A high modulus BOPP matte film and preparation method thereof

Abstract

The invention relates to a high modulus BOPP matte film, comprising an upper surface layer, a core layer and a lower surface layer which are arranged in sequence; the upper surface layer comprises 90wt%-95wt% of a high haze matte material and 5wt%-10wt% of an ethylene-norbornene copolymer, wherein the insertion rate of the norbornene is 20mol%-25mol%, and the thickness of the upper surface layer is 1.1μm-1.3μm; the core layer comprises 88wt%-95wt% of isotactic polypropylene and 3wt%-7wt% of propylene-norbornene copolymer; the lower surface layer comprises isotactic polypropylene; and a preparation method of the high modulus BOPP matte film comprises the following steps: firstly performing a longitudinal stretching, then performing a transverse stretching, and then performing a secondary longitudinal stretching. The high modulus BOPP matte film of the present invention comprises: adding 5wt%-10wt% of ethylene-norbornene copolymer to the upper surface layer and limiting the insertion rate of norbornene to 20mol%-25mol%, adding 3wt%-7wt% of propylene-norbornene copolymer to the core layer, and coordinating the production process of two longitudinal stretchings and one transverse stretching, so as to smoothly produce the high modulus BOPP matte film with an ultra-thin matte layer and uniform matte, and has the advantages of high haze, low gloss, high modulus and smooth production.

Description

Technical Field

[0001] The invention relates to the field of films, in particular to a high modulus BOPP matte film and a preparation method thereof. Background Art

[0002] Biaxially oriented polypropylene matte film is a BOPP film with low gloss, high haze and matte appearance. It is made of matte layer, core layer and bottom layer through co-extrusion biaxial stretching process. Because of its diffuse reflection matte effect and elegant temperament, the common processing methods are: plastic-plastic composite, paper-plastic composite, and it is widely used in food, gifts, books and magazines and other fields.

[0003] However, when the matting material made by blending polypropylene (PP) and high-density polyethylene (HDPE) is used, the modulus of the BOPP matting film will decrease. After the thickness of the matting layer of the film is reduced, the problem of the decrease in the modulus of the BOPP matting film will be more prominent. Generally, the modulus of the film is increased by adding stiffeners, etc. Although it can improve the modulus of the film to a certain extent, it still cannot fully meet the application needs of customers. Specifically, in the process of paper-plastic composite, downstream customers will use a disc knife to cut the film and break the film longitudinally. Due to the insufficient longitudinal modulus of the matting film, the film will have a glue edge after being broken. Summary of the invention

[0004] Based on this, the object of the present invention is to provide a high modulus BOPP matte film, by adding 5wt% to 10wt% ethylene-norbornene copolymer to the upper surface layer and limiting the insertion rate of norbornene to 20mol% to 25mol%, adding 3wt% to 7wt% propylene-norbornene copolymer to the core layer, and then coordinating the production process of two longitudinal stretching and one transverse stretching, to achieve smooth production of a high modulus BOPP matte film with an ultra-thin matte layer and uniform matte, which has the advantages of high haze, low gloss, high modulus and smooth production.

[0005] The technical solution of the present invention is achieved by the following methods:

[0006] A high modulus BOPP matte film, comprising an upper surface layer, a core layer and a lower surface layer arranged in sequence;

[0007] The upper surface layer comprises 90 wt% to 95 wt% of high haze matting material and 5 wt% to 10 wt% of ethylene-norbornene copolymer; the insertion rate of norbornene in the ethylene-norbornene copolymer is 20 mol% to 25 mol%; the thickness of the upper surface layer is 1.1 μm to 1.3 μm;

[0008] The core layer comprises 88 wt% to 95 wt% of isotactic polypropylene and 3 wt% to 7 wt% of propylene-norbornene copolymer;

[0009] The lower surface layer comprises isotactic polypropylene;

[0010] The method for preparing the high modulus BOPP matte film comprises the following steps: firstly performing a longitudinal stretching, then performing a transverse stretching, and then performing a secondary longitudinal stretching.

[0011] The high modulus BOPP matte film of the present invention is prepared by adding 5wt% to 10wt% of ethylene-norbornene copolymer to the upper surface layer and limiting the insertion rate of norbornene to 20mol% to 25mol%, adding 3wt% to 7wt% of propylene-norbornene copolymer to the core layer, and then coordinating the production process of two longitudinal stretchings and one transverse stretching to smoothly produce a high modulus BOPP matte film with an ultra-thin matte layer and uniform matte, which has the advantages of high haze, low gloss, high modulus and smooth production.

