A matte heat-sealable film and a method for preparing the same

Abstract

This invention discloses a matte heat-sealable film and its preparation method, relating to the field of heat-sealable film preparation. The matte heat-sealable film includes a heat-sealable layer, which is made of a composition comprising the following components in parts by weight: 60 parts of random copolymer polypropylene, 15 parts of low-density polyethylene, 8 parts of heat-shrinkable microsphere precursor, 5 parts of tackifying resin, 0.3 parts of slip agent, and 0.4 parts of antioxidant. The heat-shrinkable microsphere precursor is a core-shell structured microsphere, with the core layer copolymerized from isoborneol methacrylate and trimethylolpropane triacrylate, and the shell layer being a long-chain alkylsiloxane graft layer. Its average particle size is 8 μm, and its volume shrinkage rate at 130°C is 8%. This invention, by introducing the heat-shrinkable microsphere precursor to form an in-situ micro-wrinkled structure within the heat-sealable layer, can impart a matte appearance to the film without using inorganic matting agents.

Description

Technical Field

[0001] This invention belongs to the field of heat-sealing film preparation, and in particular relates to a matte heat-sealing film and its preparation method. Background Technology

[0002] Matte heat-sealable film refers to a type of functional polymer film with a surface gloss lower than ordinary plastic film, exhibiting a soft, diffuse reflective texture. Its single or double surfaces can bond and seal with paper substrates, plastic sheets, or similar films under heat and pressure, making it widely used for sealing packaging of various items. Inorganic matting agents are powder additives added to the film formulation to achieve the aforementioned matte appearance. Common types include synthetic silica, diatomaceous earth, calcium carbonate, and talc. Their mechanism of action involves particles protruding from the film surface to form a microscopic rough structure, causing diffuse reflection of incident light and thus reducing the specular reflection intensity of the film surface.

[0003] Currently, the application of matte heat-sealing films has expanded to multiple fields with specific requirements for cleanliness and surface condition. Besides food and daily chemical products, the demand for matte heat-sealing films is increasing in the packaging of precision medical devices, sterile medical dressings, high-precision electronic components, and optical lens protective films. These fields not only require packaging films with a suitable matte appearance to enhance product texture or facilitate labeling, but also place high demands on the cleanliness of the packaging materials themselves. In medical device packaging, any tiny foreign matter detached from the surface of the packaging material can adversely affect the initial contamination control or safety of the product. In the field of electronic components, especially in the transport packaging of semiconductor chips, LCD panel components, and optical lenses, dust particles detached from the film surface, if adsorbed onto the device surface, may cause short circuits, poor contact, or scratches on the optical surface, thereby affecting the performance and yield of the final product.

[0004] The preparation of existing matte heat-sealable films mainly relies on the direct addition of inorganic matting powder to the heat-sealable layer formulation. However, due to the high surface energy of inorganic particles and their limited interfacial compatibility with the organic polymer matrix, the bonding between the two is primarily physical interlocking. During the film winding, unwinding, slitting, bag-making friction, and heat-sealing processes, the inorganic particles that are not firmly interlocked on the film surface are easily peeled off from the polymer matrix, forming free dust. This dust poses a risk of diffusion and migration in cleanrooms, dust-free workbenches, or device assembly environments, and its potential pollution hazards have become one of the main obstacles restricting the widespread application of matte heat-sealable films in cleanliness-sensitive fields. Furthermore, increasing the amount of inorganic powder added to achieve low gloss further occupies the effective adhesive area of ​​the heat-seal interface, hindering the thermal movement and interdiffusion of polymer molecular chains, making it difficult to maintain stable heat-seal strength, and weakening the packaging's sealing and protective function for the contents. Therefore, the following solutions are proposed to address these issues. Summary of the Invention

[0005] The purpose of this invention is to provide a matte heat-sealing film and its preparation method. By introducing a heat-shrinkable microsphere precursor to form an in-situ micro-wrinkled structure within the heat-sealing layer, a matte appearance can be imparted to the film without the use of inorganic matting powder. This solves the problems of particle shedding and migration and impaired heat-sealing strength in existing matte heat-sealing films due to the addition of inorganic particles.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0007] This invention relates to a matte heat-sealing film, comprising a heat-sealing layer made of a composition comprising the following parts by weight: 60 parts of random copolymer polypropylene, 15 parts of low-density polyethylene, 8 parts of heat-shrinkable microsphere precursor, 5 parts of tackifying resin, 0.3 parts of slip agent, and 0.4 parts of antioxidant; the heat-shrinkable microsphere precursor is a core-shell structured microsphere, the core layer being copolymerized from isoborneol methacrylate and trimethylolpropane triacrylate, and the shell layer being a long-chain alkylsiloxane grafted layer, with an average particle size of 8 μm and a volume shrinkage rate of 8% at 130°C.

