JPS6115184B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to conductive short fibers. Conductive layers made of polymers mixed with carbon black or the like and conductive composite fibers made of ordinary polymers are well known. However, fibers containing carbon black are colored black or gray, and improvement of this drawback is desired. On the other hand, some conductive inorganic particles, such as semiconductor particles such as metal oxides, metal halides, and metal sulfides, and particles having a conductive film of metal or the above-mentioned semiconductor on the surface of the inorganic particles, have high whiteness. There are several types, and using them, it is possible to obtain conductive fibers with high whiteness. Generally, in order to obtain sufficient conductivity by mixing inorganic conductive particles into a polymer, extremely high
It is necessary to mix at a mixing ratio of 50% by weight or more, but
It is extremely difficult to uniformly mix a large amount of inorganic particles. In general, the smaller the particle size, the more sufficient conductivity can be obtained with a smaller mixing ratio; however, in reality, the smaller the particle size, the higher the agglomeration, making uniform dispersion difficult. In this way, it is extremely difficult to uniformly disperse and mix inorganic conductive particles into a polymer at a mixing ratio high enough to obtain sufficient conductivity.
It is difficult to smoothly manufacture composite fibers made of non-conductive polymers. An object of the present invention is to provide a novel conductive short fiber containing inorganic conductive particles that is easy to manufacture and has excellent conductivity. The conductive short fiber of the present invention is a composite of (a) a conductive layer made of conductive inorganic particles and a thermoplastic polymer, and a protective layer made of a fiber-forming polymer, and (b)
The degree of orientation of the crystalline part of the polymer that forms the fiber is 89
% or less, and (c) due to the lubricant, the coefficient of kinetic friction against metal and/or the coefficient of static friction against fiber is lower than that in non-lubricated state.
It is characterized by being reduced to 80% or less. Here, conductive inorganic particles (hereinafter referred to as conductive particles) have sufficient conductivity, for example, a volume resistivity (hereinafter referred to as specific resistance) when compressed at a pressure of 200 Kg/cm ² .
Fine particles of 10 ⁴ Ω・cm or less, especially 10 ² Ω・cm or less,
Examples include particles of metals, metal oxides, metal halides, etc., and inorganic particles provided with conductive films of metals, metal compounds, etc. on the surfaces thereof. Metal compounds are generally semiconductors, but those whose conductivity is dramatically enhanced by adding a small amount of activator (dopant) are particularly suitable for the purpose of the present invention. Examples of high whiteness include white inorganic particles (for example, titanium oxide, zinc oxide, silica, magnesium oxide, etc.) with a conductive film of metal or metal compound applied to the surface. Examples of conductive films include metal films such as gold, silver, aluminum, copper, and tungsten, metal oxide films such as zinc oxide, aluminum oxide, zirconium oxide, and indium oxide, and metals such as copper iodide and copper sulfide. Examples include halide or sulfide films. Fine particles made of the above metals and metal compounds are also useful in the present invention. Although metal particles generally have poor whiteness, they have excellent electrical conductivity and are useful for producing conductive fibers that require high electrical conductivity. Of course, two or more of the above-mentioned various particles can be mixed and used if necessary. The average (weight average) particle size of the conductive particles must be 1 μm or less, and particularly preferably 0.5 μm or less. On the other hand, from the point of view of dispersibility in the polymer, the average particle size is 0.01μ.
It is necessary to have a thickness of approximately 0.1 μm or more, and particularly preferably 0.1 μm or more. Overall, the average particle size is 0.1~0.5μ
m is particularly preferred, and 0.15 to 0.35 μm is most preferred. The weight average diameter of conductive particles is determined by observing the particles with an electron microscope, separating them into single particles, and measuring the diameter (average of the major axis and minor axis) of a large number of particles (1000 or more).
Determine the particle size distribution fractionated at μm intervals, and use the formula () and
Calculate by (). Particle diameter D = 6/πρW () where Ni: Number of particles in the i-th fraction Wi: Weight of particles in the i-th fraction W: Weight of particles ρ: Density of particles The thermoplastic polymer to be mixed with the conductive particles is Not particularly limited. Generally, polymers with a high degree of crystallinity (60% or more), such as polyethylene, polypropylene, polyoxymethylene, and polyethylene oxide, have better conductivity. However, such polymers are inferior in terms of heat resistance and dyeability, so polyamide, polyester, polyacrylonitrile, etc., which are often used for ordinary fibers, are preferred.