[0012] The present invention adopts a scheme of adding ethylene-norbornene copolymer to the matt layer, adopts ethylene-norbornene copolymer to replace part of high-density polyethylene as the sea phase, reduces the content of relatively soft high-density polyethylene in the matt layer, realizes sufficient thinning of the matt layer and the matt layer after thinning can be smoothly stretched in the secondary longitudinal direction, improves the stiffness of the matt layer, cooperates with the secondary longitudinal stretching, jointly improves the overall modulus of the film, and ensures the haze of the product; further, the present invention also limits the content of ethylene-norbornene copolymer to 5wt%-10wt%, further ensures the modulus of the film without affecting the appearance of the film, if the content of ethylene-norbornene copolymer is less than 5wt%, it will lead to poor fluidity of HDPE, and then lead to the problem of poor haze and uneven matting of the film; if the content of ethylene-norbornene copolymer is greater than 10wt%, the improvement of the film modulus is not obvious, but the cost will be increased. Furthermore, the present invention also limits the insertion rate of norbornene in the ethylene-norbornene copolymer to 20 mol% to 25 mol%. If the insertion rate is greater than 25 mol%, norbornene itself is a cyclic olefin containing unsaturated double bonds and has high reactivity. The unsaturated double bonds in these copolymers can be introduced into crosslinking points through various chemical reactions, which easily lead to the generation of a crosslinking structure between the ethylene-norbornene copolymer and the high-density polyethylene, and the phenomenon of uneven extinction easily occurs. If the insertion rate is less than 20 mol%, the flow properties of HDPE cannot be effectively improved, and the problem of uneven dispersion occurs, which leads to uneven extinction of the film, which is not conducive to thinning the extinction layer. If the phenomenon of uneven extinction is improved by increasing the thickness of the extinction layer, the modulus of the BOPP extinction film will be reduced, and the effect of improving the film modulus cannot be achieved.

[0013] Isotactic polypropylene usually has a higher crystallization rate and a lower crystallization temperature because the space between its linear molecular chains is relatively open, which makes the intermolecular interaction force weaker, making it easy to form a crystal structure. When a propylene-norbornene copolymer is added to the core layer, on the one hand, due to the larger volume of norbornene, it will hinder the movement of the isotactic polypropylene molecular chain, thereby reducing the crystallization rate, but increasing the crystallization temperature, which is conducive to the formation of a microcrystalline structure of isotactic polypropylene and is conducive to improving the secondary longitudinal tensile properties of the film; on the other hand, norbornene is a monomer with a special structure that can combine with isotactic polypropylene through intermolecular forces such as hydrogen bonds or van der Waals forces, enhancing the hydrogen bond interaction between adjacent molecular chains, thereby improving the binding force between molecular chains. This combination also helps to improve the crystallinity and mechanical properties of isotactic polypropylene, thereby improving the modulus of the film.

[0014] Based on this, the present invention adopts a solution of adding propylene-norbornene copolymer to the core layer. On the one hand, the propylene-norbornene copolymer increases the intermolecular force, provides a favorable nucleation environment for the melting process of isotactic polypropylene, helps to improve the crystallinity and mechanical properties of the core layer, thereby improving the modulus of the film; on the other hand, it reduces the size of the spherulites, so that the isotactic polypropylene forms a microcrystalline structure, which is beneficial to secondary longitudinal stretching, thereby improving the modulus of the film, while effectively ensuring the light transmittance of the film, and improving the production smoothness during stretching, while ensuring the thickness is flat.

[0015] On this basis, the present invention also limits the content of the propylene-norbornene copolymer to 3wt% to 7wt%, ensuring smooth film production. Specifically, if the content of the propylene-norbornene copolymer is less than 3wt%, the effect of improving the modulus of the film is not obvious, and if the content of the propylene-norbornene copolymer is greater than 7wt%, it is not conducive to biaxial stretching and is likely to cause continuous film breakage during production.

