A cross-linked POF film for chocolate packaging and its preparation method

Abstract

This invention discloses a cross-linked POF film for chocolate packaging and its preparation method, solving the problem of simultaneously achieving barrier properties, toughness, and heat-sealing performance in existing films. The film, from the outside in, consists of a wear-resistant printing layer, a composite barrier layer, and an impact-resistant heat-sealing layer. The printing layer contains nano-SiO2 masterbatch to enhance wear resistance; the composite barrier layer uses modified montmorillonite to construct a biomimetic "brick-and-mortar" structure; and the heat-sealing layer uses POE and EVA to optimize impact resistance and heat sealing. All layers are made of polyolefin material. During preparation, after raw material pretreatment and three-layer co-extrusion blow molding, performance synergy is achieved through electron beam modification cross-linking and biaxial stretching at 95-105℃. The finished product has a heat shrinkage rate of 40%-50% at 100℃, can closely conform to the shape of chocolate, and is easily recyclable due to its single material, adapting to the multi-dimensional needs of chocolate packaging.

Description

Technical Field

[0001] This invention relates to the field of thin film technology, and in particular to a cross-linked POF film for chocolate packaging and its preparation method. Background Technology

[0002] Chocolate, a popular food among consumers, is extremely sensitive to its storage environment. Cocoa butter in chocolate is prone to oxidative rancidity, leading to deterioration in taste and loss of flavor. Simultaneously, chocolate is brittle and easily broken by external impacts during transportation and sales. Therefore, chocolate packaging films must simultaneously meet two core requirements: high barrier properties and high impact resistance.

[0003] The main technical challenges currently facing the chocolate packaging market are as follows:

[0004] Insufficient barrier properties: Traditional single-layer or three-layer ordinary polyolefin heat shrinkable (POF) films usually have a high oxygen permeability (OTR), making it difficult to effectively block oxygen and unable to meet the shelf life requirements of chocolate of more than 12 months, which can easily lead to a decline in product quality during the shelf life.

[0005] Poor physical protection: In pursuit of high barrier properties, some packaging uses multilayer composite films containing high-barrier materials such as polyvinylidene chloride (PVDC) or ethylene-vinyl alcohol copolymer (EVOH). However, these materials are usually hard and have poor toughness, resulting in a decrease in the overall impact and puncture resistance of the film.

[0006] Poor adaptability to automated packaging: High-speed automated packaging machines have stringent requirements for the heat-sealing performance of films, demanding a wide heat-sealing temperature window and high heat-sealing strength. Traditional films have narrow heat-sealing windows, making them prone to poor sealing or burn-through, thus affecting production efficiency.

[0007] Therefore, developing a chocolate packaging film that integrates high barrier properties, high impact resistance, and excellent heat-sealing performance is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0008] The purpose of this invention is to provide a cross-linked POF film for chocolate packaging and its preparation method, thereby solving the problem of difficulty in balancing barrier performance stability, mechanical toughness, and low-temperature heat-sealing performance in the prior art.

[0009] To achieve the above objectives, the present invention provides a cross-linked POF film for chocolate packaging, comprising, from the outside to the inside, the following layered structure: an abrasion-resistant printing layer, a composite barrier layer, and an impact-resistant heat-sealing layer, wherein the raw material components of each layer are as follows by mass:

[0010] The wear-resistant printed layer consists of: 87-91.5 parts homopolymer polypropylene, 5-8 parts ethylene-propylene copolymer, 2-4 parts nano-SiO2 masterbatch, 0.5-1 parts maleic anhydride-grafted polypropylene, 0.1-0.3 parts antioxidant 1076, and 0.1-0.2 parts hindered amine light stabilizer.

[0011] The composite barrier layer consists of 84-89 parts of linear low-density polyethylene, 8-12 parts of organically modified nano-montmorillonite masterbatch, 1-3 parts of maleic anhydride-grafted polypropylene, 0.2-0.4 parts of crosslinking agent polyethylene glycol diacrylate, 0.2-0.5 parts of polyethylene wax, and 0.2-0.5 parts of hindered amine light stabilizer.

[0012] Impact-resistant heat-sealing layer: 74-79.5 parts random copolymer polypropylene, 15-20 parts ethylene-vinyl acetate copolymer, 3-5 parts POE elastomer, 0.5-1 part maleic anhydride grafted polypropylene, 0.1-0.2 parts antioxidant 168, and 0.1-0.15 parts hindered amine light stabilizer;

[0013] The film exhibits a thermal shrinkage rate of 40%~50% at 100℃.

[0014] Preferably, the total thickness of the film is 25-35 μm, wherein the thickness ratios of the wear-resistant printing layer, the composite barrier layer, and the impact-resistant heat-sealing layer are 15-20%, 50-55%, and 25-30%, respectively.

[0015] Preferably, the VA content of the ethylene-vinyl acetate copolymer in the impact-resistant heat-sealing layer is 15-18%; the ethylene content of the random copolymer polypropylene is 3.5-4.5%; and the POE elastomer is a high-octene POE elastomer with an octene content of 28%.

