Process for preparing polyimide having excellent high temperature stability

Abstract: This invention relates to a process for preparing polyimide having good heat-stability and excellent flowability in a molten state. The polyimide is prepared by reacting 3,3′-diaminobenzophnone or bis(3-aminophenyl)sulfone, tetracarboxylic dianhydrides such as pyromellitic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and bis(3,4-dicarboxyphenyl) ether dianhydride; and dicarboxylic anhydrides such as glutaric anhydride, 1,2-hexanedicarboxylic anhydride, phthalic anhydride and 3,4-benzophenonedicarboxylic anhydride. The molar ratio of aromatic diamine : tetracarboxylic dianhydride : dicarboxylic anhydride is 1 :(0.9 to 1.0) : (0.001 to 1.0).

Description: The present invention relates to a process for preparing a polyimide that has excellent thermal stability and processability in a molten state. Polyimide obtained by reacting tetracarboxylic dianhydride and aromatic diamine has excellent mechanical strength, dimensional stability, high temperature stability, flame resistance and electrical insulative 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 exhibiting excellent properties has 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 moulding materials, specific methods such as sinter moulding 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. D.J. Progar et al. 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 (U.S.P. 4,065,345): Furthermore, D.J. Progar et al (Int. J. Adhesion and Adhesive, 4, No. 2, 79 86, April 1984) have previously discovered a polyimide that consists primarily of recurring units of the formula: The polyimide is a high-temperature stable resin having many good properties. Polyimides exhibit excellent flowability and good processability. The fused resin, however, gradually exhibits decreased flowability which has an adverse affect on processability when the polyimide is kept at high temperatures for a long period of time, for example, long residence at high temperature in a cylinder used in injection moulding. 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 steps. SAMPE Journal, vol. 21, no. 1, Jan/Feb 1987, pages 12-20, N. Johnston et al.: "Thermoplastic Matrix Composites; LARC-TPI, Polyimidesulfone and their blends", EP-A-330 739, DE-A-2 425 163, EP-A-122 060 and EP-A-200 204 describe the preparation of polyimides the polymer chains of which are endcapped as a means for improving the melt flowability of the polyimides. These documents do not, however, describe any modification of the polymer structure itself and so only limited improvement is possible. The applicants own co-pending application EP-A-313 407 and EP-A-330 505 describe different polyimides which result from using different diamine components from those used according to the present invention. The present invention overcomes the problems and disadvantages of the prior art by providing a process for the preparation of a thermally stable polyimide. The process of the present invention in one aspect comprises reacting 3,3-diaminobenzophenone and from 0.9 to 1.0, per mole of the aromatic diamine, of at least one tetracarboxylic dianhydride represented by the formula: in which R represents a monocyclic aromatic radical, a fused polycyclic aromatic radical, a polycyclic aromatic radical wherein the aromatic rings are linked to one another by a direct bond or via a bridging group, in the presence of from 0.01 to 0.5 mole per mole of the aromatic diamine, of an anhydride represented by the formula: in which Z represents a monocyclic aromatic radical, a fused polycyclic aromatic radical, or a polycyclic aromatic radical wherein the aromatic radicals are linked to one another by a direct bond or via a bridging group, in an organic solvent at a temperature of from 0 to 250°C for from 4 to 24 hours to form a polyamic acid, and dehydrating the polyamic acid at a temperature from 100 to 400°C or chemically imidizing the polyamic acid to form a polyimide. The process of the present invention in another aspect comprises reacting 3,3-diaminobenzophenone and from 0.9 to 1.0, per mole of the aromatic diamine, of