Use of aramid single fiber in curable resin

Abstract

The present invention pertains to a composite material comprising (a) at least one type of curable resin and (b) at least one type of aramid single fiber, wherein the average diameter of the aramid single fiber (b) as determined by a centrifugal sedimentation method is less than 2000 nm. According to the present invention, an article having excellent mechanical properties can be produced.

Description

Use of aramid single fibers in curable resins

[0001] The present invention relates to the use of very thin aramid single fibers.

[0002] Aromatic polyamides (aramids) include para - aromatic polyamides such as poly - p - phenylene terephthalamide and meta - aromatic polyamides such as poly - m - phenylene isophthalamide, both of which have been put into practical use as fibers.

[0003] In addition, very thin (fine) fibers made of aromatic polyamide have also been produced. For example, Patent Document 1 describes producing aramid nanofibers with an average fiber diameter of 20 nm by discharging a strong base dimethyl sulfoxide solution of aramid into water for spinning (see Patent Document 1

[0051] -

[0053] ).

[0004] Japanese Unexamined Patent Application Publication No. 2023 - 2089

[0005] However, the aramid nanofibers obtained by the method described in Patent Document 1 are only used after being coated on a porous polyolefin substrate.

[0006] An object of the present invention is to provide a composite material containing very thin aramid single fibers.

[0007] A first aspect [1] of the present invention is a composite material containing (a) at least one curable resin and (b) at least one aramid single fiber, wherein the average diameter of the (b) aramid single fiber by centrifugal sedimentation method is less than 2000 nm.

[0008] The first aspect of the present invention includes the following embodiments.

[0009] The composite material of [1], wherein the (a) curable resin is other than an epoxy resin.

[0010] The composite material of [1], wherein the (a) curable resin is selected from the group consisting of polyimide resin, urethane resin, phenol resin, urea resin, melamine resin, fluororesin, silicone resin, alkyd resin, unsaturated polyester resin, and mixtures thereof.

[0011] [4] The composite material of [1], wherein the curable resin in (a) is a polyimide resin.

[0012] [5] A composite material according to any of [1] to [4], wherein the amount of the curable resin (a) in the composite material is 50% by mass to 99.99% by mass, based on the total mass of the composite material.

[0013] [6] A composite material according to any of [1] to [5], wherein the minimum fiber length of the aramid monofiber in (b) is greater than 10 μm.

[0014] [7] The following formula for the aramid monofiber in (b) above: x = (1 - ΔQ ｘ / ΔQ ０ )×100(%) (in the formula ΔQ ｘ ΔQ is the change in heat capacity of the aramid single fiber before and after the glass transition point. ０ A composite material of any of the following types [1] to [6], wherein the degree of crystallinity x, determined by (the change in heat capacity before and after the glass transition of amorphous aramid), is 60% or more.

[0015] [8] A composite material of any of [1] to [7], wherein the maximum fiber diameter of the aramid monofibers (b) is less than 2200 nm.

[0016] [9] A composite material of any of [1] to [8], wherein the average aspect ratio of the aramid monofibers (b) is 100 or more.

[0017]

[10] A composite material according to any of [1] to [9], wherein the glass transition temperature of the (b) aramid monofiber is greater than 300°C.

[0018]

[11] A composite material according to any of [1] to

[10] , wherein the (b) aramid monofiber is made of metaaramid.

[0019]

[12] A composite material according to any of [1] to

[11] , wherein the amount of (b) aramid monofibers in the composite material is 0.001% by mass to 20% by mass, based on the total mass of the composite material.

[0020]

[13] A composite material according to any one of [1] to

[12] , further comprising (a) the curable resin, (b) the aramid monofibers, and (c) at least one dispersant.

[0021] A second aspect of the present invention is a cured product of any of the composite materials [1] to

[13] described above, and a molded article made from the cured product.

[0022] A third aspect

[16] of the present invention is a method for producing a molded article having enhanced mechanical properties selected from the group consisting of tensile strength and elastic modulus, comprising the steps of (a) mixing at least one curable resin and (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation to obtain a composite material, and curing and molding the composite material to obtain a molded article made of a cured product of the composite material.

[0023] A fourth aspect of the present invention

[17] is the use of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, and (a) a cured product of at least one curable resin, for the enhancement of at least one mechanical property selected from the group consisting of tensile strength and elastic modulus.

[0024] A fifth aspect

[18] of the present invention is an enhancer of at least one mechanical property selected from the group consisting of tensile strength and elastic modulus for a molded article consisting of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation method, and (a) a cured product of at least one curable resin.

[0025] The composite material of the present invention contains very fine aramid fibers, and the cured product of the composite material exhibits excellent mechanical properties, at least one of which is selected from the group consisting of tensile strength and elastic modulus. Therefore, the composite material of the present invention can be used in the manufacture of articles having excellent mechanical properties.

[0026] Molded articles made from the cured material of the present invention can exhibit excellent mechanical properties.

[0027] Furthermore, the manufacturing method of the present invention can produce molded articles that have excellent mechanical properties, at least one of which is selected from the group consisting of tensile strength and elastic modulus.

[0028] Furthermore, the use and enhancer of the present invention can improve at least one mechanical property selected from the group consisting of tensile strength and elastic modulus of an article made of a cured product of a curable resin.

[0029] As a result of diligent research, the inventors have found that it is possible to provide a composite material containing extremely fine aramid monofibers.

[0030] Conventionally, reinforcing fibers such as glass fibers and carbon fibers have been blended into curable resins to improve their mechanical properties. However, the reinforcing fibers used so far have mainly been large inorganic fibers, and it was unclear whether even very thin aramid monofilaments could improve the mechanical properties of curable resins, particularly their tensile strength.

[0031] Furthermore, the inventors have found that even very fine aramid monofibers can improve at least one mechanical property selected from the group consisting of tensile strength and elastic modulus of the cured product of a curable resin by being blended with the resin.

[0032] The present invention will be described below.

[0033] The composite material of the present invention comprises (a) at least one curable resin and (b) at least one aramid monofiber, wherein the average diameter of the (b) aramid monofiber by centrifugal sedimentation is less than 2000 nm.

[0034] (Curable resin) The composite material of the present invention comprises (a) at least one curable resin. The composite material of the present invention may comprise a single curable resin or two or more curable resins.

[0035] (a) The curable resin is preferably the matrix of the composite material of the present invention. That is, in the composite material of the present invention, (a) the curable resin is preferably (b) aramid monofiber, as described later.

[0036] It is preferable that the (b) aramid monofilaments described later are dispersed in the (a) curable resin. Since the (b) aramid monofilaments are unbranched, they are easily mixed with the (a) curable resin and dispersed in the (a) curable resin.

[0037] (a) The curable resin is not particularly limited as long as it is a resin having the property of curing by the action of various energies such as heat and light, or substances such as moisture, and may be any of thermosetting resins, moisture-curing resins, or photocurable resins. In the present invention, the "curable resin" is not a thermoplastic resin.

