Long-fine aramid single fiber, manufacturing method therefor, and composite material and sheet containing long-fine aramid single fiber

Abstract

The present invention pertains to a long-fine aramid single fiber having a maximum fiber diameter of less than 2,000 nm and a minimum fiber length of greater than 10 μm, wherein the degree x of crystallinity determined by the expression x=(1-∆Qx / ∆Q0)×100 (%) (in the expression, ∆Qx is a heat capacity change amount before and after the glass transition point of the long-fine aramid single fiber, and ∆Q0 is a heat capacity change amount before and after the glass transition point of an amorphous aramid) is at least 60%. The present invention can provide an aramid single fiber which is very thin and long and has a high degree of crystallinity.

Description

Long fine aramid single fibers, their manufacturing method, and composite materials and sheets containing long fine aramid single fibers

[0001] The present invention relates to very fine and long aramid monofilaments and their production and use.

[0002] Aromatic polyamides (aramids) include para-aromatic polyamides such as polyparaphenylene terephthalamide, and meta-aromatic polyamides such as polymetaphenylene isophthalamide, both of which are in practical use as fibers.

[0003] Furthermore, very thin (fine) fibers made of aromatic polyamides have also been produced. For example, Patent Document 1 describes the production of aramid nanofibers having an average fiber diameter of 20 nm by discharging a solution of aramid in a strong base, dimethyl sulfoxide, into water and spinning the resulting mixture (see paragraphs

[0051] to

[0053] of Patent Document 1).

[0004] JP 2023-2089 A

[0005] However, the aramid nanofibers obtained by the method described in Patent Document 1 are relatively short and have low crystallinity.

[0006] An object of the present invention is to provide an aramid monofilament that is very thin, long, and has a high degree of crystallinity.

[0007] A first aspect of the present invention is a long fine aramid single fiber having a maximum fiber diameter of less than 2000 nm and a minimum fiber length of more than 10 μm, wherein the maximum fiber diameter is expressed by the following formula: x=(1−ΔQx / ΔQ ０ ) × 100 (%) (wherein ΔQx is the amount of change in heat capacity of the long fine aramid single fibers before and after the glass transition point, and ΔQ ０ The crystallinity x determined by the above method (where x is the change in heat capacity of amorphous aramid before and after the glass transition point) is 60% or more.

[0008] The long fine aramid single fibers preferably have an average fiber diameter of less than 1000 nm.

[0009] The long and fine aramid single fibers preferably have an average aspect ratio of 100 or more.

[0010] The glass transition temperature of the long fine aramid single fibers is preferably higher than 300°C.

[0011] The aramid constituting the long fine aramid single fibers is preferably meta-aramid.

[0012] A second aspect of the present invention is a method for producing the long fine aramid single fibers, comprising: a swelling step of bringing raw material aramid fibers having a soluble content in dimethyl sulfoxide of 10% by mass or more into contact with a swelling liquid containing an aprotic solvent to cause swelling; a mechanical defibration step of finely pulverizing the raw material aramid fibers by mechanical defibration; and a separation step of separating long fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm from the finely pulverized raw material aramid fibers.

[0013] The aprotic solvent is preferably dimethyl sulfoxide and / or dimethylacetamide.

[0014] The glass transition point of the long fine aramid single fibers is preferably 30° C. or more higher than the glass transition point of the raw material aramid fibers.

[0015] A third aspect of the present invention is a composite material comprising the above-mentioned long fine aramid monofilaments and a matrix resin.

[0016] It is preferable that the long fine aramid fibers are dispersed in the matrix resin.

[0017] The matrix resin is preferably an epoxy resin.

[0018] A fourth aspect of the present invention is a sheet comprising the above-mentioned long fine aramid monofilaments.

[0019] The long fine aramid monofilaments of the present invention are very thin, long, and highly crystalline.

[0020] The method for producing long and fine aramid single fibers of the present invention can produce aramid fibers that are very thin, long, and have a high degree of crystallinity. The method for producing long and fine aramid single fibers of the present invention has high productivity and is advantageous for industrial production of long and fine aramid single fibers.

[0021] The composite material or sheet of the present invention can exhibit properties derived from the long, fine aramid monofilaments contained therein.

[0022] Electron microscope photograph of the long and fine aramid single fiber obtained in Example 1 Electron microscope photograph of the long and fine aramid single fiber obtained in Example 1 Electron microscope photograph of the long and fine aramid single fiber obtained in Example 1

[0023] As a result of extensive research, the present inventors have found that a specific manufacturing method can provide an aramid single fiber that is very thin, long, and has a high degree of crystallinity.

[0024] As described in Patent Document 1

[0036] , it was thought that mechanical shearing force only partially obtains a fibril structure having a fiber diameter on the nano-order, and that it is not possible to produce aramid single fibers having a fiber diameter on the nano-order. However, the present inventors have found that it is possible to produce aramid single fibers having a fiber diameter on the nano-order by using mechanical shearing force, and also that it is possible to impart a high degree of crystallinity to the aramid single fibers thus obtained.

[0025] Hereinafter, the long fine aramid single fibers, the method for producing the long fine aramid single fibers, and the composite material or sheet containing the long fine aramid single fibers of the present invention will be described.

[0026] [Aramid] In the present invention, "aramid" refers to an aromatic polyamide. In the present invention, "aromatic polyamide" refers to a linear polymer compound in which, in terms of chemical structure, 60 mol % or more, preferably 70 mol % or more, more preferably 80 mol % or more, and even more preferably 90 mol % or more of amide bonds are directly bonded to aromatic rings.

