Hybrid tube for industrial and fluid hose products and manufacturing method thereof

Abstract

A hybrid tube structure is described for use, for example, in industrial or fluid hoses, the hybrid tube generally including coextruded inner and outer tube layers. The inner tube layer is a functional layer that provides chemical resistance to the fluid carried within the hybrid tube, and the outer tube layer is an elastic layer that provides the hybrid tube with strength and flexibility. The hybrid tube may optionally include reinforcement and / or coating layers. The hybrid tube is generally manufactured using simultaneous coextrusion of the inner and outer tube layer materials to form the hybrid tube structure. In some embodiments, the hybrid tube formed by coextrusion has sufficient structural integrity to not require a supporting mandrel and / or a post-extrusion curing step.

Description

[Technical field]

[0001] The present invention relates to a hybrid tube construction for use in, for example, industrial and / or fluid hose applications, in which the hybrid tube has an inner functional tube layer designed for chemical resistance to the fluid being conveyed, and an elastic outer tube layer formed over the functional tube layer that provides the hybrid tube with flexibility and strength capabilities. [Background technology]

[0002] Fluid power and fluid transport applications rely on tubing as a critical barrier to retain fluid and pressure. A typical tube includes an inner tube layer that contacts the fluid being transported and may further include a reinforcing and / or coating layer around the inner tube layer. The material of the inner tube layer must not degrade upon contact with the inner fluid and must be strong enough to withstand the pressure and temperature. A reinforcing layer may be provided to help the inner tube withstand higher pressures.

[0003] In addition to being chemically resistant to the fluid being conveyed and meeting the necessary pressure and temperature requirements of the fluid and the tube application, there are also processing requirements that must be met when forming the tube. For example, the inner tube must be formed to have a hollow passageway extending through it, and the tube must be prevented from collapsing during forming. Meeting this requirement requires, for example, the selection of an inner tube material that is strong enough to resist collapsing during extrusion, the addition of reinforcing layers, etc., and / or the use of a supporting mandrel during extrusion.

[0004] Thermoset polymeric materials are commonly used as the inner tube material due to their inherent flexibility, chemical resistance, and pressure resistance. However, thermosets must be cured into the final product shape (after extrusion), which requires a long time, factory floor space, and energy costs. Furthermore, uncured thermosets are flexible and prone to breaking without the use of a mandrel to support the tube during processing steps prior to curing (such as braided reinforcement layers and additional extruded layers on the inner tube).

[0005] The use of a rigid inner tube thermoplastic material without post-extrusion curing eliminates the need for a supporting mandrel liner and an autoclave curing step. Unfortunately, the benefits of processing a thermoplastic inner tube are tempered by the reduced performance of the tube compared to thermoset tubes. For example, hoses have been produced using nylon as the inner tube, but these tubes are stiffer than rubber / thermoset liner hoses, and a low durometer friction layer wrapped around the inner tube is often required to achieve compression for the joint and for the braided or added reinforcement to be properly placed over the nylon tube. The final hose product is also less flexible than traditional rubber and thermoset hoses. In a different approach, polyolefins have been used as the material for the inner tube of the hose, but they do not have sufficient resistance to the fluids (e.g., oil) being conveyed.

[0006] Therefore, what is needed is an improved tube that meets both the manufacturing and performance requirements previously described. Summary of the Invention

[0007] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary and the preceding Background are not intended to identify key features or essential features of the claimed subject matter. Moreover, this Summary is not intended to be used as an aid in determining the scope of the claimed subject matter.

[0008] In some embodiments, a hybrid tube suitable for use, for example, in industrial and fluid hoses is described, the hybrid tube comprising an inner tube layer having a hollow passage therethrough, the inner tube layer configured to be chemically resistant to fluids delivered through or present within the hollow passage, and an outer tube layer formed coaxially aligned over the inner tube layer, the outer tube layer configured to provide structure and flexibility to the hybrid tube.

[0009] In some embodiments, a method of forming a hybrid tube is described that includes co-extruding an inner tube layer material and an outer tube layer material through an annular die to form a hybrid tube with an inner tube layer having hollow passages formed therein and an outer tube layer formed coaxially aligned over the inner tube layer. The method does not require the use of a support mandrel to support the co-extruded hybrid tube after it exits the annular die, due to the hybrid tube being manufactured with sufficient structural integrity to prevent the hollow passages from collapsing after extrusion. The method does not require a post-extrusion curing step due to the overall structural integrity of the hybrid tube immediately after extrusion.

