A biaxially oriented polypropylene label film and its preparation method

Abstract

This invention relates to the field of films, and particularly to a biaxially oriented polypropylene label film and its preparation method. The label film includes a printing layer, a core layer, and a lower surface layer. The printing layer comprises a random ethylene-propylene copolymer and 3-5 wt% bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite. The core layer comprises homopolymer polypropylene, a white masterbatch, and 1.5-3.0 wt% star-shaped polyethyleneimine-polypropylene graft copolymer. Each branch arm of the polyethyleneimine star core is composed of 15-20 ethyleneimine repeating units, and the number average molecular weight of the polypropylene arms is 5000-8000 Da. This invention utilizes the unique molecular structure of the star-shaped polyethyleneimine-polypropylene graft copolymer to achieve a triple migration inhibition synergy, suppressing the migration of small molecules. Furthermore, it utilizes the oriented nanosheets of bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite to construct a triple synergistic barrier mechanism, further blocking small molecules migrating from the core layer, thus achieving relatively durable surface tension of the film.

Description

Technical Field

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

[0002] In today's booming packaging and printing industry, biaxially oriented polypropylene (BOPP) white label film has become an important substrate for various product labels due to its excellent mechanical and optical properties and good processing adaptability. Typically, it consists of a multi-layer structure, with the core layer often filled with white masterbatch (such as titanium dioxide masterbatch) to achieve an opaque white appearance, thus achieving good opacity and whiteness visual effect.

[0003] In the traditional BOPP white label film manufacturing process, to improve the interfacial compatibility between titanium dioxide and the polypropylene matrix in the core layer and to enhance the uniformity of titanium dioxide dispersion, the industry commonly uses small molecule substances (such as silane coupling agents) to coat the surface of the titanium dioxide. These small molecule silane coupling agents can improve the surface properties of titanium dioxide to a certain extent, making it easier for titanium dioxide to be uniformly dispersed in the polypropylene matrix, thereby improving the overall mechanical and optical properties of the label film.

[0004] However, traditional BOPP white label film still has some limitations in practical applications. For example, the surface tension of the printing layer drops sharply during storage and use, resulting in insufficient surface tension for subsequent printing. This significantly reduces the wetting and adhesion of ink to the film surface, easily causing ink smudging, leading to blurry printed images, and even missing prints during the printing process, affecting the integrity and accuracy of the print, among other printing quality issues. This not only reduces the visual presentation of the label but also affects the product's information delivery function and brand image, becoming a technical bottleneck restricting the application of BOPP white label film in the high-end packaging field. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide a biaxially oriented polypropylene label film and its preparation method. On the one hand, a star-shaped polyethyleneimine-polypropylene graft copolymer is added to the core layer, utilizing its unique molecular structure to achieve a triple migration inhibition synergy, reducing the migration of small molecules in the core layer. On the other hand, bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite is added to the printing layer, constructing a triple synergistic barrier mechanism through its oriented nanosheets, effectively preventing small molecules that have not been completely inhibited from further penetrating to the surface of the printing layer. Through the synergistic barrier effect of the printing layer and the core layer, the surface tension of the film printing layer is relatively durable, thereby ensuring the adhesion of ink to the surface of the printing layer.

[0006] The technical solution of the present invention is achieved in the following ways:

[0007] A biaxially oriented polypropylene label film includes a printing layer, a core layer, and a lower surface layer arranged sequentially. The printing layer comprises a random ethylene-propylene copolymer and 3-5 wt% bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite, wherein the bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite is prepared by ion exchange of sodium montmorillonite and bis(long-chain alkyl dimethyl ammonium chloride), and the long-chain alkyl group in the bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite has 16-20 carbon atoms. The core layer comprises homopolymer polypropylene, 10-20 wt% white masterbatch, and 1.5-3.0 wt% star-shaped polyethyleneimine-polypropylene graft copolymer. Each branch arm of the polyethyleneimine star core in the star-shaped polyethyleneimine-polypropylene graft copolymer is composed of 15-20 ethyleneimine repeating units, and the number average molecular weight of the polypropylene arms in the star-shaped polyethyleneimine-polypropylene graft copolymer is 5000-8000 Da. The lower surface layer comprises homopolymer polypropylene.

[0008] Through extensive experimental analysis, the inventors discovered that the sharp decrease in surface tension of existing white label films is due to the migration of small molecules from the core layer to the film surface. These small molecules are primarily silane coupling agents in the white masterbatch. While silane coupling agents can form chemical bonds with the titanium dioxide surface, during the actual coating process, some molecules adhere to the titanium dioxide surface or the outer layer of already bonded areas through physical adsorption (van der Waals forces, hydrogen bonds). These physically adsorbed molecules have strong thermal mobility and are easily desorbed or migrated under subsequent processing or usage conditions. Furthermore, the high-temperature melt extrusion and high shear force in the BOPP film production process can damage the relatively fragile physical adsorption layer and even part of the chemically bonded layer, increasing the amount of migratable small molecules. These small molecules diffuse from the core layer to the label film surface, accumulating on the printing layer surface and forming a weak interface layer, leading to a decrease in the surface tension of the printing layer. In addition, the corona treatment process further exacerbates the migration of small molecules, covering the polar groups generated by the corona treatment, causing the surface tension of the printing layer to decay below the ink adhesion critical value, resulting in defects such as ink peeling and missed printing.

