Sanitary napkin absorbent core with reduced rewet and method of making

Abstract

This invention discloses a sanitary napkin absorbent core that is not prone to backflow and its preparation method. The absorbent core includes a C-shaped folded structure and a flow-guiding material. The C-shaped folded structure is formed by folding a first absorbent fiber cloth, a water-repellent membrane, and a second absorbent fiber cloth in a C-shape, and has a bottom, a first top and a second top extending upward from both sides of the bottom, and a central groove. Superabsorbent polymer (SAP) particles are sandwiched between the water-repellent membrane and the first absorbent fiber cloth. The flow-guiding material is disposed above the C-shaped folded structure and includes a perforated membrane. The perforated membrane has multiple holes evenly distributed throughout its thickness direction, and the cross-section of the holes is an inverted trapezoid with a larger top and a smaller bottom. This invention utilizes the water-repellent membrane and absorbent fiber cloth to physically confine the SAP, improving absorbency while retaining the enclosed barrier function of the C-shaped structure. The absorbency ratio meets national standards, the backflow is low, and it is suitable for mass production.

Description

Technical Field

[0001] This invention relates to the field of disposable feminine hygiene products technology, specifically to a sanitary napkin absorbent core that is not prone to backflow and its preparation method. Background Technology

[0002] Sanitary napkins are essential disposable absorbent hygiene products for women during menstruation. Their core functional component is the absorbent core, whose structural design and material composition directly determine the product's absorption speed, backflow rate, and wearing comfort. Currently, most mainstream sanitary napkins on the market use superabsorbent polymer (SAP) as the main absorbent material to increase the product's absorbency. However, SAP has significant performance drawbacks: its absorption rate for menstrual blood is only 2-3 times, far lower than its absorption rate for pure water (approximately 200 times). To achieve sufficient absorbency, a large amount needs to be added (approximately 2g per pad), resulting in high raw material costs and the fact that it is a petroleum-based, non-biodegradable material, placing a heavy burden on the environment. Furthermore, after absorbing water and swelling, SAP gel agglomerates, easily clogging fiber pores, increasing core hardness, and reducing wearing comfort. How to ensure that the absorbent core of sanitary napkins has sufficient absorbency to meet national standards while simultaneously achieving low backflow, high breathability, and good wearing comfort has become a pressing technical challenge in this field.

[0003] To address the above issues, multi-dimensional research and development attempts have been carried out in this field.

[0004] The first category is barrier and anti-backflow technology based on water-repellent materials or folded structures. This type of solution uses a water-repellent layer around the absorbent core or a folded structure to create three-dimensional leak-proof sides, utilizing the physical barrier effect of the water-repellent material to prevent liquid backflow or side leakage. For example, Chinese patent CN213788216U discloses a close-fitting, breathable sanitary napkin whose absorbent core has a C-shaped outer wrapping layer with the opening facing upwards, enclosing the upper surface layer, middle layer, and absorbent resin within it; Chinese patent CN224166504U uses a three-fold composite water-repellent fabric layer with an "e"-shaped folded structure and a two-fold water-repellent fabric layer with a "c"-shaped folded structure, with the folded structure forming three-dimensional leak-proof sides. Furthermore, Chinese patent CN111839902B discloses an absorbent core that, by setting the effective area of ​​the layers, allows menstrual blood to diffuse outwards to the outer absorption area, forming a concave structure with a flat outer surface and a recessed inner surface to create a larger menstrual blood storage space. The applicant had previously filed an invention application for a C-shaped folded liquid-absorbing and storage layer structure. This design utilizes a water-repellent membrane to form a liquid barrier on the sides and bottom of the C-shape, but the C-shaped folded structure does not contain SAP, resulting in limited water absorption capacity. However, the absorbent core in such designs still uses SAP as the main absorbent component. While this improves the anti-backflow or anti-side leakage effect to some extent, directly filling the C-shaped structure with SAP or mixing it with fibers before filling it can easily clog the internal space of the C-shape after absorbing water and expanding, affecting subsequent liquid infiltration. Furthermore, it fails to effectively address the inherent defects of SAP hardening and gel aggregation after water absorption.

[0005] The second category is perforated membrane-based flow guidance and anti-backflow technology. Using perforated membranes as a component layer of sanitary napkins is a known technology in this field. Among them, perforated membranes with a cross-sectional shape that is larger at the top and smaller at the bottom (i.e., inverted trapezoidal or funnel-shaped holes) have unique advantages in unidirectional liquid transport: liquid can quickly enter and pass through the membrane from the larger upper opening, while the smaller lower opening, due to surface tension, makes it difficult for liquid that has passed through the membrane to flow back. This type of material was previously used as the surface layer material of sanitary napkins, utilizing its perforated structure to accelerate menstrual blood penetration and reduce surface liquid residue. For example, Chinese patent CN210644344U discloses a sanitary napkin with a non-woven perforated membrane composite surface layer, whose surface layer consists of an upper hot-air non-woven fabric layer and a lower PE perforated membrane layer, with inverted V-shaped or inverted W-shaped holes provided on the PE perforated membrane layer to prevent backflow. However, because perforated membranes are made of polymer plastic films, they can easily cause allergies or discomfort in some consumers when directly applied to human skin. Therefore, their use as surface layers has gradually been replaced by non-woven fabrics. Furthermore, perforated membranes have limited structural strength and are prone to deformation during the lamination and bonding of multiple materials.

[0006] The third category involves technologies that increase absorbency by adding independent absorbent layers. For example, a composite absorbent paper containing SAP (two layers of clean paper sandwiching SAP) is added between the perforated film and the C-shaped structure. This utilizes the high absorbency of the composite absorbent paper to improve the overall absorbency of the core. However, this approach increases the overall thickness and number of layers of the core, resulting in a relatively complex structure. This is not conducive to making sanitary napkins thinner and lighter, and the interfaces between the layers increase resistance to liquid transport, potentially affecting absorption speed.

[0007] A comprehensive analysis of the existing technologies reveals the following common shortcomings:

[0008] Firstly, while existing C-shaped folded structure solutions have an enclosed water-repellent barrier function, their internal structure may lack SAP, resulting in limited water absorption capacity, or SAP may be directly mixed with fiber materials and filled inside the C-shaped structure. In the latter case, the SAP lacks effective physical restraint after absorbing water, and the expanded gel will diffuse disorderly towards the opening of the C-shaped structure and into the fiber pores. This not only blocks the fiber pores and affects subsequent liquid infiltration, but may also cause the SAP gel to escape from the C-shaped structure, posing a safety hazard.

[0009] Secondly, although the inverted trapezoidal perforated membrane has excellent unidirectional flow guidance and anti-backflow performance, existing technologies only use it as the surface layer. Due to allergy issues, it has been abandoned by the industry. Existing technologies have not realized the technical feasibility of transferring this material to the core of the sanitary napkin as a functional layer, so that it can play a physical unidirectional flow guidance and anti-backflow role without contacting the skin. Furthermore, they have not realized the need to combine it with composite absorbent paper containing SAP and C-shaped water-repellent fold structure to build a core design concept of "physical anti-backflow + chemical high absorption" for synergistic effect.

[0010] Third, while adding an independent absorbent layer solves the problem of absorbent capacity, it increases the overall number and thickness of the core, which is not conducive to making it thinner and lighter, and the multi-layer interface increases the resistance to liquid transmission.

[0011] Fourth, the existing technology lacks a core design scheme that can both increase the water absorption capacity of SAP and effectively physically limit the water absorption and expansion of SAP by utilizing the C-shaped fold structure itself. This scheme does not require adding an additional independent water absorption layer, and can firmly position the SAP in a specific area inside the C-shaped structure, so that it is still constrained within the predetermined space after water absorption and expansion, without blocking the fiber channel or overflowing outward. Summary of the Invention

[0012] This invention aims to overcome the aforementioned technical defects of existing sanitary napkin absorbent cores and provide a sanitary napkin absorbent core that is less prone to backflow and its preparation method. Specifically, the technical problems to be solved by this invention include: overcoming the defects of existing C-shaped folded structure designs, such as ineffective utilization of SAP inside the C-shaped structure, limited absorbency, and difficulty in meeting national standards for absorbency ratio; overcoming the defects of existing designs where SAP is directly filled into the C-shaped structure, such as lack of effective physical containment after SAP absorbs and expands, easy clogging of fiber pores, and easy escape from the C-shaped structure; overcoming the sensitization problems and application limitations caused by existing inverted trapezoidal perforated membranes being used only as surface layers; and overcoming the defects of existing designs that add independent absorbent layers, resulting in an increase in the number of core layers and hindering thinness.

[0013] The inventors of this application discovered through research that by sandwiching SAP between a C-shaped folded water-repellent membrane and a water-absorbing fiber cloth, the continuous coverage of the water-repellent membrane on the bottom and sides physically limits the water absorption and expansion of the SAP. At the same time, the fiber network of the water-absorbing fiber cloth evenly fixes the SAP particles on the upper surface of the water-repellent membrane, which can effectively prevent the SAP from overflowing out of the C-shaped structure after absorbing water and expanding. The gel formed after the SAP absorbs water is sealed in the interlayer between the water-repellent membrane and the water-absorbing fiber cloth, and will not migrate outward even when squeezed. Thus, the high-capacity absorption function of the C-shaped folded structure can be achieved without adding an independent water-absorbing layer. Based on this, the purpose of this invention is to: by placing a superabsorbent polymer resin between the C-shaped folded water-repellent membrane and the first absorbent fiber cloth, the SAP is wrapped inside the groove of the C-shaped folded structure. The water-repellent membrane and the absorbent fiber cloth form an upper and lower clamping physical constraint on the SAP, which improves the water absorption capacity while retaining the enclosed barrier function of the C-shaped structure. Combined with the unidirectional flow guidance and preliminary anti-backflow effect of the inverted trapezoidal perforated membrane, a balance between high water absorption capacity and low backflow is achieved. At the same time, a simple preparation method that is compatible with existing production lines is provided.

