Pipe comprising flame-retardant layer

Abstract

The invention relates to a pipe (1) comprising a flame-retardant layer (2) and an outer layer (3), the outer layer (3) surrounding the flame-retardant layer (2), the flame-retardant layer (2) comprising a thermoplastic and expandable graphite particles, the expandable graphite particles being designed to expand above a starting temperature, the expandable graphite particles being in a non-expanded state, and the outer layer (3) comprising a thermoplastic, characterized in that a roughness average Ra of an outer face of the pipe (1) or of an outer face of the outer layer (3) is at most 200 µm or 100 µm or 50 µm.

Description

[0001] Pipe with flame retardant layer

[0002] Description:

[0003] The invention relates to a pipe comprising a flame-retardant layer and an outer layer, wherein the outer layer encloses the flame-retardant layer, wherein the flame-retardant layer comprises a thermoplastic and expandable graphite particles, wherein the outer layer comprises a thermoplastic. The invention also relates to a fluid line comprising such a pipe and a use of this pipe or this fluid line.

[0004] Such a pipe is known from EP 3 271 170 B1, wherein - depending on the embodiment - the flame-retardant layer or a reinforcement layer represents the outermost layer. The reinforcement layer comprises fibers, which may be made of aramid or glass fibers, for example. The fibers are bonded together to form a woven fabric and thus a non-homogeneous layer, which protects the underlying flame-retardant layer. The flame-retardant layer ensures that the pipe can withstand fire or heat for a few seconds or minutes longer. The flame-retardant layer is approximately 1.5 mm thick and comprises expandable graphite whose particles have an average particle size of 0.2 to 0.4 mm. The pipe is attached to a pipe connector by means of a friction fit.

[0005] A disadvantage of the known pipe, however, is its ability to provide a particularly long-lasting fluidic connection. It has been found that the mechanical connection - especially in a demanding environment such as in a land vehicle - between the known pipe and an associated pipe connector leaves much to be desired. The vibrations of a land vehicle in particular can loosen the frictional connection between the pipe and pipe connector over the years. The increasing brittleness of the pipe material also contributes to this, so that this factor also significantly weakens the frictional connection over the years. In addition to normal material aging, the brittleness is also a result of high temperatures or temperature fluctuations that act on the land vehicle or on the pipe within it.Finally, the land vehicle is exposed to moisture and therefore road salt and the like, which can also have negative consequences for the frictional connection between the pipe and the pipe connector.

[0006] The invention is therefore based on the object of creating a pipe which - especially under the adverse conditions in the case of land vehicles - enables a particularly durable and robust connection with a pipe connector and which withstands the heat for as long as possible in the event of a fire.

[0007] This object is achieved by a pipe comprising a flame-retardant layer and an outer layer, wherein the outer layer encloses the flame-retardant layer, wherein the flame-retardant layer comprises a thermoplastic and expandable graphite particles, wherein the expandable graphite particles are designed to expand above a starting temperature, wherein the expandable graphite particles are in a non-expanded state, wherein the outer layer comprises a thermoplastic, wherein a mean roughness Ra of an outer side of the pipe or of an outer side of the outer layer is at most 200, 100 or 50 pm.

[0008] The invention is based on the finding that welding the pipe to a pipe connector enables a significantly better and more durable connection between these two components. It was found that neither a flame-retardant layer with expanded graphite nor, even less, a fabric layer, and thus a non-homogeneous layer, are suitable for welding to the pipe connector.

[0009] The invention is further based on the finding that the known flame-retardant coating with expanded graphite exhibits excessive surface roughness or an excessively high mean roughness. This is because the particles of the expanded graphite create considerable roughness, which significantly increases the reject rate, especially in the preferred, delicate laser-welded joints between pipe and pipe connector. It is therefore a great advantage for the laser-welded joint that the joined surfaces form a virtually ideal form fit, so that the circumferential weld seam is truly continuous in as many pipe-to-pipe connector joints as possible.

[0010] It was further discovered that for a laser-welded joint, either the material of the line connector or that of the tube should be largely transparent to the laser radiation, while the material of the respective internal component largely absorbs the laser radiation. At the same time, automobile manufacturers generally require black-colored line connectors and tubes. Special dyes meet the requirement of transparency for (near-infrared) laser beams on the one hand and a black appearance in the visible light spectrum on the other. However, these special dyes are relatively expensive, which is why they are primarily used for line connectors. Thus, the amount of material required for line connectors is usually less than that for the tubes.

