Apparatus and method for depolymerizing polycondensation polymers

Abstract

1. A method for depolymerizing a condensation polymer into its monomeric components or derivatives thereof, the method comprising the steps of: a) providing a solid starting material containing at least one condensation polymer to be depolymerized; b) providing a depolymerization agent for the condensation polymer, the depolymerization agent being a substantially anhydrous solid; c) providing a polyhydric alcohol having at least three hydroxyl groups per molecule; d) adding the starting material, the depolymerization agent, and the polyhydric alcohol to a reaction vessel to obtain a reaction mixture; e) depolymerizing the condensation polymer by kneading and / or mixing and / or conveying and / or recycling the reaction mixture in the reaction vessel; and f) separating at least one of the monomers of the condensation polymer to be depolymerized or derivatives of the monomer from the depolymerization agent and the polyhydric alcohol during and / or after depolymerization step e).

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for depolymerizing polycondensation polymers into their monomeric components or derivatives thereof. The present invention also relates to the use of the apparatus disclosed herein for carrying out the method disclosed herein. [Background technology]

[0002] Global plastic production increased from 2 million tons in 1950 to over 390 million tons in 2021, representing a market growth rate of over 5% per year. Projections suggest that plastic production could increase to over 1.2 billion tons by 2050. At the same time, there is a growing demand to reduce greenhouse gas emissions, reduce the accumulation of plastic in nature, and move toward a more sustainable circular economy. Due to the large amount of carbon present in synthetic polymers, reusing and recycling polymers whenever possible supports the goal of reducing reliance on finite fossil resources and reducing the accumulation of plastic in nature. The class of polycondensation polymers includes some of the most widely used polymers today, such as adhesives, coatings, engineering plastics, fibers, fabrics, films, beverage bottles, food containers, automotive parts, and many high-performance polymers.

[0003] Recycling of synthetic polymers can be achieved, for example, by mechanical crushing and molding into new products. However, this cannot continue indefinitely because the quality of the product declines with each recycling cycle due to either degradation or the accumulation of contaminants. Furthermore, mechanically recycled plastics are often not approved for certain applications, such as food packaging. Furthermore, synthetic polymers are often used in combination with additional polymers and / or non-polymeric materials. Such composite materials, which contain a mixture of materials, are inherently difficult to recycle because it is difficult to separate the different materials from each other. This currently imposes a limit on the percentage of synthetic polymers that can be recycled, and as a result, composites are typically only thermally recycled on a large scale or are filled in landfills. However, this means that the material is lost from the material cycle.

[0004] To increase resource reuse and avoid value loss, return to monomer recycling allows the use of plastics that are not or no longer suitable for mechanical recycling. Depolymerization of synthetic polymers into their constituent monomer building blocks allows for their repolymerization into higher-value products.

[0005] Several methods for recovering constituent monomer building blocks, i.e., monomer components, from polycondensate-containing waste are known in the prior art. For example, European Patent Application Publication No. 0875504A1 discloses a method for depolymerizing polyamides into their monomer components in the presence of water using alkali or alkaline earth metal compounds at a pressure of 0.2 to 2.0 MPa. International Patent Application Publication No. WO2021 / 180432A1 describes a method for recycling waste essentially containing polyalkylene terephthalates and further containing polycondensates, such as polyamide 6.6, in a continuous process by combining acidic and alkaline depolymerization. Alkaline depolymerization is carried out using NaOH, water, and alkylene glycol formed as a product of polyalkylene terephthalate depolymerization. Acidic depolymerization of polyamide 6.6 is carried out in a boiling aqueous hydrochloric acid solution. However, these processes operate either at high pressure and / or high temperature and require the addition of large amounts of water, making them quite resource-intensive and uneconomical.

[0006] WO 2016 / 101938 A1 discloses a method for producing secondary polyols by recycling secondary raw materials containing waste polyisocyanurate (PIR) foam, in which crushed waste PIR foam is first blended with a polyester-based polymer, and then the mixture is mixed with a polyester-based polymer having a chain length of C6-C8 as a catalyst. 20The mixed-catalyst depolymerization process uses alkali metal carboxylates and / or organic superbases to subject the polyester to controlled glycerolysis in a microwave field. The mixed-catalyst depolymerization process proceeds in two stages: in the first stage, depolymerization of the polyester occurs in glycerol; in the second stage, depolymerization of the PIR form occurs in a mixture of residual glycerol, oligoesters, and polyols formed in the first stage. Due to the mixing of the polyol units of the polyester with the PIR, the resulting secondary polyol cannot be considered a monomer for either of these condensation polymers. Furthermore, this process requires relatively high temperatures, ranging from 180°C to 300°C. The catalyst used in this process is also relatively expensive, especially compared to the depolymerization agents disclosed herein. Summary of the Invention [Problem to be solved by the invention]

[0007] It is therefore an object of the present invention to remedy these and other shortcomings of the state of the art, in particular to provide a method and apparatus for the depolymerization of polycondensation polymers, which method and apparatus require less solvent, preferably no solvent, and are more efficient and cost-effective.

[0008] This object is achieved by a method for depolymerizing condensation polymers into their monomeric components or derivatives thereof, an apparatus for depolymerizing condensation polymers, and the use of such an apparatus for carrying out such a method according to the independent claims. Advantageous embodiments are the subject of the dependent claims. [Means for solving the problem]

[0009] This object is achieved in particular by a method for depolymerizing a condensation polymer into its monomeric components or derivatives thereof, which method comprises the following steps a) to f):

[0010] In step a), a solid starting material is provided which comprises at least one condensation polymer to be depolymerized. In particular, the solid starting material provided in step a) consists essentially of at least one condensation polymer to be depolymerized.

[0011] In step b), a depolymerization agent for the condensation polymer is provided. The depolymerization agent is a substantially anhydrous solid. In the context of this specification, the term "solid" refers to the agglomerated state of the depolymerization agent at room temperature, i.e., approximately 20 to 25°C.

[0012] In step c), a polyhydric alcohol having at least three hydroxyl groups per molecule is provided.

