Radiation crosslinked biodegradable pof heat shrinkable film and method of making

Abstract

The application discloses a kind of radiation crosslinking biodegradable POF heat shrinkable film and preparation method.The film includes three-layer co-extrusion structure, core layer is formed by electron beam irradiation of medium energy electron accelerator crosslinking network structure, gel content is 30%~70%, the concentration of degradation master batch in surface layer is higher than the concentration of degradation master batch in core layer.Preparation, surface layer and core layer raw material are blended granulation respectively, after three-layer co-extrusion, bidirectional stretching, in inert atmosphere, with the dose of 50~150kGy, electron beam irradiation crosslinking, finally winding and slitting.The film is used for cold chain food packaging, frozen medicine packaging or low-temperature environment industrial product packaging;The application is synergized by chemical crosslinking network and gradient degradation structure, while giving the film biodegradability, significantly improve low-temperature tear resistance, and realize the precise control of degradation period, while widening heat shrinkage temperature window, improve shrinkage rate, reduce packaging energy consumption.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, specifically to a polyolefin heat shrink film, and more particularly to a radiation-crosslinked biodegradable POF heat shrink film with low-temperature tear resistance and its preparation method. Background Technology

[0002] Polyolefin shrink film (POF), as a high-performance and environmentally friendly packaging material, has become a mainstream alternative to traditional polyvinyl chloride (PVC) heat shrink film due to its excellent transparency, good toughness, uniform shrinkage performance, and chlorine-free and recyclable properties. It is widely used in many fields, including food, beverages, daily chemicals, electronics, pharmaceuticals, industrial parts, and cold chain logistics. In recent years, with the continuous advancement of global plastic bans and circular economy policies, the packaging industry is accelerating its transformation towards sustainable development. POF film, with its advantages of simple material composition, easy recycling, and clean production process, has seen steady growth in market demand. However, while meeting environmental protection requirements, practical applications place higher demands on the functionality of POF films. For example, in cold chain logistics and frozen food packaging, the film needs to maintain excellent tear resistance and flexibility in extreme low-temperature environments such as -20°C or even lower to prevent packaging damage. In daily chemical products and high-end consumer goods packaging, the film is required to have high transparency and high gloss to enhance shelf appeal. In addition, brands are increasingly concerned about the biodegradability of packaging materials to cope with increasingly stringent environmental regulations. To this end, industry researchers have attempted to impart biodegradable properties to POF films by adding biodegradable masterbatches. However, the introduction of conventional biodegradable masterbatches often disrupts the molecular chain regularity of polyolefins, leading to increased brittleness and a significant decrease in tear resistance at low temperatures, creating a technical contradiction of "difficulty in achieving both degradation and performance." In existing technologies, mechanical properties are mainly improved by adjusting the molecular weight distribution of polyolefins, compounding different grades of resins, or optimizing the multilayer co-extrusion structure. However, these solutions are mostly physical modifications, with limited reinforcing effects in extreme low-temperature environments, and have failed to effectively solve the problem of synergistic regulation between degradation cycle and mechanical properties. Therefore, how to endow POF membranes with biodegradability while ensuring their excellent tear resistance at low temperatures and achieving precise control over the degradation cycle has become a technical challenge that urgently needs to be solved in this field.

[0003] CN121340747A discloses a cross-linked biodegradable POF heat-shrinkable film and its preparation method, comprising three layers of film laminated sequentially. The first and third layers contain 89.5-94% PP, 2-6% degradation masterbatch, 0.5-3% opening agent, and 1-4% slip agent. The second layer contains 30-35% LLDPE-A, 9-14.5% toughening agent, 0.5-1% degradation masterbatch, 40-45% LLDPE-B, and metallocene LLDPE. The melt index of LLDPE-A is 2.5 g / 10 min, that of LLDPE-B is 2.1 g / 10 min, and that of metallocene LLDPE is 1.0 g / 10 min. Preparation involves surface irradiation and intermediate layer irradiation. The surface irradiation uses an electron beam energy of 80 keV and a dose of 25-30 kGy, while the intermediate layer irradiation uses an electron beam energy of 300 keV and a dose of 80-100 kGy. Although this patent improves low-temperature tear resistance through the synergistic combination of LLDPEs with different melt indices and toughening agents, its reinforcement mechanism is a physical entanglement network, which still has limitations at extreme low temperatures. Furthermore, it does not involve the synergistic design of crosslinked networks and gradient degradation structures, limiting its ability to regulate the degradation cycle.

