Circular reprocessing of thermoset polyurethane foams

Abstract

Disclosed are methods for reprocessing thermoset polyurethane foams. The methods include mixing polyurethane foam particles loaded with a bond-exchange catalyst with a blowing agent and compounding the mixture. The methods comprise mechanically processing a polyurethane foam into particles; loading the particles with a bond-exchange catalyst, thereby preparing loaded particles; mixing the loaded particles with a blowing agent; compounding a mixture of the loaded particles and the blowing agent, thereby preparing a compounded substance; and extruding the compounded substance.

Description

CIRCULAR REPROCESSING OF THERMOSET POLYURETHANE FOAMSSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCHThis invention was made with government support under DE-EE0007897 awarded by the US Department of Energy. The government has certain rights in the invention.CROSS-REFERENCE TO RELATED APPLICATIONSThis application claims the benefit of and priority to pending U.S. Provisional Patent Application No. 63 / 515,794, filed on July 26, 2023. The contents of which are incorporated by reference herein in their entireties.BACKGROUND OF THE INVENTIONPolyurethane (PU) is the most common thermosetting polymer class and the sixth-most consumed plastic, accounting for 8% of annual plastic production.[1]Despite their widespread use, thermoset polyurethanes cannot be recycled by conventional melt processing and therefore reprocessing options have been limited to mechanical or chemical recycling.[2]Mechanical recycling grinds PU foam into small particles that are glued together, which had been used to produce a downcycled product, carpet underlayer, until this practice was discontinued due to concerns over toxic additives.1 51Chemical recycling of polyurethanes has been developed to recover and repolymerize PU feedstocks based on glycolysis of the carbamate bonds, but this process recovers only the polyol component.16 8|New isocyanates are therefore required to repolymerize the recovered polyols.[9,10]Given the limitations of these methods, developing polyurethane reprocessing methods that are circular, as well as energy and atom efficient, are of high priority.To improve PU recycling while maintaining its utility and properties, our group and others reported thermally reprocessable PU covalent adaptable networks (CANs) that featured Lewis acid catalysts such as dibutyltin dilaurate (DBTDL).1"161The catalysts activate carbamate exchange which impart rapid stress relaxation and flow behavior at elevated temperatures.1"161In these examples, the catalyst was introduced during the synthesis of PU films, whereas recycling of existing PU waste necessitates that the catalyst be introduced to the PU just before reprocessing.[17,18]We recently developed a procedure to reprocess commercial PU foam into asolid PU film by introducing an external catalyst to the foam.

