High barrier heat shrinkable film for food packaging and method for preparing the same

Abstract

This invention provides a high-barrier heat-shrinkable film for food packaging and its preparation method. The high-barrier heat-shrinkable film comprises, from the outside to the inside, an outer heat-sealing layer, a first adhesive layer, a middle barrier layer, a second adhesive layer, and an inner heat-sealing layer. Both the inner and outer heat-sealing layers are ethylene-vinyl acetate copolymers, wherein the VA content is 12%-18% by mass. The first and second adhesive layers are maleic anhydride-grafted linear low-density polyethylene. The middle barrier layer is a vinylidene chloride-methyl acrylate random copolymer, wherein the MA content is 5%-8% by mass, and the crystallinity of the vinylidene chloride-methyl acrylate random copolymer is 30%-40%, with a crystallite size <300nm. This invention's high-barrier heat-shrinkable film overcomes the constraints of high shrinkage rate, high barrier properties, and low haze in existing technologies, and solves the technical problems of unstable multilayer co-extrusion interfaces and easy degradation during PVDC processing.

Description

Technical Field

[0001] This invention relates to the field of polymer functional film technology for food packaging, and in particular to a high-barrier heat-shrinkable film for food packaging and its preparation method. Background Technology

[0002] Currently, for packaging needs of short-shelf-life foods such as fresh produce, chilled meat products, and prepared meals, the industry mainly uses polyolefin heat shrink film and PVDC. The core technology route is based on barrier film and multilayer composite heat shrink film, which attempts to meet the four rigid technical requirements of high balanced shrinkage rate, high oxygen barrier, low haze and high transparency, and food contact compliance. However, the existing technology system has always been unable to break through the three core theoretical bottlenecks at the level of polymer science: (1) Performance coupling constraint bottleneck: High shrinkage rate requires polymer orientation, and orientation-induced crystallization will simultaneously improve barrier and light scattering, resulting in increased haze. The three are inherently thermodynamically coupled and constrained, and the existing technology cannot achieve decoupling optimization; (2) Multilayer processing matching bottleneck: There are orders of magnitude differences in solubility parameters, shear viscosity and elongation at break between PVDC and polyolefin matrix. Multilayer co-extrusion is prone to interface instability, interlayer debonding and stretching film breakage. The existing technology cannot achieve dual matching of thermodynamics and rheology; (3) PVDC processing stability bottleneck: PVDC thermal degradation is a self-catalytic deHCl reaction, and the processing window is only about 10°C. The existing technology cannot achieve continuous production degradation control, resulting in low yield and high food safety risk.

[0003] Currently, there is no technology to solve the above problems, which means that existing heat shrink films cannot achieve synergistic optimization of high shrinkage, high barrier, and low haze. They also have defects such as poor processing stability and insufficient food compliance control, which cannot meet the needs of high-end fresh food packaging, and it is even more difficult to achieve performance synergy and stable industrial production of short-shelf-life food packaging. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a high-barrier heat-shrinkable film for food packaging and its preparation method, which can overcome the constraints of high shrinkage rate, high barrier properties, and low haze in the prior art, and solve the technical problems of unstable multilayer co-extrusion interfaces and easy degradation of PVDC during processing.

[0005] To achieve the above objectives, the present invention provides a high-barrier heat-shrinkable film for food packaging. The high-barrier heat-shrinkable film has a five-layer symmetrical co-extruded composite structure, comprising, from the outside to the inside, an outer heat-sealing layer, a first adhesive layer, a middle barrier layer, a second adhesive layer, and an inner heat-sealing layer. The outer heat-sealing layer and the inner heat-sealing layer have the same structure, both being ethylene-vinyl acetate copolymers, with a vinyl acetate mass percentage of 12%-18% in the ethylene-vinyl acetate copolymer. The first adhesive layer and the second adhesive layer have the same structure, both being maleic anhydride-grafted linear low-density polyethylene; The intermediate barrier layer is a random copolymer of vinylidene chloride and methyl acrylate, wherein the mass percentage of methyl acrylate in the random copolymer is 5%-8%, and the crystallinity of the random copolymer is 30%~40%, with a crystallite size of <300nm.

[0006] As an optional technical solution, the high-barrier heat-shrinkable film has a biaxial orientation degree ≥0.85; the high-barrier heat-shrinkable film has a transverse and longitudinal heat shrinkage rate ≥55% at 90℃ / 4s, and an oxygen permeability ≤10cc / (m²) at 23℃ / 50% RH. The haze at a thickness of 30μm (24h) is ≤8%, the interlayer peel strength is ≥3N / 15mm, and it meets the GB 4806.6-2016 food contact standard.

[0007] As an optional technical solution, the thickness ratio of each layer of the high-barrier heat-shrinkable film is as follows: the outer heat-sealing layer 20%~25%, the first adhesive layer 8%~12%, the middle barrier layer 25%~40%, the second adhesive layer 8%~12%, and the inner heat-sealing layer 20%~25%; and the total thickness of the high-barrier heat-shrinkable film is 25μm~40μm.

[0008] As an optional technical solution, the melt flow rate of the ethylene-vinyl acetate copolymer is 1.0-3.0 g / 10 min (190℃ / 2.16 kg), the density is 0.920-0.930 g / cm³, and the weight-average molecular weight is 80,000-100,000. The maleic anhydride-grafted linear low-density polyethylene has a grafting rate of 0.8%-1.5%, a melt flow rate of 1.5-2.5 g / 10 min (190℃ / 2.16 kg), and a maleic anhydride monomer residue of ≤0.05%. The melt flow rate of the vinylidene chloride-methyl acrylate random copolymer is 2.0-4.0 g / 10 min (170℃ / 2.16 kg), the weight-average molecular weight is 80,000-120,000, the molecular weight distribution index is 2.0-3.0, the glass transition temperature Tg=20~24℃, the melting temperature Tm=162~168℃, and the initial decomposition temperature ≥170℃.

[0009] As an optional technical solution, the intermediate barrier layer is provided with a composite stabilizer, which is a calcium-zinc-epoxidized soybean oil composite heat stabilizer. The amount of the composite stabilizer added is 0.4%-0.8% of the mass of the vinylidene chloride-methyl acrylate random copolymer, wherein the calcium-zinc main stabilizer accounts for 60% of the mass of the composite heat stabilizer, and the epoxidized soybean oil auxiliary stabilizer accounts for 40% of the mass of the composite heat stabilizer.

[0010] As an optional technical solution, the vinylidene chloride-methyl acrylate random copolymer and the composite heat stabilizer are mixed at high speed at 40-50°C for 4-6 minutes.

[0011] As an optional technical solution, the inner heat-sealing layer and the outer heat-sealing layer are provided with 0.1%-0.3% by weight of oleic acid amide slip agent and 0.05%-0.2% by weight of silane coupling agent modified nano silica anti-blocking agent.

