Moisture-curable polyurethane hot melt adhesive and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, and discloses a moisture-curable polyurethane hot melt adhesive and a preparation method thereof, and the moisture-curable polyurethane hot melt adhesive comprises 10-20 wt% of polyester polyol; 40 wt% to 50 wt% of polyether polyol; 20 wt% to 30 wt% of tackifying resin; 10 wt% to 15 wt% of isocyanate; the particle size of the tackifying resin is 60 to 120 meshes. According to the moisture-cured polyurethane hot melt adhesive and the preparation method thereof, through optimal design of the component proportion of the moisture-cured polyurethane hot melt adhesive and the particle size of the tackifying resin, the mechanical property, the rheological property and the appearance quality are synergistically improved, the tackifying resin with the particle size range of 60-120 meshes is selected, so that the tackifying resin has good dispersity and a proper supporting effect in a polyurethane system, and the moisture-cured polyurethane hot melt adhesive is prepared. The high-temperature bonding strength can be effectively improved, meanwhile, the problems of net blocking risk and bubbling caused by overlarge particle size are avoided, and the smoothness and production stability of an adhesive layer are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a moisture-curing polyurethane hot melt adhesive and its preparation method. Background Technology

[0002] Engineered wood products are typically made from wood or non-wood plant materials, processed mechanically into unit materials such as fibers and shavings, then bonded together with adhesives. These materials offer advantages such as low cost, high resource utilization, stable performance, and strong decorative properties, making them widely used in furniture, construction, and decoration. However, engineered wood itself has limited appearance and durability, making it difficult to meet decorative and functional requirements. Therefore, surface coatings are usually applied to improve flatness, wear resistance, and visual appeal. Currently, a common finishing method is to heat-press melamine-impregnated paper onto the surface of the board. This type of finish provides a certain decorative effect but still suffers from drawbacks such as limited color and texture options, easily damaged edge banding, and a harsh feel. To address this, the industry often laminates PVC film, PET film, or other materials over the impregnated paper to further enhance the appearance and durability. However, this introduces new challenges, such as improved adhesion and thermal and moisture stability.

[0003] In the bonding process, commonly used adhesives such as urea-formaldehyde resin, phenolic resin, EVA hot melt adhesive, and white glue have problems such as high formaldehyde release, free phenol content, and insufficient heat and water resistance, making it difficult to balance environmental protection and bonding strength. Reactive moisture-curing polyurethane hot melt adhesives (PUR) have gradually become the mainstream choice for bonding engineered wood panels due to their solvent-free nature, resistance to high and low temperatures, good water resistance, and high strength after curing. PUR cures by reacting the isocyanate-terminated prepolymer with moisture in the air or substrate to form an irreversible high-strength adhesive layer, which can significantly improve the weather resistance and environmental performance of the panel surface.

[0004] To further improve the initial tack and cohesiveness of polyurethane (PUR), tackifying resins are typically introduced into the formulation. However, existing technologies often focus only on improving bond strength and adaptability, lacking systematic research on the particle size, distribution, and compatibility of tackifying resins with the polyurethane matrix. This leads to problems such as decreased bond strength, blistering on the board surface, film peeling, and screen clogging during production, especially under high temperature or humid conditions. Furthermore, some formulations exhibit increased viscosity, reduced flowability, and yellowing during long-term storage, affecting the processing stability and appearance quality of the product.

[0005] Therefore, how to improve the resistance to damp heat, yellowing resistance and storage stability of polyurethane hot melt adhesives while ensuring initial tack and workability has become an urgent technical problem to be solved in the field of adhesives for engineered wood panel finishes. Summary of the Invention

[0006] This invention provides a moisture-curing polyurethane hot melt adhesive and its preparation method to solve the problems of reduced bonding strength, easy blistering on the board surface, poor storage stability, and easy yellowing of polyurethane hot melt adhesives in the prior art under high temperature or humid heat environment.

[0007] In a first aspect, the present invention provides a moisture-curing polyurethane hot melt adhesive, comprising, by mass fraction: Polyester polyol 10wt%-20wt%; Polyether polyol 40wt%-50wt%; Tackifying resin 20wt%-30wt%; Isocyanate 10wt%-15wt%; The particle size range of the tackifying resin is 60 mesh to 120 mesh.

[0008] The tackifying resin includes acrylic resin; preferably, the softening point of the acrylic resin is 80℃-170℃, more preferably 120℃-150℃; for example, the softening points of the acrylic resin are 80℃, 100℃, 120℃, 130℃, 140℃, 150℃, and 170℃.

[0009] This invention achieves a synergistic improvement in mechanical properties, rheological properties, and appearance quality through optimized design of the component ratio and tackifying resin particle size of moisture-curing polyurethane hot melt adhesive. The tackifying resin, with a particle size range of 60-120 mesh, is selected to provide both good dispersibility and adequate support within the polyurethane system. This effectively enhances high-temperature bonding strength while avoiding the risk of clogging and foaming caused by excessively large particle sizes, significantly improving the smoothness and production stability of the adhesive layer.

