Multi-layer composite antibacterial paper for fresh keeping of agricultural products and preparation method of multi-layer composite antibacterial paper

Abstract

The invention discloses multi-layer composite antibacterial paper for fresh-keeping of agricultural products and a preparation method of the multi-layer composite antibacterial paper, and belongs to the technical field of fresh-keeping antibacterial composite paper. The multi-layer composite antibacterial paper consists of two or more layers (three to five layers) of functional paper, and is obtained by high-temperature pressing and needling punching. The antibacterial paper prepared by the method automatically and intelligently releases 5-50ppm of sulfur dioxide or chlorine dioxide gas after contacting with water vapor generated by metabolism of agricultural products, and the gas can kill botrytis cinerea and other molds bred on the surfaces of vegetables, so that the rotting of the vegetables is reduced, the quality of the vegetables is improved, and the shelf life of the vegetables is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of fresh-keeping antibacterial composite paper, and in particular to a multi-layer composite antibacterial paper for preserving agricultural products and a preparation method thereof. Background Art

[0002] Botrytis cinerea is a typical necrotrophic plant pathogenic fungus that prefers humid environments and has a wide host range. It can parasitize on various fruits, vegetables and flowers and reproduce rapidly, thereby affecting the quality and shelf life of these fresh agricultural products after harvest, causing serious economic losses to agricultural products after harvest. It is one of the important plant pathogenic fungi in the world.

[0003] At present, in order to kill the gray mold contaminating agricultural products and extend the shelf life of fruits and vegetables, people have to use sulfur dioxide to fumigate fruits and vegetables, resulting in excessive sulfur on fruits and vegetables. Various antibacterial papers and antibacterial sheets that release sulfur dioxide slowly have been widely used for sterilizing agricultural products, but their release concentration is high, resulting in high residual concentrations on fruits and vegetables, affecting food safety.

[0004] In view of the above problems, the present invention provides a multi-layer composite antibacterial paper for preserving fresh agricultural products, especially vegetables, and a preparation method thereof. Summary of the invention

[0005] The purpose of the present invention is to provide a multi-layer composite antibacterial paper for preserving agricultural products and a preparation method thereof. After the prepared antibacterial paper comes into contact with water vapor generated by the metabolism of agricultural products, it automatically and intelligently releases 5-50ppm of sulfur dioxide or chlorine dioxide gas. The gas can kill molds such as gray mold growing on the surface of vegetables, reduce the decay of vegetables, improve the quality of vegetables, extend the shelf life, and solve the problems mentioned in the above background technology.

[0006] To achieve the above-mentioned object, the present invention provides a multi-layer composite antibacterial paper for preserving agricultural products, the composite antibacterial paper is composed of at least two paper bases coated with functional layers, the functional layers are organic acid / polyvinyl alcohol coatings or antibacterial agent precursors / sodium polyacrylate coatings, the organic acid / polyvinyl alcohol coatings and the antibacterial agent precursors / sodium polyacrylate coatings are closely attached to form an antibacterial release layer; Among them, by weight, the organic acid / polyvinyl alcohol coating contains 3-18% polyvinyl alcohol, 50-80% organic acid, and the rest is water; the antibacterial agent precursor / sodium polyacrylate coating contains 0.5-4% sodium polyacrylate, 60-80% antibacterial agent precursor, and the rest is water.

[0007] Preferably, the composite antibacterial paper is composed of two paper bases coated with functional layers, wherein the upper paper base is coated with an organic acid / polyvinyl alcohol coating; and the lower paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating.

[0008] Preferably, the composite antibacterial paper consists of three paper bases coated with functional layers, wherein the upper and lower paper bases are coated with antibacterial agent precursor / sodium polyacrylate coatings on one side, and the middle paper base is coated with organic acid / polyvinyl alcohol coatings on both sides.

[0009] Preferably, the composite antibacterial paper is composed of four layers of paper base coated with functional layers, wherein the upper paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side, the middle two paper bases are coated with an organic acid / polyvinyl alcohol paper base and an antibacterial agent precursor / sodium polyacrylate coating on both sides in sequence, and the lower paper base is coated with an organic acid / polyvinyl alcohol coating on one side.

[0010] Preferably, the composite antibacterial paper is composed of five layers of paper base coated with functional layers, wherein the upper paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side, the middle three paper bases are coated with an organic acid / polyvinyl alcohol paper base, an antibacterial agent precursor / sodium polyacrylate paper base and an organic acid / polyvinyl alcohol coating on both sides in sequence, and the lower paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side.

[0011] The present invention also provides a method for preparing the multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of paper substrate coated with organic acid / polyvinyl alcohol coating Dissolving 4-50 parts of an organic acid in 45-85 parts of a 5% polyvinyl alcohol solution, coating the organic polyvinyl alcohol solution on a paper base on a dry laminating device, and drying in a drying oven at 70-80° C. for 0.5-3 minutes to obtain a paper base coated with an organic acid / polyvinyl alcohol coating; (2) Preparation of paper substrate coated with antimicrobial agent precursor / sodium polyacrylate coating Dissolve 10-50 parts of an antimicrobial agent precursor in 50-90 parts of a 0.9% sodium polyacrylate solution, apply the coating on a paper base on a dry composite device, and dry in a drying oven at 50-60° C. for 1-3 minutes to obtain a paper base coated with an antimicrobial agent precursor / sodium polyacrylate coating; (3) Preparation of multi-layer composite antibacterial paper The coated surfaces of the paper bases obtained by (1) and (2) are laminated together, pressed and punched to obtain a multi-layer composite antibacterial paper.

[0012] Preferably, the organic acid is one of citric acid, succinic acid, tartaric acid, malic acid, and lactic acid; and the antibacterial agent precursor is one of sodium bisulfite, sodium sulfite, calcium sulfite, sodium pyrosulfite, sodium hypochlorite, calcium hypochlorite, and sodium chlorate.

[0013] Preferably, in step (1), the thickness of the organic acid / polyvinyl alcohol coating is 3-5 μm, and the average moisture content of the coating is 18-35%, wherein the moisture content of the surface layer is less than 5%, and the moisture content of the middle core layer is 20-40%.

[0014] Preferably, in step (2), the thickness of the antimicrobial agent precursor / sodium polyacrylate coating is 3-5 μm, and the average water content of the coating is 15-35%, wherein the water content of the surface layer is less than 5%, and the water content of the middle core layer is 20-40%.

