Method for producing polyester

Abstract

To provide a method for producing polyester with suppressed discoloration using chemical recycling materials.SOLUTION: The method for producing polyester includes: a first step of melting solid bis(2-hydroxyethyl terephthalate) (BHET) with a purity of 94% or higher; a second step of adding phosphorus compounds in an amount of 40 ppm to 100 ppm, in terms of phosphorus element, relative to the mass of the polyester; a third step of adding acids and alcohols; and a fourth step of polycondensing the melted BHET, acids, and alcohols to produce polyester. When one molecule of BHET is set to correspond to one molecule of acid and one molecule of alcohol, the amount of BHET is 20 mol% to 90 mol% based on the total amount of acids and 20 mol% to 90 mol% based on the total amount of alcohols. The second step is carried out before the fourth step.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a process for producing polyesters. [Background technology]

[0002] In order to reduce the environmental load and protect global resources, there is an increasing need to recycle plastic materials, and in particular, circular recycling has been attracting attention. There are two types of plastic material recycling: material recycling, in which plastic materials are melted and remolded, and chemical recycling, in which plastic materials are depolymerized and the recovered monomers / oligomers are reused as raw materials (see, for example, Patent Documents 1 and 2). Material recycling is not suitable for circular recycling, as the quality of the recycled products decreases with each recycling. To achieve circular recycling, chemical recycling, which reduces the degree of quality deterioration of recycled products, is more suitable. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-172113 A [Patent Document 2] JP 2004-290130 A [Patent Document 3] JP 2020-105435 A Summary of the Invention [Problem to be solved by the invention]

[0004] The following analysis is provided in light of the present disclosure.

[0005] In order to keep recycling activities sustainable, polyesters regenerated from raw materials derived from chemical recycling are required to be equivalent to polyesters produced from petroleum-derived monomers. However, even refined raw materials obtained by chemical recycling contain impurities, and polyesters produced from such raw materials generally discolor more than polyesters produced from petroleum-derived monomers. In particular, discoloration is more noticeable in the thick parts of molded products.

[0006] For example, recycled bis(2-hydroxyethyl terephthalate) obtained by chemical depolymerization of polyethylene terephthalate (PET) contains metals used as catalysts and by-products caused by decomposition. Therefore, even if polyester is produced by a known polyester production method using bis(2-hydroxyethyl terephthalate) with a purity of 95% or more as a monomer, the produced polyester will be colored yellow.

[0007] In the method for producing a polyester resin composition described in Patent Document 3, a phosphorus compound is added after the esterification reaction of monomers such as terephthalic acid and ethylene glycol to improve the color tone of the polyester resin. It is believed that the phosphorus compound is added as a heat stabilizer in the polymerization process. However, even if the production method described in Patent Document 3 is used, the discoloration of the regenerated product from chemical recycling using bis(2-hydroxyethyl terephthalate) cannot be improved.

[0008] Therefore, a method for producing polyester with reduced discoloration using raw materials recovered by chemical recycling is desired. [Means for solving the problem]

[0009] According to a first aspect of the present invention, there is provided a method for producing a polyester, comprising a first step of melting solid bis(2-hydroxyethyl terephthalate) having a purity of 94% or more, a second step of adding a phosphorus compound in an amount of 40 ppm to 100 ppm in terms of phosphorus element relative to the mass of the polyester, a third step of adding an acid and an alcohol, and a fourth step of polycondensing the melted bis(2-hydroxyethyl terephthalate), the acid, and the alcohol to produce a polyester. When it is assumed that one molecule of bis(2-hydroxyethyl terephthalate) corresponds to one molecule of acid and one molecule of alcohol, the amount of bis(2-hydroxyethyl terephthalate) is 20 mol% to 90 mol% relative to the total amount of acid, and 20 mol% to 90 mol% relative to the total amount of alcohol. The second step is carried out before the fourth step. Effect of the Invention

[0010] According to the polyester production method of the present disclosure, even if recycled raw materials are used, polyester with excellent color tone and heat resistance can be produced. [Brief description of the drawings]

[0011] [Figure 1] 1 is a flow chart illustrating a method for producing polyester of the present disclosure. [Diagram 2] 1 is a flow chart illustrating a method for producing polyester of the present disclosure. [Diagram 3] 1 is a flow chart illustrating a method for producing polyester of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Preferred embodiments of each of the above aspects are described below.

