Electrofusion joint and hot water supply header using the same

Abstract: An electrofusion joint in which a recess is provided in the same surface as, and adjacent to, the joining portion, which is to be melted to form the joint.

Description: The present invention relates to an electrofusion joint with which members such as polyolefin pipes, in particular, conduits for supplying hot water are joined or connected either to themselves or to the lateral side of other conduits. The present invention also relates to an electrofusion joint fitted with a connecting member for establishing connection to terminals such as nozzles on a conduit header. The present invention further relates to a header for distributing hot water from a heat source providing apparatus such as a hot water supplier to more than one site of use. Pipes made of crosslinked polyolefins such as silane-crosslinked polyethylene are used as conduits for supplying hot water. Crosslinked polyolefins perform satisfactorily at elevated temperatures as evidenced by high heat resistance and good high-temperature creep characteristic and have a shape-retaining capability. However, unlike non-crosslinked polyolefins, crosslinked polyolefins cannot be fused to members to be joined. Hence, electrofusion joins used to connect non-crosslinked polyolefin pipes are not applicable to crosslinked polyolefin pipes. It is also impossible to form electrofusion joints using crosslinked polyolefins. Therefore, crosslinked polyolefin pipes are currently joined by means of mechanical couplings, or joints that rely upon mechanical couplings are produced from crosslinked polyolefins. However, mechanical joints used in connecting hot water pipes which require high strength to withstand high operating pressures involve a complicated structure and cannot be handled efficiently in piping work. Further, compressive sealing performed with these mechanical joints causes the disadvantage that their performance will decrease with time as exemplified by deterioration in the reliability of sealing due to creeping in pipes or stress relaxation of pipes and sealants. Further, if thin-walled resin pipes are connected by conventional electrofusion joints, the joined portions will thermally deform and bulge inwwardly to restrict the fluid passage. An electrofusion joint formed of a thermoplastic resin is known that is provided with a fine orifice in the body of the joint, through which the resin that has become molten when a member of interest is joined by fusion will come out to provide a measure for checking whether said member has completely fused to the joint (see, for example, Unexamined Published Japanese Patent Application No. 61-62696). However, being formed of a kind of thermoplastic resin, this prior art electrofusion joint has the disadvantage that the molten resin coming out of the orifice has the same color as the surrounding resin of which the joint is formed and that extrusion of this resin is difficult to identify. Thus, the time at which fusion has been completed cannot be detected precisely. Further, connecting terminals such as nozzles on a conduit header are usually metallic and it is difficult to connect them to crosslinked thermoplastic resin pipes. Fig. 16 is a diagram showing a piping system for supplying hot water. Shown by 81 in the diagram is a header by which hot water from a heat source providing apparatus 82 such as a hot water supplier is distributed to a faucet 83 in the kitchen, a faucet 84 in the lavatory, a faucet 85 in the bathroom, and other outlets. The heat source providing apparatus 2 receives water from a supply pipe 86, heats it by burning the gas supplied through a gas pipe 87, and supplies the heated water through a hot water main 88 to the header 81. Header 81 is shaped of a metal in hollow form and has a single receptacle 91 on the inlet side and a plurality of receptacles of same diameter 92a, 92b, 92c, 92d, ... on the outlet side. The receptacle 91 on the inlet side is connected to the hot water main 88 by fastening means such as screws, whereas the receptacles 92a, 92b, 92c, 92d, ... on the outlet side are connected to branch pipes 93a, 93b, 93c, 93d, ... by fastening means such as screws. The branch pipes 93a, ... have such bore diameters that faucets 83, 84, 85, ... are supplied with hot water in the necessary flow rates that correspond to the capacities of the respective faucets. The conventional header 81 has had several problems on account of the fact that it is formed of metal. First, it is liable to corrosion and is heavy. Second, it is difficult to connect to plastic pipes and is not suitable for large-scale production. Further, the header 81 fitted with a plurality of receptacles of same diameter 92a, ... on the outlet side has to be connected to branch pipes 93a, ... of smaller diameter by means of reducers and this has been another factor that makes the header unsuitable for large-scale production. FR-A-2,606,487 discloses an electrofusion joint in which a meltable layer is provided around the entire internal surface of the joint. EP-A-0,333,379 discloses an electrofusion joint made entirely of non-crosslinked polyolefin or in which the layer of non-crosslinked polyolefin covers the entire internal surface of the joint. Therefore, a first aim of the present invention is to provide an electrofusion joint that uses a crosslinked thermoplastic resin such as a crosslinked polyolefin to provide enhanced heat resistance, that yet allows for fusion to insure enhanced sealing property and that can be handled with efficiency. A second aim of the present invention is to provide an electrofusion joint that has not only the features described above but also a capability for fusion without causing thermal deformation of the member to be joined. A third aim of the present invention is to provide an electrofusion joint that uses a crosslinked thermoplastic resin to provide enhanced heat resistance, that enables the operator to easily check the completion of joint fusion, thereby insuring that member to be joined is completely fused to the joint to provide enhanced sealing performance, and that can be handled with efficiency. A fourth aim of the present invention is to provide an electrofusion joint fitted with a connecting member that uses a crosslinked thermoplastic resin to provide enhanced heat resistance, that yet allows for fusion to insure enhanced sealing performance, that enables crosslinked thermoplastic resin pipes to be easily joined to connecting terminals such as nozzles on a conduit header, and that can be handled with efficiency. A fifth aim of the present invention is to provide an electrofusion joint fitted with a connecting member that has not only the features described above but also a capability for fusion without causing thermal deformation of the member to be joined. A sixth aim of the present invention is to provide an electrofusion joint that uses a crosslinked thermoplastic resin to provide enhanced heat resistance, that yet allows for fusion to insure enhanced sealing performance, that can be connected to the lateral side of a conduit at will, and that can be handled with efficiency. A seventh aim of the present invention is to provide a header for supplying hot water that is lightweight and corrosion-resistant, that can be easily connected to plastic pipes, and that is suitable for large-scale production. An eighth aim of the present invention is to provide a header for supplying hot water that is lightweight and corrosion-resistant, that can be easily connected to plastic pipes having different bore diameters, and that is suitable for large-scale production. According to the present invention there is provided an electrofusion joint comprising: a main body portion comprised of a crosslinked thermoplastic resin layer; at least one joining portion comprising a non-crosslinkable thermoplastic resin layer, and adapted to be heat-welded to a pipe, said joining portion being integrally moulded in at least one major surface of said main body portion; and an electrical heating wire provided in or on said joining portion; characterised by: a recess formed by a radially inwardly tapered surface and disposed in the surface of the main body in which the joining portion is embedded, said recess being adjacent said joining portion on the side of the extreme end of the electrofusion joint without extending to the extreme end of the electrofusion joint. Optionally according to the present invention, said crosslinked thermoplastic resin layer and the non-crosslinked thermoplastic resin layer on the inner surface thereof have different colours, which crosslinked thermoplastic resin layer has a fine orifice that extends from its outer surface to a point close to the boundary between said crosslinked thermoplastic resin layer and said non-crosslinked thermoplastic resin layer. The present invention also provides a header for distributing hot water from a heat source, comprising a header body comprising a crosslinked thermoplastic resin layer, said header body being provided with at least one inlet and at least one outlet; wherein at least one of said at least one inlet and at least one outlet of said header body comprises a joint