[0008] Furthermore, the tackifying resin is polyethylene glycol diglycidyl ether modified hydrogenated rosin pentaerythritol ester.

[0009] Furthermore, the surface of the heat-sealing layer has an in-situ self-generated uneven wrinkle structure formed by the difference in thermal shrinkage between the heat-shrinkable microsphere precursor and the matrix resin, and the film has a gloss of no more than 5.0 GU at a 60° angle and a surface roughness Ra value of 0.20μm~0.30μm.

[0010] Furthermore, the preparation method of the heat-shrinkable microsphere precursor includes the following steps:

[0011] 50 parts of isobornyl methacrylate, 30 parts of trimethylolpropane triacrylate, 10 parts of methacryloxypropyltrimethoxysilane, and 1.5 parts of azobisisobutyronitrile were mixed to form an oil phase; the oil phase was added to an aqueous phase containing polyvinyl alcohol dispersant, and the mixture was subjected to suspension polymerization at 75℃~85℃ for 6 hours to form core layer microspheres.

[0012] After the reaction is complete, the temperature is lowered to 70°C, 10 parts of long-chain alkylsiloxane are added to the system, the pH value is adjusted to 5.0, and the system is kept at this temperature for 2 hours for hydrolysis and condensation to form an organosilicon grafted shell on the surface of the microspheres.

[0013] The heat-shrinkable microsphere precursor was obtained by washing, drying, and sieving.

[0014] Furthermore, the matte heat-sealing film has a three-layer co-extruded structure, including a heat-sealing layer, a core layer, and a secondary outer layer, wherein the core layer is a homopolymer polypropylene layer.

[0015] A method for preparing a matte heat-sealable film includes the following steps:

[0016] Weigh out random copolymer polypropylene, low-density polyethylene, heat shrinkable microsphere precursor, tackifying resin, slip agent and antioxidant according to the formula, mix them evenly and put them into the extruder, and melt extrude at a temperature of 160℃~230℃.

[0017] The melt is cast onto a quenching roller at 35°C to cool and form a cast sheet. The ratio of the linear speed of the quenching roller to the melt flow rate is controlled to be 1:1.2, so that residual internal stress is generated in the preform during the rapid cooling process. Due to the difference in thermal shrinkage rate, the heat shrinkable microsphere precursor and the matrix resin induce the formation of a nanoscale concave-convex wrinkle structure on the film surface.

[0018] The casting was stretched longitudinally by 4.5 times at 110℃~120℃ and transversely by 8.0 times at 155℃~165℃, and then heat-set at 168℃ for 8 seconds to fix the wrinkle morphology.

[0019] The film is subjected to corona treatment to control the surface tension to 42 mN / m, and then wound up to obtain the matte heat-sealing film.

[0020] Furthermore, the temperature of the chilled roller is 35°C, the temperature of the melt flowing out of the die head is 240°C, and the initial thickness of the preform is 250μm.

[0021] Furthermore, the winding is performed in a clean environment, and surface static charge is eliminated by an ionizer bar, so that the residual voltage of the film is below 50V.

[0022] Furthermore, the heat-sealing temperature of the heat-sealing film is 120℃~135℃, and the heat-sealing strength with the medical dialysis paper after heat-sealing for 1 second under a pressure of 0.3MPa is not less than 8.5N / 15mm.

[0023] The present invention has the following beneficial effects:

[0024] The matte heat-sealable film prepared by this invention does not rely on inorganic matting particles for its heat-sealable layer. Instead, it induces a micro-wrinkled structure on the film surface by utilizing the difference in thermal shrinkage rates between polymer microspheres and the matrix resin, thereby producing an optical matte appearance. This structure can reduce the potential impact of particulate matter precipitation and migration, avoid the problem of hindered molecular chain diffusion at the heat-sealable interface due to the addition of fillers, and help maintain the heat-sealable bonding force. The film surface gloss is at a low level, which can reduce directional reflection spots when exposed to light. The micro-wrinkled morphology can be preserved through heat setting during the preparation process, and the subsequent heat-sealable operation will not change its matte appearance. The tackifying components in the formulation have an adhesion tendency to fibrous surfaces, while having a weak adhesion tendency to metal surfaces.