[0038] In the present invention, "curing" means both polymerization of monomers and crosslinking of polymers. For example, in polymer chemistry, polymerization of monomers, crosslinking between polymer (prepolymer) molecules formed by such polymerization, and crosslinking within the polymer (prepolymer) molecules correspond to "curing" in the present invention. In the present invention, "curing" proceeds irreversibly.

[0039] Therefore, generally, the hardness of the cured product of the (a) curable resin is often higher than that of the (a) curable resin, but in the present invention, this is not necessarily the case. Also, the cured product of the (a) curable resin does not necessarily have an absolutely high hardness. For example, the urethane sponge obtained by foaming the urethane resin described later is porous and can have flexible characteristics.

[0040] The (a) curable resin is preferably a thermosetting resin. Examples of the thermosetting resin include polyimide resin, urethane resin, phenol resin, urea resin, melamine resin, fluororesin, silicone resin, alkyd resin, unsaturated polyester resin, etc.

[0041] In one aspect of the present invention, the (a) curable resin is other than an epoxy resin. In this aspect, the (a) curable resin is preferably selected from the group consisting of polyimide resin, urethane resin, phenol resin, urea resin, melamine resin, fluororesin, silicone resin, alkyd resin, unsaturated polyester resin, and mixtures thereof.

[0042] The (a) curable resin is more preferably a polyimide resin. [[ID=回ID=19]]

[0043] The "polyimide resin" in the present invention includes polyimide after curing and a precursor (polyimide precursor) that cures to become polyimide.

[0044] The polyimide resin may be either an aromatic polyimide or an aliphatic polyimide.

[0045] The aromatic polyimide can be formed from an aromatic carboxylic acid component and / or an aromatic amine component.

[0046] As the aromatic carboxylic acid component, an aromatic tetracarboxylic acid or its anhydride is preferable. For example, pyromellitic acid or its dianhydride, 4,4'-oxydiphthalic acid or its dianhydride, benzophenone tetracarboxylic acid or its dianhydride, bis(3,4-dicarboxyphenyl)sulfone or its acid anhydride, 3,3',4,4'-biphenyltetracarboxylic acid or its dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid or its dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid or its dianhydride, 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane or its dianhydride, etc. can be mentioned. Pyromellitic acid or its dianhydride, biphenyltetracarboxylic dianhydride, benzophenone tetracarboxylic dianhydride are preferable.

[0047] As the aromatic amine component, an aromatic diamine is preferable. For example, 4,4'-bis(metaaminophenoxy)biphenyl, 3,4'-oxydianiline, 4,4'-oxydianiline, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, diaminodiphenylsulfone, diaminodiphenylamine, paraphenylenediamine, metaphenylenediamine, diaminobiphenyl, diaminoterphenyl, 1,3-bis(paraa minophenoxy)benzene, 1,3-bis(metaaminophenoxy)benzene, 4,4'-bis(paraa minophenoxy)biphenyl, 4,4'-bis(paraa minophenoxy)diphenylsulfone, etc. can be mentioned. Oxydianiline, paraphenylenediamine, benzophenone diamine are preferable.

[0048] The aliphatic polyimide can be formed from an aliphatic carboxylic acid component and / or an aliphatic amine component.

[0049] Examples of aliphatic carboxylic acid components include cyclohexane-1,2,4,5-tetracarboxylic dianhydride, cyclohexane-cis-1,2,3,4-tetracarboxylic dianhydride, cyclohexane-cis-1,2-trans-3,4-tetracarboxylic dianhydride, 5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, and 1,5-cyclooctadiene-1,2,5,6-tetracarboxylic anhydride, which has a cyclooctadiene structure. Examples include monocyclic tetracarboxylic acid dianhydrides such as tetracarboxylic acid dianhydrides; and polycyclic tetracarboxylic acid dianhydrides such as bicyclo[2.2.1]heptane-2,3,5,6-tetracarboxylic acid dianhydride, bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic acid dianhydride, 1-methyl-bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic acid dianhydride, bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic acid dianhydride, and bicyclo[3.3.0]octane-2,4,6,7-tetracarboxylic acid dianhydride.

[0050] Examples of aliphatic amine components include cyclic diamines such as 5-amino-1,3,3-trimethylcyclohexanemethylamine (isophorone diamine), 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, 1,4-cyclohexanebis(methylamine), and 1,3-cyclohexanebis(methylamine); 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-hexanediamine, 1, Examples include linear diamines such as 7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,11-undecanediamine, and 1,12-dodecanediamine; and branched diamines such as 1,2-diaminopropane, 1,2-diamino-2-methylpropane, 1,3-diamino-2-methylpropane, 1,3-diamino-2,2-dimethylpropane, 1,3-diaminopentane, and 1,5-diamino-2-methylpentane.

[0051] (a) The curable resin is more preferably an aromatic polyimide, and more preferably a fully aromatic polyimide formed from an aromatic carboxylic acid component and an aromatic amine component.

[0052] Examples of polyimide precursors include reaction intermediates of carboxylic acid and diamine components.

[0053] As a precursor for aromatic polyimides, polyamic acids (polyamic acids), which are reaction products of aromatic tetracarboxylic dianhydrides and aromatic diamines, are preferred. For example, in the reaction of pyromellitic dianhydride with 4,4'-oxydianiline, the polyamic acid of formula (1) is obtained by the following reaction.

[0054] Furthermore, the polyamic acid of formula (1) can be cyclized by heating or other means to form the polyimide of formula (2), as shown below.

[0055] The above polyimide is a cured product of its precursor, polyamic acid.

[0056] Examples of precursors for aliphatic polyimides include polyamic acids, which are reaction products of aliphatic tetracarboxylic dianhydrides and aliphatic diamines.

[0057] (a) As curable resins, polyamic acid (precursor), which is a reaction product of an aromatic tetracarboxylic dianhydride and an aromatic diamine, and aromatic polyimide, which is a cyclized product (cured product) of said polyamic acid, are particularly preferred.

[0058] The composite material of the present invention may contain (a) a curable resin in an amount of, for example, 50% by mass or more, preferably 60% by mass or more, and more preferably 70% by mass or more, based on the total mass of the composite material. On the other hand, the composite material of the present invention may contain (a) a curable resin in an amount of, for example, 99.99% by mass or less, preferably 99.95% by mass or less, and more preferably 99.9% by mass or less, based on the total mass of the composite material. Therefore, the composite material of the present invention may contain (a) a curable resin in an amount of, for example, 50% by mass to 99.99% by mass, preferably 60% by mass to 99.95% by mass, and more preferably 70% by mass to 99.9% by mass, based on the total mass of the composite material.

