CN108715642B

CN108715642B - Polyethylene preservative film and preparation method and application thereof

Info

Publication number: CN108715642B
Application number: CN201810418325.XA
Authority: CN
Inventors: 肖乃玉; 何娟; 马志芳; 李湘銮; 徐依斌; 张雪琴; 钟乐; 张文
Original assignee: Zhongkai University of Agriculture and Engineering
Current assignee: Zhongkai University of Agriculture and Engineering
Priority date: 2018-04-16
Filing date: 2018-05-04
Publication date: 2021-03-30
Anticipated expiration: 2038-05-04
Also published as: CN108715642A

Abstract

The invention discloses a polyethylene preservative film which sequentially consists of a polyethylene film, an acrylic acid film and at least one layer of titanium dioxide-chitosan composite film from bottom to top. Also discloses a preparation method of the polyethylene preservative film, application of the polyethylene preservative film in preparing a packaging material with an antibacterial effect and application of the polyethylene preservative film in preserving cold fresh meat.

Description

Polyethylene preservative film and preparation method and application thereof

Technical Field

The invention belongs to the technical field of preservative films, and particularly relates to a polyethylene preservative film and a preparation method and application thereof.

Background

In the application of polymer packaging materials, Polyethylene (PE) films are widely used due to their chemical resistance, high impact strength, excellent mechanical properties, availability in large quantities and low cost. Despite these outstanding features, PE films themselves are not antimicrobial. For this reason, many scholars have conducted extensive studies in order to investigate effective methods for preparing antibacterial PE films. The methods of antimicrobial packaging can be divided into two types. First, this can be achieved by incorporating and immobilizing antimicrobial agents into the polymer film, while others are achieved by surface modification and surface coating. By the first method, several antibacterial agents, such as sorbic anhydride, and lactic acid bacteria, can be used that have been incorporated into PE polyethylene prior to fabrication into a film. However, the preparation of PE films by this method is limited by the thermal stability of the biocide during high temperature extrusion film formation or incompatibility with the polymer. Thus, surface modification and coating techniques are more preferred, and polymer solution coatings are the most desirable means in terms of stability and adhesion for attaching antimicrobial molecules to plastic films.

Chitosan is very attractive to researchers due to its unique attributes such as natural polymers, non-toxicity, film-forming ability and biodegradability. The limitation of the application of chitosan is mainly embodied in the following two points, firstly, the application of chitosan is only effective in an acid medium due to the poor solubility of chitosan under a high pH value; secondly, the only disadvantage of chitosan membranes compared to polymers is poor mechanical properties. Thus, chitosan may be coated on plastic films for improving mechanical properties and enhancing antimicrobial activity. However, among chitosan-coated materials, the chitosan-coated PE film has a limited number of studies. Because PE is a long aliphatic chain of hydrocarbons composed only of carbon and hydrogen, the PE surface is non-polar and lacks reactive functional groups. As a result, it is difficult to use PE for applications involving adhesion, such as printing and coating.

The nano titanium dioxide has higher safety, stable property and high catalytic activity, and is easy to prepare a transparent film attached to the nano titanium dioxideOn other carrier, TiO is the most representative photocatalytic antibacterial material₂Is the most actively researched inorganic nano material at present, and is prepared from TiO₂The introduction of chitosan to prepare double-effect sterilization materials has attracted great attention. However, at present, how to incorporate TiO is desired₂The incorporation of chitosan and PE films is still a problem.

The layer-by-layer self-assembly technique is proposed by Decher et al in 1991 to form a multilayer planar film from two polyelectrolytes with positive and negative charges through alternating electrostatic adsorption. The alternate deposition technology is utilized by layer-by-layer self-assembly, has the advantages of film formation, abundant substances, simplicity, universality, high product orderliness and the like, can control the generation of the film by adjusting experimental conditions, and is easy to realize multifunctional nano-assembly films and surface modification.

At present, the layer-by-layer self-assembly technology for TiO coating is not adopted in the prior art₂Chitosan and the like are introduced into the PE film and a good antibacterial effect is obtained.

Disclosure of Invention

The invention aims to provide a polyethylene preservative film which has a good antibacterial effect.

The invention also aims to provide a preparation method of the polyethylene preservative film, which adopts a layer-by-layer self-assembly technology.

The third purpose of the invention is to provide the application of the polyethylene preservative film in preparing the packaging material with the antibacterial effect and the application in preserving the chilled meat.

The first object of the present invention is achieved by the following technical solutions: a polyethylene preservative film comprises a polyethylene film, an acrylic acid film and at least one layer of titanium dioxide-chitosan composite film.

Further, the polyethylene preservative film sequentially comprises a polyethylene film, an acrylic acid film and at least one layer of titanium dioxide-chitosan composite film from the outer layer to the inner layer.

Preferably, the polyethylene film of the present invention is a corona polyethylene film.

Compared with the common Polyethylene (PE) film, the corona Polyethylene (PE) film has stronger polarity and can generate chemical action with more substances. Most plastic films (e.g., polyolefin films) are non-polar polymers and have relatively low surface tensions, typically 29-30 mN/m. When the film, especially polyethylene film, is treated by corona, various ions generated after air ionization accelerate and impact the plastic film in the treatment device under the action of a strong electric field, so that chemical bonds of plastic molecules are broken and degraded, the surface roughness and the surface area can be increased, a large amount of ozone can be generated during discharge, the plastic molecules can be oxidized, radicals with stronger polarity such as carbonyl and peroxide are generated, and the surface energy of the plastic film is improved.

