High-barrier easy-to-uncover stretch-formed food packaging composite film and preparation method thereof

Abstract

The invention relates to the technical field of food packaging materials, and discloses a high-barrier easy-to-uncover stretch-formed food packaging composite film and a preparation method thereof. The invention relates to a high-density polyethylene film which is prepared from the following raw materials in parts by weight: 5-15 parts of copolymerized modified polyethylene glycol terephthalate, 90-95 parts of ethylene-vinyl alcohol copolymer, 5-15 parts of zinc oxide nanoparticles, 0.5-3 parts of a cross-linking agent, 80-90 parts of grafted maleic anhydride low-density polyethylene, 10-20 parts of polyvinyl alcohol and 0.5-2 parts of a cross-linking aid. By constructing the modified ethylene-vinyl alcohol copolymer barrier layer containing the zinc oxide nanoparticles, the gas shielding capability of the composite film is remarkably improved. The introduction of the nano structure forms a multi-scale diffusion barrier path in the barrier main phase, and meanwhile, a stable internal network structure is constructed through physical crosslinking, so that the air permeability fluctuation of the film layer in the use process is effectively inhibited, and the fresh-keeping performance of the film layer in packaging application is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of food packaging materials, in particular to a high-barrier, easy-to-reveal, stretch-formed food packaging composite film and a preparation method thereof. Background Art

[0002] At present, the comprehensive requirements of food packaging for barrier properties, thermoforming adaptability, and sealing convenience are increasing day by day, especially in the fields of cold chain transportation, ready-to-eat food, and precision molding packaging. High-performance composite film materials have become the focus of research and application. Traditional multi-layer composite films often face a variety of performance bottlenecks in actual use, which limits their promotion and application in high-standard food packaging scenarios.

[0003] In terms of barrier properties, although ethylene-vinyl alcohol copolymer (EVOH) is widely used in food packaging composite films due to its excellent gas barrier ability, its barrier stability is prone to fluctuations in hot and humid environments. Studies have attempted to improve its barrier properties by adding inorganic nanoparticles, but due to insufficient particle dispersion or poor interface compatibility, it is often difficult to form a uniform and stable internal microstructure, resulting in unstable barrier properties of the film during long-term use.

[0004] In the application of thermoforming, existing composite films often use ordinary polyester (such as PET) or polypropylene as the support layer, but these materials have poor dimensional stability during the hot stretching process, and are prone to interlayer peeling, deformation and other problems, making it difficult to meet the requirements of complex three-dimensional molding packaging for the synergy of the film structure. Especially in multi-layer composite structures, the interface bonding force between the support layer and the functional layer is insufficient, which can easily affect the stability of the overall composite structure.

[0005] In terms of sealing performance, traditional heat-sealing materials often pursue high-strength sealing, but high sealing strength often makes it difficult to peel off the film, affecting the user experience. Although some heat-sealing layers can be peeled off with low strength, the sealing reliability is insufficient, there is a risk of leakage, and it is difficult to balance the sealing strength and the convenience of peeling. In addition, the existing sealing layer design usually lacks a flexible control structure, and the interface stress is unevenly distributed, which can easily lead to tearing failure. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a high-barrier, easy-to-open, stretch-molded food packaging composite film and a preparation method thereof, which solves the problems of the existing food packaging composite films in terms of unstable gas barrier properties, poor thermoforming adaptability, and difficulty in balancing sealing and opening properties.

[0007] To achieve the above objectives, the present invention is implemented by the following technical scheme: a high-barrier, easy-to-open, stretch-formed food packaging composite film, measured by weight, comprises the following components;

[0008] 5–15 parts of copolymer-modified polyethylene terephthalate glycol, 90–95 parts of ethylene-vinyl alcohol copolymer, 5–15 parts of zinc oxide nanoparticles, 0.5–3 parts of crosslinking agent, 80–90 parts of maleic anhydride-grafted low-density polyethylene, 10–20 parts of polyvinyl alcohol, 0.5–2 parts of crosslinking aid.

[0009] Preferably, the comonomer of the copolymer-modified polyethylene terephthalate glycol is vinyl alcohol, the content of the vinyl alcohol comonomer is 3–10 wt%, and the glass transition temperature of the PET is 70–80 °C.

[0010] Preferably, in the ethylene-vinyl alcohol copolymer (EVOH), the content of the ethylene monomer is 27–38 mol%, and the content of the vinyl alcohol monomer is 62–73 mol%.

[0011] Preferably, the crosslinking agent is glutaraldehyde, and the crosslinking aid is trimethylolpropane triacrylate.