as described above. The present invention will be further explained hereinafter with reference to the following description of exemplary embodiments and the accompanying schematic drawings, in which: Figs. 1, 2 and 3 are partial fragmentary views of three examples of an electrofusion joint according to the first embodiment of the present invention; Fig. 4 is a partial fragmentary front view of an example of an electrofusion joint according to the second embodiment of the present invention; Fig. 5 is a cross section of an example of an electrofusion joint equipped with a connecting member according to the third embodiment of the present invention; Fig. 6 is a front view of an illustrative electrofusion joint according to the fourth embodiment of the present invention; Fig. 7 is a cross section taken on line A-A of Fig. 6; Fig. 8 is a bottom view of the electrofusion joint shown in Fig. 6; Fig. 9 is a partial fragmentary front view of the same electrofusion joint that includes a cross section taken on line B-B of Fig. 7 and which illustrates how the joint is mounted on a member to be joined; Figs. 10 and 11 are partial fragmentary front views showing how two other examples of the electrofusion joint according to the fourth embodiment of the present invention are mounted on the member to be joined; Figs. 12 and 13 are partial fragmentary front views of two examples of a header of supplying hot water according to the fifth embodiment of the present invention; Figs. 14 and 15 are partial fragmentary front views of two examples of a header for supplying hot water according to the sixth embodiment of the present invention; Fig. 16 is a diagram showing an illustrative piping system for supplying hot water. The present invention is described in detail below. In the electrofusion joint and the header for supplying hot water of the present invention, the crosslinked thermoplastic resin layer is adjacent to and integral with the non-crosslinked thermoplastic resin layer. Crosslinked thermoplastic resins such as crosslinked polyolefins do not fuse, so even if a crosslinked thermoplastic resin and non-crosslinked thermoplastic resin such as a non-crosslinked polyolefin are shaped by extrusion molding, injection molding or some other methods, the layers of the respective resins will not fuse together but separate from each other. Instead, if a crosslinkable thermoplastic resin yet to be crosslinked and a non-crosslinked thermoplastic resin are thermoformed into an integral unit and with the crosslinkeble thermoplastic resin being subsequently crosslinked, a laminate which is an integral unit of the crosslinked thermoplastic resin and the non-crosslinked thermoplastic resin will be obtained. The electrofusion joint of the present invention uses this laminate in which the crosslinked thermoplastic resin layer unit. The crosslinked thermoplastic resin has a steric structure created by crosslinking between the molecules of a thermoplastic resin. Crosslinking may be achieved with the aid of a crosslinking agent as in the case of silane-crosslinked thermoplastic resins and divinylbenzene-crosslinked thermoplastic resins. Alternatively, direct crosslinking may be effected by such means as organic peroxides and radiation. The crosslinkable thermoplastic resins yet to be crosslinked according to the present invention comprises polyolefins which have a crosslinking ability created by reacting polyolefins with the crosslinking agent, such as a silane compound, but are not yet crosslinked. The electrofusion joint and the header for distributing hot water from a heat source providing apparatus are fabricated by thermoforming such the crosslinkable thermoplastic resin layer yet to be crosslinked and the non-crosslinked thermoplastic resin layer into an integral unit. Thereafter, the crosslinkable thermoplastic resin layer yet to be crosslinked is crosslinked by including crosslinking reaction by means of a hot water treatment and the like. The non-crosslinked thermoplastic resin is selected from among non-crosslinkable thermoplastic resins such as polyolefins which generally are used as thermoplastic resins. Polyolefins are preferably used as thermoplastic resins of the crosslinkable thermoplastic resin yet to be crosslinked and the non-crosslinked thermoplastic resin in the present invention and illustrative polyolefins include homo- and copolymers of α-olefins having 2-20, preferably 2-12, carbon atoms. Preferred α-olefins include ethylene, propylene, butene-1 and 4-methylpentene-1. The present invention is hereunder described in greater detail with reference to the preferred embodiments shown in accompanying drawings. To begin with, the first embodiment of the present invention is described below with reference to Figs. 1 to 3. The electrofusion joint of the present invention is such that at least one its joining portion is fabricated by the following procedure: the non-crosslinked thermoplastic resin layer having a heating electric wire either within or on its outer or inner surface or having said wire wound onto its surface is inserted into an injection mold cavity; a crosslinkable thermoplastic resin is injected and laminated on said non-crosslinked thermoplastic resin layer; thereafter, the crosslinkable thermoplastic resin layer is crosslinked to form the crosslinked thermoplastic resin layer. The thus fabricated electrofusion joint is mounted on the joining parts of members to be joined such as thermoplastic resin (e.g. polyolefin) tubes in such a way that the non-crosslinked thermoplastic resin layer will contact said members. When an electric current is applied to the electric wire, the non-crosslinked thermoplastic resin layer will melt and fuses to the member to become an integral part of the latter. Members to be joined by the electrofusion joint of the present invention are preferably such that the surface layer or the entire portion is made of a thermoplastic resin, in particular, a miscible thermoplastic resin. Members made of non-crosslinked thermoplastic resins such as polyolefins will exhibit particularly good fusing ability. Examples of the first embodiment of the present invention are specifically described below with reference to drawings. Figs. 1, 2 and 3 are front views of electrofusion joints, with their right half shown in cross section, according to three different examples of the first embodiment. In each of these drawing, the electrofusion joint 10 has a tubular crosslinked polyolefin layer 12 forming the main body of the joint, and two joining portions that comprise a non-crosslinked polyolefin layer 13 formed as an integral part of said crosslinked polyolefin layer 12 on the inner side thereof at each of the positions where it contacts two members to be joined 15 and 16, and a heating electric wire 14 provided either within or on the outer or inner surface of said non-crosslinked polyolefin layer 13. The members 15 and 16 are pipes having a layer of a thermoplastic resin such as polyethylene. While they may be totally made of fusible thermoplastic resin, pipes formed of a heat-resistant material such as a crosslinked polyolefin should be coated with a fusible thermoplastic resin on the outer surface. The crosslinked polyolefin layer 12 has a tubular structure that surrounds the entire circumference of the members 15 and 16 in their mating portion 17 and nearby areas. The non-crosslinked polyolefin layer 13 is superposed on the inner surface of the crosslinked polyolefin layer 12 in areas at opposite ends thereof in its longitudinal direction that correspond to the members 15 and 16. The heating electric wire 14 is provided either within the non-crosslinked polyolefin layer 13 as shown in Fig. 1, or in its outer surface as shown in Fig. 2, or in its inner surface as shown in Fig. 3. The electric wire 14 is fitted with leads (not shown) so that it can be connected to an external power source also not shown. The electrofusion joint 10 having the construction described above is fabricated by the following procedure: with the heating electric wire 14 buried in or wound around the non-crosslinked polyolefin layer 13, a crosslinkable polyolefin layer containing a crosslinking agent, a crosslinking aid, a catalyst, etc. in a polyolefin is laminated over said non-crosslinked polyolefin layer 13 to form an integral unit, and thereafter, said crosslinkable polyolefin layer is crosslinked to form the crosslinked polyolefin layer 12. The thus fabricated electrofusion joint 10 is mounted in the mating portion 17 of members 15 and 16 and nearby areas in such a way that the non-crosslinked polyolefin layer 13 will contact said members 15 and 16. When an electric current is applied to the electric wire 14 from an external power source connected by leads (not shown), the non-crosslinked polyolefin layer 13 will melt and fuses to the members 15 and 16 to become an integral part of the latter. In the electrofusion joint 10 of the present invention, the tubular crosslinked polyolefin layer 12 forming the main body of the joint is responsible for imparting heat resistance to the joint. Hence, the joint is capable of retaining its own shape in an effective way even if it is used to join hot water supply pipes. Further, the