Abstract:
The invention relates to processes for preparing a polyimide, a polyimide composition prepared by the processes and polyimide composite materials containing the same.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for preparing a polyimide that has excellent thermal stability and processability in a molten state. 
     The present invention also relates to a polyimide composite material that has excellent high-temperature stability, chemical resistance and mechanical strength as well as outstanding processability. 
     A polyimide obtained by reacting a tetracarboxylic dianhydride and an aromatic diamine has excellent mechanical strength, dimensional stability, high temperature stability, flame resistance and electrical insulation properties. Such a polyimide has been used in electrical and electronic appliances, space and aeronautical instruments and transport machinery. It is expected that it will also be used in applications where high-temperature resistance is required. 
     A variety of polyimides that exhibit excellent properties have been developed. Some polyimides, however, have no distinct glass transition temperature, although they are excellent in high-temperature stability. As a result, when employed as molding materials, specific methods such as sinter molding must be used for processing. Other polyimides which are excellent in processability have low glass transition temperatures and are soluble in halogenated hydrocarbons, which renders these polyimides unsatisfactory for use in applications which require high-temperature stability and solvent resistance. Thus, these polyimides have numerous advantages and disadvantages. 
     The present inventors have previously discovered a polyimide that is excellent in mechanical strength, thermal characteristics, electrical properties and solvent resistance and exhibits high-temperature stability. The polyimide consists primarily of recurring units of the formula: ##STR1## wherein X represents a direct bond or a radical selected from the group consisting of a C 1  -C 10  divalent hydrocarbon radical, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical and a sulfonyl radical; Y 1 , Y 2 , Y 3  and Y 4  may be the same or different and represent a radical selected from the group consisting of a hydrogen atom, a lower alkyl radical, a lower alkoxy radical, a chlorine atom and a bromine atom; and R represents a tetravalent radical selected from the group consisting of an aliphatic radical having two or more carbon atoms, a cyclic aliphatic radical, a monocyclic aromatic radical, a fused polycyclic aromatic radical, and a polycyclic aromatic radical wherein the aromatic rings are linked together directly or via a bridged member. The polyimides have been disclosed in the following Japanese Laid-Open Patents: 
     Ohta et al; TOKKAISHO 61-143478 (1986). 
     Tamai et al; TOKKAISHO 62-68817 (1987), which corresponds to copending U.S. patent application Ser. No. 44,028, filed on June 30, 1986. 
     Ohta et al; TOKKAISHO 62-86021 (1987), which corresponds to copending U.S. patent application Ser. No. 44,028, filed on June 30 1986. 
     Ohta et al; TOKKAISHO 62-235381 (1987) and 
     Oikawa et al; TOKKAISHO 63-128025 (1988), which corresponds to copending U.S. patent application Ser. No. 119,042, filed on Nov. 10, 1987. 
     The polyimide is a novel high-temperature stable resin having many good properties. The polyimide exhibits excellent flowability and good processability. The fused resin, however, gradually exhibits decreased flowability which has an adverse effect on processability when the polyimide is kept at high temperatures for a long period of time, for example, long residence at high temperatures in a cylinder for injection molding. 
     Therefore, it is desirable to develop a polyimide which exhibits good flowability at lower temperatures and stable flowability for a long period of time during processing. 
     Previously produced molded products prepared by using composite materials composed of polyimides and fibrous reinforcing materials exhibit excellent mechanical strength, particularly strength retention at high temperatures and are also outstanding in solvent resistance and dimensional stability. Such molded products are desirable for use as structural members for spacecrafts and the like. 
     Polyimides generally have a high melt viscosity. Therefore composite materials containing these polyimides as matrices have required severe processing conditions as compared to those containing matrices of engineering plastics such as polycarbonate and polyethylene terephthalate, thereby causing various problems. 
     Special polyimides having low melt viscosity and excellent workability have also been known in the art. Such polyimides, however, have a low heat distortion temperature and are soluble in solvents such as halogenated hydrocarbons. Consequently, the composites containing such a resin as a matrix have caused problems in high-temperature stability and chemical resistance. 
     In order to overcome these problems, a composite material containing a polyimide having the above stated good properties and fibrous reinforcing materials has been developed, Koba et al; Japanese Laid-Open Patent TOKKAISHO 62-248635 (1987). The polyimide, however, gradually decreases in melt flowability when maintained at high temperatures for a long period. This phenomenon often inhibits continuous operation. Accordingly, the development of polyimide composite materials which are capable of operating for a long period of time and also provide excellent properties is strongly desired. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems and disadvantages of the prior art by providing processes for the preparation of polyamic acid, a thermally stable polyimide and a composite material containing the polyamide. 
     An object of this invention is to provide a method for preparing an excellent polyimide which has, in addition to its substantially outstanding properties, good thermal stability in a molten state and does not deteriorate in processability even after residence in high temperatures for a long period of time. 
     Another object of this invention is to provide a polyimide base composite material capable of being stably processed for a long period of time without giving adverse effects on the essential properties of the polyimide, such as high-temperature stability, chemical resistance and dimensional stability. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by mean of the instrumentalities and combinations, particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several exemplary embodiments of the invention and together with the description serve to explain the principles of the invention. 