at least one tetracarboxylic dianhydride represented by the formula: in which R is as defined above, in an organic solvent at a temperature of from 0 to 250°C for from 4 to 24 hours to form a polyamic acid, and thereafter adding from 0.01 to 0.5 mole, per mole of the aromatic diamine, of an anhydride represented by the formula: in which Z is as defined above, and dehydrating the polyamic acid at a temperature of from 100 to 400°C or chemically imidizing the polyamic acid to form a polyimide. The resulting polyimides have, in addition to substantial outstanding properties, good thermal stability in a molten state and do not deteriorate with respect to processability even after exposure to high temperatures for a long period of time. The aromatic diamine used in the process of this invention is 3,3'-diaminobenzophenone, although other diamines may additionally be used in the process of the invention as long as such diamines do not cause adverse effects on the properties of the polyimide produced by the process of this invention. Exemplary other aromatic diamines include, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, m aminobenzylamine, p-aminobenzylamine, bis(3-aminophenyl) ether, (3-amino-phenyl)(4-aminophenyl) ether, bis(4-aminophenyl) ether, bis(3-aminophenyl) sulphide, (3-aminophenyl)(4-aminophenyl) sulphide, bis(4-aminophenyl) sulphide, bis(3-aminophenyl)sulphoxide, (3-aminophenyl)(4bis(3-aminophenyl)sulphoxide, bis(4-aminophenyl)sulphoxide, (3-aminophenyl)(4-aminophenyl) suphone, bis(4bis(4-aminophenyl)sulphone, 3,4'-diaminobenzophenone, 4,4'-diaminobenzophenone, bis[4-(4-aminophenoxy)phenyl]methane, 1,1-bis[4-(4-aminophenoxy)phenyl]ethane, 1,2-bis[4-(4-aminophenoxy)phenyl]ethane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]butane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 4,4-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl] ketone, bis[4-(4-aminophenoxy)phenyl]sulphide, bis[4-(4-aminophenoxy)phenyl]sulphoxide bis[4-(4-aminophenoxy)phenyl]sulphone, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)phenyl]ether, 1,4-bis[4-(3-aminophenoxy)benzoyl]benzene, 1,3-bis[4-(3-aminophenoxy)benzoyl]benzene, 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-methylphenyi]-propane, 2,2-bis[4-(3-aminophenoxy)-3-methylphenyl]propane, 2-[4-(3-aminophenoxy)phenyl]-2-[4-(3-aminophenoxy)-3,5-dimenthyiphenyl]-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'-bis(3-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)-3-methylbiphenyl, 4,4'-bis(3-aminophenoxy)-3,3'-dimethylbiphenyl, 4,4'-bis(3-aminophenoxy)-3,5-dimethylbiphenyl, 4,4'-bis(3-aminophenoxy)-3,3'5,5'-tetramethylbiphenyl, 4,4'-bis(3-aminophenoxy)-3,3'-dichlorobiphenyl, 4,4'-bis(3-aminophenoxy)-3,5'-dichlorobiphenyl, 4,4'-bis(3-aminophenoxy)-3,3',5,5'-tetrachlorobiphenyl, 4,4'-bis(3-aminophenoxy)-3,3'-dibromobiphenyl, 4,4'-bis(3-aminophenoxy)-3,5-dibromobiphenyl, 4,4'-bis(3-aminophenoxy)-3,3',5,5'-tetrabromobiphenyl, bis[4-(3-aminophenoxy)phenyl]ketone, bis[4-(3-aminophenoxy)phenyl]suphide, bis[4-(3-aminophenoxy)-3-methoxyphenyl]suphide, [4-(3-aminophenoxy)phenyl][4-(3-aminophenoxy)-3,5-dimethoxyphenyl]sulphide, bis[4-(3-aminophenoxy)-3,5'-dimethoxyphenyl]sulphide and bis[4-(3-aminophenoxy)phenyl]sulphone. Exemplary tetracarboxylic dianhydrides which may be used 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',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,2',4,4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, bis(3,4-dicarboxyphenyl)sulphone dianhydride, 4,4'-(p-phenylenedioxy)diphthalic dianhydride, 4,4-(m-phenylenedioxy)diphthalic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-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. The preferred tetracarboxylic dianhydride used in the process of this invention is 3,3',4,4'-benzophenonetetracarboxylic dianhydride. The tetracarboxylic dianhydride may be used singly or in combination of two or more. Exemplary dicarboxylic anhydrides which may be used in the method of this invention include, phthalic anhydride, 2,3-benzophenonedicarboxylic anhydride, 3,4-benzophenonedicarboxylic anhydride, 2,3-dicarboxyphenyl phenyl ether