A multilayer security medium frame paper and a method for making the same

Abstract

The present application belongs to the technical field of papermaking, and particularly relates to a multi-layer anti-counterfeiting medium frame paper and a preparation method thereof. The present application aims to solve the problem of poor interlayer bonding force of the current anti-counterfeiting paper and the risk of interface separation. The present application prepares a base paper in three layers through a multi-layer composite process, uses outer layer pulp for the first layer and the third layer, uses inner layer pulp for the second layer, and embeds a security line in the second layer. The inner layer pulp comprises manila hemp pulp, polyvinyl alcohol fiber and low-melting point hot melt fiber; and the outer layer pulp comprises short cotton flock fiber and high-melting point hot melt fiber. The wet base paper prepared through the multi-layer composite process is subjected to press compounding and step drying to obtain the base paper, and is further coated with a composite coating and subjected to hot calendering to obtain the multi-layer anti-counterfeiting medium frame paper. The multi-layer anti-counterfeiting medium frame paper prepared by the present application has good interlayer bonding force, excellent water mark retention effect and good anti-counterfeiting compatibility.

Description

Technical Field

[0001] This invention belongs to the field of papermaking technology, specifically relating to a multi-layer anti-counterfeiting medium frame paper and its preparation method. Background Technology

[0002] Security paper is a special type of paper that initially used density variations, such as watermarks, to ensure its anti-counterfeiting effect. With technological advancements, the industry began incorporating special fibers into the paper pulp to achieve more complex anti-counterfeiting measures, such as fluorescent anti-counterfeiting. However, in many applications, such as paper products or banknotes, single-layer paper often falls short in terms of both anti-counterfeiting effectiveness and physical strength. Therefore, multi-layered composite security paper has emerged.

[0003] In the manufacture of multi-layered security paper, cotton and hemp fibers are commonly used natural biomass conversion materials. For implementing anti-counterfeiting technology, short-fiber cotton linters are beneficial for watermark detail, while long-fiber Manila hemp is beneficial for paper strength and feel. However, when other anti-counterfeiting designs are introduced, such as security threads embedded in the pulp, the bonding strength between these two different materials faces significant challenges. Simultaneously, the weak interlayer interfaces of the multi-layered composite paper are at risk of separation when subjected to bending, crumpling, or washing. This makes multi-layered security paper easily deconstructed and copied in practical applications, potentially causing the anti-counterfeiting performance to fail.

[0004] To address the problem of poor interlayer bonding and interface separation risk in current anti-counterfeiting paper, a multilayer anti-counterfeiting media frame paper and its preparation method are proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a multi-layer anti-counterfeiting media frame paper and its preparation method. This invention uses a multi-layer composite process to prepare the base paper in three layers. The first and third layers use outer pulp, and the second layer uses inner pulp, with the security thread embedded in the second layer. The inner pulp includes Manila hemp pulp, polyvinyl alcohol fiber, and low-melting-point hot-melt fiber; the outer pulp includes short cotton fibers and high-melting-point hot-melt fiber. The wet base paper prepared by the multi-layer composite process is then pressed, laminated, and dried in stages to obtain the base paper. After further coating with a composite coating, it undergoes hot pressing and calendering to obtain the multi-layer anti-counterfeiting media frame paper.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a multi-layer anti-counterfeiting media frame paper includes the following steps: Unless otherwise specified, the parts in this invention refer to parts by mass.

[0007] The outer pulp consists of short cotton linters. These short cotton linters are natural biomass materials derived from secondary short cotton linters, with an average fiber length of 1.2-1.5 mm, a whiteness >88%, and an average solid content of 10 wt%. After catalytic oxidation treatment, the outer pulp is supplemented with high-melting-point hot-melt fibers for later use.

