Flame-retardant hybrid structure adhesive for power battery and preparation method thereof

Abstract

The application relates to the field of power battery structure bonding technology and discloses a flame-retardant hybrid structural adhesive for power batteries and a preparation method thereof, the preparation method comprising the following steps: performing partial carbonization treatment on chitosan under an inert atmosphere to obtain carbonized chitosan; performing cation exchange reaction on a phosphate ester cation modifier and sodium type montmorillonite to obtain phosphate esterized montmorillonite; reacting phosphate ester modified polyoxypropylene glycol and 4,4'-diphenyl methane diisocyanate to obtain NCO terminated polyurethane prepolymer containing phosphate ester groups; preparing structural adhesive by dispensing the components; and applying the structural adhesive to the surface of a base material and wet curing and forming. The structural adhesive has the functions of halogen-free flame retardation through gas phase-coagulation phase double paths, continuously effective flame retardation barrier performance in the whole service life cycle and in-situ crack self-sealing capacity of the thermal runaway carbon layer, does not need to be heated and cured, and solves the problems of halogen-containing flame retardation toxicity, thermal stress damage, poor curing uniformity and decay of the flame retardation barrier with service time.

Description

Technical Field

[0001] This invention relates to the field of power battery structural bonding technology, and more specifically, to a flame-retardant hybrid structural adhesive for power batteries and its preparation method. Background Technology

[0002] In power battery modules, structural adhesives are used to bond and fix the individual cells together and between the cells and the casing. As a key structural material of the battery module, structural adhesives not only need to provide sufficient adhesive strength to maintain the integrity of the module structure, but also need to provide effective flame retardant protection and prevent the spread of flames in the event of thermal runaway. With the continuous increase in the energy density of power batteries, the risk of thermal runaway is increasing, and the requirements for the flame retardant performance of structural adhesives are becoming increasingly stringent. Existing commercial structural adhesives mainly include two categories: halogen-containing flame retardant structural adhesives and halogen-free flame retardant structural adhesives. Halogen-containing flame retardant structural adhesives rely on halogenated flame retardants such as bromine and chlorine, which have high flame retardant efficiency, but release toxic gases such as hydrogen halides during thermal runaway combustion, exacerbating the toxicity of fires, and do not comply with EU RoHS and REACH regulations. Halogen-free flame retardant structural adhesives use phosphorus-based, nitrogen-based, or inorganic mineral flame retardants, which meet regulatory requirements, but still have shortcomings in terms of the completeness of the flame retardant mechanism and service stability. In addition, high-performance structural adhesive systems (such as two-component epoxy systems) need to be heated and cured at high temperatures, which can generate significant thermal stress in the assembly of large battery modules, accelerate electrode material fatigue and affect the integrity of the separator.

[0003] Existing halogen-free flame-retardant structural adhesives commonly use inorganic mineral flame-retardant fillers with hydrophilic polar groups on their surfaces. Upon prolonged contact with organic electrolyte solvents, these fillers undergo polar interactions or swelling, leading to a gradual decrease in the interfacial bonding between the filler and the polymer matrix. This results in irreversible degradation of the flame-retardant barrier layer over service life, causing a continuous reduction in actual flame-retardant performance and posing a systemic safety hazard. Furthermore, cured adhesive layers containing multiple inorganic-organic composite flame-retardant components experience gradual accumulation of microcracks and interfacial debonding damage at the interfaces between filler components under charge-discharge thermal cycling stress. During thermal runaway, the reliability of the char layer protection decreases with service life. Summary of the Invention

[0004] This invention provides a flame-retardant hybrid structural adhesive for power batteries and its preparation method, solving the technical problems of existing structural adhesives in related technologies, such as halogenated flame retardant toxicity, thermal stress damage during heating and curing, degradation of the flame retardant barrier over service time, and decreased reliability of thermal runaway carbon layer protection.

[0005] This invention provides a flame-retardant hybrid structural adhesive for power batteries, comprising an isocyanate-terminated polyurethane prepolymer containing phosphate ester groups, phosphate-esterified montmorillonite, carbonized chitosan, aluminum hypophosphite, borosilicate glass powder, and dicalcium phosphate dihydrate; based on 100 parts by weight of the prepolymer, the composition includes 3-8 parts by weight of phosphate-esterified montmorillonite, 5-15 parts by weight of carbonized chitosan, 5-15 parts by weight of aluminum hypophosphite, 5-10 parts by weight of dicalcium phosphate dihydrate, and 5-15 parts by weight of borosilicate glass powder.

[0006] The prepolymer is prepared by reacting phosphate-modified polypropylene glycol with 4,4'-diphenylmethane diisocyanate, with free isocyanate groups retained at the ends, and phosphate groups covalently embedded in the main chain of the prepolymer.

[0007] The residual amino groups of the carbonized chitosan react with the free isocyanate groups of the prepolymer to form urea bonds, which are then covalently embedded into the polyurethane network.

[0008] The borosilicate glass powder has a softening point of 700–750°C and is chemically inert to organic electrolytes.

[0009] Preferably, the phosphate-modified polypropylene glycol has a hydroxyl value of 40–75 mgKOH / g and a phosphorus content of 3–5 wt%; the 4,4'-diphenylmethane diisocyanate has an NCO mass fraction of not less than 25%; the molar ratio of NCO groups to OH groups is 2.0–2.5; and the resulting prepolymer has an NCO mass fraction of 2–6%.

[0010] Preferably, the carbonized chitosan is prepared by heating chitosan powder with a degree of deacetylation of not less than 85% and a particle size of not more than 200 μm at 250–400 °C for 1–3 hours under nitrogen protection; the carbonized chitosan has a residual amino content of not less than 0.5 mmol / g and a particle size of not more than 100 μm.

[0011] Preferably, the phosphated montmorillonite is prepared by a cation exchange reaction between sodium montmorillonite with a cation exchange capacity of 80-120 meq / 100g and a phosphate ester cationic modifier; the phosphate ester cationic modifier contains a quaternary ammonium cation and a phosphate ester group in the same molecule, wherein at least one N-alkyl group of the quaternary ammonium cation is C 14 ~C 16 Long-chain alkyl groups with a phosphorus content of not less than 4 wt%; the XRD d(001) interlayer spacing of the obtained phosphorylated montmorillonite is not less than 1.8 nm and the particle size is not greater than 75 μm.

[0012] Preferably, the borosilicate glass powder has the following mass composition: 60-70 wt% SiO2, 10-20 wt% B2O3, 5-10 wt% Na2O, with the balance being Al2O3, and a particle size not greater than 50 μm.

[0013] Preferably, the aluminum hypophosphite has a particle size of no more than 50 μm and a purity of no less than 98 wt%; the calcium hydrogen phosphate dihydrate has a particle size of no more than 50 μm and a purity of no less than 98 wt%.

[0014] The present invention also provides a method for preparing the above-mentioned flame-retardant hybrid structural adhesive, comprising the following steps:

[0015] Step 1: Partially carbonize the chitosan powder at 250-400℃ for 1-3 hours under an inert atmosphere, then grind and sieve it after cooling to obtain carbonized chitosan.

[0016] Step 2: The phosphate ester cationic modifier is subjected to a cation exchange reaction with the sodium montmorillonite suspension. After washing, drying, grinding and sieving, phosphate esterified montmorillonite is obtained.

[0017] Step 3: Under dry nitrogen protection, phosphate-modified polypropylene glycol and 4,4'-diphenylmethane diisocyanate are stirred and reacted at 60-80°C for 2-4 hours, then cooled and degassed under vacuum to obtain NCO-terminated polyurethane prepolymer containing phosphate groups.

