Description:BACKGROUND
FIELD OF THE DISCLOSURE
[0001] Various embodiments of the disclosure relate generally to recycling polyurethane. More specifically, various embodiments of the disclosure relate to methods of recycling polyurethane to form polyurethane recycled products.
DESCRIPTION OF THE RELATED ART
[0002] Polyurethanes (PU) are segmented copolymers synthesized through the reaction between a polyol and an isocyanate. These copolymers consist of hard segments, typically formed by the reaction of isocyanates with chain extenders like diols or diamines, and soft segments generally derived from long-chain polyols. The hard segments provide rigidity and strength, while the soft segments contribute flexibility and elasticity. The proportion and chemical structure of these segments influence the nature of the resulting polyurethane, determining whether it exhibits thermoplastic or thermosetting characteristics. Typically, a higher content of hard segments favors the formation of thermoset PUs, whereas an increased proportion of soft segments leads to thermoplastic PUs. By adjusting the proportion of the hard and soft segments, polyurethanes with tailored properties, such as elasticity, hardness, tensile strength, and durability can be obtained. Due to their versatile mechanical properties, polyurethanes are extensively used across various industries. However, the extensive use of polyurethanes has led to significant waste generation, primarily from end-of-life products like foams, coatings, adhesives, and elastomers. Presently, the majority of PU waste is disposed of in landfills or incinerated, contributing to environmental pollution and resource depletion.
[0003] Physical recycling methods generally require the application of heat and pressure to produce a usable recycled material. Thermoplastic polyurethanes (TPUs) can be recycled by melting and extruding the material in the form of flakes or pellets, often combined with virgin polymers and/or compatibilizers to improve performance. In contrast, thermosetting polyurethanes (PUs) are more challenging to recycle due to their cross-linked structure. Typically, thermoset PU waste is reused in its powdered, flake, or particulate form without chemical treatment or is incorporated into a continuous matrix of another material. However, the performance of physically recycled PU products is often limited, making them suitable primarily for non-demanding applications and restricting their commercial value.
[0004] Chemical recycling of polyurethanes (PU) involves depolymerization to recover monomers or oligomers that resemble the original precursors. Various chemical recycling methods are known, such as hydrolysis, alcoholysis, aminolysis, ammonolysis, acidolysis, and glycolysis. These methods primarily target the cleavage or exchange of urethane bonds and are influenced by factors such as the choice of reactive organic compounds, temperature, pH, pressure, and catalysts. Following purification and distillation, high-purity monomers like amines and polyols can be obtained, which can be reused not only in polyurethane synthesis but also in the production of other polymers. Although chemical recycling can produce higher-value products and facilitate closed-loop recycling, it faces significant challenges. The diverse chemical structures, diversified forms (i.e., foams, bulk materials, and elastomers), molecular weights, degrees of crystallinity, crosslinking densities, and hard-to-soft segment ratios inherent in polyurethanes complicate the recycling process, impacting the quality and properties of the recycled materials. As a result, most chemical recycling methods for polyurethanes are still at the research or pilot scale, with limited industrial and commercial adoption.
[0005] Hydrothermal liquefaction (HTL) is a known thermochemical technique for recycling of polymers. HTL operates under elevated temperatures, typically around 350°C or higher to depolymerize plastics into monomers or smaller hydrocarbon fractions, often accompanied by the generation of usable energy. In contrast to chemical recycling, thermochemical recycling, such as HTL requires extreme operational conditions and specialized high-pressure reactors, including Swagelok bomb-type reactors or corrosion-resistant vessels such as those made from Hastelloy.Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.
SUMMARY
[0006] According to embodiments of the present disclosure, a method of recycling a polyurethane is provided. The method comprises a step (i) of subjecting a reaction medium comprising the polyurethane to hydrothermal treatment at a temperature between 230 °C and 300 °C, a pressure between 20 megaPascal (MPa) and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture. The method further comprises a step (ii) of cooling and separating the reaction mixture into a solid component and a liquid component. The method further comprises a step (iii) of drying the solid component to obtain a polyurethane dissociation product (PDP).
[0007] In another embodiment, the method further comprises reacting the polyurethane dissociation product with an epoxy monomer, a maleated polyolefin, or a polyurethane prepolymer to form a polyurethane recycled product (PRP).
[0008] In yet another embodiment, a method of recycling a polyurethane to produce an epoxy polymer is provided. The method comprises a step (i) of subjecting a reaction medium comprising the polyurethane to hydrothermal treatment at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture. The method further comprises a step (ii) of cooling and separating the reaction mixture into a solid component and a liquid component. The method further comprises a step (iii) of drying the solid component to obtain a polyurethane dissociation product. The method further comprises a step (iv) of reacting the polyurethane dissociation product with an epoxy monomer in presence of a catalyst at a temperature in a range of 100°C to 180°C to produce the epoxy polymer.
[0009] In yet another embodiment, a method of recycling a polyurethane to produce a crosslinked polyolefin is provided. The method comprises a step (i) of subjecting a reaction medium comprising the polyurethane to hydrothermal treatment at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture. The method further comprises a step (ii) of cooling and separating the reaction mixture into a solid component and a liquid component. The method further comprises a step (iii) of drying the solid component to obtain a polyurethane dissociation product. The method further comprises a step (iv) of extruding the polyurethane dissociation product with a maleated polyolefin at a temperature in a range of 160 °C to 200 °C to produce the crosslinked polyolefin.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a flow chart that illustrates a method of recycling a polyurethane, in accordance with an exemplary embodiment of the disclosure; and
[0011] FIG. 2 is a reaction scheme in accordance with an exemplary embodiment of the disclosure;
[0012] FIG. 3 is a flow chart that illustrates a method of preparing an epoxy polymer, in accordance with an exemplary embodiment of the disclosure;
[0013] FIG. 4 is a flow chart that illustrates a method of preparing a crosslinked polyolefin, in accordance with an exemplary embodiment of the disclosure;
[0014] FIG. 5 is a reaction scheme in accordance with an exemplary embodiment of the disclosure;
[0015] FIG. 6 is the FTIR spectra of polyurethane and polyurethane disscociated product in accordance with embodiments of the disclosure;
[0016] FIG. 7A is the thermogravimetric analysis of polyurethane in accordance with embodiments of the disclosure;
[0017] FIG. 7B is the thermogravimetric analysis of polyurethane disscociated product in accordance with embodiments of the disclosure;
[0018] FIG. 8 is the mass spectra of polyurethane disscociated product in accordance with embodiments of the disclosure;
[0019] FIG. 9 is the FTIR spectra of commercial epoxy polymer (C-EP), epoxy polymer formed from polyurethane dissociation product (PDP-EP) and epoxy monomer in accordance with embodiments of the disclosure; and
[0020] FIG. 10 is a bar chart of mechanical properties of polypropylene and crosslinked polypropylene in accordance with embodiments of the disclosure.
