Polyester manufacturing method

Abstract

The present invention provides a method for producing high-quality polyester quickly, without problems of foreign matter contamination, and with good color tone, when restarting a polyester manufacturing plant after a shutdown. [Solution] When restarting a polyester manufacturing plant after it has been shut down, the following steps 1 to 4 are performed in order. Step 1: A process in which strands A extruded from molten polyester A from die head A are cut to obtain pellets A containing more than 10 carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 2: A process in which strand B, extruded from die head A from molten polyester B, is cut to obtain pellets B containing 10 or fewer carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 3: Switching die head A to die head B Step 4: A process in which molten polyester C is extruded from die head B, and the strand C is cut to obtain pellets C.

Description

Technical Field

[0001] The present invention relates to a method for producing polyester. Specifically, it relates to a method for early production of polyester with good color tone and high quality when the operation of a polyester production plant is stopped and then restarted.

Background Art

[0002] Polyesters typified by polyethylene terephthalate and polybutylene terephthalate are crystalline thermoplastic resins having excellent mechanical and chemical properties. Among them, polybutylene terephthalate is an engineering plastic excellent in electrical properties, chemical resistance, moldability, and coloring property. Taking advantage of these features, polybutylene terephthalate is widely used as a material for electrical and electronic fields, automotive fields, high-functional films, etc.

[0003] Polyesters are industrially produced by batch or continuous methods. In the industrial production of polyesters, maintenance such as inspection and cleaning of equipment is regularly performed to safely carry out the production. To perform this regular maintenance, it is necessary to stop (shut down) the operation of the polyester production plant. After the maintenance is completed, the polyester production plant is restarted (started up) to resume the production of polyester. When restarting this polyester production plant, as the startup of the plant, it is required to quickly achieve the same quality as before shutdown for the produced polyester. The same quality as before shutdown means that there is no contamination of foreign matter in the resin and the color tone of the resin is good. Here, examples of foreign matter mixed into the resin include sublimates and cross-linked products of the charged raw materials generated in the polyester production plant before the operation stop, which undergo cross-linking reactions or carbonization reactions due to the high-temperature heat history in the polyester production process and precipitate on the inside and wall surfaces of the equipment as carbonized deposits larger than 0.3 mm.

[0004] Traditionally, when restarting a polyester manufacturing plant, cleaning of polymerization tanks and other equipment within the plant is performed during maintenance in order to quickly achieve the same quality as before the shutdown. Regarding cleaning methods for polyester manufacturing plants, a method is known in which the inside of the polymerization tank of a polyester melt polymerization apparatus is cleaned using a cleaning composition in which phosphoric acid is added to a mixed glycol consisting of polyalkylene glycol and alkylene glycol (Patent Document 1). In addition, a method is known in which esterification reaction tanks, polycondensation reaction tanks, etc. are cleaned using an aqueous solution of an acid whose acid dissociation constant in water or first acid dissociation constant in water is 0.9 or more and less than 4 (Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-80342 [Patent Document 2] Japanese Patent Publication No. 2017-095620 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, even when employing the cleaning methods disclosed in Patent Documents 1 and 2, it was not possible to quickly achieve the same quality as the polyester before the shutdown after restarting the polyester manufacturing plant, i.e., the absence of the aforementioned problem of foreign matter contamination and the same good color tone as before the shutdown. Therefore, the effects were not satisfactory. After the polyester manufacturing plant is restarted, it cannot be harvested as a product resin until the quality of the manufactured polyester stabilizes to the same level as before the shutdown. Therefore, it is desirable to produce polyester of the same quality as before the shutdown as soon as possible after the polyester manufacturing plant is restarted.

[0007] This invention has been made in view of these circumstances, and aims to provide a method for producing polyester that, when restarting a polyester manufacturing plant after it has been shut down, is free from the problem of foreign matter contamination, has good color tone and other characteristics, and can produce high-quality polyester quickly. [Means for solving the problem]

[0008] The inventors of this invention conducted extensive research to solve the above problems and, as a result, discovered that when restarting a polyester manufacturing plant after a shutdown, the problems can be solved by switching the die head after manufacturing polyester in which the number of foreign objects mixed in the polyester is below a predetermined level. This led to the completion of the present invention. In other words, the present invention relates to the following invention.

[0009] [1] In a polyester manufacturing plant, a molten polyester obtained by reacting a dicarboxylic acid component with a diol component is extruded from a die head in the form of strands, and the resulting strands are cut to obtain pellets, A method for manufacturing polyester, comprising sequentially performing the following steps 1 to 4 when restarting a polyester manufacturing plant after it has been shut down. Step 1: A process in which molten polyester A is extruded from a die head A to obtain strand A, and the obtained strand A is cut to obtain pellet A containing more than 10 carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 2: Molten polyester B is extruded from die head A to obtain strand B, and the obtained strand B is cut to obtain pellet B containing 10 or fewer carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 3: The process of switching the die head that extrudes molten polyester from die head A to die head B. Step 4: A process in which molten polyester C is extruded from die head B to obtain strand C, and the obtained strand C is cut to obtain pellet C.

[0010] [2] The method for producing polyester according to [1], wherein the molten polyester in the die head A of step 3 contains one or more carbonized precipitates of 1 mm or more per 200 g of polyester.

[0011] [3] The method for producing polyester according to [1] or [2], wherein the carbonized precipitate is a carbide of polyester.

[0012] [4] A method for producing a polyester according to any one of [1] to [3], wherein the dicarboxylic acid component is terephthalic acid, the diol component is alkylenediol, and the polyester is polyalkylene terephthalate.

[0013] [5] The method for producing polyester according to [4], wherein the alkylenediol is 1,4-butanediol and the polyalkylene terephthalate is polybutylene terephthalate.

[0014] [6] The method for producing polyester according to [4] or [5], wherein the terephthalic acid is terephthalic acid produced from naphtha produced from biological resources, terephthalic acid produced from recycled naphtha produced from waste resin, or terephthalic acid produced by depolymerization of polyester.

[0015] [7] The method for producing polyester according to [5] or [6], wherein the 1,4-butanediol is a 1,4-butanediol produced from naphtha produced from biological resources, a 1,4-butanediol produced from recycled naphtha produced from waste resin, a 1,4-butanediol produced by direct fermentation of sugar, or a 1,4-butanediol produced by hydrogen reduction of succinic acid or a succinic acid derivative produced using biological resources. [Effects of the Invention]

[0016] According to the method for producing a polyester of the present invention, when restarting the operation after the operation stop of a polyester production plant, there is no problem of foreign matter contamination, the color tone and the like are good, and high-quality polyester can be produced at an early stage.