These polymers with rather poor crystallinity tend to have rather low conductivity when mixed with conductive particles. However, in the composite staple fiber of the present invention, sufficient electrical conductivity can be obtained even when these polymers are used. The conductive layer must have sufficient electrical conductivity. Its specific resistance must be about 1×10 ⁸ Ω・cm or less, preferably 10 ⁷ Ω・cm or less, and 10 ⁶ Ω・cm
The following are most preferred. The electrical resistance per 1 cm length of a single fiber must be approximately 10 ¹³ Ω/cm or less,
It is preferably 10 ¹² Ω/cm or less, most preferably 10 ¹¹ Ω/cm or less. The polymer used for the protective layer, that is, the non-conductive layer, of the composite fiber is not particularly limited as long as it is fiber-forming. Therefore, it is preferable to use polyamide, polyester, and polyacrylonitrile polymers that are compatible with these polymers and have similar heat resistance, dyeability, etc. Further, the polymer of the conductive layer and the polymer of the protective layer may be the same or different, but from the viewpoint of adhesiveness, heat resistance, dyeability, etc., they are preferably the same or of the same type. The polymers used for these are:
For example, nylon 6, nylon 12, nylon 6
6. Polyamides such as nylon 610 and nylon 612, copolyamides containing these as components, polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene oxybenzoate, and copolyesters containing these as components, polyacrylonitrile, and copolymerized polyacrylonitrile. It will be done. The composite shape and composite ratio of the conductive layer and the protective layer are arbitrary. FIG. 2 shows an example of a three-layer composite in which a conductive layer 3 is sandwiched between two protective layers 4. Examples of other composite shapes include, for example, Japanese Patent Application Laid-Open No. 57-6762 and Japanese Patent Application No. 1983-
No. 69061, and other known methods can also be applied. The short fiber of the present invention is characterized by a low degree of orientation in the crystalline portion of the polymer. Specifically, it is a short fiber in a semi-stretched state or an unstretched state with a low stretching ratio. The advantages of having such a low orientation are (1) excellent conductivity, and (2) sufficient conductivity even when using conductive particles with slightly larger particle sizes and excellent dispersibility, which generally have poor conductivity. (3) Polymers such as polyamide, polyester, and polyacrylonitrile, which generally have poor conductivity but are preferable in terms of heat resistance and dyeability, can be used, so it is practical. A disadvantage of having low orientation is that during use, for example, during the process of blending with other fibers, the fibers are stretched by external force and their conductivity tends to decrease or disappear. Conventionally, undrawn yarns or semi-drawn yarns that can be easily drawn by external force have been impractical and have rarely been used. The present inventors focused on the above-mentioned advantages of using undrawn or semi-drawn conductive composite short fibers, improved the disadvantages, and completed the present invention. The coefficient of friction of low oriented yarns such as undrawn yarns and semi-drawn yarns is generally higher than that of fully oriented highly oriented yarns, sometimes reaching 2 to 3 times as much. However, by using an appropriate lubricant (finishing oil, etc.), this can be reduced to 80% or less, especially 70% or less, and most preferably 60% of the non-lubricated state.
It has been found that by reducing the amount below, for example, blending can be carried out without any trouble, and the decrease in conductivity due to stretching in the blending process can be greatly improved. The short fibers of the present invention can be used by blending a small amount with ordinary chargeable fibers. The blending rate is approximately 0.1 to 10% in most cases, which is an extremely low blending rate. For example, this blend can be made by mixing both chargeable fibers and conductive fibers in the form of cotton, web, and/or sliver, or by mixing them in the form of a tow (doubled yarn blend) and then crimping and shrinking them. Mixed cotton can also be obtained by cutting, etc. Once mixed at a fairly high percentage (eg 10-60%), it can be further diluted with chargeable fibers. It is necessary to reduce the coefficient of friction against metal and the coefficient of friction against fibers depending on the method of these mixing steps. The fiber of the present invention (1) is not stretched after spinning or is stretched at a low stretching ratio, for example, 80% or less of the normal stretching ratio, or/and is subjected to an appropriate heat treatment (such as shrinkage heat treatment) after stretching. (2) Apply a lubricant (hereinafter referred to as oil) to the fiber surface to reduce the coefficient of kinetic friction with metal to 80% of the non-lubricated state.
