A preparation method of cross-linked polyolefin diaphragm

Abstract

The present invention provides a cross-linked polyolefin separator and a preparation method thereof. The cross-linked polyolefin separator includes an upper surface layer (A1) and a center layer (B), forming an A1 / B structure, wherein the upper surface layer (A1) includes a first polyolefin with a molecular weight of 500,000 to 1,000,000 and a second polyolefin with a molecular weight of 300,000 to 800,000, which are copolymerized; and the center layer (B) includes a third polyolefin with a molecular weight of 500,000 to 1,000,000. The gel content of the center layer (B) is 0% to 65%. The preparation method comprises the following steps: a melt extrusion step, an annealing step, a stretching step, a coating step, a drying step, and an ultraviolet lamp irradiation step. By introducing a certain amount of polyethylene into the surface layer of the base film and adding a certain amount of catalyst to the cross-linking agent, the cross-linking degree of polypropylene can be effectively increased, the degradation of polypropylene can be reduced, and the transverse tensile strength of the dry-process separator can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery separator preparation, in particular to a preparation method of a cross-linked polyolefin separator. Background Art

[0002] As we all know, the diaphragm is an important component of lithium-ion batteries. The microporous structure, physical and chemical properties, and thermodynamic properties of the diaphragm are closely related to the overall performance of the battery. The dry stretching process has received a lot of attention and research due to its simple process and low cost. Among them, the dry uniaxial stretching diaphragm has been widely used in the fields of energy storage and low-end power batteries due to its uniform pore size distribution and good thermal shrinkage performance. Since the uniaxial stretching process only involves longitudinal stretching during the stretching process, the micropores prepared are slit-like structures, so the transverse strength is very small. During the assembly and use of the battery, it is easy to be torn by transverse mechanical forces, or because the lithium dendrites formed during the charge and discharge process pierce the diaphragm, causing the battery to short-circuit or micro-short-circuit, posing a safety hazard.

[0003] Crosslinking polyolefins improves their tensile strength, impact strength, and heat resistance. Crosslinking can also improve the poor transverse strength of polypropylene separators. Commonly used crosslinking techniques include peroxide crosslinking, silane grafting, and radiation crosslinking. However, due to the presence of tertiary carbon atoms in polypropylene, free radicals generated by the initiator during the crosslinking process attack the tertiary carbon atoms, causing β-bond chain scission and degradation. This reduces the molecular weight of the polypropylene, thereby affecting its overall performance. Summary of the Invention

[0004] The purpose of the present invention is to improve the cross-linking degree of polypropylene and thus improve the transverse tensile strength of the dry-process diaphragm.

[0005] To achieve the above-mentioned purpose, the present invention provides a cross-linked polyolefin diaphragm, characterized in that it includes an upper surface layer (A1), a center layer (B) and a lower surface layer (A2), forming an A1 / B / A2 three-layer co-extrusion structure, wherein the upper surface layer (A1) includes a first polyolefin with a molecular weight of 500,000-1,000,000 and a second polyolefin with a molecular weight of 300,000-800,000, which are copolymerized; the lower surface layer (A2) includes a first polyolefin with a molecular weight of 500,000-1,000,000 and a second polyolefin with a molecular weight of 300,000-800,000, which are copolymerized; and the center layer (B) includes a third polyolefin with a molecular weight of 500,000-1,000,000.

[0006] Preferably, the upper surface layer (A1) and the lower surface layer (A2) have the same composition.

[0007] Preferably, the weight ratio of the first polyolefin to the second polyolefin of the upper surface layer (A1) is (85-95wt%):(5-15wt%); the weight ratio of the first polyolefin to the second polyolefin of the lower surface layer (A2) is (85-95wt%):(5-15wt%).

[0008] Preferably, the thickness ratio of the upper surface layer (A1), the center layer (B) and the lower surface layer (A2) is (15:70:15) to (30:40:30).

[0009] The present invention provides another cross-linked polyolefin separator, characterized in that the central layer (B) comprises a third polyolefin with a molecular weight of 500,000-1,000,000, wherein the gel content of the central layer (B) is 0% to 65%.

