Polyurethane as hot-melt adhesive and alcohol-soluble adhesive, and preparation method and application thereof

Abstract

The application provides a polyurethane serving as a hot melt adhesive and an alcohol-soluble adhesive, and a preparation method and application thereof, and relates to the technical field of polyurethane adhesives.The polyurethane is obtained by mixing polyglycol, diisocyanate, an organic solvent and a catalyst to perform a polymerization reaction, and then mixing a chain extender (a glycol containing a 2-ureido-4-pyrimidone group and a glycol containing a bipyridine group) to perform a chain extension reaction.The 2-ureido-4-pyrimidone and the bipyridine group are introduced into the high molecular chain of the polyurethane, the hydrogen bond formed by the 2-ureido-4-pyrimidone and the bipyridine group can provide strong intermolecular cohesive force inside the adhesive, improve the strength of the material itself, and can enhance the adhesion to the surface of a base material;the bipyridine group can form a chelate structure with metal ions on the surface of a metal base material, and can enhance the bonding strength to the metal base material.The application realizes that a single polyurethane material has the dual functions of hot melt bonding and room-temperature green solvent bonding, and has excellent bonding performance and more moderate bonding conditions.

Description

Technical Field

[0001] This invention relates to the field of polyurethane adhesive technology, and more particularly to a polyurethane that can be used as both a hot melt adhesive and an alcohol-based adhesive, its preparation method, and its application. Background Technology

[0002] Polyurethane adhesives hold a significant position in the industrial bonding field due to their highly designable molecular structure, excellent mechanical properties, and good interfacial adaptability. Polyurethane adhesives include two application forms: hot melt adhesives and solvent-based adhesives. Typically, when polyurethane is used as a hot melt adhesive, it is first heated to melt and achieve fluidity, thus wetting the surfaces to be bonded; then it is cooled and cured, and its polymer chains restore cohesive strength through rapid crystallization and hydrogen bond recombination, forming a bond. Polyurethane hot melt adhesives are widely used in automotive interior assembly, electronic component packaging, and other fields due to their high-strength bonding properties. When polyurethane is used as a solvent-based adhesive, the solvent first dissolves and swells the polymer chains, reducing the system viscosity to promote wetting and penetration of the substrate; after the solvent evaporates or diffuses, the polymer segments re-entangle and solidify through intermolecular forces (such as van der Waals forces and hydrogen bonds) or crystallization, forming a bond. Polyurethane solvent-based adhesives are widely used in textile lamination, packaging lamination, and other fields due to their ease of application and good adhesion. Generally, polyurethane hot melt adhesives require melting at relatively high temperatures and for extended periods. For example, related technologies report a polyurethane hot melt adhesive that achieves a bond strength of 9.02 MPa after heating at 130°C for 30 minutes; another related technology reports a polyurethane solvent-based adhesive, which is an acetone solution of polyurethane. Due to the use of acetone solvent, it has a strong odor, and even after heating the substrate at 35°C to evaporate the solvent and curing for 12 hours, the bond strength is only 0.236 MPa. The above-mentioned polyurethanes are all used as single hot melt adhesives or single solvent-based adhesives, and their bonding performance still needs improvement. Summary of the Invention

[0003] In view of this, the purpose of this invention is to provide a polyurethane that can function as both a hot melt adhesive and an alcohol-based adhesive, as well as its preparation method and applications. The polyurethane provided by this invention achieves the dual functions of hot melt bonding and room-temperature green solvent bonding with a single material, and possesses excellent bonding performance.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0005] This invention provides a method for preparing a polyurethane that can function as both a hot melt adhesive and an alcohol-based adhesive, comprising the following steps:

[0006] Polydiol, diisocyanate, organic solvent and catalyst are mixed and polymerized to obtain a prepolymer solution;

[0007] The prepolymer solution is mixed with a chain extender to carry out a chain extension reaction, thereby obtaining the polyurethane that can be used as both a hot melt adhesive and an alcohol-based adhesive; the chain extender includes a first chain extender and a second chain extender, the first chain extender having the structure shown in Formula 1, and the second chain extender having the structure shown in Formula 2:

[0008] Formula 1, Formula 2;

[0009] In the structure shown in Formula 1, R4 is an alkyl or hydrogen, R5 is an alkyl or hydrogen, and y is a natural number from 1 to 3; in the structure shown in Formula 2, z is a natural number from 1 to 5.

[0010] Preferably, the polydiol comprises one or more of the following: polypentyl adipate diol, polybutylene adipate diol, polyhexyl adipate diol, poly-1,4-butanediol-1,6-hexanediol adipate diol, polyethylene carbonate diol, polyethylene terephthalate diol, polybutylene isophthalate diol, polytetramethylene ether diol, polyethylene oxide-propylene oxide diol, polyethylene glycol, and polypropylene glycol; the number average molecular weight of the polydiol is 500-3000 Da.

[0011] Preferably, the diisocyanate comprises one or more of hexamethylene diisocyanate, isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, terephthalic diisocyanate, tetramethylphenyldimethyl diisocyanate, 1,4-cyclohexyl diisocyanate, dicyclohexylmethane diisocyanate, and 2,2,4-trimethylhexane diisocyanate; the molar ratio of the polydiol to the diisocyanate is 0.2 to 0.8:1.

[0012] Preferably, the polymerization reaction is carried out at a temperature of 50-100°C for 1-5 hours.

[0013] Preferably, when R4 is an alkyl group, the alkyl group has 1 to 4 carbon atoms; when R5 is an alkyl group, the alkyl group has 1 to 6 carbon atoms.

[0014] Preferably, the molar ratio of the first chain extender to the second chain extender is 0.05:0.95 to 0.95:0.05; and the ratio of the total molar amount of the first chain extender and the second chain extender to the molar amount of diisocyanate used in preparing the prepolymer solution is 0.2 to 0.8:1.

[0015] Preferably, the chain extension reaction is carried out at a temperature of 50~100℃ for a time of 10~32h.

[0016] This invention provides a polyurethane that can be used as both a hot melt adhesive and an alcohol-soluble adhesive, prepared by the preparation method described above.

[0017] This invention provides the application of the polyurethane, which can function as both a hot melt adhesive and an alcohol-based adhesive, as described in the above technical solution, in the field of adhesives.