It can be manufactured by reducing the amount below. Figure 1 shows the drawing ratio of an undrawn yarn obtained by melt-spinning a conductive layer made of conductive particles with an average particle size of 0.25 μm and a thermoplastic resin, and a protective layer made of a fiber-forming polymer, and the drawing ratio of the conductive layer. A specific example of the relationship between conductivity (specific resistance) and the frictional charging voltage of a knitted fabric containing 1% of the composite fiber is shown below. As is clear from the figure, the specific resistance increases as the stretching ratio increases, exceeding 10 ⁷ Ω·cm at a stretching ratio of 2 times or more, and reaching approximately 1 ⁸ Ω·cm at a stretching ratio of 2.5 times. On the other hand, the charging voltage of a knitted fabric containing composite fibers increases rapidly when the stretching ratio exceeds 2.5 times, and does not show any antistatic properties when the stretching ratio exceeds 3 times. The upper limit of the stretching ratio that exhibits antistatic properties is about 2.6 times, and the specific resistance is about ¹⁸ Ω cm. At this time, the degree of orientation of the crystalline part of the polymer that forms the composite fiber is about 89%, and the unstretched That of the yarn (drawing ratio 1.0) was about 64%. Particle size that is generally easy to disperse in polymers and is the easiest to handle
When using conductive particles of about 0.15 to 0.5 μm,
Excellent conductivity (antistatic properties) when the degree of orientation is 89% or less, especially 89% or less, and conductivity (antistatic properties) when the degree of orientation is 90% or more.
is often lost. The degree of orientation can be measured by X-ray diffraction. That is, the half-width θ of the dispersion curve along the Debye ring of the main dispersion peak of the X-ray diffraction due to the crystal plane parallel to the fiber axis is measured and calculated using the formula (). Orientation degree (%) = 180° - θ / 180°
%) They are mixed and used by means such as blending. Therefore, it is necessary to easily and uniformly disperse it into other fibers, and to prevent as much as possible the conductivity from decreasing or disappearing due to stretching by external force during the blending process, etc., and for this purpose, the coefficient of friction must be reduced. FIG. 3 is a schematic diagram showing a method for measuring the coefficient of friction of long fibers against fibers. The sample long fiber 5 passes through a tension adjuster 6 and an inlet tension measuring device 9, is subjected to twisting friction at the twisting portion A, passes through an outlet tension measuring device 14, and is wound into a winder 16.
The rollers 7, 10, 11, 12, and 15 have fixed shafts and are made with ball bearings or the like to minimize friction. Rollers 8 and 13 are connected to a tensiometer. In the twisting part A, the fibers are twisted twice, and the opening angle B is 60°. Generally, the friction coefficient μ is expressed by equation (). μ=C logT2/T1 () However, T ₁ : Inlet tension T ₂ : Outlet tension C : Constant (when measurement conditions are constant) log : Common logarithm Inlet tension is adjusted to about 0.1 g/d by regulator 6 do. The measurement atmosphere is 25℃ and 65%RH. Measure the inlet tension and outlet tension of the thread containing oil and the thread without oil, calculate log(T ₂ /T ₁ ), and calculate the log(T ₂ /T ₁ ) of the thread containing oil and the thread without oil. log( _T2 /
The reduction or increase in the friction coefficient due to the oil agent can be evaluated based on the ratio to T ₁ ). The dynamic friction coefficient is measured at a yarn speed of 20 m/min, and the static friction coefficient is measured at a yarn speed of 2 mm/min. Although the method shown in FIG. 3 cannot measure short fibers, it is useful as a means for selecting an appropriate oil with little measurement error. That is, the short fibers of the present invention can be produced by applying the oil agent (and the amount applied) selected by the method shown in FIG. 3 to the conductive undrawn yarn or semi-drawn yarn. FIG. 4 is a schematic diagram showing a method for measuring the coefficient of friction of short fibers. This method is similar to the radar method, in which a sample short fiber 17 is placed on a roller 18, a tension T ₁ due to a load 19 is applied to one end, and the tension T ₂ at the other end is measured with a tension meter 20. The coefficient of friction against metal μM is measured using a mirror-finished (approximately 0.1S or less) chrome-plated roller 18 with a diameter of 8 mm. The coefficient of friction against fiber μF is determined by using a roller 18 in which sample fibers are stretched in a cylindrical shape around a circular frame with a diameter of about 15 mm. The coefficient of dynamic friction is the surface speed of roller 18 40
m/min, and the static friction coefficient is measured at 4 mm/min. Load 19 is 0.1 mg/d. Load 1 using the method shown in Figure 4.
