HEAT RECOVERABLE ARTICLE

Abstract: A conductive polymeric heat recoverable article is provided with electrodes (10) on its surface. The electrodes (10) can be connected to a source of electrical power to cause current to flow through the article to heat and hence to recover the article. However the arrangement is such that although the regions of the article immediately beneath the electrodes (10) may comprise heat recoverable material, those regions are not heated to a temperature sufficient to recover them. Therefore although the region of the article between the electrodes (2', 2'') recovers, the regions immediately beneath the electrodes (10) do not. So the electrodes (10) remain in contact with the article.

Description: Description Heat Recoverable Article This invention relates to an assembly and to a method for passing an electrical current through a heat recoverable article. 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, including those that comprise one or more members of a conductive polymer i.e. an organic polymer having sufficient finely divided electrically conductive material disposed therein to render it electrically conductive, are known. They may be used to provide sealing, insulating or protective coatings on substrates, for example, electrical joints and terminations. Reference may be made, for example to Pat Nos US 2,027,962, US 3,086,142, US 3,770,556, EP-0307207 and EP-0307205. Heat recoverable articles are typically recovered towards a; original configuration by means of a torch. However it is also known (see British Pat. No. 1,265, 194, US Pat No. 4,421,582, EP Pat No. 0343757 and US Pat No. 4,570,055) to make heat recoverable articles out of conductive polymers and to cause recovery of the article by passing an electric current through the conductive polymer, thus raising the temperature of the article to the recovery temperature. Current may be supplied to the ends or intermediate points of the article by any suitable method. For example, alligator clips or other conventional clamps or electrodes may be used. One problem that may be encountered is maintaining contact with the heat recoverable article during recovery. For example for a tubular. radially heat shrinkable sleeve, an annular electrode applied over the outer surface of the article may lose contact with the shrinkable sleeve part way through the shrinking process so that shrinkage is halted prematurely. Attempts have been made to overcome this problem. For example, US 4,570,055 describes an assembly comprising a conductive tubular article and two electrodes which can change their size and shape to follow the recovery of the article, e.g. they may be spring electrodes to follow the radial heat shrinkage of the article. * Another method of overcoming the problem of the electrode losing contact with the heat recoverable article during recovery, is to arrange for that part of the article which is in contact with the electrode not to recover at all. even though other parts of the article do recover One way of achieving this is described in EP-A-0307206. This describes a heat recoverable conductive polymeric tubular article in which electrodes are fixed to heat-stable support elements on the article, e.g. elongate flanges extending along the article. The flanges are not expanded or are only slightly expanded, so that they are not heat-recoverable. We have discovered another way of overcoming the problem of the electrode losing contact with the heat recoverable article. This is achieved by an arrangement in which, although the region in contact with the article may be heat recoverable, the contact region is not heated to a temperature sufficiently high to cause recovery of that region of the element. Thus a first aspect of the invention provides an assembly comprising (a) a heat recoverable electrically conductive element having a tubular shape, and (b) at least two annular electrodes which can be positioned in contact with the element so that when connected to a power supply electrical current flows through the element in a direction parallel to the length of the article thereby generating sufficient heat to cause the part of the element between the electrodes to recover, the assembly being arranged such that when the electrical current flows, the region of the element that is in contact with the electrode is not heated to a temperature sufficient to cause recovery of that region of the element. A second aspect of the invention provides a method of bonding to an elongate object such as a pipe or cable or a joint or splice therebetween using an assembly according to the first aspect of the invention, the method comprising (i) positioning the element around the object; (ii) positioning the electrodes on the element, (iii) connecting the electrodes to a power supply, so that electrical current flows in a direction parallel to the length of the article causing that part of the element that is between the electrodes to heat and to recover, but not heating that region of the element that is in contact with the electrode to a temperature sufficient to cause recovery of that region of the element. Since the region of the element that is in immediate contact with the electrodes is not heated to a temperature sufficient to cause recovery, it does not recover out of contact from the electrodes . Thus the problem of loss of contact of the electrodes during recovery is avoided. The invention involves a heat recoverable element. By this it is meant that at least part, preferably all, the element is recoverable. By recoverable is meant that the article will recover if heated to a sufficiently high temperature. Thus although the region of the element that is in contact with the electrodes may be heat recoverable (and typically is. for ease of manufacture), it is not actually recovered since the temperature of that region does not ever reach the recovery temperature. Those parts of the article that are heat recoverable are preferably made heat recoverable by use of an expansion step as known in the art. Preferably the tubular element is radially heat shrinkable, and the electrodes are positioned on the outer surface of the element. Thus the region of the element in contact with the electrodes is the region underlying the electrodes and coterminous therewith. The electrodes, at least when positioned, are annular in shape. Therefore the region of the element in contact with the electrodes is the annular region immediately beneath the electrodes. Preferably the electrodes are positioned towards opposite ends of the tubular element so that the end regions of the element do not recover, and the intermediate region