JP7644440B2 - Recycled polyester resin and method for producing same - Google Patents

Description

The present invention relates to a novel recycled polyester resin and a method for producing the recycled polyester resin. In particular, the present invention relates to a recycled polyester resin produced using recycled polyester raw materials derived from used polyester products as well as recycled polyester raw materials derived from unused polyester generated in the process of producing polyester products, which has a low amount of foreign matter mixed in 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 resistant to chemicals, and is relatively low cost, and therefore is 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 cause significant damage to the incinerator and shorten its lifespan, while if they are not incinerated, they will remain semi-permanently as they do not decompose.
In recent years, it has become a problem that once-used polyester products, such as plastic containers discarded as garbage, end up in rivers and into the ocean, where microplastics are broken down into small pieces by the action of waves and tides, accumulate in the bodies of marine organisms, and are concentrated up the food chain, adversely affecting the marine ecosystem. Plastic is a major cause of marine pollution, and there is a global movement to reduce the use of plastics and switch to biodegradable plastics.

In order to reduce the amount of plastic products used and to address environmental issues, various methods of recycling are being used to reuse resources. Regarding polyester products, methods are being considered for recycling polyester waste generated during the manufacturing process, as well as recovering products that were once on the market and discarded, and reusing them as raw materials. In particular, in recent years, textile products that have been awarded the Eco Mark, which is certified as having achieved a certain recycling rate, have become widespread.

Various methods have been proposed for recycling recycled polyester raw materials. For example, a method of adding methanol 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 of adding EG to PET scraps to depolymerize them, and then adding methanol to recover DMT (Patent Document 2), a method of depolymerizing PET scraps with EG to form oligomers, which are then used in a polycondensation reaction (Patent Document 3), and so on have been proposed.

Furthermore, problems that can arise when recycling PET bottles that have already been manufactured include additives added to the polyester resin and items attached to the bottle itself, such as caps (aluminum, polypropylene, polyethylene), inner plugs and liners (polypropylene, polyethylene), labels (paper, resins such as polystyrene, ink), adhesives, printing inks, etc.
As a pre-treatment for the recycling process, the collected PET bottles are first put through a vibrating sieve to remove sand and metal. The PET bottles are then washed, and the colored bottles are separated before being roughly crushed. Labels and other items are removed by wind separation. Aluminum pieces from the caps and other parts are removed, and the PET bottle pieces 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 going through these steps, it was difficult to separate non-polyester resins, such as polypropylene, polyethylene, and polystyrene, from the PET resin. For this reason, even if recycled polyester resin was obtained using the recycling methods described in Patent Documents 1 to 3, foreign matter derived from the non-polyester resin could not be sufficiently removed, the amount of foreign matter mixed in could not be sufficiently reduced, and a product with the same quality as virgin polyester resin could not be obtained. Moreover, the methods described in Patent Documents 1 to 3 require large costs for the installation, operation, and maintenance of the recovery equipment, and there is room for improvement in terms of practicality.

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 a repolymerization reaction is carried out. It is also shown that the resulting recycled polyester resin has a low amount of foreign matter mixed in and is excellent in operability during processing. However, even with this method, the foreign matter derived from non-polyester resins as described above cannot be sufficiently removed, and the amount of foreign matter mixed in cannot be sufficiently reduced.

In general, when manufacturing plastic bottles and the like, 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 a metal mold and air is blown into it, due to the 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 the perspective of environmental issues.

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

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

Thus, there is a great demand for copolymer polyester resins made from recycled polyester raw materials in various molded products and textile products, but a copolymer polyester resin that can fully remove not only various inorganic substances but also foreign matter derived from non-polyester resins and that can be used to produce a variety of high-quality products similar to those produced from virgin polyester resin has not yet been obtained.

Special Publication No. 42-8855 Japanese Unexamined Patent Publication No. 48-62732 Japanese Unexamined Patent Publication No. 60-248646 JP 2005-171138 A Patent No. 6297351

The present invention aims to solve the above problems and provide a recycled polyester resin that is a copolymer polyester resin made from recycled polyester raw materials derived from used polyester products or recycled polyester raw materials derived from scraps generated in the process of manufacturing polyester resins and products, and that can be used to manufacture polyester products in various forms. It also aims to provide a manufacturing 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 recycled polyester resin with a low amount of foreign matter and the same thermal stability as virgin polyester resin can be obtained by using recycled polyester raw materials and a specific manufacturing method, which led to the completion of the present invention.

