Electrofusion is widely used to connect electrofusion couplings to plastics pipes and involves the electrical heating of a thermoplastics polymer in an electrofusion coupling thereby melting the thermoplastics polymer which fuses with and forms a seal between a plastics pipe, inserted into one end of the electrofusion coupling, and the electrofusion coupling.

For the purposes of the present description, an electrofusion coupling includes any coupling or adapter wherein at least one end is configured to receive an end of a pipe and can include, but is not strictly limited to, straight couplings, elbows or bends (to facilitate changes in direction of a pipe system), adapters, flange adapters, T-fittings or Y-fittings (adapted to connect three pipe ends), saddle fittings (adapted to connect a smaller, branch pipe to a larger plastics pipe at the region of an aperture formed in the larger pipe), reducers and metal to plastic transition fittings.

Pipe systems comprising polyethylene are often used to transport fluids.

Some fluids, for example liquid organic solvents, in time permeate through the walls of the pipe systems. If the pipe system is buried in the ground, this can lead to contamination of the surrounding ground.

Alternatively if the pipe system is buried in contaminated ground, the contaminants may permeate through the walls of the pipe system and contaminate the fluid being transported through the pipe system.

It is known that high density materials have low permeability. Thus the permeability of a plastics is reduced by increasing its degree of crystallinity leading to closer packing of molecular chains. The

permeability is also reduced by modifying the chemical characteristics of plastics by, for example, the inclusion of polar groups leading to closer and stronger interactions between adjacent molecular chains leading to improved permeation resistance to non-polar fluids such as paraffins, cyclo-paraffins, benzene, toluene and xylene.

It has been observed that organic solvents such as petroleum based fluids permeate through the walls of pipe systems comprising polyethylene. The permeability of such solvents through plastics such as polyamide, which have high degrees of crystallinity and polar chemical groups, is significantly lower than that through polyethylene. Thus pipes for conveying petroleum based fluids and other organic solvents are formed with an inner layer of polyamide and an outer layer of polyethylene.

In GB 2 351 257 A, an electrofusion coupling is disclosed comprising an inner polyethylene layer and an outer polyamide layer wherein the two layers are bonded together through the use of a tie layer comprising a plastics material which has an affinity for polyethylene and polyamide.

GB 2 319 496 discloses an electrofusion coupling which incorporates a barrier layer, either comprising a blend between two plastics, one of which has better permeation performance, or comprising a discrete and separate layer.

Barrier layers comprising a blend show some adhesion to adjacent layers of the coupling. However one problem with such a barrier layer is that impermeability is limited by the discontinuous nature of the less permeable plastics in the barrier.

Barrier layers comprising a discrete and separate layer show improved impermeability relative to barrier layers comprising a blend because of the continuous nature of the barrier. Such barrier layers show poor

adhesion to adjacent layers in the coupling because of lack of chemical affinity. In order to improve the mechanical integrity of the coupling, tie layers are introduced to join the barrier layer to other layers in the coupling. However one problem with the introduction of tie layers is the greatly increased complexity of the manufacturing process and therefore increased unit costs.

In GB 2 351 257 a multi-layer electrofusion coupling is disclosed comprising an inner layer of thermoplastics polymer over which is moulded a barrier layer encapsulated by a further layer of plastics. Tie layers can be optionally included either side of the barrier layer to improve adhesion to adjacent layers and thereby improve the mechanical integrity of the coupling. Such couplings may be formed from an extruded multilayer pipe, in which case the couplings are limited to simple shapes such as straight couplers.

The provision of a tie layer, whilst being effective, makes very significant demands on the moulding process. Means need to be provided for incorporating the tie layer into the structure by moulding or other means. Whilst moulding a tie layer construction may be practical for simple shapes, it is difficult and expensive to incorporate the tie layer in more complex shapes such as tees and elbows.

The present method seeks to overcome some of the deficiencies mentioned above by eliminating the need for tie layers adjacent to the barrier layer.

Summary of the Invention In a first aspect, the present invention provides a method of manufacturing an electrofusion coupling comprising the steps of: a) providing a fusible sleeve comprising an electrically conductive coil or layer and one or more thermoplastics, the electrically

conductive coil or layer disposed at or on the surface of the fusible sleeve bore; and b) providing a barrier layer over at least a portion of the exterior surface of the fusible sleeve, the barrier layer comprising one or more plastics with improved resistance to organic solvents compared to the thermoplastics ; wherein the method additionally comprises a step of chemical pre- treatment of the exterior surface of the fusible sleeve thereby improving the adhesion of the barrier layer to the said exterior surface.