[0016] In response to the problems that customers encounter during longitudinal stretching, the present invention also adopts a process of secondary longitudinal stretching and primary transverse stretching to improve the longitudinal orientation of the BOPP film, thereby improving the longitudinal modulus of the film, and also more specifically avoiding the problem of glue edges appearing on the film after longitudinal stretching. The present invention adds a certain amount of ethylene-norbornene copolymer to the matte layer and a certain amount of propylene-norbornene copolymer to the core layer, which is conducive to the secondary longitudinal stretching of the BOPP matte film with an ultra-thin matte layer, effectively solving the problem that the BOPP matte film with an ultra-thin matte layer usually cannot be smoothly stretched in the secondary longitudinal direction or the problem of uneven thickness and film breakage after secondary stretching, further ensuring the quality and smoothness of the production of the BOPP matte film with an ultra-thin matte layer, and having a high modulus.

[0017] The present invention achieves smooth production of a high modulus BOPP matte film with an ultra-thin matte layer and uniform matte under the synergistic effect of adding a certain amount of ethylene-norbornene copolymer to the matte layer, adding a certain amount of propylene-norbornene to the core layer, and secondary longitudinal stretching and primary transverse stretching processes, thereby improving the glue edge problem of the film in downstream applications.

[0018] Furthermore, the insertion rate of norbornene in the propylene-norbornene copolymer is 40 mol% to 50 mol%. Although increasing the crystallization temperature of isotactic polypropylene in the core layer and promoting the formation of a microcrystalline structure of isotactic polypropylene is beneficial to improving the smoothness of film production, too high a crystallization temperature will cause isotactic polypropylene to form a highly perfect crystal, resulting in difficulty in biaxial stretching during film production and prone to film breakage. Therefore, the inventors also limit the insertion rate of norbornene in the propylene-norbornene copolymer to 40 mol% to 50 mol%. When the insertion rate of norbornene in the propylene-norbornene copolymer is less than 40 mol%, the effect of improving the modulus of the matt film is not obvious; further, too high an insertion rate of norbornene will lead to an increase in the molecular weight of the propylene-norbornene copolymer. The larger the molecular weight, the smaller the proportion of the propylene segment, and thus the higher the glass transition temperature. In the present invention, when the insertion rate of norbornene in the propylene-norbornene copolymer is greater than 50 mol%, the crystallization temperature of the isotactic polypropylene is too high, and more segments tend to form highly perfect crystals, which increases the difficulty of biaxial stretching and secondary longitudinal stretching, resulting in easy film breakage during production.

[0019] Furthermore, the stretching ratio of the first longitudinal stretching is 4.8 to 5 times, the stretching ratio of the transverse stretching is 6.5 to 8 times, and the stretching ratio of the second longitudinal stretching is 1.1 to 1.2 times. The use of the above corresponding stretching ratios is conducive to synergistically improving the overall modulus of the film and ensuring smooth production.

[0020] Furthermore, the lower surface layer comprises 94wt% to 96wt% of isotactic polypropylene and 4wt% to 6wt% of anti-blocking masterbatch.

[0021] Furthermore, the high haze matte material comprises 61-63 wt % of high density polyethylene and 37-39 wt % of copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0022] Furthermore, the melting point of the isotactic polypropylene is 170° C. to 180° C., and the melt index of the isotactic polypropylene is 2 g / 10 min to 4 g / 10 min.

[0023] Furthermore, during the first longitudinal stretching and the second longitudinal stretching, the temperature of the upper surface layer is 120° C. to 135° C., and the temperature of the lower surface layer is 135° C. to 155° C., and during the transverse stretching, the temperature of the upper surface layer and the temperature of the lower surface layer are both 140° C. to 180° C. The above temperature range is conducive to ensuring sufficient heating and stretching of the thick sheet and improving the modulus of the film.

[0024] Furthermore, the core layer further comprises 2wt% to 5wt% of an auxiliary agent, and the auxiliary agent is one or more of an antistatic masterbatch, a slip masterbatch, and a hydrogenated petroleum resin.

[0025] Furthermore, the effective content of the anti-blocking agent in the anti-blocking masterbatch is 6wt%, and the anti-blocking agent is fumed silica.