[0059] (Aramid monofibers) The composite material of the present invention comprises (b) at least one aramid monofiber. The composite material of the present invention may comprise a single aramid monofiber or two or more aramid monofibers.

[0060] <Aramid> In this invention, "aramid" means aromatic polyamide. In this invention, "aromatic polyamide" means a linear polymer compound in which 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more of the amide bonds are directly bonded to an aromatic ring.

[0061] 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 poly(para-phenylene terephthalamide) and its copolymers, and poly(para-phenylene)-copoly(3,4'-diphenyl ether) terephthalamide (poly(para-phenylene)-copoly(3,4'-diphenyl ether) terephthalamide). Examples of meta-aramids include poly(meta-phenylene isophthalamide) and its copolymers. These aramids are manufactured industrially, for example, by conventionally known interfacial polymerization methods, solution polymerization methods, etc., and are available commercially, but are not limited to these. In the present invention, meta-aramids are preferably selected. Meta-aramids have features such as being soluble in general-purpose amide solvents, being capable of wet molding using polymer solutions as starting materials, having excellent heat-sealability, and having good heat resistance and flame retardancy. Among metaaramids, polymetaphenylene isophthalamide is preferred because it possesses excellent moldability, heat adhesion, flame retardancy, heat resistance, and other properties.

[0062] <Monofilament> (b) Aramid monofilament is a monofilament. In this invention, "monofilament" means a fiber that is not branched, and does not mean a branched fiber such as fibrillated fiber.

[0063] (b) Aramid monofilaments are very fine (i.e., microscopic) fibers with an average diameter of less than 2000 nm, measured by centrifugal sedimentation.

[0064] The centrifugal sedimentation method is a method of measuring the particle size of an object by applying centrifugal force to a liquid containing the substance to be measured and determining the particle size of the object from its sedimentation velocity in the liquid. It is widely used for measuring the particle size of fine particles.

[0065] The settling velocity of an object in a liquid depends on its density, size, and other factors. Therefore, in centrifugal sedimentation, a calibration curve showing the relationship between settling velocity and particle size is created in advance using spherical standard particles with known density and particle size. A spherical standard particle with the same settling velocity as the substance being measured is identified, and the particle size of the substance being measured is determined from this calibration curve, taking density into consideration. Consequently, the diameter measured by centrifugal sedimentation is the spherical equivalent diameter or spherical diameter (Stokes diameter).

[0066] Particle size measurement by centrifugal sedimentation can be performed in accordance with the uniform sedimentation method of JIS Z8823-2:2016. For example, this can be done using the Partica Centrifuge centrifugal nanoparticle analyzer manufactured by Horiba, Ltd., and spherical silica standard particles.

[0067] In this invention, the average diameter determined by centrifugal sedimentation is the arithmetic mean of the Stokes diameter calculated from the area-based particle size distribution measured by centrifugal sedimentation. It should be noted that, if necessary, software can mathematically convert the area-based distribution measured by centrifugal sedimentation to various other distributions such as number-based distributions and volume-based distributions, and such conversions are well known.

[0068] (b) The average diameter of the aramid monofilament obtained by centrifugal sedimentation is preferably less than 1950 nm, more preferably less than 1900 nm, even more preferably less than 1850 nm, and even more preferably less than 1800 nm.

[0069] (b) The average diameter of the aramid monofilament obtained by centrifugal sedimentation is preferably greater than 400 nm, more preferably greater than 600 nm, even more preferably greater than 800 nm, even more preferably greater than 1000 nm, and even more preferably greater than 1200 nm.

[0070] (b) The average diameter of the aramid monofilament obtained by centrifugal sedimentation is, for example, more than 400 nm and less than 2000 nm, preferably more than 600 nm and less than 1950 nm, more preferably more than 800 nm and less than 1900 nm, even more preferably more than 1000 nm and less than 1850 nm, and even more preferably more than 1200 nm and less than 1800 nm.

[0071] (b) The maximum fiber diameter of the aramid monofilament is preferably less than 2200 nm, more preferably less than 2100 nm, and even more preferably less than 2000 nm. (b) The maximum fiber diameter of the aramid monofilament may be 1000 nm or more.

[0072] (b) The aramid monofilament preferably has a minimum fiber diameter greater than 50 nm, more preferably greater than 100 nm, and even more preferably greater than 150 nm. (b) The minimum fiber diameter of the aramid monofilament preferably is 500 nm or less, more preferably 450 nm or less, and even more preferably 400 nm or less.

[0073] (b) The maximum and minimum fiber diameters of an aramid monofilament can be determined in a set of a predetermined number of fibers. Specifically, (b) the fiber diameters of 100 aramid monofilaments can be measured by electron microscopy such as SEM, and the largest fiber among them can be taken as the maximum fiber diameter, and the smallest fiber as the minimum fiber diameter.

[0074] (b) The aramid monofiber is preferably a relatively long fiber with a minimum fiber length of more than 10 μm. (b) The minimum fiber length of the aramid monofiber 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, there is no particular upper limit to the minimum fiber length of the aramid monofiber, but it can be less than 100 μm, for example.

[0075] (b) The maximum fiber length of the aramid monofilament is preferably less than 1000 μm, more preferably less than 800 μm, and even more preferably less than 600 μm. (b) The maximum fiber length of the aramid monofilament is preferably 200 μm or more, more preferably 250 μm or more, and even more preferably 300 μm or more.

[0076] (b) The number-average fiber length of the aramid monofilaments is preferably greater than 20 μm, more preferably greater than 50 μm, and even more preferably greater than 80 μm. Also, (b) the number-average fiber length of the aramid monofilaments is preferably less than 400 μm, more preferably less than 300 μm, and even more preferably less than 200 μm. (b) The number-average fiber length of the aramid monofilaments is preferably greater than 20 μm and less than 400 μm, preferably greater than 50 μm and less than 300 μm, and even more preferably greater than 80 μm and less than 200 μm.

[0077] (b) The minimum fiber length, maximum fiber length, and number-average fiber length of an aramid monofilament can be determined in a set of a predetermined number of fibers. Specifically, (b) the fiber lengths of 100 aramid monofilaments can be measured by electron microscopy such as SEM, and the smallest fiber can be taken as the minimum fiber length, the largest fiber as the maximum fiber length, and the number-average value of the fiber lengths can be taken as the number-average fiber length.

[0078] (b) The aramid monofiber is preferably a fiber with a relatively high aspect ratio. (b) The minimum aspect ratio of the aramid monofiber is preferably 80 or higher, more preferably 90 or higher, and even more preferably 100 or higher. On the other hand, (b) the maximum aspect ratio of the aramid monofiber is not particularly limited, but can be less than 500, for example. (b) The number mean aspect ratio of the aramid monofiber is preferably 100 or higher, more preferably 150 or higher, and even more preferably 200 or higher. (b) The upper limit of the number mean aspect ratio of the aramid monofiber is not particularly limited, but can be less than 400, for example.

