WO2007129887A1

WO2007129887A1 - Thermoplastic construction elements and methods of coupling thereof

Info

Publication number: WO2007129887A1
Application number: PCT/NL2007/000122
Authority: WO
Inventors: Martin Theodoor De Groot
Original assignee: Fits Holding Bv
Priority date: 2006-05-08
Filing date: 2007-05-07
Publication date: 2007-11-15
Also published as: NL1031771C2

Abstract

The invention firstly relates to a method for coupling thermoplastic construction elements, such as sandwich panels, in which a thermoplastic element (10) comprises a thermoplastic core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16), which method comprises the steps of : 1) providing a first thermoplastic construction element (10a) with a tongue connection piece (100a); 2) providing a second thermoplastic construction element (10b) with a groove connection piece (156); and 3) positioning the tongue connection piece (100a) of the first element (10a) into the groove connection piece (156) of the second element (10b).

Description

THERMOPLASTIC CONSTRUCTION ELEMENTS AND METHODS OF COUPLING THEREOF

The present invention firstly relates to a method for coupling thermoplastic construction elements such as sandwich panels, in which a thermoplastic construction element comprises a thermoplastic foam core layer between at least two fiber-reinforced thermoplastic cover layers .

In the field, usually metal profiled sections are used in order to connect sandwich structures such as Nomex honeycomb with thermosetting or thermoplastic cover layers. When used in the transport sector, in particular in aviation and space travel, these metal profiled sections represent an appreciable portion of the total weight, which is undesirable.

Furthermore, sandwich panel constructions are generally made from flat panels, with the entire panel absorbing the load.

WO 01/19603 discloses a method for producing edge seals for sandwich panels having a honeycomb core and cover layers on both sides thereof, in which a warm pressing tool is pressed against the relevant panel edge, parallel to the main surfaces of the panel, until the adjoining core and cover layer are melted together while retaining their shape. An edge which is finished in this way can be produced as a tongue and groove joint, depending on the shape of the pressing tool used. A drawback of this known method is the fact that the tongue and groove joint which is formed as a result only allows a small transmission of force from tongue connection piece to groove connection piece and vice versa. DE-A1-196 12 127 discloses a lightweight sandwich panel made of thermoplastic material, in which the panel is provided with areas which have been compacted by means of deformation under the effect of heat, such as the edge, which serve as coupling parts for at least thermoplastically connecting such panels. It is an object of the invention to provide a coupling method and system in which the transmission of force between the construction elements is improved. Furthermore, it is an object of the invention to provide a method for manufacturing construction elements, which are suitable for use with this coupling method according to the invention.

It is yet another object of the invention to provide an assembly of such construction elements, which can be coupled to one another in a simple manner.

It is furthermore an object of the invention to provide a new and/or alternative construction method for thermoplastic construction elements having thermoplastic sandwich structures. To this end, the method for coupling thermoplastic construction elements comprises the steps of:

1) providing a first thermoplastic construction element with a tongue connection piece;

2) providing a second thermoplastic construction element with a groove connection piece, in which the wall of the groove is at least partially provided with a fiber-reinforced thermoplastic cover layer; and

3) positioning the tongue connection piece of the first sandwich panel into the groove connection piece of the second sandwich panel in such a manner that the fiber-reinforced thermoplastic cover layers of the tongue connection piece overlap the fiber-reinforced thermoplastic cover layer of the groove wall.

