CN116023699B - A polyethylene antibacterial film suitable for preserving meat products and its preparation method and application - Google Patents

Abstract

The invention discloses a polyethylene antibacterial film suitable for meat product fresh-keeping and a preparation method thereof, comprising the following steps: (1) activating the polyethylene film; (2) Arranging an antibacterial agent on the surface of a polyethylene film to obtain an antibacterial film; (3) And (3) carrying out grafting reaction of the polyethylene film and the antibacterial agent on the antibacterial film in an ultrahigh-temperature hydrogen reaction system, so that chemical bond combination is formed between the polyethylene film and the antibacterial agent, and the modified polyethylene antibacterial film is obtained. The method adopts ultrahigh-temperature hydrogen as an initiator and combines with an ultraviolet ozone technology to prepare the modified polyethylene film with antibacterial performance, solves the difficult problem of impurity removal in the traditional antibacterial packaging modification process of foods, avoids the addition of additives, and solves the problems of easy peeling, easy migration, poor stability and the like of the traditional antibacterial coating. Can be applied to the preservation of meat products such as salmon and the like and prefabricated vegetable products, can effectively delay the putrefaction degree of salmon, and is a food packaging material with better application prospect.

Description

A polyethylene antibacterial film suitable for preserving meat products and its preparation method and application

Technical Field

The invention belongs to the technical field of modified polyethylene films, and in particular relates to an antibacterial polyethylene film suitable for preserving meat products and a preparation method thereof.

Background Art

Pre-prepared meals are increasingly used in the market, and packaging materials are the last line of defense to ensure the safety of pre-prepared food in the circulation process. They directly affect the storage and transportation efficiency, shelf life, cost control, food safety and quality assurance of pre-prepared foods, and are the key development direction to break through the difficulties of the industry. Foodborne diseases caused by bacterial contamination pose a great threat and challenge to people's life safety and social and economic development. One of the most effective strategies to prevent bacterial infection is disinfection with antimicrobial agents, but how to ensure the durability of antimicrobial agents is the primary key issue.

Polyethylene (PE), as a traditional packaging material, has become one of the most widely used thermoplastic polymer materials due to its low cost, non-toxicity and excellent comprehensive performance. At present, a large number of studies have been conducted on the antibacterial film of polyethylene at home and abroad. Givi and Rojas used low-density polyethylene and typical preservative ZnO nanoparticles to prepare antibacterial films by melt blending. The physical blended material showed excellent antibacterial ability and had an antibacterial activity of more than 99.99% against Escherichia coli. However, nano antibacterial particles are very easy to migrate and pose a potential threat to human health. Zhang successfully prepared antibacterial polyethylene cling film containing capsaicin structure by precipitation polymerization and ultraviolet polymerization, and the monomer was N,N′-[(4,5-dihydroxy-1,2-phenylene)bis(methylene)]bisacrylamide (OHABA). The inhibition rate of polyethylene antibacterial films prepared by the two methods on Bacillus subtilis exceeded 90%. However, there are problems such as removal of initiators, environmental pollution, and surface damage during the production process. Ghorbani used corona discharge to treat the surface of polyethylene film to improve the adhesion of the antibacterial coating solution on the surface of polyethylene film to prepare an antibacterial film. The disk diffusion method found that the inhibition diameter of polyethylene antibacterial film on Escherichia coli and Staphylococcus aureus exceeded 10 mm. However, how to avoid excessive corona, surface damage, coating shedding, poor stability and other problems still need further optimization and improvement.

Hydrogen treatment technology is a green new process that does not require initiators, terminators and catalysts. Among them, ultra-high thermal hydrogen surface induction technology (CN102414345B) is a new type of initiator using ultra-high thermal hydrogen molecules. By precisely controlling the dose and energy of the bombarding particles, the C-H bonds are selectively broken to form a high density of carbon free radicals, and cross-linking or grafting reactions occur between carbon free radicals, thereby forming a new structure on the polymer surface. This technology can complete free radical reactions at low temperatures, and can effectively perform covalent bonding while maintaining its chemical properties and stability. Compared with traditional modification techniques such as ultraviolet irradiation, gamma-ray irradiation, and plasma treatment, the use of ultra-high thermal hydrogen surface induction technology to prepare antibacterial materials can effectively prevent defects such as the migration of antibacterial substances, coating peeling, potential toxicity of initiators, and surface damage caused by radiation. It is an important new environmental protection technology.