anhydride, 3,4-dicarboxyphenyl phenyl ether anhydride, 2,3-biphenyldicarboxylic anhydride, 3,4-biphenyldicarboxylic anhydride, 2,3-dicarboxyphenyl phenyl sulphone anhydride, 3,4-dicarboxyphenyl phenyl sulphone anhydride, 2,3-dicarboxyphenyl phenyl sulphide anhydride, 3,4-dicarboxyphenyl phenyl sulphide anhydride, 1,2-naphthalenedicarboxylic anhydride, 2,3-naphthalenedicarboxylic anhydride, 1,8-naphthalenedicarboxylic anhydride, 1,2-anthracenedicarboxylic anhydride, 2,3-anthracenedicarboxylic anhydride, and 1,9-anthracenedicarboxylic anhydride. The above mentioned dicarboxylic anhydride may be used singly or in combination. 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, the molar ratio of the tetracarboxylic dianhydride to the aromatic diamine is from 0.9 to 1.0, and the resulting polyimide has good melt viscosity. The amount of dicarboxylic anhydride simultaneously present in the reaction mixture is from 0.001 to 0.5 mole per mole of aromatic diamine. When the amount is less than 0.001 mole, heat stability in the molten state of the polyimide of this invention may not be obtained. When the molar ratio of dicarboxylic anhydride to the aromatic diamine is greater than 1.0, the mechanical properties of moulded products are not as good. More preferably the molar ratio of the dicarboxylic anhydride to the aromatic diamine is from 0.01 to 0.05 mole of dicarboxylic anhydride per mole of aromatic diamine. Suitable organic solvents which are used in the method of this 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, picoline, dimethyl sulfoxide, dimethyl sulphone, tetramethylurea, hexamethylphosphoramide, phenol, m-cresol, p-cresol, p-chlorophenol, xylenol and anisole. The organic solvents may be used singly or in combination. In one method according to this invention, the starting materials, i.e. the aromatic diamine, tetracarboxylic dianhydride and dicarboxylic anhydride are added to the organic solvent and reacted. In another method according to the invention, after reacting the aromatic diamine with the tetracarboxylic dianhydride, to form a polyamic acid, the dicarboxylic anhydride is added and reacted. The reaction temperature is preferably in the range of from 0 to 60°C. Any reaction pressure may be used and ambient pressure is sufficient to carry out the reaction. The reaction time is from 4 to 24 hours and is dependent upon the aromatic diamine, tetracarboxylic dianhydride, dicarboxylic anhydride, solvent and reaction temperature. A polyamic acid is formed by the above mentioned reaction. The resultant polyamic acid is dehydrated by heating at from 100 to 400°C or chemically imidized by using a usual imidizing agent. The polyimide obtained consists primarily of recurring units of the formula: in which R is 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 60 to 250°C. In this method, an aromatic diamine, tetracarboxylic dianhydride and dicarboxylic anhydride are suspended or dissolved in an organic solvent and reacted by heating. Thus, the formation and imidization of the polyamic acid are carried out at the same time and produce a polyimide consisting primarily of the recurring units of the above formula. When the polyimide of this invention is processed by fusion moulding, other thermoplastic resins may be incorporated in suitable amounts depending upon the application so long as no adverse effects occur contrary to the objects of this invention. Illustrative examples of thermoplastic resins which may be used with the polyimide include polyethylene, polypropylene, polycarbonate, polyarylate, polyamide, polysulphone, polyethersulphone, polyetherketone, polyphenylenesulphide, polyamideimide, polyetherimide and modified polyphenyleneoxide. Fillers which are used for usual resin compositions may be employed in an amount which has no adverse effects on the objects of this invention. Suitable fillers which may be used include, wear resistance improvers such as graphite, carborundum, quartz powder, molybdenum disulphide and fluororesins, reinforcing materials such as glass fibre, carbon fibre, boron fibre, silicon carbide fibre, carbon whisker, asbestos, metal fibre and ceramic