Film, laminate containing same, and method for producing fiber-reinforced composite material using same

Abstract

This is a film in which a powder is adhered to at least one surface of a polyvinyl alcohol film and in which the occupied area ratio for the powder with respect to the film area is 0.05-5%. The present invention provides a non-fluorine-type film that, when applied as a release film or bagging film used during the production of fiber-reinforced composite materials, exhibits an excellent workability during molding and an excellent releasability from the molded body.

Description

Film, laminate including same, and method for producing fiber-reinforced composite material using same

[0001] The present invention relates to a film, a laminate containing the film, and a fiber-reinforced composite material using the film.

[0002] Fiber-reinforced composite materials are materials in which fiber materials are reinforced by compounding them with a resin matrix, and they are lightweight yet have high strength, high rigidity, and high impact resistance. Methods for molding fiber-reinforced composite materials include pultrusion molding, resin transfer molding (RTM) molding, press molding, and autoclave (AC) molding. AC molding involves stacking prepregs (fiber materials pre-impregnated with resin) in a mold, covering them with a bagging film (a film for creating a vacuum), reducing the pressure, and then applying pressure in an autoclave to heat and cure the material. While AC molding has the advantage of being suitable for producing complex-shaped or large-sized molded bodies, its long molding cycle and limited film size that can be made into large-area products are cited as challenges.

[0003] As a release film for use in AC molding, a release film that is embossed to allow air to escape from the gap between the prepreg and the film has been proposed (Patent Document 1). While the use of this release film can shorten the molding cycle and produce high-quality molded products, the film material is limited to polyolefins or fluoropolymers. Recently, in particular, there has been concern that various fluorine compounds produced as by-products during the production of fluoropolymers may have adverse effects on the human body and the environment, and legal restrictions have been put in place to address this. Therefore, there is a demand for materials that can replace fluoropolymers.

[0004] Polyvinyl alcohol (hereinafter sometimes abbreviated as "PVA") is a crystalline hydrophilic polymer. Polyvinyl alcohol film has excellent film properties (strength, transparency, film-forming ability, oxygen gas barrier property, etc.) and is used as a raw material for polarizing films and various packaging films. It has also been disclosed that polyvinyl alcohol film is used as a release film for the production of artificial marble (Patent Document 2).

[0005] US Publication No. 2022-266479 International Publication No. 2020 / 050394

[0006] An object of the present invention is to provide a non-fluorine-based film that is excellent in workability during molding and releasability from molded articles as a release film or bagging film used in the production of fiber-reinforced composite materials.

[0007] As a result of extensive research, the present inventors discovered that a polyvinyl alcohol film having a specific amount of powder attached thereto has excellent releasability from a molded body, and further research based on this finding led to the completion of the present invention.

[0008] That is, the present invention provides the following items [1] to

[13] . [1] A film having a powder attached to at least one surface of a polyvinyl alcohol film, wherein the powder occupies 0.05 to 5% of the film area. [2] The film according to [1], wherein the polyvinyl alcohol film contains polyvinyl alcohol and a plasticizer, and the content of the plasticizer is 5 to 30 parts by mass per 100 parts by mass of the polyvinyl alcohol. [3] The film according to [1] or [2], wherein the thickness is 15 to 100 μm. [4] The film according to any of [1] to [3], wherein the average particle size of the powder is 5 to 30 μm. [5] The film according to any of [1] to [4], wherein the powder is a processed starch that has been subjected to a water-repellent treatment. [6] The film according to any of [1] to [5], wherein the degree of polymerization of the polyvinyl alcohol is 500 to 6,000 and the degree of saponification is 80 to 99.9 mol%. [7] The film according to any one of [2] to [6], wherein the plasticizer is any one selected from the group consisting of ethylene glycol, glycerin, diglycerin, propylene glycol, and diethylene glycol. [8] The film according to any one of [1] to [7], wherein at least one surface of the polyvinyl alcohol film is coated with any one selected from the group consisting of a polymer containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic acid esters, an aqueous polyvinyl alcohol solution, and a polyvinyl alcohol-based emulsion, and the powder adheres to the coated surface. [9] A laminate obtained by laminating an uncured fiber composite material with a release film made of the film according to any one of [1] to [8].

[10] A method for producing a fiber-reinforced composite material, comprising curing the uncured fiber composite material in the laminate according to [9] under reduced pressure and then peeling the release film from the cured fiber composite material.

[11] The method for producing a fiber-reinforced composite material according to

[10] , wherein the laminate is covered with a bagging film made of the film, and then vacuumed to reduce the pressure inside, and the uncured fiber composite material is cured under reduced pressure.

[12] The film according to any one of [1] to [8], which is a release film for producing a fiber-reinforced composite material.

[13] The film according to any one of [1] to [8], which is a bagging film for producing a fiber-reinforced composite material.

[0009] The present invention provides a non-fluorine-based film that is excellent in workability during molding and releasability from molded articles as a release film or bagging film used in the production of fiber-reinforced composite materials. It also provides a laminate including the film and a method for producing a fiber-reinforced composite material.

