Process for preparing pitch for matrix

Abstract: Disclosed is a process for preparing pitch for matrix, said process is characterized in that an optically isotropic pitch is formed into a fibrous pitch, and the fibrous pitch thus obtained is nitrated with a nitration reagent in the presence of an acid catalyst. The pitch for matrix obtained according to the present invention can be carbonized with a low energy, high in carbonization yield, low in softening point and excellent in moldability and impregnability.

Description: FIELD OF THE INVENTION This invention relates to a process for preparing a pitch suitable for molding or for use as a matrix in which solid material is embedded or as an impregnant, hereinafter referred to as "matrix pitch" and more particularly provides a process for preparing such a pitch which has a low temperature at which the thermopolymerization of said pitch is initiated and for increasing the carbonization yield of said pitch. BACKGROUND OF THE INVENTION Generally, matrix pitch is used for the preparation of carbon materials, for example, by impregnating carbonaceous materials such as carbon fiber, porous carbonaceous material and the like with said pitch, followed by thermopolymerization in an inert atmosphere. Such matrix pitch as mentioned above should have a high flowability at the time when it is heated for impregnation and should be carbonized in a high yield during the thermopolymerization thereof. In this connection, attempts have heretofore been made to increase the carbonization yield of matrix pitch by air oxidation, such as an air blow, of a starting pitch consisting essentially of an aromatic compound in its molten state. Using this method, however, there is a problem in that it is difficult to obtain uniform carbon materials having excellent characteristics, because the oxidation reaction of the starting pitch is slow to progress owing to insufficient contact of the pitch with oxygen and said oxidation reaction does not proceed uniformly. It is also known to pulverize the starting pitch or mold the starting pitch into a fiber form, followed by air oxidation thereof. In this method, however, there is a problem in that when the carbonization yield of the starting pitch is increased by means of sufficient oxidation, the oxidized pitch does not melt such that it can flow and, on the other hand, when the oxidation of said starting pitch is limited to such an extent that the pitch can flow, the carbonization yield of the starting pitch does not increase sufficiently. It is known to obtain petroleum resin or phenol resin suitable for use in coating materials, adhesives, pressure-sensitive adhesives, casting materials, sealing materials and the like by oxidation polymerization of the starting aromatic compound in the presence of a crosslinker such as formaldehyde and an acid catalyst. And though it may be presumed that such petroleum resin or phenol resin as referred to above is used as a raw material for the preparation of carbon materials, it is difficult to obtain desired carbon materials in a high carbonization yield. It should also be mentioned that JP-A-63139080 (Japanese Patent L-O-P Publn. No. 139080/1988) discloses a process for producing graphite, which comprises treating carbonaceous mesophase or coke with nitric acid or a mixture of nitric acid and sulfuric acid, followed by heating at a temperature of at least 2400°C. Further, JP-A-3169339 (Japanese Patent L-O-P Publn. No. 169339/1991) discloses a process for producing carbonaceous adsorbents having micro pores, which comprises treating a mesophase obtained by heat treatment of pitch at at least about 350°C or a coke with nitric acid or mixture of nitric acid and sulfuric acid, and eluting insolubles out of the carbon material by treatment with a solvent, followed by heat treatment of the thus treated product at 300-1400°C. The above-cited publications, however, fail to disclose a process for preparing matrix pitch with good impregnating properties and the like. Further, JP-B-2027302 (Japanese Patent Publn. No. 27302/1990) discloses a process for preparing pitch for preparing carbon materials, which comprises subjecting powdered pitch or fibrous pitch to an oxidizing treatment, and molding the oxidized pitch under a molding pressure of 100-500 kg/cm 2 G into a desired form, followed by carbonization. In the process for preparing the pitch disclosed in this publication, however, the oxidation treatment employed, to which the powdered or fibrous pitch is subjected, is limited solely to vapor phase oxidation. Using a process comprising vapor phase oxidation of molten pitch as illustrated above, there is a limit to the use of the resulting oxidized pitch as a carbonaceous material, particularly as a matrix pitch, because a uniform pitch could not be obtained owing to the slow oxidation of the pitch, and when the vapor phase oxidation reaction temperature was elevated in order to accelerate the oxidation reaction of the pitch, thermopolymerization of the pitch took place concurrently therewith, and hence the resulting oxidized pitch was found to have poor flowability. In