[0010] Other features of the technology described herein will become apparent after consideration of the detailed description and drawings set forth herein, but the scope of the claimed subject matter should be determined by the claims as written, and not by whether a given subject matter addresses all of the problems described in the Background or includes features described in the Summary. [Brief description of the drawings]

[0011] Non-limiting, exemplary embodiments of the disclosed technology are described with reference to the following drawings, in which like reference numerals refer to like parts throughout the individual drawings unless otherwise specified.

[0012] FIG. 1 is a perspective view of a portion of a hybrid tube constructed in accordance with various embodiments described herein.

[0013] FIG. 2 is a perspective view of a portion of a hybrid tube constructed in accordance with various embodiments described herein.

[0014] FIG. 3 is a flow chart detailing a method for manufacturing a hybrid tube in accordance with various embodiments described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The embodiments are described more fully hereinafter with reference to the accompanying drawings, which form a part of the specification and which show, by way of illustration, certain exemplary embodiments. These embodiments are disclosed in sufficient detail to enable those skilled in the art to practice the invention. However, the embodiments should not be construed as being limited to the embodiments described herein. Thus, the following detailed description should not be taken in a limiting sense.

[0016] Various embodiments of hybrid tube structures and methods of manufacturing the same are described herein, the hybrid tubes being suitable for use in industrial and / or fluid hoses. The hybrid tube embodiments described herein generally include an inner tube layer defining a hollow passageway extending the entire length of the tube, and an outer tube layer formed on the inner tube layer and coaxially aligned therewith. The inner tube layer is designed to act as a functional layer having chemical resistance to fluids conveyed through the hollow passageway of the hybrid tube. The outer tube layer is designed to act as an elastic layer providing the hybrid tube with strength and flexibility. The combination of these two layers generally provides a hybrid tube including a chemically resistant inner tube layer that does not have sufficient strength and ability to maintain the hollow passageway and is not suitable for use in industrial hoses due to high cost. Additionally, the material of the outer tube layer may be selected from materials that provide the hybrid tube with strength and flexibility, and that have insufficient chemical resistance to the conveyed fluids due to the use of the chemically resistant inner tube layer. The strength imparted to the hybrid tube by the use of the outer tube layer provides many benefits, including reducing the amount of reinforcing layers required, including eliminating the need for a supporting mandrel as part of the hybrid tube manufacturing process, eliminating a curing step as part of the hybrid tube manufacturing process, and / or eliminating the need for reinforcing layers.

[0017] 1, a hybrid tube 100 constructed in accordance with various embodiments described herein is shown including an inner tube layer 110 and an outer tube layer 120 formed on and axially aligned with respect to the inner tube layer 110. The inner tube layer 110 defines a hollow passageway 115 extending therethrough that provides a passageway for fluid to be conveyed through the hybrid tube 100. The specific dimensions of the overall hybrid tube 100, inner tube layer 110, and outer tube layer 120 are not generally limited, and in some embodiments, the outer diameter of the inner tube layer 110 is approximately equal to the inner diameter of the outer tube layer 120 such that the outer tube layer 120 resides directly on the inner tube layer 110.

[0018] In some embodiments, the thickness of the inner tube layer 110 is between 1 and 99% of the total thickness of the hybrid tube 100 (including additional and optional layers, described in more detail below), such as between about 5 and about 50% of the total thickness of the hybrid tube 100. In some embodiments, the thickness of the inner tube layer 110 is relatively small, such as about 10% of the total thickness of the hybrid tube 100. A relatively thin inner tube layer 110 can minimize the amount of inner tube layer 110 material used in the hybrid tube 100, reducing the cost of the hybrid tube 100 (if the cost of the inner tube layer 110 material is relatively high compared to other materials used in the hybrid tube 100), while still providing desirable chemical resistance.

