JP7784658B2 - 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 65% by mass or more of components derived from recycled polyester raw materials, including used polyester products and unused polyester resin generated during the process of producing polyester products, which has a low level of foreign matter contamination 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 (hereinafter sometimes abbreviated as PET) has a high melting point, is chemical resistant, and is relatively low cost, and is therefore widely used in fibers, films, and molded products such as PET bottles.
These polyester products inevitably generate waste during the manufacturing and processing stages, and are often disposed of after use, but if they are incinerated, the high heat generated will severely damage the incinerator and shorten its lifespan, while if they are not incinerated, they will remain in the furnace almost indefinitely because they do not decompose.
In recent years, plastic containers and other polyester products that have been used and discarded as garbage have been found to flow into the ocean via rivers. These microplastics are broken down into small pieces by the action of waves and tidal currents, and accumulate in the bodies of marine organisms. These plastics are concentrated in the food chain, adversely affecting the marine ecosystem, and plastics have become a major cause of marine pollution. This has led to a global movement to reduce the amount of plastic used and to switch to biodegradable plastics.

In order to reduce the amount of plastic products used and to address environmental issues, various methods of recycling resources are being used. Regarding polyester products, methods are being considered for recycling polyester scraps generated during the manufacturing process, as well as recovering products that have been discarded after being sold on the market and reusing them as raw materials. In recent years, textile products in particular have become more popular, with Eco Mark certification given to products that have achieved a certain recycling rate.

Various methods have been proposed for recycling recycled polyester raw materials. For example, there is a method 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 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 in which PET scraps are depolymerized with EG to form oligomers, which are then used in a polycondensation reaction (Patent Document 3).

Furthermore, when recycling PET bottles that have already been manufactured, problems can arise, such as additives added to the polyester resin and items attached to the bottle itself, such as caps (aluminum, polypropylene, polyethylene), inner stoppers and liners (polypropylene, polyethylene), labels (paper, resins such as polystyrene, ink), adhesives, and printing inks.
As a pre-treatment for the recycling process, recovered PET bottles are first 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. Aluminum pieces from caps and other materials are then removed and the PET bottle fragments are finely crushed. Components such as adhesives, proteins, and mold are removed by high-temperature alkaline washing, and heterogeneous components such as polypropylene and polyethylene are separated by specific gravity.

However, even after these steps, it was difficult to completely separate non-polyester resins, particularly those mentioned above, such as polypropylene, polyethylene, and polystyrene. Therefore, even if recycled polyester resin was obtained using the recycling methods described in Patent Documents 1 to 3, it was not possible to fully remove foreign matter derived from the non-polyester resin, and the amount of foreign matter mixed in was not sufficiently reduced, making it impossible to obtain a product with the same quality as virgin polyester resin. Furthermore, methods such as those described in Patent Documents 1 to 3 require significant costs for the installation, operation, and maintenance of recovery equipment, and there is room for improvement in terms of practicality.

Furthermore, the invention described in Patent Document 4 describes a method in which polyester scraps are depolymerized with ethylene glycol, filtered through a filter with an average mesh size of 10 to 50 μm, and then repolymerized. It has been shown that the resulting recycled polyester resin contains little foreign matter and is easy to process. However, even with this method, the amount of foreign matter derived from non-polyester resins, as described above, is not sufficiently removed, and the amount of foreign matter is not sufficiently reduced.

Generally, when manufacturing plastic bottles and other products, the so-called blow molding method is used, in which molten plasticized resin is extruded through a die orifice to form a cylindrical parison, which is then sandwiched between molds and air is blown into it, due to its ease of molding, high productivity, and relatively low equipment costs for molding machines and molds. Even for such blow-molded products, the use of recycled polyester resins is being considered from an environmental perspective.

Furthermore, since crystallization is likely to occur during blow molding, even if molding is possible, there is a problem in that whitening occurs and transparency becomes insufficient.
Therefore, in order to improve transparency, polyester resins have been proposed in which polyethylene terephthalate is copolymerized with other monomer components (see, for example, Patent Document 5).

Hot-melt binder fibers are also widely used to bond fibers that make up fillings for pillows and bedding, quilting padding, mattress padding, etc. Among these, copolymer polyester resins whose main components are terephthalic acid, isophthalic acid, and ethylene glycol are widely used as polyester binder fibers.

As such, there is a great demand for copolymer polyester resins made from recycled polyester raw materials in a variety of molded products and textile products. However, no copolymer polyester resin has yet been developed that can fully remove not only various inorganic substances but also non-polyester resin-derived foreign matter, making it possible to produce a variety of high-quality products equivalent to those made from virgin polyester resin.

Special Publication No. 42-8855 Japanese Unexamined Patent Publication No. 48-62732 Japanese Patent Application Publication No. 60-248646 Japanese Patent Application Laid-Open No. 2005-171138 Patent No. 6297351

Furthermore, due to growing awareness of environmental issues, there is an increasing demand for recycled polyester resins with a high recycled material content. In recycled polyester resins with a recycled material content of 65% or more, or even 80%, the above-mentioned problem of foreign matter contamination becomes significant, and the resins also have poor thermal stability.
An object of the present invention is to provide a recycled polyester resin that can be used to produce various types of polyester products, which is a copolymer polyester resin having a recycled raw material usage rate (content of components derived from recycled polyester raw materials) of 65 mass% or more and that can be used to produce various high-quality products equivalent to virgin polyester resins.An object of the present invention is also to provide a production method by which such a recycled polyester resin of the present invention can be obtained.

As a result of extensive research into the problems of the prior art, the inventors discovered that a specific manufacturing method using recycled polyester raw materials can produce recycled polyester resin with minimal foreign matter contamination and thermal stability equivalent to that of virgin polyester resin, leading to the completion of the present invention.

That is, the present invention encompasses the following (a) to (e):

(i) A recycled polyester resin containing 65% by mass or more of a component derived from at least one recycled polyester raw material, which is a) a used polyester product and b) an unused polyester resin 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 %, 50 to 98 mol % is terephthalic acid and 2 to 40 mol % is isophthalic acid,
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol is 70 mol% or more and diethylene glycol is 4 mol% or less,
(3) The carboxyl terminal group concentration is 40 equivalents/t;
(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)
(ii) The recycled polyester resin according to (i), wherein the glycol component in (2) is an ethylene oxide adduct of bisphenol A in an amount of 1 to 10 mol %.
(c) A molded product containing the recycled polyester resin described in (a) or (b).
(iv) Fibers containing the recycled polyester resin described in (a) or (b).
(e) A method for producing a recycled polyester resin using at least one recycled polyester raw material, which is a) a used polyester product and b) an unused polyester resin generated in the process of producing a polyester product, and which is characterized by including all of the following steps (1) to (4):
(1) A step of adding the recycled polyester raw material to a mixture containing ethylene glycol and isophthalic acid 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 depolymerized product having a melt viscosity of 10 to 1500 mPa s. (2) A step of passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover a filtrate. (3) A step of adding a polycondensation catalyst to the filtrate and kneading the mixture to obtain a reaction product. (4) A step of performing a polycondensation reaction on the reaction product at a temperature of 250 ° C. or higher and a reduced pressure of 1.0 hPa or lower.

The recycled polyester resin of the present invention contains 65% by mass or more of a component derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester resins generated in the process of manufacturing polyester products. However, the amount of foreign matter contamination is low, and the carboxyl end group concentration and diethylene glycol content satisfy specific ranges, resulting in excellent thermal stability. Therefore, for example, in processes for obtaining fibers by melt spinning, for obtaining sheets or films by film formation, and for obtaining molded products such as bottles, it is possible to operate the recycled polyester resin continuously for a relatively long period of time, enabling the production of various types of products with high productivity. Furthermore, because the recycled polyester resin is a copolymerized polyester resin containing copolymer components, it can be suitably used in various applications, such as molded products such as bottles that require transparency and fiber applications that require adhesive properties.
Furthermore, the method for producing the recycled polyester resin of the present invention does not require complicated steps or equipment, and it is possible to obtain the recycled polyester resin of the present invention having copolymer components with good operability and low cost, which has great practical advantages.

The present invention will be described in detail below.
The recycled polyester resin of the present invention (hereinafter sometimes referred to as the resin of the present invention) is a polyester resin containing 65% by mass or more of components derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester resins generated in the process of manufacturing polyester products.

