Water-absorbing core with double-gradient water-absorbing layer and sanitary product

Abstract

The application discloses a water-absorbing core with double-gradient water-absorbing layers and a sanitary product. The water-absorbing core is provided with at least four layers of water-absorbing layers arranged in a stepped manner from top to bottom in the main body, and each water-absorbing layer is distributed with polymer water-absorbing resin; the cross-sectional area of each water-absorbing layer increases layer by layer from the uppermost layer to the lowermost layer; the distribution density of the polymer water-absorbing resin decreases layer by layer from the uppermost layer to the lowermost layer, forming a double-gradient structure of area and density. Preferably, each water-absorbing layer except the lowermost layer is elliptical, the lowermost layer is rectangular, and each layer is generally located at the same center. The layers are connected by hot-melt fibers or spraying latex. Through the double-gradient synergistic effect, the concentration of the polymer water-absorbing resin is smoothly transitioned from the center of the core to the periphery, effectively avoiding the problems of liquid diffusion obstruction and local supersaturation caused by the concentration mutation boundary in the existing two-layer structure, improving the overall utilization rate of the polymer water-absorbing resin, and improving the absorption and anti-back seepage performance under the condition of multiple liquid loading.

Description

Technical Field

[0001] This invention relates to the field of disposable hygiene products technology, and more specifically, to an absorbent core for absorbent products such as sanitary napkins, diapers, and incontinence pads, particularly an absorbent core having multiple stepped absorbent layers with the absorbent layer area and the amount of superabsorbent polymer varying in a dual gradient. Background Technology

[0002] Disposable absorbent products (such as sanitary napkins, diapers, and incontinence pads) typically consist of a top layer, a distribution layer, an absorbent core, and a bottom layer. The absorbent core is the core functional layer that determines the product's absorbency, playing a crucial role in rapidly absorbing, evenly distributing, and reliably locking in liquids (such as urine and menstrual blood) that penetrate through the top and distribution layers. The absorbent core is usually composed of a hydrophilic fiber substrate (such as wood pulp fiber or fluffy nonwoven fabric) and a superabsorbent polymer (SAP) dispersed within it. SAP can absorb tens or even hundreds of times its own weight in liquid and form a hydrogel to lock in the liquid; it is the core material determining the core's absorbency capacity. Because SAP is relatively expensive, achieving its optimal distribution within the limited core space to maximize absorption efficiency while controlling costs has always been a key focus of technological research in this field.

[0003] During use, liquid is typically guided from the surface layer and the guide layer to the central area of ​​the absorbent core, and then diffuses outwards. Therefore, the liquid load on the central area of ​​the core is much higher than on the edge areas. If the distribution density of superabsorbent polymer (SAP) is the same in all areas of the core, the SAP in the central area will quickly become saturated due to the large volume of liquid, while the SAP in the edge areas will not be fully utilized, resulting in low overall absorption efficiency. To improve this situation, the industry has proposed a technical solution of setting differentiated SAP densities in different areas of the core. For example, Chinese utility model patent CN206548711U discloses a multi-layer core absorption structure, which includes an upper composite core and a lower composite core. The area of ​​the upper composite core is smaller than that of the lower composite core, and the distribution density of SAP in the upper composite core is higher than that in the lower composite core. This solution achieves a relatively concentrated distribution of SAP towards the central area of ​​the core through a two-layer stepped structure, which improves the utilization efficiency of SAP to a certain extent. For example, Chinese patent CN01250187.5, which has expired, discloses an elliptical convex absorbent core structure for sanitary napkins. The elliptical concentrated absorbent core is set in the middle of the sanitary napkin, which proves that the idea of ​​setting the concentrated absorbent area as an ellipse to match the liquid diffusion pattern is known in the field.

[0004] However, the aforementioned existing technologies still have room for improvement. The two-layer structure represented by CN206548711U only has two superabsorbent polymer (SAP) concentration gradients between the central and peripheral regions of the core, resulting in a "cliff-like" change in concentration from the center to the periphery. This abrupt concentration distribution has two shortcomings: First, at the boundary of the abrupt change in SAP concentration, the liquid encounters a sudden drop in absorption capacity during outward diffusion, easily causing localized liquid accumulation inside the boundary. This hinders the continuous diffusion of liquid to the periphery and, to some extent, impedes the full utilization of SAP in the peripheral region, leaving room for improvement in the overall utilization rate of SAP. Second, for more viscous bodily fluids such as menstrual blood, which have poor fluidity and are more sensitive to abrupt changes in absorption capacity, the problems of accumulation and impeded diffusion at the concentration abrupt change boundary are more pronounced, potentially leading to increased backflow on the product surface after absorption, affecting user comfort. Therefore, how to further optimize the spatial distribution of superabsorbent polymers in the core to make the transition of concentration from the center to the periphery smoother, so as to improve the overall utilization rate of superabsorbent polymers and the absorption and anti-backflow performance under multiple liquid loading conditions, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to solve the technical problem in existing absorbent core technologies where the concentration of superabsorbent polymer (SAP) is abruptly abruptly distributed between the center and the edge of the core due to only a few concentration gradients (such as a two-layer structure). This abrupt concentration distribution easily leads to the accumulation of liquid at the concentration abrupt boundary during outward diffusion, hindering the full utilization of the SAP in the edge area and reducing the overall utilization rate of the SAP. Simultaneously, localized liquid accumulation at the concentration abrupt boundary may also increase the amount of backflow after absorption, affecting user comfort, especially for viscous bodily fluids such as menstrual blood. Therefore, the purpose of this invention is to provide an absorbent core with multiple stepped absorbent layers and a dual gradient variation in area and amount of SAP. By forming a SAP concentration distribution that smoothly transitions from the center to the periphery, the resistance to concentration abruptness during liquid diffusion is reduced, thereby improving the overall utilization rate of the SAP and enhancing absorption and anti-backflow performance under multiple liquid loading conditions.