[0025] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic flowchart of a method for preparing a matte heat-sealing film according to the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Please see Figure 1 As shown, the present invention relates to a matte heat-sealing film and its preparation method, comprising the following steps:

[0030] Step S1: Preparation of specific heat-shrinkable microsphere precursors

[0031] Raw material preparation: By weight, prepare 50 parts of isobornyl methacrylate (IBOMA), 30 parts of trimethylolpropane triacrylate (TMPTA), 10 parts of methacryloyloxypropyltrimethoxysilane (KH-570), 10 parts of long-chain alkylsiloxane (C16-18 carbon atoms), 1.5 parts of azobisisobutyronitrile (AIBN), 400 parts of deionized water, and 5 parts of polyvinyl alcohol (PVA, degree of alcoholysis 88%) dispersant;

[0032] Suspension polymerization reaction: IBOMA, TMPTA, KH-570 and AIBN are mixed and stirred until completely dissolved to form an oil phase mixture;

[0033] In a 2L four-necked flask equipped with a reflux condenser, a nitrogen inlet tube and a mechanical stirrer, deionized water and PVA were added, and the mixture was heated to 65°C and stirred to dissolve, forming an aqueous phase.

[0034] The oil phase mixture was slowly added dropwise to the aqueous phase and dispersed at 400 rpm for 30 minutes to form a stable suspension droplet;

[0035] Nitrogen gas was introduced to remove oxygen from the system, and the system temperature was raised to 75°C at a rate of 1°C / min and kept at a constant temperature for 4 hours; then the temperature was raised to 85°C and the reaction was continued for 2 hours to solidify the droplets.

[0036] Surface grafting treatment: After the reaction is completed, the temperature is lowered to 70℃, and 10 parts of long-chain alkylsiloxane are slowly added dropwise to the reaction system. The pH value is adjusted to 5.0, the rotation speed is maintained at 200 rpm, and the hydrolysis and condensation are carried out for 2 hours. This process forms a flexible long-chain alkyl organosilicon coating layer on the surface of the microspheres.

[0037] Post-processing: The reaction product was centrifuged and washed three times with deionized water, then dried in a vacuum oven at 40°C for 24 hours and passed through a 200-mesh sieve to obtain a core-shell structured microsphere precursor with an average particle size of 8 μm; the volume shrinkage rate of the microsphere at 130°C was about 8%.

[0038] Step S2: Preparation of the matte heat-sealing film composition

[0039] This step describes the formulation of the heat-sealing layer composition, which constitutes the three-layer structure of the heat-sealing film, by weight:

[0040] Weighing of matrix resin: Weigh 60 parts of random copolymer polypropylene (PPR) with a melt index of 8 g / 10 min (230℃, 2.16 kg) and 15 parts of low-density polyethylene (LDPE) with a melt index of 2 g / 10 min; PPR provides basic transparency and heat resistance, while LDPE is used to adjust the low-temperature heat-sealing window.

[0041] Add 8 parts of the heat-shrinkable microsphere precursor obtained in step S1;

[0042] Add 5 parts of polyethylene glycol diglycidyl ether modified hydrogenated rosin pentaerythritol ester; this substance is an unconventional tackifying resin modifier. The hydroxyl groups formed after the epoxy groups open the ring can be polarly adsorbed to the surface of metal instruments, but shrink and detach after cooling, giving the film the property of strong adhesion to paper and non-adhesion to metal, preventing the instrument handle from sticking and being damaged during packaging.

[0043] Auxiliary functional additives: 0.3 parts erucamide slip agent, 0.2 parts antioxidant 1010, and 0.2 parts antioxidant 168 were added;

[0044] Mixing process: All the above components are put into a high-speed mixer and mixed at 800 rpm for 10 minutes at room temperature; after mixing, the material is conveyed through a closed pipeline to the loss-in-weight feed hopper of the twin-screw extruder; this step does not use any inorganic matting agents such as silica, calcium carbonate or barium sulfate.

