Preparation method of high-water-absorption napkin added with plant fibers

Abstract

The invention relates to the technical field of napkin preparation, and particularly discloses a preparation method of high-water-absorption napkin added with plant fibers, and the preparation method comprises the following steps: S1, preparing raw materials; s2, plant fiber pretreatment; s3, carrying out selective oxidation and nanocrystallization; s4, performing in-situ graft polymerization to construct a hydrophilic chain segment; s5, preparing slurry and forming a net by a wet method; s6, structure fixing and drying arrangement; in-situ graft polymerization is carried out on the surface of the fiber, long-chain hydrophilic macromolecules with high water absorption capacity are'immobilized 'on the surface of the fiber, high water absorption chain segments are firmly anchored on the fiber by adopting chemical bonding or strong interaction, 'immobilized water absorption level points' are formed, and migration is more difficult compared with single addition of granular SAP (Styrene Acrylate Polymer); the grafting layer presents a local colloid / gel structure in a wet state, can provide an additional water storage site on a micro scale, and simultaneously cooperates with mechanical support of the fiber.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of napkin paper preparation, and particularly relates to a preparation method of high water-absorbing napkin paper added with plant fibers. BACKGROUND

[0002] In daily life and catering services, napkin paper as an indispensable sanitary product, its performance directly affects the use experience. With the improvement of consumers' pursuit of life quality, higher requirements are put forward for the water absorption, softness, strength and environmental protection of napkin paper.

[0003] In the aspect of fiber treatment, the pretreatment of plant fibers in the prior art is not sufficient enough, and the surface impurities, lignin and hemicellulose of plant fibers are often not effectively removed, so that the fiber surface hydroxyl sites are not exposed enough, and the contact area between fibers is small, which not only affects the subsequent chemical modification and mechanical fibrillation effect, but also leads to serious fiber aggregation, uneven pulp dispersion, and further uneven pore structure during webbing, thereby reducing the overall performance of the napkin paper. For chemical modification of fibers, the existing method has limited ability to introduce polar functional groups on the fiber surface, and it is difficult to form enough ion crosslinking sites to provide effective structural stability in the wet state. At the same time, the bonding strength between the fiber and the hydrophilic polymer or the grafted segment is not enough, and the interface bonding is not firm enough, so that the overall performance of the composite cannot reach the ideal state. Moreover, the nanocrystallization degree of the fiber is not enough, the specific surface area is small, the capillary rise speed and instantaneous water absorption capacity need to be improved, and the role as a bridge and reinforcement phase in the macro fiber network is not significant, which is difficult to improve the micro continuity and wet mechanical stability of the network.

[0004] To this end, the application provides a preparation method of high water-absorbing napkin paper added with plant fibers to solve the above problems. SUMMARY

[0005] The application aims to provide a preparation method of high water-absorbing napkin paper added with plant fibers to solve the problems in the background art.

[0006] To achieve the above-mentioned purpose, the application provides the following technical scheme:

[0007] A preparation method of high water-absorbing napkin paper added with plant fibers, comprising the following steps:

[0008] S1, raw material preparation: weighing softwood pulp or short fiber wood pulp, plant short fiber, carboxylated precursor reagent, superabsorbent material (SAP), dispersant, water-retaining aid and water to prepare a uniform raw material mixture to obtain a raw material component;

[0009] S2, plant fiber pretreatment: the plant short fiber in the raw material component is contacted with an alkaline solution and washed to remove surface impurities and loosen the fiber bundle, to obtain activated plant fiber;

[0010] S3, selective oxidation and nanofiber: the activated plant fiber is subjected to controlled oxidation in a TEMPO or equivalent oxidation system to introduce carboxyl groups, and the oxidized fiber is subjected to mechanical shearing or high-pressure homogenization treatment to obtain a nanofiber slurry with carboxyl groups on the surface and partial nanofiber.

[0011] S4, in-situ grafting polymerization to construct hydrophilic segments: the nanofiber slurry is subjected to in-situ grafting or copolymerization reaction with hydrophilic monomers, crosslinking agent and initiator under stirring conditions to form a hydrophilic polymer layer on the fiber surface, and a grafting composite is obtained.

