Method of joining substrates

This application is a divisional of European Patent Application No. 85302326.5, published as EP-A-0157640.

This invention relates to the use of articles comprising conductive polymer compositions, for joining, repairing, reinforcing or otherwise modifying substrates, particularly pipes composed of an organic polymer.

Many methods are known for joining, repairing and reinforcing pipes and other substrates, including methods which make use of heat-recoverable articles comprising conductive polymers, which, when powered, supply the heat needed to cause recovery of the article. Reference may be made for example to U.S. Patents Nos. 4,085,286, 4,177,446, and 4,421,582, and U.K. Patent No. 1,265,194. However, all the known methods suffer from one or more disadvantages, especially for joining pipes composed or organic polymer (often referred to as "plastic pipes"). This invention includes a new advantageous method for joining plastic pipes and other substrates having at least an exterior surface composed of a polymeric composition. This method preferably makes use of a coupler which comprises a conductive polymer and which is heated by passing electrical current through the conductive polymer.

EP-A-0157640 relates to novel methods of modifying a substrate, particularly a plastic pipe, in which a heat-recoverable article comprising a conductive polymer is recovered by generating heat within the conductive polymer, and is thus joined to the substrate either directly by fusion or indirectly through an insert. The present invention relates to similar methods in which the article is not heat-recoverable.

In its first aspect, the present invention provides a method of joining together <ul id="ul0001" list-style="none"><li>(A) a substrate having an outer surface composed or a first composition which is a heat-softenable composition comprising an organic polymer, and</li><li>(B) an article which is not heat recoverable and which comprises a conductive polymer element composed of a second composition which: <ul id="ul0002" list-style="none"><li>(a) is compatible with the first composition;</li><li>(b) comprises an organic polymer and, dispersed in the polymer, a particulate conductive filler, preferably <ul id="ul0003" list-style="none"><li>(i) a matrix consisting essentially of organic polymer particles which have been sintered together so that the particles have coalesced without losing their identity, and</li><li>(ii) a particulate conductive filler which is dispersed in said matrix but is present substantially only at or near the boundaries of the coalesced particles,</li></ul> or <ul id="ul0004" list-style="none"><li>(i) a melt-formed organic polymer having a molecular weight of at least 150,000, and</li><li>(ii) dispersed in the polymer, a particulate conductive filler; and</li></ul></li><li>(c) has a Melt Flow Index of less than 0.3 g/ 10 min. at a temperature 50°C above its softening point and at a loading of 5 kg and has a Melt Flow Index of less than 3.0 g/10 min. at a temperature 50°C above its softening point and at a loading or 15 kg;</li></ul></li></ul> which method comprises <ul id="ul0005" list-style="none"><li>(1) placing the article in direct contact with the outer surface of the substrate; and</li><li>(2) generating heat within the conductive polymer element by passing electrical current therethrough while maintaining the article in direct contact with the substrate, until there is fusion between the article and the outer surface of the substrate.</li></ul>

The term "fusion", which is said to occur between the article and the outer surface of a substrate, 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 viscoelastic 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.

In a second aspect, the invention provides a method of joining together <ul id="ul0006" list-style="none"><li>(A) a substrate having an outer surface composed of a first composition, and</li><li>(B) an article which is not heat-recoverable and which comprises a conductive polymer element composed of a second composition which: <ul id="ul0007" list-style="none"><li>(a) comprises an organic polymer and, dispersed in the polymer a particulate conductive filler, preferably <ul id="ul0008" list-style="none"><li>(i) a matrix consisting essentially of organic polymer particles which have been sintered together so that the particles have coalesced without losing their identity, and</li><li>(ii) a particulate conductive filler which is dispersed in said matrix but is present substantially only at or near the boundaries of the coalesced particles,</li></ul> or <ul id="ul0009" list-style="none"><li>(i) a melt-formed organic polymer having a molecular weight of at least 150,000, and</li><li>(ii) dispersed in the polymer, a particulate conductive filler; and</li></ul></li><li>(b) has a Melt Flow Index of less than 0.3g/10 min. at a temperature 50°C above its softening point and at a loading of sky and has a Melt Flow Index of less than 3.0g/10 min. at a temperature 50°C above its softening point and at a loading of 15kg; which method comprises</li></ul></li></ul><ul id="ul0010" list-style="none"><li>(1) placing the article adjacent to the substrate;</li><li>(2) placing between the article and the substrate a heat activatable insert which when activated by heat promotes the strength of the attachment of the article to the substrate; and</li><li>(3) generating heat within the conductive polymer element by passing electrical current therethrough while maintaining the article in indirect contact with the substrate through the insert, until the insert has been activated, thus joining the article to the substrate.</li></ul>

