Biaxially oriented polyester film and secondary battery separator including the same

Abstract

The biaxially oriented polyester film according to the present invention is a biaxially oriented polyester film containing inorganic particles and polyester resin. After slitting the master roll on which the biaxially oriented polyester film is wound, the film is rewound at a winding tension of 6.5 to 8.5 kg / m, a winding surface pressure of 35 to 50 kg / m, and a winding speed of 300 to 350 m / min. The hardness of the rewound film is 90 to 95 points, which prevents the generation of wrinkles in the rolled state and prevents the wrinkles from being transferred to the separator when the film is used as an inserting sheet for a separator of a secondary battery.

Description

[Technical field]

[0001] The present invention relates to a biaxially oriented polyester film, and more specifically, to a biaxially oriented polyester film that can prevent minor wrinkles from being transferred to a secondary battery separator when used as a separator for a secondary battery by redesigning surface properties and optimizing winding conditions, and a secondary battery separator including the same. [Background technology]

[0002] Recently, as part of environmental protection and carbon dioxide reduction policies, internal combustion engine vehicles that use gasoline or diesel as fuel are being replaced by electric vehicles, the demand for electric vehicles is gradually expanding and the supply rate is increasing. As the supply rate of electric vehicles increases, the demand for secondary batteries, which are essential for electric vehicles, is rapidly increasing, and the market for secondary batteries is also growing rapidly.

[0003] Such a secondary battery is a battery composed of one or more electrochemical cells that can be charged and discharged, and can be used multiple times by recharging, making it convenient to use, and is environmentally friendly because it does not generate battery waste. In general, a secondary battery is composed of an anode and a cathode that determine the capacity and voltage of the battery and allow current to flow, an electrolyte that is a medium that helps ions to move smoothly, and a separator that blocks contact between the anode and the cathode.

[0004] In addition, as part of the process of manufacturing secondary batteries, a safety reinforced separator (SRS) is applied to the separator (PE material, soft), which is a component of the secondary battery, to enhance safety. During the rewinding process after coating, the separators can stick together, so polyester film is inserted between the separators as interleaving paper to prevent the separators from sticking together when rewinding.

[0005] However, if the polyester film used as the interleaf has severe wrinkles, the wrinkles are transferred to the separation membrane, which is a soft material, causing a problem of degraded quality of the separation membrane. In particular, although conventional polyester films are wrinkle-free in a rolled state immediately after production, they have poor durability, and the air inside the film is not sufficiently released and is trapped inside, causing wrinkles. Such wrinkles in the polyester film are transferred to the separation membrane, greatly affecting the quality of the separation membrane.

[0006] Therefore, there is a strong need to improve the problem of wrinkles in polyester films used as separator inserts for secondary batteries, and to prevent the wrinkles from being transferred to the separator. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2018-0077585 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been devised to solve the above problems, and an object of the present invention is to provide a biaxially oriented polyester film that can prevent the generation of minor wrinkles in a roll-wound state after production of the polyester film, and a secondary battery separator including the same.

[0009] The above and other objects and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment. [Means for solving the problem]

[0010] The above object is achieved by a biaxially oriented polyester film comprising inorganic particles and a polyester resin, which is wound up on a master roll and then slitted, and then rewound at a winding tension of 6.5 to 8.5 kg / m, a winding surface pressure of 35 to 50 kg / m, and a winding speed of 300 to 350 m / min, with the hardness of the roll being 90 to 95 points.

[0011] Preferably, the inorganic particles may have an average particle size of 3.0 to 5.0 μm.

[0012] Preferably, the inorganic particles are present in an amount of 0.0008 to 0.0015 wt %.

[0013] Preferably, the biaxially oriented polyester film may have a center line average roughness (SRa) of 40 to 80 nm and a ten point average roughness (SRz) of 800 to 1300 nm.

[0014] Preferably, the biaxially oriented polyester film may have a maximum roughness (SRmax) of 1100 to 1800 nm.

[0015] Preferably, the biaxially oriented polyester film has a haze of 4% or more.

