A BOPP matte film and its preparation method

Abstract

This invention relates to the field of thin films, and more particularly to a BOPP matting film and its preparation method. The matting film comprises an upper surface layer, a lower surface layer, a core layer, and a lower surface layer arranged sequentially. The lower surface layer and the upper surface layer with a thickness of 0.4~0.6μm together form a matting layer with a thickness of 2μm. The upper surface layer comprises random copolymer polypropylene and high-density polyethylene, and the lower surface layer comprises random copolymer polypropylene, high-density polyethylene, and 10~15wt% PMMA masterbatch, wherein the content of monodisperse PMMA microspheres is 10wt% and the particle size is 3.5~4.5μm. Both the core layer and the lower surface layer comprise homopolymer polypropylene. This invention, through the rational design of the PMMA microsphere particle size, the upper surface layer, and the lower surface layer thickness, makes the PMMA microspheres protrude from the surface of the upper surface layer to increase the surface roughness, thereby reducing the amount of high-density polyethylene used in the upper surface layer, reducing the precipitation of high-density polyethylene from the source, improving production efficiency, and ensuring uniform matting.

Description

Technical Field

[0001] This invention relates to the field of thin films, and in particular to a BOPP matte film and its preparation method. Background Technology

[0002] BOPP matte film is increasingly used in the packaging industry due to its soft visual appeal, and the market demand for mid-to-high-end BOPP matte film is also growing. Furthermore, with the technological iteration and development of BOPP production equipment, high-width (>10m) and high-speed (>550m / min) production lines are gradually replacing existing low-efficiency equipment due to their high production efficiency. The increased width places higher demands on the uniformity of matte finish in BOPP matte film, while the faster production speed leads to an increase in the amount of matte material extruded and a higher longitudinal stretching temperature, thereby increasing the risk of longitudinal stretching separation and die head separation of the matte layer.

[0003] Currently, the main components of the matte finish used in BOPP matte film are mixtures of binary / ternary copolymer polypropylene and high-density polyethylene (HDPE). On one hand, the melting point of copolymer polypropylene is mostly around 135-145℃, exhibiting poor temperature resistance; HDPE has a melting point of approximately 128-132℃ and an initial melting point of approximately 120-122℃, also showing poor temperature resistance. This results in poor overall temperature resistance of the matte finish component. Furthermore, the poor thermodynamic compatibility between HDPE and copolymer polypropylene leads to increased HDPE precipitation on the longitudinal stretching rollers during high-speed production, specifically during longitudinal stretching of the matte surface. This accelerates the accumulation and shedding of precipitates, leading to… This leads to increased film defects, poorer matte film thickness, and a higher risk of film breakage, thus requiring more frequent shutdowns for cleaning and severely reducing production efficiency. On the other hand, when the matte material used in BOPP matte film is produced on high-width production lines, the matte layer needs to be made thicker than 2.0μm to meet the requirements of uniform matte finish. However, a greater thickness will lead to an increase in the amount of matte material extruded, which will exacerbate the precipitation of HDPE with poor thermodynamic compatibility at the die head, increase the occurrence of scratches on the matte surface, accelerate the accumulation of precipitates, and to some extent increase the risk of film breakage. Summary of the Invention

[0004] Based on this, the purpose of this invention is to provide a BOPP matting film and its preparation method. The method involves setting an upper surface layer and a lower surface layer, with PMMA masterbatch added to the lower surface layer. By rationally designing the particle size of the monodisperse PMMA microspheres in the PMMA masterbatch, the thickness of the upper surface layer, and the thickness of the lower surface layer, the PMMA microspheres in the lower surface layer protrude on the surface of the upper surface layer, increasing the surface roughness of the upper surface layer, reducing the amount of HDPE used in the upper surface layer, thereby reducing the precipitation of HDPE in the upper surface layer from the source, improving production efficiency, and ensuring uniform matting.

[0005] The technical solution of the present invention is achieved in the following ways:

[0006] A BOPP matte film includes an upper surface layer, a secondary surface layer, a core layer, and a lower surface layer arranged sequentially. The upper surface layer and the secondary surface layer together form a matte layer with a thickness of 2 μm. The upper surface layer has a thickness of 0.4~0.6 μm. The upper surface layer comprises random copolymer polypropylene and 30~35 wt% high-density polyethylene. The secondary surface layer comprises random copolymer polypropylene, 44~50 wt% high-density polyethylene, and 10~15 wt% PMMA masterbatch. The PMMA masterbatch contains 10 wt% monodisperse PMMA microspheres with a particle size of 3.5~4.5 μm. Both the core layer and the lower surface layer comprise homopolymer polypropylene.

[0007] The BOPP matte film of this invention not only improves the problem of poor uniformity of matte finish in wide-width (>10m) and high-speed (>550m / min) production lines, but also improves the problem of excessive extrusion of HDPE from the die head and longitudinal stretching during high-extrusion production, thus improving the matte effect and increasing the production efficiency of the matte film.

