Preparation process of environment-friendly polyurethane adhesive

Abstract

The invention discloses a preparation process of an environment-friendly polyurethane adhesive, and relates to the technical field of sustainable materials, the environment-friendly polyurethane adhesive comprises castor oil-based polyol, a furyl-containing isocyanate compound, a first chain extender, a second chain extender, nano cellulose and a silane coupling agent; the castor oil-based polyol is prepared from the following components in parts by weight: 65 to 75 parts of castor oil-based polyol; 18 to 22 parts of a furyl-containing isocyanate compound; a first chain extender (castor oil amine) accounting for 10-15 parts of the mass of the prepolymer; a second chain extender (bismaleimide compound) which accounts for 5-10 parts of the mass of the prepolymer; 0.3 to 0.8 part of nano cellulose; 1.5 to 2.5 parts of a silane coupling agent; bio-based raw materials such as castor oil-based polyol are adopted to replace traditional petroleum-based raw materials, dependence on fossil resources is greatly reduced, the green chemistry concept is met, unreacted monomers and by-products are removed through the supercritical CO2 fluid technology, use of organic solvents in a traditional process is avoided, emission of volatile organic compounds is effectively reduced, and the method is suitable for industrial production. And environmental and health risks are reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of sustainable materials, and particularly relates to a preparation process of an environmentally friendly polyurethane adhesive. Background Art

[0002] Polyurethane adhesives are widely used in fields such as automotive, construction, and packaging due to their excellent adhesion performance, flexibility, and durability. However, traditional polyurethane adhesives rely on petroleum-based raw materials, and their permanent cross-linked network makes the materials non-recyclable. Moreover, a large amount of organic solvents are used in the preparation process, resulting in the emission of volatile organic compounds, which pose a threat to the environment and human health. In recent years, various preparation methods of environmentally friendly polyurethanes have been proposed, such as bio-based polyurethanes and degradable polyurethanes. Bio-based polyurethanes use vegetable oils or lignin as raw materials, which reduces the dependence on fossil resources but has poor mechanical properties and heat resistance. Degradable polyurethanes achieve degradation by introducing hydrolyzable bonds, but the degradation rate is difficult to control, and the degradation products may cause secondary pollution to the environment. In addition, organic solvents are still commonly used in existing processes, the problem of VOC emissions has not been completely solved, and the de-crosslinking temperature of the dynamic cross-linked system is relatively high, and the performance decreases significantly after multiple recoveries, which limits their practical applications. Therefore, we propose a preparation process of an environmentally friendly polyurethane adhesive. Summary of the Invention

[0003] To solve the above technical problems, the present invention is realized through the following technical solutions:

[0004] The present invention is a preparation process of an environmentally friendly polyurethane adhesive, including castor oil-based polyol, furan group-containing isocyanate compound, first chain extender, second chain extender, nanocellulose, and silane coupling agent;

[0005] By weight, wherein the castor oil-based polyol: 65 - 75 parts;

[0006] Furan group-containing isocyanate compound: 18 - 22 parts;

[0007] First chain extender (castor oil amine): 10 - 15 parts of the prepolymer mass;

[0008] Second chain extender (bismaleimide compound): 5 - 10 parts of the prepolymer mass;

[0009] Nanocellulose: 0.3 - 0.8 parts;

[0010] Silane coupling agent: 1.5 - 2.5 parts.

[0011] Further, the preparation process includes the following steps;

[0012] Step S1: React the castor oil-based polyol with furan group-containing MDI to prepare a prepolymer containing dynamic bond precursors;

[0013] Step S2: Adopt a gradient chain extension strategy to form flexible chain segments by chain extension with castor oil amine, introduce bismaleimide for dynamic crosslinking, and construct a reversible network structure;

[0014] Step S3: Utilize the high diffusivity and low surface tension of supercritical CO2 to remove unreacted monomers and low molecular weight by-products;

[0015] Step S4: Trigger the reversible reaction of dynamic bonds through heat treatment to achieve the reconstruction of the crosslinked network;

[0016] Step S5: Improve the peel strength and interfacial bonding force of the material by adding nanocellulose and silane coupling agent.