[0015] Preferably, in step (3), the specific operation of pressing and punching is: using a double-roll pressing temperature of 130-150°C and a pressure of 5-8atm to form a composite structure, and then punching holes with a pair of needle rollers, the needle rollers are densely covered with steel needles with a spacing of 2-5mm and a diameter of 0.15-0.20mm.

[0016] Two functional layers (gel layers) containing antimicrobial agent precursors and organic acids are opposite to each other, and the two paper-based coatings are hot-pressed and laminated. The water content in the surface layer (the outermost layer 0.5 microns) of the gel layer is less than 5%, which can be regarded as a "dry film". The interface layer between the antimicrobial agent functional layer and the organic acid functional layer in the attached drawings of the specification can be regarded as a dry interface; when the ambient humidity is lower than 70%, this dry interface layer cannot be swollen and destroyed by the ambient humidity, and the organic acid and antimicrobial agent precursor ions can only be confined in their respective functional layers and cannot diffuse across the dry film interface layer to the opposite gel layer; only when the preservation environment is humid When the degree exceeds 80%, the polymer material in the dry interface layer (the outermost 0.5 microns) begins to absorb water vapor in the fresh-keeping environment. When the water content of the interface layer reaches more than 20%, the ions in the two gel layers can cross the interface layer and migrate to the opposite gel layer. The organic acid and the antimicrobial agent precursor meet in the gel layer and a chemical reaction occurs. The antimicrobial agent precursor slowly produces sulfur dioxide or chlorine dioxide gas. These gases diffuse to the surface of the film through the gaps between the polymer chains in the coating substrate and are slowly released into the headspace of the fresh-keeping environment, inhibiting or killing the mold on the surface of fresh agricultural products.

[0017] For agricultural products with high respiration intensity, the humidity in the fresh-keeping environment is high, and fruits and vegetables are prone to mold growth. At this time, the high humidity environment of the antibacterial paper will swell the interface layer to a greater extent, the water content in the interface layer will increase, and the H in the organic acid layer will + The diffusion resistance of ions through the interfacial gel layer is reduced, and H +The diffusion rate and diffusion concentration of ions are high, and sulfur dioxide or chlorine dioxide gas substances will release a large concentration of sulfur dioxide or chlorine dioxide gas, inhibiting the growth of mold on fruits and vegetables; for agricultural products with low respiration intensity, they produce less water vapor through metabolism, and the humidity in the fresh-keeping environment is relatively low, so the probability of mold growth is low. At this time, the antibacterial paper has a low swelling degree of its interface layer, and the concentration of sulfur dioxide or chlorine dioxide released is also low. In this way, the antibacterial paper will intelligently and slowly release sulfur dioxide or chlorine dioxide gas according to the relative humidity of the fresh-keeping environment, that is, the metabolic characteristics of agricultural products, and intelligently release antibacterial agents just right. For some agricultural products with mechanical damage, the water or organic acid flowing out from the wound will directly contact the antimicrobial precursor / sodium polyacrylate coating layer of the cling paper (the antimicrobial paper is placed on the lower side of the agricultural product when in use, the antimicrobial precursor / sodium polyacrylate functional paper contacts the agricultural product above, and the organic acid / polyvinyl alcohol functional paper is at the bottom). After the antimicrobial functional layer in the attached figure of the specification absorbs the organic acid or water produced by the wound of fruits and vegetables, it will quickly produce sulfur dioxide or chlorine dioxide, inhibiting the formation of mold on the wound of fruits and vegetables and avoiding the occurrence of rot.

[0018] Therefore, the present invention provides the above-mentioned multi-layer composite antibacterial paper for preserving agricultural products and a preparation method thereof, which has the following technical effects: (1) The antibacterial paper prepared by the present invention has an intelligent controlled release function.

[0019] Traditional antibacterial paper or antibacterial sheet directly mixes the antibacterial agent precursor with organic acid powder, and the concentration of the antibacterial agent released after contact with water vapor is high, and the antibacterial agent release cycle is short, and fruits and vegetables are prone to sulfur residue or even bleaching. In the antibacterial paper of the present invention, the sulfur dioxide / chlorine dioxide precursor and the organic acid are respectively in two respective gel layers. When the ambient humidity is higher than 80%, the acid ions and the sulfur dioxide / chlorine dioxide precursor ions can migrate and diffuse into each other's gel layer to generate sulfur dioxide / chlorine dioxide gas. The release process is slow and controllable, the release cycle is long, and the concentration of the released sulfur dioxide / chlorine dioxide is appropriate, and no residue will be generated on the preserved fruits and vegetables, and the fruits and vegetables will not be bleached.

[0020] (2) The antibacterial paper prepared by the present invention can release the antibacterial agent evenly.

[0021] In order to enhance the swelling effect of water vapor on the interface layer, the antibacterial paper is evenly perforated with small holes by a needle roller, which is densely covered with steel needles with an interval of 2-5mm and a diameter of 0.15-0.20mm. The water vapor (water) molecules in the fresh-keeping environment migrate to the interface layer through these numerous small holes to ensure that the interface layer is evenly swelled. When the water content in the interface layer is higher than 20%, it is equivalent to the destruction of the interface layer. The ions or molecules on both sides of the interface layer can diffuse into the opposite gel layer, evenly and slowly releasing sulfur dioxide gas of appropriate concentration.

[0022] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of an antibacterial paper having an antibacterial release layer prepared in accordance with the present invention; Figure 2 A schematic diagram of the antibacterial paper with two antibacterial agent release layers prepared by the present invention; Figure 3 A schematic diagram of the antibacterial paper with three antibacterial release layers prepared in the present invention; Figure 4 A schematic diagram of the antibacterial paper with four antibacterial release layers prepared in the present invention; Figure 5 Schematic diagram of the mechanism of controlling the release of antimicrobial agents in the interface layer; Reference numerals 1. Paper base; 2. Organic acid / polyvinyl alcohol functional layer; 3. Antimicrobial agent precursor / sodium polyacrylate functional layer. DETAILED DESCRIPTION

[0024] The present invention provides a multi-layer composite antibacterial paper for preserving agricultural products and a preparation method thereof. The composite antibacterial paper of the present invention may have one to four or even more than four antibacterial release layers.

