Reprocessing of crosslinked polyurethane

Abstract

This study provides a green catalyst capable of rapid polyurethane bond exchange for direct recycling of crosslinked PU waste, and enables large-scale recycling of crosslinked PU waste by controlling the dynamic properties of the urethane bonds. [Solution] Disclosed herein are methods and compositions for reprocessing crosslinked polyurethane. The method may include machining the crosslinked polyurethane, mixing the machined crosslinked polyurethane with a solid polyurethane exchange catalyst, heating the mixture to an effective bond exchange temperature, and applying a mechanical force to the mixture for an effective bond exchange time. In another aspect, the method may include heating a polyurethane exchange catalyst and an antioxidant composition to an effective bond exchange temperature, where the antioxidant composition comprises a crosslinked polyurethane and an antioxidant, and applying a mechanical force to the polyurethane exchange catalyst and antioxidant composition for an effective bond exchange time.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Patent Application No. 63 / 373,197, filed August 22, 2022, the entire contents of which are incorporated by reference.

[0002] Statement Regarding Federally Sponsored Research This invention was made with government support under DE-EE0007897 awarded by the Department of Energy and CHE1901635, DMR1121262 and DGE1842165 awarded by the National Science Foundation. The government has certain rights in this invention. [Background technology]

[0003] Many cross-linked polymers are not recyclable. By incorporating dynamic covalent bonds into polymer networks, materials can be obtained that exhibit the recyclability typically associated with thermoplastics while possessing mechanical properties that compete with traditional static thermosets. Many dynamic covalent bonds, including imines, boronic esters, disulfides, and reversible Diels-Alder adducts, have been incorporated into cross-linked polymer networks and shown to enable reprocessing of the cross-linked polymer networks. While these approaches potentially enable new technologies and sustainable reuse of cross-linked materials, these bonds are not common in commodity polymers such as polyurethanes (PUs). Summary of the Invention [Problem to be solved by the invention]

[0004] Polyurethanes (PUs) are the sixth largest class of polymers used worldwide and are widely used in crosslinked structural designs as foams, adhesives, coatings, and structural components. Direct bulk reprocessing of PUs to produce similar valuable materials is not well developed. Due to their large-scale use, much research has focused on the repurposing and recycling of crosslinked PU waste, but most approaches rely on chemical recycling via glycolysis to generate new PU polyol oligomers or blending with thermoplastic polymers. One strategy for directly recycling these crosslinked materials is the incorporation of other dynamic linkages into PUs. Currently, few methods exist for recycling polyurethanes on an industrially viable scale. While proposed methods for reprocessing polyurethanes using dibutyltin dilaurate (DBTDL) are effective, DBTDL is a toxic compound with documented health concerns. Green catalysts capable of rapid polyurethane bond exchange are needed to mitigate these toxicity concerns and make these reprocessing methods industrially viable. Another strategy involves controlling the dynamic nature of urethane linkages, enabling large-scale recycling of these materials. [Means for solving the problem]

[0005] Disclosed herein are methods and compositions for reprocessing crosslinked polyurethane. In a first aspect, the method can include machining a crosslinked polyurethane, mixing the machined crosslinked polyurethane with a solid polyurethane exchange catalyst, heating the mixture to an effective bond exchange temperature, and applying a mechanical force to the mixture for an effective bond exchange time. The crosslinked polyurethane can include a network polymer formed from isocyanate building blocks and second building blocks having hydroxyl groups capable of reacting with the isocyanate groups of the isocyanate building blocks to form urethane bonds. Suitably, the crosslinked polyurethane is machined with the solid polyurethane exchange catalyst. The crosslinked polyurethane can be combined with an antioxidant before, during, or after machining. The disclosed technology enables solvent-free reprocessing of crosslinked polyurethanes, including crosslinked polyurethane foams. Additionally, the disclosed technology allows for reprocessing of crosslinked polyurethanes with a lower loading of polyurethane exchange catalyst compared to methods involving impregnating the crosslinked polyurethane with the polyurethane exchange catalyst.

[0006] The solid polyurethane exchange catalyst can include Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom. In some embodiments, the solid polyurethane exchange catalyst includes Zr. Suitably, the solid polyurethane exchange catalyst is Zr(acac)4 or Zr(tmdh)4. In some embodiments, the solid polyurethane exchange catalyst does not include tin. The machined crosslinked polyurethane can be mixed with less than 5 mol% of the solid polyurethane exchange catalyst relative to the carbamate.

[0007] Machining the crosslinked polyurethane can include milling the crosslinked polyurethane. The crosslinked polyurethane can be machined at a temperature below room temperature, for example, below 20° C. In some embodiments, the crosslinked polyurethane can be machined at a temperature below room temperature, such as between −200° C. and 0° C.

[0008] The method can optionally include a drying step prior to the step of heating to an effective bond exchange temperature. The mixture can be dried under vacuum at a drying temperature prior to the step of heating the mixture to an effective bond exchange temperature to remove extraneous water.

[0009] Another aspect of the present invention provides a composition for use in the disclosed method, comprising a machined crosslinked polyurethane and a solid polyurethane exchange catalyst. The solid polyurethane exchange catalyst can comprise Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom. In some embodiments, the solid polyurethane exchange catalyst comprises Zr. Suitably, the solid polyurethane exchange catalyst is Zr(acac)4 or Zr(tmdh)4. In some embodiments, the solid polyurethane exchange catalyst is tin-free. The machined crosslinked polyurethane can be mixed with less than 5 mol% of the solid polyurethane exchange catalyst relative to the carbamate.

[0010] In another aspect, the method can include heating a polyurethane exchange catalyst and antioxidant composition to an effective bond exchange temperature, where the antioxidant composition comprises a crosslinked polyurethane and an antioxidant, and applying a mechanical force to the polyurethane exchange catalyst and antioxidant composition for an effective bond exchange time. In some embodiments, the antioxidant is tris(nonylphenyl)phosphite. In some embodiments, the polyurethane exchange catalyst can be solid and can be mixed with the polyurethane composition before heating to an effective bond exchange temperature. In some embodiments, a polyurethane exchange catalyst solution containing the polyurethane exchange catalyst is infiltrated into the crosslinked polyurethane composition before heating to an effective bond exchange temperature. The polyurethane exchange catalyst can include Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom. In some embodiments, the polyurethane exchange catalyst includes Zr. Suitably, the solid polyurethane exchange catalyst is Zr(acac)4 or Zr(tmdh)4. In some embodiments, the polyurethane exchange catalyst is tin-free.

[0011] Another aspect of the present invention provides a composition for use in the disclosed method, comprising a crosslinked polyurethane, an antioxidant, and a polyurethane exchange catalyst. The polyurethane exchange catalyst can comprise Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom. In some embodiments, the polyurethane exchange catalyst comprises Zr. Suitably, the polyurethane exchange catalyst is Zr(acac)4 or Zr(tmdh)4. In some embodiments, the polyurethane exchange catalyst is tin-free. In some embodiments, the crosslinked polyurethane is machined and mixed with less than 5 mol% of the polyurethane exchange catalyst relative to the carbamate. Optionally, the polyurethane exchange catalyst is solid. [Brief explanation of the drawings]

