Preparation method of environment-friendly anti-aging packaging base paper and packaging base paper

Abstract

The preparation method of the environment-friendly anti-aging packaging base paper comprises the following steps: taking softwood pulp and dissolving pulp, pulping for the first time, adding microcrystalline cellulose, and stirring to form first pulp; adjusting the pH value of the first slurry to be neutral, adding a water-soluble chitosan-polyphenol compound, and pulping for the second time to form second slurry; adding oxidized sodium alginate into the second pulp, and pulping for the third time to form third pulp; forming raw paper according to a papermaking process; the invention further provides packaging base paper which has the characteristic of good anti-aging performance. Softwood pulp and dissolving pulp are used as papermaking raw materials, microcrystalline cellulose, a water-soluble chitosan-polyphenol compound and oxidized sodium alginate are used, a pulping process which is sequentially added according to a sequence is formed, a stable fiber network is constructed layer by layer, and a mechanism of skeleton supporting, covalent crosslinking and antioxidant end capping is formed. The anti-aging performance of the raw paper is improved.

Description

Technical Field

[0001] This invention belongs to the field of composite material technology, and particularly relates to the field of papermaking technology, specifically to a method for preparing an environmentally friendly anti-aging packaging base paper and the packaging base paper itself. Background Technology

[0002] As consumers pay more attention to product shelf life and packaging safety, packaging materials not only need to have good initial strength, but also need to maintain stable performance in harsh environments such as high temperature and high humidity. However, existing traditional packaging base paper often shows significant comprehensive performance degradation after undergoing typical accelerated thermal aging conditions (150℃, 8 hours): tear strength is significantly reduced, paper becomes brittle; whiteness is significantly reduced, paper surface color turns yellow; dimensional stability is poor, irreversible thermal shrinkage deformation occurs; micropores collapse, and average pore size is abnormally increased.

[0003] These problems not only seriously affect subsequent coating and printing processes, product appearance and packaging, and shelf life, but also restrict the applicability of traditional packaging base paper in high-requirement application scenarios such as high-temperature sterilization, heat sealing and lamination, and retort packaging.

[0004] Most existing technologies improve performance and delay aging by adding large amounts of synthetic wet strength agents or applying surface coatings, but these methods are prone to problems such as degradation difficulties and recycling obstacles, and cannot effectively solve the aging and deterioration mechanism at the fiber level. Although some have tried to improve performance by using natural antioxidants or crosslinking agents, they still have problems such as single functional components, difficulty in balancing strength and durability, mutual interference when adding multiple auxiliaries at the same time (such as charge neutralization and precipitation problems), low retention rate and uneven distribution of auxiliaries due to improper timing of addition, or lack of systematic synergistic mechanism design, resulting in limited performance improvement.

[0005] Chinese invention patent application CN119877321A discloses a method for manufacturing biodegradable food packaging paper. Specifically, it discloses the use of softwood pulp and hardwood pulp for pulping as the base fiber of the paper base material, and the combination of components such as chitosan to form packaging paper with good waterproof and oil-proof properties. However, it does not disclose the specific parameters of softwood pulp and hardwood pulp, nor does it disclose the interaction between them and subsequent antioxidants and fiber crosslinking agents.

[0006] Chinese invention patent application CN119265995A discloses a method for preparing breathable paper. Specifically, it discloses a Schiff base reaction between sodium alginate oxide and chitosan to form a coating layer. The product formed is sodium alginate oxide grafted onto the chitosan molecular chain. Specifically, chitosan first coats wood pulp fibers, and then sodium alginate oxide is added dropwise. The reaction occurs on the chitosan layer to form a composite fiber with a core-shell structure, that is, a porous membrane is coated on the fiber. The purpose is to build a loose structure on the fiber surface to facilitate gas passage. However, it does not involve the formation of a network structure by chitosan together with fibers and other components, and it does not disclose the anti-aging effect.

[0007] Chinese invention patent application CN117468263A discloses an archival paper based on the full utilization of paper mulberry bark and its preparation method. It discloses that the high crystallinity of microcrystalline cellulose can be used to increase the paper's anti-aging properties. However, it only discloses the role of microcrystalline cellulose as a filler and alkali source, which is limited to physical filling and delaying acidification. It does not disclose the correlation effect formed by the interaction between microcrystalline cellulose and antioxidants and fiber crosslinking agents.

[0008] Therefore, in order to solve the above problems, the present invention provides a method for preparing an environmentally friendly anti-aging packaging base paper and the packaging base paper itself. Summary of the Invention

[0009] This invention aims to solve the technical problems in the prior art, such as a significant decrease in tear strength, a marked reduction in whiteness, and shrinkage and deformation of paper surface pores after thermal aging. It first provides a method for preparing an environmentally friendly anti-aging packaging base paper, including the following steps:

[0010] S10: Take softwood pulp and dissolving pulp for the first pulping treatment, then add microcrystalline cellulose, stir, and form the first pulp;

[0011] S20: Adjust the pH of the first slurry to neutral, then add water-soluble chitosan-polyphenol complex, and perform a second pulping treatment to form the second slurry;

[0012] S30: Add sodium alginate oxide to the second pulp and perform a third pulping process to form a third pulp; then form the base paper according to the papermaking process.

[0013] Optionally, the dissolving pulp is a softwood dissolving pulp.

[0014] Optionally, the oven-dry mass ratio of the softwood pulp to the oven-dry mass ratio of the dissolving pulp is (80-90):(5-10).

