Composite fiber material and preparation method thereof

Abstract

The invention relates to the technical field of papermaking, and provides a composite fiber material and a preparation method thereof.The composite fiber material comprises a reinforcing layer and a functional layer which are arranged in a stacked mode; the reinforcing layer comprises matrix-fibril type composite fibers, and the matrix-fibril type composite fibers comprise synthetic fibrils and matrix fibers covering at least part of the surfaces of the synthetic fibrils; the functional layer comprises a skin-core type composite fiber, and the skin-core type composite fiber comprises a core layer and a functional skin layer wrapping at least part of the surface of the core layer. According to the invention, the interface bonding and load transmission efficiency of synthetic fibrils and a cellulose network is improved on the enhancement layer, and a high-strength bearing skeleton is constructed; the functional layer limits functional components on the surfaces of the fibers to form a stable functional skin layer, so that the functional components are prevented from forming a weak boundary layer among the fibers and weakening fiber bonding; the two layers cooperate to realize that the paper-based material keeps high strength and reduces the interlayer stripping risk while obtaining the target surface function.

Description

Technical Field

[0001] This invention relates to the technical field of papermaking materials, and more specifically, to a composite fiber material and its preparation method. Background Technology

[0002] Paper-based materials, with hydrogen bond networks constructed from cellulose fibers as the main load-bearing system, have advantages such as wide availability, renewability, and ease of processing, and are widely used in packaging, printing, and functional materials. As application scenarios expand, in addition to meeting certain mechanical properties such as tensile strength and bursting strength, paper-based materials often also need to possess surface or interface functions such as conductivity, hydrophobicity, barrier properties, and antibacterial properties.

[0003] In existing technologies, synthetic fibers or microfibrils are usually introduced into the slurry to improve the strength of the skeleton, or functional particles, functional polymers, coating, impregnation and other methods are used to impart functions; some technologies also use multi-layer molding or laminated composite structures, so that different layers undertake the tasks of mechanical reinforcement and functional realization respectively.

[0004] However, existing paper-based functionalization solutions generally suffer from a drawback: functional components or fibers in the fiber network easily interfere with the effective bonding between fibers and form weak interfaces, making it difficult for paper-based materials to maintain high strength while achieving the target function, and increasing the risk of interlayer delamination. Specifically, when functional components migrate, aggregate, or form a continuous phase between fibers, they easily form weak boundary layers at fiber-to-fiber contacts, reducing the hydrogen bonding area and interfacial load transfer efficiency. In multilayer structures, this weak interface effect may be further amplified, leading to a decrease in the bonding strength between composite layers, thus restricting the achievement of compatibility between high strength and functionalization. Summary of the Invention

[0005] One of the problems addressed by this invention is how to provide a composite fiber material.

[0006] One of the problems addressed by this invention is how to provide a method for preparing composite fiber materials.

[0007] To address at least one of the aforementioned problems, the present invention provides a composite fiber material comprising a reinforcing layer and a functional layer stacked together; the reinforcing layer comprises a matrix-fiber composite fiber, the matrix-fiber composite fiber comprising a synthetic fibril and a matrix fiber covering at least a portion of the surface of the synthetic fibril; the functional layer comprises a core-sheath composite fiber, the core-sheath composite fiber comprising a core layer and a functional sheath covering at least a portion of the surface of the core layer.

[0008] In the above technical solution, the synthetic fibrils include at least one of aramid fibers and PBO fibers; and / or the matrix fibers include at least one of microfibrillated cellulose and nanocellulose; and / or the length of the synthetic fibrils is 3-12 mm and the linear density is 0.5-3.0 dtex; and / or the mass percentage of the matrix fibers is 0.5-10% based on the oven-dry weight of the reinforcing layer fibers.

[0009] In the above technical solution, the core layer includes cellulose fibers; and / or the functional skin layer includes at least one of a conductive polymer skin layer and a hydrophobic polymer skin layer; the mass percentage of the functional skin layer is 5-35% based on the oven-dry weight of the core-skin composite fiber.

[0010] In the above technical solution, the conductive polymer skin includes at least one of poly(3,4-ethylenedioxythiophene) system conductive polymer, polyaniline, and polypyrrole system conductive polymer; and / or the hydrophobic polymer skin includes at least one of acrylic hydrophobic emulsion, AKD system, and ASA system.

[0011] This invention also provides a method for preparing a composite fiber material, comprising the following steps: S100. The synthetic fibrils and cellulose matrix fibers are mixed and treated to obtain the reinforcing layer fiber pulp. S200: Mix the cellulose core fiber with the functional skin dispersion and solidify and anchor it to obtain the functional layer fiber pulp; S300. Plasma surface activation treatment and ultrasonic treatment are performed on the reinforcing layer fiber pulp and the functional layer fiber pulp, respectively, to obtain the reinforcing layer pulp and the functional layer pulp. S400: The reinforcing layer pulp and functional layer pulp are sequentially stacked and formed into a wet paper sheet using a layered pulping and forming process. The wet paper sheet is then subjected to step-by-step hot pressing and heat preservation treatment to obtain the finished product.

[0012] In the above technical solution, in S300, the plasma surface activation treatment includes low-temperature plasma treatment with a radio frequency power of 80-400W and a treatment time of 20-180s; and / or ultrasonic treatment with an ultrasonic frequency of 20-40kHz, a power density of 0.4-1.2W / mL, a treatment time of 3-15min, and a treatment temperature controlled at 20-40℃.