[0009] The biaxially oriented polypropylene label film of this invention achieves relatively durable surface tension and improved ink adhesion on the printed layer surface through a synergistic barrier design of the printing layer and the core layer. On one hand, a star-shaped polyethyleneimine-polypropylene graft copolymer is added to the core layer, utilizing its unique molecular structure to achieve a triple migration inhibition synergy, suppressing the migration of small molecules. On the other hand, the bi-long-chain alkyl dimethyl ammonium chloride-modified montmorillonite is added to the printing layer, constructing a triple synergistic barrier mechanism through its oriented nanosheets. This effectively prevents incompletely inhibited small molecules from further penetrating to the surface of the printed layer. In other words, the core layer reduces the total amount of small molecule migration at the source, while the printing layer blocks the pathways of incompletely inhibited small molecules. This dual-layer barrier creates synergistic inhibition, resulting in high and durable stable surface tension of the printed layer of the biaxially oriented polypropylene white label film. The ink layer formed on the printed layer surface has strong adhesion, making it less prone to ink peeling and printing defects.

[0010] This invention adds 1.5-3.0 wt% of star-shaped polyethyleneimine-polypropylene graft copolymer to the core layer. This star-shaped copolymer can effectively inhibit the migration of small molecules, slow down the decay rate of the surface tension of the printing layer, and ensure the adhesion of the printing ink. This is mainly attributed to its unique molecular structure and multiple mechanisms of action. On the one hand, its core is a star-shaped polyethyleneimine rich in high-density amine groups. This strongly polar core can interact strongly with polar groups in polar small molecules such as silane coupling agents through intermolecular forces such as hydrogen bonding, electrostatic attraction, and coordination, achieving molecular-level locking of small molecules. This helps to firmly anchor small molecules to the core layer and reduce their migration from the core layer. On the other hand, the star-shaped polyethyleneimine-polypropylene graft copolymer also acts as a highly efficient interfacial compatibilizer. Through its amphiphilic structure, it bridges the originally incompatible polar small molecules with the non-polar polypropylene matrix, greatly improving interfacial compatibility, reducing phase separation tendency, and reducing the spontaneous migration of small molecules to the film surface from the source. Furthermore, the polypropylene segments grafted onto the star-shaped polyethyleneimine have good thermodynamic compatibility with the polypropylene resin matrix, allowing the entire copolymer to be uniformly dispersed and firmly embedded in the polypropylene resin matrix through physical entanglement. This physically constructs a dense three-dimensional network spatial steric barrier, increasing the tortuosity of the migration path. In summary, the star-shaped polyethyleneimine-polypropylene graft copolymer achieves effective inhibition of small molecule migration through a triple synergistic effect: "polar core anchoring of migrating substances - formation of a three-dimensional network spatial steric barrier to hinder diffusion - improved interfacial compatibility to reduce phase separation." This helps to address the problem of insufficient surface tension persistence in the printed layer, which affects subsequent processing and use. The content of the star-shaped polyethyleneimine-polypropylene graft copolymer in the core layer is 1.5-3.0 wt%. If the content of the star-shaped polyethyleneimine-polypropylene graft copolymer in the core layer is less than 1.5 wt%, the amine anchoring point density is insufficient, the migration inhibition rate does not meet the expected requirements, and it cannot effectively reduce the migration of small molecules; the surface tension of the printed layer will still significantly decrease. If the content of the star-shaped polyethyleneimine-polypropylene graft copolymer in the core layer is greater than 3.0 wt%, excessive entanglement of the star structure leads to an increase in melt shear viscosity, which can easily cause the risk of film breakage during the biaxial stretching process, affecting the smoothness of production.

[0011] Furthermore, in the star-shaped polyethyleneimine-polypropylene graft copolymer, each branch arm of the polyethyleneimine star core is composed of 15-20 ethyleneimine repeating units; the polypropylene arm has a molecular weight of 5000-8000 Da and is connected to the star core through terminal amide bonds. If the number of ethyleneimine repeating units in the branch arms of the polyethyleneimine star-shaped core is less than 15, the density of amine anchoring sites is insufficient, resulting in insufficient coverage of anchoring sites for intermolecular forces such as hydrogen bonds and electrostatic attraction with polar migrants. This leads to a low migration inhibition rate, causing the surface tension of the printed layer to still decay rapidly after the label film has been stored for a period of time. If the number of ethyleneimine repeating units in the branch arms of the polyethyleneimine star-shaped core is more than 20, the star-shaped core is too hydrophilic, reducing its compatibility with the non-polar polypropylene matrix in the core layer and causing phase separation, which is detrimental to the optical performance of the label film. If the molecular weight of the polypropylene arm is less than 5000 Da, the physical entanglement with the polypropylene matrix is ​​weak, failing to form an effective anchoring network and thus failing to effectively reduce the migration of small molecules. If the molecular weight of the polypropylene arm is greater than 8000 Da, the molecular chain entanglement density increases, leading to a significant decrease in melt flow rate and increasing the risk of film breakage during the biaxial stretching process, affecting the smoothness of production. Preferably, the number of branched arms of the polyethyleneimine star core is 3-6, which helps to ensure the amine group density on the surface of the star core to form a dense three-dimensional network spatial steric barrier, thereby obtaining a better small molecule migration inhibition effect, while also ensuring compatibility with the polypropylene matrix.

[0012] To further block the migration of small molecules that are not completely suppressed in the core layer, this invention also adds 3-5 wt% of bis-long-chain alkyl dimethyl ammonium chloride modified montmorillonite to the printing layer. Through the triple synergistic mechanism of maze effect, electrostatic targeting capture and free volume compression, it achieves efficient migration suppression, further effectively inhibiting the migration of small molecules from the core layer to the film surface, maintaining the high surface tension of the printing layer after corona treatment, and ensuring ink adhesion and printing integrity.

