Biaxially stretched polypropylene film

Abstract

The present application discloses a biaxially stretched polypropylene film. The biaxially extending polypropylene film includes an intermediate layer and a first surface layer disposed on the intermediate layer. The intermediate layer includes a first polypropylene having a first melting point, a petroleum resin, and a nucleating agent. The first melting point is higher than 135 DEG C. Based on 100 parts by weight of the total weight of the middle layer, the content of the first polypropylene is 88-99.5 parts by weight, the content of the petroleum resin is 1.5-10 parts by weight, and the content of the nucleating agent is 0.1-0.5 part by weight. The first skin layer includes a second polypropylene having a second melting point less than 135 DEG C.

Description

Technical Field

[0001] The present application relates to a biaxially stretched polypropylene film, in particular to a biaxially stretched polypropylene film suitable for high-speed bag making. Background Art

[0002] Due to the difference in the nature of the materials, the heat resistance and rigidity of biaxially oriented polypropylene (BOPP) film are lower than those of biaxially oriented polyester film. Therefore, compared with biaxially oriented polyester film, biaxially oriented polypropylene is more limited in terms of product application.

[0003] For example, biaxially oriented polypropylene film can be used for packaging materials. In the process of bag making, in order to maintain the flatness when folding the bag and the heat melting strength after bag making, the speed of using biaxially oriented polypropylene film to make packaging materials should not be too high. Specifically, the bag making speed should not exceed 170 bags per minute.

[0004] When the bag making speed is increased, the heat sealing of the biaxially stretched polypropylene film may be incomplete, resulting in insufficient heat-melting strength. In addition, when the bag making speed is increased, if the rigidity of the biaxially stretched polypropylene film is insufficient, it is easy to be skewed or wrinkled during the bag folding process.

[0005] Therefore, how to improve the bag-making speed of biaxially oriented polypropylene film by improving the materials without negatively affecting the bag-making quality has become one of the important issues that this business wants to solve. Summary of the invention

[0006] The technical problem to be solved by the present application is to provide a biaxially stretched polypropylene film in view of the deficiencies of the prior art.

[0007] In order to solve the above-mentioned technical problems, one of the technical solutions adopted in the present application is to provide a biaxially stretched polypropylene film. The biaxially stretched polypropylene film includes an intermediate layer and a first surface layer arranged on the intermediate layer. The intermediate layer includes a first polypropylene having a first melting point, a petroleum resin and a crystal nucleating agent. The first melting point is higher than 135°C. Based on the total weight of the intermediate layer of 100 parts by weight, the content of the first polypropylene is 88 parts by weight to 99.5 parts by weight, the content of the petroleum resin is 1.5 parts by weight to 10 parts by weight, and the content of the crystal nucleating agent is 0.1 parts by weight to 0.5 parts by weight. The first surface layer includes a second polypropylene having a second melting point, and the second melting point is lower than 135°C.

[0008] Furthermore, the first melting point is at least 10° C. higher than the second melting point.

[0009] Furthermore, the first polypropylene is a propylene homopolymer.

[0010] Furthermore, the second polypropylene is a propylene random copolymer, which is synthesized from ethylene monomers and propylene monomers.

[0011] Furthermore, the petroleum resin is an aromatic copolymerized hydrogenated petroleum resin.

[0012] Furthermore, the petroleum resin is a hydrogenated petroleum resin having 5 or 9 carbon atoms.

[0013] Furthermore, the weight ratio of the petroleum resin to the crystal nucleating agent is 5 to 25.

[0014] Furthermore, the crystal nucleating agent is selected from the group consisting of: fatty carboxylic acid metal compounds, aromatic carboxylic acid metal compounds, organic phosphates, sorbitol benzyl derivatives or lignin acid and its derivatives.

[0015] Furthermore, based on 100 parts by weight of the total weight of the first surface layer, the first surface layer includes 90 parts by weight to 98 parts by weight of the propylene random copolymer and 2 parts by weight to 10 parts by weight of the anti-sticking agent.

