PVDF (Polyvinylidene Fluoride) type water-based binder as well as preparation method and application thereof

Abstract

The invention discloses a PVDF (Polyvinylidene Fluoride) type water-based binder as well as a preparation method and application thereof. The PVDF type water-based binder comprises a core layer structure and a shell layer structure coated on the surface of the core layer structure, the core layer structure comprises a polyvinylidene fluoride polymer; the shell layer structure comprises a polyacrylate polymer; the reaction raw materials of the polyacrylate polymer comprise an acrylate hard monomer, an acrylate soft monomer, a carboxyl or hydroxyl containing vinyl monomer and a bifunctional vinyl cross-linking agent; the molar ratio of the acrylate hard monomer to the acrylate soft monomer to the vinyl monomer containing carboxyl or hydroxyl to the bifunctional vinyl cross-linking agent is 1: (2-5): (0.2-0.6): (0.05-0.35); the mass ratio of the core layer structure to the shell layer structure is 1: (0.4-1). The PVDF type water-based adhesive prepared by the preparation method disclosed by the invention has excellent cold pressing adhesive property, low swelling property and thermal stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a PVDF-type aqueous binder, its preparation method, and its application. Background Technology

[0002] With the increasing demand for high-energy-density, fast-charge-discharge-rate, and high-safety lithium batteries in electric vehicles and electronic devices, the requirements for battery kinetic performance are also becoming increasingly stringent. Improving the contact between the separator and the positive and negative electrode sheets can effectively shorten the migration distance of lithium ions, thereby enhancing battery kinetic performance. Currently, this is mainly achieved by coating the separator surface with an adhesive and then bonding the positive and negative electrodes to the separator through hot or cold pressing. However, conventional PVDF primarily provides adhesion through van der Waals forces, which are relatively weak. Prolonged use can easily lead to separation between the electrode sheets and the separator, resulting in deteriorated battery cycle performance and charge / discharge rates. PVDF is highly hydrophobic, dispersible poorly in water, and prone to agglomeration, leading to uneven and unstable slurries, which in turn causes problems with the uniformity of the separator coating, affecting battery performance. Modification of PVDF is usually necessary. Furthermore, PVDF has high crystallinity, requiring high-temperature and high-pressure conditions for processing as a separator adhesive, resulting in high energy consumption. Therefore, developing an aqueous adhesive that still exhibits excellent adhesion performance under cold pressing processes is crucial.

[0003] Chinese patent CN118271526A discloses a core-shell structured PVDF-modified acrylic emulsion binder, with PVDF as the core and polyacrylate as the shell. While this binder can achieve bonding under cold pressing conditions, its polyacrylate to PVDF mass ratio is greater than 2. Excessive polyacrylate content leads to a decrease in the binder's melting point, which can easily cause diaphragm damage under high-temperature conditions, posing a safety hazard. Furthermore, it is prone to swelling in the electrolyte, affecting structural stability, reducing bonding performance, and consequently impacting battery cycle life. In addition, the binder's preparation process is relatively complex, further limiting its application prospects.

[0004] Therefore, it is particularly important to develop a PVDF-based waterborne adhesive that combines excellent cold-press bonding performance, low swelling, and thermal stability. Summary of the Invention

[0005] The primary objective of this invention is to address the shortcomings or defects of existing PVDF-based water-based adhesives and provide a PVDF-based water-based adhesive. This adhesive comprises a core structure and a shell structure coating the surface of the core structure; the core structure comprises a polyvinylidene fluoride polymer; and the shell structure comprises a polyacrylate polymer. This adhesive exhibits excellent cold-press bonding performance, low swelling, and thermal stability.

[0006] Another object of the present invention is to provide a method for preparing the above-mentioned PVDF-type waterborne adhesive.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: A PVDF-type waterborne adhesive includes a core structure and a shell structure coating the surface of the core structure; the core structure includes a polyvinylidene fluoride polymer; the shell structure includes a polyacrylate polymer; the reaction raw materials of the polyacrylate polymer include acrylate hard monomers, acrylate soft monomers, carboxyl or hydroxyl-containing vinyl monomers, and a difunctional vinyl crosslinking agent; the molar ratio of the acrylate hard monomers to the acrylate soft monomers to the carboxyl or hydroxyl-containing vinyl monomers to the difunctional vinyl crosslinking agent is 1:2~5:0.2~0.6:0.05~0.35; the mass ratio of the core structure to the shell structure is 1:0.4~1.