the process for the vapor phase oxidation of pitch in its powdered or fibrous state, it is presumed that the oxidation reaction proceeds into the interior of the powder or fiber, and the pitch decreases in flowability even when it is in a heated state, and hence it is considerably difficult to increase the carbonization yield of the pitch while keeping flowability of said pitch. It is also known to obtain pitch for preparing carbon materials, by liquid phase oxidation of a mesophase or coke. In this process, however, there is a problem in that the process has low economic efficiency, because the liquid phase oxidation is prone to non-uniformity, and for obtaining a uniform oxidized pitch, a liquid phase oxidation treatment for an extended period of time is required. With a view to solving the above-mentioned problems, the present inventors carried out extensive research and have accomplished eventually the present invention on the basis of their findings. According to the preferred embodiments of the invention, a pitch having a low softening point and an optical isotropy is formed in advance into a fibrous form and the thus treated pitch may be nitrated in a short period of time even at a low reaction temperature by liquid phase nitration with a nitration reagent containing an acid catalyst and, as a result, the thus nitrated pitch is high in thermal reactivity. Hence the thermopolymerization reaction of the nitrated pitch can proceed with a low energy input and, moreover, because the nitrated pitch is thermopolymerized without evaporation of low molecular weight components at the time of thermopolymerization of the pitch, the carbonization yield of the pitch increases. The nitrated pitch thus obtained has a low softening point, has good moldability and impregnability as matrix for various composite materials, and can yield uniform carbon materials. SUMMARY OF THE INVENTION According to the present invention, there is provided a process for preparing and using a pitch suitable for molding or for use as a matrix in which solid material is embedded or as an impregnant, which process comprises melt spinning an optically isotropic pitch of which the optical anisotropy is less than 50%, to form a fibrous pitch, nitrating the fibrous pitch thus obtained with a liquid nitration reagent in the presence of an acid catalyst at a temperature up to the softening point of said optically isotropic pitch, and either molding and carbonizing the nitrated fibrous pitch, or impregnating a carbonaceous material with the nitrated fibrous pitch and carbonizing the nitrated fibrous pitch. As indicated below, the term "optically isotropic" is used in this specification to include optical anisotropy less than 50% In the present invention, it is desirable that the fibrous pitch to be nitrated has a diameter of not more than 20 µm, the nitration reagent is nitric acid, and the acid catalyst is sulfuric acid. Using the invention, there can be prepared a uniform matrix pitch of high thermal reactivity by melt spinning an optically isotropic pitch into a fibrous pitch, and nitrating the fibrous pitch thus obtained by means of a nitration reagent in the presence of an acid catalyst. DETAILED DESCRIPTION OF PREFERRED FEATURES The process for preparing and using a matrix pitch of the present invention is illustrated below in detail. In the process for preparing a matrix pitch according to the invention, an optically isotropic pitch is melt spun, according to the usual way, into a fibrous pitch with a large specific surface area, and the fibrous pitch thus obtained is nitrated by means of a nitration reagent in the presence of an acid catalyst to convert it into a pitch having a high thermal reactivity. By "pitch high in thermal reactivity" as used in the present specification is meant a pitch having a thermopolymerization initiation temperature as measured by a differential scanning calorimeter (DSC) lower by 10-100°C than that of the starting optically isotropic pitch prior to nitration thereof. Using the vapor phase oxidation of the starting pitch known per se, it was difficult to perform an effective and uniform oxidation, because said starting pitch must be heat treated at a high temperature and consequently the pitch begins to melt. In contrast thereto, according to the present invention, an optically isotropic pitch is nitrated in a liquid phase by means of a nitration reagent containing an acid catalyst, and hence even an optically isotropic pitch having a low softening point can be treated; thus it becomes possible to nitrate uniformly the starting pitch formed into a fibrous pitch, as it is, at a low temperature and in a short period of time. Further, because of its high thermal reactivity, the nitrated pitch thus obtained has an increased carbonization yield, and because of its low softening point, the nitrated pitch is easily used as an impregnant, for example, for use in impregnating carbonaceous materials therewith, thus the nitrated pitch is suitable for use in the preparation of composite materials. Furthermore, it has become possible to perform the nitration of the starting