[0014] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0015] In a first aspect, the present invention provides a sanitary napkin absorbent core that is not prone to backflow, comprising a C-shaped fold structure, superabsorbent polymer particles, and a diversion material.

[0016] The C-shaped folded structure is formed by sequentially layering and folding a first absorbent fiber cloth, a water-repellent membrane, and a second absorbent fiber cloth. The C-shaped folded structure has a bottom, a first top and a second top extending upwards from both sides of the bottom, and a central groove formed by the bottom, the first top, and the second top. The first and second absorbent fiber cloths are each independently selected from fibrous sheets with liquid absorption and storage capabilities, including but not limited to plant fiber cloth (such as natural cotton fiber cloth, bamboo fiber cloth, etc.), dust-free paper (dry-laid paper nonwoven fabric), or nonwoven fabric (such as spunlace cloth, hot-air cloth, etc.). The basis weight of the first and second absorbent fiber cloths can be selected according to the target liquid storage capacity, for example, each independently being 30–80 g / m², preferably 40–60 g / m². The materials of the first and second absorbent fiber cloths can be the same or different. A water-repellent membrane is sandwiched between the first and second absorbent fiber fabrics, forming a liquid barrier in localized areas at the bottom, first top, and second top, thereby creating a liquid barrier on the sides and below the C-shaped folded structure. The water-repellent membrane is a low surface energy microporous polymer film with air-permeable but water-impermeable properties, allowing air and water vapor to pass through while blocking liquid water penetration. The water-repellent membrane can be made of polyethylene (PE), polypropylene (PP), or polytetrafluoroethylene (PTFE), etc., and is a sanitary-grade microporous breathable membrane. Its thickness is preferably 15–40 μm, more preferably 20–30 μm; its basis weight is preferably 15–25 g / m², more preferably 18–22 g / m²; its air permeability is preferably 1000–5000 g / (m²·24h), more preferably 2000–4000 g / (m²·24h); and its hydrostatic pressure resistance is preferably ≥50 cmH₂O, more preferably ≥80 cmH₂O. The average pore size of the water-repellent membrane is preferably 0.01–2 μm. After C-folding, a "sandwich" composite structure is formed on the left and right sides, with two layers of absorbent fiber cloth sandwiching the water-repellent membrane, and a central groove with the opening facing upwards is formed in the center.

[0017] The superabsorbent polymer (SAP) particles are sandwiched between the water-repellent membrane and the first absorbent fiber cloth, or between the water-repellent membrane and the second absorbent fiber cloth, and are encapsulated within the grooves of the C-shaped folded structure. The Particle size of the SAP particles is preferably 100–500 μm, more preferably 200–400 μm; the SAP particles are uniformly distributed between the water-repellent membrane and the first absorbent fiber cloth, and the average spacing between adjacent SAP particles is 0.5–2.0 mm. The SAP particles preferably absorb 30–60 times the amount of physiological saline, 20–40 times the amount of menstrual blood, and retain more than 80% of the absorbed liquid under a pressure of 5.5 kPa. The amount of SAP particles added between the water-repellent membrane and the first absorbent fiber cloth is preferably 30–100 g / m², more preferably 50–80 g / m². SAP particles can be made from sanitary grade SAP materials known in the art, such as sodium polyacrylate superabsorbent polymer.

[0018] The flow-guiding material, disposed above the C-shaped folded structure, includes a perforated membrane. The perforated membrane has multiple holes evenly distributed throughout its thickness direction, and the cross-section of each hole is an inverted trapezoid, wider at the top and narrower at the bottom. The upper diameter of the inverted trapezoidal hole is larger than the lower diameter; preferably, the ratio of the upper to lower diameter is 1.5:1 to 3:1. When liquid contacts the perforated membrane from above, menstrual blood, under the influence of gravity, quickly enters the pores through the larger upper opening and permeates downwards through the membrane. Once the menstrual blood has passed through the membrane, due to the smaller lower opening diameter and the surface tension of the liquid, it is difficult for the blood to flow back through the perforated membrane. The pore diameter of the perforated membrane is preferably 0.3–1.5 mm, the pore spacing is preferably 1.0–4.0 mm, and the porosity is preferably 15%–35%. The perforated membrane can be heat-set to maintain its pore shape, ensuring that the inverted trapezoidal holes maintain a stable form during use.

[0019] In a preferred embodiment, the guiding material employs a multi-layer perforated membrane composite structure, comprising a perforated membrane, a hot-air nonwoven fabric, and a wood pulp layer stacked sequentially from top to bottom. The hot-air nonwoven fabric has a fluffy three-dimensional mesh structure, used to support the perforated membrane and maintain the shape of the pores. The wood pulp layer is a fluff pulp layer or a composite layer of fluff pulp and chemical fibers. The basis weight of the hot-air nonwoven fabric is preferably 15–60 g / m², and the thickness is preferably 0.5–5.0 mm. The hot-air nonwoven fabric is preferably made from core-sheath type thermally bonded composite fibers (ES fibers) through hot air molding. The basis weight of the wood pulp layer is preferably 20–80 g / m².

[0020] In the perforated membrane composite structure, hot-air nonwoven fabric is used as the elastic support layer of the perforated membrane. During the preparation process, the perforated membrane, hot-air nonwoven fabric, and wood pulp layer are bonded together by spraying adhesive and roller pressing. The hot-air nonwoven fabric is compressed during the hot pressing process but does not undergo irreversible deformation. After the hot pressing is completed, the hot-air nonwoven fabric rebounds, forming a continuous upward support force on the perforated membrane. This ensures that the inverted trapezoidal holes of the perforated membrane maintain their original shape of being larger at the top and smaller at the bottom after composite pressing and during product use, ensuring that the channels are unobstructed and that the liquid seepage rate does not decrease due to hole deformation.

[0021] Based on the above technical solution, the present invention can be further optimized as follows:

[0022] To further enhance the liquid infiltration rate, a flow-guiding layer can be added above the flow-guiding material. This flow-guiding layer is either an ES chemical fiber flow-guiding layer or a super-flow-guiding wood pulp cloth. The ES chemical fiber flow-guiding layer is made from core-sheath type thermally bonded composite fibers (ES fibers) through hot air molding, possessing high porosity and resilience. The fineness of the ES fibers is preferably 1.5–4.0 denier, and the fiber length is preferably 38–51 mm. The super-flow-guiding wood pulp cloth includes a non-woven fabric layer and a fluff pulp layer or a composite layer of fluff pulp and chemical fibers adhered beneath the non-woven fabric layer. The flow-guiding layer accelerates the horizontal diffusion and downward penetration of the liquid, allowing menstrual blood to be evenly distributed over a larger area before entering the perforated membrane, thus improving the effective utilization rate of the entire absorbent core. The basis weight of the flow-guiding layer is preferably 20–50 g / m².

[0023] The preferred method for fixing the functional layers is adhesive spraying. The lower surface of the flow-guiding material is bonded to the upper surfaces of the first and second tops of the C-shaped folded structure using adhesive spraying; when a flow-guiding layer is provided, the lower surface of the flow-guiding layer is bonded to the upper surface of the flow-guiding material using adhesive spraying. The amount of adhesive applied should not be excessive (preferably 1-5 g / m²) to avoid clogging the inverted trapezoidal holes on the perforated membrane or the fiber pores of each functional layer, thus affecting the liquid permeability.

[0024] The absorbent core of this invention achieves a balance between low backflow and high absorption through a synergistic mechanism of "physical barrier + chemical absorption": the upper perforated membrane utilizes the geometric structure of inverted trapezoidal pores to facilitate rapid infiltration of menstrual blood and provide initial anti-backflow; the lower C-shaped folded structure utilizes the fiber network of the first and second absorbent fiber cloths to absorb and store menstrual blood, while simultaneously utilizing SAP particles sandwiched between the water-repellent membrane and the first absorbent fiber cloth to achieve high-capacity chemical absorption and locking of menstrual blood—after contacting menstrual blood, the SAP particles rapidly absorb water and swell to form a hydrogel, firmly locking the liquid phase components in the menstrual blood within its three-dimensional network structure. The water retention capacity of SAP ensures that even under dynamic pressure conditions, it can still retain more than 80% of the absorbed liquid, significantly reducing the presence of free liquid; at the same time, the water-repellent membrane forms a closed-loop liquid barrier at the bottom and sides of the C-shaped folded structure, so that the gel formed after SAP absorbs water is sealed within the interlayer between the water-repellent membrane and the first absorbent fiber cloth. The expansion of SAP is physically constrained by the water-repellent membrane below and to the sides, preventing it from overflowing to the outside of the C-shaped structure or blocking the fiber channels. The three layers work synergistically to form multiple physical and chemical water-locking barriers along the menstrual blood seepage pathway, effectively blocking reverse osmosis. Meanwhile, because the perforated membrane is located inside the core rather than on the surface, its plastic material does not come into contact with human skin, completely avoiding the risk of sensitization.

[0025] Secondly, the present invention provides a method for preparing the absorbent core of a sanitary napkin that is not prone to backflow as described in the first aspect, comprising the following steps:

[0026] A C-shaped folding structure is provided, wherein the C-shaped folding structure is formed by sequentially stacking and compounding a first absorbent fiber cloth, a water-repellent membrane, and a second absorbent fiber cloth and then folding them in a C-shape. The C-shaped folding structure has a bottom, a first top and a second top extending upward from both sides of the bottom, and a central groove formed by the bottom, the first top, and the second top.

[0027] Super absorbent polymer particles are disposed between the water-repellent membrane and the first absorbent fiber cloth or between the water-repellent membrane and the second absorbent fiber cloth, such that the super absorbent polymer particles are sandwiched between the water-repellent membrane and the first absorbent fiber cloth or between the water-repellent membrane and the second absorbent fiber cloth and are wrapped inside the groove of the C-shaped fold structure.

[0028] The flow guiding material is bonded to the upper surface of the C-shaped folded structure through a second adhesive layer. The flow guiding material includes a perforated membrane with multiple holes evenly distributed on the perforated membrane through the thickness direction of the perforated membrane. The cross-section of the holes is an inverted trapezoid with a larger top and a smaller bottom.