[0008] Acrylic hard monomers improve polymer strength, acrylate soft monomers improve polymer toughness, carboxyl or hydroxyl-containing vinyl monomers improve polymer adhesion, and bifunctional vinyl crosslinking agents improve polymer network structure. This invention optimizes the shell structure obtained through polymerization by adjusting the molar ratio of acrylate hard monomers, acrylate soft monomers, carboxyl or hydroxyl-containing vinyl monomers, and bifunctional vinyl crosslinking agents, effectively improving its adhesion properties. This results in a PVDF waterborne adhesive that exhibits excellent cold-pressing adhesion and low swelling even with low polyacrylate content.

[0009] Preferably, the molar ratio of the acrylate hard monomer to the acrylate soft monomer to the carboxyl or hydroxyl-containing vinyl monomer to the difunctional vinyl crosslinker is 1:3~4.5:0.3~0.5:0.1~0.3.

[0010] Preferably, the mass ratio of the core structure to the shell structure is 1:0.5~0.9.

[0011] The PVDF aqueous binder of this invention is used on a separator. If the volume average particle size (Dv50) of the binder is too small, it can easily clog the pores of the separator, reducing the permeability of lithium ions. If the volume average particle size (Dv50) of the binder is too large, a thick coating will form when the binder is applied to the separator, affecting the electrochemical performance of the battery. Preferably, the volume average particle size (Dv50) of the PVDF aqueous binder is 100~1000 nm.

[0012] Preferably, the solid content of the PVDF-type water-based adhesive is 20-40%.

[0013] It should be noted that the glass transition temperature of the acrylate hard monomers in this invention is above 25°C, and any acrylate monomers that meet this requirement are applicable to this invention. Preferably, the acrylate hard monomers are one or more selected from methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, cyclohexyl methacrylate, vinyl acetate, or tert-butyl acrylate. More preferably, the acrylate hard monomers are one or more selected from methyl methacrylate, ethyl methacrylate, or vinyl acetate.

[0014] It should be noted that the glass transition temperature of the acrylate soft monomers in this invention is below 0°C, and any acrylate monomers that meet this requirement are applicable to this invention. Preferably, the acrylate soft monomers are one or more selected from ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octyl methacrylate, or isooctyl acrylate. More preferably, the acrylate soft monomers are one or more selected from ethyl acrylate, propyl acrylate, butyl acrylate, or amyl acrylate.

[0015] In this invention, the glass transition temperature of the acrylate hard monomer or the acrylate soft monomer can be obtained by DSC instrument testing.

[0016] Preferably, the vinyl monomer containing a carboxyl or hydroxyl group is one or more of acrylic acid, methacrylic acid, maleic acid, itaconic acid, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, or hydroxypropyl acrylate.

[0017] Preferably, the difunctional vinyl crosslinking agent is one or more of divinylbenzene, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, triethylene glycol diacrylate, polyethylene glycol dimethacrylate, or polyethylene glycol diacrylate.

[0018] Preferably, the polyvinylidene fluoride polymer is one or both of polyvinylidene fluoride homopolymer and polyvinylidene fluoride copolymer.

[0019] The vinylidene fluoride copolymer may be selected from commonly used vinylidene fluoride copolymers in the art, such as: vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-pentafluoropropylene copolymer, vinylidene fluoride-tetrafluoropropylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, or vinylidene fluoride-perfluoromethyl vinyl ether, or one or more thereof.

[0020] This method employs a seed emulsion polymerization method to prepare a PVDF-type aqueous binder with a core-shell structure. Those skilled in the art will understand that this method requires forming a seed emulsion of polyvinylidene fluoride (PVDF) polymer, and the PVDF polymer emulsion can be prepared using existing techniques. Preferably, the volume average particle size (Dv50) of the PVDF polymer emulsion is 100-450 nm. Preferably, the solid content of the PVDF polymer emulsion is 15-30%.

[0021] Preferably, the PVDF-type waterborne adhesive further includes an emulsifier and an initiator.

[0022] In this invention, commonly used emulsifiers in the prior art can be selected, such as, but not limited to, one or more of the following: sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, OP-10, Span 80, Span 20, Tween 80, Tween 20, polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, or hydroxymethyl cellulose. Preferably, the emulsifier is one or two of sodium dodecyl sulfate or OP-10.

[0023] In this invention, commonly used initiators or combinations of initiators and reducing agents in the prior art can be selected.

[0024] Specifically, the initiator is one or more of ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, and tert-butyl hydroperoxide. The reducing agent is one or more of sodium bisulfite, sodium formaldehyde sulfoxylate, or Brügmann reducing agent.