[0010] In this invention, the proportions of polyester polyol, polyether polyol, tackifying resin, and isocyanate in the formulation are rationally controlled to achieve a balance between viscosity, flowability, and crosslinking reactivity, thereby obtaining a high-strength, low-bubble, and uniformly surfaced bonding effect, which significantly improves the applicability and overall performance of hot melt adhesives in high-temperature composite processes.

[0011] In one alternative embodiment, the polyester polyol comprises one or more of linear polyester polyols and branched polyester polyols.

[0012] The linear polyester polyol is selected from one or more of crystalline polyester polyols and liquid polyester polyols; Preferably, the crystalline polyester polyol and the liquid polyester polyol have a functionality of 2 and a number-average molecular weight of 3500-5000; for example, a number-average molecular weight of 3500, 4000, or 5000.

[0013] Preferably, the linear polyester polyol is obtained by reacting a diol with a diacid or a diacid anhydride; the diol is selected from one or more of 1,2-propanediol, ethylene glycol, neopentyl glycol, diethylene glycol, 1,4-butanediol, and 1,6-hexanediol; the diacid is selected from one or more of dodecanoic acid, sebacic acid, adipic acid, isophthalic acid, and terephthalic acid; and the diacid anhydride is selected from one or more of phthalic anhydride, acetic anhydride, and succinic anhydride.

[0014] In one optional embodiment, the mass fraction of branched polyester polyol in the polyester polyol is 25wt%-50wt%. For example, the mass fraction of branched polyester polyol in the polyester polyol is 25wt%, 30wt%, 40wt%, or 50wt%.

[0015] In this invention, the branched polyester polyol contains a multi-hydroxyl structure, enabling it to react with isocyanates at more points, thereby forming a higher crosslinking density and a three-dimensional network structure in the system. Compared to systems using only linear polyester polyols, the introduction of the branched structure results in tighter bonding between the molecular chains of the colloid, enhancing the cohesive strength and interfacial adhesion of the polyurethane hot melt adhesive. This structural advantage significantly improves the adhesive strength of the material under high-temperature conditions, while also enhancing its resistance to stress relaxation and deformation.

[0016] Furthermore, the presence of multifunctional groups in branched polyester polyols helps improve the system's resistance to humid heat. A higher crosslinking density effectively inhibits the hydrolysis of polyurethane bonds by water molecules under humid and hot conditions, slowing down the material's aging rate and enabling the hot melt adhesive to maintain high bonding strength and structural stability even under prolonged high temperature and humidity conditions.

[0017] Meanwhile, the branched structure improves the storage stability of the colloid to a certain extent. Due to the enhanced steric hindrance between molecular chains, unreacted isocyanate groups in the system are less likely to undergo self-polymerization or react with trace amounts of moisture, thereby reducing viscosity growth and colloid performance degradation during storage.

[0018] In one optional embodiment, the branched polyester polyol is obtained by reacting a diol, a diacid, and a triol branching agent. In one optional embodiment, the branched polyester polyol has a hydroxyl value of 25-35 mgKOH / g; for example: 25 mgKOH / g, 28 mgKOH / g, 30 mgKOH / g, 35 mgKOH / g; In one optional embodiment, the branched polyester polyol has a number average molecular weight of 3200-4500; for example: 3200, 3800, 4000, 4500; In one optional embodiment, the branched polyester polyol is prepared as follows: (1) Add the dicarboxylic acid, diol, and triol branching agent to a reaction vessel equipped with stirring, temperature measurement, and pressure reduction functions in proportion, and add titanate catalyst. Remove air under nitrogen protection to avoid oxidation; wherein the molar ratio of dicarboxylic acid, diol, and triol branching agent is 1:1-1.5:0.03-0.05, for example, the molar ratio is 1:1:0.03, 1:1.2:0.04, or 1:1.5:0.05; (2) Heat to about 160℃-200℃ for esterification reaction. During this process, monitor the acid value. When the acid value drops to below 5mgKOH / g, the esterification is considered to be basically completed. For example, heat to 160℃, 170℃, 180℃ or 200℃. (3) Continue to raise the temperature to about 210℃-230℃ and carry out the polycondensation reaction under reduced pressure. The reaction process is controlled by periodically detecting the hydroxyl value until the target hydroxyl value is reached and then the reaction is stopped. For example, continue to raise the temperature to 210℃, 220℃ or 230℃; preferably, control the reduced pressure to -0.095MPa~-0.098MPa. (4) Cool to about 65℃-85℃, add an appropriate amount of antioxidant to improve product stability, and the branched polyester polyol is obtained; for example, cool to 65℃, 70℃, 80℃ or 85℃. Preferably, the diol includes one or more of 1,2-propanediol, ethylene glycol, neopentyl glycol, diethylene glycol, 1,4-butanediol, and 1,6-hexanediol; Preferably, the dicarboxylic acid includes one or more of dodecanoic acid, sebacic acid, adipic acid, isophthalic acid, and terephthalic acid; Preferably, the triol branching agent includes glycerol.

