Polyacrylate emulsion as well as preparation method and application thereof

Abstract

The invention provides a polyacrylate emulsion as well as a preparation method and application thereof, and relates to the technical field of organic polymerization. Specifically, polyacrylate emulsion particles in the polyacrylate emulsion are of a core-shell structure, D50 ranges from 500 nm to 700 nm, and Span ranges from 0.5 to 0.6; the core layer of the polyacrylate emulsion particle comprises a polyacrylate main chain, the shell layer comprises a branch chain grafted to the polyacrylate main chain, and the structure of the main chain and the branch chain, the number of repetitive units, substituent group selection and the like are provided. The polyacrylate emulsion has good cohesiveness and electrolyte resistance in the field of battery diaphragms, the narrow particle size distribution and the submicron particle size of the polyacrylate emulsion are beneficial to reduction of the thickness of an adhesive layer on the surface of the diaphragm, coating consistency is kept, external bonding function or wetting function components do not need to be introduced when the polyacrylate emulsion is applied to the surface of the diaphragm, and the application range is wide. Good application prospects are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of organic polymerization, and in particular, to a polyacrylate emulsion, a preparation method thereof, and an application thereof. Background Art

[0002] With the development of secondary battery technology, especially the increasing requirements for energy density in high-end power batteries, high-end 3C product batteries, etc., corresponding requirements for the thinning of diaphragms have also been put forward. The thinner diaphragm can not only reduce the overall weight of the battery and improve the energy density, but also improve the cycle performance and safety of the battery to a certain extent. However, this also brings technical challenges, such as how to ensure the mechanical strength, thermal stability, and electrolyte wettability of the diaphragm while keeping it thin, and ensure the safe and reliable operation of the battery.

[0003] Adopting new emulsion or microsphere materials for diaphragm coating is a technical means to achieve a thinner and better-performing diaphragm. In previous studies, for example, CN202211325940.9 discloses a core-shell polymer particle with a large particle size, its preparation method and application, the particle size is 1-50μm, and its structure includes a water-insoluble soft material layer and a hard material layer, which can improve the hardness of the battery core and inhibit the deformation of the battery core without affecting the air permeability; the hard material layer located on the outer layer of the particle ensures that the diaphragm coating does not become sticky after drying, while the soft material layer located on the inner layer of the particle can provide strong adhesion, so that the diaphragm and the electrode have a higher peel strength. Another example is CN202211325937.7, which discloses a functional coating for a secondary battery diaphragm, including a functional coating composition, a dispersion medium, and a dispersant; its functional coating can effectively adhere the diaphragm and the electrode without hot pressing, and can still maintain the integrity of the morphology and keep the ion channels unblocked as much as possible under certain temperature and pressure, reducing the adverse effect of increased internal resistance caused by introducing the coating. Still another example is CN202411110981.5, which discloses a polymer microsphere material, its preparation method and application, which includes a three-layer core-shell structure of a core layer, a transition layer, and a shell layer. By introducing a functional transition layer, the connectivity between the soft core layer and the hard shell layer can be effectively enhanced, making the overall core-shell structure more dense and firm; at the same time, it can also effectively solve the structural defects of excessive swelling of the polymer microspheres and easy rupture under long-term pressure.

[0004] However, the particle size of the current polymer microsphere materials is generally in the micron range, and their particles are relatively large, resulting in a relatively thick glue coating layer on the diaphragm, which is not conducive to the thinning of the diaphragm. Further research finds that if the particle size of the glue coating layer of the secondary battery can be controlled within a smaller range, on the one hand, the thickness of the glue coating layer can be effectively reduced, and on the other hand, it will not cause blockage of the pores on the surface of the diaphragm due to too small particles; in addition, the particle size distribution range is closely related to the coating uniformity of the coated diaphragm.

[0005] In view of this, the present invention is hereby provided. SUMMARY OF THE INVENTION

[0006] The first object of the present invention is to provide a polyacrylate emulsion, which is mainly used to solve the technical problem of too high particle size of the emulsion binder for conventional diaphragms. Based on the improvement of the previous application, the present invention provides a submicron emulsion with a narrow particle size distribution and a specific chemical structure and substituent groups.

[0007] The second object of the present invention is to provide a preparation method of the polyacrylate emulsion. The preparation method is based on the conventional emulsion polymerization method. By strictly restricting the nature of monomers, monomer selection and their ratio in the synthesis formula, the kinetic processes of nucleation and particle growth in emulsion polymerization are controlled, so as to obtain a product with a particle size and its distribution meeting the requirements, and there are no special requirements for equipment and process.

[0008] The third object of the present invention is to provide a secondary battery, which is mainly used to reflect the use of the polyacrylate emulsion as a functional coating layer in the diaphragm of the secondary battery.

[0009] The fourth object of the present invention is to provide a preparation method of the secondary battery.

[0010] In order to achieve the above objects of the present invention, the following technical solutions are specifically adopted: A polyacrylate emulsion, in which the polyester emulsion particles have a core-shell structure, D50 is 500 nm to 700 nm, and Span is 0.5 to 0.6; The core layer of the polyester emulsion particles includes a polyester main chain, and the shell layer includes a branched chain grafted on the polyester main chain; The structure of the polyester main chain satisfies the following chemical formula (I), and the structure of the branched chain satisfies the following chemical formula (II): (I); (II); Wherein, a, b, c, d, e, x, and y are all positive integers, and 100 ≤ a + b + c + d + e ≤ 1000, 30 ≤ x + y ≤ 300; R 01 、R 02 、R 03 、R 04 、R 05 independently include at least one of a hydrogen atom, a methyl group or a carboxymethyl group, and at least one of R 01 、R 02 、R 03 、R 04 、R 05 is a hydrogen atom; R 11 comprises at least one of a hydrogen atom or a carboxyl group; R 21 comprises at least one of a methyl group, an ethyl group or an alkoxy group, and the number of carbon atoms in the alkoxy group is 2 to 5 times the number of oxygen atoms; R 22 comprises at least one of a phenoxyalkyl group and an alkyl group having 4 to 12 carbon atoms; R 23 comprises at least one of a methyl ester group or a nitrile group; R 24 comprises at least one of a sodium salt or a lithium salt of a carboxyl group and a sodium salt or a lithium salt of a sulfonic acid group; R 25 comprises at least one of a hydroxymethyl group, an epoxy group, a methoxysilyl group or a methylene group activated by a bilateral carbonyl group; R 31 、R 32 independently comprises at least one of a hydrogen atom or a methyl group; R 41 comprises at least one of a benzene ring, a methyl ester group, a cycloalkane ester group or a nitrile group; R 42 comprises an ester group, an alkyl group having 2 to 6 carbon atoms and 1 to 5 carbon-carbon double bonds, and the carbon-oxygen double bond in the ester group and the carbon-carbon double bond form a conjugated structure.

