A co-extruded adhesive-free composite film and a method for preparing the same

Abstract

The present application relates to the field of film, especially to a kind of co-extrusion adhesive-free composite film and preparation method thereof, the co-extrusion adhesive-free composite film includes first skin, core layer, secondary skin, second skin arranged in sequence, secondary skin includes ethylene-vinyl acetate copolymer A, ethylene-acrylic acid copolymer and maleic anhydride grafted polypropylene;Second skin includes ethylene-vinyl acetate copolymer B and ethylene-acrylic acid copolymer;Wherein secondary skin and second skin include modified macroporous silica in both;The present application also provides the preparation method of the co-extrusion adhesive-free composite film.The co-extrusion adhesive-free composite film of the present application does not need to use adhesive, and is environmentally friendly and energy-saving, has good hot-pressing adhesion and anti-blocking performance, and good optical performance.

Description

Technical Field

[0001] This invention relates to the field of films, and in particular to a co-extruded adhesive-free composite film and its preparation method. Background Technology

[0002] With increasingly stringent environmental protection requirements, adhesive-free paper-plastic composite films, which do not require coating with adhesives, have been widely used in packaging, printing, and other fields. These composite films typically use biaxially oriented polypropylene film as the substrate and ethylene-vinyl acetate copolymer (EVA) as the hot-pressing adhesive layer, directly laminating it with paper under heat and pressure.

[0003] However, existing adhesive-free paper-plastic composite films still have a series of prominent problems in actual production and use. On the one hand, the EVA surface layer, as a hot-pressing adhesive layer, has strong adhesion and is prone to sticking to the corona-treated polypropylene surface (the other surface layer in the film besides the hot-pressing adhesive layer) under winding pressure. This leads to difficulties in subsequent slitting and unwinding during customer use, and may even cause film breakage, seriously affecting efficiency. Simultaneously, due to EVA's low melting point, the EVA surface layer is prone to sticking to the high-temperature stretching rollers in the longitudinal stretching section of the biaxial stretching process, forming imprints and reducing the film yield. If EVA with a higher melting point, higher softening point, and lower melt index is used, the EVA has insufficient fluidity during hot pressing and cannot fully wet and spread on the paper surface, resulting in defects such as incomplete lamination and a weak appearance. To alleviate adhesion, the common practice in the field is to add anti-adhesion masterbatch to the surface of EVA, such as micron-sized solid silica or polymethyl methacrylate microspheres. However, due to the refractive index difference between traditional solid silica and other anti-adhesion agents and the EVA matrix, and the clear interface between solid silica and the matrix, when the amount added is high, light is scattered through the film, resulting in increased haze, decreased gloss and clarity, and deterioration of the film's optical properties. This limits the application of such films in packaging fields with high optical quality requirements, such as high-definition printing and transparent windows. Summary of the Invention

[0004] The purpose of this invention is to provide a co-extruded adhesive-free composite film and its preparation method. The co-extruded adhesive-free composite film of this invention eliminates the need for adhesives, making it environmentally friendly and energy-saving. On one hand, it exhibits high bonding strength and excellent hot-pressing adhesion, which helps improve the problem of incomplete or weak film adhesion during paper-plastic lamination. On the other hand, it also possesses excellent anti-blocking properties, ensuring smooth unwinding and rewinding without sticking to the rollers during production. This effectively improves the smoothness of film slitting and subsequent application unwinding, while also preventing the degradation of the film's optical properties.

[0005] A co-extruded adhesive-free composite film includes a first surface layer, a core layer, a sub-surface layer, and a second surface layer arranged sequentially. The first surface layer and the core layer are both polypropylene layers. The sub-surface layer includes ethylene-vinyl acetate copolymer A, ethylene-acrylic acid copolymer, and maleic anhydride-grafted polypropylene. The second surface layer includes ethylene-vinyl acetate copolymer B and ethylene-acrylic acid copolymer. Both the sub-surface layer and the second surface layer include modified macroporous silica, which is prepared by adsorbing tackifying resin into the pores of the macroporous silica. The melt index of ethylene-vinyl acetate copolymer A is measured to be 50~150 g / 10 min at 190℃ and 2.16 kg, and the melt index of ethylene-vinyl acetate copolymer B is 15~25 g / 10 min.

[0006] The inventors discovered that existing adhesive-free composite films primarily use EVA as the main material for their functional layers, typically employing a three-layer co-extrusion structure (such as a polypropylene surface layer / polypropylene core layer / ethylene-vinyl acetate copolymer functional layer). During the biaxial stretching and winding stages, because the EVA functional layer is directly exposed on the film surface, it experiences severe adhesion upon contact with metal or rubber rollers on the production equipment. This results in adhesive residue on the roller surface, leading to quality defects such as scratches and dirt on the film surface, severely impacting production continuity and product yield. Simultaneously, during winding, slitting, and storage, the EVA layer remains in close contact with the other surface layer of the film for extended periods under winding tension. Due to the inherent stickiness of the EVA surface, adhesion easily occurs, causing difficulties in unwinding and film surface damage. The bonding between the functional layer and the paper relies mainly on physical anchoring, lacking stronger and more stable interfacial interactions. When the functional layer material lacks sufficient fluidity, it struggles to fully penetrate the microscopic irregularities and pores on the paper surface caused by fiber structure, ink state, or powder residue, resulting in a weak film surface or incomplete lamination after hot pressing. Using EVA with high vinyl acetate (VA) content and high melt index as the functional layer, while offering good flowability and strong affinity with paper, resulting in excellent lamination strength, also leads to severe roller sticking and adhesion problems, resulting in poor processing adaptability. Using EVA with low VA content and low melt index significantly improves anti-blocking performance and processing stability, but its flowability decreases drastically, making it difficult to effectively wet and fill the microstructure of the paper surface, thus reducing lamination strength and easily causing film blurring and incomplete bonding after hot pressing. Because current technology concentrates all adhesive functions on a single functional layer, it cannot simultaneously meet the conflicting performance requirements of high adhesive strength to paper and anti-blocking properties during production and winding, making it difficult to achieve a comprehensive improvement in the overall product performance.