[0019] Moreover, reprocessing commercial foam yielded greater film uniformity and superior tensile properties when using a twin-screw extruder instead of compression molding.A truly circular process for PU foam requires the development of foam-to-foam recycling, as the commercial demand for solid polyurethanes is more limited.|2‘2()|Refoaming a thermoset is possible when the polymer network can flow under shear stress, as foam processing requires an extensional flow of polymer for cell growth.[2 k221The catalysts associated with CANs may enable the foaming process of thermoset PU by allowing for the extensional flow by crosslink exchange during melt reprocessing. Foaming CANs has recently been reported with polymers such as ethylene-vinyl acetate, polyethylene terephthalate, and polylactic acid.122 251However, the foaming in these studies was achieved with CANs derived from thermoplastic materials, which are already reprocessable using conventional industrial processes. In contrast, a foam-to-foam reprocessing method for thermoset polyurethanes would enable the circular use of PU foam products that are otherwise landfilled or incinerated. Accordingly, there is a need in the art for foam-to-foam recycling of PU foams.BRIEF SUMMARY OF THE INVENTIONDisclosed herein are methods for reprocessing thermoset polyurethane foams. The methods comprise mechanically processing a polyurethane foam into particles; loading the particles with a bond-exchange catalyst, thereby preparing loaded particles; mixing the loaded particles with a blowing agent; compounding a mixture of the loaded particles and the blowing agent, thereby preparing a compounded substance; and extruding the compounded substance. In some instances, the compounded substance may be extruded into a mold to prepare molded polyurethane foams articles. The blowing agent may be a chemical blowing agent. Suitably, the blowing agent is selected to degrade during compound that dissolve into the compounded substance. Exemplary blowing agents include azodi carbonamide or water.BRIEF DESCRIPTION OF THE DRAWINGSNon-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in everyfigure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.Figure 1. Schematics of thermoset PU foam synthesis, dynamic bond exchange of carbamate bonds in PU network, and structure of carbamate exchange catalyst, Zr(acac)4.Figure 2. Schematics of the foam-to-foam recycling process using a twin-screw extruder (scale bar = 100 pm).Figure 3. Characterization of reprocessed PU foams at different temperatures, (a) FT-IR spectra of as-synthesized PU, ADC, and foams reprocessed at various temperatures (b) Solid state13C CP -MAS NMR spectra of pre-extrusion mixture and foams reprocessed at various temperatures, (spin rate = 10 KHz, field strength = 400 MHz) (c) DSC traces of PU containing 3 wt% Zr(acac)4 and foams reprocessed at various temperatures. The solid dot of each DSC traces indicates Tg. (heating rate = 10 °C / min) (d) Tgand gel fraction of as- synthesized foam (AS) and PU foams reprocessed at various temperatures.Figure 4. (a) The cross-sectional SEM image of foam extrudate made through the foam- to-foam process at different temperatures (160 - 220 °C) with a screw rotation speed of 10 rpm (Scale bar = 200 pm), (b) Cell number density and average cell diameter of foams at different temperatures (180 - 220 °C) with an extrusion speed of 10 rpm. (c) Cell number density and average cell diameter of foams with different screw rotation speeds at 180 °C (5 - 15 rpm).Figure 5. An extended, continuous foam-to-foam extrusion process was conducted to judge the uniformity of the sample at 180 °C and 15 rpm. (a) An image of the extruded foam with the position of the cross-section used for image analysis indicated. The part on the left of the image was extruded early. (Scale bar = 1 cm) (b) SEM image of a cross-section of the foamed extrudate at each position. (Scale bar = 100 pm) (c) Average cell diameter and (d) cell number density and bulk density at each location of 3g and 6g extrusion.Figure 6. Schematics of the foam-to-foam recycling process resembling foam injection molding.Figure 7. A foam-to-foam process resembling injection molding using a mold cavity connected to the extruder (a) An image of mold cavity connected to the extruder screw and molded foam with different cycling time (2.5 to 20 min, 180 °C, feeding time = 4.8 g PU / 5 min, Scale bar = 1 cm) (b) SEM image of a cross-section of the molded foam at different cycling time. (Scalebar = 200 pm) (c) bulk density and (d) cell number density and average cell diameter of foam at different cycling time.Figure 8. Compression test of foams under a constant compression rate (0.005 mm / s). (a) compressive stress-strain curves of foams made through different processes: small scale (3g) extrusion foaming, large scale (6g) extrusion foaming, injection foaming with 3 wt% of Zr(acac)4 and 5 wt% of Zr(acac)4, and as-synthesized foam blown with isopentane and water, (b) The expanded region of compressive stress-strain curves showing the linear region and early part of the plateau.Figure 9. Images of (a) as-synthesized PU foam (length of background grid = 0.5 inch) and (b) cross-section of as-synthesized PU foam. SEM images of (c) as-synthesized PU foam and (d) ground foam particles using kitchen blender.Figure 10. Images of continuous PU foam extrudate from twin-screw extruder with various extrusion temperatures and speeds (scale bar = 1.0 cm).Figure 11. Images of (a) the die end of the twin-screw extruder and (b) PU foam extruded from the die. (Extrusion temperature = 180 °C, Screw speed = 10 rpm) The thickness of foam extrudate was 2.51 ± 0.06 mm (arrow in Figure 1 lb), which was greater than the die thickness of 1.10 mm arrow in Figure I la).Figure 12. FT-IR spectra near (a) isocyanate bands (2285 cm'1) and (b) urea bands (1640 cm'1) of as-synthesized PU foam, ADC, and foams reprocessed at various temperatures, (a) Vibrational bands corresponding to isocyanates were not found in as-synthesized and reprocessed PU foams. The bands at 2350 cm'1are atmospheric carbon dioxide, (b) Vibrational urea bands of ADC were observed in PU foams reprocessed at 160 and 180 °C, indicating the presence of residual ADC after reprocessing.Figure 13. Full spectra of CP-MAS13C NMR of pre-extrusion mixture and foams reprocessed at various temperatures.Figure 14. Extruder torque and screw rotation speed were measured while reprocessing a mixture of thermoset PU and 8.5 wt% of ADC without addition of Zr(acac)4 catalyst. The twin- screw extruder stopped after feeding as the torque of the extruder exceeded its inherent limit, 5 N m. The ceased extrusion was verified as the screw rotation speed decreased from 10 rpm to 0 rpm when the maximum torque was reached.Figure 15. The cross-sectional SEM image of foam extrudate made through the foam-to- foam process at different screw rotation speeds (5 - 20 rpm) with an extrusion temperature at 180 °C. The screw rotation speeds for each image are (a) 5, (b) 10, (c) 15, and (d) 20 rpm.Figure 16. Solid state13C CP-MAS NMR spectra of PU foams reprocessed through foam injection molding with various cycling time, (spin rate = 10 KHz, field strength = 400 MHz) (a) Full Solid state13C CP-MAS NMR spectra, and (b) spectra near the resonance of intermediate (INTMD) and ADC.Figure 17. DSC traces of PU foams reprocessed through foam injection molding with various cycling time. The solid dot of each DSC traces indicates Tg(heating rate = 10 °C / min).Figure 18. Zgand gel fraction of PU foams reprocessed through foam injection molding with various cycling time.Figure 19. Stress relaxation plots for reprocessed PU films containing different concentration of Zr(acac)4 at 160 °C. T* decrease by increasing concentration of Zr(acac)4.Figure 20. Cross-sectional images and bulk densities of a PU film and foams (containing 3 - 5 wt% of Zr(acac)4) reprocessed by injection molding in circulation chamber of the twin-screw extruder (180 °C, feeding time = 4.8 g PU / 8 min, cycling time = 10 min). Reprocessed foams were thicker than reprocessed film, which thickness is 1.5 mm, indicating that successful volume expansion occurred in foam injection molding. The average thickness of the reprocessed foam was 3.51 ± 0.26, 3.84 ± 0.19, and 4.23 ± 0.15 mm for the samples containing 3, 4, and 5 wt% Zr(acac)4, respectively.Figure 21. Cross-sectional SEM images of reprocessed foam made via the foam injection molding process at various concentrations of Zr(acac)4 (3 - 5 wt %). A mixture of 4.8 g PU foam and 0.45 g of the chemical blowing agent was fed into extruder for 8 minutes, then cycled for 10 minutes. The operation temperature of the extruder was maintained at 180 °C for both feeding and cycling.Figure 22. Cell diameter distribution of reprocessed PU foam made via foam injection molding process at various concentrations of Zr(acac)4 (3 - 5 wt %). The cell diameter distribution shifted to larger diameters with increasing concentration of Zr(acac)4, indicating faster cell growth due to faster dynamic bond exchange.Figure 23. DSC traces of as-synthesized PU foam and PU foams reprocessed through foam injection molding with various Zr(acac)4 concentration. The solid dot of each DSC traces indicates7g (heating rate = 10 °C / min). The 7gof as-synthesized PU foam in this experiment is different from that of extrusion experiments because a different batch of PU foam was used for this experiment.Figure 24. Cross-sectional Images of (a) water-blown PU foam after synthesis and (b) water-blown PU foam after curing in a vacuum oven at 90 °C for 48 hours, and post-curing in an oven at 150 °C for 1 hour.DETAILED DESCRIPTION OF THE INVENTIONDisclosed herein is a direct and continuous foam-to-foam recycling method by leveraging the melt-processability of PU CANs.One aspect of the technology is a method for reprocessing a thermoset polyurethane foam, the method comprising mechanically processing a polyurethane foam into particles; loading the particles with a bond-exchange catalyst, thereby preparing loaded particles; mixing the loaded particles with a blowing agent; compounding a mixture of the loaded particles and the blowing agent, thereby preparing a compounded substance; and extruding the compounded substance.Two additives enabled simultaneous reprocessing and refoaming of PU CANs: a bondexchange catalyst and blowing agent. Blowing agents are substances that are capable of producing a cellular structure in a material that undergoes a phase transitions. Blowing agents include chemical blowing agents, physical blowing agents, and combinations there of. Suitably, the blowing agent may be a chemical blowing agent that can decompose at reprocessing temperatures. The blowing agent may thermally degrade