Hollow round tube

Abstract

To provide a hollow circular pipe which satisfies weight saving and multifunctionality, as well as an apparatus and method for manufacturing the hollow circular pipe.SOLUTION: A hollow circular pipe 100 includes an outer layer 220 having an inclination structure varying in the porosity in a wall thickness direction, an inner layer 210, and a coating layer 230. The inclination structure of the hollow circular pipe 100 has the porosity lower in a further side from the center of the outer layer 220 than in a closer side. The hollow tube 100 thus has multifunctionality at least including weight saving and mechanical physical properties.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a hollow tube, a hollow tube manufacturing apparatus, and a hollow tube manufacturing method. [Background technology]

[0002] In recent years, the use of pipes made of foaming resin (foam pipes) as drainage pipes in condominiums or buildings is increasing. Foam pipes have an appropriate foaming ratio and a low specific gravity of the foam, with the aim of reducing the weight of the entire pipe. Foam pipes also have the advantage of reducing the sound of running water due to the soundproofing effect of the foam.

[0003] For example, Patent Document 1 (JP 2007-283733 A) discloses a polyvinyl chloride resin foam pipe which has a polyvinyl chloride resin foam layer with few open bubbles and uniform bubbles laminated with a polyvinyl chloride resin having a substantially non-foamed structure, and which has excellent insulating properties, anti-condensation properties, etc., and which does not allow water to get between the two layers when joined to a fitting, etc.

[0004] The polyvinyl chloride resin foam pipe described in Patent Document 1 (JP Patent Publication No. 2007-283733) is an extrusion-molded polyvinyl chloride resin foam pipe having a foam layer formed between an inner skin layer and an outer skin layer, the inner skin layer having a thickness of 0.05 to 0.6 mm and a substantially non-foamed structure, the outer skin layer having a thickness of 0.2 to 1.5 mm and a substantially non-foamed structure, the cells in the foam layer are substantially not interconnected in a direction parallel to the extrusion direction, the average cell diameter in a cross section perpendicular to the extrusion direction is 30 to 150 μm, and the foam ratio of the foam layer is 2 to 5 times.

[0005] Furthermore, Patent Document 2 (JP Patent Publication No. 11-257548) discloses a sound-absorbing pipe that is excellent in workability and productivity and has a sufficient sound-absorbing effect.

[0006] The sound-absorbing pipe described in Patent Document 2 (JP Patent Publication No. 11-257548) is a sound-absorbing pipe applied to water supply pipes and drainage pipes, and is characterized in that it is composed of an intermediate pipe made of a foam formed from a polyvinyl chloride resin composition with an expansion ratio of 1.2 to 5 times, and an outer pipe and an inner pipe formed from a soft polyvinyl chloride resin composition.

[0007] Patent Document 3 (JP 2002-234066 A) discloses a method for manufacturing a three-layered resin pipe, which is composed of inner and outer layers formed from a non-foamed foamed thermoplastic resin composition and a middle layer formed from a foamed thermoplastic resin composition, so that the wall thickness of the finished product is always uniform.

[0008] The method for producing a three-layered resin pipe described in Patent Document 3 (JP Patent Publication No. 2002-234066) comprises co-extruding, from a die, an unsolidified tubular body having a three-layered structure in which a foamed thermoplastic resin composition as an intermediate layer is interposed between non-foamed thermoplastic resin compositions as inner and outer layers, foaming the foamed thermoplastic resin composition in the co-extruded unsolidified tubular body, and cooling and solidifying the thermoplastic resin. The method is characterized in that the amount of foaming agent added to the thermoplastic resin forming the intermediate layer is equal to or greater than the theoretical amount required to obtain an expansion ratio of a specified product dimension when the resin pipe is molded without restricting its radial expansion, and the resin pipe is formed by sizing to a specified finished dimension against the foaming pressure using a tube for molding the outer surface of the pipe and a core for molding the inner surface of the pipe.

[0009] Patent Document 4 (JP Patent Publication 9-222185A) discloses a composite pipe that has excellent heat insulation and heat retention properties and is free from thermal deformation of the coating layer and water absorption from the surface even when used at high temperatures, and a method for manufacturing the same.

[0010] The composite pipe described in Patent Document 4 (JP Patent Publication 9-222185 A) is a composite pipe in which a coating layer made of a polyvinyl chloride resin foam is provided on the outer peripheral surface of a pipe body made of a polyvinyl chloride resin, and the coating layer is characterized in that it comprises a foamed inner layer and a substantially non-foamed surface layer formed on the outer peripheral surface of the foamed inner layer and the inner layer and the surface layer are integrally formed.

[0011] Patent Document 5 (JP Patent Publication No. 7-217934) discloses a drainage pipe for draining water from an air conditioner that can reliably prevent condensation on its surface and is easy to install.

[0012] The drainage pipe described in Patent Document 5 (JP Patent Publication 7-217934) is made of closed-cell foamed polyvinyl chloride resin with an average foaming ratio of 2 to 7 times, and is characterized in that a skin layer having a thickness of 0.2 to 0.5 mm is provided on the inner surface of the foamed polyvinyl chloride resin, and a skin layer having a thickness of 0.2 to 1.0 mm is provided on the outer surface of the foamed polyvinyl chloride resin. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2007-283733 A [Patent Document 2] Japanese Patent Application Publication No. 11-257548 [Patent Document 3] JP 2002-234066 A [Patent Document 4] Japanese Patent Application Publication No. 9-222185 [Patent Document 5] Japanese Patent Application Publication No. 7-217934 Summary of the Invention [Problem to be solved by the invention]

[0014] As described above, various types of foam pipes have been developed. All foam pipes are usually manufactured with the aim of making the foaming ratio of the foam uniform and homogenizing the bubbles from the viewpoint of dimensional stability, etc. The expansion ratio of the foamed layer in the foamed pipe is set in consideration of the strength and weight of the entire pipe. For example, if the expansion ratio is increased, the mechanical properties such as bending strength and impact resistance of the pipe will decrease, so it is necessary to ensure the strength of the entire pipe by laminating both the inner and outer sides of the foamed layer with non-foamed resin. In order to ensure the strength, the thickness of the non-foamed resin layer needs to be large. Therefore, the weight of the entire pipe will inevitably increase, making it difficult to achieve the desired weight reduction.

[0015] An object of the present invention is to provide a hollow tube that can satisfy multifunctional requirements of at least light weight and mechanical properties, an apparatus for manufacturing the hollow tube, and a method for manufacturing the hollow tube. [Means for solving the problem]

[0016] (1) A hollow tube according to one aspect includes a tubular foam having a gradient structure with different porosity across the wall thickness.

[0017] The hollow tube of the present invention is lightweight because it contains a foam, and has excellent mechanical properties because it has a gradient structure with different porosity in the thickness direction of the foam. Thus, the hollow tube of the present invention has multiple functions, being at least lightweight and having good mechanical properties.

[0018] Multifunctionality refers to a property that combines at least light weight and mechanical properties, and may also refer to properties that exhibit sound deadening, heat retention, insulation, condensation prevention, weather resistance, etc., depending on the manner of use of the hollow tube. The mechanical properties include at least one of bending properties (flexibility), internal pressure resistance, and flatness (cushioning properties).

[0019] In the present invention, a gradient structure in which the porosity varies in the thickness direction refers to a structure that exhibits a gradient in the porosity in the thickness direction, and more specifically includes a structure in which the porosity changes gradually in the thickness direction, a structure in which the porosity changes stepwise in the thickness direction, and a mixed structure of these.

[0020] (2) The gradient structure may include a structure in which the porosity is higher on the side away from the axis of the tubular foam than on the side closer to the axis.

[0021] This provides the hollow tube with better mechanical properties (eg, resistance to internal pressure).

[0022] (3) The gradient structure may include a structure in which the porosity is lower on the side away from the axis of the tubular foam than on the side closer to the axis.

[0023] This provides the hollow tube with better mechanical properties (eg, bending properties).

[0024] (4) The gradient structure may include a stacked structure of multiple layers with different porosities. This allows for a variety of porosity gradients.

[0025] (5) The hollow tube of the present invention comprises a non-foamed inner layer on the inner peripheral surface of a tubular foam, and the wall thickness of the non-foamed inner layer tube may be 10% or more and 70% or less of the wall thickness of the tubular foam.

[0026] In the present invention, since the tubular foam has both light weight and good mechanical properties, the wall thickness of the inner layer tube can be reduced in this manner.

[0027] (6) The hollow tube of the present invention may include a non-foam covering layer on the outer periphery of the tubular foam.

[0028] This allows the outer layer of the tubular foam to be covered by the covering layer. The thickness of the covering layer may be 10% to 30% of the thickness of the tubular foam. Since the tubular foam has light weight as a mechanical property in the present invention, the thickness of the covering layer as the outer layer can be reduced in this manner.

