A method of forming a multilayer plastic pipe and a multilayer plastic pipe for conducting fluids

Abstract: According to the invention, a core pipe (2) is formed, whose properties correspond to the requirements set by the fluid to be conducted. With a suitable coating method an outer protective layer (3) is formed around the core pipe (2) of such a plastic material, which properties meet the requirements set by the prevailing environment and/or pipelaying procedure.

Description: The invention relates to a method of forming a two-layer plastic pipe for conducting fluids. The invention also relates to a plastic pipe formed by the method, as well as to a method of connecting a plastic pipe. During handling, laying and joining or plastic pipes, the pipe surface is exposed to damage. For example, modern laying methods for plastic pipes are based on drilling a tunnel in the ground for the pipe, the pipe being then passed through the tunnel e.g. to an excavation where the next pipe sections are seamed. Naturally, a pipe is hereby subjected e.g. to different bending and tensile forces. This is disadvantageous since bending, stretching and scratching of a pipe deteriorate its mechanical strength, thereby reducing its life. In addition, the lifetime of a pipe may be reduced by environmental conditions, such as diffusible materials. It is previously known to coat plastic pipes with different protective layers e.g. for transportation, use and storage. AT-365974 and WO91/00466 are directed to such multi-layered pipes. The object of the present invention is to eliminate the above disadvantages by providing a method for forming a dual layer plastic pipe. It is also an object of the present invention to provide such a pipe. Furthermore, it is an object of the present invention to provide a method of connecting a dual layer plastic pipe. A pipe according to the present invention is is easy to lay and, in addition, inexpensive in view of its properties. The method of forming a two-layer plastic pipe for conduction fluids, the two-layer plastic pipe formed by the method, and the method of connecting a dual layer plastic pipe, are characterized by what is stated in the appending claims. A pipe according to the present invention, which is provided with a protective layer, is fully protected against scratching. The invention enables e.g. the use of standard pipes in conditions to which they are not applicable as such, e.g. in laying procedures that cause vast tensile stresses or in laying in the ground pipes impervious only to internal pressure. When a single-layer pipe is formed, often a bigger or smaller amount of fillers, depending on the use or the pipe must be fixed with the plastic materail. Fillers usually have a disadvantageous effect on the mechanical properties and weldability of the pipe. In a two-layer pipe according to the present invention, the properties of the core pipe or conducting pipe are optimised for the conducting of fluids and the outer layer is designed to resist any external conditions and stress. This results in a pipe arrangement where the structure has not been compromised with, thus replacing expensive special pipes with standard pipes coated with an inexpensive outer layer tailored for the laying conditions. Welding is also easier and safer with the pipe formed by the method of the invention since e.g. seaming of pipes is not possible without that the outer layer is first removed from the area to be seamed, i.e. the pipe ends must always be clean when they are seamed, whether by welding or by any other method. The inventive concept of providing an outer layer which is removable by knocking or peeling ensures that welding is always successful and that the welding machines operate as intended. In the following the invention is described in greater detail by means of examples and with reference to the attached drawings, wherein Fig. 1 depicts a forming method and a pipe according to one embodiment of the invention, Fig. 2 depicts another embodiment of the pipe of the invention. Fig. 3 shows partly in section two pipes according to the invention placed in a welding device. Fig. 1 depicts coating of a plastic pipe 2 according to the invention with a surface layer by coextrusion. The surface layer is formed from melt plastic 1 to provide a protective layer 3 around a pipe 2 of a specified size. Coextrusion dies 5, e.g. so-called crosshead dies, are known to one skilled in the art, and coextrusion as such is not explained herein in greater detail. The protective outer layer according to the invention is made easily detachable from the core pipe by simple means, either wholly or only at the joint surfaces, such as the pipe ends. In this way, the surfaces remain in as good a condition as possible for the seaming carried out by e.g. welding. According to a preferred embodiment of the invention, these objectives are achieved by making the surface of the outer layer moderately hard, whereby it has a low adhesion, and to make the structure of the layer moderately stiff, whereby the outer layer con be detached from the pipe by peeling or by knocking. For example chalk and talc are suitable fillers in respect of achieving this effect. In a pipe 4 according to the invention the plastics raw material used for the protective outer layer 3 may be e.g. linear LDPE or even recycled plastics waste. The advantage of the linear LDPE is its high scratching and puncture resistance in view of its price, whereas the advantage of the plastics waste is its low price. When the material is selected, it is advisable to take into account that if the outer layer is made from plastics differing from the material of the core pipe with respect to the chemical structure, the adhesion between the pipe and the protective outer layer is probably lower (they can be detached more easily) than if exactly the same material is used. According to another