[0012] Non-limiting embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component shown is typically represented by a single numeral. For clarity, not every component is shown in every figure, nor are every component of each embodiment of the present invention shown unless illustration is necessary to enable those skilled in the art to understand the invention. [Figure 1] FIG. 1 shows a general scheme for the reprocessing of polyester polyurethane networks using catalysts such as Zr(acac) or Zr(tmhd). [Figure 2] (A) General scheme for casting polyester polyurethane films; (B) FT-IR (attenuated total reflectance) spectra of MDI and crosslinked films cured with either DBTDL or Zr(acac)4 catalyst; (C) DMTA of crosslinked films cured with either DBTDL or Zr(acac)4 catalyst. [Figure 3] (A) Stress relaxation analysis of films containing 0.25, 0.50, or 1.0 mol% DBTDL catalyst relative to carbamate bonds. (B) Stress relaxation analysis of films containing 0.25, 0.50, or 1.0 mol% Zr(acac)4 catalyst relative to carbamate bonds. [Figure 4] FIG. 1 illustrates a general scheme for reprocessing polyester polyurethane foam. [Figure 5] FIG. 1 shows stress relaxation analysis data for reprocessed foams with varying amounts of catalysts corresponding to DBTDL, Zr(acac)4, and Zr(tmhd)4. [Figure 6]FIG. 1 shows images of polyester polyurethane films reprocessed by multiple cycles, DMTA data for films reprocessed by multiple successive cycles, and tensile data for films reprocessed by multiple successive cycles. [Figure 7] FIG. 1 shows commercial materials reprocessed with DBTDL and Zr-based catalysts and the corresponding SRA curves. [Figure 8] FIG. 10 shows stress relaxation analysis of a sample post-incorporated with 2 mol% Zr(acac)4 by cryogenic milling. [Figure 9] FIG. 1 shows the differential scanning calorimetry corresponding to polyester PU films synthesized with 0.5 wt% TNPP and 1 mol% DBTDL catalyst or 1 mol% Zr(acac)4 catalyst. [Figure 10] Figure 1 shows images of polyester PU films synthesized with 0.5 wt% TNPP and, from top left to bottom right, 0.25, 0.35, 0.50, 0.75, or 1.0 mol% Zr(tmhd)4. [Figure 11] FIG. 1 shows FT-IR of MDI and polyester PU foams with 0.5% TNPP. [Figure 12] FIG. 1 shows differential scanning calorimetry of polyester PU foam containing 0.5 wt% TNPP. [Figure 13] This figure shows polyester PU foams reprocessed without TNPP and with the post-incorporation of DBTDL, Zr(acac)4, or Zr(tmhd)4 at 30 mg / mL, from left to right. Note that the material quality without TNPP is poor, including inhomogeneity, brittleness, and cracks visible to the naked eye. Each filament is approximately 4 mm wide and 1 mm thick. [Figure 14] Figure 1 shows the FT-IR of polyester PU foam containing 0.5% TNPP and retreated with Zr(acac)4 for five consecutive cycles. The increase in urea content can be seen by the growth of the corresponding peak at 1642 cm-1, indicated by the grey dotted line. [Figure 15]Figure 1 shows the FT-IR of polyester PU foam containing 0.5% TNPP and retreated with Zr(acac)4 for five consecutive cycles. The increase in urea content can be seen by the growth of the corresponding peak at 1642 cm-1, indicated by the grey dotted line. [Figure 16] Images of polyester PU foam cryogenically milled with 2 mol% Zr(acac)4 and the resulting powder reprocessing into films. Each film was successfully reprocessed for four consecutive cycles. [Figure 17] FIG. 10 shows dynamic mechanical thermal analysis of a sample post-incorporated with 2 mol% Zr(acac)4 by cryogenic milling. [Figure 18] FIG. 1 shows differential scanning calorimetry comparing polyester PU foam retreated with Zr(acac)4 introduced by a solvent-assisted method and with 2 mol% Zr(acac)4 introduced by cryomilling, with untreated PU film material. DETAILED DESCRIPTION OF THE INVENTION

[0013] Reprocessing or recycling crosslinked polymers by incorporating dynamic covalent crosslinks has the potential to enhance the sustainability associated with the use of these materials. Polyurethanes (PUs) are the largest class of polymers commonly used for crosslink formation, yet their direct recycling to generate similar valuable materials is underdeveloped. Examples demonstrate that various Lewis acid catalysts can selectively mediate the exchange of urethane bonds under relatively mild conditions. Incorporation of these catalysts into crosslinked polyether and polyester PUs results in crosslinked materials that release stress very quickly. Due to their dynamic nature, reprocessing these polymers, for example by compression molding, can yield materials with similar crosslink densities despite their covalent crosslink architecture.

[0014] Covalently compatible networks (CANs) offer one strategy for achieving these sustainability goals. CANs are polymer networks with dynamically exchanging bonds that allow them to reshape at elevated temperatures or other stimuli while maintaining the desired properties of the material at its use temperature. 6-9 Some functional groups give rise to dynamic networks, including esters. 9 , boronate ester 10-13 , Diels-Alder adducts 14,15 , carbonate 16 , disulfide 17,18 , siloxane 19,20 , urethane 21-23 , and other things 5,7 Many of these utilize external catalysts blended into the network. For PU, transcarbamation can be achieved by catalysts at temperatures above 140°C using methods such as compression molding, extrusion, and reaction injection molding. 23-25 Organotin compounds such as DBTDL exhibit neurotoxicity, teratogenicity, and environmental toxicity. 26-28 For CAN, it is even more important that the catalyst be safe and environmentally friendly, as it is often embedded throughout the life of the material. Exemplary PU reprocessing methods are disclosed in US 17 / 050,138, US 17,605,831, PCT / US2023 / 063276, and PCT / US2023 / 066028, the contents of which are incorporated by reference in their entirety for all purposes.

[0015] Here, the applicants are using zirconium acetylacetonate [Zr(acac)4] 33 and zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Zr(tmhd)4] 34We demonstrate the reprocessing of PU thermoset resins using non-tin Lewis acid catalysts such as (Figure 1). Here, we impregnated both synthesized and commercial thermoset PUs with these catalysts. The catalysts can be introduced from solution at high loadings, which results in reprocessable networks upon extrusion at elevated temperatures. The Lewis acid catalyst promotes carbamate exchange and enables a scalable recycling process for PU waste. The disclosed catalysts enable the reprocessing of PU foams for up to four cycles, and dynamic mechanical thermal analysis revealed that the zirconium-based catalysts can maintain the glass transition temperature and crosslink density between cycles. Therefore, non-tin catalysts, such as the disclosed zirconium catalysts, are suitable green catalysts for bulk reprocessing of PU thermoset resins.

[0016] Additionally, solvent-free methods are disclosed that allow for low catalyst loading. Additionally, these solvent-free methods allow for the elimination of solvents from reprocessing workflows. The crosslinked polyurethane can be machined, such as by cryogenic milling. A solid polyurethane exchange catalyst can be introduced into the crosslinked polyurethane before, during, or after machining of the crosslinked polyurethane. If a solid polyurethane exchange catalyst is introduced before or during machining, the crosslinked polymer and catalyst can be mixed during machining.

[0017] To aid in understanding the technology, some definitions are provided.

[0018] "Antioxidant" means an additive that protects a polymer from oxidation by controlling molecular weight changes that result in loss of physical, mechanical, and optical properties. Antioxidants can act by scavenging free radicals that are formed when the polymer is exposed to high temperatures and / or oxygen. Antioxidants can be divided into two categories: primary antioxidants, * OH and *They react with free radicals such as OR to form inactive products such as water and alcohols. Examples of primary antioxidants include sterically hindered phenols. Secondary antioxidants react with hydroperoxides to form inactive products such as alcohols. Examples of secondary antioxidants include phosphites. The use of antioxidants can enable superior physical properties of reprocessed PU. Compositions containing PU and antioxidants can be referred to as antioxidant compositions. For example, in some cases, samples lacking antioxidants were unable to be characterized due to the presence of cracks visible to the naked eye in the samples. One example of an antioxidant is tris(nonylphenyl)phosphite (TNPP).

[0019] "Block" means a portion of a macromolecule that contains many constitutional units and has at least one structural or configurational feature that is not present in adjacent portions.

[0020] "Branch" means an oligomer or polymer offshoot from a macromolecular chain.

[0021] "Branch point" means the point on a chain where a branch is attached.

[0022] "Branch unit" means a building block that includes a branch point.

[0023] "Catalyst" means a substance that increases the rate of a reaction without altering the total Gibbs energy change in the reaction. Suitably, a catalyst may be a coordination entity comprising a central atom and one or more ligands bonded to the central atom. Suitably, the central atom is a metal. "Ligand" means an atom or group bonded to the central atom.

[0024] "Chain" means all or part of a polymer, oligomeric molecule, or block that includes a linear or branched sequence of constitutional units between two boundary units, each of which may be an end group, a branch point, or another characteristic feature of a polymer.

[0025] "Compounding" means blending or mixing materials, such as any of the polyurethane compositions described herein, in a compounding apparatus. Suitably, the materials are compounded at an effective bond exchange temperature for an effective bond exchange time.

