Polyolefin cross-linked heat shrink film for fresh food packaging and preparation method of polyolefin cross-linked heat shrink film

Abstract

The invention is suitable for the technical field of heat shrink films, and provides a polyolefin cross-linked heat shrink film for fresh food packaging and a preparation method thereof.The heat shrink film is prepared from co-polypropylene, polyethylene, metallocene polypropylene, ethylene-octylene copolymer and a processing assistant.The invention further provides the preparation method of the heat shrink film. A five-layer or more-layer co-extrusion three-bubble method synchronous stretching production process is adopted, electron beam irradiation crosslinking is adopted, and the polyolefin crosslinking heat shrinkage film for fresh food packaging provided by the invention adopts a method of blending modification of low-softening-point elastomers such as metallocene polypropylene and POE, so that the shrinkage stress and initial shrinkage temperature of the film are reduced; a three-bubble method production process is adopted, the performance of the film is improved, crystal overgrowth is inhibited through a rapid cooling process, transparency and shrinkage uniformity are improved, and the mechanical property and thermal shrinkage of the film are improved through electron beam irradiation crosslinking.

Description

Technical Field

[0001] The invention belongs to the technical field of heat shrinkable films, and in particular relates to a polyolefin cross-linked heat shrinkable film for fresh food packaging and a preparation method thereof. Background Art

[0002] Fresh meat packaging films mainly include heat shrink film, body-fitting packaging film, vacuum packaging film and other types. Among them, heat shrink film is a material widely used in meat food packaging. It can shrink and wrap around the product after heating, playing a role in protecting and beautifying the product.

[0003] Skin film is a packaging material that fits snugly around meat products, ensuring excellent freshness preservation. This type of film typically features high transparency and a strong seal, effectively preventing oxidation and bacterial growth in meat products, extending their shelf life.

[0004] Vacuum packaging film achieves the purpose of preservation by removing the air inside the package. This packaging method can effectively prevent the oxidation and deterioration of meat products, and at the same time inhibit the growth of bacteria through the vacuum state. For example, nylon vacuum bags and PA / PE co-extruded films are used for packaging frozen meat products of beef and mutton, providing high barrier properties and effectively maintaining the freshness of meat. In addition, there are some special packaging films, such as anti-fog film and high-barrier PA / PE / EVOH co-extruded film. These special materials can further improve the preservation effect of meat and maintain the quality and taste of meat. For example, anti-fog film can prevent fogging on the surface of meat and maintain the clarity of the product: while high-barrier PA / PEEVOH co-extruded film, through its special material structure, provides higher barrier properties and prevents the influence of oxygen and moisture. The selection of these packaging films depends on the specific needs and preservation requirements of the product, while also considering cost and environmental factors. When using heat shrink film to package fresh meat, the shrinkage temperature must not be too high. Conventional POF film usually needs to be above 110°C to fully shrink. If the initial heat shrinkage temperature of the packaging film is too high, it can easily cause the fresh meat to deteriorate. In response to the customer's demand for using POF film to package fresh meat, a POF cross-linked film that can be used for fresh meat packaging has been developed. Summary of the Invention

[0005] The present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging and a preparation method thereof, aiming to solve the above-mentioned problems.

[0006] The present invention is achieved by providing a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 80-95 parts of copolymerized polypropylene, 50-60 parts of polyethylene, 20-30 parts of metallocene polypropylene, 15-30 parts of ethylene-octene copolymer (POE), and 5-10 parts of a processing aid. The film is prepared using a co-extrusion three-bubble synchronous stretching production process with five or more layers, cross-linked by electron beam irradiation, and modified by blending metallocene polypropylene with POE and other low-softening-point elastomers to reduce the shrinkage stress and initial shrinkage temperature of the film. The three-bubble production process not only improves the film's performance but also solves the technical problem of poor dimensional stability, such as curling during storage. Electron beam irradiation cross-linking improves the film's mechanical properties and heat shrinkage rate.