[0021] Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.
DETAILED DESCRIPTION OF EMBODIMENTS
[0022] The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
[0023] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or process steps.
[0024] As used herein, the term “or combinations thereof” means that the listed components may be used individually or in any combination thereof.
[0025] As used herein, “combinations thereof” is inclusive of one or more of the recited elements, optionally together with a like element not recited, e.g., inclusive of a combination of one or more of the named components, optionally with one or more other components not specifically named that have essentially the same function.
[0026] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
[0027] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.
[0028] The term, “functionalization” of a polymer, generally refers to the introduction of specific functional groups (for example, hydroxyl, carboxyl, amine, or anhydride) onto a polymer chain, and the resulting polymer is referred to as a “functionalized polymer”. Functionalizing the polymer modifies its chemical properties by altering reactivity, adhesion, or compatibility with other materials.
[0029] The term “crosslinking” of a polymer, generally refers to the formation of covalent bonds between polymer chains, leading to a three-dimensional network structure. The polymer formed through this process is referred to as a “crosslinked polymer”. Crosslinking improves the mechanical and thermal properties of the polymer without significantly altering its chemical properties.
[0030] As used herein, the term “copolymer” refers to a polymer derived from more than one species of monomer, where the copolymer includes repeating units of each of the monomers.
[0031] As used herein, the term “blend” refers to a mixture of two or more polymers or copolymers that have been blended to create a new material with different physical properties.
[0032] As used herein, the term “yield strength” or “yield stress’ is defined as the minimum stress at which a solid will undergo permanent deformation or plastic flow without a significant increase in the load or external force. “Elongation at yield” is the deformation of plastic material at the yield point. The yield point corresponds to a point when an increase in strain is not marked by a significant increase in stress of the material. Elongation at yield is the ability of a plastic material to resist change of shape before it deforms irreversibly. Elongation at yield is the ratio between increased length and initial length at the yield point.
[0033] According to embodiments of the present disclosure, a method of recycling a polyurethane is provided. Polyurethanes, as used herein, refer to segmented copolymers formed by reaction between a polyol and an isocyanate. Polyurethanes of the present disclosure can be thermoplastic, thermosetting, or a mixture, or blends of thermoplastic and thermosetting polyurethanes. The isocyanate compounds typically have the structure R-(NCO), where R comprises an aromatic group, or an aliphatic group. Non-limiting examples of isocyanates include diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI) (for e.g., 2,4 TDI, or 2,6 TDI), hexamethylene diisocyanate (HMDI), isophorone diisocyanate, butylene diisocyanate, trimethylhexamethylene diisocyanate, di(isocyanatocyclohexyl)methane (e.g. 4,4'-diisocyanatodicyclohexylmethane), 1,8-diisocyanato-4-(isocyanatomethyl)octane, tetramethylxylene diisocyanate, 1,5-naphtalenediisocyanate, p-phenylenediisocyanate, 1,4-cyclohexanediisocyanate, or combinations thereof.
[0034] Non-limiting examples of polyols include polyether polyols, polyester polyols, polyacrylic polyols, polycarbonate polyols, polyalkylene polyols, caprolactone polyols, polyolefin polyols, and combinations thereof. In preferred embodiments, the polyols are polyether polyols, polyester polyols, or combinations thereof. Polyether polyols comprise propylene oxide (PO), ethylene oxide (EO), or a combination of PO and EO groups or moieties in the polymeric structure of the polyols. Polyester polyols are polymers that contain both ester and hydroxyl (OH) groups in their structure and are formed by the reaction between a diacid and an alcohol.
[0035] The reaction between the polyol and the isocyanate can be catalyzed using a catalyst. Non-limiting examples of catalysts include amine catalysts or organotin catalysts. Examples of amine catalysts include tertiary amines such as triethylenediamine, N-cocomorpholine, dimethylbenzylamine, hexadecyl(dimethyl)amine, N,N-dimethylethanolamine, bis(2-dimethylaminoethyl)ether, N-ethylmorpholine, N,N,N',N'-tetramethyl-1,3-butanediamine, 1,4-diazabicyclo[2.2.2]octane, 3-dimethylamino-N,N-dimethylpropionamide, N-methylmorpholine, N,N,N',N'-tetramethylethylenediamine, triethylamine, and salts of triethylene diamine.
[0036] A chain extender may be added to control the final properties of the polyurethane, such as hardness, flexibility, or crosslinking. As used herein, “chain extenders” refer to low molecular weight compounds that react with diisocyanates to enhance the molecular weight and/or increase the proportion of the hard segment. Typical chain extenders include polyamines, such as ethylene diamine, putrescine, and diaminopropane; and low molecular weight polyols, such as ethylene glycol, butane diol, and propylene glycol, or other compounds that may react with isocyanate groups.
[0037] The polyurethane has a molecular weight in a range of 50,000 to 400,000 grams per mole (g/mole). As used herein, molecular weight is defined as the average weight of the repeating units (or monomers) that make up the polymer chain.
