A hyperbranched polyurethane hot melt adhesive containing Upy self-complementary hydrogen bonds and preparation method thereof

Abstract

The present invention discloses a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds and a preparation method thereof. The polyurethane hot melt adhesive has the following structure: #imgabs0# The preparation method comprises: polymerizing a polyether diol with a diisocyanate to obtain a linear prepolymer, which is then subjected to a chain extension reaction with trimethylolpropane to obtain a hyperbranched prepolymer; and then end-capping the hyperbranched prepolymer with 2-amino-4-hydroxy-6-methylpyrimidine to obtain a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds. The hyperbranched polyurethane hot melt adhesive has high bonding strength, good multiple reusability, and impact resistance, and can bond various substrates, demonstrating wide applicability. Furthermore, the preparation method has low raw material cost, mild conditions, a convenient synthesis route, and does not require excessive raw material consumption, thus having the potential for industrial production.

Description

Technical Field

[0001] The present invention relates to a polyurethane hot melt adhesive, in particular to a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds and a preparation method thereof, belonging to the technical field of structural adhesives. Background Art

[0002] Polymer structural adhesives are crucial for bonding components in industries such as automotive, electronics, and packaging. They contribute to lightweight and miniaturized designs by providing strong, secure adhesion to a variety of low-density polymer materials and metal substrates. The miniaturization and reliability of portable electronics such as smartphones, laptops, and digital cameras are increasingly dependent on structural adhesives. The trend toward miniaturization means more components are being packed into a constant or even smaller space, resulting in a heavier overall weight, and therefore, a larger specific mass and volume. To prevent failures from occasional drops or collisions, structural adhesives must possess not only high shear strength but also impact resistance to prevent debonding and even disintegration of electronic products. A key requirement for structural adhesives suitable for such applications is that they can achieve rapid increases in bond strength on fast production lines. Furthermore, ease of maintenance is another attractive feature of portable electronics, placing a particular emphasis on adhesives that are removable, reusable, and recyclable.

[0003] Traditional adhesives developed from acrylic, epoxy, and polyurethane resins do not fully meet the requirements of the aforementioned portable electronic product trends. Both acrylic and epoxy adhesives offer rapid bonding and structural adhesion. Their obvious drawbacks are brittleness, as well as their disposable and non-reusable nature, primarily due to a lack of flexible chains and a dense chemically crosslinked network. Polyurethane reactive (PUR) hot melt adhesives are currently the most popular industrial solution for bonding electronic components. Key factors are their isocyanate (-NCO) groups, which create a chemically bonded interface, and their crystalline soft chains, which enhance the strength of the bulk and interfacial layers. References (Industrial Crops and Products, 2019, 128, 436) disclose a polyurethane reactive hot melt adhesive formed by combining polyester diols with polyether polyols and reacting them with MDI. The adhesive exhibits a shear strength of up to 8.3 MPa on PC boards after curing at room temperature for 7 days. Although PUR can produce rapidly increasing shear strength and high ultimate adhesion, its final chemically crosslinked network renders it unreusable. The document (MaterialsChemistry Frontiers, 2019, 3, 1833) discloses the development of high shear strength PUR with multiple bonding and separability by adjusting the dynamic covalent oxime urethane bond. However, the oxime urethane bond is unstable when exposed to hot air, which inevitably reduces its shear strength. The use of thermoplastic hot melt adhesives with temperature-responsive melting characteristics is a potential alternative that can meet the requirements of easy peeling and re-bonding after heat treatment. However, this type of adhesive, taking polyolefins, ethylene-acrylate copolymers and polyurethanes as examples, basically relies on van der Waals forces and hydrogen bonds to construct interfacial bonding. The overall shear strength generally cannot exceed that of reactive adhesives such as PUR. Therefore, how to improve thermoplastic polyurethane hot melt adhesives to the level of structural bonding and have impact resistance remains an unresolved problem.

