Recycling of thermoset polyurethane foams

Abstract

Disclosed herein is a method of reprocessing a thermoset polyurethane foam. The method includes mixing polyurethane foam particles loaded with a bond exchange catalyst with a blowing agent and compounding the mixture. The method includes machining 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 the mixture of the loaded particles and the blowing agent, thereby preparing a compound; and extruding the compound.

Description

[0001] Statement on Federally Funded Research

[0002] This invention was completed with government funding granted by the U.S. Department of Energy under license number DE-EE0007897. The government owns certain rights to this invention.

[0003] Cross-reference to related applications

[0004] This application claims the benefit and priority of pending U.S. Provisional Patent Application No. 63 / 515,794, filed July 26, 2023. The entire contents of the stated patent application are incorporated herein by reference. Background Technology

[0005] Polyurethane (PU) is the most common thermoset polymer category and the sixth most consumed plastic, accounting for 8% of annual plastic production. [1] Despite their wide range of applications, thermosetting polyurethanes cannot be recycled through conventional melt processing, so their reprocessing options are limited to mechanical or chemical recycling. [2] Mechanical recycling involved grinding PU foam into small, cohesive particles for use in the production of recycled products, such as carpet backing. This practice was later discontinued due to concerns about toxic additives. [3–5] A chemical recycling process for polyurethane based on glycolysis of urethane bonds has been developed to recover and repolymerize PU raw materials, but this process only recovers the polyol component. [6–8] Therefore, new isocyanates are needed to repolymerize the recovered polyols. [9,10] Given the limitations of these methods, it is urgent to develop recyclable, energy-efficient, and atomically efficient polyurethane reprocessing methods.

[0006] To improve the recyclability of PU while maintaining its utility and performance, our group and other researchers have reported thermally reprocessable PU covalent adaptive networks (CANs) using Lewis acid catalysts, such as dibutyltin dilaurate (DBTDL). [11–16] This catalyst activates carbamate exchange, thereby imparting rapid stress relaxation and flow behavior at high temperatures. [11–16] In these examples, a catalyst is introduced during the synthesis of the PU film, while the recycling of existing PU waste requires introducing the catalyst into the PU just before reprocessing. [17,18] We have recently developed a method for reprocessing commercial PU foam into solid PU films by introducing an external catalyst into the foam.

[19] Furthermore, when a twin-screw extruder is used instead of compression molding, reprocessed commercial foams achieve higher film uniformity and superior tensile properties.

[0007] A true circular process for PU foam requires the development of foam-to-foam recycling technologies, as the commercial demand for solid polyurethane is more limited. [2,20] Refoaming of thermosetting materials is possible when the polymer network is able to flow under shear stress, because foam processing requires the stretching flow of the polymer to achieve cell growth. [21,22] Catalysts associated with CANs can achieve the stretch flow required for the foaming process of thermosetting PU through crosslinking exchange during melt reprocessing. Recently, the use of polymers such as ethylene-vinyl acetate, polyethylene terephthalate, and polylactic acid to foam CANs has been reported. [23–25] However, the foaming in these studies was achieved in CANs derived from thermoplastic materials, which can themselves be reprocessed using conventional industrial processes. In contrast, foam-to-foam reprocessing methods for thermosetting polyurethanes would enable the recycling of PU foam products that would otherwise be landfilled or incinerated. Therefore, there is a need in the art for a foam-to-foam recycling method for PU foams. Invention Overview

[0009] This document discloses a method for reprocessing thermosetting polyurethane foam. The method includes: mechanically processing the polyurethane foam into granules; loading the granules with a bond exchange catalyst to prepare loaded granules; mixing the loaded granules with a blowing agent; compounding the mixture of the loaded granules and the blowing agent to prepare a compound; and extruding the compound. In some cases, the compound may be extruded into a mold to prepare a molded polyurethane foam article. The blowing agent may be a chemical blowing agent. Suitably, a blowing agent that degrades and dissolves into the compound during compounding is selected. Exemplary blowing agents include azodicarbonamide or water.

[0010] Brief description of the attached figures

[0011] Non-limiting embodiments of the invention will be described by way of example with reference to the accompanying drawings, which are schematic and not intended to be drawn to scale. In the drawings, each identical or nearly identical illustrated component / assembly is generally represented by a single number. For clarity, not every component / assembly is labeled in every figure, nor is every component / assembly shown in every embodiment of the invention necessary for those skilled in the art to understand the invention.

[0012] Figure 1 A schematic diagram of the synthesis of thermosetting PU foam, the dynamic bond exchange of urethane bonds in the PU network, and the structure of the urethane exchange catalyst Zr(acac)4.

[0013] Figure 2A schematic diagram of a foam-to-foam recycling process using a twin-screw extruder (scale bar = 100 μm).

[0014] Figure 3 Characterization of PU foam reprocessed at different temperatures. (a) FT-IR spectra of virgin PU (freshly synthesized PU), ADC, and foam reprocessed at different temperatures. (b) Solid-state ¹³C CP-MAS NMR spectra of the pre-extrusion mixture and foam reprocessed at different temperatures (rotation speed = 10 kHz, field strength = 400 MHz). (c) DSC traces of PU containing 3 wt% Zr(acac)₄ and foam reprocessed at different temperatures; solid points on each DSC trace represent Tg (heating rate = 10 °C / min). (d) Tg and gel fraction of virgin foam (AS) and PU foam reprocessed at different temperatures.

[0015] Figure 4 (a) Cross-sectional SEM images (scale bar = 200 μm) of foam extrusions produced by foam-to-foam process at different temperatures (160–220 °C) and screw speeds of 10 rpm. (b) Cell number density and average cell diameter of foam at different temperatures (180–220 °C) and extrusion speeds of 10 rpm. (c) Cell number density and average cell diameter of foam at 180 °C and different screw speeds (5–15 rpm).

[0016] Figure 5 An extended, continuous foam-to-foam extrusion process was performed to determine the homogeneity of the samples at 180°C and 15 rpm. (a) Images of the extruded foam, indicating the locations of the cross-sections used for image analysis. The left portion of the image is in the early extrusion stage (scale bar = 1 cm). (b) SEM images of the cross-sections of the foamed extrudate at each location (scale bar = 100 μm). (c) Average cell diameter and (d) Cell number density and bulk density at each location for 3 g and 6 g extrusions.

[0017] Figure 6 A schematic diagram of a foam-to-foam recycling process similar to foam injection molding.

[0018] Figure 7 Foam-to-foam process similar to injection molding was performed using a die connected to an extruder. (a) Image of the die connected to the extruder screw and images of molded foam at different cycle times (2.5–20 min, 180 °C, feed rate = 4.8 g PU / 5 min) (scale bar = 1 cm). (b) SEM images of the cross-section of the molded foam at different cycle times (scale bar = 200 μm). (c) Bulk density and (d) Cell number density and average cell diameter of the foam at different cycle times.

[0019] Figure 8 Compression tests of foam at a constant compression rate (0.005 mm / s). (a) Compression stress-strain curves of foams produced by different processes: small-scale (3 g) extrusion foaming, large-scale (6 g) extrusion foaming, injection foaming containing 3 wt% Zr(acac)4 and 5 wt% Zr(acac)4, and raw foam foamed with isopentane and water. (b) The extended regions of the compression stress-strain curves show the early portions of the linear region and the plateau region.

