Die-cut hot melt adhesive sheet for insulating and fixing steel shell lithium battery and preparation process thereof

Abstract

The present application relates to the technical field of hot melt adhesive, in particular to a die-cut hot melt adhesive sheet for insulating and fixing steel shell lithium battery and a preparation process thereof. The die-cut hot melt adhesive sheet is made of fluorine / epoxy co-grafted elastomer granules, SEBS rubber block copolymer, tackifying resin, polyethylene wax powder, antioxidant and specific mercapto phosphonic acid and fluorine-containing mercaptan. The preparation process comprises: preparing fluorine / epoxy co-grafted elastomer granules by melt grafting, then melt mixing the granules with other components and adding functional additives step by step to obtain modified hot melt adhesive, and finally coating, compounding and die-cutting to obtain the finished product. The scheme grafts fluorine and epoxy groups on the polymer chain, and forms covalent anchoring and gradient barrier structure on the interface by subsequent click chemistry reaction, thereby significantly improving the swelling resistance, permeation resistance and long-lasting metal bonding reliability of the adhesive product in the lithium battery carbonate electrolyte environment.

Description

Technical Field

[0001] This invention relates to the field of hot melt adhesive technology, and in particular to a die-cut hot melt adhesive sheet for insulating and fixing steel-cased lithium batteries and its preparation process. Background Technology

[0002] In the manufacturing and packaging of steel-cased lithium batteries, insulating adhesives are needed to fix insulating films such as polyimide or polyester to the inner wall or specific parts of the steel casing. This ensures reliable insulation between the battery cell and the casing and prevents internal components from shifting during charge / discharge cycles or under external forces. Currently, hot-melt pressure-sensitive adhesives based on styrene-ethylene / butene-styrene block copolymers, combined with tackifying resins, waxes, and other additives, are commonly used in this field. These materials typically provide good adhesion and ease of application in their initial state.

[0003] However, lithium batteries are filled with organic electrolytes, whose main components are carbonate solvents (such as ethylene carbonate and dimethyl carbonate) and lithium salts. These electrolytes have a strong dissolving or swelling capacity for most polymers. When conventional hot melt adhesives are applied inside the battery, the carbonate electrolyte gradually penetrates into the adhesive layer. On the one hand, as a small-molecule plasticizer, the electrolyte causes the polymer matrix of the hot melt adhesive to swell and soften, severely weakening its cohesive strength and creep resistance (cold flow resistance). On the other hand, when the electrolyte penetrates to the interface between the adhesive and the adhered material (steel shell metal and polyimide insulating film), it changes the interfacial energy, disrupts the original adhesive balance, and significantly reduces interfacial adhesion. This decrease in cohesive strength and loss of interfacial adhesion caused by medium penetration occurs synergistically. Macroscopically, the hot melt adhesive has sufficient peel strength initially, but after long-term immersion in the electrolyte or storage in a high-humidity, high-temperature environment, the peel strength drops sharply, and even interfacial debonding and edge lifting occur.

[0004] A deeper problem is the significant difference in surface energy between the steel shell's metal surface and the polyimide insulating film. While conventional hot melt adhesives use tackifying resins (such as terpene resins and petroleum resins) to provide initial tack, their limited polarity makes it difficult to form a strong and durable chemical or physical bond with the high surface energy of the clean steel surface. Under the continuous action of the electrolyte, this already weak interfacial bond is more easily disrupted, allowing the electrolyte to penetrate along the steel-adhesive interface through capillary channels. This not only accelerates peeling failure but may also induce insulation failure or even short-circuit risks. Existing technologies have attempted to improve resistance to media by adding various solvent-resistant fillers or introducing fluoropolymers through physical blending, but these often result in excessively high colloidal modulus, decreased initial tack, or failure to achieve long-term stability due to poor compatibility between the additives and the matrix, or easy extraction and migration in the electrolyte.

[0005] Therefore, developing an insulating and fixing adhesive material that can fundamentally resist electrolyte swelling and penetration, while forming a durable and strong bonding interface on the steel shell surface, has become a key technical requirement for improving the reliability and safety of steel-shell lithium batteries. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a die-cut hot melt adhesive sheet for insulating and fixing steel-cased lithium batteries and its preparation process, so as to solve the problem that conventional hot melt adhesives are prone to swelling, plasticization and interface wetting damage in the carbonate electrolyte environment of steel-cased lithium batteries, which leads to a significant decrease in peel strength after immersion in electrolyte, interface lifting and the formation of insulating microchannels, making it difficult to achieve both long-lasting metal adhesion reliability and effective electrolyte barrier sealing.

[0007] To achieve the above objectives, the present invention provides a die-cut hot melt adhesive sheet for insulating and fixing steel-cased lithium batteries, comprising a hot melt adhesive layer, wherein the hot melt adhesive layer is prepared from the following raw materials by weight: 1800-2200 parts of fluorinated / epoxy co-grafted elastomer granules, 800-1200 parts of SEBS rubber block copolymer, 1500-1900 parts of tackifying resin, 150-300 parts of polyethylene wax powder, 10 parts of antioxidant 1010, 10 parts of antioxidant 168, 0.2-0.7 parts of 2-methylimidazole, 25-45 parts of 11-mercaptoundecylphosphonic acid, 5-12 parts of 1H,1H,2H,2H-perfluorooctyl mercaptan, and 8-20 parts of 1H,1H,2H,2H-perfluorodecyl mercaptan.