[0011] The invention is therefore based on the finding that the pipe is particularly preferably inserted into the pipe connector, so that the outer surface of the pipe is preferably welded to the inner surface of the pipe connector. It has been found that low mean roughness values ​​of the outer side of the pipe enable particularly good connections, in particular welded connections and very particularly laser-welded connections. This is preferably achieved in that the outer layer has at most small particles and / or the thermoplastic of the outer layer is preferably homogeneous. Non-homogeneous plastics are, for example, fabric layers or foams. The layer arrangement of the pipe according to the invention thus enables a heat-resistant pipe which enables particularly long-lasting connections with pipe connectors. As a result, the object stated above is achieved.

[0012] The term "average roughness" preferably refers to the arithmetic average roughness according to DIN EN ISO 4287:2010 and DIN EN ISO 4288:1998. To measure a measuring section, the measuring device is guided along a circumferential direction of the pipe, possibly performing several axially offset revolutions or using a spiral path.

[0013] According to a very preferred embodiment, the outer layer in a cross-section of the pipe (1) has particles or cavities or filled cavities with an average extension in the radial direction of at most 200 or 150 or 100 or 50 pm, or no particles or cavities or filled cavities at all. This contributes to a low mean roughness. For the determination, for example, a pipe section with a length of 10 mm may be examined, in particular by pCT. If fewer than 100 or 50 or 20 or 10 particles / cavities / filled cavities with an extension in the radial direction of at least 5 or 2 or 1 pm are found, the criterion “no particles at all” is deemed to be met. According to a very preferred embodiment, the thermoplastic material of the outer layer is homogeneous across the entire cross-section of the outer layer. This improves the weldability of the outer layer.The term "homogeneous" preferably refers to a structure with only one intentionally created inner and one intentionally created outer interface in a cross-section. Isolated, even very small voids (in the sub-millimeter range), for example, are not intentional. Foams, on the other hand, intentionally have bubbles and thus internal interfaces. Additive particles are also intentionally added, so that even in the case of additive particles, numerous intermediate interfaces can be found between the inner and outer interfaces of the layer. Likewise, the thermoplastic of the outer layer is not homogeneous across the entire cross-section of the outer layer if it has the shape of a fabric or a fabric layer.

[0014] The same applies if the outer layer in the form of a fabric is additionally impregnated with another plastic. A fabric layer impregnated in this way may not have any particles or cavities or filled cavities, provided that the cast material and the fabric material are considered to be a coherent plastic structure, against which the particles or cavities or filled cavities are defined. The particles or the filled cavities may consist of a material other than plastic. The particles may, in particular, be mineral in nature, while the filled cavities may, in particular, contain a gas - especially air. In the complete absence of particles or cavities or filled cavities, however, a porous plastic layer filled with another plastic may still be present.This includes, in particular, a fabric layer impregnated with a thermoplastic. However, such layers do not comprise a thermoplastic that is homogeneous across the entire cross-section of the outer layer. A thermoplastic is preferably homogeneous when at most 100, 50, or 20 structures are found within a 10 mm long tube section, which preferably have a size of at least 5, 2, or 1 pm, respectively—preferably according to pCT images.

[0015] It is particularly preferred for the pipe to be formed in one piece. The outer layer is expediently irreversibly bonded to the flame-retardant layer, either directly or indirectly via the interposition of one or more layers. The individual layers of the pipe are thus firmly bonded to one another and ensure corresponding mechanical stability. To test the irreversible bond between the layers of the pipe, it is expedient to examine a middle section of the pipe while separating any line connectors. If, for example, an outer protective pipe can be pulled off the pipe, the layer or layers of the protective pipe do not belong to the pipe or inner pipe contained therein. It may be the case, for example, that a pipe or inner pipe is integrally bonded to a line connector, whereas an outer protective pipe is not integrally bonded to the pipe but is nevertheless connected to the line connector.In these cases, the outer protective tube and the pipe or inner tube would also be indirectly connected to each other via the pipe connector(s). In this case, a pipe is only considered to be a single-piece structure if, with an arbitrarily selected middle section of the pipe and the pipe connectors removed, the layers can only be separated irreversibly and thus destructively. For example, if a pipe or inner tube with an outer protective tube is cut off from the pipe connectors, the pipe is not considered to be a single-piece structure if the protective tube can be removed from the pipe or inner tube by hand and, preferably, also pushed back on.

