HEAT-RECOVERABLE DEVICES

Abstract: Hollow heat-shrinkable conductive polymer devices, particularly coupling members (2) for joining polymeric pipes (3, 4), have end sections (12, 13) of increased wall thickness so that, when current is passed through the device, the end sections are heated to a lower temperature. This avoids overheating (and consequent weakening) of that part of the pipe which is enclosed by the end section of the device.

Description: HEAT-RECOVERABLE DEVICES BACKGROUND OF THE INVENTION Field of the Invention This invention relates to heat-recoverable devices and their use for repairing, reinforcing, joining or otherwise modifying a substrate. Introduction to the Invention It is known that heat-recoverable devices can be used to repair, reinforce, join or otherwise modify substrates of many kinds. It is also known that such devices can comprise a heating element which is composed of a conductive polymer and which, when connected to a suitable power source, will generate heat and cause recovery of the device. The heating element can itself be heat-recoverable, and/or it can be secured to a recoverable article and be such that it does not prevent recovery of the article. Reference may be made, for example, to U.S. Patents Nos. 4,085,286, 4,177,376, 4,177,446, 4,421,582, 4,570,055 and 4,575,618 and to copending commonly assigned U.S. patent applications Serial Nos. 634,242, (MP0949), 720,117 (MP0922-US2) and 720,118 (MP1039), and the equivalent published European Patent Applications Nos. 0,157,640 and 0,197,759. The disclosure of each of the patents and patent applications referred to above is incorporated herein by reference. SUMMARY OF THE INVENTION We have found, in the flex testing of joints between polymeric pipes which have been joined together by means of a heat-shrinkable coupling member which is composed of a conductive polymer and which becomes fused to the pipes (in particular as described in Serial No. 720,117), that failure of the joint occurs substantially sooner than failure of a continuous length of the pipe; this is in contrast to the results obtained in other tests, in which the joint is stronger than the pipe itself. We have also found that the failure results from cracking of the pipe under the coupling member and near the end of the coupling member. In investigating this problem, we have discovered that the pre¬ mature failure results from heating of the pipes, during the coupling operation, to a temperature sufficiently high to have a substantial adverse effect on their resistance to cracking when flexed. We have further discovered that the problem can be solved by making use of a coupling member having end sections which are heated to a lower temperature than the center section, so that weakening of the pipe through overneating takes place only in a central portion of the joint where enough of the bending stresses are carried by the coupling member that the weakening of the pipe is not significant. In a first aspect, the invention provides a radially heat-shrinkable hollow member, preferably a cylindrical coupling member which (1) is composed of a conductive polymer, (2) comprises at least two heat-shrinkable end sections, and at least two heat-shrinkable intermediate sections each of which is adjacent to a heat-shrinkable end section, and (3) can be caused to shrink by passing current longitudinally therethrough; wherein at least one of the end sections has a wall thickness which is at least 1.05 times the wall thickness of the intermediate section adjacent thereto. The increased wall thickness results in the end section(s) having a lower resistance, and this (together with other factors such as the greater heat loss from the end sections) results in the generation of higher temperatures under the intermediate sections than under the end sections(s), so that although the end sections are caused to shrink, overheating of the thermoplastic pipe is avoided. The end sections are preferably identical to each other, but can be different. The intermediate sections can be joined directly to each other, so that there is then a single intermediate section which is notionally divided into two (or more) parts which behave in the same way if the coupling member is used to join like substrates but which behave in different ways if the coupling member is used to join unlike substrates. Alternatively, the intermediate sections can be joined to each other through a center section, for example a center section which has a plurality of circumferential ribs on the interior surface thereof in order to provide improved coupling to the substrates. The heat-shrinkable members of the present invention are particularly useful in the form of cylindrical coupling members, especially for joining pipes, and the invention will be chiefly described by reference to such use. However, they can also be used for repairing or reinforcing or otherwise modifying a substrate, for example for adding a branch pipe to an existing pipe. Thus, in another aspect, the invention provides a method of joining two pipes which comprises placing the ends of the pipes inside a cylindrical coupling member as defined above, and passing current tnrough the coupling member so that it is heated and shrinks around the pipes. In one preferred embodiment, each of the pipes is composed of a thermoplastic material and the conditions of tne method are such that one of the intermediate sections becomes fused to one of the pipes and the end section adjacent thereto shrinks into compressive contact with that pipe but does not become fused tnereto, and the other intermediate section becomes fused to the other pipe and the end section adjacent thereto shrinks into compressive contact with the other pipe but does not become fused thereto. In