Use of aramid fine single fibers in thermoplastic resin

Abstract

The present invention relates to a composite material comprising (a) at least one thermoplastic resin and (b) at least one type of aramid single fibers, wherein the average diameter of the aramid single fibers (b) as determined by the centrifugal sedimentation method is less than 2000 nm. According to the present invention, it is possible to produce an article having excellent abrasion resistance by using very fine aramid single fibers.

Description

Use of aramid fine monofilaments in thermoplastic resins

[0001] This invention relates to the use of extremely fine aramid monofilaments.

[0002] Aromatic polyamides (aramids) include para-aromatic polyamides such as poly(p-phenylene terephthalamide) and meta-aromatic polyamides such as poly(metaphenylene isophthalamide), both of which are used in practical applications as fibers.

[0003] Furthermore, very fine (micron) fibers made of aromatic polyamides have also been manufactured. For example, Patent Document 1 describes the production of aramid nanofibers with an average fiber diameter of 20 nm by extruding a strong base dimethyl sulfoxide solution of aramid into water and spinning it (see Patent Document 1

[0051] to

[0053] ).

[0004] Japanese Patent Publication No. 2023-2089

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

[0006] The present invention aims to provide a composite material containing extremely fine aramid monofibers.

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

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

[0009] [2] The composite material of [1] wherein the thermoplastic resin (a) is selected from the group consisting of polyamide, polyimide, polyamideimide, polyetherimide, polyester, polycarbonate, (meth)acrylic resin, polyacrylamide, polyacrylonitrile, polyvinyl acetate, ABS resin, AS resin, polyether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polyvinyl chloride, polyvinylidene chloride, polyolefin, polystyrene, fluororesin, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

[0010] [3] The composite material of [1] wherein the thermoplastic resin (a) is selected from the group consisting of polyamide, polyimide, polyamideimide, polyetherimide, polyethylene terephthalate, polybutylene terephthalate, polyarylate, polycarbonate, polyacetal, polyphenylene ether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

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

[0012] [5] A composite material according to any of [1] to [4], wherein the amount of the thermoplastic resin (a) in the composite material is 50% by mass to 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. ０A 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 temperature of amorphous aramid), is 60% or greater.

[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 monofiber in the composite material is 0.1% 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 thermoplastic resin, (b) the aramid monofibers, and (c) at least one dispersant.

[0021] A second aspect of the present invention is a molded article

[14] made of any of the composite materials [1] to

[13] described above.

[0022] A second aspect of the present invention includes the following embodiments.

[0023]

[15] A molded product of

[14] which is a sliding member.

[0024] A third aspect

[16] of the present invention is a method for producing a molded article with enhanced wear resistance, comprising the steps of (a) mixing at least one thermoplastic 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 molding the composite material to obtain a molded article.

[0025] 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 for the purpose of improving the wear resistance of a molded article made of (a) at least one thermoplastic resin.

[0026] A fifth aspect

[18] of the present invention is an abrasion resistance enhancer for molded articles made of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, and (a) at least one thermoplastic resin.

[0027] The composite material of the present invention, which contains very fine aramid monofibers, can be used in the manufacture of articles with excellent abrasion resistance.

[0028] The molded articles of the present invention can exhibit excellent wear resistance. The molded articles of the present invention can be suitably used, for example, as sliding members.

[0029] Furthermore, the manufacturing method of the present invention can produce molded articles with excellent wear resistance.

[0030] Furthermore, the agents used and enhancers of the present invention can improve the wear resistance of molded articles made of thermoplastic resins.

[0031] A diagram illustrating the overview of the sliding abrasion test. A plan view illustrating the evaluation of the test specimen after the sliding abrasion test. A schematic diagram showing the cross-section of the test specimen after the sliding abrasion test.

[0032] As a result of diligent research, the inventors have found that it is possible to provide a composite material containing extremely fine aramid monofibers.

[0033] Conventionally, to improve the mechanical properties of articles made of thermoplastic resins, reinforcing fibers such as glass fibers and carbon fibers have been blended into the thermoplastic resin. However, the reinforcing fibers used so far have mainly been inorganic fibers with a wide fiber width, and it was unclear whether even very thin aramid single fibers could improve the mechanical properties of thermoplastic resins, particularly abrasion resistance.