[0016] According to a preferred embodiment, the outer layer is the outermost layer of the—in particular, one-piece—pipe. It is possible for a one-piece pipe or inner pipe to have an outer layer and simultaneously be enclosed by a protective pipe. Then, the outer layer of the pipe or inner pipe is also the outermost layer of the one-piece pipe or inner pipe, because the protective pipe can be pulled off the pipe or inner pipe if the pipe connectors are separated.

[0017] Particularly advantageously, the flame-retardant layer has a maximum thickness of 1.5, 1.0, 0.7, or 0.5 mm. This prevents the pipe from becoming too thick and thus too inflexible when bent. It is advantageous for the flame-retardant layer to have a thickness of at least 0.05, 0.07, or 0.10 mm. This prevents layer tears.

[0018] It is very preferred that the outer layer has a layer thickness of at least 0.06, 0.08 or 0.10 mm. Otherwise, the flame-retardant layer may tear off during extrusion due to the expandable graphite particles, and the flame-retardant layer is therefore not formed over the entire surface. The layer thickness of the outer layer is advantageously at most 1.0, 0.8 or 0.6 mm. This prevents the pipe from becoming too thick and thus too inflexible when bent. It is very preferred that the layer thickness of the outer layer is designed such that it compensates for unevenness in the underlying flame-retardant layer. The combined layer thickness of the outer layer and the flame-retardant layer is preferably at least 0.15, 0.2, 0.25 or 0.3 mm. It is advisable for the added thickness of the flame-retardant layer and the outer layer to be no more than 2.0 or 1.5 mm.1 .0 mm.

[0019] Advantageously, the weight proportion of the expanded graphite particles in the flame-retardant layer is at least 1, 3, 5, 7, or 9 wt.%. The weight proportion of the expanded graphite particles in the flame-retardant layer is advantageously at most 50, 40, or 30 wt.%. It has been found that an excessively high weight proportion of the expanded graphite particles causes excessive unevenness or excessive brittleness.

[0020] According to a preferred embodiment, the expandable graphite particles in the flame-retardant layer have an average height in the radial direction of at most 200, 150, 100, or 70 pm in a cross-section of the pipe. This ensures that the mean roughness on the outside of the pipe or the outside of the outer layer does not become too large. Advantageously, the expandable graphite particles in the flame-retardant layer have an average height in the radial direction of at least 10, 20, or 30 pm in a cross-section of the pipe. This ensures, among other things, that the expansion rate of the expandable graphite particles does not become too small.

[0021] It is advantageous for the expanded graphite particles of the flame-retardant layer to have a starting temperature of at least 130, 150, 170, 190, 210, or 230 °C. The starting temperature of the expanded graphite particles is advantageously matched, in particular, to the melting temperature of the thermoplastic material of the flame-retardant layer. Preferably, the starting temperature of the expanded graphite particles is at least 10, 20, or 30 °C higher than the melting temperature of the thermoplastic material of the flame-retardant layer.

[0022] Preferably, an expansion rate of the expandable graphite particles of the flame retardant layer is at least 10 or 20 or 30 cm 3 / g. The expansion rate of the expandable graphite particles of the flame retardant layer is advantageously at most 350, 250, 200, or 150 cm 3 / g. It was found that a very high expansion rate is required for such expandable graphite particles whose size is too large for the layer thickness of the pipe. The invention is based on the finding that, in particular, expansion rates of around 100 cm 3 / g are sufficient for intumescence on the one hand and allow sufficiently small expandable graphite particles on the other.

[0023] It is preferred that the pipe comprises an auxiliary flame retardant, wherein the auxiliary flame retardant preferably has a gas-diluting or oxygen-diluting effect, wherein the auxiliary flame retardant is preferably an inorganic flame retardant. This makes it possible for the flame-retardant layer to be additionally or even synergistically supported by the auxiliary flame retardant. It is particularly preferred that the auxiliary flame retardant contains ammonia and comprises, for example, ammonium phosphate, ammonium sulfate, and / or ammonium polyphosphate. It is possible for the auxiliary flame retardant to be located in the flame-retardant layer and / or in the outer layer. Particularly preferably, the auxiliary flame retardant is contained only in the flame-retardant layer.