[0029] (7) The tubular foam may be an inorganic foam and / or an organic foam.

[0030] Thus, the hollow tube of the present invention may be embodied in either inorganic or organic foam, or in a mixture of inorganic and organic foam.

[0031] (8) The tubular foam may be made of at least one of a polyvinyl chloride resin and an acrylonitrile-styrene resin.

[0032] In this manner, the hollow tube of the present invention can be embodied using general-purpose resins.

[0033] (9) A method for manufacturing a hollow tube according to another aspect includes a laminating step and a foaming step. In the laminating step, at least a first foamable material containing a predetermined amount of foaming agent and a second foamable material containing an amount of foaming agent different from the predetermined amount are laminated in a tubular shape to obtain a tubular laminate of foamable materials containing at least the first and second foamable materials. In the foaming step, the tubular laminate of foamable materials is foamed to obtain a tubular foam.

[0034] This produces a hollow tube containing a tubular foam with a varying porosity through its wall thickness.

[0035] (10) In the method for producing a hollow tube of the present invention, the number of layers of the tubular laminate of the expandable material may be three or more. This produces hollow tubes containing tubular foam with varying porosity gradients.

[0036] (11) In the method for manufacturing a hollow tube of the present invention, the number of layers of the tubular laminate of the foamable material is 4 or more and 10 or less, and the foamable material constituting each layer may be prepared so that the layer farther from the axis of the tubular laminate contains a greater amount of foaming agent than the layer closer to the axis of the tubular laminate.

[0037] This produces a hollow tube comprising tubular foam having a structure with a higher porosity on the side away from the axis than on the side closer to the axis.

[0038] (12) In the method for manufacturing a hollow tube of the present invention, the number of layers of the tubular laminate of the expandable material is 4 or more and 10 or less, and the expandable material constituting each layer may be prepared so that the layer farther from the axis of the tubular laminate contains a smaller amount of foaming agent than the layer closer to the axis of the tubular laminate.

[0039] This produces a hollow tube comprising tubular foam having a structure with less porosity on the side away from the axis than on the side closer to the axis.

[0040] (13) The method for producing a hollow tube of the present invention may further include a step of laminating a non-foamable material onto the tubular laminate or tubular foam.

[0041] This produces a hollow tube in which the tubular foam is laminated with the non-foam. A layer of non-foam material may be laminated to the inside and / or outside of the tubular foam.

[0042] (14) In the method for producing a hollow tube of the present invention, the foaming agent may be a solid thermally decomposable foaming agent. In this case, the amount of the foaming agent can be easily adjusted.

[0043] (15) In the method for producing a hollow tube of the present invention, the foaming agent may be an inorganic gas. In this case, good long-run performance can be maintained.

[0044] (16) A hollow tube according to still another aspect has a structure obtained by the method for producing a hollow tube according to any one of (9) to (15) above. Therefore, the hollow tube of the present invention has multiple functions, including at least light weight and good mechanical properties.

[0045] (17) A hollow tube manufacturing apparatus according to yet another aspect includes at least a first extruder for extruding a first expandable material, a first gas supplying section for supplying a foaming gas to the first expandable material, and a first molding section for molding the first expandable material into a tubular shape, a second extruder for extruding a second expandable material, a second gas supplying section for supplying a foaming gas to the second expandable material, and a second molding section for molding the second expandable material into a tubular shape so as to be laminated on the tubular first expandable material. Furthermore, the first gas supplying section and the second gas supplying section are configured to be able to independently control the gas pressure from each other.

[0046] This makes it possible to obtain a tubular foam having a gradient structure in which the porosity varies in the thickness direction. [Brief description of the drawings]

[0047] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a hollow circular tube according to a first embodiment. [Diagram 2] 4 is a graph showing a gradient structure of a foam layer of a hollow circular tube according to the first embodiment. [Diagram 3] FIG. 11 is a schematic cross-sectional view showing an example of a hollow circular tube according to a second embodiment. [Figure 4] 13 is a graph showing a gradient structure of a foam layer of a hollow circular tube according to a second embodiment. [Diagram 5] FIG. 11 is a schematic cross-sectional view showing an example of a hollow circular tube according to a third embodiment. [Figure 6] 13 is a graph showing a gradient structure of a foam layer of a hollow circular tube according to a third embodiment. [Figure 7] FIG. 11 is a schematic cross-sectional view showing an example of a hollow circular tube according to a fourth embodiment. [Figure 8]13 is a graph showing a gradient structure of a foam layer of a hollow circular tube according to a fourth embodiment. [Figure 9] FIG. 11 is a schematic cross-sectional view showing an example of a hollow circular tube according to a fifth embodiment. [Figure 10] 13 is a graph showing a gradient structure of a foam layer of a hollow circular tube in another example. [Figure 11] FIG. 2 is a schematic diagram showing an example of a hollow circular tube manufacturing apparatus. [Figure 12] FIG. 2 is a schematic diagram showing an example of a hollow circular tube manufacturing apparatus. [Figure 13] FIG. 2 is a schematic enlarged cross-sectional view of a mold for a hollow circular tube. [Figure 14] FIG. 13 is a schematic diagram showing another example of a hollow circular tube manufacturing apparatus. [Figure 15] FIG. 11 is a schematic enlarged cross-sectional view showing another example of a mold for a hollow circular tube. [Figure 16] FIG. 11 is a schematic enlarged cross-sectional view showing another example of a mold for a hollow circular tube. [Figure 17] FIG. 11 is a schematic enlarged cross-sectional view showing another example of a mold for a hollow circular tube. [Figure 18] FIG. 13 is a schematic diagram showing another example of a hollow circular tube manufacturing apparatus. [Figure 19] FIG. 11 is a schematic enlarged cross-sectional view showing another example of a mold for a hollow circular tube. [Figure 20] 1 is an X-ray CT image of a cross section of a hollow circular tube produced in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description, the same components are given the same reference numerals. Furthermore, when the reference numerals are the same, their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0049] [First embodiment] FIG. 1 is a schematic cross-sectional view (cross-sectional view taken along a plane perpendicular to the axis) showing an example of a hollow circular tube 100 according to the first embodiment.

[0050] (Outline of hollow circular pipe) 1, the hollow circular tube 100 is mainly composed of an inner layer 210, an outer layer 220, and a covering layer 230. Of these, the inner layer 210 and the covering layer 230 are not essential components.

[0051] (Inner layer) The inner layer 210 shown in FIG. 1 is made of a non-foamed hard resin. Specifically, the hard resin may be a hard polyvinyl chloride resin (PVC). More specifically, the hard polyvinyl chloride resin may be a homopolymer of a vinyl chloride monomer, for example, a chlorinated polyvinyl chloride resin, a copolymer of a vinyl chloride monomer and a polymerizable monomer other than a vinyl chloride monomer, or a graft copolymer in which a vinyl chloride monomer is grafted onto a polymer other than a vinyl chloride resin. The hard polyvinyl chloride resin may contain ultrafine rubber particles. In this case, the impact resistance can be improved. The ultrafine rubber particles may be physically or chemically bonded to the polyvinyl chloride resin.

[0052] The non-foamable resin forming the inner layer 210 is not limited to the vinyl chloride resin described above, and may be any other resin. For example, polyethylene, polypropylene, polybutene, chlorinated polyethylene, ethylene-propylene copolymer, ethylene-propylene-diene copolymer, ethylene-ethyl acrylate copolymer, ethylene-vinyl acetate copolymer, polyvinyl fluoride, polycarbonate, polyacetal, polyphenylene sulfide, polyphenylene oxide, nylon-6, nylon-6 / 6, nylon 12, polyethylene terephthalate, polybutylene terephthalate, acrylonitrile-butadiene-styrene resin, (meth)acrylic resin, etc. The above-mentioned resins may be used alone or in combination of two or more.

[0053] The thickness (wall thickness) of the inner layer 210 in the stacking direction may be 10% or more and 70% or less of the thickness of the outer layer 220 described below. The thickness of the inner layer 210 in the stacking direction may be a thickness that occupies a range with any two percentage values ​​of 10%, 20%, 30%, 40%, 50%, 60%, and 70% of the thickness of the outer layer 220 as upper and lower limits. More specifically, for example, it may be 0.3 mm or more and 10 mm or less. However, the thickness (wall thickness) of the inner layer 210 in the stacking direction may be less than 0.3 mm if there is no problem in terms of strength, and may be more than 10 mm if there is no problem in terms of application.