[0038] The polyurethane of the present disclosure comprises post-consumer recycled (PCR) polyurethane, post-industrial recycled (PIR) polyurethane, or combinations thereof. Post-consumer recycled (PCR) plastics refer to plastic waste generated by consumers after the use of plastic products. The composition of PCR plastics can vary significantly due to the diverse mix of polymers and additives used by different manufacturers. This variation in composition makes the recycling of PCR plastics more complex and challenging. In contrast, post-industrial recycled (PIR) plastics are derived from plastic waste produced during industrial and manufacturing processes and are of known composition. PIR plastics are generally easier to recycle as they typically originate from a single source and are of known composition.
[0039] FIG. 1 is a flow chart of a method 100 of recycling a polyurethane in accordance with embodiments of the present invention. At step 102, a reaction medium comprising the polyurethane is subjected to hydrothermal treatment at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture.
[0040] The reaction medium is an aqueous-based medium comprising salt(s) maintained at a pH between 0.8 and 1.5. The term “salt” as used herein refers to a compound formed by a reaction between an acid and a base, which dissociates into positive ions and negative ions in water, or in aqueous-based medium. Examples of salts include salts of alkali metals, alkaline earth metals, or combinations thereof. Non-limiting examples of salts include sodium sulphate, sodium chloride, potassium chloride, magnesium chloride, magnesium sulphate, and calcium chloride.
[0041] In one embodiment, the reaction medium is seawater. As used herein, the term “seawater” refers to any naturally occurring water source with a minimum salt content of 3% (30 grams of salt in one liter of water). Seawater differs from the aqueous-based medium comprising salt as it is a complex mixture containing a wide and variable range of ionic species, minerals, and organic matter, in addition to the above-mentioned salts. Unlike a reaction medium comprising pure salts, seawater's unpredictable composition would not intuitively suggest its effectiveness in facilitating the dissociation of polyurethane to form desired products. The present disclosure demonstrates that seawater not only supports the reaction but does so with comparable or enhanced efficiency. Surprisingly, seawater offers distinct advantages, such as cost-effectiveness, ease of availability, and environmental sustainability.
[0042] The pH of the reaction medium is adjusted to a range between 0.8 and 1.5 by adding an acid or a mixture of acids. While not wishing to be bound by any particular theory, it is believed that the acid interacts with the polyurethane (PU) matrix, causing it to swell. The swelling of the PU matrix is believed to enhance the accessibility of internal reaction sites, thereby facilitating chain scission by enabling better penetration and activity of the salt ions. Any acid, such as mineral acids or organic acids, may be utilized. However, in one embodiment, bio-based acids, also termed bio-acids, are utilized to render the process more environmentally benign. The “bio-based acid” or “bio-acid” refers to an acid derived from renewable or bio-based sources, such as plants or microorganisms, rather than fossil fuels or petrochemicals. Non-limiting examples of bio-acids include citric acid, tartaric acid, formic acid, acetic acid, gallic acid, oxalic acid, maleic acid, succinic acid, lactic acid, levulinic acid, itaconic acid, or combinations thereof. In one embodiment, the bio-acid is citric acid.
[0043] The hydrothermal treatment is performed in a hydrothermal reactor at a temperature in a range of 230°C to 300°C. The hydrothermal reactor is a pressure chamber designed to conduct chemical reactions using water as a reaction medium at high temperatures and pressures. The hydrothermal reactor is typically made of materials resistant to corrosion and capable of withstanding the conditions encountered during hydrothermal reactions. In one embodiment, the hydrothermal treatment is performed in a pressurized chamber corresponding to a pressure ranging from 20MPa to 50MPa, and the chamber is maintained at a temperature ranging between 230°C and 300°C using a heat source such as an oven or a furnace.
[0044] The hydrothermal treatment, at step 102, results in dissociation of the polyurethaneunder acidic conditions, primarily involving the cleavage of C–N and C–O bonds within the urethane linkages generating a number of monomeric and/or,oligomeric species including precursors used in the original synthesis of the polyurethane. Notably, mass spectrometry analysis confirms that the predominant dissociation products are oligomeric species.FIG. 2 illustrates representative species that have been identified, resulting from the cleavage of the C-N linkage and the C-O linkage of the urethane bond of a representative polyurethane. It is believed that the dissociation of polyurethanes predominantly involves the cleavage of C-N linkages, as indicated by the presence of multiple species corresponding to fragments derived from C-N bond scission. It should be noted that FIG. 2 is not an exhaustive representation, as the exact species formed may vary depending on the composition of the polyurethane used as the starting material for recycling. As will be appreciated, since polyurethane is a polymer, its dissociation may also lead to the formation of various oligomeric species, with molecular weights varying based on the cleavage pattern and extent of depolymerization.
[0045] The reaction mixture comprises the monomeric and/or oligomeric species formed as a result of polyurethane dissociation. The preferred breakage of C-N linkage of urethane bond to form species comprising acid, hydroxy and amine functional groups is facilitated by the presence of salt(s) in the acidic reaction medium. The monomeric and/or oligomeric species may exist in the salt form, or in hydrated form. For example, the acid may exist in salt form, as shown in FIG. 2.
[0046] At step 104, the reaction mixture is cooled and separated into a solid component and a liquid component. In one embodiment, the pressurized chamber is removed from the heat source and allowed to cool to room temperature. As used herein, the term “room temperature” refers to an ambient temperature in a range of 25 °C to 40 °C. The reaction mixture after cooling to room temperature is separated by filtration, in one embodiment, into the liquid component and the solid component. Other separation techniques, such as sedimentation followed by decantation, may be utilized. The solid component comprises the monomer and predominantly oligomeric species. The solid component is subjected to washing, preferably using water to remove any residual salts, acids, or other water-soluble impurities. The liquid component comprises water and water-soluble salts, which may be reused as the reaction medium at step 102. The liquid component of step 104 is reused as the reaction medium at step (i) by adjusting the pH between 0.8 and 1.5.
