Corrugated board based on waterproof coating and preparation method thereof

Abstract

The invention discloses a corrugated board based on a waterproof coating and a preparation method thereof, and relates to the technical field of packaging materials, the corrugated board comprises corrugated medium paper and a bionic gradient response waterproof coating coated on the surface of the corrugated medium paper; the bionic gradient response waterproof coating is composed of a waveform adaptive resin matrix, a dynamic cross-linking agent system and a reconfigurable pore filler. In order to solve the problem of cross-linking failure caused by uneven drying, asynchronous shrinkage, microcracks and unbalanced heat and mass transfer of a corrugated board due to a waveform structure, a bicontinuous gradient network is formed through a bionic gradient response waterproof coating (containing waveform adaptive resin, a dynamic cross-linking agent and reconfigurable pore filler); and in cooperation with surface energy pretreatment, electrostatic atomization coating and segmented drying processes, ridge peak / ridge valley performance balance is achieved, the drying defect and crosslinking dynamic unbalance are solved, the waterproof durability and environmental adaptability are improved, and the equipment cost is reduced by 70%-80%.

Description

Technical Field

[0001] The present invention relates to the technical field of packaging materials, in particular to a corrugated paperboard based on a waterproof coating and a preparation method thereof. Background Art

[0002] Corrugated cardboard is widely used in the packaging industry due to its light weight, high strength, and low cost. With the expansion of cold chain logistics and storage in humid environments, higher requirements are being placed on the waterproof performance of corrugated cardboard. Currently, applying a waterproof coating to improve the waterproofness of corrugated cardboard is the mainstream technology approach. However, during the drying process, the coupling of the geometric characteristics of the corrugated structure and its heat and mass transfer behavior can lead to significant performance defects, as shown below: 1. The geometric induction effect of the corrugated structure causes drying unevenness: The wavy structure of the corrugated core paper leads to significant differences in the gas-liquid interface area: the exposed area of ​​the coating at the corrugation peak is much larger than that at the corrugation valley. According to Fourier's law of heat conduction, the water evaporation rate is faster.

[0003] Under dry conditions, the moisture content of the coating decreases significantly at peaks and valleys, resulting in asynchronous shrinkage. At a microscopic level, the peaks dry rapidly, forming a dense surface layer, while the valleys, due to delayed drying, retain a porous structure. This significant density difference further exacerbates stress concentration. Nanofiber porosity significantly amplifies shrinkage stress, causing local stresses in the peak coating to approach the material's yield strength, leading to the initiation of microcracks.

[0004] 2. Nonlinear behavior of heat and mass transfer leads to performance degradation: During the drying process, the moisture content of the core paper's middle layer is higher than that of the surface, forcing internal moisture to migrate against the concentration gradient toward the coating. The dense barrier layer rapidly forms at the flute peaks, hindering moisture escape and causing it to accumulate in the flute valleys. This causes secondary swelling of the flute coating and destroys the established hydrophobic network (GB / T6544-2020, the national standard for corrugated cardboard). The problem is further exacerbated by imbalanced crosslinking reaction kinetics: While high temperatures accelerate the crosslinker's condensation reaction, above its thermal decomposition temperature, the molecular chain breakage rate exceeds the crosslinking rate, resulting in a decrease in the effective crosslink density and a significant reduction in waterproofing durability.

[0005] 3. Bottlenecks and limitations of existing technologies: Process optimization challenges: While lowering the drying temperature reduces crosslinker decomposition, it significantly increases drying time, significantly reduces production line speed, and makes it difficult to meet industrial production requirements. Increasing coating thickness can lead to coating accumulation at the valleys, creating surface defects and increasing water permeability. Existing single-channel hot air ovens cannot provide differentiated air volume at the peaks and valleys, and the cost of upgrading to multi-channel zoned temperature control equipment is prohibitive for small and medium-sized enterprises.

[0006] Material design limitations: Traditional thermosetting resins offer excellent heat resistance, but their slow crosslinking speed at low temperatures makes them incompatible with high-speed production. UV-cured coatings, due to the corrugated shadow effect, result in insufficient curing of the corrugated valleys, significantly reducing adhesion. While adding elastomer toughening agents improves elongation at break, it also lowers the glass transition temperature, reducing anti-blocking properties at high temperatures.

[0007] Lack of theoretical models: Existing drying kinetic models are only applicable to planar homogeneous materials and cannot describe the coupled processes of heat transfer on curved surfaces, mass transfer in porous media, and polymer crosslinking in corrugated structures, resulting in significant prediction errors. Traditional mechanical analysis ignores the stress amplification effect of pores, leading to inadequate microcrack risk assessment (Packaging Engineering, 2022, Study on Drying Stress of Corrugated Cardboard Coatings).

[0008] In summary, existing technologies fail to effectively decouple the geometrically induced effects of the corrugated structure from the nonlinear behavior of heat and mass transfer. This leads to defects in waterproof coatings during the drying process, such as uneven shrinkage, crosslinking failure, and microcracks. This restricts the application of corrugated cardboard in high-end waterproofing applications. Therefore, there is an urgent need to develop a new waterproof coating and preparation method that balances structural adaptability, heat and mass transfer balance, and crosslinking efficiency.

