Hot-pressing reinforcing method for reinforcing fiber interface

Abstract

The invention relates to the technical field of papermaking, and provides a hot-pressing reinforcement method for fiber interface reinforcement. The method comprises the following steps: S100, in a hot pressing area, carrying out hot pressing treatment on a wet paper web to obtain an intermediate paper web; and S200, drying and shaping the middle paper web to obtain finished paper. Impulse type pressing pressure is introduced in the medium-pressure stage of hot pressing treatment and cooperates with heating, so that an interlayer bonding area is promoted to undergo periodic high-pressure compaction and low-pressure rebound, interlayer fibers are driven to be subjected to controlled rearrangement and embedment, meanwhile, directional migration and redistribution of interface moisture are promoted, and the interface moisture content is improved. Therefore, residual water films are reduced, the effective contact area is enlarged, the interlayer bonding strength is enhanced, layering is inhibited, and the problems that the interlayer bonding strength of multiple layers of paper sheets is low, and layering is prone to occurring in subsequent processing are solved.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and more specifically, to a hot-pressing strengthening method for fiber interface strengthening. Background Technology

[0002] Multilayer paper sheets are widely used in packaging paper, paperboard, and functional composite paper products. The multilayer structure allows for the separate placement of long fibers, short fibers, fillers, and functional additives in different layers to achieve a comprehensive balance between stiffness, strength, surface properties, and cost. However, during the forming, pressing, drying, and subsequent processing such as coating, laminating, die-cutting, and folding, the interlayer interfaces often become weak areas, easily exhibiting insufficient interlayer bonding strength. This can lead to defects such as delamination, blistering, or interlayer peeling under stress concentration or repeated bending conditions. Insufficient interlayer bonding strength is usually related to factors such as insufficient effective contact area between layers, difficulty in eliminating residual water films at the interface, uneven distribution of fine particles between layers, and insufficient fiber interlocking.

[0003] Existing technologies often improve interlayer bonding strength by increasing pressing pressure, extending pressing time, increasing pressing temperature, or adding interlayer reinforcing components. However, simply using a continuous, constant, high pressing pressure can easily lead to the rapid closure of interlayer interface pores, restricting the outflow channels of interfacial moisture, and forming residual water films or closed water layers locally, thereby inhibiting the controlled rearrangement and intercalation of fibers between layers. In addition, under continuous high pressure, the dynamic adjustment space of the interlayer microstructure is insufficient, making it difficult to form a sufficient fiber interpenetration and stable bonding network at the interface. Although using chemical reinforcing components can improve bonding to a certain extent, it brings problems such as increased costs, higher requirements for system compatibility and operational stability.

[0004] Therefore, there is an urgent need for a hot-pressing reinforcement method for fiber interface strengthening to solve the above problems. Summary of the Invention

[0005] The problem addressed by this invention is how to provide a hot-pressing reinforcement method for fiber interface strengthening.

[0006] To address the above problems, this invention provides a hot-pressing reinforcement method for fiber interface strengthening, the method comprising the following steps: S100. In the hot pressing zone, the wet paper web is hot-pressed to obtain the intermediate paper web; S200. The intermediate paper web is dried and shaped to obtain the finished paper. The hot pressing process includes a pre-pressing stage P1, a medium-pressing stage P2, and a final pressing stage P3. The medium-pressing stage P2 applies pulsed pressing pressure to the wet paper web, maintaining the pressing pressure at a high value P. H With low pressure value P L Periodically switching between them, and P H With PL Satisfy: P H > P L > 0, P H / P L = 1.5 to 10.

[0007] In the above technical solution, the frequency of the pulsed pressing pressure is 0.1 to 10 Hz.

[0008] In the above technical solution, in S100, the duty cycle D of the pulsed pressing pressure is 10 to 90%; where D = t H / (t H + t L ), t H is the duration of the high-pressure value P H within one pulse period, and t L is the duration of the low-pressure value P L within one pulse period.

[0009] In the above technical solution, in S100, the pressing pressure of P1 is 0.1 to 1.0 MPa, and the temperature is 40 to 120 °C; the low-pressure value P L of P2 is 0.2 to 2.0 MPa, the high-pressure value P H is 0.5 to 4.0 MPa, and the temperature is 80 to 200 °C; the pressing pressure of P3 is 1.0 to 6.0 MPa, and the temperature is 120 to 230 °C.

