Air brake tubes

Abstract

To provide an air brake tube that can strike a balance between pressure resistance under a high temperature environment and laying workability with respect to a vehicle.SOLUTION: An air brake tube 10A has a multilayer structure, a hoop stress of 9.72 MPa or more under the 125°C environment and a tensile elasticity of less than 650 MPa at 23°C environment.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a tube for an air brake. [Background technology]

[0002] Metal piping and polyamide resin tubing are used in air brake systems used in trucks, buses, etc. The use of polyamide resin tubing is increasing from the viewpoints of weight reduction and ease of installation work.

[0003] Air brake tubing is used to distribute high-pressure air in high-temperature environments, so it must be heat-resistant and pressure-resistant. The German industrial standard DIN 74324 specifies the maximum operating temperature of 100°C and the maximum operating pressure of 1250 kPa, which are the standards with the strictest heat and pressure resistance requirements. In recent years, temperatures around air brake tubing have been rising, and the international industrial standard ISO 7628, which is designed to accommodate even higher temperature environments, specifies a maximum operating temperature of 125°C and a maximum operating pressure of 1250 kPa.

[0004] In order to improve pressure resistance at the maximum use temperature specified by ISO 7628, a multilayer tube has been proposed (Patent Document 1) that has an innermost layer made of polyamide resin, an outer layer made of polyester elastomer, and an adhesive layer provided between the innermost layer and the outer layer. In Patent Document 1, the outer layer is made of polyester elastomer, which has a higher melting point than the polyamide resin specified as a material for tubes in DIN 74324, thereby improving pressure resistance in high-temperature environments. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 08-247344 Summary of the Invention [Problem to be solved by the invention]

[0006] The polyester-based elastomer disclosed in Patent Document 1 has higher rigidity than polyamide-based resins commonly used for air brake tubing. Therefore, a multilayer tube including a layer of polyester-based elastomer as disclosed in Patent Document 1 has lower flexibility than a single-layer tube made of polyamide-based resin. As a result, installation work in vehicles such as trucks and buses becomes significantly more difficult. Conventionally, single-layer tubes made of polyamide-based resins conforming to DIN 74324, which are commonly used for air brake tubing, cannot meet the pressure resistance at 125°C specified in Category 3 of ISO 7628. On the other hand, the pressure resistance at 125°C specified in Category 3 of ISO 7628 can be met by using a polyamide-based resin with higher rigidity than the polyamide-based resins conforming to DIN 74324. However, the use of a highly rigid polyamide-based resin makes installation work in vehicles significantly more difficult.

[0007] An object of the present invention is to provide an air brake tube that is both pressure-resistant in a high-temperature environment and easy to install in a vehicle. [Means for solving the problem]

[0008] The air brake tube according to the present invention has a multi-layer structure, and has a hoop stress of more than 9.72 MPa in a 125°C environment and a tensile modulus of elasticity of less than 650 MPa in a 23°C environment. [Effects of the Invention]

[0009] According to the present invention, the hoop stress at 125°C is greater than 9.72 MPa, so the tube has higher pressure resistance at 125°C than that of a single-layer tube made of polyamide resin conforming to DIN 74324. Furthermore, the tensile modulus at 23°C is less than 650 MPa, so the tube has higher flexibility at 23°C than that of a single-layer tube made of highly rigid polyamide resin that meets the pressure resistance at 125°C specified in Category 3 of ISO 7628. Therefore, it is possible to achieve both pressure resistance in high-temperature environments and ease of installation into vehicles. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing an air brake tube according to a first embodiment. [Figure 2] FIG. 6 is a cross-sectional view showing an air brake tube according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Each embodiment shown below is an example of the present invention, and the present invention is not limited thereto. In the following description of each embodiment, similar components are designated by similar reference numerals, and description thereof will be omitted as appropriate.

[0012] 1. First embodiment 1 shows a cross-sectional view of an air brake tube 10A according to a first embodiment. The air brake tube 10A has a multi-layer structure. The multi-layer structure includes a hollow inner layer 12 and an outer layer 14A disposed outside the inner layer 12.