[0009] In view of this, a corrugated paperboard based on a waterproof coating and a preparation method thereof are provided to overcome the above problems. Summary of the Invention

[0010] The object of the present invention is to provide a corrugated board based on a waterproof coating and a preparation method thereof, so as to solve the problems raised in the above background technology.

[0011] To solve the above technical problems, the present invention provides a corrugated paperboard based on a waterproof coating, comprising a corrugated core paper and a bionic gradient responsive waterproof coating coated on the surface of the corrugated core paper; the bionic gradient responsive waterproof coating is composed of a waveform-adaptive resin matrix, a dynamic crosslinking agent system, and a reconfigurable pore filler, wherein: The waveform-adaptive resin matrix is ​​a polylactic acid-polyethylene glycol block copolymer PLA-PEG grafted with dodecafluoroheptyl methacrylate, and the molecular chain contains a hydrophobic fluorocarbon segment and a hydrophilic PEG segment; The dynamic crosslinker system includes cyclodextrin-encapsulated N-hydroxymethyl acrylamide (component A) and nano-confined isocyanate prepolymer (component B); Reconfigurable pore fillers include hollow mesoporous silica microspheres and biomimetic lignin fibers; The coating forms a double continuous gradient network on the surface of the corrugated core paper: the corrugation peak area is mainly composed of hydrophobic fluorocarbon segments with a cross-linking density of 1.2-1.5 mol / m³ and a porosity of 15-20%; the corrugation valley area is mainly composed of hydrophilic PEG segments with a cross-linking density of 0.8-1.0 mol / m³ and a porosity of 30-35%. The two form a cross-regional stress transfer channel through lignin fibers.

[0012] Furthermore, the molar ratio of the polylactic acid-polyethylene glycol block copolymer PLA-PEG to dodecafluoroheptyl methacrylate is 1:1.2, the contact angle of the graft copolymer is 110°-130°, and the critical micelle concentration is 0.8-1.2 g / L.

[0013] Furthermore, the inclusion rate of cyclodextrin-encapsulated N-hydroxymethyl acrylamide is 75-85%, and the particle size of the nano-confined isocyanate prepolymer is 20-40 nm, which is loaded in the mesoporous silica pores.

[0014] Furthermore, the hollow mesoporous silica microspheres have a particle size of 1-5 μm and a pore diameter of 5-10 nm. The surface is grafted with temperature-responsive PNIPAM polymer brushes, which undergo a volume phase change at 32°C and a swelling rate of 40-60%. The bionic lignin fibers are modified with a 5% tannic acid aqueous solution at 60°C for 2 hours. After modification, they have pH-responsive polyphenolic hydroxyl groups, which are activated at pH = 4-6.

[0015] A method for preparing corrugated paperboard based on a waterproof coating comprises the following steps: Surface energy gradient pretreatment of core paper: The corrugated core paper is immersed in an ethanol solution containing 0.5-1.0% chitosan quaternary ammonium salt and treated with 30-50kHz radio frequency plasma for 5-10 minutes to introduce amino groups into the flute peak area while retaining the original cellulose surface in the flute valley area, forming a corrugated core paper with a surface energy gradient of 10-15mN / m; Waveform-induced self-assembly coating: Using electrostatic atomization-airflow guided coating technology, the atomizing nozzle applies an 8-12kV electrostatic voltage to make the coating droplets negatively charged. In conjunction with the waveform-synchronized airflow field, the wind speed at the peak is 2-3m / s, and the wind speed at the valley is 0.5-1m / s. The surface energy difference is used to drive the droplets to selectively deposit on the waveform surface. Segmented dynamic cross-linking drying: Entering the variable gradient cross-linking drying chamber, it is regulated in three stages, including humidity-triggered pre-cross-linking, temperature gradient main cross-linking and stress self-balancing cooling.

[0016] Furthermore, the electrostatic atomization-airflow guided coating technology uses electrostatic voltage adjusted according to the core paper flute height. When the A-type flute height is 4.5-5.0mm, the droplet size decreases by 10-15μm for every 1kV increase in voltage; when the wind speed ratio is adjusted to 2:1-4:1.

[0017] Furthermore, the conditions for the humidity-triggered pre-crosslinking stage are: 25-30°C, RH=70-80%, and a treatment time of 15-20 min. The high humidity environment in the Lenggu area is used to induce cyclodextrin to release N-hydroxymethyl acrylamide, which undergoes hydroxyl crosslinking with the PEG chain segments to form an initial hydrogel network.

[0018] Furthermore, the conditions of the temperature gradient main cross-linking stage are: 40-50°C, 8-12°C temperature difference between the peak and valley, and 25-30 min of treatment time. This allows the nano-confined isocyanate in the peak region to break through the mesopore limitations and rapidly cross-link with the fluorocarbon segments. At the same time, the hollow microsphere PNIPAM brush shrinks and closes the pores, reducing the resistance to water evaporation. Due to the lower temperature in the valley region, the cross-linking rate slows down, and the hollow microspheres remain in a swollen state, forming a water release channel.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Structural adaptability: Bionic gradient response coating solves the drying defects induced by waveform geometry: 1. Differences in the balance performance of dual continuous gradient networks: The peak region is mainly composed of hydrophobic fluorocarbon segments (cross-linking density 1.2-1.5 mol / m³, porosity 15-20%), which quickly form a hydrophobic barrier (contact angle ≥125°) and accelerate water evaporation (rate increased by 20-25%); the valley region is mainly composed of hydrophilic PEG segments (cross-linking density 0.8-1.0 mol / m³, porosity 30-35%), which avoid secondary swelling through delayed cross-linking and sustained-release channels.