[0013] In step d), the starting material, depolymerization agent, and polyhydric alcohol are added to a reaction vessel to obtain a reaction mixture. In particular, the depolymerization agent is added so that the stoichiometric ratio of depolymerization agent to starting material is at least 2, preferably 2.2, based on the repeating units of the condensation polymer to be depolymerized. In particular, the polyhydric alcohol is added in a stoichiometric ratio of polyhydric alcohol to starting material of at least 1.5, preferably 1.8, based on the hydroxyl groups in the polyhydric alcohol and the repeating units of the condensation polymer to be depolymerized. The aforementioned ratios have been found to be particularly suitable in the context of the present invention for achieving high throughput rates and high quality of the resulting recycled product.

[0014] In step e), the condensation polymer is depolymerized by at least one of kneading, mixing, conveying, and recycling the reaction mixture in the reaction vessel, thereby obtaining a product mixture comprising the depolymerization agent, the polyhydric alcohol, the monomer components of the condensation polymer or derivatives thereof, and any remaining starting materials and condensation polymer.

[0015] In step f), at least one of the monomer components of the condensation polymer or a derivative of said monomer component is separated from the depolymerization agent and the polyhydric alcohol during the depolymerization step e). Additionally or alternatively, at least one of the monomer components of the condensation polymer or a derivative thereof is separated from the depolymerization agent and the polyhydric alcohol after the depolymerization step e).

[0016] In the context of this specification, the term "polycondensation polymer" refers to a polymer formed by a condensation reaction between two difunctional or polyfunctional monomers, i.e., monomer components, with the formation of a small molecule as a leaving group. In particular, the term polycondensation polymer, as used herein, also refers to polyamides, particularly polyamide 6, formed by ring-opening polymerization. A "derivative of a monomer component" is understood to be a molecule that is structurally different from, but can be converted into, the monomer used in the respective polymerization reaction. For example, a carboxylate obtained from the depolymerization method disclosed herein may be acidified to obtain the corresponding carboxylic acid monomer. An amine may be phosgenated to obtain the corresponding isocyanate monomer used in the formation of polyurethane, another example of a derivative of a monomer component. When the solid starting material essentially consists of at least one condensation polymer, it is contemplated and encompassed by the present invention that at least one condensation polymer may be added and / or contain a filler.

[0017] Surprisingly, it has been found that the use of a polyhydric alcohol having at least three hydroxyl groups per molecule results in particularly efficient depolymerization of polycondensation polymers without the use of solvents, particularly water. The substantially anhydrous depolymerization agent can be easily separated from the product mixture by filtration. Typically, at least one of the monomer components resulting from the depolymerization can be easily separated from the product mixture in gaseous form, particularly by distillation, vacuum distillation, or a gas stream of a carrier gas. The methods disclosed herein have the additional advantage that decomposition of the condensation polymer occurs at relatively low temperatures and is therefore particularly energy-efficient and cost-effective. Furthermore, the methods disclosed herein tolerate the presence of additional polymers, such as polyethylene, polypropylene, ethylene-vinyl acetate (EVA), ethylene-vinyl alcohol (EVOH), polyvinyl alcohol (PVOH), and / or polystyrene, which are often included in polycondensate-containing composites.

[0018] Specific condensation polymers that can be depolymerized in accordance with the present invention include polyesters, polyamides, polycarbonates, and polyurethanes. Preferably, the condensation polymer to be depolymerized is selected from polyesters, polyamides, and polycarbonates.

[0019] In certain embodiments of the methods disclosed herein, the at least one condensation polymer is not a polyurethane, in other words, the solid starting material provided in step a) does not comprise a polyurethane.

[0020] Polyesters can be obtained by condensation of dicarboxylic acids with dihydric alcohols and are characterized by repeating units of the following formula (1a): [ka]

[0021] In preferred embodiments of the methods disclosed herein, the polyester is at least one of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polycyclohexylene dimethylene terephthalate, polytrimethylene terephthalate, polybutylene succinate, polybutylene adipate terephthalate, and copolyesters of the foregoing polyesters. Even more preferably, the polyester is at least one of polyethylene terephthalate, polybutylene terephthalate, polyethylene terephthalate / polybutylene terephthalate copolymer, polyethylene terephthalate copolymer, and polybutylene terephthalate copolymer.

[0022] In the context of the present invention, the term "polyester" is also understood to mean biopolyesters, in particular polylactic acid, which can be formally obtained by condensation of lactic acid with loss of water and is characterized by a repeating unit of the following formula (1b): [ka]

[0023] Another condensation polymer that can be depolymerized according to the present invention belongs to the class of polyamides, which can be obtained by condensation of dicarboxylic acids with alkylenediamines and are characterized by repeating units of formula (2a): [ka]

[0024] Alternatively, the polyamide may also be obtained by head-to-tail condensation of aminocarboxylic acids or by ring-opening of the corresponding lactams, in which case the polyamide is characterized by repeating units of formula (2b): [ka] In a preferred embodiment of the method disclosed herein, the polyamide is at least one of polyamide 6, polyamide 6.9, polyamide 10, polyamide 4.6, polyamide 11, polyamide 12, polyamide 10.10, polyamide 6.6, and fully or partially aromatic polyphthalamide. In the case of polyamide 6, the monomer component is caprolactam (IUPAC: azepan-2-one). In the case of polyamide 6.6, the monomer components are hexamethylenediamine and adipic acid.

[0025] Another condensation polymer that can be depolymerized according to the present invention belongs to the class of polycarbonates, which can be obtained, for example, by the reaction of phosgene with a dihydric alcohol and are characterized by repeat units having the following formula (3): [ka]

[0026] In a preferred embodiment of the methods disclosed herein, the polycarbonate is at least one of a bisphenol A (BPA)-based polycarbonate and a poly(propylene carbonate).

[0027] Another condensation polymer that can be depolymerized according to the present invention belongs to the polyurethane class. Polyurethanes can be obtained by reacting diisocyanates with hydroxyl-rich compounds containing at least two hydroxyl groups. The amines obtained by depolymerization are derivatives of the monomeric components and can, in principle, be converted into isocyanates, i.e., the monomeric components, as known to those skilled in the art. Polyurethanes are characterized by the repeating unit of formula (4): [ka] In each of the above formulas (1) to (4), R, R', and R'' are divalent organic radicals.

