Low-free-monomer waterborne polyurethane adhesive and preparation method thereof

Abstract

The invention discloses a low-free-monomer waterborne polyurethane adhesive and a preparation method thereof, and relates to the technical field of adhesive preparation, the low-free-monomer waterborne polyurethane adhesive comprises a main chain structure composition, a functional additive, a reaction control and optimization additive and an auxiliary reagent, the main chain structure composition comprises low-monomer residual diisocyanate and polymer polyol, and the functional additive comprises a low-free-monomer residual diisocyanate and a low-free-monomer residual diisocyanate; by introducing a flexible chain segment and optimizing an emulsification process, an adhesive film has better hydrophobicity and a microcosmic phase separation structure, the water soaking resistance is greatly improved, the adhesive film has outstanding water resistance, a unique formula design ensures that an adhesive layer cannot be subjected to strength attenuation due to aging in a long-term use process, the bonding durability is excellent, and the adhesive film has a good application prospect. The preparation method has the advantages of reasonable process design, mild and controllable reaction conditions, good process applicability and good storage stability, and through stabilization treatment, the obtained emulsion has the advantages of uniform particle size, good mechanical stability and storage stability, and long shelf life.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of adhesive preparation, in particular to a low-free monomer waterborne polyurethane adhesive and a preparation method thereof. BACKGROUND

[0002] Polyurethane adhesive is widely used in artificial board processing, woodworking furniture, automotive interior, textile composite and other fields due to its excellent bonding strength, toughness, wear resistance and low temperature resistance. Although the traditional solvent-based polyurethane adhesive has excellent performance, it releases a large amount of volatile organic compounds (VOC) during production and construction, which is harmful to the environment and human health. Therefore, waterborne polyurethane (WPU) adhesive as an environmentally friendly product has become the main direction of industry development. However, the existing waterborne polyurethane adhesive still faces some key technical bottlenecks, specifically as follows: First, the problem of free isocyanate monomer residues: during synthesis, incompletely reacted diisocyanate monomers (such as TDI, MDI, HDI, etc.) remain in the product in free form. These free monomers are highly toxic and volatile, which not only harms the health of production personnel and consumers (such as irritating the respiratory tract and having a carcinogenic risk), but also makes it difficult for the product to meet the increasingly stringent environmental requirements. Second, the contradiction between performance and environmental protection: in order to reduce the content of free monomers, conventional methods such as increasing the reaction temperature, prolonging the reaction time or excessive dilution often lead to a wide molecular weight distribution of the prepolymer, uncontrolled viscosity, or increased side reactions affecting the mechanical properties and stability of the final product. At the same time, simply pursuing low VOC and low free monomer often sacrifices the heat resistance, water resistance and initial / long-term bonding strength of the adhesive. Third, the problem of storage stability: the terminal -NCO groups in the waterborne polyurethane system are unstable in the aqueous phase and prone to side reactions, resulting in short storage period and rapid performance degradation. Therefore, it is a technical problem to be solved in the field to develop a waterborne polyurethane adhesive that can effectively reduce the content of free monomers while having excellent comprehensive performance and high environmental safety. SUMMARY

[0003] The present application provides a low-free monomer waterborne polyurethane adhesive and a preparation method thereof, which can effectively solve the problems of free isocyanate monomer residues, performance and environmental protection contradiction and storage stability not being guaranteed in the background art.

