JP7736262B2 - Recycled polyester resin and method for producing recycled polyester resin - Google Patents

Description

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

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

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

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

Furthermore, while there is a large amount of recycled raw materials available in the market that are made from PET bottles and other products that have once been used in products, most of these recycled materials are primarily made of polyethylene terephthalate. There is almost no practice of collecting products that use copolymer polyester and using them as recycled raw materials.

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

However, because crystallization is likely to occur during blow molding, even if molding is possible, problems exist in that whitening occurs and transparency becomes insufficient. Therefore, to improve transparency, polyester resins have been proposed in which polyethylene terephthalate is copolymerized with other monomer components (see, for example, Patent Document 1).

Thus, there is a great demand for copolymer polyester resins made from recycled polyester raw materials in a variety of molded products, but no copolymer polyester resin has yet been developed that can be fully purified of various inorganic impurities and that can be used to produce a variety of high-quality products similar to those made from virgin polyester resin.

Patent No. 6297351

The present invention aims to solve the above problems and provide a recycled polyester resin that can be used to manufacture various types of polyester products, which 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 manufacturing process of polyester resins and products. The present invention also aims to provide a manufacturing method by which such 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 processing them in a specific manner, it is possible to obtain recycled polyester resin with minimal foreign matter contamination and thermal stability similar to that of virgin polyester resin, leading to the completion of the present invention.

That is, the present invention relates to the following recycled polyester resin and a method for producing the same.
(A) A polyester resin containing at least one recycled polyester raw material from a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products,
A recycled polyester resin characterized by satisfying all of the following (1) to (5):
(1) When the total amount of all acid components constituting the polyester is taken as 100 mol %, 80 to 98 mol % is terephthalic acid and 1 to 10 mol % is isophthalic acid;
(2) When a sheet having a thickness of 0.2 mm is produced, the number of foreign particles having a particle size of 50 μm or more per 1 ^{m2 of} the sheet is 200 or less;
(3) Haze is 2% or less;
(4) The retention of the intrinsic viscosity before and after melting when melted at a temperature of 300°C for 30 minutes is 85% or more;
(5) The concentration of carboxyl terminal groups is 40 equivalents/t or less.
(ii) A molded product containing the recycled polyester resin described in (i).
(c) A method for producing a recycled polyester resin using at least one recycled polyester raw material consisting of a) used polyester products and b) unused polyesters generated in the process of producing polyester products, characterized in that the method for producing a recycled polyester resin described in (a) is characterized by carrying out the following steps (1) to (4) in order.
(1) a step of removing metal foreign matter from recycled polyester raw materials using a metal detector;
(2) removing foreign matter from the recycled polyester raw material by passing it through an air classifier;
(3) A step of melting the recycled polyester raw material in an extruder, wherein the temperature near the inlet of the recycled polyester raw material in the extruder is set to 290 to 280°C, the temperature near the outlet is set to 270 to 260°C, and the temperature difference between the temperature near the inlet and the temperature near the outlet is set to 20 to 25°C;
(4) A process of removing foreign matter by passing the recycled polyester raw material through a filter with a filtration particle size of 10 to 25 μm.

The present invention makes it possible to provide recycled polyester resins that can be used to manufacture various types of polyester products, using recycled polyester raw materials derived from used polyester products or recycled polyester raw materials derived from polyester waste generated in the process of manufacturing polyester resins and products.

In particular, the present invention utilizes a high proportion of at least one recycled polyester raw material, consisting of a) used polyester products and b) unused polyester generated in the polyester product manufacturing process, while minimizing the amount of foreign matter mixed in and controlling the carboxyl end group concentration within a specific range, thereby providing a recycled polyester resin with excellent thermal stability and heat resistance. As a result, the process for obtaining molded products enables relatively long-term continuous operation, enabling the production of various types of products with properties equivalent to those obtained using virgin polyester resin with high productivity. Furthermore, because it is a copolymer polyester resin containing copolymer components despite being a recycled polyester resin, it can also be used favorably in molded products such as bottles that require transparency.

The method for producing recycled polyester resin of the present invention makes it possible to efficiently and reliably produce recycled polyester resin that is highly thermally stable and suitable for use in molded products such as bottles, with minimal contamination by foreign matter such as those described above.