[0015] Optionally, the average fiber length of the softwood pulp is greater than or equal to 2.5 mm, and the degree of polymerization of the dissolving pulp is greater than or equal to 800.

[0016] Optionally, the preparation method shall satisfy at least one of the following conditions (A) to (C):

[0017] (A) The freeness of the first slurry is 24°SR to 28°SR;

[0018] (B) The freeness of the second pulp is 29°SR to 33°SR;

[0019] (C) The beating degree of the third slurry is 34°SR to 36°SR.

[0020] Optionally, the microcrystalline cellulose has a particle size of 20 μm to 50 μm; the amount of microcrystalline cellulose added, based on oven-dry weight, is 3.0% to 8.0% of the oven-dry pulp weight of the total raw materials used.

[0021] Optionally, the microcrystalline cellulose is an aqueous suspension of microcrystalline cellulose with a mass concentration of 9% to 11%.

[0022] Optionally, the mass ratio of chitosan to polyphenol in the water-soluble chitosan-polyphenol complex is 1:0.8 to 1:1.5; the amount of the water-soluble chitosan-polyphenol complex added, based on oven-dry weight, is 0.5% to 1.0% of the oven-dry slurry mass of the total raw materials used.

[0023] Optionally, the water-soluble chitosan-polyphenol complex is an aqueous solution of water-soluble chitosan-polyphenol complex with a mass concentration of 0.4% to 0.6%.

[0024] Optionally, the water-soluble chitosan-polyphenol complex is a complex composed of chitosan and at least one of tannic acid, rosemary extract, or tea polyphenols; the degree of deacetylation of the chitosan is greater than or equal to 85%.

[0025] Optionally, the water-soluble chitosan-polyphenol complex is formed through self-assembly via non-covalent interactions such as electrostatic attraction, hydrogen bonding, or hydrophobic interactions.

[0026] Optionally, the amount of sodium oxidized alginate added, based on oven-dry weight, is 0.2% to 0.5% of the oven-dry pulp weight of the total raw materials used; the aldehyde content of the sodium oxidized alginate is 0.8 mmol / g to 1.5 mmol / g.

[0027] Optionally, the sodium alginate is an aqueous solution of sodium alginate with a mass concentration of 0.5% to 1.0%.

[0028] Optionally, the papermaking process is as follows: a hand-made sheet is prepared by standard sheet-making method to obtain a wet paper sheet, and the wet paper sheet is pressed and dried in sequence.

[0029] The present invention also provides a packaging base paper, which is prepared by the above-described preparation method; the packaging base paper, after being subjected to accelerated aging treatment at 150°C for 8 hours, meets at least one of the following conditions (D) to (F):

[0030] (D) Tear retention rate greater than 90%;

[0031] (E) The decrease in whiteness from the initial value is less than or equal to 4 units;

[0032] (F) The average aperture increases by less than or equal to 10% from the initial value.

[0033] The beneficial effects of this invention are as follows:

[0034] (1) By adding microcrystalline cellulose as a crystal nucleus to stabilize the fiber network and form a skeleton support effect, when the matrix fiber undergoes thermal aging shrinkage, the microcrystalline cellulose particles, due to their extremely low coefficient of thermal expansion and high rigidity, can effectively inhibit local fiber collapse and maintain the pore structure and fiber spacing. At the same time, microcrystalline cellulose is equivalent to introducing a large amount of low-reactivity highly crystalline material into the system, thereby diluting the proportion of easily degradable amorphous components, slowing down the rate of decrease in the average degree of polymerization of the entire paper sheet, and increasing the tear retention rate. Furthermore, the highly crystalline microcrystalline cellulose particles are tightly bound to the matrix fiber through hydrogen bonds, forming physical cross-linking points, which restricts the free rotation and migration of cellulose macromolecular chains, especially inhibiting the transfer and oxidation of lignin phenolic oxygen free radicals to cellulose chains, thereby slowing down the generation of conjugated carbonyl compounds and delaying the decrease in whiteness (yellowing). Moreover, the surface of microcrystalline cellulose is mainly crystalline, which is not easily subjected to acid-catalyzed hydrolysis, reducing the overall hydrophilicity of the paper and reducing the degradation of cellulose induced by moisture.

[0035] (2) By adding water-soluble chitosan-polyphenol complex, chitosan acts as a "carrier" and is adsorbed on the surface of negatively charged fibers through electrostatic interaction. On the one hand, it can significantly improve the retention rate of antioxidants, so that polyphenol components are released slowly, free radicals are removed, and lignin oxidation and yellowing are inhibited to achieve "slow-release antioxidation". On the other hand, it provides amino (-NH2) reaction sites for bridging sodium alginate.

[0036] (3) The sodium oxidized alginate molecule contains aldehyde and carboxyl groups. The aldehyde group can react with the primary amino group on the chitosan molecular chain to form a covalently cross-linked imine bond. At the same time, the carboxyl group forms multiple hydrogen bonds with the hydroxyl groups on the surface of cellulose fibers, thereby constructing a "covalent + physical" synergistic reinforcement network among the three components of "fiber-chitosan-sodium oxidized alginate", which jointly enhances the bonding force between fibers. This cross-linked structure effectively restricts the thermal motion of cellulose molecular chains at high temperatures, inhibits fiber thermal shrinkage, and reduces the formation of micropore collapse and microcracks. Sodium oxidized alginate can also work synergistically with polyphenolic antioxidants to significantly delay the decrease in whiteness of paper during thermal aging by chelating oxidizing metal ions and stabilizing free radicals.