[0047] At step 106, the solid component is dried to obtain a polyurethane dissociation product. The “polyurethane dissociation product” (PDP), as used herein, includes all monomeric and/or oligomeric species obtained by dissociation of polyurethane following method 100. The drying is performed in an oven or a furnace, with or without air-flow to facilitate removal of water. The drying temperature is maintained at a temperature well below the degradation temperature of the monomeric and oligomeric species. The drying, in one embodiment, is achieved by drying in an oven at a temperature in a range of 60 °C to 100 °C. The drying time may vary from 30 minutes to 12 hours depending on the amount of material and residual water present.
[0048] As will be appreciated, the composition of the polyurethane dissociation product (PDP) may be identified through chemical analytical techniques such as nuclear magnetic resonance (NMR) spectroscopy and infrared (IR) spectroscopy. Based on the obtained analytical data, individual monomeric species, or specific classes of species, such as acid species, may be isolated for further use as starting materials or reagents in chemical reactions. In one embodiment, the polyurethane dissociation product serves as a starting material or reagent in various chemical and polymerization reactions. Method 100 enables efficient recycling of polyurethane by generating dissociation products that can be converted into value-added products, otherwise termed as polyurethane recycled products. In one embodiment, the polyurethane dissociation product reacts with an epoxy monomer, a maleated polyolefin, or a polyurethane prepolymer to form a polyurethane recycled product (PRP).
[0049] At step 108, the polyurethane dissociation product reacts with a polyurethane prepolymer to form the polyurethane recycled product, which in this instance is recycled polyurethane. The term “polyurethane prepolymer”, as used herein, refers to a mixture of isocyanate compound and a polyol, or a product formed by reaction between a polyol and isocyanate, or combinations of these. The isocyanate compound and polyols are the same as those previously described for polyurethane formation. As discussed previously, the isocyanate compound reacts with the polyol and a chain extender to obtain PU of desired mechanical properties. The polyurethane dissociation product comprising amines and polyols serves as the chain extender.
[0050] A concentration of the polyurethane dissociation product is in a range of 30 weight percent (wt%) to 50 wt%, while a concentration of the polyurethane prepolymer to form the recycled polyurethane is in a range of 50 wt% to 70 wt%, based on total weight of reaction composition.
[0051] Suitable additives that may be used along with polyurethane dissociation product, at step 108 include, but are not limited to, surfactants, fire retardants, smoke suppressants, cross-linking agents, viscosity reducer, infra-red pacifiers, cell-size reducing compounds, pigments, fillers, reinforcements, mold release agents, antioxidants, dyes, pigments, antistatic agents, biocide agents, blowing agents, or combinations thereof.
[0052] FIG. 3 is a flowchart that illustrates a method 300 of preparing an epoxy polymer, according to embodiments of the present disclosure. The method 300 comprises steps 302, 304, 306, and 308. The steps 302, 304, and 306 are implemented identical to steps 102, 104, and 106 of the method 100, as previously described, and will not be repeated here for the sake of brevity.
[0053] At step 308, an epoxy monomer reacts with the polyurethane dissociation product (PDP) obtained at step 306 in presence of a catalyst at a temperature in a range of 100°C to 180°C to produce the epoxy polymer. Shown below is a schematic representation of reaction between an epoxy monomer (I) with polyurethane dissociation product to form an epoxy polymer (II):

[I] [II]

[0054] In epoxy polymer synthesis, the epoxy monomer reacts with a curing agent, leading to ring opening of the epoxide to obtain the epoxy polymer. Common curing agents for epoxy polymer formation include amines such as ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), isophorone diamine (IPDA), 4,4'-methylenedianiline (MDA), m-phenylenediamine, 4,4'-diaminodiphenylsulfone (DDS); acids; or anhydrides or combinations thereof. The polyurethane dissociation product comprising amines functions as the curing agent to react with the epoxy monomer to produce the epoxy polymer (also referred to as the polyurethane recycled product).
[0055] Suitable epoxy monomers comprise compounds containing at least one epoxide group. As used herein, the term “at least one” refers to having one, or more than one. The epoxy monomers include epoxy monomers of glycidyl type where the epoxy monomers are prepared by condensation reaction of a diol, diacid, or diamine with epichlorohydrin. In some embodiments, the epoxy monomer molecule may contain some rigidity by having phenylene rings in the molecule. In other embodiments, the epoxy monomer molecule may contain some flexibility by having linear or branched alkylene or poly(alkyleneoxide) unit in the molecule. Non-limiting examples of epoxy monomers include bisphenol A diglycidyl ether (DGEBA), diglycidyl ether of bisphenol F, hydrogenated bisphenol A diglycidyl ether, tetraglycidyl methylene dianiline, pentaerythritol tetraglycidyl ether, trimethylol triglycidyl ether (TMPTGE), tetrabromo bisphenol A diglycidyl ether, or hydroquinone diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, butylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A polyethylene glycol diglycidyl ether, bisphenol A polypropylene glycol diglycidyl ether, 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), tetraglycidyl diaminodiphenyl methane (TGDDM), or combinations thereof. In one embodiment, the epoxy monomer comprises 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), bisphenol A diglycidyl ether (DGEBA), tetraglycidyl diaminodiphenyl methane (TGDDM), diglycidyl ether of bisphenol F, or combinations thereof.
[0056] The epoxy monomer reacts with the polyurethane dissociation product to form the epoxy polymer in the presence of a catalyst. Non-limiting examples of catalysts include tertiary amines, such as triethylamine, and N,N-dimethylbenzylamine; Lewis acid catalysts, such as boron trifluoride and aluminum chloride; organophosphonium salts, such as tetraphenylphosphonium bromide and triphenylphosphine; and imidazole-based catalysts, such as 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole,1-(3-aminopropyl) imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, or combinations thereof.
[0057] A concentration of the polyurethane dissociation product is in a range of 25 wt% to 30 wt%, a concentration of the epoxy monomer is in a range of 70 wt% to 80 wt% and the concentration of the catalyst to form the epoxy polymer is in a range of 5 wt% to 10 wt%, based on total weight of reaction composition.