Brief Description of the Drawings

[0017] [Figure 1] It is a schematic cross-sectional view showing the structure of a die head.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example of an embodiment of the present invention, and the present invention is not limited to the following description as long as it does not exceed the gist thereof. In this specification, when the expression "~" is used, it shall be used as an expression including the numerical values or physical property values before and after it. In the present invention, the "main component" in the dicarboxylic acid component means a component contained in the component at 50 mol% or more. The same applies to the "main component" in the diol component.

[0019] The method for producing a polyester of the present invention is a method for producing a polyester in which a molten polyester obtained by reacting a dicarboxylic acid component and a diol component is discharged from a die head in a strand shape in a polyester production plant, and the obtained strand is cut to obtain pellets. When restarting the operation after the operation stop of the polyester production plant, the following steps 1 to 4 are sequentially performed. Step 1: A step of discharging molten polyester A from die head A to obtain strand A, cutting the obtained strand A, and obtaining pellet A containing more than 10 carbonized deposits larger than 0.3 mm per 200 g of pellets Step 2: A step of discharging molten polyester B from die head A to obtain strand B, cutting the obtained strand B, and obtaining pellet B containing 10 or less carbonized deposits larger than 0.3 mm per 200 g of pellets Step 3: The process of switching the die head that extrudes molten polyester from die head A to die head B. Step 4: A process in which molten polyester C is extruded from die head B to obtain strand C, and the obtained strand C is cut to obtain pellet C.

[0020] [mechanism] In the present invention, as described above, when restarting a polyester manufacturing plant after it has been shut down, the mechanism by which the die head can be switched after manufacturing polyester in which the number of foreign matter (carbonized precipitates larger than 0.3 mm) is below a predetermined level, thereby eliminating the problem of foreign matter contamination and enabling the early production of high-quality polyester with good color tone, etc., is as follows.

[0021] Normally, after a polyester manufacturing plant is shut down, the polymerization tanks and other equipment within the plant are cleaned before restarting. However, cleaning with conventional cleaning compositions alone is not sufficient, and foreign matter from before the shutdown remains on the walls of the equipment within the polyester manufacturing plant. When a polyester manufacturing plant is restarted and polyester production resumes, any foreign matter remaining inside the plant is drawn into the flow of molten polyester and moves downstream in the direction of the flow, gradually being discharged from the plant. However, larger pieces of foreign matter remain inside the die head instead of being discharged from the die plate.

[0022] In other words, in a polyester manufacturing plant, the die head that extrudes molten polyester in strand form generally has the cross-sectional shape shown in Figure 1. A die plate 3 is provided at the tip opening (discharge side opening) of the bell-shaped die head body 1A via a mesh-like distributor 2, and the molten polyester fed into the die head 1 is extruded in strand form from numerous die holes (discharge ports) 3a provided in the die plate 3. Of the foreign matter that gets mixed into the molten polyester produced after the polyester manufacturing plant is restarted and flows downstream, very small pieces (those that pass through the die hole 3a) are mixed into the discharged strand and discharged from the die head 1, but larger foreign matter that does not pass through the die hole 3a remains on the die plate 3 inside the die head 1. Therefore, once the discharge of small foreign matter is complete, almost no foreign matter remains in the discharged strand. At first glance, this state may appear to indicate that the incorporation and discharge of foreign matter has ended, but larger foreign matter remains in the die head 1. As polyester production continues, this larger foreign matter is physically crushed into smaller pieces by the molten polyester flowing into the die head 1, and passes through the die hole 3a. In other words, foreign matter begins to be mixed into the strand again. Thus, even after the removal of small foreign objects is complete, the process continues through the crushing of larger foreign objects and subsequent removal of the crushed foreign objects. As a result, after restarting operations, foreign objects will continue to be mixed into the polyester being manufactured over a long period of time.

[0023] In this invention, after the polyester manufacturing plant is restarted, small foreign matter is discharged (Step 1), and then, when the discharge of small foreign matter is almost complete (after Step 2), the large foreign matter accumulated inside the die head is removed from the die head by replacing the entire die head before it is crushed by the flow of molten polyester (Step 3). This prevents large foreign matter remaining in the die head from being crushed and discharged from the die head by molten polyester from upstream, thus eliminating the problem of foreign matter contamination and enabling the early acquisition of high-quality polyester (polyester C) with good color tone and other characteristics. The process of switching die heads will be explained later.

[0024] [Carbide precipitates larger than 0.3 mm] Before describing each process and each polyester in this invention, we will first explain the "carbide precipitates larger than 0.3 mm" that are evaluated as "foreign matter" in this invention.

[0025] In this invention, the "foreign matter" mixed into polyester is defined as "carbide precipitates larger than 0.3 mm," that is, "carbide precipitates whose maximum length is greater than 0.3 mm." As mentioned above, these carbide precipitates are polyester carbides that precipitate inside the equipment or on the equipment walls when sublimated or crosslinked raw materials generated in the polyester manufacturing plant before shutdown undergo crosslinking or carbonization reactions due to high-temperature thermal history during the polyester manufacturing process, and are usually black or brown in color. For example, if there is one or more "carbide precipitates larger than 0.3 mm," it is evaluated as "foreign matter (carbide precipitates larger than 0.3 mm) is mixed in," that is, "contains foreign matter (carbide precipitates larger than 0.3 mm)."

[0026] The number of carbide precipitates larger than 0.3 mm is the number of carbide precipitates larger than 0.3 mm contained in 200 g of polyester pellets. This can be counted using a foreign matter inspection device or by visual inspection, but from the viewpoint of detection accuracy, it is preferable to use a foreign matter inspection device. Commercially available foreign object inspection devices can be used, such as the pellet / powder foreign object inspection device "PGI-IV" manufactured by Huebrain Co., Ltd. The measurement principle of this "PGI-IV" is as follows: First, 200g of pellets are photographed one by one using a CCD camera. Image processing detects any black or brownish foreign objects larger than 0.3mm, which are then separated and counted. By performing this separation automatically, the number of foreign objects, i.e., "carbonized precipitates larger than 0.3mm," contained in 200g of polyester pellets is calculated. In this embodiment, "polyester containing carbonized precipitates (foreign matter) larger than 0.3 mm" refers to polyester in which, for example, one or more carbonized precipitates (foreign matter) larger than 0.3 mm are present per 200 g of pellets in the foreign matter inspection described above, and "polyester not containing carbonized precipitates (foreign matter) larger than 0.3 mm" refers to polyester in which the number of carbonized precipitates (foreign matter) larger than 0.3 mm per 200 g of pellets is 0 (does not contain carbonized precipitates (foreign matter) larger than 0.3 mm).

