High-initial-viscosity polyurethane hot melt adhesive and preparation method thereof

Abstract

The invention discloses a high-initial-viscosity polyurethane hot melt adhesive and a preparation method thereof, and relates to the technical field of polyurethane hot melt adhesives. The adhesive is composed of 55 to 80 parts of compound polyester polyol, 3 to 8 parts of polyether polyol, 7 to 11 parts of isocyanate, 6 to 10 parts of a modified composite tackifying system, 0.02 to 0.06 part of a catalyst, 8 to 22 parts of an ethylene-vinyl acetate copolymer, and 0.3 to 1.5 parts of an auxiliary agent. The compound polyester polyol is prepared from crystalline polyester polyol, liquid polyester polyol and amorphous polyester polyol; the modified composite tackifying system is a compound of rosin resin subjected to melt grafting modification by a silane coupling agent and hydroxylated nano composite filler. By arranging a core-shell type composite tackifying system of the rosin resin subjected to melt grafting modification by the silane coupling agent and the hydroxylated nanofiller, the problems that a modified tackifying system in the prior art is mainly simple in mixing, low in grafting rate and easy to agglomerate the nanofiller are solved, and the initial adhesion of the hot melt adhesive is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of polyurethane hot melt adhesive technology, specifically to a high initial tack polyurethane hot melt adhesive and its preparation method. Background Technology

[0002] Reactive polyurethane hot melt adhesives are produced by heating and melting into a fluid under conditions that inhibit chemical reactions, facilitating application. After the two substrates are bonded together, the adhesive layer solidifies, providing adhesion. Subsequently, with the help of moisture present in the air or adhering to the substrate surfaces, it reacts and extends chains, generating a high-molecular-weight polymer with high cohesive strength, significantly improving adhesion and heat resistance. It combines the solvent-free and rapid positioning characteristics of hot melt adhesives with the water resistance, temperature resistance, creep resistance, moisture resistance, and media resistance of reactive adhesives.

[0003] This type of adhesive uses NCO-terminated prepolymers as a base material, combined with thermoplastic resins that do not react with isocyanate groups, as well as additives such as catalysts and fillers, ensuring a long pot life and shelf life. PUR possesses high resistance to high and low temperatures, excellent low-temperature resistance, and strong adhesive strength, while also being solvent-free and non-toxic, thus having broad application prospects in industries such as furniture, automobiles, clothing, and electronics. However, these traditional polyurethane hot melt adhesives still suffer from low initial tack, especially in woodworking applications, where initial tack and other properties need improvement. Therefore, this paper proposes a high initial tack polyurethane hot melt adhesive and its preparation method to address these issues. Summary of the Invention

[0004] Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a high initial tack polyurethane hot melt adhesive and its preparation method, thus solving the problems mentioned in the background section.

[0005] Technical solution To achieve the above objectives, the present invention is implemented through the following technical solution: a high initial tack polyurethane hot melt adhesive, which, by weight, is composed of 55-80 parts of compounded polyester polyol, 3-8 parts of polyether polyol, 7-11 parts of isocyanate, 6-10 parts of modified composite tackifying system, 0.02-0.06 parts of catalyst, 8-22 parts of ethylene-vinyl acetate copolymer and 0.3-1.5 parts of additives.

[0006] The preparation process of the modified composite tackifying system is as follows: Rosin resin is added to a reaction vessel and heated to 80-90℃ to melt it. Silane coupling agent (KH550 or KH560) is added at 3%-8% of the rosin resin mass. The mixture is stirred at 500-800 rpm for 30-40 minutes to allow the silane coupling agent to undergo a melt grafting reaction with the rosin resin, ensuring a grafting rate ≥90% to achieve deep chemical modification. Subsequently, the temperature is lowered to 60-70℃, and hydroxylated nanocomposite filler is added. The hydroxylated nanocomposite filler is selected from hydroxylated nano-silica with a particle size of 10-50 nm or hydroxylated graphene with a sheet thickness of 1-5 nm. The mixture is stirred for another 60-90 minutes, while simultaneously using 300-500W ultrasonic-assisted dispersion for 15-25 minutes to ensure that the nanofiller is uniformly dispersed in the modified rosin resin, forming a stable core-shell composite system. This preparation process involves the active groups of the silane coupling agent forming covalent bonds with the carboxyl groups of the rosin resin, and then interacting with the hydroxyl groups of the hydroxylated nanofiller. This significantly improves the dispersion stability and interfacial bonding of the composite system, avoiding performance inhomogeneity caused by nanofiller agglomeration.

