CA1339458C - Methods and devices which make use of conductive polymers to join articles - Google Patents

Abstract

A tubular coupler comprises one or more tube-forming components and a laminar gasket comprising a laminar conductive polymer heating element. The coupler can be wrapped around a substrate, e.g. a pipe, with the gasket adjacent the substrate, and then brought into intimate contact with the substrate while passing current through (and thus generating heat within) the heating element.
Preferably, the substrate is composed of a polymeric material and becomes fused to the coupler. Alternatively, the gasket comprises a heat-activatable adhesive.

Description

~33945~

Field of the Invention This invention relates to methods of joining arti-cles together with the aid of conductive polymer components, and to novel articles for use in such methods.
Backqround of the Invention Conductive polymers are well known. They comprise a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black.
Conductive polymers have been widely used in electrical heaters, including heaters which are in the form of heat-recoverable articles or which are secured to heat-recoverable articles so that, by powering the heater, the article can be caused to recover. Typically, the recovery of the article re-sults in joining, repairing, reinforcing or otherwise modify-ing one or more substrates around or against which the article recovers. Recently, it has been shown that conductive poly-mers which retain substantial strength above their melting point, especially sintered polymers such as ultra high molec-ular weight polyethylene (UHMWPE), are particularly useful for modifying pipes composed of organic polymers (plastic pipes).
Reference may be made for example to U.S. Patent Nos.
3,987,276, 4,085,286, 4,177,376, 4,177,446 4,421,582, 4,455,482, 4,570,055, 4,575,618, 4,686,071, U.K. Patent Nos.
1,265,194, 1,449,539, and 2,124,439, published German Patent Application No. 3,442,674, and published European Application Nos. 157,640, 153,199, and 231,068.

~ ' In one aspect, the present invention provides a tubular coupler which can be placed around or within a substrate and joined thereto, and which comprises (1) a tubular member comprising one or more tube-forming components which are composed of an electrically insulating polymeric composition, and (2) at least one gasket which (a) is adjacent to a surface of the tubular member, and (b) comprises (i) a laminar heating element com-posed of a conductive polymer, and (ii) two electrodes which can be connected to a source of electrical power to cause current to pass through the heating element substantially parallel to the surface thereof and to generate heat within the conductive polymer;

the tubular member and the gasket being such that the coupler can be wrapped around and brought into intimate contact with the substrate, with the gasket between the tubular member and the substrate.
In another aspect, the invention provides a method of securing a tubular coupler to a tubular substrate, which method comprises (A) providing a coupler according to the first aspect of the invention (B) wrapping the tubular coupler around the substrate and bringing it into intimate contact with the substrate, with the gasket between the tubular member and the substrate; and X

133~458 (C) applying circumferential forces to the coupler and connecting the electrodes to a source of electrical power, thus generating heat which causes the coupler to become joined to the substrate.
When this specification refers to the coupler being "wrapped around" the substrate this includes processes in which the coupler is sufficiently flexible to be placed around the substrate without passing over an end of the substrate, processes in which the two or more components are assembled ln situ around the substrate to form the coupler, and processes in which the coupler is passed over one end of the substrate and is then deformed, without the use of heat, at the desired location.
By way of example, in one of several preferred embodiments of the method of the invention, two pipes are joined together in line by means of a cylindrical coupler which fits around the pipe ends. The joining of the pipes to the coupler can be carried out simultaneously or at different times.
The term "intimate contact" is used herein to denote a sufficiently close conformity between the surfaces to ensure that when the heat is generated within the conductive polymer, the heat serves its intended purpose of joining the first and second articles together and does not cause any substantial damage to the coupler or the substrate. If there are voids present at the various interfaces, they can result in over-heating which is wasteful and can be damaging, particularly when the rate at which power is generated is high.

;~r 1~3~L58 The join between the substrate and the coupler is preferably gas-tight and liquid-tight, and is preferably such that in an appropriate physical test, e.g. a flex, tension, torsion, or burst test, one of the articles fails before the join.
As will be described in detail below, the joining of the substrate and the coupler can be achieved in a variety of different ways. Preferably, the gasket becomes fused to both the substrate and the coupler. Alternatively or additionally, the gasket can have apertures therein and, as a result of the heating, the substrate and the coupler become fused directly to each other through the apertures. The gasket can consist essentially of the conductive polymer element, together with the electrodes and any other conductive elements which are used to ensure the desired generation of heat therein. In another embodiment, the gasket includes a central member, e.g.
a strip of an insulating polymeric material, and one or more conductive polymer elements are attached to the sides of the central member.
The term "fusion" is used herein to denote a process in which two polymeric substrates are joined directly to each other with the aid of heat, and there is sufficient molecular compatibility between the substrates that, under the condit-ions of the process, molecular diffusion takes place across the interface between the substrates and/or there is visco-elastic contact between the substrates as defined by J. N.
Anand in Adhesion 1 (1969), 16-23 and Adhesion 2 (1970) 16-22.
The fusion process preferably results in a bond which has a 13394~8 mechanical performance which is superior to that of at least one of the substrates. The surfaces of the substrates which have undergone fusion are said to be "fused" together. The method of the invention is particularly useful when the sub-strate and the coupler (A) are composed of polymeric composit-ions which are the same or in which at least 80~, preferably at least 90~, especially substantially 100~, of the repeating units of the polymer are the same, and (B) are joined together by (i) fusion to opposite sides of a laminar conductive poly-mer element comprising a polymeric component in which at least80~, preferably at least 90~, especially substantially 100~, of the repeating units are the same as in the substrate and the coupler, or (ii) by fusion to one or more laminar conduc-tive polymer elements, each of which comprises a polymeric com-ponent as just defined and each of which is fused to (or in the course of the method becomes fused to) a central member composed of an insulating polymeric composition in which at least 80~, preferably at least 90~, especially substantially 100~, of the repeating units of the polymer are the same as in the first and second articles. The preferred polymer in each of the components is polyethylene. The polyethylenes in the different components need not be, indeed usually will not be, the same. For example two pipes composed of melt-extruded low, medium or high density polyethylene can be joined to-gether with a coupler which is composed of the same or a different melt-extruded low, medium or high density polyethyl-ene, by means of a gasket which (a) comprises a conductive polymer based on a sintered ultra high molecular weight 1339~8 polyethylene, and (b) optionally comprises a central member which is composed of a melt-extruded low, medium or high density polyethylene.
In most cases, each of the substrate and the coupler is substantially thicker than at least a part of the gasket.
For example, the thickness of each may be from 0.05 to 6.0 inch, preferably 0.1 to 2.0 inch, and will not usually differ by a factor of more than 5, preferably not more than 3, whereas the gasket will usually be no more than 0.15 inch, preferably no more than 0.05 inch, thick when the articles are fused to opposite faces of the conductive polymer element, and no more than about 0.4 inch, often decreasing to no more than about 0.1 inch, when the gasket comprises a central member.
Especially when the substrate is composed of a non-polymeric material, e.g. is composed of concrete or a metal, or when there is insufficient compatibility between the poly-meric materials to allow fusion to take place, e.g. where one or both is composed of a thermoset material, e.g. an epoxy or other resin reinforced with glass fibers, an insert can be placed between the surfaces to be joined, the insert being such that, when heated by the conductive polymer, it undergoes a physical or chemical change which joins the surfaces to-gether. Thus the insert can for example be composed of a partially cured thermosetting resin or a hot-melt adhesive (including a sheet of a polymer which is sufficiently compatible to become fused to each of the surfaces). The insert can be secured to the gasket or to the substrate before 133~58 the gasket and the two articles are assembled, or it can be inserted in situ. For example, a sheet of polyethylene or another insulating polymeric material can be secured to one or both faces of the conductive polymer element. When the sub-strate and the coupler surfaces are compatible with each other, the gasket can have apertures therein so that the first and second surfaces can be fused directly to each other through the apertures. Such direct fusion can be in addition to fusion or other form of bonding between the substrate and the coupler.
Features which may be possessed by tubular couplers of the present invention include but are not limited to the following.
(1) The tubular member is articulated by reason of a cut made part way through the wall thereof.
(2) The tube-forming components have at least one chamfered edge, preferably two chamfered edges which can slide relative to each other.