Benzalkonium bromide (BB) is a highly efficient cationic quaternary ammonium salt. Due to electrostatic interaction, positively charged benzalkonium bromide can be adsorbed on the negatively charged bacterial surface, diffuse through the cell wall, and bind to the cytoplasmic membrane, resulting in the release of bacterial contents and causing bacterial apoptosis. The present invention intends to prepare an antibacterial material by inducing covalent fixation using ultra-high thermal hydrogen as an initiator, so that PE and BB are combined by chemical bonds, and on the premise of ensuring its stability, the PE film is endowed with excellent antibacterial properties, which has great application prospects in the field of food packaging.

Summary of the invention

The purpose of the present invention is to provide a method for preparing a polyethylene antibacterial film suitable for preserving meat products. The method uses an activated polyethylene film as a matrix, adopts ultra-high thermal hydrogen as an initiator, and covalently bonds with an antibacterial agent, benzalkonium bromide. The prepared polyethylene antibacterial film can maintain the chemical functionality and stability of the antibacterial material.

The present invention also aims to provide a polyethylene antibacterial film suitable for preserving meat products prepared by the above method.

The last object of the present invention is to provide the use of the above-mentioned polyethylene antibacterial film in the preservation of meat products.

The first object of the present invention can be achieved by the following technical solution: A method for preparing a polyethylene antibacterial film suitable for preserving meat products, comprising the following steps:

(1) Activating the polyethylene film;

(2) placing an antibacterial agent on the surface of the activated polyethylene film, and drying the film to obtain an antibacterial film;

(3) The antibacterial film is subjected to a polyethylene film-antibacterial agent grafting reaction in an ultra-high thermal hydrogen reaction system to form a chemical bond between the polyethylene film and the antibacterial agent to obtain a modified polyethylene antibacterial film.

The method of the present invention firstly uses ultraviolet ozone cleaning equipment to activate the polyethylene film, then sets the antibacterial agent on the surface of the polyethylene film after activation to obtain the antibacterial film, and then uses the ultra-high thermal hydrogen reaction system to modify the antibacterial film to form a new surface structure. In the modification process, the antibacterial film of modified polyethylene is obtained by controlling the kinetic energy of the extracted particles and the kinetic energy of the ultra-high thermal hydrogen particles and optimizing the treatment time. This new surface-modified antibacterial material is fixed on the surface by a covalent method, and the stability and safety are improved.

In the preparation method of the modified polyethylene antibacterial film:

Preferably, the activation treatment in step (1) is a surface activation treatment using ultraviolet ozone cleaning equipment to hydroxylate the surface of the polyethylene film to promote the interface fusion between the antibacterial molecules and the PE surface.

The low surface energy of the non-polar PE surface makes the bonding between polar antibacterial molecules and PE polymers poor. Therefore, in order to introduce polar groups, the PE surface was hydroxylated and activated using UV-ozone to improve the binding and grafting ability of antibacterial molecules on the PE surface.

Preferably, when the surface activation treatment is performed using ultraviolet ozone cleaning equipment, the surface activation treatment is performed at a temperature of 20 to 25° C. for 20 to 30 minutes.

Preferably, the antibacterial agent in step (2) is an ethanol solution of benzalkonium bromide, and the mass percentage of the benzalkonium bromide is 0.2% to 0.5%.

More preferably, the antibacterial agent in step (2) is an ethanol solution of benzalkonium bromide, and the mass percentage of the benzalkonium bromide is 0.5%.

Preferably, in step (2), the antibacterial agent is disposed on the surface of the polyethylene film by a dip-coating method to form an antibacterial coating.

Preferably, when the immersion and pulling coating method is adopted, the immersion time is 20 to 30 seconds, the drying time is 20 to 30 seconds, and the pulling times are 3 to 5 times.

As a preferred embodiment of the present invention, in order to make the polar antibacterial molecules evenly distributed on the polyethylene surface, ultraviolet ozone cleaning equipment is used in steps (1) to (2), the reaction temperature and reaction time are set, and after the activation treatment is completed, an antibacterial agent such as benzalkonium bromide ethanol solution is set on the polyethylene surface to obtain a polyethylene-benzalkonium bromide (PE-BB) antibacterial film; after the antibacterial film is dried in a vacuum drying oven in step (2), the PE-g-BB antibacterial film is grafted in an ultra-high thermal hydrogen reaction system in step (3).