The present invention will be described in detail below.
The recycled polyester resin of the present invention (the resin of the present invention) contains at least one recycled polyester raw material, which is a) a used polyester product and b) unused polyester generated in the process of manufacturing a polyester product. These raw materials constitute a part of the polyester that constitutes the resin of the present invention.

Examples of used polyester products in category a) above include polyester molded products (including fibers) that were once on the market and then collected after use. Typical examples include containers such as PET bottles and packaging materials. Of these, polyester products made from polyester resins copolymerized with isophthalic acid are preferred.

The unadopted polyester generated during the process of manufacturing polyester products (b) above is polyester that did not make it into a product. Examples include resin pellets that do not meet specifications, materials no longer needed during molding, fragments cut during molding, scraps (polyester scraps) generated during molding or processing, cut-up materials of transitional products generated when changing brands, and cut-up materials of prototypes and defective products. Among these, polyester resins copolymerized with isophthalic acid are preferred.

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

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

The properties of the recycled polyester raw materials in a) and b) above are not limited, and may be in the form of a) and b) above, or may be in the form of cut pieces or crushed material (powders) obtained by further processing such as cutting or crushing, or in solid forms such as molded products (pellets, etc.) obtained by molding these. More specific examples include pellets obtained by cooling and cutting melted polyester scrap, and cut pieces obtained by finely cutting polyester molded products such as PET bottles.

The recycled polyester resin of the present invention (the resin of the present invention) contains a component derived from at least one recycled polyester raw material, i.e., a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products. The content of this component in the resin of the present invention is preferably 40% by mass or more, and more preferably 50% by mass or more. If the content is less than 40% by mass, the recycling rate of the unused polyester decreases. There is no particular upper limit to the content, but according to the manufacturing method of the present invention described below, it is possible to easily obtain a recycled polyester resin containing up to 100% by mass of recycled polyester raw materials.

In the resin of the present invention, when the total amount of all acid components constituting the polyester is taken as 100 mol %, 80 to 98 mol % is terephthalic acid and 1 to 10 mol % is isophthalic acid. In other words, the resin contains terephthalic acid as the main acid component and isophthalic acid as a copolymerization component.
The copolymerization amount of isophthalic acid must be 1 to 10 mol%, and preferably 2 to 8 mol%. By copolymerizing 1 to 10 mol% of isophthalic acid, the crystallization rate of the polyester resin can be adjusted to suit various molding processes, and whitening due to crystallization during molding can be prevented. The melting point of the resulting recycled polyester resin is preferably 235 to 250°C.

If the copolymerization amount of isophthalic acid is less than 1 mol%, the resin composition will crystallize too quickly, causing the molded product to crystallize and turn white when molded, resulting in poor transparency. On the other hand, if the copolymerization amount of isophthalic acid exceeds 10 mol%, the melting point will be low and heat resistance will be poor.

The proportion of terephthalic acid in the acid component is 80 to 98 mol%, with a proportion of 85 to 97 mol% being preferred. If the proportion of terephthalic acid is less than 80 mol%, the crystallinity of the resin composition will decrease and it will likely become amorphous. On the other hand, if the proportion of terephthalic acid exceeds 98 mol%, the amount of isophthalic acid copolymerized will decrease, making it difficult to achieve the effects achieved by copolymerizing isophthalic acid in various products.

Other acid components in the resin of the present invention besides terephthalic acid and isophthalic acid include phthalic acid, phthalic anhydride, naphthalenedicarboxylic acid, adipic acid, sebacic acid, 1,4-cyclohexanedicarboxylic acid, dodecanedioic acid, and other dimer acids, as well as trimellitic anhydride, trimellitic acid, pyromellitic acid, 1,4-cyclohexanedicarboxylic acid, sebacic acid, and dimer acids. Two or more of these may be used in combination, and ester-forming derivatives of these acids may also be used.

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

In the resin of the present invention, diol components other than ethylene glycol in the total glycol components include neopentyl glycol, 1,4-butanediol, 1,2-propylene glycol, 1,5-pentanediol, 1,3-propanediol, 1,6-hexamethylenediol, diethylene glycol, 1,4-cyclohexanedimethanol, dimer diol, butylethylpropanediol, (2-methyl-1,3-propanediol, trimethylolpropane, glycerin, pentaerythritol, ethylene oxide adducts of bisphenol A or bisphenol S, and the like.

