Film material

Abstract

Provided is a technique with which it is possible to contribute to realizing a film material which is used in environments from the stratosphere to extraterrestrial environments, and is not susceptible to breaking at low temperatures. This film material (20) is used in environments from the stratosphere to extraterrestrial environments, and comprises a resin film, said resin film having an estimated elongation at break value at -100°C in any area S of at least 400,000 mm2 of at least 10% in both the MD direction and the TD direction, as calculated using a Weibull analysis which uses an elongation at break value measured at -100°C in conformity with JIS K 7127:1999 (test piece type 5).

Description

Membrane material

[0001] The present invention relates to a membrane material.

[0002] Structures with a membrane material such as a resin film stretched over a framework have characteristics that differ from structures made of metal, concrete, wood, or fiber-reinforced plastic. For example, such structures with a membrane material stretched over a framework have characteristics such as being lightweight, being easily configurable into various shapes such as curved surfaces, being easy to assemble, and being able to fold the membrane material before assembly. These characteristics are useful for items that need to be transported to outer space by rocket, such as artificial satellites.

[0003] Recently, attention has been drawn to High Altitude Platform Stations (HAPS), which function as wireless base stations in the stratosphere. HAPS enables wireless communication in mountainous areas and remote islands where wireless base stations are not yet widely established, and in cases where terrestrial wireless base stations are damaged by disasters.

[0004] HAPS is required to be capable of continuous flight for long periods of time, for example, several months or more. Therefore, the body of the HAPS must be lightweight. The above-mentioned structure, which has a framework covered with a membrane material, is lightweight and therefore useful for aircraft such as HAPS, which require a lightweight body. The use of the above-mentioned structure in aircraft wings is described, for example, in Patent Document 1.

[0005] International Publication No. 2023 / 026662

[0006] General aircraft such as jetliners fly in the troposphere at an altitude of about 10 km. In contrast, HAPSs fly in the stratosphere at an altitude of about 20 km. The stratospheric environment differs significantly from the troposphere in various respects, including temperature. Furthermore, items transported to extraterrestrial environments by rockets are used, for example, in outer space, planets, satellites, or small celestial bodies. These extraterrestrial environments also differ significantly from the troposphere. Therefore, even if a certain resin film exhibits sufficient performance in the troposphere, this does not necessarily mean that the resin film will exhibit sufficient performance in the stratosphere or extraterrestrial environments.

[0007] Furthermore, commercially available resin films are generally not designed for use in the stratosphere or extraterrestrial environments. Therefore, it is not possible to determine whether a resin film is suitable for use in the stratosphere or extraterrestrial environments from data provided by the resin film manufacturer in a catalog or the like. In fact, the inventors have confirmed that the data provided by the resin film manufacturer cannot be used to estimate the susceptibility of a resin film to fracture in a low-temperature environment.

[0008] Therefore, an object of the present disclosure is to provide a technology that can contribute to the realization of membrane materials that can be used in the stratosphere or extraterrestrial environments and are less likely to break at low temperatures.

[0009] According to one aspect of the present invention, there is provided a membrane material for use in the stratosphere or extraterrestrial environments, the membrane material comprising a resin film, the resin film having a breaking elongation of 400,000 mm or less calculated by Weibull analysis using a value of breaking elongation measured at -100°C in accordance with JIS K7127:1999 (test piece type 5). ２ There is provided a membrane material in which the estimated breaking elongation at -100°C in any of the above areas S is 10% or more in both the MD and TD directions.

[0010] According to another aspect of the present invention, the Weibull coefficient m and the scale parameter ε determined in the Weibull analysis are ０ The film material according to the above aspect is provided in which the product of is 500 or more.

[0011] According to yet another aspect of the present invention, there is provided the film material according to any one of the above aspects, wherein the resin film includes a polyimide film, an ethylene-tetrafluoroethylene copolymer, or an ultra-high molecular weight polyethylene film.

[0012] According to yet another aspect of the present invention, a mass per area of ​​50 g / m ２ The following membrane material according to any one of the above aspects is provided.

[0013] According to yet another aspect of the present invention, there is provided a membrane material according to any of the above aspects, which is a membrane of a single layer structure.

[0014] According to yet another aspect of the present invention, there is provided a structure for use in the stratosphere or extraterrestrial environments, the structure comprising a membrane material according to any of the above aspects and a framework over which the membrane material is stretched.

[0015] According to yet another aspect of the present invention, a plurality of test pieces are obtained from a resin film, and the plurality of test pieces are subjected to a tensile test in accordance with JIS K7127:1999 (test piece type 5) in a constant temperature environment of −100° C. or less to obtain a plurality of combinations of cumulative fracture probability and fracture elongation, and a Weibull modulus m or a scale parameter ε is calculated from the plurality of combinations. ０ and comparing the Weibull coefficient m with a first threshold value, or obtaining the scale parameter ε ０ and a second threshold value, the Weibull coefficient m is smaller than the first threshold value, or the scale parameter ε ０ is smaller than the second threshold value, determining that the resin film is unsuitable as a membrane material or a part thereof to be used in a stratosphere or extraterrestrial environment.

[0016] According to yet another aspect of the present invention, there is provided an evaluation method according to the above aspect, which includes acquiring the Weibull coefficient m and comparing the Weibull coefficient m with the first threshold value.

[0017] According to yet another aspect of the present invention, there is provided the evaluation method according to the above aspect, wherein the tensile test is carried out at −100° C. and the first threshold value is 6 or more.

[0018] Alternatively, according to yet another aspect of the present invention, the scale parameter ε ０ and obtain the scale parameter ε ０ and the second threshold value.

[0019] According to yet another aspect of the present invention, there is provided the evaluation method according to the above aspect, wherein the tensile test is carried out at −100° C. and the second threshold value is 70 or more.

[0020] According to yet another aspect of the present invention, there is provided the evaluation method according to any one of the above aspects, wherein the tensile test is performed in both the MD direction and the TD direction of the membrane material.