Preferably, in step (3), the antibacterial film is subjected to a polyethylene film-antibacterial agent grafting reaction in an ultra-high thermal hydrogen reaction system to form a chemical bond between the polyethylene film and the antibacterial agent, further comprising: introducing hydrogen into an electron cyclotron resonance microwave plasma source, extracting an H ⁺ proton flow, and introducing the H ⁺ proton flow into a vacuum reaction chamber filled with _H2 molecules, wherein the H ⁺ protons in the H ⁺ proton flow collide continuously and randomly with the _H2 molecules, and the collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer; and guiding the ultra-high thermal hydrogen molecules with kinetic energy to the surface of the PE-BB antibacterial film placed in the vacuum reaction chamber, wherein the ultra-high thermal hydrogen molecules selectively break the CH bonds on the surface of the PE-BB antibacterial film in a controllable manner to form carbon free radicals, and a grafting reaction occurs between the carbon free radicals, so that the polyethylene film and the antibacterial agent are chemically bonded to obtain a modified polyethylene antibacterial film.

Preferably, H ⁺ proton flux with energy of 300 to 340 eV is extracted.

Preferably, an H ⁺ proton flow is introduced into a vacuum reaction chamber filled with _H2 molecules. At a pressure of 3× ^10-3 torr, the H ⁺ protons in the H ⁺ proton flow collide with the _H2 molecules continuously and randomly. The collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer.

Preferably, a long drift tube is provided in the vacuum reaction chamber filled with _H2 molecules, the _H2 molecules are placed in the long drift tube, and the length of the long drift tube is 50 cm.

Preferably, the kinetic energy of the ultrahigh thermal hydrogen molecules is about 9 to 11 eV.

Preferably, in step (3), each extracted proton collides continuously but randomly with the _H2 molecules in the drift tube, and the collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer. In this process, due to the mass matching of H ⁺ and _H2 , H ⁺ and _H2 will collide effectively, and ultra-high thermal hydrogen with kinetic energy will be generated in the drift tube, selectively breaking the CH bonds on the surface of the organic matter in a controllable manner. In this process, without destroying the CC structure, ultra-high thermal hydrogen molecules of about 10eV are used to destroy CH, forming activation centers with high reactivity, so that adjacent small molecules adhere to each other through the activation centers, and finally form a PE-g-BB antibacterial film.

The present invention uses high-throughput ultra-high thermal hydrogen molecules as initiators to achieve covalent bonding between polyethylene (PE) and antibacterial molecule benzalkonium bromide (BB). In order to promote the interface fusion between BB and PE surfaces, the surface of the PE film is first activated using an ultraviolet ozone instrument, and then treated by an immersion and pulling coating machine, after the benzalkonium bromide ethanol solution is evenly applied to the PE surface, the ultra-high thermal hydrogen beam is used to start and regulate the free radical reaction, and the C-H bond is selectively broken by controlling the kinetic energy of hydrogen. The molecules undergo a grafting reaction after the formation of carbon free radicals, and the antibacterial molecules and the polymer matrix are chemically bonded. Since the C-H bond on the surface of the organic matter is selectively broken in a controllable manner during the ultra-high thermal hydrogen treatment process, the C-C bond will not be broken, thereby avoiding material damage caused by surface sputtering. The method solves the problems of easy migration of antibacterial agents, easy peeling of coatings, and potential toxicity of initiators in traditional modification methods, and obtains an antibacterial film with high stability.

Preferably, the grafting reaction time is 1 to 5 minutes.

The above second object of the present invention can be achieved by the following technical solution: A polyethylene antibacterial film suitable for preserving meat products is prepared by the above method.

The last object of the present invention can be achieved by the following technical solution: application of the above-mentioned polyethylene antibacterial film in the preservation of meat products.

These meat products can be high-end pre-prepared ingredients such as beef, mutton, salmon, etc.