[0025] The preparation method of the above-mentioned PVDF-type water-based adhesive is also within the scope of protection of this invention, and includes the following steps: S1. Mix the acrylate hard monomer, acrylate soft monomer, vinyl monomer containing carboxyl or hydroxyl groups and the difunctional vinyl crosslinking agent monomer evenly to obtain the shell monomer. S2. Stir the polyvinylidene fluoride polymer and emulsifier evenly, and then add the shell monomer and initiator dropwise after heating; S3. After the addition is complete, maintain the temperature for reaction, filter, and the PVDF type water-based adhesive is obtained.

[0026] As is known to those skilled in the art, oxygen is a highly efficient polymerization inhibitor in emulsion polymerization, suppressing free radical reactions. Therefore, emulsion polymerization needs to be carried out under an inert gas atmosphere. Preferably, the inert gas is nitrogen.

[0027] Preferably, the temperature for heating is 40~90℃.

[0028] To avoid explosive polymerization caused by adding the shell monomer to the reaction system in one step, the shell monomer is added to the reaction system at a certain dropping rate. Preferably, the dropping rate is 10~100 mL / h. It can be 10 mL / h, 30 mL / h, 40 mL / h, 50 mL / h, 60 mL / h, 70 mL / h, 80 mL / h, 100 mL / h, etc., and is not specifically limited.

[0029] Preferably, the heat preservation reaction time is 5-10 hours.

[0030] Compared with the prior art, the beneficial effects of the present invention include: This invention provides a PVDF-based waterborne adhesive, comprising a core structure and a shell structure coating the surface of the core structure; the core structure comprises a polyvinylidene fluoride polymer; and the shell structure comprises a polyacrylate polymer. This invention optimizes the molecular structure of the shell by adjusting the molar ratio of acrylate hard monomers, acrylate soft monomers, carboxyl or hydroxyl-containing vinyl monomers, and bifunctional vinyl crosslinking agents, thereby effectively improving the adhesive's bonding performance. The PVDF waterborne adhesive provided by this invention exhibits excellent cold-press bonding performance, low swelling, and thermal stability under low polyacrylate content conditions. Attached Figure Description

[0031] Figure 1 This is a SEM image of the PVDF-type waterborne adhesive prepared in Example 1 of the present invention.

[0032] Figure 2 This is a TEM image of the PVDF-type waterborne adhesive prepared in Example 1 of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to embodiments and comparative examples. These embodiments are merely typical descriptions of the present invention, but the present invention is not limited thereto. Unless otherwise specified, the test methods used in the following embodiments and comparative examples are conventional methods, and the raw materials and reagents used are commercially available from conventional commercial sources.

[0034] The raw materials used in each embodiment and comparative example are shown in Table 1.

[0035] Table 1. Raw material information for each embodiment and comparative example.

[0036] This invention employs a seed emulsion polymerization method to prepare a core-shell type PVDF aqueous binder. The seed emulsion can be prepared using existing technologies as a polyvinylidene fluoride copolymer emulsion or a polyvinylidene fluoride homopolymer emulsion. The preparation method of the polyvinylidene fluoride copolymer emulsion includes the following steps: S1. Add 12kg of deionized water and 12g of perfluorooctanoic acid (PFOA) to a 20L reactor, evacuate the reactor, and replace it with nitrogen until the oxygen content in the reactor is less than 20ppm. S2. Start the reactor and stir, then heat to 80°C. Add the pre-mixed vinylidene fluoride / hexafluoropropylene monomers (vinylidene fluoride to hexafluoropropylene mass ratio of 95:5) to the reactor until the pressure reaches 3.6 MPa. Use a metering pump to add 0.85 g of potassium persulfate (prepared with water to form a 10 wt% solution) and 4.48 g of diethyl malonate (prepared with water to form a 10 wt% solution) to the reactor to begin the polymerization reaction. S3. When the pressure in the reactor decreases, add monomer to the reactor to maintain the pressure. After 40 minutes of reaction, continue to add 0.35 g of potassium persulfate (prepared with water to form a 10 wt% solution) and 1.05 g of diethyl malonate (prepared with water to form a 10 wt% solution) as initiator, adding once every 40 minutes. After three consecutive additions, stop adding initiator and chain transfer agent. When the total amount of monomer added reaches 4.2 kg, stop adding monomer, stir, lower the reactor temperature, release the remaining gas in the reactor, and filter to obtain polyvinylidene fluoride copolymer emulsion.