That is, the present invention relates to the following (i) to (iv).
(i) A recycled polyester resin containing a component derived from at least one type of recycled polyester raw material, i.e., a) used polyester product and b) unused polyester generated in the process of manufacturing a polyester product, the recycled polyester resin being characterized in that it 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 %, 80 mol % or more is terephthalic acid,
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol is 80 mol% or more, cyclohexanedimethanol is 2 mol% or more and 15 mol% or less, and diethylene glycol is 4 mol% or less,
(3) The carboxyl end 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 line diameter: 0.07 mm, horizontal line diameter: 0.04 mm, filtration particle size: 12 μm) is set at the tip of the extruder, polyester resin is melted at 300° C. in the extruder, and the polyester resin 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 continuous extrusion for 12 hours thereafter is defined as the "final pressure value (MPa)". Based on these pressure values, the average pressure rise rate is calculated by the following calculation formula A:
Average pressure rise 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 type of recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in a process for producing a polyester product, the method comprising all of the following steps (1) to (3):
(1) a step of adding recycled polyester raw material to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and cyclohexane dimethanol in such an amount that the molar ratio of total glycol components/total acid components becomes 1.10 to 1.30, and performing depolymerization under heat treatment conditions of 245 to 280° C. to obtain a reaction product containing a depolymerization product;
(2) passing the reaction product through a filter having a filtration particle size of 10 to 25 μm to recover the filtrate;
(3) A step of adding a polymerization catalyst to the filtrate and carrying out a polycondensation reaction of the depolymerization at a temperature of 260° C. or higher and a reduced pressure of 1.0 hPa or lower.

The recycled polyester resin of the present invention contains at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyesters generated in the process of manufacturing polyester products, but has a small amount of foreign matter mixed in, and the carboxyl end group concentration and diethylene glycol content satisfy specific ranges, and has excellent thermal stability. Therefore, for example, in the process of obtaining fibers by melt spinning, the process of obtaining sheets or films by film formation, and the process of obtaining molded products such as bottles, continuous operation over a relatively long period of time is possible, and products of various forms can be produced with good productivity. And, since it is a copolymerized polyester resin containing a copolymerization component while being a recycled polyester resin, it can be suitably used for molded products such as bottles that require transparency and fiber applications that require adhesiveness.
Furthermore, according to the method for producing the recycled polyester resin of the present invention, it is possible to obtain the recycled polyester resin of the present invention with good operability and low cost without requiring complicated steps or equipment, and thus has great practical advantages.

The present invention will be described in detail below.
The recycled polyester resin of the present invention (the resin of the present invention) is a polyester resin containing a component derived from at least one recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in the process of manufacturing a polyester product. These components form a part of the polyester constituting the resin of the present invention.

Examples of the used polyester products of the above item a) include polyester molded products (including fibers) that were once distributed on the market and then collected after use, etc. Representative examples thereof include containers such as PET bottles and packaging materials.
The unadopted polyester generated in the process of manufacturing the polyester product of b) above is polyester that has not been commercialized. Examples of such unadopted polyester include resin pellets that do not meet the specifications, materials that are no longer needed during molding, pieces cut during molding, scraps generated during molding or processing, cut pieces of transitional products generated when the brand is changed, 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 crushing and cutting as necessary, or may be melted and pelletized. The above a) and b) may be used alone, or a mixture of the two may be used.

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 crystalline recycled polyester raw materials, particularly for the purpose of preventing the pellets from fusing together when being charged into the can or during 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 in the form of cut pieces, crushed material (powders), etc. obtained by further processing such as cutting and crushing, as well as in the form of solids such as molded bodies (pellets, etc.) obtained by molding these. More specifically, examples include pellets obtained by cooling and cutting melted polyester waste, and cut pieces obtained by finely cutting polyester molded products such as PET bottles. In addition, the above-mentioned cut pieces, crushed material (powders), etc. may be in the form of a liquid obtained by dispersing or dissolving them in a solvent. When manufacturing polyester products using these raw materials, they can be melted at a temperature above their melting point and charged into a can as a molten liquid, if necessary.

The recycled polyester resin of the present invention preferably contains 40% by mass or more, and more preferably 50% by mass or more, of the recycled polyester raw material which is at least one of the above a) and b).
If the content of the recycled polyester raw material is less than 40% by mass, the objective of considering environmental issues cannot be achieved. Although there is no particular upper limit to the content of the recycled polyester raw material, according to the production method of the present invention described later, it is possible to easily obtain a recycled polyester resin having a recycled polyester raw material content of 40 to 80% by mass.

In the resin of the present invention, when the total amount of all acid components constituting the polyester is taken as 100 mol %, 80 mol % or more is terephthalic acid, that is, the acid component is mainly terephthalic acid.
The proportion of terephthalic acid in the acid component is 80 mol % or more, and the proportion of terephthalic acid is preferably 90 mol %. If the proportion of terephthalic acid is less than 80 mol %, the crystallinity of the resin composition decreases and it tends to become amorphous, which is not preferable.

