CA1325514C - Method of joining articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of joining articles together with the aid of conductive polymer components. The conductive polymer components are neither heat-recoverable themselves nor fixed to heat-recoverable articles and preferably comprise a sintered ultra-high molecular weight polyethylene which undergoes a dimensional change when heat is generated within it. Specific components useful in joining or modifying articles such as pipes or cables are sealing strips, pressure access devices, and conductive polymer tapes.

Description

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BACKGROUND OF THE INVENTION
Eield of the Invention This invention relates to methods of joining articles together with the aid of conductive polymer components, and to novel c~nductive polymer components.
Background of the invention - Conductive polymers are well known. They comprise a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black. Conductive polymers have been widely used in electrical heaters, including heaters which are in the form of heat-recoverable articles or which are secured to heat-recoverable articles so that, by powering the heater, the article can be caused to recover. Typically, the recovery of the article results in joining, repairing, reinforcing or otherwise modifying one or more substrates around or agalnst which the article recovers. Recently, it has been shown that conductive polymers which retain substantial strength above their melting point, especially sintered polymers such as ultra high molecular weight polyethylene (UHMWPE), are particularly useful for modifying pipes composed of organic polymers (plastic pipes). Such modification may be required if, e.g., a pressure access device is to be incorporated into a pipe or supply line.
Reference may be made for example to U.S. Patent Nos.
3,987,276, 4,085,286, 4,177,376, 4,177,446, 4,421,582, 4,455,482, 4,570,055, 4,575,618, 4,686,071, U.K. Patent Nos. 1,265,194, 1,449,539, and 2,124,439, German Patent Application No. 3,442 ,674, European Patent Publication Nos. 153,199, 157,640, 197,759, 220,003, 231,068 and 251,482, and Canadlan patent applicatlon No. 535,079 filed April 21, 1987.
SU~RY OF THF INVENTION
We have now discovered that conductive polymer components can be used to join together articles even when B
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-2- ~ 32~14 t the components are neither heat-recoverable themselves nor rixed to heat-recoverable articles.
In one aspect, the present invention provides a method of joining a first article to a second article which comprises --(1) placing the first article adjacent the second article; and ~2) generating heat within a conductive polymer component which : -(a) is not heat-recoverable;
(b) is in the form of ~ tape having a ratio of external surface area to volume of at least 40 inch2/inch3;

(c) is placed between the first and second articles, or which forms at least part of the first article or at least part of the second article; and (d) expands when the heat is generated within it; :
the heat being generated within the conductive polymer component by passing electrical current through it, said electrical current not passing through either the first article or the second article;

and the first and second articles and the conductive polymer component being such that, or belng subject to physical restraint such that, the expansion of the conductive polymer component when the heat is generated wlthin it, results in the first article being joined to the second article through the conductive polymer component.
The conductive polymer component can be part or all of .
the first article and/or the second article, or it can be a qeparate component. Preferably at least one of, particularly each of, the fir~t and second articles ~as well as the conductive polymer component) is not heat- : -D : : :
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recoverable, by which we mean that if the article, or component, on its own, is heated to any temperature which is employed in the defined method and is then cooled, it does not undergo any substantial change in any dimension;
for example no dimension change~ by more than 10%. The dimensional change which the conductive polymer component undergoes is generally expansion, but other forms of dimensional change are not excluded. The term "joining" is used herein to include an improvement in an existing join, for example by fusing together surfaces which are already maintained adjacent to each other in some other way. The generation of heat within the conductive polymer is usually effected by passing current through it, but other methods can be used, for example induction heating, and ultra-sonic heating.
In another aspect, the present invention provides an article which is suitable for use as the sealing article in the method just described, which is not hèat-recoverable and which comprises (a) an element which is in the form of a tape having a ratio of external surface area to volume of at least 40 inch2/inch3 and which is composed of a conductive polymer which ~i) comprises a sintered polymeric component compoRed of ultra-high molecular weight polyethylene and, distributed in the polymeric component, a conductive particulate filler, and (il) increases in volume by at least 10% when it ls heated from 23C to the meltlng point of at least part of the polymeric component, and ~b) electrodes which can be connected to a source of electrical power and which, when so connected, cause current to flow through the conductlve B polymer element.