Compared with the prior art, the present invention has the following advantages:

(1) The present invention uses polyethylene film (PE) as a matrix, ultra-high thermal hydrogen as an initiator, and covalently bonds with the antibacterial agent benzalkonium bromide (BB). According to the mass matching principle, the C-H bond is selectively destroyed in a controlled manner to form carbon free radicals and initiate free radical reactions, thereby obtaining a PE-g-BB film. In this process, since the ultra-high thermal hydrogen treatment only destroys the C-H bond and affects the surface layer, the surface damage is minimized, the amino function of the film surface is maintained, and the bactericidal effect is maximized;

(2) The present invention is an efficient, safe, green and environmentally friendly modification method, which can effectively overcome the defects of migration of antibacterial substances, peeling of coatings, potential toxicity of initiators, surface damage caused by radiation, etc. in conventional antibacterial materials, and maintain the chemical functionality and stability of antibacterial materials. Using ultra-high thermal hydrogen as an initiator provides a new idea and approach for customizing functional polymer materials, which has practical application value in food packaging and potential biomedical fields.

(3) Therefore, the method of the present invention uses ultra-high thermal hydrogen as an initiator and combines it with ultraviolet ozone technology to produce a modified polyethylene film with antibacterial properties. This method solves the problem of impurity removal in the modification process of traditional food antibacterial packaging, avoids the addition of additives, and solves the problems of easy peeling, easy migration, and poor stability of traditional antibacterial coatings. It can be used to preserve salmon and other meat products and pre-prepared dishes, and can effectively delay the degree of spoilage of salmon and other meat products. It is a food packaging material with good application prospects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is an antibacterial graph of PE and PE-g-BB films in Example 4;

FIG2 is an XPS spectrum of the PE-g-BB film after ultrapure water treatment in Example 4, wherein the left figure is a C1s figure and the right figure is a N1s figure;

3 is an infrared spectrum of PE, BB, and PE-g-BB film treated with ultrapure water in Example 4;

FIG4 is a contact angle diagram of PE and PE-g-BB in Example 4;

FIG5 is an XRD diagram of PE and PE-g-BB films in Example 4;

FIG6 is an AFM image of a commercial PE film and a PE-g-BB film in Example 4, wherein FIGa is a commercial PE film and FIGb is a PE-g-BB film;

7 is TGA, DTG and DSC curves of the PE film and the PE-g-BB film in Example 4, wherein (a) is the TGA and DTG curves, and (b) is the DSC curve;

8 is a texture data diagram of salmon preserved with PE and PE-g-BB antibacterial films in Example 5, wherein a represents the change in the hardness of the salmon with storage time, b represents the change in chewiness with storage time, and c represents the change in adhesiveness with storage time;

FIG. 9 is a low-field nuclear magnetic resonance data diagram of salmon preserved with PE and PE-g-BB antibacterial films in Example 5.

DETAILED DESCRIPTION

In order to make the above aspects of the present invention more concise and easy to understand, the present invention is described in the following embodiments.

The present invention is further described below in conjunction with the accompanying drawings. The following examples are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the scope of protection of the present invention. The following raw materials, unless otherwise specified, are all commercially available products.

Example 1

(1) Place the PE film in an ultraviolet ozone cleaning device (model: UVO6T), set the reaction temperature to 20°C, and the reaction time to 20 min for activation treatment;

(2) After the activation treatment is completed, a 0.2% by weight benzalkonium bromide ethanol solution is coated on the PE surface by an immersion coating method, and the setting parameters of the immersion coating instrument are: immersion time 20s, drying time 20s, and pulling times 5 times, and the PE-BB antibacterial film is obtained by drying in a vacuum drying oven;

(3) The PE-BB antibacterial film was grafted with the PE-g-BB antibacterial film in an ultra-high thermal hydrogen reaction system, including: placing the PE-BB antibacterial film in a vacuum reaction chamber, waiting for the vacuum degree in the chamber to reach 9.9×10 ^-6 torr; introducing hydrogen into an electron cyclotron resonance microwave plasma source, extracting an H ⁺ ion beam with a kinetic energy of 300eV, and sending the extracted H ⁺ protons into a vacuum reaction chamber filled with H ₂ molecules (with a long drift tube with a length of 50cm). Under a pressure of 3×10 ^-3 torr, each extracted proton continuously and randomly collides with the H ₂ molecules in the vacuum reaction chamber, and a high flux of ultra-high thermal hydrogen molecules is generated through energy transfer during the collision process, and hydrogen molecules with a kinetic energy of about 9.38eV are directed to the surface of the antibacterial film. After a reaction time of 5 minutes, the PE-g-BB antibacterial film was prepared.