[0016] Preferably, the biaxially oriented polyester film has a surface peak count (SPC) of less than 0.1 mm2 in area. 2 The number of protrusions having an internal height of 0.15 μm or more may be 100 or more.

[0017] Preferably, the inorganic particles can be silica particles.

[0018] Preferably, the thickness of the biaxially oriented polyester film may be from 10 to 16 μm.

[0019] The above object is also achieved by the biaxially oriented polyester film, which is used as an interleaf of a separator for a secondary battery.

[0020] The above object is also achieved by a secondary battery separator including the biaxially oriented polyester film, and a secondary battery including the same. Effect of the Invention

[0021] The biaxially stretched polyester film according to one embodiment of the present invention has the effect of preventing the generation of wrinkles due to internal air during winding, and preventing the generation of wrinkles due to internal air during winding, by redesigning the surface characteristics of the polyester film and optimizing the winding conditions so that the air inside the wound film roll can be smoothly released.

[0022] In addition, according to the present invention, since no noticeable wrinkles are generated, the wrinkles are not transferred to the separation membrane when the sheet is used as an interleaving sheet for a secondary battery separator, and therefore, quality problems caused by wrinkles during the interleaving process for the separation membrane are prevented, thereby improving the quality of the secondary battery separator and enabling a stable supply of products.

[0023] In addition, according to the present invention, when used as an interleaf for a secondary battery separator, wrinkles are not transferred to the separator, so that a secondary battery including a highly efficient secondary battery separator can be provided.

[0024] However, the effects of the present invention are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic cross-sectional view of a biaxially oriented polyester film according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings, in order to enable those skilled in the art to easily carry out the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein.

[0027] In the drawings, the thickness of the layers and regions is exaggerated to clearly show them. Similar parts are designated by the same reference numerals throughout the specification. When a part, such as a layer, film, region, or plate, is described as being "on" another part, this includes not only the case where the part is "directly on" the other part, but also the case where there is another part between them. In contrast, when a part is described as being "directly on" another part, it means that there is no other part between them.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention belongs. In case of conflict, the present specification, including definitions, shall control. In addition, although methods and materials similar or equivalent to those described herein may be used in the practice or testing of the present invention, suitable methods and materials are described herein.

[0029] The biaxially stretched polyester film according to one embodiment of the present invention provides a film in which the size of inorganic particles used in the film production is controlled and the occurrence of minor wrinkles is improved by changing the conditions of the roll winding process. The present invention will be described in more detail below.

[0030] FIG. 1 is a schematic cross-sectional view of a biaxially oriented polyester film according to an embodiment of the present invention.

[0031] As shown in FIG. 1, the biaxially stretched polyester film according to an embodiment of the present invention includes inorganic particles 100 and polyester resin 200, and has a structure in which the inorganic particles 100 are dispersed in the polyester resin 200.

[0032] The inorganic particles 100 used in the present invention preferably include silica (SiO2), borosilicates, aluminosilicates, and alumina, and more preferably include silica particles.

[0033] In one embodiment, the inorganic particles 100 preferably have an average particle size of 3.0 to 5.0 μm. If inorganic particles 100 with a particle size of less than 3.0 μm are used, wrinkles will occur during winding and it will be difficult for air to escape in the wound roll state, resulting in minor wrinkles. On the other hand, if the inorganic particles 100 exceed 5.0 μm, the large particles will cause interlayer pressure in the film, and the filter pressure will increase in a short period of time during film production, resulting in increased production loss.

[0034] In one embodiment, the inorganic particles 100 are preferably included in an amount of 0.0008 to 0.0015 wt% based on the total weight of the biaxially stretched polyester film (100 wt%). If the content of the inorganic particles 100 is less than 0.0008 wt%, the air layer between the film layers becomes insufficient, causing surface protrusions at certain parts of the film, and these surface protrusions accumulate and become severe on the wound roll, causing processing defects, and the winding property becomes poor, and wrinkles in the MD direction may occur during winding. On the other hand, if the content of the inorganic particles 100 exceeds 0.0015 wt%, defects occur due to an increase in foreign matter caused by particle aggregation, and film breakage due to particles occurs during film stretching.