[0008] BOPP film sheets are first stretched longitudinally and then transversely, with a total stretch ratio of approximately 40-45 times. During transverse stretching, the sheet is stretched from the center outwards. The matting layer gradually roughens its surface morphology under high-speed biaxial stretching. Within a certain stretch ratio range, the higher the stretch ratio and the faster the stretching speed, the greater the surface roughening and the more pronounced the matting effect. However, high-width and high-speed BOPP matting film production lines, with their large transverse width and high speed, suffer from poor uniformity of sheet temperature distribution, resulting in a poor transverse matting effect. Therefore, the BOPP matting film of this invention employs a matting layer composed of an upper surface layer and a lower surface layer. By adding 10-15 wt% PMMA masterbatch to the lower surface layer, wherein the PMMA masterbatch contains 10 wt% monodisperse PMMA microspheres with a particle size of 3.5-4.5 μm, and by differentiating the thicknesses of the upper and lower surface layers based on the particle size of the monodisperse PMMA microspheres in the PMMA masterbatch, the harder PMMA microspheres in the lower surface layer can create a raised structure on the surface of the upper surface layer after biaxial stretching, increasing the surface roughness of the upper surface layer. This, combined with the matting effect provided by the upper HDPE layer and the lower HDPE layer, further enhances the matting effect. The matting effect provided by PE offers a multi-layered and composite matting appearance for the matting film. In addition, the matting effect provided by PMMA microspheres allows for a suitable reduction in the amount of HDPE used in both the upper and lower surface layers, reducing HDPE precipitation at the source. This reduction in HDPE usage also improves the temperature resistance of the upper surface layer, reducing burns and precipitation on the matting surface during high-speed longitudinal rolling, thus synergistically improving production efficiency. Furthermore, in addition to providing a certain matting effect, the upper surface layer can also encapsulate PMMA microspheres between the upper and lower surface layers, preventing incompatible PMMA microspheres from falling off during friction on the longitudinal rolling rollers. Fallen PMMA microspheres can cause localized bulging and film deformation during large film roll winding.

[0009] This invention uses PMMA microspheres. Firstly, the refractive index of PMMA microspheres is very close to that of PP, meaning the PMMA microspheres themselves contribute very little to scattering within the matting layer. This allows the matting effect to primarily rely on the scattering of light by the geometric morphology created by the protruding surfaces of the microspheres without affecting transparency. Secondly, PMMA microspheres are a highly cross-linked structure with a hardness far lower than silicon dioxide, so even if they protrude from the film surface, they will not scratch the film. Furthermore, limiting the particle size of the PMMA microspheres to monodisperse is beneficial for obtaining better matting uniformity. If the particle size distribution is too wide, too many large and small particles will coexist. Small particles cannot effectively achieve coarsening scattering, while excessively large particles may cause quality problems such as film breakage. Monodispersion ensures the uniformity of the surface roughness structure and the consistency of optical effects.

[0010] In this invention, PMMA microspheres are added in the form of masterbatch, which facilitates uniform dispersion on the upper surface layer. The monodisperse PMMA microspheres in the masterbatch are limited to a particle size of 3.5–4.5 μm, and the total thickness of the matting layer is 2 μm. This results in a peak height of approximately 1.5–2.5 μm from the upper surface layer to the surface itself. This peak height is greater than the visible light wavelength (approximately 0.38–0.78 μm), primarily causing strong Mie scattering. This size range efficiently scatters light across the entire visible light spectrum, achieving extremely low specular gloss. Simultaneously, because the particles are sufficiently large, they do not cause strong selective wavelength scattering, thus preventing severe bluish or whitening of the film and better maintaining transparency. If the particle size of PMMA microspheres is less than 3.5 μm, the particle size indicated by the protrusions may be less than 1.5 μm, which is not conducive to light scattering. If the particle size of PMMA microspheres is greater than 4.5 μm, the excessively large size may not be effectively covered by the random copolymer polypropylene phase (PP phase), and the PMMA microspheres may fall off during longitudinal stretching, leading to other quality problems.

[0011] Based on this, if the thickness of the upper surface layer is <0.4μm, although the roughness is higher, the PMMA microspheres are easily ground out from the upper surface layer; if the thickness of the upper surface layer is >0.6μm, the matting effect is insufficient.

[0012] Furthermore, the softening point temperature of the PMMA microspheres is 142~147℃, which is higher than the preheating and stretching temperature of the longitudinal stretching zone. This ensures that the PMMA microspheres maintain a relatively hard spherical structure. When the thick sheet comes into contact with the pressure roller under high tension, the PMMA microspheres will not easily soften, deform, or stick together. This allows for the stable creation of the required micro-uneven structure on the film surface, ensuring the consistency and efficiency of the matting effect. On this basis, if the softening point temperature of the PMMA microspheres is <142℃, the dimensional stability of the PMMA microspheres deteriorates, making it impossible to maintain a stable raised structure on the upper surface, thus reducing the matting effect. If the softening point temperature of the PMMA microspheres is >147℃, the raw material cost of the PMMA masterbatch increases while having little effect on further improving the matting effect, resulting in low cost-effectiveness.

