PA-free heat-shrinkable film for food packaging and preparation method and application thereof

Abstract

The application provides a PA-free heat-shrinkable film for food packaging and a preparation method and application thereof, and the PA-free heat-shrinkable film comprises a food contact heat-sealing layer, a reinforced puncture-resistant layer and a surface layer, the reinforced puncture-resistant layer comprises metallocene polyethylene, polyolefin elastomer, surface modified nano master batch, maleic anhydride grafted polyethylene and a first composite antioxidant; wherein the domain size of the polyolefin elastomer is 200-400 nm, the puncture resistance of the PA-free heat-shrinkable film under normal temperature, refrigeration and freezing conditions is all greater than or equal to 15 N; the transverse and longitudinal heat-shrinkage rates are all greater than or equal to 345%; and the heat-shrinkage rate attenuation after storage for 6 months at normal temperature is less than or equal to 5%. The PA-free heat-shrinkable film can be adapted to cold chain packaging of all kinds of protein food through the regulation of a trinity system of accurate regulation of phase structure, orientation-crystallization synergistic anchoring and isoviscous rheological matching, has low cost and is recyclable, and solves the problems of low-temperature brittleness, uneven shrinkage, poor stability and difficult mass production of the existing PA-free film.

Description

Technical Field

[0001] This invention relates to the field of polymer packaging materials for food contact, and particularly to a PA-free heat shrink film for food packaging, its preparation method, and its application. Background Technology

[0002] Heat shrink film, with its excellent adhesion, barrier properties, and preservation properties, has become a core material for vacuum packaging of protein foods. Traditional heat shrink films for protein food packaging mostly rely on polyamide (PA) to improve puncture resistance and dimensional stability. However, PA materials are expensive, difficult to recycle, and have poor interfacial compatibility when co-extruded with polyolefin materials. To address these shortcomings, the industry has gradually developed PA-free heat shrink film technology.

[0003] However, research has found that existing technologies do not have targeted designs for protein food packaging scenarios, and there are currently several unsolvable technical problems: (1) The low-temperature puncture resistance is seriously insufficient. At -18℃, the packaging is easily punctured by bone spikes and shells, resulting in a high breakage rate; (2) The heat shrinkage rate is low and the uniformity is poor, with large differences in the horizontal and vertical directions. Vacuum packaging is prone to air pockets and poor sealing, and residual air accelerates protein oxidation; (3) The long-term stability is extremely poor, the orientation structure is easy to loosen, and the shrinkage rate decreases significantly during the shelf life; (4) The multi-layer co-extrusion interface is unstable, the EVOH and polyolefin melt viscosity do not match, the industrial film breakage rate is high, the layer thickness is uneven, and it is difficult to achieve large-scale stable mass production; (5) It is not resistant to oil contamination and pasteurization, the heat sealing strength is low, and it is easy to delaminate and leak seal, which limits the applicable scenarios.

[0004] Existing technologies are all improvements on single performance aspects, and some PA-free heat shrink film technologies also have problems such as increased low-temperature brittleness due to nanofiller agglomeration, risks of nanomaterial migration, and lack of systematic solutions. Therefore, existing PA-free heat shrink film technologies cannot simultaneously meet the core requirements of full-temperature-range puncture resistance, high uniform shrinkage, long-term stability, food compliance, and industrial mass production, becoming a long-standing unresolved technical bottleneck in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a PA-free heat shrink film for food packaging, its preparation method, and its application. This film not only achieves puncture resistance across the entire temperature range and high uniform shrinkage rate, but also ensures a heat shrinkage rate decrease of ≤5% after 6 months of storage at room temperature; a film breakage rate of ≤1% in continuous industrial production; and a finished product qualification rate of ≥98%. Furthermore, it is suitable for the entire distribution process of protein foods; simultaneously, it reduces the cost of raw materials for protein food packaging by more than 30% and enables single-material recyclability.

[0006] To achieve the above objectives, the present invention provides a PA-free heat shrink film for food packaging. The PA-free heat shrink film has a multi-layer co-extruded structure, which includes, from the inside out, a food contact heat seal layer, a reinforced puncture-resistant layer, and a surface layer. The reinforced puncture-resistant layer includes metallocene polyethylene, polyolefin elastomer, surface-modified nano-masterbatch, maleic anhydride-grafted polyethylene, and a first composite antioxidant. The polyolefin elastomer has a domain size of 200-400nm, and the PA-free heat shrink film has a puncture resistance of ≥15N under normal temperature, refrigeration, and freezing conditions; the PA-free heat shrink film has a longitudinal and transverse heat shrinkage rate of ≥45%, and the longitudinal and transverse shrinkage rate deviation is ≤3%; the PA-free heat shrink film has a heat shrinkage rate decay of ≤5% after 6 months of storage at room temperature.

[0007] As an optional technical solution, the mass fraction of each component in the reinforced puncture-resistant layer is as follows: metallocene polyethylene 60%-70%, polyolefin elastomer 158%-2530%, surface-modified nano masterbatch 3%-6%, maleic anhydride grafted polyethylene 3%-5%, and first composite antioxidant 0.3%-0.8%.

[0008] As an optional technical solution, the metallocene polyethylene has a melt index of 0.8 g / 10 min and a density of 0.918 g / cm³; the polyolefin elastomer has a melt index of 1.0 g / 10 min, a density of 0.870 g / cm³, and a glass transition temperature Tg≈-55℃.

[0009] As an optional technical solution, the surface layer comprises the following components by mass fraction: 60-80% linear low-density polyethylene (LLDPE), 15-30% ethylene-vinyl acetate copolymer, and 3-6% surface-modified nano-masterbatch.

[0010] As an optional technical solution, the food contact heat seal layer comprises the following components by mass fraction: 92%-96% metallocene linear low-density polyethylene, 2%-4% maleic anhydride grafted polyethylene, 0.5%-1.5% oleamide, 0.5%-1.5% micron-sized silica, and 0.3%-0.8% third composite antioxidant.

[0011] As an optional technical solution, in the food contact heat seal layer, the metallocene linear low-density polyethylene has a melt index of 1.0 g / 10 min and a density of 0.915 g / cm³; the maleic anhydride grafted polyethylene has a grafting rate of 1.0% and a melt index of 2.0 g / 10 min.

[0012] As an optional technical solution, the third composite antioxidant includes a primary antioxidant 1010, a secondary antioxidant 168, and a long-lasting antioxidant 1076, wherein the primary antioxidant 1010 accounts for 0.1%-0.3% of the mass of the third composite antioxidant, the secondary antioxidant 168 accounts for 0.1%-0.3% of the mass of the third composite antioxidant, and the long-lasting antioxidant 1076 accounts for 0.1%-0.2% of the mass of the third composite antioxidant.

[0013] As an optional technical solution, the third composite antioxidant has the same composition as the first composite antioxidant.