[0007] The inventors cleverly separated the seemingly contradictory requirements of high adhesion and anti-adhesion through ingenious design. The film employs at least a four-layer structure. During the film-making and winding stages before hot-pressing lamination, modified macroporous silica in the second surface layer protrudes from the film surface, forming protrusions. Because the hardness of the modified macroporous silica is higher than that of the EVA matrix, and the increased surface roughness of the film effectively reduces the actual contact area between the film layers (the second and first surface layers in this invention), thus effectively preventing the mother roll from sticking. The core mechanism of this invention is to allow the high melt index melt of the sub-surface layer to pass through the interconnected channels of the modified macroporous silica in the sub-surface and second surface layers under hot-pressing, migrate to the surface of the second surface layer of the film, and penetrate into the paper to achieve adhesion. This process needs to be completed within an extremely short hot-pressing time, therefore requiring a high melt flow rate. The melt index of ethylene-vinyl acetate copolymer A is limited to 50~150 g / 10min. If it is lower than 50 g / 10min, the melt viscosity of the sub-layer is too high, and it cannot fully pass through the nanopores of the sub-layer and the second layer in a short time during hot pressing, resulting in insufficient migration and a weak film surface after lamination. If it is higher than 150 g / 10min, it will not be able to effectively co-extrude with other layers to form a suitable biaxially stretched thick sheet, and the polarity difference with the core layer is too large, which is not conducive to the interlayer bonding between the sub-layer and the core layer, and delamination is likely to occur during the preparation process. If the melt index of ethylene-vinyl acetate copolymer B in the second layer is too low, the melt viscosity is too high, and the flow and spreading ability at the hot pressing temperature is insufficient, which cannot fully wet the paper surface and easily leads to local incomplete lamination. Meanwhile, excessively high viscosity increases the processing difficulty of the second surface layer during co-extrusion and biaxial stretching, affecting the uniformity of film thickness. Furthermore, the high-viscosity ethylene-vinyl acetate copolymer B melt may clog the pore entrances of the modified macroporous silica, hindering the migration of the subsurface melt. While an excessively high melt index and strong fluidity of ethylene-vinyl acetate copolymer B are beneficial for spreading, they can cause adhesion to the rollers contacting the second surface layer during biaxial stretching and traction winding stages, leading to glue buildup on the rollers, affecting production smoothness, and ultimately resulting in serious film surface quality problems in the final product.

[0008] This invention uses high melt index ethylene-vinyl acetate copolymer A, ethylene-acrylic acid copolymer (EAA), and maleic anhydride-grafted polypropylene in the second surface layer, and lower melt index ethylene-vinyl acetate copolymer B and EAA in the second surface layer. A certain amount of modified macroporous silica is added to both the second and third surface layers. During the hot-pressing lamination stage, both the second and second surface layers melt and soften. Due to the extremely high fluidity of ethylene-vinyl acetate copolymer A in the second surface layer, its fluidity is higher than that of the second surface layer. Under hot pressing, the lower fluidity of ethylene-vinyl acetate copolymer B and EAA in the second surface layer first wets most of the large voids in the paper. Simultaneously, the high fluidity of the second surface layer melt migrates to the surface of the second surface layer film through the channels of the modified macroporous silica in both the second and second surface layers, then spreads and penetrates into the paper fibers to fill more voids. After cooling following hot pressing, a continuous adhesive-reinforcing interface is formed between the second surface layer and the paper surface, effectively improving defects such as incomplete lamination and a blurry film surface. The use of ethylene-vinyl acetate copolymer B with a low melt index in the second surface layer improves the problem of the second surface layer sticking to the high-temperature stretching roller during stretching due to its excessive fluidity and viscosity.

[0009] In the four-layer structure of this invention, the ethylene-vinyl acetate copolymer A in the second layer needs to penetrate the second layer to reach the paper surface. If ordinary silica is used, because it lacks internal through-channels, the melt can only migrate through the interfacial gap between the silica and the matrix, resulting in insufficient migration and failing to solve the problem of poor lamination. Macroporous silica has a through-channel structure, which is equivalent to pre-setting countless micro-channels in the second and second layers. During hot-press lamination, the melt of the ethylene-vinyl acetate copolymer A in the second layer can enter these through-channels, realizing multi-channel transport from the second layer to the film surface. At the same time, the interior of its channels is naturally filled with EVA / EAA blends with similar refractive indices, effectively reducing the effective refractive index difference between the modified macroporous silica and the second layer matrix, reducing light scattering, and helping to ensure optical performance. The inner walls of the pores in modified macroporous silica are essentially hydrophilic silanol groups, while ethylene-vinyl acetate copolymer A is a hydrophobic polymer. Therefore, the melt encounters significant frictional resistance when passing through the pores, affecting migration speed and efficiency. Pre-adsorbing a layer of tackifying resin onto the inner walls of the pores can effectively reduce the coefficient of friction between the ethylene-vinyl acetate copolymer A melt and the pore walls. This invention employs a four-layer structure to separate functions. The second outer layer, in contact with the paper, serves as an anti-adhesion and initial adhesive layer, while the inner layer is specifically designed for high-flowability filling. This balances the seemingly contradictory requirements of anti-adhesion and strong adhesion. Furthermore, the naturally incorporated modified macroporous silica, a blend of EVA / EAA with similar refractive indices, effectively reduces refractive index differences, further optimizing the optical properties of the film.