[0016] Furthermore, the biaxially stretched polypropylene film further comprises: a second surface layer, the second surface layer is disposed on the middle layer and faces away from the first surface layer. The second surface layer comprises a third polypropylene having a third melting point, and the third melting point is lower than 135°C.

[0017] One of the beneficial effects of the present application is that the biaxially stretched polypropylene film provided in the present application can improve the bag making speed of the biaxially stretched polypropylene film through the technical solutions of "the middle layer includes a first polypropylene, a petroleum resin and a crystal nucleating agent" and "the first melting point is higher than 135°C, and the second melting point is lower than 135°C".

[0018] To further understand the features and technical contents of the present application, please refer to the following detailed description and drawings of the present application. However, the drawings provided are only for reference and illustration and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic side view of the biaxially stretched polypropylene film of the present application.

[0020] Figure 2 This is a schematic diagram of the packaging structure of the biaxially stretched polypropylene film of this application. DETAILED DESCRIPTION

[0021] The following is an explanation of the implementation methods of the "biaxially stretched polypropylene film" disclosed in the present application through specific examples. Those skilled in the art can understand the advantages and effects of the present application from the contents disclosed in this specification. The present application can be implemented or applied through other different specific embodiments, and the details in this specification can also be modified and changed in various ways based on different viewpoints and applications without departing from the concept of the present application. In addition, the drawings of the present application are only simple schematic illustrations and are not depicted in actual size. It is stated in advance. The following implementation methods will further explain the relevant technical content of the present application in detail, but the disclosed content is not intended to limit the scope of protection of the present application. In addition, the term "or" used in this article may include any one or more combinations of the associated listed items depending on the actual situation.

[0022] In order to overcome the problem that the manufacturing speed of the biaxially stretched polypropylene film cannot be increased, the biaxially stretched polypropylene film of the present application is a composite structure, which includes multiple layers, and the main component of each layer is polypropylene. The present application also adjusts the composition of each layer so that the biaxially stretched polypropylene film can have both high thermal fusing strength and high rigidity.

[0023] See also Figure 1 As shown, the biaxially stretched polypropylene film 1 of the present application at least comprises an intermediate layer 11 and a first surface layer 12 disposed on the intermediate layer 11. The intermediate layer 11 and the first surface layer 12 can be integrally formed by a coextrusion process, and then the final biaxially stretched polypropylene film 1 is obtained by a stretching process.

[0024] After the biaxially stretched polypropylene film 1 is obtained, a bag making machine can be used to make the following Figure 2 The packaging bag Z shown in the figure. The biaxially stretched polypropylene film is folded in half and then heat-pressed and sealed at a distance of 1.5 cm from the edge. Therefore, a heat-pressed area 2 is formed on the folded biaxially stretched polypropylene film 1, and the heat-pressed area 2 forms a containing space inside the packaging bag Z.

[0025] Please refer to Figure 1 As shown, the main components of the middle layer 11 and the first surface layer 12 are both polypropylene. After co-extrusion, no obvious boundary is observed in the cross-sectional structure of the biaxially stretched polypropylene film 1, but for the convenience of description, they will be referred to as independent layers in the specification.

[0026] In terms of material selection, although the main components of the middle layer 11 and the first surface layer 12 are both polypropylene, the types of polypropylene are different. The main component of the middle layer 11 is the first polypropylene, and the melting point of the first polypropylene is higher than 135°C. The main component of the first surface layer 12 is the second polypropylene, and the melting point of the second polypropylene is lower than 135°C. In other words, the melting point of the first polypropylene is higher than the melting point of the second polypropylene. In some embodiments, the melting point of the first polypropylene is at least 10°C higher than the melting point of the second polypropylene.

[0027] In an exemplary embodiment, the melting point of the first polypropylene is 150° C. to 175° C., and the melting point of the second polypropylene is 115° C. to 135° C. Preferably, the melting point of the first polypropylene is 160° C. to 166° C., and the melting point of the second polypropylene is 125° C. to 135° C.

