A hot melt adhesive for bonding a drop-proof structure of a soft package lithium ion battery and a preparation method thereof

Abstract

This invention relates to the field of hot melt adhesive technology, specifically to a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries and its preparation method. The hot melt adhesive, by weight, comprises 1000 parts thermoplastic copolyester, 35-55 parts polycarbodiimide, 8-18 parts carboxylated silica filler, 6-12 parts 2-ureido-4[1H]-pyrimidinone functionalized silica filler, and 25-45 parts carboxyl-terminated flexible polycarbonate buffer segments. During preparation, the components are melt-mixed in a specific order. This hot melt adhesive, by constructing a gradient structure that synergistically integrates a covalently anchored rigid framework, a reversible hydrogen bond energy-dissipating network, and an interfacial flexible buffer layer, significantly improves the high-temperature creep resistance, thermal cycling stability, and drop impact resistance of the adhesive layer, effectively solving the problem of interfacial debonding under structural component displacement and instantaneous impact during long-term use of soft-pack batteries.

Description

Technical Field

[0001] This invention relates to the field of hot melt adhesive technology, and in particular to a hot melt adhesive for bonding anti-drop structures of soft-pack lithium-ion batteries and its preparation method. Background Technology

[0002] Pouch lithium-ion batteries are widely used in consumer electronics, electric vehicles, and other fields due to their advantages such as high energy density and flexible shape design. However, during the charge-discharge cycle and long-term storage of the battery, irreversible chemical reactions and physical deformations occur inside the cell, resulting in continuous heat accumulation and volume expansion and contraction. This creates complex long-term alternating stresses inside the battery and at the bonding interface with external drop protection structures (such as cushioning pads and positioning frames).

[0003] To address the aforementioned application scenarios, the industry commonly uses hot melt adhesives to bond and fix the PET (polyethylene terephthalate) insulating film and aluminum-plastic film of pouch cells to external buffer or positioning structures. Currently, commonly used hot melt adhesive systems, such as those based on polyolefins, polyurethanes, or polyesters, while meeting basic requirements in terms of initial bond strength, face significant challenges in long-term reliability. Firstly, the high temperatures (typically above 60°C) of the battery operating or storage environment significantly reduce the cohesive strength of most hot melt adhesives, leading to softening and creep. Under continuous shear stress, the adhesive layer accumulates irreversible plastic displacement. This minute slippage weakens the effective bonding area over long-term operation, potentially causing relative misalignment between the buffer or positioning structure and the cell, resulting in the loss of its designed function.

[0004] Secondly, pouch batteries experience frequent temperature changes during their service life, and the coefficients of thermal expansion of their materials (such as PET, aluminum foil, and hot melt adhesive) differ. Under thermal cycling, the adhesive interface is subjected to cyclic shear and peel stress. Traditional hot melt adhesives, especially systems that rely solely on physical adsorption or polarity, are prone to molecular chain rearrangement or relaxation under repeated stress, leading to a decrease in interfacial adhesion. More importantly, many hot melt adhesives experience irreversible degradation in bulk (cohesive) strength after dozens or even hundreds of thermal cycles, exhibiting thermal aging. This makes the adhesive layer brittle in the later stages of cycling, unable to effectively transfer and disperse stress.

[0005] A particularly prominent contradiction lies in protecting against instantaneous drop impacts. The core of drop-resistant structural design is to effectively absorb or disperse the impact energy through the adhesive layer when the battery cell is subjected to external impact, preventing damage to the battery cell itself. This requires the adhesive layer to have sufficiently high static strength (such as shear and peel strength) to firmly fix the structural components, while also possessing excellent dynamic toughness to dissipate impact energy. However, many existing hot melt adhesive solutions often struggle to balance these contradictory properties. Increasing static strength is often achieved by increasing crosslinking density or adding rigid fillers, but this often sacrifices the material's flexibility and impact energy dissipation capacity, causing the adhesive layer to exhibit brittle fracture under impact. Conversely, excessive pursuit of flexibility may lead to insufficient high-temperature adhesion performance, causing the structural components to fail prematurely due to creep during long-term use. Therefore, developing a novel hot melt adhesive bonding solution that can synergistically satisfy long-term heat creep resistance, excellent thermal cycling stability, and efficient drop impact energy dissipation has become a key technological bottleneck in improving the overall reliability of pouch lithium-ion batteries. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a hot melt adhesive for bonding anti-drop structures of soft-pack lithium-ion batteries and its preparation method, so as to solve the technical problems of existing hot melt adhesives for bonding anti-drop structures of soft-pack batteries, which suffer from high-temperature softening and creep, cohesive strength decay after thermal cycling, and insufficient dynamic impact energy dissipation capacity, resulting in structural component displacement accumulation and interface debonding failure during instantaneous drops.

