Silicon-containing carbide fibers having high strength and modulus of elasticity, and polymer compositions for production thereof. These fibrers are inexpensive and very useful as reinforcing fibers for composite materials comprising plastics, carbon, metals, etc. as a matrix. These polymer compositions are also useful as a matrix of the above composite materials.

Carbon fibers have light weight, high strength and high modulus of elasticity, and therefore, their utility is not only in sporting and leasure goods, but has been expanded to a wide range of fields including aircraft, automobiles and building materials.
PAN-type carbon fibers derived from polyacrylonitrile as a raw material and pitch-type carbon fibers obtained from petroleum and coal pitches as raw materials are known as the carbon fibers.
Japanese Laid-Open Patent Publication No. 223316/1984 discloses a process for producing fibers having high strength and high modulus of elasticity, which comprises
(a) hydrogenating a pitch, separating the solid from the hydrogenation product, and removing low-boiling components by distillation to obtain a hydrogenated pitch,
(b) heat-treating the hydrogenated pitch under reduced pressure to give a mesophase pitch (containing not more than 90% by weight of mesophase carbon and at least 30% of optically anisotropic fibers), and thereafter PA1 .sigma..sub.c : the tensile strength of the composite material PA1 .sigma..sub.f : the tensile strength of the fibers PA1 .sigma..sub.M : the tensile strength of the matrix PA1 V.sub.f : the volume percent of the fibers PA1 V.sub.M : the volume percent of the matrix
(c) melt-spinning the mesophase pitch, and rendering the fibers infusible and carbonize them.
International Patent Laid-Open WO87/05612 and Japanese Laid-Open Patent Publication No. 209139/1987, the corresponding Japanese priority application, discloses an organopolyarylsilane being soluble in organic solvents and comprising organosilane segments in which the skeletal portion is composed mainly of carbosilane and polysilane, said segments being connected at random via silicon-carbon linking groups.
Laid-Open International Patent WO 87/05612 and Japanese Laid-Open Patent Publication No. 215016/1987, the corresponding Japanese priority application, disclose continuous SiC-C type inorganic fibers composed of molecules having carbon and SiC as main constituents and containing 5 to 55% by weight of Si, 40 to 95% by weight of C and 0.01 to 15% by weight of 0, said inorganic fibers showing excellent thermally resistant strength and oxidation resistance with a volume resistivity of 10 to 10.sup.-3 ohms-cm.
The above laid-open specifications describe a process for producing inorganic fibers having properties intermediate between the silicon carbide fibers and carbon fibers, which comprise mixing an organic solvent-soluble component of a coal or petroleum pitch with a polysilane, and reacting the mixture under heat to synthesize an organopolyarylsilane, and spinning it and rendering the fibers infusible and curing the fibers.
However, in the above process, a pitch quite free from an organic solvent-insoluble portion is selected as one of the starting materials, and in the production of the organopolyarylsilane, the reaction is carried out under such conditions that no organic solvent-insoluble portion is formed.
Accordingly, the resulting product as a spinning material does not at all contain the above insoluble portion in the mesophase, which is said to be the most important component for development of strength by carbon fibers.
Inorganic fibers obtained by spinning, rendering the fibers infusible and curing them gives a diffraction line (002) corresponding to the graphite crystals of carbon under certain conditions, but no orientation inherent to pitch fibers is noted. Furthermore, in the process described in the above patent documents, the heat resistance of the fibers in an inert gas is enhanced as the proportion of the pitch content increases. But, on the contrary, the oxidation resistance of the fibers is decreased, and moreover, their mechanical characteristics tend to be reduced markedly.
Japanese Laid-Open Patent Publication No. 7737/1987 discloses a composite material comprising a matrix of a plastic and as a reinforcing material hybrid fibers consisting of inorganic fibers containing silicon, titanium (or zirconium), carbon and oxygen and at least one kind of fibers selected from the group consisting of carbon fibers, glass fibers, boron fibers, aramid fibers and silicon carbide fibers having carbon as a core.
Japanese Laid-Open Patent Publication No. 266666/1986 discloses a coontinuous fiber bundle for use in a composite material, said fiber bundle comprising continuous fibers of ceramics (silicon carbide, silicon nitride, alumina, etc.) or a heat-resistant material (carbon, metals, etc.) and short fibers, whiskers or powders of the same material as above adhering to the surface of the continuous fibers.
Japanese Laid-Open Patent Publication No. 195076/1985 discloses a method of improving the surface hardness and oxidation resistance of carbon fibers, which comprises adhering or contacting a silicon-containing material to or with the surface of a carbonaceous material, melting the silicon-containing material to form a modified layer composed of silicon carbide and carbon on the surface.
Japanese Laid-Open Patent Publication No. 251175/1985 discloses a process for producing a molded article composed of silicon carbide and carbon, which comprises slowly oxidizing a molded carbon article at 400.degree. to 600.degree. C. to render it light in weight and porous, and then allowing a silicon-containing material to penetrate into the pores and react at a temperature above the meling point of the silicon-containing material.
It is an object of this invention to provide novel fibers having high strength and high modulus of elasticity.
Another object of this invention is to provide fibers having high strength and high modulus of elasticity containing crystalline carbon oriented in the direction of the fiber axis and consisting essentially of silicon, carbon and oxygen.
Still another object of this invention is to provide fibers having high strength and high modulus of elasticity which when used as a reinforcing material for a composite material, shows excellent wettability with a matrix material.
Yet another object of this invention is to produce high strength and high modulus fibers which has much higher modulus of elasticity than silicon carbide fibers and excellent oxidation resistance with their oxidation resistant temperature being higher by about 200.degree. to 300.degree. C. than conventional pitch-type carbon fibers or the PAN-type carbon fibers.
A further object of this invention is to provide a polymer composition suitable for production of the fibers of this invention.
Other objects of this invention along with its advantages will become apparent from the following description.
According to this invention, the above objects and advantages of this invention are firstly achieved by fibers having high strength and high modulus of elasticity comprising
(i) crystalline carbon oriented substantially in the direction of the fiber axis,
(ii) amorphous carbon and/or crystalline carbon oriented in a direction different from the fiber axis direction, and
(iii) a silicon-containing component consisting essentially of 30 to 70% by weight of Si, 20 to 60% by weight of C and 0.5 to 10% by weight of 0, the proportions being based on the total weight of silicon, carbon and oxygen.
The above fibers of the invention (to be sometimes referred to as the first fibers of the invention) can be produced by a process which comprises preparing a spinning dope of a polymer composition comprising
(A) an organic silicon polymer resulting from random bonding of a plurality of at least one type of bond selected from the group consisting of units represented by the following formula (a) ##STR1## wherein R.sup.1 and R.sup.2, independently from each other, represent a hydrogen atom, a lower alkyl group, a phenyl group or a silyl group (--SiH.sub.3), either via methylene groups (--CH.sub.2 --) or both via methylene groups and directly,
(B) a polycyclic aromatic compound in the state of a mesophase, a premesophase or a latently anisotropic phase, and
(C) a polycyclic aromatic compound which is optically isotropic but is not in the state of a premesophase or a latently anisotropic phase, at least a part of component (A) being chemically bound to component (B) and/or component (C); spinning the spinning dope; rendering the spun fibers infusible under tension or no tension; and pyrolyzing the infusible fibers at a temperature of 800.degree. to 3,000.degree. C. in vacuum or in an inert gaseous atmosphere.
The polymer composition used in the spinning step has been provided for the first time by the present inventors, and constitutes part of the invention.
The novel polymer composition can be produced by heating the organic silicon polymer (A) and a pitch which has no excessive heat history in an inert gas, preferably at a temperature of 250.degree. to 500.degree. C., and melting the resulting reaction product at 200.degree. to 500.degree. C. together with a pitch mainly having a mesophase, a premesophase or a latently anisotropic phase.
The novel polymer composition and a process for its production will first described, and then, the above fibers of the invention and a process for production thereof.
The organic silicon polymer (A) is obtained by the random bonding of the plurality of the bond units of formula (a) via methylene groups (--CH.sub.2 --) or via methylene groups and directly.
In formula (a), R.sup.1 and R.sup.2, independently from each other, represent a hydrogen atom, a lower alkyl group, a phenyl group or a silyl group (--SiH.sub.3). Examples of the lower alkyl group are linear or branched alkyl groups having 1 to 4 carbon atoms such as methyl, ethyl, propyl and butyl groups.
The organic silicon polymer (A) can be produced, for example, by reacting dimethyldichlorosilane and metallic sodium to produce polymethylsilane, and heating the polymethylsilane at a temperature of at least 400.degree. C. in an inert gas. In this example, an organic silicon polymer in which a plurality of units of formula (a) wherein R.sup.1 and R.sup.2, independently from each other, are hydrogen and methyl are bonded randomly via methylene groups, or both via methylene groups and directly. It will be understood that when part of dimethyldichlorosilane is replaced by diphenyldichlorosilane, an organic silicon polymer is obtained which has units of formula (a) wherein R.sup.1 and R.sup.2 in formula (a), independently from each other, represent hydrogen, methyl and phenyl.
The organic silicon polymer (A) has a weight average molecular weight (Mw) of preferably 300 to 1,000, especially preferably 400 to 800.
The pitch which has no excessive heat history may be originated from petroleum or coal. In particular, distilled oils or residual oils obtained by distilling heavy oils produced by fluidized catalytic cracking of petroleums, or heat-treated products of the distilled oils or the residual oils are preferably used. These pitches are usually optically isotropic (these pitches will be called optically isotropic pitches hereafter).
Preferably, the optically isotropic pitches contain 5 to 98% by weight of components insoluble in organic solvents such as benzene, toluene, xylene and tetrahydrofuran.
These pitches are polycyclic aromatic compounds if their chemical structure is considered, and are preferably relatively high-molecular-weight compounds having a weight average molecular weight of about 100 to 3,000. The weight average molecular weight may be measured directly by gel permeation chromatography (GPC) if the pitch does not contain components insoluble in organic solvents. On the other hand, when the pitch has components insoluble inorganic solvents, the pitch is hydrogenated under mild conditions to change the organic solvent-insoluble components into organic solvent-soluble components, and the molecular weight of the treated pitch is then measured by GPC.
The organic silicon polymer (A) and the optically isotropic pitches are heated and reacted in an inert gas such as nitrogen gas or argon gas, preferably at a temperature of 250.degree. to 500.degree. C. If the reaction temperature is excessively low, the reaction (for example, the bonding of the aromatic carbons of the pitch to the organic silicon polymer) is difficult. If, on the other hand, the reaction temperature is excessively high, the decomposition of the reaction product and its conversion to a higher-molecular-weight product occur vigorously.
