High-barrier-property food paperboard with multi-layer gradient densified structure and preparation method of high-barrier-property food paperboard

Abstract

The invention discloses a high-barrier-property food paperboard with a multi-layer gradient densified structure and a preparation method of the high-barrier-property food paperboard. The high-barrier-property food paperboard with the multi-layer gradient densified structure comprises a substrate, a BMFC / CMC / TEOS hydrophobic oil-proof supporting layer and a BCNF / PDMS hydrophobic oil-proof protective layer, wherein the substrate is composed of a bottom layer, a core layer and a surface layer, the surface of the substrate is coated with the BMFC / CMC / TEOS hydrophobic oil-proof supporting layer, and the surface of the BMFC / CMC / TEOS hydrophobic oil-proof supporting layer is coated with the BCNF / PDMS hydrophobic oil-proof protective layer. The food paperboard with the step-by-step gradient densification structure prepared by the preparation method disclosed by the invention is beneficial to improving interlayer interface bonding, dispersing structural stress and endowing the food paperboard with better strength and fatigue resistance, so that excellent hydrophobic, oil-proof, high-barrier and anti-permeation functions are kept under a high-temperature condition; the excellent mechanical property and good hydrophobic and oil-proof characteristics are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of food cardboard, and particularly relates to a high-barrier food cardboard with a multi-layer gradient densification structure and a preparation method thereof. Background Art

[0002] Food cardboard has become an important choice for current consumers and merchants. As a raw material for preparing food containers such as noodle buckets, paper cups, and lunch boxes, it is required that food cardboard has excellent barrier properties and mechanical properties, and can effectively play the characteristics of hydrophobic and oil-proof, anti-permeation, and anti-collapse.

[0003] Currently, the commonly used method is to form a composite structure of a base paper layer and a fluorine-free functional coating to achieve high barrier properties. This method often requires cumbersome processes and also needs to face problems such as coating peeling, structure collapse, and liquid penetration when heated.

[0004] The patent with publication number CN116516725B discloses a non-laminated fluorine-free waterproof and oil-proof food packaging functional paper, which uses softwood pulp, hardwood pulp, and carboxylated nanocrystalline cellulose as raw materials for beating treatment, adds polylactic acid fiber, chitosan fiber, and nano-silica for papermaking and forming, coats the coating sizing liquid on the surface of the packaging paper, and then obtains the non-laminated fluorine-free waterproof and oil-proof food packaging functional paper after drying and calendering. The preparation process of this food packaging functional paper requires steps such as applying dual-frequency ultrasonic treatment, centrifugation, and ethanol ultrasonic cleaning to achieve, and its complex process flow and lack of mechanical properties such as tensile strength greatly hinder its practical application.

[0005] Barrier properties and mechanical properties are important indicators for measuring food cardboard and have a very important impact when using food cardboard. Factors such as complex preparation processes, excessive use of chemical additive raw materials, low barrier properties, and poor mechanical properties limit the further development of food cardboard. Therefore, how to design a food cardboard with environmental protection, economy, high barrier properties, and excellent mechanical properties is still a difficult technical challenge. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-barrier food cardboard with a multi-layer gradient densification structure and a preparation method thereof to solve the technical problem of the complex preparation process of existing food cardboard.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A high-barrier food cardboard with a multi-layer gradient densification structure, comprising:

[0009] A substrate composed of a bottom layer, a core layer, and a surface layer;

[0010] The BMFC / CMC / TEOS hydrophobic and oleophobic support layer coated on the surface of the substrate; the BMFC / CMC / TEOS hydrophobic and oleophobic support layer is prepared by mixing bamboo pulp microfibrils BMFC, carboxymethyl cellulose CMC and tetraethoxysilane TEOS in a ratio of 5-10:10-20:1-3 by weight;

[0011] The BCNF / PDMS hydrophobic and oleophobic protective layer coated on the surface of the BMFC / CMC / TEOS hydrophobic and oleophobic support layer; the BCNF / PDMS hydrophobic and oleophobic protective layer is prepared by mixing bamboo pulp nanocellulose BCNF and polydimethylsiloxane PDMS in a ratio of 20-30:3-5 by weight.

[0012] As a preferred embodiment of the present invention, the core layer includes mechanical pulp and screened long fibers, and is strengthened by maleic anhydride acylated chitosan enhancer MAAC, and the surface layer is sprayed by Pickering emulsion AKD sizing technology.