[0018] Preferably, when the polyurethane is used as a hot melt adhesive, the heating temperature of the polyurethane is 80~140°C;

[0019] When the polyurethane is used as an alcohol-soluble adhesive, the polyurethane is dissolved in an alcohol solvent to form an alcohol-soluble adhesive; the alcohol solvent has 1 to 3 carbon atoms, and the mass fraction of polyurethane in the alcohol-soluble adhesive is 20 to 70%.

[0020] This invention provides a method for preparing polyurethane that can function as both a hot melt adhesive and an alcohol-based adhesive, which has the following advantages compared to existing technologies:

[0021] This invention introduces urea groups and aromatic heterocyclic groups into the polyurethane chain to construct a dense and robust hydrogen bond network, which not only enhances the cohesive strength between polymer chains but also strengthens its adhesion to various substrate interfaces. Crucially, this invention introduces 2-urea-4-pyrimidinone and bipyridine groups into the polyurethane polymer chain using chain extenders (a diol containing a 2-urea-4-pyrimidinone group (Formula 1) and a diol containing a bipyridine group (Formula 2)). The specially integrated 2-urea-4-pyrimidinone and bipyridine groups can form complementary hydrogen bonds with high binding energy. This specific intermolecular force gives the polyurethane an adhesive strength exceeding that of conventional polyurethanes. Simultaneously, the abundant hydrogen bond sites ensure that the polyurethane can form numerous hydrogen bonds with alcohol solvent molecules, thereby achieving excellent dispersibility and solubility in alcohol solvents, which is the foundation for the application of alcohol sols. Furthermore, the bipyridine group not only participates in the formation of high-energy hydrogen bonds, but its nitrogen-containing heterocyclic structure can also form stable nitrogen-metal coordination bonds with various metal surfaces (such as steel and copper). These coordination bonds, combined with the strong hydrogen bond network within the polymer, form multiple and robust covalent and non-covalent interactions at the metal substrate interface, thereby further enhancing the adhesion strength and durability between the adhesive and the metal substrate. This invention, through molecular structural innovation, particularly utilizing the high-energy hydrogen bonding and metal coordination mechanism constructed from 2-ureido-4-pyrimidinone and the bipyridine group, has successfully developed a high-performance polyurethane integrating efficient hot-melt bonding, room-temperature green solvent bonding, universal interfacial bonding, and superior metal bonding performance.

[0022] This invention provides a polyurethane that can be used as both a hot melt adhesive and an alcohol-based adhesive, prepared by the method described above. The polyurethane provided by this invention achieves the dual functions of hot melt bonding and room-temperature green solvent bonding with a single material, and exhibits excellent bonding performance. Example results show that, when used as a hot melt adhesive, the polyurethane provided by this invention only requires heating at 120°C for 10 minutes to achieve a high-strength bond of 13.83 MPa to an iron substrate; and after repeated bonding five times, the bonding strength does not significantly decrease. When used as an alcohol-based adhesive, it can be coated onto the surface of an iron substrate at room temperature, and after solvent evaporation at 25°C for 10 minutes, followed by curing at 25°C and 1 MPa pressure for 1 hour, a bonding strength of 1.65 MPa with the iron substrate can be achieved. Attached Figure Description

[0023] Figure 1 The nuclear magnetic resonance spectrum of the polyurethane adhesive prepared in Example 7 when deuterated DMSO was used as a solvent was obtained by nuclear magnetic resonance spectroscopy.

[0024] Figure 2 The lap shear strength diagram of the polyurethane adhesive prepared in Example 7 when used as a hot melt adhesive to bond iron substrates under different heating temperature conditions.

[0025] Figure 3 The diagram shows the lap shear strength of the bonded iron plates at different heating times under an application temperature of 120°C when the polyurethane adhesive prepared in Example 7 is used as a hot melt adhesive.

[0026] Figure 4 The diagram shows the lap shear strength of the polyurethane adhesive prepared in Example 7 after repeated recycling when used as a hot melt adhesive.

[0027] Figure 5 The lap shear strength diagram of the bonded sample obtained by coating the polyurethane prepared in Example 7 onto the surface of an iron substrate at room temperature, curing it at 25°C and 1MPa for 1 hour after the ethanol solvent evaporated for different durations.

[0028] Figure 6 The polyurethane adhesive prepared in Example 7 was used as an alcohol-soluble adhesive. After being coated at room temperature and left to stand for 10 minutes, and then cured at 25°C and 1 MPa for 1 hour, the resulting sample bonded to the iron substrate could withstand a load of 6 kg. Detailed Implementation

[0029] This invention provides a method for preparing a polyurethane that can function as both a hot melt adhesive and an alcohol-based adhesive, comprising the following steps:

[0030] Polydiol, diisocyanate, organic solvent and catalyst are mixed and polymerized to obtain a prepolymer solution;

[0031] The prepolymer solution is mixed with a chain extender to carry out a chain extension reaction, thereby obtaining the polyurethane that can be used as both a hot melt adhesive and an alcohol-based adhesive; the chain extender includes a first chain extender and a second chain extender, the first chain extender having the structure shown in Formula 1, and the second chain extender having the structure shown in Formula 2:

[0032] Formula 1, Formula 2;

[0033] In the structure shown in Formula 1, R4 is an alkyl or hydrogen, R5 is an alkyl or hydrogen, and y is a natural number from 1 to 3; in the structure shown in Formula 2, z is a natural number from 1 to 5.

[0034] In this invention, unless otherwise specified, all raw materials involved are commercially available products or prepared in accordance with methods known to those skilled in the art.

[0035] The present invention involves mixing polydiol, diisocyanate, organic solvent (denoted as the first organic solvent) and catalyst (denoted as the first catalyst) to carry out a polymerization reaction to obtain a prepolymer solution.

[0036] In this invention, the polydiol preferably includes one or more of the following: polypentyl adipate diol, polybutylene adipate diol, polyhexane adipate diol, poly-1,4-butanediol-1,6-hexanediol adipate diol, polyethylene carbonate diol, polyethylene terephthalate diol, polybutylene isophthalate diol, polytetramethylene ether diol, polyethylene oxide-propylene oxide diol, polyethylene glycol, and polypropylene glycol; more preferably, it is polybutylene adipate diol, polyhexane adipate diol, or polytetramethylene ether diol; the number average molecular weight of the polydiol is preferably 500-3000 Da, more preferably 1000-2000 Da.