[0027] Aramids are classified into para-aramids, meta-aramids, and copolymers thereof depending on the substitution position of the amide group on the benzene ring. Examples of para-aramids include polyparaphenylene terephthalamide and its copolymers, and poly(paraphenylene)-copoly(3,4'-diphenyl ether) terephthalamide (poly(paraphenylene)-copoly(3,4'-diphenyl ether) terephthalamide). Examples of meta-aramids include polymetaphenylene isophthalamide and its copolymers. These aramids are industrially produced, for example, by conventionally known methods such as interfacial polymerization and solution polymerization, and are commercially available, but are not limited thereto. In the present invention, meta-aramids are preferably selected. Meta-aramids have the following advantages: they are soluble in general-purpose amide solvents, can be wet-molded using a polymer solution as a starting material, have excellent heat-sealing properties, and have good heat resistance and flame retardancy. Among meta-aramids, polymetaphenylene isophthalamide is preferably used because it has good moldability, heat adhesion, flame retardancy, heat resistance, and other properties.

[0028] [Long Fine Aramid Monofilament] The long fine aramid monofilament of the present invention is a monofilament. In the present invention, the term "monofilament" means a fiber without branches, and does not mean a fiber with branches such as a fibrillated fiber.

[0029] The long fine aramid single fibers of the present invention are very thin (i.e., fine) fibers having a maximum fiber diameter of less than 2000 nm. The maximum fiber diameter of the long fine aramid single fibers of the present invention is preferably less than 1900 nm, more preferably less than 1800 nm, and even more preferably less than 1700 nm. The maximum fiber diameter of the long fine aramid single fibers of the present invention may be 1000 nm or more.

[0030] The long fine aramid single fibers of the present invention preferably have a minimum fiber diameter of more than 100 nm, more preferably more than 150 nm, and even more preferably more than 200 nm.The long fine aramid single fibers of the present invention preferably have a minimum fiber diameter of 500 nm or less, more preferably 400 nm or less, and even more preferably 300 nm or less.

[0031] The long fine aramid single fibers of the present invention preferably have an average fiber diameter of less than 1000 nm, more preferably less than 950 nm, and even more preferably less than 900 nm. The long fine aramid single fibers of the present invention preferably have an average fiber diameter of more than 400 nm, more preferably more than 500 nm, and even more preferably more than 600 nm. The average fiber diameter of the long fine aramid single fibers of the present invention is preferably more than 400 nm and less than 1000 nm, more preferably more than 500 nm and less than 950 nm, and even more preferably more than 600 nm and less than 900 nm.

[0032] The maximum fiber diameter, minimum fiber diameter and average fiber diameter of the long fine aramid single fibers of the present invention can be determined for a set of a predetermined number of fibers. Specifically, the fiber diameters of 100 long fine aramid single fibers of the present invention are measured by observation with an electron microscope such as SEM, and the maximum and minimum fiber diameters among them are taken as the maximum and minimum fiber diameters, respectively, and the number average value of the fiber diameters can be taken as the average fiber diameter.

[0033] The long fine aramid single fibers of the present invention are relatively long fibers having a minimum fiber length of more than 10 μm. The minimum fiber length of the long fine aramid single fibers of the present invention is preferably more than 20 μm, more preferably more than 30 μm, more preferably more than 40 μm, more preferably more than 50 μm, and even more preferably more than 60 μm. On the other hand, the upper limit of the minimum fiber length of the long fine aramid single fibers of the present invention is not particularly limited, but can be, for example, less than 100 μm.

[0034] The maximum fiber length of the long fine aramid single fibers of the present invention is preferably less than 1000 μm, more preferably less than 800 μm, and even more preferably less than 600 μm. The maximum fiber length of the long fine aramid single fibers of the present invention is preferably 200 μm or more, more preferably 250 μm or more, and even more preferably 300 μm or more.

[0035] The average fiber length of the long fine aramid single fibers of the present invention is preferably more than 20 μm, more preferably more than 50 μm, and even more preferably more than 80 μm. The average fiber length of the long fine aramid single fibers of the present invention is preferably less than 400 μm, more preferably less than 300 μm, and even more preferably less than 200 μm. The average fiber length of the long fine aramid single fibers of the present invention is preferably more than 20 μm and less than 400 μm, preferably more than 50 μm and less than 300 μm, and even more preferably more than 80 μm and less than 200 μm.

[0036] The minimum fiber length, maximum fiber length and average fiber length of the long fine aramid single fibers of the present invention can be determined for a set of a predetermined number of fibers. Specifically, the fiber lengths of 100 long fine aramid single fibers of the present invention are measured by observation with an electron microscope such as an SEM, and the minimum and maximum fiber lengths among them are taken as the minimum and maximum fiber lengths, respectively, and the number average value of the fiber lengths can be taken as the average fiber length.

[0037] The long fine aramid single fibers of the present invention are fibers having a relatively high aspect ratio. The minimum aspect ratio of the long fine aramid single fibers of the present invention is preferably 80 or more, more preferably 90 or more, and even more preferably 100 or more. On the other hand, the maximum aspect ratio of the long fine aramid single fibers of the present invention is not particularly limited, but can be, for example, less than 500. The average aspect ratio of the long fine aramid single fibers of the present invention is preferably 100 or more, more preferably 150 or more, and even more preferably 200 or more. The upper limit of the average aspect ratio of the long fine aramid single fibers of the present invention is not particularly limited, but can be, for example, less than 400.