[0054] (Outer layer and covering layer) The outer layer 220 shown in Fig. 1 is formed by laminating on the outer periphery of the inner layer 210. The outer layer 220 is made of a foamed resin having a large number of voids (air bubbles). The resin constituting the outer layer 220 can be selected from the resins exemplified as the resin constituting the inner layer 210. The resin constituting the outer layer 220 may be the same as the resin constituting the inner layer 210 except for the inclusion of voids, or may be a different resin.

[0055] Fig. 2 is a graph showing a schematic relationship between the distance from the axis of the hollow circular tube 100 and the porosity of the outer layer 220. As shown in Fig. 2, the foamed resin constituting the outer layer 220 has a gradient structure in which the porosity gradually changes in the thickness direction. Specifically, the porosity is smaller (denser) as it is farther from the axis (closer to the outer periphery), and the porosity is larger (sparser) as it is closer to the axis (closer to the inner periphery). The porosity refers to the volume that is occupied by voids relative to the volume of the foam. Therefore, the higher the foaming rate, the higher the porosity, and the lower the foaming rate, the lower the porosity.

[0056] The porosity of the outer layer 220 may vary in any manner. For example, as shown in Fig. 2(A), the rate of change in porosity may be constant throughout the thickness, or the porosity may vary as shown in Fig. 2(B) and Fig. 2(C). In the case shown in Fig. 2(B), the variation in porosity is greater closer to the inner circumference, and in the case shown in Fig. 2(C), the variation in porosity is greater closer to the outer circumference.

[0057] The average porosity of the outer layer 220 is, for example, 50% or more and 80% or less, for example, 75%. When the porosity of the outer layer 220 is 50% or more, the light weight is good, and when the inner layer 210 is protected as in this embodiment, for example, the heat insulating performance or the condensation prevention performance is also good. When the porosity of the outer layer 220 is 80% or less, the mechanical strength of the outer layer 220 (for example, it can be measured as the flattening ratio when cracks or fissures occur in a flattening test) is easily ensured. In this way, the gradient structure can ensure excellent mechanical strength compared to the average porosity.

[0058] The thickness of the outer layer 220 is arbitrary, and is, for example, 2 mm or more, preferably 4 mm or more. By having a thickness of 2 mm or more, the gradient structure can be effectively ensured. Furthermore, by having a thickness of 4 mm or more, the gradient structure described above can be more easily provided. Although there is no upper limit to the thickness of the outer layer 220, an example is 15 mm from the viewpoint of lightness, etc.

[0059] The voids in the outer layer 220 are formed to be closed-cell foam cells that do not communicate with adjacent voids. The void size is preferably in the range of an average cell diameter of 30 μm to 150 μm. Here, the average cell diameter refers to the cell diameter when the cumulative cell diameter fraction is 50% when the cell diameters in a cross section perpendicular to the axial direction are accumulated from the smallest one. The cell diameters of the voids shown in the figure are all shown to be the same size in order to show the porosity in a schematic manner, but are not limited to such an embodiment. For example, the cell diameter may be larger in the area with a higher porosity and smaller in the area with a lower porosity.

[0060] Furthermore, the foamed resin from which the outer layer 220 is formed may further contain heat stabilizers, processing aids, lubricants, impact modifiers, fillers, antioxidants, UV absorbers, light stabilizers, pigments, etc., as necessary.

[0061] In this embodiment, the thickness of the covering layer 230 may be 5% or more and 30% or less of the thickness of the outer layer 220. The thickness of the covering layer 230 may be a thickness that occupies a range with the upper and lower limits being any two of the percentage values ​​of 5%, 10%, 15%, 20%, 25%, and 30% of the thickness of the outer layer 220. Specifically, for example, it is preferably 0.1 mm or more and 1.5 mm or less, and one example is 0.5 mm. If the thickness of the coating layer 230 is 0.1 mm or more, the strength of the hollow circular tube 100 itself can be ensured, and if the thickness is 1.5 mm or less, the portion of the coating layer 230 within the wall thickness of the hollow circular tube 100 can be reduced, and a preferable porosity can be ensured, making the hollow circular tube 100 lighter.

[0062] The covering layer 230 may be made of the same resin as the outer layer 220, except that it is non-foamed, or it may be made of a different resin.

[0063] [Second embodiment] FIG. 3 is a schematic cross-sectional view showing an example of a hollow circular tube 100a according to the second embodiment. 3, the hollow circular pipe 100a is mainly composed of an inner layer 210, an outer layer 220a, and a coating layer 230. The hollow circular pipe 100a is the same as the hollow circular pipe 100 of the first embodiment, except for the foaming mode of the outer layer 220a. The differences from the first embodiment will be described below.

[0064] Fig. 4 is a graph showing a relationship between the distance from the axis of the hollow circular tube 100a and the porosity of the outer layer 220a. As shown in Fig. 4, the foamed resin constituting the outer layer 220a has a gradient structure in which the porosity increases (lower) as it is farther from the axis (closer to the outer periphery) and decreases (higher) as it is closer to the axis (closer to the inner periphery).

[0065] The porosity of the outer layer 220a may vary in any manner. For example, as shown in Fig. 4(A), the rate of change of the porosity may be constant throughout the thickness, or as shown in Fig. 4(B) and Fig. 4(C), the porosity may vary. In the case shown in Fig. 4(B), the porosity varies more significantly closer to the outer periphery, and in the case shown in Fig. 4(C), the porosity varies more significantly closer to the inner periphery.

[0066] [Third embodiment] FIG. 5 is a schematic cross-sectional view showing an example of a hollow circular tube 100b according to the third embodiment. 5, the hollow circular pipe 100b is mainly composed of an inner layer 210, an outer layer 220b, and a coating layer 230. The hollow circular pipe 100b is the same as the hollow circular pipe 100 of the first embodiment, except for the foaming mode of the outer layer 220b. The differences from the first embodiment will be described below.

[0067] As shown in FIG. 5, the outer layer 220b has a laminated structure of foamed layers 221b, 222b, and 223c with different porosities. The foamed layers 221b, 222b, and 223c may be made of the same resin except for the different foaming ratios, or may be made of different resins. An example of a case in which the foamed layers 221b, 222b, and 223c are made of different resins is a case in which the amount of functional resin mixed in each foamed layer is different. In this case, by configuring the amount of functional resin or additive that exhibits a specific function to increase or decrease in the order of the foamed layers 221b, 222b, and 223c, it is possible to provide a gradient structure (gradient functionality) for the specific function. The specific function is not particularly limited, but may be, for example, weather resistance.

[0068] Fig. 6 is a graph showing a relationship between the distance from the axis of the hollow circular tube 100b and the porosity of the outer layer 220b. As shown in Fig. 6, the resin constituting the outer layer 220b has a gradient structure in which the porosity decreases stepwise from the inner circumference to the outer circumference in the order of the foam layers 221b, 222b, and 223b.

[0069] In the case of a laminated embodiment of a plurality of foam layers with different foaming ratios as in the present embodiment, the boundary between adjacent foam layers may be relatively clear due to the difference in porosity as shown in Fig. 6(A), but is not limited to this embodiment. The boundary between adjacent foam layers is not necessarily clear, and as shown in Fig. 6(B), the change in porosity near the boundary between the foam layers may be more gradual while maintaining a gradient structure in which the porosity changes stepwise.

[0070] In this embodiment, three foam layers having different porosities are laminated, but the number of layers is not limited to this and may be two or more.

[0071] When the foam layer is laminated in multiple layers (for example, a number of layers in a range with any two of the following numbers as upper and lower limits: 4, 5, 6, 7, 8, 9, and 10) so that the porosity gradually decreases from the inner circumference to the outer circumference, the boundaries of the foam layer pipe become unclear, and as shown in Figure 6(C), the structure may appear to be a gradient structure in which the porosity gradually changes continuously, as shown in the first embodiment.

[0072] [Fourth embodiment] FIG. 7 is a schematic cross-sectional view showing an example of a hollow circular pipe 100c according to the fourth embodiment. 7, the hollow circular pipe 100c is mainly composed of an inner layer 210, an outer layer 220c, and a coating layer 230. The hollow circular pipe 100c is the same as the hollow circular pipe 100 of the first embodiment, except for the foaming mode of the outer layer 220c. The differences from the first embodiment will be described below.

[0073] The outer layer 220c has a structure in which two outer layers 220 of the first embodiment are laminated together. As shown in Fig. 7, the foamed resin constituting the outer layer 220c has a laminated structure of foamed layers 221c and 222c having different average porosities. 8 is a graph showing a relationship between the distance from the axis of the hollow circular pipe 100c and the porosity of the outer layer 220c. The foamed layers 221c and 222c are each configured so that the porosity gradually decreases from the inner periphery to the outer periphery. Therefore, the entire thickness of the outer layer 220c includes two gradient structures in which the porosity gradually decreases from the inner periphery to the outer periphery.