     FIG. 1 and FIG. 5 illustrate the relationship between shear rate and melt viscosity of the polyimide prepared by the process of this invention; 
     FIG. 2 and FIG. 6 illustrate the relationship between the melt viscosity and numbers of repeated fusion; 
     FIG. 3 and FIG. 4 illustrate the relationship between the melt viscosity and residence time of the polyimide in the cylinder of flow tester in this invention; and 
     FIG. 7 shows an example of equipment for preparing the polyimide base composite material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a process for preparing a thermally stable polyimide consisting primarily of recurring units of the formula: ##STR2## wherein X represents a covalent bond or a radical selected from the group consisting of a C 1  -C 10  divalent hydrocarbon radical, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical and a sulfonyl radical; Y 1 , Y 2 , Y 3  and Y 4  may be the same or different and represent a radical selected from the group consisting of a hydrogen atom, a lower alkyl radical, a lower alkoxy radical, a chlorine atom and a bromine atom; and R represents a tetravalent radical selected from the group consisting of an aliphatic radical having two or more carbon atoms, a cyclic aliphatic radical, a monocyclic aromatic radical, a fused polycyclic aromatic radical, and a polycyclic aromatic radical wherein the aromatic rings are linked together directly or via a bridged member. 
     The process of the present invention comprises reacting 
     (a) An aromatic diamine represented by the formula (I): ##STR3## 
     wherein X, Y 1 , Y 2 , Y 3  and Y 4  have the same meanings as set forth above, 
     (b) a tetracarboxylic dianhydride represented by the formula (II): ##STR4## wherein R is as defined above, 
     (c) a monoamine represented by the formula (III): 
     
         Z--NH.sub.2                                                (III) 
    
     wherein Z represents a monovalent radical selected from the group consisting of an aliphatic radical, a cyclic aliphatic radical, a monocyclic aromatic radical, a fused polycyclic aromatic radical, and a polycyclic aromatic radical wherein the aromatic rings are linked together directly or via a bridged member to form a polyamide, and dehydrating or imidizing the polyamic acid to form a polyimide. 
     Preferably, the molar ratio of the aromatic diamine is from about 0.9 to about 1.0 mole per mole of tetracarboxylic dianhydride. Preferably the molar ratio of the monoamine is from about 0.001 to about 1.0 mole per mole of tetracarboxylic dianhydride. 
     Exemplary suitably aromatic diamines for use in the process of this invention include, bis[4-(3-aminophenoxy)phenyl]methane, 1,1-bis[4-(3-aminophenoxy)phenyl]ethane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2-[4-(3-aminophenoxy)phenyl]-2-[4-(3-aminophenoxy)-3-methylphenyl]propane, 2,2-bis[4-(3-aminophenoxy)-3-methylphenyl]propane, 2-[4-(3-aminophenoxy)phenyl]-2-[4-(3-aminophenoxy)-3,5-dimethylphenyl]propane, 2,2-bis[4-(3-aminophenoxy)-3,5-dimethylphenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]butane, 2,2-bis[4-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 4,4&#39;-bis(3-aminophenoxy)biphenyl, 4,4&#39;-bis(3-aminophenoxy)-3-methylbiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;-dimethylbiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,5-dimethylbiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;,5,5&#39;-tetramethylbiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;-dichlorobiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,5-dichlorobiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;,5,5&#39;-tetrachlorobiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;-dibromobiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,5-dibromobiphenyl, 4,4&#39;-bis(3-aminophenoxy)-3,3&#39;,5,5&#39;-tetrabromobiphenyl, bis[4-(3aminophenoxy)phenyl] ketone, bis[4-(3-aminophenoxy)phenyl] sulfide, bis[4-(3-aminophenoxy)-3-methoxyphenyl] sulfide, [4-(3-aminophenoxy)phenyl] [4-(3-aminophenoxy)-3,5-dimethyoxyphenyl] sulfide, bis[4-(3-aminophenoxy)-3,5-dimethoxyphenyl] sulfide and bis[4-3-aminophenoxy)phenyl] sulfone. 
     Preferably, aromatic diamines selected from the group consisting of 4,4&#39;-bis(3-aminophenoxy)biphenyl, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, bis[4-(3-aminophenoxy)phenyl] ketone, bis(4-(3-aminophenoxy)phenyl] sulfide and bis[4-(3-aminophenoxy)phenyl] sulfone are employed. The diamine compounds employed may be used singly or in combination. 
     Other diamines may be used in the process of the invention providing that such diamines do not cause an adverse effect on the properties of the polyimide produced by the process of this invention. 
     Exemplary suitable tetracarboxylic dianhydrides for use in the process of this invention include ethylenetetracarboxylic dianhydride, butanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, pyromellitic dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxypenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride, 2,2&#39;,3,3&#39;-benzophenonetetracarboxylic dianhydride, 3,3&#39;,4,4&#39;-biphenyltetracarboxylic dianhydride, 2,2&#39;,3,3&#39;-biphenyltetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl) ether dianhydride, bis(2,3-dicarboxyphenyl) ether dianhydride, bis(3,4-dicarboxyphenyl) sulfone dianhydride, 4,4&#39;-(p-phenylenedioxy)diphthalic dianhydride, 4,4&#39;-(m-phenylenedioxy)diphthalic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride and 1,2,7,8-phenanthrenetetracarboxylic dianhydride. Preferred tetracarboxylic dianhydrides for use in the process of this invention include pyromellitic dianhydride, 3,3&#39;4,4&#39;-benzophenonetetracarboxylic dianhydride, 3,3&#39;,4,4&#39;-biphenyltetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl) ether dianhydride and 4,4&#39;-(p-phenylenedioxy)diphthalic dianhydride. The tetracarboxylic dianhydride compounds employed may be used singly or in combination. 
