A coated paper and its method of manufacture and use

Abstract

The present application relates to the technical field of packaging material preparation, in particular to a kind of coating paper and its preparation method and application.The raw material of the coating paper includes base paper and sizing liquid.The raw material of sizing liquid includes tannin, epsilon-polylysine, cationic etherified starch, alkyl ketene dimer and water.The sizing liquid of the present application has the characteristics of antioxidant, bacteriostatic and hydrophobic.Compared with the base paper, the tensile index of the coating paper prepared by the sizing liquid of the present application is increased by 45%, the water contact angle can reach more than 135°, the antioxidant property to DPPH reaches more than 90%, and the blocking performance to ultraviolet light is also greatly improved.In addition, the coating paper has excellent bacteriostatic effect on staphylococcus aureus and escherichia coli, and the bacteriostatic effect on staphylococcus aureus is more significant.

Description

Technical Field

[0001] The present invention relates to the technical field of packaging material preparation, in particular to coated paper and a preparation method and application thereof. Background Art

[0002] In recent years, the concept of green and circular development has become increasingly popular. Due to the non-renewable and non-biodegradable nature of petroleum-based raw materials, the use of plastic packaging materials has been gradually restricted in various countries. Paper packaging materials have gradually attracted attention due to their wide availability of raw materials, renewable resources, lightweight, low cost, simple production process, and environmental and safety advantages. According to statistics, paper packaging consumption accounts for over 40% of the total consumption of the four traditional packaging materials: paper, plastic, glass, and metal. However, due to paper's inherent hydrophilicity and porous structure, its low wet strength and poor barrier properties hinder its widespread application. High humidity also makes paper packaging products susceptible to bacterial growth, leading to food spoilage and mold, which severely limits the multifunctional application of paper products. Summary of the Invention

[0003] Based on the above content, the present invention provides a coated paper and a preparation method and application thereof.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] One of the technical solutions of the present invention is a sizing solution, the raw materials of which include tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water;

[0006] The mass volume ratio of the tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water is (4-12) g: (0.2-1) g: (0.4-1.2) g: (0.6-1.5) g: 200 mL.

[0007] The second technical solution of the present invention is a method for preparing the sizing solution, comprising the following steps:

[0008] Dissolve tannic acid in 1 / 2 volume of water, and then add ε-polylysine to obtain a tannic acid / ε-polylysine solution;

[0009] dissolving the cationic etherified starch in the remaining 1 / 2 volume of water to obtain a cationic etherified starch solution;

[0010] mixing the tannic acid / ε-polylysine solution with the cationic etherified starch solution and gelatinizing the mixture to obtain a uniform solution;

[0011] Alkyl ketene dimer is added to the homogeneous solution to obtain the sizing solution.

[0012] The third technical solution of the present invention is the use of the sizing solution in the preparation of coated paper.

[0013] A fourth technical solution of the present invention is a coated paper, the raw materials of which include base paper and the sizing solution.

[0014] A fifth technical solution of the present invention is a method for preparing the coated paper, comprising the following steps: immersing the base paper in the sizing solution, followed by pressing and drying to obtain the coated paper.

[0015] The sixth technical solution of the present invention is the use of the coated paper in the preparation of packaging materials.

[0016] The present invention discloses the following technical effects:

[0017] Paper-based materials are highly favored due to their environmental friendliness, good biocompatibility, and low cost. However, their poor water stability, low mechanical strength, and microbial resistance limit their further widespread application. The present invention uses cationic etherified starch (CS), alkyl ketene dimer (AKD), tannic acid (TA), and ε-polylysine (ε-PL) as raw materials to prepare an antioxidant and antibacterial hydrophobic sizing solution, and uses this sizing solution to prepare biomass-based coated paper. Compared with the base paper, the tensile index of the coated paper is increased by 45%, the water contact angle can reach above 135°, the antioxidant capacity to DPPH reaches above 90%, and the UV barrier performance is also greatly improved. In addition, the antibacterial effect of the coated paper on Escherichia coli and Staphylococcus aureus was evaluated using the inhibition zone test method. The results showed that the coated paper has excellent antibacterial effects on Staphylococcus aureus and Escherichia coli, and the antibacterial effect on Staphylococcus aureus is more significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 Schematic diagram of the preparation process of TC / PL coated paper of the present invention.

[0020] Figure 2 (a) is the optical photograph of uncoated (base paper) and TC / PL coated paper; (b) is the thickness and basis weight of uncoated and TC / PL coated paper; (c) is the FT-IR spectrum of uncoated and TC / PL coated paper.

[0021] Figure 3SEM images of (a) uncoated, (b) TC / PL-0.1%, (c) TC / PL-0.3%, and (d) TC / PL-0.5%.