[0011] A method for preparing the polyacrylate emulsion comprises the following steps: S1. Prepare a pre-emulsion containing a first soft monomer, a second soft monomer, a first hard monomer, a water-soluble electrolyte monomer, an emulsifier and water, heat it up and add part of the initiator, and carry out a first heat preservation reaction; S2. Uniformly add a first crosslinking monomer to the reaction solution of S1, then add part of the initiator, and carry out a second heat preservation reaction; S3. Heat up to 75 °C to 85 °C, uniformly add a mixture containing a second hard monomer and a second crosslinking monomer to the reaction solution of S2, and simultaneously uniformly add the remaining initiator, and carry out a third heat preservation reaction to obtain the polyacrylate emulsion.

[0012] A secondary battery, the surface of the separator of which comprises the polyacrylate emulsion.

[0013] A method for preparing the secondary battery comprises the following steps: Coat the polyacrylate emulsion on both sides of the separator respectively, then assemble the positive electrode, the separator and the negative electrode to obtain a pre-cell; apply temperature and pressure to the pre-cell, and then successively carry out housing assembly, electrolyte injection and encapsulation to obtain the secondary battery.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) A submicron emulsion with a narrow particle size distribution provided by the present invention can generate a higher adhesive force on the electrode sheet while reducing the thickness of the glue coating layer on the separator when applied to the separator bonding functional component. At the same time, the emulsion of the present invention also has good electrolyte resistance and can still maintain sufficient adhesive force after being soaked in the electrolyte. When the polyacrylate emulsion of the present invention is used for separator coating, there is no need to introduce other bonding functional components or wetting functional components, and it has good separator bonding effect and air permeability.

[0015] (2) The preparation method provided by the present invention belongs to the aqueous emulsion polymerization method. The reaction conditions are mild, there are no special requirements for equipment and process, and by limiting the polymerization monomers with different water solubilities, the kinetic processes of nucleation and particle growth in emulsion polymerization are controlled, which is easy to implement.

[0016] (3) The particle size of the polyacrylate emulsion of the present invention is 500 - 700 nm. Compared with the conventional emulsion products with a particle size not exceeding 300 nm, the present invention is not easy to block the pores on the surface of the separator and can effectively reduce the adverse impact on the ion passing ability of the separator; while compared with the micron-level products with a particle size exceeding 1 μm, the present invention is beneficial to reducing the thickness of the glue coating layer on the surface of the separator, thereby avoiding the problem of over-thick battery cells.

[0017] (4) The particle size distribution width of the polyacrylate emulsion of the present invention does not exceed 0.6, that is, the emulsion particles are of uniform size, so the coating consistency is good, which is beneficial to improving the batch-to-batch stability of the performance of the separator and the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1 The particle size distribution curve of Example 1 of the present invention is provided; Figure 2 The SEM image of Example 1 of the present invention is provided. DETAILED DESCRIPTION OF THE INVENTION

[0020] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase. In addition, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0021] The first aspect of the present invention is to provide a polyacrylate emulsion, in which the polyester milk particles in the emulsion have a core-shell structure, the D50 is 500 nm to 700 nm, and the Span is 0.5 to 0.6; therefore, the polyester milk particles in the present invention have the characteristics of a narrow particle size distribution and a particle size scale in the submicron range. Another important feature of the polyester milk particles is the core-shell structure, and its core layer mainly includes the polyester main chain of the polyester milk particles, and the shell layer includes the branches grafted on the polyester main chain.

[0022] Furthermore, the structure of the polyester main chain satisfies the following chemical formula (I), and the structure of the branch satisfies the following chemical formula (II): (I); (II); Among them, for the number of repeating units and the selection of substituents in chemical formula (I) and chemical formula (II), the following characteristics are simultaneously satisfied: First, a, b, c, d, e, x, and y are all positive integers, and 100 ≤ a + b + c + d + e ≤ 1000, 30 ≤ x + y ≤ 300; it can be understood that the specific values of the above parameters can be achieved through the mass ratio of the monomers used in the preparation process; in some optional embodiments, the value of a + b + c + d + e includes but is not limited to any one or any numerical range composed of any two of 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, and the value of x + y includes but is not limited to any one or any numerical range composed of any two of 30, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300.

[0023] Second, R 01 、R02 , R 03 , R 04 , R 05 independently includes at least one of a hydrogen atom, a methyl group, or a carboxymethyl group, and R 01 , R 02 , R 03 , R 04 , R 05 at least one of them is a hydrogen atom.

[0024] It should be noted that the branched structure represented by the chemical formula (II) should be grafted onto the branched structure represented by the chemical formula (I). The specific grafting position is related to the group selection of R 01 , R 02 , R 03 , R 04 , R 05 . When the substituents in R 01 , R 02 , R 03 , R 04 , R 05 are hydrogen atoms, the carbon atom connected to the hydrogen atom forms a grafting site and generates a branched structure. Further, one end of the branched structure represented by the chemical formula (II) forms a graft with the above carbon atom, and the other end of the branched structure of the chemical formula (II) is capped by a hydrogen atom.