[0016] Preferably, the surface of the organically modified nano-montmorillonite masterbatch is grafted with PE segments, and the modifier is hexadecyltrimethylammonium bromide.

[0017] Preferably, the organic modified nano-montmorillonite masterbatch contains 15% montmorillonite, has an interlayer spacing of ≥35nm, and the dispersed particle size of montmorillonite in the linear low-density polyethylene matrix is ​​≤180nm.

[0018] Preferably, a method for preparing a cross-linked POF film for chocolate packaging includes the following steps:

[0019] a) Raw material pretreatment: Add the raw materials for the wear-resistant printing layer, composite barrier layer and impact-resistant heat-sealing layer to a high-speed mixer and stir at 700-800 r / min for 5-8 minutes at 70-75℃ until they are uniformly mixed; when mixing the raw materials for the composite barrier layer, first premix the polyethylene wax with the organically modified nano-montmorillonite masterbatch with PE segments grafted on the surface, and then add the remaining components.

[0020] b) Three-layer co-extrusion blow molding: The three pre-treated raw materials are fed into the co-extrusion die from the corresponding extruders. The extrusion temperature of the printing layer is 180-190℃, the extrusion temperature of the composite barrier layer is 170-180℃, and the extrusion temperature of the impact-resistant heat-sealing layer is 175-185℃. The thickness ratio of each layer is controlled by the flow distributor to be 15-20% for the wear-resistant printing layer, 50-55% for the composite barrier layer, and 25-30% for the impact-resistant heat-sealing layer, so as to obtain a preform with a thickness of 200-250μm.

[0021] c) Biaxial stretching: The co-extruded blown film preform is first stretched 2.5-3.0 times longitudinally at 95-100℃, and then stretched 4.0-4.5 times transversely at 100-105℃ to obtain a film with a thickness of 30-40μm.

[0022] d) Electron beam modified crosslinking: The preformed film is fed into an electron beam irradiation device and irradiated in two stages:

[0023] The first stage involves treatment with an electron beam energy of 150 keV, an irradiation dose of 3 kGy, and an irradiation temperature of 50-55℃.

[0024] The second stage involves treatment with an electron beam energy of 200keV, an irradiation dose of 5-7kGy, and an irradiation temperature of 55-60℃ to control the crosslinking degree of the composite barrier layer to be 32-38%.

[0025] e) Shaping and winding: The biaxially stretched film is corona treated to a surface dyn value of 48-52 dyn / cm, then the internal stress is eliminated by passing it through a gradient oven, and finally it is wound up with a constant tension of 30-50N to obtain the finished product.

[0026] The finished film has a thermal shrinkage rate of 40% to 50% at 100°C.

[0027] Preferably, in step a), the amount of polyethylene wax added to the composite barrier layer is 0.2-0.5 parts, nitrogen gas is introduced into the high-speed mixer for protection, the oxygen content of the nitrogen gas is <500ppm, and after the raw materials of the composite barrier layer are mixed, there are no visible agglomerates of montmorillonite.

[0028] Preferably, in step b), the co-extrusion blown film process has a blow-up ratio of 3.0-4.0:1 and the height of the frosting line is 300-500mm from the die surface.

[0029] Preferably, in step d), the electron beam irradiation device is protected by nitrogen gas, wherein the oxygen content of the nitrogen gas is <300 ppm.

[0030] Real-time monitoring of the crosslinking degree of the composite barrier layer using online gel permeation chromatography:

[0031] The degree of crosslinking is controlled in a closed loop using a semi-empirical formula:

[0032] ,

[0033] Where D is the final crosslinking degree of the composite barrier layer (%), k is the comprehensive coefficient of formulation and equipment, α is the synergistic coefficient of the first stage pre-crosslinking, β is the synergistic coefficient of the second stage main crosslinking, D1 is the first stage irradiation dose (3kGy), D2 is the second stage irradiation dose (5-7kGy), E1 is the first stage electron beam energy (150keV), E2 is the second stage electron beam energy (200keV), E0 is the reference electron beam energy (200keV), h is the actual preformed film thickness (30-40μm), h0 is the reference film thickness, T is the actual irradiation temperature (50-60℃), and T0 is the reference irradiation temperature (55℃).

[0034] The system monitors in real time. If the degree of crosslinking is greater than 38%, the second-stage irradiation dose is reduced by 0.5 kGy; if the degree of crosslinking is less than 32%, the second-stage irradiation dose is increased by 0.5 kGy.

[0035] Preferably, in step e), the corona treatment power is 420W, and the processing speed is linked to the winding speed, both being 18-20m / min; the gradient oven includes a high-temperature section and a medium-temperature section, with the high-temperature section temperature being 120-130℃ and the medium-temperature section temperature being 80-90℃, and the residence time ratio between the high-temperature section and the medium-temperature section being 1:2.

[0036] The beneficial effects of this invention are:

[0037] 1. This invention adopts a nano-montmorillonite "brick-mud" biomimetic structure, which forms a dense and orderly arrangement after biaxial stretching, effectively isolating oxygen and meeting the long shelf life requirements of chocolate, thus solving the problem of insufficient barrier properties of traditional POF films.