[0029] In the above preparation method, in the step of setting superabsorbent polymer (SAP) particles between the water-repellent membrane and the first absorbent fiber cloth, the SAP particles can be first evenly spread on the upper surface of the water-repellent membrane, and then the first absorbent fiber cloth can be laid. The spreading method can be quantitative spreading using a vibrating screen or airflow spreading to ensure the uniform distribution of SAP particles on the upper surface of the water-repellent membrane.

[0030] In the above preparation method, the C-shaped folded structure is formed through the following steps: providing a water-repellent membrane; applying an adhesive to the upper surface of the water-repellent membrane and then laying a first absorbent fiber cloth, and laying a second absorbent fiber cloth on the lower surface of the water-repellent membrane to form a laminated composite sheet; folding the laminated composite sheet upwards along its length on both sides, so that the absorbent fiber cloths on both sides are arranged opposite each other, and the water-repellent membrane is sandwiched between the first absorbent fiber cloth and the second absorbent fiber cloth, forming a C-shaped folded structure with the bottom, a first top, a second top, and a central groove. The width of the laminated composite sheet before folding is preferably 120-135 mm, the width after folding is preferably 70-80 mm, and the width of the central groove is preferably 50-65 mm. The adhesive can be a sanitary-grade environmentally friendly adhesive such as latex or hot melt adhesive, and can be applied by spraying or roller coating, with an application amount preferably of 1-5 g / m².

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. SAP is sandwiched between a water-repellent membrane and a absorbent fiber cloth, achieving a balance between physical containment and high-capacity absorption. This invention sandwiches SAP between a C-shaped folded water-repellent membrane and a first absorbent fiber cloth, encasing the SAP within the grooves of the C-shaped folds. The continuous coverage of the water-repellent membrane on the bottom and sides provides physical containment for the SAP's water absorption and expansion. The fiber network of the first absorbent fiber cloth evenly fixes the SAP particles to the upper surface of the water-repellent membrane. The gel formed after SAP absorbs water is sealed within the interlayer between the water-repellent membrane and the first absorbent fiber cloth, preventing outward migration or blockage of fiber channels even under pressure. This effectively solves the technical problems of existing technologies where SAP directly filling the C-shaped structure lacks physical containment after water absorption and expansion, easily clogging fiber pores and escaping from the C-shaped structure. The optimized SAP addition range of 30–100 g / m² ensures that the absorbency of the absorbent core meets the requirements of the GB / T 8939—2025 national standard (absorption rate ≥ 4.0 times).

[0033] 2. Optimized water retention capacity of SAP to ensure low backflow under dynamic pressure conditions. This invention limits the water retention capacity of the SAP to retain more than 80% of the absorbed liquid under a pressure of 5.5 kPa. This performance parameter ensures that the SAP can firmly lock the absorbed menstrual blood in its gel network even when subjected to pressure from human movements such as sitting or walking, making it difficult to release. This water retention requirement is directly related to the core technical effect of "non-backflow" in this invention, and not all commercially available SAPs can meet this requirement, reflecting the specific selection of SAP materials.

[0034] 3. Optimized absorption rate of SAP for menstrual blood, highly compatible with the application scenarios of sanitary napkins. This invention limits the absorption rate of SAP to 20-40 times. Because menstrual blood has a higher viscosity than saline and contains protein debris and blood cells, the absorption rate of SAP for menstrual blood is usually much lower than its absorption rate for saline. Limiting the SAP's absorption rate to 20-40 times ensures that the selected SAP has sufficient absorption capacity for menstrual blood, highly matching the application scenarios of this invention for menstrual blood absorption in sanitary napkins.

[0035] 4. Optimization of SAP distribution density to achieve a balance between water absorption capacity and liquid permeation rate. This invention limits the average spacing between adjacent SAP particles to 0.5–2.0 mm, ensuring that the distribution of SAP particles between the water-repellent membrane and the first absorbent fiber cloth is neither too sparse (leading to insufficient water absorption capacity) nor too dense (leading to gel agglomeration and clogging of fiber pores after water absorption and swelling), thus achieving a balance between water absorption capacity and liquid permeation rate. Simultaneously, limiting the SAP particle size range to 100–500 μm ensures that the SAP particles have sufficient specific surface area to guarantee a rapid water absorption rate, while avoiding the dust problem caused by excessively small particle sizes.

[0036] 5. The inverted trapezoidal perforated membrane serves as a functional layer within the core, achieving unidirectional flow guidance and avoiding the risk of sensitization. This invention moves the inverted trapezoidal perforated membrane from its traditional surface layer position to the interior of the absorbent core. Utilizing its "larger at the top, smaller at the bottom" geometric structure of the inverted trapezoidal pores, it achieves unidirectional liquid transport—menstrual blood quickly enters through the larger upper pore, passes through the membrane, and the smaller lower pore, due to surface tension, prevents backflow of the liquid, thus achieving unidirectional flow guidance and preliminary anti-backflow at the physical level. Simultaneously, since the perforated membrane no longer directly contacts human skin as a surface layer, its plastic material will not cause skin allergies or discomfort, completely avoiding the sensitization problems associated with using a perforated membrane as a surface layer. In a preferred embodiment, a multi-layered perforated membrane composite structure is used as the flow guiding material. A hot-air nonwoven fabric with a fluffy three-dimensional mesh structure is further placed between the perforated membrane and the wood pulp layer, utilizing its compression resilience to provide continuous upward support to the perforated membrane. This ensures that the inverted trapezoidal pores of the perforated membrane always maintain their original shape (larger at the top, smaller at the bottom), ensuring unobstructed flow.

[0037] 6. No need for an additional independent absorbent layer, resulting in a simpler and thinner structure. Compared to existing technologies that increase absorbency by adding an independent absorbent layer (including SAP composite absorbent paper), this invention directly incorporates the SAP within the C-shaped fold structure. The C-shaped structure's own water-repellent membrane and absorbent fiber cloth clamp and limit the SAP, eliminating the need for an additional independent absorbent layer. This reduces the overall number of layers and thickness of the core, making the sanitary napkin product thinner and lighter. Furthermore, the reduced interlayer interfaces lower liquid transport resistance, which helps improve absorption speed.

[0038] 7. High breathability, improving the wearable microenvironment. The water-repellent membrane is made of a breathable but water-impermeable microporous polymer film with a breathability of 1000~5000g / (m²·24h) and a hydrostatic pressure resistance of ≥50cmH2O. While blocking the backflow of liquid blood, it allows air and water vapor to pass through freely, ensuring unobstructed moisture evaporation channels inside the absorbent core and avoiding localized humid and hot environments that could irritate the skin.

[0039] 8. The preparation process is simple and adaptable to existing mass production processes. The preparation method provided by this invention only involves conventional processes such as multi-layer material composite, SAP spreading, folding, and stacking. Based on the existing C-shaped folding structure production process, only one SAP uniform spreading step needs to be added, without the need to introduce complex special equipment such as hot pressing, three-dimensional molding, or ultrasonic composite. The raw materials used (perforated film, absorbent fiber cloth, water-repellent film, and SAP) are all mature and commercially available materials in the sanitary materials industry. Mass production can be achieved directly using the folding, composite, and slitting equipment of existing high-speed sanitary napkin production lines. The process is simple, the yield rate is high, and the equipment modification cost is low, demonstrating good feasibility for industrialization. Attached Figure Description

[0040] Figure 1 This is an exploded view of the absorbent core of the sanitary napkin that is not prone to backflow according to the present invention, showing the decomposition state of the diversion material and the C-shaped fold structure containing SAP and the composition relationship of each layer.

[0041] Figure 2 This is a cross-sectional view of the overall assembly of the absorbent core of the sanitary napkin of the present invention, which is not prone to backflow, showing the combined state in which the guide material is placed on top of the C-shaped fold structure containing SAP.

[0042] Figure 3 This is a schematic diagram of the molding process of a C-shaped folded structure containing SAP, showing the process of layering and folding the first absorbent fiber cloth, SAP particles, water-repellent membrane, and second absorbent fiber cloth together to form a C-shaped structure. Figure 3 'a' represents the state before folding. Figure 3 b indicates the folding state. Figure 3 c represents the folded state.

[0043] Figure 4 This is a partial enlarged cross-sectional view of the perforated membrane, showing the cross-sectional shape of the evenly distributed inverted trapezoidal holes penetrating the thickness direction of the membrane (the upper diameter is larger than the lower diameter).

[0044] Figure 5 This is a schematic diagram of the working state of the absorbent core of the present invention after absorbing menstrual blood. It shows the working principle that menstrual blood passes through the guide material from above and enters the groove in the middle of the C-shaped folded structure. It is then absorbed and locked by the absorbent fiber cloth and SAP, while being blocked by the water-repellent membrane from lateral and downward backflow.

[0045] Figure 6 The assembly cross-sectional view of the embodiment of the present invention with the addition of a flow guiding layer shows the interlayer positional relationship of the flow guiding layer disposed above the flow guiding material.

[0046] Figure 7 This is a top view of the inverted trapezoidal perforated membrane, showing the uniform distribution of the inverted trapezoidal holes on the membrane surface.

[0047] Figure 8 This is a top view of the C-shaped folding structure, showing the overall shape of the C-shaped folding structure after folding and the width range of the central groove.