Examples of the used polyester products of the above category a) include polyester molded products (including fibers) that were once on the market and then collected after use, and typical examples thereof include containers such as PET bottles and packaging materials.
The unadopted polyester resin generated in the process of manufacturing polyester products (b) above is polyester that has not been commercialized, and examples thereof include resin pellets that do not meet specifications, materials that are no longer needed during molding, fragments cut during molding, scraps generated during molding or processing, cut pieces of transitional products generated when changing brands, cut pieces of prototypes or defective products, etc.

The above a) and b) are not limited in their form, and may be pelletized by further processing such as pulverization and cutting as necessary, or may be melted and pelletized.
The above a) and b) may be used alone or in combination.
The recycled polyester raw materials a) and b) above may be either crystalline or amorphous. Therefore, for example, pellets of amorphous polyester waste 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 a crystalline recycled polyester raw material, particularly for the purpose of preventing fusion between pellets during charging into the tank or the depolymerization reaction. Therefore, materials 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 scrap, 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 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 object of the present invention is to obtain a recycled polyester resin that uses a high percentage of recycled polyester raw materials. Therefore, the recycled polyester resin of the present invention contains 65% by mass or more, and preferably 80% by mass or more, of components derived from recycled polyester raw materials that are at least one of a) and b) above. Furthermore, the recycled polyester resin of the present invention can be easily obtained by the manufacturing method of the present invention described below, even when the content of components derived from recycled polyester raw materials is 80% by mass or more.

In the resin of the present invention, when the total amount of all acid components constituting the polyester is taken as 100 mol %, 50 to 98 mol % is terephthalic acid and 2 to 40 mol % is isophthalic acid. In other words, the resin contains terephthalic acid as the main acid component and isophthalic acid as a copolymerization component.
The copolymerization amount of isophthalic acid is preferably adjusted within the range of 2 to 40 mol% depending on the application of the resulting recycled polyester resin. For example, when used for molding applications, the copolymerization amount of isophthalic acid is preferably 2 to 15 mol%. Of these, 3 to 10 mol% is preferable, and 4 to 8 mol% is even more preferable. By copolymerizing 2 to 15 mol% of isophthalic acid, the crystallization rate of the polyester resin can be adjusted to one suitable for direct blow molding, and whitening due to crystallization during direct blow molding can be prevented. The melting point of the resulting recycled polyester resin is preferably 210 to 250°C.

If the copolymerization amount of isophthalic acid is less than 2 mol%, the resin composition will crystallize too quickly, causing the molded product to crystallize and turn white during direct blow molding, resulting in poor transparency. On the other hand, if the copolymerization amount of isophthalic acid exceeds 15 mol%, the resin composition will become amorphous, making it more susceptible to blocking during high-temperature drying and solid-state polymerization processes.

Furthermore, when used as a main fiber, the copolymerization amount of isophthalic acid is preferably 2 to 15 mol%, and more preferably 2 to 10 mol%. By copolymerizing 2 to 15 mol% of isophthalic acid, the main fiber has appropriate performance such as strength, making it suitable for producing products such as nonwoven fabrics and woven and knitted fabrics. The melting point of the resulting recycled polyester resin is preferably 210 to 250°C.
When used for binder fiber applications, the copolymerization amount of isophthalic acid is preferably 15 to 40 mol%, and more preferably 20 to 35 mol%. By copolymerizing 15 to 40 mol% of isophthalic acid, the melting point of the polyester resin can be lowered or it can be made amorphous, making it suitable for binder applications. The melting point of the resulting recycled polyester resin is preferably 190 to 210°C, and if it does not have a melting point, it is preferable that the glass transition temperature is 62 to 80°C.

If the copolymerization amount of isophthalic acid is less than 15 mol%, the melting point of the resin will be high, making it unsuitable for binder fiber applications. On the other hand, if the copolymerization amount of isophthalic acid exceeds 40 mol%, the crystallinity and glass transition temperature of the resulting resin composition will be too low, making it difficult to melt-spin and stretch the resin to form fibers.

The proportion of terephthalic acid in the acid component is 50 to 98 mol%, with a proportion of 60 to 97 mol% being preferred. If the proportion of terephthalic acid is less than 50 mol%, the resin composition will have reduced crystallinity and will likely become amorphous. On the other hand, if the proportion of terephthalic acid exceeds 98 mol%, the amount of isophthalic acid copolymerized will be reduced, making it difficult to achieve the effects achieved by copolymerizing isophthalic acid in various products.

Other acid components in the resin of the present invention besides terephthalic acid and isophthalic acid include phthalic acid, 5-sodium sulfoisophthalic acid, phthalic anhydride, naphthalenedicarboxylic acid, adipic acid, sebacic acid, and dimer acid. 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 70 mol % or more of all glycol components, and preferably 80 mol % or more. If the ethylene glycol content is less than 70 mol %, the resulting polyester resin will have poor crystallinity and heat resistance.

Furthermore, when the total amount of all glycol components is taken as 100 mol%, the resin of the present invention has a diethylene glycol content of 4 mol% or less, preferably 3 mol% or less. In particular, the resin of the present invention obtained by the manufacturing 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 since the diethylene glycol content is 4 mol% or less, it has excellent thermal stability. This makes it possible to efficiently produce molded products such as fibers, injection-molded articles, various blow-molded articles, sheets, and films. The lower limit of the diethylene glycol content can be, for example, approximately 0.5 mol%, but is not limited to this.

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

In particular, when the resin of the present invention is used for fiber applications, it is preferable that the ethylene oxide adduct of bisphenol A is contained as a copolymerization component in an amount of 1 to 10 mol %, and more preferably 2 to 8 mol %. Copolymerization of the ethylene oxide adduct of bisphenol A can impart shrinkability to the resulting fiber. If the copolymerization amount of the ethylene oxide adduct of bisphenol A is less than 1 mol %, the resulting fiber cannot be imparted with shrinkability. On the other hand, if the copolymerization amount of the ethylene oxide adduct of bisphenol A exceeds 10 mol %, the melting point of the polyester resin will be low and the heat resistance will be poor, which is undesirable.

In this case, when the ethylene oxide adduct of bisphenol A is contained in an amount of 1 to 10 mol % as a copolymerization component, the copolymerization amount of isophthalic acid is preferably 2 to 15 mol %, and more preferably 2 to 10 mol %.
If the shrinkability of the resulting fiber is high, when a woven or knitted fabric obtained using this fiber is heat-treated, the fabric shrinks, making it possible to obtain a high-density, compact woven or knitted fabric.

The resin of the present invention contains a polycondensation catalyst and various additives that are added depending on the application.
First, the polycondensation catalyst may be, but is not limited to, at least one of germanium compounds, antimony compounds, titanium compounds, cobalt compounds, zinc compounds, tin compounds, etc. Among these, it is particularly preferable to use at least one of germanium compounds and antimony compounds. When the transparency of the resulting recycled polyester resin is important, it is preferable to use a germanium compound. As each of the above compounds, oxides, inorganic acid salts, organic acid salts, halides, sulfides, etc. of germanium, antimony, titanium, cobalt, etc. may be used.

The amount of the polycondensation catalyst used is not particularly limited, but is preferably 1× ¹⁰ mol or more, more preferably 6× ¹⁰ mol or more, per mol 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, but is not limited thereto.

In addition, since the polycondensation catalyst contained in the recycled polyester raw material may also act as a catalyst during the polycondensation reaction, it is preferable to consider the type and content of the polycondensation catalyst contained in the recycled polyester raw material when adding the polycondensation catalyst in the polycondensation step.

Various additives that are added depending on the application include fatty acid esters that can adjust the melt viscosity, hindered phenol antioxidants, phosphorus compounds that can suppress the thermal decomposition of the resin, color adjusters that can improve the appearance of the resin, titanium compounds that increase the whiteness of the resin, and crystal nucleating agents that improve the crystallinity of the resin.

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.

Examples of the color tone adjuster include cobalt compounds such as cobalt acetate, manganese compounds such as manganese acetate, dyes (blue, purple, red), copper phthalocyanine compounds, etc. Among these, cobalt acetate and dyes are preferred from the viewpoints of the activity of the polycondensation catalyst, the physical properties of the resulting polyester resin, and cost.
Furthermore, it is preferable that the dye contains a blue dye, a red dye, a purple dye, or the like, as this improves the color tone.

