JP7721075B2 - Recycled polyester resin and method for producing recycled polyester resin - Google Patents

Description

The present invention relates to a novel recycled polyester resin and a method for producing the same. In particular, the present invention relates to a recycled polyester resin produced using recycled polyester raw materials derived from used polyester products, as well as recycled polyester raw materials derived from unused polyester generated during the process of producing polyester products, which contains little foreign matter and can be processed into various molded products in the same way as virgin polyester resin, and a method for producing the same.

Polyethylene terephthalate (hereafter sometimes abbreviated as PET) has a high melting point, is chemical resistant, and is relatively low cost, making it widely used in fibers, films, plastic bottles, and other molded products. The production of waste is unavoidable during the manufacturing and processing stages of these polyester products, and they are often disposed of after use. However, when incinerated, the high heat generated causes significant damage to incinerators and shortens their lifespan. On the other hand, when not incinerated, the waste remains semi-permanently because it does not decompose.

In recent years, plastic containers and other once-used polyester products discarded as garbage have been found to flow into the ocean via rivers, where they are broken down by the action of waves or tidal currents and accumulate in the bodies of marine organisms as microplastics. These microplastics then accumulate in the food chain, adversely affecting the marine ecosystem. Plastics have become a major cause of marine pollution, and there is growing concern around the world that plastics are a major cause of marine pollution, leading to a movement to reduce their use and switch to biodegradable plastics.

In light of these environmental issues, various methods of recycling are being used to reuse resources. For polyester products such as PET, in addition to recycling the polyester scraps generated during the manufacturing process, methods are being considered for collecting products that have been on the market and then discarded, and reusing them as raw materials. In particular, in recent years, textile products that have been awarded the Eco Mark, which is certified as achieving a certain recycling rate, have become popular.

Polyester fibers are widely used due to their excellent physical properties, weather resistance, chemical resistance, and wash-and-wear properties. Furthermore, various methods are used to impart high color development to polyester fibers with cationic dyes by copolymerizing aromatic dicarboxylic acids with metal sulfonate groups, or to further impart atmospheric dyeability by copolymerizing diol components such as 1,4-butanediol or polyalkylene glycol, or aliphatic dicarboxylic acids such as adipic acid or sebacic acid. Even with such cationic dyeable polyester fibers, the use of recycled polyester resins is being considered from an environmental perspective.

However, because recycled polyester is processed into a variety of products and then recovered after use, the regenerated polyester fibers obtained from recycled polyester are prone to coloring and discoloration. Furthermore, recycled polyester has significant variations in physical properties such as melt viscosity, molecular weight, and degree of crystallinity, and its properties are not stable between lots. As a result, the performance of the resulting fibers is less uniform, and when dyed, color spots can occur within the product or color differences can occur between packaging units.

Various methods have been proposed for recycling polyester using recycled polyester scraps generated during the manufacturing process or recovered used polyester products as raw materials. For example, a method has been proposed in which methanol is added to PET scraps to decompose them into dimethylene terephthalate (hereinafter sometimes referred to as "DMT") and ethylene glycol (hereinafter sometimes referred to as "EG") (Patent Document 1), a method has been proposed in which EG is added to PET scraps to depolymerize them, and then methanol is added to recover DMT (Patent Document 2), and a method has been proposed in which PET scraps are depolymerized with EG to form oligomers, which are then used in a polycondensation reaction (Patent Document 3).

Incidentally, impurities that can become a problem when recycling PET bottles and other products include various additives added to the polyester resin, as well as items attached to the bottle itself, such as a) caps (aluminum, polypropylene, polyethylene), b) inner stoppers, c) liners (polypropylene, polyethylene), d) labels (paper, resins such as polystyrene, ink), e) adhesives, and f) printing ink.

Typically, as a pretreatment step for the recycling process, recovered PET bottles are passed through a vibrating sieve to remove sand, metal, etc. The PET bottles are then washed, and colored bottles are separated before being roughly crushed. Labels and other materials are then removed by air separation. Furthermore, aluminum pieces from caps and other materials are removed and the PET bottle fragments are finely crushed. High-temperature alkaline washing is then performed to remove components such as adhesives, proteins, and mold, and different components such as polypropylene and polyethylene are separated based on differences in specific gravity.
However, even after these steps, it is difficult to completely separate and remove non-polyester resins, particularly polypropylene, polyethylene, polystyrene, etc., from the PET resin.

For example, even if an attempt is made to produce recycled polyester resin using the recycling methods described in Patent Documents 1 to 4, it is difficult to sufficiently remove non-polyester resin-derived foreign matter, and the amount of contaminants cannot be said to be sufficiently reduced, making it difficult to obtain a product with the same quality as virgin polyester resin. If the amount of contaminants is not sufficiently reduced, the filter pressure rise rate in the spinning or film-forming process is fast, making long-term continuous operation impossible and significantly reducing processing operability. Furthermore, methods such as those described in Patent Documents 1 to 3 require significant costs for the installation, operation, and maintenance of recovery equipment, leaving room for improvement in terms of practicality.

Special Publication No. 42-8855 Japanese Unexamined Patent Publication No. 48-62732 Japanese Unexamined Patent Publication No. 60-248646 Japanese Patent Application Laid-Open No. 2005-171138

As such, no method has yet been developed to obtain recycled polyester resin from which not only various inorganic substances but also non-polyester resin-derived foreign matter has been sufficiently removed, allowing the production of a variety of products similar to virgin polyester resin.

In particular, when attempting to produce fibers using polyester resin through melt spinning, the amount of foreign matter contamination and thermal stability have a significant impact on productivity. When producing fibers, resin is extruded through a small-diameter nozzle in the melt spinning process, and the extruded numerous filaments are collected on a roller. If necessary, they are stretched and heat-treated, and then wound up. In this case, if a resin with a high level of foreign matter contamination or poor thermal stability is used, thread breakage is likely to occur during the melt spinning, stretching/heat treatment, and winding processes, making stable production difficult.

This problem becomes even more pronounced when attempting to produce even finer fibers. For example, it has not yet been possible to produce ultrafine fibers, particularly those with a single filament size of 0.5 decitex or less, by melt-spinning recycled polyester resin produced using polyester waste as raw materials.

The main object of the present invention is therefore to solve the above problems and provide a recycled polyester resin made from recycled polyester raw materials derived from used polyester products or recycled polyester raw materials derived from scraps generated in the process of manufacturing polyester resins and products, and copolymerized with an aromatic dicarboxylic acid having a metal sulfonate group, which is particularly suitable for producing cationic dyeable polyester fibers.