during compounding into gas molecules that dissolve into the compounded substance. The gas molecules produced during the thermal degradation may be N2, CO, CO2, or any combination thereof. Exemplary blowing agents include azodicarbonamide or water.In some embodiments, the method comprises loading the particles with the bond-exchange catalyst comprising contacting the particles with a solution comprising the bond-exchange catalyst and evaporating solvent from the solution. The bond-exchange catalyst can be a carbamate exchange catalyst. Exemplary bond-exchange catalyst include zirconium(IV) acetyl acet onate.The mixture of the loaded particles and the blowing agent can be compounded at an effective bond-exchange temperature between 160 °C and 220 °C or any range therebetween. For example, the effective bond-exchange temperature may be greater than 160 °C, 165 °C, 170 °C, or175 °C and less than 220 °C, 215 °C, 210 °C, 205 °C, 200 °C, 195 °C, 190 °C, or 185 °C, including any range or value therebetween.The mixture of the loaded particles and the blowing agent can be compounded for an effective bond-exchange time to degrade the blowing agent into gas molecules during compounding that dissolve into the compounded substance. The mixture of the loaded particles and the blowing agent can be compounded for an effective bond-exchange time between 2 min and 20 min or any range therebetween. For example, the effective bond-exchange time may be greater than 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, or 10 min and less than 20 min, 19 min, 18 min, 17 min, 16 min, 15 min, 14 min, 13 min, 12 min, 11 min, or 10 min, including any range or value therebetween.When compounded with a twin-screw extruder, the mixture of the loaded particles and the blowing agent can be compounded at an effective ration rate to alter the extruded foam density, cell number, cell diameter, or combination thereof. The mixture of the loaded particles and the blowing agent can be compounded at a screw rotation speed between 5 and 15 revolutions per minute (RPM). For example, the screw rotation speed may be greater than 5, 6, 7, 8, 9, or 10 and less than 15, 14, 113, 12, or 10 RPM, including any range or value therebetween.The method can comprise blending the polyurethane foam with one or more additional polymers and compounding polyurethane foam, bond-exchange catalyst, blowing agent, and one more or additional polymers.These processes can produce recycled PU foam with a porous structure, as determined through image analysis using scanning electron microscopy (SEM). The continuity of the refoaming process was demonstrated in terms of constant cell diameter and cell number density of foams. A process mimicking injection molding of the PU CAN was demonstrated by fdling a chamber connected to the extruder with reprocessed materials and subsequently opening the lid of the chamber to achieve rapid depressurization. Compression tests of the recycled foam showed mechanical properties comparable to those of the original foams. These results demonstrate that waste PU foam can be directly recycled into a new foam while preserving its structural properties and chemical integrity.A foam-to-foam recycling process of thermoset polyurethane was developed by melt reprocessing PU CANs with added blowing agents to induce foaming following continuous extrusion. The Examples demonstrated reprocessing of PU foams prepared by polymerizing apolyester-based polyol with a diisocyanate in the presence of isopentane as a physical blowing agent and a small amount of dibutyltin dilaurate as a catalyst (Figure 1, 9a, and 9b). A bondexchange catalyst is used to enhance bond exchange rates (Figure 1). To promote the introduction of bond-exchange catalyst, the synthesized foam with an may be ground into smaller particles. The ground particles can be soaked in a solution of the bond-exchange catalyst, after which the solvent was evaporated, yielding ground PU particles containing bond-exchange catalyst. A blowing agent is incorporated into the catalyst-loaded polymer.PU reprocessing and refoaming may be performed by feeding the PU particles, bondexchange catalyst and blowing agent into a compounding held at an bond-exchange temperature allows for preparation of a PU foam extrudate (Figure 2 and Figure 10). As the PU CAN was heated, the reversible dissociation of the network allows the PU to be reprocessed via catalyzed crosslink exchange. Simultaneously, a blowing agent can be thermally degraded into gas molecules (e.g., N2, CO, and CO2), which dissolve within the polymer. As the reprocessed polyurethane is extruded, the resulting pressure drop can induce nucleation of the dissolved gas molecules and, consequently, the volumetric expansion of the extrudate and formation of a cellular structure. Alternatively, the reprocessed PU may be extruded into a mold. The reprocessed PU may also be controllably depressurized after extruding the compounded substance. This depressurization can induce nucleation of the dissolved gas molecules and, consequently, the volumetric expansion of the extrudate. Several definitions are provided to assist with the understanding of the technology."Block" means a portion of a macromolecule, comprising many constitutional units, that has at least one constitutional or configurational feature which is not present in the adjacent portions.“Blowing agent” means a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers or plastics. Blowing agents include, chemical blowing agents that degrade into gas molecules or other byproducts, physical blowing agents, or combinations thereof. Exemplary blowing agents include chemical blowing agents such as azodicarbonamide (ADC) or water."Branch" means an oligomeric or polymeric offshoot from a macromolecular chain. "Branch point" means a point on a chain at which a branch is attached."Branch unit" means a constitutional unit containing a branch point."Catalyst" means a substance that increases the rate of a reaction without modifying the overall Gibbs energy change in the reaction. Suitably the catalyst may be a coordination entity comprising a central atom and one or more ligands joined to the central atom. Suitably the central atom is a metal. "Ligand" means an atom or group joined to a central atom."Chain" means a whole or part of a macromolecule, an oligomer molecule, or a block, comprising a linear or branched sequence of constitutional units between two boundary constitutional units, each of which may be either an end-group, a branch point, or an otherwise- designated characteristic feature of the macromolecule."Compounding" means to blend or mix a substance, such as any of the polyurethane compositions described herein, within a compounding device. Suitably the substance is compounded at an effective bond-exchange temperature for an effective bond-exchange time."Compounding device" means a device for blending or mixing a substance, such as any of the polyurethane compositions described herein. In some embodiments, the compounding device is an extruder, such as a single screw or twin-screw extruder, a mixer, or a kneader. Suitably twin- screw extruders may be a co-rotating or counter-rotating twin-screw extruder. The compounding device may operate in batch or continuous service. Suitably a continuous service compound device may have an inlet, such as a feeding hopper or other suitable feeding mechanism, for introducing the substance into the compounding device, an outlet for extruding the compounded substance, and a compounding zone between the inlet and the outlet for mixing or blending the substance. Suitably the compounding zone is configured so that the substance may be compounded for an effective bond-exchange time. The compounding device may also comprise a heating element so that the substance may be compounded at an effective bond-exchange temperature."Constitutional unit" means an atom or group of atoms (with pendant atoms or groups, if any) comprising a part of the essential structure of a macromolecule, an oligomer molecule, a block, or a chain."Copolymer" means a polymer derived from more than one species of (real, implicit, or hypothetical) monomer."Covalent network" or "covalent polymer network" means a network in which the permanent paths through the structure are all formed by covalent bonds."Dynamic network" or "dynamic polymer network" or "covalent adaptable network" means a covalent network that is capable of undergoing bond-exchange reactions at a temperatureabove an effective bond-exchange temperature. A dynamic network may demonstrate viscoelastic liquid properties above the freezing transition temperature."Foam" means a multiphasic material comprising gas dispersed in a polymer. The foam may be formed by trapping pockets of gas in a solid or liquid. Foams may be prepared by physical or chemically blowing. In some embodiments, the foam may be a closed-cell foam where the gas forms discrete, completely surrounded pockets. In other embodiments, the foam may be an opencell foam where the gas pockets are interconnected. Suitably the polymer is a polyurethane ("polyurethane foam").“Homopolymer” means a polymer derived from one species of (real, implicit or hypothetical) monomer. Polymers may be made by the mutual reaction of complementary monomers. These monomers can readily be visualized as reacting to give an ‘implicit monomer’ or ‘hypothetical monomer’, the homopolymerization of which would give the actual product, which can be regarded as a homopolymer."Immiscible polymer blend" means a polymer blend that exhibits immiscibility. "Immiscibility" means an inability of a mixture to form a single phase."Inorganic polymer" means a polymer or polymer network with a skeletal structure that does not include carbon atoms. Examples include, without limitation, polyphosphazenes, polysilicates, polysiloxanes, polysilanes, polysilazanes, polygermanes, and polysulfides."Isocyanate constitutional unit" means a constitutional unit comprising at least one isocyanate group, i.e., -NCO. Suitably the isocyanate constitutional unit may comprise more than one isocyanate group such as two, three, or four isocyanate groups. In some embodiments, the isocyanate constitutional unit is an aromatic isocyanate constitutional unit. As used herein, an "aromatic isocyanate constitutional unit" means an isocyanate constitutional unit having an isocyanate group pendant from an aryl group such a phenyl or other aromatic ring."Lewis acid" means a molecular entity (and the corresponding chemical species) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base."Linear chain" means a chain with no branch points between the boundary units."Macromolecule" or "polymer molecule" means a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass."Mechanically