[0037] The preparation method of the polyvinylidene fluoride homopolymer emulsion refers to the preparation method of polyvinylidene fluoride copolymer emulsion, except that the polyvinylidene fluoride / hexafluoropropylene monomer (the mass ratio of polyvinylidene fluoride to hexafluoropropylene is 95:5) is replaced with polyvinylidene fluoride monomer.

[0038] The volume average particle size (Dv50) of the polyvinylidene fluoride homopolymer emulsion prepared in this invention is 170 nm, and the solid content is 24%; the volume average particle size (Dv50) of the polyvinylidene fluoride copolymer emulsion prepared in this invention is 185 nm, and the solid content is 24%.

[0039] The same reagents were used in parallel experiments of all embodiments and comparative examples of this invention.

[0040] Example 1 This embodiment provides a PVDF-type water-based adhesive, the preparation method of which includes the following steps: S1. According to the molar ratio of acrylate hard monomer to acrylate soft monomer to carboxyl or hydroxyl-containing vinyl monomer to difunctional vinyl crosslinking agent of 1:2.3:0.4:0.1, weigh 1.00g methyl methacrylate, 2.95g butyl acrylate, 0.52g hydroxyethyl methacrylate, and 0.20g ethylene glycol dimethacrylate, stir evenly to obtain the shell monomer; S2. Weigh 0.8% of the total mass of the shell monomers in potassium persulfate and 14g of water, and stir well to obtain the initiator solution; S3. Weigh 26.30g of polyvinylidene fluoride copolymer emulsion (solid content 24%), 0.01g of sodium dodecyl sulfate and 0.01g of OP-10 and place them in a four-necked flask. Stir for 0.5h under a nitrogen atmosphere. After stirring, heat to 75℃ and add shell monomer and initiator solution dropwise at a rate of 20mL / h. S4. After the addition is complete, keep the reaction at a constant temperature for 10 hours, cool to room temperature, and then filter with a filter cloth to obtain the PVDF type water-based adhesive.

[0041] The PVDF-type waterborne adhesive prepared in Example 1 was characterized by scanning electron microscopy, and the results are as follows: Figure 1 As shown.

[0042] The PVDF-type aqueous binder prepared in Example 1 was characterized by transmission electron microscopy, and the results are as follows: Figure 2 As shown.

[0043] from Figure 1 and Figure 2 It can be seen that the PVDF-type waterborne adhesive prepared in Example 1 has a core-shell structure.

[0044] Example 2 Examples 2-15 provide different PVDF-type waterborne adhesives, the differences from Example 1 are shown in Tables 2 and 3. The preparation methods of the PVDF-type waterborne adhesives provided in Examples 2-15 are the same as those in Example 1.

[0045] Table 2 Preparation parameters for Examples 1-8

[0046] In the table, the total shell mass is the total mass of acrylate hard monomers, acrylate soft monomers, vinyl monomers containing carboxyl or hydroxyl groups, and difunctional vinyl crosslinking agents.

[0047] Table 3. Preparation parameters for Examples 9-15

[0048] In the table, the total shell mass is the total mass of acrylate hard monomers, acrylate soft monomers, vinyl monomers containing carboxyl or hydroxyl groups, and difunctional vinyl crosslinking agents.

[0049] Comparative Examples 1-6 Comparative Examples 1-6 provide different PVDF-type waterborne adhesives, and the differences from Example 1 are shown in Table 4. The preparation methods of the PVDF-type waterborne adhesives provided in Comparative Examples 1-6 are the same as those in Example 1.

[0050] Table 4 Preparation parameters for Comparative Examples 1-6

[0051] In the table, the total shell mass is the total mass of acrylate hard monomers, acrylate soft monomers, vinyl monomers containing carboxyl or hydroxyl groups, and difunctional vinyl crosslinking agents.

[0052] Performance testing (1) Particle size test and solid content test The particle size and solid content of the PVDF-type waterborne binders prepared in each example and comparative example were tested using a laser particle size analyzer and a moisture analyzer. The test results are shown in Table 5.

[0053] (2) Cold pressing bonding performance test The PVDF-type waterborne adhesives prepared in each example and comparative example were diluted to a solid content of 12 wt%. Using an automatic spraying system, the PVDF-type waterborne adhesives were sprayed onto PE membranes and dried at 80°C for 2 minutes. The areal density of the adhesive was 1 g / m³. 2 Then, the diaphragm coated with adhesive was pressed together with the positive electrode at 25°C and 10MPa for 50s. Finally, the adhesion was tested using a universal testing machine. The test results are shown in Table 5.

