WO1993008012A1

WO1993008012A1 - Article and method for bonding to an elongate object

Info

Publication number: WO1993008012A1
Application number: PCT/GB1992/001937
Authority: WO
Inventors: Jozef Albert Gustaff Doucet
Original assignee: N.V. Raychem S.A.; Raychem Limited
Priority date: 1991-10-23
Filing date: 1992-10-22
Publication date: 1993-04-29
Also published as: EP0610279A1; GB9122512D0; JPH07500057A; FI941880A; CA2116784A1; FI941880A0

Abstract

A wraparound polymer-laminated fabric sleeve (2) that is heat-recoverable at least in part is provided with electrical heating means (8, 10, 12) along one longitudinal edge and both annular ends of the wrapped sleeve. Electrical current is passed through the heating means (8, 10, 12) to recover at least part of the sleeve and to bond it to an underlying substrate.

Description

Article and Method for bonding to an elongate object

This invention relates to a heat recoverable wraparound article for bonding to an object such as a pipe, eg to seal against ingress of water, to protect against physical damage or to make a joint between the object and another object. In particular, the invention relates to a wraparound article including heating means along at least one longitudinal edge and along the circumferential edges as considered in the wrapped configuration.

A wraparound article including heating means along three edges is known from DE-A-3712356. This describes a sheet of thermoplastic material for covering a joint between plastic pipes in which the sheet is provided with at least two parallel heating wires on three edge regions. The article is not heat recoverable.

A heat-recoverable article is one whose dimensional configuration can be made to change substantially when subjected to heat treatment. A heat-recoverable article will usually recover towards an original shape from which it has previously been deformed but the term as used herein also includes articles which adopt a new configuration when subjected to heat treatment, even if it has not previously been deformed.

Heat-recoverable articles are well known and may be used in the sealing of pipe joints, the repair of polymeric coatings on pipes and similar structures, the protection from chemical and physical attack of pipes and other structures, often metallic. They may also be used to seal cable splices.

When a joint is made between two lengths of thermally insulated steel pipe, as used for example in a district heating system, the insulation is removed (typically in the factory) from the end regions of each length of pipe in order that the insulation is not burnt when the pipes are joined, for example by welding. After the weld has been completed, the uninsulated portion of the pipe in the region of the joint must be reinsulated in order to rninimise heat loss. It is important that the insulation in the region of the joint is sealed against moisture ingress which might otherwise cause degradation of the insulation, not only in the joint region but possibly also along the pipes to some distance from the joint, and cause also corrosion of the pipes.

Severe physical constraints are placed upon the seal of the insulation at a joint between insulated pipes in a district heating system. As a result of frequent and wide temperature cycling, the pipe can be subjected to expansion and contraction cycles and to longitudinal and transverse movements. For example, it has been determined that forces of up to lON.mm^-2 can be exerted on the outer protective jacket of an insulated pipe as a result of temperature cycles experienced by a pipe in winter before it is buried. The seal must be able to withstand these cycles and movements without breaking. Furthermore, it is preferred that the seal be able to withstand the pressures exerted when the cavity around the uninsulated portion of pipe is reinsulated by foaming in situ.

One approach that has met with considerable success is the subject of GB-A-2108625. In one aspect it involves creating a flexible seal, between the insulation surrounding each pipe and a rigid casing which spans the uninsulated portion of pipe, by means of a heat- recoverable polymeric sleeve coated with a sealant. To prevent movement of the sealant coated sleeve of GB-A-2108625 relative to the pipe, the sleeve is provided with adhesive material at specified regions to bond the sleeve to the pipe insulation and to the casing.

Another approach which has met with considerable success is described in EP-B-245067. This describes a rigid system for sealing the insulation at a pipe joint which is sufficiently strong to withstand the forces encountered during installation without stretching or flexing to any significant extent, or breaking. EP-B-0245067 describes an article comprising a heat recoverable polymeric layer, a layer of bonding material for forming a bond between the recoverable material and the object, and a laminar metal heating element having a plurality of apertures through its thickness inbuilt in one of the layers. The sleeve is multiwrapped for some embodiments, especially for large pipe or cable covering, so that it builds up a thickness of, for example, from 0.5 to 3.5 mm. Increased thickness enhances the strength, which is required of a rigid seal.