[0019] In one optional embodiment, the isocyanate includes one or both of diphenylmethane diisocyanate (MDI) and isophorone diisocyanate (IPDI). Preferably, the mass ratio of diphenylmethane diisocyanate to isophorone diisocyanate is 7:3-9:1; Preferably, the diphenylmethane diisocyanate includes one or more of 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI.

[0020] This invention achieves a balance between excellent resistance to yellowing and mechanical properties by using diphenylmethane diisocyanate and isophorone diisocyanate in combination, while ensuring the reactivity and structural strength of the system.

[0021] In the compound system, MDI imparts high reactivity and rigid structure to the colloid. Its aromatic backbone can promote the regular arrangement of the polyurethane backbone, thereby improving the cohesive strength and high-temperature bonding strength of the hot melt adhesive, so that the material can maintain good mechanical support and bonding performance under high-temperature composite and long-term load conditions.

[0022] Meanwhile, IPDI belongs to the alicyclic isocyanate family, and its molecular structure does not contain aromatic rings, exhibiting excellent thermal oxidation stability and UV aging resistance. Introducing IPDI into the MDI system in an appropriate amount can significantly inhibit the oxidative discoloration reaction of aromatic structures under high temperature and light conditions, thereby reducing the yellowing index and improving the material's appearance retention and long-term environmental stability. This invention, by controlling the ratio of MDI to IPDI, balances the dual requirements of mechanical strength and yellowing resistance.

[0023] In one optional embodiment, the polyether polyol includes one or more of polyethylene oxide polyol, polypropylene oxide polyol, polyethylene oxide-propylene oxide copolymer, and polytetrahydrofuran polyol; Preferably, the polyether polyol is selected from polyoxypropylene polyol; Preferably, the molecular weight of the polyether polyol is 400-2000; for example, the molecular weight of the polyether polyol is 400, 500, 1000, 1500 or 2000.

[0024] In one optional embodiment, the moisture-curing polyurethane hot melt adhesive further includes a catalyst, the catalyst comprising one or more of bismorpholinoethyl ether and bisdimethylaminoethyl ether; Preferably, the amount of catalyst added is 100ppm-800ppm, more preferably 300ppm-600ppm, based on the total mass of polyester polyol, polyether polyol, tackifying resin and isocyanate; for example, the amount of catalyst added is 100ppm, 200ppm, 300ppm, 500ppm, 600ppm and 800ppm.

[0025] In one optional embodiment, the moisture-curing polyurethane hot melt adhesive further includes a hindered phenolic antioxidant; Preferably, based on the total mass of polyester polyol, polyether polyol, tackifying resin, and isocyanate, the amount of hindered phenolic antioxidant added is 100ppm-1000ppm, preferably 400ppm-700ppm; for example, the amount of hindered phenolic antioxidant added is 100ppm, 300ppm, 400ppm, 600ppm, 700ppm, 900ppm, or 1000ppm. Preferably, the antioxidant includes one or more of antioxidant 1010, antioxidant 1076, antioxidant 245, antioxidant 168, antioxidant T501, or antioxidant B215.

[0026] Secondly, the present invention also provides a method for preparing a moisture-curing polyurethane hot melt adhesive as described in any of the above technical solutions, comprising the following steps: S1. Mix polyester polyol, polyether polyol and tackifying resin to form a mixed system, and dehydrate the mixed system to a water content of less than or equal to 100 ppm. S2. Isocyanate is added to the dehydrated mixture at a temperature of 90℃-140℃ to react and obtain the final product.

[0027] In step S1, the polyester polyol, polyether polyol, and tackifying resin are thoroughly mixed and dehydrated until the water content is ≤100 ppm. This effectively removes trace amounts of water from the system and prevents isocyanate from reacting with water during the reaction to form urea bonds or carbon dioxide. This prevents problems such as bubbles and abnormal viscosity increases from occurring in the system, ensuring that the subsequent reaction process is stable and controllable.

[0028] In step S2, isocyanate is introduced into the reaction at a temperature range of 90 to 140°C. This temperature range can ensure the efficient reaction between isocyanate and polyol, promote the uniform formation of urethane bonds, and avoid the degradation or gelation of polyurethane segments caused by excessively high temperatures, so that the resulting product has a narrow molecular weight distribution and a moderate system viscosity.

[0029] Through the above-described process control, the entire reaction process exhibits fewer bubbles and more stable heat release. The resulting moisture-curing polyurethane hot melt adhesive has a uniform structure and low impurity content, demonstrating excellent bond strength, thermal stability, and storage stability. Furthermore, this process is simple, has a wide temperature control window, is suitable for continuous industrial production, and possesses good operability and consistency.

[0030] In one optional embodiment, step S1 includes: dehydration at a temperature of 100℃-160℃; In one optional embodiment, step S1 includes: first performing pre-dehydration at a temperature of 80℃-100℃, and then raising the temperature to 130℃-140℃ for deep dehydration; Preferably, the mixture is dehydrated to a water content of less than or equal to 50 ppm.