[0012] The present invention also provides a method for preparing the high-barrier heat-shrinkable film for food packaging as described above, the method comprising: Step S1, raw material drying pretreatment: Dry the raw material of the intermediate barrier layer to a moisture content of ≤0.05%, dry the raw materials of the outer heat-sealing layer, inner heat-sealing layer, first adhesive layer and second adhesive layer to a moisture content of ≤0.1%, and mix the raw materials of each layer evenly according to the formula; Step S2, Multi-layer co-extrusion molding: Multi-layer melt is melted and extruded using a multi-layer co-extrusion extruder. Each layer of melt is merged into a food-grade annular die through a coat hanger-type streamlined co-extrusion channel to form a molten tube blank. The shear viscosity difference between each layer of melt is ≤25%, and the wall thickness of the tube blank after one-time blow-up extrusion is controlled at 80-120μm. Step S3, rapid quenching and pre-locking of low crystallinity: adopting a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling to rapidly quench the molten tube blank, so as to accurately control the crystallinity of vinylidene chloride-methyl acrylate random copolymer at 30%-40% and the crystallite size <300nm. Step S4, high-elasticity isothermal homogenization treatment: The cooled tube blank is heated to the high-elasticity range of the vinylidene chloride-methyl acrylate random copolymer and subjected to isothermal homogenization. Step S5, viscoelastic matching synchronous bidirectional stretching: After homogenization, the tube blank enters the secondary blowing station, where it is blown laterally and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal proportions in both the transverse and longitudinal directions. Step S6, Gradient cooling and orientation locking: The film formed after stretching is immediately cooled by a two-stage gradient air cooling process. After cooling, the film thickness is controlled at 25-40μm, and the thickness uniformity error is ≤±2%. Step S7, Online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding to obtain the finished high-barrier heat shrink film.

[0013] As an optional technical solution, in step S1, the raw material of the intermediate barrier layer is vacuum dried for 4-6 hours at 60-70℃ and vacuum degree ≥-0.09MPa; the raw materials of the outer heat-sealing layer, the inner heat-sealing layer, the first adhesive layer and the second adhesive layer are dried by hot air circulation at 70-80℃ for 2-4 hours; and step S1 further includes: using a loss-in-weight metering scale to accurately control the melt extrusion amount of each layer, with a metering accuracy of ±0.5%.

[0014] As an optional technical solution, in step S2, the extrusion temperatures of each layer of melt are as follows: Extrusion temperatures for the outer heat-sealing layer and the inner heat-sealing layer: 100-110℃ in the feeding section, 115-125℃ in the plasticizing section, and 125-135℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are as follows: feeding section 110-120℃, plasticizing section 120-130℃, metering section 130-140℃. Extrusion temperatures of the intermediate barrier layer: 120-130℃ in the feeding section, 140-155℃ in the plasticizing section, and 155-165℃ in the metering section; The extrusion temperature of the annular die head is 160-162℃.

[0015] As an optional technical solution, the quenching parameters in step S3 are as follows: external air cooling adopts a double-outlet annular air ring external air cooling, with an external air cooling temperature of 15-20℃ and an air velocity of 6-8m / s; internal water cooling adopts a 1.2m immersion water cooling column internal water cooling, with an internal water cooling temperature of 10-15℃, direct cooling with deionized pure water, and a cooling rate ≥50℃ / s; after quenching, the tube blank is rapidly cooled to 15-20℃ for shaping.

[0016] As an optional technical solution, in step S4, the tube blank is heated by a dual heating box with hot air circulation and infrared auxiliary heating. The heating box is equipped with 12 sets of temperature sensors, the temperature uniformity is controlled within ±1℃, and the tube blank stay time is 8-12s.

[0017] As an optional technical solution, in step S5, the transverse inflation ratio is 3.5-4.5, the transverse stretching ratio is the same as the transverse inflation ratio, and the transverse stretching rate is 50-80 m / min; the longitudinal traction ratio is 3.5-4.5, the longitudinal stretching ratio is the same as the longitudinal traction ratio; and the difference between the transverse and longitudinal stretching ratios is ≤0.2.

[0018] As an optional technical solution, the air-cooling parameters in step S6 are as follows: the first-stage air temperature is 10-15℃, the air speed is 5-7m / s, and the temperature is rapidly reduced to 40-50℃; the second-stage air temperature is 20-25℃, the air speed is 3-5m / s, and the temperature is reduced to room temperature.

[0019] As an optional technical solution, in step S7, the corona treatment parameters are: power of 5-10kW, linear velocity of 40-60m / min, power density of 200-400J / m², treatment time of 0.01-0.05s, and surface tension after treatment ≥38mN / m. The constant tension winding parameters are as follows: a three-segment constant tension control system is used for winding, the winding tension is 3-8N, the taper is 5%-8%, the winding neatness error is ≤0.5mm, and there are no wrinkles or tensile deformations.

[0020] As an optional technical solution, the preparation method is completed entirely in a Class 10,000 food cleanroom, with food-grade 316L stainless steel used in the parts in contact with the materials, an inner wall roughness Ra≤0.4μm, and the melt residence time in the flow channel≤8min.

[0021] Compared with the prior art, the high-barrier heat-shrinkable film for food packaging and its preparation method of the present invention have the following beneficial effects: (1) Break through the performance coupling bottleneck and achieve synergistic improvement of the three core performances. This invention, through precise control of the aggregated structure, breaks through the technical biases of existing technologies and stably achieves: a lateral and longitudinal uniform shrinkage rate ≥55% at 90℃ / 4s, an oxygen permeability ≤10cc / m²·24h at 23℃ / 50%RH, a haze of ≤8% at a thickness of 30μm, and an interlayer peel force ≥3N / 15mm. Characterization by WAXD and POM shows that the PVDC film has a crystallite size <350nm and a biaxial orientation ≥0.85, demonstrating the mechanism of synergistic performance optimization at the molecular level and solving a long-standing theoretical bottleneck in the industry.

[0022] (2) Solve the problem of long-term interface stability and achieve stable industrial production. Through interfacial thermodynamic compatibility design, the interlayer peel force is ≥4N / 15mm. After 3 months of cold chain cycling at 0-4℃, the peel force retention rate is ≥85%, with no delamination or debonding. Through a three-in-one thermal degradation control system, it can achieve 72h continuous production without yellowing, black spots, or film breakage, with a yield rate of ≥95%, which is far higher than the 82% yield rate of existing technologies. The process has a wide tolerance range and is compatible with existing food film production lines.

[0023] (3) The entire process of food contact is compliant and there is no food safety risk. All raw materials and additives comply with the requirements of GB 4806.6-2016, and the residual amount of monomers and the amount of additives are strictly controlled; the total migration amount, heavy metals, vinyl chloride monomer residue and other indicators of the finished product all meet the standards; no harmful substances are generated by thermal degradation during the processing, and it can be directly used for direct contact packaging of various foods.

[0024] (4) Significantly extends the shelf life of food and has extremely high industrial value. Actual testing showed that chilled pork packaged with the high-barrier heat-shrink film of this invention, after 18 days of cold chain storage at 0-4℃, had a total bacterial count ≤10. 5 CFU / g, meeting national safety standards for fresh meat, with a shelf life 6 times that of ordinary PE shrink film and 1.5 times that of existing PVDC shrink film; low-temperature braised meat products can have a shelf life of more than 25 days, fully meeting the circulation needs of short-shelf-life foods. Attached Figure Description

[0025] 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.

[0026] Figure 1 This is a schematic diagram of the structure of a high-barrier heat-shrinkable film according to an embodiment of the present invention; Figure 2 This is a schematic flowchart of a method for preparing a high-barrier heat-shrinkable film according to an embodiment of the present invention. Detailed Implementation

[0027] 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.

[0028] 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.

[0029] Please see Figure 1 , Figure 1The diagram shown is a structural schematic of a high-barrier heat-shrinkable film according to an embodiment of the present invention. The present invention provides a high-barrier heat-shrinkable film for food packaging using a double-bubble method. The high-barrier heat-shrinkable film is a five-layer symmetrical co-extruded composite structure, which includes, from the outside to the inside, an outer heat-sealing layer 1, a first adhesive layer 2, an intermediate barrier layer 3, a second adhesive layer 4, and an inner heat-sealing layer 5.

