Polyester production method

Abstract

To provide a polyester production method which produces little amount of DEG without addition of an esterification catalyst.SOLUTION: A polyester production method subjects terephthalic acid and ethylene glycol as main raw materials to polycondensation, and includes the following three steps: (1) a step of producing bis(hydroxyethyl)terephthalate by charging only terephthalic acid, ethylene glycol and a quaternary ammonium compound into an esterification reaction tank [1], setting the concentration of the quaternary ammonium compound to 0.10 mass% or more and 0.80 mass% or less, and subjecting the materials to esterification reaction under an esterification reaction tank pressure of 0.11 MPa or more and 0.50 MPa or less; (2) a step of producing bis(hydroxyethyl)terephthalate by distilling the ethylene glycol remaining in the esterification reaction tank [1] after the step (1), then charging a terephthalic acid and ethylene glycol thereto, and subjecting the materials to second stage esterification reaction in the presence of bis(hydroxyethyl)terephthalate; and (3) a step of producing polyethylene terephthalate by transferring the bis(hydroxyethyl)terephthalate from the esterification reaction tank [1] to a polycondensation reaction tank [2] and further progressing polycondensation under reduced pressure.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing polyester with a low DEG content. [Background technology]

[0002] Polyester, particularly polyethylene terephthalate (hereinafter sometimes abbreviated as PET), is widely used in the form of fibers, films, bottles, etc., due to its excellent mechanical strength, chemical stability, gas barrier properties, etc. Generally, PET is produced from a dicarboxylic acid component such as terephthalic acid or its diester compound and ethylene glycol (hereinafter sometimes abbreviated as EG). Specifically, a PET low polymer is first formed by an esterification reaction between a dicarboxylic acid or its diester compound and ethylene glycol, and then this PET low polymer is subjected to a de-diol reaction in the presence of a polycondensation catalyst to increase the molecular weight. Among them, when dicarboxylic acids are used, the dicarboxylic acid contributes to the reaction as an autocatalyst, so that the addition of a catalyst is not required in the esterification reaction, and the generation of foreign matter and deterioration of color caused by the catalyst are suppressed, which is preferable. However, there is a problem that side reactions occur during the reaction, and various by-products such as diethylene glycol (hereinafter sometimes abbreviated as DEG) are generated. Therefore, various production methods are being considered to suppress the generation of by-products.

[0003] For example, Patent Document 1 proposes a method for producing polyester in which a titanium compound and an alkali compound are added in an esterification reaction between terephthalic acid and ethylene glycol as the main components, and describes that this method for producing polyester results in good color tone, low DEG content, and good spinnability.

[0004] Patent Document 2 proposes a method for producing polyester using terephthalic acid with a sodium atom content within a certain amount, and describes that when the DEG content is below a certain value, a polyester can be obtained that has little yellowness and has low volume resistivity when melted, allowing for high-speed film formation.

[0005] Patent Document 3 proposes a method for producing polyester in which a titanium compound is added in the transesterification reaction using a dialkyl terephthalate or a bishydroxyalkyl terephthalate and ethylene glycol as raw materials, and a germanium compound or a titanium compound is added in the polycondensation reaction. It is described that this polyester production method has a high polycondensation rate, good color tone, and a low DEG content.

[0006] Patent Document 4 proposes a method for producing polyester in which a phosphoric acid ester, a compound containing an alkali metal and / or an alkaline earth metal, and an ammonium compound or an amine compound are added in an esterification reaction between a dicarboxylic acid component mainly composed of terephthalic acid and a diol component mainly composed of ethylene glycol, and describes that the method produces polyester with good dyeability.

[0007] Patent Document 5 proposes a method for producing polyester by esterification and polymerization of bis(ethylene terephthalate) monomer with terephthalic acid, and describes that this production method makes it possible to control the DEG content to a certain value or less and the polydispersity index to within a certain value. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2004-231831 A [Patent Document 2] JP 2016-132733 A [Patent Document 3] JP 2011-26438 A [Patent Document 4] JP 2010-59232 A [Patent Document 5] JP 2020-66728 A Summary of the Invention [Problem to be solved by the invention]

[0009] DEG produced as a side reaction during polyester production is incorporated into the polymer without being distilled out of the system, and as the amount of DEG produced increases, the polymer melting point decreases, and its resistance to hydrolysis, heat and light, as well as its dyeability, decrease. Therefore, it is necessary to suppress the production of DEG during polyester production.

[0010] Furthermore, in polyester production, catalysts and additives can generate foreign matter, which can reduce the quality of the final product. Therefore, it is important to reduce the types and amounts of additives other than the raw material terephthalic acid and glycol components.

[0011] However, the technologies proposed in Patent Documents 1 and 3 are polyester production methods that add titanium compounds and alkali compounds in addition to terephthalic acid and ethylene glycol. Although the amount of DEG can be reduced, there is a concern that foreign matter derived from the additives may be generated, and it has not been possible to sufficiently achieve both a reduction in the amount of DEG and suppression of foreign matter.

[0012] In addition, the technology proposed in Patent Document 2 is a polyester manufacturing method in which a phosphorus compound is added as a heat stabilization aid in addition to terephthalic acid and ethylene glycol, which have a specified sodium atom content. Although the DEG amount can be reduced, there is a concern that foreign matter derived from the additives may be generated, and both the DEG amount and the suppression of foreign matter are not sufficiently achieved. In addition, it is necessary to pre-adjust the sodium salt of terephthalic acid so that the sodium atom content in terephthalic acid is within the specified value. This sodium salt of terephthalic acid acts as a nucleating agent, which causes a decrease in the molecular weight of the polyethylene terephthalate produced, and there is a concern that it may affect the mechanical properties and thermal properties.

[0013] In addition, the technology proposed in Patent Document 4 is a polyester production method in which an oligomer obtained by an esterification reaction of terephthalic acid and ethylene glycol is further supplemented with a phosphate ester, a compound containing an alkali metal and / or an alkaline earth metal, and an ammonium compound or an amine compound in addition to terephthalic acid and ethylene glycol. Although this method can suppress the generation of foreign matter, it has the problem of increasing the amount of DEG during oligomer production.

[0014] Furthermore, the technology proposed in Patent Document 5 is a method for producing polyester from bis(ethylene terephthalate) monomer, terephthalic acid, and ethylene glycol. Although the amount of DEG can be reduced, this is not preferable because it requires an additional step of forming the bis(ethylene terephthalate) monomer from polyethylene terephthalate by alcoholysis with glycol or by a reaction between terephthalic acid and ethylene oxide.

[0015] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for producing a polyester having a low DEG content by a simple process in which an esterification reaction and a polycondensation reaction are carried out using only terephthalic acid, ethylene glycol, and a quaternary ammonium compound. [Means for solving the problem]

[0016] As a result of extensive research to achieve the above-mentioned object, the inventors have found that it is possible to reduce the amount of DEG without adding a catalyst or the like by adding only terephthalic acid, ethylene glycol, and a specified amount of a quaternary ammonium compound to an esterification reaction tank and carrying out an esterification reaction under pressurized conditions.

[0017] The present invention has been completed based on these findings and is achieved by the following means.

