JP2025084471A - Polyester resin, molding, preform, polyester bottle, and mechanically recycled polyester resin - Google Patents

Abstract

The present invention provides a polyester resin capable of suppressing the occurrence of mold staining during molding.
[Solution] A polyester resin containing, as a main component, a polyester containing diol units and dicarboxylic acid units, the polyester resin contains a cyclic oligomer containing the diol units and the dicarboxylic acid units, and is characterized in that the ratio (C5/CT) of the peak area of a pentamer oligomer (C5) to the peak area of a trimer oligomer (CT) in the cyclic oligomer, as measured using liquid chromatography, is 0.086 or more.
[Selection diagram] None

Description

The present invention relates to a polyester resin, a molded body, a preform, and a polyester bottle made from this polyester resin, and a mechanically recycled polyester resin.

Polyester resin is a thermoplastic resin with excellent properties such as mechanical stability, chemical stability, transparency, and heat resistance. From the perspective of reducing the environmental load, a method is known in which such polyester resin is regenerated by solid-phase polymerization and mechanically recycled (for example, Patent Document 1).

Patent document 1: JP 2000-219728 A

However, with polyester resins produced using the above technology, the BHET remaining as a monomer adheres to the molds and other parts of the production equipment as dirt during molding of the polyester resin, and there is a problem in that the cost of molding increases in order to remove the dirt.

The object of the present invention is to provide a polyester resin with a reduced BHET content that can suppress the occurrence of mold contamination during molding, and a molded article made from such a polyester resin.

[1] According to aspect 1 of the present invention, there is provided a polyester resin containing, as a main component, a polyester containing diol units and dicarboxylic acid units, the polyester resin comprising a cyclic oligomer containing the diol units and the dicarboxylic acid units, and characterized in that the ratio (C5/CT) of the peak area of the pentamer oligomer (C5) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured using liquid chromatography is 0.086 or more.

[2] According to aspect 2 of the present invention, there is provided the polyester resin according to aspect 1, characterized in that the ratio (C6/CT) of the peak area of the hexamer oligomer (C6) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.058 or more.

[3] According to aspect 3 of the present invention, there is provided the polyester resin according to aspect 1, characterized in that the ratio (C7/CT) of the peak area of the heptamer oligomer (C7) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.031 or more.

[4] According to aspect 4 of the present invention, there is provided a molded article made from the polyester resin described in any one of aspects 1 to 3.

[5] According to aspect 5 of the present invention, there is provided a preform made from the polyester resin described in any one of aspects 1 to 3.

[6] According to aspect 6 of the present invention, there is provided a polyester bottle made from the polyester resin described in any one of aspects 1 to 3.

[7] According to aspect 7 of the present invention, there is provided a polyester resin according to any one of aspects 1 to 3, characterized in that the polyester resin is a mechanically recycled polyester resin.

The present invention provides a polyester resin that can suppress the occurrence of mold contamination during molding.

FIG. 1 is a flow chart showing an example of a method for producing a polyester resin according to an embodiment of the present invention. FIG. 2A is a graph showing the measurement results of 1H-NMR spectrum in Example 6. FIG. 2B is a graph showing the measurement results of the 1H-NMR spectrum in Comparative Example 1. FIG. 2C is a graph showing the results of measuring the 1H-NMR spectrum of virgin polyethylene terephthalate copolymerized with isophthalic acid resin. FIG. 3(a) is a graph showing the measurement results of the differential molecular weight distribution curve in Example 6, and FIG. 3(b) is an enlarged view of FIG. 3(a). FIG. 4(a) is a graph showing the measurement results of the differential molecular weight distribution curve in Comparative Example 1, and FIG. 4(b) is an enlarged view of FIG. 4(a).

The polyester resin in this embodiment contains a polyester containing diol units and dicarboxylic acid units as a main component, and the ratio (C5/CT) of the peak area of the pentamer oligomer (C5) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.086 or more.

