Biaxially oriented polypropylene film and preparation method thereof

Abstract

The present invention belongs to the technical field of dielectric film materials, and particularly relates to a biaxially oriented polypropylene film and a preparation method thereof. The biaxially oriented polypropylene film comprises a base film layer and a discontinuous film layer, wherein the base film layer is a polypropylene film layer, a convex structure is arranged on the working surface of the base film layer, and the discontinuous film layer is attached to the convex structure. The biaxially oriented polypropylene film of the present invention has the advantages of high bonding strength with a metal layer and good mechanical strength.

Description

Technical Field

[0001] The invention belongs to the technical field of dielectric film materials, and in particular relates to a biaxially oriented polypropylene film and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are widely used in new energy vehicles, energy storage and consumer electronics due to their high energy density and recyclability. As the energy density of new energy vehicles gradually increases and prices gradually decrease, competition among companies becomes increasingly fierce, which means that companies related to lithium batteries need to make certain technological breakthroughs in order to promote the overall progress of the lithium battery industry and win in market competition.

[0003] Traditional battery electrodes are mostly made of copper foil and aluminum foil. However, copper foil and aluminum foil have higher energy density. In order to reduce the impact of copper foil and aluminum foil on the energy density of lithium-ion batteries, polymer materials with lower density are currently used as the supporting matrix and composite current collectors with metal plating on the surface are used to replace pure metal foil, which can better improve the energy density of the battery.

[0004] The composite current collector is a "sandwich" structure, with the inner layer being a polymer layer (such as PET, PP or PI) and the two sides being metal conductive layers (such as Al or Cu). In the prior art, polypropylene is often used as the inner polymer. Compared with polyethylene terephthalate (PET) and polyimide (PI), polypropylene has the advantages of low cost, low density and acid and alkali resistance. However, the polarity of polypropylene is very weak, its surface tension is low, and its affinity with metal is poor. Therefore, the adhesion and bonding force of the interface between polypropylene and metal are poor. During the preparation of the current collector, the metal layer is easy to fall off, resulting in a low yield rate of the composite current collector.

[0005] At the same time, polypropylene has poor mechanical performance indicators and is prone to the risk of film breakage during the preparation process. Therefore, there is a need for a polypropylene film substrate for a composite current collector that can enhance the bonding strength with the metal layer while ensuring its mechanical strength. Summary of the invention

[0006] The purpose of the present invention is to provide a biaxially oriented polypropylene film and a preparation method thereof in view of the above-mentioned technical problems.

[0007] In view of this, the present invention provides a biaxially oriented polypropylene film, which includes a base film layer and a discontinuous film layer, wherein the base film layer is a polypropylene film layer, a protruding structure is arranged on the working surface of the base film layer, and the discontinuous film layer is attached to the protruding structure.

[0008] Furthermore, the thickness h0 of the base film layer is 6-12 um, and the average height h1 of the protruding structure is 0.5-1 um.

[0009] Furthermore, the density of the protrusion structures on the base film layer is greater than 100 / mm 2 .

[0010] Furthermore, the base film layer comprises the following raw materials in parts by weight:

[0011] 80-100 parts of polypropylene resin;

[0012] 10-20 parts of polyethylene mixture;

[0013] Hyperbranched polypropylene 5-10 parts;

[0014] Maleic anhydride grafted polypropylene 3-5 parts;

[0015] 3~8 parts of EPDM rubber;

[0016] Appropriate amount of additives.

[0017] Furthermore, the polypropylene resin is a mixture of homopolypropylene and copolymer polypropylene, and the polypropylene resin contains 30-60 wt % of homopolypropylene.

[0018] Furthermore, the homopolypropylene is a modified homopolypropylene after modification, and the preparation process of the modified homopolypropylene is:

[0019] First, homopolypropylene particles are mixed with a carboxyl-containing monomer modifier, and then irradiated under inert atmosphere protection, and then kept at 100-130° C. for 1-2 hours to eliminate free radicals to obtain modified homopolypropylene, wherein the added amount of the carboxyl-containing monomer modifier is 3-8% of the weight of the homopolypropylene, the irradiation ray is ultraviolet light, the irradiation dose is 1-5 kGy, and the carboxyl-containing monomer modifier is selected from one or more of acrylic acid, butenoic acid, 4-pentenoic acid, and acetic acid.