[0018] [1] A method for producing polyester by polycondensing terephthalic acid and ethylene glycol as main raw materials, the method being characterized by going through the three steps described below. (1) A process for producing bis(hydroxyethyl) terephthalate by feeding only terephthalic acid, ethylene glycol, and a quaternary ammonium compound into an esterification reaction tank [1], setting the concentration of the quaternary ammonium compound at 0.10% by mass or more and 0.80% by mass or less, and setting the pressure in the esterification reaction tank at 0.11 MPa or more and 0.50 MPa or less, and carrying out an esterification reaction. (2) After step (1), the ethylene glycol remaining in the esterification reaction tank [1] is distilled off, and then terephthalic acid and ethylene glycol are added to carry out a second-stage esterification reaction in the presence of bis(hydroxyethyl) terephthalate, thereby producing bis(hydroxyethyl) terephthalate. (3) A process in which bis(hydroxyethyl) terephthalate is transferred from the esterification reaction tank [1] to the polycondensation reaction tank [2] and polycondensation is further carried out under reduced pressure to produce polyethylene terephthalate. [2] The method for producing the polyester according to [1] above, wherein in step 1, ethylene glycol and a quaternary ammonium compound are charged into an esterification reaction tank, and then terephthalic acid or a mixed slurry of terephthalic acid and ethylene glycol is added.

[0019] [3] The method for producing a polyester according to [1] or [2] above, wherein the quaternary ammonium compound is a quaternary ammonium hydroxide.

[0020] [4] The method for producing the polyester according to any one of [1] to [3] above, wherein the reaction temperature during the esterification reaction in step 1 is 200° C. or higher and 250° C. or lower. [5] The method for producing a polyester according to any one of the above [1] to [4], wherein the amount of the quaternary ammonium compound added is 0.30% by mass or more and 0.70% by mass or less.

[0021] [6] The method for producing a polyester according to any one of the above [1] to [5], wherein in the step (1), after adding terephthalic acid, ethylene glycol, and a quaternary ammonium compound, a molar ratio of terephthalic acid to ethylene glycol calculated by the following formula is 1.4 or more and 3.5 or less. Molar ratio in esterification reaction vessel=(molar amount of EG component) / (molar amount of terephthalic acid component) Effect of the Invention

[0022] According to the present invention, a polyester resin having a low DEG content can be produced without adding an esterification catalyst. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention is a method for producing polyester by polycondensing terephthalic acid and ethylene glycol as main components, and is characterized by the following three steps: (1) A process for producing bis(hydroxyethyl) terephthalate by feeding only terephthalic acid, ethylene glycol, and a quaternary ammonium compound into an esterification reaction tank [1], adjusting the concentration of the quaternary ammonium compound to 0.10% by mass or more and 0.80% by mass or less, and setting the pressure in the esterification reaction tank to 0.11 MPa or more and 0.50 MPa or less, and carrying out an esterification reaction. (2) After step (1), the ethylene glycol remaining in the esterification reaction tank [1] is distilled off, and then terephthalic acid and ethylene glycol are added to carry out a second-stage esterification reaction in the presence of bis(hydroxyethyl) terephthalate, thereby producing bis(hydroxyethyl) terephthalate. (3) A process in which bis(hydroxyethyl) terephthalate is transferred from the esterification reaction tank [1] to the polycondensation reaction tank [2] and polycondensation is further carried out under reduced pressure to produce polyethylene terephthalate.

[0024] The production method of the present invention is a production method related to the first and second stage esterification reactions in a continuous polyester production method, and its components will be described in detail below. The present invention is not limited to the scope described below as long as it does not deviate from the gist of the present invention, and various modifications are possible within the scope of the present invention.

[0025] [Esterification reaction raw materials] In the esterification reaction of the present invention, terephthalic acid is used as the dicarboxylic acid component, and EG is used as the diol component.

[0026] Examples of dicarboxylic acids that can be used in the production of polyesters include aromatic dicarboxylic acid compounds such as terephthalic acid, isophthalic acid, and sodium 5-sulfoisophthalate, aliphatic dicarboxylic acid compounds such as adipic acid and sebacic acid, and alicyclic dicarboxylic acid compounds such as cyclohexanedicarboxylic acid, and their ester-forming derivatives, particularly dimethyl derivatives, can also be suitably used as raw materials. In particular, for the purpose of improving the mechanical strength of the polyester finally obtained, it is preferable to use terephthalic acid or its dimethyl derivative and isophthalic acid or its dimethyl derivative, and therefore terephthalic acid is used in the present invention.

[0027] The diol component that can be used in the production of the polyester is preferably selected from any one of 1,4-butanediol, 1,3-propanediol, and ethylene glycol, or a combination thereof. In particular, it is most preferable to mainly use ethylene glycol in terms of excellent dimensional stability of the resulting polyester, and therefore ethylene glycol is used in the present invention.

[0028] In the present invention, a quaternary ammonium compound is added.

[0029] Examples of the quaternary ammonium compound include hydroxides, chlorides, iodides, etc. of quaternary ammonium, and in particular, hydroxides of quaternary ammonium that are soluble in EG and have strong basicity are preferred. Examples of the hydroxides of quaternary ammonium include tetramethylammonium hydroxide, tetraethylammonium hydroxide (hereinafter sometimes abbreviated as EAH), tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, trimethylbenzylammonium hydroxide, etc., and in particular, tetraethylammonium hydroxide is preferred in terms of basicity, inhibition of foreign matter generation, and ease of availability.

[0030] [Esterification reaction under pressure (step (1))] Step (1) in the present invention is a step of producing bis(hydroxyethyl)terephthalate (hereinafter sometimes abbreviated as BHET) by charging only terephthalic acid, ethylene glycol, and a quaternary ammonium compound into an esterification reaction tank [1], adjusting the concentration of the quaternary ammonium compound to 0.10% by mass or more and 0.80% by mass or less, and carrying out an esterification reaction under pressure at a pressure of 0.11 MPa or more and 0.50 MPa or less in the esterification reaction tank.

[0031] The bis(hydroxyethyl) terephthalate in the present invention is a mixture containing not only the bis(hydroxyethyl) terephthalate monomer, which is a monomer, but also unreacted EG and oligomers of various polymerization degrees, and is different from the bis(hydroxyethyl) terephthalate monomer.

[0032] In the present invention, only terephthalic acid, ethylene glycol, and a quaternary ammonium compound are charged into an esterification reaction tank [1] to which no raw materials have been charged, and the first-stage esterification reaction is carried out under pressure (esterification reaction under pressure) to produce BHET. The method for charging terephthalic acid, ethylene glycol, and the quaternary ammonium compound is preferably a method in which ethylene glycol and the quaternary ammonium compound are charged, and then terephthalic acid or a mixed slurry of terephthalic acid and ethylene glycol is added.

[0033] Since quaternary ammonium compounds are strongly basic, they can reduce the acidity in the esterification reaction tank, which is under acidic conditions due to the carboxyl end of terephthalic acid. In addition, the production of DEG by the reaction between EGs is more likely to proceed under acidic conditions, so the addition of quaternary ammonium compounds can suppress the production of DEG. Therefore, the timing of adding quaternary ammonium compounds is preferably before the esterification reaction under pressure, and it is most preferable to add them simultaneously with EG before adding terephthalic acid. On the other hand, adding quaternary ammonium compounds after the esterification reaction under pressure, before the polycondensation reaction, or after the polycondensation reaction is not preferable because it promotes the reaction between EGs under acidic conditions during the esterification reaction, resulting in a large amount of DEG produced.