[0028] The main component of the middle layer 11 is a propylene homopolymer (first polypropylene). The propylene homopolymer has good crystallinity, which can improve the rigidity of the middle layer 11. In addition, the high crystallinity can also improve the heat-melting strength of the biaxially stretched polypropylene film 1 after heat sealing. Specifically, the melt index of the propylene homopolymer is 2 g / 10 minutes to 4 g / 10 minutes.

[0029] In order to further improve the crystallinity of the propylene homopolymer, the material of the middle layer 11 further includes petroleum resin and a crystal nucleating agent. The petroleum resin can improve the crystallinity of the amorphous phase in the propylene homopolymer, and the crystal nucleating agent can improve the crystallinity of the crystalline phase in the propylene homopolymer.

[0030] It should be noted that, in addition to improving the rigidity of the middle layer 11, improving the crystallinity of the propylene homopolymer will also improve the thermal fusing strength of the biaxially stretched polypropylene film 1. However, when the biaxially stretched polypropylene film 1 has a higher thermal fusing strength, the time required for heat sealing will also increase. When the bag making speed is increased, it may lead to insufficient heat sealing time and fail to achieve the desired heat sealing effect. Therefore, when the crystallinity of the propylene homopolymer is high to a certain extent, it may not be suitable for a high-speed bag making process.

[0031] On the other hand, when the crystallinity of the propylene homopolymer is low, although the biaxially stretched polypropylene film 1 can be heat-sealed in a shorter time, the thermal melting strength of the biaxially stretched polypropylene film 1 after bag making cannot be improved due to the material itself, and may not meet product expectations. In addition, when the crystallinity of the propylene homopolymer is low, the problem of insufficient rigidity of the intermediate layer 11 will also arise. In high-speed processes, the insufficient rigidity will negatively affect the flatness of the biaxially stretched polypropylene film 1 bag making.

[0032] Therefore, while adjusting the crystallinity of the propylene homopolymer to increase the bag making speed, it is also necessary to consider the rigidity of the biaxially stretched polypropylene film 1, the thermal melt strength after bag making, and the flatness during the bag making process. The above characteristics cannot be achieved simply by increasing the crystallinity.

[0033] The present application improves the crystallinity of the crystalline phase and the amorphous phase in the propylene homopolymer by adding petroleum resin and a crystal nucleating agent, thereby improving the thermal fusing strength of the biaxially stretched polypropylene film 1 after bag making. After the thermal fusing strength is improved, the biaxially stretched polypropylene film 1 can still achieve the expected heat sealing effect even when the bag is made at a higher speed. Therefore, the biaxially stretched polypropylene film 1 of the present application can be applied to a high-speed bag making process.

[0034] In addition, experiments have shown that if only petroleum resin is added without adding a crystal nucleating agent, or only adding a crystal nucleating agent without adding petroleum resin, the biaxially stretched polypropylene film 1 cannot have good thermal melting strength. The specific experimental data will be described later. The present application simultaneously improves the crystallinity of the crystalline phase and the amorphous phase in the propylene homopolymer, so that the biaxially stretched polypropylene film 1 can be suitable for high-speed bag making processes.

[0035] In order to adjust the crystallinity of the crystalline phase and the amorphous phase in the propylene homopolymer, the weight ratio of the petroleum resin to the crystal nucleating agent is 5 to 25. In this way, a balance can be achieved between the rigidity of the biaxially stretched polypropylene film 1, the thermal fusion strength after bag making, and the flatness during bag making.

[0036] Specifically, based on 100 parts by weight of the total weight of the intermediate layer 11 , the intermediate layer 11 includes 88 to 99.5 parts by weight of propylene homopolymer, 1.5 to 10 parts by weight of petroleum resin, and 0.1 to 0.5 parts by weight of a crystal nucleating agent.

