Sandwich panel manufacturing method

Abstract

To provide a technology that can reduce pinholes in sandwich panels.SOLUTION: The method of manufacturing a sandwich panel 100 includes the processes of: preparing a sheet-like core layer 10 having a honeycomb structure and a plurality of sheet-like prepreg 20; arranging one or more layers of prepreg 20 on each side of core layer 10 to form a laminate; arranging a release film 25 on the outermost layer of the laminate; and performing heat and pressure treatment on the laminate together with the release film 25.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a sandwich panel and a sandwich panel.

Background Art

[0002] A sandwich panel is composed of a hollow honeycomb core formed by walls with a hexagonal cross-section and a pair of skin materials such as prepregs joined to both surfaces of the honeycomb core. The sandwich panel, which has the characteristics of being lightweight and highly rigid, is used, for example, as a structural member for aircraft.

[0003] As methods for manufacturing sandwich panels, various techniques have been disclosed. As a general technique, for example, there is the technique described in Patent Document 1. In this document, from the viewpoints of achieving high quality, low cost, and reducing the remaining voids, an uncured composite honeycomb sandwich panel in which prepregs are laminated on the upper and lower surfaces of a honeycomb core is covered with a vacuum bag and placed in an autoclave, and then the inside of the vacuum bag is evacuated, and while continuing the evacuation, heating and pressurization are performed by the autoclave.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, the requirements for sandwich panels have been increasing more and more. Among them, voids remaining on the surface of the sandwich panel are called pinholes, and there is a problem that they deteriorate the appearance of the sandwich panel. Even in the technique disclosed in Patent Document 1, there is room for improvement in reducing the pinholes on the surface of the sandwich panel.

Means for Solving the Problems

[0006] The inventors of this invention conducted diligent research to reduce pinholes to a higher level and found that using a release film when laminating and integrating the honeycomb core and prepreg is effective. Specifically, they focused on the fact that the unevenness caused by the weave of the fibrous base material constituting the prepreg is one of the causes of pinholes in sandwich panels. They found that by using a release film when integrating the core layer and the prepreg, the release film can follow the unevenness and heat and pressurize the surface of the prepreg more uniformly, as a result, the manifestation of voids on the surface of the prepreg and the formation of pinholes can be suppressed, thus completing the present invention.

[0007] According to the present invention, A process of preparing a sheet-like core layer having a honeycomb structure and a plurality of sheet-like prepregs, The process involves forming a laminate by laminating one or more layers of the prepreg on each of the two sides of the core layer, and placing a release film on the outermost layer of the laminate. A step of applying heat and pressure to the laminate together with the release film, A method for manufacturing a sandwich panel is provided, which includes the following.

[0008] Furthermore, according to the present invention, A sandwich panel comprising a core layer having a honeycomb structure and one or more layers of cured prepreg provided on both sides of the core layer, A portion of the cured prepreg penetrates into the core layer, The pinhole number density observed on the outermost surface of the sandwich panel was 0 to 50 pins / cm². 2 A sandwich panel is provided. [Effects of the Invention]

[0009] The present invention provides a technology that can reduce pinholes in sandwich panels. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing an example of a sandwich panel according to this embodiment. [Figure 2] This is a cross-sectional view showing an example of a manufacturing method for a sandwich panel according to this embodiment. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the drawings. In all drawings, similar components are denoted by the same reference numerals, and their descriptions are omitted as appropriate. In this specification, the notation "a~b" in descriptions of numerical ranges means a or more and b or less, unless otherwise specified. For example, "1~5 mass%" means "1 mass% or more and 5 mass% or less." In this specification, the MD direction refers to the Machine Direction, meaning the direction of resin flow, and the TD direction refers to the Transverse Direction, meaning the direction perpendicular to the MD direction.

[0012] <Sandwich Panel> Figure 1 is a schematic diagram showing an example of a sandwich panel according to this embodiment. The sandwich panel 100 of this embodiment comprises a core layer 10 having a honeycomb structure and cured prepreg 20 40 provided on both sides of the core layer 10, with a portion of the cured prepreg 20 40 embedded within the core layer 10. More specifically, a portion of the binder resin constituting the prepreg 20 enters and hardens within the through-holes of the honeycomb structure of the core layer 10. This occurs during the manufacturing process of the sandwich panel 100, when the prepreg 20 in the B-stage state and the core layer 10 are integrated by heating and pressurizing, causing a portion of the binder resin contained in the prepreg 20 to flow into the through-holes of the core layer 10. This results in a strong bond between the prepreg 20 and the core layer 10.

