Lignin-based dynamic crosslinker, and preparation method and application thereof

Abstract

The application discloses a lignin-based dynamic crosslinking agent and a preparation method and application thereof. The preparation method of the lignin-based dynamic crosslinking agent comprises the following steps: firstly, using alkali lignin and a binary aldehyde as raw materials, performing an acetalization reaction under an acidic catalysis condition to obtain acetalized lignin; and secondly, reacting the acetalized lignin with 3-(methylamino)propylamine under the catalysis of a first catalyst to obtain the lignin-based dynamic crosslinking agent containing acetal bonds and acetal amine bonds. The crosslinking agent containing double dynamic covalent bonds is synthesized from the acetalized alkali lignin derivative. The lignin-based dynamic crosslinking agent and commercial polyurethane foam powder can be used to prepare modified polyurethane powder through mechanical chemical dispersion under the action of a catalyst. The modified polyurethane powder can be used as a high-value-added adhesive with excellent mechanical properties and strong adhesion, and the modified polyurethane powder can be converted into a recycled polyurethane sheet which can be recycled and processed multiple times through further hot pressing.

Description

Technical Field

[0001] This invention relates to the field of thermosetting plastics recycling technology, specifically to lignin-based dynamic crosslinking agents, their preparation methods, and applications. Background Technology

[0002] Polyurethane foam (PUF) is one of the world's largest-produced synthetic polymer materials. In 2022, China's production reached 11 times that of 2000, accounting for 45% of the global market. Due to its excellent properties such as high strength, high elasticity, and high abrasion resistance, it has become one of the most widely used synthetic materials of this century, showing broad application prospects in biomedicine, functional coatings, smart textiles, and flexible electronics. With its excellent thermal insulation and cushioning properties, it is widely used in building insulation, cold chain transportation, and home appliances. However, traditional polyurethane foam has a stable three-dimensional cross-linked network structure, making its commercial products difficult to biodegrade naturally. Therefore, the commercial recycling of polyurethane foam is urgently needed.

[0003] In nature, lignin, as the second most abundant natural aromatic polymer after cellulose, is rich in active groups such as phenolic and alcoholic hydroxyl groups, and can react with polyurethane. Compared with cellulose, lignin has a more complex structure and lower reactivity. Currently, it is mostly directly burned or discharged with wastewater, resulting in low resource utilization and easy environmental pollution. How to achieve high-value utilization of lignin is an urgent technical problem to be solved.

[0004] Introducing dynamic bonds has become an effective strategy for addressing the difficulties in processing and degrading thermosetting materials. The urethane bonds in polyurethane can undergo exchange reactions under specific conditions, exhibiting dynamic covalent properties. Building upon this, researchers have further introduced various dynamic bonds through molecular design, endowing materials with reprocessability, degradability, and closed-loop recycling capabilities. Existing research largely focuses on the synthesis of novel polyurethanes, while strategies for the direct high-value upgrading and recycling of commercially available polyurethane foams with complex compositions are severely lacking. In particular, how to simultaneously achieve performance enhancement and functionalization during the recycling process remains a major challenge. Summary of the Invention

[0005] The purpose of this invention is to provide a lignin-based dynamic crosslinking agent, its preparation method, and its application. The crosslinking agent, containing dual dynamic covalent bonds, is synthesized from acetalized alkali lignin derivatives. This lignin-based dynamic crosslinking agent and commercial polyurethane foam powder can be mechanically and chemically dispersed under the action of a catalyst to produce modified polyurethane powder. This modified polyurethane powder can serve as a high-value-added adhesive with excellent mechanical properties and strong adhesion. Furthermore, the modified polyurethane powder, after further hot-pressing, can be converted into recycled polyurethane sheets that can be processed and recycled multiple times.

[0006] According to a first aspect of the present invention, a method for preparing a lignin-based dynamic crosslinking agent is provided, comprising the following steps: S1: Using alkali lignin and dialdehyde as raw materials, an acetalization reaction is carried out under acidic catalytic conditions to obtain acetalized lignin. S2: Acetaldehyde-modified lignin is reacted with 3-(methylamino)propylamine under the catalysis of the first catalyst to obtain the product.

[0007] Dialdehydes have two aldehyde groups: one anchors the lignin skeleton in the acetalization reaction, and the other serves as an active site in subsequent reactions. The lignin-based dynamic crosslinking agent of this application introduces acetal and acetal-amine bonds structurally, forming a double reversible covalent bond, which can then co-construct a multiple dynamic covalent network with the urethane bonds in polyurethane foam.