[0013] Furthermore, in the SSA peak spectrum of the random copolymer polypropylene in the upper surface layer, the enthalpy value of the melting point peak at ≥140℃ accounts for 60~70%. This results in better temperature resistance of the random copolymer polypropylene and better encapsulation of HDPE in the upper surface layer and PMMA microspheres in the sub-surface layer during stretching. This helps to reduce the precipitation of HDPE during longitudinal stretching and ensure uniform extinction. If the enthalpy value of the melting point peak at ≥140℃ accounts for <60%, the temperature resistance of the random copolymer polypropylene is insufficient. If the enthalpy value of the melting point peak at ≥140℃ accounts for >70%, although better temperature resistance can be obtained, the polypropylene phase has too strong crystallinity, which is not conducive to obtaining a good extinction effect.

[0014] Furthermore, under test conditions of 230℃ and 2.16kg, the melt index of the random copolymer polypropylene in the upper surface layer was measured to be 6~8 g / 10min; under the same test conditions, the melt index of the random copolymer polypropylene in the lower surface layer was measured to be 8~10 g / 10min. By differentiating the random copolymer polypropylene in the upper surface layer and the lower surface layer, the mechanical strength of the upper surface layer is made higher than that of the lower surface layer, which is beneficial for firmly encapsulating PMMA microspheres between the two layers. Furthermore, if the melt index of the random copolymer polypropylene in the lower surface layer is <8 g / 10min, the melt viscosity of the PP phase and HDPE phase will be similar, reducing the phase separation effect and the fluidity of the matte finish. If the melt index of the random copolymer polypropylene in the lower surface layer is >10 g / 10min, the difference in melt viscosity between the two phases is too large, which is not conducive to the shear dispersion of the HDPE phase in the random copolymer polypropylene phase.

[0015] Furthermore, the density of the high-density polyethylene is 0.952-0.955 g / cm³. 3 The melt index of the high-density polyethylene (HDPE) was measured to be 8-15 g / 10 min under test conditions of 190℃ and 21.6 kg. If the melt index of HDPE is <8 g / 10 min, the fluidity of the entire matting layer is very poor, the extrusion pressure is very high, which easily leads to film breakage and an increase in impurities on the film surface. If the melt index of HDPE is >15 g / 10 min, the domain ability of HDPE deteriorates, and the island phases in HDPE as island phases are too small. These small island phases cannot produce sufficiently large undulations on the film surface, resulting in poor light scattering ability and poor matting effect on the film surface. The density of HDPE should be controlled at 0.952-0.955 g / cm³. 3 Between these two types of materials, HDPE exhibits good molecular chain regularity and crystallinity. During biaxial stretching, HDPE can more readily form hard island phases protruding from the film surface. However, if the density of high-density polyethylene is greater than 0.955 g / cm³... 3 If HDPE crystallizes too quickly or to a high degree, the "islands" of HDPE will be pulled inward strongly due to their huge crystallization shrinkage, reducing the compatibility between the HDPE phase and the PP phase. Crystal points are prone to appear on the film surface during high-speed stretching.

[0016] Furthermore, the upper surface layer also includes 2-3 wt% antioxidant masterbatch, and the lower surface layer also includes 2-3 wt% antioxidant masterbatch.

[0017] Furthermore, the melt index of the homopolymer polypropylene was measured to be 3~3.6 g / 10 min under test conditions of 230℃ and 2.16 kg.

[0018] Furthermore, the core layer also includes 1 wt% antistatic masterbatch.

[0019] The present invention also provides a method for preparing any of the above-mentioned BOPP matte films, comprising the following steps: mixing the raw materials of each layer according to the proportion and adding them into each extruder respectively; after being metered by a metering pump, the melt of each extruder is merged into a thick sheet at a T-die; the thick sheet is cooled by a chilling roller and then enters a chilling water tank and a blowing chamber; then it is stretched longitudinally, and then enters a transverse stretching area for stretching by multiples of 7.5-8.5 to obtain a film; the film is shaped, corona treated and then wound up; then it is aged and slit to obtain the BOPP matte film.

[0020] Furthermore, the extrusion temperature of the upper surface layer and the lower surface layer is 235~250℃, and the extrusion temperature of the core layer and the lower surface layer is 245~250℃; during longitudinal stretching, the preheating temperature of the upper surface layer is 130~135℃, the stretching temperature is 126~132℃, and the preheating temperature of the lower surface layer is 135~140℃, the stretching temperature is 130~135℃; during transverse stretching, the preheating temperature is 168~172℃, the stretching temperature is 160~165℃, and the setting temperature is 163~168℃.

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

[0022] 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.