non-crosslinked polyolefin layer 13 which is an integral part of the crosslinked polyolefin layer 12 fuses to the members 15 and 16 to become an integral part thereof. Hence, the joint 10 has good sealing property which is maintained for a prolonged period even if it is used to join hot water pipes. The shapes and structures of the crosslinked polyolefin layer 12 and the non-crosslinked polyolefin layer 13 are not limited to those shown in Figs. 1-3 and various modifications are possible. In the examples shown in these drawings, members 15 and 16 meet directly each other in the mating portion 17 inside the electrofusion joint 10. If desired, the members 15 and 16 may be caused to meet a projection provided on the crosslinked polyolefin layer 12. In the examples shown in Figs. 1-3, the non-crosslinked polyolefin layer 13 having the heating electric wire 14 is provided independently on either side of the mating portion 17 in the areas of contact with the members 15 and 16. Alternatively, a continuous single layer of non-crosslinked polyolefin may be provided to cover the areas of contact with the members 15 and 16 if the electric wire 14 is to be provided on either the outer or inner surface of the non-crosslinked polyolefin layer 13, it may be wound around said non-crosslinked polyolefin layer 13 or the members 15 and 16. In the examples shown in Figs. 1-3, the electrofusion joint 10 connects two members 15 and 16 in a straight line with their ends meeting each other. The present invention is not limited to these examples alone and may embrace joints having more than one portion for connecting members to be joined such as an elbow joint, a T-joint and a cross joint. A blind joint closed at one end is also included within the present invention. If joints having two or more connecting portions are used, at least one of the connecting portions must be an electrofusion joining portion that comprises the crosslinked polyolefin layer 12 and the non-crosslinked polyolefin layer 13 that is integral with said crosslinked polyolefin layer 12 and which has the heating electric wire 14. Thus, the other connecting portions may have a connecting member such as a screw secured thereto. The main body of the joint need not be totally formed of the crosslinked polyolefin layer 12 but from the viewpoint of molding, it is preferred that the main body of the joint is totally formed of the crosslinked polyolefin layer 12. In the examples described above, the non-crosslinked polyolefin layer having the heating electric wire in the joining portion is formed on the inner surface of the crosslinked polyolefin layer so that it will contact the circumference of the members to be joined. However, the present invention is not limited to this case alone and the non-crosslinked polyolefin layer may be formed on the outer surface of the cross-linked polyolefin layer in such a way that it will join the inside surfaces or ends of the members to be joined. In the examples described above, the electrofusion joint has a tubular joining portion for joining the ends of members to be joined. The present invention is not limited to this case alone and the joining portion may be shaped like a saddle so that it is capable of joining to the lateral side of a member to be joined. As described in detail on the foregoing pages, the electrofusion joint according to the first embodiment of the present invention has a joining portion comprising a non-crosslinked thermoplastic resin layer that is formed as an integral part of a crosslinked thermoplastic resin layer and which has a heating electric wire provided either within or on its outer or inner surface. Hence, this joint performs satisfactorily at elevated temperatures as evidenced by high heat resistance and exhibits good sealing property. In addition, this joint can be handled efficiently in mounting and various other operations. The second embodiment of the present invention is described below with referece to Fig. 4. The electrofusion joint according to the third embodiment of the present invention is the same as the one of the first or second embodiment except that the outer crosslinked thermoplastic resin layer and the inner non-crosslinked thermoplastic resin layer have differenct colors, which crosslinked thermoplastic resin layer has a fine orifice that extends from its outer surface to a point close to the boundary between said crosslinked thermoplastic resin layer and the inner non-crosslinked thermoplastic resin layer. When the non-crosslinked termoplastic resin layer fuses to a member to be joined, the interior of said resin layer is melted to expand by the heat supplied from the current carrying electric wire and the pressure in the molten resin layer will increase. When satisfactory conditions of fusion are reached, the melt of non-crosslinked thermoplastic resin will come out of the fine orifice to indicate the point of time at which the fusion was completed. The extruding non-crosslinked thermoplastic resin has a different color than the surrounding crosslinked thermoplastic resin layer and hence can be readily indentified. The crosslinked thermoplastic resin will not become fluid but remains as a gel even if it is melted, and only the non-crosslinked thermoplastic resin will become fluid and come out of the orifice. The second embodiment of the present invention is described below specifically with reference to Fig. 4, which is a front view of an electrofusion joint, with its right half shown in cross section, according to an example of the third embodiment. This electrofusion joint has such a structure that its right half is symmetric to its left half. Except for the colors of the outer and inner layers and the presence of a fine orifice, the electrofusion joint shown by 25 in Fig.4 has the same construction as the electrofusion joint 10 shown in Fig. 1, so the same components are identified by like numerals and will not be described in detail. In the electrofusion joint 25 shown in Fig.4, the crosslinked polyolefin layer has not only a different color than the non-crosslinked polyolefin layer 13 but also a small orifice 26 that extends from its outer surface to a point close to the boundary between said layer 12 and the non-crosslinked polyolefin layer 13. The orifice 26 is formed simultaneously with the time when the non-crosslinked polyolefin layer 13 is laminated as an integral part of a yet to be cross-linked polyolefin layer . The second polyolefin layer is then crosslinked to form the crosslinked polyolefin layer 12. After fabricating the electrofusion joint 25 in the manner described above, members to be joined 15 and 16 such as polyolefin tubes are mounted in such a way that the polyolefin layers 13 will contact the members 15 and 16. Thereafter, an electric current is applied to the electric wires 14 through leads from an external power source not shown, the non-crosslinked polyolefin layers 13 will melt and fuse to become integral parts of the members 15 and 16. When fusion occurs, the non-crosslinked polyolefin layers 13 will be melted to expand by the heat supplied from the electric wires 14 and the pressure in the melt of non-crosslinked polyolefin layers 13 will increase. When satisfactory conditions of fusion are reached, the melt of non-crosslinked polyolefin will come out of the fine orifices 26 to indicate the time at which the fusion was completed. The extruding non-crosslinked polyolefin has a different color than the crosslinked polyolefin layer 12 and hence can be readily identified. The crosslinked polyolefin layer 12 will not become fluid but remains as a gel even if it is melted and only the non-crosslinked thermoplastic resin will become fluid. Hence, the point of time at which fusion was competed can be correctly indicated, allowing the operator to stop of the application of current to the electric wires 14 at the right time. The electrofusion joints according to the first embodiment of the present invention may be used as the joint of the second embodiment as long as the outer crosslinked thermoplastic resin has a fine orifice that extends from its outer surface to a point close to the boundary between said crosslinked thermoplastic resin layer and the non-crosslinked thermoplastic resin layer having a different color. The joining portion may be tubular or shaped like a saddle as will be described hereinafter. The other joining portion which is a may be fitted with a screw or some other connecting member to establish mechanical coupling to a member to be joined. As described above in detail, according to the second embodiment of the present invention, a crosslinked thermoplastic resin layer has a non-crosslinked thermoplastic resin layer as an integral part that has a different color and which has a heating electric wire provided either within or on the outer or inner surface thereof. Further, said crosslinked thermoplastic resin layer has a fine orifice that extends from its outer surface to a point close to the boundary between the two resin layers. This allows the operator to check in an easy and exact way the point of time when the fusion of joint was completed, thereby insuring that members to be joined are completely fused to the joint. Thus, the