Corona-free prime-coat-free biaxially oriented polypropylene film as well as preparation method and application thereof

Abstract

The invention relates to the field of films, in particular to a corona-free prime coat-free biaxially oriented polypropylene film and a preparation method and application thereof, and a thick sheet for preparing the film comprises an upper surface layer, a supporting layer and a lower surface layer which are arranged in sequence; the thickness of the upper surface layer is 2-3 [mu] m, the upper surface layer comprises homo-polypropylene and 5-10 wt% of ethylene-methyl acrylate copolymer microcapsules, the microcapsules take silicon dioxide as shells and ethylene-methyl acrylate copolymers as contents, and the particle size of the microcapsules is 5-7 [mu] m; the supporting layer and the lower surface layer both comprise homo-polypropylene. According to the film disclosed by the invention, the ethylene-methyl acrylate copolymer microcapsules are added to the upper surface layer, the shells of the microcapsules are broken by virtue of tensile force during two-way stretching, the ethylene-methyl acrylate copolymer flows out and is spread on the surface of the upper surface layer to form a bottom coating, and the silicon dioxide shell is used as an anti-adhesion agent to prevent adhesion of the film during winding and unwinding; the process is simple and the production efficiency is high.

Description

Technical Field

[0001] This invention relates to the field of films, and in particular to a corona-free, primer-free biaxially oriented polypropylene film, its preparation method, and its applications. Background Technology

[0002] Existing biaxially oriented polypropylene (BOPP) films require corona treatment before printing to ensure a surface tension of at least 36 dynes due to the significant difference in polarity between the ink and the BOPP film surface. This is followed by a primer coating to further enhance the surface tension to at least 50 dynes, ensuring smooth ink spread without any missed areas.

[0003] The aforementioned processing method requires the film to undergo two additional processes: corona treatment and primer coating, resulting in a long production process, high energy consumption, and high production costs. Summary of the Invention

[0004] Based on this, the purpose of this invention is to provide a corona-free and primer-free biaxially oriented polypropylene film and its preparation method. By adding ethylene-methyl acrylate copolymer microcapsules to the upper surface layer, the huge tensile force during biaxial stretching of the film causes the microcapsule shell to rupture, allowing the ethylene-methyl acrylate copolymer inside the microcapsules to flow out and spread on the surface of the upper surface layer. During the film preparation process, a primer coating is formed, eliminating the need for additional corona treatment and primer coating processes before printing. The remaining silica shell acts as an anti-blocking agent for the film, preventing adhesion during film unwinding and rewinding. The process is simple and has high production efficiency.

[0005] The technical solution of the present invention is achieved in the following ways: A corona-free, primer-free biaxially oriented polypropylene film is disclosed. The film is prepared by biaxial stretching of a thick sheet, the thick sheet comprising an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The upper surface layer has a thickness of 2-3 μm and comprises homopolymer polypropylene and 5-10 wt% ethylene-methyl acrylate copolymer microcapsules. The ethylene-methyl acrylate copolymer microcapsules have a silica shell and ethylene-methyl acrylate copolymer as the content, and the particle size of the ethylene-methyl acrylate copolymer microcapsules is 5-7 μm. Both the support layer and the lower surface layer comprise homopolymer polypropylene.

[0006] The present invention relates to a corona-free, primer-free biaxially oriented polypropylene film with ethylene-methyl acrylate (EMA) copolymer microcapsules added to the upper surface layer. During biaxial stretching, the capsules remain intact during thick-sheet extrusion and longitudinal stretching. As the film undergoes transverse stretching, the thickness of the upper surface layer gradually decreases to less than the microcapsule particle size, exposing the microcapsules on the surface. Under the immense tensile force generated during stretching, the microcapsule shells rupture, allowing the EMA copolymer inside to flow out and spread across the upper surface. This forms a primer coating during film preparation. The remaining silica shell acts as an anti-blocking agent, preventing film adhesion during winding and unwinding. Therefore, the film can be directly printed without corona treatment or additional primer coating steps before printing, resulting in a simple process and high production efficiency. The present invention chooses EMA copolymer as the capsule filler because it contains a large number of ester bonds, which can form strong hydrogen bonds with ink, facilitating strong ink adhesion to the film surface and ensuring good printability.