[0020] Lignin fibers form cross-regional stress transfer channels, and the measured difference in drying shrinkage between peaks and valleys is ≤15% (traditional methods ≥30%), the stress concentration factor is reduced by 40-50%, and the incidence of microcracks is reduced from ≥20% of traditional methods to 0%.

[0021] 2. Dynamic crosslinker spatiotemporal mismatch crosslinking: Cyclodextrin-encapsulated N-hydroxymethyl acrylamide (component A) undergoes sustained-release cross-linking under the high humidity environment of Leng Valley, forming a hydrogel network with PEG chain segments; nano-confined isocyanate prepolymer (component B) triggers rapid cross-linking at the high temperature of Leng Peak, increasing the effective cross-linking density by 30-40%, and the waterproof durability is retained at a rate of ≥90% after 50 wet-heat cycles (65% for traditional methods).

[0022] 2. Balanced heat and mass transfer: Dynamic control mechanism optimizes the drying process: 1. Surface energy gradient driven self-assembly coating: Through chitosan quaternary ammonium salt pretreatment and plasma technology, a surface energy difference of 10-15mN / m is formed at the peak / valley. Combined with electrostatic atomization-airflow guidance technology (peak wind speed 2-3m / s, valley wind speed 0.5-1m / s), the coating droplets are selectively deposited on the corrugated surface, avoiding the accumulation of coating in the valley and excessive densification of the peak.

[0023] The segmented drying process (humidity-triggered pre-crosslinking → temperature gradient main crosslinking → stress self-balancing cooling) achieves a peak / valley crosslinking rate difference of ≤15%, and improves the uniformity of moisture migration by 60-70% (the traditional method of reverse concentration migration results in a valley moisture accumulation rate of ≥30%).

[0024] 2. Reconfigurable porous fillers to intelligently control mass transfer paths: The PNIPAM brushes on the surface of hollow mesoporous silica microspheres undergo a volume phase change at 32°C (swelling rate 40-60%). In the initial stage of drying, they shrink and open the pores to accelerate the evaporation of the peaks, and in the later stage, they swell and close the pores to inhibit the swelling of the valleys. The bionic lignin fibers form a dynamic hydrogen bond network through pH-responsive polyphenol hydroxyl groups, which can release shrinkage stress in real time (efficiency 60-70%) and guide the directional migration of water.

[0025] 3. Improved material performance: environmental adaptability and self-repairing ability: 1. Dynamic waterproof adjustment mechanism: When the humidity is ≤60%, the fluorocarbon segments are enriched to form a hydrophobic barrier, and the water absorption rate is as low as 7.2% (traditional 15.2%). When the humidity is ≥85%, the PEG segments absorb moisture and swell, increasing the porosity from 20% to 35%, allowing water to penetrate evenly (at a rate of 8-10g·m -2 ・h -1 ), to avoid local damage.

[0026] 2. Microcrack self-repair and wide temperature range stability: The polyphenol hydroxyl groups of lignin fibers form reversible hydrogen bonds with the resin matrix. The 24-hour repair rate of 0.1mm microcracks is ≥95%, and the compressive strength retention rate after repair is 98%. The temperature resistance range is extended to -10℃~60℃ (traditional thermosetting resin ≤50℃), solving the problems of low-temperature brittle cracking and high-temperature adhesion.

[0027] 4. Industrialization advantages: Equipment costs are significantly reduced: Abandoning the multi-channel partitioned temperature-controlled oven, the surface energy gradient pretreatment (cost ≤ 100,000 yuan) + electrostatic atomization airflow guided coating (equipment transformation cost ≤ 500,000 yuan) is adopted, which reduces the equipment cost by 70-80% compared with the traditional solution and is suitable for the production line transformation of small and medium-sized enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1This is a schematic diagram of a corrugated paperboard based on a waterproof coating and a preparation method thereof according to the present invention. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] See also Figure 1 , the present invention provides a technical solution: Core idea of ​​the invention: Breaking through the traditional mindset of "flat coating - uniform drying", we have constructed a biomimetic waveform-adaptive coating system. Through the triple mechanism of "structure-induced cross-linking gradient - active regulation of mass transfer paths - stress self-balancing network", we achieve: A. Utilize the surface energy difference of the waveform to drive the self-assembly of the coating components to form a natural gradient structure; B. Constructing a thermosensitive-swelling dual-responsive cross-linked network to dynamically match the differentiated drying requirements of peaks and valleys; C. Introducing a reconfigurable porous skeleton to relieve shrinkage stress in real time and guide the directional migration of water.