[0025] Thirdly, R 11 includes at least one of a hydrogen atom or a carboxyl group; R 21 includes at least one of a methyl group, an ethyl group, or an alkoxy group, and the number of carbon atoms in the alkoxy group is 2 to 5 times the number of oxygen atoms; R 22 includes at least one of a phenoxyalkyl group and an alkyl group with 4 to 12 carbon atoms. It should be noted that the phenoxyalkyl group refers to an ether group obtained by connecting through an oxygen atom, and both ends of the ether bond are a phenyl group and an alkyl group; R 23 includes at least one of a methyl ester group or a nitrile group. It should be noted that the methyl ester group satisfies the following structure (III): (III).

[0026] R 24 includes at least one of a sodium salt or a lithium salt of a carboxyl group and a sodium salt or a lithium salt of a sulfonic acid group; R 25 includes at least one of a hydroxymethyl group, an epoxy group, a methoxysilyl group, or a methylene group activated by a bilateral carbonyl group.

[0027] It should be noted that when the R 25When it includes a methoxysilyl group or a methylene group activated by bilateral carbonyl groups, according to the monomer raw materials introducing the above two functional groups (corresponding to the first crosslinking monomer described in the present invention), R 25 may also include other unlisted functional groups; for example, when introducing the methoxysilyl group with γ-methacryloxypropyltrimethoxysilane as the monomer raw material, the number of the methoxysilyl groups is 3, and at this time R 25 also includes 3 methylene groups and 1 methacrylate group. However, the methylene group and the methacrylate group are not used as typical functional groups representing R 25 and are only introduced due to the functional groups contained in the monomer raw materials.

[0028] R 31 and R 32 independently include at least one of a hydrogen atom or a methyl group; R 41 includes at least one of a benzene ring, a methyl ester group, a cycloalkane ester group or a nitrile group; R 42 includes an ester group, an alkyl group with 2 to 6 carbon atoms and 1 to 5 carbon-carbon double bonds, and the carbon-oxygen double bond in the ester group and the carbon-carbon double bond form a conjugated structure.

[0029] In addition, in some embodiments: when R 21 is a methyl group or an ethyl group, R 01 is a hydrogen atom; when R 03 is a hydrogen atom, R 23 is a nitrile group; when R 11 is a carboxyl group, R 24 is a carboxyl group.

[0030] As a preferred embodiment, in addition to the group selection of R 24 listed above, it may also include one or more of an alkyl group, a phenyl group or an amide group.

[0031] As an alternative embodiment, the particle size D50 of the polyacrylate emulsion includes but is not limited to any one of 500, 520, 550, 580, 600, 620, 650, 680, 700 (nm) or a numerical range formed by any two of them, and the particle size distribution width Span of the polyacrylate emulsion includes but is not limited to any one of 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6 or a numerical range formed by any two of them. It can be understood that the calculation method of Span conforms to the conventional understanding in the art and is obtained by dividing the difference between D90 and D10 by D50.

[0032] As a preferred embodiment, the solid content of the polyacrylate emulsion is 20% - 30%.

[0033] As a preferred embodiment, at 25 °C, the viscosity of the polyacrylate emulsion ≤ 100 mPa·s.

[0034] The second aspect of the present invention lies in providing a method for preparing the polyacrylate emulsion as described in the first aspect. In the present invention, the polyacrylate emulsion is prepared from polymerization monomers, and the polymerization monomers include a first soft monomer, a second soft monomer, a first hard monomer, a second hard monomer, a first crosslinking monomer, a second crosslinking monomer, and a water-soluble electrolyte monomer.

[0035] Specifically, in the present invention, the core-shell structure of the polyester emulsion particles is achieved through dispersion polymerization. Meanwhile, polymerization is carried out with the first soft monomer, the second soft monomer, the first hard monomer, and the water-soluble electrolyte monomer to form the main chain of the inner core layer. A crosslinking structure is formed between the main chains through the first crosslinking monomer, making the structure of the latex particles dense. Polymerization is carried out with the second crosslinking monomer and the second hard monomer and grafted onto the main chain to form the outer shell layer outside the surface of the inner core layer. It is also worth noting that in some alternative embodiments, by preferably restricting the water solubility and glass transition temperature of some of the above-mentioned monomers, the polymerization efficiency and the properties of the finished product can be effectively improved, and there are more specific descriptions below.

[0036] As a preferred embodiment, in the present invention, the first soft monomer is partially soluble in water, with a solubility in water of 10 g / L to 150 g / L, and the homopolymer of the first soft monomer is insoluble in water. It should be noted that any solubility in the present invention, without emphasizing the temperature, refers to the solubility at room temperature, that is, the solubility at 20 °C to 30 °C. As a preferred embodiment, the T of the homopolymer of the first soft monomer g ≤ 25 °C.

[0037] In the present invention, the structural units of R 01 and R 21 in chemical formula (I) are derived from the first soft monomer; the first soft monomer is an acrylate compound containing only one alkenyl group. In some preferred embodiments, the first soft monomer includes at least one of methyl acrylate, ethyl acrylate, tetrahydrofurfuryl acrylate, ethoxyethoxyethyl acrylate, or trimethylolpropane formal acrylate.

[0038] As a preferred embodiment, in the present invention, the solubility of the second soft monomer in water < 1 g / L; it can also be understood that the second soft monomer is insoluble in water. As a preferred embodiment, the T of the homopolymer of the second soft monomer g ≤ 25 °C.

[0039] In the present invention, R 02, R 22 The structural units of 22 are derived from the second soft monomer; the second soft monomer is another type of acrylate compound containing only one alkenyl group. In some preferred embodiments, the second soft monomer includes at least one of n-butyl acrylate, isooctyl acrylate, lauryl acrylate, or 2-phenoxyethyl acrylate.

[0040] As a preferred embodiment, in the present invention, the solubility of the first hard monomer in water is 10 g / L to 100 g / L, and the homopolymer of the first hard monomer is insoluble in water. As a preferred embodiment, the T of the homopolymer of the first hard monomer g ≥ 80 °C.

[0041] In the present invention, the R in chemical formula (I) 03 , R 23 The structural units are derived from the first hard monomer; as a preferred embodiment, the first hard monomer includes at least one of methyl methacrylate, acrylonitrile, or methacrylonitrile.