[0007] This weight ratio setting achieves the comprehensive goals of high initial tack, excellent cohesive strength, and stable adhesion performance in hot melt adhesives. The highest proportion of the compounded polyester polyol provides the basic adhesive framework for the hot melt adhesive while reserving sufficient space for the action of other functional components. The appropriate addition of polyether polyol balances the system's flexibility and adhesive adaptability, preventing excessive dosage from reducing cohesive strength. The amount of isocyanate is matched to the terminal hydroxyl content of the polyol to ensure sufficient crosslinking reaction. The amount of the modified composite tackifying system balances initial tack improvement with system compatibility, without compromising the structural integrity of other components. The trace addition of catalyst precisely controls the reaction rate, ensuring production efficiency while avoiding overly vigorous reactions. The amount of ethylene-vinyl acetate copolymer optimizes the adhesion effect on the substrate surface, while the small amount of additives specifically improves the storage stability and service durability of the hot melt adhesive.

[0008] The compounded polyester polyols consist of crystalline polyester polyols, liquid polyester polyols, and amorphous polyester polyols. The weight-average molecular weight (MAM) of the crystalline polyester polyols is 3200–3800, that of the liquid polyester polyols is 3200–3800, and that of the amorphous polyester polyols is 1800–2200. In the compounded polyester polyols, the weight percentage of crystalline polyester polyols is 40%–55%, that of liquid polyester polyols is 25%–35%, and that of amorphous polyester polyols is 8%–15% (total weight 55–80 parts). This formulation balances the crystallization rate, flowability, and cohesive strength of the hot melt adhesive by adjusting the proportions of the three polyesters, ensuring a simultaneous improvement in initial tack and bond stability. The combination and molecular weight limitation of the three polyester polyols achieve complementary properties. Crystalline polyester polyols impart good mechanical strength and rapid crystallization ability to hot melt adhesives, facilitating the rapid formation of initial tack. Liquid polyester polyols enhance the fluidity of the system, facilitating full contact with the substrate surface during coating. The lower molecular weight of amorphous polyester polyols allows them to fill the gaps between crystalline and liquid polyester polyols, optimizing the system's compatibility and cohesive effect. Specific molecular weight ranges ensure that the three polyols exhibit matched reactivity during the reaction, avoiding uneven reactions or performance imbalances caused by excessive molecular weight differences.

[0009] Polyether polyols include at least one of polytetrahydrofuran glycol, polypropylene glycol, and polypropylene-ethylene oxide copolymer glycol, with a weight-average molecular weight of 1800–2500. Different types of polyether polyols are suitable for different application requirements. Polytetrahydrofuran glycol has excellent hydrolysis resistance and flexibility, polypropylene glycol has moderate cost and good compatibility, while polypropylene-ethylene oxide copolymer glycol balances hydrophilicity and adhesive adaptability. The limited molecular weight range allows polyether polyols to form good molecular chain interweaving with blended polyester polyols, which improves the flexibility of hot melt adhesives without affecting their overall cohesive strength, ensuring stable adhesive performance under different temperature conditions.

[0010] The isocyanate includes at least one of isophorone diisocyanate, hexamethylene diisocyanate, and 4,4-diphenylmethane diisocyanate, with an NCO content of 30–45 wt%. Isophorone diisocyanate imparts good weather resistance and yellowing resistance to the hot melt adhesive, hexamethylene diisocyanate enhances the crosslinking density and mechanical properties after the reaction, and 4,4-diphenylmethane diisocyanate strengthens the bond strength and heat resistance. The specific NCO content ensures that the isocyanate can fully react with the terminal hydroxyl groups of the polyol to form a sufficient number of urea bonds, while reserving an appropriate amount of free NCO groups to facilitate further crosslinking through subsequent reaction with moisture in the air, thereby improving the long-term bond stability of the hot melt adhesive.