[0008] Specifically, the catalytic oxidation process is as follows: Add 1 part of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 10 parts of sodium bromide to 100 parts of outer layer pulp, adjust the pH to the range of 10-10.5, and add sodium hypochlorite aqueous solution dropwise at 25°C, wherein the amount of sodium hypochlorite added is 1.3 mmol / g dry pulp. After the addition is complete, add 5 parts of ethanol, wash with deionized water until the pulp is neutral, and readjust the pulp to an average solid content of 10 wt%. The sodium hypochlorite aqueous solution is added dropwise over 90 min, and the oxidation potential is controlled between 300-500 mV during the addition process.

[0009] Add 0.5-1.0 parts of high-melting-point hot-melt fiber to 100 parts of catalytically oxidized outer pulp. The high-melting-point hot-melt fiber is specifically PET copolymerized with IPA, with a specification of 2.0 dtex, an average length of 3-5 mm, and an average melting point of 150-160℃.

[0010] The inner pulp, including Manila hemp pulp, is a natural biomass material with an average fiber length of 5-7 mm, a Canadian standard free fiber content (CSF) of 500-600 mL, and an average solids content of 10 wt%. Polyvinyl alcohol fiber, low-melting-point hot-melt fiber, and cationic chitosan are added to the inner pulp for later use.

[0011] Specifically, 5-8 parts of polyvinyl alcohol fiber are added to 100 parts of Manila hemp pulp (dry weight). The polyvinyl alcohol fiber has a specification of 2.0 dtex and an average length of 5 mm. Subsequently, 0.8-1.5 parts of low-melting-point hot-melt fiber and 1 part of cationic chitosan are added. The low-melting-point hot-melt fiber is specifically 4080 hot-melt yarn with an average melting point of 110-120℃ in the sheath, a specification of 4 dtex, and an average length of 3-5 mm after chopping. The cationic chitosan is specifically water-soluble chitosan with a degree of deacetylation >90% and an average molecular weight of 150-200 kDa.

[0012] The base paper is prepared in three layers using a multi-layer composite process on a cylinder paper machine. Specifically, the first layer uses outer pulp, with a target dry weight of 25-30 g / m³. 2 The second layer uses inner pulp, with a target dry weight of 40-45 g / m³. 2 The third layer uses outer pulp, with a target dry weight of 25-30 g / m³. 2In the second-layer rotary screen forming area, a safety line is embedded in the second layer. The equipment operates at a speed of 100 m / min, and the pulp concentration is 0.4%.

[0013] Three layers of wet paper are pressed and laminated to obtain wet base paper. The specific operation of the press lamination is as follows: two pressing zones are used, with linear pressures of 30kN / m and 60kN / m respectively, and the outlet dryness is controlled at 45-50%.

[0014] The wet base paper is dried in stages to obtain the base paper. The specific steps of the drying process are as follows: the entire drying operation is carried out at a speed of 100 m / min. The first stage is a low-temperature drying stage: the wet base paper is dried for 30 seconds at a temperature of 115-125℃ and an operating pressure of 2.0 bar, followed by a surface curing stage. The second stage is a surface curing stage: the paper is treated at a temperature of 155-165℃ and an operating pressure of 4.5 bar for 50 seconds. The moisture content of the final dried base paper is controlled to be less than 6%.

[0015] After coating the surface of the base paper with a composite coating, it undergoes hot pressing and calendering to obtain a multi-layer anti-counterfeiting media frame paper. The coating amount of the composite coating is 2g / m² on one side. 2 .

[0016] The composite coating comprises: 60 parts waterborne polyurethane, 25 parts nanocellulose, 12 parts nanosilica, and 3 parts crosslinking agent. Specifically, the waterborne polyurethane is an aliphatic polyether polyurethane dispersion with a number-average molecular weight of 8000-12000 g / mol, acrylate end groups, carboxyl side chains, and an acid value of 25-30 mg KOH / g; the nanocellulose has an average length of 1-2 μm and an average diameter of 8-10 nm; the nanosilica has an average particle size of 12-15 nm, a refractive index of 1.46, and is surface-treated with trimethylsilane, with an average carbon content of 3.0 wt%; the crosslinking agent is hexamethoxymethylmelamine. Both the waterborne polyurethane and nanocellulose are mixed in aqueous dispersion form, with the mass parts being dry weight. The viscosity of the composite coating used for application is controlled at 50-100 mPa·s.