[0042] As a preferred embodiment, in the present invention, the homopolymer of the second hard monomer is insoluble in water, and there is no requirement for the water solubility of the second hard monomer itself. As a preferred embodiment, the T of the homopolymer of the second hard monomer g ≥ 80 °C.

[0043] In the present invention, the R in chemical formula (II) 31 , R 41 The structural units are derived from the second hard monomer; as a preferred embodiment, the second hard monomer includes at least one of methyl methacrylate, acrylonitrile, methacrylonitrile, styrene, or isobornyl methacrylate.

[0044] In the present invention, the R in chemical formula (I) 05 , R 25 The structural units are derived from the first crosslinking monomer; the first crosslinking monomer contains one alkenyl group and at least one active group capable of undergoing a crosslinking reaction. In some preferred embodiments, the active groups include carbonyl-activated methylene, epoxy group, hydroxymethyl, carbonyl-activated methylene, siloxanyl group, etc. As a more preferred embodiment, the first crosslinking monomer includes at least one of acetylacetoxyethyl methacrylate, glycidyl methacrylate, N-hydroxymethyl acrylamide, diacetone acrylamide, γ-methacryloxypropyltrimethoxysilane.

[0045] As a preferred embodiment, the second crosslinking monomer and the second hard monomer can be mutually soluble in any proportion.

[0046] In the present invention, the structural units of R in Chemical Formula (II) 32 and R 42 are derived from the second crosslinking monomer; the second crosslinking monomer contains at least two alkenyl groups. As a preferred embodiment, the second crosslinking monomer includes at least one of ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, 1,6 - hexanediol diacrylate, pentaerythritol tetraacrylate, or dipentaerythritol hexaacrylate.

[0047] In the present invention, the structural units of R in Chemical Formula (I) 04 and R 11 and R 24 are derived from the water - soluble electrolyte monomer; the water - soluble electrolyte monomer contains one alkenyl group and at least one carboxyl group or sulfonic acid group, and the carboxyl group or sulfonic acid group is neutralized with lithium hydroxide or sodium hydroxide to form the sodium salt and / or lithium salt of the carboxyl group or sulfonic acid group. As a preferred embodiment, the water - soluble electrolyte monomer includes the sodium salt and / or lithium salt of at least one of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, methallylsulfonic acid, 2 - acrylamide - 2 - methylpropanesulfonic acid, or styrenesulfonic acid.

[0048] It can be understood that the amount of the base used for neutralization should be equal to the molar amount of the acidic groups in the water - soluble electrolyte monomer. For example, when acrylic acid is used (there is only one acidic group in each molecule), the molar amount of the base (sodium hydroxide or lithium hydroxide) is equal to the molar amount of acrylic acid; when maleic acid is used (there are two acidic groups in each molecule), the molar amount of the base should be twice the molar amount of maleic acid.

[0049] As a preferred embodiment, the polymerization monomer includes the following monomer components by mass percentage: 15% - 30% of the first soft monomer, 30% - 50% of the second soft monomer, 8% - 11% of the first hard monomer, 9% - 12% of the second hard monomer, 2% - 5% of the first crosslinking monomer, 1% - 3% of the second crosslinking monomer, and 10% - 13% of the water - soluble electrolyte monomer.

[0050] As an alternative embodiment, the polymerizable monomers include the following monomer components by mass percentage: the first soft monomer 15%, 18%, 20%, 22%, 25%, 28%, 30%; the second soft monomer 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%; the first hard monomer 8%, 9%, 10%, 11%; the second hard monomer 9%, 10%, 11%, 12%; the first crosslinking monomer 2%, 3%, 4%, 5%; the second crosslinking monomer 1%, 1.5%, 2%, 2.5%, 3%; the water-soluble electrolyte monomer 10%, 11%, 12%, 13%. The mass percentages of the above monomer components can adopt any of the above-listed point values or a numerical range formed by any two point values. It should also be noted that the dosage of the water-soluble electrolyte monomer only calculates the dosage of the monomer itself and does not include the dosage of the base used to neutralize the monomer.

[0051] In the present invention, the preparation method of the polyacrylate emulsion specifically includes the following steps S1 to S3: S1. Prepare a pre-emulsion containing the first soft monomer, the second soft monomer, the first hard monomer, the water-soluble electrolyte monomer, an emulsifier and water, heat it up and add part of the initiator, and conduct the first heat preservation reaction; S2. Uniformly add the first crosslinking monomer to the reaction solution of S1, then add part of the initiator, and conduct the second heat preservation reaction; S3. Heat up to 75°C to 85°C, uniformly add a mixed solution containing the second hard monomer and the second crosslinking monomer to the reaction solution of S2, and at the same time uniformly add part of the initiator, and conduct the third heat preservation reaction to obtain the polyacrylate emulsion.

[0052] As a preferred embodiment, the emulsifier adopts an anionic-nonionic composite emulsifier, and its specific type can be selected or compounded conventionally in this field, and there is no strict limitation in the present invention; in some alternative embodiments, the emulsifier can be obtained by compounding an anionic emulsifier and a nonionic emulsifier, such as a compound of an anionic emulsifier such as sodium dodecyl sulfate and sodium dioctyl sulfosuccinate and a nonionic emulsifier such as OP-10 and ER-10; it can also be an amphoteric emulsifier having both anionic and nonionic characteristic structures in the molecular structure, such as SR-10 and SE-10.

[0053] As a preferred embodiment, the dosage of the emulsifier is 0.5 wt.% to 1.0 wt.% of the polymerizable monomers.

[0054] As a preferred embodiment, the initiator adopts a water-soluble persulfate, such as sodium persulfate, potassium persulfate or ammonium persulfate.

[0055] As a preferred embodiment, the dosage of the initiator is 0.5 wt.% to 1.0 wt.% of the polymerization monomer.

[0056] As a preferred embodiment, the usage amount of the water can be adaptively adjusted according to the solid content of the polyacrylate emulsion of the product.

[0057] As a preferred embodiment, the preparation method of the present invention is carried out in a protective gas environment; before step S1, a protective gas is introduced into the device containing the pre-emulsion to remove the oxygen in the reaction device and ensure that the reaction is carried out under an anaerobic state.

