Depolymerization method for polyester resin composition

Abstract

This invention investigates the conditions for chemically decomposing a polyester resin composition and provides a depolymerization method that allows the decomposition reaction to proceed efficiently. [Solution] This is achieved by the following depolymerization method. A method for depolymerizing a polyester resin composition, wherein the polyester resin composition and a glycol compound are added to the presence of a polyester oligomer and a depolymerization reaction is carried out, and the moisture content in the reaction system is 0.001% by weight or more and 2.0% by weight or less.

Description

Technical Field

[0001] The present invention relates to a method for depolymerizing a polyester resin composition in chemical recycling.

Background Art

[0002] Polyester is excellent in mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and is used in various applications. Among polyesters, polyethylene terephthalate (hereinafter referred to as PET) is widely used in applications that require high quality, such as optical films and release films, because of its excellent transparency and processability. However, in the case of process films such as release films, they are discarded after use, and thus, reduction of environmental impact has been demanded in recent years.

[0003] As a means of reducing environmental impact, there is thermal recycling in which waste polyester resin is burned to obtain thermal energy. However, when thermal recycling is performed, carbon dioxide is generated and the polyester raw material is lost, so it is necessary to newly use petroleum raw materials to reproduce polyester.

[0004] For recycling without newly using petroleum raw materials, there is a method called chemical recycling in which polyester is chemically decomposed into raw materials or intermediates and then repolymerized to reproduce polyester. However, since chemical recycling uses energy when decomposing polyester, the environmental load increases. Therefore, it is required to efficiently proceed with the decomposition reaction.

[0005] In response to these problems, Patent Documents 1 and 2 disclose technologies related to highly pure chemically recycled PET.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007] Patent documents 1 and 2 disclose methods for producing BHET and PET recycled from PET bottles, as well as their compositions. These production methods use catalysts to efficiently promote the decomposition reaction of polyester. However, even when catalysts are used, the decomposition reaction may be inhibited by factors such as the storage environment of the raw materials (polyester resin composition) used.

[0008] The object of the present invention is to investigate the conditions for chemically decomposing a polyester resin composition and to provide a depolymerization method in which the decomposition reaction proceeds efficiently. [Means for solving the problem]

[0009] As a result of diligent research to solve the above problems, we arrived at the method for depolymerizing the polyester resin composition of the present invention.

[0010] The object of the present invention is achieved by the following means.

[0011] (1) A method for depolymerizing a polyester resin composition, wherein the polyester resin composition and a glycol compound are added in the presence of a polyester oligomer and a depolymerization reaction is carried out, and the moisture content in the reaction system is 0.001% by weight or more and 2.0% by weight or less.

[0012] (2) A method for depolymerizing a polyester resin composition according to (1), wherein the amount of glycol compound added is in the range of 15 parts by weight or more and 1000 parts by weight or less per 100 parts by weight of the polyester resin composition.

[0013] (3) A method for depolymerizing the polyester resin composition described in (1), wherein the depolymerization reaction temperature is 190°C or higher and 240°C or lower.

[0014] (4) A method for depolymerizing the polyester resin composition according to (1), wherein the moisture content of the polyester resin composition is 0.001% by weight or more and 3.0% by weight or less.

[0015] (5) A method for depolymerizing a polyester resin composition according to (1), wherein the polyester resin composition is a polyester composition derived from used polyester products and / or waste generated in the polyester product manufacturing process.

[0016] (6) A method for depolymerizing the polyester resin composition according to (1), wherein the glycol compound is ethylene glycol.

[0017] (7) A recycled polyester resin composition obtained by polycondensing a depolymer obtained by the depolymerization method of the polyester resin composition described in (1).

[0018] (8) A method for producing polyester resin using the depolymerization method of the polyester resin composition described in (1).

[0019] (9)(7) A polyester film comprising the recycled polyester resin composition described above. [Effects of the Invention]

[0020] This invention can provide a method for efficiently advancing the depolymerization of polyester resin compositions. [Modes for carrying out the invention]

[0021] The present invention will be described in detail below.