[0010] In the above technical solution, P1 < P L < P H ≤ P3, and the equivalent average pressure P 2均 of P2 in the medium-pressure stage satisfies P1 < P 2均 < P3; where P 2均 is the time-weighted average value of the pressing pressure within one or more pulse periods. [[ID=5!]]

[0011] In the above technical solution, in S200, the drying and shaping treatment includes a first shaping section T H and a second equalization section T L executed in sequence; the temperature range of the first shaping section T H is 90 to 180 °C, and the time is 1 to 60 min; the temperature range of the second equalization section T L is 50 to 120 °C, and the time is 1 to 120 min.

[0012] In the above technical solution, the dryness of the wet paper web when entering the hot pressing area is 30 to 65 wt%.

[0013] In the above technical solution, the dryness of the press is controlled by at least one of vacuum dehydration, pressing dehydration, pre-drying, atomization humidification, steam conditioning or closed uniform humidification.

[0014] In the above technical solution, the wet paper web includes multiple layers of wet paper web, and before the multiple layers of wet paper web enter the hot pressing zone, an interlayer reinforcing component is applied to the interlayer bonding zone of the multiple layers of wet paper web.

[0015] In the above technical solution, the interlayer reinforcing component includes at least one of cationic starch, dry strength agent, wet strength resin, microfibrillated cellulose, or latex binder.

[0016] Beneficial effects This invention provides a hot-pressing enhancement method for strengthening fiber interfaces. By introducing pulsed pressing pressure during the medium-pressure stage of hot pressing, the interlayer bonding area of ​​the wet paper web undergoes a dynamic cycle of high-pressure compaction and low-pressure rebound under softened conditions. During the high-pressure stage, pressure drives the directional migration and drainage of interfacial moisture, while simultaneously promoting tight bonding between fibers. In the subsequent low-pressure stage, the interface gains a brief rebound window, which helps to release localized water-sealing and provides space for controlled rearrangement and interlocking of interlayer fibers. This synergy between periodic mechanical action and heating conditions significantly reduces residual water film at the interface and expands the effective contact area between fibers, thereby fundamentally enhancing interlayer bonding strength and effectively solving the problems of delamination and bubbling in multilayer paper sheets during subsequent processing. Compared to traditional continuous high-pressure pressing methods, the pulsed pressure mode of this method avoids the water film closure effect caused by rapid and permanent closure of pore channels. The low-pressure rebound stage facilitates the reopening of some drainage channels, producing a pulse pump-like effect, thus more efficiently and thoroughly removing moisture from the interlayer interface. Simultaneously, this dynamic compaction process promotes better bridging and distribution of components such as fine fibers within the interfacial micropores, ultimately forming a denser and more stable fiber-bonded network. Furthermore, the method of this invention exhibits excellent synergy with existing interlayer chemical reinforcement technologies. The pulsed hot-pressing process creates a more favorable physical environment for the uniform distribution and effective bonding of chemical reinforcement components in the interfacial region; the combination of these two processes produces a synergistic reinforcement effect, providing a flexible and efficient process for developing high-performance multilayer paper products. Detailed Implementation

[0017] 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.

[0018] 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.

[0019] This invention aims to provide a hot-pressing strengthening method for fiber interface strengthening, specifically addressing the problems of low interlayer bonding strength and easy delamination during subsequent processing in multilayer paper sheets, and is particularly suitable for multilayer paper sheets. The hot-pressing strengthening method of this invention includes: S100. In the hot pressing zone, the wet paper web is hot-pressed to obtain the intermediate paper web; S200: Dry and set the intermediate paper web to obtain the finished paper.

[0020] It is understood that the wet paper web of this invention refers to a paper web that still contains a certain amount of moisture before and after forming and pressing, and can undergo dehydration and structural rearrangement under pressure. Press dryness refers to the percentage of oven-dry fiber mass to the total mass of the wet paper web when it enters the hot pressing zone. Higher press dryness indicates lower moisture content; lower press dryness indicates higher moisture content. The press dryness of the wet paper web of this invention is 30-65 wt% when entering the hot pressing zone, a range that balances interfacial plasticity and dehydration controllability. If the press dryness is too low, there is excessive free water at the interface, which can easily lead to a large amount of instantaneous water migration under pulsed high pressure, causing interfacial instability and increasing the burden of water removal. If the press dryness is too high, the deformable window of the interface narrows, and the controlled rearrangement and intercalation of fibers between layers may be limited. Press dryness can be improved by vacuum dehydration, press dehydration, or pre-drying, and can also be redistributed and balanced by atomized humidification, steam conditioning, or closed-loop humidification to obtain a more stable press state and a more uniform interfacial moisture distribution.