[0045] Step S3: Multi-layer co-extrusion casting

[0046] Extruder temperature zone setting: A three-layer co-extrusion casting equipment (A / B / A structure, B layer is the core layer, and A layer is the heat-sealing layer) is used; the composition obtained in step S2 is fed into the heat-sealing layer extruder (A layer); the core layer (B layer) uses homopolymer polypropylene (PPH, melt index 3g / 10min);

[0047] A-layer extruder temperatures: feeding section 160℃, compression section 210℃, metering section 230℃, connecting body 225℃;

[0048] B-layer extruder temperatures: feeding section 180℃, compression section 230℃, metering section 245℃;

[0049] Melt extrusion and filtration: The melt is filtered through a disc melt filter with a filtration accuracy of 20μm to remove undispersed gel particles and ensure medical-grade cleanliness;

[0050] Casting cooling: The melt flows out from the die head (die lip temperature 240℃) and adheres to the low temperature quench roller at 35℃ for cooling casting;

[0051] Key control parameters: The ratio of the linear speed of the quenching roller to the melt outflow speed is controlled to be 1:1.2, that is, the melt is subjected to micro-stretching, so that the film preform generates residual internal stress during the quenching process;

[0052] Microscopic mechanism explanation: Since the microsphere precursor obtained in step S1 has a larger difference in thermal expansion / contraction coefficient than the matrix polypropylene, when it comes into contact with a 35°C cold roller, the microsphere core shrinks rapidly, while the long-chain alkylsiloxane shell on its surface has good compatibility with the PPR matrix and generates tension, resulting in the formation of an in-situ self-generated uneven wrinkle structure on the film surface at the nanoscale. This wrinkle structure diffuses light and produces a matte effect without the need to add solid particles.

[0053] Step S4: Biaxial stretching and heat treatment for shaping

[0054] Longitudinal stretching (MDO): A 250μm thick sheet obtained from casting is introduced into the longitudinal stretching machine; the preheating zone temperature is set to 120℃ and the stretching zone temperature is 110℃; within a stretching distance of 1.5 meters, the sheet is longitudinally stretched 4.5 times using the difference between fast and slow roller speeds.

[0055] Transverse stretching (TDO): The edges of the longitudinally stretched membrane are clamped and placed into a transverse stretching oven; the preheating section temperature is 165℃, and the stretching section temperature is 155℃; under the action of track expansion, the transverse stretching is 8.0 times.

[0056] Balance between heat setting and stress release: The stretched film enters the heat setting section, where the temperature is set at 168°C and the dwell time is 8 seconds;

[0057] At this temperature, the PPR matrix undergoes partial chain segment relaxation to eliminate excessive internal stress and prevent post-shrinkage. However, the microsphere precursor prepared in step S1 undergoes slight melting (softening of the surface shell) at this temperature, filling some of the micropores generated by stretching and locking the wrinkled morphology generated in step S3, so that it will not be flattened or disappear during subsequent heat sealing. This process ensures the longevity of the matte effect.

[0058] Step S5, Surface corona treatment and cleanroom winding

[0059] Corona treatment: After the film exits through the TDO outlet, the temperature drops to 40℃ and it is then subjected to a high-frequency high-voltage corona treatment machine with a power set at 45W·min / m². The purpose of the treatment is to increase the surface tension of the heat-sealing layer to 42mN / m, which facilitates the printing of barcodes. At the same time, the corona bombardment is used to slightly etch the polymer chains on the top of the wrinkles, further reducing the surface specular reflectivity.

[0060] Online thickness inspection and cleanroom control: When the film enters the clean winding room, the film thickness is monitored in real time using a non-contact beta-ray thickness gauge. The target thickness is 30μm, and the control tolerance is ±1.5μm.

[0061] Static electricity elimination and winding: The surface static charge is eliminated by an ion bar to avoid adsorbing dust particles in the air. Finally, the film is wound onto a 6-inch paper core with a constant tension of 120 N / m to obtain the finished roll film.

[0062] The specific application of this embodiment is as follows:

[0063] Example 1

[0064] This embodiment prepares a matte heat-sealing film according to the above preparation method;

[0065] 1. Preparation of heat-shrinkable microsphere precursors:

[0066] Oil phase preparation: Accurately weigh 50 parts by weight of isobornyl methacrylate, 30 parts by weight of trimethylolpropane triacrylate, 10 parts by weight of methacryloxypropyltrimethoxysilane and 1.5 parts by weight of azobisisobutyronitrile, and stir at room temperature until a uniform and transparent oil phase is formed.