[0030] The outer heat-sealing layer 1 and the inner heat-sealing layer 5 have the same structure, both being ethylene-vinyl acetate copolymer (EVA). The mass percentage of vinyl acetate (VA) in the ethylene-vinyl acetate copolymer is 12%-18%, a ratio that balances heat-sealing performance and mechanical strength. The outer heat-sealing layer 1 provides contamination-resistant heat-sealing, printability, and low-temperature puncture resistance, making it suitable for cold chain transportation. The inner heat-sealing layer 5, which comes into direct contact with food, provides heat-sealing performance resistant to grease and juice contamination, ensuring sealing strength.

[0031] The first adhesive layer 2 and the second adhesive layer 4 have the same structure, both being maleic anhydride-grafted linear low-density polyethylene (LLDPE-g-MAH). The chemical bonding between the layers is achieved through the reaction of maleic anhydride groups with the polar groups of the intermediate barrier layer 3. The first adhesive layer 2 can achieve interfacial thermodynamic compatibility between PVDC (intermediate barrier layer) 3 and polyolefin, improve the interlayer bonding force, and prevent long-term delamination and damage. The second adhesive layer 4 matches the interfacial bonding force between the inner heat-sealing layer 5 and the intermediate barrier layer 3.

[0032] The intermediate barrier layer 3 is a vinylidene chloride-methyl acrylate random copolymer (VDC-MA type PVDC). The mass percentage of methyl acrylate (MA) in the vinylidene chloride-methyl acrylate random copolymer is 5%-8%, which can reduce the crystallinity of VDC-MA type PVDC and improve its flexibility and processing stability.

[0033] The intermediate barrier layer 3 has a crystallinity of 30%~40% and a crystallite size of <300nm, balancing barrier properties and tensile strength; the high-barrier heat-shrinkable film has a biaxial orientation of ≥0.85; the high-barrier heat-shrinkable film exhibits a longitudinal and transverse heat shrinkage rate of ≥55% at 90℃ / 4s, and an oxygen permeability of ≤10cc / (m²) at 23℃ / 50% RH. The haze is ≤8% at a thickness of 30μm (24h), the interlayer peel strength is ≥3N / 15mm, and it complies with the GB 4806.6-2016 food contact standard. The high-barrier heat shrink film of this invention can be used in vacuum heat shrink packaging scenarios with stringent requirements for shrinkage resistance, oxygen barrier properties, optical performance, and food safety compliance, such as fresh and chilled meat, low-temperature braised meat products, pre-prepared dishes, and fresh aquatic products.

[0034] In one embodiment, the thickness proportions of each layer of the high-barrier heat-shrinkable film are as follows: outer heat-sealing layer 20%~25%, first adhesive layer 8%~12%, middle barrier layer 25%~40%, second adhesive layer 8%~12%, and inner heat-sealing layer 20%~25%; and the total thickness of the high-barrier heat-shrinkable film is 25μm~40μm, which is compatible with general fully automatic heat-shrink packaging equipment in the food packaging industry. Furthermore, the middle barrier layer 3 has the largest proportion, ensuring the barrier effect.

[0035] In one embodiment, the melt flow rate of the ethylene-vinyl acetate copolymer of the outer heat-sealing layer 1 and the inner heat-sealing layer 5 is 1.0-3.0 g / 10 min (190℃ / 2.16 kg), the density is 0.920-0.930 g / cm³, and the weight-average molecular weight is 80,000-100,000, to ensure processing flowability and membrane toughness.

[0036] Furthermore, the inner heat-sealing layer 5 and the outer heat-sealing layer 1 are enriched with 0.1%-0.3% by weight of oleamide slip agent and 0.05%-0.2% by weight of silane coupling agent modified nano-silica anti-blocking agent, both of which meet food contact standards. The silane coupling agent modified nano-silica anti-blocking agent exhibits good compatibility with the EVA matrix, shows no significant light scattering, and also prevents film adhesion and clumping. The addition of 0.1%-0.3% oleamide slip agent improves film surface lubrication and prevents adhesion.

[0037] In one embodiment, the first adhesive layer 2 and the second adhesive layer 4 are made of maleic anhydride-grafted linear low-density polyethylene (LLDPE-g-MAH) to achieve interfacial thermodynamic compatibility design. This is based on the principle of similar solubility parameters. By adjusting the grafting rate, the solubility parameter of the adhesive layer is increased from 16.5 (J / cm³)^0.5 of pure LLDPE to 17.8 (J / cm³)^0.5, reducing the thermodynamic difference with PVDC (polyvinylidene chloride) (the solubility parameter of PVDC is 20.8 (J / cm³)^0.5). Combined with interfacial molecular chain interdiffusion, interlayer molecular-level adhesion is achieved, thereby solving the long-term interfacial stability problem.

[0038] Furthermore, the grafting rate of maleic anhydride-grafted linear low-density polyethylene in the first adhesive layer 2 and the second adhesive layer 4 is 0.8%-1.5% to ensure adhesive strength; the melt flow rate is 1.5-2.5g / 10min (190℃ / 2.16kg) to match the processing flowability of adjacent layers; and the residual maleic anhydride monomer content is ≤0.05%, which meets food safety requirements.

[0039] In addition, in this invention, the intermediate barrier layer 3 is selected as a random copolymer of vinylidene chloride and methyl acrylate (VDC-MA type PVDC), which realizes the controllable design of the PVDC copolymer sequence structure. This is based on the free radical copolymerization reactivity ratio theory (VDC reactivity ratio r1=3.5, MA reactivity ratio r2=0.1). By controlling the feed rate of MA monomer through a semi-continuous drop-addition polymerization process, the random sequence structure of VDC-MA copolymer can be controlled, avoiding the problems of excessively high crystallinity and narrow processing window (LLDPE-g-MAH) caused by long homopolymer blocks of VDC. At the same time, it ensures the high molecular packing density of molecular chains, realizes oxygen and water vapor barrier, and is suitable for biaxial stretching process, which can take into account both barrier properties and processability.

[0040] Furthermore, the melt flow rate of the vinylidene chloride-methyl acrylate random copolymer of the intermediate barrier layer 3 is 2.0-4.0 g / 10 min (170℃ / 2.16 kg), the weight-average molecular weight is 80,000-120,000, the molecular weight distribution index is 2.0-3.0, and the processing fluidity is balanced with the film strength; the glass transition temperature Tg=20~24℃, the melting point Tm=162~168℃, and the initial decomposition temperature ≥170℃. The above temperatures are suitable for the double bubble stretching process window.

[0041] Furthermore, the intermediate barrier layer 3 contains a food-grade calcium-zinc-epoxidized soybean oil composite heat stabilizer, added at 0.4%-0.8% of the mass of VDC-MA type PVDC. The calcium-zinc primary stabilizer accounts for 60% of the composite heat stabilizer's mass, while the epoxidized soybean oil auxiliary stabilizer accounts for 40%. By selecting the food-grade calcium-zinc-epoxidized soybean oil composite heat stabilizer, the synergistic effect of HCl absorption, unstable chlorine atom substitution, and double bond addition inhibits the autocatalytic degradation of VDC-MA type PVDC due to HCl removal, ensuring an initial decomposition temperature ≥170℃ and achieving synergistic thermal stability.

[0042] Furthermore, preferably, the vinylidene chloride-methyl acrylate random copolymer and the composite heat stabilizer are mixed at high speed at 40-50°C for 5 minutes to avoid overheating of the materials.