[0020] Furthermore, the polyethylene mixture is a mixture of linear low-density polyethylene and co-olefin, the polyethylene mixture contains 70-80 wt % of linear low-density polyethylene, and the co-olefin is copolymerized by ethylene and α-olefin.

[0021] Furthermore, the discontinuous film layer comprises the following raw materials in parts by weight:

[0022] 30-40 parts of nano silicon dioxide;

[0023] 80-150 parts of polydimethylsiloxane;

[0024] 30-50 parts of vinyl-terminated polydimethylsiloxane;

[0025] 5-10 parts of alkane compound containing epoxy group;

[0026] 1-3 parts of transition metal catalyst;

[0027] Emulsifier 8-15 parts;

[0028] 60~150 parts of deionized water.

[0029] A method for preparing a biaxially oriented polypropylene film, the method for preparing the biaxially oriented polypropylene film comprises the following steps:

[0030] S1, preparation of basement membrane layer:

[0031] The raw materials of the base film layer are mixed uniformly by weight and put into an extruder, the mixed material is melt-extruded at 230-260° C. to form a melt, and the melt is extruded through a die head at 245-255° C.;

[0032] The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is -20 to 0°C, and the melt is cooled on the chilled roller to obtain a cast sheet;

[0033] The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer. The two-stage stretching process includes:

[0034] a. One-time synchronous biaxial stretching: after preheating at 130~150℃ for 3~5min, stretching is performed in the transverse and longitudinal directions at the same time. The stretching temperature is 135~145℃ and the stretching ratio is 2×2~3×3.

[0035] b. Secondary synchronous biaxial stretching: After the polypropylene film after the primary synchronous biaxial stretching is transferred to a temperature zone of 140~150℃, it is stretched in the transverse and longitudinal directions at the same time, and the stretching ratio is: 4×4~5.5×5.5;

[0036] After secondary synchronous biaxial stretching, the film is placed at 110-120°C for heat setting to obtain a base film layer;

[0037] S2, preparation of discontinuous film layer:

[0038] A transition metal catalyst, 30-40 wt% of polydimethylsiloxane, and 40-60 wt% of an emulsifier are mixed, stirred evenly, and then 20-40 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained;

[0039] The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained;

[0040] The mixed system 1 and the mixed system 2 are sequentially coated on the surface of the protruding structure in the base film layer, and then sent into an oven at 40-60°C. After drying, a non-continuous film layer is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

[0041] Furthermore, the thickness of the discontinuous film layer formed by coating is h3≤0.3 um.

[0042] The beneficial effects of the present invention are as follows: the biaxially oriented polypropylene film of the present invention has the advantages of high bonding strength with the metal layer and good mechanical strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the structure of the biaxially oriented polypropylene film of the present invention;

[0044] The markings in the figure indicate: 1. base film layer; 11. protruding structure; 2. non-continuous film layer. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present application to clearly describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments in the present application belong to the scope of protection of this application.

[0046] It should be noted that, in the present application, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises one..." does not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be noted that the scope of the method and device in the embodiment of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0047] A biaxially oriented polypropylene film comprises a base film layer 1 and a discontinuous film layer 2, wherein a protruding structure 11 is arranged on a working surface of the base film layer 1, and the discontinuous film layer 2 is attached to the protruding structure 11.

[0048] Furthermore, the base film layer 1 is a polypropylene film layer.

[0049] Furthermore, the working surface of the base film layer 1 can be one side surface in the thickness direction thereof, or can be two side surfaces in the thickness direction thereof, which can be specifically determined according to the setting requirements of the metal layer.

[0050] Further, such as Figure 1 As shown, the thickness h0 of the base film layer 1 is 6-12 um, and the average height h1 of the protruding structure 11 is 0.5-1 um.

[0051] Furthermore, when forming the composite current collector, the thickness of the metal layer attached to the biaxially oriented polypropylene film is 2-3 um.

[0052] As some examples of the present invention, the metal plating layer may be an aluminum plating layer or a copper plating layer.

[0053] Furthermore, the density of the protrusion structures 11 on the base film layer 1 is greater than 100 / mm 2 .

