JP7466889B2 - Composite Sheet - Google Patents

Description

The present invention relates to a composite sheet.

One known example of a membrane material used in buildings is one in which a fluororesin is laminated on both sides of a sheet-like fabric made of glass fiber. Since glass fiber fabric is made of tightly woven glass fibers, it has relatively good tear strength. However, since the weave of the fabric is basically made of biaxial fibers of warp and weft, there is room for improvement in the tear strength of membrane materials that contain fabric.

On the other hand, membrane structures that use such membrane materials as roofing materials may require a moderate light transmittance, taking into consideration the thermal environment and brightness inside the building. It is difficult to achieve a high light transmittance with glass fiber fabric because the opening ratio of the fabric is small. For example, the visible light transmittance of membrane materials that contain glass fiber fabric is about 10% to 20%, and it may not be possible to achieve the required light transmittance.

Japanese Patent No. 6096141 Japanese Patent Application Laid-Open No. 10-018146

The present invention was made in consideration of the above circumstances, and aims to provide a composite sheet that has appropriate visible light transmittance and high strength.

According to one aspect of the present invention, there is provided a composite sheet. The composite sheet comprises a core material of a multiaxial woven fabric, and a fluororesin-containing layer containing an ethylene-tetrafluoroethylene copolymer formed on both sides of the core material. In the thickness direction of the composite sheet, the visible light transmittance at wavelengths of 360 nm to 740 nm is in the range of 30% to 70%. The tear strength in the in-plane direction of the composite sheet is in the range of 350 N to 450 N. The core material includes fibers and an adhesive. The fibers are at least one type selected from the group consisting of glass fibers, carbon fibers, and aramid fibers. The adhesive is at least one type selected from the group consisting of acrylic acid ester polymers, vinyl acetate polymers, and copolymers of acrylic acid esters and vinyl acetate. The opening ratio of the core material is in the range of 50% to 65%.

The present invention provides a composite sheet with high visible light transmittance and excellent tear strength.

FIG. 1 is a cross-sectional view illustrating an example of a composite sheet according to an embodiment. FIG. 2 is a plan view showing an example of a triaxial woven fabric according to an embodiment of the present invention. A plan view showing an example of a four-axis woven fabric according to an embodiment of the present invention. 1 is a graph showing the results of measuring the visible light transmittance of composite sheets according to examples and comparative examples. 4 is a graph showing stress-strain curves of composite sheets according to examples and comparative examples. 1 is a graph showing the results of a tear strength test of composite sheets according to an embodiment and a comparative example.

The following describes the embodiments with reference to the drawings as appropriate. Note that common components throughout the embodiments are given the same reference numerals, and duplicate explanations will be omitted. Also, each figure is a schematic diagram to explain the embodiments and facilitate their understanding, and the shapes, dimensions, ratios, etc. may differ from the actual devices in some places, but these can be modified as appropriate by taking into account the following explanations and known technologies.

When high light transmittance is required for the membrane material, a film made only of fluororesin may be used as the membrane material. This is because, although it depends on the type of resin, a film that does not contain a core material exhibits a higher light transmittance than a film that does contain a core material. However, since a fluororesin film that does not contain a core material has low strength, in one example, a so-called creep phenomenon occurs when a certain load is applied. The creep phenomenon is a phenomenon in which a film continues to stretch while a load is applied. When constructing a membrane structure, in order to maintain the strength of the membrane structure, a load that causes creep cannot be applied. Strictly speaking, construction must be performed with a load below the tensile yield point of the membrane material. In order to construct with a load below the tensile yield point, for example, it is necessary to shorten the distance (span) between the steel or wooden beams that serve as the basis for expansion, which results in a problem of high costs for constructing the membrane structure.