[0004] CN120606576A discloses a polyolefin crosslinked heat shrink film for fresh produce packaging and its preparation method, comprising 80-95 parts of copolymer polypropylene, 50-60 parts of polyethylene, 20-30 parts of metallocene polypropylene, 15-30 parts of ethylene-octene copolymer, and 5-10 parts of processing aids. It employs a five-layer or higher co-extrusion three-bubble simultaneous stretching production process and crosslinking via electron beam irradiation. The film utilizes low-softening-point elastomers such as metallocene polypropylene and POE for blending modification to reduce shrinkage stress and initial shrinkage temperature. A rapid cooling process inhibits excessive crystal growth and improves transparency and shrinkage uniformity. While this patent also uses electron beam irradiation crosslinking technology, its improvements focus on reducing shrinkage temperature through elastomer blending modification and process optimization. It does not address biodegradability or propose a gradient degradation structure to address degradation cycle control issues, thus failing to meet increasingly stringent environmental regulations regarding the biodegradability of packaging materials.

[0005] CN121108702A discloses a biodegradable shrink film and its preparation method. Using modified polylactic acid (PLA), polybutylene terephthalate (PET), modified bio-based plasticizer, functional filler, and palm wax as raw materials, the biodegradable shrink film is prepared through processes such as melt extrusion and biaxial stretching. The modified PLA improves flexibility and heat shrinkage performance by introducing flexible 3HB segments into the PLA backbone. The modified bio-based plasticizer has plasticizing, compressive, and antibacterial functions. While this patent achieves full biodegradability of the shrink film, its substrate is a polyester material such as PLA, which is completely different from the polyolefin-based POF film in terms of material system. PLA-based films are prone to brittleness and poor tear resistance at low temperatures, making it difficult to meet the stringent low-temperature toughness requirements of cold chain packaging. Furthermore, it does not involve radiation crosslinking enhancement technology.

[0006] In summary, there is currently a lack of biodegradable POF heat shrink film products that can maintain excellent low-temperature tear resistance, achieve precise control of the degradation cycle, and have good compatibility with existing POF production lines. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings of existing biodegradable POF heat shrink film in terms of insufficient low-temperature tear resistance and difficulty in precisely controlling the degradation cycle, and to provide a radiation crosslinked biodegradable POF heat shrink film that maintains excellent tear resistance in low-temperature environments and whose degradation cycle is adjustable and controllable, as well as its preparation method.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a radiation-crosslinked biodegradable POF heat-shrinkable film, comprising a three-layer co-extrusion structure, wherein the three-layer co-extrusion structure consists of a surface layer and a core layer; the core layer is irradiated by an electron beam from a medium-energy electron accelerator to form a crosslinked network structure, and the gel content is 30-70%; the concentration of degradation masterbatch in the surface layer is higher than the concentration of degradation masterbatch in the core layer.

[0010] This invention utilizes electron beam irradiation to induce free radical coupling reactions between the polyolefin molecular chains in the core layer, forming a three-dimensional chemical cross-linked network composed of C-C covalent bonds. Unlike physical entanglement, chemical cross-linking locks the molecular chains in place at low temperatures, making relative slippage difficult. When subjected to tearing stress, the cross-linking points act as stress concentration points, uniformly dispersing energy and forcing cracks to propagate along a tortuous path, thus significantly improving low-temperature tear resistance. Simultaneously, the concentration of degradation masterbatch in the surface layer is higher than in the core layer. The surface layer, in direct contact with microorganisms, water, and oxygen, degrades preferentially, while the core layer, due to the steric hindrance effect of the cross-linked network, delays contact between the degradation masterbatch and the external environment. Furthermore, the cross-links themselves resist hydrolysis and enzymatic degradation, resulting in a slower degradation rate in the core layer compared to the surface layer. This temporal difference of "fast surface degradation and slow core degradation" enables precise control of the overall film degradation cycle.

[0011] The cross-linked network structure formed by electron beam irradiation of the core layer is the core of this invention for improving low-temperature tear resistance. When the gel content is below 30%, the cross-linked network density is insufficient, and there is a lack of sufficient chemical bonds between molecular chains. At low temperatures, the movement of molecular chains is restricted, but relative slippage can still occur. Under tear stress, cracks tend to propagate along the molecular chain interface, thus limiting the improvement in low-temperature toughness. When the gel content is in the preferred range of 30% to 70%, the cross-linking point density is moderate, forming a continuous and stable three-dimensional chemical cross-linked network. The chemical bonds do not dissociate at low temperatures, and under tear stress, the cross-linking points act as stress concentration points, uniformly dispersing energy and forcing cracks to propagate along tortuous paths. At the same time, the cross-linked network restricts the low-temperature embrittlement tendency of molecular chains, thereby significantly improving low-temperature tear resistance. When the gel content is above 70%, excessive cross-linking leads to severely restricted molecular chain mobility, resulting in excessively rigid film with decreased flexibility. Furthermore, the overly dense cross-linked network becomes a stress concentration source, while haze increases significantly, affecting optical performance.