[0008] Preferably, the die-cut hot melt adhesive sheet further includes a first release film and a second release film respectively coated on both sides of the hot melt adhesive layer.

[0009] Preferably, the first release film is a silicone oil release polyester film, and the second release film is a fluorosilicone release polyester film.

[0010] Preferably, the thickness of the silicone oil release polyester film is 45-55 μm, and the thickness of the fluorosilicone release polyester film is 70-80 μm.

[0011] Furthermore, the fluorinated / epoxy co-grafted elastomer granules are obtained by melt grafting the following components: using 2500 parts of SEBS rubber block copolymer as the matrix, adding 150-260 parts of 1H,1H,2H,2H-perfluorooctyl methacrylate, 30-80 parts of glycidyl methacrylate, and 6-10 parts of dicumyl peroxide.

[0012] Preferably, the styrene / rubber block mass ratio of the SEBS rubber block copolymer is 30 / 70, and the melt flow rate at 230℃ / 5kg is 4-6g / 10min.

[0013] Preferably, the tackifying resin is Escorez 5400 tackifying resin.

[0014] Preferably, the polyethylene wax powder is AC 617A polyethylene wax powder.

[0015] Preferably, the hot melt adhesive layer has a metal side and an outer side, wherein the side in contact with the silicone oil release polyester film is the metal side, and the side in contact with the fluorosilicone release polyester film is the outer side.

[0016] Furthermore, the present invention also provides a process for preparing a die-cut hot melt adhesive sheet for insulating and fixing steel-cased lithium batteries, comprising the following steps: (1) Preparation of fluorine / epoxy co-grafted elastomer granules: Under nitrogen protection, SEBS rubber block copolymer was melt-plasticized, and 1H,1H,2H,2H-perfluorooctyl methacrylate, glycidyl methacrylate and dicumyl peroxide were added to carry out melt grafting reaction. The material was discharged, devolatilized, crushed and granulated to obtain fluorine / epoxy co-grafted elastomer granules. (2) Preparation of modified hot melt adhesive: The fluorinated / epoxy co-grafted elastomer granules obtained in step (1) are melt-blended with SEBS rubber block copolymer, tackifying resin, polyethylene wax powder and antioxidant to obtain hot melt adhesive matrix. Then, 2-methylimidazole, 11-mercaptoundecylphosphonic acid, 1H,1H,2H,2H-perfluorooctyl mercaptan and 1H,1H,2H,2H-perfluorodecyl mercaptan are added in sequence. After volatilization, the material is discharged to obtain modified hot melt adhesive. (3) Film formation and die cutting: The modified hot melt adhesive obtained in step (2) is melt-coated onto the release surface of the silicone oil release polyester film and the coating thickness is controlled. After cooling on one side, it is combined with the fluorosilicone release polyester film to form a double release sandwich structure. After post-processing, the die-cut hot melt adhesive sheet is obtained.

[0017] Preferably, in step (2), 2-methylimidazole and 11-mercaptoundecylphosphonic acid are added to the hot melt adhesive matrix at 150°C and 50 rpm and mixed for 12-15 min; then the chamber temperature is lowered to 145°C and 1H,1H,2H,2H-perfluorooctylthiol is added and mixed for 8-10 min; then the chamber temperature is lowered to 130°C and 1H,1H,2H,2H-perfluorodecylthiol is added and mixed for 10-12 min; finally, the material is discharged after maintaining the rotation speed at 50 rpm at 130°C and being pumped to 5 kPa for 10 min for devolatilization.

[0018] Preferably, in step (3), the post-treatment is performed at 60℃ and 5kPa for 6-8 hours.

[0019] The beneficial effects of this invention are: The die-cut hot melt adhesive sheet and its preparation process provided by this invention achieve a leap in comprehensive performance under harsh electrolyte environments through unique molecular design and process steps.

[0020] First, this method significantly improves the long-term stability of adhesive products in carbonate electrolyte environments. By covalently grafting fluorinated side chains onto the elastomer backbone, a continuous, low-surface-energy solvent-resistant phase is constructed within the adhesive layer. This structure effectively inhibits the penetration and diffusion of electrolyte molecules into the adhesive layer, greatly reducing the degree of swelling after immersion, and maintaining the original cohesive strength and modulus of the adhesive. This fundamentally avoids adhesive strength degradation and decreased cold flow resistance caused by matrix plasticization.

[0021] Secondly, the solution ensures extremely high reliability and durability of the bonding interface between the metal and the insulating substrate. Utilizing simultaneously introduced epoxy reaction sites, strong metal complex groups are precisely anchored to the interface via stepwise click chemistry. This covalent anchoring method enables the bonding interface to resist wetting and erosion by the electrolyte, greatly enhancing the adhesion between the steel shell surface and the adhesive layer. Even after long-term immersion in electrolyte, the interface remains intact, effectively inhibiting edge lifting and microchannel formation, ensuring the long-lasting effectiveness of the insulation function.

