JP7784657B2 - Recycled polyester resin and method for producing recycled polyester resin - Google Patents

Description

The present invention relates to a novel recycled polyester resin with flame retardancy and a method for producing the recycled polyester resin. In particular, the present invention relates to a recycled polyester resin produced using 80% by mass or more of components derived from recycled polyester raw materials, including used polyester products and unused polyester resin generated in the process of producing polyester products, which contains little foreign matter and can be processed into various molded products in the same way as virgin polyester resin, and a method for producing the same.

Polyethylene terephthalate (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, such as polypropylene, polyethylene, and polystyrene, from PET resin. Therefore, even if recycled polyester resin was obtained using the recycling methods described in Patent Documents 1 to 3, the foreign matter derived from the non-polyester resin could not be sufficiently removed, the amount of foreign matter mixed in was not sufficiently reduced, and it was not possible 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.

In particular, when attempting to produce fibers by melt spinning polyester resin, the amount of contaminants present can have a significant impact on productivity. When producing fibers, resin is extruded through a small-diameter nozzle in the melt spinning process, and the extruded filaments are collected on a roller. If necessary, they are stretched and heat-treated, and then wound up. In this case, using resin contaminated with contaminants can easily lead to thread breakage during the melt spinning, stretching, heat treatment, and winding processes, making stable production difficult. This problem becomes even more pronounced when attempting to produce finer fibers.

Furthermore, growing awareness of environmental issues has led to an increasing demand for recycled polyester resins that use a higher percentage of recycled raw materials. Recycled polyester resins that use 80% or more recycled raw materials, or even 100%, have become more susceptible to the problem of foreign matter contamination, as described above, and also have poorer thermal stability, making it more difficult to obtain fine fibers through melt spinning.

Furthermore, in recent years, there has been an increasing demand for flame-retardant synthetic fibers and various plastic products from the perspective of fire prevention.
Various attempts have been made to impart flame retardancy to polyesters, but from the viewpoints of performance and productivity, a method of copolymerizing a flame retardant during polyester production is common. Phosphorus compounds are considered advantageous as flame retardant substances used for this purpose from the viewpoints of flame retardancy, cost, environmental pollution, and safety. However, no polyester resins have yet been proposed that have been copolymerized with such phosphorus compounds to impart flame retardancy and that have performance equivalent to that of virgin polyester, even when made from recycled polyester.

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

The present invention aims to solve the above problems and provide a recycled polyester resin that contains 80% by mass or more of components derived from recycled polyester raw materials, including used polyester products or scraps generated in the manufacturing process of polyester resins and products. The recycled polyester resin has flame retardancy and performance equivalent to that of virgin polyester resin, and can be used to manufacture various types of polyester products. The present invention also aims to provide a manufacturing method by which the recycled polyester resin of the present invention can be obtained.

The inventors conducted extensive research in light of the problems with the prior art and discovered that by using recycled polyester raw materials and manufacturing them through a specific manufacturing process, it is possible to obtain recycled polyester resin with low levels of foreign matter and with performance and flame retardancy equivalent to that of virgin polyester resin, leading to the completion of the present invention.

In other words, the present invention encompasses the following (i) to (iv):