[0007] This invention limits the amount of ethylene-methyl acrylate copolymer microcapsules to 5-10 wt%, and limits the thickness of the upper surface layer to 2-3 μm and the particle size of the ethylene-methyl acrylate copolymer microcapsules to 5-7 μm. On the one hand, selecting a microcapsule particle size of 5-7 μm ensures that the microcapsules can encapsulate a sufficient amount of ethylene-methyl acrylate copolymer, while limiting the thickness of the upper surface layer to 2-3 μm ensures that the ethylene-methyl acrylate copolymer microcapsules can be exposed on the upper surface layer after the film is stretched. On the other hand, controlling the amount of microcapsules to 5-10 wt% helps ensure that the ethylene-methyl acrylate copolymer can spread on the surface of the upper surface layer after the microcapsules rupture, and that the remaining silica shell continues to protrude from the surface of the upper surface layer as an anti-blocking agent, which helps to prevent the film from stacking and sticking together after winding, making it difficult to use.

[0008] If the amount of ethylene-methyl acrylate copolymer microcapsules is less than 5 wt%, the amount of ethylene-methyl acrylate copolymer that can be spread on the surface of the film during biaxial stretching will be low, resulting in insufficient or even incomplete thickness of the ethylene-methyl acrylate copolymer undercoat, which will affect the printability of the film. If the amount of ethylene-methyl acrylate copolymer microcapsules is greater than 10 wt%, the number of silica shells on the film surface after the microcapsules rupture will be large, which can easily cause unevenness on the film surface and even form crystal points and other appearance defects.

[0009] If the particle size of the ethylene-methyl acrylate (EMA) copolymer microcapsules is less than 5 μm, on the one hand, the amount of EMA copolymer that can be encapsulated within the microcapsules will be insufficient, resulting in insufficient or even incomplete EMA copolymer undercoating thickness on the surface of the film after stretching. On the other hand, the remaining silica shell after the microcapsule shell ruptures cannot fully protrude from the upper surface, which is detrimental to the anti-adhesion performance of the upper surface and thus hinders the smoothness of film winding and unwinding. If the particle size of the EMA copolymer microcapsules is greater than 7 μm, on the one hand, it will significantly reduce the surface smoothness of the film and even cause crystal points to appear. It is also easy to form stress weak points during biaxial stretching, which may lead to film rupture. On the other hand, it may cause the microcapsules to fail to rupture after stretching, thus failing to release the encapsulated EMA copolymer and failing to form the EMA copolymer undercoating layer. This will result in the need for corona treatment and undercoating during subsequent printing applications.

[0010] Furthermore, the ethylene-methyl acrylate copolymer has a melting point of 90°C, which gives it sufficient fluidity so that after flowing out of the microcapsules during transverse stretching, it can quickly and evenly spread onto the film surface to form an effective and uniform undercoat.

[0011] Furthermore, under test conditions of 230℃ and 2.16kg, the melt index of the homopolymer polypropylene was measured to be 2-4g / 10min, which is beneficial to ensure the efficient and smooth operation of the biaxial stretching process.

[0012] Further, the preparation method of the ethylene-methyl acrylate copolymer microcapsules includes the following steps: preparing silica hollow microspheres; mixing silica hollow microspheres, ethylene-methyl acrylate copolymer, and ethanol at a ratio of silica hollow microspheres: ethylene-methyl acrylate copolymer = 1g: (9~12)g; sealing and heating the mixture at 60°C for 24h; filtering and washing with acetonitrile to obtain the ethylene-methyl acrylate copolymer microcapsules. Controlling the feeding ratio of silica hollow microspheres: ethylene-methyl acrylate copolymer = 1g: (9~12)g ensures the content of ethylene-methyl acrylate copolymer encapsulated within the capsules.

[0013] Furthermore, the ethylene-methyl acrylate copolymer (EMA) content in the ethylene-methyl acrylate (EMA) copolymer microcapsules is 15-20 wt%, which helps ensure that the formed EMA copolymer base coating has a suitable thickness, thus guaranteeing the printability of the film. Through experimentation, the inventors have verified that the maximum EMA copolymer content in the EMA copolymer microcapsules can only reach 20 wt%. If the EMA copolymer content in the EMA copolymer microcapsules is too low, the amount of encapsulated EMA copolymer will be insufficient, resulting in insufficient or even incomplete EMA copolymer copolymer coating thickness on the film surface after stretching.

[0014] Furthermore, the support layer also includes 10-20 wt% pearlescent masterbatch.

[0015] Furthermore, the lower surface layer also includes 1-5 wt% anti-blocking masterbatch. Providing a lower surface layer with anti-blocking masterbatch helps prevent the upper and lower surfaces from sticking together during film unwinding.

