Large-size core-shell polymer particles, preparation method and application thereof

Abstract

The application discloses large-diameter core-shell polymer particles, a preparation method and application thereof. The large-diameter core-shell polymer particles have a particle size of 1-50 microns; the structure of the particles comprises a water-insoluble soft substance layer and a hard substance layer, and the weight ratio of the soft substance layer to the hard substance layer is 1:0.1-10, and the hard substance layer is located at the outer layer of the soft substance layer. The particles are prepared by using a modified aqueous suspension polymerization and / or emulsion polymerization method. The obtained particles can improve the hardness of an electric core, inhibit the deformation of the electric core, and do not affect the air permeability. Compared with general micron-level homogeneous polymer particles, the particles provided by the application have a soft core and hard shell structure, which is also beneficial to improving the performance of a diaphragm. The hard substance layer located at the outer layer of the particles ensures that the diaphragm coating is not sticky after drying, so that the diaphragms are not adhered to each other after being wound. The soft substance layer located at the inner layer of the particles can provide strong adhesion, so that the diaphragm and the pole piece have higher peeling strength.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation of a new material core-shell polymer, in particular to a large particle size core-shell polymer particle and its preparation method and application. BACKGROUND

[0002] The polymer particles with core-shell structure have a wide range of applications in various fields due to their "customized" structural characteristics. In recent years, the application of core-shell polymer particles in secondary batteries has attracted increasing attention. In addition to being used as adhesives for electrodes, separators and the like, core-shell polymer particles, especially those with soft core and hard shell structure, are more commonly used in the functional coating of battery separators. For example, such particles are uniformly coated on the surface of the ceramic layer of the battery separator, and then the separator is attached to the coated surface of the electrode sheet, and a certain temperature and pressure are applied to bond the separator and the electrode sheet, thereby improving the hardness of the battery and reducing the interfacial resistance. Generally, a ceramic layer needs to be coated on the surface of the separator first, and then the core-shell polymer particles are coated on the surface of the ceramic layer, resulting in an increase in the number of processes and an increase in process complexity.

[0003] A feasible solution is to mix the core-shell structured polymer particles with ceramic powder uniformly to prepare a polymer-ceramic mixed slurry. The separator coated with this slurry can be directly bonded with the electrode sheet under pressure. Obviously, this requires the particle size of the core-shell structured polymer particles to be large enough to form protrusions on the surface of the separator rather than being "buried" by the ceramic powder, so as to play a role. The thickness of the ceramic layer of the ceramic separator is mostly 2-3 μm, and some manufacturers have also introduced a coating process with a thickness of about 1 μm or even thinner. Therefore, the particle size of the core-shell structured polymer particles should be greater than 1 μm, and 2-7 μm is appropriate. At the same time, in order to ensure the consistency of the performance of the coated separator, the particle size distribution of the core-shell structured polymer particles should also be as narrow as possible. However, at this particle size level, the preparation of core-shell structured polymer particles presents a great challenge. In the currently disclosed technologies, more solutions are adopted for homogeneous polymer particles rather than core-shell polymer particles. For example, Chinese invention CN112940650A discloses a similar solution, which prepares a large particle size polymer particle with a glass transition temperature of 35-90℃ (paragraphs

[0015] and

[0016] of the specification) (paragraph

[0070] of the specification). The large particle size and relatively high glass transition temperature homogeneous polymer particles are simple to prepare, but the peel strength after hot pressing is not high when such particles are used as a functional coating of a separator, and the actual application effect is not good.