[0018] Step 4: Under the protection of dry nitrogen, the carbonized chitosan and phosphated montmorillonite obtained in Step 1 and Step 2, as well as aluminum hypophosphite, dicalcium phosphate dihydrate and borosilicate glass powder, are pre-dried separately and then added to the prepolymer obtained in Step 3 in proportion. The mixture is stirred and dispersed evenly and then sealed and stored at a temperature below 25°C and a relative humidity of no more than 30%.

[0019] Step 5: Apply the structural adhesive obtained in Step 4 to the surface of the substrate to be bonded, with a thickness of 0.3 to 1.0 mm on one side, and allow it to cure in an atmosphere with a temperature of 15 to 35°C and a relative humidity of 40 to 80%.

[0020] The beneficial effects of this invention are as follows: the system contains no halogen flame retardants and does not produce toxic hydrogen halide gases during thermal runaway combustion, meeting the requirements of RoHS, REACH and other regulations; the wet curing method completes cross-linking and curing at room temperature and pressure, eliminating the need for heating equipment and removing the thermal stress introduced into the battery module by high-temperature curing; the phosphate-modified montmorillonite sheets form tortuous diffusion channels in the adhesive layer, reducing the difference in the rate of moisture transport from the outside to the inside during curing, making the cross-linking density distribution of the adhesive layer more uniform; the phosphate prepolymer and carbonized chitosan form an NP synergistic condensed phase for carbonization, and aluminum hypophosphite provides gas-phase free radical quenching, achieving efficient halogen-free flame retardancy through a dual-pathway gas-phase-condensed phase; the borosilicate glass powder maintains the flame-retardant barrier function of the adhesive layer throughout its entire service life with a chemically inert glass network structure, and softens and fills the cracks in the carbon layer under high-temperature conditions, synergistically achieving in-situ self-sealing of cracks in the carbon layer with calcium phosphate dihydrate, ensuring that the reliability of thermal runaway protection does not decrease with service time. Attached Figure Description

[0021] Figure 1 This is a bar chart comparing the limiting oxygen index and UL-94 flame retardant rating of each sample in this invention.

[0022] Figure 2 This is a line graph showing the changes in LOI of each sample after immersion and aging in the organic electrolyte of this invention;

[0023] Figure 3 This is a bar chart comparing the residual weight rate of carbon layers in various samples before and after thermal cycling in this invention.

[0024] Figure 4 This is a scanning electron microscope (SEM) image of the carbon layer cross-section of the S-Full sample of this invention after carbonization (800℃) through 500 thermal cycles;

[0025] Figure 5 This is a scanning electron microscope (SEM) image of the carbon layer cross-section of the S-NoGlass sample of this invention after carbonization (800℃) following 500 thermal cycles.

[0026] Figure 6 This is a bar chart comparing the swelling rates of each layer along the thickness direction of the adhesive layer in this invention. Detailed Implementation

[0027] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0028] Example 1

[0029] This embodiment discloses a method for preparing a flame-retardant hybrid structural adhesive for power batteries, the method comprising the following steps:

[0030] Step 1 (Preparation of carbonized chitosan): Chitosan powder with a degree of deacetylation of 88% and a particle size of no more than 150 μm was placed in a tube furnace and protected with high-purity nitrogen gas (150 mL / min) of 99.9% purity. The temperature was increased to 320℃ at a heating rate of 3℃ / min and held at that temperature for 2 hours for partial pyrolysis carbonization. The outlet was connected to a washing device with an 8% NaOH aqueous solution. After cooling to room temperature in the furnace, the powder was ground and passed through a 100 μm sieve to obtain carbonized chitosan powder (the residual amino content was measured to be 0.72 mmol / g). The powder was then sealed and stored under dry conditions for later use.

[0031] Step 2 (Preparation of Phosphate-modified Montmorillonite P-MMT): Sodium-type montmorillonite with a cation exchange capacity of 100 meq / 100g and a sheet diameter-to-thickness ratio ≥50 was added to deionized water to prepare a 3% (w / w) suspension. The suspension was stirred at 70℃ and 250 r / min for 1.5 hours to induce swelling. Separately, a C-containing... 16 A phosphate ester cation modifier consisting of hexadecyl quaternary ammonium cations and phosphate ester groups (-OPO(OH)2 linked to quaternary ammonium nitrogen via a C3 alkylene chain, with a phosphorus content of 4.8 wt%) was dissolved in hot deionized water at 70°C at 1.1 times the molar amount of montmorillonite CEC. The solution was slowly added dropwise to the montmorillonite suspension over 35 minutes with stirring at 250 r / min. The mixture was then kept at 70°C with stirring for 3 hours. The mixture was filtered while hot and repeatedly washed with hot deionized water at 70°C until the difference in conductivity between the washing solution and the deionized water was no greater than 5 μS / cm. The solution was dried to constant weight at 90°C, ground, and passed through a 75 μm sieve to obtain P-MMT (XRD analysis showed a d(001) interlayer spacing of 1.94 nm), which was then sealed and stored for later use.

[0032] Step 3 (Preparation of NCO-terminated polyurethane prepolymer containing phosphate ester groups): Under dry nitrogen protection, phosphate-modified polypropylene glycol with a hydroxyl value of 57 mgKOH / g and a phosphorus content of 4wt% was added to a dry reaction vessel according to the measured amount, and the temperature was raised to 70℃ with stirring at 300 r / min; 4,4'-diphenylmethane diisocyanate with an NCO mass fraction of 25% was pre-melted into a liquid state in a 50℃ water bath and added at an NCO / OH molar ratio of 2.0, along with 0.03wt% dibutyltin dilaurate (DBTDL) of the total reactants. The reaction was stirred at 300 r / min for 3 hours at 70℃; the NCO mass fraction was measured every 30 minutes by di-n-butylamine titration (GB / T 12009.4), and the NCO mass fraction stabilized at 3.2% when the reaction was completed (the difference between two consecutive measurements was 0.06%); the temperature was lowered to room temperature, and the reaction was carried out at a gauge pressure of −0.09. Vacuum degassing at MPa for 30 minutes (with the outlet connected to an alkali absorption device) yields NCO-terminated polyurethane prepolymer, which is then sealed and stored for later use.

[0033] Step 4 (Preparation of structural adhesive): The P-MMT obtained in Step 2, the carbonized chitosan powder obtained in Step 1, aluminum hypophosphite (AHP, particle size 30μm, purity 99%), and borosilicate glass powder (mass composition: SiO2 65wt%, B2O3 15wt%, Na2O 8wt%, balance Al2O3, particle size 20μm, softening point 725℃) were dried in a vacuum oven at 90℃ (gauge pressure -0.09 MPa) for 3 hours; the dicalcium phosphate dihydrate (DCPD, particle size 30μm, purity 99%) was dried in a vacuum oven at 70℃ for 3 hours; after drying, all fillers were cooled to room temperature and immediately sealed for use. Under dry nitrogen protection, based on 100 parts by weight of the prepolymer obtained in step three, add the following fillers in batches in the following order: 5 parts by weight of P-MMT → 10 parts by weight of carbonized chitosan → 10 parts by weight of AHP → 7 parts by weight of DCPD → 10 parts by weight of borosilicate glass powder. After each filler is added, stir at 400 r / min for 12 minutes. After all fillers are added, continue stirring at 400 r / min for 45 minutes. The mixture is considered qualified when there are no visible powder agglomerates and the system has uniform flowability. Store in a sealed container at a temperature below 25°C and a relative humidity of no more than 30% (shelf life 6 months).

