Copolyamide production method

Abstract: A copolyamide in which the diamine component comprises a mixture of meta- and para-xylylene diamine may advantageously be made using a method which comprises: (1) heating a dicarboxylic acid which is adipic acid or a mixture containing 40 mol% or more of adipic acid to a temperature higher than the melting point of adipic acid but lower than a temperature 30°C below the melting point of the resulting copolyamide; (2) adding a diamine comprising 35-70 mol% paraxylylene diamine and 65-30 mol% metaxylylene diamine to the heated dicarboxylic acid until the conversion of the dicarboxylic acid approaches 90 mol%; (3) stopping addition of the diamine before the conversion of the dicarboxylic acid reaches 90 mol% and adding metaxylylene diamine or a diamine mixture of 70 mol% or more metaxylylene diamine and 30 mol% or less paraxylylene diamine until a molar ratio of the total diamines to the total dicarboxylic acid(s) becomes 0.97 to 1.03; and (4) before the conversion of the dicarboxylic acid reaches 95 mol%, heating the reaction system to a temperature higher than 30°C below the melting point of the final copolyamide and, upon termination of addition of the diamine, elevating the temperature of the reaction system to a temperature higher than the melting point of the final copolyamide product.

Description: This invention relates to a method for producing a copolyamide which contains meta- and para-xylylene diamines as the diamine component and adipic acid as the main dicarboxylic acid component. More specifically, this invention relates to a method for producing a copolyamide, which has the above homogeneous composition, is crystalline and colorless and is excellent in heat resistance, in accordance with a direct polycondensation method. EP-A-008461 discloses preparation of a polyamide by polycondensation of a dicarboxylic acid and a diamine, in two diamine-adding steps, whilst increasing the temperature. US 4018746 discloses polycondensation of dicarboxylic acids with metaxylylene and paraxylylene. EP-A-0071000 discloses polycondensation of a dicarboxylic acid (at least 80 mole % adipic acid) with a diamine (at least 70 mole % metaxylylene diamine). JP-A-14925/1989 (the term "JP-B-" as used herein means an "examined published Japanese patent application) discloses a method for producing a polyamide wherein, when a dicarboxylic acid component and a diamine component are directly mixed to carry out a polycondensation reaction under normal pressure, a reaction temperature is controlled in such a manner that a reaction start temperature higher than the melting point of the dicarboxylic acid is elevated to maintain a reaction system including a starting mixture substantially in a uniform molten state to proceed with the reaction, and the temperature of the reaction system is further elevated beyond a temperature lower by 30°C than the melting point of the resulting polyamide before a conversion reaches 95 mol% so that the reaction can proceed in the homogeneous system without losing fluidity in the system. The above production method is characterized in that the diamine is added continuously to the reaction system while the reaction system containing the formed oligoamide/polyamide is maintained in a uniform molten state, and in this method, since the reaction is conducted under normal pressure, a polycondensation reactor does not need to be a container having pressure resistance. Moreover, this production method does not require the time needed for operations to increase and reduce pressure to be applied to the reaction system and the time to distill off water used as a solvent in the case of an aqueous solution method. Therefore, compared with a conventional polyamide production method, this method can significantly reduce the time required for polycondensation. Further, the method does not require heat in order to concentrate the aqueous solution, and it allows large volumes to be reacted together, thereby enhancing productivity. Therefore, the method is an extremely advantageous method for producing polyamide. However, when this method is to be used to produce a polyamide having a high melting point and having as its diamine component more than 30 mol% of paraxylylene diamine, the paraxylylene diamine is caused preferentially to react due to its relatively low volatility. As a result, a polyamide having a higher melting point and a high content of paraxylylene group is formed. The polyamide therefore solidifies and precipitates locally in the reaction system, preventing a uniform molten state from being maintained, and making it impossible to obtain a product having stable performance. It is therefore desirable to provide a method for producing a copolyamide having a high melting point from a mixed diamine which consists of paraxylylene diamine and metaxylylene diamine and a dicarboxylic acid comprising adipic acid as the main constituent. It is also desirable to provide a method for producing a copolyamide having the above composition in an industrially advantageous manner in accordance with a direct polycondensation method, without producing a polyamide having a high melting point. It is further desirable to provide a method for producing a copolyamide having the above uniform composition, and which is crystalline, colorless and has excellent heat resistance. It is yet further desirable to provide a novel polycondensation method for producing