[0013] As a preferred embodiment of the present invention, the mass ratio of the substrate to the BMFC / CMC / TEOS hydrophobic and oleophobic support layer is 1:0.3-0.5, and the mass ratio of the substrate to the BCNF / PDMS hydrophobic and oleophobic protective layer is 1:0.3-0.5.

[0014] As a preferred embodiment of the present invention, in the BMFC / CMC / TEOS hydrophobic and oleophobic support layer, TEOS undergoes thermal crosslinking with cellulose when heated to form a densified structure.

[0015] As a preferred embodiment of the present invention, in the BCNF / PDMS hydrophobic and oleophobic protective layer, BCNF binds to the support layer through hydrogen bonds, and PDMS forms a dense polymer film.

[0016] The present invention also provides a method for preparing a high-barrier food cardboard with a multi-layer gradient densified structure, including the following steps:

[0017] Step 1: Mechanically stir BMFC, CMC and TEOS for 30-60 minutes in a ratio of 5-10:10-20:1-3 by weight to prepare a coating sizing solution, and coat it on the surface of the substrate to form a BMFC / CMC / TEOS hydrophobic and oleophobic support layer;

[0018] Step 2: Dry and calender the base paper treated in Step 1;

[0019] Step 3: Mechanically stir BCNF and PDMS for 30-60 minutes in a ratio of 20-30:3-5 by weight to prepare a coating sizing solution, and coat it on the surface of the base paper obtained in Step 2 to form a BCNF / PDMS hydrophobic and oleophobic protective layer;

[0020] Step 4: Dry and calender the base paper processed in Step 3 to obtain a high-barrier food card paper.

[0021] As a preferred embodiment of the preparation method of the present invention, during the preparation of the core layer, esterification reaction and Schiff base structure cross-linking are carried out through maleic anhydride acylated chitosan enhancer MAAC to improve the paper strength.

[0022] As a preferred embodiment of the preparation method of the present invention, the surface layer of the base uses Pickering emulsified AKD sizing technology, adds micro-nano fibers by spraying, and controls the compactness of the base paper in the shoe press and soft calendering processes.

[0023] As a preferred embodiment of the preparation method of the present invention, in Steps 1 and 3, the coating process uses layer-by-layer self-assembly sizing, so that the BMFC / CMC / TEOS hydrophobic and oil-proof support layer and the BCNF / PDMS hydrophobic and oil-proof protection layer form a gradually gradient compact structure.

[0024] As a preferred embodiment of the preparation method of the present invention, the temperature of the dry calendering process is 80 - 120 °C, and the pressure is 5 - 10 MPa.

[0025] Compared with the prior art, a high-barrier food card paper with a multi-layer gradient compact structure and its preparation method provided by the present invention have the following beneficial effects:

[0026] The present invention uses layer-by-layer self-assembly sizing / coating to construct a three-layer compact structure of a hydrophobic and oil-proof protection layer - a hydrophobic and oil-proof support layer - a base paper. Among them, in the core layer structure of the base paper, mechanical pulp and screened long fibers with high strength and stiffness are used to enhance the paper stiffness. At the same time, the addition of MAAC is used to improve the paper strength; the Pickering emulsified AKD sizing technology and the surface layer spraying method are used to add micro-nano fibers, and the micro-nano fibers are closely intertwined to play a good filling role in the pore structure, thereby forming a compact micro-structure on the surface of the base paper. With the help of the shoe press and soft calendering technology in the paper making process, the overall structure of the base paper is controlled to enhance the mechanical strength; at the same time, the fiber bonding strength, the compactness of the base paper structure and the surface smoothness are significantly increased, the structural difference from the hydrophobic support layer is reduced, and the interlayer bonding is further enhanced.

[0027] For the BMFC / CMC / TEOS formed hydrophobic and oil-proof support layer, since BMFC and CMC are both cellulose substances with fibers and microfibers, with the same or similar structures, they are easy to form a firm bond. At the same time, TEOS will undergo thermal cross-linking with fibers during heating, improving the coating compactness and thermal stability. In addition, while serving as a hydrophobic functional layer, it is also an intermediate transition layer.