[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. 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.

[0025] 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.

[0026] 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.

[0027] This invention provides a BOPP matte film, comprising an upper surface layer, a secondary surface layer, a core layer, and a lower surface layer arranged sequentially. The upper surface layer and the secondary surface layer together form a matte layer with a thickness of 2 μm. The upper surface layer has a thickness of 0.4~0.6 μm. The upper surface layer comprises random copolymer polypropylene and 30~35 wt% high-density polyethylene. The secondary surface layer comprises random copolymer polypropylene, 44~50 wt% high-density polyethylene, and 10~15 wt% PMMA masterbatch. The PMMA masterbatch contains 10 wt% monodisperse PMMA microspheres with a particle size of 3.5~4.5 μm. Both the core layer and the lower surface layer comprise homopolymer polypropylene.

[0028] Furthermore, the softening point temperature of the PMMA microspheres is 142~147℃.

[0029] Furthermore, in the SSA peak spectrum of the random copolymer polypropylene in the upper surface layer, the enthalpy value of the melting point peak at ≥140℃ accounts for 60~70%.

[0030] Furthermore, under test conditions of 230℃ and 2.16kg, the melt index of the random copolymer polypropylene in the upper surface layer was measured to be 6~8g / 10min; under test conditions of 230℃ and 2.16kg, the melt index of the random copolymer polypropylene in the lower surface layer was measured to be 8~10g / 10min.

[0031] Furthermore, the density of the high-density polyethylene is 0.952-0.955 g / cm³. 3 The melt index of the high-density polyethylene was measured to be 8~15g / 10min under test conditions of 190℃ and 21.6kg.

[0032] Furthermore, the upper surface layer also includes 2-3 wt% antioxidant masterbatch, and the lower surface layer also includes 2-3 wt% antioxidant masterbatch.

[0033] Furthermore, the melt index of the homopolymer polypropylene was measured to be 3~3.6 g / 10 min under test conditions of 230℃ and 2.16 kg.

[0034] Furthermore, the core layer also includes 1 wt% antistatic masterbatch.

[0035] The present invention also provides a method for preparing any of the above-mentioned BOPP matte films, comprising the following steps: mixing the raw materials of each layer according to the proportion and adding them into each extruder respectively; after being metered by a metering pump, the melt of each extruder is merged into a thick sheet at a T-die; the thick sheet is cooled by a chilling roller and then enters a chilling water tank and a blowing chamber; then it is stretched longitudinally, and then enters a transverse stretching area for stretching by multiples of 7.5-8.5 to obtain a film; the film is shaped, corona treated and then wound up; then it is aged and slit to obtain the BOPP matte film.

[0036] Furthermore, the extrusion temperature of the upper surface layer and the lower surface layer is 235~250℃, and the extrusion temperature of the core layer and the lower surface layer is 245~250℃; during longitudinal stretching, the preheating temperature of the upper surface layer is 130~135℃, the stretching temperature is 126~132℃, and the preheating temperature of the lower surface layer is 135~140℃, the stretching temperature is 130~135℃; during transverse stretching, the preheating temperature is 168~172℃, the stretching temperature is 160~165℃, and the setting temperature is 163~168℃.

[0037] The physical properties and testing methods of the embodiments or comparative examples of the present invention are as follows:

[0038] SSA testing method: A DSC instrument was used for testing, with a heating / cooling rate of 10 K / min and an N2 charging rate of 50 mL / min. The temperature was increased from 20℃ to 180℃, then held at 180℃ for 5 min to eliminate thermal history, followed by cooling from 180℃ to 20℃. Then, the temperature was repeatedly increased and annealed at 6℃ intervals from 20℃, with temperature settings of 166, 160, 154, 148, 142, 136, 130, 124, 118, 112, and 106℃. Finally, the temperature was increased from 20℃ to 180℃ at a rate of 5 K / min. By calculating the absorption peak area corresponding to different melting point ranges of the SSA curve, the heat absorbed by the melting of the segment components corresponding to different melting point ranges is obtained, and then normalized (divided by the sample mass) is performed to calculate the enthalpy ratio of the segment components with peaks above 140℃ in polypropylene, that is, the ratio of the heat absorbed by the melting of the segment components in a certain melting point range after normalization to the heat required for the copolymer to completely melt after normalization.

[0039] The calculation method for the longitudinal stretching cleaning cycle is the time when the defect detection instrument detects 10 defects larger than 1mm per kilometer in the longitudinal stretching area of ​​the matte surface due to high temperature precipitation of raw materials, sticking to the roller, or raw material particles falling off.

[0040] The continuous production cycle calculation method is the time required for cleaning when the number of times the thick sheet exits the mold lip, the mold lip exudate scratches the film surface, or the exudate falls off and causes the film to break reaches 6 times.

[0041] Haze determination: Tested according to GB / T2410-2008.

[0042] Gloss determination: According to GB / T8807-1988, test at a 45° angle to the matte surface.