[0022] Figure 4 Physical properties of uncoated and TC / PL coated paper: (a) tensile index; (b) breaking length; (c) whiteness; (d) UV transmittance.

[0023] Figure 5 (a) DPPH radical scavenging rate and (b) DPPH radical reaction changes of uncoated and TC / PL coated paper.

[0024] Figure 6 (a) Hydrophobic mechanism of ncoated and TC / PL coated paper; (b) Water contact angle; (c) Wetting by milk, juice, Coca-Cola, coffee, and acid and alkali solutions.

[0025] Figure 7 Inhibition zones of uncoated and TC / PL coated paper (a) Escherichia coli; (b) Staphylococcus aureus; (c) inhibition zone diameter. DETAILED DESCRIPTION

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0028] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0029] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0030] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0031] Bamboo, a fast-growing plant resource widely distributed across Asia, has been increasingly recognized as a wood substitute in recent years. Leveraging bamboo forest resources is an effective way to alleviate the current shortage of timber resources. Bamboo has a high cellulose content and a fiber length intermediate between coniferous and hardwood species, offering excellent pulping properties and making it a high-quality raw material for pulping and papermaking.

[0032] Currently, various methods are being used to enhance the water resistance and antibacterial properties of cellulose paper, including chemical modification, atomic layer deposition, and physical blending of hydrophobic and antibacterial substances. In industrial production, sizing is a common method for improving paper properties, which can be divided into surface sizing and internal sizing. Recent research on paper hydrophobicity focuses on surface sizing because it effectively retains the dosage of hydrophobic compounds. Commonly used hydrophobic paper sizing agents include ASA, AKD, and rosin size. Surface deposition of non-reactive sizing agents such as starch, polyvinyl alcohol, and chitosan on the paper surface can improve paper physical properties, including surface strength, wettability, and barrier properties. Starch and its derivatives are the most widely used ingredients in paper products, besides cellulose fibers and mineral fillers. Compared to other sizing agents, starch and its derivatives offer advantages such as low cost, good biocompatibility, and high yield, making them the most widely used sizing agent in industrial production. Among starch and its derivatives, cationic starch has seen a significant increase in usage in recent years. It not only acts as a dry strength enhancer to increase interfiber bonding, but its cationic groups also better retain anionic fibers, thereby reducing wastewater load.

[0033] Tannic acid is a natural polyphenolic compound with antioxidant, antimicrobial, anti-inflammatory, and hemostatic properties. It has been approved for clinical and food additive use, making its application in food packaging safe and feasible. Natural ε-polylysine is a typical cationic antimicrobial peptide with broad-spectrum antimicrobial activity against bacteria and fungi. It adsorbs onto the surface of microbial membranes, causing physiological damage to cells. Therefore, based on their antioxidant and antimicrobial properties, tannic acid and ε-polylysine could be used as antimicrobial additives in food packaging paper, imparting antimicrobial properties to the paper.

[0034] The present invention uses cationic starch (CS) as a sizing matrix to enhance fiber strength, alkyl ketene dimer (AKD) as a hydrophobic agent, tannic acid (TA) as an antioxidant, and ε-polylysine (ε-PL) as an antibacterial agent to prepare a bio-based paper sizing solution. Bleached bamboo pulp is used as the fiber raw material to prepare bleached bamboo pulp paper. A starch-based coating with antioxidant, antibacterial and hydrophobic properties is deposited on the surface of the bamboo pulp base paper by an impregnation method. The preparation process is shown in the following figure. Figure 1 The modified bleached bamboo pulp paper was characterized by its morphology, hydrophobicity, antioxidant properties, and mechanical strength, and its antibacterial activity was evaluated. The method of the present invention is highly efficient, simple, and environmentally friendly. The coated paper prepared using this method has broad application prospects in the food packaging field.

[0035] The first aspect of the present invention provides a sizing solution, the raw materials of which include tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water;

[0036] The mass volume ratio of the tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water is (4-12) g: (0.2-1) g: (0.4-1.2) g: (0.6-1.5) g: 200 mL.

[0037] In the present invention, if the amount of alkyl ketene dimer is too low, the sizing effect will not be achieved; if the amount of alkyl ketene dimer is too high, it will cause agglomeration and uneven dispersion in the sizing solution. Therefore, the present invention preferably limits the amount of alkyl ketene dimer to the above parameter ratio range.

[0038] In the present invention, too low a dosage of cationic etherified starch can result in low paper strength; too high a dosage can lead to excessive emulsion viscosity, increased paper sizing, and brittleness and increased rigidity. Therefore, the present invention preferably limits the dosage of cationic etherified starch to the aforementioned parameter ratio range.