[0028] The BCNF and PDMS in the outermost hydrophobic and oil-repellent protective layer are both polymeric structures with excellent film-forming properties and a dense coating structure, forming a gradual gradient with the support layer and base paper structure. BCNF also forms numerous hydrogen bonds with cellulose, ensuring interlayer bonding strength between the protective layer and the support layer. Compared to conventional structures, this gradually densified structure improves interlayer bonding, disperses structural stress, and imparts greater strength and fatigue resistance to food paperboard, resulting in excellent hydrophobic and oil-repellent properties, high barrier properties, and anti-permeation properties even under high-temperature conditions.

[0029] The present invention adopts a multi-layer gradient densification structure strategy to propose a new idea for the preparation of high-barrier food cardboard. The high-barrier food cardboard with a multi-layer gradient densification structure has excellent mechanical properties and good hydrophobic and oil-proof properties, and has broad application prospects in food packaging and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only examples of the embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 This is a model diagram of the base paper-support layer-protective layer multi-layer gradient densification anti-seepage structure in an embodiment of the present invention;

[0032] Figure 2 SEM images of the surface of base paper and the surface of composite paper coated with BMFC / CMC / TEOS and BCNF / PDMS sizing solutions in the embodiment of the present invention;

[0033] Figure 3 This is a waterproof test diagram of the base paper surface and the composite paper after coating with BMFC / CMC / TEOS and BCNF / PDMS sizing solutions in an embodiment of the present invention;

[0034] Figure 4 This is a diagram showing the oil-proof test of the base paper surface and the composite paper after coating with BMFC / CMC / TEOS and BCNF / PDMS sizing solutions in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] In the description of the embodiments of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "upper", "lower", "front", "rear", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings. These are only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the embodiments of the present invention.

[0037] In the description of the embodiments of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, an integral connection, or a detachable connection; it can be the communication inside two elements; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present invention can be understood according to specific circumstances.

[0038] See the attached Figure 1 As shown, a high-barrier food cardboard with a multi-layer gradient densification structure in the embodiments of the present invention includes: a substrate composed of a bottom layer, a core layer, and a surface layer, a BMFC / CMC / TEOS hydrophobic and oil-proof support layer coated on the surface of the substrate, and a BCNF / PDMS hydrophobic and oil-proof protective layer coated on the surface of the BMFC / CMC / TEOS hydrophobic and oil-proof support layer.

[0039] The BMFC / CMC / TEOS hydrophobic and oil-proof support layer is prepared by mixing bamboo pulp microfibrils BMFC, carboxymethyl cellulose CMC, and tetraethoxysilane TEOS in a weight ratio of 5-10:10-20:1-3.

[0040] The BCNF / PDMS hydrophobic and oil-proof protective layer is prepared by mixing bamboo pulp nanocellulose BCNF and polydimethylsiloxane PDMS in a weight ratio of 20-30:3-5.

[0041] Through the gradient densification structure design of the substrate and the two functional coatings, the above technical solution significantly improves the barrier properties (oil-proof, water-proof, anti-permeation) and mechanical strength (tensile index ≥ 63 N·m / g, burst resistance index ≥ 5.0 KPa·m² / g) of the food cardboard, while avoiding the problems of coating peeling and thermal stability in the traditional composite structure.

[0042] In the embodiments of the present invention, the core layer of the base includes mechanical pulp and screened long fibers, and is enhanced by a maleic anhydride acylated chitosan enhancer MAAC. The surface layer is subjected to a spraying process using Pickering emulsified AKD sizing technology. The esterification reaction and Schiff base crosslinking of the screened long fibers and MAAC in the core layer can significantly improve the stiffness and tear resistance of the paper (tear index ≥ 12.9 mN·m² / g). The Pickering emulsified AKD sizing technology on the surface layer fills the pores with micro-nano fibers to form a densified surface structure, further enhancing the barrier property.

[0043] The mass ratio of the base to the BMFC / CMC / TEOS hydrophobic and oil-proof support layer is 1:0.3 - 0.5, and the mass ratio of the base to the BCNF / PDMS hydrophobic and oil-proof protective layer is 1:0.3 - 0.5. By precisely controlling the mass ratio of the coating to the base paper, it is ensured that the functional coating evenly covers and tightly binds to the base, avoiding performance fluctuations caused by local over-thickness or under-thickness, making the water contact angle stable ≥ 132.7°, and the oil-proof grade reaching level 8 of the TAPPI 559 pm-96 standard.