a copolyamide having the above composition, wherein the metaxylylene diamine content in the reaction mixture is increased and the temperature is elevated at the final stage of the polycondensation reaction. Objects and advantages of the invention will become more apparent from the following description. The present invention provides a method for producing a copolyamide which comprises the steps of: (1) heating a dicarboxylic acid which is adipic acid or a mixture comprising 40 mol% or more of adipic acid and 60 mol% or less of a dicarboxylic acid other than adipic acid to a temperature higher than the melting point of adipic acid but lower than a temperature 30°C below the melting point of the copolyamide product; (2) adding a diamine comprising 35 to 70 mol% of paraxylylene diamine and 65 to 30 mol% of metaxylylene diamine to the heated dicarboxylic acid stepwise or continuously (3) stopping addition of the diamine before the conversion of the dicarboxylic acid reaches 90 mol% and adding metaxylylene diamine or a diamine mixture of 70 mol% or more of metaxylylene diamine and 30 mol% or less of paraxylylene diamine continuously or stepwise to the reaction mixture until the molar ratio of the total of the diamines to the total of the dicarboxylic acid or acids becomes 0.97 to 1.03; and (4) before the conversion of the dicarboxylic acid reaches 95 mol% heating the reaction system to a temperature higher than a temperature 30°C below the melting point of the final copolyamide product and upon termination of addition of diamine, elevating the temperature of the reaction system to a temperature higher than the melting point of the final copolyamide product. The dicarboxylic acid used in the step (1) is adipic acid or a mixture of adipic acid and other dicarboxylic acid. In the case of a mixture, adipic acid should be contained in an amount of 40 mol% or more and hence, other dicarboxylic acid should be contained in an amount of 60 mol% or less. Adipic acid is preferably contained in an amount of 60 mol% or more, more preferably 80 mol% or more. Illustrative examples of the other dicarboxylic acid other than adipic acid include aliphatic dicarboxylic acids such as succinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecane diacid and dodecane diacid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid and 2,6-naphthalene dicarboxylic acid. When the adipic acid content of the dicarboxylic acid component is below 40 mol%, particularly in the case of a mixture of an aromatic dicarboxylic acid having a high melting point, unfavorably, it is difficult to ensure the fluidity of the dicarboxylic acid component before addition of the diamine is begun. In other words, in the step (1), the dicarboxylic acid is heated to a temperature higher than the melting point of adipic acid but lower than a temperature 30°C below the melting point of the copolyamide product. Preferably, this heating brings the dicarboxylic acid into a fluid state. The term "fluid state" means that the whole dicarboxylic acid is not necessary in a molten state, but may be in a slurry state as a whole, i.e., part of the dicarboxylic acid may be in a molten state and the other part may be still in a solid state. The reactor used in the present invention is preferably a reaction can equipped with a stirrer and a dephlegmator. In the above step (1), before the dicarboxylic acid is charged into the reaction can, the reaction can is, preferably, fully evacuated with an inert gas. Thereby, a colorless copolyamide is advantageously produced. According to the production method of the present invention, it is possible to produce a copolyamide directly from a dicarboxylic acid and a diamine without using a solvent and without having a particular restriction on reaction pressure. However, a reaction is preferably carried out under normal pressure or reduced pressure from viewpoints of operation and manufacture of the reactor. In the step (2), a diamine is dropped into the heated dicarboxylic acid. The diamine contains 35 to 70 mol% of paraxylylene diamine and 65 to 30 mol% of metaxylylene diamine. Addition of the diamine is carried out continuously or stepwise. By addition of the diamine, the dicarboxylic acid and the diamine are mixed together and maintained at a temperature higher than the melting point of adipic acid to start a polycondensation reaction. To start amidation at a substantial rate, the reaction temperature is preferably elevated to a temperature higher than 160°C and set to a temperature at which oligoamide and/or polyamide produced as an intermediate are/is in a molten state and the entire reaction system can be held in a uniform fluid state. Preferred polymerization operation is performed by stirring the dicarboxylic acid in a molten state in the reaction can, adding the diamine to the dicarboxylic acid under normal pressure and maintaining the reaction mixture at a temperature higher than the melting point of the resulting oligoamide and polyamide. Addition of the diamine is performed continuously or stepwise as described above, but the addition rate of the diamine is selected in consideration of heat generated by amidation, energy consumed to distill off water generated by condensation, and a cooler and a dephlegmator for separating water generated by condensation and starting compounds and further in consideration of a predetermined reaction temperature, i.e., a temperature at which the reaction system