[0010] Furthermore, the modified macroporous silica content in the sub-layer is 0.3~0.5 wt%, and in the second layer is 0.5~0.8 wt%. The modified macroporous silica in the sub-layer forms through-channels to transport the ethylene-vinyl acetate copolymer A melt to the second layer. During co-extrusion and biaxial stretching, it forms microscopic protrusions within the sub-layer and at the interface, synergistically exerting an anti-adhesion effect with the modified macroporous silica in the second layer. When the modified macroporous silica content in the sub-layer is too low, the number of modified macroporous silica particles per unit area is insufficient, and the average spacing between them is too large. This results in a low distribution density of through-channels in the sub-layer, meaning the ethylene-vinyl acetate copolymer A melt can only be transported to the second layer through a few channels in the modified macroporous silica. The efficiency of migration to the second surface of the film is insufficient, and some areas lack a supply of high melt index EVA melt, making localized voids after lamination still likely. When the modified macroporous silica content in the sub-layer is too high, although the channel density increases, excessive modified macroporous silica will reduce the overall fluidity of the sub-layer. Agglomeration of modified macroporous silica may occur, blocking melt channels. Furthermore, excessive modified macroporous silica content increases the interfacial roughness between the sub-layer and the core layer, affecting interlayer bonding and creating more stress concentration points, easily leading to film breakage. It will also inevitably increase the haze of the product and degrade optical performance. Within this range, the distribution density of modified macroporous silica in the sub-layer is sufficient to form a continuous, interconnected melt transport network, ensuring that the ethylene-vinyl acetate copolymer A melt can be uniformly and fully transported to the second layer through the modified macroporous silica channels during hot pressing. The modified macroporous silica in the second surface layer serves to form microscopic protrusions on its interior and surface, reducing the actual contact area between film layers during winding and preventing the master roll from sticking together. Furthermore, during biaxial stretching, these protrusions reduce the direct contact area between the second surface layer and the high-temperature stretching rollers, preventing roller sticking. Simultaneously, its through-holes serve as the final channel for EVA melt migration to the film surface. Because the second surface layer directly faces the winding process and processing rollers, it has higher requirements for anti-sticking and anti-roller sticking; therefore, the modified macroporous silica content is higher than that of the second surface layer.

[0011] Furthermore, the macroporous silica has a particle size D50 of 4-6 μm and a pore size of 100-500 nm. The macroporous silica selected in this invention has an average particle size of 4-6 μm. This is beneficial in two ways: firstly, it facilitates the formation of sufficiently high protrusions on the second surface layer, effectively increasing the surface roughness of the second surface layer and enhancing its anti-adhesion effect; secondly, it facilitates the penetration of macroporous silica from the sub-surface layer into the second surface layer, promoting the efficiency of melt migration from the sub-surface layer to the film surface. The particle size of macroporous silica affects the dispersion of modified macroporous silica in films, as well as the surface protrusion height and pore length. If the particle size is too small, the protrusion height of the modified macroporous silica on the film surface is insufficient, which cannot effectively reduce the contact area between film layers, resulting in poor anti-adhesion effect and easy agglomeration of modified macroporous silica. If the particle size is too large, the protrusion of the modified macroporous silica on the film surface is too high, which prevents the film from making sufficient contact with the paper during hot pressing, resulting in poor lamination and a weak appearance. Furthermore, excessively large modified macroporous silica can easily cause the film to break during biaxial stretching. In addition, macroporous silica scatters light more strongly, increasing haze and degrading the optical properties of the film. The pore size of macroporous silica determines the flow resistance of the melt through the channels and the molecular size that can pass through. If the pore size is too small, the resistance of the melt increases, and the sub-surface melt cannot flow through the channels to the second surface in a short time during hot pressing, which is not conducive to ensuring the bonding strength with paper. On the other hand, if the pore size is too large, it will reduce the mechanical strength of the modified macroporous silica. During co-extrusion and biaxial stretching, the modified macroporous silica is prone to breakage, resulting in film defects and production problems.

[0012] Furthermore, in the sub-layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer A is 18-28 wt%, and the maleic anhydride grafting rate of the maleic anhydride-grafted polypropylene is 0.8%-1.2%; in the second layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer B is 5-10 wt%. The higher the VA content of the ethylene-vinyl acetate copolymer A in the sub-layer, the stronger the compatibility and interaction between the ethylene-vinyl acetate copolymer A and the fibers, EAA, and maleic anhydride-grafted polypropylene in the paper. When the VA content is too low, the compatibility with EAA and maleic anhydride-grafted polypropylene deteriorates, while too high a content can easily lead to adhesion to the metal roller during production. This range ensures the compatibility of the internal components of the sub-layer and its interaction with the fibers in the paper. When the VA content in the ethylene-vinyl acetate copolymer B in the second surface layer is too low, the compatibility between the ethylene-vinyl acetate copolymer B and EAA deteriorates, and phase separation may occur within the second surface layer, affecting the uniformity of the blend. Simultaneously, ethylene-vinyl acetate copolymer B with too low a VA content exhibits poor flexibility and insufficient melt spreading ability, which is detrimental to its adhesion to paper. Conversely, when the VA content in the ethylene-vinyl acetate copolymer B in the second surface layer is too high, although it is beneficial for adhesion to paper, it easily leads to roller sticking in the longitudinal stretching section of the biaxial stretching process. After winding, the master roll is more prone to sticking, affecting slitting and use. Maleic anhydride-grafted polypropylene has good compatibility with the polypropylene in the core layer, which helps ensure the interlayer bonding strength between the sub-surface layer and the core layer, preventing delamination.

[0013] Furthermore, in the sub-layer, the content of ethylene-vinyl acetate copolymer A is 74-90 wt%, the content of ethylene-acrylic acid copolymer is 5-15 wt%, and the content of maleic anhydride-grafted polypropylene is 5-10 wt%; in the second layer, the content of ethylene-vinyl acetate copolymer B is 69-85 wt%, and the content of ethylene-acrylic acid copolymer is 15-30 wt%. Ethylene-vinyl acetate copolymer A serves as the main resin of the sub-layer. When its content is too low, the proportion of the main resin is insufficient, and the overall fluidity and film-forming properties of the sub-layer will decrease; when its content is too high, the functional components of EAA and maleic anhydride-grafted polypropylene are excessively diluted, failing to fully exert their respective thickening and compatibilizing effects. The main function of ethylene-vinyl acetate copolymer B in the second surface layer is to provide anti-blocking and basic adhesive functions. When the content of ethylene-vinyl acetate copolymer B is too high, the proportion of EAA decreases accordingly. Insufficient EAA content leads to a decrease in the bonding strength between the second surface layer and the paper, while an excessively high proportion of ethylene-vinyl acetate copolymer B will worsen the compatibility between the second surface layer and the sub-surface layer, weakening the interlayer bonding strength. On the one hand, the carboxyl groups on the EAA molecular chain can form hydrogen bonds and chemical bonds with the hydroxyl groups of the fibers in the paper, which is beneficial to improving the bonding strength of the paper-plastic composite interface. On the other hand, they can interact with the maleic anhydride groups in maleic anhydride-grafted polypropylene and the ester groups in EVA, promoting the compatibility between the components within the sub-surface layer. If the EAA content in the sub-surface layer is too low, it cannot improve the interfacial bonding strength and compatibility. If the content is too high, the melt viscosity of EAA itself is higher than that of ethylene-vinyl acetate copolymer A, which reduces the overall fluidity of the sub-surface layer melt and is not conducive to ensuring hot-pressing adhesion performance. Insufficient EAA content in the second surface layer leads to decreased adhesion between the second surface layer and the paper, resulting in reduced overall peel strength. Conversely, excessive EAA content results in overly polar second surface layer, making it more prone to adhesion to the metal rollers during biaxial stretching. Insufficient maleic anhydride-grafted polypropylene (MPP) addition is detrimental to maintaining adhesion between the sub-surface layer and the core PP layer, easily causing delamination at the core / sub-surface layer interface. Excessive addition may lead to crosslinking or side reactions, causing film breakage and hindering smooth production.

