Milled carbon fiber and process for producing the same

Milled carbon fibers are provided which have a fiber cut surface and a fiber axis intersecting with each other at cross angles, the smaller one thereof being at least 65.degree. on the average. The milled carbon fibers may have a specific surface area as measured by the BET method of 0.2 to 10 m.sup.2 /g. The milled carbon fibers may be obtained by a process comprising melt spinning of mesophase pitch, infusibilization, milling of the infusibilized pitch fibers as obtained or after a primary heat treatment at low temperatures in an inert gas and a high-temperature heat treatment in an inert gas. Even when the graphite layer plane has achieved high-level growth, the above milled carbon fibers have low reactivity with a metal of high temperature or the like during the molding or use thereof because the proportion of reactive exposed surface of the inner portion of the fiber is small, so that the use of the milled carbon fibers can improve the mechanical strength and high-temperature heat resistance of the composite material.

FIELD OF THE INVENTION
 The present invention relates to milled carbon fibers. More particularly,
 the present invention is concerned with milled carbon fibers which have a
 large surface area available for contact with metals, etc., so that it is
 suitable for improving the rigidity and high-temperature heat resistance
 of metals, alloys and the like, thereby ensuring advantageous utilization
 thereof in, for example, carbon-fiber-reinforced composite materials.
 Also, the present invention is concerned with a process for producing the
 milled carbon fiber.
 BACKGROUND OF THE INVENTION
 The carbon fiber is lightweight and has high strength and rigidity, so that
 in recent years it is utilized in a wide spectrum of fields from the
 aerospace and aircraft industry to the general industries.
 For example, carbon-fiber-reinforced plastics are actually widely utilized
 as structural materials having high specific strength and specific modulus
 of elasticity. Further, carbon-fiber-reinforced metals (CFRM), such as
 carbon-fiber-reinforced aluminum alloys and carbon-fiber-reinforced
 magnesium alloys (hereinafter referred to as "CFRAl(Mg)"), have been
 developed as materials having excellent high-temperature dimensional
 stability and thermal deformation resistance, and their use is anticipated
 as a material for use in structural members for aerospace and aircraft and
 engine members for vehicles.
 However, the production of CFRAl(Mg) has encountered, for example, a
 problem such that not only is the wettability of the carbon fiber with
 molten Al (or Mg) poor but also, once the wetting is effected, the carbon
 fiber reacts with Al to thereby form Al.sub.4 C.sub.3 with the result that
 the strength of the material is lowered.
 The amount of formed Al.sub.4 C.sub.3 is connected with the type of the
 carbon fiber. That is, the carbon fiber produced by heat treating at a
 temperature of about 2000.degree. C., known as "graphitized carbon fiber",
 has a high carbon crystallization degree and a strong carbon-to-carbon
 bond to render itself stable, as compared with the carbon fiber produced
 by heat treating at a temperature of about 1500.degree. C., known as
 "carbonized carbon fiber", so that the reactivity with molten Al alloy or
 the like is poor, thereby minimizing the formation of carbides, such as
 aluminum carbide.
 Therefore, the mechanical properties of the CFRAl(Mg) are superior when the
 graphitized carbon fiber is used as reinforcement.
 The graphite crystals of the graphitized carbon fiber are generally highly
 anisotropic from the dynamical, electrical and scientific viewpoints,
 because the carbons interact each other between the graphite layer planes
 with only weak intermolecular force while the sp.sup.2 carbons are
 strongly bonded within each of the graphite layer planes (c-planes).
 In the so-called monoaxially oriented structure in which the c-planes are
 arranged parallel to the fiber axis, there may be some mutually different
 microstructures or high-order structures, depending on the type of the
 carbon fiber precursor [polyacrylonitrile (PAn), rayon, pitch, etc.].
 Of the above precursors, when mesophase pitch with greater graphitizability
 is used as a starting material, the graphitization is more readily
 promoted even at the same heat treating temperature to thereby produce
 carbon fibers having higher modulus of elasticity. Therefore, the use of
 carbon fibers of high elastic modulus derived from mesophase pitch is
 especially promising in the formation of a composite with an aluminum
 alloy and the like.
 On the other hand, from the viewpoint of moldability, the use of milled
 carbon fibers is advantageous in respect of the degree of freedom of
 molding and molding/working costs, although the molding with the use of
 lengthy carbon fibers is suitable for producing a fiber-reinforced metal
 composite having excellent mechanical properties.