[0004] Core-shell polymer particles are generally prepared through an emulsion polymerization route. The core-shell emulsion polymerization process is mature, and the products are stable and controllable. However, the particle size range of the products obtained by this method is usually 50 to 500 nm, and it is almost impossible to produce particles with larger particle sizes. Although the particles of the core-shell emulsion can be agglomerated into "secondary particles" by spray drying and other methods, which are usually powders of tens to hundreds of microns, this powder still needs to be processed to the desired particle size range by ball milling and other methods before use. The direct preparation of micron-sized polymer particles is generally achieved through dispersion polymerization, suspension polymerization, microsuspension polymerization and other methods. However, these methods are mostly used to prepare hard polymer particles with homogeneous structures (such as polystyrene microspheres, etc.), and there are relatively few reports on the preparation of core-shell polymer particles. Among them, the preparation of micron-sized polymer particles with soft core and hard shell structures is even rarer. This is because, to prepare micron-sized polymer particles with soft core and hard shell structures, it is necessary to first prepare soft particles with a sufficiently large particle size, uniform dispersion, and a glass transition temperature usually below room temperature. This step poses a great challenge to the above methods. Specifically, at the micron scale, soft polymer particles easily aggregate and adhere due to sedimentation and collisions, making it difficult to maintain stability within a few microns. The resulting product is often unevenly distributed coarse particles tens or hundreds of microns in size, or even completely "mud" precipitated from the reaction system. Therefore, further research is needed to develop stable and controllable micron-scale soft-core, hard-shell polymer particles with an appropriate particle size. Summary of the Invention

[0005] To address the problems in existing core-shell polymers, particularly soft-core hard-shell polymer particles, that fail to meet large particle size requirements or exhibit low peel strength after hot pressing, the present invention proposes large-particle core-shell polymer particles. More specifically, micron-sized core-shell polymer particles with a soft-core hard-shell structure. Furthermore, the present invention proposes a method for preparing such particles. Furthermore, the present application proposes applications of such particles, particularly in the preparation of secondary battery separators.

[0006] The large-particle core-shell polymer particles provided by the present invention have a particle size (D50) of 1 to 50 μm, preferably 1.5 to 25 μm, and more preferably 2 to 7 μm; the structure of the particles includes a water-insoluble soft material layer and a hard material layer, and the weight ratio of the soft material layer to the hard material layer is 1:0.1 to 10, preferably 1:0.5 to 5, and more preferably 1:1 to 3; the hard material layer is located on the outer layer of the soft material layer.

[0007] Furthermore, the soft material layer is a type of polymer A, whose glass transition temperature is not higher than 40°C, preferably not higher than 0°C, and more preferably not higher than -20°C; the hard material layer is a type of polymer B, whose glass transition temperature is 60°C to 250°C, preferably 80°C to 150°C, and more preferably 90°C to 120°C.

[0008] It is understandable that those skilled in the art can select from a variety of monomers and adjust their ratios to obtain polymers A and B that meet design requirements. For example, polymer A is a homopolymer and / or copolymer of soft monomers such as ethyl acrylate, butyl acrylate, and vinyl acetate, and polymer B is a homopolymer and / or copolymer of hard monomers such as styrene, acrylonitrile, and methyl methacrylate.

[0009] Furthermore, the particle structure also includes a hydrophilic functional layer. Whether to incorporate this layer into the particle structure depends primarily on the design requirements for product performance. If incorporated, the hydrophilic functional layer can be bonded to the particle in at least one of the following ways: coating the hard material layer, interpenetrating the hydrophilic functional layer with the hard material layer, interpenetrating the hydrophilic functional layer with the soft material layer, or interpenetrating both the hard material layer and the soft material layer. The introduction of the hydrophilic functional layer helps improve product stability.

[0010] Furthermore, the hydrophilic functional layer is at least one of the following substances: a water-soluble polymer, a copolymer of a water-soluble monomer and a hard material layer monomer, a copolymer of a water-soluble monomer and a soft material layer, and a copolymer of a water-soluble monomer and hard material layer monomers and soft material layer monomers.