[0013] The compatibility between the dimethyl dimethyl ammonium chloride (DMCA) and the random ethylene-propylene copolymer matrix is ​​a core prerequisite for the aforementioned mechanism to function. MDCA-modified montmorillonite changes its surface from hydrophilic to oleophilic, exhibiting good compatibility with the nonpolar matrix of the random ethylene-propylene copolymer in the printed layer. This compatibility ensures the uniform dispersion of the lamellar montmorillonite within the matrix, preventing the formation of "blocking gaps" due to agglomeration. Simultaneously, the dimethyl dimethyl ammonium chloride-modified montmorillonite can entangle with the ethylene-propylene copolymer molecular chains, promoting the directional alignment of the montmorillonite's lamellar structure along the film surface during biaxial stretching, forming a continuous and complete high-torsional-degree labyrinth barrier. This forces small molecules migrating from the core layer to bypass the lamellar edges, significantly extending the migration path and slowing the migration rate. If compatibility is insufficient, the montmorillonite lamellar structure easily agglomerates, the labyrinth structure breaks, and small molecules can still migrate rapidly through the gaps, resulting in a decreased migration inhibition rate.

[0014] Simultaneously, the quaternary ammonium cation groups on the surface of the dimethylammonium chloride-modified montmorillonite strongly capture polar small molecules through ion-dipole interactions; the amorphous regions of the dimethylammonium chloride and ethylene-propylene copolymer generate strong van der Waals forces, further compressing the free volume. The sheets act as "nano-crosslinking points," restricting molecular chain movement and synergistically inhibiting the further migration of small molecules to the printed layer surface. Good compatibility ensures effective contact between the quaternary ammonium cations and polar small molecules, avoiding the inability to effectively inhibit further migration of small molecules due to a reduction in capture sites caused by aggregation.

[0015] If the content of bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite in the printed layer is less than 3 wt%, the montmorillonite sheet structure is dispersed and discontinuous, making it difficult to form a continuous barrier network, resulting in insufficient inhibition of small molecule migration. If the content of bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite in the printed layer is greater than 5 wt%, the montmorillonite sheet structure aggregates, disrupting the continuity of the maze, which is also not conducive to inhibiting the migration of small molecules. It may even lead to stress concentration, reducing the right-angle tear strength of the film. Furthermore, excessive bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite is not conducive to the crystallization of the matrix resin, resulting in decreased crystallinity and a decline in the mechanical properties of the film.

[0016] Further, the preparation method of the bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite is as follows: using sodium-based montmorillonite as raw material and bis(long-chain) alkyl dimethyl ammonium chloride as modifier, sodium-based montmorillonite is dispersed in deionized water to form a suspension. An equimolar ratio of bis(long-chain) alkyl dimethyl ammonium chloride solution is added under high-speed stirring conditions, and ion exchange occurs during the stirring reaction. After the reaction is completed, the mixture is centrifuged, and the precipitate is washed with deionized water until no chloride ions are detected. Finally, it is vacuum dried, ground, and sieved to obtain bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite. The quaternary ammonium cations in the bis(long-chain) alkyl dimethyl ammonium chloride form strong ionic bonds with the bridging oxygen anions on the surface of the montmorillonite silica sheets, achieving a stable bond.

[0017] Furthermore, the number of carbon atoms in the long-chain alkyl group of the bis(long-chain alkyl)dimethylammonium chloride modified montmorillonite is 16-20. Limiting the number of carbon atoms in the long-chain alkyl group to 16-20 is beneficial for ensuring good compatibility between the bis(long-chain alkyl)dimethylammonium chloride modified montmorillonite and the matrix resin of the printing layer, and also facilitates the formation of a continuous and complete high-torsional-degree labyrinth barrier, thus slowing down the migration rate. If the number of carbon atoms in the long-chain alkyl group is less than 16, the steric hindrance is insufficient, the entanglement with the molecular chains of the matrix resin is weak, and the improvement in compatibility is limited, making it difficult to form an effective tortuous labyrinth structure. If the number of carbon atoms in the long-chain alkyl group is more than 20, it affects the dispersibility and self-flowability, which is not conducive to forming a uniform, high-torsional-degree labyrinth barrier, and may even lead to agglomeration, affecting the mechanical properties of the film.

[0018] Furthermore, the star-shaped polyethyleneimine-polypropylene graft copolymer is formed by grafting a polyethyleneimine star-shaped core and polypropylene arms through an amidation reaction. The polypropylene arms have good compatibility with the polypropylene resin matrix, allowing the star-shaped polyethyleneimine-polypropylene graft copolymer to be uniformly dispersed in the core layer and firmly embedded in the polypropylene resin matrix through physical entanglement. This physically constructs a dense three-dimensional network spatial steric barrier, increasing the tortuosity of the migration path of small molecules.

[0019] Furthermore, the ethylene unit content in the atactic ethylene-propylene copolymer is 3.5-4.5 wt%, and the melt index of the atactic ethylene-propylene copolymer was measured to be 7.0-10.0 g / 10 min at 230℃ and 2.16 kg. Selecting atactic ethylene-propylene copolymers within the above-mentioned ethylene content and melt index range is beneficial for ensuring that the di-long-chain alkyl dimethyl ammonium chloride-modified montmorillonite can be effectively and uniformly dispersed in the printing layer, forming an effective barrier path, while also ensuring the mechanical properties of the film printing layer, which is beneficial for the smooth production of biaxial stretching.