[0022] In the method for depolymerizing the polyester resin composition of the present invention, when adding the polyester resin composition and the glycol compound in the presence of the polyester oligomer and causing a depolymerization reaction, it is necessary that the water content in the reaction system is 0.001% by weight or more and 2.0% by weight or less. Further, as the upper limit, it is preferably 1.0% by weight or less. When the water content in the reaction system is less than 0.001% by weight, the polyester resin composition is not affected by hydrolysis, so the efficiency of the decomposition reaction decreases. When the water content exceeds 2.0% by weight, the proportion of water with a low boiling point in the reaction system increases, and in the depolymerization reaction, it becomes a reaction in a state where the evaporated substance is liquefied and refluxed to the reaction system. Therefore, even if heat is applied, the temperature in the reaction system hardly rises, and the efficiency of depolymerization decreases.

[0023] In order to reduce the water content in the reaction system, it is effective to reduce the water content in the polyester resin composition and the glycol compound. The method for reducing the water content is not limited. For example, the polyester resin composition can reduce the water content by using a hot air dryer or a vacuum dryer. Also, the glycol compound is preferably distilled to remove water. This distillation may be carried out before being introduced into the reaction system, or the water may be distilled off after being introduced into the reaction system together with the polyester resin composition.

[0024] The depolymerization reaction in the present invention is carried out in the presence of a polyester oligomer. The amount of the polyester oligomer is not particularly limited, but it is preferably 30 parts by weight or more and 50 parts by weight or less with respect to 100 parts by weight of the polyester resin composition to be subjected to the decomposition reaction. By carrying out the reaction in the presence of the polyester oligomer, it becomes possible to increase the reaction efficiency.

[0025] A polyester oligomer refers to a product obtained by subjecting a dicarboxylic acid compound such as terephthalic acid, isophthalic acid, dimethyl terephthalate or dimethyl isophthalate and a glycol compound such as ethylene glycol to an esterification reaction or an ester exchange reaction, or a product obtained by subjecting a polyester resin composition to a depolymerization reaction, but is not particularly limited thereto. However, since an oligomer obtained by an esterification reaction or an ester exchange reaction is newly produced using raw materials, it is preferable to use an oligomer obtained by subjecting a polyester resin composition to a depolymerization reaction from the viewpoint of environmental load.

[0026] The depolymerization reaction in the present invention is a decomposition reaction in which a glycolysis reaction is carried out by heating and stirring a polyester composition in the presence of a glycol compound to obtain a monomer or oligomer depolymerized composition. For example, when ethylene glycol is used as the glycol compound, the monomer obtained by decomposing polyethylene terephthalate is bis-(2-hydroxyethyl) terephthalate (hereinafter referred to as BHET).

[0027] In the present invention, from the viewpoint of the decomposition efficiency of the polyester resin composition, it is preferable to carry out the depolymerization reaction by adding the glycol compound in the range of 15 parts by weight or more and 1000 parts by weight or less with respect to 100 parts by weight of the polyester resin composition. Further, the lower limit is preferably 100 parts by weight or more. Also, the upper limit is preferably 600 parts by weight or less. When the addition amount of the glycol compound is less than the above lower limit, the amount of the glycol compound with respect to the polyester resin composition is small, and sufficient stirring in the reaction vessel cannot be performed, so the depolymerization reaction does not proceed uniformly, and local heating occurs, which may cause thermal decomposition of the polyester resin composition and deterioration of the quality of the depolymerized product. Further, when the addition amount of the glycol compound exceeds 1000 parts by weight, the cost increases by greatly exceeding the necessary and sufficient amount of glycol for the reaction, the depolymerized product deteriorates due to an increase in the reaction time, and the reactor used for the reaction may become large.

[0028] In this invention, it is preferable to carry out the depolymerization reaction in the range of 190°C to 240°C. These temperatures refer to the final temperature reached in the depolymerization reaction. Furthermore, it is preferable that the lower limit be 200°C or higher and the upper limit be 230°C or lower. If the temperature of the depolymerization reaction is below 190°C, the time required for the depolymerization reaction will be significantly longer. If it exceeds 240°C, thermal decomposition will proceed, and the quality of the depolymer may decrease.