[0038] The minimum aspect ratio, maximum aspect ratio and average aspect ratio of the long fine aramid single fibers of the present invention can also be determined for a set of a predetermined number of fibers. Specifically, the fiber lengths of 100 long fine aramid single fibers of the present invention are measured by observation with an electron microscope such as SEM, the aspect ratio of each fiber is determined from the fiber length and fiber diameter of each fiber, the minimum aspect ratio and the maximum aspect ratio are taken as the minimum aspect ratio and the maximum aspect ratio, respectively, and the number average value thereof can be taken as the average aspect ratio.

[0039] The long and fine aramid single fiber of the present invention has a relatively high degree of crystallinity. That is, the long and fine aramid single fiber of the present invention has a crystallinity of 100% or more, and the crystallinity is expressed by the following formula: x=(1−ΔQx / ΔQ ０ ) × 100 (%) (wherein ΔQx is the amount of change in heat capacity before and after the glass transition point of the long fine aramid single fibers of the present invention, and ΔQ ０ The crystallinity x determined by the above formula (where x is the amount of change in heat capacity before and after the glass transition point of amorphous aramid) is 60% or more. The crystallinity of the long fine aramid single fibers of the present invention is preferably 70% or more, and more preferably 80% or more. There is no particular upper limit to the crystallinity of the long fine aramid single fibers of the present invention, but it can be, for example, 95% or less or 90% or less.

[0040] As the amorphous aramid, an aramid that can be used is one that is completely soluble in dimethyl sulfoxide (in the absence of a base) at room temperature (25° C.).

[0041] ΔQx and ΔQ in the above formula ０ can be determined by a differential scanning calorimeter. Specifically, the temperature-dependent specific heat capacity baseline is determined by plotting the variation in specific heat capacity against temperature change on a plane with the vertical axis representing specific heat capacity and the horizontal axis representing temperature using a differential scanning calorimeter. The glass transition point is then identified from the shift point of the baseline, and the amount of change in heat capacity can be determined from the interval between baseline shifts before and after the glass transition point. The interval between baseline shifts can be determined by linearly approximating the baseline.

[0042] The measurement or determination of the crystallinity of a polymer by the above formula is known, as described, for example, in Kobunshi Ronbunshu, Vol. 31, No. 2, pp. 138-139 (1972).

[0043] The long fine aramid single fibers of the present invention preferably have a glass transition point of more than 300° C., more preferably more than 310° C., and even more preferably more than 320° C. A high glass transition point means that there are fewer amorphous portions that undergo Brownian motion and that the crystallinity is high.

[0044] The long fine aramid monofilament of the present invention is a monofilament and is unbranched, and therefore has excellent mixability with resins and the like.

[0045] Furthermore, the long fine aramid single fibers of the present invention are fine but relatively long, and therefore can exhibit superior mechanical properties when combined with a resin or the like.

[0046] Furthermore, the long and fine aramid single fibers of the present invention have high crystallinity, and therefore have a stable fiber shape, which can be well maintained against various physical or chemical actions.

[0047] [Method for producing long fine aramid single fibers] The method for producing long fine aramid single fibers of the present invention includes: a swelling step of bringing raw material aramid fibers having a soluble content in dimethyl sulfoxide of 10% by mass or more into contact with a swelling liquid containing an aprotic solvent to cause swelling; a mechanical defibration step of finely pulverizing the raw material aramid fibers by mechanical defibration; and a separation step of separating long fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm from the finely pulverized raw material aramid fibers.

[0048] The method for producing long and fine aramid single fibers of the present invention is characterized in that, instead of spinning an aramid solution, predetermined raw material aramid fibers are swelled and mechanically opened up to form single fibers.

[0049] The method for producing long and fine aramid single fibers of the present invention can produce aramid fibers that are very fine, long, and have a high degree of crystallinity.

[0050] When an aramid solution is spun, it is difficult to produce relatively long aramid fibers, but the method for producing long and fine aramid single fibers of the present invention makes it possible to easily produce relatively long aramid single fibers.

[0051] The method for producing long and fine aramid single fibers of the present invention can exhibit high productivity and is industrially advantageous. On the other hand, although it is known to produce aramid single fibers by electrospinning, electrospinning has low productivity and is disadvantageous for industrial production.

[0052] The method for producing the long and fine aramid single fibers of the present invention will be described below in order.

[0053] In the method for producing long and fine aramid single fibers of the present invention, aramid fibers having a dimethyl sulfoxide soluble content of 10% by mass (weight) or more are used as the raw material. The soluble content of the raw material aramid fibers is based on the total mass (total weight) of the raw material aramid fibers.

[0054] As the raw material aramid fibers having a soluble content in dimethyl sulfoxide of 10 mass (weight) % or more, aramid fibers that show a change in mass (weight) of 10% or more when immersed in dimethyl sulfoxide (having a purity of 99.5% or more and not containing bases such as sodium hydroxide and potassium hydroxide) at room temperature (25°C) for 72 hours can be used.

[0055] In raw aramid fibers having a dimethyl sulfoxide soluble content of 10% by mass (weight) or more, the amorphous portion is easily solvated, and the fibers are easily swollen. The dimethyl sulfoxide soluble content of the raw aramid fibers is not particularly limited as long as it is 10% by mass (weight) or more, but may be, for example, 15% by mass (weight) or more, 20% by mass (weight) or more, 25% by mass (weight) or more, or 30% by mass (weight) or more. On the other hand, the dimethyl sulfoxide soluble content of the raw aramid fibers is preferably 80% by mass (weight) or less, more preferably 70% by mass (weight) or less, and even more preferably 60% by mass (weight) or less.