     Exemplary suitable monoamines for use in the process of this invention include methylamine, ethylamine, n-propylamine, iso-propylamine, n-butylamine, iso-butylamine, sec-butylamine, tert-butylamine, n-amylamine, iso-amylamine, tert-amylamine, n-hexylamine, heptylamine, n-octylamine, 2-ethylhexylamine, nonylamine, decylamine, vinylamine, allylamine, cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, cycloheptylamine, cyclooctylamine, cyclohexylmethylamine, aniline, o-toluidine, m-toluidine, p-toluidine, 2,3-xylidine, 2,4-xylidine, 2,5-xylidine, 2,6-xylidine, 3,4-xylidine, 3,5-xylidine, o-chloroaniline, m-chloroaniline, p-chloroaniline, o-bromoaniline, m-bromoaniline, p-bromoaniline, o-nitroaniline, m-nitroaniline, p-nitroaniline, o-aminophenol, m-aminophenol, p-aminophenol, o-anisidine, m-anisidine, p-anisidine, o-phenetidine, m-phenetidine, p-phenetidine, o-aminobenzaldehyde, m-aminobenzaldehyde, p-aminobenzaldehyde, o-aminobenzotrifluoride, m-aminobenzotrifluoride, p-aminobenzotrifluoride, o-aminobenzonitrile, m-aminobenzonitrile, p-aminobenzonitrile, 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 2-aminophenyl phenyl ether, 3-aminophenyl phenyl ether, 4-aminophenyl phenyl ether, 2-aminobenzophenone, 3-aminobenzophenone, 4-aminobenzophenone, 2-aminophenyl phenyl sulfide, 3-aminophenyl phenyl sulfide, 4-aminophenyl phenyl sulfide, 2-aminophenyl phenyl sulfone, 3-aminophenyl phenyl sulfone, 4-aminophenyl phenyl sulfone, 1-naphthylamine, 2-naphthylamine, 1-amino-2-naphthol, 2-amino-1-naphthol, 4-amino-1-naphthol, 5-amino-1-naphthol, 5-amino-2-naphthol, 7-amino-2-naphthol, 8-amino-1-naphthol, 8-amino-2-naphthol, 1-aminoanthracene, 2-aminoanthracene and 9-aminoanthracene. 
     Preferred monoamines for use in the process of the invention include n-propylamine, n-butylamine, n-hexylamine, n-octylamine, cyclohexylamine, aniline, 4-aminobiphenyl, 4-aminophenyl phenyl ether, 4-aminobenzophenone, 4-aminophenyl phenyl sulfide and 4-aminophenyl phenyl sulfone. The monoamine compound employed may be used singly or in combination. 
     The molar ratio of the aromatic diamine to the tetracarboxylic dianhydride is preferably from about 0.9 to about 1.0 mole of aromatic diamine per mole of tetracarboxylic dianhydride. The molar ratio of the monoamine is preferably from about 0.001 mole to about 1.0 mole per mole of tetracarboxylic dianhydride. 
     In preparing the polyimide, the molar ratio of the aromatic diamine to the tetracarboxylic dianhydride is usually adjusted to control the molecular weight of the polyimide formed. In the method of this invention, when the molar ratio of the aromatic diamine to the tetracarboxylic dianhydride is from about 0.9 to about 1.0 a polyimide having good melt viscosity is obtained. 
     The amount of monoamine simultaneously present in the reaction is preferably in the range of from about 0.001 to about 1.0 mole per mole of tetracarboxylic dianhydride. When the amount is less than 0.001 mole, heat stability in the molten state of the polyimide, which is an object of this invention, may . not be obtained.. When the molar ratio of the monoamine to the tetracarboxylic dianhydride is greater than 1.0, the mechanical properties of the molded products are not as good. Most preferably, the molar ratio of the monoamine to the tetracarboxylic dianhydride is from about 0.01to about 0.5 mole per mole of the tetracarboxylic dianhydride. 
     Suitable organic solvents for use in the process of the invention include N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylmethoxyacetamide, N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone, N-methylcaprolactam, 1,2-dimethoxyethane, bis(2-methoxyethyl) ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl] ether, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, pyridine, dimethyl sulfoxide, dimethyl sulfone, tetramethylurea, hexamethylphosphoramide, phenol, m-cresol, p-cresol, p-chlorophenol and anisole. The organic solvents may be used singly or in combination. 
     In the method of this invention, the starting materials, e.g., the tetracarboxylic dianhydride, aromatic diamine and monoamine are added to the organic solvent and reacted. The reaction can be carried out by any of the following methods. 
     (a) After reacting tetracarboxylic dianhydride with aromatic diamine, monoamine is added and reacted. 
     (b) After reacting tetracarboxylic dianhydride with monoamine, aromatic diamine is added and reacted. 
     (c) Tetracarboxylic dianhydride, aromatic diamine and monoamine are reacted at the same time. 
     The reaction temperature is preferably in the range of from about 0° to about 250° C., most preferably 60° C. or less. 
     Any reaction pressure may be used and ambient pressure is enough to carry out the reaction. 
     The reaction time is dependent upon the tetracarboxylic dianhydride, aromatic diamine, monoamine, solvent and reaction temperature. Preferably, the reaction time is from about 4 to about 24 hours. 
     Polyamic acid is formed by the above mentioned reaction. The resultant polyamic acid is dehydrated by heating at about 100° to about 400° C. or chemically imidized by using a usual imidizing agent. The polyimide obtained consists primarily of recurring units of the formula: ##STR5## wherein X, Y 1 , Y 2 , Y 3 , Y 4  and R are as defined above. 
