axial direction. Such orientation is obtained by a process involving stretching the fibre or filament above the glass transition temperature but below its melting temperature range similarly the properties of film can be enhanced by uniaxial or by bi-axial stretching. Such stretching of the polymer produces shear-induced crystalline orientation as a result of shear induced crystallisation or recrystallisation. Orientation of this type is usually accompanied by an increase in density (crystallisation and/or tighter packing of the polymer chains) and manifests itself in characteristic X-ray diffraction patterns. The resulting product is mechanically anisotropic. and may show substantial changes in mechanical properties e.g.

increases in modulus and tensile strength, as well as changes in chemical properties, such as improved solvent resistance and dye absorption characteristics.

We have now found that it is possible to introduce crystalline orientation of this type into insulation of electric wires and cables made from certain of these polymers and thereby obtain useful improvements in the mechanical and chemical properties of the insulation, and especially improved resistance to stress cracking especially in the presence of agressive industrial fluids.

Conventional extrusion techniques in which shear deformation of the polymer occurs above the melting temperature range do not impart crystalline orientation of the type described here.

In accordance with this invention, a method of continuously manufacturing an electric wire or cable comprises: advancing a core comprising at least one conductor (with or without pre-existing insulation as required) in the direction of its length; extruding around the core an oversize tube of a crystallisable polymeric material selected from Poly (ester-imides) Poly (amide-imides) Polyolefins (including polyethylene (especially high density type) and polypropylene and copolymers) Polycarbonates Polyphenylene oxides Polysulphones Polyvinylidene fluoride Homopolymers and copolymers based on fluoro- and perfluoro-vinyl monomers and Substantially linear aromatic homopolymers and copolymers consisting of chains of mono- or polycyclic aromatic groups linked by one or more of -0 -S 502

-CR2- where R=H or alkyl or acyl groups -NHCO

-CO-Oand other groupings including those containing P or P and N; cooling the extruded tube to form a first zone downstream of the extruder in which the temperature of the tube is low enough for it to be gripped (and preferably below the glass transition temperature) and in that zone gripping the tube to advance it at a first controlled speed; reheating the tube to form a second zone downstream of the first zone in which the temperature of the tube is above the glass transition temperature but below the melting temperature range of the polymer; stretching the tube in the second zone by advancing it (preferably by gripping means downstream of the second zone) at a second controlled speed which is from one and a half to ten times the first controlled speed; and collapsing the tube onto the core in or downstream of the second zone. When the collapsing step is to take place in the second zone it will normally be indistinguishable from the stretching step effected there.

Preferred polymers of the last-mentioned group include those described in one or more of British patent specifications 971,227, 1,016,245, 1,019,266, 1,019,458, 1,078,234, 1,086,021, 1,102,679, 1,153,035, 1,153,527, 1,164,817, 1,177,183, 1,383,393, 1,387,303 and 1,414,421, 1,414,422 and 1,414,423; some of those polymers can be effectively used in the oriented state for insulation of cables designed for service at or above 1500C, despite the fact that the same polymer applied by the conventional extrusion techniques have been considered unsuitable for electric cable use because of poor resistance to stress cracking and/or poor solvent resistance.

In most cases it is desirable, to "anneal" the polymeric material after stretching by heating to a temperature above the stretching temperature but still below the melting temperature range, and often superior results can be obtained by stretching in two stages (or more) with intermediate annealing by repetition of the appropriate steps.

To obtain reproducible results, the temperature of the second zone, or each such zone if there are two or more stretching stages, and of the annealing zone, and the associated elongation (the ratio of the second controlled speed to the first) will need to be precisely maintained; obtimum values depend on the nature of the crystallisable polymer.

The method described, can be used to make a wide range of insulated wires and cables, including products suitable for use in wiring buildings, aircraft, ships and vehicles, electrical equipment and appliances, telecommunication, as well as control and industrial power cables.

The invention will be further described, by way of example, with reference to the accompanying drawings in which figures 1 and 2 are diagrams of two different production lines for operation of the method of the invention in producing insulated wires.

The line shown in Figure 1 is arranged horizontally or vertically and includes an extruder 1, wire pay-off 2 and cable take-up 3 which are conventional. The extrusion temperature, T1 is above the softening range of the polymer being used.

On emerging from the extruder crosshead 4 the wire 5 is centred in the extruded tube 6 sufficiently to prevent the tube sticking to the wire, and the tube is cooled by a cooler 7 e.g.

a water trough, until it reaches a temperature T2 preferably below the glass transition temperature of the polymer.

At this temperature the tube can be gripped by a driven-belt traction device 8 without sticking or substantial permanent deformation. The speed V1 of this traction device controls stretching of the tube 6 at the outlet of the extruder and thereby influences the wall thickness of the insulation being produced.

From the traction device 8 the tube, with the wire inside it, passes to a stretching unit 9 comprising a high density liquid bath or a fluidised bed maintained at a temperature T3 that is above the glass transition temperature of the polymer but below its melting range, from which it is pulled by another driven-belt traction device 10 located outside the stretching unit and sufficiently downstream of it to allow the tube to cool or be cooled (preferably a cooler. not shown. is used) to a temperature T4 usually below the glass transition temperature. The traction device 10 is driven at a speed V2 which is from one and a half to ten times Vl and the tube is consequently stretched and the polymer oriented in the stretching unit 9 (being the place between the two traction devices where its temperature is highest and its strength consequently lowest) and at the same time the tube is collapsed onto the wire.