Preferably, the acrylic film of the present invention is a food grade aqueous acrylic film.

Compared with a non-aqueous acrylic acid film, the aqueous acrylic acid film has the advantages of flexible formula, good water resistance, good base material adhesion, excellent aging resistance, good acid and alkali resistance and the like.

Preferably, the number of layers of the titanium dioxide-chitosan composite membrane is three.

The PE films coated with the titanium dioxide-chitosan composite films with different layers have different antibacterial effects on escherichia coli and staphylococcus aureus, the larger the antibacterial zone is, the stronger the antibacterial effect is, and the result shows that the synergistic antibacterial effect on two bacteria is stronger as the number of the layers coated with the titanium dioxide-chitosan composite films is increased, and the antibacterial effect is strongest when the number of the layers coated with the titanium dioxide-chitosan composite films is three.

Preferably, the thickness of the polyethylene film is 15-25 μm, the thickness of the acrylic film is 1-10 μm, and the thickness of the titanium dioxide-chitosan composite film is 5-20 μm.

The second purpose of the invention is realized by the following technical scheme: the preparation method of the polyethylene preservative film comprises the following steps:

(1) preparing a chitosan solution: selecting chitosan, dissolving the chitosan in an acetic acid aqueous solution, and magnetically stirring to obtain a chitosan solution;

(2) preparing a titanium dioxide solution: selecting titanium dioxide, adding the titanium dioxide into water, and performing ultrasonic vibration to prepare a titanium dioxide solution;

(3) preparing a titanium dioxide-chitosan composite solution: mixing the chitosan solution in the step (1) and the titanium dioxide solution in the step (2) in proportion to prepare a titanium dioxide-chitosan composite solution;

(4) preparing a polyethylene preservative film by layer-by-layer self-assembly: selecting a polyethylene film, coating acrylic emulsion on the polyethylene film, drying, coating at least one layer of titanium dioxide-chitosan composite solution obtained in the step (3), and drying to obtain the polyethylene preservative film.

The preparation method of the polyethylene preservative film comprises the following steps:

the mass-to-volume ratio of the chitosan to the acetic acid aqueous solution in the step (1) is preferably 0.05g to 0.2 g: 10-20 mL, wherein the mass percentage of the acetic acid water solution is preferably 0.5-2%.

The rotating speed of the magnetic stirring in the step (1) is preferably 800-1000 rpm, and the stirring time is preferably 1-2 h.

The mass percentage of the titanium dioxide solution in the step (2) is preferably 1-5%.

The volume ratio of the chitosan solution to the titanium dioxide solution in the step (3) is preferably 1: 2 to 6, preferably 1: 3.

the acrylic emulsion described in step (4) is preferably an aqueous acrylic emulsion.

After drying in the step (4), preferably coating three layers of the titanium dioxide-chitosan composite solution in the step (3).

The invention utilizes the layer-by-layer self-assembly method to prepare the polyethylene preservative film, has the advantages of film forming, abundant substances, simplicity, universality, high product orderliness and the like, can control the generation of the film by adjusting experimental conditions, and is easy to realize multifunctional nano-assembly film and surface modification.

When the multilayer film is prepared, the structure and the thickness of the film can be controlled by controlling the concentration of the solution, the assembly times and other parameters, and the components, the structure, the appearance and the functions of the film can be well regulated and controlled. In addition, due to the non-specificity of the electrostatic action, substances with different functionalities can be added to the membrane to exert their specific action.

The invention further evaluates the influence of the coating treatment of acrylic on the surface properties of the polyethylene film by Atomic Force Microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and determines the hydrophilicity of the film by measuring the water contact angle of the acrylic treated polyethylene film. The amount of chitosan deposited on the membrane is determined by the Kjeldahl method, and the application effect of the membrane in the cold fresh meat is evaluated by the water retention capacity of nuclear magnetic resonance and the total number of colonies.

The third object of the present invention is achieved by the following technical solutions: the polyethylene preservative film is applied to the preparation of packaging materials with antibacterial effects and the preservation of chilled meat.

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

(1) the polyethylene preservative film comprises a polyethylene film, an acrylic film and at least one layer of titanium dioxide-chitosan composite film, wherein the surface form of the polyethylene film can be obviously changed by coating the acrylic film on the surface of the polyethylene film, the hydrophilicity of the corona polyethylene film coated with the acrylic film is improved compared with that of a pure corona polyethylene film, more active hydrophilic groups-OH are added on the surface of the polyethylene film by coating the acrylic film on the surface of the polyethylene film, and the interaction between the chitosan and the polyethylene film treated by PAA can be enhanced by coating the water-based acrylic film on the surface of the polyethylene film;

(2) the polyethylene preservative film prepared by the self-assembly alternative deposition technology has the advantages of film formation, rich substances, simplicity, universality, high product orderliness and the like, can control the generation of the film by adjusting experimental conditions, and is easy to realize multifunctional nano-assembly film and surface modification;

(3) the construction of the self-assembled film of the chitosan and titanium dioxide composite film on the polyethylene film is a more preferable coating technology, and is an ideal way for attaching the antimicrobial molecules on the plastic film by the polymer solution coating in terms of stability and adhesiveness, and also meets the requirements of active antimicrobial packaging, namely, the requirements of 'less treatment, easy treatment, instant freshness, food transaction globalization, distribution after centralized treatment' and the like in terms of food safety and quality in the current society can be met to a certain extent.