[0004] To achieve the above purpose, the present application provides the following technical solution: a low-free monomer waterborne polyurethane adhesive, comprising a main chain structure composition, a functional additive, a reaction control and optimization additive and an auxiliary reagent. The main chain structure composition includes low monomer residual diisocyanate and polymer polyol; The low monomer residual diisocyanate adopts special HDI with ultra-low free monomer content, and the free HDI monomer content is less than 0.1%; The polymer polyol is specifically polybutylene adipate glycol PBA and polytetramethylene glycol PTMG; The functional additives include hydrophilic chain extenders and small molecule chain extenders; The hydrophilic chain extender is specifically dimethylol propionic acid DMPA, which provides a hydrophilic group carboxyl group, so that the prepolymer is stably dispersed in water to form a water-based system. The small molecule chain extender is specifically 1,4-butanediol, which adjusts the length of the molecular chain segment. The reaction control and optimization additives include high-activity capping agents, multi-functional cross-linking agents, and end group capturing agents. The high-activity capping agent is specifically methyl ethyl ketone oxime MEKO and caprolactam, which quickly seals the terminal -NCO group of the prepolymer. The multi-functional cross-linking agent is specifically trimethylolpropane TMP and glycerol, which forms a three-dimensional cross-linked network during film formation. The end group capturing agent is a small molecule amine compound, specifically diethylamine and ethanolamine, which specifically removes trace amounts of free -NCO groups and isocyanate monomers in the system. The auxiliary reagents include neutralizing agents, catalysts, and emulsifying water. The neutralizing agent is triethylamine TEA, which neutralizes the carboxyl group to form a water-soluble salt according to the molar amount of DMPA. The catalyst is dibutyltin dilaurate DBTDL, which regulates the rate of prepolymerization. The emulsifying water is deionized water, which is used as a dispersion medium to form a water-based polyurethane dispersion.

[0005] According to the above technical solution, the amount of low monomer residual diisocyanate is 25g, which provides isocyanate groups -NCO as the hard segment skeleton of the polyurethane molecular chain.

[0006] According to the above technical solution, the polybutylene adipate glycol PBA has an Mn of 1000, and the amount thereof is 150g, which introduces a rigid polyester chain segment. The polytetramethylene glycol PTMG has an Mn of 2000, and the amount thereof is 200g, which provides a flexible polyether chain segment.

[0007] According to the above technical solution, the amount of dimethylol propionic acid DMPA is 12g, and the amount of 1,4-butanediol is 5g. The amount of the high-activity capping agent is 10 g, the amount of the multi-functional cross-linking agent is 3 g, and the amount of the end group capturing agent is 2 g. The amount of the neutralizing agent is 9 g, the amount of the catalyst is 0.03 g, and the amount of the emulsifying water is 400 g.

[0008] A preparation method of a low-free-monomer waterborne polyurethane adhesive comprises the following steps: Step one, pre-polymerization reaction; Step two, capping and cross-linking; Step three, emulsification and dispersion; Step four, end group capturing; Step five, stabilization treatment.

[0009] According to the above technical solution, in step one, 200 g of polymer polyol, 25 g of low-monomer residual HDI, and 0.03 g of DBTDL catalyst are put into a four-necked flask equipped with a stirrer, a thermometer, a condenser, and nitrogen protection. Under a nitrogen atmosphere, the temperature is slowly raised to 80±5℃, and the reaction is kept for 2 hours, then 12 g of dimethylol propionic acid (DMPA) and 5 g of 1,4-butanediol are added, the DMPA is dissolved in an appropriate amount of N-methyl pyrrolidone, and the reaction is continued at 80℃ for 1.5 hours. The content of -NCO groups in the system is monitored by di-n-butylamine titration method, and when the theoretical value is reached, the pre-polymerization reaction is completed, and a -NCO pre-polymer is obtained.

[0010] According to the above technical solution, in step two, the reaction system is cooled to 60℃, 10 g of methyl ethyl ketone oxime (MEKO) is added as a high-activity capping agent, and the reaction is kept for 1 hour to close most of the -NCO groups. Then, 3 g of a multi-functional cross-linking agent is added, dissolved in a small amount of solvent, and reacted at 60℃ for 0.5 hours to introduce a preliminary cross-linking structure.

[0011] According to the above technical solution, in step three, the system is cooled to 40℃, and an equal molar amount of triethylamine (TEA) is added to the dimethylol propionic acid (DMPA) for neutralization reaction, and stirred for 15 minutes to form a carboxylate salt, providing hydrophilicity. Under the action of high-speed shearing, the speed is controlled at 8000 rpm, and 400 g of deionized water is slowly added for emulsification to obtain a waterborne polyurethane dispersion.

[0012] According to the above technical solution, in step four, 2 g of an end group capturing agent, which is a small molecule amine compound, is added dropwise to the emulsion, and the stirring is continued at 40℃ for 1 hour to ensure that it fully reacts with the residual free -NCO groups and a small amount of isocyanate monomer.