[0007] To achieve the above objectives, the present invention provides a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries. By weight, the hot melt adhesive comprises: 1000 parts of thermoplastic copolyester; 35-55 parts of polycarbodiimide; 8-18 parts of carboxylated silica filler; 6-12 parts of 2-ureido-4[1H]-pyrimidinone functionalized silica filler; and 25-45 parts of carboxyl-terminated flexible polycarbonate buffer segments.

[0008] Preferably, the hot melt adhesive further includes 3 parts antioxidant 1010 and 2 parts antioxidant 168.

[0009] Preferably, the polycarbodiimide is a multifunctional polycarbodiimide.

[0010] Preferably, the glass transition temperature of the thermoplastic copolyester is 70°C.

[0011] Preferably, the carboxylated silica filler is obtained by amination of hydrophilic fumed silica with γ-aminopropyltriethoxysilane to obtain aminated silica, followed by reaction with succinic anhydride.

[0012] Preferably, the mass ratio of aminated silica to succinic anhydride in the raw materials for preparing the carboxylated silica filler is 40:30.

[0013] Preferably, the specific surface area of ​​the hydrophilic fumed silica is 200±25 m². 2 / g.

[0014] Preferably, the 2-ureido-4[1H]-pyrimidinone functionalized silica filler is obtained by reacting hydrophilic fumed silica with a 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate, wherein the 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate is obtained by reacting 3-isocyanate-propyltrimethoxysilane with 2-amino-4-hydroxy-6-methylpyrimidine.

[0015] Preferably, the mass ratio of hydrophilic fumed silica to 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate in the raw materials for preparing the 2-ureido-4[1H]-pyrimidinone functionalized silica filler is 60:20; and the mass ratio of 3-isocyanate-propyltrimethoxysilane to 2-amino-4-hydroxy-6-methylpyrimidine in the raw materials for preparing the 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate is 25:15.

[0016] Preferably, the carboxyl-terminated flexible polycarbonate buffer segment is obtained by reacting polycarbonate diol with succinic anhydride in the presence of 4-dimethylaminopyridine.

[0017] Preferably, the mass ratio of polycarbonate diol, succinic anhydride and 4-dimethylaminopyridine in the raw materials for preparing the carboxyl-terminated flexible polycarbonate buffer segment is 100:20:1.

[0018] Furthermore, the present invention also provides a method for preparing a hot melt adhesive for bonding the anti-drop structure of a soft-pack lithium-ion battery, comprising the following steps: melt-blending a thermoplastic copolyester; adding polycarbodiimide to the molten thermoplastic copolyester and blending to obtain a modified melt; adding carboxylated silica filler to the modified melt and blending; adding 2-ureido-4[1H]-pyrimidinone functionalized silica filler to the obtained blending system and blending; adding carboxyl-terminated flexible polycarbonate buffer segments to the obtained blending system and blending, followed by extrusion granulation to obtain a hot melt adhesive for bonding the anti-drop structure of a soft-pack lithium-ion battery.

[0019] The beneficial effects of this invention are: This invention significantly improves the long-term heat resistance and creep resistance of the adhesive layer: By introducing multifunctional polycarbodiimide during the melt-mixing stage of the thermoplastic copolyester matrix, the invention cleverly achieves passivation and end-capping of the polyester molecular chain ends, as well as appropriate branching and chain extension. This treatment not only effectively inhibits the decrease in molecular weight of polyester due to end-group hydrolysis under humid and hot conditions and enhances the cohesive strength of the matrix phase, but also retains necessary residual reaction sites for the subsequent targeted grafting of functional fillers and buffer segments. The resulting modified copolyester matrix, while maintaining good polarity matching with the PET substrate, has its modulus and creep resistance above the glass transition region fundamentally enhanced, thereby ensuring the dimensional stability and adhesion reliability of the adhesive layer under the long-term operating temperature of the battery.

[0020] A spatially stable framework was constructed to suppress the accumulation of interfacial shear displacement: the carboxylated silica rigid anchoring filler designed in the scheme preferentially reacts with the residual carbodiimide groups reserved in the modified copolyester matrix during the mixing process through the carboxyl functional groups on its surface. This covalent anchoring effect firmly fixes the nanoscale silica particles to the polymer matrix by chemical bonds, forming spatially non-slip rigid anchor points. These uniformly dispersed nano-anchor points can effectively mechanically interlock the polymer molecular chains, greatly limiting the plastic flow and displacement accumulation of the adhesive layer under continuous shear stress, providing a solid skeletal support for the entire adhesive system, which is the key to achieving excellent high-temperature tack.