[0020] Further, the white masterbatch is composed of 30-50 wt% homopolymer polypropylene and 50-70 wt% surface-modified titanium dioxide. The surface-modified titanium dioxide is obtained by a gradient coating process, first coating the titanium dioxide with an inorganic material as the coating medium, and then coating it with an organic coupling agent as the coating medium. The particle size D50 of the coated surface-modified titanium dioxide is 0.2-0.4 μm. The gradient coating process includes the following steps: first, coating the titanium dioxide with an inorganic material as the coating medium, with a coating amount of 0.5-2.0 wt%, wherein the inorganic material includes silica and alumina; then, surface coating with a silane coupling agent through a high-speed mixing process, with a coating amount of 0.8-1.2 wt%. Through gradient coating treatment, the surface of titanium dioxide is transformed from hydrophilic to oleophilic; more preferably, the particle size D50 of the coated surface-modified titanium dioxide is 0.25-0.35μm.

[0021] Furthermore, the lower surface layer also includes 1-5 wt% anti-blocking agent masterbatch, wherein the anti-blocking agent in the anti-blocking agent masterbatch is at least one selected from silica, talc, and plexiglass microspheres, and the particle size D50 of the anti-blocking agent is 3-7 μm. Controlling the particle size D50 of the anti-blocking agent to 3-7 μm ensures the formation of an effective surface anti-blocking microstructure and inhibits the migration loss of the anti-blocking agent caused by localized stress concentration, thereby significantly reducing the risk of detachment under high-speed winding conditions. Preferably, the effective content of the anti-blocking agent in the anti-blocking agent masterbatch is 5 wt%.

[0022] Furthermore, the melt index of the homopolymer polypropylene was measured to be 3-6 g / 10 min at 230°C and 2.16 kg.

[0023] The present invention also provides a method for preparing any of the above-described biaxially oriented polypropylene label films, comprising the following steps:

[0024] According to the formula, the dried raw materials of each layer are transported to the extruder of each layer. The melt of each layer formed by heating and melting and fully plasticizing is co-extruded through the co-extrusion die. The co-extruded melt is cooled to obtain a thick sheet. The thick sheet is first subjected to longitudinal stretching and shaping treatment, then transverse stretching and shaping, and cooling treatment. Next, it is subjected to thickness measurement and corona treatment. After corona treatment, it is wound up to obtain a master roll, subjected to master aging treatment, and cut to obtain the finished biaxially oriented polypropylene label film roll.

[0025] Furthermore, the melt extrusion temperature of the printed layer is 180-270℃, the melt extrusion temperature of the core layer and the lower surface layer is 220-270℃, and the quenching temperature is 15-40℃; the longitudinal stretching temperature is 90-150℃, and the longitudinal stretching ratio is 4.5-5.7 times; the transverse stretching temperature is 150-170℃, and the transverse stretching ratio is 7.5-10 times; the corona power factor is 20-40 W•min / m. Through precise temperature control in each layer, uniform plasticization of the material is achieved, ensuring melt flowability and interlayer compatibility; through the synergistic optimization of temperature and stretching ratio, the film molecular chains are fully oriented in both the longitudinal and transverse directions; and by adjusting the corona discharge energy, suitable polar groups and roughness are formed on the film surface, effectively improving the surface properties of the printed layer.

[0026] To better understand and implement this invention, the invention will be described in detail below. Detailed Implementation

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] It should be understood that the embodiments of this application are not limited to the precise structures already described above, 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.

[0032] As one embodiment of the present invention, this embodiment provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer arranged sequentially; the printing layer comprises a random ethylene-propylene copolymer and 3-5 wt% bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite, wherein the bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite is prepared by ion exchange of sodium montmorillonite and bis(long-chain alkyl dimethyl ammonium chloride), and the long-chain alkyl group in the bis(long-chain alkyl dimethyl ammonium chloride) modified montmorillonite has 16-20 carbon atoms; the core layer comprises homopolymer polypropylene, 10-20 wt% white masterbatch, and 1.5-3.0 wt% star-shaped polyethyleneimine-polypropylene graft copolymer; each branch arm of the polyethyleneimine star core in the star-shaped polyethyleneimine-polypropylene graft copolymer is composed of 15-20 ethyleneimine repeating units, and the number average molecular weight of the polypropylene arms in the star-shaped polyethyleneimine-polypropylene graft copolymer is 5000-8000 Da; the lower surface layer comprises homopolymer polypropylene.

[0033] Furthermore, the number of carbon atoms in the long-chain alkyl group of the modified montmorillonite is 16-20.

[0034] Further, the star-shaped polyethyleneimine-polypropylene graft copolymer is formed by grafting a polyethyleneimine star-shaped core (with polyethyleneimine as the branch arm) to a polypropylene arm via an amidation reaction. Specifically, the preparation method of the star-shaped polyethyleneimine-polypropylene graft copolymer of the present invention is as follows: maleic anhydride-grafted polypropylene is prepared by a melt amidation reaction, in which maleic anhydride-grafted polypropylene is reacted with a polyethyleneimine star-shaped core, causing the anhydride groups of the maleic anhydride-grafted polypropylene to undergo a ring-opening reaction with the amino groups of the polyethyleneimine star-shaped core, forming a graft structure in which the polypropylene chain ends are connected to the star-shaped core via amide bonds. Preferably, the polypropylene in the maleic anhydride-grafted polypropylene is homopolymer polypropylene.

[0035] Furthermore, the content of ethylene units in the random ethylene-propylene copolymer is 3.5-4.5 wt%, and the melt index of the random ethylene-propylene copolymer is measured to be 7.0-10.0 g / 10min at 230℃ and 2.16 kg.