[0016] The present invention also provides a method for preparing the corona-free and primer-free biaxially oriented polypropylene film as described above, comprising the following steps: mixing the raw materials of each layer according to the ratio and adding them into each extruder respectively; after being metered by a metering pump, the melt of each extruder is merged into a thick sheet at a T-die; after being chilled, the thick sheet is first stretched longitudinally and then stretched transversely; after stretching, it is shaped and then wound up; then it is subjected to aging treatment and slitting to obtain the corona-free and primer-free biaxially oriented polypropylene film.

[0017] Furthermore, the melt extrusion temperature of the upper surface layer is 160-180℃, the melt extrusion temperature of the support layer is 200-220℃, the melt extrusion temperature of the lower surface layer is 220-240℃, the temperature of the chill roll is 50-80℃, the biaxial stretching temperature is 100-120℃, and the stretching ratio of longitudinal stretching and transverse stretching is 4-5 times.

[0018] The present invention also provides an application of any of the above-described corona-free and primer-free biaxially oriented polypropylene films as a printing base film.

[0019] To better understand and implement this invention, the following detailed description is provided in conjunction with the accompanying drawings. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of the upper surface layer of the corona-free and primer-free biaxially oriented polypropylene film before stretching, as described in this invention. Figure 2 This is a schematic diagram of the structure of the upper surface layer of the corona-free and primer-free biaxially oriented polypropylene film described in this invention after stretching. The composition includes: upper surface layer 1, ethylene-methyl acrylate copolymer microcapsules 2, silica hollow microspheres 3, and bottom coating layer 4. Detailed Implementation

[0021] It should be understood that the described embodiments are merely some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of the embodiments of this application.

[0022] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0023] In the following description, when referring to the accompanying 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 this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims. In the description of this application, it should be understood that the terms "first," "second," "third," etc., are used only to distinguish similar objects and are not necessarily used to describe a specific order or sequence, nor should they be construed as indicating or implying relative importance. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0024] Furthermore, in the description of this application, unless otherwise stated, "multiple" means two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0025] It should be understood that the embodiments of this application are not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from their scope. The scope of the embodiments of this application is limited only by the appended claims.

[0026] This embodiment provides a corona-free and primer-free biaxially oriented polypropylene film. The film is prepared by biaxial stretching of a thick sheet. The thick sheet includes an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thickness of the upper surface layer is 2~3μm. The upper surface layer includes homopolymer polypropylene and 5~10wt% ethylene-methyl acrylate copolymer microcapsules. The ethylene-methyl acrylate copolymer microcapsules have a silica shell and ethylene-methyl acrylate copolymer as the content. The particle size of the ethylene-methyl acrylate copolymer microcapsules is 5~7μm. Both the support layer and the lower surface layer include homopolymer polypropylene.

[0027] Please see Figure 1 and Figure 2In the present invention, when the thick sheet of the corona-free and primer-free biaxially oriented polypropylene film is biaxially stretched, the thickness of the upper surface layer 1 is gradually reduced to a size smaller than that of the ethylene-methyl acrylate copolymer microcapsules 2, so that part of the ethylene-methyl acrylate copolymer microcapsules 2 are exposed on the surface of the upper surface layer. Under the huge tensile force generated by biaxial stretching, the shell of the microcapsules is torn and cracks are generated, allowing the ethylene-methyl acrylate copolymer inside the microcapsules to flow out and spread on the surface of the upper surface layer, thereby forming a primer layer 4 during the film preparation process. The remaining shell of the silica hollow microspheres 3 acts as an anti-blocking agent for the film, preventing the film from sticking together when it is wound up. This allows the film to be printed directly without corona treatment or additional steps to form a primer layer before printing, resulting in a simple process and high production efficiency.

[0028] Furthermore, the melting point of the ethylene-methyl acrylate copolymer is 90°C.

[0029] Furthermore, the melt index of the homopolymer polypropylene was measured to be 2-4 g / 10 min under test conditions of 230℃ and 2.16 kg.

[0030] Further, the preparation method of the ethylene-methyl acrylate copolymer microcapsules includes the following steps: preparing silica hollow microspheres, mixing silica hollow microspheres, ethylene-methyl acrylate copolymer and ethanol in a ratio of silica hollow microspheres: ethylene-methyl acrylate copolymer = 1g: (9~12)g, sealing and heating at 60°C for 24h, filtering and washing with acetonitrile to obtain the ethylene-methyl acrylate copolymer microcapsules.