[0021] An efficient energy dissipation mechanism is introduced to enhance impact resistance and environmental adaptability: After establishing a rigid anchoring framework, this invention further introduces silica with 2-ureido-4[1H]-pyrimidinone groups grafted onto its surface as a reversible network filler. This group can form strong quadruple hydrogen bonds, constructing a dynamic and reversible physical cross-linked network within the adhesive layer. Under instantaneous high loads such as drop impacts, this hydrogen bond network can dissipate a large amount of impact energy through reversible breakage and recombination, preventing stress from damaging the covalent bond main network and the interfacial adhesion layer. Simultaneously, under repeated stress caused by temperature cycling, this dynamic network can adapt to interfacial strain through rearrangement, effectively buffering internal stress caused by differences in the thermal expansion coefficients of the materials, thereby significantly improving the peel strength and shear strength retention rate of the adhesive joint after rigorous thermal cycling.

[0022] This invention achieves a gradient synergy between rigid support and flexible buffering, resulting in a balanced overall performance. The final step involves introducing carboxyl-terminated flexible polycarbonate buffer segments and covalently grafting them onto the preliminarily constructed rigid anchoring framework-reversible hydrogen bond network using residual carbodiimide active sites. This post-introduction, site-specific grafting strategy ensures that the flexible segments are primarily concentrated in the interfacial region or act as flexible bridges between networks, rather than being uniformly dispersed throughout the matrix, thus preventing overall softening. Consequently, a gradient structure is formed within the adhesive layer, from a robust matrix and rigid anchoring points to a dynamic hydrogen bond network and finally to the interfacial flexible buffer layer. This structure enables the material to simultaneously possess high static strength, excellent creep resistance, outstanding impact energy dissipation capacity, and good resistance to environmental stress changes, ultimately meeting the comprehensive requirements of high strength, high durability, and high toughness for the adhesive material in the drop-proof structure of soft-pack lithium-ion batteries. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0024] Example 1: In this embodiment, the thermoplastic copolyester granules are selected from Vitel 1200BP copolyester extrusion resin of Bostik Corporation, in granular form, with a glass transition temperature of 70°C; the polycarbodiimide is selected from Stabaxol P 200 polycarbodiimide of Lanxess Corporation; and the fumed silica is selected from AEROSIL 200 G hydrophilic fumed silica of Evonik Corporation, with a specific surface area of ​​200±25m². 2 / g; The polycarbonate diol used is Covestro's Desmophen C 1100 polyester-polycarbonate diol, with a viscosity of 3200 mPa·s at 23°C and a hydroxyl content of 3.3%.