[0034] The inventors of the present invention have found that even very thin aramid single fibers can improve the abrasion resistance of articles made of the thermoplastic resin when blended with the thermoplastic resin, and have completed the present invention.

[0035] Hereinafter, the present invention will be described.

[0036] [Composite Material] The composite material of the present invention contains (a) at least one kind of thermoplastic resin and (b) at least one kind of aramid single fiber, and the average diameter of the (b) aramid single fiber by the centrifugal sedimentation method is less than 2000 nm.

[0037] (Thermoplastic Resin) The composite material of the present invention contains (a) at least one kind of thermoplastic resin. The composite material of the present invention may contain a single kind of thermoplastic resin or two or more kinds of thermoplastic resins.

[0038] It is preferable that the (a) thermoplastic resin is the matrix of the composite material of the present invention. That is, in the composite material of the present invention, it is preferable that the (a) thermoplastic resin contains the (b) aramid single fiber described later.

[0039] It is preferable that the (b) aramid single fiber described later is dispersed in the (a) thermoplastic resin. Since the (b) aramid single fiber has no branches, it is easy to mix with the (a) thermoplastic resin and disperse in the (a) thermoplastic resin.

[0040] The type of the (a) thermoplastic resin is not particularly limited, and any of general-purpose plastics, engineering plastics, super engineering plastics, or mixtures thereof may be used.

[0041] (a) The thermoplastic resin is preferably selected from polyamide, polyimide, polyamideimide, polyetherimide, polyester, polycarbonate, (meth)acrylic resin, polyacrylamide, polyacrylonitrile, polyvinyl acetate, ABS resin, AS resin, polyether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polyvinyl chloride, polyvinylidene chloride, polyolefin, polystyrene, fluororesin, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

[0042] (a) The thermoplastic resin is more preferably selected from the group consisting of polyamide, polyimide, polyamideimide, polyetherimide, polyethylene terephthalate, polybutylene terephthalate, polyarylate, polycarbonate, polyacetal, polyphenylene ether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

[0043] (a) The thermoplastic resin is more preferably a polyamide.

[0044] The polyamide is not particularly limited, and for example, those consisting of aliphatic and / or alicyclic monomers and / or aromatic monomers can be used.

[0045] Aliphatic monomers only need to have an aliphatic skeleton, for example: Aliphatic diamines (linear or branched C such as tetramethylenediamine, hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, 2-methyloctamethylenediamine, trimethylhexamethylenediamine, decamethylenediamine, dodecamethylenediamine, etc.) ２－２０ Alkylenediamine (preferably linear or branched C) ４－１２ Alkylenediamine, more preferably linear or branched C ６－９Alkylene diamine); aliphatic dicarboxylic acids (such as adipic acid, sebacic acid, 1,10-decanedicarboxylic acid, etc., linear or branched C ２－１８ Alkylene dicarboxylic acids (preferably linear or branched C ４－１０ Alkylene dicarboxylic acids, more preferably linear or branched C ４－８ Alkylene dicarboxylic acids); lactams (such as lactams of 4- to 14-membered rings (preferably 6- to 13-membered rings) such as ε-caprolactam, ω-laurolactam, etc.); and aliphatic aminocarboxylic acids (amino C such as 6-aminohexanoic acid, 11-aminoundecanoic acid, 12-aminododecanoic acid, etc.) ２－２０ Alkyl-carboxylic acids (preferably amino C ３－１６ Alkyl-carboxylic acids, more preferably amino C ５－１１ Alkyl-carboxylic acids), etc.).

[0046] As the alicyclic monomer, it only needs to have an alicyclic skeleton (cycloalkane skeleton). For example, alicyclic diamines (such as diamino cycloalkanes, di(aminoalkyl) cycloalkanes (such as diamino methyl cycloalkane, etc.)); alicyclic dicarboxylic acids (such as cycloalkane dicarboxylic acids); and alicyclic aminocarboxylic acids (such as aminocycloalkane carboxylic acids) can be mentioned.

[0047] As the aromatic monomer, it only needs to have an aromatic ring skeleton. For example, aromatic (or araliphatic) diamines (such as diamino arenes such as m-phenylenediamine, p-phenylenediamine, etc., di(aminoalkyl) arenes such as m-xylylenediamine, etc.); aromatic (or araliphatic) dicarboxylic acids (such as dicarboxy arenes such as terephthalic acid, isophthalic acid, etc.); aromatic aminocarboxylic acids (such as aminoaryl carboxylic acids such as aminobenzoic acid, etc.) can be mentioned.