[0020] Figure 9 (a) Image of raw PU foam (background grid length = 0.5 inches) and (b) image of a cross-section of raw PU foam. (c) SEM image of raw PU foam and (d) SEM image of foam particles ground using a kitchen blender.

[0021] Figure 10 Images of continuous PU foam extrusions from a twin-screw extruder at different extrusion temperatures and speeds (scale bar = 1.0 cm).

[0022] Figure 11 (a) Image of the die end of a twin-screw extruder and (b) Image of PU foam extruded from that die (extrusion temperature = 180°C, screw speed = 10 rpm). The thickness of the foam extrusion is 2.51 ± 0.06 mm. Figure 11 The arrow in b indicates that the thickness of the die head is greater than 1.10 mm. Figure 11 (The arrow in a).

[0023] Figure 12 (a) Isocyanate absorption bands (2285 cm⁻¹) of raw PU foam, ADC, and foam reprocessed at different temperatures. - ¹) the vicinity and (b) the urea absorption zone (1640 cm) - ¹) Near-field FT-IR spectra. (a) No vibrational absorption bands corresponding to isocyanates were found in virgin and reprocessed PU foam. 2350 cm⁻¹ - The absorption band at ¹ originates from carbon dioxide in the atmosphere. (b) Urea vibration absorption bands of ADC were observed in PU foam reprocessed at 160°C and 180°C, indicating the presence of residual ADC after reprocessing.

[0024] Figure 13 CP-MAS¹³C NMR full spectrum of pre-extrusion mixture and foams reprocessed at different temperatures.

[0025] Figure 14Extruder torque and screw speed were measured when reprocessing a mixture of thermosetting PU and 8.5 wt% ADC without the addition of Zr(acac)4 catalyst. After feeding, the twin-screw extruder stopped because the extruder torque exceeded its inherent limit of 5 N·m. When the maximum torque was reached, the screw speed dropped from 10 rpm to 0 rpm, confirming the extrusion stoppage.

[0026] Figure 15 Cross-sectional SEM images of foam extrusions produced by foam-to-foam process at an extrusion temperature of 180°C and different screw speeds (5–20 rpm). The screw speeds corresponding to each image are (a) 5 rpm, (b) 10 rpm, (c) 15 rpm, and (d) 20 rpm.

[0027] Figure 16 Solid-state ¹³C CP-MAS NMR spectra of PU foam reprocessed by foam injection molding with different cycle times (rotation speed = 10 kHz, field strength = 400 MHz). (a) All-solid-state ¹³C CP-MAS NMR spectrum, and (b) spectra near the resonance peaks of the intermediate (INTMD) and ADC.

[0028] Figure 17 DSC traces of PU foam reprocessed by foam injection molding with different cycle times. The solid point of each DSC trace represents Tg (heating rate = 10℃ / min).

[0029] Figure 18 Tg and gel fraction of PU foam reprocessed by foam injection molding with different cycle times.

[0030] Figure 19 Stress relaxation diagrams of reprocessed PU films containing different concentrations of Zr(acac)₄ at 160°C. It decreases with increasing Zr(acac)4 concentration.

[0031] Figure 20 Cross-sectional images and bulk density of PU films and foams (containing 3–5 wt% Zr(acac)4) reprocessed by injection molding in the circulating chamber of a twin-screw extruder (180°C, feed time = 4.8 g PU / 8 min, circulation time = 10 min). The reprocessed foam is thicker than the reprocessed film with a thickness of 1.5 mm, indicating successful volume expansion during foam injection molding. For samples containing 3 wt%, 4 wt%, and 5 wt% Zr(acac)4, the average thicknesses of the reprocessed foams were 3.51 ± 0.26 mm, 3.84 ± 0.19 mm, and 4.23 ± 0.15 mm, respectively.

[0032] Figure 21 Cross-sectional SEM images of reprocessed foams produced by foam injection molding at different Zr(acac)4 concentrations (3–5 wt%). A mixture of 4.8 g PU foam and 0.45 g chemical foaming agent was fed into an extruder for 8 min, followed by circulation for 10 min. The operating temperature of the extruder was maintained at 180 °C during both feeding and circulation.

[0033] Figure 22 Cell diameter distribution of reprocessed PU foam produced by foam injection molding at different Zr(acac)4 concentrations (3–5 wt%). With increasing Zr(acac)4 concentration, the cell diameter distribution shifts towards larger diameters, indicating faster cell growth due to more rapid dynamic bond exchange.

[0034] Figure 23 DSC traces of raw PU foam and PU foam reprocessed with different Zr(acac)4 concentrations via foam injection molding. Solid dots on each DSC trace represent Tg (heating rate = 10℃ / min). The Tg of the raw PU foam differs from that of the extrusion experiment because different batches of PU foam were used in this experiment.

[0035] Figure 24 (a) Cross-sectional images of the synthesized water-blown PU foam and (b) water-blown PU foam cured in a vacuum oven at 90°C for 48 hours and then post-cured in an oven at 150°C for 1 hour. Invention Details

[0037] This paper discloses a direct, continuous foam-to-foam recycling method utilizing the melt processability of PU CANs.

[0038] One aspect of this technology is a method for reprocessing thermosetting polyurethane foam, the method comprising: mechanically processing the polyurethane foam into granules; loading the granules with a bond exchange catalyst to prepare loaded granules; mixing the loaded granules with a blowing agent; compounding the mixture of the loaded granules and the blowing agent to prepare a compound; and extruding the compound.

[0039] Two additives enable simultaneous reprocessing and refoaming of PU CANs: a bond-exchange catalyst and a blowing agent. A blowing agent is a substance capable of creating a porous structure in a material undergoing a phase change. Blowing agents include chemical blowing agents, physical blowing agents, and combinations thereof. Suitably, the blowing agent can be a chemical blowing agent capable of decomposing at reprocessing temperatures. The blowing agent can be thermally degraded during compounding into gas molecules soluble in the compound. The gas molecules generated during thermal degradation can be N2, CO, CO2, or any combination thereof. Exemplary blowing agents include azodicarbonamide or water.

[0040] In some embodiments, the method includes loading the particles with a bond exchange catalyst, the loading comprising contacting the particles with a solution containing the bond exchange catalyst and evaporating the solvent from the solution. The bond exchange catalyst may be a carbamate exchange catalyst. Exemplary bond exchange catalysts include zirconium acetylacetonate (IV).

[0041] The mixture of the loaded particles and the blowing agent can be compounded at an effective bond exchange temperature of 160-220°C or any range thereof. For example, an effective bond exchange temperature may be greater than 160°C, 165°C, 170°C, or 175°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.

[0042] The mixture of the loaded particles and the blowing agent can be formulated for an effective bond exchange time, allowing the blowing agent to degrade into gas molecules and dissolve into the mixture during formulation. The mixture of the loaded particles and the blowing agent can be formulated for an effective bond exchange time of 2-20 min or any range therewith. For example, an effective bond exchange time can 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.

[0043] When using a twin-screw extruder for compounding, the mixture of the loaded particles and the foaming agent can be compounded at an effective screw speed to change the density, number of cells, cell diameter, or a combination thereof of the extruded foam. The mixture of the loaded particles and the foaming agent can be compounded at a screw speed of 5-15 rpm. For example, the screw speed can be greater than 5, 6, 7, 8, 9, or 10 RPM and less than 15, 14, 13, 12, or 10 RPM, including any range or value between these values.