[0010] The present invention further provides a method for preparing a cross-linked polyolefin separator, comprising the following steps:

[0011] (S1) Melt extrusion step: A third polyolefin having a molecular weight of 500,000 to 1,000,000 is fed into a melt pump B as the material for the center layer (B), and extruded to obtain a diaphragm.

[0012] Preferably, (S1) further includes: 85-95wt% of a first polyolefin with a molecular weight of 500,000-1,000,000 and 5-15wt% of a second polyolefin with a molecular weight of 300,000-800,000 are uniformly mixed as the material of the upper surface layer (A1) or the lower surface layer (A2), and the mixture is fed into melt pump A, and extruded separately with melt pump B to obtain a diaphragm having a three-layer co-extruded structure of A1 / B / A2.

[0013] (S2) Annealing step: annealing the membrane obtained in (S1).

[0014] (S3) Stretching step: The diaphragm obtained in (S2) is subjected to cold stretching and hot stretching.

[0015] (S4) Coating step: A crosslinking agent solution is separately prepared and coated on the diaphragm obtained in (S3) using a coating machine.

[0016] (S5) UV lamp irradiation step: The diaphragm coated in (S4) is placed under a UV lamp for a period of time to obtain a cross-linked polyolefin diaphragm.

[0017] Preferably, a drying step (S6) is further included before or after (S5).

[0018] Preferably, in (S4), the crosslinker solution is prepared by dissolving a certain proportion of a photoinitiator, a crosslinker, and a catalyst in a solvent.

[0019] Preferably, the catalyst includes a platinum-based catalyst, a tin-based catalyst, a chloroplatinic acid composite catalyst, a Grubbs catalyst, a ruthenium-based catalyst, a rhodium-based catalyst, a zirconium-based catalyst, a potassium isooctanoate catalyst, a bismuth catalyst, a palladium catalyst, a nickel catalyst, a zinc carboxylate, an iron carboxylate, a lead carboxylate, or a cobalt carboxylate.

[0020] Preferably, in (S4), the ratio of the photoinitiator, the crosslinking agent, and the catalyst is 4:10:(1-3).

[0021] Preferably, in (S5), the UV lamp irradiation time is 10-300 seconds.

[0022] The beneficial effect of the present invention is that by introducing a certain amount of polyethylene into the surface layer of the base film,

[0023] At the same time, adding a certain amount of catalyst to the cross-linking agent can effectively improve the cross-linking degree of polypropylene.

[0024] Reduce the degradation of polypropylene and thus improve the transverse tensile strength of dry-process diaphragms. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram illustrating the structure of the present invention.

[0026] Figure 2 It is a flow chart illustrating the preparation method of the present invention.

[0027] Figure 3A and Figure 3B FIG1 is a flow chart illustrating another preparation method of the present invention. DETAILED DESCRIPTION

[0028] In order to make the above and / or other purposes, effects and features of the present invention more obvious and understandable, the following is a special description of the present invention.

[0029] The preferred embodiment is described in detail below:

[0030] See also Figure 1The present invention provides a cross-linked polyolefin diaphragm, characterized in that it comprises an upper surface layer (A1), a center layer (B) and a lower surface layer (A2), forming an A1 / B / A2 three-layer co-extruded structure, wherein the upper surface layer (A1) comprises a first polyolefin with a molecular weight of 500,000-1,000,000 and a second polyolefin with a molecular weight of 300,000-800,000, preferably, the upper surface layer (A1) comprises a first polyolefin with a molecular weight of 500,000-600,000 and a second polyolefin with a molecular weight of 350,000-700,000. The lower surface layer (A2) is formed by copolymerizing a first polyolefin having a molecular weight of 500,000-1,000,000 and a second polyolefin having a molecular weight of 300,000-800,000. Preferably, the lower surface layer (A2) is formed by copolymerizing a first polyolefin having a molecular weight of 500,000-600,000 and a second polyolefin having a molecular weight of 350,000-700,000. The center layer (B) includes a third polyolefin having a molecular weight of 500,000-1,000,000. Preferably, the center layer (B) includes a third polyolefin having a molecular weight of 500,000-600,000.