[0013] In this embodiment, the number density of pinholes when observing the outermost surface of the sandwich panel 100 is 0 to 50 pinholes / cm². 2 That is the case. That is, since the sandwich panel 100 of the present embodiment is obtained by the manufacturing method described later, the manifestation of voids is suppressed more than in the past. As a result, the number density of pinholes on the surface is reduced. The lower the number density of pinholes, the more preferable it is, and it is preferably 40 or less per cm 2 More preferably, it is 30 or less per cm 2 Even more preferably, it is 20 or less per cm 2 Even more preferably, it is 10 or less per cm 2 Even more preferably, it is even lower. Also, it is preferable that the above-mentioned pinhole number density is achieved on both surfaces of the sandwich panel 100. The number density of pinholes is obtained by counting the holes visible when observing the surface of the sandwich panel 100 with a magnifying glass (4 to 50 times) and dividing by the observed area.

[0014] Hereinafter, each member constituting the sandwich panel 100 will be described.

[0015] [Core layer] For the core layer 10, for example, a sheet-like member obtained by impregnating a core layer base material having a honeycomb structure with a binder resin can be used. Due to the honeycomb structure, the core layer 10 can have high strength and light weight. The honeycomb structure is a known structure and is intended to be a structure in which a plurality of substantially regular hexagonal through-holes penetrating from the upper surface to the lower surface are arranged.

[0016] Examples of the base material having the honeycomb structure of the core layer 10 include those formed into a honeycomb shape by a known method using aramid fiber, paper, balsa wood, plastic, aluminum, titanium, glass, and their alloys, etc. From the viewpoint of heat resistance, it is preferable that the base material having the honeycomb structure of the core layer 10 contains aramid fiber.

[0017] The core layer 10 is preferably made of woven fiber cloth as the base material. This improves the processability into a honeycomb structure and allows for weight reduction of the sandwich panel 100. When the core layer 10 includes a woven fiber cloth, the areas where the fibers intersect are less likely to be pressurized during the manufacturing process, making it easier for voids to remain. However, in the sandwich panel 100 of this embodiment, the manufacturing method described later effectively suppresses the manifestation of voids on the surface, which can cause pinholes.

[0018] The thickness of the core layer 10 is not particularly limited, but for example, it may be 1 mm or more and 50 mm or less, 3 mm or more and 40 mm or less, or 5 mm or more and 30 mm or less.

[0019] The size of each core cell in the core layer 10 is not particularly limited, but for example, it can be 1 mm or more and 10 mm or less on one side.

[0020] The surface area (top and bottom) of the core layer 10 is not limited, but for example, it may have the surface area of ​​one sandwich panel 100, or it may have the surface area of ​​multiple sandwich panels combined. This makes it possible to cut out multiple panels by separating one sandwich panel 100 into individual pieces, thereby improving productivity. For example, the surface area (top and bottom) of the core layer 10 can be large, for example, 1 m². 2 That's fine too.

[0021] Furthermore, the core layer 10 may be subjected to various surface treatments on its interior and / or exterior to improve corrosion resistance and heat resistance.

[0022] [Prepreg] As the prepreg 20, for example, a sheet member in a B-stage state, obtained by impregnating a prepreg substrate with a binder resin, can be used.

[0023] The thickness of each sheet of prepreg 20 is adjusted as appropriate depending on the application, but is preferably 0.05 to 10 mm, and more preferably 0.1 to 5 mm.

[0024] The B-stage state refers to a state in which the reaction rate of the binder resin impregnated into the prepreg substrate, calculated from the measurement results of a DSC (Differential Scanning Calorimeter), is preferably greater than 0% and 60% or less, more preferably between 0.5% and 55%, and even more preferably between 1% and 50%.

[0025] Examples of prepreg substrates for the prepreg 20 include fiber substrates. As the above-mentioned fiber base material, aramid fibers, polyester fibers, polyphenylene sulfide fibers, carbon fibers, graphite fibers, glass fibers, and silicon carbide fibers can be used. From the viewpoint of high heat resistance, it is preferable that the above-mentioned fiber base material contains glass fibers. These fibers may be in the form of a woven fabric. When they are woven, the areas where the fibers (warp threads) and fibers (weft threads) intersect are less likely to be pressurized during the manufacturing process, making it easier for voids to remain. However, in the sandwich panel 100 of this embodiment, the manufacturing method described later effectively suppresses the manifestation of voids on the surface, which can cause pinholes.

[0026] The prepreg 20, by incorporating a fibrous base material, can further improve the heat resistance of the sandwich panel 100. Furthermore, by reducing the difference in the coefficient of thermal expansion between the prepreg 20 and the core layer 10, warping of the sandwich panel 100 can be suppressed.

[0027] In this embodiment, the binder resins used in the core layer 10 and the prepreg 20 may be the same or different, but it is preferable that they be the same binder resin in order to effectively improve adhesion and enhance flame resistance. The binder resins will be described later.