fibre, 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 sulphate, silica and calcium metasilicate, thermal conductivity improvers such as iron powder, zinc powder, aluminium 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 colouring agents. The present invention will now be further described by the following Examples and Comparative Examples and the accompanying drawings, in which: Figures 1 and 3 illustrate the relationship between melt viscosity and number of repeated fusions; and Figure 2 illustrates the relationship between melt viscosity and residence time of the polyimide in the cylinder flow tester. Example 1 To a reaction vessel equipped with a stirrer, reflux condenser and nitrogen inlet tube, 212g (1.0 mole) of 3,3'-diaminobenzophenone and 2,970g of N,N-dimethylacetamide as a solvent were charged. Then 312g (0.97 mole) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added by portions at room temperature in a nitrogen atmosphere so as not to raise the temperature of the solution and were stirred for about 20 hours at the room temperature. To the polyamic acid solution thus obtained 22.2g (0.15 mole) of phthalic anhydride were added at room temperature in a nitrogen atmosphere and further stirred for an hour. Then 202g (2 moles) of triethylamine and 306g (3 moles) of acetic anhydride were added dropwise to the solution. Yellow polyimide powder started to precipitate after about one hour from the completion of dropwise addition. The reaction mixture was further stirred for 10 hours at the room temperature. The resultant slurry was filtered, washed with methanol and dried at 180°C for 2 hours. The amount of polyimide powder obtained was 487g. The polyimide powder had a glass transition temperature of 250°C, a melting point of 298°C by DSC method and an inherent viscosity of 0.52 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 0.5g/l00ml solvent. The melt viscosity of the polyimide powder thus obtained was repeatedly measured using a Japan Polymer Society type flowtester (Trade Mark, CFT-500, a product of Shimazu Seisakusho Co.) with an orifice having diameter of 0.lcm and a length of 1cm. After being kept at 380°C for 5 minutes, the sample was extruded with a pressure of 100kg/cm². The strand obtained was crushed and extruded again. The procedure was continuously repeated 5 times. The relationship between the repetition number and the melt viscosity is illustrated by Curve A in Figure 1. Almost no variation is found in the melt viscosity even though number of repetitions is increased, which indicates good heat stability of the molten polyamide. Comparative Example 1 The same procedure as described in Example 1 was carried out without using phthalic anhydride. The amount of polyimide powder obtained was 464g and the polyimide had an inherent viscosity of 0.52dl/g. The repeated measurement of the melt viscosity was carried out on the polyimide powder thus obtained by the method described in Example 1. The results are illustrated by Curve B in Figure 1. The melt viscosity increased with an increase of in the number of repetitions. The heat stability of the molten polyimide thus obtained was inferior to that obtained in Example 1. Example 2 To the same reaction vessel as described in Example 1, 212g (1.0 mole) of 3,3'-diaminobenzophenone and 2,970g of N,N-dimethylacetamide as a solvent were charged. Then 88.8g (0.06 mole) of phthalic anhydride and 312g (0.97 mole) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added at room temperature in a nitrogen atmosphere so as not to raise the temperature of the solution and stirred for 20 hours at the room temperature. Then 202g (2 moles) of triethylamine and 306g (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 amount of light yellow polyimide powder thus obtained was 458g. The polyimide powder had a glass transition temperature of 250°C, melting point of 298°C and an inherent viscosity of 0.50dl/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 380°C. The pressure at the measurement was 100kg/cm². The results are illustrated by Curve A in Figure 2. 