[0011] Understandably, technicians can use a variety of methods to introduce a hydrophilic functional layer into the particle structure. For example, a water-soluble polymer such as PVA or PVP can be directly dissolved in the reaction system to adhere to the particle surface. Another example is that after the polymerization of the hard material layer is completed, a water-soluble monomer such as acrylic acid or acrylamide can be added to initiate polymerization, thereby obtaining a three-layer core-shell polymer structure of soft material layer-hard material layer-hydrophilic functional layer. Another example is that a hard material layer monomer and a water-soluble monomer can be mixed and simultaneously added to the reaction system to obtain a structure in which the hard material layer and the hydrophilic functional layer are interpenetrating. Another example is that a soft material layer monomer and a water-soluble monomer can be mixed and simultaneously added to the reaction system, and after a period of polymerization, the hard material layer monomer is added to obtain a structure in which the hydrophilic functional layer is interpenetrating with both the soft material layer and the hard material layer.

[0012] Furthermore, the particle structure also includes a hydrophobic functional layer. Whether to incorporate this layer into the particle structure depends primarily on the design requirements for product performance. If incorporated, the hydrophobic functional layer is located within the soft material layer and / or interspersed with the soft material layer. The inclusion of the hydrophobic functional layer helps control the particle size of the product.

[0013] Furthermore, the hydrophobic functional layer is a water-insoluble compound containing not less than 12 carbon atoms, and / or a copolymer of a water-insoluble compound containing not less than 12 carbon atoms and a soft material layer monomer, and the number of carbon atoms is preferably 12-50.

[0014] It is understood that technicians can use a variety of methods to introduce a hydrophobic functional layer into the particle structure. For example, a hydrophobic substance such as cetyl alcohol or stearate can be directly dissolved in the monomer of the soft material layer, and then polymerization can be initiated to obtain a structure in which the soft material layer is coated on the surface of the hydrophobic substance. For another example, a monomer with strong hydrophobicity (such as long-chain olefins and their derivatives, such as octadecenoic acid; or esters containing long carbon chain structures, such as lauryl acrylate, etc.) can be copolymerized with the soft material layer monomer to obtain a structure in which the hydrophobic functional layer and the soft material layer are interpenetrated.

[0015] Obviously, those skilled in the art should know that the "one layer interpenetrating with another layer" mentioned above (such as the hydrophilic functional layer and the hard material layer interpenetrating with each other) can be bonded by chemical bonds or by an interpenetrating polymer network (IPN).

[0016] Furthermore, the method for preparing the large-size core-shell polymer particles comprises the following steps:

[0017] S101. The initiator is dissolved in the monomer of the soft material layer to obtain a solution 1-1;

[0018] S102. The surfactant is dissolved in water to obtain a solution 1-2;

[0019] S103. Solution 1-1 and solution 1-2 are mixed, so that solution 1-1 is uniformly dispersed in the form of droplets in solution 1-2, and then the temperature is raised to initiate polymerization;

[0020] S104. Add the monomer of the hard material layer into the reaction system, continue polymerization, and then cool and collect the material.

[0021] Furthermore, in the step S101, the non-water-soluble compound containing not less than 12 carbon atoms is dissolved in the monomer of the soft material layer; in the step S102, the water-soluble polymer is dissolved in water; and in the step S104, the water-soluble monomer and / or the water-soluble polymer are added to the reaction system.

[0022] Obviously, the above method can be considered an improved aqueous suspension polymerization method. As a non-limiting example, the initiator is generally soluble in the monomer but insoluble in water, such as AIBN and BPO; the water-soluble polymer can be polyvinyl alcohol, polyvinyl pyrrolidone, sodium carboxymethyl cellulose, etc.; the surfactant, based on its solubility and ionization, can be anionic, cationic, nonionic, amphoteric, and mixtures thereof; based on its activity, it can be reactive, non-reactive, and mixtures thereof, etc. For example: sodium lauryl sulfate (anionic, non-reactive), sodium p-styrene sulfonate (anionic, reactive), alkylphenol polyoxyethylene ether (non-ionic, non-reactive), etc.

