A PROCESS FOR THE JOINING OF CROSSLINKED POLYOLEFIN BODIES

Abstract: The present invention provides a method of joining a body (1) of crosslinked polyolefin to another body (3) formed of crosslinked polyolefin comprising the steps of heating the surfaces of the crosslinked polyolefin bodies (1, 3) to be joined, to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin, and bringing together said heated surfaces of the bodies (1, 3) under pressure, and allowing the surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70 %, preferably less than 68%, and most preferably no greater than 65 %.

Description: A PROCESS FOR THE JOINING OF CROSSLINKED POLYOLEFIN BODIES The present invention relates to a method of joining crosslinked polyolefin bodies, such as pipes and pipe fittings. There are many applications, for example the construction of pipelines for carrying mains water or gas, which pipelines typically are buried beneath the ground, where increased durability and strength are desirable properties, and such properties can be imparted to the polyolefin by means of crosslinking. Crosslinking is a process whereby individual polymer molecules are joined together to form larger molecules, thus modifying the molecular weight distribution of the polymer mass and the polymer properties. Crosslinking may be brought about, for example, by the use of peroxide or silane crosslinking agents, by electron beam crosslinking or by azo crosslinking. Taken to its ultimate conclusion the crosslinking process would result in all the molecules within the mass being linked to one another in a network, forming one large molecule. In all practical embodiments this is not achieved and crosslinked polyolefins consist of varying numbers of molecules large and small, that may be joined together, as in a network structure, or may be entangled within or without interpenetrating formations including crystalline structures. Although at one extreme crosslinking may be said to occur when simply one molecule has been joined to another, the effect of such a crosslinking would be insignificant within a substantial polymer mass. Crosslinking is therefore relevant as a process when the bulk polymer properties have been modified in a significant way. Conventionally this would be when properties are developed that are associated with the behaviour of true thermosetting plastics i.e. loss of thermoplasticity and enhanced resistance to deformation at elevated temperatures. However, low levels of crosslinking may significantly modify certain characteristics without complete loss of thermoplastic properties. Crosslinking sufficient to significantly modify the molecular weight distribution so that a change in Melt Flow Index (MFI) may be measured, may result in changes in certain physical properties such as resistance to environmental stress cracking, fracture toughness, and creep properties. AS the crosslinking level increases these properties may continue to be enhanced although the MFI may become lower and lower until finally it becomes immeasurable. At this stage an ultra high molecular weight polyolefin has been formed and as crosslinking continues the crosslinked polymer may become increasingly of thermosetting character. The level of crosslinking may be assessed by measurement of the gel content, for example in accordance with the method of ISO/DP 10147. The term "gel content" means the residual polyolefin remaining following exhaustive extraction of the non-crosslinked polyolefin material with a hydrocarbon solvent such as xylene. The gel content can conveniently be determined using a Soxhlet extraction apparatus or by boiling in a suitable vessel. The extraction of the uncrosslinked polyolefin is carried out over a period of at least 5 hours, and up to 24 hours. An inert atmosphere of nitrogen or other means to prevent oxidative degradation may be provided. The dried residue, expressed as a percentage of the initial sample mass gives a gel content (%). Typical commercial processes for crosslinking of polyolefins will normally give maximum gel content within the range 65-90%, although crosslinked polymer of any gel content from zero to the maximum level may be produced. At crosslinking levels below the gel point (0% gel), the MFI may be used as a crude indicator of molecular weight. For example, crosslinked polyethylenes produced by electron beam crosslinking or azo crosslinking typically have maximum gel contents in excess of 60%, silane- crosslinked polyethylenes may have a maximum gel content in excess of 65% whereas maximum gel contents in excess of 70% can be achieved with peroxide crosslinking. The use of fusion welding techniques to join polyolefin bodies or articles, such as pipes, is well known and such techniques involve causing the surfaces of two polyolefin bodies to become molten, pressing the surfaces together whilst still molten, and allowing the bodies to cool and the joint to harden. The term "electrofusion" is given to the technique in which a surface of one of the polyolefin bodies has embedded therein a heating wire which, on the passage of an electric current therethrough, heats and melts the adjacent surface portion. When this is done with the relevant surface contacting