[0025] The preparation steps of the multi-layer composite antibacterial paper are as follows: (1) first prepare a paper base coated with an organic acid functional layer or an antibacterial agent precursor functional layer on one side (or two sides) on a dry composite device; apply the coating solution (or suspension) on the paper base with a scraper, and dry it in a drying oven at 50-60°C for 1-2 minutes to obtain a coated paper with a total coating thickness of 2-5 microns, an average moisture content of 18-27% in the coating, and a gradient distribution of moisture content from the core to the surface, wherein the moisture content of the outermost layer (0.5 microns) is less than 5%, and the moisture content in the middle core layer is higher; using a paper base coated with a single-sided functional layer as a raw material, coating the same coating solution on the other side (other side) of the paper base to prepare a double-sided coated functional paper, wherein both sides of the paper are coated with the same coating solution. The same coating liquid (functional layer) is coated on the side; (2) functional paper is laminated and hot pressed; the surface layer containing a small amount of water contains polyvinyl alcohol or sodium polyacrylate polymer materials, and the polymer materials make the coating surface have a certain adhesion. When the functional layers on the upper and lower functional papers are laminated on both sides and pressed by double rollers at a temperature of 100-150°C and a pressure of 5-8atm, the multi-layer functional paper is tightly pressed into one by the polymer adhesion to prepare a multi-layer antibacterial paper; (3) needle punching of antibacterial paper: the hot-pressed multi-layer antibacterial paper is passed through a needle roller and a pressing roller covered with steel needles, the steel needles are spaced 2-5mm on the roller, and the diameter of the steel needles is 0.15-0.20mm, to prepare the antibacterial paper described in the patent.

[0026] In the drawings of the specification Figure 1 The schematic diagram of antibacterial paper with an antibacterial release layer is formed by pressing two layers of functional paper together. The lower side of the upper paper base 1 is coated with an organic acid / polyvinyl alcohol functional layer 2, and the upper side of the lower paper base 1 is coated with an antibacterial precursor / sodium polyacrylate functional layer 3. The organic acid / polyvinyl alcohol functional layer 2 and the antibacterial precursor / sodium polyacrylate functional layer 3 form an antibacterial agent release layer.

[0027] In the drawings of the specification Figure 2 The schematic diagram of the antibacterial paper with two antimicrobial agent release layers is obtained by pressing and punching three layers of functional paper, the upper and lower surfaces of the middle paper base 1 are coated with the organic acid / polyvinyl alcohol functional layer 2, the upper and lower functional papers are only coated with the antimicrobial precursor / sodium polyacrylate functional layer 3 on one side, and then the functional layers 2 and 3 are placed opposite to each other and hot-pressed and punched to obtain the antibacterial paper with two antimicrobial agent release layers (four functional layers in total).

[0028] In the drawings of the specification Figure 3 Schematic diagram of antibacterial paper with three antibacterial release layers, which is obtained by pressing and punching four layers of functional paper. The two middle layers of functional paper are both coated with functional layers on both sides, and the functional papers located above and below are only coated with functional layers on one side. From top to bottom, they are: paper coated with functional layer 3 on one side, paper coated with functional layer 2 on both sides, paper coated with functional layer 3 on both sides, and paper coated with functional layer 2 on one side. Functional layers 2 and 3 are organic acid / polyvinyl alcohol and antibacterial precursor / sodium polyacrylate, respectively. After drying, functional layers 2 and 3 are placed opposite each other and hot-pressed and punched to obtain antibacterial paper with three antibacterial agent release layers (a total of six functional layers).

[0029] In the drawings of the specification Figure 4 This is a schematic diagram of antibacterial paper with four antibacterial release layers, which is obtained by pressing and punching five layers of functional paper. The three middle layers of functional paper are all coated with functional layers on both sides, and the functional papers located above and below are only coated with functional layers on one side. From top to bottom, they are: paper coated with functional layer 3 on one side, paper coated with functional layer 2 on both sides, paper coated with functional layer 3 on both sides, paper coated with functional layer 2 on both sides, and paper coated with functional layer 3 on one side. After qualified drying, functional layers 2 and 3 are placed opposite each other and hot-pressed and punched to obtain antibacterial paper with four antibacterial agent release layers (eight functional layers in total).

[0030] In the drawings of the specification Figure 5 Figure 1 is a schematic diagram of the mechanism of controlling the release of antimicrobial agents by the interface layer. The area surrounded by the dotted line is the surface layer of the functional layer. The surface layers of the two functional layers form the interface layer of the antimicrobial release layer. After the coating on the paper substrate is dried, the surface layer of the functional coating, i.e., the interface layer, is highly dry and has a water content of less than 5%. Due to the low water content of the polymer resin in the interface layer, the migration resistance of organic acid molecules and antimicrobial agent precursors in the interface layer is very high, that is, the organic acid ions (such as H +) and HSO3 - Ions cannot migrate across the interface layer to the functional layer on the opposite side; only when the antibacterial paper comes into contact with fruits and vegetables, and the functional layer absorbs the water vapor produced by the metabolism of fruits and vegetables, making the water content of the interface layer higher than 20%, can the ions in the functional layer migrate to the core layer of the other side, undergo a chemical reaction, and generate antibacterial gas.

[0031] The more antibacterial release layers of the antibacterial paper, the greater the density of the needle-punched holes, and the higher the relative humidity of the preservation environment, the higher the concentration of sulfur dioxide released by the composite antibacterial paper, and the stronger the ability to inhibit and kill fungi such as gray mold.

[0032] Citric acid and sodium bisulfite are commonly used food additives and are the preferred organic acids and sulfur dioxide precursors. Citric acid can also be replaced by organic acids such as succinic acid, tartaric acid, malic acid, and lactic acid as needed; sodium bisulfite can be replaced by precursors such as sodium sulfite, calcium sulfite, and sodium metabisulfite.

[0033] Antibacterial paper containing multiple functional layers, all sodium bisulfite functional layers, or one or several of them can also be replaced by chlorine-containing precursors such as sodium hypochlorite, calcium hypochlorite (or sodium chlorate), etc., to prepare antibacterial paper that releases hypochlorous acid (or chlorine dioxide) or composite release (simultaneously releases hypochlorous acid (or chlorine dioxide) and sulfur dioxide) to improve the antibacterial effect and broad-spectrum antibacterial properties. Sulfur dioxide, chlorine dioxide (further decomposed to produce chlorine and hypochlorous acid), and hypochlorous acid can all be used in the deep processing and preservation of fruits and vegetables, in compliance with relevant domestic and international standards.

[0034] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. Any other changes, modifications, substitutions, combinations, and simplifications made without violating the spirit and principle of the present invention should all be equivalent replacement methods and are included in the protection scope of the present invention. In addition, it should be understood that after reading the content of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to this application, and all belong to the scope of protection of the present invention.

[0035] Reference to "embodiment" herein means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present application. The term "embodiment" appearing in various places in the specification does not necessarily refer to the same embodiment, nor does it particularly limit its independence or association with other embodiments. In principle, in the present application, as long as there is no technical contradiction or conflict, the various technical features mentioned in the embodiments can be combined in any way to form a corresponding implementable technical solution.

