Pre-decomposition product of fiber-reinforced resin composite material, and method for manufacturing the same

Abstract

The present invention provides a pre-decomposition product of a fiber-reinforced resin composite material with excellent recyclability, and a method for producing the same. [Solution] A fiber-reinforced resin composite material pre-decomposition product comprising a matrix resin and reinforcing fibers, wherein when a cross-section of the pre-decomposition product comprising a matrix resin and reinforcing fibers is observed with a scanning electron microscope, the number of microcracks in the matrix resin within any 3 μm square field of view is 8 or more, with a length of 100 nm or more and 4.24 μm or less and a maximum width of 20 nm or more and 4.10 μm or less, and the total length of the boundary line between the matrix resin portion and the microcrack portion within the same field of view is 1.9 μm or more. A method for producing a fiber-reinforced resin composite material pre-decomposition product, comprising a heat treatment in which a fiber-reinforced resin composite material, in which the matrix resin is reinforced with reinforcing fibers, is immersed in a solvent at a temperature of 100°C or more and 280°C or less.

Description

Technical Field

[0001] The present invention relates to a fiber reinforced resin composite material pre-decomposition product obtained by heat-treating a fiber reinforced resin composite material and a method for producing the same.

Background Art

[0002] Fiber reinforced resin composite materials composed of fibrous fillers such as glass fibers and carbon fibers and a matrix resin are used in a wide range of fields from general household electrical appliances to aircraft and automotive parts. Most of the end materials and product wastes generated in the manufacturing process of these fiber reinforced resin composite materials are not effectively utilized, and most of them are discarded by incineration or landfill. From the perspective of effective utilization of resources and prevention of environmental pollution, the development of fiber reinforced resin composite materials with excellent recyclability is required.

[0003] As a fiber reinforced resin composite material with excellent recyclability, for example, a technique for improving the decomposability by incorporating a structure that dissociates in response to external stimuli such as heat into the matrix resin skeleton is known, but there are problems such as a decrease in weather resistance and heat resistance.

[0004] As a method for improving the recyclability without changing the matrix resin skeleton in the fiber reinforced resin composite material, for example, Patent Document 1 discloses a method for subdividing a fiber reinforced resin composite material into which cracks are introduced by ozone irradiation.

[0005] Similarly, Patent Document 2 discloses a method for subdividing a plastic-fiber composite material in which the plastic is embrittled by cooling and separating and recycling the plastic and the fiber.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

[0007] However, it is generally known that ozone-irradiated fiber-reinforced resin composite materials, as described in Patent Document 1, also damage the fibers, leading to a decrease in the physical properties of the recycled material. Furthermore, while coarse cracks of several hundred micrometers are introduced in the ozone-irradiated composite material described in Patent Document 1, it is physically difficult to generate a large number of coarse cracks per unit area, resulting in little effect on improving recyclability. In addition, although Patent Document 2 describes improving recyclability by efficiently subdividing the composite material, it has hardly considered the form of fiber-reinforced resin composite material with excellent recyclability.

[0008] Therefore, the object of the present invention is to provide a pre-decomposed fiber-reinforced resin composite material with excellent recyclability, and a method for producing the same. [Means for solving the problem]

[0009] The present invention, which solves the above problems, consists of the following embodiments (1) to (5). (1) A fiber-reinforced resin composite material pre-decomposition product comprising a matrix resin and reinforcing fibers, observed with a scanning electron microscope, wherein in any 3 μm square field of view, there are 8 or more microcracks in the matrix resin having a length of 100 nm or more and 4.24 μm or less and a maximum width of 20 nm or more and 4.10 μm or less, and the total length of the boundary between the matrix resin portion and the microcrack portion in the same field of view is 1.9 μm or more. (2) Pre-decomposed product of the fiber-reinforced resin composite material according to (1), wherein the reinforcing fibers include carbon fibers. (3) Pre-decomposition product of a fiber-reinforced resin composite material according to (1) or (2), wherein the matrix resin is a thermosetting resin. (4) A method for producing a preliminary decomposition product of a fiber-reinforced resin composite material, comprising a heat treatment in which a fiber-reinforced resin composite material, in which a matrix resin is reinforced with reinforcing fibers, is immersed in a solvent at a temperature of 100°C to 280°C. (5) A method for producing a pre-decomposition product of a fiber-reinforced resin composite material as described in (4), wherein the boiling point of the solvent is 100°C or higher. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a pre-decomposed product of a fiber-reinforced resin composite material with excellent recyclability, and a method for producing the same. [Brief explanation of the drawing]

[0011] [Figure 1] This is one of the cross-sectional images of the fiber-reinforced resin composite material obtained by scanning electron microscopy according to Example 2 of the present invention. [Modes for carrying out the invention]

[0012] Hereinafter, exemplary embodiments for carrying out each aspect of the present invention will be described in detail.