[0035] In one embodiment, the polyester resin 100 constituting the biaxially oriented polyester film may be formed by polymerizing a dicarboxylic acid and a diol, and may include polyethylene terephthalate (PET).

[0036] Here, the dicarboxylic acid may include aromatic dicarboxylic acids such as terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenylcarboxylic acid, diphenylsulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, 5-sodium sulfone dicarboxylic acid, and phthalic acid, aliphatic dicarboxylic acids such as hydroxyl, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid, alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid, and oxycarboxylic acids such as paraoxybenzoic acid. In addition, the diol may include aliphatic glycols such as ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, and neopentyl glycol, polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, and polypropylene glycol, alicyclic glycols such as cyclohexanedimethanol, and aromatic glycols such as bisphenol A and bisphenol S.

[0037] In one embodiment, the biaxially stretched polyester film according to one embodiment of the present invention preferably has a center line average roughness (SRa) of 40 to 80 nm. At this time, if the center line average roughness (SRa) is less than 40 nm, the space for air to escape between the layers during film lamination is small, causing minor wrinkles, and if it exceeds 80 nm, too many particles are introduced, increasing filter pressure during production and reducing productivity.

[0038] In one embodiment, the biaxially stretched polyester film preferably has a 10-point average roughness (SRz) of 800 to 1300 nm. If the 10-point average roughness (SRz) is less than 800 nm, the space for air to escape between layers during film lamination is reduced, resulting in the generation of wrinkles, and if it exceeds 1300 nm, too many particles are introduced, increasing the filter pressure during production and reducing productivity.

[0039] In one embodiment, the biaxially oriented polyester film preferably has a maximum roughness (SRmax) of 1100 to 1800 nm. If the maximum roughness (SRmax) is less than 1100 nm, the space for air to escape between layers during film lamination is reduced, resulting in the generation of wrinkles, and if it exceeds 1800 nm, too many particles are introduced, increasing the filter pressure during production and reducing productivity.

[0040] In one embodiment, the biaxially oriented polyester film preferably has a haze of 4% or more. If the haze is less than 4%, the particle content is accordingly low, which reduces the air release properties and causes wrinkles.

[0041] In one embodiment, the biaxially oriented polyester film has a surface peak count (SPC) of 0.1 mm2 or less. 2 It is preferable that the number of protrusions having an inner height of 0.15 μm or more is 100 or more. In this case, if the number of protrusions is less than 100, there will be less space for air to escape between the layers during film lamination, resulting in the formation of superficial wrinkles.

[0042] In one embodiment, the thickness of the biaxially oriented polyester film is preferably 10 to 16 μm. At this time, if the thickness is less than 10 μm, wrinkles will occur during roll winding, which is unfavorable for air escape, and thus minor wrinkles will occur. However, the thicker the biaxially oriented polyester film is, the less minor wrinkles will tend to occur, so the above upper limit is not limited.

[0043] A method for producing a biaxially oriented polyester film according to an embodiment of the present invention includes a first step of mixing inorganic particles and a polyester resin, extruding the mixture, and biaxially stretching the mixture to produce a master roll, a second step of slitting the mixture to a predetermined width, and a third step of rewinding the mixture at a predetermined winding tension, winding surface pressure, and winding speed to produce a rewound biaxially oriented polyester film roll.

[0044] In the first step of manufacturing a master roll by mixing inorganic particles and polyester resin, extruding and biaxially stretching the mixture, inorganic particles are mixed with polyester resin, and then extruded and biaxially stretched in MD and TD to manufacture a biaxially stretched polyester film. In this case, the average particle size of the inorganic particles is preferably 3.0 to 5.0 μm, and the content of the inorganic particles is preferably 0.0008 to 0.0015 wt% relative to 100 wt% of the total biaxially stretched polyester film. In addition, the polyester film manufactured in the first step is preferably wound in the form of a master roll for the convenience of subsequent processes.

[0045] Next, in the second step of slitting to a predetermined width, the polyester film wound in the form of a master roll is slit to a required width, i.e., a width required by a user. At this time, the width of the polyester film achieved by slitting is not particularly limited and can be freely adjusted according to the purpose of use of the polyester film and the requirements of the company that requires it.