[0007] Preferably, the following raw materials are included in parts by weight: 85-90 parts of copolymerized polypropylene, 52-58 parts of polyethylene, 23-27 parts of metallocene polypropylene, 20-25 parts of ethylene-octene copolymer (POE), and 6-9 parts of processing aid.

[0008] Preferably, the following raw materials are included in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer (POE), and 7.5 parts of processing aid.

[0009] Preferably, the processing aids include an anti-blocking agent, a lubricant, and an anti-fogging agent.

[0010] Preferably, the polyolefin cross-linked heat shrinkable film for fresh food packaging further comprises the following raw materials in parts by weight: 5-10 parts of modified polyethylene glycol, 8-14 parts of modified cellulose acetate, and 1-5 parts of epoxidized soybean oil.

[0011] Preferably, the preparation method of the modified polyethylene glycol is as follows: by weight, 10-30 parts of polyethylene glycol (PEG-400), 1-3 parts of nano-silica particles, 0.1-0.3 parts of a silane coupling agent (KH-570), 5-15 parts of acrylamide, 0.1-0.3 parts of azobisisobutyronitrile, and an appropriate amount of ethylene glycol monomethyl ether (solvent) are taken; the nano-silica particles are dispersed in the ethylene glycol monomethyl ether, ultrasonically dispersed for 30-60 minutes (temperature 20-25°C), the silane coupling agent is added, and the mixture is stirred and reacted at 60-80°C for 3-5 hours; the modified nano-silica suspension is mixed with polyethylene glycol, acrylamide monomer is added, azobisisobutyronitrile is added, the temperature is raised to 60-70°C, and the mixture is reacted under nitrogen protection for 4-8 hours. After the reaction is completed, the mixture is cooled to room temperature, centrifuged, washed with ethanol 2-3 times, and vacuum dried to constant weight (temperature 40-50°C).

[0012] Silane coupling agents can form a layer of organic functional groups on the surface of nano-silica, which, on the one hand, improves its compatibility with polyethylene glycol, and on the other hand, provides active sites for subsequent grafting reactions. Acrylamide monomers undergo polymerization under the action of the initiator and undergo grafting reactions with polyethylene glycol and the active sites on the surface of nano-silica. By introducing nano-silica particles into polyethylene glycol, the microstructure and thermal properties of polyethylene glycol can be changed. Nano-silica has a high specific surface area and surface activity, and can form a special network structure in the polyethylene glycol matrix. This structure can affect the shrinkage behavior of the heat-shrinkable film and reduce its shrinkage temperature. Grafting acrylamide can further improve the compatibility and interaction between the modified polyethylene glycol and the heat-shrinkable film substrate. The acrylamide grafted chain can act as a bridge in the polymer matrix of the heat-shrinkable film, enhancing the dispersibility and stability of the modified polyethylene glycol in the matrix. It can also affect the crystallization behavior and molecular chain movement of the matrix polymer, thereby more effectively reducing the shrinkage temperature. The surface-modified nano-silica has a stronger bond with the polyethylene glycol and acrylamide, avoiding the agglomeration of nanoparticles in the system and improving the performance of the modified polyethylene glycol.

[0013] Preferably, the preparation method of the modified cellulose acetate is as follows: 20-40 parts of cellulose acetate, 3-8 parts of maleic anhydride, 0.2-0.5 parts of catalyst (p-toluenesulfonic acid), and an appropriate amount of acetone-methanol mixed solvent (volume ratio 1:1) are taken by weight, and the cellulose acetate is dissolved in the acetone-methanol mixed solvent and stirred until completely dissolved (temperature 20-25°C). Maleic anhydride and the catalyst are added, and under nitrogen protection, the temperature is raised to 50-60°C and the reaction is carried out for 3-5 hours. After the reaction is completed, the solution is poured into water for precipitation, and the crude product is obtained by filtration, washed with distilled water 3-5 times, and vacuum dried to constant weight (temperature 40-50°C).