[0021] It should be noted that the wet paper web can be a single-layer or multi-layer structure. The method of this invention is particularly suitable for multi-layer wet paper webs, as it allows for the application of interlayer reinforcing components to the interlayer bonding areas before the multi-layer wet paper web enters the hot-pressing zone, thereby further improving the interlayer bonding point formation capability and enhancing interface continuity. The interlayer bonding area refers to the interface region at the junction of each layer of the multi-layer wet paper web; the fiber distribution, fine particle content, moisture state, and pore structure of this area determine the effective interlayer contact and bonding point formation capability.

[0022] Preferably, the interlayer reinforcing component includes at least one of cationic starch, dry strength agent, wet strength resin, microfibrillated cellulose, or latex binder, used to enhance interfacial bridging, improve the density of the fine fiber network, or introduce polymer film formation and bonding effects, thereby synergistically improving the interlayer bond strength with the pulsed hot pressing process. The interlayer reinforcing component can be applied by spraying, misting, coating, impregnation, or wet-end addition and enrichment in the interlayer bonding area. The reinforcing component should be distributed continuously or semi-continuously in the interlayer bonding area to avoid uneven penetration or operational contamination due to localized excess.

[0023] It is understood that the hot-pressing zone of this invention refers to the processing area that applies pressing pressure and provides heat to the wet paper web. It can be composed of a hot-pressing roller group, a hot-pressing plate, a belt hot-pressing device, or an equivalent structure, as long as it can achieve heating and controllable pressure loading of the wet paper web. The pressing pressure refers to the normal pressure applied to the wet paper web by the hot-pressing device, and the unit is MPa. The temperature refers to the surface temperature of the heating element in contact with the wet paper web in the hot-pressing zone. The internal temperature of the wet paper web fluctuates greatly due to the influence of evaporation heat absorption and heat transfer conditions, making it difficult to use as a uniform and controllable process setpoint. However, the surface temperature of the heating element can be stably measured and controlled by the temperature control system, and can be used as the hot-pressing heat transfer boundary condition to accurately characterize the thermal driving intensity of the hot-pressing process.

[0024] This invention involves hot-pressing a wet paper web in a hot-pressing zone to obtain an intermediate paper web. The hot-pressing process sequentially includes a pre-pressing stage (P1), a medium-pressing stage (P2), and a final-pressing stage (P3). The pre-pressing stage establishes initial contact and stabilizes interlayer positions while removing some free water, preventing interface water sealing or interlayer misalignment caused by initial high pressure. The medium-pressing stage, as a crucial strengthening stage, drives interface rearrangement and moisture migration under heating conditions through pulsed pressure. The final-pressing stage ensures structural stabilization and shaping after sufficient interface adhesion and rearrangement, providing a more uniform structural foundation for subsequent drying and setting. During the subsequent drying and setting process, as moisture is further removed, a stable bonding network is more easily formed between the layers, allowing the strengthening effect to solidify.

[0025] Furthermore, during the intermediate pressure stage P2, a pulsed pressing pressure is applied to the wet paper web, maintaining the pressing pressure at a high value P. H With low pressure value P L Periodically switching between them, and P H With P L Satisfy: P H >P L >0, P H / P L=1.5~10, the high and low pressure switching generates a sufficient pressure difference to form an effective compaction and rebound cycle, while avoiding an excessively small pressure difference that leads to an effect approaching constant pressure, or an excessively large pressure difference that leads to local over-compaction and increased control difficulty. Pulsed compression pressure causes the interlayer bonding zone to undergo periodic high-pressure compaction and low-pressure rebound. The high-pressure stage promotes the compression of interfacial voids and establishes a pressure gradient, driving the migration and discharge of free water and weakly bound water at the interface, while making it easier for components such as fine fibers to enter the interfacial micropores to form more bridging points and contact points. The low-pressure stage provides a brief rebound window, causing some micropores to reopen, releasing the local water-sealing state and releasing water drainage paths, while also facilitating controlled rearrangement and interpenetration of interlayer fibers. The repeated action of the above compaction and rebound cycle reduces the residual water film in the interlayer, increases the effective contact area, and increases the number of bonding points, thereby significantly enhancing the interlayer bonding strength and inhibiting delamination. In specific implementation, within the medium-pressure stage P2, the pressure switches between PH and PL for at least N≥2 cycles.