[0031] In some embodiments, the upper surface layer (A1) and the lower surface layer (A2) have the same composition.

[0032] In some embodiments, the weight ratio of the first polyolefin to the second polyolefin of the upper surface layer (A1) is (85-95wt%):(5-15wt%); the weight ratio of the first polyolefin to the second polyolefin of the lower surface layer (A2) is (85-95wt%):(5-15wt%).

[0033] In some embodiments, the thickness ratio of the upper surface layer (A1), the center layer (B), and the lower surface layer (A2) is (15:70:15) to (30:40:30).

[0034] The present invention further provides another cross-linked polyolefin separator, characterized in that the central layer (B) comprises a third polyolefin having a molecular weight of 500,000-1,000,000, wherein the gel content of the central layer (B) is 0% to 65%.

[0035] See also Figure 2 The present invention provides a method for preparing a cross-linked polyolefin membrane, comprising:

[0036] Next steps:

[0037] (S1) Melt extrusion step: Polypropylene with a molecular weight of 500,000 to 1,000,000 is fed into a melt pump B as an intermediate layer, and the extruder enters the die head to obtain a diaphragm.

[0038] (S2) Annealing step: annealing the membrane obtained in (S1).

[0039] (S3) Stretching step: The diaphragm obtained in (S2) is subjected to cold stretching and hot stretching.

[0040] (S4) Coating step: A crosslinker solution is separately prepared and coated on the diaphragm obtained in (S3) using a coating machine. The crosslinker solution is prepared by dissolving a certain proportion of a photoinitiator, a crosslinker, and a catalyst in an organic solvent.

[0041] (S5) UV lamp irradiation step: the diaphragm after drying in (S4) is placed under UV lamp for a period of time.

[0042] A cross-linked polyolefin separator was obtained.

[0043] In some preferred embodiments, (S1) further comprises adding 85-95 wt% of a 50-100

[0044] Polypropylene with a molecular weight of 10,000-15wt% and polyethylene with a molecular weight of 300,000-800,000 are evenly mixed in a mixer as the material for the upper surface layer (A1) or the lower surface layer (A2) and fed into melt pump A, and polypropylene with a molecular weight of 500,000-1,000,000 is fed into melt pump B as the middle layer. The extruder enters the die head and is divided into two flow paths, thereby obtaining a diaphragm with an A1 / B / A2 three-layer co-extruded structure, wherein the thickness of the diaphragm with the A1 / B / A2 three-layer co-extruded structure is (15:70:15) to (30:40:30).

[0045] See also Figure 3A and Figure 3B , before or after (S5), a drying step (S6) is further included.

[0046] In some embodiments, in (S1), the melt extrusion temperature of melt pump A is 180-230°C,

[0047] The melt extrusion temperature of melt pump B is 200-240°C, the die temperature is 220-250°C, and the cooling roller speed is 60-100 m / min; in (S2), the annealing temperature is 120-160°C; in (S3), the cold drawing temperature is 50-130°C, the hot drawing temperature is 130-160°C, and the stretch ratio is 1.5-3; in (S5), the UV lamp irradiation time is 10-300 seconds, preferably 150 seconds, and the irradiation intensity is 1000-5000 mW / cm 2 , preferably 3000mW / cm 2 ; (S6), the oven temperature is 50-100°C.

[0048] In some embodiments, in (S4), the ratio of the photoinitiator, crosslinker, and catalyst is 4:10:(1-3).

[0049] In some embodiments, the photoinitiator includes but is not limited to aromatic ketone initiators, thioxanthone derivatives, benzil derivatives, benzoin alkyl ethers, benzoin derivatives, acylphosphine oxides, alkyl phenones, benzophenones, acetophenones, anthraquinones, thioxanthones, thiol compounds, fused-ring aromatic hydrocarbons, polysilanes, acylphosphonates, aromatic acylphosphine oxides, aromatic acylphosphonates, diketones, tertiary amine photoinitiator systems, or azos.