[0044] The method may include blending polyurethane foam with one or more additional polymers, and compounding polyurethane foam, bond exchange catalyst, blowing agent and one or more additional polymers.

[0045] These processes can produce recycled PU foam with a porous structure, as determined by image analysis using scanning electron microscopy (SEM). The continuity of the refoaming process was demonstrated by the constant cell diameter and cell number density of the foam. A process simulating PU CAN injection molding was demonstrated by filling the chamber connected to the extruder with reprocessed material and subsequently opening the chamber lid for rapid decompression. Compression tests of the recycled foam showed mechanical properties comparable to the virgin foam. These results indicate that waste PU foam can be directly recycled into new foam while maintaining its structural properties and chemical integrity.

[0046] A foam-to-foam recycling method for thermosetting polyurethane was developed by melt reprocessing PU CANs with added foaming agents to induce foaming after continuous extrusion. Examples demonstrate the reprocessing of PU foam prepared by polymerizing polyester-based polyols with diisocyanates in the presence of isopentane as a physical foaming agent and a small amount of dibutyltin dilaurate as a catalyst. Figure 1 , 9 a and 9b). Using bond exchange catalysts to increase the bond exchange rate ( Figure 1 To facilitate the introduction of the bond exchange catalyst, the synthesized foam can be ground into smaller particles. The ground particles can be immersed in a solution of the bond exchange catalyst, and then the solvent can be evaporated to obtain ground PU particles containing the bond exchange catalyst. A blowing agent is then introduced into the catalyst-supported polymer.

[0047] The reprocessing and refoaming of PU can be carried out by feeding PU particles, a bond exchange catalyst, and a foaming agent into a compounding apparatus maintained at a bond exchange temperature that allows the preparation of PU foam extrusions. Figure 2 and Figure 10 When the PU CAN is heated, the reversible dissociation of the network allows the PU to be reprocessed via catalytic crosslinking exchange. Simultaneously, the blowing agent thermally degrades into gaseous molecules (e.g., N2, CO, and CO2), which dissolve within the polymer. As the reprocessed polyurethane is extruded, the resulting pressure drop induces nucleation of the dissolved gas molecules, leading to volume expansion of the extrudate and the formation of a porous structure. Alternatively, the reprocessed PU can be extruded into a die. Controlled pressure reduction can also be applied to the reprocessed PU after extrusion of the compound. This pressure reduction induces nucleation of the dissolved gas molecules, resulting in volume expansion of the extrudate. Several definitions are provided to aid in understanding this technique.

[0048] A “block” refers to a part of a macromolecule that contains many structural units, and the macromolecule has at least one compositional or structural feature that is not present in adjacent parts.

[0049] "Blowing agent" refers to a substance capable of creating a porous structure in various materials undergoing hardening or phase change (such as polymers or plastics) through a foaming process. Blowing agents include chemical blowing agents, physical blowing agents, or combinations thereof that degrade into gas molecules or other byproducts. Exemplary blowing agents include chemical blowing agents such as azodicarbonamide (ADC) or water.

[0050] "Branch" refers to the side branches of oligomerization or polymerization of macromolecular chains.

[0051] A "branched point" refers to a point on a chain that is connected to a branch.

[0052] A "branched unit" refers to a structural unit that contains branching points.

[0053] A "catalyst" is a substance that can increase the rate of a reaction without changing the total Gibbs energy change of the reaction. Suitably, the catalyst can be a coordinating entity comprising a central atom and one or more ligands attached to the central atom. Suitably, the central atom is a metal. A "ligand" is an atom or group attached to the central atom.

[0054] "Chain" refers to all or part of a macromolecule, oligomer molecule, or block, comprising a straight or branched sequence of structural units between two boundary structural units, each of which may be an end group, branching point, or other specified macromolecular feature.

[0055] "Compounding" refers to the blending or mixing of substances, such as any polyurethane composition described herein, within a compounding apparatus. Suitably, the substances are compounded for an effective bond exchange time at an effective bond exchange temperature.

[0056] "Compounding equipment" refers to equipment used for blending or mixing substances (such as any polyurethane compositions described herein). In some embodiments, the compounding equipment is an extruder, such as a single-screw or twin-screw extruder, a mixer, or a kneader. Suitably, the twin-screw extruder can be a co-rotating or counter-rotating twin-screw extruder. The compounding equipment can be operated intermittently or continuously. Suitably, a continuously operating compounding equipment may have an inlet (e.g., a feed hopper or other suitable feeding mechanism) for introducing the substance into the compounding equipment, an outlet for extruding the compound, and a compounding zone between the inlet and outlet for mixing or blending the substance. Suitably, the compounding zone is configured such that the substance can be compounded at an effective bond exchange time. The compounding equipment may also include a heating element so that the substance can be compounded at an effective bond exchange temperature.

[0057] "Structural unit" refers to an atom or group of atoms (and side-attached atoms or groups, if any) that constitutes part of the basic structure of a macromolecule, oligomer, block, or chain.

[0058] A copolymer is a polymer derived from more than one monomer (actual, implicit, or hypothetical).

[0059] A "covalent network" or "covalent polymer network" is a network in which all permanent paths running through the structure are formed by covalent bonds.

[0060] "Dynamic networks," "dynamic polymer networks," or "covalent adaptive networks" refer to covalent networks capable of bond exchange reactions at temperatures above the effective bond exchange temperature. Dynamic networks can exhibit viscoelastic liquid properties above the freeze-thaw transition temperature.

[0061] "Foam" refers to a multiphase material containing gas dispersed in a polymer. Foam can be formed by trapping air pockets in a solid or liquid. Foam can be prepared by physical or chemical foaming. In some embodiments, the foam may be a closed-cell foam, in which the gas forms discrete, completely enclosed air pockets. In other embodiments, the foam may be an open-cell foam, in which the air pockets are interconnected. Suitably, the polymer is polyurethane ("polyurethane foam").

[0062] A "homogeneous polymer" is a polymer derived from a single monomer (actual, implicit, or hypothetical). Polymers can be made by the interaction of complementary monomers. These monomers can be readily envisioned reacting to produce an "implicit monomer" or "hypothetical monomer," the homopolymer of which yields an actual product that can be considered a homopolymer.

[0063] "Incompatible polymer blends" refer to polymer blends that exhibit incompatibility. "Incompatibility" means that the mixture cannot form a single phase.

[0064] "Inorganic polymers" refer to polymers or polymer networks whose backbone structure does not contain carbon atoms. Examples include, but are not limited to, polyphosphazenes, polysilicates, polysiloxanes, polysilanes, polysilazanes, polygermanes, and polysulfides.

[0065] "Isocyanate structural unit" refers to a structural unit containing at least one isocyanate group (i.e., -NCO). Suitably, the isocyanate structural unit may contain more than one isocyanate group, such as two, three, or four isocyanate groups. In some embodiments, the isocyanate structural unit is an aromatic isocyanate structural unit. As used herein, "aromatic isocyanate structural unit" refers to an isocyanate structural unit having an isocyanate group side-attached to an aryl group (e.g., phenyl or other aromatic ring).

[0066] "Lewis acid" refers to a molecular entity (and corresponding chemical substance) that acts as an electron pair acceptor and is therefore capable of reacting with a Lewis base to form a Lewis adduct by sharing the electron pair provided by the Lewis base.

[0067] A "straight chain" refers to a chain that has no branching points between boundary units.

