Pasteurization-resistant non-stick milk packaging film based on polyester degradable plastics

Abstract

The invention relates to the technical field of polyester degradable materials, in particular to a pasteurization-resistant non-stick milk packaging film based on polyester degradable plastics. According to the invention, the chaos mixing-reactive compatibilization synergistic online thermal annealing process is adopted, so that the problems of low strength and high brittleness of conventional polyester melt are solved, and the balance of rigidity and toughness of the material is realized; by combining asymmetric surface modification and an interlayer reactive compatibilization system, the problems of contradiction between smoothness of an inner layer and adhesion of an outer layer and weak interlayer binding force are solved; dispersion of the inorganic nano-filler is optimized through a compatibilization system, and the conflict between addition of the functional filler and optical transparency is solved; the coarse structure of the inner layer is cooperated with a migration biological water repellent agent and an on-line rapid crystallization process, so that the extreme liquid repellency and the high-temperature size stability are realized, the pasteurization requirement is met, and food residues and packaging failure are avoided.

Description

Technical Field

[0001] This invention relates to the field of polyester biodegradable materials technology, specifically to a pasteurization-resistant, non-stick milk packaging film based on polyester biodegradable plastics. Background Technology

[0002] Polyester-based biodegradable plastics, with their excellent biocompatibility, mechanical strength, and controllable degradation, have become one of the core materials for solving the white pollution problem caused by traditional plastic packaging, especially in the food packaging sector where their application prospects are broad. Milk packaging films, as a key carrier in the dairy product distribution process, not only need to meet basic barrier properties to ensure the shelf life of dairy products, but also need to be compatible with the pasteurization process in dairy processing, typically involving heat treatment at 60-85℃ for several tens of minutes. Simultaneously, they must possess non-stick properties to prevent dairy product residue inside the packaging or adhesion between packaging films, affecting the user experience. Due to the stability of their molecular chain structure, polyester materials stand out among biodegradable packaging materials, becoming the preferred substrate for developing high-performance milk packaging films. Their application can effectively reduce the environmental pressure of traditional non-biodegradable milk packaging films such as polyethylene and polypropylene, aligning with the current trend of green packaging development.

[0003] However, existing milk packaging films based on polyester biodegradable plastics still face numerous technical bottlenecks in practical applications, making it difficult to simultaneously meet the comprehensive performance requirements of pasteurization resistance, non-stick properties, and long-lasting barrier properties. On the one hand, most polyester biodegradable materials are prone to slight molecular chain degradation under the warm conditions of pasteurization, leading to decreased mechanical strength, reduced barrier performance, and even deformation and damage, failing to guarantee the safety of dairy products during processing and subsequent storage. On the other hand, to improve the non-stick properties of the packaging film, existing technologies often employ surface coatings with silicone or fluorocarbon materials. However, these coatings have poor adhesion to the polyester substrate and are prone to peeling off under the humid and hot environment of pasteurization. This not only affects the non-stick effect but the peeled coating may also contaminate dairy products, posing a food safety hazard. Furthermore, some uncoated polyester packaging films have high surface polarity, easily interacting with proteins, fats, and other components in dairy products, resulting in severe adhesion and reducing the consumer experience.

[0004] Therefore, developing a milk packaging film that can maintain stable performance under pasteurization, possess long-lasting non-stick properties, and not affect the biodegradability of polyester materials has become an urgent technical problem to be solved in the field of polyester biodegradable packaging materials. This also places higher demands on the modification process and formulation design of polyester materials.

[0005] To address this, a pasteurization-resistant, non-stick milk packaging film based on polyester biodegradable plastics was proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a pasteurization-resistant, non-stick milk packaging film based on biodegradable polyester plastics. Through a chaotic mixing-reactive compatibilization synergistic online thermal annealing process, the low melt strength and brittleness of conventional polyesters are addressed, achieving a balance between material rigidity and toughness. Combining asymmetric surface modification and an interlayer reactive compatibilization system resolves the contradiction between inner layer slipperiness and outer layer adhesion, as well as the problem of weak interlayer bonding. Optimizing the dispersion of inorganic nanofillers through the compatibilization system resolves the conflict between functional filler addition and optical transparency. The rough inner layer structure, combined with migratory biohydrophobic agents and an online rapid crystallization process, achieves extreme liquid repellency and high-temperature dimensional stability, meeting pasteurization requirements and preventing food residue and packaging failure.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a pasteurization-resistant non-stick milk packaging film based on polyester biodegradable plastic, which consists of a three-layer structure: an inner layer, a middle layer, and an outer layer.