[0028] In a preferred embodiment of the method disclosed herein, no additional solvent is added to the reaction mixture before step f). In particular, alkylene glycol and water are not added to the reaction mixture before step f). This eliminates the need for solvent separation or purification, which helps reduce equipment size and costs, and in particular means that the equipment used to carry out the method can have fewer and smaller containers. Because depolymerization agents such as sodium hydroxide are relatively soluble in water but not in polyhydric alcohols such as glycerol, there is no or little dilution of the depolymerization agent in the absence of additional water, which means that less depolymerization agent can ultimately be used, or the quantitative ratio between the condensation polymer to be depolymerized and the depolymerization agent can be shifted toward a higher polymer content. This has a favorable effect on the cost and environmental balance of the method disclosed herein.

[0029] In particularly preferred embodiments of the methods disclosed herein, no additional solvent is added to the product mixture until after step f). In particular, alkylene glycol and water are not added to the product mixture until after step f). At this point, at least one of the monomer components or derivatives thereof has already been partially, preferably largely, removed from the product mixture, and thus the added solvent no longer interferes with efficient or effective separation of that monomer component from the product mixture.

[0030] Suitable depolymerization agents useful in the methods disclosed herein are alkali or alkaline earth metal oxides, alkali or alkaline earth metal hydroxides, and alkali or alkaline earth metal carbonates, or mixtures of the foregoing compounds. Examples of such compounds are sodium oxide, potassium oxide, magnesium oxide, calcium oxide, sodium ethoxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, or mixtures thereof.

[0031] In a preferred embodiment of the method disclosed herein, the depolymerization agent is at least one compound selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium ethoxide, sodium carbonate, and hydrosilane. All of the foregoing compounds are generally readily available at low cost, making the method particularly cost-effective.

[0032] It is particularly preferred that the depolymerization agent used in the methods disclosed herein is sodium hydroxide, also known as "lye" or "caustic soda." Sodium hydroxide is a strong alkali (pK a It has a pH of approximately 15.7 and is therefore generally readily available, inexpensive, recyclable, and a highly active catalyst. Sodium hydroxide absorbs water and forms a series of hydrates, NaOH·nH2O. Commercially available "sodium hydroxide" is often the monohydrate NaOH·H2O, which, despite the water bound therein, is still considered a "substantially anhydrous solid" in the context of this specification.

[0033] In a preferred embodiment of the method disclosed herein, the depolymerization agent is not a transition metal compound. This means that the reaction mixture is substantially free of transition metal compounds. Transition metal compounds or complexes are often expensive and difficult to handle due to, for example, sensitivity to high temperatures, water, or oxygen. Avoiding such compounds or complexes containing transition metals results in a less complex and less expensive process. In particular, the depolymerization agent is not zinc acetate. This means that the reaction mixture is substantially free of zinc acetate. When using acetate salts such as zinc acetate, there is a risk that the acetic acid formed during the depolymerization reaction, which is relatively volatile and odorous, may escape from the reaction vessel and cause damage. This risk can be avoided by not using zinc acetate. Therefore, the equipment used does not need to be separately protected against acetic acid vapor leakage, reducing design effort and, therefore, costs.

[0034] In a preferred embodiment of the method disclosed herein, the polyhydric alcohol has 3 to 5 hydroxyl groups per molecule. Preferably, the polyhydric alcohol is selected from the group consisting of glycerol, diglycerol, triglyceride, pentaerythritol, dipentaerythritol, and sorbitol. More preferably, the polyhydric alcohol is glycerol. Each of these compounds has a relatively small molecular size or molecular weight, yet a relatively large number of hydroxyl groups available for transesterification. As a result, despite the transesterification reaction, the viscosity of the reaction mixture during depolymerization decreases only slowly at first. This allows harsh mechanical treatment of the reaction mixture through mixing, kneading, conveying, and / or recirculation processes within the reaction vessel to contribute to the decomposition of the polycondensate-containing waste for the majority of the total depolymerization time, meaning that the depolymerization reaction can proceed more completely in a shorter time overall.

[0035] In a preferred embodiment of the method disclosed herein, the reaction mixture is heated and / or cooled to a temperature above the glass transition temperature (T g ) and its melting temperature (T m ) to a temperature lower than the glass transition temperature. Heating above the glass transition temperature displaces the polymer chains, significantly promoting and accelerating the depolymerization reaction. On the other hand, the reaction mixture should not be heated above the melting temperature of the polymer in question, as this significantly reduces the viscosity of the reaction mixture, i.e., makes the reaction mixture thinner (more fluid). Within the scope of the present invention, it has surprisingly been found that relatively harsh mechanical treatments, made possible by the kneading, mixing, conveying or recirculation processes of the reaction mixture in the reaction vessel at relatively low temperatures, in combination with the reactants used according to the present invention, result in a particularly rapid depolymerization reaction as well as a high conversion rate with low time requirements.

[0036] If the starting material contains several polycondensation polymers with different glass transition temperatures and melting temperatures, the reaction mixture is preferably heated to a temperature below the glass transition temperature (T) of all condensation polymers to be depolymerized. g) and the minimum melting temperature (T m ) is tempered to a temperature of 0.15°C. In this way, the beneficial effects described above are achieved even when the starting material comprises a mixture of different polycondensation polymers.

[0037] Differential scanning calorimetry (DSC) is a thermal analysis technique that measures the difference in the amount of heat required to raise the temperature of a test specimen and a reference specimen as a function of temperature. Both the test specimen and the reference specimen are maintained at approximately the same temperature throughout the experiment. The DSC chamber contains two specimen positions, one used for the test specimen and the other for the reference specimen. The reference specimen is typically an inert material that does not exhibit thermal transitions in the temperature range under investigation. During the measurement, both the test specimen and the reference specimen are heated at a specified rate within the chamber within a predetermined temperature range where the test specimen is expected to undergo a thermal transition. The temperature difference between the test specimen and the reference specimen is measured by a thermopile sensor, which provides a differential thermocouple voltage, which is converted by the instrument to energy per unit time using calibration constants. The results are expressed as heat flow versus temperature of the test specimen compared to the reference specimen.