As a result of extensive research into the problems of the prior art, the inventors discovered that polyester resins obtained using recycled polyester raw materials and employing a specific manufacturing method contain minimal amounts of foreign matter and have the same thermal stability as virgin polyester resins, leading to the completion of the present invention.

That is, the present invention relates to the following recycled polyester resin and a method for producing the same.
1. A recycled polyester resin containing a component derived from at least one recycled polyester raw material, which is a) a used polyester product and b) unused polyester generated in the process of manufacturing a polyester product, and which satisfies all of the following (1) to (4):
(1) When the total amount of all acid components constituting the polyester is taken as 100 mol %, 77 to 97 mol % is terephthalic acid, 1.0 to 5 mol % is an aromatic dicarboxylic acid having a metal sulfonate group, and 2 to 18 mol % is an aliphatic dicarboxylic acid having 5 to 10 carbon atoms;
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol accounts for 80 mol% or more and diethylene glycol accounts for 6.5 mol% or less;
(3) The carboxyl terminal group concentration is 40 equivalents/t or less,
(4) The average pressure rise rate is 0.6 MPa/h or less (wherein the average pressure rise rate is a value calculated by the following procedure: a pressure rise tester including an extruder and a pressure sensor is used, a stainless steel filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical wire diameter: 0.07 mm, horizontal wire diameter: 0.04 mm, filtration particle size: 12 μm) is set at the tip of the extruder, a polyester resin is melted in the extruder at 300° C., and the melt is extruded from the filter at a discharge rate of 29.0 g/min. The pressure value at the start of extrusion is defined as the “initial pressure value (MPa),” and the pressure value at the time of subsequent continuous extrusion for 12 hours is defined as the “final pressure value (MPa).” The average pressure rise rate is calculated based on these pressure values using the following calculation formula A:
Average pressure increase rate (MPa / h) = (final pressure value - initial pressure value) / 12) ... A)
2. Fibers containing the recycled polyester resin according to item 1.
3. A method for producing a recycled polyester resin using at least one recycled polyester raw material selected from a) used polyester products and b) unused polyester generated in the process of producing polyester products, the method comprising the following steps (1) to (3):
(1) a step of adding the raw materials to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and an aliphatic dicarboxylic acid having 5 to 10 carbon atoms so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.30, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a reaction product containing a depolymerization product;
(2) passing the reaction product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A method for producing a recycled polyester resin, comprising the steps of adding an aromatic dicarboxylic acid having a metal sulfonate group and a polymerization catalyst to the filtrate, and carrying out a polycondensation reaction of the depolymerization at a temperature of 260°C or higher and a reduced pressure of 1.0 hPa or lower.

The recycled polyester resin of the present invention contains at least one recycled polyester raw material: a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products. However, it contains little foreign matter, has a carboxyl end group concentration and diethylene glycol content that satisfy specific ranges, and exhibits excellent thermal stability. Therefore, for example, in a process for obtaining fibers by melt spinning, continuous operation over a relatively long period of time is possible, and ultrafine fibers with a single fiber fineness of 0.5 decitex or less can be produced with good productivity.

Furthermore, because the recycled polyester resin of the present invention is copolymerized with a specific amount of aromatic dicarboxylic acid having a metal sulfonate group, the resulting fibers are dyeable with cationic dyes under normal pressure conditions, and have excellent mechanical properties and good dyeability equivalent to those made with virgin polyester resin (no color spots within the product or color differences between packaging units).

Furthermore, the method for producing recycled polyester resin of the present invention does not require complex processes or equipment, and it is possible to obtain recycled polyester resin containing copolymer components of the present invention with good operability and low cost, offering great practical benefits.

The present invention will be described in detail below.
The recycled polyester resin of the present invention (the resin of the present invention) contains components derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products. These components constitute part of the polyester constituting the resin of the present invention.

Examples of used polyester products in category a) above include polyester molded products (including fibers) that were once on the market and then collected after use. Typical examples include containers such as PET bottles and packaging materials.

The unadopted polyester generated during the process of manufacturing polyester products (b) above is polyester that did not make it into a product. Examples include off-specification resin pellets, materials no longer needed during molding, fragments cut during molding, scraps (polyester scraps) generated during molding or processing, cut-up materials of transitional products generated when changing brands, and cut-up materials of prototypes or defective products.

The above a) and b) are not limited in their form, and may be further processed, such as crushed or cut, to form pellets as needed, or may be melted and then formed into pellets. The above a) and b) may be used alone, or a mixture of the two may be used.

The recycled polyester raw materials in a) and b) above may be either crystalline or amorphous. Therefore, for example, pellets of amorphous polyester scrap that have not been heat-treated, crystalline pellets that have been heat-treated, or a mixture of crystalline and amorphous pellets can be used. In the present invention, it is preferable to use crystalline recycled polyester raw materials, particularly for the purpose of preventing fusion between pellets during charging into the tank or the depolymerization reaction. Therefore, materials in a) or b) above that have been crystallized by heat treatment (crystallized pellets, etc.) can be preferably used.

The properties of the recycled polyester raw materials in a) and b) above are not limited, and may be in the form of a) and b) above, or may be further processed by cutting, pulverizing, etc. to produce shredded pieces, pulverized material (powders), etc., or solid forms such as molded articles (pellets, etc.) formed from these. More specific examples include pellets obtained by cooling and cutting melted polyester scraps, and shredded pieces obtained by finely cutting polyester molded products such as PET bottles. Alternatively, the raw materials may be in the form of a liquid (dispersion or solution) obtained by dispersing or dissolving the shredded pieces, pulverized material (powders), etc., in a solvent. When producing polyester products using these raw materials, they can be melted at a temperature above their melting point and charged into a can as a melt, if necessary.

The recycled polyester resin of the present invention (the resin of the present invention) contains a component derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products. The content of this component in the resin of the present invention is preferably 40% by mass or more, and more preferably 50% by mass or more. If the content is less than 40% by mass, the recycling rate of the unused polyester decreases. There is no particular upper limit to the content, but according to the manufacturing method of the present invention described below, it is possible to easily obtain a recycled polyester resin with a recycled polyester raw material content of 40 to 80% by mass.

When the total amount of all acid components constituting the polyester is taken as 100 mol %, the resin of the present invention is composed of 77 to 97.5 mol % terephthalic acid, 0.5 to 5 mol % aromatic dicarboxylic acid having a metal sulfonate group, and 2 to 18 mol % aliphatic dicarboxylic acid having 5 to 10 carbon atoms. By copolymerizing appropriate amounts of aromatic dicarboxylic acid having a metal sulfonate group and aliphatic dicarboxylic acid having 5 to 10 carbon atoms, it is possible to impart dyeability to the polyester resin with cationic dyes under normal pressure conditions.