[0031] Furthermore, the method for preparing the silica hole microspheres is a template method.

[0032] Furthermore, the content of ethylene-methyl acrylate copolymer in the ethylene-methyl acrylate copolymer microcapsules is 15~25wt%.

[0033] Furthermore, the support layer also includes 10-20 wt% pearlescent masterbatch.

[0034] Furthermore, the lower surface layer also includes 1-5 wt% anti-blocking masterbatch.

[0035] The present invention also provides a method for preparing the corona-free and primer-free biaxially oriented polypropylene film as described above, comprising the following steps: mixing the raw materials of each layer according to the ratio and adding them into each extruder respectively; after being metered by a metering pump, the melt of each extruder is merged into a thick sheet at a T-die; after being chilled, the thick sheet is first stretched longitudinally and then stretched laterally; after stretching, it is shaped and then wound up; then it is subjected to aging treatment and slitting to obtain the corona-free and primer-free biaxially oriented polypropylene film.

[0036] Furthermore, the melt extrusion temperature of the upper surface layer is 160-180℃, the melt extrusion temperature of the support layer is 200-220℃, the melt extrusion temperature of the lower surface layer is 220-240℃, the temperature of the chill roll is 50-80℃, the biaxial stretching temperature is 100-120℃, and the stretching ratio of longitudinal stretching and transverse stretching is 4-5 times.

[0037] The present invention also provides an application of any of the above-described corona-free and primer-free biaxially oriented polypropylene films as a printing base film.

[0038] The physical properties and testing methods of the embodiments or comparative examples of the present invention are as follows: Membrane surface quality: A 50×50cm area of ​​the finished film was randomly selected, and surface defects such as crystal points, bright spots, and protrusions were observed visually. The total number of defects was recorded. A defective number < 5 was rated as excellent; 5 ≤ number < 10 was rated as good; 10 ≤ number < 15 was rated as average; other conditions were rated as poor. This test was used to evaluate whether the self-made ethylene-methyl acrylate copolymer microcapsules would cause undesirable appearance on the film surface.

[0039] Surface dyne value of the upper surface layer: obtained by testing according to GB / T14216-2008.

[0040] Surface ink adhesion: 3g of black ink was applied to the upper surface of a 20×30cm film using a test coating machine. After drying for 24 hours, an ink layer was formed. 3M 600 tape was then applied to the upper surface of the film. A 2kg roller was used to roll the tape back and forth 20 times without applying additional force. Finally, the tape was pulled up at a 90° angle to observe whether the ink layer was missing. If the ink layer was completely intact, it was rated as excellent; if the defective area was less than 20% of the total tape-bonded area, it was rated as good; if the defective area was less than 40% of the total tape-bonded area, it was rated as average; and other conditions were rated as poor.

[0041] Is the base coating intact? Observe with the naked eye, and at the same time, check whether the surface ink layer is intact to determine whether the base coating is missing.

[0042] It should be noted that the percentages mentioned in the embodiments or comparative examples of this invention are all weight percentages.

[0043] In the embodiments or comparative examples of this invention, the pearlescent masterbatch used is commercially available model ZG904E, with a calcium carbonate content of 70wt% and a calcium carbonate particle size of 1.3μm.

[0044] In the embodiments or comparative examples of this invention, the anti-blocking masterbatch used is commercially available model AB6001PP, with a silica content of 10 wt% and a silica particle size of 5 μm.

[0045] In the embodiments or comparative examples of this invention, the homopolymer polypropylene used is commercially available model L5D98C, with a melt flow index of 3 g / 10 min (test conditions: 230℃, 2.16 kg). In the embodiments or comparative examples of this invention, the ethylene-methyl acrylate copolymer contents of the ethylene-methyl acrylate copolymer microcapsules are all commercially available model 1125AC, with a melting point of 90°C.