[0026] The present invention also provides a method for preparing any of the above-mentioned high modulus BOPP matte films, comprising the following steps: putting the screened raw materials of each layer component into a batching unit and stirring them evenly, entering an extruder after being metered, and after being melted and plasticized and homogenized and metered, the melt of each layer passes through a filter and then is co-extruded through a die head in multiple layers, and according to a flat film method, a cast sheet is formed and then quenched into a thick sheet, the thick sheet is first longitudinally stretched once, then transversely stretched, and then longitudinally stretched twice to form a biaxially stretched film, and after the film is cooled, one of corona treatment, flame treatment, and plasma treatment is performed, and then a film mother roll is collected, and the film mother roll is subjected to aging treatment and slitting to obtain a finished product.

[0027] For better understanding and implementation, the present invention is described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic structural diagram of a high modulus BOPP matte film according to an embodiment of the present invention;

[0029] Among them, there are upper surface layer 1, core layer 2, and lower surface layer 3. DETAILED DESCRIPTION

[0030] It should be clear that the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the embodiments of the present application.

[0031] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the embodiments of the present application. The singular forms of "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0032] When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. On the contrary, they are merely examples of devices and methods consistent with some aspects of the present application as detailed in the attached claims. In the description of the present application, it should be understood that the terms "first", "second", "third", etc. are only used to distinguish similar objects, and do not have to be used to describe a specific order or sequence, nor can they be understood as indicating or implying relative importance. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0033] In addition, in the description of this application, unless otherwise specified, "multiple" means two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0034] It should be understood that the embodiments of the present application are not limited to the precise structures described above and shown in the drawings, and various modifications and changes can be made without departing from the scope thereof. The scope of the embodiments of the present application is limited only by the appended claims.

[0035] See also Figure 1 As an embodiment of the present invention, the high modulus BOPP matte film of this embodiment adds 5wt% to 10wt% ethylene-norbornene copolymer to the upper surface layer and limits the insertion rate of norbornene to 20mol% to 25mol%, and adds 3wt% to 7wt% propylene-norbornene copolymer to the core layer, and then cooperates with the production process of two longitudinal stretching and one transverse stretching to smoothly produce a high modulus BOPP matte film with an ultra-thin matte layer and uniform matte, which has the advantages of high haze, low gloss, high modulus and smooth production.

[0036] Furthermore, the insertion rate of norbornene in the propylene-norbornene copolymer is 40 mol% to 50 mol%.

[0037] Furthermore, the stretching ratio of the first longitudinal stretching is 4.8 to 5 times, the stretching ratio of the transverse stretching is 6.5 to 8 times, and the stretching ratio of the second longitudinal stretching is 1.1 to 1.2 times.

[0038] Furthermore, the lower surface layer comprises 94wt% to 96wt% of isotactic polypropylene and 4wt% to 6wt% of anti-blocking masterbatch.

[0039] Furthermore, the high haze matte material comprises 61wt% to 63wt% high density polyethylene and 37wt% to 39wt% copolymerized polypropylene; wherein the copolymerized polypropylene can be one or more of ethylene-propylene-butylene copolymerized polypropylene and ethylene-propylene copolymerized polypropylene.

[0040] Furthermore, the melting point of the isotactic polypropylene is 170° C. to 180° C., and the melt index of the isotactic polypropylene is 2 g / 10 min to 4 g / 10 min.

[0041] Furthermore, when the first longitudinal stretching and the second longitudinal stretching are performed, the temperature of the upper surface layer is 120°C to 135°C, and the temperature of the lower surface layer is 135°C to 155°C. When the transverse stretching is performed, the temperature of the upper surface layer and the temperature of the lower surface layer are both 140°C to 180°C.

[0042] Furthermore, the core layer further comprises 2wt% to 5wt% of an auxiliary agent, and the auxiliary agent is one or more of an antistatic masterbatch, a slip masterbatch, and a hydrogenated petroleum resin.

[0043] Furthermore, the effective content of the anti-blocking agent in the anti-blocking masterbatch is 6wt%, and the anti-blocking agent is fumed silica.

[0044] The invention also provides a method for preparing any of the above-mentioned high modulus BOPP matte films, comprising the following steps: putting the screened raw materials of each layer component into a batching unit and stirring them evenly, entering an extruder after being metered, and after being melted and plasticized and homogenized and metered, the melt of each layer passes through a filter and then is co-extruded through a die head in multiple layers, and according to the flat film method, the cast film is quenched into a thick sheet, the thick sheet is first longitudinally stretched once, then transversely stretched, and then longitudinally stretched twice to form a biaxially stretched film, and after the film is cooled, it is subjected to one of corona treatment, flame treatment, and plasma treatment, and then collected into a film mother roll, and the film mother roll is subjected to aging treatment and slitting to obtain a finished product.