[0013] According to a preferred embodiment of the first aspect, the bis(2-hydroxyethyl terephthalate) melted in the first step is produced by thermal decomposition of a polyester.

[0014] According to a preferred embodiment of the first aspect, in the first step, bis(2-hydroxyethyl terephthalate) is melted under conditions equivalent to 130° C. to 160° C. under atmospheric pressure.

[0015] According to a preferred embodiment of the first aspect, the phosphorus compound is at least one selected from the group consisting of triethyl phosphate, orthophosphoric acid, and trimethyl phosphate.

[0016] According to a preferred embodiment of the first aspect, the acid is at least one selected from the group consisting of terephthalic acid and isophthalic acid, and the alcohol is at least one selected from the group consisting of ethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol.

[0017] According to a preferred embodiment of the first aspect, the polyester produced in the fourth step has a yellowness index of 14 or less.

[0018] According to a preferred embodiment of the first aspect, the method for producing a polyester further includes a fourth step of molding the polyester produced in the fourth step.

[0019] According to a preferred embodiment of the first aspect, the fourth step includes a step of esterifying bis(2-hydroxyethyl terephthalate), an acid, and an alcohol, and a step of polymerizing the esterified product obtained in the esterification step.

[0020] A method for producing a polyester according to a first embodiment of the present disclosure will be described. In the present disclosure, unless otherwise specified, the polyester may also include a molded product. Figures 1 to 3 show a flow chart illustrating the method for producing a polyester according to the present disclosure.

[0021] The method for producing polyester according to the first embodiment of the present disclosure includes a first step of melting solid bis(2-hydroxyethyl terephthalate) [also known as bis(2-hydroxyethyl terephthalate)]. In the present disclosure, bis(2-hydroxyethyl terephthalate) may be abbreviated as "BHET". The temperature at which BHET is melted is preferably a condition equivalent to 130°C to 160°C under atmospheric pressure. If the temperature is less than 130°C, unmelted BHET may be produced, which may prolong the polymerization time and deteriorate the color tone of the polyester. If the temperature exceeds 160°C, the produced polyester may be significantly discolored (the color tone may be reduced). It is preferable to melt BHET under atmospheric pressure.

[0022] In the present disclosure, BHET is not included in the alcohols, polyols, and polyhydroxy compounds shown in the present disclosure.

[0023] BHET can be a recycled compound from waste plastics, a petroleum-derived compound, or a biomass-derived compound. Examples of recycled compounds include chemically recycled compounds from waste plastics.

[0024] Examples of the chemically recycled compound include BHET obtained by thermal decomposition of polyethylene terephthalate (PET) or polyester containing PET, and BHET obtained by enzymatic decomposition of polyethylene terephthalate (PET) or polyester containing PET. It is preferable that the BHET obtained by thermal decomposition is isolated and purified rather than used directly from the thermal decomposition process. The purity of the BHET before melting (BHET used in the first step) is preferably 94% or more (impurities less than 6% by mass), and more preferably 95% or more (impurities less than 5% by mass). In addition, it is preferable that the BHET before melting (BHET used in the first step) does not contain Sb, T, Co, Mg, and Al. In the method for producing polyester of the present disclosure, when BHET is a chemically recycled compound, the color tone improvement effect is more pronounced.

[0025] The ratio of BHET added can be set appropriately. It is considered that a part of the ethylene glycol component in BHET is liberated as ethylene glycol in the esterification step and the polymerization step. It is considered that the liberated ethylene glycol is discharged from the reaction system by the reduced pressure in the polymerization step. In the present disclosure, it is assumed that one molecule of BHET corresponds to one molecule of an acid compound and one molecule of an alcohol compound as a polyester monomer.