[0026] It is understandable that the frequency of the pulsed pressing pressure in the medium-pressure stage is 0.1~10Hz. This frequency range is intended to match the rhythm of pressure switching with the time scale of moisture migration and viscoelastic rearrangement of the fiber interface in the interlayer bonding zone during hot pressing, so that each high-pressure and low-pressure stage can complete its respective function and form a sufficient number of effective cycles within the limited hot pressing residence time.

[0027] Duty cycle refers to the proportion of the high-pressure segment within the entire pulse cycle. A longer high-pressure duration facilitates interface compaction and the establishment of a pressure gradient, promoting the outward migration of free and weakly bound water at the interlayer interface. Simultaneously, it allows components such as fine fibers to more easily enter the interlayer micropores and form bridging points, thus increasing the number of interlayer contact points. A longer low-pressure duration provides a more sufficient interlayer rebound window, facilitating the reopening of partially closed micropores, releasing localized water sealing, and restoring drainage pathways. It also provides time for relative sliding, controlled rearrangement, and interpenetration of interlayer fibers, transforming the bonding state formed during the high-pressure phase into a more stable interlocking structure. A pulsed pressing pressure duty cycle (D) of 10-90% avoids insufficient compaction and water squeezing due to an excessively short high-pressure segment, or prolonged constant pressure leading to long-term pore closure and insufficient interlayer adjustment space. It also avoids insufficient opening and redistribution due to an excessively short low-pressure segment, or excessive rebound causing damage to the established bonding state due to an excessively long low-pressure segment.

[0028] Frequency and duty cycle together determine the duration of high and low pressure, providing sufficient cycles within the effective contact time of hot pressing. This allows for repeated accumulation of compaction and rebound, gradually expanding the effective contact area between layers and increasing the number of bonding points. If the frequency is too low, the number of cycles is insufficient, and the process resembles a long period of constant pressure with intermittent cycles. If the frequency is too high, each cycle is too short. The high-pressure section does not have enough time to form a stable pressure gradient and drive sufficient moisture migration, and the low-pressure section does not have enough time to restore effective pores and complete moisture redistribution. The pulse action easily evolves into high-frequency perturbations, reducing the effective compaction and rebound amplitude.

[0029] It is important to note that the hot-pressing process sequentially includes a pre-pressing stage (P1), a medium-pressing stage (P2), and a final-pressing stage (P3), with progressively increasing pressure and temperature windows. The purpose is to allow the interlayer bonding zone to undergo a continuous process from establishing contact to controlled rearrangement and finally to stable curing under thermal softening conditions. Improving the interlayer bonding strength of multilayer wet paper webs relies on two key prerequisites: first, the interlayer interface must be able to form a sufficiently large effective contact area in the wet stage; second, the residual water film at the interface must be continuously weakened, creating conditions for bonding point formation. Segmented pressure and temperature settings are designed to gradually enhance interfacial adhesion and fiber interlocking without causing interlayer misalignment or interfacial water sealing, and to stabilize the formed interfacial structure, facilitating subsequent drying, shaping, and curing of the bonded network.

[0030] The pre-compression stage (P1) uses a compression pressure of 0.1~1.0 MPa and a temperature of 40~120℃ to establish initial contact and stabilize interlayer positions, while simultaneously expelling some free water to form a controllable initial interface state. Too low a pressure results in insufficient interlayer adhesion, reducing subsequent strengthening efficiency; too high a pressure can easily lead to interlayer slippage or localized rapid compaction before the interface stabilizes, causing premature closure of interface pores, restricting water drainage channels, and increasing the probability of residual water film formation. The temperature is set within a relatively mild range to improve the deformability of the fiber wall and interface conformability without triggering severe vaporization and localized defects, making interlayer adhesion easier and providing a uniform contact foundation for subsequent strengthening stages.

[0031] The low pressure value P of P2 in the medium pressure stage L The pressure ranges from 0.2 to 2.0 MPa, with a high pressure value P. HThe pressure ranges from 0.5 to 4.0 MPa. This pressure is within a range that induces significant interfacial compaction while preserving space for rebound and rearrangement. Combined with pulse switching, the interlayer bonding zone repeatedly experiences high-pressure compaction and low-pressure rebound. Under thermal softening conditions, the high-pressure stage significantly increases the interfacial contact pressure and establishes a pressure gradient, promoting the migration and discharge of free and weakly bound water at the interface. Simultaneously, it encourages components such as fine fibers to enter the interfacial micropores, increasing bridging points. The low-pressure stage provides a brief rebound window, allowing some of the compressed micropores to reopen, releasing localized water blockage and restoring drainage pathways. It also provides time for controlled rearrangement and interpenetration of interfacial fibers through relative slippage. Temperatures of 80–200°C reduce resistance to moisture migration and enhance the plasticity of the fiber interface, making the compaction and rebound cycles more effectively translate into increased contact area and accumulation of bonding points, thereby significantly improving interfacial bonding strength and reducing the risk of delamination.