The composite sheet according to the embodiment includes not only a fluororesin-containing layer but also a core material of a multiaxial woven fabric. A multiaxial woven fabric is a woven fabric in which threads are arranged in three or more axial directions. In each axial direction, multiple thread groups are arranged at a predetermined interval along the in-plane direction. Specific examples of multiaxial woven fabrics include triaxial woven fabrics and four-axial woven fabrics.

The details of triaxial and four-axis woven fabrics will be described later, but triaxial woven fabrics have a warp group arranged along the in-plane direction, a first diagonal group arranged on the warp group so as to intersect with the warp group, and a second diagonal group arranged on the warp group or on the first diagonal group so as to intersect with the warp group and the first diagonal group. In other words, triaxial woven fabrics include a warp group, a first diagonal group, and a second diagonal group arranged in three different directions. These thread groups are not densely arranged, but are arranged at a predetermined interval. In other words, multiaxial woven fabrics such as triaxial woven fabrics have multiple openings formed by the intersection of each thread group. When visible light is incident along a direction perpendicular to the in-plane direction of the core material, the visible light mainly passes through these openings.

The fluororesin-containing layer of the composite sheet contains ethylene-tetrafluoroethylene copolymer (ETFE: ethylene tetrafluoro ethylene). ETFE is a copolymer of tetrafluoroethylene (C ₂ F ₄ ) and ethylene (C ₂ H ₄ ), and has a lamellar structure specific to amorphous plastics, unlike crystalline plastics such as PTFE, so that the fluororesin-containing layer containing ETFE has a high visible light transmittance.

The copolymerization molar ratio of ethylene:tetrafluoroethylene in ETFE is typically 1:1. However, the copolymerization ratio is not limited to 1:1. The copolymerization molar ratio of ethylene:tetrafluoroethylene is, for example, in the range of 30:70 to 70:30, and preferably in the range of 50:50 to 60:40. When the copolymerization molar ratio of ETFE is within this range, it has the effect of having low haze and high light transmittance.

The fluororesin-containing layers, including ETFE, are provided on both sides of the core material, which is a multiaxial woven fabric. The fluororesin-containing layers are also formed in the openings of the core material, and because the visible light transmittance of the fluororesin-containing layers is high, a large amount of visible light can be transmitted through the openings of the core material.

On the other hand, the threads that make up the core material do not have excellent visible light transmittance. As a result, the composite sheet according to the embodiment can achieve a moderate visible light transmittance of 30% to 70% in the thickness direction at wavelengths of 360 nm to 740 nm. In other words, the visible light transmittance is in the range of 30% to 70% at any wavelength within the range of 360 nm to 740 nm. Note that there is no particular restriction on the spacing between the thread groups that make up the multiaxial woven fabric, i.e., the distance between the threads, and the spacing between the thread groups can be set appropriately as long as the thread groups are arranged so that the composite sheet has a visible light transmittance of 30% to 70% at wavelengths of 360 nm to 740 nm.

If the visible light transmittance of the composite sheet at wavelengths of 360 nm to 740 nm is less than 30%, it may be difficult to meet the required transmittance when the composite sheet is used in a membrane structure. If the visible light transmittance exceeds 70%, for example, the transmittance of sunlight may be too high, causing the temperature inside the room to rise too much. It is preferable that the visible light transmittance at wavelengths of 360 nm to 740 nm is in the range of 40% to 60%, and more preferably in the range of 50% to 60%.

The visible light transmittance in the thickness direction of the composite sheet can be measured in accordance with JIS R 3106:1998 using a spectrophotometer V-670 manufactured by JASCO Corporation, or a device with equivalent functions. This measurement makes it possible to measure the visible light transmittance of the sheet being measured, for example, at wavelengths of 360 nm to 740 nm.