 The use of the milled carbon fibers in the fiber-reinforced metal composite
 leads to the increase of the surface area brought into contact with
 metals. The opportunity of reaction with the metals becomes high as much
 as the above increase, so that greater attention must be paid to the
 formation of carbides.
 Coating with silicon carbide or precoating with a matrix metal, such as
 aluminum, at low temperatures has been tried for the purpose of improving
 the wettability with metals and suppressing the above reaction.
 However, these conventional trials have had a drawback in that the efficacy
 is low for the cost increase involved.
 The inventors have made extensive and intensive studies with a view toward
 resolving the above problems. As a result, they have found that the
 configuration of the milled carbon fiber, especially the morphology of the
 surface thereof, has an important relationship with the formation of
 carbides with metals, and that the reaction of the milled carbon fiber
 with metals can be suppressed by improving the above configuration. The
 present invention has been completed on the basis of the above findings.
 OBJECT OF THE INVENTION
 The present invention has been made with a view toward obviating the above
 drawbacks of the prior art. Thus, the object of the present invention is
 to provide milled carbon fibers which have desirably grown graphite layer
 planes and accordingly a low reactivity with metals, so that it can
 provide a lightweight and rigid fiber-reinforced metal having excellent
 heat resistance at high temperatures, and also to provide a process for
 producing the desired milled carbon fibers.
 SUMMARY OF THE INVENTION
 The milled carbon fibers of the present invention are one produced from
 mesophase pitch, which have a fiber cut surface and a fiber axis
 intersecting with each other at cross angles, the smaller one thereof
 being at least 65.degree. on the average.
 The milled carbon fibers of the present invention preferably have a
 specific surface area as measured by the BET method of 0.2 to 10 m.sup.2
 /g.
 The process for producing milled carbon fibers according to the present
 invention comprises the steps of:
 melt spinning mesophase pitch to obtain pitch fibers;
 infusibilizing the obtained pitch fibers;
 milling the infusible pitch fibers as obtained or after a primary heat
 treatment at 250 to 1500.degree. C. in an nert gas; and
 subjecting the obtained milled fibers to a high-temperature heat treatment
 at 1500.degree. C. or higher in an inert gas.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will now be illustrated.
 The pitch as the starting material of the milled carbon fiber according to
 the present invention is optically anisotropic pitch, i.e., mesophase
 pitch. The mesophase pitch can generally be produced from petroleum, coke
 and other various raw materials. The mesophase pitch as the starting
 material for use in the present invention is not particularly limited as
 long as it is spinnable.
 The desired mesophase-base carbon fiber produced by subjecting the above
 starting pitch to spinning, infusibilization and carbonization or
 graphitization according to the customary procedure permits free control
 of the crystallization degree thereof.
 The terminology "milled carbon fiber" used herein means a carbon fiber
 which is shorter than the carbon fiber of about 1 to 25 mm generally known
 as "chopped strand" and which has a length of about 1 mm or less.
 The milled carbon fibers of the present invention have a fiber cut surface
 and a fiber axis intersecting with each other at cross angles, the smaller
 one thereof being at least 65.degree., preferably at least 70.degree.,
 still preferably at least 75.degree. on the average. The cross angle of
 the fiber cut surface and the fiber axis intersecting with each other will
 be illustrated below with reference to the appended FIGURE. The appended
 FIGURE is a schematic perspective of an end portion of the milled carbon
 fiber provided for explaining the cross angle of the fiber cut surface and
 the fiber axis of the carbon fiber intersecting with each other. As
 illustrated, the carbon fiber 1 has a fiber cut surface (s) formed by the
 milling at an end portion thereof. In the present invention, the smaller
 angle (.theta.), on the average, of the cross angles of the fiber cut
 surface (s) and the fiber axis (d) of the carbon fiber 1 intersecting with
 each other is used as the above value for numerical limitation.
 Herein, the average of the cross angle (.theta.) is an average of the cross
 angles of at least 100 milled carbon fibers. In the calculation of the
 average of the cross angle (.theta.), when the carbon fiber has suffered
 from longitudinal crack along the fiber axis (d) on the fiber cut surface
 during milling, the cross angle (.theta.) is defined to be 0.degree.. The
 average of the cross angle (.theta.) of the fiber cut surface (s) and the
 fiber axis (d) intersecting with each other can be measured by the use of
 a scanning electron microscope (SEM).