[0039] The second aspect of the present invention provides a method for preparing the sizing solution, comprising the following steps:

[0040] Dissolve tannic acid in 1 / 2 volume of water, and then add ε-polylysine to obtain a tannic acid / ε-polylysine solution;

[0041] dissolving the cationic etherified starch in the remaining 1 / 2 volume of water to obtain a cationic etherified starch solution;

[0042] mixing the tannic acid / ε-polylysine solution with the cationic etherified starch solution and gelatinizing the mixture to obtain a uniform solution;

[0043] Alkyl ketene dimer is added to the homogeneous solution to obtain the sizing solution.

[0044] In a preferred embodiment of the present invention, the pH of the cationic etherified starch solution is 4.

[0045] In some specific embodiments of the present invention, the pH value of the cationic etherified starch solution is adjusted to 4 by adding an acetate buffer solution with a pH value of 3.5.

[0046] In a preferred embodiment of the present invention, the gelatinization is specifically performed at 95° C. and a rotation speed of 500 rpm for 30 minutes.

[0047] In a preferred embodiment of the present invention, the temperature for adding the alkyl ketene dimer to the homogeneous solution is 60°C.

[0048] The third aspect of the present invention provides a use of the sizing solution in preparing coated paper.

[0049] A fourth aspect of the present invention provides a coated paper, the raw materials of which include base paper and the sizing solution.

[0050] In some specific embodiments of the present invention, the base paper is bleached bamboo pulp base paper, which is prepared by the following steps: beating the bleached bamboo pulp to a beating degree of 70°SR to obtain wet pulp, dispersing the wet pulp with a pulp deflaker and then using a paper sheet former to make a fixed amount to obtain the bleached bamboo pulp wet paper; pressing the bleached bamboo pulp wet paper and then drying it to obtain the bleached bamboo pulp base paper.

[0051] A fifth aspect of the present invention provides a method for preparing the coated paper, comprising the following steps: immersing the base paper in the sizing solution, followed by pressing and drying to obtain the coated paper.

[0052] In a preferred embodiment of the present invention, the immersion temperature is 60° C. and the time is 10 s; the pressing pressure is 0.5 MPa and the time is 10 min; and the drying temperature is 105° C. and the time is 15 min.

[0053] A sixth aspect of the present invention provides use of the coated paper in preparing packaging materials.

[0054] Unless otherwise specified, the technical solutions described in the present invention are all conventional solutions in the field, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0055] The schematic diagram of the preparation process of TC / PL coated paper of the present invention is as follows Figure 1 As shown (in the figure, CS / ε-PL-AKD represents TC / PL sizing solution).

[0056] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0057] Example 1

[0058] 1. Materials and Methods

[0059] 1.1 Materials

[0060] Bleached bamboo pulp (dry pulp) was provided by Mudanjiang Hengfeng Paper Co., Ltd. (Heilongjiang). Food-grade tannic acid was purchased from Wufeng Chicheng Biotechnology Co., Ltd. (Hubei, China). Cationic etherified starch was purchased from Hefei BASF Biotechnology Co., Ltd. (Anhui), with a viscosity of 690–1200 mPa·s (6% at 95°C) and a degree of substitution of 0.025–0.03. ε-Polylysine (food grade) was purchased from Zhejiang Xinyinxiang Bioengineering Co., Ltd. (Zhejiang). AKD emulsion with a solids content of 12.5% ​​± 0.5% and a pH of 3–4 was purchased from Dongguan Huiteng New Materials Co., Ltd. (Guangdong). LB broth agar was purchased from Shenggong Bioengineering (Shanghai) Co., Ltd. LB broth was purchased from Beijing Aoboxing Biotechnology Co., Ltd.

[0061] 1.2 Preparation of TC / PL coated paper

[0062] 1.2.1 Preparation of bleached bamboo pulp base paper (abbreviated as base paper)

[0063] Weigh 360g of absolute dry pulp and use a Valley beater to beat the bleached bamboo pulp to a beating degree of 70°SR. Wet pulp equivalent to 1.256g of absolute dry pulp is measured and dispersed in a pulp disintegrator. After that, the paper is made into a sheet with a weight of 37±1g / m 2 The round bleached bamboo pulp wet paper sheet with a diameter of 20 cm was pressed at a pressure of 0.5 MPa for 10 minutes and then dried in a plate dryer at 105° C. to obtain bleached bamboo pulp base paper, which was stored in a sealed bag.