[0014] By modifying the cellulose acetate with maleic anhydride, maleic anhydride groups are introduced into the cellulose acetate molecular chain. This can, on the one hand, change its intermolecular forces and crystallization behavior. On the other hand, the maleic anhydride groups have high reactivity and can interact with the acrylamide grafted chains in the modified polyethylene glycol, such as forming hydrogen bonds or chemical cross-linking reactions, thereby enhancing the synergistic effect between the two. The prepared modified polyethylene glycol and modified cellulose acetate can be mixed in a certain proportion and melt blended or solution blended at high temperature to ensure that the two are fully and evenly dispersed. During the blending process, the nano-silica particles in the modified polyethylene glycol can fill the molecular gaps of the modified cellulose acetate, forming a structure similar to "filling-reinforcement". At the same time, the interaction between the acrylamide grafted chains and the maleic anhydride groups can enhance the intermolecular forces and compatibility of the entire blend system.

[0015] Modified polyethylene glycol reduces the shrinkage temperature through its own structural characteristics (nano-silica and acrylamide grafting), while the molecular chain regularity of modified cellulose acetate changes after modification with maleic anhydride, which also has a certain impact on the shrinkage temperature. The synergistic effect of the two can further reduce the shrinkage temperature of the heat-shrink film. In terms of mechanical properties, modified cellulose acetate provides good tensile strength and elastic modulus, while the nano-silica particles in the modified polyethylene glycol play a reinforcing role, so that the heat-shrink film can maintain good mechanical properties while reducing the shrinkage temperature and is not easily broken or deformed during the packaging process. In terms of thermal stability, the chemical cross-linking reaction that may occur between cellulose acetate modified with maleic anhydride and polyethylene glycol grafted with acrylamide can improve the thermal stability of the heat-shrink film, making it more stable during use and less prone to problems such as premature shrinkage or uneven shrinkage.

[0016] The present invention also provides a method for preparing the above-mentioned polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following steps: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5-6, stretching temperature 80-90℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 electron beam irradiation cross-linking of the tubular membrane, with a dose of 5-8 kGy and an energy of 1.5-2 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0017] Preferably, 0.1-0.3% nitrogen-doped carbon quantum dots are added to the raw materials of S2 and mixed. When the heat shrinkable film is used, it is first vacuumed and then contracted with hot air-assisted near-infrared light. An 808nm light source is used with a power of 30-50W and an irradiation time of 5-10s. Through the photothermal conversion effect of nitrogen-doped carbon quantum dots, the absorbed light energy is converted into thermal energy, triggering the contraction and combination of the film. Through photothermal synergistic contraction, the contraction temperature is further reduced. In combination with the three-bubble process, low-temperature triggered contraction, high transparency and excellent mechanical properties of the film are achieved.

[0018] Preferably, the preparation method of the nitrogen-doped carbon quantum dots is as follows: citric acid and urea are dissolved in deionized water in a molar ratio of 1:2-3, 5% PEG-2000 is added, and the reaction is carried out at 180-200°C for 3-5 hours. After centrifugal purification, nitrogen-doped carbon quantum dots are obtained with a particle size of ≤5nm, containing carboxyl and amino groups on the surface, and the absorption peak is red-shifted to 700-900nm (near-infrared region) by nitrogen doping. The photothermal conversion efficiency is ≥85%, and PEG modification is used to improve the compatibility with polyolefins and avoid agglomeration. Electron beam irradiation crosslinking can induce nitrogen-doped carbon quantum dots to form an interfacial crosslinked network with polyolefin molecular chains, thereby improving the mechanical strength.