An important advantage of using articles comprising a conductive polymer element as specified is that such elements retain good physical strength even when heated to elevated temperatures. This permits heating to be continued for the time needed to effect fusion between the article and the substrate or to effect a desired change in an insert. When the conductive polymer composition comprises a matrix consisting essentially of sintered organic polymer particles, the particles are preferably composed of ultra high molecular weight polyethylene having a molecular weight greater than 1.5 million, particularly at least 3 million. When the conductive composition comprises a melt-formed organic polymer, the polymer preferably has a molecular weight in the range 150,000 to 600,000, more preferably in the range 200,000 to 400,000; high molecular weight polyethylene is especially preferred.

In both aspects of the invention, the article can be held against the substrate by at least one securing member.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which the Figures are diagrammatic cross-sectional illustrations of the method of the invention. <ul id="ul0011" list-style="none"><li>Figure 1 shows an assembly for patching a hole in a pipe; and</li><li>Figure 2 shows the patched pipe prepared by the method of Figure 1.</li></ul>

Figure 1 shows the repair of a plastic pipe 1 using a patch 8 which is not heat recoverable and which is composed of a sintered conductive polymer composition which is compatible with the pipe material. Plastic pipe 1 has a hole 14 therein which needs to be repaired. The patch is placed over the hole and is kept in place by means of tape 15 which is wrapped around the patch and the tape. Current is passed through the patch so that it softens and fuses to the pipe, resulting in the product shown in Figure 2 which is a cross section of line D-D of Figure 1.

The methods of the invention can be used to join, repair, reinforce or otherwise modify any type of substrate, including pipes and cables, eg. power cables, telecommunication cables and cables containing optical fibers. Particularly good results are obtained when the first composition (which provides at least an outer surface of the substrate) is a heat-softenable composition comprising an organic polymer. The term "organic polymer" is used herein to denote any polymer which contains carbon in the backbone and/or in the side chains, including for example polymers of olefinic monomers, polyamides, polyesters, polyacrylates, polyimides, polycarbonates and polysiloxanes. When using such articles and substrates, it is preferred to continue the heating of the article so that the surface of the substrate is heated and softened. This is advantageous when the substrate surface and the article come into direct contact with each other and are composed of compositions which are compatible with each other so that at least one of the heat-softened polymers can wet the other and thus cause fusion of the compositions at the interface; when using the preferred sintered conductive polymer compositions, there can be wetting and flow of a less viscous substrate composition around and possibly into the sintered article. To achieve such fusion, at least one and preferably both of the compositions should be above its softening point (in the case of crystalline polymers, above its crystalline melting point). The compatibility of different polymeric compositions depends upon a variety of factors, including both the chemical natures of the polymers and the other ingredients, e.g. fillers, of the compositions. However, those skilled in the art will have no difficulty, having regard to their own knowledge and the disclosure herein, in determining whether any two particular compositions are compatible. In many (but by no means all) cases, compatibility is the result of substantial amounts of common recurring units in the polymers of the two compositions, as, for example when at least 10%, preferably at least 30%, of the recurring units in the polymeric component of the first composition are the same as at least 10%, preferably at least 30%, of the recurring units in the polymeric component of the second composition, these percentages being based on the number of recurring units.

When the first and second compositions are not compatible with each other and/or are not heated so that bonding can occur across the interface, and/or when the substrate (or at least one of the substrates if two or more substrates are used) is not composed of a heat-softenable polymeric composition, e.g. is composed of a metal, concrete, or a thermoset polymer, e.g. a glass-reinforced plastic, a heat-activatable insert is placed between the substrate and the article. The term "insert" is used herein in a broad sense to include any article, whether self-supporting or in the form of a layer supported on one of the surfaces, which will promote the strength of the attachment of the article to the substrate, or which will provide some other desirable result at the interface, e.g. electrical insulation. The insert can be effective as a result of its physical and/or chemical effect at the interface, and can be for an example an adhesive, including a hot-melt or heat-curable adhesive; a cross-linking agent or other chemical activator for one or both of the surfaces; or a metal, elastomeric or fibrous member including an O-ring or other sealing member which fits into a slot in the article. Depending upon the nature of the insert, it may be desirable to continue to heat the article until some desired change has taken place in one or more of the insert, the substrate and the article.