Drawings

FIG. 1 is an AFM image of a performance test and results analysis section of a polyethylene preservative film of the present invention, a pure PE surface (left panel) and a PAA-coated PE film (right panel);

FIG. 2 is a comparison of contact angles of pure PE films (left panel) and PAA-PE films (right panel) in the performance test and result analysis part of the polyethylene preservative film of the present invention;

FIG. 3 is a part of performance testing and results analysis of a polyethylene cling film of the present invention, coated with PAA (a) and pure PE film in infrared spectrum as PE film (b);

FIG. 4 is a photographic image obtained after 12 hours of dyeing with amino black 10B, of the performance test and results analysis part of the polyethylene wrap film of the present invention, wherein (a) the image is a pure corona PE film, (B) the image is a PE film coated with 2% chitosan, and (c) the image is a chitosan-coated PE film treated with PAA;

FIG. 5 is a graph of the effect of the number of washing cycles on the amount of chitosan deposited on PE film (graph A) and the comparison of the amount of coated chitosan immersed on untreated and PAA coated PE films of different chitosan concentrations (graph B) in a performance test and results analysis section of the polyethylene cling film of the present invention;

FIG. 6 is a comparison of contact angles of chitosan and titanium dioxide in different proportions for the performance test and result analysis part of the polyethylene preservative film of the present invention;

FIG. 7 shows the performance test and result analysis of the PE-PAA-CTS/TIO plastic wrap of the present invention₂Infrared spectrum comparison of film (c), PE-PAA-CTS film (d), PE-CTS film (e), pure CTS film (f);

FIG. 8 is the antibacterial test and result analysis part of the polyethylene preservative film of the invention, the antibacterial effect of chitosan solution and titanium dioxide solution of different proportions on Escherichia coli is shown, A represents the volume ratio of the chitosan solution to the titanium dioxide solution is 80 μ L: 0 μ L, B represents a volume ratio of the chitosan solution to the titanium dioxide solution of 60 μ L: 20 μ L, C represents the volume ratio of chitosan solution to titanium dioxide solution of 40 μ L: 40 μ L, D represents the volume ratio of chitosan solution to titanium dioxide solution of 20 μ L: 60 μ L, E represents the volume ratio of the chitosan solution to the titanium dioxide solution of 0 μ L: 80 mu L of the solution;

FIG. 9 shows the antibacterial effect of chitosan solution and titanium dioxide solution in different proportions on Staphylococcus aureus, where A represents the volume ratio of the chitosan solution to the titanium dioxide solution of 80 μ L: 0 μ L, B represents a volume ratio of the chitosan solution to the titanium dioxide solution of 60 μ L: 20 μ L, C represents the volume ratio of chitosan solution to titanium dioxide solution of 40 μ L: 40 μ L, D represents the volume ratio of chitosan solution to titanium dioxide solution of 20 μ L: 60 μ L, E represents the volume ratio of the chitosan solution to the titanium dioxide solution of 0 μ L: 80 mu L of the solution;

FIG. 10 is the antibacterial test and result analysis part of the polyethylene preservative film of the invention, the bacteriostatic diameters corresponding to five groups of bacteriostatic experiments, A represents that the volume ratio of the chitosan solution to the titanium dioxide solution is 80 μ L: 0 μ L, B represents a volume ratio of the chitosan solution to the titanium dioxide solution of 60 μ L: 20 μ L, C represents the volume ratio of chitosan solution to titanium dioxide solution of 40 μ L: 40 μ L, D represents the volume ratio of chitosan solution to titanium dioxide solution of 20 μ L: 60 μ L, E represents the volume ratio of the chitosan solution to the titanium dioxide solution of 0 μ L: 80 mu L of the solution;

FIG. 11 is a part of the antibacterial test and result analysis of the polyethylene wrap of the present invention, in which a1 is a PE film, b1 is a PE film coated with one layer of chitosan/titanium dioxide, c1 is a PE film coated with two layers of chitosan/titanium dioxide, and d1 is a PE film coated with three layers of chitosan/titanium dioxide;

FIG. 12 is a part of the antibacterial test and result analysis of the polyethylene wrap of the present invention, in which different layers of the coated PE films have antifungal effects against Staphylococcus aureus, wherein a2 is the PE film, b2 is the PE film coated with one layer of chitosan/titanium dioxide, c2 is the PE film coated with two layers of chitosan/titanium dioxide, and d2 is the PE film coated with three layers of chitosan/titanium dioxide;

FIG. 13 shows the antibacterial testing and result analyzing part of the polyethylene wrap of the present inventionRespectively, PE film and PE-PAA-CTS/TIO at 4 DEG C₂Total number of colonies of the film-packaged pork;

FIG. 14 is the antibacterial test and result analysis part of the polyethylene wrap of the present invention, PE film (a) and PE-PAA-CTS/TIO at 4 deg.C₂Graph of nmr relaxation time T2 for film (b) packaged pork.

Detailed Description

The present invention will be further described with reference to the following examples and the accompanying drawings, but the scope of the present invention as claimed is not limited to the examples, such as the film thickness, the solution concentration, the magnetic stirring speed, etc.

Polyethylene preservative film and preparation method thereof

Example 1

The polyethylene preservative film provided by the embodiment is composed of a polyethylene film, an acrylic acid film and at least one layer of titanium dioxide-chitosan composite film.

Wherein the polyethylene film is corona treated.