[0006] To achieve the above-mentioned objectives, the present invention provides a water-absorbing core with a dual-gradient water-absorbing layer, comprising a main body, wherein at least four stepped water-absorbing layers are disposed in the main body from top to bottom, and each water-absorbing layer contains superabsorbent polymer (SAP); the cross-sectional area of ​​each water-absorbing layer increases progressively from the top layer to the bottom layer; the top layer has the highest SAP distribution density, and the SAP distribution density decreases progressively from the top layer to the bottom layer.

[0007] In this technical solution, "at least four layers" means that there are four or more absorbent layers, such as four, five, or six layers. The more absorbent layers there are, the smoother the transition of the superabsorbent polymer concentration from the center to the periphery, and the smaller the concentration abrupt change between the levels. This is more conducive to the smooth diffusion of the liquid and the full utilization of the superabsorbent polymer in the edge areas. At the same time, this invention requires each absorbent layer to meet two progressive changes: First, the cross-sectional area increases layer by layer from the top to the bottom, forming an area gradient, so that the upper absorbent layer is surrounded by the lower absorbent layer on the horizontal projection plane, and the superabsorbent polymer is spatially concentrated towards the central area of ​​the core; Second, the distribution density of the superabsorbent polymer decreases layer by layer from the top to the bottom, forming a density gradient, so that the central area has more superabsorbent polymer per unit area to cope with the concentrated liquid load, while the density of superabsorbent polymer in the peripheral area gradually decreases, forming a smooth transition of absorption capacity. The synergistic effect of area gradient and density gradient results in a continuous gradient change in the content of superabsorbent polymer per unit area from the center of the core to the periphery, rather than the abrupt change in the two-layer structure.

[0008] As a preferred embodiment, in the at least four absorbent layers, the cross-sectional profile of each absorbent layer except the bottom absorbent layer is elliptical, and the cross-sectional profile of the bottom absorbent layer is rectangular; the ellipse and the rectangle are generally located at the same center.

[0009] The elliptical shape of the upper absorbent layer serves a clear functional purpose: as liquid diffuses from the center outwards within the absorbent core, it typically forms an approximately elliptical diffusion front on the product's horizontal projection surface (since the product's conductivity along its length is generally better than its width). The elliptical shape matches this diffusion perimeter, ensuring that each elliptical absorbent layer is fully contacted and utilized when the liquid reaches its boundary. This avoids the waste that might occur with rectangular or circular shapes, where the superabsorbent polymer in the corner areas struggles to contact the liquid. The rectangular bottom absorbent layer matches the overall shape of the product (usually rectangular or oval), maximizing the use of the core's space. Furthermore, the major and minor axes of each elliptical absorbent layer increase progressively from the top to the bottom, maintaining a similar shape while expanding outwards layer by layer, achieving a smooth progression of area gradient.

[0010] As a preferred embodiment, the main material of each absorbent layer is a fluffy nonwoven fabric. Fluffy nonwoven fabric possesses excellent bulkiness, breathability, and liquid conductivity, providing an ideal dispersion carrier and liquid transport channel for the superabsorbent polymer. When both absorbent layers are made of fluffy nonwoven fabric, they are joined by hot-melt fiber bonding to reduce the amount of adhesive used and simplify the process.

[0011] In one specific embodiment, the at least four absorbent layers include a first absorbent layer, a second absorbent layer, a third absorbent layer, and a fourth absorbent layer arranged sequentially from top to bottom (i.e., a four-layer structure). The superabsorbent polymer (SAP) distribution density of the first absorbent layer is 400–600 gsm; the SAP distribution density of the second absorbent layer is 250–450 gsm; the SAP distribution density of the third absorbent layer is 150–350 gsm; and the SAP distribution density of the fourth absorbent layer is 80–250 gsm. As a more preferred embodiment, the SAP distribution density of the first absorbent layer is 450–550 gsm, the second absorbent layer is 300–400 gsm, the third absorbent layer is 200–300 gsm, and the fourth absorbent layer is 100–200 gsm. In an optimal configuration, the distribution densities of the superabsorbent polymers in each layer are 500 gsm, 350 gsm, 250 gsm, and 150 gsm, respectively.

