Method for enabling recycling of a polyester waste stream and system for applying the method

Abstract

The invention relates to a method for enabling recycling of a polyester waste stream by depolymerizing the polyester present in the polyester waste stream into a reactive oligoester, the method comprising: providing the polyester in liquid form in a reactor, and feeding an alcohol to the reactor to subject the polyester to alcoholysis to depolymerize the polyester, while adding an amount of water and an amount of carboxylic acid to form the reactive oligoester. The invention also relates to a system for using the method.

Description

Technical Field

[0001] This invention relates to the field of recycling polyester waste, particularly materials containing semi-crystalline polyesters such as polyethylene terephthalate (PET), and to systems for applying such recycling methods. Background Technology

[0002] Polyesters, such as PET commonly used in soda bottles and yarn materials used in textile production, are typically recycled. The post-consumer polyester recycling industry began due to environmental pressures to improve waste management. Another driving force behind the polyester recycling industry is the slow rate of natural decomposition of polyester products. Many polyesters are non-degradable plastics under normal circumstances because no known organism can consume their relatively large molecules. Making polyester biodegrade requires complex and expensive procedures.

[0003] The world's first attempt at recycling polyester waste (i.e., post-consumer polyester articles or materials) occurred in the 1970s, but appropriate recycling methods have developed rapidly. For example, in 2000, Australia's total PET consumption was 88,258 tonnes, of which 28,113 tonnes were recycled, a recycling rate of approximately 32%. Many researchers have reported that PET flakes should meet certain minimum requirements for successful PET recycling. The main factors affecting the suitability of post-consumer PET flakes for recycling are the level and nature of contaminants present in the flakes. Minimizing the amount of these contaminants results in better rPET (i.e., recycled PET) quality. PET is contaminated by many substances, such as acid-producing contaminants, water, coloring contaminants, acetaldehyde, and other contaminants such as detergents, fuels, and pesticides, due to the use of PET bottles to store them.

[0004] Various methods have been applied to recycle polyester waste, each with its own advantages and disadvantages. The first type of method used to recycle polyester waste is so-called energy recovery, such as pyrolysis and carbonization. Pyrolysis of polyester waste was first described in the early 1980s. It is an alternative to PET landfill disposal. Typically, polyester waste is pyrolyzed without further purification of the plastic waste. Most pyrolysis is performed to produce aliphatic and aromatic hydrocarbons as alternatives to fossil fuels or as a source of chemicals. Carbonization is the second method for pyrolyzing polyester waste.

[0005] A second type of method for recycling polyester waste involves sorting the waste directly, then using the sorted material as an additive in asphalt mastic aggregate, cementitious materials, mortar, or concrete composites. Because polyester waste may be supplied mixed with other polymers, the polyester material must be separated from these polymers before reprocessing. Therefore, several methods have been developed, including foam flotation, wet shaking, swelling, or thermomechanical processes.

[0006] In addition, there is a class of methods for recycling polyester waste based on (thermo)mechanical methods. The simplest thermomechanical recycling method is to remelt the sorted polyester waste. This method is applicable, for example, to bottle-to-bottle technology, in which sorted PET bottles are remelted into a crushed shape and reprocessed into bottles for beverage packaging. Several studies have been conducted on the thermal reprocessing of PET. During this method, the polymer is exposed to high temperatures, shear forces, and pressures. As a result, PET undergoes thermal degradation. Consequently, the thermal and mechanical properties of the reprocessed material are generally reduced. Therefore, repeated thermal reprocessing of polyester waste leads to the degradation of the material during recycling. The undesirable discoloration problem of recycled polyester materials is usually addressed by using additives. Since the collection of polyester waste often requires mixtures of polyester materials of different colors, this thermal reprocessing results in undesirable discoloration of the recycled polyester. Therefore, adding complementary colors to polyester waste has been applied to mask discoloration. Although this is an approved procedure, this method severely limits the use of recycled polyester, especially as a food packaging material.

[0007] The fourth type of method for recycling polyester waste is the so-called chemical recycling (chemical decomposition) method, in which the recovery of polyester waste is achieved by depolymerization into monomers and / or oligomers. This category can be further divided into numerous subcategories based on the type of reactants used in chemical decomposition. Through chemical decomposition, polyester can be broken down into its monomeric components, theoretically meaning that the starting point for creating new polyester polymers corresponds to the starting point from the virgin material. An example is the application of ionic liquids for depolymerization, first described around 2000. This method was developed to avoid the drawbacks of other methods, such as alcoholysis (high pressure and high temperature, and heterogeneous reaction products) or acid and alkali hydrolysis (pollution problems), thereby providing an eco-friendly degrader for polymers and enabling degradation under mild reaction conditions. However, no applications of the obtained reaction products have been described to date.

[0008] As an alternative, castor oil is used for depolymerization. This method was developed to provide a renewable alternative to petrochemical reagents (e.g., ethylene glycol) for PET depolymerization. The reaction products after depolymerization are intended for use in the preparation of polyurethane systems. However, due to the application of excessive castor oil, it appears difficult to determine the characteristic molecular weight. Furthermore, even with precise control of the reaction temperature, a heterogeneous mixture of reaction products is obtained.

[0009] The use of enzymes to degrade polyester polymers was first described in the 1970s. Similar to the use of ionic liquids and castor oil, this biochemical approach was developed to provide an eco-friendly polymer recycling process compared to conventional chemical recycling methods. However, it is quite inefficient for the complete depolymerization of polyesters, and therefore it is not possible to quantitatively recover homogeneous reaction products for reuse.

[0010] Alcohololysis for PET depolymerization was first described in the early 1990s. This method was developed to avoid the drawbacks (pollution problems) of acid and alkali hydrolysis, providing a renewable and more eco-friendly degrader for the polymer. Typically, polyesters are depolymerized with excess alcohol to produce the corresponding acid and the corresponding ester of ethylene glycol. In alcohololysis methods, the reaction with methanol is particularly important due to methanol's low price and availability. Furthermore, ethylene glycol (a diol whose uses are sometimes separately classified as "glycolysis," although it belongs to the alcoholysis category) is primarily used in reactive extrusion to produce low molecular weight oligomers. However, since the crude reaction products consist of a heterogeneous mixture of monomers, oligomers, and polymers, these oligomers must be separated and purified for further processing. Various other useful alcohols, such as pentaerythritol, 1-butanol, 1-pentanol, 1-hexanol, and 2-ethyl-1-hexanol, are described. In techniques for depolymerizing polyesters via alcoholysis, other diols besides ethylene glycol have been described, such as BHET, neopentyl glycol (NPG), tetraethylene glycol (TEEG), polyethylene glycol, polytetramethylene oxide, and terpolymers [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)]. However, in all cases, mixtures of undefined low molecular weight oligomers are typically obtained. Another significant drawback of alcoholysis is the requirement for a catalyst. The most important catalysts are zinc acetate and manganese acetate. Other catalysts include cobalt acetate and lead acetate. The need for such catalysts hinders the widespread application of alcoholysis in the recycling of polyester waste. Aminolysis and ammonialysis have been developed for polyester recovery because the amine group is more reactive than the hydroxyl group of the alcohol used for polyester alcoholysis. However, for alcoholysis, a metal catalyst is still required.