[0026] The ethylene glycol discharged from the reaction system can be recovered and reused in the production of polyesters.

[0027] The amount of BHET can be 20 mol% or more, 30 mol% or more, 40 mol% or more, or 50 mol or more, based on the total amount of the acid components and the total amount of the alcohol components, respectively. The amount of BHET can be 90 mol% or less, 80 mol% or less, 70 mol% or less, or 60 mol% or less, based on the total amount of the acid components and the total amount of the alcohol components, respectively. If the amount of BHET is more than 90 mol% based on the total amount of the acid components and the total amount of the alcohol components, respectively, discoloration of the produced polyester becomes severe.

[0028] The method for producing polyester according to the first embodiment of the present disclosure includes a second step of adding a phosphorus (P) compound. The phosphorus compound may be at least one selected from the group consisting of triethyl phosphate, orthophosphoric acid, and trimethyl phosphate. The phosphorus compound is preferably 40 ppm or more in terms of phosphorus element relative to the mass of the polyester produced. The phosphorus compound may be 50 ppm or more, 55 ppm or more, or 60 ppm or more in terms of phosphorus element relative to the mass of the polyester produced. If the phosphorus compound is less than 40 ppm (in terms of phosphorus element), the color tone of the polyester produced will be reduced. The phosphorus compound is preferably 100 ppm or less in terms of phosphorus element relative to the mass of the polyester produced. The phosphorus compound may be 90 ppm or less, 80 ppm or less, 70 ppm or less, or 60 ppm or less in terms of phosphorus element relative to the mass of the polyester produced. If the phosphorus compound is more than 100 ppm (in terms of phosphorus element), the reactivity will be reduced.

[0029] The amount of phosphorus compound to be added can be calculated using the following formula. Amount of phosphorus compound added (g) = amount of phosphorus element added (ppm) × molecular weight of phosphorus compound (g / mol) / weight of phosphorus atom (g / mol) × mass of polyester (g) × 0.000001

[0030] As shown in FIG. 1 to FIG. 3, the second step (S12, S21, S32) is performed before the fourth step (S14, S24, S33) described later. The second step can be performed during the first step. The second step can be performed after the first step and before the third step. The second step can be performed during the third step. The phosphorus compound can be added to the solid-state BHET before the start of heating (or the solid-state BHET and the phosphorus compound are placed in a container). The phosphorus compound can be added, for example, at the start of heating to melt the BHET or at the start of melting the BHET. The phosphorus compound can be added, for example, after the start of melting the BHET. The phosphorus compound can be added, for example, to the molten BHET before the addition of the acid and the alcohol. The phosphorus compound can be added, for example, to the molten BHET together with the acid and the alcohol. By adding the phosphorus compound at such a timing, the color tone of the produced polyester can be suppressed from decreasing. The phosphorus compound can make the color of the polyester lighter by adding it before the start of heating the BHET or while the BHET is melting.

[0031] The method for producing a polyester according to the first embodiment of the present disclosure further comprises a third step of adding an acid and an alcohol to the BHET. The acid and the alcohol are polycarboxylic acids and polyols (polyhydroxy compounds) that act as monomers with the BHET to form a polyester.

[0032] In the present disclosure, polycarboxylic acid refers to a compound having a plurality of carboxy groups. The polycarboxylic acid can be appropriately selected according to the target polyester. Examples of the polycarboxylic acid include at least one selected from the group consisting of terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, succinic acid, dimer acid, 1,4-cyclohexadicarboxylic acid, dimethyl terephthalate, dimethyl isophthalate, trimellitic acid, and derivatives thereof. Examples of the derivatives include ester compounds. Any of the polycarboxylic acids may be added alone, or two or more may be added in any ratio. The acid component that is insufficient with only BHET can be supplemented by adding additional polycarboxylic acid. The polycarboxylic acid is preferably at least one selected from the group consisting of terephthalic acid and isophthalic acid. The polycarboxylic acid may be any of recycled compounds from waste plastics, compounds derived from petroleum, and compounds derived from biomass.