pitch at a low temperature and in a short period of time by means of a combination of the nitration reagent and acid catalyst mentioned above. A. Optically isotropic pitch: The optically isotropic pitch used in the present invention is prepared, in the usual way, from petroleum pitch, coal pitch, etc. through treatment steps such as filtration, purification, distillation, hydrogenation, catalytic cracking, etc. By "an optically isotropic pitch" as used in the present specification is meant a pitch that is optically isotropic, or a pitch whose optical anisotropy is less than 50%, preferably less than 30%, when it is observed by means of a polarization microscope. Desirably, the optically isotropic pitch used in the present invention as the starting material has a softening point of not more than 200°C, preferably not more than 170°C. From the optically isotropic pitch having a low softening point as mentioned above, there can be prepared a nitrated matrix pitch having a low softening point. However, commonly used optically isotropic pitch having a relatively high softening point such as higher than 200°C can also be used in the present invention in the same way as mentioned above. In the present invention, moreover, a nitration reaction in a liquid phase can be performed at a temperature of about room temperature, and hence even optically isotropic pitch having a low softening point, for example, not more than 100°C, can be used as the starting pitch. B. Fibrous pitch: In the present invention, the optically isotropic pitch as mentioned above is molded into a fibrous pitch, followed by nitration. In molding the optically isotropic pitch into a fibrous form, any molding techniques may be employed so long as the optically isotropic pitch is formed thereby into a fibrous optically isotropic pitch. The fibrous pitch to be nitrated is desirably small in diameter and large in specific surface area, if possible, so that the fiber surface is sufficiently nitrated, and the nitration reaction is accelerated. The diameter of the fibrous pitch is desirably not more than 20 µm, preferably not more than 16 µm. C. Nitration: The optically isotropic pitch molded into the fibrous form in such a manner as mentioned above is then nitrated in a liquid phase. In the present invention, it is desirable to nitrate the fibrous pitch to such an extent that the infusibilization of the pitch does not progress too much. This is because, if the fibrous pitch is nitrated excessively, the resulting nitrated pitch becomes infusible, whereby the thus nitrated pitch becomes difficult to handle as a matrix pitch. The liquid phase nitration of the optically isotropic fibrous pitch of the present invention may be carried out by bringing said fibrous pitch into contact at a relatively low temperature with a nitrating solution (preferably an aqueous solution) containing an acid catalyst of given concentration and a nitration reagent. The acid catalysts usable in the present invention are not particularly restricted so long as they are in a liquid state. The acid catalysts include, for example, inorganic strong acids such as sulfuric acid, hydrochloric acid and phosphoric acid; organic strong acids such as p-toluenesulfonic acid, etc.; Lewis acids such as boron trifluoride and aluminum chloride; inorganic weak acids such as carbonic acid and boric acid; and organic weak acids such as acetic acid, propionic acid and butyric acid. Of the acid catalysts exemplified above, the preferred one is sulfuric acid because of its high catalytic activity to the nitration reaction and its water-solubility. The nitration reagent is typically nitric acid. However, there may also be used nitrates such as sodium nitrate, potassium nitrate, etc., organic nitric acid esters and nitrous acid. The amounts of the acid catalyst and the amounts of the nitration reagent used are not particularly restricted if the liquid phase nitration of the optically isotropic pitch proceeds smoothly due to the catalytic action of the acid catalyst. Generally, however, the amounts may suitably be changed according to the kind of optically isotropic pitch used, and to the shape and diameter of the fibres. More specifically, the concentration in the liquid phase of the nitration reagent is generally more than 2N, preferably from 5 to 10N, and that of the acid catalyst is desirably from 0.2 to 1.0 mol/L. If the concentration of the nitration reagent used is less than 2N, the nitration reaction does not sometimes proceed sufficiently, and if said concentration exceeds 10N, the nitration reaction sometimes proceeds excessively. The nitration of the fibrous pitch needs to be carried out in such a way that the fibrous pitch retains its shape. The nitration temperature employed is preset, taking productivity into account, and is generally from room temperature to a temperature up to the softening point of the optically isotropic pitch, preferably from room temperature to about 100°C. The nitration time employed is from 15 minutes to 5 hours, preferably from 15 minutes to about 1 hour. If the reaction temperature employed exceeds the softening point of