[0019] Compared with the prior art, the embodiments of the present application have the following beneficial effects: The polyolefin cross-linked heat shrinkable film for fresh food packaging provided by the present invention adopts a method of blending and modifying low-softening-point elastomers such as metallocene polypropylene and POE to reduce the shrinkage stress and initial shrinkage temperature of the film, adopts a three-bubble production process to improve the performance of the film, suppresses excessive crystal growth through a rapid cooling process, improves transparency and shrinkage uniformity, and improves the mechanical properties and heat shrinkage rate of the film through electron beam irradiation cross-linking.

[0020] The polyolefin cross-linked heat shrinkable film for fresh food packaging provided by the present invention further reduces the shrinkage temperature of the heat shrinkable film through the synergistic effect of adding modified polyethylene glycol and modified cellulose acetate, thereby maintaining good mechanical properties and thermal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention provides a flow chart of a method for preparing a polyolefin cross-linked heat shrinkable film for fresh food packaging. DETAILED DESCRIPTION

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this application belongs. The terms used in the specification of the application are only for the purpose of describing specific embodiments and are not intended to limit this application. The terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. The terms "first", "second", etc. in the specification and claims of this application or the above-mentioned drawings are used to distinguish different objects, not to describe a specific order.

[0023] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0024] Example 1 The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 80 parts of copolymerized polypropylene, 50 parts of polyethylene, 20 parts of metallocene polypropylene, 15 parts of ethylene-octene copolymer (POE), and 5 parts of processing aids, wherein the processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5, stretching temperature 80℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 electron beam irradiation cross-linked the tubular membrane with a dose of 5 kGy and an energy of 1.5 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0025] Example 2 The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 85 parts of copolymerized polypropylene, 52 parts of polyethylene, 23 parts of metallocene polypropylene, 20 parts of ethylene-octene copolymer (POE), and 6 parts of processing aids, wherein the processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5, stretching temperature 80℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 electron beam irradiation cross-linked the tubular membrane with a dose of 5 kGy and an energy of 1.5 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0026] Example 3 The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer (POE), and 7.5 parts of processing aids, wherein the processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5.5, stretching temperature 85℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 performed electron beam irradiation cross-linking of the tubular membrane with a dose of 6.5 kGy and an energy of 1.75 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0027] Example 4 The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 90 parts of copolymerized polypropylene, 58 parts of polyethylene, 27 parts of metallocene polypropylene, 25 parts of ethylene-octene copolymer (POE), and 9 parts of processing aids, wherein the processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 6, stretching temperature 90℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 electron beam irradiation cross-linking of the tubular membrane, dose 8 kGy, energy 2 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0028] Example 5 The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 95 parts of copolymerized polypropylene, 60 parts of polyethylene, 30 parts of metallocene polypropylene, 30 parts of ethylene-octene copolymer (POE), and 10 parts of processing aids, wherein the processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 6, stretching temperature 90℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 electron beam irradiation cross-linking of the tubular membrane, dose 8 kGy, energy 2 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0029] Example 6 (Based on Example 3, adding modified polyethylene glycol, modified cellulose acetate, and nitrogen-doped carbon quantum dots) The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer (POE), 7.5 parts of processing aids, 5 parts of modified polyethylene glycol, 8 parts of modified cellulose acetate, and 1 part of epoxy soybean oil. The processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2: mixing the raw materials and nitrogen-doped carbon quantum dots and performing melt co-extrusion, wherein the weight of the nitrogen-doped carbon quantum dots is 0.1% of the total weight of the raw materials, and the Bacillus fermentation broth extract is in the form of microcapsules (particle size ≤ 200 nm), accounting for 0.5%; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5.5, stretching temperature 85℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 performed electron beam irradiation cross-linking of the tubular membrane with a dose of 6.5 kGy and an energy of 1.75 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0030] The preparation method of the modified polyethylene glycol is as follows: 10 parts of polyethylene glycol (PEG-400), 1 part of nano-silica particles, 0.1 part of silane coupling agent (KH-570), 5 parts of acrylamide, 0.1 part of azobisisobutyronitrile, and an appropriate amount of ethylene glycol monomethyl ether (solvent) are taken by weight, the nano-silica particles are dispersed in ethylene glycol monomethyl ether, ultrasonically dispersed for 30 minutes (temperature 20°C), the silane coupling agent is added, and stirred at 60°C for 3 hours. The modified nano-silica suspension is mixed with polyethylene glycol, acrylamide monomer is added, azobisisobutyronitrile is added, the temperature is raised to 60°C, and the reaction is carried out under nitrogen protection for 4 hours. After the reaction is completed, the mixture is cooled to room temperature, centrifuged, washed twice with ethanol, and vacuum dried to constant weight (temperature 40°C).