(3) The gasket is adjacent to a flexible tubular member over a substantial area and is attached to the tubular member over a minor proportion of that area (e.g. less than 50~, preferably less than 30~, particularly less than 10~) whereby the gasket can move relative to the tubular member when the coupler is wrapped around a substrate.

(4) The gasket is fused to the tubular member.

,,'x,~

(5) The gasket comprises a conductive polymer which is (a) a sintered conductive polymer and/or (b) exhibits essentially ZTC behavior, and/or (c) has a low melt flow index above its melting point.

(6) The gasket has a conductive bridge across a central portion thereof.

(7) The gasket has conductive bridges attached to the electrodes to improve the uniformity of the heat output.
BRIEF DESCRIPTION OF THE DRAWING
This invention is illustrated in the accompanying drawing, in which Figure 1 is a cross-section through an assembly in which two pipes are being joined together via a wrap-around flexible coupler comprising a gasket secured to its inner face;
Figure 2 is a cross-section on line 5-5 of Figure 1;
Figure 3 is an "unrolled" plan view of the gasket used in Figures 1 and 2;
Figure 4 is a cross-section through an assembly in which two pipes are being joined together by a coupler having mating edges which can slide relative to each other and having a gasket secured to its inner surface;
Figure 5 is a cross-section on line 8-8 of Figure 4;
Figure 6 is an "unrolled" plan view of the gasket used in Figures 4 and 5;

X

133~8 Figure 7 is a cross-section through an articulated coupler comprising two gaskets secured to the inner face thereof; and Figure 8 is a cross-section through a two-piece coupler comprising two gaskets secured to respective inner faces of the two parts of the couplers.
DETAILED DESCRIPTION OF THE INVENTION
The following detailed description of the invention has been divided up into various sections and sub-sections.
It is to be understood that this is for clarity and conven-ience, and that relevant disclosure of a particular feature may be included in more than one section or sub-section, that the headings of the sections and sub-sections are not to be regarded as having any limiting effect, and that the disclos-ure of this specification includes all the appropriate com-binations of information found in the different sections and sub-sections. Similarly, although the various Figures and the descriptions thereof disclose specific embodiments of the invention, it is to be understood that where a specific feature is disclosed in the context of a particular Figure, such feature can also be used, to the extent appropriate, in the context of another Figure or the invention in general.
1. Location at Which Heat is Generated An important factor in the present invention is the fact that heat is generated where it is needed, i.e. at or close to the surfaces to be joined. This has distinct advan-tages over processes in which heat is generated at some rela-tively remote point such that significant thermal barriers 1~39~58 have to be overcome before the desired heating is achieved, and over processes in which heat is generated not only at the desired location but also at other locations. For example, the power consumption and heating time are reduced, and damage resulting from overheating can be eliminated or reduced. The substrate and any components of the gasket in addition to the conductive polymer and electrodes should not be such that they prevent the desired generation of heat within the conductive polymer, and they are preferably such that no significant heat is generated within them. The substrate is preferably non-conductive, e.g. composed of an insulating polymeric compo-sition; if it is conductive, it is preferably separated from the conductive polymer by a layer of a non-conductive material.
2. Shape of the Conductive Polymer Element It is in general desirable that heat should be gen-erated over a substantial area in order to provide a satis-factory bond between the articles, and that the heat should be generated in such a way that, taking into account different rates of thermal transfer in the assembly, the heating is substantially uniform in those parts of the assembly where fusion (or activation of thermally responsive insert) is desired. Non-uniform heating tends to result in non-uniform bonding and to cause overheating in the locations at which heat is generated in an attempt to supply sufficient heat to other locations at which bonding is desired. For this reason, it is preferred that the conductive polymer should be present in the form of a laminar element having a relatively wide and .,s~

thin cross-section, e.g. a ratio of width to thickness of at least 8, preferably at least 20, particularly at least 50, especially 50 to 200, e.g. 100 to 160. Thus the laminar con-ductive polymer element can be in the form of a tape, partic-ularly a flexible tape, which has a length which is for example at least 3 times, preferably at least 6 times, its width. The laminar conductive polymer element will generally be of uniform width and thickness, but can be of non-uniform width and/or non-uniform thickness, e.g. corrugated, ribbed or grooved. The average thickness of the tape is preferably .005 to 0.150 inch, particularly .01 to .05 inch. The ratio of the total surface area of the tape (i.e. including both surfaces) to the volume of the tape is preferably at least 20 inch~1, particularly at least 40 inch-1, especially 40 to 100 inch~1, e.g. 55 to 75 inch-1.
3. Electrodes The electrodes are preferably placed at opposite edge portions of a laminar conductive polymer element; they may be partially or completely embedded in the conductive polymer element or placed on a surface thereof, preferably the same surface. In this embodiment substantially all of the current flows in the plane of the laminar element, little or none of the heated portion of the l~m; n~r element is covered by the electrodes, and the heat is generated in a bonding section which has substantial width and length, and is sub-stantially free of metal. Thus the bonding section generally has an area of at least 0.25 square inch, preferably at least 1 square inch, particularly at least 4 square inch, especially at least 10 square inch, e.g. 10 to 120 square inch, and has a width which is preferably at least 0.4 inch, particularly at least 1 inch, e.g. up to 10 inch, and a length which is pre-ferably at least 1 inch, particularly at least 3 inch, espec-ially at least 10 inch, e.g. 20 to 200 inch. Much larger di-mensions can be used. For example, when joining large pipes, the bonding area can be 2,000 to 3,000 sq. inch. A large metal-free bonding section provides important benefits, because the presence in a bonding section of metal wires or strips (e.g. metal wires and strips such as have been used in prior art joining methods as resistance heaters) often reduces the overall strength of the bond.
The electrodes which are generally present in the gaskets are preferably such that substantially no heat is gen-erated within them. However, they can be of dimensions and resistivity such that substantial heat is generated within them, either uniformly along their length or in a desired pat-tern. The electrodes are preferably composed of copper, but can be composed of another metal or other material of appropriate resistivity.
The electrodes are preferably longitudinal elec-trodes (e.g. metal strips, braid, or silver paint or a combin-ation of these) applied along (or near) the edges of one or both surfaces. In a preferred embodiment, the electrodes are attached to the conductive polymer by the use of a conductive adhesive. The conductive adhesive, which is preferably in the form of a self-supporting strip composed of an organic polymer and, dispersed in said polymer, a particulate conductive 1~39~58 filler, is positioned between the tape and the electrodes or other laminar elements, and the assembly is laminated under pressure at a temperature sufficient to melt the adhesive.
Although the electrodes are preferably placed at opposite edge portions of a l~m;n~r conductive polymer ele-ment, it is also possible to place electrodes on opposite sides of a laminar conductive polymer element so that they overlap each other, thus producing current flow which is partially or, preferably, completely at right angles to the plane of the element. Under these circumstances, it is preferred that (1) the conductive polymer elements should be continuous and the electrodes apertured (e.g. the electrodes are expanded metal mesh electrodes) so that fusion can take place between the conductive polymer in the apertures and an adjacent polymeric surface, or (2) that there should be aper-tures running through the electrodes and the conductive poly-mer element so that fusion can take place, through the apertures, between polymeric surfaces on either side of the conductive polymer element. Such apertures can for example provide 30 to 90~ of each surface of the heating element.
4. The Conductive Polymer As noted above, an important factor in the present invention is the fact that heat is generated where it is needed, the conductive polymer element being sandwiched be-tween the surfaces to be joined and maintained in intimate contact with them. This has important implications for the preferred nature of the conductive polymer employed and the rate at which power is generated therein.