If the content (copolymerization amount) of aromatic dicarboxylic acid having a metal sulfonate group is less than 0.5 mol%, the number of dyeing sites (the number of reactive groups that react with the cationic dye) for the cationic dye in the entire polymer is insufficient, and when the resulting polyester copolymer is made into fibers, sufficient dyeing performance tends to be insufficient, which is undesirable. On the other hand, if the content exceeds 5 mol%, the melt viscosity of the polyester tends to become too high in the polycondensation process, making it difficult to sufficiently increase the degree of polymerization, which is undesirable. As a result, the material strength, such as the thread strength when made into fibers, tends to decrease.

Examples of aromatic dicarboxylic acids having a metal sulfonate group include 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, 5-lithium sulfoisophthalic acid, sodium sulfonaphthalenedicarboxylic acid, sodium sulfophenyldicarboxylic acid, and 5-sodium sulfoterephthalic acid. In the present invention, 5-sodium sulfoisophthalic acid is preferred from the standpoints of color development with cationic dyes, operability during melt spinning, and cost. These acids may be used as they are, or ester-forming derivatives may be used. From the standpoint of operability, etc., esters with ethylene glycol are preferably used.

If the content (copolymerization amount) of aliphatic dicarboxylic acid having 5 to 10 carbon atoms is less than 2 mol%, the dyeability with cationic dyes under atmospheric dyeing conditions when the copolymerized polyester is made into a fiber will be insufficient. On the other hand, if the content of aliphatic dicarboxylic acid components exceeds 18 mol%, the thermal stability of the copolymerized polyester will decrease, resulting in low yarn strength when made into a fiber.

Examples of aliphatic dicarboxylic acids having 5 to 10 carbon atoms include glutaric acid, adipic acid, pimelic acid, suberic acid, and sebacic acid. In the present invention, adipic acid is preferably used from the standpoints of operability and cost during melt spinning.

The proportion of terephthalic acid in the acid component is 77 to 97.5 mol%, with a preferred proportion of terephthalic acid being 85 to 90 mol%. If the proportion of terephthalic acid is less than 77 mol%, the crystallinity of the resin composition will decrease, the melting point will be lower, and operability will decrease during melt spinning and drawing. On the other hand, if the proportion of terephthalic acid exceeds 97.5 mol%, the amount of copolymerized aromatic dicarboxylic acid having a metal sulfonate group and aliphatic dicarboxylic acid having 5 to 10 carbon atoms will decrease, resulting in a reduced effect on cationic dye dyeability under normal pressure conditions.

In the resin of the present invention, examples of acid components other than terephthalic acid, aromatic dicarboxylic acid having a metal sulfonate group, and aliphatic dicarboxylic acid having 5 to 10 carbon atoms include isophthalic acid, phthalic acid, phthalic anhydride, naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, dodecanedioic acid, and dimer acids. Two or more of these may be used in combination, and ester-forming derivatives of these acids may also be used.

When the total amount of all glycol components in the resin of the present invention is taken as 100 mol %, ethylene glycol accounts for 80 mol % of all glycol components, and preferably 90 mol % or more. If the ethylene glycol content is less than 80 mol %, the resulting polyester resin will have poor crystallinity and heat resistance.

Furthermore, when the total amount of all glycol components in the resin of the present invention is taken as 100 mol %, the diethylene glycol content (copolymerization amount) is 6.5 mol % or less, and preferably 1.0 to 5 mol %. In particular, the resin of the present invention obtained by the production method of the present invention uses ethylene glycol as one of the raw materials, and diethylene glycol may be generated as a by-product during this process. The resin of the present invention has a small amount of diethylene glycol as a by-product, and by keeping the diethylene glycol content to 6.5 mol % or less, excellent thermal stability can be achieved. This makes it possible to produce fibers with a fine single yarn fineness with good productivity.

In the resin of the present invention, examples of diol components other than ethylene glycol and diethylene glycol in the total glycol components include neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexamethylenediol, 1,4-cyclohexanedimethanol, dimer diol, and ethylene oxide adducts of bisphenol A or bisphenol S.

The polymerization catalyst may be, but is not limited to, at least one of germanium compounds, antimony compounds, titanium compounds, cobalt compounds, etc. Among these, at least one of germanium compounds and antimony compounds is particularly preferred. When the transparency of the resulting recycled polyester resin is important, it is preferable to use a germanium compound. Examples of the above compounds include oxides, inorganic acid salts, organic acid salts, halides, sulfides, etc. of germanium, antimony, titanium, cobalt, etc.

The amount of the polymerization catalyst used is not particularly limited, but is preferably 5× ¹⁰ mol/unit or more, more preferably 6× ¹⁰ mol/unit or more, per mole of the acid component of the polyester resin to be produced. The upper limit of the amount used can be, for example, about 1× ¹⁰ mol/unit, but is not limited to this.

Note that the polymerization catalyst contained in the recycled polyester raw material may also act as a catalyst during the polycondensation reaction, so when adding a polymerization catalyst in the polycondensation process, it is preferable to take into consideration the type and content of the polymerization catalyst contained in the recycled polyester raw material.

In addition, during the polycondensation reaction, if necessary, the following can be added in addition to the above-mentioned polymerization catalyst: an alkali metal compound that can suppress the by-production of diethylene glycol; an alkaline earth metal compound that can generate fine internal particles during the polymerization reaction; a fatty acid ester that can adjust the melt viscosity; a hindered phenol-based antioxidant; a phosphorus compound that can suppress the thermal decomposition of the resin; and titanium oxide to improve whiteness.

Examples of alkali metal compounds include hydroxides, organic carboxylates, alcoholates, inorganic weak acid salts, etc., and specific examples include hydroxides of sodium, potassium, and lithium, aliphatic carboxylates such as acetates, methylates, ethylates, carbonates, and borates. Among these, sodium acetate and lithium acetate are particularly preferred because they are highly effective in suppressing the by-production of diethylene glycol. The amount of alkali metal compound added is preferably 5×10 ⁻⁴ to 3×10 ⁻³ mol per mol of the total acid components of the polyester. Within this range, the amount of diethylene glycol by-product can be suppressed.

Examples of alkaline earth metal compounds include hydroxides, organic carboxylates, alcoholates, and inorganic weak acid salts. Specific examples include hydroxides of beryllium, magnesium, calcium, strontium, barium, and radium, aliphatic carboxylates such as acetates, methylates, ethylates, carbonates, and borates. Among these, magnesium acetate and calcium acetate are preferred because they are widely available.