[0037] (4) Microcrystalline cellulose, water-soluble chitosan-polyphenol complex and sodium oxidized alginate are added sequentially in the pulping stage, which can synergistically regulate the timing matching of fiber dispersion, interfacial adsorption and covalent crosslinking; and combined with the pulping degree of each stage, the technical process of building a stable fiber network layer by layer is realized, forming a triple mechanism effect of "skeleton support → covalent crosslinking → anti-oxidation end sealing", thereby realizing the processing of anti-aging base paper.

[0038] (5) The raw materials used in this invention are all derived from renewable biomass resources and do not contain petroleum-based synthetic polymers or persistent organic pollutants. They are environmentally friendly raw materials, specifically including: softwood pulp and dissolving pulp, which are made from timber from sustainably managed forest land. The main component is natural cellulose, which has good biodegradability; microcrystalline cellulose, which is purified from plant cellulose by physical-chemical methods and is widely used in the field of environmental protection materials; and water-soluble chitosan-polyphenol complex, in which chitosan is derived from chitin in crustacean aquatic processing waste and polyphenols are extracted from plant-derived substances such as tea and tannins. The two react non-covalently in the aqueous phase. Self-assembled; oxidized sodium alginate, prepared by selective oxidation of sodium alginate extracted from brown algae (such as kelp), retaining the natural polysaccharide skeleton, its molecular structure contains only carbon, hydrogen, oxygen and a small amount of sodium; the above components are all natural polymers or their derivatives that are widely found in nature, and can be gradually degraded by microorganisms into carbon dioxide, water and inorganic salts in composting, soil or aquatic environments, meeting the basic requirements of green packaging materials for resource renewability and end-of-life disposal; it is common knowledge in the art that such biomass materials usually have good environmental compatibility because of their natural source and clear metabolic pathway (see: GB / T 41010–2021 "Degradation performance and labeling requirements of biodegradable plastics and products"; ISO 16620-2:2015 Plastics — Determination of biobased content — Part 2: Carbon content determination by radiocarbon analysis). Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0040] Figure 1 This is a process flow diagram for preparing the packaging base paper according to the present invention;

[0041] Figure 2 This is a schematic diagram illustrating the chemical bond connection method of "cellulose-chitosan-sodium oxidized alginate-chitosan-cellulose" in this embodiment;

[0042] Figure 3 This is a schematic diagram illustrating the mechanism by which the "fiber-chitosan-oxidized sodium alginate-chitosan-fiber" network is formed in this embodiment.

[0043] Figure 4 A photograph of the original paper prepared according to this embodiment;

[0044] Figure 5 The images show a comparison of the whiteness of the original paper prepared in Example 1 and Comparative Examples 1 to 3 in this embodiment.

[0045] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0048] In the description of this invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features; thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature; in the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0049] The weights of the relevant components mentioned in the embodiments of this invention can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is scaled up or down proportionally according to the embodiments of this invention, it is within the scope disclosed in the embodiments of this invention. Specifically, the weights mentioned in the embodiments of this invention can be well-known mass units in the chemical industry, such as μg, mg, g, kg, nm, μm, cm, mm, wt%, mPa, L, mL, etc.

[0050] The following specific examples will provide further explanation.

[0051] The method for preparing the anti-aging packaging base paper according to this embodiment includes the following steps:

[0052] The components and dosages are as follows:

[0053] Commercially available softwood pulp board: Weigh a sample with an oven-dry weight of (17.0±0.5) g, the amount of which is 85% of the oven-dry pulp weight of the total raw materials used in this embodiment; the average fiber length of the softwood pulp is 2.5 mm, which can provide mechanical support and main fiber skeleton; the brand of the softwood pulp board is CanforPulp, and the manufacturer is CanforPulp in Canada.

[0054] Commercially available dissolving pulp slabs: Weigh out a sample with an oven-dry weight of (2.0g ± 0.5g), the amount of which is 10% of the oven-dry pulp weight of the total raw materials used in this embodiment; the dissolving pulp has a degree of polymerization of 850, and 93% of its components are high-purity cellulose (α-cellulose), with a regular structure and fully exposed hydroxyl groups, making it very easy to participate in chemical cross-linking reactions and effectively maintaining the integrity of the cellulose backbone; the brand of the dissolving pulp slabs is Bioore, and the manufacturer is UPM.

[0055] Softwood pulp and dissolving pulp provide the cellulose basis for the preparation of packaging base paper and for further processing.

[0056] It is worth noting that in this embodiment, the mass ratio of softwood pulp to dissolving pulp is (80-90):(5-10), and the selection of long-fiber softwood pulp with an average length of 2.5 mm and dissolving pulp with a higher degree of polymerization of 850 can effectively enhance the function of subsequent microcrystalline cellulose, sodium alginate oxide, and water-soluble chitosan-polyphenol complex, and is more conducive to the formation of a stable cross-linking network with sodium alginate oxide; the high degree of polymerization of dissolving pulp is more conducive to the formation of chemical cross-linking reaction with sodium alginate oxide, and can effectively maintain the integrity of the cellulose backbone.

[0057] Microcrystalline cellulose: Weigh a sample with an oven-dry weight of (1.0g ± 0.1)g, and add it in the form of an aqueous suspension with a mass concentration of 10%, based on the oven-dry weight of the total raw materials used in this embodiment, which is 5wt% of the oven-dry pulp.

