ELECTROFUSION JOINT

Abstract: An electrofusion joint (1), for use in connecting plastics pipes (4), which comprises a coil electric heating wire (3) embedded in a cylindrical resin body (2), wherein (a) said resin is an adhesive resin; and/or (b) said coil (3) has a substantially unoxidized surface; and/or (c) said coil (3) is coated with a substance having affinity to both the resin and the coil.

Description: This invention relates to a pipe joint used to connect plastic pipes with each other, and particularly to an electrofusion joint wherein a coiled electric heating wire (referred to as 'coil' hereinafter) is embedded therein and which fuses and adheres at the contact surface to the pipes by thermal fusion by application of an electric current to the coil. The electrofusion joint has recently been developed as a pipe joint for plastic pipes which are used in water supply and drainage pies and a gas pipe. It comprise a molden product wherein an electric heating wire coil is embedded in a cylindrical body (see, for example, DE-A-1615-092). The pipe are inserted into the joint from both sides, and an electric current is applied to the coil to cause thermal fusion at the contact surface with the pipes to accomplish fusion/adhesion between the joint and the pipes. The joint has such advantages that the connection work may simply and rapidly be carried out, work efficiency is improved, and connection may be carried out with relatively high reliability as compared to a conventional pipe joint wherein connection of the pipes is carried out by fitting the joint to the pipes which have previously been fused by heating at the contact surface. However, there is a problem in the degree of adherence of the coil to the resin forming the cylindrical body. The thermal conductivity between the coil and the resin depends on the degree of adherence. If the degree of adherence is nonuniform, the resin is fused in some parts and is not fused in other parts. The degree of adherence is affected by the temperature of the joint (the degree of adherence decreases by the difference in the thermal expansion of the resin and the coil if the temperature rises, whereas, it increases by the difference in shrinkage when the temperature lowers), the pressure of the fused resin, the residual stress at molding and deterioration of the resin with time. Also, the resin sometimes pells from the coil by shrinkage of the resin in the process of cooling and solidification at molding. If a joint having inferior adherence between the coil and the resin is used, the coil is abnormally heated because the heat is not well absorbed by the coil. This causes degradation and finally wax formation of the peripheral resin. These cause a decrease in fusion/adhesion properties. The invention seeks to improve the fusion/adhesion properties of the joint with the pipes by an improvement of the adherence between the coil and the resin forming the electrofusion joint. According to the invention there is provided an electrofusion joint, for use in connecting plastics pipes, which comprises an electric heating wire coil embedded in a cylindrical resin body, characterised in that the coil has a substantially unoxidised surface and: (a) said cylindrical resin body is of an adhesive resin; and/or (b) said coil is coated with a substance having adhesive affinity to both the cylindrical resin body and the coil. Furthermore, the invention may effectively be used by using an adhesive resin for the pipes to be connected. Brief description of the drawings Fig. 1 is a sectional view of the electrofusion joint. Fig. 2 is a partly expanded sectional view of the same joint. Fig. 3 is a diagram illustrating a method for removing an oxidized film from the surface of a coil; and Figs 4 and 5 are diagrams illustrating methods for maintaining the resistance value of a coil. Referring to Figs 1 and 2, an electrofusion joint 1 comprises a coil 3 embedded in a cylindrical resin body. The resin body (a) may be formed of an adhesive resin but if it is not (b) the coil 3 is coated with a substance 2 having adhesive affinity to both the resin body and the coil. In use pipes 4 are inserted into each end of the electrofusion joint. First, the adhesive resin used to constitute the resin body in embodiment (a) above will be described. Polyolefins are typical resins constituting a joint and modified polyolefins and coupling agents may be illustrated as substances having affinities to the coil. Examples include polyolefin A modified by unsaturated carboxylic acids or derivatives thereof, polyolefin B modified by unsaturated epoxy monomers, and polyolefin C modified by silane monomers having olefinic unsaturated bonds. These will be described in more detail. Modified polyolefin A described above includes those prepared by random copolymerization or block copolymerization of olefin monomers and unsaturated carboxylic acid monomers, or by graft