The proportion of the pitch used in this reaction is preferably 83 to 4,900 parts by weight per 100 parts by weight of the organic silicon compound. If the proportion of the pitch used is too small, the amount of silicon carbide component in the finally obtained fibers is large, and fibers having a high modulus of elasticity are difficult to obtain. If this proportion is excessively large, the amount of the silicon carbide component formed becomes small, and fibers having excellent wetting property with respect to the matrix and excellent oxidation resistance are difficult to obtain.
The reaction product obtained by the above reaction is then heat-melted with a pitch in the mesophase, the premesophase or in a latently anisotropic state.
The mesophase pitch can be prepared by heating a petroleum or coal pitch at 300.degree. to 500.degree. C. in an inert gas, and polycondensed while the resulting light fractions are removed.
A suitable petroleum or coal pitch contains 5 to 98% by weight of components insoluble in an organic solvent such as benzene, toluene, xylene or tetrahydrofuran like the pitch used to react with the organic silicon polymer.
By heat-treating the above starting pitch either directly or after as required, components soluble in organic solvent are removed, the mesophase pitch can be obtained. The advantage of removing the organic solvent-soluble components is to facilitate the formation of a mesophase by removing the soluble components which are difficult of forming a mesophase and to obtain a pitch having high optical anisotropy and a low melting point.
The mesophase pitch is a polycyclic aromatic compound in view of its chemical structure. Preferably, it has a melting point of 200.degree. to 400.degree. C., a weight average molecular weight of 200 to 10,000 and a degree of optical anisotropy of 20 to 100%, and contains 30 to 100% of components insoluble in benzene, toluene, xylene or tetrahydrofuran. When the starting pitch is subjected to an operation of removing the organic solvent-soluble components, the mesophase pitch has a melting point of 200.degree. to 350.degree. C. and a weight average molecular weight of 200 to 8,000. The melting point can be determined by an ordinary capillary method in a nitrogen box (the same hereinafter).
The premesophase pitch can be produced by, for example, hydrogenating a petroleum or coal pitch with a hydrogen donor such as tetrahydroquinoline or hydrogenating the pitch under hydrogen pressure in the optional presence of a catalyst, and then heating the resulting hydrogenated pitch for a short period of time at high temperatures under reduced pressure.
When the hydrogenation is carried out by using tetrahydroquinoline, at least 30 parts of the quinoline is added to 100 parts by weight of the pitch, and the mixture is heated at 300.degree. to 500.degree. C.
When hydrogenation is carried out by using hydrogen, a catalyst such as a cobalt-molybdenum system or an iron oxide system and a solvent such as quinoline are optionally added to the starting pitch, and the pitch is hydrogenated at 400.degree. to 500.degree. C. under a partial hydrogen pressure of at least 10 kg/cm.sup.2. The resulting product is heat-treated at a temperature of at least 440.degree. C. under a pressure of not more than 50 mmHg for a period of not more than 60 minutes after optionally it is filtered and subjected to a treatment of removing the solvent and the light components. The treating time is determined by the treating temperature. Preferably, the treatment is performed at the highest possible temperature for the shortest possible time. Particularly, treatment for a time of not more than 15 minutes is advantageous.
The premesophase pitch is a polycyclic aromatic compound in view of its chemical structure, and preferably has a melting point of 200.degree. to 350.degree. C. and a weight average molecular weight of 600 to 6,000, and contains at least 5% of components insoluble in quinoline.
The premesophase state, as referred to herein, denotes the state which is optically isotropic at room temperature but on heating to a high temperature of at least 600.degree. C., can change to a mesophase state. The premesophase pitch alone is spun, rendered infusible, and pyrolyzed, orientation occurs in the pyrolyzing step, and high modulus fibers can be obtained in the same way as in the case of using a mesophase pitch. The advantage of using the premesophase pitch is that it can be spun at lower temperatures than when the mesophase pitch is used.
The pitch in the latently anisotropic state can be obtained by removing light fractions from a heavy oil (to be referred to sometimes as the FCC slurry oil) obtained by fluidized catalytic cracking of petroleums, heat-treating the resulting pitch at 300.degree. to 500.degree. C., and subjecting the resulting optically anisotropic mesophase pitch to a hydrogenation treatment until the mesophase contained therein changes into substantially quinoline-soluble components and the pitch as a whole forms an optically isotropic homogeneous phase.
Various known methods used for hydrogenation of the aromatic ring may be used in the hydrogenation. For example, there can be used a method involving reduction with an alkali metal, an alkaline earth metal and a compound of any of these, an electrolytic reduction method, a hydrogenation method in a homogeneous system with a complex compound catalyst, a hydrogenation method in a heterogeneous system using a solid catalyst, a hydrogenation method under a hydrogen pressure in the absence of catalyst, and a hydrogenation method using a hydrogen donor such as tetralin.
The hydrogenation may be carried out at a temperature of not more than 400.degree. C. under a pressure of not more than 200 atmospheres, although these conditions may vary depending upon the method used. The resulting hydrogenated pitch may be maintained in the heat-melted state to enhance its thermal stability.
The heating temperature at this time is preferably above the melting temperature but does not exceed 450.degree. C. Heating at high temperatures may result in the formation of a new mesophase. The formation of too large an amount of the mesophase is undesirable because it increases the softening point of the pitch.
The pitch in the latently anisotropic state is a polycyclic aromatic compound in view of its chemical structure. Preferably, it has a melting point of 200.degree. to 350.degree. C. and a weight average molecular weight of 200 to 6,000, and is soluble in quinoline.
The latent anisotropy, as used herein, denotes anisotropy which is attributed to the orientation of molecules in the direction of an external force such as a shearing force or a stretching force, which occurs upon application of the external force. For example, when this pitch is spun, rendered infusible (cured) and pyrolyzed in accordance with an ordinary method of producing pitch-type carbon fibers, fibers oriented in the direction of the fiber axis are obtained.
The pitches in the mesophase, premesophase or the latently anisotropic state may be used singly or in combination.
These pitches and the reaction product between the organic silicon polymer and the optically isotropic pitch, are melted at a temperature in the range of 200.degree. to 500.degree. C. The pitch in the mesophase, premesophase or the latently anisotropic state is used in a proportion of 5 to 50,000 parts by weight, preferably 5 to 10,000 parts by weight, per 100 parts by weight of the reaction product.
If the proportion of the pitch is less than 5 parts by weight, highly elastic pyrolyzed fibers are difficult to produce as a final product. If it exceeds 50,000 parts by weight, it is difficult to obtain final fibers having excellent wettability with respect to the matrix and excellent oxidation resistance.
Thus, according to this invention, there is provided a polymer composition, comprising (A) an organic silicon polymer, (B) a polycyclic aromatic compound in the mesophase, and (C) an optically isotropic polycyclic aromatic compound, at least a part of component (A) being chemically bound to component (B) and/or component (C) by reaction. The formation of a chemical bond can be determined by the increase of the amount of that portion of the polymer composition which is insoluble in, for example, toluene over the total amount of toluene-insoluble portions of the individual components. For example, if the polymer composition comprises 1 part by weight of the reaction product obtained between 30 parts by weight of the organic silicon polymer (A) and 70 parts by weight of component (C), and 14 parts of component (B), the amount of the insoluble portion of the polymer composition increases to about 1.03 to 1.08 times based on the total amount of the insoluble portions of the individual components. Generally, this figure tends to be larger as the total amount of components (A) and (C) bcomes larger than the amount of component (B) and the proportion of component (A) becomes larger in the total amount of components (A) and (C).
The polymer composition of this invention is composed of the constituents (A), (B) and (C), and at least a part of the silicon atoms of component (A) is bonded to the carbon atoms on the aromatic rings of component (B) and/or component (C). Preferably, the weight ratio of of component (A) to the total amount of components (B) and (C) is from 1:0.5-5,000, and the weight ratio of component (B) to component (C) is 1:0.02-4.
If the weight ratio of of component (A) to the total amount of components (B) and (C) is below 0.5, the amount of the mesophase component in the polymer composition is insufficient, and fibers obtained from the polymer composition have low strength and modulus of elasticity. If this ratio exceeds 5,000, the amount of the organic silicon in the polymer composition is insufficient, and fibers obtained from this composition have lowered oxidation resistance and tend to have reduced wettability with an FRP matrix.
If the weight raito of (C) to (B) is less than 0.02, the polymer composition has reduced spinnability in melt spinning, and its spinning becomes extremely difficult with the occurrence of fiber breakage owing to the non-uniform viscosity of the spinning dope. If the above weight ratio exceeds 4, the amount of the mesophase component in the polymer composition becomes insufficient, and fibers obtained from the composition have lowered strength and modulus.
The polymer composition of this invention contains 0.01 to 30% by weight of silicon atoms, and has a weight average molecular weight of 200 to 11,000 and a melting point of 200.degree. to 400.degree. C.
If the silicon atom content of the polymer composition is less than 0.01%, the amount of the amorphous phase composed of Si, C and O or the ultrafine beta-SiC particles in the fibers formed from the composition is too small, and therefore, no marked improvement in the wettability of the resulting fibers with respect to the FRP matrix and the oxidation resistance of the fibers is achieved. On the other hand, if the silicon atom content exceeds 30%, the high elasticity of the fibers owing to the orientation of ultrafine graphite crystals in the fibers and the improved heat resistance of the fibers in a non-oxidizing atmopshere cannot be achieved, and the resulting fibers do not at all differ from SiC fibers.
If the weight average molecular weight of the polymer composition is lower than 200, the composition does not substantially contain a mesophase. From such a composition, therefore, highly elastic fibers cannot be obtained. If its weight average molecular weight is larger than 11,000, the composition has a high melting point and becomes difficult to spin.
A polymer composition having a melting point lower than 200.degree. C. does not substantially contain a mesophase, and as-spun fibers from this composition tend to melt adhere at the time of curing, pyrolyzed fibers having high strength and modulus of elasticity cannot be obtained. If it is higher than 400.degree. C., the composition is decomposed during spinning, and becomes difficult to spin.
Preferably, the polymer composition contains 10 to 98% of components insoluble in an organic solvent such as benzene, toluene, xylene and tetrahydrofuran and has a degree of optical anisotropy at room temperature of 5 to 97%.
If the proportion of the organic solvent-insoluble portion of the polymer composition is less than 10%, or the degree of optical anisotropy of the composition is less than 5%, the mesophase is hardly oriented in the direction of the fiber axis at the time of melt-spinning the composition. Hence, even when the resulting as-spun fibers are cured and pyrolyzed, there can only be obtained fibers having low strength and low modulus of elasticity. When the composition contains more than 98% of the organic solvent-insoluble portion or has a degree of optical anisotropy of more than 97%, the amount of the mesophase in the composition becomes too large, and the composition becomes difficult to spin.
