Heat-shrinkable tube and wire harness

Abstract

A heat-shrinkable tube of the present disclosure is a heat-shrinkable tube comprising a crosslinked product of a fluororesin composition, wherein the fluororesin composition contains a vinylidene fluoride-hexafluoropropylene copolymer as a main component and a crosslinking aid, and when the fluororesin composition is heated from 0°C to a crystal melting temperature at a heating rate of 20°C / min and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the difference between the crystal melting temperature and the temperature at which the elastic modulus of the fluororesin composition reaches 1.0×107 Pa upon cooling from the crystal melting temperature is 82°C or less.

Description

Heat shrink tubing and wire harness

[0001] This disclosure relates to heat shrink tubing and wire harnesses.

[0002] Wire harnesses for aircraft, electronic components, railway vehicles, automobiles, and motorcycles utilize bundles of multiple insulated wires bound together with connecting bands or adhesive tape. Each insulated wire is generally formed by covering the outer circumference of a single conductor or a conductor made of multiple strands twisted together with an insulating layer. The connection points (joints) at the ends and in the middle of these wire bundles expose the strands, requiring electrical insulation, mechanical protection, and waterproofing. Heat-shrink tubing is used for electrical insulation, mechanical protection, and waterproofing. In particular, for waterproofing, wire harnesses equipped with heat-shrink tubing that shrinks radially (from the outer circumference of the cross-section toward the center) are used. When the heat-shrink tubing of this wire harness is heated over the connection points between insulated wires, it shrinks to conform to the shape of the connection point due to its shape memory effect, thereby protecting the connection points of wires, pipes, etc.

[0003] Polytetrafluoroethylene (PTFE) is preferably used as the material for such heat-shrinkable tubing because it has excellent heat resistance, mechanical strength, and a low coefficient of friction. However, since PTFE has a high storage modulus and is not easy to mold, a technology has been proposed to crosslink PTFE tubing by radiation irradiation to facilitate the molding process (see Patent Document 1).

[0004] International Publication No. 2010 / 038800

[0005] The heat-shrinkable tube of the present disclosure is a heat-shrinkable tube made of a crosslinked fluororesin composition, wherein the fluororesin composition contains a vinylidene fluoride-hexafluoropropylene copolymer as the main component and a crosslinking aid, and when the fluororesin composition is heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min, and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the elastic modulus of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10⁻⁶. ７ The difference between the temperature at which Pa is reached and the crystal melting temperature is 82°C or less.

[0006] Figure 1 is a diagram illustrating an example of the relationship between temperature and elastic modulus in a fluororesin composition forming a heat-shrinkable tube according to the present disclosure. Figure 2 is a schematic perspective view showing a heat-shrinkable tube according to one embodiment of the present disclosure. Figure 3 is a diagram illustrating the crystal melting temperature of a fluororesin composition. Figure 4 is a schematic side view showing a wire harness according to one embodiment of the present disclosure. Figure 5 is a graph showing the change in elastic modulus during heating from 0°C to the crystal melting temperature and during cooling in the fluororesin compositions of Test No. 2 and Test No. 15.

[0007] [Problems this disclosure aims to solve] Heat shrinkable tubes containing fluororesins such as polytetrafluoroethylene are relatively rigid, so there is a need to improve their flexibility when coating objects. On the other hand, heat shrinkable tubes are generally made by forming a tube using a resin composition by melt extrusion molding, and then crosslinking by electron beam irradiation. After electron beam irradiation, the tube is heated and expanded (increased in diameter) is performed by making the inside of the tube more pressure than the outside using an expansion die. In the expansion process, when a flexible tube made of a resin with a low crystal content is expanded, crystallization during cooling is slow, and the tube is easily stretched in the longitudinal direction due to friction with the expansion die. When the tube is easily stretched in the longitudinal direction during the expansion process, excessive shrinkage occurs when the heat shrinkable tube is heat-shrinkable, which can lead to a large rate of change in length during heat shrinkage or uneven heat shrinkage, thus reducing the heat shrinkability. When such a reduction in heat shrinkability occurs, objects to be coated, such as insulated wires, cannot be adequately coated, and there is a risk of reduced watertightness. Furthermore, heat shrink tubing requires visibility inside the tube to check the shrinkage state, solder melting, and wire connection status. Therefore, transparency is also required for heat shrink tubing.

[0008] This disclosure aims to provide a heat-shrinkable tube that is flexible and has excellent heat-shrinkability and transparency.

[0009] [Effects of this disclosure] According to this disclosure, it is possible to provide a heat-shrinkable tube that is flexible and has excellent heat-shrinkability and transparency.

[0010] [Description of Embodiments of the Disclosure] First, embodiments of the Disclosure will be listed and described.