[0014] As an optional technical solution, the preparation method of the surface-modified nanomasterbatch includes the following steps: (1) Hydrolysis of coupling agent: Anhydrous ethanol and deionized water are mixed at a volume ratio of 9:1, the pH is adjusted to 4-5 with glacial acetic acid, silane coupling agent KH570 is added, and the mixture is stirred at 50°C for 30 min to obtain hydrolysate; (2) Nanoparticle modification: Add nano-calcium carbonate or nano-silica to the hydrolysate at a solid-liquid ratio of 1:10, ultrasonically disperse at 60°C for 30 min, filter, and vacuum dry at 105°C for 2 h to obtain modified nanoparticles. (3) Masterbatch preparation: The modified nanoparticles and maleic anhydride grafted polyethylene are mixed at a mass ratio of 1:4 and granulated by extrusion through a twin-screw extruder. The extrusion temperature is 130-150℃, the screw speed is 300rpm, and the feeding speed is 15kg / h to obtain surface-modified nano masterbatch.

[0015] As an optional technical solution, an adhesive layer is also included, which is located between the food contact heat seal layer and the reinforced puncture-resistant layer, and the adhesive layer comprises the following components by mass fraction: 98%-99.5% hydrolysis-resistant maleic anhydride-grafted polyethylene, 0.5%-2% epoxy chain extender, the grafting rate of the hydrolysis-resistant maleic anhydride-grafted polyethylene is 1.2%-1.8%, and the melt index is 1.5g / 10min.

[0016] As an optional technical solution, a high-barrier barrier layer is also included, which is located between the adhesive layer and the food contact heat-sealing layer. The high-barrier barrier layer includes 10%-18% by mass of an ethylene-vinyl alcohol copolymer, wherein the ethylene molar content in the ethylene-vinyl alcohol copolymer is 32%-3844%, and the melt index of the ethylene-vinyl alcohol copolymer is 3.2-3.8 g / 10 min. The present invention also provides a method for preparing the PA-free heat-shrinkable film for food packaging as described above, the method comprising the following steps: S1, Raw material drying pretreatment: Dry each layer of raw materials to ensure that the moisture content of all raw materials is controlled below 50ppm, and weigh each layer of raw materials according to the formula ratio and mix them evenly; S2, Single-stage blown film: A multi-layer co-extrusion extruder is used to melt and extrude multiple layers of melt, which are then blown into a single-stage bubble film. The thickness of the single-stage bubble film is controlled at 60-100μm. The multi-layer co-extrusion extruder is equipped with a temperature transition channel in the die head, and the viscosity difference of each layer of melt in the confluence section is controlled by temperature gradient ≤±30%. S3, Primary gradient cooling and shaping: The primary bubble membrane is cooled and shaped using a gradient cooling process to control the crystallinity of the primary bubble membrane to be between 35% and 45%; S4, Secondary blown film: The temperature of the primary blown film is heated to the high elasticity range below the melting point of the polymer, and then stretched and blown a second time. The ratio of transverse to longitudinal stretching is controlled to be 0.9-1.1. After secondary stretching, a second film is formed with a thickness of 25-50 μm. S5, Secondary cooling and post-processing: The second tubular membrane is subjected to secondary cooling and post-processing to obtain the PA-free high puncture-resistant heat shrink film for food packaging.

[0017] As an optional technical solution, the PA-free heat shrink film further includes an adhesive layer located between the food contact heat seal layer and the reinforced puncture-resistant layer. The adhesive layer comprises the following components by mass fraction: 98%-99.5% hydrolysis-resistant maleic anhydride-grafted polyethylene and 0.5%-2% epoxy chain extender. The grafting rate of the hydrolysis-resistant maleic anhydride-grafted polyethylene is 1.2%-1.8%, and the melt index is 1.5 g / 10 min.

[0018] As an optional technical solution, the PA-free heat shrink film further includes a high-barrier barrier layer, which is located between the adhesive layer and the food contact heat seal layer. The high-barrier barrier layer includes 10%-18% by mass of ethylene-vinyl alcohol copolymer, wherein the ethylene molar content in the ethylene-vinyl alcohol copolymer is 32%-3844%, and the melt index of the ethylene-vinyl alcohol copolymer is 3.2-3.8 g / 10 min. As an optional technical solution, in step S1, the ethylene-vinyl alcohol copolymer is vacuum dried at 85-95℃ for 4-6 hours, and the polyolefin elastomer is hot-air dried at 70-80℃ for 2-3 hours.

[0019] As an optional technical solution, in step S2, the extrusion temperature of the reinforced puncture-resistant layer is 135-155℃, the extrusion temperature of the adhesive layer is 145-160℃, and the extrusion temperature of the high barrier layer is 180-195℃.

[0020] As an optional technical solution, in step S2, the length of the temperature transition channel is 150 mm, the temperature gradient of the temperature transition channel is 190℃→170℃→155℃, the confluence temperature of the confluence section is 155℃, and the shear rate of the confluence section is 500 s. - ¹.

[0021] As an optional technical solution, in step S2, the inflation ratio of the first inflation is 2.0-3.0, the traction speed is 25-40m / min, the longitudinal pre-stretch ratio is 1.5-2.0, and the transverse thickness deviation of the first bubble membrane is ≤±3%.

[0022] As an optional technical solution, in step S3, the gradient cooling process is as follows: first, the temperature is rapidly reduced by air cooling at 20-25℃ through an annular air ring with a wind speed of 4-6m / s and an air ring vent angle of 30°; then, the temperature is slowly reduced by water cooling at 15-20℃ through a constant temperature water cooling tank with a length of 2m, the traction speed is matched with the water cooling path, and the cooling rate is controlled at 10-15℃ / s.

[0023] As an optional technical solution, in step S4, the transverse inflation ratio is 3.0-4.0, the longitudinal stretching ratio is 2.0-2.5, and the thickness deviation of the second membrane is ≤±2μm.

[0024] As an optional technical solution, in step S5, the secondary cooling is to use a 18-22℃ air ring to rapidly cool to room temperature; The post-processing includes critical corona treatment and gradient tension winding. The parameters for critical corona treatment are: electrode gap of 2 mm, processing linear speed synchronized with traction speed, power density controlled at 2-3 W·min / m², surface tension of the film after treatment ≥38 dyn / cm, and surface tension ≥36 dyn / cm after 6 months of storage. The gradient tension winding uses a winding tension that decreases linearly from the inside to the outside by 3-8 N.

[0025] This invention also provides a preparation apparatus for the preparation method of the PA-free heat-shrinkable film for food packaging as described above. The preparation apparatus includes an extrusion system, a die system, a stretching system, a cooling system, and a winding system arranged along a production line. The extrusion system is a barrier-type compounding screw extruder used to output a uniformly mixed multilayer melt. The length-to-diameter ratio of the screw is 28:1. The extrusion system adopts a PID closed-loop temperature control unit with a temperature control error of ≤±1℃. The die head system is used for multi-layer melt fusion molding. The viscosity of each layer of melt is matched by temperature gradient control to output a primary bubble tube film. The die head system adopts a multi-layer co-extrusion spiral die head. The temperature transition channel is provided in the die head. The die head gap can be adjusted within a range of 0.8-2.0 mm. The stretching system adopts a double-bubble structure to achieve gradient stretching and biaxial orientation of the primary bubble membrane and output a secondary bubble membrane. The stretching system has a multi-segment independently temperature-controlled infrared heating box and a food-grade bubble stabilizing rack. The servo motor of the stretching system synchronously controls the stretching traction roller, with a speed error ≤ ±0.5m / min. The cooling system is used for gradient cooling and shaping of primary bubble membranes and rapid cooling of secondary bubble membranes, controlling the crystallinity and molecular orientation of the membrane. The winding system is used for the smooth winding of the secondary tubular membrane. The winding system adopts a tension closed-loop control gradient winding machine for gradient tension winding.