[0024] The pipe expediently comprises at least one further, preferably an inner, layer. This allows complex pipe structures. It is possible for the pipe to have a plurality of further, inner layers. The flame-retardant layer preferably encloses the at least one inner layer. It is possible for the flame-retardant layer to bear against the at least one inner layer. The at least one further, preferably inner layer can be materially connected to the flame-retardant layer. The inner layer can be the innermost layer. It is possible for a further layer or a plurality of further layers to be arranged between the innermost layer and the flame-retardant layer. A separating layer is advantageously arranged between the innermost layer and the flame-retardant layer, wherein the separating layer preferably serves to spatially decouple the flame-retardant layer and the innermost layer.

[0025] Preferably, the at least one inner layer comprises a thermoplastic. This ensures coextrusion even in the case of the at least one inner layer. The thermoplastic of the at least one further inner layer can be a polyamide, a polyester, a polyolefin, a polyurethane, or a thermoplastic elastomer. The polyester can in particular be polyethylene terephthalate (PET). It is possible for a barrier layer to be arranged between the further inner layer and the flame-retardant layer. The barrier layer can, for example, comprise an ethylene-vinyl alcohol copolymer (EVOH) or a fluoropolymer. It is advantageous for a separating layer to be arranged between the barrier layer or the innermost layer and the flame-retardant layer.

[0026] It is preferred that the pipe be manufactured at least partially, and preferably entirely, by coextrusion. The thermoplastic material of the flame-retardant layer and / or the thermoplastic material of the outer layer can be a polyamide, a polyester, a polyolefin, a polyurethane, a thermoplastic elastomer, or a blend of the aforementioned materials. PET is particularly suitable as a polyester.

[0027] According to a very preferred embodiment, the thermoplastic of the outer layer comprises a thermoplastic elastomer, preferably a thermoplastic vulcanizate and particularly preferably Santoprene. Thermoplastic vulcanizates comprise ethylene propylene diene rubber (EPDM) particles in a polypropylene matrix. It is very preferred that the thermoplastic of the flame-retardant layer is a polyolefin and preferably a polypropylene. According to a preferred embodiment, the pipe comprises two and preferably only two inner layers. Advantageously, the innermost layer comprises a thermoplastic, preferably a thermoplastic elastomer, more preferably a thermoplastic vulcanizate and particularly preferably Santoprene. It is very preferred that the second inner layer is arranged between the innermost layer and the flame-retardant layer and is a separating layer.It is advantageous if the separating layer comprises a thermoplastic, preferably a polyolefin and particularly preferably a polypropylene. It is particularly advantageous if all layers of the pipe comprise a common plastic, preferably polypropylene. The common plastic of all layers preferably serves to protect the layer composite particularly well against delamination. The material polypropylene is particularly suitable as a barrier material against aqueous solutions, for example water-glycol solutions, so that a polypropylene composite is particularly suitable as a heat exchanger pipe. Preferably, a fluid line comprises at least one aforementioned pipe according to the invention and at least one pipe connector. It is preferred that the pipe and the at least one pipe connector are welded to one another. This creates a particularly long-lasting, fluidic connection.The cable connector advantageously comprises a thermoplastic. It is particularly preferred that the thermoplastic of the cable connector is a plastic from the group “polyamide, polyester, polyolefins, polyurethane, thermoplastic elastomer” or is a blend of two or more plastics from the aforementioned group. The plastic of the cable connector preferably belongs to the same plastic group as the thermoplastic of the outer layer and / or the thermoplastic of the inner layer. If, for example, the thermoplastic of the outer layer is a polyamide, the thermoplastic of the cable connector is preferably also a polyamide. Very particularly preferably, the thermoplastic of the cable connector and the thermoplastic of the outer layer and / or inner layer belong to the same plastic type, for example PA6 or PA11 or PA12.It is possible that the thermoplastic of the pipe connector comes from the same material reservoir as the thermoplastic of the outer layer and / or the inner layer.

[0028] It is very particularly preferred that the pipe and the line connector are welded to one another and are preferably connected by laser welding. It is possible for the pipe and the line connector to be connected by means of rotational friction welding or mirror welding. Preferably, the pipe is inserted into a connecting section of the line connector. It is possible for the pipe to be pushed onto a connecting section of the line connector. The connecting section expediently comprises at least one circumferential weld seam. It is possible for the connecting section to have two or more circumferential weld seams. It is possible for the connecting section of the line connector to comprise an inner and an outer weld seam which connect the connecting section of the line connector to the pipe.