[0013] [Preliminary decomposition product of fiber-reinforced resin composite material] (Reinforced fiber) Examples of reinforcing fibers in the present invention include known reinforcing fibers such as glass fibers and carbon fibers, with carbon fibers being preferred. Since carbon fibers are materials with a negative coefficient of linear expansion in the fiber direction, when thermal energy is applied, the thermal stress caused by the difference in the coefficient of linear expansion with the resin is large, and it is possible to introduce cracks into the resin due to this thermal stress, thereby improving the recycling efficiency of fiber-reinforced resin composite materials.

[0014] Furthermore, the shapes of these reinforcing fibers can include continuous fibers, short fibers such as chopped strands, and whisker shapes.

[0015] The carbon fibers of the present invention are not particularly limited and can be any known carbon fibers, such as carbonaceous fibers or graphite fibers produced using polyacrylonitrile (PAN), pitch, rayon, lignin, hydrocarbon gas, etc. Among these, PAN-based carbon fibers, which are expected to improve mechanical properties, are preferably used.

[0016] (Matrix resin) Examples of the matrix resin of the present invention include thermosetting resins and thermoplastic resins. Examples of thermosetting resins include epoxy resins, unsaturated polyester resins, vinyl ester resins, phenolic resins, melamine resins, and urea resins, with epoxy resins and vinyl ester resins being preferred. In particular, epoxy resins cured with amine compounds have excellent properties, and fiber-reinforced resin composite materials using carbon fibers as reinforcing fibers are widely used in aircraft applications. However, the lifespan of aircraft is 20 to 30 years, and it is expected that many aircraft will reach the end of their service life in the future. Therefore, using epoxy resins cured with amine compounds as the matrix resin is preferable from the viewpoint of recycling needs. Examples of thermoplastic resins include polycarbonate resins, polypropylene resins, polyphenylene sulfide resins, polyamide resins, polyurethane resins, polyether ether ketone resins, and polyether ketone ketone resins, with polyphenylene sulfide resins, polyamide resins, polyether ether ketone resins, and polyether ketone ketone resins being preferred.

[0017] The matrix resin of the present invention may also be used with additives such as stabilizers, release agents, ultraviolet absorbers, colorants, flame retardants, flame retardant enhancers, lubricants, fluorescent whitening agents, phosphorescent pigments, fluorescent dyes, flow modifiers, impact resistance modifiers, crystal nucleating agents, inorganic and organic antibacterial agents, photocatalytic antifouling agents, and infrared absorbers.

[0018] (Preliminary decomposition product of fiber-reinforced resin composite material) The pre-decomposed product of the fiber-reinforced resin composite material of the present invention is at least composed of a matrix resin reinforced with the reinforcing fibers, and is a composite material having a certain number of microcracks in the matrix resin portion in the visual field measured by the subsequent method.

[0019] The number of microcracks having a length of 100 nm or more and 4.24 μm or less and a maximum width of 20 nm or more and 4.10 μm or less is 8 or more, preferably 10 or more, and more preferably 15 or more. Although the upper limit of the number of microcracks is not particularly limited, for example, it is preferably 4,500 or less, more preferably 1,000 or less, further preferably 100 or less, and particularly preferably 50 or less. The total length of the boundary lines between the matrix resin portion and the microcrack portion in the same visual field range is 1.9 μm or more, preferably 2.4 μm or more, and more preferably 4.8 μm or more. Although the upper limit of the total length of the boundary lines between the matrix resin portion and the microcrack portion in the same visual field range is not particularly limited, for example, it is preferably 1080.0 μm or less, more preferably 940.0 μm or less, further preferably 600.0 μm or less, and even more preferably 120.0 μm or less. The pre-decomposed product of the fiber-reinforced resin composite material having the number of microcracks within this range and the total length of the boundary lines between the matrix resin portion and the microcrack portion has high penetration efficiency of heat and decomposing agents during the decomposition of the pre-decomposed product of the fiber-reinforced resin composite material, and can greatly improve the decomposition efficiency.