The polyesters contained in the polyester resin include aromatic polyesters, wholly aromatic polyesters, polycarbonates, and aliphatic polyesters, among which aromatic polyesters are preferred. Aromatic polyesters contain diol units and dicarboxylic acid units. Diol compounds for forming diol units include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,6-hexylene glycol, cyclohexanedimethanol, and ethylene oxide adducts of bisphenol A, among which ethylene glycol is preferred. Dicarboxylic acid compounds for forming dicarboxylic acid units include aromatic dicarboxylic acids and their derivatives such as terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, and furandicarboxylic acid, among which terephthalic acid is preferred. Specific examples of polyesters include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polyethylene furanoate, among which polyethylene terephthalate, a copolymer of ethylene glycol and terephthalic acid, is preferred. The polyesters described above are not limited to those derived from petroleum raw materials, but may be polyesters derived from plant raw materials, or may be polyesters made by recycling these polyesters derived from petroleum raw materials or plant raw materials. The polyesters described above may be used alone or in combination.

The content of ethylene terephthalate units in polyethylene terephthalate is preferably 70 mol% or more, more preferably 90 mol% or more, based on the total monomer units.

Polyethylene terephthalate may contain, among all monomer units, units of ethylene glycol and dicarboxylic acids other than terephthalic acid that are copolymerizable with terephthalic acid. Examples of such dicarboxylic acids other than terephthalic acid include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid, ethylmalonic acid, adamantanedicarboxylic acid, norbornenedicarboxylic acid, cyclohexanedicarboxylic acid, decalindicarboxylic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodiumsulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid, 2,5-furandicarboxylic acid, and ester derivatives thereof. Among these, isophthalic acid is preferred. The content of units consisting of dicarboxylic acids other than terephthalic acid is preferably 30 mol% or less, more preferably 10 mol% or less, based on the total monomer units.

Polyethylene terephthalate may contain, among all monomer units, units of diols other than terephthalic acid that are copolymerizable with ethylene glycol and terephthalic acid. Examples of such diols other than ethylene glycol include 1,2-propanediol, 1,3-propanediol, butanediol, 2-methyl-1,3-propanediol, hexanediol, neopentyl glycol, cyclohexanedimethanol, cyclohexanediethanol, decahydronaphthalenedimethanol, decahydronaphthalenediethanol, norbornanedimethanol, norbornanediethanol, tricyclodecanedimethanol, tricyclodecanediethanol, tetracyclododecanedimethanol, tetracyclododecanediethanol, decalindimethanol, decalindiethanol, 5-methylol-5-ethyl-2-(1,1-dimethyl-2-hydroxyethyl) Examples of suitable diols include 2,2-bis(4-hydroxycyclohexylpropane), 2,2-bis(4-(2-hydroxyethoxy)cyclohexyl)propane, cyclopentanediol, 3-methyl-1,2-cyclopentadiol, 4-cyclopentene-1,3-diol, adamantanediol, paraxylene glycol, bisphenol A, bisphenol S, styrene glycol, trimethylolpropane, pentaerythritol, diethylene glycol, triethylene glycol, and bis-β-hydroxyethyl terephthalate (BHET), among which diethylene glycol is preferred. The content of units consisting of diols other than ethylene glycol is preferably 30 mol % or less, more preferably 10 mol % or less, based on the total monomer units.

Polyethylene terephthalate may contain other components such as additives. For example, one or more of various additives such as plasticizers, light stabilizers, antioxidants, UV absorbers, flame retardants, colorants, pigments, fillers, release agents, antistatic agents, fragrances, foaming agents, and antibacterial and antifungal agents may be blended.

The polyester resin in this embodiment contains a cyclic oligomer containing a diol unit and a dicarboxylic acid unit. Such a cyclic oligomer is a by-product in the polymerization reaction of the above-mentioned diol compound and dicarboxylic acid compound. The polyester resin in this embodiment contains a trimer oligomer (cyclic trimer; CT) and a pentamer oligomer (C5) as cyclic oligomers. For example, the trimer oligomer (CT) contained in a polyester resin containing polyethylene terephthalate as the polyester is composed of three constituent units of polyethylene terephthalate.

The content of cyclic oligomers in the polyester resin is preferably 1% by weight or less, and more preferably 0.5% by weight or less.