processed" means to mechanically alter a substance, e.g., by mechanically grinding, cutting, chopping, or applying some other form of mechanical force. Suitably, the substance such as the polyurethane compositions described herein may be mechanically processed to fragment the substance into pieces, grains, granules, or particles."Monomer" means a substance composed of monomer molecules."Monomer molecule" means a molecule which can undergo polymerization, thereby contributing constitutional units to the essential structure of a macromolecule."Monomeric unit" means the largest constitutional unit contributed by a single monomer molecule to the structure of a macromolecule or oligomer molecule."Network" means a highly ramified macromolecule in which essentially each constitutional unit is connected to each other constitutional unit and to the macroscopic phase boundary by many permanent paths through the macromolecule, the number of such paths increasing with the average number of intervening bonds; the paths must on the average be coextensive with the macromolecule."Network polymer" means a polymer composed of one or more networks."Oligomer molecule" means a molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass."Organic polymer" means a polymer or polymer network with a skeletal structure that includes carbon atoms. Examples include, without limitation, polyethers, polyesters, polycarbonates, polyacrylates, polyolefins, and polybutadienes."Polymer" means a substance composed of macromolecules."Polymer composition" means a composition comprising two or more different homopolymers. The homopolymers may have reactive chemical moieties that can undergo bond exchange. The two or more different homopolymers may be selected from two or more different classes of polymers, such as polyurethanes, polyesters, and polycarbonates. The two or more different homopolymers may be between 20 and 80 wt% the polymer composition For two different homopolymers, the weight ratio of the first homopolymer and second homopolymer may be between 20:80 and 80:20, 25:75 and 75:25, 30:70 and 70:30, 35:65 and 65:35, 40:60 and 60:40, 45:55 and 55:45, or about 50:50. The polymer composition may be a miscible or immiscible polymer blend."Polymerization" means a process of converting a monomer or a mixture of monomers into a polymer."Prepolymer molecule" means a macromolecule or oligomer molecule capable of entering, through reactive groups, into further polymerization, thereby contributing more than one constitutional unit to at least one type of chain of the final macromolecules."Polyurethane composition" means a dynamic network formed from urethane bonds that are capable of undergoing urethane bond-exchange reactions. The polyurethane compositions comprise a network urethane-containing polymer and a polyurethane exchange catalyst permeated within the network polymer. The network polymer may be formed from isocyanate constitutional units and a second constitutional unit having hydroxyl groups capable of reacting with the isocyanate group of the isocyanate constitutional unit. The mol% of the polyurethane exchange catalyst to the total isocyanate functionality may be less than or equal to 5 mol%. Suitable, the mol% may be less than or equal 4 mol%, 3 mol%, 2 mol%, 1 mol%, or less than 1 mol%. The second constitutional unit may be a prepolymer molecule or a branch unit. Suitably the second constitutional unit may function as both a prepolymer molecule and a branch unit. The prepolymer molecule is an organic polymer molecule or an inorganic polymer molecule such as a polyether, a polyester, a polycarbonate, a polyacrylate, a polyolefin, a polybutadiene, a polysulfide, or a polysiloxane having one or more hydroxyl groups capable of reacting with an isocyanate group. When the prepolymer molecule also functions as a branch unit, the prepolymer molecule has a three or more hydroxyl groups capable of reacting with isocyanate groups and typically a plurality of hydroxyl groups in proportion to the number of constitutional units of the prepolymer molecule. The network polymer may also be formed from urethane-containing monomers featuring other polymerizable groups, including but not limited to, acrylates, methacrylates, or other polymerizable olefins."Bond-exchange catalyst" means a catalyst that increases the rate of a bond-exchange reaction, such as a polyurethane bond exchange reaction. Suitable metal for the catalyst includes Sn, Bi, Fe, Zr, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, or Mo. Suitable ligands for the catalyst include, without limitation, branched or unbranched, substituted or unsubstituted carboxylates, alkyls, alkoxides, 1,3 -diketones, 1,2-diketones, sulfonates, sulfonamides, amines, diamines, carbonates, phosphates, nitrates, halides, catecholates, hydroxamates, hydroxides, or any combination thereof. The ligand may be branched or unbranched, substituted or unsubstituted.Exemplary ligands include acetylacetonate (acac), isopropoxide (OiPr), neodecanoate (neo), laurate, butyl, ethylhexanoate, and 2,2,6,6-Tetramethyl-3,5- heptanedione (tmhd), trifluoromethanesulfonate, trifluoromethanesulfonamide, cyclopentadiene, pyridine salicylidene diamine, phosphine, or any combination thereof. Exemplary catalysts include, without limitation, dibutyltin dilaurate (DBTDL), Bi(neo)3, Fefacacfl, 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."Thermosetting polymer" or "thermoset" is a polymer that is irreversibly hardened by curing from a soft solid of viscous liquid prepolymer or resin."Vitrimer" means a network polymer that can change its topology by thermally activated bond-exchange reactions that occur through an associative mechanism, such that the total number of covalent bonds in the network polymer does not decrease transiently while the bond-exchange reactions are taking place. At elevated temperatures, the bond-exchange reactions occur at an effectively rapid rate and the network polymer has properties of a viscoelastic liquid. At low temperatures, the bond-exchange reactions are slowed and the network polymer behaves like a thermosetting polymer.Foam-to-foam recycling of thermoset PU was successfully demonstrated. The addition of bond-exchange catalyst aided the reprocessing of PU by promoting bond exchange, enabling the formation of cells upon the in-situ generation of gas in the twin- screw extruder. The optimal temperature of foam-to-foam process allows for both reprocessing of PU and decomposition of chemical blowing agent occur simultaneously. Extrusion foaming can be continuously performed while maintaining bulk density, cell number density and average cell diameter, with compression properties consistent with cellular materials. Furthermore, foam injection molding using the mold cavity yielded low density foams, showing comparable compression properties to the as- synthesized PU foam, indicating that foam-to-foam process can recover the mechanical properties. Various feedstock of PU foams, such as soft and blend of soft / rigid foams, can undergo this process in the presence of bond-exchange catalyst.Methods for polyurethane reprocessing are disclosed in US 17 / 050,138, US 17 / 605,831, and PCT / US2023 / 063276, which are incorporated by reference herein for all purposes.Unless otherwise specified or indicated by context, the terms “a”, “an”, and “the” mean “one or more.” For example, “a molecule” should be interpreted to mean “one or more molecules.”As used herein, “about”, “approximately,” “substantially,” and “significantly” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” and “approximately” will mean plus or minus <10% of the particular term and “substantially” and “significantly” will mean plus or minus >10% of the particular term.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 being “open” transitional terms that permit the inclusion of additional components further to those components recited in the claims. The terms “consist” and “consisting of’ should be interpreted as being “closed” transitional terms that do not permit the inclusion additional components other than the components recited in the claims. The term “consisting essentially of’ should be interpreted to be partially closed and allowing the inclusion only of additional components that do not fundamentally alter the nature of the claimed subject matter.All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.Preferred aspects of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred aspects may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect a person having ordinary skill in the art to employ 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. Moreover, any combination of theabove-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.EXAMPLESA foam-to-foam recycling process of thermoset polyurethane was developed by melt reprocessing PU CANs with added chemical blowing agents to induce foaming following continuous extrusion. The initial PU foams were prepared by polymerizing a polyester-based polyol with equimolar methylene diphenyl diisocyanate (MDI) in the presence of isopentane as a physical blowing agent and a small amount of dibutyltin dilaurate (0.63 mol % with respect to - NCO) as a catalyst (Figure 1, 9a, and 9b). Zr(acac)4 was used as an added catalyst to enhance carbamate exchange rates and because of its lower toxicity compared to conventional tin-based catalysts (Figure 1). To promote the introduction of Zr(acac)4, the synthesized foam with an average cell diameter of 253.4 ± 119.7 pm (Figure 9c) was ground into smaller particles with an average particle diameter of 235.3 ± 150.9 pm (Figure 9d) using a kitchen blender. The ground particles were soaked in a solution of Zr(acac)4 in CH2CI2 for 4 h, after which the solvent was evaporated at 90 °C for 24 h in a vacuum oven, yielding ground PU particles containing 3 wt% Zr(acac)4. A chemical blowing agent, azodicarbonamide (ADC), was incorporated into the catalyst-loaded polymer using a vortex mixer to achieve an 8.5 wt% mixture of ADC.PU reprocessing and refoaming was performed by continuously feeding the PU granules into a twin-screw extruder held at an elevated temperature, which provided a continuous PU foam extrudate (Figure 2 and Figure 10). As the PU CAN was heated within the microcompounder, the reversible dissociation of the network allows the PU to be reprocessed via Zr-catalyzed crosslink exchange. Simultaneously, ADC is thermally degraded into gas molecules (N2, CO, and CO2), which dissolve within the polymer.[291As the reprocessed polyurethane was extruded from the die, the resulting pressure drop induced nucleation of the dissolved gas molecules and, consequently, the volumetric expansion of the extrudate and formation of a cellular structure (Figure ll).