[0056] The form of the raw aramid fiber is not particularly limited, and either branched or unbranched fibers can be used. For example, the raw aramid fiber may be in the form of aramid floc or aramid fibrid.

[0057] "Aramid floc" refers to short fibers made of aramid. The (number) average fiber length of the aramid floc is not particularly limited, but can be, for example, in the range of 1 to 50 mm, preferably in the range of 2 to 40 mm, and more preferably in the range of 3 to 30 mm. The (number) average fiber diameter of the aramid floc is also not particularly limited, but can be, for example, in the range of 10 to 30 μm, preferably in the range of 12 to 28 μm, and more preferably in the range of 14 to 26 μm.

[0058] The average fiber length and average fiber diameter of aramid flocs can be obtained by averaging the lengths and widths of a predetermined number of aramid flocs (for example, 100 flocs). Specifically, the fiber lengths and fiber diameters of 100 flocs can be measured by observation with an electron microscope such as an SEM, and the average value of the fiber lengths can be taken as the average fiber length, and the average value of the fiber diameters can be taken as the average fiber diameter.

[0059] "Aramid fibrids" are film-like or fibrous fine particles made of aramid, and are sometimes called aramid pulp (for details of aramid fibrids, see Japanese Patent Publication Nos. 1960-11851 and 1962-5752). Since fibrids have papermaking properties similar to ordinary wood (cellulose) pulp, they can be dispersed in water and then formed into a sheet using a paper machine.

[0060] The (number) average fiber diameter of the aramid fibrids is not particularly limited, but can be, for example, 4 to 40 μm, preferably 5 to 35 μm, and more preferably 6 to 30 μm.

[0061] The average fiber diameter of aramid fibrids can be obtained by averaging the widths of a predetermined number of aramid fibrids (for example, 100). Specifically, the fiber diameters of 100 aramid fibrids can be measured by observation with an electron microscope such as SEM, and the average value of the fiber diameters can be used as the average fiber diameter.

[0062] (Swelling step) In the swelling step of the method for producing long and fine aramid single fibers of the present invention, raw material aramid fibers having a soluble content in dimethyl sulfoxide of 10 mass (weight) % or more are brought into contact with a swelling liquid containing an aprotic solvent to swell them.

[0063] The aprotic solvent is not particularly limited as long as it can swell the raw material aramid fibers, but dimethyl sulfoxide, dimethyl acetamide, or a mixture of the two can be preferably used.

[0064] It is believed that the aprotic solvent breaks the hydrogen bonds between the low-crystalline portions of the raw aramid fibers, and the highly crystalline portions other than these portions are easily separated as fine fibers.

[0065] The swelling liquid may contain optional components other than the aprotic solvent, such as an acid, a base, a salt, water, etc. The amount of the optional components is not particularly limited, but may be, for example, 0.001 to 10% by mass (weight), 0.01 to 5% by mass (weight), or 0.1 to 1% by mass (weight) based on the total mass (total weight) of the swelling liquid.

[0066] The mode of bringing the raw aramid fibers into contact with the swelling liquid is not particularly limited, and for example, the raw aramid fibers can be immersed in the swelling liquid, or the swelling liquid can be sprinkled or sprayed onto the raw aramid fibers. The mode of immersing the raw aramid fibers in the swelling liquid is preferred.

[0067] The contact temperature and contact time are not particularly limited as long as the raw material aramid fibers are swelled, and may be, for example, in the range of room temperature (25° C.) to 40° C. and 10 minutes to 1 hour.

[0068] The raw aramid fibers swollen in the swelling step are subjected to a mechanical defibration step.

[0069] (Mechanical Opening Step) In the mechanical opening step of the method for producing long and fine aramid single fibers of the present invention, the raw material aramid fibers after the swelling step are opened up by mechanical opening.

[0070] It is considered that, through the swelling step, hydrogen bonds between the portions of low crystallinity in the raw material aramid fibers are cut, and the portions become easy to separate, and then, due to mechanical defibration, a physical force such as a shear force acts on the portions, causing the fine fibers to separate from the raw material aramid fibers.

[0071] The mechanical defibration can be carried out by any known method as long as the defibration is mechanical. The mechanical defibration is preferably carried out by applying a shear force to the swollen raw material aramid fibers.

[0072] The device used for mechanical fiberization is not particularly limited, and examples that can be used include devices that apply collision shear force such as planetary ball mills and bead mills, devices that apply rotational shear force such as disc refiners and grinders, devices that can perform kneading, stirring, and dispersion with high efficiency such as various kneaders and planetary mixers, high-pressure homogenizers, etc. It is preferable to use a Clearmix device (for example, one manufactured by MTECHNIQUE) equipped with a high-speed rotating rotor and a screen arranged with a small clearance.

[0073] (Separation Step) In the separation step of the method for producing long fine aramid single fibers of the present invention, long fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm are separated from the finely divided raw material aramid fibers after the mechanical defibration step.

[0074] In the separation step, long fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm are separated from the fine fibers separated from the raw material aramid fibers in the mechanical defibration step.

[0075] The separation method is not particularly limited, and can be carried out by a known method. For example, the raw material aramid fibers pulverized in the mechanical defibration step are diluted and then centrifuged, and the supernatant is filtered, whereby the long fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm can be separated from the raw material aramid fibers.

