The present invention relates to a plastic multilayer-wall curved segment for assembling pipes. It also relates to a method for manufacturing this segment. It also relates to a method for assembling pipes from same segments and also to the pipes thus assembled.

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 a view to 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 proposed. 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.

The French patent application published under the number 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).

Document WO 2009/115521 discloses, in particular, a process for producing a structured wall plastic pipe according to which at least one plastic sheet having a honeycomb structure with asymmetric cells (i.e. cells that have a cross section for which the envelope has a shape factor (ratio of its largest dimension to its smallest dimension) greater than 1) is wound around a tubular plastic mandrel, and firmly attached to the latter, so that these cells are arranged radially to the surface of the mandrel. The assembly is covered with at least one top layer which must be attached to the honeycomb sheet on the side opposite the one to which the tubular mandrel is attached. The self-supporting structured wall pipe has, for a given diameter, a high ring stiffness for a low weight.

However, this process for producing a structured wall pipe is difficult to implement since it involves the design, production and use of heavy and bulky equipment and tools, requiring significant investment, especially when, as is often the case in practice, the structured wall pipe must be assembled in the factory and transported to the work site with the transport costs that this involves since the pipes must be stacked up on the lorry, which represents a large volume of air and therefore a high transport cost per pipe.

One subject of the present invention is especially to provide elements with structured walls, which elements are much less bulky and lighter than the pipe described in document WO 2009/115521, and can be easily transported due to their subsequent assembly into pipes at the work site.

For this purpose, the invention relates to a plastic multilayer-wall curved segment for assembling pipes, said segment comprising at least one central layer (P) based on a thermoplastic polymer (PI) and said central layer having a honeycomb cellular structure, each of the outer faces of this central layer (P) being covered with at least one sheet (F) of solid structure based on a thermoplastic polymer (P2).

The expression "curved segment" is understood to define, in the present description, a solid three-dimensional structure, rounded in its circumferential direction and substantially flat in its longitudinal direction, i.e. a panel that is curved in such a way that two of its parallel edges describe arcs of a circle, the other two parallel sides being rectilinear. The curved segment according to the invention is advantageously present as an element (a fraction) of a wall of cylindrical cross section. This curved segment constitutes a portion of the final pipe to be assembled. Thus, if this curved segment represents 1/n of the circumference of the final pipe (or of a length thereof), the latter will be obtained by assembling n elements. Advantageously, n is greater than or equal to 2.

Preferably, n is less than or equal to 8, preferably less than or equal to 4.

In the present description, the term "plastic" is understood to mean any amorphous or semicrystalline 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, linear block copolymers, other block copolymers and graft copolymers.

In the present description, the term "amorphous polymer" is understood to mean any thermoplastic polymer predominantly having a disordered arrangement of the macromolecules that form it. In other words, this term is understood to mean any thermoplastic polymer that contains less than 30% by weight, preferably less than 10% by weight, of crystalline phase (that is to say, the phase characterized by a melting endotherm during differential thermal analysis (DSC) measurements).

In the present description, the expression "semi-crystalline polymer" is understood to mean any thermoplastic polymer having, in a large proportion, a chemically and geometrically regular arrangement of the macromolecules that form it. In other words, this expression is understood to mean any thermoplastic polymer that contains more than 30% by weight, preferably more than 50% by weight, of crystalline phase (that is to say the phase characterized by a melting endotherm during differential thermal analysis (DSC) measurements).

Two characteristic temperatures are generally associated with

thermoplastic polymers: they are the glass transition temperature (T _g ) and the melting temperature or temperature of fusion (T _f ). T _g is the temperature below which a polymer mass has several properties of inorganic glass, including hardness and rigidity. Above the T _g , the polymer mass has plastic or elastic properties and it is said to be in the rubbery or elastomeric state. T _f is also called the flow temperature in the case of amorphous polymers and the straightforward melting point when it is a question of semicrystalline polymers. At the T _f (which is rather, in practice, a temperature zone or range of temperatures), it is more a case of a viscous liquid.