[0064] 1.2.2 Preparation of TC / PL sizing solution

[0065] 8g of tannic acid (TA) was dissolved in 100mL of deionized water. ε-polylysine (0.2g, 0.6g, and 1g) was then dissolved in the tannic acid solution to obtain a tannic acid / ε-polylysine solution. 0.8g of cationic etherified starch was dissolved in 100mL of deionized water and the pH was adjusted to 4 using acetate buffer (pH 3.5) to obtain a cationic etherified starch solution. The tannic acid / ε-polylysine solution and the cationic etherified starch solution were mixed and gelatinized on a magnetic stirrer at 95°C and 500rpm for 30 minutes to obtain a homogeneous solution. The homogeneous solution was then cooled to 60°C and 0.6 g of AKD emulsion was added to obtain an antibacterial hydrophobic sizing solution (according to the different ε-polylysine (ε-PL) contents in the sizing solution (0.1 wt%, 0.3 wt%, 0.5 wt%), the sizing solutions were named TC / PL-0.1%, TC / PL-0.3%, and TC / PL-0.5%, respectively).

[0066] 1.2.3 Impregnated bleached bamboo pulp paper

[0067] Bleached bamboo pulp base paper was immersed in a sizing solution at 60°C for 10 seconds, pressed at a pressure of 0.5 MPa for 10 minutes, and then dried in a flat dryer at 105°C for 15 minutes to obtain TC / PL coated paper (referred to as sized paper). TC / PL coated papers prepared with different sizing solutions of TC / PL-0.1%, TC / PL-0.3%, and TC / PL-0.5% were labeled as TC / PL-0.1%, TC / PL-0.3%, and TC / PL-0.5%, respectively.

[0068] 1.3 Characterization

[0069] 1.3.1 Scanning Electron Microscopy (SEM)

[0070] The surface microstructure of the sized paper was analyzed using an Apreo S field emission scanning electron microscope (Thermo Scientific, Waltham, Massachusetts, America). All samples were sprayed with gold before testing.

[0071] 1.3.2 Fourier Transform Infrared Spectroscopy (FTIR)

[0072] The samples were analyzed using Nicolet IS10 FTIR (Perkin Elmer Inc., Waltham, MA, USA) spectroscopy at 4000-500 cm -1 Scan 32 times within the wavelength range with a resolution of 4cm -1 , the functional group changes of paper before and after sizing were tested.

[0073] 1.4 Performance

[0074] 1.4.1 Thickness and Basis Weight

[0075] All samples were stored at 23 ± 1°C and 55 ± 2% relative humidity before testing. Paper thickness was measured using a Schubert thickness gauge at three different locations on the paper. Paper mass was determined using an analytical balance, and the quantification was calculated using formula (1):

[0076] g=m / A (1)

[0077] g is the sample weight (g / m 2 ), m is the mass of the sample (g), A is the area of ​​the sample (m 2 )

[0078] 1.4.2 Mechanical properties

[0079] The paper was cut into 15 mm × 180 mm strips and the tensile strength (100 mm span) was measured using a ZLD-300 electronic tensile testing machine under standard test conditions (23 ± 1°C and 50 ± 2% relative humidity). The breaking length and tensile index were calculated using formulas (2) and (3):

[0080] Lb=(S / 9.8g)×1000 (2)

[0081] Y=(S / g)×1000 (3)

[0082] Wherein, Lb is the breaking length (km), S is the tensile strength (kN / m), g is the basis weight (g / m 2 ); Y is the tensile index (N·m / g)

[0083] 1.4.3 Whiteness and optical performance test

[0084] The whiteness of the samples was tested according to the Chinese national standard GB / T 7974-2002. The mechanical properties were averaged from three tests and the standard deviation was calculated.

[0085] The UV-visible spectra of the paper before and after sizing were measured using a Cary100 UV-visible spectrophotometer in the wavelength range of 200-600nm.

[0086] 1.4.4 Antioxidant test

[0087] The antioxidant activity of the sample was determined by scavenging DPPH free radicals. A 50 mg sample was cut into small pieces (approximately 3.0 × 3.0 mm) and immersed in 8 mL of anhydrous ethanol. Extraction was performed at 60°C for 2 h. 3 mL of the sample extract and 3 mL of a 0.1 mM DPPH solution were mixed evenly and reacted in the dark for 30 min. The absorbance at 517 nm was measured using a UV-visible spectrometer. The DPPH free radical scavenging rate was calculated using formula (4):

[0088] DPPH (%)=[1-(AB) / C]×100% (4)

[0089] A = absorbance of DPPH + paper extract, B = absorbance of paper extract + 95% ethanol, C = absorbance of DPPH + 95% ethanol.