[0013] The above technical solution also includes S400: adding retention aids and microparticles to the reinforcing layer slurry and functional layer slurry before they are applied to the wire.

[0014] In the above technical solution, the retention aid includes cationic polyacrylamide, and the mass percentage of the retention aid is 0.01-0.03% based on the oven-dry weight of the fiber; the microparticles include at least one of silica sol and bentonite, and the mass percentage of the microparticles is 0.02-0.06% based on the oven-dry weight of the fiber.

[0015] In the above technical solution, in S400, the temperature of the stepped hot pressing is 120-200℃, the linear pressure is 200-800kN / m; and / or the temperature of the heat preservation treatment is 120-160℃, and the time is 5-30min.

[0016] In the above technical solution, the cascade hot pressing involves 2-6 hot pressing stages, with the linear pressure gradually increasing from 200kN / m to 800kN / m.

[0017] Beneficial effects This invention improves a composite fiber material and its preparation method. By forming a covering, entanglement, and bridging structure on the surface of synthetic fibrils using cellulose matrix fibers, the interfacial bonding strength and load transfer efficiency between the synthetic fibrils and the cellulose network are enhanced, thereby constructing a high-strength load-bearing skeleton and suppressing fibril pull-out slippage. Simultaneously, functional components are confined to the surface of the core fiber in the functional layer to form a stable functional skin, reducing the probability of functional components forming continuous weak boundary layers between fibers, reducing interference with fiber hydrogen bonding, and improving functional durability. Furthermore, by combining layered pulping and molding with tiered hot pressing and thermal insulation consolidation, reliable interfacial consolidation between the reinforcing layer and the functional layer is achieved. While obtaining target surface functions such as conductivity and hydrophobicity, high mechanical strength is maintained, and the risk of interlayer delamination and functional layer detachment is reduced. It has good process compatibility and scalable fabrication applicability. Detailed Implementation

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, a detailed description of specific embodiments of the present invention will be provided below.

[0019] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available. Experimental methods in the following examples that do not specify particular conditions should be performed according to conventional methods and conditions, or as selected in the product instructions.

[0020] This invention aims to provide a composite fiber material and its preparation method, for preparing paper-based composite materials that combine a high-strength load-bearing skeleton with oriented surface functionality, particularly suitable for specialty paper products. The preparation method of the composite fiber material of this invention includes the following steps: S100. The synthetic fibrils and cellulose matrix fibers are mixed and treated to obtain the reinforcing layer fiber pulp. S200: Mix the cellulose core fiber with the functional skin dispersion and solidify and anchor it to obtain the functional layer fiber pulp; S300. Plasma surface activation treatment and ultrasonic treatment are performed on the reinforcing layer fiber pulp and the functional layer fiber pulp, respectively, to obtain the reinforcing layer pulp and the functional layer pulp. S400: The reinforcing layer pulp and functional layer pulp are sequentially stacked and formed into a wet paper sheet using a layered pulping and forming process. The wet paper sheet is then subjected to step-by-step hot pressing and heat preservation treatment to obtain the finished product.

[0021] In papermaking systems, synthetic fibrils, namely aramid fibrils or PBO fibrils, have high strength and modulus, but their surface is relatively inert and their compatibility with cellulose is insufficient, making it difficult to form a sufficient effective bonding area and strong interfacial bonding. As a result, when under stress, the load is difficult to be effectively transferred from the cellulose network to the synthetic fibrils, and the synthetic fibrils are more prone to pull-out slippage, thus limiting the reinforcing effect.

[0022] In view of this, in S100, synthetic fibrils and cellulose matrix fibers are mixed. The matrix fibers include at least one of microfibrillated cellulose and nanofibrillated cellulose. Taking advantage of their large specific surface area, high surface hydroxyl density, and easy entanglement into a network, the matrix fibers spread and entangle on the surface and near-surface voids of the synthetic fibrils under aqueous mixing and shearing action, forming a continuous or semi-continuous cellulose phase capping layer and bridging structure. This transforms the originally difficult-to-bond outer surface of the synthetic fibrils into a bondable interface with cellulose phase characteristics, while improving surface roughness and effective contact area. In the subsequent paper forming process, this capping layer can form a continuous bonding zone with the surrounding cellulose fibers through hydrogen bonds and micro-entanglement. When stress is applied, it first enters the capping layer and is then transferred to the synthetic fibrils through interfacial shearing. This significantly inhibits pull-out slip and improves load transfer efficiency, allowing the synthetic fibrils to truly participate in load bearing and crack bridging, thereby improving both strength and toughness.

[0023] It should be noted that the length of the synthetic fibrils is 3-12mm to balance the overlap of the skeleton and the uniformity of the forming; the linear density is 0.5-3.0dtex, which is beneficial to increase the specific surface area and promote the coverage and entanglement of the matrix fibers; the mass percentage of the matrix fibers is 0.5-10% based on the oven-dry weight of the reinforcing layer fibers, to achieve a balance between the continuity of the covering and the viscosity of the pulp and the stability of the filtration and forming, avoiding insufficient interface modification due to too low content or excessive pulp thickness, poor forming and excessive paper density due to too high content, so as to ultimately achieve the purpose of strengthening the interface of the reinforcing layer and effectively constructing the load-bearing skeleton.