[0021] According to yet another aspect of the present invention, there is provided an evaluation method including: obtaining a test piece from a resin film or a membrane material containing the resin film; supporting the test piece on a support having a hole such that the test piece covers the opening of the hole and is fixed to the support around the opening; piercing one or more indenters into a portion of the test piece covering the opening at a temperature of −100° C. or less and withdrawing the one or more indenters from this portion; then checking for the presence or absence of rupture in the portion; and if the rupture has occurred, determining that the resin film or membrane material is unsuitable as a membrane material or a part thereof for use in the stratosphere or extraterrestrial environments.

[0022] According to yet another aspect of the present invention, there is provided an evaluation method according to the above aspect, further comprising thermally shrinking the test piece fixed to the support prior to piercing the one or more indenters into the portion.

[0023] The present disclosure provides technology that can contribute to the realization of membrane materials that are used in the stratosphere or extraterrestrial environments and are less likely to break at low temperatures.

[0024] FIG. 1 is a perspective view of a structure according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view showing an example of a membrane material that may be included in the structure of FIG. 1. FIG. 3 is a cross-sectional view showing another example of a membrane material that may be included in the structure of FIG. 1. FIG. 4 is a graph showing the results of tensile tests performed on various membrane materials. FIG. 5 is a graph showing the average breaking elongation estimated when various membrane materials have a large area. FIG. 6 is a graph showing the Weibull parameter m of various membrane materials. FIG. 7 is a graph showing the Weibull parameter ε of various membrane materials. ０ 8 is a graph showing the relationship between the Weibull modulus and the fracture probability. FIG. 9 is a perspective view showing an example of a test specimen used in an evaluation method according to a third embodiment of the present invention. FIG. 10 is a diagram showing an example of the state of a membrane material contained in a test specimen after a hole has been drilled. FIG. 11 is a diagram showing another example of the state of a membrane material contained in a test specimen after a hole has been drilled. FIG. 12 is a diagram showing yet another example of the state of a membrane material contained in a test specimen after a hole has been drilled.

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are more specific embodiments of any of the above aspects. The following items can be incorporated into each of the above aspects, either singly or in combination.

[0026] Furthermore, the embodiments shown below are merely examples of configurations for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention is not limited by the materials, shapes, structures, etc. of the components described below. Various modifications can be made to the technical idea of ​​the present invention within the technical scope defined by the claims.

[0027] In the drawings, elements having the same or similar functions are denoted by the same reference numerals, and redundant explanations will be omitted. Furthermore, the drawings are schematic, and the relationship between dimensions in one direction and dimensions in another direction, and the relationship between the dimensions of one member and the dimensions of another member, etc. may differ from the actual relationship.

[0028] <1> First Embodiment The first embodiment relates to a membrane material for use in the stratosphere or extraterrestrial environments, and a structure including this membrane material for use in the stratosphere or extraterrestrial environments. The membrane material and structure will be described below with reference to the drawings.

[0029] Fig. 1 is a perspective view of a structure according to a first embodiment of the present invention, which is a structure to be used in the stratosphere or extraterrestrial environments.

[0030] Here, the range specified by "stratosphere to extraterrestrial environment" includes the region of the atmosphere from the stratosphere to outer space, outer space, and the surfaces of planets, satellites, and minor celestial bodies other than Earth. The region of the atmosphere from the stratosphere to outer space is a region of Earth's atmosphere at an altitude of 10 km or more, preferably 15 km or more, and preferably 20 km or more above the Earth's surface. Planets, satellites, and minor celestial bodies other than Earth may have surface temperatures of preferably -130°C or less, more preferably -170°C or less. Examples of planets and satellites other than Earth include Mars and the Moon, respectively. According to one example, the environment in which the structure 1 and the membrane material contained therein are used is the region of the stratosphere specified by the above altitude.

[0031] In one example, the structure 1 is an object or part thereof that flies in outer space, such as an artificial satellite, a space probe, a space passenger aircraft, or a space station. In another example, the structure 1 is a facility or part thereof that is installed on a planet other than Earth, a satellite, or a small celestial body, such as a lunar base. In yet another example, the structure 1 is a flying object or part thereof that flies in the stratosphere, such as a HAPS. Note that there are airplane types and airship types of HAPS.

[0032] The structure 1 includes a framework 10 and a membrane material 20. The framework 10 is made of a lightweight material such as carbon fiber reinforced plastic. The framework 10 defines the outer shape of the structure 1 or a part thereof.

[0033] The membrane material 20 is stretched over the framework 10. The membrane material 20 is fixed to the framework 10 by, for example, an adhesive. The membrane material 20 is a member that is exposed to the stratosphere or extraterrestrial environment.

[0034] Fig. 2 is a cross-sectional view showing an example of a membrane material that may be included in the structure of Fig. 1. Fig. 3 is a cross-sectional view showing another example of a membrane material that may be included in the structure of Fig. 1.

[0035] 2 is made of a resin film 21. In this manner, the film material 20 can have a single-layer structure.

[0036] 3 includes a resin film 21 having a first surface and a second surface opposite to the first surface, a protective layer 22 provided on the first surface, and a reinforcing layer 23 provided on the second surface. The protective layer 22 is a metal layer such as an aluminum layer provided on the first surface by vapor deposition. The reinforcing layer 23 is a layer containing fibers such as carbon fiber reinforced plastic.

[0037] The film material 20B may further include one or more other layers. For example, the film material 20B may further include an adhesive layer between the resin film 21 and the reinforcing layer 23. Alternatively, the film material 20B may have a structure in which the resin film 21 and a protective layer are sequentially provided on both sides of the reinforcing layer 23. Furthermore, one of the protective layer 22 and the reinforcing layer 23 may be omitted. In this way, the film material 20B may have a multi-layer structure.

[0038] The membrane material 20 has a mass per area of ​​100 g / m ２ Preferably, it is 50 g / m or less. ２ More preferably, it is 35 g / m or less. ２ It is more preferable that the mass per area of ​​the membrane material 20 is 5 g / m or less. A membrane material 20 having a small mass per area contributes greatly to reducing the weight of the structure 1. ２ It is preferable that the content is 15 g / m or more. ２ If the mass per area is made too small, the strength of the membrane material 20 may decrease.