With the coupling method according to the invention, a tongue and groove joint is used. The first thermoplastic construction element comprises, at least on a peripheral edge thereof, a tongue connection piece, which fits into a groove connection piece with a complementary shape of the second thermoplastic construction element. The tongue connection piece may be thinner than the main body of the relevant construction element. If the construction element is a shelf, one end or side edge thereof may serve as tongue connection piece and be accommodated in the groove connection piece of the other construction element. This groove connection piece comprises a groove, which is at least partially defined by a wall made of a fiber-reinforced thermoplastic material. Preferably, the entire groove wall, i.e. the upright walls and the bottom thereof, are made from such material. Since the fiber-reinforced thermoplastic cover layers of the tongue connection piece and of the groove connection piece overlap one another, the transmission of force of the construction is better than is the case with the abovementioned constructions. In order to facilitate the positioning in step 3, the tongue connection piece and the groove connection piece preferably have a conical shape, viewed in cross section. If desired, the thermoplastic construction elements can be fixedly connected to one another by means of an adhesive. It is possible to use a thermoplastic glue or a thermosetting glue, for example, as adhesive. Connecting the tongue connection pieces and the groove connection pieces by means of heating the thermoplastic present in the relevant cover layers will generally not be used due to the problems in connection with the transfer of heat from a stamp which is used for this purpose. Resistance welding and induction welding are examples of other possible connecting techniques. The construction elements may have many shapes. A panel which is used as construction element will generally be flat, but it is also possible to use (slightly) curved or bent panels in the context of the invention. Other examples of construction elements are shelves, reinforcing ribs, posts, jambs or fstampes for receiving panels, doors and/or windows. The reinforcing ribs, posts, jambs and fstampes are generally elongate segments of a relatively great length in relation to their width and height, with one or more of the longitudinal planes being provided with a tongue connection piece and/or groove connection piece. In addition, it is also possible to provide the head ends of such segments with a tongue connection piece, preferably a narrow section, or a groove connection piece. Tenon-mortise joints can also be used for these head ends. The shape of the segments is not limited, but a polygonal, in particular a rectangular, cross section is preferred. In the context of this description, the term "composite panel" denotes an object having a thickness which is small compared to its length and width, and which is composed of a thermoplastic core layer and a fiber-reinforced thermoplastic cover layer.

Furthermore, the expression "sandwich panel" in this description is used to denote a particular embodiment of a composite panel, namely an object having a thickness which is small compared to the length and width of the object, composed of a thermoplastic foam core layer which is arranged between two cover layers. The second cover layer does not necessarily have to comprise a fiber- reinforced thermoplastic, but can also comprise an optionally fiber- reinforced cover layer made from another material, such as a metal plate or a fiber-reinforced thermosetting material.

The term "thermoplastic sandwich panel" is understood to mean a sandwich panel having a thermoplastic foam core layer, in which both cover layers of the sandwich panel comprise a fiber-reinforced thermoplastic . An "in-situ manufactured sandwich panel" denotes such a thermoplastic sandwich panel, which is produced by means of the in- situ foaming technique, in other words an in-situ produced thermoplastic foam layer arranged between two fiber-reinforced thermoplastic cover layers. If the foam is an anisotropic foam with elongate cells, as is the case with an in-situ manufactured thermoplastic sandwich element, the length direction of the foam cells is at right angles to the main surfaces of the cover layers. With this advantageous embodiment, the orientation of the cells supports the compressive strength. In order to connect the foam core layer to the cover layers, a thermosetting or thermoplastic glue may be used. If possible, the thermoplastic which is present in the foam core, optional reinforcing layers and/or cover layers, itself acts as adhesive . A method for manufacturing a thermoplastic sandwich panel with a thermoplastic foam core by means of in-situ foaming is known, for example, from EP-A1-0636463. Such a known sandwich panel consists of a core layer, which is covered with two cover layers. A cover layer comprises one or more (fiber-reinforced) thermoplastic materials. The in-situ foaming process consists of several steps. The first step is an assembly step, during which a core web, which comprises at least one film made of a thermoplastic material, which material contains a certain amount of a suitable physical blowing agent, is positioned between, for example, two (fiber-reinforced) cover layers, which are usually made of the same thermoplastic material as the core web. Subsequently, the assembly of core web and cover layers is placed between two press platens in a press. In this position, a foaming step is performed, in which heat and pressure are added to the assembly first via the press, so that a connection between the core web and the cover layer or cover layers is produced. Then, during a foaming step, when the temperature has reached a sufficiently high level, the press is slowly opened, as a result of which the distance between the two press platens increases. This allows the blowing agent (swelling agent, solvent, chemical blowing agent and/or physically inert gas) to expand, as a result of which the material of the core web starts to foam. This expansion is usually carried out under controlled conditions. In this way, the core web is foamed, and the connection between the core web and the cover layer or layers is produced in one manufacturing step without the use of a separate or additional adhesive being required. Once a predetermined thickness of the foamed core web has been achieved, the assembly is allowed to cool off during a cooling step. The sandwich product obtained in this way comprises the foamed core web which is covered by two cover layers and connected thereto. In addition, a drying step is usually carried out. The in-situ foaming method can also be used with a composite panel.