[0014] Furthermore, the adsorption amount of the tackifying resin relative to the macroporous silica is 5-30 wt%; the tackifying resin is one or more of rosin and its derivatives, terpene resin, and petroleum resin. Rosin and its derivatives, terpene resin, and petroleum resin are all low molecular weight, low viscosity functional additives. After being pre-adsorbed onto the inner wall of the pores of macroporous silica, they can form a lubricating layer on the pore surface, which helps to reduce the frictional resistance of the high melt index EVA melt when passing through the pores and improves the efficiency of migration to the second surface layer of the film. The aforementioned rosin and its derivatives, terpene resin, and petroleum resin all have good compatibility with EVA. During hot pressing, they can be carried out of the pores along with the melt and reach the paper surface. Their own viscosity can also play an auxiliary role in tackification. In addition, rosin and its derivatives, terpene resin, and petroleum resin are all thermoplastic resins, which are stable at the biaxial stretching process temperature, do not decompose or produce volatiles, and are easily soluble in organic solvents, making it easy to achieve uniform adsorption in the pores using the impregnation method.

[0015] Furthermore, in the sub-layer and the second layer, the ethylene-acrylic acid copolymer has a melt index of 15-25 g / 10 min and a melting point of 80-105 °C, measured at 190 °C and 2.16 kg. The acrylic acid content in the ethylene-acrylic acid copolymer is 8-15 wt%. EAA mainly plays a role in chemical bonding and compatibility improvement in the sub-layer, and mainly enhances interlayer bonding in the second layer. The melt index of EAA is limited to 15-25 g / 10 min, ensuring an appropriate viscosity ratio after blending with the high melt index ethylene-vinyl acetate copolymer A. This facilitates the transport of EAA along with EVA to the paper interface for chemical bonding during migration. The melting point of EAA is limited to 80-105 °C, ensuring sufficient softening at hot-pressing temperatures (95-110 °C), enabling it to form hydrogen bonds or chemical bonds with the fibers in the paper, thus improving adhesive strength. Furthermore, the acrylic acid content within this range provides sufficient chemical bonding sites.

[0016] Furthermore, the polypropylene in both the first surface layer and the core layer is isotactic polypropylene, with an isotacticity of 95-97% and a melt flow index of 2.8-3.8 g / 10 min measured at 230°C and 2.16 kg. Using the aforementioned isotactic polypropylene as the core layer helps ensure the excellent overall mechanical properties of the film.

[0017] Furthermore, the thickness of the sub-layer is 1~2μm, and the thickness of the second layer is 2~3μm. On the one hand, this helps to ensure the hot-pressing adhesion performance of the film. On the other hand, the fact that both the sub-layer and the second layer are smaller than the particle size of the modified macroporous silica helps to form sufficiently high protrusions on the second layer, which is equivalent to increasing the surface roughness of the second layer and enhancing the anti-adhesion effect. It also helps the channels of the modified macroporous silica in the sub-layer to penetrate into the second layer, promoting the efficiency of the melt migration from the sub-layer to the film surface.

[0018] This invention also provides a method for preparing a co-extruded adhesive-free composite film, comprising the following steps:

[0019] The raw materials of each layer are fed into the batching unit, metered, and then fed into the extruder. After being melted, plasticized, homogenized, and metered, they enter the flow channel distributor and are then extruded through the die head. After being cast into a thick sheet by the chilled rollers, the sheet is then stretched longitudinally and then stretched laterally to form a film. The film is cooled by air shower, and its thickness is controlled by the trimming machine. After being treated with corona and / or flame, the film is collected into a film master roll. The film master roll is then subjected to aging treatment, slitting, and packaging to obtain the finished product.

[0020] This invention employs a highly fluid secondary layer, incorporating modified macroporous silica into both the secondary and secondary layers. This modified macroporous silica simultaneously serves as melt flow channels and an anti-blocking agent, allowing the melt to migrate to the film surface during hot pressing via the interconnected channels of the modified macroporous silica in both layers. Furthermore, the added EAA in both layers forms hydrogen or chemical bonds with the fibers in the paper, enhancing adhesion strength and overcoming the weakness of traditional single-layer EVA adhesive layers that fail to fully wet the paper. The modified macroporous silica channels are naturally filled with a blend of EVA and EAA with similar refractive indices, effectively reducing the refractive index difference between the modified macroporous silica and the EVA matrix, thus minimizing light scattering. Therefore, while maintaining excellent anti-blocking properties, the film exhibits low haze, making it suitable for high-transparency packaging applications.

[0021] To better understand and implement this invention, the invention will be described in detail below. Detailed Implementation

[0022] To facilitate understanding of the present invention, it will be described more fully below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0023] 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.

[0024] The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. 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.

[0025] The experimental methods in the following examples or comparative examples, unless otherwise specified, are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used, unless otherwise specified, are all commercially available from the conventional market.

[0026] This invention provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a sub-surface layer, and a second surface layer arranged sequentially. The first surface layer and the core layer are both polypropylene layers. The sub-surface layer comprises ethylene-vinyl acetate copolymer A, ethylene-acrylic acid copolymer, and maleic anhydride-grafted polypropylene. The second surface layer comprises ethylene-vinyl acetate copolymer B and ethylene-acrylic acid copolymer. Both the sub-surface layer and the second surface layer comprise modified macroporous silica, which is prepared by adsorbing a tackifying resin into the pores of the macroporous silica. The melt index of ethylene-vinyl acetate copolymer A is measured to be 50~150 g / 10 min at 190°C and 2.16 kg, and the melt index of ethylene-vinyl acetate copolymer B is measured to be 15~25 g / 10 min.