[0017] Furthermore, the step S1 includes the following steps:

[0018] Step S11: Dehydrate the bio-based polyol under vacuum conditions at 75 - 85 °C for 0.5 - 1.5 hours, control the moisture content ≤ 0.03 wt%; add the furan group-containing isocyanate compound to the dehydrated polyol in batches. The furan group content in the isocyanate compound is 10 - 15 mol%. Carry out a prepolymerization reaction at 60 - 70 °C under nitrogen protection. During the reaction process, monitor the isocyanate group content in real time by Fourier transform infrared spectroscopy. When the NCO content reaches 6.5 - 7.5%, terminate the reaction to obtain a prepolymer containing dynamic bond precursors;

[0019] Step S12: The bio-based polyol is a castor oil derivative with a hydroxyl value of 250 - 300 mgKOH / g;

[0020] Step S13: The preparation method of the furan group-containing isocyanate compound includes: reacting 4,4'-diphenylmethane diisocyanate with furfuryl alcohol in a molar ratio of 1:0.8 - 1.2 at 60 - 80 °C for 2 - 4 hours to obtain a modified 4,4'-diphenylmethane diisocyanate with a furan group substitution rate of 20 - 30%.

[0021] Furthermore, the step S2 includes the following steps:

[0022] Step S21: Cool the prepolymer to 35 - 45°C, add the first chain extender, which is a castor oil amine compound with an amine value of 280 - 350 mgKOH / g, and the addition amount is 10 - 15% of the prepolymer mass. React for 20 - 40 minutes at a stirring rate of 200 - 300 rpm; then raise the temperature to 55 - 65°C, add the second chain extender, which is a bismaleimide compound, and the addition amount is 5 - 10% of the prepolymer mass, and increase the stirring rate to 400 - 600 rpm, and react for 30 - 60 minutes; finally, raise the temperature to 85 - 95°C and maintain it for 10 - 20 minutes under a vacuum of -0.07 to -0.09 MPa to trigger the Diels - Alder reaction between the furanyl group and the maleimide group to form a dynamic cross - linked network;

[0023] Step S22: The gradient chain - extension reaction is divided into three stages;

[0024] Step S221, the first stage: Conduct amino chain - extension at 40 ± 2°C and 200 - 300 rpm;

[0025] Step S222, the second stage: Conduct maleimide cross - linking at 60 ± 2°C and 500 ± 50 rpm;

[0026] Step S223, the third stage: Trigger dynamic bonding through temperature jump in a vacuum environment;

[0027] The bismaleimide compound is 4 - maleimidophenylalkane, where the alkane chain length is C3 - C6.

[0028] Furthermore, the following steps are included in Step S3:

[0029] Step S31: Transfer the chain - extended product to a supercritical extraction device, introduce supercritical CO2 fluid, control the pressure to be 12 - 18 MPa, the temperature to be 40 - 50°C, the CO2 flow rate to be 5 - 10 L / min, and process for 1.5 - 2.5 hours; gradually reduce the pressure from the initial pressure to below 5 MPa through a step - by - step pressure - reduction program, and the pressure - reduction gradient is 3 - 5 MPa per stage, and each stage interval is 5 - 15 minutes;

[0030] Step S32: The pressure curve of the supercritical CO2 treatment satisfies the following relationship;

[0031] P(t) = P0 - k×t2;

[0032] In the formula, P0 is the initial pressure of 12 - 18 MPa, P(t) is the pressure value at time t, and k is the pressure - reduction rate coefficient of 0.2 - 0.5 MPa / min2;

[0033] Step S33: During the pressure reduction process, on-line gas chromatography analysis is synchronously implemented to control the residual monomer content ≤ 0.1 wt%.

[0034] Furthermore, the step S4 includes the following steps:

[0035] Step S41: Place the devolatilized material in a mold, heat it to 110 - 130 °C and hold for 15 - 25 minutes to dissociate the dynamic crosslinking network; then mold it under a pressure of 5 - 10 MPa for a holding time of 10 - 20 minutes, and cool it to 30 - 50 °C at a rate of 1 - 3 °C / min using a programmed cooling method to complete the reconstruction of the dynamic crosslinking network;

[0036] Step S42: During the thermo-reconstitution molding process, the dissociation and reconstruction of the dynamic crosslinking network are monitored in real time by differential scanning calorimetry to ensure that the dissociation peak temperature is 110 - 130 °C and the reconstruction peak temperature is 80 - 100 °C;

[0037] Step S43: The surface of the mold is pretreated with a silane coupling agent. The pretreatment method is: soak the mold in a 1 - 2 wt% γ-aminopropyltriethoxysilane ethanol solution for 30 - 60 minutes, and then dry it at 80 - 100 °C for 1 - 2 hours.