[0008] First, acetal bonds can undergo reversible hydrolysis and recombination under acidic conditions, which endows the crosslinking agent of this application with unique dynamic response characteristics—enabling the subsequently prepared recycled polyurethane sheets to have excellent reprocessability and to achieve on-demand degradation under specific stimuli. More importantly, acetal bonds have a higher activation energy than other dynamic covalent bonds, exhibiting good stability at normal operating temperatures and being less prone to creep. This precisely solves the bottleneck problem of poor dimensional stability in traditional dynamic crosslinking materials.

[0009] Secondly, lignin, as a natural aromatic polymer, possesses a rigid three-dimensional network structure and the synergistic effect of acetal bonds, which can significantly improve the thermal stability and flame retardant properties of materials. On the other hand, using alkali lignin as the crosslinking backbone, the lignin molecule contains multiple hydroxyl reaction sites, enabling multi-site crosslinking and significantly improving crosslinking density and interfacial bonding strength. Simultaneously, lignin itself possesses photothermal conversion capabilities, providing a potential pathway for crosslinking agents to trigger dynamic bond exchange under photothermal response, thereby achieving controllable recycling and reshaping of materials.

[0010] In addition, the acetal bond and the acetal-amine bond complement each other in terms of dynamic responsiveness, giving the adhesive a richer stimuli response and a more flexible controllable space.

[0011] Therefore, the crosslinking agent obtained by this application through the progressive molecular design of "dialdehyde-acetalized lignin-dual dynamic network" can achieve a synergistic unity of high adhesive strength, environmental durability and green sustainability of adhesives.

[0012] In some embodiments, the molar ratio of alkali lignin to dialdehyde in step S1 is 1:5 to 11.

[0013] In some embodiments, the dialdehyde in step S1 is an aliphatic or aromatic dialdehyde. Aliphatic dialdehydes include at least one of glyoxal, glutaraldehyde, and succinaldehyde, while aromatic dialdehydes include at least one of terephthalaldehyde, o-phthalaldehyde, iso-phthalaldehyde, 2,5-dihydroxyterephthalaldehyde, and 2,5-bis(alkoxy)terephthalaldehyde. Terephthalaldehyde is preferred due to its high reactivity and its rigid benzene ring structure, which effectively enhances the mechanical strength of the crosslinked network.

[0014] In some embodiments, hydrochloric acid is added as an acidic catalyst in step S1. The concentration of hydrochloric acid is 36–38 wt%. In other embodiments, the acidic catalyst may be a strong acid such as sulfuric acid and / or trifluoromethanesulfonic acid, which exhibits strong catalytic activity.

[0015] In some embodiments, step S1 involves continuous stirring of the reaction under a nitrogen atmosphere at 65–85°C for 4–6 hours.

[0016] In some embodiments, the alkali lignin and dialdehyde in step S1 need to be dissolved in a solvent for reaction. The solvent is 1,4-dioxane or a eutectic solvent (DES), and the eutectic solvent is a basic ternary DES, such as sodium hydroxide-ethylene glycol-water or FeCl3-polyethylene glycol 400-choline chloride.

[0017] In some embodiments, after the reaction in step S1 is completed, the product is cooled to room temperature and neutralized with sodium bicarbonate; subsequently, the reaction solution is slowly poured into n-propanol for precipitation; the solid product is collected by vacuum filtration and thoroughly washed with n-propanol (e.g., 3-5 times), and dried overnight in a vacuum oven to obtain acetalized lignin. The drying temperature in the vacuum oven is 50-60°C.

[0018] In some embodiments, in step S2, the molar ratio of the aldehyde group in the acetalized lignin to the amino group in 3-(methylamino)propylamine is 1:1 to 1.05.

[0019] In some embodiments, in step S2, the first catalyst is at least one of p-toluenesulfonic acid, iodine, and nickel chloride.

[0020] In some embodiments, the reaction temperature in step S2 is 65–85°C, and the reaction time is 24–36 hours.

[0021] In some embodiments, the acetalized lignin and 3-(methylamino)propylamine in step S2 need to be dissolved in a solvent for the reaction. The solvent is 1,4-dioxane or a eutectic solvent (DES), and the eutectic solvent is a basic ternary DES, such as sodium hydroxide-ethylene glycol-water or FeCl3-polyethylene glycol 400-choline chloride.

[0022] In some embodiments, after the reaction in step S2 is completed, the resulting solution is slowly poured into cold petroleum ether for precipitation, the product is collected by vacuum filtration, and dried overnight in a vacuum oven to obtain a lignin-based dynamic crosslinking agent. The drying temperature in the vacuum oven is 50–60°C.

[0023] According to a second aspect of the present invention, a lignin-based dynamic crosslinking agent is provided, prepared by the above-described preparation method. Furthermore, the application of this lignin-based dynamic crosslinking agent in the preparation of polyurethane foam adhesives is also provided.