(i) A recycled polyester resin containing 80% 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) An organic phosphorus compound having two or more ester-forming functional groups is contained, and the content of phosphorus atoms in the polyester resin is 1,000 to 10,000 ppm by mass;
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol is 80 mol% or more and diethylene glycol is 4 mol% or less,
(3) The carboxyl terminal group concentration is 40 equivalents/t or less,
(4) The average pressure rise rate is 0.6 MPa/h or less (wherein the average pressure rise rate is a value calculated by the following procedure: a pressure rise tester including an extruder and a pressure sensor is used, a stainless steel filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical wire diameter: 0.07 mm, horizontal wire diameter: 0.04 mm, filtration particle size: 12 μm) is set at the tip of the extruder, a polyester resin is melted in the extruder at 300° C., and the melt is extruded from the filter at a discharge rate of 29.0 g/min. The pressure value at the start of extrusion is defined as the “initial pressure value (MPa),” and the pressure value at the time of subsequent continuous extrusion for 12 hours is defined as the “final pressure value (MPa).” The average pressure rise rate is calculated based on these pressure values using the following calculation formula A:
Average pressure increase rate (MPa / h) = (final pressure value - initial pressure value) / 12) ... A)
(ii) A molded product containing the recycled polyester resin described in (i).
(c) Fibers containing the recycled polyester resin described in (a).
(iv) 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 ethylene glycol, an organic phosphorus compound having two or more ester-forming functional groups, and a recycled polyester raw material so that the molar ratio of total glycol components/total acid components is 1.05 to 1.40, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a depolymerized polymer having a melt viscosity of 10 to 1,500 mPa s;
(2) passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A step of adding a polycondensation catalyst to the filtrate and kneading to obtain a reaction product; and (4) A step of subjecting the reaction product to a polycondensation reaction of the depolymerization 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 80% 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 producing polyester products, but the amount of foreign matter mixed in is low, and the carboxyl end group concentration and diethylene glycol content satisfy specific ranges, so that it has excellent thermal stability, and also has excellent flame retardancy due to the inclusion of a phosphorus compound. 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, relatively long-term continuous operation is possible, and flame-retardant products in various forms can be produced with high productivity.
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 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) contains 80% 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-mentioned a) and b) are not limited in their form, and may be pelletized by further processing such as pulverization or cutting as necessary, or may be melted and pelletized. The above-mentioned a) and b) may be used alone or as a mixture of both.
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 80% by mass or more, and preferably 85% by mass or more, of components derived from recycled polyester raw materials that are at least one of a) and b) above. The recycled polyester resin of the present invention can be obtained by the manufacturing method of the present invention described below, but it is also possible to easily obtain a recycled polyester resin that is 100% derived from recycled polyester raw materials, i.e., is composed only of recycled raw materials.

The resin of the present invention contains an organic phosphorus compound having two or more ester-forming functional groups (hereinafter sometimes referred to as phosphorus compound (Y)), and the content of phosphorus atoms in the polyester resin is 1,000 to 10,000 ppm. From the standpoints of reactivity in the polycondensation reaction, the residual rate of the organic phosphorus compound, etc., the phosphorus compound (Y) is preferably a compound represented by the following formula (1):

where R1 represents an alkyl group or aryl group having 1 to 12 carbon atoms, R2 represents an alkyl group, aryl group, or monohydroxyalkyl group having 1 to 18 carbon atoms, or a cyclic structure linked via R1, or a hydrogen atom, R3 represents an alkyl group, aryl group, or monohydroxyalkyl group having 1 to 18 carbon atoms, or a hydrogen atom, A represents a divalent or higher hydrocarbon group, and n represents the valence of A minus 1.

Specific examples of preferred organophosphorus compounds represented by the above formula (1) include those represented by the following structural formulas (a) to (d). Among these, the compound represented by structural formula (a) is particularly preferred in terms of flame retardancy. These compounds may be used as they are or in the form of esterified products.

By including the phosphorus compound in the polyester resin so that the phosphorus atom content in the polyester resin is 1,000 to 10,000 ppm, flame retardancy can be imparted to the polyester resin, and flame-retardant molded articles and fibers can be obtained.
The content of phosphorus atoms in the polyester resin is 1,000 to 10,000 ppm, and preferably 2,000 to 7,000 ppm.

If the phosphorus atom content is less than 1,000 ppm, the flame retardancy will be poor. On the other hand, if the phosphorus atom content exceeds 10,000 ppm, the resin will become amorphous, which makes it prone to blocking and fusion during high-temperature drying, making it unsuitable for spinning or molding processes.

The content of phosphorus atoms in the polyester resin can be adjusted to fall within the above range by adjusting the amount of the phosphorus compound (Y) contained in the polyester resin.
In the present invention, examples of a method for incorporating the phosphorus compound (Y) into the polyester resin include a method in which the phosphorus compound (Y) is copolymerized during polymerization to obtain the recycled polyester resin, and a method in which the phosphorus compound (Y) is blended into the recycled polyester resin after it is obtained. More specific methods will be described in detail in the explanation of the production method.