[0043] It should be noted that the percentages mentioned in the embodiments or comparative examples of this invention are all weight percentages.

[0044] In the embodiments or comparative examples of this invention, the antioxidant masterbatch uses BASF 1010 antioxidant with an effective concentration of 10wt%; the PMMA masterbatch contains 10wt% monodisperse PMMA microspheres, and the matrix resin is the same polypropylene resin as the random copolymer polypropylene in the previous surface layer; the antistatic masterbatch contains 35wt% antistatic agent, and the matrix resin is homopolymer polypropylene.

[0045] The melt flow index of the high-density polyethylene (HDPE) in the embodiments or comparative examples of this invention is 11.6 g / 10 min, and the test conditions are 190°C and 21.6 kg; the density is 0.953 g / cm³. 3 .

[0046] In the embodiments or comparative examples of the present invention, the melt index of the random copolymer polypropylene on the upper surface layer is 6.8 g / 10 min, and the test conditions are 230°C and 2.16 kg; the melt index of the random copolymer polypropylene on the lower surface layer is 9 g / 10 min, and the test conditions are 230°C and 2.16 kg.

[0047] The homopolymer polypropylene in the embodiments or comparative examples of this invention has a melt index of 3.5 g / 10 min and is tested at 230°C and 2.16 kg.

[0048] The BOPP matte film in the embodiments or comparative examples of this invention has a width of 10.4 meters, a production speed of 550-600 m / min, and a film thickness that can be adjusted between 12-25 μm.

[0049] Example 1

[0050] This embodiment provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this embodiment includes the following steps:

[0051] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (61.5% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained.

[0052] Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 10wt% PMMA masterbatch (the effective content of monodisperse PMMA microspheres is 10wt%, the monodisperse particle size is 3.5μm, and the softening point temperature is 145℃) and 3wt% antioxidant masterbatch and mix them evenly. Feed the mixture into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating screen, and drying, the previous surface resin is obtained.

[0053] Core layer resin preparation: 99 wt% homopolymer polypropylene and 1 wt% antistatic masterbatch are mixed evenly to obtain core layer resin.

[0054] Preparation of the lower surface layer resin: 100 wt% homopolymer polypropylene was used as the lower surface layer resin.

[0055] The preparation method of the BOPP matte film in this embodiment includes the following steps: Resin for each layer is fed to four extruders via a batching system. The upper and lower surface layers are single-screw extruders, while the sub-surface layer and core layer are twin-screw extruders. The core layer extrusion temperature is 250°C, the upper and sub-surface layers are set to 240°C, and the lower surface layer is set to 245°C. The melt from the four extruders is pumped through a melt pump, passes through a narrow flow channel, and reaches the die head to co-extrude a thick sheet. The matte surface of the thick sheet is an air knife surface. The thick sheet passes through a chilling roller and enters a chilling water tank. After being blown with water, the thick sheet enters the longitudinal stretching area. The matte surface of the preheating area (…) The temperature of the top surface (the side containing the top layer) is set at 133℃, and the temperature of the bottom surface (the side containing the bottom layer) is set at 137℃. The temperature of the matte surface in the stretching zone is set at 131℃, and the temperature of the bottom surface is set at 135℃. After the thick sheet is shaped by longitudinal stretching, it enters the transverse stretching zone for 8 times stretching into a film with a film width of 10.4 meters and a production speed of 580m / min. The transverse stretching preheating temperature is set at 172℃, the stretching temperature is set at 165℃, and the shaping temperature is set at 166℃. After the film is shaped, it undergoes corona treatment and is finally wound into an aging rack for aging treatment. After slitting, BOPP matte film is obtained, which is then packaged for shipment or stored in the warehouse.

[0056] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.6µm, the thickness of the lower surface layer is 1.4µm, and the thickness of the lower surface layer is 1µm.

[0057] Example 2

[0058] This embodiment provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this embodiment includes the following steps:

[0059] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0060] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0061] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0062] The preparation method of the BOPP matte film in this embodiment is the same as that in Example 1, so it will not be described again.

[0063] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0064] Example 3

[0065] This embodiment provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this embodiment includes the following steps:

[0066] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (68.5% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0067] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0068] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0069] The preparation method of the BOPP matte 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 matte film is 15µm, of which the thickness of the upper surface layer is 0.4µm, the thickness of the lower surface layer is 1.6µm, and the thickness of the lower surface layer is 1µm.

[0071] Comparative Example 1

[0072] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0073] Preparation of the upper surface resin: 65 wt% random copolymer polypropylene (with 65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak profile), 32 wt% HDPE, and 3 wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420 rpm, and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating sieve, and drying, the upper surface resin was obtained. Preparation of the lower surface resin: 50 wt% random copolymer polypropylene, 47 wt% HDPE, and 3 wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420 rpm, and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating sieve, and drying, the lower surface resin was obtained.