[0013] According to the above technical solution, the step five removes the small amount of organic solvent in the system by vacuum distillation, and filtration to obtain the final product with a solid content of 40%, a low free monomer waterborne polyurethane adhesive.

[0014] Compared with the prior art, the beneficial effects of the present application are: 1. By adopting the dual action mechanism of high-activity capping agent and end group capturing agent, and combining with the control of specific prepolymerization reaction conditions, the free isocyanate monomer content can be reduced to a very low level, the total VOC emission is very low, the product easily meets and exceeds the most stringent national environmental protection standards such as ENF grade, and is particularly suitable for indoor decoration and other harsh air quality requirements, has extremely high environmental safety and environmental protection, introduces a multifunctional crosslinking agent to form a moderate crosslinking network in the system, which significantly improves the cohesive strength, heat resistance and solvent resistance of the adhesive, and has excellent bonding performance.

[0015] 2. By introducing flexible chain segments and optimizing the emulsification process, the adhesive film has better hydrophobicity and microphase separation structure, and the water immersion resistance is greatly improved, effectively overcoming the shortcoming of traditional waterborne polyurethane in water resistance, having outstanding water resistance, and unique formula design ensures that the adhesive layer will not have strength decay due to aging during long-term use, excellent bonding durability, and long-term bonding strength, the entire preparation method has reasonable process design, mild and controllable reaction conditions, easy to industrialize production, good process applicability and storage stability, and through stabilization treatment, the obtained emulsion has uniform particle size, good mechanical stability and storage stability, and long shelf life.

[0016] In summary, the present application solves the contradiction between low free monomer and high performance by the organic combination of raw material innovation selection, formula synergistic design and process precise control, significantly reduces the residual amount of free isocyanate monomer in the waterborne polyurethane adhesive, meets the highest level of environmental protection standards, realizes low VOC and low toxicity, and does not sacrifice or even improves the key performance of the adhesive such as heat resistance, water resistance and long-term bonding strength, and solves the contradiction between low free monomer and high performance through the synergistic design of process and formula, and obtains a product with good storage stability. BRIEF DESCRIPTION OF DRAWINGS

[0017] The accompanying drawings are included to provide a further understanding of the present application, and constitute a part of the specification, together with the embodiments of the present application, to explain the present application, and do not constitute a limitation of the present application.

[0018] In the drawings: Figure 1 is a step flow chart of the preparation method of the present application. DETAILED DESCRIPTION

[0019] The preferred embodiments of the present application are described below with reference to the accompanying drawings, and it should be understood that the preferred embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0020] The present application provides a technical solution, a low free monomer waterborne polyurethane adhesive, including main chain structure composition, functional additives, reaction control and optimization additives and auxiliary reagents; The main chain structure composition includes low monomer residual diisocyanate and polymer polyol; The low monomer residual diisocyanate uses a special HDI with ultra-low free monomer content, and the free HDI monomer content is <0.1%, specifically Wannate HDI from Wanhua, and the low monomer residual diisocyanate is used as the main raw material to reduce the introduction of free monomers from the source; The polymer polyol is specifically polybutylene adipate glycol PBA and polytetramethylene glycol PTMG, and by mixing PBA and PTMG, the soft and hard segment ratio of the adhesive molecule is adjusted, the adhesive film has excellent flexibility, initial adhesion and impact resistance, and a rigid-flexible molecular structure design is realized; The functional additives include hydrophilic chain extenders and small molecule chain extenders; The hydrophilic chain extender is specifically dimethylol propionic acid DMPA, which provides hydrophilic groups carboxyl groups, so that the prepolymer is stably dispersed in water to form a water-based system; The small molecule chain extender is specifically 1,4-butanediol, which adjusts the length of the molecular segment, optimizes the reactivity of the prepolymer and the mechanical properties of the final adhesive film; The reaction control and optimization additives include high-activity blocking agents, multi-functional cross-linking agents and end group capturing agents; The high-activity blocking agent is specifically methyl ethyl ketone oxime MEKO and caprolactam, which quickly blocks the terminal -NCO groups of the prepolymer, reduces the residual free monomers, and selects a high-activity blocking agent, so that it can react more quickly and more completely with the terminal -NCO groups of the prepolymer, and reduces the active groups remaining due to incomplete blocking; The multi-functional cross-linking agent is specifically trimethylolpropane TMP and glycerol, which forms a three-dimensional cross-linked network during film formation, improves the heat resistance, water resistance and cohesive strength of the adhesive, and introduces multi-functional cross-linking agents to enhance the mechanical strength and durability of the adhesive at the molecular level, making up for the performance loss caused by reducing free monomers; The end group capturing agent is a small molecule amine compound, specifically diethylamine and ethanolamine, which specifically removes trace amounts of free -NCO groups and isocyanate monomers in the system, ensuring that the free monomer content is <0.03%, which is below the detection limit; The auxiliary reagents include neutralizing agents, catalysts and emulsifying water; The neutralizing agent is triethylamine TEA. According to the molar amount of DMPA, the carboxyl groups are neutralized by the neutralizing agent TEA to form a water-soluble salt, which gives the emulsion stability. The catalyst is dibutyltin dilaurate DBTDL, which controls the pre-polymerization reaction rate and avoids side reactions. The emulsifying water is deionized water, which is used as a dispersion medium to form an aqueous polyurethane dispersion.