[0048] Polyamides can be obtained by polymerizing the above monomers individually or in combination of two or more. Polyamides may be homopolyamides formed from a single monomer (a single diamine and dicarboxylic acid, or a single lactam and / or aminocarboxylic acid), or they may be copolyamides formed by copolymerization of multiple monomers. Typical polyamides include, for example, aliphatic polyamides, alicyclic polyamides, and aromatic polyamides.

[0049] Aliphatic polyamides can be formed from units derived from aliphatic monomers, and examples include: - Homopolyamides of aliphatic diamines and aliphatic dicarboxylic acids (e.g., polyamide 46, polyamide 66, polyamide 610, polyamide 612); - Homopolyamides of lactams and / or aliphatic aminocarboxylic acids corresponding to lactams (e.g., polyamide 6, polyamide 11, polyamide 12); - Copolymers of multiple aliphatic monomers (e.g., copolyamide 6 / 66, copolyamide 6 / 11, copolyamide 66 / 12).

[0050] Alicyclic polyamides only need to have units derived from alicyclic monomers, and may be formed by combining aliphatic monomers and alicyclic monomers. For example, homopolyamides of alicyclic diamines and aliphatic dicarboxylic acids (such as polymers of diaminomethylcyclohexane and adipic acid) can be cited.

[0051] Aromatic polyamides only need to have units derived from aromatic monomers. For example, they can be divided into semi-aromatic polyamides, which are formed from aromatic monomers and aliphatic (or alicyclic) monomers, and fully aromatic polyamides, which are formed from aromatic monomers and do not contain either an aliphatic or alicyclic skeleton.

[0052] Examples of semi-aromatic polyamides include: • Homopolyamides of aromatic (or aromatic aliphatic) diamines and aliphatic dicarboxylic acids (e.g., polyamide MXD6 (polymer of m-xylylenediamine and adipic acid)); • Homopolyamides of aliphatic diamines and aromatic dicarboxylic acids (e.g., polyamide 6T (polymer of hexamethylenediamine and terephthalic acid), polyamide 9T (polymer of nonamethylenediamine and terephthalic acid), polyamide 10T (polymer of decamethylenediamine and terephthalic acid), polyamide 12T (polymer of dodecamethylenediamine and terephthalic acid), polyamide M5T (polymer of 2-methylpentamethylenediamine and terephthalic acid), polyamide M8T (polymer of 2-methyloctamethylenediamine and terephthalic acid), polyamide 6I (polymer of hexamethylenediamine and isophthalic acid), polymer of trimethylhexamethylenediamine and terephthalic acid, etc.); Examples include copolymers containing at least an aliphatic diamine and an aromatic dicarboxylic acid (e.g., copolyamide 6T / 66, copolyamide 6T / M5T, copolyamide 6T / 6I, copolyamide 6T / 6I / 6, copolyamide 6T / 6I / 66).

[0053] Examples of fully aromatic polyamides include: homopolyamides of aromatic diamines and aromatic dicarboxylic acids (polymers of m-phenylenediamine and isophthalic acid, polymers of p-phenylenediamine and terephthalic acid, etc.).

[0054] (a) As the thermoplastic resin, aliphatic polyamides are more preferably, and polyamide 6, polyamide 66, or mixtures thereof are particularly preferred.

[0055] The composite material of the present invention may contain (a) a thermoplastic 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 thermoplastic resin in an amount of, for example, 99% by mass or less, preferably 98.5% by mass or less, and more preferably 98% 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 thermoplastic resin in an amount of, for example, 50% by mass to 99% by mass, preferably 60% by mass to 98.5% by mass, and more preferably 70% by mass to 98% by mass, based on the total mass of the composite material.

[0056] (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.

[0057] <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.

[0058] 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, and heat resistance.

[0059] <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.

[0060] (b) Aramid monofilaments are very fine (i.e., microscopic) fibers with an average diameter of less than 2000 nm, measured by centrifugal sedimentation.

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] (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.

[0066] (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.

[0067] (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.