[0036] Unless otherwise defined, the technical terms used in this document have the same meanings as those generally understood by those skilled in the art to which this application belongs; the use of relevant terms in this document is only for describing specific embodiments and is not intended to limit this application.

[0037] Unless otherwise specified in the present invention, the reagents, instruments, and equipment used are those commonly used by technicians in this field.

[0038] The polyvinyl alcohol used in the following examples of the present invention is polyvinyl alcohol with a brand number of 2699.

[0039] Example 1 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper base: 2.5 parts (weight ratio, the same below) of polyvinyl alcohol was mixed with 47.5 parts of water, dissolved in a 90°C water bath, and a polyvinyl alcohol solution with a concentration of 5% was formed after complete dissolution. Then, 50 parts of anhydrous citric acid was added to the solution and completely dissolved in a water bath. When the temperature of the organic acid / polyvinyl alcohol (PVA) solution formed was reduced to 50°C, the coating liquid was transferred to the coating hopper of the dry laminating equipment, and the solution was coated on the paper base. The drying oven of the coating equipment was set to 80°C, and the wet coating thickness of the coating liquid was adjusted to 8.5 μm. The residence time of the paper base in the drying oven was 1.0 minute. After drying, an organic acid / polyvinyl alcohol functional layer paper base was obtained. The functional layer contained 4.8 parts of polyvinyl alcohol, 63.5 parts of citric acid, and the rest was water. The total thickness of the functional layer was 5 μm, of which the surface layer thickness was 0.5 μm, the surface layer water content was 5%, and the core layer thickness other than the surface layer had a water content of 20.8%.

[0040] (2) Preparation of sodium bisulfite / sodium polyacrylate paper base: 0.68 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 74.5 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 24.8 parts of sodium bisulfite was added to the above solution and stirred until it was completely dissolved. The coating liquid was transferred to the coating hopper of the dry laminating equipment and the solution was coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 17 μm. The residence time of the paper base in the drying oven was 2.0 minutes. After drying, a sodium bisulfite / sodium polyacrylate functional layer paper base was obtained. The functional layer contained 2.2 parts of sodium polyacrylate, 78.3 parts of sodium bisulfite, and the rest was water. The total thickness of the functional layer was 5 μm, of which the surface layer thickness was 0.5 μm, the surface layer moisture content was 5%, and the core layer thickness other than the surface layer had a moisture content of 22.5%.

[0041] (3) Preparation of sulfur dioxide release layer (single layer) antibacterial paper: Figure 1 As shown, the two single-layer coated functional papers prepared above, the citric acid / polyvinyl alcohol coating and the sodium bisulfite / sodium polyacrylate coating, are placed on opposite sides (adhered to each other), and then pressed by a double-roller at a temperature of 150°C and a pressure of 8atm to form a double-layer composite structure, and then treated with a needle-roller with a steel needle spacing of 5mm and a steel needle diameter of 0.15mm to obtain an antibacterial paper with an antibacterial agent release layer. The antibacterial paper is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity of 80-90%, and the concentration of sulfur dioxide released in the fresh-keeping environment is 20ppm, and the release period is 10 days.

[0042] Prepare 1 kg of spinach without mechanical damage, and put it together with a piece of 20×30 cm antibacterial paper prepared in this embodiment into a commercially available 40-micron polyethylene fresh-keeping bag with an outer dimension of 30×40 cm. Place it at 25°C for 2 days. After 2 days, open the fresh-keeping bag, observe the appearance, cut off the rotten leaves 1 cm below the rotten part of the leaves and weigh them, and calculate the percentage of the rotten leaves in the total weight of the vegetables.

[0043] Example 2 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper substrate (double-sided coating): The solution preparation method and the single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and a citric acid / polyvinyl alcohol functional layer of the same thickness is coated on the other side of the paper substrate.

[0044] (2) Preparation of sodium bisulfite / sodium polyacrylate paper base (single-sided coating): Two groups of sodium bisulfite / sodium polyacrylate paper bases were prepared according to the preparation method of Example 1.

[0045] (3) Preparation of sulfur dioxide release layer (double layer) antibacterial paper: Figure 2 As shown, the double-sided citric acid / polyvinyl alcohol coated functional paper prepared in step (1) is placed in the middle position, and the two groups of paper bases coated with sodium bisulfite / sodium polyacrylate functional layers on one side prepared in step (2) are placed on the upper and lower sides of the middle functional paper, and the organic acid functional layer and the sodium bisulfite functional layer are opposite (adhered to each other), and are pressed by double rollers at a temperature of 150° C. and a pressure of 8 atm, and then the three-layer composite paper is passed through a pair of needle rollers and punched with holes. The needle rollers are densely covered with steel needles with a spacing of 3 mm and a diameter of 0.20 mm. The antibacterial paper with two sulfur dioxide release layers is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%, and the concentration of sulfur dioxide released in the fresh-keeping environment is 25 ppm, and the release period is 25 days.

[0046] Example 3 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper substrate (single-sided coating): The specific preparation method is the same as that in Example 1.

[0047] (2) Preparation of citric acid / polyvinyl alcohol paper substrate (double-sided coating): The solution preparation method and single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and an organic acid / polyvinyl alcohol functional layer of the same thickness is coated on the other side of the paper substrate.

[0048] (3) Preparation of sodium bisulfite / sodium polyacrylate paper base (single-sided coating): The specific preparation method is the same as that in Example 1.

[0049] (4) Preparation of sodium bisulfite / sodium polyacrylate paper substrate (double-sided coating): The solution preparation method and single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and a sodium bisulfite / sodium polyacrylate functional layer of the same thickness is coated on the other side of the paper substrate.

[0050] (5) Preparation of sulfur dioxide release layer (three-layer) antibacterial paper: Figure 3 As shown, the following functional paper bases are placed in order from top to bottom: a paper base coated with a sodium bisulfite / sodium polyacrylate functional layer on one side (functional layer facing downward), a paper base coated with a citric acid / polyvinyl alcohol functional layer on both sides, a paper base coated with a sodium bisulfite / sodium polyacrylate functional layer on both sides, a paper base coated with a citric acid / polyvinyl alcohol functional layer on one side (functional layer facing upward), an organic acid functional layer and a sodium bisulfite functional layer facing each other (adhering to each other), and then the four-layer composite paper is pressed by a pair of needle rollers at a temperature of 150°C and a pressure of 8atm, and then the four-layer composite paper is passed through a pair of needle rollers and punched with holes. The needle rollers are densely covered with steel needles with a spacing of 4mm and a diameter of 0.20mm. The antibacterial paper with three sulfur dioxide release layers is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%, and the concentration of sulfur dioxide released in the fresh-keeping environment is 38ppm, and the release cycle is 36 days.