[0012] Preferably, the grafting amount of maleic anhydride in the maleic anhydride-grafted low-density polyethylene is 0.5–5 wt%, and the molecular weight of the low-density polyethylene is 20,000–100,000 Da.

[0013] A preparation method of a high-barrier and easily peelable stretch-forming food packaging composite film includes the following steps;

[0014] S1. Preparation of the outer base film layer: Provide a copolymer-modified polyethylene terephthalate glycol film with a thickness of 5–15 μm, an ethylene alcohol copolymerization ratio of 3–10 wt%, and a glass transition temperature of 70–80 °C, and perform corona treatment to make the surface tension reach ≥40 mN / m;

[0015] S2. Preparation of the barrier layer: Melt the ethylene-vinyl alcohol copolymer, add ZnO nanoparticles (particle size 20–80 nm, addition amount 5–15 parts by mass), and add 0.5–3 parts of crosslinking agent, mix evenly at 160–180 °C, and cool and extrude to form a barrier film layer with a thickness of 10–25 μm;

[0016] S3. Preparation of the peelable heat-sealing layer: Melt the PE-g-MAH at 130–150 °C, add 10–20 parts of PVA microemulsion, and 0.5–2 parts of crosslinking aid, and mix evenly to form a peelable heat-sealing layer with a thickness of 15–30 μm;

[0017] S4. Three-layer hot pressing and compounding: Hot press and compound the three-layer film materials obtained in steps 1) to 3) in sequence, the compounding temperature is 130–150 °C, the compounding pressure is 10–20 MPa, and at the same time introduce far-infrared radiation (160–180 °C) in the compounding area to promote crosslinking;

[0018] S5, cooling and shaping: the composite film is cooled by a cooling roller, and the temperature is controlled at 15-30°C;

[0019] S6. Thermoforming treatment: The composite film is stretched and thermoformed at 110-130°C for 0.5-2 seconds, and then cooled to obtain the final packaging film.

[0020] Furthermore, the composite film includes, from the outside to the inside: an outer base film with thermal stability and molding support, an intermediate barrier layer with gas and water vapor barrier function, and an inner heat seal / peel layer with controllable sealing and peeling functions. Each layer of material not only has its own function, but also forms a multi-combination interface through a hot-pressing cross-linking process to construct an overall mechanical support-barrier-easy-peel composite system.

[0021] Preferably, the outer base film, barrier layer and peeling heat sealing layer are preheated by hot rollers before lamination, and the preheating temperature is controlled as follows: 80-100°C for the outer base film layer, 90-110°C for the barrier layer, 90-100°C for the peeling heat sealing layer, and the preheating time is 20-60 seconds.

[0022] Furthermore, in the outer base film, copolymer-modified polyethylene terephthalate (PET) is used as the main material, and its rigidity and flexibility and polar compatibility are adjusted by introducing vinyl alcohol copolymer monomers. This copolymer structure enables PET to obtain a better bonding interface with the intermediate barrier layer while maintaining the hot tensile strength, enhances the interlayer adhesion in the subsequent composite process, and is compatible with strain synergy under thermoforming conditions. In addition, the surface tension is further increased by corona treatment, providing a physical basis for subsequent binder-free hot pressing composites. This strategy of combining copolymerization with surface treatment not only maintains the shaping effect of PET as a structural layer, but also creates interface conditions for the effective adhesion of the intermediate layer material. The intermediate barrier functional layer is mainly composed of ethylene-vinyl alcohol copolymer (EVOH), supplemented by zinc oxide nanoparticles for modification, and small molecule cross-linking agents are introduced to realize the construction of an organic-inorganic hybrid network. The high polarity and crystalline structure of EVOH provide gas barrier function, while the introduction of ZnO particles provides inorganic layered or particle barrier paths, increasing the complexity of the diffusion path, and on the other hand, improves the thermal stability of the film layer during thermal processing by physical cross-linking or interface coordination reaction with EVOH segments. The participation of the cross-linking agent further forms a micro-cross-linking network in the system, inhibiting high-temperature softening or deformation, thereby ensuring that there is no migration or peeling between layers during the thermoforming process.

[0023] Preferably, the emulsifier used in the PVA microemulsion in step S3 is nonylphenol polyoxyethylene ether or alkyl polyglycoside, and the amount of the emulsifier used is 1-3wt% of the mass of PVA.

[0024] Preferably, in the step S4, the irradiation time of the infrared radiation is 1–5 seconds, and a medium and far infrared radiation source with a wavelength of 2–4 μm is used.