[0037] In this invention, the diisocyanate preferably includes one or more of hexamethylene diisocyanate, isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, terephthalic diisocyanate, tetramethylphenyldimethyl diisocyanate, 1,4-cyclohexyl diisocyanate, dicyclohexylmethane diisocyanate, and 2,2,4-trimethylhexane diisocyanate, more preferably isophorone diisocyanate, 1,4-cyclohexyl diisocyanate, or dicyclohexylmethane diisocyanate. In this invention, the diisocyanate exhibits high reactivity, facilitating the control of the polyurethane hard segment structure.

[0038] In this invention, the molar ratio of the polydiol to the diisocyanate is preferably 0.2 to 0.8:1, more preferably 0.4 to 0.6:1, and can be 0.4:1, 0.5:1 or 0.6:1.

[0039] In this invention, the first catalyst preferably comprises one or more of organotin metal catalysts, organobismuth metal catalysts, acetylacetone metal catalysts, and organic amine catalysts, more preferably organotin metal catalysts. In the embodiments of this invention, the organotin metal catalyst is preferably CUCAT-S9 (produced by Guangzhou Yourun Synthetic Materials Co., Ltd.), which has the advantages of high catalytic activity and few side reactions. In this invention, the mass of the first catalyst is preferably 0.001~0.06% of the mass of diisocyanate, more preferably 0.01~0.06%.

[0040] In this invention, the first organic solvent preferably includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dioxane, and dichloromethane, and more preferably N,N-dimethylacetamide. This invention does not have particular requirements on the amount of the first organic solvent used, as long as it ensures the dissolution of the raw materials and the smooth progress of the reaction.

[0041] In this invention, the polydiol, diisocyanate, first organic solvent and catalyst are preferably mixed by stirring to ensure that the components are mixed evenly.

[0042] In this invention, the temperature of the polymerization reaction is preferably 50~100℃, but can be 60, 70, 80 or 90℃, and the time is preferably 1~5h, but can be 2, 3 or 4h; the polymerization reaction is preferably carried out under a nitrogen protective atmosphere.

[0043] After obtaining the prepolymer solution, the prepolymer solution is mixed with a chain extender to carry out a chain extension reaction, thereby obtaining the polyurethane that can be used as both a hot melt adhesive and an alcohol sol adhesive.

[0044] In this invention, the chain extender comprises a first chain extender and a second chain extender; the first chain extender has the structure shown in Formula 1 and is a diol containing a 2-ureido-4-pyrimidinone group, and the second chain extender has the structure shown in Formula 2 and is a diol containing a bipyridine group. In this invention, in the structure shown in Formula 1, R4 is an alkyl group or hydrogen, and R5 is an alkyl group or hydrogen; when R4 is an alkyl group, the number of carbon atoms of the alkyl group is preferably 1 to 4, and can be methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, or tert-butyl; when R5 is an alkyl group, the number of carbon atoms of the alkyl group is preferably 1 to 6, preferably 1 to 3, and can be methyl, ethyl, or propyl; y is preferably a natural number from 1 to 3, i.e., 1, 2, or 3. In this invention, in the structure shown in Formula 2, z is preferably a natural number from 1 to 5, i.e., 1, 2, 3, 4, or 5; the substitution site of the hydroxyalkyl group in Formula 2 can be 3 (3'), 4 (4'), 5 (5'), or 6 (6'), preferably 4 (4') or 5 (5'); in the embodiments of this invention, the structure shown in Formula 2 is specifically 2,2'-bipyridine-4,4'-diethanol or 2,2'-bipyridine-5,5'-diethanol. This invention uses a diol containing a 2-ureido-4-pyrimidinone group, which can provide high-density, strong hydrogen-bonding groups, which is beneficial for improving the bonding strength of polyurethane hot melt adhesives; using a diol containing a bipyridine group allows for stable coordination between the bipyridine nitrogen and the surface of the metal substrate, thereby improving the bonding strength of the polyurethane hot melt adhesive at the metal interface.

[0045] In this invention, the first chain extender having the structure shown in Formula 1 is preferably prepared by the following method: mixing the compound shown in Formula a, N,N'-carbonyldiimidazole, a second catalyst, and a second organic solvent, and carrying out a first carbamylation reaction to obtain the compound shown in Formula b;

[0046] Formula a, Formula b,

[0047] The compound shown in formula b, the amino diol shown in formula c, and a third organic solvent are mixed to carry out a second carbamylation reaction to obtain the first chain extender.

[0048] Formula c.

[0049] In this invention, the chemical name of the compound shown in Formula a is 2-amino-6-alkyl(H)-4-pyrimidinone (R4 in the compound shown in Formula a is the same as R4 in the structure shown in Formula 1). The compound shown in Formula a is preferably 2-amino-6-1(H)-4-pyrimidinone, 2-amino-6-methyl-4-pyrimidinone, 2-amino-6-ethyl-4-pyrimidinone, 2-amino-6-propyl-4-pyrimidinone, or 2-amino-6-isopropyl-4-pyrimidinone, more preferably 2-amino-6-methyl-4-pyrimidinone, 2-amino-6-ethyl-4-pyrimidinone, or 2-amino-6-propyl-4-pyrimidinone. In this invention, the molar ratio of the 2-amino-6-alkyl(H)-4-pyrimidinone to N,N'-carbonyldiimidazole is preferably 1:1~3.

[0050] In this invention, the second catalyst is preferably an organonitrile metal catalyst, specifically bis(1,5-cyclooctadiene)nickel; the mass of the second catalyst is preferably 0.02 to 0.06% of the mass of the compound shown in formula a.

[0051] In this invention, the second organic solvent preferably comprises one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dioxane, and dichloromethane, more preferably N,N-dimethylformamide. In this invention, the mass of the second organic solvent is preferably 3 to 16 times the sum of the mass of the compound shown in formula a and the mass of N,N'-carbonyldiimidazole.

[0052] In this invention, the temperature of the first carbamylation reaction is preferably 40~120℃, more preferably 80℃, and the time is preferably 2~10h, more preferably 4h; the first carbamylation reaction is preferably carried out under a nitrogen protective atmosphere. In this invention, the reaction formula involved in the first carbamylation reaction is as follows:

[0053] .

[0054] After the first amino reaction is completed, the present invention preferably further includes cooling the resulting reaction solution and filtering to collect the solid to obtain a crude product; the crude product is then washed and dried sequentially to obtain a white powder, which is the compound shown in formula b (chemical name carbonylimidazolyl-2-ureido-6-alkyl(H)-4-pyrimidinone). In the present invention, the washing reagent is preferably acetone, and the number of washings is preferably ≥3 times; the drying is preferably carried out in a vacuum oven, the drying temperature is preferably 30~70℃, more preferably 50℃, and the drying time is preferably 6~24h, more preferably 14h.