[0012] In the above four-layer scheme, the fourth absorbent layer is the bottom layer, and its main material can be either fluffy nonwoven fabric or wood pulp fiber. When the main material of the fourth absorbent layer is wood pulp fiber, it is bonded to the adjacent third absorbent layer (whose main material is fluffy nonwoven fabric) by spraying latex to ensure a reliable bond between the different material layers.

[0013] Regarding the overall parameters of the absorbent core, the total weight of the absorbent core is 150gsm-300gsm, in order to ensure absorption capacity while taking into account the need for a thinner and lighter product.

[0014] From a functional perspective, the distribution density of superabsorbent polymers in each absorbent layer decreases continuously from the top to the bottom, resulting in a smooth transition of superabsorbent polymer content per unit area from the center to the periphery of the absorbent core. This smooth transition means that the density difference of superabsorbent polymers between adjacent layers is controlled within a limited range (e.g., the density difference between adjacent layers is no greater than 200 gsm, preferably no greater than 150 gsm), ensuring that the liquid does not encounter a sharp decrease in absorption capacity when spreading from one layer to the next, and the diffusion process is smoother.

[0015] The present invention also provides a sanitary product, comprising a top layer, a diversion layer, an absorbent core, and a bottom layer, wherein the absorbent core is an absorbent core according to any of the above-described embodiments. The sanitary product includes, but is not limited to, disposable absorbent products such as sanitary napkins, panty liners, diapers, and incontinence pads.

[0016] Compared with existing technologies, the absorbent core with a dual-gradient absorbent layer provided by this invention has the following advantages: 1. High overall utilization rate of superabsorbent polymer (SAP): Through the synergistic effect of the area and density gradients of at least four absorbent layers, the concentration of SAP smoothly decreases from the center to the periphery, effectively avoiding the problems of impeded liquid diffusion and localized oversaturation on the inner side of the boundary caused by abrupt concentration changes in existing two-layer structures. This allows the liquid to diffuse more smoothly from the center to the outer area, and the SAP in the outer area can be more fully utilized. Compared to existing two-layer core structures with the same total amount of SAP, the core of this invention has a higher effective water absorption participation rate.

[0017] 2. Excellent absorption and anti-backflow performance under multiple liquid loading conditions: Due to the absence of concentration abrupt change boundaries, the liquid is less likely to continuously accumulate at a specific boundary during multiple loading processes, reducing the risk of liquid backflow to the surface due to local supersaturation. Especially for viscous body fluids such as menstrual blood, the smooth transition of concentration distribution reduces flow resistance during diffusion, making the liquid more evenly distributed in the core and the product surface drier.

[0018] 3. Elliptical upper layer design matches liquid diffusion pattern: The upper absorbent layer adopts an elliptical shape, and the size of each ellipse increases progressively with each layer. This matches the actual isochronous pattern of liquid diffusion from the center to the periphery, ensuring a uniform liquid concentration distribution at the boundaries of each ellipse. This avoids the waste of material in the four corner areas of the rectangular absorbent layer where the superabsorbent polymer is difficult to utilize effectively, further improving the utilization efficiency of the superabsorbent polymer.

[0019] 4. Flexible and adjustable structure to meet the needs of multiple scenarios: By adjusting the number of absorbent layers (four or more), the area ratio of each layer, and the density of each layer of superabsorbent polymer, the absorption gradient curve of the core can be flexibly formulated to meet the comprehensive requirements of different product types (such as daytime / nighttime use, light / medium / large absorption needs) and usage scenarios for absorption speed, absorption capacity, and comfort.

[0020] 5. Good process compatibility: The layers are mainly bonded by hot melt fiber bonding (between fluffy non-woven fabric layers) or by spraying latex bonding (between non-woven fabric and wood pulp layers). Both are mature connection technologies in the field, requiring no special equipment and easy to implement on existing hygiene product production lines, making industrial application highly feasible. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1This is a top view of the absorbent core provided in Embodiment 1 of the present invention.

[0023] Figure 2 for Figure 1 A schematic diagram of the cross-sectional structure of the absorbent core.

[0024] Figure 3 This is an exploded three-dimensional structural diagram of the absorbent core provided in Embodiment 1 of the present invention.

[0025] Figure 4 This is a schematic diagram of the liquid diffusion path of the absorbent core provided in Embodiment 1 of the present invention.

[0026] Figure 5 This is a schematic diagram of the cross-sectional structure of the absorbent core provided in Embodiment 2 of the present invention, which shows the structure of five absorbent layers.

[0027] Figure 6 This is a schematic diagram of the cross-sectional structure of the absorbent core provided in Embodiment 3 of the present invention, wherein the fourth absorbent layer is made of wood pulp fiber and has superabsorbent polymer resin distributed thereon, and the connection method between the layers of the main material is different.

[0028] Figure 7 This is a schematic diagram showing the interlayer positional relationship of the absorbent core of the present invention when applied to sanitary napkin products.

