CN119591849A - Chemical regeneration method of waste polyester - Google Patents

Abstract

The present invention discloses a chemical regeneration method of waste polyester: waste polyester is added to ethylene glycol to carry out depolymerization reaction, the obtained depolymerization reaction liquid is filtered to remove impurities, decolorized, cooled and crystallized, and separated, and the obtained solid product is a mixture of BHET and BHET dimer, which is directly subjected to polycondensation reaction to prepare regenerated polyester. The present invention first discovered that the solid impurities filtered out when adding water to purify BHET are BHET dimers, which do not need to be removed at all. The mixture of BHET and BHET dimer is directly used as a raw material for polymerization to prepare RPET, which simplifies the process and saves costs. The post-processing steps of the present invention are simple, and there is no need to add a large amount of water to purify BHET. The solvent ethylene glycol can be recycled after simple treatment. The steps of depolymerizing waste polyester to polymerizing to prepare rPET in the present invention are few, the process is short, the energy consumption is low, and the yield is high, which is of great significance for the efficient recovery of PET and the promotion of green economic technology.

Description

Chemical regeneration method of waste polyester

Technical Field

The invention belongs to the field of high polymer materials, and particularly relates to a chemical regeneration method of waste polyester.

Background

Polyesters are used in various aspects of life by virtue of their excellent physical properties, and as the range of applications of polyester products continues to expand, their yields rise year after year. On the one hand, polyester brings convenience to people's life, but also brings the disposal problem of waste polyester due to the excellent chemical stability, and if any waste is in nature, white pollution and ecological environment damage can be caused. At present, related policies are formulated for recycling in many countries and regions and across-country enterprises, such as new edition of food contact regeneration plastic regulations draft is proposed at the end of 2021 of European Union, the requirements of updating and refining on PET regeneration technology are also set by European Union through single use plastic instruction, and the recovery rate of plastic package reaches 50% by 2025 and reaches 65% by 2030.

At present, most of the recycling modes of polyester are mainly physical recycling modes, and the recycling modes are convenient, low in technical threshold and short in period. However, physical recovery has a non-negligible disadvantage that impurities or other components in the recycled materials cannot be thoroughly removed, the viscosity is reduced, the color is different, the recycled products are mainly applied to the low-end field, and the recycled products are difficult to apply to the high-end and even food-grade fields.

Chemical recycling of polyesters involves a further level of depolymerization of the polyester to monomer, followed by polymerization to polyester, resulting in Recycled PET (RPET) of quality approaching virgin polyester. Glycolysis is a mainstream PET chemical recovery technology in the industry at present, and polyethylene glycol is utilized to depolymerize PET, the reaction condition is mild compared with methanol depolymerization, but the reaction is reversible, the conversion rate is limited, and the product is generally a mixture of bis (2-hydroxyethyl) terephthalate (BHET) and other polymers, so that the separation is difficult and the cost is high.

The current research mainly focuses on the influence of catalysts on PET depolymerization rate and BHET yield, such as ionic liquid type catalysts, alkaloid catalysts and the like, and the new catalysts can slightly improve the BHET yield, but have little influence on the cost of the whole process of extracting the BHET from polyester depolymerization. In the process of preparing BHET by depolymerizing PET, the separation and purification of the rear end product is the main part of the production cost, which is neglected at present,

Specifically, the depolymerization product BHET is generally introduced with water as a solvent, and the BHET is dissolved and separated from other unreacted raw materials, such as patent CN201510511713.9 and patent CN202310257181.5, and is purified by multiple recrystallization by introducing water, so that the whole period is long, the efficiency is low, and a large amount of water is required, so that resources are consumed. In addition, since the depolymerization product also contains ethylene glycol, the separation of water and ethylene glycol at a later stage also requires higher energy consumption after water addition, resulting in a great cost. The high production costs result in excessive prices for renewable BHET products on the market, which makes common polyester plants and downstream customers prohibitive.

It is necessary to develop a new recovery and regeneration method, which is convenient and fast to use in a physical method, and the product has high quality in a chemical method, so that the cost is reduced.