[0038] 2. The impact-resistant heat-sealing layer of this invention uses POE elastomer and EPC for synergistic toughening. POE absorbs impact energy with its "island structure", while random copolymer PP improves low-temperature toughness and prevents frozen chocolate packaging from cracking. It has better impact resistance than EVOH composite film and can withstand external force collisions during transportation, significantly reducing the risk of chocolate breakage.

[0039] 3. This invention forms a uniform crosslinking network in the film through two-stage electron beam crosslinking combined with semi-empirical formula closed-loop control, which retains the appropriate fluidity of the molecular chains and ensures the rigidity of the surface layer; combined with EVA with a VA content of 15-18%, it achieves low-temperature gentle sealing. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0041] Figure 1 This is a schematic diagram of the structure of a cross-linked POF film for chocolate packaging according to the present invention. Detailed Implementation

[0042] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0043] like Figure 1 As shown, the present invention provides a cross-linked POF film for chocolate packaging, comprising, from the outside to the inside, a wear-resistant printing layer, a composite barrier layer, and an impact-resistant heat-sealing layer. The raw material components of each layer are as follows by mass:

[0044] Wear-resistant printing layer: 87-91.5 parts homopolymer polypropylene, 5-8 parts ethylene-propylene copolymer (EPC), 2-4 parts nano-SiO2 masterbatch, 0.5-1 parts maleic anhydride grafted polypropylene (PP-g-MAH), 0.1-0.3 parts antioxidant 1076, and 0.1-0.2 parts hindered amine light stabilizer (HALS). In this invention, this layer is designed with a full polyolefin composition, homologous to the middle and inner layers, laying the foundation for subsequent single-material recycling.

[0045] Homopolymer polypropylene with a melt index of 1.5-2.0 g / 10min is selected to provide the necessary rigidity and high-temperature resistance for printing. The introduction of EPC balances rigidity and toughness, preventing brittleness during film shrinkage—this is because the ethylene segments of EPC alleviate the crystallization stress of homopolymer PP, increasing the elongation at break. Nano-SiO2 masterbatch (SiO2 particle size 30-50nm) mimics the reinforcement mechanism of hard particles, improving the surface hardness of the film and solving the scratching problem in the post-printing logistics process. PP-g-MAH acts as a compatibilizer, bridging the polyolefin matrix and nanoparticles through polar groups, preventing printing defects caused by particle agglomeration. Antioxidant 1076 inhibits high-temperature processing oxidation and suppresses free radical chain reactions, ensuring that the film does not yellow during long-term use. Hindered amine light stabilizers (HALS) can capture free radicals generated by ultraviolet or electron beam irradiation, extending the anti-aging life of the film.

[0046] Composite barrier layer: 84-89 parts linear low-density polyethylene (LLDPE), 8-12 parts organically modified nano-montmorillonite masterbatch with surface-grafted PE segments, 1-3 parts maleic anhydride-grafted polypropylene, 0.2-0.4 parts crosslinking agent polyethylene glycol diacrylate (PEGDA), 0.2-0.5 parts polyethylene wax (preferably pyrolytic polyethylene wax with a softening point ≥105℃), and 0.15-0.25 parts hindered amine light stabilizer (HALS, preferably light stabilizer 770). The performance of nacreous shells far surpasses that of calcium carbonate itself, containing 95% aragonite micron-sized lamellar crystals and 5% organic matrix, exhibiting a "brick-and-mortar" ordered stacked structure. Under external force, cracks deflect and the organic matrix slips, dissipating energy, resulting in fracture toughness far exceeding that of aragonite crystals. In this invention, high aspect ratio montmorillonite lamellar crystals act as "bricks," and flexible LLDPE as "mortar," reproducing the "brick-and-mortar" ordered structure.

[0047] The LLDPE used is a metallocene-catalyzed grade (melt index 0.7-0.9 g / 10 min), with uniformly distributed branches, providing a good dispersion environment for montmorillonite and ensuring interlayer compatibility. Organically modified montmorillonite is modified with hexadecyltrimethylammonium bromide (CTAB) (which can increase the interlayer spacing of montmorillonite ≥35 nm), and PE segments are grafted onto its surface, enhancing the bonding force with the LLDPE matrix and preventing montmorillonite agglomeration. The masterbatch contains 15% montmorillonite, with a particle size ≤180 nm after dispersion. After biaxial stretching, it forms a "brick-and-mortar" ordered structure, greatly extending the oxygen diffusion path. Maleic anhydride-grafted polypropylene further improves the compatibility between montmorillonite and LLDPE, avoiding barrier "loopholes" caused by interlayer interface defects. Simultaneously, the crosslinking agent, polyethylene glycol diacrylate (PEGDA), preferably with a number average molecular weight of 400, can rapidly crosslink with polyolefin molecular chains under electron beam irradiation, forming a stable three-dimensional network that locks the "labyrinth structure" of montmorillonite while improving the thermal stability of the film. Secondly, this invention uses a crackable polyethylene wax with a softening point of 105-120℃, which has both lubricating and dispersing functions, effectively reducing the shear resistance during raw material mixing, while also assisting in the uniform dispersion of montmorillonite and preventing agglomeration. Furthermore, the hindered amine light stabilizer and the crosslinking agent polyethylene glycol diacrylate (PEGDA) work synergistically to reduce the degradation of polyolefin segments by electron beam irradiation, ensuring the mechanical properties of the crosslinked film.

