FR2728048A1 - Multi-layer pipe with outer layer, and inner layer closely enclosed by intermediate layer - Google Patents

Abstract

A pipe of multi-layer material comprises an internal layer, an external layer and an intermediate layer comprising a thin sheet closely enclosing the inner layer. Pref. the intermediate film is of metal, esp. Al or is a multi-layer assembly of a metal film with an adhesive layer on at least one face. The metal film may carry a layer based on glass fibres, or a mica coating on at least one face. The thickness of the metal film is 10-70mu. The enveloping layer is applied by helicoidal or longitudinal rolling, to give a ribbon of material on the inner layer. The outer layer is of polyamide, co-extruded with the intermediate layer already carrying an adhesive based on maleic acid, and the inner layer is of HDPE, pref. cross-linked; or the inner layer is of polyamide and the outer layer is of HDPE or is based on PVC.

Description

 The present invention relates to a pipe whose wall is made of composite material and which is intended for fluids such as fuel or windshield washer fluid, engine coolant, or even heat transfer fluid for the heating system of a vehicle. automobile.

 The pipes used today when they are no longer metallic, are essentially made of polyamide. This material has a number of drawbacks, in particular due to the alcohol which is necessarily present in the fluids conveyed. Thus, the first effect of alcohol is to cause the material to swell, which destroys its dimensional characteristics and leads to leaks or unacceptable stresses in the pipes. In addition, the alcohol dissolves some of the components of the pipe and leads them to the rest of the installation where they are a cause of fouling.

 To combat this drawback, it has been proposed to coat the inner wall of these pipes with a fine barrier layer, for example of polyolefin (polypropylene in particular) by coextrusion. In other techniques, it has also been proposed to coat the internal face of this pipe with an EVOH (ethylene vinyl alcohol copolymer) for the well-known property of these bodies to form an effective barrier, in particular to organic solvents. However, in the presence of alcohol, EVOHs tend to retain this alcohol and especially methanol.

 We have also described products in which this layer is sandwiched between a thick outer polyamide and a thin inner polyamide.

 The present invention intends to propose a pipe in which the barrier function to all the chemicals present in a fuel liquid or in an alcohol solution such as they are encountered in a motor vehicle, is ensured economically by a single intermediate layer . Indeed, the search for the impermeability of such a pipe can be satisfied by using noble raw materials but of very high price. As in this type of product, the price of the material comes in for around half the cost price, it is essential to be able to use inexpensive materials (therefore of lower performance from the point of view of waterproofing) and to apply treatments, also as cheaply as possible, to improve their poor performance.

 In addition, as some of these liquids are flammable, the invention in a preferred embodiment, has the second aim of providing a pipeline having a delay in ignition therefore in its piercing by burning or fusion greater than the current pipelines made of plastic.

 To this end, therefore, the subject of the invention is a pipe made of multilayer material, intended to convey a fluid between some of the members of a motor vehicle which comprises an internal polyethylene layer, an intermediate layer formed by a thin sheet enveloping the layer. internal and an external layer made of a synthetic material suitable for the environment. This pipe structure is remarkable essentially for its internal components, namely a polyethylene layer and a sheet covering this layer, the external layer possibly being of a material the choice depends on the external conditions to which the pipeline is subjected (polyamide, elastomer, rubber, polyester, polyolefins with or without crosslinked polyvinyl chloride mineral filler, etc.). Indeed polyethylene is a very cheap material, easy to work by extrusion in particular but whose barrier functions are relatively poor. We will also preferably choose a high density polyethylene to benefit from the best of these characteristics.

 The barrier function is then devolved to the envelope of the internal polyethylene tube. According to the invention this envelope is made from a sheet material because, in the manufacture of this sheet, there is either a rolling or calendering which is a mechanical operation extremely favorable to the compactness of the material therefore to its character waterproof, a coating that makes the sheet waterproof.

 Among the sheet materials which can be used, the choice will preferably relate to a metal, in particular aluminum. It is known in fact that the ductility of this metal makes it possible to obtain, extremely inexpensively, extremely thin sheets.

This thinness is an essential quality required by the invention to ensure very great intimacy of the contact between the envelope and the internal polyethylene tube. This contact is reinforced by an adhesive which is inserted between the surface of the polyethylene tube and the sheet, this adhesive preferably being present on one of the faces of the sheet, for example in the inactive state. Its activation is carried out by any suitable means such as the supply of heat.