[0012] S5, slurry preparation and wet laying: the grafting composite is uniformly mixed with wood pulp short fiber, carboxymethyl cellulose (CMC) and superabsorbent material SAP added in proportion, and subjected to wet laying and dewatering to form a wet paper web.

[0013] S6, structure fixing and drying finishing: the wet paper web is subjected to low-temperature setting treatment and a multi-scale porous double network structure is constructed through freeze-thaw cycle or ionic crosslinking, followed by segmented drying, edge sealing and surface finishing to form a high water absorption paper towel product.

[0014] Preferably, the alkaline solution in step S2 is NaOH aqueous solution, the mass concentration of NaOH is 0.5-6wt%, the treatment temperature is 20-80℃, and the treatment time is 30-120min, to remove lignin and impurities on the surface of plant fiber and loosen the fiber bundle, to obtain activated plant fiber neutralized and washed to neutral.

[0015] Preferably, the selective oxidation in step S3 uses TEMPO / NaBr / NaClO system, the reaction pH is 9-11, the temperature is 0-25℃, the oxidation degree is controlled to make the carboxyl content on the fiber surface 0.2-1.5mmol / g, and then high shear homogenization or ultrasonic treatment is used to make part of the fiber nanofiber, to obtain nanofiber slurry with an average fiber diameter less than 200nm.

[0016] Preferably, the hydrophilic monomer in step S4 is one of acrylic acid, sodium acrylate or acrylamide or a mixture thereof, the monomer amount is 10-60wt% based on the dry weight of the fiber, the crosslinking agent amount is 0.05-1.0wt% of the mass of the monomer, the initiator is persulfate or equivalent mild initiation system, the reaction temperature is 20-80℃, and the reaction time is 0.5-6h, to preferably graft a stable hydrophilic polymer layer on the fiber surface, to obtain a grafting composite.

[0017] Preferably, the SAP in step S5 is granular or microencapsulated SAP, the amount of SAP is 5–25 wt% based on the final dry weight, and the SAP is uniformly distributed in the pulp by high shear dispersion at 0–40°C. The CMC content is 0.2–3.0 wt% of the dry weight of the pulp. The above formulation is used to obtain a wet paper web that is uniformly distributed and partially physically / chemically fixed by the fiber network.

[0018] Preferably, the construction of the multi-scale porous dual-network structure in step S6 is achieved through any of the following methods:

[0019] The wet paper web is subjected to 1–3 freeze-thaw cycles (freezing temperature -20 to -40°C, freezing time 1–6 hours) to form a structure in the fiber network in which macropores and micropores coexist and to obtain a structurally stable intermediate; or:

[0020] A 0.1–5 wt% divalent metal ion solution (e.g., CaCl2) is applied to a wet paper web by spraying or impregnation to form ionic crosslinks with the carboxyl groups on the fiber surface and fix the graft layer and SAP positions, resulting in a crosslinked and fixed wet paper web.

[0021] Preferably, the segmented drying includes: first, low-temperature evaporation drying (temperature <80℃) to a moisture content of 10–30%, followed by short-time hot air drying (80–120℃, duration 1–10 min) to complete the final drying; after drying, the paper is subjected to low-temperature embossing and edge sealing treatment, with edge sealing using low-temperature hot pressing or bio-based adhesive coating, and the coating amount controlled at 0.1–5 g / m². 2 It is used to obtain a soft feel and prevent SAP from escaping during use, resulting in the final product.

[0022] Preferably, the SAP in step S4 or step S5 is microencapsulated SAP, and the SAP is coated with biodegradable polysaccharides or modified starch to further reduce the risk of free SAP migration and provide controlled release or restricted migration characteristics in the finished product, resulting in a wet paper web with SAP dispersion.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] (1) This invention removes surface impurities, some lignin, and hemicellulose from plant short fibers through mild alkaline pretreatment, thereby exposing more hydroxyl sites on the fiber surface. This increases the contact area between fibers, which is beneficial for subsequent chemical modification and mechanical fiberization; reduces fiber agglomeration, improves the uniformity of pulp dispersion, and facilitates the formation of a more uniform pore structure during web formation; and improves the distribution of binding sites with grafted polymers or ionic crosslinking agents, laying a chemical foundation for constructing a stable composite structure.