[0029] The line connector preferably comprises a coupling body. The coupling body is advantageously formed in one piece and particularly preferably integrally. The term "integral" preferably means production from a single injection molding. The coupling body preferably comprises a coupling section for a counterpart to be coupled to the line connector. The counterpart is expediently a plug which is inserted into the line connector. The coupling body is advantageously designed such that a counterpart can be inserted into the coupling section - preferably reversibly. The coupling body preferably comprises a / the connecting section for connecting to the pipe. The connecting section can be designed for plugging in and / or plugging on the pipe.For example, if the connecting section is designed as a circumferential groove into which the pipe is inserted, the pipe is simultaneously placed on an inner wall of the circumferential groove and inserted into an outer wall of the circumferential groove.

[0030] The line connector can in particular have a retainer. The retainer can, for example, be approximately U-shaped and have a U-base and two U-legs. According to one embodiment, the retainer can be completely circumferential. The counterpart can have a locking element, which is designed, for example, as a circumferential collar or as a circumferential groove. The locking element of the counterpart or plug advantageously engages with the retainer of the line connector during insertion of the counterpart into the line connector. According to one embodiment, the coupling body can be designed in two parts and have a connecting part and a coupling part. It is possible for the connecting part and the coupling part to be connected to one another via a locking connection or a screw connection. The line connector expediently comprises a seal.The seal may, for example, comprise one sealing ring or two sealing rings made of an elastic material.

[0031] The object stated at the outset is achieved by the use of a pipe according to the invention or a fluid line according to the invention in a land vehicle and in particular in an electric vehicle. The pipe or the fluid line is preferably used for cooling, particularly preferably for cooling a battery of an electric vehicle drive. It is particularly preferred that the pipe or the fluid line is used to carry an aqueous solution and in particular a water-glycol solution. The pipe or the fluid line can serve, for example, as a supply line to a pipe system of the battery of the electric vehicle drive. It is particularly advantageous that the pipe or the fluid line is designed to maintain the cooling of the battery of the electric vehicle drive for a few seconds or minutes longer than conventional pipes or fluid lines without a flame retardant layer, so that the passengers of the vehicle have correspondingly more time to escape orThe cooling of the battery of the electric vehicle is particularly critical, since excessively high battery temperatures cause the battery to catch fire even more quickly. The pipe or fluid line according to the invention is shown schematically below using figures of exemplary embodiments. They show:

[0032] Fig. 1 shows a cross section of the pipe according to the invention,

[0033] Fig. 2 shows a longitudinal section through a fluid line according to the invention comprising the pipe from Fig. 1 and a line connector together with an inserted counterpart,

[0034] Fig. 3 is a front view of a retainer of the cable connector and

[0035] Fig. 4A to 4C show three further variants of fluid lines, the connecting section of which is shown in longitudinal section.

[0036] According to an exemplary embodiment shown in Fig. 1, a pipe according to the invention may comprise four layers 2, 3, 26, 27 which enclose a lumen 8 of the pipe. The pipe 1 has a flame-retardant layer 2 and an outer layer 3 and may additionally comprise two inner layers 4. In this exemplary embodiment, the flame-retardant layer 2 is bonded to the outer layer 3 and the inner layer 4 by co-extrusion. In this exemplary embodiment, the inner layer 4, 26 forms the innermost layer 26 of the pipe 1, which is separated from the flame-retardant layer 2 by a preferred separating layer 27. Not shown in Fig. 1 are possible protective pipes which are designed, for example, as corrugated pipes and enclose the pipe 1 without being directly bonded to it. In this exemplary embodiment, the outer layer 3 is the outermost layer of the integrally formed pipe 1.Advantageously, all layers 2, 3, 26, 27 of the pipe 1 comprise a thermoplastic material. It is possible for the flame-retardant layer 2 to comprise a polypropylene, in particular an isotactic polypropylene. The separating layer 27 in this embodiment comprises a polypropylene, preferably an isotactic polypropylene. In this embodiment, the outer layer 3 and the innermost layer 26 may comprise a thermoplastic vulcanizate, in particular Santoprene. The material of the innermost layer 26 preferably originates from the same material reservoir as the material of the outer layer 3.