[0011] Finally, alternative chemical recycling of polyester is achieved by controlled depolymerization of polyester through the use of a defined amount of depolymerizing agent to block chain scission (see Geyer et al.).Pulse Express (eXPRESS Polymer Letters) Volume 10, Issue 7 (2016) 559–586. Compared with existing chemical methods (such as alcoholysis), this method produces polyester oligomers with a wider range of well-defined molecular weights. However, this method requires sorted polyester material that is free of contaminants.

[0012] While energy recovery and thermomechanical recovery can lead to the degradation and recycling of materials, chemical recovery requires large quantities of chemicals, and its main drawbacks are high cost, variable end products (which largely depend on the type of waste), the need for (metal) catalysts, and difficulty in removing colorants. Therefore, improved methods, systems, and processes are needed to achieve the recycling of polyester waste.

[0013] Purpose of the invention The object of this invention is to design a method and system that allows for the economical recycling of polyester waste into advanced new polyester. Summary of the Invention

[0014] To achieve the objectives of this invention, a method has been developed for recovering a polyester waste stream by depolymerizing polyester present in the polyester waste stream into a reactive low-reactivity polyester. The method includes: providing the polyester in liquid form in a reactor, and feeding an alcohol into the reactor to cause the polyester to undergo alcoholysis to depolymerize the polyester, while adding a certain amount of water and a certain amount of carboxylic acid (i.e., while depolymerizing the polyester by alcoholysis) to form the reactive low-reactivity polyester. Surprisingly, it was found that in the current alcoholysis process, even in the absence of a catalyst, when water and carboxylic acid are co-fed during alcohol-initiated depolymerization, oligomers with a large number of reactive groups (referred to as "reactive oligoesters," which have groups capable of reacting to form ester bonds) are obtained, even exceeding the expected number based on the corresponding amounts of water and acid used. In other words, there is a synergistic effect between water and carboxylic acid that leads to a highly advantageous process for obtaining oligomers that react in a relatively short time and, if desired, at a relatively low temperature to generate polyesters. This not only saves energy but also prevents or at least minimizes undesirable thermal degradation during the repolymerization of recycled polyesters. The oligomers resulting from the depolymerization reaction appear to be more susceptible to thermal degradation than those made from virgin materials. The reason for this is unclear, but the result is that known alcoholysis and repolymerization methods can ultimately lead to partial degradation of the polymer, manifested as a slight yellowing discoloration, making the material less suitable, or even unsuitable, for high-end applications. This problem can be solved by the current method.

[0015] Adding carboxylic acids during the depolymerization of polyester may seem counterintuitive for an economically viable depolymerization process, but it has been found that this actually promotes the formation of highly reactive oligomers without substantially negatively impacting the depolymerization process. Conversely, the synergistic effect with water promotes the formation of highly desirable reactive oligomers that can be used in rapid repolymerization processes, minimizing undesirable thermal degradation typically seen when repolymerizing pre-polyester products obtained from polyester waste streams.

[0016] Furthermore, the addition of water and carboxylic acid provides the opportunity to fine-tune the relative amounts of reactive groups, making the material more suitable for methods in which native compounds (alcohols, acids, oligomers, etc.) are copolymerized with recycled compounds.

[0017] Methods for depolymerizing polyesters into oligomers via alcoholysis are known from the prior art, such as those described in WO 2022 / 003084 (assigned to Cure Technology BV). Furthermore, for the repolymerization of oligomers, the mixture can be exposed to low pressure in a so-called condensation reactor. However, such condensation processes typically require long reaction times at relatively high temperatures and can therefore lead to thermal degradation. With the present invention, repolymerization can be carried out in a considerably short time, and may even eliminate the need for a pre-condensation step.

[0018] It has been found that the present invention is effective for different types of polyesters because their chemical properties are essentially the same. Correspondingly, the type of alcohol is not essential for the depolymerization process itself, and for obtaining a low-reactivity polyester for repolymerization. Different levels of depolymerization and repolymerization can be achieved (when other factors such as residence time are the same), or the conditions can be changed to achieve a predetermined level of depolymerization or repolymerization, depending on the type of polyester, the type of alcohol, and the amount thereof. This can be controlled, for example, by measuring the viscosity of the (partially depolymerized) polyester mixture, the residence time in various reactors, and other environmental factors such as temperature and pressure. For any type of polyester, reference curves can be prepared in advance at different stages of its depolymerization and repolymerization with a specific alcohol, defining, for example, the relationship between viscosity and polymerization grade, thereby obtaining the desired level of polymerization. However, diols such as ethylene glycol, 1,3-propanediol, butanediol, cyclohexanediol, and neopentyl glycol are commonly used in the art and are also advantageously used in the present invention.

[0019] It should be noted that methods in the art utilize, in part, the methods of this invention. For example, CZ 299244 (assigned to Sirek Milan) discloses a method for combined alcoholysis and hydrolysis of waste PET based on a two-stage decomposition principle to produce terephthalate and ethylene glycol. In known methods, in a first stage, PET waste is degraded by simultaneously running extrusion hydrolysis and glycolysis, and in a second stage, the melt of the resulting oligomer from reactive extrusion of PET exits the first stage in a continuous sequence, and an aqueous solution of alkali metal hydroxide and / or ammonium hydroxide is continuously applied to carry out alkaline hydrolysis in the presence of a catalyst. The resulting product (e.g., pure terephthalic acid) is then used for repolymerization.

[0020] WO 2023281531 (assigned to C. Venkatesh) describes a method for synthesizing hydroxyl and carboxyl functional oligomers and their products from recycled PET via depolymerization of waste polyester in glycerol and / or triglycerides to produce OH-functional oligomers, which are then reacted with an anhydride or polyfunctional acid to yield COOH-functional oligomers. However, the method lacks a combined alcoholysis / hydrolysis step in the presence of a carboxylic acid.

[0021] WO 2023084378 (assigned to SHPP Global Technologies BV) describes a method for converting PET to TPA (terephthalic acid) to obtain purified TPA via catalytic glycolysis (using Zn(OAc)2) at 150°C to 230°C, followed by hydrolysis (using NaOH / water) at 35°C to 100°C, and finally acidification (using HCl). However, the method lacks a combined alcoholysis / hydrolysis step in the presence of a carboxylic acid.

[0022] WO 2023060768 (assigned to NATINDUSTRIAL INNOVATION CENTER OF POLYMER MATERIALS CO LTD) describes a method for the closed-loop recovery of waste polyester to prepare recycled polyester via microwave-assisted glycolysis (in the presence of, for example, NaCl, (Na)₂CO₃, or AC). Although the polymerization process is microwave-assisted polymerization in the presence of polyols, chain extenders, and catalysts, and in the presence of diacids, there is no step involving combined alcoholysis / hydrolysis in the presence of carboxylic acids.

[0023] US 4620032 (assigned to Celanese Corporation) discloses a two-stage depolymerization method based on hydrolysis for repolymerization into higher polyesters. Specifically, in known methods, the polyester is closely mixed with a depolymerizing agent, which is one of the products of complete hydrolysis of the condensation polymer, or water. The depolymerizing agent is mixed with the polyester for a sufficient time to reduce the molecular weight of the polyester by at least 50% in the first stage. The treated lower molecular weight condensation polymer is then subjected to neutral hydrolysis in the second stage to achieve complete hydrolysis and depolymerization into a monomer material that can be used for repolymerization.

[0024] WO 9720886 (assigned to Eastman Chemical Company) discloses a one-stage batch process in which post-consumer or waste polyester is reacted with ethylene glycol to produce monomers or low molecular weight oligomers through the depolymerization of the polyester. The monomers or oligomers (as appropriate) are then purified using one or more steps including filtration, crystallization, and optional adsorbent treatment or evaporation to enable their use in subsequent repolymerization processes to obtain polyester.