Example 2

(1) Place the PE film in a UV-ozone cleaning device, set the reaction temperature to 25°C, and the reaction time to 30 min;

(2) After the activation treatment, 0.5% benzalkonium bromide ethanol solution was coated on the PE surface by the dip-coating method. The setting parameters of the dip-coating instrument were: dipping time 30 s, drying time 30 s, and pulling times 5 times, to obtain a PE-BB antibacterial film.

(3) After the PE-BB film was dried in a vacuum drying oven, the PE-g-BB antibacterial film was grafted in an ultra-high thermal hydrogen reaction system. After the film was placed in a vacuum reaction chamber, the system vacuum was waited to reach 9.9× ^10-6 torr, hydrogen was introduced into the electron cyclotron resonance microwave plasma source, and an H ⁺ ion beam with a kinetic energy of 320eV was extracted. The extracted protons were sent into a vacuum reaction chamber filled with _H2 molecules (with a long drift tube of about 50cm in length). Under a pressure of 3× ^10-3 torr, each extracted proton collided continuously and randomly with the _H2 molecules in the vacuum reaction chamber. The collision process generated a high flux of ultra-high thermal hydrogen molecules through energy transfer, and directed the hydrogen molecules with a kinetic energy of about 10eV to the surface of the antibacterial film. At this time, since the energy actually attacking the surface is greater than the energy of the CH bond breaking, the CH bond breaks to form a high density of carbon free radicals, and a grafting reaction occurs between the carbon free radicals. After a reaction time of 3 minutes, the PE-g-BB antibacterial film was prepared.

Example 3

(1) Placing the PE film in a UV-ozone cleaning device, setting the reaction temperature to 25° C. and the reaction time to 30 min for activation treatment;

(2) After the activation treatment is completed, 0.5% benzalkonium bromide ethanol solution is coated on the PE surface by an immersion coating method. The setting parameters of the immersion coating instrument are: immersion time 25s, drying time 25s, and pulling times 3 times. The PE-BB antibacterial film is obtained by drying in a vacuum drying oven;

(3) The PE-BB film was grafted with the PE-g-BB antibacterial film in an ultrahigh thermal hydrogen reaction system, including: placing the film in a vacuum reaction chamber and waiting for the system vacuum to reach 9.9× ^10-6 torr; introducing hydrogen into an electron cyclotron resonance microwave plasma source to extract an H ⁺ ion beam with a kinetic energy of 340 eV, and sending the extracted protons into a vacuum reaction chamber (with a long drift tube of 50 cm in length) filled with _H2 molecules. Under a pressure of 3×10 ^-3 torr, each extracted proton collides continuously and randomly with the H ₂ molecules in the vacuum reaction chamber. The collision process produces a high flux of ultra-high thermal hydrogen molecules through energy transfer. The ultra-high thermal hydrogen molecules are introduced into the vacuum reaction chamber to selectively break the CH bonds on the surface of the organic matter in a controllable manner. In this process, without destroying the CC structure, hydrogen molecules with a kinetic energy of about 10.6 eV are used to guide the film. At this time, the energy reaching the surface of the film is sufficient to break the CH bonds and form a high density of carbon free radicals. Grafting reactions occur between the carbon free radicals. After a reaction time of 1 minute, the PE-g-BB antibacterial film is prepared.

Comparative Example 1

(1) A PE-BB antibacterial film was prepared according to the method of Example 1;

(2) The PE-BB antibacterial film was grafted with the PE-g-BB antibacterial film in an ultra-high thermal hydrogen reaction system, including: placing the PE-BB antibacterial film in a vacuum reaction chamber, waiting for the vacuum degree in the chamber to reach 9.9× ^10-6 torr; introducing hydrogen into an electron cyclotron resonance microwave plasma source, extracting an H ⁺ ion beam with a kinetic energy of 120eV, and sending the extracted H ⁺ protons into a vacuum reaction chamber filled with _H2 molecules (with a long drift tube with a length of 50cm). Under a pressure of 3× ^10-3 torr, each extracted proton continuously and randomly collides with the _H2 molecules in the vacuum reaction chamber. The collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer, and guides the hydrogen molecules with a kinetic energy of 3.75eV to the surface of the antibacterial film. After a reaction time of 5 minutes, the antibacterial film was prepared.