[0024] According to a third aspect of the invention, a polyurethane adhesive is provided, which is prepared by mixing a powder obtained by pulverizing commercial polyurethane foam, a catalyst, and the aforementioned lignin-based dynamic crosslinking agent.

[0025] In some embodiments, the powder obtained after pulverizing commercial polyurethane foam is mixed with a lignin-based dynamic crosslinking agent and a second catalyst, and then ball-milled to obtain modified polyurethane powder, which is used as a polyurethane adhesive. By employing a ball-milling mechanochemical dispersion process, the catalyst and lignin-based dynamic crosslinking agent are uniformly dispersed in the commercial polyurethane foam powder to obtain modified polyurethane powder, which can be directly used as a polyurethane adhesive. This process not only achieves efficient loading and uniform distribution of the catalyst and crosslinking agent but also activates the surface of the foam powder, generating more crosslinking reactive sites.

[0026] In some embodiments, the amount of lignin-based dynamic crosslinking agent used is 0.5 to 5 wt. of commercial polyurethane foam.

[0027] In some embodiments, the amount of the second catalyst is 1 to 10 wt. of commercial polyurethane foam.

[0028] In some embodiments, the second catalyst is a Lewis acid or an organic base. The Lewis acid is at least one of dibutyltin dilaurate (DBTDL), iron acetylacetone, and zinc acetate; the organic base is 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and / or 1,8-diazabicyclo[5.4.0]undecene (DBU).

[0029] In some embodiments, the polyurethane foam is crushed for 5 to 10 minutes.

[0030] In some embodiments, the ball milling speed is 500–680 rpm / min, and the ball milling time is 1–2 h.

[0031] According to a fourth aspect of the invention, a recycled polyurethane sheet is provided, which is obtained by hot pressing of a polyurethane adhesive.

[0032] Polyurethane adhesives are powder adhesives. Through hot pressing molding, powder adhesives can construct a highly dense multi-layer dynamic cross-linked network structure, thereby obtaining recycled polyurethane sheets with excellent reprocessing mechanical properties.

[0033] In some embodiments, the hot-pressing temperature is 150–200°C, the pressure is 25–35 MPa, and the time is 30–60 min.

[0034] The beneficial effects of this invention are as follows: (1) The present invention uses a condensation reaction to synthesize a lignin-based dynamic crosslinking agent containing double reversible covalent bonds. The synthesis route is simple, the reaction conditions are mild, and the raw materials are renewable biomass resources, which are low in cost and environmentally friendly.

[0035] (2) This invention designs and constructs a multi-reversible dynamic cross-linking network containing hydrogen bonds, weak dynamic covalent bonds (including cyclic acetal bonds and cyclic acetal amine bonds) and strong dynamic covalent bonds (including urethane bonds) in the polyurethane molecular chain segment. When the resulting recycled polyurethane powder is used as an adhesive, it can generate multiple interactions such as hydrogen bonds, covalent bonds and coordination bonds with the surfaces of various substrates such as metals (including stainless steel, copper and aluminum), glass and wood, thereby exhibiting high adhesive shear properties and having good industrial adaptability and large-scale application prospects.

[0036] (3) This invention introduces a lignin-based dynamic crosslinking agent into polyurethane foam, constructing a multi-reversible dynamic covalent network containing cyclic acetal bonds, cyclic acetal-amine bonds, and urethane bonds under the action of a catalyst. This allows the resulting recycled polyurethane sheet to be completely reconstructed into a uniform and complete sheet after further crushing, ball milling, and hot-pressing cycles, with no obvious surface defects or structural damage. Furthermore, the recycled polyurethane sheet of this invention exhibits excellent reprocessing efficiency in secondary physical recycling, exceeding 100%, demonstrating that cyclic acetal-amine bonds can achieve a unique "the more it is processed, the stronger it becomes" effect, showcasing promising prospects for recycling applications. Simultaneously, this invention is simple to operate and can be continuously produced using existing industrial equipment, possessing excellent potential for industrial application. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the technical route of the present invention; Figure 2 The synthesis pathway diagrams of the crosslinking agents in Examples 1 and 2 of this invention are shown. Figure 3 The Fourier transform infrared spectra of the crosslinking agent and alkali lignin in Examples 1 and 2 of this invention are shown. Figure 4 The above are the proton NMR spectra of the crosslinking agent and alkali lignin in Examples 1 and 2 of this invention; Figure 5Fourier transform infrared spectra of the ultrafine polyurethane powders in Examples 1-2 and Comparative Examples 1-2 of this invention; Figure 6 The Fourier transform infrared spectra of the recycled polyurethane sheets in Examples 1-2 and Comparative Examples 1-2 of this invention are shown below. Figure 7 The tensile properties of recycled polyurethane sheets in various embodiments and comparative examples of the present invention are shown in the diagram. Figure 8 This is a diagram showing the cyclic processing tensile properties of the recycled polyurethane sheet in Embodiment 1 of the present invention; Figure 9 This is a diagram showing the cyclic processing tensile properties of the recycled polyurethane sheet in Embodiment 2 of the present invention; Figure 10 The results of adhesion tests on different substrates for the adhesives of Examples 1-2 and Comparative Example 2 of this invention are shown. Figure 11 The bonding strength results of the overlapping sheared parts of Embodiment 2 of the present invention after being immersed in acidic liquid, alkaline liquid and artificial seawater for 24 hours are shown. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, all raw materials and reagents involved in the following embodiments are commercially available.