[0038] Furthermore, the step S5 includes the following steps:

[0039] Step S51: Add 0.3 - 0.8 wt% of nanocellulose to the reconstituted polyurethane matrix. The aspect ratio of the nanocellulose is 40 - 60, and after being dispersed in ethanol, it is ultrasonically treated for 20 - 40 minutes; at the same time, add 1.5 - 2.5 wt% of a silane coupling agent and blend them at a rate of 200 - 400 rpm at 50 - 70 °C for 20 - 40 minutes to obtain the final product;

[0040] Step S52: The silane coupling agent is a mixture of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane, and the mixing mass ratio is 1:0.5 - 2;

[0041] Step S53: The blending process uses a twin-screw extruder, and the barrel temperature is controlled in sections as follows: zone 1 is 50 - 60 °C, zone 2 is 70 - 80 °C, and zone 3 is 90 - 100 °C.

[0042] The present invention has the following beneficial effects:

[0043] 1. The present invention uses bio-based raw materials such as castor oil-based polyols to replace traditional petroleum-based raw materials, significantly reducing the dependence on fossil resources, conforming to the concept of green chemistry. By using supercritical CO2 fluid technology to remove unreacted monomers and by-products, the use of organic solvents in traditional processes is avoided, effectively reducing the emission of volatile organic compounds and reducing environmental and health risks. Moreover, the design of the dynamic crosslinked network endows the material with reversible crosslinking characteristics, and through heat treatment, de-crosslinking and reconstitution can be achieved, endowing the adhesive with excellent recyclability, solving the problem of non-recyclability caused by permanent crosslinking of traditional polyurethanes, and significantly enhancing the sustainability of the material throughout its life cycle.

[0044] 2. Through the gradient chain extension strategy and the bismaleimide dynamic crosslinking technology, the present invention constructs a network structure with both flexibility and rigidity, enabling the adhesive to exhibit excellent performance in terms of peel strength, tensile properties and heat resistance. The introduction of nanocellulose further enhances the interfacial bonding force and mechanical strength through the nano-enhancement effect, overcoming the defect of insufficient mechanical properties of traditional bio-based polyurethanes. At the same time, supercritical CO2 treatment precisely removes low-molecular-weight impurities, reduces material defects, and combined with the reversible reaction characteristics of dynamic bonds, ensures that the performance does not significantly decay after multiple recycling, meeting the requirements of industrial applications for durability.

[0045] 3. The dynamic crosslinked network of the present invention realizes the reversible switching between crosslinking and de-crosslinking through the Diels-Alder reaction. The de-crosslinking temperature and reconstitution temperature are precisely regulated by differential scanning calorimetry, enabling the controlled degradation rate of the material and avoiding the secondary pollution problem caused by uncontrollable degradation products of traditional degradable polyurethanes. The synergistic effect of silane coupling agent and nanocellulose further stabilizes the material interface, ensuring stable bonding performance after multiple cycles. This process not only extends the service life of the material, but also provides technical support for the recycling and reuse of waste adhesives.

[0046] Of course, it is not necessary for any product implementing the present invention to simultaneously achieve all the above-mentioned advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings without creative efforts based on these drawings.

[0048] Figure 1 It is a schematic flow chart of the preparation process of an environmentally friendly polyurethane adhesive of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0050] Please refer to Figure 1 As shown, the present invention is a preparation process of an environmentally friendly polyurethane adhesive, including castor oil-based polyol, furan-based isocyanate compound, first chain extender, second chain extender, nanocellulose and silane coupling agent;

[0051] By weight, among them, castor oil-based polyol: 65 - 75 parts;

[0052] Furan-based isocyanate compound: 18 - 22 parts;

[0053] First chain extender (castor oil amine): 10 - 15 parts of the prepolymer mass;

[0054] Second chain extender (bismaleimide compound): 5 - 10 parts of the prepolymer mass;

[0055] Nanocellulose: 0.3 - 0.8 parts;

[0056] Silane coupling agent: 1.5 - 2.5 parts.