[0031] In this embodiment, the preparation method of the modified cellulose acetate is as follows: 20 parts of cellulose acetate, 3 parts of maleic anhydride, 0.2 parts of a catalyst (p-toluenesulfonic acid), and an appropriate amount of an acetone-methanol mixed solvent (volume ratio of 1:1) are taken by weight, and the cellulose acetate is dissolved in the acetone-methanol mixed solvent and stirred until completely dissolved (temperature 20°C). Maleic anhydride and the catalyst are added, and the temperature is raised to 50°C under nitrogen protection, and the reaction is carried out for 3 hours. After the reaction is completed, the solution is poured into water for precipitation, and the crude product is filtered to obtain a crude product, which is washed with distilled water three times and vacuum dried to a constant weight (temperature 40°C).

[0032] Preferably, the preparation method of the nitrogen-doped carbon quantum dots is as follows: citric acid and urea are dissolved in deionized water at a molar ratio of 1:2, 5% PEG-2000 is added, and the mixture is reacted at 180° C. for 3 h, and the nitrogen-doped carbon quantum dots are obtained after centrifugal purification.

[0033] Example 7 (Based on Example 3, modified polyethylene glycol, modified cellulose acetate, and nitrogen-doped carbon quantum dots were added as raw materials) The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer (POE), 7.5 parts of processing aid, 7.5 parts of modified polyethylene glycol, 11 parts of modified cellulose acetate, and 3 parts of epoxy soybean oil. The processing aid includes an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2: mixing the raw materials and nitrogen-doped carbon quantum dots and performing melt co-extrusion, wherein the weight of the nitrogen-doped carbon quantum dots is 0.2% of the total weight of the raw materials, and the Bacillus fermentation broth extract is in the form of microcapsules (particle size ≤ 200 nm), accounting for 1%; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5.5, stretching temperature 85℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 performed electron beam irradiation cross-linking of the tubular membrane with a dose of 6.5 kGy and an energy of 1.75 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0034] The preparation method of the modified polyethylene glycol is as follows: 20 parts of polyethylene glycol (PEG-400), 2 parts of nano-silica particles, 0.2 parts of silane coupling agent (KH-570), 10 parts of acrylamide, 0.2 parts of azobisisobutyronitrile, and an appropriate amount of ethylene glycol monomethyl ether (solvent) are taken by weight, the nano-silica particles are dispersed in ethylene glycol monomethyl ether, ultrasonically dispersed for 45 minutes (temperature 22.5°C), the silane coupling agent is added, and stirred at 70°C for 4 hours. The modified nano-silica suspension is mixed with polyethylene glycol, acrylamide monomer is added, azobisisobutyronitrile is added, the temperature is raised to 65°C, and the reaction is carried out under nitrogen protection for 6 hours. After the reaction is completed, the mixture is cooled to room temperature, centrifuged, washed twice with ethanol, and vacuum dried to constant weight (temperature 45°C).