[0058] As a preferred embodiment, the temperature for heating up in step S1 is 50°C to 70°C, and more preferably 60°C to 70°C.

[0059] As a preferred embodiment, the dosage of the initiator in step S1 is equivalent to 0.5 wt.% to 1.0 wt.% of the total amount of the first soft monomer, the second soft monomer, the first hard monomer, and the water-soluble electrolyte monomer.

[0060] As a preferred embodiment, the time for the first heat preservation reaction in step S1 is 30 min to 60 min.

[0061] As a preferred embodiment, the feeding time of the first crosslinking monomer in step S2 is 30 min to 90 min; it should be noted that in S2, after the feeding of the first crosslinking monomer is completed, the initiator is added.

[0062] As a preferred embodiment, the dosage of the initiator in step S2 is equivalent to 0.5 wt.% to 1.0 wt.% of the first crosslinking monomer.

[0063] As a preferred embodiment, the time for the second heat preservation reaction in step S2 is ≥6 h, preferably 6 h to 10 h.

[0064] As a preferred embodiment, the feeding time of the mixed liquid in step S3 is 3.5 h to 4.5 h; it should be noted that in step S3, the feeding of the mixed liquid and the initiator starts and ends simultaneously, and the specific feeding rate is adaptively adjusted according to the dosage and time of the reaction materials.

[0065] As a preferred embodiment, the dosage of the initiator in step S3 is equivalent to 0.5 wt.% to 1.0 wt.% of the total amount of the second hard monomer and the second crosslinking monomer.

[0066] As a preferred embodiment, the time of the third heat preservation reaction in step S3 is ≥ 2 h, preferably 2 h to 5 h.

[0067] The third aspect of the present invention lies in providing a secondary battery, the surface of whose separator includes the polyacrylate emulsion as described in the first aspect. It can be understood that the secondary battery should include positive and negative electrodes, electrolyte, separator, and other necessary or unnecessary functional elements or packaging components, etc., and those skilled in the art can make any selection and combination thereof.

[0068] For the polyacrylate emulsion, it is usually used as a functional coating layer of the separator, mainly for bonding the separator and the electrode, so as to improve the hardness of the battery cell and inhibit the occurrence of defects such as battery cell deformation and separator wrinkling. When the surface of the separator of the secondary battery contains the polyacrylate emulsion, the secondary battery can be used as an embodiment of this aspect.

[0069] As a preferred embodiment, the surface of the separator of the secondary battery includes the polyacrylate emulsion, and the surface of the separator does not include any other bonding functional components or wetting functional components; that is, when using the polyacrylate emulsion, even without using conventional adhesives and / or wetting agents, the emulsion of the present invention can still be normally coated on the surface of the separator, and the obtained separator will not deteriorate in various key properties, and even has better performance in terms of adhesion and / or air permeability.

[0070] The fourth aspect of the present invention lies in providing a method for preparing the secondary battery as described in the third aspect, which mainly includes the following steps: coating the polyacrylate emulsion on both sides of the separator, and then assembling the positive electrode, separator and negative electrode to obtain a battery cell precursor; applying temperature and pressure to the battery cell precursor, and then successively performing housing assembly, electrolyte injection and encapsulation to obtain the secondary battery.

[0071] As a preferred embodiment, the present invention does not impose any restrictions on the type of the separator, which can be a conventional polymer film such as polyethylene and polypropylene, or a modified film containing a ceramic coating, a refractory coating, etc.; the polyacrylate emulsion of the present invention can be directly coated on the surface of the base film, or on the surface of any modified coating.

[0072] As a preferred embodiment, the temperature is 25°C to 85°C, preferably 25°C to 45°C; the pressure is 1 MPa to 6 MPa, preferably 1 MPa to 3 MPa.

[0073] As a preferred embodiment, calculated on a single side, the coating amount of the polyacrylate emulsion is 0.3 g / m 2 ~1.5 g / m 2 , preferably 0.5 g / m2 ~1.0 g / m 2 。

[0074] Example S1. Charge the first soft monomer, the second soft monomer, the first hard monomer, the water-soluble electrolyte monomer (previously neutralized with sodium hydroxide), the emulsifier (SR-10), and deionized water into the reaction device, and then introduce nitrogen to remove the oxygen in the reaction device; among them, the dosage of the emulsifier is 0.75 wt.% of the total amount of monomers, and the amount of water is controlled so that the solid content of the emulsion is 25%; S2. Stir S1 well to obtain a pre-emulsion; S3. Keep stirring and heat the reaction device to 60 - 70 °C; S4. Add a part of the initiator, and the dosage is 0.75 wt.% of the total amount of the first soft monomer, the second soft monomer, the first hard monomer, and the water-soluble electrolyte monomer; S5. After reacting for 40 min, use a uniform feeding device to uniformly add the first crosslinking monomer into the reaction device, and control the feeding time to be 60 min; S6. After the addition of the first crosslinking monomer is completed, add a part of the initiator, and the dosage is 0.75 wt.% of the first crosslinking monomer, and then continue to keep the temperature for reaction for 8 h; S7. Heat the reaction device to 80 °C; S8. Mix the second hard monomer and the second crosslinking monomer, stir well to obtain a monomer mixture, and then use a uniform feeding device to uniformly add the monomer mixture into the reaction device, and control the feeding time to be 4 h; S9. During the addition of the above monomer mixture, synchronously and uniformly add the remaining initiator into the reaction device, and the dosage is 0.75 wt.% of the above monomer mixture; S10. After the addition is completed, continue to keep the temperature for reaction for 4 h, and then cool and collect the product to obtain the polyacrylate emulsion of this example.

[0075] As shown in Table 1 below, the selection of each monomer in Examples 1 - 5 is provided, and the corresponding mass percentage dosage (unit: wt.%) is provided after the compound selected for the monomer.

[0076] Table 1

[0077] Comparative Example: It is basically the same as Example 1, except that: a certain feature is replaced or modified to evaluate the role of this feature in the preparation of the product. Table 2 shows the differences between Comparative Examples 1 - 22 and Example 1.