[0046] Next, in the third step of manufacturing a rewound biaxially oriented polyester film roll by rewinding the polyester film to a predetermined width through slitting, the polyester film is rewound at a winding tension of 6.5 to 8.5 kg / m, a winding pressure of 35 to 50 kg / m, and a winding speed of 300 to 350 m / min, thereby preventing the biaxially oriented polyester film from developing wrinkles when rewound under the above conditions.

[0047] Meanwhile, the second step and the third step may be performed as separate processes, or for convenience of the process, the third step may be performed simultaneously during the slitting process of the second step.

[0048] In the above-mentioned method for producing a biaxially oriented polyester film, the same contents as those of the biaxially oriented polyester film shown in FIG. 1 are omitted.

[0049] In one embodiment, when the master roll on which the biaxially oriented polyester film is wound is slit and rewound to provide it to a user, it is preferable that the hardness of the final wound roll satisfies 90 to 95 points at a winding tension of 6.5 to 8.5 kg / m, a winding surface pressure of 35 to 50 kg / m, and a winding speed of 300 to 350 m / min. If the hardness of the roll is less than 90 points, sagging of the roll occurs and transverse wrinkles occur in the width direction of the film, and if it exceeds 95 points, minor wrinkles occur.

[0050] During rewinding as described above, if the winding tension is less than 6.5 kg / m and the winding surface pressure is less than 35 kg / m, transverse wrinkles and loss of winding surface will occur during winding onto the master roll due to the tension and surface pressure, resulting in incorrect winding and making it impossible to produce products. On the other hand, if the winding tension exceeds 8.5 kg / m and the winding surface pressure exceeds 50 kg / m, a large amount of air will get in between the different film layers, accelerating the generation of wrinkles.

[0051] In addition, if the winding speed is less than 300 m / min, the hardness of the roll increases, which causes minor wrinkles to occur, while if the winding speed exceeds 350 m / min, the winding speed becomes too fast, which increases the amount of air that gets between the different layers of the film, accelerating the occurrence of wrinkles.

[0052] As described above, the biaxially oriented polyester film according to an embodiment of the present invention is wound up and manufactured as a master roll. When the film is slit and then wound up again for later delivery to users, the film is wound up under the set conditions of winding tension of 6.5 to 8.5 kg / m, winding surface pressure of 35 to 50 kg / m, and winding speed of 300 to 350 m / min, so that air can be smoothly released from within the film, thereby preventing wrinkles from occurring over time.

[0053] As described above, the biaxially stretched polyester film according to an embodiment of the present invention can suppress the generation of minor wrinkles through the above-mentioned physical properties, and can prevent the problem of wrinkles being transferred to the separator during separator lamination processing, so that the biaxially stretched polyester film can be used as a separator interleaving paper for secondary batteries.

[0054] Therefore, it should be readily apparent to those skilled in the art that the present invention also encompasses a biaxially oriented polyester film used as an interleaf of a secondary battery separator, a secondary battery separator including the same, and a secondary battery including such a secondary battery separator.

[0055] The present invention will be described in more detail with reference to the following examples. These examples are provided to further illustrate the present invention, and the scope of the present invention is not limited to these examples. EXAMPLES

[0056] Example 1 0.00087wt% of inorganic particles (SiO2) with an average particle size of 3.75μm and 99.99913wt% of polyester resin (PET A9039, Toray Advanced Materials Co., Ltd.) are mixed. Next, the mixed polyester resin is vacuum dried and then melted at a temperature of 275-285℃ in an extruder, extruded through a feed block and a T-die (T-DIE), and cooled and solidified by contacting a cooling roll with a cast drum using an electrostatic application method (Pinning) to obtain an unstretched sheet, which is uniaxially stretched (longitudinal direction) by the difference in peripheral speed ratio between the rolls on a roll heated to a temperature above the glass transition temperature of the sheet. At this time, the stretch ratio in the longitudinal direction is 4.5 times. Next, after the uniaxial stretching, biaxial stretching is performed at 4.1 times in the transverse direction, which is perpendicular to the longitudinal direction. At this time, the stretching temperature is set to 100 to 120°C in the initial stage of the stretching zone, 120 to 140°C in the middle stage of the stretching zone, and 200 to 240°C in the stretch heat fixing zone, based on the tenter.