[0054] Furthermore, the protruding structure 11 and the base film layer 1 are prepared by integrally molding.

[0055] Furthermore, the base film layer 1 comprises the following raw materials in parts by weight:

[0056] 80-100 parts of polypropylene resin;

[0057] 10-20 parts of polyethylene mixture;

[0058] Hyperbranched polypropylene 5-10 parts;

[0059] Maleic anhydride grafted polypropylene 3-5 parts;

[0060] 3~8 parts of EPDM rubber.

[0061] Preferably, the polypropylene resin is a mixture of homopolypropylene and copolymer polypropylene, and the polypropylene resin contains 30-60 wt % of homopolypropylene.

[0062] More preferably, the homopolypropylene is modified homopolypropylene after modification.

[0063] Specifically, the preparation process of the modified homopolypropylene is as follows:

[0064] First, homopolypropylene particles are mixed with a carboxyl-containing monomer modifier, and then irradiated under inert atmosphere protection, and then kept at 100-130° C. for 1-2 hours to eliminate free radicals to obtain modified homopolypropylene, wherein the added amount of the carboxyl-containing monomer modifier is 3-8% of the weight of the homopolypropylene, the irradiation light is ultraviolet light, and the irradiation dose is 1-5 kGy.

[0065] As some examples of the present invention, the carboxyl-containing monomer modifier is selected from one or more of acrylic acid, crotonic acid, 4-pentenoic acid, and acetic acid.

[0066] Preferably, the carboxyl-containing monomer modifier is selected from one or more of acrylic acid and acetic acid.

[0067] Preferably, the homopolymer polypropylene particles are porous particles.

[0068] In the modified homopolypropylene, the carboxyl-containing monomer modifier can be attached to the surface or internal pores of homopolypropylene particles by irradiation technology, so that the surface tension and melt strength of the homopolypropylene are increased, thereby improving the surface tension and melt strength of the base film layer 1.

[0069] Preferably, the polyethylene mixture is a mixture of linear low-density polyethylene and co-olefin, and the polyethylene mixture contains 70-80 wt % of linear low-density polyethylene.

[0070] As some examples of the present invention, the copolymerized olefin is obtained by copolymerizing ethylene with a small amount of α-olefin, such as 1-butene, 1-pentene, 1-hexene, 1-octene, and the like.

[0071] This co-olefin is a copolymer formed by polymerization of two or more olefin monomers with different numbers of carbon atoms, and has mechanical properties such as high toughness, high strength, high stiffness and high heat resistance. Compared with linear low-density polyethylene, it has a more complex molecular structure, higher crystallinity and better mechanical properties. When used for the base film layer 1, during the preparation of the base film layer 1, due to the different crystallization rates of this co-olefin and other components, such as polypropylene resin and linear low-density polyethylene, the melt elasticity responds differently to the stretching action during the stretching process, and finally forms a reinforcing phase in the base film layer 1, which can improve the mechanical strength and surface roughness of the base film layer 1 at the same time.

[0072] In addition, the use of a small amount of hyperbranched polypropylene in the base film layer 1 can reduce the melt viscosity of polypropylene and increase the fluidity of the melt, thereby improving the processing performance and making the polypropylene film easier to process and shape. At the same time, hyperbranched polypropylene can increase the strength and toughness of the polypropylene film and improve the impact resistance, making the base film layer 1 more durable.

[0073] The molecular chain of maleic anhydride grafted polypropylene has polar groups, which can form good compatibility with non-polar materials such as polypropylene and polar materials, thereby improving the adhesion and compatibility of polypropylene film with other materials. At the same time, maleic anhydride grafted polypropylene can form a thin film on the surface of the polypropylene film, improve the adhesion between the film and the substrate or other materials, and enhance the adhesion performance of the base film layer 1.

[0074] The addition of EPDM rubber can increase the toughness of the polypropylene film, improve the impact resistance and tensile strength of the film, and make the film more durable. In addition, the addition of EPDM rubber can also improve the processing properties of the polypropylene film, making the film easier to process and shape.

[0075] Furthermore, the base film layer 1 further comprises the following auxiliary agents in parts by weight:

[0076] 1~3 parts of surface treatment agent;

[0077] Antioxidant 0.2~0.5 parts;

[0078] Lubricant 0.3~0.8 parts.