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Detailed Description of the Inv~ntion The conductive polymer composition preferably comprises polyethylene, particularly ultra-high molecular weight polyethylene (UHMW~'E) together with a particulate conductive filler, especially carbon black. In general, the composition preferably consists essentially of (a) a matrix consisting essentially of organic polymer particles which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate filler, preferably carbon black which is dispersed in said matrix but which is -present substantially only at or near the boundaries of the coalesced particles.
Suitable formulations are found in European Patent Publication No. 157,640. Although UHMWPE is preferred, other polymers that may be sintered include polytetrafluoroethylene, polyphenylene sulfide, and polyimides.
Such conductive polymers can be prepared by sintering a dry blend of the polymer particles and the filler. A
typical procesis 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 exceesively, followed by cooling under pres~ure. The slnterlng may be conducted in a mold to produce an article of a desired shape, or a sintered block or rod ~frequently produced by ram extrusion) may be skived on a lathe or otherwise machined or skived on a lathe to produce an article with the desired dimensions. The surface shape may be smooth or rough and the texture of the article may be changed, e.g. grooved by use of an appropriate qkivlng blade, if it is desired to increase the surface area, e.g. for heat transfer or bon,ding purposes. Irregular or rlbbed surfaces are preferred for strong welds.
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The conductive polymers suitable for use in this invention generally have a low electrical resistivity, generally below 1000 ohm-cm, particularly below 100 ohm-cm, especially below 10 ohm-cm, for example, in the range of about 0.5 to 10 ohm-cm. The desired resistivity depends upon the power source, which may be for example DC up to about 48 volts or AC of up to about 120 or 240 volts.
Sintering produces low electrical resistivity at a lower conductive filler loading than for a melt-blended product.
Thus the preferred sintered compositions for use in this invention contain less than 9%, preferably less than 7%, particularly 2-6%, by volume of carbon black or other conductive filler. Particularly preferred carbon blacks are those sold by Akzo Chemie under the tradename Ketjenblack EC and by Degussa under the tradename Printex XE-~. The low levels of carbon black help to maintain the desired physical properties of the polymer such as flexibility, good elongation, high tensile strength, good notch and impact resistance, and good chemical resistance. The conductive polymer may optionally be crosslinked.
The preferred polymers are those which maintain a relatively high viscosity above their softening or melting point; i.e. those polymers which, at a temperature 50C
above its softening point, have a melt flow index of less ;
than 0.3 g/10 min., particularly less than 0.1 g/10 min., especially less than 0.05 g/10 min. at a loading of 3 kg, and a melt flow index less than 3.0 g/10 min., particularly less than 1.0 g/10 min., especially less than 0.1 g/10 min.
at a loading of 15 kg. UHMWPE, which has a molecular weight preferably greater than l.S million, particularly greater than 3.0 million, and frequently aq high as 4 to about 6 million, is a most preferred polymer in this invention, although very high molecular weight polyethylene (VHMWPE), which has a molecular weight of about 600,000 to 1.5 million, may al~o be uqed after melt-blending.