[0074] In this embodiment, the foam layers 221c and 222c have different average porosities (see FIG. 8(A)), but the present invention is not limited to this. The foam layers 221c and 222c may have the same average porosity (see FIG. 8(B)). Even in this case, the outer layer 220c includes two gradient structures in which the porosity gradually decreases from the inner periphery to the outer periphery throughout the thickness of the outer layer 220c.

[0075] In this embodiment, the foam layers 221c and 222c may be made of the same resin, except that the average porosity is different, or may be made of different resins. An example of a case in which the foam layers 221c and 222c are made of different resins is a case in which the amount of functional resin mixed in each foam layer is different. In this case, by configuring the amount of functional resin that exhibits a specific function to increase or decrease in the order of the foam layers 221c and 222c, a gradient structure (gradient functionality) can be provided for the specific function as well.

[0076] In the present embodiment, the two foam layers 221c and 222c each having a gradient porosity structure are laminated, but the number of layers is not limited to this, and more foam layers may be laminated. The number of layers may be 2 or more, for example, 3, 4, or 5.

[0077] [Fifth embodiment] FIG. 9 is a schematic cross-sectional view showing an example of a hollow circular pipe 100d according to the fifth embodiment. 9, the hollow circular pipe 100d is mainly composed of an inner layer 210d, an outer layer 220a, and a coating layer 230. The hollow circular pipe 100d is the same as the hollow circular pipe 100a of the second embodiment except that it has the inner layer 210d instead of the inner layer 210. The differences from the second embodiment will be described below.

[0078] The hollow circular pipe 100d has a relatively thin non-foamed inner layer 210d. The inner layer 210d is provided with a thickness similar to that of the covering layer 230 (a range of 5% to 30% of the thickness of the outer layer 220a, or a range of any two of 5%, 10%, 15%, 20%, 25%, and 30%, specifically, for example, 0.1 mm to 1.5 mm, for example, 0.5 mm). The inner layer 210d and the covering layer 230 may be made of the same resin or different resins.

[0079] The hollow circular pipe 100d has an outer layer 220a with a porosity that gradually increases from the inner circumference to the outer circumference between the inner layer 210d, which is a thin non-foamed layer, and the covering layer 230, so that the cross section when cut perpendicular to the axis resembles bamboo. Therefore, the hollow circular pipe 100d has a function similar to bamboo in terms of having voids.

[0080] The present invention includes any aspect in which the above-mentioned embodiments are appropriately combined. For example, Fig. 10 is a graph showing the relationship between the distance from the axis of a hollow circular pipe and the porosity when the outer layer 220 of the first embodiment and the outer layer 220a of the second embodiment are combined. Fig. 10(A) shows an aspect in which the outer layer 220a of the second embodiment is laminated on the inner periphery side and the outer layer 220 of the first embodiment is laminated on the outer periphery side, and Fig. 10(B) shows an aspect in which the outer layer 220 of the first embodiment is laminated on the inner periphery side and the outer layer 220a of the second embodiment is laminated on the outer periphery side.

[0081] [Manufacturing of hollow circular tube according to the first embodiment] (Outline of hollow tube manufacturing equipment) 11 and 12 are schematic diagrams showing an example of a manufacturing apparatus 500 for the hollow circular tube 100. In FIG. FIG. 11 is a plan view of the manufacturing apparatus 500 for the hollow circular tube 100 as viewed from above and below, and FIG. 12 is a side view of the manufacturing apparatus 500 for the hollow circular tube 100. As shown in FIG.

[0082] As shown in Figures 11 and 12, the manufacturing apparatus 500 includes a carbon dioxide gas cylinder 510, a metering pump 520, an extruder 530 for a non-foamable resin composition, an extruder 540 for a foamable resin composition, a mold 550, a pipe outer surface adjustment device 560, a cooling water tank 570, a take-up machine 580, and a cutting machine 590.

[0083] FIG. 13 is a schematic enlarged cross-sectional view of an elliptical portion of a mold 550 in a manufacturing apparatus 500, the portion being indicated by a dotted line in the figure.

[0084] (Manufacturing method of hollow circular pipes) An example of a method for manufacturing the hollow circular pipe 100 will be described. As shown in Figs. 11 and 12, in a non-foamable resin composition extruder 530, a non-foamable resin composition 210' (see Fig. 13) is melt-kneaded and extruded into a tube shape through a straight die. The non-foamable resin composition 210' is specifically a non-foamable thermoplastic resin composition, for example, a vinyl chloride resin composition. As described later, the non-foamable resin composition 210' is laminated together with a foamable resin composition 220' (described later) in a mold 550, and then sent out of the mold 550 and solidified in a cooling water tank 570 to become the inner layer 210 (see Fig. 1).

[0085] An example of the vinyl chloride resin composition is a resin composition obtained by supplying 100 parts by weight of vinyl chloride resin, 2.0 parts by weight of lead-based heat stabilizer, 0.5 parts by weight of polyethylene wax, 2.0 parts by weight of acrylic processing aid, 0.7 parts by weight of ester wax, 3.0 parts by weight of calcium carbonate, and 0.5 parts by weight of pigment to a Henschel mixer or the like and mixing them. Furthermore, the non-foamable resin composition may contain ultrafine rubber particles. The ultrafine rubber particles may be physically or chemically bonded to the vinyl chloride resin.

[0086] In addition to the vinyl chloride resin composition described above, any resin composition may be appropriately selected by those skilled in the art to form the non-foamable resin to be formed.

[0087] Meanwhile, carbon dioxide gas as a foaming agent is supplied from a carbon dioxide gas cylinder 510 through a vent hole via a metering pump 520 into a foamable resin composition extruder 540. In the foamable resin composition extruder 540, a thermoplastic resin composition and carbon dioxide gas injected into the thermoplastic resin composition are melt-kneaded to prepare a foamable resin composition 220' (see FIG. 13). As described below, the foamable resin composition 220' is laminated together with a non-foamable resin composition 210' in a mold 550, and then sent out of the mold 550 to be foamed, and solidified in a cooling water tank 570 to form an outer layer 220 (see FIG. 1).

[0088] The foamable resin composition 220′ can be prepared by further adding a foaming agent to the non-foamable resin composition 210′ ​​that can be used to form the above-mentioned inner layer 210. The resin contained in the foamable resin composition 220′ may be the same as or different from the resin contained in the non-foamable resin composition 210′.

[0089] In this embodiment, the foaming gas carbon dioxide is used as the foaming agent, but the present invention is not limited thereto. Examples of the foaming gas include inorganic gases such as nitrogen gas, and organic gases such as butane gas and fluorocarbon gases, in addition to carbon dioxide gas. These foaming gases may be used alone or in combination of two or more kinds. Inorganic gases are preferable in that they do not need to be recovered and do not produce by-decomposition products, and therefore have excellent long-run properties.

[0090] Other examples of the foaming agent include thermal decomposition type foaming agents. Examples of the thermal decomposition type foaming agent include thermal decomposition type inorganic foaming agents such as sodium bicarbonate, ammonium bicarbonate, and ammonium carbonate, and thermal decomposition type organic foaming agents such as nitroso compounds such as N,N':dinitrosoterephthalamide, azo compounds such as azodicarbonamide and azobisisobutyronitrile, and sulfonyl hydrazide compounds such as benzenesulfonyl hydrazide and toluenesulfonyl hydrazide. These thermal decomposition type foaming agents may be used alone or in combination of two or more. In this case, the resin is foamed by utilizing the gas generated by the thermal decomposition reaction of the foaming agent. In addition, a solid thermal decomposition type foaming agent is preferable in that the content in the resin composition can be easily adjusted.

[0091] Further, as another example of the blowing agent, a solvent-based blowing agent can be mentioned. In this case, the resin is foamed by utilizing the gas generated by vaporization due to heat or pressure release. Examples of the solvent-based blowing agent include solvents with low boiling points, such as lower alcohols such as methanol and ethanol, and lower alkanes such as pentane and hexane. These solvent-based blowing agents can be used alone or in combination of two or more kinds.

[0092] The foaming agent may be a foaming gas, a thermal decomposition type foaming agent, or a solvent type foaming agent in any combination. When no foaming gas is used, the manufacturing apparatus 500 does not require the carbon dioxide gas cylinder 510 and the metering pump 520.

[0093] The amount of the blowing agent can be adjusted according to the desired expansion ratio. To obtain a foam with a larger expansion ratio or porosity, a larger amount of the blowing agent can be used, and to obtain a foam with a smaller expansion ratio or porosity, a smaller amount of the blowing agent can be used.