[0031] plan: 1. Bionic gradient response waterproof coating: 1. Core Component Design (parts by weight): Waveform-adaptive resin matrix (40-60 parts): Polylactic acid-polyethylene glycol block copolymer (PLA-PEG) grafted with dodecafluoroheptyl methacrylate. The molecular chain contains a hydrophobic fluorocarbon segment (contact angle 110°-130°) and a hydrophilic PEG segment (critical micelle concentration 0.8-1.2 g / L). The synthesis method is as follows: PLA-PEG (molecular weight 5000) is dissolved in dichloromethane (100 mL / g). Dodecafluoroheptyl methacrylate (PLA-PEG: dodecafluoroheptyl methacrylate molar ratio 1:1.2) and azobisisobutyronitrile (AIBN, 0.5% of the total reaction system weight) are added. Under nitrogen protection, the reaction is carried out at 60°C for 24 hours. After completion of the reaction, the product is precipitated and dried to obtain the graft copolymer. The copolymer produces orientation differences on the corrugated surface due to curvature induction: the fluorocarbon chain segments are enriched at the peaks to form a hydrophobic quick-drying layer, while the PEG chain segments are enriched at the valleys to form a hydrophilic sustained-release layer.

[0032] 2. Dynamic crosslinker system (15-25 parts): two-component synergistic crosslinker: Component A: N-hydroxymethyl acrylamide encapsulated by cyclodextrin (encapsulation rate 75-85%), which slowly releases cross-linking groups when the humidity is ≥60% to achieve delayed cross-linking in the valley area.

[0033] Component B: Nano-confined isocyanate prepolymer (particle size 20-40nm, loaded in mesoporous silica channels), which triggers rapid crosslinking at temperatures ≥40°C, preferentially forming initial crosslinking points at the peaks.

[0034] 3. Reconfigurable porous filler (25-35 parts): hollow mesoporous silica microspheres (particle size 1-5 μm, pore diameter 5-10 nm), surface-grafted with temperature-responsive PNIPAM polymer brushes, which undergo a volume phase transition at 32°C (swelling ratio 40-60%).

[0035] Bionic lignin fibers (50-100 μm in length) were modified with a 5% tannic acid aqueous solution (pH = 4.5) at 60 ° C for 2 hours. After modification, they have pH-responsive polyphenolic hydroxyl groups (polyphenolic hydroxyl groups are activated at pH = 4-6) and can form a dynamic hydrogen bond network with the resin matrix.

[0036] 4. Coating microstructure: A "double-continuous gradient network" forms on the surface of the corrugated core paper: the peak region is dominated by hydrophobic fluorocarbon chains, with a high cross-linking density (1.2-1.5 mol / m³) and low porosity (15-20%); the valley region is dominated by hydrophilic PEG chains, with a low cross-linking density (0.8-1.0 mol / m³) and high porosity (30-35%). The two regions form cross-regional stress transfer channels through lignin fibers. This structure can be referenced by research results on the microstructure of gradient composites in the Journal of Composite Materials. Its cross-regional stress transfer channels can effectively balance the shrinkage stress between the peak and valley regions. 2. Preparation method (waveform adaptation cross-linking-dynamic mass transfer control process): 1. Surface energy gradient pretreatment of core paper: Immerse the corrugated core paper in an ethanol solution containing 0.5-1.0% chitosan quaternary ammonium salt (pH = 5.5-6.5), and treat it with 30-50kHz radio frequency plasma for 5-10 minutes to introduce amino groups in the peak area (surface energy 55-60mN / m), and retain the original cellulose surface in the valley area (surface energy 45-50mN / m), forming a natural surface energy gradient ( =10-15mN / m).

[0037] 2. Waveform-induced self-assembly coating: using electrostatic atomization-airflow guided coating technology: 2.1. The atomizing nozzle applies an electrostatic voltage of 8-12kV to make the coating droplets (particle size 50-100μm) negatively charged; 2.2. In conjunction with the waveform synchronous airflow field (wind speed at the peak is 2-3m / s, wind speed at the valley is 0.5-1m / s), the surface energy difference is used to drive the droplets to selectively deposit on the waveform surface: 2.2.1. Peak (high surface energy): The droplets spread rapidly, and the fluorocarbon segments are preferentially adsorbed to form a dense layer.

[0038] 2.2.1. Valley (low surface energy): The droplets form a resident liquid pool, and the PEG chain segments are enriched and wrap the hollow microspheres.

[0039] The electrostatic voltage is adjusted based on the flute height of the core paper. For example, for Type A flute heights of 4.5-5.0mm, a 1kV increase in voltage reduces the droplet size by 10-15μm, ensuring proper deposition of droplets at both flute peaks and flute valleys. Adjusting the wind speed ratio to 2:1 reduces the flute valley coating thickness by 15%, but reduces the crosslink density uniformity by 5%. This trade-off must be made based on actual production needs.

[0040] In order to clarify the influence of electrostatic atomization voltage and wind speed ratio on coating performance, an orthogonal experiment was designed as follows. The data showed that parameter adjustment needs to balance coating thickness and cross-linking uniformity, see the following table (Table 1): Table 1: 3. Segmented dynamic cross-linking drying: Entering the variable gradient cross-linking drying chamber, it is regulated in three stages: 3.1. Humidity-triggered pre-crosslinking (25-30°C, RH=70-80%, 15-20min): The high humidity environment in the valley area prompts cyclodextrin to release N-hydroxymethyl acrylamide, which undergoes hydroxyl crosslinking with the PEG chain segments to form an initial hydrogel network (storage modulus 10-20kPa), fixing the valley coating morphology.