[0024] If functional components are added directly to the pulp system in the form of free particles or free polymers, they are prone to migration, aggregation or loss during papermaking and dewatering, resulting in uneven functional distribution and low retention. At the same time, free functional components may also fill the spaces between fibers to form weak interfaces, weakening fiber bonding and leading to decreased strength and interlayer delamination. Furthermore, they are prone to falling off during use, causing functional degradation.

[0025] In view of this, in S200, cellulose core fibers are mixed with and solidified with a functional cortex dispersion. This transforms the functional components from a free state to a fiber-level carrier state. First, the functional cortex dispersion is deposited on the surface of the cellulose core fibers to form a coating. Then, solidification and anchoring fix the coating structure, ensuring that the function exists on the fiber surface as a stable cortex, rather than as a free phase sandwiched between fibers. The cellulose core fiber surface has good wettability and abundant hydroxyl sites. Conductive polymer particles or hydrophobic polymer latex particles in the dispersion can more easily contact the fiber surface during mixing and achieve uniform deposition under shear and diffusion, avoiding independent agglomeration in the system and improving the uniformity of functional distribution from the source. Solidification and anchoring transform the initial deposition from reversible adsorption to stable adhesion. For hydrophobic polymer emulsions, the solidification process promotes particle synthesis and film formation, firmly adhering to the fiber surface, thus forming a continuous low surface energy coating layer. This makes it difficult for water to wet the fiber surface and achieves a durable hydrophobic effect. For AKD or ASA systems, the curing process facilitates the formation of a more stable hydrophobic structure on the fiber surface and improves its resistance to migration during subsequent use. For conductive polymer systems, such as poly(triethylenedioxythiophene), polyaniline, or polypyrrole, curing and anchoring help form a continuous or quasi-continuous conductive phase on the fiber surface and stabilize interparticle contact points, thereby making it easier to construct charge transport pathways and making the conductive or antistatic functions more durable. In addition, using cellulose as the core layer can also reduce the adverse effects on paper strength, because the core layer is still a cellulose system and can participate in the paper web bonding and support; the functional components are confined to the fiber surface to form a skin layer, without the need for a large amount of free functional phase to fill the fiber gaps, thus significantly reducing the probability of weak interface formation and reducing the risk of bonding weakening and interlayer failure caused by functionalization.

[0026] Curing and anchoring can be achieved by heating to promote the dispersion particles to synthesize a film, or by adding a crosslinking agent to crosslink and fix the functional skin layer to the reactive groups on the cellulose surface. Curing and anchoring can also be completed in conjunction with the subsequent hot pressing and heat preservation process of S400, so that the functional skin layer can further form a film and fix during the thermal process, thereby improving the abrasion resistance and migration resistance.

[0027] Preferably, the functional sheath layer accounts for 5-35% of the oven-dry weight of the core-sheath composite fiber, in order to balance functional continuity and paper bonding. If the sheath layer is too small, the coverage will be discontinuous, and it will be difficult to establish a stable conductive path or hydrophobic coverage; if the sheath layer is too large, it will easily block the effective binding sites on the cellulose surface, increase the proportion of brittle phase, and thus be detrimental to both interlayer bonding and overall strength.

[0028] In S300, before plasma surface activation treatment, the reinforcing layer fiber slurry and functional layer fiber slurry are dehydrated to form wet fiber layers or wet fiber blankets, then subjected to low-temperature plasma treatment at atmospheric pressure, followed by water re-slurrying, and then ultrasonic treatment. In the composite fiber system, insufficient fiber surface activity and fiber agglomeration coexist, leading to uneven slurry dispersion and insufficient effective bonding area between fibers. This results in uneven forming, weak interlayer bonding, and easy detachment or failure of the functional layer during subsequent layered papermaking. By combining surface activation and enhanced dispersion methods, the fiber surface is made easier to bond, and the fibers form a more uniform and controllable network in the slurry, thus providing a stable foundation for subsequent layered forming and hot-pressing consolidation.

[0029] Preferably, low-temperature plasma surface activation treatment slightly modifies the fiber surface without significantly altering the fiber's intrinsic properties. For synthetic fibrils in the reinforcing layer, aramid or PBO surfaces are typically chemically inert, with limited wetting and interfacial bonding capabilities with the cellulose system, resulting in low load transfer efficiency under stress. Low-temperature plasma introduces polar functional groups and increases surface energy on the fiber surface, while simultaneously generating micro-rough structures at the nanoscale, enhancing surface wettability and increasing the effective contact area. This facilitates the spreading and entanglement of cellulose matrix fibers on the surface, forming a denser bonding zone. For core-sheath composite fibers in the functional layer, plasma activation can improve the surface energy and wettability of the functional layer fiber's outer surface, promoting interlayer bonding and consolidation during subsequent hot pressing. It also helps to homogenize the surface state of the functional sheath and reduce the risk of migration and peeling. Furthermore, it can improve the wetting and film-forming uniformity of the functional sheath surface, making the functional phase less prone to migration or peeling during subsequent papermaking and hot pressing. In other words, the essence of plasma activation is to transform the originally low reactivity or low wettability fiber surface into a high-energy surface that is easier to bond, thereby improving the reliability of inter-fiber bonding and interlayer adhesion at the interface level.