the surface of another polyolefin body or article, fusion of the two contacting surfaces occurs and a joint results. However, it has hitherto been considered essential that at least one of the polyolefin bodies to be joined should be formed of a non-crosslinked, ie. thermoplastic, polyolefin, and in this respect reference is made to EP-A- 0333379, and US 4927184. In US 4927184 for example, it is taught that both surfaces to be welded together should be formed of a thermoplastic material. Furthermore, in EP-A-0378406, it is taught that it is impossible to form electrofusion joints using crosslinked polyolef ins . Thus, there has not previously been available, to the best of the Applicants' knowledge, a method which enables crosslinked bodies to be joined together under field conditions found, for example, in pipeline construction. It is an object of this invention to meet this need. It has now been found, unexpectedly, and contrary to the teachings in the prior art of which we are aware, that a crosslinked polyolefin body can be joined directly to another crosslinked polyolefin body by means of the electrofusion technique, provided that the aggregate gel content of the two bodies is less than 70%, preferably less than 68%, and most preferably no greater than 65%. In accordance with one aspect of the present invention there is provided a method of joining a body of crosslinked polyolefin to another body formed of crosslinked polyolefin comprising the steps of heating the surfaces of the crosslinked polyolefin bodies to be joined, to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin, and bringing together said heated surfaces of the bodies under pressure, and allowing the surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68%, and most preferably no greater than 65%. In accordance with another aspect of the present invention there is provided a method of joining a body of crosslinked polyolefin material to another body of crosslinked polyolefin material comprising the steps of heating the surfaces of both bodies at which they are to be joined to one another, to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin material, subjecting the two contacting surfaces to pressure whilst so heated, and ensuring containment or confinement of the melted surfaces of the crosslinked polyolefin bodies generally to their area of melting; and subsequently allowing the two surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68%, and most preferably no greater than 65%. It has been found that by applying a pressure, such as that provided if the two surfaces are pressed together with confinement or containment of the melt to the area of the surface melted, jointing by fusion of the two surfaces of the bodies can be achieved. The term "aggregate gel content" as used herein is the mean value of the gel content of the polyolefins at the weld interface between the two bodies, immediately prior to welding. It is preferred that the aggregate gel content is in the range 40% to 65%, more preferably 55-65%. The gel content of one polyolefin body may be in excess of 65%, for example up to 80%, whereas the gel content of the other body in such a case will be less than 65%, for example up to 50%, provided that the gel content is greater than 0%. The term "gel content" as used herein means the residual polyolefin remaining following exhaustive extraction of the crosslinked polyolefin material with the hydrocarbon solvent xylene. The gel content can conveniently be determined using a Soxhlet extraction apparatus, and typically the extraction of the polyolefin is carried out over a period of five hours with xylene. Extraction of the polyolefin with xylene is believed to remove non-crosslinked polymer chains and small aggregates of crosslinked polymer chains, leaving an insoluble matrix of cross-linked material in the form of a gel. The crystalline melting points of crosslinked polyolefins generally range from about 120° C upwards. For example, the crystalline melting points of crosslinked polyethylenes generally lie in the range 120°C to 130°C; the melting point generally increasing with crystallinity or density. Other polyolefins may have higher or lower melting points. At the crystalline melting point, the crosslinked polyolefins do not form a flowable liquid as do the thermoplastic non-crosslinked polyolefins, but rather they generally retain their shape and form a jelly-like mass. It has been found surprisingly that contrary to what is stated in, for example US 4927184, polyolefins having an aggregate gel content up to about 65% can be fusion- welded together. The bodies or articles concerned may be any of the many practical embodiments and usages of crosslinkable polyolefin bodies, and in particular may comprise polyolefin pipes and pipe fittings, or cable sheathings, for example. With some techniques of fusion welding, which may be used with the present invention, in some instances the application of the necessary pressure can be provided by expansion of the polyolefin materials during heating, provided melt containment or confinement is practised. The invention is particularly applicable when electrofusion techniques are used, in which a portion of the area of the relevant