Because the conductive polymer is surrounded by and in intimate contact with the components adjacent thereto, and because it is relatively thin, heat can be removed from it very uniformly and efficiently. It is, therefore, possible to heat the conductive polymer to a temperature well above its melting point, and to maintain it at such a temperature for a relatively long time, without danger that part of it will become degraded through overheating, thus leading to a weak spot in the bonding section. The generation of high temper-atures in this way is very useful in ensuring rapid meltingand consequent fusion of adjacent polymeric surfaces, or rapid curing of a thermosetting resin, or rapid activation of a hot melt adhesive, or the like. On the other hand, if the conduc-tive polymer becomes too fluid at the elevated temperature, it will tend to flow so as to disrupt the desired current flow and change the amount and/or the distribution of the heat generated within it. Flow of the conductive polymer out of the recess will also tend to reduce the pressure between the various elements and to result in less efficient bonding. For these reasons, it is preferred to use a conductive polymer which maintains substantial strength above its melting point, for example a polymer which, at a temperature 50~C above its softening point, has a Melt Flow Index of less than 0.3 g/10 min, especially less than 0.05 g/10 min, at a loading of 3 kg, and a Melt Flow Index of less than 3.0 g/10 min, particularly less than 1.0 g/10 min, especially less than 0.1 g/10 min, at a loading of 15 kg. Flow of the current-carrying conductive polymer can also be minimized by placing one or more barriers to such flow around the current-carrying conductive polymer.
Such a barrier can be provided for example by an adjacent and integral portion of conductive polymer through which little or no current flows, or through an adjacent barrier of an insul-ating polymer which may be separate from or integral with the conductive polymer.
Just as flow of the conductive polymer out of the recess reduces the bonding pressures available, so also ex-pansion of the conductive polymer increases the bonding pres-sures. It is, therefore, preferred to use a conductive poly-mer which increases substantially in volume as it is heated, preferably one which increases in volume by at least 10~, preferably at least 15~, when it is heated from 23~C to the melting point of at least part of the polymeric component of the conductive polymer. In many cases, the conductive polymer is heated to a temperature well above its melting point, and it will continue to expand during such heating. Preferably the conductive polymer increases in volume by at least 20~, particularly at least 25~, especially at least 30~, between room temperature and the highest temperature which it reaches during the method.
When, as is preferred, the conductive polymer be-comes fused to an adjacent polymeric surface, it is necessary for it to be heated to a sufficiently high temperature for fusion to take place. If the conductive polymer exhibits PTC
behavior, it may not reach a high enough temperature. It is preferred, therefore, that the conductive polymer should ex-hibit essentially ZTC behavior in the temperature range of X' 1339~58 operation, and in particular that it should increase in resis-tivity by a factor of less than 5, preferably less than 2, particularly less than 1.3, over a temperature range of 23~C
to 250~C and/or over a temperature range of 23~C to (Tm +
50~C), where Tm is the softening point of the composition.
Having regard to the preferred features set out above, conductive polymers which are particularly useful in the present invention comprise, and preferably consist essentially of, (a) a matrix consisting essentially of organic polymer particles which have been sintered together so that the par-ticles have coalesced without completely losing their identity, particularly particles of ultra high molecular weight polyethylene (UHMWPE), and (b) a particulate filler, preferably carbon black, which is dispersed in said matrix but which is present substantially only at or near the boundaries of the coalesced particles.
Such sintered conductive polymers are described, for example, in published European Application No. 157,640.
UHMWPE generally has a molecular weight of at least 1.5 million, preferably greater than 3.0 million, e.g. 4 to 6 mil-lion. Other polymers that may be sintered include polytetra-fluoroethylene, polyphenylene sulfide, and polyimides.
Sintered conductive polymers can be prepared by sintering a dry blend of the polymer particles and the filler.
A typical process involves compaction of the dry blend, sin-tering of the compacted blend at or above atmospheric pressure and at a temperature at which the polymer softens but does not , ~

