Structured wall plastic pipe, process for manufacturing said pipe and particular use of same

The present invention relates to a structured wall plastic pipe, a process for manufacturing said pipe, and also one particular use of this pipe. Pipes intended for transporting fluids (pressurized or unpressurized fluids) and, in particular, the pipes for underground rainwater or wastewater drainage, water supply systems, etc., can be economically produced from ductile cast iron, ceramic or concrete. However, plastic pipes are preferred, in a number of cases, to cast iron, ceramic or concrete as they are much lighter and exhibit remarkable corrosion resistance.

However, with the aim of exhibiting sufficient stiffness and/or in order to withstand high mechanical stresses as well as cast iron does, conventional plastic pipes must have a greater wall thickness, which increases their cost and makes them less competitive compared with ductile cast iron pipes. To solve this problem, several solutions have been envisaged. In particular, for pipes used in sanitation for transporting wastewater, several configurations have been developed, in particular:

• coextruded pipes comprising three PVC-based layers and more particularly: pipes of ABA type where A is rigid PVC and B rigid foamed PVC; • pipes comprising two layers produced by in-line laminating of an inner pipe, covered during passage in a forming block with a corrugated pipe; this configuration is generally based on PP;

• pipes produced based on profiles wound at high temperature around a mandrel (especially used for large diameters). These configurations have the advantage of meeting a given ring stiffness criterion with a weight that is reduced with respect to that of a cast iron, ceramic or concrete pipe, and even with respect to that of a solid plastic pipe, but that is still relatively high. For instance, ABA pipes made of PVC with a diameter of 315 mm and having a ring stiffness of 8.8 kN/m ² and which are conventionally used in wastewater drainage have a weight per meter that is greater than 10 kg

(see, in particular, Application EP 1 393 891). At comparable diameters and ring stiffnesses, the aforementioned PP structures have a weight per meter of around 5.4 kg, which is significantly more competitive.

Application FR 2 745 746 describes a structured wall material for lining pipes comprising two outer laminated layers and an intermediate layer having a honeycomb structure comprising cells that have the shape of contiguous symmetrical prisms (the cross sections of which have a circular envelope). This intermediate layer has the advantage of increasing the mechanical strength of the whole low-weight and low-cost assembly. However, its contiguous symmetrical prism structure gives it such a high stiffness that it is necessary to provide rupture lines in order to be able to produce the desired curves, which is relatively complicated from an implementation point of view and leads to poor mechanical strength (acceptable, it is true, in lining, but not for solid piping).

The object of the invention is especially to provide a self-supporting structured wall pipe having, for a given diameter, a high ring stiffness for a particularly low weight and, in particular: a ratio (ring stiffness/weight per unit length) higher than the aforementioned prior art structures. For this purpose, the invention relates to a structured wall plastic pipe comprising a tubular plastic mandrel and at least one plastic sheet having a honeycomb cellular structure wound around the mandrel in such a way that its cells are arranged radially to the surface of the mandrel, and firmly attached to the latter, this pipe being characterized in that the cells of the cellular structure are asymmetrical, i.e. have a cross section for which the envelope has a shape factor (defined as being the ratio of its largest dimension to its smallest dimension ) greater than 1.

By virtue of this feature, the sheet has been able to be wound around the tubular mandrel even when it has a relatively pronounced curve, without it being necessary to provide rupture lines (i.e. the winding may constitute a continuous structure without rupture lines/cracks).

In the context of the present invention the term "plastic" is intended to denote any material comprising at least one synthetic resin polymer.

Any type of plastic may be suitable. Particularly suitable plastics belong to the category of thermoplastics.

The term "thermoplastic" is understood to mean any thermoplastic polymer, including thermoplastic elastomers, and also blends thereof. The term "polymer" is understood to mean both homopolymers and copolymers (especially binary or ternary copolymers). Examples of such copolymers are, non-limitingly: random copolymers, block copolymers and graft copolymers.

Any type of thermoplastic polymer or copolymer, the working temperature of which is below the decomposition temperature, is suitable. Synthetic thermoplastics having a working range spread over at least 10 degrees Celsius are particularly suitable. Examples of such materials include those that exhibit polydispersion in their molecular weight.