[0030] Having established this foam-to-foam reprocessing approach, we set out to characterize the thermomechancal properties of the foams and explore how various experimental parameters impact foam quality.Spectroscopic analysis and thermal characterization of the extrudates reported on the chemical integrity of the PU after the foam-to-foam reprocessing procedure performed at 160, 180,200, or 220 °C. Fourier-transform infrared spectroscopy (FT-IR) confirmed that the carbamate C=O vibrational band (1700-1730 cm'1) remained unchanged after processing at each temperature, and isocyanate vibrational bands (2285 cm'1) were not observed (Figure 3a and 12a).

[0031] At 160 and 180 °C, a vibrational band corresponding to urea (1640 cm'1) was observed (Figure 12b). Although a peak in this region is sometimes associated with side-reactions between isocyanates and water, here it corresponds to residual ADC that did not completely decompose at the lower temperatures.

[0032] The presence of residual ADC was further confirmed by the appearance of the two N-H vibrational bands of ADC (3320-3330 and 3160-3190 cm'1), which were not observed in the foam extrudates at higher temperatures (Figure 3a).Solid-state cross-polarization magic angle spinning (CP -MAS)13C NMR was used to characterize residual ADC and its decomposition intermediate in the extrudate at each temperature (Figure 3b and Figure 13). These experiments further confirm that the PU product was formed and reveal when ADC remains in the material after the refoaming process. In extrudate processed at 160 °C, a resonance at 160.5 ppm corresponding to ADC was observed between the carbamate (154.0 ppm) and ester (173.0 ppm) resonances of the polyurethane. The ADC resonance at 160.5 ppm was similarly prominent in the pre-extrusion mixture, suggesting little to no degradation of ADC during reprocessing at 160 °C. In sample extruded at 180 °C, the 160.5 ppm ADC resonance was also observed, along with a new resonance at 163.5 ppm, which we assign to a hydrazocarbonamide intermediate. This assignment is based on known ADC decomposition pathways,133 35and the emergence of this resonance suggests partial ADC decomposition at this temperature. In contrast, extrudate processed at 200 °C showed no resonance associated with ADC, along with a more prominent intermediate peak. Finally, when the temperature was increased to 220 °C, neither ADC nor its intermediate were observed, indicating that all the ADC was decomposed. FT-IR and NMR suggest as the temperature increased, ADC decomposition increased, and a greater amount of gas was generated, with no changes to signals associated with the PU.Characterization of the 7gand gel fraction of each extrudate suggested that the crosslink density and network integrity were maintained to at least 180 °C and that temperatures above 200 °C led to network degradation (Figure 3c and 3d, and Table 2). The 7gof the PU extruded at 160 and 180 °C were 39.0 and 37.6 °C, respectively, which is similar to the / g of the pre-extrusionmixture (38.0 °C). Similarly, the gel fractions of PU extruded at 160 and 180 °C were 73.5 and 74.3 %, respectively, which is comparable to that of as-synthesized PU foam (76.6 %). The / gand gel fraction maintained after the extrusion at lower temperatures indicate that the crosslinked structure and polymer backbone were largely preserved after the process. However, at 200 °C and 220 °C, the Zg decreased to 32.2 and 30.4 °C respectively, suggesting that the network was partially degraded during reprocessing at these higher temperatures. Furthermore, the gel fraction of PU extruded at 200 and 220 °C decreased to 69.6 and 29.9 % respectively, indicating that the reduction of Tg of the extrudate was caused by the degradation of the polymer network at higher temperatures. We attributed the decrease in the Zgand loss of cross-linking density at the elevated temperatures to the oxidation of the polymer backbone and deleterious side reaction caused by the dissociation of carbamate bond at the elevated temperatures. Overall, 7gand gel fraction measurements demonstrated that the polymer backbone and crosslinks were maintained at a lower temperature process between 160-180 °C, but some degradation of crosslinked network was observed during the process at higher temperatures between 200-220 °C.Porosity analysis by SEM indicated that the optimal reprocessing temperature for refoaming was 180 °C, despite earlier NMR studies that indicated only partial ADC decomposition at this temperature (Figure 4). The 180 °C procedure provided the highest cell number density, smallest cell diameter, and the lowest foam density (Figure 4a and 4b, and Table 3). However, under the same reprocessing condition, the mixture of PU and blowing agent without the addition of Zr(acac)4 was not able to be reprocessed. Instead, the extruder reached the torque limit, indicating that the carbamate exchange catalyst is required to enable the reprocessing and refoaming process (Figure 14). Below the optimal temperature, rough surfaces without distinctive cell structure were observed in the SEM cross-section of the extrudate at 160 °C (Figure 4a), indicating that the processing temperature was too low for ADC to decompose, which is consistent with the NMR studies. On the other hand, the significant decrease of the cell number density (Figure 4b) and the increase of foam density (Table 3) signified that the foam became significantly destabilized at reprocessing temperatures above 180 °C. At 200 °C, the increase of cell diameter and decrease of cell number density demonstrated that the foam was destabilized through the coalescence of cells. When the process temperature was raised to 220 °C, the foam underwent significant gas loss, as the cells were smaller and the bulk density increased to 0.91 g cm'3, despite the complete decomposition of ADC (Table 3). The cell destabilization at highertemperatures is attributed to the compromised melt strength of polymer network (Figure 3c and 3d).The cell number density and average cell diameter were controlled by changing the screw rotation speed of the foam-to-foam process. In an extrusion foaming process using a chemical blowing agent, the generated gas molecules are dissolved in polymer at high pressures and temperatures inside the extruder. Once the polymer leaves the die, it undergoes a significant pressure drop, which causes the gases to expand in the polymer matrix and nucleate cells. In high temperature extrusion processes (i.e. 200 and 220 °C in Figure 4b), the extrudate matrices are too soft to support the expanded cells, so saturated gas molecules can escape the polymer and nucleated cells can coalesce.1361Based on this general understanding of extrusion foaming, we examined the influence of varying screw rotation speeds on foam-to-foam process at 180 °C (Figure 4c and Figure 15) and demonstrated the tunability of the foam structure. We were not able to investigate rotation speeds faster than 20 rpm, because the residence time of extrusion at 20 rpm was too short to generate gas and effectively reprocess PU granules (Figure 15). We hypothesized that bulk density of the foam would decrease upon decreasing the rotation speed from 15 to 5 rpm, because more gas will be generated during the polymer’s longer residence time in the extruder. However, the bulk density of foam was almost constant around 0.4 ~ 0.5 g cm’3(Table 3). This finding instead indicates that a large quantity of gas generated at a lower rotation speed was lost to the atmosphere due to the slower cooling rate of the foam extruded from the die.[37 391By increasing the rotation speed from 5 to 15 rpm, the average cell diameter was decreased from 32.9 ± 26.3 pm to 20.2 ± 9.9 pm. The larger cell diameter at a slower rotation speed also indicates that cell coalescence might have occurred before the foam extrudate near the die was cooled and cell structure was stabilized.

