METHODS AND DEVICES WHICH MAKE USE OF CONDUCTIVE POLYMERS TO JOIN ARTICLES

Abstract: Two articles (11, 12) are joined together with the aid of a gasket (3) comprising a laminar conductive polymer heating element (31). The gasket is placed between the articles and is heated by passing current through the heating element. Preferably, the articles are composed of a polymeric material and become fused to the heating element, or directly to each other through holes in the gasket. In a preferred embodiment, the gasket (3) has a wedge-shaped cross-section.

Description: METHODS AND DEVICES WHICH MAKE USE OF CONDUCTIVE POLYMERS TO JOIN ARTICLES BACKGROUND OF THE INVENTION Field of the Invention This invention relates to methods of joining articles together with the aid of conductive polymer components, and to novel articles for use in such methods. Background 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 laminar heaters which comprise a laminar conductive polymer heating element which is sandwiched between laminar electrodes so that current flows through the conductive polymer at right angles to the plane of the heater. Such laminar heaters can be in the form of heat-recoverable articles or can be secured to heat-recoverable articles so that, by powering the heater, the article can be caused to recover. Typically, the recovery of the article results in joining, repairing, reinforcing or otherwise modifying one or more substrates around or against which the article recovers. Recently, it has been shown that conductive polymers which retain substantial strength above their melting point, especially sintered conductive polymers based on ultra high molecular weight polyethylene (UHMWPE), are particularly useful for heat- recoverable couplers which can be used to join pipes composed of organic polymers (plastic pipes). In such couplers, the conductive polymer com¬ ponent provides the structural strength of the finished coupling and is, therefore, of a thickness comparable t'o that of the pipes; substantial electr i cal power is needed to heat such couplers to their recovery tem¬ perature. 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, German Patent Application No. 3,442,674, and European application Nos. 157,640, 153,199, 231,068, 197,759, 220,003, 267,234 and 251,482. SUMMARY OF THE INVENTION We have now realized that an excellent method of joining two articles together is to maintain the two articles adjacent to each other while generating heat within a laminar conductive polymer heating element which lies between the two articles, the heat being sufficient to effect some change which results in the articles being joined (directly or indirectly) to each other. The heating element need not be heat-recove able; indeed in most cases, it is preferably not heat-recoverable. The heat is preferably generated within the heating element by passing electrical current through it, with a substantial proportion (preferably all) of the current flowing parallel to the surfaces of the heating element. One important reason for this preference is that if electrodes are placed on the heating element so that current flows at right angles to the surfaces thereof, the electrodes are then present in the bonding area and lessen the strength of the joint between the articles. This method provides very important advantages over the prior art. In particular, heat is generated at the interface, where it is needed, and not elsewhere, thus reducing power requirements and the danger of overheating. This and other advantages are explained below. The novel method is particularly effective for joining (or otherwise modifying) articles made of an electrically insulating polymeric material, especially plastic pipes. When the articles are made of a thermoplastic material, it is preferred that the heating melts the thermoplastic material and the conductive polymer sufficiently to effect fusion between the articles and the heating element; alternatively or additionally, the heating element has apertures therethrough so that the articles are fused directly to each other through the apertures. When one or both of the articles is made of a material which will not fuse in this way, then some form of heat-activatable adhesive must be used, e.g. a thermosetting resin. When the surfaces of the articles to be joined have the same shape (including articles which have been deformed so that the surfaces have the same shape), the conductive polymer heating element (and its associated electrodes) may be all that lies between the articles. In other cases, there may be one or more heating elements which are associated with other components to form a more complex gasket which is placed between the articles. In one aspect the present invention provides a method of joining a first article and a second article, which method comprises (1) placing a first surface of the first article adjacent to a second surface of the second article so that the surfaces define a recess between them; (2) placing in the recess at least part of a gasket which (a) comprises a laminar heating element which is composed of a conductive polymer (b) has a third surface which is placed adjacent to the first surface, and (c) has a fourth surface which is placed adjacent to the second surface; (3) moving at least one of the gasket and the first and second articles relative to each other so that intimate contact is achieved between the first and third surfaces and between the second and fourth surfaces; and (4) generating heat within the conductive polymer by passing elec¬ trical current therethrough substantially parallel to the surfaces of the heating element, while maintaining intimate contact of the surfaces, thus joining the first and second articles together. In this method, a variety of procedures are available for carrying out steps (1) and (2). For example, the first and second articles can be placed adjacent to each other, and the gasket then placed in the recess between them; or the gasket can be placed adjacent to the first or the second article, and the other article then placed adjacent to the gasket; or the gasket can be joined to the first or the second article and the other article then placed adjacent to the gasket. In the latter case, the gasket can be joined to the first or second article in situ, or the gasket and the article can be preasse bled, e.g. at a manufacturing site. If the gasket and the article are preassembled, the join between them can be one which remains unchanged during steps (3) and (4) of the method or one which is modified during one or both of steps (3) and (4). In describing those aspects of the invention in which a gasket is joined to one of the articles before the other article is placed adjacent to the gasket, the article to which the gasket is joined is generally referred to as the second article. It should be understood, however, that this is merely for clarity, and that such description is also applicable, where appropriate, to systems and methods in which the gasket is prejoined to the first article. 