A POLYIMIDE HAVING A LOW ELASTIC MODULUS AND THE PRODUCTION THEREOF

Abstract: A solvent-soluble polyimide having a low elastic modulus comprises from 60 to 99 mol % of repeating units of the formula: (in which Ar is a tetravalent aromatic group; the groups X and Y are each independently a C₁-C₄ alkyl group, a halogen atom, a carbonyl group or a hydroxyl group; and ℓ and m are each 0, 1, 2 and 3); and from 1 to 40 mol % of repeating units of formula: (in which R¹ is a C₃-C₅ alkylene group or a phenylene group; R² is a C₁-C₄ alkyl group or a phenyl group; and n is an integer of 20 to 300). The polyimide is produced by reacting an appropriate aromatic diamine, silicone diamine and aromatic tetracarboxylic acid dianhydride in an organic polar solvent at 20 to 250°C.

Description: This invention relates to a polyimide having a low elastic modulus and to the preparation thereof. In general, polyimides have good heat resistance, electric characteristics and mechanical characteristics; hence they have been used in a wide range of fields as heat-resistant, insulating materials in the field of electronic equipments, films, mechanical parts and further, structural materials, etc. However, conventional highly heat-resistant polyimides i.e. totally aromatic polyimides are highly elastic and inferior in processability. Recently it has been the intent to improve the material characteristics thereof as advanced techniques including semiconductor techniques are developed, and polyimides having a low elastic modulus have been required. In order to satisfy such needs, a totally aromatic polyimide having a low elasticity has been proposed in, for example, JP-A-63-199237 and JP-A-63-199238. Further, a low elastic polyimide having a siloxane chain introduced therein has been proposed in JP-A-63-35625. JP-A-63-199237 and JP-A-63-199238 disclose totally aromatic polyimide of reduced elasticity but such polyimides have a tensile, elastic modulus around 200 Kgf/mm² which is, however, a still insufficient value. Further, according to JP-A-63-35625, a polyimide having a tensile elastic modulus below 1961 N/mm² (200 Kgf/mm²) is obtained using a silicone diamine, but its values of tensile strength and elongation are insufficient. Further, none of these polyimides can be obtained in the form of solutions. Thus, in order to obtain polyimides, it is necessary to bake a solution obtained in the form of a polyimide precursor, at a high temperature and for a long time; hence this is commercically uneconomical. JP-A-61-118424 discloses a polysiloxane chain-containing and solvent-soluble polyimide, which, however, cannot be regarded as having adequate heat resistance. As described above, a polyimide having an elastic modulus below 1961 N/mm² (200 Kgf/mm²) and superior heat-resistance and mechanical characteristics both intrinsic of polyimides cannot be found in the prior art. According to the invention there is provided a solvent-soluble polyimide having a low elastic modulus and comprising from 60 to 99 mol% of repeating units of the formula: (in which Ar is a tetravalent aromatic group; the groups X and Y are each independently a C₁-C₄ alkyl group, a halogen atom, a carboxyl group or a hydroxyl group; and ℓ and m are each 0, 1, 2 or 3); and from 1 to 40 mol % of repeating units of the formula: (in which R¹ is a C₃-C₅ alkylene group or a phenylene group; R² is a C₁-C₄ alkyl group or a phenyl group; and n is an integer of 20 to 300). The mole fraction [I] of repeating units (I), the mole fraction [II] of repeating units (II) and n preferably satisfy the relationship: 0.5 ≦ n [II] [I] + [II] ≦ 15.0 The polyimides of the invention preferably have a logarithmic viscosity number of 0.1 to 5.0 (as measured in solution in N-methyl-2-pyrrolidone of a concentration of 0.5 g/ dℓ at 30°C) and a tensile elastic modulus of 1961 N/mm² (200 Kgf/mm²) or less, preferably 1961 N/mm² (200 Kgf/mm²) to 196 N/mm² (20 Kgf/mm²); and the polyimide of the invention are prepared by reacting a diamino component comprising 60 to 99 mol % of an aromatic diamine of formula (V) and 1 to 40% by mol of a silicone diamine of formula (VI), with an aromatic tetracarboxylic acid dianhydride of formula (IV) in an organic polar solvent at 20° to 250°C: (in which Ar, X, Y, R¹, R², ℓ, m and n have the meanings defined above). The accompanying drawing shows the infrared absorption spectrum of a polyimide obtained in accordance with the present invention. The molar ratio of the aromatic tetracarboxylic acid dianhydride to the diamino compounds is usually preferred to be equimolecular or nearly equimolecular. In the process for producing a polyimide of the present invention, examples of the aromatic tetracarboxylic dianhydride used in the present invention are pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, etc. The diamino compounds used in the production process of the present invention are aromatic diamines and silicone diamines. Examples of the aromatic diamines are as follows and they may be used alone or together: etc. A portion of these aromatic diamines may be replaced by e.g. p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 4,4'-diaminophenylmethane, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,3′-diaminobenzophenone, o-toluidine, etc., and the quantity thereof used is preferably less than 20% by mol, particularly less than 10% by mol based on the total