 The milled carbon fibers having an average of the cross angle (.theta.) of
 the fiber cut surface (s) and the fiber axis (.alpha.) intersecting with
 each other which is at least 65.degree. are cylindrical in the entire
 configuration thereof and have no sharply projecting portions such as an
 acicular portion from the fiber cut surface. That is, the milled carbon
 fiber of the present invention is cylindrical in the entire configuration
 thereof, and has a fiber cut surface nearly perpendicular to the fiber
 axis, in which the graphite layer has few sharp unevennesses inside.
 The distribution of graphitization degree in the direction of the inner
 diameter of the cut surface of the carbon fiber produced from a starting
 pitch material is reported in G. Katagiri, H. Ishida and A. Ishitani,
 carbon 26, 565 (1988). This reference shows that the nearer the surface
 the portion concerned, the greater the graphitization degree and the
 higher the crystallization degree there. Also, as mentioned above, it is
 preferred that the reinforcing carbon fiber for use in the CFRM be
 graphitized for reducing the formation of carbides due to the reaction
 with molten alloys. Therefore, in the carbon fiber derived from mesophase
 pitch, it is important that the carbon with low crystallization degree
 having originally been present inside the fiber is less exposed to the
 surface of the fiber during the milling.
 On the other hand, the inventors' study and observation have revealed that
 the angle of the cutting of the carbon fiber becomes nearly parallel to
 the fiber axis, depending on the force applied to the carbon fiber during
 milling, so that the carbon fiber is cleaved along the graphite layer
 plane to thereby expose much of sharply uneven graphite layer plane
 present inside the fiber and, in extreme cases, to render the fiber
 acicular. The above average of the cross angle (.theta.) of this milled
 carbon fibers is less than 65.degree..
 The above milled carbon fibers which are extremely marked in the area of
 exposure of the graphite layer plane having originally been present inside
 the carbon fiber, the above exposure resulting from the frequent cleavages
 along the fiber axis and along the graphite layer plane during milling,
 that is, the milled carbon fibers whose average of the cross angle
 (.theta.) is less than 65.degree., are disadvantageous in molding and
 long-time use at high temperatures. This is because, when the temperature
 is high during the molding and use, the formation of carbide due to the
 contact with the metal is extremely increased, thereby gravely
 deteriorating the strength of the carbon-fiber-reinforced metal.
 This strength deterioration would be attributed to an extreme increase in
 the area of exposure of the reactive graphite layer plane having
 originally been present inside the fiber, the above exposure resulting
 from cleavage along the fiber axis during the milling, which increase
 would cause the reaction between the metal and the carbon to proceed on
 the graphite layer plane.
 For being suitable for use as metal fiber reinforcement, it is preferred
 that the milled carbon fibers of the present invention have a relatively
 small specific surface area. Specifically, it is preferred that the
 specific surface area as measured by the BET method be in the range of 0.2
 to 10 m.sup.2 /g, especially 0.3 to 7 m.sup.2 /g. The specific surface
 area of the milled carbon fibers is measured in accordance with the BET
 one-point method in sorption and desorption of nitrogen gas at a relative
 pressure of 0.3.
 When the above specific surface area is less than 0.2 m.sup.2 /g, the
 wettability of the milled carbon fibers with a metal is likely to decrease
 so as for bubbles to remain between the fibers and the metal during the
 molding, thereby deteriorating the strength properties of the
 carbon-fiber-reinforced metal.
 On the other hand, when the above specific surface area exceeds 10 m.sup.2
 /g, the surface area brought into contact with the metal is likely to be
 extremely high so as to increase the opportunity of carbide formation,
 thereby lowering the strength of the carbon-fiber-reinforced metal.
 The milled carbon fibers of the present invention have been described, and,
 hereinafter, the process for producing the milled carbon fibers will be
 described.
 The process for producing the milled carbon fibers of the present invention
 is not particularly limited as long as the value of the cross angle of the
 fiber cut surface and the fiber axis intersecting with each other is as
 described above and as, preferably, the value of the specific surface area
 as measured by the BET method is also as described above.
 The above process, for example, comprises spinning the above mesophase
 pitch to obtain pitch fibers, infusibilizing the pitch fibers, milling the
 obtained infusible pitch fibers and effecting carbonization/graphitization
 of the milled fibers.
 The pitch fiber may be spun by any of the conventional melt, centrifugal,
 vortex and other spinning techniques. Especially, the melt blow spinning
 technique is preferred, collectively taking into account the production
 costs including spinning apparatus construction and operating costs and
 the quality control including the degree of freedom in controlling fiber
 diameters.