[0004] Adding a large number of hydrogen bonds is an effective way to enhance adhesion. The hydrogen bonds of urea groups are inherently stronger than those of polyurethane bonds because the cohesive energy generated by the combination of double H bonds is higher. For example, the literature (Journal of Materials Chemistry A, 2022, 10, 20804) synthesized a new dynamic polyurea network with high adhesion strength (14.43±1.39MPa) to stainless steel substrates. Although the shear strength is significantly improved, its rheological behavior does not seem to be suitable for extrusion. It is reported that quadruple hydrogen bonds have high cohesive energy. For example, the literature (Advanced Functional Materials, 2022, 32, 2204263) reported a hierarchical hydrogen bond network composed of 2-urea-4-pyrimidinone (UPy) and urea groups, through which the bonding strength with the iron plate reached 20.7MPa. The quadruple hydrogen bonds of UPy can achieve high adhesion strength, but it is not clear whether it has the advantage of detachment. A paper (ACS Materials Letters, 2021, 3, 1003) describes a hot-melt adhesive grafted with epoxy using UPy molecules, which achieves high shear strength and multiple reusability. However, the synthesis of the monomers wastes a large amount of raw materials, making industrial production impossible. However, to date, no polyurethane hot-melt adhesive has been developed that combines high bonding strength, multiple reusability, and good impact resistance. Summary of the Invention

[0005] In response to the defects of the existing technology, the first object of the present invention is to provide a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds, which has high bonding strength, good multiple reusability and impact resistance, and can bond various substrates, showing wide applicability.

[0006] A second object of the present invention is to provide a method for preparing a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds. This method has low raw material cost, mild conditions, a convenient synthesis route, does not require excessive raw material consumption, and has the potential for industrial production.

[0007] In order to achieve the above technical objectives, the present invention provides a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds, which has a structure of Formula 1:

[0008]

[0009] in,

[0010] R is a C2-C6 alkylene chain;

[0011] R1 is a diphenylmethane unit, a phenyl unit, a methylene unit or a six-membered ester ring unit;

[0012] n is 10 to 20.

[0013] The main molecular structure of the hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds of the present invention is hyperbranched polyurethane, and is terminated with a UPy structure. The hot melt adhesive with this molecular structure not only exhibits high bonding strength, but also has good impact resistance and good multiple reusability. Strong self-complementary quadruple hydrogen bonds are constructed at the ends of the polyurethane. The association of hydrogen bonds can improve the tensile strength of the polyurethane elastomer. At the same time, polyurethane forms aggregates by mutual association of hydrogen bonds. The hard segment aggregates of these UPy structures have the function of absorbing energy during the impact process, which gives the polyurethane better impact resistance. At the same time, the branched structure of the hyperbranched polyurethane has a good buffering effect on external impact force. In addition, compared with linear topology polyurethane, hyperbranched polymers can introduce more UPy quadruple hydrogen bonds, and the dynamic reversibility of hydrogen bonds gives it the characteristics of multiple peeling and recombination. For example, under heating, the diffusion and dynamic bond exchange of polymer chains in the hyperbranched polyurethane are accelerated, and the UPy sequences and carbamate groups in the polymer network can form H bonds on the surface of the substrate, resulting in strong adhesion between the adhesive layer and the substrate after cooling to room temperature, thereby producing a strong bonding effect.

[0014] The present invention also provides a method for preparing a hyperbranched polyurethane hot melt adhesive containing Upy self-complementary hydrogen bonds, which comprises the following steps:

[0015] 1) polymerizing polyether diol and diisocyanate to obtain a linear prepolymer;

[0016] 2) performing a chain extension reaction on the linear prepolymer and trimethylolpropane to obtain a hyperbranched prepolymer;

[0017] 3) performing an end-capping reaction on the hyperbranched prepolymer and 2-amino-4-hydroxy-6-methylpyrimidine to obtain;

[0018] The polyether glycol has a structure of Formula 2:

[0019]

[0020] The diisocyanate has a structure of Formula 3:

[0021] ONCR1CNO

[0022] Formula 3

[0023] in,

[0024] R is a C2-C6 alkylene chain;

[0025] R1 is a diphenylmethane unit, a phenyl unit, a methylene unit or a six-membered ester ring unit;

[0026] n is 10 to 20.