[0079] As some examples of the present invention, the antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 168, and antioxidant 1098.

[0080] As some examples of the present invention, the surface treatment agent is a silane coupling agent, such as one or more of KH550, KH560, and KH570.

[0081] As some examples of the present invention, the lubricant is selected from one or more of ethylene bisstearamide and butyl stearate.

[0082] Furthermore, the discontinuous film layer 2 comprises the following raw materials in parts by weight:

[0083] 30-40 parts of nano silicon dioxide;

[0084] 80-150 parts of polydimethylsiloxane;

[0085] 30-50 parts of vinyl-terminated polydimethylsiloxane;

[0086] 5-10 parts of alkane compound containing epoxy group;

[0087] 1-3 parts of transition metal catalyst;

[0088] Emulsifier 8-15 parts;

[0089] 60~150 parts of deionized water.

[0090] Preferably, the viscosity of the polydimethylsiloxane is 0.5~1 cSt.

[0091] As some examples of the present invention, the transition metal catalyst is selected from transition metal catalysts such as platinum, palladium, rhodium, and nickel.

[0092] Preferably, the emulsifier is a silicone-based emulsifier.

[0093] Furthermore, the method for preparing the biaxially oriented polypropylene film comprises the steps of:

[0094] S1, preparation of basement membrane layer:

[0095] The raw materials of the base film layer are mixed uniformly by weight and put into an extruder, the mixed material is melt-extruded at 230-260° C. to form a melt, and the melt is extruded through a die head at 245-255° C.;

[0096] The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is -20 to 0°C, and the melt is cooled on the chilled roller to obtain a cast sheet;

[0097] The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer 1. The two-stage stretching process includes:

[0098] a. One-time synchronous biaxial stretching: after preheating at 130~150℃ for 3~5min, stretching is performed in the transverse and longitudinal directions at the same time. The stretching temperature is 135~145℃ and the stretching ratio is 2×2~3×3;

[0099] b. Secondary synchronous biaxial stretching: After the polypropylene film is transferred to a temperature zone of 140-150°C after the primary synchronous biaxial stretching, it is stretched in the transverse and longitudinal directions at the same time, and the stretching ratio is 4×4~5.5×5.5;

[0100] After secondary synchronous biaxial stretching, the film is placed at 110-120°C for heat setting to obtain a base film layer;

[0101] S2, preparation of discontinuous film layer 2:

[0102] A transition metal catalyst, 30-40 wt% of polydimethylsiloxane, and 40-60 wt% of an emulsifier are mixed, stirred evenly, and then 20-40 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained;

[0103] The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained;

[0104] The mixed system 1 and the mixed system 2 are sequentially coated onto the surface of the protruding structure in the base film layer, and then placed into an oven at 40-60°C. After drying, a non-continuous film layer 2 is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

[0105] Preferably, the average particle size of the nano-silicon dioxide particles is 10-100 nm.

[0106] Preferably, the thickness h3 of the discontinuous film layer 2 is ≤ 0.3 um.

[0107] It should be noted that, in the present invention, the thickness of the discontinuous film layer 2 is the average thickness of the film attached to the top of the protruding structure 11. The detection method is to randomly select several areas under a microscope, count the film thickness of the selected areas, and calculate the average value. Of course, the thickness of the discontinuous film layer 2 can also be measured with the help of computer software.

[0108] As some examples of the present invention, when both sides of the base film layer 1 are working surfaces, a chilled roller that can cool both sides of the melt simultaneously is used to cool the melt. At this time, the rotation speed of the chilled roller is 3-5 m / min.

[0109] As some examples of the present invention, when one side of the base film layer 1 is a working surface, a chilled roller that can cool one side of the melt is used to cool the melt, and the other side is cooled by room temperature air, and the rotation speed of the chilled roller is 0.5~3m / min.