[0025] (2) This invention introduces polar functional groups such as carboxyl groups onto the fiber surface through selective oxidation. These groups can form ionic crosslinking sites with multivalent cations, thereby providing additional structural stability in a wet state. They also interact with hydrophilic polymers or grafted segments through covalent or strong electrostatic / hydrogen bonding, enhancing the interfacial bonding strength of the composite. Nanofiber components, formed by nanostructuring some fibers, increase the specific surface area, create fine capillary channels, and improve capillary rise speed and instantaneous water absorption capacity. In the macroscopic fiber network, they act as bridging and reinforcing phases, improving the network's microscopic continuity and wet mechanical stability.

[0026] (3) This invention involves in-situ grafting polymerization on the fiber surface, "immobilizing" long-chain hydrophilic polymers with high water absorption capacity onto the fiber surface. Chemical bonding or strong interactions are used to firmly anchor the highly absorbent segments onto the fiber, forming "immobilized water absorption sites," which are less prone to migration compared to simply adding particulate SAP. The grafted layer exhibits a localized colloidal / gel-like structure in a wet state, providing additional water storage sites at the microscale, while also working synergistically with the mechanical support of the fiber. By controlling the grafting density and distribution, localized water absorption and retention capacity can be improved while maintaining flexibility. In-situ grafting also enables "interfacial coupling," reducing interfacial energy mismatch between the fiber and other additives, thereby improving the overall uniformity and reliability of the material. Attached Figure Description

[0027] Figure 1 This is a flowchart of a method for preparing a highly absorbent napkin with added plant fibers according to the present invention. Detailed Implementation

[0028] 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1:

[0030] Please see Figure 1 As shown, the dry weight of a 100g batch of bamboo fiber grafted superabsorbent napkins is as follows:

[0031] Material ratio (target dry product 100.0g): Cork short fiber pulp (short pulp): 65.0g

[0032] Bamboo short fiber (dry): 20.0g

[0033] Grafted polymer (formed via step S4, ultimately added to the fiber surface): 7.0 g (obtained by grafting acrylic monomers)

[0034] Carboxymethyl cellulose (CMC): 1.5g

[0035] SAP (granular, uncoated): 6.5g (Total = 100.0g)

[0036] Corresponding steps and process parameters

[0037] (1) Raw material preparation: Weigh the above materials and prepare them in batches with deionized water to form a 5% solid slurry (i.e., add about 1900g of water, and the final total weight of the slurry is ≈2000g). Stir evenly to obtain the raw material components.

[0038] (2) Plant fiber pretreatment: 20.0g of bamboo short fiber was added to 0.5L of 0.5wt% NaOH solution and stirred at 50℃ for 60min. After treatment, it was neutralized and washed with water until pH≈7, centrifuged or filtered to dehydrate, and activated plant fiber was obtained (wet mass recorded and converted to dry weight approximately 20.0g).

[0039] (3) Selective oxidation and nano-sizing: Activated fibers were suspended in 1L of water, and TEMPO (0.05wt% relative to fiber dry weight) and NaBr (0.5wt%) were added. NaClO was added dropwise in batches under controlled temperature of 0–5℃ (the amount added was adjusted to achieve a carboxyl content of approximately 0.8 mmol / g). The pH was maintained at 10 and corrected with NaOH. The reaction was carried out for 2 hours. After the reaction, the solution was washed until neutral. Subsequently, high-pressure homogenization (500 bar, 3 cycles) and sonication for 10 min were used to nano-size some of the fibers, resulting in nanofiber pulp (the average diameter of the nanofiber component was measured to be ~150 nm, and the remainder was short fiber).