[0028] [Cured product] The upper limit of the glass transition temperature (Tg) of the cured prepreg 20 is, for example, 250°C or less, preferably 220°C or less, and more preferably 200°C or less. This makes it possible to perform the heat and pressure treatment under low temperature conditions. On the other hand, the lower limit of the glass transition temperature (Tg) may be, for example, 110°C or higher, or 120°C or higher. This makes it possible to improve the thermal properties.

[0029] [Application] The sandwich panel 100 of this embodiment can be suitably used as interior panels for toilets and partitions, as well as panels for aircraft equipment such as wagon casings.

[0030] <Manufacturing method for sandwich panels> The manufacturing method of the sandwich panel 100 of this embodiment includes the following steps. (Step 1) A step of preparing a sheet-like core layer 10 having a honeycomb structure and a plurality of sheet-like prepregs 20. (Step 2) Laminating one or more layers of prepreg 20 on each side of the core layer 10 to form a laminate, and placing a release film 25 on the outermost layer of the laminate. (Step 3) A step in which the laminate is subjected to heat and pressure treatment together with the release film 25. The following describes each step.

[0031] (Step 1) Preparation of core layer 10 and prepreg 20 Prepare the substrate for the core layer. The substrate for the core layer has a honeycomb structure and is preferably made from aramid fibers. A substrate having a honeycomb structure is prepared, and a binder resin is impregnated into the substrate. Then, the substrate is dried to obtain the core layer 10.

[0032] On the other hand, a substrate for the prepreg is prepared. A fibrous substrate is an example of a substrate for the prepreg. By impregnating the substrate with a binder resin and then drying it, a prepreg 20 in the B stage state is obtained.

[0033] In the core layer 10 and prepreg 20, methods for impregnating with binder resin include, for example, a method of dissolving the binder resin in a solvent and spraying the resulting binder solution onto the binder substrate using a spraying device such as a spray nozzle; a method of immersing the binder substrate in the binder solution; a method of coating the binder substrate with the binder solution using various coaters such as a knife coater or a comma coater; and a method of transferring the binder solution to the binder substrate using a transfer roll. Among these, the method of immersing the binder substrate in the binder solution is preferred. Furthermore, while there are no particular limitations on the conditions for heating and drying, it is usually carried out at 100 to 220°C, preferably 120 to 190°C, for 2 to 10 minutes.

[0034] [Binder solution] The binder solution is prepared by dissolving a binder resin or the like in a known organic solvent, and known solvents can be used. The binder resin is preferably a thermosetting resin, such as phenolic resin, unsaturated polyester resin, epoxy resin, melamine resin, and furan resin.

[0035] The phenolic resins mentioned above tend to generate condensation water and voids easily. However, according to the sandwich panel manufacturing method of this embodiment, even when using phenolic resin, the retention of voids on the surface of the sandwich panel and the formation of pinholes can be effectively reduced. The phenolic resins mentioned above include compounds having one or more phenolic hydroxyl groups in their molecules, and examples include novolac resins such as novolac-type phenol, novolac-type cresol, and novolac-type naphthol; bisphenol resins such as bisphenol F and bisphenol A; phenol aralkyl resins such as paraxylylene-modified phenolic resins; resol-type phenolic resins such as dimethylene ether-type resol and methylol-type phenol; water-soluble phenolic resins; and compounds obtained by further methylating the above resins. Furthermore, the water-soluble phenolic resins mentioned above specifically include phenolic resins synthesized using one or more selected from lignin, lignin derivatives, lignin decomposition products, and modified products thereof. For example, a modified lignin product is disclosed in Japanese Patent Publication No. 48-22340. Lignin-modified phenolic resins are obtained by reacting lignins, phenols, and aldehydes, as described later, in the presence of a catalyst.

[0036] The lignins used in the lignin-modified phenol resin include at least one selected from lignin and lignin derivatives. Lignin, along with cellulose and hemicellulose, is a major component that forms the structure of plant bodies and is also one of the most abundant aromatic compounds in nature. Examples of lignin include alkaline lignins such as kraft lignin, soda lignin, and soda-anthraquinone lignin, as well as pulp lignin containing lignosulfonic acid; organosol lignin; high-temperature, high-pressure water-treated lignin; explosion lignin; enzymatic saccharification lignin; lignophenol; phenolized lignin; and others. The origin of lignin is not particularly limited, and examples include wood and herbaceous plants that contain lignin and form woody parts. These include coniferous trees such as cedar, pine, and cypress; broad-leaved trees such as beech, birch, oak, and zelkova; and grasses (herbaceous plants) such as rice, wheat, corn, and bamboo. Among these, lignin derived from coniferous trees is preferred from the viewpoint of mechanical properties.