[0076] The dilution medium (liquid) is preferably an aprotic solvent. Dimethyl sulfoxide, dimethylacetamide, or a mixture of the two can be preferably used as the aprotic solvent. The dilution medium may contain optional components other than the aprotic solvent, such as an acid, a base, a salt, or water, as needed. The amount of the optional components is not particularly limited, but may be, for example, 0.001 to 10% by weight, 0.01 to 5% by weight, or 0.1 to 1% by weight, based on the total mass (total weight) of the dilution medium (liquid).

[0077] The long and fine aramid single fibers of the present invention can be obtained by the method for producing the long and fine aramid single fibers of the present invention.

[0078] In particular, the method for producing long and fine aramid single fibers of the present invention can produce long and fine aramid single fibers of the present invention having a glass transition point 30°C or more higher than that of the raw material aramid fibers. A high glass transition point means that there are fewer amorphous parts that undergo Brownian motion and that the crystallinity is high. Therefore, the method for producing long and fine aramid single fibers of the present invention can produce long and fine aramid single fibers having a higher degree of crystallinity than the raw material aramid fibers.

[0079] [Composite Material or Sheet Comprising Long Fine Aramid Single Fibers] The present invention also relates to a composite material or sheet comprising the above-mentioned long fine aramid single fibers.

[0080] (Composite Material) The composite material of the present invention contains the above-mentioned long fine aramid monofilaments and a matrix resin.

[0081] It is preferable that the long fine aramid single fibers are dispersed in the matrix resin. The long fine aramid single fibers are not branched, so that they can be easily mixed with the matrix resin. Furthermore, the long fine aramid single fibers are relatively long, so that they can easily increase the mechanical strength of the composite material.

[0082] The matrix resin is not particularly limited, and a thermoplastic resin, a thermosetting resin, or a mixture of both can be used.

[0083] The type of thermoplastic resin is not particularly limited, and one or more of polyolefin resin, nylon resin, polycarbonate resin, polyester resin, polyacetal resin, ABS resin, phenoxy resin, polymethyl methacrylate resin, polyphenylene sulfide resin, polyetherimide resin, polyether ketone resin, etc. can be used.

[0084] The type of thermosetting resin is not particularly limited, and one or more of epoxy resin, unsaturated polyester resin, vinyl ester resin, etc. can be used, with epoxy resin being preferred.

[0085] The mass (weight) ratio of the matrix resin to the long fine aramid single fibers (mass (weight) of matrix resin / mass (weight) of long fine aramid single fibers) is preferably 50 / 50 to 90 / 10, more preferably 55 / 45 to 80 / 20, and particularly preferably 60 / 40 to 70 / 30, from the viewpoint of the strength of the composite material.

[0086] When the matrix resin is a thermoplastic resin, the composite material of the present invention can be obtained, for example, by a method in which the above-mentioned long fine aramid single fibers are introduced into a molten thermoplastic matrix resin, mixed and kneaded, and then extrusion molding or injection molding is carried out.

[0087] When the matrix resin is a thermosetting resin, the composite material of the present invention can be obtained, for example, by mixing the long fine aramid single fibers with the matrix resin and molding the mixture under heat. The composite material does not need to be completely cured, but it is preferably cured to the extent that the molded composite material can maintain its shape. After molding, the composite material may be further heated to be completely cured.

[0088] The composite material of the present invention can have a higher dielectric strength than the matrix resin itself, and therefore can be suitably used as an electrical insulator.

[0089] In particular, the composite material of the present invention can have a glass transition temperature higher than that of the matrix resin itself, and therefore has a more stable shape than the matrix resin, making it suitable for a variety of applications.

[0090] (Sheet) The sheet of the present invention contains the above-mentioned long fine aramid monofilaments.

[0091] The sheet of the present invention containing the above-mentioned long fine aramid monofilaments can exhibit excellent strength, electrical insulation properties, and the like.

[0092] The sheet of the present invention is preferably porous. The form of the sheet of the present invention is not particularly limited, and various forms such as woven fabric, nonwoven fabric, paper, etc. are possible, but the form of nonwoven fabric or paper is preferred.

[0093] The thickness of the sheet of the present invention is preferably 1 to 300 μm, more preferably 2 to 200 μm, and even more preferably 3 to 100 μm. The basis weight of the sheet of the present invention is 10 to 500 g / m ２ is preferred, and 20 to 400 g / m ２ More preferably, 30 to 300 g / m ２ is even more preferred.

[0094] The sheet of the present invention contains the long fine aramid single fibers in an amount of preferably 50 to 99% by mass (weight), more preferably 60 to 95% by mass (weight), and even more preferably 70 to 90% by mass (weight) of the total mass (total weight) of the sheet.

[0095] The sheet of the present invention is preferably composed mainly of the long fine aramid fibers (80% by mass (weight) or more of the total mass). When the sheet of the present invention mainly contains the long fine aramid single fibers, it is easy to suppress the thickness of the sheet and obtain a very thin sheet.

[0096] The sheet of the present invention can be produced, for example, by forming the long fine aramid single fibers into a sheet by a known method. Specifically, for example, a method of dispersing the long fine aramid single fibers, if necessary together with other fibers, etc., in a dry state to form a sheet; a method of dispersing and mixing the long fine aramid single fibers, if necessary together with other fibers, etc., in a liquid medium such as water to obtain a slurry, and then discharging the slurry onto a liquid-permeable support such as a net or belt to form a sheet, and then removing the liquid medium and drying the sheet can be used, but the latter is preferred, and among these, a so-called wet papermaking method using water as the liquid medium is preferred.