Any type of thermoplastic polymer or copolymer, the working temperature of which is below the decomposition temperature, is suitable for producing the multilayer-wall curved segment according to the invention. Synthetic thermoplastics having a working range spread over at least 10 degrees Celsius are particularly suitable. Examples of such materials include those that exhibit polydispersity of their molecular weight.

Use may especially be made of polyolefms, polyvinyl halides (such as

PVC or polyvinyl chloride) or polyvinylidene halides (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 polyolefms [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 fibres) and/or other additives (stabilizers, processing aids, etc.), which is generally referred to as a polymer composition.

Polymers (PI) advantageously used in the composition of the central layer (P) of the curved segment according to the invention are PVC and polyolefms [in particular polypropylene (PP) and high-density polyethylene (HDPE)]. PVC is particularly preferred on account of the fact that 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, preferably at least 80% by weight of vinyl chloride. PVC polymers that are very particularly preferred are vinyl chloride homopolymers.

These same polymers may advantageously be used as polymer (P2) in the composition of the sheets (F) of the curved segment according to the invention.

PVC containing a known reinforcing agent, such as glass fibres for example, in particular long, chopped and randomly distributed glass fibres, is particularly preferred for the composition of the sheets (F). A curved segment of particularly high ring stiffness is obtained when the polymer (PI) used in the composition of its central layer (P) is PVC and the polymer (P2) used in the composition of the sheets (F) is PVC to which glass fibres have been added.

It is also particularly advantageous for the nature of each of the

constituents of the curved segment according to the invention to be judiciously chosen for the purpose of enabling it to be recycled and reused. Hence, curved segments for which all the components (central layer(s) (P) and sheets (F)) are based on the same polymer, (i.e. mainly composed of the same polymer) or on compatible polymers, are preferred.

The sheets (F) covering each outer face of the central layer (P) of the curved segment according to the invention are preferably identical in thickness and in composition. If this is not the case, their thicknesses are preferably chosen so that they have comparable flexural and tensile stiffness and similar coefficients of linear expansion in the transverse and longitudinal directions.

Curved segments comprising several central layers (P) based on a thermoplastic polymer (PI), these layers (P) then being identical or different in thickness and in composition, also fall within the scope of the invention. Curved segments comprising more than one sheet (F) on each of the outer faces of the central layer (P) also fall within the scope of the invention.

The definition "layer... having a honeycomb cellular structure" (sometimes simply referred to as "honeycomb" in the account that follows), is understood to mean, in the present description, a three-dimensional structure (plate/sheet) composed of a cohesive assembly of contiguous cellular cells (also referred to more simply as "cells").

The term "cells" is understood to mean open or closed cells preferably having an asymmetric cross section (the envelope of which has a shape factor (defined as being the ratio of their largest dimension to their smallest dimension) other than 1, preferably at least equal to 1.5, even 2.5 and preferably 4) that is generally substantially oval, elliptical or hexagonal, with walls arranged in any manner, but preferably that are parallel from one cell to another. According to the invention, these cells are arranged radially, i.e. their vertical axis (the one which is perpendicular to the longitudinal axis of the curved segment) is radial (positioned substantially in the extension of a radius of the curved segment). This arrangement makes it possible to store the curved segments according to the invention more easily, with a view to their subsequent assembly into pipes. The longest walls of the cells (when the cellular structure is seen from above) may form a non-zero angle with the longitudinal axis of the curved segment. In this case, this angle is advantageously less than 30° and, preferably, less than 15°. The longest walls of the cells are however preferably substantially parallel to the longitudinal axis of the curved segment (i.e. the angle formed by these walls with this axis is less than 5°, preferably zero).

The honeycomb cellular structure that makes up the central layer of the multilayer-wall curved segment according to the invention may have been obtained by any known process. Preferably, it is obtained by extrusion of lamellae based on a molten thermoplastic polymer (PI) that 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 thermoplastic polymer (PI) 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 polymer 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 document WO 2007/020279, the content of which is incorporated by reference in the present description for everything that it discloses.