[0021] Based on the above technical solution, the low monomer residual diisocyanate has a dosage of 25 g, which provides isocyanate groups -NCO as a hard segment skeleton of the polyurethane molecular chain, and gives the adhesive high strength and durability.

[0022] Based on the above technical solution, the polybutylene adipate glycol PBA, Mn=1000, has a dosage of 150 g, which introduces a rigid polyester segment to improve the heat resistance and mechanical strength of the adhesive film. The polytetramethylene glycol PTMG, Mn=2000, has a dosage of 200 g, which provides a flexible polyether segment to enhance the flexibility and impact resistance of the adhesive film.

[0023] Based on the above technical solution, the dimethylol propionic acid DMPA has a dosage of 12 g, and the 1,4-butanediol has a dosage of 5 g. The high-activity capping agent has a dosage of 10 g, the multi-functional cross-linking agent has a dosage of 3 g, and the end group capturing agent has a dosage of 2 g. The neutralizing agent has a dosage of 9 g, the catalyst has a dosage of 0.03 g, and the emulsifying water has a dosage of 400 g.

[0024] As shown in Figure 1 A preparation method of a low free monomer aqueous polyurethane adhesive, comprising the following steps: Step one, pre-polymerization reaction; Step two, capping and cross-linking; Step three, emulsification and dispersion; Step four, end group capturing; Step five, stabilization treatment; Example one: Step one aims to build a polyurethane molecular backbone, control the content of -NCO groups, specifically in a four-necked flask equipped with a stirrer, a thermometer, a condenser and nitrogen protection, 200 g PTMG, Mn=2000, 25 g low monomer residual HDI, 0.03 g dibutyltin dilaurate DBTDL catalyst are put in; Under a nitrogen atmosphere, slowly warm up to 80±5℃, and keep the temperature for 2 hours, then add 12 g dimethylol propionic acid DMPA and 5 g 1,4-butanediol, DMPA is dissolved in an appropriate amount of N-methyl pyrrolidone, and the reaction is continued at 80℃ for 1.5 hours; The content of -NCO groups in the system is monitored by di-n-butylamine titration method. When the theoretical value of 4.5% is reached, the prepolymerization reaction is completed, and a terminal -NCO prepolymer is obtained; By precisely controlling the prepolymerization reaction conditions, temperature, time, catalyst type and amount, the reaction is ensured to be complete and the side reaction is minimized, laying a foundation for the regularity of the main chain structure and the low free monomer content.

[0025] Step two, double mechanism to reduce free monomer content, construct crosslinking network, cool the reaction system to 60°C, add 10g of methylethyl ketoxime MEKO as a high-activity capping agent, and react for 1 hour to close most of the terminal -NCO groups; Then add 3g of trimethylolpropane TMP as a multifunctional crosslinking agent dissolved in a small amount of solvent, and react at 60°C for 0.5 hours to introduce a preliminary crosslinking structure.