[0034] Step 5 (Adhesive Application and Curing): Apply the structural adhesive obtained in Step 4 to the side of the power battery cell using a scraper, with a thickness of 0.5 mm on each side. After the cell is aligned and bonded, allow it to cure under atmospheric conditions of 25°C and 60% relative humidity. The initial curing time (when the surface is no longer sticky) is 2 hours, and the complete curing time (reaching a final bond strength of over 90%) is 6 days, resulting in a polyurethane-urea elastic structural adhesive layer. No heating process is required.

[0035] Example 2

[0036] This embodiment discloses a method for preparing a flame-retardant hybrid structural adhesive for power batteries, the method comprising the following steps:

[0037] Step 1 (Preparation of carbonized chitosan): Chitosan powder with a degree of deacetylation of 85% and a particle size of no more than 150 μm was placed in a tube furnace and protected with high-purity nitrogen gas (flow rate 120 mL / min) of 99.9%. The temperature was increased to 270℃ at a heating rate of 2℃ / min and held for 1.5 hours for partial pyrolysis carbonization. A washing device with a mass fraction of 5% NaOH aqueous solution was connected to the gas outlet. After cooling to room temperature in the furnace, the powder was ground and passed through a 100 μm sieve to obtain carbonized chitosan powder (the residual amino content was measured to be 0.89 mmol / g). The powder was then sealed and stored under dry conditions for later use.

[0038] Step 2 (Preparation of Phosphate-modified Montmorillonite P-MMT): Sodium-type montmorillonite with a cation exchange capacity of 85 meq / 100g and a sheet diameter-to-thickness ratio ≥50 was added to deionized water to prepare a 2% (w / w) suspension. The suspension was stirred at 220 r / min for 1 hour at 65℃ to induce swelling. Separately, a C-containing... 14 A phosphate ester cation modifier, consisting of tetradecyl quaternary ammonium cations and phosphate ester groups (-OPO(OH)2 linked to quaternary ammonium nitrogen via a C2 alkylene chain, with a phosphorus content of 5.2 wt%), was dissolved in a 15% ethanol aqueous solution (60°C) at a molar volume of 1.0 times that of montmorillonite CEC. The solution was slowly added dropwise to the montmorillonite suspension over 40 minutes with stirring at 220 r / min. The mixture was then kept at 65°C with stirring for 2 hours. The mixture was filtered while hot and repeatedly washed with hot deionized water at 65°C until the difference in conductivity between the washing solution and the deionized water was no greater than 5 μS / cm. The mixture was dried to constant weight at 80°C, ground, and passed through a 75 μm sieve to obtain P-MMT (XRD analysis showed a d(001) interlayer spacing of 1.82 nm), which was then sealed and stored for later use.

[0039] Step 3 (Preparation of NCO-terminated polyurethane prepolymer containing phosphate groups): Under dry nitrogen protection, phosphate-modified polypropylene glycol with a hydroxyl value of 45 mgKOH / g and a phosphorus content of 3wt% was added to a dry reactor according to the metered amount, and the temperature was raised to 65℃ with stirring at 220 r / min; 4,4'-diphenylmethane diisocyanate with an NCO mass fraction of 27% was pre-melted into a liquid state in a water bath at 52℃ and added at an NCO / OH molar ratio of 2.0 (without adding a catalyst), and the reaction was stirred at 220 r / min at 65℃ for 4 hours; the NCO mass fraction was detected every 30 minutes by di-n-butylamine titration (GB / T 12009.4), and the NCO mass fraction stabilized at 2.7% when the reaction was completed (the difference between two consecutive measurements was 0.08%); the temperature was lowered to room temperature, and vacuum degassing was performed at a gauge pressure of -0.09 MPa for 30 minutes to obtain the NCO-terminated polyurethane prepolymer, which was sealed and stored for later use.

[0040] Step 4 (Preparation of structural adhesive): The P-MMT obtained in Step 2, the carbonized chitosan powder obtained in Step 1, aluminum hypophosphite (AHP, particle size 40μm, purity 98%), and borosilicate glass powder (mass composition: SiO2 60wt%, B2O3 12wt%, Na2O 6wt%, balance Al2O3, particle size 40μm, softening point 740℃) were dried in a vacuum oven at 85℃ (gauge pressure -0.08 MPa) for 4 hours; DCPD (particle size 45μm, purity 98%) was dried in a vacuum oven at 65℃ for 4 hours; after drying, all fillers were cooled to room temperature and immediately sealed for use. Under dry nitrogen protection, based on 100 parts by weight of the prepolymer obtained in step three, add the following fillers in batches in the following order: 3 parts by weight of P-MMT → 6 parts by weight of carbonized chitosan → 6 parts by weight of AHP → 6 parts by weight of DCPD → 6 parts by weight of borosilicate glass powder. After each filler is added, stir at 300 r / min for 15 minutes. After all fillers are added, continue stirring at 300 r / min for 60 minutes. The mixture is considered qualified when there are no visible powder agglomerates and the system has uniform flowability. Store in a sealed container at a temperature below 22°C and a relative humidity of no more than 25% (shelf life 6 months).

[0041] Step 5 (Adhesive Application and Curing): Apply the structural adhesive obtained in Step 4 to the inner surface of the power battery casing by dispensing, with a single-sided adhesive thickness of 0.4 mm. After the battery cells are aligned and bonded, allow it to cure statically under atmospheric conditions of 20°C and 50% relative humidity. The initial curing time (the surface is no longer sticky to the touch) is 3.5 hours, and the complete curing time is 7 days, resulting in a polyurethane-urea elastic structural adhesive layer without the need for a heating process.

[0042] Example 3

[0043] This embodiment discloses a method for preparing a flame-retardant hybrid structural adhesive for power batteries, the method comprising the following steps:

[0044] Step 1 (Preparation of carbonized chitosan): Chitosan powder with a degree of deacetylation of 92% and a particle size of no more than 200 μm was placed in a tube furnace and protected with high-purity nitrogen gas (180 mL / min) of 99.9% purity. The temperature was increased to 380℃ at a heating rate of 5℃ / min and held for 3 hours for partial pyrolysis carbonization. A washing device with a mass fraction of 10% NaOH aqueous solution was connected to the gas outlet and a forced ventilation exhaust system was simultaneously turned on. After cooling to room temperature in the furnace, the powder was ground and passed through a 100 μm sieve to obtain carbonized chitosan powder (the residual amino content was tested to be 0.53 mmol / g). The powder was then sealed and stored under dry conditions for later use.

[0045] Step 2 (Preparation of Phosphate-modified Montmorillonite P-MMT): Sodium-type montmorillonite with a cation exchange capacity of 115 meq / 100g and a sheet diameter-to-thickness ratio ≥50 was added to deionized water to prepare a 5% (w / w) suspension. The suspension was stirred at 300 r / min at 80℃ for 2 hours to allow it to swell. Separately, a C-containing... 16 A phosphate ester cation modifier consisting of hexadecyl quaternary ammonium cations and phosphate ester groups (-OPO(OH)2 linked to quaternary ammonium nitrogen via a C4 alkylene chain, with a phosphorus content of 4.1 wt%) was dissolved in hot deionized water at 80°C at 1.2 times the molar amount of montmorillonite CEC. The solution was slowly added dropwise to the montmorillonite suspension over 30 minutes with stirring at 300 r / min. The mixture was then kept at 80°C with stirring for 4 hours. The mixture was filtered while hot and repeatedly washed with hot deionized water at 75°C until the difference in conductivity between the washing solution and the deionized water was no greater than 5 μS / cm. The solution was dried to constant weight at 100°C, ground, and passed through a 75 μm sieve to obtain P-MMT (XRD analysis showed a d(001) interlayer spacing of 2.05 nm), which was then sealed and stored for later use.