[0043] In this invention, the high-barrier heat-shrinkable film is a five-layer symmetrical co-extrusion composite structure, which is formed by multi-layer co-extrusion melt extrusion. Based on the principle of polymer processing rheology, the multi-layer rheological matching design is achieved by controlling the molecular weight and comonomer content of each layer structure, so that the shear viscosity difference of each layer melt at the die processing temperature is ≤25%, avoiding the problems of unstable multi-layer co-extrusion interface and uneven layer thickness, thereby achieving stable multi-layer co-extrusion molding.

[0044] Please see Figure 2 , Figure 2The diagram shown is a flowchart illustrating a method for preparing a high-barrier heat-shrinkable film for food packaging using the double-bubble method according to an embodiment of the present invention. The present invention also provides a method for preparing the high-barrier heat-shrinkable film for food packaging using the double-bubble method as described above, the method comprising the following steps: Step S1, raw material drying pretreatment: Vacuum dry the raw material of the intermediate barrier layer to a moisture content ≤0.05% to avoid hydrolysis and decomposition of VDC-MA type PVDC; hot air dry the raw materials of the outer heat-sealing layer, inner heat-sealing layer, first adhesive layer and second adhesive layer to a moisture content ≤0.1%, and mix the raw materials of each layer evenly according to the raw material ratio; in addition, when a composite heat stabilizer is added to the intermediate barrier layer, preferably, mix the VDC-MA type PVDC and the composite heat stabilizer at high speed at 40-50℃ for 5 minutes to avoid overheating of the material; Step S2, Multi-layer Co-extrusion Molding: Five single-screw extruders are used to melt and extrude multi-layer melts corresponding to the five-layer structure. The melts of each layer converge through a coat hanger-type streamlined co-extrusion channel to a food-grade annular die, and are extruded to form a molten tube preform. The die temperature is 160~162℃, and the shear viscosity difference between each layer of melt is ≤25% to avoid interlayer slippage. The wall thickness of the preform is controlled at 80-120μm in one extrusion to allow for subsequent stretching, and the wall thickness uniformity error of the preform is ≤±3%. Step S3, rapid quenching and pre-locking of low crystallinity: A dual-loop synchronous rapid quenching process of external air cooling and internal water cooling is adopted to rapidly quench the molten tube blank, precisely controlling the crystallinity of VDC-MA type PVDC at 30%-40%, with a crystallite size <300nm; based on the theory of polymer non-isothermal crystallization kinetics, with a cooling rate of ≥50℃ / s, the PVDC semi-crystallization time t1 / 2≤10s can effectively suppress the growth of large-sized spherulites, achieving controllable microcrystalline structure, i.e., suppressing the growth of large-sized spherulites in PVDC, precisely controlling the crystallinity at 30%-40%, and forming a microcrystalline structure with a size <300nm (smaller than the wavelength of visible light, with no obvious light scattering), which retains the stretchability of the amorphous region and achieves basic barrier properties through microcrystals, achieving low crystallinity and ensuring a balance between high elasticity and barrier properties during subsequent stretching, thereby solving the essential contradiction of "crystallization-stretchability-haze".

[0045] Step S4, High-elasticity isothermal homogenization treatment: The cooled tube blank is heated to the high-elasticity range of VDC-MA type PVDC (VDC-MA type PVDC high-elasticity range 115-135℃, above Tg, below Tm-30℃) for isothermal homogenization to eliminate internal stress in the tube blank; the storage modulus E' of VDC-MA type PVDC in this temperature range is measured by DMA (Dynamic Thermomechanical Analyzer) and found to be 10. 6 -10 7Pa, loss factor tanδ=0.3-0.5, is in the optimal high elasticity stretching range, molecular chain relaxation time τ=0.1-0.5s, which matches the subsequent tensile strain rate, that is, the molecular chain has sufficient ductility and no risk of melt relaxation.

[0046] Step S5, viscoelastic matching synchronous bidirectional stretching: After homogenization, the billet enters the secondary blowing station, where it is laterally blown by compressed air and longitudinally stretched by high-speed traction rollers to achieve synchronous bidirectional stretching with equal proportions in both the transverse and longitudinal directions. Based on the principle of polymer entropy elasticity and the principle of time-temperature equivalence, in the PVDC high elasticity range (Tg~Tm-30℃), through precise matching of stretching temperature, strain rate and molecular chain relaxation time, synchronous stretching with equal proportions in both the transverse and longitudinal directions is achieved, so that the molecular chains are uniformly oriented along the biaxial direction. At the same time, by controlling the stretching rate, the formation of large-sized spherulites caused by orientation-induced crystallization is avoided. This not only obtains the high shrinkage rate brought about by the high degree of orientation, but also further improves the barrier properties through the regular stacking of oriented molecular chains, achieving a synergistic improvement in "shrinkage rate-barrier properties".

[0047] Step S6, Gradient cooling to lock orientation: The stretched film is immediately cooled by a two-stage gradient air cooling process. After cooling, the film thickness is controlled at 25-40μm, and the thickness uniformity error is ≤±2%. Through two-stage gradient cooling, the molecular orientation structure is first rapidly cooled to ensure thermal shrinkage performance. Then, the internal stress is eliminated by slow cooling to avoid orientation relaxation during storage. At the same time, the microcrystalline structure is fixed to achieve long-term performance stability.

[0048] Step S7, Food-grade online corona treatment and constant tension winding: The outer layer of the film undergoes online continuous corona treatment and constant tension winding to obtain the finished high-barrier heat-shrinkable film. The wound film roll is wrapped with food-grade PE film and stored in a constant temperature and humidity warehouse at 23℃ / 50%RH.

[0049] In other words, the present invention achieves precise control of the PVDC aggregated structure through the above-mentioned three-step method of rapid quenching and low crystallinity pre-locking, viscoelastic matching synchronous bidirectional stretching, and gradient cooling orientation locking, breaking the coupling constraint of "high shrinkage-high barrier-low haze".

[0050] Step S1 further includes: using a loss-in-weight weighing scale to accurately control the extrusion amount of each layer with a metering accuracy of ±0.5%, thereby ensuring a stable layer thickness ratio.

[0051] The intermediate barrier layer material is dried at 60-70℃ and vacuum degree ≥-0.09MPa for 4-6 hours; the outer heat-sealing layer, inner heat-sealing layer, first adhesive layer and second adhesive layer material are dried in hot air circulation at 70-80℃ for 2-4 hours.

[0052] In step S2, the extrusion temperatures of each layer are as follows: the extrusion temperatures of the outer heat-sealing layer and the inner heat-sealing layer are 100-110℃ in the feeding section, 115-125℃ in the plasticizing section, and 125-135℃ in the metering section; the extrusion temperatures of the first adhesive layer and the second adhesive layer are 110-120℃ in the feeding section, 120-130℃ in the plasticizing section, and 130-140℃ in the metering section; the extrusion temperature of the intermediate barrier layer is 120-130℃ in the feeding section, 140-155℃ in the plasticizing section, and 155-165℃ in the metering section; the co-extrusion annular die temperature is 160-162℃, and the difference in melt shear viscosity between each layer is controlled to be ≤25%, which meets the rheological matching requirements of multi-layer co-extrusion.

[0053] The rapid cooling parameters in step S3 are as follows: external air cooling with a dual-outlet annular air ring, the external air cooling temperature is 15-20℃, and the air velocity is 6-8m / s; internal water cooling with a 1.2m immersion water cooling column, the internal water cooling temperature is 10-15℃, direct cooling with deionized pure water, and a cooling rate ≥50℃ / s; after rapid cooling, the tube blank is quickly cooled to 15-20℃ for shaping, and is completely shaped into a thick-walled tube blank. The crystallinity of VDC-MA type PVDC can be detected by DSC method to ensure that it is within the target range.