[0048] Explanation of the markings on the components in the attached diagram:

[0049] 1- Sanitary napkin absorbent core that prevents backflow; 10- Diversion material; 11- Inverted trapezoidal hole; 111- Top opening; 112- Bottom opening; 12- Perforated membrane; 13- Hot air nonwoven fabric; 14- Wood pulp layer; 20- C-shaped fold structure; 21- Bottom; 22- First top; 23- Second top; 24- Central groove; 25- First absorbent fiber cloth; 26- Water-repellent membrane; 27- Second absorbent fiber cloth; 28- Laminated composite sheet; 29- Adhesive layer; 33- Super absorbent polymer particles; 40- Diversion layer. Detailed Implementation

[0050] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0051] like Figure 1 and Figure 2As shown, this invention provides a sanitary napkin absorbent core 1 that is not prone to backflow, mainly comprising two parts: a flow-guiding material 10 and a C-shaped fold structure 20. The C-shaped fold structure 20, serving as the main body for absorbing, storing, and blocking menstrual blood, is located in the lower layer of the entire absorbent core. The C-shaped fold structure 20 contains superabsorbent polymer (SAP) particles 33, possessing both high-capacity chemical absorbency and a protective physical barrier function. The flow-guiding material 10, acting as a unidirectional flow guiding and preliminary anti-backflow layer, is located above the C-shaped fold structure 20. After menstrual blood contacts the absorbent core from above, it first passes through the flow-guiding material 10, then enters the C-shaped fold structure 20 where it is quickly absorbed and locked by the absorbent fibers and SAP. The entire process works synergistically to achieve a unidirectional channel effect where menstrual blood can enter but not exit. In this invention, the "up" and "down" directions refer to the absorbent core's usage state in the sanitary napkin product—the side facing the skin is considered "up," and the side facing away from the skin is considered "down."

[0052] like Figure 3 As shown, the C-shaped folded structure 20 is formed by sequentially layering a first absorbent fiber cloth 25, a water-repellent membrane 26, and a second absorbent fiber cloth 27, with superabsorbent polymer (SAP) particles 33 sandwiched between the water-repellent membrane 26 and the first absorbent fiber cloth 25, followed by C-folding. Specifically, a water-repellent membrane 26 (such as...) is first provided... Figure 3 As shown in Figure a), an adhesive (such as latex, hot melt adhesive, or other sanitary-grade environmentally friendly adhesive; the application method can be spraying or roller coating; the application amount is preferably 1-5 g / m², more preferably 2-3 g / m²) is applied to the upper surface of the water-repellent membrane 26 to form an adhesive layer 29; then, SAP particles 33 are evenly spread on the upper surface of the water-repellent membrane 26 with the adhesive applied. The SAP can be spread by a vibrating screen for quantitative spreading or by airflow spreading to ensure the uniform distribution of SAP particles on the upper surface of the water-repellent membrane 26. In a preferred embodiment, a vibrating screen for quantitative spreading is used. This device drives the screen to vibrate through a vibrating motor, so that the SAP particles are evenly passed through the screen holes and fall onto the upper surface of the water-repellent membrane 26. The amount of SAP spread is controlled by adjusting the vibration frequency and the screen moving speed, and the control accuracy of the spread amount is preferably ±5 g / m². After the SAP particles are spread, they can be blown with compressed air or gently rolled to make the SAP particles come into fuller contact with the adhesive layer 29, ensuring that the SAP particles do not shift during subsequent folding and lamination processes.

[0053] The particle size of SAP particles 33 is preferably 100–500 μm, more preferably 200–400 μm. Within this particle size range, the SAP particles have sufficient specific surface area to ensure a rapid water absorption rate—the smaller the particle size, the larger the specific surface area and the faster the water absorption rate; while avoiding the dust and processing difficulties caused by excessively small particle size (<100 μm), and the decreased water absorption rate and uneven distribution caused by excessively large particle size (>500 μm). The amount of SAP particles 33 added between the water-repellent membrane 26 and the first absorbent fiber cloth 25 is preferably 30–100 g / m², more preferably 50–80 g / m². SAP particles 33 can be made of sanitary-grade SAP materials known in the art, such as sodium polyacrylate superabsorbent polymer (SAP). Their saline absorption capacity is preferably 30-60 times, more preferably 40-50 times; their menstrual blood absorption capacity is preferably 20-40 times, more preferably 25-35 times; and their water retention capacity is at least 80% of the absorbed liquid under 5.5 kPa pressure, more preferably at least 85%. This water retention capacity parameter reflects the SAP's water-locking performance under pressure—even when subjected to compression from sitting, walking, or other movements, the SAP can firmly lock the absorbed menstrual blood in its gel network, making it difficult to release. The water retention capacity test method for SAP particles refers to the water retention capacity determination method in the People's Republic of China Chemical Industry Standard HG / T 4069—2008 "Industrial Superabsorbent Resins," measuring the SAP's water retention rate under 5.5 kPa pressure. In this invention, the selected SAP particles have a water retention rate of 80%-95% under 5.5 kPa pressure.

[0054] SAP particles 33 are uniformly distributed on the upper surface of the water-repellent membrane 26, with the average spacing between adjacent SAP particles preferably being 0.5–2.0 mm, more preferably 0.8–1.5 mm. At this distribution density, the SAP particles will not have insufficient water absorption capacity due to excessively sparse distribution (spacing > 2.0 mm), nor will they have excessively dense distribution (spacing < 0.5 mm) cause gel agglomeration and blockage of fiber pores after water absorption and swelling. Then, a first absorbent fiber cloth 25 is laid on the water-repellent membrane 26 with SAP particles 33 spread on it, so that the SAP particles 33 are sandwiched between the water-repellent membrane 26 and the first absorbent fiber cloth 25; a second absorbent fiber cloth 27 is laid on the lower surface of the water-repellent membrane 26, thereby forming a laminated composite sheet 28 consisting of the first absorbent fiber cloth 25, SAP particles 33, adhesive layer 29, water-repellent membrane 26, adhesive layer 29 (lower surface), and second absorbent fiber cloth 27 from top to bottom. As an alternative, the stacking order of the laminated composite sheet 28 can also be adjusted to: first absorbent fiber cloth 25, adhesive layer 29, water-repellent membrane 26, adhesive layer 29 (lower surface), SAP particles 33, and second absorbent fiber cloth 27—that is, the SAP particles 33 are sandwiched between the water-repellent membrane 26 and the second absorbent fiber cloth 27. In this alternative, menstrual blood needs to pass through the water-repellent membrane 26 before reaching the SAP layer, and the liquid components in the menstrual blood need to pass through the micropores of the water-repellent membrane 26 before contacting the SAP, which affects the absorption efficiency to some extent. Therefore, the anti-backflow performance is slightly inferior to the scheme where the SAP is placed between the water-repellent membrane 26 and the first absorbent fiber cloth 25. For applications requiring higher absorption rates, it is preferable to place the SAP between the water-repellent membrane 26 and the first absorbent fiber cloth 25; for applications that prioritize the physical containment and spillage prevention of the SAP, either scheme is acceptable.

[0055] The width W1 of the laminated composite sheet 28 (i.e., the width before folding) can be set according to the required C-shaped structure size of the final product, preferably 120-135mm, more preferably 125-130mm. Within this width range, sufficient material allowance is provided for the C-shaped structure to form a central groove 24 with sufficient depth, without the overall width of the folded product exceeding the standard width range of sanitary napkins due to excessive material width. When the width W1 of the laminated composite sheet 28 is less than 120mm, the width of the folded C-shaped structure is too small, resulting in insufficient liquid storage capacity of the central groove 24; when W1 is greater than 135mm, the width of the folded C-shaped structure is too large, exceeding the standard width range of sanitary napkins, and causing significant material waste. Subsequently, as... Figure 3As shown in b, the laminated composite sheet 28 is folded upwards simultaneously on both sides along its length (as indicated by arrow A), so that the absorbent fiber cloths on both sides are positioned opposite each other, and the SAP particles 33 are wrapped inside the folded grooves of the folded structure. The water-repellent membrane 26 is sandwiched between the first absorbent fiber cloth 25 and the second absorbent fiber cloth 27 as an intermediate layer. In the folding process, the upward folding angle on both sides is preferably 90-180°, more preferably 120-150°, and the folding speed is preferably 50-200 times per minute. Within this speed range, the folding action is smooth and will not cause the SAP particles to fall off the adhesive layer, while ensuring production efficiency.

[0056] After folding, as follows Figure 3As shown in Figure c, the laminated composite sheet 28 forms a C-shaped folded structure 20 having a bottom 21, a first top 22 and a second top 23 extending upward from both sides of the bottom 21, and a central groove 24 formed by the bottom 21, the first top 22, and the second top 23. The folded width W2 (i.e., the total width of the C-shaped structure, i.e., the horizontal distance between the outer walls of the first top 22 and the second top 23) is preferably 70-80 mm, more preferably 72-78 mm. This size range matches the standard width of existing sanitary napkin products, ensuring that the C-shaped folded structure can be adapted to the specifications of existing sanitary napkin production lines. The width W3 of the central groove 24 (i.e., the horizontal distance between the inner walls of the first top 22 and the second top 23) is preferably 50-65 mm, more preferably 55-60 mm. This width range provides sufficient volume space for storing menstrual blood. The liquid storage volume of the central groove 24 is approximately W3 × folding height × product length. Under the conditions that W3 is 55-60 mm, folding height is 12-16 mm, and product length is 230 mm, the liquid storage volume of the central groove 24 can reach approximately 150-220 mL, which is sufficient to meet the menstrual blood storage needs of daytime and nighttime sanitary napkins. At the same time, it ensures that the first top 22 and the second top 23 have sufficient wall thickness and height to achieve effective water-repellent barrier function. The first top 22 and the second top 23 are both located above the bottom 21 and are connected to the bottom 21 as an integral structure. The three together form an upward-facing U-shaped or C-shaped space, namely the central groove 24. In this C-shaped fold structure, the fold height of the first top 22 and the second top 23 (i.e., the vertical distance from the upper surface of the bottom 21 to the top surface of the first top 22 or the second top 23) is preferably 10-20 mm, more preferably 12-16 mm. Within this height range, the sidewalls of the C-shaped fold structure have sufficient barrier height to prevent menstrual blood stored in the central groove 24 from overflowing through the top when the core is squeezed, while not being too high to cause an excessive increase in the overall thickness of the sanitary napkin. The SAP particles 33 are firmly wrapped inside the fold grooves of the C-shaped fold structure 20—specifically, the SAP particles 33 are sandwiched between the water-repellent membrane 26 and the first absorbent fiber cloth 25. The water-repellent membrane 26 forms a continuous physical barrier below and to the side of the SAP particles 33, and the first absorbent fiber cloth 25 forms a covering layer above the SAP particles 33. When the SAP absorbs water and expands, its expansion space is doubly constrained by the water-repellent membrane 26 and the first absorbent fiber cloth 25, preventing disorderly diffusion to the outside of the C-shaped structure.