[0037] In this embodiment, "lignin derivative" refers to a compound having a unit structure that constitutes lignin, or a structure similar to the unit structure that constitutes lignin. The lignin derivative has a phenol derivative as its unit structure. Because this unit structure has chemically and biologically stable carbon-carbon bonds and carbon-oxygen-carbon bonds, it is resistant to chemical degradation and biological decomposition.

[0038] Examples of lignin derivatives include guaiacylpropane (ferulic acid) represented by formula (A), syringylpropane (sinapic acid) represented by formula (B), and 4-hydroxyphenylpropane (coumaric acid) represented by formula (C). The composition of lignin derivatives varies depending on the biomass used as raw material. Lignin derivatives mainly containing guaiacylpropane structures are extracted from coniferous trees. Lignin derivatives mainly containing guaiacylpropane and syringylpropane structures are extracted from broad-leaved trees. Lignin derivatives mainly containing guaiacylpropane, syringylpropane, and 4-hydroxyphenylpropane structures are extracted from herbaceous plants.

[0039] [ka]

[0040] Lignin derivatives are preferably obtained by decomposing biomass. Since biomass is formed by taking in and fixing carbon dioxide from the atmosphere during photosynthesis, it contributes to suppressing the increase of carbon dioxide in the atmosphere, and by utilizing biomass industrially, it can contribute to mitigating global warming. Examples of biomass include lignocellulosic biomass. Examples of lignocellulosic biomass include the leaves, bark, branches, and wood of lignin-containing plants, as well as processed products thereof. Examples of lignin-containing plants include the broad-leaved trees, coniferous trees, and grasses mentioned above.

[0041] Examples of lignin derivatives include decomposed lignocellulose, which is a compound formed by the bonding of lignin, cellulose, and hemicellulose. Lignin derivatives may include lignin decomposition products, cellulose decomposition products, and hemicellulose decomposition products, which mainly consist of compounds having a lignin skeleton.

[0042] Lignin derivatives preferably have many reaction sites on which the curing agent acts via electrophilic substitution of the aromatic ring. Since reactivity is better when there is less steric hindrance near the reaction sites, it is preferable that at least one of the ortho and para positions of the aromatic ring containing the phenolic hydroxyl group is unsubstituted. Lignin derived from conifers and herbaceous plants is preferred, as it contains many guaiacyl and 4-hydroxyphenyl structures as aromatic units of lignin.

[0043] Furthermore, in addition to the basic structure described above, lignin derivatives may also be lignin derivatives having functional groups (lignin secondary derivatives).

[0044] Furthermore, epoxy resin is preferred in terms of improving the mechanical properties and heat resistance of the prepreg 20.

[0045] The above-mentioned furan resins are polymers or precursors (oligomers) derived from furfural or furfuryl alcohol obtained by reducing furfural as starting materials. Examples of furan resins include furfuryl alcohol type, furfuryl alcohol-furfural cocondensation type, furfuryl alcohol-aldehyde cocondensation type, furfural-ketone cocondensation type, furfural-phenol cocondensation type, furfuryl alcohol-urea cocondensation type, and furfuryl alcohol-phenol cocondensation type. Examples of modified furan resins include epoxy-modified, phenol-modified, aldehyde-modified, urea-modified, and melamine-modified types.

[0046] The thermosetting resins described above may be made from natural or synthetic materials. In particular, resins made from natural materials may be used. Examples of resins made from natural materials include lignin-modified phenolic resins and furan resins obtained from biomass-derived furfural.

[0047] Furthermore, the binder solution may contain other additives in addition to the thermosetting resin mentioned above. While not particularly limited, examples of additives include fillers such as inorganic fillers, rubber, and thermoplastic resins.

[0048] (Step 2) Formation of the laminate and placement of the release film Next, as shown in Figure 2(a), the prepared prepregs 20 are stacked on both sides of the core layer 10 to form a laminate, and a release film 25 is placed on the outermost layer of the laminate. The prepreg 20 may be a single layer or multiple layers may be laminated on one side of the core layer 10. The number of layers is not particularly limited, but it is preferable to laminate about 2 to 5 layers on one side of the core layer 10. Also, if there are multiple prepregs 20, each prepreg 20 may be made of the same material and thickness, or they may be different from one another. Furthermore, the number of layers of prepregs 20 arranged on each side of the core layer 10 may be the same (symmetrical structure centered on the core layer 10), or they may be different.

[0049] Furthermore, in this embodiment, by using the release film 25, the release film 25 can closely conform to the fine irregularities of the prepreg 20 during heating and pressing, allowing for more uniform heating and pressing, thereby suppressing the appearance of voids on the surface and the formation of pinholes. In addition, even if voids occur inside the prepreg 20, the gap at the interface between the release film 25 and the prepreg 20 is reduced, thus suppressing the appearance of voids on the surface. Furthermore, even if voids remain inside the prepreg 20, they will not become apparent on the surface of the resulting sandwich panel 100 because the prepreg 20 will harden afterward.