Disclosure of Invention

Aiming at the technical problems of poor recovery efficiency and high cost of waste polyester in the prior art, the invention aims to develop an efficient and low-cost chemical recovery and regeneration method of waste polyester.

The technical scheme adopted by the invention is as follows:

the chemical regeneration process of waste polyester includes the steps of adding waste polyester into glycol, depolymerizing, filtering, decolorizing, cooling to crystallize, separating, and polycondensing to obtain regenerated polyester.

Further, the waste polyesters include, but are not limited to, commercially available packaging polyester bottles, polyester films, and polyester fibers.

Further, the waste polyester includes PET and a copolyester based on PET, specifically, a polyester obtained by copolymerizing a terephthalic acid monomer with a diol or a diacid, or a polyester obtained by copolymerizing a terephthalic acid monomer with a diol or a diacid.

The dihydric alcohol comprises one or more of ethylene glycol, neopentyl glycol, 1,4 cyclohexanedimethanol, diethylene glycol, isosorbide, propylene glycol and 1, 4-butanediol, and the dibasic acid comprises one or more of naphthalene dicarboxylic acid, succinic acid and adipic acid.

Preferably, the waste polyester comprises substantially PET, PBT, PETG, PBAT.

Further, the depolymerization reaction is carried out under the action of a depolymerization catalyst, which is typically an oxide, hydroxide, chloride or acetate of zinc, aluminum, antimony, manganese, cobalt, sodium or potassium, preferably an oxide or acetate of zinc, manganese, sodium.

More preferably, the depolymerization catalyst is zinc acetate or zinc oxide.

Further, the addition amount of the depolymerization catalyst is 0.01% -1% of the mass of the waste polyester.

Further, the mass ratio of the waste polyester to the glycol is 1:3-10, preferably 1:4-8.

Further, the depolymerization reaction temperature is 180 ℃ to 195 ℃, preferably 185 ℃ to 190 ℃. The depolymerization reaction time is 2 to 5 hours, preferably 2.5 to 4 hours.

Further, the depolymerization reaction solution is filtered to remove impurities, and the filter pore diameter is 1-15 μm. Filtration removes unreacted PET (very small amounts) and impurities, where both BHET and BHET dimer are dissolved in ethylene glycol solvent.

Further, the filtration and impurity removal are generally carried out at a temperature of 75 ℃ to 120 ℃ (preferably 75 ℃ to 100 ℃).

Further, the decoloring can be performed by using one or two of activated carbon and ion exchange resin.

Further, the particle size of the activated carbon is preferably 100 to 500 mesh. The ion exchange resin is preferably a styrene-divinylbenzene anion exchange resin with a macroporous structure of basic groups, preferably tertiary amine groups or quaternary ammonium groups. More preferably, the ion exchange resin is a D301 resin.

Further, the addition amount of the activated carbon or the ion exchange resin is 0.05-12% of the mass of the waste polyester.

Further, the decoloring temperature is 50-100 ℃ (preferably 50-70 ℃), and the decoloring time is 0.5-5 h.

Further, the temperature at the time of crystallization is 0 ℃ to 25 ℃, preferably 0 to 10 ℃.

Further, after cooling, crystallization and separation, the obtained solid product generally has a small amount of ethylene glycol solvent, and the ethylene glycol can be pumped out in vacuum in the subsequent polycondensation reaction without drying to remove the solvent.

After crystallization and separation, the obtained filtrate is glycol solvent, wherein metal ions (catalyst of waste polyester and added depolymerization catalyst) can be purified by chelating ion exchange resin, and the obtained purified glycol solvent can be recovered and reused.

The chelating ion exchange resin can be macroporous styrene or acrylic acid cation exchange resin, preferably D401, D113 or 001 x 7 cation exchange resin, etc.

The filtrate is purified by adopting chelating ion exchange resin, so that metal ions can be removed and the color is removed, and the purified glycol can be directly recycled as no water is added in the post-treatment of the invention.

The polycondensation reaction is typically carried out under the action of a polycondensation catalyst, which is typically one or more of antimony trioxide, antimony glycol, antimony acetate, germanium oxide or germanium glycol.

Further, the addition amount of the polycondensation catalyst is 0.01-0.04% wt of the mass of the solid product.