[0036] Further, the white masterbatch is composed of 30-50 wt% homopolymer polypropylene and 50-70 wt% surface-modified titanium dioxide. The surface-modified titanium dioxide is obtained through a gradient coating process: first, titanium dioxide is coated with an inorganic material as the coating medium, and then coated with an organic coupling agent as the coating medium. The particle size D50 of the coated surface-modified titanium dioxide is 0.2-0.4 μm. Specifically, the gradient coating process includes the following steps: first, titanium dioxide is coated with an inorganic material, including silica and alumina, at a coating amount of 0.5-2.0 wt%; then, a silane coupling agent is used for surface coating via a high-speed mixing process, at a coating amount of 0.8-1.2 wt%. Through gradient coating treatment, the surface of titanium dioxide is transformed from hydrophilic to oleophilic. Preferably, the particle size D50 of the coated surface-modified titanium dioxide is 0.25-0.35 μm.

[0037] Furthermore, the lower surface layer also includes 1-5 wt% anti-blocking agent masterbatch, wherein the anti-blocking agent in the anti-blocking agent masterbatch is at least one of silica, talc, and plexiglass microspheres, and the particle size D50 of the anti-blocking agent is 3-7 μm.

[0038] Furthermore, the melt index of the homopolymer polypropylene was measured to be 3-6 g / 10 min at 230°C and 2.16 kg.

[0039] The present invention also provides a method for preparing any of the above-described biaxially oriented polypropylene label films, comprising the following steps:

[0040] According to the formula, the dried raw materials of each layer are transported to the extruder of each layer. The melt of each layer formed by heating and melting and fully plasticizing is co-extruded through the co-extrusion die. The co-extruded melt is cooled to obtain a thick sheet. The thick sheet is first subjected to longitudinal stretching and shaping treatment, then transverse stretching and shaping, and cooling treatment. Next, it is subjected to thickness measurement and corona treatment. After corona treatment, it is wound up to obtain a master roll, subjected to master aging treatment, and cut to obtain the finished biaxially oriented polypropylene label film roll.

[0041] Furthermore, the melt extrusion temperature of the printed layer is 180-270℃, the melt extrusion temperature of the core layer and the lower surface layer is 220-270℃, and the quenching temperature is 15-40℃; the longitudinal stretching temperature is 90-150℃, and the longitudinal stretching ratio is 4.5-5.7 times; the transverse stretching temperature is 150-170℃, and the transverse stretching ratio is 7.5-10 times; the corona power factor is 20-40 W•min / m. Through precise temperature control in each layer, uniform plasticization of the material is achieved, ensuring melt flowability and interlayer compatibility; through the synergistic optimization of temperature and stretching ratio, the film molecular chains are fully oriented in both the longitudinal and transverse directions; and by adjusting the corona discharge energy, suitable polar groups and roughness are formed on the film surface, effectively improving the surface properties of the printed layer.

[0042] The physical properties and testing methods of the embodiments or comparative examples of the present invention are as follows:

[0043] Melt flow index (MFR) was determined according to GB / T3682-2018.

[0044] Tensile strength (longitudinal / transverse): Tested according to GB / T1040.3-2006;

[0045] Production smoothness: qualitatively assessed through film breakage during the biaxial stretching process;

[0046] Surface tension and durability of the printed layer: After aging treatment (0 days, 30 days, 60 days, 90 days) of the prepared film, the surface tension of the printed layer (unit: dyn / cm) was tested according to GB / T14216-2008.

[0047] Ink adhesion: Printing was performed on films after aging treatment (0 days, 30 days, 60 days, 90 days) according to GB / T9286-2021;

[0048] Printing appearance: After the film has been aged (0 days, 30 days, 60 days, 90 days), printing is performed, and the presence of defects such as missing prints, smearing, and ink loss is visually observed.

[0049] The random ethylene-propylene copolymer used in the embodiments and comparative examples of this invention has a melt index of 8.5 g / 10 min (test conditions: 230℃, 2.16 kg) and an ethylene unit content of 4.0 mol.

[0050] The melt index of the homopolymer polypropylene used in the embodiments and comparative examples of this invention was 4.5 g / 10 min (test conditions: 230℃, 2.16 kg).

[0051] The preparation method of the star-shaped polyethyleneimine-polypropylene graft copolymer used in the embodiments and comparative examples of this invention includes the following steps: maleic anhydride-grafted homopolymer polypropylene anhydride groups are subjected to a melt amidation reaction with the primary amine groups of the star-shaped polyethyleneimine core. The grafting point structure is an amide bond formed by ring opening of maleic anhydride at the end of the homopolymer polypropylene chain, resulting in a star-shaped polyethyleneimine-polypropylene graft copolymer. In the star-shaped polyethyleneimine-polypropylene graft copolymer, the polyethyleneimine star core consists of four branched arms.

[0052] The white masterbatch used in the embodiments and comparative examples of the present invention is composed of 40 wt% homopolymer polypropylene and 60 wt% modified titanium dioxide filler, wherein the particle size D50 of the modified titanium dioxide is 0.3 μm, and the modified titanium dioxide is gradient coated with silica and silane coupling agent.

[0053] The preparation method of the bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite used in the embodiments and comparative examples of the present invention includes the following steps: using sodium montmorillonite as raw material and bis(long-chain) alkyl dimethyl ammonium chloride as modifier, sodium montmorillonite is dispersed in deionized water to form a suspension, and an equimolar ratio of bis(long-chain) alkyl dimethyl ammonium chloride solution is added under high-speed stirring, and the reaction is stirred to carry out ion exchange; after the reaction is completed, the mixture is centrifuged, the precipitate is washed with deionized water until no chloride ions are detected, and finally vacuum dried, ground and sieved to obtain bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite.