[0026] Step three, convert the hydrophobic prepolymer into a stable water dispersion system, cool the system to 40°C, add triethylamine TEA in an equimolar amount with dimethylolpropionic acid DMPA for neutralization reaction, stir for 15 minutes to generate carboxylate, and provide hydrophilicity; Under the action of high-speed shearing, control the rotation speed to 8000 rpm, slowly add 400g of deionized water for emulsification, and obtain a waterborne polyurethane dispersion.

[0027] Step four, deeply remove residual free -NCO and trace amounts of isocyanate monomer, ensure the deep removal of free monomer, add 2g of an aqueous ethanolamine solution to the emulsion, and continue to stir at 40°C for 1 hour to ensure that it reacts with residual free -NCO groups and trace amounts of isocyanate monomer.

[0028] Step five, aimed at improving storage stability and use performance, ensuring the performance consistency of the final product during storage and use, remove a small amount of organic solvent in the system by vacuum distillation, filter, and obtain a final product with a solid content of 40%, a low-free monomer waterborne polyurethane adhesive.

[0029] Example two: Step one, aimed at building a polyurethane molecular main chain skeleton and controlling the content of -NCO groups, specifically in a four-necked flask equipped with a stirrer, a thermometer, a condenser and nitrogen protection, add a mixture of 150g of PBA, Mn=1000 and 50g of PTMG, Mn=2000, 25g of low monomer residual HDI, and 0.03g of dibutyltin dilaurate DBTDL catalyst; Slowly increase the temperature to 80±5℃ under nitrogen atmosphere, and keep the reaction for 2 hours. Then add 12g dimethylol propionic acid (DMPA) and 5g 1,4-butanediol, and continue the reaction at 80℃ for 1.5 hours. Monitor the content of -NCO groups in the system by titration with di-n-butylamine. When the theoretical value of 4.5% is reached, the prepolymerization is completed.

[0030] Step two, reduce the content of free monomers by double mechanism, and construct crosslinking network. Cool the reaction system to 60℃, and add 10g methyl ethyl ketoxime (MEKO) as high-activity capping agent. Keep the reaction for 1 hour to make most of the -NCO groups be capped. Then add 2g glycerol as multifunctional crosslinking agent, and continue the reaction at 60℃ for 0.5 hours to introduce preliminary crosslinking structure.

[0031] Step three, convert the hydrophobic prepolymer into stable water dispersion system. Cool the system to 40℃, and add triethylamine (TEA) in equimolar amount of dimethylol propionic acid (DMPA) for neutralization reaction. Stir for 15 minutes to generate carboxylate, which provides hydrophilicity. Under high-speed shearing, control the rotating speed at 8000rpm, and slowly add 400g deionized water for emulsification to obtain waterborne polyurethane dispersion.

[0032] Step four, deeply remove residual free -NCO and trace isocyanate monomers. Add 1.5g diethylamine aqueous solution dropwise into the emulsion as end group capturing agent, and continue to stir at 40℃ for 1 hour to ensure that it fully reacts with residual free -NCO groups and trace isocyanate monomers.

[0033] Step five, aim to improve storage stability and performance by removing a small amount of organic solvent in the system through vacuum distillation, and filter to obtain the final product, a low free monomer waterborne polyurethane adhesive with solid content of 40%.

[0034] Comparative example: Step one, aim to construct polyurethane molecular backbone and control the content of -NCO groups. Specifically, put 200g polymeric polyol, 25g conventional low monomer residual HDI with free monomer content of 0.5%, and 0.03g dibutyltin dilaurate (DBTDL) catalyst into a four-necked flask equipped with a stirrer, thermometer, condenser tube, and nitrogen protection. Slowly increase the temperature to 80±5℃ under nitrogen atmosphere, and keep the reaction for 2 hours. Then add 12g dimethylol propionic acid (DMPA) and 5g 1,4-butanediol, and continue the reaction at 80℃ for 1.5 hours. The content of -NCO groups in the system was monitored by di-n-butylamine titration method. When the theoretical value of 4.5% was reached, the prepolymerization was ended, and a terminal -NCO prepolymer was obtained.