electrofusion joint according to the third embodiment of the present invention will perform satisfactorily at elevated temperatures as evidenced by high heat resistance and exhibits good sealing property. In addition, the joint can be handled with efficiency in mounting and various other operations. The third embodiment of the present invention is described below with reference to Fig. 7. The electrofusion joint according to the third embodiment is characterized in that at least one of the portions for establishing connection to other to members of interest in the electrofusion joint of the first embodiment is a connecting portion having a tubular crosslinked thermoplastic resin layer and a connecting member that is inserted inwardly or outwardly of said crosslinked thermoplastic resin layer and secured thereto either mechanically by a screw or some other means or chemically by means of an adhesive material such as a modified thermoplastic resin. Illustrative connecting members that can be used in the fourth embodiment of the present invention include metal tubes and non-metallic tubes such as ceramic tubes and resin tubes having at one end a screw or some other mechanical means which allow for connection to connecting terminals. Such connecting members are preferably secured to the crosslinked thermoplastic resin layer by means of an adhesive material such as a modified thermoplastic resin. The adhesive material is preferably provided between the connecting member and the crosslinked thermoplastic resin layer. If desired, the adhesive material may be dispersed in the crosslinked thermoplastic resin layer. Illustrative modified thermoplastic resins that can be used in the third embodiment of the present invention are polyolefins that have been modified by graft copolymerization with modifying monomers including unsaturated carboxylic acids such as acrylic acid and maleic acid, unsaturated carboxylic acid anhydrides such as maleic anhydride, amides of unsaturated carboxylic acids, and unsaturated epoxides such as glycidyl methacrylate. The content of a modifying monomer in the modified thermoplastic resin is desirably within the range of from 0.001 to 5 wt% of the modified thermoplastic resin. The electrofusion joint fitted with a connecting member according to the third embodiment of the present invention may be fabricated by the following procedure: at least two components, i.e., the non-crosslinked thermoplastic resin layer having a heating electric wire provided either within or on its outer or inner surface and a connecting member having a coating of an adhesive material such as a modified thermoplastic resin, are inserted into an injection mold cavity; a crosslinkable thermoplastic resin is injected and laminated on said non-crosslinked thermoplastic resin layer and said connecting member to form an integral assembly; thereafter, the crosslinkable thermoplastic resin layer is crosslinked to form the crosslinked thermoplastic resin layer. If desired, the adhesive material need not be coated on the connecting member but may be dispersed in the crosslinkable thermoplastic resin which is to be injected into the mold cavity. The thus fabricated electrofusion joint is used in the following manner: the connecting member in the connecting portion is joined to a connecting terminal such as a nozzle on a conduit header; at the same time, a member to be joined such as a thermoplastic resin tube is mounted in such a way that it contacts the non-crosslinked thermoplastic resin layer in the joining portion; when an electric current is applied to the electric wire from an external power source connected by a lead, the non-crosslinked thermoplastic resin layer will melt and fuses to the member of interest to become an integral part of the latter. By following this procedure, the metallic connecting terminal can be connected to the resin tube in a leak-free and reliable manner. The electrofusion joint fitted with a connecting member according to the third embodiment of the present invention is described below with reference to the preferred examples depicted in drawings. Fig. 7 is a cross section of one example of the electrofusion joint fitted with a connecting member. As shown in this diagram, the electrofusion joint indicated by 30 has the same construction as the electrofusion joint 10 shown in Fig. 1 except that the connecting portion on its left side is fitted with a connecting member 33. Hence, the same components are identified by like numerals and will not be detailed in detail. The electrofusion joint 30 shown in Fig. 7 has a tubular crosslinked polyolefin layer 12, a joining portion comprising a non-crosslinked polyolefin layer 13 formed as an integral part of said crosslinked polyolefin layer 12 on the dinner side of one end thereof at a position where it contacts a member to be joined 15, and a heating electric wire 14 provided either within or on the outer or inner surface of said non-crosslinked polyolefin layer 13, a connecting member 33 secured to the other end of said crosslinked polyolefin layer 12 by being inserted inwardly of the same, and a modified polyolefin layer 34 serving as an adhesive material which is provided between the inserted portion 33a of the connecting member 33 and the crosslinked polyolefin layer 12. The crosslinked polyolefin layer 12 has a tubular structure that surrounds the entire circumference of the member 15 and the inserted portion 33a of the connecting member 33 in their mating portion 35 and nearby areas. The non-crosslinked polyolefin layer 13 is superposed on the inner surface of the crosslinked polyolefin layer 12 in an area at one end thereof in its longitudinal direction that corresponds to the member 15. The electric wire 14 is buried in the non-crosslinked polyolefin layer 13. The connecting member 33 is a metal tube which has a screw 36 formed as a mechanical connecting portion in a projection 33b on the side opposite to the inserted portion 33a. A hexagonal flange 37 which is to be gripped with a spanner is formed midway between 33a and 33b. The modified polyolefin layer 34 extends on the entire circumference of the space between the inserted portion 33a of the connecting member 33 and the crosslinked polyolefin layer 12 so as to adhere them together The electrofusion joint 30 having the construction described above is fabricated by the following procedure: the non-crosslinked polyolefin layer 13 having the heating electric wire 14 either buried within or in its outer or inner surface or wound around its surface, and the inserted portion 33a of the connecting member 33 coated with the modified polyolefin layer 34 on the outer surface are inserted into an injection mold cavity; subsequently, a crosslinkable polyolefin containing a crosslinking agent, a crosslinking aid, etc. in a polyolefin is injected and laminated on the non-crosslinked polyolefin layer 13 and the connecting member 33 to form an integral assembly; and thereafter said crosslinkable polyolefin layer is crosslinked to form the crosslinked polyolefin layer 12. The so fabricated electrofusion joint 30 is connected at one end to a connecting terminal such as a nozzle on a conduit header by fitting the screw 36 in the projection 33b of the connecting member 33 into said terminal. Subsequently, the member to be joined 15 such as a polyolefin tube is inserted into the other end of the joint 30 inwardly of the non-crosslinked polyolefin layer 13 until it contacts the inserted portion 33a of the connecting member 33. Thereafter, an electric current is applied to the electric wire 14 from an external power source connected by a lead not shown, whereupon the non-crosslinked polyolefin layer 13 is melted and fuses to become an integral part of the member 15. As in the first embodiment, the electrofusion joint 30 described above has the tubular crosslinked polyolefin layer 12 responsible for imparting heat resistance to the joint, and hence it is capable of retaining its own shape in an effective way even if it is used to join hot water pipes. Further, the non-crosslinked polyolefin layer 13 which is an integral part of the crosslinked polyolefin layer 12 together with the connecting member 33 fuses to the member 15 to become an integral part thereof. Hence, the joint has good sealing property which is maintained for a prolonged period even if it is used to join hot water pipes. The electrofusion joint fitted with a connecting member according to the third embodiment of the present invention is applicable to every shape and structure of electrofusion joint according to the first embodiment as long as it has plurality of open ends, one of which is provided with a connecting portion having a connecting member secured thereto. As described above, the electorfusion joint according to the third embodiment of the present invention has at least a connecting portion that has a connecting member secured to a crosslinked thermoplastic resin layer, and at least a joining potion that comprises a non-crosslinked thermoplastic resin layer formed as an integral part of the crosslinked thermoplastic resin layer and a heating electric wire provided either within or in the outer of inner surface of said non-crosslinked