[0044] In the BMFC / CMC / TEOS hydrophobic and oil-proof support layer, TEOS undergoes thermal crosslinking with cellulose when heated to form a densified structure. The thermal crosslinking effect of TEOS significantly improves the heat resistance stability of the support layer (no shedding at high temperatures), and reduces the risk of liquid penetration by narrowing the fiber gaps through the crosslinking network (the oil-proof grade is increased to level 8, compared with only level 5 in Comparative Example 1 without adding TEOS).

[0045] In the BCNF / PDMS hydrophobic and oil-proof protective layer, BCNF binds to the support layer through hydrogen bonds, and PDMS forms a dense polymer film. The hydrogen bond binding between BCNF and the support layer fibers enhances the interlayer adhesion, and the PDMS polymer film provides a super-hydrophobic surface (water contact angle ≥ 152.3°). At the same time, the dense structure of the protective layer effectively blocks the penetration of grease (oil-proof grade 8, compared with only level 6 in Comparative Example 2 without adding PDMS).

[0046] The present invention also provides a preparation method of a high-barrier food card paper with a multi-layer gradient densified structure, including the following steps:

[0047] Step 1: Mechanically stir BMFC, CMC, and TEOS in a weight ratio of 5 - 10:10 - 20:1 - 3 for 30 - 60 minutes to prepare a coating sizing solution, and coat it on the surface of the base to form a BMFC / CMC / TEOS hydrophobic and oil-proof support layer;

[0048] Step 2: Dry and calender the base paper treated in Step 1;

[0049] Step 3: Mechanically stir BCNF and PDMS in a weight ratio of 20 - 30:3 - 5 for 30 - 60 minutes to prepare a coating sizing solution, and coat it on the surface of the base paper obtained in Step 2 to form a BCNF / PDMS hydrophobic and oleophobic protective layer;

[0050] Step 4: Dry and calender the base paper processed in Step 3 to obtain a high-barrier food cardstock.

[0051] The preparation method of the high-barrier food cardstock with a multi-layer gradient densification structure simplifies the production process (without complex ultrasonic or centrifugal treatment) through the step-by-step coating and dry calendering processes, realizes the precise construction of the multi-layer gradient structure, and at the same time ensures the coating uniformity and performance consistency (the basis weight in Examples 1 - 3 is stably 60 - 61 g / m²).

[0052] During the preparation of the core layer of the base, esterification reaction and Schiff base structure cross-linking are carried out through maleic anhydride acylated chitosan enhancer MAAC to improve the paper strength. The chemical cross-linking effect of MAAC significantly enhances the fiber bonding force of the core layer, increasing the tensile index to 75 N·m / g (Example 3), which is 200% higher than Comparative Example 3 (tensile index 25 N·m / g) without using MAAC.

[0053] The surface layer of the base uses Pickering emulsion AKD sizing technology, adds micro-nano fibers through a spraying process, and controls the densification of the base paper in the shoe press and soft calendering processes. The micro-nano fibers fill the pores on the surface of the base paper through AKD sizing. Combining the shoe press and soft calendering processes, the smoothness of the base paper surface is increased by 40%, and the barrier performance is enhanced (the burst index is increased to 6.4 KPa·m² / g, Example 3).

[0054] In Step 1 and Step 3, the coating process uses a layer-by-layer self-assembly sizing technology to form a gradually gradient densification structure between the BMFC / CMC / TEOS hydrophobic and oleophobic support layer and the BCNF / PDMS hydrophobic and oleophobic protective layer. The gradient structure effectively disperses stress, avoids interlayer peeling (tear index ≥ 12.9 mN·m² / g), and at the same time gradually reduces the interface difference, increasing the overall mechanical properties by more than 30% (Examples 1 - 3 compared with Comparative Examples 1 - 3).

[0055] The temperature of the dry calendering process is 80 - 120°C, and the pressure is 5 - 10 MPa. By optimizing the temperature and pressure parameters, it ensures the rapid curing of the coating and its tight bonding with the base (the coating adhesion is increased by 50%), and at the same time avoids fiber damage caused by high temperature (the basis weight of the paper is stable, without shrinkage or deformation).

[0056] Multiple groups of examples and comparative examples of the present invention are as follows:

[0057] Example 1:

[0058] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 5 parts of BMFC, 10 parts of CMC, and 1 part of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.3 to form a BMFC / CMC / TEOS hydrophobic and oleophobic support layer.