     The polyamic acid is generally formed at low temperatures and then thermally or chemically imidized. The polyimide, however, can also be obtained by simultaneously conducting the formation and thermal imidization of the polyamic acid at a temperature of from about 60° to about 250° C. In this method, a tetracarboxylic dianhydride, aromatic diamine and monoamine are suspended or dissolved in an organic solvent and reacted by heating. Thus, formation and imidization of the polyamic acid are carried out at the same time and a polyimide consisting primarily of the recurring units of the above formula is produced. 
     When the polyimide of this invention is processed by fusion molding, other thermoplastic resins may be incorporated in a suitable amount depending upon the application providing that no adverse effects are created that are contrary to the objects of this invention. Illustrative examples of the thermoplastic resins suitable for use with the polyimide of the invention include polyethylene, polypropylene, polycarbonate, polyarylate, polyamide, polysulfone, polyethersulfone, polyetherketone, polyphenylenesulfide, polyamideimide, polyetherimide and modified polyphenyleneoxide. 
     Fillers which are used for usual resin compositions may be employed in amounts which have no adverse effects on the objects of this invention. Suitable fillers include wear resistance improvers such as graphite, carborundum, quartz powder, molybdenum disulfide and fluororesins; reinforcing materials such as glass fiber, carbon fiber, boron fiber, silicon carbide fiber, carbon whisker, asbestos, metal fiber and ceramic fiber; flame retardants such as antimony trioxide, magnesium carbonate and calcium carbonate; electrical property improvers such as clay and mica; tracking resistance improvers such as barium sulfate, silica and calcium metasilicate; thermal conductivity improvers such as iron powder, zinc powder, aluminum powder and copper powder; and other miscellaneous additives such as glass beads, glass spheres, talc, diatomaceous earth, alumina, silicate balloon, hydrated alumina, metal oxides and coloring agents. 
     The present invention also relates to a polyimide composite material comprising a polyimide made by the process of the invention and a fibrous reinforcing material. Exemplary suitable fibrous reinforcing materials for us in the polyimide composite material of this invention include glass fibers such as E-glass, S-glass, T-glass, C-glass and AR-glass; carbon fibers such as polyacrylonitrile base, pitch base and rayon base carbon fibers; aromatic polyamide fibers represented by Kevler (Trademark of E. I. Du Pont de Nemours &amp; Co.); silicon carbide fibers such as Nicalon (Trademark of Nippon Carbon Co.); metallic fibers such as stainless steel fibers; alumina fibers and boron fibers. These fibers are unidirectional long fibers such as yarn, bundled unidirectional long fibers such as roving, tow and multidirectional continuous fibers such as textiles, mats and felts. The fibrous reinforcing materials may be used singly or in combination. 
     The reinforcing materials should be selected to achieve the desired properties of the molded articles such as strength, elastic modulus, elongation at break, electrical properties and density. For example, carbon fibers or glass fibers are preferred when high values are required for specific strength and specific modulus of elasticity. Carbon fibers and metal fibers are preferred when shielding property for electromagnetic field is required. Glass fibers are preferred when electrical insulation properties are required. 
     Fiber diameter and collected number of fibers depend upon the raw materials of fibers. For example, carbon fibers generally have a fiber diameter of 4 to 8 μm and a collected number of 1,000 to 12,000. Finer diameter of fibers is preferred due to favorable effect on the mechanical properties of the molded articles obtained. 
     Surface treatment of the fibrous reinforcing materials is favorable for improving affinity with the polyimide. For example, surface treatment with a silane base or a titanate base coupling agent is particularly preferred for glass fibers. 
     Preferably, the fibrous reinforcing materials are present in the composite material in an amount of from about 5 to about 85%, more preferably in an amount of from about 30 to about 70% by volume of the composite material. When the volume content of the fibrous reinforcing materials is low, a reinforcing effect cannot be expected. When the volume content is too high, the interlayer strength of molded articles decreases severely. 
     Any previously known method can be used for the preparation of the composite material from the polyimide and the fibrous reinforcing material. 
     For example, the impregnation method may be employed wherein the fibrous reinforcing material is impregnated with the polyimide in a molten state. In the fluidized bed method, impregnation is carried out by using the polyimide powder in a state of floating in the air or suspending in a liquid such as water. In the fluidized bed method, the impregnated fibrous reinforcing material is dried, if necessary, and then the polyimide in the fibrous reinforcing material is fused by heating. This process is particularly effective for providing the integrated polyimide composite material. Finer particle size is desired for the polyimide powder at the impregnation. The preferred particle size is less than the diameter of the fibrous monofilament. 
     In another method, the powder or a film of the polyimide is placed on one or both sides of the fibrous reinforcing material and pressed with heating. When the fibrous reinforcing material is a fabric, the polyimide powder or film is alternately piled up with necessary sheets of the fabric to form the desired thickness of the article to be molded and is then pressed with heating. Thus impregnation and molding can be conducted at the same time and molded articles having a uniform resin dispersion can be obtained. 
     Further methods for the fused impregnation have been representatively described in Nakakura et al;. Japanese Laid-Open Patent 61-229534, 61-229536 (1986) and Koba et al; Japanese Patent Application 62-216253 (1987). As an example of these method, the fibrous reinforcing material is impregnated by being contacted with the molten resin on the surface of a hot roll or hot belt. 
     Specifically, a fiber sheet obtained by paralleling unidirectional long fibers such as tow which are drawn out of several bobbins or a multidirectional continuous fiber is applied a given tension in the direction of take-up with a tension adjusting roll. The polyimide is fused by heating in an extruder. The fused polyimide is then extruded through a die and applied to a lower belt on the surface of a hot roll maintained at a prescribed temperature. Successively the above mentioned fiber sheet or multidirectional continuous fiber is impregnated by passing through one or more hot rolls in a sandwiched condition between the upper and lower pair of belts. The continuous impregnation method is particularly preferred. 