[0009] The raw materials for the inner layer include polylactic acid, polybutylene adipate / terephthalate, and hydrophobic nano-silica;

[0010] By adding hydrophobic nano-silica, a micro-rough surface is constructed, achieving the "lotus leaf effect" to prevent milk sticking. The production method of hydrophobic nano-silica is as follows: 10 kg of fumed silica (native particle size 20 nm, specific surface area 200 m²) is added... 2 / g) was placed in a high-speed mixer and pre-stirred at 300 rpm for 5 minutes; 50 kg of anhydrous ethanol was injected as the dispersion medium into a reactor equipped with an ultrasonic generator, fumed silica was added, and ultrasonic dispersion (power 800W) was turned on for 30 minutes to break up agglomerates; the temperature was raised to 60°C, and the prepared modified solution, consisting of 1.5 kg hexamethyldisilazane (HMDS) and 0.5 kg deionized water, was slowly added dropwise at a rate of 50 g / min; after the addition was complete, the temperature was raised. The mixture was refluxed to the ethanol reflux temperature (approximately 78°C) and mechanically stirred (1500 rpm) for 4 hours to allow HMDS to fully hydrolyze and undergo a condensation reaction with the silanol groups on the silica surface, grafting methyl groups. After the reaction, the slurry was pumped into a centrifugal spray dryer with an inlet temperature of 180°C and an outlet temperature of 105°C to quickly remove the solvent and dry the powder. The collected powder was then reactivated in a 120°C oven for 2 hours to remove residual ammonia, yielding superhydrophobic nano silica powder with a contact angle >150°.

[0011] The raw materials for preparing the middle layer include heat-resistant microfiber reinforced masterbatch;

[0012] The outer layer is prepared from a mixture of polylactic acid and modified zein.

[0013] The production method of modified zein is as follows: 100 parts of zein powder are dispersed in 600 parts of 90% ethanol aqueous solution, and the mixture is heated to 50℃ and stirred to dissolve. 15 parts of the modifier octenyl succinic anhydride (OSA) are added, and 0.1M food-grade sodium hydroxide solution is slowly added dropwise to maintain the pH of the system between 8.0 and 8.5. The reaction time is controlled at 3h, so that the hydrophobic long chain of OSA undergoes esterification / amidation reaction with the amino or hydroxyl groups of the protein molecule. After the reaction, the pH is adjusted to 4.5 to precipitate the protein, and the precipitate is collected by centrifugation. The precipitate is washed three times with deionized water, redispersed in anhydrous ethanol, and 5 parts of oleic acid are added as a plasticizer. The mixture is stirred at 60℃ for 1h to form a homogeneous modified protein solution. Most of the ethanol in this solution is removed by flash drying to obtain a light yellow hydrophobic modified protein powder for subsequent blending.

[0014] The raw materials for preparing the heat-resistant microfiber reinforced masterbatch include polylactic acid, polybutylene adipate / terephthalate, homopolymer polypropylene, maleic anhydride-grafted polylactic acid, maleic anhydride-grafted polypropylene, and talc. Maleic anhydride-grafted polypropylene and talc are added through a feed inlet. The MFI of the homopolymer polypropylene is 2-4 g / 10 min at 230℃ / 2.16 kg.

[0015] Maleic anhydride-grafted polypropylene can be purchased directly from the market with a grafting rate of 1.1%. Alternatively, it can be prepared by adding 100 parts homopolymer polypropylene (Homo-PP) and 0.3 parts composite antioxidant (B215) at the main feed inlet at a rate of 18.0 kg / hr. The temperature is set at 50°C in zone one, 170°C in zone two, 190°C in zone three, and 210°C in zone four. Simultaneously, using a melt gear pump or high-pressure metering pump, 1.5 parts molten maleic anhydride (MAH) and 0.08 parts initiator 2,5-dimethyl-2,5-bis(tert-butylperoxide)hexane (bis(2,5-dimethyl-2,5-diphenyl-2,5-dimethyl-2,5-di(tert-butylperoxide)hexane) are directly injected into the screw reactor section, with the MAH pumping rate being 0.27 kg / hr. The initiator pumping rate was 0.014 kg / hr; the temperature in zones five to seven was set at 220℃, and the residence time was extended to 2.0 minutes by using a reverse conveying element to ensure a complete grafting reaction; high vacuum exhaust (vacuum degree 0.08-0.095 MPa) was set in zone eight to thoroughly extract volatile small molecules; the temperature in zone nine was lowered to 200℃ for extrusion; the strip was cooled with 30℃ warm water, granulated, and dried in a 90℃ forced-air oven for 3 hours to obtain a high-flowability graft with a melt index (MFI) of 55 g / 10 min (230℃ / 2.16 kg) and a grafting rate of 1.1%.

[0016] Maleic anhydride-grafted polylactic acid (PLA) can be directly purchased from the market as a product with a grafting rate of 0.95%. Alternatively, it can be prepared using the following method: 100 parts of dried polylactic acid (L-PLA) granules and 0.2 parts of antioxidant 1010 are fed into a twin-screw extruder (L / D ratio L / D = 44) at a rate of 20.0 kg / hr through the main feed inlet. The cooling temperature in zone one is 40°C, the heating temperature in zone two is 150°C, the heating temperature in zone three is raised to 170°C, and the heating temperature in zone four is raised to 180°C to completely melt the PLA. Simultaneously, a high-precision liquid-phase metering pump is used to inject a mixed solution dissolved in acetone (containing 2.0 parts of maleic anhydride (MAH) and 0.15 parts of dicumyl peroxide (DCP), with a mass ratio of MAH:DCP:acetone = 20:1.5:20) into the melt at an injection rate controlled at 0.415 kg / hr. The process continues in zone five to... The temperature in zone seven is maintained at 180℃-185℃. High shear force is provided by the kneading block to initiate the grafting reaction. The residence time of the melt in this zone is 1.2 minutes. Zones eight and nine are equipped with multi-stage vacuum exhaust ports (vacuum degree 0.06-0.09MPa, absolute pressure) to remove acetone solvent and unreacted maleic anhydride monomers under high vacuum, eliminating odors. The melt is extruded into strips at 175℃ in zone ten. The extruded strips are rapidly cooled and shaped in a circulating water bath cooling tank (water temperature 15℃). Surface moisture is removed using an air knife, and the strips are cut into 3mm particles by a pelletizer. The particles are dried in a 60℃ dehumidifying dryer for 6 hours, and then vacuum-sealed in aluminum foil. The grafting rate was measured to be 0.95%, and the residual monomer content was <10ppm, meeting food-grade requirements.

