Vitrification of polyurethane

Abstract

The vitrimerized polymer composition includes a polyurethane having a partially decomposed cross-linking ligand, a catalyst, and a dynamically recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane.

Description

Technical Field

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 394,122, filed on August 1, 2022, the content of which is incorporated herein by reference in its entirety.

Background Art

[0002] Polyurethane (PU) thermosetting materials are widely used in different applications such as furniture, buildings, automobiles, sound insulation, and heat insulation. Polyurethane is a versatile material used as adhesives, coatings, elastomers, and foams. The increasing amount of PU thermosetting resin waste causes significant environmental problems, and as a result, technologies for recycling thermosetting PU have attracted significant attention. However, the recycling of thermosetting PU is limited due to their permanent cross-linked structures that inhibit melt reprocessing. Common methods for recycling these materials are mechanical recycling and chemical decomposition. In the mechanical approach, the materials are crushed and used as fillers in other applications. However, when using these recycled materials as fillers, beyond a certain limit, the mechanical properties decrease and processing is prevented due to an increase in the viscosity of the compound. In the chemical approach, PU thermosetting materials are recycled into polyols or other small molecules by catalytic glycolysis. Both of these methods are inefficient and require a high amount of energy. Therefore, it is essential to design a practical and efficient method for directly recycling PU thermosetting resin waste into similar or higher-value products.

Summary of the Invention

[0003] The embodiments described herein relate to a method for recycling thermosetting polyurethane, which has hitherto been unprocessable, by carefully selecting materials and processing conditions. Polyurethane (PU) thermosetting materials are widely used in different applications, and the recycling of large amounts of PU thermosetting resin waste remains a global challenge. We have found that organic catalysts such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) can be used in a vitrimerization process for recycling and reprocessing thermosetting rigid PU foams. The results show that the permanent cross-linked structure of the PU thermosetting foam is converted into a dynamic network by vitrimerization. The vitrimerized network can rapidly relax stress in 10 seconds at a low temperature of 120 °C. Topological rearrangement occurs mainly through a carbamate exchange reaction by a dissociation mechanism. The vitrimerized network retains high mechanical strength, with a Young's modulus of 2.7 GPa and a tensile strength of 76.4 MPa, and can be reprocessed again without the addition of extra catalyst and without loss of mechanical properties. The vitrimerized network can also be foamed by applying a small pressure at a high temperature. Advantageously, the processing conditions do not require the handling or use of solvents, thus representing a significant improvement over the approach where the catalyst is dissolved in a solution to induce swelling in the thermosetting material and facilitate the entire recycling process.

[0004] In some embodiments, a method for recycling polyurethane includes partially decomposing cross-linking ligands in the polyurethane. A catalyst is provided to the decomposed polyurethane to produce a resil vitrimer polyurethane composition. The vitrimer polyurethane composition is then processed into a vitrimerized polyurethane, which includes a dynamic recyclable network, and a portion of the catalyst forms ligands with a portion of the polyurethane.

[0005] In other embodiments, a method for recycling polyurethane includes mechanically decomposing, in part, the cross-linked structure in the polyurethane. A catalyst is mechanically mixed with the decomposed polyurethane to produce a recyclable vitrimer polyurethane composition. The vitrimer polyurethane composition is then thermally and / or mechanically processed to produce a vitrimerized polyurethane product, which contains a dynamic recyclable network, and a portion of the catalyst forms a ligand with a portion of the polyurethane through a carbamate exchange reaction.

[0006] In yet other embodiments, a method for recycling a thermoset polyurethane foam includes selecting a thermoset polyurethane foam provided as particles and / or fragments. A catalyst is provided to the particles of the thermoset polyurethane foam to produce a recyclable vitrimer polyurethane composition. The recyclable vitrimer polyurethane composition is milled in the presence of a milling medium to produce a vitrimer polyurethane, which contains a dynamic recyclable network, and a portion of the catalyst forms a ligand with a portion of the polyurethane.

[0007] In some embodiments, the selected thermoset polyurethane foam is a waste thermoset polyurethane foam.