[0068] (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.

[0069] (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.

[0070] (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.

[0071] (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.

[0072] (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.

[0073] (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.

[0074] (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.

[0075] (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.

[0076] (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.

[0077] (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.

[0078] 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.

[0079] 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.

[0080] 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).

[0081] (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 material is highly crystalline.

[0082] (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.

[0083] (b) Although aramid monofilaments are fine, when combined with resins or other materials, they can impart excellent mechanical properties.

[0084] 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.

[0085] <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.

[0086] (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.

[0087] (b) The above method for producing aramid monofilaments allows for the simple production of very fine aramid fibers.

[0088] 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.

[0089] (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.

[0090] The following describes (b) the method for producing aramid monofilaments.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] "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.

[0096] 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.

[0097] "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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] The aramid fibers, which have been swollen in the swelling process, are then subjected to the mechanical defibration process.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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).

[0115] By the above manufacturing method, (b) aramid monofilament can be obtained.

[0116] 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.

[0117] The composite material of the present invention may contain (b) aramid monofibers in an amount of, for example, 0.1% by mass or more, preferably 0.3% by mass or more, and more preferably 0.5% 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, 0.1% by mass to 20% by mass, preferably 0.3% by mass to 10% by mass, and more preferably 0.5% by mass or more and less than 5% by mass, relative to the total mass of the composite material.

[0118] (b) Despite being extremely thin, aramid monofilaments can improve the abrasion resistance of composite materials even in relatively small quantities.

[0119] (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 thermoplastic resin. The composite material of the present invention may contain a single (c) dispersant or two or more dispersants.

[0120] (c) Dispersants can improve the dispersibility of (a) aramid monofibers in (b) thermoplastic resins by, for example, improving the wettability of (a) aramid monofibers in (b) thermoplastic resins.

[0121] The type of dispersant is not particularly limited, and known dispersants such as maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene butyl acrylate copolymer, acrylic acid-modified polyethylene, acrylic acid-modified polypropylene, acrylic acid-modified ethylene vinyl acetate copolymer, maleic anhydride-modified polyolefin block copolymer, maleic anhydride-modified ethylene octen copolymer, maleic anhydride-modified styrene-ethylene-butylene styrene, and glycidyl methacrylate-modified styrene-ethylene-butylene styrene can be used as appropriate.

[0122] 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.

[0123] (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.

[0124] 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.

[0125] 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.

[0126] [Method for producing composite materials] The composite materials of the present invention can be produced by mixing or kneading (a) a thermoplastic resin and (b) aramid monofibers together with (c) other components such as a dispersant, as needed.

[0127] For example, the composite material of the present invention can be produced by adding (b) aramid monofibers to (a) thermoplastic resin that has been heated and melted, and then mixing or kneading them. The heating / melting means and the mixing / kneading means are not particularly limited, but examples include a Banbury mixer and an extruder (e.g., a single-screw or twin-screw extruder).

[0128] The heating and melting temperature should be (a) above the melting point of the thermoplastic resin, but (a) in terms of suppressing the decomposition of the thermoplastic resin, (a) when the melting point of the thermoplastic resin is Tm, the heating and melting temperature should be, for example, Tm to (Tm+50)°C, preferably (Tm+10) to (Tm+40)°C, and more preferably (Tm+20) to (Tm+30)°C.

[0129] 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.

[0130] The composite material of the present invention (a) can have a higher glass transition temperature than the thermoplastic resin. Therefore, the composite material of the present invention (a) has a more stable shape than the thermoplastic resin itself and can be suitably used in a variety of applications.

[0131] The composite material of the present invention is preferably not an adhesive.

[0132] [Molded Articles] The present invention also relates to molded articles made of the composite material of the present invention. Here, the word "made of" is synonymous with "including" unless otherwise defined. Therefore, the molded articles of the present invention may include any additional components or materials in addition to the composite material of the present invention.

[0133] The molded article of the present invention preferably contains the composite material of the present invention as a main component. The proportion 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 composite material of the present invention.

[0134] A molded article of the present invention can be obtained by molding the composite material of the present invention using a known molding method. There are no particular limitations on the shape of the molded article of the present invention, but examples include films, sheets, bags, cylinders, and cylindrical columns.