The resin of the present invention has the above-mentioned composition and also has the following characteristic values.
(a) When a sheet having a thickness of 0.2 mm is produced, the number of foreign particles having a particle size of 50 μm or more per 1 ^{m2 of} the sheet is 200 or less,
(b) a haze of 2% or less;
(c) the retention of intrinsic viscosity before and after melting when melted at a temperature of 300°C for 30 minutes is 85% or more;
(d) The concentration of carboxyl terminal groups is 40 equivalents/t 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) that, when a sheet having a thickness of 0.2 mm is produced, the number of foreign particles having a particle size of 50 μm or more per 1 ^m2 of the sheet is 200 or less. It is particularly preferable that the number of foreign particles is 160 or less. The lower limit of the number of foreign particles can be, for example, about 50 or more, but is not limited to this.

If the number of foreign particles exceeds 200, the transparency of the resulting molded product will be deteriorated due to the foreign particles themselves or due to the occurrence of spherulites with the foreign particles as nuclei.
Examples of such foreign matter that may arise from the use of recycled polyester raw materials include foreign matter derived from materials other than polyester, such as polystyrene and polyolefin, foreign matter derived from catalysts, and powder-like or froth-like high-melting-point foreign matter adhering to the surface of recycled polyester raw materials.

The method for measuring the number of foreign particles in the resin of the present invention is to produce a sheet with a thickness of 0.2 mm under the conditions of an extruder temperature of 260 to 290°C, a rotation speed of 130 rpm, a winder temperature of 30 to 50°C, and a rotation speed of 2 m/min using a fish eye counter (gel counter) manufactured by Optical Control Systems, and detect and count the number of foreign particles with a particle size of 50 μm or more per 1 ^m2 of the sheet.

The resin of the present invention has a characteristic value (b) of 2% or less, preferably 1% or less, of haze measured by the following method. The lower limit of the haze can be, for example, about 0.1% or more, but is not limited thereto. The haze in the present invention is an index of the number of spherulites caused by various foreign matters, and the smaller the haze, the less foreign matter is mixed in.
The haze was measured by molding a 5 mm thick molded plate in an injection molding machine at a temperature of 260 to 290°C and a mold temperature of 20°C, and measuring the haze using a haze measuring device (Nippon Denshoku Industries Co., Ltd. Haze Meter SH-7000).

The resin of the present invention has the characteristic value (c) of having a retention of intrinsic viscosity before and after melting at a temperature of 300° C. of 85% or more, preferably 90% or more, when melted for 30 minutes.
If the retention rate of the intrinsic viscosity is less than 85%, the thermal stability of the polyester resin will be low, and the intrinsic viscosity will decrease when molded, resulting in poor properties such as impact resistance and strength/elongation of the resulting molded product.

The intrinsic viscosity of the resin of the present invention is preferably 0.35 to 0.80, and more preferably 0.45 to 0.70. The intrinsic viscosity is measured at 20°C using a mixture of equal parts by mass of phenol and tetrachloroethane as the solvent.

The resin of the present invention has the characteristic value (d) of a carboxyl terminal group concentration of 40 equivalents/t or less, preferably 35 equivalents/t or less, and more preferably 30 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, making it possible to obtain fibers and molded articles having excellent heat resistance by various methods.
If the carboxyl terminal group concentration exceeds 40 equivalents/ton, the thermal stability of the resulting polyester resin will be reduced, the performance of the polyester will be reduced by heat treatment during molding, and the resulting molded product will have poor properties such as impact resistance and strength/elongation.

Next, the method for producing the resin of the present invention will be described. In the production method of the present invention, it is important to carry out the steps (1) to (4) in order.
(1) a step of removing metal foreign matter from recycled polyester raw materials using a metal detector;
(2) removing foreign matter from the recycled polyester raw material by passing it through a precision air classifier;
(3) A step of melting the recycled polyester raw material in an extruder, wherein the temperature near the inlet of the recycled polyester raw material in the extruder is set to 290 to 280°C, the temperature near the outlet is set to 270 to 260°C, and the temperature difference between the temperature near the inlet and the temperature near the outlet is set to 20 to 25°C;
(4) A process of removing foreign matter by passing the recycled polyester raw material through a filter with a filtration particle size of 10 to 25 μm.