[0038] Therefore, when glass transition temperature is referred to herein, it refers to the glass transition temperature (T g The following procedure is used to determine

[0039] According to ISO 11357-2, differential scanning calorimetry (DSC) is used to measure the difference between the heat flux to the test specimen and the heat flux to the reference specimen as a function of temperature and / or time. The test specimen and reference specimen are subjected to a controlled temperature program under a specified atmosphere. Five to ten milligrams of the condensation polymer whose glass transition temperature is to be determined is placed in an aluminum specimen pan closed with a corresponding aluminum lid containing a pinhole. The mass of the test specimen is selected so that 5 to 10 mg of condensation polymer resin is present in the specimen. In other words, the mass of inert fibers or fillers is not included in the mass determination. The test specimen and reference specimen are thermally cycled under dry nitrogen purge gas at the flow rate recommended by the device manufacturer, specifically 50 ml per minute. In the first step, the test specimen and reference specimen are kept isothermal at 30°C for 10 minutes. In the second step, the test specimen and reference specimen are heated from 30°C to 280°C at a heating rate of 20°C per minute. The temperature range of the measurement should be at least 50°C above and below the desired temperature range, i.e., the approximate glass transition temperature, which can be obtained, for example, from relevant reference books or by an initial DSC test run, as known to those skilled in the art. In the third step, the test specimen and the reference specimen are kept isothermal at 280°C for 10 minutes. In the fourth step, the test specimen and the reference specimen are cooled from 280°C to 30°C at a cooling rate of 20°C per minute. In the fifth step, the test specimen and the reference specimen are kept isothermal at 30°C for 10 minutes. In the sixth step, the test specimen and the reference specimen are again heated from 30°C to 280°C at a heating rate of 20°C per minute. The glass transition temperature (T g ) is obtained from the second heating run as the inflection point of the step change in heat flow plotted on a linear scale.

[0040] When melting temperature is referred to herein, it refers to the melting temperature (T m The following procedure is used to determine:

[0041] Differential scanning calorimetry (DSC) is used to determine the temperature and enthalpy of melting and crystallization of crystalline or partially crystalline polymers according to ISO 11357-3, where melting is the transition stage between a fully crystalline or partially crystalline solid state and an amorphous liquid of variable viscosity. The melting temperature (T m 5 to 10 mg of the condensation polymer for which the melting point (T) is to be determined is placed in an aluminum specimen pan closed with a corresponding aluminum lid containing a pinhole. The mass of the specimen is selected so that there is 5 to 10 mg of condensation polymer resin in the specimen. In other words, the mass of inert fibers or fillers is not included in determining the mass. Because DSC measurements of polymers are significantly affected by the thermal history and morphology of the sample and specimen, it is important to perform a preliminary heat cycle and obtain measurements from a second heat scan. Therefore, the measuring cell of the DSC device should be heated to a temperature high enough to erase the previous thermal history of the test material, especially the extrapolated final melting temperature T. efm An initial heating run is performed under nitrogen at a heating rate of 10°C / min to a temperature 30°C higher than the initial temperature. The temperature is held for 5 minutes, after which a cooling run is performed, preferably at the same rate as used for the first and second heats, to the extrapolated final crystallization temperature T efc Before the second heating run is performed, the temperature is increased to approximately 50°C below the extrapolated final melting temperature T, preferably at the same heating rate as the first heating and cooling run. efm The temperature is again held for 5 minutes until the melting temperature (T m ) is a transition, also called "melting," characterized by an endothermic peak in the DSC curve.

[0042] Preferably, the reaction mixture is heated and / or cooled to a temperature of 30°C to 280°C. More preferably, the reaction mixture is heated and / or cooled to a temperature of 60°C to 250°C. Even more preferably, the reaction mixture is heated and / or cooled to a temperature of 90°C to 75°C. Most preferably, the reaction mixture is heated and / or cooled to a temperature of 120°C to 160°C. The latter temperature range is significantly lower than known from the relevant prior art and, in addition to maintaining a relatively high viscosity of the reaction mixture, i.e., a thick liquid (low flowability), it also therefore makes it possible to save on the energy required for heating.

[0043] Heating and cooling may be achieved, inter alia, through means for tempering the reaction vessel. For example, the reaction vessel may be tempered by using fluids and means for heat transfer or dissipation to and / or from the reaction vessel.

[0044] As another example, the reaction vessel may be tempered using electromagnetic radiation, particularly microwaves. Microwaves are a form of electromagnetic radiation having wavelengths ranging from approximately 1 meter to 1 millimeter, corresponding to frequencies of 300 MHz to 300 GHz, respectively. A microwave energy source, e.g., a microwave generator, may introduce microwave energy into the reaction vessel through one or more inlet ports disposed within the reaction vessel via a waveguide to provide a heatable microwave zone within the reaction vessel. The waveguides may be coaxially and radially disposed outside and around the reaction vessel. Microwave energy may operate, in particular, in the range of approximately 300 MHz to approximately 3 GHz.

[0045] As yet another example, the reaction vessel may be tempered by induction heating. In this example, an electromagnetic induction field is provided via an induction coil to promote depolymerization of one or more condensation polymers in the starting material into their constituent monomers. It is specifically contemplated that the electromagnetic induction field is applied to a transport means of the reaction vessel, such that the transport means is coupled to the electromagnetic induction field.

[0046] When the method disclosed herein uses a second vessel, the second vessel may be temperature-controlled, i.e., tempered. The means for tempering the reaction vessel described above are also suitable for tempering the second vessel. The reaction vessel and the second reactor may share means, i.e., they may be used together. However, preferably, the reaction vessel and the second vessel each have their own means for temperature control, i.e., the reaction vessel and the second vessel may be heated and cooled independently of each other.