[0135] There are no particular limitations on the molding method used to obtain the molded articles of the present invention, but examples include molding without using a mold or die, such as molding of thin films by casting, and molding using a mold or die, such as extrusion molding, injection molding, press molding, blow molding, extrusion blow molding, stretched film molding, inflation molding, insert molding, foam molding, and transfer molding.

[0136] The molded articles of the present invention can possess excellent mechanical properties. For example, the molded articles of the present invention can possess excellent wear resistance.

[0137] Furthermore, the molded product of the present invention exhibits excellent dimensional stability.

[0138] The molded articles of the present invention can be used in a wide range of applications, but are particularly preferred for use as sliding components such as gears, bearings, crankshafts, pistons, cylinders, and rails, where wear resistance is required.

[0139] [Method for manufacturing molded articles] The present invention also relates to a method for manufacturing molded articles with enhanced wear resistance, comprising the steps of (a) mixing at least one thermoplastic 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 molding the composite material to obtain a molded article.

[0140] The above-described method applies to (a) thermoplastic resin and (b) aramid monofiber, as described in the description of (a) thermoplastic resin and (b) aramid monofiber included in the composite material of the present invention.

[0141] The mixing method in the step of obtaining a composite material by mixing (a) a thermoplastic 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 thermoplastic resin.

[0142] The temperature at which (a) the thermoplastic resin and (b) the aramid monofiber are mixed is not particularly limited, but room temperature (25°C) or higher is preferred. Simple mixing using a stirrer can be carried out at room temperature. When kneading using a kneader, extruder, etc., the temperature at which (a) the thermoplastic resin melts and flows is preferred, for example, it can be between 150°C and 250°C, and between 170°C and 200°C is preferred.

[0143] There are no particular restrictions on the type of molding used in the process of molding the composite material, but examples include casting, extrusion molding, injection molding, press molding, blow molding, extrusion blow molding, stretched film molding, inflation molding, insert molding, foam molding, transfer molding, and the like.

[0144] Molded articles manufactured by the above method can possess excellent wear resistance. In other words, molded articles with enhanced wear resistance can be manufactured by the above method.

[0145] Furthermore, molded products manufactured using the above method exhibit excellent dimensional stability.

[0146] [Use] The present invention also relates to the use of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation method for improving the abrasion resistance of a molded article made of (a) at least one thermoplastic resin. Here, the word "made of" is synonymous with "includes" unless otherwise defined. Therefore, the above molded article may include any additional components or materials in addition to (a) the thermoplastic resin.

[0147] The above molded article preferably contains (a) thermoplastic resin as its main component. The proportion of (a) thermoplastic resin in the above molded article is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on the total mass of the molded article.

[0148] The above-mentioned descriptions of (a) thermoplastic resins and (b) aramid monofibers apply to the use of (a) thermoplastic resins and (b) aramid monofibers included in the composite materials of the present invention.

[0149] The present invention can be used by incorporating (b) at least one aramid monofiber having an average diameter of less than 2000 nm, obtained by centrifugal sedimentation, into a molded article made of (a) at least one thermoplastic resin. The method of formulation is not particularly limited, but for example, (a) a thermoplastic resin and (b) aramid monofiber can be mixed to obtain a composite material, and (b) aramid monofiber can be incorporated into a molded article made of (a) a thermoplastic resin by molding the composite material. It is preferable to disperse (b) aramid monofiber in (a) a thermoplastic resin.

[0150] There are no particular restrictions on the type of molding used to obtain the above-mentioned molded products, but examples include casting, extrusion molding, injection molding, press molding, blow molding, extrusion blow molding, stretched film molding, inflation molding, insert molding, foam molding, and transfer molding.

[0151] By using the present invention, (a) the wear resistance of molded articles made of thermoplastic resin is improved. Therefore, by using the present invention, it is possible to obtain molded articles with enhanced wear resistance.

[0152] Furthermore, the use of the present invention improves the dimensional stability of molded articles made of thermoplastic resins.

[0153] [Abrasion Resistance Enhancer] The present invention also relates to an abrasion resistance enhancer for a molded article made of (a) a thermoplastic resin, which consists of (b) at least one type of aramid monofiber having an average diameter of less than 2000 nm obtained by centrifugal sedimentation. Here, the word "consists of" is synonymous with "includes" unless otherwise defined. Therefore, the above molded article may contain any additional components or materials in addition to (a) the thermoplastic resin. Furthermore, the above abrasion resistance enhancer may contain any additional components or materials other than (b) aramid monofiber. However, the above abrasion resistance enhancer may consist only of (b) aramid monofiber.