[0057] The preparation process includes the following steps:

[0058] Step S1: React castor oil-based polyol with furan-based MDI to prepare a prepolymer containing dynamic bond precursors;

[0059] Step S2: Adopt a gradient chain extension strategy, form a flexible chain segment through chain extension with castor oil amine, introduce bismaleimide for dynamic crosslinking, and construct a reversible network structure;

[0060] Step S3: Utilize the high diffusivity and low surface tension of supercritical CO2 to remove unreacted monomers and low molecular weight by-products;

[0061] Step S4: Trigger the reversible reaction of dynamic bonds through heat treatment to realize the reconstruction of the crosslinked network;

[0062] Step S5: Improve the peel strength and interfacial bonding force of the material by adding nanocellulose and silane coupling agent.

[0063] Step S1 includes the following steps:

[0064] Step S11: Dehydrate the bio-based polyol under vacuum conditions at 75 - 85 °C for 0.5 - 1.5 hours, controlling the water content ≤ 0.03 wt%; Add the furan-based isocyanate compound to the dehydrated polyol in batches. The furan group content in the isocyanate compound is 10 - 15 mol%. Carry out a prepolymerization reaction at 60 - 70 °C under nitrogen protection. During the reaction process, monitor the isocyanate group content in real time by Fourier transform infrared spectroscopy. When the NCO content reaches 6.5 - 7.5%, terminate the reaction to obtain a prepolymer containing dynamic bond precursors;

[0065] Step S12: The bio-based polyol is a castor oil derivative with a hydroxyl value of 250 - 300 mgKOH / g;

[0066] The preparation method of the furan-based isocyanate compound includes: React 4,4'-diphenylmethane diisocyanate with furfuryl alcohol in a molar ratio of 1:0.8 - 1.2 at 60 - 80 °C for 2 - 4 hours to obtain a modified 4,4'-diphenylmethane diisocyanate with a furan substitution rate of 20 - 30%.

[0067] Step S2 includes the following steps:

[0068] Step S21: Cool the prepolymer to 35 - 45 °C, add the first chain extender. The first chain extender is a castor oil amine compound with an amine value of 280 - 350 mgKOH / g, and the addition amount is 10 - 15% of the prepolymer mass. React at a stirring rate of 200 - 300 rpm for 20 - 40 minutes; Subsequently, raise the temperature to 55 - 65 °C, add the second chain extender. The second chain extender is a bismaleimide compound, and the addition amount is 5 - 10% of the prepolymer mass. Increase the stirring rate to 400 - 600 rpm and react for 30 - 60 minutes; Finally, raise the temperature to 85 - 95 °C and maintain it for 10 - 20 minutes under a vacuum degree of -0.07 to -0.09 MPa to trigger the Diels - Alder reaction between the furan group and the maleimide group to form a dynamic cross - linked network;

[0069] Step S22: The gradient chain - extension reaction is divided into three stages;

[0070] Step S221, the first stage: Carry out amino chain - extension at 40 ± 2 °C and 200 - 300 rpm;

[0071] Step S222, the second stage: Carry out maleimide cross - linking at 60 ± 2 °C and 500 ± 50 rpm;

[0072] Step S223, the third stage: Trigger dynamic bonding through a temperature jump in a vacuum environment;

[0073] Step S23: The bismaleimide compound is 4 - maleimidophenylalkane, where the alkane chain length is C3 - C6.

[0074] Step S3 includes the following steps:

[0075] Step S31: Transfer the chain - extended product to a supercritical extraction device, introduce supercritical CO2 fluid, control the pressure at 12 - 18 MPa, the temperature at 40 - 50 °C, the CO2 flow rate at 5 - 10 L / min, and process for 1.5 - 2.5 hours; gradually reduce the pressure from the initial pressure to below 5 MPa through a step - down pressure program, with a pressure reduction gradient of 3 - 5 MPa per stage and an interval of 5 - 15 minutes between each stage;

[0076] Step S32: The pressure curve of the supercritical CO2 treatment satisfies the following relationship;

[0077] P(t) = P0 - k×t2;

[0078] In the formula, P0 is the initial pressure of 12 - 18 MPa, P(t) is the pressure value at time t, and k is the pressure reduction rate coefficient of 0.2 - 0.5 MPa / min2;

[0079] Step S33: Implement on - line gas chromatography analysis synchronously during the pressure reduction process to control the residual monomer content ≤ 0.1 wt%.