[0047] In a preferred embodiment of the method disclosed herein, the method is carried out as a continuous process. The operational advantages of the process are particularly pronounced in a continuous process, as opposed to a batchwise process, because, among other things, periodic cooling of the reaction vessel to a relatively low temperature, e.g., room temperature, and subsequent heating of the reaction vessel to a relatively high temperature is avoided.

[0048] In a preferred embodiment of the method disclosed herein, the starting material, depolymerization agent, and polyhydric alcohol are added separately to the reaction vessel. In other words, the aforementioned components are added to the reaction vessel one after the other via dedicated means for adding each component or via a shared means. As will be understood by those skilled in the art, the means and shared means for adding the starting material, depolymerization agent, and polyhydric alcohol may be selected from vertical or horizontal feed ports, gravimetric or volumetric dosing systems, metering pumps, liquid dosing pumps, and injectors, depending on whether each component being added is a liquid or solid. This embodiment of the method is made possible by the fact that all components used in the method described herein are solids, and therefore the method and apparatus required to carry it out may be simpler or require less structural effort than known prior art.

[0049] In an even more preferred alternative embodiment of the method disclosed herein, the starting material, depolymerization agent, and polyhydric alcohol are added to the reaction vessel in the form of a premix containing two or more of the components of the reaction mixture. This alternative is particularly advantageous when the starting material and depolymerization agent are added in premixed form, i.e., placed together in the reaction vessel. Surprisingly, it has been found that this can increase the depolymerization conversion rate. Without wishing to be bound by theory, it is currently believed that if these components are added simultaneously to the reaction vessel, both components are co-ground and sheared during kneading, mixing, conveying, or recirculation so that the starting material softens and settles around the depolymerization agent, preventing it from melting at the temperature within the reaction vessel. This intimate mixing of the starting material and depolymerization agent increases surface reactivity and promotes saponification under essentially anhydrous conditions. Furthermore, when an extruder is used as the reaction vessel, the extruder section used to combine these components of the reaction mixture may be omitted, and the required screw length or extruder length can be reduced by using a premix, as described above.

[0050] In a preferred embodiment of the method disclosed herein, the depolymerization agent and the polyhydric alcohol are added to the reaction vessel at separate locations in the reaction vessel, which allows the reaction between the starting material and the depolymerization agent to proceed sufficiently in advance, thus allowing the depolymerization to occur in a more controlled and complete manner.

[0051] In a preferred embodiment of the method disclosed herein, the reaction vessel is a twin-screw extruder or mixer / kneader. In a twin-screw extruder, materials are mixed and simultaneously pressurized by two rotating screws within a barrel heated to a predetermined temperature. The screw length is designed according to reaction requirements, such as the need for a feed zone, mixing zone, reaction zone, high-pressure zone, vacuum zone, exhaust zone, and / or metering zone, as understood by those skilled in the art. Materials are extruded in the form of strands through extrusion orifices in a die plate located at the exit of the extruder. A kneader, also known as a "kneading extruder," contains a set of counter-rotating kneading blades and a discharge screw. The blades are mounted on a horizontal shaft within a U-shaped trough. The discharge screw is located below the blades in a separate cavity. During the mixing cycle, the blades rotate toward each other, and the mix constantly feeds new material to the mixing blades. After the mixing / kneading cycle is completed, the screw direction is reversed and the final product is extruded from the mixing zone through a discharge die for further processing. Another specific example of a kneader that can be preferably used as a reaction vessel is the BUSS® type co-kneader. Such machines are characterized by the simultaneous rotation and axial vibration of the mixing and kneading screw shaft, i.e., the conveying means, which are characterized by kneading flights. The vibrating screw shaft ensures intensive axial material exchange by multiple divisions, folding, and reorientation of the reaction mixture or product mixture, respectively. The kneading flights of the screw shaft interact with stationary kneading pins, which results in effective mixing.

[0052] Both of the above-mentioned devices, i.e., twin-screw extruders and kneading extruders, are particularly well suited for carrying out the method disclosed herein, since they allow for intensive processing by kneading, mixing, conveying, and recycling the components provided in process steps a) to c), which allows for a significant acceleration of the depolymerization, which is advantageous for the depolymerization conversion and the yield of the monomer or its derivatives obtained therefrom, respectively. BUSS®-type co-kneaders are particularly effective when the proportion of depolymerization agent and / or polyhydric alcohol is high compared to the starting material. They provide excellent distributive mixing and optimal distribution of the raw materials. The oscillating screw shaft motion allows for particularly short processing lengths.

[0053] This object is further achieved by an apparatus for depolymerizing condensation polymers. The apparatus comprises a reactor having a conveying means, a means for tempering the contents disposed in the reactor, and an outlet for discharging at least a portion of the reaction mixture after depolymerization. The apparatus further comprises means for supplying a solid starting material containing at least one condensation polymer into the reactor, means for supplying a substantially anhydrous solid depolymerization agent for the condensation polymer into the reactor, and means for supplying a polyhydric alcohol having at least three hydroxyl groups per molecule into the reactor. The conveying means is configured to at least one of knead, mix, convey, and recycle the reaction mixture in the reactor. The conveying means of the reactor may, in particular, be at least one screw-like member rotatable about its longitudinal axis.

[0054] Such an apparatus allows carrying out the processes described herein with the advantages described for each embodiment.

[0055] In particular, the apparatus disclosed herein is for carrying out the methods disclosed herein.

[0056] In a preferred embodiment of the apparatus disclosed herein, the reaction vessel is a twin-screw extruder having an extrusion tube having a length (L) and a diameter (D). Preferably, the length to diameter ratio (L / D) is no greater than 40, more preferably approximately 32. However, a length to diameter ratio (L / D) of approximately 40 is also contemplated and preferred.

[0057] The design of the reaction vessel as a twin-screw extruder with a relatively short screw length allows for a short residence time of the reaction mixture in the extruder and rapid execution of the depolymerization, while requiring little space for the extruder.