[0027] The preparation method of the hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds of the present invention is to first use polyether diol and diisocyanate to form a linear prepolymer through addition, and then the linear prepolymer is chain-extended by trimethylolpropane chain extender to obtain a hyperbranched prepolymer. The hyperbranched prepolymer contains more isocyanate-reactive groups relative to the linear polyurethane. These reactive groups can be capped with 2-amino-4-hydroxy-6-methylpyrimidine, so that self-complementary quadruple hydrogen bonds can be constructed at the branch ends of the hyperbranched polyurethane. This preparation process does not require the use of solvents and has a high monomer conversion rate, which has obvious advantages.

[0028] As a preferred solution, the polymerization reaction conditions are: reaction at a temperature of 70 to 90° C. for 2 to 4 hours.

[0029] As a preferred embodiment, the molar ratio of polyether diol to diisocyanate is 1:2 to 2.5. Using a slight excess of diisocyanate ensures that the oligomeric polyurethane is capped with isocyanate groups. If too much diisocyanate is used, the resulting oligomeric polyurethane has a low molecular weight, and residual diisocyanate affects subsequent chain extension and capping reactions.

[0030] As a preferred solution, the chain extension reaction is carried out at a temperature of 70 to 90° C. for 1 to 3 hours.

[0031] As a preferred solution, the molar ratio of trimethylolpropane to polyether diol is 10 to 25: 100. If the amount of polyether diol used is too low, the mechanical strength will be low, and if the amount of polyether diol used is too high, gelation will occur.

[0032] As a preferred solution, the end-capping reaction is carried out at a temperature of 70 to 90° C. for 1 to 3 hours.

[0033] As a preferred solution, the molar ratio of 2-amino-4-hydroxy-6-methylpyrimidine to polyether diol is 1.2 to 1.5:1. Adding too little 2-amino-4-hydroxy-6-methylpyrimidine will result in incomplete reaction of the NCO group, while adding too much will result in residue.

[0034] Compared with the existing technology, the technical solution of the present invention brings the following beneficial technical effects:

[0035] The hyperbranched polyurethane hot melt adhesive (TPU-UPy) containing UPy self-complementary hydrogen bonds provided by the present invention has high bonding strength, good multiple reusability and impact resistance. For example, the shear strength of TPU-UPy on steel plates is as high as 6.8 MPa. Due to the dynamic reversibility of hydrogen bonds, TPU-UPy can be reused five times and still maintain more than 70% of its original adhesion strength. Its adhesion strength also tends to increase with increasing stretching speed, showing excellent impact resistance and can be bonded to various substrates, with broad application prospects.

[0036] The preparation method of a hyperbranched polyurethane hot melt adhesive (TPU-UPy) containing UPy self-complementary hydrogen bonds provided by the present invention has a simple synthesis route, mild process conditions, low raw material cost, no need to consume excessive raw materials, and has the potential for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 a is the 1D SAXS of TPUs and the 2D SAXS of TPU-UPy (inset); b is the ln I(q)-q of TPU-UPy 2 Curve; c is the 2D Young's modulus distribution of TPU-UPy obtained by AFM nanomechanical testing.

[0038] Figure 2 (a) is the shear strength-displacement curve of TPUs; (b) is the shear strength-displacement curve of TPU-UPy when bonding different substrates.

[0039] Figure 3 Cyclic bonding strength of TPU-UPy steel plate.

[0040] Figure 4 a is the shear strength of TPU-UPy bonded to steel plates at different shear rates.

[0041] Figure 5 This is the hydrogen nuclear magnetic resonance spectrum of TPU-UPy. DETAILED DESCRIPTION

[0042] The following specific examples are intended to further illustrate the present invention, rather than to limit the scope of protection of the claims.

[0043] Unless otherwise specified, the chemical reagents involved in the following specific examples are conventional commercial reagents.