[0010] 1 is a cross-sectional view of a vacuum bag system used in the manufacture of fiber reinforced composite materials, according to one embodiment of the present invention.

[0011] [Film] The film of the present invention is a polyvinyl alcohol film having a powder attached to at least one surface thereof, and the ratio of the area occupied by the powder to the area of ​​the film is 0.05 to 5%. In this specification, a film without powder attached thereto will be referred to as a "polyvinyl alcohol film," and a film of the present invention with powder attached thereto will be referred to as a "film."

[0012] The mechanism of action of the present invention is presumed to be as follows: When forming irregularities by adhering powder to the surface of a polyvinyl alcohol film, by setting the amount of powder adhered within a specific range, an appropriate contact area with the prepreg is achieved during molding, resulting in good releasability from the molded product.

[0013] <PVA> In the present invention, the polyvinyl alcohol (PVA) may be one produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferred in terms of ease of PVA production, availability, cost, etc.

[0014] The vinyl ester polymer is preferably one obtained by using only one or more vinyl ester monomers as the monomer, more preferably one obtained by using only one vinyl ester monomer as the monomer, but may also be a copolymer of one or more vinyl ester monomers with other monomers copolymerizable therewith.

[0015] Examples of other monomers copolymerizable with such vinyl ester monomers include ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or salts thereof; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or salts thereof; methyl methacrylate, ethyl methacrylate, methyl ... Methacrylic acid esters such as n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof. methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, N-methylolmethacrylamide or a derivative thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate.The vinyl ester polymer may have structural units derived from one or more of these other monomers.

[0016] Among other monomers copolymerizable with these vinyl ester monomers, from the viewpoint of the mechanical properties of the film, olefins with a small number of carbon atoms such as ethylene and propylene, carboxylic acid monomers such as acrylic acid and methacrylic acid, and sulfonic acid monomers are preferred.

[0017] From the viewpoint of the mechanical properties of the polyvinyl alcohol film and suppression of hole formation, the upper limit of the proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol % or less, more preferably 10 mol % or less, and even more preferably 5 mol % or less, based on the number of moles of all structural units constituting the vinyl ester polymer.

[0018] In the present invention, the degree of polymerization of the PVA contained in the polyvinyl alcohol film is not particularly limited, but is preferably 500 to 6,000. If the degree of polymerization of the PVA is less than 500, the strength of the film may be insufficient. The lower limit of the degree of polymerization of the PVA is preferably 600 or more, more preferably 1,000 or more, and even more preferably 1,500 or more. On the other hand, if the degree of polymerization of the PVA exceeds 6,000, it may be difficult to ensure the productivity of the PVA or polyvinyl alcohol film. The upper limit of the degree of polymerization of the PVA is preferably 5,000 or less, more preferably 4,000 or less, and even more preferably 3,500 or less. Here, the degree of polymerization refers to the average degree of polymerization (Po) measured in accordance with JIS K6726-1994, and is calculated by the following formula using the intrinsic viscosity [η] (dL / g) measured in water at 30°C after resaponifying and purifying the PVA. Po = ([η]×10 ４ / 8.29) （１／０．６２）

[0019] In the present invention, the saponification degree of the PVA contained in the polyvinyl alcohol film is preferably 80 to 100 mol%. A saponification degree of 80 mol% or more can improve the strength of the polyvinyl alcohol film, and therefore the film of the present invention using the polyvinyl alcohol film. The lower limit of the saponification degree is preferably 90 mol%, more preferably 95 mol% or more, and even more preferably 99 mol% or more. From the viewpoint of PVA productivity, the upper limit of the saponification degree is preferably 99.9 mol%. Here, the saponification degree of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of structural units (typically vinyl ester monomer units) that can be converted into vinyl alcohol units by saponification contained in the PVA and vinyl alcohol units. The saponification degree of PVA can be measured in accordance with the description of JIS K6726-1994.

[0020] In the film of the present invention, one type of PVA may be used alone as the PVA, or two or more types of PVAs having different degrees of polymerization, saponification, or modification may be blended together.

[0021] In the present invention, the upper limit of the PVA content in the polyvinyl alcohol film is not particularly limited, but the lower limit of the PVA content relative to 100% by mass of the polyvinyl alcohol film is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0022] <Plasticizer> In the present invention, the polyvinyl alcohol film may contain a plasticizer. The content of the plasticizer is preferably 5 to 30 parts by mass per 100 parts by mass of PVA. By containing a plasticizer, flexibility equivalent to that of other plastic films can be imparted. As a result, mechanical strength such as impact strength and processability during secondary processing are improved.

[0023] Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, trimethylolpropane, sorbitol, etc. From the viewpoint of preventing bleeding out onto the surface of the polyvinyl alcohol film, the plasticizer is preferably any one selected from the group consisting of ethylene glycol, glycerin, diglycerin, propylene glycol, and diethylene glycol.