[0094] As shown in FIG. 13, the mold 550 is composed of a straight die 551 and a crosshead die 552. A vacuum slit VS2 is formed in the crosshead die 552. In the mold 550, the non-foamable resin composition 210' melt-kneaded in the non-foamable resin composition extruder 530 is molded into a tubular shape through the straight die 551, and the foamable resin composition 220' melt-kneaded in the foamable resin composition extruder 540 is molded into a tubular shape through the crosshead die 552 so as to be laminated on the outer periphery of the tubular non-foamable resin composition 210'. In this way, a laminated pipe containing the non-foamable resin composition 210' and the foamable resin composition 220' is prepared in the mold 550.

[0095] A non-foamable resin composition (not shown) is further laminated on the laminate pipe to form a covering layer 230 (see FIG. 1). The non-foamable resin composition for forming the covering layer 230 may be laminated in a mold 550, or may be laminated after foaming of a foamable resin composition 220′, which will be described later.

[0096] The covering layer 230 (see FIG. 1) can be formed by laminating the non-foamable resin composition as described above, but is not limited to this method. For example, when the surface of the crosshead die 552 that contacts the foamable resin composition 220' is configured as the cooling surface S2, the covering layer 230 may be formed by cooling the outer peripheral surface of the foamable resin composition 220' with the cooling surface S2 to solidify the vicinity of the outer peripheral surface prior to foaming.

[0097] The laminated pipe containing the non-foamable resin composition 210' and the foamable resin composition 220' is sent out of the mold 550, whereby the pressure is reduced and the foamable resin composition 220' is foamed. The foamed laminated pipe is sized by applying an appropriate pressure to the outer layer surface by a pipe outer surface adjustment device 560 attached to a cooling water tank 570. The pipe outer surface adjustment device 560 includes a tube for molding in contact with the outer surface of the pipe.

[0098] During foaming, a temperature adjustment unit (not shown) of the pipe outer surface adjustment device 560 cools the layer surface of the foamable resin composition 220', thereby generating a temperature gradient in the layer of the foamable resin composition 220' so that the outer periphery side is lower in temperature and the inner periphery side is higher in temperature. This makes it possible to adjust the gradient structure (see FIG. 2) of the porosity in the outer layer 220 (see FIG. 1) after foaming.

[0099] The laminated pipe sent out from the pipe outer surface adjustment device 560 is sufficiently cooled and solidified in the cooling water tank 570, and becomes the hollow circular pipe 100. In this embodiment, the refrigerant used in the cooling water tank 570 is water, but the refrigerant may be either liquid or gas as long as it is not corrosive to the product and has good fluidity and heat exchange properties. Examples of the refrigerant include air, oil, and polyethylene glycol. The hollow circular tube 100 is taken up by a take-up machine 580 and cut to an appropriate length by a cutter 590.

[0100] In the above, an example was given of forming the gradient structure of the outer layer 220 of the hollow circular pipe 100 by a method of varying the foaming ratio in the wall thickness direction by temperature control, but the method of forming the outer layer 220 is not limited to this method. In addition to the method of temperature control, the outer layer 220 can also be formed by a method of varying the foaming ratio in the wall thickness direction by stacking multiple layers with different amounts of foaming agent. The term "multiple layers" refers to layers stacked in a range with the upper and lower limits being any two of the numbers of layers, for example, 4, 5, 6, 7, 8, 9, and 10, although this may vary depending on the wall thickness of the outer layer 220 to be formed.

[0101] A method for varying the foaming ratio in the thickness direction by stacking multiple layers with different amounts of foaming agent will be described in detail in the manufacturing of a hollow circular pipe according to the second embodiment described later, but when forming the outer layer 220 by this method, the amount of foaming agent contained in each of the stacked layers is controlled so that the amount of foaming agent contained in the layers farther from the axis is less than that of the layers closer to the axis. As a result, the layers are foamed in a stacked state so that the amount of foaming agent gradually decreases from the side closer to the axis to the side farther from the axis, and the porosity appears to gradually and continuously decrease from the inner circumference side to the outer circumference side.

[0102] [Manufacturing of hollow circular tube according to the second embodiment] Fig. 14 is a schematic diagram showing an example of a manufacturing apparatus 500a for a hollow circular tube 100a (schematic diagram in plan view when the manufacturing apparatus 500a is seen from above and below). Fig. 15 is a schematic enlarged cross-sectional view of an elliptical portion of a die 550a in the manufacturing apparatus 500a, indicated by a dotted line in the figure. In the following, differences from the manufacturing of a hollow circular tube according to the first embodiment will be mainly described, and the same points will not be described.

[0103] As shown in FIG. 14, the manufacturing apparatus 500a for the hollow circular pipe 100a includes a non-foamable resin composition extruder 530 for melt-kneading the non-foamable resin composition 210' and a plurality of extruders for melt-kneading the foamable resin composition. In FIG. 14, of the plurality of extruders for melt-kneading the foamable resin composition, only two extruders, a first extruder 540a1 and a second extruder 540a2, are shown, and the other extruders are omitted. A carbon dioxide gas cylinder 510 and a metering pump 520 are connected to each of the plurality of extruders for melt-kneading the foamable resin composition, and the gas pressure is independently controllable so that different amounts of carbon dioxide gas can be dispersed in each of the plurality of extruders for the foamable resin composition 540.

[0104] In the non-foamable resin composition extruder 530, the non-foamable resin composition 210' (see FIG. 15) is melt-kneaded. In the first extruder 540a1, the foamable resin composition 221a' is melt-kneaded, and in the second extruder 540a2, the foamable resin composition 222a' is melt-kneaded. Similarly, in an extruder not shown, the foamable resin composition 223a' is melt-kneaded. The foamable resin compositions 221a', 222a', and 223a' are prepared so that the foaming gas content gradually increases in this order. In another extruder not shown, the foamable resin composition is melt-kneaded so that the foaming gas content gradually varies.

[0105] 15, the mold 550a includes a straight die 551 and crosshead dies 552, 553, and 554. The crosshead dies 552, 553, and 554 are provided with vacuum slits VS2, VS3, and VS4, respectively. The mold 550a also includes other similar crosshead dies, not shown.

[0106] The extruded non-foamable resin composition 210' and the foamable resin compositions 221a', 222a', 223a'... are molded into a tubular shape by the straight die 551 and the crosshead dies 552, 553, 554..., respectively, in the mold 550a, and are guided so that the non-foamable resin composition 210', the foamable resin compositions 221a', 222a', 223a'... are laminated in this order from the inside. In this manner, a multi-layered laminated pipe including the non-foamable resin composition 210' and the foamable resin compositions 221a', 222a', 223a'... is prepared in the mold 550a. The term "multi-layered" may vary depending on the thickness of the outer layer 220a to be formed, and means that the number of layers is within a range in which any two of the numbers of layers are upper and lower limits, for example, 4, 5, 6, 7, 8, 9, and 10.

[0107] A non-foamable resin composition (not shown) is further laminated on the laminate pipe to form a covering layer 230 (see FIG. 3). The non-foamable resin composition for forming the covering layer 230 may be laminated in a mold 550, or may be laminated after foaming of foamable resin compositions 221a', 222a', 223a', etc., which will be described later.

[0108] The covering layer 230 (see FIG. 3) can be formed by laminating non-foamable resin compositions as described above, but is not limited to this method. For example, when a crosshead die for molding the outermost layer of a foamable resin composition has a surface that contacts the foamable resin composition of the outermost layer as a cooling surface, the outer peripheral surface of the foamable resin composition of the outermost layer is cooled by the cooling surface, so that the vicinity of the outer peripheral surface is solidified prior to foaming.

[0109] When the laminated pipe containing the non-foamable resin composition 210' and the foamable resin compositions 221a', 222a', 223a',... is sent out of the mold 550a, the pressure therein decreases, and the foamable resin compositions 221a', 222a', 223a',... foam. Since the foamable resin composition is foamed in a state in which many layers are laminated in this manner, the layers blend with each other near their boundaries, resulting in an overall appearance in which the porosity increases gradually and continuously from the inner periphery to the outer periphery.

[0110] This makes it possible to form a gradient structure (see FIG. 4) in the porosity of the outer layer 220a (see FIG. 3) after foaming. By setting the thickness of each layer in consideration of the foaming ratio expected from the amount of foaming agent, the gradient structures shown in FIG. 4(A), FIG. 4(b), and FIG. 4(c) can be formed separately.

[0111] [Manufacturing of hollow circular tube according to the third embodiment] Fig. 16 is a schematic enlarged cross-sectional view of a die 550b in a manufacturing apparatus for the hollow circular pipe 100b. An example of the manufacturing apparatus for the hollow circular pipe 100b is based on the manufacturing apparatus 500a shown in Fig. 15, and is similar to the manufacturing apparatus 500a, except that the number of extruders for melt-kneading the foamable resin composition is three, and crosshead dies 552, 553, and 554 are connected to the three extruders, respectively.