[0054] In step S4, the tube blank is heated by a dual heating box with hot air circulation and infrared auxiliary heating. The heating box is equipped with 12 sets of temperature sensors, and the temperature uniformity is controlled within ±1℃. The tube blank stay time is 8-12s to ensure that the inner and outer layers are completely consistent and achieve overall uniform heating.

[0055] In step S5, the transverse inflation ratio is 3.5-4.5, the transverse stretching ratio is the same as the inflation ratio, and the transverse stretching rate is 50-80 m / min; the longitudinal traction ratio is 3.5-4.5, and the longitudinal stretching ratio is the same as the traction ratio; the difference between the transverse and longitudinal stretching ratios is ≤0.2, and the biaxial orientation degree is ≥0.85, achieving proportional stretching. Tensile strain rate. =10-20s - ¹, Matching the molecular chain relaxation time τ ( With a stretching rate of τ≈1, uniform and highly elastic stretching can be achieved, avoiding brittle fracture. Through infrared dichroism testing, the biaxial orientation degree of the stretched film is ≥0.85, providing a molecular basis for high shrinkage. At the same time, by controlling the stretching rate, the orientation-induced crystallinity is increased by ≤10%, and the crystallite size is still <350nm, avoiding the increase of haze.

[0056] The air-cooling parameters in step S6 are as follows: the first-stage air temperature is 10-15℃, the air speed is 5-7m / s, and the temperature is rapidly reduced to 40-50℃ to instantly freeze the molecular orientation structure and prevent orientation relaxation; the second-stage air temperature is 20-25℃, the air speed is 3-5m / s, and the temperature is reduced to room temperature to eliminate internal stress in the film and prevent performance degradation during storage.

[0057] In step S7, the corona treatment parameters are: power 5-10kW, linear speed 40-60m / min, power density 200-400J / m², treatment time 0.01-0.05s, and surface tension ≥38mN / m after treatment, improving printability. This avoids surface oxidation and food safety risks caused by over-treatment. The winding parameters are: a three-segment constant tension control system is used for winding, winding tension 3-8N, taper 5%-8%, winding neatness error ≤0.5mm, and no wrinkles or stretching deformation.

[0058] The entire preparation process is carried out in a Class 10,000 food-grade cleanroom. The parts in contact with the materials are made of food-grade 316L stainless steel with an inner wall roughness Ra≤0.4μm and the melt residence time in the flow channel ≤8min to avoid material degradation.

[0059] Moreover, as mentioned above, this invention also constructs a three-in-one prevention and control system of "thermal stability system - processing temperature control - flow channel non-retention design" based on the PVDC deHCl autocatalytic degradation kinetic model, which can achieve thermal degradation prevention and control: (1) food-grade calcium zinc-epoxidized soybean oil composite thermal stabilizer is selected, and autocatalytic degradation is inhibited through the synergistic effect of HCl absorption, unstable chlorine atom substitution and double bond addition; (2) the maximum temperature of VDC-MA type PVDC melt is controlled within 165℃ through gradient temperature control, which is lower than the initial decomposition temperature (170℃); (3) the die head flow channel adopts a streamlined non-retention design, and the melt residence time is ≤8min, which is far lower than the decomposition induction period of 165℃ (15min), so as to achieve 72h continuous production without degradation and without yellowing.

[0060] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0061] All ingredients used in the embodiments are food contact grade and comply with GB 4806 series standards.

[0062] In this study, the performance tests conducted on the heat-shrinkable films of the embodiments or comparative examples all adopted current and valid national standards, and the characterization of the core aggregated state structure adopted general standard methods for polymer materials, as detailed below: (1) Heat shrinkage rate: GB / T 13519-2023, 120℃ glycerol bath, 10s (2) Oxygen permeability: GB / T 31354-2014, 23℃ / 50%RH (3) Haze: GB / T 2410-2024, corresponding to the thickness of the finished product in the example. (4) Interlayer peel force: GB / T 8808-1988, T-type peel, 15mm wide sample (5) Crystallinity / thermal properties: GB / T 8808-1988, T-type exfoliation, 15mm wide sample (6) Crystal morphology: Polarizing microscope (POM), magnification 500× (7) Orientation degree: Fourier transform infrared spectrometer (FTIR), infrared dichroism method (8) Rheological properties: rotational rheometer, shear rate 10-1000s - ¹ (9) Molecular weight and distribution: Gel permeation chromatography (GPC) was used with tetrahydrofuran as the mobile phase. (10) Food contact compliance: GB 4806.6-2016 Example 1 (Special film for fresh and chilled meat) 1. Structure and composition of high-barrier heat-shrinkable film (total thickness 30μm) Outer heat-sealing layer / inner heat-sealing layer: EVA (VA content 12%, MFR 1.0g / 10min), single layer thickness 6μm, accounting for 20%; First adhesive layer / second adhesive layer: LLDPE-g-MAH (grafting rate 0.8%, MFR 1.5g / 10min), single layer thickness 3μm, accounting for 10%; Intermediate barrier layer: VDC-MA type PVDC (MA content 5%, MFR 2.0g / 10min, Tg=22℃, Tm=165℃), thickness 12μm, accounting for 40%; 0.4% composite heat stabilizer added; Both the outer heat-sealing layer and the inner heat-sealing layer contain 0.1% oleamide and 0.05% modified silica.

[0063] 2. Preparation method (1) Raw material drying pretreatment: VDC-MA type PVDC is vacuum dried at 60℃ for 6h until the moisture content is 0.04%; the remaining raw materials are hot air dried at 70℃ for 4h until the moisture content is 0.09%, and the raw materials of each layer are mixed evenly according to the formula; (2) Multi-layer co-extrusion molding: Five single-screw extruders are used to melt and extrude multi-layer melts corresponding to five-layer structures. Each layer of melt is merged into a food-grade annular die through a coat hanger-type streamlined co-extrusion channel to extrude and form a molten tube blank. Extrusion temperatures of the outer heat-sealing layer / inner heat-sealing layer (EVA layer): 105℃ in the feeding section, 120℃ in the plasticizing section, and 130℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are: 115℃ in the feeding section, 125℃ in the plasticizing section, and 135℃ in the metering section. The extrusion temperatures for VDC-MA type PVDC layers are: 125℃ for the feeding section, 150℃ for the plasticizing section, and 160℃ for the metering section; the co-extrusion ring die temperature is 162℃. The thickness of the extruded tube blank is 100μm in one extrusion, and the traction speed is 12m / min in one traction. (3) Pre-locking of low crystallinity by rapid quenching: The molten tube blank is rapidly quenched by a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling. The specific parameters are as follows: air temperature 15℃, air speed 6m / s; water temperature 10℃, cooling rate 52℃ / s; the tube blank is cooled to 15℃, the initial crystallinity of PVDC is 32%, and the microcrystal size is 280nm. (4) High-elasticity constant temperature heat treatment: The cooled tube blank is heated to the high-elasticity range of vinylidene chloride-methyl acrylate random copolymer and subjected to constant temperature heat treatment. The specific parameters are as follows: temperature 115℃, uniformity ±1℃, residence time 10s. (5) Viscoelastic matching synchronous bidirectional stretching: After the tube blank is heated, it enters the secondary blowing station, and is blown laterally by compressed air and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal ratio in the transverse and longitudinal directions. The specific parameters are as follows: transverse blowing ratio 3.5, longitudinal traction ratio 3.5, stretching rate 50m / min. (6) Gradient cooling and orientation locking: The stretched film is immediately cooled by a two-stage gradient air cooling process. The specific parameters are as follows: the first stage air temperature is 10℃ and the air speed is 5m / s; the second stage air temperature is 20℃ and the air speed is 3m / s. (7) Food-grade online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding. The specific parameters are as follows: Corona treatment: power 5kW, linear velocity 40m / min, power density 200J / m²; Winding tension 3N, taper 5%.