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The conductive polymers useful in the present invention preferably exhibit ZTC properties; i.e., in the temperature range of operation, e.g. 23-250C, the resistivity changes by a factor of less than 5, preferably less than 2, especially less than 1.
For adequate bonding to an article, the conductive -polymer preferably can achieve a bond-line temperature of at least 135C, preferably 150C, more preferably at least 200C, especially at least 250C. The desired temperature is preferably attained under normal ambient conditions in less than 30 minutes, preferably less than 15 minutes, particularly less than 5 minutes, especially 0.5 to 3 minutes, when using a power source of less than 50 volts.
Such rapid heating and high temperatures can result in short installation times and minimal damage to the conductive polymer, the substrate or surrounding material.
In general it has previously been recognized that the maximum power density which such conductive polymers were capable of sustaining was only up to about 25 watts/cm3 before damage to the polymer occurred.
The conductive polymer component ls preferably such that the power load which can be placed upon the conductive polymer (in terms of watt density), when the sealing article ls being used ln the method of the invention, can be increased to at least 50 watts/cm3, preferably at least 100 watts/cm3, and in some cases up to 200 watts/cm3 or higher. Tapes which can withstand such high power loadings are extremely useful for providing high amounts of heat in confined and limited spaces in a short amount of time using low overall power input. The term "tape" is used herein to mean material of any configuration (e.g. a strip, rod, or plece of oval or dog-bone cross-section) of the polymer whlch has a sufficiently high ratio of external heat-disslpating surface area to polymer volume (in the heat-producing portion) to provide a ratio of external surface area to polymer volume which is at least about 40 in2/in3, preferably at least about 130 in2/in3, and which can be D : ~:
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132~14 about 200in2/in3 or higher. In its most preferred form, the tape is of about 1 to 3 cm in width and up to 2 mm in thickness with longitudinal electrodes (e.g. metal strips, braid, or silver paint or a combination of these) applied along (or near) the edges of one or both surfaces. The surfaces of the tape may be covered with a dielectric layer.
UHMWPE and VHMWPE are preferred polymers for use in these tapes due to their high ~olume expansion at elevated temperatures, i.e. at least 10% by volume increase, sometimes as much as 30~v by volume at temperatures above the melting point. UHMWPE conductive polymers are preferred for pressure applications because above the melting temperature they maintain a solid elastomeric form without flowing. VHMWPE conductive polymers may be preferred when it is necessary to fill voids between the tape and the substrate. If the tape has slits or apertures cut through ~
it, and the tape melts a surface of the substrate, molten -substrate may then flow through the holes, slits, or apertures and then resolidify. Thus the tape may become fixed to the substrate without welding. The materials may thus be incompatible.
The tape may be used to seal the edges of a wrap-around sheet, to butt-weld the ends of two pipes, or to seal a coupling sleeve or pipe onto a pipe by making a lap ~oint. Alternatlvely, the tape may be wrapped around a substrate. It may be folded lengthwise and inserted into the recesses of a sealing strip to be used for heating and welding or bondlng, allowing the seallng strip to be made from a wider range of materials (e.g. non-melt-processable materials) than lf the sealing strip itself is electrically heatable.
Brlef Descri~tlon ~f the nra~ing~

The invention is further illustrated by the -accompanylng drawings, in whlch: ~
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132~514 Figure 1 shows a perspective view of a thin, flexible conductive polymer tape of substantially rectangular cross-section;
Figure 2 is a cross-sectional view illustrating a pipe lap type joint made with a coupling sleeve;
Figure 3 is a cross-sectional view illustrating a butt weld type pipe joint using a conductive polymer tape;
Figure 4 shows a conductive polymer tape inserted into the recesses of a sealing strip; and Figure 5 shows various applications for a conductive polymer tape on a cable splice.
Detailed Description of the Drawinas Figure 1 illustrates a conductive polymer tape wherein the tape 20 is printed with conductive ink electrodes 21 for good electrical contact with the conductive polymer, and electrode (or buss) wires 22 are then placed on printed ink electrodes 21. Optional dielectric layer 23 may be laminated on the tape to provide insulation and to assure good contact of the wires 22 with the lnk electrodes 21.
Optlonally, a second dielectric layer may be laminated tc the other side of the tape and the two films sealed along the edges to encapsulate the tape.
Figure 2 illustrates a lap pipe joint between pipes 31 and 32 using coupling 33. Coupling 33 has grooves 34 cut in the interior surace thereof to form a recess for containing tape 20. Electrode wires 22 can extend from the conductive polymer tape 20 through the wall of coupling 33 at any desired point. In Figure 2, the left side of the coupling on pipe 32 i9 shown before powering the tape and the right side on pipe 31 (the fused area) is shown after powerlng the tape.
Figure 3 illustrates a butt type ~olnt between pipes 41 and 42 wherein conductive polymer tape 21 is placed over B