[0051] Example 4 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of succinic acid / polyvinyl alcohol paper base: 3 parts of polyvinyl alcohol and 57 parts of water were mixed and dissolved in a 90°C water bath to form a polyvinyl alcohol solution with a concentration of 5%. Then, 10 parts of succinic acid were added to the solution and completely dissolved in a water bath. When the temperature of the organic acid / polyvinyl alcohol solution formed was reduced to 50°C, the coating liquid was transferred to the coating hopper of the dry laminating equipment and the solution was coated on the paper base. The drying oven of the coating equipment was set to 80°C, and the wet coating thickness of the coating liquid was adjusted to 15 μm. The residence time of the paper base in the drying oven was 1 minute. After drying, an organic acid / polyvinyl alcohol functional layer paper base was obtained. The functional layer contained 17.6 parts of polyvinyl alcohol, 58.8 parts of succinic acid, and the rest was water. The total thickness of the functional layer was 4 μm, of which the surface layer thickness was 0.5 μm, the surface layer water content was 5%, and the core layer thickness other than the surface layer had a water content of 27.2%.

[0052] (2) Preparation of calcium sulfite / sodium polyacrylate functional paper base: 0.78 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 85.0 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 14.2 parts of calcium sulfite with a particle size of 200 mesh was added to the above solution and stirred at high speed until it was evenly dispersed. The coating liquid was transferred to the coating hopper of the dry composite equipment and coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 30 μm. The residence time of the paper base in the drying oven was 2 minutes. After drying, a calcium sulfite / sodium polyacrylate functional layer paper base was obtained. The functional layer contained 3.3 parts of sodium polyacrylate, 68.7 parts of calcium sulfite, and the rest was water. The total thickness of the functional layer was 5 μm, of which the surface layer thickness was 0.5 μm, the surface layer moisture content was 5%, and the core layer thickness other than the surface layer had a moisture content of 32.0%.

[0053] (3) Preparation of sulfur dioxide release layer (single layer) antibacterial paper: Figure 1 As shown, the two single-layer coated functional papers prepared above, the succinic acid / polyvinyl alcohol coating and the calcium sulfite / sodium polyacrylate coating, are placed on opposite sides (adhered to each other), and then pressed by a double roller at a temperature of 150°C and a pressure of 5atm, and then passed through a pair of needle rollers; the needle rollers are densely covered with steel needles with a spacing of 2mm and a diameter of 0.20mm. This antibacterial paper with a sulfur dioxide release layer is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%. The concentration of sulfur dioxide released in the fresh-keeping environment is 8ppm, and the release period is 15 days.

[0054] Example 5 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of tartaric acid / polyvinyl alcohol functional paper base (double-sided coating): 3 parts of polyvinyl alcohol were mixed with 57 parts of water, dissolved in a 90°C water bath, and a polyvinyl alcohol solution with a concentration of 5% was formed after complete dissolution. Then 40 parts of tartaric acid were added to the solution and completely dissolved in a water bath. When the temperature of the formed organic acid / polyvinyl alcohol solution dropped to 50°C, the coating liquid was transferred to the coating hopper of the dry laminating equipment, and the solution was coated on the paper base. The drying oven of the coating equipment was set to 80°C, and the coating liquid wet coating thickness was adjusted to 8.5 μm. The residence time of the paper base in the drying oven was 1 minute. After drying, a single-sided tartaric acid / polyvinyl alcohol functional layer paper base was obtained; using the single-sided coated paper base as a raw material, a tartaric acid / polyvinyl alcohol functional layer of the same thickness was coated on the other side of the paper base by the same process to obtain a double-sided tartaric acid / polyvinyl alcohol functional layer paper base. The functional layer contains 5.3 parts of polyvinyl alcohol, 70.2 parts of tartaric acid, and the rest is water. The thickness of each functional layer is 5 microns, of which the surface layer is 0.5 microns thick and has a water content of 5%. The core layer excluding the surface layer has a water content of 28.5%.

[0055] (2) Preparation of malic acid / polyvinyl alcohol functional paper base (double-sided coating): 3.2 parts of polyvinyl alcohol were mixed with 60 parts of water, dissolved in a 90°C water bath, and a polyvinyl alcohol solution with a concentration of 5% was formed after complete dissolution. Then 36.8 parts of malic acid were added to the solution and completely dissolved in a water bath. When the temperature of the formed organic acid solution dropped to 50°C, the coating liquid was transferred to the coating hopper of the dry laminating equipment, and the solution was coated on the paper base. The drying oven of the coating equipment was set to 80°C, and the coating liquid wet coating thickness was adjusted to 9 μm. The residence time of the paper base in the drying oven was 1 minute. After drying, a single-sided coated malic acid / polyvinyl alcohol functional layer paper base was obtained; using the single-sided coated paper base as a raw material, a malic acid / polyvinyl alcohol functional layer of the same thickness was coated on the other side of the paper base by the same process to obtain a double-sided coated malic acid / polyvinyl alcohol functional layer paper base. The functional layer contains 5.1 parts of polyvinyl alcohol, 67.8 parts of malic acid, and the rest is water. The thickness of each functional layer is 5 microns, of which the surface layer is 0.5 microns thick and has a water content of 5%. The core layer excluding the surface layer has a water content of 31.5%.

[0056] (3) Preparation of sodium sulfite / sodium polyacrylate functional paper base (single-sided coating): 0.68 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 74.5 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 24.8 parts of sodium sulfite was added to the above solution and stirred until dissolved. The coating liquid was transferred to the coating hopper of the dry composite equipment and coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 15 microns. The residence time of the paper base in the drying oven was 2 minutes. After drying, a single-sided coated sodium sulfite / sodium polyacrylate functional layer paper base was obtained; then, the single-sided coated film was used as a raw material, and a sodium sulfite / sodium polyacrylate functional layer of the same thickness was coated on the other side of the paper base in the same way. The functional layers on both sides contain 2.1 parts of sodium polyacrylate, 75.3 parts of sodium sulfite, and the rest is water. The total thickness of the functional layers is 5 microns, of which the surface layer is 0.5 microns thick and has a water content of 5%. The core layer excluding the surface layer has a water content of 26.1%.