[0008] In some embodiments, the catalyst is provided at less than 15.0 wt% of the mass of the recyclable vitrimer polyurethane composition. For example, the catalyst can be provided at about 5.0 wt% to about 10 wt% of the mass of the recyclable vitrimer polyurethane composition.

[0009] In some embodiments, the catalyst includes an environmentally friendly organic catalyst, such as triazabicyclodecene.

[0010] In some embodiments, the recyclable vitrimer polyurethane composition is formed as a fine powder.

[0011] In some embodiments, the method further includes reprocessing the vitrimer polyurethane to form a recycled article. The vitrimer polyurethane can be reprocessed by heating it at a temperature lower than the melting temperature of the catalyst. For example, the vitrimer polyurethane can be reprocessed by compression molding it at a temperature lower than the melting temperature of the catalyst.

[0012] In some embodiments, the method can further include heating the recycled article to a temperature and pressure effective to foam the resil vitrimer polyurethane composition. The temperature effective to foam the resil vitrimer polyurethane composition can exceed the melting temperature of the catalyst.

[0013] In other embodiments, the recycled article can be reprocessed without the addition of a further catalyst and without loss of mechanical properties.

[0014] Other embodiments relate to recycled polyurethanes formed by the methods described herein. The recycled polyurethanes can be configured to be reprocessed without the addition of a further catalyst and without loss of mechanical properties.

[0015] Other embodiments relate to a polyurethane having a partially degraded crosslinking ligand and a vitrimerized polymer composition comprising a catalyst, the vitrimerized polymer composition including a dynamic recyclable network, and a portion of the catalyst forming a ligand with a portion of the polyurethane.

[0016] Still other embodiments relate to a vitrimerized polymer composition comprising a polyurethane having a partially decomposed crosslinked ligand and an environmentally friendly organic catalyst, the vitrimerized polymer composition comprising a dynamic recyclable network, with a portion of the catalyst forming a ligand with a portion of the polyurethane; and / or the vitrimerized polymer composition network can preferably rapidly relieve stress at a low temperature, preferably below 120°C, in 10 seconds; and / or the vitrimerized polymer composition network retains high mechanical strength, preferably with a Young's modulus of at least 2.7 GPa and a tensile strength of at least 76.4 MPa.

[0017] Still other embodiments relate to a vitrimerized polymer composition comprising a polyurethane having a partially decomposed crosslinked ligand, a catalyst, and a dynamic recyclable network, with a portion of the catalyst forming a ligand with a portion of the polyurethane.

[0018] In some embodiments, the catalyst is provided at less than 15.0 wt% of the mass of the composition. For example, the catalyst can be provided at about 5.0 wt% to about 10 wt% of the mass of the composition.

[0019] In some embodiments, the catalyst comprises triazabicyclodecene.

[0020] In some embodiments, the vitrimerized polymer composition can be in the form of a fine powder.

[0021] In some embodiments, the polyurethane is a thermosetting polyurethane foam.

[0022] In some embodiments, the vitrimerized polymer composition can be processed into an article. The article can be, for example, a compression molded article and / or a foamed article.

[0023] In some embodiments, the article can have at least one of a Young's modulus (GPa) greater than that of a thermoset polyurethane foam, a tensile strength (MPa) greater than that of a thermoset polyurethane foam, or an elongation at break (%) less than that of a thermoset polyurethane foam.

[0024] In some embodiments, the article or the vitrimerized polymer composition can be reprocessed without the addition of a catalyst and without loss of mechanical properties.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0026] Although specific embodiments are specified, it will be understood that elements from one described aspect can be combined with those from another aspect that is separately specified. Similarly, those skilled in the art will have the necessary understanding of general processes, components, and methods. This description is intended to include and disclose such general aspects even if they are not explicitly specified herein.

[0027] In this specification, the words "example" and "exemplary" mean an instance or illustration. The words "example" or "exemplary" do not indicate important or preferred aspects or embodiments. The word "or" is intended to be inclusive rather than exclusive unless the context suggests otherwise. As an example, the clause "A employs B or C" includes all inclusive arrangements (e.g., A employs B; A employs C; or A employs both B and C). As another matter, the articles "a" and "an" are generally intended to mean "one or more" unless the context suggests otherwise.