[0027] [Steps 1~4] The following describes each process and the polyester used in each process.

[0028] <Process 1> Step 1 is a process in which the polyester manufacturing plant is shut down and then restarted to produce polyester A containing carbide precipitates larger than 0.3 mm, i.e., foreign matter, which is then molded into strand A using die head A, and the resulting strand A is cut to produce pellet A containing more than 10 carbide precipitates larger than 0.3 mm.

[0029] Polyester A is a polyester containing a large number of foreign matter, with more than 10 carbonized precipitates larger than 0.3 mm per 200 g of pellets. There is no particular upper limit on the number of carbonized precipitates larger than 0.3 mm per 200 g of pellets in Polyester A.

[0030] The intrinsic viscosity of polyester A is preferably equivalent to the intrinsic viscosity of the target polyester (i.e., polyester C). The intrinsic viscosity of the target polyester is preferably 0.650 dL / g or higher, particularly 0.660 dL / g or higher, especially 0.670 dL / g or higher, and most preferably greater than 0.800 dL / g, since it is formed into strands by a die head and then cut into pellets. The target intrinsic viscosity of polyester C for manufacturing is as described below. The intrinsic viscosity of polyester can be measured by the method described in the Examples section below.

[0031] <Process 2> Step 2 is a process in which, after producing polyester A in Step 1, polyester B is produced, molded with die head A to form strand B, and then the resulting strand B is cut to produce pellet B containing 10 or fewer carbide precipitates larger than 0.3 mm per 200 g of pellets. The number of carbonized precipitates larger than 0.3 mm in 200 g of pellet B should be 10 or less. While a smaller number of carbonized precipitates larger than 0.3 mm in 200 g of pellet B tends to reduce the amount of foreign matter contamination in polyester C obtained in step 4, excessively reducing the number of carbonized precipitates larger than 0.3 mm in 200 g of pellet B would require a long time in step 2, potentially undermining the objective of the present invention, which is to produce high-quality polyester quickly. From this perspective, the number of carbide precipitates larger than 0.3 mm per 200 g of polyester B pellets is preferably 9 or less, particularly 8 or less, and especially 5 or less, with 1 or more being preferable.

[0032] The intrinsic viscosity of polyester B may be different from or the same as that of polyester A, but it is preferable that it be the same in order to stably produce polyester B.

[0033] <Process 3> Step 3 is the process of switching from die head A to die head B after the production of polyester B in which the number of carbonized precipitates larger than 0.3 mm in 200 g of pellet B is 10 or less. Die head B is an unused die head, i.e., a new or thoroughly cleaned die head with a high degree of cleanliness.

[0034] In this invention, since the die head is switched in this manner, it is preferable to have two or more switchable die heads arranged in parallel in the strand discharge section of the polyester manufacturing plant. Furthermore, it is preferable to design the system so that the destination of the molten polyester flow can be switched by valve operation or the die head can be switched by handle operation, in order to quickly switch the die head. As shown in Figure 1, the die head 1 is equipped with a die plate 3 for extruding polyester in strand form. The die plate 3 can be removed from the die head body 1A after switching. The die plate 3 has multiple die holes 3a to extrude multiple strands simultaneously. The size of the die holes 3a is equal to or greater than the diameter of the strand. Typically, the diameter of the die holes 3a is 1 mm or more, for example, about 2 to 20 mm.

[0035] After switching, when die plate A is removed from die head A, the removed die plate A contains foreign matter larger than the diameter of the distributor's pores that could not pass through the die holes along with the polyester, such as carbide precipitates larger than 1 mm, at a rate of one or more per 200 g of polyester in die head A. The switch from die head A to die head B is preferable to occur after the production of polyester B containing 10 or fewer carbide precipitates larger than 0.3 mm per 200 g of pellets, from the viewpoint of efficiently collecting carbide precipitates larger than 1 mm on die head A. As mentioned above, it is preferable to switch after the production of polyester B containing 9 or fewer carbide precipitates larger than 0.3 mm per 200 g of pellets, and it is particularly preferable to switch after the production of polyester B containing 8 or fewer carbide precipitates larger than 0.3 mm.

[0036] <Step 4> Step 4 is the process of extruding polyester C using the switched die head B to form strand C, and then cutting the resulting strand C to obtain pellet C. Polyester C is the target polyester for production in this invention, and is a polyester that is free of foreign matter and has a good color tone. The intrinsic viscosity of polyester C may be different from or equivalent to that of polyester A or polyester B, but it is preferable that it be equivalent in order to stably produce polyester C.

[0037] Regarding the color tone of polyester, the presence or absence of coloring may be confirmed by visual observation, or the b-value of the Lab color system may be measured using a commercially available colorimeter, and those with a b-value within a predetermined range may be evaluated as uncolored, and those outside the predetermined range as colored. However, from the viewpoint of preventing judgment errors by workers, evaluation based on the b-value is preferred. The b-value can be measured by the method described in the Examples section below.

[0038] A lower b-value is preferable as it results in less yellowness. From this perspective, a b-value of 2 or less is preferable. On the other hand, if the b-value is excessively low, although the yellowness is reduced, the bluish tint increases, which is undesirable. From this perspective, a b-value of -2 or higher is preferable. In other words, if the b value is between -2 and 2, the color tone can be judged as good.

[0039] When polyester C is used in compounding or injection molding, the intrinsic viscosity of polyester C is usually preferably between 0.800 dL / g and 1.300 dL / g. If the intrinsic viscosity is too low, the mechanical strength of the molded product may be insufficient, and if it exceeds 1.300 dL / g, the melt viscosity becomes high, fluidity deteriorates, and moldability tends to worsen. The intrinsic viscosity of polyester C is more preferably between 0.800 dL / g and 1.260 dL / g, and even more preferably between 0.800 dL / g and 1.200 dL / g.