To produce the first fibers of this invention from the polymer composition of this invention, a spinning dope of the polymer composition is prepared, and spun, and the resulting as-spun fibers are cured under tension or under no tension. The resulting infusible fibers are pyrolyzed in an inert gaseous atmosphere at a temperature of 800.degree. to 3,000.degree. C.
The spinning dope is prepared usually by heat-melting the polymer composition and as required, filtering the melt to remove substances detrimental to spinning, such as microgels or impurities. Its spinning is carried out by an ordinarily used synthetic resin spinning apparatus.
The temperature of the spinning dope to be spun is advantageously 220.degree. to 420.degree. C. although it varies depending upon the softening temperature of the starting composition.
As required, a spinning cylinder is mounted on the spinning appratus, and the atmosphere of the inside of the spinning cylinder is formed into an atmosphere of at least one gas selected from air, an inert gas, hot air, a hot inert gas, steam and ammonia gas, and by increasing the wind up speed, fibers having a small diameter can be obtained. The spinning speed in melt spinning can be varied within the range of 50 to 5,000 m/min. depending upon the properties of the starting composition.
The resulting as-spun fibers are then rendered infusible (cured) under tension or under no tension.
A typical method of curing is to heat the as-spun fibers in an oxidizing atmosphere. The temperature at this time is preferably 50.degree. to 400.degree. C. If the temperature is excessively low, no bridging takes place in the polymer constituting the as-spun fibers. If this temperature is excessively high, the polymer burns.
The purpose of curing is to bridge the polymer constituting the as-spun fibers to provide an insoluble and infusible three-dimensional structure and to prevent it from being melted with the adjacent fibers melt-adhering to each other in the subsequent pyrolyzing step. The gas constituting the oxidizing atmosphere at the time of curing is preferably, for example, air, ozone, oxygen, chlorine gas, bromine gas, ammonia gas or a gaseous mixture of these.
Another method of curing comprises applying gamma-ray irradiation or electron beam irradiation to the as-spun fibers in an oxidizing or non-oxidizing atmosphere optionally with heating at low temperatures.
The purpose of applying gamma-rays or electron beam irradiation is to polymerize the polymer forming the as-spun fibers to a greater degree, and thereby prevent the as-spun fibers from melting and thus losing the fiber shape.
The suitable irradiation dose of gamma-rays or electron beams is 10.sup.6 to 10.sup.10 rads.
The irradiation may be carried out under vacuum or in an atmosphere of an inert gas or an oxidizing gas such as air, ozone, oxygen, chlorine gas, bromine gas, ammonia gas or a gaseous mixture thereof.
Curing by irradiation may be carried out at a room temperature, but as required, curing may be achieved in a shorter period of time by performing the irradiation while heating at a temperature of 50.degree. to 200.degree..
The operation of curing may be carried out under tension or under no tension. The tension to be applied is preferably 1 to 500 g/mm.sup.2. Application of a tension of not more than 1 g/mm.sup.2 cannot keep the fibers taut. On the other hand, when this operation is carried out under no tension, the as-spun fibers assume a wavy form because of their shrinkage, but since this can frequently be corrected in the subsequent curing step, tension is not always essential.
The resulting infusible fibers are pyrolyzed in vacuum or in an atmosphere of an inert gas at a temperature of 800.degree. to 3,000.degree. C. The pyrolyzing can be carried out under tension or under no tension. Preferably, it is carried out under tension because if the fibers are pyrolyzed at high temperatures under a tension of, for example, 0.001 to 100 kg/mm.sup.2, inorganic fibers having high strength and little flex can be obtained.
It is presumed that in the temperature elevating process, carbonization begins to become vigorous at about 700.degree. C., and is almost completed at about 800.degree. C. To obtain higher temperatures than 3,000.degree. C., an expensive apparatus is required, and there is no industrial advantage. Hence, pyrolyzing is carried out at a temperature of 800.degree. to 3,000.degree. C.
Thus, according to this invention, there are provided high strength and high modulus fibers containing components (i), (ii) and (iii) as stated at the outset of the section "Disclosure of the Invention" are obtained.
Component (i) is crystalline carbon oriented substantially in the direction of the fiber axis. It is believed in relation to the production process described above that this carbon is derived from a polycyclic aromatic compound which is in the mesophase, or in other words, optically anisotropic.
Owing to the presence of component (i), a structure known in the art, that is, a radial structure, an onion structure, a random structure, a core-radial structure, a skin onion structure or a mosaic structure is observed in the cross section of the fibers of this invention.
Component (ii) is amorphous carbon and/or crystalline carbon oriented in a direction different from the fiber axis direction. Likewise, in relation to the production process described above, this component is believed to be derived from an optically isotropic polycyclic aromatic compound.
Crystalline carbon has a crystallite size of not more than 500 angstrom, and is an ultrafine graphite crystal oriented in the direction of the fiber axis in which by a high-resolution electron microscope having a resolution ability of 1.5 angstrom, a fine lattice image corresponding to (002) plane with an interplanar spacing of 3.2 angstrom is observed.
In the fibers of this invention, microcrystals which are three-dimensionally arranged with a small interlayer distance are effectively formed.
The silicon-containing component (iii) consisting essentially of silicon, carbon and oxygen may be an amorphous phase or an aggregation of a crystalline particulate phase consisting essentially of crystalline SiC and an amorphous SiO.sub.x (0<x.ltoreq.2) phase.
The crystalline particulate phase consisting essentially of crystalline SiC may have a particle diameter of not more than 500 angstrom.
The distributed state of silicon in the fibers can be controlled in relation to the atmosphere in which fibers are pyrolyzed for production of fibers, the size and concentration of the mesophase in the starting material. For example, if the mesophase is grown to a large size, the silicon-containing polymer is liable to be pushed out onto the fiber surface layer, and after pyrolyzing, forms a silicon-rich layer on the fiber surface.
The fibers of this invention preferably contain 0.015 to 200 parts by weight of component (iii) per 100 parts by weight of components (i) and (ii) combined, and the weight ratio of component (i) to component (ii) is 1:0.02-4.
If the proportion of component (iii) is less than 0.015 part by weight per 100 parts by weight of components (i) and (ii) combined, the resulting fibers are much the same as pitch fibers, and an improvement in oxidation resistance and wettability cannot be expected. If the proportion exceeds 200 parts by weight, fine crystals of graphite are not effectively formed, and fibers of a high modulus of elasticity are difficult to obtain.
The fibers of this invention comprises preferably 0.01 to 29% by weight of silicon, 70 to 99.9% by weight of carbon and 0.001 to 10% by weight of oxygen, especially preferably 0.1 to 25% by weight of silicon, 74 to 99.8% by weight of carbon and 0.01 to 8% by weight of oxygen, based on the total weight of silicon, carbon and oxygen.
As second fibers of this invention, the present invention provides fibers having high strength and high modulus comprising
(i) crystalline carbon oriented substantially in the direction of the fiber axis,
(ii) amorphous carbon and/or crystalline carbon oriented in a direction different from the direction of the fiber axis, and
(iii') a silicon-containing component substantially composed of 0.5 to 45% by weight of a metal selected from titanium, zirconium and hafnium, 5 to 70% by weight of Si, 20 to 40% by weight of C and 0.01 to 30% by weight of O, the proportions being based on the total weight of said metal, silicon, carbon and oxygen.
According to this invention, the second fibers of this invention can be produced by a process which comprises
preparing a spinning dope of a polymer composition comprising
(A') an organic silicon polymer resulting from random bonding of a plurality of units of at least one kind selected from the group consisting of units of the following formula (a) ##STR2## wherein R.sup.1 and R.sup.2, independently from each other, represent a hydrogen atom, a lower alkyl group, a phenyl group or a silyl group (--SiH.sub.3), and at least one unit of formula (b) ##STR3## wherein R.sup.1 is as defined above, and R.sup.3 represents -M or -OM, and M represents one equivalent of a metal selected from the group consisting of titaniuum, zirconium and hafnium, via methylene groups (--CH.sub.2 --) or both via methylene groups or directly,
(B) a polycyclic aromatic compound in the mesophase, premesophase or the latently anisotropic phase, and
(C) an optically isotropic polycyclic aromatic compound which is not in the premesophase or the latently anisotropic phase, part of component (A) being chemically bonded to component (B) and/or component (C);
spinning the spinning dope;
rendering the fibers infusible under tension or under no tension; and
pyrolyzing the resulting infusible fibers in vacuum or in an atmosphere of an inert gas at a temperature of 800.degree. to 3,000.degree. C.
The polymer composition used in the spinning step has been provided for the first time by the present inventors and constitute part of the present invention.
The novel polymer composition can be produced by heating the organic silicon polymer (A) described above in the production of the first fibers of the invention (to be sometimes referred to as the first organic silicon polymer) and an optically isotropic pitch in an inert gas at a temperature of preferably 250.degree. to 500.degree. C., then reacting the reaction product with a transition metal compound of formula EQU M.sup.1 X.sub.4
wherein M.sup.1 represents titanium, zironium or hafnium, and X may be any moiety, for example a halogen atom, an alkoxy group, or a chain-forming group such as a beta-diketone, which permits M to be bonded to the silicons of the precursor reaction product directly or through an oxygen atom by condensation, at a temperature of 100.degree. to 500.degree. C.; and heat-melting the reaction product with a pitch in the mesophase, the premesophase or the latently anisotropic state at a temperature of 300.degree. to 500.degree. C.
The first organic silicon polymer, the optically isotropoic pitch and the heating conditions therefor are as described hereinabove.
The precursor reaction product obtained by heating is then reacted with the transition metal compound M.sup.1 X.sub.4. By this reaction, the silicon atoms of the precursor reaction product may be at least partly bonded to the metal M directly or through an oxygen atom.
If the reaction temperature is low, the condensation reaction between the precursor reaction product and the compound of formula M.sup.1 X.sub.4 does not proceed. If the reaction temperature is excessively high, the cross-linking reaction through M proceeds excessively to cause gellation or the precursor reaction product itself condenses and becomes high in molecular weight. In some cases, MX.sub.4 volatilizes, and a composition for obtaining excellent fibers cannot be obtained.
The reaction product can also be prepared by reacting the reaction product obtained after the reaction of the organic silicon polymer (A) with the transition metal compound, with a pitch.
The above reaction product contains the organic silicon polymer (A') which results from random bonding of a plurality of the units represented by formula (a) to at least one unit of formula (b) through methylene groups or both through methylene groups and directly without the intermediary of methylene groups.