We have discovered that particular advantages can be achieved by providing a heat recoverable fabric wraparound sleeve, preferably impregnated or coated with polymeric material, which sleeve is provided with heating means in or on its inner surface, along at least three edges, being one longitudinal edge and both annular edges when the sleeve is in its wrapped configuration.

The existence of heat-recoverable fabric sleeves is known. Reference may be made, for example, to EP-A-0116393, GB-A- 2133639, GB-A-2133740, GB-A-2134334, GB-A-2135632, GB-A- 2135836 and GB-A-213942, the disclosures of which are incorporated herein by reference.

Thus the present invention provides a wraparound article for bonding to an elongate object such as a pipe or cable or for making, or bonding to, a joint or splice between such articles, the article comprising a layer of a fabric that is heat-recoverable, preferably heat shrinkable, at least in part, which is in the form of a sheet that can be wrapped around the object, and which is provided with electrical heating means in and/or on its inner surface when wrapped, along at least three edges of the sheet, being at least one of the longitudinal edges and both annular edges when the sheet is in its wrapped configuration. One. some or all of the heating means can be at least partially embedded in the recoverable fabric, or they can lie on the surface of the fabric layer.

The electrical heating means may be of any suitable type. For example an open mesh of the type described in EP-B-0245067 can be used. Such a structure can advantageously follow the recovery of the sleeve. In another embodiment certain fibres of the fabric may be conductive, or extra conductive fibres or wires can be inserted in the fabric and connected to a power supply to act as heating elements. For example, conductive copper wires may be inserted. Mixtures of different heating means may be used. Electrical power to the electrical heating means (which heat by I²R heating as a result of electrical current passage) may be provided by any suitable electrodes. For example, metal strips or conductive braid strips may be fixed to, or interwoven in, the fabric.

The sleeve preferably also comprises a bonding layer, on its inner surface (when wrapped) at least in the region of the heating means. Preferably the bonding layer comprises any of the materials mentioned for the bonding layer of EP-B-0245067, the disclosure of which is incorporated herein by reference.

The bonding material may be selected from materials which provide an adhesive bond and from materials which provide a fusion bond. The selection will depend on the materials of the recoverable layer and on the object to which it is to bond. Preferably the bonding material will be heat-activatable, that is for example a material which can provide a fusion bond, or if a material which provides an adhesive bond (ie, an adhesive) a hot-melt adhesive or a heat-curable adhesive. This has the advantage of providing an article with a layer of bonding material that is latent until the heating element is powered in order to cause the layer of recoverable material to recover and to activate the bonding material, and which is therefore particularly convenient during storage and during manipulation of the article prior to installation. A heat-activatable adhesive (as the term is used herein) forms a bond that is based on chemical and physical interactions which are initiated when the adhesive is in its liquid phase and are retained on solidification. Examples of hot-melt adhesives which may be used as bonding material in the article of the invention are disclosed in US- 4181775 and US-4018733, which disclose formulations comprising polyamides modified with hydrocarbon waxes, and mixtures of acidic ethylene polymers and tackifiers. Also suitable are compositions based on etheylene vinyl-acetate copolymers. blended with hydrocarbon waxes and optionally butyl rubber. A particularly preferred adhesive, especially for use on untreated surfaces and in low temperature applications, is that disclosed in GB-A-2075991, which is a blend of a polyamide, an acrylic rubber and preferably a small amount of an ethylene/acrylic acid /butyl acrylate terpolymer. In order to obtain satisfactory shear performances in such adhesives, it may be advantageous to cross-link the adhesive, although the degree of cross-linking must not be so high as to reduce the ability of the adhesive to liquify, and thereby to flow and to wet the object to be bonded, and therefore to form a bond to the object with good peel strength.

