JOINING OF POLYOLEFIN ARTICLES

Abstract: A method of joining polyolefin articles by fusion bonding, in which the surfaces of the articles are raised to the fusion temperature in a time of less than one minute, and in which there is interposed between the surfaces to be joined a polyolefin film of a thickness less than 50 microns.

Description: JOINING OF POLYOLEFIN ARTICLES This invention relates to a method of joining polyolefin articles, and more particularly to a method of joining polyolefin articles in which the surfaces of the articles are heated and joined under pressure. Many methods have been proposed for the joining of polyolefin articles such as, for example, pipes, tubes, conduits and other hollow members. A particularly suitable method for joining uncross-linked polyolefin articles, comprising very similar or identical polyolefin compositions, is fusion bonding, in which the surfaces of the articles are heated to their fusion temperature and then pressed together to form a bond. This method is widely used, for example, in the butt welding of polyolefin pipes and in electrofusion couplers. Substantial difficulties arise, however, where the polyolefin articles comprise a polyolefin of high molecular weight, and therefore low melt flow index, or where one or other of the polyolefin articles comprises a cross-linked polyolefin. In such circumstances, whilst an electrofusion coupler can still sometimes be used, as described in European Patent Application Number 0333379, butt welding using a standard butt welding machine is usually ineffective. The reason for this, as explained in European Patent Application Number 0329350, is that at the inner and outer surfaces of the weld, pronounced circumferential slit defects are present. The sharpness and location of these defects, which extend into the interface structure in the melt zone, weaken any such joint as is formed. The flaws involved are effectively infinitely sharp stress raising points. Very many proposals have been made to overcome this problem. Special butt welding methods are described in European Patent Application Number 0329349 and in GB2207882. The use of a member of uncross-linked polymeric material, interposed between the surfaces to be joined, is described in DE2026193 and in European Patent Application Number 0329350. Cross-linking of the weld region to improve the strength of the weld is disclosed in GB1582078, DE3631082, and in JP58059020, JP58059019 and JP58059018. In the latter Japanese patent publications, crosslinked polyolefin articles are joined by a method which comprises interposing a layer of cross-linkable polyolefin compound, comprising an organic peroxide and an antioxidant, between the surfaces of the crosslinked polyolefin products to be joined and heating and cross- linking the layer of cross-linkable polyolefin compound whilst holding the crosslinked polyolefin surfaces to be joined under pressure. As exemplified, this process requires the polyolefin articles to be held at temperatures of 200°C or above for periods of five to ten minutes, and it has been found in practice that these conditions can lead to severe distortion of the polyolefin articles. In the method of European Patent Application Number 0329350, not only is it necessary to hold the pipe ends at a high temperature for a considerable period of time, but also the uncrosslinked third member interposed between the surfaces of the polyolefin articles is of substantial thickness, so that the joint region has a considerably lower strength than that of the crosslinked pipes. The entire disclosures of all the abovementioned patents and patent applications are incorporated herein by reference for all purposes. According to the present invention, polyolefin articles are joined by a method in which the surfaces of the articles are heated and jointed under pressure, by interposing between the surfaces to be joined a polyolefin film of thickness less than 50 microns, preferably less than 20 microns. In a first aspect, the invention provides a method of joining polyolefin articles by fusion bonding, in which the surfaces of the articles are raised to the fusion temperature in a time of less than 1 minute, and in which there is interposed between the surfaces to be joined a polyolefin film of a thickness less than 50 microns, preferably less than 20 microns. The method of the invention can be applied to the joining of any suitable polyolefin articles, but is particularly applicable to the joining of hollow members such as, for example, pipes, tubes, and conduits. In another aspect, the invention provided an apparatus for joining polyolefin articles, which comprises: means for heating the surfaces of the polyolefin articles to a fusion temperature in a time of less than 1 minute, carrier means for holding a polyolefin film of a thickness less than 50 microns interposed between the surfaces, transport means for bringing the surfaces together with the film interposed between them, and clamping means for applying a clamping pressure between the polyolefin articles for a period of time sufficient to form a bond. The invention is particularly suitable for the joining of polyolefin articles having a low melt flow index, for example, those with a melt flow index of less than about 0.2, and such articles may comprise, for example, polyolefins of very high weight average molecular weight, for example, greater than 100,000, or crosslinked polyolefins, having a gel content of in