The acrylic film is an aqueous acrylic film.

(1) weighing 0.2g of chitosan (Saddy Biotechnology Co., Ltd, Fumon, but not limited to the Saddy Biotechnology Co., Ltd.) and dissolving the chitosan into 2% (10mL) of acetic acid solution (dissolved by distilled water), and stirring the solution for 1 hour in a magnetic stirrer (the rotating speed of magnetic stirring is 800-1000 rpm) to prepare 10mL of 2% (w/v) chitosan solution;

(2) measuring 2mL of titanium dioxide stock solution (a company, but not limited to the company, Japan) by using a pipette, dissolving the titanium dioxide stock solution in 8mL of distilled water, and simultaneously carrying out ultrasonic vibration at 30 ℃ for 20min to prepare 10mL of titanium dioxide solution, wherein the mass percentage content of the titanium dioxide solution is 1-5%;

(3) in order to impart hydrophilicity and chemical reactivity to the polyethylene PE film, the polyethylene PE film used was a corona-treated PE film (to which, however, the company shall not be limited), which was flatly spread on a glass of a coater, sequentially wiped with acetone and ethanol, and then divided into three groups, the first group being a PE film coated with only chitosan, the second group being a PE film coated with a layer of aqueous acrylic resin (commercially available product) of PAA and then chitosan, and the third group being a polyethylene film coated with acrylic PAA and then coated with a titanium dioxide-chitosan composite solution.

The volume ratio of the titanium dioxide/chitosan solution PE film prepared by different proportions is respectively chitosan CTS: titanium oxide TiO₂Is 8: 0. 6: 2. and 0: 8.

example 2

In contrast to example 1, in step (3) the chitosan CTS: titanium oxide TiO₂Is 8: 0. 4: 4 and 0: 8.

example 3

In contrast to example 2, in step (3) the chitosan CTS: titanium oxide TiO₂Is 8: 0. 2: 6 and 0: 8.

example 4

In contrast to example 1, chitosan CTS: titanium oxide TiO₂Is 2: and 6, coating two layers of chitosan and titanium dioxide composite solution and three layers of chitosan and titanium dioxide composite solution on the polyethylene PE film, and comparing the two layers of chitosan and titanium dioxide composite solution with the uncoated chitosan and titanium dioxide composite solution and the coated layer of chitosan and titanium dioxide composite solution.

Example 5

Different from the embodiment 1, the polyethylene preservative film is composed of a polyethylene film, an acrylic acid film and at least one layer of titanium dioxide-chitosan composite film from an outer layer to an inner layer in sequence, when in preparation, 0.2g of chitosan (Saddy bioscience, Inc., Qufuji city, but not limited to the company) is weighed and dissolved into an acetic acid solution with the concentration (mass percentage) of 2% (20mL), and the mass percentage of the titanium dioxide solution is 2%.

Performance test and result analysis of polyethylene preservative film

The polyethylene preservative films prepared in examples 1 to 5 were subjected to performance tests, and the results were as follows:

firstly, performance testing:

AFM testing was to demonstrateTo illustrate the effect of the coating process of PAA on PE films, first, the surface roughness of pure PE films and PE films coated with PAA were characterized using a CSPM5500 atomic force microscope, produced by beijing primitive nano instruments, and the root mean square (rms) roughness and morphology curves measured on 10 μm × 10 μm images were evaluated. Observing the surface chemical components of the untreated and treated PAA and the chitosan or chitosan/titanium dioxide coated film by attenuated total reflection-Fourier transform infrared spectrometer to obtain ATR-FTIR spectrum with wave number of 4000^-1To 650^-1Cm, in 4cm^-1Is scanned 64 times.

Amido black 10B staining test was performed to confirm the presence of chitosan deposited on PE film. First, a chitosan coated PE film was immersed in a 0.01% w/v aqueous amido black 10B solution for 12 hours. The film was then washed with distilled water to remove excess dye, and then the dispersion and distribution of the deposited chitosan was observed by an optical microscope.

The kjeldahl method measures the amount of chitosan coated on the untreated and PAA treated PE films by analysis. Membranes with precise dimensions of 6cm x 6cm were placed in a digestion flask. Concentrated H₂SO₄(5mL) and CuSO₄·5H₂O then (0.05-0.1g) was added to the digestion flask, which was then heated on a heating mantle for 2 hours. After heating, the decomposition of the film was indicated by visual observation of the color changing to dark black. Then 5 drops of H₂O₂Added to the decomposed sample and then further heated until the solution becomes transparent and colorless. And (3) carrying out a distillation step of a Kjeldahl method on the obtained solution. Twenty ml of 0.01M aqueous HCl was added to an erlenmeyer flask (200 ml) and placed at the end of the condenser. Aqueous NaOH solution (40% w/v) was added to the digested sample via a distillation column in a closed system. The ammonium ions from chitosan were distilled as ammonia gas through a stream. The ammonia gas was passed through a trapping solution (0.01M aqueous HCl), dissolved therein, and turned into ammonium ions again. Finally, the amount of ammonia was determined by titration with a standard solution (0.01M aqueous NaOH).

Amount of chitosan(g)＝((V₁M₁-V₂M₂)/1000)×161.06g/mol of chitosan

Wherein V₁And V₂HCl solution and NaOH solution, respectively, volume and Medium 1 and Medium 2 are HCl solution and NaOH solution (M), (V), respectively, at molarity₁M₁-V₂×M₂Mmol of HCl solution consumed-mmol of nitrogen).