[0025] Preferably, in the step S6, vacuum thermoforming or positive pressure gas forming is adopted for forming, the mold cavity temperature is controlled at 110–130 °C, and the forming time is 0.5–2 seconds.

[0026] The present invention provides a high-barrier and easily peelable stretch-formed food packaging composite film and a preparation method thereof. It has the following beneficial effects:

[0027] 1. By constructing a barrier layer containing zinc oxide nanoparticle-modified ethylene-vinyl alcohol copolymer, the present invention significantly improves the gas barrier ability of the composite film. The introduction of the nano-structure forms multi-scale diffusion barrier paths in the barrier matrix, and at the same time constructs a stable internal network structure through physical cross-linking, effectively suppressing the permeability fluctuation of the film layer during use and ensuring its freshness preservation performance in packaging applications.

[0028] 2. By using a modified polyethylene terephthalate containing an ethylene glycol copolymer structure as the outer supporting substrate, the composite film not only has thermal stretching adaptability but also can improve the bonding interface of the interlayer structure. This structure not only enhances the dimensional stability of the film body during the stretch-forming process but also provides a good mechanical support basis for the intermediate functional layer, which is the key link to ensure the synchronous and collaborative forming of the multi-layer structure.

[0029] 3. By introducing a polyvinyl alcohol-based regulating component into the heat-sealing / peeling layer and constructing a synergistic structure with grafted polyethylene, the present invention realizes an effective balance between the sealing strength and the peeling convenience of the composite film. This layer design takes into account polarity regulation and flexible transition, significantly optimizing the stress distribution at the sealing interface and contributing to a packaging experience with firm sealing and smooth peeling.

[0030] 4. By using the far-infrared-assisted hot pressing composite technology, the present invention promotes cross-linking reactions and interfacial fusion during the film layer composite process to form a stable multi-phase combined structure. This process enhances the structural integrity and complex adaptability between the film layers, is particularly suitable for stretch packaging scenarios with high requirements for forming shape retention and interlayer stability, and improves the adaptability and use reliability of the composite film under complex packaging processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flowchart of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0033] Please refer to the attached Figure 1 ;

[0034] Example 1:

[0035] A preparation method of a high-barrier and easily peelable stretch-forming food packaging composite film is as follows:

[0036] Raw material composition ratio (parts by mass):

[0037] Copolymerized modified polyethylene terephthalate (PET): 10 parts (ethylene glycol copolymerization ratio 6 wt%, Tg = 75 °C);

[0038] Ethylene-vinyl alcohol copolymer (EVOH): 92 parts (ethylene / vinyl alcohol ratio = 32 / 68 mol%);

[0039] Zinc oxide nanoparticles (ZnO): 10 parts (particle size about 40 nm);

[0040] Crosslinking agent (glutaraldehyde): 2 parts;

[0041] PE-g-MAH: 85 parts (grafting rate 2.5 wt%, LDPE molecular weight 50,000 Da);

[0042] Polyvinyl alcohol (PVA): 15 parts (degree of alcoholysis 90%, molecular weight about 40,000 Da);

[0043] Crosslinking aid (TAIC): 1 part

[0044] Nonylphenol polyoxyethylene ether emulsifier: 2% of the mass of PVA

[0045] S1 Preparation of the outer base film layer The PET copolymer film is extruded into a thickness of 10 μm and corona-treated to increase the surface tension to 42 mN / m.

[0046] S2 Preparation of the barrier layer

[0047] EVOH is melted at 170 °C, ZnO (10 parts) and glutaraldehyde (2 parts) are added, and after high-speed shear mixing for 5 minutes, it is cooled and extruded to form an intermediate barrier film with a thickness of 20 μm.

[0048] S3 Preparation of the peelable heat-sealing layer

[0049] The PE-g-MAH was melted at 140 °C, and the PVA microemulsion and TAIC were added and blended at high speed. Then, it was cast to form a heat-sealing layer with a thickness of 25 μm.

[0050] S4 Three-layer lamination

[0051] Each film layer was laminated by hot pressing in sequence. The temperature was set at 145 °C, the pressure was 15 MPa, and the far-infrared irradiation time was 3 seconds (wavelength 3 μm).

[0052] S5 Cooling and shaping

[0053] The temperature of the cooling roll was 25 °C, and the pressure was maintained for shaping for 10 seconds.