[0025] Step S1: Weigh 1000g of thermoplastic copolyester granules and place them in a vacuum oven. Dry them at 120℃ and -80kPa for 6 hours. After removing them, immediately transfer them to a drying oven and seal them for later use. At the same time, weigh 120g of fumed silica and place it in a vacuum oven. Dry it at 120℃ and -80kPa for 4 hours and then seal it for later use. Step S2: Add 100g of polycarbonate diol to a reaction vessel equipped with mechanical stirring and nitrogen protection, heat to 130℃ and stir at 150rpm, then add 20g of succinic anhydride and 1g of 4-dimethylaminopyridine, continue stirring at 130℃ for 3h, then lower to 120℃ and vacuum to -80kPa for 1h to obtain carboxyl-terminated flexible polycarbonate buffer segments; Step S3: Add 600g anhydrous ethanol, 60g deionized water, and 60g of the hydrophilic fumed silica dried in Step S1 to the reaction vessel. Stir and ultrasonically disperse the mixture at 400rpm for 30min at 25℃ to form a uniform suspension. Then, add 20g of γ-aminopropyltriethoxysilane and continue stirring for 30min. Subsequently, add glacial acetic acid to adjust the pH of the system to 4.5 and continue stirring for 2h. Then, raise the temperature to 70℃ and maintain the temperature for 4h. After the reaction is complete, filter and wash once with 200g anhydrous ethanol. Dry the obtained solid at 100℃ and vacuum degree -80kPa for 6h to obtain aminated silica. Then, add 400g of 2-methyltetrahydrofuran, 40g of aminated silica, and 30g of succinic anhydride to the reaction vessel. Stir and react at 400rpm for 4h at 60℃. After the reaction is complete, filter and wash with 200g of 20g of anhydrous ethanol. 2-Methyltetrahydrofuran was washed once with 200g of anhydrous ethanol, then dried at 80℃ and vacuum degree -80kPa for 8h and passed through a 200-mesh sieve to obtain carboxylated silica rigid anchoring filler. Step S4: Nitrogen gas was introduced into a dry three-necked flask for protection. 25g of 3-isocyanate-propyltrimethoxysilane was added and heated to 75°C. Then 15g of 2-amino-4-hydroxy-6-methylpyrimidine was added and the mixture was stirred at 300rpm for 3h. The temperature was controlled at 75-80°C during the reaction. After the reaction was completed, the mixture was evacuated at 80°C and a vacuum of -80kPa for 1h to obtain 2-ureido-4[1H]-pyrimidinone tetrahydrosilane intermediate. Step S5: Add 600g of anhydrous ethanol and 60g of deionized water to the reactor, add 60g of the hydrophilic fumed silica dried in step S1, stir and ultrasonically disperse at 400rpm at 25℃ for 30min, add 20g of 2-ureido-4[1H]-pyrimidinone tetrahydro-bonded silane intermediate and stir for 30min, then add glacial acetic acid to adjust the pH of the system to 4.5 and continue stirring for 2h, then raise the temperature to 70℃ and keep it at that temperature for 6h. After the reaction is completed, filter and wash once each with 200g of anhydrous ethanol and 200g of deionized water. The obtained solid is dried at 80℃ and vacuum degree -80kPa for 10h and passed through a 200-mesh sieve to obtain 2-ureido-4[1H]-pyrimidinone functionalized silica filler. Step S6: Add 1000g of the dried thermoplastic copolyester granules from Step S1 to a mixer equipped with nitrogen protection and vacuum exhaust. Set the melting zone temperature to 170℃ and mix at 200rpm until completely melted. Then, add 40g of polycarbodiimide in four portions, mixing for 5 minutes after each addition. After all the granules are added, continue mixing for 10 minutes and turn on the vacuum exhaust to -80kPa for 5 minutes to obtain a carbodiimide-modified copolyester melt. Next, add 10g of carboxylated silica rigid anchoring filler in five portions, mixing for 2 minutes after each addition. After all the fillers are added, continue mixing for 15 minutes to obtain a covalently anchored nanoparticle / copolyester composite melt. Reduce the temperature of the composite melt to 160℃ and stir at 200rpm. Add 8g of polycarbodiimide in four portions. 2-Ureido-4[1H]-pyrimidinone functionalized silica filler was added and mixed for 2 min after each addition, and then mixed for 10 min after all the filler was added. Then 30 g of carboxyl-terminated flexible polycarbonate buffer segments were added and mixed for 20 min after the addition was completed. The vacuum was then drawn to -80 kPa and maintained for 5 min. The temperature was lowered to 150 °C and stirred at 100 rpm. 3 g of antioxidant 1010 and 2 g of antioxidant 168 were added and mixed for another 5 min. The mixture was then extruded into strips, cooled in water at 25 °C, and granulated to obtain a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries.

[0026] Example 2: Compared with Example 1, this example only adjusts the mixing parameters and raw material dosage in step S6 as follows: The melting zone temperature is set to 175℃ and mixed at 220 rpm until completely melted. Then, 55g of polycarbodiimide is added in 5 portions, mixing for 5 minutes after each addition. After all the material is added, mixing continues for 10 minutes, and vacuum exhaust is activated to -80kPa and maintained for 5 minutes to obtain the carbodiimide-modified copolyester melt. Subsequently, 12g of carboxylated silica rigid anchoring filler is added in 6 portions, mixing for 2 minutes after each addition. After all the material is added, mixing continues for 15 minutes. The composite melt temperature is lowered to 160℃ and stirred at 220 rpm. 10g of polycarbodiimide is added in 5 portions. 2-Ureido-4[1H]-pyrimidinone functionalized silica filler was added and mixed for 2 min after each addition, and then mixed for 10 min after all the filler was added. 25 g of carboxyl-terminated flexible polycarbonate buffer segments were added and mixed for 20 min after addition. The vacuum was then evacuated to -80 kPa and maintained for 5 min. The temperature was lowered to 150 °C and stirred at 100 rpm. 3 g of antioxidant 1010 and 2 g of antioxidant 168 were added and mixed for another 5 min. The mixture was then extruded into strips, cooled in water at 25 °C, and pelletized to obtain a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries. The remaining conditions were the same as in Example 1.