[0037] Since thermoplastic vulcanizates also contain polypropylene, all layers of the pipe 1 in this exemplary embodiment comprise polypropylene, which is why good material bonds are achieved between the four layers 2, 3, 26, 27 of the pipe 1. Furthermore, the polypropylene composite of the pipe 1 is particularly well suited for conveying an aqueous solution—especially a water-glycol solution. Polypropylenes absorb little water and are therefore more resistant to water than, for example, the more expensive polyamides. The pipe in this exemplary embodiment may, in particular, be a supply line to a piping system of a drive battery of an electric vehicle.

[0038] According to the invention, the flame-retardant layer 2 comprises expandable graphite particles, as indicated accordingly in Fig. 1. The expandable graphite particles are preferably flake-shaped and embedded in the polypropylene of the flame-retardant layer 2. In this exemplary embodiment, the weight fraction of the expandable graphite particles in the flame-retardant layer 2 is at least 10%. The expandable graphite particles can, for example, have an average thickness of 25 μm. Due to the flake-like shape, the surface of the expandable graphite particles essentially comprises a top side and a bottom side, which can extend along a few 100 μm in both dimensions. If the expandable graphite particles were sieved, for example, 85% of the particles could pass through a mesh size of 180 μm (85% of the particles finer than 80 mesh). The expandable graphite of this exemplary embodiment may expand by 100 cm at 1000 °C. 3 / g expand.

[0039] A melting temperature of the isotactic propylene of layers 2 and 27 may be 185°C. The melting temperature of the thermoplastic vulcanizate of the outer layer 3 and the innermost layer 26 may be 165°C. It is preferred that a starting temperature of the expandable graphite particles be 210°C. This means that the temperature of the polypropylene melt in this embodiment should be set to approximately 190 to 200°C to prevent expansion of the expandable graphite particles during pipe extrusion.

[0040] In this exemplary embodiment, the outer side of the outer layer 3 has a mean roughness Ra of 50 pm. This mean roughness Ra is primarily caused by the expandable graphite particles in the flame-retardant layer 2, but is partially leveled by the thermoplastic material of the flame-retardant layer 2, because the thermoplastic material of the flame-retardant layer 2 also serves as a filler in the flame-retardant layer 2. The unevenness of the flame-retardant layer 2 is further leveled by the outer layer 3, which in this exemplary embodiment is microscopically homogeneous and, in particular, has neither gas bubbles nor particles with a maximum particle size exceeding 1 pm. It is preferred that the layer thickness of the flame-retardant layer 2 is approximately 200 pm. The layer thickness of the outer layer 3 may be 250 pm. The layer thickness of the separating layer 27 is 150 pm in this exemplary embodiment.The thickness of the innermost layer 26 may be 150 pm.

[0041] The pipe 1 shown in Fig. 1 is, according to Fig. 2, connected to a line connector 5 via a welded joint 7, 18 to form a fluid line 1, 5. The line connector 5 comprises a coupling body 11, which in this exemplary embodiment is preferably formed in one piece and more preferably integrally and is produced by injection molding from a thermoplastic material - preferably from polypropylene. Because the thermoplastic vulcanizates of the outer layer 3 and the innermost layer 26 also comprise polypropylene, this results in a particularly good welded joint 7, 18. The coupling body 11 expediently comprises a coupling section 14 and a connecting section 15. The welded joint 7, 18 is arranged in the region of the connecting section 15 in this exemplary embodiment. The welded joint 7, 18 may comprise an outer weld seam 7 and an inner weld seam 18.In this embodiment, the two weld seams 7, 18 were produced by means of rotational friction welding, wherein the pipe 1 was inserted into a pipe receptacle 17 of the pipe connector 5 or the connecting section 15.

[0042] The pipe receptacle 17 is expediently delimited in the axial direction by a stop 13 and is preferably designed as a circumferential, annular groove. The pipe receptacle 17 may, in particular, have an outer wall 20 and an inner wall 21. The inner wall 21 of this exemplary embodiment can taper on its outer side in the axially inward direction and thus in the direction of the pipe 1, so that the pipe 1 can be pushed onto the inner wall 21 particularly easily. The taper of the inner wall 21 is relatively pronounced in Fig. 2 and can be significantly less pronounced in other embodiments.

[0043] During rotational friction welding, the inserted pipe 1 is advantageously held in place while the pipe connector 5 is rotated. The rotation is so fast that the frictional heat generated in the pipe holder 17 is sufficient to melt the material at the interfaces. This creates, in particular, an outer weld seam 7 and an inner weld seam 18. Another weld seam, not shown here, may be created directly at the groove base of the pipe holder 17 or at the stop 13. Crucial for clean weld seams 7, 18 is that the unevenness of the expandable graphite particles of the flame-retardant layer 2 is compensated for by the inner layers 4 and the outer layer 3.