In the polyester resin of this embodiment, the ratio (C5/CT) of the peak area of the pentamer oligomer (C5) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured using liquid chromatography is 0.086 or more, preferably 0.088 or more. The upper limit of C5/CT is not particularly limited, but is usually 0.300 or less. In the polyester resin of this embodiment, the ratio (C5/CT) of the peak area of the pentamer oligomer (C5) to the peak area of the trimer oligomer (CT) is within the above range, so that the content of BHET is reduced and the occurrence of mold contamination during molding can be suppressed. In addition, the polyester resin of this embodiment has excellent light blocking properties, mouth crystallization suitability, and flavor properties due to the peak area ratio (C5/CT) being within the above range. In addition, when measuring using liquid chromatography, it is preferable to use a UV/Vis detector or a diode array detector (DAD) as a detector. The same applies when calculating the peak area ratio of hexamer oligomers and heptamer oligomers, which will be described later.

Although not particularly limited, the polyester resin may contain a hexamer oligomer (C6) as a cyclic oligomer. In the polyester resin, the ratio (C6/CT) of the peak area of the hexamer oligomer (C6) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured using liquid chromatography is preferably 0.058 or more, more preferably 0.060 or more. The upper limit of C6/CT is not particularly limited, but is usually 0.200 or less. By setting the peak area ratio (C6/CT) within the above range, the occurrence of mold contamination during molding of the polyester resin can be further suppressed. In addition, by setting the peak area ratio (C6/CT) within the above range, the polyester resin can be made to have better light blocking properties, mouth crystallization suitability, and flavor properties.

Although not particularly limited, the polyester resin may contain a heptamer oligomer (C7) as a cyclic oligomer. In the polyester resin, the ratio (C7/CT) of the peak area of the heptamer oligomer (C7) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured using liquid chromatography is preferably 0.031 or more, more preferably 0.032 or more. The upper limit of C7/CT is not particularly limited, but is usually 0.100 or less. By setting the peak area ratio (C7/CT) within the above range, the occurrence of mold contamination during molding of the polyester resin can be further suppressed. In addition, by setting the peak area ratio (C7/CT) within the above range, the polyester resin can be made to have better light blocking properties, mouth crystallization suitability, and flavor properties.

In the polyester resin, the ratio (A _7.5 /A _acid ) of the peak area (A _7.5 ) at 7.43 to 7.55 ppm to the peak area (A _acid ) derived from all dicarboxylic acids measured by 1H-NMR is preferably 0.00020 or more, more preferably 0.00040 or more. The upper limit of _{A 7.5} /A _acid is not particularly limited, but is usually 0.00300 or less. The peaks derived from dicarboxylic acids are present in the ranges of 7.60 to 7.75 ppm, 7.90 to 8.55 ppm, and 8.75 to 8.90 ppm, and the peak area (A _acid ) derived from all dicarboxylic acids refers to the sum of the peak areas present in all of these ranges. By controlling the ratio (A _7.5 /A _acid ) of the peak area (A _7.5 ) at 7.43 to 7.55 ppm to the peak area (A _acid ) derived from all dicarboxylic acids within the above range, the occurrence of mold staining during molding of the polyester resin can be further suppressed. Furthermore, by controlling the peak area ratio (A _7.5 /A _acid ) within the above range, the polyester resin can be made to have better light blocking properties, mouth crystallization suitability, and flavor properties.

In the polyester resin, the ratio (A _7.8 /A _acid ) of the peak area (A _7.8 ) at 7.75 to 7.85 ppm to the peak area (A _acid ) derived from all dicarboxylic acids measured by 1H-NMR is preferably 0.00010 or more, more preferably 0.00020 or more. The upper limit of A _7.8 /A _acid is not particularly limited, but is usually 0.00300 or less. By setting the ratio (A _7.8 /A _acid ) of the peak area (A _7.8 ) at 7.75 to 7.85 ppm to the peak area (A _acid ) derived from all dicarboxylic acids within the above range,
The occurrence of mold contamination during molding of the polyester resin can be further suppressed. In addition, by setting the peak area ratio (A _7.8 /A _acid ) within the above range, the polyester resin can be made to have better light-shielding properties, mouth-part crystallization suitability, and flavor properties.