[0045] The physical properties and test methods of the embodiments or comparative examples of the present invention are as follows:

[0046] The melt index (melt mass flow rate MFR) is based on GB / T3682-2018, and polypropylene and matte materials are measured at 2.16 kg and 230°C.

[0047] Glossiness is tested according to GB / T8807-88 at a 45° angle.

[0048] Haze and light transmittance are tested according to GB / T2410-2008.

[0049] The Young's modulus test method is formulated according to GB / T 1040.1-2018 "Determination of tensile properties of plastics Part 1: General principles". The specific method is as follows: Use a tensile testing machine to set the parameters as follows: gauge length is 10cm, stretching distance is 2mm, speed is 1mm / min, and the stress values ​​at two positions with a stroke of 0.05mm and 0.25mm are taken to calculate the Young's modulus.

[0050] It should be noted that the proportions described in the embodiments or comparative examples of the present invention are all weight percentages.

[0051] Example 1

[0052] This embodiment provides a high modulus BOPP matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The method for preparing each layer of resin of the high modulus BOPP matte film of this embodiment comprises the following steps:

[0053] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0054] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0055] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0056] The screened raw materials of each layer component are put into the batching unit and stirred evenly, and then enter the extruder after being measured. After being melted and plasticized and homogenized and measured, the melt of each layer passes through the filter and then is co-extruded through the die head. According to the flat film method, the film is cast and then chilled into a thick sheet. The thick sheet is first longitudinally stretched once, then transversely stretched, and then longitudinally stretched twice to form a biaxially stretched film. After the film is cooled, it is corona treated and then collected into a film mother roll. The film mother roll is aging treated and slit to obtain a finished product. In the above process flow, the upper surface layer melt extrusion temperature is: 200℃~260℃; the core layer melt extrusion temperature is: 240℃~270℃; the lower surface layer melt extrusion temperature is: 240℃~270℃; the chilling water and chilling roller temperature: 15℃~50℃; the longitudinal stretching zone temperature (upper surface layer) is 120℃~135℃; the longitudinal stretching temperature (lower surface layer) is 135℃~155℃; the transverse stretching zone temperature is 140℃~180℃.

[0057] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0058] The total thickness of the film in this embodiment is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0059] The film of this embodiment can be produced smoothly, and the obtained film has uniform matte. No glue edge appears after the film is cut along the transverse direction and pulled apart along the longitudinal direction by a disc knife.

[0060] Example 2

[0061] This embodiment provides a high modulus BOPP matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The method for preparing each layer of resin of the high modulus BOPP matte film of this embodiment comprises the following steps:

[0062] Preparation of upper surface layer resin: 90wt% high haze matting material and 10wt% ethylene-norbornene copolymer (norbornene insertion rate 20mol%) are uniformly mixed to obtain upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0063] Preparation of core layer resin: 95 wt% of isotactic polypropylene, 3 wt% of propylene-norbornene copolymer (norbornene insertion rate 50 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0064] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0065] The preparation steps of the high modulus BOPP matte film of this embodiment are the same as those of Embodiment 1, so they are not described in detail.

[0066] Among them, the first longitudinal stretching ratio is 4.9 times, the transverse stretching ratio is 8.0 times, and the second longitudinal stretching ratio is 1.2 times.

[0067] The total thickness of the film in this embodiment is 13 μm, the thickness of the upper surface layer is 1.1 μm, and the thickness of the lower surface layer is 1.3 μm.

[0068] The film of this embodiment can be produced smoothly, and the obtained film has uniform matte. No glue edge appears after the film is cut along the transverse direction and pulled apart along the longitudinal direction by a disc knife.

[0069] Example 3

[0070] This embodiment provides a high modulus BOPP matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The method for preparing each layer of resin of the high modulus BOPP matte film of this embodiment comprises the following steps:

[0071] Preparation of upper surface layer resin: 95wt% high haze matting material and 5wt% ethylene-norbornene copolymer (norbornene insertion rate 25mol%) are uniformly mixed to obtain the upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0072] Preparation of core layer resin: 91 wt% of isotactic polypropylene, 7 wt% of propylene-norbornene copolymer (norbornene insertion rate 40 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0073] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0074] The preparation steps of the high modulus BOPP matte film of this embodiment are the same as those of Embodiment 1, so they are not described in detail.