[0011] (1) The heat shrinkable tube of the present disclosure is a heat shrinkable tube made of a crosslinked fluororesin composition, wherein the fluororesin composition contains a vinylidene fluoride-hexafluoropropylene copolymer as the main component and a crosslinking aid, and when the fluororesin composition is heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the elastic modulus of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10 ７ The difference between the temperature at which Pa is reached and the crystal melting temperature is 82°C or less.

[0012] The heat-shrinkable tube is made of a crosslinked fluororesin composition, and by making the fluororesin composition mainly of a vinylidene fluoride-hexafluoropropylene copolymer, the flexibility and transparency of the heat-shrinkable tube can be enhanced. Furthermore, in the case of flexible fluororesins such as polyvinylidene fluoride, the history differs between the curve showing the decrease in elastic modulus when heated to a crystalline melt state and the curve showing the increase in elastic modulus when cooling from the crystalline melt state back to the original temperature. In the cooling stage, the rate of recovery of the elastic modulus is slow, so sufficient heat-shrinkability cannot be obtained. Therefore, the inventors focused on the fact that the rate of recovery of the elastic modulus in the cooling stage is related to the rate of recrystallization when cooling from the crystalline melt state back to the original temperature. Figure 1 is a diagram illustrating an example of the relationship between temperature and elastic modulus in a fluororesin composition forming a heat-shrinkable tube in one embodiment of the present disclosure. In the heat-shrinkable tube according to this embodiment, as shown in Figure 1, when the fluororesin composition is heated from 0°C to the crystal melting temperature under the above conditions and then cooled from the crystal melting temperature to 0°C, the elastic modulus of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10⁻⁶. ７The difference between the temperature at which Pa (10,000,000 Pa) is reached and the above-mentioned crystal melting temperature is 82°C or less. The elastic modulus of the above-mentioned fluororesin composition is 1.0 × 10⁻⁶. ７ The difference between the temperature at which Pa is reached and the crystal melting temperature, that is, the temperature from which the crystals of the fluororesin composition have completely melted, after cooling to 1.0 × 10⁻⁶ ７ By keeping the temperature difference required to return to the elastic modulus of Pa below 82°C, the recovery rate of the elastic modulus during cooling from the crystalline molten state is controlled within an appropriate range, and the crystallization rate during cooling is maintained within a good range. As a result, the stretching of the tube due to friction with the expansion die during the expansion process is reduced, and the rate of change in length during tube contraction can be controlled within a good range, thus improving heat shrinkability. Therefore, this heat shrinkable tube is flexible and has excellent heat shrinkability and transparency.

[0013] The term "main component" above means the component that is present in the largest quantity, and is present in an amount of 90% by mass or more of the total mass of the heat shrinkable tube, and in an amount of 95% by mass or more of the total mass of the fluoropolymer components.

[0014] (2) In (1) above, the vinylidene fluoride-hexafluoropropylene copolymer may include a first copolymer and a second copolymer, wherein the mass ratio of the second copolymer to the first copolymer is 20 / 80 or more and 80 / 20 or less, the melting point of the first copolymer measured in accordance with ASTM D3418 is less than 110°C, and the melting point of the second copolymer measured in accordance with ASTM D3418 is 110°C or higher. Including a first copolymer having a melting point of less than 110°C measured in accordance with ASTM D3418 in the vinylidene fluoride-hexafluoropropylene copolymer can improve the flexibility of the heat shrinkable tube. Furthermore, including a second copolymer having a melting point of 110°C or higher measured in accordance with ASTM D3418 in the vinylidene fluoride-hexafluoropropylene copolymer can improve the heat shrinkability of the heat shrinkable tube. Furthermore, by having a mass ratio of the second copolymer to the first copolymer of 20 / 80 or more and 80 / 20 or less, the flexibility and heat shrinkability of the heat shrinkable tube can be controlled within a well-balanced range.

[0015] (3) In (1) or (2) above, the content of hexafluoropropylene units in the fluororesin composition may be 6% by mass or more and 12% by mass or less. By having a content of hexafluoropropylene units in the fluororesin composition of 6% by mass or more and 12% by mass or less, the flexibility and heat shrinkability of the heat shrink tube can be controlled to a better range with a good balance.

[0016] (4) In any of (1) to (3) above, the melting point of the first copolymer measured in accordance with ASTM D3418 may be 97°C or higher and 103°C or lower. The flexibility of the heat shrink tube can be further improved by having a melting point of the first copolymer measured in accordance with ASTM D3418 of 97°C or higher and 103°C or lower.

[0017] (5) In any of (1) to (4) above, the melting point of the second copolymer measured in accordance with ASTM D3418 may be 117°C or higher and 123°C or lower. The heat shrinkability of the heat shrink tube can be further improved by having a melting point of the second copolymer measured in accordance with ASTM D3418 of 117°C or higher and 123°C or lower.