The units of formula (b) may be, for example, as follows when Ti(OC.sub.4 H.sub.9).sub.4 is used as the transition metal compound. ##STR4## The reaction temperature at this time is especially desirably 200.degree. to 400.degree. C.
The reaction product obtained by the above reaction is then heat-melted with a pitch in the mesophase, premesophase or the latent anisotropy.
It should be understood that as regards these pitches and the heat-melting conditions, the same description as that for the polymer composition used in the production of the first fibers (to be sometimes referred to as the first polymer composition) will apply.
The above polymer composition (to be sometimes referred to as the transition metal-containing reaction product or the second polymer composition) may also be produced by a process which comprises reacting the first organic silicon polymer (A) with an optically isotropic pitch, and reacting the resulting product with a polycyclic aromatic compound such as one in the mesophase and a transition metal compound successively or together.
Thus, according to this invention, there is provided a polymer composition comprising (A') an organic silicon compound, (B) a polycyclic aromatic compound such as one in the mesophase, and (C) an optically isotropic polycyclic aromatic compound, at least part of the component (A') being chemically bonded to component (B) and/or component (C).
The second polymer composition of this invention comprises the components (A'), (B) and (C), and the silicon atoms of the component (A') are at least partly bonded to the carbon atoms of the aromatic rings of component (B) and/or component (C). The weight ratio of component (A') to the total sum of components (B) and (C) is preferably 1:0.5-5,000, and the weight ratio of component (C) to component (B) is preferably 1:0.02-4.
If the weight ratio of component (A') to the total sum of components (B) and (C) is less than 0.5, the amount of the mesophase component in the second polymer composition is insufficient, and fibers obtained from this polymer have low strength and modulus of elasticity. If this ratio exceeds 5,000, the amount of the organic silicon compound in the second polymer composition becomes insufficient, and fibers obtained from this polymer have low oxidation resistance. Furthermore, the wettability of the fibers with respect to an FRP matrix tends to be low.
If the weight ratio of (C) to (B) is less than 0.02, the spinnability of the second polymer composition in its melt-spinning is degraded, and fiber breakage occurs owing to the non-uniform viscosity of the dope. Hence, the polymer composition becomes extremely difficult to spin. If the above weight ratio exceeds 4, the amount of the mesophase component in the second polymer composition is insufficient, and fibers obtained from the polymer tends to have low strength and modulus of elasticity.
Preferably, in component (A'), the ratio of the total number of units Si--CH.sub.2 to that of units Si--Si is within 1:0-20, and 0.2 to 35% of units M of the transition metal compound is contained based on the total weight of the units Si--CH.sub.2 and units Si--Si.
The second polymer composition preferably contains 0.01 to 30%, especially 0.05 to 30%, of silicon atoms, and 0.005 to 10% of M, and has a weight average molecular weight of 200 to 11,000 and a melting point of 200.degree. to 400.degree. C.
If the content of silicon atoms in the second polymer composition is less than 0.01%, the wettability of the resulting fibers with respect to an FRP matrix and the oxidation resistance of the fibers do not markedly show an improvement. On the other hand, if it exceeds 30%, the orientation of the ultrafine graphite crystals in the fibers makes it impossible to achieve high elasticity in the fibers, and an improvement in the heat resistance of the fibers in a non-oxidizing atmosphere, and the fibers do not differ at all from SiC fibers.
Since the second polymer composition contains M in addition to silicon, the composition shows a further improvement in mechanical properties, wettability with plastics. If the content of M is less than 0.005%, the above properties are scarcely exhibited. If it exceeds 10%, both a high-melting product which is extremely crosslinked and the unreacted MX.sub.4 exist in the composition, and it becomes very difficult to melt-spin a dope of the composition.
If the second polymer composition has a weight average molecular weight lower than 200, it hardly contains a mesophase, and therefore, high elasticity fibers cannot obtained from the composition. If its weight average molecular weight is larger than 11,000, the composition has a high melting point and is difficult to spin.
If the second polymer composition has a melting point lower than 200.degree. C., it does not substantially contain a mesophase, and as-spun fibers obtained by spinning this composition are liable to melt-adhere when subjected to curing. Thus, fibers having high strength and modulus of elasticity cannot be obtained. If its melting point is higher than 400.degree. C., the composition undergoes decompositon during spinning, and is difficult to spin.
Preferably, the second polymer composition contains 10 to 98% of a portion insoluble in an organic solvent such as benzene, toluene, xylene or tetrahydrofuran, and has a degree of optical anisotropy at room temperature of 5 to 97%.
If the proportion of the organic solvent-insoluble portion of the second composition is less than 10% or its degree of optical anisotropy is less than 5%, the mesophase is hardly oriented in the direction of the fiber axis when the composition is melt-spun. Accordingly, when the as-spun fibers are cured and pyrolyzed, there can only be obtained fibers having low strength and modulus of elasticity. On the other hand, when the second polymer composition contains more than 98% of the organic solvent-soluble portion, or has a degree of optical anisotropy of more than 97%, the amount of the mesophase in the composition becomes excessive, and the composition is difficult to spin.
The second fibers may be produced from the second polymer composition of this invention by quite the same process as that for producing the first fibers of this invention.
Thus, the present invention also provides fibers of high strength and elasticity comprising components (i), (ii) and (iii') described above.
The component (i) is crystalline carbon oriented substantially in the direction of the fiber axis. In relation to the above production process, this component is believed to be derived from a polycyclic aromatic compound in the mesophase, or in other words, an optically anisotropic polycyclic aromatic compound. In the fibers of this invention, a structure well known in the art is observed in a fiber cross-section owing to the presence of component (i), namely a radical structure, an onion structure, a random structure, a core-radial structure, a skin onion structure, or a mosaic structure.
The constituent component (ii) is amorphous carbon and/or crystalline carbon oriented in a direction different from the direction of the fiber axis. Likewise, in relation to the above production process, it is believed that component (ii) is derived from an optically isotropic polycyclic aromatic compound.
The crystalline carbon has a crystallite size of not more than 500 angstrom. It is in the form of ultrafine graphite crystal particles in which under a high-resolution electron microscope, a fine lattice image corresponding to (002) plane having a planar spacing of 32 angstrom and oriented in the direction of the fiber axis is observed.
In the fibers of this invention, microcrystals having a small interlayer distance and arranged three dimensionally are effectively formed.
The silicon-containing component (iii') consisting essentially of the transition metal, silicon, carbon and oxygen may be an amorphous phase, or an aggregate consisting substantially of a crystalline fine particulate phase consisting of silicon, carbon and a transition metal selected from the group consisting of titanium, zirconium and hafnium and an amorphous SiO.sub.y (0<y.ltoreq.2) and MO.sub.z (M is Ti, Zr or Hf, and 0<z.ltoreq.2).
The amorphous phase of the silicon-containing component tends to form when the pyrolyzing temperature in the production of the fibers is lower than 1000.degree. C. The aggregate of the crystalline fine particulate phase and the amorphous phase tends to form when the pyrolyzing temperature is 1700.degree. C. or higher.
The crystalline fine particulate phase consists of crystalline SiC, MC (M is as defined above), a crystalline solid solution of SiC and MC, and MC.sub.1-x (0<x<1), and may have a particle diameter of not more than 500 angstrom.
At pyrolyzing temperatures intermediate between the above temperatures, a mixture of the aggregates forms. The amount of oxygen in the fibers can be controlled by the proportion of MX.sub.4 added or the curing conditions.
The state of distribution of the component (iii') may also be controlled by the atmosphere of pyrolyzing, or the size and concentration of the mesophase in the starting material. For example, when the mesophase is grown to a large size, the component (iii') is liable to be pushed out onto the surface of the fibers.
Preferably, the fibers of this invention contain 0.015 to 200 parts by weight of component (iii) per 100 parts by weight of the components (i) and (ii) combined, and the ratio of components (i) to (ii) is 1:0.02-4.
If the amount of component (iii) is less than 0.015 part by weight per 100 parts by weight of components (i) and (ii) combined, the resulting fibers do no differ from pitch fibers, and an improvement in oxidation resistance and wettability can hardly be expected. If the above proportion exceeds 200 parts by weight, fine crystals of graphite are not effectively formed, and fibers having a high modulus of elasticity are difficult to obtain.
The fibers of this invention preferably consist of 0.01 to 30% by weight of silicon, 0.01 to 10% by weight of the transition metal (Ti, Zr or Hf), 65 to 99.9% by weight of carbon, and 0.001 to 10% by weight of oxygen, particularly preferably 0.1 to 25% by weight of silicon, 0.01 to 8% by weight of the transition metal, 74 to 99.8% by weight of carbon, and 0.01 to 8% by weight of oxygen.
The first and second fibers may be advantageously used as a reinforcing material for composite materials. Examples of such composite materials are as follows:
(1) A fiber-reinforced composite material comprising a plastic as a matrix.
(2) A fiber-reinforced composite material ceramics as a matrix.
(3) A fiber-reinforced composite material comprising carbon as a matrix.
(4) A fiber-reinforced composite material comprising a pyrolyzed product of the polymer composition of this invention as a matrix.
(5) A composite material comprising a metal as a matrix.
These examples will be described successively.
For the composite material comprising a plastic as a matrix, both the first and the second fibers of the invention can be used.
Incorporation of the fibers may be effected by, for example, a method comprising incorporating these fibers in the matrix, monoaxially or multiaxially, a method comprising using the fibers in the form of a woven fabric such as a plain-weave fabric, a satin weave fabric, a twill fabric, an imitation gauze fabric, a helical weave fabric and a three-dimensionally woven fabric, or a method comprising using the fibers as chopped fibers.
Examples of the plastic include epoxy resins, unsaturated polyester resins, phenolic resins, polyimide resins, polyurethane resins, polyamide resins, polycarbonate resins, silicone resins, fluorine-containing resins, nylon resins, polyphenylene sulfide resins, polybutylene terephthalate, ultrahigh-molecular-weight polyethylene, polypropylene, modified polyphenylene oxide resins, polystyrene, ABS resins, vinyl chloride resins, polyether-ether ketone resins and bismaleimide resins.
These plastic composite materials can be produced by methods known per se, for example, (1) a hand layup method, (2) a matched metal die method, (3) a break away method, (4) a filament winding method, (5) a hot press method, (6) an autoclave method, and (7) a continuous pulling method.
According to the hand layup method (1), the fibers are cut and spread densely on a mold. Then, the plastic containing a catalyst is coated on the spread fibers by means of a brush or a roller and allowed to cure naturally. The mold is then removed to produce a composite material.