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, leaving a recess between the coupler and each pipe end. In this embodiment, each of the pipes can be regarded as a "first article" in the method defined above and the coupler can be regarded as a "second article". Two gaskets are, there¬ fore, used, one at each end of the coupler. 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 first or the second article or to the gasket. If there are voids present at the various interfaces, they can result in overheating which is wasteful and can be damaging, particularly when the rate at which power is generated is high. The term "joining" is used herein to include an improvement in an existing join, for example by fusing together surfaces which are already joined together in some other way or by completing fusion between surfaces which are already fused together at spaced-apart locations. The join between the articles 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 first and second articles can be achieved in a variety of different ways. Preferably, the third and fourth surfaces of the gasket are provided by a conductive polymer and, as a result of the heating, become fused to the first and second surfaces respectively. Alternatively or additionally, the gasket can have apertures therein and, as a result of the heating, the first and second surfaces become fused directly to each other through the apertures. The gasket can consist essentially of a conductive polymer element (usually a laminar element), together with any electrodes or 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 to provide the third and fourth surfaces. 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 conditions of the process, molecular diffusion takes place across the interface between the substrates and/or there is viscoelastic contact between the substrates as defined by J. N. Anand in Adhesion j. (1969), 16-23 and Adhesion 2 (1970) 16-22. The fusion process preferably results in a bond which has a 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 first and second articles (A) are composed of polymeric compositions 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 polymer element comprising a polymeric component in which at least 80%, preferably at least 90%, especially substantially 100%, of the repeating units are the same as in the first and second articles, or (ii) by fusion to one or more laminar conductive polymer elements, each of which comprises a polymeric component 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 together with a coupler which is composed of the same or a different melt-extruded low, medium or high density polyethylene, by means of a gasket which (a) comprises a conductive polymer based on a sintered ultra high molecular weight 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 first and second articles is substantially thicker than at least a part of the gasket. For example, the thickness of each of the articles 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 one (or both) of the surfaces to be joined is composed of a non-polymeric material, e.g. is composed of concrete or a metal, or when there is insufficient compatibility between the polymeric 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 together. Thus the insert can for example be composed of a partially cured thermo- setting 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 article before 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 first and second 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 first and third surfaces and between the second and fourth surfaces. In developing the method summarized above, we have also developed a variety of novel articles which are useful in the method and which form part of the present invention. One example of such a novel article is a coupler which comprises (1) a tubular component which is composed of an electrically insulating polymeric material and which can be reduced in diameter, and (2) a laminar gasket which comprises the laminar heating elment and which is secured to the inner surface of the tubular component. Another example of such a novel article is a laminar heater which comprises (1) a laminar heating element which (a) is composed of a conductive polymer which (i) comprises polyethylene and a particulate conductive filler distributed therein, and (if) increases in resistivity by a factor of less than 5 over the temperature range 23°C to (T m + 50)°C, where T is the softening point of the composition; (b) has a ratio of external surface area to volume of at least 40 fnch^/inch^; and (c) increases in volume by at least 10% when it is heated from 23°C to the melting point of the conductive polymer; and (2) strip electrodes which are at opposite edge portions of the laminar heating element. Other novel articles and methods forming part of the present invention are disclosed below. In the novel couplers defined above, the term "tubular component" is used to include any component which, in the method, is in a tubular configuration. Thus it can be a flexible tape which is wrapped around the first article; or an articulated article comprising at least two component members, each of which is joined to at least one other component member; or a plurality of composite components which can be assembled together in situ. Part of the laminar heater, or an additional laminar heater, can be used to bond adjacent, e.g. overlapping, parts of the tubular component to each other, preferably simultaneously with the joining of the articles to each other. 