[0017] Preferably, the pasteurization-resistant non-stick milk packaging film is obtained by feeding the inner melt, middle melt and outer melt into a superimposed spiral die head, and then performing crystallization induction, rapid cooling and shaping and surface functionalization treatment; the surface functionalization treatment is corona treatment.

[0018] Preferably, the inner melt is obtained by melting a mixture of polylactic acid, polybutylene adipate / terephthalate, chitosan grafted stearic acid, and hydrophobic nano-silica; the middle melt is obtained by melting a heat-resistant microfiber reinforced masterbatch; and the outer melt is obtained by melting a mixture of polylactic acid and modified zein.

[0019] The production method of chitosan grafted with stearic acid is as follows: 100 parts of chitosan with a degree of deacetylation of 90% are dissolved in 5000 parts of 1% acetic acid aqueous solution and stirred until completely dissolved; 80 parts of food-grade stearic acid are dissolved in 2000 parts of anhydrous ethanol, and 20 parts of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl) are added as carboxyl activator and activated at 40℃ for 30 minutes; the activated stearic acid solution is slowly added dropwise to the chitosan solution and reacted at 50℃ and pH adjusted to 5.5-6.0 for 12 hours to form amide bonds; the reaction product is precipitated with sodium hydroxide solution, repeatedly washed with deionized water until neutral, and dialyzed to remove small molecule byproducts; finally, the wet precipitate is sent to a vacuum freeze dryer and freeze-dried at -40℃ and 10Pa for 24 hours, pulverized and passed through a 200-mesh sieve to obtain a chitosan graft with amphiphilic but slightly hydrophobic properties.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. By employing a chaotic mixing process combined with reactive compatibilization, a microfiber framework is induced in situ within the matrix by the dispersed phase. This is further enhanced by an online hot annealing process to promote matrix crystallization and lock in molecular chain orientation. This effectively overcomes the shortcomings of conventional polyester materials, such as low melt strength and high brittleness. The resulting packaging film maintains good toughness while possessing excellent tensile strength and Young's modulus, ensuring that the packaging bag maintains high stiffness and resistance to deformation under load, preventing bag breakage or brittle fracture during transportation. This successfully achieves a balance between rigidity and toughness in biodegradable structural materials.

[0022] 2. By employing an asymmetric surface modification strategy combined with an interlayer reactive compatibilizer system, the chemical integrity of the inner surface is maintained through selective high-energy treatment of the outer surface, while compatibilizers are introduced into the interlayer to induce interfacial chemical bonding. This method effectively solves the contradiction between inner layer slipperiness and outer layer adhesion, as well as the problem of weak interlayer bonding in multi-component composite films. The resulting packaging film exhibits a significant difference in surface energy between the inner and outer layers. While ensuring excellent ink adhesion and printability on the outer layer, it maintains a low coefficient of friction on the inner layer to facilitate machining, and the enhanced interlayer peel strength prevents delamination during use, ensuring the structural integrity of the composite material.

[0023] 3. By introducing a compatibilizing system to optimize the interfacial tension between inorganic nanofillers and the organic matrix, the uniform monodispersion of hydrophobic nanoparticles in the polyester matrix is ​​promoted, effectively inhibiting the formation of large-sized agglomerates that cause severe light scattering; the conflict between the addition of functional fillers and optical transparency is resolved; the resulting packaging film maintains a low haze level while possessing functional characteristics, avoiding whitening or turbidity caused by filler agglomeration, ensuring the packaging's ability to clearly display the contents, giving the film excellent optical aesthetics and visibility, and meeting the intuitive quality inspection needs of end consumers.

[0024] 4. By constructing a biomimetic micro-nano rough structure in the inner layer in conjunction with a migratory biohydrophobic agent, and combining it with a highly crystalline physical framework formed by rapid online crystallization, a stable lotus leaf effect is constructed on the surface, and a thermally stable network is established internally, achieving the dual goals of extreme liquid repellency and high-temperature dimensional stability. In practical applications, the packaging film exhibits extremely low liquid residue rates, significantly reducing food waste; simultaneously, it maintains excellent dimensional stability during high-temperature sterilization, eliminating packaging deformation or shrinkage failure caused by thermal shock, effectively meeting the stringent requirements of pasteurization processes for biodegradable packaging. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method for preparing pasteurization-resistant non-stick milk packaging film according to the present invention;

[0026] Figure 2 The results are the application performance test results of the pasteurization resistant non-stick milk packaging film of Examples 1-6, Comparative Examples 1-3 and Comparative Examples 5-6 of the present invention. Detailed Implementation

[0027] 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.