Furthermore, since the core material has groups of threads arranged in three, four or more different directions, it has excellent tear strength in various directions and can suppress creep of the composite sheet. In addition, it is possible to omit separate reinforcement even in the parts of the composite sheet where stress is applied in the tear direction. Furthermore, since the composite sheet having this core material has excellent isotropy, it is possible to reduce the risk of the composite sheet being damaged due to stress concentration. The tear strength of the composite sheet in the in-plane direction is, for example, in the range of 300N to 450N, preferably in the range of 350N to 450N, and more preferably in the range of 400N to 450N. The tear strength of the composite sheet in the in-plane direction can be measured in accordance with JIS L 1096 C method (trapezoid method).

As such, the composite sheet has moderate visible light transmittance and high tear strength, which can expand the range of applicable architectural designs and the range of design possibilities.

Figure 1 is a cross-sectional view showing an example of a composite sheet according to an embodiment. Figure 2 is a plan view showing an example of a triaxial woven fabric according to an embodiment. Figure 3 is a plan view showing an example of a four-axial woven fabric.

The composite sheet 1 shown in FIG. 1 comprises a sheet-like core material 2 and a fluororesin-containing layer 3 formed on both sides of the core material 2. The fluororesin-containing layer 3 is composed of a first fluororesin-containing layer 3a formed on the front surface of the core material 2 and a second fluororesin-containing layer 3b formed on the back surface of the core material 2. Although not shown in FIG. 1, the first fluororesin-containing layer 3a and the second fluororesin-containing layer 3b are fused together through an opening 20 in the core material 2. The opening 20 is formed by the intersection of each group of threads, as shown in FIG. 2 and FIG. 3.

The core material 2 shown in FIG. 2 includes a warp thread group 2a arranged along an in-plane direction. The in-plane direction is, for example, a direction perpendicular to the thickness direction of the composite sheet 1. The warp thread group 2a includes multiple warp threads. The multiple warp threads included in the warp thread group 2a are preferably arranged so as to be parallel to each other. The interval (pitch between threads) between the multiple warp threads included in the warp thread group 2a can be changed as appropriate depending on the application of the composite sheet, but is, for example, within a range of 5 mm to 20 mm. The multiple warp threads are preferably arranged with a certain interval between adjacent warp threads. In this way, unevenness in the visible light transmittance of the composite sheet is less likely to occur, and a uniform light transmittance can be achieved at any location.

The core material 2 further includes a first diagonal yarn group 2b arranged on the warp yarn group 2a so as to intersect with the warp yarn group 2a. The first diagonal yarn group 2b is layered on the warp yarn group 2a. The first diagonal yarn group 2b includes a plurality of first diagonal yarns. The plurality of first diagonal yarns included in the first diagonal yarn group 2b are preferably arranged so as to be parallel to each other. Here, the term "intersect" means that the warp yarn group 2a and the first diagonal yarn group 2b have at least one intersection point. The interval (pitch between the yarns) between the plurality of first diagonal yarns included in the first diagonal yarn group 2b can be appropriately changed depending on the application of the composite sheet, but is, for example, within a range of 2 mm to 20 mm. It is preferable that the plurality of first diagonal yarns are arranged at a constant interval from the adjacent first diagonal yarns. In this way, unevenness in the visible light transmittance of the composite sheet is unlikely to occur, and a uniform light transmittance can be achieved at any location.

When the composite sheet 1 is observed in a direction parallel to the thickness direction of the composite sheet 1, the first diagonal yarn group 2b preferably forms an angle of 55° to 65° with the warp yarn group 2a. In other words, the first diagonal yarn group 2b preferably forms an angle of 55° to 65° with the warp yarn group 2a in the in-plane direction. It is more preferable that the first diagonal yarn group 2b forms an angle of 60° with the warp yarn group 2a, as shown in FIG. 2.

The core material 2 further includes a second diagonal yarn group 2c arranged on the warp yarn group 2a or the first diagonal yarn group 2b so as to intersect with the warp yarn group 2a and the first diagonal yarn group 2b. In other words, the second diagonal yarn group 2c is layered on the warp yarn group 2a or the first diagonal yarn group 2b. In Figs. 1 to 3, as an example, the second diagonal yarn group 2c is layered on the warp yarn group 2a.