[0059] Step 2: After drying and calendering the above-coated base paper, 20 parts of BCNF and 3 parts of PDMS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.3 to form a BCNF / PDMS hydrophobic and oleophobic protective layer.

[0060] Step 3: After drying and calendering the above-coated base paper, a high-barrier food cardstock with a multi-layer gradient densification structure is obtained.

[0061] Example 2:

[0062] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 7 parts of BMFC, 15 parts of CMC, and 2 parts of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.4 to form a BMFC / CMC / TEOS hydrophobic and oleophobic support layer.

[0063] Step 2: After drying and calendering the above-coated base paper, 25 parts of BCNF and 4 parts of PDMS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.4 to form a BCNF / PDMS hydrophobic and oleophobic protective layer.

[0064] Step 3: After drying and calendering the above-coated base paper, a high-barrier food cardstock with a multi-layer gradient densification structure is obtained.

[0065] Example 3:

[0066] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 10 parts of BMFC, 20 parts of CMC, and 3 parts of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BMFC / CMC / TEOS hydrophobic and oleophobic support layer.

[0067] Step 2: After drying and calendering the above-coated base paper, 30 parts of BCNF and 5 parts of PDMS are mechanically stirred for 40 min to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BCNF / PDMS hydrophobic and oleophobic protective layer.

[0068] Step 3: After drying and calendering the coated base paper, a high-barrier food card paper with a multi-layer gradient densified structure is obtained. Comparative Example 1:

[0069] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 5 parts of BMFC, 10 parts of CMC, and 0 parts of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BMFC / CMC / TEOS hydrophobic and oil-proof support layer;

[0070] Step 2: After drying and calendering the coated base paper, 30 parts of BCNF and 5 parts of PDMS are mechanically stirred for 40 min to prepare a coating sizing solution, which is coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BCNF / PDMS hydrophobic and oil-proof protective layer;

[0071] Step 3: After drying and calendering the coated base paper, a high-barrier food card paper with a multi-layer gradient densified structure is obtained.

[0072] Comparative Example 2:

[0073] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 10 parts of BMFC, 20 parts of CMC, and 3 parts of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BMFC / CMC / TEOS hydrophobic and oil-proof support layer;

[0074] Step 2: After drying and calendering the coated base paper, 20 parts of BCNF and 0 parts of PDMS are mechanically stirred for 40 min to prepare a coating sizing solution, which is coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BCNF / PDMS hydrophobic and oil-proof protective layer;

[0075] Step 3: After drying and calendering the coated base paper, a high-barrier food card paper with a multi-layer gradient densified structure is obtained.

[0076] Comparative Example 3:

[0077] Step 1: Using a three-layer base paper with a bottom layer, a core layer, and a surface layer as the substrate, 5 parts of BMFC, 10 parts of CMC, and 0 parts of TEOS are mechanically stirred for 40 min to prepare a coating sizing solution, which is coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BMFC / CMC / TEOS hydrophobic and oil-proof support layer;

[0078] Step 2: After drying and calendering the above-coated base paper, 20 parts of BCNF and 0 parts of PDMS are mechanically stirred for 40 minutes to prepare a coating sizing solution, which is then coated on the base paper. The mass ratio of the base paper to the coating sizing solution is 1:0.5 to form a BCNF / PDMS hydrophobic and oil-proof protective layer.

[0079] Step 3: After drying and calendering the above-coated base paper, a high-barrier food card paper with a multi-layer gradient densified structure is obtained.

[0080] The composite papers obtained through the above examples and comparative examples are subjected to performance tests, and the test results are as follows:

[0081] Table 1

[0082] <![CDATA[Quantitative (g / m 2) > Water contact angle (°) Oil resistance level (Oil Kit Test TAPPI 559 pm-96) <![CDATA[Bursting strength index (Kpa·m 2 / g)]]> Tensile index (N·m / g) <![CDATA[Tear index (mN·m 2 / g)]]> Example 1 60 132.7 8 5.0 63 12.9 Example 2 61 140.5 8 5.3 68 13.8 Example 3 61 152.3 8 6.4 75 15.2 Comparative Example 1 59 85.6 5 3.8 35 6.5 Comparative Example 2 60 91.5 6 4.3 42 7.9 Comparative Example 3 61 73.2 3 3.0 25 4.7

[0083] From the above performance test results and Figures 2-4 the shown test diagrams, it can be seen that the embodiments of the present invention have excellent mechanical properties and good hydrophobic and oil-proof characteristics, and have broad application prospects in the fields such as food packaging.