[0026] "Compounding apparatus" refers to an apparatus for blending or mixing materials, such as any of the polyurethane compositions described herein. In some embodiments, the compounding apparatus is an extruder, such as a single-screw or twin-screw extruder, a mixer, or a kneader. Suitably, the twin-screw extruder may be a co-rotating twin-screw extruder or a counter-rotating twin-screw extruder. The compounding apparatus can operate in a batch or continuous mode. Suitably, a continuous compounding apparatus can include an inlet, such as a feed hopper or other suitable feeding mechanism, for introducing materials into the compounding apparatus, an outlet for extruding the compounded materials, and a compounding zone between the inlet and outlet for mixing or blending the materials. Suitably, the compounding zone is configured to allow the materials to be compounded for an effective bond exchange time. The compounding apparatus can also include a heating element to allow the materials to be compounded at an effective bond exchange temperature.

[0027] "Constitutional unit" means an atom or group of atoms (including pendant atoms or groups, if any) that comprise part of the essential structure of a polymeric or oligomeric molecule, block or chain.

[0028] "Covalent network" or "covalent polymer network" means a network in which all permanent pathways through the structure are formed by covalent bonds.

[0029] "Dynamic network" or "dynamic polymer network" refers to a covalently bonded network capable of undergoing bond exchange reactions above the effective bond exchange temperature. The dynamic network can exhibit viscoelastic liquid properties above the solidification transition temperature.

[0030] An "effective amount of polyurethane exchange catalyst" refers to the amount of polyurethane exchange catalyst necessary for the urethane bond exchange reaction to occur at an effective bond exchange temperature within an effective bond exchange time. In some embodiments, an effective amount of polyurethane exchange catalyst enables an effective bond exchange time of 12 minutes or less at an effective bond exchange temperature of 160°C or less. In some embodiments, the mol% of polyurethane exchange catalyst relative to the total carbamate functionality may be 5 mol% or less. Suitably, this mol% may be 4 mol% or less, 3 mol% or less, 2 mol% or less, 1 mol% or less, or less than 1 mol%. Some materials may contain small amounts of residual catalyst from their preparation, often 0.1 mol% or less. Such limited amounts of catalyst are typically insufficient to enable dynamic bond exchange within practical timeframes. Therefore, the effective amount of polyurethane exchange catalyst can be increased after synthesis by methods described herein, such as swelling or direct mechanical mixing.

[0031] "Effective bond exchange temperature" refers to a temperature above the solidification transition temperature. The "solidification transition temperature" is the temperature at which a material transitions from a viscoelastic solid to a viscoelastic liquid. The effective bond exchange temperature is below the temperature at which the dynamic network undergoes irreversible thermal instability or degradation. In some embodiments, the effective bond exchange temperature is above the solidification transition temperature and below 275°C. Suitably, the effective bond exchange temperature may be above the solidification transition temperature and below 250°C, 225°C, 200°C, 190°C, 180°C, 170°C, 160°C, 150°C, 140°C, 130°C, 120°C, 110°C, or below 110°C. In some embodiments, the effective bond exchange temperature is between 110°C and 275°C, between 140°C and 275°C, between 150°C and 250°C, between 160°C and 250°C, or between 170°C and 225°C. In some cases, it may be beneficial to dry the material comprising the crosslinked polyurethane under vacuum before heating the material to an effective bond exchange temperature. Drying can remove water, which may react with free isocyanate during reprocessing. The material can be dried at a temperature between 25°C and 100°C. Drying can be carried out for a suitable period of time. For example, the material can be dried for a period of 2 to 48 hours, 2 to 36 hours, 2 to 24 hours, or 2 to 12 hours. A vacuum is a pressure below atmospheric pressure. For example, the vacuum can be a rough vacuum (e.g., about 1000 to 1 mbar), a medium vacuum (e.g., 1 to 10 -3 mbar), or high vacuum (e.g., 10 -3 ~10 -7 mbar).

[0032] "Effective bond exchange time" refers to a time sufficient for urethane bond exchange to occur. The effective bond exchange time can be determined by monitoring the stress decay of the polyurethane composition. Suitably, the minimum effective bond exchange time can be determined as the time required for the stress relaxation modulus to relax to at least 37% (1 / e) of its initial value. In some embodiments, the effective bond exchange time is 60 minutes or less. Suitably, the effective bond exchange time can be less than 50 minutes, 45 minutes, 40 minutes, 35 minutes, 30 minutes, 25 minutes, 20 minutes, 15 minutes, 12 minutes, 10 minutes, or less than 10 minutes.

[0033] "Foam" refers to a multiphase material containing a gas dispersed in a polymer. Foams can be formed by trapping pockets of gas in a solid or liquid. Foams can be prepared by physical or chemical blowing. In some embodiments, the foam may be a closed-cell foam, in which the gas forms individual, completely enclosed pockets. In other embodiments, the foam may be an open-cell foam, in which the gas pockets are interconnected. Suitably, the polymer is a polyurethane ("polyurethane foam").

[0034] "Inorganic polymer" means a polymer or polymer network having a backbone structure that does not contain carbon atoms. Examples include, but are not limited to, polyphosphazenes, polysilicates, polysiloxanes, polysilanes, polysilazanes, polygermanes, and polysulfides.

[0035] "Isocyanate unit" refers to a unit containing at least one isocyanate group, i.e., -NCO. Suitably, the isocyanate unit can contain more than one isocyanate group, such as two, three, four, or more than four isocyanate groups. The isocyanate unit can be a prepolymer molecule or a branched unit. Suitably, the second unit can function as both a prepolymer molecule and a branched unit. In some embodiments, the isocyanate unit is an aromatic isocyanate unit. As used herein, "aromatic isocyanate unit" refers to an isocyanate unit having an isocyanate group pendant to an aryl group, such as a phenyl ring or other aromatic ring. In other embodiments, the isocyanate unit is an aliphatic isocyanate unit. As used herein, "aliphatic isocyanate unit" means an isocyanate unit having an isocyanate group pendant to an aliphatic group such as an acyclic or cyclic alkyl group, an acyclic or cyclic alkenyl group, or an acyclic or cyclic alkynyl group.

[0036] "Lewis acid" means a molecular entity (and corresponding chemical species) that is an electron pair acceptor and therefore can react with a Lewis base to form a Lewis adduct by sharing the electron pair donated by the Lewis base.

[0037] By "linear chain" is meant a chain with no branch points between boundary units.

[0038] By "macromolecule" or "polymer molecule" is meant a molecule of high relative molecular weight whose structure essentially comprises multiple repeating units actually or conceptually derived from molecules of lower relative molecular weight.

[0039] "Machine processed" means that a material is mechanically altered, for example, by mechanical milling, grinding, cutting, shredding, or any other form of mechanical force. Suitably, a material, such as the polyurethane compositions described herein, can be machined to break the material into small pieces or particles. For example, crosslinked polyurethane can be subjected to cryogenic milling, which may also be referred to as cryomilling. Cryogenically milled means that crosslinked polyurethane is reduced to small pieces or particles at a temperature below room temperature, for example, 20°C. During cryogenic milling, the material or a chamber containing the material can be cooled by any suitable cooling medium. While the examples show the use of liquid nitrogen, other cooling media, such as refrigerants, dry ice, or ice water, may also be used. Suitably, crosslinked polyurethane can be cryogenically milled at temperatures between -200°C and 0°C. The temperature can be determined by controlling the selected cooling medium. For example, the crosslinked polyurethane can be subjected to cryogenic milling at temperatures of about −200° C., −180° C., −160° C., −140° C., −120° C., −100° C., −80° C., −60° C., −40° C., −20° C., 0° C., or any range therebetween. During the mechanical processing of the crosslinked polyurethane, a solid polyurethane exchange catalyst can be mixed with the machined crosslinked polyurethane. In other embodiments, after machining, a solid polyurethane exchange catalyst can be mixed with the machined crosslinked polyurethane.

[0040] "Monomer" means a substance composed of monomer molecules.

[0041] By "monomer molecule" is meant a molecule that is capable of polymerizing, thereby providing the building blocks for the essential structure of a macromolecule.

[0042] "Monomer unit" means the largest structural unit contributed to the structure of a polymeric or oligomeric molecule by a single monomer molecule.

[0043] By "network" is meant a highly branched macromolecule in which essentially each unit is connected to each other and to macroscopic phase boundaries by many permanent pathways throughout the macromolecule, the number of such pathways increasing with the average number of intervening bonds, and these pathways should generally be coextensive with the macromolecule.

[0044] "Network polymer" means a polymer composed of one or more networks.

[0045] By "oligomeric molecule" is meant a molecule of medium relative molecular weight, the structure of which essentially comprises a number of small units actually or conceptually derived from molecules of lower relative molecular weight.