In practice, honeycombs with a low density and a longitudinal flexibility

(see cited document WO 2009/115521) are preferred. These honeycombs may advantageously be obtained by adding, to the lamella extrusion process described above, a step of stretching the honeycomb perpendicular to the extrusion direction and in line with said extrusion. Such a process is described in document WO 2007/110370, the content of which is incorporated by reference in the present description for everything that it discloses. The components (central layer(s) (P) and sheets (F)) of the curved segments according to the invention are advantageously attached (assembled) by any of the methods known for assembling together three-dimensional plastic structures. Among these methods, advantageously applied to the flat panels that are precursors of the curved segments (see below), mention may be made of welding and adhesive bonding. Supplementary details regarding these methods are provided below within the context of the description of the subject of the invention relating to the process for manufacturing a plastic multilayer-wall curved segment.

There are essentially 3 ways of manufacturing the curved segment according to the invention:

- (1): firstly the layers P and F are firmly attached and then the assembly is thermoformed;

- (2): the layer P is thermoformed and a dynamic cold press is supplied with this layer and the layer(s) F that is (are) shaped in situ and that is (are) assembled with the layer P, preferably with the aid of highly reactive adhesives, or a laser beam;

- (3): a dynamic cold press is supplied with a flexible honeycomb and the layer(s) F and they are all shaped and assembled preferably with the aid of highly reactive adhesives, or a laser beam.

The first variant is preferred as flat structures are commercially available. Therefore, in particular, the invention also relates to a process for manufacturing a plastic multilayer-wall curved segment comprising at least one central layer (P) based on a thermoplastic polymer (PI), having a honeycomb cellular structure, each of the outer faces of this central layer (P) being covered with at least one sheet (F) of solid structure made of a thermoplastic polymer (P2).

The process according to this aspect of the invention comprises the thermoforming and bending of a flat panel, the multilayer composition of which corresponds to the multilayer wall of the curved segment to be obtained. That is to say that this flat panel is composed, like the curved segment of which it is the precursor, of at least one central layer based on a thermoplastic polymer (PI), having a honeycomb cellular structure, each of the outer faces of this central layer being covered with at least one sheet of solid structure made of a thermoplastic polymer (P2).

All the definitions, mentions and limitations set out above, especially in relation to: - the expressions "plastic", "polymer", "amorphous polymer", "semicrystalline polymer", "honeycomb cellular structure", "cells";

- the polymers (PI) and (P2);

- the central layer (P) and the sheets (F);

can be applied, mutatis mutandis, to the description of the process for manufacturing the curved segment.

The components (central layer(s) (P) and sheets (F)) of the flat panel to be thermoformed are advantageously attached (assembled) by any of the methods known for assembling together three-dimensional plastic structures.

One method of assembling the flat panel which has proved advantageous consists in covering the two outer faces of the central layer (P) and/or the inner faces of the sheets (F) with a surface layer that acts as an adhesive. This adhesive layer may be deposited by coating or by coextrusion. Preferably, the adhesive layer is deposited by coextrusion, is adapted to the nature of the central layer (P) and of the inner faces of the sheets (F) and to the attachment (assembly) technology used (adhesive bonding, welding, in particular thermal welding, laser welding, UV welding, welding using electromagnetic radiation, etc.).

Furthermore, the adhesive must advantageously have a sufficient temperature resistance to withstand the temperature prevailing during the subsequent thermoforming of the flat panel.

The most widely used adhesive is generally a polymer adhesive, preferably that can be thermally activated (by heating). It may be a polyurethane, an acrylic polyester, an EVA/VAc copolymer (if the central layer is based on PVC) or a functionalized polyolefin (if the central layer is based on a polyolefm). Details on the functionalized polyolefins that can be used as a polymer adhesive can be found in the cited document WO 2009/115521.