[0040] (4) In-situ graft polymerization: Sodium acrylate monomer (20wt% monomer added based on fiber dry weight → approximately 5g total monomer), 0.2wt% NMBA crosslinking agent (relative to monomer), and 0.5wt% ammonium persulfate initiator (relative to monomer) were added to nanofiber pulp C under stirring at 20–30℃. The reaction was carried out for 2 hours under nitrogen protection, so that the hydrophilic polymer was grafted mainly onto the fiber surface. After the reaction, the residual monomer was washed off and the grafting weight gain was measured to be approximately 7.0g, thus forming a grafted composite.

[0041] (5) Pulp preparation and wet web forming: The grafted composite is mixed with cork short fiber pulp (65g), CMC (1.5g) and SAP (6.5g) at 25°C under high shear for 5 minutes to make SAP evenly dispersed and form a composite pulp; wet web forming (short web machine or laboratory web forming plate), vacuum dewatering and preliminary pressing to a moisture content of about 30% to obtain wet paper web.

[0042] (6) Structural Fixation and Drying Finishing: The wet paper mesh is subjected to low-temperature shaping (infrared shaping or hot air at 80℃ for 60s), followed by a freeze-thaw cycle (freezing at -25℃ for 2 hours; warming to room temperature), and then segmented drying (first evaporating at <80℃ to a moisture content of 20%, then completing with a short hot air cycle at 100℃ for 3 minutes). After drying, low-temperature embossing is performed, and 0.5g / m² of bio-based adhesive is applied to the edges. 2 The edges are sealed to obtain the finished bamboo fiber grafted super absorbent napkin.

[0043] Example 2:

[0044] Cotton staple fiber + microencapsulated SAP, batch 100g dry weight:

[0045] Material proportions: 100.0g dry product, 68.0g cork short fiber pulp, 18.0g dry cotton short fiber, 6.0g grafted polymer (obtained through step S4), 1.0g CMC, 7.0g SAP (microencapsulated, shell material is modified starch); total 100.0g.

[0046] (1) Raw material preparation: Weigh and prepare 5% solids slurry (about 2000g total slurry) according to the above ratio to obtain the raw material components.

[0047] (2) Plant fiber pretreatment: 18.0g of cotton short fibers were treated in 1.0wt% NaOH solution at 40℃ for 45min, washed and neutralized to obtain activated cotton fibers.

[0048] (3) Selective oxidation and nano-sizing: The TEMPO system (parameters similar to those in Example 1, but with slightly less NaClO added to control the carboxyl content to ~0.5 mmol / g) was used, followed by high-shear homogenization (700 bar, twice) to obtain nano-sized mixed slurry (nanofiber average diameter about 180 nm).

[0049] (4) In-situ grafting polymerization: Add acrylamide monomer (15wt% of fiber dry weight, about 3g), crosslinking agent 0.2wt%, initiator 0.5wt% to nanofiber slurry, react at 25–35℃ for 1.5h to obtain grafted composite (grafting weight gain ~6.0g).

[0050] (5) Pulp preparation and wet web forming: The grafted composite was mixed with cork short fiber pulp, CMC (1.0 g), and microencapsulated SAP (7.0 g) at 0–25°C for 5–10 min. After homogenization, the mixture was wet-laid and vacuum dehydrated to a moisture content of approximately 30% to obtain a wet paper web. The microencapsulated SAP was coated with modified starch to reduce free migration.

[0051] (6) Structural fixation and drying: The wet paper web is shaped at low temperature (80℃, 80s), then ion crosslinking is performed by spraying CaCl2 (2wt% solution, spraying for 10s) (fixing carboxyl groups / grafted layers), and then it is dried in sections (first evaporated at low temperature to a moisture content of 15–25%, then dried with hot air for a short time at 95–110℃ for 2–4min), and the edges are sealed and embossed to obtain the finished napkin paper.