[0055] In this invention, R5 and y in the aminodiol shown in Formula c are identical to R5 and y in the structure shown in Formula 1. The aminodiol is preferably 2-amino-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, 3-amino-1,5-pentanediol, 3-amino-3-methyl-1,5-pentanediol, 3-amino-3-ethyl-1,5-pentanediol, or 4-amino-1,7-heptanediol, more preferably 2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, or 3-amino-3-methyl-1,5-pentanediol. In this invention, the molar ratio of the compound shown in Formula b to the aminodiol is preferably 1:1.2 to 1:1.7.

[0056] In this invention, the third organic solvent preferably includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dioxane, and dichloromethane, more preferably dimethyl sulfoxide. In this invention, the mass of the third organic solvent is preferably 1 to 8 times the sum of the mass of the compound shown in formula b and the aminodiol.

[0057] In this invention, the temperature of the second carbamylation reaction is preferably 20-50°C, more preferably 30°C, and the time is preferably 8-20 h, more preferably 13 h; the second carbamylation reaction is preferably carried out under a nitrogen protective atmosphere. The reaction formula involved in the second carbamylation reaction in this invention is as follows:

[0058] .

[0059] After the second carbamylation reaction is completed, the resulting reaction solution is preferably added to a precipitating agent to obtain a white precipitate; the precipitate is filtered and dried to obtain a diol containing a -2-ureido-4-pyrimidinone group, i.e., the first chain extender. In this invention, the precipitating agent is preferably one or more of methanol, ethylene glycol, and water, more preferably water. In this invention, the drying temperature is preferably 60~100℃, more preferably 80℃, and the drying time is preferably 12~48h, more preferably 24h; the drying is preferably carried out in an oven.

[0060] In this invention, the molar ratio of the first chain extender to the second chain extender is preferably 0.05:0.95 to 0.95:0.05, more preferably 1:1 to 5:1, and can be 1:1, 2:1, 3:1, 4:1, 4.2:1, 4.3:1 or 4.5:1. The molar ratio of the first chain extender to the second chain extender affects the adhesive properties and mechanical strength of the polyurethane. The ratio of the total molar amount of the first chain extender and the second chain extender to the molar amount of diisocyanate used in preparing the prepolymer solution is preferably 0.2 to 0.8:1, more preferably 0.4 to 0.6:1, and can be 0.4:1, 0.5:1 or 0.6:1. This ratio affects the molecular weight and mechanical strength of the polyurethane.

[0061] Preferably, the first chain extender and the second chain extender are added to the prepolymer solution, and the mixture is stirred evenly before the chain extension reaction is carried out.

[0062] In this invention, the preferred temperature for the chain extension reaction is 50~100℃, which can be 60, 70, 80 or 90℃, and the preferred time is 10~32h, which can be 15, 20, 24 or 30h; the chain extension reaction is preferably carried out under a nitrogen protective atmosphere.

[0063] Following the chain extension reaction, the present invention preferably dries the resulting polymer solution to remove the solvent, thereby obtaining the polyurethane that serves as both a hot melt adhesive and an alcohol-based adhesive. In the present invention, the drying is preferably vacuum drying, and the vacuum drying is preferably carried out in a vacuum oven. In the present invention, the vacuum drying temperature is preferably 40~100℃, more preferably 60℃, and the time is preferably 24~48h, more preferably 32h.

[0064] The preparation method provided by this invention has simple steps, mild conditions, and is easy to carry out industrial production.

[0065] This invention provides a polyurethane that can be used as both a hot melt adhesive and an alcohol-soluble adhesive, prepared by the preparation method described above.

[0066] In this invention, the weight-average molecular weight of the polyurethane is preferably 2.9 × 10⁻⁶. 4 ~4.8×10 4 Da.

[0067] The etheroxy, urea, and aromatic heterocyclic groups in the polyurethane structure provided by this invention can form numerous hydrogen bonds between polymer chains and at the polymer-substrate interface, achieving adhesion. The polyurethane polymer chain contains 2-ureido-4-pyrimidinone and bipyridine groups, where the 2-ureido-4-pyrimidinone group acts as a hydrogen bond donor and the bipyridine group as a hydrogen bond acceptor, forming hydrogen bonds with higher binding energy. These hydrogen bonds not only provide strong intermolecular cohesion within the adhesive, thereby improving the material's strength, but also enhance the adhesion between the adhesive and polar groups on the substrate surface, thus increasing the bonding strength of the polyurethane adhesive. In particular, the bipyridine group, as a bidentate ligand, can form a chelate structure with metal ions on the surface of the metal substrate (forming a stable bipyridine nitrogen → metal coordination bond), further enhancing the bonding strength between the adhesive and the metal substrate. The combined effect of these groups gives the polyurethane prepared by this invention multiple advantages, including high bonding strength, mild operating conditions, and excellent adaptability to various substrates. The polyurethane has a melting point of 80~100℃ and a thermal decomposition temperature of 285~315℃. Therefore, it can be used as a hot melt adhesive after being heated to a certain temperature (such as 120℃) and has high bonding strength. At the same time, due to these two polar groups, the polyurethane can be dissolved in alcohol solvents to form alcohol-soluble adhesives, which can be used for room temperature coating and curing. Even after the alcohol evaporates, it still has a good bonding effect.

[0068] This invention provides the application of the polyurethane, which can function as both a hot melt adhesive and an alcohol-based adhesive, as described in the above technical solution, in the field of adhesives.

[0069] In this invention, when the polyurethane is used as a hot melt adhesive, the heating temperature of the polyurethane is preferably 80~140℃, and can be 120℃. In this invention, when the polyurethane is used as a hot melt adhesive, the preferred bonding method is to sandwich the polyurethane film between the substrates to be bonded, heat it at 120℃, and then cool it to room temperature; the heat treatment time can be 5~20 minutes, preferably 10 minutes.