The second diagonal yarn group 2c includes a plurality of second diagonal yarns. The plurality of second diagonal yarns included in the second diagonal yarn group 2c are preferably arranged so as to be parallel to each other. The interval (pitch between the yarns) between the plurality of second diagonal yarns included in the second diagonal yarn group 2c can be changed as appropriate depending on the application of the composite sheet, but is, for example, within the range of 2 mm to 20 mm. The plurality of second diagonal yarns are preferably arranged with a constant interval between adjacent second diagonal yarns. In this way, unevenness in the visible light transmittance of the composite sheet is less likely to occur, and a uniform light transmittance can be achieved in all locations.

When the composite sheet 1 is observed in a direction parallel to the thickness direction of the composite sheet 1, the second diagonal yarn group 2c preferably forms an angle of 55° to 65° with the warp yarn group 2a and the first diagonal yarn group 2b. In other words, the second diagonal yarn group 2c preferably forms an angle of 55° to 65° with the warp yarn group 2a and the first diagonal yarn group 2b in the in-plane direction. It is more preferable that the second diagonal yarn group 2c forms an angle of 60° with the warp yarn group 2a, as shown in FIG. 2.

The core material is preferably configured so that the warp yarn group 2a, the first diagonal yarn group 2b, and the second diagonal yarn group 2c each form an angle of 60° in the in-plane direction. That is, the core material is preferably a triaxial woven fabric. In this case, even if the volume per unit area (mass per unit area) of the core material is small, a composite sheet with excellent tear strength in various directions in the in-plane direction can be obtained. Since the volume per unit area (mass per unit area) of the core material is small, it is easy to increase the visible light transmittance. A composite sheet including a triaxial woven fabric as a core material is excellent not only in tear strength, but also in isotropy, burst resistance, shear resistance, and impact strength.

Figure 3 shows another example of a core material (multiaxial woven fabric) in which the core material is a four-axis woven fabric. In this example, the core material 2 further includes a weft group 2d in addition to the warp group 2a, the first diagonal yarn group 2b, and the second diagonal yarn group 2c. The weft group 2d includes multiple wefts. The multiple wefts included in the weft group 2d are preferably arranged parallel to each other.

The configuration of the warp group 2a, the first diagonal yarn group 2b, and the second diagonal yarn group 2c in the four-axis woven fabric is the same as that described in FIG. 2, except that the first diagonal yarn group 2b forms an angle of 45° with the warp group 2a in the in-plane direction, and the second diagonal yarn group 2c forms an angle of 45° with the warp group 2a in the in-plane direction.

In the four-axis woven fabric, the first diagonal yarn group 2b may form an angle of 40° to 50° with the warp yarn group 2a in the in-plane direction. As shown in FIG. 3, the first diagonal yarn group 2b preferably forms an angle of 45° with the warp yarn group 2a. In the four-axis woven fabric, the second diagonal yarn group 2c may form an angle of 40° to 50° with the warp yarn group 2a in the in-plane direction. As shown in FIG. 3, the second diagonal yarn group 2c preferably forms an angle of 45° with the warp yarn group 2a.

The weft group 2d is layered on one of the warp group 2a, the first diagonal group 2b, and the second diagonal group 2c so as to intersect with all of these groups. FIG. 3 shows, as an example, a case in which the weft group 2d is layered on the first diagonal group 2b. In FIG. 3, the weft group 2d forms an angle of 90° with the warp group 2a in the in-plane direction. The weft group 2d may also form an angle of 85° to 95° with the warp group 2a in the in-plane direction.

In a multiaxial woven fabric, it is preferable that the angle between the first diagonal yarn group 2b and the warp yarn group 2a is approximately the same as the angle between the second diagonal yarn group 2c and the warp yarn group 2a. Approximately the same means, for example, that the difference between these angles is within 0° to 5°. In this case, the isotropy of the composite sheet is increased, which is preferable.