[0012] The gradient degradation structure, where the concentration of masterbatch in the surface layer is higher than that in the core layer, is key to achieving precise control of the degradation cycle in this invention. When a gradient degradation structure is lacking (i.e., the concentrations of masterbatch in the surface and core layers are equal), although the cross-linking network still provides good low-temperature toughness, the excessively high concentration of masterbatch in the core layer interferes with the cross-linking reaction, reducing cross-linking efficiency. Simultaneously, the core layer degradation rate is too fast, resulting in a longer overall degradation cycle, making it difficult to achieve a balance between degradation performance and mechanical properties. When both a cross-linking network and a gradient degradation structure are present, the high concentration of masterbatch in the surface layer ensures a rapid degradation initiation rate, allowing for preferential degradation after direct contact with the external environment. The steric hindrance effect of the cross-linking network in the core layer delays contact between the masterbatch and the external environment. Furthermore, the resistance of the cross-links themselves to hydrolysis and enzymatic degradation results in a slower degradation rate in the core layer compared to the surface layer. This sequential control, where the surface layer degrades first and the core layer degrades later, allows for flexible control of the degradation cycle while maintaining excellent low-temperature toughness.

[0013] Preferably, the core layer further comprises a crosslinking accelerator and an auxiliary resin; the crosslinking accelerator is selected from one or more of triallyl isocyanurate (1,3,5-Tri-2-propenyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, abbreviated as TAIC) and trimethylolpropanetriacrylate (TMPTA), and is added in an amount of 0.5-3 parts; the auxiliary resin is selected from one or more of metallocene polyolefins, α-olefin ethylene copolymers, and vinyl elastomers, and is added in an amount of 1-10 parts.

[0014] The role of crosslinking promoters is to improve the crosslinking efficiency of electron beam irradiation, reduce the required irradiation dose, and promote the formation of a uniform crosslinked network. Triallyl isocyanurate and trimethylolpropane triacrylate are both multifunctional monomers containing multiple carbon-carbon double bonds in their molecular structure. Under electron beam irradiation, they can undergo free radical grafting and crosslinking reactions with polyolefin molecular chains, acting as a "bridge" to connect different molecular chains. The introduction of auxiliary resins can further regulate the flexibility and processing flow of the core layer, forming a synergistic reinforcing effect with the crosslinked network, while not affecting the biodegradability of the film.

[0015] Preferably, the core layer comprises the following components in parts by weight: 60-80 parts of linear low-density polyethylene (LLDPE), 1-10 parts of auxiliary resin, 0.5-3 parts of crosslinking accelerator, 2-5 parts of degradation masterbatch, and 5-10 parts of toughening agent.

[0016] In the core layer, LLDPE serves as the matrix resin, and its content directly affects the mechanical and processing properties of the film. LLDPE molecular chains possess good flexibility and tear resistance, making it the main load-bearing phase in the core layer. When the LLDPE content is below 60 parts, the matrix resin proportion is low, the crosslinking network density is relatively low, and the tensile and tear strengths of the film decrease slightly, as does the shrinkage rate. When the LLDPE content is 60-80 parts, the matrix resin proportion is moderate, and the crosslinking network forms a good composite structure with the matrix, resulting in optimal overall performance. When the LLDPE content is above 80 parts, the matrix resin proportion is high, and the crosslinking network is relatively sparse, but the inherent flexibility of LLDPE gives the film good extensibility, and the tear strength is improved, although the shrinkage rate decreases slightly.

[0017] Preferably, the electron beam irradiation dose is 50-150 kGy, more preferably 80-120 kGy.

[0018] Irradiation dose is a key process parameter determining the degree of crosslinking, i.e., gel content. During electron beam irradiation, high-energy electrons collide with polyolefin molecular chains to generate free radicals, which couple to form C-C crosslinks. If the irradiation dose is too low, the number of free radicals generated is insufficient, the crosslinking reaction is incomplete, the gel content is only 45%, the crosslinking network is discontinuous, and the improvement in low-temperature toughness and shrinkage is limited. With a moderate irradiation dose, the number of free radicals generated is moderate, the crosslinking reaction proceeds fully, forming a uniform and stable crosslinking network, with a gel content of 50%, resulting in optimal overall performance. If the irradiation dose is too high, excessive free radicals are generated, leading to over-crosslinking that severely restricts molecular chain mobility and may also trigger degradation reactions, resulting in excessively rigid films, decreased flexibility, and increased haze.