[0028] Please see Figures 1 to 2 This invention provides a pasteurization-resistant, non-stick milk packaging film based on polyester biodegradable plastics, the technical solution of which is as follows:

[0029] Example 1

[0030] The polyester raw materials underwent chaotic blending and granulation treatment, specifically as follows: A twin-screw extruder was controlled under negative pressure devolatilization, and the dried materials were melt-blended under a vacuum of 0.08 MPa. Specifically, 65 parts of L-polylactic acid granules, 10 parts of poly(butylene adipate / terephthalate), and 20 parts of homopolymer polypropylene were fed into the main feed inlet at a rate of 50 kg / hr; 4 parts of maleic anhydride-grafted polypropylene and 1 part of talc were fed into the side feed inlet at a rate of 5 kg / hr; the screw speed was set to 450 rpm; eccentric rotor chaotic blending elements were installed in zones 5 to 7 of the extruder; and the temperature gradient from zone 1 to zone 9 was set to 160℃ to 210℃. The stretching flow field induced the polypropylene to form a microfiber structure with an aspect ratio greater than 20. After processing, the melt was processed into heat-resistant microfiber-reinforced masterbatch via an underwater pelletizing system.

[0031] The raw materials entering the three-layer co-extrusion blown film unit undergo layered melting and plasticization, specifically divided into an inner layer hydrophobic plasticization stage, a middle layer heat-resistant skeleton construction stage, and an outer layer printing adaptability plasticization stage. In the inner layer hydrophobic plasticizing stage, 62 parts of polylactic acid, 30 parts of poly(butylene adipate / terephthalate), 5 parts of chitosan-grafted stearic acid, 1 part of maleic anhydride-grafted polylactic acid, and 2 parts of hydrophobic nano-silica are introduced into the first extruder. The screw speed is maintained at 60 rpm, and the temperature in each zone is controlled between 175°C and 185°C to prevent degradation of the bio-based hydrophobic agent. The target melt pressure is 10 MPa. In the middle layer heat-resistant skeleton construction stage, the heat-resistant microfiber reinforced masterbatch prepared above is introduced into the second extruder. The screw speed is maintained at 80 rpm, and the melt temperature is raised to 205°C. The die pressure is maintained at 15 MPa to prevent shrinkage of the polypropylene microfiber morphology. In the outer layer printing adaptability plasticizing stage, 50 parts of polylactic acid and 5 parts of modified zein are introduced into the third extruder. The screw speed is maintained at 50 rpm, and the processing temperature is 180°C.

[0032] Three melt streams, after being molten and plasticized, are fed into a stacked spiral die head, where a composite film is formed through a blow-up and online crystallization induction process. Specifically, the online crystallization induction process involves controlling the die head outlet temperature to 195℃, the die gap to 1.2mm, the blow-up ratio to 3.0, and the traction ratio to 4.0. After the film bubble is formed, it passes sequentially through a first cooling air ring and a far-infrared heating zone. Specifically, the first cooling air ring temperature is set to 30℃, and the airflow is 2000 m³ / h. 3 The process involves heating the membrane bubble at a rate of 1000 μm / h to allow the melt to initially solidify. The bubble then enters a 2.0-meter-long far-infrared heating zone, where the ambient temperature is controlled at 95°C. The bubble remains in this zone for 4.0 seconds, utilizing thermal energy to activate the nucleating agent and induce cold crystallization of the polylactic acid molecular chains, resulting in a mid-layer crystallinity exceeding 35%. The target mid-layer heat-resistant framework layer has a thickness of 40-50 μm.

[0033] On the co-extrusion blow molding production line, after the film bubble completes online crystallization induction, it undergoes rapid cooling and surface functionalization treatment. Specifically, the rapid cooling and surface treatment process involves blowing 15°C cold air into the film bubble through a second cooling air ring to rapidly reduce the film temperature to below 40°C, locking in the crystal structure. Subsequently, the corona treatment machine power is adjusted to 2.0kW, and corona discharge treatment is applied only to the outer surface of the film at a speed of 40m / min, achieving a surface tension of over 38mN / m to meet printing adhesion requirements; the inner layer remains untreated to maintain its hydrophobic properties. Finally, the film is wound up on a constant tension winding machine with a taper tension of 5.0kg. The specific preparation process is as follows: Figure 1 As shown.

[0034] After co-extrusion blow molding and online strengthening processes are completed, the packaging film consists of a hydrophobic modified polylactic acid layer, a polypropylene microfiber reinforced polylactic acid crystalline layer, and a printable modified polyester layer from the inside out.

[0035] Examples 2-6

[0036] Except for the following adjustments to the preparation parameters, all other parameters remained the same as in Example 1, as shown in Table 1.

[0037] Table 1. Preparation process parameters for pasteurization-resistant non-stick milk packaging film

[0038]

[0039] Comparative Examples 1-6

[0040] The following comparative settings are based on Example 1, and remain unchanged except for the following adjustments to the methods.

[0041] Comparative Example 1 had PLA-g-MAH (maleic anhydride-grafted polylactic acid) removed from its inner layer formulation, while the rest remained unchanged.

[0042] In Comparative Example 2, the intermediate formulation removed PP and 4 parts by weight of PP-g-MAH, and replaced them all with an equal amount of L-PLA.

[0043] Comparative Example 3 eliminated the online infrared annealing process, and the membrane bubble was directly cooled to room temperature through a cold air ring after extrusion.