[0013] The materials of the inner layer 12 and the outer layer 14A are selected so that the hoop stress of the air brake tube 10A in a 125°C environment is greater than 9.72 MPa, and the tensile modulus of the air brake tube 10A in a 23°C environment is less than 650 MPa.

[0014] Here, the indices of pressure resistance and flexibility will be described in detail.

[0015] <Pressure resistance> The pressure resistance index is determined based on the burst pressure of ISO 7628, a standard for use in higher temperature environments. For ISO 7628 Category 3, the burst pressure is set at 2.5 MPa at 125°C, the maximum operating temperature. The burst pressure of 2.5 MPa is converted to hoop stress using the hoop stress conversion formula specified in DIN 73378. The converted hoop stress is the pressure resistance index.

[0016] DIN73378 specifies the hoop stress conversion formula as follows: σ=p (ds) / (2 s) (Equation 1) σ: Hoop stress (MPa) p: burst pressure (MPa) d: Tube outer diameter (mm) s: Tube wall thickness (mm)

[0017] ISO 7628 and DIN 74324 specify the outer diameter and wall thickness of tubes, as well as their dimensional tolerances, for each nominal tube diameter. For commonly used tube nominal diameters, the hoop stress σ can be calculated using the DIN 73378 hoop stress conversion formula from the outer diameter and wall thickness that are most susceptible to fracture within the dimensional tolerances. The hoop stress σ corresponding to a burst pressure of 2.5 MPa is 9.72 MPa. In other words, a hoop stress of 9.72 MPa is the standard value for determining whether or not a tube satisfies pressure resistance requirements.

[0018] <Flexibility> The tensile modulus, which indicates the resistance to deformation, is used as an index of flexibility. The lower the tensile modulus, the higher the flexibility. The tensile modulus, which is the standard value for determining whether flexibility is satisfactory, is the measurement result of the tensile modulus of a single-layer tube made of polyamide 1010 (PA1010) resin that satisfies the pressure resistance specified as PHLY in DIN 74324.

[0019] To measure the tensile modulus, a sample of a single-walled tube made of polyamide 1010 resin that meets the pressure resistance specified as PHLY in DIN 74324 was cut to a length of 170 mm and left to stand for at least 24 hours in an environment of 23°C / 50% RH, and then set in a tensile testing device specified in Japanese Industrial Standard JIS K7161-1 so that the distance between the grips was 115 mm. A tensile test was started in an environment of 23°C / 50% RH in accordance with JIS K7161-1 at a test speed of 1 mm / min, and the increase in tensile load and the distance between the grips were measured.

[0020] The tensile stress is calculated from the measured tensile load and the initial cross-sectional area of ​​the tube sample using the following tensile stress calculation formula specified in JIS K7161-1. σ=F / A (Equation 2) σ: Tensile stress (MPa) F: Tensile load (N) A: initial cross-sectional area of ​​the tube sample (mm 2 )

[0021] In addition, the nominal strain is calculated from the measured increase in the grip distance and the initial grip distance using the following nominal strain calculation formula specified in JIS K7161-1. εt=Lt / L (Formula 3) εt: Nominal strain (dimensionless) L: Initial grip distance (mm) Lt: Increase in distance between grippers (mm)

[0022] The tensile modulus is calculated from the obtained tensile stress and nominal strain using the following tensile modulus calculation formula specified in JIS K7161-1. Et=(σ2-σ1) / (ε2-ε1) (Equation 4) Et: Tensile modulus (MPa) σ1: Tensile stress (MPa) when nominal strain ε1 is 0.0005 σ2: Tensile stress (MPa) when nominal strain ε2 is 0.0025

[0023] The tensile modulus of elasticity at 23°C / 50%RH for a single-layer tube made of polyamide 1010 resin that satisfies the pressure resistance specified as PHLY in DIN 74324 is 650 MPa. In other words, a tensile modulus of elasticity of 650 MPa is the standard value for whether flexibility is satisfied.