[0017] The hot-pressing and calendering process specifically involves subjecting the base paper coated with the composite coating to hot-pressing and calendering at 170℃ with a linear pressure of 100-150 kN / m and a linear speed of 100 m / min.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The outer pulp derived from short cotton linters undergoes catalytic oxidation treatment, oxidizing some of the hydroxyl groups on the cellulose surface to carboxyl groups. After this specific catalytic oxidation treatment, the negative charge on the surface of the shorter cotton fibers creates electrostatic repulsion between the fiber bundles in an aqueous environment, causing the fiber bundles to expand and increase their flexibility. This allows them to better support the embossing process of the watermark screen during subsequent forming, resulting in higher contrast and finer detail in the watermark on the multi-layer anti-counterfeiting media frame paper. Simultaneously, the negative charge in the outer pulp generates stronger ionic interactions when it comes into contact with the inner pulp, ensuring good interlayer bonding.

[0019] The inner layer pulp is based on Manila hemp pulp, with a specific amount of polyvinyl alcohol fiber introduced to effectively ensure the good mechanical strength of the multi-layer anti-counterfeiting media frame paper. High-melting-point hot-melt fibers are introduced into the outer layer pulp, while low-melting-point hot-melt fibers and cationic chitosan are introduced into the inner layer pulp. In the subsequent step-by-step drying process, during the low-temperature drying stage, the low-melting-point hot-melt fibers begin to melt, forming partial fusion between the inner and outer layers. The cationic chitosan also begins to dehydrate and solidify, with the cationic groups forming a more effective bond with the negatively charged outer layer. At the same time, the molten low-melting-point hot-melt fibers and solidified chitosan effectively fix the embedded security thread to the inner layer of the paper. During the surface curing stage, the high-melting-point hot-melt fibers melt, redistribute on the outer layer of the paper, and complete curing, effectively enhancing the tear resistance of the multi-layer anti-counterfeiting media frame paper.

[0020] Using a multi-layer composite process, outer pulp is used in the first and third layers of the cylinder paper machine, while inner pulp is used in the second layer, forming an outer layer of paper mainly composed of short fibers and an inner layer with good toughness. The three layers of wet paper are then pressed and laminated to form a three-layer physical composite structure, laying the foundation for the surface load-bearing capacity and mechanical strength of the multi-layer anti-counterfeiting media frame paper. Simultaneously, the multi-layer composite process is well-compatible with the security thread embedding process, allowing the security thread to be embedded in the inner layer of the paper and effectively bonded to the inner layer through subsequent step-by-step drying processes, thus improving the paper's anti-counterfeiting compatibility.

[0021] After the dried base paper is coated with a composite coating, it undergoes a hot-pressing and calendering process. The composite coating forms a dense film on the surface of the base paper, improving the waterproof and stain-resistant properties of the multi-layer anti-counterfeiting media frame paper. The hot-pressing and calendering process not only firmly presses this dense film onto the base paper, but also partially melts the high-melting-point hot-melt fibers in the outer layer of the paper, achieving flatness and complete curing of the multi-layer anti-counterfeiting media frame paper and improving the interlayer bonding strength. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the preparation process of the multi-layer anti-counterfeiting media frame paper in this invention. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below through some embodiments and experimental examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] Reference Figure 1 The process flow diagram shown illustrates that this invention provides a multi-layer anti-counterfeiting media frame paper and its preparation method. The technical solution is as follows: Example 1

[0025] Add 1 part of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 10 parts of sodium bromide to 100 parts of outer layer pulp, adjust the pH to 10-10.5, and add sodium hypochlorite aqueous solution dropwise at 25°C. The amount of sodium hypochlorite added is 1.3 mmol / g dry pulp. After the addition is complete, add 5 parts of ethanol, wash with deionized water until the pulp is neutral, and readjust the pulp to an average solid content of 10 wt%. The sodium hypochlorite aqueous solution is added dropwise over 90 min, and the oxidation potential is controlled between 300-500 mV during the addition.