[0026] The present invention also provides an application of the PA-free heat shrink film for food packaging as described above in the packaging of protein foods, wherein the protein foods include fresh livestock and poultry meat, aquatic protein, soy products, plant protein meat, and cooked protein foods.

[0027] Compared with the prior art, the technical solution of the present invention has the following technical effects: (1) Breakthrough in puncture resistance across the entire temperature range: Through the precise control of the ternary synergistic toughening system and phase structure, the PA-free heat shrink film of the present invention can achieve a puncture resistance of ≥15N at room temperature / refrigeration (4℃) / freezing (-18℃), and the damage rate of cold chain transportation packaging is greatly reduced; (2) High uniform shrinkage rate and long-term stability: Through the orientation-crystallization synergistic anchoring process, the PA-free heat shrink film of the present invention can achieve a heat shrinkage rate of ≥35% in both the transverse and longitudinal directions, and a shrinkage rate deviation of ≤3%; the shrinkage rate decay is ≤5% after 6 months of storage at room temperature, which completely solves the problem of orientation relaxation. (3) Significantly improved stability in industrial mass production: Through the design of equal viscosity rheological matching, the PA-free heat shrink film of the present invention can be continuously produced in the industrial industry for 72 hours with a film breakage rate of ≤1% and a finished product qualification rate of ≥98%, which solves the core pain point of industrial production of PA-free multilayer film. (4) Full-process adaptability to protein food packaging scenarios: anti-protein / oil contamination heat seal layer, heat seal strength retention rate under meat juice contamination ≥75%; after pasteurization at 85℃ for 30 min, the interlayer peel strength retention rate ≥80%, no delamination or leakage; the shelf life of fresh and chilled meat can be extended to 17 days when refrigerated at 0-4℃, and the shelf life of soy products can be extended by more than 2 times. (5) Food compliance and cost advantages: The full formula of the PA-free heat shrink film of the present invention complies with the GB 4806 series standards, the migration amount of nanoparticles is <0.01mg / kg, and there is no food safety risk; the PA material is eliminated, the raw material cost is reduced by more than 30% compared with the traditional PA type packaging film, and the polyolefin body of the whole film can be recycled as a single material, which is in line with the environmental protection trend. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the structure of a PA-free heat-shrinkable film for food packaging according to an embodiment of the present invention; Figure 2 This is a schematic flowchart illustrating a method for preparing a PA-free heat-shrinkable film for food packaging according to an embodiment of the present invention. Detailed Implementation

[0030] To provide a further understanding of the purpose, structure, features, and functions of the present invention, detailed descriptions are provided below with reference to specific embodiments.

[0031] In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0032] Please see Figure 1 , Figure 1 This is a schematic diagram of the structure of a PA-free heat-shrinkable film for food packaging according to an embodiment of the present invention. The present invention provides a PA-free heat-shrinkable film for food packaging, wherein the PA-free heat-shrinkable film has a multi-layer co-extruded structure, comprising, from the inside out, a food contact heat-sealing layer 1, a reinforced puncture-resistant layer 2, and a surface layer 3. The reinforced puncture-resistant layer 2 comprises metallocene polyethylene (mPE), polyolefin elastomer (POE), surface-modified nano-masterbatch, maleic anhydride-grafted polyethylene (PE-g-MAH), and a first composite antioxidant. The polyolefin elastomer has a phase domain size of 200-400 nm, and the PA-free heat-shrinkable film exhibits a puncture resistance ≥15N under normal temperature (23℃), refrigeration (4℃), and freezing (-18℃) conditions, as tested with a 1mm standard needle. The PA-free heat-shrinkable film has a transverse and longitudinal heat shrinkage rate ≥345%, with a transverse and longitudinal shrinkage rate deviation ≤3%. The PA-free heat-shrinkable film exhibits a heat shrinkage rate decay of ≤5% after 6 months of storage at normal temperature (23℃ / 50% RH).

[0033] Preferably, the mass fractions of each component in the reinforced puncture-resistant layer 2 are: 60%-70% metallocene polyethylene, 158%-2530% polyolefin elastomer, 3%-6% surface-modified nano-masterbatch, 3%-5% maleic anhydride-grafted polyethylene, and 0.3%-0.8% first composite antioxidant. Preferably, the metallocene polyethylene is food-grade high-toughness metallocene polyethylene, and the metallocene polyethylene has a melt index of 0.8 g / 10 min, a density of 0.918 g / cm³, and exhibits tensile strain hardening characteristics. The polyolefin elastomer has a melt index of 1.0 g / 10 min, a density of 0.870 g / cm³, and a glass transition temperature Tg≈-55℃. The reinforced puncture-resistant layer 2 adopts a ternary toughening system of mPE + POE + surface-modified nano-masterbatch + compatibilizer. High-toughness mPE is used as the continuous phase and POE is used as the dispersed toughening phase. The size of the polyolefin elastomer phase domain is controlled at 200-400nm by the compatibilizer PE-g-MAH, which is completely matched with the critical crazing size of the mPE matrix. The surface-modified nano-masterbatch can further enhance and inhibit the crazing expansion. Thus, through the crazing-shear band synergistic toughening mechanism, it can achieve high-efficiency toughening at -18℃ and still maintain a puncture resistance strength of ≥15N, thus solving the problem of low-temperature brittleness of polyolefins.

[0034] In addition, PA-free heat shrink film means that there is no PA material in the entire heat shrink film product, which reduces the raw material cost by more than 30% compared with traditional PA-type packaging film. The whole film polyolefin body can be recycled as a single material, which is in line with the trend of environmental protection.

[0035] In one embodiment, the surface layer comprises the following components by mass fraction: 60-80% linear low-density polyethylene (LLDPE), 15-30% ethylene-vinyl acetate copolymer, and 3-6% surface-modified nano-masterbatch.

[0036] In one embodiment, the food contact heat-sealing layer 1 is a layer that directly contacts protein foods. The food contact heat-sealing layer 1 uses metallocene linear low-density polyethylene (mLLDPE) as a matrix, compounded with maleic anhydride-grafted polyethylene (PE-g-MAH), oleamide, micronized silica, and a composite antioxidant. PE-g-MAH improves the wetting and spreading properties of proteins / oils, solving the problem of incomplete sealing caused by meat juice contamination. Furthermore, micronized silica provides anti-sticking properties, enabling the food contact heat-sealing layer 1 to achieve heat sealing against protein / oil contamination, oxidation resistance, and pasteurization resistance. Preferably, the food contact heat-sealing layer 1 comprises the following components by mass fraction: 92%-96% metallocene linear low-density polyethylene (mLLDPE), 2%-4% PE-g-MAH, 0.5%-1.5% oleamide, 0.5%-1.5% micronized silica, and 0.3%-0.8% a third composite antioxidant. In the food contact heat seal layer 1, the melt index of mLLDPE is 1.0 g / 10 min and the density is 0.915 g / cm³; the grafting rate of PE-g-MAH is 1.0% and the melt index is 2.0 g / 10 min.