[0024] As described above, the reference value for pressure resistance is a hoop stress of 9.72 MPa, and the reference value for flexibility is a tensile modulus of elasticity of 650 MPa. The materials for the inner layer 12 and the outer layer 14A of the air brake tube 10A are selected so that the hoop stress of the air brake tube 10A at 125°C is greater than 9.72 MPa and the tensile modulus of elasticity of the air brake tube 10A at 23°C is less than 650 MPa.

[0025] In this embodiment, the inner layer 12 is made of an unplasticized polyamide resin, and the outer layer 14A is made of a plasticized polyamide resin. As a result, the pressure resistance of the outer layer 14A, which is made of a plasticized polyamide resin, is complemented by the unplasticized polyamide resin of the inner layer 12, and the flexibility of the inner layer 12, which is made of an unplasticized polyamide resin, is complemented by the plasticized polyamide resin of the outer layer 14A.

[0026] By using an unplasticized polyamide resin for the inner layer 12, it is possible to increase the pressure resistance while reducing the thickness of the inner layer 12 compared to when the outer layer 14A is made of an unplasticized polyamide resin. Therefore, the thickness of the plasticized polyamide resin in the outer layer 14A can be increased, and the effect of the plasticized polyamide resin in providing flexibility can be enhanced.

[0027] If the ratio of the thickness of the inner layer 12 to the total thickness of the inner layer 12 and the outer layer 14A (hereinafter simply referred to as the "thickness ratio of the inner layer 12") is too large, the flexibility of the unplasticized polyamide resin of the inner layer 12 will not be complemented by the plasticized polyamide resin of the outer layer 14A, resulting in an insufficient flexibility. Therefore, the thickness ratio of the inner layer 12 is set so that the flexibility of the unplasticized polyamide resin of the inner layer 12 is complemented by the plasticized polyamide resin of the outer layer 14A. Note that if the thickness ratio of the inner layer 12 is less than 5%, it will be difficult to stably mold the inner layer 12, so the thickness ratio of the inner layer 12 is preferably 5% or more.

[0028] If the hoop stress at 125°C of the unplasticized polyamide resin of the inner layer 12 when formed into a single-layer tube is too high, the plasticized polyamide resin of the outer layer 14A will not be able to fully complement the flexibility of the unplasticized polyamide resin of the inner layer 12, resulting in an insufficient flexibility. Similarly, if the hoop stress at 125°C of the plasticized polyamide resin of the outer layer 14A when formed into a single-layer tube is too high, the plasticized polyamide resin of the outer layer 14A will not be able to fully complement the flexibility of the unplasticized polyamide resin of the inner layer 12, resulting in an insufficient flexibility. Furthermore, if the hoop stress at 125°C of the unplasticized polyamide resin of the inner layer 12 when formed into a single-layer tube is too low, the unplasticized polyamide resin of the inner layer 12 will not be able to fully complement the pressure resistance of the plasticized polyamide resin of the outer layer 14A, resulting in an insufficient pressure resistance. Therefore, the hoop stress at 125°C when made into a single-layer tube is set so that the flexibility of the unplasticized polyamide resin of the inner layer 12 is complemented by the plasticized polyamide resin of the outer layer 14A, and the pressure resistance of the plasticized polyamide resin of the outer layer 14A is complemented by the unplasticized polyamide resin of the inner layer 12.

[0029] In polyamide-based resins, the stress reduction from room temperature to high temperatures depends on the melting point or deflection temperature under load. Specifically, the lower the melting point or deflection temperature under load, the greater the stress reduction from room temperature to high temperatures. Therefore, if the melting point or deflection temperature under load of the polyamide-based resin used for the inner layer 12 and the outer layer 14A is too low, the stress reduction from 23°C to 125°C will prevent the pressure resistance in high-temperature environments from being satisfied. Therefore, the melting point or deflection temperature under load is set so that the stress reduction of the inner layer 12 and the outer layer 14A from 23°C to 125°C is small.