[0023] Furthermore, the method for preparing the large-size core-shell polymer particles comprises the following steps:

[0024] S201. The initiator is dissolved in water to obtain a solution 2-1;

[0025] S202. Mixing the monomers of the soft material layer and the solution 2-1, so that the soft material layer monomer is uniformly dispersed in the solution 2-1 in the form of droplets, and then heating to initiate polymerization to obtain a soft material layer dispersion;

[0026] S203. Mixing the monomers of the hard material layer and the soft material layer dispersion in which the initiator is pre-dissolved, so that the hard material layer monomers are uniformly dispersed in the form of droplets in the soft material layer dispersion;

[0027] S204. Continue polymerization until the reaction is completed.

[0028] Furthermore, in the step S201, the water-soluble polymer and / or water-soluble monomer are dissolved in water; in the step S202, the water-insoluble compound containing not less than 12 carbon atoms and / or surfactant are pre-dissolved in the monomer of the soft material layer; and in the step S203, the water-soluble polymer, water-soluble monomer and / or surfactant are pre-dissolved in the soft material layer dispersion.

[0029] Obviously, the above method can be regarded as an improved aqueous emulsion polymerization method. As a non-limiting example, the initiator is usually soluble in water, such as persulfate; the water-soluble polymer can be polyvinyl alcohol, polyvinyl pyrrolidone, sodium carboxymethyl cellulose, etc.; the surfactant can be classified from the perspective of its solubility and ionization, such as anionic, cationic, nonionic, amphoteric, and mixtures thereof; from the perspective of its activity, it can be reactive, non-reactive, and mixtures thereof, etc. For example: sodium lauryl sulfate (anionic, non-reactive), sodium p-styrene sulfonate (anionic, reactive), alkylphenol polyoxyethylene ether (non-ionic, non-reactive), etc.

[0030] The application of the above-mentioned large-particle core-shell polymer particles includes at least one of the following methods: directly coating the particles on the surface of the secondary battery separator; uniformly mixing the particles with other powders and then coating them on the surface of the secondary battery separator, wherein the other powders include inorganic ceramic powders and / or organic polymer powders.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The present invention provides a large-particle core-shell polymer particle that can be stably dispersed and has a narrow particle size distribution. More specifically, it is a polymer particle with a soft core and hard shell structure with a particle size (D50) of 1 to 50 μm. In terms of the preparation method, the preparation method of the polymer particles is an improved aqueous suspension polymerization and / or emulsion polymerization method, which has low equipment requirements, few steps, simple process, and is both green and environmentally friendly and economically efficient. As for the polymer particles themselves, the secondary battery diaphragm prepared using the particles can effectively form protrusions on the surface of the diaphragm because the particle size of the particles is much larger than the thickness of the diaphragm coating, so that the diaphragm can form "point bonding" with the electrode under a certain temperature and pressure, while improving the hardness of the battery core and suppressing the deformation of the battery core without affecting the permeability (i.e., ion permeability). At the same time, compared with general micron-sized homogeneous polymer particles, the soft core and hard shell structure of the particles provided by the present invention is also beneficial to improving the performance of the diaphragm. Specifically, the hard material layer on the outer layer of the particle ensures that the separator coating is non-sticky after drying, thus preventing the separators from sticking to each other after being rolled up. Meanwhile, the soft material layer on the inner layer of the particle provides strong adhesion, resulting in higher peel strength between the separator and the electrode. Furthermore, with respect to the aforementioned "certain temperature and pressure effects," by engineering the core-shell ratio and shell hardness of the particles, core-shell polymer particles can be prepared that are suitable for a wide range of temperatures and pressures. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 1 is a particle size distribution diagram of the core-shell polymer particles obtained in Example 1.

[0034] Figure 2 3 is the DSC curve of the core-shell polymer particles obtained in Example 1, where Tg1 and Tg2 are the glass transition temperatures of the soft material layer and the hard material layer, respectively.

[0035] Figure 3 is a SEM photograph of the core-shell polymer particles obtained in Example 3. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Methods not otherwise specified are understood according to the common knowledge in this field; equipment, reagents, etc. not otherwise specified are all conventional commercially available products.