[0080] Step S4 includes the following steps:

[0081] Step S41: Place the devolatilized material in a mold, heat it to 110 - 130 °C and hold for 15 - 25 minutes to dissociate the dynamic cross - linked network; then mold it under a pressure of 5 - 10 MPa, with a holding pressure time of 10 - 20 minutes, and use a programmed cooling method to cool it to 30 - 50 °C at a rate of 1 - 3 °C / min to complete the reconstruction of the dynamic cross - linked network;

[0082] Step S42: During the hot - reconstitution molding process, the dissociation and reconstruction of the dynamic cross - linked network are monitored in real - time by differential scanning calorimetry to ensure that the dissociation peak temperature is 110 - 130 °C and the reconstruction peak temperature is 80 - 100 °C;

[0083] Step S43: The mold surface is pretreated with a silane coupling agent. The pretreatment method is: soak the mold in a 1 - 2 wt% ethanol solution of γ - aminopropyltriethoxysilane for 30 - 60 minutes, and then dry it at 80 - 100 °C for 1 - 2 hours.

[0084] Step S5 includes the following steps:

[0085] Step S51: Add 0.3 - 0.8 wt% of nanocellulose with an aspect ratio of 40 - 60 to the reconstructed polyurethane matrix. After dispersion in ethanol, perform ultrasonic treatment for 20 - 40 minutes. Meanwhile, add 1.5 - 2.5 wt% of silane coupling agent and blend at a rate of 200 - 400 rpm at 50 - 70 °C for 20 - 40 minutes to obtain the final product;

[0086] Step S52: The silane coupling agent is a mixture of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane, and the mixing mass ratio is 1:0.5 - 2;

[0087] Step S53: The blending process uses a twin-screw extruder, and the barrel temperature is controlled in sections as follows: Zone 1: 50 - 60 °C, Zone 2: 70 - 80 °C, Zone 3: 90 - 100 °C.

[0088] A specific application of this embodiment is:

[0089] Step 1: Prepare a prepolymer containing dynamic bond precursors

[0090] 1. Raw material preparation:

[0091] Bio-based polyol: A castor oil derivative with a hydroxyl value of 280 mg KOH / g;

[0092] Furan-based isocyanate compound: By reacting 4,4'-diphenylmethane diisocyanate with furfuryl alcohol in a molar ratio of 1:1 at 70 °C for 3 hours, a modified MDI with a furan substitution rate of 25% is prepared;

[0093] 2. Dehydration treatment:

[0094] Place the castor oil-based polyol in a vacuum drying oven and dehydrate it at 80 °C and a vacuum degree of -0.09 MPa for 1 hour, controlling the moisture content ≤ 0.03 wt%;

[0095] 3. Prepolymerization reaction:

[0096] Transfer the dehydrated polyol to a reaction kettle, and add the modified MDI in batches under nitrogen protection, controlling the reaction temperature at 65 °C;

[0097] Monitor the isocyanate group content in real-time by Fourier transform infrared spectroscopy, and terminate the reaction when the NCO content reaches 7.0% to obtain the prepolymer;

[0098] Step 2: Gradient chain extension and dynamic crosslinking

[0099] 1. The first stage:

[0100] Cool the prepolymer to 40 °C, add 12% of the castor oil amine based on the mass of the prepolymer, and react at a stirring rate of 250 rpm for 30 minutes;

[0101] 2. Second stage:

[0102] Heat up to 60 °C, add 4-maleimidophenyl propane which is 8% of the prepolymer mass, increase the stirring rate to 500 rpm, and react for 45 minutes;

[0103] 3. Third stage:

[0104] Heat up to 90 °C, maintain for 15 minutes under a vacuum of -0.08 MPa to trigger the Diels - Alder reaction between furyl and maleimide groups to form a dynamic cross - linked network;

[0105] Step three: Supercritical CO2 purification

[0106] Transfer the chain - extended product to a supercritical extraction device, introduce CO2 fluid, set the pressure at 15 MPa, temperature at 45 °C, and the CO2 flow rate at 8 L / min, and process for 2 hours;

[0107] Pressure reduction procedure: Reduce the pressure in stages, with a 4 - MPa reduction in each stage and a 10 - minute interval until the pressure drops to 4 MPa;

