CA1052515A - Welded polymeric articles and process - Google Patents

Abstract

A B S T R A C T
The invention relates to a method of joining abutting surfaces of polymeric material. The method comprises disposing between the surfaces an insert formed of thermoplastic material having dispersed therein a heat-activatable crosslinking agent, the insert having one or more perforations therethrough, and heating the surfaces and insert under compression, to cause the insert to flow and the crosslinking agent to be activated, whereby the surfaces are joined by crosslinked weld material and the escape of gases from the weld area is facilitated by the performations.
The insert is preferably generally L-shaped in cross-section and, in a preferred embodiment, the perforations pass through the centre upright of the L-shaped insert.

Description

~OS'~5~5 ~ his invention xelates to a method o~ joining articles and to articles so joined.
Sleeves, which may be thin-walled, of polymeric materials, particularly polymeric material~ crosslinked by high energy ionizing radiation to impart form stability at melt temperatures, are frequently used to protect articles, particularly those having a tubular or otherwise regular elongate configuration. Another use ~or such sleeves is for sealing joints between lengths o~ pipes or other conduits.
Of particular utility ~or these purposes are heat-recoverable sleeves, especially heat-shrinkable sleeves that is, sleeves that possess the property of being able to shrink to a smaller diameter when heated. A heat-shrinkable sleeve is constructed with a diameter large enough so that it can be conveniently placed about the substrate whereupon it is heated causing it to shrink into close contact with the substrate. Heat-recoverable sleeve~ have been found highly ef~ective, inter ~1~3~ for producing corrosion-resistant coverings *or pipe joints as disclosed in, Ior egample, U.S. Patents Noæ. 3,297,819, 3,379,218, 3,415,287, 3,455,336, 3,530,598 and 3,770,556.
Materials suitable for making heat-recoverable sleeves are well knownO ~hus, ~or example, in U.S. Patent ~o. 3,086,242 there is described a variety OI suitable materials obtained by crosslinking, particularly by radiation, organopolymeric precursors.
~ leeves o~ relatively small diameter can conveniently be e~truded directly as tubes. ~his however is not the case with sleeves having a diameter large enough to enable them to be used to cover sections o~ pip~- of a size that might be employed in pi~elines for the transmission of oil or gas over lon~ distances. ~ecause o~ this di~icul-ty~ it has been the _ 2 - ~

~oS~5~5 practice in the past to employ ~o-called "~rap-around"
sleeves ~or large diameter pipes. Sléeves o~ this type may be made, for e~ample, by wrapping a length oP heat-recoverable material about the substrate, for example, at a joint between two lengths of pipe, and joining its ends by mechanical means that prevent the ends from separating when the sleeve is sub-jected to heat to cause it to recover. Examples o~ such sleeves are described in certain of the patents mentioned above.
It is still preferred to use a wrap-around sleeve in situations where it would be inconvenient to position a pre-formed sleeve about the substrate, for example, ~hen the end of the substrate is remote from the point where $he sleeve is desired, as might be the case when the sleeve is to be used in the repair o~ a pipeline. Nevertheless, there are many situation~ in which it would be advantageous to have available a supply of prefabricated sleeves o~ large diameter.
In order to obtain large diameter preformed sleeves it has hitherto been proposed to overlap the opposite edges of a polymeric sheet, disposing between the overlapping segments a pero~ide crosslinking agent which is then activated.
This technique has gen~rally proved unsuccessful because, apparently because of uneven crosslink formation, the edges of the overlapped sur~aces pucker severely upon expansion, presenting an unsightly weld which is also potentially subject to ~ailure because of a low peel strength. Joints have been alternatively proposed to be formed by disposing thermoplastic materials between crosslinked polymeric articles to be joined, and subsequently heating the thermoplastic substance to its melt temperature~ ~hat approach is unsuited to the production ~0 of heat-reco~erable sleeves because the process by which heat-recoverability is imparted involves e~pansion of the sleeve at lOSZS~S
high temperatures which will cause the failure of a thermo-plastic joint.
Another method by ~hich pre~ormed large diameter sleeves maU be made is described in Canadian Patent Application ~erial No. 218,597, filed 24th January, 1975 in the name of Robert James Naidoff; this method is generally carried out using non-heat-recoverable polymeric sheet material, and a sleeve obtained from this process may subsequently be processed to make it heat-recoverable. A further method is described in U.S. Patent No. 3,991,243 granted 9th Se~tember, 1976 to William G. Biddell. ~y using the latter method, heat-recover-able sleeves may be made directly from heat-recoverable sheet material.
The aforementioned Naidoff and Biddell specifieations describe, inter alia, inserting the end~ of polymeric sheet material into the channels of a generally "I"-shaped member referred to as an "insert" fabricated from a thermoplastic material containing a heat-activatable crosælinking agent followed by heating the assembly under pressure to form a weld.
~he insert, in e~fect, functions as a thermosetting member and bonds itæelf covalently to the polymeric sheet. In these methods, the length of the insert is typically substantially the same as the width of the sheet ends to be joined.
The welds formed using the inserts disclosed in the above-numbered Naidoff and Biddell specificationæ, however, are frequently marred by the presence of blisters or voids. ~hese appear to be caused either by gaseous products formed in the welding process, for example, by decomposition of the heat-activatable crosslinking agent as the assembly is heated, or by air entrapped in the welding procesæO Whatever their source, these defects can æignificantly reduce the strength of the weld.