[0022] Furthermore, this solution achieves a gradient and synergistic effect of internal material functions, resulting in superior overall performance without sacrificing processability. By controlling the order and temperature of fluorinated thiols addition, a differentiated distribution of long-chain and short-chain fluorinated components along the adhesive layer thickness direction is achieved. Short-chain components help fill the free volume of the bulk phase, improving density; while long-chain components are more easily enriched on the surface, forming a dense, low-surface-energy protective layer. This structural gradient gives the adhesive layer excellent barrier properties on the electrolyte-facing side, while maintaining good initial tack and creep resistance within the bulk phase. Simultaneously, the selected specific tackifying resin, in synergy with polyethylene wax powder, provides a suitable viscoelastic environment for the formation of the aforementioned functional structures, ensuring the process adaptability of the hot melt adhesive during coating, die-cutting, and hot-pressing applications.

[0023] Finally, the entire preparation process is highly efficient and environmentally friendly. All key modification steps are completed in the molten state, eliminating the need for additional solvents and avoiding the potential harm of solvent residues to the battery system. The process flow from elastomer grafting and functionalization modification to coating and molding is seamless, facilitating industrial-scale production and providing a reliable material solution for the manufacture of high-performance lithium batteries. Detailed Implementation

[0024] 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.

[0025] Raw materials, their sources, and key parameters: SEBS rubber block copolymer was selected from Kraton's KRATONG 1652 type (specific gravity 0.91, styrene / rubber block mass ratio 30 / 70, melt flow rate approximately 5g / 10min (230℃ / 5kg), Shore A hardness approximately 70); tackifying resin was selected from ExxonMobil's Escorez 5400 type (softening point approximately 103℃, glass transition temperature approximately 52℃, melt viscosity at 160℃ approximately 800mPa·s); polyethylene wax powder was selected from Honeywell's AC 617A type (Mettler dropping point approximately 101℃, viscosity at 140℃ approximately 180mPa·s, density 0.91); release material was selected from 3M's 3M Secondary Liner 5002 silicone oil release polyester film (thickness approximately 50μm, single-sided silicone oil release) and 3M Cotchpak. 9709 Fluorosilicone Release Polyester Film (approximately 75μm thick, single-sided fluorosilicone release).