[0027] As one implementation method, the preparation method of macroporous silica includes the following steps: Commercially available mesoporous silica gel powder (commonly known in the industry as silica gel) with an average particle size of 4-6 μm, an initial average pore size of 10 nm, and a specific surface area of ​​350 m² / g was used as a precursor. LiCl·H₂O and NaCl were dissolved in deionized water in a certain proportion (the mass of the mixed salt composed of LiCl·H₂O and NaCl was 15-30 wt% of the silica gel mass), and stirred at room temperature until completely dissolved to obtain a mixed salt solution. 100 g of the silica gel was added to this solution and ultrasonically dispersed for 30 min to allow the mixed salt solution to fully penetrate into the pores of the silica gel. The mixture was rotary evaporated at 60-80 °C until the solvent was completely evaporated to obtain mixed salt-loaded silica gel. The mixed salt-loaded silica gel was transferred to a crucible, placed in a muffle furnace, and heated to 500-650 °C at a heating rate of 5 °C / min, and calcined at this temperature for 3-7 h. After calcination, it was naturally cooled to room temperature. The calcined product was washed with 5% dilute hydrochloric acid for 30 minutes, followed by repeated washing with deionized water until no white precipitate (i.e., no chloride ion residue) was detected in the filtrate using 0.1 mol / L silver nitrate solution. Finally, it was rinsed once with anhydrous ethanol. The washed product was dried at 100℃ for 12 hours to obtain macroporous silica. The pore size can be controlled by adjusting the calcination temperature, calcination time, and the amount of mixed salt. Increasing the calcination temperature, extending the calcination time, or increasing the amount of mixed salt results in larger pore sizes; conversely, decreasing these parameters leads to smaller pore sizes.

[0028] Furthermore, the content of macroporous silica in the sub-layer is 0.3~0.5wt%, and the content in the second layer is 0.5~0.8wt%.

[0029] Furthermore, the particle size D50 of the macroporous silica is 4~6μm, and the pore size of the macroporous silica is 100~500nm.

[0030] Further, in the sub-layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer A is 18~28wt%, and the maleic anhydride grafting rate of the maleic anhydride-grafted polypropylene is 0.8%~1.2%; in the second layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer B is 5~10wt%.

[0031] Furthermore, the sub-layer contains 74-90 wt% ethylene-vinyl acetate copolymer A, 5-15 wt% ethylene-acrylic acid copolymer, and 5-10 wt% maleic anhydride-grafted polypropylene; the second layer contains 69-85 wt% ethylene-vinyl acetate copolymer B and 15-30 wt% ethylene-acrylic acid copolymer.

[0032] Furthermore, the amount of the tackifying resin adsorbed relative to the macroporous silica is 5-30 wt%; the tackifying resin is one or more of rosin and its derivatives, terpene resin, and petroleum resin.

[0033] As one implementation method, the adsorption process of the tackifying resin by the macroporous silica in the preparation of modified macroporous silica is as follows: the tackifying resin is dissolved in a volatile organic solvent (acetone or ethyl acetate) to prepare a solution with a concentration of 5-25 wt%, and then the macroporous silica is immersed in it for 3-18 hours to allow the solution to fully penetrate into the pores of the macroporous silica. After that, the solvent is removed by heating or vacuum evaporation. The tackifying resin is physically adsorbed and solidified in the nanopores. The adsorption amount is determined by thermogravimetric analysis (TGA), and the final adsorption amount of the tackifying resin is 5-30 wt% of the mass of the macroporous silica.

[0034] Furthermore, in the sub-layer and the second layer, the ethylene-acrylic acid copolymer has a melt index of 15~25g / 10min and a melting point of 80~105℃ when measured at 190℃ and 2.16kg, and the acrylic acid content in the ethylene-acrylic acid copolymer is 8~15wt%.

[0035] Furthermore, the polypropylene in the first surface layer and the core layer is isotactic polypropylene, the isotacticity of which is 95-97%, and the melt index measured at 230℃ and 2.16kg is 2.8-3.8g / 10min.

[0036] Furthermore, the thickness of the sub-layer, which has a total thickness of 12-17 μm, is 1-2 μm, the thickness of the second layer is 2-3 μm, and the thickness of the first layer is 1-2 μm.

[0037] This invention also provides a method for preparing a co-extruded adhesive-free composite film: the raw materials of each layer are fed into the batching unit, metered, and then fed into the extruder. After being melted, plasticized, homogenized, and metered, the film enters the flow channel distributor and is then extruded through the die head. The film is then cast into a thick sheet by a chilled roller, and then stretched longitudinally and then stretched laterally to form a film. The film is cooled by air shower, and its thickness is controlled by an edge trimmer. The film is then subjected to corona treatment and / or flame treatment, and then the film is rolled into a master roll. The master roll is then subjected to aging treatment, slitting, and packaging to obtain the finished product. Specifically, the raw materials for each layer are metered in the batching unit and then fed into the extruder. The extrusion temperature for the core layer and the first surface layer is controlled at 240~260℃, and the extrusion temperature for the second surface layer and the sub-surface layer is controlled at 210~240℃. After passing through the flow channel distributor, they converge at the multi-layer die head to form a multi-layer resin melt. After being cooled by a chilling roller at 25~35℃, a multi-layer resin sheet is formed. The resin sheet is then introduced into the longitudinal stretching device of the biaxial stretching equipment. The surface of the first surface layer is pre-treated at 130~135℃. The core layer and the first surface layer are heated to a stretching temperature of 110-130℃. The second surface layer and the sub-surface layer are preheated to 50-90℃ and stretched at a stretching temperature of 50-90℃, with a stretching ratio of 4.8-5.2 times. Then, the film is introduced into a transverse stretching device, preheated to 165-175℃, stretched 8-10 times at 156-160℃, and then shaped at 165-170℃. After air cooling, the film is subjected to corona or flame treatment to obtain a film master roll. Finally, the film is aged, slit, and packaged to obtain a co-extruded adhesive-free composite film.

[0038] The present invention will now be described in conjunction with specific embodiments.