[0028] The embodiments described herein relate to a method for recycling hitherto intractable thermosetting polyurethanes, such as rigid thermosetting polyurethane foams, by carefully selecting materials and processing conditions. Polyurethane (PU) thermosetting materials are widely used in different applications, and the recycling of large amounts of PU thermosetting resin waste remains a global challenge. We have found that an organic catalyst, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), can be used in a vitrimerization process for recycling and reprocessing rigid thermosetting PU foams. The results show that the permanent cross-linked structure of the PU thermosetting foam is converted into a dynamic network by vitrimerization. The vitrimerized network can rapidly relax stress at a low temperature of 120 °C in 10 seconds. Topological rearrangement occurs mainly via a dissociation mechanism by carbamate exchange reaction. The vitrimerized network retains high mechanical strength, with a Young's modulus of 2.7 GPa and a tensile strength of 76.4 MPa, and can be reprocessed again without loss of mechanical properties without the addition of extra catalyst. The vitrimerized network can also be foamed by applying a small pressure at high temperature. Conveniently, the processing conditions do not require the handling or use of solvents, thus representing a significant improvement over approaches where the catalyst is dissolved in a solution, which induces swelling in thermosetting materials and facilitates the overall recycling process.

[0029] Figure 1 shows a method for recycling rigid thermosetting polyurethane foam by vitrimerization. The rigid PU foam is first crushed into small pieces and mixed with vitrimerization catalyst particles and a suitable mill, such as a rotating drum, using steel balls and / or other suitable media. Due to the rotational movement, surely, the milling media (black circles represent steel balls) are completely mixed with the rigid PU foam fine particles and the catalyst particles. The rotation promotes mixing and, due to the collisions between the particles, fine particles, and / or milling media, the fine particles are crushed, their size is reduced, and metal-polymer ligand sites are formed. Although a rotating drum is schematically shown, any conventional milling device is sufficient, and the steel balls can be replaced by or enhanced with other common milling media (provided that the milling media itself does not disintegrate or introduce unwanted materials otherwise). The milling media must be durable enough to crush the particles and fine particles into powder and impart the energy required to form the metal-polymer ligand sites.

[0030] The catalyst can be selected based on the chemistry of the thermosetting polyurethane network. The catalyst must be selected to have a sufficiently high decomposition temperature to minimize the deactivation / loss of the material under the expected milling conditions. In some embodiments, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) can be used as a catalyst for the vitrimerization of thermosetting polyurethane foam. The catalyst can also be selected from organic catalysts such as, but not limited to, benzyldimethylamide, and benzyltrimethylammonium chloride. Other non-limiting examples of catalysts include the following: tin(II) 2-ethylhexanoate, zinc(II) acetate (Zn(OAc)2), triphenylphosphine (PPh3), dibutyltin bis(2-ethylhexanoate), dibutyltin diacetate, dibutyltin dilaurate, dibutyltin bis(2,4-pentanedionate), titanium 2-ethylhexanoate, monobutyltin oxide, and zinc octanoate.

[0031] The catalyst can be used in an amount sufficient to produce a vitrimer having the desired properties. Specific, non-limiting amounts of catalyst that have been found to be effective include 2 wt%, 5 wt%, and 10 wt% of catalyst per milled mixture (i.e., the combination of the thermosetting PU and the catalyst). Thus, the catalyst can be provided in less than 8.0 wt%, less than 9.0 wt%, less than 10.0 wt%, or less than 15 wt% and any range of values bounded by these upper and lower limits. For example, the catalyst can be provided at about 1 wt% to less than 15 wt%, about 1 wt% to about 14 wt%, about 1 wt% to about 13 wt%, about 1 wt% to about 12 wt%, about 1 wt% to about 11 wt%, about 1 wt% to about 10 wt%, about 2 wt% to about 14 wt%, about 3 wt% to about 14 wt%, about 4 wt% to about 14 wt%, about 5 wt% to about 14 wt%, about 3 wt% to about 13 wt%, about 4 wt% to about 12 wt%, or about 5 wt% to about 10 wt% of the recycled composition. Advantageously, the amount of catalyst should be minimized or, at least, selected to balance the processing time and cost (since the catalyst may be more costly to procure than the thermosetting resin waste material).