[0033] In the present disclosure, polyol or polyhydroxy compound refers to a compound having multiple hydroxy groups. Examples of polyols include at least one selected from the group consisting of ethylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,2-propanediol, 1,4-butanediol, 1,3-butanediol, diethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), polyethylene glycol, polytetramethylene glycol, and derivatives thereof. Any one of the polyols may be added alone, or two or more of them may be added in any ratio. The polyol is preferably at least one selected from the group consisting of ethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol.

[0034] The amount of polycarboxylic acid and / or polyol added in the third step can be appropriately adjusted depending on the amount of BHET added, the balance between the acid component and the alcohol component in the reaction system, etc. One molecule of BHET contains one terephthalic acid component and two ethylene glycol components. However, in the calculation of the monomer, one molecule of BHET is considered to correspond to one terephthalic acid component and one ethylene glycol component from the possible reaction mechanism.

[0035] Terephthalic acid can be 60 mol% or more, 70 mol% or more, 75 mol% or more, 80 mol% or more, 85 mol% or more, 90 mol% or more, 95 mol% or more, or 100 mol% based on the total amount of BHET and acid. Terephthalic acid can be 97 mol% or less, 95 mol% or less, 90 mol% or less, 85 mol% or less, 80 mol% or less, 75 mol% or less, or 70 mol% or less based on the total amount of BHET and acid.

[0036] The amount of isophthalic acid can be 3 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, or 20 mol% or more based on the total amount of BHET and acid. The amount of isophthalic acid can be 25 mol% or less, 20 mol% or less, 15 mol% or less, 10 mol% or less, or 5 mol% or less based on the total amount of BHET and acid.

[0037] The total amount of BHET and at least one selected from the group consisting of isophthalic acid and terephthalic acid can be 100 mol % based on the total amount of acids.

[0038] The amount of ethylene glycol can be 5 mol% or more, 10 mol% or more, 20 mol% or more, 30 mol% or more, 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, or 80 mol% or more based on the total amount of BHET and polyol. The amount of ethylene glycol can be 90 mol% or less, 80 mol% or less, 70 mol% or less, 80 mol% or less, 70 mol% or less, or 60 mol% or less based on the total amount of BHET and polyol.

[0039] Neopentyl glycol can be 5 mol% or more, 10 mol% or more, 20 mol% or more, 30 mol% or more, or 40 mol% or more based on the total amount of BHET and polyol. Neopentyl glycol can be 50 mol% or less, 40 mol% or less, 30 mol% or less, 20 mol% or less, 10 mol% or less, or 5 mol% or less based on the total amount of BHET and polyol.

[0040] The amount of 1,4-cyclohexanedimethanol can be 5 mol% or more, 10 mol% or more, 20 mol% or more, 30 mol% or more, or 40 mol% or more based on the total amount of BHET and polyol. The amount of 1,4-cyclohexanedimethanol can be 50 mol% or less, 40 mol% or less, 30 mol% or less, 20 mol% or less, 10 mol% or less, or 5 mol% or less based on the total amount of BHET and polyol.

[0041] The total amount of BHET and at least one selected from the group consisting of ethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol may be 100 mol % based on the total amount of alcohols.

[0042] The method for producing a polyester according to the first embodiment of the present disclosure further includes a fourth step of polycondensing the BHET melted in the first step and the acid and alcohol added in the third step as at least a portion of the monomers to produce a polyester.

[0043] The fourth step may include a step of esterifying BHET, an acid, and an alcohol to prepare an esterified product (oligomer), and a step of polymerizing the esterified product obtained in the esterification step. The esterification step and the subsequent polymerization step may be carried out by a known method. The esterified product referred to here is an oligomer having a molecular weight of about 10,000.

[0044] In the esterification step, for example, an ester prepolymer may be produced by direct esterification using an unsubstituted polycarboxylic acid as a starting material, or an ester prepolymer may be produced by transesterification using an esterified product such as dimethyl ester as a starting material. From the viewpoint of production efficiency, it is preferable to select a direct esterification reaction.