[0025] The present invention also relates to a system for recycling polyester waste streams, the system comprising, in a continuous sequence: 1) an extruder, 2) a depolymerization reactor and 3) a finishing polymerisation reactor, wherein the system is configured such that polyester waste is fed from the extruder to the depolymerization reactor, while alcohol, water and carboxylic acid are fed independently to the depolymerization reactor, wherein the extruder, the depolymerization reactor and the finishing polymerisation reactor are operatively coupled such that the polyester waste stream can flow continuously from the inlet of the extruder to the outlet of the finishing polymerisation reactor, and thus undergo subsequent depolymerization and repolymerization processes.

[0026] The present invention also relates to a method for polymerizing a low-reactivity polyester, the method comprising continuously feeding a liquid mixture into a post-polymerization reactor, the liquid mixture comprising the reactive low-reactivity polyester obtainable as described above. It has been found that the reactive low-reactivity polyester is suitable for direct repolymerization in a finisher reactor, eliminating the need for the step of using a conventional condensation reactor prior to feeding the mixture into the finisher.

[0027] This invention also relates to a method for obtaining a polyester polymer (also referred to simply as "polyester"), the method comprising mixing a reactive low-polymer obtained from a polyester waste stream as described above with a virgin pre-polyester material (which may be one or more monomers and / or oligomers, provided they have not previously been used to produce a polymer), and reacting the low-polymer and the virgin pre-polymer to form a polyester polymer. The ratio between the amount of reactive low-polymer obtained from post-consumption polyester via alcoholysis and the amount of virgin pre-polyester material is not critical and depends primarily on the availability and price of various materials on the market. Any ratio between these two materials—between 1:100 and 100:1, particularly between 1:10 and 10:1, such as, for example, 1:1—can yield higher-grade polyesters.

[0028] definition Polyester Polyesters are polymers in which 100 or more monomer units are linked together by ester groups. They are typically polymerized from polyols and polyacids and are primarily used in the manufacture of resins, plastics, and textile fibers. Polyesters are known to be prepared by condensation polymerization, in which monomers providing the "acid component" (including their ester derivatives) react with monomers providing the "hydroxyl component." If desired, polyesters may also contain other linking groups, such as, for example, a proportion of carbonylamino linking groups -C(=O)-NH- (i.e., amide linking groups) or -C(=O)-NR. 2- (tertiary amide linking group). Polyesters used in everyday life can be aliphatic, semi-aromatic, or aromatic. Typical examples are polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polypropylene terephthalate (PTT), polyethylene naphthalate (PEN), polyethylene furanate (PEF), and Vectran (a condensation product of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid). PET (also abbreviated as PETE or the obsolete PETP or PET-P) is the most common thermoplastic polymer resin in the polyester family, and the virgin material is considered one of the most important engineered polymers of the past few decades. It is considered an excellent material for many applications and is used in clothing fibers, liquid and food containers, thermoforming for manufacturing, and in combination with glass fibers for engineered resins. By brand name, it is also known as Terylene, Arnite, Eastapac, Mylar, Lavsan, Dacron, etc. Polyesters can contain up to 50% (w / w) of non-polyester polymer chains (e.g., 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%) while still being referred to as polyester materials. Pre-polyester It is any compound that can react alone or in combination with one or more other compounds to form a polyester. Prepolyesters contain, for example, monomeric alcohols and acids, as well as ester oligomers.

[0029] oligomers It is a short polymer with 5 to 100 repeating monomer units in its main chain. For example, PET oligomers with between 5 and 20 BHET units are any oligomers from pentamers to icosomers.

[0030] Polyester Melting temperature Above this temperature, polyester exhibits liquid properties. Since many polymers do not typically have a very defined melting point, the melting temperature can be the highest temperature over a fairly wide range, within which the polymer slowly becomes "cortical," then "sticky," and finally liquid.

[0031] Polyester wastePost-consumer materials are materials that are used at or after the end of their consumption life (i.e., the time during which a consumer uses them for practical or aesthetic purposes). These post-consumer materials are essentially composed of polyester and up to 10% (e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10) of additives such as fillers (e.g., fibrous materials or particulate matter), stabilizers, colorants, etc. native The material is a pre-consumer material, which is made directly from unused raw materials and therefore has never undergone any other processing besides its production and processing.

[0032] Depolymerization This refers to reducing the molecular weight by breaking down the original polyester molecule into shorter molecules (e.g., into oligomers).

[0033] Post-condensation polymerization reactor (A so-called rotating disc reactor (see, for example, US 3,728,083, assigned to Allied Chemical Corporation) is a reactor in which a large gas-liquid surface area is provided, increasing the interface between the liquid mixture and the gaseous headspace by at least 5 times relative to the horizontal cross-section of the reactor, in order to enable the efficient extraction of volatile substances such as alcohols.)

[0034] continuous The method is a continuous production line method for processing materials without interruption. In such methods, materials (e.g., dry bulk or fluid) are in continuous motion, undergoing chemical reactions and / or mechanical or thermal treatments. Continuous methods contrast with batch methods.

[0035] alcohol Alcohols are hydrocarbons or mixtures of such substances formed when hydrogen atoms in hydrocarbons are replaced by hydroxyl groups. Alcohols can be monohydric, dihydric (i.e., diols), etc.

[0036] Intrinsic viscosity It is a measure of the solute's contribution to the solution viscosity η; see " biology Progress in Biophysics and Molecular Biology (Harding 1997). IV (or iv) can be measured according to DIN / ISO 1628. Typically, a 1% polymer concentration is used and m-cresol is used as the solvent, where IV can be expressed in dL / g (the latter dimension is usually not presented). A practical method for determining intrinsic viscosity is to use an Ubbelohde viscometer. two continuous A phase implies that the corresponding first and second phases proceed sequentially and without interruption. However, this does not preclude one or more additional intermediate method steps from occurring between the two phases.

[0037] mixture It is a composition consisting of two or more different substances combined or blended together. Mixing two compounds does not preclude these compounds from reacting during mixing to form other compounds in the mixture.

[0038] gas-liquid interface It is a surface dimension, therefore in m 2 Units.

[0039] reactor cross-section It is its area, and therefore a measure of its projection onto a horizontal surface.

[0040] Another embodiment of the present invention In a first further embodiment of the method according to the invention, the amounts of water and carboxylic acid are each independently selected from amounts between 0.1 w / w% and 5 w / w% relative to the amount of polyester, such as 0.1 w / w%, 0.2 w / w%, 0.3 w / w%, 0.4 w / w%, 0.5 w / w%, 0.6 w / w%, 0.7 w / w%, 0.8 w / w%, 0.9 w / w%, 1.0 w / w%, 1.1 w / w%, 1.2 w / w%, 1.3 w / w%, 1.4 w / w%, 1.5 w / w%, 1.6 w / w%, 1.7 w / w%, 1.8 w / w%, 1.9 w / w%, 2.0 w / w%, 2. Amounts of 1 w / w%, 2.2 w / w%, 2.3 w / w%, 2.4 w / w%, 2.5 w / w%, 2.6 w / w%, 2.7 w / w%, 2.8 w / w%, 2.9 w / w%, 3.0 w / w%, 3.1 w / w%, 3.2 w / w%, 3.3 w / w%, 3.4 w / w%, 3.5 w / w%, 3.6 w / w%, 3.7 w / w%, 3.8 w / w%, 3.9 w / w%, 4.0 w / w%, 4.1 w / w%, 4.2 w / w%, 4.3 w / w%, 4.4 w / w%, 4.5 w / w%, 4.6 w / w%, 4.7 w / w%, 4.8 w / w%, 4.9 w / w%, and 5.0 w / w%. These amounts have been found to be advantageous for use in this invention. Preferably, the amount of water is between 0.1 w / w% and 2 w / w%, more preferably between 0.2 w / w% and 1 w / w%, and the amount of carboxylic acid is between 0.2 w / w% and 3 w / w%, preferably between 0.5 w / w% and 2 w / w%.