[0112] In the three extruders that melt-knead the foamable resin composition, foamable resin compositions 221b', 222b', and 223b' are prepared, each containing a different amount of foaming gas. The foamable resin compositions 221b', 222b', and 223b' are prepared in this order so that the content of the foaming gas decreases gradually.

[0113] The extruded non-foamable resin composition 210' and the foamable resin compositions 221b', 222b', 223b' are molded into a tubular shape by a straight die 551 and crosshead dies 552, 553, 554, respectively, in a mold 550b, and are guided so as to be laminated from the inside in the order of the non-foamable resin composition 210', the foamable resin compositions 221b', 222b', 223b'. In this manner, a laminated pipe containing the non-foamable resin composition 210' and the foamable resin compositions 221b', 222b', 223b' is prepared in the mold 550b.

[0114] A non-foamable resin composition (not shown) is further laminated on the laminate pipe to form a covering layer 230 (see FIG. 5). The non-foamable resin composition for forming the covering layer 230 may be laminated in a mold 550, or may be laminated after foaming of foamable resin compositions 221b', 222b', and 223b', which will be described later.

[0115] The covering layer 230 (see FIG. 5) can be formed by laminating the non-foamable resin composition as described above, but is not limited to this method. For example, when the crosshead die 554 for molding the foamable resin composition 223b' has a surface that contacts the foamable resin composition 223b' as a cooling surface S4, the outer peripheral surface of the foamable resin composition 223b' of the outermost layer is cooled by the cooling surface S4 to solidify the vicinity of the outer peripheral surface prior to foaming.

[0116] When the laminated pipe containing the non-foamable resin composition 210' and the foamable resin compositions 221b', 222b', and 223b' is sent out of the mold 550b, the pressure decreases and the foamable resin compositions 221b', 222b', and 223b' foam. By foaming the foamable resin composition in a three-layer laminate state in this manner, the porosity decreases stepwise from the inner periphery side to the outer periphery side. This makes it possible to form a porosity gradient structure (see FIG. 6) in the outer layer 220b (FIG. 5) after foaming.

[0117] [Manufacturing of hollow circular tube according to the fourth embodiment] Fig. 17 is a schematic enlarged cross-sectional view of a die 550c in a manufacturing apparatus for the hollow circular pipe 100c. An example of the manufacturing apparatus for the hollow circular pipe 100c is based on the manufacturing apparatus 500a shown in Fig. 14, and is similar to the manufacturing apparatus 500a, except that there are two extruders for melt-kneading the foamable resin composition, the two extruders are connected to crosshead dies 552 and 553, respectively, and the surfaces of the crosshead die 552 and the crosshead die 553 that contact the outer peripheral surface of the foamable resin composition are both configured as cooling surfaces S2 and S3.

[0118] In the two extruders that melt-knead the foamable resin composition, foamable resin compositions 221c' and 222c' are prepared, each having a different amount of foaming gas. Specifically, the foamable resin composition 221c' is prepared so as to have a smaller amount of foaming gas than the foamable resin composition 222c'.

[0119] The extruded non-foamable resin composition 210' and the foamable resin compositions 221c', 222c' are molded into a tubular shape by a straight die 551 and crosshead dies 552, 553, respectively, in a mold 550c, and are guided so as to be laminated from the inside in the order of the non-foamable resin composition 210', the foamable resin compositions 221c', 222c'. In this manner, a laminated pipe containing the non-foamable resin composition 210' and the foamable resin compositions 221c', 222c' is prepared in the mold 550c.

[0120] Furthermore, in the mold 550c, the surface of the crosshead die 552 that contacts the foamable resin composition 221c' and the surface of the crosshead die 553 that contacts the foamable resin composition 222c' are configured as the cooling surface S2 and the cooling surface S3, respectively. The foamable resin compositions 221c', 222c' are cooled by the cooling surfaces S2, S3, respectively, so that a temperature gradient occurs in each layer of the foamable resin compositions 221c', 222c', with the outer periphery being lower in temperature and the inner periphery being higher in temperature. Furthermore, the outer periphery of the foamable resin composition 222c' cooled by the cooling surface S3 solidifies prior to foaming, forming the coating layer 230 (see FIG. 7).

[0121] The laminated pipe containing the non-foamable resin composition 210' and the foamable resin compositions 221c', 222c' is sent out of the mold 550c, whereby the pressure is reduced and the foamable resin compositions 221c', 222c' are foamed. This allows the formation of a gradient porosity structure (see FIG. 8) in the foamed outer layer 220b (FIG. 7).

[0122] [Manufacturing of hollow circular tube according to the fifth embodiment] Fig. 18 is a schematic diagram showing an example of a manufacturing apparatus 500d for the hollow circular pipe 100d. Fig. 19 is a schematic enlarged cross-sectional view of an elliptical portion, indicated by a dotted line in the figure, of a die 550d in the manufacturing apparatus 500d.

[0123] As shown in Fig. 18, a manufacturing apparatus 500d for a hollow circular pipe 100d includes a plurality of extruders for melt-kneading a foamable resin composition. In Fig. 18, of the plurality of extruders, only three extruders, a first extruder 540d1, a second extruder 540d2, and a third extruder 540d3, are shown, and the other extruders are omitted.

[0124] In the first extruder 540d1, the foamable resin composition 221d' is melt-kneaded, in the second extruder 540d2, the foamable resin composition 222d' is melt-kneaded, and in the third extruder 540d3, the foamable resin composition 223d' is melt-kneaded. Similarly, in an extruder not shown, the foamable resin composition 224d' is melt-kneaded. The foamable resin compositions 221d', 222d', 223d', and 224d' are prepared so that the content of the foaming gas increases gradually in this order. Furthermore, in another extruder not shown, the foamable resin compositions are melt-kneaded so that the content of the foaming gas gradually changes.

[0125] As shown in Fig. 19, the mold 550d includes a straight die 551d1 and crosshead dies 552d, 553d, and 554d. The crosshead dies 552d, 553d, and 554d are provided with vacuum slits VS2, VS3, and VS4, respectively. The mold 550d also includes other similar crosshead dies that are not shown.

[0126] In the mold 550d, the extruded foamable resin composition 221d' is guided by the straight die 551d1, and the foamable resin compositions 222d', 223d', 224d'... are molded into a tubular shape by the crosshead dies 552d, 553d, 554d..., respectively, and are laminated from the inside in the order of the foamable resin compositions 221d', 222d', 223d', 224d'.... In this way, a laminated pipe containing the foamable resin compositions 221d', 222d', 223d', 224d'... is prepared in the mold 550d.

[0127] A non-foamable resin composition (not shown) is further laminated on the laminated pipe to form the inner layer 210d and the covering layer 230 (see FIG. 9). The lamination of the non-foamable resin composition for forming the inner layer 210d may be performed in a mold 550, or may be performed after foaming of the foamable resin compositions 221d', 222d', 223d', 224d', etc., described later, is completed. The lamination of the non-foamable resin composition for forming the covering layer 230 may also be performed in a mold 550, or may be performed after foaming of the foamable resin compositions 221d', 222d', 223d', 224d', etc., described later, is completed. In the case where the lamination of the non-foamable resin composition for forming the inner layer 210d and / or the coating layer 230 is performed in a mold 550, the manufacturing apparatus 500d further includes an extruder for extruding the non-foamable resin composition, and a die for molding the non-foamable resin composition into a tubular shape so as to be laminated with the foamable resin compositions 221d', 222d', 223d', 224d', etc. described below.

[0128] The inner layer 210d and the covering layer 230 (see FIG. 9) can be formed by laminating the non-foamable resin composition as described above, but are not limited to this method. For example, at least one of the inner layer 210d and the covering layer 230 may be formed by solidifying the foamable resin composition 221d', 222d', 223d', 224d', etc. by cooling before foaming. Specifically, when the surface of the inner surface regulating portion 551d2 provided in the mold 550a that contacts the inner peripheral surface of the foamable resin composition 221d' is configured as the cooling surface S1, the inner peripheral surface of the foamable resin composition 221d' may be cooled by the cooling surface S1 to solidify the vicinity of the inner peripheral surface prior to foaming, thereby forming the inner layer 210d. In addition, when the crosshead die for molding the outermost layer of the foamable resin composition has a surface that comes into contact with the foamable resin composition of the outermost layer configured as a cooling surface, the outer peripheral surface of the foamable resin composition of the outermost layer may be cooled by the cooling surface, thereby solidifying the vicinity of the outer peripheral surface prior to foaming, thereby forming the coating layer 230.