[0054] (3) Melting point test The melting points of the PVDF-type water-based adhesives prepared in each example and comparative example were determined using differential scanning calorimetry. The test results are shown in Table 5. The test conditions were: heating and cooling rates of 10℃ / min, and a temperature range of 25~180℃.

[0055] (4) Swelling rate test The swelling rate of the PVDF-type waterborne adhesives prepared in each example and comparative example was tested by gravimetric method, and the test results are shown in Table 5. The specific test method was as follows: the PVDF-type waterborne adhesive emulsion was dried to constant weight, pulverized, and then stirred at 60°C to dissolve in N-methylpyrrolidone (mass fraction of 10wt%). It was then dried at 100°C to constant weight to obtain a polymer film. The polymer film was cut into square sample strips with a mass of about 1g, and the mass was m0. The sample was then immersed in an electrolyte (EC:PC:DMC:EMC=35:5:55) at 70°C and kept at a constant temperature for ten days. After the surface electrolyte was absorbed with filter paper, the mass was measured as m1. The swelling rate was calculated as (m1-m0) / m0×100%.

[0056] Table 5. Test results of the PVDF-type waterborne adhesives prepared in each example and comparative example.

[0057] As shown in Table 5, by adjusting the molar ratio of acrylate hard monomers, acrylate soft monomers, carboxyl or hydroxyl-containing vinyl monomers, and difunctional vinyl crosslinking agents, the present invention enables the prepared PVDF-type waterborne adhesive to exhibit excellent cold-pressing bonding performance under low swelling conditions. When the swelling rate is 60-70%, its cold-pressing bonding performance can reach above 8.7 N / m. When the swelling rate is 70-81%, its cold-pressing bonding performance can reach above 11 N / m.

[0058] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A PVDF-type water-based adhesive, characterized in that, The PVDF-type waterborne adhesive comprises a core structure and a shell structure coating the surface of the core structure; the core structure comprises a polyvinylidene fluoride polymer; the shell structure comprises a polyacrylate polymer; the reaction raw materials of the polyacrylate polymer include acrylate hard monomers, acrylate soft monomers, carboxyl or hydroxyl-containing vinyl monomers, and a difunctional vinyl crosslinking agent; the molar ratio of the acrylate hard monomers to the acrylate soft monomers to the carboxyl or hydroxyl-containing vinyl monomers to the difunctional vinyl crosslinking agent is 1:2~5:0.2~0.6:0.05~0.35; the mass ratio of the core structure to the shell structure is 1:0.4~1.

2. The PVDF-type water-based adhesive according to claim 1, characterized in that, The volume average particle size Dv50 of the PVDF-type waterborne binder is 100~1000nm.

3. The PVDF-type water-based adhesive according to claim 1, characterized in that, The solid content of the PVDF-type water-based adhesive is 20-40%.

4. The PVDF-type water-based adhesive according to claim 1, characterized in that, The glass transition temperature of the acrylate hard monomer is above 25°C.

5. The PVDF-type water-based adhesive according to claim 1, characterized in that, The glass transition temperature of the acrylate soft monomer is below 0°C.

6. The PVDF-type waterborne adhesive according to claim 1, characterized in that, The vinyl monomer containing carboxyl or hydroxyl groups is one or more of acrylic acid, methacrylic acid, maleic acid, itaconic acid, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, or hydroxypropyl acrylate.

7. The PVDF-type water-based adhesive according to claim 1, characterized in that, The difunctional vinyl crosslinking agent is one or more of divinylbenzene, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, triethylene glycol diacrylate, polyethylene glycol dimethacrylate, or polyethylene glycol diacrylate.

8. The PVDF-type water-based adhesive according to claim 1, characterized in that, The polyvinylidene fluoride polymer is one or both of polyvinylidene fluoride homopolymer and polyvinylidene fluoride copolymer.

9. The PVDF-type waterborne adhesive according to claim 1, characterized in that, The PVDF-type waterborne adhesive also includes emulsifiers and initiators.

10. A method for preparing the PVDF-type waterborne adhesive according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1. Mix the acrylate hard monomer, acrylate soft monomer, vinyl monomer containing carboxyl or hydroxyl groups and the difunctional vinyl crosslinking agent evenly to obtain the shell monomer. S2. Stir the polyvinylidene fluoride polymer and emulsifier evenly, and then add the shell monomer and initiator dropwise after heating; S3. After the addition is complete, maintain the temperature for reaction, filter, and the PVDF type water-based adhesive is obtained.

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