[0041] In another embodiment where the polyester is based on a diol and a diacid, the carboxylic acid is a dicarboxylic acid. Thus, the carboxylic acid is matched with the acid used as the monomer to construct the polyester. Typical dicarboxylic acids used in this invention are adipic acid, succinic acid, terephthalic acid, and naphthalenedicarboxylic acid.

[0042] In another embodiment of the method according to the invention, the method includes a first stage in which the polyester waste is fed into an extruder to melt the polyester, and a subsequent second stage in which the polyester is depolymerized into the low-reactivity polyester. This two-stage setup has been found advantageously suitable for applying the method. The limitation of this embodiment is that depolymerization is completed in the second stage. However, it is not excluded that depolymerization is initiated in the first stage, for example, by initiating alcoholysis or any other depolymerization method.

[0043] In another embodiment of the method, the second stage includes a first sub-stage and a continuous second sub-stage, in which the molten polyester is subjected to alcoholysis, and in the second sub-stage, the partially decomposed polyester is further subjected to alcoholysis, while a certain amount of water and a certain amount of carboxylic acid are added. In yet another embodiment, the first sub-stage is carried out in an extruder, and the second sub-stage is carried out in a continuous stirred tank reactor (CSTR).

[0044] In another embodiment of the method itself (in which the polyester is polyethylene terephthalate (PET)), the reactive low polyester comprising more than 50 w / w, preferably more than 60 w / w, 70 w / w, 80 w / w, or even 90 w / w, is an oligomer comprising 5 to 20 bis(2-hydroxyethyl) terephthalate (BHET) units (i.e. monomer subunits), preferably 6 to 15 (BHET) units, and most preferably 8 to 10 BHET units.

[0045] In another embodiment, the reactive oligoester is fed directly into a post-condensation polymerization reactor (also referred to as a "post-condensation reactor") for repolymerization. As described above, the method of depolymerizing polyester into a liquid mixture containing oligoesters via alcoholysis is known in the art, and it is also described that the mixture can be exposed to low pressure in a so-called condensation reactor for the repolymerization of the oligomers. However, as indicated, a combination of two condensation reactors is typically required. The inventors recognize several disadvantages of the known methods. First, the repolymerization method in the condensation reactor is time-consuming, typically around 8 to 10 hours or longer. This means that the method requires a significant amount of energy and is therefore relatively expensive to implement, given that the mixture must be kept above its melting temperature. However, it is always considered an inherent and unavoidable problem: a large amount of alcohol monomer must be removed to initiate repolymerization, and the method requires time. Importantly, however, the present invention recognizes another major disadvantage, as indicated above: it appears that the oligomers produced by the depolymerization reaction are more susceptible to thermal degradation than oligomers made from virgin materials. The cause is unclear, but the result is that known methods may lead to partial degradation of the polymer, which manifests as a slight yellowing discoloration, making the material less suitable, or even unsuitable, for high-end applications.

[0046] The inventors have now surprisingly discovered that, despite the typically low intrinsic viscosity (iv) of low-molecular-weight polyesters having 5 to 20 (sub)units (i.e., approximately 0.1 dL / g), such mixtures can be repolymerized by feeding them directly into a so-called post-polymerization reactor. A common feature of conventional industrial polycondensation methods is the use of several reactors in series for multiple reaction stages, where the sequential reaction procedure includes (a) the synthesis of monomers, low-molecular-weight oligomers, and low-molecular-weight prepolymers in a conventional polycondensation reactor, and (b) the polymerization of the prepolymers into higher molecular weight polymers in a post-polymerization reactor (typically a rotary reactor; see, for example, US 3,728,083, assigned to United Chemical Dyes, Inc.), where a large gas-liquid surface area (increasing the interface between the liquid mixture and the gas phase headspace by at least 5 times relative to the horizontal cross-section of the reactor) is provided for the extraction of the final alcohol. It is well known that the first stage requires increasing the viscosity (reducing the amount of volatile monomers) to at least 0.3 iv, because the required low pressure cannot otherwise be obtained in the post-polymerization reactor. A gradual increase in viscosity to at least 0.3 is considered necessary for a stable and efficient polymerization method.

[0047] However, it has now been found that even at polymerization levels between 5 and 20 subunits and therefore very low iv (approximately 0.1), the oligomers produced by alcoholysis according to the present invention can be directly fed into such post-condensation reactors. Even more surprisingly, the total time required for volatile matter removal can be kept below 120 minutes. This means that the novel method is more economical, requires significantly less energy, and, moreover, the thermal degradation of these materials, if present, is at a very low level due to the significantly shorter time required to expose the materials to high melting temperatures (above 250°C).

[0048] The reasons why low-viscosity oligomers can be fed directly into the post-polymerization reactor and repolymerized within a relatively short time are not entirely clear. One believed reason is that oligomers are products of depolyolation, such as those known from the aforementioned '084 patent application. Another believed reason is the relatively large number of reactive groups (high carboxyl terminus, i.e., the so-called CEG number), thus ensuring rapid and sufficient repolymerization. Furthermore, the degree of polymerization of the oligomer (i.e., the number of subunits) can be important. With fewer than 5 subunits, the amount of volatiles is believed to be too high to be removed in a single-stage process, let alone in a post-polymerization reactor with a high liquid-gas interface. With more than 20 subunits, repolymerization in the post-polymerization reactor is possible, but the oligomer viscosity is too high for prepolymerization purification. This means that skipping this step when such prepolymerization purification is required may result in a lower quality product. However, if purification is not required, mixtures with a high number (e.g., up to 100) of subunits can be used for direct repolymerization in a post-condensation unit.

[0049] In the post-polymerization unit, an iv of up to 0.65 can be achieved within a relatively short time frame, making the material ideal for direct use or as a starting material for even higher-grade polyesters. The achieved iv is a matter of time. Furthermore, the design of the post-polymerization unit (pump type and capacity, outlet diameter, etc.) allows for the processing of the maximum iv that can still be achieved.

[0050] It should be noted that the post-copolymerizer consists of two or more sub-post-copolymerizers connected in series, rather than a large-volume post-copolymerizer, provided that each of these sub-post-copolymerizers has means for increasing the interface between the liquid mixture and the gaseous headspace by at least 5 times relative to the horizontal cross-section of the reactor (i.e., the corresponding sub-post-copolymerizer).

[0051] Preferably, when repolymerization is carried out directly in the post-polymerization unit, the reactive low-polymer comprises 5 to 20 subunits, more preferably 5 to 14 subunits, such as 8 to 10 subunits. Furthermore, it is preferred that the reactive low-polymer is repolymerized to a polyester having an intrinsic viscosity of at least 0.4 dL / g, preferably at least 0.5 dL / g, or even 0.6 dL / g, within less than 100 minutes, preferably less than 80 minutes, and most preferably less than 60 minutes.