[0011] The modified composite tackifying system is a core-shell composite of rosin resin melt-grafted with a silane coupling agent and hydroxylated nanocomposite filler. The silane coupling agent is selected from KH550 or KH560, with an addition amount of 3%–8% of the rosin resin mass, and the hydroxylated nanocomposite filler is added at 10%–30% of the modified rosin resin mass. Rosin resin modified with silane coupling agent KH550 introduces amino active groups, enhancing its interaction with other components, while rosin resin modified with silane coupling agent KH560 improves its binding capacity through epoxy groups. Hydroxylated nano-silica enhances the system's mechanical strength and heat resistance, while hydroxylated graphene optimizes thermal conductivity and dispersion stability. The amount of silane coupling agent added is precisely controlled to ensure the modification effect of rosin resin while avoiding compatibility problems caused by excessive addition; the amount and size parameters of hydroxylated nanocomposite filler are matched to ensure that it can be uniformly dispersed and form a core-shell structure, further improving the initial tack and cohesive strength of hot melt adhesive.

[0012] The catalyst includes at least one of dimorpholine diethyl ether, dibutyltin dilaurate, and stannous octoate, with a weight ratio of organometallic catalyst to amine catalyst of 1:1 to 2:1. Dimorpholine diethyl ether, as an amine catalyst, efficiently promotes the reaction between isocyanate and hydroxyl groups, while dibutyltin dilaurate and stannous octoate, as organometallic catalysts, exhibit high catalytic efficiency and wide applicability. Optimization of the ratio of these two types of catalysts allows for precise control of the reaction rate. The organometallic catalyst ensures rapid reaction, while the amine catalyst improves reaction uniformity, avoiding performance defects caused by excessively vigorous local reactions, and ensuring the formation of a uniform and stable cross-linked structure in the hot melt adhesive during preparation.

[0013] The VA content of the ethylene-vinyl acetate copolymer is 55–85 wt%, and the VA content is matched with the grafting rate of the modified composite tackifier system, resulting in a hydrogen bonding efficiency of ≥85%. VA units within this content range provide sufficient polar groups to form hydrogen bonds with the grafted functional groups in the modified composite tackifier system, while simultaneously enhancing the interaction between the hot melt adhesive and the polar substrate surface. The higher VA content improves the copolymer's flexibility and compatibility, allowing the hot melt adhesive to quickly wet the substrate surface after application, shortening the initial tack time, improving bonding efficiency, and ensuring the cohesive strength and peel strength after bonding.

[0014] Additives must include at least one of the following: antioxidants and stabilizers. Antioxidants inhibit oxidative degradation of hot melt adhesives during storage and use, extending their service life, while stabilizers enhance the chemical stability of the system, preventing performance changes in components after high temperatures or prolonged storage. A proper combination of these two additives ensures that the hot melt adhesive maintains stable performance under various environmental conditions, preventing problems such as cracking, aging, or adhesive failure.

[0015] The terminal hydroxyl groups of the compounded polyester polyol undergo an addition reaction with the NCO groups of the isocyanate to form a urea bond structure. The grafted functional groups in the modified composite tackifying system form hydrogen bonds with the VA units of the ethylene-vinyl acetate copolymer. At the same time, the nano-components in the modified composite tackifying system construct a three-dimensional support structure within the system, encapsulating the composite phase of the polyurethane prepolymer and the ethylene-vinyl acetate copolymer. Through chemical bonding and physical interweaving, the components are tightly bound together, ultimately endowing the hot melt adhesive with excellent initial tack (initial tack ≥ 2.8 N), cohesive strength, and bonding stability (retention rate of cohesive strength ≥ 98% within 60 hours of damp heat).

[0016] A method for preparing a high initial tack polyurethane hot melt adhesive, comprising the following steps: Step 1: Add the compounded polyester polyol and polyether polyol into the reactor and dry them for 40 to 70 minutes at 125 to 135°C and an absolute pressure not exceeding 80 Pa to obtain the dried material.