[0032] By the action of milling, the catalyst is completely mixed with the pieces of thermosetting polyurethane waste. The waste (and, presumably, the catalyst) is size-reduced to produce a fine powder mixture in 100% yield. This procedure, called "vitrimerization", produces a vitrimerized polyurethane, which can be reprocessed.

[0033] It will be understood that the fine powder describes the relative particle size. The powder has a significantly smaller average particle size and distribution compared to crushing. Both techniques are known in the art.

[0034] More specifically, the fine powder is a free-flowing particle when poured. In some embodiments, substantially all of the material passes through at least a no. 355 and / or no. 180 sieve (i.e., both according to ISO standard 565-1972), meaning that substantially all of the fine particles are smaller than the respective individual aperture sizes of 0.355 mm and / or 0.180 mm found in such sieves.

[0035] Figure 1 further shows a presentation of a hot press or compression molding procedure that can be used to form a fine powder mixture and a powder mixture into a recycled component or article. These steps are after the formation of the catalyst-ligand complex that enables recycling of the produced powder.

[0036] The vitrimer polyurethane can be reprocessed by heating the vitrimer polyurethane at a temperature lower than the melting temperature of the catalyst. For example, the vitrimer polyurethane can be reprocessed by compression molding the vitrimer polyurethane at a temperature lower than the melting temperature of the catalyst.

[0037] Figure 2 shows that the exchange reaction in the urethane bond can occur by a bonding and dissociation mechanism. The sharp drop in viscosity due to the dissociative exchange reaction increases the efficiency of reprocessing. However, the dissociation mechanism generates free isocyanate groups, which can result in secondary reactions and stable by-products, thereby potentially reducing the dynamic properties of the network.

[0038] In particular, as soon as the vitrimer-type polymer is formed, it can be reprocessed and recycled without adding more catalyst. Dynamic analysis including the following data shows that the vitrimer-type polymer exhibits properties corresponding to the original / “unused” thermosetting polyurethane foam material. In some embodiments, an article comprising a vitrimer polyurethane or a vitrimer polyurethane can have at least one of a Young's modulus (GPa) greater than that of a thermosetting polyurethane foam, a tensile strength (MPa) greater than that of a thermosetting polyurethane foam, or an elongation at break (%) less than that of a thermosetting polyurethane foam.

[0039] In some embodiments, the method can further include heating the recycled article to a temperature and pressure effective to foam the vitrimer polyurethane. The temperature effective to foam the vitrimer polyurethane may exceed the melting temperature of the catalyst. For example, as shown in FIG. 6A, by applying a force of 10 N at a temperature of 170° C., it is shown that foaming occurs in the vitrimerized network. SEM images show that the initial foam has a cell size of approximately 250 micrometers, and the cell size for the foam generated from the vitrimerized network is in the range of 200 - 500 micrometers. Further optimization will allow for adjustment of the foam density and cell structure.

[0040] Advantages of the disclosed method include the elimination of solvents. Further, ball milling can be achieved at low temperatures (i.e., without the need to provide an external heat source, typically lower than 300° C.). The milling operation can be designed to incorporate a batch or continuous feed process, and for the latter, it is necessary to control the material feed rate in combination with the milling conditions to ensure that a sufficient residence time is achieved within the mill, and the gravity-induced inclination, rotation of the milling chamber, and / or relief valve provide additional control means.