[0068] "Macromolecules" or "polymer molecules" refer to molecules with a high relative molecular mass whose structure essentially consists of multiple repetitions of units actually or conceptually derived from molecules with a lower relative molecular mass.

[0069] "Mechanically processed" means that a material is mechanically altered, for example by mechanical grinding, cutting, shredding, or applying some other form of mechanical force. Suitablely, the material (e.g., the polyurethane composition described herein) can be mechanically processed into fragments, particles, granules, or granules.

[0070] "Monomer" refers to a substance composed of monomer molecules.

[0071] "Monomer" refers to a molecule that can polymerize to contribute structural units to the basic structure of a macromolecule.

[0072] "Monomer unit" refers to the largest structural unit that a single monomer molecule contributes to the molecular structure of a macromolecule or oligomer.

[0073] A “network” refers to a highly branched macromolecule in which virtually every structural unit is connected to each other and to the macroscopic phase boundary through numerous permanent pathways that run through the macromolecule; the number of such pathways increases with the average number of intermediate bonds; these pathways must extend along with the macromolecule on average.

[0074] "Network polymer" refers to a polymer composed of one or more networks.

[0075] "Oligomer molecules" refer to molecules with medium relative molecular mass whose structure essentially consists of a small number of units that are actually or conceptually derived from molecules with lower relative molecular mass.

[0076] "Organic polymers" refer to polymers or polymer networks whose backbone structure contains carbon atoms. Examples include, but are not limited to, polyethers, polyesters, polycarbonates, polyacrylates, polyolefins, and polybutadiene.

[0077] "Polymer" refers to substances composed of macromolecules.

[0078] A "polymer composition" refers to a composition comprising two or more different homopolymers. The homopolymers may have reactive chemical moieties capable of 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 constitute 20-80 wt% of the polymer composition. For the two different homopolymers, the weight ratio of the first homopolymer to the second homopolymer may be 20:80-80:20, 25:75-75:25, 30:70-70:30, 35:65-65:35, 40:60-60:40, 45:55-55:45, or approximately 50:50. The polymer composition may be a compatible or incompatible polymer blend.

[0079] "Polymerization" refers to the process of converting monomers or mixtures of monomers into polymers.

[0080] "Prepolymer molecules" refer to macromolecules or oligomers that can enter into further polymerization through reactive groups, thereby contributing more than one structural unit to at least one chain of the final macromolecule.

[0081] "Polyurethane composition" refers to a dynamic network formed by urethane bonds capable of undergoing urethane bond exchange reactions. The polyurethane composition comprises a network of urethane-containing polymers and a polyurethane exchange catalyst permeated within the network polymer. The network polymer may be formed from isocyanate structural units and second structural units having hydroxyl groups capable of reacting with the isocyanate groups of the isocyanate structural units. The molar percentage of the polyurethane exchange catalyst relative to the total isocyanate functional groups may be less than or equal to 5 mol%. Suitably, the molar percentage may be less than or equal to 4 mol%, 3 mol%, 2 mol%, 1 mol%, or less than 1 mol%. The second structural unit may be a prepolymer molecule or a branched unit. Suitably, the second structural unit may simultaneously serve as a prepolymer molecule and a branched unit. The prepolymer molecule is an organic polymer molecule or an inorganic polymer molecule having one or more hydroxyl groups capable of reacting with isocyanate groups, such as polyethers, polyesters, polycarbonates, polyacrylates, polyolefins, polybutadiene, polysulfides, or polysiloxanes. When the prepolymer molecule also functions as a branching unit, the prepolymer molecule has three or more hydroxyl groups capable of reacting with isocyanate groups, and the number of hydroxyl groups is typically proportional to the number of structural units in the prepolymer molecule. The network polymer may also be formed from urethane-containing monomers containing other polymerizable groups (including but not limited to acrylates, methacrylates, or other polymerizable olefins).

[0082] "Bond exchange catalyst" refers to a catalyst that increases the rate of bond exchange reactions (e.g., polyurethane bond exchange reactions). Suitable metals for the catalyst include Sn, Bi, Fe, Zr, Ti, Hf, Al, Zn, Cu, Ni, Co, Mn, V, Sc, Y, Ce, or Mo. Suitable ligands for the catalyst include, but are not limited to, branched or unbranched, substituted or unsubstituted carboxyl, alkyl, alkoxy, 1,3-dione, 1,2-dione, sulfonate, sulfonamide, amine, diamine, carbonate, phosphate, nitrate, halogen, catechol, isohydroxamic acid, hydroxide, or any combination thereof. The ligand can be branched or unbranched, substituted or unsubstituted. Exemplary ligands include acetylacetone (acac), isopropoxy (OiPr), neodecanoate (neo), laurate, butyl, ethylhexanoate, 2,2,6,6-tetramethyl-3,5-heptadecanedione (tmhd), trifluoromethanesulfonate, trifluoromethanesulfonamide, cyclopentadiene, pyridine salicylaldehyde iminediamine, phosphine, or any combination thereof. 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.

[0083] "Thermosetting polymers" or "thermosetting plastics" are polymers that are irreversibly cured by curing from a soft solid of a viscous liquid prepolymer or resin.

[0084] "Vitreous body" refers to a network polymer whose topology can be altered through thermally activated bond exchange reactions via an association mechanism, preventing a temporary decrease in the total number of covalent bonds within the network polymer during the bond exchange reaction. At high temperatures, the bond exchange reaction occurs at an efficient and rapid rate, and the network polymer exhibits the properties of a viscoelastic liquid. At low temperatures, the bond exchange reaction slows down, and the network polymer behaves like a thermosetting polymer.

[0085] The above successfully illustrates the foam-to-foam recycling of thermosetting PU. Adding a bond-exchange catalyst facilitates PU reprocessing by promoting bond exchange, enabling cell formation during in-situ gas generation in a twin-screw extruder. The optimal temperature for the foam-to-foam process allows for simultaneous PU reprocessing and decomposition of the chemical foaming agent. Extrusion foaming can be continuous while maintaining bulk density, cell number density, and average cell diameter, with compressive properties consistent with porous materials. Furthermore, cavity-based foam injection molding produced low-density foam exhibiting compressive properties comparable to the original PU foam, demonstrating that the foam-to-foam process can restore mechanical properties. PU foams from various raw materials (e.g., soft and soft / rigid blends) can undergo this process in the presence of a bond-exchange catalyst.

[0086] Methods for reprocessing polyurethane are disclosed in U.S. Patents US17 / 050,138, US17 / 605,831 and PCT / US2023 / 063276, which are incorporated herein by reference for all purposes.

[0087] Unless otherwise stated or indicated by the context, the terms “a,” “an,” and “the” mean “one or more.” For example, “(a) molecule” should be interpreted as “one or more molecules.”

[0088] As used herein, “about,” “approximately,” “substantially,” and “significantly” will be understood by those skilled in the art and will vary depending on the context in which they are used. Where there is a usage that is unclear to those skilled in the art based on the context of its use, “about,” “approximately,” will mean ± ≤10% of a particular term, while “substantially” and “significantly” will mean ± >10% of a particular term.

[0089] As used herein, the terms “comprising” and “containing” have the same meaning as “including”. The terms “comprising” and “including” should be interpreted as “open-ended” transitional terms, allowing the inclusion of additional components beyond those listed in the claims. The terms “comprising” and “composed of” should be interpreted as “closed-ended” transitional terms, disallowing the inclusion of additional components beyond those listed in the claims. The term “substantially composed of” should be interpreted as partially closed, allowing only the inclusion of additional components that do not fundamentally alter the nature of the claimed subject matter.