[0034] The concentration of the quaternary ammonium compound is 0.10% by mass or more and 0.80% by mass or less with respect to the polyester obtained. The concentration of the quaternary ammonium compound is preferably 0.30% by mass or more, more preferably 0.50% by mass or more, to reduce the acidity in the system during the esterification reaction, and suppress the generation of DEG due to the reaction between EGs, which is promoted under acidic conditions, and therefore the amount of DEG can be reduced. On the other hand, the concentration of the quaternary ammonium compound is preferably 0.70% by mass or less, more preferably 0.60% by mass or less, to improve the reactivity during the esterification reaction and reduce the amount of DEG generated by thermal decomposition.

[0035] The method of adding terephthalic acid or a mixed slurry of terephthalic acid and EG to the EG and quaternary ammonium compound charged in advance may be lump-sum addition or continuous addition. In particular, continuous addition is preferred because it allows uniform mixing with the stored EG and allows the esterification reaction to proceed quickly under pressure without lowering the temperature in the system.

[0036] The slurry molar ratio when adding a mixed slurry of terephthalic acid and EG is preferably 1.05 or more and 1.40 or less. The slurry molar ratio is preferably 1.10 or more, more preferably 1.15 or more, so that the supply during the addition of the slurry can be stabilized and the esterification reaction can be carried out efficiently. On the other hand, the slurry molar ratio is preferably 1.30 or less, more preferably 1.20 or less, so that the amount of EG can be reduced and the production of DEG can be suppressed. The slurry molar ratio is represented by the following formula (1). Slurry molar ratio=(molar amount of EG component) / (molar amount of terephthalic acid component) Formula (1) When adding a mixed slurry of terephthalic acid and EG, it is preferable to add it after the inside of the esterification reaction vessel has reached a predetermined pressure and / or a predetermined temperature. By adding it after the inside of the esterification reaction vessel has reached a predetermined pressure and / or a predetermined temperature, the esterification reaction under pressure after the addition of the slurry proceeds quickly, and the production of DEG due to the reaction between EGs can be suppressed.

[0037] In the present invention, the molar ratio of EG to terephthalic acid in the esterification reaction tank after the raw material is charged is preferably 1.4 to 3.5. The molar ratio in the esterification reaction tank is preferably 3.0 or less, more preferably 2.1 or less, and even more preferably 1.8 or less, so that the amount of EG can be reduced and the production of DEG due to the reaction between EGs can be suppressed. On the other hand, if the molar ratio in the esterification reaction tank is lower than 1.4, the solubility of terephthalic acid decreases, and the esterification reaction is delayed or the reaction does not proceed, which is not preferable. The molar ratio of EG to terephthalic acid is expressed by the following formula (2). Molar ratio in esterification reaction vessel=(molar amount of EG component) / (molar amount of terephthalic acid component) Formula (2) The molar ratio in the esterification reaction tank can be adjusted by reducing the amount of EG stored or by adding EG and terephthalic acid to the esterification reaction tank all at once, and the method of adding EG all at once is preferable because it can reduce the amount of EG and reduce the molar ratio in the esterification reaction tank. Furthermore, in the case of the method of adding EG and terephthalic acid all at once, it is preferable to add terephthalic acid after adding EG and EAH, because this allows EG and terephthalic acid to be mixed uniformly and the esterification reaction to proceed quickly under pressure.

[0038] The pressure during the esterification reaction under pressure in the present invention is 0.11 MPa or more and 0.50 MPa or less. By setting the pressure to preferably 0.15 MPa or more, more preferably 0.20 MPa or more, the temperature during the esterification reaction can be increased, and the reactivity of terephthalic acid and ethylene glycol can be improved. On the other hand, by setting the pressure to preferably 0.40 MPa or less, more preferably 0.30 MPa or less, the reaction between EG can be suppressed, and the amount of DEG can be reduced.

[0039] The pressure during the esterification reaction under pressure in the present invention is preferably reached within 10 to 100 minutes after the start of heating the esterification reaction tank. The time to reach the predetermined pressure is preferably 20 minutes or more, more preferably 30 minutes or more, so that the inside of the esterification reaction tank can be gradually pressurized and bumping of ethylene glycol can be suppressed. On the other hand, the time to reach the predetermined pressure is preferably 90 minutes or less, more preferably 80 minutes or less, so that the residence time of the EG stored under pressurized conditions can be shortened and the generation of DEG due to the reaction between EGs can be suppressed.

[0040] In the present invention, the pressure control during the esterification reaction under pressure is preferably carried out when the temperature in the esterification reaction tank is 100° C. or higher. By setting the temperature at 120° C. or higher, and more preferably at 150° C. or higher, the EG vapor in the esterification reaction tank can be used to pressurize the tank to a certain level or higher, and the amount of gas to be charged from outside can be minimized.

[0041] When pressure control is performed at temperatures below 100°C, it is preferable to fill the gas from outside since there is little EG vapor.

[0042] The gas to be filled from the outside during pressure control in the present invention is preferably nitrogen. By using nitrogen, the decomposition of terephthalic acid, EG and BHET can be suppressed, and the deterioration of the color tone of the final polyester can be suppressed.

[0043] Furthermore, the pressure in the esterification reaction tank is preferably controlled to be "predetermined pressure -0.05 MPa or more and predetermined pressure +0.05 MPa or less" by charging gas from outside and / or releasing pressure in the system. By controlling the pressure to be "predetermined pressure -0.03 MPa or more and predetermined pressure +0.03 MPa or less" and more preferably to be "predetermined pressure -0.01 MPa or more and predetermined pressure +0.01 MPa or less," the pressure in the esterification reaction tank is stabilized and the production of DEG due to the reaction between EGs caused by overpressurization can be suppressed.

[0044] The temperature during the esterification reaction under pressure in the present invention is preferably 200° C. or higher and 250° C. or lower. By setting the reaction temperature at preferably 210° C. or higher, more preferably 215° C. or higher, the reaction between terephthalic acid and EG can be promoted, and BHET can be obtained in a short time. On the other hand, by setting the reaction temperature at preferably 240° C. or lower, more preferably 230° C. or lower, the reaction between EGs can be suppressed, the amount of DEG can be reduced, and the amount of residual COOH groups can also be reduced.

[0045] In the present invention, the temperature of the esterification reaction tank during the esterification reaction under pressure is preferably raised to a predetermined temperature (reaction temperature) in 60 to 200 minutes. By setting the temperature rise time to preferably 65 minutes or more, more preferably 70 minutes or more, the temperature in the esterification reaction tank can be raised to the set value, and the production of DEG can be suppressed by suppressing thermal decomposition due to overheating. On the other hand, by setting the temperature rise time to preferably 185 minutes or less, more preferably 170 minutes or less, the temperature can be raised efficiently, the residence time of the stored EG until the predetermined temperature is reached can be shortened, and the production of DEG due to the reaction between EG can be suppressed.

[0046] Furthermore, it is preferable to raise the temperature stepwise, and it is more preferable to change the rate of temperature increase at the boundary of 200°C. The boiling point of EG varies depending on the pressure in the esterification reaction tank, but is about 200°C, so it is possible to raise the temperature in the esterification reaction tank containing only EG and a quaternary ammonium compound according to the set value up to 200°C. On the other hand, above 200°C, the temperature can be raised by the boiling point of EG and the generation of BHET, so it is preferable to gradually raise the temperature in the system to promote the esterification reaction under pressure.

[0047] [EG expulsion reaction and esterification reaction in the presence of BHET (step (2))] In the step (2) of the present invention, after distilling off the unreacted ethylene glycol remaining in the esterification reaction tank [1], terephthalic acid and ethylene glycol are again charged into the esterification reaction tank [1] and a second stage esterification reaction is carried out in the presence of bis(hydroxyethyl)terephthalate to produce bis(hydroxyethyl)terephthalate. That is, a further esterification reaction is carried out in the presence of the bis(hydroxyethyl)terephthalate produced in the step (1).