One preferred method of assembling the flat panel consists in

manufacturing the honeycomb cellular structure that makes up the central layer (P) by coextrusion. One process that is particularly advantageous for this purpose is described in document WO 2006/106101, the content of which is incorporated by reference in the present description for everything that it discloses. According to one transposition of the process described in this document, the coextrusion according to the present invention aims to provide an adhesive layer on at least one of the faces of the central layer (P).

Preferably, the honeycomb cellular structure that makes up the central layer (P) is coextruded so that its cells comprise three sublayers: one, relatively thick, sublayer that forms the body of the honeycomb and two, relatively thinner, adhesive-based surface sublayers (the terms "thick" and "thin" being relative, that is to say by comparison from one layer to another), each of the surface sublayers being intended to firmly attach the central layer (P) to the sheets (F).

The coextrusion of the central layer (P) and/or of the sheets (F) may prove particularly advantageous for assembling components of the flat panel by the laser welding technique (in particular described in the

TECHNOFUTUR.Industrie awareness report of September 2006 (SD-VEILM- RAPPORTS SEMESTRIELS, Andre DETILLEUX, 14/09/06)). In this case, either the apexes of the cells of the central layer (P), or the inner faces of the sheets (F) may be covered by coextrusion of a layer of a plastic (as defined above) containing an agent having very high absorption of the IR radiation of the laser, preference being given to carbon black.

According to the invention, the manufacture of the flat panel to be thermo formed (with a view to converting it into a curved segment) may optionally be taken advantage of in order to fill at least one portion of the honeycomb cellular structure that makes up the central layer (P) with foam and thus render the resulting curved segment insulating and water-impermeable. The term "foam" is understood to mean an expanded material, the density of which has been reduced to less than 20% of its initial value (without blowing agents), and even to less than 10% of this value, for identical pressure and temperature conditions. Preferably, the material expanded is a plastic. Polyurethanes (PURs) are examples of plastic that is very suitable as a constituent material of the foam.

One advantageous process for manufacturing a flat panel, for which the honeycomb cellular structure that makes up the central layer (P) is filled with foam, is described in document WO 2006/045723, the content of which is incorporated by reference in the present description for everything that it discloses. According to this process, firstly the honeycomb cellular structure (central layer (P)) and the first sheet (F) of the flat panel, the sheet located on the ground side, are attached; in a former, this assembly and also a composition comprising a blowing agent that is poured into the cells are introduced and the second sheet (F) on the sky side is deposited on top; the composition comprising the blowing agent is then expanded; finally the flat panel is removed from the former. Alternatively, the second sheet (sky side) can be affixed to the assembly formed by the 1 ^st sheet and the foam- filled honeycomb in a separate (finishing) step.

Once it has been manufactured and its constituents have been attached, the flat panel is thermoformed and bent in order to convert it into a curved segment. The thermoforming of the flat panel in order to convert it into a curved segment can be carried out in-line with the manufacture of said panel or else at the work site prior to the assembly of the pipes from the curved segments.

In variants (1) and (2) of the general process described above, the thermoforming is preferably carried out in a heating device, the heating technique, thermal profile and thermal cycle of which can be adjusted in order to bring the thermoplastic polymer (PI) to a temperature Ti close to T _gl , T _gl being the glass transition temperature of the polymer (PI) if this polymer is

amorphous, or to a temperature ΤΊ close to T _f i, T _f i being the melting point of the polymer (PI) (measured according to the ASTM D 3417 standard) if it is semicrystalline. The term "close" is preferably understood, in order to prevent crushing of the honeycomb cellular structure that makes up the central layer (P), to mean that Ti < T _gi or ΤΊ < T _fl , preferably Ti < T _gi - 20°C or ΤΊ < T _fl - 20°C. Very particularly preferably, Ti < T _gl - 30°C and ΤΊ < T _f i - 30°C, it being understood that in order to allow the thermoforming, it is preferable that Ti > T _g i - 50°C or T' i > T _fl - 50°C.