[0037] Preferably, the total weight of the intermediate layer 11 is 100 parts by weight, and the intermediate layer 11 includes 90 to 98 parts by weight of propylene homopolymer, 1.8 to 6 parts by weight of petroleum resin, and 0.15 to 0.45 parts by weight of crystal nucleating agent. More preferably, the intermediate layer 11 includes 91 to 96 parts by weight of propylene homopolymer, 2 to 5 parts by weight of petroleum resin, and 0.2 to 0.4 parts by weight of crystal nucleating agent.

[0038] The petroleum resin is a hydrogenated petroleum resin, which can make the propylene homopolymer have appropriate crystallinity. For example, the petroleum resin can be a hydrogenated petroleum resin with a carbon number of 5 or 9, for example: the petroleum resin can be a hydrogenated isoprene resin or a hydrogenated dicyclopentadiene (DCPD) resin. In addition, the petroleum resin can also be an aromatic copolymer hydrogenated petroleum resin.

[0039] The crystal nucleus agent is an organic crystal nucleus agent, for example, the crystal nucleus agent can be a fatty carboxylic acid metal compound, an aromatic carboxylic acid metal compound, an organic phosphate, a sorbitol benzyl derivative or a lignin acid and its derivatives. In an exemplary embodiment, the aromatic carboxylic acid metal compound is sodium benzoate and bis(p-tert-butylbenzoic acid) carboxy aluminum.

[0040] In some embodiments, the material of the intermediate layer 11 further includes an antifogging agent. Taking the total weight of the intermediate layer 11 as 100 parts by weight, the amount of the antifogging agent added is 1 part by weight to 5 parts by weight.

[0041] The main component of the first surface layer 12 is a propylene random copolymer (second polypropylene). The propylene random copolymer allows the first surface layer 12 to melt quickly for heat sealing. In this way, the biaxially stretched polypropylene film 1 can be applied to a higher speed bag making process. Specifically, the melt index of the propylene homopolymer is 6 g / 10 min to 7 g / 10 min.

[0042] In the exemplary embodiment, the propylene random copolymer is synthesized from ethylene monomer and propylene monomer. Since the arrangement of monomer polymerization is not fixed, the propylene random copolymer has low crystallinity and a low melting point. Therefore, during the bag making process, the first surface layer 12 can be quickly combined, thereby improving the heat sealing effect of the biaxially stretched polypropylene film 1.

[0043] Specifically, based on the total weight of the first surface layer 12 being 100 parts by weight, the first surface layer 12 includes 90 parts by weight to 98 parts by weight of the propylene random copolymer and 2 parts by weight to 10 parts by weight of the anti-adhesive agent. Preferably, based on the total weight of the first surface layer 12 being 100 parts by weight, the first surface layer 12 includes 93 parts by weight to 97 parts by weight of the propylene random copolymer and 3 parts by weight to 7 parts by weight of the anti-adhesive agent.

[0044] The addition of the anti-adhesive agent can improve the opening property of the packaging bag. Specifically, the anti-adhesive agent is composed of silicon dioxide. The anti-adhesive agent includes silicon dioxide with a size of 2 microns to 15 microns. In an exemplary embodiment, two anti-adhesive agents with different particle sizes can be mixed, for example, an anti-adhesive agent with a particle size of 1 micron to 3 microns and an anti-adhesive agent with a particle size of 3.1 microns to 5 microns are mixed.

[0045] In order to be applied to a high-speed bag-making process, the thickness of the middle layer 11 can be 10 microns to 30 microns, the thickness of the first surface layer 12 can be 0.2 microns to 2 microns, and the total thickness of the biaxially stretched polypropylene film 1 is 10.2 microns to 34 microns. Preferably, the thickness of the middle layer 11 can be 18 microns to 19 microns, the thickness of the first surface layer 12 can be 0.5 microns to 1 micron, and the total thickness of the biaxially stretched polypropylene film 1 is 18.5 microns to 21 microns.