[0029] The moisture content of the polyester resin composition used in the present invention is preferably 0.001% by weight or more and 3.0% by weight or less. Furthermore, it is preferable that the upper limit be 1.0% by weight or less. If the moisture content is less than 0.001% by weight, the polyester resin composition will not be affected by hydrolysis, which may reduce the efficiency of the decomposition reaction. If the moisture content exceeds 3.0% by weight, the proportion of water, which has a low boiling point, will be large in the reaction system, and the depolymerization reaction will occur in a state where the evaporated product is liquefied and refluxed into the reaction system. As a result, even if heat is supplied, the temperature in the reaction system will not rise easily, which may reduce the efficiency of depolymerization.

[0030] The polyester resin composition used in the present invention preferably uses used polyester such as PET bottles, polyester films, clothing, and containers, or polyester derived from waste generated in the polyester product manufacturing process. A polyester resin composition consisting mostly of PET is preferred, but it may also contain components other than PET, such as copolymer components, as long as it does not impair the effects of the present invention.

[0031] The glycol compound used in the present invention preferably has the same structure as the glycol component constituting the polyester resin composition. Specifically, if the polyester resin composition used is PET, it is ethylene glycol. Having the same structure ensures stable quality and physical properties when producing recycled polyester resin using depolymerization. In addition, other glycol compounds may be included as long as they do not impair the effects of the invention.

[0032] In the present invention, when a depolymerization reaction catalyst is used, metal hydroxides such as sodium hydroxide, potassium hydroxide, and magnesium hydroxide, as well as metal acetate salts such as magnesium acetate, manganese acetate, cobalt acetate, calcium acetate, and lithium acetate, can be used, but the invention is not limited to these.

[0033] The depolymer obtained in this invention can be subjected to a polycondensation reaction to produce a polyester resin composition.

[0034] In this invention, the polycondensation reaction refers to the process of heating and stirring the depolymer under reduced pressure to remove the glycol compound and ultimately obtain a polyester resin composition. Conventional known catalysts can be used as the reaction catalyst in the polycondensation, and antioxidants, color inhibitors, phosphorus compounds, pigments, dyes, particles, etc., can be added as needed. Furthermore, copolymerization may be carried out using multiple types of other dicarboxylic acid components, diol components, and even hydroxycarboxylic acids, as long as the effects of this invention are not impaired.

[0035] The recycled polyester resin composition obtained by the present invention can be suitably used in fibers, films, sheets, containers, bottles, and the like by molding it. Among these, the recycled polyester resin composition obtained using the depolymerization method of the present invention is suitable for polyester films due to its excellent color tone and transparency, and is particularly suitable for biaxially oriented polyester films for process release. Films used in this application become unnecessary after release and are suitable for use as raw materials (resources) for recycling, and further recycling them back into this application is preferable from the viewpoint of a circular economy. Specific examples of such process release films include release films for the manufacture of multilayer ceramic capacitors (MLCCs), films for dry film resists, films for polarizing plates, and films for optical release, and the recycled polyester resin composition obtained by the present invention can be suitably used in these applications.

[0036] In the polyester film of the present invention, the recycled polyester resin composition obtained in the present invention preferably accounts for 3% or more by weight of the total weight, more preferably 10% or more by weight, and even more preferably 20% or more by weight. There is no particular upper limit.

[0037] The resin composition used in the polyester film of the present invention is not particularly limited in terms of components other than the recycled polyester resin composition obtained in the present invention. Conventional known resin compositions such as polyester resin compositions obtained from virgin raw materials, polyester resin compositions obtained by other chemical recycling, polyester resin compositions obtained by material recycling, and copolymerized polyester resin compositions can be used. Furthermore, the polyester film of the present invention may be a single-layer film or a laminated film with two or more layers laminated together, and the stretched form is not particularly limited, such as an unstretched film, a uniaxially oriented film, or a biaxially oriented film, but a biaxially oriented film is preferred from the viewpoint of mechanical strength.

[0038] The method for casting the polyester film of the present invention is not particularly limited, but methods such as a method in which a polyester resin composition is heated and melted in an extruder and extruded from a die onto a cooled cast drum to process it into a sheet (melt casting method), or a method in which a polyester resin is dissolved in a solvent, the solution is extruded from a die onto a support such as a cast drum or endless belt to form a film, and then the solvent is dried off from the film layer to process it into a sheet (solution casting method) can be used. In the case of a laminated film, a method in which the polyester resin of each layer to be laminated is put into a separate extruder, melted, and then combined, and co-extruded from a die onto a cooled cast drum to process it into a sheet (melt film formation method by co-extrusion) can be preferably used.