9 as T ₁ and the measured value of the tension meter 20 as T ₂ , the relationship of equation () is established, and the log of the sample containing oil agent
(T ₂ /T ₁ ) and the log (T ₂ /T ₁ ) of the sample without oil can be used to evaluate the increase or decrease in the coefficient of friction due to oil. The measurement shown in Figure 4 has a rather large error, so the measurement is repeated 20 times and evaluated as an average. The method shown in FIG. 3 is evaluated by averaging five measurements. In either case, keep the rollers and guides clean; when measuring samples that do not contain oil, wash them thoroughly before use; when measuring samples that contain oil,
Measure with only the target oil attached.
The tension is measured using a strain gauge or the like and recorded with a recorder, and the value is read when the tension is stabilized after measuring for 2 minutes or more. A sample containing no oil can be obtained by extracting and removing the oil from a sample containing an oil using a solvent or the like, taking care not to swell or shrink the fibers. Examples of oils include perilla oil (liquid paraffin, etc.),
It is obtained by blending animal and vegetable fats and oils, fatty acids, fatty acid esters, higher alcohols, leveling agents such as silicone oil, cationic activators, anionic activators, nonionic activators, and the like. In general, when there is a large amount of lubricant, the coefficient of kinetic friction against metal μdM decreases, but the coefficient of static friction against fiber μSF
tends to increase. Conversely, as the amount of anionic activator increases, μSF tends to decrease and μdM increases. Nonionic activators reduce μdM but μSF
There are many things that increase the. In order to achieve the purpose of the present invention, it is necessary to prepare and select an oil agent that reduces both μdM and μSF. Such a formulation,
For example, a continuous filament of an undrawn yarn or a drawn yarn is manufactured by using various oils at the time of making a certain yarn, and then
Comparisons can be made using the method shown in the figure. μdM can be measured by using a mirror-finished chrome-plated round bar (diameter 8 mm) in place of the roller 11 in FIG. 3 and omitting the twisted part A. The fibers of the present invention are produced by composite spinning a conductive layer component consisting of conductive particles and a thermoplastic polymer and a normal fiber-forming component by a method such as melting, wet or dry, cutting and applying an appropriate amount of an appropriate oil agent. You can get it. If necessary, stretching at a relatively low magnification, heat treatment under tension or relaxation, shrinkage treatment, dyeing, etc. can be performed. The oil may be applied at the time of spinning and winding, or may be applied in the form of a tow or staple by a dipping method or a spraying method. The oil can also be applied in the form of an aqueous emulsion, a solvent solution, or a pure oil without diluent. Generally, the amount of oil deposited (purity) is often preferably 0.1 to 1.5%, particularly about 0.3 to 0.7%. If the amount of adhesion is too large, μ
SF tends to increase. When the spinning speed is high in melt spinning etc. (1500m/mi
n or more, particularly 2000 m/min or more), the degree of orientation of the undrawn yarn is already quite high, and a phenomenon in which the antistatic property disappears when it is stretched later is observed. Even higher e.g. 3000
At a spinning speed of m/min or more, even if unstretched, the material may become excessively oriented and lose its antistatic properties. However, even if the antistatic property is lost due to excessive orientation, the degree of orientation can be reduced by appropriate shrinkage treatment (heat shrinkage, combined use of a swelling agent and heating, etc.) and the conductivity and antistatic property can be restored. There are many cases where you can get it. The short fibers of the present invention can be made of other electrically chargeable fibers such as silk, wool, cellulose acetate, polyamide,
Polyester, polyolefin, polyvinyl,
It can be mixed with various natural and artificial fibers such as polyacrylonitrile to produce various textile products such as yarn, knitted fabrics, woven fabrics, nonwoven fabrics, ropes, strings, and carpets. If the purpose is to provide normal antistatic properties, the mixing ratio (weight) of conductive composite fiber in the product is 0.1~