The acid component (100 mol%) of the resin of the present invention preferably contains 80 to 98 mol% terephthalic acid, with 85 to 97 mol% being particularly preferred. If the terephthalic acid content is less than 80 mol%, the resin composition will have reduced crystallinity and tend to become amorphous. On the other hand, if the terephthalic acid content exceeds 98 mol%, when a phosphorus compound is copolymerized, the amount of phosphorus atoms copolymerized (content) will be reduced, resulting in a reduced flame retardant effect.

Other acid components in the resin of the present invention besides terephthalic acid and phosphorus compounds include isophthalic acid, 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 80 mol % or more of all glycol components, and preferably 90 mol % or more. If the ethylene glycol content is less than 80 mol %, the resulting polyester resin will have poor crystallinity and heat resistance.

Furthermore, when the total amount of all glycol components 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.

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 mole of the acid component of the polyester resin to be produced. The upper limit of the amount used can be, for example, about 1× ¹⁰ mol, 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.

The phosphorus compound may be one other than the phosphorus compound (Y) described above. For example, phosphorus compounds such as phosphorous acid, phosphoric acid, trimethyl phosphite, triphenyl phosphite, tridecyl phosphate, trimethyl phosphate, triethyl phosphate, tridecyl phosphate, and triphenyl phosphate may be used. These may be used alone or in combination of two or more. Furthermore, the content of these compounds should be such that the phosphorus atom content in the polyester resin is within the range of 1,000 to 10,000 ppm.

Examples of the color tone adjuster include cobalt compounds such as cobalt acetate, manganese compounds such as manganese acetate, anthraquinone dye compounds, copper phthalocyanine compounds, etc. Among these, cobalt acetate is preferred from the viewpoints of polycondensation catalyst activity, 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, since this results in a good 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.
(a) a carboxyl terminal group concentration of 40 equivalents/t or less; (b) 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 (a) 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 generally preferably approximately 0.44 to 0.80. 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 a characteristic value (b) of an average pressure increase rate of 0.6 MPa/h or less, preferably 0.5 MPa/h or less, and more preferably 0.4 MPa/h or less, as measured by the following method. The average pressure increase rate in the present invention is an indicator of the amount of foreign matter derived from various inorganic substances 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, thereby suppressing the by-production of diethylene glycol and making it possible to obtain a recycled polyester resin that satisfies the characteristic values (a) and (b).

The method for producing the resin of the present invention will now be described. Methods for incorporating phosphorus compound (Y) into polyester resin include copolymerizing phosphorus compound (Y) during the polymerization process to obtain recycled polyester resin (hereinafter sometimes referred to as Production Method 1), and blending phosphorus compound (Y) into recycled polyester resin after it has been obtained (hereinafter sometimes referred to as Production Method 2).

[Manufacturing method 1]
First, a description will be given of the above-mentioned Production Method 1. By Production Method 1, a phosphorus compound can be copolymerized into the acid component constituting the polyester resin.
In the manufacturing method 1 of the present invention, it is important to carry out the steps (1) to (4) in this order.
(1) a step of adding ethylene glycol, an organic phosphorus compound having two or more ester-forming functional groups, and a recycled polyester raw material so that the molar ratio of total glycol components/total acid components is 1.05 to 1.40, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a depolymerized polymer having a melt viscosity of 10 to 1,500 mPa s;
(2) passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A step of adding a polycondensation catalyst to the filtrate and kneading to obtain a reaction product; and (4) A step of subjecting the reaction product to a polycondensation reaction of the depolymerization at a temperature of 250° C. or higher and a reduced pressure of 1.0 hPa or lower.

First, in the depolymerization step (1), ethylene glycol, an organophosphorus compound having two or more ester-forming functional groups, and a recycled polyester raw material are added so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.40, and depolymerization is carried out under heat treatment conditions of 245 to 280°C, thereby obtaining a depolymer 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, using a VISCO METER DV2T melt viscometer manufactured by Brookfield.