[0058] The preparation method of the microcrystalline cellulose aqueous suspension is as follows: microcrystalline cellulose with a particle size of 35μm is added to water, stirred for 10 minutes at 8000rpm using a high-speed shear emulsifier, and then ultrasonically dispersed for 5 minutes (optionally, the ultrasonic dispersion power is 200W, the frequency is 140kHz, and the pulse mode is preferred) to ensure that the microcrystalline cellulose is uniformly dispersed without flocculation; the microcrystalline cellulose model is MCCPH-105, and the manufacturer is Shandong Jiahe.

[0059] Water-soluble chitosan-polyphenol complex: Weigh a sample with an oven-dry weight of (0.180±0.002) g. The amount added is 0.9% of the oven-dry weight of the total raw materials used in this embodiment, based on the oven-dry weight of chitosan and polyphenols. The oven-dry weight ratio of chitosan to polyphenols is 1:1.0, and it is added in the form of an aqueous solution with a mass concentration of 0.60%.

[0060] The preparation method of water-soluble chitosan-polyphenol complex is as follows: Using a 1.5% (v / v) aqueous acetic acid solution as the solvent, chitosan with a degree of deacetylation of 90% to 92% is prepared into a 1.5% (w / w) chitosan solution; tannic acid (or rosemary extract, or tea polyphenols) is dissolved in deionized water to prepare a 1.5% (w / w) polyphenol solution; the oven-dry mass ratio of chitosan to tannic acid is controlled at 1:1.0; the polyphenol solution is slowly added dropwise to the chitosan solution under stirring at room temperature, and stirring is continued for 6 hours. 0 min, until a semi-transparent, stable aqueous complex solution is formed; adjust the pH of the system to 5.5 to 6.0 with alkali; ultrafiltration desalt (molecular weight cutoff 10 kDa) removes free acetic acid and unbound small molecules to obtain a homogeneous complex solution without flocculation or precipitation, and evaporates under reduced pressure at 33 °C, with the final solid content of the aqueous solution being 0.60% by mass; during this process, water-soluble chitosan and polyphenols are formed through self-assembly via electrostatic attraction, hydrogen bonding, or hydrophobic interactions (non-covalent interactions).

[0061] This embodiment uses a water-soluble chitosan-polyphenol complex, a natural antioxidant. Based on chitosan as a "carrier," it is adsorbed onto the surface of negatively charged fibers through electrostatic interaction. During pulping, it is not easily damaged structurally, maintains good water solubility in the pulp, does not flocculate, and can be quickly adsorbed onto the fiber surface. This provides abundant amino reaction sites for the subsequent cross-linking reaction of oxidized sodium alginate, participating in the Schiff base cross-linking of sodium alginate with oxidized sodium alginate. This method significantly improves the retention rate of antioxidants. The polyphenol components are slowly released, scavenging free radicals and inhibiting lignin oxidation and yellowing, achieving the effect of "slow-release antioxidant".

[0062] The composite structure is formed through electrostatic attraction and multiple hydrogen bonds between the protonated amino groups of chitosan and the hydroxyl groups of tannic acid polyphenols, supplemented by weak hydrophobic interactions, thereby constructing a stable nanoscale or molecular-scale composite structure in the aqueous phase. The composite process reaches the optimal equilibrium at a pH of 5.5 to 6.0, which ensures that the chitosan is fully protonated while avoiding excessive oxidation or precipitation of polyphenols.

[0063] Chitosan, model number JK-90, is manufactured by Zhejiang Jinke Biochemical Co., Ltd.; tannic acid, model number Cat.No.T0375, is manufactured by TCI Corporation of Japan.

[0064] Sodium alginate oxide: Weigh a sample with an oven-dry weight of (0.080±0.001) g. The amount added is 0.4% of the oven-dry weight of the total raw material used in this embodiment, based on the oven-dry weight. It is added in the form of an aqueous solution with a mass concentration of 0.8%. The preparation method is as follows: Take sodium alginate oxide with an aldehyde content of 1.2 mmol / g, add deionized water to prepare an aqueous solution with a mass ratio of 0.8%, put it into an ultrasonic disperser, and disperse it for 45 min under the conditions of ultrasonic frequency of 350 kHz and power of 600 W to ensure that there is no precipitation of sodium alginate oxide and obtain a clear and transparent homogeneous dispersion. The manufacturer of sodium alginate oxide is NOF Corporation of Japan.

[0065] Please see Figure 1 ; Figure 1 This is a process flow diagram for preparing the packaging base paper according to the present invention;

[0066] The preparation method of anti-aging packaging base paper is as follows:

[0067] Step S10:

[0068] The softwood pulp and dissolving pulp are subjected to a first pulping process, and then microcrystalline cellulose is added and stirred to form the first pulp.