105'h5~5 In order to avoid the occurrence of these defect~
it has previously been -proposed to use two or more inserts separated by a space or spaces which is or are thought to provide a pathway by which the gases can escape as they are formed. If properly spaced, the section~ of insert will flow together before thermosetting is complete, thereby filling the gap and ensuring that a substantially uniform bond is obtained.
~his process i~ described in Canadian Application Serial I\To.
228,638 filed 5th June, 1975 by ~iilliam Harry Humphries.
Although the use of a plurality of spaced inserts constitutes a valuable improvement in the process of insert welding polymeric sheet material, the use of a plurality of separate inserts may be somewhat inconvenient in some circum-stances. Thus, the multiple inserts must be properly aligned relative to each other and to the polymeric material to be joined if a uniform bond is to be obtained. '~his requirement necessarily increases the time required to assemble the components of the weld thereby making the use o~ a plurality of inserts less attractive for the manufacture of large numbers of sleeves.
Furthermore, the alignment of the sleeves may in some circumstances be lost in the steps of the welding process when a plurality of inserts are usedO For example, as the heated platens used to provide the pressure and heat that cause the insert to soften and flow as well as initiate the thermosetting reaction are closed, they may tend to displace the insert.
Also, inasmuch as the clearance between the platens when the opening is not large, the proper positioning of a plurality of inserts and sheet ends constitutes an additional hazard to the operator.
The present invention provides a method of joining lOSZ5~5 abutting surfaces of polymeric material which comprises disposing between the æur~aceC an insert formed of thermo-plastic material having dispersed therein a heat-activatable crosslinking agent~ the insert having one or more perforations therein whereby escape of gaseous products from the weld area is facilitated, and heating the surfaces and insert under compression, to cause the insert to flow and the cross-linking agent to be activated, whereby the surfaces are joined by cross-linked weld material.
In a preferred embodiment of the invention, butt welded articles may be made by disposing a thermoplastic "I"
beam shaped insert between the ends of thin-walled crosslinked thermoplastic polymeric sheet material to be joined, insertin~
the ends into the channels defined by the "I"-shaped insert, and heatin~ the assembly under pressure to form a weld. ~he material of the "I"-shaped member, in addition to its thermo-plastic component, further comprises a heat-activatable cross-linking agent that during weld formation initiates crosslinking within the insert and the formation of covalent bonds between the insert and polymeric sheet material.
The invention also provides an article having a weld made by the method of the invention. The article may be in the form of a sleeve, the weld line being parallel to the a~is o~ the sleeve, and the weld is preferably continuous from one end o~ the sleeve to the other and is of essentially the same thickness as the walls of the sleeve. In a sleeve made in accordance with the invention, one of the surfaces of poly-meric material may be part of a dimensionally heat-stable thin-walled segment, while another of the surfaces of polymerio ~0 material is part of a heat-recoverable thin-walled segment.
In a preferred embodiment of the invention, the sleeve is lOSZ5~5 heat-recoverable, and large diameter heat-recoverable sleeve~
in accordance with the invention may be used in an~ of the ways indicated above J for example for protecting pipe joints from corrosion.
~he method of the invention makes it possible to obtain butt-welded articles wherein the weld area is substan-tially free from voids caused by air and gaseous by-products of the weldin~ process.
The polymeric material joined by the method of the invention is preferably cro~slinked, and the modl~us of the weld material is advantageously at least equal to that of the polymeric material joined thereby. Welding is effected at temperatures sufficient to cause the thermoplastic insert material to flow and wet the surfaces to be joined, and also sufficient to actlvate the crosslinking ~ystem.