[0030] The deflection temperature under load specified in ISO 75 can be used. ISO 75 is a test method for evaluating the heat resistance of synthetic resins. ISO 75 specifies the deflection temperature under load as the temperature at which the increase in bending strain becomes 0.2% when a bending stress of 1.80 MPa is applied to a test piece and the temperature is raised at a rate of 120°C / hr.

[0031] In the air brake tube (10A), the wall thickness of the inner layer (12) made of unplasticized polyamide resin is 5% or more and 40% or less, the inner layer (12) is made of an unplasticized polyamide resin that, when made into a single-layer tube, will have a hoop stress of more than 9.72 MPa and not more than 25 MPa at 125°C, and the outer layer (14A) is made of a plasticized polyamide resin that, when made into a single-layer tube, will have a hoop stress of 9.72 MPa or less at 125°C. The unplasticized polyamide resin of the inner layer (12) and the plasticized polyamide resin of the outer layer (14A) have melting points of 190°C or more or a deflection temperature under load of 50°C or more when subjected to a bending stress of 1.80 MPa in accordance with ISO 75, so that the hoop stress of the air brake tube (10A) at 125°C is more than 9.72 MPa and the tensile modulus of the air brake tube (10A) at 23°C is less than 650 MPa. Therefore, it is possible to achieve both pressure resistance in a high-temperature environment and ease of installation in a vehicle.

[0032] 2. Second embodiment Figure 2 shows a cross-sectional view of an air brake tube 10B according to a second embodiment. The air brake tube 10B differs from the air brake tube 10A (see Figure 1) in that the outer layer 14B includes a first outer layer 16 provided on the outside of the inner layer 12 and a second outer layer 18 provided on the outside of the first outer layer 16. In other words, the air brake tube 10B has a three-layer structure consisting of the inner layer 12, the first outer layer 16, and the second outer layer 18.

[0033] In the air brake tube 10B, the materials of the inner layer 12, the first outer layer 16, and the second outer layer 18 are also selected so that the hoop stress of the air brake tube 10B in a 125°C environment is greater than 9.72 MPa and the tensile modulus of elasticity of the air brake tube 10B in a 23°C environment is less than 650 MPa.

[0034] In the air brake tube 10B, the proportion of the wall thickness of the inner layer 12 made of unplasticized polyamide resin is 5% or more and 40% or less, the inner layer 12 is made of an unplasticized polyamide resin that will have a hoop stress of more than 9.72 MPa and not more than 25 MPa when made into a single-layer tube at 125°C, and the outer layer 14B is made of a plasticized polyamide resin that will have a hoop stress of 9.72 MPa or less when made into a single-layer tube at 125°C, and the unplasticized polyamide resin of the inner layer 12 and the plasticized polyamide resin of the outer layer 14B have melting points of 190°C or more or a deflection temperature under load of 50°C or more when subjected to a bending stress of 1.80 MPa in accordance with ISO 75, so that the hoop stress of the air brake tube 10B in a 125°C environment is more than 9.72 MPa and the tensile modulus of the air brake tube 10B in a 23°C environment is less than 650 MPa. Therefore, it is possible to achieve both pressure resistance in a high-temperature environment and ease of installation in a vehicle.

[0035] Furthermore, for example, an inexpensive plasticized polyamide resin with poor chemical resistance and abrasion resistance can be used for the first outer layer 16, and a plasticized polyamide resin with better chemical resistance and abrasion resistance than the first outer layer 16 can be used for the second outer layer 18. In this case, the chemical resistance, abrasion resistance, and other properties can be improved while keeping the cost of the air brake tube 10B low. In this way, the air brake tube 10B allows for greater freedom in selecting the plasticized polyamide resin used for the outer layer 14B, enabling a wide range of designs.

[0036] 3. Working Example The present invention will be described in detail below with reference to examples. The pressure resistance and flexibility of the air brake tubes obtained in the following examples and comparative examples were evaluated as follows.