aromatic diamine component in the aspect of the characteristics of the polyimide of the present invention. Examples of the silicone diamine are as follows: In these formulas, n represents an integer of 20 to 300. The silicone diamine is an important diamine component for reducing the elastic modulus of the polyimide, and if the n value is small, it is impossible to obtain the polyimide aimed in the present invention, while if the value is too large, the reactivity of the component is inferior and the heat resistance of the resulting polyimide is far reduced. Thus, in order to achieve the object of the present invention, the n value is in the range of 20 to 300, preferably 20 to 200. Further, as to the content of the siloxane bond, it is preferred in the aspect of the characteristics of the polyimide of the present invention that there is a relationship represented by the expression (III), and it is more preferred that there is a relationship represented by the expression 1.0 ≦ n [II] [I] + [II] ≦ 10.0 Concrete examples of the organic polar solvent used in the process of the present invention are N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, etc. Further, in the case where water is byproduced in the reaction, it is preferred to add a solvent azeotropic with water such as benzene, toluene, xylene, etc. to the reaction system and remove the byproduced water to the outside of the system by azeotropy to thereby advance the reaction smoothly. It is usually preferred to feed an aromatic tetracarboxylic acid dianhydride and diamine compounds in an equimolecular ratio or in a nearly equimolecular ratio in an organic polar solvent. The quantity of the solvent used at that time is such that the concentration of the resulting polyimide is preferably 5 to 40% by weight, more preferably 10 to 30% by weight. The reaction of the aromatic tetracarboxylic acid with the diamino compounds comprises a first step reaction of forming a polyimide precursor which is an amic acid copolymer and a second step reaction of imidization by way of dehydration reaction. The production of the polyimide of the present invention is characterized by carrying out the above first step reaction and second step reaction at one stage, and the reaction temperature is usually 100° to 250°C, preferably 120° to 200°C and the reaction is complete usually in 0.5 to 50 hours. Further, it is also possible to carry out the first step reaction and the second step reaction at two stages. In this case, the first step reaction is carried out at a temperature of 80°C or lower, particularly 0° to 60°C for 0.5 to 20 hours to obtain a polyimide precursor, followed by heating the reaction solution to 120° to 200°C or carrying out imidization using an imidization catalyst such as a tertiary amine and an acid anhydride, at a temperature of 10° to 100°C to prepare the polyimide of the present invention. The logarithmic viscosity number of the polyimide of the present invention, obtained by measuring at 30°C a solution prepared by diluting the resulting polyimide solution with N-methyl-2-pyrrolidone so as to give a concentration of 0.5 g/dℓ therein, is preferably in the range of 0.1 to 5.0, more preferably in the range of 0.3 to 3.0. If the logarithmic viscosity number is too small, the mechanical strength of film or the like prepared from the resulting polyimide lowers, while if the logarithmic viscosity number is too large, it is difficult to cast the polyimide solution on glass plate or the like so that operation for film-making becomes difficult. The above logarithmic viscosity number refers to that calculated from the following equation, and the dropping time in the equation was measured by a capillary tube: Logarithmic viscosity number η inh = ℓn (t₁/t₀) C wherein t₁ refers to a time in which a definite volume of the solution drops through the capillary; t₀ refers to a dropping time of the solvent therethrough; and C refers to the concentration of the polymer expressed by gram number of the polymer in 100 mℓ of the solution. The effectiveness of the present invention consists in that a polyimide which is heat-resistant and soluble in solvents and has a low elastic modulus is provided. Further, the production process of the present invention is characterized in that in the reaction of an aromatic tetracarboxylic acid with diamino compounds, it is possible to carry out the first step reaction of forming the polyimide precursor which is an amic acid copolymer and the second step reaction of imidization by way of dehydration reaction, at one stage and also it is possible to obtain the polyimide in the form of its solution. The low elastic polyimide obtained in the present invention is useful particularly for low elastic polyimide film. Only by casting the reaction solution of the polyimide formed according to the production process of the present invention, on a smooth and flat plate such as glass plate, stainless plate, aluminum plate, steel plate, etc. and removing the solvent, it is possible to easily form a polyimide film. Thus, as compared with the conventional case where the polyimide film is formed from the solution of polyimide precursor, it is unnecessary to heat the solution at a high temperature and for a long time so that decomposition of the polymer is slight and the mechanical strengths of the film are superior. The present invention will