[0017] To remove moisture from polyols and prevent side reactions between moisture and isocyanates that could affect the performance of hot melt adhesives, a temperature range of 125–135°C is used to efficiently remove moisture without causing degradation of the polyol. A vacuum condition with an absolute pressure not exceeding 80 Pa accelerates moisture evaporation, and a dehydration time of 40–70 minutes ensures complete moisture removal, providing a pure raw material system for subsequent reactions and avoiding cross-linking structural defects caused by residual moisture.

[0018] Step two: Add the modified composite thickening system to the dried material and pre-disperse it for 40-60 minutes at 80-90℃ and 900-1300 rpm to form a three-dimensional network precursor. The pre-dispersion process uses a combination of mechanical stirring dispersion and ultrasonic-assisted dispersion, with an ultrasonic power of 300-500W and an ultrasonic time of 15-25 minutes, to ensure that the hydroxylated nanocomposite filler does not agglomerate and that the monodisperse ratio after dispersion is ≥90%.

[0019] A temperature of 80–90℃ enhances the fluidity of the system, facilitating dispersion. A high rotation speed of 900–1300 rpm breaks up the aggregates of the modified composite thickening system through shear force. Combined with the microscopic dispersing effect of ultrasonic-assisted dispersion, this ensures uniform distribution of nano-components. A dispersion time of 40–60 minutes guarantees effective dispersion, allowing the nano-components to fully contact and form a preliminary three-dimensional network structure, laying the foundation for subsequent bonding with other components while preventing damage to the core-shell structure.

[0020] Step 3: Add ethylene-vinyl acetate copolymer and additives to the three-dimensional network precursor, heat to 135-145℃, and dehydrate under vacuum for 80-110 minutes.

[0021] This process further removes moisture from the system and ensures the ethylene-vinyl acetate copolymer and additives are fully dissolved and dispersed. A temperature of 135–145°C promotes the melting and dissolution of the copolymer, ensuring its full integration with the three-dimensional network precursor. A vacuum dehydration time of 80–110 minutes thoroughly removes moisture and any volatile impurities that may be generated in the system, preventing these impurities from forming bubbles or defects in subsequent reactions and ensuring the hot melt adhesive has a uniform and stable appearance and performance.

[0022] Step 4: Cool down to 88-92°C, add isocyanate and catalyst, heat up to 105-115°C, react for 50-80 minutes under an absolute pressure not exceeding 80Pa, discharge and seal in packaging to obtain the hot melt adhesive.

[0023] A temperature of 88–92°C facilitates the addition and mixing of isocyanate and catalyst, preventing premature reaction of isocyanate due to high temperature. A reaction temperature of 105–115°C provides suitable conditions for the reaction of isocyanate and polyol, ensuring that the reaction proceeds fully. A reaction time of 50–80 minutes ensures the complete formation of the cross-linked structure. Vacuum conditions with an absolute pressure not exceeding 80Pa remove small molecule impurities generated during the reaction, preventing impurity residues from affecting the performance of the hot melt adhesive. Sealed packaging prevents the finished product from absorbing moisture from the air, ensuring stability during storage.

[0024] Beneficial effects The present invention has the following beneficial effects: (1) The high initial tack polyurethane hot melt adhesive and its preparation method, by setting up a core-shell composite tack-enhancing system of rosin resin modified by silane coupling agent melt grafting and hydroxylated nanofiller, solves the problems of simple mixing, low grafting rate and easy agglomeration of nanofiller in the existing modified tack-enhancing system, and achieves a significant improvement in the initial tack of hot melt adhesive, with a grafting rate ≥90% and a monodisperse ratio of nanofiller ≥90%, while enhancing the cohesive strength (≥3.5MPa) and interfacial bonding force.

[0025] (2) The high initial tack polyurethane hot melt adhesive and its preparation method solve the problems of slow crystallization rate, poor fluidity or insufficient compatibility caused by single polyester polyol by setting a precise compounding system of three polyester polyols: crystalline, liquid and amorphous and matching weight average molecular weight range. It achieves a balance between the crystallization rate and fluidity of hot melt adhesive, can quickly wet the substrate when coated, and has a ≥98% retention rate of polymerization strength within 60 hours of damp heat.