 As an indication, aluminum can have a thickness of the order of fifteen microns. One finds on the market, and this is also one of the advantages of the invention from the point of view of cost price, tapes of aluminum sheets pre-coated (on both sides) whose thickness is of the order of 70 microns the metal core having between 15 and 25 microns thick.

 By way of example, it is also possible to use a film of synthetic material such as cellulose hydrate (trademark cellophane) for its impermeability qualities to gases which are not soluble in water (thanks in particular to the nitrocellulose varnish, the sheet of which is usually coated). An additional advantage of this material lies in its ease in adding it to polyethylene and therefore in making its intimate contact with this material.

 Mention will also be made of the use in the context of the invention of composite films or ribbons. It may for example be an aluminum sheet coated on one side with a woven or non-woven fiberglass material or a deposit of mica. The aluminum foil can be replaced by a cellophane or polyester film.

The advantage of such a composite lies in its ability to add another function to that of the barrier of the base film, for example an anti-fire layer, that is to say retarding the piercing due to the flame. This is why the coated side of the film will face outward.

 The application of the sheet material on the polyethylene tube can be carried out by wrapping. A ribbon is therefore wrapped helically around the tube with overlap, its adhesive side facing the tube. It can also be produced by longitudinal wrapping with overlapping of the edges.

 The outer layer of the pipe is preferably produced by coextrusion of a material suitable for protecting the pipe against attacks from the external environment. The material can be a polyamide, an elastomer. Mention will in particular be made of crosslinked PVC which has excellent fire resistance. It will be advantageous, prior to this final extension, to also put in place, by coextrusion, a layer of adhesive. An adhesive based on maleic acid will be chosen for the bonding of a polyamide layer to a metal sheet.

 It is also within the framework of the invention to provide an adhesive tape on its two faces, the adhesive being dedicated to the material which it must meet.

 Other characteristics and advantages will emerge from the description of the invention.

Reference will be made to the attached drawing in which
FIG. 1 is a diagram illustrating a line for manufacturing a pipe according to the invention,
- Figure 2 illustrates by a diagram a variant of the production line.

 An extruder 1 makes it possible to manufacture a polyethylene tube 2. The polyethylene will preferably be a high density polyethylene, the outside diameter of which is of the order of a centimeter. At the exit of the extruder, the tube 2 passes through a calibration station 3 at the exit of which it is supported by a pulling device 4. At station 5 we proceed to the helical wrapping of a ribbon 6 around of the tube 2, this tape 6 being able to be an aluminum foil covered at least on its face turned towards the tube 2 with an appropriate adhesive. At the time of wrapping, this adhesive is in an inactive state and in a heating station 7 for example by infrared, located downstream of station 5, the adhesive is made active to create an adhesive bond between the metal sheet and the upper surface of the tube 2 and to create an adhesive bond between the longitudinal portions of the tape which overlap when this ribbon is wrapped around the tube 2.

 The aluminum foil has of course a width which is a function of the external diameter of the tube 2 on the one hand and the pitch of the propeller which it is desired to produce on the other hand. Its thickness is preferably around 15 y. It is not, however, outside the scope of the invention to provide a metal sheet of smaller thickness, or on the contrary of greater thickness as long as an intimate contact between the tube and the ribbon is ensured during wrapping. . The advantage of this technique lies partly in the presence on the market of an aluminum strip having one or both of its pre-adhesive faces. At the outlet of the heating element 7, the banded tube passes through a extruder head 8 where it receives a layer of an adhesive based on maleic acid, and in the head of an extruder 9 where, immediately after the deposition of maleic acid, a layer of polyamide 10 is deposited for example PA11, PA12. The composite tube then passes into a cooling unit 11 from which it is extracted by a second pulling mechanism 12.

 A multilayer tube is thus produced which has excellent qualities compared to a fluid such as a hydrocarbon to be transported. Indeed, the tightness of this tube to hydrocarbon vapors was measured and it was found that after two months of aging of the tube at 600, the "leak" was 0.30 g / m2 / hour. By way of comparison, it will be indicated that the best performances in this field of application are obtained by tubes made of noble material of the fluorocarbon polymer type, the price of which is more than three times that of the tube obtained in accordance with the invention, and are around 1 to 1.5 g / m2 and per hour.