[0048] [EG expulsion reaction] In the present invention, the ethylene glycol remaining in the esterification reaction tank [1] after step (1) is preferably removed by an EG purging reaction. The EG purging reaction is a process for removing EG at a constant pressure while heating the esterification reaction tank [1]. By removing the remaining EG by the purging reaction, it is possible to improve the reactivity and suppress the production of DEG in the second-stage esterification reaction carried out after the EG purging reaction.

[0049] The temperature during the EG removal reaction is preferably 240°C or higher and 250°C or lower. If the reaction temperature is lower than 240°C, EG distillation will be insufficient, and the amount of EG will increase in the second-stage esterification reaction, resulting in an increase in the amount of DEG produced. On the other hand, if the reaction temperature is higher than 250°C, EG will react with itself, producing DEG before EG distillation, which is easily incorporated into BHET, which is undesirable.

[0050] The pressure during the EG purge reaction is preferably 0.05 MPa or more and 0.10 MPa or less. If it is less than 0.05 MPa, the polymerization of BHET is promoted, which is not preferable. On the other hand, if the pressure is higher than 0.10 MPa, the boiling point of EG increases, which suppresses the distillation of EG, which is not preferable.

[0051] It is preferable that the EG purging reaction is carried out under normal pressure and then further under reduced pressure. By carrying out the EG purging reaction under reduced pressure after the EG purging reaction under normal pressure, the remaining EG can be further distilled off, and the reactivity of the second-stage esterification reaction carried out after the EG purging reaction can be improved and the production of DEG can be suppressed.

[0052] The EG purging reaction under normal pressure is preferably carried out for 10 to 90 minutes, and the pressure is preferably 0.10 MPa. If the reaction time is less than 10 minutes, the EG purging reaction becomes insufficient, the amount of EG in the second-stage esterification reaction increases, and the amount of DEG produced increases, which is not preferable. On the other hand, if the reaction time is longer than 90 minutes, the residence time of BHET under heating conditions is long, and DEG is produced by the reaction between the remaining EGs, which is not preferable.

[0053] The EG purging reaction under reduced pressure after the EG purging reaction under normal pressure is preferably carried out for 10 minutes or more and 90 minutes or less, and the pressure is preferably 0.05 MPa or more and less than 0.10 MPa. If the reaction time is less than 10 minutes, the EG purging reaction becomes insufficient, the amount of EG increases in the second-stage esterification reaction, and the amount of DEG produced increases, which is not preferred. On the other hand, if the reaction time is longer than 90 minutes, the residence time of BHET under heating conditions is long, and DEG is produced by reaction between the remaining EG, which is not preferred. In addition, a pressure of less than 0.05 MPa is not preferred, as it promotes polymerization of BHET. On the other hand, if the pressure is 0.10 MPa, it is not preferred, as the reaction is carried out under normal pressure conditions and distillation of EG does not proceed. [Esterification reaction in the presence of BHET] In the step (2) of the present invention, after the ethylene glycol remaining in the esterification reaction tank [1] is distilled off, terephthalic acid and ethylene glycol are added to the esterification reaction tank [1] in which bis(hydroxyethyl) terephthalate is present, and a second-stage esterification reaction is carried out to produce bis(hydroxyethyl) terephthalate.

[0054] In the present invention, in step (1), only terephthalic acid, ethylene glycol, and a quaternary ammonium compound are charged into an esterification reaction tank [1], and a first-stage esterification reaction is carried out under pressure to produce BHET. In the second-stage esterification reaction, terephthalic acid or a mixed slurry of terephthalic acid and EG is added to the esterification reaction tank [1] in which BHET is present, and an esterification reaction (hereinafter referred to as an esterification reaction in the presence of BHET) is carried out. In the esterification reaction in the presence of BHET, which is the second-stage esterification reaction, terephthalic acid or a mixed slurry of terephthalic acid and EG is added to the esterification reaction tank in which BHET is present, and the state in the esterification reaction tank before the addition of terephthalic acid or a mixed slurry of terephthalic acid and EG is different from the first-stage esterification reaction under pressure, which is carried out by adding terephthalic acid or a mixed slurry of terephthalic acid and EG to an empty esterification reaction tank [1]. It is preferable not to add a quaternary ammonium compound in the esterification reaction from the second stage onwards.

[0055] In the second-stage esterification reaction in the presence of BHET, the temperature during the esterification reaction can be increased due to the presence of BHET obtained in the first-stage esterification reaction under pressure, and the esterification reaction can be carried out efficiently. In addition, by leaving a certain amount of BHET obtained after the second-stage esterification reaction in the presence of BHET in the esterification reaction tank [1], the esterification reaction in the presence of BHET can be carried out continuously (from the third stage onwards), which is preferable.

[0056] The method of adding terephthalic acid or a mixed slurry of terephthalic acid and EG to the esterification reaction tank [1] containing the BHET obtained after the first-stage esterification reaction under pressure and the EG purge reaction may be lump-sum addition or continuous addition. In particular, continuous addition is preferred because it allows uniform mixing with BHET and the esterification reaction to proceed quickly without lowering the temperature in the system.

[0057] When adding a mixed slurry of terephthalic acid and EG, the slurry molar ratio is preferably 1.05 or more and 1.40 or less. The slurry molar ratio is preferably 1.10 or more, more preferably 1.15 or more, so that the supply during the addition of the slurry can be stabilized and the esterification reaction can be carried out efficiently. On the other hand, the slurry molar ratio is preferably 1.30 or less, more preferably 1.20 or less, so that the amount of EG can be reduced and the production of DEG can be suppressed. The slurry molar ratio is the value calculated by the above formula (1).

[0058] When adding a mixed slurry of terephthalic acid and EG, it is preferable to add it after the inside of the esterification reaction tank [1] has reached a predetermined pressure and / or a predetermined temperature. By adding it after the inside of the esterification reaction tank has reached a predetermined pressure and / or a predetermined temperature, the esterification reaction after the addition of the slurry proceeds quickly, and the production of DEG due to the reaction between EGs can be suppressed.

[0059] The temperature during the esterification reaction in the presence of BHET is preferably 240° C. or higher and 250° C. or lower. If the reaction temperature is lower than 240° C., it is difficult to carry out the esterification reaction efficiently, which may cause a deterioration in the reactivity of the subsequent polycondensation. If the reaction temperature is higher than 250° C., DEG is easily produced as a by-product.

[0060] The pressure during the esterification reaction in the presence of BHET is preferably 0.10 MPa or more and 0.50 MPa or less. If it is less than 0.10 MPa, the esterification reaction between terephthalic acid and EG is inhibited and the polymerization of BHET is promoted, which is not preferable. On the other hand, if the pressure is higher than 0.50 MPa, the reaction between EGs is promoted and the amount of DEG produced increases, which is not preferable.

[0061] It is preferable that the pressure during the esterification reaction in the presence of BHET is allowed to reach the predetermined pressure in 10 to 100 minutes. By setting the time to reach the predetermined pressure to preferably 20 minutes or more, more preferably 30 minutes or more, the inside of the esterification reaction tank [1] can be gradually pressurized and bumping of ethylene glycol can be suppressed. On the other hand, by setting the time to reach the predetermined pressure to preferably 90 minutes or less, more preferably 80 minutes or less, the residence time of EG at high temperatures can be shortened and the production of DEG due to the reaction between EGs can be suppressed.