[0078] Table 2

[0079] In Comparative Examples 8, 10, and 12, since a certain monomer was removed, the dosages of all the other monomers were increased proportionally to maintain the total monomer dosage at a constant 100 wt.%. For example, in Comparative Example 8, the dosage of the first crosslinking monomer was 4 wt.%. After removing the first crosslinking monomer, the preset total monomer dosage was 96 wt.%. To ensure that the actual total monomer dosage in Comparative Example 8 was 100 wt.%, the dosages of all the monomers other than the first crosslinking monomer would be increased by a factor of (100 / 96 =) 1.04. The same applies to Comparative Example 10 and Comparative Example 12.

[0080] Test Examples (1) The emulsion prepared in Example 1 was subjected to particle size testing and microscopic testing; it was tested using a laser particle size analyzer, and the parameter results of D10, D25, D50, D75, D90, D97, D99, volume specific surface area, weight specific surface area, residual, and Span were characterized, as Figure 1 shown; in addition, SED detection was performed to obtain an SED pattern as Figure 2 shown.

[0081] (2) The samples prepared in each of the Examples and Comparative Examples were tested, and the corresponding viscosities, D50, and Span were recorded; among them, the viscosity was tested at 25 °C using a rotational viscometer, and D50 and Span were tested using a laser particle size analyzer (the refractive index of the emulsion particles was set to 1.52, the solvent was water, and the refractive index was set to 1.33).

[0082] It should be noted that not all of Comparative Examples 1 to 22 could prepare stable and testable emulsion samples. The results of this test are recorded in Table 3 below.

[0083] Table 3

[0084] Based on Table 3, the following significant conclusions were obtained after analysis: 1) Combining Examples 1 to 5 and Comparative Examples 1 to 7, it can be seen that the water solubility of the monomers has a significant impact on the particle size and distribution width of the emulsion, and even on the stability of the emulsion itself; further, the water solubility of the monomers will affect the nucleation mechanism of emulsion polymerization, and thus affect whether the emulsion particles can grow to the target particle size, whether they can maintain the uniformity of particle size, and even whether the emulsion particles can be stably suspended in water.

[0085] 2) It can be seen from Comparative Examples 8, 9, 17, 18, and 19 that the selection of the first crosslinking monomer and its feeding method also affect the particle size and distribution width of the emulsion, as well as the stability of the emulsion. Through exploration, it is found that the first crosslinking monomer in the present invention contains a carbon-carbon double bond and an active group, enabling it to form an active layer on the surface of the core layer emulsion particles. On the one hand, this active layer can be transformed into a crosslinked layer on the surface of the emulsion particles through self-crosslinking, making the emulsion particles stable and dense. On the other hand, it can also promote the ripening of the emulsion, causing the particle size of the emulsion to further increase while narrowing the distribution range.

[0086] Among them, the so-called "ripening" means that the difference in particle size of the emulsion particles causes a difference in surface energy, which makes the smaller particles tend to migrate and adsorb onto the surface of the larger particles. The ripening process reduces the number of small particles and further increases the size of the large particles. The first crosslinking monomer makes the surface of the core layer emulsion carry active groups, which makes the ripening process easier to proceed under the action of the mutual reaction and crosslinking of the active groups.

[0087] Combined with the test results in Table 3, it can be seen that the first crosslinking monomer should not be removed (such as Comparative Example 8), nor should it be replaced with a type containing multiple carbon-carbon double bonds but no active groups (such as Comparative Example 9). If the feeding rate of the first crosslinking monomer is too fast (such as Comparative Example 17), it is easy to cause too fast crosslinking rate, resulting in emulsion instability; conversely, if the feeding rate is too slow (such as Comparative Example 18), some monomers are difficult to adsorb onto the surface of the emulsion particles, resulting in poor crosslinking effect. If the holding time after the feeding of the first crosslinking monomer is insufficient (such as Comparative Example 19), it will lead to insufficient crosslinking and ripening.

[0088] 3) In the present invention, the second hard monomer and the second crosslinking monomer together constitute the formulation of the shell layer, whose function is to form a hard and dense shell on the surface of the emulsion particles to make the particles more stable, and it does not involve the ripening of the emulsion particles (because the ripening has been completed during the synthesis process of the core layer emulsion). It can be seen from Comparative Examples 10, 11, 20, 21, and 22 that removing the second crosslinking monomer (such as Comparative Example 10) or replacing the second crosslinking monomer with a type containing a carbon-carbon double bond and an active group will result in an insufficiently dense shell layer and poor particle stability, manifested as a decrease in particle size and a broadening of the distribution. Improper feeding rate (such as Comparative Examples 20 and 21) or insufficient holding reaction time after feeding (such as Comparative Example 22) will lead to problems such as emulsion instability, poor shell layer coating effect, and incomplete reaction.

[0089] 4) The role of the water-soluble electrolyte monomer in the present invention is as follows: on the one hand, it regulates the water-oil balance of the emulsion particles, and on the other hand, it forms an electrostatic layer on the particle surface, endowing the particles with a certain electrostatic repulsion to maintain the stability of the emulsion. It can be seen from Comparative Examples 12 to 14 that removing the water-soluble electrolyte monomer (such as Comparative Example 12) or replacing it with a type that cannot be ionized in water (such as Comparative Example 13) will result in poor emulsion stability. And adding the reaction without neutralization (such as Comparative Example 14) mainly affects the emulsion stability from the aspect of reactivity ratios; the un-neutralized water-soluble electrolyte monomer tends to self-polymerize rather than copolymerize with other monomers, so it is not conducive to the water-oil balance of the emulsion particles.

[0090] 5) It can be seen from Comparative Example 15 that a pure anionic emulsifier is not conducive to the growth of the emulsion particle size; and it can be seen from Comparative Example 16 that a pure non-ionic emulsifier is difficult to maintain the stability of the emulsion; therefore, an anionic-non-ionic composite emulsifier is preferably used in the present invention.

[0091] (III) The emulsion products prepared in Example 1, Comparative Example 1, and Comparative Example 3 are used as Application Example 1, Application Example 2, and Application Example 3 respectively; in addition, the 4.7 μm-level emulsion polymer microsphere material product prepared in Example 1 of CN202411110981.5 is used as Application Example 4. Each application example is used for the preparation of the coated diaphragm of the secondary battery to evaluate the application performance.