[0097] In the wet papermaking method, an aqueous slurry containing at least the above-mentioned long fine aramid single fibers is generally fed to a papermaking machine, dispersed therein, dehydrated, squeezed out, and dried, and then wound into a sheet (wet papermaking method). As the papermaking machine, a Fourdrinier papermaking machine, a cylinder papermaking machine, an inclined papermaking machine, or a combination papermaking machine combining these can be used. Additives such as a dispersibility improver, an antifoaming agent, and a paper strength agent are used as needed during papermaking.

[0098] The present invention will be described in more detail below using examples and comparative examples, but the scope of the present invention is not limited to these examples. Each example and comparative example was evaluated by the following methods.

[0099] <Measurement of Soluble Content> 0.1 g of raw aramid fibers, long fine aramid single fibers, or amorphous aramid nanofibers was added to 10 g of dimethyl sulfoxide and immersed at room temperature (25°C) for 72 hours. When the measurement target was fibers, the fibers were suction-filtered using a membrane filter (Merck Millipore, hydrophilic PTFE, pore size: 1.0 μm (raw aramid fibers), 0.2 μm (long fine aramid single fibers and amorphous aramid nanofibers)), and then thoroughly washed with 99.0% by mass of dimethyl sulfoxide and then with deionized water. The washed filter cake was dried at 105°C for 24 hours, and the mass was used as the insoluble content to calculate the soluble content using the following formula: Soluble content (%) = (1 - insoluble content / 0.1) * 100%

[0100] <Measurement of Glass Transition Point> Long fine aramid single fibers or amorphous aramid nanofibers were washed with deionized water and dried in an oven at 105°C for 24 hours, and the change in heat capacity was measured under the following conditions, with the midpoint of the baseline shift being taken as the glass transition point. Differential scanning calorimetry: Netsch Japan Co., Ltd. DSC214-SKK27 Temperature range: 25 to 340°C during heating at 10°C / min Atmosphere: Nitrogen gas 40 ml / min Sample container: Aluminum standard container (with lid)

[0101] <Measurement of Crystallinity> The long fine aramid single fibers and amorphous aramid nanofibers were washed with dimethyl sulfoxide and deionized water in this order, and dried in an oven at 105°C for 24 hours. Thereafter, the change in heat capacity of each was measured under the following conditions. Differential scanning calorimeter: X-DSC7000 model, Hitachi High-Tech Science Corporation. Temperature range: Heating process 25 to 340°C, 10°C / min Cooling process 340 to 25°C, 20°C / min Atmosphere: Nitrogen gas, 40 ml / min Sample container: Aluminum standard container (with lid) Standard material: Sapphire The crystallinity of the long fine aramid single fibers was determined by the following formula: x = (1 - ΔQx / ΔQ ０ ) × 100 (%) ΔQx: Change in heat capacity of long fine aramid single fiber before and after the glass transition point ΔQ ０ : Heat capacity change of amorphous aramid nanofiber around the glass transition point

[0102] <Measurement of fiber diameter and fiber length> Long fine aramid single fibers or amorphous aramid nanofibers were washed with deionized water and dried in an oven at 105°C, and then measured under the following conditions, and the maximum fiber diameter, minimum fiber diameter, maximum fiber length, and minimum fiber length were determined by image analysis. Microscope: Field emission scanning electron microscope: Hitachi High-Tech Corporation SU8010 Voltage: 5 kV Number of fibers measured: 100

[0103] <Measurement of Glass Transition Point of Composite Material> A composite material containing long fine aramid single fibers or amorphous aramid nanofibers and a matrix resin was dried under reduced pressure at 80°C and -100 kPa, and then the change in heat capacity of the composite material was measured under the following conditions, and the midpoint of the baseline shift was taken as the glass transition point. Differential scanning calorimetry: Netsch Japan Co., Ltd. DSC214-SKK277 Temperature range: 25 to 140°C during heating at 10°C / min Atmosphere: Nitrogen gas 40 ml / min Sample container: Aluminum standard container (with lid)

[0104] <Measurement of Dielectric Strength of Composite Material> A composite material containing long fine aramid single fibers or amorphous aramid nanofibers and a matrix resin was formed into a 6 cm x 6 cm film, washed with deionized water, dried at 80°C and -100 kPa under reduced pressure, and measured under the following conditions: High voltage withstand voltage tester: Voltage Tester 7473, Keisoku Gijutsu Kenkyusho Co., Ltd. Rmap Up: 30 seconds Voltage range: 0 to 20 kV Under the above conditions, the dielectric strength was calculated from the breakdown voltage of the composite material and the film thickness using the following formula: Dielectric strength (kV / mm) = Breakdown voltage (kV) / Film thickness (mm)

[0105] <Measurement of soluble content of composite material> 0.1 g of a composite material containing long fine aramid single fibers or amorphous aramid nanofibers and a matrix resin was added to 10 g of dimethyl sulfoxide and immersed at room temperature (25°C) for 72 hours. This was suction filtered using a membrane filter (Merck Millipore, hydrophilic PTFE, pore size: 2.0 μm), thoroughly washed with 99.0% by mass of dimethyl sulfoxide, and then washed with deionized water. The washed filter cake was dried at 80°C for 24 hours, and the mass was used as the insoluble content to calculate the soluble content using the following formula: Soluble content (%) = (1 - insoluble content / 0.1) * 100%