containing starting compounds can be held in a uniform molten state. Generally, the time required for addition of the diamine varies depending on the size of the reaction can, while it is in a range from 0.5 to 10 hours. During this addition, water generated by condensation with the progress of the reaction is removed from the reaction system through the dephlegmator, in which steam temperature at the column top is controlled to 100 to 120°C, and a cooler. Scattered starting materials such as metaxylylene diamine and adipic acid are collected by the dephlegmator and recycled to the reaction can. As the diamine, aliphatic diamines such as tetramethylene diamine, pentamethylene diamine, hexamethylene diamine, octamethylene diamine and nonamethylene diamine; aromatic diamines such as paraphenylene diamine; alicyclic diamines such as 1,3-bisaminomethyl cyclohexane and 1,4-bisaminomethyl cyclohexane may be used within the limit of the maximum amount of 20 mol% based on the total of the diamines, in combination with the mixed-xylylene diamine as described above. In the step (2), addition of the mixed-xylylene diamine having the above composition is stopped before the conversion of the dicarboxylic acid reaches 90 mol%. In the step (3), metaxylylene diamine, or a mixture comprising 70 mol% or more of metaxylylene diamine and 30 mol% or less of paraxylylene diamine is added in place of the diamine used in the step (2) subsequently to the step (2). If a diamine containing more than 30 mol% of paraxylylene diamine is added, relatively low-volatile paraxylylene diamine is caused preferentially to react in the reaction system and a copolyamide having a high content of paraxylylene diamine and a high melting point is formed. Since the copolyamide is solidified and precipitated locally in the reaction system, an uniform molten state cannot be maintained and hence, a product having stable performance cannot be obtained. The diamine is added continuously or stepwise until a molar ratio of the total of diamines including the diamine added in the preceding step (2) becomes 0.97 to 1.03 based on the total of dicarboxylic acids. The maximum amount of paraxylylene diamine contained in the copolyamide which is formed by addition of the diamine having the above composition in the step (3) is preferably controlled to 65 mol%. If the paraxylylene diamine content of the diamine component exceeds 65 mol%, the resulting copolyamide may have a melting point higher than 300°C, and when such copolyamide is used, moldability deteriorates considerably. That is, the copolyamide obtained by the method of the invention preferably contains a diamine component which comprises 35 to 65 mol% of metaxylylene diamine and 65 to 35 mol% of paraxylylene diamine and a dicarboxylic acid component which comprises adipic acid, or 40 mol% or more of adipic acid and 60 mol% or less of other dicarboxylic acid. In the step (4), the temperature of the reaction system is elevated to a temperature higher than a temperature 30°C below the melting point of the final copolyamide product before the conversion of the dicarboxylic acid reaches 95 mol%. The step (3) addition of the diamine is continued until the molar ratio of the total of the diamines to the total of the dicarboxylic acids becomes 0.97 to 1.03. In other words, as the molecular weight of oligoamide or polyamide formed by a polycondensation reaction increases with the progress of the reaction, the melting point of the reaction product rises and the viscosity of the reaction mixture increases, thereby preventing a rise in the temperature of the contents. As a result, the reaction mixture is easily solidified. Therefore, the reaction temperature must be increased as the reaction proceeds, and the reaction product must be maintained in a uniform fluid state. In the method of the present invention, this temperature control is performed in such a manner that the temperature of the reaction system is elevated to a temperature higher than a temperature 30°C below the melting point of the final copolyamide product before the conversion of the reaction materials reaches 95 mol%. In this instance, the degree of conversion is expressed by the proportion of reacted functional groups to functional groups originally present. The degree of conversion can be confirmed from the amount of water formed during the polycondensation reaction. If the conversion is below 95 mol%, the diamine added is reacted in the reaction system in a short period of time and hence, in the actual case, the conversion can be estimated from the amount of the diamine added. When the production method of the present invention is carried out, like the conventional aqueous solution pressurizing method, it is inevitable that the diamines distill off outside the reaction system. Therefore, it is necessary to provide a polycondensation reactor with a dephlegmator. Provision of the dephlegmator can prevent effectively the diamines containing metaxylylene diamine from distilling out during reaction. As the result, a copolyamide having a predetermined molecular weight can be produced with good reproducibility by setting the molar ratio of diamine component to dicarboxylic acid component (containing adipic acid) to 0.97 to 1.03, depending on the desired molecular weight. The upper limit of the temperature of the reaction system to be controlled does not need to be particularly limited, but preferably is not higher than a