[0075] Among them, the first longitudinal stretching ratio is 4.9 times, the transverse stretching ratio is 8.0 times, and the second longitudinal stretching ratio is 1.2 times.

[0076] The total thickness of the film in this embodiment is 13 μm, the thickness of the upper surface layer is 1.3 μm, and the thickness of the lower surface layer is 1.3 μm.

[0077] The film of this embodiment can be produced smoothly, and the obtained film has uniform matte. No glue edge appears after the film is cut along the transverse direction and pulled apart along the longitudinal direction by a disc knife.

[0078] Example 4

[0079] This embodiment provides a high modulus BOPP matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The method for preparing each layer of resin of the high modulus BOPP matte film of this embodiment comprises the following steps:

[0080] Preparation of upper surface layer resin: 90wt% high haze matting material and 10wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0081] Preparation of core layer resin: 91 wt% of isotactic polypropylene, 7 wt% of propylene-norbornene copolymer (norbornene insertion rate 50 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0082] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0083] The preparation steps of the high modulus BOPP matte film of this embodiment are the same as those of Embodiment 1, so they are not described in detail.

[0084] Among them, the first longitudinal stretching ratio is 4.9 times, the transverse stretching ratio is 8.0 times, and the second longitudinal stretching ratio is 1.2 times.

[0085] The total thickness of the film in this embodiment is 12 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.0 μm.

[0086] The film of this embodiment can be produced smoothly, and the obtained film has uniform matte. No glue edge appears after the film is cut along the transverse direction and pulled apart along the longitudinal direction by a disc knife.

[0087] Example 5

[0088] This embodiment provides a high modulus BOPP matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The method for preparing each layer of resin of the high modulus BOPP matte film of this embodiment comprises the following steps:

[0089] Preparation of upper surface layer resin: 93wt% high haze matting material and 7wt% ethylene-norbornene copolymer (norbornene insertion rate 20mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0090] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 50 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0091] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0092] The preparation steps of the high modulus BOPP matte film of this embodiment are the same as those of Embodiment 1, so they are not described in detail.

[0093] Among them, the first longitudinal stretching ratio is 4.8 times, the transverse stretching ratio is 8.0 times, and the second longitudinal stretching ratio is 1.1 times.

[0094] The total thickness of the film in this embodiment is 15 μm, the thickness of the upper surface layer is 1.3 μm, and the thickness of the lower surface layer is 1.5 μm.

[0095] The film of this embodiment can be produced smoothly, and the obtained film has uniform matte. No glue edge appears after the film is cut along the transverse direction and pulled apart along the longitudinal direction by a disc knife.

[0096] Comparative Example 1

[0097] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0098] Preparation of upper surface layer resin: 100wt% high haze matting material is taken to obtain upper surface layer resin; wherein the high haze matting material comprises 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0099] Preparation of core layer resin: 95 wt% of isotactic polypropylene and 5 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0100] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0101] The preparation steps of the matte film of this comparative example are different from those of Example 1 in that a single longitudinal stretching is adopted, and the other steps are the same and thus will not be described in detail.

[0102] The longitudinal stretching ratio was 5.4 times, and the transverse stretching ratio was 8.0 times.

[0103] The total thickness of the film of this comparative example is 13 μm, and the thickness of the upper surface layer and the lower surface layer are both 2 μm.

[0104] The film of this comparative example has poor flatness, large thickness fluctuations, and is easily broken during production. When the film is cut along its transverse direction with a disc knife and pulled apart along its longitudinal direction, a glue edge appears.

[0105] Comparative Example 2

[0106] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0107] Preparation of upper surface layer resin: 100wt% high haze matting material is taken to obtain upper surface layer resin; wherein the high haze matting material comprises 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0108] Preparation of core layer resin: 95 wt% of isotactic polypropylene and 5 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0109] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0110] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0111] Among them, the first longitudinal stretching ratio is 4.8 times, the transverse stretching ratio is 8.0 times, and the second longitudinal stretching ratio is 1.2 times.

[0112] The film of this comparative example had the problem of continuous film breakage during production.

[0113] Comparative Example 3

[0114] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0115] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 35mol%) are uniformly mixed to obtain the upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0116] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0117] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0118] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0119] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0120] The total thickness of the film of this comparative example is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0121] The film of this comparative example has problems of uneven extinction and insufficient improvement in modulus. When the film is cut in the transverse direction with a disc knife and pulled apart in the longitudinal direction, a glue edge appears.