[0050] The release film 25 may have the same surface roughness on both sides, or the surface roughness may be different, as described later. If the surface roughness is different, it is preferable that the side with the higher surface roughness becomes the prepreg 20, and the side with the lower surface roughness becomes the side facing the metal plate 30, as described later.

[0051] In this embodiment, an example is described in which the release film 25 is placed on both outermost layers of the laminate, but the release film 25 only needs to be placed on at least one of the outermost layers of the laminate.

[0052] [Release film] In this embodiment, the release film 25 is placed on the outermost layer of the laminate formed by the prepreg 20 and the core layer 10. During heating and pressing, it is heated and pressed to follow the prepreg 20, and then peeled off. Furthermore, the release film 25 also has good release properties against the metal plate 30, which will be described later.

[0053] (e.g., modulus of elasticity) The release film 25 preferably has a storage modulus (frequency 100 Hz, 100°C) E' measured after heat treatment at 180°C for 120 seconds that is between 0.05 GPa and 10 GPa, and more preferably between 0.1 GPa and 5 GPa. This improves the peelability of the release film 25 from uneven surfaces after use (after heating and pressing). Specifically, by controlling the value of the storage modulus E' after the heat history to fall within the above numerical range, it becomes possible to significantly change the degree of crystallinity of the release film 25 before and after use (before and after heating and pressing), and as a result, it becomes possible to improve release properties while maintaining conformability. In addition, appropriate elasticity is obtained during the heating and pressing process, allowing for more uniform pressure to be applied, and effectively reducing the occurrence of voids in the sandwich panel 100.

[0054] Furthermore, the release film 25 preferably has a stiffness parameter G, defined by the following formula (1), of 25 to 1500, more preferably 50 to 1000, and even more preferably 100 to 800. Stiffness parameter G = Thickness of release film 25 [μm] × Storage modulus E' [GPa] (1)

[0055] The release film 25 preferably has a storage modulus at 150°C measured by a dynamic viscoelasticity measuring device (tensile mode, frequency 1 Hz, heating rate 5°C / min) of 30 MPa to 70 MPa, and more preferably 40 MPa to 60 MPa. By setting the storage modulus at 150°C to be above the lower limit, even when hot pressing is performed using the release film 25, good hardness and rigidity can be obtained throughout the release film 25, resulting in improved release properties. Furthermore, molded products with a good appearance can be obtained. On the other hand, by keeping the storage modulus at 150°C below the above upper limit, it becomes easier to achieve embedding properties while maintaining good release properties when performing hot pressing using the release film 25. Furthermore, it is possible to maintain a good appearance of the molded product.

[0056] The release film 25 preferably has a loss modulus of elasticity at 150°C measured by a dynamic viscoelasticity measuring device (tensile mode, frequency 1 Hz, heating rate 5°C / min) of 4 MPa to 20 MPa, and more preferably 5 MPa to 15 MPa. By setting the loss modulus at 150°C to be above the lower limit, even when hot pressing is performed using the release film 25, good viscosity is obtained throughout the entire release film 25, resulting in the ability to maintain release properties while also achieving embedding properties. On the other hand, by keeping the loss modulus of elasticity at 150°C below the above upper limit, it becomes easier to obtain release properties while maintaining good embedding properties during hot pressing.

[0057] The release film 25 preferably has a tanδ of 0.05 or more and 0.3 or less at 150°C, as measured by a dynamic viscoelasticity measuring device (tensile mode, frequency 1 Hz, heating rate 5°C / min), and more preferably 0.1 or more and 0.2 or less. By setting the tanδ at 150°C to be above the lower limit, even when hot pressing is performed using the release film 25, an appropriate viscoelasticity can be obtained throughout the entire release film 25, resulting in an improved balance between release properties and embedding properties.

[0058] [Loop stiffness] In the embodiment, the release film 25 preferably has a loop stiffness value in the MD direction of 45 to 90 mN, more preferably 50 to 80 mN, and even more preferably 55 to 75 mN, measured under the conditions of 23°C, 50% RH, width 10 mm, circumference 120 mm, and indentation distance 20 mm. By setting the loop stiffness value to be equal to or greater than the lower limit value mentioned above, good hardness and rigidity can be obtained throughout the entire release film 25. On the other hand, by keeping the loop stiffness value below the above upper limit, it becomes easier to achieve embedding properties while maintaining good release properties. The loop stiffness can be measured over time using a loop stiffness tester (manufactured by Toyo Seiki Co., Ltd.) under the following conditions: test specimen size: 10 mm wide x 180 mm wide, or 15 mm wide x 180 mm wide (flow direction during film formation of the release film), loop length: 120 mm, and indentation amount: 20 mm. The maximum value during this period can be defined as the "value measured by the loop stiffness test" (mN / cm).