[0041] Examples This example describes an efficient method for recycling waste rigid PU thermoset foam into a high-value-added product. The method uses a carbamate exchange reaction for the vitrimerization of thermoset rigid PU foam. Vitrimerization is an executable, cost-effective, environmentally friendly, and commercially scalable process that can open the way for thermoset recycling. An organic catalyst, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), was used for the vitrimerization of rigid PU foam. TBD is in a solid state at room temperature and can be used in a mechanochemical process. We investigated the effect of different catalyst concentrations on the mechanism (bonding and dissociation) of the exchange reaction and the final properties of the vitrimerization network.

[0042] Materials Commercially available rigid polyurethane foam was kindly provided by the Stepan Company. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) was purchased from Sigma-Aldrich and used as a catalyst.

[0043] Vitrimerization process Particles of polyurethane foam (<500 μm) were obtained by crushing small pieces of PU foam. The ultrafine powder mixture was obtained by ball milling PU microparticles and the catalyst (TBD) in a ball mill tank (Fritsch Pulverisette 6) and purged with N2. Each run for the ball milling process was 45 minutes, consisting of 8 cycles of crushing at a speed of 570 rpm for 5 minutes and intermediate cooling for 15 minutes. Compression molding of the ball-milled powder mixture was carried out at 110 °C and 20 Mpa, with preheating in the mold for 10 minutes and heating for 60 minutes to obtain a vitrimerized sample (Figure 1). Reprocessing of the vitrimerized sample was carried out using the same procedure except that no catalyst was added during ball milling.

[0044] Characterization Dynamic mechanical analysis The dynamic mechanical properties, storage modulus (E’), and tan(δ) were measured using a TA Instruments Q800. The measurements were carried out in tensile mode with a strain amplitude of 0.05%, a constant frequency of 1 Hz, and a scanning rate of 5 °C min -1 from 25 to 200 °C. The glass transition temperature (T g ) of the sample was determined by the peak of the tan(δ) curve. Dilatometry was performed in tensile and controlled force modes. Two different constant forces of 0.2 and 0.75 N were used from 25 to 200 °C with a heating rate of 5 °C min -1 . Strain was measured during the test.

[0045] Fourier transform infrared spectroscopy (FTIR) FTIR analysis was performed on an Agilent Cary 630 FTIR spectrophotometer in the spectral range of 4000 - 600 cm -1 .

[0046] Mechanical tests Stress-strain curves were obtained in tensile mode on an MTS Insight tensile device. The sample size was 1.2 mm × 5.4 mm × 15 mm (thickness, width, gauge length), and the strain rate was 5 mm min -1 .

[0047] Rheology Stress relaxation tests were performed on a TA ARES-G2 rheometer using a 25 mm parallel plate geometry on samples with an average thickness of 1.2 mm. After a 10-minute temperature equilibration, a 0.1% strain step was applied. A constant vertical force of 10 N was applied during the test to avoid the gap between the sample and the geometry.

[0048] Thermogravimetric analysis (TGA) The thermal stability was investigated using a TA Instruments Q500 with an aluminum pan. Approximately 10 mg was used for each run, and a heating rate of 10 °C min -1 was used from room temperature to 700 °C under a nitrogen flow.

[0049] Scanning Electron Microscopy (SEM) The morphology of the PU foam and vitrimerized samples was characterized using a ThermoFisher Apreo2 SEM scanning electron microscope.

[0050] Results and Discussion Network Reformation The vitrimerization process is schematically shown in Figure 1. A mixture of crushed PU foam and different concentrations of catalyst (2, 5, and 10 wt%) is ball milled under a nitrogen atmosphere. The melting temperature of TBD is 125 °C, so it should be noted that the powder was vitrimerized at 110 °C in order to avoid catalyst melting during compression molding as indicated by preheating the powder at 120 °C (Figure 8A). When the temperature is increased to 200 °C for the powder ball milled with the catalyst, sample decomposition occurs during compression molding (Figure 8B).

[0051] As shown in Figure 2, the exchange reaction in the urethane bond can occur via a bond and dissociation mechanism. The sharp drop in viscosity due to the dissociative exchange reaction increases the efficiency of reprocessing. However, the dissociation mechanism may generate free isocyanate groups, which may result in secondary reactions and stable by-products, thereby potentially reducing the dynamic properties of the network.