[0031] This invention further achieves more precise control of the system's water content by employing a segmented dehydration process, significantly improving the purity of the reaction system and the stability of subsequent products. Specifically, pre-dehydration is first carried out at 80℃~100℃, which effectively removes volatile free water and some low-boiling impurities from the system; then, deep dehydration is carried out at 130℃~140℃, which fully removes bound water and high-boiling residual water, thereby reducing the final water content of the system to ≤50 ppm.

[0032] This two-step dehydration strategy significantly suppresses the side reactions of isocyanate with water in subsequent reactions, preventing the formation of urea bonds and carbon dioxide, thus avoiding bubble formation or gelation and ensuring a stable and controllable reaction process. The fully dehydrated system not only improves the structural integrity and crosslinking uniformity of the polyurethane segments but also gives the resulting hot melt adhesive superior storage stability, viscosity retention, and resistance to damp heat.

[0033] The segmented dehydration process of this invention achieves efficient control of moisture in the polyurethane reaction system through graded temperature control and fine water removal, providing a reliable process guarantee for obtaining a dense, stable, and industrially continuous moisture-curing polyurethane hot melt adhesive.

[0034] In one optional embodiment, in step S1, dehydration is carried out under a vacuum of -0.095MPa to -0.098MPa. In one optional embodiment, in step S2, isocyanate is added to the dehydrated mixture at a temperature of 90°C-100°C to carry out the reaction.

[0035] The technical solution of this invention has the following advantages: This invention achieves a synergistic improvement in mechanical properties, rheological properties, and appearance quality through optimized design of the component ratio and tackifying resin particle size of moisture-curing polyurethane hot melt adhesive. The tackifying resin, with a particle size range of 60-120 mesh, is selected to provide both good dispersibility and adequate support within the polyurethane system. This effectively enhances high-temperature bonding strength while avoiding the risk of clogging and foaming caused by excessively large particle sizes, significantly improving the smoothness and production stability of the adhesive layer.

[0036] In this invention, the proportions of polyester polyol, polyether polyol, tackifying resin, and isocyanate in the formulation are rationally controlled to achieve a balance between viscosity, flowability, and crosslinking reactivity, thereby obtaining a high-strength, low-bubble, and uniformly surfaced bonding effect, which significantly improves the applicability and overall performance of hot melt adhesives in high-temperature composite processes.

[0037] Additional aspects and advantages of the embodiments of the present invention will be described and shown in part in the following description, or illustrated by practice of the embodiments of the present invention. Detailed Implementation

[0038] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0039] The main raw materials involved in the embodiments and comparative examples of this invention are as follows: Crystalline polyester polyol 1: molecular weight 3500, Evonik 7360; Liquid polyester polyol 2: molecular weight 5000, Evonik 7250; Polyether polyol 3A: Polyoxypropylene polyol with a molecular weight of 2000, Dongda DL-2000; Polyether polyol 3B: Polyoxypropylene polyol with a molecular weight of 1000, Dongda DL-1000; Polyether polyol 3C: Polyoxypropylene polyol, molecular weight 400, Dongda DL-400; Tackifying resin 4A: Acrylic resin, molecular weight 60,000, softening point 130℃, Rohm LP66 / 02N; Tackifying resin 4B: Acrylic resin with a molecular weight of 60,000 and a softening point of 120°C, Mitsubishi BR106. Tackifying resin 4C: Acrylic resin, molecular weight 60000, softening point 130℃, Kunler RS530; Tackifying resin 4D: Acrylic resin with a molecular weight of 55,000 and a softening point of 150°C, Evonik AC1630. Isocyanate 5A: Diphenylmethane diisocyanate, Wanhua Chemical WANNATE MDI-100; Isocyanate 5B: Isophorone diisocyanate, Wanhua Chemical WANNATE IPDI; Antioxidant 6: Tetra[β] (3,5 Second Uncle Ding Ji 4 [Hydroxyphenyl]propionate pentaerythritol ester, Rianon 1010; Catalyst 7: Bismorpholinoethyl ether, Huntsman JEFFCAT DMDEE; Branched polyester polyol 8A: 0.03 mol glycerol, hydroxyl value 25 mg KOH / g, number average molecular weight 4500; preparation method: 1 mol diacid, 1.2 mol diol and 0.03 mol glycerol are added to a reactor equipped with stirring, temperature measurement and pressure reduction devices. Based on the total mass of the materials in the reactor, 0.1% wt tetrabutyl titanate catalyst is added, and nitrogen is introduced to replace the air; the temperature is raised to 180℃ and esterified for 4 hours. During this period, the acid value is measured every 1 hour. When the acid value is ≤5 mg KOH / g, the temperature is raised to 220℃ and pressure reduction is started. The pressure is reduced to -0.095 MPa to -0.098 MPa for condensation for 2~3 hours. The hydroxyl value is measured every 30 minutes until the hydroxyl value reaches 25 mg KOH / g. The reaction is stopped, the temperature is lowered to 80℃, antioxidant BHT is added and the product is discharged to obtain branched polyester polyol 8A.