[0029] Figure 8 This is a graph comparing the absorption rates of the embodiments and comparative examples of the present invention after multiple liquid loadings. The horizontal axis represents the number of liquid loadings (from the 1st to the 5th time), and the vertical axis represents the absorption rate (mL / s). The blue curve represents the absorption rate change trend of the four-layer dual-gradient absorbent core described in Example 1, and the red curve represents the absorption rate change trend of the two-layer abrupt change structure absorbent core described in the comparative example.

[0030] Explanation of the markings on the attached diagram: 100-Absorbent core; 110-Main body; 120-First absorbent layer; 121-First elliptical outline; 130-Second absorbent layer; 131-Second elliptical outline; 140-Third absorbent layer; 141-Third elliptical outline; 150-Fourth absorbent layer; 151-Fourth rectangular outline; 160-Superabsorbent polymer particles; 170-Hot melt fiber adhesive layer; 180-Latex adhesive layer; 200-Top layer; 300-Flow guiding layer; 400-Bottom layer; L-Length direction; W-Width direction. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0032] It should be noted that in the description of this invention, the terms "upper," "lower," "front," "rear," "left," and "right," etc., indicate the orientation or positional relationship based on the natural orientation or positional relationship of the product in its usage state. For example, "upper" refers to the side facing the user's body, and "lower" refers to the side away from the user's body. The term "length direction" refers to the longitudinal direction of the product, i.e., the front-to-back direction, and "width direction" refers to the transverse direction of the product, i.e., the left-to-right direction. In this invention, unless otherwise explicitly specified and limited, the terms "set," "connect," "adhere," etc., should be interpreted broadly. "A plurality of" means at least two.

[0033] In this invention, superabsorbent polymer (SAP) refers to a well-known superabsorbent polymer material that can typically absorb tens to hundreds of times its own weight in aqueous liquid and forms a hydrogel after absorbing water, firmly locking the liquid in place. Commonly used superabsorbent polymers include, but are not limited to, sodium polyacrylate-based, polyacrylamide-based, and starch-grafted copolymer-based polymers. Loose nonwoven fabric refers to a nonwoven material with high bulk and good liquid conductivity, which can be manufactured through processes such as hot air, hot rolling, and air-laid web forming. Its fiber raw materials can be selected from ES fibers (PE / PP core-sheath bicomponent fibers), polyester fibers, polyolefin fibers, etc.

[0034] Example 1 This embodiment provides an absorbent core 100 with a dual-gradient absorbent layer, which can be used as an absorbent core in disposable absorbent products such as sanitary napkins, diapers, and incontinence pads.

[0035] Please also refer to Figure 1 (Top view) Figure 2 (Cross-section view) Figure 3 (Exploded view). The absorbent core 100 includes a main body 110, within which four stepped absorbent layers are arranged from top to bottom: a first absorbent layer 120, a second absorbent layer 130, a third absorbent layer 140, and a fourth absorbent layer 150. It should be noted that "from top to bottom" here refers to the direction from the side closer to the user's body to the side farther away from the user's body, i.e., the direction in which liquid enters the core.

[0036] Each absorbent layer contains superabsorbent polymer (SAP) particles 160. The first absorbent layer 120 has the highest SAP density, followed by the second layer 130, then the third layer 140, and the fourth layer 150 has the lowest. This means the SAP density decreases progressively from the top to the bottom, forming a density gradient. Simultaneously, the cross-sectional area of ​​each absorbent layer increases progressively from the top to the bottom. The first absorbent layer 120 has the smallest cross-sectional area, the second layer 130 has a larger cross-sectional area than the first layer 120, the third layer 140 has a larger cross-sectional area than the second layer 130, and the fourth layer 150 has the largest cross-sectional area, forming an area gradient. This area gradient, combined with the density gradient, ensures that the SAP content per unit area gradually decreases from the center to the periphery of the absorbent core, achieving a smooth transition in SAP concentration.

[0037] In this embodiment, the main material of each absorbent layer is a fluffy nonwoven fabric. The specific type of fluffy nonwoven fabric can be hot-air nonwoven fabric, and the fiber raw material is ES bicomponent composite fiber (PE / PP core-sheath structure). The PE sheath of the ES fiber has a low melting point (approximately 120-130℃) and can be used as an adhesive component during hot-melt bonding. The PP core has a high melting point (approximately 165℃) and remains solid at the hot-melt bonding temperature to maintain the fiber skeleton structure. The basis weight of each layer of fluffy nonwoven fabric can be flexibly set according to the overall core design; in this embodiment, it is preferably 15-50 gsm / layer. Adjacent layers of fluffy nonwoven absorbent fabric are connected by a hot-melt fiber adhesive layer 170. Specifically, low-melting-point hot-melt fibers (such as PE hot-melt fibers or PE / PP bicomponent hot-melt fibers) are laid between each layer of fluffy nonwoven fabric. The hot-melt fibers are heated to at least partially melt, and after cooling, adjacent layers of fluffy nonwoven fabric are bonded together. The preferred application rate of hot-melt fiber is 3-10 gsm / layer. Using hot-melt fiber bonding instead of traditional water-based adhesives can significantly reduce adhesive usage, simplify the drying process, and align with the green manufacturing trend of reducing and eliminating adhesives in the hygiene products industry.