[0039] The following are examples of specific manufacturing methods for the present invention, but are not limited thereto.

[0040] In the presence of 30 to 50 parts by weight of PET oligomer, 100 parts by weight of recovered PET raw materials such as PET bottles and PET film scraps, and 15 to 1000 parts by weight of ethylene glycol are added, and a depolymerization reaction is carried out in the temperature range of 190°C to 240°C to obtain a depolymer.

[0041] The resulting depolymer is purified as needed and added to a polymerization reactor, where it is melted at a temperature range of approximately 200°C to 250°C. A polymerization catalyst, such as antimony trioxide, is then added, and the temperature inside the apparatus is gradually increased to approximately 270°C to 300°C while the pressure inside the apparatus is reduced to 1 Torr or less. As the polymerization reaction progresses, the viscosity of the reactants increases, and when the increase in stirring torque of the reactants reaches the target value for the end of polymerization, the reaction is terminated, and the polyester is discharged from the polymerization reactor into a water tank. The discharged polyester is rapidly cooled in the water tank and chipped with a cutter to obtain a recycled polyester resin composition.

[0042] Various polyester resin compositions, including the obtained recycled polyester resin composition, are fed into extruders corresponding to each layer and heated, melted, and extruded. The layers are laminated using a confluence block and co-extruded from the die onto a cast drum cooled to a surface temperature of 10-60°C. The layers are then cooled and solidified using electrostatic force to create an unstretched film. At this time, it is preferable to filter the polyester resin melted in the extruder using a filter. Since even very small foreign matter can become large protrusions and defects in the film, it is effective to use a high-precision filter that captures 95% or more of foreign matter larger than 5 μm.

[0043] Next, the unstretched film is guided to a group of rolls heated to a temperature of 70-140°C and stretched 3-4 times in the longitudinal direction (vertical direction, i.e., the direction in which the sheet moves), and then cooled in a group of rolls heated to a temperature of 20-50°C. Subsequently, the sheet is guided to a tenter while both ends are held with clips and stretched 3-4 times in the direction perpendicular to the longitudinal direction (width direction) in an atmosphere heated to a temperature of 80-240°C. After stretching, a relaxation treatment of 0.1-5% in the longitudinal and / or width directions may also be applied. As for the biaxial stretching method, in addition to the sequential biaxial stretching method in which the longitudinal and width directions are stretched separately as described above, either the simultaneous biaxial stretching method in which the longitudinal and width directions are stretched at the same time is acceptable. [Examples]

[0044] The present invention will be described in more detail below with reference to the following examples. The physical properties in the examples were measured by the following method.

[0045] (1) Moisture content in polyester resin composition and glycol compound The moisture content was measured using coulometric titration with a Karl Fischer moisture meter (AQ-2000) manufactured by HIRANUMA Corporation. The average value of three trials was used.

[0046] (2) Moisture content in the reaction system The moisture content in the reaction system was calculated from the moisture content and amount added of the polyester resin composition and glycol compound used for depolymerization.

[0047] (3) Percentage of polyester resin composition The solution obtained from the depolymerization reaction was filtered through a 400-mesh filter to separate the insoluble material. The recovered insoluble material was vacuum-dried at 70°C under high vacuum for 8 hours, and its weight was measured. The residual rate was calculated using the weight of the polyester resin composition used in the depolymerization reaction as 100%. A high residual rate indicates that the depolymerization reaction is progressing slowly, so it was evaluated according to the following criteria, with ◎ and ○ being considered acceptable. ◎: Less than 30% ○: 30% or more but less than 50% △: 50% or more.

[0048] (4) Yield of recycled polyester resin composition The yield was calculated from the weight of the recycled polyester resin composition, with the theoretical mass of the recycled polyester resin composition obtained when the polyester resin composition used in the depolymerization reaction was completely depolymerized and then repolymerized being set as 100%.

[0049] (5) Color tone of recycled polyester resin composition The Hunter value (b value) was measured for recycled polyester resin composition chips using a color difference meter (SM Color Computer model SM-T45) manufactured by Suga Test Instruments Co., Ltd. A smaller value indicates less yellowness and a better color tone.