[0045] The direct esterification reaction can be carried out, for example, by charging the raw materials into a reaction tank equipped with a heater, a stirrer, and a distillation tube, raising the temperature while stirring under an inert gas atmosphere at atmospheric pressure, and allowing the reaction to proceed while distilling off water produced by the reaction. The direct esterification can be carried out at 240°C to 270°C under a pressure of 0 to 3 kg / cm2G, and preferably at 245°C to 255°C. The reaction time can be, for example, 3 to 7 hours.

[0046] The transesterification reaction can be carried out, for example, by charging the raw materials into a reaction vessel equipped with a heater, a stirrer, and a distillation tube, adding a reaction catalyst, and raising the temperature while stirring under an inert gas atmosphere at atmospheric pressure, and allowing the reaction to proceed while distilling off the methanol produced by the reaction. The reaction temperature can be, for example, 150°C to 270°C, and is preferably 160°C to 260°C. The reaction time is, for example, about 3 to 7 hours.

[0047] As the catalyst for the transesterification reaction, at least one metal compound can be used. Preferable metal elements include, for example, sodium, potassium, calcium, titanium, lithium, magnesium, manganese, zinc, tin, cobalt, etc. Among these, titanium and manganese compounds are preferred because they have high reactivity and the color tone of the resulting resin is good. The amount of the transesterification catalyst added is usually preferably 5 ppm to 1000 ppm, more preferably 10 ppm to 100 ppm, based on the polyester resin produced.

[0048] Following the direct esterification reaction or the transesterification reaction, a polymerization catalyst is added to the ester prepolymer, and a polycondensation reaction is further carried out until the desired molecular weight is reached. As the catalyst in the polymerization reaction, for example, germanium dioxide can be used. The catalyst addition rate can be, for example, 50 ppm to 300 ppm relative to the amount of resin produced. The polycondensation reaction can be carried out, for example, by gradually increasing the temperature and reducing the pressure in the reaction tank after adding the polymerization catalyst. It is preferable to reduce the pressure in the tank to, for example, 0.4 kPa or less, preferably 0.2 kPa or less, finally. It is preferable to increase the temperature in the tank to, for example, 250°C to 290°C, finally. The polymerization reaction can be carried out, for example, under reduced pressure until the final tank pressure is 150 Pa or less, until the desired melt viscosity is reached. Thereafter, the tank pressure can be increased to, for example, 0.5 MPa, and the reaction product can be extruded from the bottom of the tank and collected. For example, the reaction product can be extruded into water in the form of a strand, cooled, and then cut to obtain a pellet-shaped polyester resin.

[0049] The polymerization catalyst is preferably germanium dioxide, which can suppress discoloration.

[0050] The polyester of the present disclosure may be appropriately blended with various additives such as antioxidants, heat stabilizers, lubricants, antistatic agents, plasticizers, ultraviolet absorbers, and pigments, within the scope of not impairing the effects of the present disclosure. These additives may be blended in either the polymerization reaction step or the processing / molding step. Examples of the antioxidant include hindered phenol-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants, with hindered phenol-based antioxidants being particularly preferred. The amount of addition is preferably about 100 ppm to 5000 ppm.

[0051] The method for producing the polyester according to the first embodiment of the present disclosure may further include a fifth step of molding the polyester produced in the fourth step.

[0052] An example of a method for molding polyester will be described below. As a method for producing a polyester molded body, for example, injection molding can be adopted.

[0053] First, the polyester is melted. The set temperature of the heating device (e.g., a cylinder) for melting the polyester is a temperature that does not cause an unmelted body of the composition. The set temperature of the heating device is preferably 270°C or less, more preferably 200°C or less, and can be 180°C or less depending on the composition. By lowering the heating temperature, the cooling time can be shortened to increase production efficiency and suppress quality deterioration.

[0054] Secondly, the molten polyester is filled into the mold. The mold can be maintained at a predetermined temperature. The mold temperature can be set, for example, at 20°C to 60°C, preferably 30°C to 50°C. If the mold temperature is set below 20°C, a large amount of energy is required for cooling. In addition, condensation occurs on the mold, accelerating deterioration of the mold. It is preferable to cool the mold with water.