The ethylene glycol and organophosphorus compound having two or more ester-forming functional groups added to the recycled polyester raw material can be obtained by known methods or commercially available products.

In step (1), when the recycled polyester raw material is added to ethylene glycol and an organophosphorus compound having two or more ester-forming functional groups, depolymerization is carried out under heat treatment conditions of 245 to 280°C while stirring so that the molar ratio of total glycol components/total acid components becomes 1.05 to 1.40, thereby obtaining 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, the depolymerization of the recycled polyester raw material is carried out in the presence of ethylene glycol and an organic phosphorus compound, and at this time, the ethylene glycol, the organic phosphorus compound, and the recycled polyester raw material are all charged in such a manner that the molar ratio of the total glycol components/total acid components becomes 1.05 to 1.40, and then the depolymerization is carried out.

The amount of recycled polyester raw material added (amount added) is preferably 65 parts by mass or more, more preferably 70 parts by mass or more, even more preferably 80 parts by mass or more, and most preferably 85 parts by mass or more, when the total amount added is 100 parts by mass. The total amount of ethylene glycol and organic phosphorus compound added is preferably 5 to 20 parts by mass, more preferably 7 to 15 parts by mass, when the total amount added is 100 parts by mass.
If the amounts of ethylene glycol and organic phosphorus compound added are 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.

Note that when the content of components derived from recycled polyester raw materials in the recycled polyester resin obtained by the production method of the present invention is 80% by mass or more, depolymerization may be carried out in step (1) by adding ethylene terephthalate oligomer in addition to ethylene glycol, the organophosphorus compound, and the recycled polyester raw materials. 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 1.40.

As the 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.

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 ester reactor, it is preferable to carry out the depolymerization reaction in multiple batches by adding the recycled polyester raw material and ethylene glycol, or to add the recycled polyester raw material little by little to the ethylene glycol.

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, and therefore does not come into contact with air (particularly oxygen), so 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 atmospheric pressure, elevated 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 using a melt extruder in step (1), supplying ethylene glycol into the extruder allows the depolymerization reaction to proceed inside the extruder. Adding ethylene glycol while feeding the recycled polyester raw material through the melt extruder facilitates lowering the viscosity, allowing for smoother discharge from the tip nozzle. Furthermore, changing the pressure conditions to lower the viscosity also allows for a lower melt temperature (the temperature of the depolymerization reaction) in 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, resulting in poor operability and the resulting recycled polyester resin having a worse 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 manufacturing method 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.

[Manufacturing method 2]
Next, the above-mentioned Production Method 2 will be described. Production Method 2 is a method in which a recycled polyester resin is obtained and then a phosphorus compound (Y) is blended therewith, and in this case, the following methods (a) and (b) can be adopted.
(A) A method of directly blending a phosphorus compound into a recycled polyester resin (B) A method of blending a polyester resin (master resin) containing a high concentration of a phosphorus compound into a recycled polyester resin The recycled polyester resin used in production method 2 is a) used polyester product and b) a recycled polyester resin containing a component derived from at least one recycled polyester raw material of unused polyester resin generated in the process of manufacturing a polyester product, and when the total amount of all glycol components is 100 mol%, ethylene glycol is 80 mol% or more, diethylene glycol is 4 mol% or less, the carboxyl terminal group concentration is 40 equivalents / t or less, and the average pressure increase rate is 0.6 MPa / h or less. Such recycled polyester resins are preferably the recycled polyester resins described in Japanese Patent Application No. 2021-132793, and are preferably obtained by the manufacturing method of recycled polyester resins described in the same issue.

Furthermore, in the manufacturing method (b), in consideration of the global environment, it is preferable to use the recycled polyester resin described in the above-mentioned Japanese Patent Application No. 2021-132793 as the polyester resin used in the master resin.

In both manufacturing methods (a) and (b), a batch blending method can be used, in which, for example, a screw-type extruder is used to simultaneously add the recycled polyester resin and a phosphorus compound or a master resin containing a phosphorus compound to the recycled polyester resin obtained by the recycled polyester resin manufacturing method described in Japanese Patent Application No. 2021-132793, and the mixture is melted, kneaded, and pelletized; or a split blending method can be used, in which, after the recycled polyester resin is melted and kneaded, a master resin containing a phosphorus compound or a phosphorus compound is fed from another feed port of the extruder, melted, kneaded, and pelletized.