Examples of swelling agents for a core layer comprising thermoplastic foam include acetone, methyl ethyl ketone, methyl acetate, methyl propionate, nitro ethane, cyclohexane, ether, ethanol, methanol and pentane, as well as mixtures, such as ethanol/acetone and methanol/methyl acetate. Acetone is a preferred swelling agent. Examples of suitable thermoplastics for both the core layer and the matrix for the fiber-reinforced cover layer (s) comprise polyetherimide (PEI), polyethersulfone (PES), polysulfone, polyphenysulfone (PPSU) , polyketone, such as polyetheretherketone (PEEK) , liquid crystal polymers, polycarbonate (PC) , propylene (PP) , etc., as well as combinations thereof. Polyetherimide is a preferred thermoplastic. The latter is available in different grades from General Electric under the brand name Ultem. Glass fibers are preferred as reinforcement. Other inorganic fibers, such as metal fibers, carbon fibers and organic fibers, such as astampide fibers can be used in a similar manner, provided they can be deformed in order to form the tongue connection pieces and groove connection pieces. In addition to the abovementioned synthetic fibers, it is also possible to use natural fibers, provided they can withstand the for the thermoplastic sandwich panels to be produced at relatively high temperatures by deformation and having a tongue connection piece and/or groove connection piece. The fibers can be used in the form of mats, fabrics and the like. Directional fibers, in particular unidirectional fibers where the fiber direction is matched to the intended use, can also be used advantageously. The foam core layer may optionally be reinforced with fibers of the abovementioned types or with nanoparticles . Preferably, the thermoplastic material of the foam core layer is the same as the thermoplastic material in the cover layers. However, it is also possible to use combinations of different thermoplastics. Suitable examples thereof include, inter alia, PEI foam covered with cover layers made of PPSU, PS, PEEK or PC, PES or PPSU foam covered with

PPSU or PC cover layers, and PP foam covered with polyamide, such as nylon cover layers .

The abovementioned examples are similarly true for the additional reinforcing layers. These may be used as consolidated layer or as prepreg, which may consist of several layers (laminates) which are stacked on top of one another.

If desired, the core may also consist of several foam layers, which are separated by an intermediate layer, such as an optionally fiber-reinforced (thermoplastic) layer, cestampic layer or metal layer.

Other methods for manufacturing an assembly comprising a thermoplastic foam core layer and at least one fiber-reinforced thermoplastic cover layer include, inter alia, extruding a thermoplastic foam onto a fiber-reinforced thermoplastic cover layer, preferably between two such cover layers, and gluing a thermoplastic foam to a fiber-reinforced thermoplastic cover layer. The adhesive used should be able to withstand the temperatures required for deformation to form a tongue connection piece or groove connection piece to a sufficient degree. The coupling method according to the invention is suitable in particular for the modular construction of airplane interiors and the like, but can also be used for dividing industrial and office buildings, constructing lightweight furniture, such as cupboards and the like. The fact is that thermoplastic construction elements, in particular in-situ manufactured sandwich panels on the basis of, for example, PEI and PPSU, have excellent fire resistivity, very good mechanical properties, insulating properties and are thermoplastically deformable.

Furthermore, the present invention offers the possibility of producing designs using relatively thick construction elements, such as beams which are connected to one another and between which thin construction elements, such as sandwich panels and/or other plates, are positioned in the groove connection pieces of the relatively thick construction elements.

The invention also provides production techniques for manufacturing construction elements with either a tongue connection piece or a groove connection piece, or a combination thereof, along various peripheral edges. One preferred method for providing a thermoplastic construction element with a tongue connection piece, in which the thermoplastic construction element comprises a thermoplastic foam core layer between two fiber-reinforced thermoplastic cover layers, comprises the steps of

A) narrowing a peripheral edge of a thermoplastic construction element, in which both cover layers are deformed by applying pressure and heat near the peripheral edge in a direction at right angles to the respective cover layer while simultaneously locally compacting the intermediate foam core layer.