[0039] When the membrane material 20 has a single-layer structure, the mass per area of ​​the resin film 21 is equal to the mass per area of ​​the membrane material 20. In this case, or when the membrane material 20 does not include the reinforcing layer 23, the thickness of the resin film 21 is preferably in the range of 3 μm to 120 μm, and more preferably in the range of 8 μm to 60 μm.

[0040] When the membrane material 20 includes the reinforcing layer 23, the resin film 21 is made thinner than when the membrane material 20 does not include the reinforcing layer 23. In this case, the thickness of the resin film 21 is preferably in the range of 3 μm to 60 μm, and more preferably in the range of 4 μm to 30 μm.

[0041] Hereinafter, as an example, the structure 1 is assumed to be the wing of an airplane-type HAPS, and the membrane material 20 will be described in further detail.

[0042] The biggest environmental difference between the stratosphere and the ground or troposphere is the temperature environment. Taking into account diurnal and seasonal variations, the membrane material 20 used in the stratosphere is required to be in the range of approximately -100°C to approximately 100°C without causing any problems.

[0043] In flight tests of airplane-type HAPS, cases were confirmed in which large fractures occurred in the membrane material made of resin film used on the wings. Initially, the inventors speculated that this fracture was due to thermal strain caused by temperature changes in the stratosphere.

[0044] However, when the present inventors evaluated this resin film by thermomechanical analysis (TMA) and performed a tensile test at −100°C (in accordance with JIS K7127:1999 (test piece type 5)), the following facts became clear: the amount of breaking elongation in the tensile test was significantly greater than the expected amount of thermal strain. This revealed that the above-mentioned breaking mechanism was difficult to explain based on initial speculation.

[0045] As a result of further investigation, the inventors found that the strength variations in the resin film used as the membrane material and the dimensional effect that occurs when the film is used over a large area are likely to be involved in the fracture. Based on this finding, they investigated resin membrane materials that can be suitably used in airplane-type HAPS.

[0046] First, formula (1) is established based on the idea of ​​a two-parameter Weibull distribution using the cumulative fracture probability of the membrane material and the fracture elongation in the tensile test at -100°C described above. In formula (1), the cumulative fracture probability is P and the fracture elongation is ε ｆ where m is the Weibull coefficient and ε ０ is a scale parameter. In this study, the average rank method was used for the cumulative failure probability P.

[0047]

[0048] Equation (1) can be transformed into the following equation (2), where ln represents the natural logarithm: ln[-ln(1-P)]=mlnε ｆ -mlnε ０ ...(2) In formula (2), Y, X, and B are expressed as ln[-ln(1-P)] and lnε, respectively. ｆ , and -mlnε ０ The inventors plotted data obtained by measurements, which were composed of a plurality of combinations of cumulative fracture probability and fracture elongation, to obtain a graph of a linear function according to equation (3), and calculated the Weibull coefficient m and scale parameter ε for the membrane material to be evaluated based on the slope and intercept of the graph. ０ was obtained.

[0049] Next, the obtained Weibull coefficient m and scale parameter ε ０ Using this, the average breaking elongation when the area of ​​the membrane material is S is calculated as "bar ε Sｆ " was calculated by the following formula (4), where Γ represents the gamma function.

[0050]

[0051] In equation (4), the area S ０The area of ​​the central part of the dumbbell-shaped test piece used in the tensile test at -100°C, where the tensile stress actually acts, is 6 mm x 33 mm = 198 mm. ２ ) was used.

[0052] The area S is 400,000 mm ２ Preferably, the area S is 480,000 mm ２ Here, the area S is set to the assumed maximum area of ​​the membrane material that is not joined to other members such as the framework when applied to an actual HAPS. Specifically, assuming that the above structure is used as the wing of an airplane-type HAPS, the area S is set to 400 mm x 1200 mm (= 480000 mm ２ )

[0053] Based on the above, the following study was carried out: Resin films made of the following eight materials were prepared, and a tensile test (-100°C, 200 mm / min) in accordance with JIS K7127:1999 (test piece type 5) was carried out to obtain the breaking strength (MPa) and breaking elongation (%).

[0054] - Polyvinyl fluoride (thickness 25 μm) - Polyimide (thickness 25 μm) - Polyetherimide (thickness 15 μm) - Polyaramid (thickness 12 μm) - Polyphenylene sulfide (thickness 12 μm) - Polyether ether ketone (thickness 25 μm) - Ultra-high molecular weight polyethylene (thickness 30 μm) - Ethylene-tetrafluoroethylene copolymer (thickness 25 μm).

[0055] Here, for each material, three dumbbell-shaped test pieces were prepared, one with the length direction in the MD direction and the other in the TD direction.

[0056] Three dumbbell-shaped test pieces, the length of which was in the MD direction, were cut out from the middle of the resin film in the length direction in a roll form. Of these three dumbbell-shaped test pieces, one was cut out from the middle of the resin film in the width direction, and the remaining two were cut out from near both ends of the resin film in the width direction.

[0057] Three dumbbell-shaped test pieces, the length of which was the transverse direction, were also cut from the middle of the roll of resin film. Of these three dumbbell-shaped test pieces, one was cut from the middle of the width of the resin film, and the remaining two were cut from the vicinity of both ends of the width of the resin film. The test pieces thus obtained were subjected to the tensile test described above.

[0058] The MD and TD directions are two mutually perpendicular directions present in industrially produced resin films. Generally, in a resin film distributed in a roll state, the length direction is the MD direction and the width direction is the TD direction. In a resin film distributed in a rectangular or square shape, the length direction of one side is the MD direction, and the length direction of another side perpendicular to this side is the TD direction.

[0059] Depending on the manufacturing process of the resin film, the molecular orientation may differ between the MD and TD directions, which we thought could affect the tensile test values ​​and, ultimately, the strength variation. Therefore, we performed measurements in two directions for one material.

[0060] 4 is a graph showing the results of a tensile test at −100° C. In some materials, the breaking elongation is 10% or less, but there is no significant difference in breaking strength.