between the ends does recover. Different arrangements may be provided so that the region of the element that is in contact with the electrode is not heated to a temperature sufficient to cause recovery. This can be achieved, for example, by making the wall thickness of the region of the element that is in contact with the electrode (hereinafter the "contact region") thicker than the wall thickness of the region of the element that is not in contact with the electrode (hereinafter the "non-contact region). The wall thickness in each of the contact region and the non-contact regions may or may not be uniform along the length or around their circumference of those regions. Preferably the wall thickness along most, preferably all, of the length of the contact region is greater than the wall thickness along most preferably all, of the length of the non-contact region, preferably 1.2, especially 1.4, more preferably 1.5,1.6, or even 2 times as great. Generalising, since the wall thickness in each region may not be uniform around the circumference of each region, at least one, preferably any cross-section through the region of the element that is in contact with at least one of the electrodes (the contact region) is larger than preferably 1.2, more preferably 1.4, especially preferably 1.5, 1.6 or even 2 times the area of at least one, preferably any, cross-section through the region of the element that is between the said at least two electrodes (the non-contact region). In another embodiment at least one of the electrodes is of sufficient thermal mass to act as a heat sink, thereby substantially preventing the said region of the element that is in immediate contact with the electrode being heated to a temperature sufficient to cause recovery of that region. Preferably the electrode comprises a metal. Where the element is heat shrinkable the electrodes may be provided on a backing sheet which can be wrapped around the outer surface of the element. This can optionally be removed after electrical current has been supplied to the element. The backing sheet must, of course, comprise a material which allows electrical current to pass from the electrode to the heat recoverable element. Where reference is made to "the region of the element that is in contact with the electrodes" it does not mean that there has to be direct contact between the electrode and the element. What is meant is that region of the element which, if it did recover, would cause electrical contact between the electrode and the element to be broken. This is illustrated by the embodiment, within the invention, in which electrodes are provided on a backing sheet and positioned on the outer surface of a radially shrinkable element. In this case "the region of the element that is in contact with the electrode" is the annular region immediately underlying the annular electrodes (and having an outer surface coterminous with the inner surface area of the annular electrodes) but separated therefrom by the backing sheet. The assembly according to the invention may be part of a larger tubular article, which has a heat recoverable element according to the invention at one or both ends. It may have a non-recoverable, or heat-stable section between the two end heat-recoverable elements. Such a larger article may be used for example to bond to pipes or cables or joints therebetween. The larger article is particularly suitable for covering a joint between district heating pipes. The heat recoverable element may be lined with a layer of material which can bond to an elongate object positioned within the heat recoverable element. 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. In general, examples of materials that can be used as the bonding layer are described in EP-B-0245067 (B129). Specifically, 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 ethylene vinyl-acetate copolymers, blended with hydrocarbon waxes and optionally butyl rubber. 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. Polymeric materials that may be used for a fusion bonding layer include, for example polyethylene, polypropylene, polybutene. copolymers of ethylene, propylene, butene and hexene. copolymers 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. Preferably the heat recoverable element of the invention comprises a conductive polymer, that is an organic polymer and, dispersed in the polymer, a particulate conductive filler. Preferred materials are those described in EP-0307207 (RK365) and EP-0307205 (DK005) the disclosures of which are incorporated herein by reference. Typically such materials have resistivities at 25°C of less than 25 ohmcm. The preferred dimensions of the element depends on its intended use. Typically the diameter of the heat recoverable element may be in the range 90 mm to 560 mm, and the wall thickness in the range 3 mm to 10 mm, and the length in the range 250 mm to 750 mm Where the recoverable element forms one or both ends of a larger tubular article the larger article may typically have a length in the region of 0.2 to 0.7 metres Where the heat recoverable element forms part of a larger tubular article the remainder of the article may or may not also be heat recoverable. However if the electrodes are only positioned at either end of the said element(s) then electrical current will not flow through the remainder of the article so only the elemental will actually recover. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein Figures 1 and 2 show an assembly according to the invention, before and after the heat recoverable elements have been rendered heat recoverable. Figure 3 shows the assembly of Figure 2 after the elements have been recovered onto an elongate object. Figures 4 and 5 show second assembly according to the invention respectively before and after the heat recoverable elements have been rendered heat recoverable; Figure 6 shows the assembly of Figure 5, with electrodes installed; and Figure 7 shows the assembly of Figure 6 after the heat recoverable elements have been recovered onto an elongate object. Referring now to the drawings, Figure 1 shows a tubular article comprising end regions 2, a central region 4, and intermediate tapering regions 5, the end regions 2 being of smaller cross-sectional area than the intermediate region 4. The article has an increased wall thickness, that is an increased cross sectional area at either end of the end regions 2. This is illustrated by reference number 6 in the drawings. The material of the entire article comprises a conductive polymer material. The article of Figure 1 is made by moulding. Figure 2 shows the article of Figure 1 after the end regions 2 (now designated 2') have