[0040] When polyester C is used for extrusion applications of films, sheets, or filaments, the intrinsic viscosity of polyester C is preferably 1,000 to 1,600 dL / g, more preferably 1,030 to 1,500 dL / g, even more preferably 1,050 to 1,550 dL / g, particularly preferably 1,100 to 1,500 dL / g, and especially preferably 1,150 to 1,350 dL / g. If the intrinsic viscosity is less than 1,000 dL / g, the extrusion moldability deteriorates, leading to resin drawdown and molding losses, resulting in insufficient mechanical strength of extruded products such as films, or the melt viscosity becomes low, and the fluidity becomes too high, which tends to worsen the extrusion moldability. On the other hand, if the intrinsic viscosity exceeds 1,600 dL / g, the melt viscosity becomes high, the fluidity deteriorates, and the extrusion moldability tends to worsen.

[0041] The color b value of polyester C is as described above, and is preferably 2 or less and -2 or greater.

[0042] [Method for manufacturing polyester] One method for producing polyester according to the present invention is to esterify or transesterify a dicarboxylic acid component and a diol component in the presence of an esterification catalyst or a transesterification catalyst. The present invention will primarily describe a method for producing polyester using terephthalic acid as the dicarboxylic acid component and 1,4-butanediol (hereinafter sometimes abbreviated as "BDO") as the diol component, with polybutylene terephthalate (hereinafter sometimes abbreviated as "PBT") as the polyester. However, according to the present invention's method for producing polyester, polyesters other than PBT, such as polyethylene terephthalate (hereinafter sometimes abbreviated as "PET"), can be produced using diol components other than 1,4-butanediol, in a similar manner to the method described below.

[0043] In the following, polyester produced by the polyester production method of the present invention may be referred to as "the polyester of the present invention," and PBT produced by the polyester production method of the present invention may be referred to as "the PBT of the present invention."

[0044] PBT refers to a polymer having a structure in which dicarboxylic acid and diol components are ester-bonded, where the main component of the dicarboxylic acid, i.e., 50 mol% or more, is terephthalic acid, and the main component of the diol, i.e., 50 mol% or more, is BDO. The proportion of terephthalic acid in the total dicarboxylic acid is preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 95 mol% or more, and the proportion of BDO in the total diol is preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 95 mol% or more. If the terephthalic acid component or BDO is less than 50 mol%, the crystallization rate of PBT decreases, leading to deterioration of moldability.

[0045] The production method for manufacturing the PBT of the present invention may be continuous or batch, but continuous is preferred.

[0046] The PBT of the present invention can be produced by conventional methods. For example, it is produced by a process of mixing a dicarboxylic acid component mainly composed of terephthalic acid and a diol component mainly composed of BDO in a predetermined ratio under stirring to form a raw material slurry; then heating the raw material slurry under normal or reduced pressure to cause an esterification reaction to form a low-polymer polyester (oligomer); and then gradually reducing the pressure of the obtained oligomer and heating it to cause a melt polycondensation reaction to obtain PBT. As described above, the method for producing PBT according to the present invention is not limited, but one example is a method that involves the following esterification reaction step and polycondensation reaction step.

[0047] Furthermore, the terephthalic acid used as a raw material is not limited to terephthalic acid produced from fossil fuels, but may also be terephthalic acid produced from naphtha produced from biological resources, terephthalic acid produced from recycled naphtha produced from waste resins, or terephthalic acid produced by the depolymerization of polyester, or a mixture of two or more of these. Furthermore, BDO is not limited to BDO produced from fossil fuels, but may also be BDO produced from naphtha produced from biological resources, BDO produced from recycled naphtha produced from waste resins, BDO produced by direct fermentation of sugars, or BDO produced by hydrogen reduction of succinic acid or succinic acid derivatives produced using biological resources, or a mixture of two or more of these. Examples of succinic acid derivatives include succinic anhydride, succinic acid esters such as dialkyl succinate (more specifically, dialkyl succinate with an alkyl group having 1 to 4 carbon atoms, preferably 1 to 3, more preferably 1 to 2, and most preferably 1 carbon atom), etc.

[0048] <Dicarboxylic acid component> There are no particular restrictions on the dicarboxylic acid components other than terephthalic acid that can be subjected to the esterification reaction. Examples include aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-benzophenonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, and 2,6-naphthalenedicarboxylic acid; alicyclic dicarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid; and aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. These dicarboxylic acid components other than terephthalic acid may be used individually or as a mixture of two or more.

[0049] These dicarboxylic acid components may be obtained from naphtha produced from petroleum, naphtha produced from biological resources, or those obtained by depolymerization of polyester.

[0050] From the viewpoint of more effectively obtaining the effects of the present invention, the proportion of terephthalic acid in the total dicarboxylic acid components is 50 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 95 mol% or more, and may be 100 mol%.

[0051] Furthermore, when PBT is produced by a transesterification reaction using a dicarboxylic acid derivative as the dicarboxylic acid component, examples of terephthalic acid derivatives include terephthalic acid esters such as dimethyl terephthalate and ester-forming derivatives such as terephthalic acid halides. In addition, examples of dicarboxylic acid derivatives other than terephthalic acid include esters of dicarboxylic acids other than terephthalic acid and ester-forming derivatives such as dicarboxylic acid halides.

[0052] <Diol component> There are no particular restrictions on the diol components other than BDO that are subjected to the esterification reaction. Examples include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, and dibutylene glycol; alicyclic diols such as 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,1-cyclohexanedimethylol, and 1,4-cyclohexanedimethylol; xylylene glycol; polyalkylene glycols such as polyethylene glycol, polytrimethylene glycol, and polytetramethylene ether glycol; and aromatic diols such as 4,4'-dihydroxybiphenyl, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone. These diol components other than BDO may be used individually or as a mixture of two or more.

[0053] These diol components may be obtained from naphtha produced from petroleum, naphtha produced from biological resources, or from the depolymerization of polyester. They may also be obtained from the fermentation of sugars.

[0054] From the viewpoint of more effectively obtaining the effects of the present invention, the proportion of BDO in the total diol components is 50 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 95 mol% or more, and may be 100 mol%.

[0055] By using diol components other than the aforementioned BDO, such as alkylenediols like ethylene glycol, as the diol component, polyalkylene terephthalates such as PET can be produced.