A contact angle meter can be used to verify whether the membrane is hydrophilic. The film samples were cut into 2cm by 2cm squares for measurement. The syringe sucks an appropriate amount of ultrapure water, the syringe is mounted on a holder of a measuring instrument, the film sample is placed on a stage of the measuring instrument, an up-and-down control knob of the syringe is adjusted to drop a 4. mu.L drop of water on the surface of the film, an image of the drop of water is frozen within 10s, a contact angle value is calculated, and a contact angle (°) is expressed by its mean value.

Second, results and analysis

2.1 Effect of PAA coating treatment on the surface morphology of PE films

2.1.1 test results of AFM

To investigate the effect of the coating process of PAA on the surface morphology of PE films, AFM observations were used to present a three-dimensional surface view. Fig. 1 shows AFM images of PAA coated PE films and pure PE surfaces. It is evident that the PAA-coated treatment significantly changed the surface morphology of the PE film. As can be seen from fig. 1, most of the area of the surface of the untreated PE film was quite smooth, while there were many protrusions on the surface of the PAA coated treated PE film. Furthermore, the variation in surface roughness may be quantified by a root mean square (rms) roughness value, which refers to the average size of peaks and valleys within a region of interest. Lower significance numbers represent smooth surfaces. The root mean square of the untreated PE film can be calculated from fig. 1 to be 26.35 ± 7.39nm, with the PAA treated PE film increasing to 32.52 ± 8.93 nm. The results show that PAA coating removes the top layer of the PE surface and gives a strong impact on the PE surface. This phenomenon may be associated with physical or chemical removal of molecules, chain scission and degradation processes.

2.1.2 contact Angle test results

Contact angle (θ) is a variable that determines the wettability of a surface. The contact angle is an angle from a solid-liquid interface to a gas interface through the inside of a liquid at a three-phase boundary of the solid, liquid and gas, and is generally represented by θ. The contact angle meter is manufactured by Shanghai Lomb information technology Limited and is model SL 200B. The contact angle of the surface of the PE film before and after PAA treatment was tested. Theta >90 deg. is commonly referred to as non-wetting (hydrophobic); theta <90 deg. is called wetting (hydrophilic) and equilibrium contact angle is absent or 0, is called spreading. The tendency of a droplet to spread out on a flat surface increases with decreasing contact angle. Thus, a high contact angle indicates poor wetting. As can be seen from fig. 2, the contact angle of the surface of the pure PE film was 71 °, and the contact angle of the PAA-coated film was 67 °. The angle of the contact angle becomes smaller, i.e. it also indicates that the hydrophilicity of the PE film coated with PAA is increased compared to pure corona PE, which may make the surface of PE become more hydrophilic after PAA coating PE, making the wetting phenomenon more obvious.

2.2 Effect of PAA coating treatment on the surface chemical composition of PE film

As shown in FIG. 3, the corona PE film was at 723cm^-1The left and the right have stronger absorption vibration peak which is-CH₂Keys, other peaks absorbing vibrations, e.g. 1713cm^-1In the presence of an ester group C ═ O vibrating in extension and contraction, and 1238cm^-11095cm of phenolic hydroxyl group of^-1The ether bonds have obvious stretching vibration, and the PE surface becomes active and can contact with more active groups due to the fact that the corona-treated PE film surface is provided with a plurality of active groups. The surface adhesion of the printing stock is also enhanced. While the PAA-coated PE film was 2000cm^-1The absorption peak after the reaction is substantially identical to that of the PE film, but is 3571cm^-1At a distance of 3365cm^-1A new wide absorption peak appears, and intermolecular hydrogen bonds vibrate in a stretching mode, so that the PAA coating treatment enables the surface of the PE film to have more active hydrophilic groups, namely hydroxyl-OH. This also explains the phenomenon that the surface contact angle of the PAA-coated PE film is lowered and the hydrophilicity is improved compared with that of the PE film.

2.3 Effect of the coating treatment of PAA (acrylic acid) on the surface coating of Chitosan on PE films

2.3.1 dyeing results of Amido Black 10B

The deposition of chitosan on the PE film and the strong adsorption of PAA to chitosan were confirmed by staining the PE film with Amido Black 10B (AR, analytical pure) 0.01% w/v aqueous solution. Amido black 10B is an anionic dye that can interact with the amino group of chitosan. Due to the positively charged nature of chitosan, the anionic dye will be selectively adsorbed by chitosan, but not by PE.

Fig. 4 shows photographic images of pure corona films, PE films coated with chitosan, PE films coated with PAA and then with chitosan acetate solution. Clearly, no specific interaction between the pure PE membrane and the anionic dye was observed. On the other hand, when the PE was coated with chitosan, the dyeing result was not obvious, and the film was slightly bluish at the edge and almost absent in the center. On the other hand, a distinct and uniform blue color was seen on the PE film previously coated with PAA and then chitosan-coated, indicating that PAA can effectively and uniformly adsorb chitosan on the surface. The staining of PAA-coated chitosan PE occurred due to the occurrence of specific interactions between the coated chitosan and the dye molecules, confirming the successful coating of chitosan on the PAA-coated PE film.