[0048] Impact-resistant heat-sealing layer: 74-79.5 parts random copolymer polypropylene, 15-20 parts ethylene-vinyl acetate copolymer (EVA), 3-5 parts POE elastomer, 0.5-1 part maleic anhydride-grafted polypropylene, and 0.1-0.2 parts antioxidant 168. Random copolymer polypropylene (ethylene content 3.5-4.5%) has better low-temperature toughness than homopolymer PP, maintaining good flexibility under low-temperature refrigeration conditions and preventing packaging brittleness after chocolate freezing. EVA can lower the heat-sealing temperature through polar VA groups, and combined with the "rigid framework" support of the intermediate layer's gradient cross-linking structure, it widens the heat-sealing window. Too high a VA content will lead to a decrease in heat-sealing strength, while too low a content will result in a higher heat-sealing temperature; therefore, a preferred content is 15-18%. POE elastomer (octene content 25%) has a glass transition temperature below -50℃ and a "sea-island structure." The island phase of POE can absorb impact energy, providing the film's impact strength. Antioxidant 168 forms a synergistic system with 1076 in the printing layer, inhibiting free radical chain reactions and extending the film's shelf life. The addition of hindered amine light stabilizers further enhances the film's anti-aging properties, preventing performance degradation caused by oils during contact with chocolate.

[0049] Furthermore, the total thickness of the film is 25-35 μm, with the wear-resistant printing layer, composite barrier layer, and impact-resistant heat-sealing layer accounting for 15-20%, 50-55%, and 25-30% of the thickness, respectively. The composite barrier layer has the highest proportion because it performs the core barrier function, and this proportion maximizes the coverage of the montmorillonite "maze structure." At the same time, the thickness deviation of each layer is ≤±2%, avoiding imbalance of shrinkage force due to uneven local thickness, preventing wrinkles or loose adhesion during packaging, and providing a prerequisite for uniformity control in the subsequent biaxial stretching process.

[0050] Furthermore, the film has a heat shrinkage rate of 40%-50% at 100°C, and a longitudinal and transverse shrinkage rate deviation of ≤±2%, which can closely fit the shape of chocolate and avoid wrinkles and hollowness.

[0051] In one embodiment, a method for preparing a cross-linked POF film for chocolate packaging includes the following steps:

[0052] (1) Raw material pretreatment: The raw materials for the wear-resistant printing layer, composite barrier layer, and impact-resistant heat-sealing layer are added to a high-speed mixer. The high-speed mixer is protected by nitrogen gas with an oxygen content of <500ppm. The mixture is stirred at 700-800r / min for 5-8min at 70-75℃ until it is uniformly mixed. When mixing the raw materials for the composite barrier layer, polyethylene wax (addition amount of 0.2-0.5 parts) is premixed with organically modified nano-montmorillonite masterbatch grafted with PE chain segments. After mixing, there are no visible agglomerates in the montmorillonite. Then the remaining components are added. The high speed of 700-800r / min of the composite barrier layer works synergistically with polyethylene wax to break the montmorillonite agglomerates by shear force, so that the dispersed particle size is ≤200nm. This promotes the interfacial bonding between the LLDPE matrix and the montmorillonite sheets, reduces the voids between the matrix and the filler, and provides a foundation for the subsequent stretching to form an ordered brick-and-mortar structure. The matrix of the composite barrier layer is LLDPE (polyolefin). Although oxidized polyethylene wax contains polar groups such as hydroxyl and carboxyl groups (which can enhance the interaction with montmorillonite), these polar groups will form "interfacial tension" with the non-polar segments of LLDPE, resulting in decreased compatibility between the wax and the matrix. During co-extrusion, "wax precipitation" (forming a white hazy layer on the film surface) is likely to occur, which not only affects transparency but also disrupts the "labyrinth structure" of montmorillonite. In contrast, the raw material of pyrolytic polyethylene wax is polyethylene (homogeneous with LLDPE), and its molecular chain contains only non-polar methylene (-CH2-). Its compatibility with LLDPE is far superior to other types of wax. Furthermore, pyrolytic polyethylene wax has no polar groups, so there is no precipitation after co-extrusion, resulting in better film surface gloss. Moreover, the molecular chain of pyrolytic polyethylene wax is stable and will not react with free radicals generated by electron beam irradiation (avoiding the generation of low molecular weight fragments).

[0053] Similarly, nitrogen gas is introduced into the mixer to prevent LLDPE, EVA and other raw materials from oxidizing and degrading when in contact with oxygen at 70-75℃. Oxidation produces polar groups such as carbonyl groups, which not only affect the compatibility of raw materials (e.g., the interfacial bonding force between LLDPE and montmorillonite decreases after oxidation), but also generate small molecule volatiles, leading to "pinhole" defects in the subsequent film.