[0018] (6) In any of (1) to (5) above, the light transmittance at a wavelength of 550 nm may be 10% or more. In the heat shrinkable tube, the transparency of the heat shrinkable tube can be made good by having a light transmittance at a wavelength of 550 nm of 10% or more. As a method for measuring the light transmittance at a wavelength of 550 nm, for example, it can be measured using a spectrophotometer "UV-3600" manufactured by Shimadzu Corporation.

[0019] (7) In any of (1) to (6) above, the rate of change in the longitudinal length before and after heat shrinkage may be 10% or less. In such heat shrinkable tubing, a rate of change in the longitudinal length before and after heat shrinkage of 10% or less ensures that the area to be covered is sufficiently covered when heat-shrinkable, resulting in good watertightness. The rate of change in the longitudinal length represents the percentage change in the longitudinal dimension of the heat shrinkable tubing when it is heated to the shrinkage temperature. Here, "longitudinal direction" means the direction in which the tip points along the axis of the heat shrinkable tubing. The rate of change in length can be calculated using the following formula: Rate of change in length (%) = [(tube length after shrinkage - tube length before shrinkage) / tube length before shrinkage] × 100

[0020] (8) In any of (1) to (7) above, the 2% secant modulus may be 300 MPa or less. The flexibility of the heat shrink tubing can be further improved by having a 2% secant modulus of 300 MPa or less. Here, "2% secant modulus" is an indicator of flexibility, obtained by measuring the load at which a 100 mm long test piece is stretched longitudinally using a tensile testing machine at a tensile speed of 50 mm / min, dividing the load by the cross-sectional area, and multiplying that value by 50, in accordance with ASTM-D5223-92.

[0021] (9) The wire harness of the present disclosure is a wire harness comprising a plurality of wires having a conductor and an insulating layer covering the outer surface of the conductor, and a tube attached to the plurality of wires, wherein the tube is a heat shrink tube of any of (1) to (8) above.

[0022] The wire harness, by incorporating a heat-shrinkable tube, exhibits excellent adhesion to the adherend, visibility, and waterproofing of the connection points during heat shrinkage, thus possessing good electrical characteristics.

[0023] [Details of Embodiments of the Disclosure] Hereinafter, heat-shrinkable tubing and wire harnesses according to embodiments of the Disclosure will be described with reference to the drawings.

[0024] <Heat Shrink Tubing> A heat shrink tubing according to one embodiment of the present disclosure is used as a covering material to protect an object to be covered. Heat shrink tubing is a tube that shrinks in diameter when heated. More specifically, the object to be covered is protected by heating the heat shrink tubing into which the object to be covered is inserted, and covering the object with the shrinkable body of the heat shrink tubing.

[0025] The heat-shrinkable tube 1 shown in Figure 2 is formed from a single layer of cylindrical base material. The heat-shrinkable tube 1 is used for covering, for example, connection points between objects to be covered, wiring terminals, metal pipes, etc., for protection, insulation, waterproofing, corrosion prevention, etc. The heat-shrinkable tube 1 is made of a crosslinked fluororesin composition. The fluororesin composition contains a vinylidene fluoride-hexafluoropropylene copolymer as the main component and a crosslinking aid. In the heat-shrinkable tube 1, the vinylidene fluoride-hexafluoropropylene copolymer as the main component of the fluororesin composition enhances flexibility and transparency. Furthermore, the inclusion of a crosslinking aid in the fluororesin composition promotes crosslinking of the fluororesin composition, improving expandability, shrinkability during heat shrinkage (shape memory effect), and shape retention at high temperatures after shrinkage.

[0026] In the above fluororesin composition, when heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min, and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the elastic modulus of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10⁻⁶. ７ The difference between the temperature at which Pa is reached and the above crystal melting temperature is 82°C or less, and may be 65°C or less, 60°C or less, or 55°C or less. The elastic modulus of the above fluororesin composition is 1.0 × 10 ７ The difference between the temperature at which Pa is reached and the crystal melting temperature is 82°C or less, which controls the recovery rate of the elastic modulus during cooling from the crystal melting state to an appropriate range, and maintains a good crystallization rate during cooling. As a result, the stretching of the tube due to friction with the expansion die during the expansion process is reduced, and the rate of change in length during tube contraction can be reduced. Therefore, the heat shrinkable tube 1 can shrink appropriately with respect to the area to be covered, and thus has excellent heat shrinkability. Furthermore, the elastic modulus of the fluororesin composition is 1.0 × 10 when cooled from the crystal melting temperature. ７ The difference between the temperature at which Pa is reached and the above-mentioned crystal melting temperature can also be measured from the crosslinked fluororesin composition.

[0027] Figure 3 is a diagram illustrating the crystalline melting temperature of a fluororesin composition, showing the endothermic peak P of the melting curve obtained by differential scanning calorimetry (DSC). The melting point is the temperature M at which the endothermic peak reaches its maximum height. In this disclosure, the crystalline melting temperature refers to the temperature T at the intersection Q of the tangent line to the end-set temperature (endothermic termination temperature) S of the endothermic peak and the extension line of the flat portion after the end-set temperature.