Examples of dyes include blue dyes such as SOLVENT BLUE 104, SOLVENT BLUE 122, and SOLVENT BLUE 45, red dyes such as SOLVENT RED 111, SOLVENT RED 179, SOLVENT RED 195, SOLVENT RED 135, P IGMENT RED 263, and VATRED 41, and purple dyes such as DESPERSE VIOLET 26, SOLVENT VIOLET 13, SOLVENT VIOLET 37, and SOLVENT VIOLET 49, according to the color index. Among these, SOLVENT BLUE 104, SOLVENT BLUE 45, SOLVENT RED 179, SOLVENT RED 195, SOLVENT RED 135, and SOLVENT VIOLET 49 are preferred, as they do not contain halogens, which can easily cause equipment corrosion, have relatively good heat resistance at high temperatures, and have excellent color development. These can be used alone or in combination of two or more types.

Titanium oxide is preferably used as the titanium compound. Titanium oxide is commonly used as a matting agent or white pigment for polyester resins, but adding an appropriate amount of titanium oxide to the resin of the present invention is preferable because it improves the whiteness of the fibers when made into fibers and allows for the production of woven or knitted 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 polyester resin.

Nucleating agents include carbon black, calcium carbonate, synthetic silicic acid and silicates, zinc oxide, hysite clay, kaolin, basic magnesium carbonate, mica, talc, quartz powder, diatomaceous earth, dolomite powder, titanium oxide, zinc oxide, antimony oxide, barium sulfate, calcium sulfate, alumina, calcium silicate, boron nitride, etc.; low-molecular-weight organic compounds having a metal salt of a carboxyl group; high-molecular-weight organic compounds having a metal salt of a carboxyl group; and high-molecular-weight organic compounds. Among these, mica, talc, and high-molecular-weight organic compounds are preferred. These can be used alone or in combination of two or more.

The resin of the present invention has the above-mentioned composition and also has the following characteristic values.
(3) A carboxyl terminal group concentration of 40 equivalents/t or less. (4) An average pressure increase rate measured by a pressure increase tester of 0.6 MPa/h 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 (3) of a carboxyl terminal group concentration of 40 equivalents/t or less, preferably 30 equivalents/t or less, and more preferably 25 equivalents/t or less.
The resin of the present invention has a carboxyl terminal group concentration of 40 equivalents/t or less, and therefore has excellent heat resistance, and it is possible to produce molded products with excellent heat resistance with good productivity by various molding methods.

The intrinsic viscosity of the recycled polyester resin of the present invention is not particularly limited, but is typically preferably approximately 0.35 to 0.80. Furthermore, as described below, the recycled polyester resin of the present invention can also be used for molding applications by increasing the degree of polymerization through a solid-state polymerization process. In this case, the intrinsic viscosity of the resulting recycled polyester resin is preferably 0.80 to 1.25. The intrinsic viscosity (IV) is measured at 20°C using an equal mass mixture of phenol and tetrachloroethane as the solvent.

The resin of the present invention has the characteristic value (4) of an average pressure increase rate measured by the following method of 0.6 MPa/h or less, preferably 0.5 MPa/h or less, and more preferably 0.4 MPa/h or less. The average pressure increase rate in the present invention is an indicator of the amount of foreign matter derived from various inorganic substances or non-polyester resins, with a lower average pressure increase rate indicating a lower amount of foreign matter. The lower limit of the average pressure increase rate can be, for example, about 0.01 MPa/h, but is not limited to this.

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 characteristic value (4), to 0.6 MPa/h or less.

The extruders, filters, etc. used in the above measurements may be any publicly known or commercially available product, as long as they meet the requirements of this invention.

In the present invention, as shown in the examples below, a reinforcing material may be added to the filter as needed, as long as it does not substantially affect the measurement results. In the above-described method for measuring the average pressure rise rate, extremely high pressure is applied to the filter, which may cause deformation or damage to the filter if used alone. In such cases, it is preferable to support the filter with a reinforcing material. A mesh-like metal member or the like can be used as the reinforcing material. More specifically, a metal filter that is strong enough to prevent filter deformation and has a coarse mesh that does not substantially affect the measurement results can be used as the reinforcing material. Such a reinforcing material can be used by laminating it on the downstream side of the filter.

Next, a method for producing the resin of the present invention will be described. In the production method of the present invention, it is important to carry out the steps (1) to (4) in this order.
(1) A process of adding the recycled polyester raw material to a mixture containing ethylene glycol and isophthalic acid 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 depolymerized product having a melt viscosity of 10 to 1500 mPa s. (2) A process of passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover a filtrate. (3) A process of adding a polycondensation catalyst to the filtrate and kneading the mixture to obtain a reaction product. (4) A process of performing a polycondensation reaction on the reaction product at a temperature of 250 ° C. or higher and a reduced pressure of 1.0 hPa or lower.

First, in the depolymerization step (1), a mixture containing ethylene glycol and isophthalic acid is added to a recycled polyester raw material so that the molar ratio of total glycol components/total acid components is 1.05 to 1.30, and depolymerization is carried out under heat treatment conditions of 245 to 280°C, thereby obtaining a depolymerized product having a melt viscosity of 10 to 1500 mPa s.
In the production method of the present invention, the melt viscosity of the depolymerized polymer is a value measured at the heat treatment temperature during depolymerization, and is measured using a VISCO METER DV2T melt viscometer manufactured by Brookfield.

The ethylene glycol added to the recycled polyester raw material can be obtained by known methods or commercially available.

The amount of the mixture containing ethylene glycol and isophthalic acid (hereinafter sometimes referred to as mixture E) added is preferably 5 to 45 parts by mass, and more preferably 8 to 40 parts by mass, when the total amount of mixture E and the recycled polyester raw material is 100 parts by mass.
If the amount of mixture E is less than the above range, the recycled polyester raw materials are likely to block with each other when they are added, which is undesirable because it places an excessive load on the mixer.

On the other hand, if the amount of mixture E is greater than the above range, no particular problems will occur in the depolymerization reaction, but the recycle rate (proportion of components derived from recycled polyester raw materials) of the final recycled polyester resin will be low, which is undesirable.
In addition, when the resin of the present invention contains a copolymerization component other than isophthalic acid, such as an ethylene oxide adduct of bisphenol A, it is preferable to add it so that it is contained in mixture E before adding the recycled polyester raw material.

In step (1), when a mixture E containing ethylene glycol and isophthalic acid (and, if necessary, a copolymerization component other than isophthalic acid, such as an ethylene oxide adduct of bisphenol A) and a recycled polyester raw material are added, the mixture is stirred and depolymerized under heat treatment conditions of 245 to 280°C so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.30, to obtain a depolymerized product having a melt viscosity of 10 to 1500 mPa s.
This step is important in the production method of the present invention. That is, in the present invention, mixture E and the recycled polyester raw material are added to carry out depolymerization, and at this time, all components of mixture E and the recycled polyester raw material are added so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.30, and then depolymerization is carried out.

In the production method of the present invention, if the content of components derived from recycled polyester raw materials in the resulting recycled polyester resin is 65% by mass or more, depolymerization may be carried out by adding ethylene terephthalate oligomer in addition to mixture E and the recycled polyester raw materials in step (1). When adding ethylene terephthalate oligomer, it is also necessary to ensure that the molar ratio of total glycol components to total acid components is 1.05 to 1.30.

As an ethylene terephthalate oligomer, for example, an esterification reaction product of ethylene glycol and terephthalic acid can be suitably used. Furthermore, the number-average degree of polymerization of the ethylene terephthalate oligomer is not limited, but can be, for example, approximately 2 to 20.

Furthermore, even when ethylene terephthalate oligomer is added in the depolymerization step, the amount of recycled polyester raw material added in the depolymerization step is preferably 55 to 95 parts by mass, and more preferably 60 to 92 parts by mass, per 100 parts by mass of the total amount of mixture E, ethylene terephthalate oligomer, and recycled polyester raw material.

In the step (1), it is important that the melt viscosity of the depolymerized polymer obtained by the above depolymerization is 10 to 1500 mPa·s.
By adjusting the molar ratio of total glycol components/total acid components during depolymerization and obtaining a depolymerization product having a melt viscosity of 10 to 1,500 mPa s, not only various inorganic substances contained in the recycled polyester raw material but also foreign substances derived from non-polyester resins can be efficiently precipitated, and therefore, these foreign substances can be completely filtered out in the step (2).