Subsequently the covered wire 11 is heated in an annealer 12 to a temperature T5 which is higher than T3 but below the melting temperature range, before cooling to a temperature T6 for reeling in the take-up 3.

The temperature T3 and the ratio V2/V1 are chosen to obtain the required degree of orientation; in some cases T3 may usefully vary along the length of the stretching unit 9.

Figure 2 shows a production line for a two-stage drawing process; in this case the traction device 10 is driven at a speed V3 which is intermediate between V1 and V2 and does not bring the tube into contact with the wire; and on emerging from the annealer 12 the tube passes to a further driven-belt traction device 8' driven at the same speed V3 or a lower speed V4 to allow for shrinkage in the annealer. This traction device 8' is followed by a further stretching unit 9' in which stretching is completed and the tube at the same time collapsed into contact with the wire, followed by a further caterpillar device 10' and annealer 12. Temperatures T2,, T31 and T4' satisfy the conditions described forT2, T3 and T4 respectively, but need not necessarily be equal to them.

Example Bare wire was covered with a crystallisable polyester of a polyethylene glycol terephthalate type having a glass transition temperature of about 75"C by the method illustrated by Figure 1. The stretching unit 9 was filled with water at 950C and the annealer 12 (air-filled) maintained at 250-3000C (the optimum value depending on the length of the annealer, insulation thickness and line speed).

Claims

WHAT WE CLAIM IS:

1. A method of continuously manufacturing an electric wire or cable comprising: advancing a core comprising a least one conductor in the direction of its length; extruding around the core an oversize tube of a crystallisable polymeric material, selected from Poly (ester-imides) Poly amide-imides) Polyolefins (including polyethylene (especially high density type) and,polypropylene and copolymers) Polycarbonates Polyphenylene oxides Polysulphones Polyvinylidene fluoride Homopolymers and copolymers based on fluoro- and perfluoro-vinyl monomers and Substantially linear aromatic homopolymers and copolymers consisting of chains of mono- or polycyclic aromatic groups linked by one or more of -0 -S -SO2-

-CR2- where R=H or alkyl or acyl groups -NHCO

-CO-Oand other groupings including those containing P or P and N; cooling the extruded tube to form a first zone downstream of the extruder in which the temperature of the tube is low enough for it to be gripped and in that zone gripping the tube to advance it at a first controlled speed; reheating the tube to form a second zone downstream of the first zone in which the temperature of the tube is above the glass transition temperature but below the melting temperature range of the polymer; stretching the tube in the second tube by advancing it at a second controlled speed which is from one and a half to ten times the first controlled speed; and collapsing the tube onto the core in or downstream of the second zone.

2. A method as claimed in claim 1 in which the tube is advanced to stretch it by gripping means downstream of the second zone.

3. A method as claimed in claim 1 or claim 2 in which the polymeric material is annealed after the tube has been stretched.

4. A method of continuously manufacturing an electric wire or cable comprising: advancing a core comprising at least one conductor in the direction of its length; extruding around the core an oversize tube of a crystallisable polymeric material selected from Poly (ester-imides) Poly (amide-imides) Polyolefins (including polyethylene (especially high density type) and polypropylene and copolymers) Polycarbonates Polyphenylene oxides Polysulphones Polyvinylidene fluoride Homopolymers and copolymers based on fluoro- and perfluoro-vinyl monomers and Substantially linear aromatic homopolymers and copolymers consisting of chains of mono- or polycyclic aromatic groups linked by one or more of -0 -S -SO2

-CR2- where R=H or alkyl or acyl groups -NHCO

-CO-Oand other groupings including those containing P or P and N; cooling the extruded tube downstream of the extruder to form a first zone downstream of the extruder in which the temperature of the tube is low enough for it to be gripped and in that zone gripping the tube to advance it at a first controlled speed; reheating the tube to form a second downstream of the first zone in which the temperature of the tube is above the glass transition temperature but below the melting temperature range of the polymer; stretching the tube in the second zone by advancing it from the second zone at a second controlled speed, annealing the tube downstream of the second zone; cooling the annealed tube to form a third zone in which its temperature is again low enough for it to be gripped; in that zone gripping the tube to advance it at a third controlled speed; reheating the tube to form a fourth zone downstream of the third zone in which the temperature of the tube is above the glass transition temperature but below the melting range of the polymer, further stretching the tube in the fourth zone by advancing it at a fourth controlled speed which is from one and a half to ten times the first controlled speed, the second and third controlled speeds being intermediate between the first and fourth controlled speeds and the third controlled speed equal to or less than the second controlled speed; and collapsing the tube onto the core in or downstream of the fourth zone.

5. A method as claimed in any one of the preceding items in which the temperature of the tube when it is gripped and advanced is in each case below the glass transition temperature of the polymer.

6. A method of making an insulated wire or cable substantially as described with reference to Figure 1 or Figure 2.

7. A method of making an insulated wire or cable substantially as described with reference to the example.

8. An insulated wire or cable made by the method claimed in any one of the preceding claims.