[0074] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0075] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0076] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0077] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0078] Comparative Example 2

[0079] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0080] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: 35wt% random copolymer polypropylene, 44wt% HDPE, 18wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder was set to a speed of 420rpm and a barrel temperature of 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin was obtained.

[0081] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0082] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0083] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0084] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0085] Comparative Example 3

[0086] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0087] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 2.5μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder was set to a speed of 420rpm and a barrel temperature of 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin was obtained.

[0088] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0089] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0090] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0091] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0092] Comparative Example 4

[0093] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0094] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 6μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve, and drying, the previous surface resin is obtained.

[0095] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0096] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0097] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0098] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0099] Comparative Example 5

[0100] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0101] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (46% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0102] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0103] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0104] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0105] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0106] Comparative Example 6

[0107] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0108] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (75% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed inlet of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0109] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0110] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0111] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0112] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0113] Comparative Example 7

[0114] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0115] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0116] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0117] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0118] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0119] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 1µm, the thickness of the upper sub-surface layer is 1µm, and the thickness of the lower surface layer is 1µm.

[0120] Comparative Example 8

[0121] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0122] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 145℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0123] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0124] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0125] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.2µm, the thickness of the lower surface layer is 1.8µm, and the thickness of the lower surface layer is 1µm.

[0126] Comparative Example 9

[0127] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0128] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 131℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve and drying, the previous surface resin is obtained.

[0129] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0130] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0131] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0132] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0133] Comparative Example 10

[0134] This comparative example provides a BOPP matte film, comprising an upper surface layer, an upper sub-surface layer, a core layer, and a lower surface layer sequentially disposed therefrom, wherein the upper surface layer and the upper sub-surface layer together form the matte layer of the BOPP matte film. The preparation method of each resin layer of the BOPP matte film in this comparative example includes the following steps:

[0135] Preparation of the upper surface resin: 65wt% random copolymer polypropylene (65.3% of the enthalpy value of the melting point peak at ≥140℃ in the SSA peak spectrum), 32wt% HDPE and 3wt% antioxidant masterbatch were mixed evenly and fed into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed was 420rpm and the barrel temperature was set to 240℃. After extrusion, underwater cutting, vibrating screen and drying, the upper surface resin was obtained. Previous surface resin preparation: Take 40wt% random copolymer polypropylene, 44wt% HDPE, 13wt% PMMA masterbatch (monodisperse PMMA microspheres with an effective content of 10wt%, a monodisperse particle size of 4μm, and a softening point temperature of 151℃) and 3wt% antioxidant masterbatch, mix them evenly, and feed them into the feed port of a twin-screw extruder with an aspect ratio of 56. The twin-screw extruder speed is 420rpm, and the barrel temperature is set to 240℃. After extrusion, underwater cutting, vibrating sieve, and drying, the previous surface resin is obtained.

[0136] Core layer resin preparation: Same as in Example 1, so it will not be described in detail.

[0137] Preparation of the lower surface resin: Same as in Example 1, so it will not be described in detail.

[0138] The preparation method of the BOPP matte film in this comparative example is the same as that in Example 1, so it will not be described again.

[0139] The total thickness of the matte film is 15µm, of which the thickness of the upper surface layer is 0.5µm, the thickness of the lower surface layer is 1.5µm, and the thickness of the lower surface layer is 1µm.

[0140] The performance test results of the BOPP matte films of Examples 1-3 and Comparative Examples 1-10 are shown in Table 1 below.

[0141] Table 1

[0142]

[0143] The performance test data above shows that no PMMA microspheres were added to the upper surface layer of Comparative Example 1. Furthermore, to improve longitudinal stretching and die adhesion, the HDPE content (as a matting island phase) in the upper surface layer was relatively low. Therefore, the matting layer of Comparative Example 1 exhibited reduced light scattering, directly resulting in high gloss and low haze, with uneven matting occurring across the entire width. From Examples 1 to 3, when PMMA microspheres were added to the upper surface layer, the PMMA microspheres protruded to the upper surface, increasing the roughness of the matting surface and thus increasing the scattering of light. This reduced the gloss of the matting layer and increased its haze. To achieve a good matting effect, traditional BOPP matting films must increase the HDPE content. However, the bi-stretch nature of BOPP matting films and the extruder die structure result in a poorer matting effect on the sides of the film compared to the middle. Adding microspheres, however, allows for relatively uniform dispersion in the flow channel, and the melt-entrained microspheres exhibit better dispersion uniformity in the die channel than traditional high-HDPE content matting materials. Furthermore, as shown in Examples 1 to 3, increasing the size and content of PMMA microspheres significantly increases the haze of the matting film. However, Comparative Example 3 reveals that when the PMMA microsphere particle size is below 3.5 μm, the degree of microsphere protrusion from the film surface is relatively low, reducing the film surface roughness. Therefore, the matting effect is significantly lower than in Example 2, and noticeable uneven matting occurs at the edges. In Comparative Example 4, the PMMA microspheres had a particle size of 6 μm, resulting in the highest degree of film roughness and a gloss level as low as 6.3. However, the excessively high peak size caused the PMMA microspheres to be ground out of the matrix or detached from the melt, leading to increased PMMA precipitation at the die, PMMA detachment in the longitudinal stretching zone, increased extruder pressure, increased film breakage, and poor production smoothness. Similarly, in Comparative Example 2, the PMMA masterbatch addition reached 18%. While higher addition yielded better film roughness and matting effect, the PMMA microspheres had poor compatibility with HDPE and random copolymer polypropylene. Excessive addition not only increased costs but also increased PMMA precipitation in the longitudinal stretching and die, reducing production smoothness and efficiency. Therefore, the data in Table 2 summarizes that selecting PMMA microspheres in the 3.5-4.5 μm size range with a PMMA masterbatch addition of 10-15 wt% can significantly improve the matting effect while maintaining good production smoothness and efficiency.