[0110] In the step S1, since the crystallization rate of the polymer is closely related to the temperature, when the melt is cooled, the surface temperature of the melt in contact with the chilled roller drops sharply, the polymer crystallization rate is accelerated, and spherulites with larger crystal nuclei are formed; while the polymer temperature in other areas is relatively high, the crystallization rate is slow, and spherulites with smaller crystal nuclei are eventually formed; in the subsequent stretching process, since the crystal nuclei of the spherulites are very small in deformation, as the film thickness decreases, the large-sized spherulites in the cast film will be able to protrude from the surface of the base film layer, forming a protruding structure integrally formed with the base film layer on the surface of the base film layer, and this protruding structure is different from the conventional Unlike the protrusions formed by adding inorganic particles or forming by film pressing, the protrusions formed by large-sized spherulites in the present invention are integrally formed with the base film layer, have good integrity and uniformity, and the protrusion structure is more natural and stable; while in the protrusions formed by adding inorganic particles, because the combination of inorganic particles and polymer matrix is ​​not as tight as that of naturally formed spherulites, it is easy to cause a decrease in the strength and elongation of the base film layer, and the protrusions formed by molding require additional process steps and equipment, which increases the production cost and complexity, and the protrusions formed by molding are easy to recover deformation under high temperature during use and become smaller or disappear, and the stability is poor.

[0111] In the step S2, by coating the surface of the base film layer with mixed system 1 and mixed system 2, the vinyl-terminated polydimethylsiloxane therein can react to form a film under the action of a transition metal catalyst. However, due to the presence of the micron-sized protruding structure 11 on the base film layer 1, the mixed system 1 and the mixed system 2 are not easy to adhere to the low-lying planar area on the base film layer 1 where the protruding structure 11 is not formed during coating. Instead, the mixed system 1 and the mixed system 2 will preferentially adhere to and deposit on the protruding structure 11. Then, along with the volatilization of volatile components such as polydimethylsiloxane and water, the mixed system 1 and the mixed system 2 will gradually shrink, adhere to and wrap around the protruding structure 11 on the base film layer 1, forming a non-continuous film layer 2 in which most of the material is attached to the protruding structure 11. While further increasing the size of the protruding structure 11, the roughness, polarity and adhesion ability of the surface of the protruding structure 11 are improved, thereby improving the overall roughness and adhesion ability of the surface of the base film layer 1.

[0112] In addition, since the non-continuous film layer 2 is also provided with components containing polar functional groups such as alkane compounds containing epoxy groups, the surface tension of the non-continuous film layer 2 is relatively high, and a stable bond can be formed with the metal layer. The provision of nano-silicon dioxide can not only improve the surface roughness of the non-continuous film layer 2, but also enhance the strength of the non-continuous film layer 2.

[0113] The biaxially oriented polypropylene film of the present invention is illustrated by way of specific embodiments below:

[0114] Embodiment 1

[0115] A biaxially oriented polypropylene film, the preparation process of which is as follows:

[0116] S1, preparation of basement membrane layer:

[0117] The raw materials of the base film layer are mixed uniformly by weight, and then put into an extruder, and the mixed material is melt-extruded at 230°C to form a melt, and the melt is extruded through a die head at 245°C; wherein, the base film layer comprises the following raw materials by weight: 80 parts of polypropylene resin; 10 parts of polyethylene mixture; 5 parts of hyperbranched polypropylene; 3 parts of maleic anhydride grafted polypropylene; 3 parts of EPDM rubber; 1 part of surface treatment agent; 0.2 parts of antioxidant; 0.3 parts of lubricant; the polypropylene resin contains 30wt% modified homopolypropylene and 70wt% copolymer polypropylene, the modified homopolypropylene is a homopolypropylene modified by acrylic acid irradiation, the polyethylene mixture contains 70wt% of linear low-density polyethylene and 30wt% of copolymer olefin, and the copolymer olefin is copolymerized by ethylene and α-olefin;

[0118] The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is -20°C, and the melt is cooled on the chilled roller to obtain a cast sheet;

[0119] The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer. The two-stage stretching process includes:

[0120] a. One-time synchronous biaxial stretching: After preheating at 130°C for 3 minutes, stretching is performed in the transverse and longitudinal directions at the same time. The stretching temperature is 135°C and the stretching ratio is 2×2.