[0039] Example 1 Step 1: Weigh 20g of alkali lignin (Lig) and 16.93g of terephthalaldehyde (TPAL) and add them to a 250mL three-necked round-bottom flask. Simultaneously add 100mL of 1,4-dioxane and 2mL of 37wt% HCl. The flask is equipped with a reflux condenser and a stirrer. Under a nitrogen atmosphere, the reaction system is continuously stirred at 65℃ for 4 hours. After the reaction is complete, the mixture is cooled to room temperature and neutralized with sodium bicarbonate. Subsequently, the reaction solution is slowly poured into 500mL of n-propanol to precipitate and remove excess dialdehyde. The solid product is collected by vacuum filtration, washed thoroughly with n-propanol, and dried overnight in a vacuum oven at 60℃ to obtain acetalized lignin (AL) containing cyclic acetal dynamic bonds. The reaction formula is shown below. Figure 2 .

[0040] Step 2: Cut the commercial polyurethane foam into 1cm pieces 3 Weigh 300g of the cube-shaped pieces and place them in a small grinder. Cover the grinder and grind for 5 minutes. Remove and set aside (PUF powder).

[0041] Step 3: Weigh 20g of the powder obtained in Step 2, add 3wt.% organic base catalyst and 1wt.% AL by weight of the powder, and place them in a zirconia ball mill jar. Add zirconia beads and place the jar in a planetary ball mill for 1.5h at a speed of 600rpm / min. After the ball milling is completed, the modified ultrafine polyurethane powder is obtained with a particle size of about 100μm. The modified ultrafine polyurethane powder can be used directly as a powder adhesive and is designated as PT-1AL powder adhesive.

[0042] Step 4: Weigh 0.2 g of the modified ultrafine polyurethane powder from Step 3 and place it in a custom-made metal sheet (100 mm long, 25 mm wide, and 2 mm thick), with an overlap area of ​​25 mm × 12.5 mm. Then, place it in a hot press and hot press for 30 min at a temperature of 170 ℃ and a pressure of 30 MPa. The resulting lapped shear part is designated as PT-1AL lapped shear part. See the operation procedure below. Figure 1 (b).

[0043] Step 5: Weigh 3g of the modified ultrafine polyurethane powder from Step 3, and hot-press it at 170℃ for 45 minutes at a pressure of 30MPa. Then cool it to room temperature to obtain recycled polyurethane sheet, denoted as PT-1AL sheet. See the operation procedure below. Figure 1 (a).

[0044] Example 2 Step 1: The dried acetalized lignin AL obtained in Example 1 was dissolved together with 3-methylaminopropylamine (the molar ratio of aldehyde groups in acetalized lignin to primary amine groups in 3-methylaminopropylamine was approximately 1:1.05) in 100 mL of 1,4-dioxane. 20 mg of p-toluenesulfonic acid (PTSA) was added as a catalyst to this reaction system. The mixture was reacted at 85°C for 24 hours. The resulting solution was slowly poured into cold petroleum ether for precipitation. The product was collected by vacuum filtration and dried overnight in a vacuum oven at 60°C to obtain the crosslinking agent ALMA(C) containing cyclic acetals and cyclic acetal amine dynamic bonds. The reaction formula is shown below. Figure 2 .

[0045] Step 2: Cut the commercial polyurethane foam into 1cm pieces 3 Weigh 300g of the cube-shaped pieces and place them in a small grinder. Cover the grinder and grind for 5 minutes. Remove and set aside (PUF powder).

[0046] Step 3: Weigh 20g of the powder obtained in Step 2, and add 3wt.% organic base catalyst and 1wt.% ALMA(C) by weight of the powder. Place the powder in a zirconia ball mill jar, add zirconia beads, and place the jar in a planetary ball mill for 1.5h at a speed of 600rpm / min. After the ball milling is completed, the modified ultrafine polyurethane powder is obtained with a particle size of about 100μm. The modified ultrafine polyurethane powder can be used directly as a powder adhesive and is designated as PT-1ALMA(C) powder adhesive.