 The thus obtained pitch fiber is infusibilized by the conventional method.
 Although this infusibilization can be effected by heating in an oxidative
 atmosphere of air, oxygen, nitrogen dioxide or the like or treating in an
 oxidative solution of nitric acid, chromic acid or the like, practically,
 it is preferred that the infusibilization be performed by heating in air
 at temperatures ranging from 150 to 350.degree. C. in which the heating
 temperature is elevated at a heat-up rate of 3 to 10.degree. C./min.
 The infusibilized pitch fiber may directly be milled and subjected to
 high-temperature heat treatment for carbonization/graphitization.
 Alternatively, it may first be subjected to primary heat treatment at
 lower temperatures, and then milled and subjected to the high-temperature
 heat treatment.
 The milling of the infusibilized pitch fiber or the primarily heat-treated
 carbon fiber may be performed by a procedure comprising revolving a rotor
 equipped with a blade at a high speed and contacting the fiber with the
 blade to thereby cut the fiber in the direction perpendicular to the fiber
 axis. In this procedure, the milling may be performed by the use of, for
 example, the Victory mill, jet mill or cross flow mill. In the above
 procedure, the length of the milled pitch (or carbon) fiber can be
 controlled by regulating the rotating speed of the rotor, the angle of the
 blade, the size of porosity of a filter attached to the periphery of the
 rotor, etc.
 In the prior art, the milling of the carbon fiber has also been performed
 by means of the Henschel mixer, ball mill or mixing machine. This milling
 cannot be stated to be an appropriate procedure because not only does
 pressure apply to the carbon fiber in the direction of the diameter
 thereof to thereby increase the probability of longitudinal cracks along
 the fiber axis but also the milling takes a prolonged period of time.
 The above primary heat treatment prior to the milling may be performed in
 an inert gas at 250 to 1500.degree. C., preferably 400 to 1200.degree. C.,
 still preferably 600 to 1000.degree. C.
 In the carbon fiber derived from mesophase pitch, the crystallization
 degree of the carbon is increased with the increase of the heat treating
 temperature, thereby growing the graphite layer, whose plane is oriented
 parallel to the fiber axis. Thus, when heat treatment is conducted in an
 inert gas at temperatures exceeding 1500.degree. C. before milling, the
 carbon fiber is likely to suffer from cleavage and breakage along the
 graphite layer plane having grown along the fiber axis. The resultant
 milled carbon fiber is not desirable because the proportion of reactive
 broken surface area to the total surface area of the milled carbon fiber
 is high to thereby promote the reaction between the reactive carbon and
 the metal.
 The milled mesophase-pitch-based infusibilized pitch fiber obtained by
 milling directly after the infusibilization or the milled primarily
 heat-treated carbon fiber obtained by milling after the primary heat
 treatment, is subjected to a high-temperature heat treatment at
 1500.degree. C. or higher, preferably 1700.degree. C. or higher, still
 preferably 2000.degree. C. or higher.
 High-temperature heat treatment at temperatures lower than 1500.degree. C.
 is not suitable because the degree of graphitization of the milled carbon
 fiber is so low that the reaction with metals is likely to occur.
 The high-temperature heat treatment after milling causes highly reactive
 carbon exposed on the cut surface from the fiber interior during milling
 to undergo cyclization and thermal polycondensation, so that the fiber cut
 surface can be converted to the state of low reactivity.
 EFFECT OF THE INVENTION
 As described above, the milled carbon fibers of the present invention have
 a fiber cut surface and a fiber axis intersecting with each other at cross
 angles, the smaller one thereof being at least 65.degree. on the average.
 Thus, even when the graphite layer plane has achieved high-level growth,
 the above milled carbon fiber has low reactivity with a metal of high
 temperature or the like during the molding or use thereof because the
 proportion of reactive exposed surface of the inner portion of the fiber
 is small, so that the use of the milled carbon fiber can improve the
 mechanical strength and high-temperature heat resistance of the carbon
 fiber/metal composite material.
 The process for producing milled carbon fibers according to the present
 invention comprises melt spinning of mesophase pitch, infusibilization,
 milling of the infusible pitch fibers as obtained or after a primary heat
 treatment at 250 to 1500.degree. C. in an inert gas, and a
 high-temperature heat treatment at 1500.degree. C. or higher in an inert
 gas. Thus, not only can milled carbon fibers for metal reinforcement
 having low reactivity with a metal of high temperature or the like during
 the molding or use thereof so as to be suitable for improvement of the
 mechanical strength and high-temperature heat resistance of the composite
 material be provided, but also the degree of graphitization of the carbon
 fiber can be regulated by selecting appropriate temperature in the
 high-temperature heat treatment, so that materials suitable for
 intercalation into graphite layers or for application to fields where the
 crystallinity of the graphite is utilized can be obtained.