[0035] In step two, the free monomer content was reduced by a double mechanism, and a crosslinking network was constructed. The reaction system was cooled to 60°C, a small amount of methyl ethyl ketoxime (MEKO) was added as a high-activity capping agent, and the reaction was maintained for 1 hour. Most of the terminal -NCO groups were capped.

[0036] In step three, the hydrophobic prepolymer was converted into a stable water dispersion system. The system was cooled to 40°C, and triethylamine (TEA) was added in an equimolar amount with dimethylolpropionic acid (DMPA) for neutralization reaction. After stirring for 15 minutes, a carboxylate was formed, providing hydrophilicity. Under the action of high-speed shearing, the rotation speed was controlled at 8000 rpm, and 400 g of deionized water was slowly added for emulsification to obtain a waterborne polyurethane dispersion.

[0037] In step four, without adding a terminal group capturing agent, the stirring was continued at 40°C for 1 hour.

[0038] In step five, to improve storage stability and performance, a small amount of organic solvent was removed by vacuum distillation, and the final product was obtained by filtration.

[0039] The products obtained in Examples 1 and 2 and the comparative example were tested for performance, and the results are shown in the following table:

[0040] Result analysis: Environmental safety: The free HDI monomer content of Examples 1 and 2 was much lower than that of the comparative example, less than 0.03%, reaching an extremely low level, and the VOC emission was also much lower than that of the comparative example, fully demonstrating the effectiveness of the double mechanism of high-activity capping agent and terminal group capturing agent, fully meeting the ENF-level environmental protection standard; Bonding and durability: The initial bonding strength, heat resistance, and water resistance of Examples 1 and 2 were significantly better than those of the comparative example, which was due to the moderate crosslinking network introduced by the multifunctional crosslinking agent, which enhanced the cohesive strength, and the synergistic effect of flexible and rigid chain segments; Storage stability: The prepared product showed excellent storage stability due to the thorough terminal group capturing and stabilization treatment, eliminating the hidden danger of late reaction; Conclusion: Through the comparison of the above specific Examples 1 and 2 and the comparative example, it is fully demonstrated that the provided low-free monomer waterborne polyurethane adhesive and preparation method can effectively solve the problems raised in the background art, successfully achieving the integration of low VOC, low toxicity, high bonding performance, and excellent durability.

[0041] Finally, it should be noted that the above only describes the preferred examples of the present application, and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art will appreciate that modifications can be made to the technical solutions described in the foregoing embodiments, or some of the technical features thereof can be replaced equivalently, without departing from the spirit and principle of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A low-free-monomer waterborne polyurethane adhesive, characterized in that: This includes main chain structure compositions, functional additives, reaction control and optimization additives, and auxiliary reagents; The main chain structure composition includes a low-monomer residual diisocyanate and a polymeric polyol; Low-monomer residual diisocyanate uses a special HDI with ultra-low free monomer content, with free HDI monomer content <0.1%; The specific polymeric polyols are poly(1,4-butanediol adipate) diol (PBA) and polytetrahydrofuran ether diol (PTMG). The functional additives include hydrophilic chain extenders and small molecule chain extenders; Specifically, the hydrophilic chain extender is dimethylolpropionic acid (DMPA), which provides the hydrophilic carboxylic acid group, enabling the prepolymer to be stably dispersed in water and forming an aqueous system. The small molecule chain extender is specifically 1,4-butanediol, which adjusts the molecular chain segment length; The reaction control and optimization additives include highly active end-capping agents, multifunctional crosslinking agents, and end-group trapping agents; The highly active end-capping agent is specifically methyl ethyl ketone oxime (MEKO) and caprolactam, which rapidly seal the -NCO groups at the end of the prepolymer. The multifunctional crosslinking agents are specifically trimethylolpropane (TMP) and glycerol, which form a three-dimensional crosslinked network during film formation. The end-group scavengers are small molecule amine compounds, specifically diethylamine and ethanolamine, which specifically remove trace amounts of residual free -NCO groups and isocyanate monomers in the system; The auxiliary reagents include neutralizing agents, catalysts, and emulsifying water; The neutralizing agent is triethylamine (TEA). Based on the molar amount of DMPA, the neutralizing agent TEA neutralizes the carboxylic acid groups to generate a water-soluble salt. The catalyst is dibutyltin dilaurate (DBTDL), which regulates the prepolymerization reaction rate; The emulsifying water is deionized water, which serves as the dispersion medium to form an aqueous polyurethane dispersion.