[0058] The polymerization process typically proceeds via a nucleophilic ring-opening reaction of the epoxide group of the epoxy monomer by the amines present in the polyurethane dissociation product. The reaction may be conducted at temperatures ranging from room temperature (~20°C) to elevated temperatures of about 180°C, depending on the reactivity of the amine. The polymerization is accelerated through the use of the catalysts mentioned above. In one embodiment, the epoxy monomer and polyurethane dissociation product are formulated in solvent-free compositions, though polymerization in the presence of solvents such as water, acetone, methyl ethyl ketone (MEK), toluene, or xylene may be employed where necessary.
[0059] Suitable additives that may be used along with polyurethane dissociation product, at step 308 for epoxy polymer synthesis include, but are not limited to, surfactants, fire retardants, smoke suppressants, cross-linking agents, viscosity reducer, infra-red pacifiers, cell-size reducing compounds, pigments, fillers, reinforcements, mold release agents, antioxidants, dyes, pigments, antistatic agents, biocide agents, blowing agents, or combinations thereof.
[0060] FIG. 4 is a flowchart that illustrates a method 400 of preparing a crosslinked polyolefin, according to embodiments of the present disclosure. The method 400 comprises steps 402, 404, 406, 408, and 410. The steps 402, 404, and 406 are implemented identical to steps 102, 104, and 106 of the method 100, as previously described, and will not be repeated here for the sake of brevity.
[0061] At step 408, a polyolefin is extruded with a maleic anhydride and a co-grafting agent to form a maleated polyolefin. As will be appreciated, the product of this step, maleated polyolefin, serves as a reactant for the subsequent reaction in step 410 of method 400. However, step 408 operates independently of steps 402, 404, and 406 and does not follow sequentially from them in the process flow.
[0062] Non-limiting examples of polyolefins comprise polyethylene, polypropylene, polyethylene copolymers, polypropylene copolymers, ethylene-propylene copolymers, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene (BOPE), or combinations thereof. Polyethylene and/or polypropylene copolymers may contain minor amounts of one or more α-olefins or comonomers, such as butene-1, hexene-1, or octene-1.
[0063] In one embodiment, the polyolefin is post-consumer recycled (PCR) polyolefin, or post-industrial recycled (PIR) polyolefin, or combinations thereof. In another embodiment, the polyolefin is virgin polyolefin.
[0064] In embodiments where the polyolefin is PCR polyolefin, the PCR polyolefin is washed to remove any contaminants or residues and dried to remove moisture before processing. In one embodiment, the PCR polyolefin is washed with an aqueous detergent solution. The washing is followed by drying in a vacuum oven at a temperature in a range of 50°C to 80°C for a time in a range of 5 to 12 hours before use to remove moisture. Once the PCR polyolefin is washed and dried, it is cut into smaller pieces for extrusion.
[0065] The polyolefin may be in the form of film, granules, flakes, powders, pellets, or combinations thereof. Before adding the polyolefin into the extruder it may be suitably sized into desirable dimensions of the order of a few millimeters.
[0066] The co-grafting agent may be chosen to minimize a surface energy mismatch between the maleic anhydride and the polyolefin whereby an extent of grafting of the maleic anhydride to polyolefin backbone may be enhanced. Examples of co-grafting agents include styrene, alpha-methyl styrene, toluene, divinyl benzene, and dimethyl maleate. In some embodiments, the co-grafting agent is styrene. A ratio of maleic anhydride to the co-grafting agent is in a range of 1:1 to 1:1.2. In one embodiment, the ratio of maleic anhydride to the co-grafting agent is 1:1.
[0067] The extrusion is performed in an extruder such as a single-screw extruder, or a twin-screw extruder. The processing parameters of the extruder may be varied to facilitate melt extrusion of the polyolefin, maleic anhydride and the co-grafting agent by optimizing one or more of melting of the polyolefin, homogeneous mixing between the polyolefin, maleic anhydride and the co-grafting agent, and efficient reaction between the polyolefin, maleic anhydride and the co-grafting agent. Examples of such process parameters include, but are not limited to, type of extruder, geometrical design of the extruder, screw speed, residence time of material in the extruder, feed rate of the material into the extruder, temperature, and die geometry through which a product is extruded. In one embodiment, the extruder is a twin-screw extruder that facilitates enhanced mixing between maleic anhydride, co-grafting agent and the polyolefin when compared to a single-screw extruder. The extrusion may be performed at a temperature corresponding to the melting temperature of the polyolefin. In some embodiments, the melting temperature is in a range of 160°C to 200°C. In some embodiments, the residence time is in a range of 1 to 10 minutes, preferably 1 to 5 minutes. In some embodiments, screw speed is in a range of 100 to 150 rotations per minute (rpm) in a twin-screw extruder. The extrusion, at step 408, in one embodiment, is performed in a twin-screw extruder at a temperature of 180°C at screw speeds of 100-150 rotations per minute (rpm) and at a residence time in a range of 1 to 5 minutes.
[0068] The maleated polyolefin is produced upon extrusion, at step 408, where the co-grafting agent-maleic-based side chain is grafted onto C-C backbone or linked to C-C backbone of the polyolefin. As used herein, the term “maleated polyolefin” refers to a polyolefin having the co-grafting agent-maleic-based side chain grafted onto C-C backbone of the polyolefin. The grafting makes the otherwise inert C-C backbone of polyolefin polar thus making it amenable for further functionalization and derivatization.
[0069] At step 410, the polyurethane dissociation product obtained from step 406 reacts with the maleated polyolefin from step 408 to form a crosslinked polyolefin. The polyurethane dissociation product is extruded with the maleated polyolefin through an extruder at a temperature in a range of 160 °C to 200 °C to form the polyurethane recycled product, wherein the polyurethane recycled product comprises the crosslinked polyolefin.