[0020] (Cross-sectional observation) Observation using a scanning electron microscope involves observing a cross-section cut from a pre-decomposed fiber-reinforced resin composite material at a magnification of 5,000 times, observing any five fields of view where there are fiber cross-sections perpendicular to the fiber direction, and calculating, using subsequent binarization software, the number of microcracks with a length of 100 nm or more and 4.24 μm or less and a maximum width of 20 nm or more and 4.10 μm or less, and the total length of the boundary lines between the matrix resin part and the microcrack part under the condition that the field angle is 3 μm at a magnification of 30,000 times. When the field angle is larger than 3 μm at a magnification of 30,000 times, the field of view with a 3-μm angle starting from the upper right end of the observed field of view is taken as the evaluation target. Microcracks within this range are prone to the penetration of heat and decomposition agents during the decomposition of the pre-decomposed fiber-reinforced resin composite material.

[0021] [Method for manufacturing a pre-decomposed fiber-reinforced resin composite material] The pre-decomposed fiber-reinforced resin composite material is a fiber-reinforced resin composite material that has undergone treatment to improve the recyclability of the fiber-reinforced resin composite material, and can be manufactured by applying external energy to the fiber-reinforced resin composite material. Examples of external energy include physical energy such as impact, thermal energy such as cooling and heating, chemical energy such as ozone, electromagnetic energy such as microwaves, electrical energy such as an electric field, and radiation energy from radioactive substances, etc. These can be used alone or in combination. Among these external energies, thermal energy is preferred, and among thermal energies, energy by heating is even more preferred. With thermal energy, it is possible to carry out the process using general-purpose equipment compared to other energy application methods.

[0022] (Heat treatment) In the heat treatment using energy by heating as the external energy, examples of the heat medium include gases such as air and nitrogen, and liquids such as solvents. Liquids are preferred because of their high heat conduction efficiency.

[0023] Preferably, the heating temperature of the fiber-reinforced resin composite material is raised to 100°C to 280°C, and more preferably to 150°C to 250°C. Within this temperature range, the temperature is raised to near the glass transition temperature of the resin, causing significant thermal expansion while maintaining the glassy state of the resin. Thermal stress is generated between the resin and fibers in a short time due to the difference in the coefficient of linear expansion between the resin and fibers, and this stress can be utilized during recycling, thereby increasing recycling efficiency.

[0024] The temperature of the fiber-reinforced resin composite material before heating is generally room temperature, but it may undergo pretreatment steps such as cooling or heating beforehand. The lower limit temperature of the pretreatment step is not particularly limited, but for example, -200°C or higher is preferred, -100°C or higher is more preferred, and 0°C or higher is even more preferred. The upper limit temperature of the pretreatment step is preferably 90°C or lower, more preferably 80°C or lower, and even more preferably 50°C or lower. When the upper limit temperature is within this range, high thermal stress can be applied to the fiber-reinforced resin composite material by heat treatment.

[0025] The liquid used as the heat transfer medium for the heat treatment preferably has a boiling point of 100°C or higher at atmospheric pressure, more preferably 150°C or higher, and even more preferably 180°C or higher. Within this temperature range, the heat treatment process can be carried out under pressure conditions close to atmospheric pressure, which facilitates the transition to the next process after the decomposition process, resulting in excellent productivity, and is also preferable in terms of safety during production.

[0026] (Recyclability assessment) Recyclability is not limited to any method related to the decomposition of fiber-reinforced resin composites, but examples include thermal decomposition and solvent decomposition of fiber-reinforced resin composites. The resin can be decomposed by thermal or solvent decomposition of the fiber-reinforced resin composite, the resulting reinforcing fibers are washed and dried, and then subjected to thermogravimetric analysis three times. The average value of the resin decomposition rate obtained from the thermogravimetric analysis is then calculated to evaluate the recyclability. [Examples]

[0027] The present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0028] (Main ingredient) • "SumiEpoxy®" ELM434 (Tetraglycidyldiaminodiphenylmethane, manufactured by Sumitomo Chemical Co., Ltd., hereinafter also referred to as ELM434). • “jER(registered trademark)” 828 (Diglycidyl ether of bisphenol A, manufactured by Mitsubishi Chemical Corporation; hereinafter also referred to as jER828). • “jER(registered trademark)” 825 (Diglycidyl ether of bisphenol A, manufactured by Mitsubishi Chemical Corporation; hereinafter also referred to as jER825). • Epiclon® 830 (diglycidyl ether of bisphenol F, manufactured by DIC Corporation; hereinafter also referred to as EP830).