[0069] Specifically, take (20±0.5) g of oven-dried pulpboard from both softwood pulp and dissolving pulp, tear it into small pieces (approximately 1 cm² to 2 cm²), and soak the pulpboard in water at room temperature for 60 minutes to allow the fibers to fully swell. Transfer the swollen mixed pulp to a fiber debonding machine, add water to adjust the pulp concentration to 1.5%, start the L&W fiber debonding machine, and debond the pulp at 10,000 revolutions under standard laboratory conditions (refer to TAPPI). (T205 or SCAN-C28:87 standard method) to obtain a loosened pulp; place the loosened pulp in a 120-mesh filter bag and centrifuge to concentrate it, squeeze out excess water, and adjust the pulp concentration to 12%; add microcrystalline cellulose in the form of an aqueous suspension with a mass concentration of 10% to the above pulp, the amount of which is 5 wt% of the oven-dry pulp mass of the total raw materials used in this embodiment; start the PFI refiner to perform the first pulping treatment; set the disc mill gap to 0 mm to 0.2 mm, the refiner speed to 7000 rpm, and monitor the pulp beating degree in real time until it reaches 24°SR to 28°SR, at which point the microcrystalline cellulose particles have been fully embedded in the fiber gaps, and the first pulp is obtained.

[0070] The mechanism and function of using microcrystalline cellulose in this embodiment include:

[0071] (1) The crystallinity of microcrystalline cellulose is 70% to 85%, which is much higher than that of ordinary chemical pulp (40% to 60%); the decomposition temperature of microcrystalline cellulose is ≥320℃, which is much higher than that of ordinary chemical pulp (200℃).

[0072] (2) Microcrystalline cellulose acts as a crystal nucleus to stabilize the fiber network and form a skeleton support effect. When the matrix fiber undergoes thermal aging and shrinkage, the microcrystalline cellulose particles, due to their extremely low coefficient of thermal expansion and high rigidity, can effectively inhibit local fiber collapse and maintain the pore structure and fiber spacing.

[0073] (3) The amorphous region of cellulose is the main area where the aging reaction occurs. Adding microcrystalline cellulose introduces a large amount of low-reactivity, highly crystalline substances into the system, thereby diluting the proportion of easily degradable amorphous components, slowing down the rate of decrease in the average degree of polymerization of the entire paper sheet, and increasing the tear retention rate.

[0074] (4) Restricting the movement of cellulose chain segments and inhibiting oxidation chain reaction; the highly crystalline microcrystalline cellulose particles are tightly bound to the matrix fibers through hydrogen bonds, forming physical cross-linking points, which restricts the free rotation and migration of cellulose macromolecular chains, inhibits the transfer and oxidation of lignin phenolic oxygen free radicals to cellulose chains, thereby slowing down the generation of conjugated carbonyl compounds and delaying the decrease in whiteness (i.e., yellowing).

[0075] (5) The surface of microcrystalline cellulose is mainly crystalline, which reduces the overall hydrophilicity of the paper, making it less susceptible to acid-catalyzed hydrolysis and reducing moisture-induced cellulose degradation.

[0076] (6) Microcrystalline cellulose can provide a stable platform for subsequent cross-linking of oxidized sodium alginate and adsorption of water-soluble chitosan-polyphenol complex, making the overall three-dimensional network more robust.

[0077] (7) In this embodiment, microcrystalline cellulose is the first link in the triple mechanism of “skeleton support → covalent crosslinking → antioxidant end capping”, and plays the role of systemic structural support. It must be used and added in a specific order with other components. Its function has been deeply integrated into a brand-new technical system.

[0078] Step S20:

[0079] The pH of the first slurry is adjusted to neutral, and then water-soluble chitosan-polyphenol complex is added for a second pulping process to form the second slurry.

[0080] Specifically, the pulping process is paused, the pH value of the first pulp is tested, and a 1% NaOH solution is added dropwise to adjust the pH of the pulp to 6.8 to 7.2. Then, a water-soluble chitosan-polyphenol complex is added to the first pulp in the form of a 0.60% aqueous solution. The amount added is 0.9% of the total dry pulp mass of the raw materials used in this embodiment, based on the oven-dry mass of chitosan and tannic acid. The PFI refiner is restarted, with the disc gap between -0.2 mm and 0 mm, and the refiner speed at 3000 rpm, for a second pulping. The freeness of the pulp is monitored in real time until it rises to 29°SR to 33°SR. At this point, the fibers are moderately fibrillated, exposing more hydroxyl groups, and the chitosan has been fully pre-adsorbed onto the fiber surface, forming the second pulp.

[0081] The water-soluble chitosan-polyphenol complex used in this embodiment combines chitosan and polyphenols through a chemical compounding process to form a novel functional material with the following properties:

[0082] (1) Chitosan provides cationic properties, ensuring high retention (>90%) on negatively charged fibers. It provides amino (-NH2) reaction sites for subsequent bridging of oxidized sodium alginate;

[0083] (2) Polyphenols provide free radical scavenging ability and inhibit lignin oxidation and yellowing;

[0084] (3) After compounding, a sustained-release structure is formed to avoid the rapid loss of polyphenols and achieve a long-lasting antioxidant mechanism;

[0085] (4) The preparation process of the water-soluble chitosan-polyphenol complex in this embodiment is as follows: react under pH conditions of 5.5 to 6.0, and remove salt by ultrafiltration to obtain a stable, clear, and soluble composite solution that can exert the above-mentioned maximum effect; rather than directly mixing and using it, otherwise precipitation is easily generated and it cannot be applied evenly.

[0086] It is worth noting that, unlike existing technologies that use chitosan-polyphenol complexes to coat paper surfaces for localized functionalization, this embodiment introduces chitosan and polyphenols during the pulping stage, allowing them to synergistically participate in the covalent cross-linking reaction between fibers, forming a bulk reinforcing network that runs through the paper sheet. This network combines structural strengthening and antioxidant functions, overcoming the limitations of traditional surface modification.