[0054] The effective component of the anti-blocking masterbatch used in the embodiments and comparative examples of the present invention is silicon dioxide, with an effective content of 5 wt% and a particle size D50 of 5.0 μm.

[0055] It should be noted that the percentages mentioned in the embodiments or comparative examples of this invention are all weight percentages.

[0056] Example 1

[0057] This embodiment provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially arranged. The preparation method of each layer of the biaxially oriented polypropylene label film in this embodiment includes the following steps:

[0058] Preparation of printing layer resin: 97 wt% random ethylene propylene copolymer and 3 wt% dihexyldimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0059] Core layer resin preparation: 78.5 wt% homopolymer polypropylene, 20 wt% white masterbatch and 1.5 wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 15 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 5000 Da) are mixed evenly to obtain core layer resin.

[0060] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0061] The method for preparing the BOPP film in this embodiment includes the following steps:

[0062] According to the formula, the dried raw materials of each layer are transported to the extruder of each layer. The melt of each layer formed by heating and melting and fully plasticizing is co-extruded through the co-extrusion die. The co-extruded melt is cooled to obtain a thick sheet. The thick sheet is first subjected to longitudinal stretching and shaping treatment, then transverse stretching and shaping, and cooling treatment. Next, it is subjected to thickness measurement and corona treatment. After corona treatment, it is wound up to obtain a master roll, subjected to master aging treatment, and cut to obtain the finished biaxially oriented polypropylene label film roll.

[0063] In the above process, the melt extrusion temperature of the printed layer is 235℃; the melt extrusion temperature of the core layer and the lower surface layer is 250℃; the temperature of the quench water and quench roller is 40℃; the temperature of the longitudinal stretching zone is 120℃; the temperature of the transverse stretching zone is 160℃; the longitudinal stretching ratio is 5.0 times; the transverse stretching ratio is 8.0 times; and the corona power factor of the printed layer is 29.5W·min / m.

[0064] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0065] Example 2

[0066] This embodiment provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially arranged. The preparation method of each layer of the biaxially oriented polypropylene label film in this embodiment includes the following steps:

[0067] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain the printing layer resin. Preparation of core layer resin: 78 wt% homopolymer polypropylene, 20 wt% white masterbatch and 2 wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core with 4 branch arms, each branch arm composed of 18 ethyleneimine repeating units, polypropylene arm number average molecular weight 6500 Da) were mixed evenly to obtain the core layer resin.

[0068] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

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

[0070] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0071] Example 3

[0072] This embodiment provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially arranged. The preparation method of each layer of the biaxially oriented polypropylene label film in this embodiment includes the following steps:

[0073] Preparation of printing layer resin: 95 wt% random ethylene propylene copolymer and 5 wt% dieicoyldimethylammonium chloride modified montmorillonite were mixed evenly to obtain the printing layer resin. Preparation of core layer resin: 77 wt% homopolymer polypropylene, 20 wt% white masterbatch and 3 wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core with 4 branch arms, each branch arm composed of 20 ethyleneimine repeating units, polypropylene arm number average molecular weight 8000 Da) were mixed evenly to obtain the core layer resin.

[0074] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0075] The preparation method of the biaxially oriented polypropylene label film in this embodiment is the same as that in Example 1, so it will not be described again.

[0076] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0077] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0078] The preparation method of the biaxially oriented polypropylene label film in this embodiment is the same as that in Example 1, so it will not be described again.

[0079] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0080] Comparative Example 1

[0081] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0082] Preparation of printing layer resin: Take 100wt% random ethylene propylene copolymer to obtain printing layer resin.

[0083] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0084] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0085] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0086] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0087] Comparative Example 2

[0088] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0089] Preparation of printing layer resin: 98 wt% random ethylene propylene copolymer and 2 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0090] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0091] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0092] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0093] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0094] Comparative Example 3

[0095] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0096] Preparation of printing layer resin: 94 wt% random ethylene propylene copolymer and 6 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0097] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0098] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0099] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0100] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0101] Comparative Example 4

[0102] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0103] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(tetradecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0104] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0105] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0106] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0107] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0108] Comparative Example 5

[0109] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0110] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(docodimethylammonium chloride) modified montmorillonite were mixed evenly to obtain printing layer resin.

[0111] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0112] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0113] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0114] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0115] Comparative Example 6

[0116] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0117] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% montmorillonite were mixed evenly to obtain printing layer resin.

[0118] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0119] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0120] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0121] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0122] Comparative Example 7

[0123] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0124] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0125] Core layer resin preparation: Mix 80wt% homopolymer polypropylene and 20wt% white masterbatch evenly to obtain core layer resin.

[0126] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0127] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0128] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0129] Comparative Example 8

[0130] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0131] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0132] Core layer resin preparation: 79 wt% homopolymer polypropylene, 20 wt% white masterbatch and 1 wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0133] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0134] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0135] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0136] Comparative Example 9

[0137] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0138] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0139] Core layer resin preparation: 76 wt% homopolymer polypropylene, 20 wt% white masterbatch and 4 wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms with 4 branches, each branch arm is composed of 18 ethyleneimine repeating units, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain the core layer resin.

[0140] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0141] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0142] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0143] Comparative Example 10

[0144] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0145] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0146] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms with 4 branches, each branch arm is composed of 12 ethyleneimine repeating units, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain core layer resin.