[0027] Example 3: Compared with Example 1, this example only adjusts the mixing parameters and raw material dosage in step S6 as follows: The melting zone temperature is set to 165℃ and mixed at 180 rpm until completely melted. Then, 35g of polycarbodiimide is added in four portions, mixing for 5 minutes after each addition. After all the material is added, mixing continues for 10 minutes, and vacuum exhaust is activated to -80 kPa and maintained for 5 minutes to obtain the carbodiimide-modified copolyester melt. Subsequently, 8g of carboxylated silica rigid anchoring filler is added in four portions, mixing for 2 minutes after each addition. After all the material is added, mixing continues for 15 minutes. The composite melt temperature is lowered to 160℃ and stirred at 180 rpm. 6g of polycarbodiimide is added in three portions. 2-Ureido-4[1H]-pyrimidinone functionalized silica filler was added and mixed for 2 min after each addition, and then mixed for 10 min after all the filler was added. 45 g of carboxyl-terminated flexible polycarbonate buffer segments were added and mixed for 20 min after addition. The vacuum was then evacuated to -80 kPa and maintained for 5 min. The temperature was lowered to 150 °C and stirred at 100 rpm. 3 g of antioxidant 1010 and 2 g of antioxidant 168 were added and mixed for another 5 min. The mixture was then extruded into strips, cooled in water at 25 °C, and pelletized to obtain a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries. The remaining conditions were the same as in Example 1.

[0028] Example 4: Compared with Example 1, this example only adjusts the amount of raw materials in step S6 as follows: The melting temperature is set to 170℃ and mixed at 200 rpm until completely melted. Then, 45g of polycarbodiimide is added in four portions, mixing for 5 minutes after each addition. After all the material is added, mixing continues for 10 minutes, and vacuum exhaust is activated to -80 kPa and maintained for 5 minutes to obtain the carbodiimide-modified copolyester melt. Subsequently, 18g of carboxylated silica rigid anchoring filler is added in six portions, mixing for 2 minutes after each addition. After all the material is added, mixing continues for 15 minutes. The composite melt temperature is lowered to 160℃ and stirred at 200 rpm. 12g of polycarbodiimide is added in four portions. 2-Ureido-4[1H]-pyrimidinone functionalized silica filler was added and mixed for 2 min after each addition, and then mixed for 10 min after all the filler was added. Then 35 g of carboxyl-terminated flexible polycarbonate buffer segments were added and mixed for 20 min after the addition was completed. The vacuum was then drawn to -80 kPa and maintained for 5 min. The temperature was lowered to 150 °C and stirred at 100 rpm. 3 g of antioxidant 1010 and 2 g of antioxidant 168 were added and mixed for another 5 min. The mixture was then extruded into strips, cooled in water at 25 °C, and granulated to obtain a hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries. The remaining conditions were the same as in Example 1.

[0029] Comparative Example 1: The difference from Example 1 is that: in step S6, polycarbodiimide is not added (the amount of polycarbodiimide is 0g), and in the corresponding stage of "adding 40g of polycarbodiimide in 4 parts, mixing for 5 minutes after each addition, and continuing to mix for 10 minutes after all the additions", the mixing is carried out at 200rpm and the same mixing time is maintained. Then, the vacuum is turned on to exhaust to -80kPa and maintained for 5 minutes; the other conditions are the same as in Example 1.

[0030] Comparative Example 2: The difference from Example 1 is that: in step S6, no carboxylated silica rigid anchoring filler is added (the amount of carboxylated silica rigid anchoring filler is 0g), and the corresponding stage of "adding 10g of carboxylated silica rigid anchoring filler in 5 portions, mixing for 2 minutes after each addition, and continuing to mix for 15 minutes after all the filler has been added" is maintained at 170°C, 200rpm and the same mixing time; the other conditions are the same as in Example 1.

[0031] Comparative Example 3: The difference from Example 1 is as follows: In step S6, after obtaining the carbodiimide modified copolyester melt, the temperature of the composite melt is first lowered to 160°C and stirred at 200 rpm. 8g of 2-ureido-4[1H]-pyrimidinone functionalized silica filler is added in 4 portions (mixed for 2 minutes after each addition, and continued mixing for 10 minutes after all the filler is added). Then, the temperature of the composite melt is raised to 170°C and mixed at 200 rpm. 10g of carboxylated silica rigid anchoring filler is added in 5 portions (mixed for 2 minutes after each addition, and continued mixing for 15 minutes after all the filler is added). The remaining conditions are the same as in Example 1.

[0032] Comparative Example 4: The difference from Example 1 is that: in step S6, 2-ureido-4[1H]-pyrimidinone functionalized silica filler is not added (the amount of 2-ureido-4[1H]-pyrimidinone functionalized silica filler is 0g), and the corresponding stage of "adding 8g of 2-ureido-4[1H]-pyrimidinone functionalized silica filler in 4 portions, mixing for 2min after each addition, and continuing to mix for 10min after all the filler has been added" is maintained at 160℃, 200rpm and the same mixing time; the other conditions are the same as in Example 1.