[0070] In this invention, when the polyurethane is used as an alcohol sol, the polyurethane is dissolved in an alcohol solvent to form an alcohol sol. The alcohol solvent preferably has 1-3 carbon atoms and can be a straight-chain or branched fatty alcohol, specifically one or more of methanol, ethanol, n-propanol, and isopropanol, more preferably ethanol, which has low toxicity and a high evaporation rate, and is widely available and inexpensive, making it suitable for large-scale preparation and application. The mass fraction of polyurethane in the alcohol sol is preferably 20-70%, and can be 30%, 40%, or 50%. In this invention, when the polyurethane is used as an alcohol sol, the preferred bonding method is as follows: the alcohol sol is coated onto the substrate surface at room temperature (25°C), allowed to stand at room temperature to evaporate the ethanol, the coated areas of the substrate are aligned and overlapped, and then cured. The standing time can be 5-20 minutes, preferably 10 minutes, the curing temperature is preferably room temperature, the pressure is preferably 1 MPa, and the time is preferably 1 hour.

[0071] The polyurethane provided by this invention exhibits high bonding strength on both metallic substrates (such as iron, copper, and aluminum) and non-metallic substrates (such as polycarbonate resin and carbon fiber composites (CF-EP)) when used as a hot melt adhesive or alcohol-based adhesive. Furthermore, the bonding conditions are milder; it melts and achieves high-strength hot melt bonding in just 10 minutes at 120°C, improving work efficiency. Simultaneously, its unique molecular structure allows it to dissolve well in alcohol solvents (such as ethanol), forming a stable alcohol-based adhesive system. This system can achieve effective bonding by being applied at room temperature and curing within 1 hour, avoiding high-temperature energy consumption and adhering to the concept of green construction by relying on low-toxicity and environmentally friendly alcohol solvents. This invention achieves the dual functions of high-temperature rapid hot melt bonding and room-temperature green solvent bonding with a single material, providing a new solution to meet the needs of efficient operation, environmental friendliness, and flexible control of bonding modes.

[0072] To further illustrate the present invention, the following detailed description, in conjunction with examples, describes the polyurethane that can function as both a hot melt adhesive and an alcohol-soluble adhesive, its preparation method, and its applications. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0073] Example 1

[0074] In a dry 250 mL three-necked flask, 20.00 g of polytetramethylene ether diol (PTMEG1000, number average molecular weight 1000 Da, BASF Chemicals), 9.56 g of isophorone diisocyanate, 73.79 g of N,N-dimethylacetamide, and 5 mg of organotin catalyst (CUCAT-S9, Guangzhou Yourun Synthetic Materials Co., Ltd.) were added. After stirring thoroughly, the mixture was heated to 80 °C under nitrogen protection and reacted for 3 h to obtain a prepolymer solution. Subsequently, 2.56 g of diol chain extender A containing 2-ureido-4-pyrimidinone groups and 2.16 g of diol chain extender 2,2'-bipyridine-4,4'-diethanol (Shanghai Bide Pharmaceutical) containing bipyridine groups were added to the above prepolymer solution. After stirring thoroughly, the mixture was heated to 80 °C under nitrogen protection and reacted for 24 h to obtain a polymer solution. The obtained polymer solution was dried in a vacuum oven at 60 °C for 32 h to remove the solvent, yielding polyurethane.

[0075] The preparation method of the diol chain extender A containing a 2-ureido-4-pyrimidinone group is as follows:

[0076] (a) 7.53 g of 2-amino-6-methyl-4-pyrimidinone, 14.53 g of N,N'-carbonyldiimidazole, 2 mg of bis(1,5-cyclooctadiene)nickel, and 360 mL of N,N-dimethylformamide were added to a dry 1000 mL three-necked flask. After thorough mixing, the mixture was heated to 80 °C under nitrogen protection and reacted with magnetic stirring for 4 h. After the reaction was completed, the system was cooled to room temperature and filtered to obtain the crude product. The crude product was washed three times with 150 mL of acetone and then dried under vacuum at 50 °C for 14 h to obtain 12.22 g of carbonylimidazolyl-2-ureo-6-methyl-4-pyrimidinone white powder.

[0077] (b) 8.81 g of carbonylimidazolium-2-ureido-6-methyl-4-pyrimidinone and 5.49 g of 2-amino-2-methyl-1,3-propanediol were added to a dry 250 mL three-necked flask and dissolved in 110 mL of dimethyl sulfoxide. After stirring thoroughly, the mixture was heated to 30 °C under nitrogen protection and reacted with magnetic stirring for 13 h. After the reaction was completed, the reaction solution was slowly poured into deionized water to form a white precipitate; after filtration, the precipitate was dried in a vacuum drying oven at 80 °C for 24 h to obtain 9.23 g of diol chain extender A containing the 2-ureido-4-pyrimidinone group, the structure of which is shown below:

[0078] ;

[0079] Where y=1, R4 is methyl, and R5 is methyl.

[0080] 1H NMR spectrum of diol chain extender A containing 2-ureido-4-pyrimidinone group: 1H NMR (400 MHz, DMSO-d6): δ 11.51 (s, 1H), 9.61 (s, 1H), 7.35 (s, 1H), 5.79 (s, 1H), 4.81(s,2H), 3.34 (s, 4H), 2.07 (s, 3H), 1.21 (s, 3H).

[0081] Example 2

[0082] The difference from Example 1 is that 3.12g of diol chain extender B containing 2-ureido-4-pyrimidinone groups was used instead of diol chain extender A, and the mass of N,N-dimethylacetamide was 75.21g, resulting in polyurethane.

[0083] The preparation method of the diol chain extender B containing the 2-ureido-4-pyrimidinone group is as follows:

[0084] (a) 8.35 g of 2-amino-6-ethyl-4-pyrimidinone, 14.53 g of N,N'-carbonyldiimidazole, 2 mg of bis(1,5-cyclooctadiene)nickel, and 360 mL of N,N-dimethylformamide were added to a dry 1000 mL three-necked flask. After thorough mixing, the mixture was heated to 80 °C under nitrogen protection and reacted with magnetic stirring for 4 h. After the reaction was complete, the system was cooled to room temperature and filtered to obtain the crude product. The crude product was washed three times with 150 mL of acetone and then dried under vacuum at 50 °C for 14 h to obtain 13.06 g of carbonylimidazolyl-2-ureo-6-ethyl-4-pyrimidinone white powder.