First, in the step (1) of removing foreign metal matter using a metal detector, the recycled polyester raw material is subjected to removal of foreign metal matter using a commercially available metal detector. The metal detector preferably uses a magnet, and examples of such a method include attaching a magnetic bar to a pipe when the recycled polyester raw material is sent by air to the step (2) to remove foreign metal matter.
If the metal-derived foreign matter is not removed, the heat stability of the polyester will be reduced, the impact resistance of the molded product will be low, and the number of foreign matters will be large, resulting in low transparency when molded.

In the next step (2) of removing foreign matter by passing the material through an air classifier, the recycled polyester raw material that has been subjected to step (1) is further subjected to removal of foreign matter using an air classifier. As the air classifier, a gravity classifier, an inertial classifier, a centrifugal classifier, or the like can be used, and commercially available ones can be used, but it is preferable to use a classifier capable of precision classification. For example, a double cylindrical classifier manufactured by Sanko Air Equipment Co., Ltd. can be used.
By using an air classifier, powder and froth-like foreign matter adhering to the surface of recycled polyester raw materials can be separated using a nitrogen or air stream. The powder and froth-like foreign matter adhering to the chip surface often has a high melting point, and if it is left adhering without being removed using an air classifier, it will not melt during molding, resulting in molded products with poor transparency.

Furthermore, (3) in the process of melting the recycled polyester raw material in the extruder, the temperature near the extruder's recycled polyester raw material inlet is set to 290-280°C, the temperature near the outlet is set to 270-260°C, and the temperature difference between the temperature near the inlet and the temperature near the outlet is set to 20-25°C.

This step is important in the manufacturing method of the present invention. In step (3), it is believed that the foreign matter present in the recycled polyester raw material becomes susceptible to precipitation as foreign matter, with substances derived from the catalyst metal acting as nuclei. Therefore, in the next step (4), it becomes possible to remove the foreign matter from the recycled polyester raw material that could not be removed in steps (1) and (2), thereby producing a resin of the present invention that satisfies the above-mentioned characteristic values.

First, when the recycled polyester raw material is fed into the extruder, it is melted at a temperature of 290-280°C. The temperature inside the extruder is then gradually lowered, allowing substances derived from the catalytic metal melted into the resin to act as nuclei, making it easier for foreign matter to precipitate. It is important to melt the material by setting the temperature near the extruder outlet to 270-260°C and ensuring that the temperature difference between the temperature near the inlet and the temperature near the outlet is 20-25°C.

The temperature near the extruder's inlet must be set in the range of 290-280°C. If this temperature is below 280°C, the precipitation of the above-mentioned foreign matter will not proceed. On the other hand, if the temperature exceeds 290°C, the polyester resin will be more likely to decompose, the carboxyl end group concentration will increase, and the thermal stability will be poor.

The residence time of the recycled polyester raw material in the extruder is preferably 10 minutes or less, and more preferably 5 minutes or less from the viewpoint of suppressing the concentration of carboxyl terminal groups and suppressing deterioration of the color tone of the polyester.

The foreign matter precipitated in step (3) is removed in step (4). In step (4), the recycled polyester raw material is passed through a filter with a filtration particle size of 10 to 25 μm to remove the foreign matter. If a filter with a filtration particle size larger than 25 μm is used, the foreign matter in the precipitated resin cannot be sufficiently removed, and the number of foreign matter particles mentioned above will exceed 200. On the other hand, if a filter with a filtration particle size smaller than 10 μm is used, it is prone to clogging with foreign matter, and the filter life is shortened, which is cost-effective and also reduces operability.

Furthermore, filters that can be used in step (4) of the present invention include general filters, such as screen changer filters, leaf disc filters, and candle-type sintered filters.

From the viewpoint of environmental issues, molded articles containing the resin of the present invention preferably contain the resin of the present invention in an amount of 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more. The resin of the present invention contains little foreign matter, has excellent thermal stability, and has performance equivalent to that of virgin polyester resin, so there is no problem in using 100% by mass of the resin to produce molded articles.
Examples of molded articles include blow molded articles, injection molded articles, sheets, films, etc. obtained by blow molding, injection molding, stretching, etc., and fibers (multifilament yarn, monofilament yarn, long fiber, short fiber) can be obtained by melt spinning.