[0090] 1.4.5 Wettability

[0091] To investigate the influence of TC / PL coating on the surface hydrophilicity, an OCA20 contact angle tester (Dataphysics, Germany) was used to measure the water contact angle (WCA) of the paper surface at room temperature, using a 5μL test water volume. Each sample was tested at least three times, and the average value was calculated. To determine the wetting properties of TC / PL coated paper to acid and alkaline solutions, acetic acid solution with a pH of 3.5 and sodium hydroxide solution with a pH of 12 were tested on the paper surface. The wetting properties of common beverages on the coated paper were tested using milk, Orange Juice, Coca-Cola, and coffee as test solutions.

[0092] 1.4.6 Antibacterial properties

[0093] The antibacterial properties of paper were tested according to the inhibition ring test method in Section 5 of GB / T 42702-2023. Escherichia coli and Staphylococcus aureus were used as the test bacteria.

[0094] 2. Results and Discussion

[0095] 2.1 Preparation and characterization of TC / PL coated paper

[0096] Cationic etherified starch and alkyl ketene dimer can enhance the fiber strength of paper and impart liquid repellency to paper. Adding tannic acid and ε-polylysine to the sizing solution can impart excellent antioxidant and antibacterial properties to paper. TC / PL coated paper is obtained by impregnating bleached bamboo pulp base paper. The preparation process is as follows: Figure 1 shown.

[0097] Compared with bleached bamboo pulp paper, TC / PL coated paper has no obvious color change in appearance, only changing from original white to light yellow. Figure 2 As shown in (a) Figure 2Uncoated refers to bleached bamboo pulp base paper). The thickness and basis weight of base paper and TC / PL coated paper are as follows: Figure 2 As shown in (b), the thickness and basis weight of TC / PL coated paper are approximately 110 μm and 43 g / m 2 , increasing by approximately 10% and 16.2% compared to the base paper. This is because the TC / PL sizing solution not only fills the pores within the base paper fibers but also adheres to the fiber surface, increasing thickness and basis weight. Increasing ε-PL content has little effect on paper thickness and basis weight, indicating that it has little effect on the emulsion viscosity, and therefore, the amount of emulsion-fiber bonding does not change significantly.

[0098] Figure 2 (c) is the FT-IR spectra of TC / PL coated paper and bleached bamboo pulp base paper. Compared with the base paper, the TC / PL coated paper has a -1 and 1192cm -1 The new absorption peak at 1236 cm-1 corresponds to the C=C and C=O stretching in the aromatic ring of TA. -1 The stretching vibration of CN appears at 1641cm -1 Move to 1611cm -1 , which proves that TA and ε-PL interact with cellulose to produce hydrogen bonds. -1 The characteristic peak of β-ketoester bond appeared at , which indicated that the lyophobic groups of AKD and cellulose hydroxyl groups reacted to form covalent bonds during the drying process, thus enhancing the hydrophobicity of the paper.

[0099] 2.2TC / PL coated paper SEM

[0100] The surface morphology of base paper and TC / PL coated paper was characterized using scanning electron microscopy. Figure 3 As can be seen in (a), the surface of the base paper is composed of interwoven cellulose fibers, which form a network of micropores of varying sizes. After beating, bamboo pulp fibers exhibit distinct fibrils. These microfibrils provide more hydrogen bonding sites, enhancing the bonding strength of the paper. Figure 3 Panels (b)-(d) show TC / PL-coated paper. Compared to the base paper, the TC / PL sizing solution significantly fills the base paper's pores after impregnation. The pores and microfibrils decrease significantly with increasing ε-PL. Furthermore, the TC / PL sizing solution deposits on the paper fiber surface, explaining the simultaneous increase in basis weight and thickness. The emulsion's penetration into the paper clogs the fiber pores, resulting in a uniform, dense surface that imparts excellent mechanical strength and barrier properties to the coated paper.

[0101] 2.3 Mechanical properties of TC / PL coated paper

[0102] Mechanical strength is an important indicator to measure the physical properties of paper, which directly affects its application in food packaging. Figure 4 As shown in (a), the tensile index of TC / PL coated paper is increased by about 25% compared with the base paper. When the ε-PL dosage is 0.1%, the tensile index of the coated paper can reach 58N·mg -1 , compared to the original paper's 40N·mg -1 The tearing length of coated paper has also been significantly improved, such as Figure 4 As shown in (b), this may be due to the enhanced inter-fiber bonding. Tannic acid is a natural plasticizer that reduces the hardness of paper. The covalent bond between tannic acid and cellulose can increase cellulose strength, but tannic acid reduces the hydrogen bonds between cellulose molecules, generally resulting in a decrease in the tensile index and breaking length of coated paper. Therefore, cationic starch is used as a dry strength agent for coated paper. Because it has a positive charge, it can tightly bind to the negatively charged fibers. The free glucose hydroxyl groups of starch can hydrogen bond with cellulose molecules, thereby increasing the inter-fiber bonding strength. The use of cationic starch sizing can also improve the formation of paper sheets, reduce the stress unevenness caused by fiber agglomeration, and provide more uniform inter-fiber bonding. In summary, TC / PL coating improves the mechanical properties of bamboo pulp base paper.