another preferred embodiment one of the pipes is composed of a metal and the other pipe is composed of a thermoplastic material, and the conditions of the method are such that one of the intermediate sections becomes fused to the thermoplastic pipe and the end section adjacent thereto shrinks into compressive contact with the thermoplastic pipe but does not become fused thereto, and the other intermediate section and the other end section shrink into compressive contact with the metal pipe. In each of these embodiments, it is preferred that at least part of the surface of the thermoplastic pipe which becomes fused to the coupling member is heated to a temperature at which the thermoplastic material is completely melted, and the maximum temperature reached by at least part of the surface of the thermoplastic pipe which is brought into compressive contact with the end section is less than the temperature at which the thermoplastic material begins to melt. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawing, in which Figure 1 is a longitudinal cross-section through an unexpanded blank for a coupling member; Figure 2 is a transverse cross-section taken on line II-II of Figure 1; Figure 3 shows an assembly for joining two pipes by means of a coupling member made by radial expansion of a blank as shown in Figures 1 and 2. Figure 4 is a transverse cross-section through a wrap-raround repair sleeve of the invention placed around a pipe; Figure 5 is a longitudinal cross-section taken on V-V of Figure 4 (but not showing the pipe); and Figure 6 is a plan view of another coupling member of the invention. DETAILED DESCRIPTION OF THE INVENTION The invention is particularly useful when the coupling member shrinks into direct physical contact with the substrate(s) , especially when at least one of the substrates is composed of (or has at least an outer surface composed of) a thermoplastic material, and there is fusion between at least one of the intermediate secrions and the thermoplastic material. The term "fusion" is used herein to mean that sufficient molecular compatibility exists between the element and the substrate outer surface that a bond forms which will provide mechanical performance equal to or greater than that of the substrate. This may be through viscoelas ic contact as defined by J.N. Anand in Adhesion 1, 1969, pages 16 through 23 and Adhesion 2, 1970, pages 16 through 22, or through a process of molecular diffusion across the polymer/polymer interface, such that, within the interfacial region there is a continuous concentration gradient of one polymer in the other. However, the inven¬ tion can also be used when a heat-responsive insert of some kind, eg. a curable polymeric insert, a hot melt adhesive, a mastic or a solder is placed between all or part of the heat-shrinkable member and the substrate. In the preferred embodiments of the invention, wherein at least one of the substrates is composed of a thermoplastic material, it is preferred that the maximum temperature reached by the surface of the substrate underneath the end section of the coupling member is less than the temperature at which the thermoplastic material begins to melt (defined herein as the temperature at which a differential scanning calorimeter plot for the material, measured in accordance with ASTM, first departs from a straight line); and it is also preferred that at least part of the surface of the substrate underneath the intermediate section is heated to a temperature above the temperature at which the thermoplastic material is completely melted (defined herein as the peak temperature on the same plot). The thermoplastic material preferably comprises an organic polymer, eg. a crystalline polymer, which is preferably not cross-linked, but can be cross-linked. The composition can consist essentially of such a polymer and, if desired, conventional stabilizers for the polymer, or it can also contain one or more fillers, eg. in amount up to 20%, preferably not more than 10%, by weight. Particularly suitable polymers are polymers of one or more ethylenically unsaturated monomers, eg. polyolefins, including in particular low, medium and high density polyethylenes and polypropylene, and polyvinyl chloride. Particularly useful substrates are pipes, particularly pipes which are composed of the thermoplastic material, especially pipes as defined in ASTM D-2513 and D-3350; such pipes are suitable for example for transporting natural gas or water. The heat-shrinkable member is preferably composed of a conductive polymer which is compatible with the thermoplastic material of the substrate, the organic polymers in the conductive polymer and the thermoplastic material preferably comprising at least 80%, particularly at least 90%, of the same repeating units. Particularly suitable conductive polymers are based on very high molecular weight polymers, particularly a sintered mixture of an organic polymer, especially ultra high molecular weight polyethylene (UHMWPE) , and a conductive filler, particularly carbon black. The coupling member is usually monolithic, but can comprise first and second parts which are secured as a "wrap-around" around a substrate (as for example in Figures 4 and 5), in which case only part of the member need be heat-shrinkable. The term "end section" is used herein to denote a section which is nearer to an open end of the member than the adjacent intermediate section; the end section can include the open end, but does not necessarily do so - for example the open end need not necessarily be heat shrinkable. Particularly good results are obtained when the end sections of the coupling member shrink into compressive contact with the pipes. The term "compressive contact" is used to indicate that the member, after shrinkage, exerts hoop