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

(d) Number of foreign matters: Measured by the method described above.
(e) Haze 1
Using the obtained recycled polyester resin, a 5 mm thick molded plate was molded in an injection molding machine (manufactured by Nissei Plastic Industrial Co., Ltd., model name: PS-20) under conditions of a temperature of 260 to 290°C and a mold temperature of 20°C, and the turbidity was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., model name: SH-7000) and taken as the average value of n number 20. The smaller this value, the better the transparency, and if it was 2% or less, it was determined to be excellent in transparency.
(f) Haze 2
Sample pieces (20 pieces) were cut out from the obtained container, and the turbidity was measured using a haze measuring device (manufactured by Nippon Denshoku Industries Co., Ltd., model name: SH-7000) and recorded as the average value of n=20. The smaller this value, the better the transparency, and a value of 2% or less was determined to be excellent transparency.

(g) Retention of intrinsic viscosity 5 g of the obtained recycled polyester resin was placed in a test tube, the inside of the test tube was swept under a nitrogen atmosphere, and the test tube containing the resin was set in a heater at 300°C. After 30 minutes of heat treatment, the test tube was removed and the intrinsic viscosity of the resin was measured before and after the heat treatment, and the retention was calculated using the following formula.
Retention rate of intrinsic viscosity (%) = (intrinsic viscosity after heating/intrinsic viscosity before heating) x 100
(h) Moldability The thickness of the body of each of the obtained containers (100 samples) was measured, and samples with a difference in thickness between the thickest and thinnest parts of up to 0.30 mm were judged to be acceptable. Depending on the number of samples that passed, the moldability was evaluated in two stages as follows:
◯: Number of passing samples: 95 or more ×: Number of passing samples: 94 or less (i) Impact Resistance (h) Molded products that passed the moldability evaluation (100 samples) were filled with 340 ml of tap water and dropped once on a P tile from a height of 200 cm at room temperature, with the bottom and side facing downwards. The impact resistance was evaluated based on the number of molded products that did not break. Note that if the number of samples that did not break was 90 or more, the impact resistance was evaluated as good.
〇: Number of passing samples is 90 or more ×: Number of passing samples is 89 or less

Example 1
100 kg of recycled polyester raw material (unused polyester resin (pellet-like polyester waste) generated in the process of producing polyester resin, in which 4 mol % of isophthalic acid (IPA) is copolymerized when the total amount of all acid components is 100 mol %)) was passed through an air transfer pipe installed inside a 13,000 gauss magnet bar as a metal detector, with 3 magnet bars in 1 row x 3 rows, and introduced into an air classifier.
In this case, a double cylindrical air classifier manufactured by Sanko Air Equipment Co., Ltd. was used as the air classifier.
Next, the recycled polyester raw material was introduced into an extruder. At this time, the temperature near the inlet of the extruder for the recycled polyester raw material was set to 285°C, the temperature near the outlet was set to 265°C, and the temperature difference between the temperature near the inlet and the temperature near the outlet was set to 20°C, and the recycled polyester raw material was melted. Then, the melt was passed through a metering pump and a 20 μm filter to obtain a recycled polyester resin.
[Blow molded products]
The recycled polyester resin was chipped and dried, and then extruded at 260°C using a direct blow molding machine (manufactured by Tahara Co., Ltd.) to form a cylindrical parison. The parison was then clamped in a mold while still softened to form a bottom, which was then blown into a bottle. The bottom was formed when the parison reached a diameter of 3 cm and a length of 25 cm, and the bottle was then blown to obtain a 350 ml hollow container (direct blow molded product).

Examples 2 to 7, Comparative Examples 1 to 9
A recycled polyester resin was obtained in the same manner as in Example 1, except that the type of recycled polyester raw material (copolymerization amount of isophthalic acid), the presence or absence of a metal detector, the presence or absence of an air classifier, the extruder temperature, and the filtration particle size of the filter were changed.
Using the obtained recycled polyester resin, a blow-molded article was obtained in the same manner as in Example 1.

The property values of the polyester resins obtained in Examples 1 to 7 and Comparative Examples 1 to 9, as well as the haze values, moldability, and impact resistance of the blow-molded products, are shown in Table 1.