[0019] In some embodiments, the thickness of the outer tube layer 120 is between 1 and 99% of the total thickness of the hybrid tube 100 (including additional and optional layers, described in more detail below), such as between about 50 and about 99%. In some embodiments, the thickness of the outer tube layer 120 is greater than the thickness of the inner tube layer 110. Along with the specific material selected for the outer tube layer 120, the thickness selected for the outer tube layer 120 is also used to ensure that the outer tube layer 120 provides the desired strength and flexibility to the hybrid tube 100. In some embodiments, the thickness of the outer tube layer 120 is designed such that the hybrid tube 100 meets the minimum bend radius requirements of a hose product while being strong enough that the hybrid tube 100 does not require the use of a support mandrel for manufacture and is capable of withstanding the pressures experienced in hose product applications.

[0020] As previously mentioned, the primary function of the inner tube layer 110 is to provide chemical resistance to the fluid being conveyed within the hybrid tube 100. As such, the material of the inner tube layer 110 should generally be a material having the desired chemical resistance. For example, the material of the inner tube layer 110 may be selected to meet the ASTM D543 chemical resistance test standard, and other chemical resistance test standards may be used to select the material of the inner tube layer 110. In some embodiments, the material of the inner tube layer 110 is selected from thermoplastics, thermosets, ceramics, metals, and carbon. In more specific embodiments, the material of the inner tube layer 110 is selected from polyamide, high density polyethylene (HDPE), crosslinked HDPE, crosslinked low density polyethylene (LDPE), ethylene propylene diene terpolymer (EPDM) / polypropylene (PP) blends, polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and fluorinated ethylene propylene (FEP). Each of these materials provides at least sufficient chemical resistance as required for a specific application of the hybrid tube 100.

[0021] The primary function of the outer tube layer 120 is to provide strength and flexibility to the hybrid tube 100. As such, the material of the outer tube layer 120 should generally be a material having the desired strength and flexibility. In some embodiments, the material of the outer tube layer 120 is selected from thermoplastics, thermosets, ceramics, metals, and carbon. In more specific embodiments, the material of the outer tube layer 120 is selected from polyamide, high density polyethylene (HDPE), cross-linked HDPE, low density polyethylene (LDPE), polypropylene (homopolymer or copolymer), polyethylene terephthalate (PET), and polyvinylidene fluoride (PVDF). Each of these materials provides a sufficient combination of strength and flexibility to the hybrid tube 100.

[0022] Any suitable combination of inner and outer tube layer materials may be used, provided that the hybrid tube provides the desired combination of chemical resistance and improved strength and flexibility. In some embodiments, the inner tube layer material may be nylon 12 and the outer tube layer material may be a blend of one or more polyethylenes (e.g., HDPE, LDPE, LLDPE, etc.) with binders (e.g., EVA, EEA, or grafted polyethylene, etc.). In other embodiments, the inner tube layer material may be a polypropylene functional layer and the outer tube layer material may be an EPDM / PP blend.

[0023] As discussed above, the dimensions of the inner tube layer 110 and the outer tube layer 120 are selected such that the outer tube layer 120 resides directly on the inner tube layer 110. In some embodiments, the outer tube layer 120 adheres to the inner tube layer 110 to provide a structurally strong and cohesive hybrid tube. Any adhesive type agent can be used for the inner tube layer 110 and the outer tube layer 120, including multiple adhesive types. In some embodiments, the outer tube layer 120 adheres to the inner tube layer 110 by one or more of chemical bonds, intermolecular attractions, and mechanical adhesion. As described in more detail below, the particular method of adhesion may be by coextrusion used to form the hybrid tube 100.

[0024] Referring to FIG. 2, another embodiment of the hybrid tube 200 is shown, which further includes an optional reinforcing layer 230 and / or an optional covering layer 240. The inner tube layer 210 and the outer tube layer 220 are similar or identical to the inner tube layer 110 and the outer tube layer 120, respectively, as described above with reference to FIG. 1. FIG. 2 shows an additional reinforcing layer 230 molded coaxially aligned over the outer tube layer 220. The reinforcing layer 230 is provided to provide additional strength and / or structural integrity to the hybrid tube 200, and is therefore not required in embodiments where the outer tube layer 220 provides sufficient strength and structural integrity to the hybrid tube 200. In some embodiments where the reinforcing layer 230 is used, the thickness and / or amount of the reinforcing layer 230 is reduced compared to previously known tubes for industrial and / or fluid hoses due to the increased strength provided by the outer tube layer 220, which can provide reduced cost and improved manufacturability.