stress on the pipe. Preferably the hoop stress is sufficient to create a circumferential depression around a plastic pipe. It is theorized that the presence of the hoop stress helps to combat the effects of cold flow of the thermoplastic substrate which may take place over an extended time. In addition, this feature assists in satisfactorily spreading flexural and other stresses over the coupling as a whole. The greater the wall thickness of the heat-shrinkable member, the greater the hoop stress that it can exert. The reduction in wall thickness which is characteristic of the heat-shrinkable members of this invention can take place abruptly, in which case it is simple to identify the end and intermediate sections. It is preferred, however, that the reduction in wall thickness should be gradual. Thus the wall thickness can decrease gradually from each open end towards the center of the member, there being optionally a center section which lies between the intermediate sections and in which there is no such decrease in wall thickness. Where there is a gradual reduction in wall thickness over part or all of the length of the member, the junction between the end and intermediate sections is defined herein as the point at which the wall thickness is equal to the.average of the maximum and minimum wall thicknesses of the member. The wall thickness of the end section(s) is at least 1.05 times, preferably at least 1.1 times, e.g. 1.1 to 1.4 times, the wall thickness of the adjacent intermediate section; and the wall thicknesses referred to are the maximum wall thicknesses in the end and intermediate sections respectively. When two plastic pipes are joined together, the con¬ ditions are preferably such that the ends of the pipes are butt-welded to each other. Particularly under these cir¬ cumstances, it is preferred that a support member be placed' within the pipes to ensure that molten pipe material does not drip into the interior of the pipe. Referring now to the drawing. Figures 1 and 2 show an unexpanded blank which can be converted into a coupling member of the invention by a process which comprises radial expansion. In Figures 1 and 2, the blank 1 is composed of a blend of carbon black and UHMWPE which has been ram-extruded as a cylinder and then machined into the shape shown, which comprises a center section 11 of generally uniform inner and outer diameter with a plurality of small ribs 111 on its inner surface, and two tapered end sections 12 and 13 having a constant outer diameter and an inner diameter which increases from the end adjacent the center section to the open end. Figure 3 shows a coupling member 2 made by radial expan¬ sion of a blank as shown in Figures 1 and 2 (to a generally uniform inner diameter which is slightly larger than the pipes to be joined thereby) , followed by attachment of electrodes 24 and 25. The electrodes are shaped and posi¬ tioned so as to provide the desired distribution of heat within the coupling member (and hence also the pipes) when the electrodes are connected to a suitable power supply. Also shown in Figure 3 are pipes 3 and 4 to be joined by coupling member 2, and support member 5 which helps to keep the pipes in position and to prevent excessive deformation of the pipes during the coupling operation. When the electrodes 24 and 25 are connected to a suitable power supply, as shown diagrammatically in Figure 3, through leads which will maintain contact with the alectrodes during the coupling operation, the coupling member shrinks into close and conforming contact with the pipes. Figures 4 and 5 show a wrap-around heat-shrinkable repair sleeve for repairing a crack 31 in a plastic pipe 3 (shown in Figure 4 but not in Figure 5). The repair sleeve comprises longitudinal electrodes 24 and 25 and hemicy- lindrical parts 26 and 27 which are composed of heat- shrinkable conductive polymer and which are secured to each other around the pipe by means of snap-fittings 261 and 271. Part 26 comprises relatively thick end sections 262 and 263 and relatively thin center section 261 which covers the crack in the pipe. Part 27 is of uniform wall thickness. When the electrodes are connected to a suitable power source, the sleeve shrinks into contact with the pipe, the center section 261 reaching a higher temperature than the rest of the sleeve and fusing to the pipe 3, so as to seal the crack 31, and the rest of the sleeve shrinking into compressive contact with the pipe but not becoming fused thereto. Figure 6 shows a coupling member 2 which comprises a first heat-shrinkable part 27 of relatively large diameter and wall thickness, a second heat-shrinkable part 26 of relatively small diameter and wall thickness, and axially spaced circumferential electrodes 241, 242 and 243. Recovery of all or part of the member can be effected by connecting a suitable power supply to an appropriate pair of electrodes. EXAMPLE A blank for a coupling member was made in the following way. Carbon black (Ketjenblack EC 300 available from Akzo Che ie), about 14.5 parts by weight, was dried and then blended with UHMWPE (Hostalen GUR 413 available from Hoechst), about 150 parts by weight, in a Henschel blender. The resulting blend was ram-extruded at about 255°C into a rod having a diameter of about 3.26 inch. A length cut from the rod was then machined into a blank as shown in Figures 1 and 2. In the blank, the center section had a length of 1.5 inch, a uniform outer surface having a constant diameter of 2.50 inch, and an inner surface having 16 threads per inch machined thereon, each thread being 0.025 inch deep and having an inner diameter of 2.11 inch; and each end section had a length of 1 inch, a uniform outer surface having a constant diameter of 2.59 inch and an inner surface which tapers uniformly at an