[0054] In one embodiment, the nano-SiO2 masterbatch needs to be premixed with PP-g-MAH for 2 minutes. The polar groups (maleic anhydride) of PP-g-MAH are used to bind with the hydroxyl groups on the surface of SiO2 to prevent SiO2 from agglomerating in the homopolymer PP matrix.

[0055] In one embodiment, the POE elastomer and EVA need to be added to the mixer later (random copolymer PP is added first, followed by POE and EVA) to avoid excessive shearing of POE due to its low viscosity under high shear, which would cause the "island structure" to break and thus reduce the impact strength.

[0056] (2) Three-layer co-extrusion blow molding: The three pre-treated raw materials are fed into the co-extrusion die from the corresponding extruders. The thickness ratio of each layer is controlled by the flow distributor to be 15-20% for the wear-resistant printing layer, 50-55% for the composite barrier layer, and 25-30% for the impact-resistant heat-sealing layer. After melting and bonding at 175-195℃, the layers are blow molded into a film to obtain a preform with a thickness of 200-250μm. The extrusion temperature of each layer is independently controlled. The printing layer is set at 185-195℃ (to ensure complete melting and avoid excessive temperature that could lead to PP degradation, generating low molecular weight fragments and affecting film rigidity). The barrier layer is set at 175-185℃ (if the temperature is too high, the crosslinking agent PEGDA will undergo premature thermal crosslinking, causing the preform to break easily during subsequent stretching; if the temperature is too low, the LLDPE will not melt sufficiently, resulting in a decrease in interfacial bonding with montmorillonite and a decrease in interlayer peel strength). The heat-sealing layer is set at 180-190℃ (to ensure complete compatibility between random copolymer PP and EVA, while avoiding the problem of EVA decomposing to produce acetic acid due to excessive temperature, which could corrode equipment, and the problem of EVA not melting due to excessive temperature, resulting in "granular" defects).

[0057] In this application, a blow-up ratio of 3.0-4.0:1 is used (if the blow-up ratio is too low, the molecular chain orientation is insufficient, and stretching can easily lead to a sudden drop in local thickness; if the blow-up ratio is too high, the membrane bubble is prone to rupture due to excessive lateral tension, and uneven film cooling can easily cause wrinkles), combined with a frost line height of 300-500 mm (if the frost line is too high, the initial preform cooling rate is slow, the LLDPE crystal grain size increases, leading to film embrittlement; if the frost line is too low, the initial preform cools too quickly, crystallization is insufficient, and subsequent biaxial stretching can easily lead to tensile fracture due to molecular chain slippage). This combination promotes the initial orientation of the molecular chains, laying the foundation for the orientation of montmorillonite in subsequent biaxial stretching. Controlling the initial preform thickness at 200-250 μm ensures that after subsequent 2.5-4.0 times stretching, the finished product thickness can be stabilized at 25-35 μm, avoiding overstretching and film damage.

[0058] (3) Biaxial stretching: The co-extruded blown film preform is first stretched 2.5-3.0 times longitudinally at 95-100℃, and then stretched 3.5-4.0 times transversely at 100-105℃ to obtain a film with a thickness of 25-35μm. The longitudinal stretching temperature is lower than the transverse stretching temperature, which reduces longitudinal shrinkage stress; the longitudinal-to-transverse stretching ratio of 1:1.2-1:1.5 can induce montmorillonite lamellars to be highly oriented parallel to the film surface. This is the core step in reproducing the "brick-and-mortar" structure of nacre: the oriented montmorillonite lamellars make the oxygen diffusion path extremely tortuous, forming a "maze effect", and the barrier properties are significantly improved compared with the unoriented ones. The film thickness is uniform after stretching, avoiding local performance defects caused by "necking".

[0059] (4) Electron beam modified crosslinking: The preformed film is fed into an electron beam irradiation device and irradiated in two stages:

[0060] The first stage uses an electron beam energy of 150keV, an irradiation dose of 3kGy, and an irradiation temperature of 50-55℃. The low electron beam energy can only penetrate the surface of the pre-formed film. The purpose is to allow the surface molecular chains to initially cross-link and form a "rigid surface framework" to limit the excessive movement of the surface molecular chains during the high-energy irradiation in the second stage and avoid "excessive cross-linking and embrittlement of the surface".

[0061] The second stage involves treating the composite barrier layer with an electron beam energy of 200keV, an irradiation dose of 5-7kGy, and an irradiation temperature of 55-60℃. This controls the cross-linking degree of the composite barrier layer to be 32-38%. The high electron beam energy can completely penetrate the preformed membrane and act on the deep layers of the membrane blank, working synergistically with the surface framework formed by the pre-cross-linking to reduce the difference in cross-linking degree between the inner and outer layers.