[0041] 3.2. Temperature gradient main cross-linking (40-50℃, peak / valley temperature difference 8-12℃, 25-30min): Nano-confined isocyanate in the peak region breaks through the mesoporous limitation and rapidly cross-links with the fluorocarbon segments (cross-linking degree increase rate 2.5-3.0% / min). At the same time, the hollow microsphere PNIPAM brush shrinks and closes the pores, reducing the resistance to water evaporation. Due to the lower temperature in the valley region, the cross-linking rate slows down (cross-linking degree increase rate 1.0-1.5% / min), and the hollow microspheres remain swollen, forming a water release channel.

[0042] 3.3. Stress self-balancing cooling (RT, RH=30-40%, 10-15min): The dynamic hydrogen bond network of lignin fibers is utilized to capture the coating shrinkage stress in real time (stress relief efficiency 60-70%). At the same time, the PEG chain segments compensate for the volume shrinkage of the valley area by swelling through moisture absorption (linear shrinkage rate ≤1.5%).

[0043] It is necessary to add that: 1. Deconstructing geometric induction effects: 1. Surface energy gradient-guided self-assembly: Abandoning the traditional idea of ​​"artificially controlling filler distribution", the surface energy difference of the natural waveform of corrugated core paper is utilized (enhanced by plasma treatment) to drive the spontaneous orientation of amphiphilic resin molecular chains, forming a functionally differentiated natural gradient structure at the peaks / valleys. This eliminates the need for complex differentiated coating equipment and adapts the waveform geometric characteristics from the source of material design.

[0044] 2. Reconfigurable pore stress relief mechanism: Introducing a synergistic system of thermosensitive hollow microspheres and bionic lignin fibers: 2.1. Initial stage of drying (high temperature stage): The microspheres shrink and open the pores, accelerating the evaporation of water from the peaks.

[0045] 2.2. Late drying stage (cooling stage): The microspheres swell and close the pores, inhibiting the secondary swelling of the valleys.

[0046] 2.3. Lignin fiber acts as a "nano-spring" to buffer shrinkage stress in real time (the measured stress concentration factor is reduced by 40-50%), fundamentally eliminating the conditions for microcrack initiation.

[0047] 2. A disruptive design that reconstructs the laws of heat and mass transfer: 1. Dual-responsive crosslinker with spatiotemporal mismatch crosslinking: Cyclodextrin inclusion and nano-confinement technology realize the "humidity-temperature dual triggering" of the crosslinker: 1.1. Peak (high temperature, low humidity): Nano-confined isocyanate quickly cross-links to form early strength support.

[0048] 1.2. Lenggu (low temperature, high humidity): Cyclodextrin slow-release crosslinker delays the reaction and avoids high temperature decomposition.

[0049] This time-space mismatch cross-linking increases the effective cross-linking density by 30-40% (the traditional method causes a 20-30% loss in cross-linking degree due to high-temperature decomposition), and the waterproof durability is maintained at ≥90% after 50 wet-heat cycles.

[0050] 2. Active regulation network of mass transfer path: The amphiphilic resin matrix forms a "hydrophobic fast-drying channel-hydrophilic slow-release network" on the corrugated surface: Lengfeng's fluorocarbon chain segments create channels for rapid water evaporation (increasing the evaporation rate by 20-25%), but avoid excessive densification through the dynamic pores of hollow microspheres.

[0051] The PEG chain segments in Lenggu form a moisture buffer layer, which, combined with delayed cross-linking to prevent secondary swelling, increases the uniformity of moisture migration rate in the middle layer of the core paper by 60-70% (the traditional method of migrating against the concentration gradient results in a Lenggu moisture accumulation rate ≥30%).

[0052] Further explanation is needed: 1. Substantially reduced equipment costs: There is no need for a multi-channel zoned temperature-controlled oven. Only surface energy gradient pretreatment + electrostatic atomization airflow guidance is required to achieve self-assembly of the coating on the corrugated surface. The equipment modification cost is reduced by 70-80% (the modification cost of ordinary electrostatic atomization equipment is ≤ 500,000 yuan, which is 70% lower than that of traditional multi-channel ovens). It is particularly suitable for the production line transformation of small and medium-sized enterprises. For details, please refer to the production line transformation case reported in "China Papermaking" in 2024.

[0053] 2. Cross-dimensional improvement of material performance: Dynamic waterproof adjustment: When the ambient humidity is ≥85%, the PEG chain segments absorb moisture and swell, increasing the coating porosity from 20% to 35%, allowing trace amounts of water to penetrate evenly (avoiding local swelling damage); when the humidity is ≤60%, the fluorocarbon segments are enriched to form a hydrophobic barrier (contact angle ≥125°).

[0054] The changes in coating porosity and contact angle under different humidity environments are as follows, reflecting the effectiveness of the dynamic waterproofing adjustment mechanism, see the following table (Table 2): Table 2: Self-repairing microstructure: The polyphenolic hydroxyl groups of lignin fibers form reversible hydrogen bonds with the resin matrix. When the local stress exceeds 1.5 MPa, the hydrogen bonds break and absorb energy. After the stress is reduced, they rebond to achieve self-repair of microcracks (repair efficiency ≥ 80%).

[0055] The self-repairing ability of microcracks was verified by stress loading experiments. The data show that the coating of the present invention has a significant self-repairing effect, see the following table (Table 3): Table 3: Example 1: High-strength waterproof corrugated cardboard for cold chain logistics: 1. Coating formula (parts by weight): 50 parts of PLA-PEG grafted fluoroacrylic resin, 10 parts of cyclodextrin inclusion N-hydroxymethyl acrylamide, 10 parts of nano-confined isocyanate, 20 parts of hollow mesoporous silica microspheres, 15 parts of tannic acid modified lignin fiber, and 5 parts of additives.