[0062] It is preferable to control the pressure during the esterification reaction in the presence of BHET when the temperature in the esterification reaction tank is 240° C. or higher. By setting the temperature at 245° C. or higher, it is possible to pressurize the tank to a certain level or higher using the EG vapor in the esterification reaction tank [1], and it is possible to minimize the amount of gas that needs to be charged from outside. On the other hand, when pressure control is performed at a temperature below 240° C., it is preferable to charge the gas from outside because there is little EG vapor.

[0063] The gas used for pressure control is preferably nitrogen. By using nitrogen, the decomposition of terephthalic acid, EG and BHET can be suppressed, and the deterioration of the color tone of the polyester finally obtained can be suppressed. Furthermore, the pressure in the esterification reaction tank is preferably controlled to a predetermined pressure of "predetermined pressure -0.05 MPa or more and predetermined pressure +0.05 MPa or less" by filling gas from the outside and / or releasing pressure in the system. By setting the predetermined pressure to "predetermined pressure -0.03 MPa or more and predetermined pressure +0.03 MPa or less, and more preferably to "predetermined pressure -0.01 MPa or more and predetermined pressure +0.01 MPa or less", the pressure in the esterification reaction tank [1] is stabilized, the esterification reaction proceeds stably, and the generation of DEG due to the reaction between EGs caused by overpressurization can be suppressed.

[0064] [Polycondensation reaction (step (3))] The step (3) in the present invention is a step of transferring BHET from the esterification reaction tank [1] to a polycondensation reaction tank [2] after the esterification reaction in the presence of BHET in the step (2), and further promoting polycondensation in the polycondensation reaction tank [2] under reduced pressure to produce polyethylene terephthalate. By carrying out the polycondensation reaction under reduced pressure, it is possible to obtain polyethylene terephthalate having a desired intrinsic viscosity (IV).

[0065] The amount of BHET transferred from the esterification reaction tank [1] to the polycondensation reaction tank [2] is preferably 30% by weight or more and 70% by weight or less of the amount of BHET in the esterification reaction tank [1]. The amount of BHET transferred to the polycondensation reaction tank [2] is more preferably 40% by weight or more, and even more preferably 50% by weight or more, so that the amount of polyethylene terephthalate obtained by the polycondensation reaction can be secured. On the other hand, the amount of BHET transferred is more preferably 65% ​​by weight or less, and even more preferably 60% by weight or less, so that the amount of BHET remaining in the esterification reaction tank [1] increases, and when the esterification reaction in the presence of BHET is continuously carried out (from the third stage onwards), the temperature during the esterification reaction in the presence of BHET can be increased, and the esterification reaction can be carried out efficiently.

[0066] In the reaction in the polycondensation reaction tank [2], the final temperature is preferably 285 to 290°C. If the final temperature of the polycondensation reaction is less than 285°C, the polycondensation does not proceed to the desired intrinsic viscosity (IV), which is not preferred. On the other hand, if the final temperature is higher than 290°C, the thermal decomposition of the polyester obtained proceeds, which is not preferred, as the color tone of the polyester obtained in the end deteriorates.

[0067] In the present invention, the temperature is raised to the final temperature of the polycondensation reaction in 60 minutes or more and 120 minutes or less. The time for raising the temperature to the final temperature during the polycondensation reaction is preferably 70 minutes or more, more preferably 80 minutes or more, so that EG can be gradually distilled out of the system and bumping during the polycondensation reaction can be suppressed. On the other hand, the time for raising the temperature is preferably 110 minutes or less, more preferably 100 minutes or less, so that the polycondensation reaction time is not prolonged and deterioration of the color tone of the finally obtained polyester can be suppressed.

[0068] The final pressure of the polycondensation reaction in the present invention is preferably 300 Pa or less. By setting the final pressure to preferably 200 Pa or less, more preferably 100 Pa or less, the polycondensation of BHET obtained in the esterification reaction is promoted, and a polyester having a desired intrinsic viscosity (IV) can be obtained. On the other hand, if the final pressure is higher than 300 Pa, the polycondensation reaction time increases, and the color tone of the finally obtained polyester deteriorates, which is not preferable.

[0069] In the present invention, the pressure reduction to the final pressure of the polycondensation reaction is preferably carried out for 50 minutes or more and 100 minutes or less. By setting the pressure reduction time to 60 minutes or more, EG can be gradually distilled out of the system, and bumping during the polycondensation reaction can be suppressed. On the other hand, by setting the pressure reduction time to 90 minutes or less, the polycondensation reaction time is not prolonged, and deterioration of the color tone of the finally obtained polyester can be suppressed.

[0070] In the polycondensation reaction of the present invention, a catalyst must be added, and lithium compounds, manganese compounds, cobalt compounds, magnesium compounds, calcium compounds, titanium compounds, aluminum compounds, tin compounds, antimony compounds, germanium compounds, etc. are used. Examples of various metal compounds include acetates or their hydrates and oxides. In particular, in terms of bringing about good polycondensation reactivity, it is preferable to mainly use antimony trioxide, germanium dioxide, or tetra-n-butyl titanate, and to auxiliary use lithium acetate, manganese acetate, cobalt acetate, magnesium acetate, or calcium acetate.

[0071] In the production method of the present invention, it is preferable to add a phosphorus compound as a stabilizer. Specifically, phosphoric acid, trimethyl phosphate, diethyl phosphonoethyl acetate, etc. are preferable, and trivalent phosphorus compounds such as 3,9-bis(2,6-di-t-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane (PEP-36: manufactured by Asahi Denka Co., Ltd.) and tris(2,4-di-tert-butylphenyl) phosphite (IRGAFOS168: manufactured by BASF) are more preferable in terms of improving color tone and heat resistance. In order to prevent the phosphorus compound from inhibiting the esterification reaction, it is preferable to add the phosphorus compound from the completion of the esterification reaction to the start of the polycondensation reaction, and it is more preferable to add the phosphorus compound to the polycondensation reaction tank, i.e., in the step (3), in order to prevent the phosphorus compound from remaining in the esterification reaction tank.

[0072] Polyethylene terephthalate having the desired intrinsic viscosity (IV) is withdrawn from the polycondensation reactor [2]. The BHET (BHET obtained by the esterification reaction in the presence of BHET in the third or subsequent stages) is transferred back to the polycondensation reactor [2] and polycondensed, thereby allowing polyethylene terephthalate to be produced continuously.

[0073] [Characteristics of polyester] The intrinsic viscosity IV of the polyester produced by the production method of the present invention is preferably 0.65 to 0.70, more preferably 0.66 to 0.69, which can stabilize the melt spinning behavior and suppress yarn breakage and unevenness in fineness.

[0074] The intrinsic viscosity IV of the polyester was measured by dissolving the polyester in o-chlorophenol solvent to prepare solutions with concentrations of 0.5 g / dL, 0.2 g / dL, and 0.1 g / dL, and measuring the relative viscosity η at 25°C of the solution with concentration C. r was measured using an Ubbelohde viscometer, (η r -1 ) / C against C and extrapolate the results to a concentration of zero.

[0075] The DEG content in the polyester produced by the production method of the present invention can be 3.2% by mass or less, more preferably 2.9% by mass or less, and even more preferably 2.4% by mass or less, making it possible to suppress a decrease in the melting point of the polyester, and to suppress a decrease in hydrolysis resistance, heat resistance, and light fastness, as well as a decrease in dyeability.