[0040] Branched polyester polyol 8B: 0.05 mol glycerol, hydroxyl value 35 mg KOH / g, number average molecular weight 3200; preparation method: 1 mol diacid, 1.2 mol diol and 0.05 mol glycerol are added to a reactor equipped with stirring, temperature measurement and pressure reduction devices. Based on the total mass of the materials in the reactor, 0.1% wt tetrabutyl titanate catalyst is added, and nitrogen is introduced to replace the air; the temperature is raised to 180℃ and esterified for 4 hours. During this period, the acid value is measured every 1 hour. When the acid value is ≤5 mg KOH / g, the temperature is raised to 220℃ and pressure reduction is started. The pressure is reduced to -0.095MPa~-0.098MPa for condensation for 2~3 hours. The hydroxyl value is measured every 30 minutes until the hydroxyl value reaches 35 mg KOH / g. The reaction is stopped, the temperature is lowered to 80℃, antioxidant BHT is added and discharged to obtain branched polyester polyol 8B.

[0041] Screening methods for tackifying resin 4 with different particle sizes in embodiments and comparative examples of the present invention: This invention obtains samples with different particle size distributions by sieving tackifying resins 4A, 4B, 4C, and 4D from the same batch using different mesh sizes, as detailed below: The particle size range is 60 mesh to 120 mesh: the tackifying resin 4A is first sieved through a 60 mesh sieve, and the portion that passes through the 60 mesh sieve is taken. Then it is sieved through a 120 mesh sieve, and the portion that is retained on the 120 mesh sieve is taken to obtain the tackifying resin 4A with a particle size of 125μm-250μm. Particle size range of 120 mesh to 200 mesh: First, the same batch of tackifying resin 4A is sieved through a 120 mesh sieve, and the portion that passes through the 120 mesh sieve is taken. Then, it is sieved through a 200 mesh sieve, and the portion that is retained on the 200 mesh sieve is taken to obtain tackifying resin 4A with a particle size of 75μm-125μm. Particle size range of 40 mesh to 60 mesh: First, the same batch of tackifying resin 4A is sieved through a 40 mesh sieve, and the portion that passes through the 40 mesh sieve is taken. Then, it is sieved through a 60 mesh sieve, and the portion that is retained on the 60 mesh sieve is taken to obtain tackifying resin 4A with a particle size of 250μm-425μm. Particle size range of 40 mesh to 200 mesh: The same batch of tackifying resin 4A is first sieved through a 40 mesh sieve, and the portion that passes through the 40 mesh sieve is taken. Then it is sieved through a 200 mesh sieve, and the portion that is retained on the 200 mesh sieve is taken to obtain tackifying resin 4A with a particle size of 75μm-425μm.

[0042] Tackifying resins 4B, 4C, and 4D from the same batch were screened using the same screening method to obtain samples with different particle size distributions.

[0043] The main performance testing methods involved in the embodiments and comparative examples of this invention are as follows: Viscosity: Referring to GB / T 9751-1988, set the temperature of the Brookfield CAP 2000 cone-plate viscometer to 120℃, place the sample to be tested in the center of the plate, and after the sample has basically melted into a flowable liquid, press down the handle to ensure that the sample fully fills the entire space between the cone plates, clean up the excess sample, and perform the test after the temperature stabilizes.

[0044] High-temperature bonding strength: Referring to GB / T 2792-2014, at 35℃ / 60%RH, hot melt adhesive was melted using a flat laminator to bond melamine particleboard and transparent PET film. The adhesive layer thickness was approximately 0.05mm. After bonding, the film was cold-pressed using a pressure roller. After 24 hours, the bonded PET film was cut into 25mm wide strips, and the peel force at 180° was tested using a tensile testing machine. The bonding strength is the average value of the peel force.

[0045] Bubbling on the board surface: Visually inspect the bubbling after the transparent PET film is bonded to the board surface and record the number of bubbles; "basically no bubbles" means: the number of bubbles ≤ 20 / m². "Fewer bubbles" means: 20 < number of bubbles ≤ 40 bubbles / m²; "Many bubbles" means that the number of bubbles is >40 per m².

[0046] Colloidal particle condition: The hot melt adhesive was heated to a fluid state (130℃) and then quickly applied to the surface of melamine particleboard. The particle condition of the applied adhesive layer was observed, and the number and size of the particles were recorded. "Small particles, few in number" means: particle size <100 μm, particle number ≤1 particle / cm²; "smaller particles, fewer in number" means: 100 ≤ particle size ≤200 μm, particle number >1 particle / cm². "Large particle size, small number" means: particle size > 200 μm, particle number > 1 particle / cm²; Moist heat strength retention rate: In accordance with GB / T 2792-2014 standard, the bonded sample was placed in a constant temperature and humidity chamber at 40℃ / 90%RH for 7 days for aging, and the 180° peel force was tested. The damp heat strength retention rate is the percentage of the strength after aging to the strength before aging.