When the total amount of all glycol components is taken as 100 mol %, ethylene glycol is 80 mol % or more, cyclohexanedimethanol is 2 mol % to 15 mol % and diethylene glycol is 4 mol % or less.
The copolymerization amount of cyclohexane dimethanol is preferably adjusted within the range of 2 to 15 mol% depending on the application of the resulting recycled polyester resin. For example, when used for molding applications, the copolymerization amount of cyclohexane dimethanol is preferably 4 to 15 mol%. Of these, 4 to 8 mol% is preferable. By copolymerizing 2 to 15 mol% of cyclohexane dimethanol, 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 amount of cyclohexanedimethanol copolymerized is less than 2 mol%, the resin composition will crystallize quickly, causing the molded product to crystallize and turn white during direct blow molding, resulting in poor transparency. On the other hand, if the amount of cyclohexanedimethanol copolymerized is more than 15 mol%, the resin composition will become amorphous, making it more susceptible to blocking during high-temperature drying and solid-phase polymerization processes.

When the total amount of all glycol components in the resin of the present invention is taken as 100 mol %, ethylene glycol is 80 mol % or more of the total glycol components, and preferably 85 mol % or more. If the ethylene glycol content is less than 80 mol %, the crystallinity and heat resistance of the resulting polyester resin will be poor.

In addition, when the total amount of all glycol components is 100 mol%, the diethylene glycol content of the resin of the present invention is 4 mol% or less, and preferably 3 mol% or less. In particular, in the resin of the present invention obtained by the manufacturing method of the present invention, ethylene glycol is used as one of the raw materials, and diethylene glycol may be generated as a by-product. The resin of the present invention has a small amount of diethylene glycol as a by-product, and has excellent thermal stability due to the diethylene glycol content of 4 mol% or less. Therefore, it is possible to obtain molded products such as fibers, injection molded products, various blow molded products, sheets, and films with good productivity. The lower limit of the diethylene glycol content can be, for example, about 0.5 mol%, but is not limited to this.

Examples of acid components other than terephthalic acid in the resin of the present invention include isophthalic acid, phthalic acid, 5-sodium sulfoisophthalic acid, phthalic anhydride, naphthalenedicarboxylic acid, adipic acid, sebacic acid, dimer acid, etc., and two or more of these may be used in combination, or ester-forming derivatives of these acids may be used.

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

As the polycondensation catalyst, for example, at least one of germanium compounds, antimony compounds, titanium compounds, cobalt compounds, etc. can be used, but is not limited thereto. Among them, at least one of germanium compounds and antimony compounds is particularly used. When the transparency of the resulting recycled polyester resin is important, it is preferable to use a germanium compound. Examples of the above compounds include oxides, inorganic acid salts, organic acid salts, halides, sulfides, etc. of germanium, antimony, titanium, cobalt, etc.

The amount of polycondensation catalyst used is not particularly limited, but is preferably 5×10-5 moles/unit or more per mole of the acid component of the polyester resin produced, and more preferably 6×10-5 moles/unit or more. The upper limit of the amount used can be, for example, about 1×10-3 moles/unit, but is not limited to this.

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

During the polycondensation reaction, if necessary, fatty acid esters capable of adjusting the melt viscosity, hindered phenol-based antioxidants, and phosphorus compounds capable of suppressing thermal decomposition of the resin can be added in addition to the polycondensation catalyst.

Examples of fatty acid esters include beeswax (a mixture mainly 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, dipentaerythritol hexastearate, etc. Among these, glycerin monostearate, pentaerythritol tetrastearate, and dipentaerythritol hexastearate are preferred. These can 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 tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate. Ethylene glycol-bis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate], 3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1'-dimethylethyl}-2,4,8,10-tetraoxaspiro[5,5]undecane, etc. are used, but in terms 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, tridecyl phosphate, and triphenyl phosphate. These compounds 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 end group concentration of 40 equivalents/t or less; (b) an average pressure rise rate measured by a pressure rise 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 later.

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 excellent heat resistance due to the carboxyl terminal group concentration of 40 equivalents/t or less, and it is possible to produce molded products having 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 preferably about 0.44 to 0.80. In addition, the recycled polyester resin of the present invention can be used for molding applications by increasing the degree of polymerization through a solid-phase polymerization process, as described below. 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 a temperature of 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 rise 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 rise rate in the present invention is an index of the amount of foreign matter derived from various inorganic substances or non-polyester resins, and a smaller average pressure rise rate indicates a smaller amount of foreign matter. The lower limit of the average pressure rise 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, with a stainless steel filter (nominal mesh size: 1400 mesh, weave: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical line diameter: 0.07 mm, horizontal line diameter: 0.04 mm, filtration particle size: 12 μm) set at the tip of the extruder, polyester resin was melted at 300° C. in the extruder, and the polyester resin 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 continuous extrusion for 12 hours thereafter was defined as the "final pressure value (MPa)". The average pressure rise rate was calculated based on these pressure values using the following calculation formula A.
Average pressure rise 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 foreign matter derived from various inorganic substances and non-polyester resins, making it possible to reduce the average pressure rise rate measured by a pressure rise tester, which is the characteristic value of (b), to 0.6 MPa/h or less.