1 3 2 ~ ~ 1 Ll seam 44 and compression means 43 which is a silicone elastomeric band is used to compress the tape against seam 44 to maintain good thermal contact and to hold pressure on the tape when the polymer and conductive polymer tape expand upon heating.
Figure 4 shows sealing strip 10 which has pieces of conductive polymer tape 20 inserted into recesses 11 for use in heating and welding to edge portions 2 of the wrap~
around sleeve. ~-Figure 5 shows various uses for conductive polymer tape 20. A splice case 50 comprises half-shells surrounding cables 5. Cable jacket repair is shown at 51 as a simple wrap of the tape. A seal between a cable or pipe and a duct is shown at 52. A seal between a cable and a splice case end plate is shown at 53, and a seal between end plate halves is shown at 54. A longitudinal seal between the half-shells is shown at 55 and a seal between a cable and an end cap is shown at 56.
De~crlptlon of Embodiments of the I~vention This invention is illustrated by the following examples and the above drawings.
Example 1 A conductive polymer tape 20 as shown in Figure 1 was prepared by dry blending in a high speed blender 95 parts by volume of ultra high molecular weight polyethylene powder, UHMWPE, (Hostalen~ GUR-413, available from American Hoechst), having a molecular weight of about 4.0 million and an average particle size of about 0.1 mm, and 5 parts by volume of carbon black (Ket~enblack~ EC300 DJ, available from Akzo Chemie). The mixture was extruded through a ram extruder heated to 170C at a rate of 5 feet/minute and a pressure of 3000 p8i to produce a sintered rod 8 lnches (20.3 cm) in diameter. The rod was skived to produce a 0.015 inch by 0.875 inch (0.038 by 2.223 cm) tape.
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Using a silver-filled ink (Electrodag~ 504, available from Acheson Colloids), two electrode strips each 0.1875 inch (0.476 cm) wide were painted on one side of the tape 0.5 inch (1.27 cm) apart. Electrode or buss wires were prepared by flattening 30 AWG silver-coated copper wire to give a cross-section 0.003 by 0.013 inch (0.008 by 0.033 cm~, and then braiding 12 flattened wires together. Using a roll laminator operating at 1 ft/min ~30.5 cm/min), the electrode/buss wires were laid onto the ink electrode strips and a 0.012 inch (0.030 cm) layer of clear polyethylene film was laminated on the top surface of the tape over the electrode strips and buss wires and a second ~ -layer was laminated on the bottom surface of the tape and heat sealed at the edges to produce a fully sealed and insulated conductive polymer tape. This tape can be cut to length and the top layer of polyethylene film peeled back from e~ther end to expose the electrode wires for powering the tape.
~X~m~le 2 Two plastic pipes were connected in the following way.
A 3,5 inch (8.9 cm) diameter orange polyethylene coupler was injection molded. Two g~ooves 0.875 inch (2.223 cm) wide and 2 inches (5.08 cm) apart were machined on the inner circumference of the coupler. Two copper foil strips ~each 0.002 by 0.188 inch/0.005 by 0.476 cm) with adhesive on one side to hold them ln place in the grooves were placed in the grooves and the tape de~cribed in Example 1, but wlthout the buss wires or the polyethylene film layers, was placed on top of the copper strips so that the ink electrode~ on the tape would contact the copper strips.
~sing a mandrel to hold the tape in place the tape was powered at 30V/5.5A for 1 minute to melt and bond the tape to the coupler.
To connect the coupler to a pipe, the pipe was inserted into the coupler and the tape powered at 30V/5.5A
for 2 minutes. The melting of the tape to the pipe and the D
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coupler was observed through a small hole drilled through the wall of the coupler over the tape. At the point of melting the color visible through the hole changed from black to orange. These conditions were sufficient to produce an excellent void-free bond. The expansion of the conductive polymer tape and the melting of the surfaces of the pipe and coupling produces the void-free bond.