[0037] The third composite antioxidant comprises a primary antioxidant 1010, a secondary antioxidant 168, and a long-lasting antioxidant 1076. The primary antioxidant 1010 accounts for 0.1%-0.3% of the total mass of the third composite antioxidant, the secondary antioxidant 168 accounts for 0.1%-0.3% of the total mass of the third composite antioxidant, and the long-lasting antioxidant 1076 accounts for 0.1%-0.2% of the total mass of the third composite antioxidant. All the above-mentioned amounts comply with the limits required by GB 4806.6-2016. The aforementioned third composite antioxidant can withstand at least two high-temperature extrusions, pasteurization, and long-term cold chain oxidation.

[0038] In one embodiment, the first composite antioxidant in the reinforced puncture-resistant layer 2, the second composite antioxidant in the surface layer 3, and the third composite antioxidant in the food contact heat seal layer 1 have the same composition.

[0039] In one embodiment, the preparation method of the surface-modified nanomasterbatch includes the following steps: (1) Hydrolysis of coupling agent: Anhydrous ethanol and deionized water are mixed at a volume ratio of 9:1, the pH is adjusted to 4-5 with glacial acetic acid, silane coupling agent KH570 is added, and the mixture is stirred at 50°C for 30 min to obtain hydrolysate; (2) Nanoparticle modification: Add nano-calcium carbonate or nano-silica to the hydrolysate at a solid-liquid ratio of 1:10, ultrasonically disperse at 60°C (power 300W, frequency 40kHz) for 30min, filter, and vacuum dry at 105°C for 2h to obtain modified nanoparticles. (3) Masterbatch preparation: The modified nanoparticles and maleic anhydride-grafted polyethylene are mixed at a mass ratio of 1:4 and granulated by extrusion through a twin-screw extruder. The extrusion temperature is 130-150℃, the screw speed is 300rpm, and the feeding speed is 15kg / h to ensure that the nanoparticles are monodisperse and do not agglomerate, thus obtaining surface-modified nano masterbatch.

[0040] Furthermore, the PA-free heat-shrinkable film also includes an adhesive layer 4, which is located between the food contact heat-sealing layer 1 and the reinforced puncture-resistant layer 2. The adhesive layer 4 comprises the following components by mass fraction: 98%-99.5% hydrolyzable maleic anhydride-grafted polyethylene, 0.5%-2% epoxy chain extender, with a grafting rate of 1.2%-1.8% and a melt flow index of 1.5 g / 10 min for the hydrolyzable maleic anhydride-grafted polyethylene. In this embodiment, the use of hydrolyzable PE-g-MAH combined with a small amount of epoxy chain extender significantly improves the interlayer bonding force under wet heat and low-temperature cycling, ensuring an interlayer peel strength retention rate of ≥80%, thereby improving interlayer peel strength and water boiling stability.

[0041] Furthermore, the PA-free heat-shrink film also includes a high-barrier barrier layer 5, which is located between the adhesive layer 4 and the food contact heat-sealing layer 1, that is, the adhesive layer is located between the high-barrier barrier layer 5 and the reinforced puncture-resistant layer 2. The high-barrier barrier layer 5 comprises 10%-18% by mass of ethylene-vinyl alcohol copolymer (EVOH), in which the ethylene molar content is 32%-4438%. The high-barrier barrier layer 5 allows for precise control of oxygen barrier performance, ensuring an oxygen permeability ≤10cm³ / (m²·24h·0.1MPa), thus delaying protein oxidation and deterioration. Moreover, by using food-grade EVOH with different ethylene contents, the oxygen barrier properties can be adjusted according to product requirements. For example, a 32mol% ethylene content can be used for high-oxygen-barrier fresh food packaging, while a 3844mol% ethylene content can be used for moisture-resistant bean product packaging. In addition, the melt index of the ethylene-vinyl alcohol copolymer is 3.2-3.8-3.8 g / 10 min.

[0042] Moreover, the above-mentioned adhesive layer structure can solve the problem of interfacial delamination between EVOH (high barrier layer) and polyolefin elastomer (reinforced puncture-resistant layer) under humid heat / low temperature cycling, and achieve an interlayer peel force retention rate of ≥80% after boiling in water at 85°C for 30 minutes.

[0043] Furthermore, all raw materials used in this invention are food contact grade and comply with requirements such as GB 4806.6 and GB 4806.7.

[0044] Please see Figure 2 , Figure 2This is a schematic flowchart of a method for preparing a PA-free heat-shrinkable film for food packaging according to an embodiment of the present invention. The present invention also provides a method for preparing the PA-free heat-shrinkable film for food packaging as described above, the method comprising the following steps: S1, Raw material drying pretreatment: Dry each layer of raw materials to ensure that the moisture content of all raw materials is controlled below 50ppm, and weigh each layer of raw materials according to the formula ratio and mix them evenly; strictly control the moisture content to avoid squeezing out air bubbles, degradation and film breakage.

[0045] S2, One-time blown film: Multi-layer melt is obtained by melt extrusion using a multi-layer co-extrusion extruder, and a single-layer bubble film is obtained by one-time blown expansion. The thickness of the single-layer bubble film is controlled at 60-100μm. The multi-layer co-extrusion extruder is equipped with a temperature transition channel in the die head. The viscosity difference of each layer of melt in the confluence section is controlled by temperature gradient ≤±30%, that is, equal viscosity matching is achieved, thereby solving the problems of unstable interface and uneven layer thickness from the root. S3, First-stage gradient cooling and shaping: The first-stage bubble membrane is cooled and shaped using a gradient cooling process, and the crystallinity of the first-stage bubble membrane is controlled at 35%-45%. Microcrystals are used as physical cross-linking points to anchor the pre-oriented structure, taking into account both high shrinkage and long-term stability. S4, Secondary blown film: The temperature of the primary blown film is heated to the high elasticity range below the melting point of the polymer, and secondary blowing and stretching are performed. The ratio of transverse to longitudinal stretching is controlled to be 0.9-1.1 to form a second film. The thickness of the second film is 25-50 μm, and the thickness deviation is ≤±2 μm. S5, Secondary cooling and post-processing: The second tubular membrane is subjected to secondary cooling and post-processing to obtain the PA-free high puncture-resistant heat shrinkable film.

[0046] In one embodiment, in step S1, the ethylene-vinyl alcohol copolymer is vacuum dried at 85-95°C for 4-6 hours, and the polyolefin elastomer is hot-air dried at 70-80°C for 2-3 hours.

[0047] In one embodiment, in step S2, the extrusion temperature of the reinforced puncture-resistant layer is 135-155°C, the extrusion temperature of the adhesive layer is 145-160°C, and the extrusion temperature of the high-barrier barrier layer is 180-195°C.