[0056] <Other monomers> In the production of PBT according to the present invention, one or more copolymer components such as lactic acid, glycolic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, 6-hydroxy-2-naphthalenecarboxylic acid, p-β-hydroxyethoxybenzoic acid, hydroxycarboxylic acids, alkoxycarboxylic acids, monofunctional components such as stearyl alcohol, benzyl alcohol, stearic acid, benzoic acid, t-butylbenzoic acid, and benzoylbenzoic acid, and trifunctional or polyfunctional components such as tricarbaryl acid, trimetic acid, trimesic acid, pyrometic acid, gallic acid, trimethylolethane, trimethylolpropane, glycerol, and pentaerythritol can be used.

[0057] <Esterification reaction> An example of a process for obtaining an oligomer by esterifying a dicarboxylic acid component mainly composed of terephthalic acid with a diol component mainly composed of BDO is to use a single esterification reactor or a multi-stage reactor in which multiple esterification reactors are connected in series, and carry out the esterification reaction with or without a catalyst, under atmospheric pressure or reduced pressure, while removing the water and excess diol component produced in the reaction from the system, until the esterification reaction rate (the proportion of all carboxyl groups of the starting material dicarboxylic acid component that react with the diol component to undergo esterification) reaches 90% or more, to obtain the oligomer. Typically, the temperature for esterification reactions is around 210-230°C, the pressure is around 10-133 kPa, and the residence time in the reaction vessel, which corresponds to the reaction time, is around 1-4 hours.

[0058] <Polycondensation reaction process> An example of a polycondensation reaction process is a multi-stage reactor using a single molten polycondensation tank or multiple molten polycondensation tanks connected in series, for example, a fully mixed reactor with a stirring blade in the first stage, and horizontal plug-flow reactors with stirring blades in the second and third stages, in which the diol produced is distilled out of the system while heating under reduced pressure in the presence of a catalyst. Typically, the polycondensation reaction is carried out at a temperature of 210-280°C, preferably around 220-250°C, and under reduced pressure of 27 kPa or less, preferably 13 kPa or less. The reaction vessel may be a single vessel or multiple stages, but in order to suppress discoloration and deterioration and to inhibit the increase of terminal groups such as vinyl groups, it is preferable to carry out the reaction in at least one reaction vessel under high vacuum, usually 1.3 kPa or less, preferably 0.3 kPa or less. Furthermore, in the process of producing the aforementioned high-intrinsic viscosity polyester B, the intrinsic viscosity of the produced polyester B can be increased by raising the vacuum level of the polycondensation reaction to, for example, about 0.30 to 0.01 kPa. Subsequently, by lowering the vacuum level, polyester C with a lower intrinsic viscosity than polyester B can be produced.

[0059] The PBT obtained by the polycondensation reaction is usually extracted in strand form through a die head from an outlet at the bottom of the polycondensation reaction tank, and then cut with a cutter while or after water cooling to form pellets or chips (for example, about 3 to 10 mm in length). Alternatively, the molten resin is released from the polycondensation reaction tank through piping into cold water adjusted to a predetermined temperature, and then cut with a cutter to form spherical bodies (about 2 to 10 mm in diameter).

[0060] <Polycondensation catalyst> When polycondensing oligomers obtained by the esterification reaction of a diol component and a dicarboxylic acid component, a titanium compound and preferably a Group 2A metal compound of the periodic table are typically used as catalysts. These catalyst components may be used in the esterification reaction and then proceed directly to the polycondensation reaction, or they may not be used in the esterification reaction, or only the titanium catalyst may be used, with the remaining catalyst components added at the polycondensation stage. Furthermore, a portion of the final amount of catalyst to be used may be used in the esterification reaction, and additional catalysts may be added as appropriate as the polycondensation reaction progresses. In any case, in this invention, the final PBT will inevitably contain titanium and preferably a metal from Group 2A of the periodic table, the amount of which will be described later.

[0061] (Examples of titanium compounds) Specific examples of titanium compounds used as catalysts include inorganic titanium compounds such as titanium oxide and titanium tetrachloride, titanium alcoholates such as tetramethyl titanate, tetraisopropyl titanate, and tetrabutyl titanate, and titanium phenolates such as tetraphenyl titanate. These may be used individually or in combination of two or more. Among these, tetraalkyl titanate is preferred, and among those, tetrabutyl titanate is preferred.

[0062] (Amount of titanium catalyst) The titanium catalyst content in the PBT of the present invention is preferably 5 to 100 ppm by mass ratio of titanium atoms to PBT. More preferably 10 ppm or more, even more preferably 20 ppm or more, and most preferably 25 ppm or more. Furthermore, more preferably 90 ppm or less, even more preferably 80 ppm or less, particularly preferably 70 ppm or less, especially preferably 60 ppm or less, and most preferably 50 ppm or less. If the titanium content is too high, deterioration of color, hydrolysis resistance, solution haze, and increased fisheye in the resulting molded product will occur. If the titanium content is too low, polymerization will be impaired.

[0063] (Example of group 2A metal compounds) Specific examples of Group 2A metal compounds used as catalysts include various compounds of beryllium, magnesium, calcium, strontium, and barium. However, magnesium compounds and / or calcium compounds are preferred in terms of ease of handling and availability, as well as catalytic effect, and magnesium compounds, which exhibit excellent catalytic effect, are particularly preferred. Specific examples of magnesium compounds include magnesium acetate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium alkoxide, and magnesium hydrogen phosphate. Specific examples of calcium compounds include calcium acetate, calcium hydroxide, calcium carbonate, calcium oxide, calcium alkoxide, and calcium hydrogen phosphate. These Group 2A metal compounds may be used individually or in combination of two or more. Among these, magnesium acetate is preferred.

[0064] (Group 2A metal catalyst amount) The content of the Group 2A metal catalyst in the polyester of the present invention is not particularly limited, but it is preferably 3 to 50 ppm by mass ratio to PBT as Group 2A metal atoms. More preferably 5 ppm or more, and even more preferably 10 ppm or more. Furthermore, it is more preferably 40 ppm or less, even more preferably 30 ppm or less, particularly preferably 20 ppm or less, and most preferably 15 ppm or less. If the content of Group 2A metals is too high, the color tone and hydrolysis resistance will deteriorate, and if it is too low, the polymerization properties will deteriorate. When using acetate salts of Group 2A metals of the periodic table, the acetic acid source enters the reaction system, so it is preferable that the amount of Group 2A metal in PBT be 15 ppm or less.

[0065] (M / Ti ratio) The molar ratio (group 2A metal / titanium) of titanium atoms to group 2A metal atoms in the PBT of the present invention is typically 0.01 to 100, preferably 0.1 to 10, more preferably 0.3 to 3, and even more preferably 0.3 to 1.5.