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

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.

In the case of molded products, adding 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.

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

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

The 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, etc.

Furthermore, in the production of fibers, it is generally more difficult to produce multifilaments. However, the fiber of the present invention can be a multifilament having 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 described 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 levels, so that when the above-mentioned long fibers are produced, thickness variations are unlikely to occur, and highly uniform fibers can be produced with good operability.

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 such foreign matter makes it possible to efficiently produce ultrafine fibers with a single fiber fineness of 1.0 decitex or less, especially 0.6 decitex or less.

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.

When the fibers of the present invention are short fibers, they may be mechanically crimped short fibers for dry nonwoven fabrics with a fiber length of 30 to 100 mm, or they may be uncrimped short fibers for wet nonwoven fabrics with a fiber length of 3 to 20 mm that have been treated with a hydrophilic oil.

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, thereby improving single filament dispersion in water when used in wetlaid nonwoven fabric applications. The lubricant may be added at any stage of the composite fiber spinning process. Examples of addition methods include the masterbatch method and liquid color method. However, the masterbatch method is preferred due to stability during melt spinning and ease of handling of colored pigments. When using the masterbatch method, either method may be used, such as a method in which the raw material pellets are metered and mixed and then melt-spun, or a method in which separately molten polymers are metered and mixed and then spun.

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, the nonwoven fabric may be in the form of a thermal-bonded nonwoven fabric, a needle-punched nonwoven fabric, an air-laid nonwoven fabric, a spunlace nonwoven fabric, or a wet-laid nonwoven fabric (papermaking). Because the fibers of the present invention contain a phosphorus compound, they have high flame retardancy, and nonwoven fabrics using the fibers of the present invention are suitable for use as interior materials for automobiles and other vehicles used in high-temperature environments.

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.

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 and Phosphorus Atom Content 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 and glass transition temperature The obtained recycled polyester resin was measured using a PerkinElmer differential scanning calorimeter (Diamond DSC) in a nitrogen gas flow at a temperature range of 25°C to 280°C, a temperature increase (decrease) rate of 20°C/min, and a sample weight of 8 mg.
(g) Fiber manufacturing operability (yarn cut)
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 (×).
(h) Single fiber fineness: Measured according to JIS L1015 8.5.1 B method.
(i) Texture of Nonwoven Fabric The texture of the obtained nonwoven fabric was visually evaluated on the following two-level scale.
◯: The distribution of the constituent fibers was uniform with very little unevenness. ×: The distribution of the constituent fibers was non-uniform with noticeable unevenness. (j) Flame retardancy of the nonwoven fabric The flame retardancy of the obtained nonwoven fabric was evaluated in terms of limiting oxygen index (LOI) in accordance with JIS K 7201 A2. The LOI value is preferably 26% or more.