[0087] Step S30:

[0088] Sodium alginate oxide is added to the second pulp, and a third pulping process is performed to form the third pulp; then, according to the papermaking process, the base paper is formed.

[0089] Specifically, the pulping process is paused, and an aqueous solution of sodium alginate oxide with a mass concentration of 0.8% is added to the second pulp. The amount added is 0.4% of the oven-dry pulp mass of the total raw materials used in this embodiment, based on oven-dry mass. The PFI refiner is started, with a disc gap of 0 mm to 0.2 mm and a refiner speed of 5000 rpm, for a third pulping process until the freeness rises to 34°SR to 36°SR. At this point, a cross-linked network of "fiber-chitosan-sodium alginate oxide-chitosan-fiber" has been formed, resulting in the third pulp. The third pulp is then used to prepare hand-made sheets using the standard sheet-making method to obtain wet paper sheets. The wet paper sheets are then pressed and dried sequentially to obtain a basis weight of (60±1) g / m³. 2 The base paper.

[0090] This process fully utilizes the fiber state at different pulping stages to achieve a synergistic effect of physical and chemical reinforcement, significantly improving the paper's anti-aging properties.

[0091] Please see Figure 2 and Figure 3 ; Figure 2 This is a schematic diagram illustrating the chemical bond connection method of "cellulose-chitosan-sodium oxidized alginate-chitosan-cellulose" in this embodiment; Figure 3 This is a schematic diagram illustrating the mechanism by which the "fiber-chitosan-oxidized sodium alginate-chitosan-fiber" network is formed in this embodiment.

[0092] The purpose and principle of adding specific components at each pulping stage are as follows:

[0093] Microcrystalline cellulose is added at low freeness (24°SR to 28°SR) and embedded in the fiber gaps to form a rigid skeleton, providing a stable distribution platform for subsequent addition of various components and avoiding local aggregation caused by excessively loose fibers.

[0094] When the freeness reaches a moderate level (29°SR to 33°SR), a water-soluble chitosan-polyphenol complex is introduced. At this point, the fiber begins to undergo moderate fibrillation, and the number of surface active sites increases, especially the exposure of hydroxyl groups. The primary amino groups on the chitosan molecular chain can be rapidly adsorbed onto the fiber surface through electrostatic adsorption or hydrogen bonding, forming "molecular anchors" rich in reactive groups. These anchors provide clear reaction sites for the subsequently added sodium alginate oxide, greatly improving the crosslinking efficiency.

[0095] At the high freeness stage (34°SR to 36°SR) close to pulp formation, the fiber network has been basically formed, and the inter-fiber contact interface is rich and stable. When sodium alginate is added at this time, the aldehyde group (-CHO) on its molecular chain can react with the primary amino group on the chitosan molecular chain previously adsorbed on the fiber surface to form an imine bond (-CH=N-) in situ, forming an irreversible covalent cross-linked network.

[0096] Thus, a three-dimensional cross-linked network structure of "fiber-chitosan-sodium alginate-chitosan-fiber" was constructed among the fiber (provided by softwood pulp and dissolving pulp), chitosan, and sodium alginate oxide. This network effectively restricts the thermal motion of cellulose molecular chains under high-temperature conditions, significantly inhibits fiber thermal shrinkage, and reduces micropore collapse and microcrack formation. Simultaneously, sodium alginate oxide and polyphenols synergistically exert antioxidant effects by chelating Fe... 2+ / Cu 2+ It promotes the oxidation of metal ions and the scavenging of free radicals, delaying the whiteness degradation of paper during the thermal aging process, thereby improving the long-term durability of the material.

[0097] This embodiment provides a method for preparing anti-aging packaging base paper, which differs from the reversible crosslinking of borate esters relying on dynamic hydrogen bonds in the prior art. It uses sodium alginate oxide as a multifunctional crosslinking agent to generate irreversible imine bonds (-CH=N-), and its aldehyde / carboxyl group structure can construct a covalent-hydrogen bond synergistic three-dimensional network. Unlike the small molecule aldehyde crosslinking agents such as glutaraldehyde in the prior art, it has the advantages of being non-toxic and biodegradable, which can avoid free aldehyde residue and material embrittlement, and has both environmental friendliness and functionality.

[0098] Figure 2 The structural diagrams in the diagram are explained below:

[0099] (1) CS: Chitosan;

[0100] (2) OSA: Oxidized sodium alginate;

[0101] (3) Dashed line segment: hydrogen bond (multi-site);

[0102] (4) Solid line segment: electrostatic adsorption (multi-site);

[0103] (5) Double solid line segment: imine bond (multi-site).

[0104] Figure 3 The structural diagrams in the diagram are explained below:

[0105] (1) Cellulose:

[0106] The large green six-membered ring structure in the figure represents the polysaccharide backbone of cellulose, with each six-membered ring connected to an oxygen atom and a hydroxyl group (-OH) represented by a blue sphere.

[0107] (2) Chitosan (CS):

[0108] Polysaccharides containing amino groups (-NH2) are shown in the diagram. The red "N" atom represents the amino group in chitosan. The amino group participates in the formation of covalent bonds or hydrogen bonds.

[0109] (3) Oxidized sodium alginate (OSA):

[0110] White oxygen atoms (O) and red sulfur atoms (S), with the oxygen atoms partially attached to the end of the carbon chain to form -CHO (aldehyde group) or -COOH (carboxyl group).