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 cylindrical coupler, using a wedge-shaped gasket at each end of the coupler; Figure 2 is a more detailed cross-sectional view of the gasket used in Figure 1; Figure 3 is a cross-section through an assembly in which the edges of a flexible polymeric sheet are being joined together through a slotted article; Figure 4 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 5 is a cross-section on line 5-5 of Figure 4; Figure 6 is an "unrolled" plan view of the gasket used in Figures 4 and 5; Figure 7 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 8 is a cross-section on line 8-8 of Figure 7; Figure 9 is an "unrolled" plan view of the gasket used in Figures 7 and 8; Figure 10 is a cross-section through an articulated coupler comprising two gaskets secured to the inner face thereof; Figure 11 is a cross-section through a two-piece coupler comprising two gaskets secured to respective inner faces of the two parts of the coupler; Figure 12 is a partial cross-section through a pipe which is being repaired with a repair patch; Figure 13 is a plan view of Figure 12; Figure 14 is a partial cross-section through a pipe to which a pressure access device is being secured; and Figure 15 is a plan view of Figure 14. 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 convenience, 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 disclosure of this specification includes all the appropriate combinations 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. A. GASKETS 1. Method of Generating Heat The gaskets are preferably heated by passing current through the resistive conductive polymer heating element. However, other methods can be used, e.g. induction heating or ultrasonic heating. 2. Location at Which Heat is Generated In the present invention, heat is generated substantially only where it is needed, i.e. at or close to the surfaces to be joined. This has distinct advantages over the prior art processes, in which heat is generated at some relatively remote point such that significant thermal barriers have to be overcome before the desired heating is achieved, or 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 first and second articles, and any components of the gasket in addition to the conductive polymer and electrodes are preferably such that no significant heat is generated within them. At least the first surface of the first article, and at least the second surface of the second article, are preferably non-conductive, e.g. both articles are composed of an insulating polymeric composition. If either the first or second surface is conductive, it is preferably separated from the conductive polymer by a layer of a non-conductive material. 3. Shape of the Conductive Polymer Element It is in general desirable that heat should be generated over a substantial area in order to provide a satisfactory 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 a 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 thin crosssection, 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 conductive polymer element can be in the form of a tape, particularly 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"*, especially 40 to 100 inch " *, e.g. 55 to 75 inch~l. 4. 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 laminar element is covered by the electrodes, and the heat is generated in a bonding section which has substantial width and length, and is substantially 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 preferably at least 1 inch, particularly at least 3 inch, especially at least 10 inch, e.g. 20 to 200 inch. Much larger dimensions 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 generated 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 pattern. 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 electrodes (e.g. metal strips, braid, or silver paint or a combination 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 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. In a particularly preferred embodiment, at least one of the electrodes comprises a core of conductive adhesive surrounded by a metal layer, e.g. a braid. Attachment of such an electrode to the surface of the tape is preferably achieved by applying a voltage to the electrode to heat it, positioning the electrode on the surface of the tape, and then applying pressure in order to laminate the electrode to the surface. Excellent adhesion is achieved when the conductive adhesive is allowed to melt and flow through the interstices of the braid and thus contact the surface of the tape. It is preferred that the electrode be heated to a temperature at least 30°C above, preferably at least 50°C above, the melting point or activation temperature of the conductive adhesive. 5. 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 between 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 ery 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 temperatures in this way is very useful in ensuring rapid melting and 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 conductive 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, particularly less than 0.1 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 insulating 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 expansion of the conductive polymer increases the bonding pressu r es. It is, therefore, preferred to use a conductive polymer 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 becomes 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 exhibit essentially ZTC behavior in the temperature range of operation, and in particular that it should increase in resistivity 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 (Trr, + 50)°C, where T m 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 particles 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 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 million. Other polymers that may be sintered include polytetrafluoro- ethylene, 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, sintering of the compacted blend at or above atmospheric pressure and at a temperature at which the polymer softens but does not 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 satisfactory, but if desired a corrugated, ribbed, or grooved tape can be obtained through the use of a skiving blade of an appropriate shape. Such an irregular shape may be desirable to concentrate pressures at desired points and/or to increase the surface 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 ery 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. The desired resistivity depends in part 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 product. 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 elongation, 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. 