[0044] In Comparative Example 4, the intermediate formulation removed PP-g-MAH, while the rest remained unchanged.

[0045] Comparative Example 5 had its inner layer formulation with chitosan-grafted stearic acid and nano-SiO2 removed.

[0046] Comparative Example 6 changed the corona treatment to double-sided corona treatment, with a power of 2.0kW for both.

[0047] Test Example 1

[0048] The pasteurization-resistant non-stick milk packaging films prepared in Examples 1-6 and Comparative Examples 1-4 were subjected to mechanical property tests, and the test indicators were tensile strength, Young's modulus and elongation at break.

[0049] According to GB / T 1040.3-2006, blown film was cut into strips with a width of 15 mm and a total length of 150 mm. An electronic universal testing machine with an accuracy of 0.5 grade was used, and the fixture spacing (gauge length) was set to 50 mm. After conditioning for 24 hours in a standard laboratory environment (temperature 23±2℃, relative humidity 50±5%), the test was conducted. The tensile speed was set to 200 mm / min. The instrument automatically recorded the stress-strain curve and calculated the corresponding values ​​of the yield point and fracture point. Each group of samples was tested 5 times, and the arithmetic mean was taken. The test results are summarized in Table 2.

[0050] Table 2. Mechanical Performance Test Results Data Table

[0051] Group Tensile strength (MPa) Young's modulus (MPa) Elongation at break (%) Example 1 48.5 720.3 410.5 Example 2 54.2 880.6 320.1 Example 3 42.1 550.8 520.4 Example 4 49.3 760.2 385.7 Example 5 47.9 735.4 405.2 Example 6 50.4 725.1 425.8 Comparative Example 1 46.2 715.5 390.3 Comparative Example 2 35.6 410.7 550.9 Comparative Example 3 40.2 520.5 480.3 Comparative Example 4 38.1 610.2 150.6

[0052] As shown in Table 3, Examples 1 to 6 maintained an excellent balance in the three key mechanical indicators of tensile strength, Young's modulus, and elongation at break, which were significantly better than Comparative Examples 2, 3, and 4. This indicates that the microfiber reinforcement and crystallization induction process used in this invention has significant advantages in improving the rigidity of biodegradable materials, maintaining toughness, and constructing a stable framework.

[0053] Comparative Example 2 removed polypropylene and its compatibilizer from the middle layer, using only polylactic acid as the matrix. Although it had a high elongation at break, the lack of physical reinforcement from high-modulus PP microfibers resulted in a significant drop in Young's modulus from 720.3 MPa in Example 1 to 410.7 MPa, and tensile strength also fell to 35.6 MPa. This indicates that the material is too soft and lacks the stiffness and tensile load-bearing capacity required for packaging bags. Comparative Example 3 retained the PP component but omitted the crucial online infrared annealing process, causing the polylactic acid matrix to fail to complete cold crystallization. The amorphous matrix could not effectively fix the PP microfibers, resulting in a Young's modulus of only 520.5 MPa and a lower tensile strength than Example 1. This demonstrates the decisive contribution of crystallinity to the macroscopic mechanical rigidity of the material. Most notably, Comparative Example 4, although it added PP, removed the maleic anhydride-grafted polypropylene (PP-g-MAH) compatibilizer, leading to the failure of interfacial bonding between the non-polar PP dispersed phase and the polar polylactic acid matrix, preventing effective stress transfer. During the stretching process, the two-phase interface rapidly debonds, forming voids, causing the elongation at break to plummet to 150.6%, exhibiting extreme brittleness and completely losing the toughness that the packaging film should have.

[0054] In summary, this invention introduces a reactive compatibilizer through chaotic mixing and combines it with online thermal annealing technology. These two processes produce a significant synergistic effect: the compatibilizer ensures a strong chemical bond between the PP microfibers and the PLA matrix, preventing brittle fracture at the interface and guaranteeing high elongation; the online annealing process induces matrix crystallization, which, together with the PP microfiber skeleton, significantly improves Young's modulus and tensile strength, ensuring that the film achieves excellent rigidity and tensile properties while maintaining high toughness.

[0055] Test Example 2

[0056] The pasteurization-resistant non-stick milk packaging films prepared in Examples 1-6, Comparative Example 1, and Comparative Examples 4-6 were subjected to surface / interface performance tests. The coefficient of friction was tested according to GB / T 10006-2021, with a focus on the dynamic friction coefficient of the inner surface of the film (layer A, hydrophobic layer) against a standard metal plate to characterize the film's slipperiness and the physical basis of its non-stick properties. The films were cut into 63mm × 63mm samples and fixed on a slider (inner surface facing outwards), with the inner surface in contact with a standard stainless steel plate on the test bench. Under conditions of 23°C and 50% RH, the slider was moved at a speed of 100mm / min, and the dynamic friction force was recorded and the coefficient of dynamic friction was calculated.

[0057] According to GB / T 14216-2008, the wetting tension of the outer surface (C layer, printing layer) of the film was tested to characterize its printability. For a specific comparative example, the inner surface was also tested. A series of mixtures with different surface tensions (formamide and ethylene glycol ethyl ether) were prepared and coated onto the outer surface of the film in ascending order of surface tension. The film was observed to see if it ruptured or contracted into droplets within 2 seconds. The highest mixture tension value corresponding to the film remaining intact for 2 seconds was the wetting tension of the film (unit: mN / m).