As is clear from the results in Table 1, the recycled polyester resins obtained in Examples 1 to 7 had the foreign matter count, haze, intrinsic viscosity retention, and carboxyl terminal group concentration within the ranges specified in the present invention. Therefore, hollow containers obtained from these recycled polyester resins had good moldability and were also excellent in haze value and impact resistance.
On the other hand, in Comparative Example 1, the recycled polyester resin obtained did not pass through a metal detector, so it contained metal-derived foreign matter and had poor thermal stability, resulting in a high concentration of carboxyl end groups, a low retention rate of intrinsic viscosity, a large number of foreign matters, and poor haze.The molded product also had uneven thickness, poor moldability and haze, and low impact resistance.
In Comparative Example 2, the recycled polyester resin obtained did not pass through a precision classifier, and therefore contained high-melting-point impurities, resulting in a large amount of impurities and poor haze. The molded product also had uneven thickness, poor moldability, poor haze, and low impact resistance.
In Comparative Example 3, the temperature directly below the resin inlet of the extruder was high, so the resulting recycled polyester resin had poor thermal stability, a high carboxyl end group concentration, and low intrinsic viscosity retention. It also contained catalyst-derived impurities, resulting in a large amount of impurities and poor haze. The molded product also had uneven thickness, poor moldability, poor haze, and low impact resistance.
In Comparative Example 4, the temperature immediately adjacent to the nozzle of the extruder was low, so the resin solidified in the extruder, and no resin could be obtained.
In Comparative Example 5, the temperature gradient of the extruder was low at the resin inlet and high near the nozzle, so that catalyst-derived impurities remained in the resin without precipitation. The resulting recycled polyester resin had a high carboxyl end group concentration and low intrinsic viscosity retention. It also contained catalyst-derived impurities, resulting in a large amount of impurities and poor haze. The molded product also had uneven thickness, poor moldability, haze, and low impact resistance.
In Comparative Example 6, the openings of the filter were too small to allow the resin to pass through the extruder, and therefore, no resin could be obtained.
In Comparative Example 7, the filter had large openings, so catalyst-derived contaminants remained in the resin. The resulting recycled polyester resin had a high carboxyl end group concentration and low intrinsic viscosity retention. The resin also contained catalyst-derived contaminants, resulting in a large amount of contaminants and poor haze. The molded product also had uneven thickness, poor moldability, poor haze, and low impact resistance.
The recycled polyester resin of Comparative Example 8 had a low content of isophthalic acid, and therefore had high crystallinity, which caused crystallization in the molded product and resulted in poor haze.
The recycled polyester resin of Comparative Example 9 had a high content of isophthalic acid, so the resin had low heat resistance, a high concentration of carboxyl terminal groups, and a low intrinsic viscosity retention. The molded product also had uneven thickness and poor moldability and impact resistance.

Claims (1)

A method for producing a recycled polyester resin comprising at least one recycled polyester raw material consisting of a) used polyester products and b) unused polyesters generated in the process of producing polyester products, the method satisfying all of the following (1) to (5), characterized by carrying out the following steps (1) to (4) in order:
Recycled polyester resin (1) When the total amount of all acid components constituting the polyester is taken as 100 mol%, 80 to 98 mol% is terephthalic acid, 1 to 10 mol% is isophthalic acid, and when the total amount of all glycol components is taken as 100 mol%, 80 mol% or more is ethylene glycol;
(2) When a sheet having a thickness of 0.2 mm is produced, the number of foreign particles having a particle size of 50 μm or more per 1 ^{m2 of} the sheet is 200 or less;
(3) Haze is 2% or less;
(4) The retention of the intrinsic viscosity before and after melting when melted at a temperature of 300°C for 30 minutes is 85% or more;
(5) The concentration of carboxyl terminal groups is 40 equivalents/t or less.
Step (1) removing metal foreign matter from recycled polyester raw materials using a metal detector;
(2) removing foreign matter from the recycled polyester raw material by passing it through an air classifier;
(3) A step of melting the recycled polyester raw material in an extruder, wherein the temperature near the inlet of the recycled polyester raw material in the extruder is set to 290 to 280°C, the temperature near the outlet is set to 270 to 260°C, and the temperature difference between the temperature near the inlet and the temperature near the outlet is set to 20 to 25°C;
(4) A process of removing foreign matter by passing the recycled polyester raw material through a filter with a filtration particle size of 10 to 25 μm.