[0055] Third, the polyester filled in the mold is held in the mold for a specified time to form the molded product. After molding, the molded product is released from the mold. The time it takes to hold the resin from when it is poured into the mold until it is released is the cooling time (molding time). The cooling time depends on the size of the molded product, especially its thickness.

[0056] According to the method for producing polyester of the first embodiment, discoloration of polyester caused by BHET of recycled compounds can be suppressed. That is, even if BHET of recycled compounds is used as a raw material, polyester with suppressed discoloration can be produced. Molded bodies of this polyester can be molded into molded bodies having thick portions. Moreover, according to the method for producing polyester of the first embodiment, polyester with high heat resistance can be produced even if BHET of recycled compounds is used as a raw material.

[0057] According to the method for producing a polyester according to the first embodiment, it is possible to activate resource circulation through chemical recycling.

[0058] The polyester according to the second embodiment of the present disclosure will be described.

[0059] The polyester according to the second embodiment of the present disclosure may be a polyester produced by the method for producing a polyester according to the first embodiment of the present disclosure.

[0060] There are cases where it is not possible or is largely impractical to directly characterize the polyesters of the present disclosure by composition, structure, properties, etc. In such cases, it should be permitted to characterize the polyesters of the present disclosure by their method of manufacture.

[0061] The polyester of the present disclosure can have a composition according to the proportions of BHET, acid, and alcohol added in the first embodiment, and the amount of the phosphorus compound added.

[0062] The polyester of the present disclosure may have a yellowness index (YI) of 14 or less, preferably 13 or less, preferably 12 or less, and more preferably 11 or less.

[0063] Yellowness index (YI) is the L value of the Lab color system measured on polyester. * Value, a * value and b * It can be calculated from the value according to the following formula: YI=100(1.28×X-1.06×Z) / Y In the above formula, X, Y and Z are calculated from the following formula: X=98.072×L * ×a * / 17500+98.072×Y / 100 Y=L *2 / 100 Z=118.225×Y / 100-118.225×L * ×b * / 7000

[0064] The polyesters of the present disclosure may have a glass transition temperature of at least 70° C. The glass transition temperature may be measured using a differential scanning calorimeter (DSC).

[0065] Even when the polyester of the present disclosure is molded into a plate having a thickness of 7.0 mm, the haze can be kept at 1 or less. The haze can be measured in accordance with JIS K7105. EXAMPLES

[0066] The polyester and the method for producing the same according to the present disclosure will be described below using examples. The polyester and the method for producing the same according to the present disclosure are not limited to the following examples.

[0067] [Test Examples 1-19] Polyesters were produced using bis(2-hydroxyethyl terephthalate) as a monomer, which was isolated and purified by chemical recycling of polyethylene terephthalate. Tables 1 to 4 show the feed composition, production conditions, and physical properties of the produced polyesters.

[0068] The abbreviations used in the table are as follows: BHET: Bis(2-hydroxyethyl terephthalate) TPA: Terephthalic acid IPA: Isophthalic acid EG: Ethylene glycol NPG: Neopentyl glycol CHDM: 1,4-cyclohexanedimethanol

[0069] BHET is partially hydrolyzed during the reaction to release ethylene glycol. In the table below, the composition is determined as one molecule of BHET being a compound formed by esterification of one molecule of TPA and one molecule of EG.

[0070] In the following table, the purity of BHET indicates the purity of BHET used as a raw material. The contents of Sb, Ti, Co, P, Mg, and Al in the BHET used were below the detection limit. The melting temperature indicates the set temperature of the reaction vessel when melting BHET. "At the time of melting" in the "Phosphorus compound added" column indicates that the phosphorus compound was added when melting BHET. "After esterification" in the "Phosphorus compound added" column indicates that the phosphorus compound was not added when melting BHET, but was added after the esterification reaction of acid and alcohol and before the polycondensation reaction. The amount of phosphorus compound added indicates the amount of phosphorus compound added in terms of phosphorus element relative to the mass of polyester. "After melting" in the "Acid / alcohol added" column indicates that acid and alcohol were added after the melting of BHET was completed. "At the time of melting" in the "Acid / alcohol added" column indicates that acid and alcohol were added while melting BHET.

