JP7611086B2 - Thermoplastic resin film, prepreg, and prepreg laminate - Google Patents

Description

The present invention relates to a thermoplastic resin film for impregnating carbon fiber to form a prepreg, which is an intermediate material for thermoplastic carbon fiber reinforced resin, a prepreg, and a prepreg laminate using the same.

Carbon fiber reinforced resin is used in a wide range of fields, including automobiles, aircraft, and civil engineering temporary construction materials, due to its characteristics of light weight, excellent strength, and high durability. Carbon fiber reinforced resin is classified into thermosetting carbon fiber reinforced resin and thermoplastic carbon fiber reinforced resin, depending on the properties of the resin used for impregnation. Of these, thermoplastic carbon fiber reinforced resin is used particularly as a material for automobiles, due to its advantages of short molding time and recyclability by heating.

Thermoplastic carbon fiber reinforced resin is manufactured using prepreg, an intermediate material. Prepreg is obtained by opening carbon fiber tows (bundles) and impregnating the carbon fibers with thermoplastic resin, and among prepregs, those in which carbon fiber tows (bundles) are opened and aligned in one direction are called unidirectional (UD) prepregs.

For example, if the final molded product is a battery case for an electric vehicle, it will require flame retardancy when the amount of heat generated by the battery increases, solvent resistance (chemical resistance) to solvents such as ethylene carbonate, which is used as a solvent for the electrolyte in lithium-ion secondary batteries, and acetone, which is used in the painting and bonding process for the battery case, and thin-wall film formability to improve adhesion between the carbon fiber and the thermoplastic resin.

Therefore, for example, a polycarbonate resin composition has been proposed that contains 60-88% by mass of polycarbonate resin and 12-40% by mass of a phosphorus-based flame retardant, and has a melt mass flow rate (MFR) of 9-120 x 0.01 cc/sec at 240°C. It is described that such a composition can provide a polycarbonate resin composition with excellent impregnation properties for continuous fiber reinforcement and flame retardancy, and a polycarbonate prepreg with excellent flame retardancy (see, for example, Patent Document 1).

Furthermore, a flame-retardant resin composition has been proposed, which is formed by melt-kneading a thermoplastic resin, which is a diluent resin, with a flame-retardant master batch obtained by melt-kneading a thermoplastic resin such as polybutylene terephthalate resin and an organic metal phosphate (b). It is described that such a composition can provide a thermoplastic resin molded product with excellent flame retardancy (see, for example, Patent Document 2).

Also disclosed is a polycarbonate resin composition in which a polycarbonate resin is alloyed with a polyester (another crystalline resin) such as polybutylene terephthalate resin. It is described that this composition can improve the solvent resistance of the polycarbonate resin (see, for example, Patent Document 3).

International Publication No. WO 2016/186100 JP 2015-63646 A JP 2019-151711 A

However, although the polycarbonate resin composition described in Patent Document 1 has flame retardancy, because the polycarbonate resin is an amorphous resin, it has poor solvent resistance, and when used in the battery cases of electric vehicles, there is a problem that it cannot prevent corrosion caused by leakage of the electrolyte from lithium-ion secondary batteries.

In addition, the flame-retardant resin composition described in Patent Document 2 contains a flame-retardant filler (organic metal phosphate), which makes it difficult to mold thin-walled components having thicknesses of 20 μm.

In addition, in the polycarbonate resin composition described in Patent Document 3, even when the polycarbonate resin is alloyed with the polybutylene terephthalate resin, the damage spreads from the portion of the polycarbonate resin that was damaged by the solvent contained in the prepreg laminate to the entire prepreg laminate, making it difficult to improve the solvent resistance by alloying.

Therefore, the present invention has been made in consideration of the above problems, and aims to provide a thermoplastic resin film, a prepreg, and a prepreg laminate using the same that are excellent in flame retardancy and thin-film forming properties as well as solvent resistance to solvents such as ethylene carbonate and acetone.

In order to achieve the above object, the thermoplastic resin film of the present invention is a thermoplastic resin film for impregnating carbon fibers to form a prepreg, which contains at least a polyester resin and a phosphorus-based flame retardant, the phosphorus content of the phosphorus-based flame retardant relative to the entire thermoplastic resin film is 1.6% by mass or more and less than 2% by mass, and the weight loss rate after immersion in ethylene carbonate and acetone at 23°C for 30 minutes measured in accordance with JIS K 7114 is less than 5%.

The present invention can provide a thermoplastic resin film, a prepreg, and a prepreg laminate using the same that are excellent in flame retardancy and thin-film forming properties as well as solvent resistance to solvents such as ethylene carbonate and acetone.

FIG. 2 is a schematic diagram for explaining an example of a method for producing a prepreg according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining another example of the method for producing a prepreg according to an embodiment of the present invention.

The present invention will be described in detail below. Note that the present invention is not limited to the following embodiments, and can be modified as appropriate within the scope of the present invention.

<Thermoplastic resin film>
The thermoplastic resin film of the present invention is a resin film for forming a prepreg by placing it on one side and the other side of a sheet-like continuous carbon fiber, or by placing sheet-like continuous carbon fiber on one side and the other side of the thermoplastic resin film of the present invention and impregnating the carbon fiber by a film stacking method described later.

This thermoplastic resin film is a film that contains a thermoplastic resin, such as a polyester resin such as polybutylene terephthalate resin, polycarbonate resin, or phenoxy resin, a polyphenylene sulfide resin, a polysulfone resin, a polyether ether ketone resin, a polyimide resin, a polystyrene resin, or an ABS resin, and a flame retardant.