[0061] Fig. 5 is a graph showing the estimated average breaking elongation at -100°C for large-area resin films, calculated based on formula (4). Unlike the results shown in Fig. 4 obtained for small dumbbell-shaped test pieces, the breaking elongation of polyvinyl fluoride in particular is significantly smaller than that of other resins.

[0062] Table 1 below shows detailed data on polyvinyl fluoride, ultra-high molecular weight polyethylene, and ethylene-tetrafluoroethylene copolymer, which showed relatively good estimated elongation at break when used in large areas. Specifically, Table 1 shows the elongation at break at -100°C obtained for dumbbell-shaped test specimens, as well as the Weibull modulus m and scale parameter ε as Weibull parameters. ０and the estimated breaking elongation when the area is large.

[0063]

[0064] The estimated breaking elongation in the transverse direction for polyvinyl fluoride in the large-area case was 0.11%. This estimated breaking elongation was below the 1% expected strain at -100°C, and was significantly smaller than the breaking elongation at -100°C obtained for the dumbbell-shaped specimen. This suggests that membrane materials made of polyvinyl fluoride may well break due to strain caused by low temperatures when used in the stratosphere.

[0065] On the other hand, for ultra-high molecular weight polyethylene and ethylene-tetrafluoroethylene copolymer, the estimated breaking elongation in the case of a large area was significantly greater than 10% in both the MD and TD directions. Therefore, it was considered that membrane materials made of ultra-high molecular weight polyethylene or ethylene-tetrafluoroethylene copolymer are unlikely to break due to strain caused by low temperatures, even when used in the stratosphere.

[0066] It is believed that the breakage occurred when the weakest part of the membrane could no longer withstand the force applied to it. Based on this and the data in Table 1, it is inferred that there were localized areas in the polyvinyl fluoride that could not withstand the deformation caused by strain at low temperatures.

[0067] In other words, the above study revealed that when a membrane material made of a resin film is made large, there is a possibility that there are regions having lower physical property values ​​than those obtained for a small dumbbell-shaped test piece. It was also considered important that such regions do not exist, even if only a small number of them, in order to be suitably applied to HAPS.

[0068] Assuming that the estimated breaking elongation in the case of a large area reflects the physical properties of the portion that is most difficult to deform, as described above, if this value is 10% or more in both the TD and MD directions, it is considered that the film can be suitably applied to an airplane-type HAPS that flies in the stratosphere. In this study, this applies to polyimide films, polyphenylene sulfide films, ultra-high molecular weight polyethylene films, and ethylene-tetrafluoroethylene copolymers.

[0069] On the other hand, the two parameters in Weibull analysis are the Weibull coefficient m and the scale parameter ε ０ The Weibull coefficient m represents the variation in strength, and the smaller the value, the greater the variation. ０ is considered to reflect the average strength of the material. From this point of view, the Weibull modulus m and the scale parameter ε ０ If the product of m and ε is 500 or more, it is considered that there is a very low possibility of occurrence of a portion that cannot withstand deformation due to strain occurring at low temperatures, even when variations are taken into consideration. ０ In this study, polyimide film, ultra-high molecular weight polyethylene film, and ethylene-tetrafluoroethylene copolymer met the above requirements.

[0070] From the above, among the resin films used in this study, films made of polyimide, ultra-high molecular weight polyethylene, or ethylene-tetrafluoroethylene copolymer were considered to be suitable as membrane materials for HAPS. However, this does not mean that films made of polyimide, ultra-high molecular weight polyethylene, or ethylene-tetrafluoroethylene copolymer are naturally suitable as membrane materials for HAPS. In the production of resin films, even if they are produced under the same conditions, their properties vary slightly. Furthermore, the above-mentioned parameters are not control targets during production. Therefore, even films made of polyimide, ultra-high molecular weight polyethylene, or ethylene-tetrafluoroethylene copolymer may not satisfy the above requirements, and for example, confirmation by measurement is required for each production lot of the resin film.

[0071] Some of the physical properties of the films used in the above study are shown in Table 2. Specifically, Table 2 shows the thermal decomposition onset temperature, glass transition temperature, and linear expansion coefficient for each of the films used in the above study.

[0072]

[0073] As can be seen from Table 2, the physical properties of specific films are not unique, and it was therefore thought to be difficult to find a polyimide film, an ultra-high molecular weight polyethylene film, or an ethylene-tetrafluoroethylene copolymer film that was considered suitable in the above study based on these physical properties.

[0074] <2> Second Embodiment The second embodiment is an embodiment relating to an evaluation method for determining whether a resin film is unsuitable for use as a membrane material or a part thereof in the stratosphere or extraterrestrial environments. This evaluation method and the studies conducted in devising it are described below.

[0075] As is clear from the explanation of the first embodiment, an actual HAPS blade includes multiple large-area membranes, and if even one of them breaks, it will cause a major problem. Taking this into consideration, we calculated the probability that one or more membranes will break under a strain of 1%, which is the assumed strain amount at -100°C, when multiple large-area membranes are present.

[0076] Specifically, for each of the materials examined in the first embodiment, the above calculations were performed with the number of film materials set to 10, 100, and 1000. The results are shown in Table 3.

[0077]

[0078] As shown in Table 3, it is estimated that large-area membrane materials made of polyvinyl fluoride will almost certainly break if the number of membranes exceeds 100. Also, large-area membrane materials made of polyether ether ketone do not appear to pose much of a problem in Figure 5, but the probability of breakage was approximately 24% when the number was 100, and approximately 93% when the number was 1000.

[0079] FIG. 8 is a graph showing the relationship between the Weibull coefficient m and the probability of fracture when the number of large-area membrane materials is 100. From this study, it was found that when the Weibull coefficient m is 6 or more, even if the number of large-area membrane materials is 1000, there is almost no possibility of fracture occurring. In addition, the scale parameter ε ０ When a similar plot was performed using the scale parameter ε ０ The results showed that when the value is 70 or more, there is almost no possibility of breakage even when there are 1000 large-area membrane materials.

[0080] As is clear from the above explanation, by utilizing the following evaluation method, it can be determined whether or not a resin film is suitable as a membrane material or a part thereof to be used in the stratosphere or extraterrestrial environments, for example, as a membrane material or a part thereof to be used in the wings of an airplane-type HAPS.