[0090] Unless otherwise stated herein or the context explicitly contradicts it, all methods described herein may be performed in any suitable order. Any and all instances or exemplary language (such as “for example”) provided herein are intended only to better illustrate the invention and do not constitute a limitation on the scope of the invention, unless otherwise required. No language in this specification should be construed as indicating that any unclaimed element is necessary for practicing the invention.

[0091] All references cited in this article, including publications, patent applications and patents, are incorporated herein by reference to the extent that each reference is individually and specifically indicated as incorporated by reference and listed in full.

[0092] This document describes preferred aspects of the invention, including the best modes of carrying out the invention known to the inventors. Variations of these preferred aspects may become apparent to those skilled in the art upon reading the foregoing description. The inventors expect those skilled in the art to adopt such variations as appropriate, and the inventors intend to practice the invention in ways different from those specifically described herein. Therefore, the invention includes all modifications and equivalents to the subject matter described in the claims, provided that applicable law permits. Furthermore, unless otherwise stated herein or the context clearly contradicts it, any combination of all possible variations of the foregoing elements is included in the invention.

[0093] Example

[0094] A foam-to-foam recycling process for thermosetting polyurethane has been developed by adding a chemical blowing agent (to induce foaming after continuous extrusion) to PU through melt reprocessing. The initial PU foam is prepared by polymerizing a polyester-based polyol with an equimolar amount of diphenylmethane diisocyanate (MDI) in the presence of isopentane as a physical blowing agent and a small amount of dibutyltin dilaurate (0.63 mol% relative to -NCO) as a catalyst. Figure 1 , 9 (a and 9b). Zr(acac)4 was used as an added catalyst to improve the carbamate exchange rate and was less toxic than conventional tin-based catalysts. Figure 1 To facilitate the introduction of Zr(acac)4, a kitchen mixer was used to mix materials with an average pore diameter of 253.4 ± 119.7 μm. Figure 9 c) The synthesized foam was ground into smaller particles with an average particle size of 235.3 ± 150.9 μm. Figure 9d). The ground particles were soaked in a Zr(acac)4 CH2Cl2 solution for 4 hours, and then the solvent was evaporated in a vacuum oven at 90°C for 24 hours to obtain ground PU particles containing 3 wt% Zr(acac)4. Azodicarbonamide (ADC), a chemical foaming agent, was incorporated into the catalyst-supported polymer using a vortex mixer to obtain a mixture containing 8.5 wt% ADC.

[0095] The reprocessing and refoaming of PU is carried out by continuously feeding PU granules into a twin-screw extruder maintained at a high temperature, which provides continuous PU foam extrusions. Figure 2 and Figure 10 As the PU CAN is heated within a micromixer, the reversible dissociation of the network allows the PU to be reprocessed via Zr-catalyzed crosslinking exchange. Simultaneously, the ADC thermally degrades into gaseous molecules (N2, CO, and CO2), which dissolve within the polymer.

[29] When reprocessed polyurethane is extruded from the die, the resulting pressure drop induces nucleation of dissolved gas molecules, leading to volume expansion of the extrudate and the formation of a porous structure. Figure 11 ).

[30] Having established this foam-to-foam reprocessing method, we set out to characterize the thermomechanical properties of the foam and explore how various experimental parameters affect the foam quality.

[0096] Spectroscopic analysis and thermal characterization of the extrudates report the chemical integrity of the PU after foam-to-foam reprocessing procedures performed at 160, 180, 200, or 220 °C. Fourier transform infrared spectroscopy (FT-IR) confirms the presence of the urethane C=O vibrational absorption band (1700–1730 cm⁻¹) after processing at each temperature. - ¹) remained unchanged, and no isocyanate vibrational absorption band (2285 cm⁻¹) was observed. - ¹)( Figure 3 a and 12a).

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

[32] The presence of residual ADC is due to the absorption bands of the two NH vibrations of the ADC (3320-3330 and 3160-3190 cm). - The presence of these absorption bands was further confirmed, as they were not observed in foam extruders at higher temperatures. Figure 3 a).

[0097] Solid-state cross-polarized magic-angle rotating (CP-MAS)¹³C NMR was used to characterize the residual ADC and its decomposition intermediates in the extrudate at each temperature. Figure 3 b and Figure 13 These experiments further confirmed the formation of the PU product and revealed when the ADC remained in the material after the re-foaming process. In the extrudate processed at 160°C, a resonance peak corresponding to the ADC at 160.5 ppm was observed between the urethane (154.0 ppm) and ester (173.0 ppm) resonance peaks of the polyurethane. The ADC resonance peak at 160.5 ppm was equally significant in the pre-extrusion mixture, indicating that the ADC experienced little or no degradation during reprocessing at 160°C. In the sample extruded at 180°C, an ADC resonance peak at 160.5 ppm was also observed, along with a novel resonance peak at 163.5 ppm, which we attributed to the hydrazine carbonylamide intermediate. This attribution is based on known ADC decomposition pathways. [33–35] The presence of this resonance peak indicates partial decomposition of the ADC at this temperature. In contrast, the extrudate processed at 200°C did not show any ADC-related resonance peaks, and the intermediate peaks were more prominent. Finally, when the temperature was increased to 220°C, neither the ADC nor its intermediates were observed, indicating that all ADCs had decomposed. FT-IR and NMR showed that ADC decomposition increased with increasing temperature, producing more gas, while the PU-related signal remained unchanged.

[0098] Characterization of the Tg and gel fraction of each extrudate showed that crosslinking density and network integrity were maintained at least up to 180°C, while temperatures above 200°C led to network degradation. Figure 3(c, 3d and Table 2). The Tg of the PU extruded at 160 and 180 °C were 39.0 and 37.6 °C, respectively, which is similar to the Tg of the mixture before extrusion (38.0 °C). Similarly, the gel fractions of the PU extruded at 160 and 180 °C were 73.5% and 74.3%, respectively, comparable to the gel fraction of the original PU foam (76.6%). The Tg and gel fraction retained after extrusion at the lower temperatures indicate that the crosslinked structure and polymer backbone are largely retained after the process. However, at 200 °C and 220 °C, the Tg decreased to 32.2 and 30.4 °C, respectively, indicating that the network underwent partial degradation during reprocessing at these higher temperatures. Furthermore, the gel fractions of the PU extruded at 200 and 220 °C decreased to 69.6% and 29.9%, respectively, indicating that the decrease in extruded Tg was caused by degradation of the polymer network at the higher temperatures. We attributed the decrease in Tg and loss of crosslink density at high temperatures to oxidation of the polymer backbone and harmful side reactions caused by the dissociation of urethane bonds at high temperatures. In summary, Tg and gel fraction measurements indicate that the polymer backbone and crosslinks are maintained in processes at lower temperatures of 160–180 °C, but some degradation of the crosslinked network was observed in processes at higher temperatures of 200–220 °C.