[0054] S6 Thermoforming treatment

[0055] The composite film was vacuum formed at 120 °C for 1 second and then cooled and formed to obtain the final packaging film, and the peel strength was about 1.0 N / cm

[0056] Example 2:

[0057] Raw material component ratio (parts by mass):

[0058] PET: 5 parts (ethylene alcohol copolymerization ratio 3 wt%, Tg = 70 °C);

[0059] EVOH: 90 parts (ethylene / vinyl alcohol ratio = 27 / 73 mol%);

[0060] ZnO: 5 parts (particle size about 20 nm);

[0061] Glutaraldehyde: 0.5 part;

[0062] PE-g-MAH: 80 parts (grafting rate 0.5 wt%, LDPE molecular weight is 20,000 Da);

[0063] PVA: 10 parts (degree of alcoholysis 88%, molecular weight about 30,000 Da);

[0064] TAIC: 0.5 part;

[0065] Alkyl polyglycoside emulsifier: 1% of the mass of PVA;

[0066] Process steps:

[0067] S1 Preparation of the outer base film layer

[0068] The PET film was blown into a film with a thickness of 5 μm and corona treated to a surface tension of 40 mN / m.

[0069] S2 Preparation of the barrier layer

[0070] EVOH is melted at 160°C, ZnO and a crosslinking agent are added, and after stirring and mixing, it is extruded to obtain a film layer with a thickness of 10 μm.

[0071] Preparation of S3 peeling heat-sealing layer

[0072] After PE-g-MAH is melted at 130°C, PVA microemulsion and additives are added, and it is cast into a film to form a heat-sealing layer with a thickness of 15 μm.

[0073] For S4 hot pressing and laminating, the temperature is 130°C, the pressure is 10 MPa, the infrared irradiation time is 1 second, and the wavelength is 2 μm.

[0074] S5 Cooling and shaping

[0075] The temperature of the cooling roll is set at 15°C.

[0076] S6 Thermoforming treatment

[0077] Using the positive pressure air forming method, it is formed at 110°C for 0.5 seconds and then cooled. The overall peel strength of the composite film is 0.8 N / cm.

[0078] Example 3:

[0079] Raw material component ratio (parts by mass):

[0080] PET: 15 parts (ethylene alcohol copolymerization ratio 10 wt%, Tg = 80°C);

[0081] EVOH: 95 parts (ethylene / vinyl alcohol ratio = 38 / 62 mol%);

[0082] ZnO: 15 parts (particle size about 80 nm);

[0083] Glutaraldehyde: 3 parts;

[0084] PE-g-MAH: 90 parts (grafting rate 5 wt%, LDPE molecular weight 100,000 Da);

[0085] PVA: 20 parts (degree of alcoholysis 93%, molecular weight about 50,000 Da);

[0086] TAIC: 2 parts;

[0087] Nonylphenol polyoxyethylene ether emulsifier: 3% of the mass of PVA;

[0088] Process steps:

[0089] S1 Preparation of outer base film layer

[0090] The PET layer is extruded through a T-die with a thickness of 15 μm, and the surface tension is increased to 45 mN / m after corona treatment.

[0091] Preparation of S2 Barrier Layer

[0092] After EVOH is melted at 180°C, ZnO (15 parts) and glutaraldehyde (3 parts) are added, and after being mixed evenly, it is extruded into a barrier layer with a thickness of 25 μm.

[0093] Preparation of S3 Release Heat-sealing Layer

[0094] PE-g-MAH is melted at 150°C, PVA emulsion and TAIC additive are added, and a 30-μm inner heat-sealing film is calendered.

[0095] S4 Three-layer Hot Pressing and Laminating

[0096] The three-layer film is hot-pressed and laminated at 150°C, the lamination pressure is 20 MPa, and the infrared irradiation time is 5 seconds (wavelength 4 μm).

[0097] S5 Cooling and Shaping

[0098] It is rapidly cooled by a 30°C cooling roller.

[0099] S6 Thermoforming Treatment

[0100] The film material is vacuum formed at 130°C, and after the forming time of 2 seconds, it is naturally cooled. The obtained film has a peel strength of 1.2 N / cm.

[0101] Comparative Example 1: Compared with Example 1, the difference is that zinc oxide nanoparticles (ZnO) are not added, and the rest are the same.

[0102] Comparative Example 2: Compared with Example 1, the difference is that the content of ethylene-vinyl alcohol copolymer (EVOH) is 98 parts by mass, exceeding the specified range of 90–95 parts, and the rest are the same.

[0103] Comparative Example 3: Compared with Example 1, the difference is that PVA microemulsion is not added to the PE-g-MAH heat-sealing layer, and the rest are the same.