なお、上記（１）式中、Ｍはポリエステル樹脂の分子量であり、ｆ（ＬｏｇＭ）は、ポリエステル樹脂の微分分子量分布曲線を表し、ｇ（ＬｏｇＭ）は、ｆ（ＬｏｇＭ）におけるＬｏｇＭが３．４０および３．７５である２点のデータで結ばれる一次関数を表す。また、微分分子量分布曲線の縦軸は、濃度分率ｗ（％）をＬｏｇＭで微分したｄｗ／ｄＬｏｇＭである。ショルダー相関パラメータＳを上記範囲内とすることにより、ポリエステル樹脂の成形時の金型汚れの発生を抑制することができる。また、ショルダー相関パラメータＳを上記範囲内とすることにより、ポリエステル樹脂を、遮光性、口部結晶化適性、およびフレーバー性により優れたものとすることができる。 In the polyester resin, the shoulder correlation parameter S, which is derived using a differential molecular weight distribution curve obtained by GPC and is represented by the following formula (1), is preferably 0.160 or less, more preferably 0.145 or less. The lower limit of the shoulder correlation parameter S is not particularly limited, but is usually -0.200 or more.

In the above formula (1), M is the molecular weight of the polyester resin, f(LogM) represents the differential molecular weight distribution curve of the polyester resin, and g(LogM) represents a linear function connected by two data points where LogM in f(LogM) is 3.40 and 3.75. The vertical axis of the differential molecular weight distribution curve is dw/dLogM obtained by differentiating the concentration fraction w(%) with LogM. By setting the shoulder correlation parameter S within the above range, the occurrence of mold staining during molding of the polyester resin can be suppressed. In addition, by setting the shoulder correlation parameter S within the above range, the polyester resin can be made to have better light blocking properties, mouth crystallization suitability, and flavor properties.

In the polyester resin, the polydispersity Mw/Mn measured by GPC is preferably 2.470 or more, more preferably 2.490 or more. The upper limit of the polydispersity Mw/Mn is not particularly limited, but is usually 3.000 or less. By setting the polydispersity Mw/Mn within the above range, it is possible to suppress the occurrence of mold contamination during molding of the polyester resin. In addition, by setting the polydispersity Mw/Mn within the above range, it is possible to make the polyester resin more excellent in light blocking properties, suitability for crystallization of the mouth part, and flavor properties.

The acetaldehyde content of the polyester resin in this embodiment is preferably 10 ppm or less, more preferably 5 ppm or less, in the form of pellets after solid-state polymerization. Furthermore, it is preferable to reduce the amount of acetaldehyde increase when the polyester resin is molded into a molded body. By keeping the acetaldehyde content within the above range and reducing the amount of acetaldehyde increase, the effect on the flavor of the contents of the molded body when the polyester resin is molded into a molded body can be reduced, resulting in a molded body with excellent flavor properties.

In this embodiment, the intrinsic viscosity (IV) of the polyester resin is preferably 0.60 to 1.40 dL/g, and more preferably 0.70 to 1.00 dL/g.

The chromaticity b ^* of the polyester resin in the present embodiment is preferably 15.0 or more, more preferably 27.0 or more, in the form of pellets after solid-state polymerization. By setting the chromaticity b ^* of the polyester resin within the above range, a molded article made from the polyester resin can have excellent light-shielding properties.

In this embodiment, the cold crystallization peak top temperature Tc1 of the polyester resin is preferably 149°C or less. The upper limit of the cold crystallization peak top temperature Tc1 is not particularly limited, but is usually 120°C or more. By setting the cold crystallization peak top temperature Tc1 of the polyester resin within the above range, crystallization of the mouth portion can be easily performed when the polyester resin is molded into a polyester bottle or the like.

Figure 1 is a flow chart showing an example of a method for producing a polyester resin in this embodiment. As shown in Figure 1, the polyester resin in this embodiment can be produced by a method in which a polyester containing the above-mentioned diol units and dicarboxylic acid units is subjected to a thermal history control process including a first melt extrusion process, a second melt extrusion process, a crystallization process, and a solid-phase polymerization process.