Example 1
[Recycled polyester resin]
36.0 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was charged into an esterification reactor, and then 2.7 parts by mass of ethylene glycol was added while the agitator of the esterification reactor was rotating. When the internal temperature of the esterification reactor (hereinafter referred to as the "ES can") stopped decreasing, 54.0 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was added in a fixed amount over about 30 minutes, and then 5.3 parts by mass of the phosphorus compound represented by structural formula (a) and 2.0 parts by mass of ethylene glycol were additionally added.
At this time, the recycled polyester raw material was charged 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 50 mPa·s.
The obtained depolymerization product was pressure-fed to a polycondensation reactor (hereinafter referred to as a PC can) via a candle filter having a mesh size of 20 μm, which was set between the esterification reactor and the polycondensation reactor. At this time, the depolymerization product was passed through the filter, and the filtrate was collected.
In a PC can, 2.0×10 ⁻⁴ mol of antimony trioxide and 0.37 parts by mass of titanium oxide were added as a polycondensation catalyst to the filtrate per mole of the acid component of the resulting polyester resin, and the mixture was kneaded at 275° C. to obtain a reaction product.
Next, the PC can was decompressed and after 60 minutes, a polycondensation reaction was carried out at a final pressure of 0.5 hPa and a temperature of 275° C. for 4 hours to obtain a recycled polyester resin (intrinsic viscosity: 0.64).
[Fiber manufacturing]
Using the obtained recycled polyester resin, melt spinning was carried out using a spinneret with 2160 holes and a hole diameter of 0.16 mm, at a discharge rate of 312 g / min, a spinning temperature of 297 ° C, and a spinning speed of 1176 m / min. The number of yarn breaks during melt spinning was 1.5 times / ton. The obtained undrawn yarn was converged to form a 50 ktex tow and stretched under conditions of a stretching temperature of 70 ° C and a stretch ratio of 3.2 times. After stretching, the tow was heat-set on a heat drum at a temperature of 199 ° C., then an oil solution mainly composed of potassium lauryl phosphate salt was applied to the tow by spraying, the tow was heated with a heating roller at 170 ° C., then heated with steam, and introduced into a push-in crimper to impart crimping. Thereafter, the tow was cooled on a cooling conveyor without being heat-treated in a dryer, and cut to 32 mm to obtain short fibers.
[Production of nonwoven fabric]
The obtained staple fibers were used as the main fibers, and core-sheath type heat-fused fibers (staple fibers consisting of a polyethylene terephthalate core and a copolymer polyester sheath with a melting point of 110°C, with a single fiber fineness of 2.2 dtex and a fiber length of 51 mm) manufactured by Unitika Ltd. were used as the binder fibers. A carded web (basis weight 100 g/ ^m2 ) was produced using a main fiber/binder fiber mass ratio of 80/20, and heat-treated in a hot air dryer at a temperature of 150°C, an air volume of 38 ^m2 /min, and a treatment time of 1 minute to produce a nonwoven fabric with a basis weight of 100 g/ ^m2 .

Examples 2 to 13, Comparative Examples 1 to 10
[Recycled polyester resin]
The depolymerization reaction was carried out in the same manner as in Example 1, except that the amounts of ethylene terephthalate oligomer, ethylene glycol, phosphorus compound, 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, recycled 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.
[Fiber manufacturing]
Staple fibers were produced in the same manner as in Example 1 using the resulting recycled polyester resins (Examples 2 to 13, Comparative Examples 1 to 9).
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 1.

Table 1 shows the operability and property values of the recycled polyester resins and staple fibers obtained in Examples 1 to 8 and Comparative Examples 1 to 3 and 6 to 8, as well as evaluations of the nonwoven fabrics.

Example 14
[Recycled polyester resin]
36.4 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was charged into an esterification reactor, and then 3.6 parts by mass of ethylene glycol was added while the agitator of the esterification reactor was rotating. When the internal temperature of the esterification reactor (hereinafter referred to as the "ES can") stopped decreasing, 54.5 parts by mass of recycled polyester raw material (recycled PET flakes made from used products) was added in a fixed amount over about 30 minutes, and then 5.5 parts by mass of ethylene glycol was 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 1.31.
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 50 mPa·s.
The obtained depolymerization product was pressure-fed to a polycondensation reactor (hereinafter referred to as a PC can) via a candle filter having a mesh size of 20 μm, which was set between the esterification reactor and the polycondensation reactor. At this time, the depolymerization product was passed through the filter, and the filtrate was collected.
In a PC can, 1.0 × ¹⁰ mol of antimony trioxide as a polycondensation catalyst relative to 1 mol of the acid component of a polyester resin from which antimony trioxide can be obtained, 0.2 × ¹⁰ mol of cobalt acetate as a color adjuster, and 0.23 parts by mass of titanium oxide were added to the filtrate, and the mixture was kneaded at a temperature of 275°C to obtain a reaction product.
Next, the PC can was decompressed and after 60 minutes, a polycondensation reaction was carried out at a final pressure of 0.5 hPa and a temperature of 275° C. for 2.5 hours to obtain a recycled polyester resin (A) (intrinsic viscosity: 0.65).
Next, the recycled polyester resin (A) and the phosphorus compound represented by structural formula (a) were fed into an extruder at a temperature of 290°C so that 5.3 parts by mass of the phosphorus compound were used per 100 parts by mass of the obtained recycled polyester resin (A), thereby obtaining a polyester resin having an intrinsic viscosity of 0.62.
[Fiber manufacturing]
Using the obtained recycled polyester resin, short fibers were produced in the same manner as in Example 1.
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 1.