[0092] For Application Examples 1 to 4, the preparation method of the coated diaphragm of the secondary battery is as follows: 1) Prepare the spraying adhesive solution: Take the samples of each application example, add a binder and a wetting agent, and then add deionized water to dilute to a solid content of 8%, and stir well; among them, the binder is a commercially available aqueous polyacrylate emulsion-type binder, and the addition amount is 3 wt.% of the application example sample based on the total active ingredients; the wetting agent is a polyoxyethylene ether type, and the addition amount is 3 wt.% of the application example sample based on the total active ingredients; 2) Use the spray coating method to evenly coat the adhesive solution on the surface of the secondary battery diaphragm (polyethylene film, 9 μm), and control the coating amount to be 0.4 g / m 2 ; After coating, dry the diaphragm at 60 °C to obtain the coated diaphragm of the secondary battery.

[0093] Furthermore, the emulsion product prepared in Example 1 is still used as Application Examples 5 to 7, and the 4.7 μm-level emulsion polymer microsphere material product prepared in Example 1 of CN202411110981.5 is used as Application Examples 8 to 10.

[0094] For Application Examples 5 and 8, the difference from the preparation method of the coated diaphragm of the secondary battery in Application Examples 1 to 4 above is that: the addition and use of the binder are cancelled.

[0095] For Application Examples 6 and 9, the difference from the preparation method of the coated separator of the secondary battery in the above Application Examples 1 to 4 is that the addition and use of the wetting agent are cancelled.

[0096] For Application Examples 7 and 10, the difference from the preparation method of the coated separator of the secondary battery in the above Application Examples 1 to 4 is that the addition and use of both the binder and the wetting agent are cancelled.

[0097] The following application performance tests were carried out on each coated separator: 1) Dry adhesion: Take the coated separator and cut it into a spline with a width of 25 mm, a total length of 100 mm, and a coated section length of 60 mm; take another graphite negative electrode sheet of the secondary battery and cut it into a spline with a width of 27 mm and a length of 100 mm; stack the splines of the coated separator and the graphite negative electrode sheet, with the coated surface of the separator coinciding with the graphite coated surface of the electrode sheet; then use a precision hot press to apply pressure for 10 s under the conditions of 25 °C and 4 MPa to bond the separator and the electrode sheet to obtain a spline for adhesion test, and then use an electronic tensile testing machine to test its peel strength.

[0098] 2) Wet adhesion: Take the spline for adhesion test prepared in the above step 1), soak it in the secondary battery electrolyte at 60 °C for 24 h, and then use an electronic tensile testing machine to test its peel strength. This application performance is used to evaluate the influence of electrolyte immersion on the adhesion of the sample; among them, the solvent ratio of the electrolyte is EC:DEC:DMC = 1:1:1, the electrolyte is lithium hexafluorophosphate, and the concentration is 1 mol / L.

[0099] 3) Thickness before and after pressing of the coated layer: Take the coated separator and use a thickness gauge to measure its total thickness before pressing. Then use a precision hot press to apply pressure for 10 s under the conditions of 25 °C and 3 MPa, and measure its total thickness after pressing. After subtracting the base film thickness respectively, the thickness before pressing and the thickness after pressing of the coated layer are obtained. The smaller the thickness of the coated layer (especially the thickness after pressing), the more beneficial it is to improve the energy density of the battery cell.

[0100] 4) Increase in air permeability: Take the coated separator and use a Gurley air permeability tester to test its air permeability value; take another uncoated blank separator and test its air permeability value in the same way; the difference between the two is the increase in air permeability of the coated separator. The smaller the increase in air permeability, the better the ion permeability of the separator.

[0101] Record the results of the above various application performance tests in Table 4, and each result is expressed by the 95% confidence interval of the mean value.

[0102] Table 4

[0103] As can be seen from Table 4, Application Example 1 obtained based on the polyacrylate emulsion of the present invention has comprehensive advantages in terms of adhesion, thickness, and air permeability, and has a significant improvement in terms of improving the energy density of secondary batteries and even enhancing the electrochemical performance. At the same time, the distribution range of the mean values of the test results of Application Example 1 is narrower, indicating better coating consistency of the emulsion.

[0104] Furthermore, from the test results of Application Example 1 and Application Examples 5 to 7 in Table 4, it can be seen that for the emulsion product of Example 1 of the present invention, if the binder is removed when preparing the coating adhesive, the obtained adhesive can still be normally used for coating, and no significant changes are observed in the adhesion and coating thickness of the obtained coated separator, and the increase in air permeability decreases (i.e., the ion permeability increases); at the same time, if the wetting agent is removed when preparing the coating adhesive, the obtained adhesive can still be normally used for coating, no significant changes are observed in the coating thickness of the obtained coated separator, the adhesion increases, and the increase in air permeability decreases.

[0105] From the test results of Application Example 1 and Application Examples 8 to 10 in Table 4, it can be seen that when using the existing emulsion product to prepare the coating adhesive, it cannot be used independently without the binder or the wetting agent, which will cause the coating to fall off and cannot be tested and used.