[0030] In practice, plasma surface activation treatment includes low-temperature plasma treatment. This involves active particles and short-lived free radicals acting on the fiber surface to introduce polar functional groups and increase surface energy without significantly raising the system temperature. Simultaneously, it forms a nanoscale micro-rough structure, thereby improving wettability and effective contact area, and enhancing the reliability of interfacial bonding between fibers and layers. The radio frequency power is 80-400W to avoid over-etching or chain segment damage while introducing polar groups and micro-rough structures. Too low a power results in insufficient activation, while too high a power may cause fiber surface embrittlement or weaken the bulk strength. The treatment time is 20-180s to strike a balance between effective activation and avoiding over-treatment. Too short a time results in insufficient modification, while too long a time can easily lead to over-etching and affect the controllability of fiber strength and the dense structure of the paper.

[0031] Preferably, ultrasonic treatment utilizes cavitation and micro-jet shearing to strongly disperse and deagglomerate the slurry. In the reinforcing layer fiber slurry, synthetic fibrils and microfibrillated cellulose or nanofibrils all have a certain tendency to agglomerate. Agglomeration can lead to local fiber concentration, uneven pore structure, stress concentration points, and a reduction in the effective bonding area of ​​the fiber network. The instantaneous high shear generated by ultrasonic cavitation can break up fiber bundles and micro / nanofiber agglomerates, allowing the matrix fibers to more uniformly cover the surface of the synthetic fibrils and form a more continuous bridging structure, thereby improving the uniformity and reinforcement efficiency of the reinforcing layer. In the functional layer fiber slurry, the functional skin dispersion may also exhibit particle aggregation or local over-deposition. Ultrasonic treatment can make the functional components more uniformly deposited and spread on the fiber surface, reducing large particle defects and weak interface areas, and improving the continuity and durability of the functional layer. In other words, ultrasonic treatment solves the problems of dispersion and uniformity, making the system more stable from macroscopic formation to microscopic interface.

[0032] In practice, the ultrasonic frequency for ultrasonic treatment is 20-40 kHz because this range more easily creates a strong cavitation effect and provides sufficient micro-jets for shearing, suitable for breaking down fiber bundles and dispersing micro / nano components. The power density is 0.4-1.2 W / mL to ensure significant depolymerization and dispersion while avoiding excessive shearing that could lead to over-severing of fibers or damage to the functional skin structure. The treatment time is 3-15 min to ensure that depolymerization and dispersion reach a stable state without excessively damaging the fiber morphology. The treatment temperature is controlled at 20-40℃ to suppress the adverse effects of localized heating caused by ultrasound, including changes in slurry viscosity leading to re-agglomeration, premature film formation or instability of the functional dispersion, and changes in the structure and dispersion state of conductive polymers or hydrophobic systems at high temperatures, thereby ensuring dispersion quality and repeatability.

[0033] The system of this invention comprises synthetic fibrils, microfibrillated cellulose or nanocellulose, and fine polymer phases related to the functional skin layer. These fine components have large specific surface areas, high surface charges, are easily lost with white water, and are easily redispersed during molding shearing, resulting in low retention, uneven distribution within the layers, interlayer cross-mixing, and difficulty in dehydration. Ultimately, this manifests as functional instability, strength fluctuations, and increased risk of interlayer bonding. Therefore, adding retention aids and microparticles to the reinforcing layer slurry and functional layer slurry before web forming can effectively fix the fine components within the target layer without changing the main material formulation, and adjust the dehydration and molding state of the slurry to a more controllable window.

[0034] Retention aids include cationic polyacrylamide, whose core function is charge neutralization and polymer bridging. Pulp fibers, microfibrillated cellulose, nanocellulose, and many emulsion particles typically exist as negatively charged colloidal systems in the aqueous phase, exhibiting electrostatic repulsion that makes it difficult for fine components to adhere to the fiber backbone. Cationic polyacrylamide molecules, carrying a positive charge, preferentially adsorb onto the surfaces of negatively charged fibers and fine particles, reducing electrostatic repulsion. Furthermore, its long molecular chains simultaneously connect multiple particles or particles to fibers, forming a bridging flocculent structure. In this way, fine components that would otherwise easily be lost are drawn into and fixed within the fiber network, significantly improving retention and reducing the proportion of functional components lost with white water during the web dewatering stage.

[0035] Microparticles, including at least one of silica sol and bentonite, play a crucial role in transforming large, loose flocs into shear-resistant micro-flocs, while simultaneously improving dewatering and forming. When polymeric retention aids are used alone, the resulting flocs are often large in size, easily broken by shear, and this breakage leads to the redispersing of finer components, resulting in unstable retention and poor paper uniformity. Silica sol or bentonite, being high-surface-area inorganic microparticles, can interact secondaryly with fibers and particles that have already adsorbed cationic polyacrylamide, effectively locking the positive potential points on the polymer chains and promoting the formation of more microscopic connection points within the flocs. Macroscopically, this manifests as a shift from large-scale to micro-scale flocs, resulting in a denser and more shear-resistant structure. During wire dewatering, this creates more open drainage channels, thereby increasing filtration speed and reducing moisture fluctuations in the wet paper. This is particularly important for systems containing microfibrillated cellulose or nanofibrillated cellulose, as microfibrils significantly improve pulp water retention and hinder drainage. Microparticle systems can alleviate dewatering difficulties to some extent and improve wire-forming stability.