2.3.2 washing results

The effect of the coating treatment of PAA on the surface coating of PE films was determined by comparing the amount of chitosan on untreated PE films and PAA coated PE films. Both untreated and PAA coated PE films were coated with chitosan by dipping the PE films into chitosan acetate solutions with different chitosan concentrations. The amount of chitosan coated on the PE film was then determined by kjeldahl analysis. Prior to this step, a suitable number of wash cycles are performed after the chitosan is coated to remove loosely bound and unbound chitosan from the membrane surface. PE films immersed in a 2% chitosan acetate solution were used in the present invention.

The results are shown in FIG. 5, in which A represents the relationship between the number of washing cycles characterized by Kjeldahl method and the amount of chitosan deposited on the PE film. It was found that the amount of chitosan deposited on the PE film decreased slightly with the increase in the number of washing cycles, becoming constant after three washes. Therefore, the chitosan-coated PE film was washed three times, and then the amount of chitosan coated on the PE film was measured. Panel B shows a comparison of the amount of coated chitosan on untreated and PAA coated PE films impregnated at different chitosan concentrations. For untreated PE films, chitosan cannot be deposited on the film surface at any chitosan concentration. On the other hand, the amount of chitosan coated on the PAA-treated PE film increased with the increase in the chitosan concentration. These results indicate that the PAA coating treatment of PE films can enhance the interaction between chitosan and PAA treated PE films.

2.4 comparison of contact angles of Chitosan and titanium dioxide at different ratios

As can be seen from fig. 6, when only chitosan or titanium dioxide is used, the angle of the contact angle is larger than 60 degrees, the hydrophilicity is not high, and as the proportion of chitosan is reduced, the proportion of titanium dioxide is increased, and the size of the contact angle is slightly reduced. The ratio of chitosan to titanium dioxide is 1: 1 is most hydrophilic and is 38 degrees, which shows that the coating effect is better.

2.5 chemical Effect of PAA coating treatment on Chitosan and PE films

Fig. 7 shows ATR-FTIR spectra of pure chitosan coated with PAA, PE film coated with chitosan again by PAA coating treatment, and PE film coated with chitosan-titania mixed solution again by PAA coating treatment. From c, d and f, the coverage is 3307-3365 cm^-1Overlap of N-H and O-H stretching of the carbohydrate ring was observed in the large band of the range of (1). d is 2400cm when compared with e^-1The peak of carboxylic acid dimer appears, and better combination can be carried out with the hydroxyl of the chitosan. f pure chitosan is respectively 1586cm^-1Shows a characteristic absorption band corresponding to amide-based vibration at 3358cm^-1Has a characteristic peak of amido bond-NH₃At 2920cm^-1Has a strong absorption vibration peak which is hydroxyl-OH. In addition, c is the mixed coating of titanium dioxide and chitosan so as to be 2000cm^-1Only 1713cm are left^-1Ester group (e) and 1146cm^-1The phenolic hydroxyl group indicates that the titanium dioxide chitosan solution and the PAA should have chemical bond combination.

(III) antibacterial test and result analysis of polyethylene preservative film

3.1 results of the antibacterial Activity test

The antibacterial effect of chitosan/titanium dioxide can be measured by the oxford cup method, and the influence of the number of layers coated on the antibacterial effect can be observed. The specific operation is as follows: weighing 2.5g of beef extract, 5g of peptone, 2.5g of sodium chloride and 10g of agar powder, adding into 500mL of distilled water, heating to dissolve, adjusting the pH value to 7-7.2 by using sodium hydroxide, and preparing into a solid culture medium for later use. After the solid agar medium sterilized by high-pressure steam is gently shaken, the solid agar medium is poured into culture dishes, about 20mL of the culture dishes are placed on a clean bench for waiting solidification. The bacteriostatic experiment has 10 groups, five groups of data are measured by two strains respectively, and the optimal bacteriostatic proportion is tested by dividing the two strains into A, B, C, D, E groups (shown in table 1) according to different proportions of a Chitosan (CTS) solution and a titanium dioxide solution, so that 10 labels are required to be pasted, and the subsequent operation is convenient. After the solid medium solidified, 0.1mL of 10 th solution was aspirated by a sterile pipette^-1The suspension of the escherichia coli and the staphylococcus aureus is put on the surface of the suspension, and the bacteria liquid is evenly coated on the surface of the culture medium by using a sterile triangular ring. And placing the oxford cups in the middle of the solid culture medium, placing one oxford cup in a culture dish, and slightly pressurizing to ensure that the oxford cups and the culture medium are in contact without gaps. The solution to be measured is added into the Oxford cup according to the proportion of the table 1, the culture dish is put into a constant temperature incubator at 37 ℃ for 24 hours, and finally the diameter of the inhibition zone is measured and the size of the inhibition zone is observed.

TABLE 1 formulation ratio of chitosan solution and titanium dioxide solution

FIG. 8 shows the bacteriostatic effect of chitosan solution and titanium dioxide solution on Escherichia coli at different ratios. The larger the inhibition zone is, the better the inhibition effect is. The result shows that the inhibition zone of the group D is the largest, namely when the ratio of the chitosan solution to the titanium dioxide solution is 1: and 3, the antibacterial effect on the escherichia coli is optimal.

FIG. 9 shows the bacteriostatic effect of chitosan solution and titanium dioxide solution on Staphylococcus aureus. The larger the inhibition zone is, the better the inhibition effect is. The result shows that the inhibition zone of the group D is the largest, namely when the ratio of the chitosan solution to the titanium dioxide solution is 1: and 3, the antibacterial effect on the escherichia coli is optimal.