[0052] FTIR results (Figure 3) for the initial, vitrimerized, and ball milled PU foams show that by introducing the catalyst and increasing the concentration to 5 and 10 wt%, the absorbance of the urethane carbonyl group (C=O) splits at 1718 and 1650 cm -1 This split absorbance is due to the stretching vibrations of the non-hydrogen-bonded and hydrogen-bonded carbonyl groups. It has been shown that the carbonyl stretching vibration can shift by dozens of cm -1 in the lower wavenumber direction of the spectrum due to hydrogen bonding. In addition, at approximately 3340 cm -1The absorbance at [specific condition] is related to the N-H stretching vibration of the urethane group, and with the increase in catalyst concentration, hydrogen bonding of the N-H group is obtained. According to the FTIR results, with the increase in catalyst concentration, more hydrogen bonds are obtained in the structure of the vitrimerized PU foam, indicating that the mechanical properties of the vitrimerized sample can be improved.

[0053] Dynamic mechanical analysis The DMA results (Figure 4A) show similar values for the storage modulus of the vitrimerized PU foams at room temperature using different catalyst concentrations, followed by a sharp decrease with increasing temperature and reaching a plateau region at high temperatures. The storage modulus in the plateau region decreases with the increase in catalyst concentration in the vitrimerized samples, suggesting mainly a dissociation mechanism for the exchange reaction. At higher temperatures (around 170 °C), it should also be noted that the vitrimerized samples with 5 and 10 wt% catalyst show a sharp decay in the storage modulus. As shown in Figure 9, at this temperature, these vitrimerized samples start to foam. This phenomenon may indicate the occurrence of side reactions of free isocyanate groups. Such groups may be formed via the dissociation mechanism during the exchange reaction (Figure 2).

[0054] Thermal properties of the vitrimerized network The thermal behavior of the vitrimer network was investigated and compared with the initial PU form. According to the DSC results (Figure 4C), in the initial PU form, a weak transition is shown at approximately 50 °C, which is not affected by vitrimerization, and a much sharper transition is shown at approximately 90 °C, which shifts to a higher temperature (110 - 120 °C) in the vitrimerized sample. The second transition occurring at a higher temperature for the vitrimerized sample may be due to a higher crosslink density in the vitrimer network. The DSC results show the same trend as the tan-delta results shown in Figure 4B, indicating a small reduction in the glass transition temperature of the vitrimerized sample with an increase in catalyst concentration. These results repeat a reasonable dissociation mechanism of the exchange reaction at high temperatures with an increase in catalyst concentration. According to the TGA results (Figure 4D), the vitrimerized PU form is stable up to 200 °C, indicating that it is suitable for most polyurethane-based material applications.

[0055] Stress relaxation In the presence of a catalyst, topological rearrangement and stress relaxation occur for the vitrimerized sample due to the exchange reaction activated at high temperatures. As shown in Figures 5A and 5B, increasing the catalyst concentration accelerates the exchange reaction rate because more dynamic bonds are formed in the network. The Arrhenius equation τ * (T)=τ * 0(T)exp(E a / RT) is used to obtain the activation energy (Ea) for the exchange reaction. The relaxation time τ * is defined as the time to relax 63% of the initial stress. The results show that the activation energy for the vitrimer network with 2 wt% catalyst is 43 kJ / mol, and for 10 wt% TBD, it is 40 kJ / mol.

[0056] The dynamic covalent bonds in the vitrimer network introduce temperature-dependent behavior, and the chemical exchange reaction controls the viscosity. Therefore, the vitrimer can be processed without losing network integrity due to the viscosity controlled by the exchange reaction. The topological freezing point (T v) defines the viscoelastic phase transition in the vitrimer. The exchange reactions occur slowly below and above T v respectively, and fast. To ensure reproducible results, the topology freezing point measured in a dilatometry experiment carried out using two different constant forces of 0.20 and 0.75 N is approximately 90 °C in a vitrimerized sample having 10 wt% TBD (Figure 5C).