[0144] This invention incorporates a random copolymer polypropylene with improved temperature resistance into the upper surface layer. Specifically, the enthalpy (ΔHm, J / g) of the melting point peak ≥140℃ in the SSA peaks of the random copolymer polypropylene accounts for 60-70%. The SSA peak spectrum shows that higher crystallization temperatures indicate stronger chain segment crystallization ability, better structural regularity, thicker lamellar crystals, and better temperature resistance. The random copolymer polypropylene also provides better encapsulation of the high-viscosity HDPE phase, reducing the degree of high-temperature scalding and HDPE phase precipitation in the longitudinal tensile region. Conversely, a lower proportion of the enthalpy of the melting point peak >140℃ in the SSA peaks indicates poorer chain segment regularity, weaker crystallization ability, thinner lamellar crystals, lower temperature resistance of the matte layer, and increased scratch defects on the matte surface during longitudinal tensile testing. Furthermore, based on the matting principle of BOPP matting film, the weaker the crystallinity of random copolymer polypropylene, the softer the random copolymer polypropylene (as the marine phase) becomes during stretching. This makes it easier to thin under high-speed biaxial stretching, resulting in a raised HDPE phase and a better matting effect. Therefore, from Examples 1 to 3 in Table 2, it can be seen that the higher the proportion of the ≥140℃ melting point peak enthalpy of the random copolymer polypropylene in the upper layer, the better its temperature resistance. During stretching, the PP phase exhibits higher modulus and strength, further reducing the precipitation and shedding of PMMA microspheres and HDPE from the matting layer. When the proportion of the ≥140℃ melting point peak enthalpy of the random copolymer polypropylene in Comparative Example 5 is as low as 46%, its high-temperature crystallization ability is poor. When the polypropylene phase is stretched and thinned, it is more conducive to the protrusion of the HDPE phase and PMMA microspheres. The matting effect of Comparative Example 5 is slightly better than that of Example 2. However, the random copolymer polypropylene has low temperature resistance and low melt strength, which causes the PP phase to be burned by high temperature during longitudinal stretching. The PMMA microspheres are detached from the thick sheet and ground out. The HDPE phase is also ground out from the polypropylene phase with low melt strength, which accelerates the cleaning cycle of the longitudinal region. In Comparative Example 6, the enthalpy of the melting point peak at ≥140℃ in the random copolymer polypropylene accounted for as high as 75%, increasing the temperature resistance of the matting layer, extending the longitudinal stretching cleaning cycle, and reducing die exudates. However, it can be seen that the crystallinity of the random copolymer polypropylene increased, and the crystallinity difference between the random copolymer polypropylene and high-density polyethylene narrowed, which is not conducive to film roughening during stretching and reduces the matting effect. At the same time, the high crystallinity and stiffness of the thick sheets increased, resulting in increased crystallization shrinkage and large lateral sway of the thick sheets, which significantly affected the thickness flatness. Therefore, controlling the enthalpy of the melting point peak at >140℃ in the random copolymer polypropylene SSA of the upper surface layer to between 60-70% is better for balancing haze and production efficiency.

[0145] Furthermore, the matting layer of the matting film in this invention consists of an upper surface layer and a lower surface layer. The thinner the upper surface layer, the higher the protrusion of the PMMA microspheres, and theoretically the matting effect will increase. However, it also increases the risk of PMMA microspheres detaching during longitudinal stretching. As can be seen from Examples 2, 7, and 8, the upper surface layer of Comparative Example 7 is too thick, resulting in a lower degree of PMMA microsphere protrusion from the film surface, a reduced degree of film surface roughening, and a significantly worse matting effect than Example 2. The upper surface layer of Comparative Example 8 is too thin, resulting in a slight increase in haze. This is because the high protrusion of the PMMA microspheres leads to high roughness. However, the longitudinal stretching cleaning cycle of Comparative Example 8 is short because the thin upper surface layer causes the PMMA microspheres with poor surface compatibility to peel off from the polypropylene matrix resin in the PMMA masterbatch, making it easier to grind them off from the thin upper surface layer.