~he iasert used in accordance with the inv~ntion is preferably I-shaped in cross-section. Such an insert not only makes it possible substantially to avoid the problems of void formation attributable to entrapped air or gas, but i9 also convenient to use.
~he perforations in the preferred insert of the in-vention can be located through the ~lange~ of an "I"-shaped insert, but preferably extend ~rom top to bottom through the centre upright joining the flanges. The inserts can be extruded or otherwise moulded from suitable thermoplastic compositions and can then be provided with perforations by any suitable meansO ~he per~orations can be of any convenient, usually regular, configuration and are preferably either square, rec-tangular or circular in shapeO
~he number and size of the perforations giving optimum results can vary and depends upon a number of parameters 105'~S15 including tne flow characteristics of the polymeric con-stituents of the material to be joined and the inser~, on the location of the perforations in the insert and on the width and thickne~s o the material to be joined. Fixed requirements applicable in every situation thus cannot be laid down.
Nevertheless, the optimum number and arrangement o-~ perforations can readily be determined empirically by routine experiment by one skilled in the art. To achieve greater insert flow, however, it is pre~erable to use I-shaped inserts the per-forations o~ which are made through the centre upright portion - of the insert, the diameter of the perforations being at least as great as the width of the upright.
~ he insert can be as long as the polymeric sheet material to be joined is wide. Pre~erably, however, the length of the insert is slightly less than this width and in the welding process it is centred between the ends o~ the ~heet ends to leave gaps at each end that provide an additional area through which gaseous material can escape. ~he width of the gaps is pre~erably gauged to be such that in the welding proces~ they are filled as the insert flows under ~eat and pressure.
In a preferred embodiment of the invention, heat-recoverable sleeves are made ~rom heat-recoverable polymeric material or from material capable of being rendered heat-recoverable after sleeve formation. A heat-recoverable article is a dimensionally heat-unstable article having an elastic or plastic memory. Such an article can be obtained, ~or example J by heating a sheet of a suitable thermoplastic crosslinked crystalline polymer above its crystalline melting point and elongating or expanding it in one or more directionsO
The sheet is allowed to cool below the crystalline melting ~S~'~5~ 5 temperature while maintaining the deforming force. If heated without restraint above its crystalline melting point, such a sheet will exhibit its `'elastic memory`' by returning to its pre-e~panded dimenæions. For use as an element o~ a heat-recoverable sleeve, it is preferred to employ heat-recoverable material expanded in a single directionO
Polymeric materials that have been crosslinked by chemical means or by irradiation, for e~ample, with high energy electrons or nuclear radiation, are preferred for use in making heat-recoverable articles in accordance with this in-vention, although other polymers capable of having the property of heat-recoverability imparted to them can also be used. The polymeric material to be joined in accordance ~ith the in-vention must be capable of undergoing further crosslinking in the presence o~ heat-activatable crosslinking agents since, in the formation of the weld, the insert and the polymeric sheet are believed to become covalently bonded to each other, this bond formation being induced by the crosslinking agent.
Examples of polymeric materials which may be joined by the method of the invention include polyolefins;
saturated and unsaturated polyesters; polyvinyl halides; and elastomers, for example natural rubber, butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, isoprene-isobutylene copolymers, polyisoprene, polybutadiene, poly-sulphides (Thiokol)*, polychloroprene (neoprene), polysiloxanes(silicone)~ fluorocarbons (for example Viton)*, chlorosul-phonated polyethylene (for example Hypalon)*, plasticized polyvinyl chloride, and polybutene. ~Iost preferred are the polyolefins, for e~ample, polyethyle~e; poly (butene-l);
various copolymers of ethylene, propylene and butene; ethylene/
ethyl acrylate, ethylene/vinyl acetate, or ethylene/methyl * "Thiokol", "Viton" and ~l-nypalonl~ are -trade marksO