In the step (3) of adding and kneading the polycondensation catalyst and various additives depending on the application, the polycondensation catalyst and various additives can be kneaded uniformly without agglomeration.
As a result, in the polycondensation reaction of step (4), it is possible to obtain a recycled polyester resin having the characteristic values of the present invention, namely, the diethylene glycol content (copolymerization amount) and the carboxyl terminal group concentration, which are below a specific amount, and which has a low amount of foreign matter mixed in.

In step (1) of the present invention, it is desirable that the recycled polyester raw material is not decomposed into monomers by the depolymerization reaction described above, but rather into oligomers with approximately 5 to 20 repeating units.

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, if the molar ratio of total glycol components to total acid components during the depolymerization reaction is outside the above range, the various inorganic substances and non-polyester resin-derived impurities in the recycled polyester raw material are not efficiently precipitated, 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 (4), resulting in a recycled polyester resin with a high average pressure rise rate.

Furthermore, in the depolymerization reaction step (1), if the melt viscosity of the resulting depolymerization product is less than 10 mPa·s, the viscosity will be too low, which will result in liquid leakage from the joint between the piping and the filter when filtering out foreign matter, resulting in poor filtration efficiency, inability to sufficiently filter out foreign matter, and reduced productivity. On the other hand, if the melt viscosity of the depolymerization product exceeds 1500 mPa·s, the viscosity will be too high, which will increase the load on the stirring blades during the depolymerization reaction and further increase the pressure when filtering out foreign matter, resulting in reduced productivity.

In the production method of the present invention, the reactor used in step (1) 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 equipped with a distillation column that prevents ethylene glycol from being distilled 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).

When carrying out the depolymerization reaction using an esterification reactor, it is preferable to carry out the depolymerization reaction by adding the recycled polyester raw material and mixture E in multiple batches, or to add the recycled polyester raw material to mixture E in small amounts at a time.

When carrying out the depolymerization reaction, a melt extruder may be used instead of an esterification reactor. In the method using a melt extruder, the polymer is sealed and extruded in the extruder, so that it does not come into contact with air (particularly oxygen), and therefore oxidative decomposition of the polymer does not occur, and problems such as color deterioration and a decrease in viscosity of the molten polymer due to an increase in carboxyl end groups do not occur. Moreover, when a melt extruder is used, the reaction can be carried out under any pressure condition, including normal pressure, increased pressure, and reduced pressure.
The melt extruder may be either a single-screw extruder or a twin-screw extruder, but it is preferable to use a twin-screw extruder since the above-mentioned advantages can be more easily obtained.

Furthermore, when a melt extruder is used in step (1), the depolymerization reaction proceeds inside the extruder by feeding mixture E into the extruder. When feeding recycled polyester raw material, adding mixture E while passing it through the melt extruder makes it easier to lower the viscosity, allowing for smoother discharge from the tip nozzle. Furthermore, changing the pressure conditions to lower the viscosity also makes it possible to lower the melt temperature (the temperature of the depolymerization reaction) of the melt extruder.

The melt viscosity of the depolymerized polymer can be adjusted to fall within the range of the present invention by appropriately adjusting the molar ratio of total glycol components to total acid components during depolymerization, the heat treatment temperature, the heat treatment time, the pressure, etc.

The reaction temperature during depolymerization in step (1) is preferably set at an internal temperature of a reactor or melt extruder in the range of 245 to 280° C., and more preferably at an internal temperature of 250 to 280° C. If the reaction temperature during depolymerization is less than 245° C., a long period of heat treatment is required to reduce the melt viscosity of the depolymer to 1,500 mPa s or less, which deteriorates operability and results in the obtained recycled polyester resin having a worsened color tone and a high diethylene glycol content and carboxyl terminal group concentration.
On the other hand, if the reaction temperature exceeds 280°C, the melt viscosity of the depolymerization product becomes too low, and the diethylene glycol content and carboxyl terminal group concentration of the resulting recycled polyester resin become high.
The reaction time during depolymerization (the reaction time from the end of the introduction of the recycled polyester raw material) is preferably within 4 hours, and more preferably within 2 hours from the viewpoints of suppressing the amount of diethylene glycol by-produced and suppressing deterioration in the color tone of the polyester.

In step (2), the depolymerized product obtained in step (1) is passed through a filter with a filtration particle size of 10 to 25 μm to filter out foreign matter. As described above, by performing the depolymerization reaction under the conditions of step (1) to obtain a depolymerized product with a melt viscosity within a specific range, even when a large amount of recycled polyester raw material is used, not only various inorganic substances contained in these raw materials in large amounts but also foreign matter derived from non-polyester resins is efficiently precipitated. Therefore, by passing the depolymerized product through a filter with a filtration particle size of 10 to 25 μm, all of the precipitated foreign matter can be captured by the filter, and a filtrate with a low amount of foreign matter can be obtained.
If a filter with a filtration particle size larger than 25 μm is used, foreign matter in the depolymerization cannot be sufficiently removed, resulting in a large amount 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, if a filter with a filtration particle size smaller than 10 μm is used, clogging by foreign matter easily occurs, shortening the filter life, which is cost-inefficient and also deteriorates operability.

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

In step (3) of the production method of the present invention, the polycondensation catalyst as described above is added to the filtrate obtained through step (2) above, and the mixture is kneaded to obtain a reaction product. In step (3), it is preferable that additives to be added depending on various applications are also added together with the polycondensation catalyst and kneaded. The additives to be added depending on various applications can be those described above.
The temperature at which the kneading is carried out in the step (3) is preferably within a range of ±10° C. of the temperature at which the polycondensation reaction is carried out in the step (4).

Next, in step (4), the reaction product obtained in step (3) is subjected to a polycondensation reaction in a polycondensation reaction tank at a temperature of 250° C. or higher and under reduced pressure of 1.0 hPa or lower.
If the polycondensation reaction temperature is less than 250°C or the pressure during the polycondensation reaction exceeds 1.0 hPa, the polycondensation reaction time will be long, resulting in poor productivity or the polycondensation reaction will not proceed, making it impossible to obtain recycled polyester resin.
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, the color tone will deteriorate, and the amount of terminal groups (COOH) will 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 intrinsic viscosity of the recycled polyester resin (prepolymer) obtained here is preferably 0.44 to 0.80.

To further increase the intrinsic viscosity of recycled polyester resin, it is preferable to perform solid-state polymerization on the prepolymer obtained by the polycondensation reaction. The prepolymer is then cut into chips of any shape, such as dice or cylinders. The polyester chips are continuously fed into a crystallizer and crystallized at 150-190°C. They are then fed into a dryer and dried at 190°C or below for 4-16 hours. They are then fed into a preheater and heated to the solid-state polymerization temperature described below for 2-5 hours. They are then continuously fed into a solid-state polymerization machine for solid-state polymerization, thereby obtaining a polyester resin with the desired intrinsic viscosity. Solid-state polymerization is preferably carried out under an inert gas, such as nitrogen gas. Solid-state polymerization is typically carried out at temperatures between 170-230°C, preferably between 180-220°C. The polymerization time is typically between 20 and 80 hours, with the reaction occurring in the solid-state polymerization machine.

The resin of the present invention can be used to obtain molded articles (blow molded articles, injection molded articles, sheets, films, etc.) with excellent color tone and transparency using methods such as blow molding, injection molding, and stretching, and can also be used to obtain fibers by melt spinning.

Molded articles containing the resin of the present invention can be produced by applying various molding methods, such as press molding, extrusion molding, pressure molding, and blow molding, using raw materials containing the resin of the present invention. This allows for the production of a variety of parts, including containers. The resin of the present invention is particularly suited to the production of blow-molded articles because it has properties similar to those of virgin polyester resin and excellent thermal stability. Therefore, a molded article manufacturing method can be suitably employed, which includes the steps of obtaining a parison from a melt of raw materials containing the resin of the present invention and blowing gas into the parison. This allows for the production of molded articles such as containers.

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 molded articles such as those described above, thickness variations are unlikely to occur, and highly uniform molded articles can be obtained with good operational efficiency. Furthermore, as mentioned above, the resin of the present invention has an average pressure increase rate of 0.6 MPa/h or less and contains little foreign matter. Foreign matter present in the resin is expected to be inorganic matter or foreign matter derived from non-polyester resins, but the low amount of these foreign matters allows for the production of molded articles with good surface appearance and excellent impact resistance and strength.