[0052] Preferably, in the post-polymerization reactor, the pressure in the headspace is maintained below 20 mbar, and even more preferably below 10 mbar. This further reduces the time required for repolymerization. Preferably, the pressure in the headspace is maintained between 1 mbar and 3 mbar, such as 2 mbar.

[0053] In another embodiment of the method according to the invention, the post-polymerization reactor has means for increasing the interface between the liquid mixture and the gaseous headspace by at least 10 times, preferably at least 20 to 100 times, relative to the cross-section of the reactor. The increased interface between the liquid mixture and the gaseous headspace means that repolymerization can occur at a higher rate. The disadvantage is that the reactor typically has a more complex construction and requires a higher pump capacity. This means the method becomes more expensive. However, this is compensated for by a higher quality final product, which translates to a higher price.

[0054] The foregoing does not preclude any variations of the method or additional method steps. For example, additional (pre)purification / filtration steps may be added. Furthermore, the invention is not limited to a particular type of filter, although replaceable mesh filters are believed to be ideally suited to the claimed continuous method, as the filters may need to be replaced every few hours depending on the amount of particulate matter present in the polyester waste. Moreover, when applying filters, it is likely to be a cascade of two, three, or more separate continuous filters with decreasing mesh sizes to withstand the pressure differential across the filters. In addition, to ensure complete removal of all colorants and dyes, the molten material may advantageously be pumped through a bed filled with (activated) carbon particles, SiO2 particles or powder, or any other small molecule absorbent material.

[0055] It is also anticipated that copolymerization will be introduced into the repolymerization process. For example, a slurry of pure diacid and pure glycol can be prepared, and / or other monomers can be formed, which can be added to a post-polymerization unit containing a purified oligomer mixture. When the diacid and / or glycol are different from the monomers present in the polyester waste, copolymers are generated. Such monomers can be, for example, bio-based to further reduce the CO2 footprint, or can be, for example, isophthalic acid, succinic acid, neopentyl glycol to obtain other product properties of the final polyester to be produced, suitable for the intended application. Typically, the comonomers are prepolymerized before being fed into the post-polymerization unit and mixed with the recycled oligomers.

[0056] Any of the additional embodiments described above can also be embodied in the system of the present invention. The invention will now be further illustrated using the following non-limiting drawings and examples.

[0057] Examples Attached Figure Description Figure 1 An overview of depolymerization and repolymerization methods as known from the prior art is schematically depicted.

[0058] Figure 2 A typical repolymerization configuration for industrial applications is schematically depicted.

[0059] Figure 3 The schematic depiction illustrates the depolymerization and repolymerization configuration of polyester waste streams using existing technologies.

[0060] Figure 4 The depolymerization and repolymerization configuration according to the invention is schematically depicted.

[0061] Example 1 describes an experiment on the depolymerization and repolymerization of post-consumer PET sheets.

[0062] Example 2 describes a solid-state post-polymerization experiment.

[0063] Example 3 provides various examples of feeding oligomers directly into the post-polymerization unit. (The materials used are not manufactured according to the invention, but rather are materials with lower CEG values.) Detailed Implementation Figure 1 Figure 1 An outline of a depolymerization and repolymerization method as known from WO 2022 / 003084 is schematically depicted, which can itself be used to apply the method of the present invention with some modifications. In this example, the polymer PET is depolymerized and repolymerized in a continuous process through steps 1 to 8. Step 9 is an additional repolymerization step (including solid-state polymerization) to achieve a desired IV much higher than 0.6 for use in high-end PET applications.

[0064] PET is formed by reacting monoethylene glycol and terephthalic acid according to the following reaction (non-stoichiometric formula): MEG + PTA BHET + H2O BHET can then be polymerized according to the following reaction, in which MEG is formed again: BHET [PET]x + MEG The alcoholysis method is based on the fact that the above reaction is an equilibrium reaction and can shift to either side depending on the reaction conditions. Starting with PET, by adding MEG to the polyester melt, the equilibrium shifts to the left, resulting in shorter polymer chains, ultimately oligomers (less than 100 repeating BHET units, particularly less than 50, 40, 30, 20, or even 10 units) and reduced viscosity. By adding water to these oligomers, the BHET will decompose according to the above reaction. However, this is undesirable for the present invention, which aims to achieve oligomers rather than monomers.

[0065] For repolymerization starting from BHET, short chains react with each other to re-form polyester by removing MEG, for example, through the use of vacuum or nitrogen. The viscosity of the material is controlled by controlling the depolymerization rate, thereby controlling the oligomer length. In principle, this chemical property is the same for any industrial polymerization method that yields polyester or corresponding oligomers.

[0066] Now for reference Figure 1In step 1, the polyester waste (in this embodiment comprising (pure) PET carpet scraps and PET bottle flakes) is dried to a moisture content of 50 ppm (0.005 w%). This is to ensure that the total water content is controlled at a known level, thereby providing a known amount of CEG for the low-reactivity polyester. Then, in step 2, the dried waste stream is fed into a conical co-rotating twin-screw extruder. Due to the conical shape of the extruder, the opening for material feeding is larger than that of a conventional twin-screw extruder, making feeding easier and resulting in less shear due to gentler natural compression, thus causing less thermal damage to the polymer. Thermal degradation produces undesirable side reactions and the formation of end groups, degrading the quality of the final product. The extruder is operated at 280°C to allow the polyester to fully melt. In this embodiment, an injection point for MEG application (as indicated by arrow 50) is set near the end of the conical twin-screw extruder (at 10% of its length) to obtain the first step of depolymerization, thereby reducing viscosity. For this purpose, approximately 0.5% MEG (w / w) is applied. The reduction in IV also helps minimize the pressure difference in the first filtration step 3, allowing filtration with an 80-micron mesh size. The filter also acts as a static mixer to homogenize the mixture and distribute the added ethylene glycol with the molten polymer to react completely with the shorter polymer chains, thus achieving an equilibrium molecular weight distribution (dispersion grade of approximately 2). Process parameters are selected such that the MEG reacts (almost) completely and there is no longer (almost) free MEG present.

[0067] In step 4, the partially depolymerized and filtered material is fed into a single-screw extruder. Screw design (see WO 2022 / 003084) Figure 2 The aim is to maximize the percentage of MEG that can be applied (to prevent the melt from becoming heterogeneous). This maximum value is increased by adjusting common process parameters such as screw speed, pressure increase, etc. Typically, about 2% to 4% MEG is applied in this extruder (indicated by arrow 50'). The viscosity of the melt is measured at the end of the extruder. The viscosity level is controlled by an automatic control loop (…). Figure 1 (Not specified) The automatic control loop controls the level of MEG applied in the single-screw extruder. This automatic control loop produces a consistent viscosity, typically between 0.1 and 0.2 IV, independent of the starting material's IV. Due to the inherent transesterification reaction occurring in the extruder, polydispersity can be kept very low, preferably around 2 to 3, which depends primarily on the residence time in the extruder (this can be adjusted in this method by controlling the initial feed and extruder speed).

[0068] The depolymerized material is then filtered a second time in step 5. Since IV is approximately 0.15, the filter size can be reduced compared to the first filter, preferably to 40 micrometers, without the pressure differential across the filter becoming excessive.