Examples of fatty acid esters include beeswax (a mixture primarily composed of myricyl palmitate), stearyl stearate, behenyl behenate, stearyl behenate, glycerin monopalmitate, glycerin monostearate, glycerin distearate, glycerin tristearate, pentaerythritol monopalmitate, pentaerythritol monostearate, pentaerythritol distearate, pentaerythritol tristearate, pentaerythritol tetrastearate, and dipentaerythritol hexastearate. Among these, glycerin monostearate, pentaerythritol tetrastearate, and dipentaerythritol hexastearate are preferred. These may be used alone or in combination of two or more.

Examples of hindered phenol antioxidants include 2,6-di-t-butyl-4-methylphenol, n-octadecyl-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate, tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, 4,4'-butylidenebis-(3-methyl-6-t-butylphenol), tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, and 4,4'-butylidenebis-(3-methyl-6-t-butylphenol). Examples of compounds that can be used include ethylene glycol-bis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate] and 3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1'-dimethylethyl}-2,4,8,10-tetraoxaspiro[5,5]undecane, but from the standpoint of effectiveness and cost, tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane is preferred. These can be used alone or in combination of two or more.

Examples of phosphorus compounds that can be used include phosphorous acid, phosphoric acid, trimethyl phosphite, triphenyl phosphite, tridecyl phosphite, trimethyl phosphate, triethyl phosphate, tridecyl phosphate, and triphenyl phosphate. These compounds can be used alone or in combination of two or more.

Titanium oxide is commonly used as a matting agent or white pigment for polyester, but adding an appropriate amount of titanium oxide to the resin of the present invention is preferable because it improves the whiteness of the fiber when made into fibers and allows for the production of fabrics with good color tone. The amount of titanium oxide added is preferably 0.05 to 5 parts by mass per 100 parts by mass of copolymerized polyester.

The resin of the present invention has the above-mentioned composition and also has the following characteristic values.
(a) a carboxyl terminal group concentration of 40 equivalents/t or less; (b) an average pressure rise rate measured by a pressure rise tester of 0.6 MPa or less. The resin of the present invention having these characteristic values can be obtained by the production method of the present invention described below.

First, the resin of the present invention has the characteristic value (a) of a carboxyl terminal group concentration of 40 equivalents/t or less, preferably 5 to 35 equivalents/t.
The resin of the present invention has a carboxyl end group concentration of 40 equivalents/t or less, and therefore has excellent thermal stability, and can be produced with good operability even when the single filament size is small, and also has excellent yarn quality (strength, etc.) and dyeability. Excellent dyeability means that there is no color unevenness within the product or no color difference between packaging units.

The resin of the present invention has a characteristic value (b) of an average pressure increase rate of 0.6 MPa/h or less, preferably 0.5 MPa/h or less, and more preferably 0.4 MPa/h or less, as measured by the following method. The average pressure increase rate in the present invention is an indicator of the amount of foreign matter derived from various inorganic substances and foreign matter derived from non-polyester resins, and a lower average pressure increase rate indicates a lower amount of foreign matter. By having an average pressure increase rate of 0.6 MPa/h or less, it is possible to produce fibers having a single fiber fineness of 0.5 decitex or less, for example, by melt spinning. The lower limit of the average pressure increase rate can be, for example, about 0.01 MPa/h, but is not limited to this.
If the average pressure increase rate exceeds 0.6 MPa/h, the amount of foreign matter increases, making melt spinning difficult and reducing the yarn quality and performance, and also resulting in poor dyeability, similar to when the carboxyl end group concentration becomes too high.

The average pressure rise rate was measured using a pressure rise tester including an extruder and a pressure sensor. A stainless steel filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical wire diameter: 0.07 mm, horizontal wire diameter: 0.04 mm, filtration particle size: 12 μm) was attached to the tip of the extruder. A polyester resin was melted in the extruder at 300° C., and the melt was extruded from the filter at a discharge rate of 29.0 g/min. The pressure value at the start of extrusion was defined as the "initial pressure value (MPa)," and the pressure value at the time of subsequent 12-hour continuous extrusion was defined as the "final pressure value (MPa)." The average pressure rise rate was calculated based on these pressure values using the following formula A:
Average pressure increase rate (MPa / h) = (final pressure value - initial pressure value) / 12) ... A)

By employing the manufacturing method described below, the resin of the present invention can reduce the amount of contaminants derived from various inorganic substances and non-polyester resins, making it possible to reduce the average pressure increase rate measured using a pressure increase tester, which is the characteristic value of (b), to 0.6 MPa/h or less.

The intrinsic viscosity of the recycled polyester resin of the present invention is not particularly limited, but is preferably approximately 0.35 to 0.80. The intrinsic viscosity is measured at 20°C using an equal weight mixture of phenol and tetrachloroethane as the solvent.

Next, a method for producing the resin of the present invention will be described. The production method of the present invention includes the steps (1) to (3), and it is preferable to carry out the steps (1) to (3) in this order.
(1) a step of adding the raw materials to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and an aliphatic dicarboxylic acid having 5 to 15 carbon atoms so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.30, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a reaction product containing a depolymerization product;
(2) passing the reaction product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A step of adding an aromatic dicarboxylic acid having a metal sulfonate group and a polymerization catalyst to the filtrate and carrying out a polycondensation reaction at a temperature of 250°C or higher and under a reduced pressure of 1.0 hPa or lower.

First, in the depolymerization step (1), recycled polyester raw material is added to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and an aliphatic dicarboxylic acid having 5 to 15 carbon atoms so that the molar ratio of total glycol components to total acid components is 1.05 to 1.30, and depolymerization is carried out under heat treatment conditions of 245 to 280°C to obtain a reaction product containing the depolymerized polymer.

The ethylene terephthalate oligomer, ethylene glycol, and aliphatic dicarboxylic acid having 5 to 15 carbon atoms can all be known or commercially available products. They can also be produced by known production methods.

In particular, the ethylene terephthalate oligomer may be, for example, an esterification reaction product of ethylene glycol and terephthalic acid. The number-average degree of polymerization of the ethylene terephthalate oligomer is not limited, but may be, for example, about 2 to 20.

The amount of ethylene terephthalate oligomer, or a mixture containing ethylene terephthalate and an aliphatic dicarboxylic acid having 5 to 15 carbon atoms (hereinafter sometimes referred to as mixture E) is preferably about 20 to 80 mass % of the final recycled polyester resin (100 mass %), and more preferably 30 to 70 mass %.
If the amount of ethylene terephthalate oligomer is less than the above range, the recycled polyester raw material is likely to block with itself when the recycled polyester raw material is added, which is undesirable because it places an excessive load on the mixer.
On the other hand, if the amount of ethylene terephthalate oligomer is greater than the above range, no particular problems will arise in the depolymerization reaction, but the recycling rate of the finally obtained recycled polyester resin will be undesirably low.