[0019] In a second aspect, the present invention also provides a method for preparing a radiation-crosslinked biodegradable POF heat-shrinkable film as described in the first aspect, comprising the following steps: (1) blending and granulating the surface material and the core material respectively; (2) extruding the film through a three-layer co-extrusion die; (3) performing biaxial stretching; (4) performing crosslinking by irradiation with a medium-energy electron accelerator electron beam at a dose of 50-150 kGy under an inert atmosphere; and (5) winding and slitting.

[0020] This invention places the electron beam irradiation process after biaxial stretching. Biaxial stretching causes the polyolefin molecular chains to be highly oriented along the stretching direction, forming ordered crystalline and amorphous regions. Electron beam irradiation at this point causes the cross-linking reaction to occur primarily in the more mobile amorphous regions. The cross-linking points connect adjacent oriented molecular chains laterally, creating a "chemical riveting" effect. During subsequent heat shrinkage, these lateral cross-links constrain the deorientation process of the molecular chains, causing shrinkage to occur over a wider temperature range rather than being concentrated in a narrow window, thereby widening the heat shrinkage temperature window and increasing the shrinkage rate. Furthermore, the irradiation process is compatible with existing production lines, requiring no modification to the extrusion and stretching sections; only the addition of an irradiation device before winding or outsourcing the film processing is needed, facilitating industrial implementation.

[0021] Preferably, in step (4), the energy of the electron beam irradiation by the medium-energy electron accelerator is 150-300 keV.

[0022] The irradiation energy determines the penetration depth of the electron beam. For a three-layer co-extruded POF film, sufficient energy is needed to penetrate the surface layer and reach the core layer. When the irradiation energy is below 150 keV, the electron beam penetration depth is shallow, the irradiation dose received by the core layer is insufficient, the cross-linking degree of the core layer is lower than that of the surface layer, the cross-linking network is uneven, and the low-temperature toughness improvement effect is insufficient. When the irradiation energy is between 150-300 keV, the electron beam penetration depth is moderate, and it can uniformly penetrate the entire film thickness, with both the surface layer and the core layer achieving uniform cross-linking. When the irradiation energy is above 300 keV, the electron beam penetration depth is too large, and the energy utilization rate decreases.

[0023] Preferably, in step (4), the inert atmosphere is a nitrogen atmosphere.

[0024] Thirdly, the present invention also provides an application of the radiation-crosslinked biodegradable POF heat-shrinkable film as described in the first aspect in cold chain food packaging, frozen pharmaceutical packaging, or low-temperature environment industrial product packaging.

[0025] The film of this invention maintains its core layer chemical cross-linking network intact in low-temperature environments ranging from -20°C to 0°C. The cross-linking points effectively pin the molecular chains, inhibiting low-temperature brittle fracture and ensuring sufficient puncture and tear resistance when wrapping frozen foods or pharmaceuticals, preventing frostbite, dehydration, or contamination caused by packaging damage. Furthermore, after the film is discarded, the high-concentration degradation masterbatch on the surface initiates degradation in composting or the natural environment. Once the packaging protection function is lost, the core layer gradually degrades. This satisfies both the mechanical performance requirements during use and environmental regulations regarding the biodegradability of packaging materials.

[0026] Preferably, the cold chain food packaging includes frozen meat products, frozen pasta, ice cream, and frozen seafood packaging.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The radiation crosslinked biodegradable POF heat shrink film provided by the present invention, through the synergy of the core layer chemical crosslinking network and the gradient degradation structure, not only endows the film with biodegradability, but also retains more than 85% of the tear strength at -20℃. Furthermore, the crosslinking network is used to delay the degradation rate of the core layer, thereby achieving precise control of the degradation cycle within the range of 12-24 months. This solves the problems of easy brittleness and uncontrollable degradation cycle of biodegradable films in cold chain scenarios.

[0029] (2) The method for preparing radiation-crosslinked biodegradable POF heat shrink film provided by the present invention, by introducing electron beam irradiation crosslinking after biaxial stretching, widens the heat shrinkage temperature window by 15-20℃, increases the shrinkage rate to more than 72%, reduces packaging energy consumption, and the process is compatible with existing POF production lines, without the need for major modifications to the extrusion and stretching sections, which facilitates rapid industrialization.

[0030] (3) The radiation crosslinking biodegradable POF heat shrink film provided by the present invention can be used in the packaging of cold chain food, frozen medicine or industrial products in low temperature environment to maintain complete wrapping and puncture resistance in the environment from -20℃ to 0℃, while meeting the requirements of degradability and environmental protection. Detailed Implementation

[0031] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0032] It should be understood that in the description of this invention, the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the shown orientation or positional relationships and 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, and therefore should not be construed as a limitation of this invention. Those skilled in the art can understand the specific meaning of the above terms in this invention through specific circumstances.