[0036] In practice, based on the oven-dry weight of the fiber, the retention aid is 0.01-0.03% by mass, and the microparticles are 0.02-0.06% by mass, striking a balance between retention rate, forming uniformity, and dewatering speed. If the dosage is too low, the fine components are still easily lost and the distribution within the layer is unstable; if the dosage is too high, it may lead to excessive flocculation, decreased paper uniformity, and localized stress concentration, which is detrimental to strength and appearance consistency.

[0037] Optionally, in the layered pulping and forming process, staggered crosslinking can be implemented: a crosslinking agent is added to the i-th layer of pulp, and a reactive component is added to the (i+1)-th layer of pulp; based on the oven-dry weight of the corresponding fiber, the amount of crosslinking agent added is 0.3-1.5%, the amount of reactive component added is 0.5-5.0%, and the equivalent ratio of the reactive groups carried by the crosslinking agent and the reactive component is 0.8-1.2. This can improve the bonding reliability between the reinforcing layer and the functional layer, and reduce the risk of interlayer slippage and delamination failure. If the interlayer bonding of layered paper-based composite materials mainly relies on fiber bonding, hydrogen bonding, and frictional locking, the interface is prone to becoming a weak link under humid and hot environments, repeated bending, or shear loads, leading to peeling, powdering, interlayer delamination, and performance degradation caused by the detachment of the functional layer. Displacement crosslinking arranges the reaction pairs in adjacent layers, allowing the crosslinking reaction to occur preferentially in the interlayer contact area, thereby forming an interfacial crosslinking zone between the reinforcing layer and the functional layer. This elevates the interlayer connection from physical bonding to chemical consolidation, significantly enhancing the degree of integration in the thickness direction.

[0038] It is understandable that the staggered addition of crosslinking agents and reactive components, rather than simultaneous addition in the same layer, is to avoid premature reaction of the pulp before wire bonding or during dewatering, which could lead to uncontrolled forming. If both coexist in the same layer, they are prone to rapid reaction under shear and concentration conditions, inducing strong flocculation, resulting in increased pulp viscosity, agglomeration, poor uniformity, and even localized gelation, thus compromising the stability and purity of the layered pulp distribution. The staggered addition keeps the two types of components relatively separated within their respective layers, making the pulp rheology and dispersion state easier to control. At the same time, it guides the main site of the crosslinking reaction to the interlayer interface, so that chemical bonding is used more for interlayer consolidation rather than being consumed in single-layer internal crosslinking, thus balancing the improvement of interlayer strength and the maintenance of paper flexibility, while minimizing interference with the surface state of functional layers.

[0039] It is understood that through the above operations, an interfacial cross-linking region is formed between the reinforcing layer and the functional layer. This interfacial cross-linking region is formed by the reaction of cross-linking agents and reactive components located in different layers. After the layered wet paper sheets are formed, adjacent layers are tightly bonded in a wet state. The fiber cross-points at the interface are dense and water-connected, allowing the cross-linking agent and its active groups to encounter the reactive groups of the other layer under short-range migration and interfacial enrichment conditions. During the subsequent hot-pressing and heat preservation stages, the increased temperature and interfacial compaction increase the actual contact area between the fibers. The gradual removal of interfacial moisture leads to an increase in the effective concentration of reactants, thereby promoting a more complete cross-linking reaction near the interface, generating interlayer chemical bridging points and constructing a stable interfacial cross-linking region. This cross-linking region has a pinning and deflecting effect on crack propagation, inhibiting interfacial crack initiation and propagation, reducing interlayer slip, and making the structure more durable under humid heat and cyclic loading.

[0040] Furthermore, controlling the equivalence ratio of the crosslinking agent to the reactive groups carried by the reactive components within 0.8-1.2 ensures that interfacial crosslinking is both sufficient and controllable, avoiding side effects caused by insufficient reaction or excessive residue. When the equivalence ratio is too low, the crosslinking agent is relatively insufficient, the density of interfacial crosslinking points is inadequate, the crosslinking zone is discontinuous, and the improvement in interlayer strength is limited. When the equivalence ratio is too high, there is an excess of crosslinking agent or reactive components, which can easily lead to unreacted groups or small molecule residues, potentially causing migration, hygroscopicity, stickiness, or embrittlement, and may also adversely affect the surface properties and long-term stability of the functional layer. The combination of crosslinking agent and reactive components includes azide propidium crosslinking agent and a carboxyl-containing reactive component.

[0041] The paper sheet of this invention is composed of a reinforcing layer and a functional layer. The composition and interface state of the two fiber systems differ significantly. If relying solely on conventional wet pressing and natural drying, insufficient interlayer bonding, weak fixation of the functional layer, uneven pore structure, warping caused by moisture migration, and internal stress concentration can easily occur, ultimately leading to interlayer delamination, dusting, strength fluctuations, or functional degradation. In S400, the layer structure is locked through the forming stage, and then controlled hot pressing and heat holding are used to advance the interface between the two layers from physical adhesion to stable consolidation, thereby obtaining a finished product with uniform structure, strong interface, and repeatable performance.

[0042] The purpose of layered pulping is to orient the reinforcing and functional components according to their designed locations, ensuring that the reinforcing layer maintains its primary fiber network structure for load-bearing, while the functional layer maintains its primary fiber coating structure for surface function. Layered forming reduces cross-mixing of the two types of components during the dewatering process in the web section, prevents functional components from entering the reinforcing layer and weakening fiber bonding, and also prevents excessive reinforcing fibers from entering the functional layer and affecting the continuity of surface function. The resulting wet paper sheet already possesses a clearly defined layered structural foundation on a macroscopic level, providing a clear interface for subsequent hot-pressing and consolidation.