[0121] b. Secondary synchronous biaxial stretching: After the polypropylene film after the primary synchronous biaxial stretching is transferred to a temperature zone of 140°C, it is stretched in the transverse and longitudinal directions at the same time, and the stretching ratio is 4×4;

[0122] After secondary synchronous biaxial stretching, the film is placed at 110°C for heat setting to obtain a base film layer;

[0123] S2, preparation of discontinuous film layer:

[0124] The discontinuous film layer includes the following raw materials by weight: 30 parts of nano-silicon dioxide; 80 parts of polydimethylsiloxane; 30 parts of vinyl-terminated polydimethylsiloxane; 5 parts of alkane compounds containing epoxy groups; 1 part of transition metal catalyst; 8 parts of emulsifier; 60 parts of deionized water;

[0125] A transition metal catalyst, 30 wt% of polydimethylsiloxane, and 40 wt% of an emulsifier are mixed, stirred evenly, and then 20 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained;

[0126] The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained;

[0127] The mixed system 1 and the mixed system 2 are sequentially coated onto the surface of the protruding structure in the base film layer, and then placed into an oven at 40°C. After drying, a non-continuous film layer is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

[0128] Embodiment 2

[0129] A biaxially oriented polypropylene film, the preparation process of which is as follows:

[0130] S1, preparation of basement membrane layer:

[0131] The raw materials of the base film layer are mixed uniformly by weight, and then put into an extruder, and the mixed material is melt-extruded at 240°C to form a melt, and the melt is extruded through a die head at 250°C; wherein, the base film layer comprises the following raw materials by weight: 90 parts of polypropylene resin; 15 parts of polyethylene mixture; 7 parts of hyperbranched polypropylene; 4 parts of maleic anhydride grafted polypropylene; 5 parts of EPDM rubber; 2 parts of surface treatment agent; 0.3 parts of antioxidant; 0.5 parts of lubricant; the polypropylene resin contains 45wt% modified homopolypropylene and 55wt% copolymer polypropylene, the modified homopolypropylene is a homopolypropylene modified by acetic acid irradiation, the polyethylene mixture contains 75wt% of linear low-density polyethylene and 25wt% of copolymer olefin, and the copolymer olefin is copolymerized by ethylene and α-olefin;

[0132] The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is -10°C, and the melt is cooled on the chilled roller to obtain a cast sheet;

[0133] The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer. The two-stage stretching process includes:

[0134] a. One-time synchronous biaxial stretching: After preheating at 140°C for 4 minutes, stretching is performed in the transverse and longitudinal directions at the same time. The stretching temperature is 140°C and the stretching ratio is 2.5×2.5.

[0135] b. Secondary synchronous biaxial stretching: After the polypropylene film after the primary synchronous biaxial stretching is transferred to a temperature zone of 145°C, it is stretched in the transverse and longitudinal directions at the same time, with a stretching ratio of 5×5;

[0136] After secondary synchronous biaxial stretching, the film is placed at 115°C for heat setting to obtain a base film layer;

[0137] S2, preparation of discontinuous film layer:

[0138] The discontinuous film layer includes the following raw materials by weight: 35 parts of nano silicon dioxide; 100 parts of polydimethylsiloxane; 40 parts of vinyl-terminated polydimethylsiloxane; 8 parts of alkane compound containing epoxy group; 2 parts of transition metal catalyst; 10 parts of emulsifier; 100 parts of deionized water;

[0139] A transition metal catalyst, 35 wt% of polydimethylsiloxane, and 50 wt% of an emulsifier are mixed, stirred evenly, and then 30 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained;

[0140] The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained;

[0141] The mixed system 1 and the mixed system 2 are sequentially coated onto the surface of the protruding structure in the base film layer, and then placed into an oven at 50°C. After drying, a non-continuous film layer is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

[0142] Embodiment 3

[0143] A biaxially oriented polypropylene film, the preparation process of which is as follows:

[0144] S1, preparation of basement membrane layer:

[0145] The raw materials of the base film layer are mixed uniformly by weight, and then put into an extruder, and the mixed material is melt-extruded at 260°C to form a melt, and the melt is extruded through a die head at 255°C; wherein, the base film layer comprises the following raw materials by weight: 100 parts of polypropylene resin; 20 parts of polyethylene mixture; 10 parts of hyperbranched polypropylene; 5 parts of maleic anhydride grafted polypropylene; 8 parts of EPDM rubber; 3 parts of surface treatment agent; 0.5 parts of antioxidant; 0.8 parts of lubricant; the polypropylene resin contains 60wt% of modified homopolypropylene and 40wt% of copolymer polypropylene, the modified homopolypropylene is a homopolypropylene modified by butylene acid irradiation, and the polyethylene mixture contains 80wt% of linear low-density polyethylene and 20wt% of copolymer olefin, and the copolymer olefin is copolymerized by ethylene and α-olefin;