[0058] The peel strength test was conducted according to GB / T 8808-1988, testing the bonding strength between the film composite layers (mainly between layer B and layers A / C). The film was cut into strips with a width of 15 mm and a length of 200 mm. One end of the strip was manually peeled open by 50 mm beforehand. The two peeled ends were clamped in the upper and lower clamps of the tensile testing machine in a T-shaped peeling state. The strip was stretched at a speed of 300 mm / min, and the average force value (unit N / 15 mm) during the peeling process was recorded. The final test results are summarized in Table 3.

[0059] Table 3 Summary of Surface / Interface Performance Test Results

[0060]

[0061] As shown in Table 3, Examples 1 to 6 are significantly better than Comparative Examples 1, 4, 5 and 6 in terms of dynamic friction coefficient, outer surface wetting tension and interlayer peel strength. This indicates that the reactive compatibilization combined with asymmetric surface modification process adopted in this invention has obvious advantages in taking into account the smoothness and non-stickiness of the inner layer of the film, improving the interlayer bonding strength and ensuring the printability of the outer layer.

[0062] Comparative Example 1 added only hydrophobic nanoparticles to the inner layer formulation without adding maleic anhydride-grafted polylactic acid compatibilizer. This failed to effectively coat and disperse the inorganic filler in the polar polyester matrix, resulting in severe agglomeration of nano-silica. The key mechanism for constructing a uniform micro-rough surface was missing, leading to a significant increase in the dynamic friction coefficient to 0.55 and a noticeable impact on non-stick properties. Comparative Example 4 removed the PP-g-MAH compatibilizer from the middle layer. For a polyester and polyolefin composite system with significantly different polarities, the chemical bonding at the phase interface could not be guaranteed. This resulted in the core layer and skin layer being connected only by weak physical interactions, causing the interlayer peel strength to plummet to 0.8 N / 15 mm, severely damaging the structural integrity of the packaging film as a multilayer composite material. Comparative Example 5 used only the base resin without adding chitosan grafts and nano-silica. For a polyester material with inherent polarity and viscosity, the surface energy could not be reduced, resulting in a dynamic friction coefficient as high as 0.68 and a complete loss of hydrophobic and non-stick properties. Meanwhile, although Comparative Example 6 had a complete formulation, it used a double-sided corona treatment process, which caused the inner surface, which should have remained hydrophobic, to oxidize due to high-energy electron bombardment, introducing a large number of polar groups. As a result, the wetting tension of its inner surface increased abnormally, and the coefficient of friction deteriorated to 0.73.

[0063] In summary, this invention achieves a significant synergistic effect by introducing a specific reactive compatibilizing system between different layers and combining it with asymmetric surface corona treatment: the compatibilizing design of the inner and middle layers ensures the dispersion of nanofillers and the firm bonding of the multilayer structure, effectively preventing the increase of the friction coefficient and interlayer delamination; the strictly controlled asymmetric corona process avoids damage to the hydrophobic structure of the inner layer, maintaining its low surface energy; simultaneously, the high-energy treatment of the outer layer successfully constructs an ink-friendly surface, and the differential regulation of interfacial energy achieves a dual improvement in the extreme smoothness of the inner layer and the high adhesion of the outer layer. This synergistic effect ensures that the non-stick milk packaging film achieves consistent high smoothness and high printability across the entire surface while maintaining the structure without delamination.

[0064] Test Example 3

[0065] The optical performance of samples from Examples 1-6 and Comparative Examples 1-3 was tested, specifically the haze index. Following GB / T 2410-2008, flat, scratch-free thin film samples were cut to a size of 50mm × 50mm. An integrating sphere haze meter was used under a standard light source C. The instrument collects the total luminous flux transmitted through the sample (T2) and the scattered luminous flux (T4) deviating from the incident light direction by more than 2.5° through the integrating sphere. The haze value H was calculated as (T4 / T2) × 100%. Five samples were tested in each group, and the arithmetic mean was taken. The test results are summarized in Table 4.

[0066] Table 4 Summary of Optical Performance (Haze) Test Results

[0067]

[0068]

[0069] As shown in Table 4, the haze values ​​of Examples 1 to 6 were controlled within a reasonable range of 8.1% to 18.3%. Although these values ​​were higher than those of Comparative Examples 2 and 3, they were significantly lower than those of Comparative Example 1. This indicates that the reactive compatibilization and dispersion technology used in this invention effectively controlled the light scattering effect at the phase interface while introducing the necessary heat-resistant skeleton and hydrophobic filler into the matrix, thus ensuring that the packaging film has good visibility of the contents.