[0033] Comparative Example 5: The difference from Example 1 is that in step S6, after the temperature of the composite melt is reduced to 160°C, 30g of carboxyl-terminated flexible polycarbonate buffer segments are added and mixed for 20min. Then, 8g of 2-ureido-4[1H]-pyrimidinone functionalized silica filler is added in 4 portions (mixed for 2min after each addition, and mixed for 10min after all additions). Then, the vacuum is drawn to -80kPa and maintained for 5min as in Example 1, and an antioxidant is added. The remaining conditions are the same as in Example 1.

[0034] Comparative Example 6: The difference from Example 1 is that: the amount of succinic anhydride used in step S2 is 0g, so the product obtained in step S2 is polycarbonate diol; in step S6, 30g of the polycarbonate diol obtained in step S2 is used to replace 30g of carboxyl-terminated flexible polycarbonate buffer segments, and the other conditions are the same as in Example 1.

[0035] Performance testing: Sample Preparation: Hot melt adhesive particles were prepared according to Examples 1-4 and Comparative Examples 1-6, respectively. For the bonding test samples, polyethylene terephthalate (PET) sheets (2.0 mm thick) and polyethylene terephthalate (PET) films (0.10 mm thick) were used as substrates. Before the test, the substrate surfaces were wiped twice with anhydrous ethanol and dried in a 60°C hot air oven for 30 min. The hot melt adhesive particles were melted in a 160°C constant temperature oven for 30 min, and a 0.20 mm thick wet film layer of hot melt adhesive was prepared on the PET sheet using a scraper. This layer was then hot-pressed at 0.20 MPa for 30 s at 160°C onto another substrate. After standing at 25°C for 24 h, various tests were performed. For each sample, at least 5 bonding test samples were prepared for peel and shear tests, and at least 3 bonding test samples were prepared for holding and drop tests.

[0036] 180° Peel Strength (Flexible Material vs. Rigid Material): Prepared and tested according to GB / T 2790-1995. PET sheet (2.0 mm thick) was used as the rigid substrate and PET film (0.10 mm thick) as the flexible substrate. The sample width was 25 mm, the bonding length was 125 mm, and the thickness of the hot melt adhesive layer was controlled at (0.20±0.02) mm. After being placed at (23±2)℃ and (50±5)% relative humidity for 24 h, a 180° peel test was performed using an electronic tensile testing machine at a speed of 300 mm / min. The average peel force of the stable peel section of 100 mm was taken and converted into the 180° peel strength (N / 25 mm). Five samples of each type were tested and the average value was taken.

[0037] Tensile shear strength (rigid material to rigid material): Prepared and tested according to GB / T 7124-2008, using PET sheet (2.0 mm thick) as the bonded material, with a single overlap bonding area of ​​25 mm × 12.5 mm, and the hot melt adhesive layer thickness controlled at (0.20 ± 0.02) mm; after the specimens were placed at (23 ± 2) ℃ and (50 ± 5)% relative humidity for 24 h, they were stretched at a speed of 10 mm / min using an electronic tensile testing machine, the maximum load was recorded and the tensile shear strength (MPa) was calculated based on the bonding area, and 5 specimens of each type were tested and the average value was taken.

[0038] Shear tack at 80℃: The test was conducted according to GB / T 4851-2014. PET sheets (2.0 mm thick) were used to prepare bonding samples, with a bonding area of ​​25 mm × 25 mm and a hot melt adhesive layer thickness controlled at (0.20 ± 0.02) mm. After the samples were placed at (23 ± 2)℃ and (50 ± 5)% relative humidity for 24 hours, they were placed in an (80 ± 1)℃ constant temperature chamber for 30 minutes to equilibrate. A 1.0 kg weight was suspended at the free end, and timing was started. The time (min) at which the bonding surface of the sample completely failed was recorded. The longest observation time for a single sample was set to 10080 min. Samples that did not fail after 10080 min were recorded as 10080 min. Three samples of each type were tested, and the average value was taken.

[0039] Adhesion performance retention rate after temperature change cycle: Temperature change treatment was carried out according to GB / T 2423.22-2012. The 180° peel test specimens and tensile shear test specimens of each sample were placed in a temperature cycling test chamber. The temperature was set to -20℃ for 30 min, 60℃ for 30 min, and the temperature rise and fall conversion time was 10 min. The cycle was repeated 100 times. After the cycle, the samples were allowed to stand for 2 h at (23±2)℃ and (50±5)% relative humidity. The peel strength and shear strength were retested. The peel strength retention rate (%) and shear strength retention rate (%) were calculated.