the optically isotropic fibrous pitch used, no nitration as expected can be accomplished, because when nitrating the pitch, the pitch cannot retain its fibrous form and sometimes melts, whereby the surface area of the pitch sharply decreases. The nitrating liquid may include small amounts of various alcohols, low boiling silicone oil, various surfactants and emulsifier such as polyethylene glycol in order to improve the wetting between the nitrating liquid containing nitric acid and the acid catalyst, and the fibrous pitch, that is, so that the fibrous pitch will not be afloat on the surface of said nitrating liquid. The liquid phase nitration reaction of the fibrous pitch may be carried out continuously after the optically isotropic pitch has been molded into the fibrous form, or may be carried out by another process after the fibrous pitch has once been prepared. The nitration reaction of such fibrous pitch may usually be carried out using batchwise or continuous operation. The nitrated fibrous pitch thus obtained is high in thermal reactivity as mentioned above and has a thermopolymerization initiation temperature as measured by DSC lower by 10-100°C than that of the starting optically isotropic pitch prior to nitration thereof. In the present invention, if necessary, the nitrated fibrous pitch may be then subjected to a drying treatment and/or polymerization treatment in air at room temperature or under application of heat. Further, it is also possible that the nitrated fibrous pitch is subjected, if necessary, to an alkali washing treatment, thereby substituting the nitro group on the surface of the nitrated fibrous pitch with another functional group. The nitrated matrix pitch thus obtained in the above-mentioned manner is useful as a matrix of a carbon composite material made by impregnating the nitrated pitch into carbon fibers, carbon nonwoven fabrics or porous carbon materials. D. Molding, carbonization and graphitization The nitrated fibrous pitch prepared in the manner as mentioned above may easily be molded into such a molded product as a pellet useful as a matrix material by means of an extruder, if said nitrated fibrous pitch has a low softening point such as particularly not more than 200°C, preferably not more than 170°C. That is, the nitrated fibrous pitch generally has a low softening point, and excellent flowability and is particularly useful as a matrix pitch for a carbon material. It is used as it is, in the form of a molded product or in the form of various composite materials impregnated therewith. The nitrated fibrous pitch thus used is then heated to effect carbonization or graphitization to give a carbon material. The nitrated fibrous pitch obtained according to the present invention has good thermal reactivity, and on that account, thermopolymerization thereof proceeds at low temperatures. The pitch is thus polymerized in the thermopolymerization process without evaporation of low molecular weight components, and hence the carbonization yield of said pitch can be increased. In practicing the thermopolymerization process (carbonization or graphitization process), various reaction conditions may be selected according to the purposes for which the resulting carbonaceous materials are used. Generally, the thermopolymerization process is carried out by heating the molded product of the nitrated pitch or the composite material impregnated with the nitrated pitch in the presence of an inert gas such as nitrogen gas or argon gas at 400-3,000°C, preferably 700-2,500°C at a heating rate of 5-100°C/min. EFFECT OF THE INVENTION In a process for preparing a matrix pitch using conventional vapor phase oxidation, high reaction temperatures such as 200-400°C are required, and hence pitch having a softening point of not more than 200°C, particularly optically isotropic pitch is oxidized in its molten state and therefore it was difficult to perform the oxidation of the pitch efficiently and uniformly. In the present invention, however, the starting pitch is formed into fibers so as to have increased surface area, and the fibrous pitch is nitrated in a liquid phase by bringing said pitch into contact with the nitration reagent containing the acid catalyst, and hence even optically isotropic pitch having a low softening point may be nitrated uniformly and efficiently at low temperatures and in a short period of time. As a result, the nitrated pitch thus obtained has high thermal reactivity, and can be carbonized with low energy input. Therefore, the carbonization yield thereof becomes high. Further, the nitrated pitch has a low softening point, has excellent flowability at the time of heating, is easy to mold and has excellent impregnability. The nitrated pitch with high thermal reactivity is very useful as a matrix for various composite materials. Embodiment of the Invention Example 1 An optically isotropic petroleum pitch having a softening point as measured by a flow tester of 137°C was spun using a melt spinning process at a spinneret temperature of 170°C and a take-up speed of 170 m/min to obtain a fibrous pitch having a fiber diameter of 15 