A fusion bond (as the term is used herein) on the other hand is one that is based on physical interactions initiated by intimate contact between compatible materials in their visco-elastic state, and is more akin to a weld. In many (but not all) cases, compatibility of two or more polymeric compositions arises from substantial amounts of common recurring units in the polymers of the materials, such as for example when at least 50%, preferably at least 70%, of the recurring units of a polymer of a first composition are the same as at least 50%, preferably at least 70%, of the recurring units of a polymer of the other composition, these percentages being based on the total number of recurring units.

Polymeric materials that may be used for the fusion bonding layer include, for example polyethylene, polypropylene, polybutene. copolymers of ethylene. propylene, butene and hexene. copoiymers of ethylene with ethyl acrylate, vinyl acetate, acrylic acid, methacrylic acid, acrylic esters or methacrylic esters in which polyethylene predominates, blends of these polymers, and blends of these polymers with elastomers.

We have found that a sleeve of a certain thickness when made from the recoverable fabric of the present invention provides the same strength eg particularly against internal or external pressure, or against tensile stress, as a much thicker sleeve made from a recoverable sleeve of the material described in EP-B-0245067. For many applications this is very desirable since it allows the thermal mass of the article to be less than that of the heated region of the pipes so that, if the article during heat recovery reaches high temperatures, damage to the joint is minimised. This is particularly useful in the case of district heating pipe joints as explained below.

District heating pipes typically comprises a central steel pipe, an outer thick insulation and an outer polymeric coating. In joining such pipes the steel is bared and a weld made. It is then necessary to make good the insulation. This may be done by surrounding the outer polymeric coatings on either side of the joint with a casing (one or multi-part) sealed to the polymeric covering, and then immediately adding foam into the annular gap around the steel pipes in the joint region within the casing. The types of foams used typically decompose at temperatures of about 140°C or higher. Therefore if a heat recoverable sleeve is used during the reinsulation of district heating joints, either as a single part casing or as seals onto a rigid casing, then the installer must ensure that the sleeve does not heat to temperatures at which the foam will decompose. In situations where the thickness of the sleeve is high, so that the thermal mass of the sleeve is greater than that of the region of the pipe which heats during the installation, then it is very difficult to control over-heating of the sleeve and consequent damage to the foam. Thus the present application, which allows the use of thinner and hence lower thermal mass sleeves, provides significant advantages compared to the prior art solutions. Preferably the sleeve used has a wall thickness less than that of the pipes to be joined, preferably less than 0.75 times the wall thickness of the pipes to be joined. As an example, according to our invention a sleeve of approximately 1.7 mm thickness can provide approximately the same strength (when tested by axial pull out of a 160mm diameter. 0.5m length tubular article) as a double wrap sleeve according to EP-B- 0245067 of about 3.2 mm total thickness. High strength sleeves are particularly required where large pipes are used, eg of the order of 400 mm diameter or larger, particularly if subject to high stress during installations. Such stresses may be generated for example when a pipe line is being laid in soil. This typically takes several days so the joints may be exposed to night/ day temperature cycles which may create stress on the joints.

In a preferred embodiment according to the invention part only of the fabric is heat recoverable, preferably heat shrinkable, preferably radially. Preferably a central annular region of the sleeve when in its wrapped configuration is not heat recoverable. This may be achieved, for example, in a weave, by arranging for the fibres in the weft to run circumferentially and to provide rows of heat recoverable fibres at either end of the wrapped sleeve and non-heat- recoverable, ie. heat stable fibres in the central region. The use of a fabric according to the invention, particularly one having a central heat stable section has been found to be a particularly advantageous structure in applications where the article is subjected to stress eg when employed as a joint between district heating pipes, since the sleeve is able to "give" a small amount due to straightening of fibres under tension and relaxation of fibres when tension is released. Such straightening and relaxation occurs due to the nature of the fabric, in which, at least in the unrecovered state, fibres pass over and under each other in a shallow sinuisoidal configuration and hence, unless in a very tight weave or other fabric configuration, can straighten and tighten in response to stresses. The flexibility of a joint between district heating pipes provided by the use of a heat recoverable fabric also forms part of the present invention. Thus another aspect of the invention provides a method of reinsulating a bared joint between district heating pipes each of which comprises a central pipe having an outer insulation, the method comprising

(1) positioning a wraparound article either to bridge the insulation on either side of the pipes or to bridge from a pipe to a central casing positioned over the joint, the article comprising a wraparound fabric sleeve which has end annular regions which are radially heat shrinkable, and a central tubular portion which is radially heat stable, and

(2) applying heat to shrink the ends of the article,^'

whereby after shrinkage the sleeve is longitudinally flexible.