iO5'~5~5 acrylate copolymers in which re~eat u~its derived from ethylene predominate (~or example, about 80 to 90~o), and blends of such copolymers with major proportions of polyethylene itself.
method for rendering polymeric materials heat-recoverable is described in U.S. Patent ~o. 3,086,242.
As previously noted, the polymeric artioles to be joined are preferably crosslinked ~rom the outset, and pre-ferably exhibit a modulus in the range ~rom about 20 to about 60 psi (1.4 to 4.2 KgJcm2). ~s used herein, the term "modulus" is the modulus determined at 150C and 100~o elonga-tion. The determination i6 carried out at a jaw separa,tion rate o~ 2 inches (about 501 cm) per minute~ Articles to be joîned are ~referably crosslinked by high energy ionizing radiation a,t dosage levels ranging from, for example, about 5 to about 20 megarads, pre~erably from about 12 to 15 megaracls.
~he invention is es~ecially useful ~or joining thin-walled articles ~or example, sheet ranging in thickness from about 60 to about 120 mils (0.15 to 0.30 cm). An article is "thin-walled" for present pur~oses when its thickness is substantially less than its length or width.

-~05i~515 A "primary face" of an article is that surface which is the product of length and width as so defined. The invention has proved well suited to joining polymeric sheet, which may be heat-recoverable, to form sleeves whose recovered diameter is equal to or greater than 12 inches (30 cm) and commonly equal to or greater than about 18 inches (45 cm).
The thermoplastic polymeric constituent of the insert can be chosen within wide bounds, including a great variety of melt processable materials capable of being cross-linked by heat-activated systems. Since the crosslinking agent is most desirably blended with the polymeric constitu0nt of the insert in the melt phase, an important consideration in choice of insert thermoplastic is melt processability at temperature less than the heat activation temperature of the crosslinking agent. The insert thermoplastic should al-so be compatible with t'ne polymeric material being joined in the sense that they are capable of adhering and bonding to-gether under heat and pressure. The criteria by which the insert thermoplastic is to be chosen are well known to those skilled in the art. With the polyethylenic materials most preferably employed in the articles to be joined, preferred insert polymeric constituents include ethylene/vinyl acetate, ethylene/methyl acrylate and ethylene/ethyl acrylate copolymers and blends of such copolymers with low density polyethylene or minor proportions of polyethylene. Such copolymers commonly contain from about 60 to about 90 to 96 % by weight of structural units derived from ethylene, lO~'~SlS
and preferably contain -~rom about 75 or 80 to about 96f~ b~
weight of such units.
~ s examples of crosslinking agents, there may be mentioned a wide variet~ of free radical generators, for example bisazodicarboxylates, for example, ~thyl and ethyl bisazodiformates, sulphonazides, ~or example 1,10-decane bis (sulphonazide), a~odicarbonamine, 3,3'-dimethoxy-4,4'-dia-zodiphenyl and 2,2'-azobisisobutyronitrile. Mo~t preferred are peroxide crosslinking agents, for example, t-butyl per-benzoate, 2,5-dimethyl-2,5-di(t-butyl peroxy3 hexyne-3, dicumyl peroxide, benzoyl peroxide, methylethylketone peroxide, cumene hydropero~ide, t-butyl hydropero~ide and di-t-butyl peroxide.
Such agents are empolyed in e~fective crosslinking proportions, as in conventional practice. Most commonly, the agents are coated on a calcium carbonate or other particulate substrate, and are employed in proportions ranging from about ~/~ to about 2~ by weight active agent, most preferably from about ~
to about 1a~o by weight active agent, based on the weight o~ the composition used for the insert body. ~ preferred agent, which is known to experts in the art as "Varox"*, is a mixture o~ 50~ calcium carbonate and 50~o 2,5-dimethyl-2,5-di (t-butyl peroxy) hexane.
Expecially where the amount of crosslinking agent employed is at the lower end of the ranges mentioned above, andmost especially where non-peroxide crosslinking agents are used, it is preferred additionally to use in the insert material a so called "co-agent" to maximize the number of crosslinks formed through the agency of a given number of free radicals generated by the agent itself. The use of such co-agents is conventional and is discussed in, for example, ~. P. ~enaf, I & E Chem. Prod. Research & Development, 2, 1~
* "Varox" is a trade mark lOS;Z51S