[0029] (Hardening agent) • "Seika Cure (registered trademark)" S (4,4'-diaminodiphenylsulfone, manufactured by Seika Co., Ltd., also referred to as DDS below.) (Solvent for heat treatment) • Triethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd.) (Treatment solution for evaluating recyclability) • Potassium hydroxide (manufactured by Tokyo Chemical Industry Co., Ltd.) • N-methyl-2-pyrrolidone (manufactured by Kuraray Co., Ltd.) (Catalyst for evaluating recyclability) 18-Crown 6-Ether (manufactured by Tokyo Chemical Industry Co., Ltd.).

[0030] (Reinforced fiber) • Carbon fiber bundle (hereinafter also referred to as CF; manufactured by Toray Industries, Inc., "Torayca®" (registered trademark) T700SC-12k-50C)) • Glass fiber bundle (hereinafter also referred to as GF; manufactured by Nitto Boseki Co., Ltd., RS240 PU-537).

[0031] (Cross-sectional observation of fiber-reinforced resin composite material) Cross-sectional observation of the fiber-reinforced resin composite material was performed on a cross-section obtained by cutting the fabricated fiber-reinforced resin composite material perpendicular to the fiber direction. The equipment and conditions used for SEM observation are as follows. Equipment: JEOL Ltd. scanning microscope JSM-IT700HR Sample configuration: Double-sided tape was attached to a dedicated sample stand, and the cut fiber-reinforced resin composite material was fixed on top of it with the observation surface facing upwards on the sample stand. The cross-section was then coated with platinum using a sputtering device. Acceleration voltage: 15kV Observation magnification: 30,000x.

[0032] (Evaluation of microcracks) Five SEM images were obtained for each sample. From the obtained images, the fiber portion, calculated by approximating the fiber cross-section as circular, was divided from the observation field area, and the void wall portion and resin portion were binarized. Next, voids with a length of 100 nm to 4.24 μm and a maximum width of 20 nm to 4.10 μm were counted as microcracks. The total length of the boundary between the matrix resin portion and the microcrack portion within the same field of view was calculated by obtaining the perimeter length of each microcrack obtained through binarization using binarization software, and then calculating the sum of the perimeter lengths of each microcrack.

[0033] (Resin decomposition rate measurement) The resin decomposition rate was calculated by heating the fiber-reinforced resin composite material after the recyclability evaluation test described below, according to the heating program below, using the mass (M0) after heating program (a) and the smallest mass (M1) up to the end of heating program (b). Equipment: Hitachi High-Tech Science Corporation STA7000 Measurement atmosphere: Under a nitrogen stream Temperature rise program: (a) Hold at programmed temperature of 50°C for 1 minute (b) Heat from a programmed temperature of 50°C to 500°C at a heating rate of 25°C / min, and hold at 500°C for 30 minutes. Resin decomposition rate: (M0-M1) / M0×100.

[0034] (Reference example 1) ELM434 (70 parts by mass), jER828 (10 parts by mass), EP830 (20 parts by mass), and DDS (45.2 parts by mass) were mixed to form a resin raw material. This resin raw material was impregnated into carbon fiber sheets (CF) arranged in one direction and set in a mold, at 90°C. The resin was then heated from 90°C to 180°C for 60 minutes, followed by heating at 180°C for 2 hours to cure the resin, thereby obtaining a sheet-like carbon fiber reinforced polymer composite material. This will be referred to as CFRP below.

[0035] (Reference example 2) In Reference Example 1, the process was carried out in the same manner as in Reference Example 1, except that CF was changed to GF, and a sheet-like glass fiber reinforced resin composite material was obtained. This will be referred to as GFRP below.

[0036] (Example 1) [Heat treatment process] 300g of triethylene glycol was added to a 500mL round-bottom flask and heated to 150°C in an oil bath. 15g of CFRP was added and held for 10 minutes. After the flask was air-cooled, the CFRP was removed, washed with acetone, and dried in the air.

[0037] [Cross-sectional observation of fiber-reinforced resin composite material] A portion of the heat-treated CFRP was removed, and the cross-section, cut perpendicular to the fiber direction, was observed. The number of microcracks and the total length of the boundary between the matrix resin and the microcracks were calculated.

[0038] [Recyclability Assessment] Heat-treated CFRP (6g), potassium hydroxide (3g), N-methyl-2-pyrrolidone (57g), and 18-Crown 6-Ether (20g) were added to a 200mL round-bottom flask and heated in an oil bath to 200°C for 2 hours. The resulting contents were filtered at 200°C, and the solid portion containing carbon fibers was washed with NMP (100g) and deionized water (100g), and dried to recover the carbon fibers.