[0050] In some preferred embodiments, the photoinitiator includes benzoin, benzoin dimethyl ether, benzoin diethyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin butyl ether, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzophenone, diphenylacetophenone, acetophenone, α-hydroxyalkyl benzophenone, α-aminoalkyl benzophenone, bisbenzoylphenylphosphine oxide, 2,4-dihydroxybenzophenone, Michler's ketone, 4,4'-bisdiethylaminobenzophenone, 1-hydroxycyclohexylbenzophenone, 2,2-dimethoxy-2-phenylacetophenone, thiopropoxythioxanthone, isopropylthioxanthone, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, 2-hydroxy-2-methyl-phenylpropiophenone, 2,4,6-trimethylbenzoyldiphenyl 1-Hydroxy-1-[4-(methylthio)phenyl]titanium ocene, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 1,1-dichloroacetophenone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 1-chloroanthraquinone, 2,4-diethylthioxanthone, 2-chlorothioxanthone, 2,4-diisopropylthioxanthone, 2-thiobenzimidazole, 2,4,6-tris-s-triazine, pentafluorophenyl, azobisisobutylamidine hydrochloride, 2-hydroxy-methylphenylpropane-1-one, diaryliodonium salt, triaryliodonium salt, alkyliodonium salt, cumylferrocenium hexafluorophosphate, or cumylferrocenium hexafluorophosphate.

[0051] In some embodiments, the crosslinking agent includes polyamines, polyols, polyacids, polyacid anhydrides, silicones, benzenesulfonic acids, acrylates, organic peroxides, metal organic compounds, oxazoles, thiazoles, imidazoles, oxazolines, triazines, isocyanates, polyvinylbenzene, polycarbodiimides, silanes, di-tert-butyl peroxide, or urea compounds, and is not limited thereto.