The sleeve comprises a fabric, which itself preferably comprises recoverable, preferably cross-linked, fibres arranged in one of the warp or the weft of the fabric, so that the fabric is recoverable in substantially one direction. Preferably this is the cirαimferential direction when the sleeve is in a wrapped configuration.

The term "fibre" as used herein includes filaments e.g. monofilaments or multifilaments, and also staple fibres, and tapes. The fabrics used in the articles according to the invention preferably employ the heat-shrinkable fibres in the form of filaments, especially monofilaments. In another preferred embodiment the recoverability is highest, especially preferably significantly highest, in the direction of the recoverable fibres.

Cross-linking of the fibres enables the fabric to be heated above their recovery temperature without melting. Higher recovery ratios can be achieved with cross-linked fibres than would be possible with non-cross-linked stretched fibres, and in addition fabric containing cross-linked fibres is not so sensitive to overheating.

As examples, any of the fabrics described in EP-B-0116393, EP-B-0243985, EP-A-0430377, EP-A-0225152, US 4816326, US 4816309, U.K. Patent Application Publication Nos. 2133639. 2133740, 2134334, 2135632, 2135836 and 2139142 can be used in the article and method of the present invention.

Whilst the fabric may be used alone, it is often advantageous for it to be employed in conjunction with an adhesive or sealant or other polymeric material that renders it substantially impervious to fluids, the adhesive, sealant or other material either being applied in situ when the fabric is installed or applied in the factory. Thus, for example, the fabric may be impregnated with a curable adhesive composition, e.g. a curable epoxy composition and especially an optionally b-staged epoxy resin that is cured by means of a polyamide, an anhydride or an isocyanate although other materials may be used such as phenolic resins or isocyanate/phenolic resin. The resin may alternatively be laminated on to the fabric or it may be employed in particulate form as described in U.K. Patent Publication No. 2104800. Alternatively an adhesive such as a hot-melt adhesive and especially an adhesive based on a polyamide or an ethylene-vinyl acetate copolymer may be applied either to the fabric or to the substrate during installation.

Most preferably, however, the heat-recoverable fabric is used in combination with a matrix polymer layer which softens when heated to accommodate recovery of the fabric, to form a single composite structure.

The heat-recoverable fabric is preferably bonded to, more preferably embedded in, the polymer matrix material. At or above the recovery temperature of the fibres, the polymer matrix material should be capable of limited flow under pressure so that it retains the integrity of the composite structure without substantially impeding recovery of the fibres. It preferably has. at the aforesaid temperature, an elongation to break of greater than 50%. most preferably greater than 100%, and a 20% secant modulus of preferably greater than 100%, and a 20% secant modulus of preferably at least 5 X 10^"2 MPa, most preferably at least 10" * MPa, measured at a strain rate of 300%M per minute.

The ability of the matrix material to flow when heated need not necessarily apply after recovery. Thus, for example, the polymer matrix material may eventually cure to a thermoset on heating, although it is preferred that the cure rate is sufficiently slow under the recovery conditions not to affect adversely the above mentioned physical properties of the polymer matrix material during the recovery of the fibres. Thus, for example, the polymer forming the matrix material may contain grafted hydrolysable silane groups which are capable of cross-linking the material subsequently in the presence of moisture. Alternatively the matrix material may include a polymer, preferably a rubber and especially an acrylic rubber, which contains epoxy groups and a room, temperature insoluble curing agent e.g. dicyandiamide. Preferred matrix materials are described in U.K. Patent Application Publication No. 2135632. Thus a particularly preferred sleeve comprises a composite structure of a heat- recoverable fabric and a polymer matrix material characterized in that

(a) the heat-recoverable fabric comprises fibres that will recover when heated, the fibres having a recovery stress Y of at least 5 x lO^-2 MPa at a temperature above their crystalline melting transition temperature; and

(b) the polymer matrix material has an elongation/ temperature profile such that there exists a temperature (T) which is at or above the crystalline melting transition temperature of the fibres at which temperature the polymer matrix material has an elongation to break of greater than 20% and a 20% secant modulus X of at least 10"² MPa (measured at a strain rate of 300% per minutes), and at which temperature the inequality (1) is satisfied:

X (1-R) is less than one.