[0024] <Polyvinyl Alcohol Film> In the present invention, from the viewpoint of achieving both mechanical strength and handleability of the polyvinyl alcohol film, the lower limit of the content of the plasticizer in the polyvinyl alcohol film is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, relative to 100 parts by mass of PVA. On the other hand, the upper limit of the content of the plasticizer is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, even more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less, relative to 100 parts by mass of PVA.

[0025] In the present invention, the polyvinyl alcohol film can be made to contain a plasticizer by adding the plasticizer to the film-forming solution for the polyvinyl alcohol film. The content of the plasticizer in the polyvinyl alcohol film is substantially equal to the content of the plasticizer in the film-forming solution for the polyvinyl alcohol film.

[0026] In the present invention, the polyvinyl alcohol film may contain, in addition to PVA and plasticizer, components such as water, surfactants, antioxidants, UV absorbers, lubricants, fillers, crosslinking agents, colorants, preservatives, antifungal agents, water-soluble polymers other than PVA, and other polymer compounds, to the extent that the effects of the present invention are not impaired. The proportion of the total mass of the PVA and plasticizer to the total mass of the polyvinyl alcohol film is preferably within a range of 60 to 100% by mass, more preferably within a range of 80 to 100% by mass, and even more preferably within a range of 90 to 100% by mass.

[0027] In the present invention, there is no particular limitation on the thickness of the polyvinyl alcohol film, but the upper limit of the thickness is preferably 100 μm or less, more preferably 90 μm or less, even more preferably 80 μm or less, and particularly preferably 70 μm or less. When the thickness of the polyvinyl alcohol film is below the above upper limit, the film of the present invention becomes easy to handle, and the productivity of the polyvinyl alcohol film is easily ensured. On the other hand, the thickness of the polyvinyl alcohol film is preferably 15 μm or more, more preferably 20 μm or more, even more preferably 30 μm or more, particularly preferably 40 μm or more, and most preferably 45 μm or more. When the thickness of the polyvinyl alcohol film is above the above lower limit, the film of the present invention becomes easy to work with during molding. The thickness of the polyvinyl alcohol film can be measured at any 10 locations (for example, any 10 locations on a line drawn in the longitudinal direction of the polyvinyl alcohol film) and calculated as the average value.

[0028] The film of the present invention contains the plasticizer and has a thickness within the above range, which makes it easy to work with when applying and removing the film, and it can also conform to molds with complex shapes, so it can be used as a hybrid film that functions as both a release film and a bagging film.

[0029] <Powder> In the film of the present invention, a powder is adhered to at least one surface of the polyvinyl alcohol film, and the powder occupies an area ratio of 0.05 to 5% of the film area. This ensures that the powder comes into contact with the prepreg over an appropriate contact area during molding, improving releasability from the molded product. The upper limit of the area ratio is preferably 3%, more preferably 2%, and even more preferably 1%. The lower limit of the area ratio is preferably 0.1%. In the present invention, "powder adhered to at least one surface of the PVA film" means that, on one or both surfaces of the PVA film, (1) at least some particles are in electrostatic contact with the film surface, (2) at least some particles are embedded in the film surface, or (3) at least some particles are in electrostatic contact with the film surface and at least some particles are embedded in the film surface. Here, the powder occupancy ratio refers to the ratio of the total area occupied by the powder to the observed area (film area) when the film is observed under a microscope. In the present invention, the ratio of the area occupied by the powder to the area of ​​the film is a value determined by the method described in the examples below.

[0030] In the present invention, examples of powders include organic powders and inorganic powders. Examples of organic powders include starch, etc. Examples of starch include natural starches such as corn starch, potato starch, sweet potato starch, wheat starch, rice starch, tapioca starch, and sago starch; and processed starches that have been etherified, esterified, oxidized, or water-repellent treated. Among these, processed starches are preferred, and water-repellent treated starches are more preferred. Examples of inorganic powders include silica, heavy, light, or surface-treated calcium carbonate, aluminum hydroxide, aluminum oxide, titanium oxide, diatomaceous earth, barium sulfate, calcium sulfate, zeolite, zinc oxide, silicic acid, silicates, mica, magnesium carbonate, kaolin, clays such as halloysite, pyroferrite, and sericite, and talc. These may be used alone or in combination of two or more.

[0031] There is no limitation on the average particle size of the powder as long as it does not impair the effects of the present invention, but when the average particle size of the powder is within a specific range, the film surface is appropriately unevenly formed, and the peelability from the molded body is improved. Therefore, the lower limit of the average particle size of the powder is preferably 5 μm or more, more preferably 10 μm or more. On the other hand, the upper limit of the average particle size of the powder is preferably 30 μm or less, more preferably 20 μm or less. In the present invention, the average particle size of the powder is a value determined by the method described in the Examples below.