[0032] The final pressing stage (P3) involves a pressing pressure of 1.0–6.0 MPa and a temperature of 120–230 °C. Its function is to finalize and solidify the fiber network structure after the interlayer interfaces have undergone bonding and interlocking optimization in the medium-pressure stage. Appropriately increasing the pressure at this stage helps to further eliminate residual interfacial voids and improve interfacial continuity, making the fiber interlocking structure formed in the previous stage more macroscopically stable. Increasing the temperature to a higher window maintains the deformability of the fiber walls and promotes the continued migration of residual moisture, providing a more uniform structural foundation for the subsequent drying and solidification stages. Setting P3 to a high pressure level higher than or no lower than that of the medium-pressure stage ensures a progressive increase in pressure intensity throughout the process chain, avoiding structural springback and interfacial contact degradation caused by a return to lower forming strength after strengthening.

[0033] The temperature at each stage is designed to increase in stages to match the changes in the plasticity and moisture migration capacity of the fiber interface, and to reduce process risks. The wet paper web has a high moisture content; if excessively high temperatures and pressures are applied directly at the beginning, the interfacial moisture may vaporize rapidly and cause localized pressure buildup, easily leading to interfacial defects and disrupting the consistency of interlayer bonding. By gradually increasing the temperature from P1 to P3 and coordinating with progressively increasing pressure, moisture migration and structural compaction can be more controlled, resulting in a more uniform interlayer bonding and distribution of bonding points.

[0034] Equivalent mean pressure P 2均 This refers to the time-weighted average of pressure changes over one or more pulse cycles. For a two-stage pulse, when the high pressure lasts for t seconds within one cycle... H Low pressure sustained t L At that time, P 2均 =(P H ×t H +P L ×t L ) / (t H +t L), which is used to quantify the comprehensive effect of pulse pressure on interface strengthening into comparable process indicators.

[0035] The pressure relationships in each stage satisfy: P1 < P L < P H ≤ P3, and P1 < P 2均 < P3. The purpose is to construct the hot pressing process into a process chain with progressive strength and clear functional division, so that the pre-pressing stage is used to establish a stable initial interface, the medium-pressing stage is used to achieve pulse strengthening under controllable strength, and the final-pressing stage is used for structure consolidation and forming stability, avoiding the inversion of pressure intensity or the imbalance of average action intensity between stages, thereby ensuring that the interlayer bonding enhancement process is repeatable and implementable.

[0036] The pre-pressing stage P1 is set as the lowest pressure, which is used to stabilize the interlayer position and establish initial adhesion. Limiting P1 < P L , it can ensure that the necessary contact pressure is still maintained in the low-pressure section of the pulse, so that the interlayer interface can be adjusted in a controlled manner during the springback process, avoiding interlayer slip caused by the loss of restraint. In the medium-pressing stage, P L < P H is set to ensure that there is enough pressure difference for pulse switching to form an effective compaction and springback cycle. Limiting P H ≤ P3 can ensure that the pressure intensity in the final-pressing stage is not lower than the high-pressure level in the medium-pressing stage, so that the process chain maintains progressive strength from pre-pressing to strengthening and then to consolidation, avoiding springback or contact degradation of the structure in the final-pressing stage after strengthening in the medium-pressing stage. Limiting P1 < P 2均 < P3 can ensure that the overall action intensity in the medium-pressing stage is both higher than that in the pre-pressing stage to achieve strengthening, and not close to the final-pressing stage in the average sense so as not to lose the advantage of pulse springback. If P2 is close to or lower than P1 on average, it is difficult for the medium-pressing stage to drive significant interface compaction and fiber interlocking; if P2 is close to or reaches P3 on average, the overall action in the medium-pressing stage is too high, and the actual contribution of the low-pressure springback window is weakened, and there is a risk of long-term closure of pores and water sealing at the interface.