[0035] In this embodiment, the preparation method of the modified cellulose acetate is as follows: 30 parts of cellulose acetate, 5.5 parts of maleic anhydride, 0.35 parts of a catalyst (p-toluenesulfonic acid), and an appropriate amount of an acetone-methanol mixed solvent (volume ratio of 1:1) are taken by weight, and the cellulose acetate is dissolved in the acetone-methanol mixed solvent and stirred until completely dissolved (temperature 22.5°C). Maleic anhydride and the catalyst are added, and the temperature is raised to 55°C under nitrogen protection, and the reaction is carried out for 4 hours. After the reaction is completed, the solution is poured into water for precipitation, and the crude product is filtered to obtain a crude product, which is washed four times with distilled water and vacuum dried to a constant weight (temperature 45°C).

[0036] Preferably, the preparation method of the nitrogen-doped carbon quantum dots is as follows: citric acid and urea are dissolved in deionized water at a molar ratio of 1:2.5, 5% PEG-2000 is added, and the mixture is reacted at 190° C. for 4 h, and the nitrogen-doped carbon quantum dots are obtained after centrifugal purification.

[0037] Example 8 (Based on Example 3, adding modified polyethylene glycol, modified cellulose acetate, and nitrogen-doped carbon quantum dots) The embodiment of the present invention provides a polyolefin cross-linked heat shrinkable film for fresh food packaging, comprising the following raw materials in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer (POE), 7.5 parts of processing aids, 10 parts of modified polyethylene glycol, 14 parts of modified cellulose acetate, and 5 parts of epoxy soybean oil. The processing aids include an anti-blocking agent, a slip agent, and an anti-fogging agent. The preparation method of the polyolefin cross-linked heat shrinkable film for fresh food packaging is as follows: Figure 1 As shown, the following steps are included: S1 weighs the raw materials according to the ratio; S2: mixing the raw materials and nitrogen-doped carbon quantum dots and performing melt co-extrusion, wherein the weight of the nitrogen-doped carbon quantum dots is 0.3% of the total weight of the raw materials, and the Bacillus fermentation broth extract is in the form of microcapsules (particle size ≤ 200 nm), accounting for 1.5%; S3 adopts three-bubble film blowing process to produce tubular film: 1) First bubble: The melt after co-extrusion is rapidly cooled to form an initial tubular film; 2) Second bubble: synchronous inflation and stretching, inflation ratio 5.5, stretching temperature 85℃; 3) The third bubble: The stretched tubular membrane is preheated by far infrared to heat set; S4 performed electron beam irradiation cross-linking of the tubular membrane with a dose of 6.5 kGy and an energy of 1.75 MeV; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

[0038] The preparation method of the modified polyethylene glycol is as follows: 30 parts of polyethylene glycol (PEG-400), 3 parts of nano-silica particles, 0.3 parts of silane coupling agent (KH-570), 15 parts of acrylamide, 0.3 parts of azobisisobutyronitrile, and an appropriate amount of ethylene glycol monomethyl ether (solvent) are taken by weight, the nano-silica particles are dispersed in ethylene glycol monomethyl ether, ultrasonically dispersed for 60 minutes (temperature 25°C), the silane coupling agent is added, and stirred at 80°C for 5 hours. The modified nano-silica suspension is mixed with polyethylene glycol, acrylamide monomer is added, azobisisobutyronitrile is added, the temperature is raised to 70°C, and the reaction is carried out under nitrogen protection for 8 hours. After the reaction is completed, the mixture is cooled to room temperature, centrifuged, washed with ethanol three times, and vacuum dried to constant weight (temperature 50°C).

[0039] In this embodiment, the preparation method of the modified cellulose acetate is as follows: 40 parts of cellulose acetate, 8 parts of maleic anhydride, 0.5 parts of a catalyst (p-toluenesulfonic acid), and an appropriate amount of an acetone-methanol mixed solvent (volume ratio of 1:1) are taken by weight, and the cellulose acetate is dissolved in the acetone-methanol mixed solvent and stirred until completely dissolved (temperature 25°C). Maleic anhydride and the catalyst are added, and the temperature is raised to 60°C under nitrogen protection, and the reaction is carried out for 5 hours. After the reaction is completed, the solution is poured into water for precipitation, and the crude product is filtered to obtain a crude product, which is washed with distilled water 5 times and vacuum dried to a constant weight (temperature 50°C).