[0033] Unless otherwise specified, all chemical products mentioned in this article are commercially available products.

[0034] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0035] In one specific embodiment, the present invention provides a radiation-crosslinked biodegradable POF heat-shrinkable film comprising a three-layer co-extruded structure, the three-layer co-extruded structure being composed of a surface layer and a core layer. The core layer is irradiated with an electron beam from a medium-energy electron accelerator to form a cross-linked network structure, with a gel content of 30% to 70%. The concentration of degradation masterbatch in the surface layer is higher than that in the core layer. Further, the core layer also contains a cross-linking accelerator selected from one or more of triallyl isocyanurate and trimethylolpropane triacrylate, added at an amount of 0.5 to 3 parts. The core layer also contains an auxiliary resin selected from one or more of metallocene polyolefins, α-olefin ethylene copolymers, and vinyl elastomers, added at an amount of 1 to 10 parts. The core layer contains the following components in parts by weight: 60-80 parts linear low-density polyethylene, 1-10 parts auxiliary resin, 0.5-3 parts cross-linking accelerator, 2-5 parts degradation masterbatch, and 5-10 parts toughening agent. The surface layer contains the following components in parts by weight: 70-85 parts polypropylene, 8-12 parts degradation masterbatch, 1-3 parts opening agent, and 1-3 parts slip agent. The electron beam irradiation dose is 50-150 kJ / kg. kGy, preferably 80~120 kGy.

[0036] In another specific embodiment, the present invention also provides a method for preparing a radiation-crosslinked biodegradable POF heat-shrinkable film as described in any of the above embodiments, the method specifically comprising the following steps:

[0037] (1) Blend and granulate the surface material and the core material separately; (2) Extrude the casting through a three-layer co-extrusion die; (3) Perform biaxial stretching; (4) Perform crosslinking by medium-energy electron accelerator electron beam irradiation at a dose of 50~150 kGy in an inert atmosphere, the energy of which is 150~300 keV, and the inert atmosphere includes nitrogen atmosphere; (5) Rewind and cut.

[0038] In another specific embodiment, the present invention provides an application of the radiation-crosslinked biodegradable POF heat shrink film as described in any of the above embodiments, specifically applying the radiation-crosslinked biodegradable POF heat shrink film to cold chain food packaging, frozen pharmaceutical packaging, or low-temperature environment industrial product packaging; based on the above application, as a further refinement, the cold chain food packaging may include frozen meat products, quick-frozen pasta, ice cream, and frozen seafood packaging.

[0039] It should be clarified that any use of the process provided in the embodiments of the present invention or any substitution or change of conventional data falls within the protection and disclosure scope of the present invention.

[0040] Example 1

[0041] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film, comprising a three-layer co-extruded structure consisting of a surface layer and a core layer. The core layer is irradiated with an electron beam from a medium-energy electron accelerator to form a crosslinked network structure, with a gel content of 50%. The concentration of degradation masterbatch in the surface layer is higher than that in the core layer. Further, the core layer also contains a crosslinking accelerator selected from triallyl isocyanurate, added at 1.5 parts; the core layer also contains an auxiliary resin, which is a metallocene polyolefin, added at 5 parts; the core layer contains the following weight parts: 70 parts linear low-density polyethylene, 5 parts auxiliary resin, 1.5 parts crosslinking accelerator, 3.5 parts degradation masterbatch, and 7.5 parts toughening agent; the surface layer contains the following weight parts: 78 parts polypropylene, 10 parts degradation masterbatch, 2 parts opening agent, and 2 parts slip agent; the electron beam irradiation dose is 100 kGy.

[0042] This embodiment also provides a method for preparing the above-mentioned radiation-crosslinked biodegradable POF heat-shrinkable film, the method specifically including the following steps: (1) blending and granulating the surface material and the core material respectively; (2) extruding the casting through a three-layer co-extrusion die; (3) performing biaxial stretching; (4) performing medium-energy electron accelerator electron beam irradiation crosslinking at a dose of 100 kGy in an inert atmosphere, the energy of which is 225 keV, and the inert atmosphere includes a nitrogen atmosphere; (5) winding and slitting.