[0037] By restricting P 2均 and the stage pressure relationship, while maintaining the effectiveness of the pulse mechanism, it is possible to avoid the deviation of the average action intensity in the medium-pressing stage from the expected value due to improper duty cycle or frequency setting, thereby ensuring the stability and repeatability of the interlayer bonding enhancement process and forming a complete and coherent process chain with the pre-pressing and final-pressing stages.

[0038] In S200, the drying and shaping treatment includes a first shaping section T H executed sequentially and a second equalization section T LThe purpose is to effectively solidify the interlayer bonding and interlocking structure formed during the medium-pressure stage during the drying process, while reducing the risk of interlayer stress concentration and interface defects caused by the drying gradient, thereby stabilizing and improving the interlayer bonding strength and further suppressing delamination. The hot-pressing stage increases the effective contact area between layers and reduces the residual water film through compaction and springback cycles, but these interface states are still under water-containing conditions and need to be transformed into a stable internal bonding network during controlled dehydration and structural stabilization.

[0039] First shaping segment T H The temperature range is 90~180℃, and the time is 1~60min; its main function is to complete most of the dehydration and initially shape the paper web structure. During this stage, moisture inside the paper web continues to migrate outwards and evaporate, further weakening the water film at the interlayer interface, and gradually transforming the actual contact between fibers into stable bonding points. Higher temperatures are beneficial for increasing the rate of moisture migration and evaporation, allowing the interface to complete the transition from a wet to a semi-dry state in a shorter time, and fixing the formed interlayer bonding and interlocking structure. If the temperature is too low or the time is insufficient at this stage, moisture removal will be inadequate, and a large amount of water film may remain at the interface, leading to a decrease in the efficiency of bonding point formation and making it difficult to fully realize the strengthening effect of interlayer bonding. If the temperature is too high or the dehydration process is too fast, a large moisture gradient in the thickness direction may be generated, causing inconsistent shrinkage and stress concentration at the interlayer interface, which in turn increases the risk of delamination.

[0040] Second equilibrium segment T L The temperature range is 50~120℃, and the time is 1~120min. This stage primarily aims to achieve a balance between moisture and stress, ensuring a more uniform moisture distribution along the paper's thickness and releasing any drying stress that may arise during the first setting stage. In multi-layered paper, the dehydration rate often differs due to variations in fiber ratio, fine particle content, and pore structure among the layers. Without equalization, the interlayer interfaces are prone to separation due to differences in moisture content and shrinkage, increasing the probability of delamination in subsequent processing. This stage uses a relatively mild temperature to promote moisture redistribution and microstructural relaxation without triggering new, severe evaporation gradients. This results in more coordinated shrinkage in the interlayer bonding area, smoother interfacial stress, and improved interlayer bonding stability. If this time is too short, the moisture and stress within the paper web cannot be adequately balanced, potentially leading to delamination risks in subsequent processing. Conversely, if the temperature is too high, the equalization stage will lose its gentle, gradual release effect, potentially introducing a larger gradient again.

[0041] The two-stage drying and shaping process described above, along with the preceding hot pressing strengthening, forms a continuous process chain. The hot pressing stage is responsible for establishing the foundation for interlayer bonding and interlocking, and reducing residual water film at the interface; the first shaping stage rapidly crosses the critical moisture content range at a higher temperature, fixing the interface structure; the second equalization stage completes the equalization and relaxation of moisture content and stress under milder conditions, avoiding new delamination driving forces caused by uneven drying at the interlayer interface. This sequential setting of shaping followed by equalization makes the improvement in interlayer bonding strength more stable, resulting in a lower risk of delamination and more consistent product quality in subsequent processing.

[0042] Example 1 This embodiment provides a hot-pressing reinforcement method for fiber interface strengthening, the preparation method of which includes: S1. Softwood pulp and hardwood pulp are mixed at a mass ratio of 4:6, beaten to 45°SR, and a pulp concentration of 0.8% is prepared. This pulp is then formed through a multi-layer headbox to obtain a basis weight of 180 g / m³. 2 Three layers of wet paper; S2. The wet paper web's dryness upon entry is controlled at 45 wt% through vacuum dewatering and press dewatering. S3. The wet paper web is fed into a hot pressing zone consisting of a pair of heated pressure rollers for processing to obtain the intermediate paper web; wherein, in the pre-pressing stage P1, the pressure is 0.5MPa, the surface temperature of the pressure rollers is 80℃, and the time is 2s; in the intermediate pressing stage P2, a pulse pressing pressure is applied, with a high pressure value P H The low pressure value is 2.0 MPa, P. L The pressure is 0.8 MPa, the pulse frequency is 2 Hz, the duty cycle is 50%, the surface temperature of the pressure roller is 150℃, and the total residence time in this stage is 4s; in the final pressing stage, the pressure of P3 is 3.0 MPa, the surface temperature of the pressure roller is 180℃, and the time is 2s. S4. The intermediate paper web is dried and shaped to obtain the finished paper; the first shaping section T H The process was carried out in the drying cylinder group, with a surface temperature of 150℃ and a time of 15 minutes; the second equalization stage T L The process is carried out in a low-temperature drying cylinder and balancing chamber at a temperature of 80°C for 30 minutes.