[0046] The method for preparing ethylene-methyl acrylate copolymer microcapsules according to embodiments of the present invention includes the following steps: (1) Preparation of the template for the capsule. Styrene-methyl acrylate copolymer microspheres (PSMA) were prepared by dispersion polymerization as templates for the final silica microspheres. 28 g of deionized water, 140 g of anhydrous ethanol, and 3 g of polyvinylpyrrolidone were added to a 500 mL three-necked flask. The mixture was mechanically stirred at 200 rpm, heated to 75 °C, and purged with argon for 60 minutes. 2.3 g of AIBN was dissolved in a mixed solution of 74 g of styrene and 6 g of methyl acrylate, and then poured into the flask. The reaction was carried out at 75 °C for 12 hours under an argon atmosphere. After cooling for 20 minutes, the reaction solution was centrifuged. The product was washed three times with deionized water and then vacuum dried at 40 °C for 24 hours to obtain styrene-methyl acrylate copolymer monodisperse microspheres (PSMA) with a particle size of approximately 6 μm. The particle size of the resulting copolymer microspheres can be adjusted by changing the reaction temperature and the amount of polyvinylpyrrolidone added. When the reaction temperature is increased to 90°C and the amount of polyvinylpyrrolidone added is changed to 1.5g, while other reaction conditions remain unchanged, microspheres with a particle size of 7 micrometers can be obtained; when the amount of polyvinylpyrrolidone added is changed to 5g and other reaction conditions remain unchanged, microspheres with a particle size of about 5 micrometers can be obtained.

[0047] (2) Microspheres with an oligomeric siloxane shell and a PSMA core were prepared by adsorbing orthosilicic acid through hydrogen bonds on the copolymer template. PSMA was coated using the hydrolysis sol-gel method with industrial-grade 99% pure tetraethyl orthosilicate (TEOS). 5g of the monodisperse PSMA microspheres prepared in the previous step were placed in a flask, 70g of ethanol was added, and the mixture was ultrasonically dispersed for 30 minutes. Under magnetic stirring, 10g of deionized water, 4mL of ammonia, and 10g of TEOS were added. The ammonia provided an alkaline environment. Under the catalysis of the alkaline environment, TEOS first underwent a substitution reaction with water. One -OH in the water molecule replaced one -OCH2CH3 in TEOS, eventually generating orthosilicic acid and ethanol. Orthosilicic acid was adsorbed onto the PSMA microspheres under the action of hydrogen bonds and continued to undergo dehydration condensation. After 24 hours of reaction, oligomeric siloxane was finally formed on the surface of the microspheres. Then, the microspheres with oligomeric siloxane on the surface and styrene-methyl acrylate copolymer in the core were obtained by atmospheric pressure filtration.

[0048] (3) Preparation of hollow silica microspheres. The organic material (i.e., styrene-methyl acrylate copolymer) is burned off and the water in the oligomeric siloxane is removed by calcination at 600℃ for 5 hours in a muffle furnace. This yields hollow silica microspheres with a particle size of approximately 6 micrometers and micropores on the walls, obtained after removing the organic template and moisture from the silica gel. To obtain microspheres of other particle sizes, simply prepare copolymer templates of different sizes according to step one, and then repeat steps (2) and (3) above.

[0049] (4) Filling with ethylene-methyl acrylate copolymer. Take 1g of silica hollow microspheres into a 50mL single-necked flask, add 9.44g of ethylene-methyl acrylate copolymer, add 30ml of ethanol until the silica microspheres are completely submerged, seal and stir and heat to 60℃. After the capsules are fully in contact with the ethylene-methyl acrylate copolymer ethanol solution for 24 hours, filter and wash the microspheres three times with acetonitrile to obtain ethylene-methyl acrylate copolymer microcapsules. The ethylene-methyl acrylate copolymer content is 0.2g per 1g of microspheres, that is, the ethylene-methyl acrylate copolymer content in the microcapsules is 20wt%.

[0050] Example 1 This embodiment provides a corona-free, primer-free biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free biaxially oriented polypropylene film of this embodiment includes the following steps: Preparation of the upper surface resin: 95 wt% homopolymer polypropylene and 5 wt% ethylene-methyl acrylate copolymer microcapsules (particle size of 6 μm, ethylene-methyl acrylate copolymer content of 20 wt%) were mixed evenly to obtain the upper surface resin.

[0051] Preparation of support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch are mixed evenly to obtain support layer resin.

[0052] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0053] The preparation method of BOPP film in this embodiment includes the following steps: the raw materials of each layer are mixed according to the ratio and then added to each extruder. After being metered by a metering pump, the melt of each extruder is merged into a thick sheet at the T-die. After the thick sheet is chilled, it is first stretched longitudinally and then stretched laterally. After the film enters the lateral stretching, the thickness of the upper surface layer is gradually reduced to be smaller than the microcapsule particle size, so that the microcapsule part is exposed on the surface of the upper surface layer. Under the action of the huge tensile force generated by stretching, the shell of the microcapsule is torn and cracks are generated, so that the ethylene-methyl acrylate copolymer inside the microcapsule flows out and spreads on the surface of the upper surface layer, thereby forming a base coating during the film preparation process. After stretching, it is shaped and then wound up, followed by aging treatment and slitting to obtain the corona-free and base-coated biaxially oriented polypropylene film. The upper surface layer melt extrusion temperature is 160-180℃, the support layer melt extrusion temperature is 200-220℃, the lower surface layer melt extrusion temperature is 220-240℃, the quench roll temperature is 50-80℃, the biaxial stretching temperature is 100-120℃, and the stretching ratio for both transverse and longitudinal stretching is 5 times.