[0154] The above molded article preferably contains (a) thermoplastic resin as its main component. The proportion of (a) thermoplastic resin in the above molded article is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on the total mass of the molded article.

[0155] The above-mentioned description of (a) thermoplastic resin and (b) aramid monofibers in relation to the abrasion resistance enhancer applies to (a) thermoplastic resin and (b) aramid monofibers included in the composite material of the present invention.

[0156] The wear resistance enhancer of the present invention can be used by incorporating the agent into a molded article made of (a) at least one thermoplastic resin, thereby improving the wear resistance of the molded article. The method of formulation is not particularly limited, but for example, (a) a thermoplastic resin and (b) aramid monofibers can be mixed to obtain a composite material, and (b) aramid monofibers can be incorporated into a molded article made of (a) a thermoplastic resin by molding the composite material. It is preferable to disperse (b) aramid monofibers in (a) a thermoplastic resin.

[0157] There are no particular restrictions on the type of molding used in the process of molding the composite material, but examples include casting, extrusion molding, injection molding, press molding, blow molding, extrusion blow molding, stretched film molding, inflation molding, insert molding, foam molding, transfer molding, and the like.

[0158] The abrasion resistance enhancer of the present invention improves the abrasion resistance of (a) molded articles made of thermoplastic resin. Therefore, molded articles with enhanced abrasion resistance can be obtained using the abrasion resistance enhancer of the present invention.

[0159] Furthermore, the wear resistance enhancer of the present invention improves the dimensional stability of molded articles made of thermoplastic resins.

[0160] The composite material of the present invention can be used in the manufacture of various articles.

[0161] The molded articles of the present invention can possess excellent mechanical properties such as wear resistance and can be used in a wide range of applications. In particular, the molded articles of the present invention can be suitably used as sliding members such as gears, bearings, crankshafts, pistons, cylinders, and rails. The manufacturing method of the present invention can produce molded articles with such excellent mechanical properties.

[0162] The use and agent of the present invention can (a) enhance the wear resistance of molded articles made of thermoplastic resins. Furthermore, (a) improve the dimensional stability of molded articles made of thermoplastic resins.

[0163] 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.

[0164] <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.

[0165] <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 collected to obtain an aramid microfiber paste. Acetone was added to the aramid microfiber paste and mixed, and the filtrate was washed with acetone while performing suction filtration. The washed filtrate was crushed in a pulverizer and dried under reduced pressure at 110°C for 24 hours to obtain aramid microfibers. ・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) Acetone (Kanto Chemical Co., Ltd., Special Grade) 99.5% by mass

[0166] <Measurement of Fiber Diameter and Fiber Length> The dried aramid microfibers were washed with deionized water and dried in an oven at 105°C. Then, 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.

[0167] <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. The mixture was visually inspected for any undissolved polyethylene oxide and 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 mixture was heated on a hot plate at 200°C for 24 hours to measure the weight (mass) W2. 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, as measured at room temperature with a B-type viscometer, was 40 to 100 mPa.s. This allowed for the preparation of a sample for measuring the average diameter. The average diameter of fine fibers was measured using a 1.5 ml sample for average diameter measurement with a Partica Centrifuge centrifugal nanoparticle analyzer manufactured by Horiba, Ltd. 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.

[0168]

[0169] <Example 1> 10 g of the dried aramid fine fibers and 990 g of polyamide 6 resin were mixed in a resin container and dried under reduced pressure at 80°C for 12 hours to obtain a mixture. The obtained mixture was melt-kneaded in a twin-screw extruder (Japan Steel Works, Ltd., TEX-30HSS) at a cylinder temperature of 240°C and 85 rpm, and the molten mixture was discharged into a water bath to obtain a strand-like composite material. Polyamide 6 resin UBE Corporation Grade 1013B The above composite material was pulverized in a pulverizer (Horai Co., Ltd., P-1314) to form pellets of about 3 mm in size, and these were dried under reduced pressure at 80°C for 12 hours. Using the obtained pellet-like composite material, a 30 mm × 30 mm × 2.5 mm flat test piece was produced by injection molding in an injection molding machine (Nissei Plastic Industrial Co., Ltd., NEX30IV-2EG).