In step (1), the proportions of the ethylene terephthalate oligomer, ethylene glycol, aliphatic dicarboxylic acid having 5 to 15 carbon atoms, and recycled polyester raw material added may be varied as appropriate depending on the copolymerization amount of the polyester resin to be ultimately obtained, but are preferably added in the following proportions (totaling 100 parts by mass): 5 to 50 parts by mass of ethylene terephthalate oligomer, 1 to 18 parts by mass of ethylene glycol, 1 to 20 parts by mass of aliphatic dicarboxylic acid having 5 to 15 mol% of carbon atoms, and 40 to 60 parts by mass of recycled polyester raw material.
In particular, the amount of ethylene glycol added is preferably 1 to 18% by mass, and more preferably 3 to 15% by mass, relative to 100% by mass of the ethylene terephthalate oligomer in order to sufficiently proceed with the depolymerization reaction. If the amount of ethylene glycol added exceeds 18% by mass, the ethylene terephthalate oligomer tends to solidify in the reactor, and the subsequent reaction may not be able to continue.

When adding ethylene glycol or an aliphatic dicarboxylic acid having 5 to 15 mol% carbon atoms to the ethylene terephthalate oligomer in Mixture E, it is preferable to keep the temperature of the contents uniform while rotating the agitator before adding them in order to prevent the oligomer from solidifying.

When adding recycled polyester raw materials to mixture E in step (1), the depolymerization reaction is carried out under heat treatment conditions of 245 to 280°C while stirring so that the molar ratio of total glycol components to total acid components is between 1.05 and 1.30.

This step is important in the manufacturing method of the present invention. In other words, while conventional methods using recycled polyester raw materials involve depolymerization using only the recycled polyester raw material, in the present invention, the depolymerization reaction of the recycled polyester raw material is carried out in the presence of ethylene terephthalate oligomer, an aliphatic dicarboxylic acid having 5 to 15 mol% carbon atoms, and ethylene glycol, and the depolymerization reaction is carried out by adding the recycled polyester raw material in such a way that the molar ratio of ethylene terephthalate oligomer, ethylene glycol, an aliphatic dicarboxylic acid having 5 to 15 carbon atoms, and all of the components of the recycled polyester raw material is 1.05 to 1.30 (total glycol components/total acid components).

By carrying out step (1) as described above, not only various inorganic substances but also non-polyester resin-derived impurities are efficiently precipitated, allowing these impurities to be filtered out completely in the filtration step (2). Furthermore, in the polycondensation reaction of step (3), it is possible to obtain recycled polyester resin with a diethylene glycol content (copolymerization amount) and carboxyl terminal group concentration that are below specific amounts, as characteristic values of the present invention, and with a relatively low amount of impurities.

Furthermore, by depolymerizing recycled polyester raw materials in the presence of ethylene terephthalate oligomer, ethylene glycol, and an aliphatic dicarboxylic acid having 5 to 15 carbon atoms, the presence of copolymerization components makes it possible to proceed with the depolymerization reaction at a lower temperature than when depolymerization is carried out using only recycled polyester raw materials. This is a significant advantage when carried out on an industrial scale.

In the manufacturing method of the present invention, it is desirable that the recycled polyester raw material is not broken down into monomers by the depolymerization reaction, but rather into oligomers with approximately 5 to 20 repeating units. By controlling the depolymerization reaction in this way, not only various inorganic substances but also non-polyester resin-derived foreign matter are efficiently precipitated, making it possible to remove a larger amount of foreign matter.

If the molar ratio of total glycol components to total acid components during the depolymerization reaction is outside the above range, the resulting recycled polyester resin will not satisfy at least one of the carboxyl terminal group concentration and diethylene glycol content specified in the present invention, and will also have a high average pressure rise rate. This is because, when the molar ratio of total glycol components to total acid components during the depolymerization reaction is outside the above range, precipitation of various inorganic substances and non-polyester resin-derived impurities does not occur efficiently, making it impossible to filter out all of these impurities in step (2). As a result, the impurities precipitate after the polycondensation reaction in step (3), resulting in a recycled polyester resin with a high average pressure rise rate.

The reactor used in the production method of the present invention may be a commonly used esterification reactor with no particular problem in terms of volume or shape of stirring blades. However, in order to efficiently proceed with the depolymerization reaction, it is preferable that the reactor has a structure in which a distillation column is attached so as not to distill ethylene glycol out of the system.
When the recycled polyester raw material is charged, it is preferably stirred under normal pressure, and more preferably charged in a state purged with a small amount of inert gas (generally nitrogen gas).

The reaction temperature during depolymerization in step (1) is preferably set at an internal reactor temperature in the range of 245 to 280°C, and more preferably in the range of 255 to 275°C. If the reaction temperature during depolymerization is below 245°C, the reaction product will solidify, reducing operability. Even if a recycled polyester resin is obtained, the diethylene glycol content and carboxyl end group concentration will be too high. If the reaction temperature exceeds 280°C, the diethylene glycol content and carboxyl end group concentration of the resulting recycled polyester resin will be too high.

The depolymerization reaction time (the reaction time from the end of the addition of the recycled polyester raw material) is preferably within 4 hours, and more preferably within 2 hours from the standpoint of suppressing the amount of diethylene glycol by-product and suppressing deterioration in the color tone of the polyester.

In step (2), the reaction product containing the depolymerized polymer from step (1) is passed through a filter with a filtration size of 10 to 25 μm, and the filtrate is recovered. As described above, by carrying out the depolymerization reaction under the conditions of step (1), not only various inorganic substances but also foreign matter derived from non-polyester resins is efficiently precipitated. Therefore, by passing the product through a filter with a filtration size of 10 to 25 μm, the precipitated foreign matter can be filtered out, and a depolymerized polymer with a low amount of foreign matter can be obtained.

When a filter with a filtration particle size larger than 25 μm is used, foreign matter in the polymer cannot be sufficiently removed, resulting in a high content of foreign matter in the resulting recycled polyester resin. Therefore, when such a resin is used for spinning, pressure buildup in the nozzle pack and broken threads occur. On the other hand, when a filter with a filtration particle size smaller than 10 μm is used, it is prone to clogging with foreign matter and the filter life is shortened, which is cost-effective and also reduces operability.

Furthermore, filters that can be used in step (2) of the present invention include general filters without any particular problems, such as screen changer filters, leaf disc filters, and candle-type sintered filters.

In the production method of the present invention, the aromatic dicarboxylic acid having a metal sulfonate group as described above and a polymerization catalyst are added to the filtrate obtained through step (2) above, and a polycondensation reaction is carried out at a temperature of 260°C or higher and under a reduced pressure of 1.0 hPa or less.