[0129] The laminated pipe including the foamable resin compositions 221d', 222d', 223d', 224d'... is sent out of the mold 550d, whereby the pressure is reduced and the foamable resin compositions 221d', 222d', 223d', 224d'... are foamed. Since the foamable resin composition is foamed in a multi-layered state in this manner, the layers blend with each other near their boundaries, resulting in an overall appearance in which the porosity increases gradually and continuously from the inner periphery to the outer periphery. This makes it possible to form a porosity gradient structure (see FIG. 4) in the outer layer 220a (see FIG. 9) after foaming.

[0130] [Other examples] As already mentioned, the present invention includes any aspect in which the respective embodiments are appropriately combined, for example, as shown in Fig. 10. The hollow tube of this aspect can be manufactured by a person skilled in the art by appropriately combining the above-mentioned manufacturing methods.

[0131] In addition to the hollow circular tube having the above-mentioned circular cross-sectional shape, the present invention includes hollow tubes having any cross-sectional shape, such as an ellipse, a rectangle, or other polygons. The hollow tube can be produced using a forming die having a shape that can be formed into the desired cross-sectional shape, instead of the above-mentioned straight die and crosshead die.

[0132] As described above, the present invention allows a hollow tube to be manufactured by either one of the manufacturing methods of foaming a foamable resin composition extruded into a tubular shape while simultaneously creating a temperature gradient in the wall thickness direction or after creating the temperature gradient, and the manufacturing method of extruding a plurality of foamable resin compositions having different foaming agent contents so as to be layered into a tubular shape and then foaming them, or by a combination of both methods.

[0133] The present invention is not limited to the hollow tube composed of three layers, the inner layer 210, 210d and the outer layer 220, 220a, 220b, 220c, as described above. The hollow tube of the present invention may not have the inner layer 210, 210d and the covering layer 230, or may have other layers provided thereon.

[0134] The present invention is not limited to the embodiment in which the outer layers 220, 220a, 220b, and 220c are made of a foamable resin that is an organic foam, as described above. The outer layer 220, which is a foam layer, may be an inorganic foam, or a mixture of an organic foam and an inorganic foam. The inorganic foam may be appropriately selected by a person skilled in the art, and one example is a foam containing cement. In either case, when the hollow tube includes a non-foamed layer, the material of the non-foamed layer may be the same as that of the foamed layer, except that it is non-foamed, or may be different.

[0135] [Effects achieved by the embodiment and other examples] The hollow circular pipes 100, 100a, to 100d each include an outer layer 220, 220a, 220b, or 220c having a gradient structure with different porosity in the wall thickness direction, and therefore have multifunctionality that combines at least light weight and mechanical properties.

[0136] The hollow circular pipes 100a, 100c, 100d have an inclined structure in which the porosity is higher on the side farther from the axis of the outer layers 220a, 220c than on the side closer to the axis, and therefore have excellent lightweight properties. The hollow circular tubes 100, 100b, 100c have superior mechanical properties since they include a gradient structure in which the porosity is lower on the side farther from the axis of the outer layers 220, 220b, 220c than on the side closer to the axis.

[0137] The hollow circular pipes 100, 100a, -, 100d include non-foamed inner layers 210, 210d on the inner surfaces of the outer layers 220, 220a, 220b, 220c, but because the outer layers 220, 220a, 220b, 220c are lightweight and have mechanical properties, the thickness of the inner layers can be made thin like the inner layer 210d.

[0138] The outer layers 220, 220a, 220b, 220c of the hollow circular tubes 100, 100a, -, 100d can be formed by laminating multiple foamable resin compositions containing different amounts of foaming agent and then foaming them, so that the tubes can be manufactured in a manner that the porosity varies in the thickness direction.

[0139] The hollow circular pipes 100, 100a, 100b, 100d of the present invention can be manufactured by laminating three or more types of foamable resin compositions containing different amounts of foaming agent, and therefore have a variety of porosity gradients, such as the outer layers 220, 220a, 200b.

[0140] The hollow circular tube 100 can be formed by laminating and foaming resin compositions containing different amounts of foaming agent in a number of layers of 4 to 10, so that the foaming agent content is lower in the layers farther from the axis than in the layers closer to the axis. In this way, the outer layer 220 can be formed in such a manner that the porosity gradually decreases from the side closer to the axis to the side farther from the axis. The hollow spaces 100a, 100d can be formed by laminating and foaming resin compositions having different amounts of foaming agent in a number of layers of 4 to 10, so that the foaming agent content is greater in the layers farther from the axis than in the layers closer to the axis, such that the porosity of the outer layer 220 gradually increases from the side closer to the axis to the side farther from the axis.

[0141] In the method for manufacturing hollow circular pipes using the manufacturing apparatuses 500, 500a, and 500d, the foaming agent is carbon dioxide gas, so recovery of the foaming agent is not required and no by-products are generated due to foaming, thereby enabling the production to be carried out in a long run.

[0142] In the manufacturing apparatus 500a, a first extruder 540a1 that extrudes the foamable resin compositions 221a', 221b', 221c' and a second extruder 540a2 that extrudes the foamable resin compositions 222a', 222b', 222c' are each separately connected to a carbon dioxide gas cylinder 510 and a metering pump 520. Therefore, the carbon dioxide gas content of the foamable resin compositions 221a', 221b', 221c' and the foamable resin compositions 222a', 222b', 222c' can be made different, and outer layers 220a, 220b, 220c having a gradient structure with different porosity in the thickness direction can be formed.

[0143] In the manufacturing apparatus 500d, a first extruder 540d1 for extruding the foamable resin composition 221d', a second extruder 540d2 for extruding the foamable resin composition 222d', and a third extruder 540d3 for extruding the foamable resin composition 223d' are each separately connected to a carbon dioxide gas cylinder 510 and a metering pump 520. Therefore, the carbon dioxide gas contents of the foamable resin composition 221d', the foamable resin composition 222d', and the foamable resin composition 223d' can be made different, and an outer layer 220d having a gradient structure with different porosity in the thickness direction can be formed. EXAMPLES

[0144] [Example 1] In this example, a hollow circular pipe was manufactured that was composed of an inner layer made of a hard non-foaming resin, an outer layer made of a foaming resin laminated on the inner layer, and a coating layer made of a hard non-foaming resin laminated on the outer layer. Sodium bicarbonate was used as the foaming agent, and the outer layer was formed by co-extrusion lamination of four types of resin compositions with different amounts of sodium bicarbonate, with the sodium bicarbonate content increasing with increasing distance from the axis.

[0145] (Determining the amount of baking soda) A resin composition containing 2.20 parts by weight of sodium bicarbonate per 100 parts by weight of polyvinyl chloride resin was prepared in advance. The foam obtained by foaming this resin composition had an expansion ratio of 5 times. Furthermore, the amount of sodium bicarbonate used and the density of the polyvinyl chloride resin (1.44 g / cm 3 ) and the foaming efficiency of the resin composition calculated from the density of the obtained foam was 53% (volume basis). By proportional calculation based on the above results, the amount of baking soda to achieve an expansion ratio of 2 times is calculated to be 0.88 parts by weight, the amount of baking soda to achieve an expansion ratio of 3 times is calculated to be 1.32 parts by weight, the amount of baking soda to achieve an expansion ratio of 4 times is calculated to be 1.76 parts by weight, the amount of baking soda to achieve an expansion ratio of 5 times is calculated to be 2.20 parts by weight, and the amount of baking soda to achieve an expansion ratio of 7 times is calculated to be 3.08 parts by weight.

[0146] (Making hollow circular tubes) A resin composition for 2 times expansion containing 0.88 parts by weight of sodium bicarbonate (i), a resin composition for 3 times expansion containing 1.32 parts by weight of sodium bicarbonate (ii), a resin composition for 4 times expansion containing 1.76 parts by weight of sodium bicarbonate (iii), a resin composition for 5 times expansion containing 2.20 parts by weight of sodium bicarbonate (iv), and a resin composition for 7 times expansion containing 3.08 parts by weight of sodium bicarbonate (v) were prepared based on 100 parts by weight of polyvinyl chloride resin, and each was pelletized using a 40 mmφ extruder. Furthermore, a polyvinyl chloride resin composition (I) not containing sodium bicarbonate was also prepared.

[0147] A separate 40 mmφ extruder was used for each resin composition, and the rotation speed and cylinder temperature of the extruder were set to the same settings. The resin compositions (I), (i), (ii), (iii), (iv), (v), and (I) were laminated from the axis in this order by co-extrusion into a mold, and then foaming was performed outside the mold. FIG. 20 shows an X-ray CT image of a cross section of the obtained hollow circular tube wall cut along a plane including the axis. In FIG. 20, the white parts are resin. As shown in FIG. 20, the hollow circular tube produced in this example was obtained in a manner in which the lamination boundary between the multiple resin compositions was not clear, and the foamed layer sandwiched between the non-foamed resin layers was formed so that the porosity was higher on the side farther from the axis.