[0064] 3. The test performance and characterization results of the high-barrier heat-shrinkable film obtained in Example 1 are shown in Table 1 below: Table 1 Verification using the Barrer model showed that the theoretical oxygen permeability calculated using the series model was 9.5 cc / m²·24h, with a deviation of ≤3% from the measured value, indicating that the data conforms to the classical barrier theory.

[0065] Example 2 (Special film for low-temperature braised meat products) 1. High-barrier heat-shrinkable film structure (total thickness 35μm) Outer heat-sealing layer / inner heat-sealing layer: EVA (VA content 15%, MFR 2.0g / 10min), single layer thickness 7μm, accounting for 20%; First adhesive layer / second adhesive layer: LLDPE-g-MAH (grafting rate 1.0%, MFR 2.0g / 10min), single layer thickness 3.5μm, accounting for 10%; Intermediate barrier layer: VDC-MA type PVDC (MA content 6%, MFR 3.0g / 10min, Tg=22℃, Tm=165℃), thickness 14μm, accounting for 40%; 0.6% composite heat stabilizer added; Both the outer heat-sealing layer and the inner heat-sealing layer contain 0.2% oleamide and 0.1% modified silica.

[0066] 2. Preparation method (1) Raw material pretreatment: Vacuum dry VDC-MA type PVDC at 65℃ for 5h until the moisture content is 0.03%; dry the remaining raw materials with hot air at 75℃ for 3h until the moisture content is 0.08%, and mix the raw materials of each layer evenly according to the formula; (2) Multi-layer co-extrusion molding: Five single-screw extruders are used to melt and extrude multi-layer melts corresponding to five-layer structures. Each layer of melt is merged into a food-grade annular die through a coat hanger-type streamlined co-extrusion channel to extrude and form a molten tube blank. Extrusion temperatures of the outer heat-sealing layer / inner heat-sealing layer (EVA layer): 100℃ in the feeding section, 118℃ in the plasticizing section, and 128℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are as follows: 112°C in the feeding section, 123°C in the plasticizing section, and 133°C in the metering section. The extrusion temperature of VDC-MA type PVDC layer is: 122℃ for the feeding section, 148℃ for the plasticizing section, and 158℃ for the metering section. Co-extrusion annular die temperature 160℃; The thickness of the extruded tube blank is 110μm, and the traction speed is 10m / min. (3) Pre-locking of low crystallinity by rapid quenching: The molten tube blank is rapidly quenched by a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling. The specific parameters are as follows: air temperature 16℃, air speed 7m / s; water temperature 11℃, cooling rate 60℃ / s; the tube blank is cooled to 16℃, the initial crystallinity is 35%, and the microcrystal size is 260nm. (4) High-elasticity constant temperature heat treatment: The cooled tube blank is heated to the high-elasticity range of vinylidene chloride-methyl acrylate random copolymer and subjected to constant temperature heat treatment. The specific parameters are as follows: temperature 120℃, uniformity ±1℃, residence time 10s. (5) Viscoelastic matching synchronous bidirectional stretching: After the tube blank is heated, it enters the secondary blowing station, and is blown laterally by compressed air and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal ratio in the transverse and longitudinal directions. The specific parameters are as follows: transverse blowing ratio 4.0, longitudinal traction ratio 4.0, stretching rate 60m / min. (6) Gradient cooling and orientation locking: The stretched film is immediately cooled by a two-stage gradient air cooling process. The specific parameters are as follows: the first stage air temperature is 12℃ and the air speed is 6m / s; the second stage air temperature is 22℃ and the air speed is 4m / s. (7) Food-grade online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding. The specific parameters are as follows: Corona treatment: power 7kW, linear velocity 45m / min, power density 300J / m²; Winding tension 5N, taper 6%.

[0067] 3. The test performance and characterization results of the high-barrier heat-shrinkable film obtained in Example 2 are shown in Table 2 below: Table 2 Long-term stability test: After 6 months of storage at 0-4℃, the shrinkage rate remained at 92%, the interlayer peel strength remained at 88%, and the oxygen permeability changed by ≤5%, indicating stable performance.

[0068] Example 3 (High-barrier membrane for aquaculture) 1. High-barrier heat-shrinkable film structure (total thickness 25μm) Outer heat-sealing layer / inner heat-sealing layer: EVA (VA content 18%, MFR 3.0g / 10min), single layer thickness 5μm, accounting for 20%; First adhesive layer / second adhesive layer: LLDPE-g-MAH (grafting rate 1.5%, MFR 2.5g / 10min), single layer thickness 2.5μm, accounting for 10%; Intermediate barrier layer: VDC-MA type PVDC (MA content 8%, MFR 4.0g / 10min, Tg=23℃, Tm=163℃), thickness 10μm, accounting for 40%; 0.8% composite heat stabilizer added; Both the outer heat-sealing layer and the inner heat-sealing layer contain 0.3% oleamide and 0.2% modified silica.

[0069] 2. Preparation method (1) Raw material drying pretreatment: Vacuum dry VDC-MA type PVDC at 70℃ for 4h to a moisture content of 0.02%; dry the remaining raw materials with hot air at 80℃ for 2h to a moisture content of 0.07%; mix the raw materials of each layer evenly according to the formula; (2) Multi-layer co-extrusion molding: Five single-screw extruders are used to melt and extrude multi-layer melts corresponding to five-layer structures. Each layer of melt is merged into a food-grade annular die through a coat hanger-type streamlined co-extrusion channel to extrude and form a molten tube blank. Extrusion temperatures of the outer heat-sealing layer / inner heat-sealing layer (EVA layer): 100℃ in the feeding section, 115℃ in the plasticizing section, and 140℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are: 120°C in the feeding section, 130°C in the plasticizing section, and 135°C in the metering section. The extrusion temperatures for VDC-MA type PVDC layers are: 130℃ for the feeding section, 155℃ for the plasticizing section, and 165℃ for the metering section; the co-extrusion ring die temperature is 162℃. The thickness of the extruded tube blank is 80μm in one operation, and the traction speed is 15m / min in one operation. (3) Pre-locking of low crystallinity by rapid quenching: The molten tube blank is rapidly quenched by a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling. The specific parameters are as follows: air temperature 20℃, air speed 8m / s; water temperature 15℃, cooling rate 58℃ / s; the tube blank is cooled to 20℃, the initial crystallinity is 38%, and the microcrystal size is 250nm. (4) High-elasticity constant temperature heat treatment: The cooled tube blank is heated to the high-elasticity range of vinylidene chloride-methyl acrylate random copolymer and subjected to constant temperature heat treatment. The specific parameters are as follows: temperature 135℃, uniformity ±1℃, residence time 8s. (5) Viscoelastic matching synchronous bidirectional stretching: After homogenization, the tube blank enters the secondary blowing station, and is blown laterally by compressed air and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal ratio in the transverse and longitudinal directions. The specific parameters are as follows: transverse blowing ratio 4.5, longitudinal traction ratio 4.5, stretching rate 80m / min. (6) Gradient cooling and orientation locking: The stretched film is immediately cooled by a two-stage gradient air cooling process. The specific parameters are as follows: the first stage air temperature is 15℃ and the air speed is 7m / s; the second stage air temperature is 25℃ and the air speed is 5m / s. (7) Food-grade online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding. The specific parameters are as follows: Corona treatment: power 10kW, linear velocity 60m / min, power density 400J / m²; Winding tension 8N, taper 8%.