[0044] In the embodiment according to Fig. 2, the coupling body 11 comprises a fluid channel 10, which connects the connecting section 15 to the coupling section 14. The connecting section 15 may, in particular, have an axial extension which corresponds to the overlap of the pipe 1 and the coupling body 11 in the axial direction. In the longitudinal section according to Fig. 2, a counterpart 9 is inserted into the line connector 5 or into the coupling section 14. The counterpart 9 may be the tip of another pipe (not shown) or a connecting element of an assembly, for example a tank / pump / valve / battery. The counterpart 9 may have a locking element 23, which may preferably be designed as a circumferential collar, but may also be designed as a circumferential groove.

[0045] The line connector 5 expediently comprises a retainer 16, as shown by way of example in Fig. 3. The retainer 16 can be substantially U-shaped, particularly in a front view, and have two legs 25. The legs 25 can be connected to one another via a U-shaped base, which forms an actuating section 24. The retainer 16 can be inserted radially into the coupling body 11 or into the coupling section 14 from Fig. 2 by manually applying pressure to the actuating section 24. If the counterpart 9 is then inserted axially into the line connector 5, the locking element 23 of the counterpart 9 elastically spreads the legs 25 of the retainer 16 apart. As soon as the locking element 23 has passed the legs 25 of the retainer 16, the legs 25 snap back due to the elastic restoring energy, whereby the counterpart 9 is secured in the cable connector 5.

[0046] The line connector 5 may also have a seal 12, which in this exemplary embodiment comprises two sealing rings and a spacer arranged therebetween. A plug shaft 22 of the counterpart 9 is expediently designed such that a frictional connection with the seal 12 is created, thereby achieving a fluidic seal. The seal 12 can be secured in particular by a seal holder 19, so that the seal 12 is also secured in the axially outward direction. In this exemplary embodiment, an inner wall of the coupling body 11 as well as the plug shaft 22 of the counterpart 9 and the connecting section 15 together with the pipe 1 connected thereto are designed concentrically to one another and have a common center axis M. In other exemplary embodiments, the connecting section 15 and the coupling section 14 may be designed at an angle to one another.

[0047] Fig. 4 shows three variants A to C of further welded connections 6, 21 between the pipe 1 and a pipe connector 5. In the case of Fig. 4A, the connecting section 15 of the pipe connector 5 comprises a pipe receptacle 17 into which the pipe 1 is inserted. In contrast to Fig. 2, the pipe receptacle 17 has no inner wall, but only an outer wall 20 and a stop 13. Due to the lack of an inner wall of the pipe receptacle 17, no inner weld seam is created, but only a single weld seam 6. The single weld seam 6 can be produced, for example, by means of rotational friction welding.

[0048] Fig. 4B shows a further embodiment of a welded connection 21 between a pipe 1 and a pipe connector 5. In this exemplary embodiment, the connecting section 15 comprises a pipe receptacle 17 which does not have an outer wall, but does have an inner wall 21. The inner wall 21 is tapered on its outer side in an axially inward direction, so that when the pipe 1 is pushed on, the tip of the pipe 1 increasingly widens and forms an increasing frictional connection. In this embodiment, the pipe 1 can also be pushed on up to a stop 13 of the pipe connector 5 or the connecting section 15 or the pipe receptacle 17. A weld seam 6 can be created by rotational friction welding.

[0049] In Fig. 4C, the connecting section 15 comprises a pipe receptacle 17, which is essentially hollow-cylindrical and is closed off by a stop 13. The pipe receptacle 17 may, in particular, be designed such that an inner wall of the pipe 1 merges flush with an inner wall of the line connector 5. In this exemplary embodiment, the material of the line connector 5 or of the connecting section 15 is transparent to laser beams, in particular near-infrared laser beams, due to a special dye additive, but is colored black in the visible spectrum. In contrast, the material of the pipe 1 is opaque to the same laser beams, so that the laser beams are absorbed at the interface between an outer side of the pipe 1 and an inner side of the pipe receptacle 17. Since the pipe 1 is also preferably dark oris colored black with the help of conventional pigment, a uniform color appearance of the fluid line 1, 5 is achieved, while at the same time the laser beams in the pipe holder 17 can only penetrate as far as the pipe 1.

