High-molecular weight polycarbodiimide solution and methods for producing the same

Abstract: This method provides methods for producing a high-molecular weight polycarbodiimide solution from an organic diisocyanate in the presence of a carbodiimidation catalyst, wherein an alicyclic ether is used as a solvent. This invention also provides a polycarbodiimide solution with excellent stability in preservation.

Description: BACKGROUND OF THE INVENTION [Field of the invention] The present invention relates to a polycarbodiimide solution and a methods for producing the same, and particularly, to a high-molecular weight polycarbodiimide solution with excellent stability in preservation which can be obtained by appropriately choicing reaction conditions such as a solvent used in the production of the solution, and to a method for producing said polycarbodiimide solution. [Prior art] It is known that polycarbodiimides, particularly aromatic polycarbodiimides, have a high level of heat resistance, and such polycarbodiimides are therefore used as thermosetting resin materials for molding which are hot-pressed in a powder form. A method for producing said polycarbodiimides is disclosed in Japanese Patent Laid-Open No. 51-61599, and other methods are reported in D. J. Lyman et al., Die Makromol. Chem., 67, 1 (1963) and E. Dyer et al., J. Amer. Chem. Soc., 80, 5495 (1958). Each of these methods uses, as a polymerization solvent, a hydrocarbon such as benzene, xylene, decalin, toluene or the like, or, o-dichlorobenzene, chlorobenzene, cyclohexanone, a mixture of dimethyl sulfoxide and chlorobenzene (1 : 1), N-methylpyrrolidone or the like, and uses as a monomer, one of various organic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate and the like or mixtures thereof, in a concentration within the range of 10 to 25% by weight. Since the polycarbodiimides obtained by the production methods mentioned in the above documents are powdery, and the films also mentioned in the above documents are produced by extracting the polycarbodiimides from the reaction system after 10 minutes from the start of reaction and then casting it. And moreover, since gelation or precipitation occurs in the reaction system over 10 minutes after the start of the above reaction, the polycarbodiimides thus obtained cannot be used in the form of a stable solution. Namely, high-molecular weight polycarbodiimide solutions having excellent stability cannot be obtained by the above methods. Another method for producing polycarbodiimides is a method in which the terminal isocyanate groups are sealed. L. M. Alberino et al., J. Appl. Polym. Sci., 21, 1999 (1977) and T. W. Campbell, J. Org. Chem., 28, 2069 (1963) also show methods for producing polycarbodiimides from organic diisocyanates in which the molecular weight of the polycarbodiimides produced is controlled by adding the organic monoisocyanate to the reaction system. In any one of these production methods, reaction is effected by using as a solvent a hydrocarbon such as benzene, xylene, petroleum ether or the like to produce a polycarbodiimide as a precipitate product. Namely, polycarbodiimide solutions cannot be obtained by the above mentioned production methods, and it is obvious that polycarbodiimides having a high molecular weight cannot be obtained. From US-A 2,941,983 there is known a method of producing urethane-terminated polycarbodiimides by dissolving a diisocyanate or an isocyanate-terminated linear polymer or their mixtures in a suitable inert solvent, initiating chain extension by adding a catalyst for carbodiimide formation, and finally, chain-stopping the polymerizatin by introducing a primary or secondary alcohol when the polymer has reached a certain observed number average molecular weight. SUMMARY OF THE INVENTION The present invention has been achieved as a result of energetic research on reaction conditions which includes choicing a specific solvent, which has not been used in prior art, for a particular organic diisocyanate. Accordingly it is an object of the present invention to solve the problem of the above-described prior art and provide a method for easily and simply producing a high-molecular weight polycarbodiimide solution having excellent stability. To this end the present invention provides a method for producing a high-molecular weight polycarbodiimide solution from an organic diisocyanate in the presence of a carbodiimidation catalyst by using an alicyclic