[0099] Porosity analysis by SEM indicated that the optimal reprocessing temperature for re-foaming was 180°C, although previous NMR studies showed that the ADC only partially decomposed at this temperature. Figure 4 The 180°C program provides the highest cell number density, the smallest cell diameter, and the lowest foam density. Figure 4 (a, 4b, and Table 3). However, under the same reprocessing conditions, the mixture of PU and foaming agent without the addition of Zr(acac)4 could not be reprocessed. Instead, the extruder reached its torque limit, indicating the need for a urethane exchange catalyst to achieve both reprocessing and refoaming. Figure 14 Below the optimal temperature, a rough surface without obvious pore structure was observed in the SEM cross-section of the extrudate at 160°C. Figure 4 a) indicates that the processing temperature was too low, preventing the ADC from decomposing, which is consistent with NMR studies. On the other hand, the significant reduction in cell number density ( Figure 4 b) and the increase in foam density (Table 3) indicate that the foam is significantly unstable at reprocessing temperatures above 180°C. At 200°C, the increase in cell diameter and the decrease in cell number density indicate that the foam is unstable through cell coalescence. When the process temperature is increased to 220°C, although the ADC completely decomposes, the foam experiences significant gas loss because the cells are smaller and the bulk density increases to 0.91 g / cm³ (Table 3). The cell instability at higher temperatures is attributed to the impairment of the melt strength of the polymer network. Figure 3 c and 3d).

[0100] By changing the screw speed in the foam-to-foam process, the number density and average cell diameter of the cells can be controlled. In extrusion foaming processes using chemical foaming agents, the generated gas molecules dissolve in the polymer under high pressure and high temperature inside the extruder. Once the polymer leaves the die, it experiences a significant pressure drop, causing the gas to expand within the polymer matrix and form cell nuclei. In high-temperature extrusion processes (i.e., Figure 4 At 200 and 220°C (in b), the extrudate matrix is ​​too soft to support the expanding cells, so saturated gas molecules may escape from the polymer, and the nucleated cells may coalesce.

[36] Based on this general understanding of extrusion foaming, we investigated the effect of changing the screw speed at 180°C on the foam-to-foam process. Figure 4 c and Figure 15 This demonstrates the adjustability of the foam structure. We were unable to study speeds exceeding 20 rpm because the residence time during extrusion at 20 rpm is too short to generate gas and effectively reprocess the PU particles. Figure 15 We hypothesized that the bulk density of the foam would decrease as the screw speed decreased from 15 rpm to 5 rpm because the longer residence time of the polymer in the extruder would generate more gas. However, the bulk density of the foam remained almost constant at approximately 0.4–0.5 g / cm³ (Table 3). This finding suggests that a significant amount of gas generated at lower speeds is lost to the atmosphere due to the slower cooling rate of the foam extruded from the die. [37–39] By increasing the rotational speed from 5 rpm to 15 rpm, the average cell diameter decreased from 32.9 ± 26.3 μm to 20.2 ± 9.9 μm. The larger cell diameter at the slower rotational speed also suggests that cell coalescence may have occurred before the foam extruded near the die cooled and the cell structure stabilized.

[40] Meanwhile, the number density of foam cells decreased from 4.54 ± 1.18 × 10⁻⁶ at 5 rpm. 6 The number of bubbles / cm³ increased to 1.37 ± 0.66 × 10⁻⁶ at 15 rpm. 7 One bubble per cm³ Figure 4 c). This result indicates that stabilizing the cells through cooling the polymer extruder (which is promoted at faster screw speeds) has a greater effect on cell number density than on gas generation, which is proportional to residence time. Furthermore, the higher throughput of the extruder at faster screw speeds leads to a higher rate of gas pressure drop in the polymer, which may result in more cell nucleation. [41–44]

[0101] To determine the consistency of the foam produced by this process, we extruded twice the mass of PU foam compared to earlier foam experiments, obtaining foam filaments with a length of approximately 65 cm. Figure 5 In previous experiments, we qualitatively observed that the cross-sections of the front and rear portions, which constitute approximately half of the recycled extrudate, differed from those of the middle portion. We attributed this inconsistency to poor residence time control and inconsistent pressure distribution at both ends of the extrudate. To test this hypothesis, we conducted an extended extrusion experiment, expecting to provide consistent performance for the main body of the extrudate. The extended continuous extrusion was performed by reprocessing 6g (instead of 3g) of PU. The extruded foam from the 6g extrusion was 65cm long, four times the length obtained from the 3g extrusion. Figure 5 a). Qualitatively, the extrudate exhibits a foam structure at several points between 10-50 cm, but the cell structure becomes largely unstable after 60 cm, again corresponding to the polymer portion at the end of the extrusion process. Figure 5 b). The consistency of the process was quantitatively assessed by measuring the average diameter, bulk density, and cell number density at several locations in the 6g extrudate and comparing them with those of the 3g extrudate. Figure 5 c and 5d). In 3g extrusion, due to the inconsistent residence time and insufficient pressure distribution inside the extruder, the foam extrudate exhibits inconsistent cell diameter and density. Figure 5 (c and 5d). However, in extended extrusion, except for the first and last sections of the extrudate, the average diameter, cell number density, and bulk density remained at approximately 20 μm, 1 × 10⁻⁶, and 1 × 10⁻⁶, respectively. 7 pores / cm³ and 0.4g cm³ ( Figure 5 (c and 5d). We suspect that the insufficiently formed pressure distribution in the initial 5 cm and the extremely long residence time of the extrudate exceeding 60 cm resulted in a higher bulk density and lower cell number density than the constant values ​​of the parameters in the extrudate bulk. Therefore, a continuous foam-to-foam process is likely to provide a consistent cell structure, consistent with how these materials may be reprocessed in the future.

[0102] By operating the extruder in circulation mode at 180°C, allowing the polymer melt to flow into the die cavity (with a return channel connected to the extruder barrel), a process similar to injection molding was demonstrated. Figure 6 Subsequently, opening the mold cavity lid caused the entire sample to undergo a rapid pressure drop, inducing cell nucleation and generating a foam structure with significant volume expansion. Figure 6 Therefore, we obtained PU foam that maintains the shape of the mold cavity, regardless of the cycle time of the reprocessed PU CAN. Figure 7 a). By increasing the cycle time, the bulk density of the foam was significantly reduced to 0.25 g / cm³, comparable to the original foam with a bulk density of 0.18 g / cm³. Figure 7 (b and 7c). Foam produced by a cycle time of 2.5–7.5 min has a higher packing density. Figure 7 c), this is due to the partial degradation of ADC and the limited amount of gas available for cell growth. Figure 16 After cycling for 10 minutes, most of the ADC converted to intermediates, and the resulting gaseous material was trapped in the mold cavity, causing considerable volume expansion and producing low-density foam. Figure 7 c and 16). Increasing the cycle time also resulted in a slight decrease in the number density of pores, indicating aggregation between adjacent nucleated pores (c and 16). Figure 7 d). However, further material cycling exceeding 10 minutes showed that the cell number density tended to plateau, indicating that catastrophic cell instability and foam structure collapse were prevented. This trend is attributed to the preservation of polymer chain integrity, as evidenced by the Tg remaining at approximately 45°C and the gel fraction remaining at approximately 75% after the foam injection molding process. Figure 17 and 18 By increasing the cycle time from 5 min to 10 min, the average pore diameter increased from 15 μm to 40 μm because more gas was available to participate in pore growth. Figure 7 d). Therefore, foam injection molding processes with sufficient cycle time produce low-density foams with a cell number density of approximately 1 × 10⁻⁶. 7 The number of cells per cm³ has an average diameter of approximately 40 μm. This finding suggests that foam injection molding with increased pressure drop can effectively generate cell nucleation and growth while maintaining a closed-cell morphology, which is crucial for applications such as thermal insulation.