[0046] "Organic polymer" means a polymer or polymer network having a backbone structure containing carbon atoms. Examples include, but are not limited to, polyethers, polyesters, polycarbonates, polyacrylates, polyolefins, and polybutadienes.

[0047] "Polymer" means a substance composed of macromolecules.

[0048] "Polymerization" means the process of converting a monomer or mixture of monomers into a polymer.

[0049] By "prepolymer molecule" is meant a polymeric or oligomeric molecule that can be further polymerized through reactive groups, thereby providing two or more building blocks for at least one type of chain in the final polymer.

[0050] "Polyurethane composition" refers to a dynamic network formed from urethane bonds capable of undergoing a urethane bond exchange reaction. The polyurethane composition includes a urethane-containing network polymer and a polyurethane exchange catalyst infiltrated within or mixed with the network polymer. The network polymer can be formed from isocyanate building blocks and second building blocks having hydroxyl groups capable of reacting with the isocyanate groups of the isocyanate building blocks. The mol% of the polyurethane exchange catalyst relative to the total number of carbamate functional groups can be 5 mol% or less. Suitably, this mol% can be 4 mol% or less, 3 mol% or less, 2 mol% or less, 1 mol% or less, or less than 1 mol%. The second building block can be a prepolymer molecule or a branching unit. Suitably, the second building block can function as both a prepolymer molecule and a branching unit. A prepolymer molecule is an organic or inorganic polymer molecule, such as a polyether, polyester, polycarbonate, polyacrylate, polyolefin, polybutadiene, polysulfide, or polysiloxane, that has one or more hydroxyl groups capable of reacting with an isocyanate group. When the prepolymer molecule also functions as a branching unit, the prepolymer molecule has three or more hydroxyl groups capable of reacting with an isocyanate group, and typically has a number of hydroxyl groups proportional to the number of constituent units of the prepolymer molecule. Network polymers can also be formed from urethane-containing monomers characterized by other polymerizable groups, including, but not limited to, acrylates, methacrylates, or other polymerizable olefins.

[0051] "Polyurethane exchange catalyst" refers to a catalyst that increases the rate of a polyurethane bond exchange reaction. In some embodiments, the polyurethane bond exchange reaction is a carbamate exchange reaction. Suitable metals for the catalyst include Sn, Bi, Fe, Zr, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, or Mo. In some embodiments, the metal is Zr. Suitable ligands for the catalyst include carboxylate, alkoxide, 1,3-diketone, 1,2-diketone, trifluoromethanesulfonate, trifluoromethanesulfonamide, amide, sulfonate, halide, catecholate, phosphine, salicylidenediamine, carbonate, phosphate, nitrate, cyclopentadiene, pyridine, hydroxide, or any combination thereof. Exemplary ligands include acetylacetonate (acac), isopropoxide (OiPr), neodecanoate (neo), laurate, ethylhexanoate, and 2,2,6,6-tetramethyl-3,5-heptanedione (tmhd). Exemplary catalysts include, but are not limited to, dibutyltin dilaurate (DBTDL), Bi(neo)3, Fe(acac)3, Ti(OiPr)2(acac)2, Hf(acac)4, Zr(acac)4, Mn(acac)2, Bi(oct)3, Zn(tmhd)2, Zr(tmhd)4, or any combination thereof.

[0052] "Subchain" means any selected contiguous sequence of constitutional units in a chain.

[0053] A "thermosetting polymer" or "thermosetting resin" is a polymer that irreversibly hardens by curing from a soft solid, viscous liquid prepolymer or resin.

[0054] "Vitrimer" refers to a network polymer that can change its topology through thermally activated bond exchange reactions. At high temperatures, the bond exchange reactions occur at an effectively high rate, and the network polymer exhibits the properties of a viscoelastic liquid. At low temperatures, the bond exchange reactions slow down, and the network polymer behaves like a thermoset polymer.

[0055] Unless otherwise specified or indicated by context, the terms "a," "an," and "the" mean "one or more." For example, "a molecule" should be interpreted as meaning "one or more molecules."

[0056] As used herein, "about," "approximately," "greatly," and "significantly" will be understood by those skilled in the art and will vary to some extent depending on the context in which they are used. When terms are used that are not clear to those skilled in the art given the context in which they are used, "about" and "approximately" mean the particular term plus or minus 10% or less, and "greatly" and "significantly" mean the particular term plus or minus more than 10%.

[0057] As used herein, the terms "include" and "including" have the same meaning as the terms "comprise" and "comprising." The terms "comprise" and "comprising" should be interpreted as "open" transitional terms that allow for the inclusion of additional elements beyond those recited in the claims. The terms "consist" and "consisting of" should be interpreted as "closed" transitional terms that do not allow for the inclusion of additional elements other than those recited in the claims. The term "consisting essentially of" should be interpreted as partially closed, allowing for the inclusion of only additional elements that do not fundamentally alter the nature of the claimed subject matter.

[0058] Any of the methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples or exemplary phrases (e.g., "etc.") provided herein is solely for the purpose of more fully describing the invention and does not impose limitations on the scope of the invention, unless otherwise specifically claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0059] All references, including publications, patent applications, and patents, cited in this specification are hereby incorporated by reference to the same extent as if each individual reference was individually and specifically indicated to be incorporated by reference and was set forth in its entirety herein.

[0060] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of these preferred embodiments may become apparent to those skilled in the art in light of the foregoing description. The inventors anticipate that skilled artisans will adopt such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Furthermore, unless otherwise indicated herein or clearly contradicted by context, the invention includes any combination of the above-described elements in all possible variations thereof. [Example]

[0061] To determine whether Zr-based catalysts are promising candidates for PU foam reprocessing, we first used Zr(acac)4 to catalyze PU film formation, which successfully produced a crosslinked PU network suitable for reprocessing (Figure 2A). Therefore, we first characterized the stress relaxation of these Zr(acac)4-containing films by producing them without any external catalysts. Since these catalysts were subsequently introduced into already prepared PU to simulate the recycling process, no efforts were made to optimize their properties as polymerization catalysts. The thermosetting PU films in this study were synthesized from methylene diphenyl diisocyanate (MDI) and a commercially available polyester polyol (polyol 1), poly[trimethylolpropane / di(propylene glycol)-alt-adipic acid / phthalic anhydride] polyol (f = 2.5). Tris(nonylphenyl) phosphite (Tnpp), a common commercially available antioxidant additive, was also added at 0.5 wt% to reduce PU degradation. The DBTDL or Zr(acac)4 catalyst was directly introduced into the film during synthesis at 0.25, 0.50, and 1.0 mol% relative to the carbamate. All components were dissolved in 20 mL of DCM, cast into an aluminum pan, and post-cured in a vacuum oven at 90 °C for 48 h to produce a translucent, pale yellow film. A control film containing 0.5 wt% Tnpp but no catalyst was also synthesized using a similar procedure, where the film components were dissolved in anhydrous toluene, cast into a pan, and cured at 60 °C. Both DBTDL and Zr(acac)4 catalysts yielded fully crosslinked PU films (Figure 2a). Fourier transform infrared spectroscopy (FT-IR) revealed a peak at 2285 cm after the post-curing step. -1 The isocyanate stretching at 1708-1724cm disappeared. -1 It was found that carbamate stretching appeared near , indicating that the PU network was crosslinked (Figure 2B). Furthermore, dynamic mechanical thermal analysis (DMTA) showed that the film synthesized using DBTDL exhibited a T g is 35 °C, whereas for the film synthesized using Zr(acac)4, T gThe T of the PU film synthesized using DBTDL was found to be 45 °C (Figure 2C). Differential scanning calorimetry (DSC) showed that the T g The T of the PU film synthesized using Zr(acac)4 was 28°C. g The temperature was 34 °C (Figure 9). The gel fractions of these samples were 82% and 77%, respectively, indicating that the materials were crosslinked. Films incorporating Zr(tmhd)4 were also prepared, but these films were brittle and not suitable for further characterization (Figure 10), although subsequent testing showed that Zr(tmhd)4 was a promising reprocessing catalyst (see below). These data indicate that both catalysts provide a crosslinked network that allows for further evaluation of the dynamic process.