[0050] (6) Transparency of polyester film Transparency was evaluated by the haze (%) of a film molded to a thickness of 25 μm. Film haze was measured using a turbidimeter (NDH5000) manufactured by Nippon Denshoku Industries Ltd. after leaving the polyester film for 1 hour under normal conditions (temperature 23°C, relative humidity 65%). The average of three measurements was taken as the film haze of the laminated polyester film. Film haze was evaluated according to the following criteria, with ◎ and ○ indicating passing. ◎: Less than 1.0 ○: 1.0 or higher, less than 2.0 △: 2.0 or higher.

[0051] (7) Color tone of polyester film A 100mm x 100mm sample was cut from an arbitrary point on the polyester film to be measured. Using a Konica Minolta CM-3600d spectrophotometer, the sample was set so that the angle between the normal to the film plane and the incident light was 0°. The chromaticity b* at an arbitrary point on the sample was measured using transmitted light under the conditions of a target mask with a measurement diameter of φ25.4mm. The same measurement was then repeated four times, with the sample moved at least 30mm away from the center of the measurement point (measurements were taken at five arbitrary points). The chromaticity b* was evaluated according to the following criteria, with ◎ and ○ being considered passing grades. ◎: Less than 1.5 ○: 1.5 or higher, less than 3.0 △: 3.0 or higher.

[0052] (Example 1) In the presence of 30 parts by weight of PET oligomer, 100 parts by weight of recovered PET raw material with a moisture content of 0.001% by weight, 162 parts by weight of ethylene glycol with a moisture content of 0.005% by weight, and 0.5 parts by weight of sodium hydroxide as a catalyst were added. The depolymerization reaction was carried out for 2.5 hours with the final temperature set to 206°C to obtain the depolymer. The recovered PET raw material was recovered from waste process film.

[0053] The resulting depolymer was filtered through a 400-mesh filter to separate the insoluble material. The recovered insoluble material was vacuum-dried at 70°C under high vacuum for 8 hours, and when its weight was measured, the remaining PET content was 20%, which was a good result.

[0054] (Example 2) In Example 2, the moisture content of the recovered PET raw material used in the depolymerization reaction was set to 0.4% by weight, and the moisture content of the ethylene glycol was set to 1.5% by weight. Depolymerization was carried out in the same manner as in Example 1, except that the moisture content in the reaction system and the final temperature reached were changed. Compared to Example 1, Example 2 had a higher moisture content in the reaction system, resulting in an increased PET residue rate, but this was within an acceptable range.

[0055] (Examples 3-6) The moisture content of the recovered PET raw material used in the depolymerization reaction and the moisture content in the reaction system were changed, and the final temperature reached was also changed in Examples 5 and 6. Otherwise, the depolymerization was carried out in the same manner as in Example 2.

[0056] In Example 3, although the moisture content of the recovered PET raw material was low, the moisture content of the ethylene glycol was higher than in Example 1, and the moisture content in the reaction system was also higher. As a result, the efficiency of the depolymerization reaction decreased, and the residual rate increased slightly, but the results were still good.

[0057] Examples 4-6 had a high moisture content in the recovered PET raw material, and the moisture content of the ethylene glycol was also higher than in Example 1. Therefore, the high moisture content in the reaction system reduced the efficiency of the depolymerization reaction and increased the PET residue rate, but the results were still good.

[0058] (Examples 7-9) Depolymerization was carried out in the same manner as in Example 2, except that the moisture content of the ethylene glycol used in the depolymerization reaction, the moisture content in the reaction system, and the final temperature reached were changed.

[0059] Although the moisture content of the recovered PET raw material in Example 7 was higher than in Example 1, the moisture content of the ethylene glycol was as low as in Example 1, and the moisture content in the reaction system was also low. As a result, the depolymerization reaction efficiency was good and the PET retention rate was good.

[0060] In Examples 8 and 9, the high moisture content of the ethylene glycol and the high moisture content in the reaction system resulted in a slight decrease in the efficiency of the depolymerization reaction and an increase in the residual rate, but this was within an acceptable range.

[0061] (Examples 10, 11, Comparative Example 1) Depolymerization was carried out in the same manner as in Example 2, except that the moisture content of the recovered PET raw material and ethylene glycol used in the depolymerization reaction, the moisture content in the reaction system, and the final temperature reached were changed.