According to the matched metal die method (2), the fibers are impregnated with the plastic, a curing agent, a filler and a thickening agent, and then molded under heat and pressure to form a composite material. Depending upon the form of the material during the molding, either the SMC (sheet molding compound) method or the BMC (bulk molding compound) method may be selected.
According to the break away method (3), sheets of the fibers are impregnated with the plastic and precured to form prepregs. The prepregs are wound up around a tapered mandrel, and after curing, the cured composite material is pulled out. A hollow article of a complex shape can be produced by this method.
According to the filament winding methed (4), inorganic fibers impregnated with a thermosetting resin such as an epoxy resin or an unsaturated polyester resin, wound around a mandrel, and treated to cure the resin. The cured product was removed from the mandrel to form a composite material. This method is carried out by a wet procedure or a dry procedure (using a prepreg tape).
According to the hot press method (5), prepreg sheets of the fibers are stacked in one direction or at any desired angle, and the stack is heated under pressure by a hot press to form a composite material in the form of a plate.
According to the autoclave method (6), prepregs are stacked on a mold, and wrapped with a special rubber. In a vacuum condition, the stack is put in a high-pressure kettle and heated under pressure to obtain a cured composite material. This method is suitable for production of complex shapes.
According to the continuous pulling method (7), the fibers and the plastic are separately fed into a molding machine, and mixed just before a mold. On the way, the mixture is passed through a heating oven, and continuously taken up as a continuous long composite material.
The tensile strength (.sigma..sub.c) of the composite material produced from the fibers and the plastic matrix is expressed by the following equation. EQU .sigma..sub.c =.sigma..sub.f V.sub.f +.sigma..sub.M V.sub.M
In which
As shown by the above equation, the strength of the composite material becomes larger as the volume percentage of the fibers in the composite material becomes larger. Accordingly to produce a composite material having high strength, the proportion of the volume of the inorganic fibers to be combined must be increased. However, if the volume proportion of the inorganic fibers exceeds 80%, the amount of the plastic matrix correspondingly decreases, and it is impossible to fill the interstices of the hybrid fibers sufficiently with the plastic matrix. As a result, the composite material produced does not exhibit the strength shown by the above equation. If the volume proportion of the fibers is decreased, the strength of the composite material correspondingly decreases as shown by the above equation. To produce a practical composite material, it is necessary to combine at least 10% of the fibers. In the production of fiber-reinforced plastic composite materials, the volume proportion of the fibers to be combined is preferably 10 to 80%, especially preferably 30 to 60%.
The various mechanical properties in the present specification are determined by the following measuring methods.
(a) Interlayer shear strength
In the testing method for determining interlayer shear stress, a composite material containing fibers (10.times.12.times.2 mm) oriented monoaxially is placed on two pins (length 20 mm) having a radius of curvature of 6 mm. By using a presser with its tip having a radius of curvature of 3.5 mm, the composite material was compressed and the so-called 3-point bending test was carried out, and its interlayer shear stress is measured, and expressed as shear stress (kg/mm.sup.2).
(b) Tensile strength and tensile modulus in a direction perpendicular to the fibers
A composite material, 2 mm thick, reinforced monoaxially with fibers was produced, and a test piece, 19.times.127 mm, was taken from it so that the axial direction of the test piece became perpendicular to the direction of the fiber arrangement. The test piece had a thickness of 2 mm. A curvature of 125 mmR was provided in the thickness direction at the centeral portion of the test piece was finished in a thickness of about 1 mm. The pulling speed was 1 mm/min., and the tensile strength (kg/mm.sup.2) and tensile modulus (t/mm.sup.2) were determined.
(c) Flexural strength and flexural modulus in a direction perpendicular to the fibers
A composite material, 2 mm thick, reinforced monoaxially with fibers was produced, and a test piece, 12.7.times.85 mm, was taken from it so that the axial direction of the test piece became perpendicular to the direction of the fiber arrangement. The test piece had a thickness of 2 mm. A curvature of 125 mmR was provided in the thickness direction at the centeral portion of the test piece was finished in a thickness of about 1 mm. The test piece is subjected to a 3-point bending test, and the flexural strength (kg/mm.sup.2) and the flexural modulus (t/mm.sup.2) are determined.
The interlayer shear strength, the tensile strength in the direction perpendicular to the fibers and the flexural strength in the direction perpendicular to the fibers are indices showing the strength of bonding between the matrix and the fibers.
(d) Tensile strength and tensile modulus
A composite material, 2 mm thick, reinforced monoaxially with fibers was produced, and a test piece, 12.7.times.85 mm, was taken from it so that the axial direction of the test piece became perpendicular to the direction of the fiber arrangement. The test piece had a thickness of 2 mm. A curvature of 125 mmR was provided in the thickness direction at the centeral portion of the test piece was finished in a thickness of about 1 mm. The pulling speed was 1 mm/min., and the tensile strength (kg/mm.sup.2) and tensile modulus (t/mm.sup.2) were determined.
(e) Flexural strength and flexural modulus
A composite material, 2 mm thick, reinforced monoaxially with fibers was produced, and a test piece, 12.7.times.85 mm, was taken from it so that the axial direction of the test piece became perpendicular to the direction of the fiber arrangement. The test piece had a thickness of 2 mm. A curvature of 125 mmR was provided in the thickness direction at the centeral portion of the test piece was finished in a thickness of about 1 mm. The test piece was subjected to a 3-point bending test, and the flexural strength (kg/mm.sup.2) and the flexural modulus (t/mm.sup.2) were determined.
(f) Flexural impact value
Flexural impact value was measured by the Charpy testing method (JIS K7111) by three-point bending. The result was expressed by flexural impact value (kg.multidot.cm/cm.sup.2).
The flexural impact value is an index representing the strength of bonding between the plastic and the fibers, particularly an index representing the strength of resistance to instantaneous impact. If the flexural impact value is low, the resin is liable to separate from the fibers, and destruction is liable to occur owing to instantaneous impact.
The above plastic composite material has
a) an interlayer shear strength of at least 8.5 kg/mm.sup.2,
b) a tensile strength in a direction perpendicular to the fibers of at least 6 kg/mm.sup.2,
c) a flexural modulus in a direction perpendicular to the fibers of at least 8 kg/mmhu 2, and
d) a flexural impact value of at least 200 kg.multidot.cm/cm.sup.2.
Since the fibers of this invention have excellent wetting property with respect to the plastics, the fiber-reinforced plastic composite material of this invention does not particularly require surface-treatment of the fibers and has excellent strength of bonding between the fibers and the plastic. Accordingly, the present invention provides a composite material having excellent interlayer shear strength, tensile strength in a direction perendicular to the fibers, a flexural strength in a direction perpendicular to the fibers, and flexural impact value.
Since the fibers of this invention contain carbon in which the crystals are oriented, they have higher elasticity than amorphous inorganic fibers. Accordingly, plastic composite materials reinforced with the fibers of this invention have excellent tensile modulus and flexural modulus.
The fibers of this invention are produced at lower costs than conventional silicon carbide fibers because the use of an expensive organic silicon compound is decreased.
The fibers of this invention have an excellent reinforcing effect in plastic composite materials. The resulting reinforced plastic composite materials have excellent mechanical properties and can withstand in a severe environment over long periods of time. Hence, they can be used in applications in which conventional inorganic fiber-reinforced plastic composite materials cannot be used satisfactorily. For example, such reinforced materials can be used as building materials, materials for aircraft and space exploiting devices, materials for ships and boats, materials for land transportation machines and devices, and materials for acoustic machines and devices.
The first or second fibers of the invention may be hybridized with fibers selected from the group consisting of the fibers of the invention, carbon fibers, glass fibers, boron fibers, alumina fibers, silicon nitride fibers, aramid fibers, silicon carbide fibers, silicon carbide fibers having carbon as a core and Si--M--C--O fibers (M=Ti or Zr) having carbon as a core, and the resulting hybrid fibers may be used to reinforce plastic composite materials. The proportion of the fibers of this invention in the hybrid fibers is at least 10%, preferably at least 20%. If the proportion is lower than 10%, the hybrid fibers have a reduced improving effect in respect of the strength of bonding between the fibers and the plastic, the reinforcing efficiency or the mechanical properties such as fatigue strength. In other words, the hybrid fibers have a reduced improving effect on interlayer shear strength, flexural impact value and fatigue strength.
The states of hybridization of the hybrid fibers are (1) interhybridization achieved by lamination of a layer of a certain kind of fibers and a layer of another kind of fibers, and (2) interlayer hybridization achieved by hybridization within one layer, which are basic, and there are (3) combinations of these. The main combinations are of the following 6 types.
(a) Lamination of single layer tapes (alternate lamination of layers of dissimilar fibers)
(b) Sandwich-type (lamination of dissimilar layers in a sandwich form)
(c) Rib reinforcement
(d) Lamination of mix-woven tows (hybridization of dissimilar monofilaments)
(e) Lamination of mix-woven tapes (hybridization of dissimilar yarns within a layer)
(f) Mix-woven surface layer
Plastic composite materials reinforced with these hybrid fibers have the same excellent advantages as the above-described composite materials.
Fiber-reinforced composite materials including ceramics as a matrix:
Both the first and second fibers of this invention described above may be used as the reinforcing fibers.
These fibers may be directly oriented in the monoaxial or multiaxial directions in the matrix. Alternatively, they may be used as woven fabrics such as a plain weave fabric, a satin weave fabric, an imitation gauze fabric, a twill fabric, a helical weave fabric, or a three-dimensionally woven fabric, or in the form of chopped fibers.
Carbides, nitrides, oxides, or glass ceramics, for example, may be conveniently used as the ceramics. Examples of the carbide ceramics that can be used include silicon carbide, titanium carbide, zirconium carbide, vanadium carbide, niobium carbide, tantalum carbide, boron carbide, chromium carbide, tungsten carbide and molybdenum carbide. Examples of the nitride ceramics are silicon nitride, titanium nitride, zirconium nitride, vanadium nitride, niobium nitride, tantalum nitride, boron nitride, aluminum nitride and hafnium nitride. Examples of the oxide ceramics include alumina, silica, magnesia, mulite and corierite. Examples of the glass ceramics are borosilicate glass, high silica glass and aluminosilicate glass. In the case of using these ceramic matrices in the form of a powder, the powder is advantageously as fine as possible and at most 300 micrometers in maximum particle diameter in order to better the adhesion of the ceramics to the fibers.
The proportion of the fibers of this invention mixed in the matrix is preferably 10 to 70% by volume. If the above mixing ratio is less than 10% by volume, the reinforcing effects of the fibers does not appear sufficiently. If it exceeds 70%, the amount of the ceramics is small so that the interstices of the fibers cannot be filled sufficiently with the ceramics.