To enable this, in variant (1), the heating technique, thermal profile and thermal cycle of the heating device are advantageously adjusted so as to bring the thermoplastic polymer (P2) contained in the sheets (F) of solid structure of the flat panel to a temperature T ₂ such that T _g2 < T ₂ < T _g2 + 50°C (preferably, T _g2 < T ₂ < T _g2 + 30°C and very particularly preferably, T _g2 < T ₂ < T _g2 + 10°C), T _g2 being the glass transition temperature of the polymer (P2) if it is amorphous, or such that Tf2 < T ₂ < Tf2 + 50°C (preferably, T _G < T ₂ < T _G + 30°C and very particularly preferably, Te < T ₂ < Te + 10°C), Te being the melting point of the polymer (P2) if it is semicrystalline. If the sheets F on the sky and ground side are of different polymer nature, the sky and ground side indexed temperatures in the heating device must consequently be adjusted, it being understood that the temperature Ti must remain within the temperature ranges defined above.

The adjustable heating device for the thermoforming of the flat panel may be, for example, an electric oven, a liquid fuel oven or solid fuel oven, a pulsed- air oven, a pressurized heated mould, etc. The constituents of the flat panel can be heated by irradiation, by infrared radiation, etc.

After conveyance to and residence in the adjustable heating device, the hot flat panel is guided, for example, by a profiled cone, a calibration tunnel, etc. to a shaping means such as a mandrel, a cylinder, etc. advantageously divided into a heated first portion and a cooled second portion, on which the panel is held and cooled in order to be shaped into a curved segment, the uniformity of the thickness of which may advantageously be obtained by optional passage in a pulling machine.

According to another aspect, the invention also relates to a process for assembling pipes from the curved segments that constitute the main subject thereof. When the thermoforming of the flat panel, in order to convert it into a curved segment, was carried out in-line with its manufacture, the assembly of the pipes advantageously takes place at the site of their installation, where the curved segments have previously been brought to, advantageously stacked one on top of another so as to fill as much as possible of the available volume in the means of transport (lorry, wagon, etc.) used. As mentioned above, each curved segment constitutes a portion (circumferential fraction) of at least one portion of the final pipe to be assembled. Thus, if this curved segment represents 1/n of the circumference of the final pipe, the latter will be obtained by assembling n elements. Advantageously, n may range from 2 to 8, preferably from 2 to 4.

In order to form the pipe, the curved segments may advantageously be joined together, along their longitudinal edges, by clip-fastening or adhesive bonding. In order to facilitate the assembly, one of the sheets (F) may protrude beyond one of the longitudinal edges of the central layer (P) by a length λ (λ advantageously being equal to less than a quarter, preferably less than an eighth of the arc of a circle formed by the circumferential edge of the layer (P)), the sheet (F) leaving uncovered, on the side of the opposite longitudinal edge of the layer (P), a surface of this layer (P) equivalent to the same length λ. This configuration may be symmetrical or asymmetrical for each of the sheets (F) located on both sides of the central layer (P). Preferably, during the assembly, 2 adjacent curved panels are positioned so that the protruding edge of the panel located on the left is on the opposite side (for example sky side) to the protruding edge of the panel located on the right, which is therefore positioned on the ground side. The objective is that each protruding part be complementary and actually be affixed to the corresponding uncovered part (as can be imagined with segments such as the one represented in the appended Figure 1).

In other words, during the assembly, in order to guarantee good impact strength, preferably the protruding edge of the left panel covers the honeycomb of the right panel and vice versa for the latter so that the honeycombs of adjacent panels are contiguous and so that the protruding edge is firmly attached to the adjacent curved panel by welding, adhesive bonding or any other technique.

The seam between the circumferential edges of adjacent curved segments may be strengthened by addition of a bonded strip or a filling seal (for example, silicone seal, etc.) to prevent a reduction in the impact strength.