[0146] The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is 0° C. After the melt is cooled on the chilled roller, a cast sheet is obtained;

[0147] The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer. The two-stage stretching process includes:

[0148] a. One-time synchronous biaxial stretching: after preheating at 150°C for 5 minutes, stretching is performed in the transverse and longitudinal directions at the same time, the stretching temperature is 145°C, and the stretching ratio is 3×3;

[0149] b. Secondary synchronous biaxial stretching: After the polypropylene film after the primary synchronous biaxial stretching is transferred to a temperature zone of 150°C, it is stretched in the transverse and longitudinal directions at the same time, and the stretching ratio is: 5.5×5.5;

[0150] After secondary synchronous biaxial stretching, the film is placed at 120°C for heat setting to obtain a base film layer;

[0151] S2, preparation of discontinuous film layer:

[0152] The discontinuous film layer includes the following raw materials by weight: 40 parts of nano-silicon dioxide; 150 parts of polydimethylsiloxane; 50 parts of vinyl-terminated polydimethylsiloxane; 10 parts of alkane compounds containing epoxy groups; 3 parts of transition metal catalyst; 15 parts of emulsifier; 150 parts of deionized water;

[0153] A transition metal catalyst, 40 wt% of polydimethylsiloxane, and 60 wt% of an emulsifier are mixed, stirred evenly, and then 40 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained;

[0154] The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained;

[0155] The mixed system 1 and the mixed system 2 are sequentially coated onto the surface of the protruding structure in the base film layer, and then sent into an oven at 60°C. After drying, a non-continuous film layer is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

[0156] Comparative Example 1

[0157] The only difference between it and the above-mentioned second embodiment is that:

[0158] The biaxially oriented polypropylene film only includes a base film layer and does not include a discontinuous film layer.

[0159] Comparative Example 2

[0160] The only difference between it and the above-mentioned second embodiment is that:

[0161] The polypropylene resin used in the base film layer contains 45wt% of unmodified homopolypropylene and 55wt% of copolymer polypropylene.

[0162] Comparative Example 3

[0163] The only difference between it and the above-mentioned second embodiment is that:

[0164] The polyethylene mixture used in the base film layer is all linear low-density polyethylene and does not contain copolymerized olefins.

[0165] Comparative Example 4

[0166] The only difference between it and the above-mentioned second embodiment is that:

[0167] The raw material used for the discontinuous film layer does not contain an alkane compound containing an epoxy group.

[0168] Comparative Example 5

[0169] The only difference between it and the above-mentioned second embodiment is that:

[0170] The raw materials used for the discontinuous film layer do not contain nano silicon dioxide.

[0171] Comparative Example 6

[0172] The only difference between it and the above-mentioned second embodiment is that:

[0173] During the preparation of the base film layer, the temperature of the chilled roller was 60° C. The performance of the biaxially oriented polypropylene film obtained by the present invention was tested, and copper metal was magnetron sputtered on both sides of the film prepared in the above Examples 1 to 3 and Comparative Examples 1 to 6 to obtain a composite current collector, and the adhesion between the film and the metal layer was tested. The results are shown in Table 1 below:

[0174] Table 1 Performance test results of biaxially oriented polypropylene film

[0175]