First, in the first melt extrusion step, the polyester pellets obtained by polymerizing the diol compound and dicarboxylic acid are melt-extruded using an extruder after the moisture content is reduced to 50 ppm or less in a dehumidifying dryer set at 150°C in advance, and the discharged molten resin is air-cooled to form pellets. The extruder is not particularly limited, but examples include single-screw extruders, twin-screw extruders, and multi-screw extruders, and among these, twin-screw extruders are preferred. When using a twin-screw extruder, the extrusion temperature is preferably 0 to 60°C higher than the melting point of the polyester, and in particular, in the case of polyethylene terephthalate, it is 260 to 310°C, and more preferably 265 to 300°C. The screw rotation speed of the twin-screw extruder is preferably 50 to 800 rpm, and more preferably 70 to 400 rpm. The discharge rate of the twin-screw extruder is preferably 5 to 30,000 kg/h, and more preferably 10 to 10,000 kg/h.

Next, in the second melt extrusion process, the pellets obtained in the first melt extrusion process are used to perform melt extrusion again, and the discharged molten resin is air-cooled to form pellets. The extruder is not particularly limited, but examples include single-screw extruders, twin-screw extruders, and multi-screw extruders, among which twin-screw extruders are preferred. In addition, the extruder used is preferably equipped with a vacuum vent facility that can reduce the pressure to atmospheric pressure or lower. In the second melt extrusion process, when a twin-screw extruder equipped with a vacuum vent facility is used, the melt extrusion conditions are preferably the same as those in the first melt extrusion process, except that the vacuum is set to a pressure of 100 Torr or less.

Next, in the crystallization step, the pellets obtained in the second melt extrusion step are heated to crystallize the resin. The heating temperature is preferably 100 to 170°C, more preferably 130 to 150°C. Heating is preferably performed under a pressure of 200 Torr or less, or under a nitrogen gas flow, or a combination of these. The heating time is preferably 0.5 to 6 hours, more preferably 2 to 5 hours.

Next, in the solid-state polymerization step, the crystallized pellets are heated to carry out solid-state polymerization of the resin. The solid-state polymerization is preferably carried out under a pressure of 200 Torr, or under a nitrogen gas flow, or a combination of both. The temperature of the solid-state polymerization is preferably 200 to 230°C, more preferably 205 to 225°C. The heating time is preferably 2 to 24 hours, more preferably 6 to 20 hours.

As described above, the polyester resin of this embodiment can be obtained by performing the thermal history control treatment consisting of the first melt extrusion process, the second melt extrusion process, the crystallization process, and the solid-state polymerization process once or multiple times on the polyester. The number of times of thermal history control treatment is preferably 4 to 10 times, with so-called virgin polyester that has never been recycled being taken as the standard (0 times).

The method for producing the polyester resin in this embodiment is not particularly limited to the above. For example, the polyester used as a polyester bottle or the like may be mechanically recycled to control the thermal history and obtain the polyester resin in this embodiment (mechanically recycled polyester resin). The polyester used for mechanical recycling may be, for example, a polyester bottle produced from virgin polyester or a polyester bottle produced by performing mechanical recycling once or multiple times.

The polyester resin in this embodiment can be suitably used for processing into molded articles such as preforms and polyester bottles. Preforms can be manufactured by injection molding the polyester resin. Polyester bottles can be manufactured by stretch-blowing such preforms.

As described above, since the polyester resin in this embodiment is not yet processed into a molded body, the ratios of the peak area of the 5-, 6-, and 7-mer oligomers to the peak area of the trimer oligomer (CT) measured by liquid chromatography (C5/CT, C6/CT, C7/CT), the ratios of the peak areas of the 7.43-7.55 ppm and 7.75-7.85 ppm to the total dicarboxylic acid-derived peak area measured by 1H-NMR, the shoulder correlation parameter S, and the polydispersity Mw/Mn (hereinafter, these are also collectively referred to as "thermal history parameters") cannot be determined in the state of the molded body. In the flowchart shown in FIG. 1, the molded body corresponds to the state between the first melt extrusion process and the second melt extrusion process. Therefore, in order to compare the polyester resin constituting the molded body with the polyester resin in this embodiment, it is necessary to perform the second melt extrusion process, the crystallization process, and the solid-phase polymerization process on the molded body, evaluate the obtained pellets, and determine whether the thermal history parameters are within a specific range.

When the molded body is subjected to the second melt extrusion process, the crystallization process, and the solid-state polymerization process, an additional thermal history equivalent to one thermal history control process is added to the polyester resin before molding. Therefore, when the thermal history parameters evaluated after the second melt extrusion process, the crystallization process, and the solid-state polymerization process for a certain molded body are values close to the upper and lower limits of the specific range specified in this embodiment, it is preferable to evaluate the thermal history parameters of the polyester resin before molding according to the following procedure in order to confirm whether they were within the specific range.