temperature 50°C above the melting point of the final copolyamide product, depending on the heat history appropriate to the resulting product. If the temperature of the reaction system is set to the above predetermined temperature only after the conversion exceeds 95 mol%, the copolyamide product will be partially crystallized before the temperature is elevated, making it extremely difficult to bring the reaction product into a uniform fluid state. Such solidification results in an increase in viscosity and a reduction in heat conductivity of the reaction system, and may involve a risk of solidification of the entire product. Under such conditions, it is substantially impossible to produce a copolyamide on an industrial scale. Upon termination of addition of the diamine component, the temperature of the reaction system is elevated to a temperature higher than the melting point of the final copolyamide product. In other words, the temperature of the reaction system whose conversion exceeds 95 mol% and whose temperature is elevated to a temperature higher than the temperature specified by the present invention, is elevated gradually so that the reaction product containing the copolyamide can be held substantially in a uniform fluid state until termination of the reaction. As described above, although the reaction starts at a temperature higher than the melting point of the starting dicarboxylic acid and a temperature of 160°C or more at which amidation takes place at a substantial rate, it is possible from the start of the reaction to elevate the temperature of the reaction system to a temperature higher than a temperature 30°C below the melting point of the final copolyamide product. However, that is not always expedient from a viewpoint of thermal economy or the heat history of the reaction product. Therefore, it is preferred to maintain the reaction system at a temperature higher than a temperature 30°C below the melting point of the finally produced copolyamide after the conversion exceeds 40 mol%. The polycondensation reactor used in the method of the present invention does not need to be a container having pressure resistance and hence, can be installed at an extremely low cost. In addition, since the method of the present invention does not require the time needed for operation to increase or decrease pressure and the time to distill out water as a solvent which is required by the conventional aqueous solution method, it can reduce the time required for polymerization significantly. Moreover, the method does not require heat for the concentration of an aqueous solution as is required in conventional methods, and productivity can be enhanced because large volumes of reagents can be reacted in each batch. As a consequence, the present invention provides an extremely economical method for producing a copolyamide. The present invention is further illustrated with reference to the following examples. Example 1 A 50-liter flask equipped with a stirrer, dephlegmator, thermometer, dropping funnel, and nitrogen gas introduction tube was charged with 10.000 kg of adipic acid which were accurately weighed, and fully substituted with nitrogen. Further, adipic acid was molten at 170°C under a trace amount of a nitrogen gas current to achieve a uniform fluid state. To this molten adipic acid were added 7.411 kg of a mixed-xylylene diamine comprising metaxylylene diamine and paraxylylene diamine (molar ratio of 5/5) over 91 minutes under agitation. During this addition, the temperature inside the flask was elevated continuously to 255°C. Subsequently, 1.853 kg of metaxylylene diamine were added continuously in 76 minutes under agitation. During this addition, the reaction temperature was elevated continuously from 255°C to 270°C. Water which distilled out during the addition of the various xylylene diamines was removed from the system through the dephlegmator and the cooler. After termination of addition of metaxylylene diamine, the inside temperature was elevated continuously up to 277°C and the reaction was continued for 32 minutes. Thereafter, the pressure inside the reaction system was reduced continuously over 10 minutes to 600 mmHg and then the reaction was continued for a further 10 minutes. During this time, the reaction temperature was elevated continuously from 277 to 282°C. Throughout the entire reaction process, no solidification or precipitation of oligomer or polyamide was observed, and the reaction system remained in a uniform fluid state. The resulting polyamide had a relative viscosity of 2.09 (1 g of 96 % sulfuric acid solution/100 ml) and a melting point of 269°C. Example 2 A 50-liter flask equipped with a stirrer, dephlegmator, thermometer, dropping funnel, and nitrogen gas introduction tube was charged with 7.000 kg of adipic acid and 3.410 kg of terephthalic acid which were accurately weighed and fully substituted with nitrogen. Further, adipic acid was molten at 170°C under a trace amount of a nitrogen air current to achieve a uniform fluid state. To this molten adipic acid were added 7.343 kg of a mixed-xylylene diamine comprising metaxylylene diamine and paraxylylene diamine (molar ratio of 5/5) over 93 minutes under agitation. During this addition, the temperature inside the flask was elevated continuously to 245°C. Subsequently, 1.836 kg of metaxylylene diamine were added continuously over 74 minutes under agitation. During this addition, the reaction temperature