[0059] The storage modulus, loss modulus, etc. of the release film 25 in this embodiment can be adjusted by known methods, for example, by controlling the overall thickness of the release film 25, the layer structure of the release film 25, and the combination of its materials.

[0060] (Thickness) The thickness of the release film 25 is preferably 50 μm to 150 μm, more preferably 75 μm to 140 μm, and even more preferably 90 μm to 130 μm. This makes it possible to apply the material more uniformly to the release film 25, and further suppresses the appearance of voids on the surface and the formation of pinholes.

[0061] Furthermore, the thickness of the release film 25 is preferably 5 to 40%, and more preferably 10 to 30%, of the total thickness of the prepreg 20 arranged on one side of the core layer 10.

[0062] (Surface roughness) The arithmetic mean height Sa of one side of the release film 25 (the side facing the prepreg 20) is preferably 0.6 μm or more and 2.3 μm or less, and more preferably 0.7 μm or more and 2.0 μm or less. By setting the arithmetic mean height Sa to be above the lower limit, the release properties from the prepreg 20 can be improved. On the other hand, by setting the arithmetic mean roughness Sa to be below the upper limit, good conformability to the prepreg 20 can be maintained. The arithmetic mean height Sa can be measured in accordance with ISO 25178.

[0063] The ten-point average roughness Rz of one side of the release film 25 (the side facing the prepreg 20) is preferably 1 μm or more and 10 μm or less, and more preferably 2 μm or more and 7 μm or less. By setting the ten-point average roughness Rz to be above the lower limit, mold release properties can be improved. On the other hand, by setting the ten-point average roughness Rz to be below the upper limit, good conformability can be maintained. The ten-point average roughness Rz can be measured in accordance with JIS B 0601 (1994).

[0064] The average spacing Sm of the irregularities on one side of the release film 25 (the side facing the prepreg 20) is preferably 100 μm or more and 350 μm or less, more preferably 150 μm or more and 310 μm or less, and even more preferably 180 μm or more and 300 μm or less. Because the average spacing Sm of the irregularities is within the above range, air escape between the release film 25 and the prepreg 20 is good, and the occurrence of voids and wrinkles due to trapped air is suppressed. The average spacing between irregularities, Sm, can be measured in accordance with JIS B 0601 (1994).

[0065] In this embodiment, the surface roughness of the release film 25 can be adjusted by controlling the thickness of the release film 25, the manufacturing method of the release film 25, or by controlling the particle composition and particle size, which will be described later.

[0066] The resin constituting the release film 25 preferably includes one or more selected from, for example, polyalkylene terephthalate resins such as polyethylene terephthalate resin (PET), polybutylene terephthalate resin (PBT), polytrimethylene terephthalate resin (PTT), and polyhexamethylene terephthalate resin (PHT), poly-4-methyl-1-pentene resin (TPX), syndiotactic polystyrene resin (SPS), and polypropylene resin (PP). Among these, polyethylene terephthalate resin (PET), polybutylene terephthalate resin (PBT), poly-4-methyl-1-pentene resin (TPX), and polypropylene resin (PP) are more preferred.

[0067] The release film 25 of this embodiment may contain particles. The average particle size d50 is preferably 3 μm or more, more preferably 10 μm or more. On the other hand, the average particle size d50 is preferably 35 μm or less, more preferably 25 μm or less. By setting the average particle size d50 to above the lower limit, the rigidity of the release film 25 can be improved, as can the release properties from the prepreg 20. On the other hand, by setting the average particle size d50 to below the upper limit, a good balance between release properties and conformability can be achieved, and a sandwich panel 100 with a good finished appearance can be produced.

[0068] The particles are preferably inorganic particles from the viewpoint of effectively improving the rigidity of the release film 25. Examples of inorganic particles include silica such as crystalline silica, amorphous silica, and fused silica, and particles made using one or more of the group consisting of aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, calcium silicate, magnesium silicate, calcium oxide, magnesium oxide, zinc oxide, alumina, aluminum nitride, aluminum borate whiskers, boron nitride, antimony oxide, E glass, D glass, S glass, and zeolite. Only one type of inorganic particle may be used alone, or different types of particles may be used in combination. The inorganic particles may be surface-treated with a silane coupling agent or the like to improve adhesion to the resin, or core-shell type particles that have been treated with an organic coating may be used to improve dispersibility.

[0069] The particle content is preferably 1% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 20% by mass or less, relative to the entire release film 25.

[0070] In addition to the resin and particles mentioned above, the release film 25 may also contain known additives such as antioxidants, slip agents, antiblocking agents, antistatic agents, colorants such as dyes and pigments, stabilizers, and impact-improving agents such as fluororesins and silicone rubber.