[0057] (4) Preparation of sodium metabisulfite / sodium polyacrylate functional paper base (double-sided coating): 0.61 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 66.3 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 33.1 parts of sodium metabisulfite was added to the above solution and stirred until dissolved. The coating liquid was transferred to the coating hopper of the dry composite equipment and coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 13 μm. The residence time of the paper base in the drying oven was 2 minutes. After drying, a single-sided coated sodium metabisulfite / sodium polyacrylate functional layer paper base was obtained; then, the single-sided coated film was used as a raw material, and a sodium metabisulfite / sodium polyacrylate functional layer of the same thickness was coated on the other side of the paper base in the same way. The functional layers on both sides contain 1.5 parts of sodium polyacrylate, 79.9 parts of sodium metabisulfite, and the rest is water. The total thickness of the functional layers is 5 microns, of which the surface layer is 0.5 microns thick and has a water content of 5%. The core layer excluding the surface layer has a water content of 21.4%.

[0058] (5) Preparation of sulfur dioxide release layer (four layers) antibacterial paper: Figure 4As shown, the following functional paper bases are placed from top to bottom: a paper base coated with a sodium sulfite / sodium polyacrylate functional layer on one side (functional layer facing downward), a paper base coated with tartaric acid / polyvinyl alcohol functional layer on both sides (paper base), a paper base coated with a sodium pyrosulfite / sodium polyacrylate functional layer on both sides, a paper base coated with malic acid / polyvinyl alcohol functional layer on both sides, a paper base coated with a sodium sulfite / sodium polyacrylate functional layer on one side (functional layer facing upward), an organic acid functional layer and an antimicrobial agent precursor functional layer are opposite (adhered to each other), and after double-roll pressing at a temperature of 150°C and a pressure of 8atm, the four-layer composite paper is passed through a pair of needle rollers and punched with holes. The needle rollers are densely covered with steel needles with an interval of 3mm and a diameter of 0.15mm. The antibacterial paper with four sulfur dioxide release layers is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%, and the concentration of sulfur dioxide released in the fresh-keeping environment is 42ppm, and the release cycle is 60 days.

[0059] Prepare 1 kg of fennel without mechanical damage, and put it together with a piece of 20×30 cm antibacterial paper prepared in this embodiment into a commercially available 40-micron polyethylene fresh-keeping bag with an outer dimension of 30×40 cm. Place it at 25°C for 2 days. Open the fresh-keeping bag after 2 days, observe the appearance, cut off the rotten branches and leaves 1 cm below the rotten part of the branches and leaves and weigh them, and calculate the percentage of the rotten branches and leaves in the total weight of the vegetables.

[0060] Example 6 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper substrate (double-sided coating): The solution preparation method and the single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and a citric acid / polyvinyl alcohol functional layer of the same thickness is coated on the other side of the paper substrate.

[0061] (2) Preparation of sodium bisulfite / sodium polyacrylate paper substrate: The solution preparation method and single-layer coating process are the same as those in Example 1.

[0062] (3) Preparation of calcium hypochlorite / sodium polyacrylate functional paper base: 0.78 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 85.0 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was lowered to 50°C, 14.2 parts of calcium hypochlorite with a particle size of 200 mesh was added to the above solution and stirred at high speed until it was evenly dispersed. The coating liquid was transferred to the coating hopper of the dry composite equipment, and the solution was coated on the paper base. The drying oven of the coating equipment was set to 60° C., the wet coating thickness of the coating liquid was adjusted to 30 μm, and the residence time of the paper base in the drying oven was 2 minutes. After drying, a calcium hypochlorite / sodium polyacrylate functional layer paper base was obtained, wherein the functional layer contained 3.5 parts of sodium polyacrylate, 64.3 parts of calcium hypochlorite, and the rest was water. The total thickness of the functional layer was 5 μm, wherein the surface layer had a thickness of 0.5 μm, a water content of the surface layer of 5%, and a water content of 37.6% in the core layer excluding the surface layer.

[0063] (4) Preparation of antibacterial paper with two release layers that releases sulfur dioxide and hypochlorous acid simultaneously: Figure 2 As shown, the functional paper coated with citric acid / polyvinyl alcohol on both sides is placed in the middle, the paper base coated with sodium bisulfite / sodium polyacrylate functional layer on one side is placed on the upper side, the calcium hypochlorite / sodium polyacrylate functional paper base is on the lower side (functional layer facing upward), the organic acid functional layer and the fungicide precursor functional layer are opposite (fitted with each other), and after being pressed by a double-roller at a temperature of 150°C and a pressure of 8atm, a pair of needle rollers are used to punch holes. The needle rollers are densely covered with steel needles with a spacing of 4mm and a diameter of 0.15mm. The antibacterial paper is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%. The concentration of sulfur dioxide released in the fresh-keeping environment is 20ppm, and the release period is 10 days; the release concentration of hypochlorous acid is 5ppm and the release period is 5 days.

[0064] Prepare 1 kg of rapeseed without mechanical damage, and put it together with a piece of 20×30 cm antibacterial paper prepared in this embodiment into a commercially available 40-micron polyethylene fresh-keeping bag with an outer dimension of 30×40 cm. Place it at 25°C for 2 days. After 2 days, open the fresh-keeping bag, observe the appearance, cut off the rotten leaves 1 cm below the rotten part of the leaves and weigh them, and calculate the percentage of rotten leaves in the total weight of the vegetables.

[0065] Example 7 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper substrate (double-sided coating): The solution preparation method and the single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and a citric acid / polyvinyl alcohol functional layer of the same thickness is coated on the other side of the paper substrate.

[0066] (2) Preparation of sodium bisulfite / sodium polyacrylate paper substrate: The solution preparation method and single-layer coating process are the same as those in Example 1.

[0067] (3) Preparation of sodium hypochlorite / sodium polyacrylate functional paper base: 0.50 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 54.3 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 45.2 parts of sodium hypochlorite was added to the above solution and stirred until it was completely dissolved. The coating liquid was transferred to the coating hopper of the dry composite equipment and coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 9 μm. The residence time of the paper base in the drying oven was 2 minutes. After drying, a sodium hypochlorite / sodium polyacrylate functional layer paper base was obtained. The functional layer contained 0.8 parts of sodium polyacrylate, 76.3 parts of sodium hypochlorite, and the rest was water. The total thickness of the functional layer was 5 μm, of which the surface layer thickness was 0.5 μm, the surface layer water content was 5%, and the core layer thickness other than the surface layer had a water content of 26.5%.