[0071] The yellowness index (YI) was calculated from the above formula by measuring each value of the Yab color system for the produced polyester pellets using a color meter (manufactured by Nippon Denshoku).

[0072] The haze was measured by molding each composition into a plate with a thickness of 7.0 mm using an injection molding machine (manufactured by Sumitomo Heavy Industries, Ltd.) and measuring the haze of each plate using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K 7105. The plates were molded at a temperature of 180°C to 290°C depending on the composition.

[0073] The glass transition temperature was measured by weighing 10 mg of polyester and using a differential scanning calorimeter DSC (DSC2500 manufactured by TA Instruments).

[0074] The method for producing polyester in Test Example 1 will be described. BHET was placed in a reaction vessel equipped with an agitator, a thermometer, and a distillation tube, and BHET was heated at a set temperature of 160°C under nitrogen flow and atmospheric pressure. At the start of heating, 60 ppm of triethyl phosphate was added as a phosphorus compound relative to the mass of BHET. After melting BHET, TPA, IPA, and EG were added, and an esterification reaction was carried out at 250°C for 3 hours under stirring, nitrogen flow, and atmospheric pressure conditions. Next, the temperature was gradually increased while the pressure was reduced so that the final temperature was 280°C and the pressure was 0.2 hPa. Using germanium dioxide as a polymerization catalyst, the reaction was carried out until the torque value of the agitator blade corresponding to the intrinsic viscosity reached the desired value, and the polycondensation reaction was terminated. The obtained polyester resin was extracted in the form of strands from an extraction port at the bottom of the reaction vessel, cooled in a water tank, and then cut into chips.

[0075] In Test Example 17, the BHET melting step and the esterification step were not necessary, so the polymerization step was started from the beginning.

[0076] [Table 1]

[0077] [Table 2]

[0078] [Table 3]

[0079] [Table 4]

[0080] [Table 5]

[0081] In Test Examples 1 to 10 and 19 to 23, the yellowness index of the produced polyester could be reduced to 14 or less. In other words, discoloration of the polyester could be suppressed. It is considered that discoloration of the polyester can be suppressed if the recycled raw material BHET is 90 mol % or less, preferably 80 mol % or less, based on the total amount of BHET and acid and the total amount of BHET and alcohol components.

[0082] When BHET was 100 mol%, the reaction temperature was difficult to increase due to the latent heat of excess ethylene glycol, and the polymerization time was prolonged, resulting in strong discoloration.

[0083] Discoloration of the polyester was also suppressed when components other than terephthalic acid and ethylene glycol, such as isophthalic acid, neopentyl glycol, and 1,4-cyclohexanedimethanol, were copolymerized.

[0084] When the purity of BHET was low, discoloration became more pronounced and heat resistance also decreased. It is considered that the purity of BHET, which is a recycled raw material, is preferably 94% or higher, and more preferably 95% or higher.

[0085] The heating temperature when melting BHET under atmospheric pressure is preferably 130° C. or higher. If the heating temperature is lower than 130° C., unmelted BHET will be generated. The heating temperature when melting BHET under atmospheric pressure is preferably 160° C. or lower. If the heating temperature exceeds 160° C., the polyester will discolor significantly.

[0086] The phosphorus compound is preferably 40 ppm or more, more preferably 50 ppm or more, calculated as phosphorus element, relative to the mass of polyester. If the phosphorus compound is less than 40 ppm, the discoloration of polyester becomes strong. If the phosphorus compound is more than 100 ppm or less, more preferably 90 ppm or less, calculated as phosphorus element, relative to the mass of polyester. If the phosphorus compound is more than 100 ppm, the activity of the polymerization catalyst is reduced, the reactivity is reduced, and polyester cannot be produced.