Further, the polycondensation reaction of the mixture of BHET and BHET dimer includes a prepolymerization step and a final polycondensation step.

In the prepolymerization step, the prepolymerization temperature is 200-270 ℃ (preferably 230-270 ℃), the prepolymerization reaction is carried out in a vacuum environment, the pressure is reduced to 1-3 kpa from normal pressure, and the prepolymerization time is 50-120 min (preferably 90-120 min).

In the prepolymerization step, ethylene glycol in the product and generated by the transesterification reaction is removed by vacuum.

In the final polycondensation step, the reaction temperature is 275-290 ℃, the absolute pressure is 100Pa or less, and the reaction time is 1-5 hours.

In the conventional PET depolymerization method, separation and purification are required after depolymerization, and water is generally added to dissolve BHET, and water-insoluble solid matters are removed as impurities. Solid impurities are generally considered to comprise impurities, small amounts of unreacted PET, and mixtures of various types of oligomers of varying degrees of polymerization. Multiple recrystallization purifications are necessary to obtain a purer BHET, but this also results in a significant increase in cost. However, the invention discovers for the first time that in the process of depolymerizing PET polyester with ethylene glycol, products are BHET and BHET dimers. After removal of the impurities, the resulting solid product, after addition of water, yields water-insoluble impurities, actually BHET dimers, as a single species, rather than a mixture of oligomers of varying degrees of polymerization. Like BHET, dimers of BHET are also organic molecules, and can also be used as end products and as starting materials for polymerization into rPET. Therefore, as long as the product contains only BHET and pure BHET dimer, both raw materials can be used for polycondensation reaction, and the content of BHET in the mixture has no substantial influence on the quality of the final rPET. The BHET dimer which is removed as impurities before can be used as a raw material for application, and is not required to be removed at all, so that the separation and purification steps are simplified, and the recovery rate is greatly improved.

The molecular formulas of BHET and BHET dimers are shown below, respectively:

Compared with the prior art, the invention has the beneficial effects that the problems of complicated separation process and high energy consumption of the BHET and the multimer (dimer) thereof are solved, and the invention discovers that the solid impurities filtered and removed when the BHET is separated by adding water for the first time are the dimer of the BHET, so that waste is changed into valuables, the dimer impurities are not required to be removed, but the BHET and the multimer (dimer) thereof are used as raw materials to directly polymerize and prepare the RPET, the flow is simplified, and the cost is saved.

According to the invention, other solvents such as water are not introduced to highly purify BHET, the later high-energy-consumption separation step of water and glycol is omitted, and the solvent glycol can be recycled through simple treatment.

In the decoloring and metal ion removing processes, the activated carbon or the ion exchange resin has better adsorption effect on metal ions in the depolymerization solution, and the product has less metal ions and can be directly used as a raw material for polymerizing rPET.

The depolymerization agent glycol can be directly recycled after being purified by decolorization, ion adsorption and the like, and the glycol extracted in low vacuum in the prepolymerization process can also be recycled.

The method has the advantages of less steps from depolymerizing waste polyester to preparing rPET by polymerization, short flow, low energy consumption and high yield, and has important significance for high-efficiency recovery of PET and popularization of green economic technology.

Drawings

FIG. 1 is an infrared spectrum of BHET and BHET dimer.

FIG. 2 is a nuclear magnetic hydrogen spectrum of BHET.

FIG. 3 is a nuclear magnetic resonance hydrogen spectrum of BHET dimer.

Fig. 4 is a photograph of a depolymerization reaction and decoloration step of waste polyester, wherein a photograph of a raw material of a blue PET bottle chip added into a glycol solvent is shown in a drawing A, a photograph of a heated depolymerization reaction is shown in a drawing C, a photograph of a depolymerization reaction completed is shown in a drawing D, a photograph of a decolored product added with activated carbon is shown in a drawing E, and a photograph of a decolored product after filtration, cooling crystallization and drying is shown in a drawing E.

FIG. 5 is a photograph of comparative example 2 without decolorizing the product.

FIG. 6 is a schematic diagram of BHET obtained by isolation of the product of example 5.