As the number of axes increases, the multiaxial fabric tends to have higher isotropy, which improves tear resistance, burst resistance, shear resistance, and impact strength. However, as the number of axes increases, the volume per unit area of the core material (mass per unit area) tends to increase, making it difficult to increase the visible light transmittance, and the mass per unit area of the composite sheet also tends to increase. When using the composite sheet as a roofing material, it is preferable for the mass per unit area to be small. In other words, it is preferable for the composite sheet to be lightweight. However, for example, the opening rate of the core material can be adjusted by changing the arrangement interval (pitch between threads) of each thread group, so that a moderate visible light transmittance and mass of the composite sheet can be achieved even with a large number of axes.

The opening rate of the core material can be changed as appropriate depending on the visible light transmittance and tear strength required for the composite sheet, but is, for example, within the range of 50% to 65%. The opening rate of the core material can be adjusted, for example, by changing the number of axes of the multiaxial woven fabric, by changing the arrangement interval of each yarn group as described above, or by changing the tex count of each yarn. If the opening rate is too low, even if the light transmittance of the fluororesin-containing layer described below is increased, the visible light transmittance of the composite sheet at wavelengths of 360 nm to 740 nm may be less than 30%. If the opening rate is too high, the visible light transmittance of the composite sheet 1 may exceed 70%. It is more preferable that the opening rate of the core material 2 is within the range of 45% to 60%.

The opening rate of the core material 2 can be measured by using a KEYENCE Corporation shape laser microscope VK-X100 and analysis application VK-H1XA on the composite sheet. This measurement makes it possible to measure the ratio of the opening area to a given area of the core material.

In the core material 2, the area of each of the openings 20 is, for example, within the range of 0.4 cm ² to 2.0 cm ² .

The intersections where each group of threads meet are fixed, for example, with an adhesive. Since the tear strength of the composite sheet depends on the tear strength of the core material, fixing these groups of threads makes the composite sheet less likely to stretch and increases its tear strength.

There are no particular limitations on the adhesive as long as it is a room temperature drying type, but it is preferable to use an emulsion type of polymer and/or copolymer of acrylic ester, vinyl acetate, etc.

The thickness T of the core material 2 is, for example, in the range of 150 μm to 450 μm. If the thickness of the core material 2 is excessively large, this is not preferable because it tends to increase the mass per unit area of the composite sheet.

The material of the threads (fibers) constituting each thread group is not particularly limited, but may be any selected from glass fiber, carbon fiber, and aramid fiber, for example. Each thread group may contain only one type of thread, or may contain multiple types of thread. From the viewpoint of increasing visible light transmittance, it is preferable that the material of the threads constituting the thread group is glass fiber.

Because the fluororesin-containing layer laminated on the core material is highly transparent, for example, when an observer observes the surface of the composite sheet from a direction perpendicular to the surface of the composite sheet, the observer can perceive the shape of the core material. When glass fiber is used as the thread that constitutes the core material, the observer can perceive the core material as white. When carbon fiber, aramid fiber, or the like is used as the thread that constitutes the core material, the observer can perceive the core material as black. The composite sheet according to the embodiment is also excellent in design because it allows the observer to perceive the pattern formed by the core material.

The tex count of the yarns (fibers) that make up each yarn group is, for example, in the range of 10 to 40. When the composite sheet is used as a roofing material for buildings, etc., it is preferable that the tex count is in the range of 30 to 40. If the tex count is less than 10, it may not be possible to obtain sufficient strength as a core material. If the tex count exceeds 40, it is not preferable because the mass per unit area of the composite sheet tends to increase.

The fluororesin-containing layer contains ethylene-tetrafluoroethylene copolymer (ETFE). The fluororesin-containing layer may be made of ETFE. Since ETFE has excellent transparency, weather resistance, stain resistance, and mechanical strength, a composite sheet having a fluororesin-containing layer containing ETFE is suitable for use in architectural membrane materials that require high visible light transmittance. For example, it is suitable for applications that are exposed to the outdoors for long periods of time.