[0025] The specific material of the reinforcing layer 230 is generally not limited, so long as the reinforcing layer 230 material provides some additional strength to the hybrid tube 200. In some embodiments, the reinforcing layer 230 material is selected from woven, wire, ceramic, or carbon materials. The reinforcing layer is formed on the outer tube layer 200 using any suitable technique, including, but not limited to, applying the reinforcing layer 230 in a braided, spiral wound, or linear pattern. The reinforcing layer 230 material may be provided in the form of, for example, cord, filament, wire, or mesh.

[0026] 2 further illustrates the hybrid tube 200 including a cover layer 240. Although FIG. 2 illustrates the cover layer 240 formed directly on the reinforcing layer 230, it should be understood that in embodiments where the hybrid tube 200 does not include the reinforcing layer 230, the cover layer may be formed directly on the outer tube layer 220. The material of the cover layer 240 is generally selected based on the environmental conditions in which the hybrid tube 200 will be used. In some embodiments, the material of the cover layer 240 is selected from elastomeric or thermoplastic materials. Another suitable material for the cover layer 240 is a thermoplastic vulcanizate (TPV), a heat treatable material with elastomeric properties.

[0027] The hybrid tubes 100, 200 described above are generally manufactured using a co-extrusion process. Referring to Figure 3, a method 300 for manufacturing the hybrid tubes described herein is shown and generally includes a step 310 of co-extruding an inner tube layer material with an outer tube layer material to form the hybrid tube structure described above and shown in Figure 1. Generally speaking, the co-extrusion step 310 generally includes the use of a co-extruder with an annular die near which the flowable inner tube material and the flowable outer tube layer material are combined in a layered configuration (the outer tube layer material is layered over the inner tube layer material) where the combined materials are extruded through the annular die to at least partially harden the materials and form the hybrid tube structure.

[0028] Because the coextruded layers are laminated as melt or adhesive layers, material layers of different thickness, modulus, and / or melt temperature (e.g., below 50° C.) may be used with minimal or no compatibility issues. The layers of the coextruded hybrid tube are bonded by chemical bonds, intermolecular attractions, and / or mechanical adhesion as a result of being extruded under pressure. The adhesion between the inner and outer tube layers is preferably designed to be at least the minimum required to hold the layers together, either alone or in conjunction with a braided reinforcement and / or covering layer, in the application in which the hybrid tube is being used. If a reinforcement layer is used, the adhesion between the outer tube layer and the reinforcement layer is greater than the minimum required for the application.

[0029] Figure 3 illustrates optional and additional steps for adding a reinforcing layer (step 320) and / or a cover layer (330) to the hybrid tube. The reinforcing and cover layers are similar or identical to the reinforcing and cover layers described above with reference to Figure 2 and are formed on the hybrid tube using techniques known in the art. Molding a cover layer step 330 involves molding the cover layer over the reinforcing layer or, in embodiments where a reinforcing layer is not used, over the outer tube layer.

[0030] Step 340 of FIG. 3 highlights that, as with the coextrusion step of 310, depending on the materials used for the inner and outer tube layers, method 300 may be a post-extrusion process without a support mandrel and / or a curing step. In some known processes, the structural integrity of the extruded tube is weak enough that the hollow passages of the tube would collapse if the tube were not extruded over a support mandrel. The support mandrel typically has a diameter approximately equal to the diameter of the hollow passages of the tube such that the support mandrel extends through the hollow passages as the extruded tube exits the die and passes to the support mandrel. The need for a support mandrel typically increases manufacturing area, which reduces manufacturing capacity and production footprint. In the embodiments of method 300 described herein, the hybrid tube extruded in step 310 has structural integrity such that the hollow passages are maintained without the need for a support mandrel.

[0031] Similarly, step 340 highlights that method 300 may be performed without the need for a post-extrusion curing step. This is due to the materials that may be used for the inner and outer tube layers and the co-extrusion process used to produce the hybrid tube structure. In such embodiments, the hybrid tube exiting the extruder has sufficient strength that curing, for example in an autoclave, is not required.