copolymerization of unsaturated carboxylic acids to polyolefins. The constitutional unit of the unsaturated carboxylic acids or derivatives thereof include, for example, unsaturated dicarboxylic acids such as acrylic acid, methacrylic acid, α-ethylacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, endo-cis-bicyclo [2.2.1] -hepto-5-en-2,-3-dicarboxylic acid (Nadic acid® ), and methyl-endo-cis-bicyclo [2.2.1] hepto-5-en-2,3-dicarboxylic acid (methyl Nadic acid®) and derivatives of the unsaturated dicarboxylic acids such as acid halides, amides, imides, acid anhyrides and esters of said unsaturated dicarboxylic acids. The derivatives of unsaturated dicarboxylic acids include, for example, maleanil chloride, maleimide, maleic anhydride, citraconic anhydride, maleic acid monomethyl ester and maleic acid dimethyl ester. Of these, unsaturated dicarboxylic acids or acid anhydrides thereof and particularly maleic acid and acid or acid anhydrides thereof are preferred. Modified polyolefin B is also prepared by incorporating the unsaturated epoxy monomers into polyolefins by random copolymerization, block copolymerization or graft copolymerization. The unsaturated epoxy monomers represent those monomers having one or more unsaturated bonds capable of polymerization and epoxy groups in a molecule thereof. Said unsaturated epoxy monomers include, for example, unsaturated glycidyl esters shown by the general formula, (where R represents a hydrocarbon group having an ethylenic unsaturated bond capable of polymerization), unsaturated glycidyl ethers shown by the general formula, (where R represents the same as in formula [I], and X represents a divalent group represented by and epoxy alkenes shown by the general formula, (where R represents the same as in formula [I], and R′ represents a hydrogen atom or a methyl group). For example, they include glycidyl acrylate, glycidyl methacrylate, mono- and diglycidyl esters of itaconic acid, mono-, di- and triglycidyl esters of butene tricarboxylic acids, mono- and diglycidyl of esters of citraconic acid, mono- and diglycidyl esters of endo-cis-bicyclo [2.2.1]-hepto-5-en-2,3-dicarboxylic acid (Nadic acid®) mono- and diglycidyl esters of endo-cis-bicyclo [2.2.1] hepto-5-en-2-methyl-2,3dicarboxylic acid (methyl Nadic acid®), mono- and diglycidyl esters of allylsuccinic acid, glycidyl esters of p-styrene carboxylic acids, allyl glycidyl ether, 2-methylallyl glycidyl ether, styrene-p-glycidyl ether, 3,4-epoxy-1-butene, 3,4epoxy-3-methyl-1-butene, 3,4-epoxy-1-pentene, 3,4-epoxy-3-methyl-1-pentene, 5,6-epoxy-1-hexene and vinylcyclohexene monoxide. Of these, glycidyl acrylate and glycidyl methacrylate should be preferred, Modified polyolefin C is also prepared by incorporating said silane monomers into polyolefin by random copolymerization, block copolymerization or graft copolymlerization in the same manner as in modified polyolefin A and B as described above. Any silane monomer having olefin unsaturated bonds may be used, however, those silane monomers having hydrolyzable organic groupe in addition to the olefinic unsaturated bonds are preferred. For example, they are illustrated by the following general formulae:- In the formulae, R₁ and R₂ are the same or different monovalent group which have olefinic unsaturated bonds and comprises carbon atoms, hydrogen atoms and arbitrarily oxygen atoms. These groups include, for example, vinyl, allyl, butenyl, cyclohexenyl and cyclopentadienyl groups. In particular, unsaturated groups having double bonds at their ends are preferably used. In particular, unsaturated groups with end double bonds and ester groups adjacent to the bonds are used more preferably. Examples of such groups are hydrocarbon groups as expressed by the following formulae: CH₂ = C(CH₃)COO(CH₂)₃- CH₂ = C(CH₃)COO(CH₂)₂-o-(CH₂)₃- CH₂ = C(CH₃)COOCH₂OCH₂CH₂(OH)CH₂O(CH₂)₃- Of these, vinyl group is most preferred. X represents an organic group having no olefinic unsaturated bonds. It includes, for example, monovalent hydrocarbon groups such as methyl, ethyl, propyl, tetradecyl, octadecyl, phenyl, benzyl and tolyl groups. These hydrocarbon groups may partially be substituted by halogen atoms. Groups Y₁, Y₂ and Y₃ are the same or different hydrolyzable groups including, for example, alkoxy groups such as methoxy, ethoxy, butoxy and methoxyethoxy groups, alkoxyalkoxy groups, acyloxy groups such as formyloxy, acetoxy and propionoxy groups, oximes such as -ON=C(CH₃)₂, -ON=CHCH₂C₂H₅ and -ON=C(C₃H₇)₂,and substituted amino groups and alkylamino group such as -NHCH₃, -NHC₃H₇ and -N(C₃H₇)₂, and any other hydrolyzable organic groups. A preferred organic silicon compound is represented by the general formula, and particularly that wherein Y₁,Y₂ and Y₃ are the same. Of these vinyl trisalkoxysilanes are preferred. They include, for example, vinyltrimethoxysilane, vinyltriethoxysilane and vinyltris(methoxyethoxy)silane. However, vinylmethyldiethoxysilane