thermoplastic layer. Hence, this electrofusion joint fitted with a connecting member performs satisfactorily at elevated temperatures as evidenced by high heat resistance and exhibits good sealing property. In addition, this joint can be handled efficiently in such operations as mounting members to be joined. The fourth embodiment of the present invention is described below with reference to Figs. 6 to 11. The electrofusion joint according to the fourth embodiment has a first connecting portion and a second connecting portion. The first connecting portion is saddle-shaped joining portion having the construction described in connection with the first embodiment, and the second connecting portion is on the top of this saddle-shaped joining portion. The second connecting portion may be of any construction. For example, it may have the same construction as the first connecting portion which is an electrofusion joining portion comprising a crosslinked thermoplastic resin layer, a non-crosslinked thermoplastic resin layer and a heating electric wire. It may be fitted with a fastening device or some other connecting member as in the third embodiment. Alternatively, a dual structure consisting of a crosslinked thermoplastic resin layer and a non-crosslinked thermoplastic resin layer may be coupled to another electrofusion joint. The electrofusion joint according to the fourth embodiment of the present invention can be fabricated by the following procedure: to begin with, as in the first embodiment, a non-crosslinked thermoplastic resin layer shaped like a saddle which has a heating electric wire buried either within or in its outer or inner surface is positioned in a mold cavity, and a crosslinkable thermoplastic resin layer is injected and laminated in saddle form over the non-crosslinked thermoplastic resin layer, and subsequently crosslinked to form the cross-linked thermoplastic resin layer; the resulting joining portion is used as the first connecting portion; thereafter, the second connecting portion is formed as an integral part on the top of the saddle-shaped crosslinked thermoplastic resin layer in the first connecting member. The thus fabricated electrofusion joint is used in the following manner: the first connecting portion is mounted on the lateral side of a member to be joined such as a tube having a thermoplastic resin layer in such a way that the non-crosslinked thermoplastic resin layer will contact said member to be joined; then, as in the first embodiment, an electric current is applied to the heating electric wire, whereupon the non-crosslinked thermoplastic resin layer will melt and fuse to the member of interest as an integral part thereof. Subsequently, the connecting terminal of another member to be joined is connected to the second connecting portion formed on the top of the first connecting portion, and this enables the second member to be connected to any desired part of the lateral side of the first member. The second connecting portion of the electrofusion joint may be formed by repeating the procedure of fabricating a joining portion as described in connection with the first embodiment and the fabricated joining portion can be fused as an integral part of a member to be joined such as a tube having a thermoplastic resin layer. The second connecting portion having a connecting member as in the fourth embodiment may be obtained by shaping a non-crosslinked thermoplastic resin layer together with connecting member positioned in a mold cavity. The connecting member can be securely adhered to this connecting member if shaping is performed with an adhesive material such as a modified thermoplastic resin being coated in the area where the connecting member is buried. The second connecting portion thus obtained may be connected to the connecting terminal of another member to be joined by making use of mechanical connecting means such as a screw or a coupler. The second connecting portion of a dual structure may be obtained by molding a non-crosslinked thermoplastic resin layer on the circumference of a crosslinked thermoplastic resin layer as an integral part thereof by repeating the procedure described above. The resulting connecting portion has a fusible non-crosslinked thermoplastic resin layer on its circumference. This second connecting portion can be connected to another member to be joined by bringing it into end-to-end relationship with the connecting terminal of this member and covering the entire circumference of the mating portion and nearby areas with another electrofusion joint. Specific examples of the fourth embodiment of the present invention are hereunder described with reference to Figs. 6 to 11. Fig. 6 is a front view of an electrofusion joint according to an example of the sixth embodiment. Fig. 7 is a cross section taken on line A-A of Fig. 6. Fig. 8 is a bottom view of the joint shown in Fig. 6. Fig. 9 is a cross section taken on line B-B of Fig. 7 and show how the joint is mounted on a member to be joined. The electrofusion joint generally indicated by 50 has the first connecting portion 51a formed at one end as a saddle-shaped joining portion and the second connecting portion 51b integral with the first connecting portion 51a which is formed as a tubular joining portion in the center of the top of 51a. The first connecting portion 51a has a crosslinked polyolefin layer 12a in saddle shaped having an opening 52 in the central area, a non-crosslinked polyolefin layer 13a in saddle shaped formed as an integral part of the crosslinked polyolefin layer 12a on the inner surface thereof at the position where it contacts a member to be joined 15a, and a heating electric wire 14a provided within the non-crosslinked polyolefin layer 13a in saddle shape. The second connecting portion 51b has a tubular crosslinked polyolefin layer 12b formed as an integral part of the crosslinked polyolefin layer 12a, a tubular non-crosslinked polyolefin layer 13b formed as an integral part of the crosslinked polyolefin layer 12b on the inner surface thereof at the position where it contacts another member to be joined 15b, and a heating electric wire 14b provided within the non-crosslinked polyolefin layer 13b. Each of the members 15a and 15b is a pipe having a layer of thermoplastic resin such as polyethylene. While they may be totally made of a fusible thermoplastic resin, those which are made of heat-resistant materials such as crosslinked polyolefins should be coated with fusible thermoplastic resin on their outer surface. The member 15a has a bypass hole 53 in its lateral side on a position that corresponds to the opening 52. The electrofusion joint 50 having the construction described above can be fabricated by the following procedure: non-crosslinked polyolefin layers 13a and 13b having heating electric wires 14a and 14b buried therein are inserted into a mold cavity; a crosslinkable polyolefin containing a crosslinking agent, a crosslinking aid, a catalyst, etc. in a polyolefin is injected and laminated on the non-crosslinked polyolefin layers 13a and 13b to form an integral assembly; thereafter, the crosslinkable polyolefin layer is crosslinked to form crosslinked polyolefin layers 12a and 12b. The thus fabricated electrofusion joint 50 is used in the following manner: it is first mounted in such a way that the non-crosslinked polyolefin layer 13a in saddle shape will contact the lateral side of the member 15a such as a polyolefin tube ; when an electric current is applied to the electric wire 14a, the non-crosslinked polyolefin layer 13a will melt and fuse to become an integral part of member 15a. After the melting and fusing, the bypass hole 53 is formed on the lateral side of the member 15a, which is positioned corresponding to the opening 52, using a tool for making a hole. Subsequently, the end of the other member to be joined 15b is inserted into the second connecting portion 51b until it contacts the non-crosslinked polyolefin layer 13b; when an electric current is applied to the electric wire 14b, the non-crosslinked polyolefin layer 13b will fuse to become an integral part of member 15b. Thus, two members 15a and 15b will connect to each other via the bypass hole 53 and the opening 52. In the electrofusion joint 50 described above, the crosslinked polyolefin layers 12a and 12b forming the main body of the joint are responsible for imparting heat resistance to the joint. Hence, the joint is capable of retaining its own shape in an effective way even if it is used to join hot water supply pipes. Further, the non-crosslinked polyolefin layers 13a and 13b which are integral parts of the crosslinked polyolefin layers 12a and 12b, respectively, will fuse to the members 15a and 15b to become integral parts thereof. Hence, the joint has good sealing property which is maintained for a prolonged period even if it is used to join hot water pipes. Figs. 10 and 11 show other examples of the fourth embodiment and they are equivalent to cross sections taken on line B-B of Fig. 7. In each of the electrofusion joints indicated by 55 and 56, the first connecting portion 51a is composed of a joining portion that is shaped like a saddle as in the example shown in Figs. 6 to 9. In the electrofusion joint 