[0037] <Pressure resistance evaluation> The tube sample is left to stand in a 23°C / 50% RH environment for at least one hour, then placed in 125°C oil for 10 minutes, at which point the breakdown test begins. The tube sample is broken over 30 to 60 seconds, and the maximum breakdown pressure is measured. The hoop stress in a 125°C environment is calculated from the measured maximum breakdown pressure using the hoop stress conversion formula (see Equation 1) specified in DIN 73378.

[0038] The calculated hoop stress is the pressure resistance in an environment of 125°C. If the calculated hoop stress is greater than 9.72 MPa, it is marked as "Good", and if the calculated hoop stress is 9.72 MPa or less, it is marked as "Poor".

[0039] <Flexibility evaluation> A tube sample cut to a length of 170 mm is left to stand in an environment of 23°C / 50% RH for at least 24 hours, and then set in a tensile testing device specified in JIS K7161-1 so that the distance between the grips is 115 mm. In an environment of 23°C / 50% RH, a tensile test is started at a test speed of 1 mm / min in accordance with JIS K7161-1, and the tensile load and the increase in the distance between the grips are measured.

[0040] The tensile stress is calculated from the measured tensile load and the initial cross-sectional area of ​​the tube sample using the tensile stress calculation formula (see Equation 2) specified in JIS K7161-1. The nominal strain is calculated from the measured increase in the grip distance and the initial grip distance using the nominal strain calculation formula (see Equation 3) specified in JIS K7161-1. The tensile modulus is calculated from the obtained tensile stress and nominal strain using the tensile modulus calculation formula (see Equation 4) specified in JIS K7161-1.

[0041] The calculated tensile modulus is flexibility in an environment of 23°C. If the calculated tensile modulus is less than 650 MPa, it is marked as "Good", and if the calculated tensile modulus is 650 MPa or more, it is marked as "Poor".

[0042] <Example I and Comparative Example I> In Example I and Comparative Example I, air brake tubes with an outer diameter of 8 mm and a wall thickness of 1 mm were molded and evaluated for pressure resistance in a 125°C environment and flexibility in a 23°C environment. To distinguish between resin materials of the same polyamide type but with different 125°C hoop stresses, the numbers for each polyamide resin were suffixed with "-A," "-B," or "-C." This also applies to Example II, Comparative Example II, Example III, Comparative Example III, and Reference Example, which will be described later.

[0043] Example I-1 is an air brake tube having a two-layer structure with an inner layer made of unplasticized polyamide 612 resin and an outer layer made of plasticized polyamide 612-A resin, with the inner and outer layers having thicknesses of 0.25 mm and 0.75 mm, respectively (25% of the inner layer thickness). Example I-2 is an air brake tube having a two-layer structure with an inner layer made of unplasticized polyamide 6-A resin and an outer layer made of plasticized polyamide 612-A resin, with the inner and outer layers having thicknesses of 0.4 mm and 0.6 mm, respectively (40% of the inner layer thickness). Example I-3 is an air brake tube having a three-layer structure in which the inner layer is made of unplasticized polyamide 612 resin, the first outer layer is made of plasticized polyamide 612-B resin, and the second outer layer is made of plasticized polyamide 612-A resin, with the inner layer, first outer layer, and second outer layer having thicknesses of 0.2 mm, 0.2 mm, and 0.6 mm, respectively (20% of the inner layer thickness).Example I-4 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-A resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner layer and outer layer having thicknesses of 0.1 mm and 0.9 mm, respectively (10% of the inner layer thickness).