[0047] Step 4: Weigh 0.1-0.5 g of the modified ultrafine polyurethane powder from Step 3, place it in a custom-made metal sheet (100 mm long, 25 mm wide, and 2 mm thick), with an overlap area of ​​25 mm × 12.5 mm, and then place it in a hot press for 30 min, maintaining the temperature at 170℃ and the pressure at 30 MPa. The resulting overlapping sheared part is denoted as PT-1ALMA(C) overlapping sheared part.

[0048] Step 5: Weigh 3g of the modified ultrafine polyurethane powder from Step 3, place it in a hot press at 170℃ for 45 minutes at a pressure of 30MPa, and then cool it to room temperature to obtain a recycled polyurethane sheet, denoted as PT-1ALMA(C) sheet.

[0049] In some other embodiments, the dialdehyde used in preparing acetalized lignin may be selected from at least one of o-phthalaldehyde, iso-phthalaldehyde, 2,5-dihydroxyterephthalaldehyde, 2,5-bis(alkoxy)terephthalaldehyde, glyoxal, glutaraldehyde, and butyraldehyde.

[0050] In some other embodiments, the concentration of hydrochloric acid used in preparing acetalized lignin may be 36 wt%, 36.5 wt%, 37.5 wt%, or 38 wt%.

[0051] In other embodiments, the acidic catalyst used in the preparation of acetalized lignin may be a strong acid such as sulfuric acid or trifluoromethanesulfonic acid.

[0052] In other embodiments, the reaction temperature for preparing acetalized lignin can be 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, or 85°C.

[0053] In some other embodiments, the reaction time for preparing acetalized lignin can be 4.5h, 5h, 5.5h, or 6h.

[0054] In some other embodiments, the solvent used for the reaction of alkali lignin with dialdehyde can be a eutectic solvent, such as a basic ternary DES like sodium hydroxide-ethylene glycol-water or FeCl3-polyethylene glycol 400-choline chloride.

[0055] In some other embodiments, the drying temperature of acetalized lignin can be 55°C, 56°C, 57°C, 58°C, or 59°C.

[0056] In some other embodiments, the molar ratio of acetalized lignin to 3-(methylamino)propylamine can be 1:1.

[0057] In some other embodiments, the catalyst used in preparing the lignin-based dynamic crosslinking agent may be iodine or nickel chloride.

[0058] In other embodiments, the reaction temperature for preparing the lignin-based dynamic crosslinking agent can be 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, or 84°C.

[0059] In some other embodiments, the reaction time for preparing the lignin-based dynamic crosslinking agent can be 25h, 26h, 27h, 28h, 29h, 30h, 31h, 32h, 33h, 34h, 35h, or 36h.

[0060] In some other embodiments, the solvent used for the reaction of acetalized lignin with 3-(methylamino)propylamine can be a eutectic solvent, such as a basic ternary DES like sodium hydroxide-ethylene glycol-water or FeCl3-polyethylene glycol 400-choline chloride.

[0061] In some other embodiments, the drying temperature of the lignin-based dynamic crosslinking agent can be 55°C, 56°C, 57°C, 58°C, or 59°C.

[0062] In some other embodiments, when preparing the polyurethane adhesive, the amount of lignin-based dynamic crosslinking agent can be 0.5 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, or 5 wt.% of commercial polyurethane foam.

[0063] In some other embodiments, the catalyst used in preparing the polyurethane adhesive can be a Lewis acid or an organic base. The Lewis acid is at least one of dibutyltin dilaurate (DBTDL), iron acetylacetone, and zinc acetate; the organic base is 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and / or 1,8-diazabicyclo[5.4.0]undecene (DBU).

[0064] In some other embodiments, the amount of catalyst used in preparing the polyurethane adhesive may be 1 wt.%, 2 wt.%, 4 wt.%, 5 wt.%, 6 wt.%, 7 wt.%, 8 wt.%, 9 wt.%, or 10 wt.% of commercial polyurethane foam.

[0065] In some other embodiments, the polyurethane foam pulverization time can be 6 min, 7 min, 8 min, 9 min, or 10 min.

[0066] In other embodiments, the ball milling speed can be 500 rpm / min, 510 rpm / min, 520 rpm / min, 530 rpm / min, 540 rpm / min, 550 rpm / min, 560 rpm / min, 570 rpm / min, 580 rpm / min, 590 rpm / min, 610 rpm / min, 620 rpm / min, 630 rpm / min, 640 rpm / min, 650 rpm / min, 660 rpm / min, 670 rpm / min, or 680 rpm / min; the ball milling time can be 1 hour or 2 hours.