Examples 15 and 16, Comparative Examples 11 and 12
[Recycled polyester resin]
Recycled polyester resins were produced in the same manner as in Example 14, except that the amount of phosphorus compound added per 100 parts by mass of the obtained recycled polyester resin (A) was changed to that shown in Table 2.
[Fiber manufacturing]
Short fibers were produced in the same manner as in Example 14 using the obtained recycled polyester resins (Examples 15 and 16, Comparative Examples 11 and 12).
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 14.

Table 2 shows the operability and property values of the recycled polyester resins and staple fibers obtained in Examples 14-16 and Comparative Examples 11-12, as well as evaluations of the nonwoven fabrics.

Example 17
[Recycled polyester resin]
70 parts by mass of the recycled polyester resin (A) obtained in Example 14 and 30 parts by mass of the phosphorus compound represented by structural formula (a) were fed into an extruder at a temperature of 290°C to obtain a recycled polyester resin (B) containing 30% by mass of the phosphorus compound.
Subsequently, 80 parts by mass of recycled polyester resin (A) and 20 parts by mass of recycled polyester resin (B) were fed into an extruder at a temperature of 290°C to obtain a polyester resin with an intrinsic viscosity of 0.62.
[Fiber manufacturing]
Using the obtained recycled polyester resin, short fibers were produced in the same manner as in Example 1.
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 1.

Examples 18 and 19, Comparative Examples 13 and 14
[Recycled polyester resin]
Recycled polyester resins were produced in the same manner as in Example 17, except that the blending ratio of recycled polyester resin (A) and recycled polyester resin (B) was changed to that shown in Table 3.
[Fiber manufacturing]
Staple fibers were produced in the same manner as in Example 17 using the resulting recycled polyester resins (Examples 18 and 19, Comparative Examples 13 and 14).
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 17.

Example 20
[Recycled polyester resin]
A polyester resin having an intrinsic viscosity of 0.62 was obtained in the same manner as in Example 17, except that a virgin polyester resin [polyethylene terephthalate (MA-6101-3 manufactured by Unitika Ltd.)] was used instead of the recycled polyester resin (A) when obtaining the recycled polyester resin (B) in Example 17.
[Fiber manufacturing]
Using the obtained recycled polyester resin, short fibers were produced in the same manner as in Example 17.
[Production of nonwoven fabric]
Using the resulting short fibers, a nonwoven fabric was produced in the same manner as in Example 17.

Table 3 shows the operability and property values of the recycled polyester resins and staple fibers obtained in Examples 17-20 and Comparative Examples 13-14, as well as evaluations of the nonwoven fabrics.

As is clear from Tables 1 to 3, the recycled polyester resins obtained in Examples 1 to 20 had phosphorus atom content, carboxyl terminal group concentration, diethylene glycol content, and average pressure rise rate within the ranges specified in the present invention. As a result, excellent operability was achieved when producing short fibers, and fibers with excellent flame retardancy were obtained.