[0081] First, a plurality of test pieces are obtained from the resin film, such as the dumbbell-shaped test pieces described in the first embodiment.

[0082] Next, these test pieces are subjected to a tensile test in accordance with JIS K7127:1999 (test piece type 5) in a constant temperature environment of -100°C or less to obtain multiple combinations of cumulative fracture probability and fracture elongation. This temperature is preferably -100°C or less. Also, this temperature is preferably -196°C or more, and more preferably -180°C or more. According to one example, this tensile test is performed at -100°C. According to another example, this tensile test is performed at -170°C. The number of combinations obtained is preferably 3 or more, and more preferably 5 or more. There is no upper limit to this number, but considering efficiency, it is preferably 10 or less.

[0083] Next, the Weibull coefficient m or the scale parameter ε ０ Weibull coefficient m and scale parameter ε ０ can be obtained by the same method as in the first embodiment.

[0084] Next, the Weibull coefficient m is compared with the first threshold value. Alternatively, the scale parameter ε ０ and a second threshold value. Then, if the Weibull coefficient m is smaller than the first threshold value, or if the scale parameter ε ０ is smaller than the second threshold value, the resin film is determined to be unsuitable as a membrane material or a part thereof for use in the stratosphere or extraterrestrial environments.

[0085] The first and second thresholds are the Weibull modulus m and the scale parameter ε at which the fracture probability is sufficiently small, for example, 0.1% or less. ０ The first and second threshold values ​​are set appropriately depending on the expected temperature of the membrane material 20, the expected size of the membrane material 20, the expected number of membrane materials 20, and the like.

[0086] For example, when the expected minimum temperature is about -100°C, the area S of the membrane material 20 is within the above range, and the number of membrane materials 20 in the structure 1 is 30 or less, it is preferable to perform the tensile test at -100°C and set the first threshold value to 4 or more. When the number of membrane materials 20 is 31 or more and 100 or less, it is preferable to perform the tensile test at -100°C and set the first threshold value to 6 or more, for example, 20 or less. In this case, the first threshold value may be 6.

[0087] In the above case, the second threshold is preferably set to 70 or more. In this case, the second threshold is, for example, 170 or less. In this case, the second threshold may be 70.

[0088] Resin films that are not determined to be unsuitable may be determined to be suitable as membrane materials or parts thereof for use in stratospheric or extraterrestrial environments, or may be subjected to one or more other evaluations to determine whether or not they are suitable as membrane materials or parts thereof for use in stratospheric or extraterrestrial environments.

[0089] This evaluation method is extremely useful in that it allows accurate evaluation without using a membrane material 20 with a large area, such as that actually used in the structure 1.

[0090] <3> Third Embodiment: A membrane under tension may burst when subjected to a localized stimulus. For example, when a needle is pierced into a balloon, not only does a hole with a diameter approximately equal to the needle diameter appear in the rubber membrane, but the entire membrane instantly tears, resulting in a so-called burst. This is a phenomenon in which the localized tear propagates and spreads rapidly, often accompanied by a popping sound. Similarly, when a needle is pierced into a glass plate, not only a hole appears in the glass plate, but a tear (crack) spreads throughout the glass plate instantly. Similar to a plate made of a brittle material such as glass, a membrane that becomes embrittled due to a drop in temperature also breaks when localized stress is applied, and this tear propagates rapidly, resulting in a burst.

[0091] In the structure 1, the membrane material 20 is stretched with a constant tension on the framework 10. Since the structure 1 is used in the stratosphere or extraterrestrial environments, the membrane material 20 is subject to tension changes and material property changes due to temperature changes, and tension changes due to changes in wind pressure or atmospheric pressure, as well as to local stimuli such as particle collisions and discharges. Therefore, it is desirable that the membrane material 20 be resistant to rupture when subjected to local stimuli in a low-temperature environment.

[0092] There are ISO and ASTM standards for evaluating the basic physical properties and mechanical strength (e.g., tensile strength) of materials. However, although the ISO and ASTM standards specify test methods for tear strength, membrane rupture strength, and puncture strength, they do not allow evaluation of the propagation of breakage, such as bursting.

[0093] There is also a test called the Mullen burst test, which measures the strength of a membrane by hydraulically inflating a rubber membrane to burst the membrane. However, this test does not evaluate the extent of the tear, and since it uses a testing machine that includes a rubber membrane, it cannot be used at low temperatures (for example, below -80°C).

[0094] The following describes a method for evaluating the resistance of a membrane material to bursting in a low-temperature environment.

[0095] As described above, taking into consideration diurnal and seasonal variations, the membrane material 20 used in the stratosphere is required to be able to withstand temperatures ranging from about -100°C to about 100°C without causing any problems. Furthermore, since the HAPS is as large as a jumbo jet, if the structure 1 were used as the wings of the HAPS, the total area of ​​the membrane material 20 would be extremely large. Therefore, it is desirable for the membrane material 20 to have high resistance to rupture in low-temperature environments.

[0096] After extensive research, the inventors discovered that when a micro-hole is formed in a membrane material, some membrane materials remain as a micro-hole and are resistant to widening, while others are prone to large ruptures (bursts) starting from the hole. Furthermore, the inventors noted that in an airplane-type HAPS, even if a micro-hole is formed in the membrane material used for the wing due to a collision with a small object or discharge, as long as the micro-hole remains as a micro-hole, lift will not be excessively reduced and flight will not be hindered. Based on these findings, the inventors developed an evaluation method that can identify resin membrane materials that are less likely to develop long cracks when a micro-hole is formed.

[0097] In developing the evaluation method, one of the conditions was that this phenomenon be reproducible on a tabletop, for the following reasons: Some aircraft-type HAPS systems are several tens of meters long. The membrane material is fixed to the framework by adhesive or other means, but it is not uncommon for the area of ​​the part not joined to the framework to be several square meters. If an attempt were made to evaluate the presence or absence of rupture and the likelihood of such an occurrence using such a large-area membrane material, the evaluation equipment would be quite large, which would pose problems in terms of installation space, cost, etc.