[0071] The release film 25 is obtained by forming a film using a resin composition containing the above-mentioned resin, etc., using a known method such as co-extrusion, extrusion lamination, dry lamination, or inflation. Alternatively, it may be subjected to stretching. The stretching may be simultaneous biaxial stretching or sequential biaxial stretching. Furthermore, to adjust the surface roughness, embossing or other similar processes may be applied.

[0072] Furthermore, the release film 25 may be a single layer or a multilayer structure. In the case of a multilayer structure, in addition to the release layer that constitutes the release surface, it may also include layers with functions such as a cushion layer and a reinforcing layer. For example, a release film may have a multilayer structure in which a release layer, a cushion layer, and a release layer are laminated in that order.

[0073] (Step 3) Heat and pressurization treatment Next, as shown in Figure 2(b), the laminate is subjected to heat and pressure treatment with a metal plate 30 together with the release film 25. That is, heat and pressure are applied from the top and bottom surfaces of the laminated sheet-like prepreg 20 via the release film 25 to harden the prepreg 20 and integrate it with the core layer 10. This makes it possible to create a sandwich panel 100 in which the hardened prepreg 20 40 and the core layer 10 are firmly bonded. In addition, a portion of the prepreg 20 penetrates into the core layer 40, which can improve the adhesion between the prepreg 20 and the core layer 10.

[0074] The heating and pressurizing treatment is preferably performed at 110-150°C and 0.1-3.0 MPa, and more preferably at 120-140°C and 0.3-1.0 MPa. By setting the temperature and pressure of the heating and pressurizing treatment to above the above lower limit, the cured prepreg 20 and the core layer 10 can be firmly integrated. On the other hand, by keeping the temperature and pressure of the heating and pressurizing process below the above upper limits, damage to the prepreg 20 can be suppressed while ensuring proper integration.

[0075] Furthermore, in this embodiment, by using the release film 25, pressure can be applied more uniformly to the entire surface of the prepreg 20, effectively suppressing the appearance of voids on the surface of the sandwich panel 100 and the occurrence of pinholes on the surface of the sandwich panel 100. In particular, the release film 25 follows the unevenness caused by the weave of the fibrous base material of the prepreg 20, allowing for more uniform pressure to be applied and suppressing the occurrence of voids on the surface of the sandwich panel 100. Moreover, even if a part of the prepreg 20 becomes embedded in the core layer 10, causing a depression on the surface of the prepreg 20, in this embodiment, the use of the release film 25 can similarly suppress the occurrence of pinholes.

[0076] The metal plate 30 can be any known material, such as SUS plates, tin plates, aluminum plates, and magnesium plates. Furthermore, the film thickness of the metal plate 30 is not particularly limited, but may be, for example, 0.5 mm to 10 mm, 0.8 mm to 5 mm, or 1.0 mm to 2.0 mm. By keeping it within these ranges, a balance between rigidity and thermal conductivity can be achieved.

[0077] Subsequently, as shown in Figure 2(c), the release film 25 and the metal plate 30 are separated to obtain the sandwich panel 100. Furthermore, because the release film 25 has good release properties, the sandwich panel 100 is prevented from sticking to the metal plate 30, and the sandwich panel 100 can be easily removed.

[0078] The embodiments of the present invention have been described above with reference to the drawings, but these are merely examples of the present invention, and various other configurations can also be adopted. For example, the sandwich panel 100 of this embodiment may have adhesive layers between the core layer 10 and the prepreg 20, depending on the purpose. [Examples]

[0079] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way to the descriptions of these examples.

[0080] (1) Fabrication of sandwich panels <Example 1> First, a prepreg was prepared using the following procedure: Phenolic resin 1 (resol-type phenolic resin, Durez "34370") was mixed and impregnated into glass fibers (#7781, HEXCEL) to obtain a sheet-like prepreg (thickness 250 μm). This prepreg was in the B stage. Next, sandwich panels were fabricated using the obtained prepreg according to the following procedure. A laminate was obtained by placing the prepreg prepared above on both sides of a honeycomb core (aramid fiber, thickness: 10 mm, HRH-10-1 / 8-3.0 (manufactured by HEXCEL), area: 1 m x 3 m). Next, as shown in Table 1, release film 1 (TPX® film "Sumilight CEL-E960D", manufactured by Sumitomo Bakelite Co., Ltd., thickness 120 μm) was placed on the top and bottom surfaces of the obtained laminate. Then, SUS plates (thickness: 1.5 mm, Rz: 1.0 μm) were pressed against the top and bottom surfaces, and the prepreg was heated and pressurized using a mechanical press at 0.3 MPa, 130°C, and 60 minutes to harden the prepreg and integrate the core layer and the prepreg. After that, the release film 1 and the SUS plates were separated to obtain a sandwich panel.

[0081] <Example 2> A sandwich panel was obtained in the same manner as in Example 1, except that the thickness of the release film 1 was set to 60 μm.