[0069] In step 6, material with an IV of about 0.15 (0.1 to 0.2) is continuously added to the CSTR. In this CSTR, MEG (indicated by arrow 50") is also added to further depolymerize the material to the desired viscosity / oligomer length. Due to the fact that the material already has a very low (controlled) viscosity upon entry, the viscosity difference with the added MEG is large enough to make homogeneous mixing crucial. If desired, 4% to 6% MEG can be easily homogenized. Simultaneously, water (indicated by arrow 61) and a certain amount of TPA (indicated by arrow 62) are fed into the CSTR to ensure a high level of CEG obtained (see Example 1 below). Although the respective streams of water, MEG, and TPA can be separate streams, it is also anticipated that the compounds are added as one or more mixtures. The residence time in the CSTR is long enough (typically 25 to 45 minutes) to depolymerize the material to the desired oligomer length, but also sufficient time for transesterification to achieve a polydispersity of 2. Viscosity is measured at the end of the reactor, and the addition of MEG in the reactor is controlled by an automated control loop. This results in an extremely stable continuous method, almost independent of the type of starting material (IV).

[0070] In the CSTR, additional decolorization is performed by adding activated carbon, indicated by arrow 60. The activated carbon can be pre-selected to obtain optimal performance for adsorbing colorants present in the polyester waste. Following the CSTR, a low-viscosity oligomer / activated carbon mixture is pumped through a three-step microfiltration (20 / 10 / 5 microns) step 7 to remove colorant-loaded carbon particles from the oligomers. A set of three filters connected in parallel is installed so that in the event of excessive pressure differential across the filters, the melt can be pumped through one set in parallel, while simultaneously cleaning the first set of filters.

[0071] After filtration, the melt, while still at a high temperature of approximately 250°C with an IV of approximately 0.1 (the average number of subunits in the reactive oligomer mixture is approximately 10), is pumped to a post-polymerization reactor (step 30). This reactor operates under a vacuum of 2 mbar at a temperature of approximately 260°C to remove MEG, resulting in a rightward shift of the BHET / PET equilibrium, leading to the formation of a PET polymer in a short time. In the post-polymerization reactor, polyesters with an IV between 0.4 and 0.6 or higher are obtained. The polymer is removed from the reactor and pumped through a perforated template to produce polymer strips. These strips are cooled and cut into amorphous granules.

[0072] Amorphous particles can undergo an offline crystallization process, which involves subjecting the particles to temperatures between 130°C and 180°C, leading to crystallization. Partially crystallized particles undergo a solid-state polymerization method, in which the polyester is heated to a high temperature below its melting point while being subjected to a vacuum or inert gas. This initiates a solid-state additional repolymerization to achieve an IV much higher than 0.6. The resulting IV can be adjusted by processing parameters to match the IV required for the intended application, typically having any value between 0.65 and 1.0.

[0073] Figure 2 Figure 2 A typical repolymerization configuration known from the prior art for industrial applications is schematically depicted. This setup is a so-called two-reactor polycondensation technology, in which polymerization takes place in two consecutive polymerization reactors. The first reactor, indicated by reference numeral 8, is a standard polycondensation reactor. The second reactor, indicated by reference numeral 30, is a so-called post-polymerization reactor, which has means for increasing the liquid-gas interface in the reactor by at least five times relative to the reactor's footprint.

[0074] Reactor 8 is identical to the reactors known from WO 2022 / 003084. For laboratory testing purposes, the use of such a standard condensation reactor alone is usually sufficient, as time is not a major concern. However, for industrial applications, a post-polymerization reactor is always followed by the standard condensation reactor to allow for faster repolymerization. In this typical setup, an oligomer mixture with a typical IV value of approximately 0.1 is fed (22) into reactor 8, forming a gas-liquid interface 23 in the middle of the reactor. The reactor is equipped with a mixing device 21 driven by motor 20. The alcohol is pumped out via device 24. In this way, the oligomer mixture can be polymerized to a higher grade (unit number 25 or higher) to achieve an IV of at least 0.2 to 0.3, thus enabling it to be fed (25) into the post-polymerization reactor 30 to form the gas-liquid interface 230. In this example, the post-polymerization reactor is equipped with a rotary mixer 210 driven by motor 200. Because the gas-liquid interface is established in the center of the rotating disk, the rotation of the disk and the subsequent lifting of the liquid film (on both sides of the disk) into the open gas space result in a significant increase in the footprint of the gas-liquid interface relative to the post-polymerization reactor 30, in this case by approximately 10 times (a factor 10). This allows for more rapid removal of alcohol (240), resulting in a polyester stream 250 with an IV of approximately 0.6 in a relatively short time.

[0075] This two-stage setup is typically used because mixtures with a low IV of approximately 0.1 are known to be unsuitable for direct feeding into the post-polymerization unit: most of the alcohol must be removed before entering the post-polymerization unit, otherwise the low pressure cannot be achieved in the post-polymerization unit, and therefore polycondensation will hardly occur. In many cases, even in industry, a three-stage setup (adding an additional condensation reactor 8 before the post-polymerization unit 30) is used to gradually reduce the amount of alcohol and then gradually increase the IV until the mixture is suitable for feeding into the final stage of the post-polymerization unit.

[0076] Figure 3 Figure 3 A schematic depiction of a depolymerization and repolymerization configuration for a polyester waste stream using existing technology is provided. In this setup, a depolymerization technique as known from WO 2022 / 003084 is employed. Specifically, as described in the Examples section of WO 2022 / 003084 (which is incorporated herein by reference), the waste stream is fed into a first extruder 2, then into a second extruder 4, and finally into a CSTR 6. This corresponds to... Figure 1 Steps 1 to 6 are then performed. The resulting oligomer mixture with an IV of approximately 0.1 is then fed into condensation reactor 8. The remainder of the method is... Figure 2 The same as described.

[0077] Figure 4 Figure 4 The depolymerization and repolymerization configuration according to the present invention is schematically depicted. In principle, this setup is consistent with... Figure 3 The setup is the same, although the oligomer mixture from CSTR 6 is fed directly into the post-polymerization unit 30. The large number of reactive groups enables very rapid repolymerization, which is advantageous not only economically but also for achieving high-quality polyesters.

[0078] Example 1 Example 1 describes an experiment involving the depolymerization and repolymerization of post-consumer PET sheets. For example... Figure 1 The described experimental setup includes a laboratory-type post-polymerization unit capable of handling liquids with IV values ​​up to 0.3 to 0.35. In this embodiment, via the use of, Figure 1The depolymerization-repolymerization experiments conducted in the described experimental setup demonstrate the effects of adding water, TPA, and combinations thereof on the final reactive properties of the depolymerized polyester. Melting and depolymerization of the polyester material, followed by repolymerization of the liquid depolymerized polyester material, were performed at a throughput of approximately 30 kg / h under the conditions indicated below. For each experiment, the depolymerization method described in WO 2022 / 003084 was essentially used to form an intermediate liquid mixture of low-polymer with an average degree of polymerization of 15 to 20 (15 to 20 BHET subunits per molecule) prior to repolymerization in the post-polymerizer. In the first extruder, the temperature was maintained between 270°C and 280°C in each case, without the addition of MEG. In each case, 2.4% of MEG (fiber grade, purchased from Vivochem) was added to the second extruder at a temperature of approximately 265°C to 270°C. The amount of MEG added is indicated in the table below in the CSTR. Water (deionized) and terephthalic acid (TPA, purchased from Indorama Ventures) were added to the CSTR as mixtures in ethylene glycol, respectively, and calculated relative to the total amount of polyester material. The filler level in the CSTR was maintained at 40% to 50%, ensuring that the partially depolymerized polyester remained at an average temperature of 260°C to 270°C for 30 minutes. The temperature in the post-condensation unit was maintained at approximately 270°C. PET sheets (light-colored MOPET A, purchased from Morssinkhof Plastics) were used as is and therefore not pre-dried or prepared. A portion of the obtained polyester material was post-condensed overnight (approximately 16 hours) at 220°C under vacuum (<55 mbar) in an in-house laboratory-scale post-condensation setup to evaluate the effect of carboxyl end groups on the post-condensation rate, expressed as (ΔdL / g) / h (see Example 2).