     The composite material thus obtained is piled up and pressed with heating t obtain molded articles of desired shapes. 
     EXAMPLES 
     The present invention will be further clarified by the following examples which are intended to be purely exemplary of the invention. 
     Example 1 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 322 g (1.0 mole) of 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride and 4,050 g of N,N-dimethylacetamide as a solvent were charged. Then 392 g (0.98 mole) of bis[4-(3-aminophenoxy)phenyl] sulfide were added by portions at the room temperature in a nitrogen atmosphere so as not to raise the temperature of the solution and stirred for about 20 hours at room temperature. 
     To the polyamic acid solution thus obtained 8.10 g (0.08 mole) of n-hexylamine were added and further stirred for one hour. Then 202 g (2 moles) of triethylamine and 306 g (3 moles) of acetic anhydride were added dropwise to the solution. The reaction mixture was stirred for 20 hours. The resultant light yellow slurry was filtered, washed with methanol and dried at 180° C. for 8 hours under reduced pressure. The light yellow polyimide powder obtained was 661 g. The polyimide powder had a glass transition temperature of 205° C. in accordance with DSC method and an inherent viscosity of 0.62 dl/g. The inherent viscosity was measured at 35° C. in a solvent mixture (90/10 weight ratio of p-chlorophenol/phenol) at a concentration of 0.5 g/100 ml solvent. 
     The relationship between the shear rate and the melt viscosity of the polyimide powder thus obtained was measured by using a Japan Polymer Society type flowtester (Trade mark, CFT-500; a product of Shimadzu Seisakusho Co.) with an orifice having a diameter of 0.1 cm and a length of 1 cm. After being maintained at 320° C. for 5 minutes, the melt viscosity was measured at various shear rates. The relationship between melt viscosity and shear rate is illustrated by curve A in FIG. 1. 
     The melt viscosity of the polyimide was remarkably lowered when the shear rate was increased. 
     Comparative Example 1 
     The same procedures as described in Example 1 were carried out without n-hexylamine. The polyimide powder obtained weighed 652 g and had an inherent viscosity of 0.62 dl/g. By using the polyimide powder, the relationship between the melt viscosity and the shear rate was measured with the flow tester as in Example 1. The results are illustrated by curve B in FIG. 1. 
     Curve B indicates higher melt viscosity at lower shear rates (around 10 2  1/sec.) than Curve A, which means difficulty of processing in Comparative Example 1. 
     Example 2 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 322 g (1.0 mole) of 3,3&#39;-4,4&#39;-benzophenonetetracarboxylic dianhydride and 4,050 g of N,N-dimethylacetamide as a solvent were charged. Then 392 g (0.98 mole) of [4-(3-aminophenoxy)phenyl] sulfide were added by portions at room temperature in a nitrogen atmosphere so as not to raise the temperature of the solution and stirred for about 20 hours at room temperature. 
     To the polyamic acid solution thus obtained, 7.44 g (0.08 mole) of aniline were added and further stirred for one hour. Then 202 g (2 moles) of triethylamine and 306 g (3 moles) of acetic anhydride were added dropwise to the solution. The reaction mixture was stirred for 20 hours at room temperature. The resultant light yellow slurry was filtered, washed with methanol and dried at 180° C. for 8 hours under reduced pressure. The light yellow polyimide powder obtained weighed 660 g. The polyimide powder had a glass transition temperature of 205° C. and an inherent viscosity of 0.62 dl/g. 
     The melt viscosity of the polyimide powder thus obtained was repeatedly measured by using a Japan Polymer Society type flowtester. After holding at 320° C. for 5 minutes, the sample was extruded with a pressure of 100 kg/cm 2 . The strand obtained was crushed and extruded again. The procedure was continuously repeated five times. The relationship between the repeated number and the melt viscosity is illustrated by Curve A in FIG. 2. Almost no variation was found in the melt viscosity even though the repeated number increased, which indicates good heat stability of the molten polyimide. 
     Comparative Example 2 
     The same procedures as described in Example 2 were carried out without aniline. The polyimide powder obtained weighed 652 g and had an inherent viscosity of 0.62 dl/g. 
     The repeated measurement of the melt viscosity was carried out on the polyimide powder obtained by the method described in Example 2. The results are illustrated by Curve B in FIG. 2. The melt viscosity was increased with the increase of repeated number. The heat stability of the molten polyimide thus obtained was inferior to that obtained in Example 2. 
     Example 3 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 218 g (1.0 mole) of pyromellitic dianhydride and 3,830 g of N,N-dimethylacetamide were charged and 5.95 g (0.06 mole) of cyclohexylamine were added. The mixture was stirred for about 20 minutes. Then 357 g (0.97 mole) of 4,4&#39;-bis(3-aminophenoxy)biphenyl were added so as not to raise the temperature of the solution and stirred for about 10 hours at the room temperature. 
     To the solution thus obtained, 202 g (2 moles) of triethylamine and 255 g (2.5 moles) of acetic anhydride were added. The reaction mixture was further stirred for 10 hours at room temperature. The resultant light yellow slurry was filtered, washed with methanol and dried at 180° C. for eight hours under reduced pressure. The light yellow polyimide powder thus obtained weighed 523 g. The polyimide powder had a glass transition temperature of 252° C., a melting point of 378° C. in accordance with DSC method and an inherent viscosity of 0.52 dl/g. 