[0111] To further illustrate the influence of each key component of this embodiment on the performance of the prepared base paper, experimental tests were conducted. With the amounts of softwood pulp and dissolving pulp fixed, the effects of the amounts of microcrystalline cellulose, water-soluble chitosan-polyphenol complex, and oxidized sodium alginate on the performance of the prepared base paper were tested.

[0112] Extensive experiments were conducted to test different amounts of microcrystalline cellulose, water-soluble chitosan-polyphenol complex, or oxidized sodium alginate, and the preparation process was analyzed. Tests were also performed on samples of the prepared base paper. The following results were obtained:

[0113] (1) The optimal dosage range of microcrystalline cellulose is 3.0% to 8.0% (by mass); when the dosage is <3%, the skeleton support of the base paper is insufficient; when the dosage is >8%, the uniformity and flexibility of the paper are affected.

[0114] (2) The optimal dosage range of water-soluble chitosan-polyphenol complex is 0.5% to 1.0% (by mass). When the dosage is <0.5%, the resulting base paper is prone to yellowing. When the dosage is >1.0%, the material cost is high and the paper uniformity is affected.

[0115] (3) The optimal dosage range of sodium alginate is 0.2% to 0.5% (by mass). When the dosage is <0.2%, the cross-linking of the reaction process is insufficient, resulting in insufficient anti-aging of the packaging paper. When the dosage is >0.5%, excessive cross-linking will occur, leading to increased brittleness.

[0116] Please see Figure 4 and Figure 5 ; Figure 4 A photograph of the original paper prepared according to this embodiment; Figure 5 The images show a comparison of the whiteness of the original paper prepared in Example 1 and Comparative Examples 1 to 3 in this embodiment.

[0117] To further illustrate the effect of the order of adding microcrystalline cellulose, water-soluble chitosan-polyphenol complex and oxidized sodium alginate in the preparation method of this embodiment (Example 1, and Comparative Examples 1 to 4), and the effect of not adding microcrystalline cellulose (Comparative Example 5) on the performance of the prepared base paper, base papers prepared by different preparation methods as shown in the table below are provided and tested.

[0118] Table 1 - Process schemes with different addition sequences and beating windows:

[0119]

[0120] Based on the addition order and freeness window corresponding to the above embodiments or comparative examples, the base paper samples prepared were subjected to performance testing according to the following methods:

[0121] Initial tear index: determined according to GB / T 12914-2008;

[0122] Dry heat aging conditions: 8 hours in an oven at 150℃;

[0123] Tear retention rate = (tear index after aging / initial tear index) × 100%;

[0124] Whiteness determination: CIE whiteness was determined according to GB / T 7974-2013;

[0125] Whiteness reduction (Δ%) = Initial whiteness - Whiteness after aging;

[0126] Average pore size determination: determined by nitrogen adsorption-desorption isotherms, and pore structure parameters were analyzed and calculated according to ISO 15901-2:2006.

[0127] The following results were obtained after the test:

[0128] Table 2 - Process schemes for different addition sequences and beating windows:

[0129]

[0130] The test results above show that:

[0131] (1) The tear retention rate comparison shows that the addition order of Example 1 used in this embodiment, combined with the pulping window value, makes the tear retention rate reach 90.6% (tear retention rate greater than 90%), which is significantly better than Comparative Examples 1 to 4.

[0132] (2) Whiteness stability comparison: The whiteness of Example 1 used in this embodiment decreased by only 3.3 percentage points (the decrease in whiteness from the initial value is less than or equal to 4 units), and the anti-aging effect was the best; the whiteness of Comparative Examples 1 to 3 (and 4) decreased by more than 6 percentage points, especially Comparative Example 2 which was close to 8 percentage points and showed obvious yellowing.

[0133] (3) Comparison of the increase in average pore size: The average pore size of Example 1 used in this embodiment increased by 8.1% compared with the initial value (the increase in average pore size compared with the initial value is less than or equal to 10%), which is significantly lower than that of Comparative Example 1, indicating that the microstructure of the paper sheet maintained good stability during the thermal aging process.

[0134] (4) As can be seen from the comparison between Example 1 and Comparative Example 5, the increase in average pore size in Example 1 is significantly better than that in Comparative Example 5. This effect shows that microcrystalline cellulose has a significant effect on inhibiting micropore collapse and abnormal increase in average pore size during thermal aging. Comparative Example 5 did not add microcrystalline cellulose, but only introduced water-soluble chitosan-polyphenol complex and sodium alginate. Its average pore size increased by as much as 36.5% after thermal aging, which is much higher than 8.1% in Example 1. The pore size reached 389.623 Å after aging, indicating that there was obvious pore expansion and loose structure inside the paper sheet, which led to a significant decrease in tear strength. In contrast, Example 1, by preferentially adding microcrystalline cellulose, constructed a high-rigidity three-dimensional network skeleton in the early stage of pulping, which effectively maintained the integrity of the microstructure. This result fully demonstrates that the early introduction of microcrystalline cellulose is crucial for improving the thermal stability and mechanical durability of paper base materials, and can provide a stable distribution platform for subsequent addition of various components.

[0135] The above test results demonstrate that the order of addition of microcrystalline cellulose, water-soluble chitosan-polyphenol complex, and sodium alginate in this embodiment has a significant impact on the anti-aging effect, and plays an important role in coordination with the beating window.

[0136] Specifically, microcrystalline cellulose is added first under low freeness (24°SR to 28°SR). At this time, the fibers have not yet fully swelled, and the microcrystalline cellulose particles can be evenly dispersed and embedded in the fiber gaps to form an initial skeleton and provide rigid support. If added later, it is difficult to distribute evenly.

