GB2145652A - Manufacture of insulated wires - Google Patents

Abstract

An electric wire with insulation of oriented polymeric material is made by first forming a preform with a metal core and an extruded covering of an appropriate polymeric material larger than the required size. The preform is then drawn to size through one or more one reducing dies, so reducing core and covering together and producing the elongation required to effect orientation. The polymer should have a yield strength equal to or greater than that of the core metal, and the process must be performed at an appropriate temperature, but ambient temperature is often satisfactory. Given suitable polymeric materials, the finished product can be annealed and stress-relieved in one operation. The wire is preferably of copper or aluminium, and the insulation is preferably a polyparaphenylene ether-ketone, having substantially two ether linkages for each ketone linkage.

Description

SPECIFICATION Manufacture of insulated wires This invention relates to the manufacture of insulated electric wires and more particularly to the manufacture of wires insulated with oriented polymeric material.

It is known that certain physical properties of a wide range of polymers can be changed, usually for the better, by a stretching process conducted at a temperature appropriate for obtaining molecular orientation of the polymer chains; one important property being the resistance of the polymer to cracking when exposed to certain liquids, e.g. the hydraulic fluids used in the aircraft industry. In some cases oriented material may be attractive as a wire-covering material when the same polymer in its as-extruded condition is useless for this purpose.

In some cases wires are insulated by wrapping them helically or longitudinally with a film of oriented polymeric material, but even when the slow and expensive helical wrapping technique is used the tape joints are an evident source of weakness and/or inhomogeneity. Consequently we have developed certain techniques (UK Patent 1599106 and European Published Application 7814) for orientation of polymeric material initially extruded as a tube that surrounds the wire with a clearance. These techniques are effective but demand expensive equipment that is of little use for other purposes and require quite sophisticated control to ensure that both the degree of orientation and the dimensions of the insulation are within acceptable tolerances.

The method of the present invention is characterized by first forming a preform having a core of conductive metal and an extruded covering of a polymer which has a yield stress substantially equal to or greater than that of the core and which is orientable but in substantially unoriented condition, the cross-sectional areas of the core and the covering being proportional to but larger than those required in the finished insulated wire, and then drawing the preform through at least one reducing die at a temperature in the range in which the polymer can be oriented to elongate both the core and the covering and reduce their cross-sectional areas to the required values and at the same time to orient the polymeric material in the covering.

The core will usually be of copper (plain or plated with tin, nickel, or silver) or of aluminium, but other metals of sufficient ductility (e.g. tin, magnesium, lead or even sodium) could be used if desired. It will usually be circular but could be flat (with rounded corners), sector-shaped, or of other cross-section; when it is non-circular, some change in shape of the core, and possibly of the covering, during drawing may need to be allowed for in the design of the preform.

In respect of materials, the process of the invention is of broad but not universal application: the required relationship of yield stress can in some cases be achieved by heating, or cooling, the preform to an appropriate temperature.

The polymeric material is preferably selected from the semicrystalline ones listed in the specification Euuropean Patent Application Publication No. 7814, and more especially the harder and/or more heatresistant of them. More expecially preferred when the core is of copper or aluminium are the aromatic ether-ketone polymers and more especially that now sold by Imperial Chemical industries pic under the designation PEEK, which is believed to be a polyparaphenylene ether-ketone having substantially twice as many ether linkages as ketone linkages (the designation being an acronym for the non- systematic name polyether-ether-ketone). Normal compounding practices can be followed, and in some cases two or more polymers may be blended.

Another possibility is for the covering to include parts of different polymeric materials, for example concentric layers.

Preferably the covering is in intimate contact with the core but this is not essential in every case; preferably the covering is firmly adhered to the core, but this is not essential in every case either. A thin layer of a separate bonding agent could be interposed.

Subject to any reduction of cross-section that might be effected outside the range for producing orientation, the extent to which the cross-section of the preform exceeds that of the product determines the degree of orientation. A reduction in area of at least 10% is needed for useful results with most polymers, with optimum elongation values typically in the range 20-300%.

The dimensions of both parts as well as the strength of the adhesive bond, if any, between them may influence the degree of orientation that can be obtained.

It will be understood that the drawing process generates heat and the temperature will vary both longitudinally and radially in the course of reduction; it is sufficient that a substantial part of the polymeric material should be in the the required temperature range for a substantial part of the time taken to reduce it. Many polymers can be oriented by operating at ambient temperatures, but some need to reach a temperature above a glass transition temperature of the polymer; in all cases, however, the temperature should remain below the softening range of the material.

Conventional wire-drawing dies can be used, but we prefer to use a lower reduction per die than would be apropriate for drawing the metal of the core alone. For example, for wires of the order of 1 mm diameter with a copper core, a reduction of 4-8% per die is recommended compared with 15-30% for plain copper wire. A relatively low drawing speed is considered desirable, for example a speed in the range 1-10 mls when the core metal is copper.

It will be appreciated that the method so far defined produces an insulated wire in which the metal is work hardened and there may be residual stresses in the polymeric material. Sometimes, when tensile strength is of major importance, the hardness of the metal may be beneficial; in other cases the finished wire may be annealed, subject to the ability of the polymeric material to withstand the necessary heat-treatment without loss of orientation or other deleterious effect. When the wire is annealed, this will usually also relieve stresses in the polymeric material, and in some other cases a heat-treatment may be used to stress-relieve the polymeric material without annealing the wire.

Example Afully annealed tinned copper wire 0.92 mm in diameter was provided by a conventional extrusion process with a covering of the polymer identified above as PEEK with a radial thickness of 0.345 mm.

This was then drawn through a series of ten dies of diameter 1.590, 1.550,1.450,1.401, 1.354, 1.308, 1.264.1.221,1.180 and 1.140 mm, the reduction in area at each die being about 7O,o and the total elongation about 80%, in a conventional wire drawing machine operating at an output speed of 1 metre per second. The preform was fed to the machine at ambient temperature and conventional lubrication was used without any special cooling arrangement.

The dies were conventional wire drawing dies with an inlet angle of 120 leading to a parallel portion of one quarter of the diameter and a relief angle on the exit side of 7 . The product had a conductor 0.67mm in diameter and radial thickness of insulation 0.26.

Heat treatment at 200 C for 21/2 hours was effective to anneal the copper and stress-relieve the PEEK.

It will be noted from the Example that, owing to the resilience of the polymeric material, the overall dimensions of the finished wire do not corresponu exactly to the size of the last die, so that due allowance needs to be made.

Claims (4)

1. A method of making an electric wire insulated with oriented polymeric material characterised by first forming a preform having a core of conductive metal and an extruded covering of a polymer which has a yield stress substantially equal to or greater than that of the core and which is orientable but in substantially unoriented condition, the crosssectional areas of the core and the covering being proportional to but larger than those required in the finished insulated wire, and then drawing the preform through at least one reducing die at a temperature in the range in which the polymer can be oriented to elongate both the core and the covering and reduce their cross-sectional areas to the required values and at the same time to orient the polymeric material in the covering.

2. A method as claimed in Claim 1 in which the polymeric material is selected from the semicrystalline ones listed in the specification of European Patent Application Publication No. 7814.

3. A method as claimed in Claim 1 in which the wire is of copper or aluminium and the polymeric material is a polyparaphenylene ether-ketone having substantially two ether linkages for each keytone linkage.

4. A method of making an electric wire insulated with oriented polymeric material substantially as described with reference to the Example.