[0056] 2. Key parameters of preparation: Core paper pretreatment: 0.8% chitosan quaternary ammonium salt solution treatment, radio frequency plasma power 80W, treatment time 8min.

[0057] Electrostatic atomization: voltage 10kV, atomization pressure 0.4MPa, with an airflow field of peak / valley wind speed ratio of 3:1.

[0058] Drying process: Humidity pre-crosslinking: 28°C, RH=75%, 18 min.

[0059] Temperature main cross-linking: peak 45℃ / valley 35℃, temperature difference 10℃, 28min.

[0060] Stress cooling: 25℃, RH=35%, 12min.

[0061] 3. Performance test (test standard: water absorption refers to GB / T1034-2008, sample size n=50, statistical method uses 95% confidence interval; micro crack detection uses 100x optical microscope, and 50 samples are tested): Water absorption rate (25℃, RH=95%): 7.2% (traditional method 15.2%).

[0062] Drying shrinkage: Peak 1.2% / Valley 1.3% (difference rate of traditional method ≥30%).

[0063] Crosslink density: Peak 1.4 mol / m³ / Valley 1.1 mol / m³ (uniformity improved by 65%, as tested by dynamic mechanical analysis (DMA)).

[0064] Microcrack incidence: 0% (traditional method ≥20%).

[0065] Compressive strength retention rate (50 wet heat cycles): 92% (65% for traditional methods).

[0066] Example 2: High-durability corrugated cardboard for long-term storage in a humid environment: 1. Coating formula (parts by weight): 45 parts of PLA-PEG grafted fluoroacrylic resin, 12 parts of cyclodextrin inclusion N-hydroxymethyl acrylamide, 13 parts of nano-confined isocyanate, 25 parts of hollow mesoporous silica microspheres, 10 parts of tannic acid modified lignin fiber, and 5 parts of additives.

[0067] 2. Key parameters of preparation: Core paper pretreatment: 1.0% chitosan quaternary ammonium salt solution, plasma treatment for 10 min.

[0068] Electrostatic atomization: voltage 12kV, wind speed ratio 4:1, achieving a 20% increase in coating thickness in the valley area.

[0069] Drying process: Humidity pre-crosslinking: 30℃, RH=80%, 20min.

[0070] Temperature main cross-linking: peak 50℃ / valley 38℃, temperature difference 12℃, 30min.

[0071] Stress cooling: 22℃, RH=30%, 15min.

[0072] 3. Performance testing: Waterproofing time (continuous humid environment): ≥90 days (30 days for traditional methods).

[0073] Temperature range: -10℃~60℃ (traditional thermosetting resin ≤50℃).

[0074] Adhesion (valley area): Grade 5B (test standard ISO2409:2013, traditional UV-cured coating ≤3B).

[0075] Self-repairing ability: 95% of 0.1mm micro-cracks can be repaired within 24 hours.

[0076] Example 3: Verification of the endpoint value of the resin matrix (40 parts of the resin matrix, 15 parts of the cross-linking agent).

[0077] 1. Coating formula (parts by weight): 40 parts of PLA-PEG grafted fluoroacrylic resin, 8 parts of cyclodextrin inclusion N-hydroxymethyl acrylamide, 7 parts of nano-confined isocyanate, 30 parts of hollow mesoporous silica microspheres, 15 parts of tannic acid modified lignin fiber, and 3 parts of additives.

[0078] 2. Performance testing: Water absorption rate (25℃, RH=95%): 8.5%.

[0079] Crosslink density uniformity: Peak / valley crosslink density difference rate is 12% (detected by nuclear magnetic resonance (NMR)).

[0080] Contact angle (peak): 128°, critical micelle concentration 1.0g / L (in line with the scope of claims).

[0081] Example 4: Verification of the endpoint value of the resin matrix (60 parts of the resin matrix, 25 parts of the cross-linking agent).

[0082] 1. Coating formula (parts by weight): 60 parts of PLA-PEG grafted fluoroacrylic resin, 15 parts of cyclodextrin inclusion N-hydroxymethyl acrylamide, 10 parts of nano-confined isocyanate, 20 parts of hollow mesoporous silica microspheres, 10 parts of tannic acid modified lignin fiber, and 5 parts of additives.

[0083] 2. Performance testing: Drying shrinkage: Peak 1.5% / Valley 1.4% (difference rate ≤15%).

[0084] Microcrack incidence: 0%.

[0085] Cross-link density: Peak 1.5mol / m³ / Valley 1.0mol / m³ (good uniformity).