surface of the crosslinked polyolefin article has embedded therein a heating wire adapted, on the passage of an electric current therethrough, to heat and melt the adjacent surface portion. When this is done with the relevant surface contacting a surface of another crosslinked polyolefin body or article, it is possible to ensure that the surfaces are held together in such a manner as to provide confinement or containment of the molten surface of one crosslinked polyolefin body, whilst the expansion of the materials as they heat and melt provides the excess pressure required. It is to be stressed that the Applicants have found that in practical terms the required pressure is that which is required to ensure intimate contact at the melt interface between the two materials to be joined. In the case of two uncrosslinked materials being welded together, both materials would easily soften and flow at elevated temperature rapidly giving the required melt interface, but when the materials are crosslinked they will remain intractable at elevated temperature thus often requiring more care in ensuring that there is intimate contact at the melt interface. In practice in carrying out the invention, the surfaces must be held together in a pressurised melt contained condition for adequate time to enable migration of molecular chains from one crosslinked polyolefin body into the surface of the other crosslinked polyolefin and ensure adequate and satisfactory fusion or welding. It is to be understood that, although the invention is particularly applicable with electrofusion techniques, it can be carried out in any other circumstance where an appropriate pressure and melt containment of confinement can be applied. Thus, and purely by way of example, electrofusion techniques using a conductive polymer instead of wire can be employed; inductive heating of an embedded wire, metal loop, metal sheet, grid or other form within an enclosed system similar to conventional electrofusion welding techniques; other radiative heating methods using electromagnetic radiation, e.g. microwave energy, to heat either modified polymers or inserted receptive components and transfer heat; and butt (or socket) welding or joining of articles within a confined space where weld movement is restricted from flowing freely away from the weld zone, whereby there is an increase in melt pressure significantly over and above that normally encountered in butt (or socket) welding of this kind; and other methods using heating by friction or ultrasonic means but employing similar restrictions on melt movement and application of pressure may also be employed. Pipes or sheathings to which the invention may be applied would include, but not be restricted to, under floor heating and hot/cold water pipes; electrical cable sheathing; steel cable sheathing; district heating pipes and sheathing; flue pipes; industrial pipes; chemical plant pipes. The invention includes within its scope a joint between one crosslinked polyolefin body and another crosslinked polyolefin body formed by a method as herein defined. In a particular aspect the invention provides a method of joining a body of crosslinked polyolefin material to another body of crosslinked polyolefin material, one of said bodies beinα a pipe and the other being a fusion coupling sleeve having a heating element embedded therein in or adjacent a surface thereof; the process comprising heating the heating element thereby to heat the said surface of the coupling sleeve and an adjacent surface of the pipe to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin material; and ensuring containment or confinement of the melted surfaces of the crosslinked polyolefin bodies generally to the area of melting such that the surfaces fuse to form a joint; and subsequently allowing the fused surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68% and most preferably no greater than 65%. In a further aspect the invention provides a polyolefin coupling sleeve for coupling together pipes and/or pipe elements, the coupling sleeve having a heating element embedded in or adjacent a surface thereof, which surface in use is arranged to contact and fuse with a surface of a pipe or pipe element; characterised in that in at least the region of the heating element, the coupling sleeve is formed from crosslinked polyolefin wherein the gel content of the crosslinked polyolefin is greater than 0%. In order that the invention may be more readily understood, a number of embodiments of the use of the method of the present invention will now be illustrated with reference to the accompanying drawings in which:- Figure 1 is a schematic sectional elevation through one half of a first embodiment of a pipe joint in accordance with the invention; Figure 2 shows a joint for connecting cable sheathing; and Figure 3 shows a technique for mending or repairing damaged cable sheathing. Referring now to the drawings, it will be seen that in Figure 1 there is illustrated a technique for coupling two crosslinked polyethylene pipes 1, 2 by means of a crosslinked polyethylene annular coupling sleeve 3. The crosslinked polyethylene sleeve is provided with an electrical wire heating element wound helically within it