[0046] If the thickness of the intermediate layer 11 is too thin, resulting in insufficient rigidity, it may negatively affect the flatness of the biaxially stretched polypropylene film 1 after bag making. If the thickness of the intermediate layer 11 is too thick, the required heat sealing time increases, which is not conducive to accelerating the bag making process. If the thickness of the first surface layer 12 is too thin, the biaxially stretched polypropylene film 1 cannot be quickly combined during the bag making process. If the thickness of the first surface layer 12 is too thick, the thermal fusing strength will decrease.

[0047] See also Figure 1 As shown, the biaxially stretched polypropylene film 1 may further include a second surface layer 13. The second surface layer 13 is disposed on the intermediate layer 11 and faces away from the first surface layer 12. Through the design of the composite structure, the biaxially stretched polypropylene film 1 can be applied to a relatively high-speed bag-making process.

[0048] The main component of the second surface layer 13 is a propylene random copolymer (third polypropylene). The propylene random copolymer enables the second surface layer 13 to melt quickly for heat sealing. In this way, the biaxially stretched polypropylene film 1 can be applied to a higher speed bag making process.

[0049] In an exemplary embodiment, the propylene random copolymer is synthesized from ethylene monomer and propylene monomer. The melting point of the third polypropylene is lower than 135°C. That is, the melting point of the first polypropylene is higher than the melting point of the third polypropylene. In some embodiments, the melting point of the first polypropylene is at least 10°C higher than the melting point of the third polypropylene. Specifically, the melting point of the third polypropylene is 115°C to 135°C. Preferably, the melting point of the third polypropylene is 125°C to 135°C.

[0050] The second surface layer 13 includes 90 to 98 parts by weight of the propylene random copolymer and 2 to 10 parts by weight of the anti-adhesive agent, based on the total weight of the second surface layer 13 being 100 parts by weight. Preferably, the second surface layer 13 includes 93 to 97 parts by weight of the propylene random copolymer and 3 to 7 parts by weight of the anti-adhesive agent, based on the total weight of the second surface layer 13 being 100 parts by weight.

[0051] In order to be applied to a higher speed bag making process, the thickness of the second surface layer 13 may be 0.2 micrometers to 2 micrometers. Preferably, the thickness of the second surface layer 13 may be 0.5 micrometers to 1 micrometer.

[0052] [Experimental data]

[0053] In order to verify that the biaxially stretched polypropylene film 1 of the present application is applicable to a higher speed bag making process, materials of the middle layer 11, the first surface layer 12 and the second surface layer 13 are prepared and co-extruded to form the biaxially stretched polypropylene films 1 of Examples 1 to 3 and Comparative Examples 1 to 3.

[0054] The added weight of the materials of each layer in the biaxially oriented polypropylene film 1 is listed in Table 1. The thicknesses of the middle layer 11, the first surface layer 12 and the second surface layer 13 are 18 microns, 1 micron and 1 micron, respectively, so the total thickness of the biaxially oriented polypropylene film 1 is 20 microns.

[0055] The biaxially stretched polypropylene films 1 of Examples 1 to 3 and Comparative Examples 1 to 3 were fed to a bag making machine to produce packaging bags at a rate of 200 bags / minute. The setting temperature of the fuse knife in the bag making machine was 400° C. After the bags were made, the thermal melting strength test, anti-fogging test and opening test were carried out, and the results are listed in Table 2.

[0056] In the thermal melt strength test experiment, a sample with a length of 15 mm was cut at the melting point of the packaging bag. The sample was fixed to the tensile machine with a clamp, and the strength test was carried out at an angle of 180° and a tensile speed of 300 mm / min. Five samples were taken from the biaxially stretched polypropylene film 1, and the average of the five test results was listed in Table 1. In Table 1, when the thermal melt strength is greater than 18N / 15mm, it is represented by "○". When the thermal melt strength is between 16N / 15mm and 17.9N / 15mm, it is represented by "△". When the thermal melt strength is less than 15.9N / 15mm, it is represented by "×".