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 2 The same procedure as described in Example 2 was carried out without phthalic anhydride. The light yellow polyimide powder thus obtained had a glass transition temperature of 250°C and an inherent viscosity of 0.50dl/g. The heat stability of the molten polyimide was evaluated by the method described in Example 2. The melt viscosity increased with an increase in residence time. The heat stability of the molten polyimide thus obtained was inferior to that obtained in Example 2. The results are illustrated by Curve B in Figure 2. Example 3 To the same reaction vessel as described in Example 1, 212g (1.0 mole) of 3,3'-diaminobenzophenone, 315.6g (0.98 mole) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 5.92g (0.04 mole) of phthalic anhydride and 2,970g of m-cresol were charged and gradually heated with stirring in a nitrogen atmosphere. A brown transparent homogeneous solution was obtained at about 120°C. The solution was heated to 150°C. Yellow polyimide powder started to precipitate after stirring for about 20 minutes at this temperature. After stirring the reaction mixture for an additional 2 hours at 150°C, the precipitate was filtered, washed with methanol and then acetone, and dried at 180°C for 8 hours under reduced pressure. The amount of polyimide powder thus obtained was 485g and the polyimide had an inherent viscosity of 0.57 dl/g, and a glass transition temperature of 251°C. The repeated measurement of the melt viscosity was carried out on the polyimide powder thus obtained by the method described in Example 1. The temperature at the measurement was 400°C. The pressure at the measurement was 100kg/cm². The results obtained are illustrated by Figure 3. Almost no variation is found in the melt viscosity even though the number of repetitions is increased, which indicates good heat stability of the molten polyimide.

Claims: A process for preparing a polyimide comprising reacting 3,3-diaminobenzophenone and from 0.9 to 1.0, per mole of the aromatic diamine, of at least one tetracarboxylic dianhydride represented by the formula: in which R represents a monocyclic aromatic radical, a fused polycyclic aromatic radical, a polycyclic aromatic radical wherein the aromatic rings are linked to one another by a direct bond or via a bridging group, in the presence of from 0.01 to 0.5 mole per mole of the aromatic diamine, of an anhydride represented by the formula: in which Z represents a monocyclic aromatic radical, a fused polycyclic aromatic radical, or a polycyclic aromatic radical wherein the aromatic radicals are linked to one another by a direct bond or via a bridging group, in an organic solvent at a temperature of from 0 to 250°C for from 4 to 24 hours to form a polyamic acid, and dehydrating the polyamic acid at a temperature from 100 to 400°C or chemically imidizing the polyamic acid to form a polyimide. A process for preparing a polyimide comprising reacting 3,3-diaminobenzophenone and from 0.9 to 1.0, per mole of the aromatic diamine, of at least one tetracarboxylic dianhydride represented by the formula: in which R is as defined in Claim 1, in an organic solvent at a temperature of from 0 to 250°C for from 4 to 24 hours to form a polyamic acid, and thereafter adding from 0.01 to 0.5 mole, per mole of the aromatic diamine, of an anhydride represented by the formula: in which Z is as defined in Claim 1, and dehydrating the polyamic acid at a temperature of from 100 to 400°C or chemically imidizing the polyamic acid to form a polyimide. A process as claimed in Claim 1 or Claim 2 in which the tetracarboxylic dianhydride is 3,3', 4,4'-benzophenonetetracarboxylic dianhydride. A process as claimed in any preceding claim in which the dicarboxylic anhydride is phthalic anhydride. A process as claimed in any preceding claim in which the reaction temperature is no greater than 60°C.

Inventors: IIYAMA KATSUAKI, KAWASHIMA SABURO, OHTA MASAHIRO, OIKAWA HIDEAKI, TAMAI SHOJI, YAMAGUCHI AKIHIRO, IIYAMA, KATSUAKI, KAWASHIMA, SABURO, OHTA, MASAHIRO, OIKAWA, HIDEAKI, TAMAI, SHOJI, YAMAGUCHI, AKIHIRO
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1989
Main classification: C08G73
IPC codes: C08G73