connecting between electrical terminal wires 5, 6. A removable central annular internally protruding register 7 is provided to locate against the ends of the two crosslinked polyethylene pipes. In operation, after assembling the two pipes 1, 2 within the sleeve so as to engage the centre register, electrical current is passed through the wire 4 embedded in the sleeve, so that the sleeve is heated at its inner surface, whilst the polyethylene pipes are heated on their external radial surfaces, both surfaces being heated to a temperature above the crystalline melting point of the polymer. At such a temperature the surfaces become jelly-like. The heating of the component materials of the inner surface of the sleeve, and the radially outer surfaces of the juxtaposed pipes, causes a tendency to expand so that their mating jelly-like surfaces are pressed together under considerable pressure from such expansive force. Upon disconnecting the electrical terminal wires 5, 6, the sleeve 3 and the pipes 1 and 2 cool and harden and it is found that a most satisfactory joint between the pipes through the use of the sleeve is achieved. The joints are not only firm but are also fluid-tight, and can satisfactorily be subjected to pressure testing at elevated temperature. The following comparative examples illustrate the importance of the aggregate gel content in determining the strength of the welded joint. COMPARATIVE EXAMPLES Example 1 Two one metre sample lengths of 32mm SDR 11 peroxide- crosslinked polyethylene pipe having a gel content of 80% were inserted into a 32mm silane-crosslinked electrofusion coupler having a gel content of 50% (sample 1), the arrangement of pipes and electrofusion coupler being as illustrated in Figure 1. Two further one metre (identical) sample lengths of 32mm SDR 9 peroxide-crosslinked polyethylene pipe having a gel content of 80% were inserted into a 32mm silane- crosslinked electrofusion coupler having a gel content of 65% (sample 2) . Both samples were fused by passing a current at 40 volts for 60 seconds, and then allowing the fused assembly to cool. The samples were then machined into strip test pieces in order that a 180° peel test of the weld zone could be performed. Sample 2 exhibited sudden catasrophic failure along the weld line, at a moderate load of less than lOMPa. Sample 1 showed no failure until a load of >20MPa was applied, when the polymer failed by a normal tensile failure mechanism, there was no failure along the weld line. Example 2 Two further samples 1 and 2 were prepared, identical to those in Example 1. Both samples were subjected to a thermal cycling regime by pumping water through them at 80°C and 20°C for 12 hours respectively for a period of 50 cycles (external medium was air). On pressure testing at 10 bar following thermal cycling, leakage was found from sample 2 but no leakage from sample 1. The foregoing examples illustrate that electrofusion coupler/pipe combinations having an aggregate gel content of 65% (sample 1) form strong durable welded joints, whereas the welded joints formed when the aggregate gel content is as high as 72.5% are relatively weak and break down after thermal cycling. Figure 2 illustrates a connection between crosslinked polyethylene cable sheathing 11, 12, in which the connection is by means of an electrofusion technique. A sleeve 13, tapering from its central portion, of crosslinked polyethylene and having embedded an electrical heating wire 14 is fitted about the proposed connection between the sheathing 11, 12. Upon heat application, the usual process of heating the crosslinked sheathing outer surface and the sleeve inner surface adjacent to the electrical heating wire, leads to expansion pressure which enables an adequate fusion joint to take place. Electrical terminal wires 15, 16 are conveniently removed after use. Finally, Figure 3 illustrates a variation of the arrangement shown in Figure 2. In this case, a crosslinked polyethylene heat-shrink tube 23 is used for the repair and insulation of a damaged cable sheathing 24. As usual, the cable sheathing is of crosslinked polyethylene. At each end of the heat-shrink tube an appropriately shaped fillet sleeve 25 tapering to the outer diameter of the cable is inserted under the heat- shrink tubing. Electrofusion heating elements 26, 27 are embedded within the fillet sleeves 25 but separated from both ends of each. Upon passing current through the electrofusion heating elements, a fusion seal between the cable sheathing 21, 22, the fillet sleeves 25 and the heat-shrink tube 23 is obtained. The heat-shrink tube may be shrunk upon the cable sheathing and fillet sleeves either before or during the fusion jointing process. Electrical terminal wires 28, 29 may again be removed afrer use. By means of the invention, a surprisingly successful method of jointing crosslinked polyolefin bodies with other crosslinked bodies is achieved. It will be readily apparent that numerous alterations and modifications may be made to the particular embodiments illustrated in the drawings without departing from the underlying principles of the invention, and all such alterations and modifications are intended to be included within the scope of this application.