[0046] Step 3 (Preparation of NCO-terminated polyurethane prepolymer containing phosphate ester groups): Under dry nitrogen protection, phosphate-modified polypropylene glycol with a hydroxyl value of 70 mgKOH / g and a phosphorus content of 5wt% was added to a dry reaction vessel according to the measured amount. The mixture was heated to 80℃ with stirring at 400 r / min. 4,4'-diphenylmethane diisocyanate with an NCO mass fraction of 25% was pre-melted into a liquid state in a 55℃ water bath and added at an NCO / OH molar ratio of 2.5. Simultaneously, dibutyltin dilaurate with a mass fraction of 0.05wt% of the total reactants was added. The mixture was stirred at 400 r / min for 2 hours at 80℃. The NCO mass fraction was measured every 30 minutes using the di-n-butylamine titration method (GB / T 12009.4). When the reaction was complete, the NCO mass fraction stabilized at 5.1% (the difference between two consecutive measurements was 0.07%). The mixture was then cooled to room temperature and subjected to a gauge pressure of -0.09... Vacuum degassing at MPa for 30 minutes (with the outlet connected to an alkali absorption device) yields NCO-terminated polyurethane prepolymer, which is then sealed and stored for later use.

[0047] Step 4 (Preparation of structural adhesive): The P-MMT obtained in Step 2, the carbonized chitosan powder obtained in Step 1, aluminum hypophosphite (AHP, particle size 25μm, purity 99%), and borosilicate glass powder (mass composition: SiO2 70wt%, B2O3 18wt%, Na2O 9wt%, balance Al2O3, particle size 30μm, softening point 710℃) were dried in a vacuum oven at 100℃ (gauge pressure -0.09 MPa) for 2 hours; DCPD (particle size 20μm, purity 98.5%) was dried in a vacuum oven at 75℃ for 2 hours; after drying, all fillers were cooled to room temperature and immediately sealed for use. Under dry nitrogen protection, based on 100 parts by weight of the prepolymer obtained in step three, add the following fillers in batches in the following order: 8 parts by weight of P-MMT → 14 parts by weight of carbonized chitosan → 13 parts by weight of AHP → 9 parts by weight of DCPD → 13 parts by weight of borosilicate glass powder. After each filler is added, stir at 500 r / min for 10 minutes. After all fillers are added, continue stirring at 500 r / min for 30 minutes. The mixture is considered qualified when there are no visible powder agglomerates and the system has uniform flowability. Store in a sealed container at a temperature below 20°C and a relative humidity of no more than 30% (shelf life 6 months).

[0048] Step 5 (Adhesive Application and Curing): The structural adhesive obtained in Step 4 is applied by extrusion between the bottom surface of the power battery cell and the bottom plate of the casing. The thickness of the adhesive on one side is 0.8 mm. After the alignment and bonding are completed, the adhesive is left to cure under atmospheric conditions of 30°C and 70% relative humidity. The initial curing time (the surface is no longer sticky to the touch) is 1.5 hours, and the complete curing time is 5 days. A polyurethane-urea elastic structural adhesive layer is obtained without the need for heating.

[0049] Example 4

[0050] This embodiment discloses a method for preparing a flame-retardant hybrid structural adhesive for power batteries, the method comprising the following steps:

[0051] Step 1 (Preparation of Carbonized Chitosan): Chitosan powder (degree of deacetylation ≥ 85%, particle size not greater than 200 μm) is placed in a tube furnace and protected with high-purity nitrogen gas (purity ≥ 99.9%, flow rate 150 mL / min). The temperature is increased to 320℃ at a rate of 3℃ / min and held for 2 hours to allow partial pyrolysis and carbonization of the chitosan segments. The holding temperature is controlled at 320℃, which is higher than the initial thermal decomposition temperature of chitosan (approximately 200℃) but lower than the temperature required for complete carbonization (> 600℃). This causes localized dehydration, deacetylation, and thermal decomposition of the chitosan segments, forming a partially carbonized structure while retaining incompletely decomposed residual amino groups. Chitosan raw materials with a deacetylation degree ≥85% ensure sufficient free amino content before carbonization, and after the above partial carbonization treatment, still have a residual amino content of not less than 0.5 mmol / g, which meets the dosage requirements of the covalent bonding reaction in step four. During the carbonization process, the outlet of the tubular furnace is connected to a washing device with an 8% NaOH aqueous solution to absorb the organic volatiles (acetic acid, furan, etc.) generated by the pyrolysis of chitosan; the entire carbonization process is carried out under effective ventilation conditions. After the heat preservation is completed, the mixture is cooled to room temperature with the furnace, taken out, ground and sieved (sieve aperture not greater than 100μm) to obtain carbonized chitosan powder, which is then sealed and stored under dry conditions for later use.

[0052] The aforementioned partial carbonization treatment endows chitosan with two functions: firstly, the carbonized chain segments act as a nitrogen-containing organic carbon source during combustion, synergistically promoting char formation with phosphorus in the system; secondly, the retained residual amino groups can react with the isocyanate groups in the NCO-terminated polyurethane prepolymer during the preparation of the structural adhesive in step four. ) react to form urea bonds ( The carbonized chitosan is covalently embedded into a polyurethane network, making it a non-migratable reactive nitrogen-containing carbon source, which is different from the scheme of adding organic carbon sources by physical mixing.

[0053] Step 2 (Preparation of Phosphate-modified Montmorillonite P-MMT): Sodium-type montmorillonite (cation exchange capacity 100 meq / 100g, sheet diameter-to-thickness ratio ≥50) was added to deionized water to prepare a 3% (w / w) montmorillonite suspension. The suspension was stirred at 70°C and 250 r / min for 1.5 hours to allow the montmorillonite sheets to be fully dispersed.

[0054] The phosphate ester cationic modifier (containing both quaternary ammonium cationic groups and phosphate ester groups in the same molecule; the cationic group is a quaternary ammonium cation (R4N)) + ), where the N-alkyl group is C 16A hexadecyl group, with the remaining N-alkyl groups being methyl or ethyl; a phosphate ester group (-OPO(OH)2) linked to a quaternary ammonium cation molecule via a C3 alkylene chain; phosphorus content P% ≥ 4 wt%; and an amount 1.1 times the molar amount of montmorillonite's cation exchange capacity) was dissolved in hot deionized water at 70°C and slowly added dropwise to the above montmorillonite suspension under stirring at 250 r / min (dropping time not less than 30 minutes). The mixture was kept at 70°C with stirring for 3 hours, allowing the phosphate ester cation to intercalate into the montmorillonite interlayer through the cation exchange reaction, replacing the original inorganic cations in the interlayer. After the reaction, the mixture was filtered while hot and repeatedly washed with hot deionized water at 70°C. The washing endpoint was defined as the conductivity of the washing solution decreasing to a level comparable to that of the deionized water used (conductivity difference ≤ 5 μS / cm). The mixture was dried in an oven at 90°C to constant weight, ground, and sieved (sieve aperture not greater than 75 μm) to obtain phosphate-esterified montmorillonite (P-MMT), which was then sealed and stored for later use. The d(001) interlayer spacing of P-MMT was determined by X-ray diffraction (XRD). The acceptable standard is an interlayer spacing ≥ 1.8 nm (the original sodium montmorillonite is about 1.2 nm). The increase in interlayer spacing indicates that the phosphate ester cations have been successfully intercalated into the interlayer of montmorillonite.