2. The low-free-monomer waterborne polyurethane adhesive according to claim 1, characterized in that: The amount of the low-monomer residual diisocyanate is 25g, providing isocyanate groups -NCO as the hard segment backbone of the polyurethane molecular chain.

3. The low-free-monomer waterborne polyurethane adhesive according to claim 1, characterized in that: The poly(1,4-butanediol adipate) diol (PBA), with Mn=1000, is used in an amount of 150g to introduce rigid polyester segments. The polytetrahydrofuran ether diol (PTMG), with Mn=2000, is used in an amount of 200g to provide flexible polyether segments.

4. The low-free-monomer waterborne polyurethane adhesive according to claim 1, characterized in that: The amount of DMPA used is 12g, and the amount of 1,4-butanediol used is 5g; The amount of the highly active end-capping agent is 10g, the amount of the multifunctional crosslinking agent is 3g, and the amount of the end-group trapping agent is 2g. The amount of the neutralizing agent is 9g, the amount of the catalyst is 0.03g, and the amount of the emulsifying water is 400g.

5. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 1, characterized in that: Includes the following steps: Step 1, prepolymerization reaction; Step 2, End-capping and cross-linking; Step 3: Emulsification and dispersion; Step 4, End-base capture; Step 5: Stabilization treatment.

6. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 5, characterized in that: In step one, specifically, 200g of polymer polyol, 25g of low monomer residue HDI, and 0.03g of dibutyltin dilaurate (DBTDL) catalyst are added to a four-necked flask equipped with a stirrer, thermometer, condenser, and nitrogen protection. Under a nitrogen atmosphere, the temperature was slowly raised to 80±5℃ and the reaction was maintained for 2 hours. Then, 12g of dimethylolpropionic acid (DMPA) and 5g of 1,4-butanediol were added. DMPA was dissolved in an appropriate amount of N-methylpyrrolidone, and the reaction was continued at 80℃ for 1.5 hours. The content of -NCO groups in the system was monitored by di-n-butylamine titration. When the theoretical value was reached, the prepolymerization reaction ended, and the terminal -NCO prepolymer was obtained.

7. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 5, characterized in that: In step two, the reaction system is cooled to 60°C, and 10g of methyl ethyl ketone oxime (MEKO) is added as a highly active end-capping agent. The reaction is kept at this temperature for 1 hour to block most of the terminal -NCO groups. Subsequently, 3g of a multifunctional crosslinking agent was added, dissolved in a small amount of solvent, and reacted at 60°C for 0.5 hours to introduce a preliminary crosslinking structure.

8. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 5, characterized in that: In step three, the system is cooled to 40°C, and triethylamine (TEA) in an equimolar amount with dimethylolpropionic acid (DMPA) is added for neutralization reaction. The mixture is stirred for 15 minutes to generate a carboxylate, which provides hydrophilicity. Under high-speed shearing, the rotation speed is controlled at 8000 rpm, and 400g of deionized water is slowly added for emulsification to obtain an aqueous polyurethane dispersion.

9. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 5, characterized in that: In step four, 2g of end-group scavenging agent is added dropwise to the emulsion. The end-group scavenging agent is specifically a small molecule amine compound. The mixture is stirred at 40°C for 1 hour to ensure that it reacts fully with the remaining free -NCO groups and trace amounts of isocyanate monomers.

10. The method for preparing a low-free-monomer waterborne polyurethane adhesive according to claim 5, characterized in that: In step five, a small amount of organic solvent in the system is removed by vacuum distillation, followed by filtration to obtain the final product with a solid content of 40%, a low-free monomer waterborne polyurethane adhesive.

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