55 shown in Fig. 10, the second connecting portion 51b is such that the inserted portion 33a of a connecting member 33 is securely adhered by means of on adhesive layer 34 such as a modified polyolefin layer to the tubular crosslinked polyolefin layer 12b shaped as an integral part of the crosslinked polyolefin layer 12a. The connecting member 33 is a metal tube which has on its projection 33b a screw or some other means 36 by which it is joined to member 15b. A hexagonal flange 37 which is to be gripped with a spanner is formed midway between 33a and 33b. With the connecting member 33 inserted into an injection mold cavity as in the third embodiment, the electrofusion joint 55 having the construction described above is fabricated by the procedure already described in connection with the fourth embodiment. The first connecting portion 51a is connected to member 15a in the manner already described. The second connecting portion 51b is joined to the other member 15b by the joining means 36. The joining means 36 by which the connecting member 33 is joined to member 15b may assume another form such as a socket having a female screw. In the example shown in Fig. 11, the second connecting portion 51b may consist of a crosslinked polyolefin layer 12a, a tubular crosslinked polyolefin layer 12b formed as an integral part thereof, and a tubular non-crosslinked polyolefin layer 13b laminated on the circumference of layer 12b as an integral part thereof. The electrofusion joint 56 having the construction described above is fabricated by the procedure already described, with the non-crosslinked polyolefin layer 13b being shaped by lamination over the tubular crosslinked polyolefin layer 12b as an integral part thereof. The second connecting portion 51b may be connected to the member 15b by means of another electrofusion joint. Alternatively, it may be directly connected to a member constructed as an electrofusion joint. The construction of the second connecting portion 51b described above is not limited to those shown in Figs. 6 to 11. The heating electric wires 14a and 14b may be provided on either the outer or inner surface of the non-crosslinked polyolefin layers 13a and 13b. As described in detail on the foregoing pages, the electrofusion joint according to the fourth embodiment of the present invention has a joining portion comprising a crosslinked thermoplastic resin layer in saddle form, a non-crosslinked thermoplastic resin layer formed as an integral part of said crosslinked thermoplastic resin layer, and a heating electric wire provided either within or on the outer or inner surface of said non-crosslinked thermoplastic resin layer. Hence, this joint enables one member to be fused to the lateral side of another member while performing satisfactorily at elevated temperatures as evidenced by high heat resistance and exhibiting good sealing property. In addition, this joint can be handled efficiently in mounting and various other operations. The fifth embodiment of the present invention is described below with reference to Figs. 12 and 13. This embodiment provides a header for distributing hot water from a heat source providing apparatus. The header comprises a header body shaped of a crosslinked thermoplastic resin in a hollow form, a receptacle provided on the inlet side of said header body and at least one receptacle on the outlet side, at least part of which receptacles is an electrofusion joining portion of the same type as used in the first embodiment which comprises a crosslinked thermoplastic resin and a non-crosslinked thermoplastic resin having a heating electric wire provided either within or on its outer or inner surface. When the non-crosslinked thermoplastic resin layer and the heating electric wire are to be formed at one or more receptacles, the other receptacles may be fitted with connecting member commonly used to establish connection to pipes. An illustrative connecting member is a metal tube having at one end a screw or some other means for establishing coupling to connecting terminals. The connecting member is preferably secured to the crosslinked thermoplastic resin layer by means of bonding with an adhesive material such as a modified thermoplastic resin. The adhesive material is preferably interposed between the connecting member and the crosslinked thermoplastic resin layer but if desired, it may be dispersed in the crosslinked thermoplastic resin layer. The header according to the fifth embodiment of the present invention is fabricated by the following procedure: a tubular non-crosslinked thermoplastic resin layer having a heating electric wire provided either within or on its outer or inner surface is inserted into an injection mold cavity; a crosslinkable thermoplastic resin is injected and laminated on the non-crosslinked thermoplastic resin layer to form an integral unit; thereafter, the crosslinkable thermoplastic resin layer is crosslinked to form the crosslinked thermoplastic resin layer. If a connecting member is necessary, it may be attached to the completed header. Alternatively, it may be injection molded together with the other components, with it being inserted into the mold cavity after it was coated with an adhesive material such as a modified thermoplastic resin. In this latter case, the adhesive material need not be coated on the non-crosslinked thermoplastic resin layer but may be dispersed in the crosslinkable thermoplastic resin to be injected. The so fabricated header is used with the water supply main from the heat source providing apparatus being connected to the receptacle on the inlet side and branch pipes to the necessary faucets being connected to the receptacles on the outlet side. In the case where a connecting member is used, the header is installed in such a way that the connecting member is connected to the connecting terminal of a member to be joined (i.e., a pipe) whereas a plastic pipe such as a thermoplastic tube is brought into contact with the non-crosslinked thermoplastic layer. When an electric current is applied to the heating electric wire, the non-crosslinked thermoplastic resin will melt and fuse to become an integral part of the plastic pipe. This enables the header to be connected to all pipes in a leak-free and reliable manner. As in the case of members to be joined by the electrofusion joints described on the foregoing pages, pipes to be connected to the header are preferable such that the surface layer or the entire portion is made of a thermoplastic resin, in particular, a miscible thermoplastic resin. Pipes made of non-crosslinked thermoplastic resins such as polyolefins will exhibit particularly good fusing ability. The header for supplying hot water according to the seventh embodiment of the present invention is described below in a specific way by reference to the preferred examples shown in Figs. 12 and 13, which are partial fragmentary front views of two different examples of the header. The header generally indicated by 60 is composed of a hollow header body 61 and a crosslinked polyolefin layer 62 forming a tubular receptacle 91 on the inlet side and a plurality of tubular receptacles 92a, 92b, 92c and 92d on the outlet side. The header body and the receptacles are shaped of a crosslinked polyolefin into an integral unit. The receptacle 91 on the inlet side is open at one end of the header 60 whereas the receptacles 92a, ... on the outlet side which are arranged in a row are open in a direction perpendicular to the receptacle 91. All receptacles 91, 92a, 92b, 92c and 92d (in the example shown in Fig.12) or receptacles on the outlet side 92a-92d (in the example shown in Fig. 13) have the following construction: on the inner surface of the tubular crosslinked polyolefin layer 62 shaped as an integral part of the header body 61, a tubular non-crosslinked polyolefin layer 63 is laminated to become an integral part of the crosslinked polyolefin layer 62, and a coil of heating electric wire 64 is buried within the non-crosslinked polyolefin layer 63 and connected to terminals 65 and 66 to form an electrofusion joining portion of the same type as described in connection with the first embodiment. A connecting member 33 is attached to the receptacle 91 on the inlet side of the header shown in Fig. 13. The connecting member 33 is a metal tube which has a screw 36 formed on the projection 33b on the side opposite to the inserted portion 33a to serve as connecting means. A hexagonal flange 37 which is to be gripped with a spanner is formed midway between 33a and 33b. A modified polyolefin layer 34 is disposed along the entire circumference of the space between the inserted portion 33a of the connecting member 33 and the crosslinked polyolefin layer 62 to adhere them together. The header 60 having the construction described above is fabricated by the following procedure: the non-crosslinked polyolefin layer 63 having the heating electric wire 64 either buried within or on its outer or inner surface or wound around its surface is inserted into an injection mold cavity; a crosslinkable polyolefin containing a crosslinking agent, a crosslinking aid, etc. in a polyolefin is injected and laminated over the non-crosslinked polyolefin layer 63 