Use may especially be made of polyolefins, polyvinyl halides (such as PVC or polyvinyl chloride) or polyvinylidene chlorides (such as PVDF or polyvinylidene fluoride, PVDC or polyvinylidene chloride), thermoplastic polyesters, thermoplastic fluoropolymers, polyarylethersulphones such as polyphenylsulphones (PPSUs), polyketones, polyamides (PAs) and copolymers thereof. PVC and polyolefins [and in particular polypropylene (PP) and polyethylene (PE)], polyarylethersulphones such as polyphenylsulphones (PPSUs), PAs and thermoplastic fluoropolymers have given good results.

A blend of polymers or copolymers may also be used; similarly it is also possible to use a blend of polymeric materials with inorganic, organic and/or natural fillers such as, for example but non-limitingly: carbon, salts and other inorganic derivatives, natural or polymeric fibers, which is generally referred to as a polymer composition. It is also possible to use multilayer structures composed of stacked and joined layers comprising at least one of the polymers or copolymers described above.

One polymer that is often used is PVC or PE (polyethylene) and in particular high-density polyethylene (HDPE). PVC is particularly preferred because it has a high stiffness/cost ratio. The term "PVC" is understood to mean any homopolymer or copolymer containing at least 50% by weight of vinyl chloride.

The base constituent of the structured wall pipe according to the invention is a tubular mandrel, that is to say a hollow pipe-shaped article. It is constituted, at least partially and preferably completely, of a composition based on plastic(s). According to one advantageous variant, it is a pipe comprising an inner layer based on plastic(s) and a thinner surface layer that acts as an adhesive during the attachment of the honeycomb (the terms "thick" and "thin" being relative, i.e. by comparison of one layer with the other). Preferably, the adhesive layer is deposited by coextrusion, is compatible with the thick layer (mandrel/support strictly speaking) and is suitable for the attachment technology used (adhesive bonding, thermal welding, laser welding, UV welding, etc.).

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The most widely used adhesive is generally a polymer adhesive that may be in the form of a polyurethane, an acrylic polyester, an EVA/VAc or an ethylene vinyl alcohol/vinyl acetate copolymer (if the inner layer is based on PVC) or a functionalized polyolefin (if the inner layer is based on a polyolefin). The expression "functionalized polyolefin" is understood to mean any polyolefin comprising, besides units derived from olefins, functional monomer units. These may be incorporated either into the main chain of the polyolefin, or into its side chains. They may also be incorporated directly into the backbone of these main and side chains, for example by copolymerization of one or more functional monomers with the olefin monomer(s) or else may result from the grafting of one or more functional monomers to said chains, after the manufacture of the polyolefin. Several functionalized polyolefins may thus be used as a blend.

The functional monomer units of the functionalized polyolefin are chosen from carboxylic acids, carboxylic diacids and the anhydrides corresponding to these diacids. These monomer units generally result from the copolymerization or grafting of at least one unsaturated monomer having the same functional groups. Examples of monomers that can be used are, non-limitingly, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, maleic anhydride, fumaric anhydride, glycidyl acrylate and itaconic anhydride. Preferably, the functional monomer units result from the copolymerization or grafting of maleic anhydride.

According to the invention, the sheet having a (honeycomb) cellular structure also comprises at least one layer based on plastic(s). Preferably, this plastic is the same as that of the tubular mandrel (PE or PVC, the latter being preferred) or a plastic compatible with this so that the structured wall pipe can easily be recycled. Most particularly preferably, it is a multilayer structure comprising at least one (or even 2) surface layers based on an adhesive as described above.

The expression "sheet having a honeycomb cellular structure" (sometimes simply referred to as "honeycomb" in the account that follows), is in fact understood to mean a sheet (panel) composed of a cohesive assembly of contiguous cellular cells (also referred to more simply as "cells" in the present description). The term "cells" is understood to mean open or closed cells having an asymmetric cross section i.e. the envelope of which has a shape factor other than 1, preferably at least equal to 1.5, even 2.5 and preferably 4. Values at or

above 2 are especially preferred for hexagonal cells, while lower values are admitted for diamond-shaped cells (see further in the specification).