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 (3) in this order.
(1) A process for obtaining a reaction product containing a depolymerization product by adding a recycled polyester raw material to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and cyclohexanedimethanol so that the molar ratio of total glycol components/total acid components is 1.10 to 1.30, and carrying out depolymerization under heat treatment conditions of 245 to 280° C.;
(2) passing the reaction product through a filter having a filtration particle size of 10 to 25 μm to recover a filtrate;
(3) A step of adding a polymerization catalyst to the filtrate and carrying out a polycondensation reaction at a temperature of 250° C. or higher and a reduced pressure of 1.0 hPa or lower.

First, in the depolymerization step (1), recycled polyester raw material is added to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and cyclohexanedimethanol so that the molar ratio of total glycol components/total acid components is 1.10 to 1.30, and depolymerization is performed under heat treatment conditions of 245 to 280°C to obtain a reaction product containing the depolymerization product.

Ethylene terephthalate oligomer, ethylene glycol, and cyclohexane dimethanol can be any known or commercially available product. They can also be produced by known production methods.

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

The amount of the mixture containing ethylene terephthalate oligomer, ethylene glycol and cyclohexanedimethanol (hereinafter, sometimes referred to as mixture E) is preferably 20 to 80 mass% and more preferably 30 to 70 mass% in 100 mass% of the finally obtained recycled polyester resin.
If the amount of mixture E is less than the above amount, the recycled polyester raw materials are likely to block with each other when the recycled polyester raw materials 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 more than the above range, no particular problem occurs in the depolymerization reaction, but the recycling rate of the finally obtained recycled polyester resin is undesirably low.

In step (1), the proportions of ethylene terephthalate oligomer, ethylene glycol, cyclohexane dimethanol, and recycled polyester raw materials to be added may be changed as appropriate depending on the amount of isophthalic acid copolymerized in the polyester resin to be ultimately obtained, but it is generally preferable to add them in the following proportions (total of 100 parts by mass): 5 to 50 parts by mass of ethylene terephthalate oligomer, 1 to 18 parts by mass of ethylene glycol and cyclohexane dimethanol combined, and 40 to 80 parts by mass of recycled polyester raw materials.

In particular, the total amount of ethylene glycol and cyclohexanedimethanol to be added is preferably 1 to 15 parts by mass, more preferably 3 to 13 parts by mass, in order to sufficiently advance the depolymerization reaction. If the total amount exceeds 18 parts by mass, the ethylene terephthalate oligomer tends to solidify in the reactor, which may make it impossible to continue the subsequent reaction, and this is not preferred.
In the mixture E, when ethylene glycol or cyclohexanedimethanol is added to the ethylene terephthalate oligomer, it is preferable to uniformize the temperature of the contents while rotating the stirrer in order to prevent solidification of the oligomer, and then add the ethylene glycol or cyclohexanedimethanol.

In step (1), when the recycled polyester raw material is added to mixture E, 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 1.10 to 1.30.
In the production method of the present invention, this step is important. That is, in the conventional method using recycled polyester raw materials, depolymerization is carried out using only the recycled polyester raw materials, but in the present invention, the recycled polyester raw materials are depolymerized in the presence of ethylene terephthalate oligomer, ethylene glycol, and cyclohexanedimethanol, and the recycled polyester raw materials are charged and depolymerized in such a manner that all the components of the recycled polyester raw materials in the mixture E have a molar ratio of total glycol components/total acid components of 1.10 to 1.30.

By carrying out the above-mentioned step (1), not only various inorganic substances but also foreign substances derived from non-polyester resins are efficiently precipitated, so that these foreign substances can be filtered out without fail in the step (2). Then, in the polycondensation reaction in the step (3), it becomes possible to obtain a recycled polyester resin having a diethylene glycol content (copolymerization amount) and a carboxyl end group concentration that are not more than a specific amount as characteristic values of the present invention, and having a small amount of foreign substances mixed in.
Furthermore, by carrying out depolymerization of recycled polyester raw materials in the presence of ethylene terephthalate oligomer, ethylene glycol and cyclohexanedimethanol, the presence of copolymerization components makes it possible to proceed with the depolymerization reaction at a lower temperature than when depolymerization is carried out using only recycled polyester raw materials, which is a great advantage when carried out on an industrial scale.