ETFE has a low melt viscosity, so it can be melt molded. For example, by placing ETFE films on both sides of the core material described above and fusing them together by thermal lamination, the two ETFE films can be melted and integrated through the openings in the core material. In this way, the fluororesin-containing layer and the core material can be firmly attached to each other.

As long as the composite sheet has a visible light transmittance of 30% to 70% at wavelengths of 360 nm to 740 nm, the fluororesin-containing layer may contain other fluororesins than ETFE. The weight ratio of ETFE in the fluororesin-containing layer is, for example, in the range of 60% to 100%, and preferably in the range of 80% to 100%. As the fluororesin other than ETFE, for example, a fluororesin having a melt flow rate in the range of 10 g/10 min to 25 g/10 min may be used. The other fluororesin may be, for example, at least one selected from the group consisting of perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), polyvinylidene fluoride (PVDF), and tetrafluoroethylene-hexafluoropropene-vinylidene fluoride copolymer (THV).

Whether or not the fluororesin-containing layer of the composite sheet contains ETFE can be confirmed by taking into consideration the results of thermogravimetric (TG) analysis and infrared spectroscopy (IR) analysis.

The thickness of the fluororesin-containing layer formed on one side of the core material is, for example, in the range of 15 μm to 250 μm. The total thickness of the fluororesin-containing layers formed on both sides of the core material is the thickness of the composite sheet, and is, for example, in the range of 200 μm to 1000 μm. If the thickness of the fluororesin-containing layer formed on the core material is excessively small, the core material may be exposed by bending the composite sheet, resulting in reduced durability. If the thickness of the composite sheet is excessively large, the flexibility of the composite sheet tends to be poor.

The thickness of the core material and the thickness of the fluororesin-containing layer can be measured, for example, by observing the cross section of the composite sheet with a scanning electron microscope (SEM). The thickness of the fluororesin-containing layer is the thickness of the fluororesin-containing layer at the position of the opening of the core material.

The visible light transmittance can be adjusted by applying, for example, dot-like prints to the front and/or back surfaces of the fluororesin-containing layer. If the visible light transmittance of the composite sheet is excessively high, applying such prints can reduce the visible light transmittance.

The composite sheet according to the embodiment has the above-mentioned core material, so it can withstand application under a load equal to or greater than the tensile yield point of a single ETFE film that does not have a core material.

The composite sheet according to the embodiment can be applied by the lap fusion method used in the application of general membrane materials, which can reduce construction costs and space maintenance costs. Alternatively, the composite sheet can be used to enhance the aesthetics of buildings and the like, taking advantage of its high designability.

[Example]
Examples will be described below, but the embodiments are not limited to the examples described below.

Example 1
As the core material, a triaxial woven fabric (KT221H manufactured by Nittobo Co., Ltd.) in which the warp group, the first diagonal yarn group, and the second diagonal yarn group were all made of glass fiber was prepared and cut to a predetermined size. All the yarns contained in the warp group were arranged approximately parallel along the in-plane direction. In the first diagonal yarn group, all the yarns were arranged approximately parallel to form an angle of 60° with the warp group in the in-plane direction. In addition, in the second diagonal yarn group, all the yarns were arranged approximately parallel to form an angle of 60° with the warp group in the in-plane direction. The opening ratio of the core material was 52.5%, the tex count of the yarns constituting each yarn group was 40, and the interval between the multiple yarns contained in each yarn group was 10 mm for all yarn groups. The intersections of each yarn group were bonded with an acrylic adhesive.