[0043] Example 2

[0044] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film, comprising a three-layer co-extruded structure consisting of a surface layer and a core layer. The core layer is irradiated with an electron beam from a medium-energy electron accelerator to form a crosslinked network structure, with a gel content of 30%. The concentration of degradation masterbatch in the surface layer is higher than that in the core layer. Further, the core layer also contains a crosslinking accelerator selected from trimethylolpropane triacrylate, added at 0.5 parts. The core layer also contains an auxiliary resin, which is an α-olefin ethylene copolymer, added at 2 parts. The core layer comprises the following weight parts: 60 parts linear low-density polyethylene, 2 parts auxiliary resin, 0.5 parts crosslinking accelerator, 2 parts degradation masterbatch, and 5 parts toughening agent. The surface layer comprises the following weight parts: 70 parts polypropylene, 8 parts degradation masterbatch, 1 part opening agent, and 1 part slip agent. The electron beam irradiation dose is 50 kGy.

[0045] This embodiment also provides a method for preparing the above-mentioned radiation-crosslinked biodegradable POF heat-shrinkable film, the method specifically including the following steps: (1) blending and granulating the surface material and the core material respectively; (2) extruding the casting through a three-layer co-extrusion die; (3) performing biaxial stretching; (4) performing medium-energy electron accelerator electron beam irradiation crosslinking at a dose of 50 kGy in an inert atmosphere, the energy of which is 150 keV, and the inert atmosphere includes a nitrogen atmosphere; (5) winding and slitting.

[0046] Example 3

[0047] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film, comprising a three-layer co-extrusion structure consisting of a surface layer and a core layer. The core layer is irradiated with an electron beam from a medium-energy electron accelerator to form a crosslinked network structure, with a gel content of 70%. The concentration of degradation masterbatch in the surface layer is higher than that in the core layer. Further, the core layer also contains a crosslinking accelerator selected from a mixture of triallyl isocyanurate and trimethylolpropane triacrylate, added in an amount of 3 parts. The core layer also contains an auxiliary resin, which is a vinyl elastomer, added in an amount of 8 parts. The core layer comprises the following weight parts: 80 parts linear low-density polyethylene, 8 parts auxiliary resin, 3 parts crosslinking accelerator, 5 parts degradation masterbatch, and 10 parts toughening agent. The surface layer comprises the following weight parts: 85 parts polypropylene, 12 parts degradation masterbatch, 3 parts opening agent, and 3 parts slip agent. The electron beam irradiation dose is 150 kGy.

[0048] This embodiment also provides a method for preparing the above-mentioned radiation-crosslinked biodegradable POF heat-shrinkable film, the method specifically including the following steps: (1) blending and granulating the surface material and the core material respectively; (2) extruding the casting through a three-layer co-extrusion die; (3) performing biaxial stretching; (4) performing medium-energy electron accelerator electron beam irradiation crosslinking at a dose of 150 kGy in an inert atmosphere, the energy of which is 300 keV, and the inert atmosphere includes a nitrogen atmosphere; (5) winding and slitting.

[0049] Example 4

[0050] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the crosslinking promoter in the core layer is selected from trimethylolpropane triacrylate, and the amount added is 0.5 parts.

[0051] Example 5

[0052] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the amount of crosslinking promoter added to the core layer is 3 parts.

[0053] Example 6

[0054] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the linear low-density polyethylene in the core layer is 60 parts.

[0055] Example 7

[0056] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the linear low-density polyethylene in the core layer is 80 parts.

[0057] Example 8

[0058] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the polypropylene content in the surface layer is 70 parts.

[0059] Example 9

[0060] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the polypropylene content in the surface layer is 85 parts.

[0061] Example 10

[0062] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the surface layer contains 8 parts of degradation masterbatch.

[0063] Example 11

[0064] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the surface layer contains 12 parts of degradation masterbatch.

[0065] Example 12

[0066] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the electron beam irradiation dose is 50 kGy.

[0067] Example 13

[0068] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 is that the electron beam irradiation dose is 150 kGy.

[0069] Example 14

[0070] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 in the preparation method is that in step (4), the energy of the electron beam irradiation by the medium-energy electron accelerator is 150 keV.

[0071] Example 15

[0072] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference between this embodiment and Example 1 in terms of preparation method is that in step (4), the energy of the electron beam irradiation by the medium-energy electron accelerator is 300 keV.

[0073] Example 16

[0074] This embodiment provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method, which simultaneously possesses a core layer chemical crosslinking network and a gradient degradation structure where the concentration of surface degradation masterbatch is higher than that of the core layer. Specific parameters are: gel content 50%, surface degradation masterbatch concentration 10 parts, core degradation masterbatch concentration 3.5 parts, irradiation dose 100 kGy, and other parameters are the same as in Example 1.