[0057] In the anti-fog test experiment, 100 ml of water was measured and placed in a 250 ml beaker. The packaging bag was covered on the beaker and placed in a 4°C refrigerator for 10 minutes, and the attachment of water droplets on the surface of the packaging bag was observed. In Table 1, water vapor attached to the surface of the packaging bag to form a water film, and no fog was generated, which was indicated by "○". Water vapor formed water droplets on the packaging bag and could not be seen through, which was indicated by "×".

[0058] In the opening test, the bag mouth is rubbed between the thumb and index finger to check whether the bag mouth is easy to open. In Table 1, if the bag mouth is not sticky after rubbing with the fingers and can be easily opened, it is represented by "○". If the bag mouth is sticky after rubbing with the fingers and cannot be easily opened, it is represented by "×".

[0059] In Table 1, the propylene random copolymer used in the first and second surface layers is a propylene random copolymer purchased from Taiwan Plastics, model 5050 (melt index 6.5 g / 10 min, melting point 133° C.). The anti-adhesive agents used in the first and second surface layers include silica (anti-adhesive agent A, particle size 2 μm) purchased from Kenchem, model PSE-55, and silica purchased from Constant Engineering Plastics, model 1000 μm. AB 06019 PP silicon dioxide (anti-sticking agent B, particle size 4 microns). The propylene homopolymer used in the middle layer is a propylene homopolymer model PPR-FT03-S (melt index 3.1 g / 10 minutes, melting point 165°C) purchased from Sinopec Maoming Petrochemical Company (referred to as Maoming Petrochemical, Maoming Petrochemical or Sinopec). The anti-fog agent used in the middle layer is an anti-fog agent masterbatch model PAT-381 purchased from Riken Vitamin Co., Ltd. of Japan. The petroleum resin added to the middle layer is an aromatic copolymer hydrogenated petroleum resin model P125 purchased from Idemitsu Kosan Co., Ltd. of Japan. The nucleating agent added to the middle layer is a sorbitol nucleating agent.

[0060] Table 1

[0061]

[0062]

[0063] Table 2

[0064]

[0065] According to the contents of Table 1 and Table 2, the biaxially stretched polypropylene film of the present application can be applied to the high-speed (200 bags / minute) bag-making process, and has good thermal melt strength (greater than 18N / 15mm). Adding petroleum resin and crystal nucleating agent in the middle layer can improve the crystallinity of the crystalline phase and amorphous phase in the propylene homopolymer, thereby improving the rigidity and thermal melt strength. Even if the bag-making rate is increased, the packaging bag can still have good thermal melt strength, and the thermal melt strength will not decrease due to the increase in the bag-making rate.

[0066] It can be seen from the results of Examples 1 to 3 that when the relative weight ratio of petroleum resin to crystal nucleating agent is 5 to 25, the biaxially stretched polypropylene film can be more applicable to high-speed bag-making processes. Specifically, the addition of crystal nucleating agent can directly improve the crystallinity of the crystalline phase in propylene homopolymer, and the addition of petroleum resin can improve the crystallinity of the amorphous phase in propylene homopolymer, making the texture of the amorphous phase change from soft to hard. Therefore, when the addition amount of crystal nucleating agent is high, the addition amount of petroleum resin can be reduced. However, when the crystal nucleating agent is excessive, the texture of the biaxially stretched polypropylene film will be hard and brittle, and it is easy to break the film during the film-making process, which is not conducive to production.