[0076] In the present invention, the DEG of a polyester refers to a value measured and calculated by the following method. (i) As a pretreatment, a certain amount of polyester is decomposed at 260°C using 2-aminoethanol as a solvent and 1,6-hexanediol as an internal standard. (ii) After cooling, methanol is added to precipitate the polyester portion, which is then filtered. (iii) The filtrate is subjected to measurement using a gas chromatograph (GC, for example, GC-2025 manufactured by Shimadzu Corporation). (iv) Using the measured values ​​and the calibration curve obtained in (iii) above, the amount of DEG is calculated and rounded off to three decimal places.

[0077] The amount of COOH terminal groups in the polyester produced by the production method of the present invention can be 24.0 eq / t or less, more preferably 18.5 eq / t or less, and even more preferably 17.5 eq / t or less, whereby thermal decomposition during polyester production can be reduced, coloration of the polyester can be reduced, and deterioration of mechanical properties can be suppressed.

[0078] In the present invention, the amount of COOH terminal groups in the polyester refers to a value measured and calculated by the following method. (i) The obtained polyester pellets are dissolved in an o-cresol solvent. (ii) The solution is titrated with an aqueous potassium hydroxide solution using a neutralization titrator (for example, an automatic titrator "COM-A195" manufactured by Hiranuma Sangyo Co., Ltd.). (iii) Calculate the amount of COOH terminal groups from the amount dropped and round off to the second decimal place.

[0079] The melting point of the polyester produced by the production method of the present invention can be 244° C. or higher, more preferably 246° C. or higher, and further preferably 248° C. or higher, whereby the heat resistance of the polyester is improved and the polymer strength is improved.

[0080] In the present invention, the melting point of the polyester refers to a value measured and calculated by the following method. (i) As a pretreatment, the polyester is melted at 290° C. for 5 minutes in a nitrogen stream and then rapidly cooled to room temperature at a rate of 50° C. / min. (ii) The melting point is measured using a differential scanning calorimeter (DSC, for example, "Q-2000" manufactured by TA Instruments) under the following conditions. Heating rate: 16℃ / min Measurement temperature: -20℃ to 300℃.

[0081] The color tone (b value) of the polyester obtained by the production method of the present invention can be 6.5 or less. More preferably, it is 5.0 or less, and even more preferably 4.0 or less. This results in less yellowness of the polymer and serves as an indicator of less thermal decomposition during polyester production, enabling the production of white threads and films without coloring when processed into threads or films.

[0082] In the present invention, the color tone (b value) refers to the value measured by the following method. (i) The polyester is molded into chips and filled into a container made of quartz glass. (ii) Using a Hunter color difference meter (for example, "SM-Color Computer Model SM-3" manufactured by Suga Test Instruments Co., Ltd.), measurement is performed. (iii) For the b value obtained in (ii) above, rounding is performed to the first decimal place.

[0083] The polyester efficiently produced by the production method of the present invention has a low DEG content and can be suitably used for fibers, fiber structures made therefrom, films, and resin applications.

Examples

[0084] Next, the present invention will be specifically described based on examples. However, the present invention is not limited only to these examples.

[0085] [Measurement method] The evaluation methods and measurement conditions used in the examples will be described. In the measurement of each physical property, those without special description were measured based on the above methods.

[0086] <A. Measurement of the intrinsic viscosity (IV) of polyester> For the measurement of the intrinsic viscosity (IV) of polyester, an Ubbelohde viscometer was used and measured and calculated by the above method.

[0087] <B. DEG content of polyester> For the measurement of the DEG content (mass %) of the polyester, using gas chromatography (GC), such as GC-14B manufactured by Shimadzu Corporation, it was measured and calculated by the above method. It was evaluated in five grades from SS to C according to the obtained DEG amount. SS: The DEG amount is 2.00 mass % or less, there is no influence on the melting point, hydrolysis resistance, and dyeability of the polyester, and it can be suitably adopted for all applications. S: The DEG amount is 2.40 mass % or less, there is no influence on the melting point, hydrolysis resistance, and dyeability of the polyester, and it can be suitably adopted for all applications. A: The DEG amount is 2.90 mass % or less, within the range where it does not affect the melting point, hydrolysis resistance, and dyeability of the polyester, and it can be suitably adopted for all applications. B: The DEG amount is 3.20 mass % or less, it slightly affects the melting point, hydrolysis resistance, and dyeability of the polyester, but there is no significant influence, and it can be adopted for all applications. C: The DEG amount is higher than 3.20 mass %, there is a concern that it may affect the melting point, hydrolysis resistance, and dyeability of the polyester, so it is difficult to adopt in applications.

[0088] <C. COOH terminal group amount of polyester> For the measurement of the COOH terminal group amount (eq / t) of the polyester, it was measured and calculated by neutralization titration using sodium hydroxide. It was evaluated in five grades from SS to C according to the obtained COOH terminal group amount. SS: The COOH terminal group amount is 17.0 or less, there is little thermal decomposition, and there is no influence on the color tone and mechanical properties, so it can be suitably adopted for all applications. S: The COOH terminal group amount is 17.5 or less, there is little thermal decomposition, and there is no influence on the color tone and mechanical properties, so it can be suitably adopted for all applications. A: The COOH terminal group amount is 18.5 or less, there is little thermal decomposition, and within the range where it does not affect the color tone and mechanical properties, so it can be suitably adopted for all applications. B: The COOH terminal group amount is 24.0 or less, although there is some thermal decomposition, it slightly affects the color tone and mechanical properties, but there is no significant influence, and it can be suitably adopted for all applications. C: Since the amount of the COOH terminal group is more than 24.0, thermal decomposition occurs, and there is a concern that it may affect the color tone and mechanical properties, so it is difficult to be adopted in applications.

[0089] <D. Measurement of the melting point of polyester> For the measurement of the melting point (°C) of polyester, using a differential scanning calorimeter "Q-2000" manufactured by TA Instruments, it was measured and calculated by the above method. It was evaluated in five grades of SS to C according to the obtained melting point. SS: The melting point is 250 °C or higher, which has no influence on the heat resistance of polyester and can be suitably adopted for all applications. S: The melting point is 248 °C or higher, which has no influence on the heat resistance of polyester and can be suitably adopted for all applications. A: The melting point is 246 °C or higher, which is within the range that does not affect the heat resistance of polyester and can be suitably adopted for all applications. B: The melting point is 244 °C or higher, which has a slight influence on the heat resistance of polyester but no significant influence, and can be adopted for all applications. C: The melting point is lower than 244 °C, and there is a concern that it may affect the heat resistance of polyester, so it is difficult to be adopted in applications.

[0090] <E. Color tone (b value) of polyester composition> For the measurement of the color tone (b value) of polyester, using a Hunter type color difference meter, a colorimeter "SM-T" manufactured by Suga Test Instruments Co., Ltd. (tristimulus value direct reading type, 0° light receiving method), it was measured by the above method. It was evaluated in four grades of S to C according to the obtained b value. S: The b value is 4.0 or less, and there is no influence on coloring after processing into fibers, and it can be suitably adopted for all applications. A: The b value is 5.0 or less, which is within the range that does not affect coloring after processing into fibers, and it can be suitably adopted for all applications. B: The b value is 6.5 or less, and it is slightly colored after processing into fibers, but there is no significant difference from white, so it can be adopted for all applications. Since the C:b value is higher than 6.5 and there is concern about discoloration after processing into fibers, it is somewhat unsuitable for applications requiring a white color.