[0070] Comparative Example 1 removed the maleic anhydride-grafted polylactic acid compatibilizer from the inner layer formulation, causing the hydrophobic nano-silica to lose its dispersion stability in the polyester matrix and undergo severe agglomeration, forming micron-sized agglomerates larger than the visible light wavelength. These large agglomerates became strong light scattering centers, causing disordered diffuse reflection of transmitted light, resulting in a sharp increase in haze to 35.4%, manifested as severe whitening and turbidity, seriously affecting the packaging's display effect. Comparative Example 2 removed the polypropylene component from the middle layer, retaining only the more compatible polylactic acid and poly(butylene adipate / terephthalate). The system lacked the polypropylene microfiber phase with a refractive index different from the matrix, resulting in minimal interfacial scattering when light penetrated, thus reducing the haze to 6.5%. However, this low haze was achieved at the expense of the heat-resistant skeleton (as shown in the aforementioned mechanical and heat resistance tests), representing a functionally deficient type of high transparency. Comparative Example 3 omitted the online infrared annealing process. The polylactic acid matrix did not undergo cold crystallization and lacked high-refractive-index spherulites or microcrystalline structures, exhibiting a uniform amorphous morphology. Therefore, light scattering was weak, resulting in a haze value of 7.2%. While this is optically good, it indirectly confirms that the higher haze in Examples 1-6 was due to the successful induction of a large number of heat-resistant microcrystals and microfibers. Although these microstructures caused some Rayleigh scattering leading to a slight increase in haze, they are an indispensable physical basis for achieving pasteurization resistance at 85°C.

[0071] In summary, this invention introduces a specific compatibilizing system, which produces a significant synergistic effect: the compatibilizer significantly reduces the particle size distribution of the dispersed phase (nanoparticles, PP microfibers), avoiding severe light scattering caused by large particle agglomeration (as in Comparative Example 1); at the same time, the process allows for a certain degree of crystallization and microfibrillation, resulting in a moderate increase in haze, in exchange for key heat resistance and mechanical properties, synergistically ensuring that the packaging film maintains commercially acceptable optical transparency while meeting functional requirements.

[0072] Test Example 4

[0073] The pasteurization-resistant non-stick milk packaging films prepared in the examples and comparative examples were subjected to specific application performance tests; the non-stick properties were characterized by milk residue rate, heat shrinkage rate, and heat sealing temperature; the milk residue rate was used to characterize the non-stick properties. The films were then cut and heat-sealed to form films with an inner surface area of ​​200 cm². 2 The packaging bag (three-side seal). Accurately weigh the empty bag (M0), pour 200mL of commercially available whole milk into the bag, and heat-seal the bag to ensure a tight seal. Place the packaging bag in a 4°C refrigerator for 24 hours to simulate cold chain storage. After removing it, cut open the bottom corner, pour out the milk, and let it drain naturally for 30 seconds without squeezing. Accurately weigh the packaging bag with residual milk clinging to the sides (M1).

[0074] Heat shrinkage rate at 85℃ characterizes heat resistance. According to GB / T 12027-2004, a 100mm × 100mm square sample was cut, and its initial longitudinal (MD) length L0 was accurately measured. The sample was completely immersed in a constant temperature water bath at 85℃ for 30 minutes to simulate pasteurization. The sample was then removed, its surface moisture was blotted with absorbent paper, and after cooling at room temperature for 15 minutes, its longitudinal length L1 was measured again.

[0075] The heat-sealing start temperature was referenced in QB / T 2358-1998. A heat-sealing apparatus was used, with the heat-sealing pressure set to 0.2 MPa and the heat-sealing time to 1.0 s. Sealing was performed within a temperature range of 80℃ to 120℃, with an assumed heat-sealing temperature gradient of 2℃. The sealed samples were cut into 15 mm wide strips, and peel strength tests were conducted. The lowest heat-sealing temperature corresponding to a heat-sealing strength of 2.0 N / 15 mm was defined as the heat-sealing start temperature (SIT) of the film. The test results are summarized in Table 5 and... Figure 2 middle.

[0076] Table 5 Specific Application Performance Test Results

[0077] Group Milk residue rate (%) Heat shrinkage rate at 85℃ (%) SIT (°C) Example 1 0.4 1.8 105.5 Example 2 0.5 1.2 107.1 Example 3 0.3 2.5 103.3 Example 4 0.4 0.6 108.4 Example 5 0.2 1.7 106.2 Example 6 0.5 1.9 104.8 Comparative Example 1 3.8 1.9 105.7 Comparative Example 2 0.4 18.5 98.2 Comparative Example 3 0.4 15.2 101.6 Comparative Example 5 8.5 1.8 105.3 Comparative Example 6 7.9 1.8 105.1

[0078] As shown in Table 5, Examples 1 to 6 exhibited excellent overall performance in terms of milk residue rate (non-stickiness), 85℃ heat shrinkage rate (heat resistance), and heat sealing start temperature, significantly outperforming Comparative Examples 1, 2, 3, 5, and 6. This indicates that the synergistic technology solution of microfiber reinforcement-online crystallization-functionalization modification adopted in this invention has significant advantages in balancing the extreme working condition tolerance of packaging film, content emptying efficiency, and processing adaptability.