These cells generally have parallel walls from one cell to another. Although they may have a section (viewed in a plane parallel to the honeycomb surface) which is substantially oval, elliptical or hexagonal, is has been found that cells with a diamond-shaped section (i.e. a section having 6 sides parallel two by two, 4 of about the same length and 2 of a shorter length, which are the sides by which the cells are welded: see further in the specification and figure 1 and table 1 below) give better results in terms of flexibility.

Preferably, the shortest sides or welds (C on figure 1) have a length of less than a half, preferably less than a fourth and even, less than a l/7 ^th of the length of the other sides. Good results are obtained when the ratio of the length of the cells (L on figure 1) to the length of the welds or sides C (in other terms: the ration L/C with ref. to figure 1) is comprised between 3 and 90, preferably between 5 and 45.

Such honeycombs with diamond-shaped cells are particularly flexible in all directions of space i.e. they can be wound and distorted in order to match a tridimensional surface, which makes them interesting also outside the frame of the present invention, for instance as reinforcement structures in the construction of buildings, boats... and other items with complex surfaces to be reinforced. In the case of complex geometries, it is preferable to use band shaped honeycombs, for instance having a width of 50 cm or less, preferably 10 cm or less.

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According to the invention, the cells of the honeycomb sheet are arranged radially, i.e. the vertical axis of the cells (the one which is perpendicular to the plane of the sheet) is radial (positioned substantially in the extension of a radius of the pipe) and is in a plane substantially perpendicular to the axis of the pipe.

This sheet may have been obtained by any known process. However, preferably, it is obtained by extrusion of molten plastic lamellae that are intermittently welded, as will be described in greater detail below.

According to the invention, this sheet is wound around the mandrel, i.e. brought into contact with at least one part of its surface (and preferably: around the entirety of its working surface) so as to at least partially follow the curvature thereof. Preferably, they are wound by bending in the direction where their flexural modulus is the lowest, which is generally perpendicular to the longitudinal axis of the cells (the longest symmetry axis when the cellular structure is seen from above). Hence, there are essentially two possibilities:-

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- either the longitudinal axis of the cells are substantially parallel to the main axis of the sheet (i.e. its length); in this case, the cellular structure is wound around the mandrel with these axis substantially parallel to the generatrix of the pipe (i.e. like an open "sock") and its width is preferably equal to the circumference of the mandrel;

- or these axis form a non-zero angle with the main axis of the sheet and, in this case, the structure is wound substantially transversely around the mandrel, i.e. the winding angle is made so that the axis of the cells form an angle of less than 60° with respect to the generatrix of the pipe, preferably of less than 30° and particularly preferably of less than 15°. The edges of two adjacent turns

(windings) may not be contiguous over their entire length. In this case, the distance between 2 such edges is preferably at most 10 cm, preferably at most 5 cm and most particularly preferably at most 1 cm, it being understood that the turns are in fact preferably contiguous. Preferably, the axes of the cells are substantially perpendicular to the main axis of the sheet since this arrangement makes it easier to store the honeycomb and to attach it to the tubular mandrel (via transverse winding).

It should be noted that the structured wall pipe according to the invention may comprise several superposed honeycomb sheets with a view to achieving several levels of stiffness from one and the same tubular mandrel.

According to one particularly advantageous variant of the invention, the structured wall pipe comprises at least one top layer attached to the honeycomb sheet on the side opposite the one to which the tubular mandrel is attached. This variant makes it possible to give the structured wall pipe thus obtained a "solid" outer surface that allows easy connections by means of standard couplings or bends and that prevents the sheet from being damaged, especially in the case where its cells could be filled with water, which is capable of expanding it in the case of frost. To prevent this same problem, the adjacent turns are preferably contiguous. This top layer preferably comprises a compact sheet based on plastic(s) and preferably a sheet comprising a thick layer and a thinner layer, the latter preferably being based on an adhesive via which the sheet is attached to the honeycomb. Preferably, the layer of adhesive is deposited by coextrusion and is adapted to the nature of the honeycomb or of the top layer, and to the fastening (attachment) technology used. During the deposition, the sheet of the top layer may be in the molten or solid state. Preferably, it is in the solid state. There are

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essentially two variants: either the sheet of the top layer is wound around the tubular mandrel that already bears the honeycomb, or it is attached to the latter before it is fastened to the tubular mandrel. The latter variant makes it possible to simplify the process of manufacturing the structured wall pipe but requires the use of an adhesive that is elastic at the winding temperature.