About 5% is sufficient, and when particularly excellent antistatic properties are required, the mixing ratio can be about 1 to 50%. In blending, which is one of the most important methods for blending, the short fibers of the present invention have excellent uniform dispersibility into other fibers, and are less likely to reduce or lose electrical conductivity due to external force (stretching) during the blending process. , the final product has excellent antistatic properties. In the following examples, parts and percentages are by weight unless otherwise specified. Example 1 A film of conductive tin oxide (SnO ₂ ) was formed on the surface of titanium oxide (TiO ₂ ), average particle size 0.25 μm (variation range 0.20 to 0.30 μm), specific resistance 6.3 Ω・cm,
Light gray-blue conductive particles with a whiteness of 86% are designated as E1. E
The tin oxide content of No. 1 is 15%, and the antimony oxide content for activating tin oxide is 1.5%. 75 parts of particles E1, 25 parts of nylon 12 powder having a molecular weight of 14,000, and 0.5 part of magnesium stearate were mixed and melt-kneaded three times using a twin-screw kneader to obtain conductive polymer CP1. Molecular weight with 1.6% _TiO2 particles as matting agent
14,000 nylon 12 and conductive polymer CP1 were melt-spun into a structure as shown in Figure 2 at a composite ratio (volume) of 10/1. Combined both components at 260℃, diameter 0.25
It was spun from a mm orifice, cooled, and wound at a speed of 600 m/min while oiling to obtain an undrawn yarn UF1 of 60 d/4 f. The oil used for oiling was 80 parts of lauryl phosphate (potassium salt) and polyethylene glycol.
It contains 10 parts of 300 mol monostearate and 10 parts of oleic acid monoglyceride, and is used as a 5% aqueous dispersion. The amount of oil attached to undrawn yarn is
It is 0.6%. The undrawn yarn UF1 was stretched at various stretching ratios on a stretching bottle at 65° C., and its degree of orientation, conductivity, and antistatic property were measured. That is, the degree of orientation and conductivity of the obtained undrawn yarn and drawn yarn were measured by X-ray as they were, and then twisted with nylon 12 drawn yarn (non-conductive) having a 160 denier/32 filament and a strength of 4.6 g/d. Yarns twisted at several 80 T/M were knitted into a circular knitted fabric of 210 denier/54 filament nylon 6 drawn yarn at intervals of 6 mm, and the frictional charging voltage of the knitted fabric was measured. The mixing ratio of the composite yarn varies depending on the draw ratio, and is about 0.9% when the draw ratio is 3 times, about 1.3% when the draw ratio is 2 times, and about 2.5% when the draw ratio is 1 times (undrawn yarn). Frictional charging voltage can be determined by washing the sample thoroughly to remove oil, etc., and drying it.
After being left in an atmosphere of 25℃ and 33%RH for 12 hours,
Measured in the same atmosphere. That is, rub the sample strongly with a clean cotton cloth 15 times, and measure the electrostatic voltage after 1 minute. Table 1 and FIG. 1 show the relationship between the stretching ratio and the degree of orientation, electrical conductivity (specific resistance), antistatic properties, strength, and elongation. Furthermore, Table 1 shows the ratio of the coefficient of dynamic friction against metal μdM and the coefficient of static friction against fiber μSF of the yarn whose oil agent was extracted and removed with petroleum ether.

[Table] As is clear from Table 1 and Figure 1, the stretching ratio is 2.5 times or more, the degree of orientation is 90% or more, and the specific resistance is 1×10 ⁸
If it exceeds Ω・cm, the antistatic property will be lost. From the standpoint of antistatic properties, the stretching ratio is preferably 2.0 or less and the degree of orientation is 89% or less, particularly 86% or less, and the specific resistance is preferably about 10 ⁷ Ω·cm or less, particularly about 10 ⁶ Ω·cm or less. For example, at a stretching ratio of 2.02, the strength of the conductive thread is 2.
The g/d and elongation are 110%, and there is a high possibility that it will lose its conductivity if it is stretched by strong external force (such as tension). Undrawn yarn UF2 of 600d/40f was obtained under the same spinning conditions as undrawn yarn UF1, and 30 yarns were converged and towed.
Five types of short fibers were obtained by stretching at a stretching ratio of 1.0 (unstretched), 2.0, 2.45, 2.85, and 3.25, crimping by pressing, steam treatment at 95°C, and bias cutting at 39/47 mm.