[0055] Step 3 (Preparation of NCO-terminated polyurethane prepolymer containing phosphate groups): Under the protection of dry nitrogen (purity ≥99.9%), phosphate-modified polypropylene glycol (phosphate-modified polypropylene glycol (PPG), hydroxyl value 57 mgKOH / g, phosphorus content 4wt%, phosphate groups covalently bonded to the polyether backbone) containing phosphate groups is added to a dry reactor according to the specified amount, and the temperature is raised to 70℃ with stirring at 300 r / min; then, 4,4'-diphenylmethane diisocyanate (MDI, NCO mass fraction 25%), which has been melted into liquid in a water bath at 50-55℃, is added to the above-heated PPG at an NCO / OH molar ratio of 2.0, and simultaneously, dibutyltin dilaurate catalyst (0.03wt% of the total mass of reactants) is added, and the reaction is stirred at 300 r / min for 3 hours at 70℃; during the reaction, samples are taken every 30 minutes and titrated with di-n-butylamine (GB / T The NCO mass fraction of the system was determined using method 12009.4. When the NCO mass fraction dropped to 3–4% and the difference between two consecutive test results did not exceed 0.1%, the reaction was considered complete. The system was then cooled to room temperature and subjected to vacuum degassing (gauge pressure -0.09 MPa, maintained for 30 minutes; the vacuum system outlet was connected to an alkaline absorption device to prevent trace amounts of isocyanate vapor from escaping), resulting in a polyurethane prepolymer with terminal free NCO groups. The terminal free NCO groups in this prepolymer undergo a moisture-curing reaction upon contact with moisture in the air at room temperature and pressure, generating a polyurethane-urea crosslinked elastic adhesive layer without the need for an external heat source.

[0056] Step 4 (Preparation of structural adhesive): Under dry nitrogen protection, combine the P-MMT obtained in Step 2, the carbonized chitosan powder obtained in Step 1, and aluminum hypophosphite (… AHP, particle size not greater than 50μm, purity ≥98%), dicalcium phosphate dihydrate ( DCPD (diameter of carbon dioxide, particle size not greater than 50 μm, purity ≥ 98%) and borosilicate glass powder with a particle size not greater than 50 μm ( System, quality components: 65wt%, 15wt%, 8wt%, the rest is Before preparation, the auxiliary components (softening point 725℃) must be pre-dried: the above solid fillers (P-MMT, carbonized chitosan powder, AHP and borosilicate glass powder) are dried in a vacuum oven at 90℃ (gauge pressure above -0.08 MPa) for 3 hours; the pre-drying temperature of DCPD is controlled at 70℃ (not exceeding 80℃, to avoid premature removal of crystal water, which would change the chemical structure and stage pyrolysis characteristics of DCPD), and dried for 3 hours; after drying, each filler is cooled to room temperature and immediately sealed for use. Using the NCO-terminated polyurethane prepolymer containing phosphate ester groups obtained in step three as the matrix, the amount of each component is calculated based on 100 parts by weight of the prepolymer: 5 parts by weight of P-MMT, 10 parts by weight of carbonized chitosan, 10 parts by weight of AHP, 7 parts by weight of DCPD, and 10 parts by weight of borosilicate glass powder. The pre-dried fillers are added to the prepolymer in batches in the order of P-MMT, carbonized chitosan, AHP, DCPD, and borosilicate glass powder. After each addition, the mixture is mechanically stirred at 400 r / min for 12 minutes. After the mixture is evenly dispersed, the next component is added. After all components are added, the mixture is stirred at 400 r / min for 45 minutes. The dispersion is considered qualified when no visible powder agglomerates are observed and the system has uniform fluidity. The structural adhesive is then obtained. It is sealed and stored at 25°C or below, away from moisture (relative humidity ≤30%), and the shelf life does not exceed 6 months.

[0057] During the preparation process, the residual amino groups retained in the carbonized chitosan powder react with some of the NCO groups in the prepolymer to form urea bonds, embedding the carbonized chitosan into the polyurethane network via covalent bonds, thus consuming some of the NCO groups. The layered silicate sheets of P-MMT are uniformly dispersed in the prepolymer, forming tortuous diffusion channels during subsequent curing, hindering the diffusion rate of external moisture into the adhesive layer, reducing the rate difference in curing from the outside to the inside, and making the crosslinking density distribution of the adhesive layer more uniform. AHP is uniformly dispersed in the prepolymer in solid powder form, and thermally decomposes at the combustion temperature (approximately 300–400°C) to generate gaseous phosphorus-containing free radicals. (etc.), quenching flame gas phase combustion chain reaction; DCPD is uniformly dispersed in solid powder form, and under thermal runaway conditions, it sequentially performs endothermic cooling and high-temperature carbon layer inorganic integration functions in stages; borosilicate glass powder is dispersed in the adhesive layer in a chemically inert glass network structure, which replenishes the mineral filler barrier gaps degraded after electrolyte erosion during service, and softens and flows to fill carbon layer cracks under high-temperature carbon layer formation conditions (above 600-800℃). After carbonized chitosan consumes NCO groups, the prepolymer still retains a sufficient amount of free NCO groups to ensure the normal progress of wet curing crosslinking in step five.

[0058] Step 5 (Adhesive Application and Curing): Apply the structural adhesive obtained in Step 4 to the surface of the battery cell or housing substrate to be bonded by scraping, dispensing, or extrusion. The thickness of the adhesive on one side is 0.5 mm. After alignment and bonding, allow it to cure statically under atmospheric conditions of 25°C and 60% relative humidity. The free NCO groups at the ends of the prepolymer undergo a moisture curing reaction upon contact with moisture in the air. As the polyurethane-urea crosslinking network gradually forms, the initial curing time (the surface is no longer sticky) under the above conditions is about 2 hours, and complete curing (reaching more than 90% of the final bonding strength) takes about 6 days. After curing, a polyurethane-urea elastic structural adhesive layer is obtained, which realizes the bonding and fixation between battery cells and between battery cells and housing without the need for heating.

[0059] Experimental verification

[0060] Experiment 1: Verification of Halogen-Free Flame Retardant Performance via Gas-Phase-Condensed Phase Dual Path

[0061] Experimental objective: To verify the contribution of the synergistic effect of the gas phase flame retardant path (provided by AHP) and the condensed phase flame retardant path (phosphate ester prepolymer + NP synergistic char formation) to the flame retardant performance of the structural adhesive system of the present invention, and to prove that the dual-path flame retardant system is superior to the single-path system.

[0062] Sample preparation:

[0063] Based on the formulation and process of Example 1, the following four groups of samples were prepared; after each group was prepared, it was cast into a polytetrafluoroethylene (PTFE) mold to prepare free membrane samples of 130 mm × 6.5 mm × 3 mm (for LOI testing) and 125 mm × 13 mm × 3 mm (for UL-94 testing). After complete curing at 25°C and 60% relative humidity for 6 days, the samples were demolded for later use.