In the 1 ^st variant, the winding is preferably carried out so that the edges of two successive top sheets (or turns of one and the same sheet) are contiguous or partially overlap. Preferably, the 2 contiguous or overlapping edges are heat welded or adhesively bonded so as to ensure good impermeability. In order to ensure a better impermeability, the top layer may be composed of at least 2 sheets wound at opposite angles, i.e. these make between them, relative to the axis of the pipe, a similar angle but of opposite sign. In this definition, the term "similar angle" means an angle at least equal to the same angle minus 5 degrees of angle. The term "similar angle" also comprises an angle at most equal to the same angle plus 5 degrees of angle. Preferably, this term signifies an angle at least equal to the same angle minus 2 degrees of angle. Preferably, this term signifies an angle at most equal to the same angle plus 2 degrees of angle.

Similarly, when the reinforced pipe comprises more than two top sheet layers, any two adjacent layers of this pipe preferably make a similar angle, but of opposite sign with respect to the axis of the pipe.

In the 2 ^nd variant, the top sheet is attached to the cellular sheet, and this variant makes it possible to give the structured wall pipe thus obtained a top layer deposited at the same time as the cellular layer is deposited, which allows easy connections by means of standard couplings or bends. In particular, when the tubular mandrel and the honeycomb are based on

PVC, the top layer may comprise a sheet of PVC to which long, chopped and randomly distributed glass fibres have been added. Such a sheet is described in particular in Application PCT/EP2007/062787 in the name of the Applicant, the content of which is, for this purpose, incorporated by reference in the present application. Such a structure makes it possible to obtain a particularly high ring stiffness.

It is also particularly advantageous for the nature of each of the constituents of the pipe to be judiciously chosen for the purpose of allowing it to be recycled and reused as a blend in one of the layers of a new pipe. This capability should in this case be able to be guaranteed both when manufacturing a new pipe (by using manufacturing waste) and when recycling a spent pipe, at

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the end of its period of use. Hence, structured wall pipes for which all the components (tubular mandrel, honeycomb and top layer, where appropriate) are based on the same polymer (i.e. mainly composed of the same polymer) or on compatible polymers are preferred. The invention also relates to a process for manufacturing a structured wall pipe as described above, according to which at least one plastic sheet having a honeycomb structure with asymmetric cells is wound around a tubular plastic mandrel and attached to the latter in such a way that these cells are arranged radially to the surface of the mandrel (the asymmetric nature having been defined above).

As explained above, the honeycomb may be wound about a given winding angle (between the longest walls of the cells and the generatrix of the pipe).

According to one preferred variant, the honeycomb is obtained by extrusion of molten plastic lamellae which are intermittently welded. In particular, according to this variant of the invention, the honeycomb is obtained by a process according to which:

- parallel lamellae of a composition based on at least one plastic are continuously extruded, in an approximately horizontal direction, through a die having a front face provided with a plurality of parallel slots and with an insulating material, at least on the surface; and

- upon exiting the die, the spaces lying between two adjacent lamellae are subjected, in successive alternations and between two sizing units whose length is short enough for the plastic composition to remain molten, to an injection of compressed gas or of a coolant and to a vacuum, the two sides of any one lamella being, in respect of one of them, subjected to the action of the compressed gas or coolant and, in respect of the other of them, subjected to the action of the vacuum, and vice versa during the next alternation, so as to deform the lamellae and weld them together in pairs, with formation, in a plane approximately parallel to the extrusion direction, of a cellular structure whose constituent cells extend perpendicular to the extrusion direction.

Such a process is described in Application WO 2007/020279 in the name of the Applicant, the content of which is, for this purpose, incorporated by reference in the present application. The honeycomb resulting from this process may be fastened to the mandrel in line with the manufacture of the latter and/or of the honeycomb. Preferably, the honeycomb is manufactured in advance and stored in order to be wound around and fastened to the tubular mandrel in line

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with the manufacture of the latter. When the honeycomb has a longitudinal flexibility, it may be wound around a support which facilitates its storage and its subsequent winding around the mandrel.