133~ 1~8 flow excessively, followed by cooling under pressure. The sintering is preferably conducted as part of a ram extrusion process which produces a rod, and the rod is then skived on a lathe, or otherwise machined, to produce a conductive polymer tape of desired surface texture and shape. For many purposes a tape of substantially rectangular cross-section is satisfac-tory, but if desired a corrugated, ribbed, or grooved tape can be obtained through the use of a skiving blade of an appropri-ate shape. Such an irregular shape may be desirable to con-centrate pressures at desired points and/or to increase thesurface area of the tape, e.g. for heat transfer or bonding purposes.
Other conductive polymers which can be used include those which are based on the so-called very high molecular weight polyethylenes (VHMWPE) having molecular weights of less than 1.5 million and which can be processed by melt-extrusion.
VHMWPE polymers generally have molecular weights in the range of 150,000 to 600,000, preferably 200,000 to 400,000.
The resistivity of the conductive polymers used in this invention is generally quite low, e.g. below 1000 ohm-cm, particularly below 100 ohm-cm, especially below 10 ohm-cm, for example, in the range of about 0.5 to 10 ohm-cm. If the con-ductive polymer is to be electrically powered, the desired resistivity depends upon the power source, which may be of any kind, for example DC of 6 to 48 volts or AC of about 120 or 240 volts. Sintering produces low electrical resistivity at a lower conductive filler loading than for a melt-blended pro-duct. Thus the preferred sintered compositions for use in this invention contain less than 9~, preferably less than 7~, particularly 2-6~, by volume of carbon black and/or other conductive filler. Particularly preferred carbon blacks are those sold by Akzo Chemie under the tradename Ketjenblack EC
and by Degussa under the tradename Printex XE-2. The low levels of carbon black help to maintain the desired physical properties of the polymer such as flexibility, good elonga-tion, high tensile strength, good notch and impact resistance, and good chemical resistance. The conductive polymer may optionally be crosslinked, providing that this does not prevent any desired fusion thereof.
5. Methods of Assembly, and Shape of Gasket The gasket should preferably be such that when the coupler and the substrate are moved relative to each other, a good interference fit can be achieved between them, thus achieving the desired intimate contact. If the gasket is not in intimate contact with the tubular member, the relative movement produces the desired intimate contact. If the gasket has already been brought into intimate contact with the tubular member, e.g. by preassembly, the relative movement produces intimate contact between the surfaces which are not yet in intimate contact. In many cases, the requirement for intimate contact will mean that the gasket has a relatively wide and thin cross-section, e.g. a ratio of width to thick-ness of at least 4, preferably at least 10, and often substan-tially more, for example at least 50, particularly 50 to 200, especially 100 to 160. Thus the gasket may be in the form of a tape, which will generally be of uniform width and 1~3~458 thickness, but can be of non-uniform width and/or non-uniform thickness. In many cases, it will be desirable for the tape to have a length which is for example at least 3 times, preferably at least 6 times, its width, and may be much more.
6. Conformance of Articles to Each Other Durinq Assembly Relative movement of the substrate and the coupler can have the very desirable result of changing the shape of one or both of them so that they have substantially uniform dimensions. For example, an out-of-round pipe can be placed inside a round coupler, and the pipe pressed into a round shape. Alternatively, the gasket can change its shape under pressure.
7. Rate at Which Power is Generated, and Temperature Distribution One of the features of the present invention is the high rate at which power can be generated within the conduc-tive polymer (without causing substantial degradation thereof) over periods which are long enough to produce the desired fusion (or other physical or chemical effect) at the boundar-ies of the conductive polymer heating element. This makes it possible to use power sources of high voltages; however, it must be remembered that the higher the voltage, the more care-fully the duration of the heating must be controlled. Thus an AC voltage of 600 volts or more can be employed. In another method, the heater can be powered by the discharge of one or more capacitors, thus providing both a high voltage and controlled overall power consumption.

The power source and the conductive polymer element are preferably such that the power output of the element is 5 to 120, particularly 7 to 40, especially 10 to 20, watts per square inch of the total surface area of the element (includ-ing both surfaces). One of the advantages of the present in-vention is that the total energy used is limited because substantially all the heat is generated at the required site.
Thus the energy density P x t (where P is the A

power in watts per second, t is the time in seconds for which the conductive polymer heater is connected, and A is the total surface in square inch of the heater, i.e. including both surfaces thereof) is generally 10 to 100,000, preferably 50 to 25,000, particularly 100 to 5,000, for a time (t) which is generally less than 1,500 seconds, preferably less than 800 seconds, particularly less than 150 seconds, especially 10 to 120 seconds. The conductive polymer element, when contacted on one or both sides by a solid substrate, is preferably capable of withstanding a power load of at least 50 watts/cm3, particularly at least 100 watts/cm3, e.g. up to 200 watts/cm3 or even higher. The various quantities given above are at steady state conditions. When the power supply is first switched on, the quantities are usually higher, e.g. twice as high for a very short period of time, because the conductive polymer does show some increase in resistivity with temperature.
Another way of quantifying the difference between the present process and processes in which a relatively thick conductive polymer heater is used is to look at the temperature gradient across the conductive polymer element, which is relatively small in the present invention. Thus in the method of the invention, we believe that the maximum temperature difference between the center of the conductive polymer element and the outer surface of the element is generally less than 150~C, preferably less than 100~C, particularly less than 75~C, especially less than 50~C.
The temperatures which should be reached depend upon the way in which the surfaces are to be joined together and the nature of the materials at the interface. In general, the higher the temperature the better, providing that none of the materials is degraded. When fusion is desired, the temper-ature preferably exceeds the melting point of the material of each surface. When joining is achieved via an insert, the temperature must be sufficient to activate the insert. In preferred processes, a temperature of at least 135~C, preferably at least 150~C, particularly at least 200~C, especially at least 250~C, e.g. 300~C or more is achieved.
We have found that when the surfaces are to be fused together, the rate and duration of the heating are preferably such that the material of the substrate and the coupler is melted to a depth which is at least sufficient to permit mo-lecular diffusion across the interface. On the other hand, if the surface is melted to an excessive depth, this is not only wasteful of power but can also cause the formation of voids on cooling. The surface is preferably melted to a depth of 0. 02 to 0.1 inch, particularly 0. 035 to 0.08 inch, especially 0.04 133~4~8 to 0.06 inch. If desired, the depth to which melting has taken place can be monitored with the aid of holes drilled from the outside of the article and leaving a precisely known wall thickness between the bottom of the hole and the surface which is to be fusion bonded to the gasket; as soon as melting to the bottom of the hole has taken place, a molten polymeric material will be extruded through the hole. If the depth of melting is too great, it can be reduced by increasing the power and reducing the duration of the heating.

8. Use of Conductive Bridges to Control Generation of Heat In some cases, we have found, it is difficult or impossible to ensure that the whole of the gasket makes in-timate contact with another solid body so that heat is rapidly removed through the whole of the gasket surface. For example, when the ends of two pipes to be joined, unless the pipe ends are precisely mated together (which rarely happens), excessive heat may be generated in the conductive polymer which overlies the gap between the pipes. This problem can be solved by using a plurality of conductive polymer heating elements, each having its own electrodes. We have found, however, that a better solution is to provide a conductive bridge over or through that part of the conductive polymer element which is liable to become overheated. The conductive bridge can be, for example, a layer of a low resistivity material, e.g. a silver- or other metal-comprising material, which has been applied, e.g. by printing, to appropriate parts of one surface of the conductive polymer element. The bridge ensures that f'' little or no heat is generated in the conductive polymer in the bridge area. The bridge itself is preferably of low re-sistance so that substantially no heat is generated therein, but it can be chosen so as to generate a controlled amount of heat less than would be generated in the conductive polymer in the absence of the bridge. The bridge is preferably shaped so as to avoid undue current concentration; for example a bridge in the form of a peninsula preferably has a round or pointed end. The electrodes and/or the bridge can for example be pro-duced by etching a continuous metal layer on the surface of aconductive polymer element.
A conductive bridge can also be used to change the effective shape of strip electrodes running down the side of the conductive polymer element. For example, the presence of a central conductive bridge along part of the length of a heating element can cause the heating element to be substan-tially hotter in the end area which does not contain the cen-tral conductive bridge. To avoid this, conductive bridges can be placed over each of the electrodes in the end area in order to reduce the path length between them and make the heating more uniform. This is illustrated for example by Figure 6.