[0147] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0148] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0149] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0150] Comparative Example 11

[0151] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0152] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0153] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms with 4 branches, each branch arm is composed of 25 ethyleneimine repeating units, and the number average molecular weight of polypropylene arms is 6500 Da) are mixed evenly to obtain the core layer resin.

[0154] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0155] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0156] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0157] Comparative Example 12

[0158] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0159] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0160] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms with 4 branches, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 4000 Da) are mixed evenly to obtain the core layer resin.

[0161] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0162] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0163] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0164] Comparative Example 13

[0165] This comparative example provides a biaxially oriented polypropylene label film, comprising a printing layer, a core layer, and a lower surface layer sequentially disposed therefrom. The method for preparing the resins of each layer of the biaxially oriented polypropylene label film of this comparative example includes the following steps:

[0166] Preparation of printing layer resin: 96 wt% random ethylene propylene copolymer and 4 wt% bis(octadecyl)dimethylammonium chloride modified montmorillonite were mixed evenly to obtain printing layer resin.

[0167] Core layer resin preparation: 78wt% homopolymer polypropylene, 20wt% white masterbatch and 2wt% star-shaped polyethyleneimine-polypropylene graft copolymer (star-shaped core branch arms with 4 branches, each branch arm is composed of 18 ethyleneimine repeating units connected together, and the number average molecular weight of polypropylene arms is 9000 Da) are mixed evenly to obtain core layer resin.

[0168] Preparation of the lower surface resin: 97 wt% homopolymer polypropylene and 3 wt% antiblocking masterbatch are mixed evenly to obtain the lower surface resin.

[0169] The preparation method of the biaxially oriented polypropylene label film in this comparative example is the same as that in Example 1, so it will not be described again.

[0170] The total thickness of the film is 38µm, of which the thickness of the printed layer is 1.2µm and the thickness of the lower surface layer is 1.2µm.

[0171] The performance test results of the biaxially oriented polypropylene label films of Examples 1-3 and Comparative Examples 1-13 are shown in Tables 1 and 2 below.

[0172] Table 1. Test results of basic film properties and surface tension durability of printed layer

[0173]

[0174] Table 2. Test results of key printing performance after aging treatment

[0175]

[0176] From the above performance test data, we can see that:

[0177] The biaxially oriented polypropylene white label films of Examples 1-3 of this invention achieve significant improvements in printability through the synergistic design of a printing layer containing 3-5 wt% bis(long-chain) alkyl dimethyl ammonium chloride-modified montmorillonite and a core layer containing 1.5-3 wt% star-shaped polyethyleneimine-polypropylene graft copolymer. The surface tension of the printing layer remains consistently high, even after prolonged aging treatment, with minimal attenuation, effectively suppressing the negative impacts of small molecule migration. Ink adhesion consistently maintains excellent levels, with no ink smudging, missing prints, or other printing defects, indicating that stable surface tension provides a reliable guarantee for print quality. Good tensile strength and other mechanical properties, along with smooth production without film breakage, demonstrate the rationality of the formulation design and the adaptability of the process.

[0178] The printing layer of Comparative Example 1 did not contain bis-long-chain alkyl dimethyl ammonium chloride modified montmorillonite, which lacked the key role of blocking the migration of small molecules. This resulted in a rapid decrease in surface tension over time, leading to severe ink loss and printing defects after printing. This demonstrates the necessity of montmorillonite modified in the printing layer for blocking migration.

[0179] In Comparative Example 2, the content of bis(long-chain) alkyl dimethyl ammonium chloride modified montmorillonite in the printed layer was below the lower limit of 3 wt%, which prevented the formation of a continuous barrier network, accelerated the decay of surface tension, and reduced ink adhesion.

[0180] In Comparative Example 3, the content of bis-long-chain alkyl dimethyl ammonium chloride modified montmorillonite in the printed layer exceeded the upper limit of 5 wt%, which led to the agglomeration of the sheets and damaged the film structure. This not only affected the mechanical properties but also increased the risk of film breakage during production.

[0181] The printing layer of Comparative Example 4 used a double long-chain alkyl dimethyl ammonium chloride modified montmorillonite. The alkyl chain was too short, resulting in insufficient compatibility with the matrix, weakened barrier effect, and rapid decrease in surface tension, leading to smearing during printing.

[0182] The double long-chain alkyl dimethyl ammonium chloride used in the printing layer of Comparative Example 5 had excessively long alkyl chains, which affected dispersibility and resulted in poor barrier effect, but it was still better than unmodified montmorillonite. This indicates that the alkyl chain length needs to be controlled within a reasonable range to balance dispersibility and barrier effect.

[0183] The printing layer of Comparative Example 6 uses unmodified montmorillonite, which has strong hydrophilicity and poor compatibility with the matrix, easily forming barrier holes. The small molecule migration rate is fast, the surface tension decreases significantly, and the printing defects are more serious than those of Comparative Example 5, which shows the significance of dual long-chain alkyl modification in improving compatibility and barrier effect.

[0184] In Comparative Example 7, the core layer did not contain star-shaped polyethyleneimine-polypropylene graft copolymer, which could not inhibit the migration of small molecules from the source. The surface tension decreased significantly over time, and multiple areas were missed after printing, highlighting the importance of star-shaped copolymer in inhibiting migration.

[0185] In Comparative Example 8, the content of star-shaped polyethyleneimine-polypropylene graft copolymer in the core layer was less than 1.5 wt%, resulting in insufficient amine anchoring points, weak anchoring and blocking effect on small molecules, accelerated decrease in surface tension, and weakened ink adhesion.