[0038] Please refer to this carefully. Figure 1 and Figure 3 In this embodiment, the cross-sectional profile shape of each absorbent layer is specifically designed. The cross-sectional profile shapes of the first absorbent layer 120, the second absorbent layer 130, and the third absorbent layer 140 are all elliptical, while the cross-sectional profile shape of the fourth absorbent layer 150 is rectangular. Please refer to [link / reference]. Figure 1The first elliptical contour 121, the second elliptical contour 131, the third elliptical contour 141, and the fourth rectangular contour 151 are generally located at the same center (i.e., the layers are spatially superimposed and distributed around the same center). It should be noted that "generally located at the same center" in this invention means that the geometric centers of each absorbent layer are substantially coincident in the vertical direction, or that in the product's usage state, each absorbent layer is located in its central region relative to the overall contour of the core (i.e., the rectangular contour of the fourth absorbent layer); minor deviations due to manufacturing tolerances are permissible (e.g., the offset between the geometric centers of each layer does not exceed 10% of the total length of the core).

[0039] Setting the upper absorbent layers (first, second, and third layers) in an elliptical shape, rather than other shapes, has a clear functional consideration and is another important innovative feature of this invention. During the use of absorbent products, when liquid enters the absorbent core from the surface layer, although the initial entry point is usually approximately a point or small circle on the surface layer, the liquid, guided by the flow-guiding layer and through capillary diffusion between the fibers of the core itself, often presents an approximately elliptical shape as the diffusion front from the center to the periphery on the horizontal projection plane of the core. This is because the length direction of absorbent products (such as sanitary napkins) is usually significantly greater than the width direction, resulting in a longer flow path and stronger flow-guiding capacity along the length direction. This makes the isochrones of liquid diffusion (i.e., the lines connecting positions arriving at the same time) approximately a series of ellipses with their major axes along the length of the product. Setting the upper absorbent layers in an elliptical shape means that the boundary of each elliptical absorbent layer roughly coincides with the isochrones of liquid diffusion—when liquid diffuses to the boundary of a certain elliptical absorbent layer, the concentration of superabsorbent polymer (SAP) at each point along that boundary is basically the same, and the liquid has an equal chance of contacting SAP within the same layer, thereby achieving the most complete and uniform utilization of the SAP in that layer. Conversely, if the upper absorbent layer adopts a circular design, its boundary does not match the elliptical diffusion front of the liquid, making it difficult for some SAPs within the four quadrants of the circle to come into contact with the liquid. If a rectangular design is adopted, the four corner areas of the rectangle are occupied by SAPs before the liquid diffusion isochrones arrive, resulting in temporary idleness of the SAPs. The elliptical design of this invention effectively overcomes the above-mentioned mismatch problem, ensuring that each elliptical SAP distribution area can be fully and synchronously absorbed, further improving the overall utilization efficiency of the SAPs.

[0040] Furthermore, the major and minor axes of the ellipses of the first absorbent layer 120, the second absorbent layer 130, and the third absorbent layer 140 increase progressively from the top to the bottom. Specifically, the major and minor axes of the first ellipse 121 are smaller than those of the second ellipse 131, and the major and minor axes of the second ellipse 131 are smaller than those of the third ellipse 141. This progressively increasing elliptical size design ensures that the area gradient and the function of matching the liquid diffusion pattern are coordinated in three-dimensional space. Preferably, the major axis of each ellipse is parallel to the length direction L of the product, and the minor axis is parallel to the width direction W of the product, to match the characteristic of the product's length direction dominating the flow.

[0041] Regarding the setting of the superabsorbent polymer (SAP) distribution density in each layer, this embodiment adopts the following preferred values: the SAP distribution density in the first absorbent layer 120 is 450-550 gsm, more preferably 500 gsm; the SAP distribution density in the second absorbent layer 130 is 300-400 gsm, more preferably 350 gsm; the SAP distribution density in the third absorbent layer 140 is 200-300 gsm, more preferably 250 gsm; and the SAP distribution density in the fourth absorbent layer 150 is 100-200 gsm, more preferably 150 gsm. Under this design, the SAP distribution density difference between adjacent absorbent layers is controlled within the range of approximately 150 gsm (500→350→250→150), forming a relatively gentle gradient transition. Overall, the total weight of the absorbent core 100 is 150-300 gsm, preferably 200 gsm. The overall thickness is preferably 1-4 mm, more preferably 2-3 mm, to meet the requirements of product thinness.

[0042] It should be noted that the specific values ​​mentioned above are preferred choices for this embodiment and are not intended to limit the invention. In actual product design, the SAP distribution density and area ratio of each layer can be flexibly adjusted according to factors such as product type (e.g., day / night sanitary napkins, baby / adult diapers), target absorbency, and product thickness limitations. For example, for night-use sanitary napkins, the SAP distribution density of the first to fourth layers can be increased overall, or the density difference between adjacent layers can be reduced to create a smoother concentration transition.