[0039] The physical properties and their testing methods in the following embodiments are as follows: The film thickness was determined according to GB / T6672-2001; Melt index was determined according to GB / T3682.1-2018, at 2.16 kg and 190℃. The haze test was conducted according to GB / T2410-2008; The peel strength of white cardboard was determined according to GB / T8808-1988 (Method A); Method for statistical analysis of defects: A composite substrate was obtained by hot-pressing a non-adhesive composite film with black cardboard (hot-pressing temperature 100℃, sealing time 2s, hot-pressing pressure 17MPa). The substrate had an area of ​​0.01m². 2 Within a 10×10cm area of ​​the composite substrate, small white dots with a diameter ≥0.5mm were counted due to incomplete or indistinct lamination of the film on the black cardboard surface. All percentages mentioned in the following examples are weight percentages.

[0040] It should be noted that in the examples and comparative examples, the isotactic polypropylene used in the first surface layer and core layer has an isotacticity of 90% and a melt index of 3.8 g / 10 min measured at 230°C and 2.16 kg; the EAA in the second and third surface layers has a melt index of 20 g / 10 min measured at 190°C and 2.16 kg, a melting point of 90°C, and an acrylic acid content of 10 wt%; the maleic anhydride grafting rate in the maleic anhydride-grafted polypropylene in the second surface layer is 1.2%; the ethylene-vinyl acetate copolymer B in the second surface layer has a melt index of 20 g / 10 min measured at 190°C and 2.16 kg and a VA content of 10 wt%; the ordinary silica used in the comparative example is solid silica with a particle size D50 of 5 μm; In the embodiments and comparative examples of this invention, the particle size D50 of the modified macroporous silica is 5 μm. The pore size of macroporous silica can be controlled by adjusting the calcination temperature, calcination time, and the amount of mixed salt, as follows: Weigh 1.125g LiCl·H₂O and 0.75g NaCl and dissolve them in 30ml deionized water to prepare a mixed salt solution. The total mass concentration of the two salts in this solution is 0.063g / mL. Add this solution entirely to 10g silica gel and calcine at 500℃ for 4 hours to obtain macroporous silica with a pore size of 100nm. Weigh 1.5g LiCl·H₂O and 1.0g NaCl and dissolve them in 30ml deionized water to prepare a mixed salt solution. The total mass concentration of the two salts in this solution is 0.083g / mL. Add this solution entirely to 10g silica gel and calcine at 550℃ for 5 hours to obtain macroporous silica with a pore size of 250nm. Weigh 2.0g LiCl·H₂O and 1.3g NaCl and 1.5g NaCl and 1.0g NaCl and dissolve them in 30ml deionized water to prepare a mixed salt solution. The total mass concentration of the two salts in this solution is 0.083g / mL. Add this solution entirely to 10g silica gel and calcine at 550℃ for 5 hours to obtain macroporous silica with a pore size of 250nm. NaCl was dissolved in 30 ml of deionized water to prepare a mixed salt solution with a total mass concentration of 0.110 g / mL. This solution was then added to 10 g of silica gel and calcined at 600 °C for 6 h to obtain macroporous silica with a pore size of 500 nm. Alternatively, 2.5 g of LiCl·H₂O and 1.7 g of NaCl were dissolved in 30 ml of deionized water to prepare a mixed salt solution with a total mass concentration of 0.140 g / mL. This solution was then added to 10 g of silica gel and calcined at 650 °C for 7 h to obtain macroporous silica with a pore size of 1000 nm.

[0041] In the embodiments and comparative examples of this invention, rosin glycerol ester was selected as the tackifying resin. The specific preparation method of the modified macroporous silica is as follows: 15g of rosin glycerol ester was dissolved in 85g of acetone and stirred until completely dissolved to obtain a tackifying resin solution with a mass fraction of 15wt%. 100g of macroporous silica was immersed in the above solution and stirred and impregnated at room temperature for 12 hours to allow the solution to fully penetrate the pores of the macroporous silica. After impregnation, acetone was removed by rotary evaporation at 40-50℃. The resulting solid was transferred to a vacuum oven and dried at 60℃ for 12 hours to completely remove residual solvent. The dried product was gently pulverized and passed through a 200-mesh sieve to obtain modified macroporous silica with rosin glycerol ester adsorbed in the pores. Thermogravimetric analysis (TGA) determined that the adsorption amount of the tackifying resin was 15wt% (relative to the mass of macroporous silica).

[0042] The components and contents of each layer in the embodiments and comparative examples of the present invention are shown in Table 1: Table 1. Components and contents of each layer in the examples and comparative examples.

[0043] Example 1 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 89.5wt% ethylene-vinyl acetate copolymer A (VA content 18wt%, melt index measured at 190℃ and 2.16kg is 50g / 10min), 5wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 5wt% maleic anhydride-grafted polypropylene (grafting rate 1.2%), 0.5wt% modified macroporous silica (pore size of macroporous silica is 500nm, adsorption capacity of tackifying resin is 15wt%). Second surface layer: 84.5wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 15wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.5wt% modified macroporous silica (pore size of macroporous silica is 500nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment includes the following steps: The raw materials for each layer are metered and fed into the batching unit before entering the extruder. The extrusion temperature for the core layer and the first surface layer is controlled at 240℃, and the extrusion temperature for the second surface layer and the sub-surface layer is controlled at 210℃. After passing through the flow channel distributor, they converge at the multi-layer die head to form a multi-layer resin melt. After being cooled by a 35℃ chiller roller, a multi-layer resin sheet is formed. The resin sheet is then introduced into the longitudinal stretching device of the biaxial stretching equipment. The surface of the first surface layer is preheated to 130℃, and the stretching temperature of the core layer and the first surface layer is controlled at 120℃. The second surface layer and the sub-surface layer are preheated to 80℃, and the stretching temperature is controlled at 90℃, with a stretch ratio of 5.0 times. Then, it is introduced into the transverse stretching device, preheated to 170℃, stretched 9.0 times at 160℃, and then shaped at 165℃. After being cooled by air shower, it is then subjected to corona or flame treatment to obtain a film master roll. Finally, after aging treatment, slitting, and packaging, a co-extruded adhesive-free composite film is obtained.