Detailed Description

The process of the present invention is described in further detail below in connection with specific examples, the scope of the invention including but not limited to the following embodiments.

Examples 1-5 are laboratory depolymerization experiments using colored bottle flakes, examples 6-8 were preliminary scaled up in laboratory and prepared for recycled rPET, and because of the large raw material requirements, PET slices were used instead. In addition, examples 6-8 depolymerized in a 20L reactor, and depolymerized products remained on the walls of the reactor and in the cast strip head and the pipeline, resulting in unavoidable losses, and the actual product mass was greater than the weighing mass in the table. Example 9 is the preparation of regenerated rPET from BHET and multimers. Comparative example 1 employed a BHET purification process conventional in the art. The decolorizing in the step of comparative example 2 was not performed using activated carbon. In examples 1 to 8, the process of drying to remove ethylene glycol was carried out in order to compare the quality of the product, but the excess ethylene glycol in normal production was not significantly affected, and in the subsequent step of the prepolymerization, ethylene glycol was withdrawn under a low vacuum for a long period of time.

Example 1

In a 250mL beaker, 20g of blue PET bottle flakes (child ha activated beverage bottles, sheared into 8 x 8mm pieces, the same applies below), 100g of ethylene glycol, 0.04g of zinc acetate (raw material photos are shown as A in FIG. 4), and the mixture is placed on a magnetic stirring heating table, inserted with a thermocouple thermometer probe, sealed with a preservative film, reacted for 180min at 195 ℃ (depolymerization reaction process photos are shown as B in FIG. 4, and after the reaction is finished, photos are shown as C in FIG. 4), cooled to 80 ℃, filtered (aperture 10 μm), and unreacted PET and impurities are removed. Adding 0.5g of active carbon (150 meshes) into the filtrate (the photographs are shown in the D diagram of FIG. 4 when the active carbon is added for decolorization), stirring for 1.5h at 50-60 ℃, filtering again, cooling the solution to 10 ℃, standing for 12h, filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof, putting into a vacuum oven, 80 ℃ for 8h, weighing, and the photographs of the products are shown in the E diagram of FIG. 4. 0.5g of D113 resin is added into the filtrate, the mixture is stirred for 3 hours and filtered, and the filtrate is purified glycol and can be directly recycled.

Example 2

In a 250mL beaker, adding 20g of blue PET bottle flakes, 100g of ethylene glycol and 0.04g of zinc acetate, placing on a magnetic stirring heating table, reacting for 150min at 195 ℃, cooling to 80 ℃, filtering, adding 0.5g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 10 ℃, standing for 12h, filtering again, obtaining a mixture of ethylene glycol, BHET and a polymer thereof, placing in a vacuum oven, 80 ℃ and 8h, and weighing. To the filtrate was added 0.5g of D113 resin, which was stirred for 3h and filtered, the filtrate was left.

Example 3

In a 250mL beaker, adding 20g of blue PET bottle flakes, 100g of ethylene glycol and 0.04g of zinc acetate, placing on a magnetic stirring heating table, reacting for 120min at 195 ℃, cooling to 100 ℃, filtering, adding 2g of D301 resin into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 10 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placed in a vacuum oven at 80 ℃ for 8h and weighed. To the filtrate was added 0.5g of D113 resin, which was stirred for 3h and filtered, the filtrate was left.

Example 4

Adding 40g of blue PET bottle flakes, 250g of ethylene glycol and 0.08g of zinc acetate into a 500mL three-neck flask, placing the mixture on a magnetic stirring heating table, reacting for 180min at 195 ℃, cooling to 100 ℃, filtering, adding 1g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 10 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placed in a vacuum oven at 80 ℃ for 8h and weighed. 1g of D113 resin was added to the filtrate, which was stirred for 3 hours and filtered, and the filtrate was left.

Example 5

Adding 40g of blue PET bottle flakes, 300g of ethylene glycol and 0.1g of zinc acetate into a 500mL three-neck flask, placing the mixture on a magnetic stirring heating table, reacting for 180min at 195 ℃, cooling to 100 ℃, filtering, adding 1g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 10 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placed in a vacuum oven at 80 ℃ for 8h and weighed. 1g of D113 resin was added to the filtrate, which was stirred for 3 hours and filtered, and the filtrate was left.