[0037] Example 1

[0038] Preparation of core-shell polymer particles

[0039] In this embodiment, the soft material layer is styrene-butyl acrylate copolymer, and the hard material layer is methyl methacrylate-acrylonitrile copolymer. The weight ratio of the soft and hard material layers is 1:2. Cetyl alcohol is also introduced as a hydrophobic functional layer. The preparation method uses a modified aqueous suspension polymerization method.

[0040] Specifically, 0.15g of AIBN and 1g of cetyl alcohol were dissolved in a premixed mixture of 30g of styrene and butyl acrylate to obtain solution A. 1.2g of sodium octyl sulfosuccinate was dissolved in 200g of water to obtain solution B. Solutions A and B were mixed and ultrasonically shaken until a uniform dispersion formed. This dispersion was poured into a three-necked flask equipped with a reflux condenser and a stirrer, purged with nitrogen, and heated to 70°C for 1 hour. Subsequently, 60g of premixed methyl methacrylate and acrylonitrile were added, and the reaction continued for another 4 hours before collection. The product had a D50 of approximately 2.1μm.

[0041] Example 2

[0042] Preparation of core-shell polymer particles

[0043] In this example, 1.2 g of sodium octyl sulfosuccinate was replaced with 1.2 g of sodium lauryl sulfate and 2 g of polyvinyl alcohol 1799, and 1 g of cetyl alcohol was removed, with the remainder being the same as in Example 1. The product D50 was approximately 30 μm.

[0044] Example 3

[0045] Preparation of core-shell polymer particles

[0046] In this example, the soft material layer is styrene-butyl acrylate copolymer, the hard material layer is methyl methacrylate-acrylonitrile copolymer, and the weight ratio of the soft and hard material layers is 1:2. Sodium polyacrylate is introduced as a hydrophilic functional layer. The preparation method uses a modified aqueous emulsion polymerization method.

[0047] Specifically, 0.15 g of potassium persulfate and 3 g of acrylic acid are dissolved in 200 g of water, and the pH value is adjusted to 7.5±0.5 with sodium hydroxide to obtain solution A, which is poured into a three-necked flask equipped with a reflux condenser and a stirrer, and then 30 g of styrene and butyl acrylate, which are mixed in advance, are poured into the flask. Nitrogen is introduced, and the temperature is raised to 70°C for 4 h. Then, 60 g of methyl methacrylate and acrylonitrile, which are mixed in advance, are added dropwise into the flask at a constant pressure through a dropping funnel, and the dropping is completed in 2 h. At the same time, 0.3 g of potassium persulfate is added into the flask every 30 min. The reaction is continued for 2 h, and then the product is collected. The particle size (D50) of the product is about 5.5 μm.

[0048] Comparative Example

[0049] A methyl methacrylate-butyl acrylate copolymer homogeneous microsphere with a D50 of about 2 μm and a glass transition temperature of about 45°C is prepared by dispersion polymerization. Specifically, the solid content is about 20%, the solvent is an aqueous solution of ethanol with a volume fraction of about 80%; the mass ratio of methyl methacrylate to butyl acrylate is about 3:1; the dispersant is PVP-K30, and the amount is 0.2% of the monomer equivalent; and the initiator is AIBN, and the amount is 0.5% of the monomer equivalent.

[0050] Example 4

[0051] Performance Test

[0052] The products obtained in Preparation Examples 1-3 and the Comparative Example are mixed with alumina ceramic powder, binder, dispersant, wetting agent, etc. to prepare a ceramic powder-polymer particle mixed slurry, which is coated on the surface of a polyolefin-based film according to the thicknesses shown in the table below, and dried to obtain a coated separator. The 180° peeling strength of the coated separator is tested after pressure bonding with a lithium iron phosphate positive electrode sheet at a certain temperature. The pressure bonding conditions are as follows:

[0053] Example 1, 2, and Comparative Example, 85±5°C, 1.5-2.0 MPa, 15-20 s;

[0054] Example 3, room temperature, 3.5-4.0 MPa, 8-12 s.