     The melt viscosity of the polyamide was measured at a cylinder temperature of 420° C., a residence time of 5 minutes and a pressure of 100 kg/cm 2 . The melt viscosity was 2,600 poises. 
     The heat stability of the molten polyimide was evaluated by measuring the variation of melt viscosity when the residence time of the molten polyimide was changed in the cylinder of the flow tester. The cylinder temperature was 420° C. The pressure at the measurement was 100 kg/cm 2 . The results are illustrated by Curve A in FIG. 3. Almost no variation was found in the melt viscosity even though the residence time was extended, which indicates good heat stability of the molten polyimide. 
     Comparative Example 3 
     The same procedures as described in Example 3 were carried out without cyclohexylamine. 
     The light yellow polyimide powder thus obtained had a glass transition temperature of 255° C., a melting point of 378° C. and an inherent viscosity of 0.50 dl/g. The polyimide had a melt viscosity of 3,800 poises at a cylinder temperature of 420° C., a residence time of 5 minutes under a pressure of 100 kg/cm 2 . 
     The heat stability of the molten polyimide was evaluated by measuring the variation of melt viscosity when the residence time of the molten polyimide was changed in the cylinder of the flow tester. The melt viscosity was increased with the increase of residence time. The heat stability of the molten polyimide thus obtained was inferior to that obtained in Example 3. The results are illustrated by Curve B in FIG. 3. 
     Example 4 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 218 g (1.0 mole) of pyromellitic dianhydride and 3,830 g of N,N-dimethylacetamide were charged and 5.58 g (0.06 mole) of aniline were added. The mixture was stirred for about 20 minutes. Then 357 g (0.97 mole) of 4,4&#39;-bis(3-aminophenoxy)biphenyl were added so as not to raise the temperature of the solution and stirred for about 10 hours at room temperature. 
     To the solution thus obtained, 202 g (2 moles) of triethylamine and 255 g (2.5 moles) of acetic anhydride were added. The reaction mixture was further stirred for 10 hours at room temperature. The resultant light yellow slurry was filtered, washed with methanol and dried at 180° C. for eight hours under reduced pressure. The light yellow polyimide powder thus obtained weighed 523 g. The polyimide powder had a glass transition temperature of 255° C., a melting point of 378° C. and an inherent viscosity of 0.52 dl/g. 
     The heat stability of the molten polyimide was evaluated by measuring the variation of melt viscosity when the residence time of the molten polyimide was changed in the cylinder of the flow tester. The cylinder temperature was 420° C. The pressure at the measurement was 100 kg/cm 2 . The results are illustrated by Curve A in FIG. 4. Almost no variation was found in the melt viscosity even though the residence time was extended, which indicates good heat stability of the molten polyimide. 
     Comparative Example 4 
     The same procedures as described in Example 4 were carried out without aniline. 
     The light yellow polyimide powder thus obtained had a glass transition temperature of 255° C., a melting point of 378° C. and an inherent viscosity of 0.50 dl/g. 
     The heat stability of the molten polyimide was evaluated by measuring the variation of melt viscosity when the residence time of the molten polyimide changed in the cylinder of the flow tester. The melt viscosity increased with the increase of residence time. The heat stability of the molten polyimide thus obtained was inferior to that obtained in Example 4. The results are illustrated by Curve B in FIG. 4. 
     Example 5 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 218 g (1.0 moles) of pyromellitic dianhydride, 393.6 g (0.96 mole) of 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 10.34 g (0.08 mole) of n-octylamine and 3,460 g of m-cresol were charged and gradually heated with stirring in a nitrogen atmosphere. After stirring the reaction mixture for three hours at 150° C., the precipitate was filtered, washed with methanol, then acetone, and dried at 180° C. for eight hours under reduced pressure. The polyimide powder thus obtained weighed 560 g and had an inherent viscosity of 0.56 dl/g and a glass transition temperature of 228° C. The relationship between the melt viscosity and the shear rate was measured at 360° C. The results obtained are illustrated by Curve A in FIG. 5. 
     Comparative Example 5 
     The same procedures as described in Example 5 were carried out without n-octylamine. The polyimide powder thus obtained had an inherent viscosity of 0.56 dl/g. 
     The relationship between the melt viscosity and the shear rate was measured by the same conditions as in Example 5. The results obtained are illustrated by Curve B in FIG. 5. 
     Example 6 
     To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 218 g (1.0 mole) of pyromellitic dianhydride, 393.6 g (0.96 mole) of 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 7.44 g (0.08 mole) of aniline and 3,460 g of m-cresol were charged and gradually heated with stirring in a nitrogen atmosphere. After stirring the reaction mixture for three hours at 150° C., the precipitate was filtered, washed with methanol and then acetone, and dried at 180° C. for eight hours under reduced pressure. The polyimide powder thus obtained weighed 558 g and had an inherent viscosity of 0.56 dl/g and a glass transition temperature of 231° C. 
     The repeated extrusion test with the flow tester was carried out at 360° C. under the pressure of 100 kg/cm 2  by using the same conditions as described in Example 2. 
     Almost no increase in the melt viscosity of the molten polyimide thus obtained was observed by the repeated extrusion. The results are illustrated in FIG. 6. 
     Example 7 
     The polyimide powder was produced from 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride, bis[4-(3-aminophenoxy)phenyl] sulfide and n-hexylamine according to the procedure of Example 1. 
     The composite material was manufactured from the polyimide and carbon fibers by the following method. FIG. 7 illustrates an outline of the equipment used for the production. 