[0032] The particle size of the powder is not particularly limited as long as it does not impair the effects of the present invention, but removing coarse particles and fine particles is preferable because it improves the handleability during production of the polyvinyl alcohol film and allows for uniform adhesion. The particle size of the powder is preferably 99% or more, more preferably 99.9% or more, in terms of sieve passage rate. The particle size of the powder can be evaluated using a standard sieve, which can be selected depending on the particle size of the powder used, and the mesh diameter of the sieve can be, for example, 75 to 105 microns.

[0033] In the present invention, the amount of powder attached to the film is 0.1 to 100 mg / m ３ The upper limit of the amount of adhesion is preferably 50 mg / m ３ More preferably, it is 30 mg / m ３ The lower limit of the amount of adhesion is 0.5 mg / m ３ More preferably, it is 1 mg / m ３ When the amount of attached powder is equal to or less than the above upper limit, deterioration of processability during film production and molding can be easily prevented.

[0034] In the present invention, the powder is adhered to at least one side of the polyvinyl alcohol film, and may be adhered to both sides, but it is preferable that the powder be adhered to both sides, as this allows good releasability to be obtained regardless of the orientation of the film and also allows good releasability to be exhibited from the mold surface and both sides of the molded body when placed between the mold and the prepreg.

[0035] The surface of the film of the present invention may be embossed as long as the effects of the present invention are not impaired. Embossing is generally a processing method in which a film is formed and then nipped between an embossing roll and a rubber roll while applying heat and pressure.

[0036] In the present invention, at least one side of the polyvinyl alcohol film may be coated with a liquid coating agent, as long as the effects of the present invention are not impaired. Furthermore, it is preferable that the powder adhere to the coated surface of the polyvinyl alcohol film. Examples of coating agents include polymers containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic esters, aqueous polyvinyl alcohol solutions, polyvinyl alcohol-based emulsions, aqueous solutions of silicone-based water repellents, and silicone-based emulsions. Among these, any of the following is preferred: polymers containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic esters, aqueous polyvinyl alcohol solutions, and polyvinyl alcohol-based emulsions. As the polymer containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic esters, a copolymer containing styrene units and (meth)acrylic ester units is preferred. The binder used to disperse the microparticles contained in the emulsion is not particularly limited, but PVA is preferred because of its strong adhesion to the polyvinyl alcohol film, which inhibits peeling of the coating layer, and its excellent peelability from the molded product. The PVA is appropriately selected from the PVAs used in the polyvinyl alcohol film.

[0037] The average particle size of the microparticles contained in the emulsion is not particularly limited, but may be 0.020 to 0.5 μm. The microparticles are preferably made of a polymer containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic acid esters. Examples of such polymers include those mentioned above as those used in the coating agent.

[0038] In the present invention, the amount of coating in the coating process is not particularly limited, but the amount of microparticles contained in the emulsion is preferably 0.005 to 0.050 g / m per side. ２ will be adopted.

[0039] In the film of the present invention, from the viewpoint of ease of placement and handling in a fiber composite material, the loop stiffness is preferably 0.30 to 3.0 g at 23°C and 50% RH, more preferably 0.50 to 2.8 g, and even more preferably 0.50 to 1.00 g. Furthermore, a loop stiffness in the range of 0.30 to 3.0 at 20°C and 20% RH is preferred because it provides excellent handleability even at low temperatures and low humidity in winter. The method for measuring loop stiffness will be described later.

[0040] <Film Manufacturing Method> The film manufacturing method of the present invention is not particularly limited. Using a film-forming solution obtained by adding a solvent, a plasticizer, an additive, etc. to PVA to homogenize it, any method can be used to form a film, such as a casting method, a wet film-forming method (discharge into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method in which the film-forming solution is cooled to gel, and then the solvent is extracted and removed to obtain a polyvinyl alcohol film), or a combination of these methods. Or, a melt-extrusion method or an inflation molding method in which a film-forming solution is obtained using an extruder or the like and extruded through a T-die or the like to form a film. Among these, the casting method or the melt-extrusion method is preferred because it can produce a homogeneous polyvinyl alcohol film with good productivity. Below, the casting method and the melt-extrusion method for polyvinyl alcohol films will be described.

[0041] When a polyvinyl alcohol film is produced by a casting method or a melt extrusion method, a film-forming solution is cast onto a support to form a liquid film, which is then heated to remove the solvent and solidify into a film. The solidified film is peeled off from the support, dried using a drying roll or a drying oven as needed, further heat-treated as needed, and wound up to obtain a roll of long polyvinyl alcohol film.

[0042] The upper limit of the volatile content of the film-forming solution (the concentration of volatile components such as solvents removed by volatilization or evaporation during film formation) is preferably 90% by mass or less, more preferably 80% by mass or less. When the volatile content of the film-forming solution is below the above upper limit, it is possible to prevent the viscosity of the film-forming solution from becoming too low, which would impair the uniformity of the thickness of the resulting polyvinyl alcohol film. On the other hand, the lower limit of the volatile content of the film-forming solution is preferably 50% by mass or more, more preferably 55% by mass or more. When the volatile content of the film-forming solution is above the above lower limit, it is possible to prevent the viscosity of the film-forming solution from becoming too high, which would make it difficult to produce a polyvinyl alcohol film.