[0040] At the same time, the cell number density of foam increased from 4.54 ± 1.18 x 106cells cm’3at 5 rpm to 1.37 ± 0.66 x 107cells cm’3at 15 rpm (Figure 4c). This result indicates that cell stabilization through cooling of polymer extrudate, which is promoted at faster screw rotation speed, had a greater effect on cell number density than the amount of gas generated proportional to the residence time. Additionally, a higher throughput of extruder at faster screw speed resulted in a higher pressure drop rate of gas in polymer, which might have led to more cell nucleation.141 441To determine the consistency of foams generated by this process, we extruded double the mass of PU foam compared to earlier foams experiments, which yielded a foam fdament ofapproximately 65 cm in length (Figure 5). Tn the previous experiments, we qualitatively observed that the front and end portions, which consist of about half of recovered extrudate had different cross-sections than the middle portion of the extrudate. We attribute this inconsistency to the poor control of residence time and inconsistent pressure profde at each end of the extrudate. To confirm this hypothesis, we performed an extended extrusion experiment, which was expected to provide consistent properties for the bulk of the extrudate. The extended continuous extrusion was performed by reprocessing 6 g instead of 3g of PU. The extruded foam was 65 cm long for the 6 g extrusion, four times longer than that obtained from the 3 g extrusion (Figure 5a). Qualitatively, the extrudate displayed a foam structure at several points between 10 to 50 cm of the extrudate, but the cell structure was largely destabilized after 60 cm, again corresponding to the portion of the polymer at the end of the extrusion process (Figure 5b). The consistency of the process was quantitatively evaluated by measuring the average diameter, bulk density, and cell number density at several positions of the 6 g extrudate, and comparing it to the 3 g extrudate (Figure 5c and 5d). In the 3 g extrusion, the foam extrudate exhibited inconsistent cell diameter and density because of the unfixed residence time and underdeveloped pressure profile inside the extruder (Figure 5c and 5d). However, in the extended extrusion, average diameter, cell number density, and bulk density were maintained around 20 pm, 1 x 107cells cm’3and 0.4 g cm’3, respectively, except for the first and last segments of the extrudate (Figure 5c and 5d). We suspect that the undeveloped pressure profile of the initial 5 cm and the extremely long residence of the extrudate past 60 cm led to a higher bulk density and a lower cell number density compared to the constant values of parameters in the bulk of the extrudate. Therefore, it is likely that a continuous foam-to-foam process will provide a consistent cell structure, which is consistent with how these materials might someday be reprocessed.A process resembling injection molding was demonstrated by allowing the polymer melt to flow into the mold cavity, a backflow channel connected to the extrusion barrel, by operating the extruder in the circulation mode at 180 °C (Figure 6). Afterwards, opening the lid of the mold cavity resulted in the entire sample undergoing a rapid pressure drop, inducing cell nucleation and generation of foam structure with significant volumetric expansion (Figure 6). As a result, we obtained PU foams retaining the shape of mold cavity regardless of cycling time for reprocessing PU CANs (Figure 7a). By increasing the cycling time, bulk density of foam decreased significantly down to 0.25 g cm’3, comparable to the as-synthesized foam with bulk density of 0.18g cm'3(Figure 7b and 7c). Foams made through 2.5 to 7.5 min of cycling time had higher bulk density (Figure 7c) due to partial degradation of ADC and limited amounts of gas that could participate in cell growth (Figure 16). After 10 minutes of cycling, most ADCs were converted into intermediate, and the generated gas species were entrapped in the mold cavity, leading to considerable volume expansion and production of low density foams (Figure 7c and 16). Increasing the cycling time also resulted in slight reduction of cell number density, indicating that coalescence between adjacent nucleated cells had occurred (Figure 7d). However, further circulation of material beyond 10 minutes showed cell number density leveling off, suggesting that catastrophic cell destabilization and collapse of foam structure were prevented. This trend is attributed to the preserved integrity of polymer chain as suggested from the maintained 7gand gel fraction around 45 °C and 75 %, respectively, after the foam injection molding process (Figure 17 and 18). By increasing the cycling time from 5 to 10 minutes, the average cell diameter increased from 15 pm to 40 pm as more gas was available for participating in cell growth (Figure 7d). Consequently, the foam injection molding process with sufficient cycling time produced a low density foam with a high cell number density of around 1 x 107cells cm'3and an average diameter of 40 pm. This finding suggests that foam injection molding with an increased pressure drop can effectively produce cell nucleation and growth while maintaining closed cell morphology, which is essential for an application such as thermal insulation.1451The properties of the reprocessed PU foam, such as bulk density and pore size, were controlled upon the dynamic properties of the PU network, which vary with the concentration of additional Zr catalyst. Increasing the Zr(acac)4 concentration from 3 to 5 wt% in the PU network reduced the stress relaxation time from 74.5 s to 50.4 s at 160 °C, indicating that the bond exchange rate increases when more catalyst is present (Figure 19). As the Zr catalyst concentration increased from 3 to 5 wt%, the foam thickness increased from 3.51 ± 0.26 to 4.23 ± 0.16 mm and the bulk density decreased from 0.26 ± 0.04 to 0.18 ± 0.01 g / cm'3, which is very similar to the density of the original foam (Figure 20). In addition to bulk density, bond exchange rates in PU networks also affected average cell diameter and distribution. As the Zr catalyst concentration increased from 3 to 5 wt%, the average cell diameter increased from 28.3 ± 20.9 to 47.6 ± 38.3 pm, which was confirmed by a shift in the pore size distribution (Figure 21 and 22). Regardless of the concentration of the Zr catalyst, the 7gof the reprocessed foam decreased by less than 1°C compared to that of the synthesized PU foam (Figure 23). The constant TRshowed the PU networkmaintained the integrity after reprocessing, suggesting that the change in size distribution was not due to PU network degradation. Therefore, the catalyst concentration, and perhaps other parameters that result in faster urethane exchange, can be used to control the density and pore size of the reprocessed foam.Having established a foam-to-foam recycling protocol, we characterized the mechanical properties of the various reprocessed PU foams. These measurements are crucial as PU foam applications are largely determined by their mechanical properties, which are tunable based on the density of foam.14071The as-synthesized PU foam, and the PU foams made through the two foam- to-foam processes were characterized by compression tests. Each foam displayed the three distinct regions of a foam upon strain: elastic deformation, plateau, and densification (Figure 8).[48’491 The compressional modulus and yield stress of each foam were calculated accordingly from the curve.

[0050] As a result, the elastic modulus (E = 9.61 MPa) and yield stress (<JY = 0.99 MPa) of foam obtained from 3g extrusion were comparable to the elastic modulus (E = 8.55 MPa) and yield stress (<JY = 1.00 MPa) of the foam produced through extended extrusion, indicating the consistency of extrusion foaming (Table 1). However, the elastic modulus and yield stress of foam extrudates were still much higher than that of the synthesized foams blown with isopentane or water since the density of extrudate was about twice of the synthetic foams (0.18 g cm’3).