to form an integral unit; thereafter, the crosslinkable polyolefin layer is crosslinked to form the crosslinked polyolefin layer 62. The connecting member 33 may be either attached to the completed header or injection molded together with the other components, with the inserted portion 33a of the connecting member 33 being inserted into the mold cavity after it was coated with the modified polyolefin layer 34 on the outer surface. The thus fabricated header 60 is used in such a way that the water main 88 from the heat source providing apparatus 82 (see Fig. 16) is connected to the receptacle 91 on the inlet side whereas branch pipes 93a, 93b, 93c and 93d to faucets 83, 84, 85, etc. are connected to the receptacles 92a, 92b, 92c and 92d, respectively, on the outlet side. If the header is fitted with the connecting member 33 as shown in Fig. 13, said member is connected to the connecting terminal of the water main 88 whereas branch pipes 93a, 93b, 93c and 93d which are made of plastics such as polyolefins are mounted in such a way that they contact the non-crosslinked polyolefin layer 63 in receptacles 92a, 92b, 92c and 92d as shown in Fig. 16. In the example shown in Fig. 12, both the water main 88 and branch pipes 93a, 93b, 93c and 93d are connected to the receptacles 91, 92a, 92b, 92c and 92d in such a way they contact the non-crosslinked polyolefin layer 63 in these receptacles as shown in Fig. 16. Thereafter, an electric current is applied to the electric wire 64 via terminals 65 and 66, whereupon the non-crosslinked polyolefin layer 63 will melt and fuse to become an integral part of each pipe to be connected. This insures that the header 60 is positively connected to the necessary pipes in a leak-free way. The connecting member 33 is suitable for establishing connection to metal pipes whereas the electrofusion joint is suitable when a plastic pipe is to be connected. Thus, the layout of these devices can be determined in accordance with the specific pipe to be joined. The header 60 has the crosslinked polyolefin layer 62 as a member responsible for imparting heat resistance, so it is capable of retaining its own shape effectively even if it is used to join hot water pipes. Further, the non-crosslinked polyolefin layer 63 has fused to become an integral part of the crosslinked polyolefin layer 62 together with the connecting member 33, so the header will exhibit good sealing property which is maintained for a prolonged period even if it is used to join hot water pipes. In the example shown in Fig.13, the connecting member 33 is fitted to the receptacle 91 on the inlet side but it may be attached to one or more of the receptacles on the outlet side. The connecting member 33 may have connecting means other than screws. As described on the foregoing pages, the header according to the fifth embodiment of the present invention has an electrofusion joining portion that comprises a non-crosslinked thermoplastic resin layer and a heating electric wire and that is fitted to at least one of the receptacles on the header body made of a crosslinked thermoplastic resin. Because of this arrangement, the header has high heat resistance, exhibits good sealing performance, is lightweight and resistant to corrosion, can be easily connected to plastic pipes, and is suitable for large-scale production. The sixth embodiment of the present invention is described below with reference to Figs. 14 and 15. The header for supplying hot water according to the present invention comprises a header body shaped of a crosslinked thermoplastic resin in hollow form and a plurality of receptacles on the outlet side of the header body which are formed of a crosslinked thermoplastic resin as an integral part of said header body in such a way that said tubular resin layer has a bore diameter sufficient to admit the flow rate of hot water distributed to each receptacle. Preferably, the receptacle on the inlet side and the plurality of receptacles on the outlet side are each made of a laminate of the tubular crosslinked thermoplastic resin layer and a tubular non-crosslinked thermoplastic resin layer that is capable of joining by fusion. More preferably, each of these receptacles is an electrofusion joining portion that has a heating electric wire provided either within or on its outer or inner surface as in the first embodiment. In fabricating the header according to the sixth embodiment, an injection mold must be employed whose cavity configuration is such that a plurality of receptacles having different bore diameters can be molded on the outlet side. Lamination of a non-crosslinked thermoplastic resin layer on a receptacle, placement of a heating electric wire and fixation of a connecting member can be performed in entirely the same manner as in the fifth embodiment and will not be described in detail. The so fabricated header is used with the water supply main from the heat source providing apparatus being connected to the receptacle on the inlet side and branch pipes to the necessary faucets being connected to the receptacles on the outlet side. The receptacles on the outlet side having different bore diameters are connected to branches having the corresponding bore diameters. In the case where a connecting member is attached to a receptacle, it is coupled to the connecting terminal of the pipe to which said receptacle is to be connected. In the case where a non-crosslinked thermoplastic resin layer is laminated on a receptacle, a plastic pipe such as a thermoplastic resin tube is mounted in such a way that it contacts the non-crosslinked thermoplastic resin layer and thereafter connected by some other means such as an electrofusion joint. If both a non-crosslinked thermoplastic resin layer and a heating electric wire are to be provided on a receptacle, the pipe to be connected is mounted in the same way and a current is applied to the electric wire, whereupon the non-crosslinked thermoplastic resin layer will melt and fuse as an integral part of the pipe. Thus, either method insures that the header is positively connected to the necessary pipe in a leak-free way. Two different examples of the header according to the sixth embodiment of the present invention are shown in Figs. 14 and 15. The headers shown in these drawings are identical to those shown in Figs. 12 and 13, respectively, according to the fifth embodiment except that the receptacles on the outlet side of the header body have different bore diameters, so the same components are identified by like numerals and will not be described in detail. In the header generally indicated by 70 in Figs. 21 and 22, the receptacle 91 on the inlet side open at one end of the header whereas the receptacles 92a, 92b, 92c and 92d on the outlet side which are arranged in a row are open in a direction perpendicular to the receptacle 91 and have different bore diameters Da, Db, Dc and Dd that are sufficient to admit the flow rate of hot water distributed to each receptacle. The header 70 having the construction described above is used with the water main 88 from the heat source providing apparatus 82 being connected to the receptacle 91 on the inlet side and with differently sized branch pipes 93a, 93b, 93c and 93d to faucets 83, 84, 85, etc. being connected to receptacles 92a, 92b, 92c and 92d on the outlet side which have bore diameters corresponding to those of the associated branch pipes. If the header is fitted with the connecting member 33 as shown in Fig. 15, said member is connected to the connecting terminal of the water main 88 whereas branch pipes 93a, 93b, 93c and 93d which are made of plastics such as polyolefins are mounted in such a way that they contact the non-crosslinked polyolefin layer 63 in receptacles 92a, 92b, 92c and 92d as shown in Fig. 16. In the example shown in Fig. 14, both the water main 88 and branch pipes 93a, 93b, 93c and 93d are connected to the receptacles 91, 92a, 92b, 92c and 92d in such a way that they contact the non-crosslinked polyolefin layer 63 in these receptacles as shown in Fig. 23. Thereafter, an electric current is applied to the electric wire 64 via terminals 65 and 66, whereupon the non-crosslinked polyolefin layer 63 will melt and fuse to become an integral part of each pipe to be connected. This insures that the header 70 is positively connected to the necessary pipes in a leak-free way. The header 70 has the crosslinked polyolefin layer 62 as a member responsible for imparting heat resistance, so it is capable of retaining its own shape effectively even if it is used to join hot water pipes. Further, the receptacles 92a, ... one the outlet side of the header body have different bore diameters that are sufficient to admit the flow rate of hot water distributed to each receptacle, so they can be directly connected to branch pipes of different bore diameters without using reducers. Thus, the header insures effective sealing and high operational efficiency. In addition, the non-crosslinked polyolefin layer 63 has fused to become an integral part of the crosslinked polyolefin layer 62 together with the connecting member 33, so the header will exhibit good sealing property which is maintained for a prolonged period even if it is used to join hot water