[0062] Stress relaxation analysis (SRA) revealed that Zr(acac)4 is an effective exchange catalyst for solid polymers, suggesting that Zr(acac)4 may be suitable as a replacement for DBTDL. SRA performed at 160 °C on films containing 0.25, 0.50, and 1.0 mol% (relative to exchangeable carbamate bonds) of DBTDL yielded characteristic stress relaxation times (τ) of 148 s, 114 s, and 72 s, respectively. * ) was obtained (Fig. 3A). The τ values ​​corresponding to 0.25, 0.50, and 1.0 mol% Zr(acac)4 * The τ values ​​were 2724 s, 443 s, and 100 s, respectively (Fig. 3B). * These values ​​indicate that Zr(acac)4 is a suitable catalyst for mediating urethane exchange and stress relaxation. Control films containing 0.5 wt% Tnpp but no catalyst showed no significant stress relaxation and only a 6% decrease in storage modulus after 1000 s. This confirms that antioxidants do not mediate carbamate exchange under these conditions. These studies revealed the potential of Zr(acac)4 as a urethane reprocessing catalyst. Zr(acac)4 was then evaluated by incorporating the catalyst into previously prepared PU foam.

[0063] Reprocessing of PU foam using catalysts delivered from solution. The method of catalyst introduction and PU foam reprocessing is important to enable post-consumer PU reprocessing. Both Zr(acac)4 and Zr(tmhd)4 are suitable for this application and can be introduced to reprocess thermoset PU foams by twin-screw extrusion (Figure 4). Model PU foams were prepared from the same monomers as above, using isopentane as the physical blowing agent (see SI for detailed procedures). FT-IR spectroscopy of the foam and its gel fraction percentage of 90% were consistent with the formation of a crosslinked network with the expected chemical composition, indicating that the PU network was cured (Figure 11). DSC revealed a glass transition temperature (T g ) was found to be 23 °C (Figure 12). Zr catalyst was introduced into PU foam via a solvent-assisted process and compared with similar samples produced using DBTDL. Ground PU foam (100 mg / mL) was added to a CHCl solution of DBTDL, Zr(acac)4, or Zr(tmhd)4 at catalyst concentrations of 10, 20, or 30 mg / mL. The suspension was stirred for 24 h, filtered, and dried under reduced pressure to remove excess solvent. The resulting dried polymer was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) to determine the catalyst loading derived from the solution-based procedure. PU foam suspended in 30 mg / mL DBTDL contained 1.76 wt% Sn, while foam exposed to Zr(acac)4 contained 0.11 wt% Sn and 3.1 wt% Zr. Foams exposed to the Zr(tmhd)4 solution had 0.10% Sn and 1.3% Zr. These relatively high Zr loadings indicate that the foams take up the catalyst from the solution, absorbing 38% to 87% of the Zr catalyst originally present in the solution (Table 6). While this solvent-assisted catalyst introduction method yields foams with high catalyst loadings, foams with relatively low catalyst loadings have been prepared by the solventless cryogenic milling process described below.

[0064] The catalyst-containing foams were reprocessed using twin-screw extrusion at 200 °C. Samples containing DBTDL, Zr(acac)4, and Zr(tmhd)4 each yielded continuous extrudates of sufficient quality for characterization of their thermomechanical properties. Foams incorporating the antioxidant TNPP in the original polymerization yielded the highest quality samples and were rigorously characterized. Samples lacking TNPP were unable to be characterized due to the presence of fissures visible to the naked eye (Figure 14). The reprocessed foams were run through the extruder once, corresponding to a residence time of approximately 1 minute. The extrudates were collected as homogeneous, pale yellow, continuous filaments with rectangular cross-sections originating from the die (Figure 6). A PU foam containing only residual tin from the polymerization without additional catalyst was extruded as a control. This control foam partially melted under the extrusion conditions, but the extrudates were heterogeneous, discolored, and had obvious fissures visible to the naked eye, indicating that additional catalyst was required for reprocessing under these conditions. Taken together, these findings indicate that both Zr(acac)4 and Zr(tmhd)4 are promising alternatives to DBTDL for PU foam reprocessing, which motivated a more thorough investigation of the thermomechanical properties of these samples.

[0065] Both Zr-based catalysts produced reprocessed PUs with desirable thermomechanical properties and even outperformed DBTDL under some conditions via SRA. SRA of the extrudates at 160 °C revealed that both Zr catalysts promoted dynamic carbamate exchange on a rapid timescale (Figure 5 and Table 1). Overall, higher catalyst loadings resulted in shorter τ * The τ of the samples reprocessed with either Zr catalyst was less than 2 minutes. * Therefore, both Zr(acac)4 and Zr(tmhd)4 are capable of rapid carbamate exchange and are promising candidates for reprocessing catalysts. In the case of Zr(acac)4, τ *The time is fast, less than 10 s, which may be due to the high catalyst loading from the solution-based method. In contrast, the cryogenically milled sample containing a lower amount of Zr(acac)4 has a slower time, still less than 2 min, τ * Time is shown (see below). Based on extrudate quality and SRA analysis, the foam sample containing 0.5% Tnpp, which had the highest Zr catalyst loading, produced the best performing material. Therefore, these conditions were carried forward for further testing in continuous reprocessing experiments.

[0066] Evaluation of multiple reprocessing cycles using Zr-based catalysts [Table 1]

[0067] To further explore the potential of Zr-based PU reprocessing catalysts, multiple reprocessing cycle tests using Zr-based catalysts were conducted to evaluate the lifetime of promising catalysts and their continuous use in PU materials. Dynamic PU networks were reprocessed through three repeated cycles by compression molding, both with and without the addition of additional catalyst. 21,43-45 However, reprocessing of PU networks via twin-screw extrusion has not been thoroughly studied. Therefore, catalyst-containing foam samples were reprocessed at 200 °C and 50 rpm for as many cycles as possible until the PU material degraded past characterizable quality. The concentration conditions after the introduction of 30 mg / mL of catalyst resulted in the fastest exchange kinetics, so samples produced under these conditions were used. The extrudate from the previous cycle was cut into small pieces, re-fed through a cleaned extruder, and collected again for further reprocessing cycles.

[0068] Under these conditions, the sample containing Zr(acac)4 could be reprocessed for up to four cycles, and the sample containing Zr(tmhd)4 could be reprocessed for up to five cycles, with the latter experiencing less network degradation than the sample containing Zr(acac)4. For both types of catalyst samples, the color of the extrudates darkened during the later reprocessing cycles (Figure 6). SRA results revealed that for both Zr catalysts, the catalytic effectiveness and carbamate exchange rate generally decreased with increasing number of reprocessing cycles (Figures S10-S11 and Table 2). For Zr(acac)4, τ * The time steadily increases from 19 s to 69 s by the fourth reprocessing cycle. For Zr(tmhd)4, this stress relaxation trend is less clear, and τ * was 49 s, while τ * was in the range of 20 to 30 s. * The T of the Zr(acac)4 sample increased to 46 s by the fifth reprocessing cycle, indicating that some network degradation occurs during prolonged continuous reprocessing. g The crosslink density decreased from 27 to 9 °C (Fig. 6), and the crosslink density decreased from 0.095 mol / cm in the first reprocessing cycle to 0.095 mol / cm in the first reprocessing cycle with increasing number of reprocessing cycles. 3 to 0.51 mol / cm in the last reprocessing cycle 3 It was found that the T of the Zr(tmhd)4 sample increased to 1.5 (Table 7). g and the crosslink density remains constant until the last reprocessing cycle, at which time T g The temperature decreased from approximately 52–55°C to 46°C (Fig. 6), and the crosslink density was approximately 0.23–0.29 mol / cm 3 to 0.48 mol / cm 3 (Table 7). DSC shows that the T of the sample reprocessed with Zr(acac)4 g The T of the sample reprocessed with Zr(tmhd)4 decreased from 14°C to 3.1°C. gThe temperature decreased from 50 °C to 34 °C (Table 7). With both catalysts, the presence of urea and the increased degree of crosslinking could be due to a combination of network degradation due to urea formation or, particularly in the samples containing Zr(acac)4, as yet unidentified side reactions. The exact contribution of side reactions to network degradation deserves further study, but hydrolysis of the PU network is likely one of the major causes of this degradation.