[0086] In order to verify the extreme applicability of the core component ratio in the claims, the following endpoint value verification experiment was designed. The data showed that the performance stability can still be maintained under extreme conditions. See the following table (Table 4): Table 4: For comparison with existing technologies, see the following table (Table 5): Table 5: Summarize: This invention addresses core issues such as uneven drying shrinkage, cross-linking failure, and micro-cracks caused by the coupling of corrugated cardboard's wave geometry and heat and mass transfer in cold chain logistics and humid environments. It breaks through the traditional technical formula of "uniform drying of flat coatings" and constructs a bionic gradient-responsive coating system for the first time, achieving performance breakthroughs through multi-dimensional innovation: 1. Targeted solutions: 1. Corrugated surface energy gradient drives coating self-assembly: Through pretreatment with chitosan quaternary ammonium salt solution combined with radio frequency plasma technology, a surface energy difference of 10-15 mN / m is created between the peaks and valleys of the corrugated core paper (55-60 mN / m at the peaks and 45-50 mN / m at the valleys). This drives the spontaneous orientation of the amphiphilic resin molecular chains (PLA-PEG grafted fluoroacrylate). The peaks are enriched with hydrophobic fluorocarbon segments to form a fast-drying layer (contact angle above 125°), while the valleys are enriched with hydrophilic PEG segments to form a sustained-release layer. This design adapts the material to the microenvironment of the corrugated surface from the source, solving the problem of differentiated mass transfer paths caused by differences in corrugated curvature in traditional homogeneous coatings. It also avoids the shrinkage asynchrony caused by rapid drying and densification at the peaks and delayed swelling in the valleys.

[0087] 2. Humidity-Temperature Dual-Response Dynamic Crosslinking Mechanism: Utilizing a two-component system consisting of cyclodextrin-encapsulated N-methylol acrylamide (slow-release crosslinking groups at humidity ≥ 60%) and nano-confined isocyanate prepolymer (rapid crosslinking triggered at temperature ≥ 40°C), this system achieves a temporally and spatially mismatched release of the crosslinker at peaks (high temperature, low humidity) and valleys (low temperature, high humidity). Compared to traditional methods, which experience a 20-30% loss in crosslinking due to high-temperature decomposition, this mechanism increases the effective crosslink density by 30-40%, addressing the issue of decreased waterproofing durability caused by imbalanced crosslinking dynamics. Performance retention is ≥ 90% after 50 humidity-heat cycles (compared to 65% with traditional methods).

[0088] 3. Reconfigurable pore stress relief and mass transfer control network: Introducing thermosensitive hollow mesoporous silica microspheres (volume phase change swelling ratio of 40-60% at 32°C) and pH-responsive lignin fibers (polyphenol hydroxyl groups activated at pH 4-6 after tannic acid modification): In the early stage of drying, the microspheres shrink and open the pores to accelerate the evaporation of water from the peaks (the evaporation rate increases by 25%), and in the later stage, they swell and close the pores to inhibit the secondary swelling of the valleys; the lignin fibers act as "nanosprings" to buffer stress in real time, reducing the measured stress concentration coefficient by 50%, and completely eliminating the risk of microcracks caused by nanoscale pores (the microcrack incidence rate of traditional methods is ≥20%, while that of the present invention is reduced to 0%).

[0089] 4. Dynamic waterproof adjustment and environmental adaptive performance: The porosity of the coating changes dynamically with humidity: When the humidity is ≤60%, the fluorocarbon segments are enriched to form a hydrophobic barrier (contact angle ≥125°), and the water absorption rate is only 7.2% (conventional 15.2%); when the humidity is ≥85%, the PEG segments absorb moisture and swell, increasing the porosity from 20% to 35%, allowing water to penetrate evenly (penetration rate 8-10g・m -2 ・h -1 ), avoiding localized swelling damage. This humidity-triggered, bidirectional response mechanism breaks through the limitations of traditional waterproof coatings' single hydrophobic barrier, achieving environmentally adaptive waterproofing.

[0090] 5. Microcrack Self-Healing and Wide-Temperature Stability: The polyphenolic hydroxyl groups of lignin fibers form a reversible hydrogen bond network with the resin matrix. When the local stress exceeds 1.5 MPa, the hydrogen bonds break, absorbing energy. Rebonding occurs after the stress is reduced. The 24-hour repair rate for 0.1 mm microcracks is ≥ 95%, and the compressive strength retention rate after repair is 98%. Furthermore, the coating's temperature resistance is extended to -10°C to 60°C (compared to ≤50°C for traditional thermosetting resins), addressing the challenges of low-temperature brittle cracking and high-temperature adhesion associated with traditional materials, meeting the dual requirements of cold chain and high-temperature warehousing.

[0091] 6. Substantially Reduced Industrial Costs: Eliminating traditional multi-channel, zoned temperature-controlled ovens, this technology achieves self-assembly of a corrugated surface coating solely through surface energy gradient pretreatment (cost ≤ 100,000 RMB) and electrostatic atomization airflow-guided coating (equipment modification cost ≤ 500,000 RMB). This reduces equipment costs by 70-80% compared to traditional solutions. Small and medium-sized enterprises (SMEs) do not need to significantly restructure their production lines, breaking the bottleneck of existing technologies' reliance on high-end equipment.

[0092] Existing technologies attempt to mitigate uneven drying through homogeneous coating or zoned temperature control, but fail to address the fundamental contradiction of geometric-thermal coupling. This invention, however, leverages a triple mechanism—wave-shaped surface energy-induced self-assembly, dual-responsive crosslinking with spatiotemporal mismatch, and reconfigurable pore dynamics—to transform the geometric imperfections of corrugated board into functional advantages. Experimental data demonstrates that this invention achieves cross-dimensional improvements in key metrics such as shrinkage differential (≤15% vs. conventional ≥30%), moisture migration uniformity (improved by 60-70%), and equipment cost (≤30% of conventional solutions), providing a new technological path for the application of corrugated board in high-end waterproofing.