preferred embodiment of the invention, the material of the outer layer 3 has at least the same strength and/or stiffness as the material of the core pipe 2. Thus the protective outer layer can be a load-carrying component without having high requirements set for the raw material (the price must be as low as possible). Thus, cheap reinforcing and/or stiffening fillers or fibres may be mixed to the material of the protective layer. Alternatively, an outer layer 7 may be stiffened in accordance with Fig. 2, by ribbing or corrugations 6, to make the pipe 8 sufficiently stiff. The ribbed outer layer 7 may be formed e.g. in a gilled pipe machine or corrugating device. The protective outer layer can also be stiffened by foaming in a forming step, whereby the pipe also acquires a considerable thermal insulation capacity. Furthermore, the material of the outer layer 3 may advantageously be crosslinked in order to improve the stiffness and strength and the resistance of the outer layer of the pipe. In pipes reinforced in this manner, the core pipe may be a thin-walled pipe or even a hose, which is capable of resisting only internal pressure caused by the fluid to be conducted, but which would be flattened by earth pressure when laid in the ground it not properly supported. The strength properties of the outer layer may also be made different in the radial and axial directions of the pipe. In a further embodiment of the invention, slate-like mica is mixed with the material of the outer layer to improve the barrier properties of the pipe. For example, benzene compounds penetrate fairly easily through a wall of a conventional tap water pipe made from polyethene. When a mixture having good resistance (barrier properties) to the penetration of the above substances is selected as the material of the outer layer, it is easy to tailor a tap water pipe for a specified aggressive chemical environment. In yet another embodiment of the invention barium sulfate is mixed with the material of the outer layer to make the specific gravity of the pipe higher than that of water. Barium sulfate(BaSO 4 ) has a high density, wherefore the overall density of e.g. a polyethene pipe can be made higher than that of water. This feature makes the pipe suitable for arrangements where the pipe is laid in water. In an additional embodiment of the invention, easily magnetizable material or electrically conductive particles are mixed with the material of the outer layer to render the pipe detectable by a magnetic field. Examples of magnetizable material are iron oxide and barium ferrite. In this embodiment the outer layer makes it possible to render the pipe laid in the ground easily detectable by electromagnetic means. Mixed in a single-layer pipe, fillers of this kind would reduce the strength of the pipe. In still a further embodiment of the invention, electrically conductive material is mixed with the material of the outer layer to render the pipe electrically conductive. In an electrically conductive outer layer, cracks occurring in the pipe line can be detected e.g. by a cable fault finder. In addition, an electrically conductive outer casing eliminates risks caused by induced electricity e.g. in a explosive environment. The electrically conductive material may also be e.g. a copper wire embedded in the outer layer. In a still further embodiment of the invention all the necessary identifying dyes and UV stabilizing agents are mixed with the material of the outer layer. Thereby these pipe material-weakening agents need not be mixed to the material of the actual conducting or core pipe 2. During the formation of the pipe it is possible to introduce an adhesion inhibiting agent between the outer layer and the core pipe. In a preferred embodiment of the invention, a release agent, such as a low molecule weight polyethylene wax, is mixed with the material of the outer layer to facilitate the detachment of the outer layer from the core pipe 2. In another layer dipped in a bath containing liquid polymers which act as an adhesion inhibiting layer. If the core pipe is made e.g. from polyethene and the outer layer from polypropene mixed with wax, the outer layer is easy to detach from the core pipe. This is particularly advantageous when the core pipe needs to be easily replaceable (relining): the core pipe is replaced by simply pulling it out from the outer layer and inserting a new core pipe in the layer formed by the outer layer. The outer layer can also be designed to be detachable from the core pipe upon applying heat to the part of the outer layer to be removed. In a further embodiment of the invention, a thin aluminum layer is provided between the outer layer and the core pipe. The aluminum layer both facilitates the detaching of the outer layer (in contrast to known plastic aluminum laminates) and functions as a barrier layer. In practice, a thin (0.1-0.3 mm) aluminum folic is formed on the outer layer of the core pipe by adhesion or ultrasonic welding. A pipe according to the present invention is seamed by peeling or knocking the outer layer off at the area of the pipes to be seamed, and by subsequently placing the pipes to be seamed together, and by carrying out the seaming e.g. by electric welding. Thereafter the seam is protected where necessary with a layer similar to the outer layer, or left unprotected. In figure 3 is shown, by way of example, two, identical pipes 9 according to the invention, which are placed with their ends positioned against each other at 10 in an electrofusion pipe coupler device 11. The outer layers 12 of the pipes has been removed from the cores 13 at the ends of the pipes 9, in order to facilitate proper insertion and welding of the pipes. It is obvious to one skilled in the art that the embodiments of the invention are not limited to those described above, but that they may vary within the scope of the attached claims.