[0075] Comparative Example 1

[0076] This comparative example provides a radiation-crosslinked biodegradable POF heat-shrinkable film and its preparation method. The only difference from Example 1 is that the core layer was not irradiated by a medium-energy electron accelerator electron beam and did not form a crosslinked network structure, i.e., the gel content was 0%, which does not meet the requirement of forming a crosslinked network in the core layer by irradiation in Independent Claim 1.

[0077] Comparative Example 2

[0078] This comparative example provides a radiation-crosslinked biodegradable POF heat-shrinkable film, which differs from Example 1 only in composition: the core layer is a crosslinked network structure formed by electron beam irradiation of a medium-energy electron accelerator, and the gel content is 20%.

[0079] Comparative Example 3

[0080] This comparative example provides a radiation-crosslinked biodegradable POF heat-shrinkable film, which differs from Example 1 only in composition: the core layer is a crosslinked network structure formed by electron beam irradiation of a medium-energy electron accelerator, and the gel content is 80%.

[0081] Comparative Example 4

[0082] The only difference between this comparative example and Example 1 is that the concentration of the degradation masterbatch in the surface layer is equal to the concentration of the degradation masterbatch in the core layer, that is, it does not have a gradient degradation structure, but the core layer is still irradiated with an electron beam to form a cross-linked network, the gel content is 50%, and the irradiation dose is 100 kGy.

[0083] Comparative Example 5

[0084] The only difference between this comparative example and Example 1 is that the core layer was not irradiated with an electron beam, the gel content was 0%, and the concentration of degradation masterbatch in the surface layer and the core layer was equal, meaning that it did not have both a cross-linking network and a gradient degradation structure.

[0085] Comparative Example 6

[0086] The only difference between this comparative example and Example 1 is that the gel content of the core layer after electron beam irradiation is 15%, which is lower than the lower limit of 30% in Independent Claim 1, but retains the gradient degradation structure.

[0087] Comparative Example 7

[0088] The only difference between this comparative example and Example 1 is that the gel content of the core layer after electron beam irradiation is 85%, which is higher than the upper limit of 70% in Independent Claim 1, but retains the gradient degradation structure.

[0089] Performance tests were conducted on Examples 1-16 and Comparative Examples 1-7, including tear strength at -20°C, low-temperature retention rate, degradation period, and shrinkage rate. The test results are shown in Table 1.

[0090] Table 1

[0091]

[0092]

[0093] Based on the comparative analysis of the above test data, we can draw the following conclusions:

[0094] (1) The key performance indicators such as tear strength at -20℃, low temperature retention rate, degradation cycle and shrinkage rate in Examples 1 to 16 are significantly better than those in Comparative Examples 1, 5 and 6. This shows that by adopting the synergistic design of core layer chemical cross-linking network and gradient degradation structure, the present invention not only endows the film with biodegradability, but also significantly improves the tear resistance and shrinkage performance in low temperature environment, and achieves precise control of degradation cycle.

[0095] (2) Comparing Example 1 and Comparative Examples 1 to 3, it can be seen that when the gel content of Comparative Example 1 is 0%, the tear strength at -20℃ is only 0.60N. When the gel content of Comparative Example 2 is 20%, it is increased to 0.78N but still insufficient. When the gel content of Comparative Example 3 is 80%, the tear strength reaches 1.10N, but the degradation period is as long as 22 months and the haze increases to 4.5%. In contrast, when the gel content of Example 1 is 50%, the tear strength at -20℃ reaches 1.08N, the degradation period is 17 months, and the haze is only 3.8%. This shows that the preferred range of gel content of 30% to 70% can achieve the best balance between mechanical properties, degradation performance and optical properties.

[0096] (3) Comparing Example 1 with Comparative Examples 4 to 5, it can be seen that when Comparative Example 5 lacks both cross-linking network and gradient degradation structure, the tear strength at -20℃ is only 0.52N and the degradation period is only 8 months. When Comparative Example 4 only has cross-linking network, the tear strength reaches 1.05N but the degradation period is as long as 22 months. When Example 1 has both, the tear strength at -20℃ reaches 1.08N and the degradation period is moderate at 17 months, which proves that there is a significant synergistic effect between chemical cross-linking network and gradient degradation structure.

[0097] (4) Comparing Example 1 and Examples 4 to 5, it can be seen that when the amount of crosslinking accelerator added in Example 4 is 0.5 parts, the tear strength at -20℃ is 1.05N, which is slightly lower than 1.08N in Example 1. When the amount of crosslinking accelerator added in Example 5 is 3 parts, it increases to 1.11N, but the degradation period is extended to 18 months. This indicates that there is a balance between performance and cost in the range of 0.5 to 3 parts of crosslinking accelerator added, of which 1.5 parts is the better choice.