[0043] The following is combined Figure 4The liquid diffusion path and working mechanism of the absorbent core in this embodiment are described in detail. When liquid (such as menstrual blood or urine) enters the absorbent core 100 through the surface layer 200 and the guide layer 300, it first contacts the uppermost first absorbent layer 120. The first absorbent layer 120 is located in the center of the core and has the highest SAP distribution density (500 gsm), which can quickly absorb and lock in the initially concentrated influx of liquid. As the amount of liquid increases, the liquid diffuses outward from the first absorbent layer 120 into the area of ​​the second absorbent layer 130. Since the density difference between the second absorbent layer 130 (350 gsm) and the first absorbent layer 120 is limited (approximately 150 gsm), the liquid does not encounter a sharp decrease in absorption capacity when crossing the boundary between the two layers, and the diffusion process is smooth without boundary accumulation. The liquid continues to diffuse outward, successively entering the third absorbent layer 140 (250 gsm) and the fourth absorbent layer 150 (150 gsm). The SAP density change at the boundary of each layer is limited and smooth. When the liquid reaches the fourth absorbent layer 150, although its SAP distribution density is the lowest, the amount of liquid reaching the fourth absorbent layer 150 has been significantly reduced because most of the liquid has already been absorbed in the previous three layers. The relatively low SAP distribution density precisely matches the corresponding reduction in liquid load. This mechanism of gradual liquid absorption through four layers, progressing layer by layer, is the core working mechanism that distinguishes this invention from the two-layer abrupt structure of existing technologies.

[0044] The following comparative experiment further verifies the technical effect of this embodiment. The inventors prepared the following two test samples: Example Sample: A four-layer dual-gradient absorbent core prepared according to the structure and parameters described in Example 1. Specific parameters are as follows: First absorbent layer is elliptical, SAP 500 gsm; Second absorbent layer is elliptical, with an area larger than the first layer, SAP 350 gsm; Third absorbent layer is elliptical, with an area larger than the second layer, SAP 250 gsm; Fourth absorbent layer is rectangular, with an area larger than the third layer, SAP 150 gsm. Total basis weight 200 gsm.

[0045] Comparative sample: The absorbent core was prepared according to the two-layer structure described in CN206548711U. Specific parameters are as follows: upper composite core (smaller area), SAP distribution density 500 gsm; lower composite core (larger area), SAP distribution density 150 gsm. The total amount of SAP in the two layers is the same as the total amount of SAP in the four layers of the example sample (i.e., the total amount of SAP used per square meter of core is the same).

[0046] Test Method: Two samples were placed in sanitary napkins of the same size. Artificial menstrual blood simulation solution (viscosity 15–25 mPa·s (37℃)) was applied multiple times at a loading rate of 5 mL / time, with a 5-minute interval between each loading. The complete absorption time of the liquid after each loading (i.e., the time from the completion of loading to the absence of free liquid on the surface) was recorded, and the absorption rate (unit: mL / s) after multiple loadings was calculated. The test results are shown in the table below: Based on the test results, the expected trend is that the absorption rates of both are similar during the first loading, but the absorption rate of the embodiment is significantly faster than that of the comparative embodiment after the third and subsequent loadings.

[0047] Please see Figure 8 The figure shows a comparison curve of the absorption rate of the sample in the above embodiment and the comparative sample under multiple liquid loading conditions. Figure 8 In the graph, the horizontal axis represents the number of liquid loading cycles (from the 1st to the 5th), and the vertical axis represents the absorption rate (unit: mL / s). It can be seen that during the first liquid loading, the absorption rates of the example sample and the comparative sample are similar, with their curves almost overlapping at approximately 0.17-0.18 mL / s. However, as the number of loading cycles increases, the absorption rate of the comparative sample shows a rapid decreasing trend, dropping to approximately 0.05 mL / s by the 5th loading cycle; in contrast, the absorption rate of the example sample decreases more gradually, remaining at approximately 0.13 mL / s by the 5th loading cycle. This graph clearly demonstrates that the four-layer dual-gradient absorbent core of the present invention significantly outperforms the existing two-layer abrupt change structure core in maintaining its absorption rate under multiple liquid loading conditions, and this advantage becomes increasingly apparent with the increase in the number of loading cycles. The fundamental reason for this performance difference is that the abrupt change in the concentration of superabsorbent polymer (SAP) from the center to the periphery in the two-layer structure leads to impeded liquid diffusion at the concentration abrupt boundary and local oversaturation inside the boundary. After multiple loadings, the cumulative effect causes the absorption performance to continuously deteriorate. In contrast, the four-layer continuous gradient structure of the present invention effectively avoids the above problems through smooth transition. Each layer of SAP can continuously and fully participate in absorption, thus maintaining good performance.

[0048] Example 2 (Five-layer absorbent layer scheme) Please see Figure 5 This embodiment provides a water-absorbing core, which differs from Embodiment 1 mainly in the number of water-absorbing layers. In this embodiment, there are five water-absorbing layers, from top to bottom: a first water-absorbing layer, a second water-absorbing layer, a third water-absorbing layer, a fourth water-absorbing layer, and a fifth water-absorbing layer. All five water-absorbing layers satisfy the dual gradient characteristic of "increasing area layer by layer and decreasing SAP distribution density layer by layer".