Claims: A method of forming a two-layer plastic pipe (4;8) to be laid in the ground for conducting material therethrough, by forming a core pipe (2) of a plastic material, such as polyethylene; and forming an outer layer (3;7) of a plastic material around said core pipe (2) by a suitable coating method, characterised in that said outer layer is removable from the core pipe by knocking or peeling, and said plastic material of said outer layer comprises an amount of fillers and/or an amount of UV stabilising agent whereby said core pipe is protected from being damaged during storage, transportation, pipelaying procedures and during usage of said pipe in the ground, and/or whereby said pipe can be detected by the presence of the said fillers. A method according to claim 1, characterised in that said removable outer layer (3) is provided around the core pipe (2) by coextrusion. A method according to claim 1 or 2, characterised in that the barrier properties of the pipe is improved by mixing flake-like mica with the material of said removable outer layer (3). A method according to any of claims 1 to 3, characterised in that the specific gravity of the pipe is made higher than that of water by mixing barium sulfate with the material of said removable outer layer (3). A method according to any one of claims 1 to 5, characterised in that in order to render the pipe detectable by a magnetic field easily magnetizable material or electrically conductive particles are mixed with the material of said removable outer layer (3). A method according to any one of claims 1 to 4, characterised in that in order to render the pipe electrically conductive electrically conductive material is mixed with the material of said removable outer layer (3). A method according to any one of claims 1 to 6, characterised in that the stiffness of the pipe (8) is improved by providing flanges of corrugations (6) spaced from one another on said removable outer layer (7). A method according to any one of claims 1 to 7, characterised in that the stiffness and/or thermal insulation capacity of the pipe is improved by expanding the material of said removable outer layer (3). A method according to any one of the claims 1 to 7, characterised in that the stiffness and strength of the pipe is improved by crosslinking the material of said removable outer layer (3). A method according to any one of claims 1 to 9, characterised in that any identifying colouring agents and any UV stabilising agents for the pipe (4; 8) are included in the material of said removable outer protective layer (3; 7). A method according to any one of claims 1 to 10, characterised in that the removal of said removable outer layer from the core pipe (2) is facilitated by mixing a release agent, such as wax, with the material of said removable outer layer (3). A method according to any one of claims 1 to 10, characterised in that a release agent, such as wax, is added between the core pipe (2) and said removable outer layer (3), to facilitate the removal of the outer layer from the core pipe (2). A method according to any one of claims 1 to 11, characterised in that a thin aluminium layer is provided between said removable outer layer (3) and the core pipe (2) to facilitate the removal of the outer layer (3) from the core pipe (2) and to improve the barrier properties of the pipe. A two-layer plastic pipe (4; 8) to be laid in the ground for conducting material therethrough, comprising: a core pipe (2) of a plastic material, such as polyethylene; and an outer layer (3; 7) of a plastic material provided around said core pipe (2), characterised in that said outer layer is removable from the core pipe by knocking or peeling, and plastic material of said outer layer comprises an amount of fillers and/or an amount of UV stabilising agent whereby said core pipe is protected from being damaged during storage, transportation, pipelaying procedures and during usage of the pipe in the ground, and/or whereby said pipe can be detected by the presence of the said fillers. A pipe according to claim 14, characterised int that said removable outer layer (3) contains flake-like mica. A pipe according to claim 14 or 15, characterised in that said removable outer layer (3) contains barium sulfate. A pipe according to any one of claims 14 to 16, characterised in that said removable outer layer (3) contains electrically conductive particles or easily magnetizable material. A pipe according to any one of claims 14 to 17, characterised in that said removable outer layer (3) contains electrically conductive material. A pipe according to any one of claims 14 to 18, characterised in that said removal outer layer (7) comprises flanges or corrugations (6) spaced from one another. A pipe according to any one of claims 14 to 19, characterised in that said removable outer layer (3) is made of expanded plastic. A pipe according to any one of claims 14 to 20, characterised in that said removable outer layer (3) is made of cross-linked plastic material. A pipe according to any one of claims 14 to 21, characterised in that any identifying colouring agents and any UV stabilising agents for the pipe (4; 8) are included in the material of said removable outer protective layer (3; 7). A pipe according to any one of claims 14 to 22, characterised in that said removable outer layer (3) contains a release agent, such as wax. A pipe according to any one of claims 14 to 23, characterised in that it contains a layer of release agent, such as wax, between the core pipe (2) and said removable outer layer (3). A pipe according to any one of claims 14 to 24, characterised by a thin aluminium layer between said removable outer layer (3) and the core pipe (2). A method of connecting two-layer plastic pipes (9) to be laid in the ground for conducting material therethrough, characterised by the following steps: removing by knocking or peeling a region of a removable outer layer (12) of the pipe to expose an inner core (13) of said pipe at the end of said pipe; connecting the two ends of two said pipes together by electrofusion welding the exposed cores (13) of said pipes together.

Inventors: ANDERSSON MIKAEL, JAERVENKYLAE JYRI, VESTERGAARD ANDERS, ANDERSSON, MIKAEL, JAERVENKYLAE, JYRI, VESTERGAARD, ANDERS, Järvenkylä, Jyri 
Applicants: UPONOR BV, UPONOR B.V.
Application year: 1993
Main classification: B29C47
IPC codes: B32B7, F16L9, B32B1, B29C47, B29C65, B32B27, B29L23, B29L9