[0050] The laser beam or laser beams can be guided in such a way that a complete circuit is created, whereby a correspondingly circumferential weld seam 6 is achieved. Since the laser beams are relatively fine and only a moderately strong frictional connection is created between the pipe 1 and the line connector 5, a particularly uniform surface of the outside of the pipe 1 or the outer layer 3 is advantageous. The mean roughness of the outside of the pipe 1 of the embodiment according to Fig. 4C is particularly preferably less than 50 pm and, for example, 20 pm. This creates a particularly long-lasting fluidic connection between the pipe 1 and the line connector 5, while the pipe 1 simultaneously has particularly good flame protection.

[0051] List of reference symbols:

Claims

Claims:

1. Pipe (1) comprising a flame-retardant layer (2) and an outer layer (3), wherein the outer layer (3) encloses the flame-retardant layer (2), wherein the flame-retardant layer (2) comprises a thermoplastic and expandable graphite particles, wherein the expandable graphite particles are designed to expand above a starting temperature, wherein the expandable graphite particles are in a non-expanded state, wherein the outer layer (3) comprises a thermoplastic, characterized in that a mean roughness Ra of an outer side of the pipe (1) or of an outer side of the outer layer (3) is at most 200, 100, or 50 pm.

2. Pipe (1) according to claim 1, wherein the outer layer (3) in a cross-section of the pipe (1) has particles or cavities with an average extension in the radial direction of at most 200 or 150 or 100 or 50 pm or does not comprise any particles or cavities or filled cavities.

3. Pipe (1) according to claim 1, wherein the thermoplastic material of the outer layer (3) is homogeneous over the entire cross section of the outer layer (3).

4. Pipe (1) according to one of claims 1 to 3, wherein the pipe (1) is formed in one piece.

5. Pipe (1) according to one of claims 1 to 4, wherein the outer layer (3) is the outermost layer of the pipe (1).

6. Pipe (1) according to one of claims 1 to 5, wherein the flame-retardant layer (2) has a layer thickness of at most 1.0 or 0.7 mm or 0.5 mm.

7. Pipe (1) according to one of claims 1 to 6, wherein the outer layer (3) has a layer thickness of at least 0.05 or 0.07 mm.

8. Pipe (1) according to one of claims 1 to 7, wherein the weight proportion of the expandable graphite particles in the flame-retardant layer (2) is at least 1 or 3 or 5 wt.%.

9. Pipe (1) according to one of claims 1 to 8, wherein the expandable graphite particles in the flame-retardant layer (2) in a cross-section of the pipe (1) have an average height in the radial direction of at most 200, 150, or 100 pm.

10. Pipe (1) according to one of claims 1 to 9, wherein the expandable graphite particles of the flame-retardant layer (2) have a starting temperature of at least 130 or 150 or 170°C.

11. Pipe (1) according to one of claims 1 to 10, wherein an expansion rate of the expandable graphite particles of the flame retardant layer (2) is at least 10 or 20 or 30 cm 3 / g.

12. Pipe (1) according to one of claims 1 to 11, wherein the pipe (1) comprises an auxiliary flame retardant, wherein the auxiliary flame retardant preferably has a gas-diluting or oxygen-diluting effect, wherein the auxiliary flame retardant is preferably an inorganic flame retardant.

13. Pipe (1) according to one of claims 1 to 12, wherein the pipe (1) comprises at least one further, preferably an inner, layer (4), wherein preferably the flame-retardant layer (2) encloses the inner layer (4), wherein it is preferred that the flame-retardant layer (2) rests against the at least one inner layer (4).

14. Pipe (1) according to one of claims 1 to 13, wherein the thermoplastic material of the flame-retardant layer (2) and / or the thermoplastic material of the outer layer (3) comprises a polyamide, a polyester, a polyolefin, a polyurethane or a thermoplastic elastomer.

15. Fluid line, comprising at least one pipe (1) according to at least one of claims 1 to 14 and at least one line connector (5), wherein it is preferred that the pipe (1) and the at least one line connector (5) are welded to one another.

16. Use of a pipe (1) according to one of claims 1 to 14 or of a fluid line according to claim 15 in a land vehicle, in particular in an electric vehicle, in particular for cooling, preferably for cooling a battery of an electric vehicle drive.

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