[0026] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0027] Figure 1 This is a flowchart of a method for preparing a high initial tack polyurethane hot melt adhesive according to the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This embodiment 1 provides a preparation method for a high initial tack polyurethane hot melt adhesive, comprising the following components by weight: 55 parts of compounded polyester polyol, consisting of 22 parts of crystalline polyester polyol (weight average molecular weight 3200), 14 parts of liquid polyester polyol (weight average molecular weight 3200), and 19 parts of amorphous polyester polyol (weight average molecular weight 1800); 3 parts of polyether polyol, which is polytetrahydrofuran diol (weight average molecular weight 1800); 7 parts of isocyanate, which is isophorone diisocyanate (NCO content 30wt%); and 6 parts of modified composite tackifying system, which is silicone. A core-shell composite of rosin resin melt-grafted with silane coupling agent KH550 and hydroxylated nano-silica, wherein the amount of silane coupling agent added is 3% of the mass of rosin resin, the particle size of hydroxylated nano-silica is 10-30 nm, and the amount added is 10% of the mass of modified rosin resin; 0.02 parts of catalyst, composed of dibutyltin dilaurate and dimorpholine diethyl ether in a weight ratio of 1:1; 8 parts of ethylene-vinyl acetate copolymer (VA content 55wt%, suitable for grafting rate of modified composite tackifying system, hydrogen bonding efficiency 85%); and 0.3 parts of additive, which is an antioxidant.

[0030] The preparation process is as follows: Step 1: Preparation of modified composite thickening system. Rosin resin is added to a reaction vessel and heated to 80℃ to melt it. Silane coupling agent KH550 is added at 3% of the rosin resin mass. The mixture is stirred at 500 rpm for 30 minutes to allow the silane coupling agent to undergo a melt grafting reaction with the rosin resin, achieving a grafting rate of 91%. Subsequently, the temperature is lowered to 60℃, hydroxylated nano-silica is added, and stirring continues for 60 minutes. Simultaneously, ultrasonic dispersion at 300W is used for 15 minutes to ensure uniform dispersion of the nanofiller. After dispersion, the monodispersity ratio reaches 92%, forming a stable core-shell composite system for later use.

[0031] Step 2: Preparation of hot melt adhesive. Step 1: Add 55 parts of the compounded polyester polyol and 3 parts of the polyether polyol to the reactor, and dry and dehydrate for 40 minutes at 125℃ and an absolute pressure not exceeding 80Pa to obtain the dried material. Step 2: Add 6 parts of the prepared modified composite thickening system to the dried material, and pre-disperse for 40 minutes at 80℃ and a rotation speed of 900 rpm. The pre-dispersion process uses a combination of mechanical stirring dispersion and ultrasonic-assisted dispersion, with an ultrasonic power of 300W and an ultrasonic time of 15 minutes to ensure that the hydroxylated nanocomposite filler does not agglomerate and forms a three-dimensional network precursor. Step 3: Add 8 parts of ethylene-vinyl acetate copolymer and 0.3 parts of additives to the three-dimensional network precursor, heat to 135℃, and dehydrate under vacuum for 80 minutes. Step 4: Cool to 88℃, add 7 parts of isocyanate and 0.02 parts of catalyst, heat to 105℃, and react for 50 minutes at an absolute pressure not exceeding 80Pa. Discharge and seal the product for packaging to obtain the final product.

[0032] Performance testing: Initial tack reaches 2.8N, peel strength reaches 9.2MPa, cohesive strength reaches 3.5MPa, cohesive strength retention rate is 98% within 60 hours in a humid heat environment of 40℃ and 95%RH, and there is no stratification or precipitation after 24 hours of storage; grafting rate is 91%, hydrogen bonding efficiency is 85%, and the monodispersity ratio of hydroxylated nanofillers is 92%.