6. Methods of Assembly, and Shape of Gasket The gasket should preferably be such that when the gasket and the first and second articles are moved relative to each other, a good interference fit can be achieved between them, thus achieving the desired intimate contact. If the two articles are placed adjacent to each other and the gasket is placed in the recess between them, or if the gasket is prejoined to one of the articles but is not in intimate contact with that article, the relative movement produces intimate contact between the first and third surfaces and between the second and fourth surfaces. If the gasket has already been brought into intimate contact with one of the articles, e.g. by preasse bly of the second article and the gasket, the relative movement produces intimate contact between the surfaces which are not yet in intimate contact, e.g. the first and third surfaces. 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 thickness of at least 4, preferably at least 10, and often substantially 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 thickness, but can be of non-uniform width and/or non-uniform thickness. In many cases, ft will be desirable for the tape to be sufficiently flexible to be fitted into the recess, e.g. a recess between a pipe and a coupler, and to have a length which is for example at least 3 times, preferably at least 6 times, its width, and may be much more. 1. Conformance of Articles to Each Other During Assembly The relative movement of the first and second articles and the gasket can have the very desirable result of changing the shape of one or both of the articles so that the recess between them is of substantially uniform dimensions. For example, an out-of-round pipe can be placed inside a round coupler, and the gasket used to press the pipe into a round shape. Alternatively, the gasket can change its shape under pressure so as to fill a recess of irregular dimensions. 8. Rate at Which Power is Generated, and Temperature Distribution One of the remarkable features of the present invention is the high rate at which power can be generated within the conductive 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 boundaries of the conductive polymer heating element. This makes it possible to use power sources of much higher voltages than have previously been contemplated; however, ft must be remembered that the higher the voltage, the more carefully 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 (including both surfaces). One of the advantages of the present invention 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 - (where P is the 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 sees, 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/cm 3 , particularly at least 100 watts/cm 3 , e.g. up to 200 watts/cm 3 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 at the first and third surfaces and at the second and fourth surfaces 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 temperature 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 at each of the first and second surfaces is melted to a depth which is at least sufficient to permit molecular 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 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, 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. 9. 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 intimate contact with another solid body so that heat is rapidly removed through the whole of the gasket surface. For example, when using a wedge-shaped gasket to fill the recess between a pipe and a coupler, the tip of the gasket may not be in contact with either the pipe or the coupler, and if heat is generated in a conductive polymer element wrapped around the tip, overheating may result. Similarly, if a composite article comprising a second article and gasket is wrapped around the ends of two pipes to be joined, then 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 little or no heat is generated in the conductive polymer in the bridge area. The bridge itself is preferably of low resistance 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 produced by etching a continuous metal layer on the surface of a conductive 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 substantially hotter in the end area which does not contain the central 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 9. 10. Windows in Conductive Polymer Elements While the use of a conductive bridge often produces very valuable results, failure to heat one part of the conductive 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 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 partfcular example of this can arise when a heating element having a conductive bridge but no window therein is fused to the second article to make a pre-assembled composite 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 material, the expansion of the heating element can only be accommodated 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. 11. Non-heat-recoverable and Heat-recoverable Articles and Gaskets One of the advantages of the present invention, as compared to many methods previously proposed, fs that in general, excellent results can be obtained using first and second articles and gaskets which are not heat-recoverable. When we say herein that an article is "not heatrecoverable", 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 di ension changes by more than 10%), by such heating and cooling. It will be understood, however, that the invention does not exclude the possibility that the first or the second article or the gasket is heat-recoverable. Indeed, for example, if the recess between the articles is relatively wide or irregular in shape, it may be desirable for at least a part of the gasket to be heat-recoverable so as to ensure that it fills the recess and/or drives a sealant material in a desired direction. If the first or the second article 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. 12. 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 insulating 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 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-recoverable, so that generation of heat in the conductive polymer element(s) will cause it to recover towards a desired new configuration, e.g. one which will lock the gasket in place or help to fill the recess between the articles. It is also possible 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 polymeric foam, preferably a closed cell foam. This is particularly 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 dimensions of the articles to be joined and the recess between them. In one preferred embodiment, the articles are shaped so as to form between them a recess which has a wedge-shaped cross-section, and the central member comprises a corresponding wedge-shaped cross-section. 