After the film surface was cleaned with ultrapure water, when the kinetic energy of the extracted ions was 120 eV in Comparative Example 1, the characteristic peak of the antibacterial molecule BB did not exist on the polyethylene film surface. This is because according to Smith's binary elastic collision formula, the kinetic energy of the ultra-high thermal hydrogen elastic particles generated by the kinetic energy of 120 eV is 3.75 eV, which is not enough to break the C-H bond (4.2 eV). At this time, the antibacterial film prepared has BB attached to the PE surface in a physical form, and no chemical reaction occurs.

Therefore, after optimization, the kinetic energy of the extracted ions in this application is 300-340eV, the kinetic energy of the ultra-high thermal hydrogen particles generated at this time ranges from about 9-11eV, and the processing time is 1-5min. More preferably, the kinetic energy of the extracted ions in this application is 320eV, the kinetic energy of the ultra-high thermal hydrogen particles generated at this time ranges from about 10eV, and the processing time is 3min.

Example 4

To verify the antibacterial properties of the film and test the grafting effect of PE-g-BB, the film was washed with ultrapure water before the antibacterial experiment to remove the ungrafted BB. The antibacterial effect of the film was determined by calculating the survival rate of PE-g-BB film in contact with bacteria at 35°C for 24 hours using the method of ISO 22196-2011.

The sample in Example 2 was cut into a shape of 50 mm × 50 mm, and the sample surface was treated with ultraviolet light for 15 min to prevent interference from other bacteria. 400 μL of bacterial culture solution (5×10 ⁵ cfu/mL) was aspirated onto the surface of the sample, and then a sterile PE cover film (40 mm × 40 mm) was used to cover the sample using sterile forceps.

The samples were placed in an incubator at 35°C and 90% relative humidity for 0 hours and 24 hours, respectively. Then, 20 mL of sterile phosphate buffered saline (PBS) solution was slowly dripped into the sample and eluted. The eluate was incubated in sterile nutrient agar medium at 35°C for 48 hours, and the colony count was performed. The antibacterial graphs of PE and PE-g-BB films are shown in Figure 1.

The experimental data in Figure 1 show that even when the film surface is treated with ultrapure water, the PE-g-BB film surface still exhibits good antibacterial activity, with an inhibition rate of 83% against Escherichia coli and 99.99% against Staphylococcus aureus. This indicates that the antibacterial molecule BB is firmly "rooted" on the PE surface and effectively exerts its antibacterial properties.

The XPS spectrum of the PE-g-BB film (prepared in Example 2) after ultrapure water treatment is shown in FIG2 , the infrared spectra of PE, BB, and PE-g-BB film after ultrapure water treatment are shown in FIG3 , and the contact angles of PE and PE-g-BB are shown in FIG4 .

The XPS of PE-g-BB film is shown in Figure 2. The binding energies of CC and CH bonds are 284.8 eV and 285.2 eV, respectively. After UV-ozonation, the surface elemental composition shows peaks at binding energies of 286.0 eV and 288.6 eV, corresponding to C-OH and C=O, respectively, which is the result of intense UV-ozone oxidation. Polar groups are injected into the PE surface, which is consistent with the results of the contact angle experiment. In addition, the surface elemental composition of N1s shows a peak of 402 eV, which is obviously caused by the CN ⁺ group in BB, indicating that BB exists on the PE surface.

Figure 3 shows the FTIR spectra of PE film, BB powder and PE-g-BB film. Since PE film is a material without inherent heteroatom functional groups, the characteristics shown by the untreated PE film are only attributed to the specific absorption bands of non-polar hydrocarbons in the main chain and side chains, namely CC, CH vibration. Absorption peaks were observed at 2920cm ^-1 and 2850cm ^-1 , corresponding to the CH stretching vibration bands of methylene. The peaks at 1460cm ^-1 and 719cm ^-1 are attributed to the CH bending and CH vibration of methylene, respectively. After UV-ozone treatment, oxygen-containing groups were introduced. The spectrum shows that the carboxyl (C=O) peak is at 1720cm ^-1 , 1020cm ^-1 corresponds to COC, and a strong OH bond peak width is observed at 3100-3650cm ^-1 . All of these can confirm that UV-ozone treatment can indeed introduce polar groups on the surface of PE film, which provides conditions for enhancing the binding ability between benzalkonium bromide and the matrix. In addition, the characteristic peak of BB also appeared at 1380 cm ^-1 , which did not appear on the pure PE film, further indicating that BB existed on the surface of PE film.