[0048] In one embodiment, in step S2, the length of the temperature transition channel is 150 mm, the temperature gradient of the temperature transition channel is 190℃→170℃→155℃, the confluence temperature of the confluence section is 155℃, that is, the melt of the high barrier layer is cooled to 155℃ before entering the confluence section, and the melt of the reinforced puncture-resistant layer is heated to 155℃ before entering the confluence section; the shear rate of the confluence section is 500 s. -¹ Under these conditions, the viscosity difference of each layer of melt can be controlled within ±30%, which can completely solve the problem of interface instability.

[0049] In one embodiment, in step S2, the inflation ratio of the first inflation is 2.0-3.0, the traction speed is 25-40 m / min, the longitudinal pre-stretch ratio is 1.5-2.0, and the transverse thickness deviation of the first bubble membrane is ≤±3%.

[0050] In one embodiment, in step S3, the gradient cooling process is as follows: first, rapid cooling is achieved through an annular air cooling system at 20-25°C, with a wind speed of 4-6 m / s and an air outlet angle of 30°; then, slow cooling is achieved through a 304 stainless steel constant-temperature water cooling bath at 15-20°C, with the water cooling bath length being 2 m, the traction speed matching the water cooling path, and the cooling rate controlled at 10-15°C / s. These process conditions enable precise control of the film (primary bubble tube film) crystallinity within 35%-45%. This range ensures both the microcrystalline physical cross-linking anchors the orientation structure, preventing long-term relaxation, and sufficient amorphous region orientation to achieve a high thermal shrinkage rate.

[0051] In step S4, a segmented infrared heating box is used, with multiple segments (e.g., 6 segments) for independent temperature control, to heat the second membrane to a high-elasticity state 10-15°C below the polymer melting point: pure polyolefin system. Temperatures are controlled at 105-115℃, or 115-125℃ for systems containing EVOH, with a temperature control accuracy of ±1℃, to avoid disorientation or stretching that could cause film breakage. Simultaneously, the transverse blow-up ratio is controlled at 3.0-4.0, the longitudinal stretching ratio at 2.0-2.5, and the transverse-to-longitudinal stretching ratio at 0.9-1.1, achieving isotropic orientation in both directions and ensuring a shrinkage deviation of ≤3%.

[0052] In one embodiment, during secondary cooling, a 18-22°C air ring is used to rapidly cool to room temperature, and the orientation structure is anchored by rapid microcrystal growth.

[0053] In addition, the post-processing includes critical corona treatment and gradient tension winding. The critical corona treatment parameters are: electrode gap 2mm, processing linear speed synchronized with traction speed, power density controlled at 2-3W·min / m², film surface tension ≥38dyn / cm after treatment, and surface tension ≥36dyn / cm after 6 months of storage. The gradient tension winding tension decreases linearly from the inside to the outside by 3-8N. Corona treatment and gradient tension winding further fix the orientation structure, improve printing and lamination adaptability, and avoid wrinkling and deformation during winding.

[0054] Furthermore, the present invention also provides a preparation apparatus for the preparation method of the PA-free heat shrink film for food packaging as described above. The preparation apparatus includes an extrusion system, a die system, a stretching system, a cooling system, and a winding system arranged sequentially along a production line. The extrusion system is a barrier-type compounding screw extruder used to output a uniformly mixed multilayer melt. The screw has a length-to-diameter ratio of 28:1. The extrusion system includes three compounding sections and adopts a PID closed-loop temperature control unit with a temperature control error of ≤±1℃. Moreover, the extrusion system includes at least five independent temperature-controlled extruders, corresponding to the extrusion of the surface layer, the reinforced puncture-resistant layer, the adhesive layer, the high-barrier barrier layer, and the food contact heat-sealing layer, respectively. The die system is used for multi-layer melt merging and molding. It controls the viscosity matching of each melt layer through temperature gradient control to output a primary bubble film. The die system adopts a multi-layer co-extrusion spiral die, with a 150mm temperature transition channel inside. The die gap is adjustable from 0.8 to 2.0mm. The channel has a temperature gradient of 190℃→170℃→155℃ along the melt flow direction, ensuring a shear rate of 500 s⁻¹ in the merging section. - ¹The viscosity difference between each layer of melt is ≤±30%.

[0055] The stretching system adopts a double-bubble structure to achieve gradient stretching and biaxial orientation of the primary bubble membrane and output a secondary bubble membrane. The stretching system has multiple (e.g., 6-segment) independently temperature-controlled infrared heating chambers and a food-grade bubble stabilizing rack. The servo motor of the stretching system synchronously controls the stretching traction rollers with a speed error ≤ ±0.5m / min.

[0056] The cooling system is used for gradient cooling and shaping of primary bubble membranes and rapid cooling of secondary bubble membranes, controlling the crystallinity and molecular orientation of the membrane. The winding system is used for the smooth winding of the secondary tubular membrane. The winding system adopts a tension closed-loop control gradient winding machine to perform gradient tension winding. The winding tension decreases linearly from the inside to the outside at 3 to 8 N. Combined with the corona treatment system, the surface tension is ≥38 dyn / cm.

[0057] The entire preparation equipment is connected by a servo synchronous traction system, with a traction speed error of ≤±0.5 m / min, which is suitable for industrial continuous production with a die head diameter ≥600 mm and a linear speed ≥50 m / min.

[0058] Furthermore, this invention also provides an application of the PA-free heat-shrink film for food packaging as described above in the packaging of protein foods, including fresh livestock and poultry meat, aquatic protein, soy products, plant-based meat substitutes, and cooked protein foods. The application scenarios include vacuum heat-shrink packaging, cold chain transportation packaging, and pasteurization packaging. Fresh chilled meat has a shelf life of ≥17 days when refrigerated at 0-4℃, and the shelf life of soy products is extended by more than 2 times.

[0059] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0060] All ingredients used in the embodiments are food contact grade and comply with GB 4806 series standards.

[0061] The performance tests conducted in this invention strictly adhere to national / industry standards. Three parallel experiments were set up, and the results were averaged. The relative standard deviation (RSD ≤ 3%) was noted to ensure the data is authentic and repeatable. Details are as follows: (1) Basic properties: heat shrinkage rate GB / T 13519-2016, puncture resistance GB / T 37841-2019, oxygen permeability GB / T 31354-2014, interlayer peel force GB / T 8808-1988; (2) Microstructure: Crystallinity DSC test, orientation WAXD test, and domain size SEM test; (3) Long-term stability: 85℃ boiling water for 30 min damp heat test, -18℃~25℃ cycle 10 low temperature test, 23℃ / 50%RH storage for 6 months shelf life test; (4) Food compliance: GB 31604 series migration test, the food simulants are 4% acetic acid, 50% ethanol and olive oil; (5) Industrialization verification: The 600mm die head industrial production line was continuously tested for 72 hours to test the film breakage rate and finished product qualification rate.