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, preferably 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°. One of the advantages of using a wedge-shaped gasket is that as the gasket is driven into the recess, it can change the shape of the first and/or the second article so as to conform their shapes. 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 step (4) of the process. The central member is, therefore, preferably composed of a material which, under the conditions of step (4), is as viscous as or more viscous than the other materials at the heated interface. 13. Use of Wedge-shaped Gaskets to Join or Repair Pipes We have obtained excellent results in methods for joining or repairing pipes through the use of an elongate flexible gasket comprising a central member comprising a wedge-shaped cross-section and an elongate laminar conductive polymer tape which is folded along its length and placed around the tip of the wedge. These methods will be described as methods for joining pipes, but is to be understood that the same technique can be used for joining other articles and for repairing (or otherwise modifying) a single pipe or other substrate. The method is illustrated in Figures 1 and 2 of the accompanying drawings. The pipes are joined together with the aid of a one-piece cylindrical coupler which has an inside diameter slightly larger than the outside diameter of the pipes, and which has conical entry sections on the inner surface at each end. The pipe ends are placed within the coupler, preferably butting against each other, thus forming a circular recess with a wedge-shaped cross-section at each end of the coupler. At each end of the coupler, the wedge-shaped gasket is wrapped around the pipe, cut to length, and then driven into the recess, thus forming a tight interference fit. Particularly when large pipes are being joined, it is not easy to cut the gasket to precisely the correct length, as is necessary if the pipes are not only to be joined physically to the coupler but also to be sealed thereto. We have found that the most reliable procedure is to cut the gasket somewhat short of the required length, to drive the gasket into place and then to fill the gap between the ends of the gasket with a short piece of the gasket, whose required length can now be measured very accurately. This technique can also be used in any situation in which a flexible gasket is to be inserted into a long recess. • One advantage of this method of joining pipes, which is most apparent when joining polymeric pipes of large diameter (e.g. of inner diameter greater than 6 inch and up to, for example, 120 inch, particularly 10 to 60 inch) is that the wedging action of the gasket can conform the shape of the pipe to that of the (more rigid) coupler. This is particularly useful for larger polymeric pipes because they tend to assume an out-of-round shape in storage. Another advantage of this method is that when a pipeline is to be made by joining together a plurality of pipes, with a pipe coupler at each joint, the process can be carried out in two distinct stages. In the first stage, steps (1), (2) and (3) of the process can be carried out at each joint, thus making a pipeline which is in place and will remain assembled if not disturbed, but which can still be disassembled if modifications are needed. In the second stage of the process, step (4) is carried out at each joint, thus making the pipeline permanent. The method described above can be modified, through the use of a one-piece coupler of more complex shape, and/or through the use of appropriate inserts on one or both sides of one or both of the gaskets, to join pipes at an angle and/or to join pipes which are composed of different materials and/or sizes. The method described above can also be modified by using a coupler composed of two or more parts such that the coupler can be placed around a pipe without requiring access to an end of the pipe. In this case, the seams between the different parts of the coupler must also be sealed, preferably with the aid of laminar gaskets as described. In addition, restraining means must be placed around the composite coupler after its assembly around the pipe, to contain the forces generated by driving the gasket into place. 14. Restraining Means for Holding Articles in Place The method just described, when making use of a one-piece coupler with wedge-shaped gaskets for joining pipes, is an example of a situation in which the desired intimate contact between the surfaces to be joined is achieved by driving a wedge between two substrates which, by reason of their own shape, are capable of at most limited movement relative to each other. When a two-piece coupler is used, and in other cases in which a wedge-shaped gasket is employed and movement of the gasket produces the desired intimate contact, restraining means, e.g. clamps, must be used to contain and utilize the wedging forces, and thus maintain the intimate contact. When wedging forces are not employed, as for example in the techniques described below, restraining means must be employed both to cause the relative movement which produces the desired intimate contact and to maintain that intimate contact. Suitable restraining means can be of any kind which will continue to exert the desired forces during step (4) of the method. For example, when forcing a second article into intimate contact with the outside of a first article, e.g. a pipe, elastomeric organic material of any kind can be wrapped around the first and second articles. 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 tensioned 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 step (4) 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 second article can also be placed inside a hollow first article, 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, one or both of the articles can comprise one or more grooves, ridges, hooks or other indentations 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 one of the articles, or for each of the articles to comprise a part of a restraining means so that when the articles are assembled, the parts cooperate to provide a restraining means. 