[0103] 2.4 Optical properties of TC / PL coated paper

[0104] The whiteness of paper directly affects the color rendering of printed products. Paper with high whiteness can reflect almost all color light, and the color of printed products will be bright and pure. Figure 4 As shown in (c), the brightness of bleached bamboo pulp base paper can reach 78%, and the brightness of TC / PL coated paper is about 69%. Compared with bleached bamboo pulp base paper, the brightness of TC / PL coated paper decreases less.

[0105] Tannin plays an important role as a biological colorant in textiles, leather, and other materials. Tannic acid is easily soluble in water, and its aqueous solution is light yellow. Its chromophores easily bind to cellulose, so bleached paper becomes light yellow after being impregnated with TC / PL, and the whiteness of the paper is slightly reduced. The phenolic hydroxyl groups in tannic acid also have strong absorption of ultraviolet light. The UV-shielding ability of TC / PL paper and bamboo pulp paper was tested using UV-visible spectroscopy. The results are as follows: Figure 4As shown in (d), TC / PL paper blocks approximately 70% of UV rays within the 200-280nm (UV-B and UV-C) range. Within the 280-400nm (UV-A) range, the coated paper blocks approximately 50%-70%. Furthermore, the UV resistance of the paper increases with increasing ε-PL content, demonstrating a synergistic effect between ε-PL and TA in UV protection. The synergistic UV protection of tannic acid and ε-PL helps prevent UV-induced oxidation, thereby extending the shelf life of food, demonstrating significant potential and advantages in food packaging.

[0106] Oxidation resistance of 2.5TC / PL coated paper

[0107] The antioxidant properties of bleached base paper and TC / PL coated paper were evaluated by measuring their DPPH scavenging ability. Figure 5 As shown in (a). The scavenging rates of base paper, TC / PL-0.1%, TC / PL-0.3% and TC / PL-0.5% for DPPH radicals were 0%, 95.8%, 96.5% and 95.8% respectively. The change in ε-PL content did not significantly improve the antioxidant activity of the coated paper. This is because the antioxidant activity of TC / PL coated paper is mainly contributed by TA. The DPPH radical scavenging model is mainly due to the delocalization of excess electrons on the DPPH molecule, which manifests as stable free radicals and produces a deep purple color. When the DPPH solution encounters an antioxidant that can provide hydrogen atoms, the hydrogen atoms of the antioxidant react with the DPPH radicals, terminating the chain reaction of free radicals, and ultimately leading to the scavenging of the DPPH radicals and the fading of the solution to a stable light yellow. Tannic acid is a natural antioxidant. Due to its polyphenolic hydroxyl groups, it can provide a large number of hydrogen atoms to capture free radicals and terminate the oxidation reaction. Therefore, due to the presence of phenolic hydroxyl groups in TA, TC / PL coated paper can react with the DPPH solution to consume a large number of free radicals, causing the solution to fade from purple to yellow, as shown in Figure 2. Figure 5 As shown in (b), the high antioxidant properties of TC / PL coated paper can delay food spoilage and can be used as a packaging material to keep food fresh.

[0108] 2.6 Lyophobicity of TC / PL coated paper

[0109] The hygroscopicity of paper is an important reason that limits its application in food packaging. The hydrophilic properties of the base paper are significantly improved after being modified by TC / PL sizing liquid. Figure 6As shown in (b). Since cellulose fibers contain a large number of hydroxyl groups and are therefore hydrophilic, the water contact angle of bamboo pulp base paper is 0°, while the water contact angle of the modified TC / PL coated paper can reach more than 135°. There are two main reasons for this: First, it is attributed to the presence of AKD. AKD is widely used in the sizing of paper and cardboard as a paper hydrophobic agent. AKD itself has no charge and is difficult to adsorb onto the fiber through charge, so cationic starch is used as an emulsifier to give the AKD emulsion a positive charge opposite to the fiber charge, thereby increasing the retention rate of AKD on the fiber. At high temperature, AKD melts and spreads on the cellulose molecular chain. The hydrophobic groups of AKD can react with the hydroxyl groups of cellulose fibers to form β-ketoester bonds to produce a sizing effect, making the paper hydrophobic, such as Figure 6 (a). Secondly, the pores formed by the interwoven arrangement of cellulose fibers will facilitate the diffusion of water vapor, making the paper less water-resistant. Therefore, blocking the pores between fibers is also an effective way to improve the liquid resistance of paper. The TC / PL sizing solution not only reduces the gaps between fibers, but also makes the cellulose fibers and pores have excellent liquid repellency due to the liquid-repellent properties of AKD. This dual effect gives the coated paper excellent water resistance. Figure 6 As shown in (c), the TC / PL-coated paper also exhibits excellent moisture resistance to other common liquids (milk, orange juice, Coca-Cola, and coffee). These droplets exhibit a typical spherical structure on the coated paper. Furthermore, acetic acid droplets and sodium hydroxide droplets at pH 3.5 and pH 12 also remain non-wetting on the coated paper, demonstrating that the TC / PL-coated paper is also acid- and alkali-resistant. These studies demonstrate the great potential of this coated paper in applications such as paper cups and beverage containers.