On the other hand, in Comparative Example 1, the content of phosphorus atoms was as low as 740 ppm by mass, and therefore the obtained fiber had poor flame retardancy.
In Comparative Example 2, the reaction temperature during depolymerization was as high as 287° C., resulting in a high diethylene glycol content and a high carboxyl end group concentration. As a result, thermal decomposition occurred during fiber formation, which deteriorated operability, and the obtained fiber had a high heat shrinkage rate and poor formation of the nonwoven fabric.
In Comparative Example 3, the melt viscosity of the depolymerized polymer obtained in the depolymerization step was too high, making it difficult to pass through the filter in the filtration step, and the product could not proceed to the next step.
In Comparative Example 4, the melt viscosity of the depolymerization product obtained in the depolymerization step was too low, and therefore, leakage of the filtrate occurred when the depolymerization product was passed through the filter in the filtration step, and the product could not be advanced to the next step.
In Comparative Example 5, the candle filter provided between the ES can and the PC can had a low particle size of 5 μm, which caused clogging in the filtration process, making it impossible to obtain a polyester resin.
In Comparative Example 6, the filtration particle size of the candle filter installed between the ES can and the PC can was as high as 30 μm, resulting in a large amount of foreign matter being mixed in and a high average pressure increase rate. As a result, when the resulting fiber was made, the nozzle pack pressure increased and yarn breakage occurred frequently, which deteriorated operability and resulted in poor formation of the nonwoven fabric.

In Comparative Example 7, the G/A ratio during depolymerization was as low as 1.05, and therefore depolymerization was insufficient, and removal of foreign matter was insufficient, resulting in a high average pressure rise rate. In addition, the concentration of carboxyl end groups was high, which caused thermal decomposition during fiber formation, deteriorating operability and resulting in poor formation of the nonwoven fabric.
In Comparative Example 8, the content of phosphorus atoms was as high as 10,980 ppm by mass, resulting in a resin with low crystallinity, which caused fusion during drying and could not be used for the fiber process.

In Comparative Example 9, the reaction temperature during depolymerization was as low as 240° C., so that solidification of the polyester oligomer occurred during the depolymerization reaction, and a polyester resin could not be obtained. The heat treatment temperature in the depolymerization step was low, and the melt viscosity of the depolymer obtained in the depolymerization step was too high, so that it was difficult to pass the depolymer through a filter in the filtration step, and the product could not proceed to the next step.
In Comparative Example 10, the polycondensation temperature was as low as 240° C., so that the polycondensation reaction did not proceed well and a polyester resin could not be obtained.
In Comparative Example 11, the content of phosphorus atoms was as low as 730 ppm by mass, and therefore the flame retardancy of the obtained fiber was low.
In Comparative Example 12, the content of phosphorus atoms was as high as 11,060 ppm by mass, resulting in a resin with low crystallinity, which caused fusion during drying and could not be used for the fiber process.
In Comparative Example 13, the content of phosphorus atoms was as low as 630 ppm by mass, and therefore the flame retardancy of the obtained fiber was low.
In Comparative Example 14, the content of phosphorus atoms was as high as 11,040 ppm by mass, resulting in a resin with low crystallinity, which caused fusion during drying and could not be used for the fiber process.

Claims (1)

A method for producing a recycled polyester resin that satisfies all of the following (i) to (iv), using at least one recycled polyester raw material from a) used polyester products and b) unused polyester resin generated in the process of producing polyester products, and that contains 80 mass% or more of components derived from the recycled polyester raw material, and that is characterized by including all of the following steps (1) to (4):
[Recycled polyester resin]
(A) The polyester resin contains an organic phosphorus compound having two or more ester-forming functional groups, and the content of phosphorus atoms in the polyester resin is 1,000 to 10,000 ppm by mass;
(b) When the total amount of all glycol components is 100 mol%, ethylene glycol is 80 mol% or more and diethylene glycol is 4 mol% or less,
(c) the carboxyl terminal group concentration is 40 equivalents/t or less;
(D) 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 attached to 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)
[Process]
(1) a step of adding ethylene glycol, an organic phosphorus compound having two or more ester-forming functional groups, and a recycled polyester raw material so that the molar ratio of total glycol components/total acid components is 1.05 to 1.40, and performing depolymerization under heat treatment conditions of 245 to 280°C to obtain a depolymerized polymer having a melt viscosity of 10 to 1,500 mPa s;
(2) passing the depolymerized product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A step of adding a polycondensation catalyst to the filtrate and kneading to obtain a reaction product; and (4) A step of subjecting the reaction product to a polycondensation reaction of the depolymerization at a temperature of 250° C. or higher and a reduced pressure of 1.0 hPa or lower.