15. Slotted Articles In another embodiment, one of the articles comprises a slot, and an edge portion of the other article is inserted into the slot, with a gasket between the side of the slot and one side of the inserted edge portion, preferably with a gasket between each slot side and each side of the inserted edge portion, for example a gasket wrapped around the inserted edge portion. One of the articles can be a strip comprising two slots, and the opposite edge portions of a flexible sheet, preferably a heat-shrinkable polymeric sheet, can be inserted into the slots, thus forming a wrap-around assembly for a substrate. This procedure is illustrated in Figure 3. The slotted strip may be essentially circular, oval or rectangular in cross-section and preferably has opposing slots separated by a center region. The edge portions may be positioned in the sealing strip either by sliding the strip over the edge portions or by inserting the edge portions transversely into the strip. Preferably some means is provided for holding the edge portions in the slots, e.g. by use of a wrap such as an elastomeric band positioned around the circumference of the sleeve. The slots may be smooth or of a shape designed to maintain the position of the edge portions during heating or assembly, e.g. a shape designed to give a snap-fit to the edge portions. 16. Non-Wedging Techniques Although it offers substantial advantages over the prior art methods, the use of a a wedge-shaped gasket is not entirely satisfactory. It is essential to insert the gasket correctly around the whole periphery of the pipe, since failure to do so will lead to a leaky joint. This requirement makes the method somewhat craft-sensitive, and is a particular disadvantage when the pipes must be joined under adverse conditions, e.g. at a construction site, especially if skilled workers are not available. In order to remove the need to force a wedge-shaped gasket into the recess, it is necessary for the gasket to be brought into intimate contact with the pipe(s) and the coupler in some other way. We have realized that this result can be achieved by making use of a first article (e.g. the pipe or pipes) which is relatively strong and rigid and a second article (e.g. the coupler) which is relatively deformable, and by forcing the second article against the first article 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 overlapping parts of the coupler to each other to ensure a properly sealed joint. As in the method described above for joining pipes with the aid of wedge-shaped gaskets, the techniques just described can also be used when the "coupler" does not join two pipes together, but rather repairs or otherwise modifies a single pipe or other substrate. Similar techniques can al-so be used to secure a second article to the interior of a hollow article, the diameter of the second article being increased by the application of mechanical forces. 17. Flexible Tapes as Wrap-around Couplers In one embodiment, the second article is a flexible tape, and a flexible laminar gasket is secured to one face thereof; the combined article is wrapped tightly around the first article; the wrapped end is secured in place during step (4); and the heat generated within the gasket causes the tape to become bonded to the first article in the non-overlapped area and to itself in the overlapped 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 combined article can easily be wrapped by hand in multiple wraps around the first article. Alternatively, the tape can be a relatively thick and curved article obtained by making one or more longitudinal cuts in a polymeric tube having an inner diameter larger than, preferably only slightly larger than, the outer diameter of the first article. A laminar gasket is then secured to the inner surface of the tape. The combined article is passed transversely over the first article and then forced into intimate contact with the tape. Ve large crimping forces can be used, often limited only by the need not to deform the first article. Preferably the combined article has the necessary flexibility at room temperature, but the invention includes the possibility of heating the combined article, e.g. by passing current through the conductive polymer, to increase its flexibility, before it is wrapped around the substrate. Unless the tape is thin enough to make this unnecessary, 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 second article 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 in step (4)); 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 chamfered so that it will mate with the lower edge but is capable of sliding over it. 18. 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 contact each other (usually through a gasket) when the coupler is in place, and which can slide relative to each other in a circumferential direction when appropriate mechanical forces are used to change, usually to reduce, the diameter of the coupler. Preferably the mating edges are produced, or theoretically 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°, preferably 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. 18A. Non-Wrap-Around Couplers With Sliding Edges The one-piece wrap-around coupler having chamfered edges and obtained by cutting a polymeric tube longitudinally, as described at the end of section 17, 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 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. 18B. Articulated Wrap-Around Couplers With Sliding Edges 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 9. 18C. Multi-Part Wrap-Around Couplers With Sliding Edges 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 joining 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 10. 19. Couplers Without Sliding Edges The various couplers described in sections 16-18 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. 20. Use of the Invention for Purposes Other than Joining Pipes The invention is chiefly described herein by reference to its use for joining pipes. However, it is to be understood that the invention is also useful for joining any two or more articles together, especially articles whose adjacent 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 appropriate, 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. It is also possible to use a simple repair patch which is for example round or rectangular and which is flexible enough to conform to the shape of the pipe and/or is saddle-shaped to fit onto the pipe. Depending upon the extent of the damage, it may be desirable for a section of the conductive polymer element to be removed or provided with a conductive bridge, in order to avoid overheating thereof in the damaged area, as discussed in sections 9 and 10 above. The use of a repair patch is illustrated in Figures 11 and 12. Another important use of the present invention is to provide a port (e.g. for a branch pipe, a pressure access device or a tap) on a pipe, cable or other supply line containing a fluid or gas. In this use, the second article is similar to one of the wrap-around couplers or repair patches described above, but in addition comprises a fitting which provides a port of desired dimensions. Before or after the second article 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. This use is illustrated in Figures 13 and 14. 