[0062] Example 1: High-barrier film for fresh and chilled meat (mPE mass fraction 60%, POE mass fraction 15%) (1) Composition of each layer of PA-free heat shrink film (total mass 100 parts): Surface layer: 65 parts LLDPE, 30 parts ethylene-vinyl acetate copolymer, and 5 parts surface-modified nano masterbatch; Reinforced puncture-resistant layer: 76.5 parts mPE, 15 parts POE, 5 parts modified nano-calcium carbonate, 3 parts PE-g-MAH, and 0.5 parts composite antioxidant; Adhesive layer: 99.5 parts of hydrolysis-resistant PE-g-MAH, 0.5 parts of epoxy chain extender; High-barrier layer: 100 parts of EVOH (containing 32 mol% ethylene); Food contact heat seal layer: 87 parts mLLDPE, 109.5 parts PE-g-MAH, 1.5 parts oleamide, 1.5 parts micron silica, and 0.5 parts composite antioxidant. [3] (2) Preparation method: The film is prepared using the method of the present invention for preparing PA-free heat shrink film for food packaging. The specific process parameters are as follows: One-time blown film: polyolefin layer 145℃, adhesive layer 150℃, EVOH layer 190℃, die head confluence section temperature 155℃, viscosity difference between layers 8.3%; blow-up ratio 2.5, traction speed 35m / min, one-time bubble thickness 80μm; One-step gradient cooling for shaping: air cooling at 23℃ / wind speed 5m / s, water cooling at 18℃, cooling rate 12℃ / s, crystallinity 38.2%; Secondary blown film: heating temperature 120℃, blow-up ratio 3.5, traction speed 55m / min, longitudinal stretch ratio 2.2, transverse and longitudinal stretch ratio 1.0, final thickness 35μm; Secondary cooling and post-processing: Rapid cooling from 20℃ to room temperature using a 2.5W·min / m² air ring, with a gradient tension of 3-6N during winding.

[0063] (3) Core performance test results: Shrinkage rate: lateral shrinkage rate 37.2%, longitudinal shrinkage rate 36.8%, shrinkage rate deviation 0.4%; shrinkage rate decrease of 3.1% after 6 months; Puncture resistance: 19.2N at room temperature, 18.5N at 4℃, and 17.8N at -18℃; Oxygen permeability: 4.5 cm³ / (m²·24h·0.1 MPa); initial peel strength: 3.6 N / 15 mm; retention rate after boiling in water: 83%. Food compliance: All migration levels meet national standards, with nanoparticle migration <0.01mg / kg; Industrial pilot production: 0.8% film breakage rate and 98.6% finished product qualification rate in 72 hours of continuous production.

[0064] Example 2: Low-temperature resistant film for bone-in poultry meat (mPE mass fraction 65%, POE mass fraction 20%) (1) Composition of each layer of PA-free heat shrink film (total mass 100 parts): Surface layer: 78 parts LLDPE, 20 parts ethylene-vinyl acetate copolymer, and 2 parts surface-modified nano masterbatch; Reinforced puncture-resistant layer: 68 parts mPE, 22 parts POE, 4.5 parts modified nano silica, 5 parts PE-g-MAH, and 0.5 parts composite antioxidant; Food contact heat seal layer: 85 parts mLLDPE, 1312 parts PE-g-MAH, 1.5 parts oleamide, 0.5 parts micron silica, and 1 part composite antioxidant. [4] (2) Preparation method: The film is prepared using the method of the present invention for preparing PA-free heat shrink film for food packaging. The specific process parameters are as follows: One-stage blown film: extrusion temperature 140℃, blow-up ratio 2.2, traction speed 30m / min, one-stage bubble thickness 70μm; One-step gradient cooling for shaping: air cooling at 22℃ / wind speed 4.5m / s, water cooling at 16℃, crystallinity 40.5%; Secondary blown film: heating temperature 110℃, blow-up ratio 3.2, traction speed 50m / min, longitudinal stretch ratio 2.1, transverse and longitudinal stretch ratio 0.95, final thickness 30μm; Secondary cooling and post-processing: Rapid cooling to room temperature using an 18℃ air ring, corona power density of 2W·min / m², and winding with gradient tension of 3-6N.

[0065] (3) Core performance test results: Shrinkage rate: lateral shrinkage rate 35.8%, longitudinal shrinkage rate 35.2%, shrinkage rate deviation 0.6%; shrinkage rate decrease of 3.7% after 6 months; Puncture resistance: 18.9N at room temperature, 18.1N at 4℃, and 17.5N at -18℃; After 10 cycles of low temperature cycling, the puncture resistance retention rate was 93%, and the cold chain transportation damage rate was 0.6%.

[0066] Example 3: Pasteurization membrane (mPE mass fraction 70%, POE mass fraction 25%) (1) Composition of each layer of PA-free heat shrink film (total mass 100 parts): Surface layer: 78 parts LLDPE, 20 parts ethylene-vinyl acetate copolymer, 2 parts surface-modified nano masterbatch Reinforced puncture-resistant layer: 65.5 parts mPE, 25 parts POE, 6 parts modified nano-calcium carbonate, 3 parts PE-g-MAH, and 0.5 parts composite antioxidant; Adhesive layer: 99.5 parts of hydrolysis-resistant PE-g-MAH, 0.5 parts of epoxy chain extender; Barrier layer: 100 parts of EVOH (38 mol% ethylene); Food contact heat seal: 85 parts mLLDPE, 142.5 parts PE-g-MAH, 1 part oleamide, 1 part micron silica, and 0.5 parts composite antioxidant. [5] (2) Preparation method: The film is prepared using the method of the present invention for preparing PA-free heat shrink film for food packaging. The specific process parameters are as follows: One-stage blown film production: polyolefin layer 140℃, adhesive layer 150℃, EVOH layer 190℃, confluence section temperature 155℃; blow-up ratio 2.3, traction speed 32m / min, one-stage bubble thickness 75μm. Single-stage gradient cooling for shaping: air cooling at 25℃ / wind speed 5m / s, water cooling at 17℃, crystallinity 42.1%. Secondary blown film: heating temperature 115℃, blow-up ratio 3.3, traction speed 52m / min, longitudinal stretch ratio 2.15, transverse and longitudinal stretch ratio 1.05, final thickness 32μm; Secondary cooling and post-processing: Rapid cooling to room temperature using a 22℃ air ring, corona power density of 3W·min / m², and winding with gradient tension of 3-6N.

[0067] (3) Core performance test results: Shrinkage rate: lateral shrinkage rate 34.7%, longitudinal shrinkage rate 34.2%, shrinkage rate deviation 0.5%; shrinkage rate decrease of 4.1% after 6 months; Puncture resistance: 17.9N at room temperature, 17.3N at 4℃; After boiling at 85℃ for 30 minutes, the peeling force retention rate is 85%, the shrinkage rate changes by 3.4%, and the shelf life of soy products at 0-4℃ is 32 days.

[0068] Comparative Example 1: Commercially available PA-free heat shrink film.

[0069] Comparative Example 2: Implementation Example of Rheological Matching Design Using the composition of Embodiment 1 of the present invention, only the temperature transition channel of the die head is eliminated, and the barrier layer (EVOH) and the reinforced puncture-resistant layer are directly merged at 145°C, while the rest of the process remains unchanged.

[0070] Comparative Example 3: Examples of Non-Oriented Crystalline Anchoring Design Using the composition of Embodiment 1 of the present invention, only one gradient cooling process is eliminated. The first cooling is direct rapid cooling with water at 15°C, resulting in a crystallinity of 58%. The rest of the process remains unchanged.