[0043] Instead of a single high-pressure hot pressing, a stepped hot pressing method is used. The linear pressure increases gradually, allowing the wet paper sheet to primarily drain and degas during the high moisture content stage, and primarily compact and consolidate during the low moisture content stage. This balances structural uniformity and interlayer bonding strength, achieving a balance between compaction and drainage / venting, and reducing the risk of interface damage. The wet paper sheet still contains a large amount of free and bound water, and the pore structures and drainage resistance of the two layers differ. If excessive linear pressure is applied initially, water cannot drain along the thickness direction in time, easily forming localized water pockets and vapor pressure, causing blistering, microcracks, or interlayer bulging. Simultaneously, instantaneous high pressure can cause sudden collapse of the fiber network, leading to localized densification and stress concentration, making shear slip more likely at the interface, which is detrimental to interlayer bonding. Stepped hot pressing, through multiple stages of gradually increasing linear pressure, allows water to migrate and drain in stages during compaction, and allows fibers to gradually rearrange and bond, keeping the interface in a controllable state of compaction and mass transfer, thus significantly reducing the probability of delamination and structural defects.

[0044] The temperature of the tiered hot pressing is 120-200℃, and the linear pressure is 200-800 kN / m. The synergistic effect of heat and pressure is used to enhance the effective contact and bonding between fibers and layers. The heat effect reduces the viscoelastic resistance of the system, making it easier for fibers to plastically bond and rearrange interfaces during compression, significantly increasing the actual contact area between fibers. The pressure effect compresses the two fiber networks in the thickness direction, creating more close contact points between cellulose fibers. Hydrogen bonds become more stable after cooling and setting. Simultaneously, the matrix fiber covering structure within the reinforcing layer and the skin structure on the surface of the functional layer can be further adhered and fixed under hot pressing conditions. For the functional layer, hot pressing also promotes the coalescence of hydrophobic emulsion particles into a film, making the low surface energy coverage more continuous and improving friction resistance. For the conductive polymer skin layer, hot pressing helps improve the contact and continuity of the skin layer, reduces interfacial contact resistance, and makes the conductive pathway more stable. The linear pressure is gradually increased from 200kN / m to 800kN / m in order to first complete drainage and initial bonding, and then carry out strong pressure densification and interface consolidation in the low moisture content stage, so as to avoid excessive compression and structural damage in the high moisture content stage.

[0045] The insulation treatment, conducted at 120-160℃ for 5-30 minutes, aims to transform the interfacial structure induced by hot pressing from transient deformation into a stable structure, achieving thermodynamic and mass transfer equilibrium. During hot pressing, the fiber network undergoes rapid compaction, resulting in a certain degree of resilience and internal stress at the interface. The insulation stage promotes further uniform migration and slow drainage of moisture, reducing the moisture content gradient along the thickness direction and thus minimizing warpage and interlayer stress after cooling. Simultaneously, the insulation provides a time window for the formation and fixation of the functional skin, allowing for more complete and stable coverage of the hydrophobic skin, and further stabilizing the microstructure of the conductive skin, reducing functional fluctuations caused by environmental changes during subsequent use. For reactive hydrophobic systems, the insulation process also enhances the degree of reaction and fixation, thereby improving durable hydrophobic performance. Overall, the insulation treatment is equivalent to solidifying and sculpting the interfacial structure and releasing stress, resulting in stronger interlayer bonding and better dimensional stability.

[0046] Example 1 This embodiment provides a composite fiber material and its preparation method, the preparation method including: S100. Aramid fibrils with a length of 8 mm and a linear density of 1.5 dtex are dispersed in the aqueous phase to obtain a slurry with a mass concentration of 0.8%. Microfibrillated cellulose is added to make the microfibrillated cellulose content 3.0% based on the oven-dry weight of the reinforcing layer fibers. The mixture is mixed under high-speed shear conditions for 20-30 min to obtain the reinforcing layer fiber slurry. S200. Disperse bleached softwood pulp into 0.8% pulp, add PEDOT system dispersion, so that the functional skin layer mass fraction is 20% based on the oven-dry weight of the core-skin composite fiber, continue stirring for 30 minutes to allow the functional skin layer to initially form a film and be fixed, and obtain functional layer fiber pulp. S300: The reinforcing layer fiber pulp and the functional layer fiber pulp are dehydrated into wet fiber blankets, and treated with low-temperature plasma at ambient pressure under a radio frequency power of 200W for 60s. After treatment, water is added and the pulp is re-slurried to a concentration of 0.8%. Then, ultrasonic dispersion is performed at a frequency of 25kHz and a power density of 0.8W / mL for 8min, with the temperature controlled at 30℃, to obtain the reinforcing layer pulp and the functional layer pulp. S400: Before connecting to the grid, add 0.02% CPAM and 0.04% silica sol to the two pulps respectively. Use layered pulping and forming, first forming the reinforcing layer and then the functional layer to obtain a wet paper sheet. Perform stepped hot pressing on the wet paper sheet: temperature 150℃, 4 hot pressings, linear pressure of 200, 400, 600 and 800 kN / m respectively. Then keep it at 140℃ for 15 minutes to obtain the finished product.