Fig. 10 shows the diameters of inhibition zones corresponding to five groups of experiments. The larger the diameter of the inhibition zone is, the better the inhibition effect is. The group A is single chitosan, the group E is single titanium dioxide, the bacteriostatic diameter of the group E is larger than that of the group A, and the bacteriostatic ratio of the titanium dioxide is higher than that of the chitosan; B. c, D group is the synergistic effect of chitosan and titanium dioxide, the bacteriostasis diameter is larger than that of group A, which shows that the bacteriostasis of the synergistic effect of the chitosan and the titanium dioxide is higher than that of single chitosan; wherein the diameter of the inhibition zone of the group D is the largest, which indicates that when the ratio of the chitosan solution to the titanium dioxide solution is 1: 3, the bacteriostatic activity against both of the two bacteria was highest. From the results in the figure, the average bacteriostatic diameter of the escherichia coli group can be calculated to be larger than that of the staphylococcus aureus group, which shows that the two solutions have better bacteriostatic effects on escherichia coli.

3.2 antifungal Effect of PE films coated in different layers

As shown in FIGS. 11 to 12, the PE films coated with different layers of mixed deposition solutions have the bacteriostatic effect on Escherichia coli and Staphylococcus aureus, and the larger the inhibition zone is, the stronger the bacteriostatic effect is. The results show that the synergistic bacteriostasis effect on two bacteria is stronger as the number of layers coated by the mixed membrane solution of chitosan and titanium dioxide is increased, and the bacteriostasis effect is strongest when the number of the layers coated by the mixed membrane solution in four groups is three (the ratio of the chitosan solution to the titanium dioxide solution is 1: 3).

The mechanism of the antibacterial activity of chitosan relies on the interaction between positively charged molecules of chitosan and negatively charged molecules of the bacterial cell membrane. In particular, the interaction is by protonation of NH₃ ⁺The electrostatic forces between mediate the chitosan and phosphate groups in the phospholipid bilayer of the bacterial cell membrane. This interaction results in deformation of the cell membrane, thereby disrupting its function, including internal osmotic balance and cell permeability, resulting in leakage of intracellular electrolytes such as potassium ions and other low molecular weight substances such as nucleic acids and glucose. As a result, the growth of bacteria is inhibitedAnd ultimately cell death.

3.3、PE-PAA-CTS/TIO₂Application of antibacterial film in chilled meat

The research takes the total number of colonies and nuclear magnetic resonance as determination indexes and takes PE-PAA-CTS/TIO₂The antibacterial film (the volume ratio of the chitosan solution to the titanium dioxide solution is 1: 3, the number of layers is 3) is applied to the chilled meat, and the fresh-keeping effect is measured.

3.3.1 Experimental methods

3.3.1.1 chilled fresh meat sample treatment

The surface of fresh pork is washed by clean water, after water is drained, the fresh pork is evenly cut into small blocks of 5g (used for measuring nuclear magnetic resonance) and 25g (used for measuring the total number of colonies), and then the small blocks are divided into two groups. One group is packaged by PE film; the other group is packaged by an antibacterial film (the chitosan layer containing titanium dioxide contacts the surface of fresh pork). The two groups of packaged fresh pork are respectively stored in a refrigerator at 4 ℃. The total number of colonies was measured on

days

1, 3, 5, 7 and 9, and the change in water was measured by NMR.

3.1.1.2 Total colony count determination

25g of pork was taken and measured according to the method prescribed in GB4789.2-2016 (national Standard for food safety determination of Total number of colonies). Evaluation criteria: less than 10⁴cfu/g is first order freshness, at 10⁴cfu/g～10⁶The secondary freshness is more than 10 between cfu/g⁶cfu/g is deteriorated meat.

3.1.1.3 NMR measurement

The degree of freedom of moisture is reflected on the basis of the nuclear magnetic resonance relaxation time, and the moisture distribution and change of the meat are researched. Setting parameters of a hard pulse CPMG sequence, taking 5g of pork for measurement, and performing T2 inversion after the measurement is finished to obtain a relaxation time distribution state diagram.

3.3.2 results and discussion

3.3.2.1 Total colony count results analysis

The total number of the bacterial colonies of the pork can reflect the degree of the effect of the microorganisms on the pork in the refrigeration process, and the method is one of important indexes for judging the pork quality.

As can be seen from FIG. 13, the cold storage condition at 4 ℃ was maintainedThe total number of colonies on the 5 th day of pork packed with PE film was 10⁴cfu/g～10⁶Between cfu/g, belonging to secondary freshness, the total number of colonies increased very quickly from day 5 to day 7, and reached 10 by day 7⁷cfu/g, has putrefactive and deteriorated, and is higher than 10 at day 9⁷cfu/g, with obvious foul smell; the total number of bacterial colonies of the pork packaged by the antibacterial film on the 9 th day is 10⁶cfu/g, initial deterioration, indicating PE-PAA-CTS/TIO₂The film can prolong the shelf life of pork.

3.3.2.2 nuclear magnetic resonance results analysis

The change of the water content of the pork in the cold storage process can be detected by nuclear magnetic resonance, and the transverse relaxation time of the nuclear magnetic resonance is a very effective method for researching the water retention and the water distribution.