[0067] Suspension polymerization: In a 2L reactor, add 400 parts by weight of deionized water and 5 parts by weight of polyvinyl alcohol (88% degree of alcoholysis), heat to 65°C and stir to dissolve; slowly drop the oil phase into the aqueous phase and disperse at 400 rpm for 30 minutes; after purging with nitrogen to remove oxygen, heat to 75°C at a rate of 1°C / min and react for 4 hours, then heat to 85°C and react for 2 hours.

[0068] Surface grafting: The system was cooled to 70°C, and 10 parts by weight of long-chain alkylsiloxane (carbon chain length C16-C18) were added. The pH was adjusted to 5.0 with acetic acid, and the reaction was carried out at 200 rpm for 2 hours.

[0069] Post-processing: The product was centrifuged and washed, vacuum dried at 40℃ for 24 hours, and passed through a 200-mesh sieve to obtain a core-shell structured microsphere precursor with an average particle size of 8μm and a volume shrinkage rate of about 8.2% at 130℃.

[0070] 2. Preparation of matte heat-sealing film:

[0071] Ingredients: By weight, take 60 parts of random copolymer polypropylene (melt index 8 g / 10 min), 15 parts of low-density polyethylene (melt index 2 g / 10 min), 8 parts of the heat-shrinkable microsphere precursor prepared above, 5 parts of polyethylene glycol diglycidyl ether modified hydrogenated rosin pentaerythritol ester, 0.3 parts of erucamide, 0.2 parts of antioxidant 1010, and 0.2 parts of antioxidant 168; mix in a high-speed mixer at 800 rpm for 10 minutes.

[0072] Melt extrusion: The mixture is fed into the A-layer extruder, with the feeding section at 160°C, the compression section at 210°C, and the metering section at 230°C; the core layer B uses homopolymer polypropylene (melt index 3g / 10min), with corresponding temperature zones of 180°C / 230°C / 245°C.

[0073] Casting: The melt is extruded through a 240℃ die and attached to the surface of a 35℃ chilling roller. The ratio of the melt flow rate to the chilling roller linear speed is controlled to be 1:1.2 to form a casting with an initial thickness of 250μm.

[0074] Stretching and shaping: After preheating at 120℃, the cast sheet is stretched longitudinally by 4.5 times at 110℃; then it enters a transverse stretching oven, preheated at 165℃, stretched by 8.0 times at 155℃; finally, it is heat-set at 168℃ for 8 seconds.

[0075] Post-processing: The film is corona treated to a surface tension of 42 mN / m, and then destaticated by ion air in a Class 1000 clean room to obtain a finished roll film with a thickness of about 30 μm.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that no heat-shrinkable microsphere precursor is added to the heat-sealing composition; instead, an equal amount of conventional random copolymer polypropylene resin is added.

[0078] Specific procedures: The heat-sealing layer formulation consists of 68 parts random copolymer polypropylene, 15 parts low-density polyethylene, 5 parts modified hydrogenated rosin ester, 0.3 parts slip agent, and 0.4 parts antioxidant; the remaining preparation steps and process parameters are exactly the same as in Example 1.

[0079] Comparative Example 2

[0080] The difference between this comparative example and Example 1 is that the heat-shrinkable microsphere precursor is replaced with conventional inorganic matte powder, specifically commercially available silica micropowder with an average particle size of about 5 μm.

[0081] Specific operation: The heat-sealing layer formula is 60 parts random copolymer polypropylene, 15 parts low-density polyethylene, 8 parts silica micro powder, 5 parts modified hydrogenated rosin ester, 0.3 parts slip agent, and 0.4 parts antioxidant; the material mixing and extrusion casting process parameters are consistent with those in Example 1.

[0082] Comparative Example 3

[0083] The difference between this comparative example and Example 1 is that the temperature of the chiller roller in the casting cooling process is changed and set to the cooling temperature of conventional cast film, 55°C.