[0098] Therefore, in the method according to the present embodiment, a test piece obtained from a resin film or membrane material is supported on a support having a hole, and the test piece is used as a test piece. An example of the test piece is shown in FIG. 9.

[0099] 9 is a test piece 20Z cut out from a resin film or membrane material and supported on a stainless steel support 10Z having a hole 11 so that the test piece 20Z covers the opening of the hole 11 and is fixed to the support 10Z around the opening. The portion of the test piece 20Z that covers the opening of the hole 11 is a circular evaluation area 20a.

[0100] The material of the support 10Z is not particularly limited as long as it does not significantly expand, contract, or deform within the expected temperature range. Therefore, it is not limited to the stainless steel described above, and carbon or the like can also be used. However, when shrinking the membrane material described below, metals such as stainless steel are preferred from the viewpoint of heat resistance.

[0101] The support 10Z is donut-shaped with a circular hole, but the shape of the outer edge of the support 10Z is not limited to a circle and may be a rectangle, another polygon, etc. Alternatively, the support 10Z may have two or more holes 11 so as to generate two or more evaluation regions 20a.

[0102] The opening of the hole 11 is circular here, but may have other shapes. For example, the contour of the opening of the hole 11 may have a protrusion that protrudes inward. As will be described later, in this method, an indenter is thrust into the evaluation area 20a. Breakage (burst) often occurs between two parts to which stress is applied. If the line connecting these two parts is considered to be a line of force, when there is only one indenter and the opening of the hole 11 is circular, a line of force is unlikely to occur. If the contour of the opening of the hole 11 is provided with the above-mentioned protrusion, a line of force is generated between the part of the evaluation area 20a where the indenter is pressed and the part where the protrusion is in contact, making breakage (burst) more likely to occur.

[0103] The area of ​​the opening of the hole 11, i.e., the area of ​​the evaluation area 20a, is 300 mm ２ From the viewpoint of performing the evaluation on a table, this area is preferably 8000 mm ２ It is preferable that:

[0104] There are no particular limitations on the method for fixing the test piece 20Z to the support 10Z. For example, double-sided tape having adhesive or pressure-sensitive adhesive layers on both sides of a substrate, or a liquid or paste adhesive can be used for this fixation.

[0105] In this method, first, the above-mentioned specimen 1Z is prepared, and then the specimen 1Z is cooled to a temperature of −100°C or lower. This temperature is preferably −100°C or lower. Also, this temperature is preferably −196°C or higher, and more preferably −170°C or higher. In one example, this temperature is −110°C. In another example, this temperature is −170°C.

[0106] Next, at the above temperature, one or more indenters are inserted into the portion of the test piece 20Z that blocks the opening of the hole 11, i.e., the evaluation region 20a, and then pulled out from this portion. The number of indenters inserted into the evaluation region 20a may be one, as described above, or may be two or more.

[0107] When two or more indenters are inserted into the evaluation area 20a, lines of force are generated between the two or more portions of the evaluation area 20a where the indenters are in contact. The distance between the indenters is preferably within a range of 5 mm or more and less than 20 mm. If this distance is made shorter, it becomes difficult to evaluate the extent of the tear. If this distance is made longer, the effect of forming lines of force is weakened.

[0108] The indenter has a columnar shape, such as a cylinder, triangular prism, square prism, pentagonal prism, or hexagonal prism. When a prismatic indenter is used, it is easier to pierce the evaluation region 20a and it is more likely to break than when a cylindrical indenter is used. Therefore, it is preferable that the indenter has a prismatic shape. Furthermore, the tip of the indenter may be sharp, spherical, or flat, but from the perspective of the balance between pressure load and piercing ability, it is more preferable that it be flat rather than sharp or spherical.

[0109] The maximum diameter of the portion of the indenter that penetrates the evaluation area 20a affects the pressure load and penetration properties. This maximum diameter is preferably in the range of 0.5 mm to 5 mm, and more preferably in the range of 1 mm to 2 mm. If this maximum diameter is excessively large, many films cannot be penetrated into the evaluation area 20a. If this maximum diameter is small, it may be difficult to produce differences in tear propagation due to the material.

[0110] The material of the indenter is preferably one that will not deform or break due to pressure or temperature changes during piercing, and is preferably a metal material such as iron, titanium, stainless steel, or special steel containing nickel, chromium, molybdenum, or the like.

[0111] When an indenter is inserted into the evaluation area 20a and then removed from this area, the test piece 20Z either ruptures, cracks without rupturing, or develops a hole without rupturing or cracking. If the test piece 20Z ruptures, it can be determined that the membrane material or resin film is unsuitable for use as a membrane material or part thereof in the stratosphere or extraterrestrial environments.

[0112] A membrane or resin film in which a crack develops without causing the test piece 20Z to rupture may be determined to be unsuitable for use in the stratosphere or an extraterrestrial environment, or may not be determined to be unsuitable for use in the stratosphere or an extraterrestrial environment. Alternatively, only membranes or resin films in which a crack develops without causing the test piece 20Z to rupture, whose maximum crack length (or the linear distance from one end of the crack to the other end) exceeds a predetermined threshold, may be determined to be unsuitable for use in the stratosphere or an extraterrestrial environment. In this case, membranes or resin films in which a crack develops without causing the test piece 20Z to rupture, whose maximum crack length (or the linear distance from one end of the crack to the other end) is equal to or less than the threshold, may be determined to be suitable for use in the stratosphere or an extraterrestrial environment, or may be subjected to one or more other evaluations to determine whether they are suitable for use in the stratosphere or an extraterrestrial environment.

[0113] A membrane or resin film in which the test piece 20Z has developed a hole without rupture or cracking may be determined to be suitable for use as a membrane or part thereof in a stratospheric or extraterrestrial environment. Alternatively, a membrane or resin film in which the test piece 20Z has developed a hole without rupture or cracking may be subjected to one or more other evaluations to determine whether it is suitable for use as a membrane or part thereof in a stratospheric or extraterrestrial environment.