[0032] The hybrid tubes described herein, including the hybrid tube manufacturing methods described herein, provide various benefits. For example, the hybrid tube construction allows for a wide range of material combinations to produce hybrid tubes that meet the needs of various applications. In some embodiments, the techniques described herein allow for the use of chemically resistant materials (for the inner tube layer) that cannot be used because of undesirable physical properties or too high cost. Furthermore, the material of the outer tube layer can be made sufficiently stiff (flexural modulus high enough during processing) that the use of a supporting mandrel for processing is not required, and the hybrid tube can withstand high pressures with little or no reinforcing layer material. Without a focus on chemical resistance (which is addressed by the inner tube layer), the outer tube layer is very specifically designed to provide a desirable flexural modulus. This makes the hybrid tube sufficiently flexible, allowing the tube to meet the minimum bend radius requirements of the product. Due to the improved structural strength exhibited by the hybrid tubes described herein, the hybrid tubes can be formed from materials that do not require a supporting mandrel or curing in an autoclave after extrusion. This results in a manufacturing process with a smaller facility footprint and increased capacity at the production site.

[0033] The hybrid tubes described herein have potential applications in industrial, food and beverage, fluid, chemical corrosive, pneumatic, automotive, aircraft, and other fluid transport or fluid power applications. In some embodiments, the hybrid tubes described herein meet the specifications set forth for SAE100R7 and SAE100R8 hydraulic thermoplastic hoses. The hybrid tubes may be utilized to convey a variety of materials, including water, steam, corrosive liquids, acidic liquids, solvents, flammable liquids, air, gas, and oil. Although this disclosure generally discusses conveyed fluids, the hybrid tube embodiments described herein are suitable for use with any internal liquid conveyed through or held within the tube.

[0034] From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without departing from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

[0035] Although the technical subject matter has been described in language specific to certain structures and materials, it is to be understood that the invention defined in the appended claims is not necessarily limited to the particular structures and materials described. Rather, certain features are described as forms constituting the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the appended claims.

[0036] Unless otherwise indicated, all numbers and expressions expressing sizes and physical properties used in this specification (other than the claims) are understood to be modified in all instances by the term "about" or "approximately." At the very least, and without attempting to limit the application of the doctrine of equivalents to the claims, numerical parameters set forth in the specification or claims that are modified by the term "about" should be interpreted at least by considering the number of significant digits recited and applying rounding techniques. Moreover, all ranges disclosed herein should be understood to support and encompass claims reciting all subranges or individual values ​​incorporated therein. For example, a stated range of 1 to 10 should be considered to support and encompass claims reciting all subranges or individual values ​​between and / or including a minimum value of 1 and a maximum value of 10, i.e., starting from a minimum value of 1 and up to and including a maximum value of 10 (e.g., 5.5 to 10, 2.34 to 3.56, etc.) or all values ​​from 1 to 10 (e.g., 3, 5.8, 9.9994, etc.).

Claims

1. an inner tubing layer having a hollow passage therethrough, the inner tubing layer configured to be chemically resistant to fluids conveyed through or present within the hollow passage; an outer tube layer formed coaxially aligned over the inner tube layer, the outer tube layer configured to provide structure and flexibility to the hybrid tube; Hybrid tubing for industrial and fluid hoses.

2. The hybrid tube of claim 1 , wherein the thickness of the outer tube layer is greater than the thickness of the inner tube layer.

3. 10. The hybrid tube of claim 1, wherein said inner tube layer is about 5 to about 50% of the thickness of said hybrid tube, and said outer tube layer is about 50 to about 95% of the thickness of said hybrid tube.

4. 10. The hybrid tube of claim 1, wherein said inner tube layer is about 10% of the thickness of said hybrid tube.

5. 10. The hybrid tube of claim 1, wherein the material of the inner tube layer is selected from the group consisting of thermoplastics, thermosets, ceramics, metals, and carbon.

6. 10. The hybrid tube of claim 1, wherein the material of the outer tube layer is selected from the group consisting of thermoplastics, thermosets, ceramics, metals, and carbon.