[0146] In Comparative Example 9, the softening temperature of the PMMA microspheres was too low. The microspheres were flattened or stretched by the rollers, damaging their spherical structure. This resulted in a decrease in the peak height and roughness of the film surface, ultimately leading to a cloudy, uneven matte finish. In Comparative Example 10, the softening temperature of the PMMA microspheres reached 151℃. The microspheres exhibited better resistance to deformation. When the soft polypropylene matrix resin in the PMMA masterbatch was stretched, the hard PMMA microspheres acted like "stones," hindering local deformation and creating significant stress concentration at the interface. Simultaneously, the microspheres protruded from the film surface and came into direct contact with the high-speed rotating hot rollers. The hard microspheres might be "ground out" or accumulate on the roller surface, scratching the film. Therefore, the longitudinal stretching cleaning cycle was significantly shortened. Furthermore, the actual longitudinal stretching temperature of BOPP matte film is not set at 150℃ or higher. The high softening temperature of the PMMA microspheres in Comparative Example 10 did not significantly increase the haze level; instead, it increased formulation costs and reduced production efficiency. Therefore, controlling the softening point temperature of PMMA microspheres at 142-147℃ is a solution that can simultaneously achieve both matting and cost-effectiveness.

[0147] 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 BOPP matt film characterized in that, The BOPP matt film comprises an upper surface layer, an upper subsurface layer, a core layer and a lower surface layer arranged in sequence, the upper surface layer and the upper subsurface layer jointly form a matt layer, the thickness of the matt layer is 2 μm, the thickness of the upper surface layer is 0.4-0.6 μm, the upper surface layer comprises random copolymerized polypropylene and 30-35 wt% high-density polyethylene, the proportion of the peak enthalpy value of the random copolymerized polypropylene in the SSA peak spectrum of the upper surface layer with a melting point of ≥140 ℃ is 60-70%; the upper subsurface layer comprises random copolymerized polypropylene, 44-50 wt% high-density polyethylene and 10-15 wt% PMMA master batch, the PMMA master batch contains monodisperse PMMA microspheres with an effective content of 10 wt%, the particle size of the monodisperse PMMA microspheres is 3.5-4.5 μm, the softening point temperature of the PMMA microspheres is 142-147 ℃, and the core layer and the lower surface layer both comprise homopolymerized polypropylene.

2. The BOPP matt film according to claim 1, characterized in that, The random copolymerized polypropylene in the upper surface layer has a melt index of 6-8 g / 10 min under the test conditions of 230 ℃ and 2.16 kg; and the random copolymerized polypropylene in the upper subsurface layer has a melt index of 8-10 g / 10 min under the test conditions of 230 ℃ and 2.16 kg.

3. The BOPP matt film according to claim 1, characterized in that, The high density polyethylene has a density of 0.952 to 0.955 g / cm 3 The high density polyethylene has a melt index of 8 to 15 g / 10 min measured at 190 °C under a test condition of 21.6 kg.

4. The BOPP matt film according to claim 1, characterized in that, The upper surface layer further comprises 2-3 wt% antioxidant master batch, and the upper subsurface layer further comprises 2-3 wt% antioxidant master batch.

5. The BOPP matt film according to claim 1, characterized in that, The homopolymerized polypropylene has a melt index of 3-3.6 g / 10 min under the test conditions of 230 ℃ and 2.16 kg.

6. The BOPP matt film according to claim 1, characterized in that, The core layer further comprises 1 wt% antistatic master batch.

7. A process for the production of BOPP matt film as claimed in any one of claims 1 to 6, characterized in that, The method comprises the following steps: After the raw materials of the layers are uniformly mixed according to the proportions, the raw materials are respectively added into the extruders, the melts of the extruders are combined into a thick sheet through a T-shaped die, the thick sheet is cooled by a chilling roller and then enters a chilling water tank and a blowing chamber, followed by longitudinal stretching, and then enters a transverse stretching area for 7.5-8.5 times stretching to obtain a film, the film is rolled after being shaped and subjected to corona treatment, and then is subjected to aging treatment and slitting to obtain the BOPP matt film.

8. The method of claim 7, wherein the BOPP matt film is prepared by the steps of: The extrusion temperature of the upper surface layer and the upper subsurface layer is 235-250 ℃, and the extrusion temperature of the core layer and the lower surface layer is 245-250 ℃; when longitudinal stretching is performed, the preheating temperature of the upper surface layer is 130-135 ℃, and the stretching temperature is 126-132 ℃, and the preheating temperature of the lower surface layer is 135-140 ℃, and the stretching temperature is 130-135 ℃; when transverse stretching is performed, the preheating temperature is 168-172 ℃, the stretching temperature is 160-165 ℃, and the shaping temperature is 163-168 ℃.

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