[0057] Foaming of the vitrimerized network As previously described, the vitrimerized network can be foamed again by applying heat and a small pressure. Figure 6A shows that foaming occurs in the vitrimerized network by applying a 10 N force at a temperature of 170 °C. SEM images show that the initial foam has a cell size of approximately 250 micrometers, and the cell size for the foam generated from the vitrimerized network is in the range of 200 - 500 micrometers. It should be noted that the research presented here on foaming is exploratory research and process optimization has not been attempted. Further optimization will enable adjustment of the foam density and cell structure.

[0058] Mechanical properties of the vitrimerized network The mechanical properties of the vitrimerized PU foam were evaluated by a tensile test, and the results are shown in Figure 7. A significantly harder network is obtained by vitrimerization compared to the initial PU foam (Young's modulus ≈ 4 KPa, tensile strength ≈ 170 KPa) (for the sample having 10 wt% TBD, Young's modulus ≈ 2.7 GPa, tensile strength ≈ 76.45 MPa). The details of the mechanical properties are summarized in the following table. As shown in Figure 10, both the tensile strength (σ max ) and the elongation at break (e b ) increase with an increase in the catalyst concentration, due to an increase in hydrogen bonding in the network at higher catalyst concentrations as indicated by the FTIR results (Figure 3).

Table 1

[0059] The vitrimerized network was reprocessed again by crushing and ball milling without the addition of a catalyst. The results (Figure 7) show that the vitrimerized network can be reprocessed without loss of mechanical properties.

[0060] Rigid polyurethane foam can be recycled by vitrimerization using an organic catalyst (TBD). The stress relaxation results show that the vitrimerized network can rapidly relax stress. The dynamic network has a low activation energy (as low as 40 KJ / mol with 10 wt% TBD), which allows this vitrimerized material to be processed using common processing techniques such as injection molding and extrusion. Dynamic mechanical analysis shows that the carbamate exchange reaction in the vitrimerized network mainly occurs via a dissociation mechanism. The formation of free isocyanate during the exchange reaction under high temperature (170 °C) and low pressure results in foaming of the vitrimerized network. The mechanical properties of the vitrimerized network are significantly higher compared to the values previously reported in the literature for recycled PU foam. The vitrimerization process shows the potential to convert existing rigid polyurethane foam waste in the market into products with higher added value. This research can open the way to overcome the challenges in the recycling of polyurethane thermosetting resin waste, and the properties of the vitrimerized network can be adjusted with minimal environmental impact.

[0061] From the above description of the invention, those skilled in the art will recognize improvements, changes, and modifications. Such improvements, changes, and modifications within the technical scope of the art are intended to be encompassed by the appended claims. All references, publications, and patents cited in this application are hereby incorporated by reference in their entirety.

Claims

**Claim 1** A method for recycling polyurethane, comprising: partially decomposing crosslinking ligands in the polyurethane; providing a catalyst to the decomposed polyurethane to produce a recyclable vitrimer polyurethane composition; and processing the recyclable vitrimer polyurethane composition to form a vitrimerized polyurethane wherein the vitrimerized polyurethane comprises a dynamic recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane. **Claim 2** A method for recycling polyurethane, comprising: partially and mechanically decomposing the crosslinking structure in the polyurethane; mechanically mixing a catalyst with the decomposed polyurethane to produce a recyclable vitrimer polyurethane composition; and thermally and / or mechanically processing the recyclable vitrimer polyurethane composition to obtain a vitrimerized polyurethane product wherein the vitrimerized polyurethane comprises a dynamic recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane through a carbamate exchange reaction. **Claim 3** A method for recycling a thermosetting polyurethane foam, comprising: selecting a thermosetting polyurethane foam provided as particles and / or fragments; providing a catalyst to the particles of the thermosetting polyurethane foam to produce a recyclable vitrimer polyurethane composition; and milling the recyclable vitrimer polyurethane composition in the presence of a milling medium to obtain a vitrimer polyurethane wherein the vitrimer polyurethane comprises a dynamic recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane. **Claim 4** The method according to any one of claims 1 to 3, wherein the catalyst is provided in an amount of less than 15.0 wt%, preferably about 5.0 wt% to about 10 wt% of the mass of the recyclable vitrimer polyurethane composition. **Claim 5** The method according to any one of claims 1 to 4, wherein the catalyst comprises triazabicyclodecene. **Claim 6** The method according to any one of claims 1 to 5, wherein the recyclable vitrimer polyurethane composition is formed as fine powder. **Claim 7** The method according to claim 3, further comprising reprocessing the vitrimer polyurethane to form a recycled article. **Claim 8** The method according to claim 7, wherein the recyclable vitrimer polyurethane composition is reprocessed by heating the recyclable vitrimer polyurethane composition at a temperature lower than the melting temperature of the catalyst.