[0039] (Example 2) In Example 1, the procedure was carried out in the same manner as in Example 1, except that the temperature of the triethylene glycol used in the heat treatment process was set to 200°C. Cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were then performed.

[0040] (Example 3) In Example 1, the procedure was carried out in the same manner as in Example 1, except that the temperature of the triethylene glycol used in the heat treatment process was set to 250°C. Cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were then performed.

[0041] (Comparative Example 1) The heat treatment step from Example 1 was omitted, and the procedure was carried out in the same manner as in Example 1. Cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were performed.

[0042] (Example 4) In Example 2, the procedure was carried out in the same manner as in Example 2, except that 18-Crown 6-Ether was not added during the recyclability evaluation. Cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were performed.

[0043] (Example 5) In Example 4, the procedure was carried out in the same manner as in Example 4, except that GFRP was used instead of CFRP, and cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were performed.

[0044] (Comparative Example 2) In Example 4, the procedure was carried out in the same manner as in Example 4, except that the following heat history application step was performed instead of the heat treatment step, and cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were performed. Thermal history: Place CFRP (5g) in an 80°C heating furnace and hold for 30 minutes. Then, immerse the CFRP in a Dewar flask containing liquid nitrogen (300g) and hold for 5 minutes, after which return to room temperature.

[0045] (Comparative Example 3) In Example 4, the procedure was carried out in the same manner as in Example 4, except that the following heat history application step was performed instead of the heat treatment step, and cross-sectional observation and recyclability evaluation of the fiber-reinforced resin composite material were performed.

[0046] Thermal history: A refrigerant was prepared by adding dry ice to a Dewar flask containing methanol (180g) and deionized water (120g). At this time, the temperature measured by a thermocouple was -60°C. Next, CFRP (5g) was placed in an 80°C heating furnace and held for 30 minutes. Then, the CFRP was immersed in the Dewar flask containing the refrigerant and held for 5 minutes. This heating and cooling process was repeated 10 times in a cycle test, after which the sample was returned to room temperature.

[0047] From a comparison of Examples 1 to 3 with Comparative Example 1, and a comparison of Examples 4 to 5 with Comparative Examples 2 to 3, it was found that CFRP with a larger number of microcracks and a longer total length of the boundary between the matrix resin portion and the microcrack portion exhibits superior recyclability.

[0048] A comparison of Examples 4 and 5 with Comparative Example 2 revealed that CFRP, which has a longer total boundary length between the matrix resin portion and the microcrack portion, as well as a larger number of microcracks, exhibits superior recyclability.

[0049] A comparison of Examples 4 and 5 with Comparative Examples 2 and 3 revealed that CFRP subjected to heat treatment using a solvent exhibited superior recyclability compared to refrigeration treatment using liquid nitrogen or cycle testing using a refrigerant.

[0050] [Table 1]

[0051] [Table 2] [Explanation of symbols]

[0052] 1 Reinforcement fiber 2 Matrix resin 3 Microcracks [Industrial applicability]

[0053] The fiber-reinforced resin composite material obtained by the present invention, and the method for producing the preliminary decomposition product of the fiber-reinforced resin composite material, can be used to recover reinforcing fibers and resin from commonly known fiber-reinforced resin composite materials, taking advantage of their excellent recyclability.

Claims

1. A fiber-reinforced resin composite material pre-decomposition product comprising a matrix resin and reinforcing fibers, observed with a scanning electron microscope, wherein in any 3 μm square field of view, there are 8 or more microcracks in the matrix resin having a length of 100 nm or more and 4.24 μm or less and a maximum width of 20 nm or more and 4.10 μm or less, and the total length of the boundary between the matrix resin portion and the microcrack portion in the same field of view is 1.9 μm or more.

2. The preliminary decomposition product of the fiber-reinforced resin composite material according to claim 1, wherein the reinforcing fibers include carbon fibers.

3. The preliminary decomposition product of a fiber-reinforced resin composite material according to claim 1 or 2, wherein the matrix resin is a thermosetting resin.

4. A method for producing a preliminary decomposition product of a fiber-reinforced resin composite material, comprising a heat treatment in which a fiber-reinforced resin composite material, in which a matrix resin is reinforced with reinforcing fibers, is immersed in a solvent at a temperature of 100°C to 280°C.

5. A method for producing a pre-decomposition product of a fiber-reinforced resin composite material according to claim 4, wherein the boiling point of the solvent is 100°C or higher.

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