[0104] Comparative Example 4: Compared with Example 1, the difference is that far-infrared irradiation is cancelled, and only ordinary hot-pressing and laminating are used, and the rest are the same.

[0105] Experiment 1: Instructions for Oxygen Transmission Rate (OTR) Test Experiment

[0106] Experiment Purpose:

[0107] To test the oxygen transmission performance of different film samples (Example 1 and Comparative Example 1) to evaluate the influence of zinc oxide nanoparticles on the barrier performance of the composite film.

[0108] Oxygen transmission rate tester (such as MOCON OX-TRAN2 / 21);

[0109] Temperature and Humidity Control Chamber (constant at 23°C, 50% RH);

[0110] Nitrogen, High-Purity Oxygen (99.999%);

[0111] Proofing Tools, Thickness Gauges, Sealing Fixtures, etc.

[0112] Experimental Procedures:

[0113] Sample Preparation

[0114] Cut the composite films of Example 1 and Comparative Example 1 into 100 mm × 100 mm square samples, ensuring that the film surface has no wrinkles, scratches, or pinholes. Measure and record the thickness (μm) of each sample as a reference for test data normalization.

[0115] Calibrate the oxygen permeability tester using a standard film to ensure the instrument sensitivity and the reference gas flow are accurate.

[0116] Install the film sample into the test chamber, ensuring that the edges of the film sample are tightly sealed to avoid oxygen leakage or edge penetration affecting the data.

[0117] Test Temperature: 23.0 ± 0.1°C

[0118] Relative Humidity: 50% RH

[0119] Oxygen Concentration Gradient: 100% O2 (upper side) vs 0% (nitrogen lower side)

[0120] Start the test system and record the amount of oxygen permeated through the film sample per unit time, with the unit of cm 3 / (m 2 ·day·atm).

[0121] Test 3 pieces of each group of film samples, take the average value and record the individual differences to enhance the statistical significance of the comparison.

[0122] Organize the OTR data obtained from the tests of each film sample, calculate the maximum / minimum / average values, and analyze the differences in the results.

[0123] Table 1:

[0124] Sample number Test temperature (°C) Thickness (μm) OTR value Example 1-1 23.1 55.1 1.63 Example 1-2 22.9 54.8 1.59 Example 1-3 23 55.3 1.67 Comparative example 1-1 23 54.9 3.88 Comparative example 1-2 23.1 55.2 4.12 Comparative example 1-3 23 55 3.94

[0125] Summary;

[0126] The oxygen transmission rate test results show that, under the same film thickness and test conditions, the oxygen transmission rate of the comparative sample without zinc oxide nanoparticles is significantly higher than that of the film material with added nanoparticles. This indicates that zinc oxide can effectively enhance the barrier performance in the copolymerized EVOH matrix, playing a significant synergistic role. By introducing ZnO particles with a nano-size of about 40 nm, a multiphase dispersion system is formed in the film, increasing the effective path for gas molecules to pass through the film layer, thereby extending the diffusion channel and reducing the penetration rate of oxygen molecules.

[0127] This synergistic barrier effect not only depends on the shielding effect of the nanoparticles themselves, but more on the interfacial compatibility between them and the polar EVOH matrix. Under the process conditions adopted in the present invention, zinc oxide can be uniformly dispersed in the molten EVOH and form a stable structure with the assistance of a cross-linking agent, enabling it to construct a multi-level and multi-scale barrier network at the microscale. This composite structure can effectively inhibit the linear permeation behavior of gases, presenting better gas shielding performance than a single EVOH film.

[0128] In addition, compared with the problems of large particle size and poor interfacial bonding of traditional fillers, nano-sized zinc oxide matches EVOH in terms of size and polarity, and is not prone to agglomeration or precipitation, thus ensuring the stability of the barrier function of the composite film in subsequent hot pressing and thermoforming processes. It can be seen that by constructing a synergistic system of zinc oxide and EVOH, the overall barrier performance of the composite film is significantly improved without sacrificing flexibility and processability.

[0129] Experiment 2: Test instructions for thickness uniformity and interlayer adhesion

[0130] Experiment purpose:

[0131] Evaluate the influence of excessive EVOH dosage on the overall structural uniformity and interlayer bonding force of the composite film, and analyze the possible interfacial instability phenomena during film formation and compounding.

[0132] Inductive thickness gauge (accuracy ±0.1 μm);

[0133] 180° peel strength tester (complies with GB / T2790 standard);

[0134] High-resolution microscopic imaging equipment (for photographing the morphology of the peeled surface);

[0135] Standard sample cutting die and fixture.