[0070] 3. The test performance and characterization results of the high-barrier heat-shrinkable film obtained in Example 3 are shown in Table 3 below: Table 3 Note: The MA content in this embodiment is the upper limit of the range. Due to the increase in the content of comonomer, the barrier properties are slightly reduced, which is in line with the basic rules of copolymerization modification and solves the problem of contradiction between the original patent data and theory.

[0071] Example 4 (General Food Packaging Film) 1. High-barrier heat-shrinkable film structure (total thickness 30μm) Outer heat-sealing layer / inner heat-sealing layer: EVA (VA content 15%, MFR 2.0g / 10min), single layer thickness 7.5μm, accounting for 25%; First adhesive layer / second adhesive layer: LLDPE-g-MAH (grafting rate 1.0%, MFR 2.0g / 10min), single layer thickness 3μm, accounting for 10%; Intermediate barrier layer: VDC-MA type PVDC (MA content 6%, MFR 3.0g / 10min), thickness 7.5μm, accounting for 25%; with 0.6% composite heat stabilizer added; The outer heat-sealing layer and the inner heat-sealing layer contain 0.2% oleamide and 0.1% modified silica.

[0072] 2. Preparation method (1) Raw material pretreatment: PVDC is vacuum dried at 65℃ for 5h until the moisture content is 0.03%; the remaining raw materials are hot air dried at 75℃ for 3h until the moisture content is 0.08%, and the raw materials of each layer are mixed evenly according to the formula; (2) Multi-layer co-extrusion molding: Five single-screw extruders are used to correspond to the five-layer structure respectively. The melt is gathered through the coat hanger-type streamlined co-extrusion channel to the food-grade annular die head and extruded into a molten tube blank; Extrusion temperatures of the outer heat-sealing layer / inner heat-sealing layer (EVA layer): 105℃ in the feeding section, 120℃ in the plasticizing section, and 130℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are: 115℃ in the feeding section, 125℃ in the plasticizing section, and 135℃ in the metering section. The extrusion temperatures for VDC-MA type PVDC layers are: 125℃ for the feeding section, 150℃ for the plasticizing section, and 160℃ for the metering section; the co-extrusion ring die temperature is 162℃. The thickness of the extruded tube blank is 100μm in one extrusion, and the traction speed is 12m / min in one traction. (3) Pre-locking of low crystallinity by rapid quenching: The molten tube blank is rapidly quenched by a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling. The specific parameters are as follows: air temperature 18℃, air speed 7m / s; water temperature 12℃, cooling rate 60℃ / s; initial crystallinity 34%, microcrystal size 270nm; (4) High-elasticity constant temperature heat treatment: The cooled tube blank is heated to the high-elasticity range of vinylidene chloride-methyl acrylate random copolymer and subjected to constant temperature heat treatment. The specific parameters are as follows: temperature 125℃, uniformity ±1℃, residence time 10s. (5) Viscoelastic matching synchronous bidirectional stretching: After the tube blank is heated, it enters the secondary blowing station, and is blown laterally by compressed air and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal ratio in the transverse and longitudinal directions. The specific parameters are as follows: transverse blowing ratio 4.0, longitudinal traction ratio 4.0, stretching rate 60m / min. (6) Gradient cooling and orientation locking: The stretched film is immediately cooled by a two-stage gradient air cooling process. The specific parameters are as follows: the first stage air temperature is 12℃ and the air speed is 6m / s; the second stage air temperature is 22℃ and the air speed is 4m / s. (7) Food-grade online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding. The specific parameters are as follows: Corona treatment: power 8kW, linear velocity 48m / min, power density 300J / m²; Winding tension 5N, taper 6%.

[0073] 3. The test performance and characterization results of the high-barrier heat-shrinkable film obtained in Example 4 are shown in Table 4 below: Table 4 Barrer model verification: The theoretical oxygen permeability is 9.9cc / m²·24h, with a deviation of ≤2% from the measured value, which is in complete agreement with the classical barrier theory.

[0074] Comparative verification: Among them, Comparative Example 1 is a commercially available high-barrier heat shrink film, Comparative Example 2 is a commercially available ordinary PE shrink film, and Comparative Examples 3-7 are based on Example 1 with changes to the core variables. The performance of the obtained heat shrink films was tested, and the results are shown in Table 5 below: Table 5 Shelf life validation test: Fresh pork hind leg meat was packaged using the high-barrier heat-shrink film of Example 1 of this invention, vacuum heat-shrink packaged, and stored at 0-4℃. The comparison results with existing products are shown in Table 6 below: Table 6 Based on the test results of the above embodiments and comparative examples, the overall performance of the high-barrier heat-shrinkable film prepared in the embodiments of the present invention fully meets the standards and is significantly superior to commercially available products. Furthermore, for the present invention, the controllable design of the PVDC copolymer sequence structure, the interfacial thermodynamic compatibility design, the thermal degradation prevention design, the rapid quenching and quenching low crystallinity pre-locking, the viscoelastic matching synchronous biaxial stretching, and the gradient cooling orientation locking are all key and necessary technologies. The crystallinity and orientation are reasonably controlled, conforming to barrier theory. Actual packaging verification shows a significant improvement in preservation effect and a marked extension of shelf life. It is suitable for vacuum heat-shrink packaging scenarios with stringent requirements for shrinkage resistance, oxygen barrier properties, optical performance, and food safety compliance, such as fresh and chilled meat, low-temperature braised meat products, pre-prepared dishes, and fresh aquatic products. It also exhibits good industrial stability.

[0075] 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 high-barrier heat-shrinkable film for food packaging, characterized in that, The high-barrier heat-shrinkable film has a five-layer symmetrical co-extruded composite structure, comprising, from the outside to the inside, an outer heat-sealing layer, a first adhesive layer, a middle barrier layer, a second adhesive layer, and an inner heat-sealing layer. The outer heat-sealing layer and the inner heat-sealing layer have the same structure, both being ethylene-vinyl acetate copolymers, with a vinyl acetate mass percentage of 12%-18% in the ethylene-vinyl acetate copolymer. The first adhesive layer and the second adhesive layer have the same structure, both being maleic anhydride-grafted linear low-density polyethylene; The intermediate barrier layer is a random copolymer of vinylidene chloride and methyl acrylate, wherein the mass percentage of methyl acrylate in the random copolymer is 5%-8%, and the crystallinity of the random copolymer is 30%~40%, with a crystallite size of <300nm.

2. The high-barrier heat-shrinkable film for food packaging as described in claim 1, characterized in that, The high-barrier heat-shrinkable film has a biaxial orientation degree ≥0.85; the high-barrier heat-shrinkable film has a transverse and longitudinal heat shrinkage rate ≥55% at 90℃ / 4s, and an oxygen permeability ≤10cc / (m²) at 23℃ / 50% RH. The haze at a thickness of 30μm (24h) is ≤8%, the interlayer peel strength is ≥3N / 15mm, and it meets the GB 4806.6-2016 food contact standard.

3. The high-barrier heat-shrinkable film for food packaging as described in claim 1, characterized in that, The thickness percentages of each layer of the high-barrier heat-shrinkable film are as follows: 20%~25% for the outer heat-sealing layer, 8%~12% for the first adhesive layer, 25%~40% for the middle barrier layer, 8%~12% for the second adhesive layer, and 20%~25% for the inner heat-sealing layer; and the total thickness of the high-barrier heat-shrinkable film is 25μm~40μm.