[0148] [Example 2] The resin composition for 7 times expansion (v), the resin composition for 5 times expansion (iv), and the resin composition for 3 times expansion (ii) were layered in this order from the axis center using separate extruders for each resin composition, and were co-extruded into a common die while adjusting the temperature conditions so that the resin temperatures were equivalent for each composition.

[0149] The extrusion rate ratio of each foaming resin composition was set to the reciprocal ratio of the expansion ratio (i.e., 7-fold foaming resin composition (v): 5-fold foaming resin composition (iv): 3-fold foaming resin composition (ii) = 1 / 7:1 / 3:1 / 5 (volume basis)), and the take-up speed was adjusted so that the overall thickness after foaming outside the mold was 6.5 mm.

[0150] The hollow circular tube produced in this example was obtained in a state in which the lamination boundaries of the multiple resin compositions were not clearly defined. The weight per unit length was 0.15 kg / m.

[0151] [Comparative Example 1] A hollow circular tube was prepared by laminating three layers of the 5-fold foaming resin composition (iv). The weight per unit length of the resulting hollow circular tube was 0.10 kg / m.

[0152] <Bending pipe test> Example 2 The hollow circular pipe obtained in Comparative Example 1 was subjected to the following bending pipe test to measure the strain that allows bending pipe construction. In the bending pipe test, a 4m sample pipe was fixed at three points, 50 cm from one end of the sample pipe (position A), 50 cm from the axial midpoint of the sample pipe (position B), and 50 cm from the other end of the sample pipe (position C), with three pipe fasteners, and positions A and C were fixed on a reference line, and position B was moved so that the distance from the reference line was 5 cm apart, to confirm the limit of the distance from the reference line that allows pipe construction. In the bending pipe test, it was determined that cracks would occur if the distance from the reference line exceeded the limit. Table 1 shows the test results (average value of three tests). In Table 1, the mobility rate is a relative amount with the limit of the distance in Comparative Example 1 as the reference (100%).

[0153] [Table 1]

[0154] As shown in Table 1, the results of the bending pipe test were good for the sample pipe of Example 2, which had a foaming ratio gradient in which the axial center side was sparse and the outer circumferential surface side was dense.

[0155] [Example 3] The polyvinyl chloride resin composition not containing sodium bicarbonate (I), the resin composition for 2-fold expansion (i), the resin composition for 3-fold expansion (ii), the resin composition for 5-fold expansion (iv) and the polyvinyl chloride resin composition not containing sodium bicarbonate (I) were layered in this order from the axis center using separate extruders for each resin composition, and were co-extruded into a common die while adjusting the temperature conditions so that the resin temperatures were equivalent for each composition.

[0156] The extrusion rate ratio of each foaming resin composition was set to the reciprocal ratio of the expansion ratio (i.e., 2-fold foaming resin composition (i): 3-fold foaming resin composition (ii): 5-fold foaming resin composition (iv) = 1 / 2: 1 / 3: 1 / 5 (volume basis)), and the take-up speed was adjusted so that the overall thickness after foaming outside the mold was 6.5 mm. The hard layer on the axial center side and the hard layer on the outer periphery side formed from the polyvinyl chloride resin composition (I) not containing sodium bicarbonate were 2 mm and 0.5 mm, respectively.

[0157] The hollow circular tube produced in this example was obtained in a state in which the lamination boundaries of the multiple resin compositions were not clearly defined. The weight per unit length was 0.45 kg / m.

[0158] [Comparative Example 2] A hollow circular tube was produced in the same manner as in Example 3, except that the layering order from the axis was polyvinyl chloride resin composition (I) not containing sodium bicarbonate, resin composition for 3-fold expansion (ii), resin composition for 3-fold expansion (ii), resin composition for 3-fold expansion (ii) and polyvinyl chloride resin composition (I) not containing sodium bicarbonate, and the extrusion rate ratio of the resin composition for 3-fold expansion (ii) was the same.

[0159] The overall thickness after foaming was 6.5 mm, and the hard layer on the axial center side and the hard layer on the outer periphery side formed from the polyvinyl chloride resin composition (I) not containing sodium bicarbonate were 2 mm and 0.5 mm, respectively. Furthermore, the weight per unit length was 0.46 kg / m.

[0160] <Various physical property evaluations> As described above, the thickness of the foam layer of the sample tubes of Example 3 and Comparative Example 2 is 4 mm. In this case, the calculation results showed that the thermal conductivity was approximately the same (0.09 W / (mK)). Furthermore, when each sample tube was subjected to a condensation performance test, it was confirmed that the condensation performance was also equivalent.

[0161] Furthermore, an internal pressure test was performed on each sample tube. In the internal pressure test, one end of the sample tube was sealed, water pressure was applied to the inside, and the internal pressure (water resistance pressure) required to break the sample tube was measured. The results of the internal pressure test (performed three times) are shown in Table 2 below.

[0162] [Table 2]

[0163] As shown in Table 2, although the weight and condensation performance of the sample tubes were almost the same, the water pressure resistance of the sample tube of Example 3, in which the foam layer had a foaming rate gradient from dense toward the axial center to sparse toward the outer periphery, was better.

[0164] [Correspondence between each part in the embodiment and other examples and each component in the claims] Hollow circular pipes 100, 100a, -, 100d correspond to "hollow pipes", outer layers 220, 220a, 220b, 220c correspond to "tubular foams", inner layers 210, 210d correspond to "inner layers", coating layer 230 corresponds to "coating layer", manufacturing apparatuses 500a, 500d correspond to "hollow pipe manufacturing apparatus", foamable resin compositions 221a', 221b', 221c', 221d' correspond to "first foamable material", and foamable resin compositions 222a', 222b', 222c', 222d' correspond to the "second foamable material", first extruders 540a1, 540d1 correspond to the "first extruder", second extruders 540a2, 540d2 correspond to the "second extruder", carbon dioxide cylinder 510 and metering pump 520 correspond to the "first gas supply section" and "second gas supply section", crosshead die 552 and straight die 551d1 correspond to the "first forming section", and crosshead dies 553, 552d correspond to the "second forming section".

[0165] Although some embodiments of the present invention are as described above, various other embodiments can be made without departing from the spirit of the present invention. Therefore, these embodiments are merely illustrative in all respects and should not be interpreted as being limited. Furthermore, all modifications within the scope of the claims are within the scope of the present invention. [Explanation of symbols]

[0166] 100,100a,~,100d Hollow circular tube 210,210d Inner layer 220,220a,220b,220c outer layer 230 Covering layer 500a, 500d Hollow tube manufacturing equipment 221a', 221b', 221c', 221d' Foamable resin composition 222a', 222b', 222c', 222d' Foamable resin composition 540a1, 540d1 First extruder 540a2, 540d2 Second extruder 510 Carbon Dioxide Cylinder 520 Metering Pump 552,553,552d Crosshead Die 551d1 Straight die

Claims

1. A hollow tube having a circular cross section formed by co-extrusion lamination of a foamed layer and an inner layer, the inner layer is made of a polyvinyl chloride resin different from the foam layer, the foam layer is an outer layer made of a polyvinyl chloride resin, has a thickness of 4 mm or more and 15 mm or less, and is laminated on the outer peripheral side of the inner layer by coextrusion; The thickness of the inner layer is 30% or more and 70% or less of the thickness of the foamed layer.

2. A coating layer is laminated on the outside of the outer layer, The hollow circular pipe according to claim 1 , wherein the coating layer has a thickness smaller than that of the inner layer.

3. 3. The hollow circular tube according to claim 2, wherein the coating layer has a wall thickness of 0.1 mm or more and 1.5 mm or less.

4. 3. The hollow circular pipe according to claim 2, wherein the thickness of the covering layer is 20% to 30% of the thickness of the foamed layer.

5. A coating layer is laminated on the outside of the outer layer, 5. The hollow circular pipe according to claim 1, wherein said covering layer and said outer layer are made of the same vinyl chloride resin.

6. 6. The hollow circular pipe according to claim 1, wherein the inner layer contains a rubber component.

7. The inner layer is formed of a foamable resin composition, The foamable resin composition contains a polyvinyl chloride resin and a foaming agent.

7. The hollow circular tube according to claim 1.

8. A method for manufacturing a hollow circular tube according to any one of claims 1 to 7, comprising the steps of: A lamination process of laminating at least a first foamable material containing a predetermined amount of a foaming agent and a second foamable material containing an amount of a foaming agent different from the predetermined amount into a tubular shape to obtain a tubular laminate of foamable materials containing at least the first foamable material and the second foamable material; and a foaming step of foaming a tubular laminate of the first foamable material and the second foamable material to obtain the foam layer, The second foamable material is laminated on the first foamable material to a thickness of 4 mm or more and 15 mm or less. A method for manufacturing hollow tubes.

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