GB2258421A - Coating conductors with porous dielectric material - Google Patents

Abstract

Apparatus and method for coating a conductor with porous dielectric material in which the conductor (13) is led through a hollow guide (12) located in a die (10). Plastics material is extruded through a throat (22) into an expansion chamber 24 and then past a flow modifier (30) following which the plastics is deposited on the conductor emerging from the guide (12). <IMAGE>

Description

COATED SUBSTRATES This invention relates to a method and apparatus for coating substrates and in particular to a method and apparatus for providing electrical conductors with an electrically insulative outer coating: Although the present invention is applicable to coating of substrates in general, the present invention is particularly useful in the manufacture of insulation coatings for electrical cables suitable for use in data transmission where the cable insulation has to have as low a dielectric constant as possible. "Porous" solid dielectric materials which incorporate air pockets in a solid dielectric have been found to improve the electric characteristics of the cable.Furthermore, the use of such low density dielectric materials on electrical conductors greatly reduces the weight of the whole cable and the lower dielectric constant of "porous" dielectric also reduces the overall size and thickness of insulation required to achieve a given cable characteristic impedance.

The use of tetrafluoroethylene polymers, and in particular microporous sintered polytetrafluoroethylene (PTFE) as an insulation for cables is well known. For example, PCT International Publication No.W084/O0717 discloses such a material and also describes one method of manufacturing such insulative coatings. This publication discloses a technique of extruding PTFE through a die orifice, through which an electrical conductor is pulled. The coated conductor is passed through the nip of a pair of rollers which are driven in the opposite direction to the movement of the conductor.

This produces convolutions in the outer surface of the coating. The coated conductor passes through a sintering zone and over pulleys which enables one to stretch the insulation. This produces air filled voids in the insulation to achieve the desired dielectric constant.

An object of the present invention is to provide an alternative method of coating substrates, and in particular electrical conductors with a low density microporous plastics material.

According to the present invention there is provided an apparatus for coating a substrate with an expanded plastics coating which has a first surface in contact with the substrate and a second surface remote from the substrate, the apparatus comprising an extrusion die which has a throat through which an extrudable expansible plastics material is extruded, and an expansion chamber downstream of the throat for expanding the plastics material prior to applying it to the substrate, the expansion chamber having a first zone operable to allow expansion of the extrusion in one direction and having a second zone operable to allow expansion of the extrusion in a second direction and a flow modifier disposed in the expansion chamber.

In a preferred embodiment of the invention an apparatus for coating the outer surface of an elongate substrate the apparatus comprises a die member which has an inlet leading to a restricting orifice, an elongate expansion chamber downstream of the orifice and an outlet from the expansion chamber, and a hollow centre body, through which the substrate passes, extending from the inlet, through the orifice and the expansion chamber, to the outlet, said centre body cooperating with the die member to define a first annular throat at the location of said orifice and a first annular expansion zone immediately downstream of the first throat, the centre body having downstream of the fist expansion zone, a region of increased dimensions which cooperates with the die member to define a second annular throat in said expansion chamber, and the centre body having a region of reduced outer diameter, compared with said region of increased outer diameter, immediately downstream of the second throat to define said second expansion zone.

Preferably means are provided for heat treating the plastics material deposited on the substrate to sinter the plastics material.

In a further aspect of the present invention there is provided a method of coating a substrate with an expanded plastics coating which has a first surface in contact with the substrate and a second surface remote from the substrate comprising the step of extruding an extrudable, expansible, plastics material through a restricting throat and into an expansion chamber, expanding the plastics material in a first expansion zone in one direction and expanding the plastics material in a second expansion zone in another direction, the flow of plastics material through said expansion chamber being modified by a flow modifier.

In yet a further aspect of the invention there is provided a product coated by the method according to the present invention.

The present invention will now be described, by way of an example, with reference to the accompanying drawings in which: Fig.1 illustrates, schematically apparatus for applying a plastics material, on to a substrate; Figs.2 and 3 illustrate, in greater detail, the design of two extrusion tips, constructed in accordance with the present invention, for use in one embodiment of the present invention; and Fig.4 is a cross section through a coated substrate.

Referring to Fig.1 the apparatus is intended for coating the outer surface of an elongate substrate.

One such application is in the coating of an electrical conductor with a microporous dielectric coating. The apparatus of Fig.1 comprises an extrusion die 10, (shown in greater detail in Figs.2 and 3) which has-a central hollow tubular guide 12 located in the centre of the extrusion orifice of the die. The substrate 13 to be coated is fed off a supply spool 14 and passes through the guide 12 and exits from the die through the die outlet nozzle 11. The coated substrate 13 then passes through three vaporising ovens 14, 15, 16 in which outgassing of the extrudate occurs and the volatile elements are driven off. Finally the substrate passes through two sintering ovens 17, 18, in which the PTFE is sintered, and the completed product is coiled onto a driven take-up spool.

Referring in greater detail to Fig.2, the extrusion die comprises a conical inlet opening 20 which has a circular cross-section throat 22. The tapering inlet opening 20 has an included angle of approximately 300.

The throat 22 opens out to a cylindrical expansion zone 24. The transition region between the throat 22 and the zone 24 is conical with an included angle of from 200 to 40 , preferably about 300.

Located concentrically along the length of the die 10 is a central hollow tubular guide 12 which is located concentrically in the extrusion cylinder. The guide 12 is generally cylindrical with a flow modifying part 30 formed by a slightly increased diameter portion. The part 30 has a tapered region 32, 34 at each end thereof cylindrical regions The part 30 is positioned axially along the guide 12 so that it lies downstream of the throat 22 within the expansion zone 24.