[0028] The modulus of elasticity can be measured by a method conforming to the test method for dynamic mechanical properties described in JIS K7244-4:1999. More specifically, for a fluororesin composition, the modulus of elasticity (in other words, the storage modulus) at 25°C is measured using a viscoelasticity measuring device under the conditions of tensile mode, strain of 0.08%, and frequency of 10 Hz. As a viscoelasticity measuring device, for example, the "DVA-220" manufactured by IT Measurement Control Co., Ltd. can be used.

[0029] The vinylidene fluoride-hexafluoropropylene copolymer described above comprises a first copolymer and a second copolymer, wherein the melting point of the first copolymer, as measured according to ASTM D3418, may be less than 110°C, or it may be 97°C or more and 103°C or less. Including a first copolymer in the vinylidene fluoride-hexafluoropropylene copolymer, which has a melting point of less than 110°C as measured according to ASTM D3418, can improve the flexibility of the heat-shrinkable tube. Furthermore, the melting point of the second copolymer, as measured according to ASTM D3418, may be 110°C or higher, or it may be 117°C or more and 123°C or less. Including a second copolymer in the vinylidene fluoride-hexafluoropropylene copolymer, which has a melting point of 110°C or higher as measured according to ASTM D3418, can improve the heat-shrinkability of the heat-shrinkable tube.

[0030] The above vinylidene fluoride-hexafluoropropylene copolymer may contain only the second copolymer.

[0031] The vinylidene fluoride-hexafluoropropylene copolymer described above comprises a first copolymer and a second copolymer, and the lower limit of the mass ratio of the second copolymer to the first copolymer may be 20 / 80, 30 / 70, 40 / 60, or 50 / 50. A lower limit of 20 / 80 for the mass ratio of the second copolymer to the first copolymer improves the heat shrinkability of the heat shrinkable tube. On the other hand, the upper limit of the mass ratio of the second copolymer to the first copolymer may be 80 / 20, 70 / 30, 60 / 40, or 50 / 50. A upper limit of 80 / 20 for the mass ratio of the second copolymer to the first copolymer improves flexibility. A mass ratio of 20 / 80 to 80 / 20 for the mass ratio of the second copolymer to the first copolymer allows for a good balance between flexibility and heat shrinkability of the heat shrinkable tube.

[0032] The lower limit of the hexafluoropropylene unit content in the above fluororesin composition may be 5% by mass, 6% by mass, or 8% by mass. Having the lower limit of the hexafluoropropylene unit content in the above fluororesin composition within this range improves the flexibility of the heat-shrinkable tube. The upper limit of the hexafluoropropylene unit content in the above fluororesin composition may be 13% by mass, 12% by mass, 11% by mass, or 10% by mass. Having the upper limit of the hexafluoropropylene unit content in the above fluororesin composition within this range improves the heat-shrinkability of the heat-shrinkable tube. Having the hexafluoropropylene unit content in the above fluororesin composition between 6% by mass and 12% by mass allows for a good balance between the flexibility and heat-shrinkability of the heat-shrinkable tube, within a more favorable range.

[0033] The lower limit of the light transmittance of the heat shrink tubing at a wavelength of 550 nm may be 10%, 20%, 30%, or 40%. Generally, it is necessary to check the shrinkage state and the connection state of insulated wires, etc., of the heat shrink tubing, and transparency of the tubing is required from the viewpoint of improving the visibility of the inside of the heat shrink tubing. In the heat shrink tubing 1, if the light transmittance at a wavelength of 550 nm is 10% or more, the visibility of the inside of the heat shrink tubing 1 is improved, and the inside of the heat shrink tubing 1 is made easier to check.

[0034] The upper limit of the rate of change in the longitudinal length before and after heat shrinkage may be 10%, 9%, 8%, or 7%. In the heat shrinkable tube 1, if the rate of change in the longitudinal length before and after heat shrinkage is 10% or less, the heat shrinkability of the heat shrinkable tube 1 is improved, the covering performance is enhanced, and thus the watertightness is good.

[0035] The upper limit of the 2% secant modulus of the heat-shrinkable tube 1 may be 500 MPa, may be 350 MPa, may be 300 MPa, may be 250 MPa, or may be 200 MPa. By setting the upper limit of the 2% secant modulus within the above range, the flexibility of the heat-shrinkable tube can be further improved, and the bending and stretching of the portion of the object to be coated covered by the heat-shrinkable tube 1 can be improved. On the other hand, the lower limit of the 2% secant modulus of the heat-shrinkable tube 1 is not particularly limited, but may be 50 MPa. By having the 2% secant modulus of the heat-shrinkable tube 1 be 50 MPa or more, it is possible to make it less likely to cause cutting of the heat-shrinkable tube 1.