[0070] The polyurethane dissociation product functions as a crosslinker. On reactive extrusion, as described previously (step 408), diamines having terminal amine groups present in the polyurethane dissociation product attach to the maleic moieties of the same maleated polyolefin chain or between adjacent chains to form the crosslinked polyolefin. A concentration of the polyurethane dissociation product is in a range of 1 wt% to 30 wt%, while a concentration of the polyolefin to form the crosslinked polyolefin is in a range of 70 wt% to 99 wt%, based on total weight of composition extruded at step 410.
[0071] FIG. 5 is a reaction scheme 500 illustrating the formation of crosslinked polypropylene from PCR polypropylene (PCR-PP). At step 1, PCR-PP is extruded with styrene and maleic anhydride in the presence of dicumyl peroxide (DCP) an initiator, and Irganox® an antioxidant, to form a maleated polypropylene (m’-PCR-PP). At step 2, the maleated polypropylene is extruded with polyurethane dissociation product (PDP) in the presence of dicumyl peroxide (DCP), and Irganox® to form a crosslinked polypropylene (crosslinked PCR-PP). The extrusion at step 1, and step 2, are performed in a twin-screw extruder at a temperature of 180°C with a screw speed of 150 rotations per minute (rpm) and a residence time of 2 minutes (2 min).
[0072] The extruded crosslinked polyolefin obtained at step 410 may be immediately quenched in a water bath and pelletized. Such pellets can be used for subsequent molding, or shaping. The crosslinked polypolefin of the present disclosure may be shaped in the form of films, sheets, foams, particles, granules, beads, rods, plates, strips, stems, tubes, etc. via any process known to those skilled in the art. Examples of such processes include extrusion, casting, compression molding and the like.
[0073] In some embodiments, the extrusions (step 408 and step 410) may be performed in presence of additives commonly used during polyolefin processing such as UV stabilizers, surfactants, fire retardants, smoke suppressants, pigments, fillers, antistatic agents, antioxidants, heat stabilizers, and the like. Examples of additives include phenolic antioxidants, phosphite, pentaerythritol tetrakis [3- [3,5-di-tert-butyl-4-hydroxyphenyl]propionate] (Irganox® 1010), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 3,3',3',5,5',5'-hexa-tert-butyl-a,a',a'-(mesitylene-2,4,6-triyl) tri-p-cresol, 4,4'-thio-bis (3-methyl-6 tertbutylphenol, 2,2'-Thiobis(6-tert-butyl-p-cresol), thiodiethylene bis[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate]), octylthiomethyl)-o-cresol, distearylthiodipropionate, dilaurylthiodipropionate, pentaerythritol tetrakis(β-laurylthiopropionate), or combinations thereof.
[0074] Processing or recycling the polyolefin along with polyurethane dissociation product may enhance a cycle life of the resulting polyolefin. In another embodiment, processing the polyolefin may result in a polyolefin product namely, the crosslinked polyolefin that may have similar or superior mechanical properties to an initial polyolefin. The mechanical properties may be characterized in terms of yield strength and/or elongation at yield. The term “upcycling”, as used herein, refers to obtaining a polymer product that is on par or superior in mechanical properties ot a polymer it is derived from. It is a particular advantage of the present disclosure, irrespective of the additives present, such as in multilayer packaging (multilayered polymer); PCR or PIR polyolefin, or blends may be reprocessed using the disclosed method to result in an upcycled PCR or PIR polyolefin, or blends.
[0075] The present disclosure offers several advantages. The disclosed process enables recycling of both thermoset and thermoplastic polyurethanes using environmentally benign and commonly available reagents. In one embodiment, seawater is employed as the reaction medium owing to its natural abundance and ready availability. The use of seawater reduces dependency on purified water, thereby enhancing the sustainability and cost-efficiency of the process. Unlike hydrothermal liquefaction (HTL), the present process operates under milder reaction conditions and utilizes simpler reactor setups. The polyurethane dissociation product (PDP) obtained through this process may be further utilized in the synthesis of various thermosetting or thermoplastic polymers (namely, polyurethane recycled product (PRP)) as described with reference to FIGs. 1, 3, and 4.
[0076] In some embodiments, an article comprising the polyurethane recycled products of the present disclosure is provided. The article may be formed by molding, blow molding, injection molding, filament winding, continuous molding or film-insert molding, infusion, pultrusion, RTM (resin transfer molding), RIM (reaction-injection molding), 3D printing, or any other method known to those skilled in the art.
EXAMPLES
[0077] The present disclosure will now be described in greater detail by the
following non-limiting examples. It is understood that one skill in the art will envision additional embodiments consistent with the disclosure provided herein.
EXAMPLE 1
Preparation of polyurethane dissociation product (PDP)
[0078] A reaction medium was prepared by mixing 20 grams of citric acid in 100 ml of seawater with continuous stirring till a clear solution was obtained with a pH between 0.8 and 1.5. About 5 grams (g) of thermoset polyurethane (PU) was cut into pieces and added to 25 milliliters (mL) of the reaction medium and charged into a 100 ml hydrothermal pressure reactor maintained at a pressure of 20 MPa. The reactor was placed in an oven at 250°C for 24 hours, resulting in a reaction mixture. After cooling to room temperature, the reaction mixture was filtered and separated into a solid component and a recyclable liquid component, which was reused as the reaction medium. The solid component was washed with water to remove residual salts and then dried in a hot air oven at 80°C for 2 hours, yielding a dry brown solid containing polyurethane dissociation product (PDP). The reaction yield was determined to be over 90%, based on the initial weight of the polyurethane and the final weight of the resulting PDP.