SF1 to SF5 were obtained. These short fibers are blended at 0.5% with regular nylon 6, 19 denier, triangular cross section, 39/55 mm bias-cut staples (sliver method) to obtain spun yarn, and tufting method is used to create loop pile (pile height 9mm) was flocked, dyed and finished to obtain carpets CP1 to CP5.
The electrostatic voltage of the human body when walking on each carpet with leather sole shoes was measured at 25℃ and 33% RH.The results showed the degree of orientation of short fibers, specific resistance, strength and elongation, coefficient of kinetic friction against metal μdM, and resistance against fibers. Table 2 shows the results of measuring the ratio of the static friction coefficient μSF to that when the oil was extracted and removed using the method shown in FIG.

[Table] For comparison, as spinning oil agents, 70 parts of millet oil (RW.100 seconds), 8 parts of octyl phosphate diethanolamine salt, 3 parts of potassium oleate salt, 400 moles of polyethylene glycol, 10 moles dilaurate
A mixture of 10 parts of polyethylene oxide, 7 parts of nonyl phenyl ether, and 2 parts of lauryl alcohol was used as a 10% aqueous dispersion.
Short fibers CF1 to CF5 obtained in the same manner as 1 to SF5
A similar experiment was conducted with respect to the following, and the results shown in Table 3 were obtained.

[Table] In the comparative example above, the reasons why the antistatic properties of the carpet are poor are: firstly, the dispersion of the conductive fibers into other fibers is poor, resulting in poor mixing uniformity, and secondly, during the blending process. This is presumed to be due to the fact that in many cases the conductivity decreased or disappeared due to stretching. Example 2 Instead of nylon 12 in Example 1, molecular weight
Using 17000 nylon 6, the following SF1 to SF5
Short fibers SF6 to SF10 were obtained in the same manner as above. Also
Wash and remove the SF6 to SF10 oil and replace it with polyethylene glycol/polypropylene glycol equimolar random copolymer 92 with a molecular weight of 10,000.
A 5% aqueous dispersion of a mixture of 5 parts of potash octyl phosphate, and 3 parts of sorbitan monolaurate was applied using a sprayer so that the oil content was 0.6%, and dried to obtain short fibers CF1 to CF10. Table 4 shows the properties of each short fiber and the measurement results of the human body electrostatic potential of a carpet made by blending 0.5% of each short fiber.

[Table] Examples SF6 to SF9 according to the present invention are comparative example SF1
It is clear that this is superior to 0.0, CF6 to CF10.

[Brief explanation of the drawing]

Figure 1 shows a specific example of the relationship between the draw ratio and the conductivity and antistatic properties of a fiber that is a composite of a conductive layer made of inorganic conductive particles and a thermoplastic polymer, and a protective layer made of a fiber-forming polymer. . FIG. 2 is a cross-sectional view of a composite fiber showing a specific example of a composite shape of a conductive layer and a protective layer. FIGS. 3 and 4 are schematic diagrams showing a comparison or measurement method of friction coefficients.

Claims (1)

[Scope of Claims] 1 (a) A conductive layer made of conductive inorganic particles and a thermoplastic polymer and a protective layer made of a fiber-forming polymer, (b) a composite of a crystalline portion of the polymer forming the fibers. The degree of orientation is 89% or less, and (c)
A short fiber characterized by having a coefficient of dynamic friction against metal and a coefficient of static friction against fibers reduced by a lubricant to 80% or less of the non-lubricated state. 2. The short fiber according to claim 1, wherein the conductive inorganic particles are at least one selected from the group of metal particles, metal compound particles, and particles having a conductive film made of a metal or a metal compound. 3 The average diameter of conductive inorganic particles is 0.1 to 0.5 μm
The short fiber according to claim 1, which falls within the range of . 4. The short fiber according to claim 1, wherein the polymer forming the protective layer and the conductive layer is polyamide, polyester, or acrylic polymer. 5. The short fiber according to claim 1, wherein the polymers forming the protective layer and the conductive layer are the same or of the same kind. 6. The short fiber according to claim 1, wherein the short fiber is a semi-drawn yarn or an undrawn yarn drawn at a draw ratio of 2.5 times or less. 7. The short fiber according to claim 1, wherein the coefficient of dynamic friction with respect to metal and the coefficient of static friction with respect to fibers of the short fiber are reduced by a lubricant to 70% or less of the non-lubricated state.