[0040] Preferably, the preparation method of the nitrogen-doped carbon quantum dots is as follows: citric acid and urea are dissolved in deionized water at a molar ratio of 1:3, 5% PEG-2000 is added, and the mixture is reacted at 200° C. for 5 h, and the nitrogen-doped carbon quantum dots are obtained after centrifugal purification.

[0041] Comparative Example 1: Compared with Example 3, the metallocene polypropylene is replaced by copolymerized polypropylene, polyethylene, and ethylene-octene copolymer, while keeping the ratio of the three unchanged.

[0042] Comparative Example 2: Compared with Example 3, the ethylene-octene copolymer is replaced by copolymerized polypropylene, polyethylene, and metallocene polypropylene, while keeping the ratio of the three unchanged.

[0043] Comparative Example 3: Compared with Example 3, the electron beam irradiation cross-linking of the tubular membrane is performed without S4.

[0044] Comparative Example 4: Compared with Example 7, the modified polyethylene glycol was replaced by ordinary polyethylene glycol.

[0045] Comparative Example 5: Compared with Example 7, the modified cellulose acetate was replaced with ordinary cellulose acetate.

[0046] Comparative Example 6: Compared with Example 7, the modified polyethylene glycol was replaced by ordinary polyethylene glycol, and the modified cellulose acetate was replaced by ordinary cellulose acetate.

[0047] Comparative Example 7: Compared with Example 7, the nitrogen-doped carbon quantum dots are missing.

[0048] Performance Testing Test Method Initial shrinkage temperature: Instrument: Thermomechanical Analyzer (TMA, TA Instruments Q400), conditions: heating rate 10°C / min, load 0.1N, record the temperature at which the film begins to shrink.

[0049] 100℃ heat shrinkage (longitudinal / transverse): Method: Place the film sample (10cm×10cm) in a constant temperature box and heat it at 100℃ for 5 minutes. The longitudinal (MD) and transverse (TD) shrinkage rates are measured respectively.

[0050] Tensile strength: Standard: ASTM D882, using a universal testing machine (Instron 5967), tensile rate 50 mm / min.

[0051] The test results are shown in Table 1 below: Table 1 Performance test results From the above results, it can be seen that by adopting the method of blending and modifying low-softening-point elastomers such as metallocene polypropylene and POE, the shrinkage stress and initial shrinkage temperature of the film are reduced, the mechanical properties and thermal shrinkage rate of the film are improved by electron beam irradiation cross-linking, and the shrinkage temperature of the heat-shrinkable film is further reduced by synergistic effect by adding modified polyethylene glycol and modified cellulose acetate, while maintaining good mechanical properties and thermal stability. By nitrogen-doping carbon quantum dots, photothermal synergistic shrinkage is achieved, further reducing the shrinkage temperature.

[0052] It should be noted that, for the sake of simplicity, the aforementioned embodiments are described as a series of action combinations. However, those skilled in the art should be aware that the present invention is not limited by the order of the actions described, because according to the present invention, certain steps may be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in this specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present invention.

[0053] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the scope of protection of the invention. Obviously, the embodiments described are only some embodiments of the present invention, rather than all embodiments. Based on these embodiments, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in this field can still combine, add, delete or make other adjustments to the features in the various embodiments of the present invention according to the circumstances without conflict, without making creative work, so as to obtain different other technical solutions that do not deviate from the concept of the present invention in essence, and these technical solutions also fall within the scope of protection of the present invention.