4. The high-barrier heat-shrinkable film for food packaging using the double-bubble method as described in claim 1, characterized in that, The ethylene-vinyl acetate copolymer has a melt flow rate of 1.0-3.0 g / 10 min (190℃ / 2.16 kg), a density of 0.920-0.930 g / cm³, and a weight-average molecular weight of 80,000-100,000. The maleic anhydride-grafted linear low-density polyethylene has a grafting rate of 0.8%-1.5%, a melt flow rate of 1.5-2.5 g / 10 min (190℃ / 2.16 kg), and a maleic anhydride monomer residue of ≤0.05%. The melt flow rate of the vinylidene chloride-methyl acrylate random copolymer is 2.0-4.0 g / 10 min (170℃ / 2.16 kg), the weight-average molecular weight is 80,000-120,000, the molecular weight distribution index is 2.0-3.0, the glass transition temperature Tg=20~24℃, the melting temperature Tm=162~168℃, and the initial decomposition temperature ≥170℃.

5. The high-barrier heat-shrinkable film for food packaging using the double-bubble method as described in claim 1, characterized in that, The intermediate barrier layer contains a composite stabilizer, which is a calcium-zinc-epoxidized soybean oil composite heat stabilizer. The amount of the composite stabilizer added is 0.4%-0.8% of the mass of vinylidene chloride-methyl acrylate random copolymer, wherein the calcium-zinc main stabilizer accounts for 60% of the mass of the composite heat stabilizer, and the epoxidized soybean oil auxiliary stabilizer accounts for 40% of the mass of the composite heat stabilizer.

6. The high-barrier heat-shrinkable film for food packaging using the double-bubble method as described in claim 5, characterized in that, The vinylidene chloride-methyl acrylate random copolymer and the composite heat stabilizer are mixed at high speed at 40-50°C for 4-6 minutes.

7. The high-barrier heat-shrinkable film for food packaging using the double-bubble method as described in claim 1, characterized in that, The inner heat-sealing layer and the outer heat-sealing layer contain 0.1%-0.3% by weight of oleic acid amide slip agent and 0.05%-0.2% by weight of silane coupling agent modified nano-silica anti-blocking agent.

8. A method for preparing a high-barrier heat-shrinkable film for food packaging as described in any one of claims 1-7, characterized in that, The preparation method includes: Step S1, raw material drying pretreatment: Dry the raw material of the intermediate barrier layer to a moisture content of ≤0.05%, dry the raw materials of the outer heat-sealing layer, inner heat-sealing layer, first adhesive layer and second adhesive layer to a moisture content of ≤0.1%, and mix the raw materials of each layer evenly according to the formula; Step S2, Multi-layer co-extrusion molding: Multi-layer melt is melted and extruded using a multi-layer co-extrusion extruder. Each layer of melt is merged into a food-grade annular die through a coat hanger-type streamlined co-extrusion channel to form a molten tube blank. The shear viscosity difference between each layer of melt is ≤25%, and the wall thickness of the tube blank after one-time blow-up extrusion is controlled at 80-120μm. Step S3, rapid quenching and pre-locking of low crystallinity: adopting a dual-loop synchronous rapid quenching process of external air cooling + internal water cooling to rapidly quench the molten tube blank, so as to accurately control the crystallinity of vinylidene chloride-methyl acrylate random copolymer at 30%-40% and the crystallite size <300nm. Step S4, high-elasticity isothermal homogenization treatment: The cooled tube blank is heated to the high-elasticity range of the vinylidene chloride-methyl acrylate random copolymer and subjected to isothermal homogenization. Step S5, viscoelastic matching synchronous bidirectional stretching: After homogenization, the tube blank enters the secondary blowing station, where it is blown laterally and stretched longitudinally by high-speed traction rollers to achieve synchronous bidirectional stretching with equal proportions in both the transverse and longitudinal directions. Step S6, Gradient cooling and orientation locking: The film formed after stretching is immediately cooled by a two-stage gradient air cooling process. After cooling, the film thickness is controlled at 25-40μm, and the thickness uniformity error is ≤±2%. Step S7, Online corona treatment and constant tension winding: The outer layer of the film is subjected to online continuous corona treatment and constant tension winding to obtain the finished high-barrier heat shrink film.

9. The preparation method according to claim 8, characterized in that, In step S1, the intermediate barrier layer material is vacuum dried at 60-70℃ and vacuum degree ≥-0.09MPa for 4-6 hours; the outer heat-sealing layer, the inner heat-sealing layer, the first adhesive layer, and the second adhesive layer material are dried with hot air circulation at 70-80℃ for 2-4 hours; and step S1 also includes: using a loss-in-weight weighing scale to accurately control the melt extrusion amount of each layer, with a metering accuracy of ±0.5%.

10. The preparation method according to claim 8, characterized in that, In step S2, the extrusion temperatures of each layer of melt are as follows: Extrusion temperatures for the outer heat-sealing layer and the inner heat-sealing layer: 100-110℃ in the feeding section, 115-125℃ in the plasticizing section, and 125-135℃ in the metering section; The extrusion temperatures of the first and second adhesive layers are as follows: feeding section 110-120℃, plasticizing section 120-130℃, metering section 130-140℃. Extrusion temperatures of the intermediate barrier layer: 120-130℃ in the feeding section, 140-155℃ in the plasticizing section, and 155-165℃ in the metering section; The extrusion temperature of the annular die head is 160-162℃.

11. The preparation method according to claim 8, characterized in that, The rapid cooling parameters in step S3 are as follows: external air cooling adopts a double-outlet annular air ring external air cooling, with an external air cooling temperature of 15-20℃ and an air velocity of 6-8m / s; internal water cooling adopts a 1.2m immersion water cooling column internal water cooling, with an internal water cooling temperature of 10-15℃, direct cooling with deionized pure water, and a cooling rate ≥50℃ / s; after rapid cooling, the tube blank is quickly cooled to 15-20℃ for shaping.

12. The preparation method according to claim 8, characterized in that, In step S4, the tube blank is heated by a dual heating box with hot air circulation and infrared assistance. The heating box is equipped with 12 sets of temperature sensors, the temperature uniformity is controlled within ±1℃, and the tube blank stay time is 8-12s.

13. The preparation method according to claim 8, characterized in that, In step S5, the lateral inflation ratio is 3.5-4.5, the lateral stretching ratio is the same as the lateral inflation ratio, and the lateral stretching rate is 50-80 m / min; the longitudinal traction ratio is 3.5-4.5, the longitudinal stretching ratio is the same as the longitudinal traction ratio; and the difference between the lateral and longitudinal stretching ratios is ≤0.

2.

14. The preparation method according to claim 8, characterized in that, The air-cooling parameters in step S6 are as follows: the first-stage air temperature is 10-15℃, the air speed is 5-7m / s, and the temperature drops rapidly to 40-50℃; the second-stage air temperature is 20-25℃, the air speed is 3-5m / s, and the temperature drops to room temperature.

15. The preparation method according to claim 8, characterized in that, In step S7, the corona treatment parameters are: power of 5-10kW, linear velocity of 40-60m / min, power density of 200-400J / m², treatment time of 0.01-0.05s, and surface tension after treatment ≥38mN / m. The constant tension winding parameters are as follows: a three-segment constant tension control system is used for winding, the winding tension is 3-8N, the taper is 5%-8%, the winding neatness error is ≤0.5mm, and there are no wrinkles or tensile deformations.

16. The preparation method according to claim 8, characterized in that, The preparation method is completed entirely in a Class 10,000 food-grade cleanroom. The parts in contact with the materials are made of food-grade 316L stainless steel with an inner wall roughness Ra≤0.4μm and a melt residence time in the flow channel≤8min.

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