[0033] Example 2 This embodiment 2 provides a preparation method for a high initial tack polyurethane hot melt adhesive, comprising the following components by weight: 80 parts of compounded polyester polyol, consisting of 44 parts of crystalline polyester polyol (weight average molecular weight 3800), 28 parts of liquid polyester polyol (weight average molecular weight 3800), and 8 parts of amorphous polyester polyol (weight average molecular weight 2200); 8 parts of polyether polyol, which is a polypropylene oxide-ethylene oxide copolymer diol (weight average molecular weight 2500); 11 parts of isocyanate, which is 4,4-diphenylmethane diisocyanate (NCO content 45wt%); and 10 parts of modified composite tackifying system. The product comprises: a core-shell composite of rosin resin and hydroxylated graphene, which is melt-grafted modified with silane coupling agent KH560. The amount of silane coupling agent added is 8% of the mass of rosin resin, and the thickness of the hydroxylated graphene sheets is 1-3 nm, with an addition amount of 30% of the mass of the modified rosin resin; 0.06 parts of catalyst, composed of stannous octoate and dimorpholine diethyl ether in a weight ratio of 2:1; 22 parts of ethylene-vinyl acetate copolymer (VA content 85wt%, suitable for the grafting rate of the modified composite tackifying system, hydrogen bonding efficiency 92%); and 1.5 parts of additives, composed of 1.0 part of antioxidant and 0.5 parts of stabilizer.

[0034] The preparation process is as follows: Step 1: Preparation of modified composite thickening system. Rosin resin is added to a reaction vessel and heated to 90℃ to melt it. Silane coupling agent KH560 is added at 8% of the rosin resin mass. The mixture is stirred at 800 rpm for 40 minutes to allow the silane coupling agent to undergo a melt grafting reaction with the rosin resin, achieving a grafting rate of 96%. Subsequently, the temperature is lowered to 70℃, hydroxylated graphene is added, and stirring continues for 90 minutes. Simultaneously, 500W ultrasonic-assisted dispersion is used for 25 minutes to ensure uniform dispersion of the nanofiller. After dispersion, the monodispersity ratio reaches 95%, forming a stable core-shell composite system for later use.

[0035] Step 2: Preparation of hot melt adhesive. Step 1: Add 80 parts of the compounded polyester polyol and 8 parts of the polyether polyol to the reactor and dry for 70 minutes at 135℃ and an absolute pressure not exceeding 80Pa to obtain the dried material. Step 2: Add 10 parts of the prepared modified composite thickening system to the dried material and pre-disperse for 60 minutes at 90℃ and a rotation speed of 1300 rpm. The pre-dispersion process uses a combination of mechanical stirring dispersion and ultrasonic-assisted dispersion (ultrasonic power 500W, ultrasonic time 25 minutes) to ensure that the hydroxylated nanocomposite filler does not agglomerate and forms a three-dimensional network precursor. Step 3: Add 22 parts of ethylene-vinyl acetate copolymer and 1.5 parts of additives to the three-dimensional network precursor, heat to 145℃, and dehydrate under vacuum for 110 minutes. Step 4: Cool to 92℃, add 11 parts of isocyanate and 0.06 parts of catalyst, heat to 115℃, and react for 80 minutes at an absolute pressure not exceeding 80Pa. Discharge and seal the product to obtain the final product.

[0036] Performance testing: Initial tack reaches 4.2N, peel strength reaches 11.5MPa, cohesive strength reaches 5.2MPa, cohesive strength retention rate is 99% within 60 hours in a humid heat environment of 40℃ and 95%RH, and there is no stratification or precipitation after 24 hours of storage; grafting rate is 96%, hydrogen bonding efficiency is 92%, and the monodispersity ratio of hydroxylated nanofillers is 95%.

[0037] II. Comparative Example Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the modified composite thickening system was replaced with 10 parts of "a simple mixture of unmelted grafted modified rosin resin + silane coupling agent + hydroxylated graphene", without the melt grafting reaction step. The other components, dosages and preparation processes were completely consistent with Example 2.

[0038] Results: Initial tack was only 1.5 N, peel strength was 6.8 MPa, cohesive strength was 2.1 MPa, slight degumming occurred on the surface after 60 hours of damp heat resistance, and slight sedimentation occurred after 24 hours of storage; grafting rate was <30%, hydrogen bonding efficiency was 58%, and the monodisperse ratio of hydroxylated nanofillers was <60%.