[0071] Table 1. Performance Comparison of Each Embodiment and Comparative Example As can be seen from Table 1, all embodiments of the present invention have achieved the core objectives of "puncture resistance ≥15N across the entire temperature range, longitudinal and transverse shrinkage rates ≥35%, and thermal shrinkage rate attenuation ≤5% after 6 months". Moreover, the comparison between Example 1 and Comparative Example 2 shows that the isoviscosity rheological matching design of the present invention is a core necessary technical feature for stable industrial production. The comparison between Example 1 and Comparative Example 3 proves that the orientation-crystallization synergistic anchoring process of the present invention is a core necessary technical feature for balancing shrinkage rate and long-term stability.

[0072] In summary, the PA-free heat shrink film of this invention achieves a puncture resistance strength of ≥15N at room temperature / 4℃ / -18℃ through a ternary synergistic toughening system and precise phase structure control, significantly reducing the damage rate of cold chain transportation packaging. Through an orientation-crystallization synergistic anchoring process, it achieves a longitudinal and transverse heat shrinkage rate of ≥345%, with a shrinkage deviation of ≤3%. After 6 months of storage at room temperature, the shrinkage rate decreases by ≤5%, completely solving the orientation relaxation problem. Through isoviscosity rheological matching design, the film breakage rate is ≤1% and the finished product qualification rate is ≥98% during continuous 72-hour industrial production, addressing the core pain points of PA-free multilayer film industrial production. Furthermore, the PA-free heat shrink film of this invention exhibits a heat seal strength retention rate of ≥75% under protein / oil contamination and meat juice contamination; an interlayer peel strength retention rate of ≥80% after 30 minutes of pasteurization at 85℃, with no delamination or leaks; and extends the shelf life of fresh chilled meat to 17 days under 0-4℃ refrigeration, and more than doubles the shelf life of soy products, making it suitable for protein food packaging scenarios throughout the entire process. Furthermore, the complete formulation or composition of the PA-free heat shrink film of the present invention complies with the GB 4806 series standards, with nanoparticle migration amount <0.01mg / kg, posing no food safety risk; by eliminating PA materials, the raw material cost is reduced by more than 30% compared with traditional PA-type packaging films, and the all-film polyolefin body can be recycled as a single material, which is in line with the trend of environmental protection.

[0073] The present invention has been described by the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. Furthermore, the technical features involved in the different embodiments of the present invention described above can be combined with each other as long as they do not conflict with each other. It must be pointed out that the disclosed embodiments do not limit the scope of the present invention. On the contrary, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A PA-free heat-shrink film for food packaging, characterized in that, The PA-free heat shrink film is a multi-layer co-extruded structure, which includes, from the inside out, a food contact heat seal layer, a reinforced puncture-resistant layer, an adhesive layer, a barrier layer [1] and a surface layer. The reinforced puncture-resistant layer includes metallocene polyethylene, polyolefin elastomer, surface-modified nano-masterbatch and a first composite antioxidant. The polyolefin elastomer has a domain size of 200-400nm, and the PA-free heat shrink film has a puncture resistance of ≥15N under normal temperature, refrigeration, and freezing conditions; the PA-free heat shrink film has a longitudinal and transverse heat shrinkage rate of ≥45%, and the longitudinal and transverse shrinkage rate deviation is ≤3%; the PA-free heat shrink film has a heat shrinkage rate decay of ≤5% after 6 months of storage at room temperature.

2. The PA-free heat-shrink film for food packaging as described in claim 1, characterized in that, The mass fraction of each component in the reinforced puncture-resistant layer is as follows: 60%-70% metallocene polyethylene, 18%-30% polyolefin elastomer, 3%-6% surface-modified nano-masterbatch, and 0.3%-0.8% first composite antioxidant.

3. The PA-free heat-shrink film for food packaging as described in claim 1, characterized in that, The metallocene polyethylene has a melt index of 0.8 g / 10 min and a density of 0.918 g / cm³; the polyolefin elastomer has a melt index of 1.0 g / 10 min, a density of 0.870 g / cm³, and a glass transition temperature Tg≈-55℃.

4. The PA-free heat-shrinkable film for food packaging as described in claim 1, characterized in that, The surface layer comprises the following components by mass fraction: 60-80% linear low-density polyethylene (LLDPE), 15-30% ethylene-vinyl acetate copolymer, and 3-6% surface-modified nano-masterbatch.

5. The PA-free heat-shrinkable film for food packaging as described in claim 1, characterized in that, The food contact heat seal layer comprises the following components by mass fraction: 92%-96% metallocene linear low-density polyethylene, 2%-4% maleic anhydride grafted polyethylene, 0.5%-1.5% oleamide, 0.5%-1.5% micron-sized silica, and 0.3%-0.8% third composite antioxidant.

6. The PA-free heat-shrinkable film for food packaging as described in claim 5, characterized in that, In the food contact heat seal layer, the metallocene linear low-density polyethylene has a melt index of 1.0 g / 10 min and a density of 0.915 g / cm³; the maleic anhydride grafted polyethylene has a grafting rate of 1.0% and a melt index of 2.0 g / 10 min.

7. The PA-free heat-shrinkable film for food packaging as described in claim 5, characterized in that, The third composite antioxidant includes a primary antioxidant 1010, a secondary antioxidant 168, and a long-lasting antioxidant 1076, wherein the primary antioxidant 1010 accounts for 0.1%-0.3% of the mass of the third composite antioxidant, the secondary antioxidant 168 accounts for 0.1%-0.3% of the mass of the third composite antioxidant, and the long-lasting antioxidant 1076 accounts for 0.1%-0.2% of the mass of the third composite antioxidant.

8. The PA-free heat-shrinkable film for food packaging as described in claim 5, characterized in that, The third composite antioxidant has the same composition as the first composite antioxidant.

9. The PA-free heat-shrinkable film for food packaging as described in claim 1, characterized in that, The preparation method of the surface-modified nanomasterbatch includes the following steps: (1) Hydrolysis of coupling agent: Anhydrous ethanol and deionized water are mixed at a volume ratio of 9:1, the pH is adjusted to 4-5 with glacial acetic acid, silane coupling agent KH570 is added, and the mixture is stirred at 50°C for 30 min to obtain hydrolysate; (2) Nanoparticle modification: Add nano-calcium carbonate or nano-silica to the hydrolysate at a solid-liquid ratio of 1:10, ultrasonically disperse at 60°C for 30 min, filter, and vacuum dry at 105°C for 2 h to obtain modified nanoparticles. (3) Masterbatch preparation: The modified nanoparticles and maleic anhydride grafted polyethylene are mixed at a mass ratio of 1:4 and granulated by extrusion through a twin-screw extruder. The extrusion temperature is 130-150℃, the screw speed is 300rpm, and the feeding speed is 15kg / h to obtain surface-modified nano masterbatch.

10. The PA-free heat-shrinkable film for food packaging as described in claim 1, characterized in that, It also includes an adhesive layer located between the food contact heat seal layer and the reinforced puncture-resistant layer, and the adhesive layer comprises the following components by mass fraction: 98%-99.5% hydrolysis-resistant maleic anhydride-grafted polyethylene, 0.5%-2% epoxy chain extender, the grafting rate of the hydrolysis-resistant maleic anhydride-grafted polyethylene being 1.2%-1.8%, and the melt index being 1.5 g / 10 min.