[0062] Furthermore, to ensure the stability of the above performance, the degree of crosslinking is controlled through a semi-empirical formula closed-loop control:

[0063] ,

[0064] Where D is the final crosslinking degree of the composite barrier layer (%), k is the comprehensive coefficient of formulation and equipment, α is the synergistic coefficient of the first stage pre-crosslinking, β is the synergistic coefficient of the second stage main crosslinking, D1 is the first stage irradiation dose (3kGy), D2 is the second stage irradiation dose (5-7kGy), E1 is the first stage electron beam energy (150keV), E2 is the second stage electron beam energy (200keV), E0 is the reference electron beam energy (200keV), h is the actual preformed film thickness (30-40μm), h0 is the reference film thickness, T is the actual irradiation temperature (50-60℃), and T0 is the reference irradiation temperature (55℃).

[0065] The essence of electron beam crosslinking is that high-energy electrons bombard polyolefin molecular chains, breaking CH bonds to generate free radicals. These free radicals then combine to form crosslinking bonds. The final degree of crosslinking (D) depends on three core aspects: "effective utilization efficiency of electron beam energy," "energy absorption efficiency of the film layer," and "activity efficiency of molecular chain reactions." The basic logic of the formula is: Degree of crosslinking (D) = Basic coefficient of formulation and equipment × Electron beam energy input efficiency × Film layer energy absorption efficiency × Activity efficiency of molecular chain reactions. Different formulations (such as the content of crosslinking agent PEGDA, type of HALS) and equipment (energy output stability and beam uniformity of different brands of electron beam irradiators) directly affect the crosslinking efficiency. Therefore, a comprehensive coefficient (k) of formulation and equipment can eliminate inherent differences between formulation and equipment, ensuring the universality of the formula. Furthermore, this invention employs "two-stage irradiation" (pre-crosslinking + main crosslinking), the core of which is to avoid "excessive surface crosslinking + insufficient deep crosslinking." The formula, through the design of α, β, E1 / E0, and E2 / E0, precisely quantifies the energy contribution of the two stages, quantifies the total energy input, and achieves control over the electron beam energy input efficiency. Electron beam energy decreases with increasing film thickness; the thicker the film, the less energy is received at deeper layers. Temperature affects the crosslinking efficiency by influencing the "molecular chain motion speed" and "free radical lifetime." Therefore, this invention uses a film thickness correction term (h0 / h) to control the film's energy absorption efficiency to compensate for the energy attenuation caused by differences in film thickness; and uses a temperature correction term (T / T0) to adjust the molecular chain reactivity efficiency to balance the effect of temperature on crosslinking kinetics.

[0066] The system monitors in real time. If the crosslinking degree is >38%, it indicates "excessive energy input," and the second-stage irradiation dose is proactively reduced by 0.5 kGy (prioritizing the reduction of the main crosslinking dose D2, causing the main term in the formula to decrease, and the D value to quickly return to the target range). If the crosslinking degree is <32%, it indicates "insufficient energy input," and the second-stage irradiation dose is proactively increased by 0.5 kGy (prioritizing the reduction of the main crosslinking dose D2, causing the main term in the formula to increase, and the D value to rise). Therefore, based on the real-time parameters of equipment such as irradiation dose, mold thickness measurement device, and temperature monitoring device, the system can automatically adjust the irradiation power. When the crosslinking degree deviates from the target value of 32-38%, it immediately enters the "light crosslinking" or "heavy crosslinking" mode, overcoming the shortcomings of relying solely on human experience.

[0067] (5) Shaping and winding: The biaxially stretched film is corona treated to a surface dyn value of 48-52 dyn / cm, then subjected to a gradient oven to eliminate internal stress, and finally wound with a constant tension of 30-50N to obtain the finished product. Electron beam crosslinking will reduce the polarity of the film surface. After the polyolefin molecular chain is crosslinked, the polar groups such as hydroxyl and carbonyl groups on the surface are reduced. The corona treatment power of 420W can introduce polar groups (such as -COOH, -OH) on the film surface, so that the dyn value is stabilized at 48-52 dyn / cm, which meets the adhesion requirements of printing ink (the ink is easy to fall off when the dyn value is lower than 48 dyn / cm).

[0068] During biaxial stretching and cross-linking, internal stress is generated within the film, mainly due to the orientation of molecular chains and the shrinkage tendency of the cross-linking network. The rapid heating at 120-130℃ in the gradient oven allows for a brief relaxation of the molecular chains, locking in the "labyrinth structure" of montmorillonite and the cross-linking network, preventing chain retraction during cooling. The slow elimination of internal stress at 80-90℃ (dwell time ratio 1:2) prevents film shrinkage during storage. Constant tension winding at 30-50N (30-35N for 25μm, 45-50N for 35μm) prevents film wrinkling and ensures stable film feeding during automated packaging.

[0069] The present invention is further illustrated by comparison with the following embodiments and comparative examples.

[0070] Example

[0071] Raw materials: homopolymer PP (melt index 1.8 g / 10 min), EPC (ethylene content 20%), nano-SiO2 masterbatch (SiO2 content 30%, particle size 40 nm), PP-g-MAH (grafting rate 1.2%), antioxidants 1076 and 168, hindered amine light stabilizer (HALS), LLDPE (metallocene catalysis, melt index 0.8 g / 10 min), organic modified nano-montmorillonite masterbatch (CTAB modified, grafted PE segments, montmorillonite content 15%, interlayer spacing 38 nm), crosslinking agent PEGDA, polyethylene wax (pyrolysis type), random copolymer PP (ethylene content 4.0%), EVA (VA content 18%), POE elastomer (octene content 28%), all of which are commercially available food-grade raw materials.