[45]

[0103] The properties of reprocessed PU foam (e.g., bulk density and pore size) can be controlled by the dynamic properties of the PU network, which vary with the concentration of additional Zr catalyst. Increasing the Zr(acac)₄ concentration in the PU network from 3 wt% to 5 wt% reduced the stress relaxation time at 160 °C from 74.5 s to 50.4 s, indicating that the bond exchange rate increases with the presence of more catalyst. Figure 19 As the concentration of Zr catalyst increased from 3 wt% to 5 wt%, the foam thickness increased from 3.51 ± 0.26 mm to 4.23 ± 0.16 mm, and the bulk density decreased from 0.26 ± 0.04 g / cm³ to 0.18 ± 0.01 g / cm³, which is very similar to the density of the original foam. Figure 20 Besides packing density, the bond exchange rate in the PU network also affects the average pore diameter and distribution. As the Zr catalyst concentration increased from 3 wt% to 5 wt%, the average pore diameter increased from 28.3 ± 20.9 μm to 47.6 ± 38.3 μm, which was confirmed by the shift in pore size distribution. Figure 21 and 22Regardless of the Zr catalyst concentration, the Tg of the reprocessed foam decreased by less than 1°C compared to the Tg of the original PU foam. Figure 23 A constant Tg indicates that the PU network maintained its integrity after reprocessing, suggesting that the change in size distribution was not due to PU network degradation. Therefore, the catalyst concentration, along with other parameters that contribute to faster carbamate exchange, can be used to control the density and pore size of the reprocessed foam.

[0104] Having established a foam-to-foam recycling scheme, we characterized the mechanical properties of various reprocessed PU foams. These measurements are crucial because the applications of PU foam largely depend on its mechanical properties, which can be adjusted according to the foam density. [46,47] Compression tests were used to characterize the original PU foam and PU foam produced through two foam-to-foam processes. Each foam exhibited three distinct regions under strain: an elastic deformation zone, a plateau zone, and a densification zone. Figure 8 ). [48,49] Accordingly, the compressive modulus and yield stress of each type of foam were calculated from the curves.

[50] The results showed that the elastic modulus (E=9.61MPa) and yield stress (σY=0.99MPa) of the foam obtained from 3g extrusion were comparable to those of the foam produced by extended extrusion (E=8.55MPa) and yield stress (σY=1.00MPa), indicating the consistency of extrusion foaming (Table 1). However, the elastic modulus and yield stress of the foam extrudate were still much higher than those of the original foam foamed with isopentane or water because the density of the extrudate was twice that of the original foam (0.18g / cm³).

[51] Lower-density foam (0.25 g / cm³) obtained by foam injection molding with 3 wt% Zr(acac)₄ and a cycle time of 10 min showed elastic modulus and yield stress close to that of the original foam, compared to the extrusion process. However, due to the higher bulk density of the reprocessed PU foam, its compressive modulus and yield stress were still higher than those of the original foam. This result implies that achieving low-density PU foam is crucial for fully restoring the properties of the original foam after reprocessing. We were able to obtain reprocessed PU foam with reduced bulk density by introducing more urethane exchange catalyst through injection molding, and the resulting low-density foam exhibited elastic modulus and yield stress comparable to or even lower than that of the original foam. Figure 8 (See Table 1). In summary, by optimizing processing parameters, foam-to-foam processes can produce reclaimed PU foam with adjustable performance comparable to waste PU foam.

[0105] Table 1: Compression modulus and yield stress of foam under different processes and foam densities

[0106]

[0107] Table 2: Tg and gel fraction of raw foam and PU foam extrusions at different extrusion temperatures (screw speed = 10 rpm)

[0108]

[0109] Table 3: Bulk density of PU foam and PU film after reprocessing at different extrusion temperatures and speeds

[0110]

[0111] Materials and General Methods

[0112] Materials: All reagents were purchased from Sigma-Aldrich or Fisher Scientific. The polyols used for PU foam synthesis were dried at 90°C and 20 mTorr vacuum for at least 1 hour before use. Unless otherwise specified, all other reagents were used directly without further purification.

[0113] Instrumentation and Characterization

[0114] Infrared Spectroscopy: Infrared spectra were recorded on a Thermo Nicolet iS20 equipped with a ZnSe ATR accessory. The spectra are uncorrected.

[0115] CP-MAS ¹³C solid-state NMR: Solid-state NMR spectra were recorded at ambient temperature using a standard Bruker 4mm HX probe on a 400MHz Bruker Avance III.

[0116] Differential Scanning Calorimetry (DSC): DSC was performed on a TA Instruments DSC250 differential scanning calorimeter. Samples (5–10 mg) were heated to 120 °C at a rate of 30 °C / min to eliminate thermal history, then cooled to –80 °C at 30 °C / min and held at –80 °C for 5 min. The samples were then heated again to 120 °C at a rate of 10 °C / min. All data shown are from the second heating process. The glass transition temperature (Tg) was calculated from the maximum value of the heat flux versus temperature derivative.

[0117] Gel fraction: The gel fraction was obtained as follows: Approximately 50 mg of PU was immersed in 2 mL of DCM solution for two days in a sealed 4 mL vial. After two days, the solution was washed with additional DCM and filtered. The insoluble solids were recovered and dried under reduced pressure at 90 °C, then weighed. The gel fraction was reported as the average of three measurements and their corresponding standard deviations.

[0118] Bulk density: The bulk density of PU foam (ρ)泡沫 The density was calculated by dividing the mass by the volume of the PU foam. The PU foam was cut into rectangular pieces approximately 3mm × 5mm × 10mm in size using a single-edged razor, and the mass of each foam sheet was measured. The volume of each PU foam sheet was then calculated using calipers. The density was then obtained by dividing the mass by the volume, with each sample measured three times. The bulk density is reported as the average of the three replicates and its associated standard deviation. The bulk density (ρ) of reprocessed PU film without chemical foaming agents is... 薄膜 It is measured in the same manner as that used for cell number density analysis.

[0119] Porosity: The porosity (P) of PU foam is defined as... , where ρ 薄膜 It is the bulk density of the reprocessed PU film, ρ 泡沫 It is the bulk density of each type of PU foam. [52,53] Based on the bulk density measurement procedure and the foam-to-film process, the ρ used in this analysis 薄膜 The value is 1.20±0.01 g / cm³.

[0120] Scanning electron microscopy: Polyurethane foam was fixed on a flat aluminum sample stage, 5 nm of gold-palladium was deposited using a sputtering coating machine, and the image was captured using a Hitachi S4800-II SEM.

[0121] Image processing: The average cell diameter was determined as follows: More than three images of each PU foam sample were analyzed using the ImageJ program (NIH). Post-processing of the SEM images was performed by converting the SEM images to binary images using an appropriate color threshold. The boundaries of the porous structure were then identified to calculate the area of ​​each cell in the cross-sectional SEM image of the PU foam. The number-average diameter (d) was calculated by taking the average diameter of each cell, assuming that the cross-section of each cell has a circular geometry. Cell number density (N) c The number of bubbles (n) per unit image area (A) is calculated using the following formula. [54,55] d and N c The value report is the average of at least three SEM images (at least 100 cells), with the relevant standard deviation.