[0044] Comparative Example I-1 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 612 resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layers having wall thicknesses of 0.5 mm and 0.5 mm, respectively (50% of the inner layer's wall thickness). Comparative Example I-2 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-B resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layers having wall thicknesses of 0.1 mm and 0.9 mm, respectively (10% of the inner layer's wall thickness). Comparative Example I-3 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 6-B resin and the outer layer is made of plasticized polyamide 610-A resin, with the inner and outer layers having wall thicknesses of 0.4 mm and 0.6 mm, respectively (40% of the inner layer's wall thickness). Comparative Example I-4 is a single-layer air brake tube made of polyamide 1010 resin that satisfies the pressure resistance specified by PHLY in DIN 74324. Comparative Example I-5 is a single-layer air brake tube made of plasticized polyamide 612-A resin. Comparative Example I-6 is an air brake tube with a two-layer structure, with an inner layer made of unplasticized polyamide 6-C resin and an outer layer made of plasticized polyamide 12 resin, the inner and outer layers having wall thicknesses of 0.4 mm and 0.6 mm (40% of the inner layer's wall thickness), respectively. Comparative Example I-7 is a single-layer air brake tube made of plasticized polyamide 12 resin that satisfies the PHLY specified by DIN 74324.

[0045] The evaluation results of the pressure resistance and flexibility of the air brake tubes of Example I and Comparative Example I are shown in Table 1. In Table 1, the hoop stress shown in the columns for inner layer and outer layer (first outer layer and second outer layer) is the hoop stress measured in a 125°C environment when each material is made into a single-layer tube with an outer diameter of 8 mm and a wall thickness of 1 mm. Since hoop stress is a property determined only by the material and does not depend on the tube size, the same applies to Tables 2, 3, and 4 described below. The deflection temperature under load is the deflection temperature under load when a bending stress of 1.80 MPa is applied in accordance with ISO 75. The same applies to Tables 2, 3, and 4 described below.

[0046] [Table 1]

[0047] <Example II and Comparative Example II> In Example II and Comparative Example II, air brake tubes having an outer diameter of 10 mm and a wall thickness of 1.25 mm were molded, and the pressure resistance in a 125°C environment and the flexibility in a 23°C environment were evaluated.

[0048] Example II-1 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 612 resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having thicknesses of 0.31 mm and 0.94 mm, respectively (the inner layer having a thickness of 25%). Example II-2 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 6-A resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having thicknesses of 0.5 mm and 0.75 mm, respectively (the inner layer having a thickness of 40%). Example II-3 is an air brake tube having a three-layer structure in which the inner layer is made of unplasticized polyamide 612 resin, the first outer layer is made of plasticized polyamide 612-B resin, and the second outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner layer, first outer layer, and second outer layer being 0.25 mm, 0.25 mm, and 0.75 mm, respectively (20% of the inner layer thickness). Example II-4 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-A resin and the outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner layer and outer layer being 0.12 mm and 1.13 mm, respectively (10% of the inner layer thickness).

[0049] Comparative Example II-1 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 612 resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having wall thicknesses of 0.62 mm and 0.63 mm, respectively (50% of the inner layer's wall thickness). Comparative Example II-2 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-B resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having wall thicknesses of 0.12 mm and 1.13 mm, respectively (10% of the inner layer's wall thickness). Comparative Example II-3 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 6-B resin and the outer layer is made of plasticized polyamide 610-A resin, with the inner and outer layer having wall thicknesses of 0.5 mm and 0.75 mm, respectively (40% of the inner layer's wall thickness). Comparative Example II-4 is a single-layer air brake tube made of polyamide 1010 resin that satisfies the pressure resistance specified as PHLY in DIN 74324. Comparative Example II-5 is a single-layer air brake tube made of plasticized polyamide 612-A resin. Comparative Example II-6 is an air brake tube with a two-layer structure, with an inner layer made of unplasticized polyamide 6-C resin and an outer layer made of plasticized polyamide 12 resin, the inner and outer layers having wall thicknesses of 0.5 mm and 0.75 mm (40% of the inner layer thickness), respectively. Comparative Example II-7 is a single-layer air brake tube made of plasticized polyamide 12 resin that satisfies the PHLY specified in DIN 74324.

[0050] Table 2 shows the evaluation results of the pressure resistance and flexibility of the air brake tubes of Example II and Comparative Example II.

[0051] [Table 2]

[0052] Example III and Comparative Example III In Example III and Comparative Example III, air brake tubes having an outer diameter of 12 mm and a wall thickness of 1.5 mm were molded, and the pressure resistance in a 125°C environment and the flexibility in a 23°C environment were evaluated.