As the thermoplastic resin, it is preferable to use polyester resins such as polycarbonate resin, phenoxy resin, and polybutylene terephthalate resin, which have superior impact resistance, heat resistance, and recyclability compared to thermosetting resins and are low cost.

In addition, polybutylene terephthalate resin obtained by dehydration condensation of terephthalic acid (or dimethyl terephthalate) and polyhydric alcohol (1,4-butanediol) is particularly preferred because it has high solvent resistance (chemical resistance) and excellent impregnation properties into carbon fibers.

When polybutylene terephthalate resin is used as the polyester resin, the polyester resin preferably contains more than 90% by mass of polybutylene terephthalate resin, more preferably 95% by mass or more, and even more preferably 100% by mass. When the polyester resin contains more than 90% by mass of polybutylene terephthalate resin, a thermoplastic resin film and prepreg laminate having solvent resistance can be obtained.

The content of polybutylene terephthalate resin in the entire thermoplastic resin film is preferably 83% by mass or more and less than 87% by mass, and more preferably 85% by mass or more and 86% by mass or less. If the content of polybutylene terephthalate resin is less than 83% by mass, the proportion of flame retardant in the thermoplastic resin film is high, which may cause processing defects due to holes and blocking. If the content of polybutylene terephthalate resin is 87% by mass or more, the proportion of flame retardant in the thermoplastic resin film is low, which may prevent the flame retardant effect from being fully exerted.

The polybutylene terephthalate resin preferably has a melt mass flow rate (MFR) of 1 to 10 g/10 min, and more preferably 2 to 8 g/10 min. If the melt mass flow rate (MFR) is less than 1 g/10 min, mixing with the flame retardant may be insufficient, and if the melt mass flow rate (MFR) is greater than 10 g/10 min, the polybutylene terephthalate resin has a very low melt tension, so the resin may draw down immediately after extrusion from the die, making thin-wall molding difficult.

The melt mass flow rate is obtained by measuring in accordance with the provisions of JIS K 7210:1999.

The thickness of the thermoplastic resin film is preferably 10 to 100 μm, more preferably 12 to 60 μm, and even more preferably 15 to 30 μm. If the thickness of the thermoplastic resin film is 10 μm or more, impregnation failure (occurrence of voids between carbon fibers) caused by insufficient resin components relative to the carbon fibers in the prepreg can be prevented. If the thickness of the thermoplastic resin film is 100 μm or less, a decrease in the volume content (Vf value) of the carbon fibers caused by excess resin components in the prepreg can be prevented, and therefore a decrease in strength when the prepreg is laminated can be prevented. In particular, if the thickness of the thermoplastic resin film is 15 to 30 μ (for example, 20 μm), a thin film made of a resin containing hydrogen bonding components can be formed (i.e., a thin film can be formed), and therefore the hydrogen bonding components at the interface of the film can be increased, and as a result, the adhesion can be improved.

The "volume content (Vf value)" referred to here is measured in accordance with the combustion method of JIS K 7075 (Test method for fiber content and void content of carbon fiber reinforced plastics).

As the flame retardant, a phosphorus-based flame retardant is used, which has excellent dispersibility in polybutylene terephthalate resin and allows thin-wall molding. More specifically, examples of such flame retardants include phosphate esters, ammonium polyphosphate salts, cyclic phosphazenes, and polyphosphonates. These flame retardants may be used alone or in combination of two or more.

Among these, it is preferable to use cyclic phosphazenes, which have excellent heat resistance, do not decompose at the processing temperature of polybutylene terephthalate resin, and have high flame retardancy, and polyphosphonates, which are copolymers of phosphorus compounds, have little blocking, are easy to knead, and have high flame retardancy.

When cyclic phosphazene is used as a phosphorus-based flame retardant, the content of the cyclic phosphazene relative to the entire thermoplastic resin film is preferably 3% by mass or more and less than 7% by mass, and more preferably 4% by mass or more and 5% by mass or less. If the content of the cyclic phosphazene is less than 3% by mass, the content of phosphorus element is low, and therefore the flame retardancy may not be fully exhibited. Also, if the content of the cyclic phosphazene is 7% by mass or more, the proportion of the phosphorus-based flame retardant is high, and therefore processing defects due to holes and blocking may occur.

When polyphosphonate is used as a phosphorus-based flame retardant, the content of polyphosphonate in the entire thermoplastic resin film is preferably 7% by mass or more and less than 12% by mass, and more preferably 10% by mass or more and 11% by mass or less. When the polyphosphonate content is less than 7% by mass, the phosphorus content is low, and the flame retardancy may not be fully exhibited. When the polyphosphonate content is 12% by mass or more, the proportion of phosphorus-based flame retardant is high, and processing defects due to holes and blocking may occur.

In addition, it is preferable to use cyclic phosphazenes and polyphosphonates in combination because they have a very high phosphorus content and good compatibility.

It is also preferable that the phosphorus content of the phosphorus-based flame retardant is 1.6% by mass or more and less than 2% by weight relative to the entire thermoplastic resin film. If the phosphorus content of the phosphorus-based flame retardant is less than 1.6% by mass, the phosphorus content is low, and the flame retardancy may not be fully exhibited. If the phosphorus content of the phosphorus-based flame retardant is 2% by mass or more, the proportion of the phosphorus-based flame retardant relative to the entire thermoplastic resin film is high, and processing defects due to holes and blocking may occur.