9. Windows in Conductive Polymer Elements While the use of a conductive bridge often produces very valuable results, failure to heat one part of the conduc-tive polymer can produce an unbalanced physical response of the gasket as a whole which is undesirable. In some cases, therefore, it may be desirable to weaken or remove part of all X

1~394~8 of the conductive polymer in the bridge region; this may make it necessary to use a physically stronger conductive bridge than would otherwise be required.
A particular example of this can arise when a heat-ing element having a conductive bridge but no window therein is fused to the second article to make a pre-assembled com-posite coupler which is used to join two pipes. If the composite article is brought into intimate contact with the pipes by means of bands of metal or other non-elastic mater-ial, the expansion of the heating element can only be accom-modated by driving the pipes away from each other. This problem does not arise if the restraining means has sufficient elasticity to allow the expansion to take place radially. Nor does the problem arise if the heating element is not fused to the coupler and there is room for the conductive polymer in the bridge area to buckle up between the pipe ends. If there is a window in the bridge area then the expansion can be accommodated by movement of the conductive polymer into the window. Movement of this kind can be beneficial in effecting interpenetration of the materials at the interface.

10. Non-heat-recoverable and Heat-recoverable Articles and Gaskets One of the advantages of the present invention, as compared to many methods previously proposed, is that in gen-eral, excellent results can be obtained using couplers which are not heat-recoverable. When we say herein that an article is "not heat-recoverable", we mean that if the article is heated on its own to any temperature used in the method and is then cooled back to room temperature, its dimensions at room temperature are not substantially changed (e.g. no dimension changes by more than 10~), by such heating and cooling. It will be understood, however, that the invention does not exclude the possibility that substrate or the coupler, or a part of the coupler, is heat-recoverable. If the substrate or the tubular member is heat-recoverable, the gasket is unlikely to provide sufficient heat to bring about satisfactory recovery thereof, so that an additional heat source is needed.

11. Gaskets Containing central Members As briefly indicated above, the gasket can comprise one or more conductive polymer heaters combined with a central member. The central member preferably plays no part in the generation of heat. For example it can be composed of an in-sulating polymeric material, which may be reinforced by means of fibers distributed therein or by a reinforcing member which is surrounded by the polymeric material. The central member can have a single laminar conductive polymer element wrapped partially or completely around it, or can have two or more conductive polymer elements, preferably laminar, attached to appropriate parts, e.g. opposite faces, thereof.
The conductive polymer element(s) which is (are) used in combination with a central member can be combined therewith in situ, but are preferably secured thereto before being placed in the recess. The conductive polymer element(s) can be firmly secured to the central member, e.g. by fusion, before the gasket is placed in the recess. In any event, the conductive polymer element(s) must, by the time the method is ,;:

1339~58 completed, be firmly secured to, preferably fused to, the central member, so as to provide a secure bond between the first and second articles, through the gasket.
The central member can if desired be heat-recover-able, so that generation of heat in the conductive polymer element(s) will cause it to recover towards a desired new con-figuration, e.g. one which will lock the gasket in place or help to fill the recess between the articles. It is also pos-sible for part or all of the central member to be deformable, e.g. compressible, so that it can conform to the shape of the recess, and such deformation can be elastic or plastic. Thus part or all of the central member can be composed of a poly-meric foam, preferably a closed cell foam. This is particu-larly useful when sealing around a cable, e.g. a telephone cable.
The shape of the central member (and, therefore, usually the shape of the gasket as a whole) will be dictated or influenced by the shape and dimension of the substrate and the tubular member. The angle of the wedge section can be for example from 2~ to 45~, preferably 4~ to 15~, particularly 5~
to 10~, e.g. about 6~, and it can be formed by substantially straight sections which are for example 1 to 6 inches, pre-ferably 1.25 to 4 inches long; for example the central member can have a cross-section consisting of a rectangular base section 0.2 to 2 inch in length and 0.1 to 0.75 inch high, and an integral wedge section formed by straight lines 1.25 to 3 inches long which meet at a blunt tip with an included angle of 4~ to 8~.

1339~58 Especially when the central member comprises a wedge-shaped cross-section, or is otherwise used to create pressure which assists in the joining process, it is important that the central member should retain sufficient strength during the process. The central member is, therefore, preferably composed of a material which is as viscous as or more viscous than the other materials at the heated interface.

12. Restraininq Means for Holdinq Articles in Place Restraining means, e.g. clamps, may be used to pro-duce and maintain the desired intimate contact. Suitablerestraining means can be of any kind which will continue to exert the desired forces during the method. For example, when forcing the coupler into intimate contact with the substrate, e.g. a pipe, elastomeric organic material of any kind can be wrapped around the coupler. Hose clamps and the like can also be used. Techniques and materials of the kinds employed for strapping loads onto pallets, e.g. steel bands which are ten-sioned by ratcheted levers, are also useful when larger forces are required. We have also obtained excellent results using a heat-shrinkable polymeric tape which is wrapped tightly around the two articles. As a result of the heat generated in the gasket, aided if necessary by external heating, the tape shrinks during the process and thus ensures that the desired intimate contact is maintained. The shrinkage required is small, e.g. less than 10~. Many polymeric tapes, e.g.
polyimide tapes, will shrink to this extent when heated.
The coupler can also be placed inside a hollow sub-strate, e.g. a pipe, and driven into intimate contact with the inner surface thereof by an expansion means, which can for example be expanded by mechanically generated forces or by a bladder which is expanded by hydraulic or other forces.
If desired, the coupler and/or the substrate can comprise one or more grooves, ridges, hooks or other inden-tations or projections which will cooperate with a restraining means to bring the articles into intimate contact. It is also possible for the restraining means itself to be secured to the coupler or substrate or for each of the coupler and substrate to comprise a part of a restraining means so that when they are assembled, the parts cooperate to provide a restraining means.

13. Non-Wedginq Techniques We have realized that intimate contact can be ach-ieved by making use of a substrate (e.g. the pipe or pipes) which is relatively strong and rigid and a coupler which is relatively deformable, and by forcing the coupler against the substrate with one or more gaskets sandwiched between them.
An additional advantage of this technique is that the gasket or gaskets can be secured to the coupler before the coupler is placed around the substrates, thus ensuring that the gasket(s) is (are) correctly placed. The gasket or gaskets function both to bond the coupler to the substrate and to bond over-lapping parts of the coupler to each other to ensure a pro-perly sealed joint. The techniques just described can be used not only when the "coupler" joins two pipes together, but also when it repairs or otherwise modifies a single pipe or other substrate.

':
. i. ..