In the case of molded products, the inclusion of 0.1 to 1.0 mass % of a hindered phenol-based antioxidant can prevent a decrease in intrinsic viscosity and deterioration of color tone after molding.
As will be described later, in the case of a molded article, it is preferable that the composition contains a germanium compound and a cobalt compound as a polymerization catalyst, and further contains a dye.

The germanium compound is preferably contained in an amount of 5.0× ¹⁰ mol to 3.0× ¹⁰ mol per mole of the acid component of the polyester. If the amount is less than 5.0× ¹⁰ mol, it becomes difficult to obtain a polyester resin with the target degree of polymerization. On the other hand, if the amount exceeds 3.0× ¹⁰ mol, by-products produced by the reaction between the cobalt compound and the germanium compound deteriorate the stability of the polyester over time, causing a decrease in the intrinsic viscosity and deterioration in color tone of the polyester resin composition after long-term storage, which is undesirable.

Germanium compounds include germanium dioxide, germanium tetrachloride, and germanium tetraethoxide, with germanium dioxide being preferred in terms of its polycondensation catalyst activity, the physical properties of the resulting polyester resin, and cost.

The content of the cobalt compound is preferably 1.0×10 ⁻⁵ mol to 1.0×10 ⁻⁴ mol, and more preferably 2.0×10 ⁻⁵ mol to 8.0×10 ⁻⁵ mol, per mol of the acid component of the polyester.
If the amount is less than 1.0× ¹⁰ mol, the color tone of the polyester deteriorates. On the other hand, if the amount exceeds 1.0×10 ^mol , the stability of the polyester resin over time deteriorates. Examples of cobalt compounds include cobalt acetate, cobalt formate, cobalt chloride, and cobalt oxide, and cobalt acetate is preferred from the standpoints of polycondensation catalyst activity, the physical properties of the resulting polyester resin, and cost.

It is preferable to use the dyes mentioned above, and the content is preferably 30 ppm or less. If it exceeds 30 ppm, the transparency of the polyester may decrease and the color may become dull.

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 produced with high productivity.

The fibers of the present invention containing the resin of the present invention may be, for example, monofilaments, multifilaments, etc., and may be either long fibers or short fibers. Furthermore, the fibers of the present invention may be used as either main fibers or binder fibers. Furthermore, they may be fibers made solely from the resin of the present invention, or composite fibers made from the resin of the present invention together with other resins.

For example, when the fiber of the present invention is used as the main fiber, the composite fiber may be a two-component fiber formed by bonding the resin of the present invention and another resin side by side. The other resin used in such a composite fiber may be appropriately selected depending on the required fiber properties and application. In the case of a side-by-side composite fiber, the resin of the present invention (copolymerized with an ethylene oxide adduct of bisphenol A) shrinks significantly during heat treatment, resulting in the development of a three-dimensional spiral crimp. This developed spiral crimp makes the fiber of the present invention highly stretchable.
Furthermore, when the fiber of the present invention is used as a binder fiber, it may be a fully fused binder fiber using only the resin of the present invention, or a core-sheath type binder fiber using the resin of the present invention only in the sheath portion.

In both molded articles containing the resin of the present invention and fibers containing the resin of the present invention, the content of the resin of the present invention is preferably 50% by mass or more of the resin constituting the molded article or fiber, more preferably 60% by mass or more, even more preferably 80% by mass or more, and most preferably 100% by mass. If the proportion of the resin of the present invention in the resin constituting the molded article or fiber is less than 50% by mass, the rate of recycled raw materials used will be low, making it difficult to produce an environmentally friendly product.

When the fibers of the present invention are short fibers, they can have characteristic values such as a single filament fineness of 0.1 to 25.0 decitex, a strength of 0.1 to 6.0 cN/dtex, and an elongation of 20 to 500%. As mentioned above, the resin of the present invention has excellent thermal stability because the diethylene glycol content and carboxyl end group concentration are below specific amounts. Therefore, when producing short fibers such as those described above, single filament fusion during the spinning and drawing processes is suppressed, which reduces variation in single filament fineness and allows for the production of highly uniform fibers with good operational efficiency. Furthermore, because single filament fusion during the spinning and drawing processes is suppressed, single filament dispersibility in water is improved when used in wetlaid nonwoven fabric applications.

Furthermore, as mentioned above, the resin of the present invention has an average pressure increase rate of 0.6 MPa/h or less and contains a low amount of foreign matter. Foreign matter present in the resin is expected to be inorganic matter or foreign matter derived from non-polyester resins, but the low amount of such foreign matter improves the quality and strength of products such as nonwoven fabrics obtained using the fiber of the present invention.

Furthermore, because the resin of the present invention has excellent thermal stability and a low amount of foreign matter, it is possible to perform flow drawing during the drawing process, and ultrafine fibers with a single fiber fineness of 0.1 to 0.03 decitex can be obtained. Flow drawing involves drawing at a temperature between the glass transition temperature and the heat-raised crystallization temperature of the resin used, making it possible to reduce the fiber diameter while suppressing the orientation of undrawn fibers. For more stable flow drawing, it is desirable to draw in a medium that is at least 10°C higher than the glass transition temperature of the polyester used. Furthermore, it is preferable to use water or steam as the medium, and more preferably, drawing is performed in warm water between 80°C and 110°C, or in moist heat below 110°C.

Furthermore, lubricant particles may be added during melt spinning of the fibers, provided that the effects of the present invention are not impaired. The lubricant is preferably inactive to polyester, and examples include titanium oxide, silica, calcium carbonate, barium sulfate, and aluminum oxide. Titanium oxide is particularly preferred from the standpoints of smoothness and particle size distribution. Adding a lubricant can further suppress fusion of single filaments during the spinning and drawing processes, improving single filament dispersion in water when used for wetlaid nonwoven fabrics. The lubricant may be added at any stage of the fiber spinning process. Examples of addition methods include the masterbatch method and liquid color method, with the masterbatch method being preferred due to stability and ease of handling during melt spinning. When using the masterbatch method, either method may be used, such as measuring and mixing the raw material pellets and then melt-spinning them, or measuring and mixing separately melted polymers and then spinning them.

The fibers (short fibers) of the present invention can be suitably used in nonwoven fabric applications, and nonwoven fabrics obtained using the fibers of the present invention may consist solely of the fibers of the present invention, or may contain fibers other than the fibers of the present invention. For example, a nonwoven fabric may be made using the fibers of the present invention as the main fiber and fibers made of a polyester resin with a lower melting point than the resin of the present invention as the binder fiber.

Nonwoven fabrics obtained using the fibers of the present invention may be dry-laid or wet-laid, and there are no particular limitations on the basis weight of the nonwoven fabric. When the fibers of the present invention are used as the main fiber, examples of the nonwoven fabric include thermal-bonded nonwoven fabrics, needle-punched nonwoven fabrics, air-laid nonwoven fabrics, spunlaced nonwoven fabrics, and wet-laid nonwoven fabrics (papermaking). In particular, the fibers of the present invention can be used to obtain high-quality ultrafine fibers. Therefore, nonwoven fabrics using ultrafine fibers as the fibers of the present invention are of high quality, uniform, and highly porous, and are suitable for use in various industrial filters such as water treatment filters, and separators.

Here is an example of a method for producing a drylaid nonwoven fabric. In this example, the fiber of the present invention is used as the main fiber and is mixed with a binder fiber other than the fiber of the present invention to obtain a drylaid nonwoven fabric. The ratio of the fiber of the present invention to the binder fiber when mixed can be selected according to the required properties of the nonwoven fabric, but the ratio of the fiber of the present invention is preferably approximately 10 to 90% by mass. These two fibers (fibers that will become the constituent fibers) are fed into a carding machine and defibrated to produce a drylaid web. The resulting web is then subjected to a thermal bonding treatment in a continuous heat treatment machine that performs hot air treatment at a temperature at which the resin that makes up the binder fiber melts or softens, resulting in a drylaid nonwoven fabric in which the constituent fibers are integrated by thermal bonding.

Here is an example of a method for producing a wetlaid nonwoven fabric. As with drylaid nonwoven fabrics, the fibers of the present invention are used as the main fiber and are mixed with binder fibers other than the fibers of the present invention to obtain a wetlaid nonwoven fabric. The ratio of the fibers of the present invention to the binder fibers when mixed can be selected appropriately depending on the required properties of the nonwoven fabric, but the ratio of the fibers of the present invention is preferably approximately 10 to 90% by mass. These two fibers (fibers that will become the constituent fibers) are mixed and defibrated using a pulp disintegrator, and then a wetlaid web is produced in a papermaking machine. The resulting web is then subjected to a thermal bonding treatment in a continuous heat treatment machine that uses hot air at a temperature at which the resin that makes up the binder fibers melts or softens, resulting in a wetlaid nonwoven fabric in which the constituent fibers are integrated together by thermal bonding.