[0079] Although hydrophobic fillers were introduced in Comparative Example 1 during preparation, the lack of a crucial inner layer compatibilizer prevented the monodispersity of nano-silica in the polyester matrix. This resulted in the failure to form a uniform micro-nano lotus leaf structure on the surface. Instead, filler agglomeration formed physical anchors that caused adhesion to the film, significantly increasing the milk residue rate to 3.8% and drastically reducing its non-stick properties. Comparative Examples 5 and 6 represent deficiencies in formulation and process, respectively: Comparative Example 5 did not add any hydrophobic modifying components, and Comparative Example 6 used an incorrect double-sided corona treatment process that damaged the inner layer of hydrophobic groups. Both resulted in the inner surface exhibiting the intrinsic polar hydrophilic state of polyester material, with milk residue rates as high as 8.5% and 7.9%, respectively, completely losing their non-stick function. Regarding heat resistance, Comparative Example 2 removed the middle layer of polypropylene microfiber skeleton, and Comparative Example 3 omitted the online thermal annealing process. Both of these resulted in the film lacking macroscopic heat resistance support (either due to matrix softening or extremely low crystallinity). When subjected to the harsh heat shock of pasteurization at 85°C, the polymer chains undergo drastic disorientation and shrinkage, with heat shrinkage rates as high as 18.5% and 15.2%, respectively. This results in severe deformation and damage to the packaging bags, failing to meet the sterilization requirements of the food industry. In contrast, Examples 1-6, through the synergistic effect of polypropylene microfibers and induced crystallization, control the heat shrinkage rate at an extremely low level of 0.6%-2.5%.

[0080] Meanwhile, although Comparative Example 2 exhibited a lower heat-sealing starting temperature (98.2℃), this was merely due to the lack of high-melting-point PP support in its core layer, resulting in a false impression of fusibility caused by poor overall temperature resistance. Combined with its extremely poor heat shrinkage resistance, this indicates that it cannot be used in actual packaging. In contrast, Examples 1-6, while ensuring excellent heat resistance (SIT 103.3℃-108.4℃), still maintained a suitable heat-sealing window, achieving a balance between heat resistance and ease of sealing.

[0081] In summary, this invention achieves a significant synergistic effect by constructing a microfiber framework and a hydrophobic surface in the formulation, combined with an online annealing process: the microfiber framework and the highly crystalline matrix work together to ensure dimensional stability under high-temperature sterilization, effectively preventing packaging bag deformation; the precisely controlled hydrophobic surface structure avoids the adsorption of polar liquids, achieving an extremely low residue rate; at the same time, the low-temperature heat-sealing resin in the outer layer, combined with a reasonable interlayer structure, ensures that the film has high heat resistance without sacrificing heat-sealing performance, synergistically ensuring the all-round high performance of the pasteurization-resistant non-stick milk packaging film in actual filling, sterilization, and consumption scenarios.

[0082] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A pasteurization-resistant, non-sticky milk packaging film based on polyester-based degradable plastics, characterized by: The pasteurization-resistant non-sticky milk packaging film is composed of an inner layer, a middle layer and an outer layer; The preparation raw material of the inner layer comprises polylactic acid, polybutylene adipate terephthalate and hydrophobic nano-silica; The preparation raw material of the middle layer comprises heat-resistant micro-fiber reinforced master batch; The preparation raw material of the outer layer comprises the polylactic acid and modified zein mixture; The preparation raw material of the heat-resistant micro-fiber reinforced master batch comprises the polylactic acid, the polybutylene adipate terephthalate, homopolymer polypropylene and talcum powder.

2. The pasteurization-resistant non-sticky milk packaging film based on polyester-based degradable plastics according to claim 1, characterized by: The pasteurization-resistant non-sticky milk packaging film is obtained through crystallization induction, rapid cooling and shaping and surface functionalization treatment after the inner layer melt, the middle layer melt and the outer layer melt are fed into a laminating spiral die; the surface functionalization treatment is corona treatment.

3. The pasteurization-resistant non-sticky milk packaging film based on polyester-based degradable plastics according to claim 2, characterized by: The inner layer melt is obtained by melting the polylactic acid, the polybutylene adipate terephthalate, chitosan grafted stearic acid and the hydrophobic nano-silica; the middle layer melt is obtained by melting the heat-resistant micro-fiber reinforced master batch; and the outer layer melt is obtained by melting the polylactic acid and the modified zein mixture.

4. The pasteurization-resistant, non-sticky milk packaging film based on polyester-based degradable plastic according to claim 3, characterized by: The modified zein mixture is obtained by dispersing zein powder in ethanol, dissolving, adding octenyl succinic anhydride, adjusting the pH to alkaline, adjusting the pH to acid after reaction to obtain protein precipitation; The protein precipitation is collected and then dispersed in anhydrous ethanol, and then oleic acid is added, stirred, and flash dried to obtain.

5. The pasteurization-resistant non-sticky milk packaging film based on polyester-based degradable plastics according to claim 3, characterized by: The chitosan grafted stearic acid is obtained by slowly dropping the activated stearic acid solution into the chitosan solution under constant temperature reaction in acidic conditions, and then washing, dialysis and freeze-drying after precipitation.

6. The pasteurization-resistant non-sticky milk packaging film based on polyester-based degradable plastics according to claim 3, characterized by: The hydrophobic nano-silica is obtained by activating the hexamethyldisilazane treated silica.

7. The pasteurization-resistant, non-sticky milk packaging film based on a polyester-based degradable plastic according to claim 6, characterized by: The activation is oven treatment.

8. The pasteurization-resistant, non-sticky milk packaging film based on polyester-based degradable plastics according to claim 1, characterized by: The pasteurization-resistant non-sticky milk packaging film has a heat shrinkage rate of 0.6%-2.5% at 85°C, and the residual rate of milk on the pasteurization-resistant non-sticky milk packaging film after storing milk is 0.2%-0.5%.

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