angle of 5° from a diameter at the end adjacent the center section of 2.11 inch to a diameter at the other end of 1.94 inch. The blank was then heated to about 135°C, expanded by means of a tapered mandrel to a uniform internal diameter of about 2.55 inch, and cooled in water at 20°C for 15 minutes before the mandrel was removed. Electrodes were then painted on the ends of the expanded blank, using a silver- containing paint (Electrodag 504 available from Acheson Colloids Co.), to give a coupling member as shown in Figure 3, each electrode extending 0.25 inch down the length of the member and 0.12 down the diameter of the member. The coupling member was used to join together two pipes, available from Phillips as Driscopipe, 6500 series, each pipe having an inner diameter of 1.94 inch and an outer diameter of 2.375 inch and being composed of polyethylene. The pipes were positioned within the coupler over a poly¬ meric coupling member composed of "ϋltem", available from General Electric. The electrodes were connected to a 40 volt AC power supply, and after about 2.5 minutes the coupling member had shrunk into close and conforming contact with the pipes. The coupling member was disconnected from the power supply and allowed to cool. Examination of the resulting joint showed that the center section of the coupling member had fused to the pipes and that the ends of the pipes had melted and flowed so that they were butt-fused together. The end sections of the coupling member, by contrast, were not fused to the pipes, although they had shrunk into compressive contact with the pipes with such force that the pipes were slightly deformed under the ends of the coupling member. Testing of the resulting joint showed that it had excellent properties and in particular that in a rotary fatigue test, the joint had a much longer life than a joint made with a prior art coupling member whose design resulted in fusion of the pipes to each other and to the whole length of the coupling member, but which was otherwise comparable to the coupling member of the invention.

Claims: WE CLAIM: 1. A radially heat-shrinkable hollow member which (1) is composed of a conductive polymer, (2) .comprises at least two heat-shrinkable end sections, and at least two heat-shrinkable intermediate sections each of which is adjacent to a heat-shrinkable end section, and (3) can be caused to shrink by passing current longitudinally therethrough; wherein at least one of the end sections has a wall thickness which is at least 1.05 times the wall thickness of the intermediate section adjacent thereto. 2. A member according to Claim 1 wherein each of the end sections has a wall thickness which is at least 1.1 times the wall thickness of the intermediate section adjacent thereto. 3. A member according to Claim 2 wherein each of the end sections has a wall thickness which is 1.1 to 1.4 times the wall thickness of the intermediate section adjacent thereto. 4. A member according to ' Claim 2 or 3 wherein the wall thickness decreases gradually from each open end towards the center of the coupling member. 5. A member according to any one of the preceding claims which comprises a center section which lies between the intermediate sections and which has a plurality of circumferential ribs on the interior surface thereof. 6. A member according to any one of the preceding claims which is composed of a sintered mixture of ultra high molecular weight polyethylene and a conductive filler. 7. A method of joining two pipes which comprises placing the ends of the pipes inside a coupling member as defined in any one of claims 1 to & and passing current through the coupling member so that it is heated and shrinks around the pipes, thereby joining them together. 8. A method according to Claim 7 wherein one of the pipes is composed of a metal and the other pipe is composed of a thermoplastic material, and the conditions of the method are such that one of the intermediate sections becomes fused to the thermoplastic pipe and the end section adjacent thereto shrinks into compressive contact with the thermoplastic pipe but does not become fused thereto, and the other intermediate section and the other end section shrink into compressive contact with the metal pipe. 9. A method according to Claim 7 wherein each of the pipes is composed of a thermoplastic material and the conditions of the method are such that one of the intermediate sections becomes fused to one of the pipes and the end section adjacent thereto shrinks into compressive contact with that pipe but does not become fused thereto, and the other intermediate section becomes fused to the other pipe and the end section adjacent thereto shrinks into compressive contact with the other pipe but does not become fused thereto. 10. A method according to Claim 8 or 9 wherein the thermoplastic material is polyethylene, and the coupling member is composed of a conductive polymer in which the polymeric component comprises polyethylene. 11. A method according to Claim 10 wherein the coupling member is composed of a sintered mixture of ultra high molecular weight polyethylene and carbon black. 12. A method according to any one of Claims 8 to 11 wherein at least part of the surface of the thermoplastic pipe which becomes fused to the coupling member is heated to a temperature above the temperature at which the thermoplastic material is completely melted, and the maximum temperature reached by the surface of the thermoplastic pipe which is brought into compressive contact with the end section is less than the temperature at which the thermoplastic material begins to melt.

Inventors: LANGLAND GEORGE RAYMOND, LANGLAND, GEORGE, RAYMOND
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1987
Main classification: B29C65
IPC codes: B29C65, F16L13