The chemical pre-treatment step modifies the surface chemistry of the fusible layer more compatible with the surface chemistry of the barrier layer to which bonding is desired. A coupling is thereby produced with acceptable mechanical integrity, through enhanced adhesion between the barrier and fusible layers, and a complete external barrier layer.

Furthermore the present method reduces the required number of layers from 3-5 to two layers thereby greatly simplifying the manufacturing process leading to lower unit costs.

In addition, the present method permits more complex coupling shapes such as T-fittings, reducers or elbows to be produced whereas previously this was not always possible. The complex shaped couplings can now be produced with the same ease and at the same low cost as simple couplings.

The method may additionally comprise the step of providing a preform fusible sleeve around which the fusible sleeve is provided, the preform fusible sleeve comprising one or more thermoplastics. The compositions of the fusible sleeve and the preform fusible sleeve are identical or not identical.

The fusible sleeve may be provided by cutting a slot in the surface of the fusible sleeve bore and pressing the electrically conductive coil into the slot.

The method may include an additional step of providing a tie layer over the fusible sleeve before the step of chemical pre-treatment of the exterior surface of the fusible sleeve and step'b', the tie layer comprising one or more tie plastics with chemical affinity for both the exterior surface of the fusible sleeve and the barrier layer.

Any one of the fusible sleeve, the preform fusible sleeve, the tie layer or the barrier layer may be manufactured by any one of spraying, injection moulding or by application of a pre-formed film.

The thermoplastics may be a melt processable homopolymer or copolymer such as polyethylene, functionalised polyethylene, polypropylene, polyvinylidene fluoride, polyvinyl chloride, acrylonitrile- butadiene-styrene (ABS) terpolymer, poly (ethylene-vinyl alcohol), poly (ethylene-vinyl acetate), poly (ethylene-acrylic acid), poly (ethylene-ethyl acrylate), or a homopolymer or copolymer of ethylene, propylene, vinylidene fluoride, vinyl chloride, vinyl alcohol, ethyl acrylate or acrylic acid.

The plastics may be a crystalline polar thermoplastics such as a polyamide, poly (vinylidene fluoride) poly (vinylidene chloride), a polyurea, a polyurethane, a polyacrylonitrile, a polymethacrylonitrile, poly (vinyl alcohol), or a homopolymer or copolymer of an amide, vinylidene fluoride, vinylidene chloride, a urea, a urethane, acrylonitrile, methacrylonitrile or vinyl alcohol.

The tie plastics may be selected from maleic anhydride grafted polyethylene, acrylate grafted polyethylenes, poly (ethylene-vinyl acetate), poly (ethylene-vinyl alcohol), or a homopolymer or copolymer

of ethylene, vinyl acetate, vinyl alcohol, ethyl acrylate or maleic anhydride.

Maleic anhydride provides reactive sites that promote adhesion with the barrier layer and polyethylene provides compatibility with the fusible sleeve.

The chemical pre-treatment may comprise-at least one of treatment with an acid such as chromic acid, treatment by corona discharge, treatment by fluorination, flame treatment, treatment by ozone, atmospheric plasma treatment or low pressure plasma treatment with desirably at least one of oxygen, nitrogen, hydrogen or argon.

Chemical pre-treatment increases the concentration of polar groups on the exterior surface of the fusible sleeve by oxidation or fluorination and thereby improves the chemical compatibility of the fusible sleeve for the barrier layer.

Another aspect of the present invention is the provision of an electrofusion coupling manufactured according to the preceding method.

The ratio of the thickness of the fusible layer to the outside diameter of the coupling may be in the range 5: 1 to 10: 1 and is desirably 7.5 : 1. The ratio of the thickness of the preform fusible layer to the outside diameter of the coupling may be in the range 75: 1 to 150: 1 and is desirably about 100: 1. The ratio of the thickness of the tie layer to the outside diameter of the coupling may be in the range 100: 1 to 500: 1 and is desirably 200: 1. The ratio of the thickness of the barrier layer to the outside diameter of the coupling may be in the range 10: 1 to 200: 1 and is desirably 50: 1.

Brief Description of the Figures The invention will now be described in detail with reference to:

Figure 1 which shows a longitudinal cross-section of a first embodiment of electrofusion coupling in accordance with the invention; and Figure 2 which shows a longitudinal cross-section of a second embodiment of electrofusion coupling in accordance with the invention.