[0047] Yellowing index: Referring to GB / T 1865-2009, after aging with a xenon lamp for 100 hours (irradiance 0.51W / m²@340nm, 38℃, 50%RH), the yellowing index ΔE was measured using a colorimeter.

[0048] Viscosity change rate at 40℃: The hot melt adhesive was sealed in a 40℃ oven and stored for 30 days. The viscosity at 120℃ was then tested, and the viscosity change rate was calculated.

[0049] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0050] Example 1 This embodiment provides a moisture-curing polyurethane hot melt adhesive and its preparation method, specifically including the following steps: Polyester polyol, polyether polyol, tackifying resin, and antioxidant are added to a reactor and mixed. The mixture is then vacuum-dehydrated at 130°C for 2 hours. When the moisture content in the reactor is reduced to below 100 ppm, the temperature is lowered to 100°C under a nitrogen atmosphere. Isocyanate is added, and the oil bath is kept at 100°C for the reaction. The viscosity and temperature are tested every 30 minutes. The reaction is considered complete when the viscosity increase between two consecutive tests is less than 2%. After the reaction is complete, bismorpholinoethyl ether (DMDEE) catalyst is added and mixed evenly. The mixture is then discharged under nitrogen protection to obtain a moisture-curing polyurethane hot melt adhesive.

[0051] The specific raw materials involved in this embodiment are shown in Table 1 below. In addition, the present invention also provides Examples 2 to 16 and Comparative Examples 1 to 12. Referring to the process method described in Example 1, the relevant conditions are adjusted and the specific conditions are listed in Table 1. Based on these conditions, the moisture-curing polyurethane hot melt adhesives of Examples 1 to 16 and Comparative Examples 1 to 12 are prepared respectively.

[0052] In Table 1, “R value” represents the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) in the moisture-curing polyurethane hot melt adhesive; “NCO%” represents the mass fraction of isocyanate groups (–NCO) in the moisture-curing polyurethane hot melt adhesive; the addition amount of each component in Table 1 is in parts by mass.

[0053] Table 1: Formulation composition of moisture-curing polyurethane hot melt adhesives in Examples 1-16 and Comparative Examples 1-12

[0054] (Note: In the table above, "-" indicates that the substance is not added.) Example 17 This embodiment provides a polyurethane hot melt adhesive and its preparation method, specifically including the following steps: Polyester polyol, polyether polyol, tackifying resin, and antioxidant are added to the reactor and mixed. A segmented dehydration process is adopted. After pre-dehydration at 80°C for 1 hour, it is further dehydrated at 130°C for 1 hour. When the moisture in the reactor is reduced to below 50 ppm, the temperature is lowered to 100°C under a nitrogen atmosphere, and isocyanate is added. The oil bath is kept at 100°C for reaction. The viscosity and temperature are tested every 30 minutes. When the viscosity increase between two adjacent tests is less than 2%, the reaction is considered to be over. After the reaction is over, bismorpholinoethyl ether (DMDEE) catalyst is added and mixed evenly. The product is discharged under nitrogen protection to obtain the moisture-curing polyurethane hot melt adhesive.

[0055] The specific raw materials involved in this embodiment are shown in Table 2 below. In addition, the present invention also provides Examples 18 to 25 and Comparative Examples 13 to 15. Referring to the process method described in Example 17, the relevant conditions are adjusted and the specific conditions are listed in Table 2. Based on these conditions, the moisture-curing polyurethane hot melt adhesives of Examples 17 to 25 are prepared respectively.

[0056] In Table 2, “R value” represents the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) in the moisture-curing polyurethane hot melt adhesive; “NCO%” represents the mass fraction of isocyanate groups (–NCO) in the moisture-curing polyurethane hot melt adhesive; the addition amount of each component in Table 2 is in parts by mass.

[0057] Table 2: Formulation composition of moisture-curing polyurethane hot melt adhesives in Examples 17-25 and Comparative Examples 13-15

[0058] (Note: In the table above, "-" indicates that the substance is not added.) Test Example 1 The performance of the products obtained in each embodiment and comparative example was tested, and the results are shown in Tables 3 and 4 below; Table 3: Performance test results of hot melt adhesives obtained in Examples 1–16 and Comparative Examples 1–12

[0059] Table 4: Test results of hot melt adhesive properties obtained in Examples 1, 17–23 and Comparative Examples 13–15

[0060] As shown in Tables 3 and 4, the moisture-curing polyurethane hot melt adhesives obtained in the embodiments of the present invention exhibit excellent overall performance. The products of each embodiment all demonstrate moderate melt viscosity and good flowability at 120°C, resulting in stable construction operations. The bonded samples generally exhibit high high-temperature bond strength and strong interfacial bonding. Compared to the comparative examples, the bubbling phenomenon on the board surface of the embodiments of the present invention is significantly reduced, with most samples showing a "virtually bubble-free" state, indicating good defoaming and leveling properties of the colloid. Simultaneously, the resulting adhesive layer particles are fine and few in number, indicating good system dispersion uniformity.