Claims

1. A corrugated cardboard based on a waterproof coating, characterized in that: The invention comprises a corrugated core paper and a bionic gradient responsive waterproof coating coated on the surface of the corrugated core paper; the bionic gradient responsive waterproof coating is composed of a waveform-adaptive resin matrix, a dynamic cross-linking agent system and a reconfigurable pore filler, wherein: The waveform-adaptive resin matrix is ​​a polylactic acid-polyethylene glycol block copolymer PLA-PEG grafted with dodecafluoroheptyl methacrylate, and the molecular chain contains a hydrophobic fluorocarbon segment and a hydrophilic PEG segment; The dynamic crosslinker system includes cyclodextrin-encapsulated N-hydroxymethyl acrylamide (component A) and nano-confined isocyanate prepolymer (component B); Reconfigurable pore fillers include hollow mesoporous silica microspheres and biomimetic lignin fibers; The coating forms a double continuous gradient network on the surface of the corrugated core paper: the corrugation peak area is mainly composed of hydrophobic fluorocarbon segments with a cross-linking density of 1.2-1.5 mol / m³ and a porosity of 15-20%; the corrugation valley area is mainly composed of hydrophilic PEG segments with a cross-linking density of 0.8-1.0 mol / m³ and a porosity of 30-35%. The two form a cross-regional stress transfer channel through lignin fibers.

2. The corrugated paperboard based on a waterproof coating according to claim 1, characterized in that: The molar ratio of polylactic acid-polyethylene glycol block copolymer PLA-PEG to dodecafluoroheptyl methacrylate is 1:1.2, the contact angle of the graft copolymer is 110°-130°, and the critical micelle concentration is 0.8-1.2 g / L.

3. The corrugated paperboard based on waterproof coating according to claim 1, characterized in that: The inclusion rate of cyclodextrin-encapsulated N-hydroxymethyl acrylamide is 75-85%, and the particle size of the nano-confined isocyanate prepolymer is 20-40 nm, which is loaded in the mesoporous silica pores.

4. The corrugated paperboard based on a waterproof coating according to claim 1, characterized in that: The particle size of the hollow mesoporous silica microspheres is 1-5 μm, the pore diameter is 5-10 nm, and the surface is grafted with temperature-responsive PNIPAM polymer brushes, which undergo a volume phase change at 32°C and a swelling rate of 40-60%. The bionic lignin fibers are modified with a 5% tannic acid aqueous solution at 60°C for 2 hours. After modification, they have pH-responsive polyphenolic hydroxyl groups, which are activated at pH = 4-6.

5. A method for preparing corrugated cardboard based on a waterproof coating, characterized in that: The following steps are involved: Surface energy gradient pretreatment of core paper: The corrugated core paper is immersed in an ethanol solution containing 0.5-1.0% chitosan quaternary ammonium salt and treated with 30-50kHz radio frequency plasma for 5-10 minutes to introduce amino groups into the flute peak area while retaining the original cellulose surface in the flute valley area, forming a corrugated core paper with a surface energy gradient of 10-15mN / m; Waveform-induced self-assembly coating: Using electrostatic atomization-airflow guided coating technology, the atomizing nozzle applies an 8-12kV electrostatic voltage to make the coating droplets negatively charged. In conjunction with the waveform-synchronized airflow field, the wind speed at the peak is 2-3m / s, and the wind speed at the valley is 0.5-1m / s. The surface energy difference is used to drive the droplets to selectively deposit on the waveform surface. Segmented dynamic cross-linking drying: Entering the variable gradient cross-linking drying chamber, it is regulated in three stages, including humidity-triggered pre-cross-linking, temperature gradient main cross-linking and stress self-balancing cooling.

6. The method for preparing a corrugated paperboard based on a waterproof coating according to claim 5, characterized in that: Electrostatic atomization-airflow guided coating technology, the electrostatic voltage is adjusted according to the core paper flute height. When the A-type flute height is 4.5-5.0mm, the droplet size decreases by 10-15μm for every 1kV increase in voltage; when the wind speed ratio is adjusted to 2:1-4:

1.

7. The method for preparing a corrugated paperboard based on a waterproof coating according to claim 5, characterized in that: The conditions for the humidity-triggered pre-crosslinking stage are: 25-30°C, RH=70-80%, and a treatment time of 15-20 min. The high humidity environment in the Lenggu area is used to induce cyclodextrin to release N-hydroxymethyl acrylamide, which undergoes hydroxyl crosslinking with the PEG chain segments to form an initial hydrogel network.

8. The method for preparing a corrugated paperboard based on a waterproof coating according to claim 5, characterized in that: The conditions for the main cross-linking stage of the temperature gradient are: 40-50°C, a peak / valley temperature difference of 8-12°C, and a treatment time of 25-30 min. The nano-confined isocyanate in the peak region breaks through the mesopore restrictions and quickly cross-links with the fluorocarbon segments. At the same time, the hollow microsphere PNIPAM brush shrinks and closes the pores, reducing the resistance to water evaporation. Due to the lower temperature in the valley region, the cross-linking rate slows down, and the hollow microspheres remain swollen, forming a water release channel.

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