[0136] Experiment steps:

[0137] Cut the composite films obtained in Example 1 and Comparative Example 2 into specifications of 30 mm × 150 mm for thickness and peel strength tests. Take 3 samples of each film type, number and mark them.

[0138] Use an inductive thickness gauge to take 5 equally spaced measurement points along the length direction of each film sample, record the measurement data, and calculate the average thickness and standard deviation of each sample to evaluate the thickness uniformity of the film layer.

[0139] Install the film sample into a 180° peel test fixture and conduct an interlayer peel test at a speed of 100 mm / min. Record the real-time tension change during the peeling process. Record the average values of 3 repeated tests respectively.

[0140] Use a microscope or digital camera to magnify and observe the interlayer interface after peeling, and record whether there are abnormal delamination phenomena such as peeling, delamination, and holes.

[0141] Analyze the thickness distribution trend and peel strength of different samples to determine whether there are structural non-uniformities or interface weakening phenomena caused by excessive EVOH ratio.

[0142] Table 2:

[0143]

[0144]

[0145] Summary;

[0146] The experimental results show that when the dosage of ethylene-vinyl alcohol copolymer significantly exceeds the reasonable proportion range, the thickness uniformity of the composite film decreases significantly, and the interlayer peel strength also weakens greatly. The main reason is that a high proportion of EVOH increases the shrinkage rate of the film layer during the cooling process, resulting in stress mismatch in the thermal expansion and contraction behavior with other components (such as modified PET and heat-sealing layer), easily forming interface micro-strain and structural stress concentration, and ultimately leading to unstable stress release paths inside the film layer, inducing local thickness fluctuations and interface peeling.

[0147] In addition, EVOH itself is a polar polymer. When it is used in excess, it will increase the rigidity of the middle barrier layer and reduce the interfacial wettability with adjacent layers (especially non-polar heat-sealing layers). This insufficient wettability will cause weakening of the interfacial adhesion during the composite process, ultimately forming a defect band with uneven bonding force. Especially in the actual composite hot pressing process, when the excess EVOH enters the cooling stage before being fully plasticized, it is easy to cause delamination and voids at the edges of the composite layer or stress intersection areas.

[0148] In contrast, when the EVOH ratio is controlled within an appropriate range, it can cooperate with dispersed components such as ZnO to stabilize the interface structure, and while maintaining the barrier performance of the middle layer, achieve good adhesion with the upper and lower layers, and obtain a composite film with uniform structure and tight bonding.

[0149] Experiment 3: Explanation of Peel Strength Test Experiment

[0150] Experiment Purpose:

[0151] Evaluate the difference in the controllable peeling performance of the composite film when PVA microemulsion is added or not added to the heat-sealing layer, and focus on observing its effects on the stability of the peeling force and the uniformity of peeling.

[0152] Experimental instruments and materials:

[0153] Electronic tensile testing machine (peeling mode, test speed can be set);

[0154] Standard test fixture (180° peeling device);

[0155] Constant temperature sealing instrument (used to simulate sealing);

[0156] Sample knife die, digital caliper, thermometer and hygrometer, etc.

[0157] Cut strip samples with a size of 15 mm × 100 mm from the composite films obtained in Example 1 and Comparative Example 3, with a thickness of about 55 μm, and 3 samples in each group. Keep the edges neat and free of wrinkles.

[0158] Use the constant temperature sealing instrument to heat-seal the film samples in pairs at 120 °C and 0.2 MPa. The length of each heat-sealing is 80 mm, preheat for 3 seconds, seal for 1 second, and cool and fix. After cooling, cut off the unsealed part and leave the middle effective sealing area.

[0159] Put the sealed samples into the electronic tensile testing machine, set the peeling mode to 180° reverse peeling, the test speed to 100 mm / min, and record the average peeling force (unit: N / cm) within the peeling length.

[0160] Simultaneously record the peeling characteristics during the peeling process, such as whether there are phenomena such as fracture, tearing, and irregular peeling; and evaluate the degree of peeling force fluctuation and analyze the peeling consistency.

[0161] Calculate the average peeling strength of each sample and evaluate its deviation and the stability of the peeling behavior.