In the manufacturing method of the present invention, it is desirable that the recycled polyester raw material is not decomposed into monomers by the depolymerization reaction, but is decomposed into oligomers with about 5 to 20 repeating units. By controlling the depolymerization reaction in this way, not only various inorganic substances but also foreign matter derived from non-polyester resins is efficiently precipitated, making it possible to remove more foreign matter.

If the molar ratio of total glycol components/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 end group concentration and diethylene glycol content specified in the present invention, and will have a high average pressure rise rate. This is because, if the molar ratio of total glycol components/total acid components during the depolymerization reaction is outside the above range, the precipitation of various inorganic substances and foreign substances derived from non-polyester resins is not carried out efficiently, and these foreign substances cannot be filtered out completely in step (2), and the foreign substances precipitate after the polycondensation reaction in step (3), resulting in a recycled polyester resin with a high average pressure rise rate.

The reactor used in the production method of the present invention may be a commonly used esterification reactor without any particular problem in terms of volume and shape of the stirring blades. However, in order to efficiently advance the depolymerization reaction, it is preferable that the reactor has a structure equipped with a distillation column that does not distill ethylene glycol out of the system.
When the recycled polyester raw material is charged, it is preferable to do so while stirring under normal pressure, and it is more preferable to charge it in a state where it is purged with a small amount of inert gas (generally nitrogen gas is used).

The reaction temperature during depolymerization in step (1) is preferably set at an internal temperature of the reactor in the range of 245 to 280° C., and more preferably set at an internal temperature in the range of 255 to 280° C. If the reaction temperature during depolymerization is less than 245° C., the reaction product solidifies, the operability deteriorates, and even if a recycled polyester resin is obtained, the diethylene glycol content and the carboxyl end group concentration become too high. If the reaction temperature exceeds 280° C., the diethylene glycol content and the carboxyl end group concentration of the obtained recycled polyester resin become too high.
The depolymerization reaction time (the reaction time from the end of the introduction of the recycled polyester raw material) is preferably within 4 hours, and from the viewpoints of suppressing the amount of diethylene glycol as a by-product and suppressing deterioration in the color tone of the polyester, it is more preferably within 2 hours.

In the step (2), the reaction product containing the depolymerized polymer from the step (1) is passed through a filter having a filtration particle size of 10 to 25 μm to filter out foreign matter. As described above, by carrying out the depolymerization reaction under the conditions of the step (1), not only various inorganic substances but also foreign matter derived from non-polyester resins is efficiently precipitated, and therefore, by passing the reaction product through a filter having a filtration particle size of 10 to 25 μm, the precipitated foreign matter can be filtered out, and a filtrate containing a small amount of foreign matter can be obtained.
If a filter with a filtration particle size of more than 25 μm is used, foreign matter in the polymer cannot be sufficiently removed, and the resulting recycled polyester resin contains a large amount of foreign matter. Therefore, when such a resin is used for spinning, the nozzle pack is pressurized and yarn breakage occurs. On the other hand, if a filter with a filtration particle size of less than 10 μm is used, clogging due to foreign matter is likely to occur, and the filter life is shortened, which is disadvantageous in terms of cost and also deteriorates operability.

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

In the manufacturing method of the present invention, the above-mentioned polymerization catalyst is added to the filtrate obtained through the above-mentioned step (2), and a polycondensation reaction is carried out at a temperature of 260°C or higher and a reduced pressure of 1.0 hPa or less.

Then, in the polycondensation reaction tank, the polycondensation reaction is carried out at a temperature of 260° C. or higher and under a reduced pressure of 1.0 hPa or lower. If the polycondensation reaction temperature is lower than 260° C. or the pressure during the polycondensation reaction exceeds 1.0 hPa, the polycondensation reaction time becomes long, resulting in poor productivity.
The polycondensation reaction temperature is more preferably 270° C. or higher. However, if the polycondensation reaction temperature is too high, the polymer will be colored due to thermal decomposition, the color tone will deteriorate, and the amount of terminal groups (COOH) will also increase due to thermal decomposition, so 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 (prepolymer) obtained here is preferably 0.44 to 0.80.

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

Next, the prepolymer obtained by the melt polymerization reaction described above is made into chips of any shape, such as dice-shaped or cylindrical, and the polyester chips are continuously fed to a crystallizer and crystallized at a temperature of 150 to 190°C. After that, the chips are fed to a dryer and dried at a temperature of 190°C or less for 4 to 16 hours, then fed to a preheater and heated to the solid-phase polymerization temperature described below for 2 to 5 hours, and then continuously fed to a solid-phase polymerization machine for solid-phase polymerization reaction to obtain a polyester resin with the target intrinsic viscosity. Solid-phase polymerization is preferably carried out under an inert gas such as nitrogen gas. Solid-phase polymerization is usually preferably carried out at a temperature within the range of 170 to 230°C, and more preferably at a temperature within the range of 180 to 220°C. The polymerization time is within the range of 20 to 80 hours, and the reaction is carried out in the solid-phase polymerization machine.