[0079] (b) The minimum aspect ratio, maximum aspect ratio, and number-average aspect ratio of aramid monofilaments can also be determined in a set of a predetermined number of fibers. Specifically, (b) the fiber lengths of 100 aramid monofilaments can be measured by electron microscopy observation such as SEM, and the aspect ratio of each fiber can be determined from the fiber length and fiber diameter of each fiber. Among these, the smallest can be taken as the minimum aspect ratio, the largest as the maximum aspect ratio, and the number-average value of these can be taken as the number-average aspect ratio.

[0080] (b) It is preferable that the aramid monofiber has a relatively high degree of crystallinity. That is, (b) the aramid monofiber is given by the following formula x = (1 - ΔQx / ΔQ ０ ) × 100 (%) (wherein ΔQx is the change in heat capacity of (b) aramid monofiber before and after the glass transition point, and ΔQ ０ (b) The degree of crystallinity x, determined by (b) the change in heat capacity around the glass transition point of amorphous aramid, is preferably 60% or more. (b) The degree of crystallinity of the aramid monofiber is more preferably 70% or more, and even more preferably 80% or more. (b) There is no particular upper limit on the degree of crystallinity of the aramid monofiber, but for example it can be 95% or less or 90% or less.

[0081] As the amorphous aramid, an aramid that is completely soluble in dimethyl sulfoxide or N-methyl-2-pyrrolidone (in the absence of a base) at room temperature (25°C) can be used.

[0082] ΔQx and ΔQ in the above formula ０ This can be determined by differential scanning calorimeter. Specifically, the differential scanning calorimeter plots the change in specific heat capacity with respect to temperature on a plane where the vertical axis is specific heat capacity and the horizontal axis is temperature, thereby determining the temperature-dependent specific heat capacity baseline. The glass transition point is then identified by the shift location of the baseline, and the amount of change in heat capacity can be determined from the baseline shift interval before and after the transition point. The baseline shift interval can be determined by linearly approximating the baseline.

[0083] The measurement or determination of the crystallinity of polymers using the above formula is publicly known, as described, for example, in the Journal of Polymer Science, Vol. 31, No. 2, pp. 138-139 (1972).

[0084] (b) The aramid monofiber preferably has a glass transition temperature above 300°C, more preferably above 310°C, and even more preferably above 320°C. A high glass transition temperature means that there are fewer amorphous portions undergoing Brownian motion and that the crystallinity is high.

[0085] (b) Aramid monofilaments are monofilaments and unbranched, and therefore have excellent miscibility with resins and the like. (b) It is preferable that aramid monofilaments have a form that can be well dispersed in a matrix such as a resin. Therefore, (b) it is preferable that aramid monofilaments do not have the form of a fiber bundle in which multiple fibers are bundled in one direction.

[0086] (b) Although aramid monofilaments are fine, when combined with resins or other materials, they can impart excellent mechanical properties.

[0087] Furthermore, (b) when the aramid monofilament has high crystallinity, the fiber shape is stable and can be well maintained against various physical or chemical reactions.

[0088] <Method for producing aramid monofilaments> (b) The method for producing aramid monofilaments is not particularly limited, but for example, it can be produced by a method that includes: a swelling step of contacting raw material aramid fibers having a soluble content of 10% by mass or more in dimethyl sulfoxide or N-methyl-2-pyrrolidone with a swelling liquid containing an aprotic solvent to cause swelling; a mechanical defibration step of finely refining the raw material aramid fibers by mechanical defibration; and, if necessary, a separation step of separating aramid monofilaments with an average diameter of less than 2000 nm from the finely refined raw material aramid fibers by centrifugal sedimentation.

[0089] (b) The above method for producing aramid monofilaments is characterized by swelling a predetermined raw material aramid fiber and mechanically defibridating it to form monofilaments, rather than spinning an aramid solution. In other words, (b) aramid monofilaments are not fibers produced by spinning (spun fibers) or made based on such fibers.

[0090] (b) The above method for producing aramid monofilaments allows for the simple production of very fine aramid fibers.

[0091] Furthermore, while it is difficult to produce relatively long aramid fibers when spinning an aramid solution, the above method for producing (b) aramid monofilaments makes it easy to produce even relatively long (b) aramid monofilaments.

[0092] (b) The above method for producing aramid monofilaments can achieve high productivity and is industrially advantageous. On the other hand, although it is known that aramid monofilaments can be produced by electrospinning, electrospinning has low productivity and is unfavorable for industrial production.

[0093] The following describes (b) the method for producing aramid monofilaments.

[0094] Raw materials: (b) In the above method for producing aramid monofibers, aramid fibers having a soluble content of dimethyl sulfoxide or N-methyl-2-pyrrolidone of 10% by mass (weight) or more are used as raw materials. The soluble content of the raw material aramid fibers is based on the total mass (total weight) of the raw material aramid fibers.

[0095] As raw material aramid fibers with a soluble content of 10% by mass (weight) or more in dimethyl sulfoxide or N-methyl-2-pyrrolidone, aramid fibers can be used that show a mass (weight) change of 10% or more when immersed in dimethyl sulfoxide or N-methyl-2-pyrrolidone (with a purity of 99% or more and free from bases such as sodium hydroxide and potassium hydroxide) at room temperature (25°C) for 72 hours.

[0096] Raw material aramid fibers with a soluble content of 10% by mass or more in dimethyl sulfoxide or N-methyl-2-pyrrolidone are prone to solvation of the amorphous portion and swelling of the fibers. The soluble content of raw material aramid fibers in dimethyl sulfoxide or N-methyl-2-pyrrolidone is not particularly limited as long as it is 10% by mass or more, for example, it may be 15% by mass or more, 20% by mass or more, 25% by mass or more, or 30% by mass or more. On the other hand, the soluble content of raw material aramid fibers in dimethyl sulfoxide or N-methyl-2-pyrrolidone is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0097] The form of the raw material aramid fiber is not particularly limited, and either branched or unbranched fibers can be used. For example, the raw material aramid fiber may be in the form of aramid floc or aramid fibrils.

[0098] "Aramid floc" refers to short fibers made of aramid. The number-average fiber length of aramid floc is not particularly limited, but can be in the range of 1 to 50 mm, preferably 2 to 40 mm, and more preferably 3 to 30 mm. Similarly, the number-average fiber diameter of aramid floc is not particularly limited, but can be in the range of 10 to 30 μm, preferably 12 to 28 μm, and more preferably 14 to 26 μm.

[0099] The number-average fiber length and number-average fiber diameter of aramid flocs can be obtained by averaging the length and width of a predetermined number of aramid flocs (e.g., 100). Specifically, the fiber length and fiber diameter of 100 aramid flocs can be measured by electron microscopy (SEM) observation, and the number-average value of the fiber length can be used as the number-average fiber length, and the number-average value of the fiber diameter can be used as the number-average fiber diameter.