[0026] Add 0.5 parts of high-melting-point hot-melt fiber to 100 parts of dry weight outer pulp that has undergone catalytic oxidation treatment.

[0027] Add 5 parts of polyvinyl alcohol fiber to 100 parts of Manila hemp pulp by dry weight, then add 0.8 parts of low melting point hot melt fiber and 1 part of cationic chitosan.

[0028] The base paper is prepared in three layers using a multi-layer composite process on a cylinder paper machine. Specifically, the first layer uses outer pulp, with a target dry weight of 25-30 g / m³. 2 The second layer uses inner pulp, with a target dry weight of 40-45 g / m³. 2 The third layer uses outer pulp, with a target dry weight of 25-30 g / m³. 2 In the second-layer rotary screen forming area, a safety line is embedded in the second layer. The equipment operates at a speed of 100 m / min, and the pulp concentration is 0.4%.

[0029] Three layers of wet paper are pressed and laminated to obtain wet base paper. The specific operation of the press lamination is as follows: two pressing zones are used, with linear pressures of 30kN / m and 60kN / m respectively, and the outlet dryness is controlled at 45-50%.

[0030] The wet base paper is dried in stages to obtain the base paper. The specific operation of the staged drying is as follows: the speed of the entire staged drying operation is 100 m / min. Among them, the low temperature drying stage: the wet base paper is dried for 30 seconds at a temperature of 115℃ and an operating pressure of 2.0 bar, followed by the surface curing stage; the surface curing stage: the paper is treated at a temperature of 155℃ and an operating pressure of 4.5 bar for 50 seconds.

[0031] After coating the surface of the base paper with a composite coating, it undergoes hot pressing and calendering to obtain a multi-layer anti-counterfeiting media frame paper. The coating amount of the composite coating is 2g / m² on one side. 2 The composite coating consists of 60 parts waterborne polyurethane, 25 parts nanocellulose, 12 parts nanosilica, and 3 parts crosslinking agent. The hot-pressing and calendering process involves subjecting the coated base paper to hot-pressing and calendering at 170℃ with a linear pressure of 100 kN / m and a linear speed of 100 m / min.

[0032] Examples 2-16 differ from Example 1 in operating parameters, but the other process steps and the range of raw material selection are the same.

[0033] The specific changes in operating parameters are summarized in Table 1.

[0034] Table 1. Changes in operating parameters in Examples 1-16

[0035] Comparative Example 1 Unlike Example 1, no catalytic oxidation treatment was performed, but all other process parameters remained the same.

[0036] Comparative Example 2 Unlike Example 1, no cationic chitosan was added to the inner pulp, but all other process parameters were the same.

[0037] Comparative Example 3 Unlike Example 5, instead of step-by-step drying, a single-stage drying process of 60 seconds was performed using a temperature of 160°C and an operating pressure of 3.0 bar, while all other process parameters remained the same.

[0038] Comparative Example 4 Unlike Example 5, high-melting-point hot-melt fibers were used instead of low-melting-point hot-melt fibers, while all other process parameters remained the same.

[0039] Comparative Example 5 Unlike Example 5, no polyvinyl alcohol fiber was added, but all other process parameters remained the same.

[0040] Comparative Example 6 Unlike Example 9, the Manila hemp pulp in the inner pulp layer was replaced with short cotton linters, while all other process parameters remained the same.

[0041] Comparative Example 7 Unlike Example 9, the short cotton fibers in the outer pulp were replaced with Manila hemp pulp, while all other process parameters remained the same.

[0042] Comparative Example 8 Unlike Example 13, no hot pressing and calendering treatment was performed; the composite coating was simply heat-cured at 170°C, while all other process parameters remained the same.