[0110] 2.7 Antibacterial properties of TC / PL coated paper

[0111] The antibacterial activity of food packaging materials is of paramount importance in reducing microbial contamination and extending the shelf life of food. In this study, the antibacterial performance of Escherichia coli and Staphylococcus aureus was tested using the inhibition zone method. Figure 7 As shown. Figure 7 As can be seen from (a) and (b), the TC / PL coated paper has a more significant inhibitory effect on Staphylococcus aureus. Even with a dosage of 1% ε-PL, it can still show an excellent inhibitory effect. The inhibitory effect on Escherichia coli increases significantly with the increase of ε-PL content. Figure 7As shown in Figure (c), the base paper exhibited no inhibitory activity against either test bacteria. However, as the ε-PL content of the coated paper increased from 0.1% to 0.3% and then to 0.5%, the diameter of the inhibition zone for E. coli increased from 13.3 mm to 14.2 mm and then to 14.6 mm, and the diameter of the inhibition zone for Staphylococcus aureus increased from 15.1 mm to 15.0 mm and then to 15.6 mm. This is primarily due to the high positive charge on the surface of ε-PL, which can bind to cell membranes, DNA, and some functional proteins, disrupting the cell membrane structure and achieving an antibacterial effect. TA, a natural polyphenol, possesses antibacterial, anti-inflammatory, and antioxidant properties. It binds to lipids on cell membranes and certain enzymes in microorganisms, inhibiting their activity. Here, it synergistically enhances the antibacterial properties of the coated paper with ε-PL.

[0112] 3. Conclusion

[0113] Based on existing large-scale industrial production, this paper successfully prepared a biomass cationic starch-based TC / PL coated paper with high antioxidant, antibacterial and hydrophobic properties through surface sizing. CS enhances the mechanical strength of bamboo pulp fibers, and the tensile index of the coated paper can reach 58N·mg. -1 , an improvement of 45% compared to the base paper. The reaction of AKD with cellulose to form β-ketoesters greatly improves the hydrophilic properties of cellulose paper, and the water contact angle of TC / PL coated paper can reach over 135°. In addition, it has obvious non-wetting properties for milk, coffee, cola, orange juice, acidic and alkaline solutions. TA not only gives the coated paper strong antioxidant and UV shielding properties, but also synergizes with ε-polylysine to produce a significant antibacterial effect against Escherichia coli and Staphylococcus aureus. The renewability and biodegradability of plant fiber and biomass raw materials, cationic etherified starch and tannic acid, make the prepared coated paper a green and environmentally friendly new food packaging material that can be used in current industrial production and has broad application prospects.

[0114] Comparative Example 1

[0115] The bleached bamboo pulp base paper was immersed in a 30°C TC / PL-0.5% sizing solution for 10 seconds, pressed at a pressure of 0.5 MPa for 10 minutes, and then dried in a flat plate dryer at 105°C for 15 minutes to obtain TC / PL coated paper (i.e., the only difference from the TC / PL-0.5% coated paper prepared in Example 1 is that the base paper was immersed in the sizing solution at 30°C).

[0116] Results: The amount of sizing solution loaded on the coated paper prepared at this temperature was very small, and the weight of the paper increased by only 0.8 g / m2 before and after impregnation. 2 In addition, the tensile strength and breaking length of the coated paper were slightly lower than those before immersion. Due to the low sizing liquid loading, the coated paper did not show an antibacterial effect against Escherichia coli and Staphylococcus aureus.

[0117] Comparative Example 2

[0118] The bleached bamboo pulp base paper was immersed in a TC / PL-0.5% sizing solution at 80°C for 10 seconds, pressed at a pressure of 0.5 MPa for 10 minutes, and then dried in a flat plate dryer at 105°C for 15 minutes to obtain TC / PL coated paper (i.e., the only difference from the TC / PL-0.5% coated paper prepared in Example 1 is that the base paper was immersed in the sizing solution at 80°C).