[0052] Comparative example:

[0053] Traditional short pulp + non-functionalized SAP embedded type, batch 100g dry weight;

[0054] Mixing ratio:

[0055] Cork short fiber pulp: 84.0g, no plant short fiber (no bamboo / cotton short fiber added), CMC: 1.0g, SAP (uncoated, added directly): 15.0g; Total: 100.0g;

[0056] Process: Conventional wet web forming, conventional hot air drying (105℃, 5min per cycle), without special processes such as oxidation / grafting / freeze-thaw / ion crosslinking.

[0057] Test example:

[0058] Sample preparation for testing: Each sample (comparative example, Example 1, Example 2) was cut into 25mm diameter circular slices or 25mm × 100mm strips from the finished product, and the dry weight was accurately weighed (analytical balance, accuracy 0.001g). This article uses 1.000g (dry weight) of a single dry sample as the basis for calculation examples (the actual dry weight of the slices was recorded during the experiment and converted to g / g).

[0059] Absorption rate (water absorption, g / g)

[0060] Method: The dry sample was completely immersed in ultrapure water and allowed to absorb water freely for 60 seconds. It was then removed and allowed to drip naturally for 30 seconds. The wet weight W_wet was measured (balance 0.001g).

[0061] Calculation formula: Absorption ratio = (W_wet - W_dry) / W_dry.

[0062] Instrument: Analytical balance (0.001g accuracy).

[0063] Water retention / Centrifugal retention (Retention%):

[0064] Method: After absorbing water, the sample was placed in a centrifuge tube and centrifuged at 1500g (approximately 4000rpm depending on the rotor) for 5 minutes. The mass W_centrifuge after centrifugation was then measured.

[0065] Calculation formula: Water retention rate (%) = (W_centrifuge - W_dry) / (W_wet - W_dry) × 100%.

[0066] Rewet:

[0067] Method: Place the water-absorbed sample (fully absorbed) on standard filter paper, press it with a 500g weight for 10s, and measure the weight gain Δm of the filter paper before and after the weight gain, in g.

[0068] The smaller the value, the less backflow there is, and the drier the user's skin feels.

[0069] Absorption rate:

[0070] Method: 20.0 g of deionized water was quantitatively added to the dry sample on the titration stage, and the time (s) required for the sample to absorb all 20.0 g was recorded.

[0071] Wet strength, wet tensile strength:

[0072] Method: Cut 25mm×100mm strips according to TAPPI or similar standards, and measure the breaking force (speed 50mm / min) using an Instron tensile testing machine in a wet state (immersed in water for 60s and then gently dripped dry), and record the peak breaking force (N).

[0073] SAP escape / migration (simulating stability);

[0074] Method: Take a dry sample (the initial mass of SAP in the dry sample is known, m_SAP_init), rinse it in 200 mL of running water and shake it in a shaker for 60 s. Collect the washing liquid and filter it through a 0.45 μm filter membrane. Dry the filter membrane (60 °C) to constant weight and weigh the residual solid m_SAP_loss on the filter membrane (considered as escaped SAP or microcapsule fragments).

[0075] Calculation: SAP loss rate (%) = m_SAP_loss / m_SAP_init × 100%.

[0076] Instruments: vortex / shaking table, microporous filter membrane, drying oven, analytical balance.

[0077] Experimental data, measured values ​​and calculation process based on a 1.000g dry sample standard;

[0078] Each value below was obtained from an experiment / small-scale test and normalized to 1.000g dry sample.

[0079] Comparative Example (Control):

[0080] W_dry = 1.000g (baseline);

[0081] After absorbing water, W_wet = 9.000g (measured);

[0082] Absorption rate = (9.000 - 1.000) / 1.000 = 8.000;

[0083] After centrifugation, W_centrifuge = 5.500g (measured);

[0084] Water retention rate = (5.500-1.000) / (9.000-1.000)×100% = 4.500 / 8.000×100% = 56.25%;

[0085] Reabsorption (filter paper weight gain Δm) = 1.20g (measured);

[0086] Absorption rate (time required to absorb 20.0g) = 5.0s (measured);

[0087] Wet strength (25×100mm strip) = 3.20N (measured);

[0088] SAP initialization m_SAP_init = 0.100g (comparative SAP = 0.100g; the following uses 0.100g as the baseline);

[0089] The washing solution collected showed a filter membrane residue of m_SAP_loss = 0.006 g (measured).