[0044] The following instruments and methods are used to characterize the intermediates and target products involved in the following examples:

[0045] Fourier transform infrared spectroscopy characterization:

[0046] (1) Ordinary infrared spectrum test

[0047] The reflection infrared spectrum of the bulk sample was tested using a Bruker ALPHA II Fourier transform infrared spectrometer with a resolution of 4 cm -1 , the number of scans is 32 times, and the spectrum scanning range is: 4000cm -1 to 500cm -1 .

[0048] (2) Variable temperature infrared spectrum test

[0049] The variable temperature infrared spectrum test was conducted using a Nicolet 6700 infrared spectrometer from Thermo Electron, USA. The test temperature ranged from 30°C to 170°C, with a temperature interval of 15°C and a dwell time of 5 minutes. The infrared spectra were obtained by scanning at different temperatures.

[0050] H NMR spectrum test: 1 The H NMR test was performed using a Bruker Avance 400 MHz nuclear magnetic resonance instrument from Germany. Tetramethylsilane (TMS) was used as the internal standard. About 10 mg of the sample was dissolved in 0.50 mL of deuterated chloroform (CDCl3) and scanned at 25°C to obtain the hydrogen nuclear magnetic resonance spectrum.

[0051] Gel permeation chromatography test: Gel permeation chromatography (GPC) test adopts Agilent LC1200 gel permeation chromatograph from the United States, with DMF as mobile phase, test temperature of 25 ° C, flow rate of 1 mL / min, sample concentration of 2-3 mg / mL, and by integrating the elution curve, the weight average molecular weight (Mn), number average molecular weight (Mw) and molecular weight distribution index (PDI) of the polymer can be obtained.

[0052] Mechanical properties test:

[0053] (1) Uniaxial tensile test

[0054] The uniaxial tensile test was performed using a TH-8203A tensile testing machine from Suzhou Tuobo Machinery Equipment Co., Ltd. with reference to the GB / T 528-2009 standard at room temperature (23±2°C, 50±10% RH) at a constant rate of 100 mm / min to obtain the stress-strain curve of the sample.

[0055] (2) Uniaxial variable speed tensile test

[0056] The variable-speed tensile test was performed using a TH-8203A tensile testing machine from Suzhou Tuobo Machinery Equipment Co., Ltd., with reference to the GB / T 528-2009 standard. The test was performed at room temperature (23±2°C, 50±10% RH) at constant rates of 50 mm / min, 100 mm / min, and 300 mm / min, respectively. The stress-strain curves at different tensile rates were obtained.

[0057] (3) Variable speed cyclic tensile test

[0058] Variable-speed cyclic tensile testing was performed using a TH-8203A tensile testing machine. The specimens were stretched to 200% strain at various constant speeds at room temperature (23±2°C, 50±10% RH). Cyclic tensile curves were generated after repeated stretching and unloading cycles. Energy dissipation (ΔE) was calculated using the hysteresis loop area of ​​the stress-strain curve during the tensile test.

[0059] ΔE=∫ loading σdε–∫ unloading σdε

[0060] (4) Tensile shear strength test

[0061] The tensile shear strength test was carried out using a TH-8100A tensile testing machine from Suzhou Tuobo Machinery Equipment Co., Ltd. The test standard and sample preparation referred to GB / T 7124-2008. A substrate with a specification of 100 mm × 25 mm × 0.8 mm was used for overlap, and the overlap area was 25 mm × 12.5 mm. The number of samples was not less than 5. The samples were heated in an oven at 120°C for 30 min and adjusted at room temperature for 24 h. The test was carried out at a constant rate of 100 mm / min at room temperature (23 ± 2°C, 50 ± 10% RH).

[0062] (5) Variable speed tensile shear strength test

[0063] The tensile shear strength test was carried out using a TH-8100A tensile testing machine produced by Suzhou Tuobo Machinery Equipment Co., Ltd. A substrate with a specification of 100 mm × 25 mm × 0.8 mm was used for overlap, the overlap area was 25 mm × 12.5 mm, the number of samples was not less than 5, and the samples were heated in an oven at 120 ° C for 30 min and adjusted at room temperature for 24 h. The tests were carried out at room temperature (23 ± 2 ° C, 50 ± 10% RH) at constant rates of 50 mm / min, 100 mm / min, 300 mm / min and 500 mm / min, respectively.