[0186] In Comparative Example 9, the content of star-shaped polyethyleneimine-polypropylene graft copolymer in the core layer was higher than 3 wt%. The excessive entanglement of the star-shaped structure led to an increase in melt viscosity, resulting in more film breakage during production and affecting processing stability.

[0187] Comparative Examples 10-13: Because the number of ethyleneimine units or the molecular weight of polypropylene arms in the star-shaped polyethyleneimine-polypropylene graft copolymer deviates from the limits of the technical solution of this invention, its anchoring ability, compatibility with the matrix or network barrier effect decrease, surface tension decays faster, and printing performance weakens, thus demonstrating the necessity of limiting the structural parameters of the star-shaped copolymer.

[0188] In summary, this invention, through the synergistic barrier design of the printed layer and the core layer, optimizes and solves the problem of surface tension attenuation caused by the migration of small molecules in traditional BOPP white label film from the dual dimensions of "source inhibition + path blocking", significantly improving the long-term stability of printing performance.

[0189] 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 biaxially oriented polypropylene label film, characterized in that, The material comprises a printed layer, a core layer, and a lower surface layer arranged sequentially. The printed layer consists of a random ethylene-propylene copolymer and 3-5 wt% bis(long-chain) alkyl dimethyl ammonium chloride-modified montmorillonite. The bis(long-chain) alkyl dimethyl ammonium chloride-modified montmorillonite is prepared by ion exchange of sodium montmorillonite and bis(long-chain) alkyl dimethyl ammonium chloride, wherein the long-chain alkyl group in the bis(long-chain) alkyl dimethyl ammonium chloride-modified montmorillonite has 16-20 carbon atoms. The core layer comprises homopolymer polypropylene, 10-20 wt% white masterbatch, and 1.5-3.0 wt% star-shaped polyethyleneimine-polypropylene graft copolymer. The white masterbatch is composed of 30-50 wt% homopolymer polypropylene and 50- The composite composition comprises 70 wt% surface-modified titanium dioxide; the surface-modified titanium dioxide is obtained by first coating titanium dioxide with an inorganic material as the coating medium, and then coating titanium dioxide with an organic coupling agent as the coating medium in a gradient coating process; the particle size D50 of the coated surface-modified titanium dioxide is 0.2-0.4 μm; each branch arm of the star-shaped polyethyleneimine-polypropylene graft copolymer is composed of 15-20 ethyleneimine repeating units connected together, and the number average molecular weight of the polypropylene arms in the star-shaped polyethyleneimine-polypropylene graft copolymer is 5000-8000 Da; the lower surface layer comprises homopolymer polypropylene.

2. The biaxially oriented polypropylene label film according to claim 1, characterized in that: The preparation method of the modified montmorillonite with alkyl dimethyl ammonium chloride is as follows: sodium montmorillonite is used as raw material and alkyl dimethyl ammonium chloride is used as modifier. Sodium montmorillonite is dispersed in deionized water to form a suspension. An equimolar ratio of alkyl dimethyl ammonium chloride solution is added under high-speed stirring. Ion exchange occurs during the stirring reaction. After the reaction is completed, the mixture is centrifuged and separated. The precipitate is washed with deionized water until no chloride ions are detected. Finally, it is vacuum dried, ground and sieved to obtain alkyl dimethyl ammonium chloride modified montmorillonite.

3. The biaxially oriented polypropylene label film according to claim 1, characterized in that, The star-shaped polyethyleneimine-polypropylene graft copolymer is formed by grafting a polyethyleneimine star-shaped core with polyethyleneimine as the branch arm and a polypropylene arm through an amidation reaction.

4. The biaxially oriented polypropylene label film according to claim 1, characterized in that, The random ethylene-propylene copolymer contains 3.5-4.5 wt% ethylene units, and its melt index is measured to be 7.0-10.0 g / 10 min at 230°C and 2.16 kg.

5. The biaxially oriented polypropylene label film according to claim 1, characterized in that, The lower surface layer also includes 1-5 wt% anti-blocking agent masterbatch, wherein the anti-blocking agent in the anti-blocking agent masterbatch is at least one of silica, talc, and plexiglass microspheres, and the particle size D50 of the anti-blocking agent is 3-7 μm.

6. The biaxially oriented polypropylene label film according to claim 1, characterized in that, The melt index of the homopolymer polypropylene was measured to be 3-6 g / 10 min at 230℃ and 2.16 kg.

7. A method for preparing a biaxially oriented polypropylene label film as described in any one of claims 1 to 6, characterized in that, Includes the following steps: According to the formula, the dried raw materials of each layer are transported to the extruder of each layer. The melt formed by heating and melting and fully plasticizing is co-extruded through the co-extrusion die. The co-extruded melt is cooled to obtain a thick sheet. The thick sheet is first subjected to longitudinal stretching and shaping, then transverse stretching and shaping, and cooling. Next, it is subjected to thickness measurement and corona treatment. After corona treatment, it is wound up to obtain a master roll, subjected to aging treatment and slit to obtain the finished biaxially oriented polypropylene label film roll.

8. The method for preparing biaxially oriented polypropylene label film according to claim 7, characterized in that, The melt extrusion temperature of the printed layer is 180-270℃, the melt extrusion temperature of the core layer and the lower surface layer is 220-270℃, and the quenching temperature is 15-40℃; the longitudinal stretching temperature is 90-150℃, and the longitudinal stretching ratio is 4.5-5.7 times; the transverse stretching temperature is 150-170℃, and the transverse stretching ratio is 7.5-10 times; the corona power factor is 20-40 W•min / m.

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