been expanded (now referenced 2'). This makes them heat recoverable, in particular radially heat shrinkable. The presence of tapered section 5 in the moulded part facilitates the expansion process. Thus in Figure 2 only the end regions are heat recoverable. The increased wall thickness of regions 6 is not significantly affected by the expansion, although, of course, their internal diameter is increased by the expansion. Silver paint electrodes 10 are shown painted on the regions 6. An electrical power supply may be connected by e.g. alligator clips to the electrodes 10 and electrical current supplied sufficient to heat the end regions 2 to effect heat recovery. During recovery the thicker walls in regions 6 act as a heat sink so that the regions 6 are not heated to a temperature sufficient to effect recovery of regions 6, although they allow passage of electrical current sufficient to heat and to recover the tubular section between the thick walled regions 6. Figure 3 shows the article after recovery onto an elongate pipe 12. It will be seen that regions 6 have not recovered. Hence there is no risk of loss of contact between the electrode 10 and the conductive polymer region 6 below. The region between the electrodes 10 has recovered into contact with the pipe (now indicated as 2"). Figure 4 shows another tubular article according to the invention. It has end sections 22, a central section 24 and tapering sections 25, the end section 22 being of smaller cross-sectional area than central section 24. In this case there is no area of increased wall thickness corresponding to region 6 in the article of Figures 1 to 3. Again the article comprises a conductive polymer material, and is made by moulding. Figure 5 shows the article of Figure 4 after end regions 22 have expanded to render them radially heat shrinkable (now indicated as 22'). Tapered regions 25 facilitate expansion. Figure 6 shows two pairs of metal braid electrodes 26 provided on backing sheets 28 which are wrapped around respective end sections 22' of the article of Figure 5. The sheet 28 allows electrical current to flow from the electrode 26 to the conductive polymer region 30 immediately beneath it. The metal electrodes 26 have a sufficient heat mass to act as a heat sink to prevent recovery of the conductive polymer region immediately beneath them. The article is positioned around pipe 32, but not yet recovered. Electrodes 26 are then connected to an electrical power source and powered to effect recovery in a manner similar to that described for Figures 1 to 3. Figure 7 show the assembly of Figures 4 and 5 after recovery onto a pipe 32, and after removal of the electrodes 28 and backing sheet 28. It will be seen that the regions 30 of the element immediately beneath the electrodes 26 has not recovered. Thus again the problem of loss of contact during recovery is avoided. In the embodiments above, a lining of fusion bondable material of the type described hereinbefore, is included, lining the end regions 2 and 22. This is not shown in the drawings for simplicity. It is activated by the heat which effects recovery. It enhances or forms the bond to the pipes 12, 32.

Claims: CLAIMS 1. An assembly comprising (a) a heat recoverable element having a tubular shape, and (b) at least two annular electrodes which can be positioned in contact with the element so that when connected to a power supply electrical current flows in a direction parallel to the length of the article thereby generating sufficient heat to cause the part of the element between the electrodes to recover, the assembly being arranged such that when the electrical current flows, the region of the element that is in contact with the electrode is not heated to a temperature sufficient to cause recovery of that region of the element. 2. An assembly according to claim 1, wherein at least one, preferably any, cross section through the region of the element that is in contact with at least one of the electrodes (the contact region) has an area that is larger than the area of at least one, preferably any, cross-section through the region of the element that is between the said at least two electrodes. 3. An assembly according to claim 1 or 2, wherein at least one of the electrodes is of sufficient thermal mass to act as a heat sink, thereby substantially preventing the said region of the element that is in immediate contact with the electrode from being heated to a temperature sufficient to cause recovery of that region. 4. An assembly according to any preceding claim wherein two elongate electrodes are provided on a backing sheet which can be wrapped around the outer surface of the element so that the elongate electrodes form the annular electrodes around the element, and optionally removed after electrical current has been supplied to the element. 5. An assembly according to any preceding claim wherein the heat recoverable element is part of a larger tubular article. 6. An assembly according to any preceding claim 5, wherein the heat recoverable element forms one end of the larger article. 7. An assembly according to claim 6, wherein two heat recoverable elements are provided, which form both ends of the larger article. 8. An assembly according to any preceding claim in which at least one of the heat recoverable elements is heat shrinkable. 9. An assembly according to claim 8, for bonding to an elongate object positioned within the tubular element, wherein the heat recoverable element is lined with a layer of material which can bond that part of the element which recovers to the underlying elongate object. 10. A method of bonding to an elongate object such as a pipe or cable or a joint or splice therebetween using an assembly according to any preceding claim, the method comprising (i) positioning the element around the object; (ii) positioning the electrodes on the element, (iii) connecting the electrodes to a power supply, so that electrical current flows in a direction parallel to the length of the article causing that part of the element that is between the electrodes to heat and to recover, but not heating that region of the element that is in contact with the electrode to a temperature sufficient to cause recovery of that region of the element. 11. A method according to claim 10 using an assembly according to claim 9, wherein the laver of material is heat-activatable and is activated by the heat generated by the passage of electrical current.

Inventors: DOUCET JOZEF ALBERT GUSTAFF, DOUCET, JOZEF, ALBERT, GUSTAFF
Applicants: RAYCHEM LTD, RAYCHEM SA NV, N.V. RAYCHEM S.A., RAYCHEM LIMITED
Application year: 1992
Main classification: B29C65
IPC codes: B29C61, B29C65