The amount of aromatic dicarboxylic acid having a metal sulfonate group added is preferably 0.5 to 5 mol % when the total amount of all acid components constituting the polyester is taken as 100 mol %.

It is preferable to use the polymerization catalysts described above, and the amount of these catalysts added is preferably 5× ¹⁰ mol/unit or more, more preferably 6× ¹⁰ mol/unit or more, per mole of the acid component constituting the polyester.
Furthermore, during the polycondensation reaction, alkali metal compounds, alkaline earth metal compounds, fatty acid esters, hindered phenol-based antioxidants, phosphorus compounds, and titanium oxide may be added in addition to the above-mentioned polymerization catalyst.

Then, in the polycondensation reaction tank, the polycondensation reaction is carried out at a temperature of 260° C. or higher and under a reduced pressure of 1.0 hPa or lower. If the polycondensation reaction temperature is lower than 260° C. or the pressure during the polycondensation reaction exceeds 1.0 hPa, the polycondensation reaction time becomes long, resulting in poor productivity.
The polycondensation reaction temperature is more preferably 270° C. or higher. However, if the polycondensation reaction temperature is too high, the polymer will be colored due to thermal decomposition, resulting in a deterioration in color tone, and the amount of terminal groups (COOH) will also increase due to thermal decomposition. Therefore, in the present invention, the upper limit of the polycondensation reaction temperature is preferably 285° C. or lower.

The resin of the present invention may contain various additives other than the above-mentioned additives such as polymerization catalyst, antioxidant, phosphorus compound, etc., as long as the effects of the resin are not impaired.
As various additives, additives such as manganese compounds such as manganese acetate, anthraquinone dye compounds, copper phthalocyanine compounds, etc. may be contained in order to suppress coloration due to thermal decomposition of the polyester resin.

The various products obtainable using the resin of the present invention are not particularly limited as long as they contain the resin, and can be suitably used in the form of, for example, fibers, molded articles, films, etc.

Fibers containing the resin of the present invention can be produced, for example, by a manufacturing method including a step of melting and spinning a raw material containing the resin of the present invention. This makes it possible to produce ultrafine fibers with a single filament fineness of 0.8 decitex or less (preferably 0.6 to 0.3 decitex). The spinning method can be carried out according to known conditions.

As mentioned above, the resin of the present invention contains relatively little foreign matter and has properties equivalent to virgin polyester resin. This means that yarn breakage is less likely to occur during the melt spinning, drawing/heat treatment, and winding processes, and polyester fibers can be obtained with high productivity.

The fiber of the present invention containing the resin of the present invention may be, for example, either a monofilament or a multifilament, and may be either a long fiber or a short fiber.
Furthermore, the fiber of the present invention containing the resin of the present invention is not particularly limited in the shape of the single fiber constituting the fiber, and may have not only a round cross section but also an irregular cross section such as a polygonal cross section. Furthermore, not only may the single fiber be formed entirely from the resin of the present invention, but the fiber may also be a composite fiber of the resin of the present invention and a polyester resin other than the resin of the present invention (such as a virgin polyester resin or a polyester resin containing another copolymer component). Examples of composite fibers include core-sheath, side-by-side, and sea-island types.

Generally, it is more difficult to produce multifilaments when it comes to fiber production. However, because the fibers of the present invention use the resin of the present invention, which has properties equivalent to those of virgin polyester, it is possible to produce multifilaments with properties such as a single yarn fineness of 0.3 to 30 decitex, 2 to 300 single yarns, a total fineness of 5 to 350, a strength of 1 to 5 cN/dtex, and an elongation of 10 to 400%. Even among these, it is possible to obtain ultrafine fibers, which are more difficult to produce.

As mentioned above, the resin of the present invention has excellent thermal stability due to the diethylene glycol content and carboxyl end group concentration being below specific amounts. Therefore, when producing the long fibers described above, thickness variations are unlikely to occur, and highly uniform fibers can be efficiently obtained. Furthermore, as mentioned above, the resin of the present invention has an average pressure increase rate of 0.6 MPa/h or less, and is low in foreign matter. The low level of foreign matter makes it possible to produce ultrafine fibers with good operational efficiency.

In addition, the resin of the present invention can be dyed with cationic dyes under normal pressure conditions by copolymerizing an aromatic dicarboxylic acid component having a metal sulfonate group with an aliphatic dicarboxylic acid component having 5 to 10 carbon atoms. Because it has properties equivalent to fibers obtained from virgin polyester resin, dyeing does not result in color spots within the product or color differences between packaging units, and it also has excellent dyeability.

The present invention will now be described in detail with reference to examples, in which the measurement and evaluation methods for various properties and the like are as follows.
(a) Intrinsic Viscosity The intrinsic viscosity is measured at 20°C using the obtained recycled polyester resin and a mixture of equal masses of phenol and tetrachloroethane as a solvent.
(b) Composition of Polyester Resin The obtained polyester resin was dissolved in a mixed solvent of deuterated trifluoroacetic acid and deuterated chloroform in a volume ratio of 1/11, and 1H-NMR was measured using a JNM-ECZ400R/S1 NMR apparatus manufactured by JEOL Ltd., and the type and content of copolymerization components were determined from the integrated intensity of the proton peak of each component in the obtained chart.
(c) Carboxyl End Group Concentration 0.1 g of the resulting recycled polyester resin was dissolved in 10 ml of benzyl alcohol, and 10 ml of chloroform was added to the solution, followed by titration with a 1/10 N potassium hydroxide benzyl alcohol solution to determine the carboxyl end group concentration.

(d) Average pressure rise rate measured by a pressure rise tester The obtained recycled polyester resin was melted at 300°C in an extruder, and a stainless steel twill weave filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical wire diameter: 0.07 mm, horizontal wire diameter: 0.04 mm, filtration particle size: 12 μm, viscous resistance coefficient (m ⁻¹ ): 2.60 × 10 ⁷⁾ was attached to the tip of the extruder as a filter. , inertia coefficient: 5.14 × 10, manufactured by Kamijo Seiki Co., Ltd.) was set, and a reinforcing material (stainless steel plain weave wire mesh (nominal mesh size: 40 mesh, weave: plain weave, wire diameter: 0.21 mm, manufactured by Kamijo Seiki Co., Ltd.) was further laminated on the back surface (downstream side) of the same, and the filter pressure was measured using a pressure rise tester; a detector "MES-Y44D type" manufactured by Asahi Gauge Co., Ltd., with the polymer discharge rate set to 29.0 g/min. The pressure rise test using the pressure rise tester was carried out continuously for 12 hours, and the average pressure rise rate was calculated using the following formula from the initial pressure value (MPa) at the start of the pressure rise test (the minimum pressure value between 5 and 10 minutes after the polyester resin started to pass through the filter is defined as the initial pressure) and the final pressure value (MPa) after 12 hours had elapsed.
Average pressure increase rate (MPa/h) = (final pressure value - initial pressure value) / 12

(e) Melting Point: Measured using a PerkinElmer DSC-7 differential scanning calorimeter in a nitrogen stream at a temperature range of 25 to 280° C. and a heating rate of 20° C./min.
(f) Glass Transition Temperature: Measured using a PerkinElmer DSC-7 differential scanning calorimeter in a nitrogen stream at a temperature range of 25 to 280° C. and a heating rate of 20° C./min.