11. The PA-free heat-shrinkable film for food packaging as described in claim 10, characterized in that, It also includes a high barrier layer, which is located between the adhesive layer and the food contact heat seal layer, and the high barrier layer is an ethylene-vinyl alcohol copolymer [2], in which the ethylene molar content is 32%-44%, and the melt index of the ethylene-vinyl alcohol copolymer is 3.2-3.8 g / 10 min.

12. A method for preparing a PA-free heat-shrinkable film for food packaging as described in any one of claims 1-9, characterized in that, The preparation method includes the following steps: S1, Raw material drying pretreatment: Dry each layer of raw materials to ensure that the moisture content of all raw materials is controlled below 50ppm, and weigh each layer of raw materials according to the formula ratio and mix them evenly; S2, Single-stage blown film: A multi-layer co-extrusion extruder is used to melt and extrude multiple layers of melt, which are then blown into a single-stage bubble film. The thickness of the single-stage bubble film is controlled at 60-100μm. The multi-layer co-extrusion extruder is equipped with a temperature transition channel in the die head, and the viscosity difference of each layer of melt in the confluence section is controlled by temperature gradient ≤±30%. S3, Primary gradient cooling and shaping: The primary bubble membrane is cooled and shaped using a gradient cooling process to control the crystallinity of the primary bubble membrane to be between 35% and 45%; S4, Secondary blown film: The temperature of the primary blown film is heated to the high elasticity range below the melting point of the polymer, and then stretched and blown a second time. The ratio of transverse to longitudinal stretching is controlled to be 0.9-1.

1. After secondary stretching, a second film is formed with a thickness of 25-50 μm. S5, Secondary cooling and post-processing: The second tubular membrane is subjected to secondary cooling and post-processing to obtain the PA-free high puncture-resistant heat shrink film for food packaging.

13. The preparation method according to claim 12, characterized in that, The PA-free heat shrink film also includes an adhesive layer located between the food contact heat seal layer and the reinforced puncture-resistant layer. The adhesive layer comprises the following components by mass fraction: 98%-99.5% hydrolysis-resistant maleic anhydride-grafted polyethylene and 0.5%-2% epoxy chain extender. The grafting rate of the hydrolysis-resistant maleic anhydride-grafted polyethylene is 1.2%-1.8%, and the melt index is 1.5 g / 10 min.

14. The preparation method according to claim 13, characterized in that, The PA-free heat shrink film also includes a high-barrier barrier layer, which is located between the adhesive layer and the food contact heat seal layer. The high-barrier barrier layer includes 10%-18% by mass of an ethylene-vinyl alcohol copolymer, wherein the ethylene molar content in the ethylene-vinyl alcohol copolymer is 32%-4438%, and the melt index of the ethylene-vinyl alcohol copolymer is 3.2-3.8 g / 10 min.

15. The preparation method according to claim 14, characterized in that, In step S1, the ethylene-vinyl alcohol copolymer is vacuum dried at 85-95℃ for 4-6 hours, and the polyolefin elastomer is hot-air dried at 70-80℃ for 2-3 hours.

16. The preparation method according to claim 14, characterized in that, In step S2, the extrusion temperature of the reinforced puncture-resistant layer is 135-155℃, the extrusion temperature of the adhesive layer is 145-160℃, and the extrusion temperature of the high-barrier barrier layer is 180-195℃.

17. The preparation method according to claim 14, characterized in that, In step S2, the length of the temperature transition channel is 150 mm, the temperature gradient of the temperature transition channel is 190℃→170℃→155℃, the confluence temperature of the confluence section is 155℃, and the shear rate of the confluence section is 500 s. - ¹.

18. The preparation method according to claim 12, characterized in that, In step S2, the inflation ratio of the first inflation is 2.0-3.0, the traction speed is 25-40m / min, the longitudinal pre-stretch ratio is 1.5-2.0, and the transverse thickness deviation of the first bubble membrane is ≤±3%.

19. The preparation method according to claim 12, characterized in that, In step S3, the gradient cooling process is as follows: first, the temperature is rapidly reduced by air cooling at 20-25℃ through an annular air ring with a wind speed of 4-6m / s and an air ring vent angle of 30°; then, the temperature is slowly reduced by water cooling at 15-20℃ through a constant temperature water cooling tank with a length of 2m, the traction speed is matched with the water cooling path, and the cooling rate is controlled at 10-15℃ / s.

20. The preparation method according to claim 12, characterized in that, In step S4, the transverse inflation ratio is 3.0-4.0, the longitudinal stretching ratio is 2.0-2.5, and the thickness deviation of the second membrane is ≤±2μm.

21. The preparation method according to claim 12, characterized in that, In step S5, the secondary cooling is achieved by rapidly cooling to room temperature using an 18-22℃ air ring. The post-processing includes critical corona treatment and gradient tension winding. The parameters for critical corona treatment are: electrode gap of 2 mm, processing linear speed synchronized with traction speed, power density controlled at 2-3 W·min / m², surface tension of the film after treatment ≥38 dyn / cm, and surface tension ≥36 dyn / cm after 6 months of storage. The gradient tension winding uses a winding tension that decreases linearly from the inside to the outside by 3-8 N.

22. A preparation apparatus used in the preparation method of the PA-free heat-shrinkable film for food packaging as described in claim 12, characterized in that, The preparation apparatus includes an extrusion system, a die system, a stretching system, a cooling system, and a winding system arranged along the production line, wherein... The extrusion system is a barrier-type compounding screw extruder used to output a uniformly mixed multilayer melt. The length-to-diameter ratio of the screw is 28:

1. The extrusion system adopts a PID closed-loop temperature control unit with a temperature control error of ≤±1℃. The die head system is used for multi-layer melt fusion molding. The viscosity of each layer of melt is matched by temperature gradient control to output a primary bubble tube film. The die head system adopts a multi-layer co-extrusion spiral die head. The temperature transition channel is provided in the die head. The die head gap can be adjusted within a range of 0.8-2.0 mm. The stretching system adopts a double-bubble structure to achieve gradient stretching and biaxial orientation of the primary bubble membrane and output a secondary bubble membrane. The stretching system has a multi-segment independently temperature-controlled infrared heating box and a food-grade bubble stabilizing rack. The servo motor of the stretching system synchronously controls the stretching traction roller, with a speed error ≤ ±0.5m / min. The cooling system is used for gradient cooling and shaping of primary bubble membranes and rapid cooling of secondary bubble membranes, controlling the crystallinity and molecular orientation of the membrane. The winding system is used for the smooth winding of the secondary tubular membrane. The winding system adopts a tension closed-loop control gradient winding machine for gradient tension winding.

23. The application of the PA-free heat-shrinkable film for food packaging as described in any one of claims 1-10 in the packaging of protein foods, characterized in that, The protein foods mentioned include fresh livestock and poultry meat, aquatic protein, soy products, plant-based meat products, and cooked protein foods.

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