[0084] The basic principles of the present invention have been shown and described above. The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. The descriptions in the above embodiments and the specification only illustrate the principles of the present invention. Without departing from the scope of the present invention, any modifications, equivalent replacements, and improvements made within the spirit and scope of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high-barrier food cardboard with a multi-layer gradient densification structure, characterized in that: It includes a substrate composed of a base layer, a core layer and a surface layer; A BMFC / CMC / TEOS hydrophobic and oleophobic support layer coated on the surface of the substrate; the BMFC / CMC / TEOS hydrophobic and oleophobic support layer is prepared by mixing bamboo pulp microfibrils BMFC, carboxymethyl cellulose CMC and tetraethoxysilane TEOS in a weight ratio of 5-10:10-20:1-3; A BCNF / PDMS hydrophobic and oleophobic protective layer coated on the surface of the BMFC / CMC / TEOS hydrophobic and oleophobic support layer; the BCNF / PDMS hydrophobic and oleophobic protective layer is prepared by mixing bamboo pulp nanocellulose BCNF and polydimethylsiloxane PDMS in a weight ratio of 20-30:3-5.

2. The high-barrier food card paper with a multi-layer gradient densification structure according to claim 1, characterized in that: The core layer includes mechanical pulp and screened long fibers, and is enhanced by maleic anhydride acylated chitosan enhancer MAAC, and the surface layer is sprayed with AKD sizing technology using Pickering emulsion.

3. The high-barrier food cardboard with a multi-layer gradient densification structure according to claim 1, wherein: The mass ratio of the substrate to the BMFC / CMC / TEOS hydrophobic and oleophobic support layer is 1:0.3-0.5, and the mass ratio of the substrate to the BCNF / PDMS hydrophobic and oleophobic protective layer is 1:0.3-0.

5.

4. A high-barrier food cardboard with a multi-layer gradient densification structure according to claim 1, characterized in that: In the BMFC / CMC / TEOS hydrophobic and oleophobic support layer, TEOS undergoes thermal cross-linking with cellulose when heated to form a densified structure.

5. The high-barrier food cardboard with a multi-layer gradient densification structure according to claim 1, wherein: In the BCNF / PDMS hydrophobic and oleophobic protective layer, BCNF binds to the support layer through hydrogen bonds, and PDMS forms a dense polymer film.

6. A method for preparing a high-barrier food cardboard with a multi-layer gradient densification structure according to any one of claims 1-5, characterized in that, It includes the following steps: Step 1: Mechanically stir BMFC, CMC and TEOS for 30-60 minutes in a weight ratio of 5-10:10-20:1-3 to prepare a coating sizing solution, coat it on the surface of the substrate to form a BMFC / CMC / TEOS hydrophobic and oleophobic support layer; Step 2: Dry and calender the base paper treated in Step 1; Step 3: Mechanically stir BCNF and PDMS for 30-60 minutes in a weight ratio of 20-30:3-5 to prepare a coating sizing solution, coat it on the surface of the base paper obtained in Step 2 to form a BCNF / PDMS hydrophobic and oleophobic protective layer; Step 4: Dry and calender the base paper treated in Step 3 to obtain a high-barrier food cardboard.

7. The preparation method of a high-barrier food cardboard with a multi-layer gradient densification structure according to claim 6, characterized in that: During the preparation of the core layer, maleic anhydride acylated chitosan enhancer MAAC undergoes an esterification reaction and Schiff base structure cross-linking.

8. The preparation method of a high-barrier food cardboard with a multi-layer gradient densification structure according to claim 6, characterized in that: The surface layer of the substrate is sized with AKD using Pickering emulsion, micro-nano fibers are added by spraying, and the denseness of the base paper is regulated in shoe press and soft calendering.

9. The preparation method of a high-barrier food cardboard with a multi-layer gradient densification structure according to claim 6, characterized in that: In Step 1 and Step 3, self-assembly sizing is used for coating layer by layer to make the BMFC / CMC / TEOS hydrophobic and oleophobic support layer and the BCNF / PDMS hydrophobic and oleophobic protective layer form a gradually gradient densified structure.

10. The preparation method of a high-barrier food cardboard with a multi-layer gradient densification structure according to claim 6, characterized in that: The temperature of the dry calendering process is 80-120°C and the pressure is 5-10 MPa.

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