~o. ~, 202 (196~). Among conventionally employed co-agents, otherwi~e known as ~olyfunctional monomers, may be mentioned m-phenylene dimaleimide, trimethylol propane trimethacrylate, pentaerythritol tetra-acrylate, triallyl cyanurate, triallyl i~ocyanurate, allyl methacrylate, diallyl phthalate, triallyl citrate, tetra-allyl pyromellitate and triallyl mellitate.
~uch co-agents are employed in minor proportions, ~r example, 0.5 to 2~, most preferably ~rom about ~ to about 1~o by weight o~ the composition.
The quantities of crosslinking agents and co-agent are chosen bearing in mind the ~ollowing two points. Cross-linking is preferably sufficient to ensure that the modulus of the ultimate weld is at least equal to, and preferably greater than, that of the thin-walled articles Joined. Othex~lise, the weld may be subject to failure during the high temperature treatment associated with imparting the ~roperty of heat-recoverability or effecting heat~recovery. On the other hand, care must be taken to ensure that the ~eld is not crosslinked to too great an extent, with concomitant embrittlement.
Preferably, the modulus of the weld is not greater th~n about three times that o~ the article joined. It will be appreciated by those skilled in the art that for a given combination of material to be joined, the material of the body forming the insert, and the identity o~ the crosslinking agent and co-agent, the optimum ~roportions of the two last-mentioned are a matter of routine e~eriment only. Weld modulus can in ~articular cases range from about 20 to about 250 psi (1.4 to 17.5 Eg/cm2).
However, a substantial failure rate may be experienced at moduli greater than about 150 psi (10.5 ~g/cm2), and most pre~erably the weld modulus is in the range ~rom about 50 to about 100 psi t3.5 to 7.0 KgJcm2).