[0026] Example 1: Step S1: Dry 2500g of SEBS rubber block copolymer in a vacuum oven at 80℃ and 10kPa for 12h, then cool to 25℃ and seal; dry 5g of antioxidant 1010 and 5g of antioxidant 168 in a drying oven at 60℃ for 2h for later use; degas 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate and 50g of glycidyl methacrylate under nitrogen protection by bubbling for 10min each and then seal at 0-5℃ in the dark for later use; refrigerate 8g of dicumyl peroxide at 2-8℃ in the dark for later use. Step S2: Add 2500g of the dried SEBS rubber block copolymer from Step S1, 5g of antioxidant 1010, and 5g of antioxidant 168 to a conventional internal mixer closed chamber. After nitrogen purging for 3 minutes, set the chamber temperature to 170℃ and the rotor speed to 60 rpm, and mix for 3 minutes to fully plasticize it. Then, while maintaining 170℃ and 60 rpm, add 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate and 50g of glycidyl methacrylate dropwise at a uniform rate over 2 minutes. Then add 8g of dicumyl peroxide and continue mixing for 8 minutes. In the last 2 minutes of the reaction, pump the chamber to 20kPa to remove unreacted monomers and low-molecular-weight byproducts before discharging. Step S3: While hot, roll all the materials obtained in step S2 into sheets with a thickness of about 3mm and cut them into small pieces of 20mm×20mm. Place them in a vacuum oven at 80℃ and 5kPa for 8 hours to devolve, then cool them to 25℃ and pulverize them to obtain fluorine / epoxy co-grafted elastomer granules. Step S4: Weigh 2000g of the fluorinated / epoxy co-grafted elastomer granules obtained in Step S3 as the main phase for rubber formation; separately weigh 1000g of SEBS rubber block copolymer, 1730g of tackifying resin, 200g of polyethylene wax powder, 10g of antioxidant 1010, 10g of antioxidant 168, 30g of 11-mercaptoundecylphosphonic acid, 10g of 1H,1H,2H,2H-perfluorooctyl mercaptan, and 10g of... 1H,1H,2H,2H-perfluorodecyl mercaptan; the solid raw materials (1000g SEBS rubber block copolymer, 1730g tackifying resin, 200g polyethylene wax powder, 10g antioxidant 1010, 10g antioxidant 168, 30g 11-mercaptoundecylphosphonic acid) were placed together in a vacuum oven at 60℃ and 10kPa for 6h, then sealed and transferred to a drying oven for later use. The liquid thiols (10g 1H,1H,2H,2H-perfluorooctyl mercaptan and 10g 1H,1H,2H,2H-perfluorodecyl mercaptan) were allowed to stand for degassing under nitrogen protection for 10min and then sealed for later use. Step S5: Add 2000g of fluorinated / epoxy co-grafted elastomer granules dried in step S4 and 1000g of SEBS rubber block copolymer to a mixer. After nitrogen purging for 3 minutes, set the chamber temperature to 150℃ and the rotor speed to 50rpm and mix for 4 minutes to completely melt the elastomer. Then, add 1730g of tackifying resin in three batches (with a 2-minute interval between each addition to prevent local overheating and uneven coating). After the system is homogeneous, add 200g of polyethylene wax powder and continue mixing for 3 minutes. Then, add 10g of antioxidant 1010 and 10g of antioxidant 168 and mix for 2 minutes to obtain a homogeneous hot melt adhesive matrix. Step S6: In step S5, while maintaining the hot melt adhesive matrix at 150℃ and 50 rpm, first add 0.5 g of 2-methylimidazole and 30 g of 11-mercaptoundecylphosphonic acid and mix for 12 min; then lower the chamber temperature to 145℃ and add 10 g of 1H,1H,2H,2H-perfluorooctyl mercaptan and mix for 8 min; then lower the chamber temperature to 130℃ and add 10 g of 1H,1H,2H,2H-perfluorodecyl mercaptan and mix for 10 min; finally, maintain the temperature at 130℃ and the rotation speed at 50 rpm and extract to 5 kPa for 10 min for devolatilization before discharging to obtain the modified hot melt adhesive; Step S7: Place the modified hot melt adhesive obtained in step S6 into the melting tank of a hot melt coating machine. Under nitrogen protection, set the temperature of the melting tank to 125℃ and maintain the temperature for 30 minutes to defoam. Take a silicone release polyester film and pass it through the coating station with the silicone release side facing up. At 125℃, use a slit die to evenly coat the hot melt adhesive onto the silicone release surface and control the wet film thickness to 100μm. After coating, first pass it through a 20℃ cooling roller for single-sided cooling so that the area of ​​the adhesive layer near the silicone release surface rapidly increases in viscosity within 5 seconds, thus limiting molecular diffusion. Then, take a fluorosilicone release polyester film and make its fluorosilicone release surface contact the free surface of the adhesive layer. Composite the film under the pressure of a 200kPa pressure roller to form a double release sandwich structure. Continue cooling until the adhesive layer temperature is below 40℃ before winding. Place the wound roll of material in a vacuum oven at 60℃ and 5kPa for 6 hours for post-treatment and then cool. Finally, use a flat die to cut the material as required to obtain a die-cut hot melt adhesive sheet for fixing the insulation of a steel-shell lithium battery.

[0027] Example 2: Compared with Example 1, the difference in this example is as follows: In step S1, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate was adjusted to 260g, 50g of glycidyl methacrylate was adjusted to 70g, and 8g of dicumyl peroxide was adjusted to 10g; in step S2, the cavity temperature was adjusted from 170℃ to 175℃, and 10g of dicumyl peroxide was added and mixing continued for 8 minutes; in step S4, 2000g of the fluorinated / epoxy co-grafted elastomer granules of the gelling main phase was adjusted to 2200g, and... The amount of SEBS rubber block copolymer (1000g) was adjusted to 800g, the amount of tackifying resin (1730g) was adjusted to 1600g, the amount of polyethylene wax powder (200g) was adjusted to 180g, the amount of 11-mercaptoundecylphosphonic acid (30g) was adjusted to 35g, the amount of 1H,1H,2H,2H-perfluorooctyl mercaptan (10g) was adjusted to 12g, and the amount of 1H,1H,2H,2H-perfluorodecyl mercaptan (10g) was adjusted to 12g; 35g of each of these ingredients was added in step S6. 11-Mercaptoundecylphosphonic acid, 12g 1H,1H,2H,2H-perfluorooctylthiol, and 12g 1H,1H,2H,2H-perfluorodecylthiol were mixed at the temperature and time corresponding to Example 1; in step S7, the wet film thickness was adjusted from 100μm to 110μm, the single-sided cooling time was adjusted from 5s to 6s, and the pressure of the pressure roller was adjusted from 200kPa to 220kPa; the remaining conditions were the same as in Example 1.