[0052] In some preferred embodiments, the crosslinking agent includes divinylbenzene (DVB), triallyl isocyanurate (TAIC), trimethylolpropane trimethacrylate (TMPTMA), trimethylolpropane triacrylate (TMPTA), triallyl cyanurate, allyl methacrylate (AMA), triallyl cyanurate (TAC), tris(2-hydroxyethyl)isocyanuric acid triacrylate (THE I CTA), tert-butyl peroxybenzoate (TBPB), tert-butyl peroxy 3,5,5-trimethylhexanoate (TBP IN), polyethylene glycol dimethacrylate (EGDMA), diisopropyl peroxide (DCP), bis(trimethoxysilylpropyl)amine (BTMSPA), diphenylmethane diisocyanate (MD I), hexamethylene diisocyanate (HD I), toluene diisocyanate (TD I), isophorone diisocyanate (IPD I), dicyclohexylmethane diisocyanate (HMD I), lysine diisocyanate (LD I), I), hydrogen-containing polysiloxane (PMHS), diisocyanate, tetraisocyanate, melamine, trimethylol melamine, 1,4-bis-tert-butylperoxyisopropylbenzene, 2,2-bis(tert-butylperoxide)butane, tert-butyl hydroperoxide, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, diacetone acrylamide, N-hydroxymethyl acrylamide, N-hydroxyethyl acrylamide, N,N'-methylenebisacrylamide, 1,4-butanediol diacrylate, triethylene glycol di( Meth) acrylate, tetraethylene glycol di(meth) acrylate, tripropylene glycol di(meth) acrylate, tetrapropylene glycol di(meth) acrylate, polypropylene glycol di(meth) acrylate, polybutylene glycol di(meth) acrylate, tri(2-(meth) acryloyloxyethyl) isocyanurate, di(trimethylolpropane) tetra(meth) acrylate, dipentaerythritol penta(meth) acrylate, diallyl phthalate, hydroxyethyl acrylate, glycerol dimethacrylate, 1,6-Hexanediol dimethacrylate, glycidyl ether, trimethylolpropane triglycidyl ether, trimethylolpropane diallyl ether, divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, disuccinimidyl suberate, disuccinimidyl glutarate, trimethylolpropane tris(3-mercaptopropionate), formaldehyde, glyoxal, succinic dialdehyde, glutaraldehyde, crotonaldehyde, oxalic acid, malonic acid, citric acid, maleic anhydride, hexahydrophthalic anhydride, succinic anhydride, glycidyl methacrylate, trimethylolaminomethane, epichlorohydrin, trimethylolpropane, pentaerythritol, polyethylene glycol, polypropylene glycol, trimethylolethane, diethylenetriamine, triethylenetetramine, dimethylaminopropylamine, diethylaminopropylamine , triethylenetetramine, methyl orthosilicate, ethyl orthosilicate, propyl orthosilicate, diethoxysilane, dimethylsilane, 3-aminopropyltrimethoxysilane, methyltrimethoxysilane, vinyltrimethoxysilane, propyltrimethoxysilane, phenyltrimethoxysilane, vinyltriethoxysilane, vinyltri-tert-butylperoxysilane, KH550 (3-aminopropyltriethoxysilane), KH560 (γ-(2,3-epoxypropyloxy)propyltrimethoxysilane), KH530 (2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane), KH580 (γ-mercaptopropyltriethoxysilane), KH570 (methacryloyloxy functional silicon), vinyltri(β-methoxyethoxy) Silane, 2-isopropylimidazole, mercaptoacetic acid, mercaptopropionic acid, pentaerythritol tetrakis(3-mercaptopropionic acid), ethoxylated pentaerythritol tetraacrylate, propoxylated pentaerythritol tetraacrylate, (ethoxy)trimethylolpropane triacrylate, neopentyl glycol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, isobornyl methacrylate, diisopropylbenzene hydroperoxide, tert-amyl peroxy-2-ethylhexyl carbonate, 1,1-bis(tert-butylperoxy)cyclohexane, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylcumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne, peroxide Benzoyl, tert-butyl peroxymaleate, N,N'-bisacryloylcystamine, polyvinyl alcohol, phthalic acid, ammonium sulfate, ammonium persulfate, polyamide, triethanolamine, diamine hydrogen phosphate, tin tetrachloride, azobisisobutyronitrile, azobisisoheptanenitrile, 1,3,5-triaminobenzene, 1,3,5-tris(4-aminophenoxy)benzene, 3,3'-diaminobenzidine, dimethyldiallylammonium chloride, acrylamide, N-(isobutyloxy)methacrylamide, N,N'-vinylbisacrylamide, 1,3,5-triacryloylhexahydro-1,3,5-triazine, triallyl trimellitate, triallyl phosphate, triallyl phosphite, N,N'-m-phenylene bismaleimide, bismaleimide, 1,6-divinyl dodecafluorohexane, octaphenyl POSS, bis(meth)acryloyloxyethyl phosphate, hexamethylenetetramine, acrylic acid, methacrylic acid, methyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, N,N,N′,N′-tetramethylmethanediamine (TMMDA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N′-tetramethyl-1,6-hexanediamine ( TMHDA), diethanolamine, triethanolamine, 1,4-butanediol, 1,6-hexanediol, 1,3-butanediol, 1,2-diaminocyclohexane, 1,6-diaminohexane, p-phenylenediamine, toluidine, ethylenediamine, propylenediamine, butylenediamine, hexamethylenediamine, carbodiimide, dimethylol urea, xylylenediamine, polyethylene oxide, divinyl sulfone, trivinylbenzene, triallylamine, dicyclopentadiene, N,N-dimethylbenzylamine, or dicyandiamide.

[0053] In some embodiments, the catalyst includes a platinum-based catalyst, a tin-based catalyst, a chloroplatinic acid composite catalyst, a Grubbs catalyst, a ruthenium-based catalyst, a rhodium-based catalyst, a zirconium-based catalyst, a potassium isooctanoate catalyst, a bismuth catalyst, a palladium catalyst, a nickel catalyst, a zinc carboxylate, an iron carboxylate, a lead carboxylate, or a cobalt carboxylate.

[0054] In some preferred embodiments, the catalyst includes copper acetate, cobalt acetate, ferric acetate, nickel acetate, palladium acetate, ferric sulfate, ketone sulfate, tris(dibenzylideneacetone)dipalladium, Grubbs I generation catalyst, Grubbs II generation catalyst, Grubbs III generation catalyst, Hoveyda-Grubbs I I generation catalyst, tungsten hexachloride, dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, stannous octoate, zinc naphthenate, zinc octoate, or cobalt naphthenate, and is not limited thereto.