In this method according to invention, a peripheral edge is deformed over a predetermined width under the effect of heat and pressure using one or more hot deformation stamps. The deformation stamps are in this case moved substantially at right angles to the cover layers. This offers a greater freedom of choice with respect to the final shape and position of the tongue connection piece compared to the method known from WO 01/19603. If desired, this deformation step may be divided into several substeps. As a result thereof, the distance between the cover layers is locally, i.e. at the treated peripheral edge, smaller compared to the distance between them in the main body. While carrying out the method, the core layer present between the cover layers is locally slightly compacted. If desired, thermoplastic foam can be removed before or during the various steps of producing the narrowing. Preferably, this peripheral edge is finished or will be finished later, i.e. the head end of the tongue connection piece where the foam core is normally exposed, is covered with a fiber-reinforced thermoplastic layer, preferably by folding one or both cover layers, so that these overlap one another or abut one another. If desired, an additional fiber-reinforced thermoplastic layer may be used in this case. A preferred method for providing a thermoplastic construction element with a groove connection piece, in which the thermoplastic construction element comprises a thermoplastic foam core layer between two fiber-reinforced thermoplastic cover layers, comprises the steps of B) locally compacting the foam core layer between the cover layers on a face of a thermoplastic construction element in such a manner that a groove is formed in the respective face of the construction element, in which the wall of the groove is at least partially provided with a fiber-reinforced thermoplastic cover layer.

By applying heat and pressure, a molding stamp is introduced between the cover layers up to the desired depth in a face of the construction element, preferably in a head end thereof where the foam core layer is not covered by a cover layer, in order to produce the groove in the construction element. The foam core material is compacted locally, so that a recess or groove is formed, which is limited by longitudinal edges of compacted foam. The walls of the groove and/or bottom thereof are reinforced with an (additional) fiber-reinforced thermoplastic layer. In a first variant thereof, an additional fiber-reinforced thermoplastic reinforcing layer is provided over the end edge to this end, prior to step B) . During the deformation step, this is pressed into the head end with the aid of heat and pressure and thus forms the walls and/or bottom of the groove. Usually, in this case, part of the foam core layer between the additional reinforcing layer and the cover layer and/or cover layers will be pressed to form a (substantially) solid material. The reinforcement may also be obtained from the cover layers themselves by finishing the peripheral edge with the latter, prior to step B) . Advantageously, the end edge is securely clamped during step B) , so that the cover layers on the main surfaces of the sandwich panel cannot deform. Depending, inter alia, on the thickness of an element, the dimensions of the groove and the thickness of the reinforcing layer, it may be necessary to first locally compact the core layer using a heated stamp and then to press a reinforcing layer into the first formed groove using a heated molding stamp and fix the final shape of the groove by means of a consolidation stamp. If desired, it is also possible to secure an entirely preformed edge element, preferably made from a fiber-reinforced thermoplastic material, with groove connection piece in the groove formed between the cover layers. In order to secure a predeformed reinforcing layer or edge element in a groove which has already been formed, it is also possible to use a glue.

The groove connection piece may also be provided in a face of the construction element, which face is covered by a cover layer. To this end, the fiber reinforcement in the cover layer is preferably first interrupted, for example cut or milled away, after which the edges along the interruption are folded, preferably with the addition of an additional fiber-reinforced cover layer, in order to form the groove or recess with a fiber-reinforced wall.

When the thickness of the construction element is relatively small, as is the case with a thermoplastic sandwich panel, a molding stamp is preferably used in order to provide the groove in the end edge, which molding stamp is in heat-exchanging contact with a relatively large heat reservoir in order to prevent the molding stamp from cooling down too much during the deformation step. An additional heating step may - if desired - be carried out since the dimensions of the desired shape and thus of the molding stamp are such that there is insufficient through-flow of heat. In order to be able to supply the required heat in a short time, the fiber- reinforced thermoplastic layer to be inserted and, if desired, a similar layer produced by finishing the edges of one or more of the cover layers, are heated on the head ends of the thermoplastic construction element in order to be able to carry out the required desired deformations within a short period of time (in the order of seconds) , without the temperature of the stamp or the temperature during preheating becoming so high that the foam between the groove and the cover layers becomes too compacted, which is not desirable, and still sufficient heat is supplied in such a manner that the reinforcing layer can be deformed. A consolidation stamp may be used in order to fix the shape and to reduce residual stresses, preferably at a temperature which is lower than the Tg of the thermoplastic of the cover layers, but preferably higher than half the respective Tg (⁰C). If desired, it is possible allow the thickness of the panel to increase locally when providing the groove in the end edge of a thermoplastic construction element, such as a sandwich panel, by selecting a suitable molding stamp (s), so that a groove having a width corresponding to the thickness of the original sandwich panel is obtained. This allows, inter alia, another construction element, such as a shelf or sandwich panel having this thickness, i.e. without a narrowed tongue connection piece, to be introduced into this groove.