[0066] (Metal analysis method) The metal content, such as titanium atoms, in PBT can be measured using methods such as atomic emission, atomic absorption, or ICP emission after recovering the metal from the PBT using methods such as wet ashing.

[0067] (Other catalysts) In the production of PBT according to the present invention, in addition to the titanium compounds and Group 2A metal compounds of the periodic table mentioned above, reaction aids such as antimony compounds such as antimony trioxide, germanium compounds such as germanium dioxide and germanium tetroxide, manganese compounds, zinc compounds, zirconium compounds, cobalt compounds, orthophosphate, phosphite, hypophosphite, polyphosphate, phosphorus compounds such as esters and metal salts thereof, sodium hydroxide, and sodium benzoate may also be used.

[0068] <Compound> The polyester of the present invention can be compounded into a product by adding various additives or compounding materials as needed during the polyester manufacturing process or after the polyester has been manufactured. [Examples]

[0069] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention.

[0070] [Measurement and Evaluation Methods] <Intrinsic viscosity> The viscosity was determined using a fully automatic viscosity analyzer (model DT553, capillary type) manufactured by Sentec Co., Ltd., according to the following procedure. A mixture of phenol and 1,1,2,2-tetrachloroethane (mass ratio 1 / 1) was used as the solvent. At 30°C, the number of seconds for dropping the 1.0 g / dL PBT sample solution and the solvent alone was measured and calculated using the following formula. Intrinsic viscosity (dL / g)=((1+4KHη3) 0.5 -1) / (2K H C) (However, η3 = η / η0 - 1, where η is the number of seconds the sample solution falls, η0 is the number of seconds the solvent falls, C is the PBT concentration of the sample solution (g / dL), and K H K is Huggins' constant. H (0.33 was adopted.)

[0071] <Method for measuring carbide precipitates larger than 0.3 mm> Using PGI-IV from Hubrain Co., Ltd., we measured carbide precipitates larger than 0.3 mm. First, after removing fine powder and moisture from the polybutylene terephthalate to be measured using an air gun, we obtained 200 g of polybutylene terephthalate and measured the number of precipitates larger than 0.3 mm using PGI-IV from Hubrain Co., Ltd.

[0072] <Time when the number of carbide precipitates larger than 0.3 mm became zero> The time when carbide precipitates were zero was defined as the first time (8 hours) when no carbide precipitates larger than 0.3 mm were detected, after sampling polybutylene terephthalate pellets obtained every two hours following the restart of the polybutylene terephthalate production plant.

[0073] <Color tone of polybutylene terephthalate> The color tone of polybutylene terephthalate was evaluated using a color difference meter "Z-300A" manufactured by Nippon Denshoku Co., Ltd., on the Lab color system. A lower b value indicates less yellowness, which is preferable. However, when the b value falls below -2.0, although there is less yellowness, the color tone becomes undesirable due to increased blueness.

[0074] <Time taken to obtain polybutylene terephthalate with good color tone> The time required to obtain polybutylene terephthalate with good color was defined as the time when the b-value became 2 or less, after sampling of the polybutylene terephthalate pellets obtained every two hours following the complete elimination of carbide precipitates larger than 0.3 mm.

[0075] <Time when carbide precipitates larger than 0.3 mm were detected again> The time at which carbide precipitates larger than 0.3 mm were detected again was defined as the number of days from the time when polybutylene terephthalate with good color was obtained until carbide precipitates larger than 0.3 mm were detected again in the pellet.

[0076] <Measurement of the number of carbonized precipitates 1 mm or larger per 200 g of polyester in Die Head A> The number of carbide precipitates larger than 1 mm per 200 g of polyester in die head A was measured using the following procedure. After switching from die head A to die head B, die plate A of die head A was removed, and the number of carbide precipitates larger than 1 mm present on die plate A was measured and calculated proportionally as the ratio per 200 g of polyester in die head A.

[0077] <Example 1> Polybutylene terephthalate was produced using terephthalic acid manufactured from fossil fuels and 1,4-butanediol manufactured from fossil fuels, as described below. Polybutylene terephthalate was continuously produced using a polybutylene terephthalate production apparatus consisting of an esterification reactor, a vertical polycondensation reactor, a horizontal polycondensation reactor, a polymer filter, a strand discharge section with two die heads A and B (both with a die hole diameter of 7 mm) arranged in parallel and switchable by a valve, a cooling tank, and a chip cutter. After that, the production of polybutylene terephthalate in the polybutylene terephthalate production apparatus was stopped, and the polybutylene terephthalate production apparatus was cleaned with triethylene glycol.

[0078] The polybutylene terephthalate production apparatus was restarted. A slurry of terephthalic acid and 1,4-butanediol, along with a 1,4-butanediol solution of tetrabutoxytitanium, was supplied so that the titanium concentration in the resulting polybutylene terephthalate was 40 ppm by mass, and an esterification reaction was carried out to obtain an esterification reaction solution. To the obtained esterification reaction solution, a 1,4-butanediol solution of magnesium acetate was supplied so that the magnesium concentration in the resulting polybutylene terephthalate was 10 ppm by mass, and a polycondensation reaction was carried out to obtain a polycondensation reaction solution. The obtained polycondensation reaction solution was passed through a polymer filter to obtain polybutylene terephthalate at a rate of 4000 kg per hour. The obtained polybutylene terephthalate was discharged in strand form using die head A, cooled in a cooling tank, and then cut with a chip cutter to obtain polybutylene terephthalate pellets A. The intrinsic viscosity of the obtained pellet A was 0.851 dL / g, and there were 34 carbide precipitates larger than 0.3 mm in 200 g of pellets.

[0079] The operating conditions were continued, and 40 hours after restarting the polybutylene terephthalate production equipment, there were 6 carbide precipitates larger than 0.3 mm per 200 g of pellets. This polybutylene terephthalate with 6 carbide precipitates larger than 0.3 mm per 200 g of pellets was designated as polybutylene terephthalate B, and 40 hours after restarting the polybutylene terephthalate production equipment, die head A was switched to die head B. Die plate A was removed from die head A, and the number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was found to be 10.