[0055]

Claims

1. A large-size core-shell polymer particle, characterized in that: The particle size D50 of the particles is 2 to 7 μm; the structure of the particles includes a water-insoluble soft material layer and a hard material layer, the weight ratio of the soft material layer to the hard material layer is 1:1 to 3, and the hard material layer is located on the outer layer of the soft material layer; The soft material layer is a polymer A having a glass transition temperature of no more than 40°C; the hard material layer is a polymer B having a glass transition temperature of 60°C to 250°C; the polymer A is a homopolymer and / or copolymer of soft monomers such as ethyl acrylate, butyl acrylate, and vinyl acetate, and the polymer B is a homopolymer and / or copolymer of hard monomers such as styrene, acrylonitrile, and methyl methacrylate; The structure of the particles also includes a hydrophilic functional layer or a hydrophobic functional layer. The hydrophilic functional layer is bonded to the particles in at least one of the following ways: coated on the outer layer of the hard material layer, interpenetrated with the hard material layer, interpenetrated with the soft material layer, or interpenetrated with both the hard material layer and the soft material layer; the hydrophobic functional layer is located in the inner layer of the soft material layer and / or interpenetrated with the soft material layer; the hydrophilic functional layer is PVA, PVP, polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyvinyl pyrrolidone, or sodium carboxymethyl cellulose; and the hydrophobic functional layer is a water-insoluble compound containing not less than 12 carbon atoms and / or a copolymer of a water-insoluble compound containing not less than 12 carbon atoms and a monomer of the soft material layer.

2. The method for preparing large-size core-shell polymer particles according to claim 1, wherein: The steps include: S101. The initiator is dissolved in the monomer of the soft material layer to obtain a solution 1-1; S102. The surfactant is dissolved in water to obtain a solution 1-2; S103 mixing solution 1-1 and solution 1-2, so that solution 1-1 is evenly dispersed in the form of droplets in solution 1-2, and then the temperature is raised to initiate polymerization; S104. Add the monomer of the hard material layer to the reaction system and continue polymerization until the reaction is complete.

3. The method for preparing large-size core-shell polymer particles according to claim 2, wherein: The step S101 further includes: dissolving the water-insoluble compound containing no less than 12 carbon atoms in the monomer of the soft material layer.

4. The method for preparing large-size core-shell polymer particles according to claim 2, wherein: The step S102 further includes: dissolving the water-soluble polymer in water.

5. The method for preparing large-size core-shell polymer particles according to claim 2, wherein: The step S104 further includes: adding the water-soluble monomer and / or the water-soluble polymer into the reaction system.

6. The method for preparing large-size core-shell polymer particles according to claim 1, wherein: The steps include: S201. The initiator is dissolved in water to obtain a solution 2-1; S202. Mixing the monomers of the soft material layer and the solution 2-1, so that the soft material layer monomer is uniformly dispersed in the solution 2-1 in the form of droplets, and then heating to initiate polymerization to obtain a soft material layer dispersion; S203. Mixing the monomers of the hard material layer and the soft material layer dispersion in which the initiator is pre-dissolved, so that the hard material layer monomers are uniformly dispersed in the form of droplets in the soft material layer dispersion; S204. Continue polymerization until the reaction is complete.

7. The method for preparing large-size core-shell polymer particles according to claim 6, wherein: The step S201 further includes: dissolving the water-soluble polymer and / or water-soluble monomer in water.

8. The method for preparing large-size core-shell polymer particles according to claim 6, wherein: The step S202 further includes: pre-dissolving the water-insoluble compound containing no less than 12 carbon atoms and / or surfactant into the monomer of the soft material layer.

9. The method for preparing large-size core-shell polymer particles according to claim 6, wherein: The step S203 further includes: pre-dissolving the water-soluble polymer, water-soluble monomer and / or surfactant into the soft material layer dispersion.

10. The use of the large-particle core-shell polymer particles as described in claim 1 includes at least one of the following methods: directly coating the particles on the surface of a secondary battery separator; uniformly mixing the particles with other powders and then coating them on the surface of a secondary battery separator, wherein the other powders include inorganic ceramic powders and / or organic polymer powders.

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