In addition, while it is generally more difficult to produce long fibers (multifilaments) in fiber production, the fibers of the present invention can be made into multifilaments with characteristic values such as a single yarn fineness of 0.3 to 30 decitex, a single yarn count of 2 to 300, a total fineness of 5 to 350, a strength of 1 to 5 cN/decitex, and an elongation of 10 to 400%.

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 obtained with good operational efficiency. Furthermore, as mentioned above, the resin of the present invention has an average pressure increase rate of 0.6 MPa/h or less and contains little foreign matter. Foreign matter present in the resin is expected to include inorganic substances and non-polyester resin-derived foreign matter, but the low amount of these foreign matters makes it possible to produce ultrafine fibers with a single fiber fineness of 1.0 decitex or less, especially 0.6 decitex or less, with good operational efficiency.

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) Melt Viscosity of Depolymerization The melt viscosity of the depolymerization product obtained in the production step (1) was measured using a Brookfield VISCO METER DV2T melt viscometer at the same temperature as the heat treatment temperature during depolymerization.
(b) 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.
(c) 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.
(d) 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.

(e) Average pressure rise rate measured by 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

(f) 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.
(g) 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.

(h) Moldability The thickness of the body of each of the obtained containers (100 samples) was measured, and samples with a difference in thickness between the thickest and thinnest parts of up to 0.30 mm were judged to be acceptable. Depending on the number of samples that passed, the moldability was evaluated in two stages as follows:
◯: The number of passing samples was 95 or more ×: The number of passing samples was 94 or less (i) Haze Sample pieces (20 pieces) were cut out from the obtained container, and the turbidity was measured using a turbidity meter Model 1001DP manufactured by Nippon Denshoku Industries Co., Ltd. (air: haze 0%), and the average value of n=20 was taken as the average value. The smaller this value, the better the transparency, and a value of 6% or less was determined to be excellent transparency.
(j) Impact Resistance (h) Molded articles (100 samples) that passed the moldability evaluation were filled with 340 ml of tap water and dropped once on a P tile from a height of 200 cm, with the bottom and side facing downwards. The impact resistance was evaluated based on the number of molded articles that did not break. If 90 or more molded articles were dropped, the impact resistance was evaluated as good.

(k) Runnability of staple fiber production (cut yarn)
The operability was evaluated based on the number of times that yarn breakage occurred during the melt spinning process. A yarn breakage rate of less than three times per ton of spinning was evaluated as pass (◯), and a yarn breakage rate of three or more times was evaluated as fail (×).
(l) Nonwoven Fabric Strength Samples of 150 mm in MD and 50 mm in CD were cut out of the obtained nonwoven fabric, and the MD strength of the nonwoven fabric was measured using an autograph (AG-50KNI manufactured by Shimadzu Corporation) at a pulling speed of 100 mm/min and a chuck distance of 100 mm. The number of samples was n=5.
The tensile strength of the obtained nonwoven fabric was evaluated according to the following two levels.
○: Tensile strength 1500 cN or more ×: Tensile strength less than 1500 cN (m) Shrinkage (short fiber)
The resulting short fibers (Examples 30 to 32) were used to measure the dry heat dimensional change rate according to JIS L1015 8.15b as follows. A sample was prepared by attaching each end of a fiber to a piece of glossy paper with adhesive (double-sided tape) with a spatial distance of 25 mm (a paper was attached over the double-sided tape to further secure the fiber). This sample was then attached to a single fiber elasticity tester with a grip distance of 25 mm. The glossy paper was then cut, and the initial sample length (N ₀ ) was measured when a predetermined initial load (initial load = 45 mg × fineness (dtex)) was applied. After measuring the initial sample length, the fiber was attached to a heat treatment stand, hung in a hot air dryer set to 170°C, and left for 15 minutes. It was then removed, cooled to room temperature, and attached to the single fiber elasticity tester again. The distance between the grips when the initial load was applied (sample length after heat treatment (N ₁ )) was measured, and the thermal shrinkage was calculated using the following formula:
Heat shrinkage rate (%)=[1-(N ₁ /N ₀ )]×100
A shrinkage rate of 30% or more was marked "good", and a shrinkage rate of less than 30% was marked "poor".

(n) Runnability of filament manufacturing (cut yarn)
After melt spinning and collecting undrawn yarns, if the breakage rate (number of breaks/number of spindles) during the drawing process was 5% or less, the result was marked "◯", and if not, the result was marked "X".
(o) Shrinkage (long fibers)
The obtained polyester fiber was taken up in a length of 22.5 m, and the ratio of the fiber length before and after immersion in boiling water at 100°C for 30 minutes was taken as the shrinkage rate, which was calculated using the following formula. Note that when measuring the fiber length, a load of 1.3 times the fineness (den) was applied.
Shrinkage rate (%) = [(yarn length before treatment with boiling water - yarn length after treatment) / yarn length before treatment] x 100
A shrinkage rate of 15% or more was marked "good", and a shrinkage rate of less than 15% was marked "poor".

Example 1
[Recycled polyester resin]
29.6 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was charged into an esterification reactor, and then 4.0 parts by mass of ethylene glycol (EG) was added while the agitator of the esterification reactor was rotating. When the internal temperature of the esterification reactor (hereinafter referred to as "ES can") stopped decreasing, 44.4 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was added via a rotary valve over about 2 hours, and then 6.0 parts by mass of ethylene glycol and 16.0 parts by mass of isophthalic acid (IPA) were additionally added.
At this time, the recycled polyester raw material was added so that the molar ratio of total glycol components/total acid components (hereinafter sometimes referred to as "G/A") was 1.13.
Thereafter, a depolymerization reaction was carried out under heat treatment conditions at 260°C for 1 hour, to obtain a depolymerized polymer having a melt viscosity at 260°C of 60 mPa·s.
The obtained depolymerization product was then pressure-fed to a polycondensation reactor (hereinafter referred to as PC can) through a candle filter with 20 μm openings set between the esterification reactor and the polycondensation reactor. EG slurry of titanium oxide was then added so that the titanium oxide was 0.2 mass %, and 2.0 × 10 ^-4 mol of antimony trioxide as a polymerization catalyst, 0.4 × 10 ^-4 mol of cobalt acetate as a cobalt compound, and 0.96 × 10 ^-4 mol of triphenyl phosphate were added. The PC can was then reduced in pressure, and after 60 minutes, a 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 0.7.

[Production of Short Fibers]
Melt spinning was carried out using a spinneret with 2174 holes and a hole diameter of 0.35 mm, with a discharge rate of 1630 g/min, a core-sheath mass ratio of 50/50, a spinning temperature of 270°C, and a spinning speed of 1100 m/min, so that polyethylene terephthalate with an intrinsic viscosity of 0.70 was disposed as the core and the obtained polyester resin as the sheath.
The resulting undrawn yarn was converged to form a tow of 115 ktex, which was then drawn at a drawing temperature of 56°C and a draw ratio of 3.5 times. The tow was then mechanically crimped using a press-in crimper and cut to a fiber length of 51 mm to obtain a core-sheath type composite fiber having a fineness of 2.2 dtex.

[Preparation of dry nonwoven fabric]
70% by mass of Unitika's regular polyester fiber <121> 1.7T 51 mm and 30% by mass of the obtained core-sheath composite fiber (used as a binder fiber) were blended, and the fibers were fed into a carding machine (Yamato Kiko Co., Ltd., SC-500DI3HC) to produce a web so that the nonwoven fabric after heat treatment would have a basis weight of 50 g/m ^2. A dry-laid nonwoven fabric was then produced by heat treatment using a continuous heat treatment machine (Tsujii Senki Kogyo Co., Ltd., NFD-500E2) at an air volume of 57 m ³ /min and 130°C for 1 min.

Examples 2 to 13, Comparative Examples 1 to 7
[Recycled polyester resin]
The depolymerization reaction was carried out in the same manner as in Example 1, except that the amounts of ethylene glycol, isophthalic 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.
Further, polyester resins were 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 and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 1.
[Production of Short Fibers]
Heat-adhesive sheath-core composite fibers were obtained in the same manner as in Example 1, except that the obtained polyester resins (Examples 2 to 13, Comparative Examples 2 and 4) were used for the sheath portion.
[Preparation of dry nonwoven fabric]
A drylaid nonwoven fabric was produced in the same manner as in Example 1, except that the obtained heat-adhesive sheath-core type composite fiber was used.