[0106] Although the present invention has been illustrated and described with reference to specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it; those of ordinary skill in the art should understand that: without departing from the spirit and scope of the present invention, the technical solutions described in the foregoing embodiments can be modified, or some or all of the technical features can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such replacements and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A polyacrylate emulsion, characterized in that, The polyester emulsion particles in the polyacrylate emulsion have a core-shell structure, with a D50 of 500 nm to 700 nm and a Span of 0.5 to 0.6; The core layer of the polyester emulsion particles includes a polyester main chain, and the shell layer includes branches grafted to the polyester main chain; The structure of the polyester main chain satisfies the following chemical formula (Ⅰ), and the structure of the branches satisfies the following chemical formula (II): （Ⅰ）； (II); Wherein, a, b, c, d, e, x, and y are all positive integers, and 100 ≤ a + b + c + d + e ≤ 1000, 30 ≤ x + y ≤ 300; R 01 、R 02 、R 03 、R 04 、R 05 independently includes at least one of a hydrogen atom, a methyl group or a carboxymethyl group, and at least one of R 01 、R 02 、R 03 、R 04 、R 05 is a hydrogen atom; R 11 comprising at least one of a hydrogen atom or a carboxyl group; R 21 comprises at least one of methyl, ethyl or alkoxy, and the number of carbon atoms in the alkoxy is 2 to 5 times the number of oxygen atoms; R 22 comprises at least one of a phenoxyalkyl group and an alkyl group having 4 to 12 carbon atoms; R 23 comprises at least one of a methyl ester group or a nitrile group; R 24 including at least one of a sodium salt or a lithium salt of a carboxyl group and a sodium salt or a lithium salt of a sulfonic acid group; R 25 comprising at least one of hydroxymethyl, epoxy group, methoxysilyl group or methylene group activated by bilateral carbonyl groups; R 31 、R 32 each independently includes at least one of a hydrogen atom or a methyl group; R 41 comprises at least one of a benzene ring, a methyl ester group, a cycloalkane ester group or a nitrile group; R 42 including an ester group, an alkyl group having 2 to 6 carbon atoms, and 1 to 5 carbon-carbon double bonds, and a carbon-oxygen double bond in the ester group and the carbon-carbon double bonds form a conjugated structure.

2. The polyacrylate emulsion according to claim 1, wherein The polyester emulsion particles satisfy the following characteristics (a) to (d): (a) When R 21 is methyl or ethyl, R 01 is a hydrogen atom; (b)When R 03 is a hydrogen atom, R 23 is a nitrile group; (c) When R 11 is a carboxyl group, R 24 is a carboxyl group; (d)R 24 It also includes one or more of an alkyl group, a phenyl group or an amide group.

3. The preparation method of the polyacrylate emulsion according to claim 1 or 2, characterized in that, Including the following steps: S1. Prepare a pre-emulsion containing a first soft monomer, a second soft monomer, a first hard monomer, a water-soluble electrolyte monomer, an emulsifier, and water, heat it up and add part of the initiator, and carry out a first heat preservation reaction; S2. Uniformly add a first crosslinking monomer to the reaction solution of S1, and then add part of the initiator, and carry out a second heat preservation reaction; S3. Heat up to 75 °C to 85 °C, uniformly add a mixture containing a second hard monomer and a second crosslinking monomer to the reaction solution of S2, and at the same time uniformly add the remaining initiator, and carry out a third heat preservation reaction to obtain the polyacrylate emulsion.

4. The preparation method of the polyacrylate emulsion according to claim 3, characterized in that, By mass percentage, the dosages of each polymerization monomer include: The first soft monomer 15% - 30%, the second soft monomer 30% - 50%, the first hard monomer 8% - 11%, the second hard monomer 9% - 12%, the first crosslinking monomer 2% - 5%, the second crosslinking monomer 1% - 3%, and the water-soluble electrolyte monomer 10% - 13%.

5. The preparation method of the polyacrylate emulsion according to claim 3, characterized in that, Each polymerization monomer satisfies at least one of the following characteristics (A) to (G): (A) The first soft monomer includes at least one of methyl acrylate, ethyl acrylate, tetrahydrofurfuryl acrylate, ethoxyethoxyethyl acrylate, or trimethylolpropane formal acrylate; (B) The second soft monomer includes at least one of n-butyl acrylate, isooctyl acrylate, lauryl acrylate, or 2-phenoxyethyl acrylate; (C) The first hard monomer includes at least one of methyl methacrylate, acrylonitrile, or methacrylonitrile; (D) The second hard monomer includes at least one of methyl methacrylate, acrylonitrile, methacrylonitrile, styrene, or isobornyl methacrylate; (E) The first crosslinking monomer includes at least one of acetylacetoxyethyl methacrylate, glycidyl methacrylate, N-methylolacrylamide, diacetone acrylamide, or γ-methacryloxypropyltrimethoxysilane; (F) The second crosslinking monomer includes at least one of ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, 1,6-hexanediol diacrylate, pentaerythritol tetraacrylate, or dipentaerythritol hexaacrylate; (G) The water-soluble electrolyte monomer includes the sodium salt or lithium salt of at least one of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, methallylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, or p-styrenesulfonic acid.

6. The preparation method of the polyacrylate emulsion according to claim 3, characterized in that, The dosage of the emulsifier is 0.5 wt.% to 1.0 wt.% of the total amount of the polymerization monomers; And / or, the dosage of the initiator is 0.5 wt.% to 1.0 wt.% of the total amount of the polymerization monomers; And / or, the emulsifier is an anionic-nonionic composite emulsifier, and the initiator is a water-soluble persulfate.

7. The preparation method of the polyacrylate emulsion according to claim 3, characterized in that, The preparation method includes at least one of the following features (1) to (5): (1) The time of the first heat preservation reaction in step S1 is 30 min to 60 min; (2) The feeding time of the first crosslinking monomer in step S2 is 30 min to 90 min; (3) The time of the second heat preservation reaction in step S2 is ≥ 6 h; (4) The feeding time of the mixed solution in step S3 is 3.5 h to 4.5 h; (5) The time of the third heat preservation reaction in step S3 is ≥ 2 h.

8. A secondary battery, characterized in that, The surface of the separator of the secondary battery includes the polyacrylate emulsion as described in claim 1 or 2.

9. The manufacturing method of the secondary battery according to claim 8, wherein It includes the following steps: Coating the polyacrylate emulsion on both sides of the separator respectively, then assembling the positive electrode, the separator and the negative electrode to obtain a pre-cell; applying temperature and pressure to the pre-cell, and then successively performing housing assembly, electrolyte injection and encapsulation to obtain the secondary battery; Wherein, the temperature is 25 °C to 85 °C, and the pressure is 1 MPa to 6 MPa.

10. The method for preparing a secondary battery according to claim 9, characterized in that, Calculated on a single side, the coating amount of the polyacrylate emulsion is 0.3 g / m 2 ~1.5 g / m 2 .

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