[0058] In a preferred embodiment of the apparatus disclosed herein, the reaction vessel is a twin-screw extruder with a melt pump located at the extruder's outlet. The melt pump (or gear pump) allows for precise control of the output to the die and eliminates extruder screw surging. Additionally, it reduces back pressure and increases extruder output.

[0059] Melt pumps typically have two gears, usually driven by a single motor, that mesh together as they rotate. The extruder fills the gears from the negative pressure side, and the rotating gear ejects the polymer on the other side. The principle is that a well-made gear has a very precise profile, and when that gear is surrounded by an interference-fit housing, the volume of material per tooth can be very accurate and nearly constant. As the gear rotates, it meters a precise amount of plastic at the ejection side. The speed of the gear rotation is tightly controlled by an AC or DC drive designed for low variation, e.g., less than 1%.

[0060] The gear design itself can be, for example, helical, spur, or herringbone. The helical gear design has been found to be suitable for most applications, ensuring a continuous and smooth process. Furthermore, the helical gear design is characterized by low friction and good self-cleaning properties compared to the other two gear designs. As known to those skilled in the art, the melt pump should be selected according to various criteria, such as the desired throughput (kg / h), type of starting material, filler content (type and percentage of filler), and pressure range.

[0061] In a preferred embodiment of the apparatus disclosed herein, the reaction vessel is a twin-screw extruder having a melt pump located at the extruder outlet and a die plate located at the end of the extruder. The die plate is characterized by a band configuration. For a 40 mm diameter twin-screw extruder, the band may be a 3.5 x 80 mm hole in the die plate.

[0062] In a preferred embodiment of the apparatus disclosed herein, the apparatus further comprises a second vessel connected downstream to the outlet of the reaction vessel. The second vessel comprises a means for conveying the contents disposed therein and a means for tempering the contents disposed therein. In addition to carrying out the actual depolymerization, such an apparatus also allows for post-treatment or further processing of the product mixture obtained from the depolymerization reaction in the reaction vessel.

[0063] The conveying means of the second vessel may be at least one agitator, such as an axial or radial agitator. Obstacles may be installed on the inner wall of the second vessel to prevent the product mixture from rotating with the agitator and resulting in the formation of a vortex. These so-called "baffles" improve mixing efficiency at the same agitator power. Generally, the choice of agitator depends on the mixing effort, the viscosity of the medium, the desired shear strength, and the available agitator power, as will be understood by those skilled in the art.

[0064] In a preferred embodiment of the apparatus disclosed herein, the apparatus further comprises means for separating at least one of the monomer components of the condensation polymer or a derivative of said monomer component from the depolymerization agent and the polyhydric alcohol during and / or after depolymerization from the reaction vessel, such that the monomer and / or its derivative can be separated from the product mixture in the reaction vessel.

[0065] In particular, a means for separating at least one of the monomer components of the condensation polymer or a derivative of said monomer component from the depolymerization agent and polyhydric alcohol during and / or after depolymerization is a means for removing gas from the reaction vessel. In the gaseous state, the monomer and / or its derivative can be particularly easily separated from the product mixture in the reaction vessel.

[0066] Additionally or alternatively, the apparatus disclosed herein further comprising a second vessel may also comprise means for separating at least one of the monomer components of the condensation polymer or a derivative of said monomer component from the depolymerization agent and from the polyhydric alcohol during and / or after depolymerization from the second vessel, so that any monomer still remaining in the product mixture can be separated from the product mixture in the second vessel.

[0067] In particular, the means for separating at least one of the monomeric components of the condensation polymer or a derivative of said monomeric component from the depolymerization agent and the polyhydric alcohol during and / or after depolymerization is a means for removing gas from the second vessel.

[0068] This object is further achieved by the use of the apparatus disclosed herein for carrying out the methods disclosed herein.

[0069] The present invention will be further explained in more detail with the aid of the figures, in which like reference numerals are used to refer to the same or similar elements. [Brief explanation of the drawings]

[0070] [Figure 1] FIG. 1 is a schematic diagram of one embodiment of a method for depolymerizing a condensation polymer disclosed herein. [Figure 2] 1 is a schematic diagram of one embodiment of the device disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0071] FIG. 1 shows a schematic diagram of an exemplary embodiment of the method disclosed herein. In step a), starting material 4 is provided in the form of plastic bottle waste containing polyethylene terephthalate (PET) as the condensation polymer 1. Preferably, the PET bottles are first mechanically reduced in size, for example, by crushing, shredding, tearing, and / or cutting in a suitable device, such as a shredder. Non-polyester components of the bottles, particularly polyolefins such as HDPE used in the bottle caps, do not need to be separated from the polyester and can be fed directly to the reactor, as they do not interfere with the depolymerization method disclosed herein. Starting material 4 is placed in the hopper of a twin-screw extruder (not shown), which serves as the reactor, as further described in FIG. 2 below. In step b), sodium hydroxide is provided as the depolymerization agent 5. The sodium hydroxide is provided in pellet form, i.e., as a substantially anhydrous solid. In step c), glycerol is provided as the polyhydric alcohol 6. Glycerol has three hydroxyl groups per molecule. In step d), PET bottle scraps, sodium hydroxide, and glycerol are added to a twin-screw extruder, as described in more detail with respect to FIG. 2, to obtain a reaction mixture 7 consisting of the aforementioned components 4, 5, and 6. The reaction mixture 7 is depolymerized in step e) under the kneading and mixing action of the extruder screws. After a predetermined time, typically about 1 minute, the resulting product mixture 8 is conveyed from the twin-screw extruder to a separate container (not shown), as further described in FIG. 2 below. In the case of PET depolymerization, the product mixture 8 contains ethylene glycol 1a and PET as monomer components, sodium hydroxide, disodium terephthalate 1b as a derivative of the glycerol monomer component, and low-molecular-weight esters resulting from the transesterification of glycerol with PET oligomers. Finally, in step f), ethylene glycol 1a is separated from sodium hydroxide 5 and glycerol 6.Due to the difference in their respective boiling points, ethylene glycol (bp approximately 197°C) and glycerol (bp approximately 290°C) can be separated from each other and from solid or high-boiling residues by evaporation under reduced pressure. Glycerol can be reused in the process, as indicated by the dashed arrow in Figure 1. It is also conceivable that the depolymerization is carried out under conditions in which at least one of the formed monomer components 1a and 1b is extracted from the reaction mixture 7. This can be done by passing a gas, such as nitrogen or air, through the reaction. Additionally or alternatively, at least one of the formed monomer components 1a and 1b is extracted from the reaction mixture 7 under reduced pressure. In this example, the ethylene glycol 1a formed during the depolymerization reaction of step e) can be removed in a nitrogen stream passing through the extruder barrel under reduced pressure.