Example 6

Adding 2kg of PET slices, 8kg of ethylene glycol and 3.5g of zinc acetate into a 20L reaction kettle, placing the mixture on a magnetic stirring heating table, reacting for 180min at 195 ℃, cooling to 100 ℃, filtering, adding 40g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 15 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placed in a stainless steel tray, placed in a vacuum oven, at 80 ℃ for 8 hours, and weighed. 60g of D113 resin was added to the filtrate, which was stirred for 3h and filtered, the filtrate was left for use.

Example 7

Adding 2.5kg of PET slices, 12kg of ethylene glycol and 4.4g of zinc acetate into a 20L reaction kettle, placing the mixture on a magnetic stirring heating table, reacting for 180min at 195 ℃, cooling to 100 ℃, filtering, adding 50g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 15 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placing the mixture in a stainless steel tray, placing the stainless steel tray in a vacuum oven, and weighing at 80 ℃ for 8 hours. 70g of D113 resin was added to the filtrate, stirring for 3h, and filtering to obtain filtrate.

Example 8

Adding 2kg of PET slices, 12kg of ethylene glycol and 4.4g of zinc acetate into a 20L reaction kettle, placing the mixture on a magnetic stirring heating table, reacting for 150min at 195 ℃, cooling to 100 ℃, filtering, adding 50g of active carbon into the filtrate, stirring for 1h at 50-60 ℃, filtering again, cooling the solution to 15 ℃, standing for 12h, and filtering again to obtain a mixture of ethylene glycol, BHET and a polymer thereof. Placing the mixture in a stainless steel tray, placing the stainless steel tray in a vacuum oven, and weighing at 80 ℃ for 8 hours. 70g of D113 resin was added to the filtrate, stirring for 3h, and filtering to obtain filtrate.

Example 9

7.5Kg of the mixture of BHET and BHET dimer obtained in examples 6-8 is weighed and placed into a 20L reaction kettle, 1.65g of antimony trioxide catalyst is added, the kettle temperature is controlled at 35-60 ℃, nitrogen is introduced to replace air, a vacuum pump is started to slowly vacuumize to 1kPa, and the process is repeated for 3 times. And after the temperature of the reaction kettle rises to 150 ℃ and is kept constant for 35 minutes, all reactants are melted (judged according to stirring current), the temperature is continuously raised to 230 ℃, low vacuum is started, the pressure is gradually reduced from normal pressure to 1kPa within 2 hours, and meanwhile, the temperature of the reaction kettle is controlled to slowly rise to 265 ℃ so that redundant glycol in the reaction kettle is pumped out after transesterification, and meanwhile, the BHET is prevented from being pumped out due to too fast temperature rise. After the low vacuum is finished, the kettle is heated to 275 ℃ for polycondensation, the pressure is lower than 100Pa, the current reaches the target value after 118min, the reaction is finished, the material is discharged, and the granules are cut.

Comparative example 1

Adding 40g of blue PET bottle flakes, 250g of ethylene glycol and 0.08g of zinc acetate into a 500mL three-neck flask, and placing the mixture on a magnetic stirring heating table for reaction for 180min at 195 ℃;

cooling to 100 ℃, filtering, adding 600g of pure water into the filtrate, stirring for 30min, cooling to 15 ℃, keeping the temperature for 12 hours, and filtering to obtain crude BHET;

The crude BHET was dissolved with 600g of pure water at 80℃and 1g of D113 resin was added to the solution, stirred for 60 minutes, filtered, and the filtrate was cooled to 15℃and allowed to stand for 12 hours, followed by filtration again to obtain refined BHET.

The refined BHET was placed in a vacuum oven at 80 ℃ for 8h and weighed.