[0043] Here, the "volatile content of the membrane-forming solution" in this specification refers to the volatile content calculated by the following formula: Volatile content of membrane-forming solution (mass%) = {(Wa - Wb) / Wa} x 100 (where Wa represents the mass (g) of the membrane-forming solution, and Wb represents the mass (g) of the membrane-forming solution Wa (g) after drying it in an electric dryer at 105°C for 16 hours.)

[0044] There are no particular limitations on the method for preparing the film-forming solution, and examples thereof include a method in which PVA and additives such as a plasticizer and a surfactant are dissolved in a dissolution tank or the like, and a method in which PVA in a water-containing state is melt-kneaded together with a plasticizer, a surfactant, etc. using a single-screw or twin-screw extruder. Among these, the method in which PVA is dissolved in a dissolution tank or the method using a twin-screw extruder is preferred.

[0045] The prepared film-forming solution is sent to a T-die or the like through a pipe or the like, and is extruded onto a support in the form of a film through a die lip. The upper limit of the surface temperature of the support onto which the film-forming solution flows is preferably 110° C. or less, more preferably 100° C. or less, and even more preferably 95° C. or less. While the liquid film is heated and dried on the support, hot air may be blown uniformly over the entire area of ​​the non-contact side of the liquid film to adjust the drying speed.

[0046] The liquid film formed by casting the film-forming solution onto a support is dried on the support preferably to a volatile content of 5 to 50% by mass, then peeled off, and further dried as necessary. The drying method is not particularly limited, and examples include contacting the film with a drying oven or drying rolls. When drying is performed with multiple drying rolls, it is preferable to alternately contact one side of the liquid film with the drying rolls in order to uniformize the physical properties of both sides of the resulting polyvinyl alcohol film.

[0047] The polyvinyl alcohol film preferably includes a step of heat treating the film under conditions of 80 to 300°C. The upper limit of the heat treatment temperature is more preferably 280°C or less, even more preferably 260°C or less, and particularly preferably 240°C or less. When the heat treatment temperature is below the above upper limit, it becomes easier to prevent the film from becoming too hard, which deteriorates workability during molding. On the other hand, the lower limit of the heat treatment temperature is more preferably 90°C or more, even more preferably 100°C or more, and particularly preferably 105°C or more. When the heat treatment temperature is above the above lower limit, it becomes easier to obtain a film with good workability during molding.

[0048] The upper limit of the volatile content of the polyvinyl alcohol film finally obtained by the series of treatments is preferably 5% by mass or less, more preferably 4% by mass or less, and the lower limit of the volatile content of the polyvinyl alcohol film is preferably 1% by mass or more, more preferably 2% by mass or more.

[0049] The powder to be attached to the polyvinyl alcohol film is as described above, but in order to regulate the average particle size, it is preferable to pass the powder through a sieve such as a mesh beforehand.

[0050] Known powder coating methods can be used to attach powder to a polyvinyl alcohol film. Examples of powder coating methods include electrostatic powder coating and fluidized bed dip coating. Electrostatic powder coating involves using a spray gun to charge powder with static electricity, which is then used to attach the powder to a film. The amount of powder attached to a polyvinyl alcohol film can be adjusted by adjusting the charge on the powder and the temperature of the polyvinyl alcohol film. Fluidized bed dip coating involves placing powder coating in a container (fluidized bed tank) with a perforated bottom plate, and then introducing compressed air or inert gas from the bottom to create a fluidized state of the powder coating. The powder coating adheres to the polyvinyl alcohol film by passing it through the fluidized bed. The amount of powder attached to the polyvinyl alcohol film and the ratio of the powder area to the film area can be adjusted by adjusting the transport speed of the polyvinyl alcohol film as it passes through the container, the powder concentration in the container, or the temperature of the polyvinyl alcohol film. For example, pre-warming the polyvinyl alcohol film softens the film surface, making it easier for powder to adhere.

[0051] The film of the present invention may be treated with the above-mentioned liquid coating agent as needed.Methods for coating with a liquid coating agent include, for example, roll coater method, air doctor method, blade coater method, spray method, dipping method, etc., and the roll coater method and dipping method are preferred.Commercially available coating agents can be used for coating by the above-mentioned method.

[0052] After the coating liquid is applied, it is preferable to further dry it as needed. The temperature during drying can be, for example, within the range of 70 to 110°C. The drying time can be 1 to 30 seconds. Drying can be carried out under normal pressure or under reduced pressure.

[0053] The surface of the film of the present invention may be embossed as needed. The surface to be embossed may be either the surface not in contact with the support or the surface in contact with the support. Embossing can be carried out, for example, by passing the film between an embossing roll and a backup roll.

[0054] The order of the above-mentioned powder coating, application of the liquid coating agent, and embossing is not particularly limited.