[0072] In this embodiment, the three-layer raw material formula described above is followed by raw material pretreatment, three-layer co-extrusion blow molding, biaxial stretching, electron beam crosslinking, and shaping and winding, with a crosslinking degree of 35%; a film is obtained.

[0073] Comparative Example 1

[0074] This comparative example uses the same raw materials as Example 1, but uses ordinary CTAB-modified montmorillonite (without grafted PE segments). The rest of the formulation and preparation process are the same as in Example 1.

[0075] Comparative Example 2

[0076] This comparative example uses the same raw materials as Example 1, but employs single-stage electron beam irradiation. The remaining formulation and preparation process are the same as in Example 1.

[0077] Comparative Example 3

[0078] This comparative example is a standard three-layer POF film without modified montmorillonite or crosslinking system.

[0079] According to the specifications of GB / T19787-2005 "Packaging Materials - Polyhydrocarbon Heat Shrink Film" and the heat shrinkage rate at 100℃, the test was conducted, and the comparison results are as follows:

[0080]

[0081] Table 1

[0082] As shown in Table 1 above, the film's performance perfectly meets the requirements of chocolate packaging: tensile strength of 164.8 MPa in the longitudinal direction and 167.2 MPa in the transverse direction (both ≥160 MPa), elongation at break of 100.4% in the longitudinal direction and 119.0% in the transverse direction (both ≥100%), and heat shrinkage at 100℃ of 45.3% in the longitudinal direction and 46.5% in the transverse direction (in the range of 40%~50%). It can closely fit the shape of chocolate and has excellent impact resistance and heat-sealing adaptability.

[0083] Comparative Example 1, using ordinary CTAB-modified montmorillonite without grafted PE segments, exhibits poor compatibility with the LLDPE matrix, resulting in tensile strength reduced to 123.2 MPa longitudinally and 125.1 MPa transversely (far below 160 MPa). The heat shrinkage rate at 100℃ rises to 57.1% and 56.2%, respectively, easily leading to excessive shrinkage and wrinkling of the packaging and insufficient impact resistance, increasing the risk of chocolate breakage during transportation. Comparative Example 2, using single-stage electron beam irradiation, suffers from uneven cross-linking of the film layer, leading to surface embrittlement. Its tensile strength is only 101.4 MPa longitudinally and 103.7 MPa transversely (<160 MPa), with a low elongation at break. The shrinkage rates of heat shrinkage films decreased to 84.2% and 85.7% (<100%), and the heat shrinkage rates at 100℃ were only 22.4% and 23.6%, respectively. These films could not tightly wrap the chocolate, and irregularly shaped chocolate packaging was prone to loosening and bulging. Comparative Example 3 was a regular three-layer POF film without modified montmorillonite and cross-linking system. Due to the lack of a structure that effectively restrains the movement of molecular chains, the tensile strength was only 131.1 MPa in the longitudinal direction and 136.4 MPa in the transverse direction (<160 MPa). The heat shrinkage rates at 100℃ were as high as 61.1% and 62.4%, respectively. The packaging was prone to wrinkles, which affected the appearance of the chocolate and caused it to lose its protective and shaping value as a heat shrink film.

[0084] The above-disclosed embodiments are merely one or more preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art can understand that all or part of the processes for implementing the above embodiments and equivalent changes made in accordance with the claims of this application still fall within the scope of this application.

Claims

1. A cross-linked POF film for chocolate packaging, characterized in that, From the outside in, the structure consists of the following layers: a wear-resistant printing layer, a composite barrier layer, and an impact-resistant heat-sealing layer. The raw material components of each layer are as follows by mass: The wear-resistant printed layer consists of: 87-91.5 parts homopolymer polypropylene, 5-8 parts ethylene-propylene copolymer, 2-4 parts nano-SiO2 masterbatch, 0.5-1 parts maleic anhydride-grafted polypropylene, 0.1-0.3 parts antioxidant 1076, and 0.1-0.2 parts hindered amine light stabilizer. The composite barrier layer consists of 84-89 parts of linear low-density polyethylene, 8-12 parts of organically modified nano-montmorillonite masterbatch, 1-3 parts of maleic anhydride-grafted polypropylene, 0.2-0.4 parts of crosslinking agent polyethylene glycol diacrylate, 0.2-0.5 parts of polyethylene wax, and 0.2-0.5 parts of hindered amine light stabilizer. Impact-resistant heat-sealing layer: 74-79.5 parts random copolymer polypropylene, 15-20 parts ethylene-vinyl acetate copolymer, 3-5 parts POE elastomer, 0.5-1 part maleic anhydride grafted polypropylene, 0.1-0.2 parts antioxidant 168, and 0.1-0.15 parts hindered amine light stabilizer; The film has a thermal shrinkage rate of 40%~50% at 100℃. Among them, PE segments are grafted onto the surface of the organically modified nano-montmorillonite masterbatch, and the modifier is hexadecyltrimethylammonium bromide. The film is prepared by co-extrusion blow molding of the above raw materials, followed by biaxial stretching and two-stage electron beam crosslinking.