[0055] In some embodiments, the organic solvent of the cross-linking agent solution includes monohydric alcohol, polyhydric alcohol, ether, or benzene, but is not limited thereto.

[0056] In some embodiments, the organic solvent of the cross-linking agent solution includes deionized water, N,N-dimethylformamide, formamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-methylpyrrolidine, dimethyl sulfoxide, p-xylene, tetrahydrofuran, dichloromethane butanone, ethyl acetate, n-propyl acetate, n-butyl acetate, sec-butyl acetate, n-propanol, isopropanol, n-butanol, cyclohexanol, diacetone alcohol, n-hexane, petroleum ether, cyclohexane, ethyl ether, dioxane, toluene, xylene, dimethyl carbonate, ethyl acetate, chloroform, diphenyl ether, liquid paraffin, polyethylene glycol, and sulfolane.

[0057] In some preferred embodiments, the organic solvent of the cross-linking agent solution includes methanol, ethanol, or acetone.

[0058] The present invention reduces the degradation rate of polypropylene by adding a small amount of polyethylene to the raw materials for producing polypropylene diaphragms; at the same time, metal ions are introduced into the cross-linking agent formula as catalysts, which can catalyze the cross-linking effect and also act as antioxidants to inhibit free radical degradation of the polymer.

[0059] The present invention is further described in detail below through specific examples.

[0060] A cross-linked polyolefin diaphragm is provided, characterized in that it comprises an upper surface layer (A1), a center layer (B) and a lower surface layer (A2), forming an A1 / B / A2 three-layer co-extruded structure, wherein the upper surface layer (A1) comprises a first polyolefin with a molecular weight of 500,000 to 1,000,000 and a second polyolefin with a molecular weight of 300,000 to 800,000, which are copolymerized; the lower surface layer (A2) comprises a first polyolefin with a molecular weight of 500,000 to 1,000,000 and a second polyolefin with a molecular weight of 300,000 to 800,000, which are copolymerized; and the center layer (B) comprises a third polyolefin with a molecular weight of 500,000 to 1,000,000.

[0061] Example 1: A first polyolefin with a molecular weight of 550,000 is copolymerized with a second polyolefin with a molecular weight of 600,000 to form upper and lower surface layers; a third polyolefin with a molecular weight of 550,000 is used as the center layer. 100 parts by mass of an ethanol solution are weighed, 10 parts of a crosslinking agent, TAMPTA, 4 parts of a photoinitiator, benzophenone, and 1 part of a catalyst, copper acetate, are added, and the mixture is stirred to obtain a crosslinking agent solution C. Solution C is then evenly coated on the crosslinked precursor film using a coating machine, and the solvent is then dried in an oven at 70°C and irradiated under ultraviolet light for 50 seconds to obtain a crosslinked polyolefin separator. The relevant physical properties and gel content of the obtained crosslinked polyolefin separator are tested. The specific data and results can be found in Tables 1 and 2.

[0062] In Example 2, the catalyst was replaced with 1.5 parts of copper acetate catalyst, and the rest was the same as in Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0063] In Example 3, the catalyst was replaced with 2 parts of copper acetate catalyst, and the rest was the same as in Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0064] In Example 4, the catalyst was replaced with 3 parts of copper acetate, and the rest was the same as in Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0065] Example 5: The catalyst was replaced with 3 parts of copper acetate catalyst and irradiated with UV light for 100 seconds.

[0066] The rest is the same as Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0067] Example 6: The catalyst was replaced with 3 parts of copper acetate catalyst and irradiated with UV light for 150 seconds.

[0068] The rest is the same as Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0069] Example 7: The catalyst was replaced with 3 parts of copper acetate catalyst and irradiated with UV light for 200s.

[0070] The rest is the same as Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested, and the specific results can be seen in Table 2.

[0071] Example 8 was the same as Example 1 except that the molecular weights of the first and third polyolefins were changed to 450,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The physical properties and gel content of the resulting cross-linked polyolefin separator were measured, as shown in Table 2.