ether as a solvent. The present invention also provides a high-molecular weight polycarbodiimide solution obtained from an organic diisocyanate by using an alicyclic ether as a solvent in the presence of a carbodiimidation catalyst. Brief description of the Drawings Fig. 1 is a drawing which shows the IR spectrum of the polycarbodiimide solution of the present invention obtained in Example 1; Fig. 2 is a drawing which shows the IR spectrum of the polycarbodiimide solution obtained in Comparative Example 3; Fig. 3 is a drawing which shows the IR spectrum of the polycarbodiimide solution of the present invention obtained in Example 7; and Fig. 4 is a drawing which shows the IR spectrum of the polycarbodiimide solution obtained in Comparative Example 7. DESCRIPTION OF PREFERRED EMBODIMENTS The organic diisocyanate, that is used as raw material in the present invention, is expressed by the following formula: wherein R 1 and R 2 each denote a hydrogen atom or a lower alkyl or alkoxy group; and X denotes an oxygen atom or a methylene group. Examples of such compounds include 4,4'-diphenylmethane diisocyanate, 3,3'-dimethoxy-4,4'-diphenylmethane diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate, 3,3'-dimethyl-4,4'-diphenylether diisocyanate and mixtures thereof. In the present invention, reaction of each of these organic diisocyanates is effected in an alicyclic ether which is preferably a liquid at room temperature and has two ether bonds or less in its molecule. Examples of such ethers include tetrahydrofuran, dioxane, tetrahydropyran and solvent mixtures thereof. Of these ethers, tetrahydrofuran is preferable. When one of the above organic diisocyanates is used, the concentration thereof is preferably 10% by weight or less, more preferably 7% by weight or less. If a solvent other than the above-described solvents, i.e., a known polymerization solvent, is used in the present invention, gelation takes place during reaction or a polycarbodiimide is obtained in the form of precipitates and cannot be obtained in the form of a solution. It is also confirmed that the solution contains a large amount of remaining isocyanate before the gelation or the precipitation. This shows that a high molecular weight is not attained. On the other hand, when any one of the above solvents is used, if the concentration of the organic diisocyanate exceeds 10% by weight, although a polycarbodiimide solution can be obtained, in most cases, it exhibits poor stability and forms gel within 3 days when being allowed to stand at room temperature. Further, it is known that, since the amount of the isocyanate remaining in the polycarbodiimide solution increases as the concentration of the diisocyanate increases, a high molecular weight cannot be attained at a high concentration of diisocyanate. As a result of infrared absorption spectroscopy (the product obtained by a conventional method shows remarkable absorption by remaining isocyanates) or titration of isocyanate by the dibutylamine method, it was confirmed that no isocyanate remains, and no by-product is produced in the polycarbodiimide solution obtained by the production method of the present invention. And the stability of the solution was also confirmed by allowing it to stand in a refrigerator or at room temperature. As apparent from the results, the polycarbodiimide solution of the present invention obtained from an organic diisocyanate by using as a solvent an alicyclic ether in the presence of a carbodiimidation catalyst, contains no remaining isocyanate and by-product and has excellent stability. In any case of reaction, a phosphorene oxide such as 1-phenyl-2-phosphorene-1-oxide, 3-methyl-2-phosphorene-1-oxide, 1-ethyl-3-methyl-2-phosphorene-1-oxide, 1-ethyl-2-phosphorene-1-oxide or a 3-phosphorene isomer thereof is used as the carbodiimidation catalyst. In the present invention, the type and the concentration of the organic diisocyanate used as a raw material or the solvent used is taken into consideration so that a stable high-molecular weight polycarbodiimide solution can be easily and simply obtained. The polycarbodiimide solution obtained in the present invention exhibits excellent stability in preservation and can be thus used as varnish in the industrial field. In addition, the film obtained from said solution is characterized by its excellent mechanical properties, as compared with the films obtained by conventional press molding. Examples and Comparative