[0036] The average inner diameter and average thickness of the heat-shrinkable tube 1 are appropriately selected according to the application and the like. The average inner diameter of the heat-shrinkable tube 1 before heat shrinkage can be, for example, 1 mm or more and 60 mm or less. Also, the average inner diameter of the heat-shrinkable tube 1 after heat shrinkage can be, for example, 25% or more and 65% or less of the average inner diameter before heat shrinkage. Further, the average thickness of the heat-shrinkable tube 1 can be, for example, 0.1 mm or more and 5 mm or less.

[0037] The heat-shrinkable tube 1 may contain other additives as necessary. Examples of such additives include strength retainers, antioxidants, flame retardants, copper damage preventives, crosslinking aids, colorants, heat stabilizers, infrared absorbers, and ultraviolet absorbers. The content of the additive in the heat-shrinkable tube 1 may be less than 10% by mass, may be less than 5% by mass, or may be less than 3% by mass. If the content of the additive is above the above upper limit, there is a risk that the performance of the heat-shrinkable tube 1 will easily vary.

[0038] [Method for manufacturing heat-shrinkable tube] The method for manufacturing the heat-shrinkable tube can use a known method. For example, a step of extruding a fluororesin composition mainly composed of a vinylidene fluoride-hexafluoropropylene copolymer into a tube shape, a step of crosslinking the tube formed by the above extrusion step by irradiation, a step of heating the tube after the above crosslinking step, and a step of expanding the tube.

[0039] (Extrusion molding process) First, using a melt mixer or the like, a vinylidene fluoride - hexafluoropropylene copolymer, which is the resin component of the heat - shrinkable tube, and, if necessary, other additives are mixed to prepare a fluororesin composition for forming the heat - shrinkable tube. In the above fluororesin composition, a cross - linking auxiliary agent is added to the raw materials of the vinylidene fluoride - hexafluoropropylene copolymer, if necessary. Further, when the above fluororesin composition is heated from 0 °C to the crystal melting temperature at a heating rate of 20 °C / min and then cooled from the crystal melting temperature to 0 °C at a cooling rate of 20 °C / min, the difference between the temperature at which the elastic modulus of the fluororesin composition reaches 1.0×10 ７ Pa and the crystal melting temperature is 82 °C or less. The melt mixer is not particularly limited, and for example, an open roll, a Banbury mixer, a pressure kneader, a single - screw mixer, or a multi - screw mixer can be used.

[0040] Next, in this step, a fluororesin composition containing a vinylidene fluoride - hexafluoropropylene copolymer as the main component and a cross - linking auxiliary agent is extruded into a tube shape. The above fluororesin composition is extruded using a melt extrusion molding machine. Specifically, the above fluororesin composition is heated to a temperature above the melting point and melted, eluted from an extrusion die having a cylindrical space, and cooled to a temperature below the melting point with cooling water or the like and solidified to extrude the fluororesin composition into a tube shape. The dimensions of the extruded product can be designed according to the application and the like. The dimensions of the extruded product can be adjusted by the dimensions of the extrusion die and the draw - down ratio. The "draw - down ratio" is the ratio of the cross - sectional area of the extrusion die to the cross - sectional area of the tube after extrusion molding. In order to prevent surface roughness during extrusion, the draw - down ratio may be 1.2 or more.

[0041] (Crosslinking Step) In the crosslinking step, the tube formed by the above extrusion molding step is crosslinked by irradiation. In this step, by crosslinking the extruded product of the fluororesin composition, shrinkage properties (shape memory effect) when heating and shrinking at a high temperature after the expansion step and shape retention at a high temperature after shrinkage are imparted. As a method for crosslinking the fluororesin composition, for example, a method of irradiating the extruded product with radiation can be mentioned. By performing radiation irradiation after extrusion molding of the fluororesin composition, molding can be surely carried out, and the effects of radiation irradiation can be sufficiently obtained.

[0042] As the radiation used for irradiation crosslinking of the fluororesin composition, electron beams (β rays), γ rays, etc. can be mentioned. Since an electron accelerator has a low running cost, a high-output electron beam can be obtained, and control is also easy, the radiation may be an electron beam.

[0043] The radiation dose may be in the range of 30 kGy or more and 300 kGy or less. By the radiation dose being 30 kGy or more, a sufficient degree of crosslinking can be obtained. On the other hand, by the radiation dose being 300 kGy or less, the flexibility and expansion performance of the heat-shrinkable tube can be made good.

[0044] (Heating Step) In the heating step, the tube after the above crosslinking step is heated to a temperature of 120°C or more and 180°C or less. Thereby, the expansion rate of the tube after expansion can be increased. Therefore, the manufacturing cost and variation in quality of the heat-shrinkable tube can be more surely reduced.