[0053] Example III-1 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 612 resin and the outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner and outer layers being 0.38 mm and 1.12 mm, respectively (25% of the inner layer thickness). Example III-2 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 6-A resin and the outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner and outer layers being 0.6 mm and 0.9 mm, respectively (40% of the inner layer thickness). Example III-3 is an air brake tube having a three-layer structure in which the inner layer is made of unplasticized polyamide 612 resin, the first outer layer is made of plasticized polyamide 612-B resin, and the second outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner layer, first outer layer, and second outer layer being 0.3 mm, 0.3 mm, and 0.9 mm, respectively (20% of the inner layer thickness). Example III-4 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-A resin and the outer layer is made of plasticized polyamide 612-A resin, with the thicknesses of the inner layer and outer layer being 0.15 mm and 1.35 mm, respectively (10% of the inner layer thickness).

[0054] Comparative Example III-1 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 612 resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having wall thicknesses of 0.75 mm and 0.75 mm, respectively (50% of the inner layer's wall thickness). Comparative Example III-2 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 66-B resin and the outer layer is made of plasticized polyamide 612-A resin, with the inner and outer layer having wall thicknesses of 0.15 mm and 1.35 mm, respectively (10% of the inner layer's wall thickness). Comparative Example III-3 is an air brake tube having a two-layer structure in which the inner layer is made of unplasticized polyamide 6-B resin and the outer layer is made of plasticized polyamide 610-A resin, with the inner and outer layer having wall thicknesses of 0.6 mm and 0.9 mm, respectively (40% of the inner layer's wall thickness). Comparative Example III-4 is a single-layer air brake tube made of polyamide 1010 resin that satisfies the pressure resistance specified by PHLY in DIN 74324. Comparative Example III-5 is a single-layer air brake tube made of plasticized polyamide 612-A resin. Comparative Example III-6 is an air brake tube with a two-layer structure, with an inner layer made of unplasticized polyamide 6-C resin and an outer layer made of plasticized polyamide 12 resin, the inner and outer layers having wall thicknesses of 0.6 mm and 0.9 mm (40% of the inner layer thickness), respectively. Comparative Example III-7 is a single-layer air brake tube made of plasticized polyamide 12 resin that satisfies the PHLY specified by DIN 74324.

[0055] Table 3 shows the evaluation results of the pressure resistance and flexibility of the air brake tubes of Example III and Comparative Example III.

[0056] [Table 3]

[0057] As is clear from Tables 1, 2, and 3, Examples I, II, and III all satisfy the requirements for pressure resistance and flexibility, while Comparative Examples I, II, and III only satisfy one of the requirements for flexibility and pressure resistance.

[0058] In Comparative Examples I-1, II-1, and III-1, the thickness ratio of the inner layer is 50%, exceeding 40%, and therefore the flexibility of the unplasticized polyamide resin of the inner layer is not complemented by the plasticized polyamide resin of the outer layer, resulting in an insufficient flexibility.

[0059] In Comparative Examples I-2, II-2, and III-2, the unplasticized polyamide 66-B resin of the inner layer, when made into a single-layer tube, had a hoop stress of 28.5 MPa at 125°C, which is greater than 25 MPa. Therefore, the flexibility of the unplasticized polyamide resin of the inner layer cannot be fully complemented by the plasticized polyamide resin of the outer layer, and the flexibility cannot be satisfied.

[0060] In Comparative Examples I-3, II-3, and III-3, the plasticized polyamide 610-A resin of the outer layer, when made into a single-layer tube, had a hoop stress of 11.25 MPa, which was greater than 9.72 MPa, at 125°C. Therefore, the plasticized polyamide resin of the outer layer was unable to complement the flexibility of the unplasticized polyamide resin of the inner layer, and was unable to satisfy the flexibility requirement.

[0061] In Comparative Examples I-4, II-4, and III-4, since there is no outer layer made of a plasticized polyamide resin, the flexibility is not supplemented by the outer layer, and the flexibility is not satisfactory.