[0057] The first absorbent fiber cloth 25 and the second absorbent fiber cloth 27 are each independently selected from fibrous sheets with liquid absorption and storage capabilities, including but not limited to plant fiber cloth (such as natural cotton fiber cloth, bamboo fiber cloth, etc.), dust-free paper (dry-laid paper nonwoven fabric), or nonwoven fabric (such as spunlace cloth, hot-air cloth, etc.). When using natural plant fiber cloth, the biodegradable properties of plant fibers help to further improve the overall environmental performance of the product. The basis weight of the first absorbent fiber cloth 25 and the second absorbent fiber cloth 27 can be selected according to the target liquid storage capacity, for example, each independently is 30-80 g / m², preferably 40-60 g / m². Within this basis weight range, the absorbent fiber cloth has sufficient inter-fiber pores to quickly absorb menstrual blood and provide ample storage space, while maintaining the softness and foldability of the sheet, facilitating subsequent C-folding and forming operations. When the basis weight of the absorbent fiber fabric is less than 30 g / m², the fibers are too sparse, resulting in insufficient liquid absorption and storage capacity; when the basis weight is greater than 80 g / m², the material is too thick, affecting the overall thinness of the sanitary napkin. The first absorbent fiber fabric 25 and the second absorbent fiber fabric 27 can be made of the same or different materials—for example, in one specific embodiment, the first absorbent fiber fabric 25 is made of 50 g / m² dust-free paper, which has good liquid permeability, allowing menstrual blood to quickly pass through and reach the SAP particles 33; the second absorbent fiber fabric 27 is made of 40 g / m² spunlace fabric, which has high wet strength and softness, helping to maintain the morphological stability of the C-shaped fold structure.

[0058] The water-repellent membrane 26 is a low surface energy microporous polymer film with air-permeable but water-impermeable properties, allowing air and water vapor to pass through while blocking liquid water penetration. The water-repellent membrane 26 can be made of polyethylene (PE) microporous membrane, polypropylene (PP) microporous membrane, or polytetrafluoroethylene (PTFE) microporous membrane, etc., and is a sanitary-grade microporous breathable membrane. Its thickness is preferably 15–40 μm, more preferably 20–30 μm; its basis weight is preferably 15–25 g / m², more preferably 18–22 g / m²; its air permeability is preferably 1000–5000 g / (m²·24h), more preferably 2000–4000 g / (m²·24h); and its hydrostatic pressure resistance is preferably ≥50 cmH₂O, more preferably ≥80 cmH₂O. The average pore size of the micropores in the water-repellent membrane 26 is preferably 0.01–2 μm. This pore size range is much smaller than the diameter of liquid water droplets (usually above 100 μm), but much larger than the diameter of water vapor molecules (approximately 0.0004 μm), thus achieving a selective permeability function of "breathable but waterproof". The water-repellent membrane 26 within the above parameter range ensures effective blocking of menstrual blood penetration while possessing sufficient breathability and mechanical strength to withstand mechanical actions such as folding and squeezing during the production and use of sanitary napkins without breaking. It should be noted that the water-repellent membrane 26 plays a physical barrier rather than a chemical absorption role in this invention. Its enclosed covering method, sandwiched between two layers of absorbent fiber fabric, prevents any liquid stored inside the central groove 24 from penetrating the water-repellent membrane 26 and migrating outwards.

[0059] like Figure 1 , Figure 2 and Figure 4 As shown, the flow guiding material 10 is disposed above the C-shaped folded structure 20. One structural form of the flow guiding material 10 is a single-layer perforated membrane structure (e.g., Figure 1 a and Figure 2 As shown in Figure a), it includes a single-layer perforated membrane 12, on which multiple inverted trapezoidal holes 11 are evenly distributed, penetrating the thickness direction of the perforated membrane. The cross-section of the inverted trapezoidal holes 11 is larger at the top and smaller at the bottom—that is, the diameter of the hole (upper opening 111) on the upper surface of the perforated membrane 12 is larger than the diameter of the hole (lower opening 112) on the lower surface of the perforated membrane 12. Figure 4As shown in the enlarged partial cross-sectional view, the inverted trapezoidal hole 11 penetrates the entire thickness direction of the perforated membrane 12. Its upper opening 111 is located on the upper surface of the perforated membrane 12 (i.e., the side facing the direction of menstrual flow), and its lower opening 112 is located on the lower surface of the perforated membrane 12 (i.e., the side facing the hot air nonwoven fabric 13). Since the diameter D1 of the upper opening 111 is larger than the diameter D2 of the lower opening 112, the inverted trapezoidal hole 11 presents an overall funnel-shaped or truncated conical shape that is wider at the top and narrower at the bottom. This geometric structure determines the directionality of liquid transport within the channels: when menstrual blood contacts the perforated membrane 12 from above, it can smoothly enter the channels through the large-diameter upper opening 111 and flow out through the lower opening 112 under gravity. However, once the liquid passes through the membrane and enters the lower functional layer, if there is an upward backflow tendency due to compression or other reasons, the backflowing liquid needs to enter the channels through the lower opening 112. Since the diameter of the lower opening 112 is much smaller than that of the upper opening 111, the liquid needs to overcome a large surface tension resistance when entering the small-diameter opening. At the same time, the conical structure of the channels makes the direction of the capillary driving force on the rising liquid opposite to the backflow direction, thus effectively preventing the backflow of liquid that has passed through the membrane. The inner wall of the inverted trapezoidal hole 11 is a smooth conical curved surface. This smooth curved surface can reduce flow resistance and accelerate liquid passage during infiltration; while during liquid backflow, the synergistic effect of the conical curved surface and the surface tension of the liquid enhances the barrier effect. The upper diameter D1 of the inverted trapezoidal hole 11 is preferably 0.3 to 1.5 mm, more preferably 0.5 to 1.0 mm; the ratio (D1:D2) of the lower diameter D2 to the upper diameter D1 of the inverted trapezoidal hole 11 is preferably 1.5:1 to 3:1. Within this ratio range, the inverted trapezoidal hole 11 can ensure the smooth flow of menstrual blood from the top (the upper opening 111 is large enough to accommodate the inflow of menstrual blood) and provide sufficient backflow resistance (the lower opening 112 is small enough to prevent liquid backflow), thus achieving the optimization of the unidirectional flow guidance function. The spacing d between the inverted trapezoidal holes 11 on the surface of the perforated membrane 12 (i.e., the horizontal distance between the centers of two adjacent inverted trapezoidal holes 11) is preferably 1.0–4.0 mm, more preferably 1.5–3.0 mm. Within this spacing range, the distribution density of the inverted trapezoidal holes 11 on the perforated membrane 12 is moderate, ensuring a sufficient number of drainage channels per unit area of ​​membrane to achieve rapid infiltration of menstrual blood, while avoiding a decrease in membrane structural strength due to excessive density between holes. The porosity of the upper perforated membrane 12 (i.e., the ratio of the total area of ​​all inverted trapezoidal holes 11 to the total area of ​​the upper surface of the perforated membrane 12) is preferably 15%–35%, more preferably 20%–30%. Within this porosity range, the perforated membrane 12 has sufficient liquid permeation flux to match the concentrated discharge rate of menstrual blood, while maintaining sufficient membrane body area to ensure the structural integrity and tensile strength of the perforated membrane 12 itself.The upper perforated membrane 12 is made of a polymer film, such as a polyethylene (PE) film or a blend of polyethylene and ethylene-vinyl acetate copolymer (EVA), with a preferred thickness of 20–50 μm. The inverted trapezoidal holes 11 on the perforated membrane 12 can maintain their shape through a heat-setting process—that is, after the perforation process is completed, the membrane is heated and shaped, causing stress relaxation and recrystallization of the membrane molecular chains around the holes, thereby permanently fixing the shape of the inverted trapezoidal holes 11 in the membrane, ensuring that even if subjected to compression and stretching during use, the hole shape will not undergo significant deformation and lose its unidirectional flow guidance function.

[0060] In a more preferred embodiment, the flow guiding material 10 may also employ a multi-layer perforated film composite structure (such as...). Figure 1 b and Figure 2 As shown in Figure b), the composite structure comprises three main parts stacked sequentially from top to bottom: a perforated membrane 12, a hot-air nonwoven fabric 13, and a wood pulp layer 14. The perforated membrane 12, serving as a unidirectional flow guide and anti-backflow layer, is located on top of the composite structure, directly facing the direction of menstrual flow. The hot-air nonwoven fabric 13, acting as an elastic support layer and liquid transfer layer, is located below the perforated membrane 12. The wood pulp layer 14, serving as a liquid absorption and distribution layer, is located below the hot-air nonwoven fabric 13, forming the bottom layer of the composite structure. Menstrual blood, upon contact with the composite structure from above, first passes through the perforated membrane 12, then is rapidly transferred through the hot-air nonwoven fabric 13 to the wood pulp layer 14 where it is absorbed and evenly distributed, before finally being transferred downwards to the absorbent core for final storage.