[0057] A polyester film master roll was produced in this manner, and the thickness of the biaxially oriented polyester film was 12 μm.

[0058] Next, the produced polyester film master roll was slitted to a width of 1,120 mm and rewound at a winding tension of 7.6 kg / m, a winding surface pressure of 48 kg / m, and a winding speed of 340 m / min to produce a biaxially oriented polyester film roll.

[0059] Example 2 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the content of the inorganic particles was 0.0008 wt %.

[0060] Example 3 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the content of the inorganic particles was 0.0015 wt %.

[0061] Example 4 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the size of the inorganic particles was 3 μm.

[0062] Example 5 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the size of the inorganic particles was 5 μm.

[0063] Example 6 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding tension was 6.5 kg / m.

[0064] Example 7 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding tension was 8.5 kg / m.

[0065] Example 8 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding surface pressure was 35 kg / m.

[0066] Example 9 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding surface pressure was 50 kg / m.

[0067] [Comparative Example] Comparative Example 1 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the size of the inorganic particles was 2.9 μm.

[0068] Comparative Example 2 A biaxially stretched polyester film roll was manufactured in the same manner as in Comparative Example 1, except that the content of the inorganic particles was 0.0016 wt %.

[0069] Comparative Example 3 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the size of the inorganic particles was 5.1 μm.

[0070] Comparative Example 4 A biaxially stretched polyester film roll was manufactured in the same manner as in Example 1, except that the content of the inorganic particles was 0.0007 wt %.

[0071] Comparative Example 5 A film was manufactured in the same manner as in Example 1, except that the content of the inorganic particles was 0.0016 wt %.

[0072] Comparative Example 6 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding tension was 8.6 kg / m.

[0073] Comparative Example 7 A film was produced in the same manner as in Example 1, except that the winding tension was 6.4 kg / m.

[0074] Comparative Example 8 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding surface pressure was 51 kg / m.

[0075] Comparative Example 9 A film was produced in the same manner as in Example 1, except that the winding surface pressure was 34 kg / m.

[0076] Comparative Example 10 A biaxially oriented polyester film roll was produced in the same manner as in Example 1, except that the winding speed was 290 m / min.

[0077] The biaxially stretched polyester film rolls prepared in Examples 1 to 9 and Comparative Examples 1 to 10 were evaluated for physical properties through the following experimental examples, and the results are shown in Table 1.

[0078] [Experimental Example] (1) Evaluation of minor wrinkles The film rolls made of the biaxially oriented polyester films prepared in the Examples and Comparative Examples were wrapped and stored, and after 2 days, the wrapping was opened and the surface condition of the wound roll was checked with the naked eye to see whether wrinkles were generated. The length of the wound film was 8,200 m, and it was evaluated as follows. Good: No visible wrinkles across the entire width of the film Defective: Minor wrinkles across the entire width of the film

[0079] (2) Hardness of the wound roll In accordance with the JIS K 7312 standard, a precision hardness tester (model: Type C) manufactured by ASKER was placed on the surface of the wound film roll and pressed down to measure the hardness at the left and right ends of the roll once each, and the hardness values ​​were confirmed.

[0080] (3) SRa, SRz, SRmax measurement (film surface roughness) The SRa, SRz, and SRmax values ​​of the surface of each film were measured using a KOSAKA three-dimensional surface roughness measuring instrument (model: SE3500K).

[0081] (4) Number of SPC projections Using a KOSAKA three-dimensional surface illuminance measuring device (model: SE3500K), the SPC (Peak Count) of the surface of each film was measured, and the measurement area was 0.1 mm 2 The number of protrusions with a height of 0.15 μm or more was calculated.

[0082] (5) Haze measurement One film was measured using a Hazemeter (model: COH-300A) manufactured by Nippon Denshoku Industries Co., Ltd.