105'~5~5 Of course, in addition to crosslinking agent, polymeric constituent and, where used, co-agent, the insert may contain additional conventional addends, ~or example, fillers, ultra-violet stabilizers, antio~idants, and acid acceptors. ~illers, for example carbon black, calcium carbonate and the like, are employed in substantial (albeit minor~ proportions in order, inter alia, to assist in con-du¢ting heat from the surface to the interior of the insert.
Preferably, on the order of about ~ociO such filler is employed.
Specific examples of various co~positions useful as inserts for bonding crosslinked articles are disclosed in Canadian ~pplication No. 218~597 referred to previously.
A preferred insert formulation for use with sheet material containing a substantial proportion of polyethylene or polyethylene copolymer comprises:
Wt~ (~o) low density polyethylene 48 copolymer of ethylene and methyl acrylate (1 6~o) 20 20terpolymer of ethylene, vinyl acetate and methacrylic acid 5 carbon black 22.5 stabilizers 2.5 Varo~* 200 ~he method of this invention has generally been described above with reference to joining Polymeric sheet material, for examPlef in the making o-~ sleeves from a sheet of such material by joining two opposing edges~ It is un-necessary to enumerate all the diverse ap lications to which ~0 this invention can be ~ut, but it will be apparent to those skilled in the art that it can be employed in numerous other * "Varox" is a trade mark lOS;~515 instances when it is desired to join polymeric materlal, particularly material that is heat-recoverable or capable of being so rendered at the edges to be joinedO ~or example, the method of this invention can be adapted to join two or more discrete lengths of heat-recoverable sheet materi~l to make a longer length. Such a length cot~d be employed to make wrap-around sleeves in the many ways known to the prior art or its opposing end edges could be joined by the method of this invention to form a ~leeve. In another application, material that is not heat-recoverable except along the edges which are to be joined can be satisfactorily welded by the method of this invention.
If the welding ~rocess of the invention is used to form large diameter sleeves which are not heat-recoverable but to which the property of heat-recoverability can be im-parted, the welded sleeves may be heated to above the crystalline melting temperature o~ the material ~rom which the sleeve is formed and expanded in diameter.
r~he method o~ making sleeves described in detail in this specification results in a sleeve o~ relatively uni-form cross-section~ However, the method of this invention can also be applied to making sleeve~ of irregular shape. ~or example, sleeves that taper from a relatively large diameter at one end to a smaller diameter at the other can be made by the method of this invention. Similarly, generally "Y"-shaped sleeves can also be fabricated from heat-recoverable or non-heat-recoverable material using the method disclosed herein.
r~he invention will now be described, by ~ray o~
E2ample only, with re~erence to the accompanying drawings, ~0 in which:
Figure 1 is a cross-section through a thermoplastic ~05~515 in~ert, prior to com~ression bonding, taken along the length o~ two sheets to be joined;
~?i~ure 2 is a cross-section through the assembly o~ l?igure 1 during bonding;
~igure 3 is a cross-section through the assembly of ~igure 1 after bonding;
igure 4 is a perspective view of an insert which may be used in accordance with the present invention;
Figure 5 is a perspective view of a further insert;
~i~ure 6 is a top view of an assembly having a perforated insert according to the present invention;
Figure 7 is a cross-section through an assembly ~or joining the ends of polymeric sheet material, the assembly having a foraminous rein~orc-ing member; and Figure 8 is a cross-section through an insert which may be used in accordance with the in-vention.
Referring now the drawings, Figures 1 to 3 depict a welding operation taken in cross-section along -the length of the sheets to be joined. An insert 1, which is generally "I"-shaped in cro~s-section is disposed between the abutting edges oi~ polyneric sheet segrnents 2 and 3, the edges of the segments being inserted in the channels bottoming on the up-standing portion o~ the "I". To make a sleeve, the edges would be the opposite ends of a single length of m~terial.
The resulting assembly is compressed, as by heated platens 4 and 5 (~?igure 2), thus causing the polyrnerlc constitutent o~