First, the thermal history parameters of the pellets obtained by performing the second melt extrusion process, the crystallization process, and the solid-state polymerization process on the molded body are recorded as thermal history parameters for the first adjustment. Next, the pellets are subjected to a thermal history control process, and the thermal history parameters of the obtained pellets are recorded as thermal history parameters for the second adjustment. Thereafter, the pellets are subjected to a thermal history control process multiple times as necessary, and thermal history parameters for the third and subsequent adjustments are recorded. In this way, the correlation between the number of adjustments and the thermal history parameters is removed and inserted to obtain thermal history parameters corresponding to zero adjustments. By evaluating the thermal history parameters corresponding to zero adjustments, it is possible to confirm whether the thermal history parameters of the polyester resin before molding were within a specific range.

Next, the present invention will be specifically explained using examples, but the present invention is not limited to these.

Example 1
[Preparation of PET resin pellets and preforms]
30 kg of pellets of isophthalic acid copolymerized polyethylene terephthalate resin (manufactured by Shinko Synthetic Fibers Co., Ltd., isophthalic acid copolymerization ratio 1.8 mol%, IV = 0.83) were prepared and dried using a hopper dryer at 150 ° C for 5 hours. The pellets were then fed into a twin-screw extruder (manufactured by Toshiba Machine Co., Ltd., TEM26SS) and melt extrusion was performed under the conditions of an extrusion temperature of 290 ° C, a screw rotation speed of 100 rpm, and a discharge rate of 10 kg / h. The molten resin discharged in a strand shape from the extruder was air-cooled while being conveyed on a belt conveyor, and pelletized using a pelletizer (first melt extrusion process). The pellets were then fed again into the twin-screw extruder and melt extrusion was performed under the same conditions as above, except that a vacuum vent (9 Torr) was used, to obtain pellets in the same manner (second melt extrusion process).

The obtained pellets (15 kg) were heated for 5 hours at 1 Torr and 150°C using an agitation vacuum dryer (Dalton, 45MV) to perform a crystallization process. Next, the pellets after the crystallization process were heated for 13 hours at 1 Torr and 225°C using the above agitation vacuum dryer to perform a solid-phase polymerization process. In the crystallization process and the solid-phase polymerization process, the rotation speed of the agitation blade of the agitation vacuum dryer was set to 20 rpm. The obtained pellets were subjected to the first and second melt extrusion processes, the crystallization process, and the solid-phase polymerization process three more times in the same manner as above to obtain PET resin pellets. That is, the thermal history control process was performed a total of four times on the virgin pellets. In addition, a part of the obtained PET resin pellets was fed to an injection molding machine, and a barrel temperature and a hot runner temperature were set to 300°C, a mold temperature was set to 15°C, and a molding cycle was set to 32 seconds to produce a preform for a 500 mL bottle weighing 25 g. The PET resin pellets and bottle preforms were evaluated according to the following procedures. The results are shown in Table 2.

[Cyclic Oligomer Content and BHET Content]
0.2 g of PET resin pellets and bottle preforms were weighed, and 1 mL of a mixed solvent of 1,1,1,3,3,3,-hexafluoro-2-propanol and chloroform (weight ratio 1/1) was added to completely dissolve the pellets. After adding 4 mL of chloroform to the solution, 5 mL of acetonitrile was gradually added and the solution was left for 3 hours to precipitate the PET polymer. 1 mL of the supernatant of this suspension was taken out and filtered through a membrane filter with a pore size of 0.20 μm, and the filtrate was measured by high performance liquid chromatography. At the same time, the BHET standard solution was also measured, and the BHET content in the pellets and bottle preforms was calculated based on the obtained calibration curve. The measurement device used was the 1200 series manufactured by Agilent Technologies. The measurement conditions were as follows: Agilent Technologies 1290 Infinity Diode Array Detector (G4212A) was used as a detector, the detection wavelength was 254 nm, and the reference wavelength was 500 nm. Agilent Technologies ZORBAX Eclipse Plus C18 (Rapid Resolution HD 2.1×150 mm 1.8 Micron) was used as a column, and the column temperature was 40° C. The mobile phase was a 0.05 wt% phosphoric acid aqueous solution as solution I and acetonitrile as solution II, with a flow rate of 0.6 mL/min and gradient conditions in Table 1 below. The injection volume was 2 μL. Regarding the cyclic oligomers in the pellets, the peak areas of the trimers and pentamers to heptamers (CT, C5, C6, C7) were determined, and C5/CT, C6/CT, and C7/CT were calculated. The BHET content in the bottle preform is shown as a standardized value, with the BHET content in the preform made from virgin pellets taken as 1.