Table 1 shows the property values of the polyester resins obtained in Examples 1 to 13 and Comparative Examples 1 to 7. Table 2 also shows the evaluation of operability when producing staple fibers in Examples 1 to 13 and Comparative Examples 2 and 4, and the strength values of the nonwoven fabrics obtained using the staple fibers.

Example 14
[Recycled polyester resin]
The depolymerization reaction was carried out in the same manner as in Example 1, except that the amounts of ethylene glycol, isophthalic 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, polyester resins were produced in the same manner as in Example 1, except that the filtration particle size of the filter used in the step of recovering the filtrate and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 1, and 0.18 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (Irganox 1010, manufactured by BASF) was added as a hindered phenol-based antioxidant instead of titanium oxide.
The resulting polyester resin was then continuously fed to a crystallizer and crystallized at 150°C, then fed to a dryer and dried at 130°C for 10 hours, then fed to a preheater and heated to 180°C, and then fed to a solid-state polymerizer. A solid-state polymerization reaction was then carried out at 190°C for 50 hours under nitrogen gas, yielding a polyester resin with an intrinsic viscosity of 1.1.

[Blow molded products]
The recycled polyester resin was chipped and dried, and then extruded at 260°C using a direct blow molding machine (manufactured by Tahara Co., Ltd.) to form a cylindrical parison. The parison was then clamped in a mold while still softened to form a bottom, which was then blown into a bottle. The bottom was formed when the parison reached a diameter of 3 cm and a length of 25 cm, and a 350 ml hollow container (direct blow molded product) was obtained.

Examples 15 to 26, Comparative Examples 8 to 18
[Recycled polyester resin]
The depolymerization reaction was carried out in the same manner as in Example 14, except that the amounts of ethylene terephthalate oligomer, ethylene glycol, isophthalic acid, and recycled polyester raw material added during the depolymerization reaction, as well as the G/A and heat treatment temperature, were changed to those shown in Table 1.
Further, polyester resins were produced in the same manner as in Example 14, except that the filtration particle size of the filter in the step of recovering the filtrate and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 1.
Subsequently, the obtained polyester resin was subjected to a solid-state polymerization reaction in the same manner as in Example 14 to obtain a polyester resin having an intrinsic viscosity of 1.1 (1.2 in Comparative Example 9).
[Blow molded products]
Using the obtained polyester resins (Examples 15 to 26, Comparative Examples 8, 9, 11 to 14), blow molding was carried out in the same manner as in Example 14 to obtain blow molded articles.

Table 1 shows the property values of the polyester resins obtained in Examples 14 to 26 and Comparative Examples 8 to 18. Table 3 also shows the evaluation results of the moldability, haze, and impact resistance of the blow-molded articles obtained in Examples 14 to 26 and Comparative Examples 8, 9, and 11 to 14.

As is clear from Tables 1 to 3, the recycled polyester resins obtained in Examples 1 to 26 had carboxyl terminal group amounts, diethylene glycol contents, and average pressure rise rates within the ranges specified in the present invention. This resulted in excellent operability when producing short fibers, and dry-laid nonwoven fabrics with excellent nonwoven fabric strength. Furthermore, they also had excellent moldability when blow-molding, allowing blow-molded products with excellent haze and impact resistance to be obtained.

On the other hand, the polyester resin obtained in Comparative Example 1 had a low content of components derived from recycled polyester raw materials because the isophthalic acid content exceeded the upper limit. Furthermore, the crystallinity and glass transition temperature were low, making it difficult to melt-spin and stretch the resin to produce fibers. For the polyester obtained in Comparative Example 2, the heat treatment temperature during depolymerization exceeded the upper limit, causing the diethylene glycol content and carboxyl end group amount to exceed the upper limits, resulting in poor operability when obtaining short fibers. In Comparative Example 3, the filtration particle size was below the lower limit, making it difficult to pass the resin through the filter and preventing it from proceeding to the next step.

In Comparative Example 4, the upper limit of the filtration particle size was exceeded, resulting in a high average pressure rise rate for the resulting recycled polyester resin. This resulted in poor operability when obtaining short fibers, and the resulting nonwoven fabric was of poor quality.

In Comparative Examples 5 and 16, the G/A and heat treatment temperature during depolymerization were low, and the melt viscosity of the resulting depolymerization was too high, making it difficult to pass through the filter during the filtration process and preventing the next step. In Comparative Examples 6 and 17, the G/A and heat treatment temperature during depolymerization were high, and the melt viscosity of the resulting depolymerization was too low, causing filtrate leakage during the filtration process and preventing the next step. In the polyester obtained in Comparative Example 7, the G/A during depolymerization exceeded the upper limit, resulting in a high diethylene glycol content and a fast pressure rise rate, which impaired operability when producing short fibers. In the polyester obtained in Comparative Example 8, the isophthalic acid content was below the lower limit, resulting in whitening during blow molding and the resulting blow-molded article having poor appearance. In the polyester obtained in Comparative Example 9, the heat treatment temperature during depolymerization exceeded the upper limit, resulting in a high diethylene glycol content and a high amount of carboxyl end groups, and the resulting blow-molded article had poor impact resistance.

In Comparative Example 10, the filtering particle size was below the lower limit, making it difficult to pass through the filter, and the product could not proceed to the next step.
The polyester obtained in Comparative Example 11 exceeded the upper limit of the filtering particle size, resulting in a high average pressure rise rate, and the resulting blow-molded article was therefore poor in impact resistance.
In the polyester obtained in Comparative Example 12, the G/A ratio during polymerization was less than the lower limit, and therefore the amount of carboxyl terminal groups was high, the pressure rise rate was fast, and the impact resistance of the resulting blow-molded article was poor.
In the polyester obtained in Comparative Example 13, the G/A ratio during polymerization was less than the lower limit, so the amount of carboxyl terminal groups was high, the pressure rise rate was fast, and the resulting blow-molded article had poor impact resistance.
In the polyester obtained in Comparative Example 14, the G/A ratio at the time of depolymerization exceeded the upper limit, and therefore the diethylene glycol content was high, the pressure rise rate was fast, and the resulting blow-molded article had poor impact resistance.
In Comparative Example 15, the heat treatment temperature during depolymerization was below the lower limit, so that the raw material solidified and polyester could not be obtained.
In Comparative Example 18, the polycondensation temperature was below the lower limit, so the condensation reaction did not proceed and a polyester resin could not be obtained.

Example 27
[Recycled polyester resin]
34.8 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was charged into an esterification reactor, and then, with the esterification reactor's agitator running, 1.2 parts by mass of ethylene glycol (EG), 3.1 parts by mass of isophthalic acid (IPA), and 6.7 parts by mass of bisphenol A ethylene oxide adduct (BAEO) were added. Once the internal temperature of the esterification reactor (hereinafter referred to as the "ES can") stopped decreasing, 52.2 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was added via a rotary valve over approximately 2 hours, and then an additional 1.9 parts by mass of ethylene glycol was added.
At this time, the recycled polyester raw material was added so that the molar ratio of total glycol components/total acid components (hereinafter sometimes referred to as "G/A") was 1.24.
Thereafter, a depolymerization reaction was carried out under heat treatment conditions at 260°C for 1 hour, to obtain a depolymerized polymer having a melt viscosity at 260°C of 160 mPa·s.
The obtained depolymerization product was then pressure-fed to a polycondensation reactor (hereinafter referred to as PC can) through a candle filter with 20 μm openings set between the esterification reactor and the polycondensation reactor. EG slurry of titanium oxide was then added so that the titanium oxide content was 0.4 mass %, and 2.0 × 10 ^-4 mol of antimony trioxide as a polymerization catalyst, 0.4 × 10 ^-4 mol of cobalt acetate as a cobalt compound, and 0.96 × 10 ^-4 mol of triphenyl phosphate were added. The PC can was then reduced in pressure, and after 60 minutes, a melt polymerization reaction was carried out at a final pressure of 0.5 hPa and a temperature of 275°C for 3 hours, yielding a polyester resin with an intrinsic viscosity of 0.64.