excess of about 40%. The term melt flow index (MFI as used herein) has the meaning given by the test defined in British Standard BS2782 method 720A 1979: determination of melt flow rate for plastics under the conditions of 190°C and a load of 2.16 Kg, the rate of index being expressed in grams/10 minutes. Gel content in this specification is as determined by the method of ANSI/ASTMD 2765-68. By the expression "fusion bonding" as employed herein is meant a process wherein the materials in the parts to be bonded together are caused to melt and flow, or to co-crystallise, to form the bond, as occurs, for example, in heat welding, or ultrasonic, infra-red or radio frequency energy, preferably with the application of pressure. The polyolefin articles that can be joined by the method of the present invention can be similar or dissimilar and can comprise crosslinked or non- crosslinked polyolefins. Suitable polyolefins can comprise, for example, homopolymers and copolymers of alpha olefins which may be, for example, homopolymers and copolymers of ethylene, propylene, butene-1 or 4- methylpentene-1. Homopolymers and copolymers of substituted alpha olefins such as, for example, chlorinated polyethylenes, can also be used. Derivatives of alpha olefins comprising, for example, carboxylate or ester groups such as, for example, homopolymers of vinyl esters such as vinyl acetate or ethyl acrylate and copolymers of such monomers with other alpha olefins, for example, ethylene-vinyl acetate and ethylene-ethyl acrylate copolymers may also be used. The method of the invention can be of particular application in the joining of crosslinked polyolefin articles, which may, for example, have a degree of cross¬ linking of greater than 60%, and often up to from 90 to 100% gel content. The crosslinked polyolefin articles can be manufactured by any suitable known method such as, for example, moulding or extrusion, and then crosslinked, either by chemical means, for example, using organic peroxides, or silane cross-linking, or by irradiation, for example, using ultraviolet radiation, electron beam irradiation, or gamma radiation. In the method of the invention, the surfaces of the polyolefin articles to be joined are rapidly raised to the fusion temperature in a time which is insufficient to cause unacceptable distortion to the polyolefin articles. The surfaces can be prepared in the normal way, to remove dirt and oxidised material, and then heated, either by contacting the surfaces with a heated plate, for example, by the use of mirror plate heating, or by radiant energy, for example, infra-red heating. Infra-red heating is particularly preferred in many applications, as by this means it can be possible to heat a surface very rapidly without substantial conductive energy transfer rate loss or distortion from the heating source. Standard butt welding machines can be used for heating the surfaces to the fusion temperature, provided that they are capable of achieving the fusion temperature within the specified time. Preferably, the heating is localised to the surfaces of the articles, which are preferably raised to the fusion temperature in a time of less than 45 seconds, more preferably less than 30 seconds, especially from 10 to 30 seconds. By the "fusion temperature" in this specification, is meant the temperature at which the polyolefin article melts and flows, or, if the polyolefin article is crosslinked, the temperature at which the polyolefin article would melt and flow if it were not crosslinked. The polyolefin article surfaces may be heated to different temperatures if desired. Preferably the surfaces of the articles are heated to a temperature of from 150°C to 250°C, more preferably 175°C to 215°C. The polyolefin film may be interposed between the surfaces to be joined either before, during, or after the heating step. Preferably, a polyolefin film is introduced between the surfaces, and the surfaces and the film are heated simultaneously. The surfaces can then be clamped together, with the film in between, and allowed to cool. The polyolefin film can comprise any of the polyolefins mentioned hitherto, but is preferably uncrosslinked, or crosslinked only to a small extent, preferably to less than about 10% gel content. It is important that the surfaces of the polyolefin film should be substantially free of contamination, and if necessary, the film can be treated to remove any oxidised layer or dust contamination. Very thin polyolefin films are preferred, for example of thickness less than 20 microns, preferably from 5 to 15 microns. The polyolefin film will, of course, be substantially melted or fused during the joining method, and it is preferred, therefore, for the polyolefin film to have a fusion temperature which is not substantially greater than the fusion temperature of the surfaces to be joined. Where the polyolefin articles to be joined each comprise the same polyolefin, it is preferred that the polyolefin film should also comprise the same polyolefin, although this is not essential in all cases. Polyethylene films have been found to be particularly suitable for joining crosslinked polyethylene pipes and for joining polyethylene to polypropylene pipes. It is important that the polyolefin film is of a type that does not weaken the weld by oxidation or by stress cracking, and that it comprises a polyolefin which does not weaken substantially the mechanical properties of the joint. If desired, the film can comprise a