The invention also relates to a thermoplastic construction element having a tongue connection piece or a groove connection piece, respectively, and to an assembly thereof. The invention is described below with reference to the attached drawing, in which: Fig. 1 shows a first embodiment of a method for producing a sandwich panel with a groove connection piece according to the invention;

Fig. 2 shows a narrowing which fits into the groove from figure 1;

Fig. 3 shows an example of a method for manufacturing a sandwich panel with a tongue connection piece; Fig. 4 shows an embodiment of a method for producing a sandwich panel with groove connection piece;

Fig. 5 shows a method for connecting two sandwich panels; and Fig. 6 shows another embodiment of a construction element according to the invention. Fig. 1 illustrates a construction element 10 in the shape of a thermoplastic sandwich panel, which shows a thermoplastic foam core layer 12 between two fiber-reinforced thermoplastic cover layers 14 and 16. The end edge 150 is clamped between metal or plastic clamping blocks 152. A fiber-reinforced thermoplastic reinforcing layer 70 is laid on the end edge 150 in an overlapping manner. Using a heated stamp 154, this reinforcing layer 70 can be preheated. An (elongate) molding stamp 18 with a form shoe at a temperature of the melting point ± 20% of the thermoplastic which defines the shape of the groove to be produced, is positioned on the reinforcing layer 70 at the location of the foam core layer 12. By applying pressure and heat, the molding stamp 18 then is then lowered into the foam core layer 12, which is thus locally compacted. Following consolidation by means of a consolidation stamp (not shown) , a groove 156 is made in an end edge of the thermoplastic sandwich panel 10, the walls 158 and bottom 160 being formed by the reinforcing layer which has been deformed at the same time. Figure 2 shows a sandwich panel 10 with a narrow section 100a as a tongue connection piece which fits into the groove 156 of the sandwich panel from figure 1.

Figure 3 shows a possible method for manufacturing tongue connection piece 100a in a construction element. In this case a hot deformation stamp 18 is lowered from above into the element 10 in a direction at right angles to the main surface of a fiber-reinforced thermoplastic cover layer and in this case forms approximately half 104 of the required deformation for the tongue connection piece 100a. By means of a consolidation stamp (not shown), the desired shape is consolidated. The construction element is then turned over in order to produce the complementary part of the tongue connection piece 100a in an identical manner. Alternatively, two molding stamps may be used to carry out the above process simultaneously, it being desired to likewise simultaneously consolidate the shape of the tongue 100a by means of one or more consolidation stamps. Fig. 4 shows an alternative method for producing a construction element 10 with a groove connection piece 156, in which the peripheral edge 100 of the construction element 10 to be worked is firstly finished. In the case illustrated, the ends of both cover layers 14, 16 are folded towards one another, and overlap one another partially. The edges may also abut one another or be adjacent to one another at a distance. Subsequently, a molding stamp 18 is pressed into the foam core layer 12, as is described above, with the cover layer ends deforming at the same time and forming the walls 158 and in this case the bottom 160 of the groove 156 as well. It will be clear that with the usual thicknesses of panels for aviation and space travel industry, the folded sections of the cover layers do not suffice to cover an entire groove 156, so that an additional reinforcing layer 70 is usually applied in this case as well, such as that used with the method shown in figure 1.

Fig. 5 shows the method for coupling two sandwich panels 10a, b, in which a narrowed peripheral edge 100a of a first panel 10a is positioned into a groove 156 of a second panel 10b.

Fig. 6 shows a construction element in the shape of a connecting beam 10. This beam 10 is made from a segment of a thermoplastic foam 12 which is provided on both sides with a cover layer 14, 16 of a fiber-reinforced thermoplastic. After the respective cover layer is cut, both cover layers 14 and 16 and the foam 12 beneath it are provided with recesses 156 with the aid of a molding stamp and with the local interposition of an additional fiber-reinforced cover layer 70, so that the entire wall and bottom are composed of a fiber-reinforced thermoplastic material.