[0061] Hot-air nonwoven fabric 13 is disposed below the perforated membrane 12, serving as an elastic support layer and liquid transfer layer for the perforated membrane 12. The hot-air nonwoven fabric 13 has a loose three-dimensional mesh structure, formed by a large number of fibers randomly arranged in three-dimensional space and fused together at their intersections. Specifically, the hot-air nonwoven fabric 13 is preferably made from core-sheath type thermally bonded composite fibers (i.e., ES fibers) through hot-air forming. ES fibers have a core-sheath structure—the sheath is low-melting-point polyethylene (PE), and the core is polypropylene (PP) or polyester (PET). During hot-air treatment, the hot air temperature rises above the melting point of the sheath (typically 130–160°C) but below the melting point of the core. The sheath melts while the core remains solid. The molten sheath flows and fuses at the fiber intersections, forming fiber intersection fusion points after cooling, thus fixing the fiber network into a loose three-dimensional mesh structure. The core layer, as the fiber skeleton, maintains the fiber's strength and resilience, giving the material excellent compression and resilience properties. The fineness of the ES fiber is preferably 1.5–4.0 denier, more preferably 2.0–3.0 denier, and the fiber length is preferably 38–51 mm. Within this fineness and length range, the ES fiber has sufficient softness and bulkiness, and the three-dimensional network structure formed after hot air treatment has excellent liquid diffusion properties and resilience. The basis weight of the hot-air nonwoven fabric 13 is preferably 15–60 g / m², more preferably 25–40 g / m²; the thickness is preferably 0.5–5.0 mm, more preferably 1.0–3.0 mm. Within this weight and thickness range, the hot-air nonwoven fabric 13 possesses sufficient fiber density and structural strength to provide effective support while maintaining good bulk and liquid permeability. When the weight is below 15 g / m², the fibers are too sparse, resulting in insufficient support and an inability to effectively maintain the pore shape of the perforated membrane 12. When the weight is above 60 g / m², the material is too dense, affecting overall thinness and liquid penetration speed. When the thickness is below 0.5 mm, the buffer space is insufficient, limiting the support effect. When the thickness is above 5.0 mm, it affects the overall thinness of the product. The three-dimensional mesh structure of the hot-air nonwoven fabric 13 provides a large number of interconnected pores, with a porosity preferably between 85% and 95%. These pores provide channels for rapid liquid transfer and also provide space for the compression deformation of the hot-air nonwoven fabric 13.

[0062] The supporting mechanism of the hot-air nonwoven fabric 13 is as follows: During the preparation of the perforated membrane composite structure, each layer of material needs to be laminated to ensure strong interlayer adhesion. Under the pressure of lamination, the hot-air nonwoven fabric 13 is compressed, its fluffy three-dimensional network structure deforms, the pores between fibers are compressed, and the thickness decreases. However, due to the excellent resilience of the ES fiber core layer and the stable three-dimensional skeleton formed by the cross-point fusion of the fiber network, the hot-air nonwoven fabric 13 can quickly recover its original thickness after the lamination pressure is released. This rebound process generates a continuous upward supporting force on the perforated membrane 12 located above it, so that the inverted trapezoidal holes 11 of the perforated membrane 12 can still maintain their original pore shape of being larger at the top and smaller at the bottom after the lamination pressure is released. Specifically, when the compression pressure is applied, the walls of the inverted trapezoidal holes 11 in the perforated membrane 12 are compressed and tend to collapse towards the center; however, when the pressure is released, the upward supporting force generated by the rebound of the hot-air nonwoven fabric 13 pushes the perforated membrane 12 upward, causing the walls of the inverted trapezoidal holes 11 to return to their original positions, ensuring unobstructed passageways. The compression rebound rate of the hot-air nonwoven fabric 13 is preferably 70% to 95%, more preferably 80% to 90% (this compression rebound rate is determined according to the method specified in GB / T 24442.2—2009 "Determination of Compression Properties of Textiles Part 2: Isotropic Method", where the thickness recovery rate is measured after compression at 5 kPa and pressure is released). Within this rebound rate range, the hot-air nonwoven fabric 13 can recover most of its original thickness after composite compression, ensuring continuous and effective support for the perforated membrane 12. As a specific example, when the original thickness of the hot-air nonwoven fabric 13 is 2.0 mm and the compression resilience is 85%, after composite pressing (pressure of approximately 10 kPa), the thickness of the hot-air nonwoven fabric 13 can recover to approximately 1.7 mm, which is sufficient to provide effective support for the perforated membrane 12. In contrast, the cleanroom paper in the prior art is made of plant fibers bonded by hot air or adhesives. The bonding points between the fibers lack elasticity, and the fiber network undergoes irreversible collapse after compression. The compression resilience is usually less than 20% or even close to zero, which cannot provide effective support for the perforated membrane. Therefore, the inverted trapezoidal holes of the perforated membrane will be flattened and deformed after composite pressing and cannot be restored.

[0063] The wood pulp layer 14 is disposed below the hot-air nonwoven fabric 13, serving as the liquid absorption and distribution layer of the perforated membrane composite structure. The wood pulp layer 14 is either a fluff pulp layer or a composite layer of fluff pulp and chemical fibers. Fluff pulp, also known as wood pulp or paper pulp, is a fibrous material obtained from softwood or hardwood through chemical pulping, possessing excellent hydrophilicity and capillary liquid absorption capacity. The basis weight of the wood pulp layer 14 is preferably 20–80 g / m², more preferably 30–60 g / m². Within this basis weight range, the wood pulp layer 14 has sufficient liquid absorption and storage capacity while maintaining the material's softness and processability. When the wood pulp layer 14 is a composite layer of fluff pulp and chemical fibers, the chemical fibers can be selected from ES fibers, polyester fibers, or polypropylene fibers, etc., and their addition amount is preferably 1–60 g / m² to improve the bulkiness and structural stability of the wood pulp layer 14. The preferred addition ratio of chemical fibers is 5% to 30% (by mass). Within this range, it ensures that the wood pulp layer 14 has sufficient fluff pulp content to guarantee high liquid absorption capacity, while the skeletal effect of chemical fibers improves the wet strength and structural integrity of the wood pulp layer 14.

[0064] like Figure 5 As shown, the menstrual blood transfer and storage process of the absorbent core 1 of the present invention in the working state is as follows: Menstrual blood flows from the top (e.g., Figure 5After contacting the absorbent core 1 (in the direction indicated by the middle arrow L), the menstrual blood first reaches the upper surface of the guiding material 10. Since the upper opening 111 of the evenly distributed inverted trapezoidal holes 11 on the perforated membrane 12 is a large opening, menstrual blood quickly flows into the channels from the upper opening 111 under gravity and passes through the membrane, flowing out from the lower opening 112 into the lower C-shaped folded structure 20. During this process, the "larger at the top and smaller at the bottom" structure of the inverted trapezoidal holes 11 ensures rapid downward penetration of the menstrual blood, preventing liquid accumulation or lateral loss on the surface of the guiding material 10. After entering the C-shaped folded structure 20, the menstrual blood is first absorbed by the fiber network of the first absorbent fiber cloth 25 and rapidly transferred to both sides and downwards through fiber capillary action. A portion of the menstrual blood is directly absorbed and stored by the fiber pores of the first absorbent fiber cloth 25; the other portion continues to penetrate downwards, contacting the SAP particles 33 sandwiched between the water-repellent membrane 26 and the first absorbent fiber cloth 25. Upon contact with menstrual blood, SAP particles 33 rapidly absorb water and swell to form a hydrogel, firmly locking the liquid components of the menstrual blood within its three-dimensional network structure. The sodium carboxylate groups on the SAP molecular chains ionize upon contact with water, increasing the electrostatic repulsion between molecular chains and causing the network to expand. A large number of water molecules enter the network through hydrogen bonds and osmosis and are captured. This chemical water absorption process transforms free liquid in the menstrual blood into bound water in a gel state, significantly reducing the presence of free liquid and thus significantly reducing the risk of liquid being squeezed out under pressure. Because SAP particles 33 have a water retention capacity of over 80% of the absorbed liquid under 5.5 kPa pressure, even with pressure from sitting, walking, or other movements, the SAP gel can still firmly lock in the menstrual blood. Simultaneously, any residual liquid not captured by the SAP continues to permeate downwards and is further absorbed and stored by the second absorbent fiber cloth 27. After three layers of absorption—fiber absorption by the first absorbent fiber cloth 25, chemical absorption by the SAP particles 33, and fiber absorption by the second absorbent fiber cloth 27—the menstrual blood is fully locked inside the C-fold structure 20.

[0065] At this time, as Figure 5As indicated by the middle arrow R, the menstrual blood stored within the C-shaped folded structure 20 is protected by multiple anti-reverse osmosis barriers: the first barrier is the inverted trapezoidal holes 11 on the perforated membrane 12—when the menstrual blood stored in the C-shaped structure surges upward due to the pressure generated by human postures such as sitting, walking, and squatting, the liquid needs to enter the channel through the small-diameter lower opening 112 of the inverted trapezoidal hole 11, where it is effectively blocked by the greater surface tension resistance. Simultaneously, the total cross-sectional area of ​​the lower opening 112 of each inverted trapezoidal hole 11 on the perforated membrane 12 is much smaller than the total cross-sectional area of ​​the upper opening 111, further restricting the liquid's backflow channel; the second barrier is the water-repellent membranes 26 on both sides of the C-shaped folded structure 20—the first top 22 and the second... The inner sides of the top 23 are covered by a water-repellent membrane 26. Even if menstrual blood stored in the C-shaped structure flows to the side, it will be blocked by the water-repellent membrane 26 and cannot penetrate the sidewall. The third barrier is the water-repellent membrane 26 at the bottom of the C-shaped folded structure 20—the lower surface of the bottom 21 is covered by the water-repellent membrane 26, preventing menstrual blood stored in the C-shaped structure from penetrating downwards to the bottom. The fourth barrier is the gel-state water-locking of SAP particles 33—SAP has absorbed the liquid phase components in the menstrual blood and converted them into a gel state. The water in the gel state is bound by the SAP molecular network and is difficult to release even under pressure. This "chemical water-locking" barrier is particularly effective when the SAP water retention capacity is ≥80%. Under the combined action of the four barriers, the menstrual blood that has entered the C-shaped folded structure 20 is firmly locked and cannot seep out regardless of the direction of external pressure, thus achieving an extremely low reverse osmosis rate. At the same time, because the water-repellent membrane 26 has a breathable but waterproof microporous structure, the water vapor generated by the evaporation of blood inside the C-shaped folded structure 20 can pass through the micropores of the water-repellent membrane 26 and be discharged outward, ensuring a dry and breathable environment inside the core.