14. Flexible Tapes as Wrap-around Couplers In one embodiment, the coupler is a flexible tape with a flexible laminar gasket secured to one face thereof;
the coupler is wrapped tightly around the substrate; the wrapped end is secured in place; and the heat generated within the gasket causes the tape to become bonded to the first ar-ticle in the non-overlapped area and to itself in the over-lapped area. The term "tape" is used herein in a broad sense to denote any elongate article having a substantially flat cross-section, and thus includes for example articles whose unstressed configuration is flat and articles whose unstressed configuration is curved, including substantially tubular. The terms "flexible" and "wrapped" are used herein in a broad sense to denote any article which can be passed transversely over a pipe or other substrate and can then be brought into intimate contact with the substrate by the application of mechanical forces.
Thus in this embodiment of the invention, the tape can be a relatively thin flat polymeric tape such that the coupler can easily be wrapped by hand in multiple wraps around the substrate. Alternatively, the tape can be relatively thick and curved article obtained by making one or more long-itudinal cuts in a polymeric tube having an inner diameter larger than, preferably only slightly larger than, the outer diameter of the first article. A l~mi n~r gasket is then secured to the inner surface of the tape. The combined article is passed transversely over the substrate and then forced into intimate contact with it. Very large crimping X

~339458 forces can be used, often limited only by the need not to de-form the substrate. Preferably the coupler has the necessary flexibility at room temperature, but the invention includes the possibility of heating the coupler, e.g. by passing cur-rent through the conductive polymer, to increase its flexibil-ity, before it is wrapped around the substrate.
Unless the tape is thin enough to make this unneces-sary, the covered edge of the flexible tape is preferably chamfered so as to provide a smooth transition and ensure that there is not a void between the gasket and the tube-forming component in this area. When the tape is a relatively thick and inflexible one, so that it must be crimped into place, such a chamfered edge also helps the outer wrap to slide over the transition and thus achieve the desired intimate contact.
When a relatively thin and flexible polymeric tape is used, depending upon the thickness of the tape and the properties needed in the final article, the extent of the wrapping may merely be sufficient to seal the wrapped edges, or there may be multiple wraps to provide improved physical properties. Alternatively or additionally, the physical strength of the coupling can be increased by means of an outer coupler (which can optionally also serve as a restraining means); since the coupler of the invention provides a seal around the pipe(s), the outer coupler need not do so.
When a relatively thick and inflexible polymeric tape is used, e.g. one cut from a tube, the overlap area will often be confined to the chamfered lower edge of the coupler.

In that case, the overlapping edge is preferably also cham-fered so that it will mate with the lower edge but is capable of sliding over it.

15. Use of Couplers Having Sliding Edges The one-piece wrap-around coupler having chamfered edges, as just described, is one example of a preferred class of couplers, namely those comprising mating edges which con-tact each other (usually through a gasket) when the coupler is in place, and which can slide relative to each other in a cir-cumferential direction when appropriate mechanical forces areused to change, usually to reduce, the diameter of the coup-ler. Preferably the mating edges are produced, or theoreti-cally could have been produced, by cutting a tube from end to end thereof along a cutting surface which, when projected, does not pass through the center of the tube, preferably along a plane which is parallel to the axis of the tube and which forms an angle of 0~ to less than 90~, e.g. 0 to 45~, prefer-ably 0~ to 20~, especially 0~ to 10~, with a plane which is tangential to the inner surface of the tube at the point where the cutting surface cuts the inner surface of the tube.
However, other more complex shapes can also be used, for example stepped configurations.
15A. Non-Wrap-Around Couplers With Slidinq Edqes The one-piece wrap-around coupler having chamfered edges and obtained by cutting a polymeric tube longitudinally, as described at the end of section 14, can of course also be used, when there is access to the end of the pipe by passing it longitudinally over the pipe. Furthermore, in such a ,.. .. .

13~458 situation there can also be used a coupler which is of similar design but has insufficient flexibility to be used as a wrap-around article, though retaining sufficient flexibility to be forced into intimate contact with the substrate. Thus a non-wrap-around coupler can be made of more rigid material and/or have a greater wall thickness.
15B. Articulated Wrap-Around Cou~lers With Slidinq Edqes When it is desired to use, as a wrap-around coupler, a one-piece coupler which has insufficient flexibility to be passed transversely over the substrate, one way of achieving the desired result is to form at least one longitudinal line of weakness in the coupler, thus increasing its flexibility.
In such a coupler, the line of weakness in effect forms a joint, and such a modified coupler is therefore referred to herein as an articulated coupler. Preferably, measures are taken to ensure that the line of weakness does not remain in the finished coupler. In a preferred embodiment the line of weakness is produced by making a longitudinal cut partially through the wall of the coupler, e.g. through 40 to 70~ of the wall thickness, the cut preferably being made from the inside, and preferably at a substantial angle to the radial direction, e.g. at an angle of 30~ to 75~, preferably 45~ to 65~, to the radial direction. The gasket can be placed within this cut, as well as on the inside surface and one of the sliding edges of the coupler, and this will ensure that the line of weakness does not remain in the finished article. An articulated coupler of this kind is shown in Figure 7.

1339~58 15C. Multi-Part Wrap-Around Couplers With Slidinq Edqes Closely related to the articulated couplers described above are multi-part couplers comprising components which are completely separate, rather than flexibly joined to each other. Such multi-part couplers typically comprise two separate components, but there may be more, e.g. three or four, particularly when the coupler is a complex one for join-ing disparate substrates, e.g. pipes of different size and/or different materials. In such multi-part couplers there are at least two longitudinal junctions formed by abutting edges of the components. At at least one, and preferably at each, of the junctions, the abutting edges are shaped so that they can slide relative to each other, and thus accommodate a change in the diameter of the coupler. A two-part coupler of this kind is illustrated in Figure 8.

16. Couplers Without Sliding Edqes The various couplers described in sections 13-15 above are also useful in the present invention when they do not have sliding edges or other means which enable them to change in diameter when pressed into intimate contact with the substrate. However, in that case, they must be precisely sized to the substrate, and/or additional measures must be taken to ensure a satisfactory longitudinal joint along the abutting edges.

17. Use of the Invention for Purposes Other than Joining Pipes The invention is chiefly described herein by refer-ence to its use for joining pipes. However, it is to be ~.~

133g4S8 understood that the invention is also useful for joining any two or more articles together, especially articles whose ad-jacent surfaces are composed of the same polymeric composition or of different but compatible polymeric compositions and which are joined together by fusion to opposite faces of a laminar conductive polymer element. Where the invention is described herein for joining pipes, it is to be understood that the description is also applicable, to the extent appro-priate, to joining other substrates.
One important use of the present invention is in repairing damaged pipes. For this purpose, the wrap-around "couplers" described above can be used.
Another important use of the present invention is to provide a port (e.g. for a branch pipe, a pressure access de-vice or a tap) on a pipe, cable or other supply line contain-ing a fluid or gas. In this use, the coupler is similar to one of the wrap-around couplers described above, but in add-ition comprises a fitting which provides a port of desired dimensions. Before or after the coupler has been secured to the supply line (usually after), a hole is drilled through the supply line so that the port communicates with the interior of the supply line.