[0047] Example 2 This embodiment provides a composite fiber material and its preparation method, which is the same as that shown in Example 1, except that: In S100, nanocellulose is used as the matrix fiber, with a content of 1.5%; In S200, an acrylic hydrophobic emulsion is used as the functional skin layer, with a skin layer mass fraction of 15%. In S400, the hot pressing is performed in stages: the temperature is 120℃, 6 hot pressings are performed, and the linear pressures are 200, 350, 500, 600, 700 and 800 kN / m respectively; then it is held at 160℃ for 5 minutes.

[0048] Example 3 This embodiment provides a composite fiber material and its preparation method, which is the same as that shown in Example 1, except that: In S100, PBO fibrils with a length of 5 mm and a linear density of 0.8 dtex are used as synthetic fibrils, and MFC is used as the matrix fiber with a content of 5%. In S200, the AKD system is used as the functional cortical dispersion, with a cortical mass fraction of 10%. In S300, the plasma surface activation treatment power is 300W and the time is 90s; the ultrasonic treatment frequency is 20kHz, the power density is 1.0W / mL, the time is 5min, and the temperature is 30℃. In S400, the step-by-step hot pressing temperature is 180℃, and the five linear pressures are 200, 350, 500, 650, and 800 kN / m; the temperature is held at 150℃ for 20 minutes.

[0049] Example 4 This embodiment provides a composite fiber material and its preparation method. The preparation method is as shown in Example 1, except that in S200, a polyaniline system conductive polymer dispersion is used as the functional skin layer, and 0.5% of azide propidium crosslinking agent is added. After mixing for 20 minutes, it is pre-cured at 100°C for 10 minutes, and the mass fraction of the functional skin layer is 25%.

[0050] Example 5 This embodiment provides a composite fiber material and its preparation method, which is as shown in Example 1, except that: in S400, before the layered slurry forming, staggered crosslinking is performed: an azide propidium crosslinking agent is added to the reinforcing layer slurry, with an addition amount of 0.8% based on the oven-dry weight of the reinforcing layer fiber; a carboxyl-containing acrylic emulsion is added to the functional layer slurry, with an addition amount of 2.0% based on the oven-dry weight of the functional layer fiber; and the equivalent ratio of the crosslinking agent to the reactive groups carried by the reactive components is controlled to be about 1.0.

[0051] Comparative Example 1 This comparative example provides a composite fiber material and its preparation method, which is as shown in Example 1, except that MFC is not added in S100.

[0052] Comparative Example 2 This comparative example provides a composite fiber material and its preparation method, which is as shown in Example 1, except that: there is no S200, and the PEDOT system dispersion is directly added to the functional layer slurry and then formed.

[0053] Comparative Example 3 This comparative example provides a composite fiber material and its preparation method. The preparation method is as shown in Example 1, except that the reinforcing layer slurry and the functional layer slurry are mixed in one go according to the total formula and then formed into a single layer, i.e., without S400.

[0054] Comparative Example 4 This comparative example provides a composite fiber material and its preparation method, which is as shown in Example 1, except that: there is no S300, that is, no plasma treatment and ultrasonic dispersion.

[0055] Comparative Example 5 This comparative example provides a composite fiber material and its preparation method, which is as shown in Example 1, except that: in S400, CPAM and silica sol are not added before netting.

[0056] Performance testing The finished paper sheets obtained in Examples 1-5 and Comparative Examples 1-5 were cut into samples of specified sizes and conditioned at 23°C and 50% relative humidity for no less than 24 hours before testing. At least 5 parallel samples were taken for each test, and the average value of the results was taken. Quantitative analysis: Weigh a sample of a certain area and calculate the quantitative analysis. Thickness: The thickness of the paper sheet is measured under the specified clamping pressure; Tightness: Calculated by ρ=G / t; where ρ is the tightness, G is the basis weight, and t is the thickness; Table 1 As shown in Table 1, the quantitative and thickness of each sample are on the same order of magnitude, which is beneficial for subsequent horizontal comparison of mechanical and functional indicators; the overall density of the example group is slightly higher or comparable, indicating that the structure is more uniform and dense after layered hot pressing consolidation.

[0057] Tensile properties: The tensile strength T is tested using a tensile testing machine, and the tensile index is calculated; Bursting strength: Determine the bursting strength B and calculate the bursting index; Tear strength: The tear force E is tested using the tear test method, and the tear index is calculated; Table 2 As shown in Table 1, the tensile index, bursting index and tear index of Examples 1-5 are generally higher than those of the comparative examples, indicating that the reinforcing layer improves the interface bonding and load transfer efficiency, and can effectively construct a high-strength load-bearing skeleton; the strength of Comparative Examples 2 and 3 is more significantly reduced because the functional components interfere with fiber bonding or the structure is not layered.

[0058] Z-direction inward bonding strength: energy required for separation in the thickness direction as determined by equipment and methods; 180° peel strength: A peeling initiation is pre-made at the interface between the reinforcing layer and the functional layer at the edge of the sample. The two layers are clamped separately, and a tensile testing machine peels them at a constant speed in a 180° manner. The average peel force F of the stable peeling plateau section is taken, and the peel strength per unit width is calculated. Table 3 As shown in Table 3, the inward bonding and peel strength of Example Z are significantly improved, indicating that the layered slurry forming, tiered hot pressing, and heat preservation effectively promote the interface consolidation of the reinforcing layer and the functional layer; Example 5 further proves that misaligned crosslinking can improve interlayer chemical consolidation and reduce the risk of interlayer peeling.