[0066] like Figure 6As shown, in some preferred embodiments of the present invention, the absorbent core 1 further includes a flow guiding layer 40, which is disposed above the flow guiding material 10. The function of the flow guiding layer 40 is that when menstrual blood contacts the absorbent core 1 from above, the flow guiding layer 40 first diffuses the menstrual blood in the horizontal direction, so that the menstrual blood is evenly distributed over a large area before entering the flow guiding material 10. This avoids the problem of local saturation of the core and underutilization of other areas caused by localized concentrated seepage of menstrual blood, thereby improving the effective utilization rate of the entire absorbent core. The flow guiding layer 40 can be a general-purpose ES chemical fiber flow guiding layer—that is, a fluffy nonwoven fabric made of core-sheath type thermally bonded composite fiber (ES fiber) through hot air molding. The sheath of the ES fiber is low-melting-point polyethylene, and the core layer is polypropylene or polyester. After hot air treatment, the fiber intersections are fused together to form a three-dimensional network structure with high porosity and resilience, in which menstrual blood can quickly diffuse laterally along the fiber surface and penetrate downwards. The fineness of ES fibers is preferably 1.5–4.0 denier, and the fiber length is preferably 38–51 mm. Within this fineness and length range, ES fibers possess sufficient softness and bulkiness, while the three-dimensional network structure formed after hot air treatment exhibits excellent liquid diffusion performance and resilience. The flow-guiding layer 40 can also be made of superfluid wood pulp cloth—this material specifically includes a nonwoven fabric layer and a fluff pulp layer or a composite layer of fluff pulp and chemical fibers (such as ES fibers) adhered beneath the nonwoven fabric layer. The amount of nonwoven fabric layer used is 18–60 g / m², and the amount of fluff pulp used is 20–80 g / m². The flow-guiding mechanism of the super-fluidized wood pulp cloth is as follows: the liquid first reaches the nonwoven fabric layer and diffuses laterally within the nonwoven fiber network. It is then absorbed by the fluff pulp layer tightly adhering to the nonwoven fabric layer and permeates downwards. Due to the extremely strong capillary absorption capacity of the fluff pulp layer, it can quickly pull the liquid diffused on the nonwoven fabric layer into itself and transfer it downwards, thereby allowing the liquid to uniformly and rapidly infiltrate to the flow-guiding material 10. The basis weight of the flow-guiding layer 40 is preferably 20–50 g / m², more preferably 25–40 g / m², and the thickness is preferably 0.5–2.0 mm. The selection of the thickness and basis weight of the flow-guiding layer 40 needs to balance the diffusion effect and overall thinness—if the thickness is too thin, the diffusion effect is poor; if the thickness is too thick, it affects the overall thinness.

[0067] like Figure 7As shown, when viewed from above, the perforated membrane 12 has a large number of inverted trapezoidal holes 11 evenly distributed on its surface. The upper opening 111 of each inverted trapezoidal hole 11 is circular or nearly circular, and they are arranged neatly according to a certain row and column spacing (hole spacing d). The uniform distribution of the inverted trapezoidal holes 11 on the membrane surface ensures that menstrual blood can pass evenly over the entire area of ​​the perforated membrane 12 during the seepage process, avoiding localized concentrated seepage. The arrangement of the inverted trapezoidal holes 11 can be a rectangular array, a rhomboid array, or an alternating arrangement. The figure shows a rectangular array arrangement, but the invention is not limited to this; other uniform distribution forms can also achieve the same flow guiding effect. Preferably, the inverted trapezoidal holes 11 are arranged in an alternating arrangement (i.e., a honeycomb arrangement). This arrangement can achieve a higher porosity under the same hole spacing conditions, and the membrane material distribution between the holes is more uniform, which is beneficial to improving the overall structural strength of the perforated membrane 12. In one specific embodiment, the perforated membrane 12 adopts staggered inverted trapezoidal holes 11 with an upper diameter D1 of 0.8 mm and a lower diameter D2 of 0.35 mm (D1:D2≈2.3:1), a hole spacing d of 2.0 mm, and an opening rate of 28%. This combination of parameters achieves excellent anti-backflow effect while ensuring rapid infiltration.

[0068] like Figure 8 As shown, when viewed from above, the C-shaped folding structure 20 is folded upwards on both sides, forming a central groove 24 extending along its length. The width W3 of the central groove 24 is preferably 50-65 mm. The first top 22 and the second top 23 are located on the left and right sides of the central groove 24, respectively, and their top surfaces constitute part of the upper surface of the lower layer of the absorbent core 1 of the present invention. After the guide material 10 is placed above the C-shaped folding structure 20, the two sides of its lower surface are in contact with the upper surfaces of the first top 22 and the second top 23, while the central area of ​​the guide material 10 spans over the central groove 24, covering the entire opening of the central groove 24. Since the guide material 10 covers the opening of the central groove 24, menstrual blood falls directly into the interior of the central groove 24 after passing through the guide material 10 and is absorbed and stored. At the same time, the guide material 10 forms a physical barrier at the opening of the groove, preventing the menstrual blood stored inside the groove from flowing back upwards. The width of the flow guiding material 10 is preferably 60-80mm, slightly less than or equal to the width W2 of the C-shaped folded structure 20 after folding, to ensure good edge alignment between layers and avoid uneven edges affecting the product's appearance and performance.

[0069] In the preparation process of this invention, the preferred method for fixing and connecting the functional layers is adhesive spraying. Specifically, the lower surface of the flow guiding material 10 is bonded and fixed to the upper surfaces of the first top 22 and the second top 23 of the C-shaped folded structure 20 by adhesive spraying; when a flow guiding layer 40 is provided, the lower surface of the flow guiding layer 40 is bonded and fixed to the upper surface of the flow guiding material 10 by adhesive spraying. The amount of adhesive applied should not be too much (preferably 1-5 g / m², more preferably 2-3 g / m²) to avoid the adhesive clogging the inverted trapezoidal holes 11 on the perforated membrane or the fiber pores of each functional layer, affecting the liquid permeability. The preferred method of adhesive spraying is spiral spraying or fiber spraying to obtain a uniform and breathable adhesive layer. During the spray adhesive bonding process, the open time of the adhesive (i.e., the time interval between application and bonding of the two layers) is preferably controlled within 5 to 30 seconds to ensure that the adhesive completes the bonding process in its optimal tack condition. This avoids excessively long open times that could lead to skin formation on the adhesive surface, affecting bond strength, or excessively short open times that could result in insufficient wetting of the bonded surfaces, affecting bonding uniformity. The application temperature of the hot melt adhesive is preferably 130 to 170°C, more preferably 145 to 160°C. Within this temperature range, the hot melt adhesive exhibits good flowability and wetting properties, enabling it to be evenly applied to the surfaces of each layer and form a strong bond. Bonding between layers can also be achieved through ultrasonic spot welding, hot pressing, etc., but spray adhesive bonding is the simplest method and does not damage the original porous structure of each layer, making it the preferred embodiment of this invention.

[0070] All raw materials involved in this invention can be obtained through commercial channels—the perforated membrane 12 can be made of sanitary grade PE perforated membrane (the pore size, pore spacing, open area ratio, and other parameters can be customized as required), the hot air nonwoven fabric 13 can be made of sanitary grade ES hot air nonwoven fabric (the basis weight, thickness, compression resilience, and other parameters can be customized as required), the wood pulp layer 14 can be made of sanitary grade fluff pulp (the basis weight can be customized as required), the water-repellent membrane 26 can be made of sanitary grade PE microporous breathable membrane (the air permeability, hydrostatic pressure, and other parameters can be customized as required), the first absorbent fiber cloth 25 and the second absorbent fiber cloth 27 can be made of sanitary grade plant fiber cloth, dust-free paper, or spunlace nonwoven fabric, the SAP particles 33 can be made of sanitary grade sodium polyacrylate superabsorbent resin (the particle size, water absorption ratio, water retention capacity, and other parameters can be customized as required), and the flow guiding layer 40 can be made of sanitary grade ES hot air nonwoven fabric. All process steps—sizing, SAP application, web laying, folding, and adhesive spraying—can be completed using existing mature equipment on high-speed sanitary napkin production lines. There is no need to introduce complex specialized equipment such as hot-pressing, three-dimensional molding, or ultrasonic lamination equipment, demonstrating excellent industrial applicability and feasibility for large-scale mass production. Existing high-speed sanitary napkin production lines typically operate at speeds of 300–800 pieces per minute. The manufacturing process of this invention is simple, and the equipment is highly versatile, allowing for direct integration into existing production lines without significant equipment modifications or reductions in production speed. The SAP application step only requires the addition of a vibrating screen quantitative application device, which can be installed after the sizing station and before the web laying station on the existing production line, making the modification simple and cost-effective.