[0082] <Example 3> A sandwich panel was obtained in the same manner as in Example 1, except that release film 1 was replaced with release film 2 (polypropylene film "Trefan" (registered trademark), manufactured by Toray Industries, Inc., 60 μm thick).

[0083] <Example 4> A sandwich panel was obtained in the same manner as in Example 1, except that release film 1 was changed to release film 3 (polyethylene terephthalate film, 100 μm thick).

[0084] <Example 5> A sandwich panel was obtained in the same manner as in Example 1, except that phenol resin 1 used in the prepreg was changed to phenol resin 2 (lignin-modified phenol resin "LMR-02-R4" manufactured by Sumitomo Bakelite Co., Ltd.).

[0085] <Comparative Example 1> A sandwich panel was obtained in the same manner as in Example 1, except that release film 1 was not used.

[0086] (2) Physical properties of the release film The physical properties of the above release film were measured as follows. (a) The release film was heat-treated at 180°C for 120 seconds. Then, the storage modulus E' at 100°C was determined by measuring dynamic viscoelasticity (DMA) under conditions of a heating rate of 5°C / min and a frequency of 100 Hz.

[0087] (3) Evaluation and measurement of sandwich panels The sandwich panel described above was evaluated as follows. The evaluation results are shown in Table 1.

[0088] • Measuring pinholes The surface of the resulting sandwich panel was observed using a magnifying glass (4x magnification), the number of pinholes was measured, and the density was calculated.

[0089] Furthermore, observation of the cross-section of the sandwich panel revealed that a portion of the prepreg binder resin had infiltrated the core layer and hardened there.

[0090] [Table 1] [Explanation of Symbols]

[0091] 10-core layer 20 prepregs 25 Release film 30 metal plate 40 Cured product 100 sandwich panels

Claims

1. A process of preparing a sheet-like core layer having a honeycomb structure and a plurality of sheet-like prepregs, The process involves forming a laminate by laminating one or more layers of the prepreg on each of the two sides of the core layer, and placing a release film on the outermost layer of the laminate. A step of applying heat and pressure to the laminate together with the release film, Includes, A method for manufacturing a sandwich panel, wherein the release film has a stiffness parameter G, specified by the following formula (1), between 25 and 1500. Stiffness parameter G = Thickness of the release film [μm] × Storage modulus E' [GPa] (1) (In equation (1), the storage modulus E' is measured after the release film has been heat-treated at 180°C for 120 seconds, under conditions of a frequency of 1 Hz and a temperature of 100°C.)

2. A process of preparing a sheet-like core layer having a honeycomb structure and a plurality of sheet-like prepregs, The process involves forming a laminate by laminating one or more layers of the prepreg on each of the two sides of the core layer, and placing a release film on the outermost layer of the laminate. A step of applying heat and pressure to the laminate together with the release film, Includes, A method for manufacturing a sandwich panel, wherein the release film comprises one or more selected from polyethylene terephthalate resin (PET), polybutylene terephthalate resin (PBT), polytrimethylene terephthalate resin (PTT), polyhexamethylene terephthalate resin (PHT), poly-4-methyl-1-pentene resin (TPX), syndiotactic polystyrene resin (SPS), and polypropylene resin (PP).

3. A method for manufacturing a sandwich panel according to claim 2, A method for manufacturing a sandwich panel, wherein the release film has a stiffness parameter G, specified by the following formula (1), between 25 and 1500. Stiffness parameter G = Thickness of the release film [μm] × Storage modulus E' [GPa] (1) (In equation (1), the storage modulus E' is measured after the release film has been heat-treated at 180°C for 120 seconds, under conditions of a frequency of 1 Hz and a temperature of 100°C.)

4. A method for manufacturing a sandwich panel according to any one of claims 1 to 3, The aforementioned heating and pressurizing treatment is performed at 110 to 150°C and 0.1 to 3.0 MPa, in a method for manufacturing sandwich panels.

5. A method for manufacturing a sandwich panel according to any one of claims 1 to 4, The method for manufacturing a sandwich panel, wherein the prepreg is a sheet member in a B-stage state, obtained by impregnating a prepreg substrate with a binder resin.

6. A method for manufacturing a sandwich panel according to claim 5, A method for manufacturing a sandwich panel, wherein the binder resin comprises one or more selected from phenolic resin, unsaturated polyester resin, epoxy resin, melamine resin, and furan resin.

7. A method for manufacturing a sandwich panel according to claim 5 or 6, The aforementioned prepreg substrate includes a woven fiber cloth, and the method for manufacturing a sandwich panel.

8. A method for manufacturing a sandwich panel according to any one of claims 1 to 7, A method for manufacturing a sandwich panel, wherein the thickness of the release film is 50 μm or more and 150 μm or less.

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