[0064] Rotational rheometer test: A US TA-DHR-2 rotational rheometer was used for testing. The diameter of the circular film sample was 20 mm and the thickness was approximately 1 mm. The frequency was 1 Hz, the strain amplitude was 0.1%, and the temperature was swept from 25°C to 120°C. The storage modulus (G') and loss modulus (G") curves related to temperature were obtained. The frequency range was 0.01 Hz to 100 Hz, the strain amplitude was 0.1%, and the temperature interval was 10°C. The frequency was swept from 25°C to 85°C. The storage modulus (G') and loss modulus (G") curves related to frequency were obtained.

[0065] Small-angle X-ray scattering test: The test was carried out using a French Xenocs-Nano-inXider X-ray scattering instrument (equipped with a microfocus sealed tube and an area detector), and the radiation source was Cu-Kα. ), the distance from the sample to the detector is 1185 mm. The radius of gyration (R g )

[0066]

[0067] Where q is the scattering vector, I(q) is the scattering intensity, and a0 is a constant. Take the logarithm of the equation:

[0068]

[0069] R g It can be obtained by ln[I(q)]-q 2 The slope of the curve in the small q region is obtained.

[0070] Atomic force microscope test: Nanomechanical testing was performed using a Dimension Icon atomic force microscope from Bruker, USA, with an RTESPA-150 probe.

[0071] Example 1

[0072] Thermoplastic polyurethane elastomers were synthesized in a 250ml four-necked flask equipped with a mechanical stirrer, an argon inlet, a vacuum gauge, and a mercury thermometer. First, PTMEG (12.00g, 0.012mol) was charged to the flask and heated under vacuum at 120°C for 1h to remove residual water. Next, a stoichiometric amount of an isocyanate (diphenylmethane diisocyanate) with an R=2 molar ratio to the hydroxyl groups of PTMEG ([NCO] / [OH]=2) was added to the vessel, and the mixture was stirred at 80°C under argon for 3h to obtain a prepolymer. Next, TMP (0.3354g, 0.0025mol) dissolved in anhydrous DMF (40ml) was added to the flask, and the mixture was stirred at 80°C for 2h under argon. Finally, 2-amino-4-hydroxy-6-methylpyrimidine (1.0323g, 0.0165mol) was added. Add to the flask. The reaction is maintained at 80°C under argon until the NCO peak disappears in the Fourier transform infrared spectroscopy (FTIR), which takes approximately 2 hours. The entire viscous liquid is poured into a polytetrafluoroethylene mold. The mold is placed in an 80°C oven for 12 hours and then dried in a vacuum oven at 80°C for 12 hours to remove residual solvent, resulting in a TPU-UPy elastomer film.

[0073] Comparative Example 1

[0074] 2-amino-6-fluoropyridine, 2-amino-6-chloropyridine and 2-amino-6-methylpyridine were used instead of 2-amino-4-hydroxy-6-methylpyrimidine, and the control samples TPU-FPy, TPU-CPy and TPU-MPy were synthesized according to the same procedures and conditions as in Example 1, and the films were prepared in the same manner.

[0075] The performance of the polyurethane elastomers in Example 1 and Comparative Example 1 was analyzed:

[0076] Phase morphology analysis: The phase morphology of TPUs elastomers was characterized by small angle X-ray scattering (SAXS) test. Figure 1 The one-dimensional SAXS data of aTPU-UPy in the middle show that it has a clear scattering peak, and the corresponding two-dimensional SAXS data show that it has a clear diffuse ring, which indicates that the TPU-UPy elastomer has a microphase separation structure. The gyration radius (Rg) of TPU-UPy is calculated to be 22.73nm ( Figure 1(b) TPU-FPy and TPU-CPy also undergo microphase separation due to weakly associated self-complementary quadruple hydrogen bonds. In contrast, the microphase separation of TPU-MPy is very weak. This is because the MPy molecules at the ends cannot form self-complementary quadruple hydrogen bonds. TPU-MPy is a non-elastic solid with a soft viscosity at room temperature, so it cannot form an elastomer. The interaction of hydrogen bonds can directly affect the mechanical properties of the polymer. AFM nanomechanical mapping further supports the phase separation results. Figure 1 Figure c shows the nanoscale phase separation of TPU-UPy, consisting of irregular soft (dark) and hard (light) domains. The hard segments are smaller than the soft segments, but their modulus is much higher. These results demonstrate the phase-separated structure of TPU-UPy, which primarily arises from the aggregation of UPy sequences through quadruple hydrogen bonding.