[0091] [Example 1] (polyester) [Step (1): Esterification reaction under pressure] 100L of ethylene glycol (EG) was added to an empty esterification reactor, and 121g of 20% aqueous tetraethyleneammonium hydroxide (EAH) solution (0.10% by mass relative to polyester) was further added, and the temperature of the esterification reactor was started. The esterification reactor into which the stored EG and EAH had been added was heated to 200°C over 30 minutes, and then heated to 215°C over 45 minutes. During the temperature rise, the pressure was not controlled, and the reactor was slightly pressurized with EG steam. After the temperature inside the esterification reactor reached 215°C, the pressure inside the esterification reactor was pressurized to 0.2MPa, and pressure control was started so that the pressure was 0.19MPa or more and 0.21MPa or less. A mixed slurry (molar ratio: 1.20) of 130kg of terephthalic acid and 52.4L of EG was continuously added over 220 minutes, while the generated water was distilled off to carry out an esterification reaction under pressure, and bis(hydroxyethyl)terephthalate (BHET) was obtained.

[0092] [Step (2): EG Expulsion Reaction] The pressure in the esterification reactor was released over 20 minutes to 0.10 MPa (normal pressure), and then the temperature in the esterification reactor was raised from 215°C to 245°C over 40 minutes while carrying out an EG removal reaction for 45 minutes under total distillation conditions, and 85 L of EG was distilled off.

[0093] [Step (2): Esterification reaction in the presence of BHET] A mixed slurry of 130 kg of terephthalic acid and 52.4 L of EG (molar ratio: 1.20) was continuously added to the BHET remaining in the esterification reaction tank after the EG drive-off reaction over a period of 220 minutes under temperature control at 245°C and 0.10 MPa (normal pressure), while the generated water was distilled off, thereby carrying out an esterification reaction in the presence of BHET, and BHET was obtained.

[0094] [Step (3): Polycondensation reaction] 55% by weight of the BHET obtained after the esterification reaction in the presence of BHET in step (2) was transferred to a polycondensation reaction tank. After the temperature was raised to 245°C, 28 g of an 85% aqueous phosphoric acid solution as a stabilizer and 45 g of antimony trioxide as a polycondensation catalyst were added, the pressure was reduced to 50 Pa over 60 minutes, and the temperature was raised to 290°C over 90 minutes, allowing the polycondensation reaction to proceed until the desired melt viscosity was reached.

[0095] The polycondensation tank was purged with nitrogen to return the pressure to normal pressure to terminate the polycondensation reaction, and the polyester was extruded from a die in the form of a strand, cooled in a water tank, and cut into pellets to obtain polyester chips.

[0096] The resulting polyester had an intrinsic viscosity (IV) of 0.68, a DEG content of 3.00 mass%, a COOH end group content of 18.7 eq / t, a melting point of 246° C., and a color tone b value of 5.1.

[0097] [Examples 2 to 6] Polyesters were obtained in the same manner as in Example 1, except that the amount of 20% aqueous tetraethylammonium hydroxide (EAH) solution added was changed as shown in Table 1. The results are shown in Table 1.

[0098] By increasing the EAH concentration from 0.10% by mass (Example 1) to 0.50% by mass (Example 3), the amount of DEG tended to decrease from 3.00% by mass (Example 1) to 2.06% by mass (Example 3). On the other hand, by increasing the amount of EAH to 0.80% by mass (Example 6), the amount of DEG tended to increase to 3.20% by mass (Example 6). Furthermore, with an increase in the amount of DEG, there was a tendency for the amount of COOH terminals to increase, the melting point to decrease, and the b value to increase.

[0099] [Examples 7 to 11] Polyesters were obtained in the same manner as in Example 3, except that the pressure during the esterification reaction under pressure was changed as shown in Table 1. The results are shown in Table 1.

[0100] When the pressure was reduced from 0.20 MPa (Example 3) to 0.11 MPa (Example 7), the reaction time increased from 310 minutes (Example 3) to 430 minutes (Example 7), and the DEG amount tended to increase from 2.06% by mass (Example 3) to 3.20% by mass (Example 7). On the other hand, when the pressure during the esterification reaction was increased from 0.20 MPa (Example 3) to 0.50 MPa (Example 11), the reaction time decreased from 310 minutes (Example 3) to 270 minutes (Example 11), but the DEG amount tended to increase from 2.06% by mass (Example 3) to 3.10% by mass (Example 11).

[0101] [Table 1]

[0102] [Examples 12 to 17] Polyesters were obtained in the same manner as in Example 3, except that the temperature during the esterification reaction under pressure was changed as shown in Table 2. The results are shown in Table 2.

[0103] When the reaction temperature was lowered from 215°C (Example 3) to 200°C (Example 12), the reaction time increased from 310 minutes (Example 3) to 430 minutes (Example 12), and the DEG amount tended to increase from 2.06% by mass (Example 3) to 3.20% by mass (Example 12). On the other hand, when the temperature during the esterification reaction was increased from 215°C (Example 3) to 250°C (Example 17), the reaction time was shortened from 310 minutes (Example 3) to 265 minutes (Example 17), but the EG dimerization reaction proceeded under high temperature conditions, and the DEG amount tended to increase from 2.06% by mass (Example 3) to 3.18% by mass (Example 17).

[0104] [Examples 18 and 19] Polyesters were obtained in the same manner as in Example 3, except that the quaternary ammonium compound type was changed to tetramethylammonium hydroxide (Example 18) or tetrapropylammonium hydroxide (Example 19) as shown in Table 2. The results are shown in Table 2.

[0105] By changing the quaternary ammonium compound from EAH (Example 3) to tetramethylammonium hydroxide (Example 18), there was no change in the esterification reaction and polycondensation reaction status, but the change in the quaternary ammonium compound type reduced the basicity of the initial esterification reaction under pressure, so the EG dimerization reaction proceeded and the amount of DEG increased to 2.47 mass% (Example 18). On the other hand, by changing the quaternary ammonium compound from EAH (Example 3) to tetrapropylammonium hydroxide (Example 19), there was no change in the esterification reaction and polycondensation reaction status, but the change in the quaternary ammonium compound type increased the amount of BHET produced, which has a low degree of polymerization, so thermal decomposition proceeded and the amount of DEG increased to 2.50 mass% (Example 19).

[0106] [Table 2]

[0107] [Example 20] In step (2), after the EG purging reaction under normal pressure, the EG purging reaction was carried out under conditions of 245°C and 0.06 MPa (reduced pressure) for 45 minutes, and 14 L of EG was distilled off, to obtain a polyester in the same manner as in Example 3. The results are shown in Table 3.

[0108] By carrying out the EG removal reaction under reduced pressure, there was no change in the esterification reaction and polycondensation reaction conditions; however, the amount of surplus EG during the esterification reaction was reduced due to the increased amount of EG distilled off, which suppressed the EG dimerization reaction and reduced the DEG amount to 1.96 mass%.

[0109] [Example 21] A polyester was obtained in the same manner as in Example 20, except that the amount of ethylene glycol (EG) added to the empty esterification reaction tank was changed to 80 L. The results are shown in Table 3.

[0110] Reducing the amount of stored EG did not change the status of the pressurized esterification reaction, esterification reaction, or polycondensation reaction; however, the reduction in the molar ratio during the pressurized esterification reaction reduced the amount of excess EG during the pressurized esterification reaction, suppressed the EG dimerization reaction, and reduced the DEG amount to 1.89% by mass.