[0067] In other embodiments, the temperature for hot pressing the recycled polyurethane sheet can be 150°C, 155°C, 160°C, 165°C, 175°C, 180°C, 85°C, 190°C, 195°C, or 200°C; the hot pressing pressure can be 25MPa, 26MPa, 27MPa, 28MPa, 29MPa, 31MPa, 32MPa, 33MPa, 34MPa, or 35MPa; and the hot pressing time can be 30min, 35min, 40min, 50min, 55min, or 60min.

[0068] Comparative Example 1 Step 1: Cut the commercial polyurethane foam into 1cm pieces 3 Weigh out 300g of the cube-shaped pieces and place them in a small grinder. Cover the grinder and grind for 5 minutes. Remove and set aside.

[0069] Step 2: Weigh 20g of the powder obtained in Step 1, place it in a zirconia ball mill jar, add zirconia beads, and place it in a planetary ball mill for 1.5h at a speed of 600rpm / min. After the ball milling is completed, ultrafine polyurethane powder is obtained and taken out for use.

[0070] Step 3: Weigh 3g of the ultrafine polyurethane powder obtained in Step 2, place it in a hot press at 170℃ for 45 minutes at a pressure of 30MPa, and then cool it to room temperature to obtain recycled polyurethane sheet, denoted as PUF sheet.

[0071] Comparative Example 2 Step 1: Cut the commercial polyurethane foam into 1cm pieces 3 Weigh out 300g of the cube-shaped pieces and place them in a small grinder. Cover the grinder and grind for 5 minutes. Remove and set aside.

[0072] Step 2: Weigh 20g of the powder obtained in Step 1, add 3wt.% of the powder mass of organic base catalyst and place it in a zirconia ball mill jar, add zirconia beads, and place it in a planetary ball mill for 1.5h at a speed of 600rpm / min. After ball milling, ultrafine polyurethane powder with a particle size of about 100μm is obtained. Take it out for later use and mark it as PT-0 powder adhesive.

[0073] Step 3: Weigh 0.2 g of the ultrafine polyurethane powder obtained in Step 2, place it in a custom-made metal sheet (100 mm long, 25 mm wide, and 2 mm thick), with an overlap area of ​​25 mm × 12.5 mm, and then place it in a hot press for 30 min, maintaining the temperature at 170℃ and the pressure at 30 MPa. The resulting overlapping sheared part is denoted as PT-0 overlapping sheared part.

[0074] Step 4: Weigh 3g of the ultrafine polyurethane powder obtained in Step 2, place it in a hot press at 170℃ for 45 minutes at a pressure of 30MPa, and then cool it to room temperature to obtain a recycled polyurethane sheet, denoted as PT-0 sheet.

[0075] Comparative Example 3 Step 1: Cut the commercial polyurethane foam into 1cm pieces 3 Weigh out 300g of the cube-shaped pieces and place them in a small grinder. Cover the grinder and grind for 5 minutes. Remove the pieces and set aside.

[0076] Step 2: Weigh 20g of the powder obtained in Step 1, add 3wt.% organic base catalyst and 1wt.% pure lignin by weight of the powder, and place them in a zirconia ball mill jar. Add zirconia beads, place in a planetary ball mill and ball mill for 1.5h at a speed of 600rpm / min. After ball milling, the modified ultrafine polyurethane powder with a particle size of about 100μm is obtained. Take it out for later use.

[0077] Step 3: Weigh 3g of the modified ultrafine polyurethane powder obtained in Step 2, place it in a hot press at 170℃ for 45 minutes at a pressure of 30MPa, and then cool it to room temperature to obtain a recycled polyurethane sheet, denoted as PT-1Lig sheet.

[0078] Figure 3 and Figure 4 The FT-IR spectra and ¹H NMR analyses of the crosslinking agents and alkali lignin in Examples 1-2 are presented. Compared to the FT-IR spectrum of Lig, the spectrum of AL is at 1695 cm⁻¹. -1 A characteristic absorption peak appeared at [location missing]. This peak originated from the stretching vibration of the C=O group in another aldehyde group on the monomer TPAL that had not yet undergone acetal reaction with Lig, confirming the successful synthesis of AL. Furthermore, in the infrared spectrum of ALMA(C), a peak was observed at 1572 cm⁻¹. -1 NH stretching vibrations were observed nearby, and at 1126 cm⁻¹ -1 and 927 cm -1 The presence of characteristic absorption peaks for cyclic acetals indicates the simultaneous introduction of both acetal and acetal-amine bonds. ¹H NMR further confirmed these structures: AL showed a proton signal for an aldehyde matrix at 10.14 ppm; AL showed a proton signal for a cyclic acetal near 5.8 ppm; and ALMA(C) showed a proton signal for a cyclic acetal-amine near 4.38 ppm. These results collectively confirm the successful preparation of the crosslinking agent ALMA(C) of this invention.