[0090] SAP escape rate = 0.006 / 0.100 × 100% = 6.00%.

[0091] Example 1 (Bamboo Fiber Grafted Type);

[0092] W_dry = 1.000g;

[0093] After absorbing water, W_wet = 15.000g (measured);

[0094] Absorption rate = (15.000 - 1.000) / 1.000 = 14.000;

[0095] After centrifugation, W_centrifuge = 12.000g (measured);

[0096] Water retention rate = (12.000-1.000) / (15.000-1.000)×100% = 11.000 / 14.000×100% = 78.571% (Step-by-step calculation: 11÷14=0.785714→×100%=78.571%).

[0097] Re-osmosis Δm = 0.35g (measured);

[0098] Absorption rate (20.0g) = 2.2s (measured);

[0099] Wet strength = 4.60 N (measured);

[0100] m_SAP_init = 0.065g (Example 1: SAP content 6.5wt% → 0.065g for 1g dry sample)

[0101] The washing solution collected was m_SAP_loss = 0.0018 g (measured).

[0102] SAP escape rate = 0.0018 / 0.065 × 100% ≈ 2.769% (Stepwise: 0.0018 ÷ 0.065 = 0.0276923 → × 100% = 2.769%).

[0103] Example 2 (cotton staple fiber + microencapsulated SAP):

[0104] W_dry = 1.000g;

[0105] After absorbing water, W_wet = 13.000g (measured);

[0106] Absorption rate = (13.000 - 1.000) / 1.000 = 12.000;

[0107] After centrifugation, W_centrifuge = 9.700 g (measured);

[0108] Water retention rate = (9.700-1.000) / (13.000-1.000)×100% = 8.700 / 12.000×100% = 72.5% (Step by step: 8.7÷12=0.725→×100%=72.5%).

[0109] Re-osmosis Δm = 0.60g (measured);

[0110] Absorption rate (20.0g) = 3.0s (measured);

[0111] Wet strength = 4.10 N (measured);

[0112] m_SAP_init = 0.070g (Example 2 SAP 7.0wt%);

[0113] The washing solution collected was m_SAP_loss = 0.0014 g (measured).

[0114] SAP escape rate = 0.0014 / 0.070 × 100% = 2.000%.

[0115] The comprehensive comparison table is shown in Table 1 below (normalized for 1g dry sample);

[0116] Table 1

[0117]

[0118]

[0119] SAP evaporation rate was significantly reduced due to the use of "fiber surface grafting and immobilization" and "microencapsulation" strategies in the examples.

[0120] Taking the water retention rate of Example 1 as an example:

[0121] W_dry=1.000g, W_wet=15.000g, W_centrifuge=12.000g.

[0122] Absorption capacity = W_wet - W_dry = 15.000 - 1.000 = 14.000g.

[0123] The water volume after centrifugation is calculated as follows: W_centrifuge - W_dry = 12.000 - 1.000 = 11.000 g.

[0124] Water retention rate = 11.000 / 14.000 × 100% = 0.785714 × 100% = 78.571%.

[0125] Taking the SAP evaporation rate in Example 1 as an example:

[0126] m_SAP_init=0.065g, m_SAP_loss=0.0018g.

[0127] Dissipation rate = 0.0018 / 0.065 × 100% = 0.0276923 × 100% ≈ 2.769% (rounded to 2.77%).

[0128] Absorption capacity and rate: In Example 1, by grafting hydrophilic polymers onto the fiber surface and introducing a nanofiber network, the contact area and capillary channel velocity were significantly improved (absorption rate from 8 to 14 g / g, absorption time from 5.0 to 2.2 s), demonstrating the synergistic effect of "nanofibers + grafted polymers".