[0114] In the above method, the indenter is used to pierce the sample. However, the inventors have also conducted the following test without piercing the sample with the indenter. ２Test pieces 20Z were cut out from the membrane materials that were confirmed to rupture in the above large-area tests, and these test pieces 20Z were used to prepare test pieces 1Z shown in Figure 9. Next, these test pieces 1Z were left to stand for a certain period of time at low temperatures (-196°C or higher and -100°C or lower) that simulated the stratosphere, and the frequency of rupture of test pieces 20Z was investigated. However, even in the coldest environment (estimated membrane temperature: around -190°C) where the test pieces were immersed in liquid nitrogen, no rupture occurred. In other words, the above method, which does not involve piercing with an indenter, was unable to evaluate the resistance of the membrane material to rupture in a low-temperature environment.

[0115] Tests conducted by the present inventors regarding the above evaluation method of piercing with an indenter will be described below.

[0116] First, test pieces 20Z were cut out from the resin films. The test pieces 20Z were obtained from the following eight resin films.

[0117] - Polyvinyl fluoride (thickness 25 μm) - Polyimide (thickness 25 μm) - Polyetherimide (thickness 15 μm) - Polyaramid (thickness 12 μm) - Polyphenylene sulfide (thickness 12 μm) - Polyether ether ketone (thickness 25 μm) - Ultra-high molecular weight polyethylene (thickness 30 μm) - Ethylene-tetrafluoroethylene copolymer (thickness 25 μm).

[0118] These test pieces 20Z were fixed to the support 10Z, thereby obtaining test specimens 1Z. Here, the support 10Z was a circular hole with an opening diameter of 20 mm (the opening area was approximately 314 mm ２ A doughnut-shaped stainless steel plate having a hole (20Z) was used. The test piece 20Z was fixed to the support 10Z with double-sided tape over the entire periphery of the opening.

[0119] Here, a thermocouple was brought into contact with the evaluation region 20 a to enable detection of the temperature of the test piece 20 Z. In this case, if the joint between the support 10 Z and the test piece 20 Z is clamped with a clip or the like and the clip is used to fix the thermocouple, it is possible to prevent the test piece 20 Z from unintentionally peeling off from the support 10 Z during evaluation due to a decrease in the joint strength at low temperatures.

[0120] Next, specimen 1Z was placed in a low-temperature environment of -110°C. When the temperature of evaluation area 20a, monitored using a thermocouple, reached a predetermined temperature, two metal indenters were simultaneously inserted into the center of evaluation area 20a, spaced 10 mm apart. Cylindrical indenters with a maximum diameter of 1 mm and flat tips were used. These indenters were inserted into evaluation area 20a so that their midpoint coincided with the center of evaluation area 20a. Immediately after inserting the indenters into evaluation area 20a, they were removed from evaluation area 20a.

[0121] Thereafter, the test specimen 1Z was taken out into a room temperature environment, and the evaluation area 20a was visually observed and evaluated according to the following criteria: AA: No change, A: Cracks occurred, B: Rupture.

[0122] Here, "no change" refers to a state in which a hole h was formed by the piercing of the indenter, but no crack extended from the hole h, as shown in Fig. 10. "crack generation" refers to a state in which a crack Cr extended from the hole h was generated, but the maximum length of the crack Cr was less than 5 mm, as shown in Fig. 11. "rupture" refers to a state in which a crack Cr extended from the hole h was generated, and the maximum length of the crack Cr was 5 mm or more, as shown in Fig. 12.

[0123] Here, since the shortest distance from the hole h to the outline of the evaluation area 20a is approximately 5 mm, if the crack Cr reaches the outline of the evaluation area 20a, it can be determined that the crack is in a "rupture" state without measuring the length of the crack Cr. Also, here, even if two holes h are connected by a crack Cr, this crack Cr can be considered to be a connection between a crack extending from one hole h and a crack extending from the other hole h, and since at least one of these cracks can be said to be 5 mm or longer in length, it can be determined that the crack is in a "rupture" state without measuring the length of the crack Cr. The results are shown in Table 4.

[0124]

[0125] For comparison, Table 4 also shows the results of evaluations carried out in the same manner as above, except that the temperature of the test piece 20Z was set to 25°C.

[0126] None of the test pieces 20Z ruptured when the temperature was 25° C., but the test piece 20Z made of polyvinyl fluoride ruptured when the temperature was −110° C. In this way, it was confirmed that resin materials that are of concern when applied to airplane-type HAPS are difficult to extract in a normal evaluation environment, but can be extracted using the above-mentioned evaluation method.

[0127] If the membrane material is a stretched resin film, it can be heated after being attached to the framework to cause it to shrink. This allows the membrane material to be given strong tension in the structure. Assuming this situation, the above-mentioned resin films that have the property of shrinking when heated were evaluated in the same manner as above, except that the test piece 20Z attached to the support 10Z was subjected to thermal shrinkage. The results are shown in Table 4.

[0128] As shown in Table 4, there were no resin films for which the evaluation differed depending on whether or not they had been thermally shrunk. However, by using test specimen 1Z, which was obtained by thermally shrunk test specimen 20Z, for evaluation, it was possible to perform an evaluation closer to the actual application state.

[0129] From the above evaluation, it was found that, among the above resin films, those made of ethylene-tetrafluoroethylene copolymer, ultra-high molecular weight polyethylene, polyphenylene sulfide, polyether ether ketone, etc. are promising materials for aircraft-type HAPS membranes.

[0130] Next, the following evaluation was further performed on the resin film made of polyvinyl fluoride. That is, the test piece 20Z was cut out from the resin film made of polyvinyl fluoride, and the same evaluation as above was performed except that the temperatures of the evaluation area 20a were set to -60°C, -70°C, -80°C, -90°C, and 100°C. Here, five test pieces were prepared for each temperature, and evaluation was performed on all of them. The temperature accuracy was ±4°C. The results are shown in Table 5.

[0131]

[0132] Table 5 shows the number of test specimens evaluated as "A" and the number of test specimens evaluated as "B." There were no test specimens evaluated as "AA."

[0133] The polyvinyl fluoride film used in this evaluation has a continuous use temperature of -70°C or higher. As shown in Table 5, this polyvinyl fluoride film tended to burst frequently when the temperature was -80°C or lower. From the above, it was confirmed that the above evaluation method gave sufficiently valid results.