[0108] Control the residual monomer content ≤ 0.1 wt% through on - line gas chromatography analysis;

[0109] Step four: Reconstruction of the dynamic cross - linked network

[0110] Inject the devolatilized material into a mold pre - treated with γ - aminopropyltriethoxysilane on the surface, heat to 120 °C and hold for 20 minutes to dissociate the dynamic cross - linked network;

[0111] Mold and form at 8 MPa pressure, hold the pressure for 15 minutes, and then cool down to 40 °C at a rate of 2 °C / min by programmed cooling;

[0112] Real - time monitor the cross - link dissociation peak (125 °C) and reconstruction peak (90 °C) through differential scanning calorimetry;

[0113] Step five: Nano - reinforcement and interface optimization

[0114] 1. Additive blending:

[0115] Add 0.5 wt% of nanocellulose to the reconstructed polyurethane matrix, disperse it with ethanol and then ultrasonically treat for 30 minutes;

[0116] Add 2.0 wt% of silane coupling agent. The silane coupling agent is a 1:1 mixture of γ - aminopropyltriethoxysilane and γ - glycidoxypropyltrimethoxysilane, and blend at 60 °C at a rate of 300 rpm for 30 minutes;

[0117] 2. Extrusion Molding

[0118] Using a twin-screw extruder, model SHJ-30, the barrel temperature is controlled in sections: zone 1 at 55°C, zone 2 at 75°C, and zone 3 at 95°C, to complete the extrusion granulation of the final product.

[0119] In the description of this specification, the description referring to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0120] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, according to the content of this specification, many modifications and variations can be made. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A process for preparing an environmentally friendly polyurethane adhesive, characterized in that: It includes castor oil-based polyol, furanyl-containing isocyanate compound, a first chain extender, a second chain extender, nanocellulose and a silane coupling agent; In parts by weight, castor oil-based polyol: 65-75 parts; Furanyl isocyanate compound: 18-22 parts; The first chain extender (castor oil amine): 10-15 parts by weight of the prepolymer; Second chain extender (bismaleimide compound): 5-10 parts by weight of prepolymer; Nanocellulose: 0.3-0.8 parts; Silane coupling agent: 1.5-2.5 parts.

2. A process for preparing an environmentally friendly polyurethane adhesive, characterized in that: The preparation process comprises the following steps: Step S1: adding the dehydrated castor oil-based polyol to the furanyl-containing isocyanate compound to prepare a prepolymer containing a dynamic bond precursor; Step S2: adopting a gradient chain extension strategy, forming a flexible chain segment through chain extension of castor oil amine, introducing bismaleimide for dynamic cross-linking, and constructing a dynamic cross-linking network structure; Step S3: utilizing the high diffusivity and low surface tension of supercritical CO2 to remove unreacted monomers and low molecular weight byproducts; Step S4: triggering the reversible reaction of the dynamic bonds through heat treatment to achieve reconstruction of the cross-linked network; Step S5: By adding nanocellulose and silane coupling agent, the peel strength and interface bonding force of the material are increased.

3. The process for preparing an environmentally friendly polyurethane adhesive according to claim 1, characterized in that: The step S1 includes the following steps: Step S11: dehydrating the bio-based polyol at 75-85° C. for 0.5-1.5 hours under vacuum conditions, and controlling the moisture content to be ≤0.03wt%; adding a furanyl-containing isocyanate compound to the dehydrated polyol in batches, wherein the furanyl group content in the isocyanate compound is 10-15mol%, and performing a prepolymerization reaction at 60-70° C. under nitrogen protection, and monitoring the isocyanate group content in real time by Fourier transform infrared spectroscopy during the reaction, and terminating the reaction when the NCO content reaches 6.5-7.5%, thereby obtaining a prepolymer containing a dynamic bond precursor; Step S12: the bio-based polyol is a castor oil derivative with a hydroxyl value of 250-300 mgKOH / g; Step S13: The method for preparing the furanyl isocyanate compound comprises: reacting 4,4'-diphenylmethane diisocyanate and furan methanol at a molar ratio of 1:0.8-1.2 at 60-80°C for 2-4 hours to obtain modified 4,4'-diphenylmethane diisocyanate with a furanyl substitution rate of 20-30%.