[0068] (4) Preparation of antibacterial paper with two release layers that releases sulfur dioxide and hypochlorous acid simultaneously: Figure 2 As shown, the functional paper coated with citric acid / polyvinyl alcohol on both sides is placed in the middle, the paper base coated with sodium bisulfite / sodium polyacrylate functional layer on one side is placed on the upper side, the sodium hypochlorite / sodium polyacrylate functional paper base is on the lower side (functional layer facing upward), the organic acid functional layer and the fungicide precursor functional layer are opposite (fitted with each other), and after being pressed by double rollers at a temperature of 150°C and a pressure of 8atm, holes are punched through by a pair of needle rollers. The needle rollers are densely covered with steel needles with a spacing of 2mm and a diameter of 0.20mm. The antibacterial paper is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%. The concentration of sulfur dioxide released in the fresh-keeping environment is 22ppm, and the release period is 9 days; the release concentration of hypochlorous acid is 8ppm and the release period is 4 days.

[0069] Example 8 This embodiment provides a method for preparing a multi-layer composite antibacterial paper for preserving agricultural products, comprising the following steps: (1) Preparation of citric acid / polyvinyl alcohol paper substrate (double-sided coating): The solution preparation method and the single-layer coating process are the same as those in Example 1. After the single-sided coating is completed, the paper substrate is used as the substrate and a citric acid / polyvinyl alcohol functional layer of the same thickness is coated on the other side of the paper substrate.

[0070] (2) Preparation of sodium bisulfite / sodium polyacrylate paper substrate: The solution preparation method and single-layer coating process are the same as those in Example 1.

[0071] (3) Preparation of sodium chlorate / sodium polyacrylate functional paper base: 0.55 parts (molecular weight 21,000) of sodium polyacrylate was dissolved in 59.7 parts of water in a 90°C water bath. After complete dissolution, a sodium polyacrylate solution with a concentration of 0.9% was formed. When the temperature was reduced to 50°C, 39.8 parts of sodium chlorate were added to the above solution and stirred until completely dissolved. The coating liquid was transferred to the coating hopper of the dry laminating equipment and coated on the paper base. The drying oven of the coating equipment was set to 60°C, and the wet coating thickness of the coating liquid was adjusted to 10 μm. The residence time of the paper base in the drying oven was 2 minutes. After drying, a sodium chlorate / sodium polyacrylate functional layer paper base was obtained. The functional layer contained 1.1 parts of sodium polyacrylate, 79.1 parts of sodium chlorate, and the rest was water. The total thickness of the functional layer was 5 μm, of which the surface layer thickness was 0.5 μm, the surface layer water content was 5%, and the core layer thickness excluding the surface layer had a water content of 22.7%.

[0072] (4) Preparation of antibacterial paper with two release layers that releases sulfur dioxide and chlorine dioxide simultaneously: Figure 2 As shown, the functional paper coated with citric acid / polyvinyl alcohol on both sides is placed in the middle, the paper base coated with sodium bisulfite / sodium polyacrylate functional layer on one side is placed on the upper side, the sodium chlorate / sodium polyacrylate functional paper base is on the lower side (functional layer facing upward), the organic acid functional layer and the fungicide precursor functional layer are opposite (fitted with each other), and after being pressed by double rollers at a temperature of 150°C and a pressure of 8atm, holes are punched through by a pair of needle rollers. The needle rollers are densely covered with steel needles with an interval of 2mm and a diameter of 0.20mm. The antibacterial paper is placed in a room temperature environment (fruit and vegetable fresh-keeping bag or fresh-keeping box) with a relative humidity (RH) of 80-90%. The concentration of sulfur dioxide released in the fresh-keeping environment is 22ppm, and the release period is 9 days; the concentration of chlorine dioxide released is 10ppm, and the release period is 8 days.

[0073] Prepare 1 kg of mechanically undamaged lettuce, and put it together with a piece of 20×30 cm antibacterial paper prepared in this embodiment into a commercially available 40-micron polyethylene fresh-keeping bag with an outer dimension of 30×40 cm. Place it at 25°C for 2 days. After 2 days, open the fresh-keeping bag, observe the appearance, cut off the rotten leaves 1 cm below the rotten part of the leaves and weigh them, and calculate the percentage of the rotten leaves in the total weight of the vegetables.

[0074] Control group 1 Spinach was selected in the same manner as in Example 1 and placed in polyethylene fresh-keeping bags of the same specifications, except that no antibacterial paper was added. After being placed at room temperature of 25° C. for 2 days, the appearance was observed and the weight ratio of rotten leaves was calculated.

[0075] Control group 2 Fennel was selected in the same manner as in Example 5 and placed in a polyethylene fresh-keeping bag of the same specifications, except that no antibacterial paper was added. After being placed at room temperature of 25° C. for 2 days, the appearance was observed and the weight ratio of rotten branches and leaves was calculated.

[0076] Control group 3 The rapeseed was selected in the same manner as in Example 6 and placed in polyethylene fresh-keeping bags of the same specifications, except that no antibacterial paper was added. After being placed at room temperature of 25° C. for 2 days, the appearance was observed and the weight ratio of rotten leaves was calculated.

[0077] Control group 4 The romaine lettuce was selected in the same manner as in Example 8 and placed in polyethylene fresh-keeping bags of the same specifications, except that no antibacterial paper was added. After being placed at room temperature of 25° C. for 2 days, the appearance was observed and the weight ratio of rotten leaves was calculated.

[0078] The preservation effects of Examples 1, 5, 6, 8 and Control Groups 1-4 on vegetables are shown in Table 1 below.

[0079] Table 1 Effect of different antibacterial papers on the preservation of vegetables

[0080] From the results in Table 1, it can be seen that the antibacterial paper prepared in Example 1 releases sulfur dioxide gas at a concentration of 20 ppm. Compared with the control group 1, the spinach did not rot, and the spinach was fresh and tender after opening the bag 2 days later, without any peculiar smell; while the spinach in the control group 1 had more rotten leaves, darkened color, and slightly peculiar smell. This shows that the sulfur dioxide released by the antibacterial paper prepared in this patent inhibits the rotting of spinach.

[0081] The branches and leaves of fennel are thin, and they are easily broken during transportation, and the wounds are prone to rot. The antibacterial paper prepared in Example 5 released sulfur dioxide gas at a concentration of 42ppm. Compared with the control group 2, the released sulfur dioxide inhibited the rot of fennel and kept the fennel fresh and tender without odor. Under the same conditions, the fennel in the control group 2 rotted by 15%, darkened in color, and had a slight odor.