and vinylphenyldimethoxysilane may be used in the same manner. Of the modified polyolefins described above, grafted polyolefins are particularly preferred. Various known methods may be adopted to prepare a graft copolymer. They include, for example a method in which graft copolymerisation is carried out on a mixture of molten polyolefin and graft monomer, and a method in which copolymerization is carried out on a mixture consisting of polyolefin dissolved in a solvent and the graft monomer. In either method reaction should preferably be carried out in the presence of radical initiators to carry out graft copolymerization effectively. Said radical initiators include organic peroxides, organic peroxide esters and azo compounds. Electrolytically dissociating radiation and ultraviolet rays may also be used for radical generation. The grafting rate should be 1-15 parts by weight of graft monomer to 100 parts by weight of polyolefin. The polyolefins as raw material for the modified polyolefins should desirably be the same as the polyolefins constituting the pipe (e.g., polyethylene for a polyethylene pipe and polybutene-1 for a polybutene-1 pipe). The coupling agents which may be used in the invention include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(β-methoxyethoxy)silane, vinyltriacetylsilane, γ-methacryloxypropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl) ethyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, β-mercaptoethyltriethoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropyltriethoxysilane, vinyltrichlorosilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, γ-(diethylenetriamino)-propyltrimethoxysilane, γ-ureidopropyltriethoxysilane, imidazolinetriethoxysilane, γ-[N-(β-methacryloxyethyl)-N,N-dimethylammonium(chloride)] propyltrimethoxysilane, styryldiaminosilane, aminodi(trimethyl)silane, (3-diaminomethylthiopropyl)silane, bis-(3-triethoxysilylpropyl)-tetrasulfan, γ-chloropropyltriethoxysilane, etc. The adhesive resin constituting the joint mentioned above may consist of said modified polyolefins alone described above or a mixture of said modified polyolefins and unmodified polyolefins. The coil used in the invention may be made of conventionally known electric heating material such as copper alloy, nickel-chromium steel Class I and Class II and iron-chromium steel Class I and Class II, Chromel A, Constantan and Manganin. Also a single metal wire of relatively high resistance (e.g.,cerium and bismuth) may be used in addition to said alloys. The coil has a surface which is not substantially oxidized. It may be a metal wire obtained by removal of an oxidized film of passive state from the surface as well as a metal wire just after refining. The adhesiveness of the coil to the resin constituting the joint is improved by use of said coil. A method for removing an oxidized film from the surface of the metal coil includes reduction of the metal wire at the surface. For example, a method consisting of degreasing of the coil at the surface, reduction/corrosion and washing may be illustrated. A method described as follows may be illustrated as an example. That is, oily components at the surface of the metal coil are removed, for example by use of a surface active agent. Then a liquid is prepared separated into upper and lower as shown in Fig. 3 which consist of a nonoxidative strong acid solution 11 and a nonpolar solution 12 immiscible with said strong acid solution 11. A metal 13 of a high ionization tendency is put into the strong acid solution to reduce the metal surface by catalytic reaction between the metal coil 3 and nascent hydrogen generated therein. The oxidized film is removed by the reaction, and the metal coil is pulled up into the nonpolar solution 12 which comes into interfacial contact with the strong acid solution 11 to wash out the strong acid solution adhering to the coil. Then, the coil is pulled out of the nonpolar solution. Said nonoxidative strong acid solution used in the invention include, for example, hydrochloric acid. Said nonpolar solution immiscible with hydrochloric acid includes, for example, benzene and toluene. Said metal of a high inonization tendency includes, for example, zinc and magnesium. Reduction may be carried out at ordinary temperature and pressure. Next, the substance having adhesive affinity to both the cylindrical resin body and the coil in accordance with embodiment (b) of the invention will be described. This embodiment provide a means for improving the adhrence between the resin constituting the joint and the coil by covering the coil with a substance which has adhesive affinity for both the resin and the coil surface. Modified polyolefin A, B, and C described above with reference to embodiment (a) above may be used for the covering substance. Either on both of embodiment (a) and (b) may be used in accordance with the invention. Furthermore, in addition to (a) and/or (b) selection of the materials of the pipe to be connected by use of the joint of the invention is important. If the same resins as used in the joint in embodiment (a) are used, a strong pipe joint structure may be obtained by use of the joint of the invention. Of them, a mixture of modified and unmodified polyolefins is preferably used for the resin constituting the joint and/or the pipe. In addition, the surface of the coil should desirably be covered with a modified polyolefin. Thus, the adherence of the coil and the resin may be increased by an increase in the interfacial adhesive strength between the two as mentioned above. Furthermore, the resistance value of the coil should desirably be maintained constant to eliminate deterioration of the resin which is in contact with the coil in consideration of a case wherein the coil is abnormally heated in spite of the above means. As a method for maintaining the resistance value of the coil, a method in which the current is detected to adjust the voltage by the detected value (Refer to Fig.4) and a method in which intermittent heating is carried out under control of the output by the current value (Refer to Fig.5) may be illustrated as examples. The surface of the coil is desirably covered with an insulating substance. This is because if the resin is fluidized at fusion/adhesion, it sometimes causes a shift of the coil resulting in a short-circuit at the coil. If a short-circuit is made, an abnormal electric current flows to cause insufficient fusion/adhesion. The insulating substance used to cover the coil can be, for example, heat-resisting insulating varnish, inorganic or organic heat-resisting insulating paint, a heat-resisting tape, a woven fabric and yarn. Concretely, said heat-resisting insulating varnish includes shellac (tris-2-hydroxyethyl isocyanurate)-modified polyester insulating varnish, polyester imide insulating varnish, shellac-modified polyester imide insulating varnish, polyamideimide insulating varnish, polyimide insulating varnish, polyester amide imide insulating varnish, polyhydantoin-modified polyester insulating varnish, silicon resin insulating varnish and polysiloxane resin insulating varnish. Said inorganic heat resisting paint includes alkali silicate heat-resisting paint, colloidal silica heat-insulating paint, acid metal phosphoric acid type heat-resisting paint and dichromic acid phosphoric acid type heat- resisting paint. Said organic heat-resisting paint includes organic silicate heat-resisting paint and organic titanate heat-resisting paint. Said heat-resisting tape includes glass cloth and a glass fiber. If the surface of the coil is covered with an adhesive substance as described above, it is covered with the adhesive substance after covering with the insulating substances. Degreasing, reduction, etching or rinsing is necessary also in this case, in order to enhance the adhesion between the coil and the insulator. The joint according to the present invention is produced by setting a coil in a cylindrical mold and then injecting one of the described resin materials into the mold to form the body. Various illustrations have been made above, however, the best example embodies a joint in which a coil 3 covered with modified polyethylene 2 is embedded inside an electrofusion joint 1, and the resin constituting the body consists of a mixture of modified and unmodified polyethylenes. The mode of connecting polyethylene pipes 4 by means of the joint is illustrated by Fig.1. In the electrofusion joint of the invention, the interfacial adhesive strength between the coil and the resin body is reinforced, and an increase in the adherence between the two may eliminate deterioration of the resin and a decrease in the fusion/adhesion properties of the resin body caused by overheating of the coil.

Claims: 1. An electrofusion joint, for use in connecting plastics pipes, which comprises an electric heating wire coil embedded in a cylindrical resin body, characterized in that the coil has a substantially unoxidised surface and : (a) said cylindrical resin body is of an adhesive resin; and/or (b) said coil is coated with a substance having adhesive affinity to both cylindrical resin body and the coil. 2. An electrofusion joint according to claim 1 wherein said coil is covered with an insulating substance. 3. A process for joining plastics pipes which comprises inserting the end of a plastics pipe into each end of the cylindrical resin body of an electrofusion joint as claimed in claim 1 or 2, and passing an electric current through said coil to cause thermal fusion between said joint and said plastics pipes. 4. A process according to claim 3 wherein the resistance value of the electric heating wire is maintained substantially constant.

Inventors: HIRATA, MASAHIRO, MITO, KAZUNORI
Applicants: MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1988
Main classification: F16L47
IPC codes: B29C65, F16L47