Next, ETFE films (manufactured by AGC Co., Ltd.) with a thickness of about 100 μm were laminated on both sides of the core material, and the two ETFE films were melted using a heat press machine and melted and integrated through the multiple openings of the core material. The heat press conditions were as follows: the temperature during molding was 270°C to 300°C, the press pressure during molding was 5 to 18 kgf/ ^cm2 , the press holding time during molding was 40 to 90 seconds, the press pressure during cooling was 5 to 18 kgf/ ^cm2 , and the press holding time during cooling was 20 to 40 seconds. In this way, a composite sheet in which the core material and the fluororesin-containing layer were bonded was obtained. The thickness of the obtained composite sheet was 390 μm.

In this embodiment 1, the core material and the two ETFE films are melted and integrated by using a heat press machine, but they may be melted and integrated by using a heat laminating machine. The conditions for heat lamination are, for example, a heat roll temperature of 280°C to 310°C, a heat/cool roll linear pressure of 5 to 18 kgf/ ^cm2 , and a line speed of 1.0 to 4.0 m/min.

(Comparative Example 1)
In Comparative Example 1, Chukoh Flow (registered trademark) architectural membrane material FGT-800 manufactured by Chukoh Chemical Industries, Ltd. was prepared as a composite sheet. The core material contained in this composite sheet was a glass fiber cloth configured with a plain weave, and the opening ratio of the core material was 1.2%. The fluororesin-containing layers laminated on both sides of the core material were made of polytetrafluoroethylene (PTFE). The thickness of the composite sheet was 800 μm.

(Comparative Example 2)
In Comparative Example 2, a film made of ETFE not including a core material (Aflex manufactured by AGC) was prepared. The thickness of this film was 200 μm.

<Visible light transmittance measurement>
The visible light transmittance was measured for each of the sheets obtained in Example 1, Comparative Examples 1 and 2 in accordance with JIS R 3106:1998, as described in the embodiment. The measurement results are shown in the graph of FIG. 4. In the visible light transmittance graph shown in FIG. 4, the horizontal axis shows the measured wavelength (nm) and the vertical axis shows the transmittance (%). In this graph, the visible light transmittance in the visible light range (here, wavelengths of 360 nm to 740 nm) is shown at 10 nm intervals for each of the sheets according to Example 1, Comparative Examples 1 and 2.

As is clear from the graph shown in FIG. 4, the composite sheet of Example 1 had a visible light transmittance in the range of 30% to 70% at wavelengths of 360 nm to 740 nm, and is therefore considered to have sufficient light translucency and to be able to prevent the room temperature from rising excessively.

On the other hand, the composite sheet of Comparative Example 1 had a visible light transmittance of less than 20% at wavelengths of 360 nm to 740 nm, and therefore did not provide sufficient light transparency. The ETFE film of Comparative Example 2 had a visible light transmittance of more than 80% at wavelengths of 360 nm to 740 nm. Such ETFE films have too high a visible light transmittance, and as described below, the range of loads that can be applied is narrow, which can be a problem.

<Comparison of creep properties>
The creep properties were evaluated for the composite sheet according to Example 1 and the ETFE film according to Comparative Example 2. The test subject sheets and films were cut to a predetermined size, and a tensile test was carried out as follows using a universal testing machine manufactured by Shimadzu Corporation. The test conditions were a uniaxial tensile test of the test subject at a speed of 2 mm/min, and after a constant load of 120 N/3 cm was reached, the load was maintained for 5 minutes. In Fig. 5, the load (N/3 cm) applied to the test subject sheet or film is shown on the vertical axis, and the displacement (mm) corresponding to the load is shown on the horizontal axis.

It can be seen that the ETFE film of Comparative Example 2 exhibits creep after a certain load is applied.

Point Y in Figure 5 is the tensile yield point of the ETFE film of Comparative Example 2. When constructing the ETFE film of Comparative Example 2, a load equal to or greater than the tensile yield point Y cannot be applied, so that, for example, the range of loads that can be applied is limited to the narrow range shown by line segment X. This results in a problem of high costs for constructing the membrane structure.