[0062] In Comparative Examples I-5, II-5, and III-5, since there is no inner layer made of unplasticized polyamide resin, the pressure resistance is not supplemented by the inner layer, and the pressure resistance is not satisfactory.

[0063] In Comparative Examples I-6, II-6, and III-6, the unplasticized polyamide 6-C resin in the inner layer had a deflection temperature under load of less than 50°C when subjected to a bending stress of 1.80 MPa in accordance with ISO 75, and the plasticized polyamide 12 resin in the outer layer had a melting point of less than 190°C and a deflection temperature under load of less than 50°C when subjected to a bending stress of 1.80 MPa in accordance with ISO 75. Resins with low melting points or deflection temperatures under load experience a greater decrease in stress from room temperature to high temperatures compared to resins with high melting points or deflection temperatures. As a result, the pressure resistance was no longer satisfactory.

[0064] In Comparative Examples I-7, II-7, and III-7, the air brake tubes are single-layered. Furthermore, the plasticized polyamide 12 resin specified as PHLY in DIN 74324 has a hoop stress of 9.72 MPa or less at 125°C when made into a single-layer tube, a melting point of less than 190°C, and a deflection temperature under load of 1.80 MPa in accordance with ISO 75 of less than 50°C. Therefore, the pressure resistance is not satisfactory.

[0065] In Examples I, II, and III, the inner layer is made of an unplasticized polyamide resin, the outer layer is made of a plasticized polyamide resin, the thickness of the inner layer accounts for 5% to 40% of the total tube thickness, the unplasticized polyamide resin has a hoop stress of more than 9.72 MPa and not more than 25 MPa at 125°C when made into a single-layer tube, and the plasticized polyamide resin has a hoop stress of not more than 9.72 MPa at 125°C when made into a single-layer tube, and the unplasticized polyamide resin and the plasticized polyamide resin have a melting point of 190°C or higher or a deflection temperature under load of 50°C or higher when subjected to a bending stress of 1.80 MPa according to ISO 75. Therefore, both pressure resistance and flexibility are satisfied, and pressure resistance in high-temperature environments and ease of installation into vehicles are achieved.

[0066] Reference Examples 1 and 2 shown in Table 4 also satisfied both flexibility and pressure resistance. For Reference Example 1, an air brake tube having an outer diameter of 8 mm and a wall thickness of 1 mm was molded, and its pressure resistance in a 125°C environment and its flexibility in a 23°C environment were evaluated. For Reference Example 2, an air brake tube having an outer diameter of 12 mm and a wall thickness of 1.5 mm was molded, and its pressure resistance in a 125°C environment and its flexibility in a 23°C environment were evaluated.

[0067] [Table 4]

[0068] Although the embodiments and examples of the present invention have been described above, the present invention is not limited to the above-described embodiments and examples, and can be modified as appropriate within the scope of the spirit of the present invention. [Explanation of symbols]

[0069] 10A, 10B Air brake tube 12 Inner layer 14A,14B outer layer

Claims

[Claim 1] It has a multi-layer structure, The hoop stress at 125°C is greater than 9.72 MPa and the tensile modulus at 23°C is less than 650 MPa; the multilayer structure has an inner layer made of an unplasticized polyamide-based resin and an outer layer provided outside the inner layer and made of a plasticized polyamide-based resin, a ratio of the thickness of the inner layer to the total thickness of the inner layer and the outer layer is 5% or more and 40% or less, the unplasticized polyamide resin has a hoop stress of more than 9.72 MPa and not more than 25 MPa at 125°C when formed into a single-layer tube; the plasticized polyamide resin has a hoop stress of 9.72 MPa or less at 125°C when formed into a single-layer tube; The unplasticized polyamide resin and the plasticized polyamide resin have a melting point of 190°C or higher, or a deflection temperature under load of 50°C or higher when subjected to a bending stress of 1.80 MPa in accordance with ISO 75. Air brake tube.

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