[0122] Comparative Example 4

[0123] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0124] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 10mol%) are uniformly mixed to obtain upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0125] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0126] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0127] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0128] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0129] The total thickness of the film of this comparative example is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0130] The film of this comparative example has problems of low haze, uneven extinction, and insufficient improvement in modulus. When the film is cut in the transverse direction with a disc knife and pulled in the longitudinal direction, a glue edge appears.

[0131] Comparative Example 5

[0132] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0133] Preparation of upper surface layer resin: 80wt% high haze matting material and 20wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0134] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0135] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0136] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0137] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0138] The total thickness of the film of this comparative example is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0139] The film of this comparative example has too high a cost and has no significant improvement in modulus relative to the example.

[0140] Comparative Example 6

[0141] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0142] Preparation of upper surface layer resin: 98wt% high haze matting material and 2wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0143] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0144] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0145] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0146] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0147] The total thickness of the film of this comparative example is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0148] The film of this comparative example has problems such as low haze, uneven extinction, and insufficient improvement in modulus. When the film is cut in the transverse direction with a disc knife and pulled in the longitudinal direction, a glue edge appears.

[0149] Comparative Example 7

[0150] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0151] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0152] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 60 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0153] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0154] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0155] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0156] The film of this comparative example had the problem of continuous film breakage during one longitudinal stretching and transverse stretching process.

[0157] Comparative Example 8

[0158] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0159] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0160] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 30 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0161] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0162] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0163] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0164] The film of this comparative example had the problem of continuous film breakage during the secondary longitudinal stretching.

[0165] Comparative Example 9

[0166] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0167] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0168] Preparation of core layer resin: 97 wt% isotactic polypropylene, 1 wt% propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0169] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0170] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0171] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0172] The film of this comparative example had the problem of continuous film breakage during the secondary longitudinal stretching.

[0173] Comparative Example 10

[0174] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0175] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0176] Preparation of core layer resin: 88 wt% isotactic polypropylene, 10 wt% propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0177] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin.

[0178] The preparation steps of the matte film of this comparative example are the same as those of Example 1, so they are not described in detail.

[0179] The first longitudinal stretching ratio was 5.0 times, the transverse stretching ratio was 8.0 times, and the second longitudinal stretching ratio was 1.2 times.

[0180] The film of this comparative example had the problem of continuous film breakage during one longitudinal stretching and transverse stretching process.

[0181] Comparative Example 11

[0182] This comparative example provides a matte film, including an upper surface layer, a core layer and a lower surface layer arranged in sequence. The specific structure can be referred to Figure 1 The preparation method of each layer of resin of the matte film of this comparative example comprises the following steps:

[0183] Preparation of upper surface layer resin: 92wt% high haze matting material and 8wt% ethylene-norbornene copolymer (norbornene insertion rate 22mol%) are uniformly mixed to obtain upper surface layer resin; wherein, the high haze matting material includes 63wt% high density polyethylene and 37wt% copolymerized polypropylene; wherein, the copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

[0184] Preparation of core layer resin: 93 wt% of isotactic polypropylene, 5 wt% of propylene-norbornene copolymer (norbornene insertion rate 45 mol%) and 2 wt% of antistatic masterbatch were uniformly mixed to obtain core layer resin.

[0185] Preparation of lower surface layer resin: 96 wt% isotactic polypropylene and 4 wt% anti-blocking masterbatch were mixed evenly to obtain lower surface layer resin. The preparation steps of the matte film of this comparative example differ from those of Example 1 in that a single longitudinal stretching is used, and the other steps are the same and will not be described in detail.

[0186] The longitudinal stretching ratio was 5.0 times, and the transverse stretching ratio was 8.0 times.

[0187] The total thickness of the film in this comparative example is 13 μm, the thickness of the upper surface layer is 1.2 μm, and the thickness of the lower surface layer is 1.3 μm.

[0188] The film of this comparative example has the problem of insufficient improvement in modulus, and a glue edge appears after the film is cut in the transverse direction with a disc knife and pulled in the longitudinal direction.

[0189] The performance test results and composite effect test results of the high modulus BOPP matte films of Examples 1 to 5 and Comparative Examples 1 to 11 are shown in Table 1 below.