When two types of phosphorus-based flame retardants (e.g., the above-mentioned cyclic phosphazene and polyphosphonate) are used in combination, it is preferable that the total content of phosphorus elements in the two types of phosphorus-based flame retardants is 1.6 mass% or more and less than 2 weight%.

In addition, from the viewpoint of ensuring the flame retardancy of the prepreg laminate, it is preferable that the thermoplastic resin film of the present invention has a flame retardancy of VTM-2 or higher, preferably VTM-1 or higher, and more preferably VTM-0 or higher, in a VTM test conforming to the UL94 standard (UL94/VTM combustion test).

In addition, the thermoplastic resin film of the present invention preferably has a surface free energy of 30 mJ/m2 or ^more and 60 mJ/m2 or ^less . If the surface free energy is less than 30 mJ/ ^m2 , the adhesion to the carbon fiber may be significantly reduced, and a high-strength prepreg may not be produced. If the surface free energy is greater than 60 mJ/ ^m2 , blocking may occur to the extent that the thermoplastic resin film cannot be wound up, making it difficult to form the film.

In addition, in the thermoplastic resin film of the present invention, it is preferable that the ratio of hydrogen bonding components to the surface free energy is 6% or more and 35% or less. If the ratio of hydrogen bonding components is 6% or more and 35% or less, the compatibility between the thermoplastic resin film and the sizing agent components of the carbon fiber is improved, and the adhesion between the thermoplastic resin film and the carbon fiber in the prepreg is improved, making it possible to reduce the void ratio in the prepreg laminate. As a result, the stress concentration occurring around the voids can be alleviated, making it possible to improve the strength of the prepreg laminate over the long term.

The term "void ratio" used here refers to the volume ratio of the portion of the prepreg laminate that is not impregnated with thermoplastic resin, measured in accordance with the combustion method of JIS K 7075 (Test method for fiber content and void ratio of carbon fiber reinforced plastics).

In addition, because the adhesion between the thermoplastic resin film and the carbon fiber is improved, fiber feathering on the surface of the prepreg and prepreg laminate is suppressed, making it possible to provide a prepreg and prepreg laminate with excellent appearance and feel.

In order to further reduce the void ratio in the prepreg laminate, the ratio of hydrogen bonding components to the surface free energy is preferably 10% or more and 30% or less.

The amount of hydrogen bonding components is preferably 3.0 mJ/m ² or more and 20 mJ/m ² or less, and more preferably 4.0 mJ/m ² or more and 20 mJ/m ² or less.

In this way, by using the thermoplastic resin film of the present invention, a prepreg laminate with low porosity and excellent strength over a long period of time can be obtained.

The "surface free energy" can be determined by measuring the contact angle (measurement temperature: 25°C) of various droplets and calculating it based on the measured value using the OWRK theory (Owens-Wendt-Rable-Kaelble). More specifically, a droplet of diiodomethane is used as the "dispersion component" and a droplet of pure water is used as the "hydrogen bond component (= polar component)". Using a fully automatic contact angle meter, the contact angles of diiodomethane and pure water are measured in accordance with JIS R 3257 under conditions of a temperature of 25°C and a relative humidity of 50%, and the surface free energy of each of the dispersion component and the hydrogen bond component (polar component) is calculated from the sum of the calculated surface free energies, and the surface free energy [mJ/m ² ] on the surface of the thermoplastic resin film is calculated using the calculation formula of the OWRK theory, and the ratio [%] of the hydrogen bond component to the surface free energy is calculated. Each surface energy can be determined, for example, using surface free energy analysis software.

In addition, in the thermoplastic resin film of the present invention, the heat shrinkage rate in the direction (hereinafter referred to as "TD") perpendicular to the mechanical axis (longitudinal) direction (hereinafter referred to as "MD") of the film when heated for 2 minutes at a heating temperature near the melting point of the thermoplastic resin is preferably less than 7.0%, and more preferably less than 3.0%. If the heat shrinkage rate in TD is less than 7.0%, when producing a prepreg by the film stacking method described below, heat shrinkage (neck-in) does not occur in the TD of the thermoplastic resin film, and therefore it is possible to prevent the carbon fibers from moving to the center of the prepreg. Therefore, a prepreg with uniform properties in the plane can be realized.

The "heat shrinkage rate" can be calculated by fixing both ends of a 50 mm (TD) x 100 mm (MD) thermoplastic resin film in the MD along the TD with aluminum tape to a 0.3 mm thick SUS plate, placing it in an oven, opening the damper to 50% and maintaining the temperature at or near the melting point (170°C) for 2 minutes, then measuring the length (mm) of the most shrunk part in the TD of the resin film, and calculating the heat shrinkage rate in TD from the ratio to 50 mm.

The thermoplastic resin film of the present invention has a weight loss rate of less than 5% after immersion in ethylene carbonate and acetone at 23°C for 30 minutes, measured in accordance with JIS K 7114 "Plastics - Test method for determining the effect of immersion in liquid chemicals". If the weight loss rate is less than 5%, for example, when the prepreg laminate is used in a battery case for an electric vehicle, it can improve solvent resistance (chemical resistance) against solvents such as ethylene carbonate, which is used as a solvent for electrolytes for lithium-ion secondary batteries, and acetone, which is used in the painting and bonding process of the battery case.

The "weight loss rate" referred to here is calculated using the following formula (1):

[Equation 1]
Weight reduction rate [%] = [(weight of film before immersion - weight of film after immersion) / weight of film before immersion] x 100 (1)

In addition, the thermoplastic resin film of the present invention may contain, in addition to the above-mentioned thermoplastic resin, an organic metal salt, an antistatic agent, and a compatibilizer, as long as the properties of the thermoplastic resin film are not impaired.