[0080] Subsequently, the operating conditions were continued, and 102 hours after restarting the polybutylene terephthalate production equipment, the number of carbide precipitates larger than 0.3 mm in the obtained pellets became zero, indicating that polybutylene terephthalate C was obtained. The time at which the number of carbide precipitates larger than 0.3 mm became zero was 102 hours. Furthermore, 4 hours later, the b-value of the polybutylene terephthalate became 2, indicating that the time at which polybutylene terephthalate with good color tone was obtained was 106 hours. The operating conditions were continued, and even after one month, there were no carbide precipitates larger than 0.3 mm in the obtained pellets.

[0081] <Example 2> In Example 1, instead of switching from die head A to die head B when 6 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 40 hours after restarting the polybutylene terephthalate production apparatus, the process was carried out in the same manner as in Example 1, except that the process was switched from die head A to die head B when 3 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 60 hours after restarting the polybutylene terephthalate production apparatus. That is, there were 3 carbide precipitates larger than 0.3 mm in 200 g of polybutylene terephthalate B pellets. The number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was 16. The time required to obtain polybutylene terephthalate C with zero carbide precipitates larger than 0.3 mm was 80 hours from the restart of the polybutylene terephthalate production equipment. Furthermore, the time required to obtain polybutylene terephthalate with good color tone was 84 hours from the restart of the polybutylene terephthalate production equipment. The operating conditions were continued, and even after one month, there were no carbide precipitates larger than 0.3 mm in the obtained pellets.

[0082] <Example 3> In Example 1, instead of switching from die head A to die head B when 6 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 40 hours after restarting the polybutylene terephthalate production apparatus, the process was carried out in the same manner as in Example 1, except that the switch was made from die head A to die head B when 1 carbide precipitate larger than 0.3 mm was found in 200 g of pellets obtained 100 hours after restarting the polybutylene terephthalate production apparatus. That is, there was 1 carbide precipitate larger than 0.3 mm in 200 g of polybutylene terephthalate B pellets. The number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was 22. The time required to obtain polybutylene terephthalate C with zero carbide precipitates larger than 0.3 mm was 112 hours after restarting the polybutylene terephthalate production equipment. Furthermore, the time required to obtain polybutylene terephthalate with good color was 116 hours after restarting the polybutylene terephthalate production equipment. The operating conditions were continued, and even after one month, there were no carbide precipitates larger than 0.3 mm in the obtained pellets.

[0083] <Comparative Example 1> The procedure was carried out in the same manner as in Example 1, except that the die head A was not switched to die head B. The time at which the number of carbide precipitates larger than 0.3 mm became zero was 312 hours after the restart of the polybutylene terephthalate production equipment. Furthermore, the time at which polybutylene terephthalate with good color tone was obtained was 316 hours after the restart of the polybutylene terephthalate production equipment. When the operating conditions were continued, polybutylene terephthalate pellets containing carbide precipitates larger than 0.3 mm were again obtained after 10 days. This is thought to be because the large foreign matter inside die head A is physically crushed by the molten polybutylene terephthalate, becoming smaller pieces that pass through the die hole of die head A.

[0084] <Comparative Example 2> In Example 1, instead of switching from die head A to die head B when 6 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 40 hours after restarting the polybutylene terephthalate production apparatus, the process was carried out in the same manner as in Example 1, except that the switch was made from die head A to die head B when 34 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 26 hours after restarting the polybutylene terephthalate production apparatus. That is, there were 34 carbide precipitates larger than 0.3 mm in 200 g of polybutylene terephthalate B pellets. The number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was zero. The time required to obtain polybutylene terephthalate C with zero carbide precipitates larger than 0.3 mm was 208 hours after the restart of the polybutylene terephthalate production equipment. Furthermore, the time required to obtain polybutylene terephthalate with good color was 212 hours after the restart of the polybutylene terephthalate production equipment. When the operating conditions were continued, after 22 days, polybutylene terephthalate pellets containing carbide precipitates larger than 0.3 mm were again obtained, similar to Comparative Example 1.

[0085] <Comparative Example 3> In Example 1, instead of switching from die head A to die head B when 6 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 40 hours after restarting the polybutylene terephthalate production apparatus, the process was carried out in the same manner as in Example 1, except that the process was switched from die head A to die head B when 14 carbide precipitates larger than 0.3 mm were found in 200 g of pellets obtained 40 hours after restarting the polybutylene terephthalate production apparatus. That is, there were 14 carbide precipitates larger than 0.3 mm in 200 g of polybutylene terephthalate B pellets. The number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was 7. The time required to obtain polybutylene terephthalate C with zero carbide precipitates larger than 0.3 mm was 164 hours after restarting the polybutylene terephthalate production equipment. Furthermore, the time required to obtain polybutylene terephthalate with good color was 168 hours after restarting the polybutylene terephthalate production equipment. When the operating conditions were continued, after 25 days, polybutylene terephthalate pellets containing carbide precipitates larger than 0.3 mm were again obtained, similar to Comparative Example 1.

[0086] Table 1 summarizes the following for Examples 1-3 and Comparative Examples 1-3: the number of carbide precipitates larger than 0.3 mm per 200 g of pellets when switching from die head A to B, the time from restarting the polybutylene terephthalate production equipment until the number of carbide precipitates larger than 0.3 mm in the pellets became zero, the time from restarting the polybutylene terephthalate production equipment until polybutylene terephthalate with good color was obtained, and the number of days from obtaining polybutylene terephthalate with good color until carbide precipitates larger than 0.3 mm were detected in the pellets again. Table 1 shows that by switching die head A to die head B when the number of carbide precipitates larger than 0.3 mm falls below a predetermined number, the time until the number of carbide precipitates larger than 0.3 mm in the pellet becomes zero and the time until polybutylene terephthalate with good color tone is obtained is shortened, and no more carbide precipitates larger than 0.3 mm are detected after polybutylene terephthalate with good color tone has been obtained.

[0087] [Table 1]

[0088] <Example 4> Polybutylene terephthalate was produced using terephthalic acid manufactured from fossil fuels and 1,4-butanediol (hereinafter referred to as "bio-BDO") manufactured from bio-resources, as described below. Polybutylene terephthalate was continuously produced using a polybutylene terephthalate production apparatus consisting of an esterification reactor, a vertical polycondensation reactor, a horizontal polycondensation reactor, a polymer filter, a strand discharge section with two die heads A and B (both with a die hole diameter of 7 mm) arranged in parallel and switchable by a valve, a cooling tank, and a chip cutter. After that, the production of polybutylene terephthalate in the polybutylene terephthalate production apparatus was stopped, and the polybutylene terephthalate production apparatus was cleaned with triethylene glycol.