Detailed Description of the Invention The electrofusion coupling illustrated in Figure 1 is adapted to receive a first pipe end (113) at a first coupling end (111) and a second pipe end (114) at a second coupling end (112) and comprises a first sleeve (101) received within a second sleeve (102), a third sleeve (115) which receives the second sleeve (102), and an electrical resistance wire (103) positioned between an exterior surface of the first sleeve (104) and an interior surface of the second sleeve (105). The electrically conductive wire (103), which assumes a helical configuration, comprises a first (106) and second (107) end, each end terminating at a first (108) and second (109) terminal respectively and located at an exterior surface of the third sleeve (115).

The inside diameter of the coupling is 63 mm. The wall thicknesses of the first (101), second (102) and third sleeves (115) are respectively 1 mm, 6 mm and 1.5 mm.

The first and second sleeves (101, 102) comprise a polyethylene of molecular weight range 100 000 to 200 000. The third sleeve (115) comprises Nylon (RTM). The first (101), second (102) and third (115) sleeves additionally include pigments, fillers and processing additives.

The electrical resistance wire (103) comprises an alloy of copper and nickel, or copper and tin.

The electrofusion pipe coupling illustrated in Figure 2 is adapted to receive a first pipe end (213) at a first coupling end (211) and a second

pipe end (214) at a second coupling end (212) and comprises a first sleeve (201) received within a second sleeve (202), and an electrical resistance wire (203) positioned at an interior surface of the first sleeve (204). The electrically conductive wire (203), which assumes a helical configuration, comprises a first (206) and second (207) end, each end terminating at a first (208) and second (209) terminal respectively and located at an exterior surface of the second sleeve (210).

The inside diameter of the coupling is 63 mm. The wall thicknesses of the first (201) and second (202) sleeves are respectively 7 mm and 1.5 mm.

The first sleeve (201) comprise a polyethylene of molecular weight range 100 000 to 200 000. The second sleeve (202) comprises Nylon (RTM).

The first (201) and second (202) sleeves additionally include pigments, fillers and processing additives. The electrical resistance wire (203) comprises an alloy of copper and nickel, or copper and tin.

A variant of the electrofusion couplings illustrated in Figures 1 and 2 includes a tie layer between the polyethylene sleeve and the Nylon (RTM) sleeve. The tie layer comprises maleic anhydride grafted polyethylene which is available under the trade name Orevac.

The electrofusion coupling of the form illustrated in Figure 1 was manufactured by injection moulding the first polyethylene sleeve over a cylindrical mould core thereby forming a pre-form. The cylindrical mould core was then removed from the pre-form and the electrical resistance wire wound in helical fashion around the pre-form to form a wound pre- form. The second polyethylene sleeve was then injection moulded over the wound pre-form.

The radially exterior surface of the second polyethylene sleeve then underwent low pressure plasma treatment by exposing the said surface

to an ionised gas mixture comprising nitrogen and hydrogen under a partial vacuum for a period of time necessary to modify the chemical nature of the surface to promote optimum adhesion to the nylon sleeve.

The Nylon (RTM) sleeve was then injection moulded over the chemically pre-treated second polyethylene sleeve.

The electrofusion coupling of the form illustrated in Figure 2 was manufactured by winding an electrical resistance wire around a cylindrical mould core in helical fashion thereby forming a wound core.

The polyethylene sleeve was injection moulded over the wound core.

The radially exterior surface of the polyethylene sleeve then underwent low pressure plasma treatment by exposing the said surface to an ionised gas mixture comprising nitrogen and hydrogen under a partial vacuum for a period of time necessary to modify the chemical nature of the surface to promote optimum adhesion to the nylon sleeve. The Nylon (RTM) sleeve was then injection moulded over the treated sleeve of polyethylene.

The electrofusion coupling of the form illustrated in Figure 2 was also manufactured by injection moulding the polyethylene sleeve over a cylindrical mould. Treating the radially exterior surface of the polyethylene sleeve to low pressure plasma treatment by exposing the said surface to an ionised gas mixture comprising nitrogen and hydrogen under a partial vacuum for a period of time necessary to modify the chemical nature of the surface to promote optimum adhesion to the nylon sleeve. The Nylon (RTM) sleeve was then injection moulded over the treated polyethylene sleeve. The cylindrical mould core was then removed from the polyethylene sleeve. The electrical resistance wire was then inserted into the polyethylene sleeve by cutting a slot into the radially inner surface thereof and pressing the electrical resistance wire into the slot.

The variant electrofusion couplings including a tie layer were manufactured in the same manner as described hereinabove but additionally including a step of injection moulding the tie layer over the chemically treated polyethylene sleeve prior to injection moulding the Nylon (RTM) sleeve.