The organic metal salt is used to improve the flame retardancy of the thermoplastic resin film, and examples of such salts include alkali metal salts of organic sulfonic acids. These organic metal salts may be used alone or in combination of two or more.

Examples of alkali metal organic sulfonates include potassium organic sulfonates, lithium organic sulfonates, and sodium organic sulfonates.

When using an organometallic salt, from the viewpoint of impregnation into the carbon fiber, the content of the organometallic salt relative to the entire thermoplastic resin film is preferably 0.01 to 1.0 mass%, more preferably 0.05 to 0.5 mass%, and even more preferably 0.1 to 0.3 mass%.

As the antistatic agent, an organic antistatic agent having a hydrophilic group can be used, and examples of such antistatic agents include low molecular weight conductive monomers (surfactants) and high molecular weight conductive polymers. Examples of low molecular weight conductive monomers include nonionic, anionic, cationic, and amphoteric types, and examples of high molecular weight conductive polymers include polyethers and ion conductive polymers.

From the viewpoint of increasing the amount of hydrogen bonding components in the thermoplastic resin film, it is preferable to use a low molecular weight conductive monomer. Examples of low molecular weight conductive monomers include polyhydric alcohol fatty acid esters such as sorbitan fatty acid esters, sucrose fatty acid esters, and glycerin fatty acid esters, quaternary ammonium salts such as alkyltrimethylammonium chloride, and sulfonic acid compounds such as sodium alkanesulfonate. These antistatic agents may be used alone or in combination of two or more.

The use of an organic antistatic agent with a hydrophilic group increases the amount of hydrogen bonding components in the thermoplastic resin film, improving compatibility with the sizing agent components of the carbon fiber, which has a very high polarity, and as a result, improving the adhesion between the thermoplastic resin film and the carbon fiber in the prepreg.

When an antistatic agent is used, the content of the antistatic agent in the entire thermoplastic resin film is preferably 0.1 to 20% by mass, and more preferably 0.5 to 10% by mass, based on 100% by mass of the thermoplastic resin film, from the viewpoint of improving the hydrophilic effect of the film surface and the film moldability.

As the compatibilizer, random polymer type and graft block polymer type can be used. More specifically, examples include block copolymers of polystyrene and polycaprolactone, styrene/maleic anhydride/unsaturated derivative copolymers modified with caprolactone, ε-polycaprolactone grafted with maleic anhydride, and block copolymers of glycidyl methacrylate-styrene-acrylonitrile copolymer and polycarbonate.

In the present invention, the use of such a compatibilizer can improve the moldability of the thermoplastic resin film.

When a compatibilizer is used, from the viewpoint of forming a thermoplastic resin film with a desired thickness (e.g., 20 μm), the content of the compatibilizer relative to the entire thermoplastic resin film is preferably 0.05 to 5 mass %, and more preferably 0.1 to 1 mass %, of 100 mass % of the thermoplastic resin film.

The thermoplastic resin film of the present invention is produced, for example, by an inflation method, a T-die extrusion method, a calendar method, etc.

When the thermoplastic resin film of the present invention is produced by a T-die extrusion method, the cylinder temperature and die temperature of a single-screw or twin-screw extruder are set to a temperature 5°C to 150°C higher than the melting point and glass transition point of the thermoplastic resin composition, the thermoplastic resin is fed into the extruder, melt-kneaded at a screw rotation speed of 5 to 50 rpm, and extruded from a T-die to obtain a thermoplastic resin film having a thickness of 10 to 100 μm.

<Prepreg>
The prepreg of the present invention can be obtained by a film stacking method in which continuous carbon fibers are supplied to a fiber opening and impregnation machine and sandwiched between the thermoplastic resin films of the present invention, or the thermoplastic resin film of the present invention is sandwiched between continuous carbon fibers, the thermoplastic resin film is laminated onto the carbon fibers, and the carbon fibers are impregnated with the thermoplastic resin by heating and pressurizing treatment.

Figure 1 is a schematic diagram for explaining an example of a method for manufacturing a prepreg according to an embodiment of the present invention.

When manufacturing the prepreg, first, the sheet-like continuous carbon fiber CS is conveyed toward the plate heater 12 by the pair of supply rollers 11 of the opening and impregnation machine 10, and thermoplastic resin films RF1, RF2 are laminated on one side and the other side of the continuous carbon fiber CS to produce a laminate consisting of the sheet-like continuous carbon fiber CS and the thermoplastic resin films RF1, RF2. Next, this laminate is conveyed in the direction of the arrow in the figure, and passed through the plate heater 12 while being sandwiched between the first roller pair 13 and the second roller pair 14, to perform a heating and pressurizing treatment. This causes the thermoplastic resin films RF1, RF2 of the laminate to soften and be impregnated into the sheet-like continuous carbon fiber CS, and the prepreg P of the present invention can be obtained.

Also, FIG. 2 is a schematic diagram for explaining another example of a method for producing a prepreg according to an embodiment of the present invention. In this case, first, the thermoplastic resin film RF is conveyed toward the plate heater 12 by the pair of supply rollers 11 of the opening and impregnation machine 10, and sheet-like continuous carbon fibers CS1, CS2 are laminated on one side and the other side of the thermoplastic resin film RF to produce a laminate consisting of the thermoplastic resin film RF and the sheet-like continuous carbon fibers CS1, CS2. Next, this laminate is conveyed in the direction of the arrow in the figure, and passed through the plate heater 12 while being sandwiched between the first roller pair 13 and the second roller pair 14, to perform a heating and pressurizing treatment. Then, the thermoplastic resin film RF of the laminate is softened and impregnated into the sheet-like continuous carbon fibers CS, and the prepreg P of the present invention can be obtained.