[0089] The polybutylene terephthalate production apparatus was restarted. A slurry of terephthalic acid and bio-BDO, along with a bio-BDO solution of tetrabutoxytitanium, was supplied to the resulting polybutylene terephthalate to achieve a titanium concentration of 40 ppm by mass, and an esterification reaction was carried out to obtain an esterification reaction solution. To the obtained esterification reaction solution, a bio-BDO solution of magnesium acetate was supplied to the resulting polybutylene terephthalate to achieve a magnesium concentration of 10 ppm by mass, and a polycondensation reaction was carried out to obtain a polycondensation reaction solution. The obtained polycondensation reaction solution was passed through a polymer filter to obtain polybutylene terephthalate at a rate of 40 parts by mass per hour. The obtained polybutylene terephthalate was discharged in strand form using die head A, cooled in a cooling tank, and then cut with a chip cutter to obtain polybutylene terephthalate pellets A. The intrinsic viscosity of the obtained pellets A was 0.852 dL / g, and there were 36 carbide precipitates larger than 0.3 mm in 200 g of pellets.

[0090] The operating conditions were continued, and 40 hours after restarting the polybutylene terephthalate production equipment, there were 8 carbide precipitates larger than 0.3 mm per 200 g of pellets. This polybutylene terephthalate with 8 carbide precipitates larger than 0.3 mm per 200 g of pellets was designated as polybutylene terephthalate B, and 40 hours after restarting the polybutylene terephthalate production equipment, die head A was switched to die head B. Die plate A was removed from die head A, and the number of carbide precipitates larger than 1 mm per 200 g of polybutylene terephthalate in die head A was found to be 10.

[0091] Subsequently, the operating conditions were continued, and 112 hours after restarting the polybutylene terephthalate production equipment, the number of carbide precipitates larger than 0.3 mm in the obtained pellets became zero, indicating that polybutylene terephthalate C was obtained. The time at which the number of carbide precipitates larger than 0.3 mm became zero was 112 hours. Furthermore, 4 hours later, the b-value of the polybutylene terephthalate became 2, indicating that the time at which polybutylene terephthalate with good color tone was obtained was 116 hours. The operating conditions were continued, and even after one month, there were no carbide precipitates larger than 0.3 mm in the obtained pellets.

[0092] <Comparative Example 4> The procedure was carried out in the same manner as in Example 4, except that the die head A was not switched to die head B. The time at which the number of carbide precipitates larger than 0.3 mm became zero was 316 hours after the restart of the polybutylene terephthalate production equipment. Furthermore, the time at which polybutylene terephthalate with good color tone was obtained was 320 hours after the restart of the polybutylene terephthalate production equipment. When the operating conditions were continued, polybutylene terephthalate pellets containing carbide precipitates larger than 0.3 mm were again obtained after 8 days. This is thought to be because the large foreign matter inside die head A is physically crushed by the molten polybutylene terephthalate, becoming smaller pieces that pass through the die hole of die head A.

[0093] In Example 4 and Comparative Example 4, Table 2 summarizes the number of carbide precipitates larger than 0.3 mm per 200 g of pellets when switching from die head A to B, the time from restarting the polybutylene terephthalate production equipment until the number of carbide precipitates larger than 0.3 mm in the pellets became zero, the time from restarting the polybutylene terephthalate production equipment until polybutylene terephthalate with good color was obtained, and the number of days from obtaining polybutylene terephthalate with good color until carbide precipitates larger than 0.3 mm were detected in the pellets again. Table 2 shows that, not only when using 1,4-butanediol produced from fossil fuels as the raw material, but also when using 1,4-butanediol produced from bio-resources, switching die head A to die head B when the number of carbonized precipitates larger than 0.3 mm falls below a predetermined number shortens the time until the number of carbonized precipitates larger than 0.3 mm in the pellet becomes zero and the time until polybutylene terephthalate with good color tone is obtained. Furthermore, it can be seen that carbonized precipitates larger than 0.3 mm are not detected again after polybutylene terephthalate with good color tone is obtained.

[0094] [Table 2] [Explanation of Symbols]

[0095] 1 Die head 3 Die Plates 3a Die Hole

Claims

1. In a polyester manufacturing plant, a method for producing polyester involves reacting a dicarboxylic acid component with a diol component to obtain molten polyester, which is then extruded from a die head in the form of strands, and the resulting strands are cut to obtain pellets. A method for manufacturing polyester, comprising sequentially performing the following steps 1 to 4 when restarting a polyester manufacturing plant after it has been shut down. Step 1: A process in which molten polyester A is extruded from a die head A to obtain strand A, and the obtained strand A is cut to obtain pellets A containing more than 10 carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 2: Molten polyester B is extruded from die head A to obtain strand B, and the obtained strand B is cut to obtain pellet B containing 10 or fewer carbonized precipitates larger than 0.3 mm per 200 g of pellets. Step 3: The process of switching the die head that extrudes molten polyester from die head A to die head B. Step 4: A process in which molten polyester C is extruded from die head B to obtain strand C, and the obtained strand C is cut to obtain pellet C.

2. The method for producing polyester according to claim 1, wherein the molten polyester in the die head A of step 3 contains one or more carbonized precipitates of 1 mm or more per 200 g of polyester.

3. The method for producing polyester according to claim 1, wherein the carbonized precipitate is a carbide of polyester.

4. The method for producing a polyester according to claim 1, wherein the dicarboxylic acid component is terephthalic acid, the diol component is alkylenediol, and the polyester is polyalkylene terephthalate.

5. The method for producing a polyester according to claim 4, wherein the alkylenediol is 1,4-butanediol and the polyalkylene terephthalate is polybutylene terephthalate.

6. The method for producing polyester according to claim 4, wherein the terephthalic acid is terephthalic acid produced from naphtha produced from biological resources, terephthalic acid produced from recycled naphtha produced from waste resin, or terephthalic acid produced by the depolymerization of polyester.

7. The method for producing polyester according to claim 5 or 6, wherein the 1,4-butanediol is 1,4-butanediol produced from naphtha produced from biological resources, 1,4-butanediol produced from recycled naphtha produced from waste resin, 1,4-butanediol produced by direct fermentation of sugar, or 1,4-butanediol produced by hydrogen reduction of succinic acid or succinic acid derivative produced using biological resources.

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