[0111] [Examples 22 to 24] In step (1), ethylene glycol (EG) and 20% tetraethyleneammonium hydroxide (EAH) were charged into an empty esterification reaction tank, and then terephthalic acid was added all at once, the molar ratio in the esterification reaction tank was changed to 2.1 (Example 22), 1.8 (Example 23), or 1.4 (Example 24) as shown in Table 3, the temperature during the esterification reaction under pressure was changed to 230° C., and the reaction time was changed to 330 minutes, but the same procedure as in Example 20 was carried out to obtain a polyester. The results are shown in Table 3.

[0112] By reducing the molar ratio in the esterification reaction tank, the temperature and reaction time during the pressurized esterification reaction changed, but the reaction proceeded without any problems, and there was no change in the esterification reaction and polycondensation reaction status. By reducing the molar ratio during the pressurized esterification reaction, the amount of excess EG during the pressurized esterification reaction was reduced, the EG dimerization reaction was suppressed, and the DEG amount was reduced from 1.96 mass% (Example 20) to 1.35 mass% (Example 29).

[0113] [Table 3]

[0114] [Comparative Examples 1 and 2] Polyesters were obtained in the same manner as in Example 3, except that EAH was not added (Comparative Example 1) and the amount of EAH added was changed to 1.00 mass % (Comparative Example 2). The results are shown in Table 4.

[0115] In the polyester obtained in Comparative Example 1, since no quaternary ammonium compound was added, the obtained polyester had a high DEG content due to the progress of the EG dimerization reaction, and was inferior in terms of the amount of COOH terminal groups, color tone, and melting point.

[0116] The polyester obtained in Comparative Example 2 had a high concentration of quaternary ammonium compounds and strong basicity in the system, which inhibited the reaction between terephthalic acid and EG, increased the amount of BHET with a low degree of polymerization, and increased the amount of DEG produced by thermal decomposition. As a result, the polyester had a high amount of DEG and was inferior in terms of the amount of COOH terminal groups, color tone, and melting point.

[0117] [Comparative Examples 3 and 4] The polyester was obtained in the same manner as in Example 3, except that the pressure during the first-stage esterification reaction was changed to 0.10 MPa (normal pressure) (Comparative Example 3) or 0.60 MPa (Comparative Example 4). The results are shown in Table 4.

[0118] In Comparative Example 3, the esterification reaction was carried out under normal pressure, so the reaction temperature could not be increased, and the reaction time was long, resulting in the obtained polyester being inferior in DEG amount, COOH terminal group amount, color tone, and melting point.

[0119] In Comparative Example 4, the pressure during the esterification reaction was high, which facilitated the progression of EG dimerization, and the resulting polyester was inferior in DEG amount, COOH terminal group amount, color tone, and melting point.

[0120] [Comparative Examples 5 and 6] Polyesters were obtained in the same manner as in Example 3, except that the temperature during the esterification reaction under pressure was 190° C. (Comparative Example 5) or 260° C. (Comparative Example 6). The results are shown in Table 4.

[0121] In Comparative Example 5, the reaction temperature was too low, so the reaction between terephthalic acid and EG did not proceed, and polyester was not obtained.

[0122] In Comparative Example 6, the reaction temperature was high, which facilitated the progression of EG dimerization, and the resulting polyester was inferior in DEG amount, COOH terminal group amount, color tone, and melting point.

[0123] [Comparative Examples 7, 8, and 9] The polyester was obtained in the same manner as in Example 3, except that the timing of adding EAH was changed to immediately after the esterification reaction (Comparative Example 7), before the polycondensation reaction (Comparative Example 8), and after the polycondensation reaction (Comparative Example 9). The results are shown in Table 4.

[0124] In Comparative Example 7, EAH was added as a quaternary ammonium compound immediately after the esterification reaction under pressure. EAH was added to the BHET produced by the esterification reaction, and the DEG production during the reaction of terephthalic acid and EG could not be suppressed. As a result, the obtained polyester was inferior in DEG amount, COOH end group amount, color tone, and melting point.

[0125] In Comparative Example 8, EAH was added after BHET was transferred to the polycondensation reaction tank, and the production of DEG during the esterification reaction could not be suppressed, so the obtained polyester was inferior in terms of the amount of DEG, the amount of COOH terminal groups, the color tone, and the melting point.

[0126] In Comparative Example 9, EAH was added after the polycondensation reaction, and the DEG production could not be suppressed, so the obtained polyester was inferior in the DEG amount, COOH terminal group amount, color tone, and melting point.

[0127] [Comparative Examples 10 and 11] The polyester was obtained in the same manner as in Example 20 or Example 22, except that the molar ratio in the esterification reaction tank was changed to 3.7 (Comparative Example 10) or 13 (Comparative Example 11). The results are shown in Table 4.

[0128] In Comparative Example 10, the molar ratio in the esterification reaction tank was too high, which facilitated the EG dimerization reaction, and the obtained polyester was inferior in DEG amount, COOH terminal group amount, color tone, and melting point.

[0129] In Comparative Example 11, the molar ratio in the esterification reaction tank was too low, and the esterification reaction time was delayed, which caused the EG dimerization reaction to proceed, resulting in the obtained polyester being inferior in DEG amount, COOH terminal group amount, color tone, and melting point.

[0130] [Table 4]

Claims

1. A method for producing polyester by polycondensing terephthalic acid and ethylene glycol as main raw materials, the method being characterized by going through the following three steps. (1) A process for producing bis(hydroxyethyl)terephthalate by feeding only terephthalic acid, ethylene glycol, and a quaternary ammonium compound into an esterification reaction tank [1], adjusting the concentration of the quaternary ammonium compound to 0.10% by mass or more and 0.80% by mass or less, and controlling the pressure in the esterification reaction tank to 0.11 MPa or more and 0.50 MPa or less. (2) After the step (1), the ethylene glycol remaining in the esterification reaction tank [1] is distilled off, and then terephthalic acid and ethylene glycol are added to carry out a second-stage esterification reaction in the presence of bis(hydroxyethyl)terephthalate, thereby producing bis(hydroxyethyl)terephthalate. (3) A step of transferring bis(hydroxyethyl) terephthalate from the esterification reaction tank [1] to a polycondensation reaction tank [2] and further promoting polycondensation under reduced pressure to produce polyethylene terephthalate.

2. 2. The method for producing polyester according to claim 1, wherein in step (1), ethylene glycol and a quaternary ammonium compound are charged into an esterification reaction tank, and then terephthalic acid or a mixed slurry of terephthalic acid and ethylene glycol is added.

3. 3. The method for producing a polyester according to claim 1, wherein the quaternary ammonium compound is a quaternary ammonium hydroxide.

4. 3. The method for producing a polyester according to claim 1, wherein the reaction temperature during the esterification reaction in the step (1) is 200° C. or higher and 250° C. or lower.

5. The method for producing a polyester according to claim 1 or 2, wherein the amount of the quaternary ammonium compound added is 0.30% by mass or more and 0.70% by mass or less.

6. 3. The method for producing a polyester according to claim 1, wherein in the step (1), after adding terephthalic acid, ethylene glycol, and a quaternary ammonium compound, a molar ratio of terephthalic acid to ethylene glycol calculated by the following formula is 1.4 or more and 3.5 or less. Molar ratio in esterification reaction vessel=(molar amount of EG component) / (molar amount of terephthalic acid component)

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