By using a thermoplastic resin film having the above-mentioned predetermined thickness to manufacture prepregs using a film stacking method, it becomes possible to uniformly impregnate the carbon fibers with the resin components, thereby improving the yield when manufacturing the prepregs and, as a result, making it possible to reduce the manufacturing costs of the prepregs.

When impregnating carbon fibers with a thermoplastic resin film using the film stacking method, the film is pre-impregnated at a temperature near the melting point of the thermoplastic resin, and then the main impregnation is performed. At temperatures above the melting point of the thermoplastic resin, the thermoplastic resin film breaks, and at temperatures below that temperature, the pre-impregnation is insufficient. Also, if thermal shrinkage (neck-in) occurs in the TD of the thermoplastic resin film near the melting point of the thermoplastic resin, the carbon fiber may move toward the center of the prepreg, resulting in in-plane variations in physical properties. For this reason, dimensional stability of the thermoplastic resin film is required. The dimensional stability of the thermoplastic resin film is evaluated, for example, by heating for 2 minutes in an oven with the inside temperature set to a heating temperature near the melting point of the thermoplastic resin, as described above, and measuring the heat shrinkage rate.

The thickness of the prepreg is preferably 50 to 300 μm, more preferably 60 to 150 μm. If the thickness of the prepreg is less than 50 μm, the number of prepregs required to produce a prepreg laminate increases, which may result in a longer processing time and a decrease in productivity. In addition, since the thickness of the prepreg is small, the thickness of the thermoplastic resin film constituting the prepreg must also be small (for example, less than 10 μm), and impregnation failure may occur in the prepreg due to a shortage of resin components relative to the carbon fibers. On the other hand, if the thickness of the prepreg is greater than 300 μm, the thickness of the thermoplastic resin film constituting the prepreg must also be large (for example, greater than 100 μm), and therefore, in the prepreg, a decrease in the volume fraction (Vf value) of the carbon fibers due to excess resin components occurs, which may result in a decrease in the strength of the prepreg laminate.

<Prepreg Laminate>
The prepreg laminate of the present invention is produced by laminating a plurality of prepregs in a unidirectional (UD) direction in which the fibers are aligned in one direction, and then subjecting the laminate to a heat and pressure treatment. The number of prepregs to be laminated can be appropriately changed based on the thickness of the prepreg laminate. For example, when the thickness of the prepreg laminate is 1.5 to 2.5 mm, 40 to 60 prepregs can be laminated.

In addition, from the viewpoint of improving strength, the prepreg laminate of the present invention preferably has a void ratio of less than 2.0%, and more preferably 1.0% or less.

Similarly, from the viewpoint of improving the strength of the prepreg laminate of the present invention, it is preferable that the volume content (Vf value) of carbon fibers in the prepreg laminate is 40 to 60%.

The "void ratio and fiber volume content (Vf)" are calculated in accordance with the combustion method of JIS K 7075 (Test method for fiber content and void content of carbon fiber reinforced plastics).

As described above, the prepreg laminate of the present invention has excellent strength, and from the viewpoint of use as an automobile part, the bending strength is preferably 1000 MPa or more, and more preferably 1100 MPa or more.

For example, aluminum alloys have a high flexural modulus of 70 GPa or more, but from the viewpoint of enabling replacement with metals such as aluminum, it is preferable that the flexural modulus of the prepreg laminate of the present invention is 90 GPa or more.

The "flexural strength and flexural modulus" referred to here are values measured in accordance with the provisions of JIS K 7074, "Flexural test method for carbon fiber reinforced plastics."

In addition, from the viewpoint of ensuring flame retardancy that allows use, for example, as a battery case for an electric vehicle, the prepreg laminate of the present invention preferably has a flame retardancy of V-1 or higher, preferably V-0 or higher, in a V test (UL94/V combustion test) conforming to the UL94 standard.

In addition, in the prepreg laminate of the present invention, the retention of bending strength against ethylene carbonate, measured in accordance with JIS K 7070:1999 "Testing method for chemical resistance of fiber reinforced plastics", is preferably 80% or more, and more preferably 90% or more. Similarly, the retention of bending strength against acetone, measured in accordance with JIS K 7070:1999 "Testing method for chemical resistance of fiber reinforced plastics", is preferably 70% or more, and more preferably 80% or more. With this configuration, it is possible to improve the solvent resistance (chemical resistance) against ethylene carbonate and acetone.

The "bending strength retention rate" referred to here is calculated using the following formula (2).

[Equation 2]
Flexural strength retention rate [%] = (flexural strength after immersion in ethylene carbonate (or acetone) / flexural strength before immersion in ethylene carbonate (or acetone)) × 100 (2)

The "flexural strength" referred to here is the value measured in accordance with the provisions of JIS K 7074, "Flexural test method for carbon fiber reinforced plastics."

In addition, in the prepreg laminate of the present invention, the strength retention rate after water absorption measured in accordance with JIS K 7070:1999 "Testing method for chemical resistance of fiber-reinforced plastics" is preferably 80% or more, and more preferably 90% or more. With this configuration, high strength can be maintained even after water absorption.

The "strength retention rate after water absorption" referred to here is calculated using the following formula (3).