[0085] (b) 9.39 g of carbonylimidazolyl-2-ureido-6-ethyl-4-pyrimidinone and 7.69 g of 3-amino-3-ethyl-1,5-pentanediol were added to a dry 250 mL three-necked flask and dissolved in 130 mL of dimethyl sulfoxide. After stirring thoroughly, the mixture was heated to 30 °C under nitrogen protection and reacted with magnetic stirring for 13 h. After the reaction was completed, the reaction solution was slowly poured into deionized water to form a white precipitate; after filtration, the precipitate was dried in a vacuum drying oven at 80 °C for 24 h to obtain 11.24 g of diol chain extender B containing the 2-ureido-4-pyrimidinone group, the structure of which is shown below:

[0086] ;

[0087] Where y=2, R4 is ethyl, and R5 is ethyl.

[0088] 1H NMR spectrum of diol chain extender B containing 2-ureido-4-pyrimidinone group: 1 H NMR (400 MHz, DMSO-d6): δ11.49 (s, 1H), 9.58 (s, 1H), 7.31 (s, 1H), 5.74 (s, 1H), 4.70(s,2H), 3.32 (s, 4H), 3.18 (s, 4H), 2.25 (s, 4H), 1.62 (s, 3H), 1.13 (s, 3H).

[0089] Example 3

[0090] The difference from Example 1 is that the amount of diol chain extender A containing 2-ureido-4-pyrimidinone group is 1.03 g, the amount of diol chain extender 2,2'-bipyridine-4,4'-diethanol containing bipyridine group is 3.46 g, and the amount of N,N-dimethylacetamide is 73.88 g, resulting in polyurethane.

[0091] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0092] Example 4

[0093] The difference from Example 1 is that 7.15g of 1,4-cyclohexyl diisocyanate was used instead of isophorone diisocyanate, and the mass of N,N-dimethylacetamide was 70.49g, resulting in polyurethane.

[0094] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0095] Example 5

[0096] The difference from Example 1 is that 20.00g of polytetramethylene ether glycol (PTMEG2000, molecular weight 2000 Da, BASF Chemicals) was used instead of polytetramethylene ether glycol (molecular weight 1000 Da) to obtain polyurethane.

[0097] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0098] Example 6

[0099] The difference from Example 1 is that 20.00g of poly(1,4-butanediol-1,6-hexanediol adipate) (PBHA-1000, molecular weight 1000 Da, Ruichang Haige Chemical) was used instead of polytetramethylene ether glycol to obtain polyurethane.

[0100] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0101] Example 7

[0102] The difference from Example 1 is that: the diol chain extender A containing 2-ureido-4-pyrimidinone group is 4.10 g, the diol chain extender 2,2'-bipyridine-4,4'-diethanol containing bipyridine group is 0.86 g, and the mass of N,N-dimethylacetamide is 73.57 g, thus obtaining polyurethane.

[0103] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0104] Figure 1 The nuclear magnetic resonance spectrum of the polyurethane adhesive prepared in Example 7 when deuterated DMSO is used as a solvent is obtained by nuclear magnetic resonance spectroscopy.

[0105] Example 8

[0106] The difference from Example 1 is that 2.16g of 2,2'-bipyridine-5,5'-diethanol (Shanghai Bide Pharmaceutical), a diol chain extender containing a bipyridine group, was used instead of 2,2'-bipyridine-4,4'-diethanol (Shanghai Bide Pharmaceutical). Polyurethane was obtained.

[0107] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0108] Comparative Example 1

[0109] The difference from Example 1 is that 0.76g of 1,3-propanediol (Tianjin Bohua Reagent Company) was used instead of 2,2'-bipyridine-4,4'-diethanol, a diol chain extender containing a bipyridine group, and the mass of N,N-dimethylacetamide was 66.24g, resulting in polyurethane.

[0110] The preparation method of the diol chain extender A containing the 2-ureido-4-pyrimidinone group in the steps is the same as in Example 1.

[0111] Comparative Example 2

[0112] The difference from Example 1 is that 0.76g of 1,3-propanediol (Tianjin Bohua Reagent Company) was used instead of diol chain extender A containing 2-ureido-4-pyrimidinone groups, and the mass of N,N-dimethylacetamide was 66.24g, resulting in polyurethane.

[0113] Comparative Example 3

[0114] The difference from Example 1 is that 1.52g of 1,3-propanediol (Tianjin Bohua Reagent Company) was used instead of diol chain extender A containing 2-ureido-4-pyrimidinone groups and diol chain extender 2,2'-bipyridine-4,4'-diethanol containing bipyridine groups, and the mass of N,N-dimethylacetamide was 66.24g, resulting in polyurethane.

[0115] The properties of the polyurethanes prepared in Examples 1-8 and Comparative Examples 1-3 were tested:

[0116] The weight-average molecular weight and molecular weight dispersity index of polyurethane were determined by gel permeation chromatography (Waters 600E), with polystyrene as the reference sample and N,N-dimethylformamide as the mobile phase. The results are shown in Table 1.

[0117] The mechanical properties of polyurethane, including tensile strength and elongation at break, were tested using a universal tensile testing machine (Shanghai Songdun, WDW-5). The test results are shown in Table 1.

[0118] The thermal decomposition temperature of polyurethane was determined using a thermogravimetric analyzer (NETZSCH TG209, Germany), and the results are shown in Table 1.

[0119] Table 1. Test results of molecular weight, tensile properties and thermal stability of polyurethanes prepared in the examples and comparative examples.

[0120]

[0121] As can be seen from Table 1, the molecular weight of the polyurethane prepared by this invention is 2.9 × 10⁻⁶. 4 ~4.8×10 4 Within the Da range, the molecular weight is relatively large; the molecular weight dispersion index is within the range of 1.45~1.91, indicating a narrow molecular weight distribution. Examples 1-8 of this invention used chain extenders containing 2-ureido-4-pyrimidinone and bipyridine groups to synthesize polyurethane, while Comparative Examples 1 and 2 used conventional 1,3-propanediol instead of the chain extenders containing 2-ureido-4-pyrimidinone and bipyridine groups, respectively. Comparative Example 3 used conventional 1,3-propanediol entirely as the chain extender. The results show that the tensile strength and elongation at break of the polyurethane hot melt adhesives obtained in Examples 1-8 are mostly higher than those in Comparative Examples 1-3, indicating that the 2-ureido-4-pyrimidinone and bipyridine groups introduced in this invention as chain extenders helps improve the tensile properties of polyurethane. Excellent tensile strength can effectively reduce adhesive failure caused by cohesive failure, thereby improving the adhesive performance of polyurethane adhesives. Furthermore, the thermal decomposition temperature of the polyurethanes obtained in Examples 1-8 is also higher than that of the polyurethane hot melt adhesives obtained in Comparative Examples 1-3, indicating that the polyurethanes containing 2-ureido-4-pyrimidinone groups and bipyridine groups in this invention have higher thermal stability.