[0067] From the results of Example 1, when the relative weight ratio of petroleum resin to the crystal nucleating agent is 12 to 16.9, the petroleum resin and the crystal nucleating agent complement each other, and the crystallinity of the crystalline phase and the hardness of the amorphous phase in the propylene homopolymer are improved. From the results of Example 2, when the relative weight ratio of petroleum resin to the crystal nucleating agent is 17 to 25, the petroleum resin is the main factor in improving the crystallinity of the propylene homopolymer, and at this time, the hardness of the amorphous phase in the propylene homopolymer is relatively high. From the results of Example 3, when the relative weight ratio of petroleum resin to the crystal nucleating agent is 5 to 11.9, the crystal nucleating agent is the main factor in improving the crystallinity of the propylene homopolymer, and a smaller amount of petroleum resin can be added. At this time, the crystallinity of the crystalline phase in the propylene homopolymer is relatively high.

[0068] It can be seen from the results of Comparative Examples 2 and 3 that if only petroleum resin or only nucleating agent is added, the biaxially stretched polypropylene film cannot have a thermal melt strength greater than 18N / 15mm.

[0069] [Beneficial Effects of Embodiments]

[0070] One of the beneficial effects of the present application is that the biaxially stretched polypropylene film provided in the present application can improve the bag making speed of the biaxially stretched polypropylene film through the technical solutions of "the middle layer includes a first polypropylene, a petroleum resin and a crystal nucleating agent" and "the first melting point is higher than 135°C, and the second melting point is lower than 135°C".

[0071] The contents disclosed above are only preferred feasible embodiments of the present application, and are not intended to limit the protection scope of the claims of the present application. Therefore, all equivalent technical changes made using the contents of the present application specification and drawings are included in the protection scope of the claims of the present application.

Claims

1. A biaxially stretched polypropylene film, characterized in that: The biaxially stretched polypropylene film comprises: An intermediate layer, comprising a first polypropylene having a first melting point, a petroleum resin, and a crystal nucleating agent; wherein the first melting point is higher than 135° C., based on 100 parts by weight of the total weight of the intermediate layer, the content of the first polypropylene is 88 parts by weight to 99.5 parts by weight, the content of the petroleum resin is 1.5 parts by weight to 10 parts by weight, and the content of the crystal nucleating agent is 0.1 parts by weight to 0.5 parts by weight; and The first surface layer is disposed on the middle layer, wherein the first surface layer comprises a second polypropylene having a second melting point, wherein the second melting point is lower than 135°C.

2. The biaxially stretched polypropylene film according to claim 1, characterized in that: The first melting point is at least 10°C higher than the second melting point.

3. The biaxially stretched polypropylene film according to claim 1, characterized in that: The first polypropylene is a propylene homopolymer.

4. The biaxially stretched polypropylene film according to claim 1, characterized in that: The second polypropylene is a propylene random copolymer, and the propylene random copolymer is synthesized from ethylene monomer and propylene monomer.

5. The biaxially stretched polypropylene film according to claim 1, characterized in that: The petroleum resin is an aromatic copolymer hydrogenated petroleum resin.

6. The biaxially stretched polypropylene film according to claim 1, characterized in that: The petroleum resin is a hydrogenated petroleum resin having 5 or 9 carbon atoms.

7. The biaxially stretched polypropylene film according to claim 1, characterized in that: The weight ratio of the petroleum resin to the crystal nucleating agent is 5 to 25.

8. The biaxially stretched polypropylene film according to claim 1, characterized in that: The crystal nucleating agent is selected from the group consisting of: fatty carboxylic acid metal compounds, aromatic carboxylic acid metal compounds, organic phosphates, sorbitol benzyl derivatives, and lignin acid and its derivatives.

9. The biaxially stretched polypropylene film according to claim 1, characterized in that: Based on 100 parts by weight of the total weight of the first surface layer, the first surface layer includes 90 parts by weight to 98 parts by weight of the propylene random copolymer and 2 parts by weight to 10 parts by weight of the anti-sticking agent.

10. The biaxially stretched polypropylene film according to claim 1, characterized in that: The biaxially stretched polypropylene film further comprises: a second surface layer, which is disposed on the middle layer and faces away from the first surface layer; wherein the second surface layer comprises a third polypropylene having a third melting point, and the third melting point is lower than 135°C.

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