[0069] [Table 2]

[0070] The presence of urea in the reprocessed samples can be attributed to the accumulation of free amines generated by the hydrolysis of isocyanates in the PU network. 1 Evidence of urea formation was observed in the FTIR spectrum of the reprocessed sample. -1 Resonance was sometimes observed at 46While all samples reprocessed with Zr(acac)4 showed an increase in the urea signal, samples reprocessed with Zr(tmhd)4 showed evidence of urea formation only after three or more reprocessing cycles, and this signal remained mild through the fourth and fifth cycles (Figure 15). Samples loaded with lower concentrations of Zr(acac)4 solution also showed a reduced tendency for urea formation, none of which was observed by FT-IR after the first reprocessing cycle (Figure 14). Overall, both catalysts are capable of multiple reprocessing cycles. While polyester PU materials reprocessed with Zr(acac)4 exhibit faster stress relaxation times, polyester PU reprocessed with Zr(tmhd)4 exhibits greater resistance to degradation, particularly urea formation in the network. It should be noted that this degradation appears to be more pronounced with higher catalyst loadings. This is particularly true for samples reprocessed with Zr(acac)4. As a result, lower catalyst loadings are more desirable, and the reprocessability of loadings as low as 2 mol% achieved by cryogenic milling is discussed below. The properties of PU foams with lower catalyst content and their potential degradation are also discussed below.

[0071] The sustained degradation of the material with increasing reprocessing is verified by the tensile test results, particularly for samples reprocessed with Zr(acac)4. Samples reprocessed with Zr(acac)4 exhibit lower break stresses and higher break strains, both of which decrease with increasing reprocessing cycles (Table 3). From the first to the last reprocessing cycle, the average break stress decreases from 2.5 MPa to 1.5 MPa, and the break strain decreases from 102% to 24%. As a result, the Young's modulus increases from 2.3 MPa to 6 MPa. In contrast, samples reprocessed with Zr(tmhd)4 exhibit higher break stresses and lower break strains, both of which remain generally constant with increasing reprocessing cycles (Table 4). The average break stress is 30 MPa for the first reprocessing cycle and 29 MPa for the last reprocessing cycle. The strain at break is 3.9 MPa for the first reprocessing cycle and 3.2 MPa for the last reprocessing cycle. Young's modulus increases stepwise from 0.5 MPa for the first reprocessing cycle to 0.9 MPa for the last reprocessing cycle. Compared to each other, the samples reprocessed with Zr(acac)4 appear to behave like elastomeric materials, even though the original materials were crosslinked thermosets. This transition is consistent with a decrease in crosslink density in these materials, which is verified by the gel fraction results.

[0072] The samples reprocessed with DBTDL and Zr(tmhd)4 had gel fractions of 89% and 92%, respectively, while the sample reprocessed with Zr(acac)4 had a gel fraction of 47%. g The temperature is near RT, and the tensile tests were performed at RT, so there may be some scatter in the tensile properties measured at that temperature. Zr(acac)4 exhibits a large strain of about 400% at high temperatures (40°C) combined with a small fracture stress, so the samples reprocessed using Zr(acac)4 exhibited a large strain of about 400% at high temperatures (40°C) at T gFinally, the tensile test results showed that the samples reprocessed with Zr(acac)4 underwent sustained degradation and acquired properties more consistent with elastomeric materials, while the samples reprocessed with Zr(tmhd)4 exhibited a closer match to properties associated with glass-like, highly crosslinked materials.

[0073] [Table 3]

[0074] [Table 4]

[0075] To further expand the technical scope of this reprocessing method, both Zr catalysts were also tested on commercial polyester and polyether PU materials, resulting in successful reprocessing of foams into films. Following the same foaming procedure used previously, polyester PU foams were produced using Lupraphen 6601 polyol (diol, MW approximately 2000 g / mol) and Lupranate M20 isocyanate (f = 2.7). Polyether foams were produced using Pluracol 2090 (triol, MW approximately 4800 g / mol) and MDI. All foams were synthesized with 0.5 wt% Tnpp. FT-IR analysis confirmed the successful synthesis of these foams. 2285 cm -1 Disappearance of the isocyanate stretch near 1708-1724 cm -1The appearance of carbamate stretches near the nucleus indicated that the PU network had fully cured (Figures S16-S17). These polymers were subjected to the same solution-based catalyst introduction procedure described above. DBTDL, Zr(acac)4, and Zr(tmhd)4 catalysts were all tested on both polyester PU and polyether PU. While 30 mg / mL was an adequate solution concentration for polyester PU, even a solution concentration of 10 mg / mL resulted in a low-viscosity, non-extrudable liquid for polyether PU. Therefore, the catalyst was reduced to 3 mg / mL. Zr(acac)4 was able to partially reprocess the polyether PU, but this did not result in a completely homogeneous, characterizable material (Figure 5). ICP-OES revealed that after solvent-assisted catalyst post-incorporation, the commercial polyester PU foam with Zr(acac)4 contained 0.70% Sn and 1.86% Zr, the commercial polyester PU foam with Zr(tmhd)4 contained 0.28% Sn and 0.07% Zr, and the commercial polyether PU foam with Zr(tmhd)4 contained 0.02% Sn and 0.87% Zr. All samples contained less than 0.01% P. Compared to the non-commercial PU foams tested in this study, the commercial PU foams absorbed less catalyst but still had relatively high catalyst loadings.

[0076] Reprocessing of commercial polyurethanes using Zr-based catalysts introduced from solution SRA indicated that both Zr-based compounds catalyzed transcarbamation similarly to DBTDL when reprocessing parameters were optimized (Figure 7). In commercial polyester PU, Zr(acac)4, introduced at a concentration of 30 mg / mL, exhibited a τ of 50 s. * The post-introduced Zr(tmhd)4 at a concentration of 30 mg / mL has a τ of 38 s. * In polyether PU, Zr(tmhd)4, introduced at a concentration of 30 mg / mL, had a τ of 162 s. * The DBTDL introduced at a concentration of 30 mg / mL had a τ of 442 s. *(Table 5). Thus, both Zr(acac)4 and Zr(tmhd)4 catalysts promote rapid carbamate exchange in these commercial PUs under the desired reprocessing conditions. DMTA of the corresponding samples showed that the polyester PU samples reprocessed with Zr(acac)4 had a T of -35°C. g The polyester PU sample retreated with Zr(tmhd)4 had a T of -37°C. g The polyether PU sample retreated with Zr(acac) had a T of -57°C. g The polyether PU samples were found to have a lower T than the polyester PU. g Overall, both Zr catalysts function as suitable green reprocessing catalysts for industrial use, both for model and commercial materials.

[0077] [Table 5]

[0078] Reprocessing of polyurethanes with lower catalyst loads and solvent-free catalyst incorporation While PU foams incorporate catalysts from solvents, lower catalyst loadings, as low as 2 mol% relative to the carbamate, are achievable through cryogenic milling, where the material is cooled below room temperature and mechanically ground into a fine, homogeneous powder. Adding the solid catalyst directly to the PU foam during loading into the cryogenic mill allowed for precise control of catalyst content. The reprocessing experiment was repeated using a model foam containing 2 mol% Zr(acac)4, equivalent to 3 wt% Zr(acac)4. Once prepared and dried overnight at 90 °C under vacuum, the cryogenically ground catalyst-containing powder was reprocessed using the same twin-screw extrusion method used previously, resulting in clear, homogeneous, tan-colored extrudates similar to those previously produced using the solvent-assisted, post-catalyst introduction method (Figure 16). Comparing PU samples incorporating different Zr(acac)4 incorporation methods, cryogenic milling was able to controllably incorporate Zr(acac)4 into PU foam at lower loadings than solvent-assisted methods. Samples incorporating Zr(acac)4 via solvent-assisted methods consisted of 16.6 wt% catalyst or 13.5 mol% catalyst relative to carbamate, while samples with Zr(acac)4 subsequently incorporated by cryogenic milling consisted of 2.9 wt% catalyst or 2 mol% catalyst relative to carbamate. SRA results showed that the cryogenically milled sample with lower catalyst content had a longer τ of 54 s compared to 19 s for the solvent-assisted sample. * Although the sample had a longer reaction time, these samples were still capable of rapid stress relaxation at elevated temperatures (Figure 8). DMTA results indicated that samples prepared using either catalyst post-introduction method had similar properties, even at lower catalyst loadings. DMTA showed that the T of the sample prepared using the solvent-assisted method was significantly higher. g was 33 °C, and the T of the sample produced by cryogenic milling g The T for the sample prepared by the solvent-assisted method was 47°C (Figure 17 and Table 8). g The T of the sample produced by cryogenic milling was 14.0°C. gThe temperature was 32°C (Figure 18 and Table 8). The difference in material properties between the two types of samples may be due to the difference in catalyst loading and possible partial degradation of the PU feed material. 47 .