[0072] FIG. 2 shows a schematic diagram of an apparatus 100 for depolymerizing a polymer disclosed herein. The apparatus 100 comprises a reaction vessel 10 in the form of a twin-screw extruder having an extruder barrel with a length-to-diameter ratio of approximately 32. The twin-screw extruder comprises two screws (not shown) driven by a drive and transmission as conveying means 11. The screws of the twin-screw extruder 10 may be operated at a speed of, in particular, 200 to 600 RPM. The torque may be, in particular, 60% to 80%. The twin-screw extruder 10 further comprises means 12 for tempering the contents disposed within the twin-screw extruder 10. In the example shown in FIG. 2, the means 12 comprises a heating device disposed around the extruder barrel through which hot oil can be circulated, dividing the extruder barrel into eight zones Z1 to Z8 having different temperatures. For example, the temperatures of the individual zones designated Z1 to Z8 could be 130°C, 130°C, 110°C, 110°C, 90°C, 90°C, 80°C, and 80°C. The twin-screw extruder 10 shown in the example of FIG. 2 further comprises an outlet 13 connected downstream to a melt pump 17, which allows for a controlled and stable pressure of the extruded product mixture. The pump can be tempered, for example, to about 70°C. A die plate 18 is disposed downstream of the melt pump 17. In the example shown in FIG. 2, the die plate 18 features a band configuration sized as holes 3.5 x 80 mm in size and is tempered to about 70°C. The apparatus 100 further comprises a means 14 for feeding a solid starting material 4 containing at least one condensation polymer 1 to the twin-screw extruder 10. The means for feeding the solid starting material can be, in particular, a hopper. Furthermore, the apparatus 100 comprises means 15 for feeding the twin-screw extruder 10 with a substantially anhydrous solid depolymerization agent 5 for the condensation polymer 1. In particular, the substantially anhydrous solid may be introduced into the twin-screw extruder by a side feeder, in particular in section Z3 shown in Figure 2. Finally, the apparatus 100 comprises means 16 for feeding the twin-screw extruder 10 with a polyhydric alcohol 6 having at least three hydroxyl groups per molecule.Depending on the aggregation state of the polyhydric alcohol used, the means 16 may be a liquid injection feeder or a side feeder for solids. The means for feeding the polyhydric alcohol into the reaction vessel may be heated. The conveying means 11 of the twin-screw extruder 10 is configured to knead, mix, convey, and recycle the reaction mixture present in the twin-screw extruder 10. The apparatus 100 of this embodiment further comprises a stirred reactor 20 as a second vessel connected downstream to the outlet 13 of the twin-screw extruder 10. The stirred reactor 20 comprises an agitator 21 as a means for stirring the contents placed in the stirred reactor 20 and a means 22 for tempering the contents placed in the stirred reactor 20. The product mixture 8 obtained after the depolymerization described in FIG. 1 is transferred to the stirred reactor 20, where ethylene glycol 1a is separated from the product mixture 8 by a means 29 for removing gas from the stirred reactor 20. In particular, ethylene glycol 1a can be separated from the product mixture 8 by techniques such as distillation, steam distillation, and vacuum distillation. Distillation can be carried out in one or more steps. The remaining monomer components or their derivatives can be separated from the remaining product mixture 8 by recrystallization or other common purification techniques. If the depolymerization reaction is not complete, any oligomers still present in product mixture 8 can also be separated from the monomer components of the polyester by cooling product mixture 8 or adding a non-solvent for the oligomer or polyester. The oligomers will precipitate, and the monomer components can be separated by filtration. [Example]

[0073] Example According to the method disclosed herein, the following depolymerizations were carried out in a kneader as a reaction vessel. Each experiment was repeated several times.

[0074] [Table 1]

[0075] The kneader was equipped with a heating device that divided the kneader into four heating zones, with the temperatures of individual heating zones Z1 to Z4 being 90°C, 160°C, 160°C, and 160°C, respectively. The kneader further included eight kneader teeth with temperatures of 170°C, 180°C, 155°C, 150°C, 145°C, 145°C, and 145°C, respectively. Operating the kneader at an amperage of 24 A and 1.5 kW resulted in a throughput of approximately 10 kg / hour. All depolymerizations were carried out using a polyethylene terephthalate (PET) to sodium hydroxide ratio of approximately 3:1, based on the total weight of the reaction mixture, resulting in complete depolymerization of the solid starting material, i.e., the polyethylene terephthalate resin provided in step a) of the method disclosed herein. In depolymerizations 1 and 2, the reaction mixture contained the polyhydric alcohol glycerol in an amount of 10% based on the total weight of the reaction mixture. The reaction mixtures of depolymerizations 3 and 4 contained glycerol in an amount of 20% based on the total weight of the reaction mixture, and the depolymerization of the condensation polymer in step e) of the method disclosed herein by kneading the reaction mixture in the reaction vessel was carried out under more severe conditions in depolymerization 3 than in depolymerization 4 due to the higher revolutions per minute (rpm) of the kneading unit. In depolymerizations 5 to 8, each reaction mixture contained glycerol in an amount of 30% based on the total weight of the respective reaction mixture, but the duration of the depolymerization, i.e., the residence time in the kneader, was varied.