4. The process for preparing an environmentally friendly polyurethane adhesive according to claim 1, characterized in that: The step S2 includes the following steps: Step S21: cooling the prepolymer to 35-45° C., adding a first chain extender, wherein the first chain extender is a castor oil amine compound with an amine value of 280-350 mgKOH / g, and the addition amount is 10-15% of the mass of the prepolymer, and reacting for 20-40 minutes at a stirring rate of 200-300 rpm; then heating to 55-65° C., adding a second chain extender, wherein the second chain extender is a bismaleimide compound, and the addition amount is 5-10% of the mass of the prepolymer, and the stirring rate is increased to 400-600 rpm, and reacting for 30-60 minutes; finally heating to 85-95° C., maintaining for 10-20 minutes under a vacuum degree of -0.07 to -0.09 MPa, triggering a Diels-Alder reaction between the furan group and the maleimide group to form a dynamic cross-linking network; Step S22: the gradient chain extension reaction is divided into three stages; Step S221, first stage: performing amino chain extension at 40±2°C and 200-300rpm; Step S222, second stage: performing maleimide cross-linking at 60±2°C and 500±50rpm; Step S223, the third stage: triggering dynamic bonding by temperature jump in a vacuum environment; Step S23: The bismaleimide compound is 4-maleimidephenylalkane, wherein the alkane chain length is C3-C6.

5. The process for preparing an environmentally friendly polyurethane adhesive according to claim 1, characterized in that: The step S3 includes the following steps: Step S31: transferring the chain-extended product to a supercritical extraction device, introducing supercritical CO2 fluid, controlling the pressure to 12-18 MPa, the temperature to 40-50°C, the CO2 flow rate to 5-10 L / min, and treating for 1.5-2.5 hours; gradually reducing the pressure from the initial pressure to below 5 MPa through a staged depressurization procedure, with a depressurization gradient of 3-5 MPa per stage, and an interval of 5-15 minutes between each stage; Step S32: The pressure curve of supercritical CO2 treatment satisfies the following relationship: P(t) = P0-k×t2; Wherein, P0 is the initial pressure 12-18MPa, P(t) is the pressure value at time t, and k is the pressure reduction rate coefficient 0.2-0.5MPa / min2; Step S33: During the depressurization process, online gas chromatography analysis is simultaneously performed to control the residual monomer content to ≤ 0.1 wt%.

6. The process for preparing an environmentally friendly polyurethane adhesive according to claim 1, characterized in that: The step S4 includes the following steps: Step S41: placing the devolatilized material in a mold, heating it to 110-130°C and maintaining it for 15-25 minutes to dissociate the dynamic cross-linking network; then, compression molding is performed at a pressure of 5-10MPa for 10-20 minutes, and programmed cooling is performed at a rate of 1-3°C / min to cool it to 30-50°C to complete the reconstruction of the dynamic cross-linking network; Step S42: During the thermal reconstruction molding process, the dissociation and reconstruction of the dynamic cross-linked network are monitored in real time by differential scanning calorimetry to ensure that the dissociation peak temperature is 110-130° C. and the reconstruction peak temperature is 80-100° C.; Step S43: the surface of the mold is pretreated with a silane coupling agent, wherein the pretreatment method is: immersing the mold in a 1-2 wt % γ-aminopropyltriethoxysilane ethanol solution for 30-60 minutes, and then drying at 80-100° C. for 1-2 hours.

7. The process for preparing an environmentally friendly polyurethane adhesive according to claim 1, characterized in that: The step S5 includes the following steps: Step S51: adding 0.3-0.8wt% of nanocellulose, wherein the aspect ratio of the nanocellulose is 40-60, to the reconstructed polyurethane matrix, dispersing the nanocellulose with ethanol, and then ultrasonically treating the nanocellulose for 20-40 minutes; adding 1.5-2.5wt% of a silane coupling agent, and blending the nanocellulose at 50-70°C and 200-400rpm for 20-40 minutes to obtain a final product; Step S52: the silane coupling agent is a mixture of γ-aminopropyltriethoxysilane and γ-glycidyloxypropyltrimethoxysilane, with a mixing mass ratio of 1:0.5-2; Step S53: The blending process uses a twin-screw extruder, and the barrel temperature is controlled in sections: 50-60°C in zone 1, 70-80°C in zone 2, and 90-100°C in zone 3.

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