[0043] Comparative Example 9 Unlike Example 13, the temperature of the hot pressing and calendering process was adjusted to 150°C, while all other process parameters remained the same.

[0044] Comparative Example 10 Unlike Example 13, no composite coating was applied, but all other process parameters remained the same.

[0045] Experimental Example 1 The optical opacity and interlayer adhesion of the watermarks printed on the anti-counterfeiting frame paper products prepared in Examples 1-4 and Comparative Examples 1-2 were tested, and the relevant results are summarized in Table 2.

[0046] The optical opacity test method for printed watermarks refers to the relevant test methods in GB / T 1543 standard. The opacity (%) of each sample after watermark printing is tested and recorded. For anti-counterfeiting frame paper, the higher the opacity, the better its anti-counterfeiting performance after watermark loading.

[0047] The interlaminar bond strength test method refers to the relevant test method in GB / T 26203 standard. The interlaminar bond strength (J / m) of each specimen is tested and recorded. 2 ).

[0048] Table 2. Optical opacity and interlayer adhesion of the printed watermarks on the anti-counterfeiting frame papers prepared in Examples 1-4 and Comparative Examples 1-2.

[0049] As shown in Table 2, Examples 1-4 are significantly better than Comparative Examples 1 and 2 in terms of optical opacity and interlayer adhesion of the printed watermark, indicating that Examples 1-4 have obvious advantages in improving the watermark load-bearing capacity and paper structural strength.

[0050] Comparative Example 1, without catalytic oxidation treatment, had unmodified short cotton fibers, resulting in insufficient fiber softness and reduced load-bearing capacity for watermark embossing during subsequent molding, thus exhibiting the lowest optical opacity. Simultaneously, due to the lack of negative charge in the outer pulp layer, its interfacial bonding with the inner pulp layer was also the worst. Comparative Example 2, without the addition of cationic chitosan, lacked the key component for ion interaction. Although its outer layer underwent oxidation treatment to ensure good optical opacity, its interlayer bonding was significantly lower than that of the examples, further demonstrating the crucial role of cationic chitosan in interlayer bonding in the complete scheme of the examples.

[0051] In summary, this invention performs a specific catalytic oxidation treatment on the outer layer pulp of short cotton fibers, partially oxidizing the hydroxyl groups on the surface of cellulose to carboxyl groups. This causes the negative charge on the surface to expand the fiber bundles, increasing their softness and significantly improving the watermark contrast and layering of the multi-layer anti-counterfeiting media frame paper. Furthermore, these negative charges generate stronger ionic interactions with the cationic chitosan subsequently introduced into the inner layer pulp, synergistically ensuring excellent interlayer bonding.

[0052] Experimental Example 2 The anchoring strength of the security thread and the longitudinal folding endurance of the anti-counterfeiting frame paper products prepared in Examples 5-8 and Comparative Examples 3-5 were tested, and the relevant results are summarized in Table 3.

[0053] The test method for the anchorage strength of the safety line is as follows: referring to the tension method of GB / T 453 standard, the safety line is subjected to constant-speed loading tension, and the minimum tensile force (N) pulled out of the safety line is recorded. The smaller the minimum tensile force, the worse the anchorage strength of the safety line.

[0054] The test method for longitudinal folding endurance is as follows: refer to the test method of GB / T 457 standard, test the number of folds the specimen can withstand, and record the maximum number of folds the specimen can withstand while maintaining its original shape.

[0055] Table 3. Anchoring strength and longitudinal folding endurance of the anti-counterfeiting frame paper products prepared in Examples 5-8 and Comparative Examples 3-5.

[0056] As shown in Table 3, Examples 5-8 exhibit significant advantages in both anchoring strength and longitudinal flexural strength of the safety line.