18. Overheatinq and Insulating Carriers We have found that when two heating elements are ad-jacent to each other in a section having a relatively small amount of the coupler and/or substrate, overheating can occur and/or there can be current flow between the adjacent heating elements (as for example in the tip of the edge portions 212 i3~g4~8 in Figure 8). This problem can be alleviated by placing an insulating barrier over the edge of one or both of the heating elements in the relevant section.

19. ReentrY
In many cases, especially when the surfaces have been joined by fusion, repowering the heating element(s) makes it possible to separate the first and second articles from each other.
The invention is illustrated in the accompanying drawing. Before discussing the various Figures, it should be noted that in each of them, the thicknesses of the conductive polymer element, the electrodes, the conductive bridges, and the various restraining means are exaggerated in the interests of clarity. In each of the Figures, the conductive polymer is a sintered mixture of ultrahigh molecular weight polyethylene and carbon black.
Figures 1 and 2 show two polyethylene pipes 11 and 12 which are being joined together by means of a flexible wrap-around coupler which comprises a sheet 21 of polyethylene and a laminar heater comprising a l~m' n~r conductive polymer element 31.
The inner edges of the sheet 31 and element 21 are secured together and are chamfered at 212 to provide a smooth transition area in the wrapped assembly. Sheet 31 and element 21 are secured to each other only in the region of the inside edge, so that they can move relative to each other as the coupler is wrapped around the pipes. As best shown in the plan view of the "unrolled" coupler in Figure 3, there are X

electrodes 33 and 34 secured to the surface of the heating element 31 along the edges thereof. There is also a conductive bridge 35 secured to one face of the heating element so that heat is not generated in the area of the heating element which covers the gap between the pipes.
After the flexible coupler has been wrapped around the pipes, using a sufficient number of wraps to obtain a final joint of desired strength, it is then secured in place by overlapping wraps of a heat-shrinkable polyimide tape 41.
The heater is then powered, thus fusing the coupler to the pipes and to itself and shrinkage of the tape 41.
Figures 4 and 5 show two polyethylene pipes 11 and 12 which are being joined together by means of a coupler which comprises a polyethylene member 21 and a laminar heater com-prising a conductive polymer element 31. The member 21 is obtained by making a longitudinal cut in a length of a poly-ethylene pipe, the cut being angled so that the resulting cut edges 212 and 213 are chamfered so that they can slide over each other to increase or reduce the diameter of the coupler.
The member 21 is flexible enough to permit such sliding motion, but not flexible enough to be opened up for use as a wrap-around coupler. The l~m; n~r heater is fused to the inside of the member 21 and one of the cut edges thereof. As best shown in the plan view of the "unrolled" coupler in Figure 6, there are electrodes 33 and 34 secured to the sur-face of the heating element 31, and a center section has been removed from the heating element in the area of the heating element which covers the gap between the pipes; a conductive ,~

1~39~58 bridge 35 provides for current flow across the center section between the electrodes. The electrodes 33 and 34 and the center section 35 are shaped so that the current density is substantially the same throughout the heating element.
The ends of the pipes 11 and 12 are placed within the coupler, which is then reduced in diameter by means of metal bands 41 which are crimped around the coupler. The heater is then powered, thus fusing the coupler to the pipes and to itself.
If two pipes are coupled together in the way just described, but without removing the center section of the heater, then the expansion of the heated parts of the heater, closely confined as it is by the inflexible metal bands and the pipes, has a tendency to force the pipes apart.
Figure 7 shows an articulated coupler which com-prises a polyethylene member 21 and two laminar heaters comprising conductive polymer elements 311 and 312. The member 21 is obtained by making two longitudinal cuts in a length of a polyethylene pipe. One cut passes all the way through the pipe wall and produces edges 212 and 213 which are chamfered so that the cut edges can slide over each other to reduce the diameter of the coupler. The other cut, diametric-ally opposite the first, passes part way through the pipe wall, leaving a hinge section 214 which permits the member to be opened up so that it can be passed transversely over a pipe. The laminar heaters are secured to the inner faces and the cut edges of the member 21.

1339~5~
Figure 8 is very similar to Figure 7, except that both longitudinal cuts pass all the way through the pipe wall, so that the coupler is in two separate parts.
The invention is further illustrated by the follow-ing Examples. The terms "Hostalen GUR-413" and "Ketjenblack ED 300 DJ", which are used to describe materials used in Examples, are Trademarks.
Example 1 A conductive polymer composition was prepared by dry blending in a high speed blender 95 parts by volume of ultra high molecular weight polyethylene powder, UHMWPE (Hostalen GUR-413, available from American Hoechst), having a molecular weight of about 4.0 million and an average particle size of about 0.1 mm, and 5 parts by volume of carbon black (Ketjenblack EC 300 DJ, available from Akzo Chemie). The mixture was extruded through a ram extruder heated to 170~C at a rate of 5 feet/minute and a pressure of 3000 psi to produce a sintered rod 8 inches (20.3 cm) in diameter. The rod was skived to produce a flexible tape 0.030 inch (0.076 cm) thick and 4.0 inch (10.2 cm) wide, and an element 36.5 by 6 by 0.3 inches (92.7 by 15.2 by 0.076 cm) was cut from the tape.
Electrodes were attached 0.5 inch (1.3 cm) from each long edge and a broad strip of silver paint (1 by 34.5 inch/2.5 by 87.6 cm) was painted down the center of the element, starting at one end. This produced two 2-inch wide heating zones 34.5 by 2 inch, separated by a 1 inch "cold zone" and a terminal 5 by 2 inch (12.7 by 5.1 cm) heating zone at one end of the heating X

~333 i58 element. To improve the uniformity of heating, silver paint was painted in a pattern which decreased the width of the terminal heating zone to 4 inches (10.2 cm).
A 6.75 inch-long (17.1 cm) coupler as shown in Figure 5 was prepared. A polyethylene pipe which had a 10 inch (25.4 cm) inner diameter (ID) and a 12 inch (30.5 cm) outer diameter (OD) was cut through its thickness on a tan-gential line at the ID of the pipe which was perpendicular to a radial line through the pipe. The laminar heating element was secured to the coupler as follows. The heating element was placed around the inner diameter of the coupler with the terminal heating zone in the region of the cut through the coupler and with the ends of the electrodes protruding from the coupler; a pipe coated with PTFE was inserted into the coupler and the heating element was powered at 150 volts AC
for 30 seconds; after cooling, the PTFE-coated pipe was removed.
Two 10-inch OD polyethylene pipes (0.75 inch/l.9 cm thick) were inserted into the coupler and clamps were attached on the outside of the coupler. The heating element was powered at 125 volts AC for 115 seconds to fuse the heating element to the pipes.
Example 2 A heating element was prepared as in Example 1 ex-cept that it was cut into two pieces, one 20.5 inches (52.1 cm) long and the other 18.9 inches (48.0 cm) long. Each piece had an appropriate heating zone at one end.