The resin of the present invention can be used to obtain molded products (blow molded products, injection molded products, sheets, films, etc.) with excellent color tone and transparency by employing blow molding, injection molding, stretching methods, etc., 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, compressed air molding, and blow molding using raw materials containing the resin of the present invention. This makes it possible to provide various parts, including containers. The resin of the present invention is particularly suitable for producing blow molded articles because it has properties similar to those of virgin polyester resin and has excellent thermal stability. Therefore, a manufacturing method for a molded article that includes a step of obtaining a parison from a melt of raw materials containing the resin of the present invention and a step of blowing gas into the parison can be preferably adopted. This makes it possible to produce molded articles such as containers.

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 a specific amount. Therefore, when obtaining the above-mentioned molded product, thickness unevenness is unlikely to occur, and molded products with high uniformity can be obtained with good operability. Furthermore, as described above, the resin of the present invention has an average pressure increase rate of 0.6 MPa/h or less and has a low amount of foreign matter mixed in. 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 matter makes it possible to obtain molded products 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.

In the case of a molded product, it is preferable to contain a germanium compound and a cobalt compound as a polymerization catalyst. It is preferable to contain 5.0×10 ⁻⁵ mol to 3.0×10 ⁻⁴ mol of the germanium compound per mol of the acid component of the polyester. If it is less than 1.0×10 ⁻⁵ mol, it is difficult to obtain a polyester resin with a target polymerization degree. On the other hand, if it exceeds 3.0×10 ⁻⁴ mol, the by-products produced by the reaction of the cobalt compound and the germanium compound deteriorate the stability of the polyester over time, and the intrinsic viscosity of the polyester resin composition decreases and the color tone deteriorates after long-term storage, which is not preferable.

Germanium compounds include germanium dioxide, germanium tetrachloride, germanium tetraethoxide, etc., and germanium dioxide is preferred in terms of polymerization catalyst activity, physical properties of the resulting polyester resin, and cost.

The content of the cobalt compound must be 1.0×10 ⁻⁵ mol to 1.0×10 ⁻⁴ mol, and 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 1.0×10-5 mol, the color tone of the polyester deteriorates. On the other hand, if the amount exceeds 1.0× ^10-4 mol, the stability over time of the polyester resin deteriorates. Examples of the cobalt compound include cobalt acetate, cobalt formate, cobalt chloride, and cobalt oxide, and from the viewpoints of polymerization catalyst activity, physical properties of the resulting polyester resin, and cost, cobalt acetate is preferred.
In the case of fibers containing the resin of the present invention, the fibers can be produced, for example, by a production method including a step of melting a raw material containing the resin of the present invention and spinning it. This makes it possible to produce ultrafine fibers having a single fiber fineness of, for example, 0.8 decitex or less (preferably 0.6 to 0.3 decitex). The spinning method and the like 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 those of virgin polyester resin, so yarn breakage is unlikely to occur during the melt spinning, drawing/heat treatment, and winding processes, and polyester fibers can be obtained with good productivity.

The fibers of the present invention containing the resin of the present invention may be, for example, monofilaments, multifilaments, etc., and may be long fibers, short fibers, etc.

The present invention will now be described in detail with reference to examples, in which the measurement and evaluation methods for various property values and the like are as follows.
(a) Intrinsic Viscosity The intrinsic viscosity is measured at 20° C. using the resulting recycled polyester resin and an equal mass mixture of phenol and tetrachloroethane as a solvent.
(b) Composition of polyester resin The obtained recycled polyester resin was dissolved in a mixed solvent of deuterated trifluoroacetic acid and deuterated chloroform in a volume ratio of 1/11, and 1H-NMR was measured using a JNM-ECZ400R/S1 NMR apparatus manufactured by JEOL Ltd., and the type and content of copolymerization components were determined from the integrated intensity of the proton peak of each component in the obtained chart.
(c) Carboxyl End Group Concentration 0.1 g of the resulting recycled polyester resin was dissolved in 10 ml of benzyl alcohol, 10 ml of chloroform was added to the solution, and the resultant was titrated with a 1/10 N potassium hydroxide benzyl alcohol solution to determine the carboxyl end group concentration.
(d) 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, weaving method: twill weave, vertical mesh: 165 mesh, horizontal mesh: 1400 mesh, vertical line diameter: 0.07 mm, horizontal line diameter: 0.04 mm, filtration particle size: 12 μm, viscous resistance coefficient (m-1): 2.60 × 107, inertial resistance coefficient: 5.14 × 10, manufactured by Kamijo Seiki Co., Ltd.) was set as a filter at the tip of the extruder, and a reinforcing material (stainless steel plain weave wire mesh (nominal mesh size: 165 mesh, horizontal mesh size: 1400 mesh, vertical line diameter: 0.07 mm, horizontal line diameter: 0.04 mm, filtration particle size: 12 μm, viscous resistance coefficient (m-1): 2.60 × 107, inertial resistance coefficient: 5.14 × 10, manufactured by Kamijo Seiki Co., Ltd.) was further set on the back side (downstream side) of the filter. After laminating a filter made of a polyester resin (fiberglass: 40 mesh, weave: plain weave, wire diameter: 0.21 mm, manufactured by Kamijo Seiki Co., Ltd.), the polymer discharge rate was set to 29.0 g/min, and the filter pressure was measured using a pressure rise tester; a detector "MES-Y44D type" manufactured by Asahi Gauge Co., Ltd. The pressure rise test using the pressure rise tester was performed continuously for 12 hours, and the average pressure rise rate was calculated 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, using the following formula.
Average pressure rise rate (MPa/h)=(final pressure value−initial pressure value)/12