[0045] (Expansion Process) In the expansion process, the heated tube is expanded. Note that the tube expansion may be performed simultaneously with heating. As for the method of expanding the tube, a known expansion method that is normally used in the manufacture of conventional heat shrinkable tubes can be used. For example, a method can be used in which compressed air is introduced into the extruded product while it is heated to a temperature above its melting point, or by reducing the pressure from the outside to expand it to a predetermined inner diameter, and then it is cooled to fix the shape. The heat shrinkable tube is obtained by fixing the shape of the expanded extruded product. As for this fixing method, for example, a method of cooling to a temperature below the melting point of the base resin component can be mentioned. The heat shrinkable tube is obtained by expanding the extruded product in this way and fixing its shape. Note that the tube is expanded so that, for example, the average inner diameter of the tube becomes about 1.4 to 4 times its original size.

[0046] The expansion ratio may be 2.5 times or more, or 3 times or more. The dimensions during expansion can be controlled by the size of the expansion die.

[0047] This heat-shrinkable tubing offers flexibility, excellent heat-shrinkability, and transparency. Therefore, it is suitable for applications such as protecting, insulating, waterproofing, and corrosion-resistant insulation wires or cables. Specifically, it can be applied to wire splices and wire harnesses. It can also be used in combination with adhesive layers and crimp sleeves to form crimp terminals.

[0048] <Wire Harness> The heat shrink tubing can be used for the protection, insulation, waterproofing, and corrosion prevention of wires in, for example, fluororesin wires where the insulating layer covering the conductor is fluororesin, fluororubber wires where the insulating layer is fluororubber, PE wires or PE cables where the insulating layer is polyethylene (PE), and PVC wires or PVC cables where the insulating layer is polyvinyl chloride (PVC). For example, the heat shrink tubing can be applied to a wire harness. Specifically, the wire harness comprises a plurality of wires having a conductor and an insulating layer covering the outer surface of the conductor, and a tube attached to the plurality of wires, wherein the tube is made of the heat shrink tubing. That is, the tube is made by heat shrinking the heat shrink tubing.

[0049] Figure 4 shows an example of applying the heat-shrinkable tube 1 to a wire harness 50. The wire harness 50 shown in Figure 4 consists of multiple wires 30 bound together by a tube 1a, which is made by heat-shrinking the heat-shrinkable tube 1 shown in Figure 2, and a multi-pin connector 31 is provided at the ends of the multiple wires 30. The wires 30 are insulated wires or cables such as fluororesin wires, polyethylene wires, or PVC wires. In the wire harness 50, the tube 1a can contribute to the protection, insulation, waterproofing, and corrosion prevention of the connection parts. In the wire harness 50, the tube 1a not only serves to bind each wire 30 together, but also serves to protect each individual wire 30. There is no restriction on the number of wires 30 that can be covered by the heat-shrinkable tube 1, and the heat-shrinkable tube 1 can be used for connecting one wire or multiple wires.

[0050] The wire harness, by incorporating a heat-shrinkable tube, exhibits excellent adhesion to the adherend, visibility, and waterproofing of the connection points during heat shrinkage, thus possessing good electrical characteristics.

[0051] [Other Embodiments] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is not limited to the configurations of the embodiments described herein, but is indicated by the claims, and all modifications within the meaning and scope of the claims are intended to be included.

[0052] The wire harness of this disclosure can be used as a so-called flat harness, which is a bundle of multiple wires arranged in a planar shape, or it can be in other forms.

[0053] The wire harness of this disclosure also includes wire splices. A wire splice is formed by twisting together the conductors of a pair of wires and covering the connection with a heat-shrinkable tube. The wires are the same as those in the wire harness described above. In such a wire splice, the tube can contribute to the protection, insulation, waterproofing, and corrosion prevention of the connection. A wire splice only needs to have a heat-shrinkable tube covering the connection between wires, and may be formed by connecting one wire to multiple wires, connecting multiple wires to each other, or by connecting the ends of multiple wires together, as in wiring termination, or in other forms.

[0054] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0055] <Heat Shrink Tubing No. 1 to No. 19> Single-layer heat shrink tubing No. 1 to No. 19 was formed using the following procedure.

[0056] [Fluoropolymers] The following four types of fluoropolymers were used. The hexafluoropropylene (HFP) unit content [%] in the above fluoropolymers was determined by solid-state NMR (Nuclear Magnetic Resonance) measurement. (1) Fluoropolymer vinylidene fluoride-hexafluoropropylene copolymer (1) (VDF-based copolymer (1): melting point 100°C, HFP unit content 13%) (2) Fluoropolymer vinylidene fluoride-hexafluoropropylene copolymer (2) (VDF-based copolymer (2): melting point 120°C, HFP unit content 8%) (3) Fluoropolymer vinylidene fluoride-hexafluoropropylene copolymer (3) (VDF-based copolymer (3): melting point 142°C, HFP unit content 5%) (4) Fluororubber ternary fluororubber FKM (vinylidene fluoride / hexafluoropropylene / tetrafluoroethylene: Mooney viscosity 60, HFP unit content 15%) The Mooney viscosity mentioned above is the Mooney viscosity (ML(1+4)) at 100°C, measured according to JIS-K6300-1:2013 "Unvulcanized rubber - Physical properties - Part 1: Method for determining viscosity and scorch time using a Mooney viscometer".