NMR Historical Correlation Peaks
The PET resin pellets were dissolved in a mixed solvent of deuterated trifluoroacetic acid and deuterated chloroform (volume ratio 8/2), and the 1H-NMR spectrum was measured using an NMR device (JEOL, 400SS). The peak area (A _acid ) derived from all dicarboxylic acids, the area of NMR history correlation peak 1 (A _7.5 ), and the area of NMR history correlation peak 2 (A _7.8 ) were determined, and A _7.5 /A _acid and A _7.8 /A _acid were calculated. Here, the peaks derived from all dicarboxylic acids refer to the total peak areas in the ranges of 7.60 to 7.75 ppm, 7.90 to 8.55 ppm, and 8.75 to 8.90 ppm, the NMR history correlation peak 1 refers to the peak area in the range of 7.43 to 7.55 ppm, and the NMR history correlation peak 2 refers to the peak area in the range of 7.75 to 7.85 ppm. FIG. 2A is a graph showing the measurement results of the 1H-NMR spectrum in Example 6, and FIG. 2B is a graph showing the measurement results of the 1H-NMR spectrum in Comparative Example 1. In addition, for reference, the 1H-NMR spectrum was similarly measured for virgin isophthalic acid copolymerized polyethylene terephthalate resin that was not subjected to thermal history control treatment. FIG. 2C is a graph showing the measurement results of the 1H-NMR spectrum for virgin isophthalic acid copolymerized polyethylene terephthalate resin.

[Polydispersity (Mw/Mn) and Shoulder Correlation Parameter S]
A differential molecular weight distribution curve was measured using a high-speed GPC device (Tosoh, HLC-8320GPC) to determine the polydispersity (Mw/Mn). As a sample, a solution in which PET resin pellets were dissolved in a mixed solution of 1,1,1,3,3,3,-hexafluoro-2-propanol and chloroform (volume ratio 1/49) was used. Chloroform was used as the mobile phase, and a TSKgel SuperMultiporeHZ-M column manufactured by Tosoh Corporation was used as the column. The measurement temperature was 40°C. Standard polystyrene (Tosoh, PStQuickMP-M) was used as the molecular weight standard. In addition, the shoulder correlation parameter S expressed by the above formula (1) was obtained from the differential molecular weight distribution curve. The vertical axis of the differential molecular weight distribution curve was dw/dLogM, which was obtained by differentiating the concentration fraction w (%) by LogM. Fig. 3(a) is a graph showing the measurement results of the differential molecular weight distribution curve in Example 6, and Fig. 3(b) is an enlarged view of Fig. 3(a). Also, Fig. 4(a) is a graph showing the measurement results of the differential molecular weight distribution curve in Comparative Example 1, and Fig. 4(b) is an enlarged view of Fig. 4(a).

[b ^* value]
The b ^* value of the PET resin pellets and preforms was measured using an SM color computer (manufactured by Suga Test Instruments Co., Ltd.) A larger b ^* value indicates better light-shielding properties.

[Cold crystallization peak top temperature Tc1]
Using 5 mg of PET resin pellets as a sample, the cold crystallization peak top temperature Tc1 was measured using a differential scanning calorimeter (PerkinElmer, Diamond DSC). The lower the cold crystallization peak top temperature, the better the mouth crystallization suitability when the PET resin pellets are molded into a molded product. The measurement conditions were as follows.
Step 1: Hold at 20°C for 5 minutes. Step 2: Increase temperature from 20°C to 290°C at 10°C/min. Step 3: Hold at 290°C for 5 minutes. Step 4: Decrease temperature from 290°C to 20°C at 300°C/min. Step 5: Hold at 20°C for 10 minutes. Step 6: Increase temperature from 20°C to 290°C at 10°C/min. Tc1 was calculated from the peak top temperature (°C) of the cold crystallization peak in step 6.