     Carbon fibers BESFIGHT HTA-7-3K (Trade mark of Toho Rayon Co.) drawn from 100 bobbins (1), and 100 tows (2) were paralleled to one direction with an aligner (3) and passed through tension adjusting rolls (4, 5, 6) to make a fiber sheet (7) having a width of 150 mm. 
     Separately, a polyimide powder fused by heating in an extruder (not indicated in FIG. 7) was extruded from a die (8) and applied to the surface of a lower belt (10) with a thickness of 70 μm. The lower belt (10) was heated to 350° C. by three rolls (9) contacting with the lower belt (10), and the upper belt (12) was also heated to 350° C. by three rolls (11) contacting with the upper belt (12). Then the above obtained fiber sheet was sandwiched between the upper and lower belts, and passed through three impregnation rolls (13) at a rate of 20 cm/min under a tension of 150 kg. The impregnation rolls (13) had a diameter of 240 mm and were heated to 350° C. The polyimide impregnated carbon fiber composite material (14) thus obtained was cooled, passed through take-up rolls (15, 16, 17) and wound up by the winding shaft (18). 
     The composite material thus obtained had a width of 150 mm and a thickness of 0.13 mm. 
     Twenty sheets of the above polyimide composite material were unidirectionally piled up and hot pressed at 340° C. for 20 minutes under a pressure of 30 kg/cm 2  to obtain a plate having dimensions of 200×200×2.5 (thickness) mm. Volume percentage of the fibrous reinforcing material (hereinafter abbreviated as Vf), void ratio, flexural strength and flexural modulus were measured on the plate thus obtained. The plate had a Vf of 60 %, void ratio of 1 % or less, flexural strength of 190 kg/mm 2  and flexural modulus of 12,100 kg/mm 2 . These results indicated good properties of the plate. Vf and void ratio were calculated from the density and weight percentage of the fibrous reinforcing material of the plate. Flexural strength and flexural modulus were measured in accordance with JIS K 7230. 
     Example 8 
     The polyimide powder was produced from 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride, bis[4-(3-aminophenoxy)phenyl] sulfide and aniline according to the procedure of Example 2. 
     The same procedures as described in Example 7 were carried out for the manufacture of the composite material from the polyimide powder thus obtained and carbon fiber. 
     The operation for the manufacture of the composite material was continued for five hours. No change was observed during the operation on the flowability of molten polyimide. The resultant composite material could be smoothly wound without fiber breakage. The composite material thus obtained had a width of 150 mm and a thickness of 0.13 mm. 
     Twenty sheets of the above obtained polyimide composite material were then unidirectionally piled up and hot pressed at 340° C. for 20 minutes under a pressure of 50 kg/cm 2  to produce a plate having dimensions of 200×200×2.5 (thickness) mm. The plate had a Vf of 60 %, void ratio of 1 % or less, flexural strength of 198 kg/mm 2  and flexural modulus of 12,100 kg/mm 2 . These results indicated good properties of the plate. 
     Comparative Example 6 
     The polyimide powder was produced from 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride and bis[4-(3-aminophenoxy)phenyl] sulfide according to the procedure of Comparative Example 2. 
     The same procedures for the production of the composite material as described in Example 7 were carried out using the polyimide powder obtained. After 30 minutes, molten polyimide caused gelation and release of fibers from the upper and lower belts became difficult. Finally the operation was stopped. A part of the composite material formed in this operation was molded by the same procedures as described in Example 8. Its physical properties were evaluated. The molded plate had a Vf of 60 %, void ratio of 7.2 %, flexural strength of 90 kg/mm 2  and flexural modulus of 6,100 kg/mm 2 . The extremely low strength and modulus resulted from a significant decrease in flowability and insufficient defoaming. 
     Example 9 
     A polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and cyclohexylamine according to the procedure of Example 3. A composite material was obtained from the polyimide powder and carbon fiber by the procedures of Example 7 except that the impregnation temperature was changed to 420° C. The manufacturing operation of the composite material was continued for 5 hours. No change was observed during the operation on the flowability of molten polyimide. The composite material thus obtained was hot pressed to give a plate by the same procedures as described in Example 7 except that the temperature was 400° C. The plate obtained had a flexural strength of 184 kg/mm 2  and a flexural modulus of 11,300 kg/mm 2 . 
     Comparative Example 7 
     The polyimide powder was produced from pyromellitic dianhydride and 4,4&#39;-bis(3-aminophenoxy)biphenyl according to the procedures of Comparative Example 3. The composite material was obtained and molded by the same procedures as described in Example 9. The plate thus obtained had a Vf of 60%, void ratio of 7.3%, flexural strength of 91 kg/mm 2  and flexural modulus of 5,500 kg/mm 2 . The extremely lower strength and modulus resulted from the significant decrease in flowability and insufficient defoaming of polyimide. 
     Example 10 
     The polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and aniline according to the procedures of Example 4. The composite material was obtained from the polyimide powder thus obtained by the same procedures as described in Example 7 except that the impregnation temperature was changed to 420° C. The manufacturing operation was continued for five hours. No change was observed during the operation on the flowability of molten polyimide. The composite material thus obtained was hot pressed to give a plate by the same procedures as described in Example 8 except that the temperature was changed to 400° C. The plate obtained had a flexural strength of 187 kg/mm 2  and a flexural modulus of 11,800 kg/mm 2 . 
     Examples 11-14 
     The polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and cyclohexylamine according to the procedures of Example 3. 