[0054] The film prepared in this embodiment has an intact and smooth base coating, and a total film thickness of 44µm, of which the upper surface layer is 3µm thick, the lower surface layer is 3µm thick, and the base coating is 3µm thick.

[0055] Example 2 This embodiment provides a corona-free, primer-free biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free biaxially oriented polypropylene film of this embodiment includes the following steps: Preparation of the upper surface resin: 92 wt% homopolymer polypropylene and 8 wt% ethylene-methyl acrylate copolymer microcapsules (particle size 6 μm, ethylene-methyl acrylate copolymer content 20 wt%) were mixed evenly to obtain the upper surface resin. Preparation of the support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch were mixed evenly to obtain the support layer resin.

[0056] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0057] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this embodiment is the same as that in Example 1, so it will not be described again.

[0058] The film prepared in this embodiment has an intact and smooth base coating. The total thickness of the film is 44µm, of which the thickness of the upper surface layer is 3µm, the thickness of the lower surface layer is 3µm, and the thickness of the base coating is 5µm.

[0059] Example 3 This embodiment provides a corona-free, primer-free biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free biaxially oriented polypropylene film of this embodiment includes the following steps: Preparation of the upper surface resin: 90 wt% homopolymer polypropylene and 10 wt% ethylene-methyl acrylate copolymer microcapsules (particle size 6 μm, ethylene-methyl acrylate copolymer content 20 wt%) were mixed evenly to obtain the upper surface resin. Preparation of the support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch were mixed evenly to obtain the support layer resin.

[0060] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0061] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this embodiment is the same as that in Example 1, so it will not be described again.

[0062] The film prepared in this embodiment has an intact and smooth base coating. The total thickness of the film is 44µm, of which the thickness of the upper surface layer is 3µm, the thickness of the lower surface layer is 3µm, and the thickness of the base coating is 6µm.

[0063] Example 4 This embodiment provides a corona-free, primer-free biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free biaxially oriented polypropylene film of this embodiment includes the following steps: Preparation of the upper surface resin: 92 wt% homopolymer polypropylene and 8 wt% ethylene-methyl acrylate copolymer microcapsules (particle size 5 μm, ethylene-methyl acrylate copolymer content 20 wt%) were mixed evenly to obtain the upper surface resin. Preparation of the support layer resin: 85 wt% homopolymer polypropylene and 15 wt% pearlescent masterbatch were mixed evenly to obtain the support layer resin.

[0064] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0065] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this embodiment is the same as that in Example 1, so it will not be described again.

[0066] The film prepared in this embodiment has an intact and smooth base coating, and a total film thickness of 44µm, of which the upper surface layer is 3µm thick, the lower surface layer is 3µm thick, and the base coating is 3µm thick.

[0067] Example 5 This embodiment provides a corona-free, primer-free biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially. The thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free biaxially oriented polypropylene film of this embodiment includes the following steps: Preparation of the upper surface resin: 92 wt% homopolymer polypropylene and 8 wt% ethylene-methyl acrylate copolymer microcapsules (particle size 7 μm, ethylene-methyl acrylate copolymer content 20 wt%) were mixed evenly to obtain the upper surface resin. Preparation of the support layer resin: 90 wt% homopolymer polypropylene and 10 wt% pearlescent masterbatch were mixed evenly to obtain the support layer resin.

[0068] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0069] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this embodiment is the same as that in Example 1, so it will not be described again.

[0070] The film prepared in this embodiment has an intact and smooth base coating. The total thickness of the film is 44µm, of which the thickness of the upper surface layer is 3µm, the thickness of the lower surface layer is 3µm, and the thickness of the base coating is 5µm.

[0071] Comparative Example 1 This comparative example provides a corona-free, primer-free, biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially, wherein the thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free, biaxially oriented polypropylene film of this comparative example includes the following steps: Preparation of the upper surface resin: 85 wt% homopolymer polypropylene and 15 wt% ethylene-methyl acrylate copolymer microcapsules (particle size of 6 μm, ethylene-methyl acrylate copolymer content of 20 wt%) were mixed evenly to obtain the upper surface resin.