[0039] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that the compounded polyester polyol consists only of 56 parts of crystalline polyester polyol (weight average molecular weight 3800) and 24 parts of liquid polyester polyol (weight average molecular weight 3800), and amorphous polyester polyol. The other components, dosages and preparation processes are completely consistent with those of Example 2.

[0040] Results: Initial tack 2.3 N, peel strength 7.5 MPa, cohesive strength 2.8 MPa, cohesive strength decreased by 30% after 60 hours of damp heat resistance, no delamination after 24 hours of storage but poor system compatibility; hydrogen bonding efficiency 72%, hydroxylated nanofiller monodispersity ratio 81%.

[0041] III. Experimental Examples Experimental Example 1 Test objective: To verify the core bonding performance, modification effect and stability of the examples and comparative examples, focusing on the role of initial tack and the modified system.

[0042] Test items and methods: Initial tack: The minimum initial tack force that stops the steel ball from rolling is measured and recorded using the rolling ball method. Cohesive strength: The cohesive breaking strength of the hot melt adhesive was determined using the tensile shear method; Grafting rate: The grafting rate of modified rosin resin was determined by chemical titration. Moist heat resistance: After being placed in an environment of 40℃ and 95% relative humidity for 60 hours, the cohesive strength retention rate was measured after the test; Storage stability: Seal and place in a 25°C room temperature environment for 24 hours, observe whether there is stratification or sedimentation, and at the same time measure the change rate of initial tack.

[0043] As shown in the table below: In this experiment, Examples 1 and 2 showed significantly better initial tack and cohesive strength than the comparative example, with grafting rates exceeding 90%, and good resistance to damp heat and storage stability. Example 2, due to its use of the upper limit of component dosage and a complete core-shell composite system, exhibited the best performance, demonstrating that the melt-grafted modified composite tackifying system can effectively improve core performance. Comparative Example 1, lacking melt grafting modification, had an extremely low grafting rate and significantly reduced performance; Comparative Example 2, lacking amorphous polyester polyol, suffered from compromised system compatibility and a marked decrease in resistance to damp heat.

[0044] Experiment Example 2 Test objective: To verify the construction adaptability, dispersion effect and long-term performance of the examples and comparative examples, focusing on practical application needs.

[0045] Test items and methods: Coating smoothness: Apply the adhesive using a hot melt glue machine at a cylinder temperature of 100℃ and a glue hose temperature of 105℃. Observe whether the coating process is smooth. If there is no blockage or stringing, it is considered qualified. Monodisperse ratio of nanofillers: The proportion of nanofillers in monodisperse state after dispersion is determined by laser particle size analyzer. Curing time: After coating, the time required to achieve an initial bond strength of ≥3MPa when bonded to a wood substrate was recorded. Peel strength after natural aging: The adhesive sample was placed in a room temperature environment of 25°C for 6 months for natural aging, and the peel strength was measured. Water resistance: The adhesive sample was immersed in deionized water at 25°C for 24 hours, and the peel strength retention rate was measured.

[0046] As shown in the table below: In this experimental example, both Example 1 and Example 2 exhibit excellent application smoothness, with a monodisperse ratio of over 90% for the nanofiller, short curing time, and stable long-term performance. Example 2 demonstrates superior overall performance and is suitable for industrial mass production. Comparative Example 1, due to severe agglomeration of the nanofiller, shows significant degradation in application and long-term performance; Comparative Example 2, due to insufficient system compatibility, exhibits decreased curing efficiency and water resistance. This demonstrates that the component design, modification process, and dispersion requirements of this scheme are fully adaptable to practical application scenarios.