(g) Runability of filament fiber production (cut yarn)
The case where the number of times of yarn breakage during 24 hours of continuous melt spinning was 3 times/(day/spindle) or less and there was no single yarn breakage during the drawing process was marked with "○", and the other cases were marked with "×".
(h) Yarn Quality Tenacity and elongation were measured in accordance with JIS L-1013 using an autograph DSS-500 manufactured by Shimadzu Corporation at a gripping distance of 10 cm and a pulling speed of 10 cm.
(i) Dyeability (L value after dyeing)
The obtained multifilament yarn was knitted into a cylindrical knitted fabric using a knitting machine (manufactured by Koike Kikai Seisakusho, number of needles: 300, diameter of kettle: 3.5 inches).
The produced cylindrical knitted fabric was scoured at 60°C for 20 minutes, then dyed at 100°C for 60 minutes under normal pressure under the dyeing conditions below and air-dried. Next, a small pin tenter was used to heat-set the fabric at 150°C for 1 minute, and four-ply sample pieces were prepared. The L value of this sample piece was measured with a color difference meter to evaluate dyeability. The lower the L value, the darker the color of the fiber, and the better the dyeability. An L value of 35 or less was considered acceptable.
The 30 dyed cylindrical knitted fabrics were visually inspected for the presence or absence of dye streaks or dye spots, and the number of non-defective fabrics having neither dye streaks nor dye spots was counted and evaluated as follows:
◯: The number of good products is 27 or more ×: The number of good products is 26 or less (dyeing conditions)
Dye: Astrazon Blue 0.5% o.m.f.
Leveling agent: acetic acid 0.2 mL/L
Sodium acetate 0.2g/L
Bath ratio: 1:50

Example 1
[Recycled polyester resin]
A slurry of terephthalic acid (TPA) and ethylene glycol (EG) (TPA/EG molar ratio = 1/1.6) was supplied to an esterification reactor and reacted at a temperature of 250°C and a pressure of 50 hPa to obtain an ethylene terephthalate oligomer (number average degree of polymerization: 5) with an esterification reaction rate of 95%.
An esterification reactor was charged with 41.5 parts by mass of ethylene terephthalate oligomer, followed by the addition of 2.0 parts by mass of adipic acid and 4.5 parts by mass of ethylene glycol (EG) as aliphatic dicarboxylic acids to obtain a mixture E. Thereafter, 52 parts by mass of recycled polyester raw material (pelletized polyester waste generated in the process of producing polyester resin) was quantitatively added via a rotary valve over a period of approximately 2 hours.
At this time, the recycled polyester raw material was added so that the molar ratio of total glycol components to total acid components (hereinafter sometimes referred to as G/A) was 1.17, and then the depolymerization reaction was carried out under heat treatment conditions of 260°C for 1 hour.
The obtained depolymerization product was pressure-fed to a polycondensation reactor (hereinafter referred to as a PC can) through a candle filter with 20 μm openings set between the esterification reactor and the polycondensation reactor, and then mixed with 3.6 parts by mass of diethylene glycol ester of 5-sodium sulfoisophthalic acid as an aromatic dicarboxylic acid component having a metal sulfonate group, 0.05 parts by mass of titanium oxide, 2.0× ¹⁰ mol/unit of antimony trioxide and 5.0× ¹⁰ mol/unit of germanium dioxide as polymerization catalysts, 0.4×10 mol/unit of cobalt acetate and 8.5× ¹⁰ mol/unit of triethyl phosphate as cobalt compounds, 1.5× ¹⁰ mol/unit of lithium acetate as an alkali metal compound, and 12.2× ^{10 mol} /unit of magnesium acetate as an alkaline earth metal compound. The PC can was then decompressed and, after 60 minutes, the melt polymerization reaction was carried out at a final pressure of 0.5 hPa and a temperature of 280°C for 3 hours, yielding a polyester resin with an intrinsic viscosity of 0.52 (which was then extruded into chips in the usual manner).

[Production of Long Fibers]
The polyester resin thus obtained was dried and then spun at a spinning temperature of 273°C using a spinneret having 48 nozzles, followed by cooling and application of an oil, followed by winding at a speed of 1395 m/min to obtain an undrawn yarn, which was then drawn at a draw ratio of 2.5, a roll heater temperature of 80°C, a plate heater temperature of 150°C, and a drawing speed of 600 m/min, followed by winding to obtain an 84 dtex/48 filament multifilament yarn (drawn yarn).

Examples 2 to 10, Comparative Examples 1 to 11
[Recycled polyester resin]
The depolymerization reaction was carried out in the same manner as in Example 1, except that the amounts of ethylene terephthalate oligomer, ethylene glycol, adipic acid, and recycled polyester raw material added during the depolymerization reaction, G/A, and heat treatment temperature were changed to those shown in Table 1.
In addition, a polyester resin was produced in the same manner as in Example 1, except that the filtration particle size of the filter in the step of recovering the filtrate, the heat treatment temperature in the polymerization reaction step, and the amounts of 5-sodium sulfoisophthalic acid diethylene glycol ester and lithium acetate added were changed to those shown in Table 1.
[Fiber manufacturing]
Using the obtained polyester resin, a multifilament yarn was obtained in the same manner as in Example 1.

Example 11 (Example of obtaining ultrafine fibers)
A 45 dtex/84 filament multifilament yarn (drawn yarn) was obtained in the same manner as in Example 1, except that the recycled polyester resin obtained in Example 2 was used and a spinneret (hole diameter 0.15 mm, number of holes 84) was used.

Example 12
Melt spinning and drawing were carried out in the same manner as in Example 11, except that the recycled polyester resin obtained in Example 6 was used, to obtain a multifilament yarn having a fineness of 45 dtex.

Comparative Examples 12 to 13
When melt spinning was carried out in the same manner as in Example 11 using the recycled polyester resins obtained in Comparative Examples 1 and 2, single yarn breakage and broken yarns frequently occurred during spinning, and multifilament yarns could not be obtained.