[0079] The experiments are indicated in Table 1 below (percentages are weight percentages relative to the amount of polyester; iV is in dL / g and CEG is in meq / kg; nd = undetermined). Experiment 1 is a comparative experiment in which no water and TPA were added to the partially depolymerized PET sheets. The high carboxyl terminus number was approximately 20 to 25 meq / kg. Small amounts of water (see Experiments 2 and 3) did not significantly change the CEG amount, which remained in the range of 20 to 25 meq / kg.

[0080] Table 1 Post-consumer PET flake depolymerization and repolymerization

[0081] When 0.5 wt% water was added, the carboxyl end groups of the obtained polymer increased to over 40 meq / kg (Experiments 4 and 5). The addition of 2.0 wt% TPA resulted in a considerable increase in CEG quantity, see Experiments 6 and 7. Based on these results, it was estimated that approximately the same CEG level, around 45 meq / kg, would be obtained when combining 0.4 wt% water and 1.7 wt% TPA. However, it was surprisingly found that in this case using post-consumer PET flakes, a significantly higher CEG quantity was obtained, approximately 65 to 70 meq / kg, while maintaining a solution viscosity of approximately (ca.) 0.30 dL / g (Experiments 8 and 9). Furthermore, the L*a*b values ​​appeared to be at the low end, indicating very low thermal degradation.

[0082] Example 2 Example 2 describes a solid-state post-polymerization experiment. In this example, the effect of carboxyl end groups of some polyester materials, such as those obtained from virgin PET pellets and PET sheets, on the post-condensation rate (SSP rate) in the solid state was evaluated.

[0083] When using virgin PET and without adding water to the depolymerization process, the CEG quantity obtained was 23 meq / kg. The solid-state polymerization rate (SSP) was 0.020 (ΔdL / g) / h. By adding 0.75% water, the CEG quantity increased to 68, and this higher value resulted in a significant increase in the SSP rate to 0.036 (ΔdL / g) / h. This shows that increasing the CEG quantity has a positive impact on the SSP rate.

[0084] The same results were found for post-consumer PET. Here, the addition of TPA alone (2.0 w%) resulted in a CEG quantity of 52 meq / kg, consistent with that shown above in Example 1, resulting in an SSP rate of 0.027 (ΔdL / g) / h, compared to 0.020 (ΔdL / g) / h in the control group.

[0085] Experiments show that by adding water, TPA, or a combination thereof to (partially) depolymerized polyester materials, carboxyl end groups can be adjusted, while obtaining polymer materials with comparable solution viscosities. The increase in carboxyl end groups further demonstrates a significant impact on the repolymerization process, such as the post-condensation rate to achieve the desired solution viscosity.

[0086] Example 3 Example 3 provides various examples of directly feeding PET oligomers into the post-polymerization unit. As the reactive oligomers used in this example were not prepared using the water and TPA combination addition method according to the invention, it is therefore believed that these reactive oligomers have a lower CEG count than those shown in Experiments 8 and 9 of Example 1 above. Therefore, this example merely illustrates that a mixture of oligomers with low intrinsic viscosity obtained by depolymerizing post-consumer polyesters via alcoholysis can be directly fed into the post-polymerization unit. Reactive oligomers with high CEG counts that can be obtained using the method of the invention can even reduce the residence time in the post-polymerization unit, as polymerization can proceed at even higher reaction rates.

[0087] In this embodiment, based on such Figure 3 The methods and uses of the prior art described Figure 4 A direct comparison was made between the setup methods (i.e., where a mixture of oligomers resulting from the depolymerization of polyester waste streams is fed directly into a post-polymerizer). For this comparison, a laboratory-type post-polymerizer capable of handling liquids with IV values ​​as high as 0.3 to 0.35 was used. It was estimated that approximately twice the time would be required to achieve an IV of about 0.6 in an industrial post-polymerizer. However, this additional time expectation is fully compensated by the fact that pressures as low as 1 to 2 mbar are available in industrial post-polymerizers, compared to approximately 10 mbar in laboratory settings. This means that the expected time required to achieve an IV of about 0.3 in a laboratory post-polymerizer is the same as the time required to achieve an IV of about 0.6 in an industrial post-polymerizer.

[0088] For the comparative experiments, a polyester material composed of PET granules (RamaPET N180; IV 0.8 dL / g) was melted and depolymerized with monoethylene glycol into a low-polyester liquid mixture with an average polymerization grade between 5 and 20 (5 to 20 BHET subunits per molecule), at a throughput of approximately 30 kg / hr. Since the CSTR fill level was maintained at approximately (ca.) 48%, the average residence time of the partially depolymerized polyester material in the CSTR was 30 minutes. In both the depolymerization and repolymerization tests, virgin polyester material was used as is and therefore not pre-dried.

[0089] In the first experiment, the resulting oligomers were in Figure 2 The method is then aggregated in the settings, therefore it is similar to... Figure 3 The methods shown are consistent. For example... Figure 4As shown, in the second series of experiments, the resulting oligomers were repolymerized by directly feeding the oligomer mixture into the post-polymerization unit. In this latter series, the amount of MEG used, as in the depolymerization method, was varied to obtain oligomers with different degrees of polymerization. In this regard, it should be noted that using approximately 10% MEG results in x (degree of polymerization = number of subunits in the oligomer) of approximately 8 to 10; 7.5% MEG results in x of approximately 11 to 12; 4.5% to 6% MEG results in x of approximately 15 to 16; and 2.5% MEG results in x of approximately 20.

[0090] The melting and depolymerization of the polyester material, and the subsequent repolymerization of the liquid depolymerized polyester material, were performed under the conditions depicted in Table 1. For each of these experiments, the depolymerization method as described in WO 2022 / 003084 was used. In the first extruder, the temperature was maintained at approximately 288°C in each case, without the addition of MEG. In each case, 2.4% MEG was added to the second extruder, and the temperature was maintained at approximately 268°C. In the CSTR, the amount of MEG added was indicated in the table below. The temperature in the decondensation reactor was approximately 258°C. In the post-condensation reactor, the temperature was maintained at approximately 271°C.

[0091] Table 2 Depolymerization and Repolymerization Experiments

[0092] In Experiment 1, for Extruder II and CSTR, partial depolymerization of the molten virgin polyester material proceeded smoothly within a timeframe of approximately 20 to 25 minutes in the presence of MEG relative to PET at a total amount of 10 w / w. Intermediate sampling prior to repolymerization showed the formation of a white depolymerized polyester material with a degree of polymerization (DP) of approximately 8 to 10 (or a molecular weight Mw of 1500 to 1950 Daltons). The liquid depolymerized polyester oligomers... Figure 2 Repolymerization was carried out in the specified setting, and after approximately 9 hours of polymerization, a light yellow liquid of recycled polyester material with an intrinsic viscosity IV of 0.31 dL / g was obtained (Lab value above the threshold). The yellow coloration indicates significant thermal degradation.