[0190] Table 1

[0191]

[0192] It can be seen that not all films can be smoothly produced by the secondary longitudinal stretching process. The smoothness of the production of the secondary longitudinal stretching process needs to be based on the appropriate film formula. This application prepares a BOPP matte film with an ultra-thin matte layer that can be smoothly longitudinally stretched for a second time by limiting the content of ethylene-norbornene copolymer and its norbornene insertion rate in the upper surface layer, and the content of propylene-norbornene copolymer and its norbornene insertion rate in the core layer. Under the specific selection of component formula and process, a high-modulus BOPP matte film with uniform matte is obtained by synergistic effect, which effectively improves the problem of glue edge of the film after the customer is longitudinally stretched.

[0193] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, and the present invention is also intended to include these modifications and modifications.

Claims

1. A high modulus BOPP matte film, characterized in that: It includes an upper surface layer, a core layer and a lower surface layer arranged in sequence; The upper surface layer comprises 90wt% to 95wt% of high haze matting material and 5wt% to 10wt% of ethylene-norbornene copolymer; the high haze matting material comprises 61wt% to 63wt% of high density polyethylene and 37wt% to 39wt% of copolymerized polypropylene, and the insertion rate of norbornene in the ethylene-norbornene copolymer is 20mol% to 25mol%; the thickness of the upper surface layer is 1.1μm to 1.3μm; The core layer comprises 88 wt% to 95 wt% of isotactic polypropylene and 3 wt% to 7 wt% of propylene-norbornene copolymer, wherein the insertion rate of norbornene in the propylene-norbornene copolymer is 40 mol% to 50 mol%; The lower surface layer comprises isotactic polypropylene; The method for preparing the high modulus BOPP matte film comprises the following steps: firstly performing a longitudinal stretching, then performing a transverse stretching, and then performing a secondary longitudinal stretching.

2. The high modulus BOPP matte film according to claim 1, characterized in that: The stretching ratio of the first longitudinal stretching is 4.8 to 5 times, the stretching ratio of the transverse stretching is 6.5 to 8 times, and the stretching ratio of the second longitudinal stretching is 1.1 to 1.2 times.

3. The high modulus BOPP matte film according to claim 1, characterized in that: The lower surface layer includes 94wt% to 96wt% isotactic polypropylene and 4wt% to 6wt% anti-blocking masterbatch.

4. The high modulus BOPP matte film according to claim 1, characterized in that: The copolymerized polypropylene is ethylene-propylene copolymerized polypropylene.

5. The high modulus BOPP matte film according to claim 1 is characterized in that: The melting point of the isotactic polypropylene is 170° C. to 180° C., and the melting index of the isotactic polypropylene is 2 g / 10 min to 4 g / 10 min.

6. The high modulus BOPP matte film according to claim 1, characterized in that: When performing the first longitudinal stretching and the second longitudinal stretching, the temperature of the upper surface layer is 120°C~135°C, and the temperature of the lower surface layer is 135°C~155°C. When performing the transverse stretching, the temperature of the upper surface layer and the temperature of the lower surface layer are both 140°C~180°C.

7. The high modulus BOPP matte film according to claim 1, characterized in that: The core layer further comprises 2wt% to 5wt% of an auxiliary agent, wherein the auxiliary agent is one or more of an antistatic masterbatch, a slip masterbatch, and a hydrogenated petroleum resin.

8. The method for preparing a high modulus BOPP matte film according to claim 3, characterized in that: The effective content of the anti-blocking agent in the anti-blocking masterbatch is 6wt%, and the anti-blocking agent is fumed silica.

9. A method for preparing the high modulus BOPP matte film according to any one of claims 1 to 8, characterized in that: The following steps are involved: The screened raw materials of each layer component are put into the batching unit and stirred evenly, and then enter the extruder after being measured. After being melted and plasticized and measured by homogenization, the melt of each layer passes through the filter and then is co-extruded through the die head. According to the flat film method, the cast sheet is quenched into a thick sheet. The thick sheet is first longitudinally stretched once, then transversely stretched, and then longitudinally stretched twice to form a biaxially stretched film. After the film is cooled, it is subjected to one of corona treatment, flame treatment, and plasma treatment, and then collected into a film mother roll. The film mother roll is subjected to aging treatment and slitting to obtain the finished product.

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