~.' l~g458 The coupler as shown in Figure 8 and 6.75 inch long was prepared by cutting a 12-inch OD pipe into two pieces using a cut as described in Example 1 and its mirror image.
The heating element pieces were perfused to the inner diameter of each segment prior to fusing the heating element to two 10-inch OD polyethylene pipes.
Example 3 A coupler as shown in Figure 7 and 6.75 inch long was prepared as described in Example 2 except that a second tangential cut, of an angle corresponding to that of the first cut, was made into the coupler's inner diameter, but not all the way through the thickness of the coupler. As a result, the coupler could be opened to produce a segmented coupler with two shell-forming surfaces. Appropriately sized heating elements were fused to the inner surface of the coupler prior to making connection to two 10-inch OD polyethylene pipes.
Example 4 A laminar heating element as described in Example 1 was prepared without the silver paint strip down the center.
The element was fused to a 0.030 inch (0.076 cm) strip of high density polyethylene by powering the heating element while in contact with the polyethylene strip to produce a gasket. The gasket was used to connect two 10-inch OD polyethylene pipes by wrapping the gasket around the outer diameter of the pipes, applying clamping means around the gasket, and powering the heating element.

b

Claims (28)

1. A tubular coupler which can be placed around or within a substrate and joined thereto, and which comprises (1) a tubular member comprising one or more tube-forming components which are composed of an electrically insulating polymeric composition, and (2) at least one gasket which (a) is adjacent to a surface of the tubular member, and (b) comprises (i) a laminar heating element composed of a conductive polymer, and (ii) two electrodes which can be connected to a source of electrical power to cause current to pass through the heating element substantially parallel to the surface thereof and to generate heat within the conductive polymer;

the tubular member and the gasket being such that the coupler can be wrapped around and brought into intimate contact with the substrate, with the gasket between the tubular member and the substrate.

2. A coupler according to claim 1, wherein the tubular member is not heat-recoverable.

3. A coupler according to claim 2 wherein the tube-forming components comprise longitudinally extending chamfered edges which can slide relative to each other to reduce the diameter of the coupler.

4. A coupler according to claim 3 wherein a part of the gasket lies between the chamfered edges when the coupler is in intimate contact with the substrate.

5. A coupler according to claim 3 or 4, wherein each of the tube-forming components has been produced by cutting a tube from end to end thereof along a cutting surface which, when projected, does not pass through the center of the tube.

6. A coupler according to claim 1, 2 or 3, wherein the tubular member comprises first and second members each of which has chamfered edges and each of which has a gasket adjacent to its inner surface.

7. A coupler according to claim 1 or 2, wherein the tubular member comprises a single tube-forming component in the form of a flexible tape.

8. A coupler according to claim 1, 2 or 3 which comprises a conductive bridge which passes over or through a bridged conductive polymer section which is (i) integral with the heating element and (ii), in the absence of the bridge, would be heated when the electrodes are connected to the power source.

9. A coupler according to claim 1, 2 or 3 which comprises a conductive bridge which passes over or through a bridged conductive polymer section which is (i) integral with the heating element and (ii), in the absence of the bridge, would be heated when the electrodes are connected to the power source, said conductive bridge also including a window section which does not pass over or through a conductive polymer section.

10. A coupler according to claim 3 wherein the heating element is in the form of a tape having a ratio of external surface area to volume of at least 40 inch2/inch3.

11. A coupler according to claim 10 wherein said ratio is at least 130 inch2/inch3.

12. A coupler according to claim 11 wherein said ratio is at least 200 inch2/inch3.

13. A coupler according to claim 1, 2 or 3 wherein the conductive polymer comprises (a) a polymeric component which consists of particles of an organic polymer which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate conductive filler which is present only at or near the boundaries of the coalesced particles.

14. A coupler according to claim 1, 2 or 3 wherein the conductive polymer comprises (a) a polymeric component which consists of particles of ultra high molecular weight polyethylene which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate conductive filler which is present only at or near the boundaries of the coalesced particles.

15. A method of joining a tubular coupler to a tubular substrate, which method comprises (A) providing tubular coupler which comprises (1) a tubular member comprising one or more tube-forming components which are composed of an electrically insulating polymeric composition, and (2) at least one gasket which (a) is adjacent to a surface of the tubular member, and (b) comprises (i) a laminar heating element composed of a conductive polymer, and (ii) two electrodes which can be connected to a source of electrical power to cause current to pass through the heating element substantially parallel to the surface thereof and to generate heat within the conductive polymer;

(B) wrapping the tubular coupler around the substrate and bringing it into intimate contact with the substrate, with the gasket between the tubular member and the substrate; and (C) applying circumferential forces to the coupler and connecting the electrodes to a source of electrical power, thus generating heat which causes the coupler to become joined to the substrate.

16. A method according to claim 15 wherein the substrate is a hollow conduit composed of an electrically insulating composition.

17. A method according to claim 16 wherein the coupler is placed around the ends of two conduits which are coupled together in line by the method.

18. A method according to claim 16 wherein the coupler is placed around a conduit so that it covers a damaged portion of the conduit.

19. A method according to claim 16 wherein the coupler is placed around the conduit at an intermediate position between the ends of the conduit and comprises a fitting for providing a port into the conduit, and the method includes creating the port in the conduit after the coupler has been joined thereto.

20. A method according to claim 19 wherein the conduit is a telephone cable.

21. A method according to claim 17, 18 or 19 wherein the conduit is composed of an electrically insulating thermoplastic polymeric composition, and the heat generated within the conductive polymer causes the thermoplastic polymer to melt, and thus to join the conduit to the coupler.

22. A coupler according to claim 15, 16 or 17, wherein the tubular member is not heat-recoverable.

23. A coupler according to claim 15, 16 or 17 wherein each of the tube-forming components comprise longitudinally extending chamfered edges which can slide relative to each other to reduce the diameter of the coupler, and step (B) includes sliding the chamfered edges relative to each other to bring the coupler into intimate contact with the substrate.

24. A method according to claim 15, 16 or 17 wherein the heating element is in the form of a tape having a ratio of external surface area to volume of at least 40 inch2/inch3.

25. A method according to claim 15, 16 or 17 wherein the conductive polymer comprises (a) a polymeric component which consists of particles of an organic polymer which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate conductive filler which is present only at or near the boundaries of the coalesced particles.

26. A method according to claim 15, 16 or 17 wherein the conductive polymer comprises (a) a polymeric component which consists of particles of ultra high molecular weight polyethylene which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate conductive filler which is present only at or near the boundaries of the coalesced particles.

27. A method according to claim 15, 16 or 17 wherein the coupler comprises a conductive bridge which passes over or through a bridged conductive polymer section which is (i) integral with the heating element and (ii), in the absence of the bridge, would be heated when the electrodes are connected to the power source.

28. A method according to claim 15, 16 or 17 wherein the coupler comprises a conductive bridge which passes over or through a bridged conductive polymer section which is (i) integral with the heating element and (ii), in the absence of the bridge, would be heated when the electrodes are connected to the power source, said conductive bridge also including a window section which does not pass over or through a conductive polymer section.