[0100] "Aramid fibrid" refers to film-like or fibrous fine particles made of aramid, and is sometimes called aramid pulp (see Japanese Patent Publication No. 35-11851, Japanese Patent Publication No. 37-5752, etc., for more information on aramid fibrid). Since fibrid has papermaking properties similar to ordinary wood (cellulose) pulp, it can be dispersed in water and then formed into a sheet using a paper machine.

[0101] The number-average fiber diameter of the aramid fibrils is not particularly limited, but can be, for example, 4 to 40 μm, preferably in the range of 5 to 35 μm, and more preferably in the range of 6 to 30 μm.

[0102] The number-average fiber diameter of aramid fibrils can be obtained by averaging the widths of a predetermined number of aramid fibrils (e.g., 100). Specifically, the fiber diameters of 100 aramid fibrils can be measured by electron microscopy (SEM) observation, and the number-average value of these fiber diameters can be used as the number-average fiber diameter.

[0103] Swelling step: (b) In the swelling step of the above method for producing aramid monofilaments, raw aramid fibers having a soluble content of 10% by mass (weight) or more in dimethyl sulfoxide or N-methyl-2-pyrrolidone are brought into contact with a swelling liquid containing an aprotic solvent and swelled.

[0104] The aprotic solvent is not particularly limited as long as it can swell the raw material aramid fibers, but dimethyl sulfoxide, N-methyl-2-pyrrolidone, dimethylacetamide, or mixtures thereof can be preferably used.

[0105] It is thought that the aprotic solvent breaks the hydrogen bonds between the less crystalline parts of the raw aramid fibers, making it easier to separate the more crystalline parts into fine fibers.

[0106] The swelling liquid may contain other optional components besides aprotic solvents such as acids, bases, salts, and water, as needed. The amount of optional components is not particularly limited, but for example, it may be 0.001 to 10% by mass, 0.01 to 5% by mass, or 0.1 to 1% by mass, based on the total mass (total weight) of the swelling liquid.

[0107] The manner in which the raw aramid fibers are brought into contact with the swelling liquid is not particularly limited. For example, the raw aramid fibers can be immersed in the swelling liquid, or the swelling liquid can be sprayed or misted onto the raw aramid fibers. The method of immersing the raw aramid fibers in the swelling liquid is preferred.

[0108] The contact temperature and contact time are not particularly limited, as long as the raw aramid fibers swell. For example, they can be in the range of room temperature (25°C) to 40°C and 10 minutes to 1 hour.

[0109] The aramid fibers, which have been swollen in the swelling process, are then subjected to the mechanical defibration process.

[0110] Mechanical defibration process: (b) In the mechanical defibration process of the above method for producing aramid monofilaments, the raw aramid fibers after the swelling process are refined by mechanical defibration. By refining, aramid monofilaments are obtained.

[0111] It is believed that after the aforementioned swelling process, hydrogen bonds between the less crystalline portions in the raw aramid fibers are broken, making these portions easier to separate. Then, through mechanical defibration, physical forces such as shear force are applied, separating the fine fibers from the raw aramid fibers.

[0112] Mechanical defibration can be carried out by known methods, as long as the defibration is mechanical. It is preferable to apply shear force to the swollen raw aramid fibers during mechanical defibration.

[0113] The equipment used for mechanical defibration is not particularly limited, and for example, devices that apply impact 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, and high-pressure homogenizers can be used. It is preferable to use a ClearMix device (for example, one manufactured by MTECHNIQUE) equipped with a high-speed rotating rotor and screens arranged with a small clearance.

[0114] Separation process: If the average diameter of the fine fibers obtained in the above mechanical defibration process is less than 2000 nm by centrifugal sedimentation, the fine fibers can be used as (b) aramid single fibers as is. Otherwise, a separation process can be carried out to separate the (b) aramid single fibers from the fine fibers.

[0115] In the separation process, aramid monofilaments are separated from the fine fibers obtained in the mechanical defibration process by centrifugal sedimentation so that the average diameter is less than 2000 nm.

[0116] The separation method is not particularly limited and can be carried out by known methods. For example, by diluting the fine fibers obtained in the mechanical defibrillation process, centrifuging them, and filtering the supernatant, aramid single fibers with an average diameter of less than 2000 nm can be separated from the fine fibers by centrifugal sedimentation.

[0117] An aprotic solvent is preferred as the dilution medium (liquid). Dimethyl sulfoxide, N-methyl-2-pyrrolidone, dimethylacetamide, or mixtures thereof are preferably used as aprotic solvents. The dilution medium may optionally contain other optional components besides the aprotic solvent, such as acids, bases, salts, or water. The amount of these optional components is not particularly limited, but can be, for example, 0.001 to 10% by mass, 0.01 to 5% by mass, or 0.1 to 1% by mass, based on the total mass (total weight) of the dilution medium (liquid).

[0118] By the above manufacturing method, (b) aramid monofilament can be obtained.

[0119] In particular, the above manufacturing method can produce (b) aramid monofilaments having a glass transition temperature that is 30°C or more higher than that of the raw material aramid fibers. A high glass transition temperature means that there are fewer amorphous portions undergoing Brownian motion and that the material is highly crystallin. Therefore, the above manufacturing method can produce (b) aramid monofilaments with a higher degree of crystallinity than the raw material aramid fibers.

[0120] The composite material of the present invention may contain (b) aramid monofibers in an amount of, for example, 0.001% by mass or more, preferably 0.005% by mass or more, and more preferably 0.01% by mass or more, relative to the total mass of the composite material. On the other hand, the composite material of the present invention may contain (b) aramid monofibers in an amount of, for example, 20% by mass or less, preferably 10% by mass or less, and more preferably less than 5% by mass, relative to the total mass of the composite material. Therefore, the composite material of the present invention may contain (b) aramid monofibers in an amount of, for example, 0.001% by mass to 20% by mass, preferably 0.005% by mass to 10% by mass, and more preferably 0.01% by mass or more and less than 5% by mass, relative to the total mass of the composite material.

[0121] (b) Despite being extremely thin, aramid monofilaments can improve the mechanical properties of cured composite materials even in small quantities.

[0122] (Dispersant) The composite material of the present invention may further contain (a) an at least one dispersant that enhances the dispersibility of (b) aramid monofibers in a curable resin. The composite material of the present invention may contain a single (c) dispersant or two or more dispersants.

[0123] (c) Dispersants can improve the dispersibility of (a) aramid monofibers in (b) curable resin by, for example, improving the wettability of (a) aramid monofibers in (b) curable resin.

[0124] The type of dispersant is not particularly limited, and known dispersants such as epoxy group-containing compounds, isocyanate compounds, surfactants (e.g., cationic surfactants), silicone oils, and waxes can be used as appropriate.