[0071] Performance Verification Test: Pig blood (whose viscosity and composition are similar to human menstrual blood, and is a widely used simulated liquid in the performance testing of sanitary napkin absorbent materials) was used to test the absorption rate, backflow, and absorption ratio of the absorbent core of this invention. The test method refers to the determination method of backflow and absorption ratio in the national standard GB / T 8939—2025 "Sanitary Napkins (Patrolleys)". Specifically: Five absorbent core samples with a length of 230mm were cut, and the weight of 10 sheets of φ110mm medium-speed filter paper was recorded as m1. 5±0.1ml of pig blood at 23±1℃ was measured as the test solution. The sample was laid flat on the worktable, and a point was marked in the center of the sample as the pouring point. Holding a graduated cylinder, the test solution was poured onto the sample surface at a height of about 10mm above the pouring point, and the timing was started at the same time. The time required for the solution to be completely absorbed by the sample surface was recorded as the absorption rate. Five minutes later, a φ110mm medium-speed filter paper of known weight was placed on the sample surface, and then a φ100mm standard pressure block weighing 1.2kg was pressed onto the filter paper. After applying pressure for 10 seconds, the standard pressure block was removed, and the weight of the filter paper was recorded as m2. The backflow rate (g) = m2 - m1. The absorption rate test was conducted according to the method specified in GB / T 8939—2025. The sample was immersed in physiological saline for a certain period of time and then weighed to calculate the absorption rate. The test results show that the absorbent core of this invention (with a SAP addition of 60g / m² in the C-shaped folded structure and a SAP water retention capacity of 88% at 5.5kPa pressure) can absorb 5ml of pig blood in 4-7 seconds, and the backflow rate can be as low as 0.10-0.25g / m², which is far superior to the national standard GB / T 8939—2025 requirement for sanitary napkin backflow rate ≤3.0g. Meanwhile, the absorption rate test results show that its saline absorption rate can reach 14 to 20 times, meeting the requirement of GB / T 8939—2025 for a sanitary napkin absorption rate of ≥4.0 times. In the dynamic pressure test (simulating the alternating compression state of a human body in sitting and walking postures—tested with alternating cyclic loads at a frequency of about 1Hz and a pressure of 0.5 to 2.0kPa, for 100 cycles), the absorbent core of this invention exhibits stable and continuous low backflow performance. After 100 cycles of alternating pressure, the backflow amount can still be controlled at an extremely low level of 0.15 to 0.35g / m², proving that this invention achieves a long-term stable low backflow effect while ensuring that the absorbent capacity meets national standards through a multi-synergistic anti-backflow mechanism of "inverted trapezoidal pore unidirectional flow guidance (perforated membrane) + high-capacity chemical absorption and water retention locking (SAP inside the C-shaped structure, water retention capacity ≥80%) + enclosed physical barrier (C-shaped water-repellent folded structure)".Compared to the control sample without SAP within the C-fold structure (absorption ratio of approximately 6–8 times, failing to meet the requirement of ≥4.0 times in GB / T 8939—2025), the water absorption ratio of this scheme is significantly improved. Compared to the control sample where SAP is simply filled inside the C-shaped groove without being sandwiched between the water-repellent membrane and the absorbent fiber cloth (reverse seepage of approximately 0.8–1.2 g / m²), this scheme reduces reverse seepage by approximately 75%–88% by sandwiching SAP between the water-repellent membrane and the absorbent fiber cloth, utilizing the enclosed barrier of the water-repellent membrane and the water retention capacity of SAP. Furthermore, in a comparative test using SAP with a water retention capacity of 70%, the reverse seepage was approximately 0.5–0.7 g / m², while using SAP with a water retention capacity of ≥80%, the reverse seepage could be as low as 0.10–0.25 g / m², demonstrating the significant impact of optimizing the water retention capacity of SAP on the reverse seepage effect. Furthermore, for SAP samples with a water retention capacity of over 90%, the reverse osmosis rate could be reduced to 0.08–0.18 g / m², demonstrating superior anti-reverse osmosis performance. Samples with an SAP addition of 80 g / m² showed an absorption rate of 18–22 times, while the reverse osmosis rate could still be controlled at 0.12–0.28 g / m². This indicates that under the protection of the physical confinement of the C-shaped folded structure and the enclosed barrier of the water-repellent membrane of this invention, even with a high SAP addition, excellent low reverse osmosis performance can still be maintained, verifying the effectiveness of the "physical confinement + chemical absorption" synergistic mechanism of this invention.

[0072] In summary, the non-reverse-leakage sanitary napkin absorbent core and its preparation method provided by this invention, through the composite design of the guide material 10 and the SAP-containing C-shaped fold structure 20, constructs a core system with synergistic effect of "physical barrier + chemical absorption"—the upper perforated membrane 12 utilizes the geometric structure of the inverted trapezoidal holes to achieve rapid downward penetration and unidirectional flow of menstrual blood, while the lower SAP-containing C-shaped fold structure 20, on the one hand, utilizes the enclosed barrier of the water-repellent membrane 26 to achieve physical locking of menstrual blood, and on the other hand, utilizes the barrier sandwiched between the water-repellent membrane 26 and the first absorbent fiber cloth. The SAP between 25 and 25 achieves high-capacity chemical absorption of menstrual blood (the optimized range of SAP addition of 30-100 g / m² ensures that the water absorption ratio meets national standards, and the SAP water retention capacity ≥80% ensures low backflow under dynamic pressure conditions). At the same time, the water-repellent membrane 26 and the first absorbent fiber cloth 25 form an upper and lower clamping physical constraint on the water absorption and expansion of SAP. The SAP is wrapped inside the groove of the C-shaped fold structure 20. After water absorption and expansion, it is constrained by the double constraint of the water-repellent membrane 26 and the fiber cloth and will not overflow or block the fiber channel. Since the perforated membrane 12 is located inside the core rather than the surface layer, the sensitization problem when the perforated membrane is used as the surface layer is completely avoided. The hot air nonwoven fabric layer 13 below the perforated membrane 12 can also support the inverted trapezoidal holes 11, thereby ensuring the flow guiding effect. This invention achieves extremely low backflow while ensuring that the absorbency meets national standards, and at the same time maintains the thinness, high breathability and good softness of the core. Moreover, all raw materials and process steps can be adapted to existing high-speed sanitary napkin production lines, and it has good prospects for industrialization and promotion.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A sanitary napkin absorbent core that is not prone to backflow, characterized in that, include: The C-shaped folding structure is formed by sequentially layering and folding a first absorbent fiber cloth, a water-repellent membrane, and a second absorbent fiber cloth. The C-shaped folding structure has a bottom, a first top and a second top extending upward from both sides of the bottom, and a central groove formed by the bottom, the first top, and the second top. Superabsorbent polymer particles are sandwiched between the water-repellent membrane and the first absorbent fiber cloth or between the water-repellent membrane and the second absorbent fiber cloth, and the superabsorbent polymer particles are wrapped inside the grooves of the C-shaped folded structure. A flow guiding material is disposed above the C-shaped folded structure. The flow guiding material includes a perforated membrane, on which multiple holes are evenly distributed, penetrating the thickness direction of the perforated membrane. The cross-section of the holes is an inverted trapezoid with a larger top and a smaller bottom.

2. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 1, characterized in that, The flow guiding material adopts a multi-layer perforated membrane composite structure, including a perforated membrane, a hot air nonwoven fabric and a wood pulp layer stacked from top to bottom; the hot air nonwoven fabric is a fluffy three-dimensional mesh structure, which is used to support the perforated membrane to maintain the shape of the holes.

3. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 1, characterized in that, The superabsorbent polymer particles have a particle size of 100-500 μm, are uniformly distributed between the water-repellent membrane and the first absorbent fiber cloth, and have an average spacing of 0.5-2.0 mm between adjacent superabsorbent polymer particles.

4. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 1, characterized in that, The superabsorbent polymer particles have an absorption capacity of 30 to 60 times for physiological saline and 20 to 40 times for menstrual blood, and retain more than 80% of the absorbed liquid under a pressure of 5.5 kPa. The amount of superabsorbent polymer particles added between the water-repellent membrane and the first absorbent fiber cloth is 30 to 100 g / m².

5. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 1 or 2, characterized in that, The perforated membrane has a pore diameter of 0.3–1.5 mm, a pore spacing of 1.0–4.0 mm, and an opening rate of 15%–35%; the ratio of the upper diameter to the lower diameter of the inverted trapezoidal hole is 1.5:1–3:

1.

6. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 2, characterized in that, The hot-air nonwoven fabric is made of core-sheath type thermally bonded composite fiber through hot air molding. The sheath of the core-sheath type thermally bonded composite fiber is low-melting-point polyethylene, and the core is polypropylene or polyester. The wood pulp layer is a fluff pulp layer or a composite layer of fluff pulp and chemical fiber. The basis weight of the wood pulp layer is 20-80 g / m².

7. The non-reverse-leaking absorbent core of a sanitary napkin according to claim 1, characterized in that, It also includes a flow guiding layer, which is disposed above the flow guiding material. The flow guiding layer is an ES chemical fiber flow guiding layer or a super-flowable wood pulp cloth. The super-flowable wood pulp cloth includes a non-woven fabric layer and a fluff pulp layer or a composite layer of fluff pulp and chemical fibers attached below the non-woven fabric layer.

8. A method for preparing a sanitary napkin absorbent core that is not prone to backflow according to any one of claims 1-7, characterized in that, Includes the following steps: A C-shaped folding structure is provided, wherein the C-shaped folding structure is formed by sequentially stacking and compounding a first absorbent fiber cloth, a water-repellent membrane, and a second absorbent fiber cloth and then folding them in a C-shape. The C-shaped folding structure has a bottom, a first top and a second top extending upward from both sides of the bottom, and a central groove formed by the bottom, the first top, and the second top. Super absorbent polymer particles are disposed between the water-repellent membrane and the first absorbent fiber cloth or between the water-repellent membrane and the second absorbent fiber cloth, such that the super absorbent polymer particles are sandwiched between the water-repellent membrane and the first absorbent fiber cloth or between the water-repellent membrane and the second absorbent fiber cloth and are wrapped inside the groove of the C-shaped fold structure. The flow guiding material is bonded to the upper surface of the C-shaped folded structure through a second adhesive layer. The flow guiding material includes a perforated membrane with multiple holes evenly distributed on the perforated membrane through the thickness direction of the perforated membrane. The cross-section of the holes is an inverted trapezoid with a larger top and a smaller bottom.

9. The preparation method according to claim 8, characterized in that, In the step of setting superabsorbent polymer particles between the water-repellent membrane and the first absorbent fiber cloth, the superabsorbent polymer particles are first evenly sprinkled on the upper surface of the water-repellent membrane, and then the first absorbent fiber cloth is laid.

10. The preparation method according to claim 8, characterized in that, The C-shaped folded structure is formed by the following steps: providing a water-repellent membrane; applying an adhesive to the upper surface of the water-repellent membrane and then laying a first absorbent fiber cloth, and laying a second absorbent fiber cloth on the lower surface of the water-repellent membrane to form a laminated composite sheet; folding the laminated composite sheet upwards along its length on both sides, so that the absorbent fiber cloths on both sides are arranged opposite each other, and the water-repellent membrane is sandwiched between the first absorbent fiber cloth and the second absorbent fiber cloth, forming a C-shaped folded structure with the bottom, the first top, the second top, and the central groove.

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