Subsequently, the edges of the cover layers 14, 16 are folded over at the head ends, so that the head ends are also covered with a fiber-reinforced thermoplastic layer.

Claims

C L A I M S

1. A method for coupling thermoplastic construction elements, in which a thermoplastic construction element (10) comprises a thermoplastic foam core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16), which method comprises the steps of:

1) providing a first thermoplastic construction element (10a) with a tongue connection piece (100a) ;

2) providing a second thermoplastic construction element (10b) with a groove connection piece (156) , in which the wall of the groove is at least partially provided with a fiber-reinforced thermoplastic cover layer; and

3) positioning the tongue connection piece (100a) of the first element (10a) into the groove connection piece (156) of the second element (10b) in such a manner that the fiber-reinforced thermoplastic cover layers of the tongue connection piece overlap the fiber-reinforced thermoplastic cover layer of the groove wall.

2. The method as claimed in claim 1, in which the tongue connection piece (100a) and the groove connection piece (156) are connected to one another.

3. The method as claimed in claim 1 or 2, in which the tongue connection piece and the groove connection piece (100a, 156) have a conical shape, viewed in cross section.

4. The method as claimed in one of the preceding claims, in which the bottom of the groove of the groove connection piece is formed by a fiber-reinforced thermoplastic cover layer.

5. A method for providing a thermoplastic construction element (10a) with a tongue connection piece (100a), in which the thermoplastic construction element (10) comprises a thermoplastic foam core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16), in particular as in step 1) of claim 1, which method comprises the steps of:

A) narrowing a peripheral edge (100) of a thermoplastic construction element (10), in which both cover layers (14, 16) are deformed by applying pressure and heat near the peripheral edge

(100) in a direction substantially at right angles to the respective cover layer (14, 16) while simultaneously locally compacting the intermediate foam core layer (12) .

6. The method as claimed in one of the preceding claims 4-5, in which the peripheral edge (100) is finished.

7 The method as claimed in claim 5 or 6, in which the tongue connection piece (100a) has a conical shape, viewed in cross section.

8. A method for providing a thermoplastic construction element (10b) with a groove connection piece (156) , in which the thermoplastic construction element (10) comprises a thermoplastic foam core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16), in particular as in step 2) of claim 1, which method comprises the steps of

B) locally compacting the core layer (12) between the cover layers (14, 16) on a face (150) of a thermoplastic construction element (10) in such a manner that a groove is formed in the respective face (150) of the construction element, in which the wall of the groove is at least partially provided with a fiber-reinforced thermoplastic cover layer.

9. The method as claimed in claim 8, in which an additional fiber-reinforced thermoplastic reinforcing layer (70) is provided on the respective face (150), prior to step B) .

10. The method as claimed in claim 8, in which the wall of the groove comprising a fiber-reinforced thermoplastic cover layer is formed from one or the two cover layers (14, 16) of the construction element during step B) .

11. The method as claimed in one of the claims 8-10, in which the peripheral edge (100) is finished, prior to step B).

12. The method as claimed in one of the preceding claims 8-11, in which the construction element is a sandwich panel and said face an end edge.

13. The method as claimed in one of the preceding claims 8-12, in which the end edge (150) is securely clamped during step B) .

14. The method as claimed in claim 9, in which the additional fiber-reinforced thermoplastic reinforcing layer (70) is preheated.

15. A thermoplastic construction element, comprising a thermoplastic foam core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16), in which at least one face of the construction element is provided with a groove connection piece (156) , in which the wall of the groove is at least partially provided with a fiber-reinforced thermoplastic cover layer.

16. A thermoplastic construction element, comprising a thermoplastic foam core layer (12) between two fiber-reinforced thermoplastic cover layers (14, 16) , in which at least one peripheral edge of the sandwich panel is provided with a tongue connection piece (100a) .

17. The thermoplastic construction element as claimed in claim 16, in which the tongue connection piece (100a) has a fiber-reinforced edge.

18. An assembly of a thermoplastic construction element as claimed in claim 15 and a thermoplastic construction element as claimed in claim 16 or 17.