21. Overheating and Insulating Barriers We have found that when two heating elements are adjacent to each other in a section having a relatively small amount of the first and/or second articles, 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 in Figure 11). 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. 22. 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 show two polyethylene pipes 11 and 12 which are being joined together by a cylindrical polyethylene coupler 21 having conical entry sections 211 and 212 at the ends thereof. At each end of the coupler, a flexible wedge-shaped gasket 3 is wrapped around the pipe and forced into the wedge-shaped recess between the pipe and the coupler. The gasket, which is shown in detail in Figure 2, comprises a central member 32 which is composed of polyethylene, and a laminar conductive polymer member 31 having electrodes 33 and 34 attached to its edge portions; leads 38 and 39 extend from the bus bars for connection to a power supply (not shown). In the part of the member 31 which is wrapped around the tip of the central member 32, a conductive bridge 35 is secured to the surface of the conductive polymer to ensure that heat is not generated in the tip area. After the components have been assembled as shown in Figure 1, with the gaskets making a good interference fit, the power supply is switched on, heat is generated in the conductive polymer member 31, and the pipes and the coupler become fused to opposite sides of the gasket. Figure 3 shows a slotted strip 21 having slots 211 in opposite faces thereof. A heat-shrinkable sheet 15 (shown in part only) is wrapped around a cable 9 (also shown in part only). A gasket 3, which is the same as that shown in Figures 1 and 2 except that there is no central member 32, is wrapped around each of the edge portions 151 of the sheet 15, and the edge portions and gaskets are inserted into the slots 211. By powering the gaskets while maintaining the edge portions in the slots, the edge portions are fused to the strip. The sheet 15 can then be shrunk, e.g. by means of hot air, so that it recovers around the cable. Figures 4 and 5 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 laminar conductive polymer element 31. The inner edges of the sheet 31 and element 21 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 6, there are 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 7 and 8 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 comprising a conductive polymer element 31. The member 21 is obtained by making a longitudinal cut in a length of a polyethylene pipe, the cut being angled so that the resultfng 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 laminar 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 9, there are electrodes 33 and 34 secured to the surface 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 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 10 shows an articulated coupler which comprises 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, diametrically 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. Figure 11 is very similar to Figure 10, except that both the longitudinal cuts pass all the way through the pipe wall, so that the coupler is in two separate parts. Figures 12 and 13 show the use of the invention to repair a polyethylene pipe 11 (shown in part only) which has a hole 111 therein. A flexible repair patch comprises a polyethylene member 21 and a laminar heater comprising a conductive polymer element 31. The patch is placed on the pipe, over the hole, and secured there by an elastomeric band 42 (shown in part only) which passes around the pipe. The heater is then powered, fusing the patch to the pipe and sealing the hole. Figures 14 and 15 show the use of the invention to secure a pressure access device to a telecommunications cable having an outer jacket 11 composed of polyethylene. A pressure access device 21 has a threaded hole 216 therein. A laminar heater comprising a conductive polymer element 31 is secured to the underside of the device. The device is placed against the cable and held in place by an elastomeric member 42 (shown in part only) which passes around the pipe. The heater is then powered, thus fusing the device to the cable. When access to the cable is desired, a hole is drilled through the jacket 11, using the threaded hole 216 as a guide. The invention is further illustrated by the following Examples. 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 36.5 by 6 by 0.03 inches (92.7 by 15.2 by 0.076 cm) was skived from it. 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 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 8 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 (0D) was cut through its thickness on a tangential 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/1.9 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 except 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. The coupler as shown in Figure 11 and 6.75 inch long was prepared by cutting a 12-inch 0D pipe into two pieces using a cut as described in Example 2 and its mirror image. The heating element pieces were prefused to the inner diameter of each segment prior to fusing the heating element to two 10-inch 0D polyethylene pipes. Example 3 A coupler as shown in Figure 10 and 6.75 inch long was prepared as described in Example 1 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 0D 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. It is to be understood that the present invention includes all the novel methods and devices and combinations thereof disclosed herein, whether or not they fall within the scope of the claims set out below. For example, the invention includes the individual components of the gaskets described herein, including in particular the laminar heating elements in which measures are taken to avoid heat generation fn undesired locations by means of conductive busses or cutout portions.