In the production of the ceramic composite materials, it is possible to use a binder (sintering aid) for sintering the powdery ceramic matrix to a high density and/or a binder for increasing the adhesion of the powdery ceramic matrix to the fibers.
The former binder may be ordinary binders used at the time of sintering the carbide, nitride, oxide and glass ceramics. For example, boron, carbon and boron carbide may be cited as a binder for silicon carbide. Examples of binders for silicon nitride are aluminum oxide, magnesium oxide, yttrium oxide and aluminum oxide.
Preferred examples of the latter binder include organic silicon polymers such as diphenylsiloxane, dimethylsiloxane, polyborodiphenylsiloxane, polyborodimethylsiloxane, polycarbosilane, polydimethylsilazane, polytitanocarbosilane and polyzirconocarbosilane, and organic silicon compounds such as diphenylsilanediol and hexamethyldisilazane.
The binder for increasing the adhesion of the powdery ceramic matrix to the inorganic fibers, when heated, is converted mainly into SiC or Si.sub.3 N.sub.4 which reacts on the surface of the powdery ceramic matrix to form a new carbide, nitride or oxide. Consequently, the adhesion of the powdery ceramic matrix to the inorganic fibers becomes very superior. These organic silicon compounds or polymers, like the ordinary binders, act to increase the sinterability of the powdery ceramic matrix. Accordingly, the addition of these binders is very advantageous to the production of composite materials having high strength. Where a strong adhesion between the powdery ceramic matrix and the fibers can be obtained, it is not necessary to add binders.
The amount of the binders may be one sufficient for producing an effect of the addition.
Usually, it is preferably 0.5 to 20% by weight based on the powdery ceramic matrix.
The ceramic composite materials reinforced with the fibers of this invention can be produced, for exmple, by the following methods.
There are various methods of obtaining aggregates of the powdery ceramic matrix and the fibers. The aggregate can be obtained relatively easily, particularly by embedding the fibers in a mixture of the powdery ceramic matrix or ceramics and a binder, a method of alternatingly arranging the fibers and the powdery ceramic matrix or the above mixture, or a method comprising arranging the fibers, and filling the interstices of the fibers with the powdery ceramic matrix or the above mixture.
Sintering of the aggregates may be effected, for example, by a method comprising compression molding the aggregate by using a rubber press, a mold press, etc. under a pressure of 50 to 5,000 kg/cm.sup.2, and sintering the resulting molded product in a heating furnace at 800.degree. to 2400.degree. C., or by a method which comprises sintering the aggregate at a temperature of 800.degree. to 2400.degree. C. by hot pressing while it was compressed under a pressure of 50 to 5,000 kg/cm.sup.2.
The above sintering methods may be carried out in an atmosphere, for example an inert gas as nitrogen, argon, carbon monoxide or hydrogen or in vacuum.
As shown in Example 102, in the production of the above fiber-reinforced ceramic composite material, a precursor of the fibers (precursor fibers before curing may be used instead of the fibers.
By subjecting the resulting sintered composite material to a series of treatments to be described below at least once, a sintered body having a higher density can be obtained. Specifically, a sintered body having a higher density can be obtained by a series of treatments of immersing the sintered body under reduced pressure in a melt of the organic silicon compound or polymer, or if desired, in a solution of the above compound or polymer to impregnate the melt or solution in the grain boundaries and pores of the sintered body, and heating the sintered body after impregnation. The impregnated organic silicon compound of polymer changes mainly into SiC or Si.sub.3 N.sub.4. They exist in the brain boundaries and the pores of the composite sintered body. They reduce the cores and at the same time, form a firm bond in the ceramic matrix, and thus increases the mechanical strength of the product.
The mechanical strengh of the resulting sintered body may also be increased by coating the organic silicon compound or polymer either as such or a solution of it in an organic solvent to clog the pores, or by coating it on the surface of the sintered product and then heat-treating the coated sintered body by the same method as above.
The organic solvent which may be used as required may be, for example, benzene, xylene, hexane, ether, tetrahydrofuran, dioxane, dchloroform, methylene chloride, ligroin, petroleum ether, petroleum benzine, dimethyl sulfoxide and dimethylformamide. The organic silicon compound or polymer is dissolved in the organic solvent and can be used as a solution having a lower viscocity.
The heat-treatment is carried out at 800.degree. to 2400.degree. C. in an atmosphere of at least one inert gas selected from nitrogn, argon and hydrogen or in vaccum.
The series of impregnation or coating operations may be repeated any number of times so long as these operations are possible.
In the production of the fiber-reinforced ceramic composite material, the form of the starting ceramic and the method of producing the composite are not to be limited to those described above, and ordinary forms and methods used may be employed without any inconvenience.
For example, a fine powder obtained by the sol-gel method and a precursor polymer convertible to the ceramics by pyrolyzing may be used as the starting ceramics. When the reinforcing fibers are short fibers, injection molding, extrusion molding and casting may be employed as the molding method. By jointly using HIP (hot isostatic pressing) in pyrolyzing, the performance of the composite material may be increased. On the other hand, excellent composite materials may also be obtained by vapor-phase methods such as CVD and CVI.
The fracture toughness K.sub.Ic, of the ceramic composite material to that of the matrix alone containing no fibers is about 2 to 7, and the ceramic composite material has a reduction rate of flexural strength (to be referred to as a "flexural strength reduction rate"), measured by a thermal shock fracture resistance method, of less than about 10%.
The fracture toughness (K.sub.Ic) is measured by the IF method (Indentation Fracture Method) described in J. Am. Ceram. Soc. 59, 371, 1976) of A. G. Evan.
The flexural strength reduction rate is determined from the flexural strength of a sample (obtained by heat-treating a piece, 3.times.3.times.30 mm, cut out from the ceramic composite material at a temperature of 800.degree. to 1,300.degree. C. in air or nitrogen for 20 minutes, immediately then immersing it in water at 25.degree. C., and then drying it) measured by a three-point flexural strength testing method, and that of the ceramic composite material not subjected to the above heat-treatment.
The initial rate of fiber degradation induced by reaction to be simply referred to as the "degradation rate" is determined as follows:
The inorganic fibers, silicon carbide fibers or alumina fibers are embedded in the powdery ceramic matrix and then heated in an argon atmosphere at a predetermined temperature (the temperature used at the time of producing the composite material) for 5 minutes. The fibers are then taken out, and their tensile strength is measured. The difference between the measured tensile strength and the tensile strength of the fibers before the treatment is divided by the heating time (seconds), and the quotient is defined as the above "degradation rate".
As compared with conventional ceramic composite materials reinforced with carbon fibers, the above ceramic composite material can be used at high temperatures in an oxidizing atmosphere. Furthermore, as compared with ceramic composite materials reinforced with other fibers, the increase of K.sub.Ic in the above ceramic composite material greatly improves the inherent brittleness or the inherent nonuniformity of mechanical strength of the above ceramic composite material. Accordingly it is suitable for use as a structural material. The improvement of high temperature impact strength enables the above ceramic matrix composite material to be used in an environment where variations in temperature from high to low temperatures are great. The fibers of this invention are stable to the ceramic as a matrix, and fully achieves the inherent purpose of reinforcement with fibers.
Fiber-reinforced composite materials including carbon as a matrix:
Both the first and the second fibers of this invention can be used as the reinforcing fibers.
These fibers may be directly oriented in the monoaxial or multiaxial directions in the matrix. Alternatively, they may be used in woven fabrics such as a plain weave fabric, a satin weave fabric, an imitation gauze fabric, a twill fabric, a helical weave fabric, or a three-dimensionally woven fabric, or in the form of chopped fibers.
The proportion of the fibers of this invention mixed in the matrix is preferably 10 to 70% by volume. If the above mixing ratio is less than 10% by volume, the reinforcing effects of the fibers does not appear sufficiently. If it exceeds 70%, the amount of the ceramics is small so that it is difficult to fill the interstices of the fibers sufficiently with the ceramics.
Carbonaceous material for matrices of ordinary C/C composites may be used as materials for matrices of the above composite materials. Examples include materials which can be converted to carbon by pyrolyzing, for example, thermosetting resins such as phenolic resins and furan resin, and thermoplastic polymers such as pitch, moldable materials such as carbon powder or a mixture of carbon powder and the above resins. When carbon powder is used as a carbonaceous material for matrix, the use of a binder is more effective for increasing the adhesion of the matrix to the fibers.
Examples of the binder are organic silicon polymers such as diphenylsiloxane, dimethylsiloxane, polyborodiphenylsiloxane, polyborodimethylsiloxane, polycarbosilane, polydimethylsilazane, polytitanocarbosilane and polyzirconocarbosilane and organic silicon compounds such as diphenylsilanediol and hexamethyldisilazene.
The aggregate of the carbonaceous material and the fibers may be molded, for example, by a method comprising carbon powder optionally containing the binder to the reinforcing fibers, and molding the mixture by using a rubber press, a mold or a hot press, or a method comprising impregnating a solution of a thermosetting or thermoplastic resin in a bundle of the fibers or a woven fabric of the fibers, drying and removing the solvent, and molding the prepreg sheets by an ordinary method of molding an ordinary FRP, or a method comprising laminating prepreg sheets on a mold, and molding them by a hot press.
The resulting molded article, if required, is rendered infusible, and then in an inert atmosphere, heated at 800.degree. to 3000.degree. C. to carbonize the matrix component.
The resulting fiber-reinforced material may directly be used in various applications. Alternatively, it may be further repeatedly subjected to a step of impregnating it with a melt or solution of a thermoplastic or thermosetting resin and carbonizing the coated material to give a higher density and a higher strength. In particular, where mechanical properties are required, the density of the material can be effectively increased by a vapor-phase method such as CVI.
In the fiber-reinforced carbon material obtained, the reinforcing fibers are the fibers of this invention having high strength and high modulus, and have improved adhesion to the carbon matrix. Accordingly, the resulting fiber-reinforced carbon material has high strength, modulus and tenaciousness and also excellent practical mechanical properties such as abrasion resistance.
Accordingly, the resulting composite materials may advantageously be used in various kinds of brakes and heat-resistant structural materials.
Fiber-reinforced composite materials including a sintered body matrix produced from the polymer composition of the invention:
These composite materials include a composite material comprising the first fibers of the invention as the reinforcing fibers and a carbonized product of the first polymer composition of the invention as the matrix; a composite material comprising the first fibers of the invention as the reinforcing fibers, and a carbonized product of the second polymer composition of the invention as the matrix; a composite material comprising the second fibers of the invention as the reinforcing fibers and a carbonized product of the first polymer composition of the invention as the matrix; and a composite material comprising the second fibers of the invention as the reinforcing fibers and a carbonized product of the second polymer composition of the invention as the matrix.