[0125] The composite material of the present invention may contain (c) a dispersant in an amount of, for example, 0.001% by mass or more, preferably 0.01% by mass or more, and more preferably 0.1% by mass or more, relative to the total mass of the composite material. On the other hand, the composite material of the present invention may contain (c) a dispersant in an amount of, for example, 20% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less, relative to the total mass of the composite material. Therefore, the composite material of the present invention may contain (c) a dispersant in an amount of, for example, 0.001% by mass to 20% by mass, preferably 0.01% by mass to 15% by mass, and more preferably 0.1% by mass to 10% by mass, relative to the total mass of the composite material.

[0126] (Additives) The composite material of the present invention may further contain other additives. These additives can be used individually or in combination of two or more.

[0127] Other additives that can be used include various known additives, such as (b) various inorganic fibers other than aramid monofibers, such as glass fibers and carbon fibers, inorganic fillers such as silica and mica, colorants (dyes, pigments, etc.), conductive agents, flame retardants (phosphorus-based flame retardants, halogen-based flame retardants, inorganic flame retardants, etc.), flame retardant aids, plasticizers, lubricants, stabilizers (antioxidants, ultraviolet absorbers, light stabilizers, heat stabilizers, etc.), mold release agents, antistatic agents, flow regulators, leveling agents, defoamers, antibacterial agents, preservatives, etc.

[0128] The composite material of the present invention may contain other additives in an amount of, for example, 0.001% to 20% by mass, preferably 0.01% to 15% by mass, and more preferably 0.1% to 10% by mass, relative to the total mass of the composite material.

[0129] [Method for producing composite materials] The composite materials of the present invention can be produced by mixing or kneading (a) a curable resin and (b) aramid monofibers together with (c) other components such as dispersions, as needed.

[0130] For example, the composite material of the present invention can be produced by adding (b) aramid monofibers to (a) a curable resin that has fluidity before curing, and mixing or kneading them. The mixing and kneading means are not particularly limited, but examples include a Henschel mixer, mixing roller, kneader, Banbury mixer, and extruder (e.g., a single-screw or twin-screw extruder).

[0131] (b) In terms of the dispersibility of aramid monofibers, (a) it is preferable that the curable resin is liquid at room temperature (25°C). For example, if (a) the curable resin is a polyimide resin, then (a) the curable resin is preferably a polyimide precursor in solution form.

[0132] The mixing and kneading time is not particularly limited, but can be, for example, 1 minute to 2 hours, preferably 5 minutes to 1.5 hours, and more preferably 10 minutes to 1 hour.

[0133] The composite material of the present invention hardens into a cured product upon the action of various energies such as heat and light, or substances such as moisture. Hardening by heating is preferred. Since the composite material of the present invention is fluid before hardening, the shape of the cured product can be easily designed. Therefore, the composite material of the present invention and its cured product can be suitably used in a variety of applications.

[0134] The composite material of the present invention is preferably not an adhesive.

[0135] [Molded Articles] The present invention also relates to molded articles made of cured composite materials of the present invention. Here, the word "made of" is synonymous with "including" unless otherwise defined. Therefore, molded articles of the present invention may include any additional components or materials in addition to the cured composite materials of the present invention.

[0136] The molded article of the present invention preferably contains a cured product of the composite material of the present invention as its main component. The proportion of the cured product of the composite material of the present invention in the molded article of the present invention is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the molded article, and it is particularly preferable that the molded article of the present invention consists solely of the cured product of the composite material of the present invention.

[0137] The molded article of the present invention can be obtained by molding and curing the composite material of the present invention using a known molding method. There are no particular restrictions on the shape of the molded article of the present invention, but examples include films, sheets, cylinders, and cylindrical columns.

[0138] There are no particular limitations on the molding method used to obtain the molded article of the present invention, but examples include molding without using a mold or die, such as molding a thin film by casting, and molding using a mold or die, such as injection molding, press molding, casting, vacuum molding, and transfer molding.

[0139] The molded articles of the present invention can possess excellent mechanical properties. For example, the molded articles of the present invention have excellent tensile strength, elastic modulus, and the like.

[0140] Furthermore, the molded product of the present invention exhibits excellent dimensional stability.

[0141] The molded articles of the present invention can be used in a wide range of applications, but are particularly preferred for use in vehicle bodies and engine parts for automobiles, railway vehicles, bicycles, etc., civil engineering and construction materials, aircraft and ship parts, etc., where high strength and high impact resistance are required.

[0142] [Method for Manufacturing Molded Articles] The present invention also relates to a method for manufacturing molded articles having enhanced mechanical properties, selected from the group consisting of tensile strength and elastic modulus, comprising the steps of (a) mixing at least one curable resin and (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation to obtain a composite material, and curing and molding the composite material to obtain a molded article made of a cured product of the composite material.

[0143] The description of (a) curable resin and (b) aramid monofibers included in the composite material of the present invention applies to (a) curable resin and (b) aramid monofibers according to the above method. Here, the word "consists of" is synonymous with "includes" unless otherwise defined. Therefore, the molded article may include any additional components or materials in addition to the cured product.

[0144] The above molded article preferably contains as its main component a cured product of a composite material containing (a) a curable resin and (b) aramid monofibers. The proportion of the above cured product in the above molded article is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the molded article. It is particularly preferable that the above molded article consists solely of the above cured product.

[0145] The mixing method in the step of obtaining a composite material by mixing (a) a curable resin and (b) aramid monofibers is not particularly limited, and known methods can be used. The mixing method may be simple mixing using a stirrer or the like, or kneading using a kneader, extruder or the like. It is preferable to disperse (b) aramid monofibers in (a) the curable resin.

[0146] (b) In terms of the dispersibility of aramid monofibers, (a) it is preferable that the curable resin is liquid at room temperature (25°C). For example, if (a) the curable resin is a polyimide resin, then (a) the curable resin is preferably a polyimide precursor in solution form.

[0147] The temperature at which (a) the curable resin and (b) the aramid monofiber are mixed depends on the type of (a) curable resin, but if (a) the curable resin is a thermosetting resin, a temperature that does not cause heat curing, such as room temperature (25°C), is preferred.

[0148] There are no particular restrictions on the type of molding used in the process of curing and molding the composite material, but examples include casting, injection molding, press molding, casting, vacuum molding, and transfer molding.

[0149] Molded articles manufactured by the above method can possess excellent mechanical properties. For example, the above method can be used to produce molded articles in which at least one mechanical property selected from the group consisting of tensile strength and elastic modulus is enhanced.

[0150] Furthermore, molded products manufactured using the above method exhibit excellent dimensional stability.

[0151] [Use] The present invention also relates to the use of a molded article comprising (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, and (a) a cured product of at least one curable resin, for the enhancement of at least one mechanical property selected from the group consisting of tensile strength and elastic modulus. Here, the word "comprising" is synonymous with "including" unless otherwise defined. Therefore, the molded article may include any additional components or materials in addition to the cured product.