[0044] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0045] Example 2 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 81.7wt% ethylene-vinyl acetate copolymer A (VA content 28wt%, melt index measured at 190℃ and 2.16kg is 150g / 10min), 10wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 8wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.3wt% modified macroporous silica (pore size of macroporous silica is 100nm, adsorption capacity of tackifying resin is 15wt%). Second surface layer: 78.2wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 21wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.8wt% modified macroporous silica (pore size of macroporous silica is 100nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0046] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0047] Example 3 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 25 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0048] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0049] Example 4 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 18 wt%, melt index measured at 190℃ and 2.16 kg is 50 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0050] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0051] Example 5 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 28 wt%, melt index measured at 190℃ and 2.16 kg is 150 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0052] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0053] Example 6 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 25 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 100 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 100nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0054] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0055] Example 7 This embodiment provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 25 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 500 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 500nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this embodiment is the same as that in Example 1, so it will not be described again.

[0056] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0057] Comparative Example 1 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% ordinary silica; Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg), 30wt% ethylene-acrylic acid copolymer (acrylic acid content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg, melting point is 90℃), 0.7wt% ordinary silica; The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0058] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0059] Comparative Example 2 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% ordinary silica; Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0060] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0061] Comparative Example 3 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg), 30wt% ethylene-acrylic acid copolymer (acrylic acid content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg, melting point is 90℃), 0.7wt% ordinary silica; The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0062] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0063] Comparative Example 4 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 25 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 99.3wt% ethylene-vinyl acetate copolymer B (VA content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%), no ethylene-acrylic acid copolymer added; The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0064] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0065] Comparative Example 5 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride-grafted polypropylene (grafting rate 1.2%), 0.4 wt% macroporous silica (pore size 250 nm); the macroporous silica is not pre-adsorbed with tackifying resin; Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% macroporous silica (pore size 250nm); wherein the macroporous silica is not pre-adsorbed with tackifying resin; The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0066] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0067] Comparative Example 6 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 75 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 1000 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg), 30wt% ethylene-acrylic acid copolymer (acrylic acid content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 1000nm, adsorption capacity of tackifying resin is 15wt%).

[0068] The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0069] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0070] Comparative Example 7 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content 10%, melt index measured at 190℃ and 2.16kg is 20g / 10min), 30wt% ethylene-acrylic acid copolymer (acrylic acid content 10wt%, melt index measured at 190℃ and 2.16kg is 20g / 10min, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%). The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0071] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0072] Comparative Example 8 This comparative example provides a co-extruded adhesive-free composite film, comprising a first surface layer, a core layer, a second surface layer, and a third surface layer arranged sequentially. The components of each layer are as follows: First surface layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Core layer: 100wt% isotactic polypropylene (isotacticity 95%, melt index 3.8g / 10min); Sub-layer: 74.6 wt% ethylene-vinyl acetate copolymer A (VA content 28 wt%, melt index measured at 190℃ and 2.16 kg is 180 g / 10 min), 15 wt% ethylene-acrylic acid copolymer (acrylic acid content 10 wt%, melt index measured at 190℃ and 2.16 kg is 20 g / 10 min, melting point is 90℃), 10 wt% maleic anhydride grafted polypropylene (grafting rate 1.2%), 0.4 wt% modified macroporous silica (pore size of macroporous silica is 250 nm, adsorption capacity of tackifying resin is 15 wt%). Second surface layer: 69.3wt% ethylene-vinyl acetate copolymer B (VA content is 10%, melt index is 20g / 10min measured at 190℃ and 2.16kg), 30wt% ethylene-acrylic acid copolymer (acrylic acid content is 10wt%, melt index is 20g / 10min measured at 190℃ and 2.16kg, melting point is 90℃), 0.7wt% modified macroporous silica (pore size of macroporous silica is 250nm, adsorption capacity of tackifying resin is 15wt%).

[0073] The preparation method of the co-extruded adhesive-free composite film in this comparative example is the same as that in Example 1, so it will not be described again.

[0074] The total thickness of the film is 15 μm, the thickness of the sub-layer is 1.5 μm, the thickness of the second layer is 2 μm, and the thickness of the first layer is 2 μm.

[0075] The performance test data of the co-extruded adhesive-free composite films prepared in Examples 1-7 and Comparative Examples 1-8 are shown in Table 2: Table 2 Performance test data of co-extruded adhesive-free composite films prepared in Examples 1-7 and Comparative Examples 1-8

[0076] Based on the above performance test data, we can conclude that: The co-extruded adhesive-free composite film of the present invention does not require the use of adhesives, is environmentally friendly and energy-saving. On the one hand, it has high bonding strength and good hot-pressing adhesion, which helps to improve the problem of incomplete or weak film bonding when paper-plastic lamination. On the other hand, it also has excellent anti-blocking properties, and the winding and unwinding process is smooth and does not stick to the rollers during production. It can effectively improve the smoothness of film slitting and subsequent application unwinding, and will not cause the film's optical properties to deteriorate.

[0077] The film prepared in Comparative Example 1 uses ordinary silica for both the sub-layer and the second layer. The melt in the sub-layer can only migrate to the surface of the second layer of the film through the interfacial gap between silica and the substrate. The amount of melt migration is seriously insufficient, and the fibers in the paper cannot be fully filled. This results in low peel strength, many defects, a blurry film surface, high haze, and poor optical performance.

[0078] In the film prepared in Comparative Example 2, the sub-layer was made of ordinary silica and the second layer was made of modified macroporous silica. An effective through channel could not be formed between the sub-layer and the second layer, which was not conducive to the transport of melt from the sub-layer to the second layer. This resulted in low film peel strength, more defects, and high haze.

[0079] In the film prepared in Comparative Example 3, the sub-layer was made of modified macroporous silica and the second layer was made of ordinary silica. Although the melt in the sub-layer could enter the channels of the modified macroporous silica, when it reached the interface between the sub-layer and the second layer, it encountered the ordinary silica in the second layer, and the migration of the melt was hindered, resulting in low film peel strength, more defects, and higher haze.

[0080] Comparative Examples 2 and 3 show that modified macroporous silica must be used simultaneously in both the sub-layer and the second layer to form a complete and effective through channel.