[0076] While depolymerizations 1 to 8 all resulted in complete depolymerization of the solid starting material, i.e., polyethylene terephthalate resin provided in step a) of the method disclosed herein, the product mixtures obtained in step e) of the method disclosed herein for depolymerizations 1 to 3 were found to be hard or malleable and therefore relatively difficult to further process. Without wishing to be bound by theory, it is hypothesized that the high hardness of the product mixtures is related to the crystallization of the formed disodium terephthalate. On the other hand, the properties of the product mixtures obtained in depolymerizations 5 to 8, particularly their hardness, were found to be relatively unpredictable. For example, the product mixture obtained in depolymerization 8 was very thin, almost water-like, and therefore had a relatively low viscosity, while the product mixture obtained in depolymerization 6 was significantly harder. While not wishing to be limited to this interpretation, the difference in viscosity of the resulting product mixtures is thought to be related to the residence time of the reaction mixture in the kneader. However, the observed lack of process stability makes the depolymerization conditions selected for depolymerizations 5 to 8 less favorable for a continuous process. Interestingly, it has been found that the pH of the product mixture obtained in step e) of the method disclosed herein can be used as a parameter to follow or determine the progress of depolymerization, and a product mixture pH of 10 to 12 indicates complete depolymerization.

Claims

1. 1. A process for depolymerizing a condensation polymer (1) into its monomeric components (1a, 1b) or derivatives thereof, said process comprising: a) providing a solid starting material (4) containing, in particular consisting of, at least one condensation polymer (1) to be depolymerized; b) providing a depolymerization agent (5) for the condensation polymer (1), wherein the depolymerization agent (5) is a substantially anhydrous solid; c) providing a polyhydric alcohol (6) having at least three hydroxyl groups in one molecule; d) adding the starting material (4), the depolymerization agent (5) and the polyhydric alcohol (6) to a reaction vessel (10) to obtain a reaction mixture (7); e) depolymerizing the condensation polymer (1) by kneading and / or mixing and / or conveying and / or recycling the reaction mixture (7) in the reaction vessel (10) to obtain a product mixture (8); f) separating at least one of the monomer components (1a, 1b) of the condensation polymer (1) or a derivative of the monomer component from the depolymerization agent (5) and the polyhydric alcohol (6) during and / or after the depolymerization step e); A method comprising:

2. 2. The method of claim 1, wherein the condensation polymer (1) is selected from the group consisting of polyesters, polyamides, polyurethanes and polycarbonates, preferably wherein the condensation polymer (1) is selected from the group consisting of polyesters, polyamides and polycarbonates.

3. 3. The method of claim 2, wherein the polyester is at least one of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polycyclohexylene dimethylene terephthalate, polytrimethylene terephthalate, polylactic acid, polylactic acid, polybutylene succinate, polybutylene adipate terephthalate, and copolymer polyesters of the foregoing polyesters, preferably the polyester is at least one of polyethylene terephthalate, polybutylene terephthalate, polyethylene terephthalate / polybutylene terephthalate copolymer, polyethylene terephthalate copolymer, and polybutylene terephthalate copolymer.

4. 3. The method of claim 2, wherein the polyamide is at least one of polyamide 6, polyamide 6.9, polyamide 10, polyamide 4.6, polyamide 5.6, polyamide 11, polyamide 12, polyamide 10.10, polyamide 6.6, and wholly or partially aromatic polyphthalamide.

5. The method of claim 2, wherein the polycarbonate is at least one of a bisphenol A-based polycarbonate and a poly(propylene carbonate).

6. 10. The process according to any one of the preceding claims, wherein prior to step f), no additional solvent, in particular alkylene glycol and water, is added to the reaction mixture (7).

7. 10. The method of any one of the preceding claims, wherein the depolymerization agent (5) is at least one compound selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium ethoxide, sodium carbonate, and hydrosilane, preferably the depolymerization agent is sodium hydroxide.

8. 10. The method according to any one of the preceding claims, wherein the depolymerization agent (5) is not a transition metal compound, in particular the depolymerization agent is not zinc acetate.

9. 10. The method according to any one of the preceding claims, wherein the polyhydric alcohol (6) has 3 to 5 hydroxyl groups in one molecule, preferably the polyhydric alcohol (6) is selected from the group consisting of glycerol, diglycerol, triglyceride, pentaerythritol, dipentaerythritol, and sorbitol, more preferably the polyhydric alcohol (6) is glycerol.

10. The reaction mixture (7) is heated to the glass transition temperature (T) of the condensation polymer (1) to be depolymerized, in particular through a means (12) for tempering the reaction vessel (10). ｇ ) and its melting temperature (T ｍ 2. The method of any one of the preceding claims, wherein the mixture is heated and / or cooled to a temperature lower than the temperature of the preheated or cooled mixture, preferably to a temperature of from 30°C to 280°C, more preferably to a temperature of from 60°C to 250°C, even more preferably to a temperature of from 90°C to 175°C, and most preferably to a temperature of from 120°C to 160°C.

11. 10. The method of any one of the preceding claims, wherein the method is carried out as a continuous process.

12. 10. The method according to any one of the preceding claims, wherein the starting material (4), the depolymerization agent (5) and the polyhydric alcohol (6) are added to the reaction vessel (10) separately or in the form of a premix containing two or more of the components (4, 5, 6) of the reaction mixture (7).

13. 10. The method according to any one of the preceding claims, wherein the depolymerization agent (5) and the polyhydric alcohol (6) are added to the reaction vessel (10) at separate locations in the reaction vessel (10).

14. 10. The method according to any one of the preceding claims, wherein the reaction vessel (10) is a twin-screw extruder or a mixer-kneader.

15. 15. Use of an apparatus (100) comprising a reaction vessel (10) having a conveying means (11), a means (12) for tempering the contents placed in the reaction vessel (10), an outlet (13), a means (14) for feeding a solid starting material (4) containing at least one condensation polymer (1) into the reaction vessel (10), a means (15) for feeding a substantially anhydrous solid depolymerization agent (5) for the condensation polymer into the reaction vessel (10), and a means (16) for feeding a polyhydric alcohol (6) having at least three hydroxyl groups per molecule into the reaction vessel (10), wherein the conveying means (11) is configured to knead and / or mix and / or convey and / or recycle the reaction mixture in the reaction vessel (10) for carrying out the method according to any one of claims 1 to 14.

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