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Claims (19)

1. A method of joining a first article to a second article which comprises (1) placing the first article adjacent the second article; and (2) generating heat within a sealing article comprising a conductive polymer component which (a) is not heat-recoverable;

(b) is in the form of a tape having a ratio of external surface area to volume of at least 40 inch2/inch3;

(c) is placed between the first and second articles, and (d) expands when the heat is generated within it;

the heat being generated within the conductive polymer component by passing electrical current through it; said electrical current not passing through either the first article or the second article; and the first and second articles and the conductive polymer component being such that, or being subject to physical restraint such that, the expansion of the conductive polymer component, when the heat is generated within it, results in the first article being joined to the second article through the conductive polymer component.

2. A method according to Claim 1 wherein neither the first article nor the second article is heat-recoverable.

3. A method according to Claim 1 or 2 wherein the sealing article is not secured to the first article or to the second article before heat is generated therein.

4. A method according to Claim 1 or 2 wherein the sealing article forms part of the first or second article.

5. A method according to Claim 1 or 2 wherein at least one of the first and second articles is a hollow conduit composed of an insulating polymeric composition.

6. A method according to Claim 1 wherein at least one of the first and second articles is composed of an electrically insulating thermoplastic polymeric composition, and the heat generated within the conductive polymer causes the thermoplastic polymer to melt, and thus to join the first and second articles.

7. A method according to Claim 6 wherein each of the first and second articles becomes fused to the sealing article.

8. A method according to Claim 1 wherein the first article is a hollow conduit and the second article (i) is in the form of a saddle which becomes joined to the hollow conduit around a part only of the circumference of the conduit and (ii) comprises means for receiving a tap which provides a port for the passage of fluid into or out of the conduit.

9. A method according to Claim 8 wherein the hollow conduit is a telephone cable.

10. A method according to any Claim 1 or 2 wherein two first articles, each of which is a hollow conduit having an exterior surface portion which is composed of an electrically insulating thermoplastic polymeric composition, are connected in line by joining each of them to a second article (i) which is in the form of a cylinder, and (ii) into which the ends of the hollow conduits are placed, said second article, while the heat is being generated within the conductive polymer component, being surrounded by a restraint member.

11. A method according to Claim 1 wherein the conductive polymer comprises (a) a polymeric component which consists of particles of an organic polymer which have been sintered together so that the particles have coalesced without completely losing their identity, and (b) a particulate conductive filler which is present only at or near the boundaries of the coalesced particles.

12. A method according to Claim 11 wherein the organic polymer particles are composed of ultra high molecular weight polyethylene.

13. A method according to Claim 12 wherein each of the articles is composed of polyethylene.

14. A method according to Claim 1 or 2 wherein the sealing article comprises dielectric layers laminated to each surface of the conductive polymer component.

15. A method according to Claim 1 wherein that conductive polymer component has an external surface area to volume of at least 130 inch2/inch3.

16. A method according to Claim 15 wherein said ratio is at least 200 inch2/inch3.

17. An assembly comprising first and second articles which have been joined together by a method as claimed in Claim 1.

18. An article which is suitable for use as the sealing article in a method as claimed in Claim 1, which is not heat-recoverable, and which comprises:

(a) an element which is in the form of a tape having a ratio of external surface area to volume of at least 40 inch2/inch3 and which (i) comprises a sintered polymeric component composed of ultra high molecular weight polyethylene and, distributed in the polymeric component, a conductive particulate filler, and (ii) increases in volume by at least 10% when it is heated from 23°C to the melting point of at least part of the polymeric component, and (b) electrodes which can be connected to a source of electrical power and which, when so connected, cause current to flow through the conductive polymer element.

19. An article according to Claim 18 which further includes dielectric layers which are laminated to the surfaces of the conductive polymer element.