be described in more detail by way of Examples, but it should not be construed to be limited thereto. In addition, the physical properties of the resulting film were evaluated as follows: Tensile test: measured at 20°C according to the testing method of ASTM D638. Five % thermal decomposition temperature: measured by thermogravimetric analyzer (TGA) manufactured by Seiko Denshi Co., Ltd., at a temperature-raising rate of 10°C/min. in nitrogen gas. Glass transition temperature: measured by thermomechanical analyzer (TMA) manufactured by Seiko Denshi Co., Ltd., at a temperature-raising rate of 10°C/min. under a load of 10 g and sought from the inflection point of the resulting curve. Example 1 Into a 500 mℓ capacity, four-necked, separable flask were fed 4,4'-diamino-3,3',5,5'-tetramethyl diphenyl methane 13.86 g (0.003 mol), pyromellitic anhydride (21.81 g, 0.100 mol), N-methyl-2-pyrrolidone (250 g) and toluene (50 g) as an azeotropic solvent, followed by refluxing toluene at 180°C in nitrogen current while agitating the mixture, reacting the mixture for 6 hours while continuously removing water formed by the reaction by azeotropy and then removing toluene to the outside of the reaction system to obtain a uniform and viscous polyimide solution. A portion of the solution was diluted with N-methyl-2-pyrrolidone to prepare a solution of 0.5 g/dℓ and its logarithmic viscosity number was measured to give 1.12 dℓ/g. The infrared spectra of the resulting polyimide are shown in Fig. 1. Absorptions based on imide ring formation were observed at 1,780⁻¹ cm and 720⁻¹ cm. The polyimide solution was coated on a glass plate by means of Comma Coater (trademark of Hirano Kinzoku Co., Ltd.) to form a thin film, followed by drying the resulting plate at 100°C for one hour in a hot air drying oven, fixing the dried film onto a metal frame, and subjecting it to heat treatment at 250°C for 30 minutes to obtain a polyimide film of 50 µm. The tensile physical properties of the film were as follows: elastic modulus 1412 N/mm² (144 Kgf/mm²); strength 75,5 N/mm² (7.7 Kgf/mm²); and elongation at break 65%. Further, the polyimide was a superior product having a 5% thermal decomposition temperature of 469°C and a glass transition temperature of 420°C. Examples 2 - 5 and Comparative examples 1 and 2 Example 1 was repeated except that the composition of acid anhydride and diamines was changed to those listed in Table 1. The logarithmic viscosity numbers of the resulting polyimides are listed in Table 1. Further, heat treatment was carried out in the same manner as in Example 1. The tensile physical properties and the 5% thermal decomposition-initiating temperatures of the resulting films are also shown in Table 1. Comparative example 3 Into the same apparatus as in Example 1 were fed 4,4'-diaminodiphenyl ether (19.02 g, 0.095 mol), pyromellitic dianhydride (21.81 g, 0.100 mol), N-methyl-2-pyrrolidone (250 g) and toluene (50 g), followed by carrying out the same procedure as in Example 1. As a result, a portion of the resulting polymer began to precipitate in the vicinity of 140°C and further the heating reaction was continued, but no uniform polyimide solution was obtained. As seen from Table 1, the heat-resistant resins of Examples 1 to 5 obtained according to the present invention have an elastic modulus of 1961 N/mm² (200 Kgf/mm²) or lower and sufficient values of mechanical properties for films. Further, it is seen that a polyimide having a 5% thermal decomposition temperature of 400°C or higher, a glass transition temperature of 300°C or higher and a superior solubility in solvents, never observed in conventional polyimides.

Claims: A solvent-soluble polyimide having a low elastic modulus and comprising from 60 to 99 mol % of repeating units of the formula: (in which Ar is a tetravalent aromatic group; the groups X and Y are each independently a C₁-C₄ alkyl group, a halogen atom, a carboxyl group or a hydroxyl group; and ℓ and m are each 0, 1, 2 or 3); and from 1 to 40 mol % of repeating units of formula: (in which R¹ is a C₃-C₅ alkylene group or a phenylene group; R² is a C₁-C₄ alkyl group or a phenyl group; and n is an integer of 20 to 300). A polyimide as claimed in claim 1 in which the mole fraction [I] of repeating units of formula (I), the mole fraction [II] repeating units of formula (II) and n satisfy the relationship: 0.5 ≦ n [II] [I] + [II] ≦ 15.0 A polyimide as claimed in claim 1 or claim 2 having a logarithmic viscosity number of 0.1 to 5.0 (as measured in N-methyl-2-pyrrolidone at a concentration of 0.5 g/dℓ at 30°C) and a tensile elastic modulus of 196 N/mm² (200 Kgf/mm²) of less, preferably 1961 N/mm² (200 Kgf/mm²) to 196 N/mm² (20 Kgf/mm²). A process for producing a polyimide as claimed in claim 1 which process comprises reacting a diamino component comprising 60 to 99 mol % of an aromatic diamine of the formula: (in whch X, Y, ℓ and m have the meanings defined in claim 1) and 1 to 40 mol % of a silicone diamine of the formula: (in which R¹, R² and n have the meanings defined in claim 1); with an aromatic tetracarboxylic acid dianhydride of the formula: (in which Ar has the meaning defined in claim 1), in an organic polar solvent at 20 to 250°C.

Inventors: KAWAMOTO, TAKAO, KIKUTA, KAZUTSUNE, KONOTSUNE, SHIRO
Applicants: CHISSO CORPORATION
Application year: 1989
Main classification: C08G73
IPC codes: C08G77, C08G73