[0055] The film produced in this manner may be further subjected to humidity conditioning, cutting of both ends (edges) of the film, etc., as necessary, and then wound into a roll on a cylindrical core and moisture-proof packaged to form a product. The film of the present invention may also be further subjected to coating, embossing, humidity conditioning, cutting of both ends (edges) of the film, slitting, etc., as necessary. These treatments may be performed either before or after the step of adhering powder to the polyvinyl alcohol film after production of the polyvinyl alcohol film, and the order of each treatment is not limited.

[0056] [Laminate] The laminate of the present invention comprises an uncured fiber composite material and a release film made of the film of the present invention laminated together. Preferably, the film of the present invention is laminated on both sides of the uncured fiber composite material. Furthermore, if a mold release agent is applied to the mold, the film of the present invention may be laminated on the side of the uncured fiber composite material that does not face the mold. That is, the film of the present invention has excellent releasability from the molded product, allowing the fiber-reinforced composite material to be easily removed from the laminate of the present invention after curing. Furthermore, since the film of the present invention is a non-fluorine-containing film, the film peeled off after curing can be incinerated. Examples of fiber materials for the fiber composite material include woven fabrics of carbon fiber, glass fiber, aramid fiber, polyethylene fiber, etc., with carbon fiber woven fabrics being preferred because the resulting fiber-reinforced composite material has excellent mechanical properties and is lightweight. Examples of resins used in fiber-reinforced composite materials include epoxy, unsaturated polyester, vinyl ester, bismaleimide, phenol, cyanate, polyimide, etc., with epoxy resins being preferred because of their ease of availability and the excellent properties of the resulting fiber-reinforced composite material. The film may be laminated on a prepreg in which a fiber material is pre-impregnated with a resin, or a fiber material laminated with a film may be impregnated with a liquid resin, as in vacuum-assisted RTM.

[0057] [Method for Producing a Fiber-Reinforced Composite Material] A fiber-reinforced composite material can be produced by curing the uncured fiber composite material in the laminate under reduced pressure and then peeling the release film from the cured fiber composite material. In this case, it is preferable to cover the laminate with a bagging film and then evacuate the interior to reduce the pressure, and then cure the uncured fiber composite material under reduced pressure. Specifically, a vacuum bag system, as described below, can be used. The film of the present invention is preferably used as the bagging film. Resin curing can be carried out in an oven using only vacuum bag pressure. Resin can also be cured at high temperature and high pressure in an autoclave to achieve a higher degree of solidification of the fiber layer. Curing can also be carried out via a catalyst, with or without the application of high temperature. The curing temperature is preferably 100 to 200°C. The film of the present invention is preferably used as a release film or bagging film for producing such fiber-reinforced composite materials.

[0058] [Release Film or Bagging Film] In molding methods using a mold on one side, such as AC molding, oven molding, and vacuum-assisted RTM, a vacuum bag system is used to press the fiber composite material placed in the mold against the mold, remove air trapped between the layers, and achieve the desired resin-fiber ratio so that the composite material exhibits the desired mechanical properties. A vacuum bag system is a system for performing molding using a vacuum bag (e.g., AC molding, etc.). A vacuum bag is a shape that can be evacuated to create a vacuum inside. The vacuum bag may be, for example, bag-shaped, but is not limited to this. The vacuum bag may be composed of, for example, only a bagging film, or it may be composed of a bagging film and other components (e.g., a mold, etc.). In a vacuum bag system, the fiber composite material, peel ply, release film, and breather are layered in this order, and the bagging film is sealed to the mold with sealing tape to cover them. In some cases, the peel ply may be omitted. The bagging film is fitted with a suction port to remove air from the internal space between the bagging film and the mold, and a vacuum is then applied by connecting the suction port to a vacuum pump via a hose. The breather is a nonwoven fabric that provides an escape route for the air when the internal space is evacuated. The release film is placed so that the breather and bagging film can be removed from the resulting fiber-reinforced composite material after curing. A release film can also be placed between the mold and the fiber composite material to make it easier to peel the resulting fiber-reinforced composite material from the mold after curing.

[0059] The film of the present invention can be used as a release film when molding the above-mentioned fiber-reinforced composite material, and therefore, a method of using it as a release film is also an aspect of the present invention. Since the film of the present invention has excellent releasability from the molded product, by using it as a release film or by employing a method using the release film of the present invention, the fiber-reinforced composite material can be easily removed after curing. Furthermore, since the film of the present invention is a non-fluorine-containing film, the film peeled off after curing can be incinerated.

[0060] The film of the present invention can be used as a bagging film when molding fiber-reinforced composite materials, and its use as a bagging film is also an aspect of the present invention. More specifically, it can be used to create a sealed state for reducing the pressure of the bagging film in molding methods that cure resins under reduced pressure, such as AC molding, oven molding, and vacuum-assisted RTM. In particular, the film of the present invention contains the plasticizer and has a thickness within the above range, which provides excellent workability during molding and allows it to conform to molds and molded articles with complex shapes, resulting in high-quality molded articles. Furthermore, while conventional bagging films made from other materials tend to harden during low temperatures and low humidity in winter, resulting in poor workability during molding, the film of the present invention, with its appropriate plasticizer content and thickness, provides excellent workability even at low temperatures and low humidity.