[0072] Example 9 was the same as Example 1 except that the molecular weights of the first and third polyolefins were changed to 700,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The physical properties and gel content of the resulting cross-linked polyolefin separator were measured, as shown in Table 2.

[0073] In Example 10, the molecular weight of the second polyolefin was changed to 350,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The remaining conditions were the same as in Example 1. The physical properties and gel content of the resulting cross-linked polyolefin separator were tested, as shown in Table 2.

[0074] Example 11 was the same as Example 1 except that the molecular weight of the second polyolefin was changed to 850,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The physical properties and gel content of the resulting cross-linked polyolefin separator were tested, and the specific results are shown in Table 2.

[0075] Example 12 was the same as Example 1 except that the molecular weight of the third polyolefin was changed to 500,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The physical properties and gel content of the resulting cross-linked polyolefin separator were tested, and the specific results are shown in Table 2.

[0076] Example 13 was the same as Example 1 except that the molecular weight of the third polyolefin was changed to 600,000, the catalyst was replaced with 3 parts of copper acetate, and UV irradiation was performed for 150 seconds. The physical properties and gel content of the resulting cross-linked polyolefin separator were tested, and the specific results are shown in Table 2.

[0077] In Comparative Example 1, no catalyst was added, and the UV lamp was irradiated for 200 seconds. The rest was the same as in Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested. The specific results can be seen in Table 2.

[0078] In Comparative Example 2, the catalyst was replaced with 3 parts of a non-metallic catalyst organophosphorus compound and the UV light was irradiated for 200 seconds. The rest was the same as in Example 1. The relevant physical properties and gel content of the obtained cross-linked polyolefin separator were tested. The specific results can be seen in Table 2.

[0079] Comparative Example 3: The physical properties of the uncrosslinked precursor film were tested.

[0080] Table 1

[0081] First Polyolefin (10,000) Second polyolefin (10,000) The third polyolefin (10,000) Photoinitiator crosslinking agent catalyst UV light irradiation(s) Example 1 55 60 55 4 10 1 50 Example 2 55 60 55 4 10 1.5 50 Example 3 55 60 55 4 10 2 50 Example 4 55 60 55 4 10 3 50 Example 5 55 60 55 4 10 3 100 Example 6 55 60 55 4 10 3 150 Example 7 55 60 55 4 10 3 200 Example 8 45 60 45 4 10 3 150 Example 9 70 60 70 4 10 3 150 Example 10 55 35 55 4 10 3 150 Example 11 55 85 55 4 10 3 150 Example 12 55 60 50 4 10 3 150 Example 13 55 60 60 4 10 3 150 Comparative Example 1 55 60 55 4 10 - 200 Comparative Example 2 55 60 55 4 10 3 200 Comparative Example 3 55 60 55 - - - -

[0082] Table 2

[0083]

[0084]

[0085] The gel content can directly reflect the degree of cross-linking of polyolefins. It can be seen from Table 2 that

[0086] Without a crosslinker, polyolefins exhibit no crosslinking, resulting in a gel content of zero. However, when no catalyst is added to the crosslinker solution, or when a non-metallic catalyst is used, the gel content is low. However, the addition of a catalyst significantly increases the gel content, and this increases continuously with increasing catalyst content. It should be noted that UV irradiation time also affects the degree of crosslinking. As shown in Table 2, the gel content reaches its maximum at 150 seconds of UV irradiation, but further exposure decreases the degree of crosslinking.

[0087] The gel content is calculated using a Soxhlet extractor, and the specific test method is as follows:

[0088] (1) Weigh 0.5 ± 0.05 g of the cross-linked product and place it on a clean, dry 100-120 mesh copper mesh. Weigh the copper mesh in advance and record it as m1. Weigh the mass of the product plus the copper mesh and record it as m2, accurate to 1 mg.

[0089] (2) Take a certain amount of xylene and place it in a round-bottom flask (the mass of xylene to the sample is not less than 500:1, and ensure that the sample and the sieve are completely placed in the solvent). Turn on the heating device and heat the xylene to boiling. Reflux the sample for 10 hours, and control the reflux rate at 20-40 drops / min.