[0093] In Experiments 2 through 16, various continuous depolymerization-repolymerization experiments were conducted, omitting the standard condensation reactor and directly feeding the liquid oligomer mixture into the post-polymerization unit, thus eliminating the contribution of the prepolymerization reactor. The resulting liquid recycled polyester material was cooled in a water bath and cut into solid polyester particles. In these experiments, in addition to assessing the total time required to reach the same degree of polymerization as achieved in the first experiment, the effects of the amount of MEG during the depolymerization step and the residence time in the post-polymerization unit on the physical properties of the obtained polymer, expressed as intrinsic viscosity and color value, were investigated. The conditions and settings of the individual reactors in the continuous depolymerization-repolymerization production line, along with the relevant results for intrinsic viscosity (IV) and color value (L*a*b*), are listed in the tables.

[0094] Experiments 2 and 3 in the table show that in the CSTR, the amount of MEG was reduced to 5.1 w / w%, and subsequently, the total amount of MEG was reduced to 7.5 w / w%. A residence time of approximately 1 hour in the post-polymerization reactor yielded recycled polyester material from the liquid polyester oligomers. This 1-hour residence time was significantly shorter than the 5 hours in Experiment 1, except that the prepolymerization reactor was omitted. Polyester particles with an intrinsic viscosity of approximately 0.3 dL / g were obtained from the recycled polyester material (at the viscosity level of recycled polyester material obtained in a repolymerization method including a prepolymerization reactor and a post-polymerization reactor; Experiment 1), but the time was significantly reduced and the coloration was significantly reduced, indicating less thermal degradation.

[0095] The residence time in the post-polymerizer was reduced to 0.5 hours (see Experiments 4 to 8), while maintaining a total MEG amount of 7.5 w / w% for partial depolymerization in Extruder II and CSTR to generate polyester oligomers, resulting in recycled polyester pellets with a higher intrinsic viscosity than those in Experiments 2 and 3. The reduced residence time in the post-polymerizer improved the coloration of the recycled polyester pellets, as indicated by a lower b* value of approximately 5.

[0096] In experiments 9 and 10-14, the amount of MEG in the CSTR was reduced to 3.4 w / w% and 1.7 w / w, respectively, thereby reducing the total amount of MEG used in the partial depolymerization reaction. 5.8 w / w%. These polyester oligomers were repolymerized in a post-polymerization unit for 1 hour and then granulated to obtain recycled polyester granules with an intrinsic viscosity (varying around 0.33 dL / g) slightly higher than the intrinsic viscosity in previous experiments, and also with a higher b* value.

[0097] In Experiments 15 and 16, increasing the residence time of the repolymerization method to 2 hours was investigated. Furthermore, the amount of MEG in the CSTR was reduced to 0 w / w%. Repolymerization of depolymerized polyester materials with DPs of 20 to 25 after 2 hours showed a further increase in the intrinsic viscosity of the obtained recycled polyester particles to 0.35 dL / g. The polyester particles appeared more yellow than those in Experiments 4 to 9, as confirmed by the higher b* value of approximately 9.

[0098] These experiments demonstrate that faster repolymerization can be achieved when only a post-polymerizer is used in the repolymerization of polyester oligomers. The recovered polyester particles exhibit comparable viscosity with significantly shorter residence times. The color values ​​of the recovered polyester particles indicate that the shorter the residence time of the polyester oligomers in the post-polymerizer, the less thermal degradation occurs.

Claims

1. A method for recovering a polyester waste stream by depolymerizing polyester present in the stream into a less reactive polyester, the method comprising: The polyester is provided in liquid form in a reactor, and an alcohol is fed into the reactor to cause the polyester to undergo alcoholysis to depolymerize the polyester, while a certain amount of water and a certain amount of carboxylic acid are added to form the low-reactivity polyester.

2. The method according to claim 1, characterized in that, The specified amount of water and the specified amount of carboxylic acid are each independently selected from amounts between 0.1 w / w% and 5 w / w% relative to the polyester.

3. The method according to claim 1 or 2, characterized in that, The specified amount of water is between 0.1 w / w% and 2 w / w%, preferably between 0.2 w / w% and 1 w / w%, and the specified amount of carboxylic acid is between 0.2 w / w% and 3 w / w%, preferably between 0.5 w / w% and 2 w / w%.

4. The method according to any one of the preceding claims, wherein the polyester is based on a diol and a diacid, characterized in that, The carboxylic acid is a dicarboxylic acid.

5. The method according to any one of the preceding claims, characterized in that, The method includes a first stage in which the polyester waste is fed into an extruder to melt the polyester, and a continuous second stage in which the polyester is depolymerized into the reactive low-polymer.

6. The method according to claim 5, characterized in that, The second stage includes a first sub-stage and a continuous second sub-stage, in which the molten polyester is subjected to alcoholysis, and in the second sub-stage, the partially decomposed polyester is further subjected to alcoholysis, while a certain amount of water and a certain amount of carboxylic acid are added.

7. The method according to claim 6, characterized in that, The first sub-stage is carried out in an extruder, and the second sub-stage is carried out in a continuous stirred tank reactor (CSTR).

8. The method according to any one of the preceding claims, wherein the polyester is polyethylene terephthalate (PET), characterized in that, The reactive oligomers comprising more than 50 w / w, preferably more than 60 w / w, 70 w / w, 80 w / w, or even 90 w / w, are oligomers comprising 5 to 20 bis(2-hydroxyethyl) terephthalate (BHET) units, preferably 6 to 15 (BHET) units, and most preferably 8 to 10 BHET units.

9. The method according to any one of the preceding claims, characterized in that, The reactive low-reactivity polyester is fed directly into a post-condensation polymerization reactor for repolymerization.

10. The method according to claim 9, characterized in that, The reactive low-reactivity polyester comprises 5 to 20 subunits, preferably 5 to 14 subunits, such as 8 to 10 subunits.

11. The method according to any one of claims 9 and 10, characterized in that, The low-reactivity polyester is repolymerized in less than 100 minutes, preferably less than 80 minutes, most preferably less than 60 minutes, to a polyester having an intrinsic viscosity of at least 0.4 dL / g, preferably at least 0.5 dL / g, or even 0.6 dL / g.

12. A method for polymerizing low-polymers, comprising feeding a liquid mixture in a continuous manner into a post-condensation polymerization reactor, the liquid mixture comprising the reactive low-polymer obtainable by the method according to any one of claims 1 to 8.

13. A method for obtaining a polyester polymer, the method comprising mixing the reactive low-polyester obtained from a polyester waste stream by the method according to any one of claims 1 to 8 with a virgin pre-polyester material, and reacting the low-polyester and the virgin pre-polyester to form the polyester polymer.

14. A system for recycling polyester waste streams, the system comprising, in a continuous sequence: 1) an extruder, 2) a depolymerization reactor and 3) a post-polymerization reactor, wherein the system is configured such that polyester waste is fed from the extruder to the depolymerization reactor, while alcohol, water and carboxylic acid are fed independently to the depolymerization reactor, wherein the extruder, the depolymerization reactor and the post-polymerization reactor are operatively connected such that the polyester waste stream can flow continuously from the inlet of the extruder to the outlet of the post-polymerization reactor.

15. The system according to claim 14, characterized in that, The depolymerization reactor includes a first sub-reactor and a second sub-reactor, wherein at least a portion of the alcohol is fed to the first sub-reactor, and the remaining portion of the alcohol, along with the water and the carboxylic acid, is fed to the second sub-reactor.

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