Y R

wherein R is the mean effective volume fraction of heat-recoverable fibres in the composite structure along a given direction based on the total volume of the composite structure, or relevant portion thereof.

The fabric can be knitted, woven, non-woven, braided, or the like. It is preferred that the fabric is a woven fabric. The fabric can be woven in any suitable pattern, for example, twill, broken twill, satin, sateen, Leno, plain, hop sack, sack and various weave combinations, in single or multiple ply weaves for example two or three ply weaves. The fabric may be knitted if desired, either warp knitted or weft knitted. Where the fabric is a braid the terms "warp" and "weft" are not strictly applicable, but when used in this Specification they can be considered to relate to braids by arbitary selection of fibre directions.

The fabric may contain only heat-recoverable fibres, or as is preferred it may contain both heat -recoverable fibres and non heat- recoverable fibres. It may also comprise conductive fibres which may provide the heating means. The fibres may be arranged in any pattern in accordance with the invention, but it is especially preferred that all the fibres extending in the direction of the recoverable fibres are recoverable, and that all the other fibres are non-heat-recoverable. Where the fabric is woven it preferably has heat-recoverable fibres in one direction and dimensionally heat stable fibres in the other direction so that the fabric as a whole is recoverable in a single direction only. Where the fabric is knitted, if the fabric is made solely from heat recoverable fibres it will be recoverable in two dimensions, but if, as is preferred it is knitted from a heat-stable fibre and a heat-recoverable fibre that is either warp or weft inserted, it will be recoverable in only one direction. The recoverable fibres preferably form part of the fabric itself. The recoverable fibres may instead be additional, and be incorporated, for example, inserted after production of the basic fabric.

Similarly, if conductive fibres are included, they may form part of the fabric itself or be additional, and incorporated, for example inserted after production of the basic fabric.

The fibres are preferably formed from a polymeric heat- recoverable material which recover when heated to over and above their recovery temperature. By "the recovery temperature" of polymeric heat-recoverable materials is meant that temperature at which the recovery of the polymeric material will go substantially to completion. In general, the recovery temperature will be the crystalline melting transition temperature if the polymer is crystalline or the glass transition temperature if the polymer is amorphous.

The heat-recoverable fibres are preferably formed from a polymeric material that imparts good physical properties and, in particular, good creep resistance to the fibres. Olefin polymers such as polyethylene and ethylene copolymers, polyamides. polyesters, acrylic polymers and other polymers may be employed and preferably those that are capable of being cross-linked. A particularly preferred polymeric material for the fibres is based on polyethylene having a density of from 0.94 to 0.97/gms/cc, a weight average molecular weight Mw of from 80 X 10³ to 200 X 10³ and a number average molecular weight Mn of from 15 X 10³ to 30 X 10³.

Preferably the recovery temperature of the fibres is 60°C or more, most preferably from 80°C to 250°C such as, for example, 120 to 150°C. When the fibre is cross-linked by irradiation it is convenient to incorporate the cross-linking step into manufacture of the fibre. The fibre can be extruded, stretched at a temperature below its melting temperature, preferably by an amount of from 800 to 2000%, then subjected to irradiation to effect cross-linking. A less preferred way of making the fibre is to extrude the fibre, irradiate to cross-link, then heat the fibre, preferably to above its melting temperature, stretching the fibre, and then cooling the stretched fibre. High density polyethylene fibres are preferably irradiated with a dose of from about 5 to about 35 megarads, preferably from about 5 to about 25 megarads and in particular from about 7 to about 18 megarads, especially from 10 to about 18 megarads. Usually the gel content of the cross-linked fibre is greater than 20%, preferably greater than 30%, most preferably greater than 40%. In practice, gel contents greater than 90% are not easily achievable. Fibres produced in this way can have a high strength after recovery.