[Equation 3]
Strength retention after water absorption [%] = (strength after immersion in water / strength before immersion in water) × 100 (3)

The "strength" referred to here refers to the value measured in accordance with the provisions of JIS K 7074, "Bending test method for carbon fiber reinforced plastics."

As explained above, the present invention can provide a prepreg laminate that has excellent solvent resistance and flame retardancy, and can suppress deterioration of strength even in a humid usage environment. In addition, it is lighter and stronger than aluminum alloys, so it can be used as a substitute for metals.

The present invention will be described below based on examples. Note that the present invention is not limited to these examples, and these examples can be modified or changed based on the spirit of the present invention, and are not excluded from the scope of the present invention.

The materials used in the preparation of the thermoplastic resin film are shown below.
(1) Polybutylene terephthalate resin 1 (manufactured by Polyplastics Co., Ltd., product name: 700FP, MFR: 7 g/10 min)
(2) Polybutylene terephthalate resin 2 (manufactured by Sobic, product name: Valox 357, MFR: 8 g/10 min, containing 10 to 50% by mass of polycarbonate resin, flame retardant and chemical resistant grade)
(3) Polybutylene terephthalate resin 3 (manufactured by BASF, product name: B4500, MFR: 21 g/10 min)
(4) Polybutylene terephthalate resin 4 (manufactured by Polyplastics Co., Ltd., product name: 201NF, MFR: 9.4 g/10 min, containing flame retardant filler (diameter: 5 to 20 μm))
(5) Polybutylene terephthalate resin 5 (manufactured by Mitsubishi Engineering Plastics Corporation, product name: SEF-500, MFR: 5 g/10 min, containing flame retardant filler (diameter: 2 to 20 μm))
(6) Polybutylene terephthalate resin 6 (manufactured by Mitsubishi Engineering Plastics Corporation, product name: 5710N2, MFR: 7 g/10 min, containing flame retardant filler (diameter: 2 to 20 μm))
(7) Polycarbonate resin 1 (manufactured by Sumika Polycarbonate Co., Ltd., product name: CR3440, MFR: 7 g/10 min, containing 10 to 50% by mass of polybutylene terephthalate resin, chemical resistant grade)
(8) Polycarbonate resin 2 (manufactured by Teijin Limited, product name: MN4400Z, MFR: 6 g/10 min, flame retardant grade)
(9) Polyamide resin film (manufactured by Mitsubishi Chemical Corporation, product name: Diamiron C-Z)
(10) Phosphorus-based flame retardant 1: cyclic phosphazene (manufactured by Fushimi Pharmaceutical Co., Ltd., trade name: Rabitol FP110, phosphorus element content: 3.4% by mass)
(11) Phosphorus-based flame retardant 2: polyphosphonate (manufactured by FRX Polymers, product name: Nofia HM1100, phosphorus element content: 10.6% by mass)
(12) Organic metal salt (organic potassium sulfonate, manufactured by Mitsubishi Materials Corporation, trade name: potassium nonafluorobutanesulfonate)

Example 1
<Preparation of Thermoplastic Resin Film>
First, polybutylene terephthalate resin, phosphorus-based flame retardant, and organic metal salt were blended in the ratios (all by mass%) shown in Table 1, mixed in a blender, and then fed into a twin-screw extruder (manufactured by Japan Steel Works, Ltd.) to extrude the molten resin into strands, which were then cooled and pelletized using a pelletizer. Next, the cylinder temperature and die temperature of the single-screw extruder were set to 230°C, and the pellets were extruded from a T-die to obtain a thermoplastic resin film having a thickness of 20 μm. The total content of phosphorus element in the two types of phosphorus-based flame retardants (phosphorus-based flame retardant 1 and phosphorus-based flame retardant 2) was 1.6% by mass.

<Evaluation of Solvent Resistance of Thermoplastic Resin Film>
Next, in accordance with JIS K 7114 (Plastics - Test method for determining the effect of immersion in liquid chemicals), the weight loss rate of the obtained thermoplastic resin film was measured using the above-mentioned formula (1), and the solvent resistance (chemical resistance) to solvents (ethylene carbonate and acetone) was evaluated.

More specifically, a 60 mm square sample of the obtained thermoplastic resin film was taken and used as a sample. It was immersed in 20 mL of solvent (ethylene carbonate and acetone) for 30 minutes at room temperature (23°C) and the weight change rate was measured. The results are shown in Table 1.

<Flame retardancy evaluation (UL94/VTM combustion test)>
Next, the flame retardancy of the obtained thermoplastic resin film was evaluated according to the VTM test (UL94/VTM combustion test) conforming to the UL94 standard. For the flame retardancy in TD, a film of 50 mm (MD) x 200 mm (TD) was used as a sample, and for the flame retardancy in MD, a film of 50 mm (TD) x 200 mm (MD) was used as a sample. The results achieved in both MD and TD are shown in Table 1.

<Preparation of prepreg>
Next, the obtained resin film and carbon fiber tow (manufactured by Toray Industries, Inc., product name: T700SC-1200) were supplied to a fiber spreading/impregnation machine 10 shown in FIG. 2, the thermoplastic resin film was sandwiched between continuous carbon fibers, and a heating/pressurization treatment was carried out at a temperature of 230° C. and a rate of 10 m/min, thereby obtaining a prepreg having a width of 130 mm and a thickness of 65 μm.