[0057] Comparative Example 5, without the addition of polyvinyl alcohol fiber, exhibited a significantly lower maximum folding endurance than the Example, indicating that polyvinyl alcohol fiber is a key component ensuring the good mechanical strength of the multi-layer anti-counterfeiting media frame paper. Comparative Example 3, by replacing step-by-step drying with single-stage high-temperature treatment, resulted in a substantial decrease in the minimum tensile strength of the security thread. Comparative Example 4, which used high-melting-point hot-melt fiber instead of low-melting-point hot-melt fiber in the inner layer, showed the worst anchoring strength. This demonstrates the crucial synergistic effect of the low-melting-point hot-melt fiber and the step-by-step drying process in the original formula.

[0058] In summary, this invention introduces a specific amount of polyvinyl alcohol fiber, which, together with Manila hemp pulp, ensures the product's basic mechanical strength. Simultaneously, the low-melting-point hot-melt fibers and cationic chitosan in the inner pulp layer, along with a specific stepwise drying process, produce a significant synergistic effect: the low-temperature drying stage precisely activates the low-melting-point hot-melt fibers, causing them to melt; at the same time, the cationic chitosan begins to dehydrate and solidify. This synergistic effect of the two substances effectively fixes the embedded security thread to the inner layer of the paper. The subsequent surface curing stage activates the high-melting-point hot-melt fibers in the outer layer, enhancing the tear resistance of the multi-layered anti-counterfeiting media frame paper. This precise matching of materials and processes achieves a balance between high mechanical strength and high anchoring force.

[0059] Experimental Example 3 The watermark opacity and tear resistance of the anti-counterfeiting frame paper products prepared in Examples 9-12 and Comparative Examples 6-7 were tested. The relevant results are summarized in Table 4.

[0060] For the test method of optical opacity of printed watermarks, refer to Experimental Example 1.

[0061] The test method for tear resistance is as follows: refer to the relevant test method of GB / T 455 standard, measure and record the maximum tear strength (gF) of the specimen. The higher the maximum tear strength, the better its toughness.

[0062] Table 4. Watermark opacity and tear resistance of the anti-counterfeiting frame papers prepared in Examples 9-12 and Comparative Examples 6-7

[0063] As shown in Table 4, Examples 9-12 performed excellently in both watermark opacity and tear resistance, significantly outperforming Comparative Examples 6 and 7.

[0064] Comparative Example 6, which replaced the Manila hemp pulp in the inner layer with short cotton fibers, showed a significantly lower maximum tear strength than the Example, indicating that the inner layer Manila hemp pulp is a key component ensuring good paper toughness. Comparative Example 7, which replaced the short cotton fibers in the outer layer with Manila hemp pulp, resulted in a substantial decrease in opacity, further demonstrating that using short-fiber pulp in the outer layer is essential for achieving excellent surface load-bearing properties.

[0065] In summary, this invention employs a multi-layer composite process, using outer pulp in the first and third layers and inner pulp in the second layer. This creates a significant synergistic effect, forming an outer paper layer primarily composed of short fibers and an inner paper layer with good toughness. This structural design lays the foundation for the surface load-bearing capacity and mechanical strength of the multi-layer anti-counterfeiting media frame paper. Simultaneously, this process is well-compatible with security thread insertion, improving the paper's anti-counterfeiting compatibility.

[0066] Experiment Example 4 The water absorption and interlayer bonding strength of the anti-counterfeiting frame paper products prepared in Examples 13-16 and Comparative Examples 8-10 were tested, and the relevant results are summarized in Table 5.

[0067] For the test method of interlayer bonding strength, refer to Experiment Example 1.

[0068] The water absorption test method refers to the relevant test method of GB / T 1540 standard, and the water absorption rate of the sample (g / m³) is recorded. 2 (60s) The lower the water absorption rate, the better the durability in practical applications.

[0069] Table 5. Water absorption and interlayer bonding strength of the anti-counterfeiting frame paper products prepared in Examples 13-16 and Comparative Examples 8-10

[0070] As shown in Table 5, Examples 13-16 exhibited significant advantages in both water absorption rate and interlayer bonding strength.