Characterization of PU and PDP
[0079] PU and PDP were analyzed by Fourier Transform Infrared (FTIR) spectroscopy performed on a Perkin Elmer Spectroscope (PES) in attenuated total reflectance (ATR) mode in the scanning range 650-4000 cm-1. FIG. 6 is the FTIR spectra 600 of PU and PDP, where PU IR spectrum is indicated as 600A, and PDP IR spectrum is indicated as 600B. In the PDP IR spectrum (600B), an intense broadening of the peak around 3100-3500 cm-1 was observed corresponding to overlapped N-H and O-H stretching frequency which is not a prominent band in the IR spectrum of PU (600A). Peaks at 1511 and 761cm-1 correspond to overlapped N-H bending and C-H stretching, and N-H wagging, 1377 cm-1 peak corresponds to -OH bending, and a carbonyl peak was observed at 1706 cm-1. In 600B, as PDP is a low molecular weight oligomer FTIR results indicate that sodium ion (Na+) (mostly present in seawater) selectively cleaves the C-N and C-O bonds to yield amine and acid functional groups (as represented in Reaction scheme FIG.2). When compared to PU IR spectrum of 600A, peaks corresponding to 1511 cm-1 and 1073 cm-1 are less intense or absent in PDP, confirming cleavage of C-N and C-O bonds.
[0080] Thermogravimetric analysis (TGA) was performed to investigate the thermal stability of PDP and PU using a TA Instruments Q500 instrument. The analyses were conducted over a temperature range from 40°C to 800°C at a heating rate of 10°C per minute. The PU and PDP thermal decomposition data are depicted in FIG. 7A and FIG. 7B, respectively. Derivative Thermogravimetry (DTG) plots shown in FIG. 7A and 7B, are the first derivatives of the TGA plots, representing weight loss or gain of a sample as a function of temperature or time. The initial degradation started at 120.39℃ for PDP (FIG. 7B), where weight loss was observed due to elimination of the labile oxygen functional group. Td5, corresponding to 5 wt% loss, was observed at 143℃. Next, at 294.48 ℃, another degradation was observed, which could be the breakage of the amine bond. However, maximum degradation was observed at 451.02 ℃ due to the complete breakdown of oligomers. For PU (FIG. 7A), initial degradation starts at 163.08℃, and maximum degradation occurs at 330.21℃, possibly due to degradation of the main chain backbone.
[0081] High-resolution mass spectrometry (HRMS) was performed to determine elemental compositions and identify various species present in PDP. FIG. 8 is the mass spectrum 800 of PDP. From the elemental analysis data, chemical structures corresponding to various species were identified as indicated in FIG. 8 (as represented in Reaction scheme FIG.2). The species identified confirmed the cleavage of C-N bonds and C-O bonds of PU to obtain the PDP. Further, the presence of sodium ion (Na+) on some of the species confirmed the role of salt in cleavage of C-N and C-O bonds of polyurethane.
EXAMPLE 2
Preparation of epoxy polymer
[0082] Two epoxy polymer samples were prepared. The first sample corresponds to commercial epoxy polymer (C-EP) formulated with an epoxy monomer-to-curing agent ratio of 100:24. The second sample (PDP-EP) was prepared using the polyurethane dissociation product (PDP) from Example 1 as the curing agent, with an epoxy monomer to PDP to catalyst ratio of 100:24:5. The catalyst used was 1-(3-aminopropyl) imidazole (API). The epoxy monomer in both cases was bisphenol A diglycidyl ether (DGEBA). Both samples were cured in an oven at 140°C for 2.5 hours.
Characterization of epoxy polymer
[0083] FTIR spectra of the commercial epoxy polymer (C-EP), polyurethane dissociation product (PDP) curing agent, with the epoxy monomer (PDP-EP), along with the epoxy monomer (EP) were recorded, as shown in FIG. 9. IR spectrum 900A corresponds to the epoxy monomer (EP), spectrum 900B corresponds to commercial epoxy polymer (C-EP), and IR spectrum 900C corresponds to the polyurethane dissociation product curing agent with the epoxy monomer (PDP-EP) in FIG. 9. Both the IR spectra (900B and 900C) had characteristic peaks at, 1109 cm-1 corresponding to C-O stretching, 1608 cm-1 corresponding to C=C stretching, 1733 cm-1 corresponding to C=O, 2862-3100 cm-1 corresponding to CH2, and 3400 cm-1 corresponding to OH stretching, respectively. However, the absence of an oxirane peak at 914 cm-1 in IR spectra 900B and 900C, implies the complete polymerization in C-EP and PDP-EP.
[0084] The thermogravimetric analyses of C-EP and PDP-EP were performed. The primary degradation of C-EP started at 298 ℃, whereas for PDP-EP it started at a temperature of around 238 ℃, and maximum degradation started at about 310 ℃. The higher primary degradation temperature of C-EP can be ascribed to the higher network formation when compared to PDP-EP.
[0085] Mechanical properties of C-EP and PDP-EP were tested according to ASTM D638 (type V) stress-strain properties of the samples using a Tinius Olsen 1ST Universal Testing Machine at room temperature using Test Parameters of Load Cell: 5 kN; Preload Force: 0.1 N; Cross Head Speed: 50mm/minutes; Gauge Length: 15mm; Number of Samples: 5 per batches for consistency; and Sample Dimension: 50 mm length x 4.1 mm width x 2.1 mm thickness.
[0086] The samples were placed between clamps of the Universal Testing Machine - Tensile Testing Module such that the edges of the samples were parallel to the direction of the load. The grips were tightened to hold the samples securely within the jig. The test samples were then pulled apart at a tensile speed of 50 mm/minute until they broke. The tensile strengths of C-EP and PDP-EP were found to be 64(±2) MPa and 55(±4) MPa, respectively. The higher tensile strength of C-EP is attributed to the higher crosslinked network, formed by the reaction between the curing agent containing aliphatic tetramine and the epoxy group of DGEBA. On the other hand, PDP contains primary and hydroxyl groups, which can react with the epoxy of DGEBA, but the extent of crosslinking when compared with C-EP is low, as reflected by the lower tensile strength of PDP-EP.
EXAMPLE 3
Preparation of crosslinked polypropylene
[0087] Maleic anhydride (MA), styrene (co-grafting agent), PCR Polypropylene (PCR PP), dicumyl peroxide (DCP) as catalyst, Irganox® 1010 (antioxidant) were mixed and extruded through a DSM Xplore twin-screw micro-compounder with a 15 cm³ capacity. The micro-compounder featured a co-rotating conical screw design and a recirculation channel, allowing for precise control over residence time. The extrusion was executed at a temperature of 180 °C with a screw speed of 150 rpm and a residence time of 2 minutes. Table 1 presents the composition for the preparation of maleated polypropylene.