[0083] [Table 1]

[0084] As can be seen from Table 1, in the case of Examples 1 to 9, 0.0008wt to 0.0015wt% of inorganic particles having an average particle size of 3.0 to 5.0μm are added to a polyester resin, and the polyester film master roll is made by extruding and biaxially stretching it. After slitting, the tension is set to 6.5 to 8.5kg / m, the surface pressure is set to 35 to 50kg / m, and the winding speed is set to 300 to 350m / min to manufacture a rewound roll, thereby providing a biaxially stretched polyester film with improved wrinkles.

[0085] In contrast, in the case of Comparative Example 1, the size of the inorganic particles was small, and the roll hardness was high, resulting in the generation of minor wrinkles. It can be seen that the SRmax was below the minimum range, the number of protrusions was less than 100, and the haze was less than 4%.

[0086] In addition, in Comparative Example 2, even though small inorganic particles were added, the roll hardness was high, causing minor wrinkles, and the SRmax was below the minimum range.

[0087] In addition, in Comparative Example 3, the inorganic particle size was too large, so SRa, SRz, and SRmax all exceeded their maximum values, and the roll hardness was low, so the physical properties of the produced film became soft, and the roll sagged to the bottom, causing poor winding.

[0088] In addition, in Comparative Example 4, the size of the inorganic particles is the same as in Example 1, but the content is only 0.0007 wt%, which is less than the standard. As a result, SRa, SRz, and SRmax are all below the minimum values, the number of protrusions is less than 100, the haze is less than 4%, and the roll hardness is high, resulting in the occurrence of minor wrinkles.

[0089] In Comparative Example 5, the hardness of the roll was too low, the film became soft, the roll sagged to the bottom side, and poor winding occurred. The pressure on the filter during production increased excessively, and productivity decreased.

[0090] In Comparative Example 6, the winding tension after slitting is set high at 8.6 kg / m, which increases the hardness of the roll and causes minor wrinkles.

[0091] In Comparative Example 8, the winding tension is the same as in Example 1, but the winding surface pressure is set high, resulting in increased roll hardness and the occurrence of minor wrinkles.

[0092] In Comparative Example 10, the winding speed was set low, resulting in increased roll hardness and the occurrence of minor wrinkles.

[0093] In particular, apart from the results shown in Table 1, it was confirmed that in Comparative Examples 7 and 9, which had low winding tension or winding surface pressure, winding defects such as poor winding cross section, pressing and folding occurred when the film was produced and wound.

[0094] Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also belong to the scope of the present invention.

Claims

1. A biaxially oriented polyester film comprising inorganic particles and a polyester resin, The master roll on which the biaxially oriented polyester film is wound is slit, and then rewound at a winding tension of 6.5 to 8.5 kg / m, a winding surface pressure of 35 to 50 kg / m, and a winding speed of 300 to 350 m / min. The hardness of the rewound roll is 90 to 95 points. The inorganic particles have an average particle size of 3.0 to 5.0 μm; The biaxially oriented polyester film contains 0.0008 to 0.0015 wt % of the inorganic particles.

2. 2. The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film has a center line average roughness (SRa) of 40 to 80 nm and a ten point average roughness (SRz) of 800 to 1,300 nm.

3. 3. The biaxially oriented polyester film of claim 2, wherein the biaxially oriented polyester film has a maximum roughness (SRmax) of 1100 to 1800 nm.

4. 2. The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film has a haze of 4% or more.

5. The biaxially oriented polyester film has a surface peak count (SPC) of 0.1 mm2 ２ 2. The biaxially oriented polyester film according to claim 1, wherein the number of protrusions having an inner height of 0.15 μm or more is 100 or more.

6. 2. The biaxially oriented polyester film according to claim 1, wherein the inorganic particles are silica particles.

7. 2. The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film has a thickness of 10 to 16 μm.

8. The biaxially oriented polyester film according to claim 1 , which is used as an interleaf of a separator for a secondary battery.

9. A secondary battery separator comprising the biaxially oriented polyester film according to claim 1 .

10. A secondary battery comprising the secondary battery separator of claim 9.

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