 EXAMPLES
 The present invention will further be illustrated with reference to the
 following Examples, which should not be construed as limiting the scope of
 the invention.
 Example 1
 A starting material of optically anisotropic petroleum mesophase pitch
 having a softening point of 280.degree. C. was melted and drawn through a
 nozzle comprising a 3 mm wide slit and, arranged therein, a line of 1500
 spinning orifices each having a diameter of 0.2 mm while injecting hot air
 through the slit, thereby obtaining pitch fibers. The spinning was
 conducted at a pitch discharge rate of 1500 g/min, a pitch temperature of
 340.degree. C., a hot air temperature of 350.degree. C. and a hot air
 pressure of 0.2 kg/cm.sup.2 G.
 The spun pitch fibers were collected on a belt having a collection zone of
 20-mesh stainless steel net while sucking fiber carrying air from the back
 of the belt.
 The resultant collected fiber mat was heated in air while elevating the
 temperature from room temperature to 300.degree. C. at an average heat-up
 rate of 6.degree. C./min to thereby infusibilize the fiber mat.
 Part of the thus obtained infusibilized mesophase-pitch-based fibers were
 milled with the use of a cross flow mill to obtain milled infusibilized
 fibers, which were successively graphitized at 2650.degree. C. in argon.
 An SEM observation of the thus obtained milled carbon fibers derived from
 mesophase pitch showed that the smaller cross angle of the fiber cut
 surface and the fiber axis intersecting with each other was 87.degree. on
 the average, and that the specific surface area of the milled carbon
 fibers was 1.5 m.sup.2 /g.
 The average length of the milled carbon fibers was 750 .mu.m.
 The thus obtained milled carbon fibers and a powdery aluminum alloy
 containing 4.5 wt.% of magnesium were uniformly mixed in a weight ratio of
 25:75, and charged into a metal mold.
 The charged mixture was held at 450.degree. C. for 30 min, and hot-press
 molded under a pressure of 1000 kg/cm.sup.2 for 20 min into a test
 specimen of 2 mm in thickness, 10 mm in width and 70 mm in length.
 This test specimen was subjected to the 3-point bending test according to
 JIS (Japanese Industrial Standard) R7601, and the bending strength was
 determined to be 18 kg/mm.sup.2.
 Another test specimen was prepared in the same manner as above, heated at
 600.degree. C. for 5 hr, and subjected to the above bending test. The
 bending strength was 17 kg/mm.sup.2, which indicated that there was
 substantially no strength deterioration.
 Example 2
 Another part of the fibers infusibilized in Example 1 were successively
 subjected to a primary heat treatment at 1250.degree. C. in nitrogen,
 milling and a high-temperature heat treatment at 2500.degree. C. in argon.
 The obtained milled carbon fibers had an average smaller cross angle of
 82.degree., a specific surface area of 6.8 m.sup.2 /g, and an average
 fiber length of 700 .mu.m.
 A test specimen of fiber-reinforced aluminum alloy was prepared from the
 milled carbon fibers derived from mesophase pitch, and the bending test
 thereof was performed in the same manner as in Example 1.
 The bending strengths measured immediately after molding and after
 successive heating for the predetermined period were 17 kg/mm.sup.2 and 15
 kg/mm.sup.2, respectively.
 Comparative Example 1
 Still another part of the fibers infusibilized in Example 1 were
 successively subjected to a high-temperature heat treatment at
 2500.degree. C. and milling. An SEM observation showed that many of the
 milled fibers suffered from longitudinal cracks along the fiber axis, that
 the average smaller cross angle was 57.degree., and that the cut surfaces
 were markedly uneven.
 The milled fibers had a specific surface area of 12.3 m.sup.2 /g and an
 average fiber length of 650 .mu.m. The 3-point bending test was conducted
 in the same manner as in bending test was conducted in the same manner as
 in Examples 1 and 2. The bending strength immediately after the test
 specimen molding was 15 kg/mm.sup.2 which could stand comparison with
 those of the Examples. However, the bending strength after successive
 heating at 600.degree. C. was 7 kg/mm.sup.2, which indicated an extreme
 deterioration of the bending strength.