[0079] conclusion The development of a scalable PU recycling method is essential to reduce the millions of tons of PU waste generated annually, especially thermoset PU waste. In this study, we identified suitable green catalysts for twin-screw extrusion and bulk reprocessing of thermoset polyester PU. These Zr catalysts produced high-quality, homogeneous extrudates for both model and commercial PU materials. SRA characterization revealed that these Zr catalysts performed dynamic carbamate exchange comparable to DBTDL. Furthermore, these Zr catalysts enabled continuous reprocessing of model PU materials for up to 4–5 cycles. Although the model PU network deteriorated with increasing reprocessing cycles, SRA, DMTA, FT-IR, and tensile test characterization confirmed that materials reprocessed with Zr(tmhd)4 exhibited greater resistance to urea formation and decreased crosslinking than materials reprocessed with Zr(acac)4. Zr-based catalysts were also demonstrated to effectively reprocess commercial thermoset PUs, polyester PUs, and polyether PUs.

[0080] Materials and General Methods Materials: Purchased reagents were obtained from Sigma-Aldrich or Fisher Scientific. Lupranate M diol, Pluracol 2090 triol, and Pluracol 6601 diol were provided by BASF (Wyandotte, MI). Polyols were dried at 90 °C under 20 mTorr vacuum for at least 30 minutes before use in film or foam synthesis. All other reagents were used without further purification unless otherwise specified. Dichloromethane (CHCl) and toluene were purchased from Fisher Scientific and purified using a custom-built alumina column-based solvent purification system.

[0081] Instrumentation: Infrared spectra were recorded on a Thermo Nicolet iS10 equipped with a ZnSe ATR attachment. Spectra were uncorrected.

[0082] Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC250 differential scanning calorimeter. Samples (5-10 mg) were heated at a rate of 10 °C / min to at least 150 °C to erase the thermal history, cooled at 10 °C / min to -80 °C, and then heated to at least 120 °C. All data shown are from the second heating run. The glass transition temperature (T g ) was calculated from the maximum of the derivative of the heat flow with respect to temperature.

[0083] Dynamic mechanical thermal analysis (DMTA) was performed on a TA Instruments RSA-G2 analyzer (New Castle, Delaware) using a rectangular (approximately 0.75 mm (T) × 5 mm (W) × 20 mm (L) with a 10 mm gauge length). The axial force was adjusted to 0 N, and a 30% strain adjustment was set with a minimum strain of 0.05%, a maximum strain of 5%, and a maximum force of 1 N to prevent the sample from bending or exceeding a certain strain. The force tracking mode was also set so that the axial force was twice the oscillatory force. The temperature was then ramped from 30 °C to 160 °C at a rate of 5 °C / min with an oscillatory strain of 0.05% and an angular frequency of 6.28 rad / s. -1 (1 Hz). g was calculated from the maximum value of the loss modulus (E").

[0084] Stress relaxation analysis (SRA) was performed on a TA Instruments RSA-III analyzer (New Castle, Delaware) using rectangular films (approximately 1.0 mm (T) x 4 mm (W) x 5 mm (L) and a 9 mm gauge length). SRA experiments were performed under strain control at a specific temperature (160 °C). The samples were allowed to equilibrate at this temperature for approximately 2-5 minutes, after which the axial force was adjusted to 0 N. Each sample was then subjected to an instantaneous 5% strain. The stress decay was monitored while maintaining a constant strain (5%) until the stress relaxation modulus relaxed to at least 37% (1 / e) of its initial value. This was performed three times consecutively for each sample. The activation energy (E a ) was determined using methodologies described in the literature.

[0085] Twin-screw extrusion was performed using a Thermo Scientific HAAKE MiniLab 3 Micro Compounder. All samples were discharged directly from the extruder at 200°C with a screw speed of 50 revolutions per minute. The residence time in the extruder was approximately 1 minute. The extrudate was forced through a rectangular die approximately 4 mm wide and 1 mm thick to obtain a continuous film.

[0086] Cryogenic milling was performed in a RETSCH CryoMill. All samples were milled at a frequency of 30 Hz for approximately 10 minutes per cycle (actual grinding time was 4 minutes) between two cryogenic cycles.

[0087] Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed by Robertson Microlit Analytical Testing Lab (Ridgewood, NJ). P, Zr, and / or Sn content was assessed in the samples. The standard ICP-OES procedure provided in Robertson Microlit's technical report was then performed in triplicate. In this procedure for percent level determination, a sample volume containing an estimated 10–50 ppm of the target element was loaded into a digestion flask along with digestion acid. The sample was then heated until fully digested, cooled, quantitatively transferred to an appropriate volumetric flask, and diluted with deionized water to reach the desired final concentration. Corresponding blank and "reagent spike" solutions (if necessary) were prepared. Additional details are available upon contacting Robertson Microlit.

[0088] Synthesis procedure [ka] Synthesis of crosslinked polyester polyurethane films containing reprocessed catalysts: In a 20 mL vial, polyol 1 (3.6 g, 18 mmol -OH), TNPP (0.03 g, 0.5 wt% of the total film), and catalyst were dissolved in anhydrous DCM and vortexed until completely dissolved. The catalyst was either DBTDL at 0.25, 0.50, or 1.0 mol% relative to the carbamate (21, 42, or 85 mg), or Zr(acac)4 at 0.25, 0.50, or 1.0 mol% relative to the carbamate (28, 55, or 110 mg). MDI (2.25 g, 9 mmol) was added to the polyol and catalyst solution and vortexed until completely dissolved. The resulting solution was cast into a 150 mL aluminum pan and allowed to gel for 24 hours. The films were then post-cured in a vacuum oven at 90 °C under a vacuum of 20 mTorr for 48 hours.

[0089] This same procedure was attempted to synthesize films containing 0.5 wt% TNPP and 0.25, 0.35, 0.50, 0.75, and 1.0 mol% Zr(tmhd)4. However, these films were heterogeneous, very brittle, and phase-separated, especially at relatively high catalyst loadings, making them unsuitable for characterization.

[0090] Synthesis of films without catalyst: In a 20 mL vial, polyol 1 (3.6 g, 18 mmol -OH) was dissolved in 8.0 mL of anhydrous toluene. MDI (2.25 g, 9 mmol) was added to the solution and vortexed until completely dissolved. The resulting solution was cast into a 150 mL aluminum pan, covered with aluminum foil, and heated on a hot plate at 60 °C for 18 h to promote gelation. The films were then post-cured in a vacuum oven at 90 °C under a vacuum of 20 mTorr for 4 h at 100 °C, 2 h at 140 °C, and 12 h at 60 °C.

[0091] Synthesis of crosslinked polyester polyurethane foam: In a plastic cup, poly[trimethylolpropane / di(propylene glycol)-alt-adipic acid / phthalic anhydride] polyol (10 g, 50 mmol -OH), blowing agent isopentane (300 mg), and dibutyltin dilaurate (111 mg, 0.02 mol% relative to MDI) were mixed. Ground solid 4,4'-methylenebis(phenylisocyanate) (MDI) (6.26 g, 25.0 mmol) was added and mixed vigorously. The mixture was allowed to gel and rise for 1 hour. After 24 hours, the resulting polymer foam was cured in an oven at 150 °C for 1 hour to ensure complete crosslinking.

[0092] PU film: FT-IR (solid, ATR) 3307 (NH stretching), 2917, 1708 (C=O stretching), 1597, 1529 (NH deformation), 1457, 1412, 1377, 1308, 1219, 1066, 1017, 816, 766 cm -1 .

[0093] [ka] Synthesis of crosslinked polyester polyurethane foam containing reprocessed catalyst: Polyol 1 (10 g, 50 mmol -OH), blowing agent isopentane (300 mg), and DBTDL (111 mg, 0.02 mol% relative to MDI) were mixed in a plastic cup. Ground solid MDI (6.26 g, 25.0 mmol) was added and mixed vigorously. The mixture was allowed to stand for 1 hour to gel and rise. After 24 hours, the resulting polymer foam was cured in an oven at 150°C for 1 hour to ensure complete crosslinking.