[0072] Preparation of support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch are mixed evenly to obtain support layer resin.

[0073] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0074] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this comparative example is the same as that in Example 1, so it will not be described again.

[0075] The base coating of the film prepared in this comparative example is incomplete. The total thickness of the film was measured to be 44µm based on the thickest part of the base coating, of which the thickness of the upper surface layer was 3µm, the thickness of the lower surface layer was 3µm, and the thickest part of the base coating was 2µm.

[0076] Comparative Example 2 This comparative example provides a corona-free, primer-free, biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially, wherein the thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free, biaxially oriented polypropylene film of this comparative example includes the following steps: Preparation of the upper surface resin: 99 wt% homopolymer polypropylene and 1 wt% ethylene-methyl acrylate copolymer microcapsules (particle size of 6 μm, ethylene-methyl acrylate copolymer content of 20 wt%) were mixed evenly to obtain the upper surface resin.

[0077] Preparation of support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch are mixed evenly to obtain support layer resin.

[0078] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0079] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this comparative example is the same as that in Example 1, so it will not be described again.

[0080] The base coating of the film prepared in this comparative example is incomplete. The total thickness of the film was 44µm when the thickest part of the base coating was measured, of which the thickness of the upper surface layer was 3µm, the thickness of the lower surface layer was 3µm, and the thickest part of the base coating was 0.1µm.

[0081] Comparative Example 3 This comparative example provides a corona-free, primer-free, biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially, wherein the thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free, biaxially oriented polypropylene film of this comparative example includes the following steps: Preparation of the upper surface resin: 92 wt% homopolymer polypropylene and 8 wt% ethylene-methyl acrylate copolymer microcapsules (particle size of 10 μm, ethylene-methyl acrylate copolymer content of 20 wt%) were mixed evenly to obtain the upper surface resin.

[0082] Preparation of support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch are mixed evenly to obtain support layer resin.

[0083] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0084] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this comparative example is the same as that in Example 1, so it will not be described again.

[0085] The base coating of the film prepared in this comparative example is incomplete. The total thickness of the film was measured to be 44µm based on the thickest part of the base coating, of which the thickness of the upper surface layer was 3µm, the thickness of the lower surface layer was 3µm, and the thickest part of the base coating was 2µm.

[0086] Comparative Example 4 This comparative example provides a corona-free, primer-free, biaxially oriented polypropylene film, comprising a primer layer, an upper surface layer, a support layer, and a lower surface layer arranged sequentially, wherein the thick sheet used to prepare the film comprises the upper surface layer, support layer, and lower surface layer arranged sequentially. The method for preparing each resin layer of the corona-free, primer-free, biaxially oriented polypropylene film of this comparative example includes the following steps: Preparation of the upper surface resin: 92 wt% homopolymer polypropylene and 8 wt% ethylene-methyl acrylate copolymer microcapsules (particle size of 2 μm, ethylene-methyl acrylate copolymer content of 20 wt%) were mixed evenly to obtain the upper surface resin.

[0087] Preparation of support layer resin: 80 wt% homopolymer polypropylene and 20 wt% pearlescent masterbatch are mixed evenly to obtain support layer resin.

[0088] Preparation of the lower surface resin: 95 wt% homopolymer polypropylene and 5 wt% anti-blocking masterbatch are mixed evenly to obtain the lower surface resin.

[0089] The preparation method of the corona-free and primer-free biaxially oriented polypropylene film in this comparative example is the same as that in Example 1, so it will not be described again.

[0090] The base coating of the film prepared in this comparative example is incomplete. The total thickness of the film was measured to be 44µm based on the thickest part of the base coating, of which the thickness of the upper surface layer was 3µm, the thickness of the lower surface layer was 3µm, and the thickest part of the base coating was 0.5µm.

[0091] The performance and appearance quality evaluation results of the corona-free and primer-free biaxially oriented polypropylene films of Examples 1-5 and Comparative Examples 1-4 are shown in Table 1 below.

[0092] Table 1

[0093] The following analysis is based on the test results and Table 1.

[0094] The test results from Examples 1 to 5 show that, within the dosage range specified in this application: increasing the dosage of microcapsules can increase the surface dyne value of the upper surface layer and optimize ink adhesion, but will reduce the film quality of the upper surface layer; adjusting the particle size of microcapsules within a certain range will cause certain fluctuations in the surface dyne value, but will not have a significant impact on the film quality.