[0134] The above-described evaluation method can be used to select promising materials from multiple candidate materials, and can also be used to evaluate the fracture probability of a certain material in detail.

[0135] This evaluation method can be modified in various ways. For example, the hole 11 provided in the support 10Z does not need to penetrate the support 10Z, but may be a hole with a bottom. The hole 11 does not need to be located at the center of the support 10Z in a plan view, as long as there is a support region surrounding the opening. The width of this support region is preferably 10 mm or more from the viewpoint of reliably fixing the test piece 20Z to the support 10Z.

[0136] The above evaluation of a certain resin film may be performed using only one test piece 20Z obtained from the resin film, or may be performed using multiple test pieces 20Z obtained from the resin film. In the latter case, the evaluation can be performed with higher accuracy.

[0137] The number of indenters piercing the test piece 20Z does not have to be 2. As described above, the number of indenters may be 1, or 3 or more.

[0138] As described above, by piercing the test piece 20Z with two indenters almost simultaneously, evaluation can be performed with high accuracy even when the number of test pieces 20Z is small.

[0139] On the other hand, when only one indenter is used, the frequency of rupture in the above evaluation may be slightly reduced unless some modification is made to the shape and dimensions of the tip of the indenter or the outline shape of the opening of the hole 11. However, even in this case, by obtaining multiple test pieces 20Z from a single resin film and performing the above evaluation on all of these test pieces 20Z, accuracy and results roughly similar to those obtained when two indenters are used were obtained.

[0140] In the resin film made of polyvinyl fluoride, the opening of the hole 11 is a circle with a diameter of 20 mm (approximately 314 mm ２ ) and a cylindrical indenter with a maximum diameter of 1 mm and a flat tip, and performing the above evaluation at −110°C, an evaluation method in which two indenters were simultaneously inserted into the test piece 20Z at 10 mm intervals resulted in the test piece 20Z bursting in all five tests. On the other hand, when the same evaluation method as above was used except that the number of indenters was one, three test sets each consisting of five tests were performed, and three, four, and four test pieces 20Z burst in the first test set, the second test set, and the third test set, respectively. Thus, unless some modification was made to the shape and dimensions of the indenter tip or the contour shape of the opening of the hole 11, the number of test pieces 20Z that burst was reduced when the number of indenters was one. However, by repeating the test, it is possible to evaluate the likelihood of bursting with high accuracy.

[0141] Furthermore, by appropriately changing the shape and dimensions of the tip of the indenter or the contour shape of the opening of hole 11, highly accurate evaluation is possible even if only one indenter is used and the test is conducted only once, for example.

[0142] 1...structure, 1Z...test specimen, 10...framework, 10Z...support, 11...hole, 20...membrane material, 20A...membrane material, 20a...evaluation area, 20B...membrane material, 20Z...test piece, 21...resin film, 22...protective layer, 23...reinforcing layer, Cr...crack, h...hole

Claims

1. A membrane material for use in the stratosphere or extraterrestrial environments, comprising a resin film, the resin film having a breaking elongation of 400,000 mm or more calculated by Weibull analysis using the value of breaking elongation measured at -100°C in accordance with JIS K7127:1999 (test piece type 5). ２ A film material in which the estimated breaking elongation at -100°C in any of the above areas S is 10% or more in both the MD and TD directions.

2. The Weibull coefficient m and scale parameter ε identified in the Weibull analysis ０ 2. The membrane material according to claim 1, wherein the product of [mathematical formula - see original document] and [mathematical formula - see original document] is 500 or more.

3. The film material according to claim 1 or 2, wherein the resin film comprises an ethylene-tetrafluoroethylene copolymer, a polyimide film, or an ultra-high molecular weight polyethylene film.

4. Mass per area is 50g / m ２ 4. The membrane material according to claim 1, wherein:

5. The membrane material according to any one of claims 1 to 4, which is a membrane of a single layer structure.

6. A structure for use in the stratosphere or extraterrestrial environments, comprising a membrane material according to any one of claims 1 to 5 and a framework on which said membrane material is stretched.

7. Obtaining a plurality of test pieces from a resin film; subjecting the plurality of test pieces to a tensile test in accordance with JIS K7127:1999 (test piece type 5) in a constant temperature environment of -100°C or less to obtain a plurality of combinations of cumulative fracture probability and fracture elongation; and determining the Weibull modulus m or the scale parameter ε from the plurality of combinations. ０ and comparing the Weibull coefficient m with a first threshold value or obtaining the scale parameter ε ０ and a second threshold value, the Weibull coefficient m is smaller than the first threshold value, or the scale parameter ε ０ is smaller than the second threshold value, determining that the resin film is unsuitable as a membrane material or a part thereof to be used in the stratosphere or extraterrestrial environments.

8. The evaluation method according to claim 7, further comprising the steps of: acquiring the Weibull coefficient m; and comparing the Weibull coefficient m with the first threshold value.

9. The evaluation method according to claim 8, wherein the tensile test is carried out at −100° C., and the first threshold value is 6 or more.

10. The scale parameter ε ０ and obtain the scale parameter ε ０ The evaluation method according to claim 7 , further comprising comparing the first threshold value with the second threshold value.

11. The evaluation method according to claim 10, wherein the tensile test is carried out at −100° C. and the second threshold value is 70 or more.

12. An evaluation method according to any one of claims 7 to 11, wherein the tensile test is carried out in both the machine direction and the transverse direction of the membrane material.

13. An evaluation method comprising: obtaining a test piece from a resin film or a membrane material containing said resin film; supporting said test piece on a support having a hole so that said test piece covers the opening of said hole and is fixed to said support around said opening; piercing one or more indenters into the part of said test piece covering said opening at a temperature of -100°C or less and withdrawing said one or more indenters from said part; then checking for the presence or absence of rupture in said part, and if said rupture has occurred, determining that said resin film or membrane material is unsuitable as a membrane material or part thereof for use in the stratosphere or extraterrestrial environments.

14. The evaluation method according to claim 13, further comprising heat shrinking the test piece fixed to the support prior to piercing the portion with the one or more indenters.

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