µm. Separately, a liquid phase nitration bath charged with 200 ml of an aqueous solution having a nitric acid concentration of 5 N and a sulfuric acid concentration of 0.5 mol/L was prepared. In the nitration bath prepared above was immersed 2 g of the above-mentioned fibrous pitch at room temperature for 3 hours to obtain a nitrated pitch at a reaction yield of 105.3% by weight. The nitrated pitch thus obtained was taken out of the nitration bath, followed by water-washing and drying. The thus obtained pitch had a softening point of 170°C. Differential scanning calorie determination of the said nitrated pitch was carried out, and the endothermic peak which is the initiation point of the thermopolymerization reaction appeared at 395°C. The said pitch was heated up to 800°C in a nitrogen stream at a heating rate of 10°C/min, whereby a carbon material was obtained at a carbonization yield of 69.4% by weight (73.1% by weight based on the fibrous pitch). Comparative Example 1 A fibrous pitch was obtained in the same manner as Example 1. Subsequently, this fibrous pitch was heated immediately, without subjecting it to liquid phase nitration, in a nitrogen stream up to 800°C at a heating rate of 10°C/min, whereby a carbon material was obtained at a carbonization yield of 50.8% by weight. Differential scanning calorimetry of the fibrous pitch was carried out, and the endothermic peak appeared at 441°C. Example 2 An optically isotropic petroleum pitch having a softening point as measured by a flow tester of 80°C was spun using a melt spinning process at a spinneret temperature of 110°C and a take-up speed of 170m/min to obtain a fibrous pitch having a fiber diameter of 15µm. Subsequently, a liquid phase nitration bath charged with 200 ml of an aqueous solution having the nitric acid concentration of 8.0N and the sulfuric acid concentration of 0.5 mol/L was prepared. In the nitration bath prepared above was immersed 2g of the above-mentioned fibrous pitch at room temperature for 3 hours to obtain a nitrated pitch at a reaction yield of 117.5% by weight. This nitrated pitch was taken out, followed by water-washing and drying. The pitch thus obtained had a softening point of 150°C. This nitrated pitch was heated up to 800°C in a nitrogen stream at a heating rate of 10°C/min, whereby a carbon material was obtained at a carbonization yield of 70.3% by weight (62.6% by weight based on the fibrous pitch). Comparative Example 2 Following the same procedure as in Example 2, a fibrous pitch was obtained. Subsequently, this fibrous pitch was heated immediately, without subjecting it to liquid phase nitration, in a nitrogen stream up to 800°C at a heating rate of 10°C/min, whereby a carbon material was obtained at carbonization yield of 22.8% by weight. Example 3 A UD substrate formed from uni-directionally arranged petroleum pitch based carbon fibers was impregnated with the nitrated pitch prepared in Example 1 at a temperature of 250°C and a pressure of 10 mmHg to obtain a molded product containing 60% by weight of the carbon fibers. The molded product was carbonized at 2,300°C in an argon stream, whereby a carbon/carbon (c/c) composite having a tensile strength of 350 MPa and a tensile modulus of 200 GPa was obtained. Comparative Example 3 Preparation of a c/c composite was attempted in the same manner as in Example 3 using an optically isotropic petroleum pitch having a softening point of 137°C as used in Example 1 and without subjecting it to nitration, but it was too low in carbonization yield to obtain the c/c composite as aimed at.

Claims: A process for preparing and using a pitch suitable for molding or for use as a matrix in which solid material is embedded or as an impregnant, which process comprises melt spinning an optically isotropic pitch of which the optical anisotropy is less than 50%, to form a fibrous pitch, nitrating the fibrous pitch thus obtained with a liquid nitration reagent in the presence of an acid catalyst at a temperature up to the softening point of said optically isotropic pitch, and either molding and carbonizing the nitrated fibrous pitch, or impregnating a carbonaceous material with the nitrated fibrous pitch and carbonizing the nitrated fibrous pitch. The process as claimed in claim 1 wherein the fibrous pitch has a diameter of not more than 20 µm. The process as claimed in claim 1 or 2 wherein the nitration reagent is nitric acid and the acid catalyst is sulfuric acid. The process as claimed in claim 1, 2 or 3 wherein the nitration is carried out under such conditions that the reaction temperature is from room temperature to the softening point of the optically isotropic pitch, the reaction time is from 15 minutes to 5 hours, and the concentration in the liquid phase of the nitration reagent and of the acid catalyst is 2-10 N and 0.2-1.0 mol/L, respectively.

Inventors: KAWAMURA TOSHIFUMI, MAEDA TAKASHI, KAWAMURA, TOSHIFUMI, MAEDA, TAKASHI, Kawamura, Toshifumi, c/o Petoca, Ltd., Maeda, Takashi, c/o Petoca, Ltd.,
Applicants: PETOCA LTD, PETOCA LTD.
Application year: 1993
Main classification: C04B35
IPC codes: C10C3, C04B35, C01B31, D01F9, D01D5, C08J5