[0049] The newly added fourth absorbent layer is an intermediate transition layer with an elliptical cross-sectional shape and an area between the upper layer (the third absorbent layer) and the lower layer (the fifth absorbent layer). Its SAP distribution density is also between that of the third and fifth absorbent layers, preferably 180-220 gsm, more preferably 200 gsm. The SAP density difference between adjacent absorbent layers is further reduced to within about 100 gsm (preferably 50-100 gsm), resulting in a smoother concentration gradient transition.

[0050] This embodiment demonstrates a specific extension of the overarching concept of "at least four layers," indicating that the technical solution of this invention is not limited to four layers. The more layers, the smoother the concentration transition, the more efficient the liquid diffusion, and the higher the SAP utilization efficiency. The cost of increasing the number of layers is a slight increase in manufacturing steps. Therefore, in actual product design, those skilled in the art can flexibly choose between four and seven layers based on the principle of cost-efficiency balance.

[0051] Example 3 (The fourth absorbent layer uses a wood pulp fiber solution) Please see Figure 6 This embodiment provides a water-absorbing core, which differs from Embodiment 1 mainly in the main material of the fourth water-absorbing layer 150 and the interlayer connection method.

[0052] In this embodiment, the main material of the first absorbent layer 120, the second absorbent layer 130, and the third absorbent layer 140 is still a loose nonwoven fabric (such as hot-air nonwoven fabric), and the layers are connected by hot-melt fiber bonding, the same as in Embodiment 1. The main material of the fourth absorbent layer 150 is wood pulp fiber. The wood pulp fiber can be formed into a wood pulp fiber layer through processes such as air-laid web formation, carding web formation, or wet web formation, and its basis weight is preferably 30-100 gsm. The fourth absorbent layer 150 also has uniformly distributed superabsorbent polymer particles 160, with a distribution density range the same as in Embodiment 1 (100-200 gsm, preferably 150 gsm). The superabsorbent polymer particles 160 can be uniformly applied to the interior of the wood pulp fiber layer during the wood pulp fiber layer forming process through air-laid mixing or a powder-spreading device.

[0053] Since the main material of the third absorbent layer 140 is fluffy nonwoven fabric (synthetic fiber base), while the main material of the fourth absorbent layer 150 is wood pulp fiber (natural fiber base), the connection method between the two needs to be adapted to the adhesive properties between different materials. In this embodiment, the third absorbent layer 140 and the fourth absorbent layer 150 are bonded together by spraying latex to form a latex adhesive layer 180. The latex type can be water-based acrylic emulsion, water-based polyurethane emulsion, or EVA emulsion, etc., and is applied by atomized spraying to allow the latex to penetrate into the fiber gaps between the two adjacent layers. The amount of latex applied should be controlled as much as possible to reduce the amount of adhesive used, for example, controlled to 3gsm–10gsm. The latex adhesive layer is only applied between heterogeneous layers of fluffy nonwoven fabric and wood pulp fiber; for homogeneous fluffy nonwoven fabric layers, hot-melt fiber bonding is still preferred.

[0054] This embodiment demonstrates the flexibility in selecting the material for the fourth absorbent layer. Wood pulp fiber possesses excellent natural hydrophilicity and a high specific surface area, which helps absorb and store liquids, especially when used as the bottom layer of the core, providing a final line of defense against downward-permeating liquids. Loose nonwoven fabric and wood pulp fiber can be flexibly combined according to product performance requirements and cost considerations.

[0055] In other embodiments of the present invention: 1. The difference in superabsorbent polymer density between two adjacent absorbent layers is no greater than 200 gsm, preferably no greater than 150 gsm, and more preferably 50-100 gsm; (see Examples 1 and 2) 2. The number of absorbent layers can be any value between four and seven; (see Example 1 and Example 2) 3. The major axes of the ellipses of the first absorbent layer, the second absorbent layer, and the third absorbent layer all extend along the length of the product, and the minor axes extend along the width of the product. The ratio of the length of the major axis to the length of the minor axis within the same layer is 1.5:1 to 4:1. 4. Each layer of fluffy nonwoven fabric is hot-air nonwoven fabric, and the fiber raw material is ES bicomponent composite fiber (PE / PP core-sheath structure). The hot-melt fiber in the hot-melt fiber bonding layer is PE hot-melt fiber or PE / PP bicomponent hot-melt fiber; (see Example 1) 5. The amount of hot-melt fiber laid in the hot-melt fiber adhesive layer is 3-10 gsm / layer; (see Example 1) 6. The amount of latex applied between the third and fourth absorbent layers is 3 gsm–10 gsm; (see Example 3) 7. The overall thickness of the absorbent core is 1-4mm, preferably 2-3mm; (see Example 1) 8. The superabsorbent polymer (SAP) is unevenly distributed in at least one absorbent layer, with a higher distribution density in the central region than in the edge regions (i.e., an in-plane density gradient exists within the same absorbent layer). For example, in the first absorbent layer, the SAP distribution density at the center of the ellipse can be 500 gsm, gradually decreasing to approximately 400 gsm towards the edge of the ellipse. This in-plane gradient design can complement the smooth transition of the interlayer gradient, further reducing abrupt concentration changes during liquid diffusion within the layer.