To describe these composite materials comprehensively, the "first and second" qualifying the fibers and the polymer compositions will be omitted hereinafter.
A fiber-containing molded article is produced by, for example, a method comprising adding a powder of the polymer composition to a fabric of the fibers such as a plain weave fabric, a satin weave fabric, an imitation gauze fabric, a twill fabric, a helical woven fabric or a three-dimensionally woven fabric, a method comprising impregnating the fabric with a solution or slurry of the polymer composition, removing the solvent, drying the impregnated fabric, and heat-molding the prepreg sheet, or a method comprising melt-kneading the short fibers or chopped fibers with the polymer composition and molding the mixture by compression or injection molding. At this time, the content of the fibers in the molded article is preferably 10 to 70% by volume. The polymer composition of this invention as such may be used in this step. However, since it is not necessary to fiberize the polymer composition further, the ratio of silicon to carbon may be set within a slightly broader range than in the case of the composition of this invention.
The proportions of the optically isotropic pitch used may be adjusted to 10 to 4,900 parts by weight per 100 parts by weight of the organic silicon polymer. The proportion of the mesophase pitch may be adjusted to 5 to 50,000 parts by weight per 100 parts by weight of the reaction product of the organic silicon polymer and the isotropic pitch.
In the production of the fiber-containing molded article, the polymer composition may be used as a mixture of it with a calcined inorganic powder obtained by pyrolyzing the polymer composition at 800.degree. to 1,000.degree. C. in an inert atmosphere.
This calcined powder preferably consists essentially of 0.01 to 69.9% of Si, 29.9 to 99.9% of C and 0.01 to 10% of O if it does not contain a transition metal compound. If it contains a tansition metal, it preferably consists essentially of 0.005 to 30% of the transition metal, 0.01 to 69.9% of Si, 29.9 to 99.9% of C and 0.01 to 10% of O.
Then, as required, the product is subjected to a curing treatment.
The methods of curing in the production of the fibers of this invention may be directly used to perform this treatment.
The molded article rendered infusible is pyrolyzed at a temperature of 800.degree. to 3,000.degree. C. in vacuum or in an inert gas to give a composite material containing a matrix composed of carbon, silicon and oxygen, which is carbonized and fiber-reinforced.
It is presumed that in the process of heating, carbonization begins to be vigorous at about 700.degree. C., and is nearly compeleted at about 800.degree. C. It is preferred therefore to perform pyrolyzing at temperatures of 800.degree. C. or above. To obtain temperatures higher than 3,000.degree. C. requires expensive equipment, and pyrolyzing at high temperatures above 3,000.degree. C. is not practical from the viewpoint of cost.
The step of curing may be omitted by greatly decreasing the temperature-elevation rate for carbonization in this step or by using a shape retaining jig for the molded article, or a shape retaining means such as a powder head. By performing the molding with a high temperature hot press, a high-density composite can be obtained in one step.
The fiber-reinforced carbonaceous composite material obtained by pyrolyzing and carbonization contains some open pores. If required, it may be impregnated with a molten liquid, solution or slurry of the polymer composition and then pyrolyzed and carbonized after optionally it is cured. This gives a composite having a higher density and higher strength. The impregnation may be effected by any of the molten liquid, solution and slurry of the polymer composition. To induce permeation into fine open pores, after the composite material is impregnated with the solution or slurry of the polymer composition, it is placed under reduced pressure to facilitate permeation into the fine pores. Then, it is heated while evaporating the solvent, and subjected to a pressure of 10 to 500 kg/mm.sup.2. As a result, the molten liquid of the polymer composition can be filled in the pores.
The resulting impregnated material can be cured, pyrolyzed and carbonized in the same way as above. By repeating this operation 2 to 10 times, a fiber-reinforced composite material having a high density and high strength can be obtained.
This fiber-reinforced carbonaceous composite material is characterized by having high strength, high modulus of elasticity and excellent tenaciousness since, the reinforcing fibers have high strength and modulus of elasticity, and improved adhesion to the carbon matrix.
Furthermore, it has excellent oxidation resistance and abrasion resistance attributed to the effect of the siliicon carbide component contained in the fibers and the matrix. Accordingly, this composite material have excellent mechanical properties, oxidation resistance and abrasion resistance, and is useful as various types of brakes and thermally resistant structural materials.
Fiber-reinforced composite materials including a metal as a matrix:
The first and second fibers of this invention may be used directly as the reinforcing fibers. They may also be used as fibers to which at least one adhering material selected from the group consisting of fine particles, short fibers and whiskers of thermally resistant materials.
First, a method of adhering at least one adhering material selected from the group consisting of fine particles, short fibers and whiskers of thermally stable materials to the surface of the fibers of this invention provided as continuous filaments will be described.
Examples of the thermally stable materials are metals, ceramics and carbon.
Specific examples of the metals as the thermally stable materials are steel, stainless steel, molybdenum and tungsten. Specific examples of the ceramics include carbides such as SiC, TiC, WC and B.sub.4 C, nitrides such as Si.sub.3 N.sub.4, BN and A1N, borides such as TiB.sub.2 and ZrB.sub.2 and oxides such as Al.sub.2 O.sub.3, B.sub.2 O.sub.3, MgO, ZrO.sub.2 and SiO.sub.2. Other examples of the ceramics include polycarbosilane compositions, polymetallocarbosilane compositions, and calcination products of the first and second polymer compositions of this invention.
The form of the adhering material differs depending upon the combination of it with the continuous inorganic filaments or the required properties. The short fibers or whiskers of the adhering material desirably have an average particle diameter 1/3,000 to 1/5 of that of the continuous filaments and an aspect ratio of from 50 to 1,000. The fine particles desirably have an average diameter 1/5,000 to 1/2 of that of the continuous fibers.
The amount of the adhering material to be applied to the continuous fibers differs depending upon the properties of both, and the use of the fiber-rein-forced composite produced. In the case of using it for fiber-reinforced metals, the volume ratio of the adhering material based on the continuous filaments is preferably about 0.1 to 500%.
The adhering materials may be used singly or in combination. For example, when the fibers of this invention are to be used for reinforcing Al containing Co, Si, Mg and Zn, it is especially preferable to apply the fine particles to the neighborhood of the surface of the continuous fibers and apply the short fibers and/or the whiskers to the outside of the fine particles in order to prevent microsegregation of the added elements on the surface of the continuous filaments. In this case, the suitable ratio of the fine particles to the short fibers and/or the whiskers is from 0.1:5-40:1.
It is preferred to immerse the continuous filaments in a suspension of the adhering material because it is simple and has a wide range of application.
FIG. 1 shows one example of the outline of an apparatus used in the production of the above fibers.
A bundle 4 of continuous filaments (a woven fabric from the continuous filament bundle may be used instead of the continuous filament bundle) wound on a bobbin 5 is unwound, conducted by movable rollers 6 and 7, and passed through a liquid 3 in which the adhering material is suspended. Then, it is pressed by press rollers 8 and 9 and wound up on a bobbin 10. In the resulting filament bundle or fabric, the adhering material adheres to the surface of every individual continuous filament. There may be one treating vessel 1 containing a treating liquor 3. For various modified methods, two or more tgeating vessels containing treating liquors of different compositions respectively may be used.
To promote the adhesion of the adhering material to the continuous filaments, ultrasonic vibration 2 may be applied to the treating liquor 3. In the case of applying two or more kinds of the adhering material to the continuous filaments, the treating liquor may be a suspension of the fine particles and the short fibers and/or whiskers, or it is possible to use two treating vessels one containing a suspension of the fine particles as the treating liquor and the other containing a suspension of the short fibers and/or whiskers as the treating liquor. In the latter case, the sequence of immersing the continuous filament bundle or the woven fabric may start with the suspension of the fine particles or the suspension of the short fibers and/or whiskers.
Since the fibers having the adhering material are composed of a continuous filament bundle in which the adhering material adheres to the surface of every individual filament of the invention having high strength and high modulus of elasticity, these continuous filaments can be uniformly dispersed in the composite material, and the volume ratio of the fibers can be controlled to a very broad range. Furthermore, the contact among the continuous filaments decreases, and the resulting composite material has a uniform composition. This brings about the advantage of improving the mechanical properties such as strength of the composite material.
The reinforcing fibers may be applied to the matrix by, for example, arranging the fibers themselves in the monoaxial or multiaxial direction, or used in the form of various woven fabrics such as a plain weave fabric, a satin weave fabric, an imitation gauze fabric, a twill fabric, a helical woven fabric or a three-dimensionally woven fabric, or in the form of chopped fiber, to give the composite material of this invention.
Metals that can be used in this invention may be, for example, aluminum, aluminum alloys, magnesium, magnesium alloys, titaniuum, and titanium alloys.
The mixing proportion of the reinforcing fibers in the matrix is preferably 10 to 70% by weight.
The composite material can be produced by the following methods of producing ordinary fiber-reinforced metal composite materials. There are (1) a diffusion bonding method, (2) a melting permeation method, (3) a flame spraying method, (4) an electrolytic deposition method, (5) an extrusion and hot roll method, (6) a chemical vapor-phase deposition method, and (7) a sintering method.
(1) According to the diffusion bonding method, a composite material of reinforcing fibers and a matrix metal can be produced by aligning the reinforcing fibers and wires of the matrix metal alternately in one direction, covering the upper and lower surfaces of the arrangement with a thin coating of the matrix metal, or covering only the lower surface of it with the above thin coating and the upper surface of it with a powder of a mixture of the matrix metal and an organic binder to form a composite layer, laminating a plurality of such composite layers, and consolidating the laminate under heat and pressure.
The organic binder desirably volatilizes and dissipates before it is heated to a temperature at which it forms a carbide with the matrix metal. For example, CMC, paraffins, resins and mineral oils may be used.
Alternatively, the composite material may also be produced by bonding and coating a mixture of the matrix metal powder and the organic binder to the surfaces of the reinforcing fibers, aligning and laminating a plurality of layers of such fibers, and consolidating the laminate under heat and pressure.
(2) According to the melting permeation method, the composite material can be produced by filling the interstices of the aligned reinforcing fibers with molten aluminum, aluminum alloy, magnesium, magnesium alloy, titanium or titanium alloy. Since the wettability of the metal-coated fibers with the matrix metal is good, the interstices of the aligned fibers can be filled uniformly with the matrix metal.
(3) According to the flame spraying method, a tape-like composite material can be produced by coating the matrix metal on the surface of aligned reinforcing fibers by plasma flame spray or gas flame spray. It may be used directly, or a plurality of the tape-like composite materials are laminated and subjected to the diffusion bonding method (1) to produce a composite material.