Any suitable material may be used for any non heat-fibres included in the fabric. As examples there may be mentioned glass fibre, polyester.Rayon (Trade Mark), cotton and tinsel metal.

Part of all of the heat recovery fabric may be lined with a bonding layer as aforesaid such as an adhesive for bonding to objects and/or bonding the overlapping longitudinal edges of the article, in the wrapped configuration, to each other. Alternatively bonding may be by fusion. The preferred steps and order in which bonding is carried out is described in respect of the method according to the invention.

Thus another aspect of the invention provides a method of covering an elongate object or bonding two such objects in end to end relationship using a wraparound article according to the first aspect of the invention, the method comprising connecting the heating means to a power supply to heat and to recover those regions into contact with the underlying object. Preferably the article is wrapped around the object so that longitudinal edges overlap and electrical power applied initially to the electrical heating means on the at least one longitudinal edge to bond the edges to each other. Then, the longitudinal seal having been made, heat is preferably applied to the heating means at the ends to complete the seal. In a preferred method for joining two elongate objects or for covering a joint between the two elongate objects, the article is wrapped around a piece of release paper positioned around one of the elongate objects, before heat is applied to the heating means on the longitudinal edge. The underlying pipe or cable therefore acts as a support. The longitudinally sealed article is then preferably slid from the release paper, which is removed. Then electrical power is preferably supplied to the annular heating means at each end of the wrapped article to complete the seal. Where the article is used to join two elongate objects, it is preferably wrapped around a piece of release paper on one of the objects to one side of the joint, before heat is applied to the heating means on the longitudinal edge. The article can then simply be pushed over the joint before the electrical power is supplied to the annular heating means to complete the seal. This is made possible because of the presence of the release paper.

The method according to the invention is particularly applicable where the objects are pipes or cables or joints therebetween, preferably district heating pipes or joints therebetween.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:-

Figure 1 shows a first article according to the invention:

Figures 2 and 3 show sequential steps in the installation of Figure 1: Figures 4 and 5 show other embodiments of article according to the invention

Referring to the drawings Figure 1 shows a first article according to the invention. The article comprises a fabric sleeve 2 which is laminated with a polymer matrix (not shown). The sleeve 2 has longitudinal edges 4 and circumferential edges 6 viewed when the sleeve is in its wrapped configuration as shown. The longitudinal edges 4 can be overlapped in order to hold the wraparound article in a tubular closed configuration. The sleeve also comprises three heating elements 8, 10 and 12 in the form of copper meshes extending along one longitudinal edge and both circumferential or annular edges of the sleeve 2. The mesh heater is also covered on its inside surface with a bonding layer as described in EP-B-0245067, which bonds the mesh heater to the fabric sleeve 2, and in use, will bond the sleeve 2 to the underlying elongate object. In the drawings the mesh heaters are not shown in detail, and the bonding layer is not shown (for simplicity). The article also comprises two elongate electrodes 14 extending along each of the longitudinal edges of sleeve 2, and further elongate electrodes 16 extending around the circumferential edges of the sleeve 2.

The construction of the fabric is such as to provide circumferential shrinkage at the ends only of the sleeve. Thus in two circumferential zones at the ends of the sleeve 2, beneath the mesh heaters 10 and 12, the sleeve comprises heat shrinkable polyethlene fibres extending in the circumferential direction, while in the circumferential direction in the central region between the two end regions of the sleeve 2 the sleeve comprises non-shrinkable polyethylene fibres. In the longitudinal direction throughout, the fabric comprises a mixture of glass fibres and non-shrinkable polyethylene fibres. The effect of this construction is that if electrical current is applied to the mesh heater it causes the shrinkable polyethylene fibres at the end regions in contact with mesh heaters 10 and 12 to shrink, while the central region between the end regions of the sleeve does not shrink. When electrical current is applied to the mesh heater 8 it causes the bonding layer adjacent to that mesh heater to bond to the overlapping edges of the sleeve, although there is no longitudinal shrinkage.