[0119] Results: The coated paper prepared at this temperature exhibited a uniform coating, excellent mechanical properties, and significant antibacterial activity against Escherichia coli and Staphylococcus aureus. However, testing revealed a significant decrease in the hydrophobicity of the coated paper, with a water contact angle of 103.8°. This is due to the hydrolysis of AKD at high temperatures to form β-ketoacids, which impairs the sizing effect of AKD.

[0120] Comparative Example 3

[0121] Weigh 9g of tannic acid (TA) and dissolve it in 100mL of deionized water to obtain a tannic acid solution. Weigh 0.8g of cationic etherified starch and dissolve it in 100mL of deionized water. Adjust the pH to 4 using acetate buffer (pH 3.5) to obtain a cationic etherified starch solution. Mix the tannic acid solution and cationic etherified starch solution and gelatinize on a magnetic stirrer at 95°C and 500rpm for 30 minutes to obtain a homogeneous solution. Then, cool the homogeneous solution to 60°C and add 0.6g of AKD emulsion to obtain sizing solution A.

[0122] The bleached bamboo pulp base paper was immersed in sizing solution A at 60°C for 10 seconds, pressed at a pressure of 0.5 MPa for 10 minutes, and then dried in a flat plate dryer at 105°C for 15 minutes to obtain coated paper A.

[0123] The same effect verification as in Example 1 was performed on coated paper A. The results showed that coated paper A exhibited similar hydrophobic, antioxidant, UV resistance, and mechanical properties as in Example 1, but only exhibited a significant antibacterial effect against Staphylococcus aureus, and had no antibacterial effect against Escherichia coli.

[0124] Comparative Example 4

[0125] Weigh 9 g of ε-polylysine and dissolve it in 100 mL of deionized water to obtain an ε-polylysine solution. Weigh 0.8 g of cationic etherified starch and dissolve it in 100 mL of deionized water. Adjust the pH to 4 using acetate buffer (pH 3.5) to obtain a cationic etherified starch solution. Mix the ε-polylysine solution and cationic etherified starch solution and gelatinize on a magnetic stirrer at 95°C and 500 rpm for 30 minutes to obtain a homogeneous solution. Then, cool the homogeneous solution to 60°C and add 0.6 g of AKD emulsion to obtain sizing solution B.

[0126] The bleached bamboo pulp base paper was immersed in sizing solution B at 60°C for 10 seconds, pressed at a pressure of 0.5 MPa for 10 minutes, and then dried in a flat plate dryer at 105°C for 15 minutes to obtain coated paper B.

[0127] The same effect verification as in Example 1 was performed on coated paper B. The results showed that the mechanical strength of coated paper B decreased slightly, and the hydrophobic properties remained almost unchanged, but its antioxidant and UV resistance decreased significantly, and it showed a weak antibacterial effect against Escherichia coli and Staphylococcus aureus.

[0128] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing coated paper, characterized in that: The following steps are involved: The base paper is immersed in the sizing solution, and then pressed and dried to obtain the coated paper; The raw materials of the sizing solution include tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water; The mass volume ratio of the tannic acid, ε-polylysine, cationic etherified starch, alkyl ketene dimer and water is (4-12) g: (0.2-1) g: (0.4-1.2) g: (0.6-1.5) g: 200 mL; The immersion temperature is 60° C. and the time is 10 s; the pressing pressure is 0.5 MPa and the time is 10 min; and the drying temperature is 105° C. and the time is 15 min.

2. The method for preparing coated paper according to claim 1, characterized in that: The preparation method of the sizing solution comprises the following steps: Dissolve tannic acid in 1 / 2 volume of water, and then add ε-polylysine to obtain a tannic acid / ε-polylysine solution; dissolving the cationic etherified starch in the remaining 1 / 2 volume of water to obtain a cationic etherified starch solution; mixing the tannic acid / ε-polylysine solution with the cationic etherified starch solution and gelatinizing the mixture to obtain a uniform solution; Alkyl ketene dimer is added to the homogeneous solution to obtain the sizing solution.

3. The method for preparing coated paper according to claim 2, characterized in that: The pH of the cationic etherified starch solution is 4.

4. The method for preparing coated paper according to claim 2, characterized in that: The gelatinization is specifically carried out at 95° C. and a rotation speed of 500 rpm for 30 min.

5. The method for preparing coated paper according to claim 2, characterized in that: The temperature at which the alkyl ketene dimer was added to the homogeneous solution was 60°C.

6. A coated paper, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 5.

7. Use of the coated paper according to claim 6 in preparing packaging materials.

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