[0084] Specific operation: The heat-sealing layer formula and the heat-shrinkable microsphere precursor used are completely consistent with those in Example 1; however, in the casting step, the temperature of the chilling roller is adjusted from 35°C to 55°C, and the ratio of melt flow rate to chilling roller linear speed is kept constant at 1:1.2; the parameters of subsequent stretching, heat setting and post-treatment steps are the same as those in Example 1.

[0085] Comparative Example 4

[0086] The difference between this comparative example and Example 1 is that the microsphere precursor used in the heat-sealing layer was not subjected to long-chain alkylsiloxane surface grafting treatment, that is, it was only a copolymer microsphere of isobornyl methacrylate and trimethylolpropane triacrylate.

[0087] Specific procedures: The surface grafting treatment step was omitted during the microsphere preparation process to obtain pure acrylate copolymer microspheres; the heat-sealing film preparation process was the same as in Example 1.

[0088] Performance testing methods and standards

[0089] The film rolls prepared in Example 1 and Comparative Examples 1-4 were cut into samples and subjected to the following tests:

[0090] Gloss test: The gloss of the film surface was measured using a 60° angle gloss meter in accordance with GB / T 8807 standard;

[0091] Surface roughness test: The arithmetic mean deviation Ra value is measured using an optical profilometer according to ISO 4287 standard;

[0092] Heat seal strength test: According to QB / T 2358 standard, the film heat seal and medical dialysis paper are heat sealed at 135℃ and 0.3MPa pressure for 1 second, and a 15mm wide strip is cut and a T-type peel test is performed on a tensile testing machine;

[0093] Visual evaluation of reflection under shadowless lamp: The packaged sample was placed under a shadowless lamp (100,000 Lux) simulating an operating room environment, 50cm away from the lamp head. Three testers visually evaluated whether there were visible light spots or glaring reflections in the window area. The results were recorded as "slight / obvious / relatively obvious reflection".

[0094] Particulate matter shedding test (dust migration): Refer to the relevant cleanliness requirements for medical packaging, place the film heat-sealed face down, drag it 100mm on black glossy photographic paper under a 1kg weight pressure, and visually inspect the photographic paper for any white scratches or particle transfer marks.

[0095] Results Analysis and Comparison

[0096] Table 1. Performance test results of thin films in each embodiment and comparative example.

[0097]

[0098] Discussion of Results:

[0099] As can be seen from the data in Table 1:

[0100] Regarding matte finishes (gloss and reflectivity):

[0101] Example 1 has a gloss of 5.1 GU and only slight reflection under shadowless lamp; Comparative Example 1 has the highest gloss (13.5 GU) because no roughening components were added, and the reflection is more obvious, indicating that relying solely on process stress is insufficient to form an effective matte surface.

[0102] Comparative Example 2 added silica, and its gloss (4.8 GU) was similar to that of Example 1, also exhibiting matte properties; however, it should be noted that it left slightly visible white marks in the particle shedding test, which is a risk of precipitation due to insufficient compatibility between the inorganic powder and the matrix resin.

[0103] Comparative Example 3 showed that due to the increased temperature of the chilling roller, the temperature difference driving force between the polypropylene matrix and the microspheres decreased, the surface micro-wrinkles did not develop sufficiently, the gloss increased to 9.8 GU, and the matte effect was weakened.

[0104] Comparative Example 4 used untreated microspheres, which had a weaker interfacial bond with the polypropylene matrix. Although some wrinkles could be formed during casting, the overall uniformity was slightly worse, and the gloss was slightly higher than that of Example 1.

[0105] Regarding heat seal strength:

[0106] The heat seal strength of Example 1 is 8.6 N / 15 mm, which is at a relatively high level; the heat seal strength of Comparative Example 2 is reduced to 7.2 N / 15 mm. This is because the hard silica particles occupy the heat seal interface area, which hinders the mutual diffusion and fusion of polyolefin molecular chains under hot pressing.

[0107] The heat-sealing strength of Comparative Examples 1, 3, and 4 is comparable to or similar to that of Example 1, indicating that the microsphere precursor introduced in this invention and its preparation process do not adversely affect the thermal adhesion performance of the film.

[0108] Overall performance:

[0109] Example 1 achieved a matte finish with low gloss while maintaining high heat-sealing strength, and performed well in the particle shedding test with no dust migration. This indicates that by introducing organic microspheres with specific shrinkage characteristics and surface grafting structures into the heat-sealing layer, and combining them with low-temperature casting-induced stress, it is possible to balance the relationship between matte appearance, heat-sealing performance, and cleanliness.