[0082] The rot of rapeseed mainly starts from the freshly cut roots or the wounds of damaged leaves. The antibacterial paper prepared in Example 6 simultaneously releases 20 ppm of sulfur dioxide and 5 ppm of hypochlorous acid, which inhibits the growth of mold on the cuts or damaged parts of rapeseed, and the proportion of rotten leaves is zero; in the control group 3, the rapeseed rotted 9%.

[0083] The leaves of lettuce are thin and easily mechanically damaged. In the polyethylene fresh-keeping bag of the control group 3, 17% of the lettuce was rotten and had a slight odor. However, the antibacterial paper in Example 8 can release 22ppm of sulfur dioxide and 10ppm of chlorine dioxide at the same time, which effectively inhibits the growth of mold on the lettuce, and no rot occurs, maintaining the bright green color.

[0084] In all control groups, the relative humidity reached more than 99%. Because there was no antibacterial agent, high humidity caused mold growth and rot was inevitable. In the embodiment, the relative humidity in the fresh-keeping bag was stabilized at 84-88%. This is because the film-forming materials polyvinyl alcohol and sodium polyacrylate in the antibacterial paper have the function of absorbing moisture and controlling the relative humidity in the fresh-keeping bag. When the vegetable metabolism (respiration and transpiration) produces more water, causing the humidity in the fresh-keeping bag to exceed 90%, the two water-absorbing polymers in the antibacterial paper will absorb part of the water vapor and store it in the coating. When the humidity in the fresh-keeping bag environment is low (the metabolic rate of vegetables is weakened in the later stage), the polymer material will release the stored water and maintain an equilibrium humidity of 84-88%. In the embodiment, the vegetables did not rot. In addition to the antibacterial effect of the antibacterial agent, the regulating effect of the polymer coating in the antibacterial paper on humidity is also very important.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A multi-layer composite antibacterial paper for preserving agricultural products, characterized in that: The composite antibacterial paper is composed of at least two paper bases coated with functional layers, wherein the functional layers are organic acid / polyvinyl alcohol coatings or antibacterial agent precursors / sodium polyacrylate coatings, and the organic acid / polyvinyl alcohol coatings and the antibacterial agent precursors / sodium polyacrylate coatings are closely attached to form an antibacterial release layer; Among them, by weight, the organic acid / polyvinyl alcohol coating contains 3-18% polyvinyl alcohol, 50-80% organic acid, and the rest is water; the antibacterial agent precursor / sodium polyacrylate coating contains 0.5-4% sodium polyacrylate, 60-80% antibacterial agent precursor, and the rest is water.

2. The multi-layer composite antibacterial paper for preserving agricultural products according to claim 1, characterized in that: The composite antibacterial paper consists of two paper bases coated with functional layers, wherein the upper paper base is coated with an organic acid / polyvinyl alcohol coating; and the lower paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating.

3. The multi-layer composite antibacterial paper for preserving agricultural products according to claim 1, characterized in that: The composite antibacterial paper consists of three paper bases coated with functional layers, wherein the upper and lower paper bases are coated with antibacterial agent precursor / sodium polyacrylate coatings on one side, and the middle paper base is coated with organic acid / polyvinyl alcohol coatings on both sides.

4. The multi-layer composite antibacterial paper for preserving agricultural products according to claim 1, characterized in that: The composite antibacterial paper consists of four paper bases coated with functional layers, wherein the upper paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side, the middle two paper bases are coated with an organic acid / polyvinyl alcohol paper base and an antibacterial agent precursor / sodium polyacrylate coating on both sides in sequence, and the lower paper base is coated with an organic acid / polyvinyl alcohol coating on one side.

5. The multi-layer composite antibacterial paper for preserving agricultural products according to claim 1, characterized in that: The composite antibacterial paper consists of five paper bases coated with functional layers, wherein the upper paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side, the middle three paper bases are coated with an organic acid / polyvinyl alcohol paper base, an antibacterial agent precursor / sodium polyacrylate paper base and an organic acid / polyvinyl alcohol coating on both sides in sequence, and the lower paper base is coated with an antibacterial agent precursor / sodium polyacrylate coating on one side.

6. A method for preparing a multi-layer composite antibacterial paper for preserving agricultural products according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) Preparation of paper substrate coated with organic acid / polyvinyl alcohol Dissolving 4-50 parts of an organic acid in 45-85 parts of a 5% polyvinyl alcohol solution, coating the organic polyvinyl alcohol solution on a paper base on a dry laminating device, and drying in a drying oven at 70-80° C. for 0.5-3 minutes to obtain a paper base coated with an organic acid / polyvinyl alcohol coating; (2) Preparation of paper substrate coated with antimicrobial agent precursor / sodium polyacrylate coating Dissolve 10-50 parts of an antimicrobial agent precursor in 50-90 parts of a 0.9% sodium polyacrylate solution, apply the coating on a paper base on a dry composite device, and dry in a drying oven at 50-60° C. for 1-3 minutes to obtain a paper base coated with an antimicrobial agent precursor / sodium polyacrylate coating; (3) Preparation of multi-layer composite antibacterial paper The coated surfaces of the paper bases obtained by (1) and (2) are laminated together, pressed and punched to obtain a multi-layer composite antibacterial paper.

7. The method for preparing a multi-layer composite antibacterial paper for preserving agricultural products according to claim 6, characterized in that: The organic acid is one of citric acid, succinic acid, tartaric acid, malic acid, and lactic acid; the antibacterial agent precursor is one of sodium bisulfite, sodium sulfite, calcium sulfite, sodium pyrosulfite, sodium hypochlorite, calcium hypochlorite, and sodium chlorate.

8. The method for preparing a multi-layer composite antibacterial paper for preserving agricultural products according to claim 6, characterized in that: In step (1), the thickness of the organic acid / polyvinyl alcohol coating is 3-5 μm, and the average moisture content of the coating is 18-35%, wherein the moisture content of the surface layer is less than 5%, and the moisture content of the middle core layer is 20-40%.

9. The method for preparing a multi-layer composite antibacterial paper for preserving agricultural products according to claim 6, characterized in that: In step (2), the thickness of the antimicrobial agent precursor / sodium polyacrylate coating is 3-5 μm, and the average water content of the coating is 15-35%, wherein the water content of the surface layer is less than 5%, and the water content of the middle core layer is 20-40%.

10. The method for preparing a multi-layer composite antibacterial paper for preserving agricultural products according to claim 6, characterized in that: In step (3), the specific operation of pressing and punching is: using a double-roll pressing temperature of 130-150°C and a pressure of 5-8atm to form a composite structure, and then punching holes with a pair of needle rollers, on which steel needles with a spacing of 2-5mm and a diameter of 0.15-0.20mm are densely distributed.

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