Claims: CLAIMS 1. A method of joining a body of crosslinked polyolefin to another body formed of crosslinked polyolefin comprising the steps of heating the surfaces of the crosslinked polyolefin bodies to be joined, to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin, and bringing together said heated surfaces of the bodies under pressure, and allowing the surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68%, and most preferably no greater than 65%. 2. A method of joining a body of crosslinked polyolefin material to another body of crosslinked polyolefin material comprising the steps of heating the surfaces of both bodies at which they are to be joined to one another, to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin material, subjecting the two contacting surfaces to pressure whilst so heated, and ensuring containment or confinement of the melted surfaces of the crosslinked polyolefin bodies generally to their area of melting; and subsequently allowing the two surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68%, and most preferably no greater than 65%. 3. A method according to any one of the preceding Claims wherein the joining of the two bodies of crosslinked polyolefin is brought about by electrofusion. 4. A method of joining a body of crosslinked polyolefin material to another body of crosslinked polyolefin material, one of said bodies being a pipe and the other being a fusion coupling sleeve having a heating element embedded therein in or adjacent a surface thereof; the process comprising heating the heating element thereby to heat the said surface of the coupling sleeve and an adjacent surface of the pipe to a temperature in excess of the crystalline melting point of the said crosslinked polyolefin material; and ensuring containment or confinement of the melted surfaces of the crosslinked polyolefin bodies generally to the area of melting such that the surfaces fuse to form a joint; and subsequently allowing the fused surfaces to cool and harden, wherein the aggregate gel content of the two bodies at the surfaces thereof is less than 70%, preferably less than 68% and most preferably no greater than 65%. 5. A method according to any one of the preceding claims wherein the aggregate gel content is in the range 40% to 65% . 6. A method according to Claim 5 wherein the aggregate gel content is in the range 55% to 65%. 7. A method according to any one of the preceding Claims wherein the polyolefin is a polyethylene. 8. A method of joining a crosslinked polyolefin body directly to another crosslinked polyolefin body by means of electrofusion, wherein the aggregate gel content of the two bodies is less than 70%, preferably less than 68%, and most preferably no greater than 65%. 9. A polyolefin coupling sleeve for coupling together pipes and/or pipe elements, the coupling sleeve having a heating element embedded in or adjacent a surface thereof, which surface in use is arranged to contact and fuse with a surface of a pipe or pipe element; characterised in that in at least the region of the heating element, the coupling sleeve is formed from crosslinked polyolefin wherein the gel content of the crosslinked polyolefin is greater than 0%. 10. A polyolefin coupling sleeve according to Claim 9 wherein the gel content of the crosslinked polyolefin is up to 80 % by weight of the crosslinked polyolefin. 11. A polyolefin coupling sleeve according to Claim 9 or Claim 10 wherein the polyolefin coupling sleeve is formed substantially entirely of crosslinked polyolefin. 12. A polyolefin coupling sleeve according to any one of Claims 9 to 11 wherein the embedded heating element is an electrical wire heating coil. 13. A coupling sleeve according to any one of Claims 9 to 13 which is an annular coupling sleeve for coupling together two crosslinked polyolefin pipes in an end to end manner. 14. A joint between one crosslinked polyolefin body and another crosslinked polyolefin body formed by a method as defined in any one of Claims 1 to 8.

Inventors: HILL DAVID JOHN, HILL, DAVID, JOHN
Applicants: HILL DAVID JOHN, VICTAULIC PLC
Application year: 1993
Main classification: B29C65
IPC codes: B29C65, F16L47