[0122]

[0123] Stress relaxation analysis: Stress relaxation analysis (SRA) was performed on a TA Instruments RSA-G2 analyzer (NewCastle, DE) using rectangular thin films (approximately 1.5 mm (thickness) × 10 mm (width) × 15 mm (length), with a gauge length of 5 mm). SRA experiments were strain-controlled at 160 °C. The samples were equilibrated at this temperature for approximately 10 min, and then the axial force was adjusted to 0 N. An instantaneous strain of 5% was then applied to each sample. 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.

[0124] Synthesis and Engineering Procedures

[0125] Synthesis of crosslinked polyester polyurethane foam: This procedure is adapted from a previous report.

[56] Add poly[trimethylolpropane / di(propylene glycol)-adipic acid / phthalic anhydride]polyol (average Mn approx. 500 g / mol, hydroxyl functionality = 2.5, 15 g, 75 mmol -OH), foaming agent isopentane (450 mg), dibutyltin dilaurate (300 mg, 0.63 mol relative to -NCO), and tri(nonylphenyl) phosphite (750 mg, 3 wt% of polyurethane foam) to a plastic cup. Add ground solid 4,4'-methylene bis(phenyl isocyanate) (MDI) (9.4 g, 37.5 mmol) and mix vigorously. Let the mixture stand for one hour to gel and expand. Transfer the resulting block polymer to an aluminum tray (104 mm diameter × 15 mm height) and place it in a vacuum oven at 90 °C and 20 mTorr for immediate expansion and curing for 48 hours. Cur the foam at 150 °C for 1 hour to ensure complete crosslinking.

[0126] Synthesis of water-blown polyester polyurethane foam: Poly[trimethylolpropane / di(propylene glycol)-adipic acid / phthalic anhydride] polyol (average Mn approx. 500 g / mol, hydroxyl functionality = 2.5, 15 g, 75 mmol-OH), water (0.15 g, 8.3 mmol), dibutyltin dilaurate (300 mg, 0.63 mol relative to -NCO) and tri(nonylphenyl) phosphite (750 mg, 3 wt% of polyurethane foam) were added to a plastic cup. Ground solid 4,4'-methylene bis(phenyl isocyanate) (MDI) (9.4 g, 41.7 mmol) was added and vigorously mixed. The mixture was allowed to stand for one hour to gel and expand. The resulting block polymer was transferred to an aluminum tray (104 mm diameter × 15 mm height) and placed in a vacuum oven at 90 °C and 20 mTorr for immediate expansion and curing for 48 hours. After curing the foam at 150°C for 1 hour, ensure complete cross-linking.

[0127] The catalyst was introduced into the PU foam after synthesis: the raw PU foam was cut into rectangular blocks of approximately 10mm × 10mm × 10mm, and then ground into smaller particles using a kitchen mixer. 20g of the ground foam particles were soaked in 60mL of a dichloromethane solution of zirconium acetylacetonate [Zr(acac)4] at a specific concentration (10.3, 14.0, or 17.7 mg / mL) for 4 hours. The swollen particles were then placed in a vacuum oven at 90°C and 20mTorr for 24 hours to remove the dichloromethane, yielding ground PU particles containing a specific concentration (3, 4, or 5 wt%) of Zr(acac)4.

[0128] Foam-to-foam process using a twin-screw extruder: Using a vortex mixer, 3 grams of ground PU foam granules containing 3 wt% Zr(acac)4 were mixed with 0.28 g of azodicarbonamide (ADC 8.5 wt%) for 1 min. The ground PU foam was continuously fed into a twin-screw extruder (HAAKE MiniLab3, Thermo Scientific) at specific operating temperatures (160, 180, 200, 220 °C) and screw speeds (5–20 rpm). The estimated residence time at 10 rpm was 5 min. The material was then extruded through a 1 mm thick die and air-cooled. For an extended foam-to-foam process, 6 grams of ground PU foam granules were extruded using the same concentration of azodicarbonamide at 180 °C and a screw speed of 15 rpm.

[0129] Foam-to-foam process using injection molding connected to a twin-screw extruder: Using a vortex mixer, 4.8 g of ground PU foam particles containing a specific concentration (3, 4, or 5 wt%) of Zr(acac)4 are mixed with 0.45 g of ADC (8.5 wt%) for 1 min. The ground PU foam is continuously fed into the twin-screw extruder in circulation mode, filling the return channel connected to the twin-screw extruder. For experiments with different residence times, feeding is completed within 5 min; for experiments with different catalyst concentrations, feeding is completed within 8 min. After feeding, the material is continuously circulated at a screw speed of 10 rpm for a specific time (2.5–20 min). Afterward, the extruder cover is opened, and the resulting foam is collected.

[0130] Foam-to-film process for measuring bulk density: 4.8 g of ground PU foam granules containing a specific concentration (3, 4, or 5 wt%) of Zr(acac)4 are continuously fed into a twin-screw extruder in circulation mode, where the material fills the return channel connected to the twin-screw extruder. Feeding is carried out for 5 min at 180°C and a screw speed of 30 rpm. After feeding, the material is continuously circulated at a screw speed of 10 rpm for 5 min. Afterward, the extruder cover is opened, and the reprocessed PU film is collected.

[0131]

Claims

1. A method for reprocessing thermosetting polyurethane foam, comprising: Polyurethane foam is mechanically processed into granules; The particles are loaded with a bond-exchange catalyst to prepare the loaded particles; The loaded particles are mixed with a foaming agent; The loaded particles are mixed with the foaming agent to prepare a compound; and The mixture is extruded.

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

3. The method according to any one of claims 1-2, further comprising reducing the pressure of the compound after extruding the compound.

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

5. The method according to any one of claims 1-4, wherein the foaming agent is thermally degraded into gas molecules dissolved in the mixture during the compounding process.

6. The method according to any one of claims 1-5, wherein the foaming agent is thermally degraded into gas molecules selected from N2, CO, CO2, or any combination thereof.

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

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

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

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

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

12. The method according to any one of claims 1-11, wherein the mixture of the loaded particles and the foaming agent is compounded to allow for effective bond exchange time, so that the foaming agent degrades into gas molecules dissolved in the mixture during the compounding process.

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

14. The method according to any one of claims 1-13, wherein the mixture of the loaded particles and the foaming agent is compounded using a twin-screw extruder at a screw speed of 5-15 rpm.

15. The method according to any one of claims 1-14, further comprising blending the polyurethane foam with one or more additional polymers, and compounding the polyurethane foam, bond exchange catalyst, blowing agent and one or more 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, the feeder being configured to feed the mixture of the polyurethane foam particles, the bond exchange catalyst, and the blowing agent into the twin-screw extruder, the twin-screw extruder being configured to compound the mixture of the polyurethane foam particles, the bond exchange catalyst, and the blowing agent and extrude the compound through a die or into a mold.

17. The foam-to-foam recycling system of claim 16, comprising the mold configured to reduce the pressure of the compound after it has been extruded from the twin-screw extruder.

18. The foam-to-foam recycling system according to claim 16 or 17, 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 an effective bond exchange temperature of 160-220°C.

19. The foam-to-foam recycling system according to 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 of 2-20 min.

20. The foam-to-foam recycling system according to 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 speed of 5-15 rpm.

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

22. The foam-to-foam recycling system according to any one of claims 16-21, wherein the foaming agent is thermally degraded into gas molecules dissolved in the mixture during the compounding process.

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

24. The foam-to-foam recycling system according to any one of claims 16-23, wherein the foaming agent is azodicarbonamide or water.

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

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

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