[0064] (1) S-Full (complete formulation, corresponding to Example 1): 100 parts by weight of NCO-terminated polyurethane prepolymer containing phosphate ester groups (PPG hydroxyl value 57 mgKOH / g, phosphorus content 4wt%; MDI NCO mass fraction 25%; NCO / OH molar ratio 2.0; DBTDL 0.03wt%; prepolymer NCO mass fraction 3.2%) as the matrix, and 10 parts by weight of P-MMT, 10 parts by weight of carbonized chitosan (degree of deacetylation 88%, carbonized at 320℃ / 2h, residual amino 0.72 mmol / g), 10 parts by weight of AHP, 7 parts by weight of DCPD, and 10 parts by weight of borosilicate glass powder (SiO2 65wt% / B2O3 15wt% / Na2O 8wt%, softening point 725℃) were added, and the mixture was prepared according to the process in step four.

[0065] (2) S-NoAHP (removal of vapor phase flame retardant components): The formula is the same as S-Full, but AHP (10 parts by mass) is removed, while the remaining components and processes remain unchanged.

[0066] (3) S-NoCS (removal of NP synergistic carbon source): The formula is the same as S-Full, but carbonized chitosan (10 parts by mass) is removed, and the remaining components and processes remain unchanged.

[0067] (4) S-NoFR (no flame retardant filler standard): contains only NCO-terminated polyurethane prepolymer, without adding any flame retardant filler, and the rest of the process remains unchanged.

[0068] Test method:

[0069] (1) Limiting oxygen index (LOI): determined according to GB / T 2406.2-2009, with sample size of 130 mm × 6.5 mm × 3 mm, 5 parallel samples for each group, and the average value is taken.

[0070] (2) Vertical flammability rating (UL-94): Determined according to GB / T 2408-2008, with a sample size of 125 mm × 13 mm × 3 mm. Five parallel samples are tested in each group, and the V-0 / V-1 / V-2 / NR rating is determined according to the standard.

[0071] Experimental results:

[0072] Table 1-1 LOI and UL-94 test results for each sample

[0073]

[0074] Note: t1 is the average extinction time of each parallel sample after the first ignition, and t2 is the average extinction time of each parallel sample after the second ignition; GB / T 2408-2008 stipulates that the extinction time of each ignition should not exceed 10 s for V-0 level and should not exceed 30 s for V-1 level; In S-NoAHP and S-NoCS, some parallel samples had t1 or t2 exceeding 10 s, which did not meet the V-0 judgment condition, so they were rated as V-1; NR indicates that it is not flame retardant and the sample continues to burn to the clamping end.

[0075] Figure 1 Comparison of limiting oxygen index and UL-94 flame retardant rating for each sample.

[0076] Conclusion: The S-Full (complete dual-pathway system) achieved a LOI of 33.8% and a UL-94 V-0 rating. The flame retardant properties of S-NoAHP (LOI 26.4%, V-1) and S-NoCS (LOI 27.1%, V-1) (removing AHP) and S-NoCS (LOI 27.1%, V-1) (removing carbonized chitosan) both showed significant decreases. Experimental results demonstrate that the gas-phase flame retardant pathway (AHP providing PO· radical quenching) and the NP-synergistic condensed-phase char-enhancing effect (synergistic effect of carbonized chitosan and phosphate ester prepolymer) both make irreplaceable contributions to achieving the V-0 rating; the absence of either component prevents the achievement of the V-0 rating.

[0077] Experiment 2: Verification of the stability of the flame-retardant barrier after long-term immersion in organic electrolyte

[0078] Experimental objective: To verify the effectiveness of borosilicate glass powder as a chemically inert barrier component in maintaining the flame-retardant barrier function of the adhesive layer under long-term corrosive conditions of organic electrolyte, and to demonstrate that the flame-retardant performance of the formulation containing glass powder is more stable over service time than that of the formulation without glass powder.

[0079] Sample preparation:

[0080] Based on the formulation and process of Example 1, the following two groups of samples were prepared:

[0081] (1) S-Full (complete formulation, same as S-Full in Experiment 1): prepared according to the formulation of Example 1, cast into a PTFE mold to form a free membrane sample of 130 mm × 6.5 mm × 3 mm, and demolded after complete curing at 25℃ / 60%RH for 6 days.

[0082] (2) S-NoGlass (removal of borosilicate glass powder): The formula is the same as S-Full, but the borosilicate glass powder (10 parts by weight) is removed. The remaining components (5 parts P-MMT, 10 parts carbonized chitosan, 10 parts AHP, 7 parts DCPD) and process remain unchanged. The free membrane sample of 130 mm × 6.5 mm × 3 mm is cast in a PTFE mold and demolded after being completely cured at 25℃ / 60%RH for 6 days.

[0083] Electrolyte immersion aging treatment:

[0084] The fully cured free membrane samples (130 mm × 6.5 mm × 3 mm) were immersed in a 1 mol / L LiPF6 / ethylene carbonate (EC): dimethyl carbonate (DMC) mixed electrolyte solution of 1:1 (volume ratio) and immersed in a sealed constant temperature oven at 40℃ for 0 days, 7 days, 14 days, 21 days and 30 days respectively. After the period, the samples were taken out, the residual electrolyte on the surface was wiped with anhydrous ethanol, and the samples were dried in a vacuum oven at 60℃ for 12 hours. The LOI was determined according to GB / T 2406.2-2009 (5 parallel samples per group, average value was taken).

[0085] Experimental results:

[0086] Table 2-1 Changes in LOI of each sample after immersion in organic electrolyte for different times

[0087] Note: The initial LOI of S-NoGlass (30.4%) was lower than that of S-Full (33.8%) because the glass powder itself also carries part of the physical barrier function; the decline in flame retardant performance was evaluated by the LOI reduction (relative to their respective day 0). After 30 days of immersion, the LOI of S-Full decreased by 1.4%, and that of S-NoGlass decreased by 6.3%.

[0088] Figure 2 Comparison of LOI changes after immersion and aging in organic electrolyte

[0089] Conclusion: After immersion in organic electrolyte at 40℃ for 30 days, the LOI of S-Full only decreased from 33.8% to 32.4%, a decrease of 1.4%; the LOI of S-NoGlass decreased from 30.4% to 24.1% during the same period, a decrease of 6.3%, which is already below the 26% flame retardant reference line, indicating a significant degradation of the flame retardant barrier function.

[0090] The difference in LOI reduction between the two groups of samples (8.3%, see the last column of Table 2-1) continued to increase with immersion time, indicating that when there is no glass powder, inorganic mineral fillers such as P-MMT undergo interfacial degradation under long-term erosion by organic electrolyte, and the flame retardant barrier effect continues to decline; while the chemically inert glass network structure of borosilicate glass powder is not eroded by electrolyte, maintaining the long-term effectiveness of the physical barrier of the adhesive layer, and ensuring the continuous effectiveness of the flame retardant barrier function of the adhesive layer throughout the entire service life of the power battery.

[0091] Experiment 3: Verification of carbon layer integrity and in-situ crack sealing capability after charge-discharge thermal cycling pre-damage

[0092] Experimental objective: To verify the in-situ crack sealing function of borosilicate glass powder in carbon layer and the high-temperature inorganic integration function of DCPD carbon layer, and to demonstrate that after pre-damage from charge-discharge thermal cycling, the carbon layer protection reliability of the complete formulation containing the above two components is significantly better than that of the control formulation lacking the above components.