7. 2. The hybrid tube of claim 1, wherein the material of the inner tube layer is selected from the group consisting of polyamide, high density polyethylene (HDPE), crosslinked HDPE, crosslinked low density polyethylene (LDPE), ethylene propylene diene terpolymer (EPDM) / polypropylene (PP) blends, polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and fluorinated ethylene propylene (FEP).

8. 2. The hybrid tube of claim 1, wherein the material of the outer tube layer is selected from the group consisting of polyamide, high density polyethylene (HDPE), cross-linked HDPE, low density polyethylene (LDPE), polypropylene, polyethylene terephthalate (PET), and polyvinylidene fluoride (PVDF).

9. 10. The hybrid tube of claim 1, wherein the inner tube layer is adhered to the outer tube layer by one or more of chemical bonds, intermolecular attractions, and mechanical adhesion.

10. The hybrid tube of claim 1 further comprising a reinforcing layer formed coaxially aligned over said outer tube layer.

11. 11. The hybrid tube of claim 10, wherein the reinforcing layer is applied to the outer tube layer in a braided, spiral wound, or linear pattern.

12. 11. The hybrid tube of claim 10, wherein the material of the reinforcing layer is selected from the group consisting of fabric, wire, ceramic, or carbon material.

13. The hybrid tube according to claim 10, further comprising a covering layer formed coaxially on the reinforcing layer.

14. 11. The hybrid tube of claim 10, wherein the material of the coating layer is selected from the group consisting of elastomers and thermoplastics.

15. The hybrid tube of claim 1 further comprising a cover layer formed coaxially aligned over said outer tube layer.

16. 16. The hybrid tube of claim 15, wherein the material of the coating layer is selected from the group consisting of elastomers and thermoplastics.

17. The hybrid tube of claim 1 , wherein the hybrid tube does not have a reinforcing layer.

18. The inner tube layer material and the outer tube layer material are co-extruded through an annular die to form a hybrid tube having an inner tube layer having a hollow passage formed therein and an outer tube layer formed in coaxial alignment over the inner tube layer. A method for forming a hybrid tube.

19. 20. The method of claim 18, wherein the material of the inner tube layer is selected from the group consisting of polyamide, high density polyethylene (HDPE), crosslinked HDPE, crosslinked low density polyethylene (LDPE), ethylene propylene diene terpolymer (EPDM) / polypropylene (PP) blends, polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and fluorinated ethylene propylene (FEP).

20. 20. The method of claim 18, wherein the material of the outer tube layer is selected from the group consisting of polyamide, high density polyethylene (HDPE), cross-linked HDPE, low density polyethylene (LDPE), polypropylene, polyethylene terephthalate (PET), and polyvinylidene fluoride (PVDF).

21. 20. The method of claim 18, wherein after the coextruded hybrid tube exits the annular die, it is transferred to a support mandrel, the support mandrel extending through the hollow passage of the hybrid tube.

22. 20. The method of claim 18, wherein the coextruded hybrid tube exits the annular die freely without the support of a support mandrel.

23. 20. The method of claim 18, wherein the coextruded hybrid tube exiting the annular die has sufficient structural integrity such that the hollow passages do not collapse after exiting the annular die.

24. 20. The method of claim 18, wherein the method does not include a curing step after co-extrusion of the inner tube layer material and the outer tube layer material through the annular die.

25. 20. The method of claim 18, further comprising adding a reinforcing layer to the coextruded hybrid tube, the reinforcing layer being formed over the outer tube layer and coaxially aligned with the inner tube layer and the outer tube layer.

26. 26. The method of claim 25, wherein the material of the reinforcing layer is selected from the group consisting of a fabric, a wire, a ceramic, or a carbon material.

27. 26. The method of claim 25, further comprising applying a coating layer to the hybrid tube, the coating layer being formed over the reinforcing layer and coaxially aligned with the inner tube layer, the outer tube layer, and the reinforcing layer.

28. 28. The method of claim 27, wherein the material of the cover layer is selected from the group consisting of elastomers or thermoplastics.

29. 20. The method of claim 18, further comprising applying a coating layer to the hybrid tube, the coating layer being formed over the outer tube layer and coaxially aligned with the inner tube layer and the outer tube layer.

30. 30. The method of claim 29, wherein the material of the cover layer is selected from the group consisting of elastomers or thermoplastics.

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