<Preparation of prepreg laminate>
Next, 48 pieces of the prepreg were cut to the size of a press die (200 mm square), and stacked so that the carbon fibers were aligned in the same direction. This laminate was then placed in a press die and subjected to a heat press treatment at a temperature of 245° C. and a pressure of 5 MPa for 15 minutes to produce a prepreg laminate with a thickness of 2.0 mm.

<Evaluation of Solvent Resistance of Prepreg Laminate>
Next, in accordance with JIS K 7070:1999 (Testing methods for chemical resistance of fiber reinforced plastics), the bending strength retention of the obtained prepreg laminate was calculated using the above-mentioned formula (2), and the solvent resistance (chemical resistance) to solvents (ethylene carbonate and acetone) was evaluated.

More specifically, the prepreg laminate was cut to a size of 120 mm (in the fiber direction) x 60 mm (perpendicular to the fiber direction), the end faces of the test pieces were protected with glass fiber tape, and the pieces were immersed in the solvent for 10 days. When the solvent was ethylene carbonate, the immersion was performed at 50°C, and when the solvent was acetone, the immersion was performed at 0°C. After immersion, the pieces were cut to a size of 100 mm (in the fiber direction) x 15 mm (perpendicular to the fiber direction) and used as samples to measure bending strength. The results are shown in Table 1.

<Flame retardancy evaluation (UL94/V combustion test)>
Next, the flame retardancy of the obtained prepreg laminate was evaluated according to the V test (UL94/V flame test) in accordance with the UL94 standard. The prepreg laminate was cut out to a size of 125 mm (in the fiber direction) x 13 mm (in the direction perpendicular to the fiber direction) and used as a sample. The results are shown in Table 1.

<Measurement of bending strength>
Next, in accordance with JIS K 7074 (bending test method for carbon fiber reinforced plastics), the bending strength of the obtained prepreg laminate was calculated by a four-point bending method.

More specifically, samples were cut from the obtained prepreg laminate to a width of 15 mm and a length of 100 mm, and a four-point bending test was performed using a tensile tester (Shimadzu Corporation, product name: Autograph 5000) equipped with a four-point bending jig under the following conditions: crosshead speed 5.0 mm/min, support span 81 mm, indenter span 27 mm, support diameter 4 mm, and indenter diameter 10 mm, to measure the bending strength [MPa]. The results are shown in Table 1.

<Evaluation of Water Absorption Resistance>
Next, in accordance with JIS K 7070:1999 (Testing method for chemical resistance of fiber reinforced plastics), the strength retention rate of the obtained prepreg laminate after water absorption was calculated using the above-mentioned formula (3) to evaluate the water absorption resistance.

More specifically, the prepreg laminate was taken out to a size of 120 mm (in the fiber direction) x 60 mm (perpendicular to the fiber direction), the end faces of the test pieces were protected with glass fiber tape, and the pieces were immersed in pure water at room temperature for 10 days. After immersion, the pieces were cut to a size of 100 mm (in the fiber direction) x 15 mm (perpendicular to the fiber direction) and used as samples to measure bending strength. The results are shown in Table 1.

(Examples 2 to 3, Comparative Examples 1 to 13)
Thermoplastic resin films, prepregs, and prepreg laminates having the thicknesses shown in Tables 1 and 2 were produced in the same manner as in Example 1 described above, except that the materials forming the thermoplastic resin film, the blending ratios of each material, and each processing temperature were changed to those shown in Tables 1 and 2.

Then, in the same manner as in Example 1 described above, a solvent resistance evaluation, a flame retardancy evaluation (UL94/VTM combustion test), a solvent resistance evaluation of the prepreg laminate, a flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and a water absorption resistance evaluation were performed. The results are shown in Tables 1 and 2.

In Comparative Example 1, since the content of phosphorus-based flame retardant 1 (cyclic phosphazene) was 7% by mass or more, processing defects due to holes occurred, and a thermoplastic resin film could not be produced. Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In addition, in Comparative Example 2, since the content of phosphorus-based flame retardant 2 (polyphosphonate) was 12 mass% or more, processing defects due to holes occurred, and a thermoplastic resin film could not be produced. Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In addition, in Comparative Example 4, the phosphorus element content of the phosphorus-based flame retardant relative to the entire thermoplastic resin film was 2.0 mass%, so processing defects due to blocking occurred and it was not possible to produce a thermoplastic resin film. Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In Comparative Example 7, since the MFR of the polybutylene terephthalate resin was greater than 10 g/10 min (21 g/10 min), the resin drew down immediately after extrusion from the die, and a thermoplastic resin film could not be produced. Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In addition, in Comparative Example 8, since the polybutylene terephthalate resin contains a flame retardant filler (diameter: 5 to 20 μm), it was not possible to produce a thermoplastic resin film having a thickness of 20 μm or less (i.e., thin-wall molding was not possible). Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In Comparative Example 9, since the polybutylene terephthalate resin contains a flame retardant filler (diameter: 2 to 20 μm), it was not possible to produce a thermoplastic resin film having a thickness of 20 μm or less (i.e., thin-wall molding was not possible). Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

In addition, in Comparative Example 10, since the polybutylene terephthalate resin contains a flame retardant filler (diameter: 2 to 20 μm), it was not possible to produce a thermoplastic resin film having a thickness of 20 μm or less (i.e., thin-wall molding was not possible). Therefore, solvent resistance evaluation, flame retardancy evaluation (UL94/VTM combustion test), solvent resistance evaluation of the prepreg laminate, flame retardancy evaluation (UL94/V combustion test), bending strength measurement, and water absorption resistance evaluation were not performed.