[0071] Comparative Example 10, without the composite coating, had a much higher water absorption rate than the Example, indicating that the dense film formed by the composite coating on the surface of the base paper is key to improving the waterproof and stain-resistant properties of the multi-layer anti-counterfeiting media frame paper. Comparative Example 8, without thermo-calendering, and Comparative Example 9, with insufficient thermo-calendering temperature, both had interlayer bonding forces much lower than the Example, further demonstrating that a specific thermo-calendering process is crucial for achieving the performance of the multi-layer anti-counterfeiting media frame paper.

[0072] In summary, this invention coats the dried base paper with a composite coating and then performs a hot-pressing and calendering process. These two processes produce a significant synergistic effect: the composite coating provides the foundation for waterproof and stain-resistant properties; while the specific high-temperature hot-pressing and calendering process not only firmly bonds this dense film to the base paper but also partially melts the high-melting-point hot-melt fibers in the outer layer of the paper. This synergistic effect achieves flatness and complete curing of the multi-layer anti-counterfeiting media frame paper, significantly improving the interlayer bonding strength.

[0073] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a multi-layer anti-counterfeiting media frame paper, characterized in that: The preparation method is as follows: After the outer layer pulp is subjected to catalytic oxidation treatment, high melting point hot melt fiber is added; wherein, the outer layer pulp includes short cotton fibers; the catalytic oxidation treatment specifically involves: adding 2,2,6,6-tetramethylpiperidine-1-oxy free radical and sodium bromide to the outer layer pulp, then adding sodium hypochlorite aqueous solution dropwise, adding ethanol after the dropwise addition is completed, and then washing and re-adjusting the pulp; Polyvinyl alcohol fiber, low-melting-point hot-melt fiber, and cationic chitosan are added to the inner layer pulp; wherein, the inner layer pulp includes Manila hemp pulp; the average melting point range of the high-melting-point hot-melt fiber is 150-160℃; and the average melting point range of the sheath of the low-melting-point hot-melt fiber is 110-120℃. Using a multi-layer composite process, the first layer uses the outer pulp, the second layer uses the inner pulp, and the third layer uses the outer pulp to obtain three layers of wet paper sheets; the three layers of wet paper sheets are then pressed and laminated to obtain wet base paper; The wet base paper is dried in stages to obtain the base paper; the press-compression process involves two pressing zones; the staged drying specifically includes: a low-temperature drying stage, in which the wet base paper is dried at a temperature of 115-125℃; and a surface curing stage, in which the paper is treated at a temperature of 155-165℃. The base paper is coated with a composite coating and then subjected to hot pressing and calendering to obtain the multi-layer anti-counterfeiting media frame paper. The hot-pressing and calendering process specifically involves subjecting the base paper coated with the composite coating to a linear pressure of 100-150 kN / m at 170°C. Hot pressing and light treatment are performed at a linear speed of 100 m / min; The base paper is prepared in three layers using a multi-layer composite process on a cylinder paper machine; specifically, the first layer uses the outer pulp, with a target dry weight of 25-30 g / m³. 2 The second layer uses the inner layer pulp, with a target dry weight of 40-45 g / m³. 2 The third layer uses the aforementioned outer pulp, with a target dry weight of 25-30 g / m³. 2 In the second layer of the circular mesh forming area, the safety line is embedded in the second layer.

2. The method for preparing a multi-layer anti-counterfeiting media frame paper according to claim 1, characterized in that: The composite coating comprises: waterborne polyurethane, nanocellulose, nanosilica, and a crosslinking agent; wherein the crosslinking agent is specifically hexamethoxymethylmelamine.

3. A multi-layer anti-counterfeiting media frame paper, characterized in that: The multi-layer anti-counterfeiting medium frame paper is prepared by the preparation method according to any one of claims 1-2; the multi-layer anti-counterfeiting medium frame paper includes a first layer, a second layer and a third layer, wherein the first layer and the third layer are prepared using outer pulp, and the second layer is prepared using inner pulp.

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