[Production of Long Fibers]
The obtained polyester resin was melt-spun using a spinneret with 12 holes and a hole diameter of 0.3 mm under the conditions of a discharge rate of 13 g/min, a spinning temperature of 290°C, and a spinning speed of 1395 m/min. The obtained undrawn yarn was drawn under the conditions of a heat setting temperature of 117°C and a draw ratio of 3.0 to obtain a polyester fiber having a fineness of 33 dtex/12f.

Examples 28 to 37, Comparative Examples 20 to 27
[Recycled polyester resin]
A depolymerization reaction was carried out in the same manner as in Example 27, except that the amounts of ethylene glycol, isophthalic acid, the ethylene oxide adduct of bisphenol A, and the recycled polyester raw material added during the depolymerization reaction, G/A, and the heat treatment temperature were changed to those shown in Table 4. Furthermore, a polyester resin was produced in the same manner as in Example 27, except that the filtration particle size of the filter in the step of recovering the filtrate and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 4.
[Production of Long Fibers]
Using the obtained polyester resin, polyester fibers were produced in the same manner as in Example 27.

Table 4 shows the property values of the polyester resins obtained in Examples 27-39 and Comparative Examples 20-27, as well as an evaluation of the operability during fiber production and the shrinkage of the resulting fibers.

As is clear from Table 4, the polyester resins obtained in Examples 27 to 39 had carboxyl terminal group amounts, diethylene glycol contents, and average pressure rise rates within the ranges specified in the present invention. As a result, polyester fibers with excellent operability and shrinkability were obtained.

Example 38
[Recycled polyester resin]
A depolymerization reaction was carried out in the same manner as in Example 27, except that the amounts of ethylene terephthalate oligomer, ethylene glycol, isophthalic acid, an ethylene oxide adduct of bisphenol A, and a recycled polyester raw material added during the depolymerization reaction, as well as the G/A and heat treatment temperature, were changed to those shown in Table 4. Furthermore, a polyester resin was produced in the same manner as in Example 27, except that the filtration particle size of the filter in the step of recovering the filtrate and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 4.
[Production of short fibers (composite fibers)]
Side-by-side composite fibers (undrawn yarns) were spun using polyethylene terephthalate with an intrinsic viscosity of 0.70 and the resulting recycled polyester resin using a round spinneret with 1,038 holes, with a throughput of 386 g/min, a mass ratio of the two polyester resins of 50/50, a spinning temperature of 300°C, and a spinning speed of 900 m/min. The resulting undrawn yarns were stretched to a draw ratio of 3.60 times at a drawing temperature of 73°C, then subjected to a tension heat treatment at 140°C, mechanically crimped in a stuffing box (12 crimps/25 mm), then applied with a finishing oil, and cut to a fiber length of 44 mm to obtain composite fibers with a single fiber fineness of 1.3 dtex.

Example 39
[Recycled polyester resin]
A depolymerization reaction was carried out in the same manner as in Example 27, except that the amounts of ethylene terephthalate oligomer, ethylene glycol, isophthalic acid, an ethylene oxide adduct of bisphenol A, and a recycled polyester raw material added during the depolymerization reaction, as well as the G/A and heat treatment temperature, were changed to those shown in Table 4. Furthermore, a polyester resin was produced in the same manner as in Example 27, except that the filtration particle size of the filter in the step of recovering the filtrate and the heat treatment temperature in the polymerization reaction step were changed to those shown in Table 4.
[Production of Short Fibers]
Side-by-side composite fibers (undrawn yarns) were spun using polyethylene terephthalate with an intrinsic viscosity of 0.70 and the resulting recycled polyester resin using a spinneret with 706 holes and a hole diameter of 0.30 mm, with a throughput of 312 g/min, a mass ratio of the two polyester resins of 50/50, a spinning temperature of 272°C, and a spinning speed of 1,100 m/min. The resulting undrawn yarns were stretched at a stretching temperature of 73°C to a stretch ratio of 3.40, then subjected to a tension heat treatment at 140°C, mechanically crimped in a stuffing box (12 crimps/25 mm), and then applied with a finishing oil. The yarns were then cut to a fiber length of 44 mm, yielding composite fibers with a single fiber fineness of 1.1 dtex.

Example 40
[Production of Short Fibers]
A composite fiber having a single fiber fineness of 1.3 dtex was obtained in the same manner as in Example 38, except that polyethylene terephthalate having an intrinsic viscosity of 0.70 was used as a base polymer, and a master batch (pigment MB) having iron oxide (red, yellow) and carbon black kneaded therein was mixed so that the colored pigment concentration in the base polymer was 0.2 mass %, and the recycled polyester resin obtained in Example 38 was used.

Example 41
[Production of Short Fibers]
Using a spinneret with 1014 holes and a hole diameter of 0.35 mm, melt spinning was carried out at a throughput rate of 563 g/min, a core/sheath mass ratio of 50/50, a spinning temperature of 272°C, and a spinning speed of 1120 m/min, with polyethylene terephthalate having an intrinsic viscosity of 0.70 as the core and the polyester resin obtained in Example 5 as the sheath. The undrawn yarn obtained was converged and drawn at a drawing temperature of 54°C and a draw ratio of 3.4. Next, after applying an oil, the drawn yarn was squeezed to a moisture content of approximately 18% by mass and cut to a length of 5 mm using a drum cutter, yielding a core-sheath composite fiber with a single yarn fineness of 1.7 dtex.
[Preparation of wet-laid nonwoven fabric]
Next, the obtained core-sheath type composite fiber was used as a binder fiber, and short-cut polyethylene terephthalate fibers (N801 1.6T5 manufactured by Unitika Ltd.) with a single filament fineness of 1.6 dtex and a length of 5 mm were used as the main fiber. These fibers were dispersed in water at a binder fiber/main fiber (mass ratio) of 40/60, and a paper web was obtained using a cylinder paper machine. Thereafter, a wetlaid nonwoven fabric was produced by heat treatment using a continuous heat treatment machine (NFD-500E2 manufactured by Tsujii Senki Kogyo Co., Ltd.) under conditions of an air volume of 57 ^m3 /min and 130°C x 1 min.

Example 42
[Production of Short Fibers]
Using only the polyester resin obtained in Example 5, melt spinning was carried out using a spinneret with 720 holes and a hole diameter of 0.25 mm under conditions of a discharge rate of 350 g/min, a spinning temperature of 275°C, and a spinning speed of 730 m/min.
The obtained undrawn yarn was converged to form a 50 ktex tow, and then an oil was applied thereto, and the tow was squeezed so that the moisture content was approximately 18% by mass, and then cut into lengths of 5 mm using a drum cutter to obtain heat-bondable short fibers with a fineness of 6.6 dtex.
[Preparation of wet-laid nonwoven fabric]
A wetlaid nonwoven fabric was prepared in the same manner as in Example 41, except that the obtained heat-adhesive staple fibers were used as binder fibers.

Example 43
[Production of short fibers and wet-laid nonwoven fabrics]
A thermally adhesive staple fiber having a fineness of 6.6 dtex was obtained in the same manner as in Example 42, except that the polyester resin obtained in Example 4 was used.
A wetlaid nonwoven fabric was produced in the same manner as in Example 41, except that the obtained heat-adhesive staple fibers were used as binder fibers.

As is clear from Table 5, the side-by-side type composite fibers obtained in Examples 38 to 40 had excellent shrinkage properties and could be obtained with good operability.
Both the core-sheath type and single type thermal adhesive fibers obtained in Examples 41 to 43 could be obtained with good operability, and the wetlaid nonwoven fabrics obtained by using these fibers as binder fibers had excellent nonwoven fabric strength.

Claims (1)

A method for producing a recycled polyester resin in which , when the total amount of all acid components constituting the polyester is taken as 100 mol %, 50 to 98 mol % is terephthalic acid and 2 to 40 mol % isophthalic acid, is produced using at least one recycled polyester raw material consisting of a ) used polyester products and b) unused polyester resin generated in the process of producing polyester products, the method comprising all of the following steps (1) to (4):
(1) A step of adding the recycled polyester raw material to a mixture containing ethylene glycol and isophthalic acid 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 depolymerized product having a melt viscosity of 10 to 1500 mPa s. (2) A step of passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover a filtrate. (3) A step of adding a polycondensation catalyst to the filtrate and kneading the mixture to obtain a reaction product. (4) A step of performing a polycondensation reaction on the reaction product at a temperature of 250 ° C. or higher and a reduced pressure of 1.0 hPa or lower.