was elevated continuously from 245°C to 260°C. Water which distilled out during addition of the various-xylylene diamines was removed from the system through the dephlegmator and the cooler. After termination of addition of metaxylylene diamine, the inside temperature was elevated continuously up to 265°C and the reaction was continued for 32 minutes. Thereafter, the pressure inside the reaction system was reduced continuously over 10 minutes to 600 mmHg and then the reaction was continued for a further 14 minutes. During this time, the reaction temperature was elevated continuously from 265 to 271°C. Throughout the entire reaction process, no solidification or precipitation of oligomer or polyamide was observed, and the reaction system remained in a uniform fluid state. The resulting polyamide had a relative viscosity of 1.98 (1 g of 96 % sulfuric acid solution/100 ml) and a melting point of 257°C. Comparative Example 1 A 50-liter flask equipped with a stirrer, dephlegmator, thermometer, dropping funnel and nitrogen gas introduction tube was charged with 10.000 kg of adipic acid which was accurately weighed and fully substituted with nitrogen. Further, adipic acid was molten at 170°C under a trace amount of a nitrogen air current to achieve a uniform fluid state. To this molten adipic acid were added 7.967 kg of a mixed-xylylene diamine comprising metaxylylene diamine and paraxylylene diamine (molar ratio of 6/4) over 110 minutes under agitation. During this addition, the inside temperature was elevated continuously to 262°C. Subsequently, 1.297 kg of the mixed xylylene diamine were added continuously over 68 minutes under agitation. During this addition, the reaction temperature was elevated continuously from 262 to 270°C. Water which distilled out during addition of the mixed-xylylene diamine was removed from the system through the dephlegmator and the cooler. After termination of addition of the mixed-xylylene diamine, the inside temperature was elevated continuously to 275°C and the reaction was continued for 32 minutes. Thereafter, the pressure inside the reaction system was reduced continuously over 10 minutes to 540 mmHg and then the reaction was continued for a further 10 minutes. During this time, the reaction temperature was elevated continuously from 275 to 282°C. In the latter stage of the addition, solidification and precipitation of oligomer or polyamide was observed, and the reaction system lost its uniform fluid state. The resulting polyamide resulted in a product having lack of a uniform quality and the main constituent of the product had a relative viscosity of 2.39 (1 g of 96 % sulfuric acid solution/100 ml) and a melting point of 269°C.

Claims: A method for producing a copolyamide which comprises the steps of: (1) heating a dicarboxylic acid which is adipic acid or a mixture comprising 40 mol% or more of adipic acid and 60 mol% or less of a dicarboxylic acid other than adipic acid to a temperature higher than the melting point of adipic acid but lower than a temperature 30°C below the melting point of the copolyamide product; (2) adding a diamine comprising 35 to 70 mol% of paraxylylene diamine and 65 to 30 mol% of metaxylylene diamine to the heated dicarboxylic acid stepwise or continuously; (3) stopping addition of the diamine before the conversion of the dicarboxylic acid reaches 90 mol%, and adding metaxylylene diamine or a diamine mixture of 70 mol% or more of metaxylylene diamine and 30 mol% or less of paraxylylene diamine continuously or stepwise to the reaction mixture until the molar ratio of the total of the diamines to the total of the dicarboxylic acid or acids becomes 0.97 to 1.03; and (4) before the conversion of the dicarboxylic acid reaches 95 mol%, heating the reaction system to a temperature higher than a temperature 30°C below the melting point of the final copolyamide product, and upon termination of addition of diamine, elevating the temperature of the reaction system to a temperature higher than the melting point of the final copolyamide product. The method of claim 1, wherein the dicarboxylic acid is a mixture of 60 mol% or more of adipic acid and 40 mol% or less of one or more other dicarboxylic acid. The method of claim 2, wherein the dicarboxylic acid is a mixture of 80 mol% or more of adipic acid and 20 mol% of less of one or more other dicarboxylic acid. The method of any one of the preceding claims, wherein the other dicarboxylic acid is terephthalic acid. The method of any one of the preceding claims, wherein a polycondensation reaction between the dicarboxylic acid and the diamine is performed under normal pressure. The method of claim 1, wherein the resulting copolyamide contains a diamine component which comprises 35 to 65 mol% of metaxylylene diamine and 65 to 35 mol% of paraxylylene diamine and a dicarboxylic acid component which comprises adipic acid or 40 mol% or more of adipic acid and 60 mol% or less of other dicarboxylic acid. The method of any one of the preceding claims, wherein the reaction system is heated to a temperature higher than a temperature 30°C below the melting point of the final copolymer after the conversion of the dicarboxylic acid exceeds 40 mol%.

Inventors: HARADA MASAHIRO, INABA YASUO, HARADA, MASAHIRO, INABA, YASUO, HARADA, MASAHIRO, C/O HIRATSUKA RES. LAB., INABA, YASUO, C/O HIRATSUKA RES. LAB.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1995
Main classification: C08G69
IPC codes: C08G69