The guide 12 is preferably made of stainless steel and it is best to select a tube with an inside diameter which is a close fit to the substrate 13 to be coated and should preferably have a thin wall thickness. The flow modifying part 30 of the guide is preferably positioned about 5 mm from the throat 22. The flow modifier 30 is dependent in length upon the overall size of the tooling. All surfaces of the guide and the die are preferably highly polished.

In use, a substrate 13, for example an electrical conductor is fed through the centre of the guide 12 and through the vaporising ovens 14, 15, over the guide pulley 38, through the vaporising oven 16 and through the sintering oven 17 and 18 to the take-up capstan 39 and take-up spool (not shown).

Fluoropolymer extrudate comprising fluoropolymer paste manufactured from fluoropolymer powder and a hydrocarbon lubricant is supplied under pressure to the inlet of the die so that it issues through the throat 22 into the expansion zone 24. The fluoropolymer swells as it leaves the throat 22 and the outer surface of the fluoropolymer extrudate becomes convoluted in the zone 24. The fluoropolymer in the form of a tube, then passes over the increased diameter portion 30 of the guide where the inside surface of the fluoropolymer tube also becomes convoluted.

The fluoropolymer extruded tube exits from zone 24 through the outlet nozzle 11 and is pulled down on to the substrate 13 by controlling the speed of the latter relative to the speed that the extrudate exits from die outlet 11. The speed of the substrate 13 relative to the speed of the extrudate affects the porosity (density) of the final coating.

The creation of voids in the extruded tube is achieved by pulling the coated conductor against the action of the restrictions caused by the throat 22 and the flow modifier 30. The convoluted surfaces on the outside of the extrudate are caused by the die-swell at the throat and the retarding action of the outlet nozzle 11. The convoluted surfaces on the inside of the extrudate are caused by die-swell and the retarding action of the flow modifier. Thus when exiting from the nozzle 11 the extrudate resembles corrugations of a bellows but, because the conductor is travelling faster than the extrudate the "bellows" are opened out thus creating voids 40 which are illustrated in Fig.4.

The extrudate is gradually drawn down on to the substrate 13 as it passes through the vaporising ovens.

The temperature settings of the vaporising ovens are important. The first two vaporising ovens are preferably less than 1800C - the third vaporising oven is preferably less than 1800C. If too high a temperature is used in the ovens 14, 15, 16 the extrudate may become too weak and break. Also, because the extruded fluoropolymer is porous, the extrusion aid tends to diffuse to the surface -at low temperatures so very high temperatures are best avoided. Any remaining extrusion aid is vaporised in the third oven 16.

The fluoropolymer is sintered by passing the substrate through sintering ovens 17, 18 set at about 325-335 C. If too high a temperature is used in the sintering ovens, the voids in the extrudate close-up causing loss of porosity. If too low a temperature is used in the sintering ovens 17, 18, the finished insulation will be weak and fibrous and the cable will be susceptible to random electrical faults when the cable is spark tested.

Referring to Fig.3 there is shown another embodiment of die constructed in accordance with the present invention in which the flow modifying portion 30 is of almost the same diameter as that part of the central guide 12 located in the throat 22. Immediately upstream of the flow modifier 30 the tooling is of reduced diameter 35 and immediately downstream of the flow modifier 30 the tooling is of reduced diameter 36.

In operation, the die of Fig.3 operates in a similar way to that of Fig.2. The extrudate swells outwardly immediately downstream of the throat 22 and swells inwardly immediately downstream of the flow modifier 30. In this way a more uniform expansion of the coating is achieved just prior to depositing it on to the substrate 13.

The invention is not limited to the above described embodiments. Many variations can be made.

Claims (10)

1. Apparatus for coating a substrate with an expanded plastics coating which has a first surface in contact with the substrate and a second surface remote from the substrate, the apparatus comprising an extrusion die which has a throat through which an extrudable expansible plastics material is extruded, and an expansion chamber downstream of the throat for expanding the plastics material prior to applying it to the substrate, the expansion chamber having a first zone operable to allow expansion of the extrusion in one direction and having a second zone operable to allow expansion of the extrusion in a second direction and a flow modifier disposed in the expansion chamber.

2. Apparatus as claimed in Claim 1, and comprising a hollow body through which the substrate can pass extending from the inlet through the orifice and the expansion chamber to the outlet.

3. Apparatus as claimed in Claim 2, wherein the hollow body defines an annular gap at the throat of the extrusion die.

4. Apparatus as claimed in Claim 2 or Claim 3, wherein flow modifier comprises a region of enlarged dimensions on the hollow body.

5. Apparatus as claimed in Claim 4, wherein the hollow body comprises a region of reduced dimensions as compared to the flow modifier, to define the second expansion zone.

6. Apparatus as claimed in any of Claims 2 to 5, wherein the hollow body terminates upstream of the outlet from the expansion chamber so that plastics material can be deposited on the substrate before the plastics material is extruded through said outlet.

7. Apparatus as claimed in any preceding claim and comprising means for heat treating the plastics material deposited on the substrate in order to sinter said plastics material.

8. A method of coating a substrate with an expanded plastics coating which has a first surface in contact with the substrate and a second surface remote from the substrate comprising the step of extruding an extrudable, expansible, plastics material through a restricting throat and into an expansion chamber, expanding the plastics material in a first expansion zone in one direction and expanding the plastics material in a second expansion zone in another direction, the flow of plastics material through said expansion chamber being modified by a flow modifier.

9. A method as claimed in Claim 8, wherein the plastics material is subjected to heat treatment to sinter the plastics material after deposition on the substrate.

10. A product produced by the method as claimed in Claim 8 or Claim 9.