[0090] (3) Carefully remove the copper mesh, place it in a 120°C oven to dry for 6 h, and weigh it, recording it as m3;

[0091] (4) The formula for calculating gel content is as follows:

[0092]

[0093] From the tensile and needle punching data in Table 2, it can be seen that after the polypropylene diaphragm is cross-linked,

[0094] In particular, the strength in the transverse direction is significantly improved, and the higher the degree of cross-linking, the higher the strength; the needle punching strength is also significantly improved with the increase of the degree of cross-linking.

[0095] However, the above is only a preferred embodiment of the present invention, and it cannot be used to limit the scope of patent protection of the present invention; therefore, any simple equivalent changes and modifications made according to the scope of patent protection of the present invention and the contents of the specification still fall within the scope of patent protection of the present invention.

Claims

1. A cross-linked polyolefin separator, characterized in that: The invention comprises an upper surface layer (A1), a center layer (B) and a lower surface layer (A2), which are arranged in sequence as the upper surface layer (A1), the center layer (B) and the lower surface layer (A2), forming an A1 / B / A2 three-layer co-extrusion structure, wherein the upper surface layer (A1) comprises a first polyolefin with a molecular weight of 500,000-1,000,000 and a second polyolefin with a molecular weight of 300,000-800,000; the center layer (B) comprises a third polyolefin with a molecular weight of 500,000-1,000,000; the lower surface layer (A2) comprises The first polyolefin having a molecular weight of 500,000-1,000,000 is copolymerized with the second polyolefin having a molecular weight of 300,000-800,000; the first polyolefin is polypropylene, the second polyolefin is polyethylene, and the third polyolefin is polypropylene; the weight ratio of the first polyolefin to the second polyolefin in the upper surface layer (A1) is (85-95wt%):(5-15wt%), and the weight ratio of the first polyolefin to the second polyolefin in the lower surface layer (A2) is (85-95wt%):(5-15wt%).

2. The cross-linked polyolefin membrane according to claim 1, characterized in that: The upper surface layer (A1) and the lower surface layer (A2) have the same composition.

3. The cross-linked polyolefin membrane according to claim 1, characterized in that: The gel content of the center layer (B) is 0% to 65%.

4. A method for preparing a cross-linked polyolefin separator, comprising the following steps: (S1) melt extrusion step: a third polyolefin with a molecular weight of 500,000-1,000,000 is put into melt pump B as the material of the center layer (B), 85-95wt% of a first polyolefin with a molecular weight of 500,000-1,000,000 and 5-15wt% of a second polyolefin with a molecular weight of 300,000-800,000 are uniformly mixed as the material of the upper surface layer (A1) or the lower surface layer (A2) and put into melt pump A, and after extrusion with melt pump B, a diaphragm with a three-layer co-extrusion structure of A1 / B / A2 is obtained, wherein, The first polyolefin is polypropylene, the second polyolefin is polyethylene, and the third polyolefin is polypropylene; (S2) Annealing step: annealing the membrane obtained in (S1); (S3) stretching step: cold stretching and hot stretching the diaphragm obtained in (S2); (S4) coating step: preparing a crosslinking agent solution and coating the crosslinking agent solution on the separator obtained in (S3) using a coating machine; and (S5) UV lamp irradiation step: The diaphragm coated in (S4) is placed under a UV lamp for a period of time to obtain a cross-linked polyolefin diaphragm.

5. The preparation method according to claim 4, characterized in that: A drying step (S6) is further included before or after (S5).

6. The preparation method according to claim 4, characterized in that: In (S4), the crosslinker solution is prepared by dissolving a certain proportion of a photoinitiator, a crosslinker, and a catalyst in a solvent.

7. The preparation method according to claim 6, characterized in that: The catalyst includes a platinum-based catalyst, a tin-based catalyst, a chloroplatinic acid composite catalyst, a Grubbs catalyst, a ruthenium-based catalyst, a rhodium-based catalyst, a zirconium-based catalyst, a potassium isooctanoate catalyst, a bismuth catalyst, a palladium catalyst, a nickel catalyst, a zinc carboxylate, an iron carboxylate, a lead carboxylate, or a cobalt carboxylate.

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