[0056] Please see Figure 7 The absorbent core 100 provided by this invention can be applied to various disposable hygiene products. Taking a sanitary napkin as an example, the product includes, from top to bottom, a top layer 200, a diversion layer 300, an absorbent core 100, and a bottom layer 400. The top layer 200 is the layer that contacts the user's skin and is usually made of a hydrophilic nonwoven material, allowing liquid to pass through quickly. The diversion layer 300 is located between the top layer 200 and the absorbent core 100, and is used to initially diffuse the liquid absorbed by the top layer 200 in the horizontal direction before conducting it downward to the absorbent core 100. The absorbent core 100 adopts the absorbent core described in any embodiment of this invention. The bottom layer 400 is made of a liquid-impermeable material (such as a PE film) to prevent liquid leakage. It should be noted that in the application of the absorbent core 100, the direction of its area gradient (i.e., large area facing down and small area facing up) is consistent with the direction of liquid entry (from top to bottom). Therefore, the uppermost layer (first absorbent layer) of the absorbent core 100 is close to the guide layer 300, and the lowermost layer (fourth or more absorbent layers) is close to the bottom layer 400.

[0057] In addition to the four-layer structure mentioned above, diaper products typically also include auxiliary structures such as three-dimensional protective covers and elastic waistbands. The application of the absorbent core in diapers is similar to that in sanitary napkins; the absorbent core 100 is sandwiched between the diversion layer 300 and the bottom layer 400.

[0058] The absorbent core provided by this invention can also be used in other absorbent products such as incontinence pads. Its application method has the same interlayer positional relationship as the above-mentioned products, and will not be described in detail here.

[0059] The above description is merely a preferred embodiment of the present invention and does not limit the scope of the patent. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A water-absorbing core with a dual-gradient water-absorbing layer, comprising a main body, characterized in that, The main body is provided with at least four stepped absorbent layers from top to bottom, and each absorbent layer contains superabsorbent polymer resin. The cross-sectional area of ​​each of the absorbent layers increases progressively from the top layer to the bottom layer; The distribution density of superabsorbent polymer (SAP) is highest in the top absorbent layer, and decreases layer by layer from the top to the bottom absorbent layer.

2. The absorbent core according to claim 1, characterized in that, Of the at least four absorbent layers, the cross-sectional profile of each absorbent layer except the bottom absorbent layer is elliptical, and the cross-sectional profile of the bottom absorbent layer is rectangular; all the ellipses and the rectangles are generally located at the same center.

3. The absorbent core according to claim 2, characterized in that, The major and minor axis dimensions of each of the elliptical absorbent layers increase progressively from the top layer to the bottom layer.

4. The absorbent core according to claim 1, characterized in that, The main material of each absorbent layer is a fluffy non-woven fabric; when the main material of two adjacent absorbent layers is fluffy non-woven fabric, they are connected by hot melt fiber bonding.

5. The absorbent core according to claim 1, characterized in that, The at least four absorbent layers include a first absorbent layer, a second absorbent layer, a third absorbent layer, and a fourth absorbent layer arranged sequentially from top to bottom, wherein: The distribution density of the superabsorbent polymer in the first absorbent layer is 400–600 gsm. The distribution density of the superabsorbent polymer in the second absorbent layer is 250–450 gsm. The distribution density of the superabsorbent polymer in the third absorbent layer is 150–350 gsm. The distribution density of the superabsorbent polymer in the fourth absorbent layer is 80–250 gsm.

6. The absorbent core according to claim 5, characterized in that, The distribution density of the superabsorbent polymer in the first absorbent layer is 450-550 gsm. The distribution density of the superabsorbent polymer in the second absorbent layer is 300-400 gsm. The distribution density of the superabsorbent polymer in the third absorbent layer is 200-300 gsm. The distribution density of the superabsorbent polymer in the fourth absorbent layer is 100-200 gsm.

7. The absorbent core according to claim 5, characterized in that, The main material of the fourth absorbent layer is wood pulp fiber or fluffy non-woven fabric; when the main material of the fourth absorbent layer is wood pulp fiber, it is bonded to the adjacent third absorbent layer by spraying latex.

8. The absorbent core according to claim 1, characterized in that, The distribution density of superabsorbent polymer in each absorbent layer decreases continuously from the top layer to the bottom layer, so that the superabsorbent polymer content per unit area transitions smoothly from the center to the periphery of the absorbent core.

9. The absorbent core according to any one of claims 1-7, characterized in that, The total weight of the absorbent core is 150gsm-300gsm.

10. A hygiene product, comprising a top layer, a diversion layer, an absorbent core, and a bottom layer, characterized in that, The absorbent core is the absorbent core according to any one of claims 1 to 9.

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