[0087] The timing for adding phosphorus is preferably when BHET is melted. If phosphorus is added after BHET is melted and before the polymerization reaction, the polyester will be significantly discolored.

[0088] The timing for adding the acid and alcohol other than BHET may be either when melting the BHET or after melting the BHET.

[0089] The glass transition temperatures of polyesters produced using petroleum-derived monomers are shown in Table 6. The polyesters produced under the conditions of the present disclosure, even when using recycled BHET, have glass transition temperatures equivalent to those shown in Table 6 and are considered to have sufficient heat resistance.

[0090] [Table 6]

[0091] In Test Example 10, the haze was not measured because the crystallinity was high.

[0092] The polyester and its manufacturing method of the present invention have been described based on the above-mentioned embodiments and examples, but are not limited to the above-mentioned embodiments and examples, and may include various modifications, changes and improvements to each disclosed element (including elements described in the claims, specification and drawings) within the scope of the present invention and based on the basic technical idea of ​​the present invention. Furthermore, various combinations, substitutions or selections of each disclosed element are possible within the scope of the claims of the present invention.

[0093] Further objects, objectives and modes (including modifications) of the present invention will become apparent from the entire disclosure of the present invention including the claims.

[0094] With respect to numerical ranges set forth herein, unless otherwise stated, any numerical value or range falling within that range should be construed as being specifically set forth herein.

[0095] A part or all of the above-described embodiments may be described as, but are not limited to, the following supplementary notes. Each supplementary note may be combined with each claim in the claims. [Appendix 1] A polyester produced by the polyester production method of the present disclosure. [Appendix 2] The polyester described in the appendix has a yellowness index of 14 or less. [Appendix 3] The polyester described in the appendix has a glass transition temperature of 70°C or higher. [Appendix 4] A polyester as described in the appendix, having a haze of 1 or less at a thickness of 7.0 mm. [Industrial Applicability]

[0096] The polyester production method of the present disclosure can be applied to the production of polyester from chemically recycled raw materials.

Claims

1. A first step of melting solid bis(2-hydroxyethyl terephthalate) having a purity of 94% or more; a second step of adding a phosphorus compound in an amount of 40 ppm to 100 ppm in terms of phosphorus element relative to the mass of the polyester; a third step of adding an acid and an alcohol; and a fourth step of polycondensing the molten bis(2-hydroxyethyl terephthalate), the acid and the alcohol to produce a polyester; When it is defined that one molecule of bis(2-hydroxyethyl terephthalate) corresponds to one molecule of the acid and one molecule of the alcohol, the amount of bis(2-hydroxyethyl terephthalate) is 20 mol % to 90 mol % with respect to the total amount of the acid and is 20 mol % to 90 mol % with respect to the total amount of the alcohol, The method for producing a polyester, wherein the second step is carried out before the fourth step.

2. 2. The method for producing a polyester according to claim 1, wherein the bis(2-hydroxyethyl terephthalate) melted in the first step is produced by thermal decomposition of a polyester.

3. 3. The method for producing a polyester according to claim 1, wherein in the first step, bis(2-hydroxyethyl terephthalate) is melted under conditions corresponding to 130° C. to 160° C. under atmospheric pressure.

4. The method for producing a polyester according to any one of claims 1 to 3, wherein the phosphorus compound is at least one selected from the group consisting of triethyl phosphate, orthophosphoric acid, and trimethyl phosphate.

5. The acid is at least one selected from the group consisting of terephthalic acid and isophthalic acid; The method for producing a polyester according to any one of claims 1 to 4, wherein the alcohol is at least one selected from the group consisting of ethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol.

6. The method for producing a polyester according to any one of claims 1 to 5, wherein the polyester produced in the fourth step has a yellowness index of 14 or less.

7. The method for producing a polyester according to any one of claims 1 to 6, further comprising a fifth step of molding the polyester produced in the fourth step.

8. the fourth step is a step of esterifying bis(2-hydroxyethyl terephthalate), the acid and the alcohol; and a step of polymerizing the esterified product obtained in the esterification step.

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