[0062] In Examples 10 and 11, the moisture content of the recovered PET raw material and ethylene glycol was low, and the moisture content in the reaction system was also low, resulting in improved efficiency of the depolymerization reaction and a good PET retention rate.

[0063] In Comparative Example 1, the moisture content of the recovered PET raw material and ethylene glycol was low, resulting in a moisture content of less than 0.001% by weight in the reaction system. Because the PET was no longer affected by hydrolysis, the efficiency of the depolymerization reaction decreased, the PET residue rate increased, and the result was unsatisfactory.

[0064] (Examples 12-15) The amount of ethylene glycol added to the depolymerization reaction, the water content in the reaction system, and the final temperature reached were changed. Otherwise, the depolymerization was carried out in the same manner as in Example 2.

[0065] In Examples 12 and 13, the low water content in the reaction system improved the efficiency of the depolymerization reaction, resulting in a good residual rate.

[0066] Examples 14-16 showed good results, although the efficiency of the depolymerization reaction was slightly reduced and the PET retention rate increased due to the relatively high water content in the reaction system.

[0067] (Examples 17, 18, Comparative Example 2) Depolymerization was carried out in the same manner as in Example 2, except that the amount of PET oligomer in the depolymerization reaction and the water content in the reaction system were changed.

[0068] Although the amount of PET oligomer was increased in Examples 17 and 18, the water content in the reaction system remained relatively low. Therefore, the depolymerization reaction efficiency was similar to that of Example 2, and the PET retention rate did not change significantly, which was within an acceptable range.

[0069] Comparative Example 2 failed because the depolymerization was carried out under conditions where PET oligomers were absent, resulting in reduced reaction efficiency and an increased PET residue rate.

[0070] (Example 19) Depolymerization was carried out in the same manner as in Example 2, except that the final temperature reached in the depolymerization reaction was changed. Lowering the final temperature to 180°C reduced the efficiency of the depolymerization reaction and increased the PET retention rate, but the results were still good.

[0071] (Example 20) The amount of ethylene glycol added to the depolymerization reaction, the moisture content in the reaction system, and the final temperature reached were changed. Otherwise, the depolymerization was carried out in the same manner as in Example 2. The relatively low moisture content in the reaction system and the increase in the final temperature reached to 244°C improved the efficiency of the depolymerization reaction and reduced the residual rate.

[0072] (Example 21) The depolymerization was carried out in the same manner as in Example 2, except that the depolymerization reaction was modified to be performed without a catalyst. Due to the absence of a catalyst, the reaction efficiency was lower and the residual rate increased compared to Example 2, but this was within an acceptable range.

[0073] (Comparative Example 3) Depolymerization was carried out in the same manner as in Example 1, except that the moisture content of the recovered PET raw material and ethylene glycol used in the depolymerization reaction, the amount of ethylene glycol added, the moisture content in the reaction system, and the final temperature reached were changed. Due to the high moisture content in the reaction system, the reaction efficiency decreased significantly and the residual rate increased, resulting in failure.

[0074] (Evaluation of recycled polyester resin compositions) The depolymers obtained in Examples 1-21 and Comparative Examples 1-3 were subjected to polycondensation reactions using the methods described below to obtain polyester resin compositions, which were then evaluated.

[0075] The entire amount of depolymer was added to the polymerization reactor and melted at 200°C. Antimony trioxide was then added as a polycondensation catalyst at a concentration of 0.01 wt% (assuming the resulting polyester resin composition is 100 wt%). The temperature inside the apparatus was then gradually increased to 290°C while the pressure inside the apparatus was reduced to 1 Torr or less. As the polymerization reaction progressed, the viscosity of the reactants increased, and the reaction was terminated when the increase in stirring torque of the reactants reached the target value for the end of polymerization. The mixture was then discharged from the polymerization reactor into a water tank to obtain the recycled polyester resin composition.

[0076] The yields of the recycled polyester resin compositions obtained in Examples 1 to 21 were within an acceptable range, and the yields in Examples 1, 11 to 13, and 20 were particularly good. On the other hand, in Comparative Examples 1 to 3, the efficiency of the depolymerization reaction was low, resulting in low yields of the recycled polyester resin compositions.