[0028] Example 3: Compared with Example 1, the difference in this example is as follows: In step S1, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate was adjusted to 150g, 50g of glycidyl methacrylate was adjusted to 30g, and 8g of dicumyl peroxide was adjusted to 6g; in step S2, the cavity temperature was adjusted from 170℃ to 165℃, and after adding 6g of dicumyl peroxide, mixing was continued for 10min; in step S4, 2000g of the fluorinated / epoxy co-grafted elastomer granules of the gelling main phase was adjusted to 1800g, and... The amount of SEBS rubber block copolymer (1000g) was adjusted to 1200g, the amount of tackifying resin (1730g) was adjusted to 1900g, the amount of polyethylene wax powder (200g) was adjusted to 150g, the amount of 11-mercaptoundecylphosphonic acid (30g) was adjusted to 25g, the amount of 1H,1H,2H,2H-perfluorooctyl mercaptan (10g) was adjusted to 8g, and the amount of 1H,1H,2H,2H-perfluorodecyl mercaptan (10g) was adjusted to 8g; 25g of each of these were added in step S6. 11-Mercaptoundecylphosphonic acid, 8g of 1H,1H,2H,2H-perfluorooctylthiol, and 8g of 1H,1H,2H,2H-perfluorodecylthiol were mixed at the temperature and time corresponding to Example 1. In step S7, the wet film thickness was adjusted from 100μm to 90μm, the single-sided cooling time was adjusted from 5s to 4s, and the pressure of the pressure roller was adjusted from 200kPa to 180kPa. The remaining conditions were the same as in Example 1.

[0029] Example 4: Compared with Example 1, the difference in this example is as follows: in step S1, 50g of glycidyl methacrylate was adjusted to 60g; in step S4, 1730g of tackifying resin was adjusted to 1700g, 200g of polyethylene wax powder was adjusted to 300g, and 10g of 1H,1H,2H,2H-perfluorodecyl mercaptan was adjusted to 15g; in step S6, 15g of 1H,1H,2H,2H-perfluorodecyl mercaptan was added at 130℃ and mixed for 12min; in step S7, the wet film thickness was adjusted from 100μm to 120μm, the single-sided cooling time was adjusted from 5s to 7s, the pressure of the pressure roller was adjusted from 200kPa to 250kPa, and the vacuum oven post-treatment time was adjusted from 6h to 8h; the remaining conditions are the same as in Example 1.

[0030] Example 5: Compared with Example 1, the difference in this example is as follows: in step S1, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate was adjusted to 230g, and 8g of dicumyl peroxide was adjusted to 9g; in step S4, 2000g of fluorinated / epoxy co-grafted elastomer granules in the main phase of the gelation was adjusted to 2100g, 1000g of SEBS rubber block copolymer was adjusted to 900g, and 30g of 11-mercaptoundecylphosphonic acid was adjusted to 45g; in step S6, 45g of 11-mercaptoundecylphosphonic acid was added at 150°C and mixed for 15min, and then 1H,1H,2H,2H-perfluorooctyl mercaptan and 1H,1H,2H,2H-perfluorodecyl mercaptan were added and mixed at the temperature corresponding to Example 1; the remaining conditions were the same as in Example 1.

[0031] Example 6: Compared with Example 1, the difference in this example is as follows: In step S1, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate was adjusted to 180g, and 50g of glycidyl methacrylate was adjusted to 80g; in step S4, 1730g of tackifying resin was adjusted to 1500g, 10g of 1H,1H,2H,2H-perfluorooctyl mercaptan was adjusted to 5g, and 10g of 1H,1H,2H,2H-perfluorodecyl mercaptan was adjusted to 20g; in step S6, 5g of 1H,1H,2H,2H-perfluorooctyl mercaptan was added at 145℃ and mixed for 10min, and 20g of [unspecified ingredient] was added at 130℃. 1H,1H,2H,2H-perfluorodecylthiol was mixed and kneaded for 12 min; in step S7, the wet film thickness was adjusted from 100 μm to 80 μm, the single-sided cooling time was adjusted from 5 s to 3 s, and the pressure of the pressure roller was adjusted from 200 kPa to 300 kPa; the remaining conditions were the same as in Example 1.

[0032] Comparative Example 1: The difference from Example 1 is that: in step S2, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate and 50g of glycidyl methacrylate are not added dropwise, and 8g of dicumyl peroxide is not added; in step S5, before adding 10g of antioxidant 1010 and 10g of antioxidant 168, 200g of 1H,1H,2H,2H-perfluorooctyl methacrylate and 50g of glycidyl methacrylate are added all at once and mixed for 10 minutes, so that the two monomers are physically blended into the hot melt adhesive matrix; the remaining conditions are the same as in Example 1.

[0033] Comparative Example 2: The difference from Example 1 is that 50g of glycidyl methacrylate is not added in steps S1 and S2; the other conditions are the same as in Example 1.

[0034] Comparative Example 3: The difference from Example 1 is that 30g of 11-mercaptoundecylphosphonic acid is not weighed in step S4, and 30g of 11-mercaptoundecylphosphonic acid is not added in step S6; the other conditions are the same as in Example 1.

[0035] Comparative Example 4: The difference from Example 1 is that in step S6, 10g of 1H,1H,2H,2H-perfluorooctyl mercaptan and 10g of 1H,1H,2H,2H-perfluorodecyl mercaptan are added simultaneously and mixed for 18 minutes at a chamber temperature of 145°C, without cooling to 130°C in sequence; the other conditions are the same as in Example 1.

[0036] Comparative Example 5: The difference from Example 1 is that, instead of adding 10g of 1H,1H,2H,2H-perfluorooctylthiol in step S6, 20g of 1H,1H,2H,2H-perfluorodecylthiol is added; the other conditions are the same as in Example 1.