~OS~5~5 the in~ert to ~low and wet adjoining sur~aces of the segments to be joined as well as activating the crosslinking agent.
Preferably, dams (not shown) approximating in thickness to the compres~ed thickness of the polymeric sheet are dispos~d at each end of the area to be welded and inhibit flow of the melted insert material begond the weld ends.
Once crosslinking bet~een the melted insert and the polymeric sheet has been ef~ected, the platen may be cooled to reduce the tem~erature, whereafter the upper ~laten 4 is withdrawn, freeing the formed joint (~igure 3~. ~he platens may be conventionally heated, either electrically or with steam. Preferably, heating and cooling are e*fected by sequentially forcing steam and cooling water through passage-ways formed in the platen itself, With reference to ~igure 3, it will be noted that the resulting butt-weld 6 is contiguous both with the aligned edges 7 and 8 of sheets ~ and 3 and with regions of their primary sur~aces immediately adjacent to those edgeæ. In this fashion, the weld is made continuous past the aligned edges of the polymeric sheet and the cloth in the foraminous member tif used - see below), so that failure along the interfaces between the weld and edges 7 and 8 is less likely.
To this end, the insert is so configured and positioned as to ensure that those regions of the primary sheet surfaces are wetted during melting of the insert material. ~he generally "I"-shaped insert depicted in ~igures 1 to 3 is desi~ned with this in mind.
Perforated inserts having generally "I" beam sha~ed profiles which may be used in accordance with the present in-vention are illustrated in ~igures 4 and 5. In ~igure 4,insert member 13 is shown having perforations 14 through the :105iZS~5 flanges 15 and 16 forming the channels of the "I"-shaped memberO As shown in ~ ure 4, the perforations are paired, althou~h it will be appreciated that other relationships, for example, a staggered arrangement, will be satisfactory.
Figure 5 shows a pre~erred perforated insert, wherein a generally "I"-shaped member 17 is provided with perforations 18 through the upright portion 19 connecting flanges 20 and 21.
Unexpectedly, it has been found that by u~ing a perforated insert as illustrated, for example, in ~igure 4 or ~igure 5 in a butt welding assembly, a top view of which is shown in ~igure 6, substantial advantages may be obtained, even though the use of a perforated insert 22 does not provide any unimpeded channels by which the gaseous matter leading to voids or blisters can escape as the weld is formed. Althougrh the applicants do not wish to be bound by any hypothesis, it is thought that the gaseous products or entrapped air gather within the perforations as the platens are closed and heating beginsO As the insert flows to fill the perforations, the air is displaced from between the insert and platens. ~ccordingly, when the perforations pass through the centre upright of the insert, the diameter of the perforations should in general be at least as great as the width of the upright in order to provide a means by which air trapped ~ithin the channels ca~
migrate into the perforations. Preferably, as shown in Fi~ure 6, they are wider.
When the sheet material to be joined by the process Or the invention is heat-recoverable, a foraminous rein-forcing member, or example~ member 23 shown in ~`igure 7 is preferably used in conjunction with the insert. Member 22 may comprise, for e~ample, a length of fibre glass cloth. ~he manner in which reinforcing members may be employed and the iOS;~515 adva~tages attainable by their use are fully set out in U.~.
Patent l~o. 3,991,24~ need not be repeated hereO
With reference no~1 to Figure 8, there is shown, in cross section, a pre~erred insert 24 for use in accordance with this inventionO An end of the heat-recoverable sheet i~
~irst inserted in the channel formed by converging flanges 25 of the generally "I'l-shaped member. ~he insert may have a reinforcement attached to it, ~or example, by using double-faced tape or adhesive to fa,cilitate alignment of the com-ponents of the assembly between the platens. ~he resultingsubassembly is inserted between the platens and the free end of the sheet inserted into the other channel of the "I"-shaped member. ~he diverging flanges 26 o~ this i~ert form an opening which is pre~erably wider th~n the thickness of the sheet and thereby facilitate the insertion.
~he ~ollowing Example illustrates the invention:
A perforated insert was used to butt weld sheet material having the ~ollowing composition:

Wt. (%~
Hytrel* 5555 2500 Vistalon* 702 24.0 Alathon* 3170 30.0 aarbon black 15.0 Finely divided carbon black 2.0 I~,N-m-phenylenedimaleimide 0.5 Polycarbodiimide 1.5 Acid accepter 1.5 Irganox* 1010 0.5 Eytrel* 5555 is a segmented polyether ester having a shore D hardness o~ 55, available from the Dupont ~oO