[0094] The same procedure as above was used for foams synthesized from commercially available polyols and isocyanates provided by BASF. Polyester PU foams were synthesized using Pluracol 6601 diol (13.3 g, approximately 3 kg / mol), Lupranate M20 (4.05 g, 360 g / mol, approximately 2.7 isocyanates per molecule), DBTDL (37 mg), and isopentane (100 mg). Polyether PU foams were synthesized using Pluracol 2090 triol (12.34 g, approximately 4.8 g / mol), MDI (2.09 g, 250 g / mol, two isocyanates per molecule), DBTDL (37 mg), and isopentane (100 mg). There were no variations on the procedure used with Polyol 1, except that the MDI was ground more finely in a mortar and pestle to synthesize the polyether PU foams.

[0095] PU foam: FT-IR (solid, ATR) 2932, 1723 (C=O stretching), 1596, 1530 (NH deformation), 1511, 1456, 1412, 1377, 1307, 1219, 1124, 1063, 1017, 816, 767, 745, 705 cm -1 .

[0096] Post-synthesis introduction of catalysts into model PU films or foams: Polymer foam (10.0 g) was suspended in CHCl (100 mL). To this suspension, catalysts [DBTDL, Zr(acac) or Zr(tmhd)] were added to obtain the desired solution concentrations (10, 20, or 30 mg / mL). The resulting suspension was stirred overnight, after which the solvent-swollen polymer was isolated by filtration and dried in a vacuum oven at 90 °C and 20 mTorr for 24 h.

[0097] [Table 6]

[0098] [Table 7]

[0099] [Table 8]

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Claims

1. A method for reprocessing crosslinked polyurethane, comprising the steps of: machining the crosslinked polyurethane; mixing the machined crosslinked polyurethane with a solid polyurethane exchange catalyst; heating the mixture to an effective bond exchange temperature; and applying mechanical force to the mixture for an effective bond exchange time.

2. The method according to claim 1, wherein the crosslinked polyurethane is machined together with the solid polyurethane exchange catalyst.

3. (i) The solid polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and ligands coordinated to the metal atoms, and optionally the solid polyurethane exchange catalyst contains Zr (for example, Zr(acac) ４ Or Zr(tmdh) ４ );or (ii) The process of machining the crosslinked polyurethane includes milling the crosslinked polyurethane, and optionally the machining of the crosslinked polyurethane at a temperature of less than 20°C or between -200°C and 0°C; or (iii) Mix the machined crosslinked polyurethane with a solid polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate; or (iv) An antioxidant composition comprising the crosslinked polyurethane and an antioxidant is machined, wherein the antioxidant is optionally tris(nonylphenyl)phosphite; or (v) Dry the mixture at a drying temperature under vacuum before the step of heating the mixture to the effective bond exchange temperature; or (vi) The crosslinked polyurethane is a crosslinked polyurethane foam; or The method according to claim 1 or 2, which is any combination thereof.

4. The method according to claim 3, wherein the solid polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom.

5. The method according to claim 4, wherein the solid polyurethane exchange catalyst contains Zr.

6. The solid polyurethane exchange catalyst is Zr(acac) ４ or Zr(tmdh) ４ The method according to claim 5.

7. The method according to claim 3, wherein the step of machining the crosslinked polyurethane includes milling the crosslinked polyurethane.

8. The method according to claim 7, wherein the crosslinked polyurethane is machined at a temperature of less than 20°C.

9. The method according to claim 7, wherein the crosslinked polyurethane is machined at a temperature of -200°C to 0°C.

10. The method according to claim 3, wherein the machined crosslinked polyurethane is mixed with a solid polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate.

11. The method according to claim 3, wherein the antioxidant composition comprising the crosslinked polyurethane and the antioxidant is machined.

12. The method according to claim 11, wherein the antioxidant is tris(nonylphenyl)phosphite.

13. The method according to claim 3, wherein the mixture is dried at the drying temperature under vacuum before the step of heating the mixture to the effective bond exchange temperature.

14. The method according to claim 3, wherein the crosslinked polyurethane is a crosslinked polyurethane foam.

15. The method according to claim 3, wherein the method comprises any two, any three, any four, any five, or all six of (i) to (vi).

16. A polyurethane composition comprising the machined crosslinked polyurethane described in claim 1 and a solid polyurethane exchange catalyst.

17. The polyurethane composition according to claim 16, wherein the solid polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom.

18. The polyurethane composition according to claim 17, wherein the solid polyurethane exchange catalyst contains Zr.

19. The solid polyurethane exchange catalyst is Zr(acac) ４ or Zr(tmdh) ４ The polyurethane composition according to claim 18.

20. The polyurethane composition according to any one of claims 16 to 19, wherein the machined crosslinked polyurethane is mixed with a solid polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate.

21. A polyurethane composition according to any one of claims 16 to 19, further comprising an antioxidant.

22. The polyurethane composition according to claim 21, wherein the antioxidant is tris(nonylphenyl)phosphite.

23. The polyurethane composition according to claim 21, wherein the machined crosslinked polyurethane is mixed with a solid polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate.

24. A method for reprocessing a crosslinked polyurethane, the method comprising the steps of: heating a polyurethane exchange catalyst and an antioxidant composition to an effective bond exchange temperature, wherein the antioxidant composition comprises the crosslinked polyurethane and the antioxidant; and applying mechanical force to the polyurethane exchange catalyst and the antioxidant composition over an effective bond exchange time, wherein the antioxidant is optionally tris(nonylphenyl)phosphite.

25. The method according to claim 24, wherein the polyurethane exchange catalyst is a solid and is mixed with the polyurethane composition before the step of heating to the effective bond exchange temperature.

26. The method according to claim 24, wherein the polyurethane exchange catalyst solution containing the polyurethane exchange catalyst is permeated into the crosslinked polyurethane before the step of heating it to the effective bond exchange temperature.

27. (i) The polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and ligands coordinated to the metal atoms, and optionally the polyurethane exchange catalyst contains Zr (for example, Zr(acac) ４ Or Zr(tmdh) ４ );or (ii) The polyurethane exchange catalyst is solid, the crosslinked polyurethane is machined, the solid polyurethane exchange catalyst and the machined crosslinked polyurethane are mixed before the step of heating to the effective bond exchange temperature, and optionally the crosslinked polyurethane is machined at a temperature of less than 20°C or a temperature of -200°C to 0°C; or (iii) The antioxidant is tris(nonylphenyl)phosphite; or (iv) The crosslinked polyurethane is mixed with a polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate; or (v) The crosslinked polyurethane is a crosslinked polyurethane foam; or The method according to any one of claims 24 to 26, which is any combination thereof.

28. The method according to claim 27, wherein the polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom.

29. The method according to claim 28, wherein the polyurethane exchange catalyst contains Zr.

30. The polyurethane exchange catalyst is Zr(acac) ４ or Zr(tmdh) ４ The method according to claim 29, which is

31. The method according to claim 27, wherein the polyurethane exchange catalyst is solid, the crosslinked polyurethane is machined, and the solid polyurethane exchange catalyst and the machined crosslinked polyurethane are mixed before the step of heating to the effective bond exchange temperature.

32. The method according to claim 31, wherein the crosslinked polyurethane is machined at a temperature of less than 20°C.

33. The method according to claim 31, wherein the crosslinked polyurethane is machined at a temperature of -200°C to 0°C.

34. The method according to claim 27, wherein the antioxidant is tris(nonylphenyl)phosphite.

35. The method according to claim 27, wherein the crosslinked polyurethane is mixed with a polyurethane exchange catalyst in an amount of less than 5 mol% relative to the carbamate.

36. The method according to claim 27, wherein the crosslinked polyurethane is a crosslinked polyurethane foam.

37. The method according to claim 27, wherein the method comprises any two, any three, any four, or all five of (i) to (v).

38. A polyurethane composition comprising the crosslinked polyurethane described in claim 24, an antioxidant, and a polyurethane exchange catalyst that has permeated into the crosslinked polyurethane.

39. The polyurethane composition according to claim 38, wherein the antioxidant is tris(nonylphenyl)phosphite.

40. The polyurethane composition according to claim 38 or 39, wherein the polyurethane exchange catalyst comprises Zr, Bi, Fe, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, Mo, or Sn, and a ligand coordinated to the metal atom.

41. The polyurethane composition according to claim 40, wherein the polyurethane exchange catalyst contains Zr.

42. The catalyst is Zr(acac) ４ or Zr(tmhd) ４ The polyurethane composition according to claim 41.

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