[0077] The recycled polyester resin compositions obtained using the depolymerization methods of Examples 1-11, 14-19, 21, and Comparative Examples 1-3 showed good color tone b values. On the other hand, the recycled polyester resin compositions obtained using the depolymerization methods of Examples 12, 13, and 20 showed slightly higher color tone b values, but these were within an acceptable range.

[0078] (Evaluation of polyester films containing recycled polyester resin compositions) The regenerated polyester resin compositions obtained from the depolymers in Examples 1 to 21 were prepared as laminated polyester films using the method described below and evaluated.

[0079] For the first and third layers, 90 parts by weight of the recycled polyester resin composition made from the depolymer obtained in Example 1 and 10 parts by weight of polyester composition B (virgin PET composition) were blended and dried under reduced pressure at 160°C for 2 hours before being fed into the extruder for the first and third layers. Similarly, for the second layer, 80 parts by weight of the recycled polyester resin composition made from the depolymer obtained in Example 1 and 20 parts by weight of polyester composition B (virgin PET composition) were blended and dried under reduced pressure at 160°C for 2 hours before being fed into the extruder for the second layer. Each raw material was melted at 280°C in the extruder, and the layers were combined and laminated using a lamination block to form a three-layer laminate in the order of the first, second, and third layers. Subsequently, the sheets were co-extruded from the die onto a casting drum with a surface temperature of 25°C, and cooled and solidified by electrostatic adhesion to create a laminated unstretched sheet with three layers. Next, the sheet was preheated in a group of heated rolls, then stretched 3.3 times in the longitudinal direction (vertical direction, i.e., the direction in which the sheet moves) at 90°C, and then cooled in a group of rolls at 25°C to obtain a uniaxially oriented film. Holding both ends of the obtained uniaxially oriented film with clips, it was stretched 3.5 times in the direction perpendicular to the longitudinal direction (width direction) in a 110°C heating zone inside a tenter. Subsequently, it was heat-set at a temperature of 230°C for 10 seconds in a heat treatment zone inside the tenter. After uniform slow cooling in the cooling zone, the ends of the film that were held by the tenter clips were cut off, and this was wound up to obtain a polyester film with a thickness of 25 μm.

[0080] The polyester films using the regenerated polyester resin compositions derived from the depolymers of Examples 1-11, 14-19, and 21 showed good transparency and color tone. On the other hand, the polyester films using the regenerated polyester resin compositions derived from the depolymers of Examples 12, 13, and 20 showed slight deterioration in transparency and color tone, but were still within an acceptable range for use as optical polyester films.

[0081] [Table 1]

[0082] [Table 2] [Industrial applicability]

[0083] The polyester depolymer, recycled polyester resin composition, and polyester film obtained in this manner are useful for optical applications, agricultural materials, horticultural materials, fishing materials, civil engineering and construction materials, stationery, medical supplies, automotive parts, electrical and electronic components, and other applications, and are particularly suitable for process release films where high quality is required.

Claims

1. A method for depolymerizing a polyester resin composition, wherein the polyester resin composition and a glycol compound are added to the presence of a polyester oligomer and a depolymerization reaction is carried out, and the moisture content in the reaction system is 0.001% by weight or more and 2.0% by weight or less.

2. The method for depolymerizing a polyester resin composition according to claim 1, wherein the amount of glycol compound added is in the range of 15 parts by weight or more and 1000 parts by weight or less per 100 parts by weight of the polyester resin composition.

3. A method for depolymerizing a polyester resin composition according to claim 1, wherein the depolymerization reaction temperature is 190°C or higher and 240°C or lower.

4. A method for depolymerizing a polyester resin composition according to claim 1, wherein the moisture content of the polyester resin composition is 0.001% by weight or more and 3.0% by weight or less.

5. A method for depolymerizing a polyester resin composition according to claim 1, wherein the polyester resin composition is a polyester composition derived from used polyester products and / or waste generated in the polyester product manufacturing process.

6. A method for depolymerizing a polyester resin composition according to claim 1, wherein the glycol compound is ethylene glycol.

7. A recycled polyester resin composition obtained by polycondensing a depolymer obtained by the depolymerization method of the polyester resin composition described in claim 1.

8. A method for producing a polyester resin using the depolymerization method of the polyester resin composition described in claim 1.

9. A polyester film comprising the recycled polyester resin composition described in claim 7.

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