[0110] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0111] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A matte heat-sealable film, characterized in that, The product includes a heat-sealing layer made of a composition comprising the following parts by weight: 60 parts random copolymer polypropylene, 15 parts low-density polyethylene, 8 parts heat-shrinkable microsphere precursor, 5 parts tackifying resin, 0.3 parts slip agent, and 0.4 parts antioxidant; the heat-shrinkable microsphere precursor is a core-shell structured microsphere, the core layer being copolymerized from isoborneol methacrylate and trimethylolpropane triacrylate, and the shell layer being a long-chain alkylsiloxane grafted layer, with an average particle size of 8 μm and a volume shrinkage rate of 8% at 130°C.

2. The matte heat-sealing film according to claim 1, characterized in that, The tackifying resin is polyethylene glycol diglycidyl ether modified hydrogenated rosin pentaerythritol ester.

3. The matte heat-sealing film according to claim 1, characterized in that, The surface of the heat-sealing layer has an in-situ self-generated uneven wrinkle structure formed by the difference in thermal shrinkage between the heat-shrinkable microsphere precursor and the matrix resin. The matte heat-sealing film has a gloss level of no more than 5.0 GU at a 60° angle and a surface roughness Ra value of 0.20μm to 0.30μm.

4. The matte heat-sealing film according to claim 1, characterized in that, The preparation method of the heat-shrinkable microsphere precursor includes the following steps: 50 parts of isobornyl methacrylate, 30 parts of trimethylolpropane triacrylate, 10 parts of methacryloxypropyltrimethoxysilane, and 1.5 parts of azobisisobutyronitrile were mixed to form an oil phase; the oil phase was added to an aqueous phase containing polyvinyl alcohol dispersant, and the mixture was subjected to suspension polymerization at 75℃~85℃ for 6 hours to form core layer microspheres. After the reaction is complete, the temperature is lowered to 70°C, 10 parts of long-chain alkylsiloxane are added to the system, the pH value is adjusted to 5.0, and the system is kept at this temperature for 2 hours for hydrolysis and condensation to form an organosilicon grafted shell on the surface of the microspheres. The heat-shrinkable microsphere precursor was obtained by washing, drying and sieving.

5. A matte heat-sealing film according to claim 1, characterized in that, The matte heat-sealing film has a three-layer co-extruded structure, including a heat-sealing layer, a core layer, and a secondary outer layer, wherein the core layer is a homopolymer polypropylene layer.

6. A method for preparing a matte heat-sealing film as described in any one of claims 1-5, characterized in that, Includes the following steps: Weigh out random copolymer polypropylene, low-density polyethylene, heat shrinkable microsphere precursor, tackifying resin, slip agent and antioxidant according to the formula, mix them evenly and put them into the extruder, and melt extrude at a temperature of 160℃~230℃. The melt is cast onto a quenching roller at 35°C to cool and form a cast sheet. The ratio of the linear speed of the quenching roller to the melt flow rate is controlled to be 1:1.2, so that residual internal stress is generated in the preform during the rapid cooling process. Due to the difference in thermal shrinkage rate, the heat shrinkable microsphere precursor and the matrix resin induce the formation of a nanoscale concave-convex wrinkle structure on the film surface. The casting was stretched longitudinally by 4.5 times at 110℃~120℃ and transversely by 8.0 times at 155℃~165℃, and then heat-set at 168℃ for 8 seconds to fix the wrinkle morphology. The film is subjected to corona treatment to control the surface tension to 42 mN / m, and then wound up to obtain the matte heat-sealing film.

7. The method for preparing the matte heat-sealing film according to claim 6, characterized in that, The temperature of the quenching roller is 35°C, the temperature of the melt flowing out of the die head is 240°C, and the initial thickness of the preform is 250μm.

8. The method for preparing the matte heat-sealing film according to claim 6, characterized in that, The winding is performed in a clean environment, and surface static charge is eliminated by an ionizer bar, so that the residual voltage of the film is below 50V.

9. The method for preparing the matte heat-sealing film according to claim 6, characterized in that, The heat-sealing temperature of the heat-sealing film is 120℃~135℃, and the heat-sealing strength with medical dialysis paper after heat-sealing for 1 second under 0.3MPa pressure is not less than 8.5N / 15mm.

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