Sample Name PCR PP
(wt %) Maleic anhydride
(wt %) Styrene
(wt %) DCP
(wt %) Irganox®
(wt %)
maleated PP 78.38 10 10.62 0.5 0.5
Table 1
[0088] A crosslinked PP was made by crosslinking the maleated PP batches with PDP (from Example 1) by subjecting them to melt extrusion at 180 °C and 150 rpm for 2 minutes residence time. Table 2 presents the composition of maleated PP and PDP for the preparation of crosslinked polypropylene. Samples of crosslinked polypropylene molds were prepared by injection molding for tensile testing by maintaining the barrel temperature at 180°C while the molds remained at ambient temperature. Four dog bone-shaped specimens were fabricated using an injection pressure of 14 bar pressure.
Sample Name maleated PP
(wt%) PDP
(wt%)
crosslinked PP 85 15
Table 2
Characterization of PP and crosslinked PP
[0089] Mechanical properties of PCR PP and crosslinked PP were tested according to ASTM D638 (type V) stress-strain properties of the samples using a Tinius Olsen 1ST Universal Testing Machine at room temperature using Test Parameters of Load Cell: 5 kN; Preload Force: 0.1 N; Cross Head Speed: 50mm/minutes; Gauge Length: 15mm; Number of Samples: 5 per batches for consistency; and Sample Dimension: 50 mm length x 4.1 mm width x 2.1 mm thickness. FIG. 10 is a bar chart 1000 of mechanical properties, namely, yield strength and elongation at yield, of PCR PP and crosslinked PP. In FIG. 10, 1000A corresponds to yield strength and elongation at yield of PCR PP and 1000B corresponds to yield strength and elongation at yield of crosslinked PP. Crosslinked PP exhibited higher yield strength of about 33 MPa and almost similar elongation at yield (Ey) of about 12% in comparison to PCR PP which exhibited yield strength of 27 MPa before failure. The higher yield strength or mechanical stability of crosslinked PP can be attributed to a highly crosslinked structure when compared to PCR PP.
[0090] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims.
, Claims:CLAIMS
We Claim:
1. A method (100) of recycling a polyurethane comprising steps of:
(i) subjecting a reaction medium comprising the polyurethane to hydrothermal treatment (102) at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture;
(ii) cooling and separating the reaction mixture into a solid component and a liquid component (104); and
(iii) drying the solid component to obtain a polyurethane dissociation product (106).
2. The method (100) of recycling the polyurethane as claimed in claim 1 further comprising reacting the polyurethane dissociation product with an epoxy monomer, a maleated polyolefin, or a polyurethane prepolymer to form a polyurethane recycled product.
3. The method (100) of recycling the polyurethane as claimed in claim 2, wherein reacting the polyurethane dissociation product with the epoxy monomer comprises reacting in presence of a catalyst at a temperature in a range of 100°C to 180°C to form the polyurethane recycled product, wherein the polyurethane recycled product comprises an epoxy polymer.
4. The method (100) of recycling the polyurethane as claimed in claim 2, wherein reacting the polyurethane dissociation product with the maleated polyolefin comprises extruding at a temperature in a range of 160 °C to 200 °C to form the polyurethane recycled product, wherein the polyurethane recycled product comprises a crosslinked polyolefin.
5. The method (100) of recycling the polyurethane as claimed in claim 1, wherein the liquid component from step (ii) is reused as the reaction medium at step (i) by adjusting the pH between 0.8 and 1.5.
6. The method (100) of recycling the polyurethane as claimed in claim 1, wherein the polyurethane is a reaction product of a diisocyanate and a polyol.
7. The method (100) of recycling the polyurethane as claimed in claim 1, wherein the reaction medium comprises one or more salts.
8. The method (100) of recycling the polyurethane as claimed in claim 1, wherein the reaction medium is seawater.
9. The method (100) of recycling the polyurethane as claimed in claim 1, wherein separating the reaction mixture comprises filtering the reaction mixture to obtain the solid component and the liquid component.
10. The method (100) of recycling the polyurethane as claimed in claim 1, wherein drying the solid component comprises drying in an oven at a temperature in a range of 60 °C to 100 °C.
11. The method (100) of recycling the polyurethane as claimed in claim 1, wherein the pH of the reaction medium is maintained by adding a bio-acid, wherein the bio-acid comprises citric acid, tartaric acid, formic acid, acetic acid, gallic acid, oxalic acid, maleic acid, succinic acid, lactic acid, levulinic acid, itaconic acid, or combinations thereof
12. A method (300) of recycling a polyurethane to produce an epoxy polymer comprising steps of:
(i) subjecting a reaction medium comprising the polyurethane to hydrothermal treatment (302) at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture;
(ii) cooling and separating the reaction mixture into a solid component and a liquid component (304);
(iii) drying the solid component to obtain a polyurethane dissociation product (306); and
(iv) reacting the polyurethane dissociation product with an epoxy monomer (308) in presence of a catalyst at a temperature in a range of 100°C to 180°C to produce the epoxy polymer.
13. The method (300) as claimed in claim 12, wherein the reaction medium comprises seawater.
14. A method (400) of recycling a polyurethane to produce a crosslinked polyolefin comprising steps of:
(i) subjecting a reaction medium comprising the polyurethane to hydrothermal treatment (402) at a temperature between 230 °C and 300 °C, a pressure between 20 MPa and 50 MPa, and a pH between 0.8 and 1.5 to produce a reaction mixture;
(ii) cooling and separating the reaction mixture into a solid component and a liquid component (404);
(iii) drying the solid component to obtain a polyurethane dissociation product (406); and
(iv) extruding the polyurethane dissociation product with a maleated polyolefin at a temperature in a range of 160 °C to 200 °C to produce the crosslinked polyolefin (410).
15. The method (400) as claimed in claim 14, wherein the reaction medium comprises seawater.