[0106] Example 1: Preparation of Long Fine Aramid Single Fibers 2 g of raw aramid fibers were dried in an oven at 105°C for 24 hours, added to 198 g of dimethyl sulfoxide to swell, and then decomposed for 60 minutes in a homomixer (Primix Corporation, MARKII2.5 type) at 10,000 rpm. This was then decomposed for 20 minutes in a clear mixer (M Technique Co., Ltd., CLM0.8) at 10,000 rpm to obtain a long fine aramid single fiber dispersion. The obtained long fine aramid single fiber dispersion was suction filtered through a membrane filter with a pore size of 0.2 μm and dried in an oven at 105°C. The maximum fiber diameter, minimum fiber diameter, maximum fiber length, minimum fiber length, soluble content, crystallinity, and glass transition temperature of the long fine aramid single fibers were measured (see Figures 1 to 3 and Table 1). Meanwhile, the obtained dispersion of long fine aramid single fibers was centrifuged at 16,000 G, the dimethyl sulfoxide was removed by decantation, and the precipitate was collected. N,N'-dimethylacetamide was added to the above precipitate, mixed, and centrifuged again at 16,000 G, followed by decantation to collect the precipitate. This procedure was repeated three times to obtain a paste of long fine aramid single fibers. The solid content concentration of the obtained paste was 10 mass %. Raw material aramid fiber: TAK. Inc. m-Aramid Floc, soluble content: 20 mass % Dimethyl sulfoxide: Fujifilm Wako Pure Chemical Industries, Ltd., special grade, 99.0 mass % Dimethylacetamide: Fujifilm Wako Pure Chemical Industries, Ltd., special grade, 99.0 mass %

[0107] Example 2: Preparation of a Composite Material Containing 10% by Mass of Long Fine Aramid Single Fibers. 10 g of dimethylacetamide and 10 g of acetone were mixed as a solvent, and 51.09 g of EPICLON 850 (main component), 28.27 g of TD-2131 (curing agent), and 0.794 g of 2P4MHZ-pw (curing accelerator) were added and stirred until the mixture was uniform in appearance. 80.1 g of the paste containing the long fine aramid single fibers prepared in Example 1 was then added. The resulting composition was stirred and degassed using a shear mixer (SK-300SII, manufactured by KAKUHUNTER) and cast onto a PET film. The cast PET film was heated on a hot plate at 100°C for 1 hour, and the pre-cured composition was peeled off from the film and cured in an oven at 170°C for 2 hours. The resulting composite material was washed with deionized water, dried, and its insulation resistance was measured (see Table 2). Bisphenol A epoxy resin EPICLON850 (DIC Corporation) Phenolite resin TD-2131 (DIC Corporation) 2-phenyl-4-methylimidazole 2P4MHZ-pw (Shikoku Chemicals Corporation) Acetone Fujifilm Wako Pure Chemical Industries, purity 99.5% by mass Dimethylacetamide Fujifilm Wako Pure Chemical Industries, special grade 99.0% by mass

[0108] Example 3: Preparation of a Composite Material Containing 30% by Mass of Long Fine Aramid Single Fibers. 10 g of dimethylacetamide and 10 g of acetone were mixed as a solvent, and 51.09 g of EPICLON 850 (main component), 28.27 g of TD-2131 (curing agent), and 0.794 g of 2P4MHZ-pw (curing accelerator) were added and stirred until the mixture was uniform in appearance. 240.3 g of the paste containing the long fine aramid single fibers prepared in Example 1 was then added. The resulting composition was stirred and degassed using a shear mixer (SK-300SII, manufactured by KAKUHUNTER) and cast onto a PET film. The cast PET film was heated on a hot plate at 100°C for 1 hour, and the pre-cured composition was peeled off from the film and cured in an oven at 170°C for 2 hours. The resulting composite material was washed with deionized water, dried, and the soluble content and glass transition temperature were measured (see Table 2). Bisphenol A epoxy resin EPICLON850 (DIC Corporation) Phenolite resin TD-2131 (DIC Corporation) 2-phenyl-4-methylimidazole 2P4MHZ-pw (Shikoku Chemicals Corporation) Acetone Fujifilm Wako Pure Chemical Industries, purity 99.5% by mass Dimethylacetamide Fujifilm Wako Pure Chemical Industries, special grade 99.0% by mass