[0079] Figure 5 The Fourier transform infrared spectra of the ultrafine polyurethane powders in Examples 1-2 and Comparative Examples 1-2 are shown. Figure 6 The Fourier transform infrared (FTIR) spectra of the recycled polyurethane sheets from Examples 1-2 and Comparative Examples 1-2 are shown. The spectra of all sheets and powders are obtained by passing through a 1600 cm⁻¹ region. -1 The C=C bonds on the benzene ring were normalized to compare changes in different treatment groups. From Figure 5 It can be seen that at 3400 cm -1 The absorption peak found at 3313 cm⁻¹ is attributed to the -OH vibration, while the peak at 3313 cm⁻¹ is attributed to the -OH vibration. -1 and 1520 cm -1 The absorption peak found at 1724 cm⁻¹ corresponds to the NH vibration, and at 1724 cm⁻¹... -1 C=O from a urethane bond was found at 1225 cm⁻¹. -1 At this location, a distinct and strong absorption band was observed, corresponding to the absorption band of the ether group (COC) in the polyether polyurethane, marking a characteristic peak of the polyether polyol. (Comparison) Figure 5 and Figure 6 It can be observed that the main chemical structure of the cured sheet remains unchanged, providing a basis for its multi-processing properties. However, [the text abruptly shifts to a seemingly unrelated topic:] ...appearing in the powder at 2260–2280 cm... -1The isocyanate peak at the point disappeared after processing, indicating that the residual isocyanate groups in the commercial polyurethane foam reacted with the crosslinking agent during reprocessing.

[0080] Recycled polyurethane sheet cyclic processing tensile test The recycling performance of the recycled polyurethane sheets from each embodiment and comparative example was studied. Using the same treatment method, without adding any additional catalysts or crosslinking agents, the already hot-pressed recycled polyurethane sheets were pulverized and ball-milled again, and then subjected to the same hot-pressing process to obtain secondary recycled sheets. The process is as follows: Figure 1 As shown in (a), tensile properties of recycled polyurethane sheets and secondary recycled sheets were tested. The sheet tensile tests were performed using an MTSE44.104 electronic universal testing machine. Test conditions: tensile rate of 10 mm / min. -1 The data acquisition frequency was 100Hz. Before testing, the samples needed to be cut into thin strips of 30 mm × 10 mm × 1 mm. The tensile strength, elongation at break, and reprocessing efficiency of the final samples were the average of at least three parallel samples. The reprocessing efficiency was calculated using the following formulas: Tensile strength reprocessing efficiency = (Tensile strength of reprocessed material / Tensile strength of original material) × 100%; Elongation at break reprocessing efficiency = (Elongation at break of reprocessed material / Elongation at break of original material) × 100%. The test results are shown in Table 1 and... Figures 7-9 .

[0081]

[0082] After crushing, ball milling and hot pressing, recycled polyurethane sheet fragments can be completely reconstructed into uniform and complete sheets with no obvious defects or structural damage on the surface, proving the feasibility of dynamic thermochemical multiple-cycle recycling. Figure 7 The results show that the tensile properties of PT-1ALMA(C) sheet are significantly higher than those of PT-0 sheet, indicating that the recycled polyurethane sheet obtained in this invention has excellent processing properties. Table 1 and Figures 7-9 The results show that systems with different dynamic bond structures exhibit differentiated secondary reprocessing efficiencies: PT-1AL sheets (containing a single cyclic acetal bond) have a simple structure and fewer side reactions, achieving a reprocessing efficiency of 96%; while PT-1ALMA(C) sheets (containing cyclic acetal bonds and cyclic acetal-amine bonds) exhibit unique performance enhancement, with a reprocessing efficiency exceeding 100%. This is presumably because the cyclic acetal-amine bonds can undergo ring-opening rearrangement or form new crosslinking points with adjacent groups under hot-pressing conditions, thereby compensating for performance losses caused by degradation and even achieving performance surpassing. These results demonstrate that this recycled polyurethane sheet has excellent reprocessing efficiency in secondary physical recycling, with the cyclic acetal-amine bonds achieving a unique "the more it's processed, the stronger it becomes" effect, showing promising prospects for recycling applications.