[0061] As shown in Table 3, when the particle size of the added tackifying resin is in the range of 120 mesh to 200 mesh, the high-temperature bonding strength of the resulting hot melt adhesive system is significantly lower. This indicates that although the tackifying resin has good dispersibility in the polyurethane system within this particle size range, its physical support and filling effect in the colloid is limited due to the small particle size, making it difficult to effectively improve the overall mechanical strength of the system.

[0062] When the tackifying resin particle size ranges from 40 to 60 mesh and the particle size is 250 to 425 μm, the high-temperature bonding strength of the resulting hot melt adhesive system is improved, but the number of colloidal particles increases significantly and the particle size is larger, resulting in a phenomenon of "more bubbles" on the board surface. This indicates that within this particle size range, the tackifying resin has poor solubility in the polyurethane system, and large, undispersed particles are prone to remain under the same high-temperature dispersion conditions. These large particles not only easily clog the filter screen during production, causing difficulties in discharge, but also form a localized uneven structure in the colloid, weakening the interfacial forces such as van der Waals forces between the adhesive layer and the substrate.

[0063] When the tackifying resin particle size ranges from 40 mesh to 200 mesh, with a wide particle size distribution (75–425 μm), both large and small resin particles coexist in the system. The dissolution rates and dispersion characteristics between these particles differ significantly, making it difficult to form a uniform dispersion within the polyurethane matrix. Larger particles are less likely to dissolve completely during heating and dispersion, easily forming localized aggregates in the colloid; while smaller particles tend to over-dissolve, weakening the system's supporting structure. This combination leads to an imbalance in the internal structure of the colloid, a significant increase in the number of particles, and uneven distribution, resulting in decreased adhesive layer density and discontinuous stress transmission. Ultimately, this manifests as lower high-temperature bonding strength, poorer board surface smoothness, and a tendency to exhibit surface defects such as a grainy texture and localized bubbles. Overall, the bonding performance is significantly inferior to systems with concentrated and uniformly dispersed particle sizes.

[0064] In this embodiment of the invention, a tackifying resin with a particle size range of 60-120 mesh and a particle size of 125-250 μm is selected. Tackifying resin particles within this particle size range can fully dissolve and uniformly distribute within the system under high-temperature conditions. This effectively fills the microscopic voids within the polyurethane matrix, enhancing the density and cohesive strength of the adhesive layer, while avoiding structural inhomogeneity and bubble problems caused by excessive dissolution of fine particles or residual coarse particles. Consequently, the resulting hot melt adhesive system exhibits higher interfacial bonding strength and better surface smoothness during high-temperature bonding. The production process is characterized by smooth material discharge and stable colloid, significantly improving the material's high-temperature applicability, composite quality, and industrial reliability.

[0065] As shown in Table 4, Example 17 exhibits significant improvements in storage stability and resistance to damp heat compared to Example 1, which did not employ the segmented dehydration process. This invention effectively controls the moisture content of the system through a segmented dehydration process, reducing isocyanate side reactions and uneven crosslinking caused by residual moisture, thereby significantly improving the storage stability of the moisture-curing polyurethane hot melt adhesive.

[0066] Furthermore, comparing Examples 20, 21, and 22 with Example 17, Examples 20-22 introduced branched polyester polyols into their formulations, while Example 17 did not. It is evident that the wet heat strength retention rate of Examples 20-22 was improved. This demonstrates that the introduction of branched polyester polyols resulted in a higher crosslinking density within the system. The branched polyester polyols contain multiple hydroxyl active sites, enabling them to react with isocyanates to generate a denser and more stable polyurethane crosslinking network structure, thereby enhancing the cohesive strength and interfacial bonding of the colloid. The improved crosslinking structure not only enhances the adhesive's bonding capacity under high-temperature conditions but also reduces performance degradation caused by hydrolysis or chain segment relaxation in wet and hot environments, resulting in excellent wet and hot stability and high-temperature bonding performance of the hot melt adhesive system.

[0067] As shown in Table 4, Examples 18, 19, and 23 used a compound isocyanate system, while Example 17 used a single diphenylmethane diisocyanate (MDI). The comparative results indicate that the compound isocyanate system significantly improved yellowing resistance while maintaining high adhesive strength, demonstrating that the compound isocyanate system can effectively inhibit thermal oxidative discoloration without sacrificing mechanical properties.

[0068] When the content of diphenylmethane diisocyanate in the isocyanate system is too low, the high-temperature adhesive strength of the system decreases, indicating that an excessively low MDI content will weaken the structural stability and mechanical support of the polyurethane backbone. Therefore, this invention selects a mass ratio of diphenylmethane diisocyanate to isophorone diisocyanate of (7:3) to (9:1) to achieve a balance between yellowing resistance and adhesive strength, thereby obtaining a moisture-curing polyurethane hot melt adhesive with optimal overall performance.