[Measurement of acetaldehyde (AA)]
1.0 g of the PET resin pellets and the crushed samples of the preform crushed by the frozen crushing device were weighed in a glass bottle, and 5.0 mL of pure water was added and sealed. This suspension was heated for 60 minutes in an oven adjusted to a temperature of 120°C, and then cooled in ice water. 1.0 mL of the supernatant of the suspension was collected, and 0.2 mL of a 0.1% concentration of 2,4-dinitrophenylhydrazine-phosphoric acid solution was added thereto and allowed to stand for 30 minutes. The supernatant after standing was filtered through a membrane filter with a pore size of 0.20 μm, and the filtrate was measured by high performance liquid chromatography. At the same time, a standard solution of acetaldehyde was also measured, and the acetaldehyde content in the PET resin pellets and the preform was calculated based on the obtained calibration curve. The change in acetaldehyde content (ΔAA) before and after injection molding was obtained by subtracting the acetaldehyde content in the PET resin pellets from the acetaldehyde content in the preform. The smaller the change in acetaldehyde content, the smaller the increase in acetaldehyde when the PET resin pellets are molded into a product, which indicates that the effect on the flavor of the contents of the molded product is suppressed and the product has excellent flavor properties.

<Examples 2 to 7>
PET resin pellets and preforms were obtained in the same manner as in Example 1, except that the number of times the thermal history control treatment was performed on the pellets was changed to 5 to 10 times in total, and evaluation was performed in the same manner.

<Comparative Examples 1 to 3>
Except for the fact that the number of times of the thermal history control treatment for the pellets was set to 1 to 3 times in total, PET resin pellets were obtained and evaluated in the same manner as in Example 1. For Comparative Example 1, a preform was further produced using the PET resin pellets and evaluated in the same manner as in Example 1.

<Comparative Example 4>
Pellets of isophthalic acid-copolymerized polyethylene terephthalate resin that had never been subjected to thermal history control treatment were evaluated in the same manner as in Example 1. In addition, a preform was obtained using pellets that had not been subjected to thermal history control treatment, and was evaluated in the same manner as in Example 1.

As shown in Table 2, the PET resin pellets of Examples 1 to 7, in which the ratio (C5/CT) of the peak area of the pentamer oligomer (C5) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.086 or more, and the preforms molded using the same have a low BHET content and can suppress the occurrence of mold staining during molding. In addition, the PET resin pellets and preforms of Examples 1 to 7 have excellent light-shielding properties because of their large b ^* values, excellent mouth crystallization suitability because of their low Tc1, and excellent flavor properties because of their small ΔAA. On the other hand, the PET resin pellets of Comparative Examples 1 to 4, in which the C5/CT is less than 0.086, and the preforms molded using the same were all inferior in terms of the occurrence of mold staining, light-shielding properties, mouth crystallization suitability, and flavor properties.

Claims (7)

A polyester resin containing, as a main component, a polyester containing a diol unit and a dicarboxylic acid unit,
the polyester resin comprises a cyclic oligomer containing the diol unit and the dicarboxylic acid unit,
A polyester resin characterized in that the ratio (C5/CT) of a peak area of a pentamer oligomer (C5) to a peak area of a trimer oligomer (CT) in the cyclic oligomer is 0.086 or more as measured by liquid chromatography. The polyester resin according to claim 1, characterized in that the ratio (C6/CT) of the peak area of the hexamer oligomer (C6) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.058 or more. The polyester resin according to claim 1, characterized in that the ratio (C7/CT) of the peak area of the heptamer oligomer (C7) to the peak area of the trimer oligomer (CT) in the cyclic oligomer measured by liquid chromatography is 0.031 or more. A molded article made from the polyester resin according to any one of claims 1 to 3. A preform made from the polyester resin described in any one of claims 1 to 3. A polyester bottle made from the polyester resin according to any one of claims 1 to 3. The polyester resin according to any one of claims 1 to 3, characterized in that the polyester resin is a mechanically recycled polyester resin.

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