The film alone used in Comparative Example 2 has creep properties, and therefore can only be applied with a load below the tensile yield point according to the Building Standards Act. However, the composite sheet of Example 1 can be applied with a load of up to about 1300 N/3 cm, which reduces the cost of constructing membrane structures and the like, and allows for a wider range of building designs.

<Tear strength test>
The tear strength of each of the composite sheets according to Example 1 and Comparative Example 1 was measured in accordance with JIS L 1096 C method (trapezoid method). The test speed was 50 mm/min. As the results, a graph comparing the tear strength of the sheets according to Example 1 and Comparative Example 1 is shown in FIG. 6. In FIG. 6, the horizontal axis shows the stroke (mm) during the test, and the vertical axis shows the tear strength (N).

When calculating the tear strength, the average of the five highest points was calculated for each example. As a result, the tear strength of Example 1 was 437 N. The tear strength of Comparative Example 1 was 340 N. In other words, the tear strength of the composite sheet of Example 1, which includes a triaxial fabric as the multiaxial fabric, was superior to that of the composite sheet of Comparative Example 1, which includes a plain woven glass cloth.

As is clear from the above results, the composite sheet of Example 1 had excellent tear strength and sufficient visible light transmittance.

The present invention is not limited to the above-mentioned embodiment, and can be modified in various ways without departing from the gist of the present invention. The embodiments may be combined as appropriate, and in that case, the combined effect can be obtained. Furthermore, the above-mentioned embodiment includes various inventions, and various inventions can be extracted by combinations selected from the multiple components disclosed. For example, if the problem can be solved and the effect can be obtained even if some components are deleted from all the components shown in the embodiment, the configuration from which the components are deleted can be extracted as an invention.
The invention as set forth in the claims of the present application as originally filed is set forth below.
[1] A composite sheet comprising a multiaxially woven core material and a fluororesin-containing layer containing an ethylene-tetrafluoroethylene copolymer formed on both sides of the core material,
The composite sheet has a visible light transmittance in the thickness direction of the composite sheet at wavelengths of 360 nm to 740 nm in the range of 30% to 70%.
[2] The composite sheet according to [1], wherein the opening ratio of the core material is within a range of 50% to 65%.
[3] The composite sheet according to [1] or [2], wherein the fluororesin-containing layer is made of an ethylene-tetrafluoroethylene copolymer.
[4] The composite sheet according to any one of [1] to [3], having a thickness in the range of 200 μm to 1000 μm.
[5] The composite sheet according to any one of [1] to [4], wherein the tear strength in the in-plane direction is within the range of 350 N to 450 N.

1...composite sheet, 2...core material, 2a...warp group, 2b...first diagonal group, 2c...second diagonal group, 2d...weft group, 3...fluororesin-containing layer, 20...opening.

Claims (4)

A composite sheet comprising a multiaxially woven core material and a fluororesin-containing layer containing an ethylene-tetrafluoroethylene copolymer formed on both sides of the core material,
In the thickness direction of the composite sheet, the visible light transmittance at wavelengths of 360 nm to 740 nm is within a range of 30% to 70%,
The in-plane tear strength is in the range of 350N to 450N.
The core material includes fibers and an adhesive,
The fiber is at least one selected from the group consisting of glass fiber, carbon fiber, and aramid fiber,
the adhesive is at least one selected from the group consisting of an acrylic acid ester polymer, a vinyl acetate polymer, and a copolymer of an acrylic acid ester and a vinyl acetate;
The composite sheet has an opening ratio of the core material in the range of 50% to 65% . 2. The composite sheet according to claim 1 , wherein the fluororesin-containing layer comprises an ethylene-tetrafluoroethylene copolymer. 3. The composite sheet according to claim 1, having a thickness in the range of 200 μm to 1000 μm. The composite sheet according to any one of claims 1 to 3 , wherein the core material is made of glass fiber, and the composite sheet has a visible light transmittance in a thickness direction thereof at a wavelength of 360 nm to 740 nm in a range of 50% to 70%.