[0061] The film of the present invention can be used as both a release film and a bagging film when molding a fiber-reinforced composite material. By using the film of the present invention as both a release film and a bagging film, material management can be simplified and the types of materials used can be reduced.

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The evaluation methods used in the following examples and comparative examples are shown below.

[0063] <Powder Occupation Area Ratio and Average Particle Size> A sample of the film obtained in the following Examples or Comparative Examples was cut to 5 cm x 5 cm and sandwiched between metal frames. The surface opposite the observation surface was wiped with a dry paper wiper, and then observed at 500x magnification using a Keyence VHX-6000. The observed image was subjected to brightness extraction (extraction target: bright area, threshold: 0, small particle removal setting: ON) using the automatic area measurement function, and "brightness unevenness removal (weak) + overall blurred image" was selected. For the selected image, the ratio of the powder area to the film area on the screen and the average particle size (the average of the minimum and maximum diameters of each particle was used as the average particle size of the particle) were automatically calculated. The average value of n5 data was calculated.

[0064] <Workability (Release Film)> In the molding tests of the following Examples and Comparative Examples, the workability of the film when used as a release film was evaluated as follows: A: Good. B: Workable, but the film was too soft or hard to handle. C: Difficult to place.

[0065] <Peel Force> The peelability from the molded article was measured using a tensiometer equipped with a clip as follows. 5 cm of the release film was peeled from the short side of the fiber-reinforced composite material laminated on both sides thereof obtained in the following Examples or Comparative Examples, and the peel force at 90° peeling was measured by clamping the material with a clip and pulling it in the vertical direction. The smaller the peel force, the better the peelability.

[0066] <Workability (bagging film)> In the molding tests of the following Examples and Comparative Examples, the workability of the film when used as a bagging film was evaluated as follows: A: Good. B: Workable, but the film was too thin or hard to handle. C: Unacceptable.

[0067] <Film Flexibility (Loop Stiffness)> The flexibility of the films obtained in the following Examples and Comparative Examples was measured using a "Loop Stiffness Tester No. 581" manufactured by Toyo Seiki Seisakusho. After conditioning the film at 23°C and 50% RH for at least 16 hours, samples were cut into 15 mm x 200 mm strips and fixed to the tester so that the loop length was 55 mm. The tester was pressed so that the distance between the pressing plate and the tester was 10 mm, and the crush resistance 1 minute after the start of operation was measured. Measurements were conducted on each sample n5, and the average value was calculated. Additionally, as an index of workability at low temperatures and low humidity in winter, the crush resistance was also measured using the loop stiffness tester at 20°C and 20% RH.

[0068] [Example 1] A film-forming solution with a volatile content of 68%, consisting of 100 parts by mass of PVA (degree of polymerization 1,700, degree of saponification 99.9 mol%), 13 parts by mass of glycerin as a plasticizer, 0.1 parts by mass of lauric acid diethanolamide as a surfactant, and water, was melt-extruded from a T-die onto a metal roll at 90°C, dried, and then heat-treated with a heat-treatment roll at a surface temperature of 120°C to obtain a polyvinyl alcohol film with a thickness of 50 μm. Methyl methacrylate / styrene copolymer was coated on both sides of the obtained polyvinyl alcohol film by a dipping method at a concentration of 0.020 g / m per side. ２ The coating solution was prepared by adding an emulsion of methyl methacrylate / styrene copolymer (methyl methacrylate unit / styrene unit (molar ratio) = 50 / 50) with an average particle size of 0.1 μm (methyl methacrylate / styrene copolymer particles dispersed in water at a concentration of 30% by mass) to an aqueous PVA solution containing 1.5% by mass of PVA with a degree of polymerization of 2400 and a degree of saponification of 99.2 mol% at a mass ratio of [PVA] / [methyl methacrylate / styrene copolymer] = 3 / 1. Next, water-repellent processed starch was used as the powder to be applied to one side of the coated film by electronic spraying, and then the powder was applied to the other side in the same manner to obtain a film. The powder occupied an area of ​​0.17% of the film area, and the average particle size was 13.1 μm. The flexibility of the obtained film was measured by the above-mentioned method at 23° C., 50% RH and at 20° C., 20% RH, and was found to be 0.56 g and 2.75 g, respectively.

[0069] The resulting film was used as a release film and a bagging film for a molding test. Three sheets of prepreg (fiber material: 3K plain weave carbon fiber, resin: 130°C-cured epoxy resin) of the same size were placed on a 20 cm x 27.5 cm cut film, and a film of the same size was then placed on top of them to obtain a laminate. The workability when placing the release film was good. A breather of the same size was then placed on the resulting laminate, which was then covered with a bagging film equipped with a suction port, sealed with sealing tape, and vacuumed. The workability when covering the laminate with the bagging film was good. The laminate was placed in an autoclave and left at 80°C for 30 minutes under normal pressure, then cured at 130°C and 3 atmospheres for 60 hours. The bagging film was removed, yielding a fiber-reinforced composite material with release films laminated on both sides of a 20 cm x 27.5 cm size. The peelability of the release film from the resulting fiber-reinforced composite material was measured and found to be 0.29 g.