[0109] Comparative Example 1: Preparation of Amorphous Aramid Nanofibers 3 g of potassium hydroxide and 4 g of water were stirred and added to 2 g of raw aramid fiber dried in an oven at 105°C for 24 hours and 198 g of dimethyl sulfoxide. The mixture was heated to 70°C and stirred for 60 minutes, and then 100 g of dimethyl sulfoxide was added to prepare an aramid fiber dispersion. The resulting aramid fiber dispersion was added to a container containing 2 L of deionized water while stirring. The deionized water was neutralized with sulfuric acid until the pH reached 7±0.1 as measured by a pH meter (HORIBA, Ltd. LAQUAtwin). The mixture was centrifuged at 16,000 G, the supernatant was removed by decantation, and 200 g of deionized water was added. The mixture was then centrifuged again at 16,000 G. The supernatant was removed by decantation, and 200 g of deionized water was added to obtain an amorphous aramid nanofiber dispersion. The obtained amorphous aramid nanofiber dispersion was dried, and the maximum fiber diameter, minimum fiber diameter, maximum fiber length, minimum fiber length, soluble content, and glass transition point were measured. Since the soluble content of the amorphous aramid nanofiber was 100%, the crystallinity of the amorphous aramid nanofiber was set to 0 (see Table 1). Meanwhile, 220 g of the obtained amorphous aramid nanofiber dispersion was dried in an oven at 105°C, and 198 g of dimethylacetamide was added and mixed until uniformly dispersed. The mixture was centrifuged at 16,000 G and decanted to obtain an amorphous aramid nanofiber paste. The solid content of the obtained paste was 10% by mass. Raw material aramid fiber: TAK. Inc. m-Aramid Floc soluble content 20% by mass Dimethyl sulfoxide Fujifilm Wako Pure Chemical Industries, Ltd. Special grade 99.0% by mass Dimethylacetamide Fujifilm Wako Pure Chemical Industries, Ltd. Special grade 99.0% by mass Potassium hydroxide Fujifilm Wako Pure Chemical Industries, Ltd. Special grade Purity 85.0% by mass Sulfuric acid Fujifilm Wako Pure Chemical Industries, Ltd. Special grade Purity 95.0% by mass

[0110] Comparative Example 2: Preparation of Composite Material Containing 10% by Mass of Amorphous Aramid Nanofibers A composite material was prepared in the same manner as in Example 2, except that the paste of amorphous aramid nanofibers obtained in Comparative Example 1 was used.

[0111] Comparative Example 3: Preparation of Composite Material Containing 30% by Mass of Amorphous Aramid Nanofibers A composite material was prepared in the same manner as in Example 3, except that the paste of amorphous aramid nanofibers obtained in Comparative Example 1 was used.

[0112] The maximum fiber diameter, minimum fiber diameter, maximum fiber length, minimum fiber length, soluble content, crystallinity and glass transition temperature (Tg) of the long fine aramid single fibers of Example 1 and the amorphous aramid nanofibers of Comparative Example 1 are shown in Table 1 below.

[0113]

[0114] The dielectric strengths of the composite materials of Example 2 and Comparative Example 2, and the glass transition points and soluble contents of Example 3 and Comparative Example 3 are shown in Table 2 below.

[0115]

[0116] The long fine aramid single fibers obtained in Example 1 had a maximum fiber diameter of 1,960 nm, a minimum fiber diameter of 300 nm, a maximum fiber length of 360 μm, a minimum fiber length of 70 μm, and a crystallinity of 80%, and were confirmed to be unbranched single fibers by SEM observation ( FIGS. 1 to 3 ). On the other hand, the amorphous aramid nanofibers obtained in Comparative Example 1 had a maximum fiber diameter of 200 nm, a minimum fiber diameter of 10 nm, a maximum fiber length of 10 μm, and a minimum fiber length of 0.1 μm. Furthermore, the insulation resistance (dielectric strength) of Examples 2 and 3, which are composite materials containing the long fine aramid single fibers and a matrix resin, was as high as 27.5 kV / mm, the glass transition temperature rose to 130°C or higher, and the soluble content was 0 mass%. On the other hand, the insulation resistance (dielectric strength) of Comparative Examples 2 and 3, which were composite materials containing amorphous aramid nanofibers and a matrix resin, was low at 6.0 kV / mm, the glass transition point was almost unchanged from that of the raw material epoxy resin (glass transition point: 93°C), and the soluble content was 12 mass%.

Claims

1. A long fine aramid single fiber having a maximum fiber diameter of less than 2000 nm and a minimum fiber length of more than 10 μm, which is expressed by the following formula: x = (1 - ΔQ ｘ / ΔQ ０ )×100(%) (in the formula ΔQ ｘ is the change in heat capacity of the long fine aramid single fiber before and after the glass transition point, ΔQ ０ The crystallinity x determined by the above-mentioned method (wherein x is the amount of change in heat capacity before and after the glass transition point of amorphous aramid) is 60% or more.

2. The long and fine aramid single fiber according to claim 1, having an average fiber diameter of less than 1000 nm.

3. The long and fine aramid single fiber according to claim 1, having an average aspect ratio of 100 or more.

4. The long and fine aramid single fiber according to claim 1, having a glass transition point of more than 300°C.

5. The long fine aramid filaments of claim 1, wherein said aramid is a meta-aramid.

6. A method for producing long and fine aramid single fibers according to claim 1, comprising: a swelling step of contacting raw aramid fibers having a soluble content in dimethyl sulfoxide of 10% by mass or more with a swelling liquid containing an aprotic solvent to cause swelling; a mechanical defibration step of finely pulverizing the raw aramid fibers by mechanical defibration; and a separation step of separating long and fine aramid single fibers having a fiber diameter of less than 2000 nm and a fiber length of more than 10 μm from the finely pulverized raw aramid fibers.

7. The process according to claim 6, wherein the aprotic solvent is dimethylsulfoxide and / or dimethylacetamide.

8. The method according to claim 6, wherein the glass transition point of said long fine aramid single fibers is at least 30° C. higher than the glass transition point of said raw material aramid fibers.

9. A composite material comprising the long fine aramid monofilament according to claim 1 and a matrix resin.

10. The composite material according to claim 9, wherein said long fine aramid monofilaments are dispersed in said matrix resin.

11. The composite material of claim 9, wherein said matrix resin is an epoxy resin.

12. A sheet comprising the long fine aramid monofilaments according to claim 1.

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