[0093] Sample preparation:

[0094] Based on the formulation and process of Example 1, the following three groups of samples were prepared, and each sample was fully cured at 25°C / 60%RH for 6 days:

[0095] (1) S-Full (complete formulation, same as Experiment 1): prepared according to the formulation of Example 1, with a glue layer thickness of 0.5 mm.

[0096] (2) S-NoDCPD (removal of DCPD): The formulation is the same as S-Full, but DCPD (7 parts by weight) is removed, while the remaining components and processes remain unchanged, and the adhesive layer thickness is 0.5 mm.

[0097] (3) S-NoGlass (removal of borosilicate glass powder): The formula is the same as S-Full, but borosilicate glass powder (10 parts by weight) is removed, while the other components and processes remain unchanged, and the adhesive layer thickness is 0.5 mm.

[0098] Thermal cycling pre-damage treatment:

[0099] The fully cured adhesive layer sample (50 mm × 50 mm × 0.5 mm, bonded to an aluminum plate) was placed in a programmable temperature control chamber and subjected to 500 cycles (approximately 21 days) at a temperature of 40℃ (30 min) - 80℃ (30 min) per cycle to simulate the pre-damage conditions of a power battery charge-discharge thermal cycle. After the cycle was completed, the sample was removed and designated as the "post-cycle" sample; the control sample was designated as the "pre-cycle" sample (without thermal cycling treatment).

[0100] Carbonization treatment and determination of residual weight of carbon layer:

[0101] The samples from both the "before cycling" and "after cycling" groups (with the adhesive layer peeled off from the aluminum plate and taken as a separate adhesive layer) were cut into pieces approximately 30 mm × 10 mm × 0.5 mm. These pieces were placed in a tube furnace under a protective atmosphere of high-purity nitrogen (flow rate 150 mL / min) and heated to 800℃ at a rate of 10℃ / min. After holding at this temperature for 30 min, the furnace was allowed to cool to room temperature. The mass before and after carbonization was accurately measured, and the residual weight percentage of the carbon layer (mass fraction, %) was calculated. Three parallel samples were taken from each group, and the average value was used.

[0102] Scanning electron microscopy (SEM) characterization:

[0103] Two groups of carbonized samples, S-Full (after cycling) and S-NoGlass (after cycling), were selected. Cross sections were prepared by liquid nitrogen brittle fracture along the carbon layer thickness direction, fixed to the sample stage with conductive adhesive, and sputtered with gold. The cross-sectional morphology of the carbon layer was observed using a scanning electron microscope (5 kV accelerating voltage, secondary electron mode), focusing on characterizing the distribution of internal cracks and the overall density of the carbon layer. The same cross section was also observed in a backscattered electron mode (BSE) to distinguish the distribution of the glass phase (rich in Si and B, high brightness) and the organic carbon phase (low brightness) using atomic number contrast.

[0104] Experimental results:

[0105] Table 3-1 Residual weight of carbon layer before and after thermal cycling for each sample

[0106]

[0107] Note: The decrease in carbon layer residual weight rate reflects the degree of impact of thermal cycling pre-damage on the integrity of carbon layer quality. The smaller the decrease, the less the pre-damage affects the reliability of carbon layer protection.

[0108] Figure 3 Comparison of carbon layer residual weight percentage of each sample before and after thermal cycling;

[0109] Figure 4 SEM image of the carbon layer cross section after S-Full cycling;

[0110] Figure 5 This is a SEM image of the carbon layer cross-section after S-NoGlass cycling.

[0111] Conclusion: After 500 thermal cycles of pre-damage treatment, the residual weight of the carbon layer in S-Full (containing glass powder and DCPD) decreased by only 0.9%, and the carbon layer remained dense with cracks essentially sealed (e.g., Figure 4 As shown); the residual weight of the S-NoGlass carbon layer decreased by 3.9%, and obvious through cracks (5-20 μm, as shown in the SEM cross-section) were visible. Figure 5 As shown, the reliability of carbon layer protection decreases with the degree of pre-damage.

[0112] Experimental results demonstrate that after softening at high temperatures (>700℃), borosilicate glass powder dynamically fills the microcracks at the carbon layer interface with viscous flow. The calcium pyrophosphate generated by DCPD reacts with silicate and borate residues in a solid phase to form a calcium-phosphorus composite inorganic phase. The two work together to achieve in-situ sealing of the carbon layer cracks, proving the effectiveness of the in-situ crack sealing mechanism.

[0113] Experiment 4: Verification of the improvement of wet curing uniformity by phosphated montmorillonite

[0114] Experimental objective: To verify the effect of the tortuous diffusion channels formed by phosphated montmorillonite (P-MMT) sheets on the inhibition of the gradient of water diffusion rate from the outside to the inside during the wet curing process, and to demonstrate that the crosslinking density distribution of the adhesive layer containing P-MMT is significantly better than that of the formulation without P-MMT in the thickness direction.

[0115] Sample preparation:

[0116] Based on the formulation and process of Example 1, the following two groups of samples were prepared:

[0117] (1) S-Full (complete formula, same as Experiment 1): Prepared according to the formula of Example 1, a free film with a thickness of 0.5 mm was prepared in a polytetrafluoroethylene (PTFE) mold by scraping, and then demolded after being sealed and cured at 25℃ / 60%RH for 6 days.

[0118] (2) S-NoMMT (removal of P-MMT): The formula is the same as S-Full, but P-MMT (5 parts by mass) is removed. The remaining components (10 parts carbonized chitosan, 10 parts AHP, 7 parts DCPD, 10 parts borosilicate glass powder) and processes remain unchanged. The preparation method and curing conditions are the same as S-Full.

[0119] Crosslinking density gradient determination (swelling method):

[0120] After curing for 7 days (complete curing), the 0.5 mm adhesive layer was cut into 4 layers along its thickness direction using a cryostat (-50℃): Layer 1 (outer layer) 0–0.1 mm, Layer 2 0.1–0.2 mm, Layer 3 0.2–0.3 mm, and Layer 4 (inner layer) 0.3–0.5 mm. Approximately 20 mg of sample (accurately weighed) was taken from each layer and placed in 2 mL of toluene. The sample was sealed and soaked at 70℃ for 24 hours. After removing the sample and blotting off the surface solvent with filter paper, the mass after swelling was immediately weighed, and the swelling rate was calculated. :

[0121]

[0122] The higher the crosslinking density, the lower the swelling rate; the smaller the consistency of the swelling rate along the thickness direction (measured by relative standard deviation RSD), the more uniform the curing. Three parallel samples were taken from each layer of each group, and the average value was taken.

[0123] Experimental results:

[0124] Table 4-1 Swelling rate of each layer along the thickness direction of each sample (characterized by cross-linking density)

[0125]

[0126] Note: The lower the swelling ratio, the higher the crosslinking density; the smaller the RSD, the more uniform the crosslinking density distribution along the thickness direction; the RSD of each layer of S-Full is 2.1%, which is close to uniform curing; the RSD of each layer of S-NoMMT is 17.7%, with a significant difference in crosslinking density between the surface layer and the inner layer, and the curing of the inner layer is significantly delayed.

[0127] Figure 4 Comparison of swelling rates of each layer along the thickness direction of the adhesive layer

[0128] Conclusion: The swelling rate of each layer of S-Full (containing 5 parts by mass of P-MMT) along the thickness direction is 142% to 149%, and the relative standard deviation (RSD) is only 2.1%, indicating that the crosslinking density is highly uniformly distributed along the thickness direction; the swelling rate of each layer of S-NoMMT increases from 138% in the outer layer to 209% in the inner layer, with an RSD of 17.7%, indicating that the curing of the inner layer is significantly slower than that of the outer layer.