[0040] Drop test of anti-drop structure simulation component: Drop test was conducted according to GB / T 4857.5-1992. Anti-drop structure simulation component was prepared by bonding a 50mm×50mm×2.0mm PET sheet to a 50mm×50mm×0.10mm PET film. The thickness of the hot melt adhesive layer was controlled at (0.20±0.02)mm. A counterweight was fixed on the back of the PET sheet so that the total mass of the simulation component was (500±5)g. After the sample was placed at (23±2)℃ and (50±5)% relative humidity for 24h, it was dropped freely from a height of 1.0m. The drop surface was a steel plate with a thickness of ≥20mm. The sample was dropped 10 times in the order of "front, back, long side, short side, corner". After each drop, the sample was visually inspected and the bonded area was covered with a 5mm square transparent mesh film to count the debonding area. The maximum debonding area (%) after 10 drops was recorded. Three samples of each type were tested and the average value was taken.

[0041] Table 1 Performance test results of each embodiment and comparative sample

[0042] Data Analysis: As can be seen from the data in Table 1 of the embodiments, the hot melt adhesive prepared by the present invention for bonding the anti-drop structure of soft-pack lithium-ion batteries maintains a high level in terms of 180° peel strength and tensile shear strength, while exhibiting excellent high-temperature shear holding capacity. Furthermore, it maintains a high adhesion performance retention rate after temperature change cycling, and the maximum debonding area after a drop is at a low level. Further analysis of the differences in formulation and process in the embodiments reveals that increasing the amount of polycarbodiimide results in more complete end-group passivation and branching enhancement effects, making the cohesive strength and thermal creep resistance of the copolyester phase more prominent, thus being more conducive to shear bearing capacity and high-temperature holding capacity. Increasing the amount of carboxylated silica rigid anchoring filler increases the number of non-slip nodes formed by covalent anchoring, which can more effectively suppress the accumulation of shear displacement and stabilize the interface skeleton. The introduction of 2-ureido-4[1H]-pyrimidinone functionalized silica filler, through tetrahydrogenation… The reversible bridging provides an energy dissipation channel, making it easier to maintain the structural integrity of the adhesive layer under thermal cycling and impact loads. The carboxyl-terminated flexible polycarbonate buffer segments are added after the reversible network is formed and the residual carbodiimide is used to achieve end covalent grafting, which helps to improve the interface deformation coordination and impact energy dissipation without significantly weakening the high-temperature adhesion. This achieves a comprehensive balance of strength, heat creep resistance, thermal cycling stability and drop debonding resistance, which meets the application requirements of soft-pack lithium-ion batteries for both long-term stress loading and instantaneous drop impact.

[0043] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, when polycarbodiimide is not added in step S6, the 180° peel strength, tensile shear strength, and 80° shear holding power are all significantly reduced, the performance retention rate after temperature change cycles decreases, and the maximum debonding area after drop is significantly increased. The main reason is that the lack of end-group passivation and branching reinforcement effect of polycarbodiimide results in insufficient stabilization of the copolyester phase molecular chain, making it difficult to effectively suppress high-temperature softening creep and shear displacement accumulation. In addition, insufficient residual reactive sites weaken the subsequent sequential reaction fixation of carboxylated silica rigid anchoring filler and carboxylated end-capped flexible polycarbonate buffer segments, making it difficult to establish the overall structure of covalent anchoring skeleton, reversible energy dissipation network, and flexible buffer graft, leading to more easy interface slippage and debonding failure during drop impact.

[0044] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 2 and 3, when carboxylated silica rigid anchoring filler is not added in step S6, or when 2-ureido-4[1H]-pyrimidinone functionalized silica filler is added in advance, thus restricting the preferential covalent anchoring of the carboxylated silica rigid anchoring filler, the 80℃ shear adhesion and retention rate after thermal cycling of the samples both decrease to varying degrees, and the maximum debonding area increases after the drop. The reason is that the carboxylated silica rigid anchoring filler needs to preferentially covalently anchor with the residual carbodiimide before the formation of the reversible network in order to form spatially non-slip nanonodes and provide a stable framework for subsequent hydrogen bond bridging; when this sequence of anchoring first and bridging later is broken or the anchoring nodes are missing, the role of the reversible network is more inclined to physical energy consumption and it is difficult to simultaneously constrain interface displacement, thus making displacement accumulation more likely to occur under thermal cycling and continuous shear load. This shows that covalent anchoring and quadruple hydrogen bond bridging are not simply superimposed, but rather involve a synergistic mechanism that uses a stable framework to support reversible energy consumption.

[0045] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, when 2-ureido-4[1H]-pyrimidinone-functionalized silica filler is not added in step S6, the maximum debonding area after drop is significantly increased, and the debonding retention ability after temperature change cycles decreases more significantly. The main reason is that without the reversible bridging of quadruple hydrogen bonds, the adhesive layer has insufficient energy dissipation channels under impact load, and stress is more likely to concentrate and extend along the interface to form debonding; at the same time, during the expansion and contraction and modulus fluctuation caused by temperature cycling, the buffering and reconstruction effect of the reversible hydrogen bond network on the micro-slippage of the interface is missing, making the interface debonding attenuation more likely to occur under the peeling condition. This shows that 2-ureido-4[1H]-pyrimidinone-functionalized silica filler not only provides energy dissipation, but also stabilizes the network topology together with covalent anchoring nodes, thereby achieving a synergistic improvement in impact resistance and thermal cycling stability.