(e) Melting point, (f) 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 heating (cooling) rate of 20° C./min, and a sample weight of 8 mg.
(g) Moldability The thickness of the body of the obtained container (100 samples) was measured, and the containers with a difference in thickness between the thickest and thinnest parts of up to 0.30 mm were judged to pass. Depending on the number of samples that passed, the moldability was evaluated in two stages as follows.
◯: The number of passing samples is 95 or more. ×: The number of passing samples is 94 or less.

(h) 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 was calculated for n=20. The smaller this value, the better the transparency, and a value of 6% or less was judged to be excellent in transparency.
(i) Impact resistance (g) Molded products that passed the moldability evaluation (100 samples) were filled with 340 ml of tap water, and the molded products were dropped once on a P tile from a height of 200 cm at room temperature with the bottom surface of the molded product facing down and once with the side surface facing down. The impact resistance was evaluated based on the number of molded products that did not break. If the number of molded products that did not break was 90 or more, the impact resistance was evaluated as good.

Example 1
[Recycled polyester resin]
A slurry of terephthalic acid (TPA) and ethylene glycol (EG) (TPA/EG molar ratio = 1/1.6) was supplied to an esterification reactor and reacted under conditions of a temperature of 250°C and a pressure of 50 hPa to obtain ethylene terephthalate oligomer (number average degree of polymerization: 5) with an esterification reaction rate of 95%.
40.0 parts by mass of ethylene terephthalate oligomer was charged into an esterification reactor, followed by adding 2.7 parts by mass of cyclohexanedimethanol (CHDM) and 3.3 parts by mass of ethylene glycol (EG) to obtain a mixture E. Thereafter, 54 parts by mass of recycled polyester raw material (pellet-shaped polyester waste generated in the process of producing polyester resin) was added in a fixed amount over about 2 hours via a rotary valve.
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 1.15. Thereafter, the depolymerization reaction was carried out under heat treatment conditions of 260° C. for 1 hour.
The obtained depolymerization product was pumped into a polycondensation reactor (hereinafter referred to as PC can) through a candle filter having a mesh size of 20 μm, which was set between the esterification reactor and the polycondensation reactor. Germanium dioxide was then added at 1.0× ¹⁰ mol/unit as a polymerization catalyst, cobalt acetate as a cobalt compound, and tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (manufactured by ADECA: Adekastab AO-60) as a hindered phenol-based antioxidant in amounts of 0.2 mass% based on the obtained polyester resin. 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 4 hours, to obtain a polyester resin having an intrinsic viscosity of 0.7.
The polyester resin thus obtained was then continuously fed to a crystallizer and crystallized at 150° C., fed to a dryer and dried at 130° C. for 10 hours, sent 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 to obtain a polyester resin having an intrinsic viscosity of 1.1.

[Blow Molded Products]
The obtained recycled polyester resin was chipped and dried, and then extruded at 260°C using a direct blow molding machine (manufactured by Tahara) to form a cylindrical parison, which was then sandwiched between metal dies while still in a softened state to form a bottom, and then blown into a bottle. At this time, the bottom was formed when the parison had a diameter of 3 cm and a length of 25 cm, and a 350 ml hollow container (direct blow molded product) was obtained by blow molding.

Examples 2 to 8, Comparative Examples 1 to 9
[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, cyclohexanedimethanol, 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.
Subsequently, the obtained polyester resin was subjected to a solid-phase polymerization reaction in the same manner as in Example 1 to obtain a polyester resin having an intrinsic viscosity of 1.1.
[Blow Molded Products]
The obtained polyester resin was used to carry out blow molding in the same manner as in Example 1 to obtain a blow molded product.