Figures 2 and 3 are sequential steps showing the installation of the article.

In Figure 2 the sleeve 2 is shown wrapped around two district heating pipes 18 and 20 which have been joined together by a weld 21. Each of the district heating pipes 18 and 20 comprises an inner steel pipe 22 and an outer insulation 24. The steel pipes 22 are welded at 21, but the insulation 24 around that region has being bared. It is that region which it is necessary to reinsulate. The sleeve 2 is positioned to one side of the weld 21. It is positioned around a release paper 26 wrapped around the outer insulation 24 of one of the pipes 18. The longitudinal edges of the sleeve 2 are overlapped, and then electrodes 16 are connected to a supply of power so that electrical current flows through the mesh heater 8, thereby heating the bonding layer adjacent that mesh heater 8, and thereby forming a seal between the longitudinal edges of the fabric sleeve 2.

Fabric sleeve 2 is therefore at this stage in the shape of a tubular sleeve, and this is then slid from the release paper 26 over the weld region 21 which is to be reinsulated. The advantage of forming the longitudinal seal to one side of the joint over a release paper 26 is that the pipe to one side of the joint provides a support for the sleeve while the longitudinal seal is made. The longitudinal seal could be made with the sleeve 2 positioned around the bared region around weld 21, but if this were done it would be desirable to include an insert support member while the longitudinal seal were made.

After the sleeve 2 has been slid from the release paper into position over the bared region around weld 21, electrical power needs to be supplied to effect heat shrinkage of the end regions of the sleeve 2. When the sleeve has been slid into position the edges of sleeve 2 overlap the insulation 24 on either side of the bared weld region. Electrodes 16 are then disconnected from the power supply and electrodes 14 connected to opposite terminals on the power supply. This causes electrical current to flow through mesh heaters 10 and 12 at either end of the sleeve 2. This heat effects shrinkage of the recoverable fibres in those regions of the fabric sleeve 2, thereby effecting radial heat shrinkage of the ends of the sleeve 2. Also the heat applied activates the bonding layer adjacent the mesh heaters 10 and 12 in the end regions of the sleeve, allowing the sleeve 2 to form a tight bond to the insulation 24 on either side of the bared weld region 21. Thus the fabric sleeve 2 is sealed to the district heating pipes 18 and 18 on either side of the weld 21. The sealed system is shown clearly in Figure 3.

Figure 4 shows an alternative article according to the present invention. This differs from the article shown in Figure 1 in that the mesh heaters 8, 10 and 12 are replaced by conductive fibres or wires eg. copper wires extending along the longitudinal edge and around the ends of the sleeve 2. These are indicated as 8', 10' and 12'. These conductive wires or fibres may be interwoven into the fabric 2, or they may form part of the fabric 2 itself. Thus in the end regions of the sleeve 2 the fabric comprises polyethylene heat shrinkable fibres, in the circumferential direction and in the central region, between the ends of sleeve 2, polyethlene non-shrinkable fibres are arranged in the circumferential direction. Glass fibres extend in the longitudinal direction throughout. As for the embodiment of figures 1-3 this means that the central region between the two zones at the ends will not be heat recoverable. Similarly along the longitudinal edge of the sleeve where the mesh heater was positioned in the article of Figure 1, the mesh is replaced by conductive wires eg copper wires or by conductive fibres.

This fibre arrangement can be easily manufactured by a weave in which the weft extends longitudinally and the warp extends circumferentlally around the sleeve. For the embodiments of Figure 4 the electrodes 14 and 16 are positioned as in Figure 1. In this case when electrodes 16 are connected to a power supply they cause heating of the copper wire or conductive fibres 8^l along the longitudinal edges of the sleeve 2. When electrodes 14 are connected to the power supply they cause heating of the copper wires or conductive fibres 10', 12' extending around the circumference at the ends of the sleeves. The installation steps of the sleeve of Figures 4 are the same as that for the sleeve of Figure 1 to 3, the longitudinal seal being formed before the circumferential seals.