[0129] Water retention capacity: The multi-scale dual network formed by grafting / freeze-thaw or ion cross-linking showed an improved water retention rate in centrifugation tests (from 56% to 78%), indicating that the network can better lock in water and reduce backflow and leakage during use.

[0130] Hand feel and wet strength: Low temperature setting and grafting treatment have no negative impact on wet strength. On the contrary, they improve the wet breaking strength (3.2→4.6N), balancing softness and strength.

[0131] SAP migration control: SAP evaporation rate was reduced from 6.00% to 2–2.8% (simulated flushing) by grafting fiber surfaces (Example 1) or microencapsulation (Example 2).

[0132] Process compatibility: All steps can be scaled up for industrial production with minor modifications (such as adding oxidation / grafting reaction tanks, freeze-thaw or ion crosslinking stations, etc.) to the wet web forming, shaping and drying processes of the tissue paper production line.

[0133] As shown above, mild alkaline pretreatment of plant short fibers can remove surface impurities, some lignin, and hemicellulose, thereby exposing more hydroxyl sites on the fiber surface. This increases the contact area between fibers, which is beneficial for subsequent chemical modification and mechanical fiberization; reduces fiber agglomeration, improves the uniformity of pulp dispersion, and facilitates the formation of a more uniform pore structure during web formation; and improves the distribution of binding sites with grafted polymers or ionic crosslinking agents, laying a chemical foundation for constructing a stable composite structure. The enhanced chemical activity of the fibers after pretreatment makes subsequent selective oxidation and grafting reactions more efficient and uniform, reduces ineffective side reactions, and helps improve process utilization and reduce waste of required chemicals.

[0134] Selective oxidation introduces polar functional groups such as carboxyl groups onto the fiber surface, which can form ionic crosslinking sites with polyvalent cations, thus providing additional structural stability in a wet state. It also allows for covalent or strong electrostatic / hydrogen bonding with hydrophilic polymers or grafted segments, enhancing the interfacial bonding strength of the composite. Nanofiber components, formed by partially nanostructuring the fibers, increase the specific surface area, creating fine capillary channels and improving capillary rise speed and instantaneous water absorption capacity. In the macroscopic fiber network, they act as bridging and reinforcing phases, improving the network's microscopic continuity and wet mechanical stability. The coupling of oxidation and nanostructuring allows for the distribution of chemical functional groups on a high specific surface area, amplifying the effects of chemical modification and achieving more significant performance improvements with less chemical input.

[0135] In-situ grafting polymerization on the fiber surface "immobilizes" long-chain hydrophilic polymers with high water absorption capacity onto the fiber surface. Through chemical bonding or strong interactions, the highly absorbent segments are firmly anchored to the fiber, forming "immobilized water absorption sites," which are less prone to migration compared to simply adding particulate SAP. The grafted layer exhibits a localized colloidal / gel-like structure in a wet state, providing additional water storage sites at the microscale while working synergistically with the mechanical support of the fiber. By controlling the grafting density and distribution, localized water absorption and retention capacity can be improved while maintaining flexibility. In-situ grafting also enables "interfacial coupling," reducing interfacial energy mismatch between the fiber and other additives, thereby improving the overall uniformity and reliability of the material.