The property values of the polyester resins obtained in Examples 2 to 8 and Comparative Examples 1 to 9, as well as the moldability, haze, and impact resistance of the blow molded products, are shown in Table 1.

As is clear from Table 1, the recycled polyester resins obtained in Examples 1 to 8 had excellent moldability when blow molding because the amount of carboxyl terminal groups, the content of diethylene glycol, and the average pressure rise rate were within the ranges specified in the present invention.
The hollow containers obtained in Examples 1 to 8 had low haze and excellent transparency, and also had excellent impact resistance.

On the other hand, in Comparative Example 1, since the G/A ratio during the depolymerization reaction was high, the diethylene glycol content was high and the thermal stability was poor, and since the amount of CHDM copolymerized was only 1 mol%, the crystallinity was high, and the obtained hollow container had poor haze, moldability, and impact resistance. Furthermore, since the G/A ratio was high, removal of foreign matter was insufficient and the pressure rise rate was fast.
In Comparative Example 2, the depolymerization in step (1) was performed under heat treatment conditions at 285°C, so the resulting recycled polyester resin had a high carboxyl end group concentration and a high diethylene glycol content. In addition, because both the carboxyl end group concentration and the diethylene glycol content were high, thermal decomposition of the recycled polyester resin occurred during molding, resulting in poor moldability and poor impact resistance of the resulting hollow container. In Comparative Example 3, the filtration particle size of the candle filter installed between the ES can and the PC can was 5 μm, so clogging occurred during filtration, and the process could not proceed to the polycondensation reaction, and a resin could not be obtained.

In Comparative Example 4, the filtration particle size of the candle filter installed between the ES can and the PC can was 30 μm, so the obtained recycled polyester resin had a large amount of foreign matter mixed in and a high average pressure rise rate. Therefore, the obtained hollow container had high haze, poor transparency, and poor impact resistance. In Comparative Example 5, the G/A during the depolymerization reaction was low, so the obtained recycled polyester resin had a high carboxyl end group concentration and a high average pressure rise rate. In addition, because the carboxyl end group concentration was high, thermal decomposition of the recycled polyester resin occurred during molding, causing deterioration in moldability, and the obtained hollow container had poor haze and impact resistance. In Comparative Example 6, the G/A of the depolymerization was high at 1.35, so the obtained recycled polyester resin had a high carboxyl end group concentration and a high diethylene glycol content, causing poor thermal stability, and also had a high average pressure rise rate. Therefore, moldability was deteriorated, and the obtained hollow container had poor haze and impact resistance. In Comparative Example 7, the depolymerization temperature was low, so the depolymerization of the recycled polyester did not proceed sufficiently, and a recycled polyester resin could not be obtained. In Comparative Example 8, the polycondensation reaction temperature was low, so the polycondensation reaction did not proceed sufficiently, and a recycled polyester resin could not be obtained.

Claims (4)

A recycled polyester resin comprising a component derived from at least one recycled polyester raw material, i.e., a) used polyester product and b) unused polyester generated in the process of manufacturing the polyester product, and characterized in that the recycled polyester resin satisfies all of the following (1) to (4):
(1) When the total amount of all acid components constituting the polyester is taken as 100 mol %, 80 mol % or more is terephthalic acid,
(2) When the total amount of all glycol components is 100 mol%, ethylene glycol is 80 mol% or more, cyclohexanedimethanol is 2 mol% or more and 15 mol% or less, and diethylene glycol is 4 mol% or less,
(3) The carboxyl end 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 line diameter: 0.07 mm, horizontal line diameter: 0.04 mm, filtration particle size: 12 μm) is set at the tip of the extruder, a polyester resin is melted at 300° C. in the extruder, and the polyester resin 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 continuous extrusion for 12 hours thereafter is defined as the "final pressure value (MPa)". Based on these pressure values, the average pressure rise rate is calculated by the following calculation formula A:
Average pressure rise rate (MPa / h) = (final pressure value - initial pressure value) / 12) ... A) A molded product containing the recycled polyester resin described in claim 1. Fiber containing the recycled polyester resin according to claim 1. 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) unused polyester generated in a process for producing a polyester product, the method comprising all of the following steps (1) to (3):
(1) A process of adding a recycled polyester raw material to a mixture containing ethylene terephthalate oligomer, ethylene glycol, and cyclohexanedimethanol so that the molar ratio of total glycol components/total acid components is 1.10 to 1.30, and performing depolymerization under heat treatment conditions of 245 to 280° C. to obtain a reaction product containing a depolymerization product;
(2) A step of passing the reaction product through a filter having a filtration particle size of 10 to 25 μm and recovering the filtrate.
(3) A step of adding a polymerization catalyst to the filtrate and carrying out a polycondensation reaction of the depolymerization at a temperature of 260° C. or higher and a reduced pressure of 1.0 hPa or lower.