[0129] Experimental results demonstrate that the tortuous diffusion channels formed by the P-MMT sheet in the adhesive layer effectively reduce the rate gradient of moisture transport from the outside to the inside during wet curing, significantly improve the curing uniformity of the adhesive layer, make the mechanical properties of the adhesive layer along the thickness direction more uniform, and enhance its durability under charge-discharge cycle bending fatigue.

[0130] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A flame-retardant hybrid structural adhesive for power batteries, characterized in that, It is composed of isocyanate-terminated polyurethane prepolymer containing phosphate ester groups, phosphate-modified montmorillonite, carbonized chitosan, aluminum hypophosphite, borosilicate glass powder, and dicalcium phosphate dihydrate; based on 100 parts by weight of the prepolymer, it contains 3-8 parts by weight of phosphate-modified montmorillonite, 5-15 parts by weight of carbonized chitosan, 5-15 parts by weight of aluminum hypophosphite, 5-10 parts by weight of dicalcium phosphate dihydrate, and 5-15 parts by weight of borosilicate glass powder; the prepolymer is composed of phosphate-modified polypropylene glycol and... The prepolymer is prepared by reacting diphenylmethane diisocyanate, retaining free isocyanate groups at the ends, with phosphate groups covalently embedded in the prepolymer backbone; the residual amino groups of the carbonized chitosan react with the free isocyanate groups of the prepolymer to form urea bonds, which are covalently embedded in the polyurethane network; the borosilicate glass powder has a softening point of 700–750°C and is chemically inert to organic electrolytes; the phosphate-esterified montmorillonite is prepared from materials with a cation exchange capacity of… Sodium-type montmorillonite with a sheet diameter-to-thickness ratio of not less than 50 is prepared by cation exchange reaction with a phosphate ester cationic modifier; the phosphate ester cationic modifier contains a quaternary ammonium cation and a phosphate ester group in the same molecule, wherein the quaternary ammonium cation has at least one N-alkyl group. Long-chain alkyl groups, with the remaining N-alkyl groups being methyl or ethyl, and phosphorus content not less than [amount missing]. The phosphate ester group is The XRD d(001) interlayer spacing of the obtained phosphated montmorillonite is not less than Particle size not greater than .

2. The flame-retardant hybrid structural adhesive according to claim 1, characterized in that, The phosphate-modified polypropylene glycol has a hydroxyl value of 40–75 mgKOH / g and a phosphorus content of 3–5 wt%; the 4,4'-diphenylmethane diisocyanate has an NCO mass fraction of not less than 25%; the molar ratio of NCO groups to OH groups is 2.0–2.5; and the NCO mass fraction of the resulting prepolymer is 2–6%.

3. The flame-retardant hybrid structural adhesive according to claim 1, characterized in that, The carbonized chitosan is prepared by heating chitosan powder with a degree of deacetylation of not less than 85% and a particle size of not more than 200 μm at 250–400 °C for 1–3 hours under nitrogen protection; the carbonized chitosan has a residual amino content of not less than 0.5 mmol / g and a particle size of not more than 100 μm.

4. The flame-retardant hybrid structural adhesive according to claim 1, characterized in that, The mass composition of the borosilicate glass powder is as follows: 60-70 wt% 10-20 wt% 5-10 wt%, balance is The particle size is no greater than 50 μm.

5. The flame-retardant hybrid structural adhesive according to claim 1, characterized in that, The aluminum hypophosphite has a particle size of no more than 50 μm and a purity of no less than 98 wt%; the calcium hydrogen phosphate dihydrate has a particle size of no more than 50 μm and a purity of no less than 98 wt%.

6. A method for preparing the flame-retardant hybrid structural adhesive according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step Step 1: Partially carbonize chitosan powder at 250–400°C for 1–3 hours under an inert atmosphere, then grind and sieve after cooling to obtain carbonized chitosan. Step 2: Perform a cation exchange reaction between a phosphate ester cationic modifier and a sodium montmorillonite suspension, followed by washing, drying, grinding, and sieving to obtain phosphate-modified montmorillonite. Step 3: Under dry nitrogen protection, react phosphate-modified polypropylene glycol with 4,4'-diphenylmethane diisocyanate at 60–80°C with stirring for 2–4 hours, then cool and degas under vacuum to obtain phosphate-containing... NCO-terminated polyurethane prepolymer with ester groups; Step 4: Under dry nitrogen protection, the carbonized chitosan and phosphated montmorillonite obtained in Step 1 and Step 2, as well as aluminum hypophosphite, dicalcium phosphate dihydrate and borosilicate glass powder, are pre-dried separately and then added to the prepolymer obtained in Step 3 in proportion. The mixture is stirred and dispersed evenly. It is then sealed and stored at a temperature below 25°C and a relative humidity of no more than 30%, with a shelf life of no more than 6 months; Step 5: The structural adhesive obtained in Step 4 is applied to the surface of the substrate to be bonded, with a single-sided adhesive thickness of 0.3–1.0 mm. It is then moisture-cured at an atmospheric temperature of 15–35°C and a relative humidity of 40–80%. The surface is not tacky for 1–4 hours, and the complete curing time is 5–7 days.

7. The preparation method according to claim 6, characterized in that, In step one, the inert atmosphere is high-purity nitrogen gas with a purity of not less than 99.9%, with a flow rate of 100-200 mL / min; the temperature is increased to 250-400℃ at a heating rate of 2-5℃ / min; the outlet is connected to a washing device with a mass fraction of 5-10% NaOH aqueous solution; after cooling to room temperature, it is ground and passed through a 100μm sieve.

8. The preparation method according to claim 6, characterized in that, In step two, the sodium montmorillonite suspension has a mass concentration of 2-5%, and is stirred and swollen at 200-300 r / min at 60-80℃ for 1-2 hours; the amount of phosphate ester cationic modifier is 1.0-1.2 times the molar amount of montmorillonite cation exchange capacity, and is slowly added dropwise to the montmorillonite suspension over a period of not less than 30 minutes, and the mixture is kept at 60-80℃ and stirred for 2-4 hours; the washing endpoint is defined as the difference between the conductivity of the washing solution and the conductivity of deionized water not exceeding 5 μS / cm; the mixture is dried to constant weight at 80-100℃ and then ground through a 75μm sieve.

9. The preparation method according to claim 6, characterized in that, In step three, the molar ratio of NCO groups to OH groups is 2.0–2.5, the stirring rate is 200–400 r / min, and the NCO mass fraction is determined by di-n-butylamine titration according to GB / T 12009.4 every 30 minutes. When the NCO mass fraction drops to 2–6% and the difference between two consecutive test results is not greater than 0.1%, the temperature is lowered to room temperature and vacuum degassed for 30 minutes under a gauge pressure of -0.09 MPa. In step four, carbonized chitosan, phosphated montmorillonite, aluminum hypophosphite, and borosilicate glass powder are dried in a vacuum oven at 80–100℃ for 2–4 hours, and dicalcium phosphate dihydrate is dried in a vacuum oven at 60–80℃ for 2–4 hours. Each component is added to the prepolymer in the following order: phosphated montmorillonite, carbonized chitosan, aluminum hypophosphite, dicalcium phosphate dihydrate, and borosilicate glass powder. After each addition, 300–500 ml of the prepolymer is added. Stir at 300-500 r / min for 10-15 minutes. After all components have been added, continue stirring at 300-500 r / min for 30-60 minutes.

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