The insert may be provided as a lining on the inside surface of the article, or it may be provided and installed, separately, for example in the form of a tape which is wrapped around the substrates.

In some embodiments of the invention, the heat generated within the article can cause heat-softenable material from a substrate, or from a separate insert, or from both, to fill a gap between substrates. Preferably a support member is also included adjacent the substrates, substantially to prevent the softened material flowing through, rather than filling. the gap between the substrates. The support member may also be provided with a stop to space the substrates to provide the gap therebetween. As examples of inserts that may be used there may be mentioned (i) resins, especially thermosetting resins, for example epoxy resins, which are preferably selected so as to cure on heating to form a good bond between the substrates. and (ii) heat-activatable adhesives, for example hot melt adhesives. The resin or adhesive is preferably reinforced by fibers or whiskers. This method is particularly advantageous since it enables joints to be made between substrates that are incompatible with the article. If reinforcing is not required, the article may be removed from the formed joint. The method is also advantageous, since by appropriate choice of material of the insert the properties of the joint region, for example the electrical continuity of the joint, may be controlled. For example where the insulation at the ends of part of insulated electric cables has been bared to allow the cables to be electrically connected, the method may be used to join the cut back insulation. Electrical continuity or discontinuity of the joint can be achieved by appropriate selection of a conductive or non-conductive insert. For some applications an insert comprised of two materials, for example one conductive, one non-conductive, may be selected to achieve the desired properties at the joint region.

Where the surface of the pipes, or an insert, or both, flow to fill the gap between the substrates, the article is preferably arranged such that those portions of the pipe or insert adjacent the article do not soften and therefore act as dams substantially to prevent egress of softened material out of the ends of the article. This may be achieved, for example, by shaping the article such that it has internal annular grooves at each of its ends.

The article may be any suitable shape depending on the nature of the substrate(s). In one embodiment the article is generally tubular, preferably cylindrical.

When the substrate (or one or more of the substrates) comprises a heat softenable polymeric composition of another material whose functional properties can be adversely affected by excessive heating, care must be taken to limit the heat generated within the conductive polymer to an amount that does not have an adverse effect. Thus in the case of a pipe composed of heat-softenable polymeric material, heating should be discontinued before the pipe becomes distorted (as a result of pressure from the coupler or as a result of shrinkage of an oriented pipe) to an extent which has a substantial adverse effect; preferably the interior surface of the pipe remains substantially unchanged.

In one very useful embodiment of the invention, a coupler is used to join two or more pipes. The pipes can be the same or different in size and can be the same or different in composition. The ends of the pipes can be joined, or one or more pipes can be connected as branches to another pipe, usually of larger diameter. In one preferred method of the invention, two pipes, each composed of a heat-softenable organic polymer composition, preferably two identical pipes, are joined in-line, using a hollow coupler.

When joining pipes together, it may be desirable to make use of a hollow internal support member which is placed inside one or more of the pipes. The support member, for example, can function solely to maintain substantially the original internal dimensions of the pipes (e.g. when joining thin-walled plastic pipes which could otherwise be distorted by the coupler). Alternatively, the support member can increase the strength of the coupling by modifying the shape of the pipes, e.g. through the presence of circumferential ribs or other protruberances on the external surface of the support member or through use of a support member whose center section is of smaller size than its end sections. The support member can also provide an adhesive at the function of the pipes and/or bond chemically to one or both of the pipes. The support member can also provide a stop a against which the end(s) of the pipe(s) can be butted, or can help to align the pipes (or otherwise maintain them in a desired spatial relationship) . The support member is preferably shorter than the coupler, so that if the joint is subsequently flexed, the bending forces are not concentrated at the ends of the support member. The support member can be secured to the article so as to provide one or more pockets into which the substrate(s) can be inserted.

The invention is particularly useful for joining two or more pipes, with at least one of the pipes being composed of, a composition based on an organic polymer, in particular polyethylene, polypropylene, or polyvinyl chloride, for example polyethylene pipes used to distribute natural gas or water, e.g. irrigation pipes.