[0098] (5) Comparing Example 1 and Examples 12 to 13, it can be seen that in Example 12, the gel content is 45%, the tear strength at -20℃ is 1.02N, the shrinkage rate is 68%, and the degradation period is 15 months when the irradiation dose is 50kGy. In Example 13, the gel content is 65%, the tear strength at -20℃ is 1.12N, the shrinkage rate is 75%, and the degradation period is 20 months when the irradiation dose is 150kGy. In contrast, the indicators in Example 1 are balanced when the irradiation dose is 100kGy, indicating that the irradiation dose can be flexibly selected according to the target application scenario within the range of 50 to 150kGy.

[0099] (6) Comparing Example 1 and Examples 6 to 7, it can be seen that in Example 6, when the core layer LLDPE content is 60 parts, the tear strength at -20℃ is 1.02N and the shrinkage rate is 70%. In Example 7, when the core layer LLDPE content is 80 parts, the tear strength at -20℃ is 1.12N and the shrinkage rate is 73%. In Example 1, when the core layer LLDPE content is 70 parts, the overall performance is balanced, indicating that good results can be achieved in the range of 60 to 80 parts of LLDPE content.

[0100] In summary, the radiation-crosslinked biodegradable POF heat-shrinkable film provided by this invention, through the synergistic effect of the core layer chemical crosslinking network and the gradient distribution of degradation masterbatch on the surface layer above the core layer, not only endows the film with biodegradability but also significantly improves its tear resistance at low temperatures. This design utilizes the crosslinking network to slow down the degradation rate of the core layer and combines it with gradient concentration differences to achieve flexible control of the degradation cycle. At the same time, the electron beam irradiation crosslinking process broadens the heat-shrinking temperature window and improves the shrinkage rate, enabling the film to meet the comprehensive requirements of cold chain packaging for mechanical properties, environmental characteristics, and processing adaptability.

[0101] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A radiation-crosslinked biodegradable POF heat-shrinkable film, characterized in that, It includes a three-layer co-extruded structure, which consists of a surface layer and a core layer; The core layer is irradiated with an electron beam from a medium-energy electron accelerator to form a cross-linked network structure, with a gel content of 30-70%. The concentration of degradation masterbatch in the surface layer is higher than that in the core layer.

2. The radiation-crosslinked biodegradable POF heat-shrinkable film according to claim 1, characterized in that, The core layer further comprises a crosslinking accelerator and an auxiliary resin; the crosslinking accelerator is selected from one or more of triallyl isocyanurate and trimethylolpropane triacrylate, and the amount added is 0.5-3 parts; the auxiliary resin is selected from one or more of metallocene polyolefin, α-olefin ethylene copolymer, and vinyl elastomer, and the amount added is 1-10 parts.

3. The radiation-crosslinked biodegradable POF heat-shrinkable film according to claim 1, characterized in that, The core layer comprises the following components in parts by weight: 60-80 parts of linear low-density polyethylene; 1-10 parts of auxiliary resin; Crosslinking accelerator 0.5-3 parts; 2-5 parts of degradation masterbatch; 5-10 parts toughening agent.

4. The radiation-crosslinked biodegradable POF heat-shrinkable film according to claim 1, characterized in that, The surface layer comprises the following components in parts by weight: 70-85 parts of polypropylene; 8-12 parts of degradation masterbatch; 1-3 parts of opening agent; 1-3 parts of slip agent.

5. The radiation-crosslinked biodegradable POF heat-shrinkable film according to claim 1, characterized in that, The electron beam irradiation dose is 50-150 kGy, preferably 80-120 kGy.

6. A method for preparing the radiation-crosslinked biodegradable POF heat-shrinkable film according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Blend and granulate the surface material and the core material separately; (2) Extrude the casting through a three-layer co-extrusion die; (3) Perform biaxial stretching; (4) Perform cross-linking by medium-energy electron accelerator electron beam irradiation at a dose of 50-150 kGy under an inert atmosphere; (5) Rewind and cut.

7. The preparation method according to claim 6, characterized in that, In step (4), the energy of the electron beam irradiation of the medium-energy electron accelerator is 150-300 keV.

8. The preparation method according to claim 6, characterized in that, In step (4), the inert atmosphere includes a nitrogen atmosphere.

9. The application of the radiation-crosslinked biodegradable POF heat-shrinkable film according to any one of claims 1-5 in cold chain food packaging, frozen pharmaceutical packaging, or low-temperature environment industrial product packaging.

10. The application according to claim 9, characterized in that, The cold chain food packaging includes frozen meat products, frozen pasta, ice cream, and frozen seafood packaging.

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