pipes. In accordance with the sixth embodiment of the present invention, a header body and receptacles on its outlet side having different bore diameters are shaped as an integral unit by molding of a crosslinked thermoplastic resin. Thus, the resulting header has high heat resistance, is lightweight and resistant to corrosion, can be easily connected to plastic pipes having different bore diameters, and is suitable for large-scale production. If a non-crosslinked thermoplastic resin layer and even a heating electric wire are provided at the receptacle on the inlet side or the plurality of receptacles on the outlet side, a header can be obtained that exhibits high sealing performance, particularly for a prolonged period, and that allows for easy connection to pipes. EXAMPLES The present invention is hereunder described in greater detail with reference to the following examples. (Example 1) A medium density polyethylene colored by using a coloring agent was used as the non-crosslinked thermoplastic resin 13 of the present invention. The colored medium density polyethylene crosslinked by using 1.5% by weight of a crosslinking agent (vinyl trimethoxysilane) was used as the crosslinked thermoplastic resin 12 of the present invention. A Ni-Cr alloy wire (0.3 mm ) was used as the material of the heating electric wire 14. The electrofusion joint 20 having a structure as shown in Fig. 4, to which the tubes 15 and 16 to be connected having an adaptable caliber of 8 to 21.5 mm can be joined, was fabricated in accordance with the above-described methods. The tubes 15 and 16 to be connected, which were made of the same materials and have an adaptable caliber, were inserted at one end into grooves on opposite sides of the electrofusion joint 20. Thereafter, an electric current was applied to the above-described heating electric wires 14 which have been connected to an external electric power for about 20 seconds by controlling the voltage in order to give an appropriate energy density. For example, conditions for the application of electric current in the case of an adaptable caliber (outer diameter) of the tubes being 13 mm were as follows: resistance of the heating electric wire 14, 14.5 Ω; voltage, 40 V; and the application time, 20 seconds. When sealing properties of the joining portion were checked after completion of the connection, no leakage was found for at least 100 hours under a pressure of 10 kg/cm 2 at 90°C, which confirmed similar sealing properties of the joint to those of the pipes to be connected. It was confirmed also that the joint can be handled more efficiently in mounting and other various operations than in the case of joining using wrenches and the like. (Example 2) A modified polyethylene 34 was prepared as the adhesive material for use in the connecting member 33 made of brass, by performing graft copolymerization of a medium density polyethylene and maleic acid by means of a melt kneading. Using the modified polyethylene thus prepared and the same resin materials and electric heating wire materials as described in Example 1, the electrofusion joint 40 equipped with the connecting member having the structure as shown in Fig. 10 wherein the adaptable caliber of the pipe 15 to be connected is 8 to 21.5 mm was fabricated in accordance with the above-described methods. The pipe 15 to be connected made of the same material having an adaptable caliber was inserted into the joint, and the joining portion was connected by applying electric current to the above-described electric heating wires 14 in the same manner as described in Example 1. When sealing properties of the joining portion were checked after completion of the connection, it was confirmed that the joint had similar sealing properties to those of the pipes to be connected as the results described in Example 1. It was also confirmed that the joint can be handled more efficiently in mounting and other various operations than in the case of joining using wrenches and the like as described in Example 1.

Claims: An electrofusion joint comprising: a main body portion comprised of a crosslinked thermoplastic resin layer; at least one joining portion comprising a non-crosslinkable thermoplastic resin layer, and adapted to be heat-welded to a pipe, said joining portion being integrally moulded in at least one major surface of said main body portion; and an electrical heating wire provided in or on said joining portion; characterised by: a recess formed by a radially inwardly tapered surface and disposed in the surface of the main body in which the joining portion is embedded, said recess being adjacent said joining portion on the side of the extreme end of the electrofusion joint without extending to the extreme end of the electrofusion joint. An electrofusion joint of claim 1, wherein said crosslinked thermoplastic resin comprises a crosslinked polyolefin; and said non-crosslinkable thermoplastic resin comprises a non-crosslinkable polyolefin resin. An electrofusion joint of claim 1 or 2, wherein said joining portion is embedded in said main body portion of the joint on its interior major surface so that the joining portion may be heat-welded to the outer surface of the tubular member. An electrofusion joint of claim 1, 2 or 3, wherein one end of said main body has a tubular configuration; and said joining portion is provided on said tubular end, and embedded in one major surface of said main body. An electrofusion joint of claim 1 or 2, wherein one end of said main body has a saddle-shaped configuration (12c). An electrofusion joint according to claim 3, wherein said crosslinked thermoplastic resin constituting said main body (12) has a colour different from that of said non-crosslinkable thermoplastic resin constituting said joining portion (13) embedded in the interior surface of said main body; and said main body has an orifice (26) extending from the exterior surface of said main body to a point in the vicinity of the boundary between the main body and the joining portion, such that, upon heating of the joining portion, a portion of the softened non-crosslinkable thermoplastic resin of the joining portion is urged by the pressure produced by the heating to rise within said orifice to enable the state of the joining portion to be estimated by visual inspection from the exterior of the main body. An electrofusion joint according to any one of the preceding claims, wherein said main body portion has a tubular configuration at the end opposite to said end provided with said joining portion; and said electrofusion joint further comprises a connecting member (33) fitted onto and fixedly secured inside or outside of said tubular end of the main body, said connecting member being provided with a mechanically connecting means (36). The electrofusion joint according to claim 7, wherein said connecting member is firmly secured to said tubular end of the main body portion, by an intervening layer (34) comprising a modified thermoplastic resin. A method of manufacturing an electrofusion joint having a main body portion (12) comprising a crosslinked thermoplastic resin layer and at least one joining portion (13) comprising a non-crosslinkable thermoplastic resin layer which is to be heat-welded to a tubular member, said joining portion being embedded in interior surface of said main body; the method comprising the steps of disposing in an injection mould said at least one joining portion having an electrical heating wire (14) disposed thereon or embedded therein; and injecting into said mould an uncrosslinked crosslinkable thermoplastic resin and crosslinking said uncrosslinked crosslinkable thermoplastic resin to form said main body portion comprising said crosslinked thermoplastic resin and having a recess adjacent said joining portion on the side of the extreme end of the electrofusion joint without extending to the extreme end of the electrofusion joint. and formed by a radially inwardly tapered surface. A header for distributing hot water from a heat source, comprising a header body (61) comprising a crosslinked thermoplastic resin, said header body being provided with at least one inlet (91) and at least one outlet (91a...d); wherein at least one of said at least one inlet and at least one outlet of said header body comprises a joint according to any one of claims 1 to 8. A header according to claim 10 wherein the or each said outlet of said header body has an inner diameter determined in accordance with the amount of the hot water to be distributed therethrough.

Inventors: MITO KAZUNORI, NAGATANI FUMIO, NAKAOKA MIKIO, NAKASHIBA AKIO, NISHIMURA HIROYUKI, SHIBABUCHI TOSHIO, MITO, KAZUNORI, NAGATANI, FUMIO, NAKAOKA, MIKIO, NAKASHIBA, AKIO, NISHIMURA, HIROYUKI, SHIBABUCHI, TOSHIO, Mito, Kazunori, Mitsui Petrochemical Ind. Ltd., Nagatani, Fumio, Mitsui Petrochemical Ind. Ltd., NAKAOKA, MIKIO, SHINWA SANGYO CO., LTD., NAKASHIBA, AKIO, OSAKA GAS CO., LTD., NISHIMURA, HIROYUKI, OSAKA GAS CO., LTD., SHIBABUCHI, TOSHIO, SHINWA SANGYO CO., LTD.
Applicants: MITSUI CHEMICALS INC, OSAKA GAS CO LTD, SHINWA SANGYO KK, MITSUI CHEMICALS, INC., OSAKA GAS CO., LTD., SHINWA SANGYO CO., LTD.
Application year: 1990
Main classification: F16L47
IPC codes: F16L47, B29C65