Table 1 shows the property values of the recycled polyester resins obtained in Examples 1 to 10 and Comparative Examples 1 to 11, and the property values and evaluation results of the multifilament yarns obtained in Examples 1 to 12, Comparative Examples 1 to 2, and 4 to 9.

As is clear from Table 1, the recycled polyester resins obtained in Examples 1 to 10 had carboxyl terminal group amounts, diethylene glycol contents, and average pressure rise rates within the ranges specified in the present invention. Therefore, multifilament yarns could be obtained with good spinning operability.
The fibers obtained in Examples 1 to 12 were excellent in properties such as strength and elongation, and had uniform dyeability by normal pressure dyeing. Among them, the fibers obtained in Examples 11 and 12 had a single yarn fineness of less than 0.6 dtex, but could be obtained with good spinning operability and had excellent properties and dyeability equivalent to those of the fibers obtained in Examples 1 to 10.

On the other hand, in Comparative Example 1, the amount of 5-Na sulfoisophthalic acid diethylene glycol ester added was as low as 0.8 parts by mass, and therefore the amount of copolymerization of 5-Na sulfoisophthalic acid in the obtained recycled polyester resin was as low as 0.5 mol %, and therefore the obtained fiber had poor dyeability.
In Comparative Example 2, the heat treatment temperature during depolymerization was as high as 285° C., and therefore the obtained recycled polyester resin had a high diethylene glycol content and a high carboxyl end group concentration. As a result, thermal decomposition occurred when the polyester resin was made into fibers, which deteriorated operability and resulted in poor dyeability of the obtained fibers.
In Comparative Example 3, the filtration particle size of the candle filter provided between the ES can and the PC can was as low as 5 μm, so clogging occurred and a polyester resin could not be obtained.

In Comparative Example 4, the filtration particle size of the candle filter installed between the ES can and the PC can was as high as 30 μm, so the resulting recycled polyester resin contained a large amount of foreign matter and had a high average pressure increase rate. As a result, when the resin was made into fibers, the nozzle pack pressure increased and yarn breakage occurred frequently, which deteriorated operability and resulted in poor strength and dyeability of the resulting fibers.
In Comparative Example 5, the G/A ratio during the depolymerization reaction was as low as 1.01, and therefore the resulting recycled polyester resin had a high carboxyl end group concentration and a high average pressure rise rate. As a result, thermal decomposition occurred when the polyester resin was made into fibers, which deteriorated operability and resulted in fibers with poor strength and dyeability.
In Comparative Example 6, the G/A ratio during the depolymerization reaction was as high as 1.35, and therefore the obtained recycled polyester resin had a high diethylene glycol content, which resulted in low crystallinity and thermal stability, a high concentration of carboxyl terminal groups, poor operability when processed into fibers, and poor strength and dyeability of the obtained fibers.

In Comparative Example 7, the amount of adipic acid added was low at 1.0 part by mass, and therefore the amount of adipic acid copolymerized in the obtained recycled polyester resin was low at 1.4 mol %, and the obtained fiber had poor dyeability.
In Comparative Example 8, the amount of 5-Na sulfoisophthalic acid diethylene glycol ester added was as high as 9.5 parts by mass, which resulted in an excessively high melt viscosity during the polycondensation step, making it impossible to sufficiently increase the degree of polymerization. As a result, the resulting recycled polyester resin had a high diethylene glycol content and a high carboxyl end group concentration, which resulted in poor operability when made into fibers, and the resulting fibers were inferior in both yarn quality and dyeability.
In Comparative Example 9, the amount of adipic acid added was as high as 20.0 parts by mass, and therefore the resulting recycled polyester resin had a high adipic acid content of 27.3 mol %, which resulted in low thermal stability, a high diethylene glycol content, and a high carboxyl terminal group concentration. As a result, the operability when made into fibers was poor, and the resulting fibers were poor in both yarn quality and dyeability.

In Comparative Example 10, the heat treatment temperature during depolymerization was as low as 240° C., so the reaction product solidified and a polyester resin could not be obtained.
In Comparative Example 11, the polycondensation reaction temperature was as low as 250° C., so that the polycondensation reaction did not proceed and a polyester resin could not be obtained.

Claims (3)

A recycled polyester resin comprising a component derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products, and characterized in that the recycled polyester resin satisfies all of the following (1) to (4):
(1) When the total amount of all acid components constituting the polyester is taken as 100 mol %, 77 to 97 mol % are terephthalic acid, 1.0 to 5 mol % are aromatic dicarboxylic acid having a metal sulfonate group, and 2 to 18 mol % are aliphatic dicarboxylic acid having 5 to 10 carbon atoms;
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol accounts for 80 mol% or more and diethylene glycol accounts for 6.5 mol% or less;
(3) The carboxyl terminal group concentration is 40 equivalents/t or less,
(4) The average pressure rise rate is 0.6 MPa/h or less (wherein the average pressure rise rate is a value calculated by the following procedure: a pressure rise tester including an extruder and a pressure sensor is used, a stainless steel filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical wire diameter: 0.07 mm, horizontal wire diameter: 0.04 mm, filtration particle size: 12 μm) is set at the tip of the extruder, a polyester resin is melted in the extruder at 300° C., and the melt is extruded from the filter at a discharge rate of 29.0 g/min. The pressure value at the start of extrusion is defined as the “initial pressure value (MPa),” and the pressure value at the time of subsequent continuous extrusion for 12 hours is defined as the “final pressure value (MPa).” The average pressure rise rate is calculated based on these pressure values using the following calculation formula A:
Average pressure increase rate (MPa/h) = (final pressure value - initial pressure value) / 12) ... A). Fiber containing the recycled polyester resin described in claim 1. A method for producing a recycled polyester resin according to claim 1, characterized in that the method comprises the following steps (1) to (3): a) using at least one recycled polyester raw material selected from a) used polyester products and b) unused polyesters generated in the process of producing polyester products, and the method comprises the following steps (1) to (3):
(1) A process of adding recycled polyester raw material to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and aliphatic dicarboxylic acid having 5 to 10 carbon atoms so that the molar ratio of total glycol components/total acid components is 1.05 to 1.30, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a reaction product containing a depolymerization product.
( 2) A step of passing the reaction product through a filter having a filtration particle size of 10 to 25 μm to recover a filtrate. (3) A step of adding an aromatic dicarboxylic acid having a metal sulfonate group and a polymerization catalyst to the filtrate and carrying out a polycondensation reaction of the depolymerization at a temperature of 260° C. or higher and a reduced pressure of 1.0 hPa or lower.