[0083] Adhesive performance testing Taking the bonding of stainless steel as an example, the performance of the adhesive was evaluated by testing the shear strength under uniaxial tensile strain. The test method was as shown in the national standard GB / T7124-2008. The specific polyurethane powder adhesive properties obtained are shown in Table 2 and... Figure 10 .

[0084]

[0085] Table 2 shows the lap shear strength of the adhesives and PT-0 powder of Examples 1 and 2 on various substrates such as stainless steel, copper, aluminum, and wood. Notably, the PT-1ALMA(C) powder adhesive system exhibited the best adhesion performance on all tested metal substrates, especially on stainless steel surfaces, where the lap shear strength reached 15.36 ± 0.92 MPa, significantly superior to other samples. This excellent adhesion performance can be attributed to the unique molecular structure of the ALMA(C) crosslinking agent, which is rich in cyclic acetal and cyclic acetal-amine bonds, introducing a large number of highly electronegative N and O atoms. The lone pairs of electrons carried by these atoms can form stable coordinate bonds with the exposed metal atoms on the metal substrate surface, and simultaneously form a high-density hydrogen bond network with the oxide layer and hydroxyl groups on the metal surface. This synergistic effect of coordinate bonds and hydrogen bonds constructs a strong chemical-physical interfacial bonding layer between the adhesive and the metal substrate, greatly enhancing the interfacial interaction force, thus endowing the PT-1ALMA(C) powder adhesive with the best adhesion performance among all metal substrates. Similarly, the N and O in PT-1ALMA(C) powder adhesive can also form a large number of hydrogen bonds with glass and wood substrates. Therefore, PT-1ALMA(C) powder adhesive exhibits strong adhesion to a variety of substrates, demonstrating its broad-spectrum adhesive properties.

[0086] Chemical corrosion resistance test The PT-1ALMA(C) lap-jointed sheared parts from Example 2 were immersed in acidic solution (pH=3), alkaline solution (pH=12), and artificial seawater (20 wt.% NaCl) for 24 hours. Their adhesive strength was then tested, and the results are shown in the table below. Figure 11 As shown.

[0087]

[0088] From Table 3 and Figure 11It can be seen that after the PT-1ALMA(C) lap shear sample was soaked in acidic solution (pH=3), alkaline solution (pH=12) and artificial seawater (20 wt.% NaCl) for 24 hours, its bonding strength remained stable at over 8 MPa, demonstrating excellent chemical corrosion resistance and ensuring its reliable application under complex working conditions.

[0089] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

[0090] The above descriptions are merely some embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a lignin-based dynamic crosslinking agent, characterized in that, Includes the following steps: S1: Using alkali lignin and dialdehyde as raw materials, an acetalization reaction is carried out under the action of an acidic catalyst to obtain acetalized lignin. S2: The acetalized lignin is reacted with 3-(methylamino)propylamine under the catalysis of the first catalyst to obtain a lignin-based dynamic crosslinking agent containing acetal bonds and acetal-amine bonds.

2. The method for preparing the lignin-based dynamic crosslinking agent according to claim 1, characterized in that, In step S1, the molar ratio of alkali lignin to dialdehyde is 1:5 to 11.

3. The method for preparing the lignin-based dynamic crosslinking agent according to claim 1, characterized in that, In step S2, the molar ratio of the aldehyde group in acetalized lignin to the amino group in 3-(methylamino)propylamine is 1:1 to 1.

05.

4. The method for preparing the lignin-based dynamic crosslinking agent according to claim 1, characterized in that, The dialdehyde is at least one of terephthalaldehyde, o-phthalaldehyde, iso-phthalaldehyde, 2,5-dihydroxyterephthalaldehyde, 2,5-bis(alkoxy)terephthalaldehyde, glyoxal, glutaraldehyde, and succinaldehyde.

5. The method for preparing the lignin-based dynamic crosslinking agent according to claim 1, characterized in that, The acidic catalyst is at least one of hydrochloric acid, sulfuric acid, and trifluoromethanesulfonic acid.

6. The method for preparing the lignin-based dynamic crosslinking agent according to claim 1, characterized in that, The first catalyst is at least one of p-toluenesulfonic acid, iodine, and nickel chloride.

7. A lignin-based dynamic crosslinking agent, characterized in that, Prepared by the method according to any one of claims 1 to 6.

8. The application of the lignin-based dynamic crosslinking agent according to claim 7 in the preparation of polyurethane foam adhesives.

9. A polyurethane adhesive, characterized in that, It is prepared by mixing powder obtained from commercial polyurethane foam pulverization, a second catalyst, and the lignin-based dynamic crosslinking agent as described in claim 7.

10. A recycled polyurethane sheet, characterized in that, It is prepared by hot pressing of the polyurethane adhesive according to claim 9.

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