[0057] [Crosslinking Aids] The following two additives were used as crosslinking aids: (1) Triallyl isocyanurate (TAIC) (2) Trimethylolpropane trimethacrylate (TMPTMA)

[0058] A fluoropolymer composition was prepared using the above-mentioned fluoropolymer and crosslinking agent. The content of the fluoropolymer and additives is shown in Table 1. "-" indicates that the corresponding component was not used.

[0059] [Manufacturing of Heat Shrinkable Tubes] First, tubes were formed by melt extrusion molding using a fluororesin composition prepared by mixing a vinylidene fluoride-hexafluoropropylene copolymer resin containing the fluorine content listed in Table 1 with triallyl isocyanurate as a crosslinking aid. An extrusion die was used for the extrusion molding. Extrusion molding was performed at a die temperature of 180°C, a draw ratio of 3, and a linear speed of 10 m / min. Next, the molded bodies extruded by this molding die were crosslinked by electron beam irradiation under conditions of irradiation dose of 100 kGy. After irradiation, the tubes were heated to 160°C and expanded by creating a higher pressure inside the tube than outside using an expansion die, thereby obtaining heat shrinkable tubes No. 1 to No. 19 with an outer diameter of 29.7 mm and an inner diameter of 26.7 mm.

[0060] From the state in which the crystals of fluororesin compositions No. 1 to No. 19 have completely melted, they are cooled to 1.0 × 10⁻⁶ ７ The temperature difference required to reach the elastic modulus was measured using the following procedure. First, a differential scanning calorimeter (product name "DSC8500", manufactured by Perkin Elmer) was used to heat a 5 mg sample of each fluororesin composition from -50°C to 300°C at a heating rate of 10°C / min under a nitrogen atmosphere, and the heat of fusion was measured. Then, the crystal melting temperature was determined from the endothermic peak using the method described above. Next, each fluororesin composition was heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min, and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min. When the elastic modulus of the fluororesin composition was cooled from the crystal melting temperature to 1.0 × 10⁻⁶, ７ The difference [°C] between the temperature at which (10,000,000) Pa is reached and the above-mentioned crystal melting temperature was determined. Figure 5 shows the change in elastic modulus during heating and cooling from 0°C to the crystal melting temperature for the fluororesin compositions of Test No. 2 and Test No. 15.

[0061] [Evaluation] The following evaluations were conducted on heat shrink tubing No. 1 to No. 19.

[0062] (Percentage change in longitudinal length before and after heat shrinkage) The percentage change in longitudinal length before and after heat shrinkage [%] was calculated by measuring the length of the tube after shrinkage and the length of the tube before shrinkage and using the following formula: Percentage change in length [%] = [(Length of tube after shrinkage - Length of tube before shrinkage) / Length of tube before shrinkage] × 100 A percentage change in longitudinal length of 10% or less before and after heat shrinkage is considered good.

[0063] (Heat Shrinkability) The heat shrinkability of each heat shrink tube was evaluated using the following procedure. First, the insulating layer was removed from the center of an insulated wire with an outer diameter of 10 mm and a length of 100 mm in a longitudinal range of 50 mm to expose the conductor. Next, each heat shrink tube was placed over the exposed conductor and heated at 200°C for 5 minutes to heat shrink it. The length of heat shrink tube required to cover the 50 mm conductor without exposing it was then measured. To determine whether the 50 mm conductor had been covered without exposure, it was visually checked to see if any conductor was exposed from the end. For all 20 heat shrink tubes, the minimum length required to cover the conductor without exposure was measured and evaluated using the following four evaluation criteria. If the evaluation is A, B, or C, the heat shrinkability of the heat shrink tube is good. A: The required tube length for covering is 55 mm or less. B: The required tube length for covering is more than 55 mm but 60 mm or less. C: The required tube length for covering is more than 60 mm but 62 mm or less. D: The required tube length for covering is more than 62 mm.

[0064] (2% secant modulus) The 2% secant modulus [MPa] of each heat shrinkable tube obtained was measured in accordance with ASTM-D5223-92. Specifically, a 100 mm long test specimen was pulled longitudinally using a tensile testing machine at a tensile speed of 50 mm / min. The value obtained by dividing the load at 2% elongation by the cross-sectional area was measured, and this was multiplied by 50 to obtain the 2% secant modulus (MPa (1 kg / mm²)). ２ Let it be 9.8 MPa. Note that a 2% secant modulus means that the smaller the measured value, the more flexible the heat shrink tubing is.