[0037] Comparative Example 6: The difference from Example 1 is that, instead of adding 10g of 1H,1H,2H,2H-perfluorodecylthiol in step S6, 20g of 1H,1H,2H,2H-perfluorooctylthiol is added; the other conditions are the same as in Example 1.

[0038] Performance testing: Sample Preparation and Numbering: The die-cut hot melt adhesive sheets obtained in Examples 1-6 and Comparative Examples 1-6 were selected as test samples, respectively labeled as Samples 1-6 and Comparative Examples 1-6. After peeling off the silicone oil release polyester film and the fluorosilicone release polyester film, the side originally in contact with the silicone oil release polyester film was defined as the metal side, and the side originally in contact with the fluorosilicone release polyester film was defined as the outer side. For the adhesion test, a steel plate with a thickness of 0.20 mm and a polyimide film with a thickness of 25 μm were cut... All samples were cut to 200mm × 25mm. The hot melt adhesive sheet was placed between the steel plate and the polyimide film and hot-pressed for 10 seconds at 130℃ and 0.5MPa in a flatbed hot press. After that, the samples were removed and left to stand at 25℃ for 24 hours to obtain the steel plate / hot melt adhesive / polyimide composite sample. When used for intrinsic characterization and electrolyte immersion test, the hot melt adhesive sheet after removing the release film was cut into 30mm × 30mm samples and dried in a vacuum oven at 60℃ and 5kPa for 2 hours. After that, the samples were cooled and sealed for later use.

[0039] Electrolyte immersion swelling and extractability: The weighing and immersion evaluation method was performed according to GB / T 1690-2010. The test liquid was an electrolyte solution of 1 mol / L lithium hexafluorophosphate dissolved in a mixed solvent of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (volume ratio 1:1:1). The initial mass m0 (accuracy 0.1 mg) of the dried 30 mm × 30 mm hot melt adhesive sheet sample was weighed at 25 °C, placed in a sealed glass bottle, and electrolyte solution (the mass of electrolyte solution was 50 times the mass of the sample) was added. After immersion in a constant temperature oven at 60 °C for 168 h, the sample was removed and subjected to dust-free treatment. The paper was lightly pressed to remove residual liquid from the surface and the mass m1 after soaking was measured at 25℃. The mass change rate Δm = (m1 - m0) / m0 × 100% was calculated. Subsequently, the soaked sample was placed in a vacuum oven at 60℃ and 5kPa for 6 hours to dry, then cooled and the mass m2 after drying was measured. The extract mass fraction E = (m0 - m2) / m0 × 100% was calculated. The differences in low swelling and low migration ability between the example sample and the comparative sample in the electrolyte environment were compared by Δm and E. The test results are recorded in Table 1.

[0040] 180° peel strength and immersion retention rate: According to GB / T 2790-1995, the above-mentioned steel plate / hot melt adhesive / polyimide composite samples were tested on a universal testing machine using a 180° peel method. The peel width was 25 mm, the clamp spacing was 100 mm, the peel speed was 300 mm / min, and the test environment was 25℃. Five samples were tested in parallel for each sample. The average peel force in the stable peel stage was recorded and converted into peel strength P0 (N / 25 mm). Subsequently, the composite samples of the same batch were immersed in an electrolyte solution for electrolyte swelling and extractability testing. The immersion conditions were 60℃ for 168 h. After removal, the surface residual liquid was absorbed with lint-free paper and placed at 25℃ for 30 min. The immersion peel strength P1 (N / 25 mm) was then tested under the above conditions, and the peel retention rate R = P1 / P0 × 100% was calculated. The test results are recorded in Table 1.

[0041] Adhesion retention (static shear retention) and cold flow resistance: According to GB / T 4851-2014, a steel plate (thickness 0.20mm) was used as the substrate. Hot melt adhesive sheets of 25mm×25mm were cut and hot-pressed with the steel plate at 130℃ and 0.5MPa for 10s to form an overlapping bonding area. After being placed at room temperature for 24h, the samples were vertically suspended in a 70℃ constant temperature chamber, a constant load of 1000g was applied and the time was recorded as t (min) when the sample displacement exceeded 2mm or completely detached. Three samples were tested in parallel for each sample and the average value was taken. The test results are recorded in Table 1.

[0042] Initial tack (ring method): The test was conducted according to GB / T 31125-2014. The hot melt adhesive sheet after removing the release film was cut into strips with a width of 25 mm and a length of 175 mm and made into ring specimens. The test bench was made of standard stainless steel plate and wiped with anhydrous ethanol. It was placed at 25℃ for 30 min. The test machine brought the ring specimen into contact with the steel plate at a speed of 300 mm / min and immediately pulled it off at the same speed. The maximum pull-off force F (N) was recorded. Each sample was tested in parallel 5 times and the average value was taken. The test results are recorded in Table 1.