[0069] This invention achieves a comprehensive improvement in the performance of moisture-curing polyurethane hot melt adhesives by optimizing the formulation and process parameters. Specifically, selecting tackifying resins with a particle size range of 60-120 mesh and a particle size of 125-250 μm ensures good dispersibility while providing adequate filling and support, resulting in higher adhesive strength at high temperatures. Due to the moderate particle size and good solubility, the colloid contains virtually no visible particles, significantly improving the foaming phenomenon during high-temperature bonding of composite boards and films, and avoiding screen clogging caused by large particles during production. This significantly enhances the high-temperature applicability and production stability of the hot melt adhesive.

[0070] Furthermore, a segmented dehydration process was adopted to reduce the residual moisture in the system, and a compound system of branched polyester polyol, diphenylmethane diisocyanate and isophorone diisocyanate was introduced to achieve higher crosslinking density and molecular structure stability, so that the resulting hot melt adhesive exhibits excellent comprehensive performance in terms of storage stability, resistance to damp heat and yellowing resistance.

[0071] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A moisture-curable polyurethane hot melt adhesive characterized by: By mass fraction, comprising: Polyester polyol 10wt%-20wt%; Polyether polyol 40wt%-50wt%; Tackifying resin 20wt%-30wt%; Isocyanate 10wt%-15wt%; The particle size of the tackifying resin is 60-120 mesh.

2. The moisture-curable polyurethane hot melt adhesive according to claim 1, characterized in that: The polyester polyol comprises one or more of linear polyester polyol and branched polyester polyol; Preferably, the mass fraction of branched polyester polyol in the polyester polyol is 25wt%-50wt%.

3. The moisture-curable polyurethane hot melt adhesive according to claim 2, characterized in that: The branched polyester polyol is obtained by reacting dihydric alcohol, diacid and trihydric alcohol branching agent; Preferably, the hydroxyl value of the branched polyester polyol is 25-35mgKOH / g, and / or the number average molecular weight of the branched polyester polyol is 3200-4500; Preferably, the dihydric alcohol comprises one or more of 1,2-propanediol, ethylene glycol, neopentyl glycol, diethylene glycol, 1,4-butanediol, 1,6-hexanediol, and / or the diacid comprises one or more of dodecanedioic acid, sebacic acid, adipic acid, isophthalic acid, terephthalic acid, and / or the trihydric alcohol branching agent comprises glycerol.

4. The moisture-curable polyurethane hot melt adhesive according to any one of claims 1 to 3, characterized in that: The isocyanate comprises one or both of diphenylmethane diisocyanate and isophorone diisocyanate; Preferably, the mass ratio of diphenylmethane diisocyanate to isophorone diisocyanate is (7:3)-(9:1).

5. The moisture-curable polyurethane hot melt adhesive according to any of claims 1 to 4, characterized in that The polyether polyol comprises one or more of polyoxyethylene polyol, polyoxypropylene polyol, polyoxyethylene-oxypropylene copolymer, and polytetrahydrofuran polyol; Preferably, the number average molecular weight of the polyether polyol is 400-2000.

6. The moisture-curable polyurethane hot melt adhesive according to any of claims 1 to 5, characterized in that The moisture-curable polyurethane hot melt adhesive further comprises a catalyst, which comprises one or more of dimorpholinylethyl ether and bisdimethylaminoethyl ether; Preferably, the addition amount of the catalyst is 100ppm-800ppm, preferably 300ppm-600ppm, based on the total mass of the polyester polyol, polyether polyol, tackifying resin and isocyanate.

7. The moisture-curable polyurethane hot melt adhesive according to any of claims 1 to 6, characterized in that The moisture-curable polyurethane hot melt adhesive further comprises a hindered phenolic antioxidant; Preferably, the addition amount of the hindered phenolic antioxidant is 100ppm-1000ppm, preferably 400ppm-700ppm, based on the total mass of the polyester polyol, polyether polyol, tackifying resin and isocyanate.

8. A process for the preparation of moisture-curable polyurethane hot-melt adhesives according to any one of claims 1 to 7, characterized in that: Comprising the following steps: S1, mixing polyester polyol, polyether polyol and tackifying resin to form a mixed system, and dehydrating the mixed system to a water content of less than or equal to 100ppm; S2, adding isocyanate to the dehydrated mixed system at a temperature of 90-140℃ to react, and obtaining the moisture-curable polyurethane hot melt adhesive.

9. The method for preparing a moisture-curing polyurethane hot melt adhesive according to claim 8, characterized in that: The step S1 comprises dehydrating at a temperature of 100-160℃; Alternatively, the step S1 comprises first pre-dehydrating at a temperature of 80-100℃, and then deep dehydrating at a temperature of 130-140℃; Preferably, the mixed system is dehydrated to a water content of less than or equal to 50ppm.

10. A method for preparing a moisture-curing polyurethane hot melt adhesive according to claim 8 or 9, characterized in that: In the step S2, the isocyanate is added to the dehydrated mixture at a temperature of 90-100°C.

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