modified polyolefin material, for example, an anhydride modified polyethylene, or the film can comprise a cross¬ linking agent which reacts during or after the fusion bonding step to cross-link the polyolefin film and, possibly, the adjacent surfaces of the polyolefin articles. The film can comprise a monolayer, or a laminated multilayer construction. Modified polyolefin polymer films are particularly useful in joining polyolefin articles of dissimilar composition. Suitable modified polyolefins polymers include, for example, alpha olefin polymers and copolymers comprising up to 10% by weight of an ethylinically unsaturated carboxylic acid or an anhydride thereof, such as, for example, acrylic acid, maleic acid, itaconic acid, and succinic acid, or their anhydrides, as copolymer or graft copolymer components. Cross-linking agents which can be admixed into the polyolefin film include, for example, organic peroxides, which may be used in an amount of from 0.1 to 0.6 parts per 100 parts polyolefin. Typical peroxides include, for example, dibutyl peroxide, dicumyl peroxide, 2,5- dimethyl-2, 5 di-(t-butylperoxy)-hexene-3, l,3-bis-(t- butylperoxy-iso-propyl )-benzene, and similar cross¬ linking agents. The polyolefin film may contain an antioxidant, or conventional processing aids as desired. The polyolefin film may be oriented or non-oriented, but where biaxially oriented films are used, these should preferably not shrink substantially during the fusion bonding process. As previously mentioned, it is important to ensure that the surfaces of the film are clean. This can be achieved, for example, by appropriate surface treatment, or by the provision of "peel" layers which can be removed immediately before fusion bonding. Another possibility is to produce the film by injection moulding, since it has been found in practice that such methods are more likely to produce an acceptable fusion surface without the oxidised layers which are often formed by blown film extrusion processes. Where the film has a multilayer structure, the opposed surface layers can comprise dissimilar polyolefin polymers which are compatible with the surfaces of the articles to be joined. This is particularly useful when bonding polyolefin articles comprising dissimilar polyolefin polymers, since the film can then be applied between the dissimilar surfaces such that each of the dissimilar article surfaces faces a similar film surface, thereby adding to the strength of the fusion bond. If desired, the film can be heated before it is applied to the surfaces of the polyolefin articles. The opposed surface layers of a multilayer film can be heated to different temperatures, as required. After the surfaces have been joined, a clamping pressure can be applied if necessary for a period of time sufficient to achieve the desired bond. Pressures in the range of from about 0.02 MPa to 0.2 MPa are preferred, especially pressures from about 0.05 MPa to 0.1 MPa. The time for which the pressure is applied is preferably from about 30 seconds to about 2 minutes, especially from about 1 to 1.5 minutes, after which the joint region is allowed to cool, with or without removal of the clamping pressure. If necessary, a fluid coolant, such as air or water, may be used to promote more rapid cooling of the joint region. An embodiment of an apparatus and a method according to the invention will now be described, by way of example only, with reference to the accompanying Drawing in which: Figure 1 shows in diagrammatic sectional side elevation an apparatus for joining two polyolefin pipes. Referring to Figure 1, there are shown pipe ends 1 and 2, the pipes being made from cross-linked polyethylene, which are supported in axial alignment by transport means (not shown). Interposed between the pipe ends 1, 2 is a film 3, formed from 10 micron thick uncross-linked polyethylene, which is supported by means of a film carrier 4. Infra-red heaters 5 and 6 are positioned on each side of the film, between the film and the adjacent pipe end. In the method of the invention, the end surfaces 7 and 8 of the pipe ends 1 and 2 are first cleaned and abraded to remove any contamination or oxidation and then moved into position as shown. The infra-red heat sources 5 and 6 are then energised, heating the surfaces 7 and 8 and the film 3. As indicated in Figure 1, the infra-red heat source 5 raises the surface 7 of the pipe end 1 to a temperature Tl, and the surface 9 of the film 3 to a temperature T2. In a similar fashion, the infra-red heater 6 raises the surface 8 of the pipe end 2 to a temperature T4, and the surface 10 of the film 3 to a temperature T3. Tl, T2, T3 and T4 can be the same or different. Any variations in temperature can be achieved by placing the surface to be heated at an appropriate distance from its adjacent heat source. Where the film 3 is a monolayer, in this case a polyethylene film, T2 and T3 will usually be identical. Where the film has a laminate, multilayer construction, it may be desirable that T2 and T3 should be different temperatures. When the surfaces 7, 8, 9 and 10 have reached the desired fusion temperatures, which occurs within 10 to 30 seconds, the infra-red heaters are removed and the pipe ends 1 and 2 moved axially by the transport means into contact with the film 3. The transport means applies a clamping pressure on the joint for about one minute, after which the pressure is removed and the joint is allowed to cool. It is found that a very strong butt joint is obtained, with substantially no distortion of the pipe ends. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims: CLAIMS 1. A method of joining polyolefin articles by fusion bonding, in which the surfaces of the articles are raised to the fusion temperature in a time of less than one minute, and in which there is interposed between the surfaces to be joined a polyolefin film of a thickness less than 50 microns. 2. A method according to Claim 1, in which the polyolefin articles each comprise a pipe, tube or conduit. 3. A method according to Claim 1 or 2, in which the polyolefin articles each comprise a polyolefin material having a melt flow index of less than 0.2. 4. A method according to any of the preceding claims, wherein the polyolefin articles each comprise a homopolymer or a copolymer of ethylene, propylene, butene-1 or 4-methylpentene-l . 5. A method according to any of the preceding claims, wherein the polyolefin articles are cross-linked. 6. A method according to Claim 5, wherein at least one of the polyolefin articles comprises cross-linked polyethylene. 7. A method according to any of the preceding claims, in which the surfaces of the articles are raised to the fusion temperature by infra-red heating. 8. A method according to any of the preceding claims, in which the surfaces of the articles are raised to the fusion temperature in a time of from 10 to 30 seconds. 9. A method according to any of the preceding claims, in which the surfaces of the polyolefin articles are heated to different temperatures. 10. A method according to any of the preceding claims, in which the surfaces of the polyolefin articles are heated to a temperature or temperatures of from 150° to 250°C. 11. A method according to any of the preceding claims, in which the polyolefin film is interposed between the surfaces of the articles to be joined, and the surfaces and the film are heated simultaneously. 12. A method according to any of the preceding claims, in which the polyolefin film comprises a homopolymer or a copolymer of ethylene, propylene, butene-1 or 4-methylpentene-l. 13. A method according to any of the preceding claims, in which the polyolefin film is substantially uncross-linked. 14. A method according to any of the preceding claims, in which the polyolefin film has a thickness of less than 20 microns. 15. A method according to any of the preceding claims, in which the polyolefin film has a thickness of 5 to 15 microns. 16. A method according to any of the preceding claims, wherein the polyolefin articles comprise cross- linked polyethylene and the polyolefin film comprises uncross-linked polyethylene. 17. A method according to any of the preceding claims, wherein the polyolefin articles are of dissimilar composition and the polyolefin comprises a modified polyolefin polymer. 18. A method according to Claim 17, wherein the modified polyolefin polymer is an alpha olefin polymer or copolymer comprising up to 10% by weight of an ethylenically unsaturated carboxylic acid or an anhydride thereof. 19. A method according to any of the preceding claims, in which the polyolefin film comprises a cross¬ linking agent. 20. A method according to any of the preceding claims, in which the film is provided with clean surfaces, by appropriate surface treatment, by the provision of "peel" layers which are removable immediately before fusion bonding, or by producing the film using injection moulding techniques. 21. A method according to any of the preceding claims, in which the film has a laminated multilayer structure, the opposed surface layers comprising dissimilar polyolefin polymers which are compatible with the surfaces of the polyolefin articles to be joined. 22. A method according to any of the preceding claims, wherein the polyolefin articles are clamped together, with the polyolefin film in between, and wherein the clamping pressure is in the range of from 0.02 MPa to 0.2 MPa. 23. A method according to Claim 22, in which the clamping pressure is applied for a time of from 30 seconds to 2 minutes. 24. A method according to Claim 22 or 23, in which the joint is allowed to cool, with or without removal of the clamping pressure. 25. A method of joining polyolefin articles substantially as hereinbefore described. 26. An apparatus for joining polyolefin articles, which comprises: means for heating the surfaces of the polyolefin articles to a fusion temperature in a time of less than one minutes; carrier means for holding a polyolefin film of a thickness of less than 50 microns interposed between the surfaces; transport means for bringing the surfaces together with the film interposed between them; and clamping means for applying a clamping pressure between the polyolefin articles for a period of time sufficient to form a bond. 27. An apparatus according to Claim 26, wherein the heating means comprises one or more infra-red heaters. 28. An apparatus according to Claim 26 or 27, substantially as hereinbefore described with reference and as illustrated in the accompanying Drawing. 29. An apparatus for joining polyolefin articles substantially as hereinbefore described. 30. A joint between polyolefin articles produced by a method according to any of Claims 1 to 25, or by an apparatus according to any of Claims 26 to 29.

Inventors: HOVING LARS GORAN, HOVING, LARS, GORAN
Applicants: HOVING LARS GORAN, UPONOR BV, HOVING, LARS, GORAN, UPONOR B.V.
Application year: 1996
Main classification: C08J5
IPC codes: B29C35, B29C65, C08J5