[0043] Breakdown strength: Breakdown strength was tested according to GB / T 1408.1-2016; 10 layers of hot melt adhesive sheets after removing the release film were stacked and hot-pressed for 30s at 130℃ and 0.5MPa in a flatbed hot press, followed by cold pressing for 60s at room temperature to obtain an insulating sheet with a thickness of 1.00mm. The sheet was then cut into 100mm×100mm samples and equilibrated for 24h at 25℃ and 50% relative humidity. The breakdown test was conducted using ball-to-ball electrodes with a voltage rise rate of 500V / s. The breakdown voltage was recorded and the breakdown strength Eb (kV / mm) was calculated based on the sample thickness. The test results are recorded in Table 1.

[0044] Table 1 Performance test results of the examples and comparative examples

[0045] Data Analysis: As can be seen from the data in Table 1, the die-cut hot melt adhesive sheet prepared by this invention exhibits low swelling, low extractables, high peel retention rate after immersion in electrolyte, stable tack, and high breakdown strength under electrolyte immersion conditions. In particular, with the introduction of perfluorinated side chains and the increase in the degree of fixation, the absorption of electrolyte and the release of migratable components by the hot melt adhesive sheet are simultaneously reduced, making the bulk phase of the adhesive layer less susceptible to plasticization by the electrolyte, thereby inhibiting interfacial bubbling and cohesive softening. Simultaneously, the anchoring groups on the metal side enhance chemical adsorption and energy dissipation at the steel plate interface, making it easier to form a stable adhesive failure path rather than instantaneous interfacial debonding during peeling. Furthermore, the stepwise introduction of long-chain and short-chain perfluorothiols allows for the simultaneous achievement of low surface energy shielding and dense bulk filling within the same system: the surface layer is more conducive to preventing electrolyte penetration, while the bulk phase is more conducive to reducing free volume and suppressing cold flow. Therefore, initial tack, peel strength retention rate, and high-temperature shear retention can be synergistically improved, demonstrating comprehensive performance advantages.

[0046] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when only the perfluorinated monomer and undecylphosphonic acid mercapto group are added through physical blending, without graft modification of the hydrogenated styrene-ethylene / butene-styrene rubber block copolymer and subsequent click fixation, the swelling and extract after electrolyte immersion are significantly increased, leading to a significant decrease in peel strength and retention rate after immersion. The main reason is that the small molecules in the physical blend are more likely to migrate and dissolve in the electrolyte, forming microscopic porosity and compatibility fluctuations within the adhesive layer. The electrolyte is more likely to diffuse along the interface and bulk phase, resulting in weakened cohesion and interfacial stress concentration. Therefore, graft modification and click fixation not only provide low surface energy but also improve electrolyte stability by inhibiting migration and densification, demonstrating a clear structure-performance synergy.

[0047] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, when the glycidyl methacrylate grafting step is omitted, resulting in the lack of epoxy anchoring sites that can react with thiols, even with the addition of perfluorothiols and undecylphosphonic acid thiol groups, it is more difficult to form a stable covalently fixed structure. This is manifested in higher extract content and decreased peel retention rate after soaking. The main reason is that the lack of epoxy anchoring sites weakens the continuous structure of bulk fixation-interfacial orientation: thiols and phosphonates are more likely to be distributed in a free state and redistributed during hot pressing or soaking, resulting in discontinuous surface barrier and insufficient bulk cold flow resistance. This result indicates that the introduction of reaction sites through grafting does not simply increase polarity, but provides a structural basis for subsequent stepwise click fixation, thereby achieving a simultaneous improvement in electrolyte resistance and adhesive strength.

[0048] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, when the undecylphosphonic acid mercapto group is not added and the fluorinated side chain reacts solely with the thiol network, both the initial peel strength and the peel strength after immersion are significantly reduced, and the retention rate is at a low level. The main reason for this is that the steel plate surface requires stronger anchoring groups in an electrolyte environment to resist interfacial solubilization and capillary penetration; without phosphonic acid anchoring, the interface mainly relies on van der Waals interactions and mechanical interlocking, making it more susceptible to interfacial debonding under swelling and stress. This result indicates that the barrier and anti-wetting properties provided by the fluorinated side chain and the interfacial anti-peel properties provided by the phosphonic acid anchoring are complementary in the same hot melt adhesive sheet.

[0049] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, when 1H,1H,2H,2H-perfluorodecylthiol and 1H,1H,2H,2H-perfluorooctylthiol were added simultaneously instead of in stages, although the initial peel strength did not change significantly, the retention rate and extract after electrolyte immersion showed an unfavorable trend. The main reason is that simultaneous addition makes it easier to form randomly distributed reaction products and localized enrichment zones, making it difficult to form a gradient structure between the surface and bulk phases, with an outer layer of low surface energy shielding and an inner layer of dense filling to suppress cold flow. Under the action of the electrolyte, the non-uniform structure is more prone to generating microscopic channels and interface defects, leading to accelerated penetration and adhesion attenuation. Therefore, in-stage addition is not a matter of process simplification, but rather an important means of achieving multi-scale structural control, bringing synergistic gains in surface barrier and bulk phase stability.