When such a process is used, in order to act on the shape factor (L/l with ref. to figure 1), it is possible to increase/decrease L by increasing/decreasing the time during which vacuum is applied to the cells (i.e. increasing/decreasing the time between 2 welds) and/or to increase/decrease 1 by modifying the distance between lamellae and/or to stretch the honeycomb.

In diamond- shaped cells are required, these can be obtained by first extruding a honeycomb with bad (poor quality) welds, which is afterwards stretched perpendicularly to the extrusion direction so that these welds are opened to some extent, reducing the length of the "C" sides (where the welds are located: see above). Welds of poor quality can be obtained by reducing the temperature and/or pressure of the weld. Most particularly preferably, the honeycomb is manufactured by coextrusion and most particularly preferably according to the process described in Application WO 2006/106101 in the name of the Applicant (the content of which is, for this purpose, incorporated by reference in the present application), said coextrusion aiming to provide an adhesive layer on at least one of its faces. Preferably, the honeycomb is coextruded so that its cells comprise three layers: a (relatively thick) central layer constituting the body of the honeycomb and two (relatively thinner) adhesive-based surface layers, one adhesive layer being intended for fastening the honeycomb to the mandrel and the other one for fastening a top sheet to the honeycomb (where appropriate). The adhesives used are the same as those described above for the structured wall pipe.

According to one particular form of the process according to the invention, these are thermally- activated adhesives (adhesives that are activated by heating).

Using an adhesive of this type offers the advantage of limiting the duration of heating of the components to that strictly necessary for developing the adhesive effect, without degrading their mechanical properties and their shape under the effect of heat.

As explained previously, honeycombs with longitudinal flexibility are preferred. Such honeycombs may especially be obtained by adding, to the processes described above, a step of stretching the honeycomb perpendicular to the extrusion direction and in line with said extrusion. Such a process is

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described in Application WO 2007/110370 in the name of the Applicant, the content of which is, for this purpose, incorporated by reference in the present application.

According to the invention, the tubular mandrel may also have been manufactured by coextrusion (conventional technique in this field) in order to obtain an adhesive-based surface layer and the same is true for the top sheet, where appropriate.

There are essentially two possibilities for fastening the honeycomb to the tubular mandrel when the latter is obtained by (co)extrusion: - either the mandrel is calibrated and cut to the desired length in line, and then fitted with the honeycomb subsequently;

- or the mandrel is calibrated and fitted with the honeycomb in line, then cut to the desired length.

The 1 ^st variant has several advantages: - when the honeycomb has a longitudinal flexibility and is wound around a support, it makes it possible to rotate the mandrel rather than the winding of the honeycomb, which is more practical, especially for large diameter pipes.

It is sufficient to provide a manual or automatic initiation for the fastening of the start of the winding around the rotating pipe; - it allows the flaring of the mandrel (i.e. the flared deformation of its ends so as to provide a joint between adjacent structured wall pipes during their assembly as a network);

- it makes it possible to vary the number of honeycomb layers, and by doing so to obtain pipes with varying ring stiffnesses based on one and the same mandrel.

Whichever variant is chosen, it is advantageous to pre-equip the honeycomb (and in the case of a structure having a stack of honeycombs: at least the last honeycomb, which is at the surface of the structured wall pipe) with a top layer as described previously. Most particularly preferably, the honeycomb comprises at lest one top layer attached to the sheet on the side opposite the one intended to be attached to the tubular mandrel, this layer comprising a sheet attached to the honeycomb using an adhesive, preferably a flexible adhesive (i.e. an adhesive that is flexible/elastic at ambient temperature).

The structured wall pipes according to the invention maybe used in any application where high ring stiffness presents an advantage. They are particularly suitable for the discharge of wastewater or rainwater.

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Whatever they are used for, but in particular for wastewater or rainwater drainage, before the pipes are connected together (generally via specific couplings or by flare fitting), it may be advantageous to fill the honeycomb cells visible on the edge with a foam or a plugging material before applying the couplings and/or making the joint.

Finally, the invention also concerns a honeycomb sheet suitable for a (process for making a) structured wall pipe as described above, and which comprises cells made of deformed and welded plastic lamellae which have a diamond- shaped cross section, i.e. a section having 6 sides parallel two by two, 4 of about the same length and 2 of a shorter length, through which sides the cells are welded to each other.