The articles used in the present invention comprise, and may consist essentially of, an element composed of a conductive polymer composition. Heat is generated within the conductive polymer composition by passing electrical current through the element. Conductive polymer compositions are well known and comprise an organic polymer component and, dispersed in the organic polymer component, a particulate conductive filler. The term "particulate" is used herein to include particles of any shape, including particles of high and low aspect ratios, e.g. spherical, plate-like and acicular. The conductive polymers used in the present invention preferably have resistivities at 23°C in the range of 1.5 to 100 ohm.cm, with particularly preferred values within this range being dependent on the dimensions of the article, the electrode placement and the power source. Preferably these factors are selected so that a satisfactory result is obtained in a relatively short time, e.g. within 10 minutes, preferably within 5 minutes. Suitable power sources include for example DC voltages or 6, 12, 24 or 48 volts, for which resistivities of 1 to 10 ohm.cm are usually preferred, as well as voltages of 36-40, 110-120 or 220-240 volts AC, for which higher resistivities, e.g. 50-100 ohm.cm, are appropriate. The preferred dimensions of the articles will of course depend on their intended use. It is often desirable that the article should continue to generate heat at a high rate even at the elevated temperature which it reached during the method, for example to promote rapid softening of the outer surface of a plastic pipe without softening the inner surface of the pipe. For this reason, it is preferred that the conductive polymer composition should not exhibit PTC behavior at such elevated temperature; a change in resistivity by a factor of less than 5, preferably less than 2, is preferred.

Many conductive polymer compositions are known, and most of them are prepared by a melt-mixing process. The filler loadings needed to produce the desired levels of resistivity result in relatively poor physical properties. We have surprisingly found that when melt-processed high molecular weight polyethylene is used, having a molecular weight in the range 150,000 to 600,000, the change in the physical properties that occurs as the filler loading is increased is less than that which occurs for lower molecular weight polyethylene, and that, for example, expansion ratios of 1.5 may be achieved. The quantity of carbon black or conductive filler required to provide a given level of resistivity is preferably less than 26 weight percent, particularly less than 24 weight percent, especially 18-24 weight percent.

When a sintered conductive polymer composition is used, it comprises, and preferably consists essentially of, <ul id="ul0012" list-style="none"><li>(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, and</li><li>(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.</li></ul>

Such conductive polymers can be prepared by sintering a dry blend of the polymer particles and the conductive 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 quantity of conductive filler required to provide a given level of resistivity is much less than in a melt-blended product. Thus the preferred sintered compositions for use in this invention contain less than 9%, preferably less than 7%, particularly 2 to 6%, by volume or carbon black or other conductive filler. Care must be used in selecting the carbon black (or other filler) in order to achieve the desired level of resistivity at these loadings. Excellent results have been obtained using Ketjenblack EC, a trademark of Akzo Chemie.

The polymer used in the preferred melt-formed high molecular weight composition or in the sintered conductive polymer compositions is one which maintains a relatively high viscosity at the sintering temperature. It is preferred to use a polymer which, at a temperature 50°C above its softening point, has a Melt Flow Index of less than 0.1 g/10 min, especially less than 0.05 g/10 min, at a loading of 5 kg, and a Melt Flow Index of less than 1.0 g/10 min, particularly less than 0.1 g/10 min, at a loading of 15 kg. Particularly good results have been obtained using ultra high molecular weight polyethylene (UHMWPE) especially such polyethylene having a molecular weight greater than about 1.5 million, particularly greater than about 3.0 million. Another polymer which behaves similarly when sintered is polytetrafluoroethylene (PTFE). Other polymers which can be sintered, but which are less viscous under sintering conditions than is preferred, are polyphenylene sulfide (PPS) and polyimides.

Molecular weights may be measured by a variety of methods. The molecular weight values quoted in the present invention for high molecular weight polyethylene (not the ultra high polymer) were measured using the following high temperature gel permeation chromoatography method:

The polyethylene was ground into a powder, and to 30 mg. of the powder 10 ml of 1.2.4 trichlorobenzene containing an antioxidant to prevent polymer degradation. The solution was heated for 15 minutes to dissolve the polyethylene while stirring under nitrogen flow. The solutions were filtered and then placed in the high temperature gel permeation chromatography instrument at a temperature 145°C (as supplied by Maters Model 150C). The eluent was monitored by differential refractometry. The chromatography instrument was calibrated using standard samples supplied by the U.S. National Bureau of Standards.

Materials other than polyethylene can also be used according to the present invention. As examples of other materials there may be mentioned other polyolefins, for example polypropylene and olefin copolymers; polyesters; polyvinylidene fluoride; perfluoroalkoxypolymers; copolymers of ethylene and tetrafluoroethylene; fluorinated ethylene propylene polymers; copolymers of ethylene and chlorotrifluoroethylene; and polyarylenes, for example polyetheretherketones. The preferred range of molecular weight may be different for each different polymer type, in order to achieve the desired properties to make the polymer suitable for joining, repairing, reinforcing or otherwise modifying substrates according to the present invention.