[0170] <Comparative Example 1> Polyamide 6 resin was dried under reduced pressure at 80°C for 12 hours. Using the obtained polyamide 6 resin, a 30mm x 30mm x 2.5mm flat test piece was produced by injection molding using an injection molding machine (Nissei Plastic Industrial Co., Ltd., NEX30IV-2EG).

[0171] <Abrasion Test> A sliding abrasion test was conducted in accordance with JIS K7218-A. Figure 1 shows an overview of the sliding abrasion test.

[0172] A ring-on-plate sliding abrasion tester (Orientec EFM-III-EN) was used for the test, with a sliding speed V of 0.5 m / s and a sliding distance of 3000 m. The ring (made of carbon steel for machine structures, S45C, with a surface roughness Ra = 0.4 μm and a sliding surface area of ​​200 mm²) was used. 2 The device was brought into contact with each test specimen of Example 1 and Comparative Example 1 under a load P of 100N (apparent surface pressure p of 0.5MPa) and rotated.

[0173] Subsequently, the depth direction between A and A' on the ring-shaped sliding surface shown in Figure 2 on the test specimen was observed with a laser microscope (Keyence Corporation, VK-X200) to determine the wear cross-sectional shape, and the wear area S was determined from the wear cross-sectional shape of the test specimen. For example, in the case of the wear cross-sectional shape shown in Figure 3, the intersection points of the straight line along the inclination of the non-wear portion wall and the straight line along the wall surface of the wear portion wall are defined as A and A', and the wear area S can be determined by integrating from the straight line between A and A' to the bottom of the wear portion.

[0174] The amount of wear was then calculated using the following formula (1). The "circumference D of the sliding part" is shown in Figure 2. In addition, the specific wear amount was calculated from the sliding distance and load using formula (2).

[0175] Amount of wear = Wear area S × Circumference of sliding part D ... (1) Specific wear amount = Amount of wear ÷ (Load × Sliding distance) ... (2) The results are shown in Table 2 below.

[0176]

[0177] As is clear from Table 2 above, the composite material of Example 1 containing aramid microfibers exhibited superior abrasion resistance compared to the material of Comparative Example 1 which did not contain aramid microfibers.

Claims

1. A composite material comprising (a) at least one thermoplastic 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.

2. The composite material according to claim 1, wherein the thermoplastic resin (a) is selected from the group consisting of polyamide, polyimide, polyamideimide, polyetherimide, polyester, polycarbonate, (meth)acrylic resin, polyacrylamide, polyacrylonitrile, polyvinyl acetate, ABS resin, AS resin, polyether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polyvinyl chloride, polyvinylidene chloride, polyolefin, polystyrene, fluororesin, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

3. The composite material according to claim 1, wherein the thermoplastic resin (a) is selected from the group consisting of polyamide, polyimide, polyamideimide, polyetherimide, polyethylene terephthalate, polybutylene terephthalate, polyarylate, polycarbonate, polyacetal, polyphenylene ether, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyethernitrile, polysulfone, polyethersulfone, polyphenylene sulfide, and mixtures thereof.

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

5. The composite material according to claim 1, wherein the amount of the thermoplastic resin (a) in the composite material is 50% by mass to 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 monofiber before and after the glass transition point, as described above (b). ０ The composite material according to claim 1, wherein the degree of crystallinity x, determined by (the change in heat capacity before and after the glass transition point 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 0.1% 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) a thermoplastic resin, and (c) at least one dispersant.

14. A molded article made of a composite material according to any one of claims 1 to 13.

15. A molded article according to claim 14, which is a sliding member.

16. A method for producing a molded article with enhanced wear resistance, comprising the steps of (a) mixing at least one thermoplastic 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 molding the composite material to obtain a molded article.

17. Use of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation method for enhancing the abrasion resistance of a molded article made of (a) at least one thermoplastic resin.

18. Abrasion resistance enhancer for molded articles made of (b) at least one aramid monofiber having an average diameter of less than 2000 nm by centrifugal sedimentation, and (a) at least one thermoplastic resin.

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