2. The cross-linked POF film for chocolate packaging according to claim 1, characterized in that, The total thickness of the film is 25-35 μm, wherein the thickness ratios of the wear-resistant printing layer, the composite barrier layer, and the impact-resistant heat-sealing layer are 15-20%, 50-55%, and 25-30%, respectively.

3. The cross-linked POF film for chocolate packaging according to claim 1, characterized in that, The VA content of the ethylene-vinyl acetate copolymer in the impact-resistant heat-sealing layer is 15-18%; the ethylene content of the random copolymer polypropylene is 3.5-4.5%; and the POE elastomer is a high-octene POE elastomer with an octene content of 28%.

4. The cross-linked POF film for chocolate packaging according to claim 1, characterized in that, The organically modified nano-montmorillonite masterbatch contains 15% montmorillonite, has an interlayer spacing of ≥35nm, and has a montmorillonite dispersion particle size of ≤180nm in the linear low-density polyethylene matrix.

5. A method for preparing a cross-linked POF film for chocolate packaging according to any one of claims 1-4, characterized in that, Includes the following steps: a) Raw material pretreatment: Add the raw materials of wear-resistant printing layer, composite barrier layer and impact-resistant heat-sealing layer to a high-speed mixer and stir at 700-800 r / min at 70-75℃ for 5-8 minutes until they are mixed evenly. When mixing the raw materials for the composite barrier layer, polyethylene wax is first premixed with organically modified nano-montmorillonite masterbatch with PE segments grafted onto the surface, and then the remaining components are added. b) Three-layer co-extrusion blow molding: The three pre-treated raw materials are fed into the co-extrusion die from the corresponding extruders. The extrusion temperature of the printing layer is 180-190℃, the extrusion temperature of the composite barrier layer is 170-180℃, and the extrusion temperature of the impact-resistant heat-sealing layer is 175-185℃. The thickness ratio of each layer is controlled by the flow distributor to be 15-20% for the wear-resistant printing layer, 50-55% for the composite barrier layer, and 25-30% for the impact-resistant heat-sealing layer, so as to obtain a preform with a thickness of 200-250μm. c) Biaxial stretching: The co-extruded blown film preform is first stretched 2.5-3.0 times longitudinally at 95-100℃, and then stretched 4.0-4.5 times transversely at 100-105℃ to obtain a film with a thickness of 30-40μm. d) Electron beam modified crosslinking: The preformed film is fed into an electron beam irradiation device and irradiated in two stages: The first stage involves treatment with an electron beam energy of 150 keV, an irradiation dose of 3 kGy, and an irradiation temperature of 50-55℃. The second stage involves treatment with an electron beam energy of 200keV, an irradiation dose of 5-7kGy, and an irradiation temperature of 55-60℃ to control the crosslinking degree of the composite barrier layer to be 32-38%. e) Shaping and winding: The cross-linked film is corona treated to a surface dyn value of 48-52 dyn / cm, then the internal stress is eliminated by passing it through a gradient oven, and finally it is wound with a constant tension of 30-50N to obtain the finished product. The finished film has a thermal shrinkage rate of 40% to 50% at 100°C.

6. The method for preparing a cross-linked POF film for chocolate packaging according to claim 5, characterized in that, In step a), the amount of polyethylene wax added to the composite barrier layer is 0.2-0.5 parts. Nitrogen gas is introduced into the high-speed mixer for protection. The oxygen content of the nitrogen gas is <500ppm. After the raw materials of the composite barrier layer are mixed, there are no visible agglomerates of montmorillonite.

7. The method for preparing a cross-linked POF film for chocolate packaging according to claim 5, characterized in that, In step b), the co-extrusion blown film process has a blow-up ratio of 3.0-4.0:1 and the height of the frosting line is 300-500mm from the die surface.

8. The method for preparing a cross-linked POF film for chocolate packaging according to claim 5, characterized in that, In step d), the electron beam irradiation device is protected by nitrogen gas, with an oxygen content of <300 ppm. Real-time monitoring of the crosslinking degree of the composite barrier layer using online gel permeation chromatography: If the degree of crosslinking is >38%, reduce the second-stage irradiation dose by 0.5 kGy; if the degree of crosslinking is <32%, increase the second-stage irradiation dose by 0.5 kGy.

9. The method for preparing a cross-linked POF film for chocolate packaging according to claim 5, characterized in that, In step e), the corona treatment power is 420W, and the processing speed is linked to the winding speed, both being 18-20m / min. The gradient oven includes a high-temperature section and a medium-temperature section. The temperature of the high-temperature section is 120-130℃, and the temperature of the medium-temperature section is 80-90℃. The ratio of the residence time in the high-temperature section to that in the medium-temperature section is 1:2.

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