[0152] The above molded article preferably contains the above cured material as its main component. The proportion of the above cured material in the above molded article is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the molded article. It is particularly preferable that the above molded article consists solely of the above cured material.

[0153] The above-mentioned descriptions of (a) curable resin and (b) aramid monofibers apply to the use of these materials.

[0154] The present invention can be used by curing a composite material obtained by blending (b) at least one type of aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation with (a) at least one type of curable resin to obtain a molded product. The blending method is not particularly limited, but for example, (b) aramid monofiber can be blended into (a) curable resin by simple mixing using a stirrer or the like, or by kneading using a kneader, extruder or the like. It is preferable to disperse (b) aramid monofiber in (a) curable resin.

[0155] There are no particular restrictions on the type of molding used to obtain the aforementioned molded product, but examples include casting, injection molding, press molding, casting, vacuum molding, and transfer molding.

[0156] By using the present invention, (a) the mechanical properties of a molded article made of a cured product of a curable resin are improved. For example, by using the present invention, a molded article can be obtained in which at least one mechanical property selected from the group consisting of tensile strength and elastic modulus is enhanced.

[0157] Furthermore, the use of the present invention improves the dimensional stability of molded articles made from cured curable resins.

[0158] [Mechanical Property Enhancer] The present invention also relates to an enhancer of at least one mechanical property selected from the group consisting of tensile strength and elastic modulus for a molded article consisting of (b) at least one aramid monofiber having an average diameter of less than 2000 nm obtained by centrifugal sedimentation, and (a) a cured product of at least one curable resin. Here, the word "consists of" is synonymous with "includes" unless otherwise defined. Therefore, the molded article may include any additional components or materials in addition to the cured product. Furthermore, the mechanical property enhancer may include any additional components or materials other than (b) aramid monofiber. However, the mechanical property enhancer may consist only of (b) aramid monofiber.

[0159] The above molded product preferably contains the above cured product as its main component. The proportion of the above cured product in the above molded product is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the molded product.

[0160] The above-mentioned mechanical property enhancers, specifically (a) curable resins and (b) aramid monofibers, are described in the same way as the composite materials of the present invention.

[0161] The mechanical property enhancer of the present invention can be used by blending the agent with (a) at least one curable resin, and can enhance at least one mechanical property selected from the group consisting of tensile strength and elastic modulus of a molded article made from the cured product of the (a) curable resin. The method of blending is not particularly limited, but for example, (b) aramid monofibers can be blended with (a) curable resin by simple mixing using a stirrer or the like, or by kneading using a kneader, extruder or the like. It is preferable to disperse (b) aramid monofibers in (a) curable resin.

[0162] (a) There are no particular restrictions on the type of curing molding used to obtain a molded product made from a cured resin, but examples include casting, injection molding, press molding, casting, vacuum molding, and transfer molding.

[0163] The mechanical property enhancer of the present invention improves the mechanical properties of a molded article made of a cured product of a curable resin. For example, the mechanical property enhancer of the present invention can be used to obtain a molded article in which at least one mechanical property selected from the group consisting of tensile strength and elastic modulus is improved.

[0164] Furthermore, the mechanical property enhancer of the present invention improves the dimensional stability of molded articles made from cured curable resins.

[0165] The composite material of the present invention can be used in the manufacture of various articles.

[0166] The molded articles of the present invention can possess excellent mechanical properties, for example, excelling in at least one mechanical property selected from the group consisting of tensile strength and elastic modulus. Therefore, the molded articles of the present invention can be suitably used as vehicle bodies and engine parts for automobiles, railway vehicles, bicycles, etc., civil engineering and construction materials, aircraft and ship parts, etc., where high strength and high impact resistance are required. The manufacturing method of the present invention can produce molded articles with such excellent mechanical properties.

[0167] The use and agent of the present invention can enhance (a) at least one mechanical property selected from the group consisting of tensile strength and elastic modulus of a molded article made of a curable resin. Furthermore, (a) the dimensional stability of the molded article made of a curable resin is improved.

[0168] The present invention will be described more specifically below using examples and comparative examples, but the scope of the present invention is not limited to the examples.

[0169] <Measurement of Soluble Content of Raw Aramid Fiber> 0.1 g of raw aramid fiber was placed in 10 g of dimethyl sulfoxide and immersed at room temperature (25°C) for 72 hours. This was filtered by suction using a membrane filter (Merck Millipore, hydrophilic PTFE, pore size: 1.0 μm), then thoroughly washed with 99.0% by mass of dimethyl sulfoxide, and finally washed with deionized water. The washed filtrate was dried at 150°C for 24 hours, and its mass was used as the insoluble content. The soluble content was calculated using the following formula: Soluble content (%) = (1 - Insoluble content / 0.1) * 100% The soluble content of the raw aramid fiber was 20% by mass.

[0170] <Preparation of Aramid Microfibers> Raw material aramid fiber: 2 g was dried in an oven at 105°C for 24 hours, then added to 198 g of dimethyl sulfoxide and swelled. The mixture was broken down 60 times at 10000 rpm using a homomixer (Primix, MARK II 2.5), and then broken down 20 times at 10000 rpm using a clear mixer (M-Technique, CLM0.8) to obtain an aramid microfiber dispersion. The obtained aramid microfiber dispersion was centrifuged at 16000 G, the dimethyl sulfoxide was removed by decantation, and the precipitate was recovered. N,N-dimethylacetamide was added to the precipitate and mixed, and the mixture was centrifuged again at 16000 G, decanted, and the precipitate was recovered. This process was repeated three times, and the aggregate was broken down using a spatula or similar tool to finally obtain an aramid microfiber dispersion (solid content concentration: 3 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 • N,N-Dimethylacetamide (Fujifilm Wako Pure Chemical Industries, Ltd., Special Grade) 99.5% by mass

[0171] <Measurement of Fiber Diameter and Fiber Length> The final aramid microfiber dispersion (solid content concentration: 3% by mass) obtained above was filtered by suction using a membrane filter with a pore size of 0.2 μm, dried in an oven at 105°C, and then measured under the following conditions. The maximum fiber diameter, minimum fiber diameter, maximum fiber length, and minimum fiber length were determined by image analysis. Each aramid microfiber was a single fiber. Microscope: Field emission scanning electron microscope: Hitachi High-Tech Corporation SU8010 Voltage: 5 kV Number of samples measured: 100 The results are shown in Table 1 below.