[0122] Test Example 1

[0123] The polyurethane adhesives prepared in the examples and comparative examples were used as hot melt adhesives, along with two sets of commercially available thermoplastic polyurethane hot melt adhesives (3M-2665 and Fuller-EH9695B), to bond different types of substrates. The selected substrates included iron, copper, aluminum, polycarbonate, and carbon fiber composites (CF-EP), all with dimensions of 100mm × 25mm × 2mm. The specific procedure was as follows: the adhesive film was first cut into rectangular sheets of 25mm × 12.5mm × 100μm, clamped between two substrates of the same size, and heated at 120℃ for 10 min, then cooled to room temperature to prepare test samples. Finally, all the obtained samples were placed on a universal tensile testing machine (Shanghai Songdun, model WDW5) for lap shear strength testing, and the test results are shown in Table 2.

[0124] Table 2. Overlap shear strength of polyurethane adhesives prepared according to examples and proportions as hot melt adhesives, and two commercially available polyurethane hot melt adhesives on different substrates.

[0125]

[0126] As shown in Table 2, the polyurethane adhesives prepared in Examples 1-8, when used as hot melt adhesives, exhibit significantly higher bonding strength on various substrates (including iron, copper, aluminum, polycarbonate resin, and carbon fiber composites (CF-EP)), especially on metallic substrates, compared to the two commercially available polyurethane hot melt adhesives and Comparative Examples 1-3. Example 7 demonstrates the best bonding performance on almost all substrates, exhibiting the best overall effect, particularly with a shear strength of 13.83 MPa on iron substrates. Furthermore, the comparison between Comparative Examples 1-3 reveals that the 2-ureido-4-pyrimidinone group structure enhances the bonding strength on all tested substrates, while the bipyridine group structure specifically enhances the bonding strength on metallic substrates.

[0127] Test Example 2

[0128] The polyurethane film prepared in Example 7 was cut into rectangular sheets of 25mm × 12.5mm × 100μm and sandwiched between two iron substrates of the same size (100mm × 25mm × 2mm). The heating temperatures were 80℃, 100℃, 120℃, and 140℃, with a heating time of 10min. The bonded test samples were then cooled to room temperature to obtain test samples under different heating temperature conditions. The lap shear strength of the test samples was tested using a universal tensile testing machine (Shanghai Songdun, WDW-5). The test results are shown in Table 3 and... Figure 2 As shown.

[0129] Table 3. Overlap shear strength of Test Example 2 under different heating temperatures.

[0130]

[0131] Figure 2 The polyurethane adhesive prepared in Example 7 is used as a hot melt adhesive to bond lap shear strength to iron substrates under different heating temperatures. (See Table 3 and...) Figure 2 It can be seen that when the polyurethane adhesive prepared in Example 7 is used as a hot melt adhesive, the lap shear strength is the highest when the adhesive application heating temperature is 120°C.

[0132] Test Example 3

[0133] The difference from Test Example 2 is that the heating temperature was 120℃, and the heating times were 5 min, 10 min, 15 min, and 20 min, respectively, to obtain test samples under different heating time conditions. The lap shear strength of the samples was tested using a universal tensile testing machine, and the test results are shown in Table 4. Figure 3 As shown.

[0134] Table 4 shows the lap shear strength of Test Example 3 under different heating times.

[0135]

[0136] Figure 3 The graph shows the lap shear strength of the bonded iron plates when the polyurethane adhesive prepared in Example 7 is used as a hot melt adhesive at an application temperature of 120°C and different heating times. Figure 3 As can be seen from Table 4, the polyurethane hot melt adhesive prepared in Example 7 exhibits the highest lap shear strength when heated for 10 minutes. (This is a summary of Tables 3 and 4.) Figure 2 and Figure 3 When the polyurethane adhesive prepared in Example 7 is used as a hot melt adhesive, the optimal bonding effect is achieved at a heating temperature of 120°C and a heating time of 10 minutes. Compared with the generally higher heating temperatures and longer heating times required for existing hot melt adhesives, the polyurethane hot melt adhesive obtained in this invention has milder operating conditions.

[0137] Test Example 4

[0138] Prepare an iron substrate with dimensions of 100mm × 25mm × 2mm. Cut the polyurethane film obtained in Example 7 into a rectangle of 25mm × 12.5mm × 100μm and sandwich it between two iron substrates of the same size. Heat the sample at 120℃ for 10 minutes. After heating, cool the bonded test sample to room temperature to obtain the test sample. The initial lap shear strength of the sample is determined using a universal tensile testing machine. After testing the lap shear strength, the test sample is heated again at 120℃ for 10 minutes, cooled to room temperature, and the lap shear strength is tested again. This process is repeated multiple times to verify the adhesion of the polyurethane adhesive as a hot melt adhesive after repeated recycling. The test results are shown in Table 5 and... Figure 4 As shown.

[0139] Table 5 shows the lap shear strength obtained by repeatedly bonding the same sample after heating it at 120°C for 10 minutes, as described in Test Example 4.

[0140]

[0141] Figure 4 The image shows the lap shear strength of the polyurethane adhesive prepared in Example 7 after repeated cyclic use as a hot melt adhesive. Figure 4 As can be seen from Table 5, the polyurethane hot melt adhesive obtained in Example 7 has good recyclability. After five consecutive uses, it can still maintain 91.69% of the lap shear strength of the first use, indicating that the polyurethane adhesive prepared in this invention has good recyclability when used as a hot melt adhesive.

[0142] Test Example 5

[0143] 13.75 g of the polyurethane film prepared in Example 7 was weighed and dissolved in 35 mL of ethanol to prepare an alcohol-soluble adhesive with a mass concentration of 33.23%. The resulting adhesive was applied to the surface of an iron substrate (size: 100 mm × 25 mm × 2 mm) at room temperature, with a coating area of ​​25 mm × 12.5 mm. The substrate was then allowed to stand at room temperature for 5 min, 10 min, 15 min, or 20 min to evaporate the ethanol. The coated areas of the substrate were aligned and overlapped, and cured at 25 °C and 1 MPa for 1 hour. After curing, test samples corresponding to different evaporation times were obtained. The lap shear strength of the samples was determined using a universal tensile testing machine (Shanghai Songdun, WDW-5). The test results are shown in Table 6 and... Figure 5 As shown.