Comparative example 2

In a 500mL three-neck flask, 40g of blue PET bottle flakes, 300g of ethylene glycol and 0.1g of zinc acetate are added, the mixture is placed on a magnetic stirring heating table for reaction for 180min at 195 ℃, cooled to 100 ℃, filtered, the solution is cooled to 10 ℃, and then left stand for 12h, and the mixture of ethylene glycol, BHET and polymers thereof is obtained after filtering again. Placed in a vacuum oven at 80 ℃ for 8 hours and weighed, the resulting photograph of the product is shown in fig. 5. 1g of D113 resin was added to the filtrate, which was stirred for 3 hours and filtered, and the filtrate was left.

The weights of the products obtained in examples 1 to 8 and comparative examples are shown in the following table. Wherein the products of examples 1-8 are a mixture of BHET and dimer, and the product of comparative example is BHET.

Yield data it can be seen that the yield of the product of the invention is higher. And the separation step is simple, and no additional water is needed, so that the separation of glycol and water is not needed, the purification steps are few, and the production efficiency is high.

The yield of example 3 is low, probably because the reaction time is only 120min and the depolymerization reaction is not complete. Therefore, the reaction time of the invention is preferably 150-180 min.

The product prepared in example 5 was separated, water was added, BHET was dissolved in water, dimer was not dissolved in water, and after separation, infrared detection was performed, and the spectrum was shown in FIG. 1. In FIG. 1, the upper panel shows dimer spectra, and the lower panel shows BHET spectra.

In example 5, the mass ratio of BHET to dimer was found to be about 6:1 by separation, with BHET and dimer having masses of 42.5944g and 6.5875g, respectively (separation process loss, BHET loss being greater). Wherein the BHET is shown in fig. 6.

The nuclear magnetic hydrogen spectrum of BHET is shown in figure 2, and the hydrogen spectrum accords with BHET literature data.

The nuclear magnetic hydrogen spectrum of the BHET dimer is shown in FIG. 3, and the hydrogen shift peak area ratio is approximately 4:2:2:2:1, which is the same as the hydrogen ratio of the BHET dimer. It can be seen from both fig. 1 and fig. 3 that the multimeric product is actually a dimer purer, rather than a mixture.

The regenerated PET slices of example 9 were tested and the test results are shown in the following table.

Intrinsic viscosity dL/g	Carboxyl terminated mmol/kg	DEG%	Color value L	Color value b
					0.653	32.34	1.092	73.16	4.6

Claims (10)

1. A chemical regenerating process for the waste polyester includes such steps as adding the waste polyester to glycol, depolymerizing, filtering, removing impurities, decoloring, cooling, crystallizing, separating, and polycondensing.

2. The method of claim 1, wherein the depolymerization reaction is carried out in the presence of a depolymerization catalyst that is an oxide, hydroxide, chloride or acetate of zinc, aluminum, antimony, manganese, cobalt, sodium or potassium.

3. The method of claim 1, wherein the mass ratio of waste polyester to glycol is 1:3-10.

4. The method of claim 1, wherein the depolymerization reaction is carried out at a temperature of 180 ℃ to 195 ℃ for a period of 2 hours to 5 hours.

5. The method of claim 1, wherein the decolorizing is performed with one or both of activated carbon and ion exchange resin at a temperature of 50 ℃ to 100 ℃.

6. The method of claim 1, wherein the temperature during crystallization is 0 ℃ to 25 ℃.

7. The method of claim 1, wherein the filtrate obtained after cooling crystallization and separation is ethylene glycol solvent, and the obtained purified ethylene glycol solvent is recovered and reused by purification with chelating ion exchange resin.

8. The method of claim 1, wherein the polycondensation is performed with a polycondensation catalyst that is one or more of antimony trioxide, antimony glycol, antimony acetate, germanium oxide, or germanium glycol.

9. The method according to claim 1, wherein the polycondensation reaction of the mixture of BHET and BHET dimer comprises a prepolymerization step and a final polycondensation step, wherein the prepolymerization step is carried out at a prepolymerization temperature of 200 ℃ to 270 ℃ in a vacuum environment, the pressure is reduced from normal pressure to 1 to 3kpa, and the prepolymerization time is 50 to 120min.

10. The method according to claim 9, wherein in the final polycondensation step, the reaction temperature is 275 to 290 ℃, the absolute pressure is 100Pa or less, and the reaction time is 1 to 5 hours.