[0172] <Measurement of average diameter> 0.6 g of polyethylene oxide and 99.4 g of dimethyl sulfoxide were added to a beaker and stirred at 70°C for 10 minutes. After visually checking for any undissolved polyethylene oxide, the mixture was stirred until the temperature dropped below 30°C to obtain a dimethyl sulfoxide thickener. The weight (mass) W1 of the above-mentioned fine fiber paste before drying was measured, and the weight (mass) W2 was measured after heating on a hot plate at 200°C for 24 hours. The solid content concentration of the above-mentioned fine fiber paste was calculated using the following formula: Solid content concentration (%) = W2 / W1 * 100%. Dimethyl sulfoxide and the above-mentioned dimethyl sulfoxide thickener were added to adjust the mixture so that the solid content concentration of the above-mentioned fine fiber paste was 0.05 mass% to 1.0 mass%, and the viscosity of the above-mentioned fine fiber paste measured at room temperature with a B-type viscometer was 40 to 100 mPa.s. A sample for number-average fiber diameter measurement was obtained. The average diameter of fine fibers was measured using a 1.5 ml sample of number-average fiber diameter analysis device manufactured by Horiba, Ltd., Partica Centrifuge. The centrifugal particle size distribution analyzer was calibrated using silica standard particles. • Polyethylene oxide (Sumitomo Seika Co., Ltd., viscosity-average molecular weight 1,700,000~2,200,000) • Silica standard particles (Thermo Scientific, No. 8100) The results are shown in Table 1 below.

[0173]

[0174] <Example 1> To the aramid microfiber dispersion (solid content concentration: 3% by mass) obtained above, paraphenylenediamine (PPD) was added, and the mixture was stirred in nitrogen gas at 60°C for 1 hour, then allowed to cool for 30 minutes to obtain a PPD-mixed aramid microfiber dispersion at room temperature. Meanwhile, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was placed on a glass dish and dried at 240°C for 24 hours to obtain completely dehydrated BPDA. The obtained completely dehydrated BPDA was added to the PPD-mixed aramid microfiber dispersion in nitrogen gas while being cooled in an ice bath, and stirred for more than 24 hours to obtain a polyimide precursor / aramid microfiber dispersion. The obtained polyimide precursor / aramid microfiber dispersion was coated onto a polyethylene terephthalate film and dried at 80°C for 1 hour to prepare a precursor film. The obtained precursor film was heated at 100°C for 30 minutes, 150°C for 30 minutes, 200°C for 30 minutes, 250°C for 30 minutes, and 300°C for 1 hour to obtain a polyimide / aramid microfiber composite film. The amounts of aramid microfiber dispersion, PPD, and BPDA used were adjusted so that the amount of aramid microfibers in the polyimide / aramid microfiber composite film was 0.05% by mass. • Paraphenylenediamine (Fujifilm Wako Pure Chemical Industries, Ltd., Grade 1, 97.0% by mass) • 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (Fujifilm Wako Pure Chemical Industries, Ltd., Special Grade, 99.0% by mass)

[0175] <Example 2> A polyimide / aramid microfiber composite film was obtained in the same manner as in Example 1, except that the amounts of aramid microfiber dispersion, PPD, and BPDA used were adjusted so that the amount of aramid microfibers in the polyimide / aramid microfiber composite film was 0.01% by mass.

[0176] <Comparative Example 1> A polyimide film was obtained in the same manner as in Example 1, except that an aramid microfiber dispersion was not used.

[0177] <Tensile Test> Tensile tests were performed using the films of Example 1, Example 2, and Comparative Example 1, and the tensile strength, breaking strain, and modulus of elasticity were measured. Each film was shaped at 40 mm x 5 mm. The tensile testing machine used was a Shimadzu AGS-1KNX, with a chuck distance of 20 mm and a crosshead speed of 2 mm / min. The modulus of elasticity was determined from the tensile stress at strains of 0.05% and 0.25% of each film using the following formula (1). Formula (1): Modulus of elasticity = (Stress at 0.25% strain - Stress at 0.05% strain) / (Strain 0.25% - Strain 0.05%) The results are shown in Table 2.

[0178]

[0179] As is clear from Table 2 above, the composite materials of Example 1 and Example 2, which contain aramid microfibers, had superior mechanical properties compared to the material of Comparative Example 1, which does not contain aramid microfibers.

Claims

1. A composite material comprising (a) at least one curable resin and (b) at least one aramid monofiber, wherein the average diameter of the aramid monofiber (b) by centrifugal sedimentation is less than 2000 nm.

2. The composite material according to claim 1, wherein the curable resin in (a) is other than an epoxy resin.

3. The composite material according to claim 1, wherein the curable resin (a) is selected from the group consisting of polyimide resin, urethane resin, phenolic resin, urea resin, melamine resin, fluororesin, silicon resin, alkyd resin, unsaturated polyester resin, and mixtures thereof.

4. The composite material according to claim 1, wherein the curable resin (a) is a polyimide resin.

5. The composite material according to claim 1, wherein the amount of the curable resin (a) in the composite material is 50% by mass to 99.99% by mass, based on the total mass of the composite material.

6. The composite material according to claim 1, wherein the minimum fiber length of the aramid monofiber in (b) is greater than 10 μm.

7. The following equation for the aramid monofiber of (b) above: x = (1 - ΔQ ｘ / ΔQ ０ )×100(%) (in the formula ΔQ ｘ ΔQ is the change in heat capacity of the aramid single fiber before and after the glass transition point. ０ The composite material according to claim 1, wherein the degree of crystallinity x, determined by (where is the change in heat capacity before and after the glass transition temperature of amorphous aramid), is 60% or more.

8. The composite material according to claim 1, wherein the maximum fiber diameter of the aramid monofibers (b) is less than 2200 nm.

9. The composite material according to claim 1, wherein the average aspect ratio of the aramid monofibers (b) is 100 or more.

10. The composite material according to claim 1, wherein the glass transition temperature of the aramid monofiber (b) is greater than 300°C.

11. The composite material according to claim 1, wherein the (b) aramid monofiber is made of metaaramid.

12. The composite material according to claim 1, wherein the amount of (b) aramid monofibers in the composite material is 0.001% by mass to 20% by mass, based on the total mass of the composite material.

13. The composite material according to claim 1, further comprising (a) an agent that enhances the dispersibility of (b) aramid monofibers in a curable resin, and (c) at least one dispersant.

14. A cured product of a composite material according to any one of claims 1 to 13.

15. A molded article made from the cured product described in claim 14.

16. A method for producing a molded article having enhanced mechanical properties selected from the group consisting of tensile strength and elastic modulus, comprising the steps of (a) mixing at least one curable resin and (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation to obtain a composite material, and curing and molding the composite material to obtain a molded article made of a cured product of the composite material.

17. Use of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, for the purpose of enhancing at least one mechanical property selected from the group consisting of tensile strength and elastic modulus, of a molded article made of (a) a cured product of at least one curable resin.

18. An enhancer for at least one mechanical property selected from the group consisting of tensile strength and elastic modulus, of a molded article consisting of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, and (a) a cured product of at least one curable resin.

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