As shown in Table 1, in the thermoplastic resin films of Examples 1 to 3, the phosphorus content of the phosphorus-based flame retardant relative to the entire thermoplastic resin film is 1.6% by mass or more and less than 2% by mass, so it is possible to obtain a thermoplastic resin film with flame retardancy that meets VTM-2 in the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard without processing defects due to holes and blocking (i.e., excellent thin-film formability).

In addition, the thermoplastic resin films of Examples 1 to 3 showed a weight loss of less than 5% after immersion in ethylene carbonate and acetone at 23°C for 30 minutes, as measured in accordance with JIS K 7114, demonstrating that they have excellent solvent resistance (chemical resistance) to solvents (ethylene carbonate and acetone).

The prepreg laminates of Examples 1 to 3 also have flame retardancy that meets V-0 in the V test (UL94/V combustion test) based on the UL94 standard, and the retention of bending strength measured in accordance with JIS K 7070:1999 (chemical resistance test method for fiber-reinforced plastics) is 80% or more, indicating that they have excellent solvent resistance (chemical resistance) to solvents (ethylene carbonate and acetone).

In addition, the prepreg laminates of Examples 1 to 3 have excellent strength, with a bending strength of 1000 MPa or more, and the strength retention rate after water absorption is 90% or more, indicating that high strength can be maintained even after water absorption.

On the other hand, in the thermoplastic resin film of Comparative Example 3, the phosphorus element content of the phosphorus-based flame retardant relative to the entire thermoplastic resin film is less than 1.6% by mass (1.5% by mass), so in the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard, it was found to be VTM-not, and it is clear that a thermoplastic resin film having flame retardancy conforming to VTM-2 cannot be obtained.

In addition, the resin film of Comparative Example 5 does not contain a flame retardant, and the phosphorus content of the phosphorus-based flame retardant in the entire thermoplastic resin film is less than 1.6 mass%, so in the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard, it is VTM-not, and it is clear that a thermoplastic resin film with flame retardancy conforming to VTM-2 cannot be obtained.

In addition, in the resin film of Comparative Example 6, the compatibility between the resin and phosphorus-based flame retardants, etc. was poor, and the thickness of the film was uneven due to poor processing, so the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard resulted in a VTM-not, and it is clear that a thermoplastic resin film with flame retardancy conforming to VTM-2 cannot be obtained. In addition, the polybutylene terephthalate resin 2 used in Comparative Example 6 contains 10 to 50 mass% of polycarbonate resin, which is an amorphous resin, and the polybutylene terephthalate resin is 90 mass% or less of the polyester resin, so the weight loss rate measured in accordance with JIS K 7114 is 5% or more (16.1%), and it is clear that the solvent resistance (chemical resistance) to acetone is poor.

In addition, in the resin film of Comparative Example 11, since the polycarbonate resin is an amorphous resin, the weight loss rate measured in accordance with JIS K 7114 is 5% or more, and it is clear that the solvent resistance (chemical resistance) to solvents (ethylene carbonate and acetone) is poor.

In addition, the resin film of Comparative Example 12 does not contain a flame retardant, and the phosphorus content of the phosphorus-based flame retardant in the entire thermoplastic resin film is less than 1.6 mass%, so in the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard, it is VTM-not, and it is clear that a thermoplastic resin film with flame retardancy conforming to VTM-2 cannot be obtained.

In addition, the resin film of Comparative Example 13 is made of polyamide resin, does not contain a flame retardant, and the phosphorus content of the phosphorus-based flame retardant in the entire thermoplastic resin film is less than 1.6 mass%, so in the VTM test (UL94/VTM combustion test) in accordance with the UL94 standard, it is VTM-not, and it is clear that a thermoplastic resin film with flame retardancy that meets VTM-2 cannot be obtained.

As described above, the present invention is suitable for a thermoplastic resin film for impregnating carbon fibers to form a prepreg, a prepreg, and a prepreg laminate using the same.

REFERENCE SIGNS LIST 10 Fiber opening and impregnation machine 11 Supply roller pair 12 Plate heater 13 First roller pair 14 Second roller pair CS, CS1, CS2 Carbon fiber P Prepreg RF, RF1, RF2 Thermoplastic resin film

Claims (6)

A thermoplastic resin film for impregnating carbon fibers to form a prepreg,
Contains at least a polyester resin and a phosphorus-based flame retardant,
The phosphorus content of the phosphorus-based flame retardant relative to the entire thermoplastic resin film is 1.6% by mass or more and less than 2% by mass,
The weight loss rate after immersion in ethylene carbonate and acetone at 23° C. for 30 minutes, as measured in accordance with JIS K 7114, is less than 5%;
The phosphorus-based flame retardant comprises a cyclic phosphazene and a polyphosphonate;
The content of the cyclic phosphazene relative to the entirety of the thermoplastic resin film is 4% by mass or more and 5% by mass or less,
A thermoplastic resin film, characterized in that the content of the polyphosphonate relative to the entirety of the thermoplastic resin film is 10% by mass or more and 11% by mass or less . The thermoplastic resin film according to claim 1, characterized in that the polyester resin contains more than 90% by mass of polybutylene terephthalate resin. 3. The thermoplastic resin film according to claim 1, further comprising an organic metal salt. The thermoplastic resin film according to any one of claims 1 to 3 , characterized in that it has flame retardancy that satisfies VTM-2 or higher in a VTM test in accordance with the UL94 standard. A prepreg, comprising carbon fibers impregnated with the thermoplastic resin film according to any one of claims 1 to 4 . A prepreg laminate comprising the prepreg according to claim 5 laminated thereon.

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