[0176] The embodiments of the present application are described above in conjunction with the accompanying drawings. In the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present application, ordinary technicians in this field can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A biaxially oriented polypropylene film, characterized in that: The biaxially oriented polypropylene film comprises a base film layer (1) and a discontinuous film layer (2), wherein the base film layer (1) is a polypropylene film layer, a protruding structure (11) is arranged on the working surface of the base film layer (1), and the discontinuous film layer (2) is attached to the protruding structure (11); The discontinuous film layer (2) comprises the following raw materials in parts by weight: 30-40 parts of nano silicon dioxide; 80-150 parts of polydimethylsiloxane; 30-50 parts of vinyl-terminated polydimethylsiloxane; 5-10 parts of alkane compound containing epoxy group; 1-3 parts of transition metal catalyst; Emulsifier 8-15 parts; 60~150 parts of deionized water; The method for preparing the biaxially oriented polypropylene film comprises the steps of: S1, preparation of basement membrane layer: The raw materials of the base film layer are mixed uniformly by weight and put into an extruder, the mixed material is melt-extruded at 230-260° C. to form a melt, and the melt is extruded through a die head at 245-255° C.; The melt is guided to a chilled roller for cooling and forming, the temperature of the chilled roller is -20 to 0°C, and the melt is cooled on the chilled roller to obtain a cast sheet; The cast sheet is guided to a synchronous biaxial stretching film-forming machine for two-stage stretching to obtain a base film layer. The two-stage stretching process includes: a. One-time synchronous biaxial stretching: after preheating at 130~150℃ for 3~5min, stretching is performed in the transverse and longitudinal directions at the same time. The stretching temperature is 135~145℃ and the stretching ratio is 2×2~3×3. b. Secondary synchronous biaxial stretching: After the polypropylene film after the primary synchronous biaxial stretching is transferred to a temperature zone of 140~150℃, it is stretched in the transverse and longitudinal directions at the same time, and the stretching ratio is: 4×4~5.5×5.5; After secondary synchronous biaxial stretching, the film is placed at 110-120°C for heat setting to obtain a base film layer; S2, preparation of discontinuous film layer: A transition metal catalyst, 30-40 wt% of polydimethylsiloxane, and 40-60 wt% of an emulsifier are mixed, stirred evenly, and then 20-40 wt% of deionized water is slowly added, and after shearing and emulsification, a mixed system 1 is obtained; The vinyl-terminated polydimethylsiloxane, nano-silicon dioxide, alkane compound containing epoxy group, remaining polydimethylsiloxane and emulsifier are mixed in the formula, stirred evenly, and then the remaining deionized water is slowly added, and after shearing and emulsification, a mixed system 2 is obtained; The mixed system 1 and the mixed system 2 are sequentially coated on the surface of the protruding structure in the base film layer, and then sent into an oven at 40-60°C. After drying, a non-continuous film layer is formed on the surface of the base film layer to obtain the biaxially oriented polypropylene film.

2. The biaxially oriented polypropylene film according to claim 1, characterized in that: The thickness h0 of the base film layer (1) is 6-12 um, and the average height h1 of the protruding structure (11) is 0.5-1 um.

3. The biaxially oriented polypropylene film according to claim 1 or 2, characterized in that: The density of the protruding structures (11) on the base film layer (1) is greater than 100 / mm 2 .

4. The biaxially oriented polypropylene film according to claim 1, characterized in that: The base film layer (1) comprises the following raw materials in parts by weight: 80-100 parts of polypropylene resin; 10-20 parts of linear low-density polyethylene; Hyperbranched polypropylene 5-10 parts; Maleic anhydride grafted polypropylene 3-5 parts; 3~8 parts of EPDM rubber; 1~3 parts of surface treatment agent; Antioxidant 0.2~0.5 parts; Lubricant 0.3~0.8 parts.

5. The biaxially oriented polypropylene film according to claim 4, characterized in that: The polypropylene resin is a mixture of homopolypropylene and copolymer polypropylene, and the polypropylene resin contains 30-60wt% of homopolypropylene.

6. The biaxially oriented polypropylene film according to claim 5, characterized in that: The homopolypropylene is modified homopolypropylene after modification, and the preparation process of the modified homopolypropylene is as follows: First, homopolypropylene particles are mixed with a carboxyl-containing monomer modifier, and then irradiated under inert atmosphere protection, and then kept at 100-130° C. for 1-2 hours to eliminate free radicals to obtain modified homopolypropylene, wherein the added amount of the carboxyl-containing monomer modifier is 3-8% of the weight of the homopolypropylene, the irradiation ray is ultraviolet light, the irradiation dose is 1-5 kGy, and the carboxyl-containing monomer modifier is selected from one or more of acrylic acid, butenoic acid, 4-pentenoic acid, and acetic acid.

7. The biaxially oriented polypropylene film according to claim 1, characterized in that: The thickness h3 of the discontinuous film layer (2) formed by coating is ≤0.3 um.

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