[0051] The lower density foam (0.25 g cm'3) obtained through foam injection molding with 3 wt% of Zr(acac)4 and cycling time of 10 minutes showed an elastic modulus and yield stress close to that as-synthesized foam compared to extrusion process. However, due to the higher bulk density of the reprocessed PU foam, the compressive modulus and yield stress was still higher than that of the as-synthesized foam. This result implies the importance of achieving low-density PU foams to fully recover the properties of synthesized foams after reprocessing. We were able to achieve reprocessed PU foam having reduced bulk density via injection molding process by incorporating more carbamate exchange catalyst, and the resulting low-density foam demonstrated comparable or even lower elastic modulus and yield stress than the as-synthesized foams (Figure 8 and Table 1). Overall, the foam-to-foam process can produce a recycled PU foam with comparable and tunable properties to the end-of-life PU foam by optimizing the processing parameters.Table 1. Compressive modulus and yield stress of foams depending on the processes and density of foams.Bulk Elastic Modulus YieldFoaming Process Density Porosity (Compression) Stress[g cm'3] E [MPa] OY [MPa]3g Extrusion Foaming 0.44 63 % 9.61 ± 2.11 0.99 + 0.386g Extrusion Foaming 0.37 69 % 8.55 ± 2.67 1.00 ± 0.49Foam Injection Molding with 3 0.25 79 % 2.73 ± 1.38 0.23 ± 0.04 wt% of Zr(acac)4Foam Injection Molding with 5 0.18 85 % 0.94 ± 0.13 0.14 ± 0.03 wt% of Zr(acac)4As-synthesized PU Foam 0.18 85 % 1.88 ± 0.85 0.21 ± 0.09(Physical Blowing)As-synthesized PU Foam 0.18 85 % 1.62 ± 0.41 0.21 ± 0.04(Chemical Blowing)Table 2. Tgand gel fraction of as-synthesized foam and PU foam extrudates at various extrusion temperatures (Screw Rotation Speed = 10 rpm).Condition Ts(°C) Gel Fraction (%)Foam as synthesized 38.0 76.6 + 3.7160 °C 39.0 73.5 + 1.5180 °C 37.6 74.3 ± 0.5200 °C 32.2 69.6 + 0.6220 °C 30.4 29.9 + 2.1Table 3. Bulk density of reprocessed PU foam and PU fdm at various extrusion temperatures and speeds.Screw Rotation SpeedExtrusion Temperature (°C) Bulk Density (g / cm3)(rpm)160 °C 10 0.61 + 0.05180 °C 10 0.44 + 0.02200 °C 10 0.59 + 0.01220 °C 10 0.91 + 0.06180 °C 5 0.42 + 0.03180 °C 15 0.46 + 0.06180 °C 20 0.68 + 0.11Materials and General Methods Materials All reagents were purchased from Sigma-Aldrich or Fisher Scientific. Polyols were dried at 90 °C under 20 mTorr vacuum for at least 1 hours prior to use for synthesis of PU foams. All other reagents were used without further purification unless otherwise specified.Instrumentation and CharacterizationsInfrared Spectroscopy Infrared spectra were recorded on a Thermo Nicol et iS20 equipped with a ZnSe ATR attachment. Spectra were uncorrected.CP-MAS13C Solid-State NMR Solid-state NMR spectra were recorded on a 400 MHz Bruker Avance III using a standard Bruker 4 mm HX probe at ambient temperature.Differential Scanning Calorimetry Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC250 Differential Scanning Calorimeter. Samples (5-10 mg) were heated at a rate of 30 °C / min to 120 °C to erase thermal history, cooled to -80 °C at 30 °C / min, then left at -80 °C for 5 min. The sample was again heated at a rate of 10 °C / min to 120 °C, and all data shown are taken from the second heating ramp. The glass transition temperature (Tg) was calculated from the maximum value of the derivative of heat flow with respect to temperature.Gel Fraction Gel fractions were obtained by submerging approximately 50 mg of PU into a 2 mb solution of DCM for two days, in a sealed 4 mb vial. After two days, the solution was washed with additional DCM and filtered. The insoluble solid was recovered, and dried at 90 °C under reduced pressure, prior to weighing. Gel fractions are reported as an average of triplicate measurements, along with the corresponding standard deviation.Bulk Density The bulk density of the PU foam (Pfoam) was calculated by dividing the mass by the volume of the PU foam. PU foams were cut into rectangular shapes with dimensions of approximately 3 mm x 5 mm x 10 mm using a single-edged razor blade and the mass of each foam slice was measured. Then the volume of PU foam slice was calculated by measuring each dimension using a vernier caliper. Then the density was obtained by dividing mass with volume, and each sample was done in triplicates. Bulk density was reported as an average of the triplicates, with an associated standard deviation. The bulk density of the reprocessed PU film (pfum) without a chemical blowing agent was measured in the same way for cell number density analysis.Porosity Porosity (P) of PU foam is defined as P = [1 —dfoaml x 100 % where Pfumis L “solid J the bulk density of reprocessed PU film and Pfoamis the bulk density of each PU foam.152,331The value of Pfumused in this analysis was 1.20 ± 0.01 g / cm3according to the bulk density measurement procedure and foam-to-film process.Scanning Electron Microscopy Polyurethane foams were secured to a flat aluminum sample holder, coated with 5 nm of gold-palladium using a sputter coater, and imaged with a Hitachi S4800-II SEM.Image processing Average cell diameter was determined by analyzing more than three images for each PU foam sample using ImageJ program (NIH). Post-processing of SEM images was carried out by converting SEM images to binary images using appropriate color thresholds. The boundary of cellular structures was then identified to calculate the area of each cell in cross- sectional SEM image of PU foams. The number average diameter (d) was calculated from taking the average diameter of each cell, assuming that each cross-section of cell has a circular geometry. The cell number density (Nc) was calculated from number (n) of cells per area of image (A) using the following equation.[?4’55]d and Nc values were reported as averages of at least three SEM images at least 100 cells, with associated standard deviations.Stress Relaxation Analysis Stress relaxation analysis (SRA) was performed on a TA Instruments RSA-G2 analyzer (New Castle, DE) using rectangular films (ca. 1.5 mm (T) x 10 mm (W) x 15 mm (L) and a Gauge length of 5 mm). The SRA experiments were performed with strain control at 160 °C. The samples were allowed to equilibrate at this temperature for approximately 10 minutes, after which the axial force was then 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 had relaxed to at least 37% (1 / e) of its initial value.Synthetic and Engineering ProceduresSynthesis of Crosslinked Polyester Polyurethane Foam This procedure was adapted from previous reports.