[0162] Table 3:

[0163]

[0164]

[0165] Summary;

[0166] The test results of Experiment 3 showed that the heat-sealing layer after adding the polyvinyl alcohol (PVA) microemulsion exhibited more stable and gentle peeling characteristics. Its peeling strength was controlled within a relatively low and consistent range, and the peeling process was smooth without any fracture or discontinuous peeling phenomenon. This performance advantage indicates that PVA not only plays an auxiliary role in regulating the adhesiveness in the heat-sealing layer of the composite film but also significantly improves the controllability of the peeling behavior, and it is a key regulatory factor for achieving an easily peelable structure.

[0167] Mechanistically, as a polar polymer, PVA can form microscale homogeneous distribution regions in the heat-sealing layer. This structure helps to construct an interfacial bonding structure with moderate heat-sealing strength during hot-press sealing, while reducing the overall rigid matching stress between layers, thus making the peeling process easier to control. In contrast, for the samples without PVA, due to the large difference in interfacial polarity, the structure in the heat-sealing area is prone to excessive adhesion, resulting in non-continuous peeling phenomena such as tearing and fracture during peeling, significant fluctuations in peeling strength, and unstable peeling effects.

[0168] In the present invention, the reasonable control of the dosage and dispersion method of PVA not only does not affect the integrity of the heat-sealing layer itself but also makes the peeling behavior transition from the original "sticking - fracture" type to the "continuous - smooth" type by improving the microstructure distribution.

[0169] Experiment 4: Explanation of the heat-sealing strength stability test experiment

[0170] Experimental purpose:

[0171] To evaluate the strength performance of the heat-sealing layer at different heat-sealing temperatures, judge the stability and temperature tolerance of its sealing performance, and reflect the adaptability of the film material in actual packaging applications.

[0172] Constant-temperature heat-sealing instrument (with adjustable pressure and temperature control system);

[0173] Electronic tensile testing machine (peeling mode);

[0174] Constant-humidity environmental chamber (23°C, 50% RH, for sample pretreatment);

[0175] Testing fixtures and standard knife dies.

[0176] Experimental procedure:

[0177] Cut the film materials of Example 1 and Comparative Example 4 into 15 mm × 120 mm. The samples were left standing for 12 hours under constant temperature and humidity conditions before sealing to eliminate the influence of the environment.

[0178] Select 4 typical heat-sealing temperature points: 100°C, 110°C, 120°C, 130°C. The other heat-sealing parameters remain the same: sealing pressure 0.2 MPa, sealing time 1 s, preheating 2 s, and cut the middle effective area after cooling.

[0179] Perform a 180° peel test on the sealed film sample on an electronic tensile testing machine, record the average peel force (unit: N / cm) at each temperature, and test 3 times at each temperature point.

[0180] Simultaneously record the hot sealing status, such as whether there are phenomena such as local coking, poor sealing, sealing blistering or slippage.

[0181] Sort out the peel strength corresponding to each temperature point, and analyze the stability of its heat sealing strength with temperature change and the width of the sealing window.

[0182] Table 4:

[0183]

[0184]

[0185] Summary:

[0186] The results of Experiment 4 show that the heat sealing structure constructed in the examples exhibits stable and continuous heat sealing strength response within a relatively wide temperature range. When the heat sealing temperature is increased from 100 °C to 130 °C, the peel strength shows a linear upward trend and tends to be plateaued between 120 °C and 130 °C, reflecting a relatively wide heat sealing process window. This stability stems from the reasonable ratio and polarity adjustment strategy of the key components in the heat sealing layer, enabling the heat sealing material to achieve appropriate melting and interfacial fusion at different temperatures, thus ensuring the balanced release of energy and movement of polymer chain segments during the sealing process without problems such as adhesion, coking or slippage.

[0187] In terms of the structural mechanism, the heat sealing layer design of the present invention takes into account the coordination of polarity compatibility and thermoplastic response, and uses PVA microemulsion as a polarity regulator, enabling the heat sealing layer to form a flexible chain segment connection region during the heating stage, effectively suppressing the interfacial non-uniformity caused by local over-melting. This flexible interface formation mechanism is significantly different from the problems of sealing coking or poor sealing caused by concentrated melting points and steep thermal responses in conventional heat sealing structures. By regulating the distribution and coordination of polar and non-polar components in the heat sealing structure, the present invention realizes the unity of structural stability and thermal response balance.

[0188] Compared with the comparative sample, its peel strength fluctuates greatly at the same heat sealing temperature, and there are phenomena of excessive sealing and scorching and tearing in the high temperature zone, reflecting the insufficient adaptability of its heat sealing system to temperature changes.