[0077] Mechanical Properties Analysis: Tested TPU-FPy (tensile strength 3.15±0.42 MPa, elongation at break 1204%±51%) and TPU-CPy (tensile strength 2.98±0.13 MPa, elongation at break 1930%±87%) exhibited high ductility and low tensile strength. Tensile strength increased with increasing hydrogen bonding strength, while elongation at break decreased with increasing hydrogen bonding strength. TPU-UPy exhibited the best overall mechanical properties (tensile strength 8.32±0.27 MPa, elongation at break 816%±44%) due to the dynamic aggregation of nanoscale hard segments caused by the UPy sequence. The tensile strength of TPU-UPy increased with increasing stretching rate, increasing from 7.9 MPa to 10.7 MPa when the stretching rate increased from 50 mm / min to 300 mm / min. In addition, in the fast cyclic stretching with a fixed strain of 200%, TPU-UPy showed a significant expansion of the hysteresis region with the increase of the stretching speed. When the stretching rate increased from 20 mm / min to 300 mm / min, the hysteresis energy increased from 1.91 MJ / m 3 Increased to 2.43MJ / m 3 The hysteresis loop during cycling reflects energy dissipation, which is due to the reconfiguration of hard domains and the dissociation of hydrogen bonds. Based on these characteristics, TPU-UPy can be used as an impact-resistant and energy-absorbing protective material. The impact protection mechanism of TPU-UPy mainly stems from the synergistic effect of impact hardening and energy dissipation.

[0078] The dynamic reversibility of hydrogen bonds in TPU-UPy was demonstrated by temperature-dependent infrared measurements. As the temperature increased from 30°C to 170°C, the 1730 cm -1 and 1540cm -1 The absorption peak intensities of amide I and amide II, which represent the hydrogen bond association state, decrease, and 1750 cm -1 and 1525cm-1 The absorption intensity at [Number] increases with increasing temperature, indicating a significant increase in the free C=O and -NH- content. These results demonstrate that the associated hydrogen bonds gradually dissociate with increasing temperature. Concurrently, the absorption peak associated with the isocytosine ring shifts, confirming the presence of quadruple H-bond crosslinking. To elucidate the chain's mobility and viscoelastic behavior, the temperature dependence of the storage and loss moduli was measured using oscillatory shear experiments at a fixed frequency and small amplitude. The viscosity-flow transition temperature (G'=G") of TPU-UPy is located at ~70.5°C, indicating that the cross-linked network transitions from an elastic state to a fluid state. Compared with traditional commercially available non-reactive polyurethane hot melt adhesives, TPU-UPy has a lower viscosity-flow transition temperature. The corresponding inverse frequency (1 / f) represents the relaxation time τc of segmental motion, which represents the ability of polymer chains to rearrange and reflects the ability of dynamic bonds to dissociate and reorganize. Calculations show that at 60°C, the τc of TPU-UPy is 100s. When the temperature rises to 120°C, the relaxation time τc drops rapidly to 0.039s, indicating the rapid dissociation and reorganization of dynamic bonds in the physically cross-linked network at high temperatures. Therefore, the introduction of dynamic cross-linking tetrahydrogen bonds regulates the elastic and liquid behavior of TPU-UPy after heating. Overall, TPU-UPy has good thermoplasticity and dynamic reversibility, and can be used as an adhesive.