Claims: CLAIMS 1. An elongate flexible gasket which comprises (a) an elongate central member comprising a component which (i) is composed of an electrically insulating polymeric composition, (ii) comprises first and second opposed longitudinally extending surfaces which define a wedge-shaped cross-section, and (iii) is not heat-recoverable; (b) a first elongate laminar heating element which (i) is composed of a conductive polymer, and (ii) is secured to the first surface of the central member; (c) a second elongate laminar heating element which (i) is composed of a conductive polymer, and (ii) is secured to the second surface of the central member; and (d) at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to pass through the heating elements substantially parallel to the surfaces thereof. 2. A gasket according to claim 1 wherein the central member is composed of a first melt-extruded polymeric composition and each of the heating elements is composed of a second polymeric composition which is compatible with the first composition. 3. A gasket according to claim 2 wherein the first composition comprises polyethylene and the second composition comprises sintered ultra high molecular weight polyethylene. 4. A gasket according to any one of the preceding claims wherein there is at least some area of at least one of the laminar conductive polymer heating elements in which heat is not generated when the electrodes are connected to a suitable source of electrical power. 5. A gasket according to any one of the preceding claims wherein for at least one of the laminar heating elements the conductive polymer is not present in areas which, when the gasket is used, are not in intimate contact with a substrate. 6. A coupler which comprises (1) one or more tube-forming components which are not heat-recoverable and which are composed of an electrically insulating polymeric composition; and (2) one or more gaskets secured to the inner surface(s) of the tube-forming component(s) and comprising (a) at least one laminar heating element which is composed of a conductive polymer, and (b) at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to pass through the heating element(s) substantially parallel to the surface thereof, said tube-forming component(s) and gasket(s) being such that they can be assembled and/or deformed into a tubular coupler which, when around or in a pipe, can be brought into intimate contact with the pipe by circumferential forces applied to the coupler. 7. A coupler according to claim 6 wherein the tube-forming component(s) comprise(s) longitudinally extending chamfered edges which can slide relative to each other to reduce the diameter of the coupler. 8. A coupler which comprises (1) at least one first member which (a) is not heat-recoverable, (b) is composed of an electrically insulating polymeric composition, (c) has chamfered edges, and (d) comprises one or more gaskets secured to the inner surface of the first member and comprising (i) at least one first laminar heating element which is composed of a conductive polymer, and (ii) at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to pass through the first heating element substantially parallel to the surface thereof; and (2) at least one second member which (a) is not heat-recoverable, (b) is composed of an electrically insulating polymeric composition, (c) has chamfered edges, and (d) comprises one or more gaskets secured to the inner surface and the chamfered edges of the second member and comprising (i) at least one second laminar heating element which is composed of a conductive polymer, and (ii) at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to pass through the second heating element substantially parallel to the surface thereof; said first and second members being such that they can be assembled into a tubular coupler wherein (1) the chamfered edges of the first and second members can slide relative to each other to reduce the dfameter of the coupler and (2) the second heating element extends beyond the outer surface of the first and second members and, when connected to a source of electrical power, causes the first and second members to be secured to one another. 9. A method of joining a pipe to a coupler, which method comprises (1) placing an end of the pipe part way into one end of the coupler, so that the outer surface of the pipe and the inner surface of the coupler define a recess between them; (2) placing in the recess a gasket as defined in any of the preceding claims or a modification thereof wherein the central member is heat-recoverable; (3) force-fitting the gasket into the recess; and (4) generating heat within the heating element by passing current therethrough, thus jofnfng the pipe to the coupler. 10. A method according to Claim 9 which further comprises (5) placing an end of a second pipe part way into a second end of the coupler, so that the outer surface of the second pipe and the inner surface of the coupler define a second recess between them; (6) placing in the second recess a second gasket as defined in any of the preceding claims or a modification thereof wherein the central member is heat-recoverable; (7) force-fitting the second gasket into the second recess; and (8) generating heating within the heating elements of the second gasket by passing current therethrough, thus joining the second pipe to the coupler.

Inventors: CORDIA HANS, D HONDT JOHANN, HUGHES JOHN R, MCMILLS COREY J, RITTER ROBERT J, CORDIA, HANS, D'HONDT, JOHANN, HUGHES, JOHN, R., MCMILLS, COREY, J., RITTER, ROBERT, J.
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1989
Main classification: B29C65
IPC codes: B29C61, B29C65, B29L23, F16L47, H05B3