_ 19 _ * "Hytrel", "Vistalon", "Alathon" and "Irganox" are trade marks ~ o~Sl5 Vistalon* 702, available from th~ Exxon Co., is an ethylene-propylene rubber comprising about 70~0 ethylene. Alathon*
3170 is an ethylene/vinyl acetate (18%) copolymer o~ melt index 2.5 at 190C and 2160 gm. Polycarbodiimide is a stabilizer again~t hydrolysi~. ~he acid accepter employed tLectro* 78) was a tetraba~ic lead ~umarate. Irgano~* 1010 is an antio~idant of the ~ormula tetrakis [methylene-3-(3',5'-di-t-butyl-4'-hydro~yphenyl)propionate] methane.
3heet material o~ this composition 24 inche~ wide and 60 milæ thick was joined using ~n insert hav~gthe compo~ition previously described as pre~erred ~or the insert. ~he insert had a cross-section as sho~n in ~igure 8 and was provided with perforations through each flange, the perforations being 3/16 inch in diameter and separated by ~ inch (measured centre to ce~tre). ~he insert wa~ 2~ inches long to provide 1/4 inch gap~ between the insert and a dam at either end. A~ter welding, no void~ were visible. Small pits resulting ~rom lncomplete filling of the perforations ~ere, however, visible.
A similar insert having 1/4 inch perforatio~s separated by ~ inch (measured centre to centre) through the centre of the insert was used to weld the same sheet material resulting in a weld with no visible pits or void~. The~e E~amples demonstrate that void-free sleeves can be obtained by the method o~ this inventionO

- 2~ -"Vistalon", "Alathon", "Lectro" and "Irgano~" are trade mark~.

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of joining abutting surfaces of polymeric material which comprises disposing between the surfaces an insert formed of thermoplastic material having dispersed therein a heat-activatable crosslinking agent, the insert having one or more perforations there through, and heating the surfaces and insert under compression at a temperature sufficient to cause the insert to flow and wet the surfaces and to cause the crosslinking agent to be activated, whereby the surfaces are joined by crosslinked weld material and the escape of gases from the weld area is facilitated by the perforations.

2. A method as claimed in claim 1, wherein the insert flows completely to fill the perforations.

3. A method as claimed in claim 1, wherein the insert is generally I-shaped in cross-section.

4. A method as claimed in claim 1, wherein the perforations pass through the centre upright of the insert.

5. A method as claimed in claim 4, wherein the diameter of each of the perforations is at least as great as the width of the upright.

6. A method as claimed in claim 3, wherein the perforations pass through the flanges of the insert.

7. A method as claimed in claim 1, wherein the modulus of the weld material is at least equal to that of the polymeric material joined thereby.

8. A method as claimed in claim 1, wherein the modulus of the weld material is at most three times that of the polymeric material joined thereby.

9. A method as claimed in claim 1, wherein the polymeric material is crosslinked.

10. A method as claimed in claim 1, wherein the surfaces are the edges of a sheet.

11. A method as claimed in claim 1, wherein the surfaces are opposite edges of a single sheet.

12. A method as claimed in claim 10, wherein the insert is so disposed between the edges that, on heating, the melted insert engages the primary surface of the sheet adjacent to the edges.

13. A method as claimed in claim 12, wherein the insert is generally "I"-shaped in cross-section, and the edges contact the upright of the "I" prior to heating.

14. A method as claimed in claim 1, wherein the thermoplastic material of the insert also comprises a poly-functional monomer.

15. A method as claimed in claim 1, wherein the crosslinking agent is a peroxide.

16. A method as claimed in claim 10, wherein the sheet comprises polyethylene, and the thermoplastic material of the insert is selected from the group comprising ethylene/
vinyl acetate, ethylene/methyl acrylate, ethylene/ethyl acrylate copolymers, and a blend of one or more such copolymers with a minor proportion of polyethylene.