[0059] Sheet resistance: The sheet resistance was measured on the surface of the functional layer using a four-probe method. Five points were measured at different locations on each sample and the average was taken. Wear durability: Dry friction wear was performed on the functional surface of the sample: 1000 cycles, load 500g, stroke 50mm. Sheet resistance was measured before and after wear, and the sheet resistance retention rate was calculated. Table 4 As shown in Table 4, the sheet resistance of the embodiment increases less with wear and has a higher retention rate, indicating that the core-sheath type composite fiber confines and anchors the conductive components to the fiber surface to form a stable conductive skin, reducing the performance degradation caused by the formation of weak boundary layers and shedding of free conductive phases between fibers. In contrast, although the initial sheet resistance of Comparative Example 2 may be lower due to the presence of free conductive components, it deteriorates significantly after wear.

[0060] Static water contact angle: 2-5 μL of deionized water was dropped onto the surface of the functional layer, and the contact angle (°) was recorded at 5 s. Five points were measured for each sample and the average was taken. Cobb 60 water absorption value: The water absorption in 60 seconds is determined by the Cobb method; Wear durability: After wear under the same or equivalent conditions as conductivity durability, the contact angle and Cobb value are tested again; and the contact angle retention rate is calculated. Table 5 As shown in Table 5, the embodiment has a higher contact angle, a lower Cobb water absorption value, and a higher retention rate after wear, indicating that the functional skin forms a continuous and stable low surface energy coating layer on the fiber surface and has better anti-migration and anti-wear capabilities; the durability of the comparative example 2 is significantly reduced because the functional components are free or form a weak boundary layer between fibers.

[0061] Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A composite fiber material, characterized in that, The composite fiber material includes a reinforced layer and a functional layer stacked together. The reinforcing layer includes a matrix-fiber composite fiber, which includes synthetic fibrils and matrix fibers covering at least a portion of the surface of the synthetic fibrils. The functional layer includes a core-sheath composite fiber, which includes a core layer and a functional sheath layer covering at least a portion of the surface of the core layer.

2. The composite fiber material according to claim 1, characterized in that, The synthetic fiber includes at least one of aramid fiber and PBO fiber; and / or The matrix fiber includes at least one of microfibrillated cellulose and nanocellulose; and / or The synthetic fibrils have a length of 3-12 mm and a linear density of 0.5-3.0 dtex; and / or The mass percentage of the matrix fiber is 0.5-10% based on the oven-dry weight of the fiber in the reinforcing layer.

3. The composite fiber material according to claim 1, characterized in that, The core layer comprises cellulose fibers; and / or The functional skin layer includes at least one of a conductive polymer skin layer and a hydrophobic polymer skin layer. Based on the oven-dry weight of the core-sheath composite fiber, the functional sheath layer accounts for 5-35% of the total mass.

4. The composite fiber material according to claim 3, characterized in that, The conductive polymer skin includes at least one of the following: poly(3,4-ethylenedioxythiophene) conductive polymer, polyaniline, and polypyrrole conductive polymer; and / or The hydrophobic polymer skin includes at least one of acrylic hydrophobic emulsion, AKD system, and ASA system.

5. A method for preparing a composite fiber material as described in any one of claims 1-4, characterized in that, Includes the following steps: S100. The synthetic fibrils and cellulose matrix fibers are mixed and treated to obtain the reinforcing layer fiber pulp. S200: Mix the cellulose core fiber with the functional skin dispersion and solidify and anchor it to obtain the functional layer fiber pulp; S300. The reinforcing layer fiber slurry and the functional layer fiber slurry are subjected to plasma surface activation treatment and ultrasonic treatment, respectively, to obtain the reinforcing layer slurry and the functional layer slurry. S400: The reinforcing layer pulp and the functional layer pulp are sequentially stacked and formed into a wet paper sheet using a layered pulping and forming process. The wet paper sheet is then subjected to step-by-step hot pressing and heat preservation treatment to obtain the finished product.

6. The preparation method according to claim 5, characterized in that, In S300, The plasma surface activation treatment includes low-temperature plasma treatment, with a radio frequency power of 80-400W and a treatment time of 20-180s; and / or The ultrasonic treatment is performed at a frequency of 20-40 kHz, a power density of 0.4-1.2 W / mL, a treatment time of 3-15 min, and a treatment temperature of 20-40℃.

7. The preparation method according to claim 5, characterized in that, S400 also includes: adding retention aids and microparticles to the reinforcing layer slurry and the functional layer slurry before they are applied to the web.

8. The preparation method according to claim 7, characterized in that, The retention aid includes cationic polyacrylamide, and the mass percentage of the retention aid is 0.01-0.03% based on the oven-dry weight of the fiber. The microparticles include at least one of silica sol and bentonite, and the mass percentage of the microparticles is 0.02-0.06% based on the oven-dry weight of the fiber.

9. The preparation method according to claim 5, characterized in that, In S400, The temperature of the stepped hot pressing is 120-200℃, and the linear pressure is 200-800 kN / m; and / or The heat preservation treatment is carried out at a temperature of 120-160℃ for 5-30 minutes.

10. The preparation method according to claim 9, characterized in that, The stepped hot pressing consists of 2-6 hot pressing stages, with the linear pressure gradually increasing from 200kN / m to 800kN / m.

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