The XPS in Figure 2 and the infrared experiment in Figure 3 show that after the PE surface is washed with ultrapure water to remove the ungrafted BB, the characteristic peak of benzalkonium bromide still exists on the PE-g-BB film, proving that BB has been successfully fixed on the surface of the PE film. Combined with the antibacterial experiment in Figure 1, it can be seen that due to the covalent fixation of PE and BB, microorganisms can be eliminated only by contact, showing significant antibacterial activity.

As shown in Figure 4, the original contact angle of PE is 90.8±0.7°. After 30 minutes of ozone treatment, the contact angle drops to 60.0±1.2°. This is because the UV ozone treatment introduces polar groups into the surface of the PE film, making it hydrophilic. After ultra-high thermal hydrogen modification, when the surface of the PE-g-BB film is washed with ultrapure water, the contact angle can still be maintained at 36.5±1.1°. This phenomenon is due to the presence of BB and other polar groups, which can interact strongly with water, resulting in a decrease in the contact angle.

XRD was used to further analyze the structural changes of PE film caused by covalent grafting of the ultra-high thermal hydrogen reaction system. Figure 5 shows the untreated PE film and PE-g-BB film. The XRD spectrum of the PE film is mainly formed by two crystalline diffraction peaks at 21.6° and 23.8°. Compared with the pure PE film, the PE-g-BB film shows a unique BB characteristic diffraction peak at 26.9°. Since the ultra-high thermal hydrogen reacts on the surface, the structure of the BB and the polymer substrate will not be destroyed, so the BB can be retained on the PE surface and show good antibacterial properties.

Figure 6 shows the AFM three-dimensional surface images of PE film and PE-g-BB film. Before modification, the surface of the PE film was relatively rough, with a root mean square roughness (Rq) of 2.58 nm. Ozone treatment resulted in the addition of polar groups, which improved the binding of the water-soluble molecule BB to the surface of the PE film, allowing BB to evenly cover the PE surface. The results show that the surface roughness after modification was reduced to 0.49 nm. Since BB is an ionic quaternary ammonium salt, it repels the non-polar matrix PE, and the stalactite-like structure perpendicular to the matrix plane can be seen in the AFM image of Figure 6 (b). It further shows that the treatment with ultra-high thermal hydrogen does not damage the film surface and increase the roughness.

The thermal properties of PE film and PE-g-BB film were studied by TGA and DSC. Figure 7 shows the TGA and DTG curves and DSC curves of PE film and PE-g-BB film. Since the ultra-high thermal hydrogen treatment selectively destroys the C-H bond, it causes a free radical reaction, induces the grafting reaction of PE and BB and the self-crosslinking reaction of PE. As a result of this process, a unique network structure will be produced on the surface of PE, which increases the thermal stability of PE film to a certain extent. In addition, the grafting reaction mainly occurs on the surface of PE film and does not significantly change the stability of the crystalline region in the PE matrix.

The oxygen and water vapor permeability coefficients of the films before and after modification are shown in Table 1. After ultra-high thermal hydrogen treatment, the formation of carbon free radicals leads to a grafting reaction on the PE surface. Due to the network structure formed by the successful grafting of BB, the intermolecular space becomes smaller and the structure of the PE film becomes more compact, thereby reducing the oxygen permeability of the film. The grafting reaction between BB and PE during ultra-high thermal hydrogen treatment makes the molecular structure more compact, which also reduces the free volume in the polymer matrix and improves the water vapor barrier performance.

Table 1 Oxygen and water vapor permeability coefficients

Example 5

Texture is not only an important sensory characteristic of food, but also one of the indicators that affect consumer acceptance. In order to test the preservation performance of the antibacterial film, salmon was used as the preservation object to measure the texture changes of salmon during storage. As the storage time of salmon increases, the myofibrillar protein in the fish meat denatures and hydrolyzes under the action of endogenous enzymes and microorganisms, resulting in the deterioration of the texture of salmon. Figure 8 reflects the texture changes of salmon during storage. Obviously, the hardness, chewiness and adhesiveness of salmon decrease with the extension of storage time (as shown in Figures a, b and c in Figure 8, respectively), while the quality of salmon in the PE-g-BB film treatment group is always higher than that in the ordinary PE film treatment group. Because the PE-g-BB film has excellent antibacterial properties and a low oxygen permeability coefficient, it can effectively reduce the damage of microorganisms to muscle tissue, inhibit bacterial reproduction and oxidation reactions, delay the decomposition of protein and fat, and maintain the texture of salmon.