[0070] [Example 2] A film was produced in the same manner as in Example 1, except that the amount of plasticizer was changed to 9 parts by mass per 100 parts by mass of PVA, and a molding test was carried out. The results are shown in Table 1.

[0071] [Example 3] A film was produced in the same manner as in Example 1, except that the methyl (meth)acrylate / styrene copolymer was not applied, and a molding test was carried out. The results are shown in Table 1.

[0072] [Example 4] A film was produced in the same manner as in Example 1, except that the methyl (meth)acrylate / styrene copolymer was not applied and a powder mixture of water-repellent processed starch and talc in a ratio of 1:1 was used, and a molding test was carried out. The results are shown in Table 1.

[0073] [Example 5] A film was produced in the same manner as in Example 1, except that the thickness of the polyvinyl alcohol film was 25 μm, and a molding test was performed. Because the film was too thin and soft, it was difficult to handle when placing the release film and when covering the laminate with the bagging film. The results are shown in Table 1.

[0074] [Example 6] A film was prepared in the same manner as in Example 1, except that no plasticizer was added, no methyl (meth)acrylate / styrene copolymer was applied, and a 1:1 mass mixture of water-repellent processed starch and talc was used as the powder, and a molding test was conducted. The film was too hard, making it difficult to handle when placing a release film. In addition, the film was too hard to cover the laminate without leaking, making it difficult to handle as a bagging film. Therefore, a molding test was conducted using the film of Example 1 as a bagging film instead. The results are shown in Table 1.

[0075] Comparative Example 1 A film was produced in the same manner as in Example 1, except that no powder was applied and no methyl (meth)acrylate / styrene copolymer was applied, and a molding test was carried out. When measuring the peel strength, the film did not peel properly and broke, making it impossible to measure the peel strength. The results are shown in Table 1.

[0076] [Reference Example 1] A molding test was carried out in the same manner as in Example 1, except that a polyvinyl fluoride (PVF) film having a thickness of 25 μm was used as the release film instead of the film of the present invention. The evaluation results of the release film are shown in Table 1.

[0077] [Reference Example 2] A molding test was carried out in the same manner as in Example 1, except that a nylon (Ny) film having a thickness of 50 μm was used as the bagging film instead of the film of the present invention. The evaluation results of the bagging film are shown in Table 1.

[0078]

[0079] The films of the examples have excellent peelability from molded articles because the surface of the polyvinyl alcohol film is coated with a powder at a specific occupied area ratio. Furthermore, the films of Examples 1 to 4, in which the polyvinyl alcohol film contains a plasticizer and has a thickness within a specific range, also have excellent workability during molding.

Claims

1. A polyvinyl alcohol film having powder attached to at least one surface thereof, the ratio of the area occupied by said powder to the area of ​​the film being 0.05 to 5%.

2. The film according to claim 1, wherein the polyvinyl alcohol film contains polyvinyl alcohol and a plasticizer, and the content of the plasticizer is 5 to 30 parts by weight per 100 parts by weight of the polyvinyl alcohol.

3. The film according to claim 1 or 2, having a thickness of 15 to 100 μm.

4. The film according to claim 1 or 2, wherein the powder has an average particle size of 5 to 30 μm.

5. The film according to claim 1 or 2, wherein the powder is a modified starch that has been treated to be water repellent.

6. The film according to claim 1 or 2, wherein the polyvinyl alcohol has a degree of polymerization of 500 to 6,000 and a degree of saponification of 80 to 99.9 mol %.

7. The film according to claim 2, wherein the plasticizer is any one selected from the group consisting of ethylene glycol, glycerin, diglycerin, propylene glycol, and diethylene glycol.

8. The film according to claim 1 or 2, wherein at least one surface of the polyvinyl alcohol film is coated with any one selected from the group consisting of a polymer containing at least one monomer unit selected from the group consisting of styrene and (meth)acrylic acid esters, an aqueous polyvinyl alcohol solution, and a polyvinyl alcohol emulsion, and the powder adheres to the coated surface.

9. A laminate comprising a release film made of the film according to claim 1 or 2 and an uncured fiber composite material.

10. A method for producing a fiber-reinforced composite material, comprising curing the uncured fiber composite material in the laminate described in claim 9 under reduced pressure, and then peeling the release film from the cured fiber composite material.

11. The method for producing a fiber-reinforced composite material according to claim 10, further comprising covering the laminate with a bagging film made of the film, and then drawing a vacuum to reduce the internal pressure, and curing the uncured fiber composite material under reduced pressure.

12. The film according to claim 1 or 2, which is a release film for the production of fiber-reinforced composite materials.

13. The film according to claim 1 or 2, which is a bagging film for the production of fiber-reinforced composite materials.

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