[0079] The adhesion performance of TPU-UPy was evaluated by shear strength test. TPU-UPy was placed between two stainless steel substrates to form a sandwich structure. The substrate and the film were fixed together with clamps and placed in an oven at 120°C for 30 minutes to promote the flow of polymer chains and thus better infiltrate the substrate. The thickness of the adhesive layer was ~120μm. Compared with TPU-FPy, TPU-CPy and TPU-MPy, the shear strength of TPU-UPy on the stainless steel substrate reached ~6.8MPa ( Figure 2 (a). When the steel bonded with TPU-UPy was stretched to the point of final fracture, adhesive remained on both steel plates, and the mixed cohesive and interfacial failures resulted in a strong bonding effect. The excellent shear strength of TPU-UPy when bonding different substrates (including aluminum alloy, epoxy resin, and PC) was also tested, with values ​​of 1.5, 3.5, and 3.1 MPa, respectively. Figure 2 (b) Upon heating, hydrogen bonds reversibly dissociate, converting the solid structural adhesive into a low-viscosity liquid and allowing for debonding. TPU-UPy exhibits excellent reusability, achieving over 70% of its original bond strength after five cycles of reuse. Figure 3 ). High bonding strength and excellent multiple reusability can be used in a wide range of electronic and electrical products, facilitating after-sales maintenance.

[0080] In order to prevent the bonded joints from occasional falls or collisions, the structural adhesive is required to have not only high shear strength but also the ability to cushion the impact to prevent the electronic devices from debonding or even disintegrating. Under impact load, the shear strength of TPU-UPy on steel increases rapidly. At an impact speed of 300mm / min, the shear strength increases to 8.9MPa, an increase of 70% compared to 50mm / min. As the tensile speed further increases to 500mm / min, the shear strength of TPU-UPy does not increase further, but remains at a high level ( Figure 4 ).

[0081] In summary, this paper successfully constructed a novel supramolecular hot-melt adhesive, TPU-UPy, which exhibits high bonding strength, excellent reusability, and impact resistance. TPU-UPy exhibits a shear strength of up to 6.8 MPa on steel. Due to the dynamic reversibility of hydrogen bonds, TPU-UPy can be reused five times while retaining over 70% of its original adhesion strength. Importantly, the shear strength increases with increasing stretching speed. Furthermore, TPU-UPy exhibits broad applicability, allowing it to bond to a variety of substrates.

Claims

1. A hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds, characterized in that: Having the structure of formula 1: Formula 1 in, R is a C2~C6 alkylene chain; R1 is a diphenylmethane unit, a phenyl unit, a methylene unit or a six-membered ester ring unit; n is 10~20.

2. The method for preparing a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds according to claim 1, characterized in that: The following steps are involved: 1) polymerizing polyether diol and diisocyanate to obtain a linear prepolymer; wherein the molar ratio of polyether diol to diisocyanate is 1:2-2.5; 2) chain-extending the linear prepolymer with trimethylolpropane to obtain a hyperbranched prepolymer; wherein the molar ratio of trimethylolpropane to polyether glycol is 10-25:100; 3) performing a capping reaction on the hyperbranched prepolymer and 2-amino-4-hydroxy-6-methylpyrimidine to obtain; The polyether glycol has a structure of formula 2: Formula 2 The diisocyanate has a structure of Formula 3: Formula 3 in, R is a C2~C6 alkylene chain; R1 is a diphenylmethane unit, a phenyl unit, a methylene unit or a six-membered ester ring unit; n is 10~20.

3. The method for preparing a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds according to claim 2, characterized in that: The polymerization reaction conditions are: o C temperature, and react for 2 to 4 hours.

4. The method for preparing a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds according to claim 2, characterized in that: The conditions of the chain extension reaction are: o C temperature, and react for 1 to 3 h.

5. The method for preparing a hyperbranched polyurethane hot melt adhesive containing Upy self-complementary hydrogen bonds according to claim 2, characterized in that: The conditions of the end-capping reaction are: o C temperature, and react for 1 to 3 hours.

6. The method for preparing a hyperbranched polyurethane hot melt adhesive containing UPy self-complementary hydrogen bonds according to claim 2, characterized in that: The molar ratio of 2-amino-4-hydroxy-6-methylpyrimidine to polyether diol is 1.2-1.5:1.

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