Low-field nuclear magnetic resonance is an effective method for detecting water migration and changes in muscle structure in food. The freshness of salmon can be determined by observing the changes in the water state of salmon during storage. Figure 9 shows the _T2 transverse relaxation time distribution image of salmon during storage. As the storage time increases, due to the growth and reproduction of microorganisms and the action of endogenous enzymes, the muscle tissue is destroyed, the bound water content in salmon continues to decrease, and the immobile water migrates to free water. Salmon stored with PE-g-BB antibacterial film can effectively reduce the conversion of immobile water to free water, improve the water holding capacity of myofibrils, reduce the reproduction of microorganisms, and protect the integrity of muscle fibers.

The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims (6)

1. A method for preparing a modified polyethylene antibacterial film suitable for preserving meat products, characterized in that it comprises the following steps: (1) Activating the polyethylene film; (2) placing an antibacterial agent on the surface of the activated polyethylene film, and drying the film to obtain an antibacterial film; (3) subjecting the antibacterial film to a polyethylene film-antibacterial agent grafting reaction in an ultra-high thermal hydrogen reaction system, so that a chemical bond is formed between the polyethylene film and the antibacterial agent, thereby obtaining an antibacterial film of modified polyethylene; The activation treatment in step (1) is to use ultraviolet ozone cleaning equipment to perform surface activation treatment; when using ultraviolet ozone cleaning equipment to perform surface activation treatment, the surface activation treatment is performed at a temperature of 20-25° C. for 20-30 minutes; The antibacterial agent in step (2) is an ethanol solution of benzalkonium bromide, and the mass percentage of the benzalkonium bromide is 0.2-0.5%; In step (3), the antibacterial film is subjected to a polyethylene film-antibacterial agent grafting reaction in an ultra-high thermal hydrogen reaction system to form a chemical bond between the polyethylene film and the antibacterial agent, further comprising: introducing hydrogen into an electron cyclotron resonance microwave plasma source, extracting an H ⁺ proton flow, and introducing the H ⁺ proton flow into a vacuum reaction chamber filled with _H2 molecules, wherein the H ⁺ protons in the H ⁺ proton flow collide continuously and randomly with the _H2 molecules, and the collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer; guiding the ultra-high thermal hydrogen molecules with kinetic energy to the surface of the PE-BB antibacterial film placed in the vacuum reaction chamber, wherein the ultra-high thermal hydrogen molecules selectively break the CH bonds on the surface of the PE-BB antibacterial film in a controllable manner to form carbon free radicals, and a grafting reaction occurs between the carbon free radicals to form a chemical bond between the polyethylene film and the antibacterial agent, thereby obtaining a modified polyethylene antibacterial film; A H ⁺ proton flow with an energy of 300-340 eV is extracted; the kinetic energy of the ultra-high thermal hydrogen molecules is 9-11 eV. 2. The method for preparing a modified polyethylene antibacterial film suitable for preserving meat products according to claim 1 is characterized in that: in step (2), the antibacterial agent is arranged on the surface of the polyethylene film by a dip-pull coating method to form an antibacterial coating; when the dip-pull coating method is adopted, the dipping time is 20-30 s, the drying time is 20-30 s, and the pulling times are 3-5 times. 3. The method for preparing a modified polyethylene antibacterial film suitable for preserving meat products according to claim 1 is characterized in that: an H ⁺ proton flow is introduced into a vacuum reaction chamber filled with _H2 molecules, and at a pressure of 3× ^10-3 torr, the H ⁺ protons in the H ⁺ proton flow collide with the _H2 molecules continuously and randomly, and the collision process generates a high flux of ultra-high thermal hydrogen molecules through energy transfer, and a long drift tube is provided in the vacuum reaction chamber filled with _H2 molecules, and the _H2 molecules are placed in the long drift tube, and the length of the long drift tube is 50 cm. 4. The method for preparing a modified polyethylene antibacterial film suitable for preserving meat products according to claim 3, characterized in that the grafting reaction time is 1 to 5 minutes. 5. A modified polyethylene antibacterial film suitable for preserving meat products, characterized in that it is prepared by the method of any one of claims 1 to 4. 6. Use of the modified polyethylene antibacterial film according to claim 5 in preserving meat products.