Examples of the present invention are described below. (Example 1) 50 g of 4,4'-diphenylmethane diisocyanate (referred to as MDI hereinafter) was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 700 ml of tetrahydrofuran (THF) at 67°C for 16 hours to produce a polycarbodiimide solution. Fig. 3 shows the infrared absorption spectrum of the thus-obtained polycarbodiimide solution. There is no absorption at 2260 cm -1 corresponding to remaining isocyanate. This reveals that the reaction proceeded to an extent, in which no isocyanate remained, to produce a high-molecular weight polycarbodiimide solution. The solution obtained was stable for 1 month at room temperature. (Example 2) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 500 ml of tetrahydrofuran at 67°C for 8 hours to obtain a polycarbodiimide solution. The thus-obtained solution was stable for 3 weeks at room temperature. (Example 3) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 600 ml of tetrahydropyran at 88°C for 7 hours to obtain a polycarbodiimide solution. The thus-obtained solution was stable for 1 month in a refrigerator. (Example 4) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 500 ml of dioxane at 100°C for 5 hours to obtain a polycarbodiimide solution. The thus-obtained solution was stable for 1 month in a refrigerator. (Example 5) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 500 ml of a solvent mixture consisting of tetrahydrofuran and dioxane (1 : 1) at the reflux temperature -of the solvent mixture for 11 hours to obtain a polycarbodiimide solution. The thus-obtained solution was stable for 3 weeks at room temperature. (Example 6) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 600 ml of a solvent mixture consisting of tetrahydrofuran, dioxane and tetrahydropyran (2 : 1 1)) at the reflux temperature of the solvent mixture for 10 hours to obtain a polycarbodiimide solution. The thus-obtained solution was stable for 3 weeks at room temperature. (Comparative Example 1) 100 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 300 ml of tetrahydrofuran. However, the reaction solution gelled after 5 hours had passed, and a polycarbodiimide solution could not be obtained. (Comparative Example 2) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 500 ml of xylene at 120°C. However, precipitates were produced after 3 hours had passed, and a polycarbodiimide solution could not be obtained. (Comparative Example 3) 50 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.1 g of a carbodiimidation catalyst in 500 ml of tetrachloroethylene at 120°C. However, precipitates were produced after 3 to 4 hours had passed, and a polycarbodiimide solution could not be obtained. (Comparative Example 4) 150 g of MDI, which was the same as that used in Example 7, was reacted in the presence of 0.38 g of a carbodiimidation catalyst in 500 ml of tetrahydrofuran at 67°C. The reaction was stopped before gelation took place. Fig. 4 shows the infrared spectrum of the reaction product. Absorption was observed at 2260 cm -1 corresponding to remaining isocyanates. This reveals that polymerization insufficiently proceeded. The solution obtained gelled after being allowed to stand for 1.5 days at room temperature.

Claims: A method for producing a high-molecular weight polycarbodiimide solution from an organic diisocyanate in the presence of a carbodiimidation catalyst, wherein an alicyclic ether is used as a solvent, characterized in that said organic diisocyanate is expressed by the following formula: (wherein R 1 and R 2 each denote a hydrogen atom, a lower alkyl or alkoxy group; and X denotes an oxygen atom or a methylene group), and its concentration is 10% by weight or less, and that tetrahydrofuran, dioxane, tetrahydropyran or a solvent mixture thereof is used as said alicyclic ether and that the reaction proceeds through completion to the extent that no remaining diisocyanate is detected by infrared absorbtion spectroscopy or titration. A method for producing a high molecular weight polycarbodiimide solution according to claim 1, wherein the organic diisocyanate is 4,4'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate or a mixture thereof.

Inventors: AMANO SATOSHI, IMASHIRO YASUO, AMANO, SATOSHI, IMASHIRO, YASUO, AMANO, SATOSHI, C/O NISSHINBO IND., INC., IMASHIRO, YASUO, C/O NISSHINBO IND., INC.
Applicants: NISSHIN SPINNING, NISSHINBO INDUSTRIES, INC.
Application year: 1990
Main classification: C08G18
IPC codes: C08G18