9. The method according to claim 7, wherein the recyclable vitrimer polyurethane composition is reprocessed by compression molding the recyclable vitrimer polyurethane composition at a temperature lower than the melting temperature of the catalyst.

10. The method according to any one of claims 7 to 9, further comprising heating the recycled article to a temperature and pressure effective to foam the recyclable vitrimer polyurethane composition.

11. The method according to claim 10, wherein the temperature effective to foam the vitrimer polyurethane is higher than the melting temperature of the catalyst.

12. The method according to any one of claims 7 to 11, wherein the recycled article is recyclable again without addition of a catalyst and without loss of mechanical properties.

13. The method according to claim 3, wherein the selected thermosetting polyurethane foam is a waste thermosetting polyurethane foam.

14. Recycled polyurethane formed by the method according to any one of claims 1 to 13.

15. The recycled polyurethane according to claim 14, configured to be reprocessed without addition of an additional catalyst and without loss of mechanical properties.

16. A vitrimerized polymer composition comprising a polyurethane having a partially decomposed crosslinked ligand and a catalyst, wherein the vitrimerized polymer composition comprises a dynamically recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane.

17. A vitrimerized polymer composition comprising a polyurethane having a partially decomposed crosslinked ligand and an environmentally friendly organic catalyst, wherein the vitrimerized polymer composition comprises a dynamically recyclable network, and a part of the catalyst forms a ligand with a part of the polyurethane; and / or the vitrimerized polymer composition network can rapidly relax stress at a low temperature, preferably below 120 °C, preferably in 10 seconds; and / or The vitrimerized polymer composition network retains high mechanical strength, preferably with a Young's modulus of at least 2.7 GPa and a tensile strength of at least 76.4 MPa, the composition.

18. A vitrimerized polymer composition comprising a polyurethane having a partially decomposed crosslinking ligand, a catalyst, and a dynamically recyclable network, wherein a part of the catalyst forms a ligand with a part of the polyurethane, the composition.

19. The catalyst occupies less than 15.0 wt% of the vitrimerized polymer composition, preferably the catalyst occupies from about 5.0 wt% to about 10 wt% of the vitrimerized polymer composition, the vitrimerized polymer composition according to any one of claims 16-18.

20. The catalyst comprises triazabicyclodecene, the vitrimerized polymer composition according to any one of claims 16-19.

21. The vitrimerized polymer composition according to any one of claims 16-20, which is in the form of fine powder.

22. The polyurethane is a thermosetting polyurethane foam, the vitrimerized polymer composition according to any one of claims 16-21.

23. The vitrimerized polymer composition according to any one of claims 16-22, which is processed into an article.

24. The article is a compression molded article, the vitrimerized polymer composition according to claim 23.

25. The article is a foamed article, the vitrimerized polymer composition according to claim 23.

26. The article has at least one of a Young's modulus (GPa) greater than that of the thermosetting polyurethane foam, a tensile strength (MPa) greater than that of the thermosetting polyurethane foam, or an elongation at break (%) less than that of the polyurethane, the vitrimerized polymer composition according to claim 23.

27. The vitrimerized polymer composition according to any one of claims 16-26, which is processable without addition of a catalyst and without loss of mechanical properties.

28. An article comprising the recycled thermosetting polyurethane foam according to claim 14 or the vitrimerized polymer composition according to any one of claims 16-27.

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