Claims

1. A polyolefin cross-linked heat shrinkable film for fresh food packaging, characterized in that: The invention comprises the following raw materials in parts by weight: 80-95 parts of copolymerized 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. The invention adopts a five-layer or more co-extrusion three-bubble synchronous stretching production process during preparation, and is cross-linked by electron beam irradiation. The processing aids include an anti-blocking agent, a lubricant, and an anti-fogging agent.

2. The polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 1, characterized in that: The invention comprises the following raw materials in parts by weight: 85-90 parts of copolymerized polypropylene, 52-58 parts of polyethylene, 23-27 parts of metallocene polypropylene, 20-25 parts of ethylene-octene copolymer and 6-9 parts of processing aid.

3. The polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 2, characterized in that: The invention comprises the following raw materials in parts by weight: 87.5 parts of copolymerized polypropylene, 55 parts of polyethylene, 25 parts of metallocene polypropylene, 22.5 parts of ethylene-octene copolymer and 7.5 parts of processing aid.

4. The polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 1, characterized in that: The invention also includes the following raw materials in parts by weight: 5-10 parts of modified polyethylene glycol, 8-14 parts of modified cellulose acetate, and 1-5 parts of epoxidized soybean oil.

5. The polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 4, characterized in that: The preparation method of the modified polyethylene glycol is as follows: polyethylene glycol, nano-silica particles, silane coupling agent, acrylamide, azobisisobutyronitrile, and ethylene glycol monomethyl ether are taken, the nano-silica particles are dispersed in the ethylene glycol monomethyl ether, ultrasonically dispersed for 30-60 minutes, the silane coupling agent is added, and the mixture is stirred and reacted at 60-80° C. for 3-5 hours, the modified nano-silica suspension is mixed with polyethylene glycol, acrylamide monomer is added, azobisisobutyronitrile is added, the temperature is raised to 60-70° C., and the mixture is reacted under nitrogen protection for 4-8 hours. After the reaction is completed, the mixture is cooled to room temperature, centrifuged, washed with ethanol 2-3 times, and vacuum dried to constant weight.

6. The polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 5, characterized in that: The preparation method of the modified cellulose acetate is as follows: cellulose acetate, maleic anhydride, a catalyst, and an acetone-methanol mixed solvent are taken, the cellulose acetate is dissolved in the acetone-methanol mixed solvent, and the mixture is stirred until completely dissolved. The maleic anhydride and the catalyst are added, and the mixture is heated to 50-60° C. under nitrogen protection and reacted for 3-5 hours. After the reaction is completed, the solution is poured into water for precipitation, and the crude product is filtered to obtain a crude product, which is washed with distilled water 3-5 times and vacuum dried to a constant weight.

7. The method for preparing the polyolefin cross-linked heat shrinkable film for fresh food packaging according to any one of claims 1 to 6, characterized in that: The steps include: S1 weighs the raw materials according to the ratio; S2 mixes the raw materials and performs melt co-extrusion; S3 adopts three-bubble film blowing process to produce tubular film: S4 electron beam irradiation cross-linking of the tubular membrane; After S5 irradiation, the tubular membrane is split, pulled, aged, slit, and rolled into finished products.

8. The method for preparing a polyolefin cross-linked heat shrinkable film for fresh food packaging according to claim 7, wherein: 0.1-0.3% nitrogen-doped carbon quantum dots are added to the raw materials of S2. When the heat shrinkable film is used, vacuum is first evacuated, and then hot air is used to assist near-infrared light to trigger shrinkage. An 808nm light source is used with a power of 30-50W and an irradiation time of 5-10s.

9. The method for preparing a polyolefin cross-linked heat-shrinkable film for fresh food packaging according to claim 8, wherein the nitrogen-doped carbon quantum dots are prepared by dissolving citric acid and urea in deionized water at a molar ratio of 1:2-3, adding 5% PEG-2000, reacting at 180-200°C for 3-5 hours, and centrifuging and purifying to obtain the nitrogen-doped carbon quantum dots.

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