(4) According to the electrolytic deposition method, a composite material can be produced by electrolytically depositing the matrix metal on the surface of the reinforcing fibers, laminating a plurality of the composite materials, aligning them, and subjecting the lamination to the diffusion bonding method (1).
(5) According to the extrusion and hot roll method, a composite material can be produced by aligning the reinforcing fibers in one direction, sandwiching the aligned reinforcing fibers between foils of the matrix metal, optionally passing the sandwich structure between heated rolls to bond the fibers and the matrix metal.
(6) According to the chemical vapor deposition method, a composite material can be produced by placing the fibers in a heating furnace, introducing a gaseous mixture of, for example, aluminum chloride and hydrogen to thermally decompose the gas to deposit aluminum metal on the surface of the fibers, and laminating the metal-deposited fibers, and subjecting the laminate to the diffusion bonding method (1).
(7) According to the sintering method, a composite material can be produced by filling a powder of the matrix metal in the interstices of aligned fibers, and sintering the resulting product under pressure or without pressure.
The tensile strength (.sigma..sub.c) of the composite material produced from the inorganic fibers and the metal matrix is represented by the above equation (see the above description on the composite material including a plastic matrix).
As shown by the above equation, the strength of the composite material becomes higher as the volume proportion of the reinforcing fibers in the composite material becomes larger. Hence, to produce a composite material having high strength, it is necessary to increase the volume proportion of the reinforcing fibers. However, if the volume proportion of the reinforcing fibers exceeds 70%, the amount of the metal matrix is small so that the intersices of the reinforcing fibers cannot be fully filled with the metal matrix. Hence, the composite material produced cannot exhibit the strength shown by the above equation. If the volume proportion of the reinforcing fibers in the composite material is decreased, the strength of the composite material decreases as shown by the above equation. To obtain a composite material having practical utility, it is mecessary to combine at least 10% of the reinforcing fibers. Accordingly, if the volume proportion of the reinforcing fibers is limited to 10 to 70% by volume in the production of the fiber-reinforced metal composite material, the best result can be obtained.
In the production of the composite material, it is necessary to heat the metal to a temperature near the melting point or a higher temperature as stated above, and combine it with the reinforcing fibers. Thus, the reduction of fiber strength by the reacion of the reinforcing fibers with the molten metal gives rise to a problem. But when the fibers of this invention are immersed in the molten metal, the abrupt degradation seen in ordinary carbon fibers is not observed, and therefore, a composite material having excellent mechanical strength can be obtained.
The methods of measuring the various mechanical properties used in this invention will be described.
(a) Initial rate of degradation induced by reaction
(1) In the case of metals and alloys having a melting point of not more than 1200.degree. C.
The fibers are immersed for 1, 5, 10, and 30 minutes respectively in a molten metal heated to a temperature 50.degree. C. higher than the melting point of the metal. Then, the fibers are extracted, and their tensile strength is measured. From the results obtained, a reaction degradation curve showing the relation between the immersion time and the tensile strength of the fibers is determined. From a tangent at an immersion time of 0 minute, the initial rate of degradation induced by reaction (kg/mm.sup.2 -sec.sup.-1) is determined.
(2) In the case of metals and alloys having a melting point higher than 1200.degree. C.
The fibers are laminated to a metal foil. The laminate is placed under vacuum, heated to a temperature of (the melting point of the metal foil).times.(0.6-0.7), and maintained under a pressure of 5 kg/mm.sup.2 for 5, 10, 20 and 30 minutes, respectively. Then, the fibers are extracted, and their tensile strength is measured.
From the results, the initial rate of degradation induced by reaction is determined by the same procedure as in (1).
(b) Ratio of fiber strength reduction
The fiber strength at an immersion time and a maintenance time of 30 minutes in (a) above is determined. The ratio of fiber strength reduction is calculated by dividing (the initial strength--the fiber strength determined above) by the initial strength.
The initial rate of reduction by reaction shows the degree of the reaction between the fibers and the matrix in the production of a fiber-reinforced metal within a short time. The smaller this value, the better the affinity between the fibers and the matrix and the larger the fiber reinforcing effect.
(c) Interlayer shear strength test
The same as the method described above with respect to a composite material comprising plastics as a matrix.
(d) Fatigue test
A round rod (10 mm in diameter.times.100 mm in length) is produced from a composite material in which the inorganic fibers are aligned monoaxially. The axial direction of the composite material is the longitudinal direction of the rod. The rod is worked into a test specimen for a rotational bending fatigue test. The specimen is subjected to a rotational bending fatigue test with a capacity of 1.5 kgm, and its fatigue strength after 10.sup.7 times is measured and defined as the fatigue.
The ratio of the fatigue strength and the tensile strength is an index showing the strength of bonding between the matrix and the fibers.
Since the degradation of the fiber strength due to the reaction with the molten metal is little in the fibers of this invention, the fiber-reinforced metal composite materials including the fibers of this invention have excellent tensile strength and other mechanical properties, high modulus of elasticity and excellent thermal resistance and abrasion resistance. Accordingly, they are useful as various material in various technological fields such as synthetic fibers, synthetic chemistry, machine industry, construction machinery, marine and space exploitation, automobiles and foods.
According to this invention, a carbonized sintered body can be produced from a polymer composition by the following procedure.
Examples of the polymer composition that can be used at this time are the first and second polymer compositions of the invention, and polymer compositions having a slightly broader chemical composition than the polymer compositions of this invention, which are described with reference to the description of fiber-reinforced composite materials comprising a carbonized product of the polymer composition of the invention as a matrix.
The polymer composition or a mixture of the polymer composition and its calcination product is first finely pulverized, and can be molded by using a method of molding an ordinary carbonaceous material. The calcination may be carried out at a temperature of 800.degree. to 1300.degree. C.
The molding method can be selected from the molding methods for ordinary carbonaceous material by considering shape, size, use of the molded product and the productivity of molding. For example, for production of articles of the same shape with good productivity, a dry mold press method is suitable. To obtain molded articles of a slightly complex shape, an isostatic molding method (rubber press molding method) is suitable. For molding a molten mass of the above polymer, a hot press molding method, an injection molding method and an extrusion molding method are suitable.
In the case of molding the mixture of the polymer composition and its calcination product, the proportions of the polymer composition and its calcination product may be properly determined by considering the shape, use and cost of the molded article to be obtained.
The molded article is then subjected to an curing treatment.
A typical method of curing is to heat the molded article in an oxidizing atmosphere. The curing temperature is preferably 50.degree. to 400.degree. C. If the curing temperature is excessively low, bridging of the polymer does not take place. If this temperature is excessively high, the polymer burns.
The purpose of curing is to render the polymer constituting the molded article in the three-dimensional infusible insoluble bridged state and to have the molded article retain its shape without melting during carbonization in the next step. The gas constituting the oxidizing atmosphere during curing may be, for exampe, air, ozone, oxygen, chlorine gas, bromine gas, ammonia gas, or mixtures of these gases.
An alternative method of curing which may also be used comprises applying gamma-ray irradiation or electron beam irradiation to the molded article in an oxidizing or non-oxidizing atmosphere while as required heating it at low temperatures.
The purpose of gamma ray or electron beam irradiation is to prevent the matrix from melting and losing the shape of the molded article by further polymerizing the polymer constituting the molded article.
The suitable irradiation dose of gamma rays or electron beams is 10.sup.6 to 10.sup.10 rads.
The irradiation may be carried out in vacuum, in an inert gas atmosphere or in an atmosphere of an oxidizing gas such as air, ozone, oxygen, chlorine gas, bromine gas, ammonia gas or mixtures of these.
Curing by irradiation may also be carried out at room temperature. If required, by performing it while heating at a temperature of 50.degree. to 200.degree. C., the curing may be achieved in a shorter period of time.
The molded article rendered infusible is then pyrolyzed and carbonized at a temperature of 800.degree. to 3000.degree. C. in vacuum or in an inert gas.
It is presumed that in the heating process, carbonization begins to become vigorous at about 700.degree. C., and is nearly completed at about 800.degree. C. Hence, the pyrolyzing is preferably carried out at a temperature of at least 800.degree. C. To obtain temperatures higher than 3000.degree. C., expensive equipment is required. Accordingly, pyrolyzing at high temperatures higher than 3000.degree. C. is not practical in view of cost.
The curing step may be omitted by making the temperature elevation rate for carbonization in this step very slow, or by using a jig for retaining the shape of the molded article or a shape retaining means such as a powder head. Alternatively, by using a high temperature hot press method in this molding step, the next step may be omitted.
As required, the resulting carbonaceous material may be impregnated with a melt, solution or slurry of the polymer solution, and pyrolyzed for carbonization. This further increases the density and strength of the carbonaceous material.
For impregnation, any of the melt, solution and slurry of the polymer composition may be used. To facilitate permeation into fine open pores, the carbonaceous material after impregnation with the solution or slurry of the polymer composition is placed under reduced pressure to facilitate permeation into the fine pores, heated while evaporating the solvent, and pressed under 10 to 500 kg/cm.sup.2 thereby to fill the melt of the polymer composition into the pores.
The carbonaceous material impregnated with the polymer composition may be cured, pyrolyzed and carbonized in the same way as in the previous step. By repeating this operation 2 to 10 times, a carbonaceous material having high density and high strength can be obtained.
The state of existence of Si, C and O in the silicon-containing component corresponding to the constituent (iii) of the first fibers in the resulting carbonaceous material can be controlled by the carbonization temperature in the above-mentioned step.
When it is desired to obtain an amorphous material consisting substantially of Si, C and O, it is proper to adjust the carbonization temperature to 800.degree. to 1000.degree. C. If it is desired to obtain a material consisting substantially of beta-SiC and amorphous SiO.sub.x (0<x.ltoreq.2), temperatures of at least 1700.degree. C. are suitable.
When a mixture of the aggregates is desired, temperatures intermediate between the above temperatures may be properly selected.
The amount of oxygen in the carbonaceous material of this invention may be controlled, for example, by the curing conditions in the above curing step.
The state of existence of Si, M, C and O in the silicon-containing component corresponding to component (iii) of the second fibers may be controlled likewise.
The resulting carbonaceous material contains a silicon carbide component very uniformly dispersed and integrated in carbon. The presence of this component promotes microcrystallization of carbon at low temperatures, inhibition of consumption of carbon by oxidation, and the increase of hardness.
The carbonaceous material, therefore, has excellent mechanical properties, oxidation resistance and abrasion resistance and can be advantageously used as various types of brakes and thermally stable structural materials.

Carbon fibers having high strength and high modulus of elasticity and polymer composition for their production