 FIG. 2 illustrates an alternative means to the wrapping device 5 for wrapping the polyethylene tube 2 with a metal foil which is presented here in the form of a tape 14 whose width is a function of the circumference of the tube 2 so that when this ribbon 14 is wound longitudinally around the tube, in particular by means of a form gauge 15, the longitudinal edges of ribbon 14 overlap one another. The shape of the gauge 15 will be studied so that the tape is tightly applied against the tube 2, this tape being able to have a pre-adhesive face which will make it possible to join the overlapping edges at the outlet of the gauge 15. This technique makes it possible to produce a longer tube length, with identical ribbon length, than by wrapping.

 In variants of the invention, it is entirely possible to use a sheet material which is not a metallic strip. One can for example use a cellophane tape (registered trademark), which is known to be impermeable in particular to vapors insoluble in calf, this impermeability mainly due to the varnish applied to the cellophane. This cellophane tape can be associated with a layer of polyethylene which, when activated by heat, will constitute the means of adhesion of this tape to the tube also made of polyethylene. Materials in complex sheet can be associated either by wrapping or by longitudinal winding with the polyethylene tube 2. These complex ribbons can include a metal sheet with one face, preferably the face which will be turned towards the outside of the tube when the ribbon is installed, is provided with a layer of woven and / or non-woven fiberglass or a protective layer of the mica projection type. It is also possible to add polyester fibers to these metal ribbons. This creates a structural reinforcement of the tube and a barrier to the action of an external flame on the tube. This constitutes an important advantage of these complexes, taking into account the fact that the fluid conveyed by the tube is generally very flammable.

By way of example, it will be indicated that for a tube with an outside diameter of the order of 1 to 3 centimeters, the internal layer of high density polyethylene is between 0.4 and 0.8 mm thick, the layer intermediate, depending on whether it is a simple metal sheet or whether it is constituted by a complex multilayer ribbon, is of a thickness which can vary between 15 and 100 tL
Finally, the outer layer of elastomer or polyamide has a thickness of between 0.3 and 0.5 mm.

 It is interesting to note that the sheet material used in small thicknesses does not interfere with the mechanical work that is done to this tube (tulipage, radial mechanical expansion, etc.) to ensure the insertion of end caps. connection or cooperation of connection devices with tube ends. In this respect, the tube according to the invention behaves like a conventional tube. This allows it to be implemented with traditional means whose handling and use are perfectly mastered.

 It will also be mentioned, when the sheet material is metallic, the fact that the tube becomes electrically conductive, which makes it possible to connect it to a grounding and evacuation device for example of static electricity. Finally, the fact that the inner tube is made of polyethylene makes it possible to subject the tube to a treatment process at the end of manufacture or during its manufacture consisting in crosslinking the internal layer of this tube, which improves its performance from the point of view permeability to liquids and hydrocarbon or alcoholic gases.

Claims (13)

 1. Pipe made of multilayer material intended to convey a motor vehicle fluid, characterized in that it comprises an internal layer (2) of polyethymene, an intermediate layer formed of a material (6, 14) in thin sheet intimately enveloping the inner layer (2) and an outer layer (10) of synthetic material suitable for the environment.  2. Pipe according to claim 1, characterized in that the intermediate layer (6, 14) is a metal sheet.  3. Pipe according to claim 2, characterized in that the thickness of this metal layer is between 10 and 70 p.  4. Pipe according to claim 3, characterized in that the metal is aluminum.  5. Pipe according to one of the preceding claims, characterized in that the intermediate layer comprises a multilayer complex comprising at least one metal sheet and a layer of adhesive on at least one of its faces.  6. Pipe according to claim 5, characterized in that the metal sheet is provided on one of its faces with a layer based on fiberglass.  7. Pipe according to claim 5, characterized in that the metal sheet is provided on one of its faces with a coating of mica.  8. Pipe according to one of the preceding claims, characterized in that the envelope layer is produced by helical wrapping with covering of a ribbon of the thin sheet material on the internal polyethylene layer.  9. Pipe according to one of claims 1 to 8, characterized in that the envelope layer is produced by longitudinal rolling with covering of a ribbon of the thin sheet material on the internal polyethylene layer.  10. Pipe according to claim 2, characterized in that the outer layer is made of coextruded polyamide on the intermediate layer previously covered with an adhesive based on maleic acid.  11. Pipe according to one of the preceding claims, characterized in that the internal layer is made of high density polyethylene.  12. Pipe according to one of the preceding claims, characterized in that the internal layer is made of crosslinked high density polyethylene.  13. Pipe according to one of the preceding claims, characterized in that the thicknesses of each layer are between 0.4 and 0.8 mm for the inner layer, 0.015 and 0.1 mm for the intermediate layer, 0.3 and 0.5 mm for the outer layer.