[0056] To a plastic cup was added polyftrimeth ylolpropane / di(propylene glycol)-c / / z-adipic acid / phthalic anhydride] polyol (average Mn~ 500 g / mol, hydroxyl functionality = 2.5, 15 g, 75 mmol -OH), blowing agents isopentane (450 mg), and dibutyltin dilaurate (300 mg, 0.63 mol % with respect to -NCO), and tris(nonylphenyl) phosphite (750 mg, 3 wt% to polyurethane foam). Ground solid 4,4'-methylenebis(phenyl isocyanate) (MDI) (9.4 g, 37.5 mmol) was added and mixed vigorously. The mixture was allowed to sit for one hour to gel and rise. The resulting bulk polymer was transferred to an aluminum pan (104 mm diameter x 15 mm height) and placed in a vacuum oven at 90 °C at 20 mTorr to immediately expand and cure for 48 hours. The foam was post cured at 150 °C for 1 hour to ensure full cross-linking.Synthesis of Water-blown Polyester Polyurethane Foam To a plastic cup was added poly[trimethylolpropane / di(propylene glycol )-a / / -adi pic acid / phthalic anhydride] polyol (average Mn~ 500 g / mol, hydroxyl functionality = 2.5, 15 g, 75 mmol -OH), water (0.15 g, 8.3 mmol), and dibutyltin dilaurate (300 mg, 0.63 mol % with respect to -NCO), and tris(nonylphenyl) phosphite (750 mg, 3 wt% to polyurethane foam). Ground solid 4,4'-methylenebis(phenyl isocyanate) (MDI) (9.4 g, 41.7 mmol) was added and mixed vigorously. The mixture was allowed to sit for one hour to gel and rise. The resulting bulk polymer was transferred to an aluminum pan (104 mm diameter x 15 mm height) and placed in a vacuum oven at 90 °C at 20 mTorr to immediately expand and cure for 48 hours. The foam was post cured at 150 °C for 1 hour to ensure full cross-linking.Post-synthetic introduction of catalyst to PU foam As-synthesized PU foams were cut into rectangular shapes with dimensions of approximately 10 mm x 10 mm x 10 mm, then ground into smaller particles using a kitchen blender. Twenty grams of ground foam particle was soaked in 60 mL of a certain concentration (10.3, 14.0, or 17.7 mg / mL) of Zirconium acetyl acetonate [Zr(acac)4] solution in dichloromethane for 4 hours. The swollen particles were then placed in a vacuum oven at 90 °C at 20 mTorr for 24 hours to remove di chloromethane to yield ground PU particle containing of a certain concentration (3, 4, or 5 wt%) of Zr(acac)4.Foam-to-foam process using twin-screw extruder Three grams of ground PU foam particle containing 3 wt% of Zr(acac)4 were mixed with 0.28 g of azodicarbonamide (ADC 8.5 wt%) for 1 minute using a vortex mixer. The ground PU foams were continuously fed into the twin-screw extruder (HAAKE MiniLab 3, Thermo Scientific) at a certain operating temperature (160, 180, 200, 220 °C) and a screw rotation speed (5 to 20 rpm). The residence time was estimated to be 5 min for 10 rpm. The material was then flushed through a 1 mm thick film die and air cooled. For the extended foam-to-foam process, six grams of ground PU foam particle was extruded using the same concentration of azodicarbonamide at 180 °C with a screw rotation speed of 15 rpm.Foam-to-foam process using injection molding connected to twin-screw extruder 4.8 g of ground PU foam particle containing a certain concentration (3, 4, or 5 wt%) of Zr(acac)4 were mixed with 0.45 g of ADC (8.5 wt%) for 1 minute using a vortex mixer. The ground PU foams were continuously fed into the twin-screw extruder under the circulation mode, in which the material filled a backflow channel connected to the twin-screw extruder. 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Claims

CLAIMSWe claim:

1. A method for reprocessing a thermoset polyurethane foam, the method comprising: mechanically processing a polyurethane foam into particles; loading the particles with a bond-exchange catalyst, thereby preparing loaded particles; mixing the loaded particles with a blowing agent; compounding a mixture of the loaded particles and the blowing agent, thereby preparing a compounded substance; and extruding the compounded substance.

2. The method of claim 1, wherein the compounded substance is extruded into a mold.

3. The method of any one of claims 1-2 further comprising decreasing pressure on the compounded substance after extruding the compounded substance.

4. The method of any one of claims 1-3, wherein the blowing agent is a chemical blowing agent.

5. The method of any one of claims 1-4, wherein the blowing agent thermally degrades during compounding into gas molecules that dissolve into the compounded substance.

6. The method of any one of claims 1-5, wherein the blowing agent thermally degrades into gas molecules selected from N2, CO, CO2, or any combination thereof.

7. The method of any one of claims 1-6, wherein the blowing agent is azodicarbonamide or water.

8. The method of any one of claims 1-7, wherein loading the particles with the bondexchange catalyst comprising contacting the particles with a solution comprising the bond-exchange catalyst and evaporating solvent from the solution.

9. The method of any one of claims 1-8, wherein the bond-exchange catalyst is a carbamate exchange catalyst.

10. The method of any one of claims 1-9, wherein the bond-exchange catalyst is zirconium(IV) acetyl acetonate.

11. The method of any one of claims 1-10, wherein the mixture of the loaded particles and the blowing agent is compounded at an effective bond-exchange temperature between 160 °C and 220 °C.

12. The method of any one of claims 1-11 , wherein the mixture of the loaded particles and the blowing agent is compounded for an effective bond-exchange time to degrade the blowing agent into gas molecules during compounding that dissolve into the compounded substance.

13. The method of any one of claims 1-12, wherein the mixture of the loaded particles and the blowing agent is compounded for an effective bond-exchange time between 2 min and 20 min.

14. The method of any one of claims 1-13, wherein the mixture of the loaded particles and the blowing agent is compounded with a twin-screw extruder at a screw rotation speed between 5 and 15 revolutions per minute (RPM).

15. The method of any one of claims 1-14 further comprising blending the polyurethane foam with one or more additional polymers and compounding polyurethane foam, bondexchange catalyst, blowing agent, and one more or additional polymers.

16. A foam-to-foam recycling system comprising a feeder and a twin-screw extruder, wherein the feeder has a mixture of polyurethane foam particles, a bond-exchange catalyst, and a blowing agent therein and the feeder is configured to feed the mixture of polyurethane foam particles, the bond-exchange catalyst, and the blowing agent are feed into the twin-screw extruder, wherein the twin-screw extruder is configured to compound the mixture of the polyurethane foam particles, the bond-exchange catalyst, and the blowing agent and extrude a compounded substance through a die or into a mold.

17. The foam-to-foam recycling system of claim 16 comprising the mold, wherein the mold is configured to decrease pressure on the compounded substance after the compounded substance is extruded from the twin-screw extruder.

18. The foam-to-foam recycling system of claim 16 or 17, wherein the twin-screw extruder is configured to compound the mixture of the polyurethane foam particles, the bondexchange catalyst, and the blowing agent at an effective bond-exchange temperature between 160 °C and 220 °C.

19. The foam-to-foam recycling system of any one of claims 16-18, wherein the twin-screw extruder is configured to compound the mixture of the polyurethane foam particles, the bond-exchange catalyst, and the blowing agent for an effective bond-exchange time between 2 min and 20 min.

20. The foam-to-foam recycling system of any one of claims 16-19, wherein the twin-screw extruder is configured to compound the mixture of the polyurethane foam particles, the bond-exchange catalyst, and the blowing agent at a screw rotation speed between 5 and 15 revolutions per minute (RPM).

21. The foam-to-foam recycling system of any one of claims 16-20, wherein the blowing agent is a chemical blowing agent.

22. The foam-to-foam recycling system of any one of claims 16-21, wherein the blowing agent thermally degrades during compounding into gas molecules that dissolve into the compounded substance.

23. The foam-to-foam recycling system of any one of claims 16-22, wherein the blowing agent thermally degrades into gas molecules selected from N2, CO, CO2, or any combination thereof.

24. The foam-to-foam recycling system of any one of claims 16-23, wherein the blowing agent is azodi carbonamide or water.

25. The foam-to-foam recycling system of any one of claims 16-24, wherein the bondexchange catalyst is a carbamate exchange catalyst.

26. The foam-to-foam recycling system of any one of claims 16-25, wherein the bondexchange catalyst is zirconium(IV) acetylacetonate.

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