[0189] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-barrier and easily peelable stretch-formed food packaging composite film, characterized in that By mass parts, it includes the following components; 5–15 parts of copolymer-modified polyethylene terephthalate, 90–95 parts of ethylene-vinyl alcohol copolymer, 5–15 parts of zinc oxide nanoparticles, 0.5–3 parts of crosslinking agent, 80–90 parts of maleic anhydride-grafted low-density polyethylene, 10–20 parts of polyvinyl alcohol, and 0.5–2 parts of crosslinking aid.

2. The high-barrier and easily-peeled stretch-formed food packaging composite film according to claim 1, wherein The comonomer of the copolymer-modified polyethylene terephthalate is vinyl alcohol, the content of the vinyl alcohol comonomer is 3–10 wt%, and the glass transition temperature of the PET is 70–80 °C.

3. The high-barrier and easily peelable stretch-formed food packaging composite film according to claim 1, wherein In the ethylene-vinyl alcohol copolymer (EVOH), the content of ethylene monomer is 27–38 mol%, and the content of vinyl alcohol monomer is 62–73 mol%.

4. The high-barrier and easily-peelable stretch-formed food packaging composite film according to claim 1, wherein The crosslinking agent is glutaraldehyde, and the crosslinking aid is trimethylolpropane triacrylate.

5. The high-barrier and easily peelable stretch-formed food packaging composite film according to claim 1, wherein The grafting amount of grafted maleic anhydride in the maleic anhydride-grafted low-density polyethylene is 0.5–5 wt%, and the molecular weight of the low-density polyethylene is 20,000–100,000 Da.

6. A method for preparing a high-barrier and easily peelable stretch-formed food packaging composite film, which is used to prepare the high-barrier and easily peelable stretch-formed food packaging composite film according to any one of claims 1-5, characterized in that, It includes the following steps; S1. Preparation of the outer base film layer: Provide a copolymer-modified polyethylene terephthalate film with a thickness of 5–15 μm, the copolymerization ratio of vinyl alcohol is 3–10 wt%, and the glass transition temperature is 70–80 °C, and perform corona treatment to make the surface tension reach ≥40 mN / m; S2. Preparation of the barrier layer: Melt the ethylene-vinyl alcohol copolymer and add ZnO nanoparticles (particle size 20–80 nm, addition amount 5–15 mass parts), and add 0.5–3 parts of crosslinking agent, mix evenly at 160–180 °C, and cool and extrude to form a barrier film layer with a thickness of 10–25 μm; S3. Preparation of the peelable heat-sealing layer: Melt the PE-g-MAH at 130–150 °C, add 10–20 parts of PVA microemulsion, and 0.5–2 parts of crosslinking aid, and blend evenly to form a peelable heat-sealing layer with a thickness of 15–30 μm; S4. Three-layer hot pressing and lamination: Laminate the three-layer film materials obtained in steps 1) to 3) in sequence by hot pressing, the lamination temperature is 130–150 °C, the lamination pressure is 10–20 MPa, and at the same time introduce far-infrared radiation (160–180 °C) in the lamination area to promote crosslinking; S5. Cooling and shaping: Cool the laminated film through a cooling roller, and control the temperature at 15–30 °C; S6. Thermoforming treatment: Perform stretch thermoforming on the laminated film at 110–130 °C, the forming time is 0.5–2 seconds, and then cool and form to obtain the final packaging film.

7. The preparation method of the high-barrier and easily-peelable stretch-formed food packaging composite film according to claim 6, characterized in that, Before lamination, the outer base film, the barrier layer and the peelable heat-sealing layer are preheated by a hot roller respectively, and the preheating temperature is controlled as follows: the outer base film layer is 80–100 °C, the barrier layer is 90–110 °C, the peelable heat-sealing layer is 90–100 °C, and the preheating time is 20–60 seconds.

8. The preparation method of the high-barrier and easily peelable stretch-formed food packaging composite film according to claim 6, characterized in that In step S3, the emulsifier used in the PVA microemulsion is nonylphenol polyoxyethylene ether or alkyl glycoside, and the dosage of the emulsifier is 1–3 wt% of the mass of PVA.

9. The preparation method of the high-barrier and easily peelable stretch-formed food packaging composite film according to claim 6, characterized in that, In step S4, the irradiation time of the infrared radiation is 1–5 seconds, and a medium and far infrared radiation source with a wavelength of 2–4 μm is used.

10. The preparation method of the high-barrier and easily peelable stretch-formed food packaging composite film according to claim 6, characterized in that, In step S6, vacuum thermoforming or positive pressure gas forming is used for forming, the mold cavity temperature is controlled at 110–130 °C, and the forming time is 0.5–2 seconds.

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