[0144] Table 6. Overlap shear strength of Test Example 5 at room temperature after different evaporation times.

[0145]

[0146] Figure 5The lap shear strength diagram of the bonded samples obtained by coating the polyurethane prepared in Example 7 onto the surface of an iron substrate at room temperature, followed by curing at 25°C and 1 MPa for 1 hour after different ethanol solvent evaporation times, is shown in Table 6. Figure 5 It can be seen that the polyurethane adhesive prepared in Example 7, as an alcohol-soluble adhesive, exhibits the highest bonding strength when the evaporation time at room temperature is 10 minutes after application. Compared with the longer curing time and lower bonding strength required by existing polyurethane adhesives, the polyurethane adhesive obtained in this invention, as an alcohol-soluble adhesive, not only shortens the curing time but also enhances the bonding strength.

[0147] Test Example 6

[0148] The polyurethane adhesive prepared in Example 7 was used as an alcohol-soluble adhesive and bonded to different types of substrates with two commercial polyurethane adhesive products (Loctite UK 2043 and EMFIMASTIC PU40). The bond strength was compared between the two. The selected substrates included iron, copper, aluminum, polycarbonate, and carbon fiber composite (CF-EP), all with dimensions of 100mm × 25mm × 2mm. The specific operation was as follows: 13.75g of the polyurethane film prepared in Example 7 was weighed and dissolved in 35mL of ethanol to prepare an alcohol-soluble adhesive with a mass concentration of 33.23%. The obtained adhesive and the two commercial polyurethane adhesives were coated onto the surface of each substrate at room temperature, with a coating area of ​​25mm × 12.5mm. After standing at room temperature for 10min, the coated areas of the substrates were aligned and overlapped, and cured at 25℃ and 1MPa for 1 hour. After curing, test samples of different adhesives were obtained, and the lap shear strength of the samples was determined using a universal tensile testing machine (Shanghai Songdun, WDW-5). The test results are shown in Table 7.

[0149] Table 7. Overlap shear strength of the polyurethane adhesive prepared in Example 7 as an alcohol-based adhesive and two commercially available polyurethane adhesives on different substrates.

[0150]

[0151] It can be seen that when the polyurethane adhesive prepared in Example 7 is used as an alcohol-soluble adhesive, its bonding strength to metal substrates is better than that of the two commercial polyurethane adhesives mentioned above; however, its strength to non-metal substrates is slightly lower than that of the two commercial polyurethane adhesives.

[0152] Using the polyurethane adhesive prepared in Example 7 as a hot melt adhesive, the sample bonded to the iron substrate after heating at 120°C for 10 minutes could withstand the load of an 80kg adult.

[0153] Figure 6The polyurethane adhesive prepared in Example 7 was used as an alcohol-soluble adhesive. After being coated at room temperature and left to stand for 10 minutes, and then cured at 25°C and 1 MPa for 1 hour, the resulting sample bonded to the iron substrate could withstand a load of 6 kg.

[0154] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a polyurethane that can function as both a hot melt adhesive and an alcohol-based adhesive, characterized in that, Includes the following steps: Polydiol, diisocyanate, organic solvent and catalyst are mixed and polymerized to obtain a prepolymer solution; The prepolymer solution is mixed with a chain extender to carry out a chain extension reaction, thereby obtaining the polyurethane that can be used as both a hot melt adhesive and an alcohol-based adhesive; the chain extender includes a first chain extender and a second chain extender, the first chain extender having the structure shown in Formula 1, and the second chain extender having the structure shown in Formula 2: Formula 1, Formula 2; In the structure shown in Formula 1, R4 is an alkyl or hydrogen, R5 is an alkyl or hydrogen, and y is a natural number from 1 to 3; in the structure shown in Formula 2, z is a natural number from 1 to 5.

2. The preparation method according to claim 1, characterized in that, The polydiol comprises one or more of the following: polypentyl adipate diol, polybutylene adipate diol, polyhexane adipate diol, poly-1,4-butanediol-1,6-hexanediol adipate diol, polyethylene carbonate diol, polyethylene terephthalate diol, polybutylene isophthalate diol, polytetramethylene ether diol, polyethylene oxide-propylene oxide diol, polyethylene glycol, and polypropylene glycol; the number average molecular weight of the polydiol is 500~3000 Da.

3. The preparation method according to claim 1 or 2, characterized in that, The diisocyanate includes one or more of hexamethylene diisocyanate, isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, terephthalic diisocyanate, tetramethylphenyldimethyl diisocyanate, 1,4-cyclohexyl diisocyanate, dicyclohexylmethane diisocyanate, and 2,2,4-trimethylhexane diisocyanate; the molar ratio of the polydiol to the diisocyanate is 0.2 to 0.8:

1.

4. The preparation method according to claim 1, characterized in that, The polymerization reaction is carried out at a temperature of 50~100℃ for 1~5 hours.

5. The preparation method according to claim 1, characterized in that, When R4 is an alkyl group, the alkyl group has 1 to 4 carbon atoms; when R5 is an alkyl group, the alkyl group has 1 to 6 carbon atoms.

6. The preparation method according to claim 1 or 5, characterized in that, The molar ratio of the first chain extender to the second chain extender is 0.05:0.95 to 0.95:0.05; the ratio of the total molar amount of the first chain extender and the second chain extender to the molar amount of diisocyanate used in preparing the prepolymer solution is 0.2 to 0.8:

1.

7. The preparation method according to claim 1, characterized in that, The chain extension reaction is carried out at a temperature of 50~100℃ for a time of 10~32h.

8. The polyurethane that can be used as both a hot melt adhesive and an alcohol-soluble adhesive, prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the polyurethane of claim 8, which can function as both a hot melt adhesive and an alcohol-based adhesive, in the field of adhesives.

10. The application according to claim 9, characterized in that, When the polyurethane is used as a hot melt adhesive, the heating temperature of the polyurethane is 80~140℃; When the polyurethane is used as an alcohol-soluble adhesive, the polyurethane is dissolved in an alcohol solvent to form an alcohol-soluble adhesive; the alcohol solvent has 1 to 3 carbon atoms, and the mass fraction of polyurethane in the alcohol-soluble adhesive is 20 to 70%.

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