     The polyimide composite materials were obtained from the resultant polyimide powder by carrying out the same procedures as described in Example 9 except that the kinds of fibrous reinforcing materials and the applied thickness of the molten polyimide to the belt were changed as illustrated in Table 1. A polyimide composite material wa obtained. Several composite materials as illustrated in Table 1, were piled up and processed by the same procedures as described in Example 9. The properties of the plate obtained were shown in Table 1. 
     Examples 15-16 
     The polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and cyclohexylamine according to the procedures of Example 3. 
     The same procedures as described in Example 9 were carried out except that the kinds of fibrous reinforcing materials and the applied thickness of the molten polyimide to the belt were changed as illustrated in Table 1, and the tension was changed to 30 kg. Thus polyimide composite materials were obtained. Several composite materials as illustrated in Table 1 were piled up and processed by the same procedures as described in FIG. 9. The properties of the plate thus obtained are illustrated in Table 1. 
     Examples 17-20 
     The polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and aniline according to the procedures of Example 4. 
     The same procedures as in Example 10 were carried out except that the kinds of fibrous reinforcing materials and the applied thickness of the molten polyimide to the belt were changed as illustrated in Table 1. Polyimide composite materials were obtained. Several composite materials as illustrated in Table 1 were piled up and processed by the same procedures as in Example 10. Properties of the plate thus obtained are illustrated in Table 1. 
     Examples 21-22 
     The polyimide powder was produced from pyromellitic dianhydride, 4,4&#39;-bis(3-aminophenoxy)biphenyl and aniline according to the procedures of Example 4. 
     The same procedures as Example 10 were carried out except that the kinds of fibrous reinforcing materials and the applied thickness of the molten polyimide to the belt were changed as illustrated in Table 1, and the tension was changed to 30 kg. Polyimide composite materials were obtained. Several composite materials were piled up as set forth in Table 1 and processed by the same procedures as in Example 10. Properties of the plate thus obtained are illustrated in Table 1. 
     
                                           TABLE 1__________________________________________________________________________Fibrous Reinforcing Material                     Volume                          Number                               Flexural                                     FlexuralApplied              content                          of piled                               strength                                     modulusExampleThickness      Material              Form   (%)  sheet                               (kg/mm.sup.2)                                     (kg/mm.sup.2)__________________________________________________________________________11   200   Carbon fiber              Tow    30   12   93    6,20012    40   Carbon fiber              Tow    70   26   216   13,00013   105   Glass fiber              Roving 60   12   151   5,10014    80   Aromatic              Roving 60   12   76    5,600      polyamide fiber15   100   Carbon fiber              Plain woven                     60   16   85    7,000              fabric16    70   Glass fiber              Plain woven                     60   22   66    2,500              fabric17   200   Carbon fiber              Tow    30   12   94    6,10018    40   Carbon fiber              Tow    70   26   213   13,20019   105   Glass fiber              Roving 60   12   146   5,30020    80   Aromatic              Roving 60   12   78    5,800      polyamide fiber21   100   Carbon fiber              Plain woven                     60   16   89    7,100              fabric22    70   Glass fiber              Plain woven                     60   22   65    2,700              fabric__________________________________________________________________________ 
    
     Example 23 
     An aluminum frame having inside dimensions of 30 cm×30 cm and a thickness of 1.0 mm was placed on a heat resistant release film 50μm thick. 5 g of the polyimide powder obtained in Example 3 were uniformly dispersed within the frame. The frame was removed and a carbon fiber fabric BESFIGHT W-3101 (Trademark of Toho Rayon Co.) was placed on the dispersed polyimide powder. 5 g of the polyimide powder were then uniformly dispersed on the carbon fiber. A commercial heat resistant release film was placed on the dispersed polyimide powder and pressed in a metal mold at 400° C. for 10 minutes at a pressure of 70 kg/cm 2 . After cooling to 250 C. at the same pressure, the molded product was taken out of the mold and the release films were removed. 
     The composite material thus obtained was divided into six portions, piled up and processed by the procedures described in Example 9. The resultant plate had a Vf of 60 %, flexural strength of 85 kg/mm 2  and flexural modulus of 7,100 kg/mm 2 . 
     Example 24 
     The same procedures as described in Example 23 were carried out except that the polyimide powder obtained in Example 4 was used in place of the polyimide powder obtained in Example 3 and the pressure was changed to 50 kg/cm 2 . 
     The plate thus obtained had a Vf of 60 %, flexural strength of 87 kg/mm 2  and flexural modulus of 6,900 kg/mm 2 . 
     Example 25 
     A polyimide powder was produced from pyromellitic dianhydride, 2,2-bis[4-(3-aminophenoxy)phenyl]propane and n-octylamine according to the procedures of Example 5. 
     The polyimide powder was subjected to an impregnation process by the same procedures as described in Example 7 except that the impregnation temperature was changed to 350° C. 
     The polyimide composite material thus obtained was hot pressed by the same procedures as Example 7 except that the temperature was changed to 330° C. The plate obtained had a flexural strength of 176 kg/mm 2  and a flexural modulus of 10,600 kg/mm 2 . 
     Example 26 
     A polyimide powder was produced from pyromellitic dianhydride, 2,2-bis[4-(3-aminophenoxy)phenyl]propane and aniline according to the procedure of Example 6. 
     The polyimide powder was subjected to an impregnation process by the same procedures as described in Example 7 except that the impregnation temperature wa changed to 350° C. The operation was continued for 5 hours. No change was observed on the flowability of the molten polyimide resin during the operation. Then the resultant composite material was hot pressed by the same procedures as in Example 8 except that the temperature was changed to 330° C. The plate thus obtained had a flexural strength of 181 kg/mm 2  and a flexural modulus of 11,100 kg/mm 2 .