[0043] Example 2 This embodiment provides a hot-pressing reinforcement method for fiber interface strengthening, the preparation method of which is as shown in Example 1, except that: in S3, the high pressure value P2 is P H The low pressure value is 3.0 MPa, P. L The pressure is 0.5 MPa, the pulse frequency is 5 Hz, the duty cycle is 70%, and the surface temperature of the pressure roller is 170℃. The total residence time in this stage is still 4 seconds.

[0044] Example 3 This embodiment provides a hot-pressing reinforcement method for fiber interface strengthening, the preparation method of which is as shown in Example 1, except that: In S2, the wet paper web's feed dryness is increased to 60 wt% by intensified pressing. In S3, P1 has a pressure of 0.6 MPa and a temperature of 100℃; the high pressure value of P2 is P H The low pressure value is 2.2 MPa, P. L The pressure is 0.5 MPa, the frequency is 1 Hz, the duty cycle is 60%, and the surface temperature of the pressure roller is 180℃; the pressure of P3 is 3.5 MPa and the temperature is 200℃.

[0045] Example 4 This embodiment provides a hot-pressing reinforcement method for fiber interface strengthening. The preparation method is as shown in Example 1, except that after S1, it further includes: before the wet paper web enters the hot-pressing zone, using a spraying device to uniformly spray 0.5% cationic starch solution onto the interface bonding area of ​​the two layers, with the addition amount being 1.0% based on the oven-dry fiber weight.

[0046] Comparative Example 1 This comparative example provides a hot-pressing reinforcement method for fiber interface strengthening. The preparation method is as shown in Example 1, except that in S3, instead of pulse pressure, a constant pressing pressure of 1.4 MPa is applied, the surface temperature of the pressure roller is 150°C, and the duration is 4 seconds.

[0047] Comparative Example 2 This comparative example provides a hot-pressing reinforcement method for fiber interface strengthening, the preparation method of which is as shown in Example 1, except that: In S3, the wet paper web passes through a set of hot press rolls with a constant pressure of 2.0 MPa and a roll surface temperature of 150°C. The total pressing time is 8 seconds, which includes the pre-press stage P1, the medium-press stage P2, and the final-press stage P3. In S4, a one-stage drying process is used, drying at 150℃ for 45 minutes.

[0048] Comparative Example 3 This comparative example provides a hot-pressing reinforcement method for fiber interface strengthening, the preparation method of which is shown in Comparative Example 2. The difference is that there is no S2, that is, the wet paper web does not pass through the hot-pressing zone and directly enters the drying section.

[0049] Performance testing The finished papers obtained in Examples 1-4 and Comparative Examples 1-3 were subjected to the following performance tests under standard atmospheric conditions of 23±1℃ and 50±2% relative humidity. The results are shown in Table 1: Internal bond strength: Tested according to national standard GB / T 26203-2010 "Determination of internal bond strength of paper and paperboard (Scott type)"; Thickness direction tensile strength: The thickness direction (Z direction) tensile strength of the paper sample was tested according to the constant rate tensile method in the national standard GB / T 12914-2018 "Determination of tensile strength of paper and paperboard". Interlayer peel strength: Referring to the national standard GB / T 2791-1995 "Test method for peel strength of adhesives", after the multi-layer paper sample is separated into layers, it is peeled at a 180° angle to test its peel strength; Thickness and density: The thickness is measured and the density is calculated in accordance with the national standard GB / T 451.3-2002 "Determination of thickness of paper and paperboard"; Tensile index: The tensile strength of paper in the longitudinal direction (MD) is tested according to the national standard GB / T 12914-2018, and the tensile index is calculated. Table 1 As shown in Table 1, the interlayer bonding indices of Examples 1-4 are significantly higher than those of Comparative Examples 1-3. This indicates that the pulsed hot-pressing enhancement method of the present invention can effectively improve interlayer bonding and suppress delamination. Comparative Example 1 uses the same average pressure as Example 1 but is a constant pressure, and its interlayer bonding performance is significantly lower than that of Example 1, and even lower than that of Comparative Example 2 which uses a higher peak pressure. This proves that the pulsed pressure cycling mechanism has unique advantages in promoting interfacial moisture migration, fiber rearrangement and intercalation, and its effect cannot be compared with simply increasing the average pressure or peak pressure. Example 4 obtained the best interlayer bonding performance by applying cationic starch in synergy with pulsed hot-pressing, indicating that chemical reinforcement and the physical strengthening method of the present invention can be effectively combined to achieve further performance improvement.