[0047] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0048] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A high initial tack polyurethane hot melt adhesive, characterized in that, By weight, it consists of 55-80 parts of compounded polyester polyol, 3-8 parts of polyether polyol, 7-11 parts of isocyanate, 6-10 parts of modified composite thickening system, 0.02-0.06 parts of catalyst, 8-22 parts of ethylene-vinyl acetate copolymer and 0.3-1.5 parts of additives; The compounded polyester polyols include crystalline polyester polyols, liquid polyester polyols, and amorphous polyester polyols; The modified composite thickening system is a composite of rosin resin modified by silane coupling agent melt grafting and hydroxylated nanocomposite filler. The modification process is that the rosin resin reacts with the silane coupling agent in the molten state at 80-90℃ for 30-40 minutes, with a grafting rate ≥90%, and then is compounded with nanocomposite filler. The hydroxylated nanocomposite filler is selected from hydroxylated nano-silica with a particle size of 10-50 nm or hydroxylated graphene with a sheet thickness of 1-5 nm; the additives include at least one of antioxidants and stabilizers. The terminal hydroxyl groups of the compounded polyester polyol undergo an addition reaction with the NCO groups of the isocyanate to form a urea bond structure. The grafted functional groups in the modified composite tackifying system form hydrogen bonds with the VA units of the ethylene-vinyl acetate copolymer. The nanocomposite filler constructs a three-dimensional support structure within the system, resulting in an initial tack of ≥2.8N for the hot melt adhesive and a creep strength retention rate of ≥98% within 60 hours of humid heat.

2. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The compounded polyester polyol is composed of crystalline polyester polyol, liquid polyester polyol and amorphous polyester polyol, wherein the weight average molecular weight of the crystalline polyester polyol is 3200-3800, the weight average molecular weight of the liquid polyester polyol is 3200-3800, and the weight average molecular weight of the amorphous polyester polyol is 1800-2200.

3. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The polyether polyol includes at least one of polytetrahydrofuran diol, polypropylene oxide diol, and polypropylene oxide-ethylene oxide copolymer diol, and its weight-average molecular weight is 1800 to 2500.

4. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The isocyanate includes at least one of isophorone diisocyanate, hexamethylene diisocyanate, and 4,4-diphenylmethane diisocyanate, and the NCO content is 30-45 wt%.

5. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The silane coupling agent in the modified composite tackifying system is selected from KH550 or KH560, and the amount of silane coupling agent added is 3% to 8% of the mass of rosin resin. The amount of hydroxylated nanocomposite filler added is 10% to 30% of the mass of modified rosin resin.

6. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The catalyst includes at least one of dimorpholine diethyl ether, dibutyltin dilaurate, and stannous octoate, and the weight ratio of the organometallic catalyst to the amine catalyst is 1:1 to 2:

1.

7. The high initial tack polyurethane hot melt adhesive according to claim 1, characterized in that, The VA content of the ethylene-vinyl acetate copolymer is 55-85 wt%, and the VA content is matched with the grafting rate of the modified composite tackifying system, so that the hydrogen bonding efficiency is ≥85%.

8. A method for preparing a high initial tack polyurethane hot melt adhesive, characterized in that, The preparation of a high initial tack polyurethane hot melt adhesive according to any one of claims 1-7 comprises the following steps: Step 1: Add the compounded polyester polyol and polyether polyol into the reactor and dry them at 125-135℃ and absolute pressure ≤80Pa for 40-70 minutes to obtain the dried material. Step 2: Add the modified composite thickening system to the dried material and pre-disperse it for 40-60 minutes at 80-90℃ and 900-1300 r / min to form a three-dimensional network precursor. The pre-dispersion process must ensure that the hydroxylated nanocomposite filler does not agglomerate. Step 3: Add ethylene-vinyl acetate copolymer and additives to the three-dimensional network precursor, heat to 135-145℃, and vacuum dehydrate for 80-110 min; Step 4: Cool down to 88-92℃, add isocyanate and catalyst, heat up to 105-115℃, react for 50-80 minutes under absolute pressure ≤80Pa, discharge and seal packaging to obtain the hot melt adhesive.

9. A method for preparing a high initial tack polyurethane hot melt adhesive according to claim 8, characterized in that, The pre-dispersion process in step two uses a combination of mechanical stirring dispersion and ultrasonic-assisted dispersion. The ultrasonic power is 300-500W and the ultrasonic time is 15-25min. After dispersion, the monodispersity ratio of the hydroxylated nanocomposite filler is ≥90%.

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