[0095] In Comparative Example 1, the amount of ethylene-methyl acrylate copolymer microcapsules was excessive, the number of silica shells on the upper surface was excessive, the film surface was uneven, some crystal points appeared, and the film surface quality was poor.

[0096] In Comparative Example 2, the amount of ethylene-methyl acrylate copolymer microcapsules was too small, resulting in a small amount of ethylene-methyl acrylate copolymer that could spread on the film surface during biaxial stretching. Consequently, the ethylene-methyl acrylate copolymer undercoat was incomplete, and the surface dyne value and ink adhesion were both poor.

[0097] In Comparative Example 3, the ethylene-methyl acrylate copolymer microcapsules had excessively large particle size, poor film surface smoothness, crystal points, and poor film quality. Furthermore, most of the microcapsules did not rupture after stretching, resulting in a small area of ​​the upper surface layer forming the base coating. The improvement in the dyne value of the upper surface layer was not significant, and the ink adhesion was poor.

[0098] In Comparative Example 4, the particle size of the ethylene-methyl acrylate copolymer microcapsules was too small, resulting in a small amount of ethylene-methyl acrylate copolymer that could spread onto the film surface during biaxial stretching. Consequently, the ethylene-methyl acrylate copolymer undercoat was incomplete, and the surface dyne value and ink adhesion were both poor.

[0099] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and the present invention also intends to include these modifications and variations.

Claims

1. A corona-free, primer-free biaxially oriented polypropylene film, characterized in that, The film is prepared by biaxial stretching of a thick sheet, the thick sheet comprising an upper surface layer, a support layer, and a lower surface layer arranged sequentially; the thickness of the upper surface layer is 2~3μm, the upper surface layer comprises homopolymer polypropylene and 5~10wt% ethylene-methyl acrylate copolymer microcapsules, the ethylene-methyl acrylate copolymer microcapsules having a silica shell and ethylene-methyl acrylate copolymer as the content, the particle size of the ethylene-methyl acrylate copolymer microcapsules being 5~7μm; the support layer and the lower surface layer both comprise homopolymer polypropylene.

2. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The melting point of the ethylene-methyl acrylate copolymer is 90°C.

3. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The melt index of the homopolymer polypropylene was measured to be 2-4 g / 10 min under test conditions of 230℃ and 2.16 kg.

4. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The preparation method of the ethylene-methyl acrylate copolymer microcapsules includes the following steps: preparing silica hollow microspheres, mixing silica hollow microspheres, ethylene-methyl acrylate copolymer and ethanol in a ratio of silica hollow microspheres: ethylene-methyl acrylate copolymer = 1g: (9~12)g, sealing and stirring at 60°C for 24h, then filtering and washing with acetonitrile to obtain the ethylene-methyl acrylate copolymer microcapsules.

5. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The content of ethylene-methyl acrylate copolymer in the ethylene-methyl acrylate copolymer microcapsules is 15~25wt%.

6. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The support layer also includes 10-20 wt% pearlescent masterbatch.

7. The corona-free, primer-free biaxially oriented polypropylene film according to claim 1, characterized in that, The lower surface layer also includes 1-5 wt% anti-blocking masterbatch.

8. A method for preparing a corona-free, primer-free biaxially oriented polypropylene film as described in any one of claims 1 to 7, characterized in that, Includes the following steps: After the raw materials of each layer are mixed according to the formula, they are added to each extruder. After being metered by the metering pump, the melt of each extruder is merged into a thick sheet at the T-die. After the thick sheet is chilled, it is first stretched longitudinally and then stretched laterally. After stretching, it is shaped and then wound up. Then it is subjected to aging treatment and slitting to obtain the corona-free and primer-free biaxially oriented polypropylene film.

9. The method for preparing a corona-free, primer-free biaxially oriented polypropylene film according to claim 8, characterized in that, The melt extrusion temperature of the upper surface layer is 160-180℃, the melt extrusion temperature of the support layer is 200-220℃, the melt extrusion temperature of the lower surface layer is 220-240℃, the temperature of the chill roll is 50-80℃, the biaxial stretching temperature is 100-120℃, and the stretching ratio of transverse stretching and longitudinal stretching is 4-5 times.

10. An application of the corona-free, primer-free biaxially oriented polypropylene film as described in any one of claims 1 to 7, characterized in that, Used as a printing base film.

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