The assembly of the curved segments into pipes may take place by use of extruded profiles; in this case the arc of each curved part must be calculated so as to take into account the thickness of the profiles.

For the subsequent assembling together of lengths of pipes in order to form longer pipes, use may be made of profiles or sleeves that assemble 2 pipes end to end. Alternatively, the flat panels may be belled during the thermoforming thereof. The term "belled" should be understood here in its conventional sense, which means that, at one of the circumferential edges (sections) of the thermoformed panel, a curved segment or a socket is formed which has the general shape of a fragment (lobe) of a bell into which the pipe to be joined is slid. This belling makes it possible to avoid the use of profiles or sleeves. To achieve this belling, when the flat panel is sufficiently softened by the thermoforming, it is advantageously placed in another heating device which shapes the bell fragment on the panel. A groove, into which a rubber seal which ensures leaktightness may be placed, may be hollowed out in this bell fragment which will enable the subsequent interlocking together, along their

circumferential edges, of successive curved segments in order to form pipes.

If the flat panel to be belled is rectangular, during the belling the belled part will describe an arc having a length equal to that of the arc of the main curved part of the panel. However, in the belled part, the inner sheet describes an arc about the outer circumference of the pipe, i.e. about a cylindrical surface having a greater diameter. As a result, during the assembly of n curved panels in order to manufacture the pipe, two adjacent bell lobes will not be perfectly contiguous, the resulting volume void having to be sealed by suitable means such as a profile, bead of foam or bead of flexible polymer. One way of solving this problem is to produce panels for which the width, at the belling, is adjusted in order to prevent this problem. Preferably, the inner diameter of the pipe at the belling is equal to the outer diameter of the pipe outside of the belled zone. Another way is to adapt the belling process so that the belled part is slightly drawn, transversely, in order to increase the length of the arc of the belled part so that 2 adjacent belled parts are contiguous.

If it is advisable to prevent the infiltration of liquid (water, etc.) into the honeycombs of the central layer and/or between the latter and the sheets (F) at the location of the assembling together of the shells or of the assembling together of lengths of pipes, this may be achieved by the placement of a seal, by filling at least one portion of the honeycomb cellular structure with foam (see above) or else by using coupling profiles that ensure the leaktightness.

One particular embodiment of the multilayer-wall curved segment according to the invention is illustrated with reference to the drawing

accompanying the present description. This drawing consists of appended Figures la and lb, schematically representing a typical embodiment of this segment.

Figure la represents a perspective view of a portion of a curved segment according to the invention.

Figure lb represents an elevation view, along the cutting plane

A - A, of the left portion of the same curved segment as that represented in perspective in Figure la.

Figure la represents a perspective view of a portion of a curved segment according to the invention, composed of 2 sheets (Fi) and (F ₂ ) based on PVC to which glass fibres have been added that are coextruded with a central layer (P) based on PVC. The assembly was thermo formed and bent after attachment of the constituents of the precursor flat panel by laser welding, via the apexes of the hexagonal cells (a), drawn in the longitudinal direction, of the central layer (P), which are covered by coextrusion of a layer (not represented) of a plastic, identical to or different from the matrix of the central layer and containing carbon black which absorbs the laser beam.

In order to facilitate the subsequent assembly of the pipes from these segments, the sheet (F ₂ ) protrudes from the longitudinal edge of the central layer (P) by a length λ, the sheet (Fi) leaving uncovered, on the side of the opposite

longitudinal edge of the layer (P), a surface of this layer (P) equivalent to the same length λ.

Figure lb represents an elevation view, along the cutting plane A - A, of the left portion of the same curved segment as that represented in perspective in Figure la.

The multilayer- wall curved segments according to the invention may be used for the manufacture of pipes used in any application where high ring stiffness presents an advantage. These pipes make good, light and stiff, substitutes for concrete pipes. They are particularly suitable for the discharge of wastewater or rainwater.