Curves a1, a3, a5, a7, a9 and b1, b3, b5, b7 and b9 corresponding to the a diagram and the b diagram in FIG. 14 respectively represent the PE film and the PE-PAA-CTS/TIO on

days

1, 3, 5, 7 and 9₂Nuclear magnetic resonance curve of film-wrapped pork. The graph can see that the two groups of graphs both have four peaks, according to the results of NMR relaxation time and peak value of the test sample, the four peaks of the relaxation time T2 are respectively T20, T21, T22 and T23, and T20 and T21 can be considered as bound water bound with macromolecules such as protein; t22 represents the non-mobile water present in the muscle between myofibrils and membranes, which accounts for the majority of the water in the muscle; t23 represents free-flowing water present in the extracellular space. The relaxation time of T20 and T21 has no obvious rule, and the longer the relaxation time of T22 is, the more the non-flowing water is converted into free water, and the poorer the water retention property is; the longer the T23 relaxation time, the higher the free water content and the poorer the water retention. The abscissa corresponding to the peak top of the four peaks represents the relaxation time of each component water, respectively.

TABLE 24 ℃ PE film (a) and PE-PAA-CTS/TIO₂Relaxation times T22, T23 of film (b) packaged pork

As can be seen from table 2, the T23 relaxation time of group a pork increases at

days

3, 5, and 7 under refrigeration at 4 ℃, indicating that the non-mobile water is converted into free water and the water retention of the pork decreases, and the T22 relaxation time increases at day 9 indicating that the total amount of free water increases and the water retention of the pork continues to decrease; the T23 relaxation time of the pork in the b group is increased on 5 th and 9 th days, which shows that the water retention of the pork is reduced when the non-flowing water is converted into the free water, and the T22 relaxation time is increased on 5 th, 7 th and 9 th days, which shows that the total amount of the free water is increased and the water retention of the pork is continuously reduced. Comparing the relaxation time of two groups of experiments per day, the relaxation time of group a is longer than that of group b, which shows that the water retention of group b is better, and the fresh-keeping effect of the group b packaged by the PE-PAA-CTS/TiO2 film is better.

Chitosan was successfully coated on the PE film by increasing the surface activity of the PE film through a PAA coating treatment before the chitosan was coated on the PE film. Modification of the film surface by the coating treatment of PAA effectively increases the surface roughness and the resulting oxygen-containing polar functional groups (including C ═ O and — OH) on the PE film surface. As a result, the film maintains the hydrophilicity of the surface, and the coating of PAA allows chitosan to be better adsorbed to the PE surface as a result of the positive and negative electro-adsorption of chitosan with PAA, thereby achieving the coating of chitosan on the PE film. The amount of chitosan coated on the PE film was determined after removing loosely bound chitosan by washing the chitosan coated PE film three times in water. Therefore, only chitosan chemically bonded to the PE surface remains on the PE film.

The research results of the invention show that the PAA coating treatment is an effective technology for improving the adhesion between the chitosan and the PE film. The chitosan-coated corona-treated PE film shows strong antibacterial activity against gram-negative bacteria, namely escherichia coli and gram-positive staphylococcus aureus, when the volume of a titanium dioxide solution with the mass percentage of 2 percent and the volume of chitosan with the mass percentage of 2 percent are 3: 1, the antibacterial effect is best, the coating effect is good, and the hydrophilicity is high.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents and are included in the scope of the present invention.

Claims

1. A polyethylene preservative film is characterized in that: the polyethylene preservative film sequentially comprises a polyethylene film, an acrylic acid film and three layers of titanium dioxide-chitosan composite films from an outer layer to an inner layer, wherein the polyethylene film is a corona polyethylene film, and the acrylic acid film is a food-grade water-based acrylic acid film;

(4) preparing a polyethylene preservative film by layer-by-layer self-assembly: selecting a polyethylene film, coating acrylic emulsion on the polyethylene film, drying, coating the titanium dioxide-chitosan composite solution obtained in the step (3), drying, and repeating the steps of coating the titanium dioxide-chitosan composite solution obtained in the step (3) and drying twice to obtain the polyethylene preservative film;

the chitosan solution prepared in step (1) is as follows: weighing 0.2g of chitosan, dissolving the chitosan into 10mL of 2% acetic acid solution by mass percent, and stirring the solution in a magnetic stirrer for 1 hour to prepare 10mL of 2% chitosan solution;

the titania solution prepared in step (2) is as follows: dissolving 2mL of titanium dioxide stock solution into 8mL of distilled water, and simultaneously carrying out ultrasonic vibration at 30 ℃ for 20min to prepare 10mL of titanium dioxide solution;

the volume ratio of the chitosan solution to the titanium dioxide solution in the step (3) is 1: 3.

2. the polyethylene wrap as claimed in claim 1, wherein: the thickness of the polyethylene film is 15-25 mu m, the thickness of the acrylic film is 1-10 mu m, and the thickness of the titanium dioxide-chitosan composite film is 5-20 mu m.

3. The method for preparing a polyethylene preservative film according to claim 1, which comprises the steps of:

(4) preparing a polyethylene preservative film by layer-by-layer self-assembly: and (3) selecting a polyethylene film, coating the acrylic emulsion on the polyethylene film, drying, coating the titanium dioxide-chitosan composite solution obtained in the step (3), drying, and repeating the steps of coating the titanium dioxide-chitosan composite solution obtained in the step (3) and drying twice to obtain the polyethylene preservative film.

4. The method for preparing polyethylene preservative film according to claim 3, wherein: the rotating speed of the magnetic stirring in the step (1) is 800-1000 rpm.

5. Use of the polyethylene wrap of claim 1 for the preparation of packaging material with antimicrobial effect and for the preservation of chilled meat.