[0050] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, when only 1H,1H,2H,2H-perfluorodecylthiol or only 1H,1H,2H,2H-perfluorooctylthiol are used, it is difficult to simultaneously achieve a better balance in terms of swelling, extract, retention rate after soaking, and tackiness. The main reason is that long-chain perfluorothiols tend to reduce surface energy and form a surface barrier, but their contribution to the free volume of the bulk phase and defect filling is limited; short-chain perfluorothiols are more likely to enter the bulk phase and improve compactness and initial tack, but their low surface energy barrier is insufficient, and the electrolyte may still penetrate along the interface. The examples employ a stepwise combination of long-chain and short-chain perfluorothiols, enabling surface barrier and bulk phase stability to be synergistically achieved within the same system, exhibiting a typical synergistic effect of 1+1 greater than 2.

[0051] 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 die-cut hot melt adhesive sheet for insulating and fixing steel-cased lithium batteries, comprising a hot melt adhesive layer, characterized in that, The hot melt adhesive layer, by weight, is prepared from the following raw materials: 1800-2200 parts of fluorinated / epoxy co-grafted elastomer granules, 800-1200 parts of SEBS rubber block copolymer, 1500-1900 parts of tackifying resin, 150-300 parts of polyethylene wax powder, 10 parts of antioxidant 1010, 10 parts of antioxidant 168, 0.2-0.7 parts of 2-methylimidazole, 25-45 parts of 11-mercaptoundecylphosphonic acid, 5-12 parts of 1H,1H,2H,2H-perfluorooctyl mercaptan and 8-20 parts of 1H,1H,2H,2H-perfluorodecyl mercaptan; The fluorinated / epoxy co-grafted elastomer granules are obtained by melt grafting the following components: 2500 parts of SEBS rubber block copolymer as the matrix, 150-260 parts of 1H,1H,2H,2H-perfluorooctyl methacrylate, 30-80 parts of glycidyl methacrylate and 6-10 parts of dicumyl peroxide are added.

2. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 1, characterized in that, The die-cut hot melt adhesive sheet also includes a first release film and a second release film respectively applied to both sides of the hot melt adhesive layer.

3. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 2, characterized in that, The first release film is a silicone oil release polyester film, and the second release film is a fluorosilicone release polyester film.

4. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 1, characterized in that, The styrene / rubber block mass ratio of the SEBS rubber block copolymer is 30 / 70, and the melt flow rate at 230℃ / 5kg is 4-6g / 10min.

5. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 1, characterized in that, The tackifying resin is Escorez 5400 tackifying resin.

6. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 1, characterized in that, The polyethylene wax powder is AC 617A polyethylene wax powder.

7. The die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 1, characterized in that, The hot melt adhesive layer has a metal side and an outer side, wherein the side in contact with the silicone oil release polyester film is the metal side, and the side in contact with the fluorosilicone release polyester film is the outer side.

8. A process for preparing a die-cut hot melt adhesive sheet for insulating and fixing a steel-cased lithium battery according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Preparation of fluorine / epoxy co-grafted elastomer granules: Under nitrogen protection, SEBS rubber block copolymer was melt-plasticized, and 1H,1H,2H,2H-perfluorooctyl methacrylate, glycidyl methacrylate and dicumyl peroxide were added to carry out melt grafting reaction. The material was discharged, devolatilized, crushed and granulated to obtain fluorine / epoxy co-grafted elastomer granules. (2) Preparation of modified hot melt adhesive: The fluorinated / epoxy co-grafted elastomer granules obtained in step (1) are melt-blended with SEBS rubber block copolymer, tackifying resin, polyethylene wax powder and antioxidant to obtain hot melt adhesive matrix. Then, 2-methylimidazole, 11-mercaptoundecylphosphonic acid, 1H,1H,2H,2H-perfluorooctyl mercaptan and 1H,1H,2H,2H-perfluorodecyl mercaptan are added in sequence. After volatilization, the material is discharged to obtain modified hot melt adhesive. (3) Film formation and die cutting: The modified hot melt adhesive obtained in step (2) is melt-coated onto the release surface of the silicone oil release polyester film and the coating thickness is controlled. After cooling on one side, it is combined with the fluorosilicone release polyester film to form a double release sandwich structure. After post-processing, the die-cut hot melt adhesive sheet is obtained.

9. The preparation process of the die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 8, characterized in that, In step (2), 11-mercaptoundecylphosphonic acid is added to the hot melt adhesive matrix at 150°C and 50 rpm and mixed for 12-15 min; then the chamber temperature is lowered to 145°C and 1H,1H,2H,2H-perfluorooctylthiol is added and mixed for 8-10 min; then the chamber temperature is lowered to 130°C and 1H,1H,2H,2H-perfluorodecylthiol is added and mixed for 10-12 min; finally, the material is discharged after maintaining the rotation speed at 50 rpm at 130°C and being pumped to 5 kPa for 10 min for devolatilization.

10. The preparation process of the die-cut hot melt adhesive sheet for insulating and fixing steel-shell lithium batteries according to claim 8, characterized in that, In step (3), the post-treatment is performed at 60℃ and 5kPa for 6-8 hours.

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