Figure 5 shows a further embodiment of article according to the invention. The fibre and wire structure of Figure 5 is similar to that of Figure 4, ie it includes heating wires 8^*, 10' and 12' interwoven in the fabric structure along the edges. However in this case where the wires or conductive fibres 8' overlap with the wires or conductive fibres 10' and 12' they make electrical contact with each other (in the embodiment of Figure 4 they do not). This is shown at corners 28 and 30 of Figure 5. Thus in this case, only two electrodes 32 and 34 are provided, being connected to fibres 10' and 12' respectively, at the opposite corners of the wrapped sleeve 2 from corners 28 and 30. When the sleeve is wrapped and power supplied to the electrodes, electrical current therefore flows, say from electrode 32, through conductive wires or fibres 10', then through conductive fibres or wires 8', and finally through conductive fibres 12^*. Thus by this arrangement simultaneous longitudinal and circumferential seals are made.

It is also envisaged that a U-shaped mesh heater could be used in the embodiment of Figure 5 to provide heating means 8', 10' and 12', in place of the conductive fibres or wires.

Claims

1. A wraparound article for bonding to an elongate object such as a pipe or cable or for making, or bonding to, a joint or splice between such articles, the article comprising a layer of a fabric that is heat-recoverable at least in part, which is in the form of a sheet that can be wrapped around the object, and which is provided with electrical heating means in and/or on its inner surface when wrapped, along at least three edges of the sheet, being at least one of the longitudinal edges and both annular edges when the sheet is in its wrapped configuration.

2. An article according to claim 1, wherein at least one of the heating means is at least partially embedded in the surface of the sheet.

3. An article according to claim 1, wherein at least one of the heating means lies on the surface of the fabric layer.

4. An article according to any preceding claim, wherein at least one of the heating means comprises conductive wire or fibres forming part of, or inserted into the interstices of the fabric.

5. An article according to any preceding claim, having a wall thickness in the range 2 to 15mm.

6. An article according to any preceding claim which is heat recoverable only in part.

7. An article according to claim 6, wherein a central tubular region of the sleeve, when in its wrapped configuration is not heat-recoverable.

8. A method of covering an elongate object or bonding two such objects in end to end relationship using a wraparound article according to any preceding claim, the method comprising connecting the heating means to a power supply to heat and to recover those regions into contact with the underlying object.

9. A method according to claim 8, in which the article is wrapped around the object so that longitudinal edges overlap and electrical power applied initially to the electrical heating means on the at least one longitudinal edge to bond the edges to each other.

10. A method according to claim 9. for joining two pipes or cables, or for covering a joint between two pipes, or cables wherein the article is wrapped around a piece of release paper, preferably on one object to one side of the joint before heat is applied to the heating means on the longitudinal edge.

11. A method according to claims 9 or 10, wherein electrical power is then supplied to the annular heating means at each end of the wrapped article.

12. A method according to claim 11, when dependent on claim 10, wherein the article is pushed over the joint before the electrical power is supplied to the annular heating means.

13. A method according to any of claims 10 to 12, wherein the objects are pipes or cables or joints therebetween, preferably district heating pipes or a joint therebetween.

14. A method according to any of claims 10 to 13, wherein the thermal mass of the article is less than the thermal mass of that part of the pipe which is heated by the heating means.

15. An method according to any preceding claim, wherein the wall thickness of the article is less than the wall thickness of the pipe, preferably less than 0.75 times the thickness of the pipe.

16. An article or method according to any preceding claim, which is at least partially heat shrinkable, preferably radially heat shrinkable when the article is in its wrapped configuration.

17. A method of reinsulating a bared joint between district heating pipes each of which comprises a central pipe having an outer insulation, the method comprising

( 1 ) positioning a wraparound article either to bridge the insulation on either side of the pipes or to bridge from a pipe to a central casing positioned over the joint the article comprising a wraparound fabric sleeve which has end annular regions which are radially heat shrinkable, and a central tubular portion which is radially heat stable, and

(2) applying heat to shrink the ends of the article,

whereby after shrinkage the sleeve is longitudinally flexible.