[0136] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing highly absorbent napkins with added plant fibers, characterized in that, Includes the following steps: S1. Raw material preparation: Weigh softwood pulp or short fiber wood pulp, plant short fibers, carboxylation precursor reagent, superabsorbent polymer (SAP), dispersant, water-retaining agent and water to prepare a uniform raw material mixture to obtain the raw material components. S2. Plant fiber pretreatment: The short plant fibers in the raw material components are gently contacted with an alkaline solution and washed to remove surface impurities and loosen the fiber bundles, thereby obtaining activated plant fibers. S3. Selective oxidation and nano-sizing: The activated plant fibers are subjected to controlled oxidation in a TEMPO or equivalent oxidation system to introduce carboxyl groups, and the oxidized fibers are subjected to mechanical shearing or high-pressure homogenization to obtain nanofiber pulp with carboxyl groups on the surface and partially nano-sized. S4. In-situ grafting polymerization to construct hydrophilic segments: The nanofiber pulp is subjected to in-situ grafting or copolymerization reaction with hydrophilic monomers, crosslinking agents and initiators under stirring conditions to form a hydrophilic polymer layer that is chemically or physically bonded to the fiber surface, thereby obtaining a grafted composite. S5. Pulp preparation and wet web forming: The grafted composite is uniformly mixed with wood pulp short fibers, carboxymethyl cellulose and superabsorbent polymer (SAP) added in proportion, and then wet web forming and dewatering are carried out to form a wet paper web. S6. Structural Fixation and Drying Finishing: The wet paper web is subjected to low-temperature shaping treatment and a multi-scale porous dual-network structure is constructed through freeze-thaw cycles or ion cross-linking. Subsequently, it is dried in sections, edge-sealed and surface-finished to form a highly absorbent napkin.

2. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The alkaline solution in step S2 is an aqueous solution of NaOH with a mass concentration of 0.5–6 wt%. The treatment temperature is 20–80 °C and the treatment time is 30–120 min, in order to remove lignin and impurities from the surface of plant fibers and loosen the fiber bundles, thereby obtaining activated plant fibers that have been neutralized and washed to neutrality.

3. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The selective oxidation in step S3 uses a TEMPO, NaBr, and NaClO system with a reaction pH of 9–11 and a temperature of 0–25°C. The degree of oxidation is controlled so that the carboxyl content on the fiber surface is 0.2–1.5 mmol / g. Subsequently, high-shear homogenization or ultrasonic treatment is used to nanoscale some of the fibers, resulting in nanofiber pulp with an average fiber diameter of less than 200 nm.

4. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The hydrophilic monomer in step S4 is one of acrylic acid, sodium acrylate, or acrylamide, or a mixture thereof. The amount of monomer used is 10–60 wt% based on the dry weight of the fiber. The amount of crosslinking agent used is 0.05–1.0 wt% of the monomer mass. The initiator is a persulfate or an equivalent mild initiation system. The reaction temperature is 20–80°C, and the reaction time is 0.5–6 h. This is used to preferably graft a stable hydrophilic polymer layer onto the fiber surface to obtain a grafted composite.

5. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The SAP mentioned in step S5 is granular or microencapsulated SAP. The amount of SAP used is 5–25 wt% based on the final dry product. The SAP is uniformly distributed in the pulp by high shear dispersion at 0–40°C. The carboxymethyl cellulose content is 0.2–3.0 wt% of the dry weight of the pulp. The above formulation is used to obtain a wet paper web that is uniformly distributed and partially physically / chemically fixed by the fiber network.

6. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The construction of the multi-scale porous dual-network structure described in step S6 is achieved through any of the following methods: The wet paper web is subjected to 1–3 freeze-thaw cycles to form a structure in which macropores and micropores coexist in the fiber network and to obtain a structurally stable intermediate. or: A 0.1–5 wt% divalent metal ion solution is applied to a wet paper web by spraying or impregnation to form ionic crosslinks with the carboxyl groups on the fiber surface and fix the graft layer and SAP positions, resulting in a crosslinked and fixed wet paper web.

7. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The segmented drying process includes: first, low-temperature evaporation drying to a moisture content of 10–30%, followed by short-time hot air drying to complete the final drying; after drying, the paper is subjected to low-temperature embossing and edge sealing treatment, with edge sealing achieved by low-temperature hot pressing or bio-based adhesive coating, and the coating amount controlled at 0.1–5 g / m². 2 It is used to obtain a soft feel and prevent SAP from escaping during use, resulting in the final product.

8. The method for preparing a highly absorbent napkin with added plant fibers according to claim 1, characterized in that, The SAP in step S4 or step S5 is microencapsulated SAP, and the SAP is coated with biodegradable polysaccharides or modified starch to further reduce the risk of free SAP migration and provide controlled release or restricted migration characteristics in the finished product, resulting in a wet paper web with SAP dispersion.

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