[0081] The second surface layer of the film prepared in Comparative Example 4 did not contain EAA. However, the carboxyl groups on the acrylic acid segments in EAA can form hydrogen bonds and chemical bonds with the hydroxyl groups of the fibers in the paper. Although the sub-surface layer used high melt index EVA and modified macroporous silica, and the sub-surface layer melt could migrate through the channels, the limited adhesion between the second surface layer and the paper still resulted in low peel strength, a blurry film surface, and many defects.

[0082] The film prepared in Comparative Example 5 uses macroporous silica without adsorption and tackifying resin, which reduces the migration amount and efficiency of the subsurface melt to a certain extent. Therefore, the peel strength is still lower than that of Examples 1-7, but some subsurface melt can still migrate to the film surface. The final result is that the peel strength reaches 4.0 N / 15 mm, which is significantly better than Comparative Examples 1-3, and the number of defects is reduced to 1.

[0083] The film prepared in Comparative Example 6 uses macroporous silica with a pore size of 1000 nm. The modified macroporous silica with ultra-large pores provides a channel with extremely low resistance for the subsurface melt. However, the mechanical strength of the ultra-large pore silica is low. The modified macroporous silica is easy to break during co-extrusion and biaxial stretching, resulting in film defects and production problems.

[0084] The film prepared in Comparative Example 7 uses ethylene-vinyl acetate copolymer A with a low melt index for the sub-layer. It has low peel strength and cannot effectively improve the problem of the film surface appearing blurry and incomplete after lamination, resulting in a large number of defects.

[0085] The film prepared in Comparative Example 8 uses ethylene-vinyl acetate copolymer A with a higher melt index for the sub-layer. Although the peel strength meets the requirements, it is difficult to form a suitable biaxially stretched thick sheet during the film preparation process, and delamination is prone to occur.

[0086] Compared to existing technologies, the co-extruded adhesive-free composite film of this invention, based on a first surface layer and a core layer, incorporates a high-flowability secondary surface layer containing high melt index EVA, EAA, maleic anhydride-grafted polypropylene, and modified macroporous silica, as well as a second surface layer containing modified macroporous silica. The modified macroporous silica in the second surface layer protrudes from the film surface during film formation and winding, providing anti-adhesion and ensuring smooth unwinding during subsequent processing. Simultaneously, during the hot-press lamination stage, the high-flowability melt of the secondary surface layer migrates to the film surface through the interconnected channels of the modified macroporous silica in both the secondary and second surface layers, spreading and penetrating into the fibers of the paper, resulting in high lamination strength and improving the problem of a blurry or non-solid film surface after lamination. Furthermore, the film prepared by the method of this invention exhibits low haze and excellent optical properties, making it suitable for high-transparency packaging applications.

[0087] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0088] The above-described embodiments 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 these modifications and improvements are all within the scope of protection of the present invention. Therefore, the present invention also intends to include these modifications and variations.

Claims

1. A co-extruded adhesive-free composite film, characterized in that: The material comprises, in sequence, a first surface layer, a core layer, a sub-surface layer, and a second surface layer. The first surface layer and the core layer are both polypropylene layers. The sub-surface layer comprises ethylene-vinyl acetate copolymer A, ethylene-acrylic acid copolymer, and maleic anhydride-grafted polypropylene. The content of ethylene-vinyl acetate copolymer A in the sub-surface layer is 74-89.5 wt%, the content of ethylene-acrylic acid copolymer is 5-15 wt%, and the content of maleic anhydride-grafted polypropylene is 5-10 wt%. The second surface layer comprises ethylene-vinyl acetate copolymer B and ethylene-acrylic acid copolymer. The content of ethylene-vinyl acetate copolymer B in the second surface layer is 69-89 wt%. 84.5 wt%, with the content of ethylene-acrylic acid copolymer being 15-30 wt%; both the sub-layer and the second layer contain modified macroporous silica, the particle size D50 of which is 4-6 μm and the pore size of which is 100-500 nm; the modified macroporous silica is prepared by adsorbing tackifying resin into the pores of the macroporous silica; wherein, the melt index of ethylene-vinyl acetate copolymer A is 50-150 g / 10 min and the melt index of ethylene-vinyl acetate copolymer B is 15-25 g / 10 min, measured at 190 °C and 2.16 kg.

2. The co-extruded adhesive-free composite film according to claim 1, characterized in that: The modified macroporous silica has a content of 0.3~0.5wt% in the sub-layer and 0.5~0.8wt% in the second layer.

3. The co-extruded adhesive-free composite film according to claim 1, characterized in that: In the sub-layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer A is 18~28wt%, and the maleic anhydride grafting rate of the maleic anhydride-grafted polypropylene is 0.8~1.2%; in the second layer, the vinyl acetate content in the ethylene-vinyl acetate copolymer B is 5~10wt%.

4. The co-extruded adhesive-free composite film according to claim 1, characterized in that: The amount of the tackifying resin adsorbed relative to the macroporous silica is 5-30 wt%; the tackifying resin is one or more of rosin and its derivatives, terpene resin, and petroleum resin.

5. The co-extruded adhesive-free composite film according to claim 1, characterized in that: In the second and third layers, the ethylene-acrylic acid copolymer has a melt index of 15-25 g / 10 min and a melting point of 80-105 °C at 190 °C and 2.16 kg, and the acrylic acid content in the ethylene-acrylic acid copolymer is 8-15 wt%.

6. The co-extruded adhesive-free composite film according to claim 1, characterized in that: The polypropylene in the first surface layer and the core layer is isotactic polypropylene, with an isotacticity of 95-97% and a melt index of 2.8-3.8 g / 10 min measured at 230°C and 2.16 kg.

7. The co-extruded adhesive-free composite film according to claim 1, characterized in that: The thickness of the sub-layer is 1~2μm, and the thickness of the second layer is 2~3μm.

8. A method for preparing a co-extruded adhesive-free composite film as described in any one of claims 1-7, characterized in that: Includes the following steps: The raw materials of each layer are fed into the batching unit, metered, and then fed into the extruder. After being melted, plasticized, homogenized, and metered, they enter the flow channel distributor and are then extruded through the die head. After being cast into a thick sheet by the chilled rollers, the sheet is then stretched longitudinally and then stretched laterally to form a film. The film is cooled by air shower, and its thickness is controlled by the trimming machine. After being treated with corona and / or flame, the film is collected into a film master roll. The film master roll is then subjected to aging treatment, slitting, and packaging to obtain the finished product.

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