[0065] (Flexibility) The flexibility of each heat shrink tube was measured by fixing one end of a 100 mm long heat shrink tube to a wall so that the tube was horizontal and kept in a straight line. Then, the other end of the heat shrink tube was pushed downward with a force of 0.1 N and the distance it deflected downward was measured. The flexibility was evaluated using the following four levels: A, B, and C indicate good flexibility of the heat shrink tube. A: Deflection of 12 mm or more. B: Deflection of 9 mm or more but less than 12 mm. C: Deflection of 5 mm or more but less than 9 mm. D: Deflection of less than 5 mm.

[0066] (Light transmittance at a wavelength of 550 nm) The light transmittance at a wavelength of 550 nm [%] was measured using a Shimadzu UV-3600 spectrophotometer. A light transmittance of 10% or more is considered good.

[0067] (Visibility) The visibility of each heat shrink tubing was evaluated by assessing the visibility of the characters printed on an insulated wire with an outer diameter of 10 mm when the tubing was covered with it. The evaluation criteria were as follows, in two stages: An evaluation of A indicates good visibility of the heat shrink tubing. A: The characters are visible. B: The characters are not visible.

[0068] The evaluation results are shown in Table 1.

[0069]

[0070] From the results in Table 1, the fluororesin composition consists of a crosslinked body containing a vinylidene fluoride-hexafluoropropylene copolymer as the main component and a crosslinking aid. When the fluororesin composition is heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min, and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the modulus of elasticity of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10⁻⁶. ７ In tests No. 1 to No. 16, where the difference between the temperature at which Pa is reached and the crystal melting temperature is 82°C or less, good results were obtained in all aspects: the rate of change in longitudinal length before and after thermal shrinkage, thermal shrinkage processability, 2% secant modulus, flexibility, light transmittance at a wavelength of 550 nm, and visibility.

[0071] On the other hand, test No. 17, which consisted of a crosslinked fluororesin composition without a crosslinking aid, failed to expand during the expansion process. When cooled from the above crystal melting temperature, the elastic modulus of the fluororesin composition was 1.0 × 10⁻⁶. ７ In Test No. 18, where the difference between the temperature at which Pa was reached and the above-mentioned crystal melting temperature exceeded 82°C, the rate of change in the longitudinal length before and after thermal shrinkage and the thermal shrinkage processability were poor. In Test No. 19, where the content of vinylidene fluoride-hexafluoropropylene copolymer in the fluororesin composition was 50% by mass, visibility was poor and transparency could not be obtained.

[0072] The results above demonstrate that the heat-shrinkable tubing possesses flexibility, excellent heat-shrinkability, and superior transparency.

[0073] 1 Heat shrink tubing 1a Tube 30 Wire 31 Multi-pin connector 50 Wire harness

Claims

1. A heat-shrinkable tube made of a crosslinked fluororesin composition, wherein the fluororesin composition contains a vinylidene fluoride-hexafluoropropylene copolymer as the main component and a crosslinking aid, and when the fluororesin composition is heated from 0°C to the crystal melting temperature at a heating rate of 20°C / min, and then cooled from the crystal melting temperature to 0°C at a cooling rate of 20°C / min, the elastic modulus of the fluororesin composition after cooling from the crystal melting temperature is 1.0 × 10⁻⁶ ７ A heat-shrinkable tube in which the difference between the temperature at which Pa is reached and the crystal melting temperature is 82°C or less.

2. The heat shrinkable tube according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer comprises a first copolymer and a second copolymer, the mass ratio of the second copolymer to the first copolymer is 20 / 80 or more and 80 / 20 or less, the melting point of the first copolymer as measured in accordance with ASTM D3418 is less than 110°C, and the melting point of the second copolymer as measured in accordance with ASTM D3418 is 110°C or higher.

3. The heat-shrinkable tube according to claim 1 or claim 2, wherein the hexafluoropropylene unit content in the fluororesin composition is 6% by mass or more and 12% by mass or less.

4. The heat-shrinkable tube according to any one of claims 1 to 3, wherein the melting point of the first copolymer, as measured in accordance with ASTM D3418, is 97°C or higher and 103°C or lower.

5. The heat-shrinkable tube according to any one of claims 1 to 4, wherein the melting point of the second copolymer, as measured in accordance with ASTM D3418, is 117°C or higher and 123°C or lower.

6. The heat-shrinkable tube according to any one of claims 1 to 5, wherein the light transmittance at a wavelength of 550 nm is 10% or more.

7. The heat-shrinkable tube according to any one of claims 1 to 6, wherein the rate of change in the longitudinal length before and after heat shrinkage is 10% or less.

8. The heat-shrinkable tube according to any one of claims 1 to 7, wherein the 2% secant modulus is 300 MPa or less.

9. A wire harness comprising a plurality of wires having a conductor and an insulating layer covering the outer surface of the conductor, and a tube attached to the plurality of wires, wherein the tube is a heat shrinkable tube as described in any one of claims 1 to 8.

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