[0046] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, when the carboxyl-terminated flexible polycarbonate buffer segments are added before the formation of the reversible network, or when polycarbonate diols are used to replace the carboxyl-terminated flexible polycarbonate buffer segments and the terminal carboxyl groups are missing, the 80°C shear tackiness and the retention rate after thermal cycling of the samples both decrease, and the maximum debonding area after drop further increases. The reason for this is that the subsequent introduction and site-specific grafting are key to the function of the flexible segments: when the carboxyl-terminated flexible polycarbonate buffer segments are added after the formation of the reversible network and the residual carbodiimide is used for terminal covalent grafting, a spatial gradient structure can be formed and a buffer can be provided near the interface without significantly softening the whole; while adding them in advance will make the flexible segments more likely to participate in blending and interface wetting during high-temperature mixing, thereby weakening the effective construction of the subsequent quadruple hydrogen bond network and nano-anchored framework, resulting in more obvious high-temperature creep and displacement accumulation. In the case of polycarbonate diol substitution, the lack of terminal carboxyl groups makes effective covalent fixation with the residual carbodiimide difficult. The flexible component is more likely to exist in a non-stationary manner, amplifying the risk of interfacial slippage under thermal cycling. Therefore, significant temporal and chemical fixation synergies exist between flexible segments, reversible networks, and covalent anchoring.

[0047] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries, characterized in that, The hot melt adhesive comprises, by weight parts: 1000 parts thermoplastic copolyester; 35-55 parts polycarbodiimide; 8-18 parts carboxylated silica filler; 6-12 parts 2-ureido-4[1H]-pyrimidinone functionalized silica filler; and 25-45 parts carboxylated end-capped flexible polycarbonate buffer segments.

2. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The hot melt adhesive also includes 3 parts antioxidant 1010 and 2 parts antioxidant 168.

3. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The polycarbodiimide is a multifunctional polycarbodiimide; the glass transition temperature of the thermoplastic copolyester is 70°C.

4. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The carboxylated silica filler is obtained by amination of hydrophilic fumed silica with γ-aminopropyltriethoxysilane to obtain aminated silica, followed by reaction with succinic anhydride; the mass ratio of aminated silica to succinic anhydride in the raw materials for preparing the carboxylated silica filler is 40:

30.

5. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The specific surface area of ​​the hydrophilic fumed silica is 200±25m². 2 / g.

6. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The 2-ureido-4[1H]-pyrimidinone functionalized silica filler is obtained by reacting hydrophilic fumed silica with a 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate, which is obtained by reacting 3-isocyanate-propyltrimethoxysilane with 2-amino-4-hydroxy-6-methylpyrimidine.

7. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 6, characterized in that, In the preparation of the 2-ureido-4[1H]-pyrimidinone functionalized silica filler, the mass ratio of hydrophilic fumed silica to 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate is 60:20; in the preparation of the 2-ureido-4[1H]-pyrimidinone tetrahydrobonded silane intermediate, the mass ratio of 3-isocyanate-propyltrimethoxysilane to 2-amino-4-hydroxy-6-methylpyrimidine is 25:

15.

8. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 1, characterized in that, The carboxyl-terminated flexible polycarbonate buffer segment is obtained by reacting polycarbonate diol with succinic anhydride in the presence of 4-dimethylaminopyridine.

9. The hot melt adhesive for bonding the anti-drop structure of soft-pack lithium-ion batteries according to claim 8, characterized in that, The mass ratio of polycarbonate diol, succinic anhydride and 4-dimethylaminopyridine in the raw materials for preparing the carboxyl-terminated flexible polycarbonate buffer segment is 100:20:

1.

10. A method for preparing a hot melt adhesive for bonding a drop-proof structure of a soft-pack lithium-ion battery according to any one of claims 1-9, comprising the following steps: melt-blending a thermoplastic copolyester; adding polycarbodiimide to the molten thermoplastic copolyester and blending to obtain a modified melt; adding carboxylated silica filler to the modified melt and blending; adding 2-ureido-4[1H]-pyrimidinone functionalized silica filler to the obtained blending system and blending; adding carboxyl-terminated flexible polycarbonate buffer segments to the obtained blending system and blending, followed by extrusion granulation to obtain a hot melt adhesive for bonding a drop-proof structure of a soft-pack lithium-ion battery.

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