[0137] Secondly, a water-soluble chitosan-polyphenol complex is added under medium beating conditions (28°SR to 32°SR). The fibers have been moderately split and bristled, exposing more hydroxyl groups, which allows chitosan to be fully pre-adsorbed onto the fiber surface, providing -NH2 reaction sites for subsequent sodium alginate bridging.

[0138] Finally, add the fiber crosslinking agent sodium alginate, which is added when the pulp is almost formed. At this time, the fiber network has been basically formed. The aldehyde group (-CHO) of sodium alginate can bridge the two fibers that have adsorbed chitosan, thereby forming a covalent network.

[0139] This application verification fully demonstrates that the anti-aging base paper prepared by this invention, through the construction of a stable fiber network structure layer by layer, possesses synergistic effects of long-lasting anti-aging capability and green environmental friendliness.

[0140] It should be noted that, for those skilled in the art, appropriate adjustments can be made to the component categories, component amounts, preparation parameters, process parameters, etc., without departing from the concept of the present invention, to adapt to preparation requirements and industrial processing conditions, without any creative effort required; therefore, any simple substitutions or deductions made in accordance with the component conditions, preparation principles, process routes, and interface control strategies disclosed in the present invention should fall within the protection scope of the present invention.

[0141] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments; therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

[0142] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of this invention; thus, if these modifications and variations of this invention fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing an environmentally friendly, anti-aging packaging base paper, characterized in that, The preparation method includes the following steps: S10: Take softwood pulp and dissolving pulp for the first pulping treatment, then add microcrystalline cellulose, stir, and form the first pulp; S20: Adjust the pH of the first slurry to neutral, then add water-soluble chitosan-polyphenol complex, and perform a second pulping treatment to form the second slurry; S30: Add sodium alginate oxide to the second pulp and perform a third pulping process to form a third pulp; then form the base paper according to the papermaking process.

2. The preparation method according to claim 1, characterized in that, The dissolving pulp is a softwood dissolving pulp.

3. The preparation method according to claim 1 or 2, characterized in that, The oven-dry mass ratio of the softwood pulp to the oven-dry mass ratio of the dissolving pulp is (80-90):(5-10).

4. The preparation method according to claim 3, characterized in that, The average fiber length of the softwood pulp is greater than or equal to 2.5 mm, and the degree of polymerization of the dissolving pulp is greater than or equal to 800.

5. The preparation method according to claim 1, characterized in that, The preparation method shall satisfy at least one of the following conditions (A) to (C): (A) The freeness of the first slurry is 24°SR to 28°SR; (B) The freeness of the second pulp is 29°SR to 33°SR; (C) The beating degree of the third slurry is 34°SR to 36°SR.

6. The preparation method according to claim 1, characterized in that, The microcrystalline cellulose has a particle size of 20 μm to 50 μm; the amount of the microcrystalline cellulose added, based on oven-dry weight, is 3.0% to 8.0% of the oven-dry pulp weight of the total raw materials used.

7. The preparation method according to claim 1 or 6, characterized in that, The microcrystalline cellulose is an aqueous suspension of microcrystalline cellulose with a mass concentration of 9% to 11%.

8. The preparation method according to claim 8, characterized in that, The mass ratio of chitosan to polyphenol in the water-soluble chitosan-polyphenol complex is from 1:0.8 to 1:1.5; the amount of the water-soluble chitosan-polyphenol complex added, based on oven-dry weight, is from 0.5% to 1.0% of the oven-dry slurry mass of the total raw materials used.

9. The preparation method according to claim 1 or 8, characterized in that, The water-soluble chitosan-polyphenol complex is an aqueous solution of water-soluble chitosan-polyphenol complex with a mass concentration of 0.4% to 0.6%.

10. The preparation method according to claim 1 or 8, characterized in that, The water-soluble chitosan-polyphenol complex is a complex composed of chitosan and at least one of tannic acid, rosemary extract or tea polyphenols; the degree of deacetylation of the chitosan is greater than or equal to 85%.

11. The preparation method according to claim 10, characterized in that, The water-soluble chitosan-polyphenol complex is formed through self-assembly via non-covalent interactions such as electrostatic attraction, hydrogen bonding, or hydrophobic interactions.

12. The preparation method according to claim 1, characterized in that, The amount of sodium oxidized alginate added, based on oven-dry weight, is 0.2% to 0.5% of the oven-dry pulp weight of the total raw materials used; the aldehyde content of the sodium oxidized alginate is 0.8 mmol / g to 1.5 mmol / g.

13. The preparation method according to claim 1 or 12, characterized in that, The sodium alginate is an aqueous solution of sodium alginate with a mass concentration of 0.5% to 1.0%.

14. The preparation method according to claim 1, characterized in that, The papermaking process is as follows: hand-made sheets are prepared by standard sheet-making method to obtain wet paper sheets, and the wet paper sheets are pressed and dried in sequence.

15. A type of packaging paper, characterized in that, The packaging base paper is prepared by the preparation method according to any one of claims 1 to 14; After the packaging paper is subjected to accelerated aging treatment at 150°C for 8 hours, it must meet at least one of the following conditions (D) to (F): (D) Tear retention rate greater than 90%; (E) The decrease in whiteness from the initial value is less than or equal to 4 units; (F) The average aperture increases by less than or equal to 10% from the initial value.

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