[0050] The method of this invention improves the interlayer bonding, while also increasing the paperboard density, reducing its thickness, and improving the overall tensile index. This shows that the method does not damage the overall mechanical properties of the paper when optimizing the interlayer structure; on the contrary, it makes the structure more compact and uniform.

[0051] In summary, the hot-pressing enhancement method provided by this invention, by introducing pulsed pressing pressure and combining segmented hot pressing with two-stage drying, can improve the interlayer bonding strength of multi-layer paper sheets and effectively solve the problem of easy delamination in subsequent processing. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of this invention; therefore, the scope of protection of this invention should be determined by the scope defined in the claims.

Claims

1. A hot-pressing reinforcement method for fiber interface strengthening, characterized in that, The method includes the following steps: S100. In the hot pressing zone, the wet paper web is hot pressed to obtain an intermediate paper web; S200: The intermediate paper web is dried and shaped to obtain the finished paper; The hot pressing process includes a pre-pressing stage P1, a medium-pressing stage P2, and a final pressing stage P3. The medium-pressing stage P2 applies pulsed pressing pressure to the wet paper web, maintaining a high pressing pressure value P. H With low pressure value P L Periodically switching between them, and P H With P L satisfy: P H >P L >0,P H / P L =1.5~10。 2. The hot-pressing enhancement method according to claim 1, characterized in that, The frequency of the pulsed pressure is 0.1~10Hz.

3. The hot-pressing enhancement method according to claim 1, characterized in that, In S100, the duty cycle D of the pulsed pressing pressure is 10~90%; Where D=t H / (t) H +t L ), t H The high voltage value P within one pulse cycle H The duration, t L The low voltage value P within one pulse cycle L The duration.

4. The hot-pressing enhancement method according to claim 1, characterized in that, In S100, The pressing pressure of P1 is 0.1~1.0MPa, and the temperature is 40~120℃; The low pressure value P of P2 L The pressure ranges from 0.2 to 2.0 MPa, with a high pressure value P. H The pressure ranges from 0.5 to 4.0 MPa, and the temperature ranges from 80 to 200℃. The pressing pressure of P3 is 1.0~6.0MPa, and the temperature is 120~230℃.

5. The hot-pressing enhancement method according to any one of claims 1 to 4, characterized in that, P1 <P L <P H ≤P3, and the equivalent average pressure P of the intermediate pressure stage P2. 2均 Satisfy P1 <P 2均 <P3; Among them, P 2均 It is the time-weighted average of the pressure over one or more pulse cycles.

6. The hot-pressing enhancement method according to claim 1, characterized in that, In S200, the drying and shaping process includes a first shaping segment T executed sequentially. H Second Equilibrium Segment T L ; First shaping segment T H The temperature range is 90~180℃, and the time is 1~60min; Second equilibrium segment T L The temperature range is 50~120℃, and the time is 1~120min.

7. The hot-pressing enhancement method according to any one of claims 1 to 6, characterized in that, The wet paper web has a pressing dryness of 30-65 wt% when it enters the hot press zone.

8. The hot-pressing enhancement method according to claim 7, characterized in that, The pressure dryness is controlled by at least one of vacuum dehydration, pressing dehydration, pre-drying, atomization humidification, steam conditioning, or closed uniform humidification.

9. The hot-pressing enhancement method according to any one of claims 1 to 8, characterized in that, The wet paper web comprises multiple wet paper webs, and before the multiple wet paper webs enter the hot pressing zone, interlayer reinforcing components are applied to the interlayer bonding areas of the multiple wet paper webs.

10. The hot-pressing enhancement method according to claim 9, characterized in that, The interlayer reinforcing component includes at least one of cationic starch, dry strength agent, wet strength resin, microfibrillated cellulose, or latex binder.

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